Tissue factor (TF) is an essential enzyme activator that forms a catalytic complex with FVII(a) and initiates coagulation by activating FIX and FX, ultimately resulting in thrombin formation. TF is found in adventitia of blood vessels and the lipid core of atherosclerotic plaques. In unstable coronary syndromes, plaque rupture initiates coagulation by exposing TF to blood. Biologically active TF has been detected in vessel walls and circulating blood. Elevated intravascular TF has been reported in diverse pro-thrombotic syndromes such as myocardial infarction, sepsis, anti-phospholipid syndrome and sickle-cell disease. It is unclear how TF circulates, although it may be present in pro-coagulant microparticles. We now report identification of a form of human TF generated by alternative splicing. Our studies indicate that alternatively spliced human tissue factor (asHTF) contains most of the extracellular domain of TF but lacks a transmembrane domain and terminates with a unique peptide sequence. asHTF is soluble, circulates in blood, exhibits pro-coagulant activity when exposed to phospholipids, and is incorporated into thrombi. We propose that binding of asHTF to the edge of thrombi contributes to thrombus growth by creating a surface that both initiates and propagates coagulation.

Abstract-Thrombosis occurs in a dynamic rheological field that constantly changes as the thrombus grows to occlusive dimensions. In the initiation of thrombosis, flow conditions near the vessel wall regulate how quickly reactive components are delivered to the injured site and how rapidly the reaction products are disseminated. Whereas the delivery and removal of soluble coagulation factors to the vessel is thought to occur via classic convection-diffusion phenomena, the movement of cells and platelets to the injured wall is strongly augmented by flow-dependent cell-cell collisions that enhance their ability to interact with the wall. In addition, increased shear conditions have been shown to activate platelets, alter the cellular localization of proteins such as tissue factor (TF) and TF pathway inhibitor, and regulate gene production. In the absence of high shearing forces, red cells, leukocytes, and platelets can form stable aggregates with each other or cells lining the vessel wall, which, in addition to altering the biochemical makeup of the aggregate or vessel wall, effectively increases the local blood viscosity. Thus, hemodynamic forces not only regulate the predilection of specific anatomic sites to thrombosis, but they strongly influence the biochemical makeup of thrombi and the reaction pathways involved in thrombus formation. T hrombosis is thought to begin with an event such as plaque rupture, vessel damage, or dysfunctioning endothelium, resulting in the exposure of active tissue factor (TF) on the vessel wall. 1 The exposed TF binds circulating factor VII/VIIa (fVII/fVIIa) with high-affinity (Kd Ͻ10 pM), forming a reactive vessel surface that proteolytically cleaves fIX and fX. The rate of this surface-bound reaction depends not only on biochemical factors (number and surface density of TF:VIIa complexes, intrinsic kinetic activity, and local phospholipid composition 2 ) but on the rate at which the substrates fIX and fX are transported by the flowing medium to the reactive surface 3 and the rate at which product is removed. 4 Moreover, the local accumulation of reaction product may be critical in overpowering endogenous inhibitors and successfully initiating coagulation.Although convective flow forces dominate the axial movement of blood components (coagulation factors, platelets, red cells, leukocytes, and inhibitors) throughout most of the body, in the few microns approaching the vessel wall where thrombosis is thought to initiate, the frictional drag of the vessel wall causes the axial flow velocity to slow to 0, and the convective and diffusive motion of blood components becomes comparable. 5 In this initial coagulation reaction, blood flowing parallel to the wall convectively transports coagulation factors (fIX and fX) from upstream to a region close to the reactive site, and Brownian motion mediates the radial movement toward the reactive wall. In the case of platelets adhering to the wall, a similarly defined adhesion rate is often inferred. 6 The bound proteins fIX and fX become proteolyt...

No abstract

Background-Several studies suggest a role for an increased circulating pool of tissue factor (TF) in atherothrombotic diseases. Furthermore, certain cardiovascular risk factors, such as diabetes, hyperlipemia, and smoking, are associated with a higher incidence of thrombotic complications. We hypothesized that the observed increased blood thrombogenicity (BT) observed in patients with type 2 diabetes mellitus may be mediated via an increased circulating tissue factor activity. We have extended our study to smokers and hyperlipidemic subjects. Methods and Results-Poorly controlled patients with type 2 diabetes mellitus (nϭ36), smokers (nϭ10), and untreated hyperlipidemic subjects (nϭ10) were studied. Circulating TF was immunocaptured from plasma, relipidated, and quantified by factor Xa (FXa) generation in the presence of factor VIIa. BT was assessed as thrombus formation on the Badimon perfusion chamber.