Visualization of the plasmin receptor on sections of human mammary carcinoma cells

In previous studies we observed the presence of an antigen reacting with anti-plasminogen serum on tumor cells found in sections of human colorectal and mammary carcinomas, using immunofluorescence. This antigen could be plasminogen, or plasmin or both. These results led us to surmise that carcinoma cells, and not normal epithelial cells, had a receptor for plasmin and plasminogen. We demonstrated the existence of such a receptor on cells of established tumor cell lines of colonic and mammary origin. We observed that binding of plasmin(ogen) to its receptor was inhibited by lysin and its analogs. We undertook to characterize the plasmin receptor on sections of human mammary carcinomas (18 series of sections, originating from 15 infiltrating ductal carcinomas). In our immunofluorescence studies, we observed that the fluorescence induced by anti-plasminogen serum was suppressed by pre-treatment of the sections with tranexamic acid, a lysin analog. Furthermore, when parallel sections were incubated with tranexamic acid, then with solutions of plasminogen, tumor cells were made fluorescent again. Another important finding was the strong increase of labelling observed when sections were incubated with plasminogen or plasmin before reaction with anti-plasminogen serum. This labelling disappeared when tranexamic was added to plasminogen. As a whole, these results confirm the existence of plasmin receptors on tumor cells in mammary carcinoma sections. They also show that these receptors are not saturated in vivo and may bind, in vitro, additional amounts of ligand.