Reciprocal relationships between insulin resistance and endothelial dysfunction: insights from therapeutic...

Endothelial dysfunction contributes to cardiovascular diseases that are also characterized by insulin resistance. Insulin resistance is a hallmark of metabolic disorders including Type 2 diabetes, obesity, and the metabolic syndrome that are also characterized by endothelial dysfunction. Metabolic actions of insulin to promote glucose disposal are augmented by vascular actions of insulin in endothelium to stimulate production of the vasodilator nitric oxide (NO). Indeed, NO-dependent increases in blood flow to skeletal muscle account for 25% to 40% of the increase in glucose uptake in response to insulin stimulation. PI 3-kinase-dependent insulin signaling pathways in endothelium related to production of NO share striking similarities with metabolic pathways in skeletal muscle that promote glucose uptake. Other distinct non-metabolic branches of insulin signaling pathways regulate secretion of the vasoconstrictor endothelin-1 (ET-1) in endothelium. Metabolic insulin resistance is characterized by pathway-specific impairment in PI 3-kinase-dependent signaling that in endothelium may cause imbalance between production of NO and secretion of ET-1 leading to decreased blood flow that worsens insulin resistance. Therapeutic interventions in both animal models and human studies demonstrate that improving endothelial function ameliorates insulin resistance while improving insulin sensitivity ameliorates endothelial dysfunction. Taken together, cellular, physiological, clinical, and epidemiological studies strongly support a reciprocal relationship between endothelial dysfunction and insulin resistance that helps to link cardiovascular and metabolic diseases. In this review, pathophysiological mechanisms that couple endothelial dysfunction with insulin resistance will be discussed with an emphasis on important therapeutic implications for the metabolic syndrome.