RESUMEN
Histone modification and chromatin remodeling are important events in response to DNA damage, and Polycomb group (PcG) proteins, catalyzing H3K27 methylation, are involved. However, the biological function and mechanism of PcG in DNA damage are not fully understood. Additionally, downstream effectors in hepatocellular carcinoma (HCC) remain unclear. The present study investigated the biological and mechanistic roles of PcG in the DNA damage response induced by chemotherapeutic drugs in HCC. It was found that chemotherapy drugs, such as epirubicin (EPB) and mitomycin C (MMC), effectively blocked expression of PcG in p53-wild-type HepG2 cells but not in PLC/PRF5 and Hep3B cells with p53 mutation or deletion. PcG-related target genes involved in DNA damage were identified, including p53, Ataxia telangiectasia mutated (ATM) and Forkhead box O3 (FOXO3). Moreover, targeting PcG-induced p53 expression was associated with increased drug sensitivity in HCC cells. shRNA targeting enhancer of zeste homolog 2 (EZH2) or its inhibitor GSK126 significantly promoted chemotherapeutic drug-induced genotoxicity and increased HepG2 cell chemosensitivity. Mechanistically, chromatin immunoprecipitation (ChIP) assays confirmed that PcG binds to the ATM promoter and inhibits its expression through covalent modification of H3K27me3. Herein, we establish a potential chemotherapy association with GSK126, and the findings suggest this link might represent a new strategy for increasing the sensitivity of HCC to chemotherapeutic agents.
RESUMEN
UNLABELLED: Alterations of polycomb group (PcG) genes directly modulate the trimethylation of histone H3 lysine 27 (H3K27me3) and may thus affect the epigenome of hepatocellular carcinoma (HCC), which is crucial for controlling the HCC cell phenotype. However, the extent of downstream regulation by PcGs in HCC is not well defined. Using cDNA microarray analysis, we found that the target gene network of PcGs contains well-established genes, such as cyclin-dependent kinase inhibitors (CDKN2A), and genes that were previously undescribed for their regulation by PcG, including E2F1, NOTCH2, and TP53. Using chromatin immunoprecipitation assays, we demonstrated that EZH2 occupancy coincides with H3K27me3 at E2F1 and NOTCH2 promoters. Interestingly, PcG repress the expression of the typical tumor suppressor TP53 in human HCC cells, and an increased level of PcG was correlated with the downregulation of TP53 in certain HCC specimens. Unexpectedly, we did not find obvious H3K27me3 modification or an EZH2 binding signal at the TP53 promoters, suggesting that PcG regulates TP53 expression in an H3K27me3-independent manner. Finally, the reduced expression of PcGs effectively blocked the aggressive signature of liver cancer cells in vitro and in vivo. IMPLICATIONS: Taken together, our results establish the functional and mechanistic significance of certain gene regulatory networks that are regulated by PcGs in HCC.