Activation of androgen receptor (AR) in castrated hosts may play a critical role in prostate cancer progression to androgen-independence. Multiple mechanisms of AR activation, including stimulation by tyrosine kinases, have been postulated. We and others have recently shown involvement of activated cdc42-associated tyrosine kinase Ack1 in advanced human prostate cancer. We identify AR as a downstream effector of Ack1 in prostate cancer progression and characterize the molecular basis for interplay between Ack1 and AR in prostate cancer cells. Activated Ack1 promotes androgen-independent growth of prostate xenograft tumors and enhances AR recruitment to the androgen responsive enhancers and induces androgen target gene expression at suboptimal androgen concentrations. Activated Ack1 binds and phosphorylates AR protein at Tyr-267 and Tyr-363, both located within the transactivation domain. Mutation of Tyr-267 completely abrogates and mutation of Tyr-363 reduces potentiation of AR transcriptional activity by activated Ack1. These AR point mutants exhibit decreased recruitment and DNA binding to the androgen responsive enhancer after androgen treatment. In addition, we also show that heregulin induces Ack1 activation and AR tyrosine phosphorylation. Ack1 knockdown inhibits heregulin-dependent AR phosphorylation and androgen-stimulated gene expression. Treatment with androgen or heregulin each stimulates recruitment of both AR and Ack1 proteins to the androgen responsive enhancer and treatment with both markedly enhances recruitment of these proteins to the enhancer. Moreover, in 8 of 18 human primary androgen-independent prostate tumor samples, tyrosine-phosphorylated AR protein was detected and was correlated with detection of tyrosine-phosphorylated Ack1. Tyrosine-hosphorylated AR and Ack1 proteins were not detected in androgen-dependent tumors or benign prostate samples. This study suggests that Ack1 may be an intracellular transducer of receptor tyrosine kinase signaling that enhances AR transcriptional function and plays a role in promoting androgen-independent progression of prostate cancer. Ack1 represents a potential therapeutic target in advanced prostate cancer.
98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA