EFFECT OF THROMBIN ON RELEASE OF PLASMINOGEN-ACTIVATOR INHIBITOR-1 FROM CULTURED PRIMATE ARTERIAL SMOOTH-MUSCLE CELLS

Plasminogen activator inhibitor-1 (PAI-1) is the major inhibitor for p lasmin formation promoted by tissue and urokinase plasminogen activato rs. The present study demonstrates that thrombin increase PAI-1 antige n, biological activity, and gene expression in cultured baboon aortic smooth muscle cells (BASMC). Thrombin elevates PAI-1 antigen in condit ioned medium of BASMC within 10 min of the treatment, with the peak in crease after 30 min of the treatment. Overexpression of PAI-1 gene was detected in the cultures exposed to thrombin for at least 60 min. PAI activity in conditioned medium increased in the cultures treated with thrombin for at least 4 h. The thrombin-induced early increase of PAI -1 antigen (up to 30 min of the stimulation) was blocked by hirudin (a specific inhibitor of thrombin), mimicked by trypsin and not suppress ed by cycloheximide (a protein synthesis inhibitor). The majority of m etabolically labeled PAI-1 associated with BASMC was present in extrac ellular matrix. The level of extracellular matrix-associated PAI-1 was reduced 40% by 30 min of thrombin treatment. Our results suggest that thrombin not only increases PAI-1 transcription but also proteolytica lly cleaves PAI-1 from the extracellular matrix of vascular SMC. PAI-1 released by thrombin from the extracellular matrix may not alter PAI activity in extracellular fluid but may reduce the storage of PAI-1 in the extracellular matrix of vascular smooth muscle cells.