AR is expressed mainly in androgen target tissues such as the prostate, skeletal muscle, liver, skin and CNS, with the highest expression observed in the prostate, adrenal gland and epididymis, determined by real-time PCR [19
]. Unlike ER, which has two isoforms, ERα and ERβ, as distinct gene products [20
], only one AR gene has been identified in humans. Both T and its active metabolite, DHT, are endogenous androgens that activate the AR.
Although T is the major form of circulating androgen, most is converted to DHT by 5α-reductase (type 1 and type 2) in the prostate, skin and liver [21
]; a small amount of T (0.2%) is also converted to estradiol by aromatase (). Both DHT and estradiol are active ‘metabolites’, so T is thought to have three modes of action. It might directly act through the AR in target tissues in which 5α-reductase is either not present or present at very low levels, it might be converted to DHT (5–10% in blood, >90% in prostate) [22
] by 5α-reductase before binding to AR, and it might be aromatized to estrogen that acts through the ER [1
]. Because DHT is a potent androgen that binds to the AR with a higher affinity than T [23
], the conversion of T to DHT is considered a natural ‘amplification’ process in ‘DHT-dependent’ tissues because 5α-reductase is not expressed in all androgen-target tissues. Type 2 5α-reductase is present mainly in the prostate whereas type 1 5α-reductase is found mainly in liver and skin [24
]. Both isozymes are expressed at a much lower level in other peripheral tissues, including skeletal muscle, and the CNS. By contrast, estrogen has a major role in regulating metabolic processes [25
], mood and cognition [27
], cardiovascular disease [28
], sexual function including libido [30
], and bone turnover in men [31
At the cellular level, unbound AR is located mainly in the cytoplasm where it is associated with a complex of heat shock proteins (HSPs), mainly through the interactions with the ligand binding domain (LBD) [33
]. On agonist binding, AR goes through a series of conformational changes, the HSPs dissociate from the AR, and the transformed AR undergoes dimerization, post-translational modification and translocation to the nucleus. The translocated receptor then binds to various androgen response elements (AREs), and recruits other transcription regulators (including co-activators and co-repressors) [34
] and transcriptional machinery [35
], which ensures the transactivation of AR-regulated gene expression. This process is the classical genomic function of AR.
In addition to the genomic pathway, a non-genomic AR pathway has been reported in oocytes [36
], skeletal muscle cells [37
], osteoblasts [38
] and prostate cancer cells [40
]. Compared to the genomic pathway, the non-genomic actions of steroid receptors are characterized by the rapidity of action (seconds to ~1 hour) and interaction with plasma membrane-associated signaling pathways [42
]. Nevertheless, the structural basis for non-genomic activity is the direct interaction between AR and cytosolic proteins from different signaling pathways [43
]. However, the physiological role of the non-genomic effects of the AR are yet to be defined.
Regardless of the pathway, AR activity involves ligand-modulated conformational change of the receptor, which further affects the interactions between AR and other proteins, either co-regulators and transcription factors (genomic pathway) or signaling cascade proteins (non-genomic pathway). Although the interactions between AR and other proteins are crucial for AR functions, not all protein–protein interactions are characterized. The interactions between AR and co-regulators are best understood, and are mediated by different functional domains of the receptor [1
], mainly through the N-terminal domain (NTD) and LBD, and, occasionally, the DNA-binding domain [34
]. Structural biology studies [44
indicate that ligand binding might regulate LBD-mediated interactions directly by affecting the conformation and/or surface topology of the LBD. In addition, AR might undergo N/C (N-terminal and C-terminal) interaction on agonist binding, so ligand binding might also regulate the conformation of the NTD and its interaction with co-regulators indirectly.
Most of the interactions between the LBD and the co-regulators identified so far have been mapped to a specific surface region, called activation function 2 (AF2), of the LBD. Both co-activator signature motifs, including LxxLL and FxxLF, and NTD motifs (F23
) interact specifically with the AF2 and regulate AR transcriptional activation. Several regions of the NTD [45
], including AF1, have also been shown to be important for the recruitment of co-regulators, but the interface is not yet defined because of the lack of detailed structural information on the NTD. AR agonist-binding appears to preferentially recruit co-activators, whereas antagonist binding seems to preferentially recruit corepressors [46
]. It has been shown that co-repressor binding motifs (NCoR box) also interact specifically with the AF2 region in either the unbound AR or antagonist-bound AR [46
]. It is clear that ligand binding regulates the conformation of the LBD and subsequent protein–protein interactions, which makes it possible to regulate AR action by introducing ligand-specific conformational changes in the receptor.