Abstract 4545: The E3 ubiquitin ligase COP1 controls STAT3 turnover and its loss leads to increased STAT3 stabilization and activation in prostate cancer

The E3 ubiquitin ligase COP1 acts as a tumor suppressor and is deleted in a small percentage of prostate cancers. We reported that COP1 was repressed in prostate tumors through miRNA-mediated silencing, which represents an alternative and more frequent event than genetic deletion. COP1 controls ubiquitination and turnover of c-Jun and ETV1 and its loss has been associated with over-expression of these transcription factors. In this study, we identify STAT3 as a novel substrate of COP1 and report that concomitant deregulation of COP1 and STAT3 leads to prostate cancer progression. In a panel of prostate cancer cell lines, low expression of COP1 was associated with increased level of STAT3 protein and a more aggressive phenotype. Knockdown of COP1 in normal prostate epithelial RWPE-1 cells increased total (tSTAT3) and phosphorylated STAT3 (pSTAT3) and promoted tumorigenic properties. Conversely, over-expression of COP1 in DU145 cells, expressing low level of COP1, reversed the transformed phenotype and reduced tSTAT3 and pSTAT3 level and activation. COP1/STAT3 anti-correlation suggested that STAT3 was a substrate of COP1 for ubiquitination and degradation by the ubiquitin-proteasome system. Consistently, the proteasome inhibitor PS-341 prevented down-regulation of STAT3 in response to COP1 in DU145 cells. Furthermore, COP1 knockdown delayed significantly STAT3 protein turnover in RWPE-1 cells, indicating that COP1 regulated STAT3 degradation in these cells. Co-IP showed that COP1 and STAT3 directly interacted in RWPE-1 and DU145 cells. Interestingly, co-IP with the wild type and phosphorylation defective Y705F mutant STAT3 showed that the interaction with COP1 did not depend on STAT3 phosphorylation. Furthermore, the level of ubiquitinated STAT3 in RWPE1 cells was reduced after COP1 knockdown, whereas increased in DU145 cells after COP1 over-expression. These data provided evidence of COP1-dependent ubiquitination and degradation of STAT3 in normal prostate epithelial cells and, conversely, STAT3 accumulation and activation in prostate cancer cells with loss of COP1. To assess the clinical relevance of these findings, we examined the level of COP1, tSTAT3 and pSTAT3 by IHC in primary prostate tumors (n = 136) from patients with long-term clinical follow up. Low COP1 levels were significantly associated with high tSTAT3 expression (Fisher test p