LACRIMAL GLAND PKC ISOFORMS ARE DIFFERENTIALLY INVOLVED IN AGONIST-INDUCED PROTEIN SECRETION

In the present study, we have synthesized and N-myristoylated peptides derived from the pseudosubstrate sequences of protein kinase C (PKC)- alpha, -delta, and -epsilon [Myr-PKC-alpha-(15-28), Myr-PKC-delta-(142 -153), and Myr-PKC-epsilon-(149-164)], three isoforms present in rat l acrimal gland, and a peptide derived from the sequence of the endogeno us inhibitor of protein kinase A [Myr-PKI-(17-25)]. Lacrimal gland aci ni were preincubated for 60 min with the myristoylated peptides (10(-1 0) to 3 x 10(-7) M), then protein secretion was stimulated with a phor bol ester, phorbol 12,13-dibutyrate (10(-6) M); vasoactive intestinal peptide (10(-8) M); a cholinergic agonist, carbachol (10(-5) M); or an alpha-adrenergic agonist, phenylephrine (10(-4) M), for 20 min. In in tact lacrimal gland acini, Myr-PKC-alpha-(15-28) inhibited phorbol 12, 13-dibutyrate-induced protein secretion. This effect was not reproduce d by the acetylated peptide or by the myristoylated PKI, which inhibit ed vasoactive intestinal peptide-induced protein secretion, a response mediated by protein kinase A. Carbachol-induced protein secretion was inhibited by all three peptides. In contrast, phenylephrine-induced p rotein secretion was inhibited only by Myr-PKC-epsilon-(149-164), wher eas Myr-PKC-alpha-(15-28) and Myr-PKC-epsilon-(142-153) had a stimulat ory effect. None of these myristoylated peptides affected the calcium increase evoked by cholinergic or alpha 1-adrenergic agonists. We conc luded that phorbol ester- and receptor-induced protein secretion invol ve different PKC isoforms in lacrimal gland.