IMMUNOELECTRON MICROSCOPY OF THE RECEPTOR FOR UROKINASE PLASMINOGEN-ACTIVATOR AND CATHEPSIN-D IN THE HUMAN BREAST-CANCER CELL-LINE MDA-MB-231

Receptors for urokinase-type plasminogen activator (uPAR) are present on the surface of many cell types and appear to be the key determinant controlling extracellular proteolysis catalyzed by the urokinase-type plasminogen activator (uPA). Receptor-bound uPA may be inhibited by t he specific inhibitors PAI-1 and PAI-2, and the complex thus formed ma y subsequently be internalized and degraded in lysosomes. Biochemical evidence has recently indicated that also uPAR is internalized with th e uPA/uPAI complex. We report here the subcellular localization of uPA R and cathepsin D in the MDA-MB-231 human breast cancer cell line stud ied by immune-electron microscopy of ultrathin cryosections using sing le or double immunostaining techniques. Cell surface uPAR was preferen tially localized at cell-cell junctions; cytoplasmic uPAR was inside l arge vesicles of different morphology and in flat Golgi saccules. A nu mber of vesicles also contained cathepsin D. The uPAR was exclusively membrane-bound at the cell surface and in cytoplasmic vesicles without cathepsin D. In lysosomal vesicles with both cathepsin D and u-PAR, u PAR was probably degraded as it was observed in the luminal contents.