Abstract

Angiotensin II (AII) is a growth factor that stimulates protein synthesis and induces cellular hypertrophy in aortic smooth muscle cells (SMC). This trophic effect is mediated by the AT1 subtype of AII receptors. However, very little is known about the cellular signaling pathways involved in this response. In the present study, we examined the role of protein tyrosine phosphorylation in the growth-promoting effects of AII on rat aortic SMC. The addition of AII to quiescent aortic SMC induced tyrosine phosphorylation of multiple substrates, as revealed by antiphosphotyrosine immunoblotting. This response was blocked by preincubation with the AT1-selective antagonist losartan. To explore the functional role of this signaling pathway, we performed experiments with two mechanistically distinct tyrosine kinase inhibitors. Treatment of quiescent aortic SMC with genistein and herbimycin A abolished the stimulatory effect of AII on overall protein tyrosine phosphorylation. Similarly, the two inhibitors prevented AII-induced tyrosine phosphorylation of the cytoskeletal protein paxillin. Under the same conditions, incubation with genistein or herbimycin A did not interfere with AII binding to the AT1 receptor and did not significantly affect AII-stimulated inositol-1,4,5-trisphosphate production and Ca2+ mobilization. In parallel to their selective action on tyrosine phosphorylation, both genistein and herbimycin A completely inhibited AII-stimulated protein synthesis in a dose-dependent manner. In contrast, the two inhibitors were much less potent in preventing the trophic effect of phorbol-12-myristate 13-acetate in these cells. We further demonstrate that genistein and herbimycin A did not prevent mitogen-activated protein kinase activation and c-fos gene induction, which is consistent with the notion that these downstream effectors do not link AII-induced tyrosine phosphorylation to protein synthesis. These results provide evidence that tyrosine phosphorylation has a critical role in cellular hypertrophy and is involved in AII action in vascular SMC.