Synthesis and secretion of the fibrinolytic components, including alpha 2-antiplasmin, by a human hepatoma cell line.

The capacity of the human hepatoma cell line Hep G2 to synthesize and secrete components of the fibrinolytic system was examined. Although fibrinogen, plasminogen, and alpha 2-antiplasmin were present in culture supernatants, histidine-rich glycoprotein was not detected. Albumin was monitored for comparative purposes and was also secreted. Intracellular levels of the fibrinolytic proteins were not detected, suggesting that once synthesized, these proteins were rapidly secreted by the cell. The fibrinogen, plasminogen, alpha 2-antiplasmin, and albumin secreted by the cells were immunochemically identical to their plasma counterparts. The concentration of these four molecules increased exponentially with time in the culture medium when normalized to total cellular protein. The kinetics of their secretion were similar, but the amounts of each protein differed. On day 8 the culture medium contained 29.6, 0.64, 0.39, and 0.59 micrograms/ml albumin, fibrinogen, plasminogen, and alpha 2-antiplasmin, respectively, whereas the time required for doubling the concentrations of the proteins in the medium ranged from 2.20 to 2.49 days. A detailed characterization of alpha 2-antiplasmin demonstrated that this inhibitor was synthesized by the cells. The molecular weight of intrinsically labeled alpha 2-antiplasmin was 69,000. The secreted inhibitor was biologically active, since it could inhibit plasmin cleavage of a chromogenic substrate, and this inhibition was neutralized by monospecific antibodies to alpha 2-antiplasmin. In addition, intrinsically labeled alpha 2-antiplasmin and the plasma molecule behaved in an identical manner with respect to their capacity to form a complex with plasmin. These studies suggest that the Hep G2 cell line may provide a model for assessing the regulation of synthesis and secretion of components of the human fibrinolytic system.