Secretion of plasminogen activator by the human macrophage‐like cell line, GCT: separation from colony‐stimulating and erythropoiesis‐enhancing activities

Summary. The human macrophage‐like cell line, GCT, elaborates monokines such as colony‐stimulating activity (CSA) and erythropoiesis‐enhancing activity (EEA) which stimulate the growth of primitive blood progenitors in culture. These cells also secrete a fibrinolysis activator (FA), which can be identified if cells are cultured in serum‐free medium. FA was found to have a similar molecular weight to CSA and EEA by gel filtration but could be separated from them by ion exchange chromatography. Subcellular fractionation of GCT cells indicated that fibrinolytic activity was present in the cell membranes and cytosol, whereas CSA and EEA were present only in the cytosol. FA resembled urokinase in molecular weight and its strict requirement for plasminogen as a substrate. Double immunodiffusion of GCT activator and urokinase against anti‐urokinase antiserum resulted in a line of identity, and incubation of activator with antiserum resulted in loss of its fibrinolytic activity. Thus, GCT activator was similar, if not identical to the plasminogen activator, urokinase.