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 PPARgamma 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 PPARgamma was studied in Chinese hamster ovary cells expressing insulin receptors (CHO.T), PPARgamma, and reporter genes. Both TZD and insulin independently stimulated PPARgamma-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 PPARgamma; transactivation was markedly increased in cells co-transfected with constitutively active MKK1. Both insulin and constitutively active MKK1 also stimulated 32P incorporation into PPARgamma in vivo. The conclusions are: 1) Insulin synergizes with a PPARgamma ligand and can activate the receptor in a ligand-independent fashion. 2) PPARgamma 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 PPARgamma function.