Role of tyrosine kinases in extracellular matrix-mediated modulation of arterial smooth muscle cell phenotype.

Fibronectin (FN) promotes the modulation of freshly isolated arterial smooth muscle cells (SMCs) from a contractile to a synthetic phenotype by interacting with integrins on the cell surface. This process is characterized by a structural and functional transformation of the cells, including a reorganization of the cytoskeleton, the formation of a large secretory apparatus, and the acquisition of proliferative capacity. In this study we have investigated the role of integrin signaling through tyrosine kinases in the structural changes that occur in SMCs during primary culture on FN. A gradual increase in phosphotyrosine staining in focal adhesions and a concomitant increase in tyrosine phosphorylation of proteins including focal adhesion kinase were observed. In contrast, cells seeded on laminin formed few focal adhesions, and tyrosine phosphorylation of proteins was less than in cells cultured on FN. Treatment of cells cultured on FN with the tyrosine kinase inhibitor genistein strongly suppressed focal adhesion formation, cell spreading, and cytoskeletal reorganization. In addition, electron microscopic analysis demonstrated that the phenotypic modulation was slowed down. These results indicate that the ability of extracellular matrix components to promote a change in the phenotypic properties of SMCs depends on the assembly of focal adhesions with associated tyrosine kinase activity.