INVOLVEMENT OF A TYROSINE KINASE PATHWAY IN THE GROWTH-PROMOTING EFFECTS OF ANGIOTENSIN-II ON AORTIC SMOOTH-MUSCLE CELLS

Angiotensin II (All) is a growth factor that stimulates protein synthe sis and induces cellular hypertrophy in aortic smooth muscle cells (SM C). This trophic effect is mediated by the AT(1) subtype of All recept ors. However, very little is known about the cellular signaling pathwa ys involved in this response. In the present study, we examined the ro le of protein tyrosine phosphorylation in the growth-promoting effects of All on rat aortic SMC. The addition of All to quiescent aortic SMC induced tyrosine phosphorylation of multiple substrates, as revealed by antiphosphotyrosine immunoblotting. This response was blocked by pr eincubation with the AT(1)-selective antagonist losartan. To explore t he functional role of this signaling pathway, we performed experiments with two mechanistically distinct tyrosine kinase inhibitors. Treatme nt of quiescent aortic SMC with genistein and herbimycin A abolished t he stimulatory effect of All on overall protein tyrosine phosphorylati on. Similarly, the two inhibitors prevented All-induced tyrosine phosp horylation of the cytoskeletal protein paxillin. Under the same condit ions, incubation with genistein or herbimycin A did not interfere with All binding to the AT(1) receptor and did not significantly affect Al l-stimulated inositol-l,4,5-trisphosphate production and Ca2+ mobiliza tion. In parallel to their selective action on tyrosine phosphorylatio n, both genistein and herbimycin A completely inhibited All-stimulated protein synthesis in a dose-dependent manner. In contrast, the two in hibitors were much less potent in preventing the trophic effect of pho rbol-12-myristate 13-acetate in these cells. We further demonstrate th at genistein and herbimycin A did not prevent mitogen-activated protei n kinase activation and c-fos gene induction, which is consistent with the notion that these downstream effecters do not link All-induced ty rosine phosphorylation to protein synthesis. These results provide evi dence that tyrosine phosphorylation has a critical role in cellular hy pertrophy and is involved in All action in vascular SMC.