FIBROBLASTS SPREAD ON IMMOBILIZED FIBRIN MONOMER BY MOBILIZING A BETA(1)-CLASS INTEGRIN, TOGETHER WITH A VITRONECTIN RECEPTOR ALPHA(V)BETA(3) ON THEIR SURFACE

Human and murine fibroblasts were found to spread far more avidly on f ibrin monomer monolayers than on immobilized fibrinogen, indicating th at removal of fibrinopeptides by thrombin is a prerequisite for the fi brin-mediated augmentation of cell spreading. In fact, cell spreading was not efficiently augmented on monolayers of a thrombin-treated dysf ibrinogen lacking the release of fibrinopeptide A due to an A alpha Ar g-16 --> Cys substitution. Since a synthetic Arg-Gly-Asp (RGD)-contain ing peptide inhibited the fibrin-mediated cell spreading, subsequent d issociation of the carboxyl-terminal globular domain of the A alpha-ch ains appears to render the RGD segments accessible to the cell-surface integrins. In support of this, fibrin-augmented cell spreading was in hibited by an antibody recognizing a 12-kDa peptide segment with gamma Met-89 at its amino terminus, which is located in close association w ith the RGD segment at A alpha 95-97 in the helical coiled-coil interd omainal connector. The fibrin-mediated augmentation of cell spreading was inhibited not only by an antibody against human vitronectin recept or (LM 609) but also by an antibody against the beta(1) subunit of int egrin (mAb13), suggesting that the beta(1)-class integrin together wit h a vitronectin receptor, alpha(v) beta(3), is mobilized onto the surf ace of fibroblasts upon contact with the fibrin monomer monolayer.