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Evaluation and Treatment of Gender-Dysphoric/Gender Incongruent Adults

ABSTRACT

 

Gender dysphoria refers to the suffering due to an incongruence between one’s sex assigned at birth and one’s self-perceived gender. Primary care physicians often play an important role in diagnosis and initiation of treatment of gender dysphoria. However, gender dysphoria is preferentially diagnosed by a specialized psychologist or psychiatrist. This does not imply that gender dysphoria in itself is a mental disorder, but co-morbidity needing attention, is frequently present. The prevalence of transgender people who receive medical treatment has steeply increased in the last decades. The current prevalence is estimated at 1:2,800 for transwomen (male sex assigned at birth, female gender identity) and 1:5,200 for transmen (female sex assigned at birth, male gender identity). Treatment of transgender people often includes gender-affirming hormonal therapy and/or surgery, and is optimally provided by a multidisciplinary team consisting of psychologists, endocrinologists, plastic surgeons, gynecologists, urologists, otorhinolaryngologists, and/or dermatologists. Medical treatment usually improves the quality of life of transgender people, but might also have side-effects such as increased risk for cardiovascular disease or hormone-sensitive tumors. There is still little known about the optimal therapy (for specific transgender subpopulations) and its long-term side-effects. Nowadays, guidelines are mainly based on clinical experience instead of evidence. However, transgender medicine is a growing field and the increasing number of good quality studies are helping to improve care of transgender people. In this contribution we mainly focus on what is known about the side-effects of hormonal therapy. In addition, we provide information about surgical and fertility preservation options for transgender people. We conclude this contribution with remarks about special conditions such as older age and unsupervised hormone use.

 

INTRODUCTION

 

Gender identity is the personal sense of one's own gender. A significant incongruence between one’s physical phenotype and one’s gender identity is defined as gender dysphoria. While care for transgender people has long been based on a binary understanding of gender (male versus female), the existence of non-binary or gender queer genders is getting increasing attention. Non-binary or gender queer peopleidentify with a gender that is neither exclusively male nor female, but is composed of both, oscillates between genders, is situated beyond the binary, or rejects the binary (1). Gender dysphoria is usually diagnosed by a specialized psychologist, which does not imply that it is a mental disorder per se. People who have a male sex assigned at birth but have developed a female gender identity are called transwomen and people who have a female sex assigned at birth but experience a male gender identity are called transmen.The prevalence of transgender people who seek medical treatment has dramatically increased in the last years, and a recent Dutch study estimated a prevalence of 1:2,800 for transwomen and 1:5,200 for transmen (2). While the etiology is complex and probably multifactorial, the most widely believed hypothesis is that transgender people have experienced a sex-atypical differentiation of the brain during fetal development (3,4).

 

To many physicians, transgender medicine is a novel area of medicine. Most physicians need intensive interaction with a transgender individual to empathize the suffering, and to arrive at the insight that hormonal and surgical treatment might alleviate gender dysphoria-related distress. It is usually assumed that medical interventions can only be justified when objective, identifiable pathological processes account for human suffering. The role that biological factors play in the development of gender identity is still not solidly established but an increasingly number of reports provide evidence that androgens play a role in the development of gender identity. The association is not absolute, and the information is not robust enough to draw solid conclusions. Nevertheless, it has been demonstrated that gender-affirming hormonal therapy and/or surgery, which is only recommended in people with well-documented gender dysphoria (5), contributes to an improved well-being (6). Medical treatment is preferably provided by a multidisciplinary team consisting of psychologists, endocrinologists, plastic surgeons, gynecologists, urologists, otorhinolaryngologists, and/or dermatologists.

 

When providing transgender care, physicians are confronted with a lack of systematically and accurately collected data. Transgender people are not a well-defined group, and show considerable heterogeneity, for instance in age of onset, in the degree of suffering, and in the wish for treatment. The attention that transgender people receive in medicine is certainly improving, however transgender people still experience health disparities. There is a dearth of research and evidence-based guidelines for treatment, and the specific health needs for transgender people are understudied. It is problematic to include sufficient numbers of participants to perform statistically robust studies with regard to the best transgender hormonal therapy, and the long-term side-effects of treatment. Nowadays, physicians with extensive clinical experience have drafted guidelines based primarily on empiric observation. Fortunately, the number of good quality studies is currently increasing.

 

HORMONAL THERAPY

 

In transwomen, hormonal therapy usually consists of estrogens ± antiandrogens (e.g. cyproterone acetate, spironolactone, or GnRH analogues) (9,10). Estrogen in the form of estradiol is recommended to minimalize the risk of venous thrombosis and cardiovascular disease. Since progestogens, other than progestogenic antiandrogens, have not been proven to have an additional effect on feminization in transwomen, they are usually not recommended (5,9,10), although they might have beneficial effects in cisgender women (11). In transmen, hormonal therapy usually consists of testosterone only (10). See Table 1 for an overview of the hormonal options for transwomen, and Table 2 for the hormonal options for transmen. For details on the hormonal regimens for transwomen and transmen the Endocrine Society Clinical Practice Guideline is an excellent source of information (10).

 

Table 1. Hormonal Regimens Regularly Used in Transwomen

Estrogens

Recommended Dose

Oral estradiol

2-6 mg daily

Intramuscular estradiol valerate/cypionate

10-20 mg/1-2 weeks

Estradiol patch*

50-100 mcg/24 hours

Estradiol gel*

0.75 mg-2.25 mcg daily

Antiandrogens**

 

Cyproterone acetate

10-50 mg daily

Spironolactone

50-200 mg daily

GnRH analogue

Varies per preparation

* Transdermal preparations are recommended in transwomen
aged ≥40 years or in those with an increased cardiovascular risk
** Antiandrogens are discontinued after orchiectomy

 

Table 2. Hormonal Regimens Regularly Used in Transmen

Testosterone

Recommended dose

Intramuscular enanthate or cypionate*

Intramuscular mixed esters (Sustanon®) Intramuscular undecanoate   

100–200 mg/2 weeks

250 mg/2-3 weeks
1000 mg/12 weeks OR  
 750 mg / 8 weeks**

Testosterone gel

 25-100 mg/day

Testosterone patch

 2 or 4 mg/day

Preparation availability will differ between countries

* Due to associated serum testosterone peaks, these injections may not the best option for transmen who develop polycythemia (12)
** The above are the conventional dosages for cismen. To virilize transmen 50-75% of these dosages are usually sufficient

 

Effects of Hormonal Therapy

 

TRANSWOMEN

 

In transwomen, hormonaltherapy induces feminization such as breast growth, skin softness, fat redistribution, and a decrease of body hair (9,10). Figure 1 shows the expected effects of hormonal therapy in transwomen, and the period over which achievement of maximum effects could be expected (9,10,13). Most of the hormone-induced effects start within the first months of treatment. It is important to realize that transwomen might not achieve the desired breast size; one year of hormonal therapy in transwomen usually results in less than an AAA cup size (13). As a result, many transwomen choose for breast augmentation surgery (14). It is also important to note that for complete permanent removal of (facial) hair, additional laser skin treatments are required. Furthermore, hormonal therapy does not induce voice changes. Therefore, transwomen who desire raising of their voice pitch may benefit from referral to a speech therapist (10).

Figure 1. Hormone Effects and the Term of Maximum Expected Effects in Transwomen

TRANSMEN

 

In transmen, hormonal therapy induces masculinization such as an increase in facial and body hair, an increase in muscle mass and strength, a masculinized voice, and a cessation of the menstruation. Figure 2 shows the expected effects of hormonal therapy in transmen, and the term that maximum effects could be expected (10). Most of the hormone-induced effects start within the first months of treatment. In most cases the menses cease during testosterone therapy. However, some transmen who experience persistent vaginal bleeding need additional therapy such as the progestin lynesterol or GnRH analogues, which suppress gonadotropin secretion.

Figure 2. Hormone Effects and Term of Maximum Expected Effects in Transmen

Hormonal Effects on Bone Health

 

Before puberty, the size and volume of the skeleton are similar in the two sexes. But upon the rise of androgens during puberty, a higher peak bone mass is attained in boys than in girls. Bone mass accrual, bone growth and maintenance of skeletal integrity in adulthood are critically determined by sex hormone production. In both sexes, hypogonadism leads to loss of bone. In men a significant role of estrogens in bone metabolism has been demonstrated. It is of note that testosterone is partially aromatized to estradiol, and it is well established that estradiol also plays a pivotal role in the bone health of men (15,16). These mechanisms underscore that hormonal therapy in transgender people will affect bone metabolism.

 

TRANSWOMEN

 

Unexpectedly, transwomenhave a lower bone mineral density than controls before starting with hormonal therapy. This might be explained by a less active lifestyle and/or a lower vitamin D status (17,18). As remarked above, in both sexes’ estrogens are important in the maintenance of bone health in adulthood. Initial studies examining the effect of long‐term hormonal therapy on bone mineral density showed contradictory results. However, most of these studies were small cross-sectional studies in which a baseline difference in bone mineral density was not taken into account (19–21). The most recent cohort study in transwomen showed that hormonal therapy does not have negative effects on bone mineral density, and that the lower bone mineral density in transwomen found in previous studies is solely based on a baseline lower bone mineral density (18). Therefore, it would be worthwhile to give lifestyle advice regarding physical exercise, adequate vitamin D status, and calcium intake to transwomen.

 

In postmenopausal women, bone mineral density depends on estrogens derived from aromatization of ovarian androgen production (22). Many transwomen have undergone orchiectomy in the process of their transition. Their status could probably be compared with a surgically-induced menopause in a ciswoman. Women with a surgically-induced menopause experience rapid bone loss during the first five years after oophorectomy. Based on this information, it has become clear that complete discontinuation of hormonal therapy in transwomen above the age of 50 leads to a profound loss of bone strength. Therefore, it is advisable to not discontinue estrogen therapy in older transwomen (23).

 

TRANSMEN

 

In contrast to transwomen, transmen show no lower bone mineral density before starting hormonal therapy (24,25). Furthermore, no negative effects of testosterone therapy on bone mineral density have been found (18,20,24).

 

Hormonal Effects on Cardiovascular Health

 

Cardiovascular disease is a prominent cause of morbidity and mortality in both women and men. Sex is known to affect one’s risk for cardiovascular disease. Men have a higher (age-adjusted) risk of strokes and acute coronary events than women (26–28). Strokes are 33% more incident in men than in women (28), and acute coronary events 172% (29). In addition, during reproductive age, women have a 100% higher risk of venous thromboembolic events than men (30,31). These data suggest that sex hormones play a role in the occurrence of cardiovascular events. Based on this information, it is surprising that recent studies found that estrogen therapy (without progestogen) increases the risk for developing strokes in menopausal women (32). There is also evidence that suggests a relationship between testosterone therapy and an increased risk of cardiovascular events (33–35). If exogenous sex hormones indeed have impact on the cardiovascular system, this might have consequences for transgender people receiving hormonal therapy. At the Amsterdam University Medical Center, the Netherlands, we have analyzed the development of cardiovascular disease in the population of the Gender Clinic. The Gender Clinic started in 1972 and there is now a large cohort of transgender people being followed-up including a growing number of older transgender people. The findings are summarized below.

 

TRANSWOMEN

 

Upon analysis of our transgender population in 1989 and in 1997 (36,37), cardiovascular disease, other than venous thromboembolism, was not increased in transwomen compared to cismen. However, the most recent evidence from 2011 and 2018 shows that transwomen receiving hormonal therapy have an increased risk for strokes and venous thromboembolism (but not acute coronary events) compared to cismen (38,39). The current estimated incidence rate for strokes in transwomen on hormonal therapy is 127 per 100,000 person-years, which is 80% higher than in cismen. The current estimated incidence rate for venous thromboembolic events is 320 per 100,000 person-years, which is 355% higher than in cismen (38). While the increased cardiovascular risk in transwomen was initially attributed to the usage of ethinylestradiol, recent studies found that transwomen who use other types of estrogens also have an increased risk for strokes and venous thromboembolism (38–40). The hypercoagulable effect of hormonal therapy (41)may be one of the mediators of the increased cardiovascular risk in transwomen.

 

Possibly, venous and arterial cardiovascular side-effects become more prominent past the age of 40-50 years, and in people with cardiovascular risk factors. While strong evidence is currently lacking, transdermal estradiol might be preferred over oral estrogens in these transwomen (9,42,43). In addition, one should be aware that progestogenic antiandrogens (e.g. cyproterone acetate) may further increase one’s risk for venous thromboembolism (44), and should therefore be continued no longer than necessary. Modifiable cardiovascular risk factors such as lipid concentrations, glucose concentrations, and blood pressure should be regularly monitored and treated in accordance with guidelines for ciswomen.

 

TRANSMEN

 

As in transwomen, first analyses from our center did not show an increased risk of cardiovascular disease in transmen using testosterone (36,37,45). However, the most recent analysis shows an increased risk for acute coronary events in transmen receiving testosterone, with a current estimated incidence rate of 100 per 100,000 person-years, which is 269% higher than the rate in ciswomen (38). The increased risk of acute coronary events in transmen receiving testosterone may be (partly) explained by the testosterone-induced combination of increases in hematocrit, thromboxane, triglycerides, and low-density lipoprotein cholesterol, and a decrease in plasma high-density lipoprotein cholesterol concentrations (35,46,47). Although, the design of the study made it impossible to draw any conclusions about a causal relationship we recommend to regularly monitor cardiovascular risk factors in transmen on testosterone therapy.

 

Hormonal Effects on Tumor Risk

 

Malignant neoplasms are the second leading cause of death worldwide (48). The risk for certain types of tumors differs between men and women. While this is obvious for neoplasms that develop in sex-specific organs such as the ovaries or the prostate, it is also the case for other types of tumors such as those of the meninges (49)and thyroid gland (50). Some of these differences are attributed to the exposure of sex hormones. Combined hormonal therapy in postmenopausal women has been found to increase the risk of breast cancer and death from lung cancer (51). In women with polycystic ovary syndrome a higher risk for endometrial cancer has been described, which is probably explained by the prolonged endometrial exposure to unopposed estrogen that results from anovulation (52). Testosterone therapy in hypogonadal men is not clearly associated with an increased cancer risk, but breast cancer risk in prostate cancer patients who receive estrogen therapy seems 3.91 times higher than in prostate cancer patients not receiving estrogen therapy (53). If exogenous sex hormones indeed are able to induce cell proliferation, this might have consequences for transgender people receiving hormonal therapy. It is good to keep in mind that transwomen and transmen remain susceptible to cancers of reproductive organs that are no longer in alignment with their gender. For example, postsurgical transwomen, and attending physicians, might not recognize their persisting risk of prostate cancer (the prostate is not removed during vaginoplasty). In addition, transmen who have not undergone removal of the uterus still have risk for cervical cancer. It is also important to realize that transgender people may opt out of cancer screening and examinations because of emotional or physical distress associated with the discordance between their experienced gender and their birth assigned sex.

 

To date, large-scale studies investigating neoplasms in transgender people are scarce and the literature mainly consists of case reports. One of the first reviews was presented in 2008 (54), and an extensive, high quality review appeared in 2017 (55). A cautious comparison of the two reports helps us to provide insight into the neoplasm-related morbidity and mortality in transgender people.

 

TRANSWOMEN

 

Breast Cancer

 

Estrogen in combination with antiandrogen therapy in transwomen stimulate the development of breast lobules, ducts, and acini which are histologically identical to those of ciswomen (56). While for a long time it was believed that the risk of breast cancer in transwomen receiving hormonal therapy was not higher than those of men (57,58), most recent evidence show that transwomen receiving hormonal therapy do have a 46-fold higher risk for breast cancer compared to men (59). As became clear in the Women’s Health Initiative study, addition of progestin to estrogen leads to an increase of the risk of breast cancer in women (60). Although evidence regarding breast cancer and the usage of the progestogenic cyproterone acetate is lacking, the above described data suggest that cyproterone acetate should be continued no longer than necessary. In addition, based on the most recent study that shows a much higher risk of breast cancer in transwomen compared to men, it is reasonable to recommend transwomen on hormonal therapy to participate in population-based breast cancer screening programs (9).

 

Prostate Cancer

While in the past, estrogens have been used to treat prostate cancer, estrogen and its related compounds have also been suggested as potential causative agents (61). Current literature, suggests that prostate cancer is very rare among transwomen. The few cases that have been reported in transwomen were in those who had not been screened for prostate cancer before starting hormonal therapy. Consequently, it remained unclear whether the prostate cancer was already present before hormonal therapy had been initiated (62). While prostate cancer has been rarely reported, underdiagnosis is possible due to lack of close prostate monitoring. Based on available evidence it does not seem necessary to screen transwomen in a different way to cismen, for which population-based screening is not recommended. But similarly, a transwoman with a first-degree male relative with prostate cancer should be made aware of her increased risk and prostate cancer [PSA] testing should be discussed to allow informed decision making. However, when interpreting PSA values in this context, it has to be kept in mind that suppression of testosterone by antiandrogens or due to gonadectomy lowers PSA values. A cross-sectional study of Wierckx et al. (63)found median PSA levels of 0.003 ng/mL with an interquartile range of 0.03 to 0.09 in a group of 50 postoperative transwomen using hormonal therapy on an average of 10 years. Therefore a serum level of PSA >1.0 ng/mL may already be a reason for suspicion in transwomen (64).

 

Prolactinoma

Serum prolactin concentrations usually rise slightly in response to estrogen administration and more so by cyproterone acetate (65,66). Based on case reports, it was initially believed that prolactin concentrations in transwomen had to be regularly monitored because of their increased risk for prolactinomas. Surprisingly, a very recent cohort study suggests that the occurrence of prolactinomas in transwomen using hormonal therapy is not higher than that in ciswomen, and that regular prolactin checks are not necessary (67). However, cyproterone acetate should be continued no longer than necessary.

 

Meningioma

Several meningiomas have been reported in transwomen. The current estimated incidence rate of this type of tumor is 33 per 100,000 person-years. This incidence rate is 4 times higher than the incidence rate in ciswomen and 12 times higher than the incidence rate in cismen (67,68). It has been suggested that the occurrence of meningiomas in transwomen is mainly related to cyproterone acetate usage as progesterone receptors are abundantly expressed in human meningiomas (67). Since the occurrence of meningiomas is still rare in transwomen, regular screening for this type of tumor seems not necessary. It is recommended to continue cyproterone acetate no longer than necessary.

 

Other Types of Cancer

As sexually transmitted infections may be more prevalent in transwomen, tumors related to sexually transmitted infections, such as Kaposi sarcoma or anal cancer, may also occur more often. Indeed, disproportionately high incidences of these types of tumors have been found in the transgender population (55,69). Some case reports have been published on cancer in surgically constructed organs like the neo-vagina in transwomen (70,71). While the incidence of these types of tumors seem to be very low it is important to be aware of this possibility.

 

TRANSMEN

 

Breast Cancer

Cases of breast cancer have been reported in transmen before mastectomy (59,72,73). It is important to know that because of cosmetic reasons not all glandular tissue is removed during a mastectomy in transmen. Indeed, several cases of breast cancer have been reported in transmen who already had received mastectomy (59,73–75). The incidence of breast cancer in transmen who have received mastectomy seems higher than in cismen, but much lower than in ciswomen (59). While physicians and transmen have to be aware of their risk of breast carcinoma after mastectomy, it seems unnecessary for transmen to participate in the screening programs for women. However, for transmen with a genetic predisposition for breast cancer, more radical forms of mastectomy could be considered.

 

Endometrial Cancer

Not all transmen choose to remove their uterus. Menstruation usually ceases in transmen receiving testosterone therapy. Testosterone can be converted into estradiol, which may induce proliferation of the endometrium. These mechanisms may induce a higher risk of endometrial cancer in transmen. Women with polycystic ovary syndrome who do not menstruate and suffer from hyperestrogenism, have a thicker endometrium and a higher risk of endometrial cancer (76). It is also possible that the risk for endometrial cancer in transmen using testosterone is lower due to complete atrophy of the endometrium (55). There is currently only 1 case of endometrial cancer reported in a transman using testosterone (77). But it is important to know that, until recently, many countries required removal of female sex organs before transmen could change their sex on the birth certificate. Therefore, long-term follow-up data about testosterone receiving transmen with a uterus are lacking. This makes it impossible to draw hard conclusions. Nevertheless, in transmen with non-cyclic vaginal blood loss, we recommend to perform a vaginal ultrasound.

 

Cervical Cancer

Transmen in whom the uterus has not been removed have a risk of cervical carcinoma. Human papilloma virus is the most important risk factor for developing cervix carcinoma. Studies in ciswomen show that testosterone may also be a risk factor (78). To date, only 2 cases of cervical carcinoma in transmen have been described (77,79). Again, it is important to keep in mind that, until recently, many countries required removal of female sex organs before a transman could change the sex on the birth certificate, which makes the data available limited. As there is no evidence for a decreased risk of cervical carcinoma in transmen, it seems reasonable for transmen with a uterus to participate in screening/HPV vaccination programs for ciswomen. It is important to inform transmen about the need for this screening as they probably do not receive invitations from screening organizations.

 

Ovarian Cancer

Endometrial epidermal growth factor receptor, which is stimulated by testosterone, is commonly found in ovarian cancer cells, and its expression has been associated with poor prognosis (80). However, whether testosterone therapy increases the risk for ovarian cancer in transmen has not been elucidated yet. To date, 3 cases of ovarian cancer have been reported in transmen using testosterone (81,82). Future studies need to provide more evidence about the risk of gynecological cancers in transmen. Until then, screening for ovarian cancer seems unnecessary.

 

EVALUATION OF TRANSGENDER PEOPLE RECEIVING HORMONAL THERAPY

 

Since hormonal therapy is associated with several side-effects it is recommended that medical conditions which can be exacerbated by hormonal therapy are addressed before the start of therapy. During hormonal therapy it is advisable to regularly measure hormone concentrations and maintain them in the normal physiological range. For transwomen estradiol levels between 100 to 200 pg/mL (367 pmol/L to 734 pmol/L) and testosterone levels of <50 ng/dL (<2 nmol/L) are recommended. For transmen, the testosterone level is dependent on the specific assay, but is typically 320 to 1000 ng/dL (11 nmol/L to 35 nmol/L) (10). However, the peak testosterone level after a short acting testosterone injection is often (much) higher than 1200 ng/dL (42 nmol/L). It is also recommended to regularly measure glucose concentrations, lipid panel, and blood pressure during hormonal therapy in both transwomen and transmen, hematocrit in transmen, and electrolytes in transwomen receiving spironolactone (in the first year at baseline and 3 and 12 months, hereafter every 6 months to 2 years).

 

SURGERY

 

Many transgender people choose to have surgery in addition to hormonal therapy. There are several surgical options. While some types of surgery affect fertility, such as vaginoplasty in transwomen or oophorectomy/hysterectomy in transmen, others do not, such as breast surgery in both transwomen and transmen. For surgery which affects fertility, most guidelines recommend the usage of gender-affirming hormones for at least 12 months, which is based on expert consensus that 12 continuous months of living in the experienced gender role is needed for transgender individuals to experience and socially adjust in the desired gender role (5,10).

 

Surgical Options in Transwomen

 

ORCHIECTOMY

Orchiectomy can be performed independently or as part of a vaginoplasty. Orchiectomy is a relatively low-risk procedure (83). After orchiectomy the antiandrogens are no longer necessary and can discontinued.

 

VAGINOPLASTY

 

During a vaginoplasty an orchiectomy (if not previously performed) is performed, in combination with an amputation of the penis, the creation of a neovaginal cavity with lining, the reconstruction of a urethral meatus, and the creation of labia and clitoris. The most frequent procedure is penile inversion vaginoplasty during which the penile skin is used a pedicled flap for the vaginal lining. Since the amount of penile skin is limited, the penile skin flap is often combined with a scrotal skin flap. To prevent hair in the posterior lining of the vagina, hair removal therapy is desirable before penile inversion vaginoplasty. An alternative for penile inversion vaginoplasty is intestinal vaginoplasty, which is a good option in cases in which insufficient skin is available (for example in transwomen who have received puberty blockers during adolescence (83)).

 

BREAST AUGMENTATION

 

Many transwomen choose for breast augmentation since they are not satisfied with their hormone-induced breast growth. The breast augmentation procedure does not differ from an breast augmentation in ciswomen (83).

 

FACIAL FEMINIZATION SURGERY

 

Facial feminization surgery includes a wide range of craniomaxillofacial surgical procedures which are designed to create more feminine facial features. Overall, facial feminization surgery seems a highly efficacious and beneficial procedure for transwomen (84).

 

VOCAL CORD SURGERY

 

Surgically shortening of the vibrating vocal cords or increasing the vocal cord tension can raise the voice pitch in cases that voice therapy does not achieve the desired effect (83).

 

CHONDROLARYNGOPLASTY

 

During chondrolaryngoplasty (tracheal shaving) the prominence of the thyroid cartilage is reduced. Chondroplasty can be performed alone or in combination with vocal cord surgery. Reduction of the Adam’s apple can have positive effects on the psychological well-being of transwomen (85).

 

Surgical Options in Transmen

 

SUBCUTANEOUS MASTECTOMY

 

Most transmen choose mastectomy. A mastectomy in transmen which is performed for aesthetic reasons differs from a mastectomy in ciswomen which is performed because of breast cancer. During the mastectomy in transmen not all glandular tissue is removed. In addition, (more) attention has to be paid to reduction and adequate positioning of the nipple areola complex, destruction of the inframammary fold, and minimization of scars (83).

 

HYSTERECTOMY

 

Many transmen desire uterus extirpation with or without a salpingo-oophorectomy for gender affirmation, pelvic pain, persistent vaginal blood loss, or cancer-risk reduction. It is preferred to use a vaginal approach instead of a transabdominal approach although this could be technically challenging as many transmen have not experienced penetrative intercourse and are on testosterone therapy (86). Transmen who also want a colpectomy can also choose to have a robot-assisted laparoscopic hysterectomy (with salpingo-oophorectomy) in combination with a robot-assisted laparoscopic colpectomy (87).

 

COLPECTOMY

 

Transmen may choose for a colpectomy (removal of the vaginal epithelium) for several reasons, such as unwanted vaginal discharge in general or as result of sexual arousal, or the wish for phalloplasty with urethral extension. There are two options for colpectomy, the vaginal approach and the robot-assisted laparoscopic colpectomy in combination with hysterectomy and salpingo-oophorectomy. The robot-assisted procedure seems to be safer than the vaginal approach (87). 

 

PHALLOPLASTY

 

Phalloplasty is the surgical creation of a full-size penis. It is a difficult surgical procedure with high rates of complications such as urethral stenosis. An ideal phallus has sufficient length for vaginal penetration, has sensibility, and, if desired, enables the transman to urinate in standing position. Multiple flaps have been used to create the phallus, but for penile reconstruction the free radial forearm flap remains the gold standard (83,88).

 

METADOIDIOPLASTY

 

During a metoidioplasty a microphallus is created by using the testosterone-induced hypertrophied clitoris. While a metadoidioplasty gives a lower risk for complications than a phalloplasty, it cannot be guaranteed that voiding in the standing position is possible. In addition, vaginal penetration will not be possible (83,88).

 

FERTILITY PRESERVATION

 

It is estimated that about 47% of transgender individuals would like to have a child to whom they are genetically related (89). Gender affirmation therapy, both hormonal and surgical, is an indication for fertility preservation since hormonal therapy adversely affects fertility, and surgery may include gonadal removal. While the adverse effects of hormonal therapy may be reversible when the therapy is ceased, it is important to discuss fertility preservation options with a transgender individual before the start of hormonal therapy (90). In transwomen the percentage that would have frozen sperm if this option was offered, varied from 13% in asexual or heterosexual (being attracted to men) transwomen to 56% in homosexual (being attracted to women) and bisexual transwomen (91). It is estimated that about 37% (92)of the transmen wish to have their gametes preserved before any gender affirming therapy. For transgender adolescents it is important to involve parents in the fertility preservation counseling as they play an important role in exploring options for their children and usually have to give their consent to interventions. A recent study found that parents overall did not emphasize the importance of their child having children to whom they are genetically related but they agreed that that fertility preservation counseling is relevant (93).

 

Transwomen

 

Semen cryopreservation using specimens obtained from masturbation or penile vibratory stimulation is technically the most easy, reliable and inexpensive method for fertility preservation in transwomen. However, for some transwomen this option may not be possible because of the psychologically distress induced by this procedure, or the difficulties in erection and ejaculation. Alternatives are electro-stimulation or surgical sperm retrieval, or in case of azoospermia, testicular sperm extraction (90). The obtained sperm can be used to fertilize the partner of the transwoman if this partner is female. In case of a male partner a gestational surrogate is needed for fertilization.

 

Transmen

 

For transmen fertility options are embryo cryopreservation, oocyte cryopreservation, and ovarian tissue cryopreservation. As long as the ovaries and uterus are in situ, it is also possible for a transgender man to become pregnant spontaneously. Since testosterone therapy may be dangerous for fetal development it is important that testosterone therapy be discontinued before the transman becomes pregnant. In contrast to the other options, ovarian tissue cryopreservation is more experimental and not widely available. For embryo creation or to fertilize a preserved oocyte, sperm from an intimate partner or an anonymous donor, is needed. When a transgender man does not want to carry the child, a gestational surrogate is also needed. However, surrogacy for transgender individuals is still not widely available due to ethical and legal issues. Furthermore it is important to note that all fertility options in transmen are sooner or later accompanied with controlled ovarian hyperstimulation, which could be very distressing for a transman (90).

 

SPECIAL TOPICS

 

Hormonal Therapy in Older Transgender People

 

Some transgender people start the transition to their experienced gender at an older age (often after a long time of struggling), even past the age of 50 or 60 years (23,94). The majority of these people are currently transwomen (94), but the number of transgender individuals, and especially transmen is currently rapidly growing, possibly due to a greater tolerance and acceptability in society (2). There is no evidence that the manifestations of biological effects of sex hormones will be less in the elderly than in younger people (13,95,96). Age itself should not be regarded as a contraindication to start with hormonal therapy, but the risks of side-effects may be higher at an older age (97).

 

Unsupervised Use of Hormonal Therapy

 

Ideally, the indication for hormonal therapy is the result of psychological assessment that concludes that medical treatment will bring relief to an individual suffering from gender dysphoria (98). However, it is not uncommon that transgender people self-medicate. The use of health care facilities specialized in gender care may be unaffordable, difficult to assess due to long waiting lists, or people are unwilling to undergo psychodiagnostic assessment of their gender problems. Hormones are relatively easy to obtain, and peer groups and the internet provide (sometimes misguided) information on their use (99–101). Frequently, contraceptive pills containing ethinylestradiol with its associated risks (45,102)are used in high doses. Our knowledge about self-medication in the transgender population is very limited, but a study has indicated increased side-effects with illicit use of hormones (101). Another study found 23% of applicants for treatment had used sex hormones already. Remarkably, 32% were transwomen and 6% transmen. The individuals who had used self-prescribed hormones had much less knowledge about appropriate use and potential side-effects (103). Physicians should be aware of illicit hormone use and intervene when necessary.

 

CONCLUSIONS

 

Transgender care is a challenging, multidisciplinary, and developing field in medicine. The transgender population is rapidly growing and the existence of non-binary or gender queer genders gets increasingly more attention.Before the start of any type of therapy, the physician needs to discuss the pros and cons of the several treatment options so that the transgender individual can make a well-considered decision. Due to the increase of high-quality studies, hormonal and surgical therapy will probably be further optimized in the (near) future. Therefore, it is important for physicians who provide transgender care to stay up-to-date with the latest literature. In addition, as the waiting lists may be growing due to the rapidly the increasing number of new applications to gender clinics, physicians should be aware of a growing number of transgender individuals who use self-prescribed hormones.

 

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Lipid and Lipoprotein Metabolism in Liver Disease

ABSTRACT

 

The liver plays a central role in lipid metabolism, serving as the center for lipoprotein uptake, formation, and export to the circulation. Alterations in hepatic lipid metabolism can contribute to the development of chronic liver disease, such as nonalcoholic fatty liver disease (NAFLD) and add to the progression of other chronic liver disease, as occurs in hepatitis C. Moreover, chronic liver disease can impact hepatic lipid metabolism leading to alterations in circulating lipid levels contributing to dyslipidemia. This chapter discusses the interplay between lipid metabolism and chronic liver diseases focusing on NAFLD, alcoholic liver disease, hepatitis C, hepatitis B, cholestatic liver disease, and cirrhosis.

NONALCOHOLIC FATTY LIVER DISEASE

 

Case Presentation

 

A 60-year-old woman with a past medical history significant for hypertension, dyslipidemia and diabetes mellitus presents for management of newly diagnosed nonalcoholic steatohepatitis (NASH). She has a strong family history of coronary artery disease and a personal history of dyslipidemia characterized by a serum triglyceride level of 220 mg/dl, low-density lipoprotein (LDL) cholesterol of 180 mg/dl, high-density lipoprotein (HDL) cholesterol of 50 mg/dl and total cholesterol of 274 mg/dl. Based on these values, her primary physician has recommended she start a lipid lowering medication. However, with her history of liver disease she is uncertain whether she can safely take lipid-lowering medications.

 

Introduction

 

Nonalcoholic fatty liver disease (NAFLD) is the most common cause of chronic liver disease in the United States, affecting up to a third of adults (1,2). NASH is the progressive form of NAFLD and can lead to cirrhosis, hepatocellular carcinoma, and the need for liver transplantation. In addition to significant morbidity and mortality from end-stage liver disease, NAFLD confers an increased risk of cardiovascular disease (CVD) (3). CVD is the leading cause of mortality among individuals with NAFLD (4). The dyslipidemia of NAFLD may be one of several important and modifiable CVD risk factors.

 

Changes in Lipoprotein Metabolism and Clinical Manifestations

 

DEVELOPMENT OF STEATOSIS

 

NAFLD is characterized in part by steatosis, excess lipid deposition as lipid droplets within hepatocytes. These lipid droplets consist largely of triglycerides and are the result of an imbalance of hepatic lipid handling. Steatosis can occur when one or more of the following conditions is present; 1) excess delivery of free fatty acids (FFA) to the liver from adipose tissue, 2) increased de novo lipogenesis (DNL) within the liver, 3) decreased oxidation of fatty acids within hepatocytes and 4) impaired export of triglycerides from the liver in the form of very-low density lipoproteins (VLDL).

 

Excess FFA Delivery to the Liver

 

When excess adiposity and insulin resistance are present, FFA release from adipocytes is increased (5). Upon release FFA are then delivered via the circulation to the liver and may overwhelm the liver’s capacity to oxidize or export lipids, contributing to the development of steatosis. The fatty acid translocase FAT/CD36 mediates uptake of FFA into the liver and is upregulated in human and experimental NAFLD, which may contribute to steatosis (6,7,8).

 

Increased DNL

 

Hyperinsulinemia, often seen in the setting of obesity and the metabolic syndrome, can also contribute to DNL as the result of increased transcriptional activities of sterol regulatory element binding protein (SREBP) 1c- and peroxisome proliferator-activated receptor (PPAR)-γ (5,9,10). Increased circulating glucose levels also mediate lipogenesis via cholesterol regulatory element binding protein (ChREBP) activation (11). The increased synthesis of lipids within the liver can lead to accumulation within hepatocytes and can promote the development of steatosis.

 

Insufficient Export of Hepatic Triglycerides

 

Export of triglycerides from the liver requires the formation of VLDL and when VLDL formation is impaired steatosis can develop. VLDL are formed when triglycerides are complexed to apolipoprotein B100 (apoB100) via the action of microsomal triglyceride transfer protein (MTP). Steatosis can develop when any of the components of VLDL formation are missing or impaired. Genetic or pharmacologic alteration of MTP or the truncation or absence of ApoB100 can lead to steatosis (12-16). In addition, ApoB100 levels can be decreased by FFA accumulation. FFA accumulation within the liver can lead to chronic stress of the hepatocyte endoplasmic reticulum (ER). Increased ER stress results in increased ApoB100 degradation, decreasing the ability of the liver to export triglycerides and potentially worsen steatosis.

 

Complete VLDL assembly and secretion relies on several additional steps. Following the formation of nascent VLDL particles, further lipidation is needed to create mature VLDL particles. The process of this lipidation is not well understood but may rely on fusion with lipid droplets (17). Interruption of this process of lipid mobilization from lipids droplets to VLDL may also contribute to the development of steatosis (18). Recent genetic studies have shown a strong link between a polymorphism in the gene patatin-like phospholipase domain-containing 3 (PNPLA3) and NAFLD. This coding region polymorphism (I148M) reduces hepatic VLDL secretion, possibly by interfering with triglyceride mobilization and results in hepatic steatosis (19-21). However, conflicting data indicates there may be a compensatory increase in VLDL export in some NAFLD patients, although this increase is insufficient to counterbalance the elevated hepatic triglyceride content (22). The transmembrane 6 superfamily 2 (TM6SF2) E167K variant results in decreased hepatic VLDL secretion and is associated with NAFLD, fibrosis and cirrhosis in the setting of decreased LDL and triglyceride levels. This variant is associated with progressive liver disease but a decreased risk of cardiovascular disease (23,24). Familial hypobetalipoproteinemia (FHBL) is a condition characterized by diminished levels of functional ApoB100, resulting in impaired VLDL export and the development of hepatic steatosis. Magnetic resonance spectroscopy studies have shown liver fat content in individuals with FHBL to be five times greater than in controls (25,26). Progress to steatohepatitis, cirrhosis and hepatocellular carcinoma (HCC) has been noted in this population (27,28,29,30).

 

Hepatic Accumulation of Free Cholesterol

 

The degree of hepatic free cholesterol accumulation in NAFLD correlates with presence and severity of cytologic ballooning (31). Decreased expression of ATP-binding cassette (ABC) A1 and ABCG8 cholesterol efflux proteins, may disrupt transfer of cholesterol from hepatocytes, driving up hepatocyte cholesterol (32,33). There is conflicting evidence regarding changes to hepatic uptake of LDL in individuals with NAFLD, with some studies indicating upregulation of LDL receptors resulting in cholesterol overloading (34).

 

CHANGES IN LIPID METABOLISM

 

Dyslipidemia is frequent in adults with radiographic and biopsy-proven NAFLD and is characterized by hypertriglyceridemia, increased LDL particle concentrations, decreased LDL particle size, and decreased HDL levels (35). High ratios of total cholesterol or triglyceride to HDL-cholesterol are associated with NAFLD (36). In addition, non-HDL-cholesterol (non-HDL-C), a composite measure of apolipoprotein-B containing lipoproteins and an important marker of CVD risk, is elevated in individuals with NASH (19). NASH is also characterized by alterations in lipoprotein subfractions. Lipoprotein subfraction assays measure lipoprotein particle size, density and composition. NASH is characterized by large VLDL particle size and decreased LDL and HDL particle size (35). However, there is conflicting data on the association between NASH and VLD particle size (17,18). Furthermore, increased levels of LDL-III and IV particles, atherogenic forms of LDL, and reduced HDL2b levels, a cardioprotective lipoprotein, are observed in NASH (36,37). Fortunately, resolution of NASH is associated with increases in HDL, decreases in triglycerides, and increases in mean LDL particle diameter and the frequency of LDL phenotype A (39).

 

Insulin Resistance

 

Insulin resistance is a fundamental aspect of NAFLD and can result in many of the alterations in lipid metabolism and circulating lipid levels seen in NAFLD.

 

INSULIN RESISTANCE INCREASES CIRCULATING LDL, VLDL AND TRIGLYCERIDES LEVELS

 

Insulin resistance can increase circulating VLDL and triglyceride levels via several mechanisms. Insulin resistance leads to a loss of suppression of MTP transcription, which increases the efficiency of VLDL assembly (40,41). Insulin resistance also impacts VLDL levels by decreasing lipoprotein lipase (LPL) levels. LPL is an enzyme found on the endothelial cells within muscle and adipose tissue. LPL hydrolyzes triglycerides from circulating VLDL and facilitates triglyceride delivery to muscle and adipose tissues. In the setting of insulin resistance, LPL is downregulated decreasing the clearance of VLDL from the circulation and increasing circulating VLDL levels (42).

 

Insulin resistance can also act via ApoCIII levels to increase circulating VLDL and triglyceride levels. ApoCIII, a lipoprotein found on VLDL, inhibits LPL and can decrease VLDL clearance from the circulation (43). In the setting of insulin resistance, ApoCIII levels are increased, leading to decreased VLDL/triglyceride clearance and resulting in hypertriglyceridemia and increased VLDL levels. ApoCIII also appears to modulate plasma triglyceride levels via LPL-independent mechanisms. In patients with LPL deficiency due to familial chylomicronemia syndrome, administration of an ApoCIII mRNA inhibitor for 13 weeks reduced plasma triglycerides by 56-86% (44).

 

Insulin resistance also impacts LDL metabolism via upregulation of hepatic lipase and increased LDL receptor degradation. Hepatic lipase is an enzyme that remove triglycerides from intermediate-density lipoproteins (IDL) leading to the development of smaller, denser low-density lipoproteins. In NAFLD and insulin resistance, hepatic lipase levels are upregulated leading to increased levels of small, dense LDL (sdLDL) (45). Insulin can also increase circulating LDL levels via its effects on the LDL receptor. Insulin upregulates proprotein convertase subtilisin/kexin type 9 (PCSK9), a protein that can bind and degrade the LDL receptor (46). Upregulation of PCSK9 leads to decreased LDL receptor availability on hepatocytes and increased circulating LDL levels.

 

INSULIN RESISTANCE DECREASES CIRCULATING HDL LEVELS

 

Insulin resistance decreases circulating HDL levels by interfering with HDL particle assembly. HDL is formed within plasma at the surface of the hepatocyte and requires the interaction of ApoA-1 and ABCA1 (47). Nascent HDL particles are formed when ApoA-1, secreted by the liver or released from other lipoproteins, is lipidated by ABCA1 with phospholipids and free cholesterol. Insulin resistance hampers HDL formation by promoting the phosphorylation and degradation of ABCA1 and by reducing ABCA1 activity (48). In addition to hampering HDL production, insulin resistance may interfere with reverse cholesterol transport. Insulin resistance can result in the formation of particularly triglyceride-rich HDL particles via the action of cholesterol ester transfer protein (CETP) (49). Triglyceride-rich HDL are taken up more rapidly by the liver and may result in lower circulating HDL levels.

 

Management

 

Diet and exercise are the foundations of the management of both NAFLD and the dyslipidemia of NAFLD. Small studies have indicated that both a low carbohydrate diet as well as the Mediterranean diet may improve serum lipid levels and NAFLD (50-52). Further, adherence to a Mediterranean diet reduces the development of CVD (53). As CVD is a cause of considerable morbidity and mortality in NAFLD patients, adherence to a Mediterranean diet may have multiple benefits.

 

Routine aerobic exercise, defined as 30 minutes of moderate exercise most days of the week, can result in significant improvements in lipid levels and may improve hepatic lipid content (54,55). Individuals with NAFLD should be advised to participate in regular, aerobic exercise.

 

LIPID LOWERING MEDICATIONS

 

HMG-CoA Reductase Inhibitors

 

When diet and exercise are insufficient in individuals with NAFLD, HMG-CoA reductase inhibitors or “statins” are recommended. Statins play an important role in both the primary and secondary prevention of CVD and should be used in patients with NAFLD and dyslipidemia. Compared to placebo, statins have been shown, in a post-hoc analysis of the Greek Atorvastatin and Coronary Heart Disease Evaluation (GREACE) study, to significantly reduce cardiovascular events in individuals with NAFLD (56). Statins have also been shown to exert a protective effect on liver histology in patients with NAFLD/NASH, with dose-dependent reduction in steatosis, steatohepatitis and fibrosis stages F2-F4, although protection against steatohepatitis in the presence of the I148M PNPLA3 risk variant did not reach statistical significance (57).

 

It is important to note that while there remains a concern among physicians about statin hepatotoxicity, the incidence of statin-induced hepatotoxicity in the general population is extremely low and is not increased in individuals with NAFLD or NASH (58-60). Apprehension among physicians may partly account for the current under prescribing of statins in patients with NAFLD (61,62).

 

Omega-3 Fatty Acids

 

Omega-3 fatty acids can be used in patients with NAFLD for the treatment of isolated hypertriglyceridemia or when statins alone are insufficient to control triglyceride levels. Omega-3 fatty acids act to reduce hepatic VLDL secretion and lower serum triglyceride levels. Doses of up to 4 grams daily can decrease triglycerides by 25-35% (63). Omega-3 fatty acids may reduce radiographic steatosis and several randomized controlled trials (RCTs) of omega-3 fatty acids are ongoing to determine their impact on NASH histology (64-66).

 

Cholesterol Absorption Inhibitors

 

A further class of drugs which may hold promise are the cholesterol absorption inhibitors, of which ezetimibe has been most extensively studied. A recently conducted RCT involving 32 NAFLD patients found that ezetimibe use led to significant improvement in fibrosis stage and ballooning score (67). Of note, Loomba et al. reported no significant impact of ezetimibe on liver fat content, as assessed by magnetic resonance imaging proton density-fat fraction and liver biopsy (68). The influence of ezetimibe on the various stages of NAFLD pathogenesis remains to be fully characterized. Further large-scale RCTs are warranted to explore ezetimibe’s potential as a component of NAFLD/NASH therapy alongside statins.

 

LIPID TREATMENT GOALS

 

We recommend that patients with NAFLD adhere to the Cholesterol Clinical Practice Guidelines from the American Heart Association and American College of Cardiology released in 2018. The guidelines recommend that all adults with any form of CVD or an LDL ≥ 190 mg/dL should be treated with high intensity statins for a goal 50% reduction in LDL. Patients aged 45-70 years with diabetes with LDL < 189 mg/dL or patients with > 7.5% global 10-year CVD-risk should receive moderate intensity statins for a goal 30-50% reduction in LDL. A specific target LDL is no longer formally recommended.

 

Return to Case

 

For our patient with NAFLD it would be both safe and important for her to take lipid-lowering medication to manage her dyslipidemia and reduce her risk of a CVD development. She would benefit from administration of a statin of either moderate or high intensity, based on the outcome of risk assessment.

 

Table 1. Key Points- Non-Alcoholic Fatty Liver Disease

NAFLD is associated with insulin resistance which results in atherogenic dyslipidemia characterized by increased small dense LDL and triglyceride levels and decreased HDL levels.

The dyslipidemia of NAFLD may contribute to the increased risk of CVD observed in individuals with NAFLD

Patients with NAFLD and NASH should be treated for their dyslipidemia to reduce their CVD risk.

Individuals with NAFLD can be treated with statins without increased risk of hepatotoxicity.

 

ALCOHOLIC LIVER DISEASE

 

Case Presentation

 

An obese 48-year-old man with a past medical history significant for coronary heart disease, hypertension, and diabetes mellitus presents for management of newly diagnosed hepatic steatosis. He has a family history of coronary artery disease. He admits to consuming 3 glasses of wine per night during the week and an additional two per evening on weekends. His fasting plasma triglyceride concentration is 350 mg/dl, his LDL cholesterol is 130 mg/dl, and HDL cholesterol is 55 mg/dl. The alanine aminotransferase level (ALT) is modestly elevated at 55 IU/ml. He would like to know whether he has NAFLD and whether you recommend continuing his current alcohol intake to protect against CVD, especially since he was told that his good cholesterol was elevated.

 

Introduction

 

Alcoholic liver disease (ALD) accounts for nearly half of cirrhosis-related mortality in the United States (69). A hallmark feature of ALD is hepatic steatosis, which develops in more than 90% of heavy drinkers. However, less than one third of these individuals develop complications that include alcoholic hepatitis, cirrhosis and HCC (69). Risk factors for disease progression include female sex, obesity, drinking patterns, dietary factors, non–sex-linked genetic factors, and cigarette smoking (70,71). Alcohol also synergizes with other etiologies of chronic liver disease, including NAFLD and viral hepatitis to accelerate progression (69). Hypertriglyceridemia is the primary dyslipidemia associated with alcohol ingestion (72), and a J-shaped association exists between alcohol intake and CVD (73), which may reflect a parallel effect of plasma triglycerides (72). Although its contribution to metabolic syndrome is unclear, alcohol intake appears to interact with obesity to further increase plasma triglyceride concentrations (72).

 

Changes in Lipoprotein Metabolism and Clinical Manifestations

 

DEVELOPMENT OF STEATOSIS

 

As with NALFD, the development of steatosis in response to alcohol is multifactorial. Alcohol impairs the β-oxidation of fatty acids by mitochondria, promotes de novolipogenesis in the liver, and increases fatty acid uptake. As is the case in NALFD, VLDL secretion is also increased due to alcohol.

 

Excess FFA Delivery to the Liver

 

As is the case for NAFLD, fatty acids from extrahepatic sources appear to contribute to hepatic steatosis. In addition to increasing mobilization of fatty acids from adipose tissue (74), alcohol intake augments the supply of lipids to the liver from the small intestine in the form of chylomicron remnants (75).

 

Increased DNL

 

Increased DNL contributes to alcohol-related steatosis by direct and indirect mechanisms (69). The alcohol metabolite acetaldehyde increases transcription of SREBP1c, which upregulates transcription of lipogenic genes. Alcohol-induced endoplasmic reticulum stress and inflammation leads to increased processing of the SREBP1c protein within hepatocytes. Alcohol also inhibits proteins that suppress lipogenesis. The protein deacetylase Sirtuin 1 (SIRT1), plays a central role (76). Suppression of SIRT1 by alcohol leads to hyperacetylation of a group of molecules, including those that promote lipogenesis. Inhibition of adenosine monophosphate kinase (AMPK) contributes, because AMPK-mediated phosphorylation of SREBP1c reduces transcriptional activity. AMPK also phosphorylates and inhibits acetyl-CoA carboxylate (ACC), the rate-limiting step in lipogenesis.

 

Impaired Oxidation and Degradation of Fatty Acids

 

Alcohol decreases mitochondrial fatty acid oxidation principally by decreasing activity of the transcription factor peroxisome proliferator activated receptor (PPAR) α. This occurs in response to increased NADH/NAD+ ratios and decreased AMPK activity, among other factors (69). PPARα promotes the transcription of genes that mediate fatty acid oxidation. Alcohol intake may also inhibit autophagy (69), which plays an important role removing lipids from the liver (77).

 

Insufficient Export of Hepatic Triglycerides

 

Alcohol increases VLDL secretion (72,78), apparently by increasing the transcription of MTP (74). The increased in export of hepatic triglycerides is insufficient to offset the accumulation due to increases in fatty acid uptake and synthesis in the setting of decreased oxidation.

 

HYPERTRIGLYCERIDEMIA

 

Increased VLDL secretion contributes to hypertriglyceridemia that is observed in the setting of alcohol consumption. This is exacerbated by decreased expression of LPL (79), which promotes clearance of VLDL triglycerides into muscle and fat tissue. There is also an interaction between alcohol consumption and genetic polymorphisms in apoCIII, which circulates in the plasma and functions to inhibit lipoprotein lipase activity (80).

 

CIRCULATING HDL LEVELS

 

Alcohol increases HDL lipids and apolipoproteins in patterns that depend upon amount of consumption: Moderate consumption tends to increase plasma concentrations of smaller HDL particles, whereas heavier consumption favors larger HDL particles (81). Alcohol interacts with HDL metabolism in multiple steps, which can ultimately lead to increased reverse cholesterol transport, the process by which cellular cholesterol is transported to the liver for elimination into bile (81,82). Heavier alcohol consumption impairs CETP activity, so the typical inverse relationship observed under circumstances associate with NAFLD is not necessarily observed in the setting of alcohol use and HDL may be increased as well (72,83). Moderate alcohol consumption also appears to enhance the anti-inflammatory and anti-oxidant properties of HDL particles (81).

 

CIRCULATING LDL LEVELS

 

The effects of alcohol on plasma LDL cholesterol concentrations is less consistent than observed for HDL, with different patterns observed in different populations, which may be attributable to genetic polymorphisms with these populations (81).

 

Management

 

Although considerable anecdotal evidence exists to support a CVD benefit of moderate alcohol consumption, insufficient data are available to translate this concept into a clinical recommendation. In the setting of alcohol-related hepatic steatosis, cessation of drinking, along with therapeutic lifestyle modifications, are the mainstays of therapy.

 

Return to Case

 

The diagnosis of NAFLD is based on the absence of significant alcohol consumption. For a man, the upper limit of alcohol intake is 2 drinks per day. This means that this patient cannot be categorized simply as NAFLD, although the coexistence of alcoholic liver disease and NAFLD is likely in this patient. He is at high risk for CVD, so should be managed accordingly, including lipid lowering therapy with statins. His alcohol consumption should be reduced to less than 2 drinks per day, which may help reduce his fasting triglyceride concentrations. He should not be falsely reassured by his elevated HDL cholesterol concentration.

 

Table 2. Key Points- Alcoholic Liver Disease

The consumption of alcohol is a common cause of excess fat accumulation in the liver.

There are multiple mechanisms by which alcohol promotes hepatic steatosis.

Alcohol can increase plasma HDL cholesterol concentrations and fasting triglyceride concentrations.

Although modest alcohol consumption is associated with reduced CVD risk, this cannot be recommended due to other potential adverse effects, including alcoholic liver disease.

 

VIRAL HEPATITIS-- C

 

Case Presentation

 

A 65-year-old woman with a past medical history of CVD and untreated genotype 1 chronic hepatitis C presents for management of CVD. Her lipid levels are notable for an LDL of 99. She has read that since her LDL is below the recommended level for patients with CVD she would not benefit from lipid lowering therapy. What would you advise her?

 

Introduction

 

Hepatitis C virus (HCV) is a positive-strand RNA virus of the family Flaviviridaethat can lead to chronic infection as well as the development of cirrhosis, HCC, and the need for liver transplantation. Chronic HCV (CHC) infection impacts between 130 and 170 million individuals worldwide (84).

 

Changes in Lipoprotein Metabolism

 

HCV replication is intricately linked with host cell lipids and impacts host lipid metabolism. Circulating HCV virions complex with host lipoproteins and form lipoviroparticles (85). This lipid composition is a prerequisite for maintenance of viral particle morphology and HCV infectivity (86,87,88,89). For example, lipids on the virion surface shield viral envelope epitopes, protecting them from antibody engagement (90). Lipoviroparticles can enter hepatocytes via multiple receptors including the hepatocyte LDL receptor (which may also facilitate the replication step of the HCV cycle (91)) and utilizes cell surface molecules including Niemann-Pick C1-like 1 (NPC1L1), a receptor for cholesterol resorption, and scavenger receptor class B member 1 (SRB1), which acts to promote cholesterol uptake from lipoproteins, and interacts with HCV envelope glycoprotein E2 to promote HCV entry (92,93,94). LDL receptor and SRB1 appear to have a redundant role in HCV entry (95). Several apolipoproteins influence HCV uptake: apoC1 interacts with HCV glycoproteins to promote infection, and apoE mediates initial attachment between virus and hepatocyte. Hepatocyte VLDL receptor mediates an additional HCV entry mechanism, involving E2 and apoE, with increased VLDL receptor expression conferring greater susceptibility to infection (96). Formation of the HCV core protein involves interaction with host cytosolic lipid droplets and interaction with diacylglycerol O-acetyltransferase 1, a host enzyme involved in triglyceride synthesis. HCV replication also interacts with host cholesterol synthesis within hepatocytes. The host protein FBL2 undergoes geranylgeranylation, an intermediate of the cholesterol synthesis pathway (97). When this pathway is interrupted, the HCV replication complex is extinguished (98). Finally, HCV secretion from hepatocytes involves complexing with apoE-containing host lipoproteins in the form of VLDL or HDL (99).

 

Clinical Manifestations

 

Like NAFLD, HCV infection is associated with the development of hepatic steatosis. However, unlike NAFLD, HCV is also associated with hypolipidemia. CHC infection is associated with significantly lower host LDL and total cholesterol levels than in uninfected controls (100). Treatment is associated with increases in both LDL and cholesterol levels in patients with HCV who achieve a cure, defined as a sustained virologic response (SVR). Changes in host serum lipids are also seen in patients with acute HCV. Acute HCV infection is associated with a decrease in total cholesterol, LDL and non-HDL-cholesterol from pre-infection levels. In addition, total cholesterol, LDL, triglycerides and non-HDL-C progressively decline over a 10-year period following HCV seroconversion, after adjusting for BMI and FIB-4 score (101). In patients who achieved viral clearance, either spontaneous or treatment-induced, total cholesterol, LDL and non-HDL-C increased significantly from infection levels. In an important proportion of patients with both acute and chronic infection, post-viral clearance lipid levels exceed pre-infection levels (102).

 

While HCV infection is associated with a decrease in LDL and non-HDL-C, important CVD risk factors, HCV infection is associated with an increased overall risk of CVD (103,104). When non-HCV infected individuals with similar lipid levels are compared to those with CHC, HCV infection independently confers an increased risk of acute myocardial infarction (AMI), with a more pronounced increase seen in younger individuals (105). Further, lipid-lowering therapy among individuals with CHC was associated with a greater reduction in AMI risk than uninfected persons with similar lipid levels. Therefore, lipid levels may not accurately reflect CVD risk in patients with CHC.

 

Management

 

Lipid treatment goals for individuals with CHC are not well established. We recommend that patients with CHC adhere to the Cholesterol Clinical Practice Guidelines from the American Heart Association and American College of Cardiology released in 2018 (106). Retrospectively-collected data links statin use to improved liver-related outcomes, with higher likelihood of achieving SVR, and lower rates of fibrosis progression, cirrhosis development, HCC incidence, and mortality amongst patients with CHC (107,108,109,110). Simon et al. identified that atorvastatin and fluvastatin have the most significant anti-fibrotic benefit, compared with simvastatin, pravastatin, lovastatin or no statin use (111). It is important to note that for individuals who have achieved an SVR after HCV treatment, lipid levels often increased to or above pre-infection levels. Induction of SVR using DAA therapy led to pro-atherogenic lipid changes (increased total cholesterol, LDL, LDL/HDL ratio, and non-HDL-C), irrespective of DAA regimen or fibrotic stage, with a parallel reduction in insulin resistance. The balance of these effects with respect to CVD risk remains to be determined (112). Hashimoto et al. found greater increases in serum LDL-cholesterol (LDL-C) levels in patients undergoing therapy with ledipasvir/sofosbuvir compared to daclatasvir/asunaprevir. Decline in HCV core protein was also independently associated with rises in LDL-C (113). Thus, practitioners should be mindful to monitor post-treatment lipid levels and treat appropriately.

 

Return to Case

 

For our patient with CHC and CVD it would be important for her to take a lipid-lowering medication to reduce her risk of a second CVD event. Based on the guidelines, she would benefit from high intensity statin therapy, with a goal of decreasing LDL cholesterol by >50%.

 

Table 3. Key Points- Hepatitis C

The hepatitis C virus interacts with host lipids for hepatocyte entry, viral replication and secretion.

HCV infection decreases host serum LDL and total cholesterol levels.

HCV infection is still associated with an increased risk of AMI and treatment with statins reduces this risk.

Treatment of HCV results in increase in serum lipid levels to at least pre-infection levels.

 

VIRAL HEPATITIS –HEPATITIS B

 

Introduction

 

Approximately 240 million individuals are chronically infected with the hepatitis B virus (HBV) (114). Like HCV, chronic HBV infection can lead to cirrhosis and hepatocellular carcinoma.

 

Lipoprotein Metabolism in Hepatitis B

 

HBV interacts with host lipid metabolism in several important ways including during viral cell entry and formation of a vital viral protein, the HBV surface antigen. HBV uses the Na+-taurocholate cotransporting polypeptide (NTCP), a peptide that normally allows for hepatocyte uptake of host bile acids, to gain access to hepatocytes (115). HBV binding to NTCP impairs the ability of NTCP to promote hepatocyte uptake of bile acids. This results in an increase in conversion of cholesterol to bile acids.

 

The formation of the HBV surface antigen within hepatocytes relies in part on host cell cholesterol (116). The surface antigen particle is synthesized in the membrane of the hepatocyte endoplasmic reticulum (ER) and is associated with the host ER lipid bilayer. Association with the lipid bilayer helps make the particle resistant to degradation by cellular proteases. The surface antigen is then transported to the ER lumen and exported from the hepatocyte as a lipoprotein particle. Approximately 25% of the surface antigen complex is composed of host lipids including phosphatidylcholine, triglycerides, cholesterol and cholesterol esters (116).

 

HBV infection may also alter lipogenic gene expression. Two studies have demonstrated increased in lipogenic gene expression in HBV-infected transgenic mice compared to uninfected mice. HBV-infected transgenic mice have increased gene expression of SREBP2, 3-hydroxy-3-methylglutaryl-coenzyme A reductase, LDL receptor, fatty acid synthase, and ATP citrate lyase, all of which play a role in either cholesterol metabolism or fatty acid synthesis (117,118). Oehler et al also found that in HBV infected humanized mice, gene expression of human apolipoprotein A1, a lipoprotein found in HDL which plays a role in reverse cholesterol transport and PPAR-gamma which regulates adipocyte differentiation and fatty acid storage, was significantly enhanced.

 

HBV-infected transgenic mice also demonstrate elevated levels of 7α-hydroxylase (hCYP7A1), which promotes bile acid formation from cholesterol. In liver biopsy samples from patients with chronic HBV infection, hCYP7A1 was significantly induced when compared to uninfected controls. These findings suggest that HBV replication may impact cholesterol metabolism.

 

Clinical Manifestations

 

Data on the impact of HBV infection on circulating lipid levels in humans is limited. HBV infection may be associated with lower triglyceride levels than in uninfected patients (119), however its influence on HDL remains ambiguous. Hsu et al performed a case control study comparing 322 individuals with chronic HBV infection to 870 age-matched, uninfected controls. Individuals with HBV infection were found to have significantly lower triglyceride and HDL levels when compared to controls. In a second retrospective cohort of 122 individuals with chronic HBV, HBV DNA levels was inversely proportional to serum triglyceride levels but no relationship was seen with HDL levels (119). Amongst a cohort of non-diabetic patients, HBsAg-seropositivity was inversely correlated with hypertriglyceridemia and low serum HDL cholesterol. Hence, chronic HBV infection may favorably impact lipid profiles, which could partly account for the inverse relationship between HBsAg-seropositivity and metabolic syndrome seen in this cohort (120). Similarly, Joo et al. demonstrated that in patients who were initially free of dyslipidemia, HBsAg-positivity was associated with lower risk of developing dyslipidemia during an average follow up of 4.46 years (121).

 

Circulating lipid levels may be predictive of clinical outcomes in HBV-infected patients. Chen et al. found that average plasma apolipoprotein A-V level was decreased amongst 209 non-survivors of HBV-acute on chronic liver failure versus 121 survivors (122).

 

Like HCV, chronic HBV infection is frequently associated with hepatic steatosis. Between 25% and 51% of patients with HBV are found to have steatosis on imaging or biopsy (123). However, while concurrent steatosis is common in HBV infections, steatohepatitis is not frequently described. Further, the pathogenesis of steatosis in HBV is not well understood and may be related to co-existing metabolic factors such as body mass index (BMI) and insulin resistance rather than the viral infection itself (124).

 

Management

 

As data on the impact of HBV infection on circulating lipids is limited there are no formal guidelines for dyslipidemia management in this population. Clinicians should be mindful of a possible decrease in HDL in this population and follow standard guidelines from the American Heart Association and American College of Cardiology on lipid management. Recent studies have shown reduced risk of cirrhosis development (125), decompensation (125, 126, 127, 128), mortality (126, 127) and portal hypertension (126) amongst statin users compared to non-users with chronic HBV- and HCV-related hepatitis. Furthermore, statin use was associated with a 32% reduced HCC risk. Concomitant use of statin and nucleos(t)ide analogue led to an additive chemopreventive effect (129). Large-scale RCTs to comprehensively evaluate statins as a means of protection against disease progression in patients with viral hepatitis are warranted.

 

Table 4. Key Points- Hepatitis B

HBV infection may interact with host lipids and enhance lipogenic gene expression

The clinical manifestations of HBV on host lipids are not well studied but HBV infection may decrease serum triglyceride and HDL levels.

Management of patients with HBV and dyslipidemia should be guided by standard recommendations for the treatment of dyslipidemia.

 

CHOLESTATIC DISEASES

 

Case Presentation

 

A 58-year-old woman is referred with primary biliary cirrhosis (PBC) for the management of an elevated plasma total cholesterol of 450. She reports symptoms only consistent with mild and intermittent pruritis. She is currently taking ursodeoxycholic acid. Her physical is notable for xanthelasma under the eyes.

 

Introduction

 

Bile is the route for cholesterol elimination from the body. Plasma cholesterol is taken up by the liver in the form of apolipoprotein B-containing lipoproteins (i.e. remnant lipoproteins and LDL) by receptor-mediated endocytosis or by selective uptake of HDL cholesterol (130). Cholesterol is eliminated by conversion to bile salts and by biliary secretion. Biliary obstruction, notable when due to cholestatic diseases, can interfere with cholesterol elimination leading to hypercholesterolemia. This occurs most commonly in the setting of primary biliary cirrhosis (PBC), which is an autoimmune-mediated destruction of intrahepatic bile ducts. It can also occur in primary sclerosing cholangitis (PSC), in which there is inflammatory stricturing of larger bile ducts by poorly understood mechanisms.

 

Lipoprotein Metabolism in Cholestasis

 

Hypercholesterolemia associated with cholestasis is largely attributable to the formation of lipoprotein X, an atypical lipoprotein particle. Lipoprotein X comprises principally unesterified cholesterol and phospholipids (131), resembling the cholesterol-phospholipid vesicles that are secreted by the liver into bile (132). The principal proteins associated with lipoprotein-X are apoC and albumin contained within the core (133,134). The lipids of the particle comprise a sphere, with an aqueous core. Lipoprotein-X is devoid of apoB. It appears to be formed due to the secretion of biliary-type particles into plasma in the setting of obstruction to bile flow (135), although defects in plasma cholesterol esterification may also contribute (131). Lipoprotein X has similar characteristics as LDL including density, so that its presence in plasma requires electrophoretic separation (136).

 

Plasma total cholesterol concentrations are increased in PBC in proportion to disease severity, with elevations that can be striking and exceed 1,000 mg/dl, and can be a rare cause of pseudohyponatremia (133). Where these elevations are primarily attributable to lipoprotein-X, apolipoprotein B concentrations may also be elevated due abnormal lipoprotein metabolism associated with liver disease (131,133). Serum metabolomics analysis of patients with PBC revealed elevated levels of VLDL and LDL compared to controls (137). HDL cholesterol concentrations are elevated in the early stages of PBC and tend to decline as the disease progresses (138), apparently because of increased circulating hepatic lipase activity that promotes HDL catabolism (131). Patients with more advanced PBC exhibit increased plasma triglycerides (131), presumably attributable to decreased hepatic lipase activity (138).

 

Plasma lipids in PSC have been less well characterized than in PBC. In a small series (139), the hypercholesterolemia was more modest than generally observed in PBC, but did increase in concert with disease severity. HDL cholesterol levels tended to be high, and triglyceride elevations were uncommon.

 

Clinical Manifestations

 

An important consideration has been whether the lipid abnormalities associated with cholestatic diseases confer increased CVD risk. This has been studied more extensively in PSC in the form of prospective trials (138,140). Although each had limitations, collectively there was no suggestion of increased atherosclerotic events, which is in keeping with the relative absence of elevations in atherogenic particles There is also evidence in vitroto suggest that lipoprotein-X may be atheroprotective by reducing oxidation of LDL (136). In patients with PBC, the presence of xanthelasma does not appear to connote an increased CVD burden (138).

 

As with PBC, the cholesterol elevations associated with PSC do not tend to confer CVD risk. None was observed in the small series cited previously, but it was acknowledged that patients were young enough that excess CVD complications would not have been expected (139). Lipid levels ultimately fell in patients who had progressed to cirrhosis and hepatic failure.

 

Management

 

Due to the overall lack of clinical evidence, the management of hypercholesterolemia associated with cholestasis lacks formal recommendations. In PBC, ursodeoxycholic acid (UDCA) slows the progression of disease and prolongs survival (141).Chronic UDCA administration also reduces plasma LDL concentrations. In PBC patients, statin therapy is generally safe and is effective at lowering LDL cholesterol in PBC patients (58,142-144). At present, UDCA is generally not recommended in the management of PSC (145), and data are lacking regarding lipid-lowering therapies in these patients. Of note, some patients with obstructive jaundice are treated with bile acid binders to reduce pruritus and not primarily to reduce plasma cholesterol concentrations.

 

Return to Case

 

For our patient with PBC, the presence of lipoprotein-X may be confirmed by lipoprotein electrophoresis. The possible contribution of atherogenic particles may be estimated by the measurement of the plasma apoB concentration. The institution of statin therapy should be based on standard estimates of CVD risk.

 

Table 5. Key Points- Cholestatic Disease

Plasma total cholesterol concentrations are commonly elevated in the setting of cholestasis.

Lipoprotein-X is an abnormal lipoprotein that circulates in patients with cholestasis and is primarily responsible for the elevations in plasma total cholesterol concentrations.

Elevations in plasma cholesterol concentrations due to cholestasis do not appear to confer excess CVD risk.

Patients with cholestatic disorders may be candidates for lipid lowering therapy if they are otherwise at risk for CVD.

 

CIRRHOSIS

 

Introduction

 

Cirrhosis is the common advanced histologic endpoint for chronic liver diseases in which the formation of fibrotic nodules in the liver often obscured the etiology of the responsible disease process. The clinical correlates range widely from well-compensated liver function with no apparent clinical manifestations to advanced decompensated liver disease with portal hypertension, with complications that include hepatic encephalopathy, esophageal varices, and ascites. Moreover, the development of cirrhosis confers increased risk of HCC.

 

Changes in Lipoprotein Metabolism and Clinical Manifestations

 

The changes in lipoprotein metabolism associated with cirrhosis generally reflect the degree of impairment of hepatic function. In one study (146), plasma concentrations of total cholesterol, HDL cholesterol, LDL cholesterol, and VLDL cholesterol varied with increases in prothrombin time and decreases in albumin, which reflect hepatic synthetic function.These findings are in general agreement with other studies (147). Lipoprotein compositions are also altered in the setting of cirrhosis, with LDL particles enriched with triglycerides and deficient in cholesteryl esters, and HDL particles enriched with triglycerides, free cholesterol and phospholipids (147). These changes are secondary to characteristic abnormalities in plasma enzymes that remodel lipoproteins, including lecithin-cholesterol acyl transferase (LCAT), hepatic lipase, and phospholipid transfer protein (PLTP) (147). HDL-C and enzymes involved in HDL maturation and metabolism are decreased in patients with cirrhosis. There is a shift in the composition of HDL in those with cirrhosis towards the larger HDL2 subclass, with a reduction in small HDL3 particles. The latter is associated with diminished cholesterol efflux capacity which in turn independently predicts 1-year mortality (148).

 

Hepatocellular carcinoma can occur in the setting of cirrhosis and may be associated with alterations in plasma lipids (149-151). In instances of hypercholesterolemia, the increase may be driven by elevated rates of cholesterol synthesis and cellular levels of 3-hydroxy-3-methylglutarylcoenzyme A. It is unclear whether this hypercholesterolemia confers increased CVD risk (152,153).

 

CVD risk is dependent upon the etiology of cirrhosis, at least in part due to the association of type 2 diabetes. Cirrhosis due to NASH, HCV, and alcoholic liver disease increases the risk of type 2 diabetes, which is not observed in cholestatic liver diseases and presumably contributes to CVD risk (147,154). Statin therapy may be safely administered in patients with compensated cirrhosis and increased CVD risk (58). In patients with non-cholestatic cirrhosis, low HDL cholesterol serves as a liver function test that is an indicator of poor prognosis, increasing the risk of cirrhotic death (155).

 

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Nonalcoholic Fatty Liver Disease: The Overlooked Complication of Type 2 Diabetes

* Denotes equal effort

ABSTRACT

 

Nonalcoholic fatty liver disease (NAFLD) is a common complication of obesity and type 2 diabetes mellitus (T2DM).  Most times it is an unrecognized comorbidity to the primary care provider and endocrinologist. Today it is the most common chronic liver disease in developed countries. It is characterized by insulin resistance and hepatic triglyceride accumulation in the absence of co-existing etiologies, such as excessive alcohol consumption, viral hepatitis, medications or other etiologies for hepatic steatosis.  Its more severe form of the disease with steatohepatitis (NASH) is associated with hepatocyte injury (necrosis and inflammation) and frequently with fibrosis. Although it appears to be an indolent condition, with few symptoms and often normal plasma aminotransferases, NASH is a leading cause of end-stage liver disease and hepatocellular carcinoma (HCC), and significantly increases the risk of developing cardiovascular disease (CVD) and T2DM. The pathogenesis of NASH remains poorly understood, and likely to be multifactorial, but insulin-resistant adipose tissue plays an important role. The natural history of NAFLD is incompletely understood, but risk factors for disease progression include weight gain, obesity and T2DM, as well as the severity of fibrosis stage at diagnosis.  Diagnostic algorithms are evolving but we offer an approach that integrates for the non-hepatologist plasma biomarkers, imaging, and the role of liver biopsy for the management of these complex patients.  At the present time, early screening -with biomarker panels or a liver ultrasound, ideally with transient elastography- is reserved for high-risk patients (i.e., obese patients with T2DM or elevated plasma AST/ALT levels or evidence of steatosis at a random liver exam) until more accurate non-invasive methods are available.  A liver biopsy should be considered on a case-by-case basis, to identify those at risk of NASH-cirrhosis, working in close collaboration with a hepatologist. Treatment should include a comprehensive approach with lifestyle modification and therapeutic agents tested in RCTs, such as vitamin E (in patients without diabetes) or pioglitazone for patients with or without diabetes.   Pioglitazone, given its low-cost as a generic medication, long-standing track record of efficacy in NASH, and cardiometabolic benefits, is likely to be for NASH what metformin has become for the management of T2DM. However, proper patient selection and close monitoring is needed.  In addition, a number of new pharmacological agents are being studied in phase II/III trials and future management will involve the use of combination therapy, as for other chronic metabolic conditions. In summary, endocrinologists need to be aware of the severe metabolic and liver-specific complications of NASH and establish early-on a long-term management plan. Screening will likely take place in the same way as for diabetic retinopathy or nephropathy.  A better understanding of its natural history and pathogenesis of NASH, combined with improved diagnostic and treatment options, will likely place endocrinologists at the forefront of the management efforts to prevent end-stage liver disease in patients with NASH. 

 

INTRODUCTION

 

Nonalcoholic fatty liver disease (NAFLD) is a chronic liver condition that is on the rise. It has become the most common chronic liver condition in many parts of the world. It encompasses a wide spectrum of disease with different clinical implications. NAFLD means that there is evidence of liver steatosis, either by imaging or histology, in the absence of secondary causes of hepatic fat accumulation such as significant alcohol consumption, chronic use of steatogenic medication, or another established chronic liver disease.  Between 40 to 50% of patients that are obese have NAFLD and this rises to about 70% if they have type 2 diabetes mellitus (T2DM). In its simplest form, known as isolated steatosis or NAFL (nonalcoholic fatty liver), there is triglyceride accumulation of ≥5% without evidence of hepatocellular injury in the form of hepatocyte ballooning or evidence of fibrosis.  Although the natural history of this condition remains uncertain, and may possibly progress to more severe disease, at the present moment NAFL is considered to be associated with limited risk of liver morbidity. However, it is associated with insulin resistance so that the liver can be seen as a “mirror” of metabolic health (i.e., in obesity steatosis being a reflection of insulin resistance, and in particular, of adipose tissue dysfunction) and with an increased risk of cardiovascular disease (CVD). In its more severe form, known as nonalcoholic steatohepatitis or NASH, steatosis ≥5% is associated with hepatocyte injury with necrosis (“ballooning”) and lobular inflammation, with or without fibrosis.

 

Steatohepatitis is often a progressive disorder in T2DM associated with the development of fibrosis that can eventually lead to cirrhosis. Liver fibrosis is defined by its severity in stages ranging from absence of fibrosis (stage F0) to mild (stage F1), moderate (stage F2, with zone 3 sinusoidal fibrosis plus periportal fibrosis), and “advanced” fibrosis referring specifically to stages 3 (bridging fibrosis) or 4 (cirrhosis). Having fibrosis is the most important histological feature of NAFLD associated with long-term mortality. Fibrosis also predisposes patients to hepatocellular carcinoma (HCC).

 

Type 2 diabetes mellitus has been a well-established factor in the progression of NAFLD to more severe forms, including a higher incidence of HCC (1-3). However, in clinical practice NAFLD still remains an under-recognized complication of T2DM, unlike the other microvascular and macrovascular complications.

 

As discussed later, isolated steatosis and NASH may carry an increased risk of CVD and being the most common cause of death in patients with NAFLD, independent of other metabolic comorbidities. It is important for endocrinologists and primary care physicians to recognize that NAFLD in T2DM has been shown to be associated with adverse metabolic changes resulting in increased atherosclerotic disease and cardiovascular consequences (4,5).

 

NAFLD: KEY CONCEPTS

 

The incidence of NAFLD is rising, paralleling that of obesity and diabetes mellitus. There has been extensive research in the area of NAFLD, especially over the past two decades. However, given the lack of highly reliable noninvasive diagnostic methods, the burden of NAFLD probably remains overlooked. By liver ultrasound, studies have demonstrated the prevalence of NAFLD to be 24% in United States, whereas using blood testing alone this is underestimated at just 13% (6). By the gold-standard magnetic resonance imaging and spectroscopy (1H-MRS), the prevalence of NAFLD in the general population is estimated to be 34% (7).

 

Unfortunately, imaging techniques cannot adequately evaluate for hepatocellular necrosis or inflammation (i.e. NASH). Studies that have utilized a liver biopsy to confirm the diagnosis of NAFLD have shown that 59% of patients with NAFLD have NASH, this being much higher in obese individuals (6). Moreover, recent studies have reported that about 18% of unselected patients with T2DM have moderate-to-severe (F2-F4) fibrosis (6,8).

 

NAFLD often progresses to steatohepatitis (NASH), especially in patients with T2DM. NASH is hallmarked by hepatocellular necrosis, lobular inflammation and often fibrosis. Many studies have now documented that patients with NASH and fibrosis have the worst mortality (9). As fibrosis progresses, cirrhosis develops. This rate of progression to cirrhosis is highly variable and dependent on age, BMI, blood pressure control, presence of T2DM, and degree of steatohepatitis (10). The three most relevant risk factors are obesity (excessive BMI or visceral obesity), T2DM, and presence of moderate to severe fibrosis (11). However, given the high heterogeneity in disease progression one must admit that the precise factors leading to cirrhosis remain unclear.

 

NASH is currently the second most common indication for liver transplantation, after hepatitis C. It is predicted to be the leading indication for liver transplantation in the next decade given the rise in incidence (8). The annual incidence of HCC in NAFLD – related cirrhosis is about 1% (1,8,12,13). Nonalcoholic steatohepatitis related cirrhosis is currently the third leading cause for HCC, after HCV and alcohol-related liver disease. Importantly, those with NASH- related HCC that undergo liver transplantation are more likely to have a higher BMI and higher rate of T2DM (13). In one study, it has also been demonstrated that HCC can develop in NASH in the absence of cirrhosis (14).

 

NAFLD and Cardiovascular Disease

 

Many factors lead to cardiovascular disease in patients with T2DM and NAFLD. For instance, they have increased intrahepatic triglyceride accumulation and insulin resistance. This is associated with increased hepatic VLDL secretion and a decrease in the peripheral clearance of triglyceride-rich lipoproteins. This results in a proatherogenic profile, which includes hypertriglyceridemia, low HDL-C, and an increase in small, dense LDL particles, plus a state of subclinical inflammation (8).

 

These patients also often have more severe hepatic insulin resistance leading to progressive deterioration of glycemic control (9). Hepatic insulin resistance is associated with hyperinsulinemia from increased insulin secretion and decreased insulin clearance (3,15). Hyperinsulinemia per sehas been associated with atherogenesis in animal models of disease and in epidemiological studies.  Chronic hyperinsulinemia also causes downregulation of insulin signaling pathways and acquired insulin resistance in short-term clinical studies in humans (11). In this context, hyperglycemia is more severe and also appears to contribute to CVD. Endothelial dysfunction also has been shown to cause increased cardiovascular risk in patients with NAFLD (16). Early left ventricular “diastolic dysfunction” (or heart failure with preserved ejection fraction or HFpEF) has been noted in patients with NAFLD and well controlled T2DM independent of other risk factors (17). Patients with NAFLD are often found to have a significantly worse carotid intima-media thickness with increased atherosclerotic disease when compared with clinically matched patients without NAFLD. This has been correlated in some studies with an advancing degree of steatosis, inflammation, and/or fibrosis (18,19). In NASH with cirrhosis, CVD is the leading cause of mortality (1,8,20).

 

Thus, it is not unexpected that in NAFLD many studies have reported a higher rate of CVD (Tables 1 and 2).  In addition to insulin resistance and hyperinsulinemia, NAFLD and CVD cluster with other common risk factors, including hypertension, hyperlipidemia, T2DM, obesity and inflammation (8). The evidence of the association between NAFLD and increased CVD often persists even after adjusting for traditional cardiovascular risk factors (Tables 1 and 2) (9,21).This suggests that the presence of NAFLD may independently increase an individual’s cardiovascular risk, but whether this is worse in patients with steatohepatitis compared to those just having isolated steatosis remains controversial. It should also be noted that many investigatorshave failed to see the association of NAFLD with CVD after adjusting for traditional cardiovascular risk factors, as detailed in Tables 1 and 2.

 

Table 1. Cardiovascular Disease in Patients with NAFLD without Type 2 Diabetes

Author (year)

NAFLD vs controls

(n)

Diagnosis of NAFLD

Primary endpoint

Increased CVD

Adjusted CV risk

Study design

Villanova et al (22)

80

Liver biopsy

Endothelial function 

Yes

Yes

Prospective case-control, Cross-sectional

Brea et al (23)

80

Ultrasound (US)

Carotid intima-media thickness test (CIMT)

Yes

No

Case-control, Cross-sectional

Adams et al (24)

420

US, CT, MRI or Liver biopsy

All cause and CV mortality

Yes

No

Retrospective cohort, Longitudinal

Volzke et al (25)

4222

Ultrasound

CIMT

Yes

No

Case-control, Cross-sectional

Ekstedt et al (26)

129

Liver biopsy

All cause and CV mortality

Yes *

Yes

Retrospective cohort, Longitudinal

Mirbagheri et al (27)

171

Ultrasound

Coronary angiography

Yes

Yes

Cross-sectional

Hamaguchi et al (28)

1221

Ultrasound

CV events

Yes

Yes

Prospective cohort, Longitudinal

Schindhelm et al (29)

1439

ALT

CV events

Yes

Yes

Retrospective cohort, Longitudinal

Fracanzani et al (30)

375

Ultrasound

CIMT

Yes

Yes

Case-control, Cross-sectional

Goessling et al (31)

2812

AST, ALT

CV events

Yes

No

Retrospective cohort, Longitudinal

Aygun et al (32)

80

Liver biopsy

CIMT

Yes

Yes

Prospective case-control, cross-sectional

Haring et al (33)

4160

Ultrasound

All cause and CV mortality

Yes**

Yes

Retrospective cohort, Longitudinal

Rafiq et al (34)

173

Liver biopsy

All cause and CV mortality

No***

No

Retrospective cohort, Longitudinal

Salvi et al (35)

220

Ultrasound

Arterial stiffness by carotid-femoral pulse wave velocity

Yes

Yes

Case-control, Cross-sectional

Soderberg et al (36)

118

Liver biopsy

All cause and CV mortality

Yes

Yes

Retrospective cohort, Longitudinal

Zhou et al (37)

3324

Ultrasound

All cause and CV mortality

Yes

Yes

Retrospective cohort, Longitudinal

Stepanova et al (38)

11,613

Ultrasound

All cause and CV mortality

Yes

No

Retrospective cohort, Longitudinal

Lee et al (39)

1442

Ultrasound

Arterial stiffness by brachial-ankle pulse wave velocity

Yes

Yes

Case-control, Cross-sectional

Kozakova et al (40)

1012

Fatty Liver Index

CIMT

Yes

Yes

Cross-sectional

Kim et al (41)

4023

Ultrasound

Coronary artery calcification score by CT

Yes

Yes 

Cross-sectional

Hallsworth et al (42)

38

MR spectroscopy

LV dysfunction by cardiac MRI

Yes

Yes

Case-control, Cross-sectional

Colak et al (43)

72

Liver biopsy

Endothelial function by flow mediated dilation (FMD) and CIMT

Yes

Yes

Observational case-control, cross-sectional

Pisto et al (44)

988

Ultrasound

CV events

Yes

No

Retrospective cohort, Longitudinal

Ekstedt et al (45)

2515

Liver biopsy

CV events

Yes

No****

Retrospective cohort, Longitudinal

Zeb et al (46)

4119

CT

CV events

Yes

Yes

Prospective cohort, Longitudinal

Fracanzani et al (47)

273

Ultrasound

CIMT

Yes

Yes

Prospective cohort, Longitudinal

Wong et al (48)

612

Ultrasound

CV events, Coronary artery stenosis by angiogram

Yes*****

No

Prospective cohort, Longitudinal

*Patients with NASH but not with only steatosis had increased cardiovascular mortality.

       

** NAFLD was associated with increased all cause and cardiovascular mortality in men only.

       

***Compared NASH vs non-NASH NAFLD patients, no difference in overall mortality was found,

but liver mortality was significantly different, with higher rates in NASH patients. Overall, most

common causes of death reported were cardiovascular disease, malignancy and liver related deaths.

****No increased CV risk when diabetics were excluded.

         

***** Patients with NAFLD were more likely to have significant coronary artery stenosis at baseline,

and more likely to undergo percutaneous coronary intervention; however, no increased association

of NAFLD with CV events during follow up.

 

 

           
               

Table 2. Cardiovascular Disease in Patients with NAFLD with Type 2 Diabetes

Author

NAFLD vs controls

(n)

Diagnosis of NAFLD

Primary Endpoint

Increased CVD

Adjusted CV Risk

Study Design

Targher et al (49)

200

Ultrasound

CIMT

Yes

Yes*

Cross-sectional

Targher et al (50)

2103

Ultrasound

CV events

Yes

Yes

Longitudinal

McKimmie et al (51)

623

CT

CIMT and coronary artery calcium score

No

No

Cross-sectional

Petit et al (52)

101

MR spectroscopy

CIMT

No

No

Prospective, Cross-sectional

Adams et al (53)

337

Liver US, CT or biopsy

All-cause mortality and CVD

No

No

Longitudinal

Poanta et al (54)

56

Ultrasound

CIMT

No

No

Case-control, Cross-sectional

Bonapace et al (55)

50

Ultrasound

LV diastolic dysfunction

Yes

Yes

Cross-sectional, Prospective

Dunn et al (56)

2343

CT

CV mortaility

No

No

Retrospective cohort, Longitudinal

Khashper et al (57)

93

CT

Coronary artery calcium score

No

No

Prospective, Cross-sectional

Kim et al (58)

4437

Ultrasound

CIMT

Yes

Yes

Cross-sectional

Idilman et al (59)

273

CT

Coronary artery calcium score

Yes**

Yes

Prospective, Cross-sectional

Silaghi et al (60)

336

Ultrasound

CIMT

No

No

Cross-sectional

Kwak et al (61)

213

Ultrasound

Coronary artery calcium score

Yes***

Yes

Cross-sectional

Mantovani et al (62)

222

Ultrasound

LV diastolic dysfunction

Yes

Yes

Cross-sectional

*CV risk remained significant after adjustment for other traditional cardiovascular risk factors, however did not remain significant after adjustment for HOMA-IR.                                     

**Only significant association was between NAFLD and significant CAD (defined as more than or equal to 50% stenosis in at least one coronary artery).                                   

***Only significant association in patients with NAFLD and A1C > 7% but not in lower A1C.                                                                     

NAFLD and Chronic Kidney Disease

  

The presence of NAFLD and NASH with fibrosis have been recently associated with chronic kidney disease (CKD), and more severe forms of fatty liver disease correlate with worse and progressive stages of CKD. In most studies, CKD has been defined as having an estimated glomerular filtration rate (eGFR) < 60 ml/min/1.73m2or increased albuminuria/proteinuria (20,63,64). In a case control study by Targher et al, the severity of liver histology in patients with biopsy-proven NASH was found to be independently associated with the degree of worsening eGFR (65).

 

A cross-sectional study of Japanese patients with biopsy-proven NAFLD showed an increased prevalence of CKD with worsening liver histology. They found that overall, 14% of patients with NAFLD had evidence of CKD. Of the patients with biopsy proven NASH, 21% had the presence of CKD; and of the patients with NAFLD with no evidence of NASH, only 6% had CKD (64). This was higher than in patients without NAFLD or NASH.  The pathophysiology of this association is not well understood, but the increased atherogenicity associated with NAFLD is likely a contributing factor (20). A more recent meta-analysis also showed a higher prevalence of CKD in patients with NASH when compared with patients with NAFLD without NASH, and a higher prevalence of CKD in patients with advanced fibrosis when compared with patients with lower degree of fibrosis (63).

 

NAFLD and Polycystic Ovarian Syndrome

 

Women with polycystic ovarian syndrome (PCOS) have been found to have an increased prevalence of NAFLD. This association has been present even after adjusting for other factors associated with the metabolic syndrome, such as BMI, hypertension, and type 2 diabetes mellitus (66,67). Evidence of hyperandrogenism, especially with testosterone level > 3 nmol/L has been associated with increased risk of NAFLD in women with PCOS (66,68).  

 

PATHOGENESIS

 

Of note, the pathogenesis of NASH is poorly understood in humans. Most proposed mechanisms at the molecular level have only been observed in cell systems or animal models, but not confirmed in humans. Animal models of NASH are far from ideal in resembling human disease (69). Often treatments that are promising in animal models are in discordance with results in humans – indeed, most treatments that have resolved NASH, and even fibrosis, in mice have failed so far in large RCTs.  A detailed description of the potential pathways leading to steatohepatitis exceeds the scope of this review, therefore we refer the reader to recent in-depth reviews involving a broad spectrum of mechanisms involved in the development of NASH and liver fibrosis (11,69-72).  In Figure 1(below) we propose a schematic representation of the factors and many pathways leading to NASH and fibrosis.

Figure 1: Pathogenies of NAFLD, adapted from Cusi K (11).
PNPLA3=patatin-like phospholipase domain-containing protein 3. TM6SF2=transmembrane-6 superfamily member 2. GCKR=glucokinase regulator. HSD17B13=hydroxysteroid 17-beta dehydrogenase 13. NAFLD=non-alcoholic fatty liver disease. HDL-C=high-density lipoprotein cholesterol. LDL-C=low-density lipoprotein cholesterol. VLDL=very low-density lipoprotein. CETP=Cholesteryl ester transfer protein.

Development of Steatosis

 

Clinical studies have shown that the source of intrahepatic triglycerides in NAFLD is about two-thirds from free fatty acids originating from adipose tissue. However, higher rates of de novolipogenesis (DNL) are also observed in obesity and T2DM (73). In obesity, adipocytes adapt to chronic excess energy supply by undergoing hypertrophy and hyperplasia. This is likely a protective adaptation to allow for an increase in adipocyte storage capacity and ameliorate the potential for ectopic triglyceride accumulation in tissues with limited ability to do so such as the liver, skeletal muscle, pancreas and others. When these adaptive mechanisms are overwhelmed by a chronic excess in nutrient supply, the chronic flux of FFAs promotes a state of “lipotoxicity” across different tissues (11). Adaptation to chronic overnutrition occurs at the expense of developing adipose tissue insulin resistance and triggering mechanisms that attract macrophage accumulation and activation in fat and systemic subclinical inflammation. Moreover, it has been shown that hypertrophic adipocytes share a gene expression pattern that is similar to macrophages and produce adipocytokines similar to those produces by foam cells (74). Adipocytokines have a key role to play in the pathogenesis of insulin resistance by inhibiting insulin signaling pathways via action of insulin receptor substrate (IRS)-1 and c-Jun N terminal kinase (JNK) pathways. Insulin resistance and inflammation is also triggered by the generation of reactive oxygen species, and lipid intermediates such as diacylglycerol (DAG) (75), ceramides (76, 77) and acylcarnitines (77).

 

Normally, insulin decreases gluconeogenesis and increases hepatic synthesis of fatty acids and triglycerides.  Based on animal models of T2DM, it has been postulated that there may be a selective hepatic insulin resistance to glucose metabolism pathways (i.e., inhibition of gluconeogenesis) while preservation of insulin sensitivity at lipid synthetic pathways (78, 79). Selective insulin resistance in the gluconeogenic pathway would explain (at least in part) how hyperinsulinemia may attempt to normalize glucose metabolism at the expense of driving triglyceride synthesis, as hepatic lipid synthetic pathways retain a normal insulin sensitivity, explaining the etiology of both hyperglycemia and hypertriglyceridemia in diabetes.  More recently, Perry et al (80) reported that the major mechanism by which insulin suppresses hepatic glucose production appears to be through a reduction in hepatic acetyl CoA by suppression of lipolysis in white adipose tissue (WAT). This is associated with a reduction in pyruvate carboxylase flux. Of interest, insulin’s ability to inhibit hepatic acetyl CoA and lipolysis is lost in high-fat-fed rats, a phenomenon reversible by IL-6 neutralization and inducible by IL-6 infusion (80).

 

However, the above relationship between hyperinsulinemia and steatosis does not completely explain the role of both factors in patients with NASH. In subjects with hepatic steatosis, increasing insulin levels only have a modest correlation with the severity of intrahepatic triglyceride accumulation (81) and there is no relationship between hyperinsulinemia or hepatic steatosis with the severity of inflammation, hepatocyte ballooning (injury), or fibrosis (15, 81).This is despite patients with NASH having worse hyperinsulinemia compared to patients with isolated steatosis (NAFL). This suggests that other mechanisms play a role in human disease.

 

Lipotoxicity has been extensively studied in skeletal muscle, where accumulation of ectopic triglycerides promotes the formation of toxic lipid metabolites (i.e., such as DAGs) that are closely associated with impairment in insulin signaling. Lipid infusions in healthy subjects have shown that at levels of plasma FFAs typically seen in obesity and NAFLD, there is suppression of insulin signaling and hence development of insulin resistance (82). Lipotoxicity has also reported in pancreatic beta-cells in humans.  Normally, FFAs are the main energy source in the fasting state, with a switch to using glucose as the primary fuel after a meal. However, chronically elevated plasma FFA concentrations impair insulin secretion in subjects that are genetically prone to T2DM (83).

 

NAFLD has been shown to also be a heritable disease (72, 84-90). Nuclear receptors such as peroxisome proliferator-activated nuclear receptors (PPAR) play a key role in hepatic lipid metabolism, however results on association of PPAR and severity of NAFLD have been variable (75). Studies have shown that first-degree relatives of subjects with NAFLD are more susceptible to develop chronic liver disease as compared to the general population (72, 84). Patatin-like phospholipase domain-containing protein 3 (PNPLA3) gene polymorphism has been shown to be associated with worse hepatic steatosis and a worse long-term prognosis in patients with NASH (85). PNPLA3 is usually involved in hydrolysis of hepatocyte triglycerides. This polymorphism results in a loss of function mutation resulting in accumulation of intrahepatic triglycerides. Recently, it has been described that accumulation of PNPLA3 on lipid droplets is the basis of associated hepatic steatosis observed with this polymorphism (86).

 

Another commonly described polymorphism involves transmembrane 6 superfamily member 2 (TM6SF2) which normally plays a role in interaction between triglycerides and Apolipoprotein B during the extrahepatic secretion of very low-density lipoprotein (87). This polymorphism results in increased hepatic triglyceride deposition, and lower circulating lipoproteins. Recent studies show this polymorphism is associated with higher risk of NAFLD but lower cardiovascular risk (87). A loss of function mutation in the glucokinase regulator (GCKR) gene locus has been implicated in the accumulation of hepatic fat (88,89).   Normally, GCKR is involved in controlling the glucose influx into hepatocytes and hence regulating DNL. A protective splice variant HSD17B13 has also been identified. HSD17B13 encodes the lipid droplet protein hydroxysteroid 17-beta dehydrogenase 13 (HSD17B13) (90). This allele was associated with a reduced risk for progression from steatosis to steatohepatitis and fibrosis. Interestingly, it also seems to mitigate the effects of PNPLA3 polymorphism.  Finally, an interesting observation is that individuals with familial hypobetalipoproteinemia (FHBL) are prone to NAFL but are characterized by very low levels of plasma low-density lipoprotein (LDL) cholesterol that is protective against CVD (91).

 

However, at the present time, genetic testing is not recommended in clinical practice as it remains unclear how the presence of a given mutation should modify current management of NASH (92).

 

Development of Hepatocyte Injury and Steatohepatitis: Role of Mitochondrial Dysfunction

 

It should be emphasized that the mechanisms leading to steatohepatitis in humans remain unknown. With limited exceptions that point to subtle defects in mitochondrial function in the liver of subjects with NAFLD and/or T2DM (reviewed in ref. 71), almost all of the available information has been extrapolated from cell culture studies or animal models of NASH.  It is also unclear if NASH is always heralded by isolated steatosis, and what are the drivers of disease.  While there is an increasing recognition that NASH is an heterogeneous disease affecting obese and non-obese individuals, disease progression is often with associated with obesity/weight gain and T2DM. Obvious limitations in obtaining sufficient liver tissue for molecular studies, as well as ethical challenges for performing paired liver biopsies before and after a given intervention, have greatly hampered our ability to make significant progress in understanding the pathogenesis of NASH in humans.  However, factors associated with overnutrition and insulin resistance likely play a role in the maladaptation of mitochondrial oxidative function that leads to inefficient oxidative flux, accumulation of lipotoxic intermediates and the progression from isolated steatosis to NASH (71,93). As mentioned above, genetic factors may also regulate lipid droplet accumulation that may exacerbate disease progression.  Many other trigger factors associated with endoplasmic reticulum (ER) stress, oxidative stress and inflammasome activation have been described.  However, the exact temporal relationship and sequence of events remains elusive.

 

Normally, there is a close regulation between beta-oxidation, hepatic tricarboxylic acid (TCA) cycle activity, ketogenesis and ATP synthesis. Normally, FFAs influx is efficiently dealt through beta-oxidation. However, in states of chronic overfeeding, beta-oxidation can over time become relatively ineffective, resulting in the accumulation of hepatocyte ceramides and DAGs (as well as acylcarnitines), as seen in states of hepatic steatosis (71,75-77). As summarized in Figure 2, the current working hypothesis in NASH is that overactive hepatic TCA cycle carries the risk of overloading the mitochondrial electron transport chain and hence promoting not only the formation of toxic metabolites but the production of reactive oxygen species (ROS) and other inflammatory mediators. In this setting, it is believed that inflammatory pathways are triggered which then lead to hepatocyte necrosis and chronic inflammation, Kupffer cell activation and recruitment, as well as hepatic stellate cell activation. This disruption of the normal equilibrium between hepatocyte and its microenvironment (i.e., in particular with Kupffer cells and hepatic stellate cells, the latter promoting fibrogenesis) seems to determine the degree of hepatocyte injury and the triggering of downstream pathways that lead to cirrhosis, as reviewed in-depth elsewhere (70).  However, while many recent interventions successful in animal models have failed in humans, it is of interest that there is a  correlation between successful treatment for NASH in humans (with GLP-1RA or pioglitazone [8]) with studies in vivowith such interventions that restore hepatocyte TCA function and reduce intracellular toxic lipids (94, 95), giving support to the hypothesis of increased mitochondrial FFA flux as a potential therapeutic target for patients with NASH. 

Figure 2. Hepatic Mitochondrial Oxidative Dysfunction during NASH (71). Adipose tissue insulin resistance results in increased lipolysis and higher flux of FFAs into the liver (1), resulting in high rates of hepatic triglyceride accretion (2). Initial breakdown of FFA in the liver proceeds through b-oxidation, generating two-carbon units of acetyl-CoA (3). During hepatic insulin resistance, disposal of acetylCoA units through ketogenesis undergoes an early compensatory induction in simple steatosis, but is impaired in NASH (4). In spite of FFA overload, hepatic insulin resistance and steatosis result in beta-oxidation being inefficient and incomplete as evident from accumulating levels of hepatic ceramides, DAGs, and long-chain acylcarnitine (5). However, complete oxidation of acetyl-CoA units through the mitochondrial TCA cycle continues unabated during simple steatosis and NASH (6), potentially to meet the energetic demands of maintaining high rates of gluconeogenesis (7). The chronically elevated oxidative flux through TCA cycle during NASH has the potential to uncouple hepatic TCA cycle activity from mitochondrial respiration (8) by disrupting the mitochondrial electrochemical gradient and to impair ATP synthesis (9). This mitochondrial milieu could be a chronic source of ROS generation (10) and cellular inflammation, and could be a target for therapeutic manipulations. Abbreviations: Cer, ceramides; CoA, coenzyme A; DAGs, diacylglycerols; FFAs, free fatty acid; NASH, nonalcoholicsteatohepatitis; PEP, phosphoenolpyruvate; ROS, reactive oxygen species; TCA, tricarboxylic acid.

However, linking NASH only to altered mitochondrial flux is obviously an oversimplification of a complex web of many factors at play.  Other pathways that have been implicated in hepatocyte injury and the development of NASH, although rather broadly, include cholesterol accumulation in hepatocytes (96) and a tangled web involving activation of apoptotic pathways with ER stress and abnormal unfolded protein response (97), as well as defects in autophagy (98). Recently, inflammasome activation has gained attention as it integrates many cytoplasmic signals into danger-associated molecular patterns (DAMPs) from diverse sources such as intracellular lipids to the gut microbiome (97, 98).

 

Diet and gut microbiota have been repeatedly implicated to play a role in the pathogenesis of NAFLD.  In particular, fructose appears to play a role in NASH by stimulating DNL and suppressing -oxidation of FFAs, hence leading to hepatocyte injury (99). Many studies have shown that excess fructose consumption, usually as sugar-sweetened beverages with sucrose (converted to fructose and glucose after ingestion), is associated with development of NAFLD and NASH.   Obesity is also associated with a change in gut microbiota that produce more reactive oxygen species and are involved in triggering a variety of inflammatory pathways (100). However, the causative role of the gut microbiome in the development of T2DM or NAFLD remains overall poorly understood (101).  

 

Development of Liver Fibrosis

 

Here too the data in humans is scarce and largely limited to in vitroand in vivoevidence. Potential mechanisms linked to the development of NASH have focused on hepatocyte apoptosis with the release of a broad spectrum of cytokines (e.g., interleukins [-1, -2, -18], hedgehog ligands, TNF-, TGF-, and many others) (11, 97, 98). Wang et al (102) have identified one such pathway (the transcriptional activator TAZ) that appears to play an important profibrogenic role in NASH in a mouse model of NASH. Taken together, this extensive signaling network, triggered by injured hepatocytes, activates nearby Kupffer cells that induce hepatic stellate cells to become myofibroblasts and increase the production of matrix proteins that result in cirrhosis over time. Genetics also appear to play a role as the PNPLA3-I148M variant may not only modify lipid droplet metabolism but have a direct role on stellate cell function in NASH (103). Recently, Lindén et al (104) reported a reduction in liver inflammation and fibrosis in a Pnpla3 knock-in 148M/M mutant mice (with a human PNPLA3 I148M mutation) with a liver-targeted GalNAc3-conjugated antisense oligonucleotide (ASO)-mediated that silenced Pnpla3 expression.

 

At a clinical level, a recent study examined factors associated with disease progression in a large (n = 475 patients) clinical trial (105).  The main factor associated with clinical disease progression is severity of fibrosis at baseline and greater increases in hepatic collagen content, level of alpha-smooth muscle actin, and Enhanced Liver Fibrosis score overtime.  Over a follow-up period of 96 weeks, progression occurred in 22% of patients with bridging fibrosis (F3), while liver-related clinical events occurred in 19% of patients with cirrhosis.  

 

Beyond liver histology, from a clinical perspective, practitioners must keep in mind that obesity and T2DM remain the two major risk factors for liver disease progression which calls for screening and early intervention.

 

DIAGNOSIS

 

Having T2DM is associated with a much greater risk of NAFLD with approximately 70% of patients with T2DM having NAFLD when MRI-based techniques are used, as well as higher risk of having more advanced forms of the disease, such as fibrosis and cirrhosis (6,8,106). Despite all the current evidence, there is lack of awareness in primary care physicians and endocrinologists to evaluate patients with prediabetes or type 2 diabetes mellitus for NAFLD. Even if suspected to have NAFLD based on clinical characteristics, there is currently a lack of further investigations being undertaken as non-invasive biomarkers of the disease and even imaging, are not as reliable as wished and not available at every clinic. The widely accepted thought by primary care physicians, as well as many endocrinologists, is to not pursue any confirmatory testing to assess for the presence or degree of fibrosis, as it is believed to seldom change their management, except to re-emphasize lifestyle modifications and weight loss.  However, few healthcare providers are aware about the efficacy of lifestyle changes and some currently available pharmacological agents to revert NASH, and even fibrosis, if done early and before the development of end-stage liver disease.

 

Early detection and treatment of NAFLD can lead to better histological and metabolic outcomes, including CVD, and improve overall morbidity and mortality. NAFLD is a diagnosis of exclusion, so it is imperative to eliminate all other causes of liver disease (such as, alcoholic liver disease, medication induced toxicity, viral or autoimmune hepatitis, hemochromatosis, alpha 1 antitrypsin deficiency, Wilson’s disease, other) prior to the diagnosis of NAFLD. Often management may require referral to hepatology and developing multidisciplinary teams (107, 108). Once NAFLD is diagnosed, there needs to be further testing to evaluate for the presence and severity of fibrosis (8).

 

Blood Tests

 

Plasma aminotransferases are considered an insensitive marker for the presence of NAFLD. It has been shown that the prevalence of NAFLD may be as high as 50% in patients with T2DM and “normal” (≤40 IU/L) plasma aminotransferases using 1H-MRS for the detection of hepatic steatosis (109). Of note, 56% of these patients had a diagnosis of NASH on liver biopsy, highlighting that reliance on ALT/AST alone may be an inadequate approach for the systematic detection of NASH in endocrinology or primary care clinics (50). Maximos et al (110) have reported a comparable degree of NASH in patients with normal vs. abnormal levels of plasma aminotransferases, emphasizing the non-reliability of plasma aminotransferases as clinical biomarkers for presence or severity of disease, a finding consistent with the literature by others (6,9,111).  Factors affecting elevation of plasma aminotransferases included adipose tissue insulin resistance and intra-hepatic triglyceride content, rather than hepatic insulin resistance (110). There is some evidence to suggest lowering the optimal threshold for considering plasma alanine transferase (ALT) as normal to be ≤30 U/L in men and ≤19 U/L in women (112). This increases the sensitivity of this screening method. Plasma ALT is usually more elevated than AST in the presence of NAFLD and NASH, unless there is advanced disease or cirrhosis, when AST usually increases.

 

Significant efforts have been made in finding the ideal biomarker panel for the diagnosis of NAFLD/NASH. Simple metabolic algorithms such as fatty liver index (using measures, such as BMI, waist circumference, triglyceride levels, and GGT) used for diagnosis of NAFLD have not been shown to be very reliable when compared with more accurate and advanced techniques, such as 1H-MRS (113). It is not a test for the diagnosis of inflammation or fibrosis (114).

 

Several biomarker clinical scores (using different measures, such as AST, ALT, BMI, platelets, albumin, T2DM) have been developed to evaluate for the presence and degree of liver fibrosis (8). These tests are listed in the Table 3. Among these, only the NAFLD fibrosis score and FIB-4 have been confirmed across a broad spectrum of populations and considered the most reliable for the exclusion of advanced fibrosis (115). It is apparent that these scores are only able to distinguish relatively well between the two extremes – a population without evidence of NAFLD and a population with advanced fibrosis (F3-4). Most times, results fall in an intermediate or undetermined range, thus are not able to accurately classify patients in the spectrum of mild (F1) to moderate (F2) disease (9). These scores are also limited for use in population without T2DM. They have not been shown to be very reliable in this specific high-risk population of patients with T2DM (9).

 

Table 3. Biomarkers Available for use in Diagnosis of Advanced Fibrosis (Stages 3 or 4). Modified from reference (8)

Test

Parameters included

number

PPV

NPV

Patients unable to be classified “grey zone”

NAFLD fibrosis score

Age, BMI, diabetes, AST/ALT ratio, platelets, albumin

733

82%

88%

24%

Fibrotest

Age, sex, total bilirubin, GGT,

a2-macroglobulin, apolipoprotein A1, haptoglobin

267

60%

98%

32%

FIB-4 index

Age, AST and ALT, platelets

541

80%

90%

30%

BARD score

BMI, diabetes, AST/ALT ratio

827

43%

96%

N/A

NAFIC score

Ferritin, type IV collagen, insulin

619

36%

99%

15%

Hepascore

Age, sex, total bilirubin, GGT,

a2-macroglobulin, hyaluronic acid

242

57%

92%

11%

N/A, not applicable; NPV, negative predictive value; PPV, positive predictive value.

No independent validation cohort included in the study.

 

Some commercially available tests based on a metabolomic profile have been tested as a novel means to evaluate for NAFLD or NASH and recently tested in patients with type 2 diabetes mellitus. These tests have shown some promise to distinguish between normal liver and NAFLD and also able to detect NASH in people without diabetes. However, when applied to a population with T2DM, these tests have not been as accurate as expected to predict presence of NAFLD, NASH or fibrosis (115, 116). There is an increasing interest in assessing the utility of novel biomarkers, such as plasma fragments of propeptide of type III procollagen (PROC3) for the detection of liver fibrosis in patients with T2DM.  A recent study reported that PRO-C3 performed well (overall similarly to APRI or FIB-4) but with the added value of predicting histological changes in fibrosis stage with treatment (117). However, more studies are needed to determine its real value to monitor therapy.  At the present time, available genetic tests include PNPLA3 and TM6SF2 and a few others (as described above), but they are not routinely performed at this time and limited to academic centers for research only. This is likely to change in the near future as more sophisticated genetic testing becomes available.

 

In summary, clinicians may use plasma aminotransferases or simple panels such as FIB-4 or NAFLD fibrosis score to identify patients at the highest risk of having NASH with advanced fibrosis (F3-4) in the clinic, but knowing that while the specificity may be acceptable (“rule out” advanced fibrosis or cirrhosis) the sensitivity is rather low.  A screening strategy should include the above and imaging as described below as ultrasound and/or controlled attenuation parameter (CAP) have better sensitivity for the diagnosis of steatosis. The 2019 American Diabetes Association (ADA) guidelines for the first time recommend screening to identify liver fibrosis in patients with prediabetes or T2DM with elevated plasma aminotransferases and/or steatosis (118).

 

Imaging Modalities

 

MEASUREMENT OF INTRAHEPATIC TRIGLYCERIDES

 

Liver Ultrasound

 

Ultrasound is a relatively low-cost technique that is widely availability. Because of this it is routinely used for the diagnosis of NAFLD. However, it should be noted that the sensitivity of the test can be widely variable due to differences in operator technique and devices available, definition of steatosis, use of different echographic parameters to define steatosis, as well as the heterogeneity of the liver disease. While in one meta-analysis liver ultrasound was found to have a pooled sensitivity of 84.8% and specificity of 93.6% to detect hepatic steatosis of more than 20-30% (119), this literature is largely from liver clinics where disease severity is greater (i.e., more steatosis and better performance) but may not reflect the setting of primary care physicians or endocrinologists.  More relevant was also the fact that the investigators calculated the sensitivity to diagnose moderate-to-severe fatty liver from the absence of steatosis, without considering mild-to-moderate NAFLD. However, clinicians are faced with many patients with NAFLD that have only mild-to-moderate intrahepatic triglycerides, emphasizing the importance of having simple imaging tools that can make the correct diagnosis in the clinic.

 

In a study by Bril et al (120), the authors compared in 146 patients the performance of ultrasound using a score from five echographic parameters for steatosis or liver fat quantified by1H-MRS. They used as the gold-standard histology (liver biopsy). They reported that the performance of liver ultrasound (parenchymal echo alone) was relatively poor but improved to an acceptable level when compared to 1H-MRS when enhanced by the five echographic parameter score for steatosis was utilized. The greatest sensitivity of the ultrasound test was reached at a hepatic steatosis content of at least 12.5%. Below this threshold, the test was unreliable. Technological improvements may enhance in the near future the performance of liver ultrasound and its value in the management of patients with NAFLD.

 

Controlled Attenuation Parameter (CAP)

 

CAPis a relatively new imaging methodology to quantify steatosis. It is based on the principle that intrahepatic triglycerides delay ultrasound waves, so that when travelling through tissue with steatosis they will be attenuated when compared to normal liver tissue.  The diagnostic range of CAP is from 100 to 400 dB/m. The higher the value the more suggestive of the presence of steatosis. The sensitivity of the test to diagnose hepatic steatosis was 68.8% and specificity was 82.2% in a meta-analysis of patients with biopsy-proven steatosis (121). Usually the cut-off of ≥280 dB/m is used to establish the diagnosis of steatosis. As discussed below, one advantage of CAP is that in addition to being a simple and useful point-of-care tool (often available in liver clinics), the estimation of CAP can be performed simultaneously with that of the liver stiffness measurement (LSM; FibroscanÒ) and from the same liver region of interest, significantly facilitating clinical management although the test has its limitations when liver fat is only mildly elevated.

 

MR Spectroscopy

 

MR-based techniques have been the most accurate procedure to quantify liver triglyceride content (122). The use of 1H-MRS has proven to be very accurate for quantification of intrahepatic triglyceride content, with the results correlating well with steatosis on histology (120). MR spectroscopy derived proton density fat fraction (MR-PDFF) has recently evolved into a simpler and easier to standardize method for multicenter studies examining the effect of liver steatosis of new agents for the treatment of NASH (9,108,114). It has shown better diagnostic and grading capabilities for liver steatosis when compared with controlled attenuation parameter modality using transient elastography (122). However, MR spectroscopy remains an expensive test available mostly in academic centers, and requires special expertise for performance and analysis of the test (108).

 

MEASUREMENT OF LIVER FIBROSIS

 

Liver Stiffness Measurement (LSM; FibroscanÒ)

 

Liver fibrosis can be assessed in the clinic or bedside by measuring the “stiffness” of the liver.  The LSM is estimated by using vibration controlled transient elastography or VCTE (FibroscanÒ) to assess presence and severity of fibrosis. This modality also allows for a reasonably accurate quantification of the degree of fibrosis and hence, prognosis (108,92).  It is a quick (10 minutes), easy, and economical tool for assessment, however the test requires a 3-hour fast, and in obese people liver fibrosis cannot be always estimated and performance is worse (particularly when BMI ≥40 kg/m2) (108). At present, this test is not FDA-approved to be performed in patients with a pacemaker or during pregnancy.

 

MR Elastography

 

This modality is based on the same principle of liver “stiffness” as VCTE but it is a MR-based technique that has a sensitivity of 86% and specificity of 91% for assessment of degree of fibrosis (108). It is shown to be superior to VCTE, especially in diagnosis of early as well as advanced stages of fibrosis and cirrhosis. It is however much more expensive, requires special expertise to perform, and the current availability is limited. It also needs to take into account patient’s size and weight, any metal implants, as well as anxiety and claustrophobia during the procedure (92,108).

 

Liver Biopsy

 

Liver biopsy remains the gold-standard for diagnosis of NASH and for assessing the degree/severity of fibrosis (94). It is the only modality to reliably distinguish between steatosis alone from NASH and advanced fibrosis and to eliminate other etiologies of liver disease 108,123-125).

 

The degree of liver disease on histopathology is graded on a score that has been developed, called the NAFLD activity score (NAS). NAS score ranges from 0-8 and includes three parameters that are graded separately – steatosis (0-3), hepatocellular ballooning (0-2), and lobular inflammation (0-3). The degree or stage of fibrosis is graded separately from 0-3. These scores and staging ranges allow for a more accurate and reproducible way of monitoring of disease (108,123,125). However, there are limitations involving liver biopsies as well due to the inter-pathologist variability in interpretation of grades and degree of steatosis, inflammation and fibrosis (92,124).

 

Despite all the current advances, there remains an urgent need for development of more cost-effective and reliable methods for non-invasive screening of NAFLD to ensure early and prompt diagnosis for the best treatment outcomes.

 

TREATMENT

 

The aim of treatment for patients with NASH is to delay or reverse the progression of fibrosis and improve NASH-related morbidity/mortality due to hepatic (cirrhosis and HCC) and extra-hepatic complications, mainly cardiovascular disease (CVD). Currently there is no pharmacotherapy approved by regulatory agencies for the treatment of NAFLD, although pioglitazone is recommended by the current guidelines as a choice for patients with or without T2DM, and vitamin E for patients without diabetes (92,124). The FDA has accepted 2 endpoints as valid ones for drug approval in clinical trials: a) Resolution of the histologicalfindings that define NASH (necroinflammation) without worsening of fibrosis, and b) Reversal of ≥1 fibrosis stage without worsening of steatohepatitis/NASH (124,126,127). Despite the many ongoing efforts to find novel pharmacological agents the first-line of treatment will always be lifestyle modification including diet, exercise and weight loss (92,124), to combat insulin resistance and the relatedconditions like diabetes and obesity so closely related to NAFLD (128-130). 

 

Weight loss: Lifestyle, Bariatric Surgery and Weight Loss Agents

 

Numerous studies have shown the beneficial effect of weight loss to improve hepatic steatosis. It has been reported that weight loss not only improvesliver steatosis and other histological features of NASH (including fibrosis) but can decrease insulin resistance and blood pressure as well as improve atherogenic dyslipidemia (elevated LDL-C and triglycerides, low HDL-C) (92,131). In a meta-analysis of eight trials including 373 patients, improvement in hepatic steatosis was seen in patients who lost ≥5% of body weight, while NAFLD activity score (NAS) improvement was associated with weight loss of ≥7% body weight (132). In another randomized well-controlled trial paired with liver biopsy, weight loss and exercise program resulted in improvement of NASH. Moreover, this study showed that the magnitude of weight loss correlated strongly with improvement in histology (133). However, even with intensive multidisciplinary lifestyle interventions, more than half of patients were unable to achieve the weight loss target (weight loss of ≥7% body weight) which makes patient compliance the mainconcern (132). Despite the presence of multiple studies that correlates weight loss with the improvementof histological disease in NASH, little is known about the long-termeffect (i.e. beyond 1 year) of weight loss on liver histology (8).

 

Weight reduction of 10% by lifestyle modification may cause a significant regression of fibrosis (133,134). A greater and a more sustained over time decrease in weight loss with improvement in steatohepatitis, and even fibrosis, can be achieved by bariatric surgery (92,135,136). In a systematic review that included 21 observational studies of bariatric surgery in patients with NASH, an improvement in steatosis was reported in 18 studies, decreased inflammation was reported in 11 studies and improvement in fibrosis was reported in 6 studies (137). Only four studies reported some (minor) worsening of fibrosis (137). However, most bariatricsurgery studies have some limitations: these include small size, lack of proper standardization of preoperative low-caloric diet, frequent dropouts, and often no standardized time after the repeat postoperative liver biopsy. Finally, there are no randomized clinical trials (RCT) that compare bariatric surgery versus conservative management in patients with NASH with liver histology as the primary endpoint (137,138). Weight loss agents had no specific liver benefit (131), but can help with weight control and cause improvement in plasma aminotransferases and liver histology (139,140).

 

Adding regular moderate-intensity aerobic exercise/resistance training is highly encouraged as a lifestyle intervention for NAFLD. Exercise not only improves steatosis but the high cardio-metabolic risk profile, even in the absence of significant weight loss (92,124,141). In an uncontrolled study of 293 patients paired with liver biopsies, one year of structured exercise (walking 200 min/week) combined with ahypocaloricdiet improved hepatic steatosis and necroinflammation (133). In order to sustain weight loss, most dietary recommendations for NAFLD reflect a combination of hypocaloric diet (500–1000 kcal/day energy deficit) with exercise (92,134).

 

Heavy alcohol consumption should be avoided by patients with NAFLD and NASH.  Heavy drinking is defined as four standard drinks on any day or more than 14 drinks per week in men, or more than three drinks on any day or seven drinks per week in women (92).  There are no longitudinal studies reporting the effect of ongoing alcohol consumption on disease progression or the natural history of NAFLD or NASH.

 

Pharmacological Agents with Evidence from RCTs for the Treatment of NASH

 

Pharmacologic treatment has been extensively studied for patients with NASH with or without diabetes mellitus. For patients with NASH and T2DM, the typical initial therapy is with metformin. However, randomized controlled trials did not show improvement in liver histology (92,142).

 

Given that insulin resistance is a core feature in the pathogenesis of NAFLD/NASH, thiazolidinediones (TZDs), targeting the transcription factor PPAR gamma in adipose tissue and other tissues, has been tested in several RCTs in patients with NASH (3).  Pioglitazone at the molecular level modulates glucose and lipid metabolism and improves adipose tissue and hepatic insulin signaling and insulin sensitivity, collectively leading to improved liver histology in patients with NASH (143-149).However, the exact mechanism of action in humans is unknown and likely involves other pathways, for instance, activation of a mitochondrial pyruvate carrier (MPC) and/or PPAR alpha effects that may enhance mitochondrial fatty acid oxidation.  A recent study in vitro and in vivosuggested effects independent of activation of MPC (150). Of note, when rosiglitazone was comparedto placebo in patients with NASH it did not show any improvement beyond a reduction in steatosis as hepatocyte necrosis, lobular inflammation and fibrosis were unchanged (151). This suggests that improvement in fibrosis is not necessarily due to PPAR gamma as rosiglitazone is strictly a PPAR gamma agonist while pioglitazone is a considered a weaker agonist that also has PPAR alpha activity. Of note, different PPAR gamma activators do not modulate function or increase the expression of identical genes. The expression profiles can vary, which can explain differential effects via PPAR gamma activation.

 

Pioglitazone has been the agent most studied to date in patients with and without diabetes and biopsy-proven NASH (143-149), as recently reviewed in-depth along with other medications to treat diabetes regarding their effect in NAFLD (152). Resolution of NASH with pioglitazone treatment has been fairly consistent across studies of 6 to 36 month duration and ranges from ~47% (or 29% placebo-subtracted) in patients without diabetes with pioglitazone 30 mg/day for 24 months (94), to ~60% (or ~40% placebo-subtracted) with pioglitazone 45 mg/day in those with prediabetes or T2DM treated for 6 to 36 months (143,148, 149).  Taken together, these results suggest that pioglitazone might play a role in modifying disease progression and its natural history in patients with or without diabetes.

 

In addition, pioglitazone may improve the cardiometabolic profile of patients with NASH by reducing progression to diabetes and CVD.  Many patients with obesity and NAFLD/NASH have (often undiagnosed) prediabetes. Pioglitazone has proven effective for the prevention of diabetes in subjects with prediabetes (153) and shown to ameliorate cardiovascular events in patients with metabolic syndrome or prediabetes with a history of a stroke.Recently, the IRIS study reported the effect of pioglitazone in patients that had taken ≥80% of the prescribed medication reduced stroke by 36%, acute coronary syndromes by 53%, and the combined endpoint of stroke/MI/hospitalization for heart failure by 39% (154).

 

However, it remains puzzling that for a population with such a high cardiovascular risk from having obesity, T2DM and NASH, the cardiometabolic benefits of pioglitazone are frequently dismissed because of potential side effects that can be mitigated with close monitoring: bone loss, weight gain (3-5%) (most usually associated with improved insulin action on adipose tissue, not edema),or lower extremity edema in ~5% but higher if on amlodipine or high-dose insulin (152,155). Consistent with diabetes prevention and CVD reduction (156-160), patients become more metabolically healthy despite weight gain (143,149). While pioglitazone improves left ventricular function in healthy patients with T2DM (161), it may trigger heart failure in patients who have fluid retention and subclinical (undiagnosed) heart failure with preserved left ejection fraction (HFpEF), also known as “diastolic dysfunction” (≤1%) (155).  Obese patients with T2DM and NASH are more prone to HFpEF (162). Therefore, in our experience, this can be avoided if pioglitazone is not prescribed to poor candidates, such as those with long-standing history of severe CVD that could be associated with heart failure, baseline presence of unexplained shortness of breath or lower extremity edema, severe obesity (BMI ≥40 kg/m2), or longstanding diabetes on high-dose insulin.  Concomitant use of amlodipine, that is often already associated with lower extremity edema, should also be avoided.  The clinician suspecting HFpEF may consider ruling this condition out before initiating therapy.  Options to this end are ordering a transthoracic echocardiogram or plasma N-terminal (NT)-pro hormone B-type natriuretic peptide (NT-proBNP), the non-active prohormone from BNP. Both BNP and NT-proBNP are released in response to changes in cardiac pressure with plasma levels increasing when heart failure develops or worsens (162).

 

There is significant controversy about the risk of bladder cancer with pioglitazone and unlikely ever to be resolved given the overall low frequency of bladder cancer in the general population.  A recent 10-year prospective study was negative for bladder cancer (163) and there was no association found in a recent meta-analysis comparing patients who had been ever vs. never users of pioglitazone, but there was a small but significant association with 1–2 years (HR = 1·28 [1·08–1·55]) and >2 years (HR = 1·42 [1·14–1·77]) of exposure (164). In absolute terms, bladder cancer developed in <0.3% of patients both exposed and not exposed to pioglitazone. The numbers needed to treat for one additional case of bladder cancer ranged from 899 to 6380 (median of 2540), while the benefit for CVD and NASH ranged from 4–256 and 2–12, respectively.

 

Taken together, pioglitazone is an evidence-based treatment option for patients with and without diabetes and NASH (92). It is also a generic medication recommended by the current ADA and EASD guidelines as a low-cost option, along with sulfonylureas, for the management of T2DM.  Pioglitazone is likely to become for patients with NASH what metformin is for the management of T2DM, an inexpensive and effective option offering liver histological and cardiometabolic benefit and likely to be combined with novel therapeutic agents under development.

 

Glucagon-like peptide 1 (GLP-1) receptor agonists are another group of pharmacologic agents widely used for the treatment of diabetes that also have significant cardiometabolic benefits. A recent review summarized the many studies that have tested GLP-1RAs in patients with NAFLD (152). Typically, treatment is associated with weight loss and a decrease in plasma aminotransferases and hepatic steatosis.  In the only study to date examining their role in NASH, Armstrong et al (165) randomized 52 patients with NASH to receive either liraglutide or placebo for 48 weeks. NASH resolved in nine patients (39%) who received liraglutide compared to two patients (9%) in the placebo group (RR 4.3; 95% CI 1.0-17). Patients who received liraglutide were less likely to have progression of fibrosis (9 versus 36 percent; RR 0.2; 95% CI 0.1-1.0). These results are consistent with most other controlled and uncontrolled trials with liraglutide and other GLP-1RAs that have consistently led to weight loss and a reduction hepatic steatosis on imaging and in plasma aminotransferases in patients with NAFLD (166). In contrast, DPP-IV agents have largely been ineffective in RCTs in NAFLD (166).

 

The sodium–glucose cotransporter 2 (SGLT2) inhibitors have a significant role in the management of patients with T2DM (167). They promote weight loss, reduce the risk of CKD and of heart failure, and decrease overall rates of cardiovascular events in patients with T2DM (168). Several studies in animal models of NAFLD have reported that this class of agents reverses hepatic steatosis and necroinflammation. Early studies reported improvements in plasma aminotransferases and hepatic steatosis (152).Recent controlled RCTs have reported a (modest) reduction in hepatic steatosis on imaging with canagliflozin (169) and dapagliflozin (170) in patients with T2DM and NAFLD.  These findings combined with their attractive properties of weight loss and decreasing diabetic comorbidities would make them potentially valuable for combination therapy (i.e., pioglitazone) for patients with NAFLD, as shown from combination therapy trials in patients with T2DM (171-173).

 

Finally, it is important to mention vitamin E as it has been examined in RCTs for the treatment of NASH in patients with (149) and without (147) T2DM.   In a study in patients with NASH but without diabetes, vitamin E showed improvement in the primary outcome, but had borderline efficacy for resolution of NASH (considered today a more relevant outcome) compared to placebo (36% vs. 21%; p = 0.05) and numerically appeared as less significant compared to pioglitazone (47%; p = 0.001 vs. placebo) (147).Recently, Bril et al (149) found that vitamin E alone appeared to not be as effective in patients with T2DM, as it failed to meet the primary outcome of a two-point reduction in the NAFLD activity score from two different parameters, without worsening of fibrosis.  However, when vitamin E was combined with pioglitazone more patients on combination therapy achieved the primary outcome versus placebo (54% vs. 19%, P = 0.003) although the efficacy did not seem to be greater than that with pioglitazone alone in previous trials (143, 148). Resolution of NASH occurred in both groups compared with placebo (combination group: 43% vs. 12%, P = 0.005; vitamin E alone: 33% vs. 12%, P = 0.04).

 

Other relevant group of agents tested in NASH include the lipid-loweringdrugs (e.g. statins, colesevelam, omega 3 fatty acids, fibrates and niacin), whichhave not shown much success when studied in clinical trials in patients with NASH (174-179).

 

The Future: Many Agents on the Horizon for NASH

 

Given the rapid evolution of the field, with constant new drugs entering the arena of trials and others failing, we to refer the reader to recent in-dept reviews on the topic (180,181). Many pharmacological agents are being tested in phase 2 and phase 3 trials targeting a broad spectrum of pathways involved in the pathogenesis of NASH. Therapeutic targets of significant interest include farnesoid X receptor (FXRs), which regulate hepatic glucose and lipid metabolism (182). In the FLINT trial (183), in which obeticholic acid (manufactured by Intercept) was compared to placebo there was some evidence of histological improvement, including a mild effect on fibrosis that was recently confirmed in the Interim Analysis of the Phase 3 REGENERATE trial but showed no improvement in resolution of NASH (184). Unfortunately, a significant number of patients complain of pruritus and there was a worsening of dyslipidemia that can be mitigated by co-administration of statins (183).  Several novel FXR compounds are in development (180,181).

 

As discussed, PPAR nuclear receptors play a key role in insulin sensitivity. In the light of their roles in NAFLD and NASH several combined PPAR agonists have been studied. Elafibranor (manufactured by Genfit), is a dual receptor PPAR-α/δagonist that improves in insulin resistance and glucose/lipid metabolism (185). In the GOLDEN trial, a phase study 2b study, elafibranor 120 mg/day for a year led to a modest improvement in resolution of NASH compared to placebo in the subgroup with worse steatohepatitis (186).Another PPAR agonist is lanifibranor, a panPPAR agonist (PPAR-α/δ/ γ), is currently undergoing phase 2 clinical trials in NASH (187).Saroglitazar (by Zydus), is a dual PPAR-α/γ agonist with a predominant PPAR-α activity, reverses steatohepatitis in experimental NASH models (188)and is undergoing clinical trials. A phase 2b RCT of MSDC-0602K by Cirius is expected to report results in late 2019 for the treatment of NASH. It is a compound designed to minimize PPAR gamma binding activity but to maintain binding affinity to a second cellular target of all TZDs that has been identified as the mitochondrial target of the TZDs (mTOT) or mitochondrial pyruvate carrier (MPC) (189). Other insulin-sensitizers in earlier stages of development include PXL-065 (by Poxel), an enantiomer of pioglitazone, and CHS-131 (by Coherus) a compound with PPARγ activity tested earlier in patients with T2DM.

 

Other pharmaceutical compounds being tested for the treatment of NASH aim at a variety of potential pathways. We will mention only a few examples for the reader to appreciate the broad spectrum of targets being studied. Aramchol (by Galmed) is a novel compound that downregulates stearoyl-CoA desaturase 1 (SCD1), a key enzyme involved in triglyceride biosynthesis (190). Inhibition of de novolipogenesis (increased in NASH) by an inhibitor of acetyl-CoA carboxylase (ACC), the rate limiting enzyme in this pathway, is also being studied in RCTs in patients with NASH (GS-0976, Gilead) (191,192). Fibroblast growth factor (FGF)-19 functions as a hormone that regulates bile acid metabolism with effects on glucose and lipid metabolism (193).NGM282 (NGM Biopharmaceuticals) is an engineered analogue of FGF-19 for the treatment of NASH with promising early results (194). Several companies are testing analogues of FGF21 that have significant metabolic effects on glucose and lipid metabolism as well as hepatic fat (180, 181). Thyroid hormone receptor (THR) β-selective agonists, appear to specifically target the liver and improve steatohepatitis in animal models and early clinical trials in patients with NASH (195,196). Many other agents are being tested at this time.

 

CONCLUSION

 

Endocrinologists must be aware that NAFLD is a potentially severe disease in patients with T2DM, due to both its hepatic and extrahepatic complications.  In 2019 the ADA included for the first time in its recommendations to implement regular screening for advanced fibrosis in all patients with prediabetes or T2DM with evidence of elevated plasma aminotransferases or steatosis, so an early diagnosis can prevent long-term complications (118).This is the first step of management while being aware of the significant need for accurate and cost-effective diagnostic modalities and for continued research efforts for new treatments.

Figure 3 is a suggested algorithm to be used for endocrinologists and primary care settings when evaluating a patient with prediabetes or T2DM for the possibility of having NASH. 

In the future, we anticipate that patients with T2DM will be routinely screened for NASH in the same way they are today for diabetic retinopathy or nephropathy.

Figure 3. Management of patients with prediabetes or type 2 diabetes mellitus and suspected NAFLD. Based on figure from reference (8). *High risk patients include patients with type 2 diabetes > 10 years, A1c > 8.5%, triglycerides > 250mg/dl, evidence of steatosis based on MR imaging or controlled attenuation parameter (CAP), or genetic testing (PNPLA3 and/or TM6SF2).

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Glucagon Physiology

ABSTRACT

 

Glucagon is a peptide hormone secreted from the alpha cells of the pancreatic islets of Langerhans. Hypoglycemia is physiologically the most potent secretory stimulus and the best-known action of glucagon is to stimulate glucose production in the liver and thereby to maintain adequate plasma glucose concentrations. However, glucagon is also involved in hepatic lipid and amino acid metabolism and may increase resting energy expenditure. Based on satiety-inducing and food intake-lowering effects of exogenous glucagon, a role for glucagon in the regulation of appetite has also been proposed. This chapter provides an overview of the structure, secretion, degradation and elimination of glucagon, and reviews the actions of glucagon including its role in glucose metabolism and its effects on lipolysis, ketogenesis, energy expenditure, appetite and food intake. Finally, the role of glucagon in the pathophysiology of diabetes, obesity and hepatic steatosis is discussed and emerging glucagon-based therapies for these conditions are outlined.

 

INTRODUCTION

 

Glucagon secreted from pancreatic alpha cells in the islet of Langerhans plays an important role in maintaining glucose homeostasis by stimulating hepatic glucose production (1). Thus, in contrast to the glucose-depositing nature of insulin action, glucagon acts as a glucose-mobilizing hormone. In line with these opposed actions, high plasma glucose concentrations stimulating insulin secretion from pancreatic beta cells, inhibit glucagon secretion whereas low plasma glucose concentrations represent one of the most potent glucagon secretory stimuli. Accordingly, normal plasma glucose concentrations depend largely on the balanced secretion of insulin and glucagon from the pancreatic beta cells and alpha cells, respectively. The hyperglycemic effect of glucagon was described as early as 1922 by Kimball and Murlin who discovered a hyperglycemic factor in pancreatic extracts and called this factor “the glucose agonist”, hence the name glucagon (2). In the 1950s glucagon was purified and crystallized at Eli Lilly and Co., and shortly after, the amino acid sequence of the peptide was determined (3). This led to the development of medical use of glucagon for the treatment of severe insulin-induced hypoglycemia (4,5). The development of a radioimmunoassay for the detection of glucagon in 1959 spurred further investigations of glucagon physiology and its role in health and disease (6). It was discovered that patients with diabetes exhibit increased glucagon levels which led to the “bihormonal hypothesis” stating that the combination of hypoinsulinemia and hyperglucagonemia constitutes a central pathophysiological determinant for diabetic hyperglycemia (7). Since then it has become evident that glucagon not only acts by increasing hepatic glucose production but affects overall energy homeostasis in times of limited energy supply by stimulating lipid and protein catabolism, reducing appetite and food intake and increasing energy expenditure.

 

STRUCTURE AND SYNTHESIS OF GLUCAGON

 

Glucagon is a 29-amino acid peptide hormone predominantly secreted from the alpha cells of the pancreas. It is derived from the precursor proglucagon which can be processed into a number of related peptide hormones (Fig. 1). Proglucagon is expressed in pancreatic islet alpha cells, intestinal enteroendocrine L cells, and to a minor extent in neurons in the brain stem and hypothalamus (8,9). Processing of proglucagon is undertaken by the processing enzymes prohormone convertase 1/3 (PC1/3) and prohormone convertase 2 (PC2), respectively. In the pancreas, PC2 processes proglucagon to glucagon while processing of proglucagon in the intestine and the brain is undertaken by PC1 leading to the formation of glucagon-like peptide 1 (GLP-1) and glucagon-like peptide 2 (GLP-2) (9).

Figure 1. Tissue specific processing of proglucagon. In the pancreas proglucagon is processed into glucagon, glicentin-related pancreatic polypeptide (GRPP), intervening peptide 1 (IP1), and major proglucagon fragment (MPGF) by the processing enzyme prohormone convertase 2 (PC2). In the intestine and in the brain proglucagon is processed by prohormone convertase 1/3 (PC1/3) into glucagon-like peptide 1 (GLP-1), glucagon-like peptide 2 (GLP-2), oxyntomodulin, intervening peptide 2 (IP2), and glicentin.

 

GLUCAGON SECRETION

 

Glucagon is secreted in response to hypoglycemia, prolonged fasting, exercise and protein-rich meals (10). Glucagon release is regulated through endocrine and paracrine pathways; by nutritional substances; and by the autonomic nervous system (11). Glucagon secretion occurs as exocytosis of stored peptide vesicles initiated by secretory stimuli of the alpha cell. Stimulatory regulators of glucagon release include hypoglycemia, amino acids and the gut hormone glucose-dependent insulinotropic peptide (GIP), whereas hyperglycemia and GLP-1 inhibit glucagon release. Additionally, glucagon release is inhibited in a paracrine fashion by factors like somatostatin, insulin, zinc and possibly amylin. Glucagon may regulate its own secretion indirectly via stimulatory effect on beta cells to secrete insulin (12,13). In contrast to glucose, non-glucose regulators of glucagon secretion seem to mediate their action through changes in cAMP levels rather than through the calcium-dependent pathway outlined below (14,15).

 

Regulation of Glucagon Secretion by Glucose

 

The most potent regulator of glucagon secretion is circulating glucose. Hypoglycemia stimulates the pancreatic alpha cell to release glucagon and hyperglycemia inhibits glucagon secretion (Fig. 2) (11). The cellular mechanism behind this glucose-dependent regulation of glucagon secretion involves uptake of glucose by the glucose transporter 1 (GLUT1) in the cell membrane of pancreatic alpha cells and subsequent glycolysis which ultimately generates adenosine triphosphate (ATP) in the mitochondria of the alpha cell. Thus, the intracellular ATP level in the alpha cell reflects plasma glucose levels. Hypoglycemia and resulting low intracellular ATP levels in the alpha cell close ATP-sensitive potassium channels (KATP-channels) whereby the efflux of potassium (K+) is reduced. This causes a depolarization of the cell membrane which, in turn, opens voltage-dependent Ca2+ channels allowing influx of Ca2+. This increases intracellular Ca2+ levels, the primary trigger for exocytosis of glucagon granules from the alpha cells (Fig. 2). Conversely, increasing circulating glucose levels increase glucose influx to the alpha cell generating an increase in intracellular ATP concentration, which opens KATP-channels. This leads to a membrane potential that closes voltage-dependent Ca2+ channels thereby preventing Ca2+ influx and glucagon secretion (12).

Figure 2. Glucose-dependent glucagon secretion from the alpha cell. During hypoglycemia intracellular glucose concentration falls with a subsequent reduction in glycolysis-generated adenosine triphosphate (ATP) in the mitochondria of the cell. This closes ATP-sensitive potassium (K+) channels and the intracellular K+ concentration rises, which depolarizes the cell membrane, thus opening voltage-dependent Ca2+ channels allowing influx of Ca2+. Increase in intracellular Ca2+ concentration triggers secretion of glucagon through exocytosis. ∆Vm, change in membrane potential (i.e. depolarization of the cell membrane).

 

Glucagon Concentrations in The Circulation

 

In normal physiology, circulating glucagon concentrations are in the picomolar rangeIn the fasting state with plasma glucose levels around 5 mmol/l, glucagon is secreted in basal levels resulting in plasma concentrations below 20 pmol/l (16–18). Basal glucagon secretion balances the effect of basal insulin secretion resulting in a steady-state between glucose uptake and endogenous glucose production in the fasted state; i.e. stable blood glucose concentrations. During exercise or in case of hypoglycemia, circulating glucagon levels may increase dramatically to 3-4 times basal levels increasing the glucagon to insulin ratio (12,19,20) (Fig. 3).

Figure 3. Glucagon concentrations in response to hypoglycemia, euglycemia, and hyperglycemia.

 

GLUCAGON RECEPTOR SIGNALLING

 

The effects of glucagon are mediated through binding to and activation of the glucagon receptor. The glucagon receptor is a seven transmembrane G protein-coupled receptor (Fig. 4) predominantly expressed in the liver, but also found in varying amounts in the kidneys, heart (controversial), adrenal glands, adipose tissue (controversial), gastrointestinal tract, and pancreas (21). The main mode of intracellular signaling involves activation of Gs and Gq. Gs activation stimulates adenylyl cyclase which produces cyclic adenosine monophosphate (cAMP) that activates protein kinase A (PKA). The activated PKA migrates to the nucleus and activates transcription factors like cAMP response element-binding protein (CREB) through phosphorylation. This enables CREB to bind to response elements of target genes resulting in the recruitment of coactivators and ultimately promoting gene expression. Activation of Gq by glucagon leads to activation of phospholipase C (PLC) and subsequent increase in inositol 1,4,5-triphosphate (IP3), which signals to enhance release of calcium from the endoplasmic reticulum. This, in turn, activates downstream signaling cascades including CREB-regulated transcription co-activator (CRTC2) which enhance CREB-dependent gene expression. In addition to the CREB-CRTC2 pathway, glucagon may signal through various other pathways reviewed in detail elsewhere (1,12,22).

Figure 4. Examples of the two most well-described intracellular pathways involved in glucagon-induced regulation of target gene expression: the PKA and the IP3 pathways. AC, adenylyl cyclase; CRTC2, CREB-regulated transcription co-activator; CREB, cAMP response element-binding protein; IP3, inositol 1,4,5-triphosphate; PIP2, phosphatidyl-inositol-4,5-bisphosphate; PKA, protein kinase A; PLC, phospholipase C.

 

DEGRADATION AND ELIMINATION OF GLUCAGON

 

The degradation of glucagon is mainly facilitated by receptor-mediated endocytosis and proteolysis by the ubiquitous enzyme dipeptidyl peptidase 4 (22,23). Consistent with the relative receptor expression, the liver and kidneys seem to represent the two main organs removing glucagon from the circulation. The circulating half-life of glucagon in plasma is reported to be between four to seven minutes in humans (24,25).

 

GLUCAGON ACTIONS

 

Glucagon Increases Hepatic Glucose Production

 

Glucagon controls plasma glucose concentrations during fasting, exercise and hypoglycemia by increasing hepatic glucose output to the circulation. Specifically, glucagon promotes hepatic conversion of glycogen to glucose (glycogenolysis), stimulates de novo glucose synthesis (gluconeogenesis), and inhibits glucose breakdown (glycolysis) and glycogen formation (glycogenesis) (Fig. 5) (26). Hepatic glucose production is rapidly enhanced in response to a physiological rise in glucagon; achieved through stimulation of glycogenolysis with minor acute changes in gluconeogenesis (27,28). This ability of glucagon is critical in the life-saving counterregulatory response to severe hypoglycemia. Additionally, it is a key factor in providing adequate circulating glucose for brain function and for working muscle during exercise (28). During prolonged fasting, glycogen stores are depleted, and gluconeogenesis takes over (29). The hyperglycemic property of glucagon is enhanced when hepatic glycogen levels are high and diminished when hepatic glycogen levels are low in conditions of fasting or liver diseases like cirrhosis (12).

Figure 5. Regulation of glucose metabolism by glucagon in the liver. Glucagon increases hepatic glucose production by stimulating glycogenolysis and glycogenogenesis (green arrows) while inhibiting glycolysis and glycogenesis (red arrows).

 

Glucagon Stimulates Break-Down of Fatty Acids and Inhibits Lipogenesis in the Liver

 

Glucagon promotes formation of non-carbohydrate energy sources in the form of lipids and ketone bodies. Thereby, glucagon contributes to a stable energy homeostasis during conditions where energy supply is limited (fasting) or in states of increased energy demand (e.g. exercise or cold exposure) (12). Specifically, in times of energy demand, glucagon enhances break-down of fatty acids to acetyl-coenzyme A molecules (beta-oxidation) in the liver. These intermediates are either reduced to generate ATP in the tricarboxylic acid cycle or converted to ketone bodies (ketogenesis) – a process also stimulated by glucagon. (30–34). Furthermore, glucagon signaling inhibits de novo lipogenesis by inactivating the enzyme that catalyzes the first step in fatty acid synthesis from other substrates like carbohydrates (34).  

 

Glucagon Promotes Break-Down of Amino Acids

 

During prolonged fasting, glucagon stimulates formation of glucose from amino acids (via gluconeogenesis) by upregulating enzymes involved in the process. However, the rate-limiting step of the process depends on the supply of gluconeogenic amino acids from muscle or dietary intake, a process not controlled by glucagon (35). In addition to enter gluconeogenesis, amino acids are deaminated to generate ATP in the liver. Glucagon is involved in this process by promoting the conversion of ammonia – a toxic biproduct from deamination – to urea, which is excreted in the urine. Thereby glucagon reduces ammonia levels in the blood (36). Disruption of glucagon action by inhibition of the glucagon receptor (37) leads to increased plasma levels of amino acids and pancreatic alpha cell hyperplasia, which in turn, leads to glucagon hypersecretion. This suggests that glucagon and amino acids are linked in a feedback loop between the liver and the pancreatic alpha cells (35).

 

Glucagon Reduces Food Intake

 

Acute administration of glucagon has been shown to reduce food intake and diminish hunger (38,39). Conversely, preprandial inhibition of glucagon signaling increases food intake in rats (40,41) providing evidence for a role of glucagon in the regulation of appetite. It is somewhat counterintuitive that glucagon should reduce food intake given that glucagon levels are typically elevated upon fasting and decrease upon feeding. Thus, the observed effect upon glucagon administration (in supraphysiological concentrations) could partly be due to cross-reactivity with the GLP-1 receptor (which normally result in suppression of food intake) (12). The mechanism behind glucagon’s potential appetite-reducing effect is not fully understood but could arise from hepatic metabolic changes induced by glucagon or from glucagon working directly in the central nervous system (31).

 

Glucagon Increases Energy Expenditure

 

In addition to a potential effect of glucagon on food intake, evidence suggests that glucagon contributes to a negative energy balance by stimulating energy expenditure. In humans, this effect has been observed in studies in which glucagon infusion resulted in increases in resting energy expenditure (42–44). However, the effect of endogenous glucagon on resting energy expenditure remains unclear. Also, the exact mechanisms behind the increase in resting energy expenditure elicited by exogenous glucagon remain to be determined. It has been speculated that glucagon activates brown adipose tissue (12), however this was recently challenged in an in vivo study that found no direct effect of glucagon on brown adipose tissue (43). Rodent studies indicate that the actions of glucagon to increase energy expenditure might be indirectly mediated partly by fibroblast growth factor 21 (FGF21) as glucagon-induced increase in energy expenditure is abolished in animals with FGF21 receptor deletion (45).

 

Glucagon May Regulate Heart Rate and Contractility

 

Infusion of high doses of glucagon increases heart rate and cardiac contractility (46). In fact, infusion of glucagon in pharmacological doses (milligram) is often used in the treatment of acute cardiac depression caused by calcium channel antagonist or beta-blocker overdoses (47) despite limited evidence (48). In comparison, glucagon concentrations within the normal physiological range do not appear to affect heart rate or contractility (49) and any physiological role of endogenous glucagon in the regulation of pulse rate remains questionable. This is supported by studies investigating the effect of glucagon receptor antagonist for the treatment of type 2 diabetes in which no effect of pulse rate were observed (50). Nevertheless, whether increased glucagon concentrations have a sustained effect on the heart remains unknow. Of note, most studies use bolus injections of glucagon which cause only a transient increase in heart rate and contractility (potentially reflecting the rapid elimination of glucagon from circulation) (48). Taken together, it remains uncertain whether glucagon has a place in the treatment of heart failure or hold a cardioprotective effect in healthy subjects.

Figure 6. Organ specific actions of glucagon. GIP, glucose-dependent insulinotropic polypeptide

 

GLUCAGON PATHOPHYSIOLOGY

 

Glucagon in Type 2 Diabetes

 

Patients with type 2 diabetes exhibit an impaired regulation of glucagon secretion which contributes importantly to diabetic hyperglycemia. Specifically, type 2 diabetes is characterized by elevated levels of glucagon during fasting while suppression of glucagon in response to oral intake of glucose is impaired or even paradoxically elevated (Fig. 7) (16,51,52). The mechanisms behind hyperglucagonemia are not fully understood but is usually explained by a diminished suppressive effect of insulin on alpha cells due to hypoinsulinemia and insulin resistance at the level of the alpha cells (53,54). Interestingly, subjects with type 2 diabetes, who exhibit a hyperglucagonemic response to oral glucose, respond with a normal suppression of glucagon after intravenous glucose administration (16). This suggests that diabetic hyperglucagonemia may not be explained by alpha cell resistance to glucose and/or insulin. Accordingly, hormones secreted from the gastrointestinal tract may play an important role (55,56). It has recently been confirmed that glucagon can be secreted from extrapancreatic tissue (demonstrated in experiments with totally pancreatectomized subjects) (17). This supports the notion that postprandial hypersecretion of glucagon in patients with type 2 diabetes might be of extrapancreatic origin.

Figure 7. Schematic illustration of plasma glucagon concentrations in patients with type 2 diabetes and in normal physiology (healthy subjects). Type 2 diabetes is characterized by elevated fasting plasma glucagon levels and impaired suppression of plasma glucagon levels in response to oral glucose.

 

Glucagon in Type 1 Diabetes

 

Traditionally type 1 diabetic hyperglycemia has been explained by selective loss of beta cell mass and resulting decrease in insulin secretion. However, emerging evidence indicate that glucagon plays a major role in type 1 diabetes pathophysiology. The glucagon dyssecretion that characterizes patients with type 1 diabetes is associated with two clinical manifestations: Postprandial hyperglucagonemia and impaired glucagon counterregulation to hypoglycemia (57). Data regarding fasting plasma glucagon concentrations in type 1 diabetes are inconsistent (57,58). Thus, the general notion that glucagon hypersecretion plays a role in type 1 diabetes hyperglycemia is mainly based on elevated postprandial glucagon concentrations (57). The explanation behind this is unclear, although a common explanation is, that in type 1 diabetes the postprandial increase in plasma glucose is not followed by an increase in insulin secretion from beta cells, which in normal physiology would inhibit glucagon secretion. The absence of that restraining signal from endogenous insulin could result in an increase in glucagon secretion from alpha cells after a meal (Fig. 8) (12,57). However, like in type 2 diabetes, subjects with type 1 diabetes preserve their ability to suppress glucagon after intravenous glucose administration. This suggest that inappropriate glucagon secretion in type 1 diabetes occurs as a consequence of the oral administration, possible via glucagonotropic signaling from the gut or due to glucagon secretion directly from the gut, rather than as a consequence of dysfunctional alpha cell sensing to hyperglycemia and/or lack of paracrine inhibition by insulin (17,55,59).

Figure 8. Schematic illustration of plasma glucagon concentrations in patients with type 1 diabetes and in normal physiology (healthy subjects). Type 1 diabetes is characterized by elevated concentrations of glucagon in response to a meal or oral glucose intake.

 

Hypoglycemia is a frequent and feared side effect of insulin therapy in type 1 diabetes and it represents a common barrier in obtaining glycemic control (60). In normal physiology hypoglycemia is prevented by several mechanisms: 1) Reduced insulin secretion from beta cells diminishing glucose uptake in peripheral tissues; 2) increased glucagon secretion from alpha cells increasing hepatic glucose output; and 3) increased sympathetic neural response and adrenomedullary epinephrine secretion. The latter will stimulate hepatic glucose production and cause clinical symptoms that enables the individual to recognize hypoglycemia and ultimately ingest carbohydrates (57,61,62). In type 1 diabetes, insulin-induced hypoglycemia fails to elicit adequate glucagon responses compromising counterregulation to insulin-induced hypoglycemia; a phenomenon which seems to worsen with the duration of type 1 diabetes. This defect likely involves a combination of defective alpha cells and reduced alpha cell mass (57,63).

 

Glucagon in Obesity and Hepatic Steatosis

 

Dysregulated glucagon secretion is not only observed in patients with type 2 diabetes but also in normal glucose-tolerant individuals with obesity (64) and patients with non-alcoholic fatty liver disease (NAFLD) (65,66). This suggests that dysregulated glucagon secretion may represent hepatic steatosis rather than dysregulated glucose metabolism. Interestingly, fasting hyperglucagonemia seems to relate to circulating amino acids in addition to hepatic fat content (65). This hyperaminoacidemia suggests that impairment of amino acid turnover in the liver and ensuing elevations of circulating amino acids constitutes a feedback on the alpha cell to secrete more glucagon with increasing hepatic amino acid turnover and ureagenesis needed for clearance of toxic ammonia from the body. This feedback loop is proposed as “the liver-alpha cell axis” (35,55,65,67).

 

PERSPECTIVES

 

The implication of hyperglucagonemia in obesity and NAFLD has renewed the scientific interest in actions of glucagon and the role of glucagon in the pathophysiology of these metabolic disorders. Clearly, glucagon may represent a potential target for treatments of obesity and NAFLD. A simple way to restrain the undesirable hyperglycemic effect of glucagon while realizing its actions on lipolysis and energy expenditure could be by co-treating with a glucose-lowering drug. This may be done by mimicking the gut hormone oxyntomodulin which acts as a ligand to both the glucagon and the GLP-1 receptor. Accordingly, treatment with dual glucagon/GLP-1 receptor agonists in subjects with type 2 diabetes and obesity improves glycemic control, reduces body weight (68) and ameliorates NAFLD (69).

 

SUMMARY AND CONCLUSIONS

 

Glucagon is a glucoregulatory peptide hormone that counteracts the actions of insulin by stimulating hepatic glucose production and thereby increases blood glucose levels. Additionally, glucagon mediates several non-glucose metabolic effects of importance for maintaining whole-body energy balance in times of limited nutrient supply. These actions include mobilization of energy resources through hepatic lipolysis and ketogenesis; stimulation of hepatic amino acid turnover (and related ureagenesis). Also, glucagon has been shown to increase energy expenditure and inhibit food intake, but whether endogenous glucagon is involved in the regulation of these processes remains uncertain. Glucagon plays an important role in the pathophysiology of diabetes as elevated glucagon levels observed in these patients stimulate hepatic glucose production, thereby contributing to diabetic hyperglycemia. Investigations of glucagon’s role in non-glucose metabolism and metabolic physiology and pathophysiological processes are ongoing and may reveal new treatment targets based on glucagon actions.

 

ACKNOWLEDGEMENTS

 

Figure 1,2,4,5,6 created by adopting templates from Servier Medical Arts, Les Laboratoires Servier (https://smart.servier.com/). Used under Creative Commons License 3.0.

 

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