Previous studies from others and us have implicated the SOX9 transcription factor in prostate cancer (PCa), and our recent identification of SOX9 as a downstream effector of ERG in TMPRSS2:ERG fusion positive PCa further strongly supports a major role for SOX9. We hypothesize that SOX9 contributes to both TMPRSS2:ERG fusion positive and negative PCa through its regulation of multiple genes that mediate functions including ductal morphogenesis and maintenance of stem/progenitor cells. Our overall goals are to elucidate the regulation, actions and therapeutic implications of SOX9 expression in PCa. Aim 1 focuses on further mechanisms that regulate SOX9 expression in fusion positive and negative PCa or precursor lesions, and in particular stromal growth factors including FGFs, HGF and Wnts. We hypothesize that the aberrant expression of SOX9 driven by these mechanisms may be an early event that can be targeted therapeutically, and may be a predictive biomarker for responses to therapies that incorporate androgen deprivation. Aim 2 will build on our preliminary SOX9 ChIP- seq and transcriptional profiling studies to identify the critical genes andpathways regulated by SOX9 in PCa. Significantly, our data indicate that SOX9 directly positively regulates the expression of multiple genes involved in Wnt signaling through the canonical Wnt/b-catenin/TCF pathway and an alternative Wnt/b-catenin/YAP1 pathway, and that it negatively regulates Wnt5a. Aim 2 will also extend our preliminary studies indicating that SOX9 regulates a series of chemokines including IL-8 that can stimulate inflammatory responses and angiogenesis. Finally, Aim 3 will use mice with prostate specific overexpression of SOX9 and clinical samples to evaluate the biological significance of SOX9 regulated genes and pathways identified in Aim 2. The specific aims are: 1) Identify molecular mechanisms regulating SOX9 expression in PCa, 2) Identify genes and pathways directly regulated by SOX9 in PCa cells, and 3) Determine the role of SOX9 regulated genes in PCa development in mice with prostate specific PTEN loss and in patient samples.