LOSS OF CELL VIABILITY DRAMATICALLY ELEVATES CELL-SURFACE PLASMINOGENBINDING AND ACTIVATION

The plasminogen activation cascade is focused at the cell surface by v irtue of the presence of plasminogen and plasminogen activator recepto rs. We have utilized flow cytometric plasminogen (plg) binding and act ivation assays to examine both plasminogen binding and activation on t he surface of specific subpopulations of U937 cells (viable, apoptotic , and dead cells). A direct relationship was found to exist between ce ll. viability (propidium iodide uptake) and the magnitude of lysine-de pendent plasminogen binding, with apoptotic and dead subpopulations of cells binding up to 100-fold more plasminogen than viable cells. Desp ite the high level of lysine-dependent plasminogen binding on dead cel ls, plasminogen activation was minimal due to low levels of cell-surfa ce urokinase plasminogen activator. Plasminogen activation readily occ urred on the surface of apoptotic cells because of a dramatic increase in both lysine-dependent plasminogen binding and endogenous urokinase plasminogen activator. These results indicate that colocalization of plasminogen and urokinase plasminogen activator are paramount for plas minogen activation to proceed on the cell surface. Our data also stron gly implicate the involvement of the plasminogen activation cascade in apoptosis, especially on urokinase plasminogen activator-expressing c ell types. The current study clearly supports the important role of fl ow cytometry in cellular plasminogen binding and activation studies. ( C) 1998 Academic Press.