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Mol. Cell. Biol. doi:10.1128/MCB.00073-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Evidence of Endogenous Mu Opioid Receptor Regulation by Epigenetic Control of the Promoters

Department of Pharmacology, University of Minnesota Medical School, Minneapolis, Minnesota 55455

^* To whom correspondence should be addressed. Email: hwang025{at}umn.edu.

	Abstract

The pharmacological effect of morphine as a painkiller is mediated mainly via the mu opioid receptor (MOR) and dependent on the number of MOR in the cell surface membrane. While several studies have reported that the MOR gene is regulated by various cis- and trans-acting factors, many questions remain unanswered regarding in vivo regulation. The present study shows that epigenetic silencing and activation of the MOR gene is achieved through coordinated regulation at both the histone and DNA levels. In P19 mouse embryonal carcinoma cells, expression of the MOR was greatly increased after neuronal differentiation. MOR expression could also be induced by a demethylating agent (5-aza-dC) or HDAC inhibitors in the P19 cells, suggesting involvement of DNA methylation and histone deacetylation for MOR gene silencing. Analysis of CpG DNA methylation revealed that the proximal promoter region was unmethylated in differentiated cells compared to its hypermethylation in undifferentiated cells. In contrast, the methylation of other regions was not changed in either cell type. Similar methylation patterns were observed in mouse brain. In vitro methylation of the MOR promoters suppressed promoter activity in the reporter assay. Upon differentiation, the in vivo interaction of MeCP2 was reduced in the MOR promoter region, coincident with histone modifications that are relevant to active transcription. When MeCP2 was disrupted using MeCP2 siRNA, the endogenous MOR gene was increased. These data suggest that DNA methylation is closely linked to the MeCP2-mediated chromatin structure of the MOR gene. Here we propose that an epigenetic mechanism consisting of DNA methylation and chromatin modification underlies the cell stage-specific mechanism of MOR gene expression.