Evidence that insulin secretion influences SNAP-25 through proteasomal activation

Regulation of SNARE proteins by glucose in pancreatic islets is complex and insufficiently clarified. We aimed to study effects of glucose per se separate from enhancing effects on exocytosis. A 24 h culture of rat islets at elevated glucose (27 mmol/L) increased t-SNARES (SNAP-25, syntaxin) (Western blotting). Co-culture with diazoxide, which inhibits glucose-induced insulin secretion, reversed these effects. Effects on SNAP-25 were similar in human and rat islets. Effects of diazoxide were mimicked by blocking secretion with somatostatin (rat islets). Blocking secretion by cooling abolished both glucose and diazoxide effects on SNAP-25. Total SNAP-25 mRNA as well as isoforms α and β were increased by 24-h elevated glucose. Diazoxide failed to reverse the glucose effects on mRNA. However, effects of diazoxide on SNAP-25 protein were nullified by proteasome inhibitors (ALLN, MG-132, and epoxomicin) but not by lysosomal inhibition (NH4Cl). Exocytosis per se modifies SNAREs by a process linked to proteasomal activation.