Ligand-independent Activation of Peroxisome Proliferator-activated Receptor-γ by Insulin and C-peptide in Kidney Proximal Tubular Cells

Peroxisome proliferator-activated receptor γ (PPARγ) has key roles in the regulation of adipogenesis, inflammation, and lipid and glucose metabolism. C-peptide is believed to be inert and without appreciable biological functions. Recent studies suggest that C-peptide possesses multiple functions. The present study investigated the effects of insulin and C-peptide on PPARγ transcriptional activity in opossum kidney proximal tubular cells. Both insulin and C-peptide induced a concentration-dependent stimulation of PPARγ transcriptional activity. Both agents substantially augmented thiazolidinedione-stimulated PPARγ transcriptional activity. Neither insulin nor C-peptide had any effect on the expression levels of PPARγ. GW9662, a PPARγ antagonist, blocked PPARγ activation by thiazolidinediones but had no effect on either insulin- or C-peptide-stimulated PPARγ transcriptional activity. Co-transfection of opossum kidney cells with dominant negative mitogen-activated protein kinase kinase significantly depressed basal PPARγ transcriptional activity but had no effect on that induced by either insulin or C-peptide. Both insulin- and C-peptide-stimulated PPARγ transcriptional activity were attenuated by wortmannin and by expression of a dominant negative phosphatidylinositol (PI) 3-kinase p85 regulatory subunit. In addition PI 3-kinase-dependent phosphorylation of PPARγ was observed after stimulation by C-peptide or insulin. C-peptide effects but not insulin on PPARγ transcriptional activity were abolished by pertussis toxin pretreatment. Finally both C-peptide and insulin positively control the expression of the PPARγ-regulated CD36 scavenger receptor in human THP-1 monocytes. We concluded that insulin and C-peptide can stimulate PPARγ activity in a ligand-independent fashion and that this effect is mediated by PI 3-kinase. These results support a new and potentially important physiological role for C-peptide in regulation of PPARγ-related cell functions.