We previously demonstrated that noncytolytic butanol extraction of B16 melanoma cells can increase the number of experimental lung metastases, and that brief incubation of the extracted cells with the extracted moieties reduces metastatic phenotype. This study examined the possibility that the extracted components are endogenous inhibitors of tumor cell surface-associated, degradative enzymes. The activity was found to be tumor associated, since only tumor extracts could reduce the number of experimental lung metastases of a variety of solid tumors. The activity in crude butanol extracts of B16-F1 that modulated the metastatic phenotype of extracted B16-F10 was partially purified by preparative isoelectric focusing and high-performance gel permeation chromatography. Incubation of extracted B16-F10 cells with low (Mr 2,000–10,000) molecular weight materials focusing in the pH 5.6 to 5.8 region of the preparative isoelectric focusing gradient significantly reduced the number of experimental lung foci. Ampholines alone had no effect. Evidence that the extracted moiety might be an endogenous enzyme inhibitor was obtained with the use of the subendothelial matrix degradation assay, wherein B16-F10 cells digest 35S-labeled heparan sulfate proteoglycan. The same materials that reduced the metastatic potential of butanol-extracted B16-F10 cells also inhibited extracellular matrix degradation by 30 to 85%, as well as the activity of partially purified heparanase (endo-β-glucuronidase). The metalloproteinase inhibitor 1,10-phenanthroline and the heparanase inhibitor heparin partially (30 to 50%) blocked extracellular matrix degradation. Conversely, inhibitors of serine, thiol, acid, and other proteases had little or no effect on extracellular matrix degradation. These data provide evidence that an endogenous, heat-stable inhibitor of cell surface degradative enzymes such as heparanase may play a role in hematogenous metastasis, and support the hypothesis that butanol extraction activates some of these surface enzymes by removing the endogenous inhibitors.

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Supported by USPHS Grants CA38500 and CA41524, awarded by the National Cancer Institute, and by the University of Texas Cancer Foundation.

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