A myristoylated pseudosubstrate peptide inhibitor of protein kinase C: Effects on glucose- and carbachol-induced insulin secretion

We have used synthetic pseudosubstrate peptide inhibitors of protein kinase C (PKC) to re-examine the role of conventional isoforms of PKC in the insulin secretory response of intact rat islets of Langerhans to glucose and to the cholinergic agonist carbachol (CCh). One peptide was modified by N-terminal myristoylation (PKC-myr(20-28)) to allow its use in intact beta-cells. Maximal inhibition of PKC activity in vitro required 10-fold less of this peptide (PKC-myr(20-28)) than of its non myristoylated analogue. The maximum inhibitory concentration of PKC-myr(20-28) had little effect on islet protein kinase A or Ca2+/calmodulin kinase activities. PKC-myr(20-28) (25-100 mu M) caused a dose-dependent inhibition of phorbol myristate acetate (PMA)-induced insulin secretion from intact rat islets but non-myristoylated peptides had little effect on the secretory response to PMA. A concentration of PKC-myr(20-28) (100 mu M) which maximally inhibited PMA-induced insulin secretion, also inhibited the secretory response to CCh, but did not affect glucose-stimulated insulin secretion from intact islets. These results indicate that myristoylation of pseudosubstrate peptides increases their potency as inhibitors and that PKC-myr(20-28) is a selective and cell-permeant inhibitor of PMA-sensitive isoforms of PKC. They also suggest that the activation of PMA-sensitive PKC isoforms mediates the stimulatory effects of CCh, but is not obligatory for glucose-induced insulin secretion from pancreatic beta-cells.