The urokinase receptor (uPAR) was originally identified as the membrane receptor of the serine protease urokinase (uPA), thereby implicated in the plasminogen activation cascade and regulation of pericellular proteolysis. Later on, vitronectin was showed to be another major ligand providing uPAR with a role in cell adhesion. Other unrelated ligands have been subsequently reported including for example factor XII and SRPX2 expanding the functions of uPAR to unexpected biological areas such as the initiation of the coagulation cascade or the regulation of language development. Due to its glycosylphosphatidylinositol (GPI) anchor, uPAR has no intracellular domain and thus exerts its signaling capacity through lateral interactions with other components of the plasma membrane that actually mediate uPAR-induced signals. As yet, a total 42 proteins interacting directly with uPAR can be numbered comprising 9 soluble ligands and 33 lateral partners. The fact that uPAR interacts with members of three major families of membrane receptors i.e. G protein-coupled receptors, receptor tyrosine kinases, and integrins implies that the actual number of components constituting the uPAR interacome is extremely high. For example, 156 factors belong to the integrin adhesome. Moreover, in the light of the wide diversity of the components of the uPAR interactome, uPAR appears to be an essential player of major biological systems including the blood coagulation, complement and plasma kallikrein-kinin cascades. This review describes the soluble ligands and lateral partners of the uPAR interactome, the mechanisms regulating uPAR interactions and their proved and/or potential biological functions.
Keywords: Urokinase receptor/uPAR, cell migration, cell proliferation, cell adhesion, integrin, receptor tyrosine kinase, G protein-coupled receptor, signal transduction, glycosylphosphatidylinositol, zymogen, caveolins, neutropenia, angiogenesis, MAP kinase pathway, cellular signalling