Role of the Urokinase Receptor in Facilitating Extracellular Matrix Invasion by Cultured Colon Cancer1

There is increasing evidence that urokinase secreted by tumor cells can be bound to a cell surface receptor retaining its full potential to activate plasminogen and subsequently cleave basement membrane con stituents. This study was undertaken to discriminate between soluble and cell surface bound urokinase as a potential mediator of in vitro invasion by cultured colon cancer. Extracellular matrix invasion by a colon cancer cell line GEO, characterized as being a poor secretor of urokinase and having few receptors « I(IJreceptors/cell) was not augmented when these cells were made to secrete up to 8 times as much urokinase, in response to an exogenous urokinase gene driven by the Rous sarcoma virus long terminal repeat promoter. The majority of the plasminogen activator (>95%) appeared in the culture medium, this reflecting the low numbers of binding sites displayed by CEO cells. In contrast, the cell line l K T 116 equipped with 10 times as many binding sites, (>105/cell), the majority of which are occupied with endogenous ligand, was an efficient invader of the extracellular matrix. Inhibition of urokinase binding to the cell surface receptors using an antibody to the A chain of the plasminogen activator reduced invasion by 65%. The cell line RKO is equipped with 3 x 10s receptors/cell, 15% of which are tagged with endogenous uroki nase. Pretreatment of these cells with a concentration range of urokinase known to result in the majority of these binding sites being charged with the plasminogen activator led to a dose dependent increase in extracellular matrix invasion. Together, these data suggest that for cultured colon cancer, at least, invasion is a function of the amount of cell surface receptor bound urokinase.