Expression and function of IGF-I and insulin receptors in human micro- and macrovascular cells

Insulin-like growth factors and insulin are phylogenetically closely related polypeptides and have many structural and biological similarities. Low levels of circulating insulin-like growth factor-I (IGF-I), diabetes and insulin resistance have all been implicated in the pathogenesis of cardiovascular disease, but the mechanisms involved are still not clear. Furthermore little is known about direct effects of insulin-like growth factor-I (IGF-I) and insulin on human microand macrovascular cells. In these studies we investigated the expression and function of insulin-like growth factor-I receptors (IGF-IR) and insulin receptors (IR) in human microand macrovascular endothelial cells and in human coronary artery smooth muscle cells. Our results showed expression of both IGF-IR and IR in human dermal microvascular (HMVEC), aortic (HAEC) umbilical vein (HUVEC) and coronary artery (HCAEC) endothelial cells as well as in human coronary artery smooth muscle cells (HCASMC). The gene expression of IGF-IR was considerably greater than that of IR. Ligand binding studies confirmed that the IGF-IR was considerably more abundant than the IR and that insulin and glargine interacted with the IGF-IR with one thousandand one hundred times less potency, respectively, than IGF-I itself. The presence of IGF-IR and IR proteins and activation of their β-subunits was revealed by immunoprecipitation and Western blot analysis in human macrovascular endothelial cells and in coronary artery smooth muscle cells. At physiological concentrations (≤10-9 M) IGF-I and insulin activated their cognate receptors. The presence of hybrid insulin receptor/ insulin-like growth factor-I receptor (hybrid IR/IGF-IR) was shown through detection of IGF-IR and IR β-subunits on the same membrane by Western blot after immunoprecipitation with specific antibodies against either IGF-IR or IR, implying coprecipitation of the IGF-IR