Blocking ligand occupancy of the αVβ3 integrin inhibits insulin-like growth factor I signaling in vascular smooth muscle cells

Blocking alpha V beta 3 integrin occupancy results in attenuation of the cellular migration response to insulin-like growth factor I (IGF-I), To determine whether integrin antagonists alter other IGF-I-stimulated biologic actions, quiescent smooth muscle cells (SMCs) were exposed to echistatin and their ability to respond to IGF-I was determined. Echistatin (10(-7) M) inhibited IGF-I-stimulated DNA synthesis by 80%, and the protein synthesis response also was inhibited. Therefore blocking occupancy of alpha V beta 3 inhibited multiple target cell actions of IGF-I, To determine whether blocking alpha V beta 3 occupancy could alter IGF-I receptor-mediated signal transduction, the ability of IGF-I to stimulate phosphorylation of insulin receptor substrate-1 (IRS-1) was analyzed. A 10-min exposure to 100 ng/ml of IGF-I resulted in a substantial increase in phosphorylated IRS-1, and echistatin (10(-7) M) blocked the IGF-I-induced IRS-1 phosphorylation response. Echistatin also attenuated downstream signaling because the capacity of the p85 subunit of phosphatidylinositol-3 kinase (PI-3 kinase) to bind to IRS-1 was blocked. In contrast, exposure of SMCs to vitronectin (1.0 mu g/cm(2)) or thrombospondin (0.25 mu g/cm(2)), two known ligands for alpha V beta 3, resulted in enhancement of the IGF-I-stimulated IRS-1 response. To determine whether these effects were caused by alterations in receptor kinase activity, the IGF-I receptor was immunoprecipitated and then analyzed for phosphotyrosine. Echistatin (10-7 M) significantly reduced IGF-I-stimulated tyrosine phosphorylation of the IGF-I receptor beta subunit, We conclude that occupancy of the alpha V beta 3 integrin is necessary for IGF-I to fully activate the kinase activity of the IGF-I receptor and phosphorylate IRS-1, Activation of the alpha V beta 3 receptor results in an interaction with the IGF-I signal transduction pathway, which modulates SMCs responsiveness to IGF-I.