UROKINASE-TYPE PLASMINOGEN ACTIVATION IN 3 HUMAN BREAST-CANCER CELL-LINES CORRELATES WITH THEIR IN-VITRO INVASIVENESS

In order to invade and spread cancer cells must degrade extracellular matrix proteins. This degradation is catalysed by the concerted action of several enzymes, including the serine protease plasmin. Several ex perimental studies have shown that inhibition of plasmin formation red uces cancer cell invasion and metastasis, indicating a critical role o f this proteolytic pathway in these processes. In order to further stu dy the role of plasmin in cancer progression, we have characterized ur okinase-type plasminogen activator (uPA) mediated plasmin formation in three human breast cancer cell lines. Using monoclonal antibodies aga inst uPA and its receptor uPAR, we have investigated the contribution of uPA and uPAR to invasive capacity in an in vitro invasion assay. MD A-MB-231 BAG cells were found to express high protein levels of uPA, u PAR and PAI-1. MDA-MB-435 BAG cells produced low amounts of uPA, PAI-1 and moderate amounts of uPAR, whereas MCF-7 BAG cells showed low leve ls of uPA, uPAR and PAI-1 protein. In a plasmin generation assay MDA-M B-231 BAG cells were highly active in mediating plasmin formation, whi ch could be abolished by adding either an anticatalytic monoclonal ant ibody tn:nPA (clone 5) or an anti-uPAR monoclonal antibody (clone R3), which blocks binding of uPA to uPAR. The two other cell lines lacked the capacity to mediate plasmin formation. In the Matrigel invasion as say the cells showed activity in this order: MCF-7 BAG < MDA-MB-435 BA G < MDA-MB-231 BAG. Testing MDA-MB-231 BAG cells in the Matrigel invas ion assay revealed that invasion could be inhibited in a dose-dependen t manner either by the clone 5 uPA antibody or by the clone R3 uPAR an tibody, suggesting that the cell surface uPA system is actively involv ed in this invasive process. It is concluded that these three cell lin es constitute a valuable model system for in vitro studies of the role of cell surface uPA in cancer cell invasion and has application in th e search for novel compounds which inhibit mechanisms involved in uPA- mediated plasmin generation on cancer cells.