THE UROKINASE RECEPTOR IS A MAJOR VITRONECTIN-BINDING PROTEIN ON ENDOTHELIAL-CELLS

We have previously demonstrated that vitronectin (VN), a morphoregulat ory protein in the vessel wall, is internalized and translocated to th e subendothelial matrix by an integrin-independent mechanism (J. Histo chem. Cytochem. 41, 1823-1832, 1993). The cell surface component which mediates the initial contact of VN with endothelial cells is defined here. The specific binding of VN to endothelial cells demonstrated the following properties: a threefold increase after phorbol ester treatm ent; 85% inhibition by pretreatment of cells with phosphatidylinositol -phospholipase C to release glycolipid-anchored surface proteins; a 90 % inhibition by urokinase (u-PA) receptor blocking antibody, u-PA incr eased VN binding to cells due to an eightfold increase in the affinity of VN for the u-PA receptor. Structure-function studies showed that t he amino-terminal fragment of u-PA, devoid of any proteolytic activity , mediated this effect. Active plasminogen activator inhibitor-1 (PAI- 1), but not inactivated PAI-1, inhibited VN binding to cells and displ aced VN that was prebound to endothelial cell monolayers. Similarly, V N binding to purified (immobilized) u-PA receptor, but not to integrin , was enhanced by u-PA and inhibited by PAC-1. Hence, the binding of s oluble VN to endothelial cell surfaces is mediated by the u-PA recepto r, and the relative concentrations of u-PA and PAI-1 are able to regul ate the strength of this interaction. Endothelial cell adhesion to imm obilized VN was found to be integrin-mediated without any involvement of the VN-uPA-receptor system. Hence, the interaction of VN with the u -PA receptor may be involved in the regulation of cellular processes n ecessary for endothelial cell invasion and migration at VN-rich extrac ellular matrix sites. (C) 1996 Academic Press, Inc.