Project Summary: Advanced metastatic castration-resistant prostate cancer (CRPC) is an aggressive disease with high mortality rate, primarily resulting from the transcriptional addiction driven by Androgen Receptor (AR) signaling. The evolutionarily conserved multi-subunit Mediator complex plays a central role in the regulation of transcription by virtue of its ability to functionally bridge gene-specific transcription factors with the RNA polymerase II- associated basal transcription machinery. MED1 is a key component of the Mediator complex and is responsible for targeting and anchoring the complex to a broad range of nuclear receptors, including AR. We have identified phosphorylation of MED1 catalyzed by CDK7 transcriptional kinase is required for its interaction with AR and as a rate-limiting step in AR-mediated transcription. The underlying hypothesis of this proposal is that the CDK7 mediated phosphorylation of MED1 is necessary for the formation and stability of MED1-AR complex at the chromatin in both naïve and anti-androgen refractory CRPC which could be targeted by CDK7 specific inhibitors. The goals of this grant application are to investigate the mechanistic basis of MED1-AR interaction further, and evaluate the CDK7 specific inhibitors in reversing the AR-dependent transcriptional addiction in advanced prostate cancer. The three specific aims of the projects are: Specific Aim 1: Investigate the role of p-MED1 in hyper-activation of AR-signaling Specific Aim 2: Investigate the mechanism of increased p-MED1 in enzalutamide refractory PCa. Specific Aim 3: Establish the efficacy of CDK7 inhibitor in clinically relevant naïve and refractory CRPC models in vivo.