Damian Sendler: The principal activator of the coagulation cascade is tissue factor (TF). Endothelial cells (ECs) and blood cells, such as monocytes, do not express TF under normal circumstances. During endotoxemia and sepsis, however, bacterial lipopolysaccharide (LPS) promotes TF expression in monocytes, resulting in disseminated intravascular coagulation. TF expression is induced by a number of stimuli in ECs in vitro, but it is unknown how much TF is expressed by the endothelium in vivo. Various intracellular signaling pathways including the transcription factors NF-B, AP-1, and Egr-1 are involved in LPS-induced TF gene expression in monocytic cells and ECs. VEGF, on the other hand, stimulates the expression of TF genes in ECs through the transcription factors NFAT and Egr-1. Similarly, via NFAT and Egr-1, oxidized phospholipids (oxPAPC) promote TF expression in ECs and potentially monocytes. Thromboxane (TX) A2 has now been added to the list of stimuli that cause TF gene expression in monocytes and endothelial cells. Inhibition of the TX-prostanoid (TP) receptor lowers TF expression in ECs triggered with TNF and monocytes activated with LPS, suggesting that a TP receptor antagonist could be effective in lowering pathogenic TF expression in the vasculature.
Damian Jacob Sendler: The primary starter of the coagulation cascade is TF, which is a transmembrane protein1. When the vasculature is damaged, the TF that surrounds it comes into contact with blood. This causes the development of the TF:FVIIa complex, which activates both FX and FIX, resulting in the production of thrombin, fibrin deposition, and platelet activation1. Pericytes and adventitial fibroblasts, which are found within and around the blood vessel wall, express TF on a constant basis2,3. It’s been suggested that TF produced by various cell types produces a hemostatic membrane that prevents bleeding following a vascular injury2. TF is also expressed by vascular cells like monocytes and ECs in pathologic situations like sepsis4. Disseminated intravascular coagulation (DIC) and thrombosis can result from this expression. TF expression by monocytes could be part of the innate immune response, and it’s likely that the host is trying to stop harmful organisms from spreading. TF is expressed by various cell types within atherosclerotic plaques, including macrophage-derived foam cells 5, in atherosclerosis. After a plaque ruptures, TF is thought to have a role in the formation of a thrombus.
Damian Sendler
Dr. Sendler: TF is not expressed by circulating blood cells in normal circumstances2. However, TF expression was observed to be low in a few CD14-positive monocytes in one study6. In vitro and in vivo, LPS stimulation of monocytes and monocytic cells promotes TF expression2,6–9. In endotoxemic mice, we and others have shown that TF expression by hematopoietic cells leads to coagulation activation10,11. Several agonists, including LPS, IL-1, TNF-, thrombin, and VEGF, have been shown to stimulate TF expression on ECs12–26 in vitro. TF expression by ECs in vivo, on the other hand, has only been reported in a few publications. TF and the EC marker von Willebrand factor were reported to be co-localized in the splenic microvasculature of septic baboons in one investigation, but not in the ECs of pulmonary vessels4. TF protein was also detected on ECs in LPS-treated mice and rabbits27,28. TF protein was recently discovered on ECs at branch sites of the aorta in septic baboons29. The TF protein was found to be functional when it co-localized with fibrin deposition29. The presence of TF on ECs, however, was limited to granular aggregates, some of which were also positive for the leukocyte marker P-selectin glycoprotein ligand-1 (PSGL-1)29. This shows that TF could be delivered to activated ECs in vivo via leukocyte-derived microparticles. We and others, in contrast to previous findings, did not detect TF expression by ECs in LPS-treated mice, rats, or rabbits30–33, which could be due to the relative sensitivity of the various techniques employed to detect TF expression. Furthermore, TF expression on ECs could have a role in signaling rather than coagulation activation. In mice models of endotoxemia and sickle cell disease, the effect of EC-specific deletion of the TF gene on coagulation activation was investigated. We discovered that a lack of TF in ECs had no effect on coagulation activation in either model34,35. However, we discovered a decrease in IL-6 expression in the sickle cell disease model35. In non-hematopoietic cells, an FXa inhibitor or a defect in the protease-activated receptor (PAR)-2 suggested that TF on ECs contributes to the stimulation of IL-6 production via FXa activation of PAR-2.
By studying a series of plasmids with varying lengths of the promoter cloned upstream of the luciferase reporter gene, an LPS response element (LRE) in the human TF promoter was discovered. This element contains an NF-B site and two AP-1 sites36 and spans 56 bp (227 to 172). The NF-B site is required for the LRE36 to function properly. The NF-B site, which has a C instead of a G at position 139, does not match the B consensus sequence and binds c-Rel-p65 heterodimers rather than prototypic p50-p65 heterodimers40. Functional interactions between c-fos/c-jun and c-Rel-p65 heterodimers14 were discovered to be involved in transcriptional activation of the TF gene14. Furthermore, nucleotide gap between the proximal AP-1 and B sites affected LPS induction of the TF gene. Physical interaction between c-fos/c-jun and c-Rel/p65 heterodimers may need this 15-bp spacing in the human, murine, and porcine promoters41. Alternatively, the conserved spacing and specified DNA bending between the AP-1 and B sites may be critical for c-fos/c-jun and c-Rel/p65 interaction with TATA box binding protein and transcription factor IIB within the basal transcriptional machinery41. Egr-1 is necessary for optimal LPS activation of the TF promoter42, according to another research. LPS stimulation of TF gene expression was reduced when the Egr-1 sites in the TF promoter were mutated or the ERK 1/2 pathway, which stimulates Egr-1 gene expression, was inhibited42.
Damian Jacob Markiewicz Sendler: TXA2 is an eicosanoid that plays a role in inflammation. By binding to the TP receptor55, it stimulates a range of cell types, including monocytes and ECs. A TP receptor agonist (U46619) enhanced TF expression in ECs25, according to a study published in this issue of Vascular Pharmacology. U46619 promotes MCP-1 expression in ECs56, according to a prior study. The TP receptor triggers the activation of AP-1 and NF-B56 via a PKC-dependent mechanism. TNF-induced leukocyte adhesion was dramatically reduced by injection of a TXA2 synthase inhibitor (OKY-046) and in TP receptor knockout mice in a mouse model of microcirculatory dysfunction in the liver, showing that TP receptor signaling may increase hepatic damage produced by TNF-57. The phenotype of TP deficient mice was more severe than that of TX synthase deficient mice, implying that ligands other than TXA2 may activate the TP receptor55. TXA2 activation of the TP receptor adds to the effects of TNF- in vivo, according to this study. Del Turco and colleagues discovered that inhibiting the TP receptor decreased TNF-induced TF expression in ECs25. Importantly, TNF or platelet-activating factor (PAF) activation increases TXA2 synthesis in HUVECs58–60. Del Turco and colleagues, on the other hand, concluded that the reduction of TNF-induced TF expression by inhibiting the TP receptor was not attributable to ECs producing TXA2 or prostanoids because they saw no effect after treating the cells with ASA or indomethacin25. One source of worry is that TXA2 metabolic product TXB2 levels were only tested for 24 hours. Other ligands that activate the TP receptor55 may also be expressed by the cells.
Damian Jacob Sendler
TF is a blood coagulation-initiating cellular receptor. It is expressed constitutively in various extravascular cell types and inducible in several vascular cell types, including monocytes and endothelial cells (ECs). More research is needed to determine the specific mechanism of TNF-induced TP receptor activation, as well as to examine the impact of this activation in various cell types and in vivo in various pathologic scenarios. The discovery that the TP receptor is a key inducer of TF expression in ECs is exciting since antagonizing the TP receptor could lead to new treatments for inflammatory disorders involving TF expression, such as sepsis and atherosclerosis. Del Turco and colleagues utilized terutroban, a TP receptor antagonist, which was compared to ASA in a randomized controlled trial (PERFORM)64 on patients with recent ischemic stroke or transient ischemic episodes.
Damien Sendler: The primary outcome, which was a composite of fatal or non-fatal ischemic stroke, fatal or non-fatal myocardial infarction, or other vascular mortality, revealed no significant differences. With the assumption that a key function of the medicine is the inhibition of TF expression, one possible explanation for the negative result is that there is little benefit to be gained after the ischemia event. However, it would be fascinating to see TP receptor antagonists tested in the prevention of stroke and coronary artery disease, as well as in the treatment of DIC and other thrombotic disorders linked to monocyte TF expression.