Characterization of plasminogen as an adhesive ligand for integrins αMβ2 (Mac-1) and α5β1 (VLA-75)

Plasminogen (Pg) has been implicated in many biologic processes involving extracellular proteolysis. We investigated whether Pg, by virtue of its capacity to be deposited within the extracellular matrix, can serve as a ligand for cell surface integrins. We report here that Pg supports cell adhesion by engaging integrins alpha(M)beta(2) and alpha(5)beta(1). The immobilized Glu-Pg, but not its derivatives with the N-terminal peptide lacking, plasmin and Lys-Pg, supported efficient adhesion that was abolished by anti-alpha(M)beta(2) and anti-alpha(5)beta(1) integrin-specific monoclonal antibodies (mAbs). In addition, lysine binding sites of Glu-Pg contributed to cell adhesion inasmuch as tranexamic acid and epsilon-aminocaproic acid inhibited cell adhesion. The involvement of alpha(M)beta(2) and alpha(5)beta(1) in adhesion to Glu-Pg was demonstrable with blood neutrophils, U937 monocytoid cells, and genetically engineered alpha(M)beta(2)-transfected human embryonic kidney (HEK) 293 cells. In alpha(M)beta(2), the alpha(M)I-domain is the binding site for Glu-Pg because the "I-less" form of alpha(M)beta(2) did not support cell adhesion and the recombinant alpha(M)I-domain bound Glu-Pg directly. In comparison with cell adhesion, the binding of soluble Glu-Pg to cells and the concomitant generation of plasmin activity was inhibited by anti-alpha(5)beta(1) but not by anti-alpha(M)beta(2). These findings identify Glu-Pg as an adhesive ligand for integrins alpha(M)beta(2) and alpha(5)beta(1) and suggest that alpha(5)beta(1), may participate in the binding of soluble Glu-Pg and assist in its activation.