The androgen receptor (AR) is activated in prostate cancer patients undergoing androgen ablative therapy and mediates growth of androgen-insensitive prostate cancer cells, suggesting it is activated by nonandrogenic factors. We demonstrate that activated ␣ subunit of heterotrimeric guanine nucleotide-binding G s protein activates the AR in prostate cancer cells and also synergizes with low concentration of androgen to more fully activate the AR. The G␣ s activates protein kinase A, which is required for the nuclear partition and activation of AR. These data suggest a role for G␣ s and PKA in the transactivation of AR in prostate cancer cells under the environment of reduced androgen levels.Prostate cancer, the most common noncutaneous malignant transformation in American men (1), starts as an androgen-dependent lesion in the prostate gland that can be successfully treated with surgical removal of the tumor or local radiation (2). Locally advanced and metastatic diseases are treated with endocrine therapies aimed to (i) decrease circulating androgen levels via chemical or physical castration, (ii) block androgen receptor (AR) 1 activation with anti-androgens, or (iii) achieve both (3). The hormonal therapies cause only a temporary shrinkage in tumor mass, and the cancer invariably reappears in the form of androgen-insensitive (AI) disease. Despite decades of intense basic and clinical research, to date there is no cure for AI prostate cancer.Accumulating evidence suggests that the AR itself is involved in the transition of prostate cancer from androgen-dependent to AI (4). AI prostate tumors express AR-regulated genes, including human kallikrein 2 and 3 (5), also known as prostate-specific antigen (PSA), suggesting that the AR is activated in these tumors despite the continued presence of the hormonal therapies. In vitro, the AR can be activated by factors other than androgens (6, 7), and in animal models increased AR expression was reported to be the only change associated with progression to hormone refractory disease (8). In humans, the AR is overexpressed in up to one-third of AI prostate carcinomas, suggesting that it could respond to low serum levels of androgens (9). Also, AI lesions exhibit frequent mutations in the AR that may allow it to be activated by other androgens or even anti-androgens (10). Hence, AR is a focal point in progression of the prostate cancer to advanced stage. However, the mechanisms regulating AR activation in the presence of low androgen concentrations and their significance to the progression to AI disease remain unclear.Transition of the prostate cancer to the AI stage is associated with increased expression of plasma membrane-localized G protein-coupled receptors (GPCRs) (11), which mediate cellular responses to a diverse array of extracellular molecules, including lipid and peptide growth factors (12). Human prostate cancer biopsies show elevated expression of the GPCRs for endothelin (13), bradykinin (14), and lysophosphatidic acid, 2 compared with benign specimens. The cancer...