The nature of lipopolysaccharide-stimulated monocyte tissue factor activity

There are several contradictory hypotheses that attempt to explain changes in cell tissue factor (TF) activity upon treatment with various agents ('encryption-decryption'). We evaluated the influence of lipopolysaccharide stimulation on expression of TF antigen and activity on/in THP-1 human leukemia monocytic cells. Prior to stimulation, there were 240 ± 60 TF molecules/cell on the cell surface and 510 ± 180 molecules/cell in lysates (n = 8). Upon stimulation, TF antigen increased 10-fold on the cell surface and 16.5-fold in lysates. Coincidently, the intact cell factor (F)Xa generation by TF/FVIIa increased 11-fold. Correspondingly, TF-induced clotting activity increased 35.7 ± 4.9-fold. The KM of the TF/FVIIa complex formed on the THP-1 surface and cell lysates for FX was 0.73 ± 0.07 and 0.41 ± 0.02 μmol/l and the kcat 59.4 ± 1.8 and 44.6 ± 0.1 s, respectively. For isolated and relipidated THP-1 cell TF, the efficiency of FXase was lower (KM = 1.54 μmol/l and kcat = 12.0 s). A similar comparison of isolated/relipidated vs. cell-surface TF in the synthetic coagulation proteome and corn trypsin inhibitor blood showed that the cell-surface TF-induced thrombin generation is more than 100-fold more efficient than that induced by purified/relipidated TF. These data indicate that the increase in monocytic cell TF activity upon stimulation is primarily related to an increased expression of TF protein, and that significantly higher specific activity of TF presented on cells than that of purified and relipidated protein suggests the presence of the cell membrane components which substantially enhance TF function.