Potentiation of insulin secretion by phorbol esters is mediated by PKC-α and nPKC isoforms

Abstract

Culturing clonal β-cells (HIT-T15) overnight in the presence of phorbol ester [phorbol myristate acetate (PMA)] enhanced insulin secretion while causing downregulation of some protein kinase C (PKC) isoforms and most PKC activity. We show here that this enhanced secretion required the retention of PMA in the cell. Hence, it could not be because of long-lived phosphorylation of cellular substrates by the isoforms that were downregulated, namely PKC-α, -βII, and -ε, but could be because of the continued activation of the two remaining diacylglycerol-sensitive isoforms δ and μ. The enhanced secretion did not involve changes in glucose metabolism, cell membrane potential, or intracellular Ca2+handling, suggesting a distal effect. PMA washout caused the loss of the enhanced response, but secretion was then stimulated by acute readdition of PMA or bombesin. The magnitude of this restimulation appeared dependent on the mass of PKC-α, which was rapidly resynthesized during PMA washout. Therefore, stimulation of insulin secretion by PMA, and presumably by endogenous diacylglycerol, involves the activation of PKC isoforms δ and/or μ, and also PKC-α.