Accelerated endothelial dysfunction in mild prediabetic insulin resistance: the early role of reactive oxygen species

Insulin resistance is well established as an independent risk factor for the development of type 2 diabetes and cardiovascular atherosclerosis. Most studies have examined atherogenesis in models of severe insulin resistance or diabetes. However, by the time of diagnosis, individuals with type 2 diabetes already demonstrate a significant atheroma burden. Furthermore, recent studies suggest that, even in adolescence, insulin resistance is a progressive disorder that increases cardiovascular risk. In the present report, we studied early mechanisms of reduction in the bioavailability of the antiatheroscerotic molecule nitric oxide ( NO) in very mild insulin resistance. Mice with haploinsufficiency for the insulin receptor (IRKO) are a model of mild insulin resistance with preserved glycemic control. We previously demonstrated that 2-mo-old ( Young) IRKO mice have preserved vasorelaxation responses to ACh. This remained the case at 4 mo of age. However, by 6 mo, despite no significant deterioration in glucose homeostasis ( Adult), IRKO mice had marked blunting of ACh-mediated vasorelaxation [IRKO maximum contraction response ( E-max) 66 +/- 5% vs. wild type 87 +/- 4%, P < 0.01]. Despite the endothelial dysfunction demonstrated, aortic endothelial nitric oxide synthase ( eNOS) mRNA levels were similar in Adult IRKO and wild-type mice, and, interestingly, aortic eNOS protein levels were increased, suggesting a compensatory upregulation in the IRKO. We then examined the potential role of reactive oxygen species in mediating early endothelial dysfunction. The superoxide dismutase mimetic Mn(III) tetrakis(1-methyl-4-pyridyl) porphyrin pentachloride (MnTMPyP) restored ACh relaxation responses in the Adult IRKO ( Emax to ACh with MnTMPyP 85 +/- 5%). Dihydroethidium fluorescence of aortas and isolated coronary microvascular endothelial cells confirmed a substantial increase in endothelium-derived reactive oxygen species in IRKO mice. These data demonstrate that mild insulin resistance is a potent substrate for accelerated endothelial dysfunction and support a role for endothelial cell superoxide production as a mechanism underlying the early reduction in NO bioavailability.