The effect of endothelium specific insulin resistance on endothelial function in vivo

Abstract

Objective: Insulin resistance (IR) is an independent risk factor for the development of cardiovascular atherosclerosis. A key step in the development of atherosclerosis is endothelial dysfunction, manifest by a reduction in bioactivity of nitric oxide (NO). IR is associated with endothelial dysfunction; however, the mechanistic relationship between these abnormalities and the role of impaired endothelial insulin signalling versus global IR remains unclear.

Research design and Methods: To examine the effects of IR specific to the endothelium, we generated a transgenic mouse with endothelium-targeted overexpression of a dominant negative mutant human insulin receptor (ESMIRO). This receptor has a mutation (Alanine-Threonine¹¹³⁴) in its tyrosine kinase domain which disrupts insulin signalling. Humans with the Thr¹¹³⁴ mutation are IR. We performed metabolic and vascular characterisation of this model.

Results: ESMIRO mice had preserved glucose homeostasis and were normotensive. They had significant endothelial dysfunction as evidenced by blunted aortic vasorelaxant responses to acetylcholine and calcium ionophore. Furthermore, the vascular action of insulin was lost in ESMIRO mice and insulin induced eNOS phosphorylation was blunted. Despite this phenotype, ESMIRO mice demonstrate similar levels of eNOS mRNA and protein expression to wild type.

Results: Acetylcholine-induced relaxation was normalised by the superoxide dismutase mimetic, MnTMPyP. Endothelial cells of ESMIRO mice showed increased superoxide generation as well as increased mRNA expression of the NADPH oxidase isoforms Nox2 and Nox4.

Conclusion: Selective endothelial IR is sufficient to induce a reduction in NO bioavailability and endothelial dysfunction, secondary to increased generation of reactive oxygen species. This arises independent of a significant metabolic phenotype.