Peroxisome Proliferator-activated Receptor-γ Represses GLUT4 Promoter Activity in Primary Adipocytes, and Rosiglitazone Alleviates This Effect

Abstract

The synthetic thiazolidinedione ligands of peroxisome proliferator-activated receptor-␥ (PPAR␥) improve insulin sensitivity in type II diabetes and induce GLUT4 mRNA expression in fat and muscle. However, the molecular mechanisms involved are still unclear. We studied the regulatory effects of PPAR␥ and its ligands on GLUT4 gene expression in primary rat adipocytes and CHO-K1 cells cotransfected with PPAR␥ and the GLUT4 promoter reporter. PPAR␥1 and PPAR␥2 repressed the activity of the GLUT4 promoter in a dose-dependent manner. Whereas this repression was augmented by the natural ligand 15⌬-prostaglandin J 2 , it was completely alleviated by rosiglitazone (Rg). Ligand binding-defective mutants PPAR␥1-L468A/E471A and PPAR␥2-L496A/ E499A retained the repression effect, which was unaffected by Rg, whereas the PPAR␥2-S112A mutant exhibited a 50% reduced capacity to repress GLUT4 promoter activity. The ؊66/؉163 bp GLUT4 promoter region was sufficient to mediate PPAR␥ inhibitory effects. The PPAR␥/retinoid X receptor-␣ heterodimer directly bound to this region, whereas binding was abolished in the presence of Rg. Thus, we show that PPAR␥ represses transcriptional activity of the GLUT4 promoter via direct and specific binding of PPAR␥/retinoid X receptor-␣ to the GLUT4 promoter. This effect requires an intact Ser 112 phosphorylation site on PPAR␥ and is completely alleviated by Rg, acting via its ligand-binding domain. These data suggest a novel mechanism by which Rg exerts its antidiabetic effects via detaching PPAR␥ from the GLUT4 gene promoter, thus leading to increased GLUT4 expression and enhanced insulin sensitivity.

Peroxisome proliferator activated receptor gamma activity is low in mature primary human visceral adipocytes

“…It has earlier been demonstrated that the insulin sensitive GLUT4 (now known as solute carrier family 2 [facilitated glucose transporter], member 4 [SLC2A4]) promoter is repressed by PPARG in primary adipocytes [22]. Consequently, the low PPARG activity in the intra-abdominal fat cells is in line with the high levels of GLUT4 in intraabdominal as compared with subcutaneous adipocytes that were reported by us earlier [23].…”

Improved insulin sensitivity and adipose tissue dysregulation after short-term treatment with pioglitazone in non-diabetic, insulin-resistant subjects

“…However, a recent study has shown that the PPAR/retinoic X receptor heterodimer exerts an inhibitory effect on GLUT4 promoter activity, and that thiazolidinedione binding to the receptor alleviates this repression [37]. However, other mechanisms could also be involved.…”

Regulation of glucose transporter type 4 isoform gene expression in muscle and adipocytes

“…They showed how a PPAR-g ligand, rosiglitazone, ameliorates insulin resistance in NIDDM. Rosiglitazone increases GLUT4 mRNA level in the diabetic model, whereas 15-deoxy-D12,14-prostaglandin J2 (15 D-PGJ2) represses GLUT4 gene expression in myocytes and adipocytes (121). PPAR-g-mediated suppression occurs through the binding of the PPAR-g/RXR-a heterodimer to the PPRE in the GLUT4 promoter, and the repression is relieved by synthetic ligands through the ligand-binding domain.…”

“…Its ligand regulates glucose homeostasis and lipid metabolism (120). Armoni et al (121) identified a PPAR-g response element (PPRE) in the 766/þ163 bp region of the GLUT4 promoter, suggesting that PPAR-g down-regulates GLUT4 gene transcription through direct binding to the PPRE. However, with regard to the regulation of GLUT4 gene expression, some conflicting results were obtained from the experiments using natural and synthetic ligands.…”