Integrin function is central to inflammation, immunity, and tumor progression. The urokinase-type plasminogen activator receptor (uPAR) and integrins formed stable complexes that both inhibited native integrin adhesive function and promoted adhesion to vitronectin via a ligand binding site on uPAR. Interaction of soluble uPAR with the active conformer of integrins mimicked the inhibitory effects of membrane uPAR. Both uPAR-mediated adhesion and altered integrin function were blocked by a peptide that bound to uPAR and disrupted complexes. These data provide a paradigm for regulation of integrins in which a nonintegrin membrane receptor interacts with and modifies the function of activated integrins.

Abstract. Induction of the urokinase type plasminogen activator receptor (uPAR) promotes cell adhesion through its interaction with vitronectin (VN) in the extracellular matrix, and facilitates cell migration and invasion by localizing uPA to the cell surface. We provide evidence that this balance between cell adhesion and cell detachment is governed by PA inhibitor-1 (PAl-l). First, we demonstrate that uPAR and PAL1 bind to the same site in VN (i.e., the amino-terminal somatomedin B domain; SMB), and that PAI-1 competes with uPAR for binding to SMB. Domain swapping and mutagenesis studies indicate that the uPAR-binding sequence is located within the central region of the SMB domain, a region previously shown to contain the PAl-l-binding motif. Second, we show that PAI-1 dissociates bound VN from uPAR and detaches U937 cells from their VN substratum. This PAL1 mediated release of cells from VN appears to occur independently of its ability to function as a protease inhibitor, and may help to explain why high PAI-1 levels indicate a poor prognosis for many cancers. Finally, we show that uPA can rapidly reverse this effect of PAI-1. Taken together, these results suggest a dynamic regulatory role for PAI-1 and uPA in uPAR-mediated cell adhesion and release.

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The urokinase plasminogen activator (uPA) interacts with its cell surface receptor (uPAR), providing an inducible, localized cell surface proteolytic activity, thereby promoting cellular invasion. Evidence is provided for a novel function of cell surface-associated uPA⅐uPAR. Specifically, induction of cell surface expression of uPA⅐uPAR by growth factors or phorbol ester was necessary for vitronectin-dependent carcinoma cell migration, an event mediated by integrin ␣v␤5. Cell migration on vitronectin was blocked with either a soluble form of uPAR, an antibody that disrupts uPA binding to uPAR, or a monoclonal antibody to ␣v␤5. Moreover, plasminogen activator inhibitor type 2 blocked this migration event but did not affect adhesion, suggesting a direct role for uPA enzyme activity in this process and that migration but not adhesion of these cells is regulated by uPA⅐uPAR. Growth factor-mediated induction of uPA⅐uPAR on the carcinoma cell surface promotes a specific motility event mediated by integrin ␣v␤5, since cells transfected with the ␤3 integrin subunit expressed ␣v␤3 and migrated on vitronectin independently of growth factors or uPA⅐uPAR expression. This relationship between ␣v␤5 and the uPA⅐uPAR system has significant implications for regulation of motility events associated with development, angiogenesis, and tumor metastasis.

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