Exploitation of Androgen Receptor Signaling by Guanine Nucleotide Exchange Factor Vav3 in Castration Resistant Prostate Cancer

Advanced or metastatic prostate cancer is treated by androgen deprivation; however, patients inevitably relapse with castration resistant prostate cancer (CRPC). CRPC remains dependent on androgen receptor (AR) signaling, which may include constitutive, ligand-independent action of naturally occurring AR splice variants. For example, the AR splice variant AR3 (also termed AR-V7) is expressed in CRPC and is linked to poor prognosis. Vav3, a Rho GTPase guanine nucleotide exchange factor, is an AR coactivator that is up-regulated in human prostate cancer, compared to benign tissue, and in pre-clinical models of CRPC. Vav3 confers castration-resistant growth to androgen-dependent human prostate cancer cells. Despite the importance of AR coactivators in promoting CRPC, the potential role of these regulatory proteins in modulating AR splice variant activity is unknown. We examined the contributions of Vav3 to AR activity in two CRPC cell lines that naturally express relatively high levels of Vav3 and AR3. Vav3 or AR3 knockdown greatly attenuated cell proliferation, soft agar growth and ligand-independent AR activity. Vav3 potently enhanced the transcriptional activity of AR3 and several other clinically relevant AR splice variants including ARv567es, AR1/2/3/2b, AR1/2/3/CE1, and AR1/2/3/CE2. Vav3 knockdown resulted in lowered nuclear AR3 levels while total AR3 levels remained similar. Conversely, overexpression of Vav3 resulted in increased nuclear AR3. Co-immunoprecipitation revealed that AR3 and Vav3 interact yet Vav3 and full length AR failed to show convincing interaction under the same conditions. Mutation studies demonstrated that Vav3 enhances AR3 activity by a mechanism distinct from AR coactivation. The Diffuse B-cell lymphoma (DH) domain of Vav3 is critical for coactivation of AR3 but not full length AR. Overexpression of the DH domain disrupted Vav3-AR interaction and impaired Vav3-mediated enhancement of AR3 activity but not full length AR activity. Furthermore, disruption of AR3-Vav3 interaction led to decreased anchorage dependent and independent growth of castration resistant prostate cancer cells expressing both AR3 and Vav3. Additional studies were directed at identifying AR3 and Vav3 target genes that promote growth and survival of prostate cancer cells. These novel data demonstrating physical and functional interactions between Vav3, a unique AR coactivator, and all forms of AR provide insights into the mechanisms by which Vav3 exploits and enhances AR signaling in the progression to CRPC