Regulation of tissue factor coagulant activity on cell surfaces

Abstract

Tissue factor (TF) is a transmembrane glycoprotein and an essential component of the factor VIIa-TF enzymatic complex that triggers activation of the coagulation cascade. Formation of TF-FVIIa complexes on cell surfaces not only trigger the coagulation cascade but also transduce cell signaling via activation of protease-activated receptors. Tissue factor is expressed constitutively on cell surfaces of a variety of extravascular cell types, including fibroblasts and pericytes in and surrounding blood vessel walls and epithelial cells, but is generally absent on cells that come into contact with blood directly. However, TF expression could be induced in some blood cells, such as monocytes and endothelial cells, following an injury or pathological stimuli. Tissue factor is essential for hemostasis, but aberrant expression of TF leads to thrombosis. Therefore, a proper regulation of TF activity is critical for the maintenance of hemostatic balance and health in general. TF-FVIIa coagulant activity at the cell surface is influenced not only by TF protein expression levels but also independently by a variety of mechanisms, including alterations in membrane phospholipid composition and cholesterol content, thiol-dependent modifications of TF allosteric disulfide bonds, and other post-translational modifications of TF. In this article, we critically review the key literature on mechanisms by which TF coagulant activity is regulated at the cell surface in the absence of changes in TF protein levels with specific emphasis on recently published data and provide the authors' perspective on the subject.

Keywords: cholesterol; cryptic; decryption; encryption; factor VIIa; glycosylation; phospholipids; protein disulfide isomerase; tissue factor.

Figure 1

A schematic representation of how exposure of phosphatidylserine (PS) to the outer leaflet of plasma membrane may increase TF coagulant activity at the cell surface. In cells expressing TF, FVIIa binds TF, but majority of TF-FVIIa complexes on the cell surface unable to interact with substrate factor X, therefore they remain coagulant inactive. Upon a stimulus, PS (depicted in red color circles) from the inner leaflet of the plasma membrane would translocate to the outer leaflet. TF or TF-FVIIa complexes directly interact with PS head groups and then undergo conformational changes to recognize the substrate factor X. Factor X binding to PS via its Gla domain may also play a role for efficient activation of factor X by TF-FVIIa complexes on the cell surface.

Figure 2

Proposed mechanisms of PDI-mediated regulation of TF coagulant activity at the cell surface. As per one of the proposed models, Cys¹⁸⁶-Cys²⁰⁹ disulfide bond of TF is critical for its coagulant activity and post-translational modifications of this allosteric disuflide bond such as reduction, S-nitrosylation and glutathionation keep TF in a cryptic state. PDI can regulate the coagulant activity of TF through formation/breaking or reshuffling of this disulfide bond by its oxido-reductase or isomerase activity. PDI could also regulate TF coagulant activity through S-nitrosylation and/or glutathionation of thiols in TF. In a second model, PDI through its oxido-reductase activity modulates PS dynamics at the cell surface. PDI reductase activity helps in maintaining a low exposure of PS at the cell surface by affecting both flippase and floppase activities. Inhibition of PDI at the cell surface increases PS (shown as red color circles) exposure, which facilitates the conversion of cryptic TF to the coagulant active form. Both PDI-mediated disulfide bond switch and PDI-induced changes in the lipid environment may occur simultaneously and their net effect may dictate TF activity status at the cell surface.

Figure 3

Transition of cryptic TF to coagulant active form involves transformation of dimeric TF to monomeric form. Dimeric TF lacks the coagulant activity as it can only bind to FVIIa but not to the substrate factor X. A stimulus that is capable of exposing PS (depicted in red color circles) at the cell surface also leads to transformation of dimeric TF to monomeric form that is capable of interacting with the substrate (the figure is a modified version of an earlier depiction by Key and Bach [133]).