Glucose-induced phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) in isolated rat pancreatic islets

In order to further evaluate the role of protein kinase C activation in glucose-induced insulin secretion, the extent of phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) was examined in freshly isolated rat pancreatic islets prelabeled with [³²P]orthophosphate. The islets were incubated with either 2.75 mM glucose alone, 2.75 mM glucose + 1 μM phorbol myristate acetate, 20 mM glucose, or 20 mM glucose + 50 nM staurosporine. After stimulation, the homogenized islets were processed by immunoprecipitation with a specific polyclonal anti-MARCKS antibody, followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Densitometric analysis of autoradiograms revealed that phorbol myristate acetate caused a 3.78 ± 0.97-fold increase in MARCKS phosphorylation over control. In the islets exposed to 20 mM glucose, an increase of 3.43 ± 0.46-fold over control was observed. In islets exposed to G20 + 50 nM staurosporine, MARCKS phosphorylation was inhibited by 90 ± 4% compared with control islets exposed to 20 mM glucose alone. Islets similarly treated (but incubated without ³²P) were examined by immunocytochemistry using an α-PKC-specific monoclonal antibody and visualized by confocal immunofluorescence microscopy. The α-PKC redistributed from the cytosol to the plasma membrane in the β-cells of islets exposed to 20 mM glucose. In separate experiments, unlabeled but similarly treated islets were shown to respond with a 5-7-fold increase in insulin secretion in static incubation. Thus, when freshly isolated rat pancreatic islets are exposed to stimulatory glucose concentrations, they exhibit both a translocation of α-PKC and a significant increase in the extent of phosphorylation of MARCKS protein. These data suggest that α-PKC is activated during glucose-induced insulin secretion.