Studies on plasminogen activator and other proteases in subcultured human vascular cells

Plasminogen activator activity (PAA) of human vascular cells in culture was quantitated in an assay system in which the conversion of purified plasminogen to plasmin was measured by activity against a soluble low-molecular-weight plasmin substrate. Reliable detection of PAA required cell lysis and use of membrane-disrupting detergents. After such treatment, PAA was found only in the 100,000 g subcellular fractions of human aortic smooth muscle and mixed populations of rabbit aortic cells. No corresponding activity was found in any fraction from human umbilical vein endothelial cells. In contrast, PAA was detected in significant amounts in intact mouse embryo fibroblasts (3T3 cells) used as controls. Very low levels of enzymatic activity against a spectrum of substrates preferentially amidolyzed by serine proteases were also demonstrable in human aortic smooth muscle or 3T3 fibroblast subcultures. Neither whole cell homogenates nor subcellular fractions demonstrated the previously described acid-labile inhibitor of PAA when assayed in this system. The above findings suggest that significant differences in PAA exist between various cell types and vascular segments. In addition, the subcellular localization and quantitatively low levels of PAA and other serine proteases found in human arterial and venous cells may reflect the presence of membrane-associated enzymes whose biological role is restricted to local homeostasis.