Permissive action of protein kinase C-ζ in insulin-induced CD36- and GLUT4 translocation in cardiac myocytes

Abstract

Insulin stimulates cardiac long-chain fatty acid (LCFA) and glucose uptake via translocation of human homolog of rat fatty acid translocase (CD36) and GLUT4 respectively, from intracellular membrane compartments to the sarcolemma, a process dependent on the activation of phosphatidylinositol-3 kinase. To identify downstream kinases of insulin signaling involved in translocation of CD36 and GLUT4 in the heart, we tested i) which cardiac protein kinase C (PKC) isoforms (alpha, delta, epsilon or zeta) are activated by insulin, and ii) whether PKC isoform-specific inhibition affects insulin-stimulated substrate uptake in the heart. Insulin-stimulated LCFA and glucose uptake were completely blunted by inhibition of PKC-zeta, but not by inhibition of conventional or novel PKCs. Concomitantly, translocation of CD36 and GLUT4 to the sarcolemma was completely blunted upon inhibition of PKC-zeta. However, insulin, in contrast to the diacylglycerol-analog phorbol-12-myristate-13-acetate (PMA), did not induce membrane-attachment of the conventional and novel PKCs-alpha, -delta, and -epsilon. PKC-zeta was already entirely membrane-bound in non-stimulated cells, and neither insulin nor PMA treatment had any effect on the subcellular localization of PKC-zeta. Furthermore, insulin treatment did not change phosphorylation of PKC-alpha, -delta, and -zeta or enzymatic activity of PKC-zeta towards a PKC-zeta substrate peptide. It is concluded that PKC-zeta, but not any other PKC isoform, is necessary for insulin-induced translocation of GLUT4 and CD36. However, PKC-zeta is already fully active under basal conditions and not further activated by insulin, indicating that its role in insulin-stimulated uptake of both glucose and LCFA is permissive rather than regulatory.