Tumor cells (human choriocarcinoma SMT-cc1 cells and human promyeloid leukemia U937 cells) express urinary trypsin inhibitor (UTI)-like immunoreactive substance. Because of the prominent inhibitory effects of exogenous UTI on tumor cell invasive and metastatic potential, it is important to determine whether there is endogenous UTI production by these cells. Most of cell-associated UTI is located on the cell surface. Immuno-precipitates of the particulate fraction contained three different polypeptide chains including M(r) >200 kDa, 125 kDa, and a polydisperse band of M(r) 40 kDa, which is confirmed to be UTI by amino acid sequence analysis. In SMT-cc1 cells pulse-labeled with [S-35]sulfate, a polyclonal antibody against UTI precipitated labeled proteins of >200 kDa, 125 kDa, 93 kDa, 69 kDa, and 40 kDa. Upon chase, very low levels of 40 kDa band could be detected in lysates. On the other hand, although UTI was immunoprecipitated from U937 cells and the cells were immunohistochemically positive with anti-UTI antibody, UTI remained unlabeled even after long incubation of the cells with [S-35]sulfate. This suggests that SMT-cc1 cells may produce endogenous UTI, but U937 cell-associated UTI, which may not be generated by U937 cells themselves, may be taken up onto the cell surface from the serum. SMT-cc1 cells were invasive on the Matrigel in an in vitro assay. Exogenously applied UTI efficiently inhibits tumor cell invasion in a dose-dependent manner. Furthermore, addition of anti-UTI antibody to the cells enhanced cell-associated caseinolytic activity. Anti-UTI antibody produced biphasic, concentration-dependent changes in tumor cell invasion. Although invasion by the cells was enhanced by addition of lower concentrations (<0.5 mu g/ml) of anti-UTI antibody to the in vitro assay system, higher concentrations (>0.5 mu g/ml) of antibody blocked tumor cell invasion, suggesting that too much cell-associated proteolytic activity may cause uncontrolled matrix degradation. Our results suggest that certain tumor cells express UTI on their cell surface and that membrane-bound UTI may control proteolysis and contribute to prevent the excessive fibrinolysis in condition such as tumor invasion.

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