VITRONECTIN CONCENTRATES PROTEOLYTIC ACTIVITY ON THE CELL-SURFACE ANDEXTRACELLULAR-MATRIX BY TRAPPING SOLUBLE UROKINASE RECEPTOR UROKINASECOMPLEXES

Urokinase-type-plasminogen activator (uPA) and its receptor are locali zed in the vessel wall where they are involved in cellular activation and remodelling processes. Besides the cell surface glycolipid (GPI)-a nchored urokinase receptor (uPAR), which binds uPA with high affinity, recent evidence points to the existence of soluble uPAR (suPAR), as w ell. In the present study, the origin, binding mechanism, and cellular effects of suPAR were examined. Under basal conditions human vascular smooth muscle cells (HVSMC), human umbilical vein endothelial cells ( HUVEC), and monocytic cells released 0.1 to 2 ng/mL suPAR, which was i ncreased twofold to fivefold after phorbol ester (PMA) stimulation, as measured by a function-dependent enzyme-linked immunosorbent assay (E LISA). suPAR alone did not bind to HVSMC or HUVEC, but reduced cellula r uPA binding by 50% to 70%. However, after removal of GPI-uPAR with p hosphatidylinositol-specific phospholipase C, suPAR dose-dependently i ncreased uPA binding by fourfold to fivefold. This increase in binding was completely inhibited by vitronectin (VN) and by a monoclonal anti body against VN, but not by other matrix proteins or antibodies. Thus, VN-mediated uPA binding to cells was regulated by the ratio of solubl e to surface-associated uPAR. In a uPAR-deficient cell line (LM-TK-), suPAR increased uPA binding up to 10-fold, whereas the truncated recep tor lacking the amino-terminal uPA-binding domain was ineffective. The formation of a ternary uPA/suPAR/VN-complex on the cell surface and t he free extracellular matrix could be inhibited by a monoclonal antibo dy against VN, as well as by plasminogen activator inhibitor-1 (PAI-1) . Moreover, VN-mediated binding of the uPA/suPAR-complex led to a five fold increase in plasminogen activator activity. Through this novel pa thway, VN concentrates the uPA/suPAR-complex to cell surfaces and extr acellular matrix sites, leading to the accumulation of plasminogen act ivator activity required for cell migration and tissue remodelling pro cesses. (C) 1998 by The American Society of Hematology.