REGULATION OF DISTINCT POOLS OF PROTEIN-KINASE-C-DELTA IN BETA-CELLS

Previous studies from our laboratory have demonstrated the presence of several isoforms of protein kinase C (PKC), Ca2+-independent and Ca2-dependent, in both whole islets and tumor-derived beta cells. In the basal state, a major proportion of the isoform was found in the crude membrane fraction with smaller amounts found in both the cytosolic and cytoskeletal fractions. Whole islets showed a similar distribution of the isoform. These studies were done to analyze the effects of insuli n secretagogues on the distribution of PKC delta to different cellular pools in isolated insulinoma beta cells. The phorbol ester, phorbol 1 2-myristate 13-acetate (PMA), produced a transient association of PKC delta with the beta cell cytoskeleton along with sustained decreases i n cytosolic enzyme and transient increases in membrane enzyme. Neither glucose nor carbachol could acutely affect the subcellular distributi on of PKC delta. Oleic acid decreased the amount of the enzyme associa ted with the cytoskeleton and led to a sustained decrease of cytosolic enzyme and a transient increase in membrane enzyme. Oleic acid was al so able to prevent the increase in cytoskeletal enzyme induced by PMA. Both oleic acid and PMA potentiated glucose-induced insulin release b ut oleic acid, in contrast to PMA, was unable to initiate insulin rele ase in the presence of substimulatory concentrations of glucose. These data demonstrate that different activators of PKC may have different effects on localization of the enzyme within the cells and suggest tha t there are at least three apparently distinct pools of PKC delta with in the beta cell which may be important in insulin secretion or other aspects of beta cell function. (C) 1996 Wiley-Liss, Inc.