Studies on the roles of HMGB2 and REST in cancer cells

Abstract

High-mobility group box (HMGB) proteins are ubiquitous, abundant nuclear proteins and have diverse functions in the cell. In the present study, we explored roles of HMGB2 in HPV positive cancer and hepatocellular carcinoma.We investigated the effect of HMGB2 on the stability of p53 protein in HeLa cells. Overexpression of HMGB2 led to accumulation of the p53 protein, whereas HMGB2 knockdown with siRNA resulted in a substantial decrease in the p53 protein level. The HMGB2-dependent increase of p53 stability was specific for HPV-positive HeLa cells as HCT116 and MCF7 cell lines did not demonstrate this response. Co-expression of HMGB2 and HPV E6 prevented HPV E6 protein-mediated ubiquitination and degradation of p53. FACS analysis exhibited that HeLa cells transfected with HMGB2 displayed decreased cell proliferation, with a concomitant increase of the p53 protein and arrest of the cell cycle, predominantly in G1 phase. Our findings collectively suggest that HMGB2 could stabilize p53 by interfering with E6/E6AP-mediated p53 degradation in HPV-positive HeLa cells. We investigated the expression of high mobility group box 2 (HMGB2) in patients with hepatocellular carcinoma (HCC) and its clinical effects with underlying mechanisms. HMGB2 mRNA levels were measured in 334 HCC patients by real-time reverse transcription PCR and HMGB2 protein levels in 173 HCC patients by immunohistochemical studies. The HMGB2 expression level was measured by western blotting for three HCC cell lines. To clarify the precise role of HMGB2 on cell proliferation, we performed in vitro analysis with expression vectors and small interfering RNAs. HMGB2 mRNA and protein expression were significantly higher in HCC than in non-cancerous surrounding tissues and showed a positive correlation. HMGB2 overexpression was significantly correlated with shorter overall survival (OS) time both at mRNA (P = 0.0054) and protein level (P = 0.023). Moreover, HMGB2 mRNA level was an independent prognostic factor for OS in a multivariate analysis (P = 0.0037). HMGB2-knockdown by siRNAs decreased cell proliferation and overexpression of HMGB2 by expression vectors diminished cisplatin and etoposide-induced cell death. Our clinical and in vitro data suggest that HMGB2 plays a significant role in tumor development and prognosis of HCC. These results can partly be explained by altered cell proliferations by HMGB2 associated with the anti-apoptotic pathway. REST is a neuronal gene silencing factor ubiquitously expressed in non-neuronal tissues. REST is additionally believed to serve as a tumor suppressor in non-neuronal cancers. Conversely, recent findings on REST-dependent tumorigenesis in non-neuronal cells consistently suggest a potential role of REST as a tumor promoter. Here, we have uncovered for the first time the mechanism by which REST contributes to cancer cell survival in non-neuronal cancers. We observed abundant expression of REST in various types of non-neuronal cancer cells compared to normal tissues. The delicate roles of REST were further evaluated in HCT116 and HeLa, non-neuronal cancer cell lines expressing REST. REST silencing resulted in decreased cell survival and activation of the DNA damage response (DDR) through a decrease in the level of TRF2, a telomere-binding protein. These responses were correlated with reduced colony formation ability and accelerated telomere shortening in cancer cells upon the stable knockdown of REST. Interestingly, REST was down-regulated under oxidative stress conditions via ubiquitin proteasome system, suggesting that sustainability of REST expression is critical to determine cell survival during oxidative stress in a tumor microenvironment. Our results collectively indicate that REST-dependent TRF2 expression renders cancer cells resistant to DNA damage during oxidative stress, and mechanisms to overcome oxidative stress, such as high levels of REST or the stress-resistant REST mutants found in specific human cancers, may account for REST-dependent tumorigenesis.