Expression of urokinase plasminogen activator and the urokinase plasminogen activator receptor in myeloma cells

Binding of urokinase (uPA) to its receptor (uPAR; CD87) focuses proteolytic activity on the cell surface and this system is of importance in malignant matrix degradation and tumour invasion. By immunocytochemistry and flow cy tometry, we found that primary myeloma cells and myeloma cell lines express ed uPA and uPAR. Soluble uPA was present in cell line supernatants and lysa tes in low concentrations. In cell lines, uPA and uPAR were located both on the cell surface and intracellularly, but the expression of both proteins was low. Higher levels of uPAR was detected on the cell surface of primary myeloma cells. When primary myeloma cells were gated by CD45 expression, st ronger expression was found on immature CD45(+) cells than on mature CD45(- /dim) cells. Finally, both myeloma cell lines and primary cells were able t o cleave a uPA-specific substrate showing that the uPA system is functional ly active. We conclude that myeloma cells are able to produce uPA and uPAR. This opens up a possible role of the uPA system in myeloma cell invasion a nd in the proteolytic digestion of bone matrix.