The geldanamycins are potent inhibitors of the hepatocyte growth factor/scatter factor-Met-urokinase plasminogen activator-plasmin proteolytic network

The Met receptor tyrosine kinase and its ligand, hepatocyte growth factor/s catter factor (HGF/SF), have been implicated in human tumor development and metastasis. HGF/SF induces the expression of urokinase plasminogen activat or (uPA) and the uPA receptor (uPAR), important mediators of cell invasion and metastasis. We have developed a cell-based assay to screen for inhibito rs of this signaling system using the induction of endogenous uPA and uPAR and the subsequent conversion of plasminogen to plasmin as the biological e nd point. Assay validation was established using a neutralizing antiserum t o HGF/SF and a uPA inhibitor (B428), as well as inhibitors of the MKK-MAPK1 /2 pathway, shown previously to be important in the induction of uPA and uP AR, Using this assay, we found several classes of molecules that exhibited inhibition of HGF/SF-dependent plasmin activation. However, we discovered t hat certain members of the geldanamycin family of anisamycin antibiotics ar e potent inhibitors of HGF/SF-mediated plasmin activation, displaying inhib itory properties at femtomolar concentrations and nine orders of magnitude below their growth inhibitory concentrations. lit nanomolar concentrations, the geldanamycins down-regulate Met protein expression, inhibit HGF/SF-med iated cell motility and invasion, and also revert the phenotype of both aut ocrine HGF/SF-Met transformed cells as well as those transformed by Met pro teins with activating mutations. Thus, the geldanamycins may have important therapeutic potential for the treatment of cancers in which Met activity c ontributes to the invasive/metastatic phenotype.