TISSUE FACTOR EXPRESSION IN HUMAN ARTERIAL SMOOTH-MUSCLE CELLS - TF IS PRESENT IN 3 CELLULAR POOLS AFTER GROWTH-FACTOR STIMULATION

Tissue factor (TF) is a transmembrane glycoprotein that initiates the coagulation cascade. Because of the potential role of TF in mediating arterial thrombosis, we have examined its expression in human aortic a nd coronary artery smooth muscle cells (SMC), TF mRNA and protein were induced in SMC by a variety of growth agonists. Exposure to PDGF AA o r BE for 30 min provided all of the necessary signals for induction of TF mRNA and protein. This result was consistent with nuclear runoff a nalyses, demonstrating that PDGF-induced TF transcription occurred wit hin 30 min, A newly developed assay involving binding of digoxigenin-l abeled FVIIa (DigVIIa) and digoxigenin-labeled Factor X (DigX) was use d to localize cellular TF, By light and confocal microscopy, prominent TF staining was seen in the perinuclear cytoplasm beginning 2 h after agonist treatment and persisting for 10-12 h. Surface TF activity, me asured on SMC monolayers under flow conditions, increased transiently, peaking 4-6 h after agonist stimulation and returning to baseline wit hin 16 h, Peak surface TF activity was only similar to 20% of total TF activity measured in cell lysates, Surface TF-blocking experiments de monstrated that the remaining TF was found as encrypted surface TF, an d also in an intracellular pool. The relatively short-lived surface ex pression of TF may be critical for limiting the thrombotic potential o f intact SMC exposed to growth factor stimulation, In contrast, the en crypted surface and intracellular pools may provide a rich source of T F under conditions associated with SMC damage, such as during atherosc lerotic plaque rupture or balloon arterial injury.