Characterization of the cellular binding site for the urinary trypsin inhibitor

A highly purified urinary trypsin inhibitor (UTI) inhibits not only tumor cell invasion in an in vitro assay but also production of experimental and spontaneous lung metastasis in an in vivo mouse model. UTI is present both in the lysate of tumor cells (human choriocarcinoma SMT-cc1 cells, human promyeloid leukemia U937 cells, and murine Lewis lung carcinoma 3LL cells) and human neutrophils. In each medium from tumor cells, most of the cell-associated UTI is on the cell surface. Cell-binding experiments employing cell enzyme-linked immunosorbent assay and flow cytometry indicated that tumor cells (SMT-cc1 and 3LL cells) have specific binding sites for UTI on their cell surface. UTI binds rapidly and with relatively high affinity to SMT-cc1 and 3LL cells. UTI is bound to a specific surface receptor that is incompletely saturated. U937 cells and neutrophils did not show any specific binding to UTI, since UTI receptors on the cell surface of U937 cells and neutrophils were completely saturated with endogenous UTI. UTI forms yielded cross-linked 150- and 80-kDa ligand receptor complexes with cultured SMT-cc1 cells, suggesting molecular masses of 110 and 40 kDa for the UTI receptors. Purification of the two UTI receptor proteins by ligand affinity chromatography (ligand-blotting analysis) yielded two bands when analyzed by SDS-polyacrylamide gel electrophoresis, corresponding in electrophoretic mobility to those calculated by cross-linking analysis. The results reported here showing that some tumor cells carry UTI receptors are in line with a possible role of surface-bound UTI in modulating plasmin activity to the close environment of the cell surface and in processes like prevention of tumor cell invasion and metastasis.