PROJECT SUMMARY PROJECT 3 Prostate cancer with a metastatic castration-resistant phenotype is a leading cause of cancer-related deaths among men in the United States and worldwide, affecting African Americans disproportionately. The cellular and molecular mechanisms that contribute to this deadly disease remain poorly understood, obstructing the development of an effective therapy that prolongs patients’ survival while preserving the quality of life. The primary goal of this research will be to uncover the mechanism of molecular and functional interactions between the transcriptional regulatory protein YAP1 and SCML2, a subunit of the Polycomb repressive complex 1, and to demonstrate that YAP1 and SCML2 cooperate to contribute to invasive prostate cancer. Using mass spectroscopy-based proteomic approaches, we have identified SCML2 as a binding partner of the nuclear YAP1 protein complexes from prostate cancer cell lines. Our data showed that androgen hormone signaling regulates YAP1 and SCML2 interaction in castration-sensitive and castration- resistant cell lines. Our studies also showed that silencing SCML2 in castration-sensitive cells resulted in androgen-independent cell growth and led to enzalutamide resistance. However, silencing SCML2 in castration-resistant cells had the opposite effect when exposed to androgen. In addition, our data showed that castration-resistant cells express significantly elevated SCML2 compared to castration-sensitive prostate cancer cells, and the SCML2 gene amplification could contribute to advanced disease. These observations support the overarching hypothesis that SCML2 and YAP1 cooperate to contribute to invasive prostate cancer. The primary objectives of this study are to uncover the mechanism of biochemical and functional interactions between the Hippo pathway effector YAP1 and Polycomb group protein SCML2 in prostate cancer cells (Aim 1) to determine how altered-SCML2 activity changes the growth behaviors of prostate tumor cell ex vivo and in vivo (Aim 2), and to evaluate the pattern of SCML2 expression in prostate cancer disparity tissues and correlated the results with the clinical progression of prostate cancer and race (Aim 3). This study will provide critical mechanistic insights into the functional and molecular interactions between the YAP1 and SCML2 regulating cell migration and metastasis. Thus, uncovering a new mechanism of the aggressive disease may benefit patient care through improved disease surveillance and targeted selection of patients for more effective targeted therapy, which may reduce disparities in prostate cancer mortality among men from African American communities. Lastly, YAP1 and SCML2 play a critical role in the biology of many cancers with poor outcomes, and therefore, knowledge gained through this research can benefit other cancers with poorer outcomes.