Abstract 141: Epigenetic silencing of miR-122 in hepatocellular carcinoma

Abstract

Abstract It has been reported that miRNAs potently influence cellular function through the regulation of extensive gene expression networks and most tumors are characterized by globally diminished miRNA expression. However, the potential mechanism of microRNA expression change in tumorigenesis remains to be further investigated. In this study, we focus on the epigenetic regulation of miRNA expression in hepatocellular carcinoma. We first performed miRNA microarray analysis in HepG2 cells, which reveals significant changes in miRNA expression profiling with 23 upregulated miRNAs and 15 downregulated miRNAs induced by the treatment with 5′aza-2′deoxycytidine (5′-aza, DNA methylation inhibitor) and 4-phenylbutyric acid (PBA, histone deaceltylation inhibitor). Interestingly, miR-122, the most abundant miRNA in the liver, is upregulated by 5′-aza and PBA, which is validated by qRT-PCR analysis in both HepG2 and Huh7 cells. In addition, miR-122 expression is increased by the overexpression of PPARγ or the treatment of PPARγ agonist (15-keto PGE2 or 15d-PGJ2).We then demonstrate that peroxisome proliferator-activated receptor gamma (PPARγ) and retinoid X receptor alpha (RXRα) bind to conserved DR-1 and DR-2 (peroxisome proliferator-activated response element) in the miR-122 promoter region and these bindings are significantly increased by 5′-aza and PBA with the biotinylated oligonicleotides precipitation assay. However, 5′-aza and PBA inhibit the binding of N-CoR (nuclear receptor corepressor) or SMRT (silencing mediator for retinoid and thyroid hormone receptor) to DR-1 or DR-2. And we found that 5′-aza and PBA inhibit PPARγ/N-CoR/SMRT protein complex formation in HepG2 cells with the immunoprecipitation assay. Furthermore, we observe that SUV39H1(H3K9 methyl transferase) protein is decreased by the co-treatment of 5′aza and PBA; and the suppression of endogenous SUV39H1 expression with SUV39H1siRNA increases miR-122 expression. Taken together, our results suggest that histone acethylation and demethylation lead to the release of PPAR-gamma from PPAR-gamma/N-CoR/SMRT protein complex to activate miR-122 transcription and to the inhibition SUV39H1 protein expression for the elimination of histone methylation to further facilitate miR-122 transcription. In conclusion, Epigenetic silencing is one of the most important mechanisms for diminished miR122 expression in hepatic carcinogenesis. 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 141. doi:1538-7445.AM2012-141