Protein kinase C activation by phorbol esters induces chromaffin cell cortical filamentous actin disassembly and increases the initial rate of exocytosis in response to nicotinic receptor stimulation

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Nicotinic stimulation and high K + depolarization of bovine chromaffin cells cause disassembly of cortical filamentous actin networks. Previous work from our laboratory has demonstrated that disassembly of actin filaments is Ca 2+-dependent, precedes exocytosis and occurs in cortical areas of low cytoplasmic viscosity which are the sites of exocytosis. It has also been suggested that protein kinase C is involved in catecholamine secretion from chromaffin cells. Therefore, the possibility that protein kinase C activation might be implicated in cortical filamentous actin disassembly was investigated. Here we report that phorbol myristate acetate, a protein kinase C activator, causes cortical filamentous actin disassembly. Short-term phorbol ester treatment does not alter the morphology of chromaffin cells; however, l h after phorbol ester exposure an increase in cell flattening and membrane ruffling is observed. Phorbol ester-induced cortical filamentous actin disassembly is inhibited by protein kinase C activity inhibitors, is independent of extracellular Ca 2+ and has a slower time course than that induced by either nicotinic receptor stimulation or K +-depolarization. Phorbol ester effects are likely to be mediated by activation of protein kinase C and not by any changes in intracellular Ca 2+ levels, as indicated by measurements of Ca 2+ transients. Pretreatment of chromaffin cells with phorbol myristate acetate increases the initial rate of nicotine-evoked catecholamine release. Nicotine-induced cortical actin filament disassembly and catecholamine secretion are partially (29–40%) inhibited by pretreatment of cells with either calphostin C, staurosporine or sphingosine. The results suggest that protein kinase C may be involved in the reorganization of the cortical actin filament network priming the cells for release by removing a barrier to secretory granule mobility. However, its role in exocytosis is modulatory but not essential.