Abstract
BACKGROUND: Adipose tissues serve not only as a store for energy in the form of lipid, but also as endocrine tissues that regulates metabolic activities of the organism by secreting various kinds of hormones. Peroxisome proliferator activated receptor gamma (PPARgamma) is a key regulator of adipocyte differentiation that induces the expression of adipocyte-specific genes in preadipocytes and mediates their differentiation into adipocytes. Furthermore, PPARgamma has an important role to maintain the physiological function of mature adipocyte by controlling expressions of various genes properly. Therefore, any reduction in amount and activity of PPARgamma is linked to the pathogenesis of metabolic syndrome. RESULTS: In this study, we investigated the contribution of epigenetic transcriptional regulatory mechanisms, such as DNA methylation, to the expression of the PPARgamma gene, and further evaluated the contribution of such epigenetic regulatory mechanisms to the pathogenesis of metabolic syndrome. In 3T3-L1 preadipocytes, the promoter of the PPARgamma2 gene was hypermethylated, but was progressively demethylated upon induction of differentiation, which was accompanied by an increase of mRNA expression. Moreover, treatment of cells with 5'-aza-cytideine, an inhibitor of DNA methylation, increased expression of the PPARgamma gene in a dose-dependent manner. Methylation in vitro of a PPARgamma promoter-driven reporter construct also repressed the transcription of a downstream reporter gene. These results suggest that the expression of the PPARgamma gene is inhibited by methylation of its promoter. We next compared the methylation status of the PPARgamma promoters in adipocytes from wild-type (WT) mice with those from two diabetic mouse models: +Leprdb/+Leprdb and diet-induced obesity mice. Interestingly, we found increased methylation of the PPARgamma promoter in visceral adipose tissues (VAT) of the mouse models of diabetes, compared to that observed in wild-type mice. We observed a concomitant decrease in the level of PPARgamma mRNA in the diabetic mice compared to the WT mice. CONCLUSION: We conclude that the expression of PPARgamma gene is regulated by DNA methylation of its promoter region and propose that reduced expression of PPARgamma owing to DNA methylation in adipocytes of the VAT may contribute to the pathogenesis of metabolic syndrome.