Phorbol 12-myristate 13-acetate inhibits the antilipolytic action of insulin, probably via the activity of protein kinase Cε

Protein kinase CβI (PKCβI) mediates insulin signaling and attenuates β1-adrenoceptor-stimulated lipolysis. In this work, the effect of the PKC activator phorbol 12-myristate 13-acetate (PMA) on the antilipolytic action of insulin was determined by analyzing lipolysis induced by a β3-adrenoceptor agonist CL 316243. PMA inhibited the insulin antilipolytic action. The pan-PKC inhibitors GF 109203X and chelerythrine inhibited the PMA effect, but the PKCα/β inhibitors Gö 6976 and CGP 53353 did not. Exposure of cells to PMA downregulated PKCs α, βI, and δ within 3 h and PKCε within 12 h. The effect of PMA on insulin action greatly diminished when PKCε was downregulated. Inhibitors of phosphatidylinositol 3-kinase (PI3-K), Akt, and phosphodiesterase 3B (PDE3B) diminished the PMA effect. PMA inhibited insulin-stimulated phosphorylation of Tyr in insulin receptor β subunit and Ser/Thr in Akt. These data suggest that PMA inhibits the antilipolytic signal mediated by the insulin receptor, PI3-K, Akt, and PDE3B. The most probable target of PMA is PKCε.