Binding of urokinase to its receptor promotes migration and invasion of human melanoma cells in vitro.

Previously, we reported that urokinase-type plasminogen activator (uPA) plays a pivotal role in extracellular matrix dissolution by malignant melanoma cells. Here, we demonstrate that a highly metastatic melanoma cell line (M24met) that secretes uPA expresses high levels of the uPA receptor (uPAR), 2.4 x 10(6) binding sites/cell with a KD of 5.67 x 10(-10) M. The receptor was identified as a 55,000-60,000 kDa cell surface protein. Although M24met cells secrete uPA, they are unable to efficiently utilize this enzyme for invasion, unless it is bound to its receptor. This contention is based on the finding that an antibody against uPAR (monoclonal antibody 3936) inhibited invasion of M24met cells through a reconstituted basement membrane (Matrigel) up to 33%, while a reduction of uPA catalytic activity by its plasminogen activator inhibitor-2 resulted in 46% inhibition of invasion. Furthermore, uPAR is involved in signal transduction events in M24met cells, since both uPA and its amino-terminal fragment stimulated the migration of melanoma cells toward Matrigel, resulting in maximal increases of 32 and 72%, respectively. Our results indicate that both uPA and uPAR are involved in melanoma metastasis and that uPAR contributes to at least two important steps in this process, matrix dissolution and migration.