Endothelial Insulin and IGF-1 Receptors: When Yes Means NO

Cardiovascular (CV) disease is the leading cause of morbidity and mortality and a major driver of health care costs in patients with type 2 diabetes. Observational studies suggest that insulin resistance and hyperglycemia independently predict atherosclerosis (1,2). However, recent clinical trials have been disappointing in that intensive glycemic control does not reduce the risk of CV events in individuals with diabetes (3). Consequently, it has been suggested that therapies targeting hyperinsulinemia and/or insulin resistance (e.g., metformin) may lead to CV risk reduction (2). In addition to its metabolic actions, insulin has important vascular actions that stimulate endothelial production of nitric oxide (NO), an anti-inflammatory and antiatheroslcerotic molecule (4). In turn, endothelial insulin resistance leads to diminished glucose disposal, endothelial dysfunction, and atherosclerosis. Strategies that ameliorate endothelial insulin resistance may simultaneously augment metabolic and vascular actions of insulin, thereby reducing CV risk. However, molecular mechanisms regulating endothelial insulin action are still unclear. Elegant studies from various laboratories have elucidated insulin signaling pathways that regulate NO production in the endothelium (4). Insulin binding to its receptor increases receptor tyrosine kinase activity and results in phosphorylation of insulin receptor (IR) substrate (IRS)-1 and sequential activation of phosphatidylinositol 3-kinase (PI3K) and 3-phosphoinositide-dependent protein kinase (PDK)-1. PDK-1, in turn, activates Akt, which then directly phosphorylates endothelial NO synthase (eNOS) at Ser1177, resulting in increased eNOS activity and NO production (Fig. 1). Although less potent, IGF-1, like insulin, activates the PI3K-Akt-eNOS pathway and stimulates NO production in endothelial cells (5,6). Human endothelial cells express IR, IGF-1 receptors (IGF-1R), and hybrid receptors (IR/IGF-1R) composed of heterodimers containing a ab-chain of the IR associated with a ab-chain of the IGF-1R (7). IGF-1R are more abundant (10-fold higher) than IR (5,8). IR/IGF-1R have a low affinity for insulin, but they bind IGF-1 with the same affinity as IGF-1R. However, because of the low binding affinity of insulin to IGF-1R, physiological concentrations of insulin (100–500 pmol/L) selectively activate IR to release NO and increase microvascular perfusion in vivo (6). At supraphysiological concentrations, insulin and IGF-1 cross-react with each other’s receptors, albeit at a significantly lower affinity than with their own receptors (7). In nonendothelial cells, IGF-1R expression modulates insulin signaling by altering the levels of hybrid receptors (7). Whether or not a similar dynamic affects insulin signaling in the endothelium was unknown. In this issue of Diabetes, Imrie et al. (9) demonstrate a novel role for IGF-1R in modulating insulin signaling in the endothelium. They evaluated endothelial insulin sensitivity in mice overexpressing human IGF-1R in the endothelium (hIGFREO). Aorta from hIGFREO displayed reduced basal NO release and enhanced responsiveness to vasoconstrictors. Although basal total and active eNOS levels were similar, neuronal NO synthase (nNOS) expression in endothelial cells from hIGFREO was lower when compared with wild-type mice. In hIGFREO, endothelial levels of IR/IGF-1R were increased and associated with reduced insulin, but not IGF-1–stimulated NO production and eNOS activation. Data from the current study extend and confirm previous reports from this group that have demonstrated that reducing IGF-1R and IR/IGF-1R results in improved endothelial insulin sensitivity in insulin-resistant mice (10). Taken together, these novel findings suggest that IGF-1R negatively affects insulin-stimulated NO production in the endothelium by modulating the amount of IR/IGF-1R. How does endothelial IGF-1R expression influence insulin signaling? Current models assume that IR, IGF-1R, and IR/IGF-1R are formed by random dimerization of receptor monomers (7). Consequently, relative distribution of the receptor species is determined by the monomeric ratio of IGF-1R and IR. Higher IGF-1R expression is associated with increased formation of IR/IGF-1R and a lower proportion of IR holoreceptors. Conversely, decreasing IGF-1R levels lowers the amount of IR/IGF-1R and thus a higher proportion of IR is available for ligand binding. This phenomenon does not appear to be cell-specific, since similar findings are observed in vascular smooth muscle cells, adipocytes, and osteoblasts (11–13). Likewise, fibroblasts from individuals with heterozygous IGF-1R mutation, Arg59Ter, manifest reduced IGF-1R as well as hybrid receptor expression and augmented insulin signaling (14). Thus, isolated changes in IR number may be sufficient to alter the strength of PI3K-Akt-eNOS signaling (Fig. 1). It is also possible that higher numbers of IR/IGF-1R may diminish coupling efficiency of IR to postreceptor signaling intermediates and reduce insulin responsivity. In the study by Imrie et al., insulin-stimulated eNOS phosphorylation/activation was reduced in endothelial cells from hIGFREO mice. However, insulin-stimulated Akt activation was unaffected. Thus, the molecular mechanisms From the Clinical Endocrine Section, Diabetes, Endocrinology, and Obesity Branch, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland; the Departments of Internal Medicine and Medical Pharmacology and Physiology, University of Missouri School of Medicine, Columbia, Missouri; and the Harry S. Truman Memorial Veterans’ Hospital, Columbia, Missouri. Corresponding author: Ranganath Muniyappa, [email protected]. DOI: 10.2337/db12-0654 2012 by the American Diabetes Association. Readers may use this article as long as the work is properly cited, the use is educational and not for profit, and the work is not altered. See http://creativecommons.org/licenses/by -nc-nd/3.0/ for details. See accompanying original article, p. 2359.