Studies on rabbit natural and recombinant tissue factors: intracellular retention and regulation of surface expression in cultured cells

Tissue factor structure and function

Tissue factor (TF) is an integral membrane protein that is essential to life. It is a component of the factor VIIa-TF complex enzyme and plays a primary role in both normal hemostasis and thrombosis. With a vascular injury, TF becomes exposed to blood and binds plasma factor VIIa, and the resulting complex initiates a series of enzymatic reactions leading to clot formation and vascular sealing. Many cells, both healthy, and tumor cells, produce detectable amounts of TF, especially when they are stimulated by various agents. Despite the relative simplicity and small size of TF, there are numerous contradictory reports about the synthesis and presentation of TF on blood cells and circulation in normal blood either on microparticles or as a soluble protein. Another subject of controversy is related to the structure/function of TF. It has been almost commonly accepted that cell-surface-associated TF has low (if any) activity, that is, is "encrypted" and requires specific conditions/reagents to become active, that is, "decrypted." However there is a lack of agreement related to the mechanism and processes leading to alterations in TF function. In this paper TF structure, presentation, and function, and controversies concerning these features are discussed.

Posttranslational modifications of tissue factor

Tissue factor (TF), a membrane protein, is an initiator of blood coagulation in vivo. In this review we discuss how posttranslational modifications affect activity and other properties of TF. Glycosylation of the extracellular domain and the composition of carbohydrates at three glycosylation sites have an influence on TF activity in the extrinsic FXase by increasing the rate of FX proteolysis. No influence of TF glycosylation on the activity of the FVIIa/TF complex towards small synthetic substrates was observed, suggesting that glycosylation has no effect on TF interaction with FVIIa. There are no published data suggesting a direct influence of phosphorylation or palmitoylation in the cytoplasmic domain on TF procoagulant activity. There has been a debate in the recent literature related to the role and formation of the Cys¹⁸⁶-Cys²⁰⁹ disulfide bond. Published opinions from various laboratories range from this bond being essential for the expression of cell TF activity to having no role in it. Overall, it is clear that some modifications of TF have an effect on TF procoagulant activity, signaling functions and trafficking. The influences of other modifications are debatable.

Tumor cells expressing tissue factor influence the migration of smooth muscle cells in a catalytic activity-dependent way

The expression of tissue factor (TF) in tumors reportedly exacerbates the aggressiveness of several types of cancers. The shedding of TF-containing membrane particles is believed to influence the ability of tumors to expand and metastasize, and these microparticles may also be harmful in the onset of disseminated intravascular coagulation in specific cancers. Furthermore, the intracellular signaling that is elicited after the formation of the TF / coagulation factor VIIa complex at the cell membrane modulates the activity of adhesion molecules and mitogen-activated protein (MAP) kinases. To evaluate whether TF overexpression in tumor cells modulates its shedding and neighboring stromal cells by its catalytic or intracellular activity, TF-GFP (green fluorescent protein) and a tailless form (TFDeltaC-GFP) were stably expressed in the rat Morris hepatoma and human HT1080 fibrosarcoma cell lines. Both TF proteins were efficiently produced by tumor cells and functionally active, and their clotting activity could be blocked by the active site-inhibited factor VIIa (ASIS). TF-expressing tumorigenic cells produced a soluble factor that increased the migration of arterial smooth muscle cells in vitro. This effect was abrogated by ASIS and the PAR-1 receptor antagonist ATAP-2, showing that it is dependent on the proteolytic activity of the TF ligand factor VIIa and the thrombin-activated cell membrane receptor. We propose that TF-containing microparticles that are released in the culture medium by tumor cells influence the migratory behavior of neighboring stromal cells, thus aiding the cancer cell's tumorigenic potential.

Factor VIIa interaction with tissue factor and endothelial cell protein C receptor on cell surfaces

Factor VIIa (FVIIa) is the enzyme that triggers activation of the clotting cascade that eventually leads to fibrin deposition and platelet activation. Association of FVIIa with its cellular receptor, tissue factor (TF), which greatly increases FVIIa enzymatic activity, is essential for the effective initiation of the coagulation pathway. FVIIa also complexes with endothelial cell protein C receptor (EPCR), but this association does not increase the enzymatic activity of FVIIa. This article reviews current knowledge of FVIIa interaction with TF and EPCR on cell surfaces with a specific focus on how these interactions may contribute to FVIIa and TF clearance, thereby regulating TF-FVIIa activity.

Stimulation of P2 (P2X7) receptors in human dendritic cells induces the release of tissue factor-bearing microparticles

Receptors for extracellular nucleotides are the focus of increasing attention for their ability to cause release of plasma membrane vesicles (microparticles, MPs). Here, we show that monocyte-derived human dendritic cells (DCs) stimulated with a P2X7 receptor (P2X7R) agonist undergo a large release of MPs endowed with procoagulant activity. Functional and Western blot studies revealed that MPs contain the membrane-bound form of tissue factor (TF), a glycoprotein acting as essential cofactor of activated factor VII and triggering blood coagulation. Quiescent DCs express the membrane-bound (full length), as well as truncated alternatively spliced TF forms. DC reactivity to anti-TF Abs disappeared almost completely on stimulation with ATP or benzoyl ATP (BzATP), as shown by immunoblot and confocal microscopy analysis. Concurrently, TF reactivity and activity appeared in the vesicular fraction, indicating that MPs are important carriers for the dissemination of full-length TF form. Activity of MP-bound TF, comparable to that of relipidated recombinant TF, was dose dependently inhibited by the addition of a specific anti-human TF antibody. We infer that a large fraction of this protein, and its procoagulant potential, are "deliverable" after physiological or pathological stimuli. These findings might have implications for triggering and propagating coagulation in healthy and atherosclerotic vessels.

Cellular localization and trafficking of tissue factor

Tissue factor (TF) is the cellular receptor for clotting factor VIIa (FVIIa). The formation of TF-FVIIa complexes on cell surfaces triggers the activation of coagulation cascade and cell signaling. In the present study, we characterized the subcellular distribution of TF and its transport in fibroblasts by dual immunofluorescence confocal microscopy and biochemical methods. Our data show that a majority of TF resides in various intracellular compartments, predominantly in the Golgi. Tissue factor at the cell surface is localized in cholesterol-rich lipid rafts and extensively colocalized with caveolin-1. FVIIa binding to TF induces the internalization of TF. Of interest, we found that TF-FVIIa complex formation at the cell surface leads to TF mobilization from the Golgi with a resultant increase in TF expression at the cell surface. This process is dependent on FVIIa protease activity. Overall, the present data suggest a novel mechanism for TF expression at the cell surface by FVIIa. This mechanism could play an important role in hemostasis in response to vascular injury by increasing TF activity where and when it is needed.