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Improved glucose-stimulated insulin secretion by intra-islet inhibition of protein-tyrosine phosphatase 1B expression in rats fed a high-fat diet

Background: Insulin resistance of pancreatic β-cell itself may be a potential link between systemic insulin resistance and impaired insulin secretion in Type 2 diabetes. Protein tyrosine phosphatase 1B (PTP1B) dephosphorylates tyrosine residues in insulin receptors (IR) and IR substrate (IRS) proteins, and thereby inhibits insulin signaling. Thus the impact of PTP1B expression on β-cell insulin pathway may affect insulin secretory function. Aim: The aim of the present study was to investigate the effects of intra-islet inhibition of PTP1B expression on glucose-stimulated insulin secretion and potential mechanisms in rats fed a high-fat diet (HFD). Materials and methods: Twenty 10-week-old Sprague Dawley rats were randomly assigned to a regular diet (RD) or a HFD for 8 weeks. At the end of the 8th week, fasting glucose, fasting insulin concentration and lipid profile were measured and an oral glucose tolerance test was done after 12-h fast. Then islet isolation was performed for static incubation and perifusion. Recombinant adenoviruses containing siPTP1B (Ad-siPTP1B), or siControl (Ad-siControl) sequences were constructed using AdEasy™ system. Islets were transfected and then assigned to the Ad-siPTP1B group, the Ad-siControl group, and mock control group. Real-time RT-PCR and Western blot were used to evaluate the expression level of PTP1B. Western blot of glucose transporter 2 (GLUT-2) and glucokinsase were also done to investigate the β-cell glucose-sensing apparatus. Islets were incubated with Krebs-Ringer bicarbonate containing 2.8 mmol/l glucose then 16.7 mmol/l glucose to evaluate glucose-stimulated insulin secretion (GSIS). Islet perifusion was also performed to evaluate kinetics of insulin release in vitro. Results: HFD rats manifested modest glucose intolerance compared with RD group. And PTP1B expression in isolated islets of rats in the HFD group was higher than that of the RD group. GSIS was impaired in islets of HFD rats (2.3±0.5-fold as basal for HFD vs 8.1±1.3-fold for RD; p<0.05). Ad-siPTP1B treatment resulted in 73% decrease in PTP1B mRNA levels and 61% decrease in PTP1B protein compared with islets treated with Ad-siControl (p<0.05). Simultaneously, PTP1B inhibition resulted in 4.7±0.8-fold increase of GSIS from basal (vs 1.9±0.1 -fold for Ad-siControl, p<0.05). Perifusion showed notable improvement of first-phase insulin secretion by Ad-siPTP1B treatment. Significant decrease of both GLUT-2 (by 49.8%) and glucokinase (GCK, by 43.7%) were found in the HFD group when compared with the RD group, while up-regulation of both GLUT-2 (by 98%) and GCK (by 62%) was achieved after PTP1B inhibiton by Ad-siPTP1B. Conclusions: Intra-islet PTP1B is an important physiological regulator of glucose-induced insulin release and the characteristics of PTP1B inhibitors in insulin secretion could make it a potential novel therapeutics for protection of β-cell secretory function in Type 2 diabetes.