Decreased expression of t-SNARE, syntaxin 1, and SNAP-25 in pancreatic β-cells is involved in impaired insulin secretion from diabetic GK rat islets -: Restoration of decreased t-SNARE proteins improves impaired insulin secretion

The physiological role of soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) proteins in insulin exocytosis has been reported in pancreatic beta-cells, To determine whether the beta-cells of GK rats, a nonobese rodent model of type 2 diabetes, exhibit abnormalities in their SNARE proteins, we studied the expression and function of target (t)-SNAREs, syntaxin 1A, and synaptosomal-associated protein of 25 kDa (SNAP-25) in GK rat islets, Although insulin release and insulin content of islets isolated from 12-week-old GK rats were reduced, the proinsulin biosynthetic rate was about twofold higher than that in control rat islets, and no change in the preproinsulin mRNA level was observed. Pulse-chase experiments suggested the increased degradation of insulin in GK rat islets, Immunoblot analysis revealed that protein levels of syntaxin 1A and SNAP-25 in oh rat islets decreased to similar to 60% of the levels in control rat islets. We then examined whether the restoration of the decreased expression of t-SNAREs to the normal level in GK rat islets affected insulin secretion. Restoration was achieved by the overexpression of syntaxin 1A and SNAP-25 via the recombinant adenovirus-mediated gene transduction system, which recovered levels of these proteins to almost control levels. Glucose-stimulated insulin release from Adex1CA syntaxin 1A and Adex1CA SNAP-25-infected GK rat islets increased up to similar to 135 and 200%, respectively, of those from uninfected GK rat islets, although no difference in basal (2.2 mmol/l glucose) insulin release was evident between them, We conclude that decreased expression of t-SNAREs in GK rat islets is in part the defect responsible for impaired insulin secretion.