Release of active tissue factor by human arterial smooth muscle cells

Tissue factor (TF), the initiator of coagulation, is thought to function pr edominantly at the cell surface. Recent data have suggested that active TF is present extracellularly in atherosclerotic plaques, the arterial wall, a nd the blood. This study was conducted to determine whether smooth muscle c ells (SMCs), a major source of arterial TF, could generate extracellular TF . Active TF accumulated in the medium of cultured human SMCs, representing approximate to 10% of that measured in the underlying cells at 24 hours, pl atelet-derived growth factor, phorbol ester, and tumor necrosis factor-or c aused approximate to 3-fold increases in TF activity in the medium. Release of TF into the medium was dependent on the presence of the TF transmembran e domain but not the cytoplasmic domain. Antibodies to TF precipitated most of the activity from the culture medium, whereas antibodies to the beta(1) -integrin subunit precipitated approximate to 33% Of the activity. Treatmen t with detergent or phosphatidylserine:phosphatidylcholine did not increase activity, suggesting that all TF released by SMCs was in the appropriate l ipid milieu and not encrypted. Western blotting showed that the medium cont ained full-length TF protein. Fluorescent cytometry showed that extracellul ar TF was present largely in particles less than or equal to 200 nm, which had a density of 1.10 g/mL. We hypothesize that active extracellular TF fou nd in the injured arterial wall and atherosclerotic plaques derives, in par t, from SMC microparticles.