Analysis of plasminogen activator inhibitor type 2 (PAI-2) expression on the cell surface of monocytic cell lines and human peripheral blood leukocytes

Plasminogen activator inhibitor type 2 (PAI-2) is one of the inhibitors of the plasminogen cascade, a powerful proteolytic cascade involved in many physiological processes. The main aim of this thesis was to evaluate if PAI-2 could be detected on the cell surface of monocytic cell lines and human peripheral blood leukocytes, which could regulate the activity of cell surface urokinase type plasminogen activator (uPA). By flow cytometry PAI-2 could only be detected very weakly on the surface of U937 cells. By ELISA and Western blotting, monomeric non-glycosylated PAI-2 was very abundant in cytosol enriched preparations from U937, MM6 and HaCaT cells, and present in lower amounts in plasma membrane enriched preparations. In addition, Jurkat cells were observed not to express PAI-2 antigen or mRNA. Since PAI-2 could not be detected on the surface of cells, the second aim of this thesis was to determine if PAI-2 could localise to the extracellular matrix (ECM) surface by transglutaminase activity in an in vitro fibrin clot model. By autoradiography it was observed that three species of PAI-2 associated with a fibrin clot: monomeric PAI-2, PAI-2 cross-linked to the fibrin and a high molecular weight PAI-2. Addition of 125IuPA revealed that the only species of PAI-2 that was active in the fibrin clot, was the monomer. Therefore, rather than inhibiting plasminogen activation as a cell surface moiety, it seems more likely that PAI-2 inhibits plasminogen activation by localising to the surface of the ECM.