Activation of αVβ3 on Vascular Cells Controls Recognition of Prothrombin

Regulation of vascular homeostasis depends upon collaboration between cells of the vessel wall and blood coagulation system. A direct interaction between integrin αVβ3 on endothelial cells and smooth muscle cells and prothrombin, the pivotal proenzyme of the blood coagulation system, is demonstrated and activation of the integrin is required for receptor engagement. Evidence that prothrombin is a ligand for αVβ3 on these cells include: (a) prothrombin binds to purified αVβ3 via a RGD recognition specificity; (b) prothrombin supports αVβ3-mediated adhesion of stimulated endothelial cells and smooth muscle cells; and (c) endothelial cells, either in suspension and in a monolayer, recognize soluble prothrombin via αVβ3. αVβ3-mediated cell adhesion to prothrombin, but not to fibrinogen, required activation of the receptor. Thus, the functionality of the αVβ3 receptor is ligand defined, and prothrombin and fibrinogen represent activation- dependent and activation-independent ligands. Activation of αVβ3 could be induced not only by model agonists, PMA and Mn2+, but also by a physiologically relevant agonist, ADP. Inhibition of protein kinase C and calpain prevented activation of αVβ3 on vascular cells, suggesting that these molecules are involved in the inside-out signaling events that activate the integrin. The capacity of αVβ3 to interact with prothrombin may play a significant role in the maintenance of hemostasis; and, at a general level, ligand selection by αVβ3 may be controlled by the activation state of this integrin.