Abstract 11552: The α5β1 Integrin Mediates Insulin-like Growth Factor-1 Induction of Aortic Smooth Muscle Alpha-Actin Expression Through a PI3K/mTORC2/eIF4E Translational Mechanism

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In arterial remodeling, smooth muscle cell (SMC) loss of contractile phenotype and increased proliferation may result in vessel stenosis or a greater propensity for aneurysm formation and rupture. Here, we show that insulin-like growth factor-1 (IGF-1) induces contractile phenotype of human aortic SMCs in culture, and we investigate the signal transduction mechanisms responsible using Western blot. IGF-1 induced a dose-dependent increase in protein expression of the contractile marker alpha-actin (αSMA) (2.4-fold-increase, 18hr serum-free vs. 100ng/mL IGF-1, p<0.001), without affecting ACTA2 mRNA levels (real time RT-PCR, 4-18 hrs, p=NS). Moreover, IGF-1 upregulation of αSMA expression was not blocked by actinomycin D but was inhibited by cycloheximide (p<0.01), thereby demonstrating posttranscriptional and translation-dependent regulation by IGF-1, respectively. Furthermore, IGF-1-induced αSMA expression was not blocked with PD98059 or rapamycin (50 or 100 nM), but was completely inhibited with LY294002 (p<0.001), 500 nM rapamycin (p<0.01), and Torin 1, thereby indicating that the ability of IGF-1 to induce SMC differentiation was independent of Erk1/2 and mTORC1, but dependent on PI3K- and mTORC2-signaling. A prolyl-hydroxylase-inhibitor (EDHB) or a function-blocking antibody against the α2β1 integrin (BHA2.1) had no effect on IGF-1-upregulation of αSMA expression. However, both EDHB and BHA2.1 significantly attenuated IGF-1-induced expression of the proliferation marker PCNA, thereby suggesting that collagen and the α2β1 integrin promote IGF-1-induced proliferation. Conversely, a blocking antibody against the α5β1 integrin (JBS5) did not block IGF-1-induced PCNA expression but completely inhibited IGF-1 induction of αSMA expression (p<0.01) and abolished IGF-1-induced phosphorylation of a master regulator of cap-dependent protein translation, eukaryotic initiation factor-4E (eIF4E) (p<0.001). Therefore, we concluded that IGF-1 induced αSMA expression via a PI3K/mTORC2/eIF4E translational mechanism that is dependent on the α5β1 integrin. Overall, these findings provide evidence that the α5β1 and α2β1 integrins mediate IGF-1-signaling to govern IGF-1-induced translational regulation of SMC phenotype.