Abstract
Incretins such as glucagon-like peptide-1 and gastric inhibitory polypeptide/glucose-dependent insulinotropic peptide are known to potentiate insulin secretion mainly through a cAMP/protein kinase A (PKA) signaling pathway in pancreatic beta-cells, but the mechanism is not clear. We recently found that the cAMP-binding protein cAMP-GEFII (or Epac 2), interacting with Rim2, a target of the small G protein Rab3, mediates cAMP-dependent, PKA-independent exocytosis in a reconstituted system. In the present study, we investigated the role of the cAMP-GEFII--Rim2 pathway in incretin-potentiated insulin secretion in native pancreatic beta-cells. Treatment of pancreatic islets with antisense oligodeoxynucleotides (ODNs) against cAMP-GEFII alone or with the PKA inhibitor H-89 alone inhibited incretin-potentiated insulin secretion approximately 50%, while a combination of antisense ODNs and H-89 inhibited the secretion approximately 80-90%. The effect of cAMP-GEFII on insulin secretion is mediated by Rim2 and depends on intracellular calcium as well as on cAMP. Treatment of the islets with antisense ODNs attenuated both the first and second phases of insulin secretion potentiated by the cAMP analog 8-bromo-cAMP. These results indicate that the PKA-independent mechanism involving the cAMP-GEFII--Rim2 pathway is critical in the potentiation of insulin secretion by incretins.
Highlights
Blood glucose levels are precisely controlled by insulin release from pancreatic -cells
We recently found that the cAMP-binding protein cAMP-GEFII, interacting with Rim2, a target of the small G protein Rab3, mediates cAMP-dependent, protein kinase A (PKA)-independent exocytosis in a reconstituted system
The Effects of cAMP-GEFII Are Dependent on Intracellular Calcium as Well as cAMP—To determine whether the effect of cAMP-GEFII on cAMP-potentiated insulin secretion requires a rise in intracellular calcium concentrations ([Ca2ϩ]i), we examined the effects of 8-Br-cAMP (1 mM), high Kϩ (60 mM), and their combination, in the presence of 2.8 mM glucose, on insulin secretion in pancreatic islets treated with control ODNs or antisense ODNs
Summary
Blood glucose levels are precisely controlled by insulin release from pancreatic -cells. These data indicate that both the GLP-1and GIP-potentiated insulin secretions depend critically on cAMP production in pancreatic -cells. There were no differences in expression of protein levels between control ODN-treated and antisense ODN-treated pancreatic islets (Fig. 3A), indicating that the antisense ODNs are specific for cAMP-GEFII.
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