Enhancement of glucagon secretion in mouse and human pancreatic alpha cells by protein kinase C (PKC) involves intracellular trafficking of PKCα and PKCδ

Protein kinase C (PKC) regulates exocytosis in various secretory cells. Here we studied intracellular translocation of the PKC isoenzymes PKC alpha and PKC delta, and investigated how activation of PKC influences glucagon secretion in mouse and human pancreatic alpha cells. Glucagon release from intact islets was measured in static incubations, and the amounts released were determined by RIA. Exocytosis was monitored as increases in membrane capacitance using the patch-clamp technique. The expression of genes encoding PKC isoforms was analysed by real-time PCR. Intracellular PKC distribution was assessed by confocal microscopy. The PKC activator phorbol 12-myristate 13-acetate (PMA) stimulated glucagon secretion from mouse and human islets about fivefold (p < 0.01). This stimulation was abolished by the PKC inhibitor bisindolylmaleimide (BIM). Whereas PMA potentiated exocytosis more than threefold (p < 0.001), BIM inhibited alpha cell exocytosis by 60% (p < 0.05). In mouse islets, the PKC isoenzymes, PKC alpha and PKC beta 1, were highly abundant, while in human islets PKC eta, PKC epsilon and PKC zeta were the dominant variants. PMA stimulation of human alpha cells correlated with the translocation of PKC alpha and PKC delta from the cytosol to the cell periphery. In the mouse alpha cells, PKC delta was similarly affected by PMA, whereas PKC alpha was already present at the cell membrane in the absence of PMA. This association of PKC alpha in alpha cells was principally dependent on Ca2+ influx through the L-type Ca2+ channel. PKC activation augments glucagon secretion in mouse and human alpha cells. This effect involves translocation of PKC alpha and PKC delta to the plasma membrane, culminating in increased Ca2+-dependent exocytosis. In addition, we demonstrated that PKC alpha translocation and exocytosis exhibit differential Ca2+ channel dependence.