Oxidative stress within the vascular endothelium, due to excess generation of reactive oxygen species (ROS), is thought to be fundamental to the initiation and progression of the cardiovascular complications of type 2 diabetes mellitus. The term ROS encompasses a variety of chemical species including superoxide anion (O₂^•−), hydroxyl radical (OH⁻) and hydrogen peroxide (H₂O₂). While constitutive generation of low concentrations of ROS are indispensable for normal cellular function, excess O₂^•− can result in irreversible tissue damage. Excess ROS generation is catalysed by xanthine oxidase, uncoupled nitric oxide synthases, the mitochondrial electron transport chain and the nicotinamide adenine dinucleotide phosphate (NADPH) oxidases. Amongst enzymatic sources of O₂^•− the Nox2 isoform of NADPH oxidase is thought to be critical to the oxidative stress found in type 2 diabetes mellitus. In contrast, the transcriptionally regulated Nox4 isoform, which generates H₂O₂, may fulfil a protective role and contribute to normal glucose homeostasis. This review describes the key roles of Nox2 and Nox4, as well as Nox1 and Nox5, in glucose homeostasis, endothelial function and oxidative stress, with a key focus on how they are regulated in health, and dysregulated in type 2 diabetes mellitus.

Rationale: A hallmark of type 2 diabetes is insulin resistance, which leads to increased endothelial cell (EC) production of superoxide and a simultaneous reduction in the availability of the vasoprotective signaling radical NO. We recently demonstrated in preclinical models that type 2 diabetes simultaneously causes resistance to IGF-1 (insulin-like growth factor-1)–mediated glucose lowering and endothelial NO release. Objective: To examine the effect of insulin and IGF-1 resistance specifically in ECs in vivo. Methods and Results: We generated mice expressing mIGF-1Rs (mouse IGF-1 receptors), which form nonfunctioning hybrid receptors with native IRs (insulin receptors) and IGF-1R, directed to ECs under control of the Tie2 promoter-enhancer. Despite EC insulin and IGF-1 resistance, mIGFREO (mutant IGF-1R EC overexpressing) mice had enhanced insulin and IGF-1–mediated systemic glucose disposal, lower fasting free fatty acids, and triglycerides. In hyperinsulinemic-euglycemic clamp studies, mIGFREO had increased glucose disposal and increased glucose uptake into muscle and fat, in response to insulin. mIGFREO had increased Nox (NADPH oxidase)-4 expression due to reduced expression of the microRNA, miR-25. Consistent with increased Nox4, mIGFREO ECs generated increased hydrogen peroxide (H ₂ O ₂ ), with no increase in superoxide. Treatment with catalase—a H ₂ O ₂ dismutase—restored insulin tolerance to WT (wild type) levels in mIGFREO. Conclusions: Combined insulin and IGF-1 resistance restricted to the endothelium leads to a potentially favorable adaptation in contrast to pure insulin resistance, with increased Nox4-derived H ₂ O ₂ generation mediating enhanced whole-body insulin sensitivity.

The insulin receptor and insulin like growth factor-1 receptor are heterodimers consisting of two extracellular α-subunits and two transmembrane β-subunits. IR αβ and IGF1R αβ hemi-receptors can heterodimerize to form hybrids composed of one IR αβ and one IGF1R αβ. Widely distributed in mammalian tissues, in contrast to IR and IGF1R the physiological function of hybrids is unclear. To identify tool compounds that inhibit hybrid formation we performed a high-throughput small molecule screen based on a homology model of hybrid structure. Our studies unveil a first in class quinoline-containing heterocyclic small molecule that reduces hybrids by >50% in human umbilical vein endothelial cells with no effect on IR or IGF1R expression. Downstream of IR and IGF1R our small molecule led to reduced expression of the negative regulatory p85α subunit of phosphatidylinositol 3-kinase, an increase in phosphorylation of its downstream target Akt and enhanced insulin and shear-induced phosphorylation of Akt. We show that hybrids have a role in human endothelial cell physiology distinct from IR and IGF1R.