THROMBIN STIMULATES EXPRESSION OF TISSUE-TYPE PLASMINOGEN-ACTIVATOR AND PLASMINOGEN-ACTIVATOR INHIBITOR TYPE-1 IN CULTURED HUMAN VASCULAR SMOOTH-MUSCLE CELLS

The effect of thrombin on the fibrinolytic potential of human vascular smooth muscle cells (SMC) in culture was studied. SMC of different or igin responded to thrombin treatment with a dose and time dependent in crease in tissue-type plasminogen activator (t-PA) and plasminogen act ivator inhibitor type-1 (PAI-1) levels in both cell lysates and condit ioned media with maximum effects achieved at 10-20 IU/ml thrombin. PAI -1 antigen levels also increased in the extracellular matrix of thromb in treated SMC. PAI-2 levels in cell lysates of such SMC were not affe cted by thrombin. The effect was restricted to active thrombin, since DFP-thrombin and thrombin treated with hirudin showed no increasing ef fect on t-PA and PAI-1 levels in SMC. Enzymatically active thrombin al so caused a four-fold increase in specific PAI-1 mRNA and a three-fold increase in t-PA mRNA. Furthermore we demonstrated the presence of hi gh and low affinity binding sites for thrombin on the surface of SMC w ith a K(D) = 4.3 x 10(-10) M and 9.0 x 10(4) sites per cell and a KD = 0.6 x 10(-8) M and 5.8 x 10(5) sites per cell respectively. Thrombin could come in contact with SMC in case of vascular injury or following gap formation between endothelial cells. Our data support the idea th at besides its known proliferative effect for SMC, thrombin could also modulate their fibrinolytic system.