Rosiglitazone induction of Insig-1 in white adipose tissue reveals a novel interplay of peroxisome proliferator-activated receptor γ and sterol regulatory element-binding protein in the regulation of adipogenesis

Abstract

Insulin-induced gene 1 (INSIG-1) is a key regulator in the processing of the sterol regulatory element-binding proteins (SREBPs). We demonstrated that Insig-1 is regulated by peroxisome proliferator-activated receptor γ (PPARγ) providing a link between insulin sensitization/glucose homeostasis and lipid homeostasis. Insig-1 was identified as a PPARγ target gene using microarray analysis of mRNA from the white adipose tissue of diabetic (db/db) animals treated with PPARγ agonists. Insig-1 was induced in subcutaneous (9-fold) and epididymal (4-fold) fat pads from db/db mice treated for 8 days with the PPARγ agonist rosiglitazone (30 mg/kg/day). This in vivo response was confirmed in differentiated C3H10T1/2 adipocytes treated with rosiglitazone. To elucidate the molecular mechanisms regulating INSIG-1 expression, we cloned and characterized the human INSIG-1 promoter. Co-expression of PPARγ and RXRα transactivated the INSIG-1 promoter in the presence of PPARγ agonists. This induction was attenuated when a dominant negative PPARγ construct was transfected into cells. Furthermore, a PPARγ antagonist repressed the transactivation of the INSIG-1 promoter-reporter construct. Truncations of the promoter resulted in the identification of a PPAR response element that mediated the regulation of the promoter. We demonstrated with recombinant proteins that the PPARγ/RXRα heterodimer binds directly to this PPAR response element. In addition to regulation by PPARγ/RXRα, we demonstrated that the INSIG-1 promoter is regulated by transcriptionally active SREBP. The sterol response element was identified 380 base pairs upstream of the transcriptional start site. These findings suggest that the regulation of Insig-1 by PPARγ agonists could in turn regulate SREBP processing and thus couple insulin sensitizers with the regulation of lipid homeostasis.