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 lacrimal gland, and a peptide derived from the sequence of the endogenous inhibitor of protein kinase A [Myr-PKI-(17-25)]. Lacrimal gland acini were preincubated for 60 min with the myristoylated peptides (10(-10) to 3 x 10(-7) M), then protein secretion was stimulated with a phorbol ester, phorbol 12,13-dibutyrate (10(-6) M); vasoactive intestinal peptide (10(-8) M); a cholinergic agonist, carbachol (10(-5) M); or an alpha 1-adrenergic agonist, phenylephrine (10(-4) M), for 20 min. In intact lacrimal gland acini, Myr-PKC-alpha-(15-28) inhibited phorbol 12,13-dibutyrate-induced protein secretion. This effect was not reproduced by the acetylated peptide or by the myristoylated PKI, which inhibited 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 protein secretion was inhibited only by Myr-PKC-epsilon-(149-164), whereas Myr-PKC-alpha-(15-28) and Myr-PKC-delta-(142-153) had a stimulatory effect. None of these myristoylated peptides affected the calcium increase evoked by cholinergic or alpha 1-adrenergic agonists. We concluded that phorbol ester- and receptor-induced protein secretion involve different PKC isoforms in lacrimal gland.