Abstract 3939: Hypoxia-induced BRCA1 phosphorylation and degradation

Abstract

Abstract Hypoxia is a key feature of tumor microenviroment. We have demonstrated that hypoxia downregulates BRCA1 at the transcriptional level through promoter binding by E2F4/p130 complexes and that hypoxia can produce BRCA1 gene silencing via epigenetic modifications. We also demonstrated that downregulation of BRCA1 by hypoxia leads to decreased DNA repair capacity, establishing a mechanism by which hypoxia can drive genetic instability in cancer cells. In addition to transcriptional regulation, BRCA1 has been found to be phosphorylated at S988 in a Chk2-dependent manner by both ionizing radiation and hypoxia exposure. In this study, we have been able to confirm that other residues of BRCA1 are phosphoylated during hypoxia exposure, including S1387, S1423, S1466, and S1524. Intriguingly, the phosphoryation of BRCA1 only happens under severe hypoxic condition, such as 0.01% oxygen, but not under moderate hypoxia (1% oxygen condition). Furthermore, the BRCA1 phosphoylation events induced by hypoxia can be attenuated by an ATM inhibitor, KU55933, indicating that ATM is playing a role in hypoxia-induced BRCA1 phosphorylation at multiple sites. We also observed that the decrease in BRCA1 protein levels in hypoxia is partially blocked by MG132, a proteasome inhibitor. Consistent with this, by transient transfection of exogenous BRCA1, we found that hypoxia induces BRCA1 unbiquitination in MCF-7 cells. Together, our data suggest that hypoxia-induced downregulation of BRCA1 is in part mediated through the ubiqutin-proteasome pathway. Our findings indicate a new mechanism of hypoxia induced regulation of BRCA1 by modulating its stability, suggesting that hypoxia-induced downregulation of BRCA1 is through multiple mechanisms. Future studies will investigate the mechanism of hypoxia-induced BRCA1 phophorylation and the relation between hypoxia-induced BRCA1 phosphorylation and BRCA1 degradation. To understand more about regulation of BRCA1 by hypoxia could lead to new therapies to target hypoxic tumors. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3939. doi:1538-7445.AM2012-3939