TISSUE DISTRIBUTION OF SOLUBLE AND RECEPTOR-BOUND UROKINASE IN HUMAN BREAST-CANCER USING A PANEL OF MONOCLONAL-ANTIBODIES

Current evidence regarding the regulation of urokinase-dependent extra cellular proteolysis indicates that plasminogen activation is a surfac e-associated process. We have compared the histological localization o f urokinase plasminogen activator (uPA) and urokinase plasminogen acti vator receptor (uPAR) in breast cancer sections using a panel of monoc lonal antibodies. First, the ability of six different anti-uPA monoclo nal antibodies to recognize pro-uPA, uPA, and in vitro-formed complexe s of uPA with either soluble uPAR or with plasminogen activator inhibi tor type 1 was compared. Then the reactivity of the anti-uPAR antibodi es was tested, and the occurrence of an uPA receptor of about M(r) 55, 000 in samples from breast carcinoma was assessed by immunoprecipitati ng the uPA receptor from an in vitro I-125-labeled tumor extract. Immu nocytochemical data from adjacent sections of 10 tumor specimens showe d that antibodies recognizing free and bound uPA mostly stain the cyto plasm and the membrane of epithelial tumor cells in confined areas of the tumor and some fibroblast-like stromal cells. Acid pretreatment of tumor sections, which removes receptor-bound uPA, causes a strong red uction of the immunocytochemical reactivity of epithelial tumor cells, whereas staining of fibroblast-like cells is not considerably affecte d. Consistent with these results, epithelial tumor cells were mostly u nreactive to anti-uPAR antibodies unless pretreated with acidic buffer , whereas fibroblastlike stromal cells showed a faint but acid-resista nt staining with all anti-uPARs. In conclusion, these results show tha t occupied uPA receptors are definitely present on the membrane of epi thelial tumor cells and suggest the occurrence of uPA-uPAR-dependent p roteolytic activity on the surface of breast cancer cells.