PROTEOLYTIC CLEAVAGE OF THE UROKINASE RECEPTOR SUBSTITUTES FOR THE AGONIST-INDUCED CHEMOTACTIC EFFECT

Physiological concentrations of urokinase plasminogen activator (uPA) stimulated a chemotactic response in human monocytic THP-1 through bin ding to the urokinase receptor (uPAR). The effect did not require the protease moiety of uPA, as stimulation was achieved also with the N-te rminal fragment (ATF), while the 33 kDa low molecular weight uPA was i neffective, Coimmunoprecipitation experiments showed association of uP AR with intracellular kinase(s), as demonstrated by in vitro kinase as says, Use of specific antibodies identified p56/p59(hck) as a kinase a ssociated with uPAR in THP-1 cell extracts, Upon addition of ATF, p56/ p59(hck) activity was stimulated within 2 min and returned to normal a fter 30 min, Since uPAR lacks an intracellular domain capable of inter acting with intracellular kinases, activation of p56/p59(hck) must req uire a transmembrane adaptor, Evidence for this was strongly supported by the finding that a soluble form of uPAR (suPAR) was capable of ind ucing chemotaxis not only in THP-1 cells but also in cells lacking end ogenous uPAR (IC50, 5 pM) However, activity of suPAR required chymotry psin cleavage between the N-terminal domain D1 and D2 + D3, Chymotryps in-cleaved suPAR also induced activation of p56/p59(hck) in THP-1 cell s, with a time course comparable with ATF, Our data show that uPA-indu ced signal transduction takes place via uPAR, involves activation of i ntracellular tyrosine kinase(s) and requires an as yet undefined adapt or capable of connecting the extracellular ligand binding uPAR to intr acellular transducer(s).