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Methyl-CpG Binding Domain Protein 2 Represses Transcription from Hypermethylated π-Class Glutathione S-Transferase Gene Promoters in Hepatocellular Carcinoma Cells*

J. Bakker,Xiaohui Lin,W. Nelson

Published 2002 in Journal of Biological Chemistry

ABSTRACT

During the pathogenesis of human hepatocellular carcinoma (HCC), the CpG island encompassing the π-class glutathione S-transferase gene (GSTP1) becomes hypermethylated. Repression of transcription accompanying CpG island hypermethylation has been proposed to be mediated by methyl-CpG binding domain (MBD) proteins. We report here that inhibition of transcription from hypermethylatedGSTP1 promoters in Hep3B HCC cells, which fail to expressGSTP1 mRNA or GSTP1 polypeptides, appears to be mediated by MBD2. Treatment of Hep3B cells with 5-azadeoxycytidine (5-aza-dC), a methyltransferase inhibitor, activated GSTP1expression, whereas treatment with trichostatin A, a histone deacetylase inhibitor, had little effect. To more precisely assess the contribution of the pattern of GSTP1 CpG island methylation on GSTP1 mRNA expression, Hep3B cells were treated for 72 h with 5-aza-dC and then subjected to limiting dilution cloning. Bisulfite sequencing was used to map the methylation patterns of the GSTP1 promoter region inGSTP1-expressing and -non-expressing clones. In the clone that expressed GSTP1 mRNA determined by Northern blot analysis and quantitative reverse transcriptase (RT)-PCR, widespread demethylation of at least one GSTP1allele was evident. Chromatin immunoprecipitation experiments revealed the presence of MBD2, but not Sp1, at the GSTP1 promoter in Hep3B cells. In contrast, Sp1 was detected at the GSTP1promoter in a GSTP1-expressing Hep3B 5-aza-dC subclone. To test whether MBD2 might be responsible for the inhibition ofGSTP1 transcription from hypermethylated GSTP1promoters, siRNAs were used to reduce MBD2 polypeptide levels in Hep3B cells. SssI-catalyzed methylation ofGSTP1 promoter sequences resulted in diminished luciferase reporter activity after transfection into Hep3B cells. However, when hypermethylated GSTP1 promoter sequences were transfected into Hep3B cells that had been treated with siRNA-targetingMBD2 mRNA, no repression of luciferase reporter expression was evident. These findings implicate MBD2 in the repression of GSTP1 expression associated with GSTP1 CpG island hypermethylation in HCC cells.

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