Characterization of plasminogen as an adhesive ligand for integrins α_Mβ₂ (Mac-1) and α₅β ₁

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 α_Mβ₂ and α₅β 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-α_Mβ₂ and anti-α₅β₁ integrin-specific monoclonal antibodies (mAbs). In addition, lysine binding sites of Glu-Pg contributed to cell adhesion inasmuch as tranexamic acid and ε-aminocaproic acid inhibited cell adhesion. The involvement of α_Mβ₂ and α₅β₁ in adhesion to Glu-Pg was demonstrable with blood neutrophils, U937 monocytoid cells, and genetically engineered α_Mβ₂-transfected human embryonic kidney (HEK) 293 cells. In α_Mβ₂, the α_MI- domain is the binding site for Glu-Pg because the "l-less" form of α_Mβ₂ did not support cell adhesion and the recombinant α_MI-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- α₅β₁ but not by anti- α_Mβ₂. These findings identify Glu-Pg as an adhesive ligand for integrins α_Mβ₂ and α₅β₁ and suggest that α₅β ₁ may participate in the binding of soluble Glu-Pg and assist in its activation.