EFFECT OF TRANSFORMING GROWTH FACTORβ(TGFβ) ON THE FIBRINOLYTIC SYSTEM OF CULTURED BOVINE AORTIC ENDOTHELIAL CELLS (BAEs)

TGFβ influences cell growth and differentiation in a variety of normal and transformed cells and was recently shown to have growth inhibitory activity toward cultured endothelium. The fibrinolytic activity of endothelial cells changes dramatically with their growth state. Experiments were thus performed to determine the effect of TGFβ on the fibrinolytic system of these cells. Confluent BAEs were incubated with increasing concentrations of purified human or porcine TGFβ for various times in the presence or absence of serum. Conditioned medium (CM) and extracellular matrix (ECM) were prepared and assayed for tissue-type plasminogen activator (tPA), urokinase-type PA (uPA) and PA inhibitor-1 (PAI-1) by immunological methods and by fibrin and reverse fibrin autography. TGFβ induced a dose and time dependent increase in PAI-1 antigen and activity. The half-maximal effect was observed at 0.5 ng/ml while the maximal effect was at 2-5 ng/ml and resulted in 100-fold increase of PAI-1 in CM and at least a 20-fold increase in ECM. ³⁵S-methionine labeling experiments indicated that these effects (1) could be detected in CM within 3-4 hrs, (2) resulted from an increased rate of synthesis of PAI-1, (3) were also observed following transient exposure (1-2 h) of the cells to TGFβ, and (4) were relatively specific for PAI-1. TGFβ treated cells had normal amounts of tPA activity but greatly reduced levels of uPA activity. Human serum had a synergistic effect on PAI-1 production by these cells, but neither epidermal growth factor nor platelet derived growth factor altered their response to TGFβ. TGFβ also stimulated PAI-1 production by cultured human umbilical vein endothelial cells. The release of TGFβ by platelets may thus suppress the fibrinolytic system of the vascular wall.