Phorbol ester stimulation of insulin release and secretory-granule protein phosphorylation in a transplantable rat insulinoma.
The effects of tumour-promoting phorbol esters on protein-phosphorylation reactions and secretion in rat insulinoma tissue were investigated with the objective of assessing the possible role of Ca2+- and phospholipid-dependent protein kinases (protein kinase C) in insulin release. 4 beta-Phorbol 12-myristate 13-acetate (TPA) was a potent secretagogue at concentrations above 0.1 microM. TPA-induced release was inhibited by adrenaline or omission of Ca2+ from the extracellular medium and was augmented by theophylline. These findings suggested that TPA activated an exocytotic process. TPA enhanced the Ca2+- and phospholipid-dependent phosphorylation of histone III-S by a soluble protein fraction of the tissue. Endogenous phosphorylation reactions involving soluble and secretory-granule membrane proteins were also stimulated by TPA in tissue homogenates and reconstituted subcellular fractions. Histone phosphorylation and the granule-protein phosphorylation reactions showed similar concentration-dependencies for activation by both Ca2+ and TPA, thus indicating that the same enzyme was involved. It is concluded that the phosphorylation of cytosolic and membrane protein substrates by protein kinase C may be important in the stimulus-secretion coupling mechanism of insulin release.
D007516:Adenoma, Islet Cell / Q000378:metabolism*; D000818:Animals; D003594:Cytoplasmic Granules / Q000187:drug effects / Q000378:metabolism; D004591:Electrophoresis, Polyacrylamide Gel; D066298:In Vitro Techniques; D007328:Insulin / Q000378:metabolism*; D007340:Insulinoma / Q000378:metabolism*; D008565:Membrane Proteins / Q000378:metabolism*; D010190:Pancreatic Neoplasms / Q000378:metabolism*; D010704:Phorbols / Q000494:pharmacology*; D010766:Phosphorylation; D011493:Protein Kinase C; D011494:Protein Kinases / Q000378:metabolism; D051381:Rats; D013268:Stimulation, Chemical; D013755:Tetradecanoylphorbol Acetate / Q000494:pharmacology*
D007328:Insulin; D008565:Membrane Proteins; D010704:Phorbols; D011494:Protein Kinases; D011493:Protein Kinase C; D013755:Tetradecanoylphorbol Acetate