Phosphatidylinositol 3-kinase is required for insulin-like growth Factor-I-Induced vascular smooth muscle cell proliferation and migration

Insulin-like growth factor-I (IGF-I) plays an important role in regulating vascular smooth muscle cell (VSMC) proliferation and directed migration. Th e mitogenic and chemotactic actions of IGF-I are mediated through the IGF-I receptor, but how the activation of the IGF-I receptor leads to these biol ogical responses is poorly understood. In this study, we examined the role of phosphatidylinositol 3-kinase (PI3 kinase) in mediating the mitogenic an d chemotactic signals of IGF-I. IGF-I treatment resulted in a significant i ncrease in phosphotyrosine-associated PI3 kinase activity in cultured prima ry VSMCs. To determine whether insulin receptor substrate (IRS)-1, -2, or b oth are involved in IGF-I signaling in VSMCs, cell lysates were immunopreci pitated with either an anti-IRS-l or an anti-IRS-2 antibody, and the associ ated PI3 kinase activity was determined. IGF-I stimulation resulted in a si gnificant increase in IRS-1- but not IRS-2-associated PI3 kinase activity, suggesting that IGF-I primarily utilizes IRS-1 to transmit its signal in VS MCs. The IGF-I-induced increase in IRS-I-associated PI3 kinase activity was concentration dependent. At the maximum concentration (50 ng/mL), IGF-I in duced a 60-fold increase. This activation occurred within 5 minutes and was sustained at high levels for at least 6 hours. IGF-I also caused a concent ration-dependent and long-lasting activation of protein kinase B (PKB/Akt). Inhibition of PI3 kinase activation by LY294002 or wortmannin abolished IG F-I-stimulated VSMC proliferation and reduced IGF-I-directed VSMC migration by approximate to 60%. These results indicate that activation of PI3 kinas e is required for both IGF-I-induced VSMC proliferation and migration.