Summary Prostate cancer (PCa) is the most commonly diagnosed non-skin cancer and the third leading cause of cancer deaths in American men. While early-stage PCa can be effectively treated with surgery and radiation therapy, metastatic prostate cancer remains a challenge and androgen deprivation therapy (ADT) is the mainstay treatment. Although a majority of advanced PCa initially responds well to ADT, they ultimately develop resistance and become castration- resistant, called CRPC. CRPC tumors no longer need androgen to grow but often remain dependent on an aberrantly active androgen receptor (AR). Understanding the mechanisms to this androgen-independent AR activation may shed significant light on effective strategies to eradicate CRPC. EZH2 is an enzymatic subunit of the Polycomb Repressive Complex 2 (PRC2) that catalyzes histone H3 lysine 27 trimethylation to suppress gene expression, a role that can be effectively targeted by enzymatic EZH2 inhibitors. Recent evidence suggests that EZH2 may have PRC2-independent roles in activating gene expression. However, critical gaps remain as to what the target genes are, how EZH2 activates them, and how they work in concert with the epigenetic roles of EZH2 to promote CRPC. Our preliminary results showed that AR is a direct target of EZH2-mediated transcriptional activation. This activation is likely mediated by EZH2- interacting co-activator NF90. Our preliminary data further showed that NF90 is up-regulated in CRPC and exhibits oncogenic functions. The roles of NF90, however, have never been studied in PCa. We hypothesize that NF90 cooperates with EZH2 in transcriptional activation of AR and promotes CRPC progression. To test these hypotheses, three Specific Aims are proposed. Aim 1 will determine how NF90 interacts with EZH2 protein to mediate AR gene transcription. Aim 2 will determine how NF90 regulates EZH2 chromatin recruitment and target gene activation using ChIP-seq and RNA-seq assays, characterize downstream genes/pathways of NF90, and examine the expression of NF90 in primary PCa tissues to determine its correlation with EZH2 expression and clinical outcomes. Lastly, Aim 3 will characterize the roles of NF90 in prostate cancer in vitro and in vivo and, most importantly, test the efficacy of a new combinatorial therapeutic approach that simultaneously blocks the dual roles of EZH2 using enzymatic EZH2 inhibitor GSK126 and AR antagonist enzalutamide in PCa patient-derived xenograft (PDX) models.