Androgen receptor (AR)

The androgen receptor (AR) is a member of the nuclear receptor superfamily of ligand-activated transcription factors, encoded by the AR gene on chromosome Xq11–12. It is built from an N-terminal domain carrying most of its transcriptional activity and a polymorphic polyglutamine tract, a central zinc-finger DNA-binding domain, and a C-terminal ligand-binding domain — a modular arrangement that matters clinically, since nearly every AR-directed drug engages one domain and can be escaped by altering another. Upon binding cognate androgens such as testosterone and its more potent metabolite dihydrotestosterone (DHT), AR undergoes a conformational change, dissociates from cytoplasmic chaperone complexes, and translocates to the nucleus, where it binds androgen response elements (AREs) in target gene promoters and enhancers to drive transcriptional programs governing cell proliferation, survival, and differentiation. Loss-of-function mutations cause androgen insensitivity syndrome, and expansion of the polyglutamine tract causes spinal and bulbar muscular atrophy, so both too little AR activity and an aberrant receptor produce distinct disease. Beyond its classical role in male sexual development and the prostate, AR exerts context-dependent effects in hair follicles, the central nervous system, immune cells, and a variety of epithelial cancers.

AR is best recognized as the central oncogenic driver in prostate cancer, where sustained androgen signaling fuels tumor growth even under systemic androgen deprivation; prostate cancer that progresses despite castrate androgen levels or androgen-deprivation therapy is termed castration-resistant prostate cancer (CRPC). CRPC does not proceed by abandoning the receptor but by restoring its signal: AR amplification and overexpression that respond to residual ligand, gain-of-function mutations that let antagonists or adrenal steroids act as agonists, intratumoral androgen synthesis, constitutively active splice variants — notably AR-V7, which lacks the ligand-binding domain altogether and so is indifferent to drugs that target it — and cistrome reprogramming that redirects AR to non-canonical transcriptional networks.

Therapy follows the same logic in reverse, attacking ligand supply and receptor function at separate points: gonadal suppression, abiraterone blocking the CYP17A1 step of androgen synthesis including within the tumor, and receptor antagonists such as enzalutamide that prevent nuclear translocation and DNA binding. Because escape routes converge on restoring AR output, degraders that remove the protein and agents directed at the N-terminal domain are being pursued to defeat variants no ligand-binding-site drug can reach. Prolonged suppression also selects for tumors that stop depending on AR entirely, emerging as neuroendocrine prostate cancer. Emerging evidence implicates AR in tumor immunology, metabolic regulation, and conditions such as androgenetic alopecia (AGA), broadening therapeutic interest well beyond prostate cancer.