Involvement of αvβ3 integrin in mediating fibrin gel retraction

Abstract

Platelet integrin αIIbβ3 (GPIIb-IIIa) plays important roles in platelet-mediated clot retraction. However, little is known about the mechanisms of clot retraction mediated by nucleated cells. In this report, we demonstrate that another member of the β3 integrin family, αvβ3, is involved in clot retraction mediated by nucleated cells. Retraction of fibrin clots was observed using a human melanoma cell line, C32TG, which contains no αIIbβ3 complex. This retraction was inhibited by RGD-containing peptide, monoclonal anti-β3, and anti-αvβ3 antibodies. Immunoelectron microscopic studies revealed a direct interaction between β3 integrin and fibrin fibers at an early stage of clot retraction. We found that another human embryonal cell line, 293, which is known to express αvβ1, but no αvβ3, lacks fibrin gel retractile activity. Upon transfection of β3 DNA into 293 cells, the β3 subunit formed a complex with an endogenous αv subunit. The β3-bearing transfectants were found to retract fibrin gels, which was specifically inhibited by anti-β3 antibody. In addition, a point mutation at Asp119 in the β3 ligand binding domain abolished the clot retractile activity of 293 transfectants, indicating the requirement of αvβ3 ligand-binding activity. Our findings suggest that αvβ3 is involved in mediating the interaction between the three-dimensional fibrin network and nucleated cells and in promoting "post-receptor occupancy" events.