The role of the androgen receptor in the development of prostatic hyperplasia and prostate cancer

The androgen receptor (AR) is an androgen-inducible transcription factor characterized by a modular primary structure, with each module representing a distinct functional unit. After its interaction with androgens, the cytoplasmic AR is activated and translocated to the nucleus where it binds to target genes at the androgen responsive element(s) and recruits coregulators to form a multiprotein complex that interacts with transcriptional mediators and the basal transcription machinery to regulate gene transcription. Androgens play an essential role in the morphogenesis and physiology of the normal prostate. The etiology of benign prostatic hyperplasia (BPH) and prostatic neoplasia, which can progress to adenocarcinoma, is androgen-dependent, and reduction/obliteration of androgen action in the prostate has been the therapy of choice for BPH and prostate cancer. After androgen withdrawal and antiandrogen treatment, the androgen responsive prostate cancer cells cease to proliferate and undergo apoptosis, causing tumor regression. However, relapses are seen invariably, when tumors emerge as androgen-independent and apoptosis-resistant. Gene amplification and amino acid substitutions in the AR are detected at a high frequency in recurrent tumors. These changes confer growth advantage to the tumor cells due to either hypersensitivity of AR to low, castrate-level androgens or a realignment of the receptor conformation, leading to altered ligand specificity that enables antiandrogens, adrenal androgens and non-androgen steroids act agonistically to increase AR activity. Persistence of signaling by the wildtype AR in therapy-resistant tumors is due to the increased receptor activity caused by cross talk of AR with multiple intracellular signaling cascades, especially the growth factor activated MAP kinase/ERK and PI3 kinase/Akt pathways. Ablation of AR function using antisense oligodeoxynucleotides, ribozymes or small interference RNAs (RNAi) holds promise as future approaches to the successful treatment of hormone-refractory, apoptosis-resistant prostate tumors.