Angiotensin II stimulation of rapid paxillin tyrosine phosphorylation correlates with the formation of focal adhesions in rat aortic smooth muscle cells

Angiotensin II is a potent vasoconstrictor that has been also implicated in vascular hyperproliferative diseases, including atherosclerosis and restenosis following angioplasty. Treatment of cultured, serum-starved rat aortic smooth muscle cells with angiotensin II causes rapid protein tyrosine phosphorylation that precedes cell mitogenesis. We have identified two of the phosphoproteins as paxillin (75 kilodaltons) and the tyrosine kinase pp125(Fak), both components of actin-associated focal adhesion sites. Angiotensin II stimulated a 5-fold increase in the tyrosine phosphorylation of paxillin and a smaller (1.5-fold) increase in pp125(Fak) tyrosine phosphorylation. Paxillin tyrosine phosphorylation was evident within 1 minute, and was maximal after 10 minutes. Similar elevated protein tyrosine phosphorylation levels of paxillin were obtained with exposure of the rat aortic smooth muscle cells to peptides endothelin-1 and α-thrombin that function, as angiotensin II, through binding to members of the seven transmembrane domain G protein coupled receptors. Angiotensin II treatment also stimulated the production of a well-ordered actin-containing stress fiber network and prominent paxillin-containing focal adhesions. The focal adhesions stained intensely with anti-phosphotyrosine antibody suggesting the tyrosine phosphorylation of paxillin and cytoskeletal reorganization were tightly coupled. Angiotensin II receptor occupancy has been shown previously to lead to protein kinase C activation. However, compared to angiotensin II stimulation, a smaller, delayed increase in paxillin tyrosine phosphorylation was observed following direct protein kinase C activation by the phorbol ester phorbol 12-myristate-13-acetate. Paxillin tyrosine phosphorylation was selective for certain agonists since no increase in tyrosine phosphorylation of this protein was observed following exposure to the potent mitogen PDGF. Thus, actin-based cytoskeletal changes involving sites of cell adhesion to the extracellular matrix may play an important role in normal and pathophysiologic smooth muscle cell growth regulation in response to certain angiotensin II-type vasoactive agonists.