Urokinase-Dependent Proteolysis in Cultured Colon Cancer Is Directed by Its Receptor

High mortality rates associated with many malignancies are, invariably, a consequence of the spread of the disease to distant sites. Since, at the present time, there are no effective ways of combating tumor cells that have relocated to secondary sites, the prognosis of those patients afflicted with metastatic lesions is bleak. Because of this, many investigators have argued that therapeutic advances against the disease will depend on a greater knowledge of the mechanisms that underscore tumor dissemination. Although the process of tumor metastases is highly complex, and poorly understood at the present time, it is widely accepted that breach of the basement membrane represents one of the first steps in this direction. ' This structure is made up primarily of type IV collagen and the glycoprotein laminin. Acquisition of an invasive phenotype is thought to depend, in part, on the ability of the tumor cells to elaborate one or more hydrolases that can solubilize these basement membrane components, thereby removing an impediment to their movement. ' In this regard, there is a substantial body of evidence implicating the plasminogen activator (PA) urokinase (UK) in tumor invasion and metastases. UK has been reported to be elevated in several malignancies, including colon cancer. Moreover, the presence of the PA is most dramatic at the leading edge of these tumors, suggesting a role in the invasive process. Perhaps the most compelling evidence implicating UK in tumor dissemination are the findings that UK antibodies inhibit lung colonization by melanoma and HEp3 tumor cells. UK probably promotes tumor cell invasion via its ability to convert the inert zymogen plasminogen into the widely acting serine protease plasmin. The latter enzyme can degrade laminin and type IV collagen directly or indirectly via the activation of stromelysin and type IV collagenase, respectively. Recent studies have indicated that tumor cells can bind UK in a saturable manner, this being ascribed to the presence of specific receptors on the surface of the cells. The receptor has been purified and cloned. Expression of the binding site in receptor-negative cells resulted in the specific and saturable binding of the PA, the latter retaining its activity, as is evident from the conversion of plasminogen into plasmin. The role of these binding sites in facilitating the actions of UK is unclear at the present time. The display of these receptors on tumor cells could, arguably, promote unregulated proteolysis by any one of the following mechanisms: Tumor cells equipped with these binding sites could concentrate the PA at the cell surface, this leading to a build-up of plasmin in the immediate vicinity of the cell. Alternatively, receptor binding of UK could alter the kinetics of plasminogen activation in favor of plasmin production. Thirdly, the binding site could sequester the UK in a location that is inaccessible to inhibitors, such as plasminogen activator inhibitor type 1 (PAI-1), which might otherwise neutralize the PA. These proposals are, of course, not mutually exclusive. We have chosen to concentrate our efforts on trying to understand the proteolytic mechanisms responsible for basement membrane laminin solubilization, arguing that the breach of this barrier is one of the first steps of tumor cell invasion and, ultimately, metastases. Toward this end, we have made use of a bank of benign and malignant, human colonic cell lines that have been extensively characterized by this laboratory and others (Table 1).