Membrane thrombomodulin levels are decreased during hypoxia and restored by cAMP and IBMX

Venous valve hypoxia as a possible mechanism of deep vein thrombosis: a scoping review

INTRODUCTION

The pathogenesis of deep vein thrombosis (DVT) has been explained by an interplay between a changed blood composition, vein wall alteration, and blood flow abnormalities. A comprehensive investigation of these components of DVT pathogenesis has substantially promoted our understanding of thrombogenesis in the venous system. Meanwhile, the process of DVT initiation remains obscure. This systematic review aims to collect, analyze, and synthesize the published evidence to propose hypoxia as a possible trigger of DVT.

EVIDENCE ACQUISITION

An exhaustive literature search was conducted across multiple electronic databased including PubMed, EMBASE, Scopus, and Web of Science to identify studies pertinent to the research hypothesis. The search was aimed at exploring the connection between hypoxia, reoxygenation, and the initiation of deep vein thrombosis (DVT). The following key words were used: "deep vein thrombosis," "venous thrombosis," "venous thromboembolism," "hypoxia," "reoxygenation," "venous valve," and "venous endothelium." Reviews, case reports, editorials, and letters were excluded.

EVIDENCE SYNTHESIS

Based on the systematic search outcome, 156 original papers relevant to the issue were selected for detailed review. These studies encompassed a range of experimental and observational clinical research, focusing on various aspects of DVT, including the anatomical, physiological, and cellular bases of the disease. A number of studies suggested limitations in the traditional understanding of Virchow's triad as an acceptable explanation for DVT initiation. Emerging evidence points to more complex interactions and additional factors that may be critical in the early stages of thrombogenesis. The role of venous valves has been recognized but remains underappreciated, with several studies indicating that these sites may act as primary loci for thrombus formation. A collection of studies describes the effects of hypoxia on venous endothelial cells at the cellular and molecular levels. Hypoxia influences several pathways that regulate endothelial cell permeability, inflammatory response, and procoagulation activity, underpinning the endothelial dysfunction noted in DVT.

CONCLUSIONS

Hypoxia of the venous valve may serve as an independent hypothesis to outline the DVT triggering process. Further research projects in this field may discover new molecular pathways responsible for the disease and suggest new therapeutic targets.

Tissue Injury Protection: The Other Face of Anticoagulant Treatments in the Context of Ischemia and Reperfusion Injury with a Focus on Transplantation

Organ transplantation has enhanced the length and quality of life of patients suffering from life-threatening organ failure. Donors deceased after brain death (DBDDs) have been a primary source of organs for transplantation for a long time, but the need to find new strategies to face organ shortages has led to the broadening of the criteria for selecting DBDDs and advancing utilization of donors deceased after circulatory death. These new sources of organs come with an elevated risk of procuring organs of suboptimal quality. Whatever the source of organs for transplant, one constant issue is the occurrence of ischemia-reperfusion (IR) injury. The latter results from the variation of oxygen supply during the sequence of ischemia and reperfusion, from organ procurement to the restoration of blood circulation, triggering many deleterious interdependent processes involving biochemical, immune, vascular and coagulation systems. In this review, we focus on the roles of thrombo-inflammation and coagulation as part of IR injury, and we give an overview of the state of the art and perspectives on anticoagulant therapies in the field of transplantation, discussing benefits and risks and proposing a strategic guide to their use during transplantation procedures.

Reduced Glomerular Endothelial Thrombomodulin Is Associated with Glomerular Macrophage Infiltration in Diabetic Nephropathy

The endothelial glycoprotein thrombomodulin regulates coagulation, inflammation, and apoptosis. In diabetic mice, reduced thrombomodulin function results in diabetic nephropathy (DN). Furthermore, thrombomodulin treatment reduces renal inflammation and fibrosis. Herein, thrombomodulin expression was examined in human kidney samples to investigate the possibility of targeting thrombomodulin in patients with DN. Glomerular thrombomodulin was analyzed together with the number of glomerular macrophages in 90 autopsied diabetic cases with DN, 55 autopsied diabetic cases without DN, and 37 autopsied cases without diabetes or kidney disease. Thrombomodulin mRNA was measured in glomeruli microdissected from renal biopsies from patients with DN and nondiabetic controls. Finally, glomerular thrombomodulin was measured in diabetic mice following treatment with the selective endothelin A receptor (ET_AR) blocker, atrasentan. In diabetic patients, glomerular thrombomodulin expression was increased at the mRNA level, but decreased at the protein level, compared with nondiabetic controls. Reduced glomerular thrombomodulin was associated with an increased glomerular influx of macrophages. Blocking the ET_AR with atrasentan restored glomerular thrombomodulin protein levels in diabetic mice to normal levels. The reduction in glomerular thrombomodulin in diabetes likely serves as an early proinflammatory step in the pathogenesis of DN. Thrombomodulin protein may be cleaved under diabetic conditions, leading to a compensatory increase in transcription. The nephroprotective effects of ET_AR antagonists in diabetic patients may be attributed to the restoration of glomerular thrombomodulin.

Impact of anticoagulation on the effectiveness of loop diuretics in heart failure with cardiorenal syndrome and venous thromboembolism

The limitations of diuretics in the treatment of acute decompensated heart failure (ADCHF) are progressive worsening of renal function and resistance to up-titrated doses. The occurrence of resistance to loop diuretics in patients with ADCHF is associated with worsening prognosis and increased mortality. In this study, we report two patients with ADCHF and resistance to loop diuretics suspected for venous thromboembolism, suggesting that heparin administered to ADCHF patients treated for venous thromboembolism with cardiorenal syndrome decreases right-ventricular overload and improves renal function. To our knowledge, these are the first reported cases describing restored responsiveness to loop diuretics in ADCHF patients after additional heparin administration.

Basic mechanisms and pathogenesis of venous thrombosis

In 1856 Virchow proposed a triad of causes for venous thrombosis, postulating that stasis, changes in the vessel wall or changes in the blood could lead to thrombosis. We now know that abnormally high levels of some coagulation factors and defects in the natural anticoagulants contribute to thrombotic risk. Among these, factor V Leiden, which renders factor Va resistant to activated protein C, is the most prevalent with approximately 5% of the Caucasian population having this genetic alteration. These genetically controlled variants in coagulation factors work in concert with other risk factors, such as oral contraceptive use, to dramatically increase thrombotic risk. While these abnormalities in the blood coagulation proteins are associated with thrombotic disease propensity, they are less frequent contributors to thrombosis than age or cancer. Cancer increases thrombotic risk by producing tissue factor to initiate coagulation, by shedding procoagulant lipid microparticles or by impairing blood flow. Age is the strongest risk factor for thrombosis. Among possible reasons are fragility of the vessels potentially contributing to stasis, increased coagulation factor levels, impaired function of the venous valves, decreases in the efficacy of natural anticoagulants associated with the vessel wall, increased risk of immobilization and increased risk of severe infection.

Increase in Thrombomodulin Concentrations After Pulmonary Thromboendarterectomy in Chronic Thromboembolic Pulmonary Hypertension

STUDY OBJECTIVES

The objectives of the study were as follows: (1) to identify differences in endothelial dysfunction and altered hemostasis in patients with chronic thromboembolic pulmonary hypertension (CTEPH) compared with patients with acute pulmonary thromboembolism (APTE) uncomplicated by pulmonary arterial hypertension, by measuring the concentrations of thrombomodulin (TM), a receptor for thrombin and a major anticoagulant proteoglycan on the endothelial membrane, and other plasma factors of coagulation and fibrinolysis; and (2) to examine the effects of thromboendarterectomy on TM levels as a parameter of endothelial cell injury leading to abnormal hemostasis as well as to examine the clinical significance of TM as a marker of endothelial injury.

DESIGN

Prospective comparison of concentrations of TM and other plasma parameters among patients with CTEPH or APTE and control subjects.

PARTICIPANTS

We studied 22 healthy subjects (ie, control subjects), 22 patients who had been clinically stabilized after APTE, and 44 patients with CTEPH. In 21 of the patients with CTEPH, measurements were repeated after they had undergone pulmonary thromboendarterectomy.

MEASUREMENTS AND RESULTS

Plasma concentrations of soluble TM in patients with CTEPH were measured and compared with those in patients with APTE. The mean (+/- SD) TM concentration in the CTEPH group (2.5 +/- 0.7 ng/mL) was significantly lower than that in the control group (4.0 +/- 0.6 ng/mL; p < 0.05). In contrast, the mean plasma TM concentration in the APTE group (4.6 +/- 1.9 ng/mL) was similar to that in the control group. After patients underwent pulmonary thromboendarterectomy, the mean TM concentration increased from 2.0 +/- 0.4 to 2.9 +/- 0.7 ng/mL (p < 0.05). In the CTEPH group, the plasma TM concentration was negatively correlated with mean pulmonary arterial pressure and total pulmonary resistance (p < 0.05).

CONCLUSIONS

A decreased plasma TM concentration may reflect pulmonary vascular endothelial dysfunction leading to altered anticoagulant and fibrinolytic function in CTEPH, which rarely develops after APTE. Plasma TM measurements may be useful in distinguishing CTEPH with severe pulmonary hypertension from recurrent APTE.

Hypertension associated with reduced plasma thrombomodulin levels and a hypercoagulable state in rats

The plasma thrombomodulin (TM) level, an indicator of systemic endothelial cell damage, was measured in spontaneously hypertensive rats (SHR), deoxycorticosteron acetate (DOCA)-induced hypertensive rats and normotensive Wistar-Kyoto (WKY) rats to clarify its changes in hypertension. Plasma TM levels, measured by enzyme-linked immuno-sorbent assay, decreased with aging (5-20-weeks-old) in both SHR and WKY, and they were lower in SHR than age-matched WKY in all ages examined. Deoxycorticosteron acetate-induced hypertensive WKY also showed decreased TM levels compared with normotensive WKY. Accelerated coagulation and fibrinolysis shown by the increases in thrombin-antithrombin complex (TAT) and D-dimer levels were observed in both groups of hypertensive rats. These results suggest that hypertension may decrease plasma TM levels and induce a hypercoagulable state in rats.

Endothelial cell activation, injury, damage and dysfunction: separate entities or mutual terms?

The loss of well-regulated endothelial cell functioning is followed by adverse changes in a variety of physiological systems, such as the expression of adhesion molecules, maintenance of adequate blood vessel tone and haemostasis. Therefore, a full understanding of endothelial cell biology is essential if the losses of normal function of these systems are to be avoided. The viewpoint presented in this paper suggests that a spectrum between endothelial cell health and disease can be drawn: midway between these two extremes is immunological activation (by, for example, cytokines), which is reversible. Endothelial cell damage or injury (which may be the result of chronic inflammatory activation, hypercholesterolaemia, and/or smoking) are invariably associated with clinical conditions such as hypertension and oedema (and, ultimately, thrombosis and infarction), and are more difficult to reverse. A better understanding of the events, including apoptosis, that lead to vascular dysfunction may be useful in developing our understanding of vascular biology.

Increase in expression and activity of thrombomodulin in term human syncytiotrophoblast microvilli

A comparative study of thrombomodulin (TM), a potent natural anticoagulant, was performed in first trimester and term human placentae. Immunoreactive TM was observed on fetal vascular endothelium and syncytiotrophoblast at both gestational ages. Staining was stronger in term than in early placentae, particularly along the microvillous apical membrane of the syncytiotrophoblast. Similarly, a higher level of TM mRNA was detected by RT-PCR (P<0.02) and Northern blot analysis in extracts of whole term placentae. The localization of TM on syncytial microvilli was confirmed by electron microscopy after immunogold labelling. When isolated microvilli were compared at both gestational ages; a significant 2.3-fold increase in TM protein was observed in term microvilli as compared to first trimester microvilli by Western blot analysis (P<0.005) and ELISA (P<0.05). This higher level of TM in term microvilli was associated with an increase in its ability to activate protein C, from 3.7 +/- 1.2 to 8.7 +/- 4.2 mOD/min/microg protein +/- s.d. (P<0.01) in first trimester and term microvilli, respectively. The modulation of biologically active TM at the syncytial membrane exposed to maternal blood according to the length of gestation suggests that TM may be involved both in maternal haemostasis within the intervillous spaces, and also in the trophoblast differentiation process.

Thrombomodulin-dependent anticoagulant activity is regulated by vascular endothelial growth factor

Thrombomodulin (TM) is a cell-surface receptor that plays a critical role in endothelial cell anticoagulant activity through its cofactor role in the thrombin-catalyzed activation of human protein C. In this study, we examined the effect of vascular endothelial growth factor (VEGF), a potent angiogenic factor, on surface anticoagulant activity and thrombomodulin expression. We show that thrombin-dependent activation of human protein C, measured on the endothelial cell surface, increased from 50 to 80% following exposure of cells to VEGF for 24 h. The effect was concentration dependent with the half-maximal stimulatory effect at approximately 100 pM. This increase in thrombin-dependent aPC generation correlated with a proportional and concentration-dependent increase in the level of cell-surface TM antigen. Both the total cellular TM antigen and the total cellular TM mRNA levels increased approximately 2.5-fold in VEGF-treated cells suggesting that most if not all of the regulation was at the message level. We further show that VEGF blocked IL-1 beta-induced suppression of both TM surface antigen and mRNA and was similarly capable of antagonizing the down-regulation of TM by TGF-beta and from cell activation by LPS. Our data suggest that VEGF regulation of TM may contribute to mechanisms that would maintain local hemostasis during angiogenesis and revascularization and could play a role in minimizing loss of vessel anticoagulant function during inflammatory processes.

Developmentally regulated gene expression of thrombomodulin in postimplantation mouse embryos

Embryonic lethality of thrombomodulin-deficient mice has indicated an essential role for this regulator of blood coagulation in murine development. Here, the embryonic expression pattern of thrombomodulin was defined by surveying beta-galactosidase activity in a mouse strain in which the reporter gene was placed under the regulatory control of the endogenous thrombomodulin promoter via homologous recombination in embryonic stem cells. The murine trophoblast was identified as a previously unrecognized anatomical site where TM expression is conserved between humans and mice and may exert a critical function during postimplantation development. Targeted reporter gene expression in mesodermal precursors of the endothelial cell lineage defined thrombomodulin as an early marker of vascular differentiation. Analysis of the thrombomodulin promoter in differentiating ES cells and in transgenic mice provided evidence for a disparate and cell type-specific gene regulatory control mechanism in the parietal yolk sac. The thrombomodulin promoter as defined in this study will allow the targeting of gene expression to the parietal yolk sac of transgenic mice and the initiation of investigations into the role of parietal endoderm in placental function.