Project Summary: Prostate Cancer (PCa) accounts for ~30000 deaths annually in the USA. There is urgent, yet unmet, need for identification and characterization of new targets for therapeutic intervention in PCa. Our goal is to address this vital knowledge gap by characterizing the role of Prostate Derived Ets Transcription Factor (PDEF) in PCa, thereby reducing the prostate cancer-related deaths. Our central hypotheses are that, “PDEF suppresses CRPC phenotype and PCa metastasis in part by modulating AR cistrome and in part by restricting lineage plasticity and as such may help sensitize CRPC men to current therapies”. This novel paradigm, implies that loss of PDEF tumor progression and therapy resistance in part by allowing re-targeting of AR to non-canonical AR cistrome, and in part by promoting lineage plasticity and emergence of CRPC, is a groundbreaking conceptual advancement, which holds translational promise in identifying new therapeutic targets in CRPC, for which there is no cure to date. Our hypotheses are driven by our novel observations that there is graded decrease in PDEF levels in prostate cancer cells with increasing aggressive phenotype, and that PDEF inhibits cell migration, invasion in vitro, and tumor metastasis in vivo. We discovered that PDEF negatively enriches gene sets associated with cell migration and tumor metastasis. We observed that PDEF inhibits expression of sets of genes associated with EMT, NEPC and Stemness, and that there is decreased PDEF during PCa progression in experimental (TRAMP) mouse model. Moreover, we observed that PDEF promotes distinct transcription profiles related with canonical AR cistrome and luminal differentiation. We now propose to test our hypothesis in three specific aims: AIM 1) To evaluate the role of PDEF in modulating AR cistrome, AIM 2) To determine the role of PDEF in cellular plasticity and resistance to AR pathway inhibitors, and; AIM3) To evaluate if prostate cancers with PDEF loss respond more favorably to combination of AR targeted therapies (Enzalutamide (Enz) and EZH2 inhibitor (EPZ-6438) as compared to Enz alone. The accomplishment of the goals proposed herein should substantially advance our understanding of the mechanisms by which PDEF inhibits prostate cancer progression and metastasis. Our proposed investigation is highly relevant to the mission of VA and is likely to have a significant impact on saving lives of Veterans by characterizing novel targets for intervention against prostate cancer in the immediate future.