Impaired Gene and Protein Expression of Exocytotic Soluble N-Ethylmaleimide Attachment Protein Receptor Complex Proteins in Pancreatic Islets of Type 2 Diabetic Patients

Abstract

Exocytosis of insulin is dependent on the soluble N-ethylmaleimide attachment protein receptor (SNARE) complex proteins in the B-cells. We assessed insulin release as well as gene and protein expression of SNARE complex protein in isolated pancreatic islets of type 2 diabetic patients (n = 4) and nondiabetic control subjects (n = 4). In islets from the diabetic patients, insulin responses to 8.3 and 16.7 mmol/l glucose were markedly reduced compared with control islets (4.7 ± 0.3 and 8.4 ± 1.8 vs. 17.5 ± 0.1 and 24.3 ± 1.2 μU · islet⁻¹ · h⁻¹, respectively; P < 0.001). Western blot analysis revealed decreased amounts of islet SNARE complex and SNARE-modulating proteins in diabetes: syntaxin-1A (21 ± 5% of control levels), SNAP-25 (12 ± 4%), VAMP-2 (7 ± 4%), nSec1 (Munc 18; 34 ± 13%), Munc 13-1 (27 ± 4%), and synaptophysin (64 ± 7%). Microarray gene chip analysis, confirmed by quantitative PCR, showed that gene expression was decreased in diabetes islets: syntaxin-1A (27 ± 2% of control levels), SNAP-25 (31 ± 7%), VAMP-2 (18 ± 3%), nSec1 (27 ± 5%), synaptotagmin V (24 ± 2%), and synaptophysin (12 ± 2%). In conclusion, these data support the view that decreased islet RNA and protein expression of SNARE and SNARE-modulating proteins plays a role in impaired insulin secretion in type 2 diabetic patients. It remains unclear, however, to which extent this defect is primary or secondary to, e.g., glucotoxicity.