REGULATION PLASMINOGEN ACTIVATION BY HUMAN U937 PROMONOCYTIC CELLS

Urokinase-type plasminogen activator (uPA) is initially produced by ce lls as a single-chain precursor (pro-uPA), which can be cleaved by the serine protease plasmin to form the two-chain molecule uPk This latte r protease efficiently converts the inactive zymogen plasminogen into plasmin. Cell surface binding of both pro-uPA and plasminogen is known to enhance the rate of plasminogen activation. It has been postulated that this may be due, in part, to an enhanced plasmin-mediated feedba ck activation of pro-uPA. This study directly demonstrates the enhance ment by cells of this feedback. activation loop by showing that uPA is generated more rapidly from pro-uPA and plasminogen in the presence o f human promonocytic U937 cells than it is under fluid-phase condition s. Moreover, the enhanced activation of pro-uPA and plasminogen observ ed in the presence of cells was significantly less susceptible to inhi bition by alpha(2)-antiplasmin. Finally, the presence of cells not onl y potentiated the production of plasmin, as measured using a plasmin-s pecific peptide substrate, it also potentiated the cleavage of a natur al protein substrate, I-125-labeled recombinant interferon-gamma, even in the presence of alpha(2)-antiplasmin or alpha(2)-macroglobulin. Th ese results demonstrate that cell-associated plasmin mediates a positi ve feedback amplification of plasminogen activation and thereby potent iates the proteolysis of natural plasmin substrates, even in the prese nce of plasma protease inhibitors.