Mechanism of tissue factor activation on cells

The activation of tissue factor (TF) procoagulant activity (PCA) on the surface of cells may be the first step in the extrinsic pathway of blood coagulation. The latent TF PCA on intact cells is expressed following mechanical disruption or exposure to ionomycin, a Ca²⁺ ionophore. Within seconds of ionophore addition, an increase in TF PCA of more than 100-fold is observed. The ionophore effect is blocked by pretreating the cells with calmodulin inhibitors. Complexes of TF and activated factor VII (FVIIa) form on both untreated and ionophore-treated cells. However, pseudosubstrate inhibitors bind only to TF-FVIIa on ionophore-treated cells. When proteins on unperturbed cells are crosslinked with 3-3'- dithiobis(sulphosuccinimidylpropionate), crosslinked homodimeric TF is produced. This TF crosslinking is prevented by first treating the cells with ionomycin. Thus, there is an apparent change in TF quaternary structure that is coincident with the ionophore-induced change in TF PCA. These results suggest a simple mechanism for the activation of TF PCA on cells which is triggered by Ca²⁺ influx into the cytosol. A Ca²⁺-binding protein, possibly calmodulin, appears to be an essential link in the signal transduction pathway going from increased cytosolic Ca²⁺ to increased expression of TF PCA on the cell surface. This activation of TF PCA may result from exposure of an essential macromolecular-substrate-binding site on the TF-FVIIa complex that is the direct result of a change in TF quaternary structure, i.e. the conversion of cryptic TF dimers to procoagulant TF monomers.