PKCα Is Activated but Not Required during Glucose-Induced Insulin Secretion from Rat Pancreatic Islets

Abstract

The role of protein kinase C (PKC) in glucose-stimulated insulin secretion (GSIS) is controversial. Using recombinant adenoviruses for overexpression of PKCα and PKCδ, in both wild-type (WT) and kinase-dead (KD) forms, we here demonstrate that activation of these two PKCs is neither necessary nor sufficient for GSIS from batch-incubated, rat pancreatic islets. In contrast, responses to the pharmacologic activator 12-O-tetradecanoylphorbol-13-acetate (TPA) were reciprocally modulated by overexpression of the PKαWT or PKCαKD but not the corresponding PKCδ adenoviruses. The kinetics of the secretory response to glucose (monitored by perifusion) were not altered in either cultured islets overexpressing PKCαKD or freshly isolated islets stimulated in the presence of the conventional PKC (cPKC) inhibitor Go6976. However, the latter did inhibit the secretory response to TPA. Using phosphorylation state-specific antisera for consensus PKC phosphorylation sites, we also showed that (compared with TPA) glucose causes only a modest and transient functional activation of PKC (maximal at 2-5 min). However, glucose did promote a prolonged (15 min) phosphorylation of PKC substrates in the presence of the phosphatase inhibitor okadaic acid. Overall, the results demonstrate that glucose does stimulate PKCα in pancreatic islets but that this makes little overall contribution to GSIS.