Integrin-type fibronectin receptors of rat arterial smooth muscle cells: isolation, partial characterization and role in cytoskeletal organization and control of differentiated properties

The spreading of freshly isolated rat arterial smooth muscle cells (SMCs) on a substrate of fibronectin (FN) is associated with marked changes in fine structure and function of the cells, collectively referred to as a modulation from a contractile to a synthetic phenotype. Recent studies have indicated that this process is mediated via an interaction between the minimal cell-attachment sequence of FN (RGDS) and cell surface receptors. Here, we report the isolation of such receptors by sequential chromatography on affinity columns of wheat germ agglutinin (WGA) and a 105-kDa cell-binding fragment of FN (105-kDa fragment). The receptor was composed of two proteins with electrophoretic mobilities in SDS-polyacrylamide gels of 160 and 115 kDa under nonreducing conditions and 150 and 130 kDa under reducing conditions. Immunoprecipitation of surface-labeled cells with a rabbit antiserum against the beta chain of the rat hepatocyte FN receptor similarly yielded two proteins of 160 and 115 kDa. In metabolically labeled cells an additional component of 105 kDa was precipitated, presumably representing a precursor of the 115-kDa protein. Immunocytochemical studies demonstrated that SMCs grown on laminin formed FN fibrils and actin filament bundles in close alignment with cell surface receptors after a few days of culture. In cells seeded on the 105-kDa fragment, the receptors were already arranged in parallel with actin filaments on the first day of culture. Later on, the cells secreted FN and laid down FN fibrils along the receptors on the cell surface and the actin filament bundles in the cytoplasm. Taken together, the findings indicate that arterial SMCs are equipped with FN receptors that belong to the integrin family of proteins and consists of alpha (160-kDa) and beta (115-kDa) subunits. The receptor complexes apparently play an important role in determining the differentiated characteristics of the cells, possibly by mediating a linkage between the extracellular matrix and the cytoskeleton.