Cytotoxicity of bacterial proteases in various tumor cells mediated through alpha 2-macroglobulin receptor

The binding and cytotoxicity of a complex of fluorescein isothiocyanate-labeled 56K protease and alpha 2-macroglobulin (alpha 2M) were determined by using various human and rodent tumor cell lines. The binding was higher at 37 degrees C than at 4 degrees C; a rapid and progressive uptake that was time dependent was noted at 37 degrees C, whereas no uptake was observed at 4 degrees C, which indicated temperature-dependent internalization. The binding was highest in the fibroblastic and adenocarcinoma cells, and lowest in squamous and epidermoid cells. The Scatchard plots for the binding isotherms were linear, with an apparent Kassoc 1.17 to 2.99 x 10(-8) M for those cells with high alpha 2M receptor. The number of binding sites (alpha 2M receptor) per cell was 1.3 to 4.75 x 10(6). Values for squamous/epidermoid cells were much lower or undetectable. Fluorescent antibody staining indicated that MCF-7 and other cells with alpha 2M receptor internalized the protease-alpha 2M complex, whereas B-16 melanoma, which has little alpha 2M receptor on the cell surface, did not. Furthermore, when the cytotoxicity of this complex was compared with that of different cell lines, the cells with high rates of uptake of the complex required only a low concentration of the protease and vice versa. These results suggest a possible mechanism of cytotoxic action of protease: alpha 2M receptor-mediated endocytosis of the complex followed by destruction of cellular integrity after regeneration of proteolytic activity. Thus, cells with more alpha 2M receptor require only a low dose for cytotoxic action when compared with cells with little alpha 2M receptor.