Actions of androgens are mediated by the androgen receptor (NR3C4; NCIB). This ligand dependent transcription factor belongs to the superfamily of nuclear receptors. This family includes receptors for steroid hormones, thyroid hormones, all-trans and 9-cis retinoic acid, 1,25 dihydroxy-vitamin D, ecdysone and peroxisome proliferator-activated receptors (15, 16, 17). In addition an increasing number of nuclear proteins have been identified with a protein structure homologous with that of nuclear receptors, but without a known ligand. These so-called "orphan" receptors form an important subfamily of transcription factors acting either in the absence of any ligand or with as yet unknown endogenous ligands (18). Comparative structural and functional analysis of nuclear hormone receptors has revealed a common structural organization in 4 different functional domains: a NH2-Terminal Domain, a DNA-Binding Domain, a Hinge Region and a Ligand Binding Domain [Figure 5].
Figure 5. Sequence homologies between the human Androgen Receptor (hAR), human Progesterone Receptor (hPR), human Glucocorticoid Receptor (hGR), human Mineralocorticoid Receptor (hMR) and the human Estrogen Receptors α (hERα) and β (hERβ).
The current model for androgen action involves a multi step mechanism as depicted in Figure 6. Upon entry of testosterone into the androgen target cell, binding occurs to the androgen receptor either directly or after its conversion to 5α-dihydrotestosterone. Binding to the receptor is followed by dissociation of heat shock proteins in the cytoplasm, simultaneously accompanied by a conformational change of the receptor protein resulting in a transformation and a translocation to the nucleus. Upon binding in the nucleus to specific DNA-sequences the receptor dimerizes with a second molecule and the homodimer entity recruits further additional proteins (e.g. coactivators, general transcription factors, RNA-polymerase II) resulting in specific activation of transcription at discrete sites on the chromatin.
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.
![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.](./figures/figure6.png)
Interestingly androgen signaling via the androgen receptor can also occur in a non-genomic, rapid and sex-nonspecific way by crosstalk with the Scr, Raf-1, Erk-2 pathway [Figure 6] (19, 20). The classical androgen receptor is also involved in androgen-mediated induction of Xenopus oocyte maturation via the (MAPK)-signalling cascade in a transcription independent way (21).
Since cloning of the human androgen receptor cDNA our insights into the mechanism of androgen action have been increased tremendously. Only one androgen receptor cDNA has been identified and cloned, despite the two different ligands (22, 23, 24, 25). The concept of two hormones and one receptor to explain the different actions of androgens has been generally accepted and, according to the information available from the human genome project, it is very unlikely that additional genes exist coding for a functional nuclear receptor with androgen receptor-like properties (17).
The androgen receptor gene is located on the X-chromosome at Xq11.2 -12 [approx. 186 kb] and codes for a protein with a molecular mass of approx. 110 kDa [Figure 7] (26, 27). The gene consists of 8 coding exons and the structural organization of the coding exons is essentially identical to those of the genes coding for the other steroid hormone receptors (e.g. exon/intron boundaries are highly conserved) [Figure 7] (28, 29). As a result of differential splicing in the 3' - untranslated region two androgen receptor mRNA species (8.5 and 11 kb, respectively) have been identified in several cell lines (30). There is no structural indication in the androgen receptor mRNA for any preferential use of either of the two transcripts and neither for a specific function, but it can be speculated that tissue specific factors may determine which transcript is present in which androgen target tissue. In the human prostate and in genital skin fibroblasts predominantly the 11 kb size mRNA is expressed.
Figure 7. Human androgen receptor gene was mapped to the long arm of the X-chromosome. The human androgen receptor protein (919 amino acid residues) is encoded by 8 exons. Analogous to other nuclear receptors the protein consists of several distinct functional domains: the NH2-terminal domain containing two polymorphic stretches, the DNA-binding domain, the hinge region and the ligand binding domain.
The androgen receptor DNA - and ligand-binding domains have a high homology with the corresponding domains of the other members of the steroid receptor subfamily.
There is a remarkably low homology between the androgen receptor NH2-terminal domain and that of the other steroid receptors [Figure 5, see above] (31, 32, 33, 34, 35, 36). A poly-glutamine stretch, encoded by a polymorphic (CAG)nCAA - repeat is present in the NH2-terminal domain [Figure 8] (37). The length of the repeat has been used for identification of X-chromosomes for carrier detection in pedigree analyses (38, 39).
Figure 8. Variation in the polyglutamine stretch encoded by the (CAG)nCAA repeat in exon 1. The normal range of this stretch is 9 - 38 glutamine residues, while in the motor neuron disease spinal/bulbar muscular atrophy (also called Kennedy's disease) the (CAG)nCAA repeat in exon 1 is expanded and codes for more than 40 Glutamine residues.
Variation in length (9 - 38 glutamine residues) is observed in the normal population and has been suggested to be associated with a very mild modulation of androgen receptor activity (40). This assumption is based on in vitro experiments after transient transfection of androgen receptor cDNA's containing (CAG)nCAA - repeats of different lengths (41, 42). Translating this finding to the in vivo situation it can be envisaged that either shorter or longer repeat lengths can result in a relevant biologic effect during lifetime. This concept has been explored in epidemiological studies of men with prostate cancer or infertility. With respect to prostate cancer, a clear picture has not emerged and controversy persists. In several studies, shortening of the (CAG)nCAA repeat length in exon 1 of the androgen receptor gene was found to correlate with an earlier age of onset of prostate cancer, and a higher tumor grade and aggressiveness (43, 44, 45). However, in other epidemiological studies in prostate cancer patients these associations were not confirmed (46, 47).
In several investigations in male infertile patients an association was found between a longer (CAG)nCAA repeat and the risk of defective spermatogenesis (48, 49, 50). This suggests that a less active androgen receptor, due to a moderate expanded repeat length, may be a factor in the etiology of male infertility.
The (CAG)nCAA - repeat in exon 1 of the androgen receptor gene is expanded in patients with spinal and bulbar muscular atrophy (SBMA) and varies between 38 and 75 repeat units [Figure 8] (40, 51). Spinal and bulbar muscular atrophy is characterized by progressive muscle weakness and atrophy. Clinical symptoms usually manifest in the third to fifth decade and result from severe depletion of lower motornuclei in the spinal cord and brainstem (40, 52, 53). In addition, SBMA patients frequently exhibit endocrinological abnormalities including testicular atrophy, infertility, gynecomastia, and elevated LH, FSH and estradiol levels. Sex differentiation proceeds normally and characteristics of mild androgen insensitivity appear later in life.