single-chainuPA and uPA activity to the metastatic processwasexam

We have carried out enzymatic, immunofluorescence, and surface iodination studies which show that B16 melanoma cells express the single chain form of the urokinase type plasminogenactivator (uPA) on their cell surface,and that these cells are capableof plasminogen-dependent fibronectin degradation. The significance of the expression of surface single-chainuPA and uPA activity to the metastatic processwasexam ined by preincubating melanoma cells with uPA modulating agents fol lowedby i.v. injection ofthe cells into miceandenumerationof pulmonary nodules 17 days later. B16 cells that had been pretreated with anti-uPA immunoglobulins that were inhibitory to uPA activity invariably showed significantly decreased numbers of metastases compared to controls. On the contrary,pretreatmentwith pIaSIUin, whichis not only the product of the uPA catalyzedreactionbut is also able to convertsingle-chain uPAto uPA,significantlyincreasedthe numbersof metastases.Control treatments, which included normal rabbit and mouse immunoglobulins, monovalentnoninhibitoryanti-uPA Fab fragments, and various mono clonaland polyclonalantibodiesdirectedagainstother B16 cell surface antigens, didnotaffectthe metastaticpotential of the cells.Divalent inhibitory anti-uPA F(ab)@fragments, on the contrary, inhibited metas tasis as efficiently as intact IgG. The results support the hypothesis that proteolysis of extracellular matrix componentsby cell surface-localized uPAmaybe a criticalstep duringthe processof tumorcell invasionand metastasis.