Involvement of Integrin in Mediating Fibrin Gel Retraction (*)

Abstract

Platelet integrin αβ₃ (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, α_vβ₃, 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 αβ₃ complex. This retraction was inhibited by RGD-containing peptide, monoclonal anti-β₃, and anti-α_vβ₃ 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 α_vβ₁, but no α_vβ₃, lacks fibrin gel retractile activity. Upon transfection of β₃ DNA into 293 cells, the β₃ subunit formed a complex with an endogenous α_v 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 Asp in the β₃ ligand binding domain abolished the clot retractile activity of 293 transfectants, indicating the requirement of α_vβ₃ ligand-binding activity. Our findings suggest that α_vβ₃ is involved in mediating the interaction between the three-dimensional fibrin network and nucleated cells and in promoting “post-receptor occupancy” events.