INTEGRIN RECEPTORS ON AORTIC SMOOTH-MUSCLE CELLS MEDIATE ADHESION TO FIBRONECTIN, LAMININ, AND COLLAGEN

Extracellular matrix receptors on vascular smooth muscle cells help in anchoring the cells during contraction and in promoting cellular migration after vessel injury. We found that rat aortic smooth muscle cells attach to surfaces coated with fibronectin, laminin, and collagen types I and IV. Cell attachment to these substrates appears to be mediated by members of the β1 integrin family of extracellular matrix receptors. Antibodies to the β1 subunit not only demonstrated the presence of integrin complexes in focal adhesion plaques but also blocked cell adhesion to the different substrates. Ligand-affinity chromatography and sodium dodecyl sulfate-polyacrylamide gel electrophoresis isolated a series of receptor complexes that were recognized by antisera to β1 integrin receptors. Each of the receptors appeared to be a heterodimer in which one of several α subunits shared a common 120-kDa (nonreduced) β1 subunit protein. The rat aortic smooth muscle cells had one α subunit (150 kDa nonreduced, 140 kDa reduced) that bound exclusively to fibronectin. There was a second α subunit (150 kDa nonreduced, 160 kDa reduced) that bound exclusively to collagen type I. In addition, there was a third α subunit (185 kDa nonreduced, 200 kDa reduced) that was promiscuous and bound to collagen types I and IV as well as to laminin; the 185-kDa α subunit appeared to bind to collagen more efficiently than it did to laminin. Thus, smooth muscle cells express multiple integrin receptors with different ligand specificities that appear to mediate cell interactions with the extracellular matrix.