RESUMO
Enhancer of zeste homolog 2 (EZH2) catalyses histone H3 lysine 27 trimethylation (H3K27me3) to silence tumour-suppressor genes in hepatocellular carcinoma (HCC) but the process of locus-specific recruitment remains elusive. Here we investigated the transcription factors involved and the molecular consequences in HCC development. The genome-wide distribution of H3K27me3 was determined by chromatin immunoprecipitation coupled with high-throughput sequencing or promoter array analyses in HCC cells from hepatitis B virus (HBV) X protein transgenic mouse and human cell models. Transcription factor binding site analysis was performed to identify EZH2-interacting transcription factors followed by functional characterization. Our cross-species integrative analysis revealed a crucial link between Yin Yang 1 (YY1) and EZH2-mediated H3K27me3 in HCC. Gene expression analysis of human HBV-associated HCC specimens demonstrated concordant overexpression of YY1 and EZH2, which correlated with poor survival of patients in advanced stages. The YY1 binding motif was significantly enriched in both in vivo and in vitro H3K27me3-occupied genes, including genes for 15 tumour-suppressive microRNAs. Knockdown of YY1 reduced not only global H3K27me3 levels, but also EZH2 and H3K27me3 promoter occupancy and DNA methylation, leading to the transcriptional up-regulation of microRNA-9 isoforms in HCC cells. Concurrent EZH2 knockdown and 5-aza-2'-deoxycytidine treatment synergistically increased the levels of microRNA-9, which reduced the expression and transcriptional activity of nuclear factor-κB (NF-κB). Functionally, YY1 promoted HCC tumourigenicity and inhibited apoptosis of HCC cells, at least partially through NF-κB activation. In conclusion, YY1 overexpression contributes to EZH2 recruitment for H3K27me3-mediated silencing of tumour-suppressive microRNAs, thereby activating NF-κB signalling in hepatocarcinogenesis.
Assuntos
Carcinoma Hepatocelular/metabolismo , Inativação Gênica , Neoplasias Hepáticas/metabolismo , MicroRNAs/metabolismo , NF-kappa B/metabolismo , Fator de Transcrição YY1/metabolismo , Animais , Apoptose , Sítios de Ligação , Carcinoma Hepatocelular/genética , Carcinoma Hepatocelular/patologia , Carcinoma Hepatocelular/virologia , Linhagem Celular Tumoral , Proliferação de Células , Metilação de DNA , Proteína Potenciadora do Homólogo 2 de Zeste , Regulação Neoplásica da Expressão Gênica , Histonas/metabolismo , Humanos , Neoplasias Hepáticas/genética , Neoplasias Hepáticas/patologia , Neoplasias Hepáticas/virologia , Lisina , Metilação , Camundongos Nus , Camundongos Transgênicos , MicroRNAs/genética , Complexo Repressor Polycomb 2/genética , Complexo Repressor Polycomb 2/metabolismo , Regiões Promotoras Genéticas , Interferência de RNA , Transdução de Sinais , Fatores de Tempo , Transativadores/genética , Transativadores/metabolismo , Transfecção , Carga Tumoral , Regulação para Cima , Proteínas Virais Reguladoras e Acessórias , Fator de Transcrição YY1/genéticaRESUMO
EZH2 is the histone H3 lysine 27 methyltransferase of polycomb-repressive complex 2. It transcriptionally silences cohorts of developmental regulators in stem/progenitors and cancer cells. EZH2 is essential in maintaining stem cell identity by globally repressing differentiation programs. Analogously, it plays a key role in oncogenesis by targeting signaling molecules that control cell differentiation. Emerging data indicate that EZH2 promotes cancer formation and progression through epigenetic activation of oncogenic signaling cascades and inhibition of pro-differentiation pathways. Genome-wide mapping analysis has been expanding the repertoire of target genes and the associated signaling pathways regulated by EZH2. Better understanding of the molecular basis of such regulations in various cancer types will help establish EZH2-mediated epigenetic silencing as a therapeutic target.