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Adolescent Bariatric Surgery

ABSTRACT

 

The prevalence of adolescent obesity has rapidly increased over the past several decades. With this increase, there has also been a rise in the prevalence of complications of obesity leading to premature mortality. While lifestyle and medical management remain a part of the initial management of obesity, these therapies have been shown to be inferior when compared to metabolic and bariatric surgery (MBS) for adolescents with severe obesity. A multidisciplinary approach is recommended to evaluate medically eligible candidates for MBS, prepare patients for surgery, and guide postoperative management. Laparoscopic sleeve gastrectomy (SG) and Roux-en-Y gastric bypass (RYGB) are the most common MBS procedures performed in both adolescent and adult patients. Postoperative hospital stays are generally short and long-term routine follow-up with the MBS team is recommended to monitor weight loss, resolution of complications of obesity, and to monitor for postoperative complications. Most adolescent MBS studies demonstrate average percent body mass index loss between 25-29% after surgery. This is also associated with resolution or improvement of most complications of obesity at rates that are similar or superior to adult studies. Resolution and prevention of type 2 diabetes mellitus (T2DM) after MBS is a particularly compelling reason to pursue surgical treatment due to the complications from T2DM that occur over a patient’s lifetime as well as the overall burden of health-related costs. These adverse consequences of T2DM can be mitigated by early use of MBS. MBS is generally well tolerated. Complication rates are similar to adult patients therefore it is recommended to refer patients for MBS whenever they are medically qualified. Most common short-term (<30 days) complications include leak, bleeding, and surgical site infections. Most common long-term (>30 days) complications are nutritional deficiencies.

 

INTRODUCTION

 

The prevalence of worldwide overweight and obesity in adolescents has more than quadrupled since 1975. Currently, it is estimated that over 14 million children age 2-19 years suffer from obesity in the United States alone (1, 2). Adolescents with obesity are at risk for developing significant comorbidities including insulin resistance, type 2 diabetes mellitus (T2DM), hypertension, dyslipidemia, obstructive sleep apnea, nonalcoholic fatty liver disease, depression, polycystic ovarian syndrome, impaired quality of life, cardiovascular disease, and longer term, certain malignancies (3-9). Similar to obesity, the prevalence of T2DM has been increasing dramatically (3). Obesity is a major risk factor the development of T2DM with overweight adolescents having close to a three times greater risk of developing T2DM when compared to adolescents with normal weight (10-12). Additionally, obesity in adolescence is associated with persistent obesity into adulthood, increased risk for obesity related comorbidities, and premature mortality in adulthood (13-15). Lifestyle and medical management remain the first-line treatment for adolescent obesity; however, current evidence suggests that pharmacotherapy, dietary, and behavioral modifications rarely lead to long-term weight loss in adolescents with severe obesity (16-18). The use of metabolic and bariatric surgery (MBS) in adolescents with severe obesity and complications of obesity has been shown to have superior results in both efficacy and durability (19).

 

PREOPERATIVE EVALUATION

 

Multidisciplinary Program

 

A multidisciplinary approach is recommended when considering MBS for an adolescent (20, 21). At a minimum, this includes a bariatric surgeon with adolescent experience, pediatrician, dietitian, nurse, and pediatric psychologist. It is also important the core providers have access to additional pediatric specialists including anesthesiologists, radiologists, and appropriate specialists to aid the management of complications of obesity (e.g., pulmonology, endocrinology, gastroenterology/hepatology). Adolescents undergoing preoperative work-up should be evaluated for the presence and severity of complications of obesity. Additionally, it is important for the multidisciplinary team to determine a potential patient and caregivers’ ability to assess the risks and benefits of surgery as well as to adhere to postoperative requirements including daily vitamin regimens and attending postoperative visits.

 

Patient Selection

 

BODY MASS INDEX (BMI)

 

The following criteria have been recommended by multiple panels of experts for consideration of weight loss surgery in adolescents under 18 years old: (4, 19)

  • BMI ≥ 120 percent of the 95th percentile for BMI for age or BMI ≥ 35kg/m2, whichever is lower, with complications of obesity that has a significant effect on health (Table 1).
  • OR -
  • BMI ≥ 140 percent of the 95th percentile of BMI for age or BMI ≥ 40 kg/m2, whichever is lower

 

Table 1. Qualifying Comorbidities for Consideration of MBS in Adolescents (4).

Obstructive sleep apnea (apnea-hypoxia index > 5)

Type 2 diabetes mellitus

Idiopathic intracranial hypertension

Nonalcoholic steatohepatitis

Blount’s disease

Slipped capital femoral epiphysis

Gastroesophageal Reflux Disease

Hypertension

 

CONTRAINDICATIONS

 

Contraindications to adolescent MBS are listed in Table 2.

 

Table 2. Contraindications to Adolescent MBS

Medically correctable cause of obesity

Ongoing substance abuse problem (within the preceding year)

Medical, psychiatric, psychosocial, or cognitive condition that prevents adherence to postoperative dietary and medication regimens or impairs decisional capacity

Current or planned pregnancy within 18 months of the procedure

Inability for patient or caregivers to comprehend risks and benefits of surgical weight loss procedure

 

AGE

 

A recent retrospective review of the Metabolic and Bariatric Surgery Accreditation and Quality Improvement Program (MBSAQIP) data registry from 2015 to 2018 demonstrated that adolescents and young adults only represented 3.7% of total MBS cases performed suggesting significant underutilization within this population (22). Multiple studies have evaluated the safety and efficacy of MBS in younger adolescents. Current evidence suggests there are no significant clinical differences in outcomes between MBS in younger (e.g., <16 years) versus older adolescents (e.g., ≥16 years)(23-27). It is therefore not recommended to limit access to MBS based on patient’s age, physical maturity (e.g., bone age), or pubertal status. These findings have prompted increase advocacy for the use of MBS in the adolescent population by the American Academy of Pediatrics (19).

 

TYPES OF SURGERY

 

Sleeve Gastrectomy

 

A laparoscopic sleeve gastrectomy (SG) results in the removal of the greater curvature of the stomach resulting in a smaller, tubular stomach that has a reduced capacity (Figure 1). Given the procedure is less complex than the Roux-en-Y gastric bypass (RYGB) and has less risk for micronutrient deficiencies, it is an appealing option for adolescents. SG currently accounts for approximately 80% of bariatric procedures in adolescents (22, 28-30). A sleeve gastrectomy may also be converted to RYGB in the event additional MBS is indicated or in the setting of postoperative medically refractory gastroesophageal reflux disease (GERD).

Figure 1. Sleeve Gastrectomy

Roux-en-Y Gastric Bypass

 

Laparoscopic Roux-en-Y gastric bypass involves creating a small, proximal gastric pouch which is separated from the remnant stomach and anastomosed to a Roux-limb of small bowel 70-150cm distally (Figure 2). The RYGB results in similar weight loss when compared to SG and dramatically improves glycemic control (29, 31). The incidence of postoperative GERD is significantly less following RYGB compared to SG, making the procedure an attractive option for adolescents with GERD at baseline (32).

Figure 2. Roux-en-Y Gastric Bypass

 

Others

 

Additional procedures including intragastric balloons are not currently approved by the United States Food and Drug Administration (FDA) for use in adolescents. Adjustable gastric bands have been previously used in the adolescent population; however, they have fallen out of favor due inferior efficacy compared to SG and RYGB (33).

 

POSTOPERATIVE MANAGEMENT

 

Inpatient  

 

Average inpatient stay is typically 1-3 days following both a SG and RYGB (34, 35). Patients are monitored for immediate postoperative complications including a leak, bleeding, and venous thromboembolism (VTE). Following discharge, patients are seen at regular postoperative visits to monitor body weight, nutritional status, and to manage complications of obesity.

 

Diet

 

Following a SG or RYGB, patients are gradually progressed from a high protein liquid diet to incorporating small volumes of regular food. Patients are encouraged to eat three to four protein-rich meals a day while avoiding carbohydrate rich foods. Supplemental sugar-free fluids between meals are also essential following surgery in order to avoid dehydration. Patients are typically encouraged to avoid excessive fluids with meals in order to minimize nausea and maximize nutritional intake with meals due to the restrictive component of both procedures.

 

Postoperative nausea is not uncommon following surgery but typically self resolves. Meals high in carbohydrates or sugar can result in dumping syndrome or weight regain following surgery. Some providers recommend limiting carbonated or caffeinated beverages following MBS based on theoretical concerns, however there is minimal evidence to support this apprehension. Similar to non-operative weight loss recommendations, general recommendations including exercising for 30 to 60 minutes daily, drinking sugar-free fluids, and portion-controlled protein rich meals are the same. Overall, it is recommended that patient and caregiver meet with a dietitian prior to discharge to develop a plan tailored to patient’s specific nutritional needs. Regular follow-up visits with a dietitian are also recommended to assist with postoperative weight management and to monitor for nutritional deficiencies.

 

Nutritional Supplements and Monitoring

 

Although SG may be associated with a decreased risk of nutritional deficiencies when compared to RYGB, lifelong supplementation with vitamins and minerals is recommended following both operations (Table 3). Patients are particularly at risk for deficiencies in iron, vitamin B12, and vitamin D. Additionally, lifelong annual monitoring of nutritional and micronutrient status is recommended with annual laboratory testing (Table 3). Adjustments in supplements may need to be made over time as specific deficiencies emerge. 

 

Table 3. Nutritional Supplementation and Monitoring Recommendations (36)

Nutritional Supplements

Standard multivitamin with folate or iron, or prenatal vitamin if female (once or twice daily)

Vitamin B12, 500mcg sublingually daily, or 1000mcg intramuscularly month

Calcium, 1200 to 1500mg daily (measured as elemental calcium) with 800 to 1000 international units of vitamin D.

Annual Nutritional Monitoring

Complete blood cell count with differential

Serum iron and ferritin

Red blood cell folate, serum vitamin B12, and serum homocysteine

Serum thiamin (vitamin B1)

Hepatic panel (including albumin, total protein, serum aminotransferase levels, gamma-glutamyl transpeptidase, and alkaline phosphatase

Calcium, 25-hydroxyvitamin D, and parathyroid hormone

Dual-energy x-ray absorptiometry (DXA) scan to monitor bone density (optimal frequency not yet established)

 

Pregnancy Prevention

 

Pregnancy should be avoided for 12 to 18 months following MBS to allow patients to achieve weight maintenance and to avoid potential micronutrient deficiencies which may affect both patient and fetus (37). Obesity can result in decreased fertility secondary to irregular menstruation and ovulatory dysfunction (38, 39). Weight loss after MBS has been shown to result in more regular ovulation and improved fertility (40, 41). In a retrospective review of 47 adolescents who underwent MBS surgery, seven pregnancies occurred, six of them within 10 to 22 months following surgery (42). While all six deliveries were healthy and at term, the twofold higher than anticipated pregnancy rate highlights the need for contraception counseling following MBS.

 

Multiple studies have evaluated the efficacy of hormonal contraceptive methods in patients with elevated BMIs and no definitive association was found between higher BMI and effectiveness of hormonal contraceptives (43). Due to concern for malabsorption after intestinal bypass procedures and the subsequent potential for decreased oral contraceptive efficacy, the American College of Obstetricians and Gynecologist recommend using non-oral forms of hormonal contraception in patients who have undergone malabsorptive MBS (44). Additionally, oral contraceptives are associated with increased risk of venous thromboembolism (VTE) which may be worrisome for adolescents with elevated BMIs who already have a higher predisposition for VTE (45, 46).

 

Intrauterine devices (IUDs) are an appealing option following MBS in adolescent patients as they are one of the most effective contraception methods, do not increase risk of VTE, and can be placed at the time of surgery (47). Levonorgestrel-releasing IUDs have the added benefit of promoting amenorrhea which could help reduce the risk of iron deficiency anemia following surgery (48). Regardless of the form of contraception selection, adolescents should be counseled on safe sex practices including the use of barrier protection against sexually transmitted infections.

 

Adolescent patients who become pregnant following MBS should be counseled on adequate nutritional intake with close monitoring of iron, folate, and vitamin B12 levels. Additionally, one must be cautions when screening for gestational diabetes in pregnant patients who have undergone MBS. In a study of a 119 post-bariatric surgery pregnant patients, oral glucose tolerance test resulted in hypoglycemia in 83% of patients with history of RYGB and 55% of patients with history of SG (49). Alternative methods for screening such as capillary blood glucose measurements are therefore recommended (50, 51).

 

Comorbidity Reassessment

 

Regular reassessment of complications of obesity should occur at routine intervals in the postoperative phase to monitor for resolution or need for continued management. Patients with T2DM should be evaluated by their endocrinologist every three months. Repeat polysomnography are generally obtained between three to six months after surgery for patients previously on continuous positive airway pressure therapy (52, 53). Twenty-four-hour blood pressure monitoring can also be repeated three months after surgery to demonstrate resolution or persistence of hypertension. Medication may be restarted if blood pressure is consistently ≥120 mmHg systolic or ≥80 mmHg diastolic. Patients with biopsy proven nonalcoholic fatty liver disease may be re-biopsied 12 months after surgery to document regression. Finally, patients’ mental health needs should be re-evaluated by a pediatric psychologist at 6 and 12 months after surgery.

 

In the setting of weight regain, patients should be monitored for complications of obesity. There is emerging evidence however, that some complications of obesity may be weight dependent and others non-weight dependent (54). Some surgeons will routinely obtain an upper gastrointestinal contrast study at 12 months after surgery or as needed to assess anatomy which may lead to weight regain. Anatomical abnormalities that may contribute to weight regain include a dilated gastric sleeve or gastrogastric fistula.

 

Follow Up

 

Close follow up with the multidisciplinary team including the bariatric surgeon, pediatrician, dietitian, and pediatric psychologist is strongly recommended. Patients are typically followed by a pediatrician to ensure ongoing continuity of care. It is important for the core providers to have access to pediatric specialists including endocrinology, gastroenterology/hepatology, and pulmonology as needed in those with complications of obesity that require ongoing monitoring or management. Additionally, a gynecologist for contraception counseling may be required for female patients. The transition from pediatric to adult medicine can be challenging in patients with chronic medical conditions and frequently requires assistance from multiple members of the team for transition care coordination and preparation as well as to ensure adequate communication, support, and education (55-57). 

 

OUTCOMES

 

Percent BMI Loss

 

Both SG and RYGB have resulted in clinically significant weight loss in adolescents. The efficacy of both procedures appears to be similar in the adolescent population with potentially slightly greater weight loss following RYBG. In a large, multicenter analysis of 177 adolescents who underwent RYGB and 306 adolescents who underwent SG, there was a three-year postoperative average percent BMI loss of -29% (95% CI, -26 to -33) and -25% (95% CI, -22 to -28) for RYGB and SG, respectively (29). Similar results were seen in the largest prospective study to date of 228 adolescents undergoing either RYGB or SG with an average 28% reduction in BMI at three years following RYGB compared to 26% reduction following SG (53). Smaller, long-term studies demonstrate the durability of weight loss with MBS with an average percent BMI loss of 29% in adolescents undergoing RYGB up to 12 years after surgery(58, 59). 

 

Complications of Obesity

 

TYPE 2 DIABETES MELLITUS  

 

Multiple studies have demonstrated improved glycemic control, even remission as well as prevention of T2DM following MBS, making a compelling case of MBS as a treatment for T2DM (31, 59-63). The Teen-Longitudinal Assessment of Bariatric Surgery (Teen-LABS) is a prospective, observational study of pediatric patients undergoing MBS at 5 children’s hospitals in the United States with 3 and 5 year of follow-up data published to date. Of the 242 adolescents with obesity who underwent MBS, 29 had T2DM. By 3 years after the procedure, remission of T2DM occurred in 95% (95% CI, 85-100) of participates who had diabetes at baseline with no new cases of T2DM in those without the condition at baseline (53). Additionally, 19 participants had prediabetes at baseline with a 76% (95% CI, 56-97) rate of remission at 3 years (53). These remission rates in Teen-LABS were compared to adults who underwent MBS. Among those who underwent RYGB, adolescents were more likely to have remission of T2DM at 5 years with a remission rate of 86% compared to 53% in adults (64).

 

Similar findings were demonstrated in another study of 226 adolescents undergoing SG, of which 23% of patients were found to have T2DM. Eighty-five percent of patients with T2DM were on medication for diabetes prior to surgery and 89% achieved normal fasting plasma glucose and hemoglobin A1c levels without the use of medication postoperatively (52).

 

In an effort to compare surgical versus medical therapy for T2DM in adolescents with severe obesity, data from participants with T2DM enrolled in the Teen-LABS study were compared to participants of similar age and racial distribution from the Treatment Options of Type 2 Diabetes in Adolescents and Youth (TODAY) studies. Teen-LABS participants underwent MBS. TODAY participants were randomized to metformin alone or in combination with rosiglitazone or intensive lifestyle intervention, with insulin therapy given for glycemic progression. At two years, mean hemoglobin A1c concentration decreased from 6.8% to 5.5% in patients who underwent MBS compared to an increase from 6.4% to 7.8% in those enrolled in the TODAY study. Compared to baseline, average BMI decreased by 29% in Teen-LABS participants while the average BMI increased by 3.7% in TODAY participants (65). Cardiovascular disease (CVD) risk reduction was also explored in a secondary analysis of this study and despite higher pretreatment risk for CVD, treatment with MBS resulted in reduction of estimated CVD that were sustained at 5-year follow-up where medical therapy was associated with an increase in risk of CVD in adolescents with T2DM and severe obesity (66).

 

While these initial results are promising of the beneficial effects of MBS for the treatment of T2DM, no studies have prospectively compared the efficacy of MBS with that of medical therapy for the treatment of T2DM in adolescents with obesity. Additionally, the majority of initial MBS data in adolescents were from those who underwent RYGB which is no longer the primary MBS procedure performed in adolescents due to its inferior safety profile. In 2019, the National Institute of Health funded the Surgical or Medical Treatment for Pediatric T2DM (ST2OMP) trial which will compare SG to advanced medical therapy (67, 68).

 

OTHER COMORBIDITIES

 

In the Teen-LABS study described above, a mean 74% (95% CI, 64 to 84) remission of hypertension (HTN), 66% (95% CI 57 to 74) remission of dyslipidemia, and 86% (95% CI 72 to 100) resolution of abnormal kidney function was found at 3 years (53). In a secondary analysis of Teen-LABS and TODAY data, medical management of adolescents with obesity was associated with higher odds of diabetic kidney disease when compared to MBS (69). Greater weight loss after MBS in adolescents has also been associated with greater remission of T2DM, HTN, and dyslipidemia (54, 70). In a comparison of adolescents and adults who underwent RYGB, adolescents were more likely to have remission of HTN at 5 years compared to adults (68% vs 41%) (64).

 

Additional studies have demonstrated a 66% to 84% remission of obstructive sleep apnea as well as improvements in liver disease and polycystic ovarian syndrome (8, 52, 71). Improvements in functional mobility as well as reduction in musculoskeletal pain have also been well described (72, 73).

 

Mental Health

 

Multiple studies have reported higher rates of depression, emotional and behavioral disorders, and suicidal ideation in adolescents with obesity (74-77). Additionally, binge and loss of control eating is prevalent among more than one quarter of adolescents with overweight and obesity (78, 79). A recent prospective study demonstrated that undergoing MBS in adolescence did not heighten or lower the risk of suicidal thoughts or behaviors following the initial 4 years after surgery (80). While still unclear whether obesity leads to psychopathology, or vice versa, the association highlights the need for appropriate psychological services in the pre- and postoperative period (74).

 

MBS can lead to improvements in psychosocial outcomes, although the improvements appear too often be transient. In the TEEN-Labs study, quality of life measured by the Impact of Weight on Quality of Life and Short Form 36 Health Survey improved after MBS (53, 73). Several studies have demonstrated improved depressive and anxiety symptoms in the months following MBS, although the results were not maintained after the first postoperative year (81, 82). In a multisite study assessing two year follow up of psychopathology prevalence in adolescents undergoing MBS, most patients retained their symptomatic or non-symptomatic psychopathology status at two years, although remission of symptoms was more prevalent than the development of new symptoms (83). These results emphasize the need for long-term psychosocial monitoring following MBS as well as early treatment in those with psychopathology

 

Short-Term Complications

 

Short-term complications (<30 days after surgery) in adolescents undergoing MBS are similar to those seen in adults. Early postoperative complications, though rare, include surgical site infections, bleeding, leak, strictures, and pulmonary embolism. In a retrospective review of 21,592 adolescents and young adults who underwent SG or RYGB between 2015 and 2018, 3.7% of patients required readmission, 1.1% of patients required reoperation, and 3.3% required percutaneous, endoscopic, or other intervention (22). Major complications were rare; the most common complication was bleeding (0.4%), followed by leak (0.4%), and deep surgical site infections (0.2%). RYGB was associated with higher rates of reoperation (2.1% vs. 0.8%), readmission (6.3% vs. 3.0%), and serious complications (5.5% vs. 1.8%) compared to SG. Mortality occurred in 0.05% of patients and there were no differences in mortality noted between groups (22). In an additional retrospective review of 483 adolescents (SG n=306, RYGB n=177) no perioperative deaths occurred and the rate of major adverse events were too rare for statistical comparison. VTEs occurred in only 0.4% of patients and failure to discharge in 30 days was observed in 0.7% of patients (29).

 

Multiple studies have also suggested that MBS may be safer in adolescents when compared with adults. In a large study evaluating perioperative outcomes of MBS between 309 adolescents and 55,192 adults, the overall 30-day complication rate was significantly lower in adolescents (5.5%) as compared with adults (9.8%). No in-hospital mortalities occurred in the adolescent group compared to 0.2% the adult group. The 30-day morbidity for adolescents following SG was zero compared to 4.3% following RYGB (84). In an additional study evaluating 1047 adolescents,10,429 college-aged individuals, and 24,841 young adults who underwent SG or RYGB, there were no differences in 30-day complication rates between age groups (85).

 

Long-Term Complications

 

NUTRITIONAL DEFICIENCIES  

 

Long-term complications after MBS in adolescents are primarily nutritional. Patients are particularly at risk for deficiencies in iron, vitamin B12, and vitamin D. Iron deficiency is common in premenopausal females due to menstruation. Some patients may require iron infusion if oral supplementation is not adequate. Symptomatic thiamine deficiency following MBS is rare, however can have serious consequences (86-88).These risks are higher for patients who undergo RYGB compared to SG due to potential malabsorption. In a Teen-LABS study evaluating nutritional deficiencies at 5 years postoperatively, low serum ferritin levels were seen in 71% of patients who underwent RYGB compared to 45% following a SG indicating iron deficiency (86). Iron deficiency anemia can occasionally be severe in adolescent women following MBS which can be compounded by menstruation and challenges in recognizing symptoms therefore daily supplementation and routine nutritional monitoring is essential following MBS.

 

Vitamin B12 deficiency was seen in approximately 12% of patients after either procedure. Approximately 40% of patients had low vitamin D levels at baseline with no significant change at follow up. Parathyroid hormone concentrations increased in patients undergoing RYGB from an average baseline concentration of 44 pg/ml to 59 pg/ml at 5 years with the risk of abnormal parathyroid hormone nearly sixfold higher after RYGB compared with SG (86). Elevated parathyroid hormone is utilized as a surrogate for calcium deficiency and raises concerns about long-term bone health. In adolescents, reduced bone mass has been noted two years after MBS although the bone mass remains appropriate for the patients’ age and new body weight (89). Concerns of growth retardation after MBS have been refuted and the most recent adolescent ASMBS guidelines have removed the recommendation of patients reaching physical maturity prior to MBS (4, 24).

 

The risk of nutritional deficiencies decreases with adherence to prescribed micronutrient supplements and increases with pregnancy (86). Given the high prevalence of nutritional deficiencies, lifelong micronutrient supplementation is required following surgery. One concern emphasized in the adolescent population is adherence to regular multivitamin use. In a prospective study of 41 adolescents who underwent MBS, multivitamin adherence was only 29.8%  23.9 (90).

 

WEIGHT REGAIN

 

Current data demonstrates adequate maintenance of weight loss at 5-9 years (29, 91). Several studies have suggested a long-term weight loss advantage in adolescents undergoing RYGB compared to SG, although there is still insufficient evidence to directly compare long-term outcomes of both procedures (29, 53, 92, 93). More research is needed to fully understand the mechanisms behind long-term weight maintenance after MBS.

 

OTHER COMPLICATIONS  

 

Cholelithiasis is a common complication due to rapid weight loss following MBS in both adolescents and adults. In the Teen-LABS study, cholecystectomy was required within three years in 9.9% of adolescents who underwent RYGB and 5.1% who underwent SG (53). Five percent of Teen-LABS participants required other abdominal operations including lysis of adhesions, gastrostomy, ventral hernia repair, or internal hernia repair (53). Symptoms of GERD, nausea, bloating, and diarrhea can also increase following MBS. During five years of follow up, the incidence of GERD increased from 2% to 8% in adolescents who underwent RYGB and from 11% to 24% in those who underwent SG. At five years postoperatively, the SG group had more than fourfold greater odds of having gastrointestinal distress symptoms when compared to RYGB (32). Dumping syndrome can been seen after both procedures, however it is much more common after RYGB compared to SG (94, 95). The incidence of dumping syndrome (~12%) in adolescents after RYGB was similar to adult patients two years after surgery(96).

 

There are no current established guidelines for surveillance of Barrett’s esophagus after SG for adolescent patients, however routine screening is recommended for adult patients after SG, therefore it would be prudent for adolescent patients to undergo intermittent surveillance also as the length of possible GERD exposure is theoretically longer (97). Similarly, there are no established guidelines for monitoring of bone density following use of MBS in adolescence, but due to inadequate vitamin D levels and rising PTH at least in those who underwent RYGB, periodic monitoring with DEXA may be prudent.

 

Emerging Evidence

 

Current evidence evaluating the outcomes and efficacy of adolescent MBS is generally limited to ≤10 years of follow up. Smaller, long-term studies with data available for 7 to 12 post-operatively in patients who primary underwent RYGB demonstrate the durability of weight loss and similar rates of complications, although inference is limited due to small sample sizes with reduced attrition rates (59, 60, 98, 99). Characteristics including study size, length of follow up, and attrition rate of available studies on MBS published from 2012 to present are available in Table 4. As SG is now the most predominate MBS procedure performed in the Unites States long-term data with this procedure is required. While some longitudinal studies are ongoing (Table 4), there remains a paucity of long-term data in the adolescent population.

 

Table 4. Characteristics of Studies on MBS, 2012 – Present

Author; Year

Study Design

Sample Size (N)

Type of MBS

 

 

RYBG     SG

Longest follow up

N

(1 yr)

N

(3 yr)

N

(5 yr)

Comments

Inge;

2018 (29)

RO 

483

177

306

5 yr

466 (96%)

153 (32%)

41 (8%)

The PCORnet bariatric study (2005 – 2015)

Olbers; 2017 (58)

CC

81

81

0

5 yr

81

(100%)

n/a

81 (100%)

Adolescent Morbid Obesity Surgery (AMOS) study

Inge; 2017(59)

PO

74

74

0

12.5 yr

n/a

n/a

58 (81%)

Adolescent Bariatric Surgery at 5 Plus Years (FABS-5+) study (2001-2007); mean follow up 8.0 yr

Inge; 2016 (53)

PO 

228

161

67

3 yr

205 (90%)

194 (85%)

n/a

Teen-Longitudinal Assessment of Bariatric Surgery (Teen-LABS) study; (2007-2012)

Vilallonga; 2016 (60)

RO

19

19

0

10.2 yr

n/a

n/a

n/a

Mean follow up 7.2 years; (2003-2008)

Al-Sabah; 2015 (61)

RO

125

0

135

4 yr

54 (40%)

n/a

n/a

2 yr follow up: 46 (34%); (2008-2012)

Cozacov; 2014 (98)

RO

18

8

10

7 yr

15 (83%)

10 (56%)

n/a

7 yr follow up: 3 (17%); (2002 – 2011)

Messiah; 2013 (71)

PO

454

454

0

1 yr

108 (24%)

n/a

n/a

Bariatric Outcomes Longitudinal Database (BOLD) (2004-2010)

Alqahtani; 2012 (25)

RO

108

0

108

2 yr

41 (38%)

n/a

n/a

2 yr follow-up: 8 (7%); (2008 – 2011)

Nijhawan; 2012 (99)

RO

25

25

0

9 yr

n/a

n/a

20 (80%)

Study dates not provided

RO- Retrospective observational; CC- Case-control; PO- Prospective observational

 

DISCUSSION

 

Surgical weight loss is an appropriate consideration for adolescents with severe obesity and/or complications of obesity who have failed to lose weight through conservative management. It is essential that adolescents undergoing evaluation for MBS due so in the context of a multidisciplinary program with specific expertise in adolescent medicine and MBS. SG and RYGB are safe and effective treatment options in adolescents. Weight loss outcomes are comparable between SG and RYGB. Both procedures also result in substantial improvement in complications of obesity, including T2DM. SG appears to have an improved safety profile when compared to RYGB and is now the most common adolescent bariatric procedure performed in the United States. Emerging evidence demonstrates advantages of earlier surgical intervention in those with obesity including improved weight loss, increased resolution of comorbidities, and decreased adverse events when compared to adults (64, 100). Perioperative complications in adolescents undergoing MBS are similar to those in adults but occur less frequently (84, 85). Long-term complications are primarily nutritional and life-long vitamin and mineral supplementation is recommended. Regular follow up is required following MBS and it is important for patients to have access to appropriate medical, dietary, and psychological care.

 

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Hyperparathyroidism in Chronic Kidney Disease

ABSTRACT

 

Chronic kidney disease (CKD) is associated with a mineral and bone disorder (CKD-MBD) which starts early in the course of the disease and worsens with its progression. The main initial serum biochemistry abnormalities are increases in fibroblast growth factor 23 (FGF23) and parathyroid hormone (PTH) and decreases in 1,25 dihydroxy vitamin D (calcitriol) and soluble α-Klotho (Klotho), allowing serum calcium and phosphate to stay normal. Subsequently, serum 25 hydroxy vitamin D (calcidiol) decreases and in late CKD stages hyperphosphatemia develops in the majority of patients. Serum calcium may stay normal, decrease, or increase. More recent reports showed that sclerostin, Dickkopf-1, and activin A also play an important role in the pathogenesis of CKD-MBD. Both the synthesis and the secretion of PTH are continuously stimulated in the course of CKD, resulting in secondary hyperparathyroidism. In addition to the above systemic disturbances, downregulation of vitamin D receptor, calcium-sensing receptor, and Klotho expression in parathyroid tissue further enhances PTH overproduction. Last but not least, miRNAs have also been shown to be involved in the hyperparathyroidism of CKD. The chronic stimulation of parathyroid secretory function is not only characterized by a progressive rise in serum PTH but also by parathyroid gland hyperplasia. It results from an increase in parathyroid cell proliferation which is not fully compensated by a concomitant increase in parathyroid cell apoptosis. Parathyroid hyperplasia is initially of the diffuse, polyclonal type. In late CKD stages it often evolves towards a nodular, monoclonal or multiclonal type of growth. Enhanced parathyroid expression of transforming growth factor-α and its receptor, the epidermal growth factor receptor, is involved in polyclonal hyperplasia. Chromosomal changes have been found to be associated with clonal outgrowth in some, but not the majority of benign parathyroid tumors removed from patients with end-stage kidney disease. In initial CKD stages skeletal resistance to the action of PTH may explain why low bone turnover predominates in a significant proportion of patients, together with other conditions inhibiting bone turnover such as reduced calcitriol levels, sex hormone deficiency, diabetes, Wnt inhibitors, and uremic toxins. High turnover bone disease (osteitis fibrosa) occurs only later on, when increased serum PTH levels are able to overcome skeletal PTH resistance. The diagnosis of secondary uremic hyperparathyroidism and osteitis fibrosa relies mainly on serum biochemistry. X-ray and other imaging methods of the skeleton provide diagnostically relevant information only in severe forms. From a therapeutic point of view, it is important to prevent the development of secondary hyperparathyroidism as early as possible in the course of CKD. A variety of prophylactic and therapeutic approaches are available, as outlined in the final part of the chapter.

 

INTRODUCTION

 

Chronic kidney disease (CKD) is almost constantly associated with a systemic disorder of mineral and bone metabolism, at present named CKD-MBD (1). According to this definition, the disorder is manifested by either one or a combination of biochemical abnormalities (abnormal calcium, phosphate, PTH, or vitamin D metabolism), bone abnormalities (abnormal bone turnover, mineralization, volume, linear growth, or strength) and vascular or other soft tissue calcification. More recently, the underlying pathophysiology has become more complex, with the progressive awareness that fibroblast growth factor 23 (FGF23), a-Klotho (subsequently called "Klotho") as well as the Wnt-b-catenin signaling pathway also play an important role (see below). CKD-MBD generally becomes apparent in CKD stage G3, i.e., at a glomerular filtration rate between 60 and 30 ml/min x 1.73 m2. Initially, it is characterized by a tendency towards hypocalcemia, fasting normo- or hypophosphatemia, and diminished plasma 25OH vitamin D (calcidiol) and 1,25diOH vitamin D (calcitriol) concentrations, together with a progressive increase in plasma FGF23 and intact parathyroid hormone (iPTH), a decrease in plasma soluble Klotho (2–5) and the development of renal osteodystrophy. Renal osteodystrophy often presents initially as adynamic bone disease and subsequently transforms into osteitis fibrosa or mixed bone disease (6). Pure osteomalacia is seen only infrequently. The low bone turnover observed in a significant proportion of patients in early stages of CKD could be due to the initial predominance of bone turnover inhibitory conditions such as resistance to the action of PTH, reduced serum calcitriol levels, sex hormone deficiency, diabetes, inflammation and malnutrition, and uremic toxins leading to the repression of osteocyte Wnt-β-catenin signaling and increased expression of Wnt antagonists such as sclerostin, Dickkopf-1, and secreted frizzled-related protein 4 (7,8). According to this scenario, high turnover bone disease occurs only later on, when sufficiently elevated serum PTH levels are able to overcome the skeletal resistance to its action. Even at that stage, over suppression of PTH by the administration of excessive calcium and/or vitamin D supplements can again induce adynamic bone disease (9). Nephrologists became progressively aware of the fact that the abnormally high serum phosphorus levels in late CKD stages, associated with either hyperparathyroidism or (mostly iatrogenically induced) hypoparathyroidism, may be detrimental to the patients not only in terms of abnormal bone structure and strength, but also in terms of the relative risk of soft-tissue calcifications and cardiovascular as well as all-cause mortality (10–13). As regards serum PTH levels, observational studies have consistently reported an increased relative risk of death in patients with CKD stage G5 and PTH values at the extremes, that is less than two or greater than nine times the upper normal limit of the assay (14,15). For PTH values within the range of two to nine times the upper normal limitreports of associations with relative risk of cardiovascular events or death in patients with CKD have been inconsistent. Of note, however, a report in elderly men in the community showed a strong association between plasma iPTH in the normal range and cardiovascular mortality (16).

 

SECONDARY HYPERPARATHYROIDISM IN CKD – SEQUENCE OF PLASMA BIOCHEMISTRY CHANGES IN EARLY CKD STAGES (Figure 1)

Figure 1. Schematic view of the time profile of disturbances in mineral hormones and bone turnover with progression of chronic kidney disease (CKD). From Drueke & Massy (6).

Phosphate Retention

The precise sequence of metabolic anomalies in incipient CKD leading to secondary hyperparathyroidism remains a matter of debate. Many years ago, it was postulated that a retention of phosphate in the extracellular space due to the decrease in glomerular filtration rate and the accompanying reduction in plasma ionized calcium concentration was the primary event in the pathogenesis of secondary hyperparathyroidism. These anomalies would only be transient and a new steady state would rapidly be reached, with normalization of plasma calcium and phosphate in response to increased PTH secretion and the well-known inhibitory effect of this hormone on the tubular reabsorption of phosphate (“trade-off hypothesis” of Bricker and Slatopolsky) (17). However, this hypothesis has become less attractive since it was demonstrated that plasma phosphate is only rarely elevated in early CKD, and phosphate balance was found to be not positive but negative, at least in rats with moderate-degree CKD (18). Most often, plasma phosphate remains normal until CKD stages G4-G5 (2,19). It may even be moderately diminished in CKD (20).  Oral phosphate absorption remains normal in early stages of experimental CKD (18), and urinary phosphate excretion after an oral overload in patients with mild CKD was actually found to be accelerated (20). Nonetheless, one could argue that in early kidney failure normal or even subnormal concentrations of plasma phosphate might be observed after a slight, initial plasma phosphate increase following phosphate ingestion and stimulation of the secretion of FGF23 and PTH, which in turn could overcorrect plasma phosphate rapidly, due to a more potent inhibition of tubular phosphate reabsorption. However, a more recent study identified slight increases of plasma phosphate in a large US population sample (NHANES III) with CKD stage G3 as compared to a healthy control population without evidence of kidney disease (21). Probably both the time of plasma phosphate determinations during the day as well as subtle changes in circulating and local factors involved in the control of phosphate balance determine the actual plasma level of phosphate in patients with CKD.

Fibroblast Growth Factor 23 (FGF23) and Klotho

 

FGF23 is recognized at present as a major, if not the most important player in the control of phosphate metabolism. It is mainly produced by osteocytes and osteoblasts. It decreases plasma phosphate by reducing tubular phosphate reabsorption similar to, but independent of PTH. Moreover, in contrast to PTH it decreases the renal synthesis of calcitriol. To activate its receptors FGFR-1 and FGFR-3 on tubular epithelial cells it requires the presence of Klotho (or more precisely α-Klotho), which in its function as a co-receptor confers FGF receptor specificity for FGF23 (22). Although initially Klotho expression was found only in the distal tubule, it has subsequently been demonstrated to occur in the proximal tubule as well. In line with this finding, ablation of Klotho specifically from the distal tubules certainly resulted in a hyperphosphatemic phenotype, but to a lesser degree than in systemic or whole nephron Klothoknockout models (23). The regulation of FGF23 production and its interrelations with PTH, calcitriol, calcium, phosphate, and Klotho are complex, being only progressively unraveled. Isakova et al. provided evidence that serum FGF23 increased earlier than serum iPTH in patients with CKD (4). This observation is also supported by experiments in an animal model of CKD and the use of anti-FGF23 antibodies (24). However, the authors of a subsequent large-scale population study took issue with the claim that the increase in circulating FGF23 preceded that of PTH (25). Klotho expression in kidney, Klotho plasma levels, and Klotho urinary excretion decrease with progressive CKD (26,27). The presence of Klotho is required to allow FGF23 to exert its action in the kidney. In addition, Klotho also exerts FGF23 independent effects. It acts from the tubular luminal side as an autocrine or paracrine enzyme to regulate transporters and ion channels. By modifying the Na-phosphate cotransporter NaPi2a it can enhance phosphaturia directly (28). However, its purported glycosidase activity has been put into question recently (29).  The issue then arises which comes first in CKD, an increase in FGF23 or a decrease in Klotho? The answer remains a matter of debate (30). Some studies showed that secreted soluble Klotho levels decrease before FGF23 levels increase (31,32) but the sequence of events may differ depending on experimental models and diverse clinical conditions (33). CKD is probably the most common cause of chronically elevated serum FGF23 levels (34). FGF23 production in bone is increased by phosphate, calcitriol, calcium, PTH, Klotho, and iron. Not all of these effects are necessarily direct. The effect of PTH clearly is both direct, via stimulation of PTH receptor-1 (PTH-R1) (35) and the orphan nuclear receptor Nurr1 (36), and indirect, via an increase in calcitriol synthesis (37). On the other hand, FGF23 inhibits PTH synthesis and secretion although in CKD this effect is mitigated by reduced Klotho and FGFR-1 expression in parathyroid tissue (38–40).

 

The increase in circulating FGF23 with the progression of CKD is independently associated with serum phosphate, calcium, iPTH, and calcitriol (41,42). Despite its direct inhibitory action on the parathyroid tissue FGF23 contributes to the progression of secondary hyperparathyroidism by reducing renal calcitriol synthesis and subsequently decreasing active intestinal calcium transport. Figure 2 shows the complex interrelations between serum FGF23, Klotho, phosphate, calcium, calcitriol, and parathyroid function in CKD.

Figure 2. Chronic kidney disease-associated mineral and bone disorder (CKD-MBD). Complex interactions between phosphate, FGF23, FGF receptor-1c (FGFR1c), Klotho, 1,25diOH vitamin D (calcitriol), renal 1α 25OH vitamin D hydroxylase (1α hydroxylase), vitamin D receptor (VDR), calcium, Ca-sensing receptor (CaSR), and parathyroid hormone (PTH). From Komaba & Fukagawa (43), modified.

Calcium Deficiency

 

In early CKD stages, disturbances of calcium metabolism may already be present. They include a calcium deficiency state due to a negative calcium balance resulting from low oral calcium intakes and impaired active intestinal calcium absorption (although a positive calcium balance can be induced by the ingestion of high amounts of calcium-containing phosphate binders) (44,45), a tendency towards hypocalcemia due to skeletal resistance to the action of PTH (46), and reduced calcium-sensing receptor (CaSR) expression in the parathyroid cell. All these factors contribute to the development of parathyroid over function (46,47). Their relative importance increases with the progression of CKD. It also depends on individual patient characteristics such as the underlying type of nephropathy, comorbidities, dietary habits, and amount of food intake.

 

Inhibition of Calcitriol Synthesis 

 

The progressive loss of functioning nephrons and increased production of FGF23 are mainly responsible for the reduction in renal calcitriol synthesis, favoring the development of parathyroid over function. Although PTH in turn stimulates renal tubular 1α-OH vitamin D hydroxylase activity resistance to its action probably attenuates this counter-regulatory mechanism. Whether the direct inhibition of 1α-OH vitamin D hydroxylase activity by FGF23 is more powerful than its stimulation by PTH depends on several other additional factors such as the presence of hyperphosphatemia, metabolic acidosis, and uremic toxins. The marked disturbances of the calcitriol synthesis pathway probably explain the long reported direct relation in CKD patients between plasma calcidiol and calcitriol, and between plasma calcitriol and glomerular filtration rate (48). Such relations are not observed in people with normal kidney function.

 

Yet another hypothesis is based on the observation that calcidiol does not penetrate into proximal tubular epithelium from the basolateral side, but only from the luminal side. The complex formed by calcidiol and its binding protein (DBP) is ultrafiltered by the glomerulus, subsequently enters the tubular epithelium from the apical side via the multifunctional brush border membrane receptor megalin, and then serves as substrate for the renal enzyme, 1α-OH vitamin D hydroxylase for calcitriol synthesis (Figure 3) (49).  Reduced glomerular filtration leads to a decrease in calcidiol-DBP complex transfer into the proximal tubular fluid and hence reduced availability of calcidiol substrate for luminal reabsorption and calcitriol formation. However, the validity for the human situation of this mechanism established in the mouse has subsequently been questioned since 1α-OH vitamin D hydroxylase expression was found not only in proximal, but also in distal tubular epithelium of human kidney, that is in tubular areas in which megalin apparently is not expressed (50).

Figure 3. Schematic representation of the role of megalin in renal tubular 25 OH vitamin D reabsorption. Megalin is a multifunctional brush border membrane receptor expressed in the proximal renal tubule. It enables endocytic reabsorption of 25 OH vitamin D (calcidiol) filtered by the glomerulus and the subsequent synthesis of 1,25 diOH vitamin D (calcitriol) by mitochondrial 1-a 25 OH vitamin D hydroxylase. After Nykjaer et al (49).

Finally, the concentration of plasma calcidiol is diminished in the majority of patients with CKD (51,52). The reasons for this vitamin D deficiency state include insufficient hours of sunshine or sun exposure especially in the elderly, skin pigmentation, intake of antiepileptic drugs (like in general population), and in addition enhanced urinary excretion of calcidiol complexed to vitamin D binding protein (DBP) in the presence of proteinuria, and loss into the peritoneal cavity in those on peritoneal dialysis treatment. All these factors may also contribute to the reduction in calcitriol synthesis (53). However, low plasma calcidiol has also been postulated to be a risk factor per se for secondary hyperparathyroidism, as suggested by an observational study in Algerian hemodialysis patients with insufficient exposure to sunshine (54) and the observation that calcidiol is able to directly suppress PTH synthesis and secretion in bovine parathyroid cells in vitro, although with much less potency than calcitriol (55).

 

SECONDARY HYPERPARATHYROIDISM IN CKD – PLASMA BIOCHEMISTRY CHANGES IN ADVANCED CKD STAGES (Figure 1)

 

The above-mentioned roles of relative or absolute deficiency states of calcium and vitamin D are steadily gaining importance with the progression of CKD, and phosphate becomes a major player.

 

Role of Hyperphosphatemia

 

In CKD stages G4-G5 hyperphosphatemia becomes an increasingly frequent feature (19), due to phosphate retention caused by the progressive loss of functioning nephrons and the increasing difficulty in augmenting glomerular phosphate ultrafiltration and to further reduce its tubular reabsorption when it is already maximally inhibited by high serum FGF23 and PTH levels.

 

FGF23 Excess and Klotho Deficiency

 

Circulating FGF23 may reach extremely high, maladaptive concentrations in patients with end-stage kidney disease (ESKD) (56). In parallel, a reduction of Klotho expression is observed in kidney and parathyroid tissue, as well as of soluble Klotho in the plasma and urine of patients and animals with CKD (26,27,30). The reduction is particularly marked in advanced stages of CKD. The resulting resistance to the action of FGF23 in kidney and parathyroid tissue favors hyperparathyroidism (see below).

 

The uremic syndrome itself could also play a role. In addition to phosphate many other so-called uremic toxins, that is substances which accumulate in the uremic state, are known to interfere with vitamin D metabolism and action (57,58). Indoxyl sulfate has been shown to participate in the pathogenesis of skeletal resistance to the action of PTH (59), in addition to direct inhibitory effects on bone turnover (60).

 

MECHANISMS INVOLVED IN THE PATHOGENESIS OF SECONDARY HYPERPARA-THYROIDISM

 

Generally speaking, there are at least two major different mechanisms which determine the magnitude of secondary hyperparathyroidism in CKD. The first is an increase in PTH synthesis and secretion, and the second an increase in parathyroid gland mass, mostly due to enhanced cell proliferation (hyperplasia), and to a lesser degree also an increase in cell size (hypertrophy) (see schematic representation in Figure 4). Whereas acute stimulation of PTH synthesis and/or release generally occurs in the absence of cell growth stimulation, these two processes appear to be tightly linked whenever there is chronic stimulation. The main factors involved in the control of the two processes are again calcitriol, calcium, and phosphate whereas the direct effects of FGF23 appear to be essentially limited to the control of PTH synthesis and secretion. In the following, the disturbances of the mechanisms controlling parathyroid function will be discussed subsequently for each of these three factors, although there are numerous interactions between them. Subsequently, the influence of other factors and comorbid conditions related to CKD will be presented.

Figure 4. Pathogenesis of secondary hyperparathyroidism. Schematic representation of parathyroid hormone (PTH) synthesis and secretion (upper part) and parathyroid cell proliferation and apoptosis (lower part), as regulated by a number of hormones and growth factors.

Calcitriol

 

The above-mentioned decrease in plasma calcitriol aggravates hyperparathyroidism via several mechanisms. The first is direct and results from an insufficient inhibition of PTH synthesis due to low circulating calcitriol levels and a disturbed action of calcitriol at the level of the preproPTH gene. It is well established that calcitriol, after forming a complex with its receptor, vitamin D receptor (VDR) and heterodimerizing with the retinoic acid receptor (RXR), directly inhibits preproPTH gene transcription by binding to a specific DNA response element (VDRE) located in the 5’-flanking region of the gene. In CKD, in addition to low extracellular concentrations of calcitriol, at least two other factors interfere with calcitriol’s action on the preproPTH gene (61). The first factor is a reduced expression of the VDR gene in hyperplastic parathyroid tissue of CKD patients (62). This reduction is particularly marked in nodular, as compared to diffusely hyperplastic parathyroid tissue. The second factor is reduced binding of calcitriol to VDR, slowed nuclear migration of the calcitriol–VDR complex, and less efficient inhibitory action on the preproPTH gene, in association with the uremic state (58,63). Of note, extracellular Ca2+ concentration [Ca2+e] appears to play a role in the regulation of VDR expression. In rat parathyroid glands, low [Ca2+e] reduced VDR expression independently of calcitriol, whereas high [Ca2+e] increased it (64). Hypocalcemia may attenuate by this mechanism the feedback of increased plasma calcitriol concentrations on the parathyroids.

 

The second level at which calcitriol regulates PTH gene expression involves calreticulin. Calreticulin is a calcium binding protein which is present in the endoplasmic reticulum of the cell, and also may have a nuclear function. It regulates gene transcription via its ability to bind a protein motif in the DNA-binding domain of nuclear hormone receptors of sterol hormones. Sela-Brown et al. proposed that calreticulin might inhibit vitamin D's action on the PTHgene, based on in vitro and in vivo experiments (65). They fed rats either a control diet or a low calcium diet, which led to increased PTH mRNA levels despite high serum calcitriol levels that would be expected to inhibit PTH gene transcription. Their postulate that high calreticulin levels in the nuclear fraction might prevent the effect of calcitriol on the PTH gene was strongly supported by the observation that hypocalcemic rats had increased levels of calreticulin protein in parathyroid nuclear fraction. This could explain why hypocalcemia leads to increased PTH gene expression despite high serum calcitriol levels, and might also be relevant for the refractoriness of secondary hyperparathyroidism to calcitriol treatment observed in many patients with CKD.

 

The third mechanism of calcitriol’s action could be indirect, via a stimulatory effect on parathyroid CaSR expression, as shown by Brown et al (66) and subsequently confirmed by Mendoza et al (67).

 

The fourth mechanism is again a direct one. It concerns the well-known inhibitory effect of vitamin D on cell proliferation and the induction of differentiation towards mature, slowly growing cells. A decrease in plasma calcitriol and a perturbed action at molecular targets favors abnormal cell growth. This is the case with parathyroid tissue as well, and parathyroid hyperplasia ensues (68). The importance of vitamin D in the pathogenesis of parathyroid hyperplasia of experimental uremia has first been shown by Szabo et al (69). These authors administered increasing doses of calcitriol to rats either at the time of inducing chronic kidney failure or at a later time point, when uremia was already well established. They were able to prevent parathyroid cell proliferation entirely when calcitriol was given in initial CKD stages, but not when given later on. Fukagawa et al showed that pharmacologic doses of calcitriol repressed c-myc expression in the parathyroid tissue of uremic rats and suggested that the hormone might suppress parathyroid hyperplasia by this pathway (70). In contrast, Naveh-Many et al. (71) failed to observe such an antiproliferative effect of calcitriol in parathyroid cells of uremic rats but they administered the hormone for only three days. Such short-term administration may not have been sufficient for an efficacious suppression of cell turnover.

 

To answer the question of a possible direct calcitriol action on parathyroid cells, several studies were performed in experimental models in vitro. Nygren et al. (72) showed in primary cultures of bovine parathyroid cells, maintained in short-term culture, that these cells underwent significant increases both in number and size in response to fetal calf serum, and that the addition of 10-100 ng/mL calcitriol almost completely inhibited cell proliferation whereas cell hypertrophy was unaffected. Kremer et al (73) subsequently confirmed in the same parathyroid cell model that calcitriol exerted an anti-proliferative action. They further suggested that this inhibition occurred via a reduction of c-myc mRNA expression. One report showed an inhibitory action under long-term culture conditions (up to 5 passages) of the effect of calcitriol on bovine parathyroid cell proliferation (74). Our group subsequently confirmed such a direct antiproliferative effect of calcitriol in a human parathyroid cell culture system derived from hyperplastic parathyroid tissue of patients with severe secondary uremic hyperparathyroidism (75) (Figure 5).

Figure 5. Antiproliferative effect of 1,25 diOH vitamin D on parathyroid cells. Reduction of parathyroid cell proliferation in response to increasing medium 1,25diOH vitamin D (calcitriol) concentrations in the incubation milieu of a human parathyroid cell culture system, with parathyroid cells derived from hyperplastic parathyroid tissue of patients with severe secondary uremic hyperparathyroidism. From Roussanne et al (75).

A fifth mechanism is the potential association between parathyroid function and vitamin D receptor (VDR) polymorphism. Fernandez et al (76) separated hemodialysis patients with same serum calcium and time on dialysis treatment into two groups, according to their serum iPTH levels, namely low PTH (<12 pmol/L) or high PTH (>60 pmol/L). They found that the BB genotype and the B allele were significantly more frequent in the low PTH than in the high PTH group (32.3 % vs 12.5 %, and 58.8% vs 39.1%, respectively). This information suggests that VDR gene polymorphism influences parathyroid function in CKD. Similar results have been reported by an Italian group (77) and in a large sample of Japanese hemodialysis patients (78). In this latter study, after excluding patients with diabetes and patients with a dialysis vintage of less than ten years, the authors observed lower plasma iPTH levels in ESKD patients with BB than with Bb or bb alleles. A relationship between Apa I polymorphism (A/a alleles) and the severity of hyperparathyroidism has also been sought in Japanese hemodialysis patients (79). Plasma PTH levels in AA and Aa groups were approximately half that of the aa group. However, other groups found no difference in PTH levels for various VDR polymorphisms (80–82). Moreover, although in some clinical conditions VDR polymorphism may be associated with variations of the half-life of the VDR gene transcript (83) or of VDR function (84), there has been no report showing that in uremic patients with secondary hyperparathyroidism the density of parathyroid cell VDR varies with different VDR genotypes. In addition, although VDR genotypes may have some influence on the degree of parathyroid cell proliferation, the mechanism by which this could occur remains unknown.

 

Finally, Egstrand et al recently provided experimental evidence for the role of a circadian clock operating in parathyroid glands. This clock and downstream cell cycle regulators were shown to be disturbed in uremic rats, potentially contributing to dysregulated parathyroid proliferation in secondary hyperparathyroidism (85).

 

Calcium

 

It has long been known that [Ca2+e] is the primary regulator of PTH secretion. Small changes in serum Ca2+ concentration result in immediate changes of PTH release which are short-lived or long-lived, depending on the velocity of the restoration of serum Ca2+ towards normal. Thus, postprandial urinary calcium excretion was increased in patients with CKD as it was in healthy volunteers, but only in the patients was this accompanied by significantly reduced serum Ca2+ and increased PTH levels (86). The inverse relation between Ca2+ and PTH in the circulation obeys a sigmoidal curve (87). While the majority of in vitro studies have reported a decreased responsiveness of hyperplastic parathyroid cells to changes in [Ca2+e] in vivo studies have not always confirmed this. Such discrepant findings are likely due to different methods used to assess the dynamics of PTH secretion (88).

 

Several in vitro studies have shown that the set point of calcium for PTH secretion (that is the Ca2+ concentration required to produce half maximal PTH secretion) is greater in parathyroid cells from primary adenomas and secondary (uremic) hyperplastic parathyroid glands than in normal parathyroid cells (89). Such a relatively poor response to [Ca2+e] should contribute to the increased PTH levels observed in uremic patients with secondary hyperparathyroidism.

 

We and others have demonstrated that both primary parathyroid adenoma and secondary uremic, hyperplastic parathyroid gland tissue exhibit a decrease in the expression of CaSR protein (90,91). In secondary uremic hyperparathyroidism, there is a significant decrease of CaSR in diffusely growing hyperplastic tissue, with the decrease being even more marked in nodular areas (characteristic of advanced hyperparathyroidism with autonomously growing cells) (90) (Figure 6). Since changes in intracellular Ca2+ elicited by hyper or hypocalcemia depend on the expression and activity of the CaSR, any decrease explains, at least in part, an impaired intracellular calcium response to [Ca2+e] and hence a reduced inhibitory effect of the cation on PTH secretion. Several factors contribute to the downregulation of CaSR expression and/or activity in CKD including reduced calcitriol levels (66,67), low magnesium levels (92), dietary phosphate (probably indirect action) (93), and metabolic acidosis (94). However, raising extracellular phosphate has been recently shown to also exert a direct inhibitory action on parathyroid cell CaSR activity of isolated human parathyroid cells resulting in an increase in PTH secretion (95). Almaden et al studied calcium-regulated PTH response in vitro, using respectively primary parathyroid adenoma and uremic hyperplastic tissue, the latter either of the nodular or the diffuse type (96). They found that in primary adenoma tissue PTH secretion was less responsive to an increase in [Ca2+e] than in uremic hyperplastic parathyroid tissue; among the latter, nodular tissue was less responsive than diffusely hyperplastic tissue. The decreased secretory response to Ca2+ observed in nodular uremic hyperplasia may be explained by the markedly reduced CaSR expression in CKD, as demonstrated by Gogusev et al (90). This decrease can be overcome, at least partially, by PTHrp, as shown by Lewin et al (97), who observed that the administration of PTHrp significantly stimulated the impaired secretory capacity of the parathyroid glands of uremic rats in response to hypocalcemia. Of note, this observation also implies that the PTH/PTHrp receptor is expressed on the parathyroid cell.

Figure 6. Calcium-sensing receptor (CaSR) expression in normal and hyperplastic parathyroid glands. Normal parathyroid tissue (in blue), secondary (2°) hyperparathyroidism from dialysis patients (glands with diffuse hyperplasia in yellow; glands with nodular hyperplasia in green), and primary (1°) adenomatous hyperparathyroidism from patients with conserved kidney function (in orange). Decreased expression of both CaSR protein and mRNA in the majority of hyperplastic glands, with a particularly marked decrease in nodular type secondary uremic hyperparathyroidism. After Gogusev et al (90).

The shift of the calcium set point to the right in dialysis patients in vivo has been a much less constant finding than the right shift observed in the above-mentioned studies in uremic parathyroid tissue in vitro. While in CKD patients with a mild to moderate degree of hyperparathyroidism the set point was most often found to be normal, an altered set point was observed in presence of severe parathyroid over function with hypercalcemia (98). This anomaly could at least in part be due to CaSR down-regulation. As regards CKD patients with less severe parathyroid over function, a considerable controversy took place regarding the results of in vivo assessments of parathyroid gland function (99,100). In part, disparities among study results reflected technical differences in experimental methods and/or variations in the mathematical modeling of PTH secretion in vivo (101).  Another difficulty in interpreting the results of in vivo dynamic tests of parathyroid gland function relates to the issue of parathyroid gland size. Because there is a basal, or non-suppressible, component of PTH release from the parathyroid cell even at high [Ca2+e], excessive PTH secretion may result solely from increases in parathyroid gland mass (98). This can theoretically occur in the absence of any defect in calcium sensing at the level of the parathyroid cell.  Since parathyroid gland hyperplasia is present to some extent in nearly all patients with CKD stages G3-G5, alterations in PTH secretion due to increases in parathyroid gland mass cannot readily be distinguished from those attributable to changes in calcium-sensing by the parathyroid cell using the four-parameter model for in vivo studies (100).

 

The role of calcium in parathyroid cell proliferation is less clear than is generally assumed. Calcium deficiency, in the presence or absence of hypocalcemia, together with vitamin D deficiency or reduced production of calcitriol, probably is a major stimulus of parathyroid hyperplasia. Naveh-Many et al showed that calcium deprivation, together with vitamin D deficiency, greatly enhanced the rate of parathyroid cell proliferation in normal rats and also in rats with CKD, using the cell cycle-linked antigen, PCNA (71). The concomitant decrease in CaSR expression in CKD, as observed in parathyroid glands of both dialysis patients and uremic rats (90,102), should theoretically enhance parathyroid tissue hyperplasia further. Indirect support for this contention came from the observation that the administration of the calcimimetic compound NPS R-568, a CaSR agonist, led to the suppression of parathyroid cell proliferation in rats with CKD (103). However, in the study by Naveh-Many et al the dietary regimen was poor in both calcium and vitamin D. In contrast, when feeding normal rats on a calcium-deficient diet alone, in the absence of concomitant vitamin D deficiency, Wernerson et al observed parathyroid cell hypertrophy, not hyperplasia (104).

 

The question whether the effect of calcium is direct or indirect remains therefore unsolved at present. It can only be answered by in vitro studies. For a long time, available culture systems using normal parathyroid cells did not allow the maintenance of functionally active cells for prolonged time periods. They were all characterized by a rapid and significant loss of PTH secretion within 3 to 4 days (105–107). One culture model has been described, using bovine parathyroid cell organoids, which maintained the ability to modulate PTH secretion in response to [Ca2+e] and tissue-like morphology for 2 weeks (108). However, only one long-term study using bovine parathyroid cells demonstrated a release of bioactive bovine PTH but with reduced sensitivity to [Ca2+e] (109). Other reports showed that the rapid decrease in PTH responsiveness of cultured bovine parathyroid cells to changes in [Ca2+e] was associated with a marked reduction in CaSR expression (110,111). Yet other parathyroid cell-derived culture models proposed in the literature were in fact devoid of any PTH secretory capacity (112,113).

 

To study direct effects of [Ca2+e] on the parathyroid cell in vitro, we developed a functional human parathyroid cell culture system capable of maintaining regulation of its secretory activity and the expression of extracellular CaSR mRNA and protein for several weeks. For this purpose, we used parathyroid cells derived from hyperplastic parathyroid tissue of hemodialysis patients with severe secondary hyperparathyroidism (114). In a subsequent study with this experimental model, we surprisingly obtained evidence that parathyroid cell proliferation index, as estimated by [3H]-thymidine incorporation into an acid-precipitable fraction as a measure of DNA synthesis, could be directly stimulated by high [Ca2+e] in the incubation medium, compared with low [Ca2+e] (75) (Figure 7).

Figure 7. Effect of medium calcium concentration on parathyroid cell proliferation. Stimulatory effect on parathyroid cell proliferation (measured by KI-67 staining method) of high medium calcium concentrations in the incubation milieu of a human parathyroid cell culture system derived from hyperplastic parathyroid tissue of patients with severe secondary uremic hyperparathyroidism. From Roussanne et al (75).

We confirmed this finding in independent experiments using the cell cycle-linked antigen Ki-67 to determine parathyroid cell proliferation. However, the addition of the calcimimetic NPS R-467 to the incubation medium led to a decrease in cell proliferation (Figure 8).

Figure 8. Inhibitory effect of calcimimetic on parathyroid cell proliferation. Human parathyroid cells derived from hyperplastic parathyroid tissue of patients with severe secondary uremic hyperparathyroidism were maintained in high medium calcium incubation milieu, and exposed to increasing concentrations of calcimimetic NPS R-467. Determination of cell proliferation by [3H]-thymidine incorporation method. After Roussanne et al (75).

Of interest, calcimimetics have subsequently been shown to upregulate the expression of both CaSR (67,115) and VDR (67) in parathyroid glands of uremic rats. In an attempt to unify our apparently contradictory in-vitro observations with respect to findings made in vivo, we proposed the following hypothesis. The effect of calcium on parathyroid cell proliferation could occur along two different pathways, via two distinct mechanisms. Inhibition of proliferation would occur via the well-known parathyroid CaSR-dependent pathway, whereas stimulation of proliferation would occur via an alternative pathway (Figure 9). Note that the parathyroid tissue samples used in our study stemmed from uremic patients with long-term ESKD and severe secondary hyperparathyroidism. Since such parathyroid tissue generally exhibits decreased CaSR expression, it is possible that the number of CaSR expressed in the parathyroid cell membranes of our culture model was insufficient to inhibit cell proliferation. Of note, the human CaSR gene has two promoters and two 5’ untranslated exons; therefore, the alternative usage of these exons leads to production of multiple CaSR mRNAs in parathyroid cells (116). The expression of CaSR mRNA produced by one of the two promoters of CaSR gene is specifically reduced in parathyroid adenomas, suggesting a role in PTH hypersecretion and proliferation. Moreover, the membrane-bound 550-kD Ca2+-binding glycoprotein megalin, belonging to the low-density lipoprotein receptor superfamily, has been identified in parathyroid chief cells as another putative calcium-sensing molecule which could be involved in calcium-regulated cellular signaling processes as well (117). Based on these observations, one can postulate that parathyroid cells express multiple CaSR-like molecules. Consequently, if the well-known parathyroid CaSR is downregulated, parathyroid cell proliferation induced by increases in [Ca2+e] may occur via a different type of CaSR. Another possibility is an alteration in post-receptor signal transduction that could occur in hyperparathyroid states or under cell culture conditions. Our observations are in line with findings by Ishimi et al. which were incompatible with a direct effect of low [Ca2+e] in the pathogenesis of parathyroid hyperplasia (74). However, any extrapolation from such in vitro observations to the in vivo setting should be done with caution, and further work is needed to define the precise pathway(s) by which calcium regulates parathyroid tissue growth.

Figure 9. Hypothesis of the regulation of parathyroid cell proliferation by extracellular [Ca2+]. 1) Inhibitory pathway via the calcium-sensing receptor (CaR). 2) Stimulatory pathway via an unknown transmembrane transduction mechanism. Physiologically, pathway 1 predominates over pathway 2. In presence of parathyroid hyperplasia with calcium-sensing receptor down-regulation pathway 2 could become dominant and favor parathyroid cell proliferation over suppression. After Roussanne et al (75).

Phosphate

 

Hyperphosphatemia is associated with increased PTH secretion. The stimulation of PTH release occurs via direct and indirect mechanisms. The initially proposed indirect mechanism, which remains true according to present knowledge, is via a decrease in plasma Ca2+ concentration (see above). Hyperphosphatemia also leads to an inhibition of the renal synthesis of calcitriol, probably mostly via stimulation of FGF23 production.

 

A direct action of phosphate on PTH secretion by the parathyroid cell has long been suspected. However, it has been formally demonstrated in vitro only in 1996 (118–120). This demonstration required the use of either intact parathyroid glands (from rats) (Figure 10) or parathyroid tissue slices (from cows) whereas it had not been possible to obtain such direct stimulation using the classic model of isolated bovine parathyroid cells. Elevating plasma phosphate concentration in the incubation milieu of experimental models using intact (or partially intact) parathyroid tissue leads to a stimulation of PTH secretion within some hours, in the absence of any change in [Ca2+e]. It can however be abrogated by an increase in cytosolic Ca2+ concentration (121).

Figure 10. Direct inhibition of parathyroid hormone (PTH) secretion by phosphate. Intact parathyroid glands obtained from normal rats were maintained in culture and exposed to increasing in phosphate concentrations in the incubation medium. After Almaden et al (121).

Silver’s group reported subsequently that phosphate, like calcium, regulates pre-pro-PTH gene expression post-transcriptionally by changes in protein-PTH mRNA interactions at the 3'-UTR which determine PTH mRNA stability. They identified the minimal sequence required for protein binding in the PTH mRNA 3'-UTR and determined its functionality. They found that the conserved PTH RNA protein-binding region conferred responsiveness to calcium and phosphate and determined PTH mRNA stability and levels (122). Thus, a low calcium diet increased stability, whereas a low phosphate diet decreased stability of PTH mRNA (123) (Figure 11). The PTH mRNA 3’-untranslated region-binding protein was subsequently identified by this research group as adenylate-uridylate-rich element RNA binding protein 1 (AUF1) (124).

Figure 11. Post-transcriptional regulation of PTH mRNA stability by calcium, phosphate, and kidney failure. Pre-pro-PTH gene expression is modulated via changes in protein-PTH mRNA interactions at the 3'-UTR region which determine PTH mRNA stability. Low calcium diet increases stability, whereas low phosphate diet decreases stability of PTH mRNA. PTH mRNA protective factor AUF1 in yellow, PTH mRNA degrading endonuclease in orange. After Yalcindag et al (123).

In addition to its stimulatory effect on PTH secretion a high phosphate diet also rapidly induces parathyroid over function and hyperplasia, as shown in experimental animal models (125). Subsequent studies showed that hyperphosphatemia induced by phosphate-rich diets in animals with CKD induced parathyroid hyperplasia even when changes in plasma Ca2+ and calcitriol concentration were carefully avoided, pointing to a direct effect of phosphate on cell proliferation (71,120). Conversely, early dietary phosphate restriction in the course of CKD was capable of preventing both PTH over secretion and parathyroid hyperplasia (71,120,126). Interestingly, dietary phosphate restriction following phosphate overload in rats led to an immediate decrease in PTH secretion despite no regression of parathyroid gland size (127).

 

Our group wished to know whether the stimulatory effect of phosphate on parathyroid cell proliferation was direct or indirect. To answer this question, we used the above described in vitro model of human parathyroid cells maintained in long-term culture (114). We could show that cell proliferation index was directly stimulated by high phosphate concentrations in the incubation medium, compared with low phosphate concentration (75) (Figure 12). These experiments demonstrated that phosphate is capable of stimulating not only PTH secretion, but also of inducing parathyroid tissue hyperplasia by a direct mode of action.

Figure 12. Direct stimulatory effect of extracellular phosphate on parathyroid cell proliferation. Response of parathyroid cell growth to increasing phosphate concentrations in the incubation milieu of a human parathyroid cell culture system derived from hyperplastic parathyroid tissue of patients with severe secondary uremic hyperparathyroidism. Determination of cell proliferation by [3H]-thymidine incorporation method. After Roussanne et al (128).

FGF23 plays an important role in the control of plasma phosphate. Elevated FGF23 in CKD allows efficient inhibition of proximal tubular phosphate reabsorption and maintenance of plasma phosphorus in the normal range. However, since hyperphosphatemia directly stimulates PTH secretion, its correction by FGF23 indirectly leads to a reduction of PTH release, in addition to the direct inhibitory action of FGF23 on parathyroid secretory activity (see above). 

 

FGF23 and Klotho

 

As mentioned before FGF23 directly inhibits PTH synthesis and secretion via its action on parathyroid FGFR-1 (129). FGF23 also increases parathyroid CaSR and VDR expression, further contributing to the suppression of PTH by this hormone (Canalejo 2010). In advanced stages of CKD FGF23’s effect is partially or even completely abolished owing to downregulation of the expression of its receptor and co-receptor Klotho (38–40). Of interest, in early stages of CKD there could be an initial upregulation of FGFR-1 and Klotho, with enhanced PTH secretion in response to FGF23 via an Na+/K+ -ATPase driven pathway (130). Subsequent findings suggested a function for Klotho in suppressing PTH biosynthesis and parathyroid gland growth, even in the absence of CaSR (131). Moreover, they pointed to a physical interaction between Klotho and CaSR. Specific deletion of CaSR in parathyroid tissue led to elevated serum PTH levels and parathyroid gland hyperplasia, and additional deletion of Klotho in parathyroid glands exacerbated this condition. However, a recent review concluded that role of parathyroid Klotho remains controversial (132).

 

MicroRNAs

 

More recently, Shilo et al provided evidence for the important role of microRNAs (miRNAs) in the physiological regulation of parathyroid function, and its dysregulation in the secondary hyperparathyroidism of CKD (133,134). The authors found an abnormal regulation of many miRNAs in experimental uremic hyperparathyroidism supporting a key role for miRNAs in this condition. Specifically, their studies showed that inhibition of the abundant let-7 family increased PTH secretion in normal and uremic rats, as well as in mouse parathyroid organ cultures. Conversely, inhibition of the upregulated miRNA-148 family prevented the increase in serum PTH of uremic rats, and inhibition of let-7 family also reduced PTH secretion in parathyroid cultures. Thus, miRNA dysregulation represents yet another crucial step in the pathogenesis of secondary hyperparathyroidism.

 

Other Factors and Conditions

 

As already pointed out above the uremic state with its accumulation of numerous uremic toxins is another long suspected, albeit yet ill-defined factor in the pathogenesis of secondary hyperparathyroidism. Recently, several pieces of evidence have been provided in favor of a role of the uremic state which interferes with the binding of calcitriol to VDR (58) and with the nuclear uptake of the hormone-receptor complex (63). This should have consequences not only for PTH synthesis and secretion, but also for parathyroid cell proliferation. Another mechanism of excessive proliferation involves the mTOR pathway, which has been shown to be activated in secondary hyperparathyroidism (135). Inhibition of mTOR complex 1 by rapamycin decreased parathyroid cell proliferation in vivo and in vitro.

Patients with diabetes receiving dialysis therapy have relatively low plasma PTH levels, as compared to those without diabetes. The high incidence of low bone turnover in uremic patients with diabetes (136–139) has been attributed to low levels of biologically active PTH, possibly via an inhibition of PTH secretion or a modification of the PTH peptide by the accumulation of advanced glycation end-products such as pentosidine (140) or else oxidation of PTH (141,142). However, experimental studies have demonstrated that the metabolic abnormalities associated with diabetes can also directly decrease bone turnover, independent of PTH (143). In general, patients with low bone turnover tend to develop hypercalcemia when on a normal or high dietary calcium intake, probably due to the diminished skeletal capacity of calcium uptake. This in turn tends to reduce plasma PTH. Thus, not only does hypoparathyroidism promote adynamic bone disease but adynamic bone disease also favors hypoparathyroidism. Another issue is whether in patients with diabetes abnormalities such as hyperglycemia and insulin deficiency or resistance may directly affect parathyroid function. In an in vitro study using dispersed bovine parathyroid cells, high glucose and low insulin concentrations suppressed the PTH response to low Ca2+ concentration (144). These results are compatible with the view that diabetes directly inhibits parathyroid function. However, when uremic rats were fed on a high phosphate diet to induce secondary hyperparathyroidism, the presence of diabetes did not affect the development of parathyroid over function (143).

 

Aluminum bone disease is generally associated with low serum PTH levels (145,146) and a decreased PTH response to stimulation by hypocalcemia (147,148). In aluminum intoxicated patients, high amounts of aluminum are also found in parathyroid tissue (149). The relatively low PTH levels may reflect either an inhibition of PTH secretion by the hypercalcemia commonly observed in this condition (150) or a direct inhibitory effect of aluminum on parathyroid cell function (151). Direct toxic effects of the trace element have also been demonstrated in studies in vitro (152,153).Observations made in experimental animals and results of clinical studies have been less clear. Whereas some experiments indicated that aluminum overload did not decrease plasma PTH levels in vivo (152,153), other experiments reported a decrease (154,155). Whatever the mechanisms involved, subsequent clinical data clearly showed that the introduction of an aluminum-free dialysis fluid and the discontinuation of aluminum contamination of the dialysate or aluminum removal with deferoxamine resulted in an increase in plasma PTH levels and in PTH response to hypocalcemia (156). Thus, although there appears to be an association between aluminum toxicity and parathyroid gland function, the interaction is complex.

 

Post-Receptor Mechanisms Involved in Polyclonal Parathyroid Tissue Growth

 

As pointed out above, calcitriol reduces parathyroid cell proliferation by decreasing the expression of the early gene, c-myc. This gene modulates cell cycle progression from G1 to S phase. A decrease in plasma calcitriol and/or a disturbance of its action at the level of the parathyroid cell, which are both frequently observed in uremic patients, may cause disinhibition of c-myc expression and progression into the cell cycle. Another mode of action involves the cyclin kinase inhibitor p21WAF1. Calcitriol has long been shown to induce the differential expression of p21WAF1 in the myelo-monocytic cell line U937 and to activate the p21 gene transcriptionally in a VDR-dependent, but p53-independent, manner, thereby arresting parathyroid growth (157). Slatopolsky’s group further showed that the administration of calcitriol to moderately uremic rats enhanced parathyroid p21 expression and prevented high phosphate-induced increase in parathyroid TGF-α content (157). In addition, they found that calcitriol altered membrane trafficking of the epithelial growth factor receptor (EGFR), which binds both EGF and TGF-α, and down-regulated EGFR mediated growth signaling (158). Induction of p21 and reduction of TGF-α content in the parathyroid glands also occurred when uremia-induced parathyroid hyperplasia was suppressed by high dietary Ca intake. The mechanisms by which a phosphate-rich diet and hyperphosphatemia induce parathyroid hyperplasia, and conversely a phosphate-poor diet and hypophosphatemia inhibit parathyroid tissue growth have also been examined by this group in a detailed fashion. Thus, Dusso et al showed that feeding a low phosphate diet to uremic rats increased parathyroid p21 gene expression through a vitamin D-independent mechanism (159). When administering a high phosphate diet, p21 expression was not suppressed. In this condition, they observed an increase in parathyroid tissue TGF-α expression and a direct correlation between this expression and parathyroid cell proliferation rate. This finding is in line with the previous observation by our group of de novo TGF-α expression in severely hyperplastic parathyroid tissue of patients with ESKD (160). The inducer of TGF-α gene transcription could be activator protein 2α (AP2), whose expression and transcriptional activity at the TGF-α promoter is increased in the secondary hyperparathyroidism of CKD (161).

 

Although these findings provide more insight into the pathways by which changes in phosphate intake, and ultimately variations in extracellular phosphate concentration, control parathyroid tissue growth the exciting question of the transmembrane signal transduction mechanism and subsequent nuclear events triggered by phosphate remains yet to be answered.

 

In addition to p21 and TGF-α, a variety of other growth factors and inhibitors are probably involved in polyclonal parathyroid hyperplasia. Thus, PTHrp has been proposed as a possible growth suppressor in the human parathyroid (162). PTHrp, and probably PTH itself, also exert an inhibitory effect on PTH secretion by acting via a negative feedback loop on PTH-R1 which appears to be expressed in the parathyroid cell membrane as well (97). Table 1summarizes various changes in gene and growth factor expression, which are potentially involved in the parathyroid tissue hyperplasia of secondary uremic hyperparathyroidism. Gcm2 has been identified as a master regulatory gene of parathyroid gland development, since Gcm2 knockout mice lack parathyroid glands (163). Correa et al. found high Gcm2 mRNA expression in human parathyroid glands in comparison with other non-neural tissues and under expression in parathyroid adenomas but not in lesions of HPT secondary to uremia (164). Gcm2 expression itself is regulated by Gata3, and Gata3, in cooperation with Gcm2 and MafB, stimulates PTH gene expression, by interacting with the ubiquitous transcription factor SP1 (165). MafB probably plays a role in uremic hyperparathyroidism as well. Thus, stimulation of the parathyroid by CKD in MafB+/-mice resulted in an impaired increase in serum PTH, PTH mRNA, and parathyroid cell proliferation (166,167).

 

Table 1. Changes in Gene and Growth Factor Expression Potentially Involved in Parathyroid Tissue Hyperplasia of Secondary Uremic Hyperparathyroidism

Early immediate genes and receptor/coreceptor genes

-Enhanced c-myc gene expression (Fukagawa et al, Kidney Int 1991; 39: 874-81)

-Decreased calcium-sensing receptor (CaSR) gene expression (Kifor et al, J Clin Endocrinol Metab 1996; 81: 1598-1606. Gogusev et al, Kidney Int 1997; 51: 328-36)

-Decreased vitamin D receptor (VDR) gene expression (Fukuda et al, J Clin Invest 1993; 92: 1436-42)

-Decrease in parathyroid Klotho and FGFR1c gene expression (Galitzer et al, Kidney Int 2010; 77: 211-8. Canalejo et al, JASN 2010; 21: 1125-35. Komaba et al, Kidney Int 2010; 77: 232-8)

Gene polymorphisms

-Vitamin-D receptor (VDR) gene polymorphism (Olmos et al, Methods Find Exp Clin Pharmacol 1998; 20: 699-707. Fernandez et al, J Am Soc Nephrol 1997; 8: 1546-52. Tagliabue et al, Am J Clin Pathol 1999; 112: 366-70)

Growth factors and cell cycle inhibitors

=Increased acidic growth factor (aFGF) gene expression (Sakaguchi, J Biol Chem 1992; 267: 24554-62)

-Decreased parathyroid hormone-related peptide (PTHrp) gene expression (Matsushita et al, Kidney Int 1999; 55: 130-8)

-De novo transforming growth factor-α (TGF-α) gene expression (Gogusev et al, Nephrol Dial Transplant 1996; 11: 2155-62)

-Induction of TGF-α by high phosphate diet (Dusso et al, Kidney Int 2001; 59: 855-865)

-Insufficient inhibition of cyclin kinase inhibitor p21WAF1 (Dusso et al, Kidney Int 2001; 59: 855-65); p21WAF1can be induced by calcitriol (Cozzolino et al, Kidney Int 2001; 60: 2109-2117)

-mTOR activation and rpS6 phosphorylation (Volovelsky et al, JASN 2016; 27: 1091–1101)

Gene mutations: association with monoclonal or multiclonal growth

-Mutation of menin gene (Falchetti et al, J Clin Endocrinol Metab 1993; 76: 139-44. Tahara et al, J Clin Endocrinol Metab 2000; 85: 4113-7. Imanishi et al, J Am Soc Nephrol 2002;13:1490-8)

-Mutation of Ha-ras gene (Inagaki et al, Nephrol Dial Transplant 1998; 13: 350-7)

-No involvement of VDR or CaSR gene mutations (Degenhardt et al, Kidney Int 1998; 53: 556-61. Brown et al, J Clin Endocrinol Metab 2000; 85: 868-72)

 

SECONDARY HYPERPARATHYROIDISM IN CKD – MECHANISMS INVOLVED IN THE TRANSFORMATION OF POLYCLONAL TO MONOCLONAL PARATHYROID GROWTH

 

In severe forms of secondary hyperparathyroidism nodular formations within diffusely hyperplastic tissue are a frequent finding (168). This observation probably corresponds to the occurrence of a monoclonal type of cell proliferation within a given tissue, which initially exhibits polyclonal growth. Clonal, benign tumoral growth was initially shown by Arnold et al using chromosome X-inactivation analysis method (169), and subsequently confirmed by other groups (170,171). After the initially diffuse, polyclonal hyperplasia, with the progression of CKD towards ESKD foci of nodular, monoclonal growth may arise within one or several parathyroid glands which eventually may transform to diffuse monoclonal neoplasia leading to an aspect comparable to that of primary parathyroid adenoma. Several different clones often coexist in same patient, and sometimes even in a single parathyroid gland. Figure 13 shows the progression from polyclonal to monoclonal and/or multiclonal parathyroid hyperplasia (172). It also shows corresponding changes in ultrasonographic features.

Figure 13. Schematic representation of the transformation of parathyroid hyperplasia from polyclonal to nodular, monoclonal/multiclonal growth with the progression of CKD towards ESKD. After Tominaga et al (172).

Acquired mutations of tumor enhancer or tumor suppressor genes are almost certainly involved in the development of such cell clones but precise knowledge about acquired genetic abnormalities remains limited (170). To identify new locations of parathyroid oncogenes or tumor suppressor genes important in this disease, Imanishi et al performed both comparative genomic hybridization (CGH) and genome-wide molecular allelotyping on a large number of uremia-associated parathyroid tumors (173). One or more chromosomal changes were present in 24% of tumors, markedly different from the values in common sporadic adenomas (28%), whereas no gains or losses were found in 76% of tumors. Two recurrent abnormalities were found, namely gain of chromosome 7 (9% of tumors) and gain of chromosome 12 (11% of tumors).  Losses on chromosome 11, the location of the MEN1 tumor suppressor gene, occurred in only one uremia-associated tumor (2%), as compared to 34% in adenomas. The additional search for allelic losses with polymorphic microsatellite markers led to the observation of recurrent allelic loss on 18q (13% of informative tumors). Lower frequency loss was detected on 7p, 21q, and 22q. Interestingly, the cyclin D1 oncogene, activated and overexpressed by clonal gene rearrangement or other mechanisms in 20-40% of parathyroid adenomas (174,175) has not been found to be overexpressed in uremia-associated tumors (175).

 

Another interesting question was if somatic genes played a major role in the normal regulation of parathyroid function, such as the CaSR and VDR genes.  The expression of these two genes was found to be decreased in the hyperplastic parathyroid tissue of uremic patients (62,90,91). The decrease was particularly marked in nodular areas, as compared to diffuse areas of parathyroid gland hyperplasia. Moreover, in uremic rats the decrease in CaSR expression was inversely related to the degree of parathyroid cell proliferation (93). However, the search for mutations or deletions of the VDR gene or the CaSR gene in uremic hyperparathyroidism has remained unsuccessful (170,176,177). The question remains unsolved whether the downregulation of CaSR and VDR expression is a primary event or whether it is secondary to hyperplasia.

 

Whether benign parathyroid tumors may evolve towards malignant forms is still subject to debate. Since in patients on dialysis therapy parathyroid carcinoma is a rare event (178–180), malignant transformation of clonal parathyroid neoplasms is probably exceptional.

Genome-wide allelotyping and CGH have directly confirmed the presence of monoclonal parathyroid neoplasms in uremic patients with refractory secondary hyperparathyroidism whereas the candidate gene approach has led to only modest results. Somatic inactivation of the MEN1 gene does contribute to the pathogenesis of uremia-associated parathyroid tumors, but its role in this disease appears to be limited, and there is probably no role for DNA changes of the CaSR and VDR genes. Recurrent DNA abnormalities suggest the existence of new oncogenes on chromosomes 7 and 12, and tumor suppressor genes on 18q and 21q, involved in uremic hyperparathyroidism. Finally, patterns of somatic DNA alterations indicate that markedly different molecular pathogenetic pathways exist for clonal outgrowth in severe uremic hyperparathyroidism, as compared to common sporadic parathyroid adenomas. Our group did not find a correlation between the presence of microscopically evident nodules and the clonal character of resected parathyroid tissue, and appearances of several glands with histologic patterns of diffuse hyperplasia also were unequivocally monoclonal in the absence of detectable nodular formations, suggesting that the current criteria for pathological diagnosis do not reflect the genetic differences among these two histopathological types (169).

 

Parathyroid Cell Apopotosis

 

It remains uncertain whether reduced apoptosis rates can also contribute to parathyroid tissue hyperplasia (68,181,182). One research group examined this issue in rats with short-term kidney failure (5 days). They were unable to detect apoptosis in hyperplastic parathyroid glands (183). However, this failure could be due to lack of sensitivity of the employed methods.

 

Negative findings in rats, with no identifiable apoptotic figures at all in parathyroid glands (68,182,183), contrast with subsequent positive observations in rats by others (184,185) and with personal observations of significant apoptotic figures in hyperplastic parathyroid glands removed from uremic, severely hyperparathyroid patients during surgery (186). In our study of human parathyroid glands from patients with ESKD approximately ten times higher apoptotic cell numbers were observed than in normal parathyroid tissue, using Tunel method (Figure 14) (186).

Figure 14. Increased proportion of apoptotic (TUNEL positive) cells in parathyroid glands from patients with primary or secondary uremic hyperparathyroidism, as compared to normal parathyroid tissue. After Zhang et al (186).

Of note, the uremic state appears to stimulate apoptosis in other cell types as well such as circulating monocytes (187), possibly via the well-known increase of cytosolic Ca2+ which has been observed in a variety of cell types in kidney failure (188), and also possibly via the noxious effect of bioincompatible dialysis membranes used for renal replacement therapy (189). The observed enhancement of parathyroid tissue apoptosis could compensate, at least in part, for the increase in parathyroid cell proliferation observed in secondary uremic hyperparathyroidism.

 

SECONDARY HYPERPARATHYROIDISM IN CKD – REGRESSION OF PARATHYROID HYPERPLASIA?

 

Whether regression of parathyroid hyperplasia occurs in animals or patients with advanced stages of CKD remains a matter of debate. According to some authors regression must be an extremely slow process, if it occurs at all (71,182). This is in sharp contrast to the rapid reversibility of excessive PTH secretion in uremic rats which was observed after normalization of renal function by kidney transplantation (190), although parathyroid mass probably did not rapidly decrease in this acute experimental model.

 

The issue of regression is of clinical importance. As an example, if a patient on dialysis therapy has a dramatic increase in total parathyroid mass there is practically no chance to obtain gland mass regression after successful kidney transplantation. In this condition it would seem appropriate to perform a surgical parathyroidectomy prior to transplantation. If, however significant regression of hyperplasia can occur as an active or passive process, namely by enhanced apoptosis or reduced proliferation, prophylactic surgery could be avoided. That regression of parathyroid hyperplasia secondary to vitamin D deficiency can occur has been convincingly demonstrated many years ago in experiments done in chicks (191). The administration of cholecalciferol to these birds that had developed an increase in parathyroid gland mass when fed a rachitogenic, vitamin D-free diet for 8-10 weeks led to a significant (50%) reduction in gland weight. Calcitriol failed to achieve same effect at a low, albeit hypercalcemic, dose but was capable of reducing gland mass at a higher dose. However, in an experimental dog model no parathyroid mass regression was found when the animals were first treated with a low-calcium, low-sodium, and vitamin D deficient diet for two years and subsequently a normal diet for another 17 months (192). In uremic animals, evidence for or against the possibility of regression of increased parathyroid tissue mass remains sparse and inconclusive.

 

The calcimimetic drug NPS R-568 was shown to decrease parathyroid cell proliferation and to prevent parathyroid hyperplasia in 5/6th nephrectomized rats; however, it was unable to entirely revert established hyperplasia (183,193). In apparent contrast, Miller et al showed that in rats with established secondary hyperparathyroidism, cinacalcet administration led to complete regression of parathyroid hyperplasia (194). The cinacalcet-mediated decrease in parathyroid gland size was accompanied by increased expression of the cyclin-dependent kinase inhibitor p21. However, these were short-term experiments over an 11-week time period. Interestingly, the prevention of cellular proliferation with cinacalcet occurred despite increased serum phosphorus and decreased serum calcium levels.

 

In patients with primary hyperparathyroidism spontaneous remission of parathyroid over function has been observed in rare instances, caused by parathyroid “apoplexy” due to tissue necrosis (195). The diagnosis of parathyroid tissue necrosis is more difficult to ascertain in secondary than in primary forms of hyperparathyroidism because the hyperplasia of the former is not limited to a single gland.

 

Regression of parathyroid hyperplasia in hemodialysis patients in response to intravenous calcitriol pulse therapy for 12 weeks has been reported by Fukagawa et al using ultrasonography (196). These authors observed a significant decrease in mean gland volume from 0.87 to 0.51 cm3 of this time period, together with a reduction in serum iPTH of more than 50%. In contrast, Quarles et al who also examined parathyroid gland morphology in hemodialysis patients in vivo in response to intermittent intravenous or oral calcitriol treatment for 36 weeks failed to observe a decrease in parathyroid gland size as assessed by high resolution ultrasound and/or magnetic resonance imaging (197). Mean gland size was 1.9 and 2.1 cm3 before and 3.3 and 2.3 cm3 after oral and intravenous calcitriol therapy, respectively. The authors achieved an overall maximum average serum PTH reduction of 43% over this time period. There were marked differences between these two studies which may explain the apparently diverging results. Hyperparathyroidism probably was more severe in the latter than in the former. Although initial mean serum iPTH levels were similar, serum phosphorus was higher and the decrease in serum PTH achieved in response to calcitriol was less marked in the latter. Moreover, parathyroid mass was more than double. In another study, Fukagawa et al examined the possible relation between parathyroid size and the long-term outcome after calcitriol pulse therapy, by subdividing patients into different groups according to initial parathyroid gland volume assessments (198). In two hemodialysis patients with detectable gland(s), in whom the size of all parathyroid glands as well as PTH hypersecretion regressed to normal, secondary hyperparathyroidism remained controllable for at least 12 months after switching to conventional oral active vitamin D therapy. In contrast, in seven hemodialysis patients, in whom the size of all parathyroid glands did not regress to normal by calcitriol pulse therapy, secondary hyperparathyroidism relapsed after switching to conventional therapy although PTH hypersecretion could be controlled temporarily. Similarly, Okuno et al. showed in a study in hemodialysis patients that plasma PTH levels and the number of detectable parathyroid glands decreased in response to the active vitamin D derivative maxacalcitol (22-oxacalcitriol) given for 24 weeks only when the mean value of the maximum diameter of one of the parathyroid glands was less than 11.0 mm, but not when it was above that value (199).

 

Taken together, these findings suggest that the degree of parathyroid hyperplasia, as detected by ultrasonography, is an important determinant for regression in response to calcitriol therapy. It is probable, although not proven, that the type of hyperplasia, namely monoclonal/multiclonal vs polyclonal growth, is even more important as regards the potential of regression than the mere size of each gland.

 

Figure 2 (see above) summarizes in a schematic view the main mechanisms involved in the abnormal PTH synthesis and secretion and in parathyroid tissue hyperplasia. Figure 2 further points to the possible counterregulatory role of apoptosis.

 

ALTERED PTH METABOLISM AND RESISTANCE TO PTH ACTION

 

PTH metabolism is greatly disturbed in CKD. Normally, most of full-length PTH1-84 is transformed in the liver to the biologically active N-terminal PTH1-34 fragment and several other, inactive C-terminal fragments. The latter are mainly catabolized in the kidney and the degradation process involves solely glomerular filtration and tubular reabsorption, whereas the N-terminal PTH1-34 fragment undergoes both tubular reabsorption and peritubular uptake, as does the full-length PTH1-84 molecule (200). Tubular reabsorption involves the multifunctional receptor megalin (201).

 

With the progression of CKD, both pathways of renal PTH degradation are progressively impaired. This leads to a marked prolongation of the half-life of C-terminal PTH fragments in the circulation (202–204) and their accumulation in the extracellular space. Moreover, there is no peritubular metabolism of PTH1-84 in uremic non-filtering kidneys, in contrast to peritubular uptake by normal, filtering kidneys (205). Hepatic PTH catabolism appears however to be unchanged in CKD since uremic livers and control livers released equal amounts of immunoreactive C-terminal PTH fragments (205).

 

A decreased response to the action of PTH may be another factor involved in the stimulation of the parathyroid glands in CKD. A diminished calcemic response to the infusion of PTH has long been reported, suggesting that PTH over secretion was necessary to maintain eucalcemia. The skeletal resistance to PTH has been attributed to variousmechanisms, including impaired vitamin D action in association with hyperphosphatemia, overestimation of true PTH(1-84) by assays measuring iPTH (see below), accumulation of inhibitory PTH fragments, oxidative modification of PTH, increase in circulating osteoprotegerin and sclerostin levels, administration of active vitamin D derivatives and calcimimetics, and altered PTH-R1 expression (7,141,206,207). Concerning the latter mechanism, studies have suggested the presence of PTH receptor isoforms in various organs of normal rats. Downregulation of PTH-R1 mRNA has been observed in various tissues in uremic rats (208–211) and also in osteoblasts of patients with end-stage renal disease (212). However, the issue of PTH-R1 expression in bone tissue remains a matter of controversy since onegroup found it to be upregulated in patients with moderate to severe renal hyperparathyroid bone disease (213). A recent study claimed that inhibition of PTH binding to PTH-R1 by soluble Klotho could represent yet another mechanism of PTH resistance (214). This observation would be compatible with the presence of upregulated, yet biologically inactive PTH-R1.

 

Other mechanisms involved in the control of the normal balance between bone formation and resorption and their response to PTH are the Wnt-β-catenin signaling pathway and its inhibition by sclerostin and Dickkopf-related protein 1 (Dkk1) (7,56,215), and the activin A pathway with its inhibition by a decoy receptor (216). Wnt-β-catenin inhibitors are expressed predominantly in osteocytes. Whereas reduced activity of sclerostin and Dkk1 leads to increased bone mass and strength, the opposite occurs in animal models with overexpression of both sclerostin and Dkk1. In CKD, circulating levels of both Wnt—β-catenin inhibitors have generally been found to be increased (56), and serum sclerostin was found to correlate negatively with serum PTH (217,218), and PTH has been shown to blunt osteocytic production of this Wnt inhibitor (219). Since high PTH and sclerostin levels coexist in CKD this raises the suspicion that sclerostin contributes to PTH resistance in CKD (7).  Calcitonin and bone morphogenetic proteins stimulate, whereas PTH and estrogens suppress the expression of sclerostin and/or Dkk1 (220,221). Bone formation induced by intermittent PTH administration to patients with osteoporosis could be explained, at least in part, by the ability of PTH to downregulate sclerostin expression in osteocytes, permitting the anabolic Wnt signaling pathway to proceed (222).In patients with ESKD sclerostin is a strong predictor of bone turnover and osteoblast number (223). Serum levels of sclerostin correlate negatively with serum iPTH in such patients. Sclerostin was superior to iPTH for the positive prediction of high bone turnover and number of osteoblasts. In contrast, iPTH was superior to sclerostin for the negative prediction of high bone turnover and had similar predictive values as sclerostin for the number of osteoblasts. Serum sclerostin levels increase after parathyroidectomy (7). As regards activin A, a member of the transforming growth factor-b superfamily, Hruska’s group has demonstrated increased serum levels and systemic activation of its receptors in mouse models of CKD (216). In humans, serum activin A levels increase already at early stages of CKD, before elevations in intact PTH and FGF23, equally pointing to a role in CKD-MBD and PTH resistance (224).  

 

Interesting new pathways have recently been identified by Pacifici’s group. First, they used various mouse models to demonstrate a permissive activity of butyrate produced by the gut microbiota, required to allow stimulation of bone formation by PTH (225). Butyrate’s effect was mediated by short-chain fatty acid receptor GPR43 signaling in dendritic cells and by GPR43-independent signaling in T cells. Second, the group showed that intestinal segmented filamentous bacteria (SFB) enabled PTH to expand intestinal TNF+ T and Th17 cells and thereby increase their egress from the intestine and recruitment to the bone marrow to cause bone loss (226). Figure 15 shows these recently detected pathways involving the gut microbiota.

Figure 15. Importance of intestinal microbiota for PTH action in bone. The stimulation of bone anabolism by PTH requires butyrate formation by short chain fatty acid (SCFA) producing gut bacteria. CKD probably reduces its production. Butyrate increases the frequency of regulatory T (Treg) cells in the intestine and in the bone marrow and potentiates the capacity of intermittently administered PTH to induce the differentiation of naïve CD4+ T cells into Tregs, a population of T cells which induces conventional CD8+ T cells to release Wnt10b. This osteogenic Wnt ligand activates Wnt signaling in osteoblastic cells and stimulates bone formation. Butyrate enables intermittent PTH dosing to expand Tregs via GPR43 signaling in dendritic cells (DCs) and GPR43 independent targeting of T cells. Butyrate may also affect bone remodeling by modulating osteoclast genes. The stimulation of bone resorption by PTH requires the presence of segmented filamentous bacteria (SFB) within gut microbiota for the production of Th17 cells in intestinal Peyers' plaques. Continuously elevated PTH levels lead to TNF+ T cell expansion in the gut and the bone marrow via a microbiota-dependent, but SFB independent mechanism. Furthermore, intestinal TNF producing T cells are required for PTH to increase the number of intestinal Th17 cells, and TNF mediates the migration of intestinal Th17 cells to the bone marrow. This migration depends on upregulation of chemokine receptor CXCR3 and chemokine CCL20. Bone marrow Th17 cells then induce osteoclastogenesis by secreting IL-17A, RANKL, TNF, IL-1, and IL-6. From Massy & Drueke (227).

It will be interesting to examine the hypotheses that the excessive bone resorption associated with secondary hyperparathyroidism in CKD is at least partially due to either insufficient intestinal butyrate availability, excessive intestinal SFB activity, or both (227).

 

SECONDARY HYPERPARATHYROIDISM IN CKD – CLINICAL FEATURES

 

In most patients with ESKD, even advanced secondary hyperparathyroidism remains a clinically silent disease. Clinical manifestations are generally related to severe osteitis fibrosa and to the consequences of hypercalcemia and/or hyperphosphatemia.

 

Osteoarticular pain may be present. When patients become symptomatic, they usually complain of pain on exertion in skeletal sites that are subjected to biomechanical stress. Pain at rest and localized pain are rather unusual and suggest other underlying causes. Severe proximal myopathy is seen in some patients, even in the absence of vitamin D deficiency. These symptoms and signs are more frequent in patients who suffer from mixed renal osteodystrophy, resulting from a combination of parathyroid over function and vitamin D deficiency. Skeletal fractures may occur after only minor injury. They may also develop on the ground of cystic bone lesions, the so-called “brown tumors”, which occur for still unknown reasons in a small number of uremic patients with secondary hyperparathyroidism. Rupture of the patella or avulsion of tendons may be seen in advanced cases.

 

Uremic pruritus is most often associated with an elevated Ca x P product although other factors may also be involved. Related symptoms and signs are the red eye syndrome due to the deposition of calcium in the conjunctiva, cutaneous calcification, and pseudogout. The latter is a form of painful arthralgia of acute or subacute onset caused by intra-articular deposition of radio-opaque crystals of calcium pyrophosphate dehydrate.

 

The syndrome of “calciphylaxis” is an infrequent manifestation of cutaneous and vascular calcification in uremic patients which may occur in association with secondary hyperparathyroidism, although this association is by no means constant. It is characterized by a rapidly progressive skin necrosis involving buttocks and the legs, particularly the thighs. It can produce gangrene and may be fatal. It occurs as the result of arteriolar calcification and has also been termed “calcific uremic arteriolopathy” to reflect more accurately the nature of the lesion (228). Of interest, a post-hoc analysis of the EVOLVE trial in chronic hemodialysis patients recently showed that cinacalcet administration, which allows improved PTH control, resulted in a significant decrease in the incidence of calcific uremic arteriolopathy as compared to placebo (229).

 

SECONDARY HYPERPARATHYROIDISM IN CKD -- DIAGNOSIS

 

The biochemical diagnosis relies on the determination of plasma iPTH. This is also true for primary hyperparathyroidism. In patients with CKD, it has however become apparent in recent years that there are several limitations to the measurement of iPTH, in addition to the usual day-to-day variations in healthy people (230). Physiological iPTH plasma values are not normal for uremic patients since values in the normal range are often associated with low bone turnover (adynamic bone disease) whereas normal bone turnover may be observed in presence of elevated plasma intact PTH levels (231–234). It is currently unclear to what extent this is due to imperfections in the PTH assays used (see below), PTH receptor status, post-receptor events, non-PTH-mediated changes in bone metabolism (e.g., supply of vitamin D or its metabolites, supply of estrogens or androgens), or a combination of these factors.

 

The accumulation of a large non (1-84) molecular form of PTH, which is detected by iPTH (so-called "intact" PTH) assays, has been described in patients with CKD (235). The large PTH fragment was tentatively identified as hPTH(7-84) (236). This finding is of importance in the interpretation of PTH values, since true hPTH(1-84) represents only about 50-60% of the levels detected by the currently used intact PTH assays, and since PTH(7-84) antagonizes PTH(1-84) effects on serum calcium and on osteoblasts (237). Moreover, the secretory responses of hPTH(1-84) and non-hPTH(1-84) to changes in [Ca2+e] are not proportional for these two PTH moieties (87). Moreover, a large variability has been found between different assay methods used for plasma PTH measurement in patients with CKD, recognizing PTH(7-84) with various cross-reactivities (238). Varying plasma sampling and storage conditions may further complicate the interpretation of PTH results provided by clinical laboratories (239). The development of assays which detect full-length (whole) human PTH, but not amino-terminally truncated fragments (240), was initially considered as a major progress in this field. To further improve the assessment of uremic hyperparathyroidism and the associated increase in bone turnover Monier-Faugere et al proposed to calculate the ratio of PTH-(1-84) to large C-PTH fragments (241). The usefulness in the clinical setting of the whole PTH assay and of the ratio of whole PTH to PTH fragments has however not been convincingly established for the diagnosis of parathyroid over function in adult (242,243) or pediatric (244) dialysis patients. From a practical point of view, it must be pointed out that at present measurement of PTH with third-generation assays is not widely available. Another potential issue is the presence of oxidized, inactive PTH in the circulation of patients with CKD, with concentrations much higher than those of iPTH (141), although there were large interindividual variations (141). Whereas one study showed a U-shaped association of non-oxidized, but not oxidized, PTH with survival in patients on hemodialysis therapy (245), a subsequent study done in CKD stage 2-4 patients found iPTH, but not non-oxidized PTH, to be associated with all-cause death in multivariable analysis (246). The reasons for these apparently opposite findings are unclear. The assertion that PTH oxidation is a vitro artifact has been disproven recently (247). Based on personal findings, Hocher and Zeng postulated that oxidized and non-oxidized PTH should be measured separately to correctly evaluate the degree of severity and clinical relevance of parathyroid over function in CKD (142). However, in a very recent study Ursem et al observed a strong correlation between serum non-oxidized PTH and total PTH in patients with ESKD (248). Most importantly, they found that both histomorphometric and circulating bone turnover markers exhibited similar correlations with non-oxidized PTH and total PTH. The authors therefore concluded that non-oxidized PTH is not superior to total PTH as a biomarker of bone turnover in ESKD. However, presently available methods do not enable a precise distinction between biologically active and inactive PTH forms, be it through oxidative or other post-translational modifications of the hormone (249). Most importantly, we will hopefully be able in the future to rely not only on serum PTH but also on appropriate direct markers of bone structure and function for the assessment of renal osteodystrophy and on markers of cardiovascular disease related to secondary hyperparathyroidism (250).

 

Bone x-ray diagnosis is impossible in mild to moderate forms of secondary hyperparathyroidism, but relatively easy in severe forms. Nevertheless, to date x-ray diagnosis is rarely used in routine clinical praxis. Typical lesions include resorptive defects on the external and internal surfaces of cortical bone, with the resorption particularly pronounced on the subperiosteal surface. Resorption within cortical bone enlarges the Haversian channels, resulting in longitudinal striation; resorption at the endosteal surface causes cortical thinning. These lesions can be generally detected first in the hand skeleton, most characteristically at the periosteal surface of the middle phalanges (Figure 16).

Accelerated bone deposition at this site (periosteal neostosis) can also be seen. Another characteristic feature is resorptive loss of acral bone (acro-osteolysis), in particular at the terminal phalanges, at the distal end of the clavicles, and in the skull (‘pepper-pot’ aspect) (Figure 17). Whereas cortical bone is progressively thinning, the mass of spongy bone tends to increase, particularly in the metaphyses. The latter phenomenon results in a characteristic sclerotic aspect of the upper and lower thirds of the vertebrae, contrasting with rarefaction of the center (‘rugger jersey spine’). Osteosclerosis is also commonly seen in radiographs of the metaphyses of the radius and tibia.

Figure 16. Periosteal resorption and small vessel calcification in severe secondary uremic hyperparathyroidism. (a) X-ray aspect of periosteal resorption within cortical bone of middle phalanges of the hand, indicative of osteitis fibrosa, and extensive finger artery calcification in a CKD stage 5 patient with severe secondary hyperparathyroidism. (b) One year after surgical parathyroidectomy: complete bone lesion healing and disappearance of arterial calcification.

Figure 17. X-ray pepper-and-salt aspect of the skull in in a chronic hemodialysis patient with severe secondary hyperparathyroidism.

In addition to the skeletal lesions, radiographs often reveal various types of soft tissue calcification. These comprise vascular calcifications, i.e., calcification of intimal plaques (aorta, iliac arteries) (Figure 18a), as well as diffuse calcification (Mönckeberg type) of the media of peripheral muscular arteries (Figure 18b) (251).

Figure 18. Massive intima (a) and media (b) calcification of hypogastric artery in a chronic hemodialysis patient.

Of interest, media calcification of digital arteries can entirely regress after surgical parathyroidectomy (Figure 16). Calcium deposits may also be seen in periarticular tissue or bursas and may exhibit tumor-like features (Figure 19).

Figure 19. X-ray feature of a tumor-like periarticular calcification in the shoulder of a chronic hemodialysis patient with adynamic bone disease due to aluminum intoxication.

Since the development of electron-beam computed tomography (EBCT) and multiple slice computed tomography (MSCT) more reliable means have become available to assess quantitatively vascular calcification and its progression in uremic patients (252). However, these techniques are not universally available and they are costly. Moreover, they do not allow a distinction between arterial intima and media calcifications. Such a distinction can be obtained by radiograms of the pelvis and the thigh, combined with ultrasonography of the common carotid artery. Using these simple methods, London et al could show that hemodialysis patients with arterial media calcification had a longer survival than hemodialysis patients with arterial intima calcification, but in turn their survival was significantly shorter than that of hemodialysis patients without calcifications (253). Of note, both severe hyperparathyroidism and marked hypoparathyroidism favor the occurrence of the two types of calcifications in patients with ESKD (254–256). In contrast to permanent elevations in serum PTH, the intermittent administration of PTH1-34 has been shown to decrease arterial calcification in uremic rats (257) and in diabetic mice with LDL receptor deletion (258). This observation tends to demonstrate that normal parathyroid function is required not only for the maintenance of optimal bone structure and function, but also as an efficacious defense against soft tissue calcification, and that intermittent PTH administration may not only improve osteoporosis (259), but also reduce vascular calcification, at least in experimental animals.

 

SECONDARY HYPERPARATHYROIDISM IN CKD – TREATMENT

 

Medical Management

 

Presently available options of medical treatment should take into account the levels of plasma biochemistry and x-ray findings, and as a more recently recognized parameter also the dimensions of the largest parathyroid glands, as assessed by ultrasonography. A gland diameter of 5-10 mm or more is considered as being indicative of autonomous growth, which often is resistant to medical treatment (198).

 

Schematically, there are five major medical treatment options which can be combined in some cases, but not in others, namely the restriction of phosphate intake and/or the administration of calcium supplements, oral phosphate binders, vitamin D derivatives, and calcimimetics (260,261). In dialysis patients the weekly dose of renal replacement therapy is an additional important factor. An optimal dialysis technique allows controlling hyperphosphatemia, and providing enough calcium to avoid PTH stimulation by hypocalcemia during dialysis sessions.

 

When trying to control hyperparathyroidism it is important to avoid both hypocalcemia and hypercalcemia and to reduce or correct hyperphosphatemia as well. In patients with controlled plasma phosphate, this can be achieved by giving either calcitriol or one of its synthetic analogs, or by administering oral calcium supplements. For a long time, calcitriol or alfacalcidol was the preferred therapy in uremic patients with high to very high plasma intact PTH values and normal to moderately elevated plasma calcium levels, when plasma phosphate did not exceed recommended levels, namely 1.5 mmol/L for CKD stages 3-4 and 1.8 mmol/L for CKD stage 5, according to the K/DOQI guidelines of 2003 (262). However, the administration of active vitamin D derivatives often induces hypercalcemia and/or hyperphosphatemia. The KDIGO CKD-MBD guideline of 2009 (263) and its -update in 2017/2018 (264,265) suggest "maintaining iPTH levels in CKD stage 5D patients (i.e., patients receiving dialysis therapy) in the range of approximately two to nine times the upper normal limit for the assay, to keep serum calcium normal, and to decrease serum phosphorus towards the normal range." Thus, the recommended iPTH target range has become larger than with the prior K/DOQI guidelines. The KDIGO guideline further suggests that marked changes in iPTH levels in either direction within the newly defined, broadened range should "prompt initiation or change in therapy to avoid progression to levels outside of this range." The updated guideline recommends that patients with CKD stages G3a-G5 not on dialysis whose levels of intact PTH are progressively rising or persistently above the upper normal limit for the assay be evaluated for modifiable factors, including hyperphosphatemia, hypocalcemia, high phosphate intake, and vitamin D deficiency (grade 2C recommendation).

 

Vitamin D and Active Vitamin D Derivatives

 

A satisfactory degree of vitamin D repletion should probably be aimed at in case of vitamin D deficiency since the majority of patients with CKD have at least some degree of vitamin D deficiency (51,266). Relative vitamin D depletion has been shown to be an independent risk factor for secondary hyperparathyroidism in hemodialysis patients (54). Repletion with native vitamin D may lead to improved control of secondary hyperparathyroidism in patients with CKD not yet on dialysis (267) and in those treated by dialysis (268) but a beneficial effect has not been observed in a subsequent meta-analysis (269). Vitamin D repletion may allow optimal bone formation, help to avoid osteomalacia, and exert numerous other positive effects due to the pleiotropic actions of vitamin D, but most of these presumably positive actions remain a matter of debate (269,270). Most importantly, randomized controlled trials with native vitamin D or calcidiol have not been performed so far to evaluate hard clinical outcomes of patients with CKD.

 

As regards the administration of active vitamin D sterols during the course of CKD, the updated KDIGO guideline suggests that calcitriol and vitamin D analogues not be routinely used in patients with CKD G3a-G5 (Grade 2C recommendation). It further states that it is reasonable to reserve the use of these agents for patients with CKD G4-G5 with severe and progressive hyperparathyroidism (264,265).

 

To correct secondary hyperparathyroidism of moderate to severe degree the oral administration of active vitamin D derivatives is generally more efficient than that of native vitamin D. In hemodialysis patients, calcitriol or its analogs can be given either orally or intravenously. The oral administration can be on a daily basis (for instance 0.125 to 0.5 µg of calcitriol) or as intermittent bolus ingestions (for instance 0.5 to 2.0 µg of calcitriol for each dose) whereas the intravenous administration is always intermittent (also 0.5 to 2.0 µg of calcitriol or more per injection). The route and mode of administration of calcitriol or alfacalcidol probably play only a minor role. Since the highly active 1α-hydroxylated vitamin D derivatives can easily induce hypercalcemia, intensive research has focused on the development of various non-hypercalcemic analogs, including the natural vitamin D compound 24,25(OH)2 vitamin D3, 22-oxa-calcitriol (maxacalcitol), 19-nor-1,25(OH)2 vitamin D3 (paricalcitol), and 1α-(OH) vitamin D2 (hectorol). Despite numerous studies done in many patients, none of them has been shown to be entirely devoid of inducing increases in plasma calcium or phosphate, and none has been demonstrated thus far to be superior to calcitriol or alfacalcidol in the long run in controlling secondary hyperparathyroidism (271,272). An observational study by Teng et al. showed that paricalcitol administration to a large cohort of hemodialysis patients conferred a remarkable (16%) survival advantage over the administration of calcitriol (273). Numerous subsequent observational studies reported a survival benefit, either comparing treatment with active vitamin D derivatives to no treatment, or novel active vitamin D derivatives to calcitriol in CKD patients not yet on dialysis (274) or those receiving dialysis treatment (275–277). Another observational study conducted in hemodialysis patients, however, did not find a survival advantage with paricalcitol, as compared to calcitriol (278). In the absence of randomized controlled trials, it is impossible to conclude that paricalcitol treatment is superior to calcitriol or alfacalcidol in terms of patient survival. Findings of observational studies can only be considered as hypothesis-generating. They need to be confirmed by a properly designed prospective investigation (279).

 

Calcimimetics

 

The introduction of the calcimimetic cinacalcet into clinical practice led to a change in the above treatment strategy since it enables parathyroid over function control without increasing plasma calcium or phosphorus. Calcimimetics modify the configuration of the CaSR, a receptor cloned by Brown et al in 1993 (280) They make the CaSR more sensitive to [Ca2+e] in contrast to the so-called calcilytics which decrease its sensitivity, as schematically shown in Figure 20.

Figure 20. Schematic representation of the modulation of the calcium-sensing receptor (CaSR) by calcimimetics and calcilytics. CaSR is expressed on cell membrane. Calcimimetics increase its sensitivity to calcium ions whereas calcilytics decrease it.

Initial acute studies in chronic hemodialysis patients showed that the calcimimetic cinacalcet was capable of reducing plasma PTH within hours, immediately followed by a rapid decrease in plasma calcium and a minor decrease in plasma phosphate (281–283). In addition, calcimimetics can also reduce parathyroid cell proliferation. Both short-term and long-term studies performed in rats and mice with CKD showed that the administration of the calcimimetic NPS R-568, starting at the time of CKD induction, allowed the prevention of parathyroid hyperplasia (183,193,284). This effect is probably due to a direct inhibitory action on the parathyroid cell, as shown by our group in an experimental study in which we exposed human uremic parathyroid cells to the calcimimetic NPS-R467 (75). An interesting finding of a yet unexplained mechanism and significance is the observation that calcimimetic treatment led to an approximately 5-fold increase in the proportion of oxyphil cells, as compared to chief cells, in parathyroid glands removed from CKD patients with refractory hyperparathyroidism (285–287). Of note, oxyphil cells also exhibited higher CaSR expression than chief cells in such glands (288).

 

Perhaps more important from a clinical point of view, the administration of calcimimetics enabled an improvement of osteitis fibrosa (103), halted the progression of vascular calcification both in uremic animals (284,289) and probably also in dialysis patients (290), prevented vascular remodeling (291), improved cardiac structure and function (292), and prolonged survival (293) in uremic animals with secondary hyperparathyroidism.

 

The long-term administration of cinacalcet to chronic hemodialysis patients proved to be superior to optimal standard therapy in controlling secondary uremic hyperparathyroidism, in that it was able to induce not only a decrease in plasma PTH but also in plasma calcium and phosphate (294–297). Figure 21 shows the superior control of severe secondary hyperparathyroidism by cinacalcet as compared to placebo treatment with standard of care (298). The initial daily dose is 30 mg orally, which can be increased up to 180 mg if necessary. Cinacalcet is generally well tolerated, with the exception of gastrointestinal side effects, which however cease in the majority of patients with time. Since its administration generally leads to a decrease in serum calcium, a close follow-up is required, at least initially, to avoid hypocalcemia with possible adverse clinical consequences. Cinacalcet can be associated with calcium-containing and non-calcium containing phosphate binders and also with vitamin D derivatives. For PTH lowering a combination therapy may lead to more complete correction than single drug treatment because of less side-effects and greater efficacy in the control of parathyroid over function (299,300).

Figure 21. Effect of cinacalcet on need of parathyroidectomy in patients on hemodialysis therapy. In the EVOLVE trial, parathyroidectomy was performed in 140 (7%) cinacalcet-treated and 278 (14%) placebo-treated patients. Key independent predictors of parathyroidectomy included younger age, female sex, geographic region, and absence of history of peripheral vascular disease. One hundred and forty-three (7%) cinacalcet-treated and 304 (16%) placebo-treated patients met the biochemical definition of severe, unremitting (tertiary) hyperparathyroidism. Considering the pre-specified biochemical composite or surgical parathyroidectomy as an endpoint, 240 (12%) cinacalcet-treated and 470 (24%) placebo-treated patients developed severe, unremitting hyperparathyroidism (298).

The subsequent development of an intravenously active calcimetic led to another series of clinical studies aimed at controlling secondary hyperparathyroidism in patients on hemodialysis with an easy access to parenteral drug administration, thereby reducing oral pill overload. Two randomized controlled trials were conducted in such patients with moderate to severe secondary hyperparathyroidism, evaluating the efficacy and safety of the intravenous calcimimetic, etelcalcetide as compared to placebo (301). Thrice weekly administration of active drug after hemodialysis led to a greater than 30% reduction in serum PTH compared with less than 8.9% of patients receiving placebo. The reduction in PTH was rapid and sustained over 26 weeks. Treatment with etelcalcetide lowered serum calcium in the majority of patients, with overt symptomatic hypocalcemia reported in 7%. Adverse events occurred in 92% of etelcalcetide-treated and 80% of placebo-treated patients. Nausea, vomiting, and diarrhea were more common in etelcalcetide-treated patients, as were symptoms potentially related to hypocalcemia. A subsequent double-blind, double-dummy randomized controlled trial compared intravenous etelcalcetide to oral cinacalcet in patients on hemodialysis with moderate to severe secondary hyperparathyroidism (302). It showed that the use of etelcalcetide was not inferior to cinacalcet in reducing serum PTH concentrations over 26 weeks. In addition, etelcalcetide met several superiority criteria, including a greater reduction in serum PTH concentrations from baseline, and more potent reductions in serum concentrations of FGF23 and two markers of high-turnover bone disease.

 

How about hard patient outcomes? The randomized controlled trial EVOLVE examined the question whether better control of secondary uremic hyperparathyroidism by cinacalcet, as compared to placebo treatment with standard of care, reduced the incidence of cardiovascular events and mortality (298). The study enrolled 3803 patients receiving long-term hemodialysis therapy. Using intention-to-treat analysis the study outcome was negative (Figure 22, upper part). However, after adjustment for age and other confounders, and also when using lag-censoring analysis (Figure 22, lower part), there was a nominally significant reduction in the primary cardiovascular endpoint including mortality in the cinacalcet treatment group in whom serum PTH, calcium, and phosphate were better controlled than in the placebo treatment group. Moreover, a post-hoc lag-censoring analysis of EVOLVE further showed that the incidence of clinically ascertained fractures was lower in the cinacalcet than the placebo arm (303).

Figure 22. Effect of cinacalcet on cardiovascular outcomes of patients on hemodialysis therapy. The randomized controlled trial EVOLVE examined the question whether a better control of secondary uremic hyperparathyroidism by cinacalcet, as compared to placebo treatment with standard of care, reduced the incidence of cardiovascular events and mortality. The study enrolled 3803 patients receiving long-term hemodialysis therapy. Using intention-to-treat analysis the study outcome was negative (upper part of Figure). However, with lag-censoring analysis there was a nominally significant reduction in the primary composite cardiovascular endpoint in the cinacalcet treatment group in whom serum PTH, calcium, and phosphorus were better controlled than in the placebo treatment group (lower part of Figure). From Chertow et al (298).

Phosphate Binders, Inhibitors of Intestinal Phosphate Absorption, Oral Phosphate Restriction, and Phosphate Removal by Dialysis

 

Calcium-containing phosphate binders should be given, preferentially during or at the end of phosphate-rich meals, to patients with CKD and uncontrolled hyperphosphatemia who have no hypercalcemia or radiological evidence of marked soft tissue calcifications. In these latter cases non-calcium-containing phosphate binders should be preferred (see below). The administration of calcium salts alone such as calcium carbonate or calcium acetate may be sufficient for the control of hyperphosphatemia in many instances, particularly in patients with CKD stages G3-G5 not yet on dialysis. At the same time these calcium salts will prevent serum iPTH from rising in the majority of patients (304). They may however lead to calcium overload (44,45) and excessive PTH over suppression, resulting eventually in adynamic bone disease (305). In hemodialysis patients, the efficacy and tolerance of this treatment may be enhanced by the concomitant use of low-calcium dialysate, for instance a calcium concentration of 1.25 mmol/L, especially if plasma intact PTH levels are not very high. However, long-term studies have shown that the continuous use of a dialysate calcium of only 1.25 mmol/L requires close monitoring of plasma calcium and PTH because of the risk of inducing excessive PTH secretion (306,307). A dialysate calcium concentration between 1.25 and 1.5 mmol/L is more appropriate in terms of optimal calcium balance and control of secondary hyperparathyroidism (308). The use of a low calcium dialysate also may require higher doses of active vitamin D derivatives (309) or cinacalcet (310) for the control of secondary hyperparathyroidism. Of note, the use of a low calcium bath favors hemodynamic instability during the hemodialysis session (311) and the occurrence of sudden cardiac arrest (312,313). In CAPD patients, the use of calcium carbonate, in the absence of vitamin D, together with a reduction of the dialysate calcium concentration from 1.75 to 1.45 mmol/L prevents the occurrence of hypercalcemia in most patients (314). However, the addition of daily low-dose alfacalcidol may lead to hypercalcemia, despite a further reduction of dialysate calcium to 1.0 mmol/L.

 

The development of calcium-free, aluminum-free oral phosphate binders such as sevelamer-HCl (315–317), sevelamer carbonate (318,319), lanthanum carbonate (320–322), sucroferric oxyhydroxide (323) and ferric citrate (324) allows controlling hyperphosphatemia without the potential danger of calcium overload. Their phosphate binding capacity is roughly equivalent to that of Ca carbonate or calcium acetate. Sevelamer offers in addition the advantage to lower serum total cholesterol and LDL-cholesterol and to increase serum HDL-cholesterol, to slow the progression of arterial calcification in dialysis patients (316), and possibly to improve survival in such patients (325). The administration of sevelamer is probably more efficient in halting the progression of vascular calcification than calcium carbonate or calcium acetate but this remains a matter of debate (14,326,327). The administration of lanthanum carbonate to uremic animals has been shown to also reduce progression of vascular calcification (328,329), but studies in patients with CKD have led to variable results (330–332). The effects of calcium-free, aluminum-free phosphate binders on serum iPTH are variable, depending on baseline iPTH and concomitant therapies. In general, iPTH levels are higher in response to these binders than to calcium-containing phosphate binders (Figure 23) (333,334).

Figure 23. Effect of oral calcium vs. sevelamer on serum intact PTH (iPTH) in CKD. In this 54-week, randomized, open-label study the effects of sevelamer hydrochloride on bone structure and various biochemical parameters were compared to that of calcium carbonate in 119 patients on long-term hemodialysis therapy. Serum iPTH was consistently lower with calcium carbonate than with sevelamer treatment. From Ferreira et al (333).

The administration of aluminum-containing phosphate binders should be avoided because of their potential toxicity. They may be given in some treatment resistant cases, but only for short periods of time (263).

 

Another approach chosen to control hyperphosphatemia and therefore to prevent or delay the development of secondary hyperparathyroidism is pharmacologic interference with active intestinal phosphate transport by oral inhibitors of the phosphate/sodium cotransporter NaPi2b, using either already available drugs such as niacin or nicotinamide (335–337), or recently developed novel inhibitors such as tenapanor (338,339). The rather disappointing results of available studies have not let so far to their introduction into clinical practice (340).

 

Dietary phosphate intake should be assessed and diminished, if possible. Special attention should be given to the avoidance of foods containing phosphate additives (341). The spontaneous reduction of protein intake with age probably explains the often better control of serum phosphate in elderly as compared to younger patients with ESKD, and this may contribute to the relatively lower PTH levels of the former and their propensity to develop adynamic bone disease (342). However, when reducing dietary phosphate intake and concomitantly protein intake, one has to take care to avoid the induction of a protein malnutrition state. Restricting dietary protein intake excessively may lead to greater mortality (343). In dialysis patients, an attempt should always be made as well to improve the efficiency of the dialysis procedure.

 

A better correction of metabolic acidosis by bicarbonate-buffered dialysate, as compared to acetate-buffered dialysate, probably helps to delay the progression of osteitis fibrosa in hemodialysis patients (344). One possible mechanism for the beneficial role of acidosis correction is an increase in the sensitivity of the parathyroid gland to plasma ionized calcium (345).

 

Current recommendations for the medical treatment and prevention of patients with CKD-MBD, including secondary hyperparathyroidism, can be found in the 2009 KDIGO CKD-MBD                                                                                                                                                                                                                                                                                      guideline (263) and its recent update (264,265). It must be pointed out though that there is no definitive proof of a beneficial effect of phosphate lowering on patient-level outcome (247).

 

Local Injection of Alcohol and Active Vitamin D Derivatives

 

Since in advanced forms of secondary hyperparathyroidism the hyperplasia of parathyroid glands is asymmetrical, with some glands being grossly enlarged and others remaining relatively small, local injection of ethanol (346,347) or active vitamin D derivatives (348,349) has been proposed as an alternative therapy in patients who become resistant to medical treatment. However, the direct injection technique has not reached widespread use in clinical practice outside of Japan. Other research groups have been unable to obtain convincing results (350,351).

 

Despite major advances in the medical treatment of CKD-MBD the achievement of the targets for plasma calcium, phosphate, Ca x P product, and PTH, as recommended by the K/DOQI guidelines (262), was found to be far from being optimal in the DOPPS patient population for the years 2002-2004 (352). It was actually rare in the hemodialysis patients of this international cohort to fall within recommended ranges for all four indicators of mineral metabolism, although consistent control of all three main CKD-MBD parameters calcium, phosphate, and PTH was found to be a strong predictor of survival in hemodialysis patients in an observational study (353). A recent report on chronic hemodialysis patients in France confirmed that a satisfactory control of serum calcium, phosphate, or PTH was achieved in less than 20% among them (354).

 

Surgical Treatment

 

Surgical correction remains the final, symptomatic therapy of the most severe forms of secondary hyperparathyroidism, which cannot be controlled by medical treatment (355). The most important goal remains to prevent or correct the development of major clinical complications associated with this disease. The presence of severe parathyroid over function must be ascertained by clinical, biochemical and radiological evidence. In general, neck surgery should only be done when plasma iPTH values are greatly elevated (> 600-800 pg/mL), together with an increase in plasma total alkaline phosphatases (or better bone-specific alkaline phosphatase), and only after one or several medical treatment attempts have remained unsuccessful in decreasing plasma iPTH with cinacalcet (in dialysis patients only) and/or active vitamin D derivatives or if their use is relatively or absolutely contraindicated, namely in presence of persistent hypercalcemia, marked hyperphosphatemia, or severe vascular calcifications. Bone histomorphometry examination is rarely needed. Clinical symptoms and signs such as pruritus and osteoarticular pain are non-specific and therefore not good criteria for operation by their own. Similarly, an isolated increase in plasma calcium and/or phosphate, even in case of coexistent soft tissue calcifications, is not a sufficient criterion alone for surgical parathyroidectomy. However, in the presence of a persistently high plasma PTH the latter disturbances may facilitate the decision to proceed to surgery. The results can be spectacular, including in rare instances the complete disappearance of soft tissue calcifications from small peripheral arteries (see Figure 15b). A concomitant aluminum overload should be excluded or treated, if present, before performing surgery.

 

Two main surgical procedures are generally used, either subtotal parathyroidectomy or total parathyroidectomy with immediate auto-transplantation. There is no substantial difference of operative difficulties and treatment results between the two procedures. We found that the long-term frequency of recurrent hyperparathyroidism was similar (356). One group of authors claimed superiority of total parathyroidectomy without reimplantation of parathyroid tissue in terms of long-term control of parathyroid over function, tolerance, and safety (357), but this claim has been questioned by us and others (358–360). We do not recommend the performance of total parathyroidectomy without auto-transplantation in uremic patients since permanent hypoparathyroidism and adynamic bone disease may ensue, with possible harmful consequences especially for those patients who subsequently undergo kidney transplantation.

 

As regards the prevalence of parathyroidectomy it was very high before the turn of the century and did not change significantly between 1983 and 1996. According to a survey from Northern Italy in 7371 dialysis patients (361) it was 5.5% in the all patients together but increased with duration of RRT, from 9.2% after 10-15 years to 20.8% after 16-20 years of dialysis therapy. A recent survey from the US showed that parathyroidectomy rates were much lower in the first decade of the 21st century. It decreased from 7.9% in 2003 to a nadir of 3.3% in 2005 - most likely due to the commercial introduction of cinacalcet, then increased again to 5.5% through 2006, and subsequently remained stable until 2011 (362). The authors concluded that despite the use of multiple medical therapies rates of parathyroidectomy in patients with secondary hyperparathyroidism did not decline in recent years. These findings are in contrast with a Canadian study (363) and the international Dialysis Outcomes and Practice Patterns Study (DOPPS) (364). The Canadian study, although restricted to a single province (Quebec), showed a sustained reduction in parathyroidectomy rates after 2006. The DOPPS reported that prescriptions of active vitamin D analogs and cinacalcet increased and that parathyroidectomy rates decreased. Difference in medical treatment modalities between geographic regions and different modes of data analysis may at least partially account for these apparent discrepancies. This is illustrated by the observation that parathyroidectomy rates in Japan fell abruptly after the advent of cinacalcet to approximately 2%, with median serum iPTH around 150 pg/mL between 1996 and 2011 (364).

 

Parathyroidectomy was associated with higher short-term mortality, but lower long-term mortality among chronic dialysis patients in the US (365). Whether presently available therapeutic and prophylactic measures taken to attenuate secondary hyperparathyroidism play an important role in reducing cardiovascular morbidity and mortality among patients with ESKD remains a matter of debate. The EVOLVE trial points to better clinical outcomes with a more efficient control of parathyroid over function by cinacalcet than by optimal standard treatment but the results, although suggestive, must still be considered as not definitively conclusive (298,303).

 

ACKNOWLEDGEMENTS

 

The author wishes to thank Ms Martine Netter, Paris for expert assistance in Figure design.

 

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Hypophysitis

ABSTRACT

 

Hypophysitis is an inflammation of the pituitary gland and is a rare cause of hypopituitarism. It can be primary (idiopathic) or secondary to sella and parasellar lesions, systemic diseases, or drugs (mainly immune checkpoint inhibitors). Primary hypophysitis has five histologic variants: lymphocytic, granulomatous, xanthomatous, IgG4-related, and necrotizing. Lymphocytic hypophysitis is the most common form; it is likely an autoimmune disease and is more frequently observed in females during pregnancy or postpartum. Granulomatous hypophysitis is the second most common variant and possible secondary causes of granulomatous infiltration of the pituitary should be excluded before concluding that a case of granulomatous hypophysitis is idiopathic. Xanthomatous, necrotizing, and IgG4-related hypophysitis are very rare and the latter is often the manifestation of a systemic disease with multi-organ involvement (IgG4-related disease). Immune checkpoint inhibitors are monoclonal antibodies increasingly used for the treatment of solid and hematological malignancies. They cause a T-lymphocyte activation and proliferation that lead to the anti-tumor response, and may cause autoimmune manifestations known as part of what is called “immune-related adverse events”. A significant number of patients treated with immune checkpoint inhibitors develop immune-related hypophysitis and require prompt diagnosis and treatment. Regardless of the etiology, patients with hypophysitis present with various signs and symptoms caused by the pituitary inflammation that can lead to hypopituitarism and compression of sella and parasellar structures. Contrary to other causes of hypopituitarism, adrenocorticotropic hormone and thyroid-stimulating hormone deficiencies are very frequent in the early stages of hypophysitis and must be identified immediately. The diagnosis of hypophysitis is based on clinical, laboratory, and radiological data; while pituitary biopsy is the gold standard test for diagnosing primary hypophysitis, it should be reserved only for selected cases. Magnetic resonance imaging is the technique of choice for suspected hypophysitis, and the main differential diagnoses are pituitary adenomas in adults, germinomas, and Langerhans cell histiocytosis in adolescents, and metastases in those receiving immune checkpoint inhibitors. The mainstay of treatment of patients with hypophysitis is pituitary hormone replacement. Those with severe signs and symptoms of sella compression should be treated with high-dose glucocorticoids, which usually cause an excellent initial response, although relapse of the pituitary inflammation is common. Pituitary surgery should be considered in patients who do not respond to glucocorticoids and have progressive and debilitating symptoms. Pituitary fibrosis and atrophy often develop in the late stage of the disease, with persistent hypopituitarism.

 

 

INTRODUCTION

 

Hypophysitis is a generic term that includes a variety of conditions that cause inflammation of the pituitary gland. It is an infiltrative cause of hypopituitarism and can cause symptoms related to sella compression and pituitary hormone deficiencies.

 

Hypophysitis can be classified according to the anatomic location of pituitary involvement (adenohypophysitis, infundibulo-neurohypophysitis, or panhypophysitis) and the cause (primary or secondary forms) (Table 1) (1-4). The primary forms are characterized by an idiopathic inflammatory process confined to the pituitary gland, while the secondary forms are triggered by a definite etiology (drugs and intracranial or systemic diseases). Five histologic variants of primary hypophysitis have been described: lymphocytic, granulomatous, xanthomatous, IgG4-related, and necrotizing (Table 1). Lymphocytic hypophysitis is the most common form of hypophysitis and occurs most commonly in women during late pregnancy and the postpartum period. However, thanks to the increasing use over the last two decades of monoclonal antibodies inhibiting immune checkpoints for the treatment of several solid and hematological malignancies, new immune-related adverse events have emerged, with hypophysitis being a relatively common occurrence.

 

Table 1. Classification of Hypophysitis

CAUSE: PRIMARY AND SECONDARY HYPOPHYSITIS

· Primary hypophysitis:

o Isolated

o Associated with autoimmune diseases:

•  Polyglandular autoimmune syndromes

•  Autoimmune thyroiditis (Hashimoto thyroiditis)

•  Autoimmune adrenalitis

•  Type 1 diabetes mellitus

•  Lymphocytic parathyroiditis

•  Idiopathic inflammatory myopathy

•  Systemic lupus erythematosus

•  Sjogren’s syndrome

•  Rheumatoid arthritis

•  Primary biliary cirrhosis

•  Atrophic gastritis

•  Optic neuritis

•  Myocarditis

•  Temporal arteritis

•  Bechet’s disease

•  Retroperitoneal fibrosis

•  Erythema nodosum

•  Idiopathic thrombocytopenic purpura

•  Dacryoadenitis

•  Autoimmune thrombocytopenia

•  Autoimmune encephalitis

· Secondary hypophysitis:

o Drugs:

•  Immune checkpoint inhibitors

•  Interferon-α

•  Ribavirin

•  Ustekinumab

o Sella and parasellar diseases*:

•  Germinoma

•  Rathke’s cleft cyst

•  Craniopharyngioma

•  Pituitary adenoma

•  Primary pituitary lymphoma

o Systemic diseases:

•  IgG4-related disease**

•  Sarcoidosis

•  Granulomatosis with polyangiitis (Wegener’s granulomatosis)

•  Langerhans cell histiocytosis

•  Erdheim-Chester’s disease

•  Rosai-Dorfman disease

•  Inflammatory pseudotumor

•  Tolosa-Hunt syndrome

•  Takayasu’s arteritis

•  Cogan’s syndrome

•  Crohn’s disease

o Thymoma and other malignancies (anti-Pit-1 antibody syndrome)

o Infections:

•  Bacteria (Mycobacterium tuberculosis; Treponema pallidum; Tropheryma whipplei; Borrelia; Brucella)

•  Viruses (Cytomegalovirus; Herpes simplex; Varicella-zoster virus; Influenza viruses; Coronavirus; Enterovirus; Coxsackie; Tick-Borne encephalitis virus; Hantavirus)

•  Mycoses (Aspergillus; Nocardia; Candida albicans; Pneumocystis jirovecii)

•  Parasites (Toxoplasma gondii)

ANATOMIC LOCATION OF PITUITARY INVOLVEMENT

· Adenohypophysitis: the inflammation involves the anterior pituitary. It accounts for ~65% of cases of primary hypophysitis

· Infundibulo-neurohypophysitis: the inflammation involves the posterior pituitary and the stalk. It accounts for ~10% of cases of primary hypophysitis

· Panhypophysitis: the inflammation involves the entire gland. It accounts for ~25% of cases of primary hypophysitis

HISTOPATHOLOGY FORMS OF PRIMARY HYPOPHYSITIS

· Lymphocytic hypophysitis (68%)

· Granulomatous hypophysitis (19%)

· IgG4-related (plasmocytic) hypophysitis (8%)**

· Xanthomatous hypophysitis (4%)

· Necrotizing hypophysitis (<1%)

· Mixed forms (lymphogranulomatous; xanthogranulomatous)

* The infiltrate focuses around the lesion rather than diffuse in the entire gland. This secondary form of pituitary infiltration is generally a histopathological finding and patient’s signs and symptoms are otherwise related to the primary sella and parasellar mass.

** IgG4-related hypophysitis can be isolated, but is often a manifestation of systemic disease with the involvement of multiple organs.

 

PRIMARY HYPOPHYSITIS

 

Primary hypophysitis is a rare disease, with just over 1300 published cases so far (5). The incidence is estimated to be ~1 in 9 million/year (4,6), and hypophysitis accounts for ~0.4% of pituitary surgery cases (2). Five histologic variants of primary hypophysitis have been described, and there are mixed forms as well. Table 2 summarizes the epidemiological and histopathological features of these variants (2,5,7-9). Primary hypophysitis, apart from the rare IgG4-related and necrotizing variants, occurs more frequently in young females. The clinical manifestations of all forms of primary hypophysitis are similar and are linked to the degree of pituitary involvement and the associated hormonal deficiencies.

 

Table 2. Characteristics of the Various Forms of Primary Hypophysitis

 

Lymphocytic

Granulomatous

IgG4-related

Xanthomatous

Necrotizing

Prevalence

The most common subtype (68%*).

The second most common subtype (19%*).

Rare (8%*). Higher prevalence in Japan and Korea.

Very rare (4%*).

Extremely rare (<1%).

Gender predominance

Female, ~3:1

Female, ~3:1

Male, ~2:1

Female, ~3:1

Male, ~2:1

Association with pregnancy

Yes. ~70% of patients present during pregnancy or postpartum.

No

No

No

No

Mean age at presentation

4th decade (women).

5th decade (men).

5th decade

7th decade (men).

2nd-3rd decade (women).

4th decade

Six cases reported (aged 12, 20, 33, 39, 40, and 52).

Histopathology

Diffuse lymphocyte infiltration (primarily T cells) of the pituitary gland. Lymphoid follicles can be observed and occasional plasma cells, eosinophils, and fibroblasts may also be present. Pituitary fibrosis and atrophy may occur in later stages of the disease.

Large numbers of multinucleated giant cells and histiocytes with granuloma formation.

Extensive gland infiltration by plasma cells with a high degree of IgG4 positivity. Storiform fibrosis is observed**. Pituitary fibrosis and atrophy occur in later stages of the disease, if not treated.

Foamy histiocytes (lipid-rich macrophages) without the presence of granulomas. Plasma cells and small round mature lymphocytes are also observed. Pituitary fibrosis may be seen in later stages of the disease.

Diffuse non-hemorrhagic necrosis with surrounding lymphocytes, plasma cells and eosinophils.

* Prevalence derived by published cases after excluding those where the pathologic variant is unknown. Forty-one cases with mixed histology findings have been published.

** Storiform fibrosis: dense, wire-like strands of fibrotic collagen deposition radiating outward from a central point.

 

Lymphocytic Hypophysitis

 

Lymphocytic hypophysitis is the most common histologic variant of primary hypophysitis (4,5,8). It shows a striking temporal association with pregnancy, with ~70% of cases in women presenting during pregnancy or postpartum. Most patients present in the last month of pregnancy or in the first 2 months after delivery (4). Lymphocytic hypophysitis is believed to have an autoimmune etiology. This is supported by the lymphocytic infiltration of the pituitary, the link with pregnancy, the frequent association with other autoimmune diseases (Table 1), the frequent finding of pituitary antibodies in these patients (see below), the association with particular human leukocyte antigen alleles (1), the improvement of symptoms in response to immunosuppressive drugs, and animal models of primary hypophysitis (10).

 

Granulomatous Hypophysitis

 

Granulomatous hypophysitis is the second most common subtype of primary hypophysitis and its etiology is unknown. Before concluding that a case of granulomatous hypophysitis is “primary” (i.e., idiopathic), known possible causes of granulomatous infiltration of the pituitary should be excluded. Possible secondary causes of granulomatous hypophysitis include tuberculosis, sarcoidosis, syphilis, Langerhans’ histiocytosis, granulomatosis with polyangiitis (formerly known as Wegener’s granulomatosis), and Rathke’s cleft cyst rupture (see “Hypophysitis secondary to sella and parasellar disease” and “Hypophysitis secondary to systemic disease” below). (11).

 

 

IgG4-related hypophysitis can be isolated (primary hypophysitis) but is often a manifestation of systemic disease with involvement of multiple organs (14,15). Some authors include IgG4-related hypophysitis among the histologic variants of primary hypophysitis, while others report this among the secondary forms of hypophysitis. Considering that the diagnosis and management does not change according to the classification used, we will discuss the features of IgG4-related hypophysitis in this section.

 

The etiology of this disease is poorly understood and may involve autoimmunity and/or an abnormal tolerance to unspecified allergens and infectious agents (16,17). IgG4-related disease is diagnosed more frequently in older males and is characterized by a dense lymphoplasmacytic infiltration with a predominance of IgG4-positive plasma cells in the affected tissue and storiform fibrosis in the more advanced stages of the disease (Table 2). One or (more frequently) multiple organs can be affected including lymph nodes, pancreas, liver, salivary and lacrimal glands, retroperitoneum, aorta, pericardium, thyroid, lungs, kidneys, skin, stomach, prostate, ovaries, and the pituitary gland (17-19). Overall, the prevalence of pituitary involvement in IgG4-related disease is believed to be low (2-8%) (20). Nonetheless, a recent cohort study from Japan screened 27 patients with IgG4-related pancreatitis via pituitary MRI and found 1 case of hypophysitis with hypopituitarism and 4 cases of empty sella (21). Patients with pituitary abnormalities were more likely to have multi-organ disease. If confirmed by large-scale studies, these findings would advocate for screening for hypophysitis especially in patients with multiple IgG4-related organ involvement.

 

IgG4-related disease is considered a rare cause of hypophysitis, although a Japanese group reported a strikingly high prevalence of IgG4-related hypophysitis in 170 consecutive patients with hypopituitarism/central diabetes insipidus and a clinical diagnosis of hypophysitis (4% and 30% respectively) (22). Moreover, Bernreuther et al. reviewed retrospectively 29 cases of biopsy-proven primary hypophysitis previously diagnosed as “lymphocytic” or “not otherwise specified, non-granulomatous” and found that 41.4% of cases fulfilled the criteria for IgG4-related hypophysitis, suggesting that this entity might be more frequent than previously thought (23). Two recent reviews of the literature found that the epidemiology of IgG4-related hypophysitis may differ according to sex: affected men were older, more likely to have systemic disease and higher IgG4 serum levels; women were younger and often presenting with isolated pituitary disease, lower IgG4 serum levels, and a concomitant diagnosis of other autoimmune diseases (24,25).

 

The diagnosis of IgG4-related hypophysitis is confirmed by characteristic histopathologic findings at pituitary biopsy. However, pituitary biopsy is an invasive procedure and other criteria can be used to establish the diagnosis (Table 3)(26).

 

Table 3. Diagnostic Criteria for IgG4-related Hypophysitis

Criteria

Established diagnosis

Criterion 1

PITUITARY HISTOPATHOLOGY: Mononuclear infiltration of the pituitary gland, rich in lymphocytes and plasma cells, with >10 IgG4-positive cells/high-power field. *

CRITERION 1

 

or

 

CRITERIA 2 + 3

 

or

 

CRITERIA 2 + 4 + 5

Criterion 2

PITUITARY MRI: Sella mass or thickened pituitary stalk.

Criterion 3

OTHER INVOLVEMENT: Biopsy-proven involvement in other organs.

Criterion 4

SEROLOGY: Serum IgG4 level >140 mg/dL (1.4 g/L).

Criterion 5

RESPONSE TO TREATMENT: Shrinkage of the pituitary mass and symptom improvement with corticosteroids.

* Low level of infiltration may be seen if the patient is receiving treatment with glucocorticoids (27)

 

It should be considered that patients with IgG4-related hypophysitis have multi-organ involvement in 60-90% of cases. Therefore, they should receive an extensive evaluation for establishing the extent of the disease after the initial diagnosis. The diagnostic work-up should include physical examination, laboratory evaluation, and whole-body imaging (19).

 

Xanthomatous Hypophysitis

 

The pituitary shows cystic-like areas of liquefaction infiltrated by lipid-rich macrophages. It has been suggested that many cases of xanthomatous hypophysitis may represent an inflammatory response to components of a ruptured Rathke’s cleft cyst (see “Hypophysitis secondary to sella and parasellar disease” below) (12,13).

 

Necrotizing Hypophysitis

 

Necrotizing hypophysitis has been reported in six patients (of which only five histology-proven) (28-30). Five patients presented with diabetes insipidus and some degree of anterior pituitary dysfunction was described in all reported cases. Frontal headache at presentation was reported in three patients (28,29). One patient presented with photophobia (29). Five patients were treated surgically and all but one had persistent postoperative panhypopituitarism and central diabetes insipidus (28-31).

 

Clinical Presentation of Primary Hypophysitis

 

The signs and symptoms at diagnosis, as well as the pituitary hormone abnormalities depend on the degree of pituitary involvement (Table 4) (4,5,8).

 

Primary hypophysitis more frequently involves the anterior pituitary and patients typically present with severe headaches, visual disturbances due to chiasmal compression, and symptoms of adrenal insufficiency. Contrary to other causes of hypopituitarism, impaired adrenocorticotropic hormone (ACTH) and thyroid-stimulating hormone (TSH) secretion is very frequent in the early stages of primary hypophysitis, putting these patients at increased risk of life-threatening adrenal insufficiency. A large case series from Germany has highlighted that secretion of gonadotropins is also impaired very frequently in these patients (32). Growth hormone (GH) deficiency and hyperprolactinemia can also occur.

 

Less frequently, the inflammation can involve primarily the posterior pituitary and the stalk. Patients with infundibulo-neurohypophysitis typically present with diabetes insipidus and other pituitary hormone deficiencies are less common. As expected, signs of both anterior and posterior pituitary involvement coexist in panhypophysitis (that is, inflammation of the entire gland).

 

Table 4. Clinical Presentation and Prevalence of Pituitary Hormone Abnormalities at Diagnosis in Patients with Primary Hypophysitis According to the Degree of Pituitary Involvement

SIGNS AND SYMPTOMS AT DIAGNOSIS

Adenohypophysitis

(~65% of cases)

Infundibulo-neurohypophysitis

(~10% of cases)

Panhypophysitis

(~25% of cases)

All forms *

· Headache: 53%

· Visual disturbances: 43%

· Adrenal insufficiency: 42%

· Hyperprolactinemia: 23%

· Hypothyroidism: 18%

· Hypogonadism: 12%

· Lactation failure: 11%

· Polydipsia/polyuria: 1%

· Polydipsia/polyuria: 98%

· Headache: 13%

· Adrenal insufficiency: 8%

· Hyperprolactinemia: 5%

· Hypogonadism: 3%

· Visual disturbances: 3%

· Hypothyroidism: 0%

· Lactation failure: 0%

· Polydipsia/polyuria: 83%

· Headache: 41%

· Adrenal insufficiency: 19%

· Visual disturbances: 18%

· Hypothyroidism: 17%

· Hyperprolactinemia: 17%

· Hypogonadism: 14%

· Lactation failure: 5%

· Headache: 48%

· Adrenal insufficiency: 38%

· Polydipsia/polyuria: 34%

· Visual disturbances: 32%

· Hypogonadism: 21%

· Hyperprolactinemia: 20%

· Hypothyroidism: 16%

· Lactation failure: 8%

PITUITARY HORMONE ABNORMALITIES AT DIAGNOSIS

Adenohypophysitis

(~65% of cases)

Infundibulo-neurohypophysitis

(~10% of cases)

Panhypophysitis

(~25% of cases)

All forms

· ACTH deficiency: 56%

· TSH deficiency: 44%

· FSH/LH deficiency: 42%

· GH decreased: 26%

· Hyperprolactinemia: 25%

· Hyperprolactinemia: 23% ***

· ADH deficiency: 0%

· ADH deficiency: 98%

· FSH/LH deficiency: 8% **

· Hyperprolactinemia: 5% ***

· Hyperprolactinemia: 0%

· ACTH deficiency: 0%

· TSH deficiency: 0%

· GH decreased: 0% **

· ADH deficiency: 95%

· GH decreased: 51%

· FSH/LH deficiency: 47%

· ACTH deficiency: 46%

· Hyperprolactinemia: 40% ***

· TSH deficiency: 39%

· Hyperprolactinemia: 16%

· ADH deficiency: 63%

· ACTH deficiency: 60%

· FSH/LH deficiency: 55%

· TSH deficiency: 50%

· Hyperprolactinemia: 39%

· GH decreased: 37%

Abbreviations: ACTH, adrenocorticotropic hormone; ADH, antidiuretic hormone; FSH, follicle-stimulating hormone; GH, growth hormone; LH, luteinizing hormone; TSH, thyroid-stimulating hormone.

* Other possible symptoms at diagnosis include weight gain (~20%) and temperature dysregulation (rare) (32,33).

** Some case series have reported a high prevalence of GH and FSH/LH deficiency in patients with infundibulo-neurohypophysitis (34).

*** Hyperprolactinemia may be related to stalk compression (disconnection hyperprolactinemia) or to the immune-mediated destruction of prolactin-secreting cells.

 

Granulomatous hypophysitis can be associated with more severe symptoms than lymphocytic hypophysitis, with two case series documenting more frequent occurrence of headache, chiasmal compression, and hypopituitarism (32,35). A review of the literature found that the most common symptoms of granulomatous hypophysitis at presentation were headache (61%), visual changes (40%), polyuria/polydipsia (27%) and cranial nerve palsies (27%); panhypopituitarism and diabetes insipidus were found in 49% and 27% of cases, respectively (11). Cases of compression of the cavernous part of the internal carotid artery have also been described (36).

 

Clinical data regarding xanthomatous and IgG4-related hypophysitis are less robust due to the rarity of these variants. Gutenberg et al. found that xanthomatous hypophysitis did not cause chiasmal compression and was associated with a low risk of diabetes insipidus and a less severe anterior pituitary hormone impairment than lymphocytic or granulomatous hypophysitis (FSH/LH and GH deficiencies are more common than TSH and ACTH deficiencies) (35). IgG4-related hypophysitis involves frequently both the pituitary and the stalk (~65%) and causes panhypopituitarism, anterior hypopituitarism and central diabetes insipidus in ~50%, ~25% and ~18% of cases, respectively (37). Cases of intrachiasmal abscess and spreading to the cavernous sinus have also been reported (38,39).

 

Primary hypophysitis is rare in children, with less than 100 cases reported in the literature of which only a few were biopsy-proven (40-42). The clinical presentation, however, seems to differ from adults. A review of the literature showed that the most common presenting symptoms in children are caused by antidiuretic hormone (ADH) deficiency (85%) (42). GH deficiency is found in 76% of cases, while FSH/LH, TSH and ACTH deficiencies were less common than in adults (32%, 29% and 20%, respectively). Headaches and visual disturbances were also rarely reported (17% and 8% of cases, respectively) (42). As central diabetes insipidus and growth retardation are the most common presenting symptoms in children with primary hypophysitis, the more frequent intracranial germinomas and Langerhans cell histiocytosis, as well as craniopharyngiomas, have to be considered in the differential diagnosis (43). Moreover, children with a presumptive diagnosis of hypophysitis are at risk of developing germinomas later in life (up to 3 years after the initial diagnosis) and require extended follow-up (42,44). Germinomas are also a documented cause of secondary hypophysitis (see “Hypophysitis secondary to sella and parasellar disease” below).

 

Imaging and Differential Diagnosis of Primary Hypophysitis

 

Magnetic resonance imaging (MRI) of the sella region typically shows an enlarged pituitary. In order to avoid unnecessary surgery, primary hypophysitis needs to be differentiated from other sella and parasellar masses (Table 5)(45), with pituitary adenomas being the most frequent differential diagnosis in adults.

 

Table 5. Differential Diagnosis of Hypophysitis

SELLA AND PARASELLAR MASSES

·   Pituitary adenomas (including pituitary apoplexy);

·   Pituitary metastases: the differential diagnosis is particularly important in patients with suspected hypophysitis and malignant tumors receiving immune checkpoint inhibitors;

·   Other sella and parasellar tumors (e.g., craniopharyngiomas, germinomas, gliomas, lymphomas, meningiomas, pituicytomas, chordomas, teratomas, dermoids and epidermoids);

·   Rathke’s cleft cyst;

·   Abscesses.

OTHER

·   Physiological hypertrophy of the pituitary in children and adolescents (especially pubertal females) and perimenopausal women;

·   Pituitary hyperplasia associated with pregnancy;

·   Sheehan’s syndrome at onset;

·   Thyrotropic hyperplasia associated with severe, untreated primary hypothyroidism.

 

Primary hypophysitis typically presents as a homogeneous pituitary enlargement with intense and homogeneous enhancement post-gadolinium and no deviation of the stalk (Figure 1); these and other features can help differentiate between primary hypophysitis and pituitary adenomas at MRI (Table 6) (1,4,46,47). Gutenberg et al. developed a score using variables such as age, association with pregnancy, and MRI findings to distinguish hypophysitis from pituitary adenomas with high accuracy (47). Further differential diagnoses, especially for lymphocytic hypophysitis, are the physiologic pituitary enlargement associated with pregnancy and Sheehan’s syndrome, although these patients have no history of obstetric hemorrhage (48,49). A cautious balance between radiological, clinical, and laboratory findings is necessary to reach the correct diagnosis and avoid inappropriate treatment (50).

 

 Table 6. Differential Imaging Characteristics of Primary Hypophysitis and Pituitary Adenomas

MRI

Primary hypophysitis

Pituitary adenoma

Pre-gadolinium

ACUTE / SUB-ACUTE PHASE:

·   Homogeneous pituitary enlargement with symmetrical suprasellar expansion;

·   Suprasellar extension with compression and displacement of chiasm;

·   Stalk thickened but not deviated; *

·   Loss of bright spot of the neurohypophysis in case of involvement of the posterior pituitary. **

 

CHRONIC PHASE:

·   Pituitary atrophy;

·   Empty sella.

·   Microadenoma (<1cm): unilateral, asymmetric endosellar mass;

·   Macroadenoma (>1cm): expanding, not homogeneous pituitary mass with asymmetrical suprasellar expansion;

·   Compression and displacement of chiasm (macroadenoma);

·   Contralateral deviation of the stalk;

·   The bright spot of the neurohypophysis can be usually seen. **

Post-gadolinium

·   Intense and homogeneous enhancement of the pituitary mass. Cystic areas have been described, especially in the xanthomatous variant;

·   Dural tail sign can be present (thickening of the enhanced dura that resembles a tail extending from a mass). ***

·   Slight, delayed and not homogeneous enhancement. Cystic and necrotic areas are frequently observed in macroadenomas;

·   Dural tail usually absent. ***

Abbreviations: MRI, magnetic resonance imaging.

* An enlarged pituitary stalk can also be found in other intracranial pathologies (e.g., sarcoidosis, metastases, Langerhans cell histiocytosis, germinoma, craniopharyngioma, astrocytoma, pituitary adenoma, lymphoma, tuberculosis, Erdheim-Chester’s disease) (51).

** The bright spot may be absent in up to 20% of healthy subjects (especially the elderly).

*** The dural tail sign is not specific to hypophysitis. It can be observed in meningioma (most frequently) and other intracranial pathologies (e.g. lymphoma, chloroma, metastasis, multiple myeloma, glioblastoma multiforme, aspergillosis, chordoma, schwannoma, pleomorphic xanthoastrocytoma, hemangiopericytoma, granulomatosis with polyangiitis, sarcoidosis, medulloblastoma, eosinophilic granuloma, pituitary adenoma, pituitary apoplexy, Erdheim-Chester’s disease) (52)

Figure 1. Magnetic resonance imaging findings in a case of primary hypophysitis. Panel A) T1-weighted image, sagittal section. Panel B) T1-weighted image, coronal section. Panel C) T1-weighted image post-gadolinium, sagittal section. Panel D) T1-weighted image post-gadolinium, coronal section. A homogeneous enlargement of the pituitary with thickening of the stalk can be seen. The mass shows intense and homogeneous enhancement post-gadolinium.

Autoantibodies in Primary Hypophysitis

 

Several authors have assessed the presence and utility of serum autoantibodies (pituitary and/or hypothalamic antibodies) in patients with primary hypophysitis:

 

  • An autoimmune etiology for lymphocytic hypophysitis was suggested by the presence of pituitary antibodies that may recognize α-enolase, GH, the pituitary gland-specific factors 1a and 2 (PGSF1a and PGSF2), regulatory prohormone-processing enzymes commonly produced in the pituitary gland (PC1/3, PC2, CPE and 7B2), secretogranin II, chromosome 14 open reading frame 166 (C14orf166), the corticotroph-specific transcription factor TPIT, and chorionic somatomammotrophin (HCS) (53-61). Several techniques have been used to detect pituitary antibodies in primary hypophysitis (ELISA, radioligand assay, immunoblotting, and immunofluorescence) and the prevalence of antibody-positive hypophysitis is 11-73% depending from the antigen(s) tested and the technique used (7,62). However, the pathogenic role of these autoantibodies is unclear and they are not specific to hypophysitis. For example, pituitary antibodies were identified by indirect immunofluorescence in ~45% of patients with biopsy-proven hypophysitis, but were also found in the serum of patients with isolated central diabetes insipidus (35%), germinomas (33%), isolated anterior hormone deficiencies (29%), prolactinomas (27%), Rathke’s cleft cysts (25%), craniopharyngiomas (17%), non-functioning pituitary tumors (13%), GH-secreting pituitary tumors (12%), and healthy subjects (5%) (62-65). They can also be found in patients with autoimmune endocrine disorders, especially Hashimoto thyroiditis (63). However, indirect immunofluorescence using human pituitary gland as a substrate and showing a granular cytosolic staining pattern was most commonly found in patients with hypophysitis and isolated hormone deficiencies (62); therefore, the finding of this staining pattern can be useful to clinicians in establishing a diagnosis of hypophysitis;

 

  • The detection of hypothalamic antibodies targeting corticotropin-releasing hormone (CRH)-secreting cells in some patients with GH/ACTH deficiency but with pituitary antibodies targeting only GH-secreting cells indicates that an autoimmune aggression to the hypothalamus can be responsible for some cases of lymphocytic hypophysitis (66). Consequently, not only pituitary but also hypothalamic autoimmunity may contribute to anterior pituitary dysfunction in a subset of patients with primary hypophysitis;

 

  • A search for ADH antibodies in patients with primary hypophysitis may help identifying patients who are prone to developing autoimmune central diabetes insipidus (67). These antibodies alone are not a good diagnostic marker for posterior pituitary involvement, but may serve as a predictive marker of gestational or post-partum diabetes insipidus (68,69);

 

  • Anti-Rabphilin antibodies have been proposed to be a biomarker for lymphocytic infundibulo-neurohypophysitis (70). Rabphilin is involved in the release of hormones or neurotransmitters and is expressed mainly in the brain, including the posterior pituitary and hypothalamus where ADH is present. Whether anti-Rabphilin antibodies are a cause of central diabetes insipidus or a result of infundibulo-neurohypophysitis is unknown. However, anti-Rabphilin antibodies are detected in 76% of patients with infundibulo-neurohypophysitis and 11% of patients with lymphocytic hypophysitis. In contrast, these antibodies are absent in patients with sella/suprasellar masses without lymphocytic hypophysitis, suggesting that this antibody may serve as a biomarker for the diagnosis of infundibulo-neurohypophysitis and may be useful for the differential diagnosis in patients with central diabetes insipidus (45);

 

  • Primary hypophysitis can eventually evolve in pituitary fibrosis and atrophy, documented at imaging as an “empty sella”. Lupi et al. have found pituitary antibodies in 6% of patients with an empty sella not linked to previous head trauma. In this cohort, the presence of pituitary antibodies also correlated with the presence of hypopituitarism (71);

 

  • Antibodies recognizing GH and one peptide from proopiomelanocortin (POMC) have been described in a patient with IgG4-related hypophysitis (72).

 

Natural History of Primary Hypophysitis

 

Primary hypophysitis can be self-limiting and spontaneous remission may occur (Figure 2). Considering the low prevalence of the disease, however, robust data regarding the natural history of primary hypophysitis are lacking (54). Moreover, most of the literature regards lymphocytic hypophysitis, while data from other histology subtypes are less robust. A review of 76 cases of primary hypophysitis from Germany has shown that patients not receiving any active treatment had improvement, stability or progression of the pituitary involvement at MRI in 46%, 27% and 27% of cases, respectively; pituitary deficiencies improved, remained stable or worsened in 27%, 55% and 18% of patients, respectively (73). A previous study by Khare et al. showed that spontaneous resolution of sella compression symptoms occurred in all patients managed conservatively and that 33% had complete or partial recovery of pituitary function (74). Park et al. also reported similar findings (75).

 

Primary hypophysitis frequently evolves to fibrosis and pituitary atrophy in the chronic stages of the disease, which often impair pituitary function (Figure 2). The evolution to empty sella has also been shown in a mouse model of autoimmune hypophysitis (76). Caturegli et al. reported that only 4% of patients had spontaneous remission with recovery of pituitary function, while most patients will require long-term replacement of one or more pituitary axes (4,54). Whether medical treatment with glucocorticoids has a positive impact on the natural history of primary hypophysitis is still a matter of debate.

Figure 2. Natural History of Primary Hypophysitis.

Most of the published case series mainly focus on the more frequent lymphocytic hypophysitis. Granulomatous hypophysitis can cause more severe signs and symptoms (headache, chiasmal compression and anterior/posterior hypopituitarism). Xanthomatous hypophysitis seems to cause sella compression and pituitary dysfunction less frequently. IgG4-related hypophysitis can cause various degree of involvement of the anterior pituitary, posterior pituitary and the stalk. Necrotizing hypophysitis is extremely rare and is associated with a high risk of panhypopituitarism and diabetes insipidus. The chronic stage of the disease is most likely related to the extent of damage of the pituitary. Some authors have suggested that some cases of lymphocytic hypophysitis may evolve to the granulomatous form, as mixed forms can rarely be observed. A death rate of 7% has been described in large case series of patients with primary hypophysitis and is probably related to unrecognized acute adrenal insufficiency.

 

Diagnosis and Treatment of Primary Hypophysitis

 

Pituitary biopsy is the gold standard to confirm the diagnosis of primary hypophysitis. This procedure, however, should be considered only in equivocal cases and when the outcome of the biopsy is expected to change the therapeutic management, and should be performed by a neurosurgeon with extensive expertise in pituitary surgery.

 

Due to the rarity of the disease, the management of hypophysitis is controversial. An algorithm in line with the more recent literature is reported in Figure 3. Initial evaluation of patients with suspected hypophysitis involves clinical and laboratory assessment. Patients with a suspicion of hypophysitis based on biochemical results should undergo a pituitary MRI, as well as visual assessment to check visual fields and acuity. Secondary causes of hypophysitis and other sella/parasellar masses should be considered in the differential diagnosis.

 

The mainstay of treatment of primary hypophysitis is pituitary hormone replacement (77,78). As outlined above, ACTH production is frequently impaired at presentation, and most patients will require glucocorticoid replacement. This should be started before thyroxine replacement (if TSH deficiency is present as well) to avoid precipitating acute adrenal insufficiency.

 

Conservative management is recommended for primary hypophysitis unless symptoms are severe and progressive. The only exception to this rule is IgG4-related hypophysitis that – like other manifestations of the disease – should be promptly treated to revert symptoms and prevent irreversible fibrosis (79,80). The mainstay of treatment are glucocorticoids, which often cause remission of symptoms within a few weeks. A typical starting dose is prednisone 30-40 mg/day (or equivalent), which should be continued for 2-4 weeks, and then tapered gradually over 2-6 months (19). However, some patients may benefit from long-term maintenance glucocorticoid therapy (with or without a steroid-sparing agent), especially in case of extensive multi-organ involvement. Relapse is possible and multiple courses of high-dose glucocorticoids are often necessary. Rituximab has also been used in patients with poor response to glucocorticoids (19,81,82). A case of IgG4-related hypophysitis successfully treated with azathioprine has also been reported (83).

 

High-dose glucocorticoids are the first-line treatment to improve the swelling of the pituitary and improve the symptoms related to significant sella compression. Anterior pituitary function can recover after pulse corticosteroid therapy, although >70% of patients will require long-term replacement with one or more hormones (4); central diabetes insipidus rarely recovers. Honegger et al. documented excellent initial responses to high-dose glucocorticoids, with radiological improvement, stability and progression in 65%, 31% and 4% of cases, respectively (73). However, these patients carried a higher risk of side effects (weight gain, psychiatric symptoms, peripheral edema, diabetes mellitus and glaucoma) and relapse of the pituitary inflammation was documented in 38% of cases. Relapses occurred 2-17 months after starting pulse steroids and the risk or relapse did not correlate with either initial glucocorticoid dose or treatment duration (73). Hormone deficiencies improved with glucocorticoids only in 15% of patients, while they remained stable or worsened in 70% and 15% of cases, respectively (73). Lupi et al. performed a review of the literature and found somewhat better outcomes with medical therapy, reporting pituitary mass reduction in 84% of cases, improving anterior pituitary function in 45%, and restored posterior pituitary function in 41% after high-dose glucocorticoids and/or azathioprine, with a relatively low risk of relapse (14%) (84). Recently, Chiloiro et al. found in a small prospective double-arm study that high-dose glucocorticoid treatment – compared with simple observation – was associated with higher rates of hypophysitis resolution and pituitary function recovery (85). The authors also showed that positive pituitary antibodies, a diagnosis of diabetes insipidus and secondary hypogonadism at the time of presentation, and specific MRI findings (a thicker pituitary stalk, a smaller pituitary volume, and the evidence of posterior pituitary involvement at MRI including absent bright spot) predicted better clinical outcomes following glucocorticoid therapy. These findings should be confirmed in a larger prospective cohort.

 

Whether central diabetes insipidus is an unfavorable prognostic factor for response to glucocorticoids is unclear. The abovementioned study by Chiloiro et al. suggests better outcomes in patients with central diabetes insipidus at the time of hypophysitis diagnosis (85); however, Lupi et al. found that patients with concomitant anterior and posterior pituitary dysfunction responded poorly to glucocorticoids, which were unable to revert the hypopituitarism (86). Glucocorticoid therapy was also found to be less effective in granulomatous or xanthomatous hypophysitis (35). In glucocorticoid-resistant cases and when high-dose glucocorticoids cause unacceptable side effects, immunosuppressive drugs such as azathioprine, methotrexate, and cyclosporin A have been used successfully. However, the long-term effects are unclear (1). Rituximab has also been employed to treat steroid-refractory hypophysitis (36,87-89).

 

Surgery should be considered only in cases with serious and progressive deficits of the visual field, visual acuity, or nerve paralysis not responsive to medical treatment. Surgery generally improves sella compression in the short term; however, Honegger et al. observed progression/relapse of the disease in 25% of patients after a mean follow-up of 3 years (73). Pituitary function improved only in 8% of patients after surgery, and the rates of resolution of chiasmal compression were also low (73). Further supporting the limited role of surgery in the management of hypophysitis, two small observational studies found that surgery did not impact significantly on the resolution of neurological symptoms or hormonal deficits during follow-up (90,91).

 

Stereotactic radiotherapy (radiosurgery) has been effectively employed in selected patients who have failed medical treatment or suffer from repeated recurrence of lymphocytic hypophysitis (92,93).

 

Figure 3. Diagnosis and management of primary hypophysitis. 1 Check random ACTH and cortisol if acute adrenal insufficiency is suspected. Consider confirmatory testing (e.g., Synacthen) if equivocal or borderline results. The Synacthen test can give false-positive results in the early stages of central adrenal insufficiency. During pregnancy and in patients receiving oral estrogens, the rise of corticosteroid-binding globulin (CBG) leads to falsely elevated cortisol levels and the normal reference ranges and stimulated cortisol cut-offs do not apply. 2 Pituitary surgery can also provide histology for definitive diagnosis.

DRUG-INDUCED HYPOPHYSITIS: IMMUNE CHECKPOINT INHIBITORS

 

Immune checkpoint inhibitors are monoclonal antibodies increasingly used for solid and hematological malignancies (94). They block several regulators of the immune activation (immune checkpoints), enhancing the host’s immune response to tumor cells (Figure 4). These drugs have shown a favorable toxicity profile and significant anti-tumor activity but, because of their mechanism of action, new typical side-effects have emerged (immune-related adverse events, irAEs) (Figure 4) (95,96).

 

Figure 4. Mechanism of Action of Immune Checkpoint Inhibitors. Tumor antigens are presented to T-cells by antigen-presenting-cells (APCs) via the interaction of the major histocompatibility complex (MHC) and the T-cell receptors, representing the primary signal for activating T-cells. Another costimulatory signal involving interaction between B7 on APCs and CD28 on T-cells is needed to complete T-cell activation and expansion (Panel A). Several co-receptors act as negative modulators of immune response at different molecular checkpoints. The CTLA-4 is induced in T-cells at the time of their initial response to antigen. CTLA-4 is transported to the cell surface proportionally to the antigen stimulation; it binds to B7 with greater affinity than CD28, resulting in specific T-cell inactivation (Panel B). The PD-1/PD1-L1 pathway is not involved in initial T-cell activation: it regulates inflammatory responses in peripheral tissues sustained by already activated effector T-cells. Activated T-cells up-regulate PD-1 and inflammatory signals in the tissue induce the expression of PD1-L1s, which downregulate the activity of T-cells, protecting normal tissues from collateral destruction; this mechanism is also exploited by tumor cells to evade the immune system response (Panel B). Monoclonal antibodies that block either CTLA-4 or PD1/PD1-L1 increase cytotoxic T-cell activity by expanding T-cell activation and proliferation (Panel C). The eventual T-cell reactivation is responsible for the both anti-tumor response and the immune-related adverse events associated with these drugs.

irAES mirror the immune response reactivation induced by immune checkpoint inhibitors and may predict better survival and response to the treatment of the underlying malignancy (97-100). irAEs can affect multiple organs and systems, including the pituitary and other endocrine glands (Table 7) (101).

 

Table 7. Immune-Related Adverse Events Associated with Immune Checkpoint Inhibitors

ENDOCRINOPATHIES

OTHER SYSTEMS AND ORGANS

PITUITARY: Hypophysitis.*

 

THYROID: Thyroiditis (both hypo- and hyperthyroidism); Euthyroid Graves’ ophthalmopathy.

 

ADRENAL GLANDS: Adrenalitis.*

 

PANCREAS: Insulinopenic diabetes mellitus.

SKIN: Rash/inflammatory dermatitis; Bullous dermatoses; Stevens-Johnson syndrome; Toxic epidermal necrolysis; Drug rash with eosinophilia and systemic symptoms syndrome; Drug-induced hypersensitivity syndrome; Acute generalized exanthematous pustulosis; Alopecia areata; Vitiligo; Psoriasis.

 

GASTROINTESTINAL SYSTEM: Colitis; Hepatitis; Pancreatitis.

 

LUNGS: Pneumonitis.

 

MUSCOSKELETAL SYSTEM: Arthritis; Polymyalgia-like syndrome; Myositis; Vasculitis.

 

KIDNEY: Nephritis.

 

CARDIOVASCULAR SYSTEM: Myocarditis; Pericarditis; Arrhythmias; Heart failure; Vasculitis; Venous thromboembolism.

 

NERVOUS SYSTEM: Guillain-Barré syndrome; Myasthenia gravis; Peripheral neuropathy; Autonomic neuropathy; Aseptic meningitis; Encephalitis; Transverse myelitis.

 

HEMATOLOGY: Autoimmune hemolytic anemia; Acquired thrombotic thrombocytopenic purpura; Hemolytic uremic syndrome; Aplastic anemia; Lymphopenia; Immune thrombocytopenia; Acquired hemophilia.

 

EYE: Uveitis; Iritis; Episcleritis; Blepharitis.

* Immune checkpoint inhibitors can cause both primary adrenal insufficiency (rarer) and secondary adrenal insufficiency (more frequent).

 

Epidemiology

 

Hypophysitis may occur as a complication during treatment with immune checkpoint inhibitors. Ipilimumab, a monoclonal antibody against the cytotoxic T lymphocyte antigen-4 (CTLA-4) is the drug that has been more strongly associated with this immune-related adverse event (Table 8) (5,102-112). The overall incidence of hypophysitis is 12% in patients treated with anti-CTLA-4 antibodies and 0.5% in patients treated with anti-programmed death 1 (PD1) antibodies (113,114).

 

Table 8. Immune Checkpoint Inhibitors and the Risk of Hypophysitis

Category

Drug

Approved and off-label indications

Incidence of reported hypophysitis in clinical studies

Anti-CTLA-4

(70% of published hypophysitis cases)

Ipilimumab

Unresectable or metastatic melanoma; Adjuvant treatment in melanoma; Relapsed hematologic cancer.

Up to 17.4% (G3-G4: 0.3-17.4%)

Tremelimumab

Malignant mesothelioma; Hepatocellular carcinoma. This drug is not FDA approved.

0-2.6% (G3-G4: 1%)

Anti-PD1 (24% of published hypophysitis cases)

Nivolumab

Metastatic colorectal cancer; Recurrent or metastatic squamous cell head and neck cancer; Hepatocellular carcinoma; Classical Hodgkin’s lymphoma; Unresectable or metastatic melanoma; Adjuvant treatment in melanoma; Progressive, metastatic non-small cell lung cancer; Progressive small cell lung cancer; Advanced renal cell cancer; Urothelial carcinoma; Platinum-resistant ovarian cancer.

0-3% (G3-G4: 0.5%)

Pembrolizumab

Metastatic or recurrent locally advanced gastric cancer; Recurrent or metastatic squamous cell head and neck cancer; Relapsed or refractory classical Hodgkin’s lymphoma; Relapsed chronic lymphocytic leukemia; Unresectable or metastatic melanoma; Unresectable or metastatic microsatellite instability-high cancer; Metastatic non-small cell lung cancer; Metastatic, non-squamous Non-small cell lung cancer (in combination with Pemetrexed and Carboplatin); Locally advanced or metastatic urothelial carcinoma; Advanced Merkel cell carcinoma.

0-4.8% (G3-G4: 0-2.4%)

Dostarlimab

Mismatch repair deficient recurrent or advanced endometrial cancer; Mismatch repair deficient recurrent or advanced solid tumors.

No cases published. Hypophysitis is listed as a possible adverse reaction in <10% of treated patients in the product information.

Cemiplimab

Cutaneous squamous cell carcinoma.

1 case reported

Toripalimab

Melanoma; several solid malignancies (development stage).

1 case reported

Geptanolimab (still in development stage)

Peripheral T-cell lymphoma; Alveolar soft part sarcoma; Cervical cancer; Non-Hodgkin's lymphoma; Liver cancer; Colorectal cancer; Non-small cell lung cancer.

1 case reported

Anti-PD1-L1 (2% of published hypophysitis cases)

Atezolizumab

Metastatic non-small cell lung cancer; Locally advanced or metastatic urothelial carcinoma.

1% (G3-G4: 1%)

Avelumab

Metastatic Merkel cell carcinoma; Locally advanced or metastatic urothelial carcinoma; Advanced non-small cell lung cancer.

1 case reported

Durvalumab

Advanced non-small cell lung cancer; Locally advanced or metastatic urothelial carcinoma.

1 case reported

Combination therapy

(4% of published hypophysitis cases)

Ipilimumab + Nivolumab

Unresectable or metastatic melanoma; Progressive small cell lung cancer; Non-small cell lung cancer; Advanced renal cell cancer; Malignant mesothelioma; Recurrent glioblastoma.

Up to 12.8% (G3-G4: 1.5-8.7%)

Ipilimumab + Pembrolizumab

Advanced melanoma; Advanced renal cell carcinoma.

0-9.1% (G3-G4: 0-6%)

Durvalumab + Tremelimumab

Advanced non-small cell lung cancer.

0%

Abbreviations: CTLA-4, cytotoxic T lymphocyte antigen-4; FDA, Food and Drug Administration; G3, grade 3 immune checkpoint inhibitor-induced hypophysitis (see Table 11); G4, grade 4 immune checkpoint inhibitor-induced hypophysitis (see Table 11); PD1, programmed death 1; PD1-L1, programmed death 1 Ligand 1.

 

Pathogenesis

 

The pathogenesis of anti-CTLA-4 antibody-induced hypophysitis involves type II and IV hypersensitivity, as well as the humoral immune response (Figure 5). This has been suggested by histopathological findings of patients with hypophysitis following treatment with Ipilimumab (alone or in combination with Nivolumab or Pembrolizumab), evidence of pituitary antibodies in the serum of these patients, association with specific human leucocyte antigens, and animal models of anti-CTLA-4-induced hypophysitis (8,15,115-118).

 

Evidence regarding the pathophysiology of anti-PD1/PD1-L1 antibody-induced hypophysitis is scant, but immune response reactivation most likely targets ACTH-secreting cells because of the very frequent isolated ACTH deficiency (5). Kanie et al. recently postulated that ectopic expression of ACTH in the tumor may contribute to some cases of anti-PD1/PD1-L1 antibody-induced hypophysitis, as a form of paraneoplastic syndrome (119). Furthermore, Bellastella et al. identified a higher prevalence of anti-pituitary and anti-hypothalamus antibodies in patients with cancer treated with anti-PD1/PD1-L1 agents (120). In a small longitudinal study, the same group also found that more than half of patients who start anti-PD1/PD1-L1 treatment developed anti-pituitary or anti-hypothalamus antibodies after 9 weeks of treatment, with a concomitant increase prolactin and a reduction in ACTH and IGF-1 levels compared to baseline (120). These preliminary results need to be validated in a larger cohort, but the presence of anti-hypothalamus antibodies would suggest that – at least in some patients – hypothalamic autoimmunity might contribute to the development of anti-PD1/PD1-L1 antibody-induced pituitary dysfunction.

 

Figure 5. Proposed pathogenesis of anti-CTLA-4 antibody-induced hypophysitis. Anti-CTLA-4 antibody-induced hypophysitis accounts for ~70% of immune-checkpoint induced hypophysitis cases. The CTLA-4 antibody binds to pituitary CTLA-4 antigen, inducing complement activation and infiltration with macrophages and other inflammatory cells, leading to phagocytosis and enhanced antigen presentation. Subsequently, autoimmune type IV hypersensitivity reactions start, with infiltration of the anterior pituitary by autoreactive T lymphocytes that eventually leads to pituitary cytotoxicity and inflammation. Moreover, patients with anti-CTLA-4 antibody-induced hypophysitis develop pituitary antibodies that predominantly recognize TSH- FSH- and ACTH-secreting cells. Pituitary cytotoxicity in anti-PD1/PD1-L1 antibody-induced hypophysitis presumably affects mostly ACTH-secreting cells, as isolated ACTH deficiency is the most common occurrence in these patients.

Clinical Characteristics

 

There are important differences between primary hypophysitis and immune checkpoint-induced hypophysitis (Table 9)(5,8,103). The latter does not have a female predominance (8,121) and seems to present more frequently with hypopituitarism at diagnosis. Both forms of hypophysitis are more commonly associated with an initial deficit of ACTH, FSH/LH and TSH, but symptoms of adrenal insufficiency and confirmed ACTH deficiency are much more common in patients with immune checkpoint-induced hypophysitis (8,113,114). Central diabetes insipidus can occur in a substantial share of primary hypophysitis cases (i.e., the infundibulo-neurohypophysitis and panhypophysitis variants), while it is extremely rare in immune-checkpoint induced hypophysitis. Pituitary enlargement and visual impairment are much more common in primary hypophysitis, while the size of the pituitary may appear normal in immune checkpoint inhibitor-induced hypophysitis (in absence of a baseline pituitary MRI) and optic chiasm involvement is rare (5,8).

 

Table 9. Comparison Between Primary and Immune Checkpoint Inhibitor-Induced Hypophysitis

Characteristics

Primary hypophysitis

Immune checkpoint inhibitor-induced hypophysitis

Etiology

Autoimmune.

Immune response reactivation.

Epidemiology

·   More prevalent in young females (female:male ratio ~3:1), apart from the rare IgG4-related form that is more common in older males.

·   The onset of the lymphocytic subtype is strongly associated with late pregnancy and the post-partum period.

·  The epidemiology is most likely influenced by the underlying malignancy.

·  0.5-12% of treated patients develop hypophysitis, depending on the drug used.

·  The female:male ratio is ~1:4 and the mean age at onset is ~60 years (older male patients appear to be the group carrying the higher risk).

·  No prior cancer therapy is associated with higher risk of developing hypophysitis.

Time after the initiating event

Unknown. The median duration of symptoms before clinical presentation

is varies according to the anatomic location of the pituitary involvement:

· Adenohypophysitis (during pregnancy): 4 months;

· Adenohypophysitis (outside of pregnancy): 12 months;

· Infundibulo-neurohypophysitis: 3 months;

· Panhypophysitis: 4 months.

· Ipilimumab: median time to onset 9-11 weeks (range 1-35); *

· Pembrolizumab: median time to onset 16 weeks (range 1-52); *

· Nivolumab: median time to onset 21-22 weeks (range 6-48); *

· Ipilimumab + Nivolumab: median time to onset 11-12 weeks (range 3-32). *

Symptoms at presentation

· Headache: 48%

· Adrenal insufficiency: 38%

· Polydipsia/polyuria: 34%

· Visual disturbances: 32%

· Hypogonadism: 21%

· Hypothyroidism: 16%

·Adrenal insufficiency: 81% **

·Headache: 45%

·Hypothyroidism: 18%

·Hypogonadism: 11%

·Visual disturbances: 6%

·Polydipsia/polyuria: 2%

Pituitary hormone abnormalities

· ADH deficiency: 63%

· ACTH deficiency: 60%

· FSH/LH deficiency: 55%

· TSH deficiency: 50%

· GH decreased: 37%

· Hyperprolactinemia: 39%

·   ACTH deficiency: 96% **

·   TSH deficiency: 63%

·   FSH/LH deficiency: 59%

·   GH decreased: 19%

·   Hyperprolactinemia: 11%

·   ADH deficiency: 4%

Abnormal MRI at presentation

97% of cases

64% of cases ***

Histopathology

Marked infiltration of lymphocytes of the pituitary gland, typically accompanied by scattered plasma cells, eosinophils and fibroblasts, and in later disease stages by fibrosis.

T-cell infiltration and IgG-dependent complement fixation and phagocytosis.

Treatment

Usually good response to glucocorticoids.

Good response to glucocorticoids of the symptoms related to sella compression.

Outcome

Variable: from complete recovery to persistent hypopituitarism.

Pituitary enlargement (if present) eventually resolves. TSH and FSH/LH deficiencies often recover, while central adrenal insufficiency persists almost invariably.

Abbreviations: ACTH, adrenocorticotropic hormone; ADH, antidiuretic hormone; CTLA-4, cytotoxic T lymphocyte antigen-4; FSH, follicle-stimulating hormone; LH, luteinizing hormone; GH, growth hormone; MRI, magnetic resonance imaging; TSH, thyroid-stimulating hormone.

* Data from the prescribing information of Ipilimumab, Pembrolizumab and Nivolumab.

** Anti-PD-1/PD1-L1 antibody-induced hypophysitis typically presents with isolated ACTH deficiency, while CTLA-4 antibody-induced hypophysitis more frequently leads to multiple hormone deficiencies (Table 10).

*** MRI abnormalities are transient, can be subtle and precede clinical symptoms in ~50% of cases. Anti-PD-1/PD1-L1 antibody-induced hypophysitis typically lacks MRI changes and causes no mass effect symptoms (Table 10).

 

The onset of immune checkpoint-induced hypophysitis varies according to the drug used (Table 9); early onset has been reported and it can appear also several months after the initiation of the immunotherapy (122,123). The risk of hypophysitis with Ipilimumab appears to be dose-dependent, with a higher prevalence in those receiving 10 mg/kg vs. 3 mg/kg (124-126). Conversely, patients receiving concomitant cytotoxic chemotherapy or with brain radiotherapy-pretreated metastases might be protected from the risk of developing hypophysitis, presumably through immune cell depletion (95,127).

 

Patients with immune checkpoint-induced hypophysitis typically present with nonspecific symptoms of adrenal insufficiency like fatigue, headache, myalgia, nausea, vomiting, reduced appetite, light-headedness, and dizziness, whilst symptoms of other anterior pituitary hormone deficiencies are less common at the time of diagnosis (Table 9) (113,114). Manifestations of adrenal insufficiency often overlap with those of the underlying malignancy but must not be overlooked because of the risk of developing life-threatening adrenal crisis. Visual disturbances are very rare (the pituitary enlargement, if present, is often minor and transient) and central diabetes insipidus is extremely uncommon (95,113,114,128,129). Other less frequent symptoms include confusion, hallucinations, memory loss, labile moods and depression (including suicidal ideation), insomnia, temperature intolerance, and chills (130,131). Importantly, up to 45% of patients can be asymptomatic and are diagnosed only at laboratory evaluation, highlighting the importance of regular monitoring (123,132).

 

Associated irAEs have been reported in about half of patients with immune checkpoint inhibitor-induced hypophysitis (133). By far, the most common associated irAE was thyroiditis (~30%), followed by colitis (~20%), skin reactions (~15%), pneumonitis (~5%), and hepatitis (~5%) (133).

 

Patients with anti-CTLA-4 antibody-induced hypophysitis tend to have a more diverse clinical presentation than those with anti-PD-1/PD1-L1 antibody-induced hypophysitis. The latter typically presenting later during treatment, with severe isolated ACTH deficiency (which frequently leads to hyponatremia at the time of diagnosis), and no significant pituitary enlargement both clinically and radiologically. Also, treatment discontinuation is less frequently required in patients with anti-PD-1/PD1-L1 antibody-induced hypophysitis (Table 10) (5,81,114,134-136).

 

Table 10.  Comparison Between Anti-CTLA-4 and Anti-PD1/PD1-L1 Antibody-Induced Hypophysitis

Characteristics

Anti-CTLA-4 antibody-induced hypophysitis

Anti-PD1/PD1-L1 antibody-induced hypophysitis

Number of cases reported

192 (74% males)

69 (72% males)

Mean time to onset (95% CI)

10.5 weeks (9.8-11.2)

Anti-PD1: 27.0 weeks (20.9-33.1)

Anti-PD1-L1: 27.8 weeks (0-58.0)

Mean doses to onset

3.4 doses

10.3 doses

Symptoms at presentation

· Adrenal insufficiency: 75%

· Headache: 60%

· Hypothyroidism: 21%

· Hypogonadism: 16%

· Visual disturbances: 8%

· Polydipsia/polyuria: <1%

·Adrenal insufficiency: 91%

·Hypothyroidism: 7%

·Headache: 4%

·Polydipsia/polyuria: 3%

·Hypogonadism: 0%

·Visual disturbances: 0%

Pituitary hormone abnormalities at presentation *

· ACTH deficiency: 95%

· TSH deficiency: 85%

· FSH/LH deficiency: 75%

· GH decreased: 27%

· Hyperprolactinemia: 7%

· ADH deficiency: 2%

·   ACTH deficiency: 97%

·   Hyperprolactinemia: 20%

·   FSH/LH deficiency: 13%

·   TSH deficiency: 4%

·   GH decreased: 3%

·   ADH deficiency: 3%

Prevalence of hyponatremia at presentation **

39% of cases

62% of cases

Abnormal MRI at presentation

81% of cases

18% of cases

Discontinuation of the immune checkpoint inhibitor

· No: 56%

· Yes, temporarily: 3%

· Yes, permanently: 41%

· No: 70%

· Yes, temporarily: 20%

· Yes, permanently: 10%

Outcome

· Long-term hypopituitarism: 89%

· Pituitary function recovery after treatment: 5%

· Spontaneous resolution: 1%

· Death: 5%

· Recurrence after treatment: 0%

· Long-term hypopituitarism: 90%

· Pituitary function recovery after treatment: 6%

· Spontaneous resolution: 0%

· Death: 4%

· Recurrence after treatment: 0%

* Pituitary hormone deficiencies can be isolated or combined (especially in the case of anti-CTLA-4 antibody-induced hypophysitis). ACTH + TSH deficiency is the most frequent combination observed in these patients.

** Most likely related to cortisol deficiency. It can be a clue to the diagnosis.

Abbreviations: ACTH, adrenocorticotropic hormone; ADH, antidiuretic hormone; CTLA-4, cytotoxic T lymphocyte antigen-4; FSH, follicle-stimulating hormone; LH, luteinizing hormone; GH, growth hormone; MRI, magnetic resonance imaging; PD1, programmed death 1; PD1-L1, programmed death 1 Ligand 1; TSH, thyroid-stimulating hormone.

 

According to the degree of symptoms and of the severity of the disease, immune checkpoint-induced hypophysitis is graded 1 to 4 (Table 11) (78). Grade 3 toxicity or worse (including death) has been described in 2-10% of reported hypophysitis cases (137,138).

Abbreviations: ADL, activities of daily living.

Table 11. Grading of Immune Checkpoint Inhibitor-Induced Hypophysitis

Grade

Description

Grade 1

Asymptomatic or mild symptoms.

Grade 2

Moderate symptoms, able to perform ADL.

Grade 3

Severe symptoms, medically significant consequences, unable to perform ADL.

Grade 4

Severe symptoms, life-threatening consequences, unable to perform ADL.

Grade 5

Death.

 

Diagnosis and Management

 

An algorithm for diagnosis and management of immune checkpoint-induced hypophysitis in line with the more recent literature is shown in Figure 6. Patients should be regularly monitored with clinical assessment and hormonal tests during treatment with immune checkpoint inhibitors. Almost invariably, patients who develop hypophysitis have ACTH deficiency and cases of fatal acute adrenal insufficiency have been reported (139); this highlights the importance of pituitary function assessment at baseline and during treatment, also in asymptomatic patients (140). If there is a strong suspicion of adrenal insufficiency on clinical grounds (e.g. G3-G4 symptoms), glucocorticoid replacement should be started without delay (141). TSH deficiency is also very common (>60% of patients). Indeed, a fall in serum TSH and free T4 have been suggested to be early signs of immune checkpoint inhibitor-induced hypophysitis and can be a clue to the diagnosis (141-144). A recent paper has identified antibodies against two autoantigens (anti-GNAL and anti-ITM2B) that may aid in the diagnosis of and predict the risk of developing immune checkpoint-induced hypophysitis (145). Moreover, Kobayashi et al. evaluated the usefulness of pituitary antibodies and human leukocyte antigen alleles in predicting immune checkpoint-induced pituitary dysfunction. The authors showed distinct and overlapped patterns of pituitary antibodies and human leukocyte antigen alleles between patients who developed hypophysitis (n=5) or isolated ACTH deficiency (n=17) (117). The usefulness of anti-GNAL and anti-ITM2B antibodies, pituitary antibodies, and human leukocyte antigen alleles as biomarkers in the clinical setting needs to be validated in larger cohorts of patients.

 

Patients with suspected drug-induce hypophysitis should undergo a pituitary MRI and visual assessment (141,146). The importance of obtaining pituitary imaging was recently highlighted in a retrospective study by Nguyen et al., where 33% of hypophysitis cases would have been missed if no MRI were carried out (143). Also, pituitary MRI is important for the differential diagnosis of other pituitary lesions, in particular metastases (Table 12). Faje et al. reported that ~50% of patients with immune checkpoint-induced hypophysitis presented with diffuse pituitary enlargement at MRI before the onset of clinical symptoms (98). 18F-FDG PET performed as part of the staging of the underlying malignancy can show intense radiotracer uptake and may precede clinical symptoms and biochemical abnormalities (147,148); however, its routine use for the diagnosis of hypophysitis is not recommended.

 

Current guidelines on the management of immune-checkpoint induced hypophysitis suggest clinicians to consider with holding treatment in G1-G2 hypophysitis until the patient is stabilized on hormone replacement (101). We believe that patients with immune checkpoint inhibitor-induced hypophysitis should not stop treatment unless they develop severe and progressive symptoms (G3-G4 hypophysitis). In fact, this type of hypophysitis if often self-limiting and most of patients do not show progression of sella compression. Therefore, the decision whether to withhold a treatment that can have a significant impact on the progression-free survival of the underlying malignancy should be balanced carefully. When G3-G4 hypophysitis is suspected, a course of high-dose corticosteroids given during the acute phase may result in inflammation reversal and ameliorate the compression of sella and parasellar structures. Whether high-dose glucocorticoids have an impact on the anti-tumor effect of immune checkpoint inhibitors is uncertain. Earlier evidence suggested a neutral effect on survival (99,149,150); however, a study from Faje et al. questioned this, showing reduced survival among patients with melanoma treated with high-doses glucocorticoids for Ipilimumab-induced hypophysitis (100,126). Nonetheless, treatment should not be delayed in patients with severe symptoms of sella compression.

 

The resolution of the neuroradiological abnormalities is usually observed within 2 months (128,130). Treatment with high-dose glucocorticoids, however, does not restore ACTH deficiency and most patients will require long-term replacement (Table 10) (5,123). On the other hand, thyroid and gonadal deficiencies often recover and the need for hormone replacement needs to be reassessed in the long term (123,124,143,151,152). In addition, patients developing irAEs can be severely ill and can present with a “euthyroid sick syndrome” and/or a “sick eugonadal syndrome” that can affect the interpretation of the laboratory results (130).

 

Figure 6. Diagnosis and Management of Immune Checkpoint Inhibitor-Induced Hypophysitis. 1 Some authors suggest laboratory evaluation before the first infusion, then at 8 weeks for patients receiving Ipilimumab (i.e., prior to cycle 3) and then at week 16 if there are no interim signs/symptoms suggestive of hypophysitis. Other authors recommend laboratory evaluation for hypophysitis prior to each infusion of immune checkpoint inhibitors in the first 12-16 weeks of treatment, in order to pick up early or late onset of the disease. 2 Check random ACTH and cortisol if acute adrenal insufficiency is suspected. Exclude recent glucocorticoid use and concomitant treatment that may alter serum cortisol measurement (e.g., oral estrogens). As a guide, in patients that are unwell serum cortisol >450 nmol/L makes the diagnosis of adrenal insufficiency unlikely. Adrenal insufficiency is possible if morning cortisol 200-450 nmol/L or random cortisol 100-450 nmol/L; consider confirmatory testing with Synacthen, although this can give false-positive results in the early stages of central adrenal insufficiency. Adrenal insufficiency is likely if morning cortisol <200 nmol/L or random cortisol <100 nmol/L and patients should be started on hormone replacement. These cut-offs should be seen only as a guide and need to be adapted to local laboratory assays and reference ranges. Patients receiving immune checkpoint inhibitors can also develop adrenalitis and primary adrenal insufficiency. These patients have high ACTH and renin/aldosterone should be measured to investigate mineralocorticoid deficiency. 3 IGF-1 is valuable to confirm changes from baseline that may suggest new-onset hypophysitis. However, further tests to prove GH deficiency are not required because these patients would not be treated (active malignancy). 4 Pituitary MRI is normal in ~20% and ~80% of hypophysitis cases associated with anti-CTLA-4 and anti-PD1/PD1-L1 antibodies, respectively. Therefore, normal imaging does not exclude hypophysitis. MRI changes can be very subtle (Table 11). 5 We believe that patients with immune checkpoint inhibitor-induced hypophysitis should not stop treatment unless they develop severe and progressive symptoms (G3-G4 hypophysitis). Once the acute symptoms of hypophysitis have resolved, restarting treatment with immune checkpoint inhibitors is not contraindicated. Adequately treated, long-term hypopituitarism is not a contraindication to restarting immune checkpoint inhibitors.

An important differential diagnosis in patients with suspected drug-induced hypophysitis and a sella mass are pituitary metastases (Table 12) (95,141,153-155). The early studies on immune checkpoint inhibitors mainly assessed their efficacy in patients with advanced melanoma. Pituitary metastases are rare in melanoma (~2.5% of pituitary metastasis cases reported in the literature); however, these drugs are increasingly used for other malignancies including lung cancer, which accounts for ~25% of pituitary metastases (153). Central diabetes insipidus in immune checkpoint inhibitor-induced hypophysitis is extremely rare; therefore, a sella mass associated with diabetes insipidus is strongly suggestive of a metastasis.

 

Table 12. Differential Diagnosis of Immune Checkpoint Inhibitor-Induced Hypophysitis and Pituitary Metastases

Characteristics

Immune checkpoint inhibitor-induced hypophysitis

Pituitary Metastases

Clinical presentation

·   Central diabetes insipidus is extremely rare;

·   Anterior pituitary insufficiency is very common (chiefly ACTH, FSH/LH and TSH deficiency).

·   Headache is a frequent presenting symptom.

·   Central diabetes insipidus is the most common hormonal abnormality (~45%);

·   Cranial nerve deficits due to involvement of the chiasm and the cavernous sinus are common (22-28%);

·   Anterior pituitary insufficiency has been described in ~24% of patients;

·   Headache and retro-orbital pain have been reported in ~16% of patients;

Imaging

MRI:

·   Mild-to-moderate diffuse enlargement of the pituitary (up to 60-100% of the baseline size). Pituitary height typically does not exceed 2 cm. Pituitary enlargement resolves in most cases over the course of weeks/months. Empty sella can develop in the long term.

·   Extension into the cavernous sinus or above the sellar diaphragm is uncommon.

·   Homogeneous (more frequent) or heterogeneous enhancement (less frequent) post-gadolinium;

·   Suprasellar extension with compression and displacement of chiasm is uncommon;

·   The pituitary stalk may be thickened but not deviated;

·   The posterior pituitary is preserved in most of cases.

MRI:

·   Sella or suprasellar mass;

·   Isointense or hypointense mass on T1WI, with a usually high-intensity sign on T2WI;

·   Homogeneous enhancement post-gadolinium, although hemorrhage, necrosis and areas of cystic degeneration can be observed;

·   Thickened and enhancing pituitary stalk is possible, but it is typically less common than immune checkpoint inhibitor-induced hypophysitis;

·   Presence of other brain metastases (~15%);

·   Invasion of the cavernous sinus, chiasm, or hypothalamus (~14%)

·   Loss of bright spot of the neurohypophysis (~13%);

·   Dumbbell-shaped mass (~11%);

·   Sphenoid sinus invasion (~9%);

 

CT: may show bony destruction.

Abbreviations: CT, computed tomography; MRI, magnetic resonance imaging; T1WI, T1 weighted images; T2WI, T2 weighted images.

 

DRUG-INDUCED HYPOPHYSITIS: OTHER DRUGS

 

Reversible or irreversible hypopituitarism may be a rare side effect following treatment with interferon-α, and interferon-α/ribavirin combination therapy has been associated with cases of granulomatous hypophysitis with anterior pituitary dysfunction (156-160). The anti-interleukin-12 and -23 monoclonal antibody ustekinumab (used in the treatment of psoriasis) has been associated with a case of hypophysitis with panhypopituitarism (161).

 

HYPOPHYSITIS SECONDARY TO SELLA AND PARASELLAR DISEASE

 

Pituitary inflammation can be triggered by sella and parasellar disease. The infiltrate is mainly lymphocytic or xanthogranulomatous and focuses around the lesion rather than diffusing to the entire gland (4).

 

Germinoma

 

Germinomas are rare brain tumors predominantly affecting prepubertal children. They are highly immunogenic tumors and can induce a strong immune response that can involve the pituitary leading to secondary hypophysitis (162-169). Histologically, lymphocytic or granulomatous hypophysitis is seen in ~80% and ~20% of cases linked to germinomas, respectively (169).

 

Germinomas arising in the sella and parasellar region are difficult to differentiate from hypophysitis in children because of similar clinical features (diabetes insipidus + GH deficiency + visual disturbances). This differentiation, nevertheless, is critical for patient care due to different treatments of the two diseases. Biopsy-proven cases of primary hypophysitis are extremely rare in children and adolescents (41); therefore, in children below 10 years a germinoma should be considered the most likely diagnosis.

 

Tumor markers such as α-fetoprotein, β-human chorionic gonadotropin, or placental alkaline phosphatase in the cerebrospinal fluid may be useful for diagnosing germinoma. However, a pituitary biopsy is the gold standard for differentiating the two conditions, although germinomas can have a marked lymphocytic infiltrate that can outnumber the neoplastic cells making differential diagnosis difficult (168). If germinoma is part of the histologic differential diagnosis, markers for germinomas such as Oct3/4, PLAP and NANOG may be useful.

 

Finally, it should be noted that the hypopituitarism caused by sella germinomas can precede for years a visible pituitary mass, so that prolonged symptomatic periods prior to diagnosis are common (168).

 

Rathke’s Cleft Cyst

 

The rupture of Rathke’s cleft cyst can cause hypophysitis associated with visual disturbances, headache and hypopituitarism including – very frequently – central diabetes insipidus (170-175). Histopathology can show lymphocytic, granulomatous, xanthomatous or mixed forms of hypophysitis (174). Some authors have suggested that many cases of xanthomatous hypophysitis may actually be related to rupture of Rathke’s cleft cysts (12,13).

 

Other Sella and Parasellar Masses

 

Cases of secondary hypophysitis have been described in association with craniopharyngiomas (176), pituitary adenomas (177-182) and primary pituitary lymphomas (177,183).

 

HYPOPHYSITIS SECONDARY TO SYSTEMIC DISEASE

 

Sarcoidosis

 

Sarcoidosis is a multisystem inflammatory disease of unknown origin characterized by the formation of non-caseating granulomas that can involve all organ systems. The central nervous system can be affected in 5-15% of patients (neurosarcoidosis) and may be the presenting feature of the disease (184). Granulomas can involve the pituitary, hypothalamus and the stalk in ~0.5% of patients with sarcoidosis, resulting in varying grade of hypopituitarism (185,186). ~60% of the cases reported in the literature are males presenting in the 3rd and 4th decade. Central diabetes insipidus, FSH/LH deficiency and hyperprolactinemia are among the most frequent hormone abnormalities (187). Patients with sarcoidosis and hypothalamic-pituitary involvement tend to have more frequent multi-organ involvement, as well as neurosarcoidosis and sinonasal involvement (187).

 

Granulomatosis with Polyangiitis

 

Granulomatosis with polyangiitis (previously known as Wegener’s Granulomatosis) is an antineutrophil cytoplasmic autoantibody (ANCA)-associated vasculitis of unknown etiology with multisystem involvement and formation of necrotizing granulomas and vasculitis in small- and medium-sized blood vessels. Pituitary involvement is a rare and usually late manifestation of the disease (186,188,189), but it can also be the presenting complaint (190,191). Secondary hypogonadism and central diabetes insipidus are the most common endocrine abnormalities; diabetes insipidus can recover after adequate treatment of the underlying vasculitis, while anterior pituitary dysfunction is permanent in the majority of patients (192).

 

Langerhans Cell Histiocytosis

 

Langerhans cell histiocytosis is a rare disease mainly occurring in childhood, involving clonal proliferation of myeloid Langerhans cells that can infiltrate multiple organs (bones, skin, lymph nodes, lungs, thymus, liver, spleen, bone marrow, and central nervous system including the pituitary). Patients often carry the BRAF V600E mutation in the clonal myeloid cells (193).

 

The most common endocrine abnormality in patients with Langerhans cell histiocytosis is hypothalamic-pituitary infiltration causing central diabetes insipidus. These patients usually have multi-organ and cranio-facial involvement, although localized disease of the hypothalamic-pituitary region has been reported (194,195). Up to 40% of patients develop symptoms consistent with diabetes insipidus within the first four years, particularly if there is multisystem involvement and proptosis (196-198). Anterior pituitary hormone deficiency is also possible at diagnosis and during follow up (194,199).

 

Langerhans cell histiocytosis and germinoma are the most common cause of central diabetes insipidus in children and adolescents; therefore, germinoma should always been considered in the differential diagnosis (200).

 

The definitive diagnosis of Langerhans cell histiocytosis is the biopsy-proven infiltration of the pituitary with Langerhans cells with eosinophils, neutrophils, small lymphocytes, and histiocytes. However, pituitary biopsy is invasive and the diagnosis can be suggested by the presence of the characteristic histopathologic features in other tissues when a multisystem disease is present. For patients with suspected disease isolated to the pituitary, identification of BRAF-V600E in the peripheral blood or cerebrospinal fluid can support the diagnosis and rule out germinoma, although it does not distinguish Langerhans cell histiocytosis from Erdheim-Chester disease (see below) (201).

 

When hypophysitis secondary to Langerhans cell histiocytosis is suspected but pituitary biopsy is not available, it is reasonable to initiate therapy empirically with a plan to follow disease response with MRI. Treatment options include prednisone, alone or in combination with vinblastine, cladribine and vemurafenib, alongside desmopressin and other pituitary hormone replacements to treat hypopituitarism.

 

Erdheim-Chester Disease

 

Erdheim-Chester’s disease is a rare multisystem histiocytic disorder, most often seen in adults, which may be confused with Langerhans cell histiocytosis. Histiocytic infiltration leads to xanthogranulomatous infiltrates of multiple tissues (bones, skin, lungs, facial, orbital and retro-orbital tissue, retroperitoneum, cardiovascular system and cerebral nervous system including the pituitary). Long bone pain and symmetric osteosclerotic lesions suggest this diagnosis, which is confirmed by tissue biopsies showing histiocytes with non-Langerhans features. Patients often carry the BRAF V600E mutation in the clonal myeloid progenitor cells (193).

 

Pituitary involvement may manifest as central diabetes insipidus and anterior hypopituitarism, which typically persist even with radiographic regression of the disease. As for Langerhans cell histiocytosis, the definitive diagnosis of Erdheim-Chester’s disease is the finding of the typical histologic features at pituitary biopsy, which can be supported by the finding of the BRAF V600E mutation. Treatment options include vemurafenib, interferon-α, dabrafenib, trametinib, cobimetinib, cladribine, cyclophosphamide and glucocorticoids.

 

Rosai-Dorfman Disease

 

Pituitary involvement has been described in Rosai-Dorfman disease, a rare histiocytic disorder. Patients may have both anterior pituitary dysfunction, central diabetes insipidus and visual disturbances (202,203).

 

Inflammatory Pseudotumor

 

The inflammatory pseudotumor is a rare inflammatory disorder commonly involving the lung and orbit. It can be isolated or associated with the IgG4-related disease (204). Pituitary infiltration is a rare manifestation and patients can present with anterior and posterior hypopituitarism. The inflammatory pseudotumor can also spread to the sphenoid sinus, the cavernous sinus and the optic chiasm (205-207).

 

Tolosa-Hunt Syndrome

 

Tolosa-Hunt syndrome is a painful ophthalmoplegia caused by idiopathic retro-orbital inflammation involving the cavernous sinus or the superior orbital fissure. Histology shows nonspecific granulomatous or nongranulomatous inflammation. Patients with pituitary involvement present with anterior and posterior hypopituitarism, diplopia and retro-orbital pain (often unilateral) (208-212).

 

Other Systemic Diseases

 

Cases of secondary hypophysitis have been described in association with Takayasu’s arteritis (granulomatous hypophysitis) (213), Cogan’s syndrome (214) and Crohn’s disease (215,216). A case of isolated ACTH deficiency in a patient with Crohn’s disease has also been published (217).

 

OTHER CAUSES OF SECONDARY HYPOPHYSITIS

 

Thymoma and Other Malignancies (Anti-Pit-1 Antibody Syndrome)

 

Pit-1 is essential for the differentiation, proliferation, and maintenance of somatotrophs, lactotrophs, and thyrotrophs in the pituitary (218). Yamamoto et al. described three cases of acquired combined TSH, GH, and PRL deficiency, with circulating anti-Pit-1 antibodies (219). Cytotoxic T-cells that react against Pit-1 are likely the cause of anti-Pit-1 antibody syndrome (220-222). All these patients later developed thymomas that express Pit-1. Removal of the thymoma resulted in a decline in antibody titer, suggesting that aberrant expression of Pit-1 in the thymoma plays a causal role in the development of this syndrome (223). A handful of cases of anti-Pit-1 antibody syndrome not associated with thymoma have since been published. The malignancies causing this paraneoplastic syndrome included diffuse large B-cell lymphoma of the bladder and a metastatic cancer of unknown origin (222,224). Based on these cases, Yamamoto et al. have proposed diagnostic criteria for anti-PIT-1 hypophysitis (Table 13).

 

Table 13. Diagnostic Criteria for Anti-PIT-1 Hypophysitis

Criteria

Probable diagnosis

Established diagnosis

Criterion 1

Acquired specific GH, PRL, and TSH deficiency. *

CRITERION 1

CRITERION 1

 

and

 

CRITERION 2

Criterion 2

Presence of anti-PIT-1 antibody or PIT-1-reactive T cells in the circulation.

Criterion 3

Coexistence of thymoma or malignant neoplasm. **

* The secretion of other pituitary hormones is not impaired. The MRI of the pituitary is typically normal, but a slight atrophy of the anterior pituitary can be observed.

** Criterion 3 may help the diagnosis and clarify pathogenesis but may not be necessarily obvious at the time of diagnosis.

Infections

 

Infections of the pituitary are a rare cause of hypophysitis and hypopituitarism (225). They can affect either exclusively the pituitary area or as a part of disseminated infections. Risk factors are diabetes mellitus, organ transplantation, human immunodeficiency virus infection, non-Hodgkin lymphoma, chemotherapy, and Cushing’s syndrome. They can occur by (186):

 

  • Hematogenous spread in immuno-compromised hosts;
  • Contiguous extension from adjacent anatomical sites (meninges, sphenoid sinus, cavernous sinus and skull base);
  • Previous infectious diseases of the CNS of different etiologies;
  • Iatrogenic inoculation during trans-sphenoidal surgery.

 

However, in the majority of cases of pituitary abscess an obvious cause cannot be identified.

Tuberculosis can cause granulomatous involvement of the hypothalamus, the pituitary or the stalk. Tubercular meningitis and hypothalamic-pituitary involvement seem to affect mostly anterior pituitary function (226).

 

Several viruses can cause meningitis, meningoencephalitis and encephalitis that can involve the hypothalamic-pituitary region. Partial or complete hypopituitarism may develop as a result (186). A study by Leow et al. has shown that ~40% of patients with severe acute respiratory syndrome (SARS)-associated with Coronavirus infection can develop reversible central adrenal insufficiency, suggesting a possible inflammation of the pituitary in these patients (227). Hantavirus can also cause viral hypophysitis with pituitary ischemia and hemorrhage as part of the hemorrhagic fever with renal syndrome (HFRS), leading to partial or complete hypopituitarism, including diabetes insipidus (186,228,229).

 

Mycoses with hypothalamic-pituitary involvement are extremely rare. Patients frequently present with central diabetes insipidus and anterior pituitary dysfunction (mainly FSH/LH deficiency and hyperprolactinemia) (186).

 

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Benign Prostate Disorders

ABSTRACT

 

Benign prostatic hyperplasia (BPH) is among the commonest urological abnormalities affecting the aging male. The cause of the increase in prostatic volume is multifactorial, but current research has implicated hormonal aberrations. Clinical assessment of the patient is integral to determining the optimal treatment strategy. Exclusion of prostatic cancer and complications of BPH are critical prior to the commencement of conservative and non-invasive strategies. Recently, the introduction of pharmaceutical agents has changed the landscape of management of BPH. Alpha-blockers, 5-alpha reductase inhibitors, and phosphodiesterase-5 inhibitors provide significant symptomatic improvement for BPH, particularly when used in combination. Invasive surgical therapies remain the gold standard for refractory and complicated BPH disease. Advances in technology have provided new methods to perform prostatectomy including: bipolar resection, laser resection, ablation, enucleation or vaporization. Newer, minimally invasive measures have been introduced in an attempt to limit patient morbidity, specifically operative complications, sexual and urinary function. While results are promising, these emerging therapies have limited long-term data. The purpose of the current chapter is to provide an overview of the current knowledge of benign prostatic hyperplasia.

 

INTRODUCTION

 

The prostate is an organ linked inextricably to the endocrine system. During the development of the prostate, the epithelium and mesenchyme are under the control of testicular androgens, and interact to form an organized secretory organ. Furthermore, the endocrine system plays a key mechanistic role in many prostate diseases, and many therapies for prostatic diseases are aimed at the manipulation of the endocrine system. The gland resides in the true anatomical pelvis and forms the most proximal aspect of the urethra. It has been stated that the prostate gland is the male organ most commonly afflicted with either benign or malignant neoplasms (1). Therefore, it is an organ with which every physician and surgeon needs to be familiar. We will focus on BPH, the most prevalent of benign disorders affecting the prostate.

 

EMBRYOLOGY

 

The development, growth and cytodifferentiation of the prostate are androgen-dependent and occur via embryonic cell-to-cell interactions between the mesenchyme (undifferentiated connective tissue) that induce epithelial development while the epithelium induces mesenchymal differentiation (2).

 

In the developing prostate, urogenital sinus mesenchyme acting under the influence of testicular androgens induces ductal morphogenesis, the expression of epithelial androgen receptors, regulates epithelial proliferation and specifies the expression of prostatic-lobe specific secretory proteins. The developing prostatic epithelium reciprocally induces the differentiation and morphological patterning of smooth muscle in the urogenital sinus mesenchyme (2). In the prostate, it is traditional to consider androgens as promoters of growth, while activin and tumor growth factor-beta1 (TFG-β1) are regarded as potent growth inhibitors. These factors do not act independently, however, and cross-talk occurs between the signaling pathways at a sub-cellular level (3).

 

The first step in development of the prostate begins with the urogenital sinus mesenchyme signaling to the epithelium, causing it to form epithelial buds. Androgens then induce bud elongation, branching and epithelial differentiation (3). Prenatally, the androgen receptor (AR) is expressed only in the mesenchyme, not in the epithelium. Initial epithelial development is thus controlled via paracrine interactions where activation of stromal androgen receptors stimulates growth factors and induces growth in adjacent prostatic epithelial cells (4).

 

At the 5th week, the mesonephric (Wolffian) duct opens onto the lateral surface of the urogenital sinus and gives rise to the ureteric bud (Figure 1). By the 7th week, the growth of the urogenital sinus involves the progressive incorporation of the terminal part of the mesonephric duct into the wall of the urogenital sinus. They eventually open into the Mullerian tubercle that is the future verumontanum of the prostate. At their termination the paramesonephric (Mullerian) ducts fuse and are surrounded by the mesonephric ducts. At 10 weeks, prostatic epithelial buds begin to arise from the circumference of the urethra, around the orifice of the paramesonephric ducts. They develop predominantly on the posterior surface of the junction of the mesonephric ducts, forming two concentrations, above and below them (5).

Figure 1. The embryological origin and development of the prostatic urethra and the prostate, adapted from Delmas (5).

During the fetal period at about 6 months, multiple outgrowths arise from the prostatic portion of the urethra, particularly the posterior surface of the urethra, and grow into the surrounding mesenchyme. Glandular epithelium of the prostate differentiates from the endodermal cells of the urethra, and outgrowths of glandular epithelium protrude into the associated mesenchyme differentiate into the dense stroma and smooth muscle fibers of the prostate. In contrast, the prostatic glandular epithelium outgrowths situated on the anterior surface regress and are replaced by fibromuscular tissue. This region becomes the future anterior commissure of the prostate 5,6).

 

ANATOMY

 

According McNeal’s model of the prostate (7), four different anatomical zones may be distinguished that have anatomo-clinical correlation (Figure 2):

 

  • The peripheral zone: is the area forming the postero-inferior aspect of the gland and represents 70% of the prostatic volume. It is the zone where the majority (60-70%) of prostate cancers form.
  • The central zone: represents 25% of the prostate volume and contains the ejaculatory ducts. It is the zone which usually gives rise to inflammatory processes (e.g., prostatitis).
  • The transitional zone: this represents only 5% of the total prostatic volume. This is the zone where benign prostatic hypertrophy occurs and consists of two lateral lobes together with periurethral glands. Approximately 25% of prostatic adenocarcinomas also occur it this zone.
  • The anterior zone: predominantly fibromuscular with no glandular structures.

 

The prostate weighs approximately 20g by the age of 20 and has the shape of an inverted cone, with the base at the bladder neck and the apex at the urogenital diaphragm (8). The prostatic urethra does not follow a straight line as it runs through the center of the prostate gland but it is actually bent anteriorly approximately 35 degrees at the verumontanum (where the ejaculatory ducts join the prostate) (9).

Figure 2. 1= Peripheral Zone, 2= Central Zone, 3= Transitional Zone, 4= Anterior Fibromuscular Zone. B= Bladder, U= Urethra, SV= Seminal Vesicle (adapted from Algaba (10)).

HISTOLOGY

 

The prostate consists of stromal and epithelial elements. Smooth muscle cells, fibroblasts and endothelial cells are in the stroma and the epithelial cells are secretory cells, basal cells and neuroendocrine cells (Figure 3).

Figure 3. Histology of a prostate gland affected by benign prostatic hyperplasia.

The columnar secretory cells are tall with pale to clear cytoplasm. These cells stain positively with prostate-specific antigen (PSA) (11). Basal cells are less differentiated than secretory cells and are devoid of secretory products such as PSA (12). Finally, neuroendocrine cells are irregularly distributed throughout ducts and acini, with a greater proportion in the ducts. The prostate has the greatest number of neuroendocrine cells of any of the genitourinary organs (13).  Glands are structured with open and closed cell types with the open type facing the inside of the duct having a monitoring role over its contents. Most cells contain serotonin, but other peptides that are present include somatostatin, calcitonin, gene-related peptides and katacalcin (11). The cells co-express PSA and prostatic acid phosphatase. Their function is unclear, but it is speculated that these cells are involved with local regulation by paracrine release of peptides (11). Prostatic ducts and acini are distinguished by architectural pattern at low power magnification. The prostate becomes more complex with ducts and branching glands arranged in lobules and surrounded by stroma with advancing age.

 

Figure 4. Diagram outlining the structure of the prostate gland with regard to ducts, glandular cells and their relationship to blood vessels.

PHYSIOLOGY

 

At present, there is only limited knowledge of all of the secretory products of the prostate and how these relate to reproduction and infertility. However, the main role of the prostate as a male reproductive organ is to produce prostatic fluid that accounts for up to 30 per cent of the semen volume. Prostatic fluid promotes sperm motility, and it is a milky, alkaline fluid containing PSA, citric acid, calcium, zinc, acid phosphatase and fibrinolysin among its many constituents (Table 1) (14).  During ejaculation, alpha-adrenergic stimulation of prostatic smooth muscle expresses seminal fluid containing sperm from the ampulla of the vas deferens into the posterior urethra (15). Interestingly, abnormal growth of the prostate is only experienced by humans and dogs, and why other mammals are spared is a mystery (16).

 

Table 1.  The Composition of Human Semen (adapted from Ganong (17))

Color

White, opalescent

Specific Gravity

1.028

pH

7.35-7.50

Volume

3ml

SPECIFIC COMPONENTS OF SEMEN

Gland/Site

Volume in ejaculate

Features

Testis/Epididymis

0.15ml (5%)

Average approximately spermatozoa 80 million/ml

Seminal Vesicle

1.5-2ml (50-65%)

Fructose (1.5-6.5 mg/ml) phosphorylcholine ergothioneine, ascorbic acid, flavins prostaglandins, bicarbonate

Prostate

0.6-0.9ml (20-30%)

Spermine, citric acid, cholesterol, phospholipids, fibrinolysin, fibrinogenase, zinc, acid phosphatase, prostate-specific antigen

Bulbourethral Glands

< 0.15ml (<5%)

Clear mucus

 

ENDOCRINE CONTROL OF PROSTATIC GROWTH

 

Intraprostatic signaling systems are important for the regulation of cell proliferation and extracellular matrix production in the prostatic stroma. Central to this premise is the balance between factors such as TGF-β1, that induces extracellular matrix production, suppresses collagen breakdown and cell proliferation and factors such as fibroblast growth factor 2 and insulin-like growth factors that are mitogenic in the stromal compartment (18). Other endocrine pathways are being investigated, and there is a growing body of evidence suggesting an abnormality in the insulin-like growth factor axis is playing a role in the pathogenesis of BPH (19).

 

Testosterone

 

Prostatic epithelial cells express the androgen receptor (20). From the beginning of embryonic differentiation to pubertal maturation and beyond, androgens are a prerequisite for the normal development and physiological control of the prostate (21). Androgens help maintain the normal metabolic and secretory functions of the prostate, and they are also implicated in the development of BPH and prostate cancer. Androgens do not act in isolation, and other hormones and growth factors are being investigated (22).

 

Androgens also interact with prostate stromal cells which release soluble paracrine factors that induce growth and development of the prostatatic epithelium (4). These paracrine pathways might be critical in regulating the balance between proliferation and apoptosis of prostate epithelial cells in the adult (22).

 

The appropriate balance between testosterone and its 5-alpha reduced metabolites is key to normal prostate physiology (note the metabolic pathways for androgen metabolism are described in Endotext, Endocrinology of Male Reproduction, Androgen Physiology, Pharmacology and Abuse, D Handelsman). The metabolism of testosterone to dihydrotestosterone (DHT) and its aromatization to estradiol are recognized as the key events in prostatic steroid response. 

Figure 5. Conversion of testosterone to dihydrotestosterone (DHT) by 5alpha reductase

Testosterone, to be maximally active in the prostate, must be converted to DHT by the enzyme 5-alpha reductase (Figure 5) (23). DHT has a much greater affinity for the androgen receptor than does testosterone, and DHT accumulates in the prostate even when circulating concentrations of testosterone are low (24,25). Based on rat studies, DHT is about twice as potent as testosterone at equivalent androgen concentrations (26). Therefore, prostatic DHT concentrations may remain similar to those in young and elderly men, despite the fact that serum testosterone concentrations generally decline with age (23). In the prostate, the total level of testosterone is 0.4 ng/g and the total of DHT is 4.5 ng/g (27). The total serum concentration of testosterone in the blood is approximately 10 times higher than DHT (17). Circulating DHT, by virtue of its low serum plasma concentration and tight binding to plasma proteins, is of diminished importance as a circulating androgen affecting prostate growth (16). Intra-prostatic androgens are remarkably independent of serum concentrations (28), and circulating androgen concentrations often do not correlate with intraprostatic concentrations (29).

 

Estrogen

 

A role for estrogens in the prostate pathology of the ageing male appears likely with accumulating evidence that estrogens, alone or in combination with androgens, are involved in inducing aberrant growth and/or malignant change. Animal models have supported this hypothesis in the canine model, where estrogens “sensitize” the ageing dog prostate to the effects of androgen (30). The evidence is less clear in humans. Estrogens in the male are predominantly the products of peripheral aromatization of testicular and adrenal androgens (31). While the testicular and adrenal production of androgens declines with ageing, concentrations of total plasma estradiol do not decline. This has been ascribed to the increase in fat mass with ageing (the primary site of peripheral aromatization) and to an increased aromatase activity with ageing. However, free or bioavailable estrogens may decline due to an increase in sex hormone binding globulin, which could translate to lower intraprostatic concentrations of the hormone. The potentially adverse effects of estrogens on the prostate might be due to a shift in the intra-prostatic estrogen: androgen ratio with ageing. 

 

Estrogen, which acts through estrogen receptors (ER) alpha and beta, has been implicated in the pathogenesis of benign and malignant human prostatic tumors (32-34). As stated above, BPH is thought to originate in the transitional zone (TZ) and prostate cancer the peripheral zone (PZ) of the prostate. Receptor studies have found ER-alpha and ER-beta types distributed in human normal and hyperplastic prostate tissues, using in situ hybridization and immunohistochemistry. ER-alpha expression was restricted to stromal cells of the PZ. In contrast, ER-beta was expressed in the stromal and epithelial cells of PZ as well as TZ. These findings suggest that estrogen might play a crucial role in the pathogenesis of BPH through ER-beta (33). Investigations are ongoing and could result in a new range of therapies directed against BPH and prostate cancer. Dietary phytoestrogens (in soy and other vegetables) or selective estrogen receptor modulators are currently being investigated with regard to their role in the development of BPH and prostate cancer (31). Such ER modifiers might oppose some of the effects of natural estrogen by modulating ER receptors, thus reducing the local impact of androgens that need active ER receptors, effectively making them anti-androgenic compounds, but this hypothesis requires more investigation (35).

 

BENIGN PROSTATIC HYPERPLASIA (BPH)

 

BPH is an age-related and progressive neoplastic condition of the prostate gland (36). BPH can only be diagnosed definitively by histology. BPH in the clinical setting is characterized by lower urinary tract symptoms (LUTS, see below and Table 2). There is no causal relationship between BPH and prostate cancer (37). Clinically apparent BPH has a significant effect on quality of life, particularly its effects on nocturia and bladder dysfunction. The overall prevalence of BPH is 10.3%, with an overall annual incidence rate of 15 per 1000 man-years, increasing with age (3 per 1000 at age 45-49 years, to 38 per 1000 at 75-79 years). For a symptom-free man at age 46, the 30-year risk of clinical BPH is 45% (38). The true prevalence and incidence of clinical BPH will vary according to the criteria used to describe the condition; however, it has been estimated that the prevalence of BPH is rising due to increases in modifiable risk factors such as obesity (39). It is crucial to acknowledge that LUTS can exist without signs of BPH – as the symptoms can be caused by variations in the sympathetic nervous stimulation of prostatic smooth muscle, variability of prostatic anatomy (viz., enlarged median lobe of the prostate), and the variable effects of bladder physiology from the obstruction and aging.

 

There have been several studies demonstrating the fact that clinical BPH is a progressive disease. The Olmsted county study (40) showed that with each year there were deteriorations in symptom scores, peak flow rates, and increases in prostate volumes based on transrectal ultrasound scanning (TRUS). The risk of acute urinary retention (AUR) increased with flow rates below 12 ml/sec and with glands greater than 30ml. Studies have also demonstrated that those with larger prostates (>40 ml) and with serum PSA greater than 1.4 ng/ml were more likely to develop acute urinary retention (41). Treatment, however, has changed with the advent of effective non-surgical therapies. Between 1992-1998 there has been a significant lengthening of the period between first diagnosis of LUTS secondary to clinical BPH and surgery, associated with the earlier and increased use of specific medical treatments (42). From the patients perspective, the goals of therapy are to improve quality of life, reduce symptoms, and avoid surgery while ensuring safety from the complications of BPH (43).

 

Risk Factors for BPH

 

The only clearly defined risk factors for BPH are age and the presence of circulating androgens. BPH does not develop in men castrated before the age of forty (44),  but other factors may influence the prevalence of clinical disease. These include the following:

 

GENETICS

 

Clinical BPH appears to run in families. If one or more first degree relatives are affected, an individual is at greater risk of being afflicted by the disorder (45). In a study by Sanda et al (46), the hazard-function ratio for surgically treated BPH amongst first degree relatives of the BPH patients as compared to controls was 4.2 (95% CI, 1.7 to 10.2). The incidence of BPH is highest and starts earliest in blacks than Caucasians and is lowest in Asians (37). Interestingly, despite having larger prostate glands, the age-adjusted risk of BPH was the same for blacks as for whites (RR = 1.0, 95% Cl 0.8-1.2) (47). Furthermore, in an Asian population, men presenting with BPH are likely to have higher symptom scores than blacks or Caucasians (48).

 

DIET

 

Diet has been reported as a risk factor for the development of BPH. Large amounts of vegetables and soy products in the diet may explain the lower rate of BPH in Asia when compared to countries with Western, non-Asian diets. In particular, certain vegetables and soy are said to be high in phytoestrogens, such as genestin, that have anti-androgenic effects by an undetermined mechanism on the prostate in vitro (49).

 

Studying migrant populations with their heterogeneous environmental exposures increases the probabilities of identifying potential risk factors for BPH. Therefore, the association of alcohol, diet, and other lifestyle factors with obstructive uropathy was investigated in a cohort of 6581 Japanese-American men, examined and interviewed from 1971 to 1975 in Hawaii. After 17 years of follow-up, 846 incident cases of surgically treated obstructive uropathy were diagnosed with BPH. Total alcohol intake was inversely associated with obstructive uropathy (p < 0.0001). The relative risk was 0.64 (95% confidence interval: 0.52-0.78) for men drinking at least 25 grams of alcohol per month compared with nondrinkers. Among the 4 sources of alcohol, a significant inverse association was present for beer, wine, and sake, but not for spirits. No association was found with education, number of marriages, or cigarette smoking. Increased beef intake was weakly related to an increased risk (p = 0.047), while no association was found with the consumption of 32 other food items in the study (50).

 

METABOLIC SYNDROME

 

There is a growing body of evidence supporting the association between obesity or metabolic syndrome and the development of BPH. The risk of BPH appears to be independently associated with the individual components of BPH including central obesity, hyperinsulinemia, insulin resistance and dyslipidemia. Despite this, the precise causation of this association has not been clearly identified. Recent studies have suggested that in this setting, BPH is a consequence of the metabolic syndrome-associated metabolic derangements, altered sex hormone concentrations and lowered sex-hormone binding globulin concentrations (51). One study found in a cohort of 415 men, that indicators of metabolic syndrome (abnormal concentrations of insulin resistance, subclinical inflammatory state, and sex hormone globulin changes) were significantly associated with increased risk of BPH (52).  Mechanisms associated with metabolic syndrome have been discussed as possible targets for future therapies for BPH (53).

 

CHRONIC INFLAMMATION

 

There is strong evidence to suggest that inflammation and inflammatory markers are involved in the pathogenesis of BPH. Inflammation within the prostate can be caused by several factors including bacteria, virus, autoimmune disease, diet, metabolic syndrome, and hormone imbalances (53). This leads to the activation of inflammatory cells, release of cytokines, expression of growth factors, and ultimately abnormal proliferation of epithelial and stromal cells of the prostate. Proliferation induces a cycle of hypoxia and recruitment of more growth factors resulting in increased prostate volume and BPH (54). The REDUCE study of 8224 prostate biopsy samples of men with BPH found that 77.6% had cells of chronic inflammation on histology (55).

 

Anti-inflammatory medications have been studied in combination with BPH medications. A meta-analysis of three randomized controlled trials (n=183) in this area found that non-steroidal anti-inflammatory drugs improved IPSS scores by a mean of 2.89 points and increase peak urine flow by a mean of 0.89m/s (56).

 

OTHER RISK FACTORS

 

It has not been possible to delineate any other risk factors for BPH such as coronary artery disease, liver cirrhosis, or diabetes mellitus. Traditionally it has been believed that there is no causal relationship between malignant and benign prostatic hypertrophy (37) and recent data from large trials continue to support this premise (57).  Alternative theories have emerged but more data directly linking association with causality are required (58).

 

PATHOPHYSIOLOGY OF BPH

 

Natural History

 

BPH is a histological diagnosis, but its clinical manifestations occur after growth has occurred to such a degree and in such a strategic location within the gland, namely the transitional zone, that it impairs bladder emptying and results in LUTS. One can consider the natural history of BPH as involving two phases:

 

(i) The pathological or first phase of BPH is asymptomatic and involves a progression from microscopic to macroscopic BPH. Microscopic BPH will develop in almost all men if they live long enough but in only about half will progress to macroscopic BPH. This would suggest that additional factors are necessary to cause microscopic to progress to macroscopic BPH (59). The pathological phase involves development of hyperplastic changes in the transitional zone of the prostate (60). While there is wide variability in prostate growth rates at an individual level, prostate volume appears to increase steadily at about 1.6% per year in randomly selected community men (61).

 

(ii) The clinical or second phase of BPH involves the progression from pathological to ‘clinical BPH’ that is synonymous with the development of LUTS. Only about one half of patients with macroscopic BPH progress to develop clinical BPH (59). BPH consists of mechanical and dynamic components and it is these components that are responsible for the progression from pathological to clinical BPH (62). In clinical BPH, the ratio of stroma to epithelium is 5:1 whereas in the case of asymptomatic hyperplasia the ratio is 2.7:1. A significant contribution is therefore made by stroma to the infravesical obstruction of BPH (63).

 

DISEASE MANIFESTATIONS OF BPH

 

Lower Urinary Tract Symptoms (LUTS)

 

Lower urinary tract symptoms (LUTS) are highly prevalent and the majority of LUTS in men is produced by BPH, but may be contributed to by a variety of conditions (Figure 6). LUTS are traditionally divided into voiding or obstructive and storage or irritative symptoms (Table 2). Voiding symptoms are more common, however it is storage symptoms that are most bothersome and have a greater impact on a patient's life (64,65). The prevalence of clinical BPH rises with age and approximately 25% of men age 40 or over will suffer from LUTS (66).

 

Figure 6. Interaction of the many factors involved in the pathogenesis of LUTS. Other causes of LUTS (top right) include all of the differential diagnoses included in Table 5 (see below).

Hesitancy

Poor stream

Intermittent stream

Straining to pass urine 
Prolonged micturition 
Sense of incomplete bladder emptying 
Terminal dribbling

Urinary frequency

Urgency

Urge incontinence

Nocturia

 

In the past, LUTS suggestive of bladder outflow obstruction (BOO) secondary to BPH were referred to as ‘prostatism’, once other causes such as a urinary tract infection or prostate cancer were excluded. The pathology behind the symptoms was thought to be obstruction due to prostatic gland enlargement alone. However, it is now recognized that voiding/obstructive symptoms result from direct urinary flow obstruction whilst storage/irritative symptoms appear to be due to secondary bladder dysfunction (67). Thus, LUTs occurs after the prostate enlargement causes obstruction, and the bladder voiding is secondarily affected leading irritable symptoms (Figure 7).

 

Figure 7. Diagrammatic representation of BPH with the enlarged prostate transition zone causing obstruction of the prostatic urethra and the secondary changes in the bladder leading to hypertrophy of the detrusor muscle (copyright Nathan Lawrentschuk 2012).

 

This concept has been further refined in that obstructive symptoms are thought to result not only from mechanical obstruction due to glandular enlargement, but also dynamic obstruction secondary to contraction of the smooth muscle of the prostate, urethra and bladder neck. This dynamic obstruction is a result of sympathetic nervous system mediated stimulation of alpha-1 adrenoceptors. Storage symptoms appear to be caused by detrusor instability related to detrusor muscle changes in response to obstruction, such as bladder wall hypertrophy and collagen deposition in the bladder (68,69). Adrenoceptors may be further sub-divided into alpha1A and alpha1D subtypes, with alpha1A predominant in the prostate and alpha 1D in the bladder. Thus, blockade of alpha1A may be necessary for reduction of obstruction whereas the blockade of alpha1D may be required to relieve storage symptoms (70) (see below).

 

It has been suggested that the etiology of LUTS related to BPH is even more complex than outlined above, with extra-prostatic mechanisms such as bladder wall ischemia and changes in the central nervous system being implicated (71). Normal lower urinary tract function is complex, and theoretically any disruption of the pathway for micturition (Figure 8 below) may lead to LUTS (72).

 

It is worth noting the relationship between LUTS and sexual dysfunction, with sexual dysfunction being highly prevalent in men with LUTS

By sexual dysfunction, we refer to decreased libido, erectile dysfunction, decreased ejaculation and other ejaculation disorders. Kassabian

expands on the relationship and agrees with Leilefeld et al

in suggesting that the relationship is coincidental and both are common in the ageing male.

 

Figure 8- Normal micturition pathways (reproduced with permission from Physiology and pathophysiology of lower urinary tract symptoms, Drugs of Today, Vol 37, p. 7, Michel MC(72)).

 

COMPLICATIONS OF BPH

 

The complications of BPH are summarized in Table 3.

 

Table 3. Common Complications of BPH

·       Urinary retention

·       Recurrent Urinary Tract Infections

·       Bladder Calculi

·       Hematuria

·       Secondary bladder instability

·       Renal Impairment

 

Urinary Retention (Acute and Chronic)

 

As the prostate volume increases with age, the likelihood of acute urinary retention (AUR) and symptom severity both increase while urinary flow rates fall. In one study of more than 2000 men, those with a maximum urinary flow rate (Qmax) <12 ml/s had a 4 times greater risk for AUR than did men with a Qmax >12ml/s (76). AUR is usually painful and necessitates the insertion of a per urethral indwelling or suprapubic urinary catheter.

 

If the urinary retention is not dealt with in a timely fashion, the detrusor muscle becomes distended and damaged, contributing to poor detrusor function and an inability to adequately empty the bladder. The retention of urine becomes painless over time, and the sequelae of retained urine such as recurrent UTI, calculi, and renal impairment may develop.

Furthermore, a situation of overflow incontinence may develop whereby the bladder automatically empties once the volume reached exceeds its new, larger capacity. The passage of urine is typically uncontrolled, and this may often be the first presentation for someone with advanced BPH. The bladder remains full despite the emptying, which is only partial.

 

In situations of chronic urinary retention, relieving the bladder outflow obstruction might not restore normal detrusor function. These patients often need to use intermittent self-catheterization or have permanent drainage to keep their bladder empty and to reduce damage to the upper urinary tract.

 

Recurrent Lower Urinary Tract Infection (UTI)

 

The best host defense against infection in the lower urinary tract might be normal flow of urine and bladder emptying. In BPH, bladder outflow obstruction results in disruption of this mechanism with retention and pooling of urine in the bladder, giving organisms the opportunity to multiply rather than be flushed out. Despite this logical assumption, there is little evidence in the literature to support this theory. Nevertheless, men with significant clinical BPH are probably at risk of UTI, and men with UTI should be assessed for signs of BPH.

 

Bladder Calculi

 

In developed countries, the most prevalent cause of bladder calculi is bladder outlet obstruction owing to BPH (77). Of those who undergo prostate surgery for BPH, approximately 2% of all patients are found to have bladder stones (78). Stones occur in this situation due to urinary stasis combined with high urinary solute concentrations, which leads to crystal precipitation (79). Chronic infection with urease-producing organisms may predispose to the development of stones and rarely stones pass from the upper tract to act as a nidus in the bladder (79) . Bladder calculi associated with BPH remains an absolute indication for transurethral resection of the prostate (TURP) (80,81) because of the risk or recurrence of stone formation. However, the necessity of surgery is being challenged by the expanding use of medical management in treating BPH (81).

 

Hematuria

 

The incidence of hematuria with BPH is uncertain, however; in a retrospective review of almost 4000 patients undergoing TURP, Mebust et al (80) noted that hematuria was an indication for surgery in 12% of patients. It is hypothesized that BPH, with its increased acinar and stromal cell proliferation, stimulates increased vascularity via angiogenesis. These new and prolific vessels may be easily disrupted leading to recurrent bleeding (82). This is supported by Foley et al (83) who found the microvessel density to be higher in those patients with BPH having hematuria after histological studies. It is also hypothesized that 5-alpha reductase inhibitors might reduce angiogenesis and theoretically reduce the risk prostate bleeding. Finasteride has been suggested as an option in treating the problem of hematuria (84-86).

 

Detrusor (Bladder) Instability

 

The definition of detrusor instability is the development of a detrusor contraction which exceeds 15cm H2O at a bladder volume of less than 300ml (87). Detrusor instability is not a specific term related to BPH, but implies LUTS secondary to detrusor pathology. These symptoms are normally storage related and consist of urgency, frequency, urge incontinence, and nocturia. In BPH, the normal dynamics of the bladder are altered due to detrusor muscle stretching due in turn to retention of urine and contraction against an obstructed outlet. Although not completely understood, some of the detrusor instability may be related to changes at the adrenoceptors level, rather than just from obstruction and its consequences alone. In normal bladder physiology, beta-adrenoceptors are believed to be involved in the relaxation of the bladder during storage of urine (71). In some patients, however, the administration of noradrenaline leads to contraction of the detrusor muscle which may be blocked by an alpha-1 adrenoceptor antagonist (88). This implies the presence of alpha-adrenoceptors in the detrusor muscle in at least some patients. Furthermore, alpha-adrenoceptor antagonists have been shown to relieve storage and voiding symptoms in men without obstruction and storage symptoms in women (71,89-92). Alpha adrenoceptor subtypes in the human bladder are predominantly of the alpha1D and alpha1A type. In animal models, the alpha1D receptors become more abundant with bladder obstruction (93), and it may be speculated that this is the case in humans and that these receptors, once up-regulated, play a role in storage symptoms (71).

 

Renal Insufficiency

 

Renal insufficiency results from obstructive uropathy secondary to the bladder outlet obstruction of BPH. In an analysis of patients receiving treatment for BPH, 13.6% (range 0.3-30%) had renal insufficiency (78). Certainly, an abnormal creatinine is an indication to further investigate the upper urinary tract with imaging. Obviously, other concurrent causes of renal insufficiency need to be excluded. Those patients with renal insufficiency undergoing surgery are at increased risk (25%) of postoperative complications such as acute renal failure and urosepsis compared to patients without (17%) insufficiency (80).

 

HISTORY

 

A comprehensive medical history must be evaluated and should include the use of a voiding diary, the International Prostate Symptom Score (IPSS) and a discussion of the role of PSA testing (94).  An outline of the evaluation and treatment options for LUTS is shown in Table 4 and is discussed in greater depth below (95,96). Previous urological disease should be documented including previous urological surgery, UTI, bladder or renal calculi, renal disease and penoscrotal pathology. Any risk factors for surgery such as diabetes mellitus, immunosuppression, ischemic heart disease, respiratory problems, smoking as well as a comprehensive list of medications should be noted. Medications with anti-cholinergic properties should be noted, as these may contribute to the patient’s symptoms. The use of antihypertensives must be noted as any alpha-blocker treatment initiated could potentially cause severe hypotension.

 

As discussed in the section on differential diagnosis, consideration needs to be given to neurologic causes of voiding dysfunction such as stroke or Parkinson’s disease.

 

Table 4. A Summary of Diagnosis and Treatment Options in BPH

EVALUATION of LUTS

ESSENTIAL

1. History

2. Digital Rectal Exam (DRE)

3. Urinalysis 
4. Serum creatinine 
5. PSA, if > 10-year life expectancy 
6. International Prostate Symptom Score (IPSS) or AUA symptom index

SELECTED 
1. Uroflowmetry 
2. Imaging - especially if hematuria, UTI, urolithiasis 
3. Post Void Residual (PVR) estimation 
4. +/-Pressure flow studies 
5. +/-Cystoscopy

TREATMENT OPTIONS

MEDICAL THERAPY

1. Phytotherapy

2. Monotherapy:

       a. Alpha blockers

       b. 5-alpha reductase inhibitors

       c. PDE5 Inhibitors

3. Combination therapy:

       a. Alpha blocker + 5-alpha reductase inhibitor

       b. PDE5 inhibitor + alpha blocker (experimental)

SURGERY 
1
. Invasive surgery

       a. Transurethral resection of the prostate (TURP) 
       b. Laser prostatectomy/treatment 

       c. Open prostatectomy 
2. Minimally invasive measures

       a. Transurethral Incision of the prostate (TUIP) 

       b. Thermo ablative strategies (TUMT, TUNA)

       c. Chemical ablative (PRX-302, NX-1207, TEAP)

       d. Mechanical (Urolift, prostatic stent)

       e. Others (prostatic artery embolization, histotripsy, Rezum, aquablation)

 

International Prostate Symptom Score (IPSS)

 

The American Urologic Association (AUA) Symptom Index was developed as a standardized instrument to assess the degree of bladder outlet obstruction in men (89). It is widely used and consists of seven questions that assess emptying, frequency, intermittency, urgency, weak stream and straining with each graded with a score of 0-5. Total score ranges 0-35. The index categorizes patients as:

  1. Mild (score £7)
  2. Moderate (score 8-19)
  3. Severe (score 20-35).

 

The International Prostate Symptom Score (IPSS) is a modification of the AUA Symptom Index adding a single question assessing the quality of life or bother score based on the patient’s perception of the problem (Figure 9) (97). Both the AUA and IPSS questionnaires, although not specific for BPH, prostate volume, urinary flow rate, post-void residual volume, or bladder outlet obstruction, have been validated and are sensitive enough to be to be used in the evaluation of symptoms and selection of treatment (98-100). Many would argue that the score is the primary determinant of whether or not a patient proceeds to further treatment. Further, these questionnaires are a valuable objective measure when determining the response to treatments for BPH.

 

Figure 9. International Prostate Symptom Score (IPSS) Sheet (101,102)

 

EXAMINATION

 

General appearance is of importance, especially in identifying those with neurological disease (e.g., past stroke, Parkinson’s disease) or other major co-morbidities (obesity, severe osteoarthritis, diabetes) that may impact on treatment or further investigation. An abdominal examination should identify those in marked urinary retention, any abnormal masses, and previous surgical scars. A careful assessment of the scrotum and its contents as well as the penis is also warranted to exclude any other pathology. The digital rectal examination (DRE) is important in identifying prostatic abnormalities, including clinically apparent prostate carcinoma (103). Prostate size, texture, and tenderness should all be assessed, as should anal tone. Any nodules should be carefully noted. Constipation may also be a contributing factor to urinary retention and anal tone should also be recorded.

 

DIFFERENTIAL DIAGNOSIS OF BPH

 

It is important to acknowledge that the diagnosis of BPH often relies on surrogate measures until a histological diagnosis is confirmed. These range from clinical (symptom scores), physiological (uroflowmetry), anatomical (prostatic volume on DRE or TRUS) and biochemical (PSA values) measurement. Although all of these measurements capture some component of BPH, none of them is specific for BPH (104). Surrogate measures are likely to represent a continuum of disease severity without the existence of a threshold. Thus, differential diagnoses need to always be considered and where appropriate, excluded. In table 5 below, some of the more obvious differential diagnoses are listed, but will not be examined in detail.

 

Table 5. Differential Diagnoses for LUTS

Inflammatory Conditions

 

1. Urinary Tract Infection

2. Prostatitis

3. Bladder Calculi 
4. Interstitial Cystitis 
5. Tuberculous Cystitis

Neoplastic Conditions

 

1. Prostate cancer

2. Bladder transitional cell carcinoma (usually CIS)

3. Urethral cancer

Neurological Conditions

1. Parkinson's disease

2. Stroke

3. Multiple Sclerosis 
4. Cerebral Atrophy 
5. Shy-Drager Syndrome

Other Causes of Urinary Obstruction

1. Urethral stricture

2. Severe phimosis

3. Bladder neck dyssynergia 
4. External sphincter dyssynergia

 

PROSTATITIS

 

Prostatitis is a common condition that must be excluded from other causes of LUTS and is a common cause of visits to primary care physicians and urologists. It may present as an acute bacterial infection or may be chronic, occasionally progressing to a debilitating illness. In practice, the clinical diagnosis of prostatitis depends on the history and physical examination, but there is no characteristic physical finding or diagnostic laboratory test. Patients with prostatitis experience considerable morbidity and may remain symptomatic for many years. Unfortunately, there is limited understanding of the pathophysiology and optimal treatment for most patients. Prostatitis has been sub-classified and an abbreviated version is shown in Table 6.

 

·       Table 6. The National Institute of Health (USA) Consensus Classification of Prostatitis Syndromes

·       Acute bacterial prostatitis

·       Chronic bacterial prostatitis

·       Chronic prostatitis/chronic pelvic pain syndrome

·       Inflammatory

·       Non-inflammatory

·       Asymptomatic inflammatory prostatitis

 

Acute Prostatitis

 

Clinical features suggestive of acute prostatitis (Type 1, in Table 6 above) include dysuria and urinary frequency as well as perineal pain (Table 7). Systemic symptoms such as fever, rigors, myalgia and sweats are often a feature. On examination, the patient is normally febrile, and may be overtly septic depending on the infection severity. A digital rectal exam finds an extremely tender prostate, which is often intolerable to the patient. An abscess is occasionally palpated.

 

Table 7. Clinical Symptoms in Prostatitis (adapted from Lobel (105))

Genital symptoms

1. Dribbling

2. Inguinal pain

3. Testicular pain

4. Retropubic pain 
5. Perineal pain 
6. Urethral Burning

General Symptoms

1. Backache

2. Sweating

3. Tiredness

4. Cold feet

 

Investigations should include a mid-stream urine sample for microscopy, culture for bacteria, and antibiotic sensitivity. The most common organisms are typical uropathogenic bacteria such as Escherichia coli (E. coli). Blood cultures for bacteria and antibiotic sensitivity should also be considered. Prostatic massage is usually contraindicated in patients with acute prostatitis due to pain and the risk of precipitating sepsis. A treatment regime is highlighted in Table 8.  If there is failure to respond to therapy, evaluation for a prostatic abscess using a transrectal ultrasound scan or computed tomography scan may be required. If necessary, perineal or transurethral drainage of an abscess may be undertaken. At least 4 weeks of antibiotic therapy is recommended in all patients to try to prevent chronic bacterial prostatitis. Following resolution of acute prostatitis, the urinary tract should be investigated for any structural problems (106,107).

 

Table 8. Treatment of Acute Prostatitis

1. Hydration

2. Rest and hospitalization if severe

3. Empirical therapy with antibiotic until urine culture and sensitivities available

4. For patients requiring parenteral therapy antibiotics covering the likely organisms: broad spectrum cephalosporins, for example, cefuroxime, cefotaxime, or ceftriaxone plus gentamicin

5 Oral treatment according to sensitivities.: quinolones, such as ciprofloxacin or norfloxacin.  For patients intolerant of, or allergic to, quinolones: trimethoprim or co-trimoxazole;

6. Analgesics, such as non-steroidal anti-inflammatory drugs Suprapubic catheterization if catheterization needed - per urethral catheters may precipitate abscess formation

 

Chronic Prostatitis

 

As the presentation may be localized to the genital region or non-specific (see Table 7) a careful history and examination along with specialized diagnostic tests are needed to identify this condition. Investigations may involve prostatic massage to express organisms and/or white blood cells for analysis. Urine sample collection is often done in phases to aid in the localization process: first void urethral urine; mid-stream bladder urine; post-prostatic massage sample. Urine microscopy and quantitative culture is then undertaken. Semen analysis for excessive white blood cell numbers may also be indicative of chronic prostatitis. Serum PSA concentrations are often elevated in acute prostatitis or in an active phase of chronic prostatitis. Trans-rectal ultrasound might be considered but not recommended to differentiate the different forms of chronic prostatitis. Urinary tract localization procedures (culture of first void urethral urine; mid-stream bladder urine; post-prostatic massage samples of urine correlating to urethra, bladder and prostate) although theoretically correct, are often not used in clinical practice (106,107).

 

The various classifications of chronic prostatitis are listed in Table 6. Patients with chronic bacterial prostatitis (type II prostatitis) experience recurrent episodes of bacterial urinary tract infection caused by the same organism, usually E. coli, another Gram-negative organism, or enterococcus. Between symptomatic episodes of bacteriuria, lower urinary tract cultures can be used to document an infected prostate gland as the focus of these recurrent infections. Acute and chronic bacterial prostatitis represent the best understood, but least common, prostatitis syndromes (106,107).

 

Unfortunately, more than 90% of symptomatic patients have chronic prostatitis/chronic pelvic pain syndrome (type III). This term recognizes the limited understanding of the causes of this syndrome for most patients and the possibility that organs other than the prostate gland may contribute to this syndrome. Urological pain (normally in the perineum or associated with voiding or intercourse) is now recognized as a primary component of this syndrome. Active urethritis, urogenital cancer, urinary tract disease, functionally significant urethral stricture, or neurological disease affecting the bladder must be excluded. Patients with the inflammatory subtype (type IIIA) of chronic prostatitis/chronic pelvic pain syndrome have leukocytes in their expressed prostatic secretions post prostate massage urine or in semen.

 

In contrast, patients with the non-inflammatory subtype of chronic prostatitis (type III B) have no evidence of inflammation. In essence, they have no evidence of active infection nor of inflammation on available investigative techniques taken at a particular point in time. Repeat investigations are therefore done to be sure adequate sampling has been undertaken. This condition may be difficult to treat and requires intensive counselling, information and reassurance to the patient to be successfully managed (107).

 

Finally, asymptomatic inflammatory prostatitis (type IV) is diagnosed in patients who have no history of genitourinary tract pain complaints. It is often an incidental finding on prostatic biopsy done for other reasons (e.g., a raised PSA). Treatment is usually not required.

 

Treatment of Chronic Prostatitis

 

All patients should have investigations as outlined above. A summary of treatment options is shown in Table 9. Those patients with chronic prostatitis secondary to bacterial infection (type II) require a prolonged course of antibiotics (often up to three months) and should then be re-cultured to ensure eradication of the organism. Some urologists argue that these patients should also have investigation of their urinary tract by way of cystoscopy and at minimum, an ultrasound to ensure no anatomical abnormality that may be responsible.

 

Patients with asymptomatic prostatitis (IV) require no treatment but those with the inflammatory (IIIA) and non-inflammatory (IIIB) are more difficult.  Patients with type IIIA disease have excessive leukocytosis in their specimens but no bacteria. However, because their symptoms may be due to a pathogen that is difficult to isolate, a further course of antibiotics (6-12 weeks) with coverage of chlamydia and ureaplasma should be given (105). If this antibiotic course is not therapeutic, then a focus should be on anti-inflammatory medications (which may be used in conjunction with the course of antibiotics). If anti-inflammatory treatment fails, then patients should be treated as below, for type IIIB.

 

Current treatment for Type IIIB patients requires multiple therapies. Triple-therapy involves high dose alpha-blocker (3 month minimum), analgesia, and muscle relaxant (benzodiazepines). Initially, a narcotic analgesic should be changed to a non-steroidal anti-inflammatory (NSAID) if a response occurs after 2 weeks. The NSAID should be continued for at least 6 weeks, but stopped if there is no response at 2 weeks. If the triple treatment fails, other avenues must be explored, including biofeedback, relaxation exercises, psychotherapy, and lifestyle changes (soft cushions, cease bike-riding). The focus is on improving quality of life and minimizing symptoms, not curing the disease (105).

 

·       Table 9. Management and Treatment of Chronic Prostatitis

·       Oral and written patient education

·       Pharmacological treatment for chronic bacterial prostatitis chosen according to antimicrobial sensitivities include quinolones such as ciprofloxacin; ofloxacin; norfloxacin. For those allergic to quinolones: minocycline; doxycycline; trimethoprim-sulfamethoxazole; co-trimoxazole; in many regions, trimethoprim sulfamethoxazole is first line therapy because of better safety profile than quinolones.

·       Other treatments for chronic bacterial prostatitis: radical transurethral prostatectomy or total prostatectomy in carefully selected patients.

·       Empirical treatments for chronic abacterial prostatitis

·       Treat as for chronic bacterial prostatitis with a quinolone or tetracycline

·       Alpha blockers: terazosin, doxazosin, alfuzosin, tamsulosin, silodosin

·       Non-steroidal anti-inflammatory drugs

·       Stress management. Referral for psychological assessment as appropriate; diazepam. Note: benzodiazepines are considered but not recommended in clinical practice because of dependency

·       Adequate follow-up and counselling, often with professional support

·       Cernilton (pollen extract)

·       Bioflavonoid quercetin

·       Transurethral microwave thermotherapy

 

INVESTIGATIONS OF LUTS

 

As outlined by Tubarro et al (94), the aim of investigations for LUTS should be threefold: (1) to evaluate the possible relationship between prostatic enlargement, lower urinary tract symptoms and signs of bladder outlet obstruction; (2) to quantify the severity of benign prostatic enlargement-related symptoms and signs and (3) to rule out the presence of a prostate cancer.

 

Urinalysis

 

Urinalysis is used to screen for urinary tract infection as a cause of LUTS in order to identify those with microscopic or macroscopic hematuria. A formal urine culture may be undertaken if the analysis was suspicious for infection.

 

Post-Void Residual Urine Volume (PVRU)

 

Although there is a high degree of intra-individual variation in the PVRU, it may still provide valuable information with regard to bladder emptying. Although it does not distinguish adequately between bladder outlet obstruction or poor detrusor function, it can identify a bladder emptying problem and be used as a marker for improvement.  Due to its inability to differentiate between causes, the United States guidelines on BPH suggest it is an optional investigation (78). Greater than 300ml is considered a potential risk factor for upper urinary tract dilatation and renal impairment (108).  The PVRU does have the advantage of being used as a monitoring investigation in those opting for non-surgical therapy for BPH. It is readily and quickly performed in the office or hospital setting using portable ultrasound equipment.

 

Laboratory Investigations

 

Serum creatinine is recommended by most guidelines for the investigation of BPH and an elevated serum creatinine would be an indication to evaluate the upper urinary tract (96).

Serum PSA has several implications in the diagnosis and management of BPH, including (1) providing a prediction of the prostate volume (2) providing the prediction of disease course, and (3) providing a risk assessment for prostate cancer. Indeed, in multiple placebo arms of large double-blind clinical trials, the serum PSA is an independent predictor of the risk of acute urinary retention and progression to BPH-related surgery (109). While the PSA provides useful information in the aforementioned domains, in clinical practice the main utility of PSA testing in the setting of LUTS is to exclude prostate malignancy. In patients presenting with isolated LUTS, current guidelines suggest its use only if a diagnosis of prostate cancer will change management or if the PSA can assist in decision-making in patients at high risk of BPH progression. 

 

Upper Urinary Tract Imaging

 

Urinary tract ultrasound or computerized tomography are appropriate modalities. Most would consider upper tract imaging as mandatory if hematuria is present and recommend it if there was a history of urolithiasis, urinary tract infection, or renal insufficiency. Intravenous pyelography still has a role in certain cases, as other modalities do not outline the anatomy of the collecting system with such definition (94).

 

Urodynamics

 

Urodynamics is a general term for a collection of investigations useful in quantifying the activity of the lower urinary tract during micturition (110). Complete pressure-flow urodynamics are complex and usually involve fluoroscopy, video recording, bladder and rectal pressure measurement, as well as an assessment of urine flow. The simplest urodynamics are pressure-flow studies, requiring only voiding into a measuring device to obtain flow rates, and may easily be done in the office setting.

 

With regard to the investigation and diagnosis of conditions underlying LUTS, when considering inexpensive, safe, and completely reversible treatments, one may opt to avoid urodynamics studies initially. However, when considering irreversible, expensive, or potentially morbid therapy, such studies are considered mandatory. Many patients will not have urodynamics studies based on the first premise above (110). However, in reality, many surgeons and physicians will have simple pressure-flow studies readily available and will perform these as part of an initial consultation. More complex studies require time and are costly, and so should be reserved for particular situations as discussed below.

 

Urinary Flow Rate (Uroflowmetry)

 

Uroflowmetry is considered by some as the single most useful urodynamic technique for the assessment of obstructive uropathy. The purpose of the uroflow examination is to record one or more micturitions that are representative of the patient’s usual voiding pattern. Therefore, more than one micturition is often required and it is necessary to confirm with the patient if the flow was better, worse or about the same as their normal pattern, otherwise intra-individual variability may lead to false assumptions (111). The study may be performed in the office or as part of other urodynamic studies in the laboratory or operating suite.

 

Figure 10 indicates the most common urinary flow parameters measured. Of these, the peak flow rate is the most closely correlated with the extent of outflow obstruction (Table 10). Total voiding time is prolonged in obstruction and has a reduced Qmax. Poor detrusor contractility is impossible to distinguish from bladder outflow obstruction on uroflowmetry so other urodynamics investigations such as a cytometry are indicated.

 

Figure 10. Uroflowmetry in a normal individual- diagram above and actual reading below (Table 10).

 

Table 10. Interpretation of Uroflowmetry Results.

Flow rate- Qmax

Interpretation

>15ml/sec

Unlikely to be significant obstruction

<10ml/sec

Likely to be significant obstruction or weak detrusor activity

10-15ml/sec

Equivocal

 

Urodynamics- Pressure-Flow Studies

 

Various measurements may be used to define detrusor pressures and urethral sphincter pressures as an aid to diagnosis in specific circumstances. This is relevant in patients with LUTS who have had a stroke (or other neurologic disease) where bladder function may have sensory deficits or unstable detrusor contractions that may need alternate management. Nevertheless, detrusor instability is not considered a negative factor with respect to the outcome of BPH surgery (94), provided it is adequately managed. Some have even suggested that the detection of detrusor instability in patients with LUTS is only of minor diagnostic importance (112).

 

Urethrocystoscopy

 

The performance of this investigation depends on patient history and proposed surgical intervention. It is necessary where there is a history of microscopic or macroscopic hematuria to exclude bladder tumors or stones. A history or suspicion of urethral strictures, bladder tumors, or prior lower urinary tract surgery should also prompt this investigation. Surgeons may also use urethroscystoscopy when planning different surgical treatments or invasive therapies.

 

Transrectal Ultrasound Scanning (TRUS)

 

Compared to TRUS, methods of determining prostate size such as DRE, urethrocystoscopy, and retrograde urethrography are poor (113). It is often conducted in unison with biopsies of the prostate for suspected carcinoma, but is also a useful tool for assessing the size of an enlarged prostate so that the best mode of management may be undertaken, such as open versus endoscopic surgery.

 

OVERVIEW OF TREATMENT OF BPH

 

The primary aim of any treatment for BPH in the vast majority of men is to relieve bothersome obstructive and irritative symptoms (114) (Table 2). Treatment is often undertaken on an elective basis for such patients. Those in whom complications of BPH occur have treatment done urgently as a matter of course. A range of treatment options are available and may be tailored to the needs of every individual, taking into account their disease manifestations, success rates of treatment, possible complications, and patient preference.

 

WATCH AND WAIT/LIFESTYLE CHANGE

 

Many men who present with LUTS are often seeking a full assessment of their prostatic health rather than immediate treatment of symptoms that may not be exceptionally bothersome. People with mild symptoms may wish to pursue lifestyle changes as a way of improving their quality of life but with the option of review if such measures fail or symptoms worsen. Furthermore, when an adequate history is taken, hidden agendas such as fear of prostate cancer may even be revealed and fears allayed.

 

Often drinking habits may be responsible for symptoms such as nocturia, where considerable fluid volumes are consumed in the evening. Reducing fluid intake may diminish nocturia and evening urgency. Furthermore, caffeine and alcohol acting as diuretics can further exacerbate LUTS. Simple shifts in daily fluid intake may fulfil patient expectations and result in satisfactory outcomes. Voiding diaries are useful for making patients aware of drinking habits and may be the catalyst for initiating and monitoring changes. Bladder retraining (by using timed voiding, strengthening pelvic floor exercises, and monitoring oral intake) is also an option in some individuals, once a voiding diary has been examined.

 

Medications may also play a role with LUTS. Measures such as diuretic restriction in evenings often prevents nocturia and frequency, provided the diuretic can be taken earlier in the afternoon.

 

It is important to discuss options with the patient and that they he be made aware that the possibility of damage to their upper urinary tract or to the detrusor muscle may result if their symptoms deteriorate and they do not seek medical attention.

 

PHYTOTHERAPY FOR BPH

 

Phytotherapy, or the use of plant extracts, is becoming widely used in the management of many medical conditions including BPH (Table 11) (115). Often these agents are promoted to aid “prostatic health” and a significant proportion of men try them. Factors also contributing to their widespread use include the perception that they are supposedly ''natural'' products; the presumption of their safety (although this is not adequately proven); their alleged potential to assist in avoiding surgery, and even the unproven claim that they may prevent prostate cancer. The widespread availability of these products (without prescription) in vitamin shops, supermarkets, pharmacies, and over the internet has contributed to their usage and reflects the demand for these phytotherapeutic agents. The mechanisms of action are poorly understood but have been proposed to be (1) anti-inflammatory, (2) inhibitors of 5-alpha reductase, and more recently (3) through alteration in growth factors (116).

 

Phytotherapy, although promising, lacks long-term, good quality clinical data (117). Nevertheless, because there is a large placebo effect associated with treatment of voiding symptoms, the use of herbal products that have few or no side effects may be a reasonable first-line approach for many patients (118). However, patients should be counselled that the efficacy, mechanisms of action and long-term effects of these agents are not known and they must be aware of the limitations before proceeding (119).

 

The most popular phytotherapeutic agents are extracted from the seeds, barks and fruits of plants. Products may contain extracts from one or more plants and different extraction procedures are often used by manufacturers. Thus, the composition and purity of products may differ even if they originated from the same plant. Basic research on one product may not be easily transferred to another making the gathering of data and giving of advice difficult (120).

 

Table 11. Phytotherapy Used in the Treatment of Benign Prostatic Hyperplasia

Phytotherapeutic plant extract

Proposed Mechanism of action

Saw palmetto- fruit

(Serenoa repens)

Antiandrogenic, Anti-inflammatory

African plum- bark

(Pygeum africanum)

Antiandrogenic, potential growth factor manipulation, anti-inflammation actions

Pumpkin- seed

(Cucurbita pepo)

Phytosterols are thought to be amongst the active compounds

Cernilton- pollen

(Secale cereal, Rye)

Inhibition of alpha-adrenergic receptors

South African star grass- root

(Hypoxis rooperi)

Antiandrogenic, alteration in detrusor function

Stinging nettle- root

Steroid hormone manipulation reducing prostate growth

Opuntia- flower

(Cactus)

Unknown

Pinus- flower

(Pine)

Unknown

 

Saw Palmetto Berry (Serenoa repens)

 

Extracts from the berries of the American dwarf palm (saw palmetto) are the most popular and widely available plant extracts used to treat symptomatic BPH today (121,122). At least eight possible mechanisms of action for saw palmetto have been advocated including anti-androgenic properties, anti-inflammatory properties, induction of apoptosis to name a few (120). Several studies have found that saw palmetto suppresses growth and induces apoptosis of prostate epithelial cells by inhibition of various signal transduction pathways (123). However, it is most commonly believed that saw palmetto works as a naturally occurring weak 5-alpha reductase inhibitor, blocking the conversion of testosterone to DHT, as demonstrated in several in vitro studies (118, 124-127). Thus, saw palmetto may be expected to reduce prostate size. While demonstrated in animal models (128), this is not the case in several trials using saw palmetto in men with BPH (129,130). The only trial to show in vivo effects of saw palmetto involved needle biopsies of the prostate gland, before and after treatment with saw palmetto or placebo. Although the mechanism is unclear, there was a significant increase in prostatic epithelial contraction in the saw palmetto group (131).

 

Clinical evidence reporting the use of saw palmetto is conflicting. In a meta-analysis of 18 randomized studies relating to saw palmetto extracts, almost 3000 men with BPH were studied and the authors concluded that “the evidence suggests that saw palmetto improves urologic symptoms and flow rates but that further research is needed using standardized preparations to determine long term effectiveness” (115). When analyzing flow rate and symptom score alone from this meta-analysis, the effect of Seronoa repens (the scientific name of saw palmetto was to increase the flow rate by a further 2.28 ml/sec (standard error, SE, 0.29) over placebo which gave an increase of 1.09 ml/sec (SE 0.45). Serenoa repens also reduced the IPSS by 4.7 (SE 0.41), which is comparable to that found with finasteride and tamsulosin monotherapy (132).

 

Conversely, a recently published Cochrane review concluded Serenoa repens was no more effective than placebo for treatment of urinary symptoms consistent with BPH (133). This update of a prior review, nine new trials involving 2053 additional men (a 65% increase) were included. The main comparison was again Serenoa repens versus placebo where three trials were added with 419 subjects and three endpoints (IPSS, peak urine flow, prostate size). Overall, 5222 subjects from 30 randomized trials ranging from four to 60 weeks were assessed. The vast majority were double blinded and treatment allocation concealment was adequate in just over half the studies.

 

In summary, some saw palmetto studies have shown improved symptom scores compared to placebo but generally no change in flow rates (134). However, large reviews cast doubt on its efficacy. In general, there is a real paucity of well performed, adequately powered, and placebo-controlled trials in the use of phytotherapy in clinical BPH. It is generally well tolerated at a dose of 320mg/day, but its efficacy has not been compared with alpha-blockers regarding efficacy, and has not been shown to reduce complications of BPH with long term use. Finally, the product quality and purity cannot always be assured.

 

African Plum Tree (Pygeum africanum)

 

Extracts come from the bark of the African plum tree. It is hypothesized, based on in vitro observation, that it acts on the prostate through inhibition of fibroblast growth factors, has anti-estrogenic effects, and inhibits chemotactic leukotrienes. No strong clinical data exists of its efficacy although trials are in progress (116,119).

 

Pumpkin Seed (Cucurbita pepo)

 

Dried or fresh seeds have been taken to relieve symptoms. Phytosterols are thought to be amongst the active compounds. Side effects have not been reported but evidence is lacking with no current clinical trials (135).

 

Rye Pollen (Secale cereale)

 

This is prepared from rye grass pollen extract. In a systematic review summarizing evidence from randomized and clinically controlled trials (114), rye pollen was found to be well tolerated but only achieved modest improvement in symptom outcomes and did not significantly improve objective measures such as peak and mean urinary flow rates. Again, several mechanisms of action have been proposed including an improvement in detrusor activity, a reduction in prostatic urethral resistance, inhibition of 5-alpha reductase activity, and an influence on androgen metabolism in the prostate (119).

 

Other Extracts

 

South African Star Grass (Hypoxis rooperi), Opuntia (Cactus flower), stinging nettle, and Pinus (Pine flower) have also been studied and used, however the data numbers are small and the types of trials do not allow conclusions to be drawn at this stage (116).

 

MEDICAL THERAPY FOR BPH – MONOTHERAPY AGENTS

 

In 1986, Caine (62) proposed that infravesical obstruction in men with symptomatic BPH comprised both static and dynamic components. The static component of obstruction is related primarily to the mechanical obstruction caused by the enlarging prostatic adenoma whereas the dynamic component is principally determined by the tone of the prostatic smooth muscle. Two avenues for pharmacotherapy have therefore evolved, namely shrinking the prostate tissue or relaxing the smooth muscle of the prostate. Prostatic smooth muscle tone is under the influence of the autonomic nervous system. Thus, any pharmacologic agent that may interfere with the functioning of this system could alter resistance in smooth muscle tone and resulting symptoms.

 

Medical therapy is now first-line treatment for most men with symptomatic BPH. They are non-invasive, reversible, cause minimal side effects, and significantly improve symptoms (81,136). With these recommendations, the rates of prescriptions for the medical management for BPH have increased drastically over the past decade (137,138). This increased interest has further led to the development of safer, more efficacious agents.

 

Alpha-Blockers

 

There are 3 main components to clinically significant BPH: static, dynamic and detrusor muscle components as outlined above. The dynamic component is associated with an increase in smooth muscle tone of the prostate. These smooth muscle cells contract under the influence of noradrenergic sympathetic nerves, thereby constricting the urethra (139). Prostatic tissue contains high concentrations of both alpha1 and alpha2 adrenoceptors – 98% of the alpha1 adrenoceptors are associated with stromal elements of the prostate (140). Thus alpha1-receptor blockers relax smooth muscle, resulting in relief of bladder outlet obstruction that enhances urine flow (87). Different subtypes of alpha1 receptors have been identified, with alpha1A predominating. Two alpha1A-adrenoceptors generated by genetic polymorphism have been identified with different ethnic distributions but similar pharmacologic properties (36).

 

It was demonstrated in 1978 that phenoxybenzamine, a non-selective alpha1/alpha2 blocker, was effective in relieving the symptoms of BPH (141).  Side effects were significant and included dizziness and palpitations. Many of the side effects of the alpha-blockers were mediated by alpha2-receptors (142).Thus, alpha1 selective antagonists such as terazosin, doxazosin, and  prazosin and were developed that had fewer side effects than phenoxybenzamine (67). Doxazosin, alfuzosisn, and terazosin have gained favor in clinical practice because they are longer acting than prazosin. Due to side effects, many alpha1 selective antagonists need to be titrated and are often started at the lowest dose and built up over time to the maximal dose or a dose where clinical effects are satisfactory.

 

More recently, highly uro-selective alpha1A selective agents have been introduced including tamsulosin and silodosin. Due to the uro-selective nature, there is significant reduction in risk of systemic side-effects when compared to the less selective agents. However, the increased potency of these agents results in an increased compromise to bladder neck function and as a result, increases the risk of ejaculatory dysfunction.

 

PRAZOSIN

 

Prazosin (titrated up to 5mg day) has been shown to significantly increase flow rates by 36-59% compared to placebo 6-28% but 17% of men discontinued the drug due to side effects such as dizziness (21%), headache (14%), syncope (3.4%) and retrograde ejaculation (13%).

 

ALFUZOSIN  

 

Alfuzosin (5 mg bid or 10 mg daily) has shown symptom score reduction of 31-65% (compared to placebo 18-39%) and flow rate increases of 22-54% (compared to placebo 10-30%). Hence the results were similar to those of prazosin but with only 3-7% discontinuations due to dizziness (3-7%), headache (1-6%) and syncope (<1 %) (143,144).

 

TERAZOSIN  

 

Terazosin (2-10mg) had a symptom score reduction of 40-70% (compared to placebo 16-58%) and improved flow rates 19-40% (placebo 5-46%). Between 9-15 % of men discontinued the drug, related to dizziness (10-20%), headache (1-7%), asthenia (7-10%), syncope (0.5-1.0%), and postural hypotension (3-9%). Thus, terazosin was effective and superior to placebo in reducing symptoms and increasing the peak urinary flow rate. The effect of terazosin on the peak urinary flow rate was apparent in studies as soon as 8 weeks of therapy. Most importantly, the effect of terazosin on symptoms and peak urinary flow rate was independent of the baseline prostate size for the range of prostate volumes reported (145).

 

DOXAZOSIN  

 

Doxazosin (4-12mg/day) is a selective alpha1-adrenoceptor antagonist, and produced a significant increase in maximum urinary flow rate (2.3 to 3.6 ml. per sec) at doses of 4 mg, 8 mg and 12 mg, and in average flow rate compared with placebo. The increase in maximum flow rate was significantly greater than placebo within 1 week of initiating therapy and the drug significantly decreased patient-assessed total, obstructive, and irritative BPH symptoms. Blood pressure was significantly lower with all doxazosin doses compared with placebo. Adverse events, primarily mild to moderate in severity, were reported in 48% of patients on doxazosin compared to 35% on placebo, with only 11% discontinuing treatment (a similar number to placebo). The main side effects were dizziness (15-24%), headache (12%) and hypotension (5-8%), and abnormal ejaculation (0.4%) (146,147).

 

TAMSULOSIN  

 

Tamsulosin (0.4 mg once or twice daily dose) is a selective alpha blocker for the alpha1A subtype which predominates in the human prostate, having 12 times more affinity for the receptors in the prostate than in the aorta thereby reducing side effects mediated through blood vessels receptors. Symptom scores were reduced by 20-50% (placebo 18-30%), flow rates improved 20-45% (placebo 5-15%) but only 3-7% of men discontinued drug because of dizziness (3-20%), headache (3-20%), syncope (0.3%), and retrograde ejaculation (5-10%). The rate of retrograde ejaculation was much higher than alfuzosin but the blood pressure lowering side effects are less with tamsulosin(148). There are different formulations including extended release with lower pharmacological peaks and troughs which may offer fewer side effects.

 

SILODOSIN  

 

Silodosin (8mg daily) is a highly selective blocker for the alpha1A receptor subtype. It has the highest affinity for alpha1A receptors of the medications discussed here. Symptoms scores were reduced by 40-50% (placebo 20-30%), flow rates improved by 17-30% (placebo 5-14%). Despite these favorable urinary outcomes, a significant proportion of patients experienced ejaculatory dysfunction (13-23%). These rates are higher compared to tamsulosin, however discontinuation rates secondary to ejaculatory dysfunction remains at 1-2%. Typical side effects include thirst (10%), loose stools (9%) and dizziness (5%) (149,150). Similar results were found in a recent meta-analysis of silodosin. Compared to tamsulosin, the combination of 13 studies found silodosin showed little to no difference in urological symptom scores and quality of life whilst increasing sexual adverse events. The same results were reported when silodosin was compared to naftopidil and alfuzosin (151).

 

Several meta-analyses have demonstrated that all non-selective alpha1-adrenoceptor antagonists seem to have similar efficacy in improving symptoms and flow rates (152). The difference between non-selective alpha 1-adrenoceptor antagonists is related to their side effect profile. Overall, alfuzosin appear to be better tolerated than doxazosin, terazosin and prazosin(153). More recent analyses suggest that the highly uro-selective alpha1A blockers are more efficacious compared to non-selective alpha blockers with regards to urinary symptoms and urine flow improvement (154-156). Further, these highly selective agents appear to have a favorable systemic side-effect profile at a cost of ejaculatory function when compared to non-selective alpha blockers.

 

Table 12. Commonly Used Alpha-Blockers

Group

Drug

Nonselective alpha blockers

·       Phenoxybenzamine

·       Nicergoline

·       Thymoxamine

Selective alpha1 blockers

·       Prazosin

·       Alfuzosin

Super-selective alpha1A blockers

·       Tamsulosin

·       Silodosin

Long-acting alpha1 blockers

·       Terazosin

·       Doxazosin

 

5-Alpha Reductase Inhibitors

 

The enzyme 5-alpha reductase is crucial in the amplification of androgen action in the prostate by modulating the conversion of testosterone to DHT (Figure 5). Within the prostate, 90% of testosterone is converted to DHT (78,157). There are 2 isoforms of the enzyme 5-alpha reductase which are encoded by separate genes (158). Type 1 isoenzyme is expressed highly in the skin, liver, hair follicles, sebaceous glands, and prostate whereas type 2 is responsible for male virilization of the male fetus, and in adulthood resides in prostate, genital skin, facial and scalp follicles (159,160). Inhibitors of these enzymes potentially decrease serum and intra-prostatic DHT concentrations, thus reducing prostatic tissue growth.

 

FINASTERIDE

 

Finasteride was the first of these to be studied in humans and shown to decrease DHT concentrations (161). It acts predominantly on the type 2 isoenzyme of 5-alpha reductase. There is some evidence that patients on finasteride experience fewer serious complications associated with the progression of BPH compared with those prescribed an alpha blocker, such as acute urinary retention or undergoing BPH-related surgery, but more prospective data is needed (162). Finasteride reduces serum DHT concentrations by 65-70% and prostatic concentrations by 85-90%, although the intraprostatic concentrations of testosterone are reciprocally elevated as the testosterone is not being converted to DHT.

 

Because 5-alpha reductase inhibitors work by reducing prostatic tissue volume, baseline prostate size has a significant impact on its efficacy with larger glands (>50ml) being likely to respond (163,164). After treatment for one year with finasteride, there was a significant decrease (17-30%) in total gland size with the greatest size reduction in the periurethral component of the prostate, which has the greatest impact on obstructive symptoms (78,165,166). There was a 60-70% decrease in serum DHT concentration, a 25% decrease in prostate volume, and a symptom score reduction of 13-30% (vs placebo 4-20%). Urinary flow rate improved 7-20% (vs placebo 3-15%) and was more pronounced with prostates > 40ml. The side effect profile included a decreased libido in 10%, ejaculatory dysfunction in 7.7%, and impotence in 15.8%. But adverse events resulted in only 4% of patients discontinuing treatment (117,167). There was a 50% reduction in the risk of AUR and in the need for surgery (30%).  Finasteride has also found a role in the treatment of BPH-related hematuria although its role in reduction of perioperative bleeding is not well defined (84,117).

 

More recently, as part of the Prostate Cancer Prevention Trial involving over 18,000 men, it was concluded that finasteride delays the appearance of prostate cancer whilst reducing the risk of urinary problems. However, there was a reported increased risk of high-grade prostate cancer leading to the discontinuation of this study. This point remains controversial as some believe due to the gland shrinking that sampling was altered and by virtue of a smaller area the likelihood of finding an aggressive tumor was increased (168). In any case, the benefits in terms of improved LUTS needs to be weighed against the potential sexual side effects and potential small but significant increased risk of high-grade prostate carcinoma (169,170) and compared to the option of using adrenoreceptor blockers. Despite the findings, more evidence is needed before advising patients to cease finasteride. However, they do need to be counselled on the small, but significant risks of developing aggressive prostate cancer (171).

 

DUTASTERIDE  

 

Dutasteride unlike finasteride blocks both the type I and Type II 5-alpha reductase isomers showing a 60-fold greater inhibition of the type 1 isoenzyme than finasteride plus activity against the type 2 isoform (23,117). In terms of monotherapy, a one year randomized, double-blinded comparison of finasteride and dutasteride in men with BPH (EPICS: Enlarged Prostate International Comparator Study) found a trend for dutasteride improvement over finasteride in IPSS (International Prostate Symptom Score) that did not reach statistical significance (abstract)  (172). Another non-randomized comparative trial with 240 patients, published only in abstract form, showed a small improvement in AUASI and Qmax for dutasteride (173). However, dutasteride and finasteride have never been compared in long-term therapy, either as monotherapy or in combination with an alpha-blocker. These medications appear to exert continued effects beyond 1 year so comparison after only 1 year is likely to be premature.

 

The tolerability of 5-alpha reductase inhibitors in most studies has been excellent with the most relevant adverse effects being related to sexual function. They include reduced libido, erectile dysfunction, and, less frequently, abnormal ejaculation (74,174). Specifically for dutasteride in the Combat study (175), in the monotherapy arm of 1623 patients the side effect were: erectile dysfunction (6.0%) ; retrograde ejaculation (0.6%); altered (decreased) libido (2.8%); ejaculation failure (0.5%); semen volume decreased (0.3%); loss of libido (1.3%); breast enlargement (1.8%); nipple pain (0.6%); breast tenderness (1.0%), and dizziness (0.7%).

 

As with finasteride, the REduction by DUtasteride of prostate cancer Events (REDUCE) trial now fully reported has demonstrated similar results to the PCPT trial in reducing prostate cancer (176). Again, a higher risk of developing more aggressive cancer was demonstrated- but in this study it was not statistically significant. Indeed, some organizations such as the Canadian Urological Association have been dismissive of this point in recent guidelines (171). Needless to say, careful counselling of men regarding this issue is again required, particularly for younger men who will be on dutasteride for many years.

 

5-Alpha Reductase Inhibitors to Reduce Hematuria and Intraoperative Hemorrhage for Prostate Surgery

 

While considered an off-label use, there is some evidence that suggests that 5-alpha reductase inhibitors may be useful in the setting of (177):

  • Recurrent hematuria secondary to BPH
  • To reduce gland size and/or impact on angiogenesis to reduce intraoperative bleeding for prostate surgery.

 

No large randomized trials exist but an extensive summary of the literature is available (177).

 

Phosphodiesterase 5 Inhibitors

 

Phosphodiesterase 5 (PDE5) inhibitors (e.g., sildenafil, tadalafil and vardenafil) have been used predominantly to treat erectile dysfunction in men. However, recent data suggest they are effective for the treatment of LUTS secondary to BPH. Specifically, the cyclic nucleotide monophosphate cyclic GMP represents an important mediator in the control of the lower urinary tract outflow region (bladder, urethra). PDE5 inhibitors exert effects by several mechanisms including: calcium-dependent relaxation of endothelial smooth muscle, alteration of the spinal micturition reflex pathways, and increased blood flow to the lower urinary tract. PDE inhibitors are regarded as efficacious, have a rapid onset of action, and favorable effect-to-side-effect ratio (178).

 

The rationale for using tadalafil for BPH stems from the following three observations: first, the prevalence of LUTS, BPH, and erectile dysfunction (ED) increases with age; second, phosphodiesterase-5 inhibition mediates smooth muscle relaxation in the lower urinary tract; and third, early evidence demonstrates that PDE5 inhibitors such as tadalafil are successful in treating LUTS and ED (179). Results of several randomized controlled trials have demonstrated reproducible reductions in IPSS, symptoms, and improved quality of life compared to placebo. Data suggests tadalafil 5mg improves IPSS by 22-37% and the improvement occurs within one week of commencement, with a duration of 52 weeks (180). The adverse event profile was acceptable and consistent with that previously reported in men with ED (blurred vision, headache, back ache, nausea, etc.), with discontinuation rates of 2%. Not unexpectedly, in the same study tadalafil significantly improved the International Index of Erectile Function-Erectile Function score in sexually active men with erectile dysfunction at twelve weeks. Meta-analytical data confirms these findings suggesting that PDE5 inhibitors improve IPPS and erectile function, with no significant effect on maximal urinary flow rate (181). Other PDE5 inhibitors are being studied including sildenafil and vardenafil (178). The theoretical advantage is treating BPH and erectile dysfunction with one agent (182). To date, tadalafil is the only PDE5 inhibitor that is FDA approved for use for the treatment of BPH. Data on the long-term effects on symptoms and disease progression is not available at present.

 

Anticholinergic Medications

 

High-level evidence suggests that for selected patients with bladder outlet obstruction due to BPH and concomitant detrusor overactivity, combination therapy with an alpha-receptor antagonist and anticholinergic can be helpful (183).Such agents help particularly with the irritative urinary symptoms of frequency and urgency. Caution is recommended, however, when considering these agents in men with an elevated residual urine volume or a history of spontaneous urinary retention (171).

 

Botulinum Toxin A Injection

 

Injection of botulinum toxin A into the prostate is a novel treatment for LUTS secondary to BPH. First reported in 2003 (184), trans-perineal injection of 100 units of botulinum toxin into each lobe of the prostate under trans-rectal guidance is required. In this randomized controlled trial, thirty patients demonstrated significant improvement in IPSS (65% decrease) and serum PSA (51% decrease) compared to controls, who had injections of saline without botulinum toxin A, at a median follow-up of 20 months. Subsequent long-term follow-up of 77 patients up to 30 months has shown similar results – significant reduction in IPSS (approximately 50% lower), significant improvement in maximum flow rate (approximately 70% higher), and significant reduction in serum PSA values (approximately 50% lower).Importantly, no adverse events were noted (185).

 

Summary of Monotherapy Medical Treatment

 

The first line of medical treatment is an alpha-blocker, as the majority of patients treated have a prostate volume of less than 40ml. In men with larger prostates (greater than 40cc), a 5-alpha reductase inhibitor (e.g., finasteride or duasteride) alone or in combination with an alpha-blocker would be appropriate. Patients who are likely to respond to 5-alpha reductase inhibition will do so at the same relative magnitude as an alpha-blocker, but it will take a longer period of time (months as opposed to weeks). There is likely to be a 20-30 reduction in symptoms and a 1-2ml per second increase in urinary flow (167). Side-effect profiles of medical treatments are also important, as discussed above. For example, with regard to sexual function, tamsulosin and silodosin have an increased risk of retrograde ejaculation and finasteride increases sexual dysfunction (74). These may be important factors in choosing therapies. Finally, the emergence of PDE5 inhibitors for the treatment of men with LUTS secondary to BPH alters the landscape with an ability to treat men with BPH and ED with one agent. Multiple randomized trials and associated meta-analyses demonstrate the reproducible benefits of PDE5 inhibitors on urinary and erectile function.

 

MAJOR STUDIES OF MEDICAL TREATMENT OF BPH

 

The medical treatment of clinical BPH has come under increasing scrutiny through larger trials that have become imperative for their introduction into clinical practice. Some of these larger trials have been selected and are discussed below.

 

Veterans Affairs Study

 

In the Veterans Affairs Cooperative Studies Benign Prostatic Hyperplasia Study Group (186), a total of 1,229 subjects with clinical BPH were randomized to 1 year of placebo, finasteride, terazosin or drug combination. The primary outcome measures were the AUA symptom score and the peak urinary flow rate. The percentage of subjects who rated improvement as marked or moderate with placebo, finasteride, terazosin and combination was 39, 44, 61 and 65%, respectively, only the latter two were superior to placebo. There was no significant relationship between baseline prostate volume and treatment response to finasteride or with the other treatments (terazosin or combination). There was a significant but weak relationship between change in AUA symptom score and peak flow rate in the finasteride and combination groups. The symptom responses with terazosin were not related to peak flow rate or baseline prostate volume. In men with clinical BPH, finasteride and placebo are equally effective, while terazosin and combination are significantly more effective. In men with clinical BPH and large prostates, the advantage of finasteride over placebo in terms of symptom reduction, impact on bother due to symptoms and quality of life was small at best, while the advantage of terazosin (alone or in combination with finasteride) over finasteride alone and placebo was highly significant. The authors concluded that alpha1 blockers, such as terazosin, should be first line medical treatment for BPH(186). Another arm of this study observing surgical treatment versus watchful waiting is discussed below.

 

PLESS Study

 

The Proscar Long-term Efficacy and Safety Study (PLESS) was a 4-year, randomized, double-blind, placebo-controlled trial assessing the efficacy and safety of finasteride 5mg (Proscarä) in 3040 men, aged 45 to 78 years, with symptomatic BPH, enlarged prostates on TRUS volume criteria, and no evidence of prostate cancer (187,188). Finasteride use reduced the risk of developing acute urinary retention by 57% and the need for BPH-related surgery by 55% (189) in comparison to placebo. A modified AUA symptom score was used (because trial was undertaken prior to formal AUA being developed) and showed a statistically significant reduction in mean score of 3 for finasteride and 1.2 for placebo, starting at a level of 15 for both groups (188). Compared with placebo, men treated with finasteride experienced an increased incidence of new drug-related sexual adverse events (erectile dysfunction, decreased libido, ejaculation disorder) only during the first year of therapy with 4% of men discontinuing because of such events (187).

 

PREDICT Trial

 

The Prospective European Doxazosin and Combination Therapy (PREDICT) Trial was constructed to evaluate the efficacy and tolerability of the selective alpha1-adrenergic antagonist doxazosin and the 5-alpha reductase inhibitor finasteride, alone and in combination, for the symptomatic treatment of benign prostatic hyperplasia. It was a prospective, double-blind, placebo-controlled trial involving 1,095 men aged 50 to 80 years. The dose of finasteride was 5 mg/day. Doxazosin was initiated at 1 mg/day, and titrated up to a maximum of 8 mg/day over approximately 10 weeks according to the response of the maximal urinary flow rate (Qmax) and IPSS. An intent-to-treat analysis of 1,007 men showed doxazosin and doxazosin plus finasteride combination therapy produced statistically significant improvements in total IPSS and Qmax compared with placebo and finasteride alone. Finasteride alone was not significantly different statistically from placebo with respect to Qmax or total IPSS. The treatments were generally well tolerated. They concluded that doxazosin was effective in improving urinary symptoms and urinary flow rate in men with benign prostatic hyperplasia, and was more effective than finasteride alone or placebo. The addition of finasteride did not provide further benefit to that achieved with doxazosin alone (146).

 

MTOPS Study and Predictors of Clinical Progression

 

The Medical Therapy of Prostatic Symptoms (MTOPS) study is a double-masked, placebo-controlled, multi-center, randomized clinical trial with 4 study arms 1) placebo; 2) doxazosin (4 to 8 mg); 3) finasteride (5 mg) and 4) combination of both doxazosin and finasteride. 3,047 men were randomized equally to the 4 groups (190). Baseline parameters analyzed included age at randomization, transrectal ultrasound (TRUS) volume, AUA symptom score, Qmax, PVRU, and PSA. Reduction in the risk of BPH progression was analyzed by one covariate at a time regression models of absolute risk of BPH progression versus baseline covariates. Groups compared were combination versus doxazosin, combination versus finasteride and finasteride versus doxazosin (190).

 

In the main finding, disease progression, defined as an increase in AUASS (American Urology Association Symptom Score- similar to IPSS to score LUTS) of 4, AUR, renal insufficiency, recurrent UTIs and urinary incontinence, was prevented equally by doxazosin and finasteride with an even greater effect when both medications were combined. In conflict with the Veterans Affairs and PREDICT studies, finasteride alone did improve overall symptoms and peak urine flow compared to placebo at 4 years and with even more so when combined with doxazosin (190). This finding coincides with the long-term open label ARIA dutasteride study described above showing a cumulative symptom benefit of treatment up to 4 years (191)

 

A small part of the MTOPS study focused on the routinely available measure of serum PSA, in an attempt to predict a patient’s future risk of BPH clinical progression, acute urinary retention and BPH-related invasive therapy, permitting an informed decision concerning the value of medical therapy over watchful waiting (192). In MTOPS, 737 patients were assigned to placebo and followed for an average of 4.5 years. Clinical progression of BPH was pre-defined as either a 4-point increase in AUA symptom score, acute urinary retention, incontinence, renal insufficiency, or recurrent UTI. The need for BPH-related invasive therapy was a secondary outcome. These data are summarized in Table 13, where those having a lower PSA, had a lower rise in symptom score, and a reduced risk of acute urinary retention or invasive treatment compared to those with a higher PSA. The sub-group with the highest baseline PSA was also likely to have larger prostate glands, making the findings intuitive. However, as with many such findings, translating an individual PSA to a population study is difficult, as other factors will determine progression or regression of symptoms, not just PSA.

 

Table 13. Progression of BPH Symptomatically of Placebo Group only, to AUR or Further Intervention Based on Baseline PSA (adapted from Kaplan et al).

Baseline PSA tertiles (ng/ml)

Progression of symptom score (points)

Acute urinary retention Risk over study period

Invasive Treatment Risk

<1.2

3.10

0.18

0.6

1.2-2.5

3.47

0.35

1.33

>2.5

7.21

1.46

2.13

 

CombAT Trial

 

Combination therapy with a 5-alpha reductase (dutasteride) and the alpha blocker, tamsulosin, in men with moderate-to-severe benign prostatic hyperplasia and prostate enlargement was also further studied in the Combination of Avodart™ and Tamsulosin™ (CombAT) trial. The rationale was the same as those outlined for the MTOPS trial. In summary, it is a 4-year, global, multicenter, randomized, double-blind, parallel-group study designed to investigate the benefits of combination therapy with the dual 5-ARI dutasteride and the alpha-blocker tamsulosin compared with each monotherapy in improving symptoms and long-term outcomes in men with moderate-to-severe symptoms of BPH and prostate enlargement. Symptoms and long-term outcomes (AUR and surgery) were assessed as separate primary endpoints at two and four years, respectively. Eligible patients were at least 50 years old with prostate volume ≥30 cm3 and PSA level ≥1.5 ng/mL. Almost 5,000 men were enrolled (193). Perhaps the only criticism is the lack of placebo control arm in the study. 

 

The results at four years (194) demonstrated that combination therapy was superior to tamsulosin monotherapy but not dutasteride monotherapy at reducing the relative risk of AUR or BPH-related surgery. Combination therapy was also significantly superior to both monotherapies at reducing the relative risk of BPH clinical progression. Combination therapy provided significantly greater symptom benefit than either monotherapy. Safety and tolerability were reasonable and in line with expectations for both medications.  Certainly, at four years the CombAT data supports the long-term use of dutasteride and tamsulosin combination therapy in men with moderate-to-severe LUTS due to BPH and prostatic enlargement.

 

EMERGING COMBINATION THERAPY REGIMES

 

With the increasing body of evidence supporting the use of PDE5 inhibitors in the setting of BPH, a number of trials support its use in combination therapy. To date, there are smalls studies of alfuzosin and tadalafil, tamsulosin and sildenafil, and tamsulosin and vardenafil. These early studies suggest that combination therapy is more effective than monotherapy for urinary and erectile function with a good safety profile (195,196). A meta-analysis of 11 randomized controlled trials (n=855) looked at alpha blockers with or without PDE5 inhibitors. This analysis found men receiving PDE5 inhibitors had a mean improvement of 1.66 points on IPSS, mean increase of 0.94 ml/s maximum urinary flow rate, and improved erectile function (197). Larger series with longer-term follow up is required to definitively define the role of these combination therapies in current practice.

 

Summary of Combination Therapy for Men with BPH

 

In the larger studies where the standard endpoint of prostate symptom score was measured, a greater impact of dutasteride over tamsulosin was observed.  Considering urinary flow rate (Qmax), combination therapy outperformed dutasteride in those with PSA and prostate volumes above the 75 percentile. Clearly, those with larger prostates and higher PSAs derive a greater benefit with dutasteride coinciding with the size reduction impact of this drug.

 

In summary, the results of the MTOPS and CombAT trials both suggest combination therapy is better than 5-alpha reductase monotherapy at the 4-year mark. The higher incidence of adverse effects, the increased cost of combination therapy, and the need for prolonged therapy argue for a reductionist medical approach to this condition. One recent small study investigated the discontinuation of 5-alpha reductase inhibitors in patients on combination therapy and found prostate regrowth and worsening of symptoms after 1 year of cessation, emphasizing the importance of 5-alpha reductase inhibitors in prolonged therapy (198). In an opposing design, the SMART trial (Symptom Management After Reducing Therapy) observed the effect of removing the alpha blocker (tamsulosin) after 6 months of combined therapy with dutasteride (199). With I-PSS as the primary outcome, the investigators found that 77% of patients had symptoms that were the same or better after only 3 months of alpha blocker removal. In reference to the CombAT study, the effects of dutasteride continue past two years suggesting that removal of the alpha blocker at later time points may be even less noticeable. However, the CombAT study is quite powerful as it does demonstrate that the natural history of BPH is not altered by taking alpha blocker alone. The rate of AUR and need for surgery was unaltered at about 18%, and thus while tamsulosin helps LUTS, it does not alter disease progression. Combination therapy did lead to reductions in prostate volume, and changed natural history to reduce rates of AUR and surgery. The largest benefit was in the men with the largest glands.

 

The emergence of newer agents including PDE5 inhibitors gives rise to an increasing number of combination-therapies under investigation. Long-term follow-up is required on these newer combinations. As such, combination treatment will continue to shape the management of BPH for years to come.

 

SURGICAL TREATMENT OF BPH

 

Invasive Surgical Therapies

 

Traditionally prostatectomy by an open approach or TURP has been considered the gold-standard for refractory or complicated BPH (indications listed in Table 14). At present approximately 90% of prostatectomies are done by TURP. Open prostatectomy should be considered when a gland is estimated to weigh more that 75g, where large bladder calculi exist that may not be dealt with endoscopically, where large bladder diverticula requiring repair exist, if complex urethral conditions or when orthopedic abnormalities prevent positioning in lithotomy for TURP. Contraindications to open prostatectomy include a small fibrous gland, prostate adenocarcinoma, previous prostatectomy or other surgery of the pelvis preventing access (200).

 

Table 14. Indications for Prostatectomy

·       Acute Urinary Retention

·       Recurrent or persistent urinary tract infections

·       Significant bother from LUTS secondary to bladder outflow obstruction not responding to medical therapy

·       Recurrent hematuria known to be of prostatic origin

·       Bladder Calculi

 

TRANSURETHRAL RESECTION OF THE PROSTATE (TURP)

 

TURP remains the most common surgical treatment for BPH (201) and remains the ‘gold standard’ by which other surgical (and even medical) treatments are measured (74). TURP involves either regional or general anesthesia, with most patients spending a minimum of one night in the hospital. TURP involves surgically debulking the periurethral and transitional zones of the prostate to relieve obstruction. Debulking is done by electrocautery in the standard TURP through endoscopic instruments introduced into the urethra and bladder. Tissue is resected in small pieces until the hyperplastic tissue is removed and a new channel for passage in the prostatic urethra created in the capsule left behind, much like fashioning a pumpkin for a Halloween jack o’lantern. Despite using electrocautery, there are mild to severe degrees of hemorrhage, depending on the gland size. However, transfusions are rarely needed and the procedure is relatively free of life-threatening complications and most patients experience satisfactory resolution of their micturition symptoms. Studies on urinary peak flow rates and invasive pressure flow have demonstrated the superiority of TURP over minimally invasive therapies (202). Complications of TURP include failure to void (6%), hemorrhage requiring transfusion (1-4%), clot retention (3%), infection (2%), bladder neck contracture or urethral stricture (6%), transurethral resection syndrome (2%), and rarely incontinence (80,203,204). 

 

TURP is plagued by the potential for morbidity, specifically: retrograde ejaculation, erectile dysfunction, and urinary incontinence. Retrograde ejaculation is reported to occur in almost all patients undergoing TURP as the normal bladder neck mechanism which contracts to allow antegrade ejaculation is surgically resected. Counselling prior to surgery must include a discussion of the impact on sexual performance and also fertility. Erectile dysfunction (ED) may be associated with TURP either via thermal nerve injury or emotional stress and was reported in early studies at a rate of 4-40%. This has now been shown to be an overestimation (74,206). The rate of ED in the AUA Cooperative study was found to be 13% in 1,000 men (80), however this must be compared to increases of around 20% of ED in untreated groups with BPH. Although ED is often quoted as a side effect of TURP, Kassabian concluded that TURP (or even any other surgical therapy) did not appear to have a long-term effect on erectile function or libido (74). Incontinence is infrequent and typically is a result of intra-operative damage to the external urinary sphincter. Large pooled analysis revealed rates of incontinence following TURP of around 1% (207).

 

There has been one randomized controlled trial (Veterans Affairs Cooperative Study Group, see above) comparing TURP to “watchful waiting” or reassurance (203). This demonstrated that TURP showed greater benefit with 66% of patients having a decrease in symptoms post TURP compared to 28% who were undergoing ‘watchful waiting”. 

 

One significant modification to the standard TURP using monopolar cautery with glycine as an irrigant has been the use of bipolar cautery using normal saline as the irrigant. The latter has been termed bipolar transurethral resection in saline (TURIS). Glycine alters serum osmolality when absorbed through venous channels in the prostate as the system is under pressure which potentially leading to hyponatremia and also glycine directly itself has an impact on the nervous system. This syndrome is termed “TURP syndrome” and dictates that monopolar resections should be abandoned at around the one-hour mark or when significant venous breach occurs. The TURIS or bipolar technologies thus have the advantage of the ability to carry out resections for a longer time due to very few issues with absorption of saline systemically as opposed to glycine. Large series meta-analysis illustrated comparable efficacy and morbidity profiles when compared to monopolar TURP (207,208). A further modification of the bipolar technology is so called plasmakinetic vaporization where some data is emerging (209,210). This vaporizes rather than resects the prostate tissue. Compared to TURP, plasmakinetic energies results in similar improvements to IPSS up to 12-month follow up (211).

 

LASER THERAPY FOR BPH – ENUCLEATION OR VAPORIZATION OF THE PROSTATE  

 

There are several evolving therapies for BPH involving various lasers including Nd-YAG, Holmium, and now Thulium lasers. These laser energies may be utilized in various methods to resect, enucleate, or vaporize the prostate. Laser as an energy source has an advantage over standard electrocautery by being relatively bloodless and does not carry the risk of hyponatremia, which may rarely occur via absorption of irrigation fluid in a standard TURP (212).

 

Photoselective Vaporization of the Prostate (PVP)

 

The characteristic 532-nm wavelength laser is selectively absorbed by hemoglobin within prostatic tissue (213,214). Introducing this energy to the prostate results in selective vaporization of prostatic tissue, with effective hemostasis and relatively little tissue coagulation (1.5 – 0.3mm margin). Initially launched as a 60W prototype, the laser was ultimately introduced to the urology community as an 80W system that has been the predominant device used in clinical trials. This first generation used an Nd:YAG laser beam passed through a potassium-titanyl-phosphate (KTP) crystal, halving the wavelength (to 532nm), doubling the laser's frequency, and resulting in a green light. In 2006, the 120W lithium triborate laser (LBO) laser was introduced using a diode pumped Nd:YAG laser light that is emitted through an LBO instead of aKTP crystal, resulting in a higher-powered 532 nm wavelength while still using the same 70-degree deflecting, sidefiring, silica fiber delivery system. More recently, a 180W version has been released (215). This increase in energy corresponds with reduced lasering and operating time (216). Two-year data from the GOLIATH trial illustrates that the 180W version provides durable symptom improvement that is comparable to traditional TURP (217).

 

Compared to TURP, PVP has been shown to have an improved side-effect profile, time of catheterization, hospital stay, and improvement in urinary flow rate (218,219). Clinically, the advantage of PVP is that the length of stay in hospital is usually under 24 hours and it can be performed on anticoagulated patients. Outcomes have demonstrated a reduced frequency and severity of clinical complications, however it was limited to smaller prostate sizes (215).

 

In summary, several laser wavelengths (Potassium titanyl phosphate [KTP], Holmium:Yttrium aluminum garnet [Ho:YAG], Thulium), and delivery systems (end-firing; side-firing; interstitial) are available for PVP, and each has particular characteristics and potential advantages (171,219). In current practice, the use of 532nm 180W PVP (Greenlight) lasers is becoming increasingly more common due to significantly reduced operative times.  

 

Holmium:YAG Laser for Enucleation (HoLEP) or Resection of the Prostate (HoLRP)

 

This laser may be used to enucleate the prostate and remove the tissue in pieces (HoLEP) or to vaporise the tissue (HoLRP). HoLRP is an operation involving laser resection of the prostate tissue via an endoscope, similar to a standard TURP using electrocautery as outlined above. The fragments of prostate tissue are made small enough to irrigate out prior to detachment from the prostate (220). HoLEP again uses a Holmium laser but the laser acts like a finger would at an open prostatectomy, shelling out tissue until it floats in the bladder. The tissue is then morcellated and extracted. This technique may be safely used in large prostate glands (those weighing >100g) as an alternative to open prostatectomy as discussed below (212). Initial studies have demonstrated that HoLEP improved flow rates by 56-119% and by TURP 96-127%, and symptom scores reduced in both groups by 60%. Further, these studies reported a reduced length of hospital stay, clot retention rates, the occurrence of hyponatremia, strictures but had a slightly higher risk of reoperation (221-223). Pooled data of recent randomized trials suggest HoLEP results in significantly improved maximal flow rate, IPSS, transfusion rate at a cost to operative time (224). Patients are usually kept in hospital a little longer with the Holmium:YAG compared to the PVP technique. However, the Holmium:YAG laser has a longer track record. The disadvantage is that treatment with the Holmium:YAG is quite a complex procedure to learn as it widely resects all the prostatic tissue. HoLRP with its inherent wavelength and laser properties is not photoselective for prostate tissue and as such causes more coagulation and necrosis and has not been popular as a therapeutic intervention.

 

Thulium Laser

 

A two-micron continuous-wave is produced with a wavelength of 2013nm. This wavelength is close to the water absorption peak in tissue. This provides several advantages including excellent hemostasis with minimal thermal injury to surrounding tissue. Tissue may be incised accurately or vaporized depending on the settings utilized. Initial reports in 2005 reported the use of a 50-Watt Thulium: YAG (Tm:YAG) laser (225). More recently an improved 120 W laser has been produced, allowing for up to 1.08g of vaporization per minute (226). With the high degree of accuracy of focal ablation, various resection techniques have been reported including: Tm laser resection of the prostate-tangerine technique (TmLRP-TT), Tm vaporization (ThuVaP), Tm vaporesection (ThuVaRP), Tm vapoenucleation (ThuVEP), Tm enucleation (ThuLEP). Combinations of the available techniques allow prostate removal rates to be increase to 2-3 grams per minute (227).

 

When compared to TURP, TmVaRP offered similar urinary symptom improvement however TURP was superior in improving max voiding velocity post operatively (228). Furthermore, no improvements in reduced blood loss or decreased length of hospital stay were observed (228). Similarly, a meta-analysis of four clinical trials comparing ThuVEP and HoLEP showed both lasers were effective in reducing BPH symptoms but found ThuVEP to have slightly reduced blood loss and shorter lasting urinary incontinence post procedure (229). This evidence largely suggests that the choice between TURP, HoLEP, and thulium laser is based on availability and surgeon experience.

 

Visual Laser Thermoablation of the Prostate (VLAP)

 

Alternate minimally invasive laser therapies such as VLAP rely on deep thermal coagulation of the prostate by Nd:YAG laser with later necrosis and sloughing of the prostate tissue (230). They are not photoselective for prostate tissue and do not vaporize the tissue as PVP lasers do. They require prolonged catheterization and have a failure rate of around 10% as reported by Chacko et al in a randomized trial in 2001 (231). Such therapies differ from debulking surgery and require a post-procedure period for resolution of symptoms with the advantages being lack of general or regional anesthesia. Durability and tolerability remain issues for such therapies with re-treatment rates between 10 and 49% (202). Certainly, further studies, using randomization, larger sample sizes, and comprehensive measures of outcomes and adverse events, are still needed to better define the role of laser techniques for treating benign prostatic obstruction (221).

 

OPEN SIMPLE PROSTATECTOMY

 

This is the oldest, most invasive therapy for BPH (232). This form of surgery was the standard for men with BPH for over a century however was often associated with complications and prolonged hospital stays. The number of simple prostatectomies being performed has declined since the introduction of TURP and laser energies.

 

It is commonly done through a transvesical approach, but may be done retropubically. Early complications of this operation include hemorrhage, blood transfusion, sepsis, and urinary retention with the most common late complication being bladder neck stricture (2-3%) (200). TURP has lower perioperative morbidity but open prostatectomy produces equivalent, if not superior improvement with a similar or lower re-operation rate (233). Sexual dysfunction is not likely to be altered by the surgery (74,234) however ED is still quoted at 3-5% risk (200). Retrograde ejaculation occurs in 90% patients. Other complications of surgery such as deep vein thrombosis, myocardial infarction, and stroke are less than 1% (200).

 

SUMMARY OF INVASIVE SURGICAL TECHNIQUES

 

Over the past decade, significant advances have been made regarding the invasive management for BPH. Traditionally, TURP has been reserved for refractory or complicated BPH. However, recent advances in laser technologies have resulted in a marked uptake in the use of laser prostatectomy. Novel approaches utilizing laser energy allows for the enucleation, resection, or ablation of prostatic tissue. Multiple meta-analyses demonstrate equivocal efficacy when comparing TURP and laser prostatectomy. In light of this information, in patients where prostatectomy is indicated it is reasonable to proceed with either of the energy sources discussed above based on surgeon and patient preferences.

 

Minimally Invasive Surgical Therapies (MIST)

 

Minimally invasive therapies for BPH have evolved in the past decade with the goal being to achieve symptomatic improvement that is durable, without the morbidity associated with surgery or the long-term side effects or compliance issues associated with medical therapies (202). The aim of such treatments is to achieve results similar to TURP but with minimal anesthesia, hospitalization, and morbidity. An overview of earlier randomized controlled trials in 2000 by Tubaro et al (235) comparing minimally invasive and invasive modalities of treatment found re-treatment rates to be higher in the minimally invasive group. They concluded that at the time, none of the minimally invasive treatments were superior to TURP from a cost and benefit standpoint and that TURP remains the gold standard of treatment.

 

More recently, an increasing number of therapeutic options have been developed to improve durability without limitation to the minimally-invasive approach. Multiple ablative (thermo or chemical) or mechanical options have been introduced with early data available. Accordingly, current practice suggests a markedly increasing use of MIST, particularly in the younger patients (236). While the precise role of MIST is not clear, some view such treatments as in-between medical and TURP and we await long term data on all proposed therapies.

 

TRANSURETHRAL INCISION OF THE PROSTATE (TUIP)

 

A similar approach to a TURP is used except that no surgical debulking is undertaken. Between one and three incisions are made into the prostate at the level of the bladder neck back almost to the insertion of the ejaculatory ducts. This releases the “ring” of BPH tissue at the bladder neck, creating a larger opening. There is a reduced risk of morbidity such as hemorrhage. In some instances, ejaculation may be preserved in younger men, especially if one incision is made. The procedure only works if the tissue in the periurethral area is not too bulky, otherwise a “ball-valve” mechanism of adenoma may develop. Therefore, TUIP should be recommended to men with smaller prostates (237). Laser may be used for incisions of the prostate, as well as standard electrocautery (212). Some studies have shown TUIP to have similar IPSS outcomes to TURP but lower urine peak flow rate. Understandably, TUIP has also been shown to give better outcomes in terms of ejaculatory function (238).

 

THERMO-ABLATIVE THERAPIES

 

Thermoablation is the principle underlying the several minimally invasive available treatments that have been introduced thus far (239) and these include transurethral microwave thermotherapy (TUMT), transurethral electrovaporization of the prostate (TUVP), and transurethral needle ablation (TUNA). Collectively, these therapies have been shown to have similar or decreased efficacy when compared to TURP but have a slightly better morbidity profile at this stage. Longer follow up data will determine the true efficacy and risk profiles for these thermo-ablative therapies.

 

Transurethral Microwave Therapy (TUMT)

 

An intraurethral antenna emits microwave radiation and delivers heat to a targeted region of the prostate. Histologically, this results in well-controlled coagulative necrosis. A number of series have been published reporting outcomes following TUMT. Multiple studies have compared TUMT versus TURP, which have demonstrated the sustained effect of mild symptom improvement when compared to TURP. A recent review reported a reduced efficacy when compared to TURP with regards to IPSS improvement at 12 months (65% decrease compared to 77% with TURP) and urinary flow rate (70% increase compared to 119% with TURP) (240,241). Retreatment rates are high, ranging between 10-22% compared to 4-8% following TURP. Despite this limitation to efficacy, TUMT provides significant benefits when compared to TURP including improved sexual function, hospitalization, hematuria, transfusions rates (242). Because of lower effectiveness compared to TURP, TUMT is considered a second line option at this stage (243).

 

Transurethral Electrovaporization of the Prostate (TUVP)

 

TUVP uses heat from a monopolar or bipolar high voltage electrical current to vaporize tissue (237). Theoretically this technique could have an ablative as well as coagulative effect. To date, a meta-analysis of randomized controlled trials comparing TUVP and TURP have shown no significant differences in IPSS, quality of life or post void residual volumes. Similar rates of complications have also been found however this is limited by short follow up durations (244,245). Furthermore, TUVP did not lead to a reduction in postoperative morbidity or shorter hospital stays (246).

 

Transurethral Needle Ablation (TUNA)

 

Radiofrequency ablation between two electrodes results in thermal ablation and resulting coagulative necrosis of tissue. Several randomized trials have been performed with only short-to-midterm follow up available. As with other forms of MIST, concerns regarding durability are present. A 5 year follow up demonstrated that 58% of patients had maintained symptom control, however 21% needed re-treatment (247). Meta-analytical data confirms an improved IPSS and urinary flow rate at one-year, however to a significantly lower magnitude when compared to TURP (248). Similar to TUMT and TUVP, TUNA has a favorable morbidity profile when compared to TURP.

 

MECHANICAL THERAPIES

 

Urolift

 

Prostatic urethral lift (PUL) is a novel procedure that is characterized by the placement of non-absorbable implants within the prostatic urethra. When placed correctly, these implants provide anterolateral traction to the lateral lobes of the prostate without necessitating tissue ablation. Advantages of PUL are that it is a short, simple procedure that can be done under local anesthesia and has low complication rates. However, the presence of an obstructing median lobe poses a hurdle for procedure due to the inability to place an implant to the median lobe safely. This exclusion criteria prevents a large portion of men with BPH from undergoing this procedure. The BPH6 was a randomized controlled trail that prospectively compared the PUL with TURP. This study reported that PUL improves IPSS to 52% compared to 72% following TURP. Maximal urinary flow rates improved to a modest degree (41% compared to 144% following TURP). Interestingly the preservation of native prostatic tissue results in preserved erectile and ejaculatory function (249). Pooled analysis of available studies confirm these modest improvements in urinary and sexual function (250,251). Further, this procedure is well-tolerated and is performed in the outpatient setting under local anesthetic in a vast majority of cases. Morbidity is representative of typical MIST procedures with small proportions of patients reporting dysuria, urinary tract infection, and hematuria. Durability is among the main concern surrounding this procedure. Only three-year data has been published at present, reporting a modest IPSS improvement (252). Further comparative robust studies are required to determine the role of the PUL in current practice.

 

Intra-Prostatic Stents

 

In keeping with the principles of minimal invasion, a stent or coil is placed into the urethra at the point of maximal obstruction under local anesthesia, endoscopic and radiographic guidance. Stents may be temporary/biodegradable or permanent. Although effective in the short term, they do have a significant complication rate raising concerns over safety and large randomized controlled trials are needed to establish their long-term efficacy and their true role in the management of BPH (204,253,254).

 

Transurethral Ethanol Ablation of the Prostate (TEAP)

 

Deep intra-prostatic injection of pure ethanol results in chemical ablation of the prostate. Of the limited studies available, 4 year follow up suggests sustained response in 73% of patient, with 23% requiring retreatment. More robust comparative data is required prior to more formal recommendations for the use of TEAP.  

 

Fexapotide Triflutate – NX-1207

 

NX-1207 is injected into the transition zone of the prostate to ablate the tissue, but the precise mechanism by which NX-1207 acts has not been published to date. Trials of NX-1207 have shown a mean improvement of 5.7 points on IPSS score for men receiving one injection compared to placebo at mean 43 months follow up. When compared to men taking oral BPH medications, fewer in the NX-1207 group (8% vs 27%) required additional BPH intervention at 3 years (255). Long-term follow up of men receiving this chemical show durable reductions in symptom scores to 6.5-year follow-up (256). NX-1207 is well tolerated, with low rates of mild hematuria, dysuria and infection. No sexual dysfunction or incontinence has been reported for either agent.

 

Topsalysin - PRX-302

 

PRX-302 is a genetically modified bacterial pro-toxin that is activated by PSA within the prostatic tissue and forms transmembrane cellular pores that lead to apoptosis. Like TEAP and NX-1207, PRX-302 is injected into the transition zone of the prostate. PRX-302 results in a transient reduction in symptoms score that do not appear to be maintained at 12-month follow-up (257). Similar to NX-1207, it is well tolerated, with low rates of mild hematuria, dysuria and infection. No sexual dysfunction or incontinence has been reported.

 

Botulinum Toxin subtype A (botox)

 

Botox is a toxin produced by the bacterium Clostridium Botulinum. Its mechanism of action for intraprostatic injections is poorly understood however theories include glandular necrosis and the blockage of alpha-adrenergic receptors resulting in smooth muscle relaxation (258). Phase 2 single arm studies have shown that intraprostatic botox has minimal side effects but has a re-treatment rate as high as 29% (185)

 

OTHER THERAPIES

 

Aquablation

 

Aquablation involves a transrectal ultrasound guided, robot-assisted, high velocity saline stream. This results in the ability to ablate glandular tissue without the requirement of heat. Real-time monitoring is available and allows the surgeon to ensure sparing of the prostatic capsule. Early studies have demonstrated its safety and feasibility. The WATER trial showed that aquablation was not inferior to TURP for improving IPSS scores at 6 months follow up with slightly improved rates of anejaculation (259). Longer follow up data from this study is needed to prove long term efficacy and assess long term complication rates.  

 

Prostatic Artery Embolization (PAE)

 

PAE is performed by a trained interventional radiologist. Unilateral or bilateral prostatic arteries are injected with an embolic agent - which is typically ethanol-based. With increasing experience, technical success has increased to greater than 90%. A metanalysis of 13 studies including 1,254 men found that PAE demonstrated a mean 16.2 increase in IPSS score and improved quality of life that remained statistically significant after 3 years follow up. Transient dysuria and urinary frequency were reported in 10% and 16% of men, respectively. Post embolization syndrome was reported in 3.6% of men and only three cases of major post-operative complications were recorded (260).

 

More recently, two-year follow up data from a randomized controlled trial of PAE vs TURP was published. Reduction in IPSS score was similar in both arms however TURP men showed better urinary flow, post void residual volume, reduced prostate volume but more erectile dysfunction. 21% of men who had PAE required subsequent TURP within the 2-year period. PAE adverse events were less frequent than TURP but distribution within the severity classes were similar (261).

 

Water Vapor Therapy - Rezum

 

Rezum uses radiofrequency to create thermal energy in the form of water vapor. This vapor is delivered transurethrally under cystoscopy to the prostate and causes instant cell necrosis through cell membrane disruption. It is frequently performed under local anesthetic in the outpatient setting. Two retrospective studies showed improvement in IPSS, urinary flow and post void residual volume at 6 and 12 months follow up (262,263). A randomized trial of Rezum vs sham procedure showed a mean improvement of 7 points on IPSS score and an improvement in quality of life in the Rezum group. Peak urinary flow rate was improved by 6.2ml/s in the rezum group and was sustained at 12 months (264). Morbidity is minimal and is in-line with those experienced following alternate MISTs (265).

 

Histotripsy

 

Histotripsy is the use of extracorporeal ultrasound energy that produces extreme pressure changes within the prostatic tissue. This pressure changes result in localized clusters of microbubbles which cause mechanical fractionation. Collapse of these microbubbles leads to cellular destruction and prostatic cavitation (266). The method of prostate injury allows the procedure to be monitored through ultrasound in a real-time setting. One safety and feasibility trial has been published to date reporting three cases of transient urinary retention, 1 case of minor anal abrasion, and one case of microscopic hematuria out of 25 men. No serious intraoperative complications occurred (267).

 

SUMMARY OF MINIMALLY INVASIVE THERAPIES

 

A myriad of minimally invasive therapies (MIST) has been developed to reduce the morbidity of surgical BPH management. Current evidence in MIST is characterized by improvements in symptom and urinary flow rates similar or slightly less than TURP with high rates of retreatment. Despite this, these procedures are very well tolerated and may be performed as an outpatient. Further, these therapies are highlighted by the significant reduced risk of sexual dysfunction. Some consider that MIST might be suitable in younger patients that are willing to accept less urinary improvement to preserve sexual function. Elderly or co-morbid men might also benefit given many of these procedures can be performed under local anesthetic or in the outpatient setting. However, the precise role for MIST has not become clear to date. It is clear that MISTs are emerging and will likely become a prevalent treatment option in the management of BPH.  

 

MEASURING OUTCOMES AND EFFECTIVENESS OF TREATMENT

 

When considering the effectiveness of any treatment for BPH, one must consider the efficacy and tolerability of invasive or medical therapies (i.e., the effect on both subjective symptoms and urinary flow and incidence of adverse effects), the long-term effectiveness, the impact on daily life activities (quality of life) and the costs (268). Large scale randomized controlled trials provide information on the tolerability and efficacy of treatment options and evidence-based databases such as Cochrane reviews, may further analyze evidence-based data from multiple trials.

 

CONCLUSION

 

In conclusion, BPH is a common urological condition that is increasing in incidence in conjunction with the aging male population. If left untreated, BPH can lead to lower urinary tract obstructive symptoms that can significantly affect the quality of life of men.

 

As outlined in this chapter, the diagnosis of BPH begins with a detailed history of presenting complaint and interrogation of any lower urinary tract signs or symptoms. Questionnaires such as the IPSS score can help quantify the severity of these symptoms along with a uroflowmetry and PVR scan. A urinalysis, serum creatinine, and serum PSA should be ordered to investigate for prostate cancer, UTI, and renal failure. Imaging with USS or CT is not indicated unless there is concurrent hematuria, UTI, or urolithiasis.

 

Several options are now available for the treatment of BPH. Non-surgical management consists of medications such as alpha-blockers, 5-alpha reductase inhibitors, and PDE5 inhibitors which are available as monotherapy or in combination. BPH refractory to medical management is treated with surgical management which includes invasive and minimally invasive procedures. TURP remains the most widely used procedure for surgical BPH management and simple prostatectomies are reserved for larger prostates, complicated BPH or if TURP cannot be performed. These two procedures form the gold standard of treatment. Laser treatments when available offer good patient outcomes and have potential benefits when compared to TURP. The decision for either of these modalities of treatment is still largely dependent on availability and surgeon experience. The majority of minimally invasive treatment options are still experimental however may have a potential benefit for carefully selected men.

 

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Obesity In The Elderly

ABSTRACT

 

As the proportion of population above age 65 grows, so too increases the prevalence of those individuals who are obese. This phenomenon of an elderly population with obesity is the source of much research and debate with regards to treatment recommendations. It appears that older individuals on the extreme ends of the BMI spectrum, those who are underweight and those who are morbidly obese, have an increased risk of mortality. One major concern in the treatment of obese, elderly individuals is that many may have sarcopenic obesity which can be worsened with weight loss where some degree of lean body mass loss is inevitable. While various methods of weight loss may be recommended in some elderly who are obese, it is clear that any chosen method should be accompanied by a resistance training program in order to preserve muscle mass.

 

INTRODUCTION

 

The aging population in the U.S. is expected to more than double by 2050, increasing from 40.2 million to 88.5 million people (1). In tandem with this increase in elderly individuals is the high prevalence of those who are both elderly and obese. The significance of the increasing number of elderly individuals with obesity in terms of appropriate care and associated healthcare costs is the source of much debate.

 

PREVALENCE

 

Approximately 35% of adults in the U.S. aged 65 and over between 2007-2010 were obese as defined by body mass index (BMI, weight in kilograms over height in meters squared). In crude numbers this represents over 8 million adults aged 64-74 years and almost 5 million adults aged 75 and over (1). For individuals aged 75 and over there is a lower prevalence of obesity (27.8%) compared to those aged 65-74 years (40.8%) (1). A growing number of elderly are residing in nursing home (NH) facilities, and in line with this trend, researchers are examining the prevalence of obesity in NH facilities and its impact on healthcare utilization. Between 2000 and 2010, the prevalence of moderate to severe obesity in NHs increased from 14.7% to 23.9% (2). The rapid growth of the elderly population, which can largely be attributed to the aging baby boomers, will mark a change in the population’s composition in terms of sex ratios and ethnic diversity. Sex ratios of the population are projected to shift to include a larger share of elderly men (3). Moreover, the racial and ethnic make-up of this elderly cohort of patients is expected to develop to include more Hispanic individuals and a larger proportion of racial groups other than white. Between 2010 and 2050, the number of Hispanic people 65 years and older will increase from 2.9 to 17.5 million and the number of non-Hispanic individuals 65 years and older will increase from 37.4 to 71 million (3). These numbers of elderly individuals with obesity are also expected to increase as the population ages. Paradoxically, increased longevity does not necessarily translate to extra years spent in healthy living but may in fact result in more years spent in chronic poor health.

 

PATHOPHYSIOLOGY

 

Aging is accompanied by alterations in body composition. Fat free mass composed mostly of skeletal muscle declines by 40% between ages 20 and 70 years (4). Following age 70, both fat free mass and fat mass decrease together. With aging, there is also a redistribution of fat mass mainly in the visceral component but deposits are also observed in skeletal muscle and liver. The balance between energy intake and energy expenditure determines body fat mass. In the elderly, energy intake does not appear to increase significantly or may even decrease over time; therefore, decreased energy expenditure plays an important role in increasing fat mass with aging (4). After the age of 20, resting metabolic rate decreases by 2-3% per decade mainly due to a loss of fat free mass (4).  In addition to a decrease in resting metabolic rate, physical activity declines and there is an increase in sedentary time, which accounts for approximately half the loss in total energy expenditure with aging (4).

 

The redistribution of body fat centrally leads to the production of pro-inflammatory cytokines (5). Pro-inflammatory cytokines such as tumor necrosis factor alpha (TNF-α) and interleukin 6 (IL-6) lead to muscle loss and sarcopenia due to their catabolic effects (6). This loss of muscle mass leads to adverse outcomes such as decreased mobility and increased frailty.

 

Endocrinologic changes that occur with aging also play a role in the pathophysiology of obesity including a decrease in growth hormone, testosterone, and DHEA in addition to resistance to leptin and insulin.

 

HEALTHCARE OUTCOMES: THE POSITIVE AND THE NEGATIVE

 

Limitations To BMI Measurements

 

The American College of Cardiology and the American Heart Association define adults as overweight if BMI ≥ 25 kg/m2 and obese as BMI ≥ 30 kg/m2 regardless of age range. Accurately assessing obesity outcomes in the elderly can be a challenge given the drawbacks of defining obesity by BMI. Other methods have been utilized including hydrostatic densitometry (underwater weighing), dual-energy x-ray absorptiometry (DXA), and waist circumference. Given that BMI can either underestimate or overestimate body fat mass in the elderly and the fact fat deposition in the elderly tends to be accumulated intraabdominally, measurement of waist circumstance may be a better way of assessment. Despite its drawbacks, most studies analyzing healthcare outcomes in the obese elderly have utilized BMI as an assessment tool.

 

The Obesity Paradox

 

According to existing studies and meta-analyses, a higher BMI can be protective in the elderly. In an analysis of 13 observational studies from 1966 to 1999 examining cardiovascular mortality in non-hospitalized subjects aged 65 and above, a U-shaped curve was observed with an increase in right curve only when BMI was above 31-32 kg/m2 (7). A subsequent meta-analysis showed that BMI in overweight range did not confer an increased risk of mortality and a BMI in moderately obese range was only associated with a modest increase in mortality risk by 10% independent of gender, disease and smoking status (8). In a large, multi-ethnic study of community dwelling men and women aged 65 and above, the lowest hazard ratios (HRs) for mortality were seen in individuals with BMI 25 to less than 30 and BMI 30 to less than 35. HRs for mortality were increased when BMI was below 25 or higher than 35 (9). Similarly, in a large study of mortality in over 10,000 patients with type 2 diabetes mellitus and a median age of 63 years followed for a median of 10.6 years, a lower mortality risk was observed in overweight (BMI ≥ 25 kg/m2) and a higher mortality risk in those who were underweight (BMI ≤ 18.5 kg/m2) or obese (BMI ≥ 30 kg/m2) (10). A subsequent systemic review and meta-analysis evaluating the association of BMI with all-cause and cardiovascular mortality in subjects with type 2 diabetes mellitus, showed a strong non-linear relationship between BMI and all-cause mortality in both men and women. The lowest risk was seen in those with BMI 31-35 kg/m2 and 28-31 kg/m2. Lower BMI values were associated with higher mortality in both sexes (11). Combining available data suggests that BMI < 25 and > 35 kg/m2 is associated with higher mortality (41) (Figure 1).

Figure 1. BMI and Mortality in Elderly

While there are positive effects of obesity including increased energy reserve and prevention of malnutrition, protection from bone mineral density loss and osteoporosis, and delay in cognitive decline, there are also potential biases which may account for the obesity paradox seen in the elderly. The survival effect is one such bias which postulates that the remaining living elderly with obesity are more resistant to the complications of obesity compared to those who were perhaps more susceptible and therefore died earlier. Many studies are epidemiologic in design with the limitation of reverse causation where an overestimation of mortality risk can occur if unintentional weight loss due to an underlying disease occurs prior to BMI measurements and are then compared to the BMI of healthy group. Finally, cohort effects can be seen as subjects in different environments practicing different lifestyles are compared to one another (12).

 

One of the most significant complications of obesity in the elderly is the metabolic syndrome. This clustering of risk factors including increased waist circumference, hypertension, dyslipidemia, and glucose intolerance increases the likelihood of diabetes and cardiovascular disease. Obesity can stress the joints leading to joint dysfunction and mobility impairment as well as lead to pulmonary dysfunction and obstructive sleep apnea. Certain cancers are associated with higher BMIs including breast, uterine, colon and leukemia.

 

Weight Loss

 

Numerous population-based studies have found that weight loss in older persons is associated with increased mortality (13, 42, 43, 44). This is also true in diabetes (14). Obviously, a part of this may be due to the disease itself causing weight loss, but a number of studies have used different approaches to control for this. The negative effects of weight loss are muscle loss (sarcopenia), the protective effect of fat (on hip fracture for example), lipolysis leading to accelerated atherosclerosis, and fat loss leading to release of fat-soluble toxins into circulation (15). Fat and protein loss can also lead to drug toxicity due to the alteration of the pharmacokinetics of medications that are either fat-soluble or protein-bound (15). The benefits of weight loss need to be weighed against the risks in older persons (Figure 2).

 

Figure 2. Risk and Benefits of Weight Loss in the Elderly

 

Sarcopenic Obesity

 

Diet-induced weight loss in both younger and elderly adults consist of 75% fat tissue loss and 25% is fat free mass loss (16, 17). Hypothetically, in the elderly with obesity the loss of lean body mass is buffered by the already increased muscle mass. This proved to be a falsely reassuring concept when sarcopenic obesity was first described in the early 2000s. Sarcopenia is defined as the loss of skeletal mass and function and leads to frailty, disability, and loss of independence in the elderly. Elderly individuals with obesity have the unique difficulty in that although weight gain causes increased lean body mass and fat mass, the increased muscle mass is of poor quality. In a study by Villareal and colleagues, 52 obese elderly adults, 52 nonobese frail adults and 52 nonobese, nonfrail subjects matched for age and sex were compared. Elderly adults with obesity showed lower muscle quality compared with the other two groups in addition to reduced functional performance, aerobic capacity, strength, balance, and walking speed (18). In essence, the elderly with obesity cohort were sarcopenic and their increased adiposity proved deleterious. Subsequent studies have continued to demonstrate that sarcopenic obesity is associated with and precedes the onset of instrumental activities of daily living (IADLs) disability in community dwelling elderly (19). However, elderly subjects who are obese with increased muscle mass have better outcomes compared to those with low muscle mass. Determining which individuals who are elderly and obese have sarcopenia is important clinically and can be accomplished inexpensively and easily by measuring muscle strength via handgrip dynamometry or gait speed. The brief SARC-F questionnaire (Table 1) can also be used to identify obese individuals with poor muscle function (20). Another method for measuring and monitoring skeletal muscle mass is the use of creatine (methyl-d3) creatine dilution. In this noninvasive test, an oral tracer dose of D3-creatine is given and then subsequently measured in a fasting morning urine sample. Creatine dilution is a better measure of functional muscle mass than DXA (21).

 

Table 1. SARC-F Questionnaire

Component

Question

Scoring

Strength

How much difficulty do you have in lifting and carrying 10 pounds?

None = 0

Some = 1

A lot of unable = 2

Assistance in walking

How much difficulty do you have walking across a room?

None = 0

Some = 1

A lot, use of aids, or unable = 2

Rise from a chair

How much difficulty do you have transferring from a chair or bed?

None = 0

Some = 1

A lot or unable without help = 2

Climb stairs

How much difficult do you have climbing a flight of 10 stairs?

None = 0

Some = 1

A lot or unable = 2

Falls

How many times have you fallen in the past year?

None = 0

1-3 falls = 1

4 or more falls = 2

Score: ≥ 4 predictive of sarcopenia

 

TREATMENT

 

 

Select elderly individuals with obesity and BMI ≥ 30 kg/m2 who either have metabolic derangements or functional impairment may be recommended for weight loss provided that muscle and bone loss can be avoided (22).

 

Lifestyle Changes: Dietary Changes & Physical Exercise

 

Weight loss can be achieved alone by a moderate caloric deficit of 500-1000 kcal/day which leads to 1-2 pounds lost per week and 8-10% over 6 months (4).  However, dietary changes should be prescribed in conjunction with an exercise program consisting of aerobic, resistance and balance training to promote functionality and improve frailty (23). In a study of 107 frail elderly subjects with obesity randomized to control, diet group with 500-750 kcal deficit with 1 gm protein/kg/day, and a multi-component exercise and diet group, the combined exercise and diet group was more effective. The combined group had better physical performance scores, functional status, and aerobic capacity. Subjects also lost less lean body mass and bone mineral density compared to the diet group (24). Additionally, lifestyle interventions can reduce disease burden. In the Diabetes Prevention Program, men and women ≥ 65 years with obesity were more likely to achieve 7% weight loss compared to their younger (age ≤ 45 years) counterparts with obesity, at 3 years, 63% and 27% respectively. For every kilogram lost through diet and physical activity, the incidence of T2DM was decreased by 16% over a 3-year period (25).

 

In order to prevent muscle catabolism, elderly individuals with obesity with or at risk for sarcopenic obesity should be counseled on a less restrictive caloric deficit of 200-500 kcal/day combined with a recommended protein intake of 1.0-1.5 gm/kg assuming normal renal function.

 

Pharmacotherapy

 

There is limited data on safety and efficacy of weight loss medications in the elderly as they have largely been excluded from clinical trials. The FDA has approved five medications for chronic weight management: Semaglutide, Liraglutide, Naltrexone/Bupropion, Phentermine/Topiramate, and Orlistat. Additionally, metformin has been studied as a weight loss medication in obese, non-diabetic subjects. There is also a study in progress of elderly Japanese patients with type 2 diabetes assessing the efficacy and safety of empagliflozin, a sodium-glucose cotransporter-2 inhibitor (SGLT2i), known to cause weight loss (EMPA-ELDERLY). In this population, the effects on skeletal muscle mass, muscle strength, and physical performance will be assessed in subjects age 65 and older with type 2 diabetes on Empagliflozin (26). Overall, drug interactions, affordability, efficacy, and safety are all potential drawbacks to pharmacotherapy for weight loss in the elderly. However, there are no studies of outcomes of anorectic drugs used with exercise to protect muscle and bone.

 

SEMAGLUTIDE  

 

The weekly injectable glucagon-like peptide (GLP-1) receptor agonist was approved in 2021 for chronic weight management in adult patients with BMI of 30 kg/m2 or greater or 27 kg/m2 or greater plus a weight-related comorbid condition (hypertension, type 2 diabetes or dyslipidemia) as an adjunct to reduced calorie diet and increased physical activity. In the clinical trials, 233 (8.8%) of patients were between 65 and 75 years and 23 (0.9%) were 75 years or older and no differences in safety or efficacy were observed (27).

 

LIRAGLUTIDE

 

Liraglutide, a daily injectable GLP-1 receptor agonist was approved at doses of 3mg daily for weight loss by the FDA in 2014 for chronic weight management. This incretin-based therapy appears to have a short-term effect on decreasing gastric emptying but a long lasting central anorectic effect leading to a mean weight loss of 5.8kg in clinical studies (28, 29). The concern surrounding any weight loss in the elderly is the loss of skeletal muscle mass and sarcopenia. In a small study of elderly subjects who were either overweight or obese with type 2 diabetes mellitus treated with liraglutide 3mg daily in addition to metformin, reductions in fat mass and android fat were observed with the beneficial effect of preserved muscle tropism (30). A multicenter randomized, double-blind, parallel-group study of subjects with type 2 diabetes mellitus aged 18-80 years evaluated the effects of Liraglutide (as monotherapy or in combination with metformin) at various doses approved for treatment of diabetes mellitus (0.6mg, 1.2mg, 1.8mg daily) compared to individuals treated with Glimepiride or placebo. Mean body weight was reduced from baseline in all liraglutide treatment arms (up to 3.2 kg) and reduced fat tissue mass (1.0-2.4 kg) more than lean mass (1.5 kg) while glimepiride increased the mass of one or both tissue types (31). CT assessment also confirmed that reductions in fat tissue mass occurred in both abdominal subcutaneous and visceral fat compartments (31).

 

CONTRAVE

 

Contrave, the combination of naltrexone, an opioid antagonist, and bupropion, an aminoketone antidepressant, was FDA approved in 2014 for chronic weight management. Only 2% (62 of 3,239 subjects) in the Contrave clinical trials were over age 65 years and none older than 75 years (32). Data is lacking in terms of safety in older individuals, but given potential for neuropsychiatric disturbances, seizures, increased blood pressure and heart rate; extreme caution should be observed with this medication in the elderly.

 

QSYMIA

 

The combination of phentermine, a sympathomimetic amine anorectic, and topiramate extended release, an antiepileptic rug was FDA approved for chronic weight management in 2012. A small proportion of the subjects (254 total, 7%) studied in Qsymia clinical trials were aged 65 and older (33). While no differences in safety or effectiveness were observed, the adequate study numbers are also lacking. Given the side effect profile including risk of increased heart rate, acute myopia and secondary angle closure glaucoma, cognitive impairment and elevated creatinine, caution should be taken with starting this medication in elderly. Lower doses should be chosen and potential drug-drug interactions evaluated.

 

ORLISTAT

 

Orlistat acts as a pancreatic and gastric lipase inhibitor and leads to a 6.5-7.5 lb loss at one year. Its major side effects include steatorrhea, flatulence, fecal incontinence and malabsorption of fat-soluble vitamins. It appears to be equally efficacious with similar tolerance in a both the younger and elderly population (34).

 

METFORMIN

 

Metformin, a biguanide antidiabetic medication developed in the 1950s, may be a safe option to achieve modest weight loss even in nondiabetic individuals. In a small study of middle-aged nondiabetic subjects with obesity, metformin 2500mg daily without further caloric restriction or increased physical activity requirement resulted in a mean weight loss of 5.8 +/- 7kg (5.6+/-6.5%) compared to untreated controls (35). It may therefore be an efficacious and cost-effective strategy in elderly persons pending further studies.

 

Bariatric Surgery 

 

According to the NIH, bariatric surgery procedures including sleeve gastrectomy, laparoscopic adjustable gastric banding (LAGB), Roux-en-Y gastric bypass (RYGB), and biliopancreatic diversion with or without duodenal switch are potential options for individuals with obesity between ages 18 and 64 with BMI ≥ 40 kg/m2 or BMI ≥ 35 kg/m2 with additional co-morbidities. The American Diabetes Association has recommended lower BMI cutoffs of ≥ 30 kg/m2 for select individuals with uncontrolled hyperglycemia despite medical therapy (36). A retrospective review at a major surgical center in the U.S., found that of the 393 older patients (age > 65 years) who underwent bariatric surgery, older subjects had a higher comorbid burden compared to younger patients but exhibited comparable complication rates to patients under the age of 65 (37).  In a systematic review of over 8,000 patients aged 60 years and older who underwent bariatric surgery, outcomes (resolution of hypertension, diabetes, lipid disorders) and complication rates were similar to a younger population, independent of type of procedure (38).  While age should not necessarily be a barrier to recommending bariatric, this must be balanced against the limited existing data from pooled results of mostly small studies. Furthermore, bariatric surgery in the young and the elderly should always be coupled with resistance exercise.

 

Cryolipolysis

 

Cryolipolysis is FDA approved for treatment of focal fat deposits in the flanks, abdomen and thighs. In this procedure, fat cells are destroyed through a process of thermal reduction by which temperatures below normal but above freezing induce apoptosis-mediated cell death (39). Damaged adipocytes are then removed via an inflammatory response (39). This procedure has the advantage of being less invasive, does not require anesthesia with no downtime. In a retrospective review of a single surgery center with 528 subjects with age ranging from 18-79 years, the procedure was well tolerated with no adverse events and only 3 cases of mild or moderate pain reported to resolve in 4 or fewer days (40). However, there are limitations regarding the evaluation of the literature on this procedure thus far, including short follow-up time (typically 2-3 months), variability in cooling intensity factor (CIF) applied, differences in the evaluation of efficacy, and differences in the duration of procedure.

 

CONCLUSION

 

The landscape of the population is certainly changing and is marked by two significant trends: an increasingly elderly population and an ongoing obesity epidemic. This will undoubtedly impact families, social structures, and healthcare costs. How to appropriately care for these individuals will be the subject of much debate and further research. Physicians will need to balance the potential danger of weight loss in older persons against the complications of obesity to decide on the best patient centered approach. One clear recommendation is that all weight loss regimens in the elderly need to be coupled with a comprehensive resistance exercise program.

 

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Pathogenesis of Type 2 Diabetes Mellitus

ABSTRACT

 

Numerous distinct pathophysiologic abnormalities have been associated with type 2 diabetes mellitus (T2DM).  It is well established that decreased peripheral glucose uptake (mainly muscle) combined with augmented endogenous glucose production are characteristic features of insulin resistance. Increased lipolysis, elevated free fatty acid levels, along with accumulation of intermediary lipid metabolites contributes to further increase glucose output, reduce peripheral glucose utilization, and impair beta-cell function.  Adipocyte insulin resistance and inflammation have been identified as important contributors to the development of T2DM. The presence of non-alcoholic fatty liver disease [NAFLD] is now considered an integral part of the insulin resistant state.  The traditional concepts of “glucotoxicity” and lipotoxicity, which covers the process of beta cell deterioration in response to chronic elevations of glucose and lipids, has been expanded to encompass all nutrients [‘nutri-toxicity”].  The delayed transport of insulin across the microvascular system is also partially responsible for the development of tissue insulin resistance.   Compensatory insulin secretion by the pancreatic beta cells may initially maintain normal plasma glucose levels, but beta cell function is already abnormal at this stage, and progressively worsens over time.  Concomitantly, there is inappropriate release of glucagon from the pancreatic alpha-cells, particularly in the post-prandial period. It has been postulated that both impaired insulin and excessive glucagon secretion in T2DM are secondary to an “incretin defect”, defined primarily as inadequate release or response to the gastrointestinal incretin hormones upon meal ingestion.  To a certain extent, the gut microbiome appears to play a role in the hormonal and metabolic disturbances seen in T2DM.  Moreover, hypothalamic insulin resistance (central nervous system) also impairs the ability of circulating insulin to suppress glucose production, and renal tubular glucose reabsorption capacity may be enhanced, despite hyperglycemia.   These pathophysiologic abnormalities should be considered for the treatment of hyperglycemia in patients with T2DM. 

 

NORMAL GLUCOSE HOMEOSTASIS

 

In the post-absorptive state (10-12 hour overnight fast), the majority of total body glucose disposal takes place in insulin independent tissues (1).  Under basal conditions, approximately 50% of all glucose utilization occurs in the brain, which is insulin independent and becomes saturated at a plasma glucose concentration of about 40 mg/dl (2). Another 25% of glucose uptake occurs in the splanchnic area (liver plus gastrointestinal tissues) and is also insulin independent (3). The remaining 25% of glucose metabolism in the post-absorptive state takes place in insulin-dependent tissues, primarily muscle (4,5). Basal glucose utilization averages ~2.0 mg/kg.min and is precisely matched by the rate of endogenous glucose production (1,3-7). Approximately 85% of endogenous glucose production is derived from the liver, and the remaining amount is produced by the kidney (1,8,9). Approximately one-half of basal hepatic glucose production is derived from glycogenolysis and one-half from gluconeogenesis (9,10).

 

Following glucose ingestion, the balance between endogenous glucose production and tissue glucose uptake is disrupted. The increase in plasma glucose concentration stimulates insulin release from the pancreatic beta cells, and the resultant hyperinsulinemia and hyperglycemia serve (i) to stimulate glucose uptake by splanchnic (liver and gut) and peripheral (primarily muscle) tissues and (ii) to suppress endogenous glucose production (1,3-7,11-14).

 

Hyperglycemia, in the absence of hyperinsulinemia, exerts its own independent effect to stimulate muscle glucose uptake and to suppress endogenous glucose production in a dose dependent fashion (14-16). The majority (~80-85%) of glucose that is taken up by peripheral tissues is disposed of in muscle (1,3-7,11-14), with only a small amount (~4-5%) being metabolized by adipocytes (17). Although fat tissue is responsible for only a fraction of total body glucose disposal, it plays a very important role in the maintenance of total body glucose homeostasis (see below). Insulin is a potent inhibitor of lipolysis and even small increments in the plasma insulin concentration exerts a potent anti-lipolytic effect, leading to a marked reduction in the plasma free fatty acid level (18). The decline in plasma FFA concentration results in increased glucose uptake in muscle (19) and contributes to the inhibition of endogenous glucose production (16,20). Thus, changes in the plasma FFA concentration in response to increased plasma levels of insulin and glucose play an important role in the maintenance of normal glucose homeostasis (21,22).

SITE OF INSULIN RESISTANCE IN TYPE 2 DIABETES (T2DM)

The maintenance of whole-body glucose homeostasis is dependent upon a normal insulin secretory response by the pancreatic beta cells and normal tissue sensitivity to the independent effects of hyperinsulinemia and hyperglycemia (i.e., the mass-action effect of glucose) to augment glucose uptake. In turn, the combined effects of insulin and hyperglycemia to promote glucose disposal are dependent on three tightly coupled mechanisms: (i) suppression of endogenous (primarily hepatic) glucose production; (ii) stimulation of glucose uptake by the splanchnic (hepatic plus gastrointestinal) tissues; and (iii) stimulation of glucose uptake by peripheral tissues, primarily muscle (1,4,14). Muscle glucose uptake is regulated by flux through two major metabolic pathways: glycolysis (of which ~90% represents glucose oxidation) and glycogen synthesis.

 

Hepatic Glucose Production

 

In the overnight fasted state, the liver of healthy subjects produces glucose at the rate of ~1.8-2.0 mg.kg-1.min-1 (1,3,4,6,18,54). This glucose flux is essential to meet the needs of the brain and other neural tissues, which utilize glucose at a constant rate of ~1-1.2 mg.kg-1.min-1 (2,169).  Brain glucose uptake accounts for ~50-60% of glucose disposal during the post-absorptive state and this uptake is insulin independent. Therefore, brain glucose uptake occurs at the same rate during absorptive and post-absorptive periods and is not altered in T2DM (214).  Following glucose ingestion, insulin is secreted into the portal vein and glucagon release is inhibited, and this new hormonal ratio is carried to the liver, where it suppresses hepatic glucose output. If the liver does not perceive this insulin signal and continues to produce glucose, there will be two superimposed inputs of glucose into the body, one from the liver and another from the gastrointestinal tract, and marked hyperglycemia will ensue.

 

In subjects with T2DM and mild to moderate fasting hyperglycemia (140-200 mg/dl, 7.8-11.1 mmol/L) basal endogenous glucose production [EGP] is increased by ~0.5 mg/kg.min. Consequently, during the overnight sleeping hours (i.e., 2200 h to 0800 h), the liver of an 80-kg individual with diabetes and modest fasting hyperglycemia adds an additional 35 g of glucose to the systemic circulation. The increase in basal EGP is closely correlated with the severity of fasting hyperglycemia (1,3,4,6,18,54,157-159,162). Thus, in T2DM with overt fasting hyperglycemia (>140 mg/dl, 7.8 mmol/l), an excessive rate of EGP and glucose output is the major abnormality responsible for the elevated fasting plasma glucose concentration. The close relationship between fasting plasma glucose concentration and EGP has been demonstrated in numerous studies (164-166,170-174).

 

In the post-absorptive state, the fasting plasma insulin concentration in subjects with T2DM is 2-4-fold greater than in subjects without diabetes. Because hyperinsulinemia is a potent inhibitor of EGP (1,3,4-6,16,18,164,165,175), hepatic resistance to the action of insulin must be present in the post-absorptive state to explain the excessive output of glucose. Hyperglycemia per se also exerts a powerful suppressive action on EGP (15,167,175-177). Therefore, the liver, primarily, also must be glucose resistant with respect to the inhibitory effect of hyperglycemia to suppress glucose output, and this has been well documented (15,167,178,179).

 

Using the euglycemic insulin clamp technique in combination with tritiated glucose, the dose response relationship between endogenous glucose production and the plasma glucose concentration has been defined by Groop, DeFronzo, et al (18). The following points should be emphasized: (i) first, the dose-response curve relating inhibition of EGP to the plasma insulin concentration is quite steep, with an effective dose for half-maximal insulin concentration (ED50) of ~30-40 µU/ml; (ii) in individuals with T2D the dose response curve is shifted to the right, indicating the presence of hepatic resistance to the inhibitory effect of insulin on hepatic glucose production. However, at plasma insulin concentrations within the high physiologic range (~100 µU/ml), the hepatic insulin resistance can be largely overcome and a near normal suppression of EGP can be achieved; (iii) the severity of the hepatic insulin resistance is related to the severity of the diabetic state. In T2DM with mild fasting hyperglycemia, an increment in plasma insulin concentration of 100 µU/ml causes a complete suppression of EGP. However, in diabetic subjects with more severe fasting hyperglycemia, the ability of the same plasma insulin concentration to suppress EGP is impaired (18). These results suggest that there is an acquired component of hepatic insulin resistance and that this defect becomes progressively worse as the diabetic state decompensates over time.

 

The glucose released by the liver in the post-absorptive state can be derived from either glycogenolysis or gluconeogenesis (6,16,176). Studies employing the hepatic vein catheter technique have shown that the uptake of gluconeogenic precursors, especially lactate, is increased in subjects with T2DM (180). Consistent with this observation, radioisotope turnover studies, using lactate, alanine, and glycerol have shown that ~90% of the increase in HGP above baseline can be accounted for by accelerated gluconeogenesis (181,182). More recent studies employing 13C-magnetic resonance imaging (183) and D2O (184,185) have confirmed the important contribution of accelerated gluconeogenesis to the increase in HGP. An increased rate of glutamine conversion to glucose also has been shown to contribute to the elevated rate of gluconeogenesis in subjects with T2DM (186), which may be, in part, derived from renal gluconeogenesis (8). The mechanisms responsible for the increase in hepatic gluconeogenesis include hyperglucagonemia (187), increased circulating levels of gluconeogenic precursors (lactate, alanine, glycerol) (181,188), increased FFA oxidation (18,162,189), enhanced sensitivity to glucagon (190) and decreased sensitivity to insulin (1,4.18,164,165). Although the majority of evidence indicates that increased gluconeogenesis is the major cause of the increase in EGP in subjects with T2DM (181- 186), it is likely that accelerated glycogenolysis also contributes to it (181,191). 

 

The presence of both direct and indirect effects of insulin in suppressing EGP and release into the circulation were recently demonstrated in animals using intra-portal and systemic insulin infusions (430). The results provided evidence that, in addition to a direct action of insulin on hepatic enzymes, the inhibition of adipose tissue lipolysis represents an important mechanism by which insulin regulates the rate of gluconeogenesis.  This is therefore, accomplished indirectly, by controlling the supply of free fatty acids, which are essential to the process of glucose synthesis de novo.  The rate-limiting step in achieving fast and complete inhibition of adipose tissue lipolysis is the transendothelial transport of insulin across tissue capillaries.  Additional data obtained during systemic infusions of free fatty acids and in experiments where adipocyte lipolytic factors were manipulated, together with observation in mice lacking hepatic Foxo1 & Akt1/2 signaling have confirmed this indirect action of insulin on gluconeogenesis (430-432).  These findings have generated the hypothesis that in patients with T2DM, insulin may be transported slowly across tissue capillaries, which delays the inhibition of lipolysis with subsequent impairment of the suppression of EGP.

 

On the other hand, animal studies (431), where insulin was infused directly into the portal vein, mimicking normal insulin secretory pattern, showed that there is complete and swift inhibition of EGP.  These observations were confirmed when plasma glucagon and fatty acid levels were clamped at basal values, and in conditions where brain insulin action was blocked.  Authors conclude that the direct hepatic effect of insulin in the regulation of EGP is more relevant and that, the indirect effect is redundant in physiological conditions.  Acute insulin suppression of endogenous gluconeogenesis is largely an indirect effect mediated by the inhibition of adipose tissue lipolysis, which reduces delivery of non-esterified fatty acids and glycerol to the liver.  The major direct effect of insulin on hepatic glucose metabolism is the regulation of glycogen metabolism.  Hyperglycemia and hyperinsulinemia are required to maximally stimulate net hepatic glycogenesis.  In T2DM, lipid-induced hepatic insulin resistance, high rates of adipose tissue lipolysis and hyperglucagonemia impair glucose metabolism in the liver (432).

 

Because of the inaccessibility of the liver in man, it has been difficult to assess the role of key enzymes involved in the regulation of gluconeogenesis (pyruvate carboxylase, phosphoenol- pyruvate carboxykinase), glycogenolysis (glycogen phosphorylase), and net hepatic glucose output (glucokinase, glucose-6-phosphatase). However, considerable evidence from animal models of T2DM and some evidence in humans have implicated increased activity of PEPCK and G-6-Pase in the accelerated rate of hepatic glucose production (192-194).

 

Recently, changes in hypothalamic insulin signaling have been shown to affect endogenous glucose production. The activation of the insulin receptor in the third cerebral ventricle is capable of suppressing glucose production, independent of plasma insulin or other counter-regulatory hormones.  Conversely, central antagonism to insulin signaling impairs the ability of circulating insulin to inhibit glucose production (6A).  These observations have raised the possibility that hypothalamic insulin resistance contributes to hyperglycemia in T2DM.

The Role of the Kidney

The kidney also has been shown to produce glucose and estimates of the renal contribution to total endogenous glucose production have varied from 5% to 20% (8,9,195). These varying estimates of the contribution of renal gluconeogenesis to total glucose production are largely related to the methodology employed to measure glucose production by the kidney (196). One unconfirmed study suggests that the rate of renal gluconeogenesis is increased in T2DM with fasting hyperglycemia (197). Arguing against this possibility are studies employing the hepatic vein catheter technique which have shown that all of the increase in total body EGP (measured with 3-3H-glucose) in T2DM can be accounted for by increased hepatic glucose output (measured by the hepatic vein catheter technique) (3). A more relevant aspect on the role of the kidney in the dysregulation of glucose homeostasis in diabetes is the maintenance of hyperglycemia, which results from a maladaptive enhancement of the tubular glucose transport threshold (9A, 9B). It has been hypothesized that in response to an elevated glucose load presented to the proximal tubular lumen, the sodium glucose co-transporter system increases its reabsorptive capacity by upregulating the SGLT-2 expression and kinetics (9C).  However, more recent studies conducted in humans who underwent unilateral nephrectomy were not able to confirm the over-expression of either SGLT-2 or SGLT-1 proteins in proximal renal tubules of patients with T2DM compared to non-diabetic controls (433, 434).  The augmented tubular glucose transport described in patients with type 1 and type 2 diabetes may result from a functional enhancement of the activity of these co-transporters.  The elevated renal threshold to plasma values between 220-250 mg/dl for the excretion of glucose into the urine in these patients, thus may be secondary to a sustained hyperglycemia. If this is confirmed, the maladaptive process of recycling a substantial amount of glucose back into the peripheral circulation may be attenuated with near-normoglycemia, possibly reversible. In any case, this contribution of the kidney to hyperglycemia in diabetic patients represents one additional pathogenic mechanism that has been underappreciated.

Peripheral (Muscle) Glucose Uptake

Muscle is the major site of glucose disposal in man (1,3-5,14). Under euglycemic hyperinsulinemic conditions, approximately 80% of total body glucose uptake occurs in skeletal muscle (1,3-5). Studies employing the euglycemic insulin clamp in combination with femoral artery/vein catheterization have examined the effect of insulin on leg glucose uptake in subjects with T2DM and control subjects (3). Since bone is metabolically inert with regards to carbohydrate metabolism and adipose tissue takes up less than 5% of an infused glucose load (17,198,199), muscle represents the major tissue responsible for leg glucose uptake.

 

In response to a physiologic increase in plasma insulin concentration (~80-100 μU/ml), leg (muscle) glucose uptake increases linearly, reaching a plateau value of 10 mg/kg leg wt per minute (3). In contrast, in lean subjects with T2DM, the onset of insulin action is delayed for ~40 min and the ability of the hormone to stimulate leg glucose uptake is markedly blunted, even though the study is carried out for an additional 60 min in the group with T2DMto allow insulin to more fully express its biological effects (3). During the last hour of the insulin clamp study, the rate of glucose uptake was reduced by 50% in the group with T2DM (3). These results provide conclusive evidence that the primary site of insulin resistance during euglycemic insulin clamp studies performed in subjects with T2DM resides in muscle tissue. Using the forearm and leg catheterization techniques (13,153,200,202), a number of investigators have demonstrated a decreased rate of insulin-mediated glucose uptake by peripheral tissues. The use of positron emission tomography (PET) scanning to quantitate leg glucose uptake in subjects with T2DM has provided additional support for the presence of severe muscle resistance to insulin in diabetic subjects (203).   

 

Vascular and Myocardial Insulin Resistance

 

The first and rate-limiting step in insulin-mediated glucose disposal is the transit of insulin from the plasma to the muscle.  Crossing of insulin from the circulation into the muscle interstitium is governed by vascular endothelium.  The transendothelial transport depends on the insulin receptor binding to the endothelial cell membrane and requires the activation of the nitric oxide synthase.  The transport of insulin across the endothelial cell layer appears to involve a complex vesicular trafficking process, which is saturable.  Insulin is known to promote capillary vasodilation particularly in the postprandial period to facilitate entry and distribution of fuel substrates, including glucose.  Several studies sampling lymph and interstitial glucose, using dialysis techniques, have suggested that a delay in insulin transfer from the plasma to the tissue may play an important role in the development of insulin resistance (427-429).  Thus, impairment of insulin action may be secondary to a decrease in capillary density [chronic situations] or to a defective increase in blood flow or micro-capillary recruitment [acute conditions] (429). These abnormalities have been described in obese insulin-resistant and in the skin flow response of patients with diabetes. 

 

Myocardial insulin resistance translates to abnormal intracellular signaling and reduced glucose oxidation rates in animal models of obesity (435).  It adversely affects myocardial mechanical function and tolerance to ischemia and reperfusion.  The heart is a dynamic organ that requires continuous energy in the form of ATP in order to meet contractile demands.  This is achieved via a constant supply of blood-borne oxidizable substrates.  The majority of ATP is derived from fatty acid oxidation [60-70%].  Glucose and lactate extracted from the circulation account for the remainder 30-40%.  However, when blood glucose and insulin levels are elevated, such as immediately after a meal, glucose becomes the major fuel for myocardial oxidation and, it may represent up to 70% of the total substrate oxidation by cardio-myocytes.  Long–chain fatty acids are taken up by the heart proportionately to circulating levels, via a passive facilitated transport.  Once inside the cytosol, they are degraded into acetyl-CoA moieties that enter the mitochondrial oxidative phosphorylation process.  The excess fatty acids are re-esterified to form diacyl- and triacyl-glycerides and, these lipid intermediates are stored in the form the myo-cellular lipid pool.  Glucose enters the myocardial cells both via GLUT-1 passive and insulin-stimulated GLUT-4 active transport.  These are dictated by myocardial contraction demands and circulating insulin levels.  Intracellular glucose is phosphorylated and, either stored as muscle glycogen or anaerobically oxidized to pyruvate. Under normal oxygen delivery, pyruvate is converted to acetyl-CoA, which enters mitochondrial oxidation.  In conditions of ischemia, low oxygen forces the conversion of pyruvate into lactate (435).

 

It is believed that myocardial insulin resistance with typical defects in glucose transport and oxidation develops, in part, because of an excess supply of fatty acids.  In addition to a direct competition with glucose utilization, there is evidence that the accumulation of intracellular lipid intermediates interferes with insulin signaling.  The molecular defects responsible for the insulin resistance in the cardio-myocytes are analogous to the skeletal muscle.  The local generation of reactive oxygen species and other elements also participate in obstructing insulin action. Although the cellular and metabolic manifestations may be similar, the consequences of insulin resistance in the heart muscle tends to express with lower tolerance for ischemia and poor mechanical function.  Consequently, patients with insulin resistance are susceptible to earlier and more severe cardiovascular complications.   

 

Splanchnic (Hepatic) Glucose Uptake

 

In humans, it is difficult to catheterize the portal vein, and glucose disposal by the liver has not been examined directly. Using the hepatic vein catheterization technique in combination with the euglycemic insulin clamp, the contribution of the splanchnic (liver plus gastrointestinal) tissues to overall glucose homeostasis has been examined in lean subjects with T2DM with mild to moderate fasting hyperglycemia (3). In the post-absorptive state, there is a net release of glucose from the splanchnic area (i.e., negative balance) in both control and subjects with T2DM, reflecting glucose production by the liver. In response to insulin, splanchnic glucose output is promptly suppressed (reflecting the inhibition of HGP) and, by 20 min, the net glucose balance across the splanchnic region declines to zero (i.e., there was no net uptake or release) (3). After 2 h of sustained hyperinsulinemia, there is a small net uptake of glucose (~0.5 mg.kg- 1.min-1) by the splanchnic area (i.e., positive balance). This uptake is virtually identical to the rate of splanchnic glucose uptake observed in the basal state, indicating that the splanchnic tissues, like the brain, are insensitive to insulin at least with respect to the stimulation of glucose uptake (3,5,6,175). There was no difference between diabetic and control subjects for glucose taken up by the splanchnic tissues at any time during the insulin clamp study (3).

 

The results of these studies illustrate another important point: namely, that under conditions of euglycemic hyperinsulinemia, very little of the infused glucose is taken up by the splanchnic (and therefore hepatic) tissues (3,5,6,175). During the insulin clamp, the rate of whole-body glucose uptake averaged 7 mg.kg-1.min-1, and of this, only 0.5 mg.kg-1.min-1 or 7%, was disposed of by the splanchnic region. Because the difference in insulin-mediated total body glucose uptake between the T2DM and control groups during the euglycemic insulin clamp study was 2.5 mg.kg-1.min-1, from a purely quantitative standpoint it is obvious that a defect in splanchnic (hepatic) glucose removal never could account for the magnitude of impairment in total body glucose uptake following intravenous glucose/insulin administration. However, after glucose ingestion, the oral route of administration and the resultant hyperglycemia conspire to enhance splanchnic (hepatic) glucose uptake (6,7,11,12,16,26,175) and, under these conditions, diminished hepatic glucose uptake has been shown to contribute to the impairment in glucose tolerance in T2DM (see discussion below) (6,204,205).

 

Summary: Whole Body Glucose Utilization

 

Insulin-mediated whole body glucose utilization during the euglycemic insulin clamp represents essentially skeletal muscle glucose utilization. There is a noticeable decrease for glucose taken up in the body in T2DM patients compared with non-diabetic subjects. On the other hand, net splanchnic glucose uptake, quantitated by the hepatic venous catheterization technique, is similar in both groups and averaged 0.5 mg.kg-1.min-1. Adipose tissue glucose uptake accounts for less than 5% of total glucose disposal (17,198,199). Brain glucose uptake, estimated to be 1.0-1.2 mg.kg-1.min-1 in the post-absorptive state (2,169,206), is unaffected by hyperinsulinemia (169). Muscle glucose uptake (extrapolated from leg catheterization data) in control subjects accounts for ~75-80% of the total glucose uptake (1,3,4). In subjects with T2DM, the largest part of the impairment in insulin-mediated glucose uptake is accounted for by a defect in muscle glucose disposal. Even if adipose tissue of subjects with T2DM took up absolutely no glucose, it could, at best, explain only a small fraction of the defect in whole body glucose metabolism.

 

Glucose Disposal during OGTT

 

In everyday life, the gastrointestinal tract represents the normal route of glucose entry into the body. However, the assessment of tissue glucose disposal following glucose ingestion presents a challenge because of the difficulties in quantitating the rate of glucose absorption, suppression of hepatic glucose production, and organ (liver and muscle) glucose uptake. Moreover, because the plasma glucose and insulin concentrations are changing simultaneously, it is difficult to draw conclusions about insulin secretion or insulin sensitivity.

 

To address these issues, Ferrannini, DeFronzo, and colleagues (7,11,12,205) administered oral glucose to healthy control subjects in combination with hepatic vein catheterization to examine splanchnic glucose metabolism. The oral glucose load and endogenous glucose pool were labeled with [1-14C] glucose and [3-3H] glucose, respectively, to quantitate total body glucose disposal (from tritiated glucose turnover) and endogenous HGP (difference between the total rate of glucose appearance, as measured with tritiated glucose, and the rate of oral glucose appearance, as measured with [1-14C] glucose).

 

During the 3.5 h after glucose (68 g) ingestion: (i) 19 g, or 28%, or the oral load was taken up by splanchnic tissues; (ii) 48 g, or 72%, was disposed of by peripheral (non-splanchnic) tissues; (iii) of the 48 g taken up by peripheral tissues, the brain (an insulin-independent tissue) accounted for ~15 g (~1 mg.kg-1.min-1), or 22%, of the total glucose load (12); (iv) basal HGP declined by 53%. Similar percentages for splanchnic glucose uptake (24%-29%) and suppression of HGP (50%-60%) in normal subjects have been reported by other investigators (13,204,207-209). The contribution of skeletal muscle to the disposal of an oral glucose load has been reported to vary from a low of 26% (207) to a high of 56% (208), with a mean of 45% (11,13,207-209). These results emphasize several important differences between oral and intravenous glucose administration. After glucose ingestion: (i) EGP is less completely suppressed, most likely due to activation of local sympathetic nerves that innervate the liver (210); (ii) peripheral tissue (primarily muscle) glucose uptake is quantitatively less important; (3) splanchnic glucose uptake is quantitatively much more important.

 

In individuals with T2DM (12,204,205,211,212) the disposition of an oral glucose load is significantly altered. The disturbance in glucose metabolism is accounted for by two factors: (i) decreased tissue glucose uptake and (ii) impaired EGP suppression. Splanchnic glucose uptake is similar in diabetic and control groups. Inappropriate suppression of EGP accounted for nearly one-third of the defect in total-body glucose homeostasis, while reduced peripheral (muscle) glucose uptake accounted for the remaining two-thirds. Since hyperglycemia per se enhances splanchnic (hepatic) glucose uptake in proportion to the increase in plasma glucose concentration (24,175), the splanchnic glucose clearance (SGU/plasma glucose concentration) is markedly reduced in all subjects with T2DM following glucose ingestion. Using a combined insulin clamp/OGTT technique, impairment in glucose uptake by the splanchnic tissues in subjects with T2DM has been demonstrated directly (213).

 

The gastrointestinal incretin hormones, which are produced in response to nutrient intake and potentiate the stimulus to insulin secretion in the postprandial period have been implicated as additional factors in the pathogenesis of T2DM (4A,28-30). The combined actions of glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic peptide (GIP) can account for most of the incretin effect in normal subjects (4B). Recent demonstration that in T2DM the incretin effect is impaired, diminished or absent (4B) has rekindled interest in the potential role of these gastrointestinal peptides in the abnormal handling of glucose by splanchnic tissues and perhaps, in the decline in beta-cell insulin secretion.

 

When viewed in absolute terms, most studies have demonstrated that the total amount of glucose taken up by all tissues of body over the 4-hour period following the ingestion of an oral glucose load is normal (13) or slightly decreased (204,205,211). However, this occurs at the expense of postprandial hyperglycemia. Thus, the efficiency of glucose disposal, i.e., the glucose clearance (tissue glucose uptake/plasma glucose concentration), is severely reduced. It should be emphasized that it is not the absolute glucose disposal rate, but rather the increment in glucose disposal above baseline that determines the rise in plasma glucose concentration above the fasting value. Every published study (13,204,205,211) has demonstrated that the incremental response in whole-body glucose uptake is moderately to severely reduced in individuals with T2DM. Similar results have been reported for forearm muscle glucose uptake (13,201,202,208,209), pointing out the important contribution of diminished muscle glucose disposal to impaired oral glucose tolerance in T2DM.

 

In summary, results of the OGTT indicate that both impaired suppression of EGP and decreased tissue (muscle) glucose uptake contribute approximately equally to the glucose intolerance of T2DM. The efficiency of the splanchnic (hepatic) tissues to take up glucose (as reflected by the splanchnic glucose clearance) also is impaired in individuals with T2DM.

 

Summary of Insulin Resistance in T2DM

 

Insulin resistance involving both muscle and liver are characteristic features of the glucose intolerance in individuals with T2DM. In the basal state, the liver represents a major site of insulin resistance, and this is reflected by overproduction of glucose despite the presence of both fasting hyperinsulinemia and hyperglycemia. This accelerated rate of hepatic glucose output is the primary determinant of the elevated fasting plasma glucose concentration in T2DM. Although tissue (muscle) glucose uptake in the post-absorptive state is increased when viewed in absolute terms, the efficiency with which glucose is taken up (i.e., the glucose clearance) is diminished. After glucose infusion or ingestion (i.e., in the insulin stimulated state) both decreased muscle glucose uptake and impaired suppression of HGP contribute to the insulin resistance. Following glucose ingestion, the defects in insulin-mediated glucose uptake by muscle and the suppression of glucose production by insulin contribute approximately equally to the disturbance in whole-body glucose homeostasis in T2DM. However, under euglycemic hyperinsulinemic conditions, EPG is largely suppressed and impaired muscle glucose uptake is primarily responsible for the insulin resistance.

DYNAMIC INTERACTION BETWEEN INSULIN SENSITIVITY AND INSULIN SECRETION IN T2DM

 

Subjects with T2DM manifest abnormalities both in tissue (muscle, fat, and liver) sensitivity to insulin and in pancreatic insulin secretion. To understand how these two metabolic disturbances interact to produce the full-blown diabetic condition, it is necessary to quantitate insulin action and insulin secretion in the same individual over a wide range of insulin sensitivity. This dynamic interaction is demonstrated graphically by results obtained in healthy, lean, young normal glucose tolerant women who received a euglycemic insulin clamp (1 mU.kg-1.min-1) and were stratified into quartiles based upon the rate of insulin-mediated glucose disposal (49).

 

Insulin secretion was measured independently on a separate day with a +125 mg/dl hyperglycemic clamp. Insulin resistance and insulin secretion were strongly and positively correlated (r=0.79, p<0.001).  Women who were the most insulin resistant (quartile 1) had the highest fasting plasma insulin concentrations and highest early and late phase plasma insulin responses. Similar results relating the plasma insulin response and the severity of insulin resistance have been reported in normal glucose tolerant subjects with the minimal model technique (46,47) and the insulin suppression test/oral glucose tolerance test (214).

 

A number of groups have examined the dynamic interaction between insulin secretion and insulin sensitivity in subjects with T2DM (1,4,34,35,38,39,42,46-48,58-61,150,162).

DeFronzo (4) studied lean (ideal body weight < 120%) and obese (ideal body weight > 125%) subjects with varying degrees of glucose tolerance as follows: Group I-obese subjects (n=24) with normal glucose tolerance; Group II-obese subjects (n=23) with impaired glucose tolerance; Group III-obese subjects (n=35) with overt diabetes, subdivided into those with a hyperinsulinemic response and those with a hypoinsulinemic response during a 100-gram OGTT; Group IV-normal weight subjects with T2DM (n=26); Group V-normal weight subjects (n=25) with normal glucose tolerance. All subjects ingested 100 g of glucose to provide a measure of glucose tolerance and insulin secretion. Whole-body insulin sensitivity was quantitated with the euglycemic insulin (~100 µU/ml) clamp technique, which was performed with indirect calorimetry to quantitate rates of glucose oxidation and non-oxidative glucose disposal. The later primarily reflects glycogen synthesis (215).

 

In normal weight subjects with T2DM, insulin-mediated whole-body glucose uptake was reduced by 40-50% and the impairment in insulin action resulted from defects in both oxidative and non-oxidative glucose metabolism (4). Obese individuals without T2DM were as insulin resistant as the normal-weight subjects with T2DM (4). Defects in both glucose oxidation and glucose storage contributed to the insulin resistance in the obese nondiabetic group. From the metabolic standpoint, therefore, obesity and T2DM closely resemble each other.

 

Similar results concerning reduced whole-body insulin sensitivity in individuals with obesity and T2DM have been reported by other investigators (160,161,166,216-218). Despite nearly identical degrees of insulin resistance, normal-weight subjects with T2DM manifested fasting hyperglycemia and marked glucose intolerance, whereas the obese individuals without diabetes had normal or only minimally impaired oral glucose tolerance (4). This apparent paradox is explained by the plasma insulin response during the OGTT. Compared with control subjects, the obese group without diabetes secreted more than twice as much insulin, and this was sufficient to offset the insulin resistance. In contrast, in normal-weight subjects with T2DM, the pancreas, when faced with the same challenge, was unable to augment its secretion of insulin sufficiently to compensate for the insulin resistance. This imbalance between insulin supply by the beta-cells and the insulin requirement by tissues resulted in a frankly diabetic state, with fasting hyperglycemia and marked glucose intolerance.

 

The fact that plasma insulin response to the development of insulin resistance typically is increased during the natural history of T2DM does not mean that the beta cell is functioning normally. To the contrary, recent studies (4C) have demonstrated that the onset of beta-cell failure occurs much earlier and is more severe than previously appreciated.

 

Recognizing that simply measuring plasma insulin response to a glucose challenge does not provide a valid index of beta cell function, a series of studies were conducted in subjects with normal glucose tolerance (NGT), impaired glucose tolerance (IGT) and T2DM, using an oral glucose tolerance test to evaluate the increment in insulin secretion in response to an increment in plasma glucose. A euglycemic insulin clamp to measure insulin sensitivity was also performed to address the adjustment of the beta cell to the body’s sensitivity to insulin.

 

Thus, the results yielded a better measure of beta-cell function expressed per increment of plasma glucose and corrected for the degree of insulin resistance, the so-called disposition index [ΔI/ΔG ÷IR]. These data revealed a substantial decrease in beta-cell function, most evident in individuals with IGT who had lost anywhere from 60 to 85% of the total insulin secretory capacity.

 

When obesity and diabetes coexist in the same individual, the severity of insulin resistance is only slightly greater than that in either the normal-weight diabetic or nondiabetic obese groups (4), and the magnitude of the defects in glucose oxidation and non-oxidative glucose disposal are similar in all obese and diabetic groups. Although hyperinsulinemic and hypoinsulinemic obese diabetic subjects were equally insulin resistant, the severity of glucose intolerance is worse in the hypoinsulinemic group, and this was related entirely to the presence of severe insulin deficiency.

 

In the obese nondiabetic subjects, tissue sensitivity to insulin is markedly reduced, but glucose tolerance remains perfectly normal because the beta cells are able to augment their insulin secretory capacity appropriately to offset the defect in insulin action. As the obese individual develops impaired intolerance, there is a further reduction in insulin-mediated glucose disposal, which is due primarily to a decrease in glycogen synthesis. However, there is only a small additional impairment in glucose tolerance, because the beta cells are able to further augment their secretion of insulin to counteract the deterioration in insulin sensitivity. The progression of the obese, glucose intolerant person to overt diabetes is heralded by a decline in insulin secretion without any worsening of insulin resistance. The obese diabetic has tipped over the top of Starling's curve of the pancreas and is now on the descending portion. Even though the plasma insulin response is increased compared to nondiabetic control subjects, it is not elevated appropriately for the degree of insulin resistance and there is evidence that there is ~80% of beta-cell functional loss by the time of diagnosis in diabetic subjects. The beta cell insulin response during the OGTT is best represented by the change in plasma insulin over the change in plasma glucose concentration, taking into consideration the degree of insulin resistance for each individual, the so-called Insulin Secretion /Insulin Resistance Index or Disposition Index as shown in Figure 1 below.

Figure 1- Log normalization of the relationship between 2-hour plasma glucose and Insulin Secretion/ Insulin Resistance in subjects with normal glucose tolerance (NGT), impaired glucose tolerance (IGT) and patients with type 2 diabetes (T2DM). There is a linear decline in the insulin secretory capacity with the development of the disease, such that by the time clinical diabetes with hyperglycemia become evident, the loss of beta-cell secretion of insulin is below 5% of NGT controls.

 

The natural history of T2DM described above is consistent with results in humans and monkeys published by other investigators (33-39,42,43,59-61,98,150). In lean subjects with a wide range of glucose tolerance, Reaven et al (42) demonstrated that the progression from normal to impaired glucose tolerance was marked by the development of severe insulin resistance, which was counterbalanced by a compensatory increase in insulin secretion. The onset of T2DM was associated with no (or only slight) further deterioration in tissue sensitivity to insulin.  Rather, insulin secretion declined and the impairment in beta cell function was paralleled by a decrease in glucose tolerance.  A similar sequence of events has been documented prospectively in Pima Indians (34-39,58,60), in Caucasians (1,4,41,42,44,47,59, 162, 219), Pima Indians (34-39,58,60,219), and Pacific Islanders (33,62,220) and, is consistent with the development of T2DM in the rhesus monkeys (48).  As monkeys grow older, they become obese and develop a diabetic condition closely resembling human T2DM. The earliest detectable abnormality in this primate model is a decrease in tissue sensitivity to insulin. Because of a compensatory increase in insulin secretion, the fasting plasma glucose concentration and glucose tolerance remain normal.

 

The studies detailed above indicate that insulin resistance is an early and characteristic feature of the natural history of T2DM in high-risk populations. Overt diabetes develops only in those individuals whose beta cells are unable to appropriately augment their secretion of insulin to compensate for the defect in insulin action. It should be recognized, however, that there are well-described populations with T2DM in whom insulin sensitivity is normal at the onset of diabetes, whereas insulin secretion is severely impaired (81-83). How frequently this occurs in a typical patient with T2DM remains to be determined. This insulinopenic variety of diabetes appears to be more common in African-Americans, elderly subjects, and in lean Caucasians.  In this later group, it is important to exclude type 1 diabetes, since ~10% of Caucasians with older onset diabetes are islet cell antibody and/or GAD positive (220).

 

Primary Hypersecretion of insulin

 

An alternative view to explaining the “state of insulin resistance” is the notion that primary beta cell overstimulation results in insulin hypersecretion.  This leads to the development of obesity and insulin resistance, and then, to beta cell exhaustion (436).  In a model that presupposes beta cell hypersecretion as the initial manifestation of beta cell dysfunction, insulin sensitivity is modulated by insulin secretion.  When beta cell hypersecretion occurs, the responsiveness of insulin-sensitive tissues to insulin is downregulated and, these tissues become insulin resistant.  The latter becomes necessary to maintain normal glucose tolerance, without the adverse outcome of hypoglycemia.  However, considering that beta cell hypersecretion is primary and ‘fixed’, when insulin sensitivity is acutely improved, hypoglycemia would be expected to ensue.  In either case, the demonstration of the existence of a feedback loop that regulates glucose metabolism has made it clear that assessment of the adequacy of beta cell function requires knowledge of both the degree of insulin sensitivity and the magnitude of the insulin response. 

 

When considering the feedback loop governing glucose metabolism, in the face of increased insulin secretion, insulin resistance should develop as a protective measure to maintain normal glucose concentrations without hypoglycemia. This is supported by observations in patients with insulinomas, in whom the risk of hypoglycemia is reduced by the downregulation of insulin action with the development of insulin resistance (437).  Further support for this downregulation of insulin action comes from studies in healthy individuals with normal glucose tolerance in whom insulin resistance developed during 3–5 days of chronic physiologic hyperinsulinemia, achieved by insulin infusion balanced by glucose infusion to prevent hypoglycemia (438).  Higher basal insulin levels have been documented in individuals with obesity and impaired glucose tolerance before the development of T2DM and, identified as a risk factor for diabetes.  However, in these studies, OGTT glucose levels were already higher in those who progressed and could be a confounder. Thus, although studies provide some support for the concept of a potential independent pathogenic role of primary hyperinsulinemia in dysglycemia a stronger, more definitive proof is still missing. 

 

Therefore, while it is clear that T2DM is a heterogeneous condition characterized by beta cell failure, whether beta cell dysfunction or primary hyperinsulinemia is the early event in the pathogenesis of dysglycemia is now up for debate. Although there is sufficient evidence in humans (and animal models) to support the principal defect as being early beta cell dysfunction associated with reduced insulin secretion, it is incumbent on the proponents of the primary hyperinsulinemia hypothesis to undertake further studies to make their case more forcefully. Improved understanding of whichever mechanism underlies beta cell dysfunction should allow us to provide better preventative and therapeutic interventions for T2DM.

 

Delayed Insulin Clearance in Diabetes

 

The role of delayed (or decreased) insulin clearance as a contributor to insulin resistance and to the development of T2DM has been studied.  Insulin availability in the systemic circulation is determined by the rate of beta cell secretion and its rate of hepatic/peripheral/renal clearance.  Insulin levels modulate expression and activity of the insulin receptors in target tissues, which ultimately determines insulin action. The main site of insulin clearance is the liver that removes approximately 50% of endogenous insulin with the remainder being cleared by the kidneys and the skeletal muscle.  Receptor-mediated insulin endocytosis is the primary mechanism by which insulin is removed from the circulation and inactivated.  Upon binding to its receptor, the insulin-receptor complex is internalized through the formation of clathrin-coated vesicles, and is delivered to the endosomes; the acidification of the endosomes then allows the dissociation of the hormone from its receptor and their sorting in different directions. Most of the internalized insulin is next targeted to lysosomes where it is degraded, whereas a smaller fraction remains intact. Both degradation products and intact insulin are segregated in recycling vesicles and released from cell.  Defects in the intracellular processing of insulin have been reported in cells from insulin resistant individuals and reduced insulin clearance has been observed in individuals with IGT.  More recently, it has also been demonstrated that reduced insulin clearance predicts the development of T2DM independently of confounding factors.  There is evidence in animal model of fat-induced insulin resistance supporting the idea that decreased insulin clearance may serve as a compensatory mechanism to alleviate b-cell stress from excessive demand in these conditions of insulin resistance (439).  The extent to which delayed insulin clearance is responsible for the advancement of insulin resistance and its role in the pathogenesis of T2DM remains unknown.    

 

Nutrient-induced Stress on Insulin Secretion

 

There is growing support to the theory that an excess of calorigenic nutrients ingested over time presents the pancreatic islet beta-cells with an overwhelming burden, which might lead to toxic hormonal and metabolic adaptations.  It is well recognized that the short-term effects of glucose, lipids and amino acids perfusing the beta cells in the endocrine pancreas include the stimulation of insulin biosynthesis and secretion.  Excessive exposure to these nutrients is believed to over-stimulate the beta cells with a constant and uninterrupted demand for insulin release and, possibly induce changes in tissue insulin sensitivity.  Chronically, abundant nutritional intake will trigger augmented insulin secretion and insulin resistance, both of which have been shown to contribute to the pathogenesis of T2DM.  Eventually, there is altered glucose sensing and depletion of insulin stores.  The de-differentiation, with beta cell death that follows is likely to play a role in the progression of the disease.  Thus, the traditional concepts of “glucotoxicity” and lipotoxicity”, which defines the process of beta cell deterioration in response to chronic elevation of glucose and lipids in the pericellular milieu, has now been expanded to encompass all nutrients [‘nutri-toxicity”].

 

The biochemical mechanisms underlying beta cell adaptation and failure associated with “nutri-toxicity” are not entirely clear, but appear to be related to oxidative stress.  Various pathways in the cytosol, endoplasmic reticulum [ER] and mitochondria are involved, which tend to affect the insulin secretory capacity of the beta cell.  In conditions of mild-to-moderate “nutri-stress”, such as in overweight/obesity, there is exaggerated basal and nutrient stimulated insulin secretion.  Slightly elevated blood glucose concentration, hyperinsulinemia and insulin resistance become progressively more evident.  Obesity with beta cell failure and T2DM result when there is more advanced and prolonged nutri-stress”.  The metabolic machinery of the beta cell is overwhelmed and, there is mitochondrial and ER dysfunction, which result in severe oxidative stress.  As a consequence, insulin synthesis and secretion become impaired and there is intra- cellular accumulation of toxic metabolites with beta cell de-differentiation and death (440).

 

 

ROLE OF THE ADIPOCYTE IN THE PATHOGENESIS OF T2DM

 

The majority (>80%) of persons with T2DM in the US are overweight (221). Both lean and especially obese persons with T2DM are characterized by day-long elevations in the plasma free fatty acid concentration, which fail to suppress normally following ingestion of a mixed meal or oral glucose load (222). Free fatty acids (FFA) are stored as triglycerides in adipocytes and serve as an important energy source during conditions of fasting. Insulin is a potent inhibitor of lipolysis, and restrains the release of FFA from the adipocyte by inhibiting the enzyme hormone sensitive lipase. In patients with T2DM the ability of insulin to inhibit lipolysis (as reflected by impaired suppression of radioactive palmitate turnover) and reduce the plasma FFA concentration is markedly reduced (17). It is now recognized that chronically elevated plasma FFA concentrations can lead to insulin resistance in muscle and liver (1,4,19,21,22,51,162,223,224) and impair insulin secretion (22,225,226). Thus, elevated plasma FFA levels can cause/aggravate three major pathogenic disturbances that are responsible for impaired glucose homeostasis in individuals with T2DM and the "triumvirate" (muscle, liver, beta cell) was joined by the "fourth musketeer" (227) to form the "disharmonious quartet". In addition to FFA that circulate in plasma in increased amounts, individuals with T2DM and obese individuals without T2DM have increased stores of triglycerides in muscle (228,229) and liver (230,231) and the increased fat content correlates closely with the presence of insulin resistance in these tissues. Triglycerides in liver and muscle are in a state of constant turnover and the metabolites (i.e., fatty acyl CoAs) of intracellular FFAs have been shown to impair insulin action in both liver and muscle (1,4,92). This sequences of events has been referred to as "lipotoxicity" (1,4,22,93). Evidence also has accumulated to implicate "lipotoxicity" as an important cause of beta cell dysfunction (22,93) (see earlier discussion).

 

Adipocyte Inflammation and Insulin Resistance

 

Increased risk of developing T2DM is found in patients who have chronic, low-grade adipocyte inflammation and who are also insulin resistant (441).  The mechanisms of adipose tissue inflammation and the related insulin-resistant state are complex.  Visceral adiposity is known to be highly active in releasing numerous inflammatory cytokines [Adipokines] that are strongly implicated in the genesis of tissue insulin resistance and T2DM.  Adipokines provide an important link between obesity and insulin resistance IR.  Adiponectin is a unique adipokine that is inversely related to the metabolic syndrome, T2DM, and atherosclerosis.  Adiponectin increases fatty acid oxidation while reducing glucose production in liver, and ablation of the adiponectin gene in mice induces insulin resistance and T2DM.  Adiponectin is also anti-inflammatory; it suppresses tumor necrosis factor (TNF) actions in nonalcoholic fatty liver disease and inhibits nuclear factor kappa-beta [NFκB and monocyte adhesion to endothelial cells.  Human resistin is an adipokine secreted by infiltrating inflammatory cells in human adiposity and can stimulate synthesis and secretion of other cytokines in adipocytes and endothelial cells.  Leptin, a well-known adipokine, normally functions centrally to suppress appetite, but most obese patients are leptin resistant and have increased circulating leptin.  In obesity, hyperleptinemia contributes to inflammation through modulation of T-cell and monocyte functions.  A role for retinol-binding protein 4 [RBP-4], a more recently described adipokine has been proposed to be linked to inflammation.

 

Visfatin is a novel adipokine that is increased in obesity, is pro-inflammatory, and has an insulin-mimetic effect via binding to the insulin receptor.  A member of the lipocalin family, lipocalin-2, also known as neutrophil gelatinase–associated lipocalin, modulates inflammation and is another adipokine that is elevated in the adipose tissue of obese mouse models and in the plasma of obese and insulin-resistant humans.  In vitro studies suggest that lipocalin-2 induces insulin resistance in adipocytes and hepatocytes. The plasma level of another member of the lipocalin family, lipocalin-type prostaglandin D synthase, serves as a biomarker of coronary atherosclerosis.  Thus, multiple adipose-secreted factors that are capable of impairing the cellular action of insulin have been suggested to be involved in the development of insulin resistance and facilitate the development of T2DM.

 

It should be recognized that nutritional fatty acids can modulate the inflammatory response, particularly via NFκB activity, and promote insulin resistance.  Further-more, inflammatory modulation of adipocyte differentiation increases free fatty acid release. The mechanisms of free fatty acid-associated insulin resistance include protein kinase C (PKC) activation, endoplasmic reticulum stress, and increased oxidative burden.  Free fatty acids also inhibit insulin receptor substrates [IRSs] and induce insulin resistance in skeletal muscle and liver.  Increased fatty acid flux from adipose tissue to liver causes hepatic insulin resistance by increasing gluconeogenesis, glycogenolysis, and glucose-6-phosphatase expression and activity, and by enhancing lipogenesis and triglyceride synthesis attributable to activation of the transcription factor sterol-CoA regulatory element binding protein.  Finally, free fatty acids cause endothelial insulin resistance and damage by impairing insulin and nitric oxide–dependent signaling, thus contributing to the vascular injury observed in adiposity.

 

The initial insult in obese individuals that triggers inflammation and systemic insulin resistance may occur through recruitment of macrophages and innate immune antigen activation of inflammatory receptors in the membrane.  This can be perpetuated with secretion of chemokines, retention of macrophages in adipose, and secretion of adipokines.  The inflammatory milieu induces adipocyte inflammatory cascades, such as the NFκB pathway, via activation of various kinases, and this modulates adipocyte transcription factors, attenuates insulin signaling, and increases the release of pro-inflammatory adipokines and free fatty acids. Inflammatory attenuation of adipocyte differentiation further exacerbates adipose dysfunction. These paracrine and endocrine adipose inflammatory events induce a systemic inflammatory and insulin-resistant state, favoring the development of T2DM.

 

FFA and Muscle Glucose Metabolism

 

Four decades ago, Randle (232) proposed that increased FFA oxidation restrains glucose oxidation in muscle by altering the redox potential of the cell and by inhibiting key glycolytic enzymes. The excessive FFA oxidation: (i) leads to the intracellular accumulation of acetyl CoA, a potent inhibitor of pyruvate dehydrogenase (PDH), (ii) increases the NADH/NAD ratio, causing a slowing of the Krebs cycle, and (iii) results in the accumulation of citrate, a powerful inhibitor of phosphofructokinase (PFK). Inhibition of PFK leads to the accumulation of glucose-6-phosphate (G-6-P) which in turn inhibits hexokinase II. The block in glucose phosphorylation causes a buildup of intracellular free glucose which restrains glucose transport into the cell via the GLUT4 transporter. The resultant decrease in glucose transport was postulated to account for the impairment in glycogen synthesis, although a direct inhibitory effect of fatty acyl Co-As on glycogen synthase also has been demonstrated (233). This sequence of events via which accelerated plasma FFA oxidation inhibits muscle glucose transport, glucose oxidation, and glycogen synthesis is referred to as the "Randle Cycle" (232). It should be noted that the same scenario would ensue if the FFA were derived from triglycerides stored in muscle (228,229) or from plasma (222).

 

Felber and coworkers (59,159,162,234,235) were amongst the first to demonstrate that in obese non-diabetic and diabetic humans, basal plasma FFA levels and lipid oxidation (measured by indirect calorimetry) are increased and fail to suppress normally after glucose ingestion. The elevated basal rate of lipid oxidation was strongly correlated with a decreased basal rate of glucose oxidation, as well as with reduced rates of glucose oxidation and non-oxidative glucose disposal (glycogen synthesis) following ingestion of a glucose load. Further validation of the Randle Cycle in man has come from studies employing the euglycemic insulin clamp. In normal subjects, physiologic hyperinsulinemia (80-100 μU/ml) causes a 60-70% decline in plasma FFA concentration and a parallel decline in plasma FFA and total body lipid oxidation (18). When Intralipid is infused concomitantly with insulin to maintain or increase the plasma FFA concentration/oxidation, both glucose oxidation and non-oxidative glucose disposal are inhibited in a dose dependent fashion (223). Using magnetic resonance imaging, it has been shown that the FFA-induced inhibition of non-oxidative glucose disposal reflects impaired glycogen synthesis (236). The inhibitory effect of elevated plasma FFA levels can be observed at all plasma insulin concentrations, spawning the physiologic and pharmacologic range (223).

The inhibitory effect of an acute elevation in plasma FFA concentration on muscle glucose metabolism is time dependent. Thus, the earliest (within 2 hours) observed abnormality is a defect in glucose oxidation (237), as would be predicted by operation of the Randle cycle (232). This is followed (between 2-3 hours) by defects in glucose transport and phosphorylation and eventually (after 3-4 hours) by impaired glycogen synthesis.

 

Biochemical/Molecular Basis of FFA-Induced Insulin Resistance

 

The original description of the Randle cycle was formulated based upon experiments performed in rat diaphragm and heart muscle (232). More recent studies performed in human skeletal muscle suggest that mechanisms in addition to those originally proposed by Randle are involved in the FFA-induced insulin resistance. Thus, several groups (236,238,239) have failed to observe a rise in muscle G-6-P and citrate concentrations when insulin-stimulated glucose metabolism was inhibited by an increase in the plasma FFA concentration. Elevated plasma FFA levels also failed to inhibit muscle phosphofructokinase activity. Thus, while increased FFA/lipid oxidation and decreased glucose oxidation are closely coupled, as originally demonstrated by Randle, mechanisms other than product (i.e., elevated intracellular G-6-P and free glucose concentrations) inhibition of the early steps of glucose metabolism must be invoked to explain the defects in glucose transport, glucose phosphorylation and glycogen synthesis.

 

Studies in humans and animals have shown a strong inverse correlation between insulin- stimulated glucose metabolism and increased intramuscular lipid pools, including triglyceride (240-242), diacyl-glycerol (DAG) (243,244), and long chain fatty acyl CoAs (FA-CoA) (245). An acute elevation in plasma FFA concentration leads to an increase in muscle fatty acyl CoA and DAG concentrations. Both long chain fatty acyl CoAs and DAG activate PKC theta (243), which increases serine phosphorylation with subsequent inhibition of IRS-1 tyrosine phosphorylation (246,247). Consistent with this observation, two groups have shown that in human muscle elevated plasma FFA levels inhibit insulin-stimulated tyrosine phosphorylation of IRS-1, the association of the p85 subunit of PI-3 kinase with IRS-1, and activation of PI-3-kinase (248,249). Direct effects of long chain fatty acyl CoAs on glucose transport (250), glucose phosphorylation (251), and glycogen synthase (233) also have been demonstrated in muscle. Lastly, increased muscle ceramide levels (secondary to increased long chain fatty acyl CoAs) have been shown to interfere with glucose transport and to inhibit glycogen synthase in muscle via activation of PKB (252). In summary, elevated plasma FFA concentrations can induce insulin resistance in muscle via multiple mechanisms involving alterations in a variety of intracellular lipid signaling molecules which exert their inhibitory effects on multiple steps (insulin signal transduction system, glucose transport, glucose phosphorylation, glycogen synthase, pyruvate dehydrogenase, Krebs cycle) involved in glucose metabolism.

 

Fatty Liver Disease in T2DM

 

As the epidemics of obesity increases worldwide in conjunction with T2DM, there is a parallel and proportionate increase in the prevalence of nonalcoholic fatty liver disease (NAFLD).  A subtype of NAFLD, which can be characterized as nonalcoholic steato-hepatitis (NASH) is a potentially progressive liver disease that can lead to cirrhosis, hepatocellular carcinoma, liver transplantation, and death.  NAFLD is also associated with extrahepatic manifestations such as chronic kidney disease, cardiovascular disease and sleep apnea.  Despite this important burden, we are only beginning to understand its pathogenesis and the contribution of environmental and genetic factors to the risk of developing the progressive course of fatty liver disease.  Of interest, however, despite the fact that the risk of liver-related mortality and the advancement to liver fibrosis are increased in patients with NAFLD, the leading cause of death is cardiovascular disease (442-443).

 

NAFLD and NASH are stages of fatty liver disease that are associated with obesity, insulin resistance, T2DM, hypertension, hyperlipidemia, and metabolic syndrome.  In these individuals, a net retention of lipids within hepatocytes, mostly in the form of triglycerides, is a prerequisite for the development of fatty liver disease.  The primary metabolic abnormality leading to lipid accumulation (steatosis), however, is not well understood, but it could potentially result from insulin resistance and alterations in the uptake, synthesis, degradation or secretory pathways of hepatic lipid metabolism. Insulin resistance represents the most reproducible factor in the development of fatty liver disease.  There is also some evidence that lipids synthesis de novo”, a process derived from excess non-utilized carbohydrates accumulated in hepatocytes contributes to the intracellular lipid pool.  Once an excessive amount of lipids accumulate inside the hepatocytes, a steatotic liver develops.  This makes the cellular architecture of the liver vulnerable to further injury, when challenged by additional insults. There is a presumption that progression from simple, uncomplicated steatosis to steato-hepatitis to advanced fibrosis results from two operating “hits” due to: i) insulin resistance with further accumulation of fat within hepatocytes, and ii) generation of reactive oxygen species due to lipid peroxidation with cytokine production and Fas ligand induction.  The oxidative stress and lipid peroxidation are key factors in the development and progression from steatosis to more advanced stages of liver damage.  In addition, this sequence of events reflects similar systemic processes, which worsen tissue insulin resistance with impairment of insulin secretion and accelerated atherogenesis, related primarily to the pro-inflammatory state (442).

 

FFA and Blood Flow

 

Insulin is a vaso-dilatory hormone and the stimulatory effect of insulin on muscle glucose metabolism has been shown to result from: (i) a direct action of insulin to augment muscle glucose metabolism, and (ii) increased blood flow to muscle (253,254). The vaso-dilatory effect of insulin is mediated via the release of nitric oxide from the vascular endothelium (255). In insulin resistant conditions, such as obesity and T2DM, some investigators have suggested that as much as half of the impairment in insulin-mediated whole body and leg muscle glucose uptake is related to a defect in insulin's vaso-dilatory action (253,254), although the link between insulin-mediated vasodilation and increased blood flow, as well as the underlying mechanisms have been challenged by others (256, 256A). More recent studies employed contrast-enhanced ultrasonography using 1-methyl-xantine to demonstrate that insulin infusion promotes capillary recruitment in healthy individuals. These data have suggested that there is a time-dependent effect of insulin on regional blood flow redistribution with capillary pre-sphincter relaxation preceding vasodilation and consequent increase in skeletal muscle glucose metabolism (256B). These observations also provided a partial explanation for the discrepant findings reported on the topic of insulin, fatty acids and vasodilatation.

 

Because T2DM and obesity are insulin resistant states characterized by day-long elevation in the plasma FFA concentration (222) and impaired endothelium dependent vasodilation (253), investigators have examined the effect of increased plasma FFA levels on limb blood flow and muscle glucose uptake (257,258). In healthy, non-diabetic subjects an acute physiologic increase in plasma FFA concentration inhibited metha-choline (endothelium dependent) but not nitroprusside (endothelium independent) stimulated blood flow in association with an impairment in insulin-stimulated muscle glucose disposal. In subsequent studies, the inhibitory effect of FFA on insulin-stimulated leg blood flow was shown to be associated with decreased nitric oxide availability (259). FFA elevation also inhibits nitric oxide production in endothelial cell cultures by decreasing nitric oxide synthase activity (259). Since the IRS-1/PI-3 kinase signal transduction pathway is involved in the regulation of nitric oxide synthase activity (260), one could hypothesize that FFA-induced inhibition of the insulin signal transduction pathway is responsible for the blunted vaso-dilatory response to the hormone.

 

FFA and Hepatic Glucose Metabolism

 

The liver plays a pivotal role in the regulation of glucose metabolism (1,4,6,11,16,205). Following carbohydrate ingestion, the liver suppresses its basal rate of glucose production and takes up approximately one-third of the glucose in the ingested meal (12,24,25,205).

Collectively, suppression of glucose production and augmentation of hepatic glucose uptake account for the maintenance of nearly one-half of the rise in plasma glucose concentration following ingestion of a carbohydrate meal.  Hepatic glucose production is regulated by a number of factors, of which insulin (inhibits) and glucagon and FFA (stimulate) are the most important. In vitro studies have demonstrated that plasma FFA are potent stimulators of endogenous glucose production and do so by increasing the activity of pyruvate carboxylase and phosphoenolpyruvate carboxy-kinase, the rate limiting enzymes for gluconeogenesis (261,262).  FFA also enhances the activity of glucose-6- phosphatase, the enzyme that ultimately controls the release of glucose by the liver (263).

 

In normal subjects, increase plasma FFA levels stimulate gluconeogenesis (264,265), while a decrease in plasma FFA concentration reduces gluconeogenesis (264). It has been shown that a significant portion of the suppressive effect of insulin on hepatic glucose production is mediated via inhibition of lipolysis and a reduction in circulating plasma FFA concentrations (16,266,267).  Moreover, FFA infusion in normal humans under conditions that simulate the diabetic state (268) and in obese insulin-resistant subjects (269) enhances hepatic glucose production, most likely secondarily to stimulation of gluconeogenesis.  In subjects with T2DM, the fasting plasma FFA concentration and lipid oxidation rate are increased and are strongly correlated with both the elevated fasting plasma glucose concentration and basal rate of hepatic glucose production (18,51,59,162,190,270). The relationship between elevated plasma FFA concentration, FFA oxidation, and hepatic glucose production in obesity and T2DM is explained as follows: (i) increased plasma FFA levels, by mass action, augment FFA uptake by hepatocytes, leading to accelerated lipid oxidation and accumulation of acetyl CoA. The increased concentration of acetyl CoA stimulates pyruvate carboxylase, the rate limiting enzyme in gluconeogenesis (261,262), as well as glucose-6-phosphatase, the rate-controlling enzyme for glucose release from the hepatocyte (263); (ii) the increased rate of FFA oxidation provides a continuing source of energy (in the  form of ATP) and reduced nucleotides (NADH) to drive gluconeogenesis; (iii) elevated plasma FFA induce hepatic insulin resistance by inhibiting the insulin signal transduction system (244- 248). In patients with T2DMthese deleterious effects of elevated plasma FFA concentrations occur in concert with increased plasma glucagon levels (181,190,271), increased hepatic sensitivity to glucagon, and increased hepatic uptake of circulating gluconeogenic precursors.

 

The Role of Gut Microbiome

 

Recently the potential role of the gut microbiome in metabolic disorders such as obesity and T2DM has been identified (444).  Obesity is associated with changes in the composition of the intestinal microbiota, and the obese microbiome seems to be more efficient in harvesting energy from the diet.  Lean male donor fecal microbiota transplantation (FMT) in males with the metabolic syndrome resulted in a significant improvement in insulin sensitivity in conjunction with an increased intestinal microbial diversity, including a distinct increase in butyrate-producing bacterial strains.  Such differences in gut microbiota composition might function as early diagnostic markers for the development of T2DM in high-risk patients.  Products of intestinal microbes such as butyrate may induce beneficial metabolic effects through enhancement of mitochondrial activity, prevention of metabolic endotoxemia, and activation of intestinal gluconeogenesis via different routes of gene expression and hormone regulation. There is currently an enormous effort in trying to better understand, amongst other things, whether bacterial products (like butyrate) have the same effects as the intestinal bacteria that produce it, in order to ultimately pave the way for more successful interventions for obesity and T2DM.   Rapid development of the currently available techniques, including the use of fecal transplantations, has already shown promising results, so there is hope for novel therapies based on the microbiota in the future.

 

Summary: FFA and the Pathogenesis of Obesity and T2DM

 

n obese individuals and in the majority (>80%) of subjects with T2DM, there is an expanded fat cell mass and the adipocytes are resistant to the anti-lipolytic effects of insulin (18). Most individuals with obesity or T2DM are characterized by visceral adiposity (272) and visceral fat cells have a high lipolytic rate, which is especially refractory to insulin (273). Not surprisingly, both T2DM and obesity are characterized by an elevation in the mean day-long plasma FFA concentration. Elevated plasma FFA levels, as well as increased triglyceride/fatty acyl CoA content in muscle, liver, and beta cell, lead to the development of muscle/hepatic insulin resistance and impaired insulin secretion.

 

THE OMNIOUS OCTET

Figure 2. Summary of the Eight Principal Mechanisms Contributing to Hyperglycemia in Patients with Type 2 Diabetes

 

The eight principle known causes leading to hyperglycemia through the pathogenesis of T2DM are summarized in Figure 2. It is already established that decreased peripheral glucose uptake combined with augmented endogenous (hepatic) glucose production are characteristic features of insulin resistance. Increased lipolysis with accumulation of intermediary lipid metabolites contributes to further enhance glucose output while reducing peripheral utilization. Compensatory insulin secretion by the pancreatic beta-cells eventually reaches a maximum and, then it progressively deteriorates. Concomitantly, there is inappropriate release of glucagon from the pancreatic alpha-cells, particularly in the post- prandial period. It has been postulated that both impaired insulin and excessive glucagon secretion in T2DM are facilitated by the “incretin defect”, defined primarily as inadequate response of the gastrointestinal “incretin” hormones to meal ingestion in addition to islet-cell resistance to the potentiating action on insulin-secretion by these gastrointestinal peptides. Moreover, considering that hypothalamic insulin resistance (central nervous system) with an elevated sympathetic drive, typically seen in patients with T2DM also impair the ability of circulating insulin to suppress glucose production. The fact that renal tubular glucose reabsorption capacity is enhanced in diabetic patients also contributes to the development and maintenance of chronic hyperglycemia.  Thus, the time has arrived to advance the concept from the “triumvirate” to the “omnious octet” (4A). Further, recent observations have recognized that a chronic low-grade inflammation with activation of the immune system are involved in the pathogenesis of obesity-related insulin resistance and T2DM (4D). Adipose tissue, liver, muscle and pancreas are themselves sites of inflammation in presence of obesity. Infiltration of macrophages and other immune cells as well as the presence of pro-inflammatory cytokines in these tissues has been associated with insulin resistance and beta-cell impairment. The possibility that endothelial dysfunction and changes in vascular capillary permeability affect peripheral insulin action has also been raised (4E). These pathogenic mechanisms must be taken into account when deciding for the treatment of hyperglycemia in patients with T2DM.

 

CELLULAR MECHANISMS OF INSULIN RESISTANCE

 

The stimulation of glucose metabolism by insulin requires that the hormone must first bind to specific receptors that are present on the cell surface of all insulin target tissues (1,274-277). After insulin has bound to and activated its receptor, "second messengers" are generated and these second messengers initiate a series of events involving a cascade of phosphorylation- de-phosphorylation reactions (1,274-280) that eventually result in the stimulation of intracellular glucose metabolism. The initial step in glucose metabolism involves activation of the glucose transport system, leading to influx of glucose into insulin target tissues, primarily muscle (1,281,282). The free glucose, which has entered the cell, subsequently is metabolized by a series of enzymatic steps that are under the control of insulin. Of these, the most important are glucose phosphorylation (catalyzed by hexokinase), glycogen synthase (which controls glycogen synthesis), and phosphofructokinase (PFK) and PDH (which regulate glycolysis and glucose oxidation, respectively).

 

Insulin Receptor/Insulin Receptor Tyrosine Kinase

 

The insulin receptor is a glycoprotein consisting of two alpha subunits and two beta subunits linked by disulfide bonds (1,274-277). The alpha subunit of the insulin receptor is entirely extracellular and contains the insulin-binding domain. The beta subunit has an extracellular domain, a transmembrane domain, and an intracellular domain that expresses insulin- stimulated kinase activity directed towards its own tyrosine residues (1,274-277). Insulin receptor phosphorylation of the beta subunit, with subsequent activation of insulin receptor tyrosine kinase, represents the first step in the action of insulin on glucose metabolism (274- 277). Mutagenesis experiments have shown that insulin receptors devoid of tyrosine kinase activity are completely ineffective in mediating insulin stimulation of cellular metabolism (283,284). Similarly, mutagenesis of any of the three major phosphorylation sites (at residues 1158, 1163, and 1162) impairs insulin receptor kinase activity, resulting in a decrease in the acute metabolic and growth promoting effects of insulin (283,285).

 

Insulin Receptor Signal Transduction

 

Following activation, insulin receptor tyrosine kinase phosphorylates specific intracellular proteins, of which at least nine have been identified (282). Four of these belong to the family of insulin-receptor substrate proteins: IRS-1, IRS-2, IRS-3, IRS-4 (the others include Shc, Cbl, Gab-1, p60dok, and APS). In muscle IRS-1 serves as the major docking protein that interacts with the insulin receptor tyrosine kinase and undergoes tyrosine phosphorylation in regions containing amino acid sequence motifs (YXXM or YMXM).  When phosphorylated, these serve as recognition sites for proteins containing src-homology 2 (SH2) domains (where y = tyrosine, M = methionine, and x - any amino acid) (274,275).  Mutation of these specific tyrosines severely impairs the ability of insulin to stimulate glycogen synthesis and DNA synthesis, establishing the important role of IRS-1 in insulin signal transduction (282). In liver, IRS-2 serves as the primary docking protein that undergoes tyrosine phosphorylation and mediates the effect of insulin on hepatic glucose production, gluconeogenesis and glycogen formation (287). In adipocytes, Cbl represents another substrate which is phosphorylated following its interaction with the insulin receptor tyrosine kinase, which is required for stimulation of GLUT 4 translocation.

 

Phosphorylation of Cbl occurs when the CAP/Cbl complex associates with flotillin in caveolae, or lipid rafts, containing insulin receptors (288,289).

 

In muscle, the phosphorylated tyrosine residues on IRS-1 mediate an association between the two SH2 domains of the 85-kDa regulatory subunit of phosphatidylinositol 3-kinase (PI3-kinase), leading to activation of the enzyme (274-284,290,291). PI3-kinase is a heterodimeric enzyme comprised of an 85-kDa regulatory subunit and a 110-kDa catalytic subunit. The latter catalyzes the 3-prime phosphorylation of phosphatidylinositol (PI), PI-4-phosphate, and PI-4,5- diphosphate, resulting in the stimulation of glucose transport (274-277). Activation of PI3-kinase by phosphorylated IRS-1 also leads to activation of glycogen synthase (274,275), via a process that involves activation of PKB/Akt and subsequent inhibition of kinases such as GSK-3 (292) and activation of protein phosphatase 1 (PP1) (293). Inhibitors of PI3-kinase impair glucose transport (274-277,294) by interfering with the translocation of GLUT 4 transporters from their intracellular location (281,282) and block the activation of glycogen synthase (295) and hexokinase (HK)-II expression (296). The action of insulin to increase protein synthesis and inhibit protein degradation also is mediated by PI-3 kinase and involves the activation of mTOR (297,298). mTOR controls translation machinery by phosphorylation and activation of p70 ribosomal S6 kinase (p70rsk) (297) and phosphorylation of initiation factors (299). Insulin also promotes hepatic triglyceride synthesis via increasing the transcription factor steroid regulatory element-binding protein (SREBP)-1c (300), and this lipogenic effect of insulin also appears to be mediated via the PI3-kinase pathway (274).

 

Other proteins with SH2 domains, including the adapter protein Grb2 and Shc, also interact with IRS-1 and become phosphorylated following exposure to insulin (274-276,301). Grb2 and Shc serve to link IRS-1/IRS-2 to the mitogen-activated protein (MAP) signaling pathway, which plays an important role in the generation of transcription factors (274,275). Following the interaction between IRS-1/IRS-2 and Grb2 and Shc, Ras is activated, leading to the stepwise activation of Raf, MEK, and ERK.  Activated ERK then translocates into the nucleus of the cell, where it catalyzes the phosphorylation of transcription factors.  These promote cell growth, proliferation, and differentiation (274-276,301-303).  Blockade of MAP kinase pathway prevents stimulation of cell growth by insulin but has no effect on the metabolic actions of the hormone (304-306).

 

Under anabolic conditions insulin stimulates glycogen synthesis by simultaneously activating glycogen synthase and inhibiting glycogen phosphorylase (307-309). The effect of insulin is mediated via the PI3 kinase pathway which inactivates kinases, such as glycogen synthase kinase-3 and activates phosphatases, particularly protein phosphatase 1 (PP1). It is believed that PP1 is the primary regulator of glycogen metabolism (307-310). In skeletal muscle, PP1 associates with a specific glycogen-binding regulatory subunit, causing the activation [de-phosphorylation] of glycogen synthase; PP1 also inactivates [phosphorylates] glycogen phosphorylase.  The precise steps that link insulin receptor tyrosine kinase/PI 3-kinase activation to the stimulation of PP1 have yet to be defined. Some evidence suggests that p90 ribosomal S6- kinase may be involved in the activation of glycogen synthase (274). Akt also has been shown to phosphorylate and thus inactivate GSK-3 (292). This decreases glycogen synthase phosphorylation, leading to the enzyme activation (292). A number of studies have convincingly demonstrated that inhibitors of PI3-kinase also inhibit glycogen synthase activity and abolish glycogen synthesis (274,293,310). From the physiological standpoint, it makes sense that activation of glucose transport and glycogen synthase should be linked to the same signaling mechanism to provide a coordinated stimulation of intracellular glucose metabolism.

 

Insulin Signal Transduction Defects in T2DM

 

Both receptor and post-receptor defects have been shown to contribute to insulin resistance in individuals with T2DM. Some, but not all studies have demonstrated a modest 20-30% reduction in insulin binding to monocytes and adipocytes from patients with T2DM (1,311- 316). This reduction is due to a decreased number of insulin receptors without change in insulin receptor affinity. In addition to the decreased number of cell-surface receptors, a variety of defects in insulin receptor internalization and processing have been described (314,315).

 

However, some caution should be employed in interpreting these studies. Muscle and liver, not adipocytes, represent the major tissues responsible for the regulation of glucose homeostasis in vivo and insulin binding to solubilized receptors obtained from skeletal muscle biopsies and liver has been shown to be normal in obese and lean diabetic individuals when expressed per milligram of protein (312,313,316-318). Moreover, a decrease in insulin receptor number cannot be demonstrated in over half of subjects with T2DM (319,320), and it has been difficult to demonstrate a correlation between reduced insulin binding and the severity of insulin resistance (321,322). The insulin receptor gene has been sequenced in a large number of patients with T2DM from diverse ethnic populations using denaturing-gradient gel electrophoresis or single- stranded conformational polymorphism analysis, and, with very rare exceptions (323), physiologically significant mutations in the insulin receptor gene have not been observed (324,325). This excludes a structural gene abnormality in the insulin receptor as a cause of common T2DM.

 

Insulin receptor tyrosine kinase activity has been examined in a variety of cell types (skeletal muscle, adipocytes, hepatocytes, and erythrocytes) from normal-weight and obese diabetic subjects. Most (278,301,312,313,320,326-328), but not all (317,329) investigators have found reduced tyrosine kinase activity that cannot be explained by alterations in insulin receptor number or insulin receptor binding. However, near-normalization of the fasting plasma glucose concentration, (by weight loss) has been reported to correct the defect in insulin receptor tyrosine kinase activity (330). This observation suggests that the defect in tyrosine kinase is acquired and results from some combination of hyperglycemia, defective intracellular glucose metabolism, hyperinsulinemia, and insulin resistance - all of which improved after weight loss. A glucose-induced reduction in insulin receptor tyrosine kinase activity has been demonstrated in rat fibroblast culture in vitro (331). Insulin receptor tyrosine kinase activity assays are performed in vitro, and the results of these assays could provide misleading information with regard to insulin receptor function in vivo. To circumvent this problem, investigators have employed the euglycemic hyperinsulinemic clamp in combination with muscle biopsies and anti- phospho-tyrosine immunoblot analysis (301). Such analysis yields a "snap shot" of the insulin- stimulated tyrosine phosphorylation state of the receptor in vivo. The results of these studies have demonstrated a substantial decrease in insulin receptor tyrosine phosphorylation in both obese nondiabetic and subjects with T2DM (301,328). When insulin-stimulated insulin receptor tyrosine phosphorylation was examined in normal-glucose-tolerant or impaired- glucose-tolerant individuals at high risk of developing T2DM, a normal increase in tyrosine phosphorylation of the insulin receptor has been observed (332). These observations are consistent with the concept that impaired insulin receptor tyrosine kinase activity in patients with T2DM is acquired secondarily to hyperglycemia or some other metabolic disturbance.

 

A physiologic increase in the plasma insulin concentration stimulates tyrosine phosphorylation of the insulin receptor and IRS-1 in lean healthy subjects to 150-200% of basal values (280,301,328,332,333). In obese subjects without T2DM, the ability of insulin to activate these two early insulin receptor signaling events in muscle is reduced, while in subjects with T2DM insulin has no significant stimulatory effect on either insulin receptor or IRS-1 tyrosine phosphorylation (301). The association of p85 protein and PI3-kinase activity with IRS-1 also is greatly reduced in obese non-diabetic and subjects with T2DM compared to lean healthy subjects (301,328- 334). Insulin also failed to increase the association of the p85 subunit of PI3-kinase with IRS-2 in muscle, indicating that T2DM is characterized by a combined defect in IRS-1 and IRS-2 function (301,328). The decrease in insulin stimulation of the association of the p85 regulatory subunit of PI3-kinase with IRS-1 is closely correlated with the impairment in muscle glycogen synthase activity and in vivo insulin-stimulated glucose disposal (301). Defective regulation of PI3-kinase gene expression by insulin also has been demonstrated in skeletal muscle and adipose tissue of subjects with T2DM (335). In animal models of diabetes, an 80% decrease in IRS-1 phosphorylation and a greater than 90% reduction in insulin-stimulated PI3-kinase activity have been reported (336).

 

In the insulin resistant, normal glucose tolerant offspring of two parents with T2DM, IRS-1 tyrosine phosphorylation and the association of p85 protein/PI3-kinase activity with IRS-1 are markedly decreased despite normal tyrosine phosphorylation of the insulin receptor; these insulin signaling defects are correlated closely with the severity of insulin resistance, measured with the euglycemic insulin clamp technique (332). In summary, a defect in the association of PI3-kinase with IRS-1 and its subsequent activation appears to be a characteristic abnormality in T2DM, is closely correlated with in vivo muscle insulin resistance, and is unrelated to a disturbance in insulin receptor tyrosine phosphorylation. Several groups (337,338) have reported that a common mutation in the IRS-1 gene (Gly 972 Arg) is associated with T2DM, insulin resistance, and obesity, but the physiologic significance of this mutation remains to be established (339).

 

The profound insulin resistance of the PI3-kinase signaling pathway contrasts markedly with the ability of insulin to stimulate MAP kinase pathway activity in insulin-resistant individuals with T2DM and in individuals with obesity without T2DM (301,328). Hyperinsulinemia increases MEK1 activity and ERK1/2 phosphorylation and activity to the same extent in lean healthy individuals as in patients with insulin resistance and obesity without T2DM and patients with T2DM (301,328). This finding of selective insulin resistance is similar to that recently observed in vasculature of Zucker fatty rats (340). Two possible reasons for this difference are alternate insulin signaling pathways and differential signal amplification. With regard to the former, the MAP kinase pathway can be activated either through Grb2/Sos interaction with IRS-1/IRS-2 or with Shc. Because IRS-1 tyrosine phosphorylation is dramatically reduced in the diabetics, it is possible that insulin activation of the MAP kinase pathway in vivo primarily occurs through Shc activation. There is evidence from in vitro studies to support this concept (341). Like ERK and MEK activity, insulin increased Shc phosphorylation to the same extent in lean and obese nondiabetic and subjects with T2DM (301). These results indicate that, in T2DM, insulin induces sufficient activation of the insulin receptor tyrosine kinase to increase Shc phosphorylation normally. It also is possible that differential signal amplification in the PI3-kinase and MAP kinase pathways can explain their differing susceptibilities to the effects of insulin resistance.

 

Maintenance of insulin stimulation of the MAP kinase pathway in the presence of insulin resistance in the PI3-kinase pathway may be important in the development of insulin resistance. ERKs can phosphorylate IRS-1 on serine residues (342), and serine phosphorylation of IRS-1 and the insulin receptor itself has been implicated in de-sensitization insulin receptor signaling (343). Continued ERK activity, when IRS-1 function already is impaired, could lead to a worsening of insulin resistance. Thus, subjects with T2DM or obesity have inappropriately high MAP kinase activity. One also could postulate that insulin resistance in the metabolic (PI3- kinase) pathway, with its compensatory increase in beta cell function and hyperinsulinemia, leads to excessive stimulation of the MAP kinase pathway in vascular tissue (301,302). This would result in the proliferation of vascular smooth muscle cells, increased collagen formation, and increased production of growth factors and inflammatory cytokines, possibly explaining the accelerated rate of atherosclerosis in individuals with T2DM (340A, 340B).

 

Glucose Transport

 

Activation of the insulin signal transduction system in insulin target tissues leads to the stimulation of glucose transport. The effect of insulin is brought about by the translocation of a large intracellular pool of glucose transporters (associated with low-density microsomes) to the plasma membrane (281,282,344). There are five major, different facilitative glucose transporters with distinctive tissue distributions (281,282,345,346) (Table 1). GLUT4, the transporter regulated by insulin is found in insulin-sensitive tissues (muscle and adipocytes), has a Km of ~5 mmol/l, which is close to that of the plasma glucose concentration, and is associated with HK-II (347- 349).  In adipocytes and muscle, its concentration in the plasma membrane increases markedly after exposure to insulin, and this increase is associated with a reciprocal decline in the intracellular GLUT4 pool.  GLUT1 represents the predominant glucose transporter in the insulin- independent tissues (brain and erythrocytes), but also is found in muscle and adipocytes. It is located primarily in the plasma membrane, where its concentration changes little after the addition of insulin. It has a low Km (~1 mmol/l) and is well suited for its function, which is to mediate basal glucose uptake. It is found in association with HKI (347-349).  GLUT2 predominates in the liver and pancreatic beta-cells, where it is found in association with a specific hexokinase, HKIV (347-350).  In the beta-cell, HKIV is referred to as gluco-kinase (350,351). GLUT2 has a high Km, (~15-20 mmol/l) and, as a consequence, the glucose concentration in cells expressing this transporter rises in direct proportion to the increase in plasma glucose concentration. This characteristic allows these cells to respond as glucose sensors.  In summary, each tissue has a specific glucose transporter and associated hexokinase, which allows it uniquely to carry out its specialized function to maintain whole-body glucose economy.

 

Table 1. Classification of Glucose Transport and HK Activity According to their Tissue Distribution and Functional Regulation

Organ

Glucose transporter

HK computer

Classification

Brain

GLUT1

HK-I

Glucose dependent

Erythrocyte

GLUT1

HK-I

Glucose dependent

Adipocyte

GLUT4

HK-II

Insulin dependent

Muscle

GLUT4

HK-II

Insulin dependent

Liver

GLUT2

HK-IVL

Glucose sensor

GK beta-cell

GLUT2

HK-IVB (glucokinase)

Glucose sensor

Gut

GLUT3-symporter

-

Sodium dependent

Kidney

GLUT3-symporter

-

Sodium dependent

 

Glucose transport activity in patients with T2DM uniformly has been found to be decreased in adipocytes (281,282,320,351,352) and muscle (281,282,354-356). In adipocytes from humans with T2DM and rodent models of diabetes, there is a severe reduction in GLUT4 mRNA and protein, and the ability of insulin to elicit a normal translocation response and to activate the GLUT4 transporter after its insertion into the cell membrane is impaired (281,282,320,353,357). In contrast, muscle tissue obtained from lean and obese subjects with T2DM exhibits normal or increased levels of GLUT4 mRNA expression and normal levels of GLUT4 protein (358-361). Moreover, acute (2- 4-h) physiological hyperinsulinemia does not increase the number of GLUT4 transporters in muscle in either healthy subjects or subjects with T2DM (358-361). Several studies have demonstrated an increase in muscle GLUT4 mRNA levels in response to insulin in control subjects (333,360), but not in subjects with T2DM (360), suggesting insulin resistance at the level of gene transcription. However, the physiological significance of the blunted increase in muscle GLUT4 mRNA levels in subjects with T2DM is unclear, since both basal and insulin- stimulated GLUT4 protein levels are normal. Large populations of subjects with T2DM have been screened for mutations in the GLUT4 gene (362,363). Such mutations are very uncommon and, when detected, have been of questionable physiologic significance.

 

The results summarized above indicate that the gene (GLUT4) encoding the major insulin- responsive glucose transporter and its transcriptional/translational regulation are not impaired in T2DM. However, in contrast to the normal expression of GLUT4 protein and mRNA in muscle of subjects with T2DM, every study that has examined adipose tissue has reported reduced basal and insulin-stimulated GLUT4 mRNA levels, decreased GLUT4 transporter number in all subcellular fractions, diminished GLUT4 translocation, and impaired intrinsic activity of GLUT4 (281,282,353,361,364). These observations demonstrate that GLUT4 expression in humans is subject to tissue-specific regulation. Although insulin does not increase GLUT4 expression in muscle, it stimulates the translocation of GLUT4 transporters from their intracellular location to the cell membrane (354,365,366). In humans with T2DM, the ability of insulin to stimulate GLUT4 translocation in muscle is impaired (354,367). Using a novel triple- tracer technique, the in vivo dose-response curve for the action of insulin on glucose transport in forearm skeletal muscle has been examined in nondiabetic and subjects with T2DM (368-370). Insulin-stimulated inward muscle glucose transport is severely impaired in subjects with T2DM who are studied under euglycemic conditions. The defect in glucose transport cannot be overcome by repeating the insulin clamp at each subject's normal fasting glucose (hyperglycemia) level. Since the number of GLUT4 transporters in the muscle of subjects with T2DM is normal (358-361), impaired GLUT4 translocation (281,354,367) and decreased intrinsic activity of the glucose transporter (366,371) must be responsible for the defect in muscle glucose transport. Impaired in vivo muscle glucose transport in T2DM also has been demonstrated using MRI (372) and PET (373).

 

Glucose Phosphorylation

 

Glucose phosphorylation and glucose transport are tightly coupled phenomena (374). Isozymes of hexokinase (HKI-HKIV) catalyze the first committed intracellular step of glucose metabolism, the conversion of glucose to glucose-6-phosphate (G-6-P) (347-350,375) (Table 1). HKI, HKII, and HKIII are single-chain peptides that have a number of properties in common, including a very high affinity for glucose and product inhibition by G-6-P. HKIV, also called gluco-kinase, has a lower affinity for glucose and is not inhibited by G-6-P. Gluco-kinase (HKIVB) is believed to be the glucose sensor in the beta-cell, while HKIVL plays an important role in the regulation of hepatic glucose metabolism.

 

In both rat (375-377) and human (333,348,378-380) skeletal muscle, HKII transcription is regulated by insulin. HKI also is present in human skeletal muscle, but it is not regulated by insulin (378). In response to physiological euglycemic hyperinsulinemia, HKII cytosolic activity, protein content, and mRNA levels increase by 50-200% in healthy non-diabetic subjects (378,380) and this is associated with the translocation of hexokinase II from the cytosol to the mitochondria (381). In contrast, insulin has no effect on HK-I activity, protein content, or mRNA levels (378).

 

In forearm muscle, insulin-stimulated glucose transport (measured with the triple tracer technique) has been shown to be markedly impaired in lean subjects with T2DM (370). However, since the rate of intracellular glucose phosphorylation was impaired to an even greater extent, insulin caused an increase in the intracellular free glucose concentration. By performing the insulin clamp at each subject’s normal level of fasting hyperglycemia, normal rates of whole- body glucose disposal and a normal rate of glucose influx into muscle was elicited. However, the rate of intracellular glucose phosphorylation increased only modestly; consequently, there was a dramatic rise in the free glucose concentration within the intracellular space that is accessible to glucose. These observations indicate that in individuals with T2DM, while both glucose transport and glucose phosphorylation are severely resistant to the action of insulin, impaired glucose phosphorylation (HKII) appears to be the rate-limiting step for insulin action. A similar pattern of impaired muscle glucose phosphorylation and transport is present in the insulin-resistant, normal glucose-tolerant offspring of two diabetic parents (382). These results are consistent with dose-response studies using PET to evaluated glucose phosphorylation and transport in skeletal muscle of subjects with T2DM (373). They also are consistent with 31P-NMR studies (383) which demonstrate that, during hyperinsulinemia, muscle G-6-P concentrations decline in subjects with T2DM versus control subjects. However, subsequent studies using 31P-NMR in combination with 1-14C-glucose suggest that the defect in insulin-stimulated muscle glucose transport exceeds the defect in glucose phosphorylation and is responsible for the decline in muscle glucose-6-P concentration (372). Because of methodologic differences, the results of the triple tracer (370) and MRI (372) studies cannot be reconciled at present. Nonetheless, observations from these studies are consistent in demonstrating that the defects in glucose phosphorylation and glucose transport in muscle are established early in the natural history of T2DM and cannot be explained by glucose toxicity (91). Clear evidence that HKII activity is crucial for glucose uptake derives from studies in transgenic mice who overexpress HKII. In this model, HKII over-expression increased both insulin- and exercise-stimulated muscle glucose uptake (384).

 

In healthy nondiabetic subjects, physiologic hyperinsulinemia for as little as 2-4 hours increases muscle HKII activity, gene transcription, and translation (333,378). In lean subjects with T2DM insulin-stimulated HKII activity and mRNA levels are markedly reduced compared to controls (383,385). Decreased basal muscle HKII activity and mRNA levels (385) and impaired insulin-stimulated HKII activity (379,380,386,387) in subjects with T2DM have been reported by other investigators. A decrease in insulin-stimulated muscle HKII activity also has been described in individuals with IGT (388). Because of its central role in insulin-mediated muscle glucose metabolism, several groups have looked for point mutations in the HKII gene in individuals with T2DM (388-390). Although several nucleotide substitutions have been found, none have been located close to the glucose and ATP binding sites and none have been associated with insulin resistance. Thus, an abnormality in the HKII gene is unlikely to explain the inherited insulin resistance in common variety T2DM.

 

Glycogen Synthesis

 

After glucose is phosphorylated by hexo-kinase II, it either can be converted to glycogen or enter the glycolytic pathway. Of the glucose that enters the glycolytic pathway, ~90% is oxidized. At low physiologic plasma insulin concentrations, glycogen synthesis and glucose oxidation are of approximately equal quantitative importance. With increasing plasma insulin concentrations, glycogen synthesis predominates (18,391). If the rate of glucose oxidation (determined by indirect calorimetry) is subtracted from the rate of whole-body insulin-mediated glucose disposal (determined from the insulin clamp), the difference represents non-oxidative glucose disposal (or glucose storage) (17,360), which primarily reflects glycogen synthesis (1,4,162,216,392).  Glucose conversion to lipid accounts for <5% of total body glucose disposal (18,198,199) and, less than 5-10% of the glucose taken up by muscle is released as lactate (5,393,394).  

 

Reduced insulin-stimulated glycogen synthesis is a characteristic finding in all insulin-resistant states, including obesity, diabetes, and the combination of obesity plus diabetes (1,4,18,43,59,159,162,218,219,377,393-395). Impaired glycogen synthesis also represents the major cause of insulin resistance in obese subjects with normal or only slightly impaired glucose tolerance (1,4,162,218,393,395,396).  Thus, the inability of insulin to promote glycogen synthesis is a characteristic and early defect in the development of insulin resistance in both obesity and T2DM. The emergence of overt diabetes with fasting hyperglycemia is associated with a major reduction in insulin-mediated non-oxidative glucose disposal (glycogen synthesis) in all ethnic groups (1,4,18,162,377,396).  Impaired glycogen synthesis also has been demonstrated in the normal-glucose-tolerant offspring of two diabetic parents (43,397), in the first-degree relatives of people with T2DM (41,398,399), and in a normoglycemic twin of a monozygotic twin pair in which the other has T2DM (101).

 

Using NMR imaging spectroscopy, a decrease in insulin-stimulated incorporation of [1H, 13C]- glucose into muscle glycogen of subjects with T2DM has been demonstrated directly (215). In T2DM, there was a marked lag in the onset of insulin-stimulated glycogen synthesis that was similar to the delay in insulin-mediated leg muscle glucose uptake. The rate of glycogen synthesis in subjects with T2DM was decreased by ~50%, paralleling the decrease in total glucose uptake by leg muscle (3).  Also, impaired muscle glycogen synthesis accounted for essentially all of the defect in whole body glucose disposal.

 

In summary, an abundance of convincing evidence demonstrates that impaired glycogen synthesis is the major metabolic defect in normal glucose tolerant subjects with obesity, in individuals with IGT, and in patients with overt diabetes. Moreover, numerous studies have documented that the earliest detectable metabolic abnormality responsible for the insulin resistance in normal glucose tolerant individuals who are destined to develop T2DM is impaired glycogen synthesis (4,41,43,101,382,392,399,400).

 

Glycogen synthase is the key insulin-regulated enzyme which controls the rate of muscle glycogen synthesis (307,308,310,379,401,402). Insulin enhances glycogen synthase activity by stimulating a cascade of phosphorylation/de-phosphorylation reactions (307,308,361-363,403) (see above discussion of insulin receptor signal transduction), which ultimately lead to activate PP1 (also called glycogen synthase phosphatase) (307,308,310,402). The regulatory subunit (G) of PP1 has two serine phosphorylation sites, called site 1 and site 2.  Phosphorylation of site 2 by cAMP-dependent kinase (PKA) inactivates PP1, while phosphorylation of site 1 by insulin activates PP1, leading to the stimulation of glycogen synthase (307,308,402,404). Phosphorylation of site 1 of PP1 by insulin in muscle is catalyzed by insulin-stimulated protein kinase 1 (ISPK-1) (309,405), which is part of a family of serine/threonine protein kinases termed ribosomal S6-kinases.  Because of their central role in muscle glycogen formation, considerable attention has focused on the three enzymes glycogen synthase, PP1, and ISPK-1 in the pathogenesis of insulin resistance in T2DM.

 

Glycogen synthase exists in an active (dephosphorylated) and an inactive (phosphorylated) form (307-310). Under fasting conditions, total glycogen synthase activity in subjects with T2DM is reduced and the ability of insulin to activate glycogen synthase is severely impaired (301,384,406-410). An impaired ability of insulin to activate glycogen synthase also has been demonstrated in the normal glucose tolerant relatives of individuals with T2DM (400).

Insulin-mediated activation of glycogen synthase and insulin-stimulated glycogen synthase gene expression has been shown to be impaired in cultured myocytes and fibroblasts from subjects with T2DM (411,412). Studies in insulin-resistant nondiabetic and diabetic Pima Indians have documented that the ability of insulin to activate muscle PP1 (glycogen synthase phosphatase) is severely reduced (413). PP1 dephosphorylates glycogen synthase, leading to its activation. Therefore, a defect in PP1 appears to play an important role in muscle insulin resistance (309).

 

The effect of insulin on glycogen synthase gene transcription and translation in vivo has been studied extensively. Most studies (378,414,415) have shown that insulin does not increase glycogen synthase mRNA or protein expression in human muscle studied in vivo. However, glycogen synthase mRNA expression is decreased in muscle of patients with T2DM (415,416), explaining in part the decreased glycogen synthase activity in this disease. However, the major abnormality in glycogen synthase regulation in T2DM and other insulin resistant conditions is its lack of de-phosphorylation and activation by insulin as a result of insulin receptor signaling abnormalities (see previous discussion). The glycogen synthase gene (417) has been the subject of intensive investigation. An association between glycogen synthase gene markers and T2DM has been demonstrated in Japanese, French, Finnish, and Pima Indian populations. However, DNA sequencing has revealed either no mutations (418) or rare nucleotide substitutions (419,420) that cannot explain the defect in insulin-stimulated glycogen synthase. Nonetheless, the association between the glycogen synthase gene and T2DM (418) suggests that another gene close to the glycogen synthase gene may be involved in the development of T2DM. The genes encoding the catalytic subunits of PP1 (421) and ISPK-1 (422) have been examined in insulin-resistant Pima Indians and Danes with T2DM. Several silent nucleotide substitutions were found in the PP1 and ISPK-1 genes in the Danish population; the mRNA levels of both genes were normal in skeletal muscle (422). No structural gene abnormalities in the catalytic subunit of PP1 were detected in Pima Indians (422). Thus, neither abnormalities in the PP1 and ISPK-1 genes nor abnormalities in their translation can explain the impaired enzymatic activities of glycogen synthase and PP1 that have been observed in vivo. Similarly, there is no evidence that an alteration in glycogen phosphorylase plays any role in the abnormality in glycogen formation in T2DM (423). In summary, glycogen synthase activity is severely impaired in patients with T2DM and in insulin-resistant normal glucose tolerant individuals who are predisposed to develop T2DM. However, the defect cannot be explained by an abnormality in the genes encoding glycogen synthase or is promoter or by other key genes - PP1 or ISPK-1 - involved in the regulation of glycogen synthase activity.

 

Glycolysis/Glucose Oxidation

 

Glucose oxidation accounts for ~90% of total glycolytic flux, while anaerobic glycolysis accounts for the other 10% (393,394). Two enzymes, phosphofructokinase (PFK) and pyruvate dehydrogenase (PDH) play pivotal roles in the regulation of glycolysis and glucose oxidation, respectively. In individuals with T2DM the glycolytic/glucose oxidative pathway has been shown to be impaired in many individuals with T2DM (393,394). Although one study suggested that the activity of PFK is modestly reduced in muscle biopsies from subjects with T2DM (424), the majority of evidence indicates that the activity of PFK is normal (407,412,417). Insulin has no effect on muscle PFK activity, mRNA levels, or protein content in either nondiabetic or diabetic individuals (417). PDH is a key insulin-regulated enzyme whose activity in muscle is acutely stimulated by a physiological increment in the plasma insulin concentration (415). Three previous studies have examined PDH activity in patients with T2DM. Insulin-stimulated PDH activity is decreased in isolated subcutaneous human adipocytes from patients with T2DM (425) and in skeletal muscle from subjects with T2DM undergoing euglycemic hyperinsulinemic clamps (426). However, when patients with T2DM had muscle biopsies during hyperglycemic hyperinsulinemic clamps, activation of PDH by insulin was normal (409), in concert with normalized rates of muscle glucose uptake. These results suggest that insulin stimulation of PDH activity is influenced by glycolytic flux.

 

Both obesity and T2DM are associated with accelerated FFA turnover and oxidation (1,4,18,162), which would be expected, according to the Randle cycle (232), to inhibit PDH activity and consequently glucose oxidation (see prior discussion). Thus, any observed defect in glucose oxidation or PDH activity could be acquired secondarily to increased FFA oxidation and feedback inhibition of PDH by elevated intracellular levels of acetyl-CoA and reduced availability of NAD. Consistent with this observation, the rates of basal and insulin- stimulated glucose oxidation have been shown to be normal in the normal glucose tolerant offspring of two parents with T2DM (43) and in the first-degree relatives of subjects with T2DM (41,423), while it is decreased in subjects with overt T2DM (1,4,393,394,427). Studies examining PHD activity in muscle tissue from lean diabetic subjects with mild fasting hyperglycemia are needed before the role of this enzyme in the development of insulin resistance in T2DM can be established or excluded.

 

In summary, post-binding defects in insulin action primarily are responsible for the insulin resistance in T2DM. Diminished insulin binding, when present, is small, occurs in individuals with IGT or very mild diabetes, and results secondarily from downregulation of the insulin receptor by chronic sustained hyperinsulinemia. In patients with T2DM and overt fasting hyperglycemia, post-binding defects are responsible for the insulin resistance. A number of post-binding defects have been documented, including diminished insulin receptor tyrosine kinase activity, insulin signal transduction abnormalities, decreased glucose transport, reduced glucose phosphorylation, and impaired glycogen synthase activity. The glycolytic/glucose oxidative pathway appears to be largely intact and, when defects are observed, they appear to be acquired secondarily to enhanced FFA/lipid oxidation. From the quantitative standpoint, impaired glycogen synthesis represents the major pathway responsible for the insulin resistance in T2DM, and family studies suggests that a defect in the glycogen synthetic pathway represents the earliest detectable abnormality in T2DM. Recent studies link the impairment in glycogen synthase activation to a defect in the ability of insulin to phosphorylate IRS-1, causing a reduced association of the p85 subunit of PI 3-kinase with IRS-1 and decreased activation of the enzyme (PI 3-kinase).

 

Mitochondrial Dysfunction

 

In obesity and T2DM, impaired oxidation, reduced mitochondrial contents, lowered rates of oxidative phosphorylation and the production and release of excessive reactive oxygen species (ROS) have been reported. Mitochondrial biogenesis is regulated by various transcription factors such as peroxisome proliferator-activated receptor γ coactivator-1α (PGC-1α), peroxisome proliferator-activated receptors (PPARs), estrogen-related receptors (ERRs), and nuclear respiratory factors (NRFs).  Mitochondrial fusion is promoted by mitofusin 1 (MFN1), mitofusin 2 (MFN2) and optic atrophy 1 (OPA1), while fission is governed by the recruitment of dynamin-related protein 1 (DRP1) by adaptor proteins, such as mitochondrial fission factor (MFF), mitochondrial dynamics proteins of 49 and 51 kDa (MiD49 and MiD51), and fission 1 (FIS1).  Phosphatase and tensin homolog (PTEN)-induced putative kinase 1 (PINK1) and PARKIN promote DRP1-dependent mitochondrial fission, and the outer mitochondrial adaptor MiD51 is required in DRP1 recruitment and PARKIN-dependent mitophagy.  Several molecular abnormalities affecting these critical aspects of mitochondrial dynamics have been identified in obese individuals and in patients with T2DM (446). 

 

The generation of new mitochondria, mitochondrial biogenesis, is presumed to be defective in patients with T2DM because the expressions of PGC-1α and its targeted genes are reduced.  They are associated with an impaired ability to produce mitochondrial ATP and increased ROS production from the electron transport chain.  There is some preliminary evidence that stimulation of mitochondrial biogenesis by pharmacological activation targeting these molecules is beneficial in the treatment of T2DM and obesity.  The accumulation of damaged or depolarized mitochondria in pancreatic β cells is associated with oxidative stress and favors subsequent development of diabetes.  Mitochondria in pancreatic β cells are continuously recruited in the fusion and fission processes.  In a cultured pancreatic β cell line (INS-1), high levels of glucose- and palmitate-induced mitochondrial fusion arrested and reduced respiratory function.  In INS1 cells, mitochondria with fission demonstrated reduced Δψ and decreased levels of the fusion protein OPA1. The inhibition of fission machinery proteins using DRP1 and FIS1 RNAi resulted in decreased mitochondrial autophagy, the accumulation of oxidized mitochondrial proteins, reduced respiration, and impaired insulin secretion.  All of these suggest that selective fission of damaged mitochondria is followed by their removal by autophagy.  In another study, INS-1 cells were treated with palmitate and high glucose, and the fragmentation of mitochondria with reduced fusion activities was observed. The application of FIS1 RNAi that shifts the dynamic balance to favor fusion is able to prevent mitochondrial fragmentation, maintain mitochondrial dynamics, and prevent apoptosis.  Thus, although not entirely elucidated, abnormal mitochondrial fusion and fission dynamics in the pancreatic β cells may play an important role in beta cell dysfunction and the progression of T2DM.

 

Obesity and T2DM are associated with impaired skeletal muscle oxidation, reduced mitochondrial contents, and lowered rates of TCA cycle enzymes and OXPHOS.  Patients with T2DM and obesity also demonstrated reduced expression of MFN2, which may be related to the reduced function of mitochondria in skeletal muscle.  In 17 subjects with obesity, 12 weeks of exercise improved insulin sensitivity and fat oxidation.  Skeletal muscle biopsy in these patients revealed that decreased phosphorylation and reduction of DRP1 at serine 616 were negatively correlated with increases in fat oxidation and insulin sensitivity (447).  In this same study, there was a trend towards an increase in the expression of both MFN1 and MFN2.  Studies in hepatocytes have recently demonstrated that the role of MAMs in calcium, lipid, and metabolite exchange is altered in obesity and T2DM.  Although the ER and mitochondria play distinct cellular roles in the process of intermediary metabolism, obesity is known to lead to a marked reorganization of MAMs, which results in mitochondrial calcium overload, reduced respiratory function, and augmented oxidative stress (448).  In contrast, disrupting the integrity of MAMs by knocking out cyclophilin D leads to hepatic insulin resistance through the disruption of inter-organelle Ca2 transfer, ER stress, mitochondrial dysfunction, lipid accumulation, the activation of c-Jun N-terminal kinase and PKCε.  In addition to the beta-cell and skeletal muscle defects described earlier, these altered molecular pathways in the liver represent potential targets for new pharmaceutical intervention to be explored in future studies including individuals with obesity and patients with T2DM.

 

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Adrenocortical Carcinoma

ABSTRACT  

Adrenocortical carcinoma (ACC) is a rare endocrine malignancy arising from the adrenal cortex often with unexpected biological behavior. It can occur at any age, with two peaks of incidence: in the first and between fifth and seventh decades of life. Although ACC are mostly hormonally active, precursors and metabolites may be also produced by dedifferentiated and immature malignant cells. Distinguishing the etiology of an adrenal mass, between benign adenomas, which are quite frequent in general population, and malignant carcinomas with dismal prognosis is challenging. However, recent advances in genomic, pathology, and staging allow the development of standardization of pathology reporting and refinement of prognostic grouping for planning treatment of the patients with ACC. Besides, no single histopathological as well as no single imaging method, hormonal work-up, or immunohistochemical labelling can definitively prove the diagnosis of ACC. Over several decades’ great efforts have been made in finding novel reliable and available diagnostic and prognostic factors including steroid metabolome profiling or target gene identification. Preliminary data show that for localized ACC, molecular makers (gene expression, methylation, and chromosome alterations) could predict cancer recurrence. Nevertheless, many of these markers need further validation and some are difficult to be widely applied in clinical settings. ACC is frequently diagnosed in advanced stages and therapeutic options are unfortunately limited. Surgery remains the “gold standard’ treatment. The management of patients with ACC requires a multidisciplinary approach. Immunotherapy in advanced ACC has been investigated in different studies however, the reported rates of overall response rate and progression free survival were generally poor. Thus, new biological markers that could predict patient prognosis and provide individualized therapeutic options are required.

CLINICAL RECOGNITION

Adrenocortical cancer (ACC) is a rare disease with an annual incidence of 0.7-2 cases per million per year and two distinct age distribution peaks, the first occurring in early adulthood and the second between 40-50 years with women being more often affected than men (55-60%) (1,2). Although the great majority of ACCs are sporadic in origin, they can also develop as part of familial syndromes the most common being the Beckwith-Wiedeman syndrome (11p151 gene, IGF-2 overexpression), the Li-Fraumeni syndrome (TP53 gene germline and somatic mutation), the Lynch syndrome (MSH2, MLH1, MSH6, PMS2, EPCAM  genes), the multiple endocrine neoplasia (MEN) 1 (MEN1 gene), and familial adenomatous polyposis (FAP gene, catenin somatic mutations),  neurofibromatosis type 1 (NF1 gene) and Carney complex (PRKAR1A  gene) syndromes (Table 1) (1-4). In recent years several multi-center studies have shed light on the pathogenesis of ACC but ‘multi-omic’ studies have recently revealed that only a minority of ACC cases harbor pathogenic driver mutations. 

 

Table 1. Clinical and Genetic Features of Familial Syndromes Associated with ACC

Genetic disease

Gene and chromosomal involvement

Organ involvement

Beckwith-Wiedemann syndrome

CDKN1C mutation

KCNQ10T1, H19 (epigenetic defects) 11p15 locus alterations

IGF-2 overexpression

Macrosomia, macroglossia, hemihypertrophy (70%), omphalocele, Wilm’s tumor, ACC (15-

20% adrenocortical tumors)

Li-Fraumeni syndrome P53(17p13)

Soft tissue sarcoma, breast cancer, brain

tumors, leukemia, ACC

Multiple Endocrine Neoplasia syndrome 1

Menin (11q13)

Parathyroid, pituitary, pancreatic, bronchial tumors

Adrenal cortex tumors (30%, rarely ACC)

Familial Adenomatous polyposis

APC (5q12-22)

Multiple adenomatous polyps and cancer colon and rectum

Periampullary cancer, thyroid tumors,

hepatoblastoma, rarely ACC

SBLA syndrome

Sarcoma, breast and lung cancer, ACC

Neurofibromatosis

NF1

Six or more light brown dermatological spots ("café au lait spots

At least two neurofibromas

Carney Complex

PRKAR1A

Lentigines, Atrial Myxoma, and Blue Nevi

 

The clinical features of sporadic ACCs are due to hormonal hypersecretion and/or tumor mass and spread to surrounding or distant tissues. An increasing number of cases (≈ 10-15%) are increasingly been diagnosed within the group of incidentally discovered adrenal masses (incidentalomas). However, the likelihood of an adrenal incidentaloma being an ACC is rather low (2, 6, 7). Approximately 50-60% of ACCs exhibit evidence of hormonal hypersecretion, usually that of combined glucocorticoid and androgen secretion (Table 2). Nearly 30-40% of patients with primary ACC present with a mass-related syndrome as abdominal or dorsal pain, a palpable mass, fever of unknown origin, signs of inferior vena cava (IVC) compression, and signs of left-sided portal hypertension. Rarely, complications such as hemorrhage or tumor rupture may also occur. Lately the number of patients that are identified while being investigated for an adrenal incidentaloma is rapidly increasing (5).

 

In biochemical studies the first step is the measurement of steroid hormones which is initially guided by the clinical presentation. According to the ESMO-EURACAN (European Society for Medical Oncology—the European Reference Network for rare adult solid cancers) Clinical Practice Guidelines from 2020 in cases of suspected ACC, an extensive steroid hormone work-up is recommended assessing gluco-, mineralo-, sex- and precursor-steroids (6-8). For all adrenal masses, diagnosis of pheochromocytoma should be excluded by measuring plasma-free or urinary-fractionated metanephrines to avoid intraoperative complications.

 

Table 2. Signs and Symptoms of ACC and Recommended Testing for Confirmation of

Hypersecretory Syndromes

Symptoms/Signs

Hormonal testing (ENSAT 2005, ESMO-EURACAN - Clinical Practice Guidelines 2020)

Hypercortisolism

Centripetal fat distribution.

Skin thinning – striae.

Muscle wasting – myopathy. Osteoporosis.

Increased blood pressure (BP),

Diabetes Mellitus

Psychiatric disturbance

Gonadal dysfunction

Overnight dexamethasone suppression test (1mg)

24-hour free cortisol,

Basal ACTH (plasma),

Basal cortisol (serum)

[for diagnosis minimum 3 out of 4 tests)

Androgen hypersecretion

Hirsutism

Menstrual irregularity – infertility

Virilization (baldness, deepening of the voice, clitoris hypertrophy)

DHEA-S

Androstenedione

Testosterone

17-hyrdoxy-progesterone (17OHPG)

Mineralocorticoid hypersecretion

Increased BP

Hypokalemia

Potassium (serum)

Aldosterone to renin ratio

Estrogen hypersecretion

Gynecomastia (men)

Menorrhagia (post-menopausal women)

17β-estradiol

Non-hypersecretory syndrome

 

PATHOPHYSIOLOGY

Although studies of hereditary neoplasia syndromes have revealed various chromosomal abnormalities related to ACC development the precise genetic alterations involved are still unknown. Most common mutations implicated in sporadic ACC are insulin-like growth factor 2 (IGF2), catenin (CTNNB1 or ZNRF3) and TP53 mutations (1, 4, 9). The Wnt/β- catenin constitutive activation and insulin growth factor 2 (IGF2 overexpression) are the most important implicated genetic pathways. Germline TP53 mutations and dysregulation of the Gap 2/mitosis transition and the insulin-like growth factor 1 receptor (IGF1R) signaling have also been described. Steroidogenic factor 1 (SF1) plays an important role in adrenal development and is frequently overexpressed in ACC. Recently, ACC global -omics profiling studies revealed frequent detected genetic and epigenetic alterations, including loss of heterozygosity at 17p13, alterations at the 11p15 locus, and mutations in TP53, CTNNB1, ZNRF34, CDKN2A, RB1, MEN1, PRKAR1A, RPL22, TERF2, CCNE1, and NF1 genes. Decreased expression of MLH1, MSH2, MSH6, and/or PMS2 consistent with high microsatellite instability/mismatch repair protein deficiency (MSI-H/MMR-D) status have also been reported (4, 9).

DIAGNOSIS AND DIFFERENTIAL DIAGNOSIS

A palpable mass causing abdominal pain in the presence of IVC syndrome is highly suggestive of an ACC. This is substantiated further by the presence of symptoms/signs of combined hormonal secretion (cortisol and androgens), virilizing or rarely feminizing symptoms/signs confirmed with the use of specific endocrine testing (Table 2). As the majority of ACCs are relatively large (size > 8cm, weight >100g) at diagnosis, specific imaging features are used to distinguish them from other adrenal lesions (1, 2, 3). If adrenal imaging indicates an indeterminate mass other parameters should be considered including tumor size > 4 cm, combined cortisol/androgen hormone excess, rapidly developing symptoms and/ rapid tumor growth and/or young age (e.g., < 40 years) at presentation, that all might point to an ACC (1, 2, 3, 5, 7).

 

Other adrenal lesions that need to be considered in the differential diagnosis are myelolipomas, adrenal hemorrhage, lymphoma, adrenal cysts, metastases, and mainly adrenal adenomas, the majority of which have distinctive imaging features. There is no role for biopsy in a patient who is considered suitable for surgery of the adrenal mass (5, 6, 7).

 

Computerized Tomography (CT) imaging of the adrenals is the major tool showing a unilateral non- homogenous mass, > 5cm in diameter, with irregular margins, necrosis, and occasionally calcifications. Due to the low-fat content X-Ray density is high (>20 Hounsfield Units, HU); in a series of 51 ACC none had a density of less than 13 Hounsfield Units (HU) (6-8). However, a recent study including almost 100 ACCs showed that increasing the unenhanced CT tumor attenuation threshold to 20 HU from the recommended 10 HU increased specificity for ACC at 80% [95% CI 77.9–82.0] vs. 64% [61.4–66.4] while maintaining sensitivity at 99% [94.4– 100] vs. 100% [96.3–100]; (PPV 19.7%, 16.3–23.5) [EURINE-ACT study] (10). The presence of enlarged aorto-caval lymph nodes, local invasion, or metastatic spread, are highly suggestive of ACC. For 3-6 cm size lesions, measuring CT-related tumoral density before and after contrast administration, and estimating washout percentage can be helpful; less than 50% after 15 minutes, is associated with >90% specificity (7, 8). On Magnetic Resonance Imaging (MRI), ACC appears hypo or isointense in relation to the liver on T1-weighted images, and following gadolinium enhancement and chemical shift techniques the diagnostic accuracy obtained can be as high as 85-100% (7, 8). Recently Positron Emission Tomography (PET) imaging with 18F-fluoro-2deoxy-D-glucose (18FDG-PET) has been proposed as possibly the best second-line test to assess indeterminate masses by unenhanced CT exhibiting 95-100% sensitivity and 91-94% specificity that increases further when fused with CT imaging. Furthermore, 18FDG-PET can also be used as a staging procedure identifying metastatic adrenal disease missed by conventional imaging studies including CT of the chest (7, 8). With the proper implementation of imaging studies there is no need for any adrenal biopsy.

HISTOPATHOLOGICAL DIAGNOSIS

The expression of steroidogenic factor 1(SF1) is a valid marker to document the adrenal origin (distinction of primary adrenocortical tumors and non-adrenocortical tumors) with a sensitivity of 98% and a specificity of 100%. If this marker is not available, a combination of other markers can be used which should include inhibin-alpha, melan-A, and calretinin. European Network for the Study of Adrenal Tumors (ENSAT) has shown that KI-67 is the most powerful prognostic marker in both localized and advanced ACC indicative of aggressive behavior and that higher Ki-67 levels are consistently associated with a worse prognosis (2, 6, 11, 12). Weiss system, based on a combination of nine histological criteria that can be applied on hematoxylin and eosin-stained slides for the distinction of benign and malignant adrenocortical tumors, is the best validated score to distinguish adenomas from ACC although with high inter-observer variability. A reticulin algorithm has been used for diagnosis of ACC which involves an abnormal//absent reticulin framework and at least one of the three following criteria (tumor necrosis, presence of venous invasion and mitotic rate of >5/50 high power field) (12). Studies have proposed the use of proliferative index (Ki-67 index > 5%) and IGF2 over-expression to confirm the diagnosis of ACC (11, 12). It is important to note that no single microscopic criterion on its own is indicative of malignancy and there is subjective variability in the interpretation.

PROGNOSIS

As survival depends on stage at presentation several different classification histopathological systems have evolved with the reported 5-year survival using the ENSAT system being 82% for stage I, 61% for stage II, 50% for stage III, and 13% for stage IV disease (Table 3) (6-8). Tumor size remains an excellent predictor of malignancy as tumors > 6cm have a 25% chance of being malignant compared to 2% of those with a size < 4cm. As there is no single distinctive histopathological feature indicative of malignancy the Weiss score has been used with a score >3 being suggestive of malignancy and recently Ki-67 labelling index >10%.  A relatively new system published by a European group in 2015 is the Helsinki score which relay on mitotic rate, necrosis and Ki-67 index (3x mitotic count [>5/50 high power fields] + 5x presence of necrosis + Ki-67 proliferative index) of ACC and focus on the predicting diagnosis as well as prognosis of ACC. A Helsinki score >8.5 is associated with metastatic potential and warrant the diagnosis of ACC (12). Altered reticulin pattern, Ki-67% labelling index and overexpression of p53 protein were found to be useful histopathological markers for distinguishing benign adrenocortical tumors from ACCs; however only pathological p53 nuclear protein expression was found to reach statistically significant association with poor survival and development of metastases, although in a small series of patients (11).

 

 Table 3. Staging System for ACC Proposed by the International Union against Cancer (WHO 2004) and the European Network for the Study of Adrenal Tumors (ENSAT).

Stage

WHO 2004

ENSAT 2008

I

T1,N0,M0

T1,N0,M0

II

T2,N0,M0

T2,N0,M0

III

T1-2,N1,M0

T3,N0,M0

T1-2,N1,M0

IV

T1-4,N0-1,M1

T3,N1,M0

T4,N0-1,M0

T1-4,N0-1, M1

M0: No distant metastasis, M1: Presence of distant metastasis, N0: No positive lymph nodes, N1: Positive lymph node(s), T1: Tumor ≤5cm, T2: Tumor > 5cm, T3: Tumor infiltration to surrounding tissue, T4: Tumor invasion into adjacent organs or venous tumor

thrombus in vena cava or renal vein.

 

The median overall survival (OS) of all ACC patients is about 3-4 years (6, 7). The prognosis is, however, relatively heterogeneous. Complete surgical resection provides the only means of cure (6, 7, 13, 14). In addition to radical surgery, disease stage, proliferative activity/tumor grade, and cortisol excess are independent prognostic parameters (6, 7, 13, 14). Five-year survival rate is 60-80% for tumors confined to the adrenal space, 35-50% for locally advanced disease, and significantly lower in case of metastatic disease ranging from 0% to 28% (6-8). ENSAT staging is considered slightly superior to the Union for International Cancer Control (UICC) staging. Additionally, the association between hypercortisolism and mortality was consistent. As Ki-67 has been shown to be related with prognosis in both localized and advanced ACC, threshold levels of 10% and 20% have been considered for discriminating low from high Ki-67 labelling index; however, it is not clear whether any single significant threshold can be determined. Patients with stage I-III disease treated with surgical resection had significantly better median overall survival (OS) (63 vs. 8 months; p= 0.001). In stage IV disease, better median OS occurred in patients treated with surgery (19 vs. 6 months; p=0.001), and post-surgical radiation (29 vs. 10 months; p=0.001) or chemotherapy (22 vs. 13 months; p=0.004) (6-8, 13, 14). Overall survival varied with increasing age, higher comorbidity index, grade, and stage of ACC at presentation. There was improved survival with surgical resection of the primary tumor, irrespective of disease stage; post- surgical chemotherapy or radiation was of benefit only in stage IV disease (7, 13). The 5 year-survival of adult patients from multiple datasets with ACC after surgery range from 40% to 70%. The estimated five-year overall survival rate for patients with ACC in recent cohorts is slightly less than 50% (7, 13).

 

Preliminary data also shows that for localized ACC, molecular makers (gene expression, methylation, and chromosome alterations) could predict cancer recurrence (15, 16). Nevertheless, many of these markers need further validation and some are difficult to be widely applied in clinical settings. The development of genomics has led to a new classification of ACC by two independent international cohorts; one from ENSAT network in Europe (15) and the other from the Cancer Genome Atlas consortium in America, Europe and Australia (16), with two distinct molecular subgroups, C1A and C1B being associated with poor (5-year survival rate of 20%) and good overall prognosis (5-year survival rate of 91%), respectively. The C1B group is characterized by low mutation rate, and a very low incidence of mutations of the main driver genes of ACC whereas the C1A group is characterized by high mutation rate and driver gene alterations. This group is further divided into a subgroup of aggressive tumors showing hypermethylation at the level of the CpG islands located in the promoter of genes (“CIMP phenotype”).

 

THERAPY

 

The management of patients with ACC requires a multidisciplinary approach with initial complete surgical resection in limited volume disease (stages I, II and occasionally III). Mitotane (1,1-dichloro-2(o- chlorophenyl)-2-(p-chlorophenyl) ethane [o,p’DDD]) is the only currently available adrenolytic medication achieving an overall response of approximately 30%.

 

Surgery

 

The aim of surgery is to achieve a complete margin-negative (R0) resection as patients with an R0 resection have a 5-year survival rate of 40-50% compared to the < 1year survival of those with incomplete resection (7, 14). Patients with stage III tumors and positive lymph nodes can have a 10-year OS rate of up to 40% after complete resection. When a preoperative diagnosis or high level of suspicion of ACC exists, open surgical oncological resection is recommended as locoregional lymph removal might improve diagnostic accuracy and therapeutic outcome. However, the wide range of reported lymph node involvement in ACC (ranging from 4 to 73%) implies that regional lymphadenectomy is neither formally performed by all surgeons nor accurately assessed or reported by all pathologists (7, 14). Laparoscopic adrenalectomy should be considered for tumors with size up to 6 cm without any evidence of local invasion. Routine locoregional lymphadenectomy should be performed with adrenalectomy for highly suspected or proven ACC and it should include (as a minimum) the peri-adrenal and renal hilum nodes. Preservation of the tumor capsule is essential whereas involvement of the IVC or renal vein with tumor thrombus is not a contraindication for surgery. However, even following an apparently complete surgical resection, 50-80% of patients develop locoregional or metastatic recurrence. Although such patients may be candidates for aggressive surgical resection, routine debulking is not recommended except for control of hormonal hypersecretion (6, 7, 14). Ablative therapies particularly targeting hepatic disease are used to decrease tumor load and the hypersecretory syndromes. Individualized treatment decisions are made in cases of tumors with extension into large vessels based on multidisciplinary surgical team. Such tumors should not be regarded ‘unresectable’ until reviewed in an expert center.

 

Mitotane

 

Mitotane has traditionally been used for ACCs obtaining a partial or complete response in 33% of cases mainly by metabolic transformation within the tumor and through oxidative damage. Besides its cytotoxic adrenal action mitotane also inhibits steroidogenesis.

 

Adjuvant mitotane treatment is proposed in those patients without macroscopic residual tumor after surgery but who have a perceived high risk of recurrence (stage III, KI-67%>10%). However, patients at low/moderate risk of recurrence (stage I-II, R0 resection, and Ki-67 ≤ 10%) do not benefit significantly from adjuvant mitotane (results from ADIUVO trial) (17). When indicated mitotane should be initiated within six weeks and not later than 3 months (7, 14). Adjuvant mitotane should be administrated for at least 2 years, but no longer than 5 years. However, the optimal duration of adjuvant mitotane treatment still remains unsolved and mainly dependents on personal preferences and expertise. According to a recent study the present findings do not support the concept that extending adjuvant mitotane treatment over two years is beneficial for patients with ACC at low risk of recurrence (18).

 

The tolerability of mitotane may be limited by its side effects mainly nausea, vomiting, neurological (ataxia, lethargy), hepatic and rarely hematological toxicity (7, 18). Measurement of serum mitotane levels, targeting a range of 14-20 mg/l, seems to correlate with a therapeutic response while minimizing toxicity using variable dosing regimens (6-8). Mitotane causes hyperlipidemia and increased hepatic production of hormone binding globulins (cortisol, sex hormone, thyroid and vitamin D) increasing total hormone concentration while impairing free hormone bioavailability. The induction of hepatic P450- enzymes by mitotane induces the metabolism of steroid compounds requiring high dose glucocorticoid and mineralocorticoid replacement.

 

Hormonal excess causes significant morbidity in ACC patients. Although mitotane reduces steroidogenesis it has a slow onset of action necessitating the use of other medications targeting adrenal steroidogenesis (ketoconazole, metyrapone, aminoglutathemide, and etomidate). As adrenal insufficiency may occur close supervision is required to titrate adrenal hormonal replacement therapy.

 

Cytotoxic Chemotherapy

 

In metastatic ACC or when progression on mitotane, systematic therapy is recommended. Although cisplatin containing regimens have shown some responses, most studies lack enough power and comparisons between different regimens are only few. The most encouraging results originate from the combinations of etoposide, doxorubicin and cisplatin with mitotane (EDP-M) achieving an overall response of 49% of 18 months duration (FIRMA-CT study) (19). This regimen was equally effective as first line treatment or after failing of the combination of streptozotocin with mitotane and is the currently the preferred scheme. In patients who progress under mitotane monotherapy, EDP treatment is also recommended (7, 19). The combination of gemcitabine with capecitabine is used for patients failing EDP- and for non-responding patients. Targeted therapies with tyrosine kinase inhibitors (mainly sunitinib) have only been suggested albeit results have not been great. Although initially promising, treatment with IGF-1R antagonists did not prove to be efficacious suggesting that combination therapies may be the way forward.

 

Radiation Therapy

 

Radiotherapy has a role in symptomatic metastatic disease particularly bone disease with positive responses in up to 50% - 90% of cancer patients. It is a local therapy which is mainly recommended in incomplete resection, recurrent or metastatic disease.

 

Immunotherapy

 

Several Immunotherapy agents that have been evaluated in clinical trials for metastatic ACC patients including the immune checkpoint inhibitors pembrolizumab, nivolumab, and avelumab which are monoclonal antibodies directed toward PD-L1, the ligand-binding partner of PD-1, that is expressed on tumor cells. Four immune checkpoint inhibitors (pembrolizumab, avelumab, nivolumab, and ipilimumab) have investigated the role of immunotherapy in advanced ACC. Despite, the different primary endpoints used in these studies, the reported rates of overall response rate and progression free survival were generally poor (Table 4) (20-23). Three main potential markers of response to immunotherapy in ACC have been described: Expression of PD-1 and PD-L1, microsatellite instability, and tumor mutational burden (20). However, none of these parameters has been validated in prospective studies. Several mechanisms may be responsible of immunotherapy failure, and a greater knowledge of these mechanisms might lead to the development of new strategies to overcome the immunotherapy resistance.  

 

Two clinical trials using the PD-1 inhibitor pembrolizumab as monotherapy in ACC have been reported (20, 21). The observed median PFS and OS in the first study were 2.1 and 24.9 months, respectively. Six patients in the study had microsatellite-high and/or mismatch repair deficient status (MSI-H/MMR-D), for which pembrolizumab is an FDA approved therapy. In the second study, 5 of 14 patients (36%), developed stable disease at 27 weeks and 2 exhibited a partial response. Nivolumab monotherapy was tested in a phase II trial (22). The best response observed in this trial was 1 out of 10 patients with an unconfirmed partial response and 2 out of 10 patients developing stable disease (22). Avelumab has been evaluated in a phase 1b clinical trial. in patients with metastatic ACC who had progressed after first-line platinum-based therapy (23). In this trial including 50 patients, 3 patients showed a partial response and 21 (42%) patients stable disease (42%). Median PFS and OS were 2.6 and 10.6 months, respectively.

 

Table 4. Studies that have Investigated Immunotherapy in Patients with ACC

Molecule

Phase

Population (n)

Prior systemic treatment

Results

Pembrolizomab 200 mg every 3 w (35 cycles)

39

28

PFS=2.1, OS=24.9 ORR (RECIST)=23%

Pembrolizomab 200 mg every 3 w (35 cycles)

II

16

16

SD at 27 w=36%, ORR(RECIST)=14%

Nivolumab 240 mg every 2 w

II

10

10

PFS=1.8, ORR=11%

Avelumab 10 mg/kg every 2 w (±mitotane)

Ib

50

50

PFS=2.6, OS=10.6, ORR=6%

ORR= objective response rates; PFS= progression-free survival; SD= stable disease; OS= overall survival

 

Combinations therapies with these agents are also evolving. Pembrolizumab along with the VEGF-targeted multi-kinase inhibitor lenvatinib was used in a small retrospective case series including 8 heavily pre-treated patients (the median number of prior lines of systemic therapy was4) with progressive or metastatic ACC (24). The median PFS in these patients was 5.5 months (95% CI 1.8–not reached). Two (25%) patients showed a partial response, one (12.5%) patient had stable disease, and five (62.5%) patients developed progressive disease.

 

Other immunotherapies that have been evaluated include the monoclonal antibodies figitumumab and cixutumumab directed against the ACC-expressed insulin-like growth factor 1 (IGF-1) receptor, the recombinant cytotoxin interleukin-13-pseudomonas exotoxin A, and autologous tumor lysate dendritic cell vaccine (25, 26). All of these agents have shown modest clinical activity. Figitumumab in particular was evaluated in a phase I trial. in 14 patients with metastatic ACC. The best response to treatment observed in this trial was stable disease seen in 8 of 14 patients. Toxicities were generally mild and included hyperglycemia, nausea, fatigue, and anorexia. Similarly, to figitumumab, cixutumumab (IMC-A12) was evaluated in combination with mitotane in a phase II trial as first-line therapy for patients with advanced or metastatic ACC (26). The study was terminated early due to slow accrual and limited efficacy. The recorded PFS was only 6 weeks (range: 2.66–48), and in 20 evaluable patients, the best ORR was a partial response (PR) in one patient and stable disease in a further seven. Toxicities observed included grade 4 hyperglycemia and hyponatremia and one grade 5 multiorgan failure.

 

Immune Modulators

 

The potential utility of thalidomide treatment in ACC was evaluated in a retrospective cohort study of the European Networks for the Study of Adrenal Tumors registry (27). In this study, 27 patients with progressive to mitotane or metastatic ACC were treated with 50–200 mg thalidomide daily. The best response noted was stable disease in two patients, while the remaining 25 patients had progressive disease. The median PFS was 11.2 weeks, with a median OS of 36.4 weeks. Thalidomide was generally well tolerated, with fatigue and gastrointestinal upset being the most commonly observed side effects.

 

Evolving Therapies

 

Active areas of research in this field include combinations of immune checkpoint inhibitors, combination tyrosine kinase and immune checkpoint inhibitors, cancer vaccines, and glucocorticoid receptor antagonists combined with immune checkpoint inhibitor therapies (http://www.clinicaltrials.gov). Targeting mTOR pathway alone with everolimus did not produce significant responses. An extended phase I study of the anti-IGF-1R monoclonal antibody cixutumumab with an mTOR inhibitor showed a partial but short-lived response. Other potential targets are antagonists of β-catenin and Wnt signaling pathway and SF-1 inverse agonists. The application of radionuclide therapy using 131I-metomidate has recently been explored. However, despite recent advances in dysregulated molecular pathways in ACCs, these findings have not yet been translated into meaningful clinical benefits.

 

FOLLOW-UP

 

Patients who have undergone an apparently curative resection should be followed up regularly using endocrine markers and abdominal imaging. After complete resection, radiological imaging every 3 months for 2 years and then every 3-6 months for a further 3 years is proposed. 18FDG-PET should be performed at regular intervals to detect recurrent disease in high-risk patients. Patients on mitotane therapy should be regularly monitored measuring serum mitotane levels ensuring adequate replacement therapy. In case of recurrence not amenable to surgical excision patients should be enrolled in prospective clinical trials.

 

CONCLUSION

 

Besides considerable accumulated knowledge on the genetic profiling the pathogenesis of ACC is still not delineated although groups of patients with a worse outcome could be identified. Stage of the disease remains a strong predictor of overall survival whereas new evolving biomarkers need to be further validated. Imaging with 18FDGPET is an integral part of the staging procedure but the available medical therapies for patients with advanced disease have not shown a major impact on patients' prognosis. Further research is needed to identify high risk patients and formulate efficacious therapies for patients with advanced disease.

 

REFERENCE

 

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Update On Pancreatic Transplantation In The Management Of Diabetes

ABSTRACT

Pancreas transplantation is the most effective therapeutic option that can restore insulin independence in beta-cell penic recipients with diabetes. Because of life-long immunosuppression and the initial surgical risk, pancreas transplantation is a therapeutic option only in selected patients with diabetes. Based on renal function, candidates for pancreas transplantation can be classified into three categories: uremic patients, post-uremic patients (following a successful kidney transplantation), and non-uremic patients. Uremic patients are best treated by a simultaneous kidney-pancreas transplantation. Post-uremic patients can receive a pancreas after kidney transplantation. Non-uremic patients can receive a pancreas transplant alone, if diabetes is poorly controlled resulting in hypoglycemia unawareness, and in the presence of evolving chronic complications of diabetes. Results of pancreas transplantation have improved over time and are currently non-inferior to those of renal transplantation alone in recipients without diabetes. A functioning pancreatic graft can prolong patient survival, dramatically improves quality of life of recipients, and may ameliorate the course of chronic complications of diabetes. Unfortunately, because of ageing of the donor population and lack of timely referral of potential recipients, the annual volume of pancreas transplants is declining. Considering that the results of pancreas transplantation depend on center volume, and that adequate center volume is required also for training of newer generations of transplant surgeons, centralization of pancreas transplantation activity should be considered. The recent world consensus conference on pancreas transplantation provides an independent appraisal of the impact of pancreas transplantation on modern management of diabetes as well as expert guidelines for the practice of pancreas transplantation.

INTRODUCTION

Transplantation of an immediately vascularized pancreas allograft (PTx) is currently the most effective therapy to consistently restore insulin-independence in beta-cell depleted recipients with diabetes (1-3). Islet cell transplantation may achieve the same result, especially in patients who require fewer insulin units (4-5). As compared with PTx, islet cell transplantation is associated with lower procedure-related morbidity but requires the same immunosuppression, may necessitate multiple donors, and insulin-independence, when achieved, is not often maintained long-term (1-5). However, results reported very recently from centers of excellence show, that in properly selected patients, islet cell transplantation may achieve insulin-independence rates similar to those of PTx (6).

Unfortunately, PTx is not indicated in all insulin-dependent patients with diabetes because of the initial risk associated with surgery (7) and the need for life-long immunosuppression (8). In the appropriate recipient, however, PTx prolongs survival, especially when associated with kidney transplantation (9,10), restores near-normal metabolic control (11-14), improves the course of secondary complications of diabetes (11,12,15-26) and dramatically improves quality of life (27).

PTx includes several approaches. In the most common scenario a pancreas allograft is transplanted simultaneously with a kidney in patients with insulin-dependent diabetes and end stage diabetic nephropathy (simultaneous pancreas-kidney transplantation; SPK). Grafts are typically obtained from a single deceased donor. Alternatively, a cadaver pancreas can be transplanted simultaneously with a living donor kidney (SCPLK) (28), or a segmental pancreas graft and a kidney graft can be donated from the same live donor (SLPK) (29). The pancreas can also be transplanted alone (PTA), in pre-uremic recipients, or after a successful kidney transplant (PAK), in post-uremic recipients. When the pancreas is transplanted without a kidney from the same donor, the graft is considered to be solitary because renal function cannot be used to anticipate rejection in the pancreas (so called “sentinel kidney” function) (30). In rare circumstances the pancreas is transplanted in the setting of multivisceral organ transplantation (31). This type of PTx is not considered in this review, since it is not performed in the typical recipient with diabetes to primarily reverse diabetes, but rather for technical reasons in the context of a multiorgan graft required to address specific, and rare, conditions requiring this extreme type of transplantation.

THE BURDEN OF DIABETES

Thanks to the availability of exogenous insulin therapy, Type 1 diabetes has changed from an immediately fatal disease to a chronic disease. Sub-optimal metabolic control, coupled with genetic predisposition (32-34), can lead to the development of severe secondary complications many years after the diagnosis of diabetes. These complications are associated with significant morbidity and reduce life expectancy of affected individuals. Patients with diabetes who have poor metabolic control despite intensive insulin therapy and/or who develop progressive secondary complications can benefit from PTx as near-physiologic metabolism is re-established. These complications include: retinopathy, nephropathy, neuropathy, and cardiovascular disease. Diabetic nephropathy is the leading indication to PTx, as either SPK or PAK.

Diabetes mellitus is a group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion, insulin action, or both (35). Diabetes mellitus can be classified into four types: type 1 (resulting from autoimmune destruction of beta-cells, and accounting for 5-10% of all cases), type 2 (caused by relative insulin deficiency in the setting of insulin resistance, typically associated with obesity, and representing some 90% of the cases), gestational diabetes (first diagnosed during pregnancy), and a heterogeneous group identified as “other specific types” (35).

In nearly all countries diabetes has a high, and continuously growing, prevalence (36,37). In Western countries, these figures are mainly due to changes in life style, including diet high in saturated fats and decreased physical activity, eventually leading to obesity. Regarding type 1 diabetes, which accounts for most potential recipients of PTx, the prevalence of the disease in the United States is estimated to be 1,250,000 persons, with an annual incidence of 35,000 new cases (38).

Diabetes causes significant morbidity and increases mortality in affected individuals (35,39). The risk of heart disease and stroke is increased 3 to 5-fold, and 50-70% of patients with diabetes die of these events. Fifteen years after the onset of diabetes, diabetic retinopathy is present in the majority of patients. Eventually, 20-30% of patients with diabetes will develop severe visual impairment over the years. Reduction in the incidence of diabetic nephropathy among patients with type 1 diabetes, by approximately 10%, was overcompensated by a 20% increase in the incidence of this complication in patients with type 2 diabetes, leading to a net increase of the prevalence of diabetic nephropathy among dialyzed patients and confirming diabetic nephropathy as the leading cause of end-stage renal failure (39). Incidence of end-stage renal disease in patients with diabetes is higher compared to the patients without diabetes, with a relative risk of 6.2 in the white population and 62.0 among Native Americans. Diabetic neuropathy, in its several forms, affects up to 50% of people with diabetes. In combination with reduced blood flow, neuropathy in the feet increases up to 25-fold the chance of foot ulcers and of several fold eventual limb amputation (40).

TREATMENT GOALS IN DIABETES

There is a large amount of evidence recommending that glycated hemoglobin (HbA1c) should be maintained below 7.0% to reduce the incidence of microvascular disease (35,41). However, the effects of intensive diabetes management on the occurrence of macrovascular complications remains somewhat elusive, tending to be more evident in type 1 diabetes (42), as compared with type 2 diabetes (43,44). More stringent metabolic control (e.g., HbA1c 6.0–6.5%), when achieved without significant hypoglycemia or other adverse effects of treatment, can be preferred in patients with short disease duration, long life expectancy, and without significant cerebrovascular disease (41). On the other hand, less tight metabolic control (e.g., HbA1c 7.5–8.0%) can be accepted in patients at risk of severe hypoglycemia and/or with limited life expectancy, advanced vascular complications, or extensive comorbid conditions (41).

INDICATIONS FOR PANCREAS TRANSPLANTATION AND CANDIDATE SELECTION

PTx is performed to restore an endogenous source of servoregulated insulin production in beta-cell penic patients with diabetes. In technically successful PTx, restoration of beta-cell mass is consistently and reproducibly expected to induce insulin-independence, although at the price of significant surgical morbidity and life-long immunosuppression (2,45). In most patients with diabetes there is a clear advantage in receiving a pancreas graft, when also a kidney graft is needed to reverse end-stage renal failure. Moreover, PTx is indicated in selected patients with complicated and/or labile diabetes, when the risk of surgery and immunosuppression is surpassed by the ongoing risk of ineffective insulin therapy (2,45,46).

Based on these principles, the prototype recipient for PTx is a patient with type 1 diabetes without detectable c-peptide, poor metabolic control and/or progressive secondary complications of diabetes. However, selected patients with type 2 diabetes with high insulin needs, low to mild insulin resistance, and non- or mildly obese, may achieve insulin-independence after PTx and enjoy results similar to those of patients with type 1 diabetes (2,45,46).

Since failure of conventional, insulin-based, therapy is required to become eligible for PTx, most recipients have a 20- to 25-year history of diabetes. By this time, most recipients have developed end-stage nephropathy and also require a kidney transplant. Ideally, these patients should receive an SPK transplant because diabetic nephropathy is associated with high mortality rate, and 75% of insulin-dependent patients with diabetes do not survive longer than 5 years with dialysis (47-49). SPK improves patient survival versus either dialysis or deceased donor kidney transplantation (9,10,50).

In fragile recipients deemed not suitable for SPK, renal transplantation from a live donor is an attractive possibility either as definitive treatment or as a bridge to PAK. Actually, live donor renal transplantation may be worthily pursed also in patients otherwise eligible for SPK because of organ shortage (2,45,46). SCPLK provides an additional transplant opportunity, since it still exploits the benefits of live donation for the kidney but does not require the sequential PAK to correct the diabetes. The main disadvantages of SCPLK are the fact that the pancreas is a solitary graft, and that live renal donation cannot be programmed as it has to be performed when the deceased donor pancreas graft becomes available. To do so, three surgical teams have to work simultaneously (one for the deceased donor, one for the live donor, and one for the transplant) making organization and coordination quite complex (28). Considering that correction of uremia is key in these patients (10), but that ideal donors suitable for SPK are becoming extremely rare (51), when a deceased donor is available a kidney alone transplantation (KTA) should be considered as a valid alternative to leaving the patient with end-stage renal disease while waiting for a SPK donor, who may never become actually available. After KTA, PAK could allow correction of diabetes, thus preventing recurrence of diabetic nephropathy in the renal graft in the long-term period. Paradoxically, surgical complications associated with PAK could jeopardize renal function in the short-term period making the indication for PAK a matter of debate especially in terms of baseline renal function. Although there is no agreed cut-off of renal function to safely proceed with PAK, a stable renal function with a creatinine clearance of at least 60 ml/min/1.73 m2, and a negative urine analysis are all considered important criteria (2,46,52).

According to the American Diabetes Association, PTA may be an option in selected patients with diabetes who have recurrent hypoglycemia unawareness, and/or have medical or psychological problems with insulin therapy (52). Normal or near-normal renal function is also required because the anticipated long-term beneficial effects of sustained insulin-independence on diabetic nephropathy may be surpassed by accelerated deterioration of renal function caused mostly by the nephrotoxic effects of immunosuppressants (22,50,53). Additional evidence shows that also patients with progressive complications (i.e., reversible nephropathy, progressive retinopathy, and severe neuropathy) may improve significantly with PTA (13,20). Although the impact of PTA on patient survival is still debated (54,55), in suitable recipients, PTA improves the course of diabetic retinopathy (18), diabetic neuropathy (13), and diabetic nephropathy (22,50,53), and reduces the level of cardiovascular risk (13,15).

Each patient eligible for PTx is, by definition, at high risk for cardiovascular disease, making cardiac and vascular work up key in this transplant population. In recipients of solitary pancreas grafts (PAK and PTA) accurate estimate of renal function is also mandatory, as the risk of renal dysfunction/failure is reduced when the GFR is ≥ 60-70 mL/min (56). The decision to pursue a solitary PTx should hence be well balanced against the inherent risks of PTx. On the contrary, insulin-dependent patients with diabetes have in SPK their ideal treatment modality. The evaluation process in these patients should explore all possible venues to permit transplantation because continued dialysis is associated with short survival. Unfortunately, many patients are already too sick when they are first referred for transplantation and cannot be offered the chance of SPK.

Although type-2 diabetes is often characterized by obesity and peripheral insulin resistance, recent studies have demonstrated that the old paradigm is no longer generally applicable. Several studies showed improved glycemic control after pancreas transplantation in subsets of patients with type 2 diabetes, especially if body mass index is less than 35 kg/m2 (57).

CURRENT PANCREAS TRANSPLANTATION ACTIVITY

According to the International Pancreas Transplant Registry (IPTR) and the US Organ Procurement and Transplantation Network (OPTN) approximately 51,000 PTx have been performed worldwide (> 31,000 from the United States and >20,000 from other countries) (51,56). Considering that reporting to these registries is mandatory only for US Centers, the real number of PTx performed worldwide exceeds reported registry figures.

According to IPTR data, the total number of PTx steadily increased in the United States until 2004 (peaking at a total of 1484) but has since declined substantially with fewer than 1000 procedures performed in 2014 and in 2015. The overall amount of pancreas transplants decreased slightly, from 1027 in 2018 to 1015 in 2019(56). This remains considerably higher than the nadir of 947 reported in 2015, with a slight decrease attributed to declining in PTAs (124 to 99) and PAKs (68 to 44) from 2018 to 2019. In fact, SPKs continued to increase, from 835 to 872, the highest annual number of SPKs performed in the last decade.

The reasons for the decline in PTx activity are not immediately understood. In the history of solid organ transplantation good results, such as those currently achieved by PTx, typically portend higher volumes. Decline in PTx volumes coincided with a reduction in the number of active PTx centers with only 11 Institutions performing ≥ 20 PTxs per year and most centers performing < 5 PTxs annually (51). The outcome of PTx is known to be influenced by center volume (58). Additionally, lower PTx volumes per center are expected to reduce the opportunities for training of younger generations of transplant physicians and surgeons, thus potentially worsening future outcomes of PTx and further reducing the volumes of PTx, in a vicious circle.

The reason for the current decline in PTx activity is multifactorial. Some factors are historical, such as limited referral of potential recipients (51), and incomplete procurement of pancreas grafts from otherwise suitable donors (59). Other factors, however, are newer and less correctable with educational or training programs for healthcare professionals (60). These factors include the progressive ageing of donor population (61), the increasing number of obese donors (62), and the growing proportion of cerebrovascular accidents as a cause of brain death (61). The combination of these epidemiologic factors makes the “ideal” pancreas donor (age ≤ 40 years, low BMI, death due to trauma, short stay in the intensive care unit, and hemodynamic stability without, or with low dose, vasoactive amines) extremely rare (63). These factors, along with the duration of cold graft storage, are summarized in the Pancreas Donor Risk Index (63). This index, conceived to optimize the use of all grafts suitable for PTx, has instead promoted additional donor selection and further reduced the number of PTx (64). Although it is known that PTx can be pursued using marginal donors with good results (65,66), most centers are not willing to accept this type of donor, as their use may be associated with higher rates of early graft failure.

IMPACT OF COVID-19 PANDEMIC ON PANCREAS TRANSPLANTATION

The global coronavirus disease 2019 (COVID-19) pandemic caused by the SARS-CoV-2 virus reduced the worldwide transplant activity due to the overload of the health system and concern for patient safety. Since the first few months of the pandemic, the transplant community worked on characterizing infection, morbidity, and mortality from COVID-19 in the transplanted or waitlisted patient comparing outcomes to the general population. According to a worldwide survey, pancreas transplant activity declined shortly after the beginning of the COVID-19 pandemic because of both a reduction in patient referrals and utilization of deceased donors (67). There are limited clinical data on COVID-19 in PTx recipients, including a few case reports (68,69) and small series (70-73). As detailed in a recent review, COVID-19 in PTX recipients was mostly managed by reduction of immunosuppression with withdrawal of antimetabolites. Despite lower immunosuppression, the risk of rejection and graft loss does not appear to be clearly increased (74).

PANCREAS TRANSPLANTATION FROM DONORS AFTER CARDIAC DEATH

Shortage of suitable brain-dead donors (DBD), has forced the transplant community to explore the venue of donation after cardiac death (DCD). Based on Maastricht criteria (64) there are four categories of DCD donors. PTx is pursued in type 3 DCD donors, also known as controlled DCD donors. In this category of donors, cardiac arrest is awaited following withdrawal of ventilatory support in patients with fatal brain injuries who are not expected to progress to brain death (64). The use of this type of donors is associated with high organizational needs and may be influenced by national attitudes and regulations (65), but the results of PTx are quite encouraging making this source of grafts worth of further exploration (75-78).

In a recent systematic review and meta-analysis, Shahrestani and Co-workers identified 18 studies on PTx from DCD donors. No difference was noted in allograft survival (hazard ratio, 0.98; 95% confidence interval [95% CI], 0.74-1.31; p= 0.92), and recipient survival up to 10 years after PTx between DBD and DCD donors (hazard ratio, 1.31; 95% CI, 0.62-2.78; p= 0.47). The odds ratio for vascular thrombosis was 1.67 times higher in PTx from DCD organs (95% CI, 1.04-2.67; p= 0.006), but this difference was not evident in PTx from a subgroup of DCD who were treated with heparin (78).

GRAFT PROCUREMENT, PRESERVATION, AND TRANSPLANTATION TECHNIQUES

The history of pancreas transplantation has been shaped by developments in surgical techniques (7) and advancements in immunosuppressive regimens (79). It is now accepted that pancreas grafts are composed by the entire gland with an attached duodenal segment and that the organs are procured with minimal dissection in the donor during the heart beating period. A single arterial conduit is prepared at the back-table, usually by anastomosing the peripheral branches of a Y-shaped donor iliac graft to the cut ends of the superior mesenteric and splenic arteries (80). In rare circumstances, a segmental pancreas graft made of the body and tail of the gland, can be transplanted. This type of graft is used when there are concerns on perfusion of the pancreatic head/duodenum to allow PTx in otherwise “difficult to transplant” recipients, such as patients with high immunization titers. A segmental pancreas graft is also used from live donors (29). Pancreas grafts are highly sensitive to ischemia-reperfusion injury (63). Despite the incidence of surgical complications not significantly increasing until 20 hours of preservation (81), most centers now prefer to maintain the period of cold storage to ≤ 12 hours (82).

At the moment, the gold standard for pancreas graft preservation is static cold storage using the University of Wisconsin solution (83). When the period of cold storage is not exceedingly long also Celsior (84) and histidine-tryptophan-ketoglutarate (85) can be accepted. The use of histidine-tryptophan-ketoglutarate has been associated with higher rates of graft pancreatitis (86). Reduction of perfusion volumes are thought to prevent these complications. IGL-1 in a newer preservation solution, but data on PTx are yet scarce (87). As with other organs, machine perfusion is being explored also for pancreas allografts. The potential of this innovative preservation strategy in PTx remains to be established (88).

Regarding transplantation techniques, it is quite surprising that none was clearly shown to be superior over the other procedures (89). Despite this, some surgical techniques have become very popular and are currently considered standard procedures for PTx. The main variations in PTx technique regard the site for venous drainage (either systemic or portal) and the site for exocrine drainage (either urinary or enteric). In enterically drained grafts other major variations are the use of a Roux-en-Y isolated loop or the creation of a direct anastomosis between the donor duodenum and the recipient small bowel (90), duodenum (91-94), or stomach (95).

The combination of systemic venous effluent and enteric exocrine drainage is currently prevalent (7) as the alleged metabolic and immunologic advantages of portal venous drainage have not been unambiguously proven (96). Bladder drainage along with the inclusion in the graft of a duodenal segment (97 PTx is not employed very frequently at the present time because of frequent urologic and metabolic complications.

The greatest innovation in surgical technique is the description of laparoscopic, robotic-assisted, PTx. The initial experience by Boggi et al (98,99) was recently duplicated at the University of Illinois at Chicago (100). This makes PTx a minimally invasive procedure and is associated with obvious advantages but has high organizational needs, and requires surgeon and team training in advanced robotic procedures.

IMMUNOSUPPRESSIVE PROTOCOLS

Current state-of-the art immunosuppression in PTx was recently reported in a review article (101) and practice recommendations were provided by the proceedings of the first world consensus conference on pancreas transplantation (WCCPTx) (102-103).

Although the immunologic outcome of PTx has improved over the years, rejection still occurs quite frequently (from 20-30% in SPK to around 40% in PTA) (104). Accordingly, the use of T-cell depleting antibody induction is still preferred in some 90% of recipients, while an anti-interleukin-2 receptor antibody alone is used in the remaining 10%. In last two decades, maintenance immunosuppression regimens have employed tacrolimus and mycophenolate in over 80% of the patients (105-106). The use of cyclosporine and/or mammalian target of rapamycin has been mostly considered in the setting of switching in case of documented side effects related to the standard regimen (107) Steroids may be withdrawn or minimized to avoid their side effects, including the risk of glucose intolerance (108-109). The recent evidence that development of donor specific antibodies occurs in PTx and is associated with worse immunologic outcome, further compounds the field and could require the adoption of newer protocols for the treatment of antibody-mediated rejection such as a combination of anti CD20, intravenous immunoglobulins, and protease inhibitors (110). Early experiences suggest that switch from calcineurin inhibitors to belatacept, a T-cell co-stimulation blocker used to prevent acute rejection in adult renal transplant recipients, may reduce nephrotoxicity without evidence of increased risk of kidney or pancreas rejection (111,112). Belatacept may represent an important strategy for preservation of renal and pancreatic function after SPK transplantation, either as first-line or rescue therapy. A trial in primary SPK transplantation (NCT01790594), using belatacept for induction and for maintenance, in combination with mycophenolate mofetil and low dose calcineurin inhibitors, with early steroid withdrawal, was recently completed.

According to a recent review no major improvement in immunosuppressive regimens used for PTx was achieved during the last 20 years. Most PTx patients receive induction with depleting antibodies and maintenance with a combination of a calcineurin inhibitor (with tacrolimus being more prevalent than cyclosporine) plus mycophenolate and steroid maintenance. Newer drug combinations and well-designed prospective studies are needed to further improve the outcome of PTx (101).

POST-TRANSPLANT COMPLICATIONS

PTx carries the highest risk of post-transplant complications among all solid organ transplants, as a consequence of the medical complexity of recipients with diabetes and the susceptibility of pancreas allografts to develop vascular thrombosis and pancreatitis. Occurrence of post-operative complications reduces the rate of graft survival, with allograft pancreatectomy being required in some 5% of PTx recipients, but does not affect patient survival (113). Life-threatening complications still occur in approximately 3% of recipients, mostly because of development of an arterial pseudoaneurysm or an arteroenteric fistula (114).

In the long–term, malignancies as well as bacterial, viral, and fungal infections remain a significant cause of mortality and morbidity (114). Among a cohort of 360 SPK transplants, overall 5-year patient survival was 84%, but 25 recipients (6.9%) developed malignant tumors. Almost one-fourth of the cancers were skin tumors and 5 patients developed post-transplant lymphoproliferative disorders (PTLD) (106). According to the SRTR/Annual Data Report the cumulative incidence of PTLD at 4 years is 2.3% after PTA, 0.9% after SPK, and 1.1% after PAK. The higher frequency of PTLD in PTA patients is likely related to their increased immunosuppression and higher rates of acute rejection (104,116,117). The incidence of other cancers is 3- to 4-fold higher compared with the background population (115).

PATIENT AND GRAFT SURVIVAL

According to the International Pancreas Transplant Registry, 5- and 10-year graft function rates in 21,383 PTx, performed from 1984 to 2009, are 73 and 56%, respectively, for SPK; 64 and 38%, respectively, for PAK; and 53 and 36%, respectively, for PTA (1).

Cardiovascular and/or cerebrovascular events are the leading cause of recipient death either short- (<3 months post-transplant) and long-term (>1-year post-transplant) (118). In patients with type 1 diabetes, SPK has been shown in several studies to increase the observed versus expected lifespan, as compared with a kidney transplant alone (119,120). According to a large study of 13,467 patients, using data from the US Scientific Renal Transplant Registry and the US Renal Data System, the patient survival rate at 10 years post-transplant was significantly higher in recipients of a SPK than of a KTA from a deceased donor. In fact, recipients of a SPK had the greatest longevity (23.4 years), as compared with 20.9 years for recipients of a KTA from a living donor and 12.8 years for recipients of a KTA from a deceased donor (10,121).

In recipients of PAK, evidence shows that the PTx improves long-term patient and kidney graft survival rates. Also, glomerular filtration rates are significantly higher after PAK than after KTA (122). In recipients of PTA who have brittle diabetes mellitus, the mortality rate at 4 years is lower than that in the waiting list candidates (123). Earlier reports stating a survival disadvantage for recipients of solitary pancreas transplants (PTA and PAK) compared with patients on the waiting list for a transplant now seem to be unsubstantiated (54).

Pancreas graft survival rate is based on insulin independence. In the past decade, unadjusted graft survival rates at 1 year were 89% for SPK, 86% for PAK and 82% for PTA. Equivalent figures at 5 years were 71%, 65%, and 58%, respectively (118). More recently, 10-year actual insulin independence rates have been reported to exceed 80% in SPK and 60% in PTA (12,13).

The greatest improvements are seen in the gains over time in the estimated half-life (50% function) of pancreas grafts. The estimated half-life is now 14 years for SPK, and 7 years for both PAK and PTA. Moreover, the estimated half-life has increased to 10 years in recipients of PAK or PTA with a functioning pancreas graft at 1-year post-transplant. The longest pancreas graft survival time, by category, has been 26 years (SPK), 24 years (PAK) and 23 years (PTA) (124).

The leading cause of pancreas loss is rejection (125,126). Autoimmunity is also increasingly recognized as a cause of graft failure (127,128). The diagnosis of pancreatic rejection is based on laboratory markers and imaging techniques, but core biopsy remains the final diagnostic tool. In SPK, a rise in serum creatinine can be a surrogate for pancreas rejection suspicion; however, discordant kidney and pancreas rejection have been described (129). An increase in serum amylase and lipase, although not specific, can be an initial sign of pancreatic immune-activation. Hyperglycemia occurs only in cases of severe beta-cell dysfunction or destruction, and therefore it is a late marker of rejection. Guidelines for the diagnosis of PTx rejection have been recently updated with major implementation for the identification of antibody mediated rejection (130). Pancreatic antibody mediated rejection is a combination of serological and immunohistological findings consisting of donor specific antibody detection, morphological evidence of microvascular injury, and C4d staining in interacinar capillaries. The newest Banff schema recognizes different patterns of immunoactivation, including the recurrence of autoimmune diabetes that is characterized by insulitis and/or selective beta-cell destruction. Among the different causes of graft loss, recent studies have proven that despite immunosuppression, the recurrence of autoimmune disease is not a rare event (129). Historical experience with segmental PTx in identical twins showed that, without immunosuppression, autoimmune destruction of beta cells occurs early after PTx (131). Immunosuppression prevents such recurrence in most, but not in all, patients (127).

Graft failure of any organ has a negative impact on patient survival. In recipients of SPK, kidney graft loss increases the relative risk of death by a factor of 17.6 and pancreas graft loss by a factor of 3.1. In recipients of PAK, kidney graft loss increases the relative risk of death by a factor of 4.3 and pancreas graft loss by a factor of 4.1. In recipients of PTA, pancreas graft loss increases the relative risk of death by a factor of 4.1 (132).

EFFECTS OF PANCREAS TRANSPLANTATION ON ACUTE DIABETES COMPLICATIONS

The excess mortality seen in type 1 diabetes is largely related to diabetes and its comorbidities. Acute complications are represented by hyperglycemic syndromes (most commonly ketoacidosis, less frequently the hyperosmolar syndrome) and hypoglycemia induced by exogenous insulin therapy. They contribute to 80% of all early (<10-year diabetes duration) deaths, and for a 15% of deaths thereafter. Most early acute deaths result from diabetic ketoacidosis (often at diabetes onset or after an acute illness), whereas later acute deaths tend to result from hypoglycemic episodes (133,134). Successful PTx restores a regulated endogenous insulin production and eliminates the need for exogenous insulin administration. As such, no acute diabetic complication is seen in patients with fully functioning pancreatic graft. In addition, PTx improves hypoglycemia counter-regulation, by improving catecholamine and glucagon responses to glucose lowering. These improvements are stable and long-lasting, and have been shown up to 19 years from the grafting (135). Recently, the use of beta cell replacement therapy has been discussed for patient with problematic hypoglycemia, defined as two or more episodes per year of severe hypoglycemia or as one episode associated with impaired awareness of hypoglycemia (136). In such cases, if appropriate educational and technological interventions are not sufficient to improve the condition, PTx is indicated (136). It is therefore reasonable to consider PTx in patients with type 1 diabetes who are at proven risk for serious episodes of insulin-induced hypoglycemia and who demonstrate refractoriness to conventional medical management (135,136).

EFFECTS OF PANCREAS TRANSPLANTATION ON CHRONIC DIABETES COMPLICATIONS

Chronic diabetes complications are a major burden of the disease, dramatically contributing to deterioration of quality of life and reduced survival in the population with type 1 diabetes (137). They can be broadly separated into two categories: microvascular and macrovascular. The first ones are due to damage of small vessels involving eyes, kidneys and nerves, while the others are related to damage in larger blood vessels.

Diabetic Retinopathy

Diabetic retinopathy (DR) is the most common, highly specific microvascular complication of diabetes, with prevalence strongly related to duration of diabetes and the levels of glycemic control. Numerous studies have been performed to elucidate the role of PTx on the clinical course of this complication. Initial work (138,139) found that SPK with subsequent normalization of blood glucose concentrations did not play a role in preventing or reversing retinal damage, but more recent studies support the view that PTx has beneficial effects. In a study conducted on 48 successful SPK, a careful eye examination was performed before and up to 60 months after grafting, with standardized classification of DR (19). The results showed, compared with a group of non-transplanted, matched patients with type 1 diabetes, that SPK recipients had a significantly higher rate of improvement or stabilization of the retinal lesions, depending on the severity of retinopathy at the time of transplantation. A report describing 112 patients with functioning SPK showed an improvement and/or stabilization in 73.5% patients with non-proliferative retinopathy, with an important decrease in the number or ophthalmologic procedures after a period of 4 years (140). Regarding the role of PTA, the course of DR was studied prospectively in PTA recipients and in non-transplanted patients with type 1 diabetes, with a follow-up of almost 3 years (18). The PTA and non-PTA groups consisted respectively of 33 (follow-up: 30 +/- 11 months) and 35 patients (follow-up: 28 +/- 10 months). Best corrected visual acuity, slit lamp examination, intraocular pressure measurement, ophthalmoscopy, retinal photographs, and in selected cases angiography were performed by the authors. At baseline, 9% of PTA and 6% of non-PTA patients had no diabetic retinopathy, 24 and 29% had non-proliferative diabetic retinopathy (NPDR), whereas 67 and 66% had laser-treated and/or proliferative diabetic retinopathy (LT/PDR), respectively. No new case of diabetic retinopathy occurred in either group during follow-up. In the NPDR PTA group, 50% of patients improved by one grading, and 50% showed no change. In the LT/PDR PTA, stabilization was observed in 86% of cases, whereas worsening of retinopathy occurred in 14% of patients. In the NPDR non-PTA group, diabetic retinopathy improved in 20% of patients, remained unchanged in 10%, and worsened in the remaining 70%. In the LT/PDR non-PTA group, retinopathy did not change in 43% and deteriorated in 57% of patients. Overall, the percentage of patients with improved or stabilized diabetic retinopathy was significantly higher in the PTA group (18). Therefore, although cases of early deterioration of diabetic retinopathy have been reported after pancreas transplantation (141), current evidence indicates delay of development and/or increased rate of stabilization of this complication following functioning pancreatic graft (142,143).

Diabetic Kidney Disease

Type 1 diabetes mellitus patients present a high risk of developing renal complications. Diabetic kidney disease, or CKD attributed to diabetes, occurs in 20 – 40% of patients with diabetes and is the leading cause of end-stage renal disease (ESRD) (144). Progression to ESRD in this patient population has important prognostic implications (48,145) and proves to be resistant to most nephroprotective therapeutic measures (146). As discussed above, simultaneous pancreas-kidney transplantation (SPK) in T1D patients is associated with improved patient survival compared to solitary cadaveric renal transplantation (10,121,147,148). Regarding the survival of the grafted kidney, the SPK approach generally guarantees better results compared with the cadaveric donor kidney only transplant. In long-term results (>10 years), the kidney graft survival rate in SPK is equal or better compared to that observed with a living donor solitary renal transplantation (149). Successful long-term normoglycemia as obtained by a functioning pancreas can also prevent recurrence of diabetic glomerulopathy in the kidney graft, as shown histologically by comparing renal biopsies from SPK or PAK versus kidney transplant alone (follow-up 1 to 6 years, approximately). In addition, SPK has been reported to be associated with better creatinine levels and reduced urinary albumin excretion in SPK patients, compared to kidney alone grafted individuals (150). Along similar lines, in patients with type 1 diabetes and long-term normoglycemia after successful SPK transplantation, kidney graft ultrastructure and function were better preserved compared with LDK transplantation alone (151). Altogether, the available information indicates that pancreas transplantation plays a role in protecting the grafted kidney and preventing the recurrence of diabetic nephropathy in renal allografts.

In the case of PTA, the effects on the native kidneys are not fully established yet. Currently available immunosuppressive drugs are nephrotoxic, and this places pancreas transplantation recipients, like other solid organ recipients (152), at risk for post-transplant nephropathy (153,154). Gruessner et al. (155) showed that a serum creatinine level above 1.5 mg/dL, recipient age below 30 years and or tacrolimus levels > 12 mg/dl at 6 months were significantly associated with the development of overt renal failure after PTA. However, in another study (156) no significant deterioration of renal function was observed at 1 year after PTA in patients with glomerular filtration rate (GFR) of about 50 ml/min. Initial work from our group showed no significant change in creatinine concentration and clearance and an improvement in proteinuria at 1 year after PTA (22). More recently, we reported the results achieved in 71 PTA recipients 5 years after transplantation (13,20). In this series proteinuria improved significantly, and only one patient developed ESRD. In the 51 patients with sustained pancreas graft function, kidney function (serum creatinine and glomerular filtration rate) decreased over time with a slower decline in recipients with pretransplant eGFR less than 90 ml/min in comparison to those with pretransplant eGFR greater than 90 ml/min; this finding is possibly due to the correction of hyperfiltration following normalization of glucose metabolism. However, another study (157) reported an accelerated decline in renal function after PTA in the patient population with lower pretransplant GFR. Important information on this issue has been provided by a study conducted with 1135 adult recipient of first PTA (55). The authors have subdivided their series of recipients into three groups, depending on the eGFR (ml/min/1.73 m2): ≥ 90 (n: 528), 60-89 (n: 338) and < 60 (n: 269). The patients were followed up to 10 years and the outcome was ESRD, according to the need for maintenance dialysis or kidney transplantation. The results indicated that at 10 years the cumulative probability of ESRD was 21.8%, 29.9% and 52.2% in recipients with pre-transplant eGFR ≥ 90, 60-89 and < 60 ml/min/1.73 m2, respectively (55). Overall, data available indicates the renal function before PTA as a major factor affecting post-transplantation evolution of the function of the native kidneys. The course of diabetic nephropathy after pancreas transplantation has also been characterized histologically (158-160). Fioretto et al. (161) performed protocol biopsies in patients who had received a successful PTA and found that, whereas 5 years after transplant the histologic lesions of diabetic nephropathy were unaffected, at 10 years reversal of diabetic glomerular and tubular lesions was evident. The histologic reversibility of diabetic nephropathy was previously shown in the case of transplantation of human cadaveric kidneys into recipients without diabetes (162,163) and is supported by the current favorable outcome of deceased diabetic donor kidneys (164). Of interest, a recent study has shown that mortality in PTA recipients who develop ESRD is similar to that found in type 1 diabetic patients on dialysis (165). Therefore, current evidence indicates that normoglycemia ensuing after successful pancreas transplantation prevents and may even reverse diabetic nephropathy lesions in native kidneys and kidney grafts. This has to be balanced with the potential nephrotoxic effects of immunosuppression.

Diabetic Neuropathy

Diabetic neuropathy affects approximately 50% of T1D patients and is associated with reduced survival (166,167). All types of pancreas transplantation may have beneficial effects on diabetic neuropathy (sensory, motor, and autonomic) (168-172). Navarro et al. (171) compared the course of diabetic neuropathy in 115 patients with a functioning pancreas transplantation (31 SPK, 31 PAK, 43 PTA without and 10 PTA with subsequent kidney transplantation) and 92 control patients over 10 years of follow-up. Using clinical examination, nerve conduction studies, and autonomic function tests, the authors found significant improvements in the transplanted groups (similar across the different subgroups) (171). Allen et al. demonstrated a gradual, sustained, and late improvement in nerve action potential amplitudes, consistent with axonal regeneration and partial reversal of diabetic neuropathy, in SPK recipients. Two distinct patterns of neurological recovery were analyzed: conduction velocity improved in a biphasic pattern, with a rapid initial recovery followed by subsequent stabilization. In contrast, the recovery of nerve monophasic amplitude continued to improve for up to 8 years (170). Similarly, we found a significant improvement in Michigan Neuropathy Screening Instrument scores (173), vibration perception thresholds, nerve conduction studies, and autonomic function tests in a series of PTA patients with long-term follow-up (13,20). The beneficial effects of pancreas transplantation on cardiac autonomic neuropathy were also reported by Cashion et al. (174) using 24 h heart rate variability monitoring. However, spectral analysis of heart rate variation was performed by Boucek et al. (175), but without significant findings. Interestingly, Martinenghi et al. (172) monitored nerve conduction velocities in five patients who underwent SPK, reporting a significant improvement which was strictly dependent on pancreas graft function. Nerve regeneration is defective in patients with diabetes (166). In a case report, Beggs et al. (176) performed sequential sural nerve biopsies after PTA and found histologic evidence of nerve regeneration. Quantification of nerve fiber density in skin biopsies (177-179) or in gastric mucosal biopsies obtained during endoscopy (180) is an interesting tool to assess diabetic neuropathy. However, Boucek et al. (181,182) did not find any significant improvement in intraepidermal nerve fiber density after pancreas transplantation. In contrast, Mehra et al. used corneal confocal microscopy, a noninvasive and well validated imaging technique (183,184), and were able to find significant small nerve fiber repair within 6 months after pancreas transplantation. These latter findings have been recently confirmed (26). Lately, it has been observed that successful pancreas transplantation improved cardiovascular autonomic neuropathy (185). However, the impact of pancreas transplantation on late, serious autonomic neurological complications (gastroparesis, bladder dysfunction) is still unsettled.

Cardiovascular Disease

Patients with diabetes present an increased risk for cardiovascular morbidity and mortality, mainly due to diffuse coronary atherosclerosis and diabetic cardiomyopathy (132). After SPK, cardiovascular events remain a primary cause of morbidity and mortality (186), both in the immediate postoperative period (187) and in the long term (188). Preoperative cardiovascular assessment is mandatory to select patients who may maximally benefit from transplantation (189,190), which could also include myocardial perfusion scintigraphy (191).

In SPK recipients, improvement in macrovascular disease (including cerebral vasculopathy and morphology) and cardiac function has been generally observed. A retrospective study of cardiovascular outcomes after SPK and cadaveric kidney-alone transplantation (192) showed cardiovascular death rate (acute myocardial infarction, acute heart failure, lethal arrhythmias, acute pulmonary edema) of 7.6% in SPK, 20.0% in kidney alone and 16.1% in dialyzed patients. In the same study, SPK was associated with improved left ventricular ejection fraction, left ventricular diastolic function, blood pressure, peak filling rate to peak ejection rate ratio and endothelial dependent dilation of the brachial artery (193,194). A study by Biesenbach et al compared SPK and KTA: after 10 years from the procedure, in the SPK group the authors showed a significant lower frequency of vascular complications which included myocardial infarction (16% vs. 50%), stroke (16% vs. 40%) and amputations (16% vs. 30%). In addition, when the cardiovascular outcomes after SPK or living donor kidney-alone transplantation were compared, it was found that SPK was associated with reduced long-term cardiovascular mortality especially in a long term follow up (195). Less information is available regarding the effects of PTA on the cardiovascular system. In a single center experience with 71 consecutive PTA followed for 5 years, clinical cardiac evaluation and doppler echocardiographic examinations were performed. The authors observed that left ventricular ejection fraction increased significantly, and several parameters of diastolic function improved (13). Most of these findings were confirmed after 8 years from transplant (11). As for the effects of PTx on the peripheral arteries, the available information suggests that this type of transplantation neither aggravates nor improves peripheral vascular disease events or progression (196). However, some authors have reported that SPK is protective against atherosclerotic risk factor and progression, prothrombotic state, endothelial function and carotid intima media thickness independent of significant changes in other risk factor (197).

FIRST WORLD CONSENSUS CONFERENCE ON PANCREAS TRANSPLANTATION

The first WCCPTx was held in Pisa (Italy) October 18-19, 2019. Based on the analysis and discussion of 597 studies, an independent jury provided 49 jury deliberations concerning the impact of pancreas transplantation on the treatment of patients with diabetes, using the Zurich-Danish model, while a group of 51 experts, from 17 countries and 5 continents, provided 110 recommendations for the practice of PTx. Consensus was reached after two online Delphi rounds with a final voting at the consensus conference on Pisa. Each recommendation received a GRADE rating (Grading of Recommendations, Assessment, Development and Evaluations) and was validated using the AGREE II instrument (Appraisal of Guidelines for Research and Evaluation II). Quality of evidence was assessed using the SIGN methodology (Scottish Intercollegiate Guidelines Network).

The WCCPTx conveys several important messages. First, both SPK and PTA can improve long-term patient survival. Second, PAK increases the risk of mortality only in the early period after transplantation, but is associated with improved life expectancy thereafter. Third, all types of PTx dramatically improve of quality of life of recipients. Fourth, depending on severity at baseline, PTX has the potential to improve the course of chronic complications of diabetes. Fifth, SPK transplantation should be performed before initiation of dialysis or shortly thereafter, as time on dialysis has negative prognostic implications for patients with diabetes. As a consequence, kidney grafts should be preferentially allocated to patients listed for an SPK transplant (102-103).

CONCLUSIONS

As shown by the WCCPTx, PTx has a high therapeutic index, when correctly indicated and performed at proficient centers. Therefore, all possible efforts should be made to make this important treatment option available in a timely manner to all suitable recipients.

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126. Troxell ML, Koslin DB, Norman D, Rayill S, Mittalhenkle A. Pancreas allograft rejection: analysis of concurrent renal allograft biopsies and posttherapy follow-up biopsies. Transplantation 2010;90:75-84.
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128. Occhipinti M, Lampasona V, Vistoli F, Bazzigaluppi E, Scavini M, Boggi U, et al. Zinc transporter 8 autoantibodies increase the predictive value of islet autoantibodies for function loss of technically successful solitary pancreas transplant. Transplantation 2011;92:674-7.
129. Shapiro R, Jordan ML, Scantlebury VP, Vivas CA, Jain A, McCauley J, et al. Renal allograft rejection with normal renal function in simultaneous kidney/pancreas recipients: does dissynchronous rejection really exist? Transplantation 2000;69:440-1.
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136. Paty BW, Lanz K, Kendall DM, Sutherland DE, Robertson RP. Restored hypoglycemic counterregulation is stable in successful pancreas transplant recipients for up to 19 years after transplantation. Transplantation 2001;72:1103-7.
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138. Scheider A, Meyer-Schwickerath E, Nusser J, Land W, Landgraf R. Diabetic retinopathy and pancreas transplantation: a 3-year follow-up. Diabetologia 1991;34:S95-6.
139. Zech JC, Trepsat D, Gain-Gueugnon M, Lefrancois N, Martin X, Dubernard JM. Ophthalmological follow-up of type 1 (insulin dependent) diabetic patients after kidney and pancreas transplantation. Diabetologia 1991;34 (Suppl 1):S89-91.
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148. Lindahl JP, Hartmann A, Horneland R, Holdaas H, Reisaeter AV, Midtvedt K, et al. Improved patient survival with simultaneous pancreas and kidney transplantation in recipients with diabetic end-stage renal disease. Diabetologia 2013;56(6):1364–71
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153. Fioretto P, Najafian B, Sutherland DE, Mauer M. Tacrolimus and cyclosporine nephrotoxicity in native kidneys of pancreas transplant recipients. Clin J Am Soc Nephrol. 2011;6(1):101-6
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162. Abouna GM, Al-Adnani MS, Kremer GD, Kumar SA, Daddah SK, Kusma G. Reversal of diabetic nephropathy in human cadaveric kidneys after transplantation into nondiabetic recipients. Lancet. 1983;2(8362):1274-6
163. Abouna GM, Adnani MS, Kumar MS, Samhan SA. Fate of transplanted kidneys with diabetic nephropathy. Lancet. 1986;1(8481):622-3
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Diffuse Hormonal Systems

ABSTRACT

 

Neuroendocrine (NE) cells are rare epithelial cells that, in addition to having an endocrine function, express markers and peptides otherwise associated with neurons and the central nervous system. NE cells can be found as either single cells or small clusters of cells dispersed throughout the parenchymal surface epithelium of different tissues, including the lung, the intestine, and the pancreas. The observation that NE cells, which are dispersed throughout the body in different tissue sites, are often innervated and secrete bioactive compounds that can act both locally and systemically, led to the idea of the diffuse neuroendocrine system, a diffuse hormonal system composed of NE cells. NE cells perform important endocrine functions. Furthermore, NE cells are implicated in several human diseases. In particular, a group of rare tumors that presumably arise from NE cells, neuroendocrine neoplasms (NENs), have sparked a great deal of interest in NE cell biology. NENs can arise in almost all tissues but they show the highest incidence in the lung and the gastroenteropancreatic (GEP) system. In this chapter, we will outline what is currently known about NE cell differentiation and function, focusing specifically on NE cells of the lung, pulmonary neuroendocrine cells (PNECs), and the most prominent NE cells of the GEP system: enteroendocrine cells (EECs) of the small intestine and stomach and pancreatic endocrine cells. We will also discuss the potential role of these specific NE cells in the context of tissue injury. Finally, we will provide a brief overview of NEN biology with regards to NENs arising in the lung and GEP system.  

 

INTRODUCTION

 

Neuroendocrine (NE) cells are epithelial cells that, in addition to having an endocrine function, express markers and peptides otherwise associated with neurons and the central nervous system (1,2). NE cells can be defined by the presence of dense secretory granules and the expression of general NE markers including chromogranin A and synaptophysin. The first identified NE cells were the enterochromaffin (EC) cells of the small intestine, whose distinctive shape and histological properties piqued the interest of scientists during the late nineteenth and early twentieth centuries. In particular, dense secretory granules within NE cells hint to their endocrine function. These secretory granules also make NE cells reactive to chromium and silver, which makes them easy to identify using histological staining methods. Throughout their history, NE cells in the intestine have been referred to in the literature with a variety of names recalling their distinct reactions to histological stains: clear cells (did not pick up conventional stains), chromaffin cells (reacted to chromium salts), argentaffin cells (affinity for silver stains), and Kulchitsky cells (in honor of one of the scientists who studied them) (3,4). 

 

Following the description of NE cells in the intestine, histological studies revealed the presence of NE cells not only throughout the intestinal mucosa, but also in other epithelial tissues (1–8). NE cells can be found as either single cells or small, often innervated clusters of cells dispersed throughout the parenchymal surface epithelium of different tissues, including the small intestine, the lung, and the urogenital tract. As is the case for the cells of the pancreatic islets of Langerhans, the C cells of the thyroid, and adrenal medullary cells, NE cells can also form distinct clusters of cells within endocrine glands.

 

In 1938, drawing on his histological studies of NE cells in the pancreas and intestine, Friederich Feyrter proposed that NE cells comprise a diffuse neuroendocrine system. Nearly 30 years later, Anthony Pearse refined this idea of the diffuse neuroendocrine system by showing that NE cells, much like neurons, are able to metabolize amines and produce polypeptide hormones. Thus, the concept of a diffuse neuroendocrine system that functions as a diffuse hormonal system and is composed of cells dispersed throughout the body that, through the secretion of bioactive compounds, communicate in a coordinated fashion with their surroundings and with the nervous system solidified (8,9). Famously, Pearse also suggested that NE cells are all derived from the neural crest. This hypothesis, however, was later disproved by several elegant lineage tracing experiments. With the exception of the cells of the adrenal medulla, the extra-adrenal paraganglia, and C cells of thyroid, which are indeed derived from the neural crest, different types of NE cells are derived from the epithelial progenitors of their respective tissue sites (3).

 

While the specific function of pancreatic islet cells, the cells of the adrenal medulla, and C cells of the thyroid, for example, have been well-established both in terms of their contribution to specific organ function and the maintenance of homeostasis; the specific functions of other NE cells are less well defined. Furthermore, the list of polypeptide hormones and neuropeptides secreted by NE cells is continuously being updated and further refined. Studies that elucidate the developmental differentiation trajectories of NE cells from different tissues have expanded the list of common NE marker genes so that it no longer includes only hormones and neuropeptides but also lineage specific transcription factors. A summary of common NE markers is provided in Table 1.

 

Table 1. Common NE Markers

NE Marker

Function

Associated NE cell types

ASCL1

Transcription factor

PNECs, some gastric EECs

NEUROD1

Transcription factor

GEP EECs

INSM1

Transcription factor

All NE cells

Chromogranin A (CHGA)*

Secretory protein

All NE cells

Synaptophysin (SYP)*

Synaptic vesicle glycoprotein

All NE cells

NCAM1 (a.k.a. CD56)*

Cell adhesion molecule

All NE cells

UCHL1 (a.k.a. PGP9.5)*

Deubiquitinating enzyme

All NE cells

Neuron Specific Enolase (NSE)*

Metabolic enzyme

All NE cells

* Indicates markers used in clinical diagnosis

 

Much of the interest in NE cell biology has been initiated by observations that have been made regarding their behavior in disease. In particular, a group of rare tumors that presumably arise from NE cells, neuroendocrine neoplasms (NENs), have sparked a great deal of interest in NE cell biology inasmuch as this might relate to the genesis and peculiar clinical behavior of some of these tumors. NENs have been observed in almost all tissues, and consist of well-differentiated neuroendocrine tumors (NETs), tumors that proliferate and progress slowly, and neuroendocrine carcinomas (NECs), poorly differentiated tumors that have a poor prognosis (10). While NENs were initially classified according to the embryological origin of their tissue site of incidence (i.e., foregut, midgut, or hindgut), they are now referred to according to their specific tissue site of origin and, in the case of tumors that elicit hormonal syndromes, according to the primary hormone they secrete.

 

NENs show the highest incidence in the lung and the gastroenteropancreatic (GEP) system (Figure 1). For this reason, as outlined above, while there are many different kinds of NE cells arising in many different tissue sites, for the purposes of this chapter, we will focus on the NE cells of the lung, pulmonary neuroendocrine cells (PNECs), and the most prominent NE cells of the GEP system: enteroendocrine cells (EECs) of the small intestine and stomach, and pancreatic islet cells or pancreatic endocrine cells (pECs). These cells are key components of the body’s diffuse hormonal system (Table 2).

 

Figure 1. Occurrence of the most common types of neuroendocrine neoplasms. The occurrence of the main types of neuroendocrine tumors presented as the percentage of all NENs (315,441). GI-NENs represent the largest subgroup of NENs, followed by lung and pancreatic NENs. Subtypes not listed in this figure include NENs from the thyroid, kidney, adrenal gland, breast, prostate and skin.

 

 

Table 2. Types of NE cells and Their Tissue Site

NE cell type

Tissue site

Predominant hormone

PNEC

lung

numerous (see text)

Alpha cells

Pancreas

Glucagon

Beta cells

Pancreas

Insulin

Gamma/ PP cells

Pancreas

PPY

Delta cells

Pancreas

Somatostatin

Epsilon cells

Pancreas (during development)

Ghrelin

G cells

Stomach, duodenum, pancreas

Gastrin

D cells

Stomach, small intestine

Somatostatin

Enterochromaffin cells (EC) cells

Stomach, small intestine, colon

Serotonin (5-HT)

EC-like (ECL) cells

Stomach

Histamine

X and X/A cells

Stomach (mainly), small intestine

Ghrelin

L-I-N lineage cells

Small intestine (distal), Colon (L cells)

GLP-1, GLP-2, PYY, serotonin (L-cells), CCK, serotonin (I cells), NTS (N cells)

K cells

Small intestine (proximal)

GIP, serotonin

 

PULMONARY NEUROENDOCRINE CELLS (PNECs)

 

Pulmonary neuroendocrine cells (PNECs), the neuroendocrine cells of the lung, are prominent constituents of the diffuse neuroendocrine system. Although PNECs account for only 0.5% of the lung epithelium, their distinct morphological and histological staining properties, which are shared with the majority of NE cells, led to their prominence in early histological studies of the lung (11). First described in 1949 as “helle zellen” (‘bright cells’ in German), it was later appreciated that PNECs contain secretory granules and that they produce and secrete bioactive compounds, including serotonin (12–14). PNECs were thus added to the growing list of NE cells that make up the diffuse neuroendocrine system.

 

PNECs are found both as single cells within the lung parenchyma and as small clusters, called neuroendocrine bodies (NEBs) (Figure 2A) (15). In mammalian lung tissue, NEBs are distinctly located next to airway bifurcation points of the branching airways. Solitary PNECs, on the other hand, show a more divergent pattern of localization between species. Whereas in mice, solitary PNECs are mostly found in the trachea, in human lung tissue, they can be found throughout the airways. A distinct feature of PNECs is their direct innervation (16).

 

Figure 2. Pulmonary Neuroendocrine Cells (PNECs) in the upper airways. (A) Schematic depicting epithelial cell types found in the upper airways. Solitary PNECs and innervated Neuroendocrine Bodies (NEBs) are shown. (B) Diagram of signaling and transcription factor interactions that regulate PNEC differentiation.

Although the precise function of PNECs is not well-defined, their preservation across evolution -- similar cell types are found in fish gills and in all air-breathing vertebrates -- suggest an important physiological function (17–19). Furthermore, the position at airway bifurcation points of PNECs, their contact with the airway lumen, and innervation suggest a role in airway sensing. The bioactive compounds, hormones, and neuropeptides that PNECs secrete are known to affect oxygen sensing, pulmonary blood flow and bronchial tonus, and lung immune responses. The bioactive compounds secreted by PNECs include serotonin, calcitonin, calcitonin gene-related peptide (CGRP), gastrin-releasing peptide (GRP), chromogranin A, gamma aminobutyric acid (GABA), and synaptophysin, clearly suggesting an endocrine function for these cells (20–23). A summary of the hormones and neuropeptides expressed by PNECs and other NE cells discussed in this text is included in Table 3.

 

Table 3. NE Cell Expressed Hormones

Hormone

Associated

NE cell types

Reported function in

described NE cell types

Calcitonin

PNEC

pulmonary blood flow and bronchial tonus

Calcitonin gene-related peptide (CGRP)

PNEC

vasoregulation, bronchoprotection, immune cell recruitment

Gastrin releasing peptide (GRP)

PNEC

Regulates mucus and cytokine production upon inflammation or inflammation-associated processes

Gamma aminobutyric acid (GABA)

PNEC and pECs

Regulates mucus and cytokine production upon inflammation or inflammation-associated processes

Somatostatin (SST)

PNECs and GEP NE cells

 (D cells)

Inhibits secretion of insulin, glucagon, PYY, serotonin, and gastrin

Vasoactive intestinal polypeptide (VIP)

PNECs and GEP EECs

Acts as a neurotransmitter, an immune regulator, a vasodilator, and a secretagogue

Histamine

PNECs and Gastric EECs

(ECL cells)

Stimulates gastric acid secretion

Ghrelin (GHRL)

PNECs, Gastric EECs (X/A cell), and pECs (Epsilon cells)

Stimulates appetite, promotes gluconeogenesis and increased gastric acid secretion

Cholecystokinin (CCK)

PNECs and GI EECs

 (I cells)

Stimulates gallbladder contraction, pancreatic enzyme secretion, gut motility, satiety, and inhibits acid secretion

Serotonin (5-HT)

PNECs and GI EECs

(EC cells)

Regulates vasodilation and smooth muscle contraction

Secretin (SCT)

GI EECs

Stimulates the release of bicarbonate and water to neutralize gastric acid

Gastric inhibitory peptide (GIP)*

GI EECs (K cells)

Inhibits insulin secretion and to a lesser extent gastric acid secretion

Substance P (neuropeptide)

GI EECs (EC cells)

Regulates intestinal motility and mucosal permeability

Glucagon-like peptide 1 (GLP-1)

Intestinal EECs (L cells)

Stimulates insulin secretion and inhibits glucagon secretion

Neurotensin (NTS)

Intestinal EECs (N cells)

Inhibits gastric acid secretion

Glucagon (GCG)

pECs (alpha cells)

Increases blood glucose levels by stimulating glucose production and inhibiting glycogen storage by the liver

Insulin (INS)

pECs (beta cells)

Stimulates uptake of blood glucose by other tissues

Gastrin

pECs (G cells)

Stimulates gastric acid secretion

Pancreatic polypeptide (PPY)

pECs

(PP/gamma cells)

Inhibits glucagon and somatostatin

* Gastric inhibitory peptide (GIP) is also known as glucose-dependent insulinotropic polypeptide

 

PNECs in Development: Specification, Differentiation, and NEB Formation

 

PNECs are the first differentiated cell type to appear in the developing lung (15). The timing of PNEC differentiation in human lungs has not been comprehensively delineated but several studies have reported the appearance of PNECs in human fetal airways at 8 - 9 weeks of gestation (24). In mice, the fetal epithelial progenitor cells that give rise to mature PNECs appear for the first time at around embryonic day (E) 12.5 (25). These cells are defined by expression of the basic helix loop helix (bHLH) lineage transcription factor, ASCL1, which is required for their specification. Mice that carry null alleles of Ascl1 do not have PNECs (26,27).

 

PNEC lineage specification by expression of Ascl1 is followed by two key events that appear to happen in parallel: the formation of NEBs and the maturation of early Ascl1 expressing cells to fully differentiated PNECs. Using a lineage trace of Ascl1-positive cells in mouse embryonic lung, Kuo and Krasnow observed the first signs of PNEC clustering at around E13.5 to E14, followed by the appearance of bonafide NEBs, which contained mature PNECs at around E15.5 to E16 (25). In human fetal lungs PNEC clustering was first observed at about 9 to 10 weeks of gestation (28).

 

Although one might imagine that the formation of NEBs is likely achieved through proliferation of nascent PNECs, a different mechanism has been shown to be at play. Sparse lineage tracing of early fetal Ascl1-positive PNECs in mice using a multi-color lineage reporter showed that NEBs contained either different colored cells or a single labeled cell and multiple unlabeled cells, arguing that the PNECs in NEBs are not clonal (25). Live cell imaging of fetal mouse lung tissue showed that NEBs are formed through the migration and subsequent aggregation of PNECs at airway branchpoints (25,29). This process appears to be regulated by Slit-Roundabout (ROBO) signaling, a pathway more classically associated with axonal guidance (30). PNECs express the ROBO receptor and the lungs of mice where the Slit-ROBO pathway has been disrupted by mutation of either the Slit ligands or the ROBO receptor itself, have fewer NEBs and more solitary PNECs than the lungs of wildtype mice (21).

 

Concomitant with NEB formation, the transcriptional events initiated by expression of Ascl1 culminate in the emergence of fully differentiated, functional PNECs. In particular, expression of Ascl1 in lung progenitors induces expression of the zinc finger transcription factor, INSM1. PNECs in mice carrying mutant alleles of Insm1 fail to express the mature PNEC markers, CGRP and UCHL1 (ubiquitin C-terminal hydrolase L1, a.k.a. PGP9.5). While the INSM1 targets that mediate the PNEC maturation process have not been delineated, INSM1 has been shown to directly repress the bHLH transcription factor and Notch target gene, Hes1 (Figure 2B) (31).

 

HES1 and other Notch signaling components play crucial roles in repressing the differentiation and specification of PNECs and, thereby, in mediating the NE versus non-NE cell fate choice. The lungs of mice in which Hes1 has been conditionally deleted in early lung progenitors have more ASCL1-positive cells and, in particular, fewer solitary PNECs, showing instead more and larger NEBs compared to the lungs of wildtype mice (29). The increased size of NEBs in Hes1-deficient lungs suggests a mechanism of cell fate specification through Notch-mediated lateral inhibition whereby Notch is activated in PNEC neighboring cells through binding of Notch receptors to the Notch ligands expressed on the surface of PNECs themselves. The activated Notch signaling in these PNEC neighboring cells induces expression of Hes1, which in turn represses the PNEC fate.

 

PNECs express the Notch ligands Dll1, Dll4, Jag1, and Jag2 shortly after their specification during lung development and the cells surrounding PNECs in NEBs express Notch receptors (32). Genetic loss of either all three Notch receptors or Dll1 and Dll4 Notch ligands in the developing mouse lung leads to a dramatic increase in the number and size of NEBs, phenocopying conditional deletion of Hes1 (32,33). In cultures of human airway cells derived from induced pluripotent stem cells, inhibition of Notch signaling leads to increased numbers of PNECs (34,35).

 

As we will discuss in other parts of this text, two other bHLH transcription factors, NEUROG3 and NEUROD1 have been shown to play central roles in the differentiation of EECs and pancreatic islet cells. In contrast, as of yet, there is little evidence that these transcription factors are decisive for PNEC specification or differentiation. To date, Neurog3 expression has not been described in PNECs. While Neurod1 expression has been observed in some PNECs in both fetal and adult mouse lung, a limited number of studies have investigated its specific role in the PNEC lineage (27,36). Neurod1-null mouse lungs from mice less than 2 weeks old showed decreased numbers of solitary PNECs and more NEBs compared to lungs from age-matched wild type mice. However, this difference normalized once mutant mice were 6 weeks old (36). As will be discussed later on in this text, NEUROD1 is a marker of a subtype of the high-grade lung NEN, small cell lung cancer (SCLC), suggesting it might also play a role in normal PNEC biology.

 

Reactive PNEC Proliferations: PNECs in the Response to Lung Injury

 

Multiple studies have shown that PNEC numbers and NEB size are altered in several human disease conditions. Increased numbers of PNECs have been observed in the lungs of patients with COPD, asthma, cystic fibrosis, and some forms of pneumonia. Other pathological conditions associated with increased PNECs include sudden infant death syndrome (SIDS), bronchopulmonary dysplasia (BPD), and congenital diaphragmatic hernias (CDH) (15). Studies in animal models and PNEC culture systems provide experimental evidence that PNECs respond to environmental stimuli by both proliferation and/or secretion of bioactive compounds.

 

The early observation that murine PNECs proliferate in response to a common form of experimental lung injury, naphthalene administration, and that this proliferation precedes epithelial lung regeneration, led to the hypothesis that PNECs are multipotent stem cells that aid in lung regeneration (37). Indeed, lineage tracing studies of PNECs following naphthalene induced lung injury showed that a rare subpopulation of PNECs, termed NEstem, can function as stem cells in this context (38). In response to naphthalene, NEstem cells proliferate and sometimes migrate to the site of injury where they dedifferentiate (lose NE identity) and take on other lung cell fates. The process of dedifferentiation and reprogramming was shown to be mediated by Notch signaling, recalling the role of this pathway in PNEC fate specification, and by EZH2 (38,39). Nonetheless, NEstem cells are not solely responsible for regenerating the lung after injury, as they were shown to contribute only to a small portion of the regenerated surface epithelium (38). Furthermore, genetic ablation of PNECs does not abrogate lung regeneration following naphthalene injury (23,39). 

 

In thinking about PNECs as components of a diffuse hormonal system, two questions arise from the studies of PNECs in the context of naphthalene lung injury. The first is, how do PNECs detect injury? Club cells, which express cytochrome P450 2F2 (Cyp2f2), metabolize naphthalene to a toxic metabolite and the accumulation of this toxic metabolite leads to cell death specifically in these cells (40). Given that PNECs proliferate at time points shortly after peak Club cell injury, it is likely that they are responding to the Club cell injury and not to the naphthalene itself. Consistent with this hypothesis, selective ablation of Club cells using genetic ablation techniques also resulted in PNEC proliferation (41). Nonetheless, injury associated signals that are specific to PNECs and their responses have not been identified.

 

The second question that arises is, besides functioning as stem cells, do PNECs have an endocrine or paracrine/autocrine function in the context of lung injury? Although this question has not been explored in the naphthalene injury model, evidence from other model systems and from human diseases suggest that PNECs respond to some forms of lung injury or disease through the secretion of bioactive compounds. Cigarette smoke, a common culprit of lung injury, provides a good example. Bronchoalveolar lavage (BAL) fluid from smokers has increased levels of peptides secreted by PNECs, implicating these cells in the cellular response to cigarette smoke in humans (42). The PNEC-secreted bioactive compounds associated with this response include GABA and GRP, and both of these molecules have been implicated in inflammation and inflammation-associated processes (42,43). It is likely that PNEC proliferation is also involved in the response to cigarette smoke and its primary component, nicotine. Increased PNECs have been observed in rats exposed to cigarette smoke pre- and postnatally and in rhesus monkeys exposed to nicotine prenatally (44,45).

 

Pointing to a critical role for PNECs in oxygen sensing in the lung, hypoxia and hypoxia-mimicking genetic modifications have been shown to result in higher numbers of PNECs in mice, rats, rabbits, and guinea pigs (22,46–49). Shortly after showing that the distinct dense cored vesicles of PNECs carried serotonin, Lauweryns and Cokelaere went on to show that this serotonin was secreted by PNECs upon exposure to hypoxia (14,49). This finding was further refined and shown to be dependent on changes in intracellular Ca2+ concentrations using cultured rabbit and hamster lung slices (50,51). Serotonin release by PNECs is likely a physiologically relevant functional response to hypoxia as serotonin has been shown to induce vasoconstriction of pulmonary arteries (52).

 

Increased expression in PNECs of the neuropeptide, CGRP, has also been linked to hypoxia (46,53). CGRP has been implicated in promoting alveolar regeneration and in mediating immune cell responses in the lung (21,54). Importantly, results from a study by Shivaraju et al. linked the expression of CGRP by PNECs to the hypoxia-induced regenerative response of epithelial cells in the trachea. When the authors ablated PNECs and exposed mice to hypoxia, they observed a defective regenerative response that could be rescued by intranasal administration of CGRP (46).

 

PNECs appear to also respond to hyperoxia-induced lung injury. Patients with BPD, a chronic lung disease associated with oxygen supplementation of premature infants, have increased numbers of GRP-expressing PNECs (55). In a baboon model of BPD, some of the lung defects associated with the disease could be prevented by treatment of the animals with a GRP blocking antibody, demonstrating that GRP is directly linked to the disease phenotype (56). GRP also plays a role in the lung’s response to viral pneumonia and in the fibrotic response to radiation therapy (57,58).

 

There is clear evidence for a close interplay between PNECs and immune cells. In particular, the effector molecules secreted by PNECs can recruit and activate different populations of immune cells. In one of the first studies to show this, researchers developed a mouse model of CDH, a birth defect that results in pulmonary hypoplasia and pulmonary hypertension (21). CDH is associated with both a heightened immune response and increased numbers of PNECs (59). To study CDH, since point mutations in SLIT and ROBO genes are associated with the disease, Branchfield et al. generated mice with lung-specific deletions of the roundabout receptors, Robo1 and Robo2. When Robo1 and Robo2 were deleted in the entire lung epithelium the authors noted elevated immune cell infiltration in the lung, thus mimicking one of the features of CDH. When Robo1 and Robo2 were deleted only in PNECs, the mice displayed the same phenotype, directly linking the defect to PNECs. Interestingly, ROBO1- and ROBO2-deficient PNECs have increased levels of CGRP and knockout of the gene encoding CGRP partly reversed the immune and lung phenotypes of mice deficient for ROBO1 and ROBO2 in the lung epithelium.  

 

A potential immune regulatory role for PNECs is also suggested by the observation that PNEC numbers are elevated in patients with asthma and that more chromogranin A-positive PNECs are seen in guinea pigs after allergen sensitization and challenge (60). Mice deficient of PNECs due to deletion of Ascl1 in the lung epithelium, show a dampened response to allergen challenge -- reduced goblet cell hyperplasia and reduced immune cell infiltration -- and this was tied directly to reduced levels of PNEC-derived GABA and CGRP, respectively (61).

 

Diseases of Primary PNEC Hyperplasia: NEHI and DIPNECH

 

Up to now we have highlighted instances of increased PNEC number or NEB size that appear to be consequent to or at least associated with some forms of acute or underlying lung injury. These examples are instructive in that they point to a role for PNECs and the molecules they secrete in mediating the response to external stimuli and injury in the lung. Save for lung NENs, which will be discussed in further detail later in this text, there are two notable clinical instances of primary -- as opposed to reactive -- PNEC proliferation that have no known etiology and are not associated with common pathogenic triggers: neuroendocrine cell hyperplasia of infancy (NEHI) and diffuse idiopathic neuroendocrine hyperplasia (DIPNECH).

 

NEHI is a rare pediatric lung disease consisting histologically of hyperplastic GRP-positive and serotonin-positive PNECs in the distal lung epithelium of otherwise normal lung tissue. Symptoms are usually first noted between 6 to 8 months of life and include tachypnea, retractions, crackles and hypoxemia (62). In some cases, patients with NEHI show an inconspicuous, patchy pattern of inflammation or fibrosis, generally assumed to be a consequence of the increased PNEC numbers rather than its cause (63). Interestingly, despite increased PNEC numbers in the lungs of patients with NEHI, from a study on a small patient cohort (5 patients), it appeared that PNECs were not actively proliferating in the lungs of these patients as no Ki67 and GRP double positivity was observed (63). Unfortunately, treatment for patients with NEHI are currently limited to supportive oxygen supplementation and, in some cases, additional nutritional support. The majority of NEHI patients show gradual improvement of symptoms and the disease is not associated with mortality. Nonetheless, recent reports show that some patients experience abnormal lung function persisting into adulthood (62,64,65). While the etiology of this disease remains unknown, there are indications of a genetic basis for the disease. One study identified four families with multiple members diagnosed with NEHI and showing an autosomal dominant pattern of inheritance (66). Another study identified a heterozygous mutation in the NKX2.1 gene in members of a family with a history of childhood lung disease consistent with NEHI (67).

 

DIPNECH is a rare syndrome with adult onset consisting histologically of increased PNECs in the small bronchi and bronchioles and confined to the basement membrane, appearing as scattered PNECs, small nodules, or a linear proliferation of PNECs (68). These features are often seen in concomitance with what are referred to as tumorlets, PNEC proliferations that extend beyond the basement membrane but are less than 5 mm in diameter (69). Other histological features include fibrosis, chronic inflammatory cell infiltrate, and constrictive obliterative bronchiolitis. The majority of patients with DIPNECH are women and the disease is not associated with smoking or other lung diseases. Patients diagnosed with DIPNECH often present with symptoms including cough, exertional dyspnea and an obstructive or mixed obstructive/restrictive defect on pulmonary function test. A small number of patients with DIPNECH are diagnosed due to incidental findings (69).

 

DIPNECH was first recognized and formally defined in 1992 by Aguayo et al., who described the symptoms and histological features of 6 DIPNECH patients (42). While DIPNECH is considered a disease of primary rather than reactive PNEC proliferation, cases associated with parathyroid gland hyperplasia, acromegaly and pituitary adenoma, multiple endocrine neoplasia type I syndrome, and pulmonary adenocarcinoma have been reported (70–72). The World Health Organization (WHO) classifies DIPNECH as a preinvasive, possibly preneoplastic condition (73). Most patients with DIPNECH have multiple PNEC nodules, sometimes including both tumorlets and frank low grade lung NET (carcinoid) tumors (70). In contrast to NEHI where Ki67-positive PNECs were not observed, the PNEC proliferations in DIPNECH patients contain some Ki67-positive cells (63,74). While patients with DIPNECH most often follow a clinical course showing stability or slowly progressing functional decline, a small subset of patients have rapidly progressive disease including progression to respiratory failure or metastatic carcinoid tumors (70,75). To date, there is no standard of care for DIPNECH and the most effective treatment strategy for patients with DIPNECH are somatostatin analogues (SSAs), which have shown effectiveness in improving symptoms of cough and dyspnea in some patients (76–78). Considering that it has been well-established that SSAs inhibit the secretion of bioactive compounds from gastrointestinal NETs, the effectiveness of SSAs in treating cough and dyspnea in patients with DIPNECH suggests these symptoms are caused by the secretion of bioactive compounds by DIPNECH PNECs (79).

 

Lung NENs

 

Lung NENs account for 20-25% of all lung cancers and for 25-30% of NENs from all tissue sites (80,81). As is the case for NENs in general, lung NENs comprise both low grade, well-differentiated NETs and high grade, poorly differentiated NECs. Lung NENs can thus be subdivided into the high-grade carcinomas, small cell lung cancer (SCLC) and large cell neuroendocrine carcinomas (LCNEC), and the low-grade tumors, atypical carcinoids (AC), classified as intermediate grade, and typical carcinoids (TC), classified as low grade.   

 

SMALL CELL LUNG CANCER (SCLC)

 

The most common lung NEN, SCLC, accounts for 79% of all lung NENs and 30% of all lung cancers and is also the best studied among lung NENs (82). Consistent with its classification as NEC, SCLC is a highly aggressive tumor with a high rate of metastasis and a 10-year survival rate of only 1-2% (83). Among patients with SCLC, 97% have a history of smoking (18). While rare, patients with SCLC sometimes experience paraneoplastic endocrine syndromes, most commonly a syndrome of inappropriate antidiuretic hormone (SIADH) and ectopic Cushing’s syndrome (84). Studies of SCLC biology have been aided by a collection of tumor-derived cell lines, several patient-derived xenograft (PDX) models, and genetically engineered mouse models (GEMMs) of the disease (85–87). These preclinical model systems have allowed scientists to address questions in two key areas: the cell of origin of SCLC and molecular signatures predictive of therapeutic vulnerabilities.

 

Genetically, SCLC is a relatively homogeneous disease -- RB1 and TP53 are both almost universally lost in patient tumors (88). Conditional simultaneous genetic deletion of Rb1 and p53 in the mouse epithelium results in tumors that recapitulate many of the key features of the human disease at both the histological and molecular levels (87,89–91). Targeting the deletion of Rb1 and p53 to specific epithelial cell types in the lung provided definitive evidence that Cgrp-expressing PNECs are a cell of origin for tumors in this model (23,92). However, PNECs are not the only epithelial lung cell type that can be a cell of origin for mouse SCLC. A separate study showed that an, as of yet, unidentified CGRP-negative cell that is also negative for the canonical markers of two other common lung epithelial cell types gives rise to mouse SCLC lesions that are molecularly distinct from those initiated in CGRP-positive PNECs (93).

 

Genomic analysis of human and mouse SCLC primary tumors and cell lines has revealed commonly mutated genes and pathways, most notably loss of PTEN, NOTCH, and histone modification genes, and amplification of MYC family oncogenes (88,90,91). Several studies focused on metastasis in mouse SCLC have highlighted the role of NFIB in driving progression of some of these tumors, and data from human patients with SCLC support the clinical relevance of these findings (94–96). These studies, in combination with preclinical testing in mouse models and cell lines have suggested some degree of patient stratification. In particular, high expression of MYC is associated with tumor sensitivity to Aurora Kinase inhibitors (97).

 

The standard treatment regimen for patients with SCLC is a combination therapy of a platinum agent combined with etoposide (82). Despite a clinical response to these therapies in the majority of patients, almost all patients will then experience tumor recurrence (83). The analysis of the transcriptomes of both mouse and human SCLC tumors and cell lines has identified 4 molecular subtypes of SCLC, defined by their expression (or lack of expression) of 3 lineage-specific transcription factors: ASCL1 high, NEUROD1 high, POU2F3 high, and a fourth subtype that has low expression of NE transcription factors and has been proposed to be defined by expression of YAP1. A more recent study suggests a classification in which this fourth subtype is defined by expression of immune checkpoint genes and human leukocyte antigens (98,99). It has been hypothesized that the cell of origin for the POU2F3 high subtype of SCLC might be the pulmonary tuft cell, another chemosensory cell type in the lung distinct from PNECs (100,101).

 

Importantly, several studies using preclinical models of SCLC suggest that these molecular classifications can be used to stratify patients according to potential therapeutic vulnerabilities (102). Adding complexity to this schema, single cell RNAseq studies of mouse SCLC and of xenografts derived from circulating tumor cells from SCLC patients suggest that different molecular subtypes might represent different stages of progression where tumors begin in an ASCL1-high state and progress towards a non-NE state and that individual tumors might comprise cells belonging to different subtypes (101,103). Several studies have also shown other forms of intratumor heterogeneity in SCLC that have implications for patient therapy (104–106). Other new therapies suggested for SCLC include tricyclic antidepressants, therapies that target specific metabolic vulnerabilities, and therapies targeting the GNAS/ PKA/PP2A signaling axis (107–109).

 

LARGE CELL NEUROENDOCRINE CARCINOMA (LCNEC)

 

Pulmonary LCNEC is less common than SCLC, accounting for 16% of all lung NENs (82). Like SCLC, pulmonary LCNECs are highly metastatic and are associated with smoking history and with an overall 5-year survival rate ranging from 15% to 25% (110). In contrast to SCLC, however, there are relatively few preclinical models for LCNEC and we know much less about the basic biology of this disease. This might partly explain why guidelines for treating patients with LCNEC are rather rudimentary (82,111). 

 

Pathohistological analysis of tumors from GEMMs of SCLC found that a portion of the mouse tumors in these models had a histological pattern consistent with LCNEC. While these LCNEC tumors only accounted for 10% of the tumors from the GEMM in which only Rb1 and p53 were conditionally deleted in the lung epithelium, they were much more prominent in the GEMM in which Rb1, p53, and Pten were conditionally deleted specifically in CGRP-expressing PNECs (87). A different GEMM, based on loss of Rb1 and expression of mutant p53 alleles, also develops both SCLC and LCNEC mouse tumors (112). These GEMMs have, thus far, not been used to explore the biology specifically of LCNEC and doing so might present some technical challenges. Recently, the first GEMM specifically for LCNEC was reported. In this model, Rb1, p53, Pten, and Rbl1 were simultaneously deleted in the mouse lung epithelium, resulting in a tumor spectrum consisting primarily of LCNEC and low-grade NETs (113).   

 

The majority of insights into LCNEC have been provided by molecular analysis of primary patient tumor samples. The most comprehensive analysis, consisting of whole exome sequencing of 60 LCNEC tumors and RNA-sequencing expression analysis of 69 LCNEC tumors, highlighted the existence of two major molecular subtypes of LCNEC (114). Type I LCNECs had a higher rate of alterations in TP53 and STK11/KEAP1 and an NE expression profile defined by high expression of ASCL1 and DLL3 and low expression of NOTCH. Type II LCNECs had frequent mutations in RB1and TP53, therefore resembling SCLC at the genomic level. The expression profile of type II LCNECs, however, was distinct from SCLC and instead was defined as NE low, with low expression of ASCL1 and DLL3 but high expression of NOTCH.

 

The description of these two molecular subtypes for LCNEC highlights a clinical conundrum relating to the treatment of patients with LCNEC: should they be treated with SCLC chemotherapy regimens or with chemotherapy regimens for non-NE non-small cell lung cancer (NSCLC) (111)? The report of an SCLC-like subtype of LCNEC (type I) and a NSCLC-like subtype of LCNEC (type II), might suggest a way to stratify patients for different chemotherapy regimens. In line with this idea, a retrospective analysis of LCNEC cases found that patients whose tumors either had wildtype RB1 or showed expression of RB1 protein had a better outcome when treated with a NSCLC chemotherapy regimen as opposed to a SCLC chemotherapy regimen (115).   

 

Other therapies beyond traditional chemotherapy regimens are also being explored for patients with LCNEC. One example that relates to patient stratification according to LCNEC subtype, which was defined in part by differential patterns of DLL3 expression, involves therapeutic strategies that use DLL3 to target tumor cells. Given that DLL3 is also expressed by some SCLC tumors, this also represents a potential therapeutic opportunity in SCLC. Although a DLL3-antibody conjugated to the DNA-damaging pyrrolobenzodiazepine dimer toxin did not provide a survival benefit in 2 phase 3 clinical trials, other DLL3 targeting approaches are being developed (116). In addition, several studies have uncovered potentially targetable molecular alterations in some LCNEC tumors, including activating EGFR mutations, FGFR1 amplifications, activating BRAF mutations, ALK rearrangements, and mutations affecting BDNF/TrkB signaling (114,117–119). Given that the majority of these targetable mutations have been identified in LCNEC tumors with wildtype RB1, the question remains as to how best to treat patients with RB1 mutant LCNEC (120).

 

LUNG NETs: TYPICAL AND ATYPICAL CARCINOIDS

 

Lung NETs comprise low grade typical carcinoids (TC) and intermediate grade atypical carcinoids (AC), accounting for 5% and 0.5% of all lung NENs, respectively (82). TC and AC tend to present in younger patients than LCNEC and SCLC, and the majority of patients are women and non-smokers (121). Although the majority of lung NETs are sporadic and non-functional, a small percentage of patients with Lung NETs, 5% of TC and < 2% of AC, present with paraneoplastic syndromes including those associated with adrenocorticotropic hormone (ACTH), growth hormone releasing hormone (GHRH), histamine, and serotonin (122,123). Some of these are more commonly associated with metastatic lesions of TC or AC (123). Approximately 5% of lung NETs are associated with the familial cancer syndrome caused by germline mutations in multiple endocrine neoplasia gene type I (MEN1). Interestingly, 5% to 10% of lung NETs are also associated with tumor multiplicity, a feature which might suggest a connection with either an unappreciated familial predisposition syndrome or with premalignant conditions such as DIPNECH (80,121).

 

The overall 10-year survival rates for stage I TC and AC are comparable ranging from 98% to 91%, respectively. In the case of stage IV tumors, 10-year survival for TC patients is 49% but is only 18% for patients with AC (124). A distinguishing feature of TC and AC is their relatively slow growth. Indeed, the pathological criteria for diagnosing carcinoids are the number of mitoses per mm2 and the presence of absence of necrosis: < 2 mitoses per mm2 and no necrosis for a diagnosis of TC, 2 to 10 mitoses per mm2 and demonstration of necrosis for a diagnosis of AC (82). Morphologically, carcinoids typically contain small cells that show nested, rosette, and trabecular growth patterns with peripheral palisading (125).

 

Complete surgical resection is the most common treatment for patients with TC and AC, and for the majority of these patients’ surgery is associated with a favorable survival prognosis. Unfortunately, however, a fraction of carcinoid tumors metastasizes, and tumor recurrence (even after apparent curative resection) has been reported in 1 to 6% and 14 to 29% of patients with TC and AC, respectively. Due to a highly variable time to relapse for patients with recurrence (0.2 to 12 years), the recommended follow-up period is 15 years (121,126–129). The reported incidence for lymph node metastasis for TC and AC is variable with rates ranging from 12% to 17% for patients with TC and from 35% to 64% for patients with atypical carcinoid (80,130). The incidence of distant metastases for both TC and AC is 3% and 21%, respectively (129,131).

 

The incidence of tumor recurrence and metastasis calls attention to a clinical need for systemic treatment options for patients with TC and AC tumors that are unresectable, as well as for the need for effective adjuvant therapy options that can be offered to patients after surgery. Unfortunately, standard chemotherapy and radiotherapy regimens have proven to be mostly ineffective in this patient population (80). The only treatment option shown to improve progression-free survival in patients with advanced and progressive TC and AC is the mTOR inhibitor everolimus (132).

 

Other therapeutic options for patients with TC and AC that are not currently considered standard of care due to limited clinical trials, include somatostatin (SST) analogues and peptide receptor radionuclide therapy (PRRT), and temozolomide with or without capecitabine. Given that pulmonary carcinoids can express SST receptors, patients with these tumors can be potentially considered for palliative treatment with unlabeled or radiolabeled SST analogues.

 

The lack of clarity regarding standard of care for patients with unresectable, metastatic, or recurrent TC and AC, points to an unmet need for not only new and effective systemic treatment strategies for these patients, but also for clear patient stratification criteria for predicting the probability of a response of a given patient tumor to specific therapeutic options. Furthermore, given the broad range of tumor malignancy for TC and AC, biomarkers that can predict the potential for tumor progression and metastasis or recurrence are also needed. Efforts to address these clinical needs have been hindered by both difficulties in performing molecular characterization of these tumors and by a dearth of preclinical models representative of the disease. Only a handful of cell lines exist for TC and AC and only one GEMM for TC and AC has been reported (133,134).

 

In contrast to SCLC and LCNEC, pulmonary carcinoids have a low tumor mutational burden, have very few recurrent or characteristic mutations, and rarely contain “driver” mutations in known oncogenes. The most commonly mutated gene in pulmonary carcinoids is MEN1, and up to 5 to 13% of patients with germline mutations of this gene are diagnosed with pulmonary carcinoids (135–137). The most commonly mutated class of genes in pulmonary carcinoids are chromatin remodeling genes, a category that includes MEN1, PSIP1, and ARID1A. Though prevalent in SCLC and LCNEC, mutations in RB1 and TP53 are rare in pulmonary carcinoids (135,136). Recurrent copy number alterations have also been identified in pulmonary carcinoids, including in genes that would imply targetable therapeutic vulnerabilities such as, EGFR, MET, PDGFRB, AKT1/PKB, PIK3CA, FRAP1, RICTOR, KRAS, and SRC (136,138).

 

Transcriptional and methylation analysis of primary pulmonary carcinoids has also revealed distinct subclasses of these tumors. Using multi-omics factor analysis (MOFA), Alcala et al. identified 3 molecular clusters, termed A1, A2, and B (139). While most of the tumors in clusters A1 and A2 were TC, tumors in cluster B were primarily classified as ACs. Tumors in cluster B had high expression of ANGPTL3 and ERBB4, were enriched for mutations in MEN1, and were associated with a worse overall survival. Consistent with a worse prognosis for patients with tumors in cluster B, tumors in this cluster also showed universal downregulation of the orthopedia homeobox protein gene, OTP, whose expression has previously been associated with an improved prognosis in patients with pulmonary carcinoids (126). A separate study performed a similar multi-omic analysis of an independent set of pulmonary carcinoids and also identified 3 molecular subtypes that they termed LC1, LC2, and LC3 (140). The concordance between the molecular subtypes identified in these two studies was shown through integration of the two datasets, further validating the use of these molecular classifications for pulmonary carcinoids (141). 

 

The molecular analysis of pulmonary carcinoids has provided evidence that supports the idea that a fraction of lung NENs may actually fall into a category that lies between G2 ACs and G3 NECs in terms of malignancy. While such a category is recognized in GEP-NENs and is termed well-differentiated G3 NET, its existence has only recently been suggested for lung NENs (142). The study by Alcala et al. identified a subgroup of ACs, termed “supra-carcinoids,” that showed the morphologic characteristics of pulmonary carcinoids, but whose transcriptional profile was closer to that of LCNECs (139). In their analysis of the transcriptional profiles of a series of LCNECs and ACs, Simbolo et al. identified 3 molecular clusters, C1, enriched for LCNECs, C3 enriched for ACs, and C2, which was mixed in terms of number of ACs and LCNECs and which showed intermediate molecular features (143). Finally, an earlier study by Rektman et al. had identified 2 examples of what they referred to as “carcinoid-like” LCNEC tumors -- tumors that showed a clear carcinoid-like morphology and a molecular profile consistent with ACs (low tumor mutational burden and mutation in MEN1) but that had been classified as LCNEC due to a high proliferation rate (118). 

 

The supra-carcinoids in the Alcala et al. study showed a higher expression of MKI67 than other carcinoids in the series, supporting an idea that has been purported in the literature concerning a potential role for percent Ki67 positivity in identifying pulmonary carcinoids more likely to be associated with a poor prognosis (144–146). Typically, ACs show a Ki67 positivity rate of less than 20%. However, some tumors diagnosed as AC show rates between 20 and 50% (147,148). Likewise, as indicated by the “carcinoid-like” LCNEC tumors in the Rekhtman et al. study, some tumors that would otherwise be considered ACs, are diagnosed instead as LCNEC due to having a high proliferation rate (118,149). Furthermore, the comparison of proliferation rates between matched primary stage IV pulmonary carcinoids and metastases indicated an increased proliferation rate in 35% of the metastases, suggesting increased proliferation as a feature of progression (148). This idea is further supported by the observation that Ki67 positivity was heterogeneous in the analyzed tumors with some regions of the tumors showing hot-spots of increased proliferation compared to the rest of the tumor. Beyond Ki67 positivity, a list of defining features of supra-carcinoids or borderline pulmonary carcinoids/neuroendocrine carcinomas has yet to be established.

 

ENDOCRINE CELLS IN THE GASTROENTEROPANCREATIC TRACT

 

Together, the organs connected throughout the mouth to the anus are known as the gastrointestinal (GI) tract, and when the pancreas is included, these organs are collectively referred to as the gastroenteropancreatic (GEP) tract. Throughout the GEP tract endocrine cells can be found as either solitary cells, as is the case in the GI tract, or as innervated clusters, as is the case in the pancreas.

 

Throughout the gastrointestinal (GI) tract the solitary endocrine cells, which have a slender, elongated shape, are referred to as enteroendocrine cells (EECs). This classification helps to distinguish them from endocrine cells of other organs e.g., lung and pancreas. Despite representing only 1% of the gut epithelial cells, the large size of the intestinal epithelium makes it the body’s largest endocrine organ (150,151).

 

Compared to the slender EECs of the GI tract, the pancreatic endocrine cells, dispersed as clusters (known as islets of Langerhans) throughout the organ, have a more pyramidal or round-oval shaped appearance. An adult human has millions of islets, which collectively correspond to roughly 2% of the pancreatic epithelium (152,153). These islets are highly vascularized, a feature that enables pancreatic hormones to travel via the bloodstream to reach their target organs. In fact, the pancreatic hormones act both locally and systemically, eliciting responses throughout the body, consequently affecting the overall metabolic state of the organism. The sections below will provide an overview of the endocrine cells of the GEP tract as components of both the diffuse neuroendocrine system and the body’s diffuse hormonal systems. 

 

GASTRIC ENDOCRINE CELLS

 

The first major organ of the GI tract is the stomach, which, in humans, can be divided into 4 functionally distinct compartments. From proximal to distal; the cardia is the connective region between the esophagus and the stomach, the fundus stores undigested food and gases, the corpus is the largest compartment and performs the digestive action of the stomach, and, finally, the pylorus regulates gastric emptying (Figure 3A) (154,155).

Figure 3. Gastric enteroendocrine cells. (A) Schematic showing the anatomical differences between the murine and human stomach (B) Schematic depicting epithelial cell types found in the gastric pylorus and corpus glands. Solitary gastric enteroendocrine cells are shown in orange. (C) Diagram of signaling and transcription factor interactions that regulate gastric enteroendocrine cell differentiation.

Histologically, the stomach comprises tubular-shaped mucosal invaginations containing a pit region of primarily surface mucous cells, and a gland region. The latter is further subdivided into the isthmus, neck, and base (Figure 3B). The primary differentiated cell types of the stomach are: mucus-producing pit cells, chief cells, which secrete digestive enzymes, acid-secreting parietal cells, gastric tuft cells, whose function is ill defined, and gastric EECs. These cells are continuously formed throughout life, albeit at different rates, by progenitor cells located in the isthmus of the gland region (156,157). With the exception of the cardia, which primarily contains pit cells and scattered parietal cells, gastric EECs can be found in all of the compartments of the stomach. In the corpus and fundus EECs are located in the lower third of the glands. EECs in the pylorus are located in the neck region (158).                                                  

 

Gastric EECs are divided into the following 5 main subtypes defined by their predominantly expressed hormone: G cells (gastrin), D cells (somatostatin), enterochromaffin (EC) (serotonin), EC-like (ECL) cells (histamine), and X/A cells (ghrelin) (Figure 3C) (159). While some gastric EEC subtypes overlap with those found in the intestine, comparison of duodenal EECs and gastric EECs by single cell RNA sequencing (scRNA-seq) suggested a distinct gastric EEC expression profile. These differences are most likely reflective of the tissue specific microenvironment of these EECs, the stimuli they are exposed to, and their different functions (160).     

 

As will be outlined in subsequent parts of this text, differentiation of intestinal EECs requires expression of the master bHLH transcription factor, NEUROG3. While NEUROG3 is important for gastric EECs, its role appears to be EEC subtype specific. Most gastric EECs are also dependent on ASCL1, the same transcription factor that initiates PNEC specification. Studies from two different groups, which independently generated Neurog3 null mice showed that while these mice lacked D-cells and G-cells and had decreased numbers of EC cells, ECL and X/A cells were unaffected (161,162). Ascl1 null mice displayed a similar but not identical phenotype, showing lack of D, G and EC cells and severely decreased numbers of X/A cells (163). ECL cells were not examined in Ascl1 null mice. Thus, some gastric EECs, such as D- and G- cells are dependent on both NEUROG3 and ASCL1, while others, such as X/A cells, are dependent on ASCL1 but not NEUROG3. EC cells appear to be entirely dependent on ASCL1, and only partially dependent on NEUROG3 (163). Ascl1 is not expressed in intestinal EECs from the mouse. Nonetheless, scRNA-seq of human intestinal EECs identified expression of ASCL1, suggesting a role for this transcription factor not only in gastric EECs but also in human intestinal EECs (164).

 

The EEC hormone most clearly associated with gastric function is gastrin, secreted by G-cells located in the pyloric compartment of the stomach. These cells are also found in the duodenum, but their function has been best studied in the stomach (160,164). Gastrin secreted into the bloodstream by G-cells binds to its receptor expressed by ECL cells in the corpus thereby stimulating them to secrete histamine, which, in turn, stimulates neighboring parietal cells to secrete gastric acid. Given that parietal cells themselves also express the gastrin receptor, gastrin release by G-cells can also directly stimulate parietal cells to secrete gastric acid (165–167). The other hormones produced by the gastric EECs are also secreted by other gastroenteropancreatic endocrine cells (GEP-ECs) and will be discussed in later sections.

 

INTESTINAL AND COLONIC ENDOCRINE CELLS

 

Architecture and Cell Types of the Intestine

 

The gut can be divided into the small and large intestine (also known as the colon). The two primary functions performed by the small intestinal epithelium are 1) to form a barrier against the continuous chemical and mechanical insults induced by the undigested food, microorganisms, and toxins present in the intestinal lumen, and 2) to absorb nutrients from ingested food (150). This latter process occurs with an exceptionally high efficiency made possible by the large surface area generated by the intestinal epithelium’s folded structure. Protrusions known as villi contain differentiated non-mitotic cells, while invaginations known as crypts contain proliferative, self-renewing stem cells and their epithelial niche cells, Paneth cells. The colon lacks villi and its primary function is water absorption and movement of the stool. The continuous damage experienced by the intestinal epithelium necessitates a high cellular turnover to maintain organ function. The intestinal stem cells, marked by the expression of leucine-rich G-protein-coupled receptor 5 (LGR5), continuously replace lost cells via rapid division which regenerates the epithelium within 4-5 days (150).

 

The differentiated cells that originate from the LGR5+ stem cells can be divided into two main functional categories, absorptive (enterocytes and microfold cells) and secretory (EECs, goblet, Paneth, and tuft cells) (Figure 4A). Intestinal EECs secrete hormones in response to stimuli such as nutrients from digested food and metabolites produced by the gut microbiota (168,169). The stimuli that EECs respond to can be both mechanical and chemical, and the hormones they produce are secreted both locally and into the bloodstream, thereby allowing them to act not just locally but also systemically. Gut hormones regulate important functions such as digestion, nutrient absorption, appetite, and gastric as well as gut motility (170).

 

Figure 4. Intestinal enteroendocrine lineage specification. (A) Schematic of the intestinal epithelium. Solitary enteroendocrine cells (EECs) are depicted in purple. (B) Diagram of signaling and transcription factor interactions that regulate intestinal enteroendocrine cell differentiation. Differentiated EEC subtypes are highlighted with yellow circles.

Factors Regulating Commitment to the Secretory Lineage  

 

The rapid division of LGR5-positive stem cells gives rise to progenitor cells, the majority of which differentiate as they migrate upwards along the crypt-villus axis. The following section describes the differentiation of intestinal EECs and highlights some of the essential regulatory factors and signaling pathways that direct this process.

 

NOTCH SIGNALING  

 

The first step in becoming an EEC is commitment to the secretory lineage, a process that is initiated by the transcription factor, ATOH1 (Protein atonal homolog 1). At the crypt bottom, active Notch signaling prevents the differentiation of stem and progenitor cells. Cell-cell contact between Notch ligand (Dll1 and Dll4) expressing Paneth cells and stem/progenitor cells results in the expression of the Notch target gene, HES1, which in turn represses the secretory cell fate by repressing ATOH1 (171,172). Hence, commitment to the secretory lineage requires inactivation of Notch signaling.

 

Inactivation of Notch signaling is concomitant with loss of contact with Paneth cells. Due to the high ratio of progenitor to Paneth cells in the crypt, not all progenitors can simultaneously be in touch with a Paneth cell and this results in stochastic loss of Paneth cell-stem cell contact. Additionally, as new progenitors are continuously generated by the stem cells in the crypt, older progenitors are pushed upward along the crypt-villus axis, causing them to lose contact with the Notch ligand-presenting Paneth cells. The resultant loss of active Notch in these cells enables expression of ATOH1 and commitment to the secretory lineage (173,174). Mice lacking Atoh1 do not have any secretory cells (173). In contrast, mice with null alleles of Hes1 have excessive numbers of secretory cells.

 

Factors Regulating Commitment to the EEC Lineage         

 

Transient expression of NEUROG3 commits Atoh1-expressing secretory progenitors to the EEC fate. Whereas Neurog3 knockout mice completely lack EECs, overexpression of Neurog3 leads to increased numbers of EECs and decreased numbers of goblet cells (162,175,176). Homozygous NEUROG3 mutations have been identified in children with generalized malabsorption and reduced numbers of intestinal EECs (177). Downstream targets of NEUROG3 include other transcription factors important for EEC differentiation such as NEUROD1, PAX4/6, NKX2.2, INSM1, and PDX1 (162,178–181). NEUROG3 is also implicated in cell cycle control: Neurog3 expression in mouse pancreatic endocrine progenitors leads to upregulation of the cell cycle inhibitor, Cdkn1a, and consequent cell cycle exit (182). Consistent with the idea that cell cycle exit biases secretory progenitors to the EEC lineage, inhibition of either epidermal growth factor receptor (EGFR) or mitogen activated protein kinase (MAPK) signaling induced quiescence of intestinal stem cells in organoid culture. When this quiescence was reversed by reactivation of these pathways, the resulting organoids had an increased proportion of EECs (183).

 

Specification of the Different EEC Lineages  

 

While most progenitors generated in the crypt immediately begin migrating upwards, those primed to become EECs remain in the crypt for anywhere between 48h and 60h (184). During this time, these cells become committed to one of several divergent differentiation trajectories, each of which results in a different EEC subtype. Prior to leaving the crypt, EEC-committed progenitors have already started to express and secrete their lineage-defining hormones. The time required to produce a specific hormone varies between the different EEC lineages and this may explain why some EEC-committed progenitors remain in the crypt longer than others (184). 

 

Altogether, intestinal EECs produce more than 20 different hormones. The earliest classification of EECs was based on immunostainings and consisted of the following 8 EEC lineages as defined by the main hormone they were found to express: enterochromaffin (EC) cells that secrete serotonin (5-hydroxytryptamine, 5-HT), I cells that secrete cholecystokinin (CCK), K cells that secrete gastric inhibitory peptide (GIP), L cells that secrete glucagon-like peptide 1 (GLP-1), X cells that secrete ghrelin (GHRL), S cells that secrete secretin (SCT), D cells that secrete somatostatin (SST), and N cells that secrete neurotensin (NTS) (185).

 

At the time that this classification was first proposed, it was believed that EECs belonging to a given subtype predominantly expressed the hormone that defined that EEC subtype. Thus, for example, it was believed that EC cells only predominantly expressed serotonin, and L cells only predominantly expressed GLP-1. However, new techniques for performing hormone co-stainings using multiple antibodies, fluorescent hormone reporter mice, and transcriptome-based sequencing of EECs have all led to the observation that some EECs express multiple subtype-defining hormones (186–190). The question thus arose, did these multihormonal EECs represent previously unidentified and distinct EEC subtypes? Or were they simply cells caught in a transition state along the EEC subtype differentiation trajectory? The latter would suggest that EECs are capable of hormonal plasticity and are therefore capable of transitioning from expression of one lineage defining hormone to another.

 

RESOLVING LINEAGE IDENTITY AND HORMONE SWITCHING  

 

One of the first lines of evidence that EECs might undergo hormone switching was the observation that, while serotonin-producing EC cells were rapidly labeled after a single pulse of radioactive thymidine, secretin-producing S cells were labeled much later and only after multiple injections of the isotope (191,192). Thus, it was concluded that serotonin-expressing cells but not secretin-expressing cells had the ability to self-renew and that secretin cells did not differentiate before reaching the villus. Based on this data, one could postulate that serotonin-expressing cells might become secretin-expressing cells once they reach the villus.

 

A lineage relationship between EECs localized in the crypt and EECs localized in the villus was first suggested in 1990 by Roth and Gordon based on an immunohistochemical study in which it was observed that cells expressing substance P (encoded by the Tac1 gene) but not secretin were found in the crypt, cells expressing both substance P and secretin were found in the bottom of the villus, and cells expressing secretin but not substance P were found exclusively at the top of the villus. The majority of substance P-expressing and secretin-expressing cells co-expressed serotonin. The authors thus concluded that these hormones were sequentially expressed along the crypt villus axis (186). The fact that substance P-expressing cells were labeled by the thymidine analogue, BrdU, faster than secretin-producing cells further supported this idea of sequential expression of substance P and secretin by the same EEC (193). Functional evidence for dynamic hormone-switching in EECs was provided first by cell ablation studies showing that ablation of one EEC subtype led to decreased numbers of other EEC subtypes (194). Subsequently the generation of a novel mouse Neurog3 reporter allele, Neurog3Chrono, enabled more definitive delineation of dynamic hormone-expression patterns of single EECs (184).

 

In the Neurog3Chrono mouse, two fluorescent reporter proteins, an unstable mNeonGreen and highly-stable tdTomato, are expressed concurrently with endogenous Neurog3. As a result, the ratio of red to green fluorescence of a given EEC provides real-time information about the age of that cell relative to when it expressed Neurog3 (184). ScRNAseq of EECs from Neurog3Chrono mice provided definitive evidence that many EECs switch the hormone they produce throughout the course of their lives and furthermore suggested a more simplified EEC subtype classification consisting of five mature EEC lineages. One of these five lineages was the one proposed by Roth and Gordon of substance P expressing cells that give rise to serotonin-expressing cells and then to secretin-expressing cells. This lineage was also confirmed independently by lineage tracing of Tac1-expressing cells in the mouse intestinal epithelium (195). 

 

Two key observations from the Neurog3Chrono study substantiated a simplified EEC lineage classification. First, all EEC lineages except for SST-expressing D-cells began to express secretin upon entering the villus, thus rendering the S-cell lineage obsolete. Second, L-, I- and N- cells were shown to belong to a single lineage. Prior to this, observations from a mouse model of L-cell ablation had led the Schwartz lab to propose that L- and N- cells were part of the same lineage (194). The Neurog3Chrono study showed that, when located at the bottom of the crypt, L-cells secrete GLP-1 but, upon reaching the upper regions of the crypt, begin to express the I-cell defining hormone, CCK, and, finally, upon reaching the villus region, they begin to express the N-cell defining hormone, NTS. Thus, the EEC lineages or subtypes could be reduced to the following 5 main lineages: enterochromaffin (EC) cells that secrete Serotonin, K cells that secrete GIP, X cells that secrete GHRL, D cells that secrete SST, and LIN cells that secrete GLP-1, CCK and NTS (Figure 4B) (184).

 

ENTEROCHROMAFFIN (EC) CELLS

 

EC cells are the most prevalent EEC subtype and they can be found in all regions of the intestine (196,197). They are slender, triangularly-shaped cells that can have protrusions extending towards the luminal surface of the intestine (198,199). EC cells are defined by their expression of both serotonin and tryptophan hydroxylase 1 (TPH1), an enzyme that catalyzes the rate limiting step in the biosynthesis of serotonin (189). Serotonin in EC cells is stored in pleomorphic granules and is released in response to chemical and mechanical stimuli. Although commonly associated with brain development and regulation of mood and stress, 95% of the body’s serotonin is produced by the intestine where it regulates functions such as motility, fluid secretion, and vasodilation (200–202). The response of EC cells to different types of stimuli is mediated, in part, through their expression of various receptors, including the olfactory receptor, OLFR558, which acts as a sensor for microbial metabolites, and the transient receptor potential A1 (TRPA1), a receptor-operated ion channel that detects dietary irritants (203). A subset of EC cells expresses the mechanosensitive channel Piezo2, which converts mechanical forces to secretion of fluids and serotonin (204).

 

Given the important functions exerted by serotonin, it is not surprising that EC cells are implicated in several GI pathologies (202). Consistent with the observation that most SI-NETs express serotonin, intestinal EC cells are thought to be the cell of origin for these tumors. An early hint that SI-NETs might indeed arise from EC cells was provided by a Immunohistochemical study in which serial sections of an entire ileal SI-NET tumor were stained for several EEC markers, including serotonin. The authors observed aggregates of proliferating EC cells within crypts in close proximity to the tumor, which the authors speculated were indicative of where the tumor had originated (205). A later study made the same observation in a larger cohort of SI-NET samples from eight different patients, supporting the idea that aberrant proliferation of EC cells within the crypt is associated with SI-NET formation (206).

More recently, it has been suggested that the cell of origin of SI-NETs is not necessarily a fully differentiated EC cell, but rather an EC cell that expresses not only the EC cell markers, TPH1 and CHGA, but also markers of reserve stem cells (207). As we will discuss later in this text (see, EECs can act as reserve niche and stem cells), lineage tracing of both progenitor cells and of cells expressing mature EC cell markers in the mouse intestinal epithelium has shown that EC cells can adopt a stem cell fate upon injury (208). This observation suggests a plausible scenario whereby, if an EC cell were to acquire a genomic (or other) aberration capable of driving NET genesis, it could be long lived enough to indeed give rise to a tumor. Consistent with this hypothesis, Sei et al. identified human EC cells co-expressing the EC cell marker TPH1, and markers associated with both canonical and reserve stem cells, within crypt EC cell microtumors in tissue sections from patients with familial SI-NETs (209,210).

 

WHAT CAUSES HORMONE SWITCHING?  

 

Growth factor gradients change from high WNT, high EGF, high NOTCH, and low BMP, at the crypt bottom, to increasingly high BMP, low WNT, low EGF, and low NOTCH along the villus. Thus, upon leaving the crypt, EEC progenitors are exposed to increasingly different signaling environments. It was therefore attractive to speculate that these signaling gradients could induce hormone switching in EECs.

 

Adult stem cell (ASC) derived mouse and human intestinal organoids lack mesenchymal cells and are therefore not exposed to growth factor gradients but instead experience a constant environment determined by the media composition (211). Consequently, the ASC-derived intestinal organoid system provides researchers with a controlled, in vitro setting in which the signaling environment that intestinal cells experience can be modulated. Under expansion conditions that mimic the crypt environment and are optimized to promote stem cell maintenance, the EECs in mouse small intestinal organoids display a crypt hormone profile. However, when BMP4 was added to the culture media to mimic the villus environment, the EECs in the organoids expressed Secretin suggesting they had taken on a villus-like profile (195). The molecular mechanisms governing the hormone switching from GLP-1 to CCK to NTS observed in the L-I-N lineage remain to be unraveled but are likely to similarly involve cellular signals that differ along the crypt-villus axis.

 

Non-Neoplastic EEC Hyperplasia  

 

The previous sections described the formation of intestinal EECs. These cells and the hormones they secrete play a central role in regulating processes that are important for maintaining organismal function and energy homeostasis. It is therefore not surprising that EECs are implicated in a number of human disease conditions. Increased plasma level of EEC hormones and, in some cases, direct evidence of increased numbers of specific intestinal EEC subtypes have been reported in inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), lymphocytic colitis, Celiac disease, H. pylori infection, and Giardia infection (212–215). A genome-wide association study (GWAS) identified a strong association between a single nucleotide polymorphism (SNP) in the promoter of the EEC transcription factor, PHOX2B, and Crohn’s Disease (CD), a form of IBD (216). The mechanism leading to intestinal EEC hyperplasia in these conditions is not clear, though there is some evidence that inflammatory cytokines are involved (213). Consistent with this idea, treatment of mice with IFN and TNF led to increased numbers of Chromogranin A-positive colonic EECs (217). Likewise, IL-13 has been linked to the response of EC cells to enteric parasite infection (218).

 

It is plausible that the observed changes in EEC numbers in these conditions are not necessarily or not only a result of the disease pathology, but are also mediators of the pathology itself. There is clear evidence for interplay between EECs, the immune system, sensory neurons, and commensal bacteria (168,219). Together with goblet cells, EECs have been shown to secrete the cytokine IL-17C in patients with IBD (220). EEC hormones have been shown to have immunomodulatory functions (219). Some EECs are directly innervated and one study showed that serotonin-expressing intestinal ECs form synapses with nerve fibers through which they can modulate nerve fiber activity (203). EECs express functional toll-like receptors (TLR) and can thereby interact with commensal bacteria by responding to the metabolites they produce (221). Furthermore, as EEC hormones are known to also act systemically, the consequences of disease associated alterations in their abundance or function are not limited to the GI tract. Most notably, GLP-1 and GIP, which amplify glucose stimulated insulin secretion, are less effective in patients with type 2 diabetes (T2D) and GLP-1 receptor agonists are currently used to treat T2D and obesity (185).

 

The most well-documented examples of non-neoplastic EEC hyperplasia are hyperplasias of gastric and duodenal EECs. These include: ECL cell hyperplasia and G-cell hyperplasia. ECL-cell hyperplasia is associated with chronic excessive gastrin production, hypergastrinemia, resulting most commonly from achlorhydria due to chronic atrophic gastritis (CAG), gastrin-producing tumors in Zollinger-Ellison syndrome (ZES), and long-term proton-pump inhibitor (PPI) treatment (222). Lineage tracing of gastrin/cholecystokinin-2 receptor (CCK2R)-expressing cells in mice showed that CCK2R-expressing ECL cells in the isthmus but not the base of the stomach proliferated in response to PPI-induced hypergastrinemia (223). ECL cell hyperplasia only rarely progresses to neoplastic gastric ECL NETs. G-cell hyperplasia is most commonly observed as a secondary change associated with CAG in patients with pernicious anemia. In rare cases, it has also been observed in patients with peptic ulcer disease in conjunction with decreased numbers of a different gastric EEC, the SST producing D-cell. G-cell hyperplastic lesions do sometimes progress to G-cell NETs, gastrinomas (222).

 

EECs CAN ACT AS A RESERVE NICHE AND STEM CELLS

 

The microenvironment in which LGR5-positive intestinal stem cells reside is known as the stem cell niche and includes Paneth cells. The stem cell niche presents the stem cells with cellular signals that prevent them from differentiating and help preserve their self-renewal capacity. Loss of Paneth cells or severe injury such as irradiation, chemotherapy treatment, or surgery can cause loss of intestinal stem cells. In these situations, several different epithelial cell populations have been shown to act as reserve stem cells (224–228). The first studies indicating that EECs could act as reserve stem cells followed the fate of secretory progenitors (224–228). As this progenitor population also contained progenitor cells fated to become Paneth or goblet cells, the contribution specifically of EECs could not be explicitly determined. A later study that used lineage tracing of a population of EEC committed secretory progenitors expressing markers of differentiated EECs showed that these cells can act as reserve stem cells, capable of both contributing to intestinal regeneration after irradiation and generating organoids in vitro (228).

 

Lineage tracing of mouse intestinal cells expressing either NEUROD1 or Tryptophan hydroxylase (TPH1; the rate limiting enzyme in Serotonin biosynthesis and considered an EC cell marker) showed that these cells, similar to the EEC committed secretory progenitors mentioned above, had the ability to contribute to homeostasis and to regeneration following irradiation induced injury (208). Given that the EEC-committed progenitors in the earlier study expressed Neurod1, and that a subset of these cells also expressed Tph1, it is unclear whether the Tph1 lineage-traced cells with regenerative capacity in this study were indeed mature differentiated EECs or the same multi-potent EEC committed secretory progenitors described before. More recently, it was shown that, upon genetic Paneth cell ablation, mature EECs replaced the ablated Paneth cells, serving as Notch-presenting niche cells for intestinal stem cells (229).

 

PANCREATIC ENDOCRINE CELLS

 

Located in the upper left abdomen behind the stomach, the adult pancreas, although being one anatomical entity, originates from two individual buds that arise from either side of the distal foregut endoderm during early embryonic development. As development progresses, these buds fuse to form the final glandular composite organ consisting of two compartments, one exocrine and one endocrine, which contain functionally and morphologically distinct cell types. Acinar cells secrete digestive enzymes which are transported to the duodenum by mucin-secreting ductal cells, and together these two cell types comprise the exocrine compartment. The endocrine compartment comprises 5 different pancreatic endocrine cell (pEC) types, each of which secretes a specific hormone: alpha (glucagon), beta (insulin), gamma/ PP (pancreatic polypeptide), delta (SST) and epsilon cells (ghrelin). As is the case for EEC-derived hormones, pancreatic hormones act both locally and systemically. Indeed, some pancreatic hormones are transported throughout the body via the bloodstream and instruct other organs to, among other things, release or store glucose from the blood. Hence, the pECs play a key role in nutrient metabolism, digestion, and glucose homeostasis (230).

 

Development and Differentiation of Pancreatic Endocrine Cells

 

In contrast to the GI tract where EECs are generated throughout the life of the organism, the pECs are almost exclusively generated during the fetal development of the organ (231). This process is orchestrated by an interplay of regulatory transcription factors. Whereas some of these transcription factors are expressed constitutively, others play a transient yet essential role in mediating pEC lineage commitment and differentiation (see table 4). Murine models have made invaluable contributions to our understanding of pEC generation. In rodents, pancreatic development is divided into 3 transition phases, each outlined below (Figure 5).

 

Table 4. NE Cell Lineage Transcription Factors

Transcription Factor

Associated NE cell types

Role (specifically in NE cells)

ASCL1

PNECs and gastric EECs

Lineage specification (26,27,163)

NEUROG3

GEP NE cells

Lineage specification (transiently expressed) (161,162,175,176,232,233)

NEUROD1

PNECs*, Intestinal EECs (L-I-N lineage), and pECs (beta cells)

Specification of some NE cell subtypes (not defined in lung, secretin-expressing cells and I-cells in the intestine, beta cells in the pancreas) (27,36,255,407,408)

ATOH1

(a.k.a. MATH1)

Intestinal EEC precursors

Specification of the intestinal secretory cell fate (precedes EEC lineage commitment) (173)

INSM1

PNECs and GEP EECs**

NE cell maturation (31,160,180)

GFI1

PNECs and Intestinal EECs

Maturation of CGRP+ PNECs (409); lineage specification (via inhibition of EEC fate in intestinal secretory progenitors) (410,411)

PDX1

GEP EECs (G cells, some intestinal EECs, and pECs

Patterning and specification of gastric and duodenal EECs (G cells, SST- and GIP-expressing duodenal EECs) (412,413); Maintenance of beta cell maturity (414,415)

ISL1

GEP EECs (some gastric EECs**, non EC intestinal EECs, and pECs)

Specification of non-EC intestinal EECs (416); maturation and survival of pECs in development (417,418); maintenance of adult beta cell function (419)

RFX3

pECs

pEC differentiation during development (420); differentiation and maintenance of mature beta cells in the adult (421)

RFX6

GEP EECs (gastric EECs**, some intestinal EECs, and pECs)

Differentiation of intestinal EECs (K, L, X, I, and EC cells) (184,422); pEC specification and differentiation in development (423,424); Regulation of insulin expression in beta-cells (425)

PAX4

GEP EECs (some gastric EECs, duodenal EECs, some pECs)

Differentiation of EECs in the duodenum, of EC cells and D cells in the stomach (178), and of beta and delta cells in the pancreas (426,427)

PAX6

GEP EECs (some gastric EECs, some duodenal EECs, some pECs)

Differentiation of D and G cells in the stomach, of K cells in the duodenum (178), and of alpha and epsilon cells in the pancreas (428,429)

NKX2.2

Some intestinal EECs and some pECs

Regulation of cell fate within the intestinal EEC population (promotes EC, L-I-N, and K lineages) (179) and within the pEC population (promotes beta cell, alpha cell, and PP cell lineages) (252)

NKX6.1

Some gastric EECs and some pECs

Differentiation of EC cells in the stomach**(430) and of beta cells in the pancreas (253)

NKX6.2

Some pECs

Differentiation of alpha, beta, and PP cells in the pancreas (431,432)

NKX6.3

Some gastric EECs

Differentiation of G cells in the stomach (433)

MAFA

Some pECs

Maintenance of beta cell identity in the adult pancreas (257,434)

MAFB

Some pECs

Differentiation of alpha and beta cells in the developing pancreas (246,247); Maturation of alpha and beta cells in the adult pancreas (247,256)

ARX

GEP NE cells (some gastric EECs, some intestinal EECs, and some pECs)

Differentiation of G cells in the stomach (435), of L-, I-, and K cells in the intestine (435), and of alpha cells in the developing pancreas (244,245); Maintenance of alpha cell identity in the adult pancreas (436)

FOXA2

Some intestinal EECs and some pECs

Differentiation of L cells and D cells in the intestine (437); Maintenance of adult beta cell maturation in the pancreas (438,439); Maturation of alpha cells in the developing pancreas (440)

HHEX

Some intestinal EECs and some pECs

Differentiation of and maintenance of delta cell identity in the pancreas (262)(implicated in delta cells of the intestine (184,195))

* Role in PNECs no well defined

** Role in gastric EECs implicated by expression pattern

 

Figure 5. Rodent pancreas development. Schematic depicting the development of the murine pancreas. Islets of Langerhans containing the hormone-secreting endocrine cells are shown in blue and green.

PRIMARY TRANSITION

 

The first transition phase starts at around embryonic day (E) 9.0-9.5 with the formation of the pancreatic buds and ends at around E12.5 with the start of branching morphogenesis. During this period, the dorsal bud gives rise to the first pancreatic endocrine progenitor cells. As is the case for intestinal EECs, the formation of pECs depends on the expression of the bHLH transcription factor NEUROG3, which is transiently expressed at a relatively low level starting at ~E9.0-E11.0. Mice with targeted disruption of Neurog3 lack pECs and die postnatally of diabetes (232). Consistent with this data in mice, human induced pluripotent stem cells (iPSCs) with null alleles of NEUROG3 fail to differentiate into pECs (233). Following this first pulse of Neurog3 expression, a limited number of pancreatic progenitors becomes committed to the pEC lineage, and some of these cells begin to express glucagon (234).

 

SECONDARY TRANSITION

 

Branching morphogenesis continues throughout the first half of the second transition, (~E12.5-E15.5), during which the plexus expands and segregates into tip and trunk domains. From E16.5-E18.5 the epithelial plexus is remodeled into a ductal network with a tree-like structure. Inter- and intralobular ducts branch off from the main pancreatic duct, which connects the pancreas to the common bile duct. During this period, multipotent pancreatic progenitor cells, which make up the majority of cells at the beginning of this transition phase, become either unipotent, fated to become acinar cells at the tip, or bipotent, with the potential to form endocrine and ductal cells. These lineage commitments coincide with a second, more pronounced wave of Neurog3 expression that peaks at around E14.5-E15.5, and then rapidly decreases at around E17.5(235). This second wave of Neurog3 expression commits progenitors to the pEC fate and primes them towards one of the five pEC types.

 

TERTIARY TRANSITION

 

During the tertiary transition, individual progenitor cells that have now become fated to the pEC fate by the second, higher pulse of Neurog3 expression delaminate from the ductal network through what is thought to be asymmetric cell division and/or epithelial to mesenchymal transition (EMT). These pECs then migrate throughout the mesenchyme until they encounter other pEC-fated cells, aggregate with the help of Neural cell adhesion molecule (NCAM), and form oval shaped proto-islets (236,237). This process is reminiscent of the way, during lung development, PNECs form NEBs. The formation of fully mature islets requires their vascularization by endothelial cells and their innervation by neurons, both processes that take place from E16.5 onwards, and during the first weeks postnatally.

 

POSTNATAL

 

At birth the majority (more than 80%) of the pECs originates from endocrine progenitors and the remainder results from proliferation of preexisting pECs within the islets.

 

DIFFERENCES IN PANCREATIC ENDOCRINE CELL EVELOPMENT BETWEEN RODENTS AND HUMANS

 

Although studies of human pEC development are limited, they have uncovered a few main differences compared to rodents. First, there seems to be only one pulse of NEUROG3 during the formation of human pECs (compared to two in the mouse), which occurs at around 8 weeks of gestation. Second, the premature proto-islet structures described above are formed earlier in human development (at 12 weeks post conception) (238,239). Finally, the mature islet architecture differs between rodents and humans (Figure 6). Rodent islets have a central core of beta cells, which are also the most common of all the islet cells (60–80%). The remainder of the endocrine cells, alpha cells (15–20% of the cells of the islet), delta cells (<10% of islet cells), and gamma/ PP cells (<1% of cells) surround the beta cells in a circular structure known as the mantle (240). Human islets do not display the same organized structure as rodent islets. Instead, there is a salt and pepper pattern where the different endocrine cells are randomly scattered within the islets. The proportion of the different pECs also differs as human islets have proportionally less beta but more alpha cells compared to rodent islets, consisting of circa 30% alpha cells, 60% beta cells, about 10% delta cells, and rare <1% gamma/PP cells and epsilon cells (241,242).

 

Figure 6. Differences between the murine and human pancreas. Schematic showing the anatomical differences between the murine and human pancreas in terms of both organ and islet architecture.

 

Pancreatic Islet Cells and Hormones – All with Their Own Function

 

This section describes the molecular and cellular characteristics of the different subtypes of pECs and their specific hormones and function during adult homeostasis (Figure 7).

Figure 7. Pancreatic endocrine cells. Diagram of signaling and transcription factor interactions that regulate pancreatic endocrine cell differentiation.

ALPHA CELLS, GLUCAGON

 

Alpha cells secrete glucagon and are the second most common pEC subtype. Glucagon contributes to maintaining homeostatic blood glucose levels by stimulating glucose production and inhibiting glycogen storage by the liver. The glucagon receptor, GCGR, is widely expressed by multiple organs besides the liver, and glucagon mediates multiple other physiological processes including amino acid metabolism, glomerular filtration by the kidney, lipolysis, and gastric motility (243).

 

The key factor promoting commitment of pEC progenitors to alpha cells is the transcription factor ARX. Deletion of Arxin Pdx1-expressing progenitors in the mouse leads to a complete loss of alpha cells, accompanied by a compensatory increase in the number of beta and delta cells, which results in the same total numbers of pECs in mutant and wildtype mice (244). Similarly, overexpression of Arx in the embryonic mouse pancreas or in developing islet cells results in an increase in the number of alpha and PP cells while diminishing the number of beta and delta cells (245). While ARX is required for alpha cell specification, the transcription factor, MAFB, is required for their final maturation and hormone expression. Mice with null alleles of MafB show a 50% reduction in insulin and glucagon positive cells (246). Moreover, adult mice in which MafB has been conditionally deleted in either Neurog3-expressing pEC progenitors or in mature pECs had reduced numbers of glucagon-positive cells (247).

 

BETA CELLS, INSULIN

 

The most well-known and abundant cell type of the Islets of Langerhans is the beta cell. Beta cells produce insulin which lowers the blood glucose levels via direct and indirect effects on target tissues. Binding of insulin to its receptor facilitates glycolysis in liver hepatocytes and skeletal muscle cells and promotes lipogenesis in the liver and white adipose tissue. Additionally, insulin inhibits hepatic gluconeogenesis and glucagon production by alpha cells (248–251).

 

Some of the most important transcription factors involved in the differentiation of beta cells are NKX2.2, NKX6.1, NEUROD1, MAFA, and MAFB. In mice lacking the homeodomain transcription factor NKX2.2, beta cell precursors fail to fully mature or produce insulin and, as a consequence, the mutant mice develop hyperglycemia and die shortly after birth (252). NKX2.6 lies downstream of NKX2.2, and in mice lacking Nkx6.1 beta cell neogenesis during the second transition is blocked, and the development of fully differentiated beta cells is prevented (253). NEUROD1 is also required for beta cell maturation (254). Neurod1 null mice have reduced beta cell numbers and fail to develop mature islets (255). Differentiating beta cells express both MafA and MafB, but fully mature beta cells in the mouse only express MafA. Conditional deletion of MafB in the mouse pancreas delayed beta cell maturation (247). Notably, MAFB is expressed in adult human beta cells and while human pluripotent stem cells with MAFB knockout are capable of pEC differentiation, they favor delta cell and PP cell specification at the expense of beta cells (256). MAFA is not essential for beta cell development but is required for insulin secretion and for maintenance of the beta cell identity (257).

 

DELTA CELLS, SOMATOSTATIN

 

Pancreatic delta cells, which make up ~5-6% of all islet cells, are responsible for the production of SST. Like insulin and glucagon, SST is a peptide hormone and cleavage of the precursor pro-hormone gives rise to two active isoforms, a long form that acts primarily in the central nervous system and a short form that acts on the organs of the GEP system. Delta cells predominantly secrete the short isoform of SST in response to a variety of stimuli including acetylcholine, glutamate, GLP-1, and urocortin3 produced by beta cells, ghrelin produced by epsilon cells, and high blood glucose levels (258). Within the islets, SST binds to one of its five different receptors on the surface of beta cells and alpha cells, thereby inhibiting the secretion of insulin and glucagon (259,260). SST also acts on cholangiocytes in the liver, inhibiting their secretion of fluids and thereby mediating bile flow (261).

 

The delta cell-specific transcription factor, haematopoietically expressed homeobox (HHEX), is required for both differentiation of delta cells during embryogenesis and maintenance of their identity in the adult. Deletion of Hhex in mouse pEC progenitors during development led to loss of delta cells by E16.5. Likewise, deletion of Hhex in adult mouse pancreatic delta cells led to a reduced number of delta cells and reduced secretion of SST (262). The phenotype of mice lacking HHEX in the pancreas speaks to the hormonal interplay between pECs: reduced SST secretion in mutant mice led to increased hormone secretion from alpha and beta cells in response to different stimuli. Indeed, despite not being expressed in beta cells, HHEX has been repeatedly identified through GWAS studies as a locus conferring susceptibility to T2D (263).

 

EPSILON CELLS, GHRELIN  

 

Epsilon cells, described for the first time in 2002, were the last islet cell type to be discovered. The number of pancreatic epsilon cells is highest during embryogenesis, comprising up to 10% of islet cells at mid-gestation, but their numbers then decrease such that they make up only a bit more than 1% of islet cells in the adult pancreas (264). Scattered throughout the islets, epsilon cells produce the growth hormone secretagogue ghrelin. The name of this hormone comes from the Proto-Indo-European root “ghre-”, meaning "to grow," which seems appropriate for a hormone that was discovered in efforts to identify the ligand for the growth hormone secretagogue receptor (GHS-R) (265). Also known as the hunger hormone, ghrelin is a 28 amino acid peptide that needs to be post-translationally acetylated before it can bind to its receptor, GHS-R, and exert its function on its target cells in an expanding list of target organs (266).

 

Consistent with its stimulatory effect on food intake, ghrelin concentrations are highest during fasting. This function is primarily mediated by ghrelin secreted by gastric X cells. The other hallmark functions of ghrelin are stimulating fat deposition and stimulating growth hormone release from the pituitary. The list of functions attributed to ghrelin is still growing and includes regulating glucose and energy homeostasis, cardioprotection, muscle atrophy, and bone metabolism (267). Secretion of ghrelin is stimulated by glucagon and is inhibited by glucose, insulin, leptin, and GLP-2(268–270). In the pancreas, ghrelin secreted by epsilon cells binds to the GHS-R on delta cells, thereby inducing them to secrete SST, which in turn inhibits the secretion of insulin by beta cells (271,272). There is also some evidence that ghrelin promotes the survival and proliferation of beta cells (264).

 

A complex interaction between NKX2.2 and NEUROD1 appears to be involved in epsilon cell specification during development (264). However, the transcription factor most closely linked specifically to epsilon cells is PAX6, which appears to inhibit epsilon cell formation. Ghrelin-expressing cells are increased in the developing pancreas of Pax6 knock-out mice at the cost of alpha cells (273). Likewise, when Pax6 was deleted in the adult pancreas of mice, the same increase in epsilon cell numbers was observed, concomitant with loss of beta cells, alpha cells, and delta cells (274). A later study demonstrated that, upon loss of Pax6 expression, beta and alpha cells began to express ghrelin (275).

 

PP (GAMMA) CELLS, PANCREATIC POLYPEPTIDE  

 

PP cells comprise less than 1-2% of the islets and the majority are located in the head of the pancreas. Perhaps due to their scarcity in the islets, not much is known about the genetic determinants of PP cell specification and maturation. Signals from the vagus nerve, enteric neurons, and arginine following meal intake stimulate PP cells to release pancreatic polypeptide (PP) (276). This hormone has the opposite effect of ghrelin and is referred to as the satiety hormone (277). Transgenic mice that overexpress PP show reduced weight gain and decreased fat mass, and long-acting PP analogues are being explored for the treatment of human obesity (278,279). Within the pancreatic islet, PP indirectly affects insulin secretion by inhibiting glucagon and SST secretion via a receptor expressed on alpha and delta cells, respectively (280,281).

 

Other members belonging to the same peptide hormone family as PP are neuropeptide Y (NPY) and peptide YY (PYY) which are produced in the GI tract. PYY is produced by L cells in the intestine and colon and its function mimics that of PP in the pancreas (282).

 

Pancreatic Endocrine Cells in Injury Repair

 

Injury to the pancreatic epithelium in the form of surgery, inflammation or metabolic trauma disrupts homeostasis by causing extensive cell loss. Failure in injury repair can result in organ dysfunction and cause clinical symptoms.

 

The youthful pancreas has a high potential to regenerate damaged tissue following injury (283,284). In contrast to the GI tract, however, where this capacity to regenerate is maintained throughout life, the regenerative capacity of pECs is limited in adult tissue (285,286). Through observations made in rodent injury models, mostly looking at beta cells, it has become clear that the endocrine pancreas employs multiple strategies to regain homeostasis following injury.

 

REPLICATION OF PRE-EXISTING BETA CELLS  

 

The ability of pre-existing beta cells to replicate in response to injury was first shown by the Melton lab. Using a beta-cell-specific lineage trace they found that the newly generated beta cells that arose following partial pancreatectomy originated from pre-existing ones (287). Using the same injury model, but an unbiased thymidine analogue based labelling technique, a different group came to the same conclusion (288).

 

NEOGENESIS OF BETA CELLS FROM A STEM/PROGENITOR CELL    

 

An alternative hypothesis is that beta cell regeneration in the adult pancreas involves a process called neogenesis, which involves the formation of de novo beta cells derived from a stem or progenitor cell expressing Neurog3. Initial studies of neogenesis were based on in vivo pulse labelling with the thymidine analogue, BrdU. In one study, IFN-gamma induced beta cell depletion resulted in the appearance of budding duct/islet-like structures containing proliferative ductal cells as well as newly formed acinar and endocrine cells (289). Similar observations were made in two different models of pancreatic injury in rats, where regeneration involved marked proliferation of ductal cells, some of which expressed ductal and endocrine markers, followed by the formation of new pECs and islet structures (290,291). Researchers from the Heimberg lab found that, subsequent to pancreatitis induced by pancreatic duct ligation in mice, the region of the pancreas undergoing regeneration contained NEUROG3-positive cells, a portion of which also expressed ductal markers (292). Finally, a lineage trace of ductal cells in mice showed that, following injury, ductal, acinar, and endocrine cells all contained the lineage trace (293). Altogether, these studies suggest that, following injury, new pECs can be derived from a progenitor cell in the pancreas ductal epithelium, whose differentiation to the pEC fate is dependent on transient expression of Neurog3.

 

TRANS-DIFFERENTIATION OF NON-BETA CELLS INTO BETA CELLS        

 

Finally, a landmark study from the lab of Pedro L. Herrera, provided evidence for yet another strategy employed by the pancreatic endocrine compartment to regenerate.  Prior to inducing genetic ablation of adult beta cells, the authors induced a lineage trace of glucagon-producing alpha cells. In the initial months following beta cell ablation, mice required supplementation with insulin. However, at 6 months post-ablation, mice no longer required supplemental insulin and their pancreata showed increased beta cell mass and beta cells that expressed both insulin and glucagon. At early time points following ablation, the majority of the regenerated beta cells in these mice contained the alpha cell lineage trace, arguing that the beta cells had resulted from trans-differentiation of the alpha cells (294). This provided new evidence of endocrine cell plasticity.

 

Pancreatic Endocrine Cell Hyperplasia

 

In the previous section we discussed observations relating to pEC regeneration in the context of conditions that lead to direct loss of pECs. Given defining pEC characteristics such as direct innervation and the role of pECs in mediating a number of physiological processes, it is likely that pECs also respond to stimuli produced in the context of other pathological conditions. As we have discussed in previous sections of this text, EEC and PNEC hyperplasia have each been observed in the context of inflammation or injury of their respective tissue sites. Likewise, an increase in the number pECs has been observed as a response to some pathological conditions relating to the pancreas (295,296). Moreover, focal endocrine hyperplasias have been observed incidentally in the pancreas of up to 10% of screened adults at autopsy (297).

 

The pEC mass is normally 1% and 3% of the total pancreatic mass in adults and infants, respectively. If this increases to more than 2% or 10% of the total pancreatic mass in adults or infants, respectively, it is defined as pEC hyperplasia (222,297,298). Some instances of pEC hyperplasia involve a general increase in the size of pancreatic islets that results in an increase in overall islet cell numbers but not in a change in the relative frequencies of one pEC subtype versus another. However, the majority of pEC hyperplasia are associated with an increase in the number of a specific subtype of pEC, most commonly alpha and beta cells. Hyperplasia of other pEC subtypes including the rare gamma/PP cells have also been reported. Morphologically, pEC hyperplasia either appears as large islets (larger than 250 mm in diameter) or as budding structures protruding from the ductal epithelium (297). The latter budding structures are reminiscent of the structures described above in mouse models of pEC injury and are suggestive of pEC neogenesis.

 

BETA CELL HYPERPLASIA

 

Beta cell hyperplasia is commonly observed in patients with insulin resistance and early T2D and is likely a physiological response to these conditions. Other clinical conditions in which beta cell hyperplasia is implicated include persistent hyperinsulinemic hypoglycemia of infancy (PHHI) and non-insulinoma pancreatogenous hypoglycemia syndrome (NIPHS) (299–302). These conditions are associated with dysregulated insulin secretion and hypoglycemia in infants and neonates or in adults, respectively.

 

The pancreas of patients with PHHI is characterized by the presence of either focal beta cell hyperplasia, resulting in a focal increase in islet size or, more commonly, of diffuse beta cell hyperplasia in which enlarged islets and small, irregularly shaped endocrine cell clusters are found throughout the pancreas (297). The percentage of beta cells present within the islets of patients with PHHI is increased such that they account for 70-90% of the islet (222). Increased proliferation of not just beta cells but also of ductal and centroacinar cells has been reported in the pancreas of patients with PHHI (303). PHHI is caused by mutations in ABCC8 and KCNJ11, which encode for subunits of the ATP-sensitive potassium channel involved in insulin secretion, as well as by mutations in genes affecting beta cell metabolism such as glucokinase (GCK), glutamate dehydrogenase (GLUD1), and short chain fatty acid hyroxyacyl dehydrogenase (SCHAD) (304).

 

NIPH is defined by postprandial hypoglycemia and, unlike PHHI, the genetic cause has not been clearly identified. The pancreas of patients with NIPH exhibits an increase in both number and size of the islets, and contains endocrine cells budding from the ductal epithelium (305,306). Symptoms of both PHHI and NIPH can be resolved through either partial or near total pancreatectomy.

 

ALPHA CELL HYPERPLASIA

 

Alpha cell hyperplasia (ACH) is a rare condition most commonly caused by mutations in the gene that encodes for the glucagon receptor, GCGR (307,308). The number of islets in patients with ACH is increased and the islets vary in size, are often larger, and contain a high proportion of alpha cells (309). Patients with ACH often, though not always, also present with hyperglucagonemia, and multiple pancreatic NETs. The multifocality of ACH and pancreatic NET lesions in these patients, and the observed presence of large islets showing signs of morphological transition from ACH to glucagonomas suggests that ACH lesions can progress to frank glucagonomas (308,309). Interestingly, these patients do not display features of glucagonoma syndrome due to their GCGR mutations. Mice with germline null mutations in Gcgr, or with liver-specific deletion of Gcgr also develop ACH that can progress to glucagonomas. Pharmacologic interruption of glucagon signaling in mice also leads to ACH. In these models, ACH appears to be primarily driven by alpha cell proliferation, though it is possible that transdifferentiation of other pECs or of ductal cells is also involved. It is interesting to note that the clear link between GCGR function and ACH implies a signaling feedback loop that regulates both the number of glucagon-producing alpha cells and their secretion of glucagon. One of the signals that is likely to contribute to ACH is amino acids. Serum amino acid levels are increased in patients with ACH and transcriptomic analysis of mouse models of ACH have shown altered expression patterns for genes involved in amino acid catabolism and transport (310).

 

GAMMA/PPCELL HYPERPLASIA

 

Compared to alpha and beta cell hyperplasia even less is known about PP cell hyperplasia, which occurs very rarely. As with alpha and beta cell hyperplasia the number and size of the pancreatic islets in patients with PP cell hyperplasia is increased and contain a high proportion of PP cells (311–313). In 50% of the reported PP hyperplasia cases, the patients had suffered from gastrinoma or ZES. In addition, there is no correlation between PP cell number and PP serum levels. It is possible that PP hyperplasia arises as an effect of gastrinomas (297). There are also no genetic changes directly related to the onset of this condition.

 

GEP NEUROENDOCRINE NEOPLASMS (GEP-NENs)

 

GEP-NENs encompass all NENs that arise along the GEP tract and account for 55 to 70% of NENs from all tissue sites (82,314,315). GEP-NENs comprise both well-differentiated NETs and poorly differentiated NECs. In addition, some GEP-NENs occur as GEP-mixed neuroendocrine/non-neuroendocrine neoplasms (GEP-MiNEN) that can be either well- or poorly- differentiated and are characterized by their mixed morphology showing endocrine and non-endocrine features. Based on proliferation (measured by mitotic count and Ki67 index) GEP-NETs are further subdivided into low (G1), intermediate (G2), and high (G3) grade NETs (82). G3 well-differentiated NETs are associated with a poor prognosis and show a decidedly more aggressive clinical behavior than G1 and G2 GEP-NETs. G3 well-differentiated NETs are more commonly observed in the pancreas than in other GEP tissue sites. GEP-NECs can be subdivided into small cell NEC and large cell NECs.

 

GEP-NECs

 

GEP-NECs comprise 10-20% of all NENs, with roughly 38% arising in the GI tract (colon, anus, rectum) and 23% in the pancreas (316). GEP-NETs and GEP-NECs display different mutational profiles and are therefore considered separate disease entities. Indeed, the most common site for NECs is the large bowel, whereas the most common site for NETs is the ileum (316). In addition, the observations that some GI-NECs show features resembling adenocarcinomas or squamous cell carcinomas and that up to 40% of NECs show non-endocrine features, have led some researchers to hypothesize that GEP-NECs are more closely related to non-endocrine tumor types than to high grade (G3) NETs (82,317,318).

 

The importance of the two genes RB1 and TP53 in the genesis of NECs is highlighted by the fact that these genes are commonly mutated in both lung-NECs and GEP-NECs. TP53 mutations have been identified in 20-73% of all GEP-NECs (319–323) while RB1 mutations have been identified in 44-86% of all GEP-NECs (322,324–326). The limited number of studies that have performed genomic analyses of GEP-NECs have also identified other genes that are commonly mutated in GEP-NECs, including KRAS, SMAD4 and APC (318,327,328). In addition, GEP-NECs are characterized by frequent and severe chromosomal abnormalities (79).

As discussed previously, generating a GEMM of SCLC was achieved through conditional tissue-specific deletion of Rb1 and p53 in the mouse lung epithelium (89). In contrast, attempts to generate GEMMs of RB1; TP53 mutant GEP-NECs by deletion of Rb1 and p53 in targeted mouse GEP tissues have proven to be less straightforward and have given mixed results (329). Conditional deletion of Rb1 and p53 in renin-expressing mouse pECs led to the generation of highly aggressive, metastatic, glucagon-producing tumors (330). Given the expression of glucagon in combination with the aggressive course of the tumors developed by this GEMM, however, it is unclear whether they are a more suitable model for sporadic glucagonomas or for pancreatic NECs. In human patients, pancreatic NECs are almost exclusively non-functional, i.e., they do not secrete symptom-inducing hormones (331). In the RIP-Tag2 GEMM, instead, SV40 T-antigen is expressed in beta cells, thereby effectively abrogating the Rb1 and p53 pathways in these pECs. RIP-Tag2 mice primarily develop aggressive insulinomas and, to a lesser extent, poorly differentiated pancreatic NECs (332). The RIP-Tag2 model has been instrumental in delineating stepwise aspects of pancreatic NEN tumorigenesis and in identifying therapeutic strategies for these tumors in patients (329). Nonetheless, whether this GEMM can be used as a reliable model for pancreatic NECs is unclear. 

 

In addition to GEMMs, cell lines and, more recently, GEP-NEC patient-derived tumor organoid lines have been generated (333,334). Patient-derived tumor organoids, 3D long-term cultures of tumor cells, can be expanded long-term, can be cryopreserved, and have been shown to be representative of the patient tumor tissue from which they were derived at both the genetic and phenotypic levels (335). Recently, a genetically engineered organoid model of GEP-NECs was generated by CRISPR/Cas9 mediated compound knock-out of RB1 and TP53 combined with overexpression of 6 transcription factors in otherwise normal colon organoids (333). Interestingly, in the absence of transcription factor overexpression, compound knock-out of RB1 and TP53 was not sufficient to generate GEP-NECs from these cells.   

 

Together with genomic analyses of GEP-NECs, studies using preclinical models of GEP-NECs have been informative. Nonetheless, to date, strategies for stratifying patients in terms of the molecular characteristics of their tumors and the likely response of their tumors to specific therapies are lacking. Currently, the primary treatment strategies for GEP-NECs use platinum agents combined with etoposide, based on the relative effectiveness of this approach in treating SCLC, which is a more common and better studied NEC subtype (336–338). Overall, the response rate for GEP-NECs to first line therapy is 40-60%. Upon relapse or the tumors becoming refractory, there are no well-established second-line therapies. This is reflected in a median survival of 38 months in patients with localized disease and only 5 months in patients with metastatic disease (339). Other potential treatment strategies for patients with GEP-NECs that are currently undergoing clinical trials are the mTOR inhibitor everolimus and some forms of immunotherapy (336–338).                                               

 

GEP-NETs

 

GEP-NETs represent 80-90% of all GEP-NENs and comprise many different tumor types. GEP-NETs are classified as functional or nonfunctional depending on whether they secrete symptom-causing hormone peptides. Functional GEP-NETs, which can arise throughout the GEP-tract, include gastrinomas and insulinomas (10). GEP-NETs are highly heterogeneous with regards to their biological behavior and clinical presentation, course, and prognosis. The most common tissue sites of primary GEP-NETs are the small intestine, the rectum, the colon, the pancreas (12.1%), and the appendix (315). In general, G1 and G2 GEP-NETs are associated with high 5-year survival rates ranging from 75 to 79% and from 62 to 74%, respectively. The 5-year survival rate for patients with G3 well-differentiation NETs, on the other hand, shows more variability between NETs of the intestine (40%) and NETs of the pancreas (7%) (340). Notably, the recognition of the category of G3 well-differentiated pancreatic NET by the WHO in 2017 has had important implications for patients and clinicians, as it highlighted the fact that some pancreatic NETs, despite showing morphological features and differentiation more commonly associated with low-grade NETs, show aggressive behavior more similar to that of NECs (82,331).

 

The most prevalent sites of origin for GEP-NETs show regional differences that are likely reflective of differences in both environmental and genetic factors. GI-NETs arising from the small intestine or colon are most common in the USA, small intestinal or pancreatic NETs are more common in Europe, while in Asia gastric and rectal NETs are most prevalent (341). The most well studied GEP-NETs are pancreatic NETs and SI-NETs.

 

Gastric NETs (G-NETs) are relatively rare and only account for 4 to 6% of NENs (342,343). These tumors are classified into one of four categories according to their clinical characteristics. Most G-NETs are type I tumors, which are associated with CAG and arise as multiple small nodules. These tumors rarely metastasize. Type II G-NETs are very similar to type I tumors but are commonly associated with MEN1 syndrome (in conjunction with gastrin producing pancreatic NETs) or ZES. Type I and type II G-NETs arise as a consequence of excessive gastrin. Type III G-NETs are sporadic tumors that are not associated with other gastric conditions and present as large, solitary lesions. Finally, type IV tumors, G-NECs, are both the very rare and the most malignant. These tumors arise sporadically and are poorly differentiated. While type I, II, and III G-NETs are ECL cell tumors, type IV G-NECs arise from other endocrine cell types (344).

 

Small intestinal NETs (SI-NETs) are the most common neoplasm arising in the small intestine (345). They include NETs arising in the jejunum and ileum, with the ileum being the major site of incidence (346). A unique feature of ileal SI-NETs is that they are multifocal in 10-20% of cases, with the different tumors arising independently (347). Moreover, ileal SI-NETs have a high rate of metastasis, with >50% of ileal SI-NET patients presenting with metastases at the time of diagnosis (348). Finally, the majority of SI-NETs produce serotonin, causing many of these to induce carcinoid syndrome, the most common functional hormonal syndrome of patients with NETs (349). Clinical symptoms of carcinoid syndrome include watery diarrhea, flushing, hypotension, breathlessness, wheezing, and loss of appetite, all attributable to not just increased serum levels of serotonin but also of prostaglandins, histamine, bradykinin, and tachykinins (9,350). Carcinoid syndrome is most often observed when NETs, having metastasized to the liver, secrete their biologically active compounds directly into the systemic circulation (351). Carcinoid syndrome can cause carcinoid heart disease in which elevated serum serotonin levels cause fibrosis in the heart. Symptoms of carcinoid syndrome attributable to serotonin can be ameliorated through the administration of serotonin-inhibiting therapies (340,352).

 

Of note, duodenal NETs are sometimes considered separately from SI-NETs as they more closely resemble gastric and pancreatic NETs in terms of their mutational profile (353). Whereas the serotonin producing EC cells are thought to be the cell of origin of SI-NETS, duodenal NETs more commonly express gastrin and somatostatin (described in more detail later in this chapter) and are therefore thought to arise from G and D cells (354,355).

 

Pancreatic NETs (PanNETs) are the best studied subtype of GEP-NET and are the only subtype for which the category of high grade G3 well-differentiated NET has been officially recognized (356). While some well-differentiated PanNETs are functional and consist of a single hormone-producing cell type, the majority of PanNETs are non-functional and contain a mixture of cells expressing markers for the different pEC types (357,358). PanNETs are thought to arise from differentiated endocrine cells of the islets of Langerhans. However, a recent study based on next generation DNA methylation analysis suggests that they may also arise from the exocrine pancreas (359).

 

Familial GEP-NETS

 

While the majority of the GEP-NETs arise sporadically, approximately 5% occur as part of a hereditary cancer predisposition syndrome (360). The most common of these syndromes are associated with the development of duodenal and PanNETs and are caused by germline mutation in one of five different genes: MEN1, tuberous sclerosis complex 1 (TSC1), tuberous sclerosis complex 2 (TSC2), Von Hippel–Lindau (VHL), and neurofibromatosis type 1 (NF1) (361). Of these, MEN1 is the gene most strongly implicated in PanNETs. MEN1 encodes for the protein menin, which acts as a scaffold for both transcription factors and chromatin-modifying enzymes (362–364). Thus, although its exact function is yet to be determined, menin has been suggested to play a role in multiple processes including DNA damage repair, cell cycle regulation, histone methylation, and mTOR pathway activity (365).

 

Familial SI-NETs are far rarer and have been associated with germline mutations in CDKN1B, inositol polyphosphate multikinase (IPMK), and MutY DNA glycosylase gene (MUTYH) (366–369). Notably, germline mutations in CDKN1B have been shown to be causative of the MEN4 familial cancer syndrome in which patients develop parathyroid, pituitary, and, more rarely, SI-NETs (323). Finally, there is one example of a single consanguineous family in which several family members were affected by hypergastrinemia and consequent gastric ECL NETs. These familial G-NETs were shown to be caused by germline inactivating mutations in the gene for a proton pump expressed by parietal cells and involved in gastric secretion, ATP4a (370).

 

Genetics of Sporadic GEP-NETs

 

Although rare, familial cancer syndromes associated with NETs have pointed to genes and pathways that are also important for the genesis of sporadic NETs. Genetic studies have revealed that somatic inactivation by mutation, chromosomal alteration, or epigenetic silencing of genes such as MEN1, TSC2, and VHL, each associated with a familial NET syndrome, are found in approximately 40%, 35%, and 25% of sporadic pancreatic NETs (371,372). Familial SI-NET syndromes are rarer, and of the causative genes for these syndromes, only mutations in CDKN1Bhave also been identified in 9% of sporadic SI-NETs (373). Of note, the largest whole genome analysis of pancreatic NETs to date, uncovered germline mutations in CDKN1B and MUTYH in pancreatic NETs from patients that had no family history of the disease, therefore implicating alterations in these genes also in the genesis of pancreatic NETs and, perhaps, of GEP-NETs in general (371,372). Incidentally, germline variants in pancreatic NET samples in this cohort were also identified in checkpoint kinase 2 (CHEK2), and BRCA2 (371,372).    

 

Molecular analysis of pancreatic NETs also uncovered alterations in genes that are not implicated in familial NET syndromes. In particular, Jiao et al. identified recurrent mutations in PTEN and PIK3CA in pancreatic NETs. This study also identified recurrent mutations in the chromatin remodeling enzymes DAXX (death domain associated protein) and ATRX (α thalassemia/mental retardation syndrome X-linked), in 25% and 18% of pancreatic NETs (374). DAXX and ATRX function together in a complex that deposits histone H3.3 at different sites including telomeres and mutations in ATRX/DAXX were mutually exclusive in pancreatic NETs (372). ATRX/DAXX mutations in pancreatic NETs were correlated with chromosomal instability and the activation of a telomerase independent telomere maintenance mechanism, alternative lengthening of telomeres (ALT) (375,376). Perhaps it is thus not entirely surprising that ATRX/DAXX mutations are more often found in G3 pancreatic NETs than G1/G2 pancreatic NETs and are associated with reduced patient survival (323,375). Other mutations found in G3 pancreatic NETs and associated with shorter survival times are mutations in ARID1A and CDKN2a (377,378).

 

Whereas several driver mutations have been identified in pancreatic NETs, identifying a clear genetic driver of SI-NETs has been more difficult. Instead, the development of sporadic SI-NETs seems to be dependent on chromosomal aberrations. Loss of chromosome (chr) 18 is observed in more than 60% of SI-NETs. While the functional importance of this chromosomal loss in SI-NETs has not been determined, one study identified allelic loss of BCL2, CDH19, DCC, and SMAD4 (all on chr 18) in 44% of SI-NETs (379). Loss of chr 9 and 16 or gain of chr 4, 5, 7, 14 or 20 are also recurrently observed in SI-NETs, albeit at lower frequencies (353,380–383). Targeted mutational and copy number analysis of metastatic SI-NETs has identified recurrent mutations in APC, CDKN2C, BRAF, KRAS, PIK3CA, and TP53, though at relatively low frequencies (ranging from 4 to 10%) (328,379,384).

 

Although genomic studies demonstrate that SI-NETs and pancreatic NETs have distinct genomic profiles, a common pathway alteration stands out -- activation of mTOR/PI3K signaling. In pancreatic NETs the pathway is recurrently activated by frequent loss of negative regulators of the pathway (PTEN, TSC1/2) and by recurrent activating mutations in PIK3CA. Likewise, this pathway is implicated in SI-NETs by the observation of recurrent activating mutations in KRAS, BRAF, and PIK3CA as well as, more commonly, of frequent copy number gains in components of this pathway, including SRC and mTOR itself (323). Frequent alteration of CDKN family genes also appears to be a common feature of both GEP-NET subtypes, suggesting deregulation of the cell cycle is also important for NETs. Finally, though not discussed in this text, aberrant methylation patterns are likely to contribute to GEP-NETs in general and some studies have suggested that methylation patterns can be used to stratify GEP-NETs with implications for patient prognosis (385–387).

 

Treatments for GEP-NETs

 

The main alternatives used to treat GEP-NETs for which surgical resection is not possible are hormone analogs, PRRT, the mTOR inhibitor everolimus, and for pancreatic NETs, sunitinib. Chemotherapy mainly targets proliferative cells and is therefore predominantly used on NECs and G2 pancreatic NETs.

 

Somatostatin analogues (SSAs) such as octreotide and lanreotide bind to the somatostatin receptors (SSTR1-5) and mimic the natural hormone’s ability to exert an inhibitory function on the cell’s hormone secretion. To prolong their effect, these analogues have been synthetically engineered to have an increased half-life compared to the endogenous hormone. Administration of SSAs is common in the treatment of functional GEP-NETs as they prevent the tumors from secreting excessive amounts of hormones and thus alleviate clinical symptoms (388,389). Although GEP-NETs can express all five somatostatin receptors, the majority express SSTR2. Binding of SSAs to SSTR2 has been shown to decrease proliferation and lead to disease stabilization in GEP-NET patients (390–392). In addition, SSAs can also be coupled to radioactive isotopes and used for targeted radiological therapy. The internalization of the radioactive isotope by the cancer cells causes DNA damage and cell death (393,394).

 

As discussed previously, the mTOR-AKT pathway plays an important role in GEP-NETs. Consistent with this, inhibitors of this pathway such as everolimus have been shown to have a positive effect (132,395,396). Cell type-specific drugs have also been used in the treatment of pancreatic NETs and one of the oldest drugs is streptozotocin, a beta cell specific cytotoxic agent that has been used for almost four decades (397). Streptozotocin selectively enters beta cells via the glucose transporter, GLUT2, causing DNA damage resulting in cell death (398). Finally, inhibitors of receptor tyrosine kinases, vascular endothelial growth factor and its receptor have been effective in the treatment of GEP-NETs, most notably, pancreatic NETs as they are often highly vascularized (399,400).

 

CONCLUSIONS

 

When comparing neuroendocrine cells from different tissues, multiple recurrent themes can be identified. For one, their development relies on the expression of bHLH transcription factors ASCL1 and NEUROG3. Animal models have highlighted the differential roles played by these two transcription factors in the formation and differentiation of NE cells of the diffuse NE system. Whereas ASCL1 drives PNEC lineage commitment in the lung, NEUROG3 is essential for the formation of the intestinal EECs and pECs. Both ASCL1 and NEUROG3 appear to be important for the development of gastric EECs. Another interesting difference can be seen in the expression pattern of these transcription factors. Whereas NEUROG3 is transiently expressed during the formation of GEP endocrine cells, ASCL1 is constitutively expressed in lung progenitor and mature PNECs. The mechanistic differences and biological reason behind these differential dependencies on either ASCL1 or NEUROG3 remain to be determined.

 

Second, NE cells of the diffuse NE system appear to be involved in the response to some forms of injury, as indicated by their increased numbers in some disease conditions. Importantly, comparing NE cells during development or homeostasis to NE cells in disease conditions has the potential to uncover aberrations in common pathways and regulatory factors that contribute to or mediate the pathological state. This kind of analysis is likely to provide candidate targets for the development of new (targeted) therapies.

 

Another recurrent theme in the biology of neuroendocrine cells from different tissues, is the crosstalk between NE cells and their environment. This crosstalk can be seen both in the response of NE cells to environmental stimuli and in the direct influence they can exert both locally and systemically through the bioactive compounds they secrete. Finally, the ideas of NE cell plasticity and NE cell heterogeneity are ones that have not been fully explored but might be important to their function. As an example of the former, although EECs are named after their predominantly expressed hormone, they can also be multi-hormonal and even change their hormone expression depending on their location within the tissue. While EEC hormonal switching is just starting to be discovered within the intestinal tract, it is likely that a similar degree of plasticity in hormone expression exists for other NE cell types. Indeed, some hints of plasticity between endocrine cells can also be seen in pancreatic endocrine cells, where some studies imply trans-differentiation of alpha cells to beta cells under certain conditions. Regarding NE cell heterogeneity, it is interesting to note that, whereas the initial studies of NE cells were driven by observations about their shared features, with the advancement of single-cell sequencing technologies, the more recent era of NE cell research has highlighted a previously underappreciated heterogeneity of different tissue-specific NE cell populations (34,35,164,183,401,402). NE cell heterogeneity highlights the plasticity and dynamic nature of these cells as they respond to external stimuli and microenvironmental signals in a context-specific manner.

 

With regards to NENs, next generation sequencing efforts have started to characterize the mutational landscape of NENs. So far, these have been mostly focused on pancreatic and lung NENs. Similar studies of NENs originating from other organs would be of value as they could provide new insights into NEN biology. Importantly, candidates discovered in genome-wide genomic studies need to be validated to determine whether they represent drivers or passengers throughout disease progression. Additionally, their exact mechanistic function is of importance if they are to become targets for drug development. For NENs with a lower mutational burden and few recurrent mutations, epigenome, non-coding or protein level changes may play a more significant role in disease initiation and progression. These possibilities are just starting to be explored.

 

A significant challenge in studying NENs is the development of model systems that can be used to study both basic as well as translational NEN biology. Currently, the most commonly used models include GEMMs, cell lines, and patient derived xenografts (PDX). Although these model systems have provided invaluable knowledge about NEN biology they also have their limitations. GEMMs have been instrumental in the study of SCLC and some very specific subtypes of pancreatic NETs (RIP-Tag2 mice and Men1 mutant mice). Nonetheless, the current GEMMs for different NENs do not cover the entire range of different tumor types encompassed by NENs. Furthermore, species differences require comparisons and/or end stage studies in human derived model systems.

 

Cell lines, which while being robust and relatively cheap to culture, suffer from genetic changes occurring over time and may not recapitulate the full tumor heterogeneity.

 

PDX models circumvent those limitations while providing analysis in an in vivo environment, but have a very low engraftment rate of <10% and tedious logistics (403). Organoids derived from healthy tissue provide researchers with the means to study normal NE cell differentiation via the establishment of differentiation protocols (183,195) and offer the potential to model NEN disease progression by stepwise genome engineering, as has been achieved for other tumor types (333,404–406). Moreover, in vitro drug screens on patient-derived tumor organoids have the potential to aid personalized treatment. However, to date only a handful of NEN organoid lines have been established. Of note, most of those organoid lines represent NECs, rather than NETs (333,334).

 

Although NEN biology is starting to be explored in more detail, much remains to be discovered. A combination of both basic and translational research will be needed to provide the biological insights necessary to significantly be able to improve or establish novel clinical treatment options for NENs.

 

ACKNOWLEDGEMENTS

 

We thank Lisanne den Hartigh for help with making figure 5 and Joep Beumer for helpful discussions. Figures 1 - 4 and 6-7 were created with BioRender.com. This work was supported by an EMBO long-term fellowship (AALTF 332-2018 to A.A.R.) and funding from the Neuroendocrine Tumor Research Foundation under a Petersen Accelerator Award (H.C. and T.D.). 

 

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Hypertension in Diabetes

ABSTRACT

 

The coexistence of diabetes and hypertension is known to have a multiplicative effect on adverse clinical outcomes with respect to both microvascular and macrovascular disease. Effective management of diabetes should therefore include a multifaceted approach combining optimal control of blood pressure and lipids with appropriate glycemic control. The pathophysiology of hypertension in diabetes involves maladaptive changes in the autonomic nervous system, vascular endothelial dysfunction, enhanced activation of the renin-angiotensin-aldosterone system, immune function alterations, and harmful environmental factors. Multiple high-quality randomized controlled trials have shown improvement in morbidity with lowering of elevated blood pressure in people with diabetes. Attention must be paid to individual risk factors and co-morbidities with a goal of less than 130/80 mm Hg in most patients with diabetes who are at higher risk of cardiovascular disease (CVD) than those without diabetes.  Good glycemic control, optimizing weight, and promotion of exercise as well as lessening harmful environment factors such as air pollution exposure are integral components of the approach to blood pressure control in these patients. Judicious selection of therapy and consideration of relevant side-effect profiles is paramount. The potential for both beneficial and detrimental drug interactions must be kept in mind and drug combinations should be chosen after due deliberation. Angiotensin converting enzyme inhibitors and angiotensin receptor blockers remain preferred agents for initiation of antihypertensive therapy, while combined use of these agents is not recommended due to poor renal outcomes. With the advent of newer antidiabetic agents such as SGLT inhibitors and GLP1 receptor agonists, consideration should be given to their antihypertensive, renal, and cardiovascular disease lowering properties when initiating therapy for glycemic control.

 

INTRODUCTION

 

Statistics from the Centers for Disease Control and Prevention (CDC) and National Health and Nutritional Examination Survey (NHANES) database show that the incidence of Type 2 diabetes mellitus (T2DM) has risen steeply in the last few decades. It is estimated that diabetes affects 34.2 million people in US 10.5% of US population. 73.6% of individuals with diabetes aged 18 years or more have hypertension. The coexistence of hypertension and diabetes in a large population of patients is not coincidental; individuals with T2DM often display a constellation of metabolic derangements termed the cardiometabolic or cardiorenal metabolic syndrome (1). This syndrome comprises a cluster of CVD risk factors including T2DM, hypertension, dyslipidemia, central obesity, and chronic kidney disease. The coexistence of hypertension and diabetes in these individuals substantially increases the risk for cardiovascular disease (CVD), cerebrovascular accident (CVA), retinopathy, and nephropathy (2). The rising prevalence of obesity and sedentary lifestyles in the US are the major driver of both diabetes and hypertension and the resulting health care costs are a serious public health concern. Increasingly, the role of environmental factors such as food deserts and environmental pollution in the promotion of diabetes, hypertension, and CVD is being appreciated. These harmful environmental factors especially affect minorities and other disadvantaged populations.

 

The increasing prevalence of T2DM in the general population has expectedly been paralleled by a rise in microvascular and macrovascular complications. Despite major advances in healthcare delivery, diabetes mellitus continues to be the leading cause of blindness, end stage renal disease (ESRD), and non-traumatic lower limb amputations in the US as well as the seventh leading cause of death as of 2017 (1). While optimal glycemic control remains paramount in the prevention of microvascular complications (retinopathy, nephropathy, and neuropathy), concurrent cardiometabolic derangements such as hypertension and dyslipidemia play a pivotal role in the initiation and progression of macrovascular disease (ischemic heart disease, stroke, and peripheral vascular disease) (3). Effective management of diabetes should therefore include a multifaceted approach combining optimal control of blood pressure and lipids with appropriate glycemic control (4). This chapter will focus on the management of hypertension in patients with diabetes.

 

PATHOPHYSIOLOGY OF HYPERTENSION IN DIABETES

 

The pathophysiology of hypertension in diabetes can be traced to maladaptive changes and complex interactions between the autonomic nervous system, a maladaptive immune system, enhanced activation of the renin-angiotensin-aldosterone system (RAAS) as well as adverse environmental factors. The factors listed below play a major role in the pathogenesis of hypertension and have been targeted for therapeutic interventions (2,5).

 

Sedentary Lifestyle, Excessive Caloric Intake and Insulin Resistance

 

Sedentary lifestyle and excessive caloric intake can lead to increased adiposity which has been associated with increased risk of worsening insulin resistance. Insulin resistance has been linked in turn to an increased vascular oxidative stress, inflammation, and endothelial dysfunction characterized by diminished vascular nitric oxide bioactivity, all of which promote vascular stiffness resulting in a persistent elevation of blood pressure and the promotion of CVD (6,7).

 

Elevated Intravascular Volume

 

Intravascular volume is strongly influenced by total body sodium content. Sodium is the major extracellular cation in human beings, and possesses osmotic activity which helps determine effective arterial blood volume. A mismatch between sodium intake and sodium loss can result in a positive sodium balance. The ensuing increase in intravascular sodium concentration stimulates an influx of water along the osmotic gradient, thus raising intravascular volume. Elevated intravascular volume consequently increases venous return to the heart boosting cardiac output in accordance with the Frank Starling Law, and this process eventually leads to elevated arterial pressure (8). There is also increasing evidence that increased activation of sodium inward transport in endothelial cells contribute to increased vascular stiffness and elevated blood pressure in states of obesity and insulin resistance as exists in most patients with T2DM (7).

 

Increased blood pressure (BP) from intravascular volume expansion is typically corrected by a rise in glomerular filtration and compensatory urinary salt excretion. This phenomenon of increased salt excretion in a state of elevated blood pressure has been termed pressure natriuresis. Unfortunately, this mechanism alone cannot correct persistently elevated blood pressure, principally because of secondary changes within the kidney microvasculature and maladaptive changes within the glomerular apparatus itself that lower glomerular filtration and stimulate sodium reabsorption. These changes are most apparent in overt chronic kidney disease (CKD) and end stage renal disease (ESRD), both of which are characterized by concurrent volume overload and sustained hypertension. Hypertension in CKD/ESRD is often difficult to control and requires restoration of normal vascular volume, which can be achieved by means of diuretics or dialysis (8,9).

 

Premature Vascular Aging

 

Changes in vessel lumen elasticity affect the ease with which blood can flow through arteries. Minimal reductions in lumen diameter can lead to exponentially increased resistance to blood flow. Patients with hypertension often demonstrate structural and functional changes that adversely alter the lumen of small arteries and arterioles. The vascular remodeling, low grade inflammation, vascular fibrosis and stiffening seen with hypertension in individuals with diabetes can arise as a response to elevated BP. Patients with diabetes thus manifest accelerated premature vascular aging characterized by impaired endothelial mediated relaxation, enhanced vascular smooth muscle contraction and resistance as well as vascular stiffness (7). These maladaptive vascular changes both contribute to the development of hypertension and accelerate the harmful effects of hypertension on vessel integrity (8,10).

 

Autonomic Nervous System Dysregulation

 

The autonomic nervous system is an important determinant of BP. Both the sympathetic and the parasympathetic systems are involved in the regulation of BP. Increased sympathetic activity leads to an increase in heart rate, force of contraction of ventricles, peripheral vascular resistance, and fluid retention. These physiological actions combine to promote BP elevation. Decreased parasympathetic outflow also results in increased heart rate and relative sympathetic hyperactivity thus contributing to an elevation in BP. Dysregulation of these pathways is seen with central obesity, insulin resistance, and sleep apnea. Hypertension associated with these disorders is often accompanied by increased sympathetic activity, an activated RAAS, and resistant hypertension. Furthermore, activation of the sympathetic nervous system also promotes insulin resistance and risk of T2DM. The autonomic dysfunction seen with T2DM can also contribute to these changes and thus worsen hypertension. The relevance of these pathways in the pathogenesis of hypertension and diabetes is demonstrated by the observation that interruption of the central sympathetic outflow by renal denervation is associated with improved insulin sensitivity, better glycemic control, and reductions in BP (2,8).

 

Renin Angiotensin Aldosterone System (RAAS)

 

The RAAS pathway plays a central role in maintaining normal BP. RAAS activation is closely linked to the pathogenesis of hypertension via the cardiovascular and renal effects of elevations, particularly of plasma aldosterone level.  Angiotensin II is a potent vasoconstrictor and acts directly to increase vascular smooth muscle tone. Angiotensin II also stimulates secretion of aldosterone, which promotes sodium and water retention, leading to elevated blood pressure through volume expansion.  Obesity is associated with elevated plasma aldosterone levels, even in the absence of elevation of angiotensin levels. This elevation is thought to be related, in part, to secretion of aldosterone releasing factors from the expanded adipose tissue (7). Understanding the physiology of RAAS is essential as it is the principal target for angiotensin converting enzyme (ACE) inhibitors, angiotensin II receptor blockers (ARBs), and increasingly mineralocorticoid receptor antagonists, which are cornerstones of BP management in individuals with diabetes (8).

 

Renin is a proteolytic enzyme secreted by the juxtaglomerular cells in the kidney. Renin cleaves circulating angiotensinogen to angiotensin I. ACE within the lung capillaries then converts angiotensin I into angiotensin II. Production and release of renin is tightly regulated by many interdependent factors such as renal perfusion pressure, sodium chloride concentration in distal tubule of nephron, and stimulation of renin secreting cells by the sympathetic nervous system.

 

Physiologic activation of RAAS is seen with renal hypoperfusion due to hypovolemia. Release of renin from the juxtaglomerular apparatus results in a cascade of events, culminating in increased production of angiotensin II. Angiotensin II then raises blood pressure through direct vasoconstriction and by stimulation of aldosterone secretion leading to sodium and water retention and restoration of intravascular volume (8).

 

Obesity and insulin resistance are associated with inappropriate activation of RAAS and the sympathetic nervous system. Increased adiposity has been linked with high levels of plasma aldosterone suggesting that RAAS may be chronically overactive in obesity (7). Angiotensin II and aldosterone have been shown to inhibit insulin metabolic signaling in classical insulin sensitive tissues and this likely plays a role in impaired endothelial-mediated vascular relaxation and the development of hypertension. Angiotensin II and aldosterone may also promote insulin resistance through non-genomic mechanisms such as activation of serine kinases and increased serine phosphorylation of insulin receptor substrate 1, reduced phosphatidylinositol 3-kinase engagement and protein kinase B stimulation, diminished insulin metabolic signaling, and impaired nitric oxide mediated vascular relaxation. (11,12). Increasingly it is recognized that elevated aldosterone in conjunction with hyperinsulinemia, as often exists in obesity and insulin resistance, promote vascular stiffness and associated increases in hypertension and CVD (7).

 

Renal Dysfunction

 

Renal dysfunction appears to share a reciprocal relationship with hypertension in diabetic individuals. While hypertension itself is recognized as a risk factor for chronic kidney disease in the setting of diabetes, it is important to note that diabetic nephropathy also contributes to development of hypertension. This reciprocal relationship is most obvious in type 1 diabetics without pre-existing hypertension. Longitudinal studies have shown that microalbuminuria precedes hypertension in this population, and the prevalence of hypertension rises progressively with worsening kidney disease, approaching 90% in type 1 diabetics with end stage renal disease. Proposed mechanisms include volume expansion secondary to increased renal sodium reabsorption, peripheral vasoconstriction arising from endothelial dysfunction, dysregulated activation of the RAAS, upregulation of endothelin1, and downregulation of nitric oxide (13).

 

Role of Innate and Adaptive Immunity

 

There is emerging evidence that innate immunity and acquired immunity are involved in angiotensin II and aldosterone-induced hypertension and vascular disease (6). Animal studies suggest that intact T cell function is required for full expression of these adverse effects and that T cells and macrophages mediate the oxidative injury associated with these effects. On the other hand, the protective properties of T regulatory cells in animal models suggests a potential therapeutic role for these cells, although at this time such interventions are limited to the research setting.

 

Environmental and Socioeconomic Factors

 

There are marked disparities in hypertension between White and Black Americans. This disparity is increasing despite higher levels of awareness and treatment of their hypertension amongst Black Americans as compared to their White counterparts (14). This disparity has been magnified with the recent Covid-19 pandemic with disproportionate levels of morbidity and mortality amongst communities of color. Some have suggested that this disparity in Covid outcomes are related to similar environmental, economic, and social inequities as those that promote hypertension, obesity, and diabetes (15).

 

Foods that are traditionally considered healthy and promoted as components of the DASH diet (16) are often unavailable to people living in these communities due to either lack of access or reasons of affordability. Instead, they become consumers of cheap high salt and high caloric foods, a process that naturally leads to obesity and hypertension (15). Furthermore, lack of safe outdoor spaces discourages exercise and targeted advertising increases poor health decisions such as smoking. These effects are further reinforced by a study of Black and White Americans living in the same environmental setting (long term integrated neighborhoods).  In the Exploring Health Disparities in Integrated Communities-South Western Baltimore (EHDIC-SWB) study it was found that although the odds ratio for hypertension was higher in Blacks in the sample population, it was decreased by roughly 30% as compared to NHANES data. The authors concluded that social and environmental exposure explained a substantial proportion of race differences in persons with hypertension and diabetes (17).

 

BLOOD PRESSURE MEASUREMENT AND MONITORING

 

Accurate measurement of BP is key for both diagnosis and effective management of hypertension. BP measurement is most often conducted in the medical office, where it can be performed either through the auscultatory technique of listening to Korotkoff sounds or the oscillometric technique employed in automated devices. Use of oscillometric devices has largely replaced the auscultatory method primarily for reasons of convenience and concerns over inter-observer variability with manual measurements. However, it is important to remember that even automated measurements can be erroneous if certain precautions are not taken. Measurements should be made in the seated position after the patient has rested for 3-5 minutes, and preferably with an empty bladder. No exertion, physical exercise, eating, smoking or exposure to stress for at least 30 minutes before BP reading. Three readings within a period of 2 weeks will be ideal. The device used should be calibrated regularly to ensure reliable readings. Improper cuff size is a common source of erroneous readings. It is recommended that cuff bladder length be equal to the patient’s arm circumference measured at the midpoint of acromion and olecranon process and the width be equal to about one-half of the arm circumference. Use of a cuff that is too small is more common because of the rising incidence of obesity and results in overestimation of blood pressure. Despite using all these precautions, there can be significant variability between individual readings and American Heart Association (AHA) recommends obtaining at least two readings during each clinic visit (18).

 

Ambulatory blood pressure monitoring (ABPM) is a fully-automated non-invasive modality that involves placement of a blood pressure cuff on the non-dominant arm with measurements every 15 to 30 minutes over the course of a 24-hour period.  Compared to in-office blood pressure measurement, ABPM has higher prognostic value for target organ damage and cardiovascular outcomes (19). The primary advantage of ABPM lies in its comprehensive nature unlike office monitoring that relies on single measurements. This format permits detection of distinct blood pressure patterns such as sustained, white-coat, masked, and nocturnal hypertension, as well as non-dipping or reverse-dipping patterns that cannot be detected with office measurements alone. These patterns are associated with varying cardiovascular outcomes and must therefore be managed quite differently. White-coat hypertension denotes a situation wherein office measurements are in the hypertensive range but ABPM readings are consistently normal. This phenomenon is attributed to the effect of an observer at the time of measurement; it is associated with minimal cardiovascular risk and is not an indication for antihypertensive therapy. It should be noted however that individuals with white-coat are at elevated risk for developing sustained hypertension and should therefore be monitored periodically. On the other hand, masked hypertension refers to a situation where office measurements are normal but ABPM shows readings in the hypertensive range. This phenomenon is associated with increased cardiovascular risk comparable to that seen with sustained hypertension. Importantly, masked hypertension is more common in diabetic individuals and obese patients. It is assumed that these patients benefit from aggressive antihypertensive therapy although no randomized controlled trials have been performed to confirm such expectations (20).

 

Blood pressure normally displays a physiologic circadian rhythm, dipping by more than 10% during the night relative to daytime readings.  Patients in whom blood pressure drops by less than 10% are said to have a non-dipping pattern. This non-dipping pattern is more prevalent in diabetic individuals and has been associated with cardiovascular autonomic neuropathy. Its contribution to progression of diabetic complications is more controversial. Hyperglycemia itself can influence the normal nocturnal dip through its effect on circulating plasma volume, blood flow distribution and renal hemodynamics (21).

 

BLOOD PRESSURE TARGETS IN PATIENTS WITH DIABETES

 

The importance of rigorous blood pressure control in prevention of diabetes-related morbidity cannot be overemphasized. This holds true for macrovascular as well as microvascular complications and is supported by a mounting body of evidence. The United Kingdom Prospective Diabetes Study (UKPDS), showed 44, 32, and 34 percent reductions in risks for stroke, diabetes related deaths, and retinopathy respectively with blood pressure reduction (target blood pressure <150/85 mm Hg). A linear relationship between systolic blood pressure reduction and adverse outcomes was seen in readings as low as 120 mm Hg (22,23).

 

The Hypertension On Target (HOT) trial showed a reduction in CVD with lowering of diastolic blood pressure. Interestingly however, this benefit was only seen in patients with diabetes, suggesting the need for establishing a different and perhaps more aggressive blood pressure target in this population subgroup (24).

 

The Action in Diabetes and Vascular Disease: Preterax and Diamicron MR Controlled Evaluation (ADVANCE) trial was the first study designed specifically to address blood pressure control in subjects with diabetes. The results were impressive, showing significant reduction in microvascular events, cardiovascular deaths, and all-cause mortality with aggressive reduction in both systolic and diastolic blood pressure (mean achieved blood pressure of 134/74 mm Hg versus 140/76 mm Hg) (25).

 

Major medical societies including the American Diabetes Association (ADA) recommend a target blood pressure of less than 130/80 mm Hg for patients with diabetes. The first trial to seek justification for this recommendation was the Normotensive Appropriate Blood Pressure Control in Diabetes (ABCD) trial. Although no specific blood pressure target was pursued, the mean attained blood pressure of 128/75 mm Hg in the intensive treatment group, was under the systolic target of 130 mm Hg. Over a follow up of five years, no significant difference was seen in creatinine clearance (primary outcome) or cardiovascular events when compared to the placebo group (mean blood pressure 137/81). The intensive treatment group did manifest significant reductions in progression of retinopathy, albuminuria, and absolute risk of stroke (26,27).

 

The notion of a systolic blood pressure goal of less than 130 mm Hg was challenged by the ACCORD blood pressure trial. This large randomized control trial compared a systolic target of <120 mm Hg (intensive therapy) to a systolic target of <140 mm Hg (standard therapy). With more than 4500 patients and a mean follow up of 4.7 years, no significant difference was seen between the two groups in terms of combined CVD outcomes (heart attack, stroke, and cardiovascular death). Importantly, similar to the results of the ABCD trial, a 40 percent reduction was seen in stroke risk (28). This study was confounded by factors that do not allow for recommendations based on the outcomes of this study.

 

The most recent large-scale randomized control trial that examined a lower systolic blood pressure goal was the Systolic Blood Pressure Intervention Trial (SPRINT). This trial compared the benefit of treatment to a systolic blood pressure target of less than 120 mm Hg (intensive-treatment group) with the treatment to a target of less than 140 mm Hg (standard-treatment group). At 1 year, the intensive-treatment group had a mean systolic blood pressure of 121.4 mm Hg versus the standard-treatment group with a mean systolic blood pressure of 136.2 mm Hg. The results showed significantly lower rates of fatal and nonfatal cardiovascular events and death from any cause in the intensive-treatment group. Serious adverse events possibly or definitely related to the intervention were statistically more frequent in the intensive-treatment group with a hazard ratio of 1.88 (P<0.001). This study included 9361 participants with a median follow up of 3.26 years; however, patients with diabetes were excluded. The SPRINT trial therefore supports a lower goal but cannot be applied directly to the diabetic population because of its study design (29).

 

Some experts have suggested that the ACCORD trial was underpowered to show a significant difference for the primary endpoint. A recently pooled analysis merged the data from the SPRINT and ACCORD trials and looked at the same primary endpoint that was used in SPRINT. The primary endpoint differed from the ACCORD trial in that it included unstable angina and acute decompensated heart failure in addition to myocardial infarction, stroke and CVD death. The final analysis showed a significant favorable effect for the intensive treatment group in both patients with and without diabetes. This suggests that there may not be a differential beneficial effect of intensive blood pressure lowering (i.e., to less than 130/80 mm Hg) in patients with T2DM (30). It must also be noted that both the SPRINT and ACCORD trials involved BP measurements under strictly controlled conditions that would be expected to yield lower readings compared to conventional clinic settings. This observation raises questions about whether more liberal targets might be used in real world settings to achieve comparable cardiovascular benefits.

 

In conclusion, multiple high-quality randomized controlled trials have shown improvement in morbidity with correction of elevated BP in people with diabetes. Patients with T2DM appear to be particularly susceptible to the deleterious effects of hypertension in initiation and progression of CVD. In the treatment of hypertension in patients with diabetes attention must be paid to individual risk factors, co-morbidities, and patient preferences when considering lower treatment targets. A lower blood pressure target, for instance, might be more appropriate for a young person who would likely benefit from a reduction in stroke risk and reduced progression of retinopathy without experiencing unwanted side effects of hypotension, syncope, and hyperkalemia that are encountered more commonly in the older population and those with multiple co-morbidities.

 

Key outcome studies and results are summarized in table 1.

 

Table 1. Key Outcome Studies and Results

Outcome Study

Intervention

Results

United Kingdom Prospective Diabetes Study (UKPDS)

Blood pressure reduction

(< 150/85 mmHg)

 

44 % risk reduction in stroke

 

32 % risk reduction in

diabetes related deaths

 

34 % risk of retinopathy

 

Hypertension On Target Trial (HOT)

Lower diastolic blood pressure

 

Reduction in CVD

 

Action in Diabetes and Vascular Disease: Preterax and Diamicron MR Controlled Evaluation trial (ADVANCE)

Reduced systolic and diastolic blood pressure     

(134/74mmHg vs 140/76mmHg)

 

Reduction in microvascular events, cardiovascular deaths, and all-cause mortality

 

Normotensive Appropriate Blood Pressure Control in Diabetes Trial (ABCD)

Intensive blood pressure control

(128/75mmHg vs 137/81mmHg)

 

Reduction in progression of retinopathy, albuminuria, and absolute risk of stroke

 

No difference in creatinine clearance or cardiovascular events

 

 

ACCORD Study Group Trial

Intensive blood pressure control

(Systolic target <120mmHg vs <140mmHg)

 

40% risk reduction for stroke

 

No difference for combined CVD outcomes (heart attack, stroke, and cardiovascular death)

 

Systolic Blood Pressure Intervention Trial (SPRINT)

Intensive blood pressure control

(Systolic target <120mmHg vs <140mmHg)

 

Achieved mean blood pressure 121.4mmHg vs 136.2mmHg

 

Reduced rates of fatal and nonfatal cardiovascular evens and death

 

Increased adverse events related to intensive group

 

 

TREATMENT OF HYPERTENSION

 

Treatment of hypertension in patients with diabetes is challenging as these patients can develop resistant hypertension. Moreover, individuals with diabetes have a higher incidence of cardiac and renal comorbidities that can lower tolerance to aggressive antihypertensive therapy. An effective treatment regimen must therefore address all aspects of the complex metabolic derangements seen in this population group (4).

 

This section will focus chiefly on the treatment of hypertension in association with T2DM. We will examine treatment strategies by drug class, critically reviewing the advantages and disadvantages of each. The importance of accurately measuring BP and using proper techniques needs to be emphasized, especially considering the lifelong implications for the patient. Once a diagnosis of hypertension has been established in a patient with diabetes, it is imperative that aggressive treatment be initiated in a timely manner. It is also worth noting that with some exceptions, the degree of blood pressure reduction achieved is of greater importance than the class of antihypertensive employed.

 

The various classes of antihypertensive drug that are commonly employed in diabetic individuals are summarized in table 2. The overall approach to hypertension in a diabetic patient is outlined in figure 1.

Figure 1. Approach to hypertension in the diabetic patient

 

Table 2. Summary of Antihypertensive Agents with Emphasis on Patients with Diabetes

Class with representative examples

Preferred use

Notable side effects

Contraindications

Effect on insulin resistance and/or glycemic control

ACE inhibitor*

·     Lisinopril

·     Ramipril

·     Benazepril

Diabetics

Also preferred in:

·     Proteinuric CKD

·     HFrEF

·     Established CAD

·     Hyperkalemia

·     Acute kidney injury (up to 25% rise in creatinine is expected)

·     Angioedema

·     Cough

·     Teratogenicity

·    Pregnancy

·    Avoid concomitant use with aliskiren or ARB

Improved

ARB

·     Telmisartan

·     Valsartan

·     Losartan

·     Irbesartan

·     Candesartan

Diabetics who are intolerant of ACE inhibitors

Also preferred in:

·     Proteinuric CKD

·     HFrEF

·     Established CAD

·     Hyperkalemia

·     Acute kidney injury (up to 25% rise in creatinine is expected)

·     Teratogenicity

·     Pregnancy

·     Avoid concomitant use with aliskiren or ACE inhibitor

Improved

Direct renin inhibitor**

·     Aliskiren

Diabetics with proteinuric CKD who are intolerant of both ACE inhibitors and ARBs

·     Hyperkalemia

·     Acute kidney injury

·     Teratogenicity

·     Pregnancy

·     Avoid concomitant use with ACE inhibitor or ARB

Unknown

Thiazide-like diuretic

·     Chlorthalidone

·     Indapamide

·     HCTZ

Hypervolemic or edematous patients

Must be used before diagnosing “resistant hypertension”

·     Photosensitivity

·     Hyponatremia

·     Hypokalemia

·     Hypomagnesemia

·     Hyperuricemia

·     Orthostatic hypotension

·     Pregnancy

·     Use with caution in cirrhotic patients (risk of hyponatremia)

·     Ineffective in advanced CKD-GFR<30

Worsened with HCTZ

Indapamide has positive effect

Dihydropyridine calcium channel blocker*

·     Nicardipine

·     Amlodipine

Patients who are already on preferred agents but not at target blood pressure

·     Peripheral edema

·     None but should not be initiated until other preferred agents have been started

Neutral

Beta adrenergic blocker

·     Carvedilol

·     Nebivolol

·     Metoprolol

Preferred in:

·     History of myocardial infarction

·     HFrEF

·     Orthostatic hypotension

·     Acute decompensation of heart failure

·     Bronchospasm

·     Hypoglycemia unawareness

·     Depression

·     Impotence

·     Avoid in active bronchospasm, vasospastic disorders

·     Avoid if pheochromocytoma suspected (until adequate alpha blockade)

·     Use with caution in PVD

Worsened with non-vasodilating agents like metoprolol and not with Carvedilol and nebivolol

Mineralocorticoid receptor blocker

·     Spironolactone

·     Eplerenone

·     Finerenone

Preferred in:

·     HFpEF and HFrEF

·     Resistant hypertension

·     Primary aldosteronism

·     Hyperkalemia

·     Gynecomastia (with spironolactone)

·     Avoid in pregnancy

·     Caution if using with ACE, ARB or renin inhibitors

Improved with spironolactone, unknown with other agents

Preferred agents within each class are bolded. Preference is based on available evidence from randomized control trials.

*All agents in this class are considered equivalent

**Only agent currently approved in this class

Abbreviations: ACE: Angiotensin converting enzyme; ARB: Angiotensin receptor blocker; CAD: Coronary artery disease; CKD: Chronic kidney disease; HCTZ: Hydrochlorothiazide; HFpEF: Heart failure with preserved ejection fraction; HFrEF: Heart failure with reduced ejection fraction; PVD: Peripheral vascular disease. GFR: Glomerular Filtration rate

 

Lifestyle Modification

 

Lifestyle modification is a very important and often overlooked aspect of treatment of diabetes and hypertension. Changes to lifestyle that appear to have health benefits include:

 

  • Reducing salt intake to less than 1.5 g/day
  • Increasing consumption of fruits and vegetables (8-10 servings per day)
  • Increasing consumption of low- fat dairy products (2-3 servings per day)
  • Increasing activity levels/ engaging in regular aerobic physical activity (e.g., brisk walking 30 min/day)
  • Losing excess weight
  • Avoiding excessive alcohol consumption (less than 2 drinks (30 ml ethanol)/day for men and less than 1 drink (15ml of ehanol)/day for women)

 

Lifestyle modification may be used as a sole treatment modality in patients with BP <140/90, but ideally should be combined with pharmacotherapy in patients with systolic blood pressure (SBP) ≥ 140 and/or diastolic blood pressure (DBP) ≥ 90 mm Hg. It is generally agreed that lifestyle modification has modest antihypertensive effects, yielding an effective blood pressure reduction of 5-10 mm Hg. Nevertheless, ancillary benefits of improved cardiovascular fitness, reduced adiposity, and the possibility of future reduction in medication doses make such interventions an indispensable part of the management of these patients.

 

Angiotensin Converting Enzyme Inhibitors

 

ACE inhibitors inhibit the angiotensin converting enzyme and thus prevent conversion of angiotensin 1 to angiotensin II. This along with other mechanisms leads to decreased peripheral resistance and lowering of BP. ACE inhibitors selectively dilate the efferent renal arterioles and therefore lower intraglomerular pressure. This hemodynamic effect is reno-protective in patients with diabetic kidney disease. An acute rise in serum creatinine may occur at the onset of ACE inhibitor therapy. Elevation of serum creatinine by up to 30% above baseline is acceptable and does not mandate stopping therapy but does underscore the need for careful monitoring. The beneficial effects of ACE inhibitors on renal and cardiac function are widely recognized (31,32,33) and these agents are prescribed almost reflexively as initial antihypertensive treatment in patients with concomitant diabetes and hypertension (34). However, it must be noted that the primary advantage of ACE inhibitors over other classes of antihypertensive agents, lies in their proven ability to slow the progression of proteinuria.

 

ACE inhibitors possess a favorable side effect profile and are well-tolerated in general. Use of these agents is not associated with adverse alterations in lipid profile, glucose levels, and uric acid levels, such as those seen with other antihypertensive agents. As noted above creatinine elevation is frequently observed and should not require cessation of therapy unless excessive. On the other hand, dry persistent cough, another common side effect, is a reasonable cause for discontinuation of therapy. Patients with long standing diabetes, diabetic nephropathy, and hyporeninemic hypoaldosteronism/ type 4 renal tubular acidosis can develop hyperkalemia with these drugs. Angioedema – a severe hypersensitivity reaction more commonly observed in the African American population, is also associated with ACE inhibitor use and the drug should be permanently discontinued in such patients. Further, ACE inhibitors have teratogenic potential by interfering with fetal kidney development and caution must be exercised while using ACE inhibitors in females of child bearing age (33).

 

Due to their potential benefits and favorable risk benefit profile, ACE inhibitors have been established as the benchmark by which newer classes of antihypertensive agents are judged, especially in patients with diabetes and diabetic kidney disease.

 

Angiotensin Receptor Blockers (ARBs)

 

ARBs exert similar salutary effects as ACE inhibitors, by displacing angiotensin II from its receptor. The main advantage of ARBs over ACE inhibitors is the lower incidence of cough and angioedema with their use. The ONTARGET trial compared the ARB telmisartan to the ACE inhibitor Ramipril and the combined use of these drugs. This trial established general non-inferiority of telmisartan compared to ramipril with regards to BP control as judged by outcomes such as cardiovascular deaths, myocardial infarction, stroke, and hospitalization for heart failure. Additionally, the telmisartan arm had substantially lower rates of cough and angioedema. Data from the ONTARGET trial also showed that although both telmisartan and ramipril offered equivalent renal protection, the combined use of these two drugs led to inferior renal outcomes (35). A combination of ACE inhibitors and ARBs is therefore not recommended at this time. As with ACE inhibitors, hyperkalemia remains a potential adverse effect. The risk of hyperkalemia can be attenuated by combining these agents with other medications like thiazide or loop diuretics which promote urinary potassium loss. ARBs used to cost substantially more than ACE inhibitors but the advent of generic ARB’s have addressed this concern. Today ARBs are a popular choice for treatment of hypertension, and for prevention of renal complications in patients with diabetes, and are the preferred treatment in patients who develop a cough with ACE inhibitors (34).

 

Diuretics

 

Diuretics transiently decrease blood pressure by boosting renal sodium excretion and consequently lowering plasma volume. Overtime, these changes in volume status revert back to normal, but the antihypertensive effect persists due to a decrease in peripheral vascular resistance. Hydrochlorothiazide (HCTZ) and related sulfonamide compounds (chlorthalidone) are effective for blood pressure management in patients with mild to moderate hypertension and eGFR >50. In patients with eGFR <30, loop diuretics or a combination of loop diuretics and thiazides are more efficacious (34).

 

Data from the Swedish Trial in Old Patients with Hypertension-2 (STOP Hypertension-2) trial demonstrated that diuretics were as efficacious as ACE inhibitors or calcium channel blockers (CCBs) in lowering BP and reducing cardiovascular mortality in patients with diabetes (36).

 

Use of diuretics is associated with metabolic derangements like hypokalemia, hyperglycemia, and hyperuricemia. Once again, the risk of hypokalemia associated with diuretic use can be mitigated by combining a diuretic with medications, like an ACE inhibitor, ARB, potassium-sparing diuretic, or aldosterone antagonist (37). Patients with T2DM and concomitant hypertension also demonstrate impaired nocturnal BP dipping compared to patients without diabetes. Chlorthalidone, with its longer duration of action and higher potency might be a better choice to treat hypertension in this subgroup of patients (38).

 

Calcium Channel Blockers (CCBs)

 

CCBs are sub-classified as Dihydropyridines (DHPs) (amlodipine, felodipine, isradipine, nicardipine, nifedipine) and non-DHPs (NDHPs) (verapamil, diltiazem). DHPs exert their antihypertensive activity through peripheral vasodilatation, without significantly affecting cardiac conduction and contractility. NDHPs also have a modest antihypertensive effect, but they affect cardiac automaticity and conduction, and hence are primarily used for management of arrhythmias (34).

 

The strongest evidence for CCB use over other classes of antihypertensive drugs comes from the Avoiding Cardiovascular Events through Combination Therapy in Patients Living with Systolic Hypertension (ACCOMPLISH) trial. This trial was designed to compare benazepril plus amlodipine to benazepril plus hydrochlorothiazide in subjects with hypertension and a high risk of cardiovascular events, and showed fewer cardiovascular events in the CCB/ACE combination arm when compared to the ACE/Diuretic combination arm (39). These results are not in line with those of ALLHAT trial which found that ACE inhibitors, CCBs and alpha-blockers were not superior to thiazide diuretics for either BP control or improvement of cardiovascular or renal outcomes (40). Regardless of these conflicting results, the available evidence positions calcium channel blockers in line with ACE/ARBs and thiazides for treatment of hypertension in patients at high risk for cardiovascular events. CCB’s are well tolerated by most patients. Common side effects include headache, peripheral edema, and flushing (41).

 

Adrenergic Receptor Antagonist

 

The adrenergic receptor antagonists have been sub-classified into three categories: beta-blockers, alpha-blockers, and combined alpha and beta-blockers. Alpha-beta blockers like carvedilol and labetalol produce greater reductions in BP compared to pure beta blockers (34).

 

Beta-blockers have gained popularity due to mortality benefits in patients with heart failure and in patients who have sustained a myocardial infarction. Despite lack of robust evidence, beta- blockers are widely used for primary prevention of myocardial infarction as well. Use of beta-blockers can be associated with precipitation of bronchospasm, worsening peripheral arterial disease, sexual dysfunction, and worsening of glycemic control. Of particular concern is the decreased perception of hypoglycemia symptoms in patients with diabetes (42).

 

Beta-blockers are also known to alter insulin resistance and lipid metabolism – properties that are especially relevant in diabetic individuals. However, these effects vary across individual drugs and are more often seen with older non-vasodilatory beta-blockers such as atenolol, metoprolol and propranolol. For instance, a randomized control trial comparing metoprolol and carvedilol in patients with T2DM demonstrated that metoprolol was associated with worsened glycemic control compared to carvedilol at doses titrated to achieve comparable BP control (43). The same study also revealed that carvedilol had beneficial impacts on lipid profile with lowering of total cholesterol, triglycerides and non-HDL cholesterol. In contrast, metoprolol use was associated with increased need for lipid lowering therapy with statins (44). Similarly, labetalol and nebivolol, highly selective beta-1-blockers with nitric oxide dependent vasodilatory properties have been shown to improve insulin resistance (45). Unopposed activation of the alpha-adrenergic system has been proposed as a putative mechanism (46). Nebivolol also decreases cellular stiffness and stimulates endothelial cell growth causing improved endothelial function (47).

 

Mineralocorticoid Receptor Antagonists 

 

Steroidal mineralocorticoid receptor (MR) antagonists (spironolactone and eplerenone) and new non-steroidal antagonists such as finerenone are particularly efficacious in those with resistant hypertension, which is more common in persons with obesity and diabetes (7). They also lower mortality in patients with heart failure by blocking the deleterious effects of aldosterone on cardiac remodeling. Addition of finerenone to patients receiving ACE inhibitors or ARB reduced urinary albumin excretion compared to placebo. The FIDELIO-DKD trial also showed improved cardiovascular outcomes and reduced progression of kidney disease with finerenone (48).

 

Hyperkalemia is a common side effect of steroidal MR antagonists, and monitoring for hyperkalemia is of particular importance, as MR antagonists are often added to an ACE inhibitor or an ARB. This is less of a problem with the newer non-steroidal MR antagonists (49). Gynecomastia and menstrual irregularities are other potential adverse effects seen with spironolactone. Eplerenone is a more selective aldosterone antagonist and it seldom causes anti-androgenic effects. It is likely that the newer non-steroidal MR antagonist will negate many of these concerns, and they will likely be increasingly used for treatment of hypertension in patients with diabetes.

 

Direct Renin Inhibitors 

 

Aliskiren, a first in class direct renin inhibitor was approved by FDA in 2007. It is an effective antihypertensive agent and provides end-organ protection, but its exact place in the hypertension treatment algorithm remains uncertain. Aliskiren improves left ventricular hypertrophy, and shows synergism when used in combination with ARB. Its side effect profile is similar to ARBs and monitoring of potassium levels is recommended (34). The Aliskiren Trial in Type 2 Diabetes Using Cardiovascular and Renal Disease Endpoints (ALTITUDE) trial was a randomized control trial evaluating the efficacy of Aliskiren in combination with ACE inhibitors or ARBs in patients with T2DM. It was prematurely halted because of increased cardiovascular events and safety concerns. Additionally, there was more hyperkalemia and hypotension with the combination (50). It is possible that these adverse events were related to use of combination therapy analogous to the ONTARGET trial. At this time, Aliskiren should not be used in combination with ACE inhibitors or ARBs for management of hypertension in patients with T2DM. Aliskiren may be used for its antiproteinuric effect in patients who are intolerant of both ACE inhibitors and ARBs.

 

DIABETES MEDICATIONS WITH ANTIHYPERTENSIVE EFFECTS

 

Several anti-diabetic medications possess modest antihypertensive properties. These should be kept in mind especially in patients concurrently receiving antihypertensive drugs who may experience hypotensive symptoms if caution is not exercised. On the other hand, several of these drugs provide cardiovascular protection, likely in part from their antihypertensive effects that make them attractive options for patients at increased cardiovascular risk. Anti-diabetic medications with these properties include thiazolidinediones, dipeptidyl diphosphatase (DPP-4) inhibitors, glucagon-like peptide 1 (GLP-1) receptor agonists, and sodium glucose cotransporter 2 (SGLT 2) inhibitors. Of these classes, GLP-1 receptor agonists appear to exert the largest effect on blood pressure (51).

 

In a metanalysis of 16 randomized control trials comparing the GLP-1 agonists exenatide and liraglutide to placebo as well as other antihyperglycemic agents, BP reduction was seen. Against placebo, exenatide lowered systolic BP by approximately 6 mm Hg. Similarly, a mean reduction of about 5 mm Hg in systolic BP was seen with liraglutide versus placebo (52). A randomized control trial studying the hemodynamic effects of dulaglutide also showed a reduction in systolic BP regardless of baseline readings (53). The other classes of anti-hyperglycemic medications have shown reductions in systolic BP of less than 5 mm Hg (51).

 

Due to their sodium and volume lowering properties, the SGLT2 inhibitors were looked at early on for effects on BP. In phase 2 and 3 trials, canagliflozin and dapagliflozin showed modest reductions in BP, just under 4 mm Hg (54,55). A recent meta-analysis confirmed this finding across all other major SGLT2 inhibitors currently on the market with a mean reduction of 3.6/1.7 mm Hg in blood pressure as compared to placebo. This reduction is comparable to that seen with low dose hydrochlorothiazide (56). The exact mechanism of BP reduction is not completely understood but is postulated to be mediated by osmotic diuresis, natriuresis, and weight loss (57,58). Importantly, these agents have been shown to have CVD and renal disease reducing properties in patients with diabetes (54,59,60).

 

IMPACT OF COMORBIDITIES ON CHOICE OF ANTIHYPERTENSIVE REGIMEN

 

Despite advances in diagnosis and management, a significant proportion of diabetic individuals develop microvascular and macrovascular complications throughout their lifetime. Indeed, many patients present with advanced complications at diagnosis. These comorbidities must be considered when choosing an antihypertensive regimen because of ancillary benefits and potential for harm. Proteinuria and chronic kidney disease are most responsive to ACE inhibitors and ARBs and these agents are considered standard of care for such patients. On the other hand, beta-blockers have demonstrated benefit in the settings of established coronary artery disease and heart failure with reduced ejection fraction (HFrEF) but have no proven mortality benefit in their absence. Their use should therefore be restricted to the appropriate settings. Beta-blockers may exacerbate peripheral arterial disease due to reflex vasoconstriction and are best avoided in such patients. Beta-blockers should also be avoided in patients with a history of brittle diabetes and frequent hypoglycemia because of their ability to mask symptoms of hypoglycemia and thus contribute to hypoglycemia unawareness. Mineralocorticoid antagonists have shown proven benefits in HFrEF and should be included in antihypertensive regimens for diabetics with heart failure.

 

RESISTANT HYPERTENSION

 

Resistant hypertension is defined as BP greater than 140/90 mm Hg despite a therapeutic strategy that includes appropriate lifestyle modifications along with a diuretic and two other antihypertensive drugs from different classes, administered at optimal doses. It poses a special therapeutic challenge for endocrinologists. It is important to keep in mind that a number of other conditions need to be excluded before diagnosing resistant hypertension. Medication non-adherence must always be ruled out and barriers such as cost and side effects should be addressed. White coat hypertension can be remarkably resistant to therapy or alternatively be associated with intolerable side effects at home, leading to medication non-adherence and can be assessed by means of ABPM. Finally, secondary causes of hypertension should be looked for. The list of causes for secondary hypertension is extensive and includes such diverse disorders as renal artery stenosis, hyperaldosteronism, obstructive sleep apnea, and illicit drug use. Of special interest to the consulting endocrinologist are the various endocrine disorders that manifest with hypertension including primary hyperaldosteronism, pheochromocytomas and paragangliomas, Cushing’s syndrome, and hypo- and hyperthyroidism. Many of these disorders are characterized by distinct clinical presentations, and an exhaustive and expensive evaluation should be discouraged in the absence of supportive signs and symptoms. Obstructive sleep apnea deserves special mention because of its close association with diabetes and obesity and must always be considered in patients with resistant hypertension. Once diagnosed, secondary hypertension is often amenable to specific therapies with immediate improvement in BP.

 

After confirming a diagnosis of resistant hypertension and excluding possible secondary causes, pharmacological therapy with addition of mineralocorticoid receptor antagonists is typically the most effective intervention. These agents are effective in patients with T2DM when added to existing treatment with an ACE inhibitor or ARB, diuretic and calcium channel blocker. Mineralocorticoid receptor antagonists also reduce proteinuria and have additional cardiovascular benefits as noted above. However, adding a mineralocorticoid receptor antagonist to a regimen that already includes an ACE inhibitor or ARB increases the risk for hyperkalemia. Therefore, these patients need regular monitoring of serum creatinine and potassium.

 

SPECIAL CONSIDERATIONS IN TYPE 1 DIABETES

 

Patients with type 1 diabetes currently make up about 5% to 8% of the total diabetes population in the US (1). In contrast to patients with T2DM, patients with type 1 diabetes typically develop renal disease before developing hypertension. Longitudinal studies of type 1 diabetics consistently show development of proteinuria prior to onset of hypertension (13). However, once hypertension has developed, it accelerates the course of microvascular and macrovascular disease similar to patients with T2DM. Unfortunately, there is limited data in type 1 diabetics. A randomized trial has demonstrated that an ACE inhibitor protects against deterioration in renal function in insulin-dependent diabetic nephropathy and is significantly more effective than blood-pressure control alone (69). Therefore, guidelines for antihypertensive therapy in these patients are extrapolated from patients with T2DM, such as a preference for therapy with an ACE inhibitor or ARB. Furthermore, as tight glycemic control with insulin is the cornerstone of management of these patients, beta-blockers should be avoided because of their propensity to promote hypoglycemia and their ability to mask symptoms of hypoglycemia (61).

 

Perhaps the most distinctive aspect of hypertension in type 1 diabetes relates to the role of glycemic control in its prevention. Data from the Diabetes Control and Complications Trial (DCCT) and the Epidemiology of Diabetes Intervention and Complications (EDIC) trials showed that intensive therapy reduced incident hypertension by 24% over a 15 year follow up period (62). Interestingly the reduction in incident hypertension was not seen while the subjects were actually on intensive control but only appeared years later, suggesting that the connection between hyperglycemia and hypertension is not direct but rather is mediated through chronic complications of diabetes such as diabetic nephropathy.

 

COVID-19 AND ANTIHYPERTENSIVE THERAPY IN INDIVIDUALS WITH DIABETES

 

The ongoing novel SARS-CoV-2 coronavirus (COVID-19) pandemic has disproportionately affected individuals with multiple medical comorbidities. For instance, a large observational study from China showed that up to 23.7% of patients with severe infection had hypertension and 16.2% had diabetes compared to just 13.4% and 5.7% respectively, of patients with non-severe infection (63). The higher incidence of adverse outcomes seen with COVID-19 in patients with hypertension and diabetes is now believed to be related not only to the direct immunosuppressive effects of these comorbidities but also to common underlying socioeconomic themes such as lack of access to quality healthcare and healthy foods (64).

 

Further research on COVID-19 infection in this subset of patients led to questions on the role of ACE inhibitors and ARBs in its pathogenesis. Specifically, the observation that the novel coronavirus binds to human cells via the angiotensin converting enzyme 2 raised concerns that medications like ACE inhibitors and ARBs that increase levels of this enzyme might accelerate infection with the novel coronavirus. However, at this time there are no clinical data to support this hypothesis and the European Society of Cardiology Council on Hypertension, the American College of Cardiology (ACC)/ American Heart Association (AHA)/ Heart Failure Society of America (HFSA) and the American Society of Hypertension have all released policy statements strongly recommending that patients continue treatment with their usual antihypertensive regimen. Therefore, at this time, recognizing the multiple benefits obtained with these classes of medications in patients with diabetes or hypertension, it is not advisable to discontinue therapy simply because of COVID-19 infection (65).

 

RECENT GUIDELINES

 

The high blood pressure clinical practice guidelines released by the ACC/AHA Task Force in 2017 redefined hypertension as a blood pressure greater than 130/80 mm Hg and eliminated the category of pre-hypertension altogether. By lowering the threshold for diagnosis, this new definition immediately reclassifies a large proportion of individuals with diabetes as hypertensive, and consequently raises the incidence and prevalence of hypertension in the diabetic population. These guidelines recommend that pharmacologic therapy be initiated in patients with diabetes who have a blood pressure of greater than 130/80 mm Hg as it is assumed that they have an increased risk of cardiovascular disease. In the general population it is recommended that the 10-year atherosclerotic cardiovascular disease (ASCVD) risk be calculated. Pharmacotherapy should be initiated in those with an ASCVD risk of greater than ten percent when the blood pressure is greater than 130/80 mm Hg while the remainder can be treated with lifestyle modification alone (66).

 

The position statement on cardiovascular risk management in diabetes released by the American Diabetes Association (ADA) in 2021 retains the traditional cut off of 140/90 mm Hg for diagnosis of hypertension among individuals with diabetes. Just like the ACC/AHA guidelines, the ADA guidelines incorporate the ASCVD risk calculator in their treatment algorithm. The ADA guidelines differ however, in that the score is used to determine the target blood pressure. Thus, individuals with diabetes who have a score below fifteen percent have a target blood pressure of less than 140/90 mm Hg while individuals with a score greater than fifteen percent should aim for less than 130/80 mm Hg if such a goal can be safely achieved. This approached is based on observations from the SPRINT and other trials that the absolute benefit from BP reduction correlated with absolute baseline cardiovascular risk. These guidelines also emphasize the importance of individualized treatment targets and considering patient preferences and provider judgement when setting blood pressure goals (67).

 

Both the ACC/AHA guidelines and the ADA guidelines recommend pharmacologic therapy with two drugs belonging to different classes in patients with stage 2 hypertension, defined as a blood pressure greater than 160/100 mm Hg. This recommendation is based on evidence from multiple trials showing that combination therapy is safe and more efficacious than monotherapy in achieving blood pressure control. Combination therapy also leads to faster lowering of blood pressure and accelerates achievement of target levels, minimizing target organ damage in patients with stage 2 hypertension. The ADA guidelines also support use of single pill fixed dose combinations to maximize patient adherence. However, it should be noted that single pill combinations are often difficult to titrate, leading to suboptimal dosing of one component because of intolerance to maximal dosing of the other. This is especially relevant for ACE-inhibitors, ARBs and beta-blockers that show dose dependent benefits and should always be up titrated to maximally tolerated doses.

 

SUMMARY

 

Adequate treatment of hypertension in patients with diabetes is critical for prevention of end-organ damage and limiting the massive socioeconomic burden imposed by these disorders.

However, despite an abundance of evidence supporting tight control of blood pressure in diabetic individuals, it is sobering to note that BP targets are not met in the majority. Indeed, a larger retrospective registry-based study showed that as recently as 2018 only 48% of adults with diabetes were able to achieve a blood pressure of less than 130/80 mm Hg (68). Barriers to achieving good control include poor access to quality healthcare, lack of awareness among patients and providers, and concerns about side effects of tight control especially among older and frail individuals.

 

Judicious selection of therapy and consideration of relevant side-effect profiles is paramount. The potential for both beneficial and detrimental drug interactions should be kept in mind and drug combinations should be chosen after due deliberation. ACE inhibitors and ARBs continue to enjoy a special place in the management of hypertension in patients with diabetes and remain the preferred agents in this population subgroup. Combined use of these agents, however, is not recommended due to poor renal outcomes and hyperkalemia. The ancillary antihypertensive effects of antidiabetic medications should also be considered when designing an optimal regimen.

 

Goal blood pressure in patients with diabetes remains a subject of active discussion. This is reflected in the divergent recommendations offered by major organizations as noted above. While the evidence for lowering of blood pressure to a target of 140/90 mm Hg is unequivocal, the benefits of further intensification of therapy are less clear and must be balanced against the risk of adverse events such as falls, electrolyte abnormalities, and renal failure. Moreover, BP measurement protocols applied in trial settings can yield lower readings than comparable measurements in real world clinic settings, raising questions of whether such tight control is truly needed. A nuanced approach based on cardiovascular risk factors, comorbidities and patient preferences is encouraged.

 

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