Insulin- and mitogen-activated protein kinase-mediated phosphorylation and activation of peroxisome proliferator-activated receptor gamma

Peroxisome proliferator-activated receptor (PPAR) gamma plays an important role in adipocyte differentiation and the regulation of adipocyte gene expression. Insulin also serves to promote adipogenesis. We report that insulin and a PPAR gamma ligand (thiazolidinedione (TZD)) stimulate in a synergistic manner the expression of an adipocyte-specific gene (aP2) in rat adipocytes and 3T3-L1 cells. Potential cross-talk between insulin signaling and PPAR gamma was studied in Chinese hamster ovary cells expressing insulin receptors (CHO.T), PPAR gamma, and reporter genes. Both TZD and insulin independently stimulated PPAR gamma-mediated transactivation of aP2 promoter-luciferase reporter genes; both agents combined resulted in a synergistic effect. Co-transfection of CHO.T cells with dominant-negative mitogen-activated protein (MAP) kinase-kinase (MKK1) abrogated both insulin- and TZD-mediated activation of PPAR gamma; transactivation was markedly increased in cells co-transfected with constitutively active MKK1. Both insulin and constitutively active MKK1 also stimulated P-32 incorporation into PPAR gamma in vivo. The conclusions are: 1) insulin synergizes with a PPAR gamma ligand and can activate the receptor in a ligand-independent fashion. 2) PPAR gamma is phosphorylated in vivo by insulin stimulation or activation of the MAP kinase pathway. 3) MAP kinase is an important mediator of cross-talk between insulin signal transduction pathways and PPAR gamma function.