Endothelial cells synthesize and secrete urokinase-type plasminogen activators (u-PA) in response to various stimuli. To modulate the local expression of u-PA activity both intravascularly and pericellularly during angiogenesis, endothelial cells express both inhibitors (primarily PAI-1) and receptors (u-PAR) for u-PA. The interaction of u-PA with receptors on the surface of endothelial cells may play an important role in the regulation of fibrinolysis and cell migration. Using radioligand binding studies, we and others have demonstrated that human umbilical vein endothelial cells (HUVEC) express high affinity receptors for urokinase on the cell surface. We have demonstrated that single chain urokinase (scu-PA, prourokinase) binds only via the growth factor domain, while two chain high molecular weight urokinase (tcu-PA) can bind to the receptor or to cell- or matrix-associated PAI-1. We have isolated a approximately 46 kDa glycoprotein from HUVEC using affinity chromatography which retains the ability to specifically bind u-PA. At least three post-translational modifications appear to occur including removal of an N-terminal signal peptide; N-linked glycosylation, and C-terminal cleavage with addition of a phosphatidylinositol-glycan moiety which links the externally oriented protein to the cell surface. Using the polymerase chain reaction and published sequence information of the u-PAR cloned from a transformed fibroblast cDNA library, we amplified cDNAs of u-PAR from HUVEC and PMA-treated U937 cells. The specificity of the cDNAs was confirmed by restriction mapping and direct sequence analysis. Using these probes and radioligand binding studies we have demonstrated that at least two independent protein kinase pathways exist in endothelial cells for upregulating u-PA receptor expression. Down regulation of receptors may be pathophysiologic in thrombotic disorders whereas up regulation may be important in promoting wound healing, vascular repair, and protection from thrombosis. Up regulation could be harmful as well in such conditions as pathologic neovascularization (e.g., diabetic retinopathy) and in tumour metastasis as well as in tumour-related angiogenesis. Understanding the control and functional significance of u-PA binding to cells in general will hopefully enable the design of therapies to optimize the beneficial aspects and minimize any harmful effects of this interaction. Changes in the local expression of u-PA, PAI-1 and u-PAR by endothelial cells may affect the extent of angiogenesis or the degree of local intravascular fibrinolysis, which might be critical in conditions such as unstable angina and myocardial infarction.
