PPARγ gene expression is autoregulated in primary adipocytes: ligand, sumoylation, and isoform specificity

Being a key-factor in glucose homeostasis, PPARγ transcriptional activity (TA) is of high importance. However, its mediation by ligands and post-translational modifications in insulin target tissues are unclear. We investigated effects of rosiglitazone (Rg) and sumoylation on PPARγ-TA by overexpressing expression vectors and promoter-reporters for PPARγ1 and PPARγ2 in primary rat adipocytes. Wild type (WT) PPARγ1 and PPARγ2 dose-dependently repressed transcription from their promoters to a maximum of 40-50%. PPARγ2 mutants defective in either MAP-kinase phosphorylation (S112A) or the ligand-binding domain (LBD; P495L, L496A/E499A) exhibited decreased repression of PPARγ2 promoter. Rg enhanced repression by S112A, but not by LBD-defective mutants. Sumoylation-defective PPARγ1 mutants K77R and K365R repressed PPARγ2 promoter activity similar to WT, while Rg enhanced repression by K77R but not by K365R. Sumoylation-defective PPARγ2 mutants K107R and K395R exhibited impaired TA and impaired responsiveness to Rg. GLUT4 promoter, previously shown by us to be repressed by WT-PPARγ1 and WT-PPARγ2, was similarly repressed by both sumoylation-defective PPARγ1 mutants, while both sumoylation-defective PPARγ2 mutants exerted reduced repression. Surprisingly, Rg alleviated only WT-PPARγ2-induced repression, while augmenting that induced by WT-PPARγ1 and all sumoylation-defective mutants. Promoter and chromatin immunoprecipitation analyses revealed that PPARγ2 autorepression involves its direct binding to its promoter. In concert with effects at promoter level, Rg decreased endogenous level of PPARγ2 mRNA, while increasing that of GLUT4. We suggest a hypothetical model for PPARγ gene regulation in primary adipocytes that is isoform-specific and Rg/sumoylation-dependent. These findings are important due to the role of PPARγ and Rg in insulin sensitivity.