Hyperglycemia Regulates the Sensitivity of Vascular Cells to IGF-I Stimulation

Abstract

Insulin-like growth factor-I (IGF-I) stimulates a coordinated growth response in all tissues. However, following injury, increases in IGF-I locally can stimulate regional changes in cell growth that occur during the repair process. Atherosclerosis is a disease in which local increases in IGF-I synthesis have been shown to stimulate arterial smooth muscle cell proliferation, and this is believed to be an important component of early proliferative lesion development. In addition to stimuli that enhance atherosclerotic lesion progression by increasing IGF-I concentrations, such as increases in oxidized LDL and advanced end glycosylation products, changes in IGF-I sensitivity occur in response to variables that are known to increase the risk of coronary artery disease, such as hyperglycemia. Following induction of hyperglycemic stress, smooth muscle cells become sensitized to stimulation of cell proliferation by IGF-I. The mechanism for this increased sensitivity involves cooperative signaling through the αVβ3 integrin receptor. Specifically, when smooth muscle cells are exposed to hyperglycemic conditions, they markedly increase their synthesis and secretion of αVβ3 ligands such as thrombospondin, osteopontin and vitronectin. These proteins in turn activate αVβ3 signaling, which then functions in coordination with IGF-I receptor-linked signaling mechanisms to enhance cell migration and proliferation. The pathway that mediates this effect requires Shc phosphorylation, which leads to MAP kinase activation. Inhibition of either of these processes will retard the smooth muscle cell response to IGF-I. For Shc to be phosphorylated in response to IGF-I requires activation of Src kinase, which directly phosphorylates Shc. Correspondingly, Src kinase activation requires ligand occupancy of the αVβ3 integrin; therefore, blocking ligand occupancy results in an inability to generate an increase in phosphoShc and hence no induction of MAP kinase. Having discovered the components of this mechanism, we wished to determine whether inhibiting ligand occupancy of αVβ3 during hyperglycemia would result in attenuation of MAP kinase activation and lesion proliferation. We prepared a monoclonal antibody to the active site on αVβ3 that was the principle binding site for αVβ3 ligands. This antibody disrupted not only ligand occupancy but also activation of the αVβ3- linked signaling pathway and inhibited IGF-I stimulated smooth muscle replication in cells cultured in hyperglycemic conditions.

To determine if this mechanism was operative in vivo, pigs were made diabetic and received an infusion of this antibody for three months. The antibody resulted in a 68% reduction in lesion area in the diabetic animals, whereas lesions that received a control antibody had no change. Importantly, the antibody also inhibited activation of the αVβ3-linked signaling pathway as well as IGF-I stimulated Shc and MAP kinase activation. These studies clearly indicate that, under hyperglycemic conditions, there is cooperative signaling between the αVβ3 and the IGF-I receptor, leading to enhanced SMC proliferation. The findings in experimental animals suggest that blocking activation of this pathway may be a successful therapeutic approach for inhibiting the atherosclerosis that occurs in diabetes.