Figure 6. Simplified model of androgen action in an androgen target cell. The key protein is the androgen receptor, which binds testosterone directly or its active metabolite 5α-dihydrotestosterone (DHT). After dissociation of heat shock proteins (hsp) the receptor enters the nucleus via an intrinsic nuclear localization signal. Upon steroid hormone binding, which may occur either in the cytoplasm or in the nucleus, the androgen receptor binds as homodimer to specific DNA elements present as enhancers in upstream promoter sequences of androgen target genes. The next step is recruitment of coactivators, which can form the communication bridge between receptor and several components of the transcription machinery. The direct and indirect communication of the androgen receptor complex with several components of the transcription machinery (e.g. RNA-polymerase II [RNA-Pol II], TATA box binding protein [TBP], TBP associating factors [TAF’s], general transcription factors [GTF’s]) are key events in nuclear signaling. This communication triggers subsequently mRNA synthesis and consequently protein synthesis, which finally results in an androgen response. A non-genomic pathway involving the classical androgen receptor via cross-talk with the Src/Raf-1/Erk-2 pathway is also known.