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Hyperglycemia Alters the Responsiveness of Smooth Muscle Cells to Insulin-Like Growth Factor-I

Division of Endocrinology, Department of Medicine, University of North Carolina, Chapel Hill, North Carolina 27599-7170

Address all correspondence and requests for reprints to: Laura A. Maile, Ph.D., 5029 Burnett Womack, CB 7170, University of North Carolina, Chapel Hill, North Carolina 27599-7170. E-mail: laura_maile{at}med.unc.edu.

IGF-I stimulation of smooth muscle cell (SMC) migration and proliferation requires Vß3 ligand occupancy. We hypothesized that changes in the levels of extracellular matrix proteins induced by alterations in glucose concentrations may regulate the ability of SMCs to respond to IGF-I. IGF-I stimulated migration and proliferation of SMCs that had been maintained in 25 mM glucose containing media, but it had no stimulatory effect when tested using SMCs that had been grown in 5 mM glucose. IGF-I stimulated an increase in Shc phosphorylation and enhanced activation of the MAPK pathway in SMCs grown in 25 mM glucose, whereas in cells maintained in 5 mM glucose, IGF-I had no effect on Shc phosphorylation, and the MAPK response to IGF-I was markedly reduced. In cells grown in 25 mM glucose, the levels of Vß3 ligands, e.g. osteopontin, vitronectin, and thrombospondin, were all significantly increased, compared with cells grown in 5 mM glucose. The addition of these Vß3 ligands to SMCs grown in 5 mM glucose was sufficient to permit IGF-I-stimulated Shc phosphorylation and downstream signaling. Because we have shown previously that Vß3 ligand occupancy is required for IGF-I-stimulated Shc phosphorylation and stimulation of SMC growth, our data are consistent with a model in which 25 mM glucose stimulates increases in the concentrations of these extracellular matrix proteins, thus enhancing Vß3 ligand occupancy, which leads to increased Shc phosphorylation and enhanced cell migration and proliferation in response to IGF-I.

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