Androgen receptor

Effect on development

In some cell types, testosterone interacts directly with androgen receptors, whereas, in others, testosterone is converted by 5-alpha-reductase to dihydrotestosterone, an even more potent agonist for androgen receptor activation. Testosterone appears to be the primary androgen receptor-activating hormone in the Wolffian duct, whereas dihydrotestosterone is the main androgenic hormone in the urogenital sinus, urogenital tubercle, and hair follicles. Hence, testosterone is responsible primarily for the development of male primary sexual characteristics, whereas dihydrotestosterone is responsible for secondary male characteristics.

Androgens cause slow epiphysis, or maturation of the bones, but more of the potent epiphysis effect comes from the estrogen produced by aromatization of androgens. Steroid users of teen age may find that their growth had been stunted by androgen and/or estrogen excess. People with too little sex hormones can be short during puberty but end up taller as adults as in androgen insensitivity syndrome or estrogen insensitivity syndrome.

Also, AR knockout-mice studies have shown that AR is essential for normal female fertility, being required for development and full functionality of the ovarian follicles and ovulation, working through both intra-ovarian and neuroendocrine mechanisms.

Maintenance of male skeletal integrity

Via the Androgen receptor, androgens play a key role in the maintenance of male skeletal integrity. The regulation of this integrity by androgen receptor (AR) signaling can be attributed to both osteoblasts and osteocytes.

Genomic

The primary mechanism of action for androgen receptors is direct regulation of gene transcription. The binding of an androgen to the androgen receptor results in a conformational change in the receptor that, in turn, causes dissociation of heat shock proteins, transport from the cytosol into the cell nucleus, and dimerization. The androgen receptor dimer binds to a specific sequence of DNA known as a hormone response element. Androgen receptors interact with other proteins in the nucleus, resulting in up- or down-regulation of specific gene transcription. Up-regulation or activation of transcription results in increased synthesis of messenger RNA, which, in turn, is translated by ribosomes to produce specific proteins. One of the known target genes of androgen receptor activation is the insulin-like growth factor I receptor (IGF-1R). Thus, changes in levels of specific proteins in cells is one way that androgen receptors control cell behavior.

One function of androgen receptor that is independent of direct binding to its target DNA sequence, is facilitated by recruitment via other DNA-binding proteins. One example is serum response factor, a protein that activates several genes that cause muscle growth.

Androgen receptor is modified by post translational modification through acetylation, which directly promotes AR mediated transactivation, apoptosis and contact independent growth of prostate cancer cells. AR acetylation is induced by androgens and determines recruitment into chromatin. The AR acetylation site is a key target of NAD-dependent and TSA-dependent histone deacetylases and long non coding RNA.

Non-genomic

More recently, androgen receptors have been shown to have a second mode of action. As has been also found for other steroid hormone receptors such as estrogen receptors, androgen receptors can have actions that are independent of their interactions with DNA. Androgen receptors interact with certain signal transduction proteins in the cytoplasm. Androgen binding to cytoplasmic androgen receptors can cause rapid changes in cell function independent of changes in gene transcription, such as changes in ion transport. Regulation of signal transduction pathways by cytoplasmic androgen receptors can indirectly lead to changes in gene transcription, for example, by leading to phosphorylation of other transcription factors.

Gene

In humans, the androgen receptor is encoded by the AR gene located on the X chromosome at Xq11-12.

AR deficiencies

The androgen insensitivity syndrome, formerly known as testicular feminization, is caused by a mutation of the androgen receptor gene located on the X chromosome (locus:Xq11-Xq12). The androgen receptor seems to affect neuron physiology and is defective in Kennedy's disease. In addition, point mutations and trinucleotide repeat polymorphisms has been linked to a number of additional disorders.

CAG repeats

The AR gene contains CAG repeats which affect receptor function, where fewer repeats leads to increased receptor sensitivity to circulating androgens and more repeats leads to decreased receptor sensitivity. Studies have shown that racial variation in CAG repeats exists, with African-Americans having less repeats, Caucasians having an intermediate number of repeats, and Asians having the longest. The racial trends in CAG repeats parallels the incidence and mortality of prostate cancer in these groups.

Isoforms

Two isoforms of the androgen receptor (A and B) have been identified:

Domains

Like other nuclear receptors, the androgen receptor is modular in structure and is composed of the following functional domains labeled A through F:

Splice variants

AR-V7 is an androgen receptor splice variant that can be detected in circulating tumor cells of metastatic prostate cancer patients. and is predictive of resistance to some drugs.

As a drug target

The androgen receptor (AR) is important for therapeutic target in prostate cancer, thus many different inhibitors have been developed, primarily targeting the ligand binding domain of the protein, while inhibitors that target the N-terminal domain of the protein are still under development. AR ligands can either be classified based on their structure (steroidal or nonsteroidal) or based on their ability to activate or inhibit transcription (agonists or antagonists).

AR inhibitors

AR antagonists: flutamide, nilutamide, bicalutamide, enzalutamide, apalutamide, cyproterone acetate, megestrol acetate, chlormadinone acetate, spironolactone, canrenone, drospirenone, ketoconazole, topilutamide (fluridil), cimetidine.

AR agonists

See Selective androgen receptor modulator

Interactions

Androgen receptor has been shown to interact with: