| Androgen_recep | |||||||||
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| crystal structure of the human androgen receptor ligand binding domain bound with an androgen receptor nh2-terminal peptide, ar20-30, and r1881 | |||||||||
| Identifiers | |||||||||
| Symbol | Androgen_recep | ||||||||
| Pfam | PF02166 | ||||||||
| InterPro | IPR001103 | ||||||||
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The androgen receptor (AR), also known as NR3C4 (nuclear receptor subfamily 3, group C, member 4), is a type of nuclear receptor[6] that is activated by binding of either of the androgenic hormones testosterone or dihydrotestosterone [7] in the cytoplasm and then translocating into the nucleus. The androgen receptor is most closely related to the progesterone receptor, and progestins in higher dosages can block the androgen receptor.[8][9]
The main function of the androgen receptor is as a DNA-binding transcription factor that regulates gene expression;[10] however, the androgen receptor has other functions as well.[11] Androgen regulated genes are critical for the development and maintenance of the male sexual phenotype.
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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.[12] 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.[13] 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.[14]
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.[15]
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.[16] 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 insulin-like growth factor I (IGF-1).[17] 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.[18]
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.[11][19] 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.
In humans, the androgen receptor is encoded by the AR gene located on the X chromosome at Xq11-12.[20][21]
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).[22] The androgen receptor seems to affect neuron physiology and is defective in Kennedy disease.[23][24] In addition, point mutations and trinucleotide repeat polymorphisms has been linked to a number of additional disorders.[25]
Two isoforms of the androgen receptor (A and B) have been identified:[26]
Like other nuclear receptors, the androgen receptor is modular in structure and is composed of the following functional domains labeled A through F:[28]
Androgen receptor has been shown to interact with:
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