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 PKCα and PKCδ, 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, PKCα and PKCβ1, were highly abundant, while in human islets PKCη, PKCε and PKCζ were the dominant variants. PMA stimulation of human alpha cells correlated with the translocation of PKCα and PKCδ from the cytosol to the cell periphery. In the mouse alpha cells, PKCδ was similarly affected by PMA, whereas PKCα was already present at the cell membrane in the absence of PMA. This association of PKCα 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 PKCα and PKCδ to the plasma membrane, culminating in increased Ca²⁺-dependent exocytosis. In addition, we demonstrated that PKCα translocation and exocytosis exhibit differential Ca²⁺ channel dependence.