BIOLOGICAL ACTIVATION OF PRO-HGF (HEPATOCYTE GROWTH-FACTOR) BY UROKINASE IS CONTROLLED BY A STOICHIOMETRIC REACTION

Hepatocyte growth factor (HGF) is a paracrine inducer of morphogenesis and invasive growth in epithelial and endothelial cells. HGF is secre ted by mesenchymal cells as an inactive precursor (pro-HGF). The cruci al step for HGF activation is the extracellular hydrolysis of the Arg( 494)-Val(495) bond, which converts pro-HGF into alpha beta-HGF, the hi gh-affinity ligand for the Met receptor. We previously reported that t he urokinase-type plasminogen activator (uPA) activates pro-HGF in vit ro. We now show that this is a stoichiometric reaction, and provide ev idence for its occurrence in tissue culture. Activation involves the f ormation of a stable complex between pro-HGF and uPA. This complex was isolated from the in vitro reaction of pure uPA with recombinant pro- HGF, as well as from the membrane of target cells, after sequential ad dition of uPA and pro-HGF. On the cell membrane, the uPA HGF complex w as bound to the Met receptor. Monocytic cell lines, and primary monocy tes after adhesion, activated efficiently pro-HGF both on their surfac e and in the culture medium. This activation was inhibited by anti-cat alytic anti-uPA antibodies, and occurred by a stoichiometric reaction. The stoichiometry of the activation reaction suggests that the biolog ical effects of HGF can be titrated in vivo by the level of uPA activi ty. Adequate amounts of uPA can be locally provided by the macrophages , which would condition the tissue microenvironment by rendering HGF b ioavailable to its target cells.