Diabetes impairs insulin-induced endothelium-dependent relaxation by reducing nitric oxide (NO) production. GLP-1, an incretin hormone, has been shown to prevent the development of endothelial dysfunction. In this study, we hypothesized that GLP-1 would improve the impaired insulin-induced relaxation response in diabetic mice. We also examined the underlying mechanisms.
Using aortic rings from ob/ob mice, an animal model of obesity and type 2 diabetes, and lean mice, vascular relaxation responses and protein expressions were evaluated using insulin, GLP-1, and pathway-specific inhibitors to elucidate the mechanisms of response. In parallel experiments, β-arrestin2 siRNA-transfected aortas were treated with GLP-1 to evaluate its effects on aortic response pathways.
When compared to that of untreated ob/ob aortas, GLP-1 increased insulin-induced vasorelaxation and NO production. AMPK inhibition did not alter this vasorelaxation in both GLP-1-treated lean and ob/ob aortas, while Akt inhibition reduced vasorelaxation in both groups, and co-treatment with GLP-1 and insulin caused Akt/eNOS activation. Additionally, GLP-1 decreased GRK2 activity and enhanced β-arrestin2 translocation from the cytosol to membrane in ob/ob aortas. β-Arrestin2 siRNA decreased insulin-induced relaxation both in lean aortas and GLP-1-treated ob/ob aortas.
We demonstrated that insulin-induced relaxation is dependent on β-arrestin2 translocation and Akt activation via GLP-1-stimulated GRK2 inactivation in ob/ob aortas. We showed a novel cross-talk between GLP-1-responsive β-arrestin2 and insulin signaling in diabetic aortas.

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