Design and validation of a new immunoassay for soluble forms of thrombomodulin and studies on plasma

Revisiting the activated protein C-protein S-thrombomodulin ternary pathway: Impact of new understanding on its laboratory investigation

Although suspected conceptually in the 60 s, Protein C and Protein S activities in hemostasis were investigated and reported from the mid-80 s, followed by the discovery of Thrombomodulin, an endothelial cell membrane associated protein, playing the most important heamostatic role. These 3 proteins act in regulating thrombogenesis and protecting against thrombo-embolic events. When blood is activated, any trace of circulating thrombin is captured by Thrombomodulin in the microcirculation, making thrombin become an anticoagulant through its capacity to activate Protein C to Activated Protein C, which operates as a sentinel in blood coagulation, in the form of a complex with free Protein S, to block any new blood activation site, and more especially circulating activated Factors V and VIII. Protein S not only acts as the Activated Protein C cofactor, but also as the cofactor of Tissue Factor Pathway Inhibitor. In addition, it has some functions in the complement pathway through its binding to C4b-BP. Another capability of activated protein C is to lower fibrinolytic activity, as the Activated Protein C Inhibitor is also known as Plasminogen Activator Inhibitor 3. The Protein C-Protein S system becomes less efficient in the presence of mutated Factor V (Factor V-Leiden or other variants), which is resistant to its inactivating effect. Other pathologies linked to this system concern the development of allo- or auto-antibodies to Protein S or to thrombin, which can generate severe thrombotic complications in affected patients. Some antithrombotic drugs have originated from this regulatory system. Protein C or Protein S concentrates are used for treating deficient patients. Activated Protein C (especially in patients with sepsis) or Thrombomodulin are proposed as antithrombotic medications. Most importantly, congenital or acquired Protein C or Protein S deficiencies are associated with severe recurrent thrombotic events. From the clinical standpoint most of the patients are heterozygous, as homozygosity is almost incompatible with life in the absence of a continuous and efficient treatment. Laboratory investigation of this highly complex system involves many different specialized assays for measuring these 3 proteins' activities, their antigenic content or their genetic sequence. The Protein S in-vitro anticoagulant activity is weak and contrasts with its high antithrombotic role in-vivo, showing that diagnostic assays have not yet succeeded in reproducing all the natural mechanisms for evidencing the anticoagulant role of Protein S. This paradoxal notion is discussed and illustrated in this manuscript as well is a revisit of the major characteristics and pathophysiological functions of the Protein C-Protein S-Thrombomodulin system; the associated pathologies; and the main laboratory tools available for clinical diagnosis. In respect to future perspectives, we also focused on developing more significant and relevant assays, especially for Protein S, thanks to the understanding of its biological roles.

Association of the Thrombomodulin Gene c.1418C>T Polymorphism With Thrombomodulin Levels and With Venous Thrombosis Risk

Objective—

To investigate the association of the THBD c.1418C>T polymorphism, which encodes for the replacement of Ala455 by Val in thrombomodulin (TM), with venous thromboembolism (VTE), plasma soluble TM, and activated protein C levels. In addition, human umbilical vein endothelial cells (HUVEC) isolated from 100 umbilical cords were used to analyze the relation between this polymorphism and THBD mRNA and TM protein expression.

Approach and Results—

The THBD c.1418C>T polymorphism was genotyped in 1173 patients with VTE and 1262 control subjects. Levels of soluble TM and activated protein C were measured in 414 patients with VTE (not on oral anticoagulants) and 451 controls. HUVECs were genotyped for the polymorphism and analyzed for THBD mRNA and TM protein expression and for the ability to enhance protein C activation by thrombin. The 1418T allele frequency was lower in patients than in controls ( P <0.001), and its presence was associated with a reduced VTE risk, reduced soluble TM levels, and increased circulating activated protein C levels ( P <0.001). In cultured HUVEC, the 1418T allele did not influence THBD expression but was associated with increased TM in cell lysates, increased rate of protein C activation, and reduced soluble TM levels in conditioned medium.

Conclusions—

The THBD 1418T allele is associated with lower soluble TM, both in plasma and in HUVEC-conditioned medium, and with an increase in functional membrane–bound TM in HUVEC, which could explain the increased activated protein C levels and the reduced VTE risk observed in individuals carrying this allele.

Soluble thrombomodulin levels in plasma of multiple sclerosis patients and their implication

Thrombomodulin (TM) on the cell-surface of cerebrovascular endothelial cells (CECs) is released into blood upon CEC damage. TM promotes activation of protein C (APC), an anticoagulant, anti-inflammatory, neuroprotective molecule that protects CECs and impedes inflammatory cell migration across the blood-brain barrier (BBB). Multiple sclerosis (MS) is associated with CEC damage and BBB dysfunction. We evaluated soluble TM (sTM) levels as a biomarker of BBB integrity and whether glatiramer acetate (GA) influenced sTM levels in MS patients. sTM levels quantified by 2-site ELISA from sera of healthy controls and systemic lupus erythematosus (SLE) patients (CEC-damage positive control) were compared with levels from patients with relapsing-remitting (RRMS) or secondary-progressive MS (SPMS), stratified as: RRMS/GA/no relapse, RRMS/GA/in relapse, RRMS no GA/no relapse, RRMS/no GA/in relapse; and SPMS/no GA. Additionally, soluble endothelial protein C receptor (sEPCR) levels were assessed in the non-stratified MS group, SLE patients, and controls. sTM levels were highest in RRMS patients taking GA with or without relapse, followed in decreasing order by SLE, RRMS/no GA/in relapse, SPMS, RRMS/no GA/no relapse, healthy controls. sEPCR levels were highest in MS patients, then SLE, then controls. sTM may be a useful biomarker of BBB integrity in RRMS patients. Further evaluation of sEPCR is needed. The finding that the highest sTM levels were in RRMS patients taking GA is interesting and warrants further investigation.

Hemostatic Factors as Risk Markers for Intracerebral Hemorrhage

Background and Purpose— Abnormalities in the hemostatic system may cause hemorrhagic complications. The aim of the present study was to examine whether total concentrations of tissue plasminogen activator (tPA), plasminogen inhibitor-1 (PAI-1), tPA/PAI-1 complex, von Willebrand factor (VWF), and soluble thrombomodulin were associated with a first-ever intracerebral hemorrhage (ICH).

Methods— This prospective study was an incident case-referent study nested within the Västerbotten Intervention Program and the Northern Sweden Monitoring Trends and Determinants in Cardiovascular Disease (MONICA) cohorts. By 2000, ≈74 000 subjects had been screened, and 39 ICH cases were defined according to the World Health Organization MONICA criteria. A total of 78 matched controls were selected from the same cohort.

Results— The average time from screening to the ICH event was 5.1 years. tPA/PAI-1 complex, systolic and diastolic blood pressures, and hypertension were associated with ICH in the univariate analysis. In the multivariate model, only hypertension (odds ratio [OR], 3.96; 95% confidence interval [CI], 1.27 to 12.36) and the tertile with the highest level of VWF compared with the lowest tertile (OR, 0.27; 95% CI, 0.08 to 0.90) were independently associated with ICH. The OR for the combined exposure to hypertension and low levels of VWF was 8.95, indicating a possible synergistic interaction. No associations were observed for smoking, cholesterol, body mass index, PAI-1, tPA, and soluble thrombomodulin.

Conclusions— This study showed that hypertension and low concentrations of VWF were independently associated with ICH. Furthermore, we observed a possible synergistic interaction between low levels of VWF and hypertension, suggesting 2 different pathways in the development of ICH.

Diabetic ketosis activates lymphomonocyte-inducible nitric oxide synthase

AIMS

Inappropriate production of nitric oxide (NO) may be responsible for the haemodynamic disturbances of diabetic ketoacidosis. We investigated whether this metabolic condition is associated with increased plasma nitrate (the stable oxidation product of NO) levels and NO synthase gene expression in lymphomonocytes.

RESEARCH DESIGN AND METHODS

Plasma nitrate concentrations, lymphomonocyte-inducible nitric oxide synthase (iNOS) gene expression, tumour necrosis factor-alpha (TNF-alpha) and soluble thrombomodulin were measured in 11 Type 1 diabetic patients at baseline, during mild ketosis and after euglycaemia was re-established.

RESULTS

During diabetic ketosis plasma nitrate concentrations were higher (18 (16-21) vs. 9 (7-11) micro mol/l; (95% lower-upper confidence interval) P < 0.05) than at baseline. At baseline lymphomonocyte iNOS mRNA expression and iNOS protein levels were undetectable, but in ketosis both were increased (both at P < 0.0001). After recovery from ketosis, NO3 concentration, iNOS mRNA, and iNOS expression (270 +/- 36%, mean +/- sd) decreased but not significantly. No significant changes were observed in either TNF-alpha or soluble thrombomodulin levels between the three conditions.

CONCLUSIONS

Diabetic ketosis is associated with increased nitrate levels and the activation of iNOS expression in circulating lymphomonocytes, but it does not affect either the proinflammatory cytokine TNF-alpha or a marker of endothelial dysfunction such as thrombomodulin. Our data support the hypothesis that, during diabetic ketosis, alterations in NO homeostasis are present in circulating lymphomonocytes.

Thrombomodulin expression in endothelial cells after contact with bone cement

The expression of thrombomodulin after contact with CMW 1 bone cement extracts was studied in human umbilical vein endothelial cells. Cement extracts after 1 h and 7-day curing induced no significant variations in thrombomodulin antigen levels and in mRNA expression. Significant increase of thrombomodulin was observed when endothelial cells were treated with all-trans retinoic acid (ATRA). ATRA induced the increase of thrombomodulin also in cells incubated with cement extracts. These results suggest that CMW 1 bone cement does not impair the expression of thrombomodulin in endothelial cells.

Effect of alpha-lipoic acid on the progression of endothelial cell damage and albuminuria in patients with diabetes mellitus: an exploratory study

Oxidative stress plays a central role in the pathogenesis and progression of late microangiopathic complications (diabetic nephropathy) in diabetes mellitus. Previous studies suggested that treatment of diabetic patients with the antioxidant alpha-lipoic acid reduce oxidative stress and urinary albumin excretion. In this prospective, open and non-randomized study, the effect of alpha-lipoic acid on the progression of endothelial cell damage and the course of diabetic nephropathy, as assessed by measurement of plasma thrombomodulin and urinary albumin concentration (UAC), was evaluated in 84 patients with diabetes mellitus over 18 months. Forty-nine patients (34 with Type 1 diabetes, 15 with Type 2 diabetes) had no antioxidant treatment and served as a control group. Thirty-five patients (20 with Type 1 diabetes, 15 with Type 2 diabetes) were treated with 600 mg alpha-lipoic acid per day. Only patients with an urinary albumin concentration <200 mg/l were included into the study. After 18 months of follow up, the plasma thrombomodulin level increased from 35.9+/-9.5 to 39.7+/-9.9 ng/ml (P<0.05) in the control group. In the alpha-lipoic acid treated group the plasma thrombomodulin level decreased from 37.5+/-16.2 to 30.9+/-14.5 ng/ml (P<0.01). The UAC increased in patients without alpha-lipoic acid treatment from 21.2+/-29.5 to 36.9+/-60.6 ng/l (P<0.05), but was unchanged with alpha-lipoic acid. It is postulated that the significant decrease in plasma thrombomodulin and failure of UAC to increase observed in the alpha-lipoic acid treated group is due to antioxidative effects of alpha-lipoic acid, and if so that oxidative stress plays a central role in the pathogenesis of diabetic nephropathy. Furthermore, progression of the disease might be inhibited by antioxidant drugs. A placebo-controlled study is needed.

Thrombomodulin prolongs thrombin-induced extracellular signal-regulated kinase phosphorylation and nuclear retention in endothelial cells

On endothelial cells, thrombin binds to thrombomodulin (TM), an integral membrane-bound glycoprotein, and to protease-activated receptors (PARs). Thrombin binding to TM modulates endothelial cell and smooth muscle cell proliferation mediated through PAR1. We studied the phosphorylation and nuclear translocation of extracellular signal-regulated kinases (ERKs) 1 and 2 in human umbilical vein endothelial cells activated by thrombin. Thrombin and thrombin receptor-activating peptide (TRAP)-induced DNA synthesis were significantly inhibited by PD98059, an inhibitor of ERK phosphorylation. Immunoblots of phosphorylated ERKs (pERKs) and immunocytochemical studies of pERK localization revealed differences in the signal generated by thrombin and TRAP. After a short activation (15 minutes), the phosphorylation and the intracellular localization of pERKs were the same with the 2 agonists. After 4 hours, however, pERKs were visualized in the nuclei of thrombin-activated cells but barely detectable in TRAP-activated cells. Moreover, after 4 hours, the pERKs were visualized in the nuclei of cells stimulated by TRAP in the presence of a thrombin mutant that bound to TM, whereas they were around the nuclei in cells stimulated by thrombin in the presence of a monoclonal antibody preventing thrombin binding to TM. The results demonstrate that ERKs are involved in human umbilical vein endothelial cell DNA synthesis mediated by PAR agonists, that the duration of pERK nuclear retention is in inverse ratio to the mitogenic response, and that in addition to its role in the regulation of blood coagulation, TM acts as a thrombin receptor that modulates the duration of pERK nuclear retention and cell proliferation in response to thrombin.

Hemostatic disturbances in patients with systemic inflammatory response syndrome (SIRS) and associated acute renal failure (ARF)

Endothelial damage plays a central role in the development of an SIRS-related Multiple Organ Dysfunction Syndrome (MODS) as a consequence of the establishment of a hemostatic imbalance between coagulation and fibrinolysis systems. Until now, sepsis is the SIRS model that has been most studied. The aim of this study was to assess the endothelial damage and the hemostatic imbalance in early stages of an SIRS of different origins, and to study if there are any differences in these disturbances between infectious and noninfectious SIRS. The endothelial damage and hemostatic changes were studied in 40 patients with SIRS (with less than 12 h of evolution) and an acute renal failure. Infectious SIRS was diagnosed in 19 cases and noninfectious SIRS in the remaining 21 patients. Patients with SIRS presented significantly higher values (p<0.001) for factors related to endothelial damage [von Willebrand factor (vWF), thrombomodulin, tissue plasminogen activator (t-PA), and plasminogen activator inhibitor type 1 (PAI-1) antigen], hypercoagulability [prothrombin fragment 1+2 (F1+2) and thrombin-antithrombin complexes (TAT)], and fibrinolysis (D-dimer and PAI activity) with respect to the control group. However, although the group with infectious SIRS presented higher values for all the factors except for the t-PA and D-dimer with respect to SIRS of other origins, none of these differences reached statistical significance (p>0.05). Our data show that patients with SIRS and associated acute renal failure, irrespective of the origin (infectious or noninfectious), show signs of intense endothelial damage and hypercoagulability throughout the process.

High prevalence of mild hyperhomocysteinemia in patients with abdominal aortic aneurysm

PURPOSE

In vitro studies have recently demonstrated that homocysteine interacts with the aortic wall by inducing both elastolysis and endothelial perturbation. The aim of this study was to evaluate homocysteine plasma levels and their relationships with aortic diameter and endothelial damage in patients with abdominal aortic aneurysm.

SUBJECTS AND METHODS

Fifty-eight consecutive male patients (mean age, 69.5 +/- 6.6 years; age range, 49-78 years) who underwent abdominal aortic aneurysm surgery were enrolled in the study. Twenty-two of 58 patients had no clinical or instrumental evidence of atherosclerosis. Sixty control subjects were age matched and sex matched with the patients. In all of the subjects, we evaluated total homocysteine and thrombomodulin plasma levels and the distribution of the C677T methylenetetrahydrofolate reductase gene mutation.

RESULTS

Hyperhomocysteinemia was found in 26 (48%) of the 58 patients with abdominal aortic aneurysm, and homocysteine plasma levels were significantly higher in patients than in control subjects (15.7 +/- 6.5 micromol/L vs 9.6 +/- 3.9 micromol/L; P <. 0001). In addition, the subgroup of patients with abdominal aortic aneurysm who did not show evidence of atherosclerosis showed homocysteine plasma levels significantly higher than those in the controls (14.8 +/- 6.1 micromol/L vs 9.6 +/- 3.9 micromol/L; P <. 001). A larger aneurysmal size was detected in hyperhomocysteinemic patients than in those with normal homocysteine plasma levels (5.09 +/- 0.84 cm vs 5.79 +/- 1.5 cm; P <.05). The genotype distribution of the C677T methylenetetrahydrofolate reductase mutation was as follows: TT 21%, TC 55%, and CC 24% in the patients; TT 10%, TC 58%, and CC 32% in the controls. Moreover, in patients a significant correlation (P <.005) between homocysteine plasma level and 677TT methylenetetrahydrofolate reductase genotype was found. Thrombomodulin plasma levels were significantly higher (P <.00005) in patients (median, 30 ng/mL; range, 10-164 ng/mL) than in controls (median, 19 ng/mL; range, 13-44 ng/mL), and thrombomodulin levels were significantly higher (P <.005) in hyperhomocysteinemic patients (median, 39.5 ng/mL; range, 15-164 ng/mL) than in normohomocysteinemic patients (median, 27.5 ng/mL; range, 10-85 ng/mL). In addition, in patients with abdominal aortic aneurysm, a direct significant correlation (P <.005) was found between homocysteine and thrombomodulin.

CONCLUSIONS

These data indicate an association between the presence of AAA in patients selected for surgical treatment of AAA and elevated homocysteine plasma levels and suggest that homocysteine may induce endothelial perturbation and stimulation in these patients.

Tissue factor pathway inhibitor levels in patients with homocystinuria

Thrombotic events are a well-recognized complication of homocystinuria. However, the mechanisms involved in the atherogenic and thrombotic effects of homocyst(e)ine remain incompletely understood. The objective of this study was to determine the role of endothelial cell activation/damage as indicated by levels of thrombomodulin, tissue factor and tissue factor pathway inhibitor, and factor VII activity in patients with homocystinuria. Six patients with homocystinuria, nonresponsive to pyridoxine, treated only with trimethylglycine (betaine) were injected with a bolus of 20 IU/kg body weight of unfractionated commercial heparin to induce the release of tissue factor pathway inhibitor from the vascular endothelium. Tissue factor, thrombomodulin, and factor VII activity were measured by enzyme-linked immunosorbent assay and clotting assay before heparin administration. Tissue factor pathway inhibitor antigen and activity were measured before and 5 minutes after the bolus of heparin. Levels of homocyst(e)ine were elevated (patients: 144.2+/-19.2 micromol/L; controls: 10.2+/-0.9 micromol/L); however, levels of thrombomodulin, tissue factor, and tissue factor pathway inhibitor antigen were not statistically different from the control group. In contrast, tissue factor pathway inhibitor activity showed a significantly increased level (patients: 2.09+/-0.34 U/L; controls: 1.14+/-0.20 U/L; p<0.05) that was correlated with homocyst(e)ine. Factor VII activity was significantly decreased (patients: 64.7+/-5.1%; controls: 91.4+/-4.7%; p<0.05) and inversely correlated with homocyst(e)ine. After heparin the patients released higher amounts of tissue factor pathway inhibitor antigen and activity compared with the control group; however, the difference was not statistically significant. Although not treated with antithrombotic drugs, none of the patients had any thromboembolic complications after starting betaine. In addition to betaine treatment, the enhanced factor pathway inhibitor antigen activity observed in this small series of patients suggests that factor pathway inhibitor antigen may play an additional, as yet unexplained, role in this genetic disorder.

G-33A mutation in the promoter region of thrombomodulin gene and its association with coronary artery disease and plasma soluble thrombomodulin levels

Thrombomodulin is an endothelial glycoprotein that decreases thrombin activity and activates protein C. A recent study has shown that G-33A promoter mutation of the thrombomodulin gene occurs particularly in Asians. In this study, we analyzed the distribution of G-33A mutation in the promoter region of the thrombomodulin gene in the Chinese population and determined whether the mutation might be a risk for coronary artery disease (CAD). In addition, the influence of this mutation on plasma soluble thrombomodulin levels in patients with CAD was also examined. We studied 320 consecutive patients (mean age 63 years; 73% men) with CAD and 200 age- and sex-matched control subjects. Screening for thrombomodulin G-33A promoter mutation was conducted using polymerase chain reaction, single-strand conformation polymorphism, and direct deoxyribonucleic acid sequencing. The frequency of the G-33A mutation (GA+AA genotypes) was significantly higher in the CAD group (23.8% vs 15.5%, odds ratio [OR] 1.70, p = 0.031). Multiple logistic regression analysis showed that the mutation was an independent risk factor (OR 1.81, p = 0.016) for CAD, as was hypertension (OR 1.44, p = 0.040), diabetes mellitus (OR 2.50, p <0.001), and smoking (OR 2.15, p <0.001). In CAD patients with GG genotype, the soluble thrombomodulin level increased with the extent of CAD (36 +/- 15 vs 47 +/- 18 vs 55 +/- 36 ng/ml in 1-, 2-, or 3-vessel CAD, p <0.001). However, in CAD patients with G-33A mutation, there was no difference between the levels of soluble thrombomodulin (39 +/- 17 vs 37 +/- 15 vs 42 +/- 18 ng/ml, p = NS) in 1-, 2-, or 3-vessel CAD. Our observations suggest that there is a significant association of the G-33A mutation in thrombomodulin gene with CAD, and this mutation may influence the soluble thrombomodulin levels in patients with CAD.

C-reactive protein in healthy subjects: associations with obesity, insulin resistance, and endothelial dysfunction: a potential role for cytokines originating from adipose tissue?

C-reactive protein, a hepatic acute phase protein largely regulated by circulating levels of interleukin-6, predicts coronary heart disease incidence in healthy subjects. We have shown that subcutaneous adipose tissue secretes interleukin-6 in vivo. In this study we have sought associations of levels of C-reactive protein and interleukin-6 with measures of obesity and of chronic infection as their putative determinants. We have also related levels of C-reactive protein and interleukin-6 to markers of the insulin resistance syndrome and of endothelial dysfunction. We performed a cross-sectional study in 107 nondiabetic subjects: (1) Levels of C-reactive protein, and concentrations of the proinflammatory cytokines interleukin-6 and tumor necrosis factor-alpha, were related to all measures of obesity, but titers of antibodies to Helicobacter pylori were only weakly and those of Chlamydia pneumoniae and cytomegalovirus were not significantly correlated with levels of these molecules. Levels of C-reactive protein were significantly related to those of interleukin-6 (r=0.37, P<0.0005) and tumor necrosis factor-alpha (r=0.46, P<0.0001). (2) Concentrations of C-reactive protein were related to insulin resistance as calculated from the homoeostasis model assessment model, blood pressure, HDL, and triglyceride, and to markers of endothelial dysfunction (plasma levels of von Willebrand factor, tissue plasminogen activator, and cellular fibronectin). A mean standard deviation score of levels of acute phase markers correlated closely with a similar score of insulin resistance syndrome variables (r=0.59, P<0.00005), this relationship being weakened only marginally by removing measures of obesity from the insulin resistance score (r=0.53, P<0.00005). These data suggest that adipose tissue is an important determinant of a low level, chronic inflammatory state as reflected by levels of interleukin-6, tumor necrosis factor-alpha, and C-reactive protein, and that infection with H pylori, C pneumoniae, and cytomegalovirus is not. Moreover, our data support the concept that such a low-level, chronic inflammatory state may induce insulin resistance and endothelial dysfunction and thus link the latter phenomena with obesity and cardiovascular disease.

Soluble plasma thrombomodulin levels in patients with chronic myeloproliferative disorder

The plasma levels of soluble thrombomodulin (TM) were measured in 44 patients with chronic myeloproliferative disorder, 15 with polycythemia vera (PV), 29 with essential thrombocythemia (ET), and a group of 62 matched healthy controls. The younger patients had significantly lower TM levels (mean: 15.6 +/- 4.8 ng/mL) than the older patients (mean: 28.6 +/- 8.2 ng/mL, p < .001). Moreover, a significant negative correlation between platelet counts and plasma TM levels in healthy persons was noted (r = 0.317, p < .05). The only significant difference we found in plasma TM levels between patients and controls or among patients was between the young patients with ET (mean: 29.0 +/- 19.2 ng/mL) and young healthy controls (mean: 15.6 +/- 4.8 ng/mL). It is possible that younger ET patients with more active platelets are more susceptible to earlier vascular damage. The lack of any significant difference compared with the older patient population supports this hypothesis.

Thrombomodulin: an overview and potential implications in vascular disorders

Thrombomodulin (TM), a high affinity thrombin receptor present on endothelial cell membrane, plays an important role as a natural anticoagulant. It acts as a cofactor of thrombin-catalyzed activation of protein C, and inhibits the procoagulant functions of thrombin. TM is also located in other cells (keratinocytes, osteoblasts, macrophages,...) where it might be involved in cell differentiation or in inflammation. In the presence of cytokines, activated neutrophils and macrophages, endothelial TM is cleaved enzymatically, releasing soluble fragments which circulate in the blood and are eliminated in urine. Plasma TM level (pTM) can be measured using a two-site enzyme-linked immunosorbent assay (ELISA). pTM level is regarded as a molecular marker reflecting injury of endothelial cells. It is often increased in case of diffuse endothelial damage as in disseminated intravascular coagulation, diabetic microangiopathy, Plasmodium falciparum and rickettsial infections. pTM is also a predictive marker of hypertensive complications in pregnancy. In several systemic inflammatory diseases, pTM levels are correlated to the activity of the disease.

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 and induced tissue factor expression on monocytes as markers of diabetic microangiopathy: a prospective study on hemostasis and lipoproteins in insulin-dependent diabetes mellitus

Vascular complications are the main cause of morbidity in diabetes mellitus. To evaluate lipoprotein and hemostatic parameters and their relationship with clinically detectable microangiopathy, we studied 58 insulin-dependent diabetes mellitus patients and 60 controls matched for age, sex, and body mass index. Thirteen patients presented clinically detectable microangiopathy (8 retinopathy and 5 both retinopathy and microalbuminuria). A cross-sectional study of lipid profile, coagulation parameters, and a flow-cytometric evaluation of tissue factor expression in normal monocytes induced by patient plasma were performed. Patients were re-evaluated for microangiopathy in a 3-year median follow-up. Patients showed triglyceride enrichment in low (P = 0.00002) and high density lipoproteins (P = 0.004) and increased levels of D-dimer (P < 0.00001), prothrombin fragment 1 + 2 (P < 0.00001), and thrombin-antithrombin III complex (P = 0.0001). Patients with clinically detectable microangiopathy had increased type 1 plasminogen activator inhibitor (P = 0.00001), thrombomodulin (P = 0.02), and induced monocyte tissue factor expression (P < 0.00001). Nine patients developed clinically detectable microangiopathy in the follow-up and the only predictive variable was increased induced tissue factor expression. In conclusion, in these patients elevated thrombin and fibrin generation reflects a hypercoagulable state but clinically detectable microangiopathy seems related to endothelial cell injury markers and to increased induced tissue factor expression on monocytes.

Effluent levels of thrombomodulin predict early graft function in clinical liver transplantation

Thrombomodulin is a surface protein on vascular endothelial cells that serves as a binding site for thrombin and plays an important role as an anticoagulant factor. We correlated plasma thrombomodulin levels with early graft function after liver transplant in 58 recipients. Blood samples were collected at the following time points: before surgery, just before reperfusion, 30, 60, 120 min after reperfusion, and post-operative day 1. The first and last 20 cc of caval effluent were also collected. Plasma thrombomodulin levels were measured by a sandwich enzyme-binding assay in the blood samples; tissue expression was determined by immunohistochemistry. Poor early graft function was defined as peak aspartate aminotransferase (AST) or alanine aminotransferase (ALT) >2500 U/l during the first 3 post-operative days and prothrombin time >16 s on post-operative day 2. Thrombomodulin levels in the first 20 cc of caval effluent ranged from 1.33 to 91 FU/ml and showed a significant positive correlation with ischemic time, intra-operative blood transfusion requirement, and early graft function. In grafts with high effluent thrombomodulin (>20 FU/ml, n=12), the incidence of poor early graft function and primary nonfunction was 66.7% and 25.0%, respectively; in grafts with low effluent thrombomodulin (<20 FU/ml, n=46), graft function was not impaired. By immunohistochemistry, thrombomodulin was detected in large vessels of every donor liver. Sinusoidal cells, however, showed positive staining only in livers with poor early graft function. Effluent thrombomodulin levels reflect the extent of preservation injury and might be a useful marker for predicting graft function after liver transplantation.

On the mechanism of the antifibrinolytic activity of plasma carboxypeptidase B

The precursor of plasma carboxypeptidase B (pCPB) also known as thrombin-activable fibrinolysis inhibitor can be converted by thrombin to an active enzyme capable of eliminating C-terminal Lys- and Arg-residues from proteins. The activation is about 1000-fold more efficient in the presence of thrombomodulin (TM). We investigated the antifibrinolytic potency of maximally activated pCPB in plasma and explored the antifibrinolytic mechanism of pCPB. During clotting of plasma in the presence of 3.3 NIH units/ml thrombin and 1 microg/ml soluble TM, more than 80% pro-pCPB was converted into the active form causing an increase of plasma carboxypeptidase activity from 100 units/liter (constitutive activity ascribed to plasma carboxypeptidase N) to 430 units/liter as measured with furoylacroleyl-alanyl-arginine substrate. Under these conditions, lysis of a plasma clot induced by a range of tissue-type plasminogen activator (t-PA) concentrations (0.2-2 microg/ml) was retarded more than 4-fold. A considerable retardation of fibrinolysis was observed upon addition of as little as 12 ng/ml soluble TM, a concentration comparable with physiological concentrations of soluble TM in human plasma. The presence of Ca2+ appeared to be a critical requirement for effective activation of pro-pCPB by thrombin-TM in plasma. Plasminogen-binding sites (C-terminal lysines) on the surface of a plasmin-treated fibrin clot were eliminated within 1-3 min by plasma with maximally activated pCPB, as studied in a recently described model involving fluorescence microscopy. Confocal fluorescence microscopy showed that in the absence of TM plasminogen strongly accumulated on fibrin fibers during t-PA-induced lysis of a plasma clot. In the presence of TM (and a concomitant pro-pCPB activation), lysis was slow and was not accompanied by accumulation of plasminogen on the fibers. In conclusion, generation of active pCPB during clotting of plasma in the presence of Ca2+ and TM leads to a retardation of plasma clot lysis in a wide range of t-PA concentrations, from low to therapeutic, and to a fast elimination of plasminogen-binding sites on partially degraded fibrin. This is a likely mechanism for the antifibrinolytic effect of active pCPB.

Thrombomodulin is synthesized by human mesangial cells

Thrombomodulin (TM), an endothelial receptor for thrombin, endowed with a powerful anticoagulant activity, plays an important role in the antithrombogenicity of the vascular endothelium. Its presence within the human renal glomerulus is already known but was thought to be only endothelial. We looked for TM expression in human mesangial cells (MC), both in situ, in freshly prepared glomeruli, and in primary culture. Both fresh and cultured MC were strongly reactive for TM by immunocytochemical methods. Total TM antigen measured on MC lysates and surface TM activity on MC were 0.292 +/- 0.075 ng/mg of cellular proteins and 1.20 +/- 0.02 pmole of activated protein C/min/mg of cellular proteins, respectively. As shown by the presence of numerous transcripts detected by in situ hybridization, TM was shown to be synthesized by MC in vivo and in culture. The synthesis of active TM by both endothelial and mesangial cells within the renal glomerulus stresses the importance of its role in maintaining renal hemostatic equilibrium, and sheds some light on the conflicting reports of TM over- and underexpression in glomerulopathies to open a new field for investigation.

[Thrombomodulin: physiology and clinical applications (excluding systemic diseases)]

Thrombomodulin (TM) is a proteoglycan present on the surface of endothelial and mesothelial cells where it acts as a strong anticoagulant. TM is also located in other cells (keratinocytes, osteoblasts, mononuclear phagocytes...) where it might be involved in cell differentiation or inflammatory processes. The damage of endothelial cells releases cleavage fragments. Plasma TM appears to be a marker of endothelium damage. Plasma TM has been investigated in several disorders: it is usually increased in the case of diffuse endothelial damage.

Neutrophil elastase: a determinant of endothelial damage and reperfusion injury after liver transplantation?

Reperfusion injury has been implicated in the development of primary graft dysfunction (PGD) after liver transplantation. Neutrophil migration and activation may be involved in the pathogenesis of this injury. We studied neutrophil activation and its role in the etiology of PGD by measuring neutrophil elastase by radioimmunoassay, in serial blood samples of 19 patients before, during, and for 24 hr after transplantation. In a subgroup of patients, we also measured soluble thrombomodulin at the same time points as a marker of endothelial damage. The pretransplant elastase level was significantly raised (40.13+/-4.84 ng/ml, mean+/-SEM) compared with levels of healthy controls (18.7+/-5.6 ng/ml, P<0.05). A marked increase in elastase activity followed reperfusion, with a peak at 2 hr (370+/-50.5 ng/ml, P<0.01). Thereafter, there was a decline, but elastase remained elevated at 24 hr (186+/-60.94 ng/ml). The mean increase in neutrophil elastase after reperfusion correlated significantly with markers of graft function (P<0.05) and with the mean rise in soluble thrombomodulin (P=0.042), which increased from a pretransplant level of 81.2+/-11.32 to 186+/-50.4 ng/ml, 6 hr after reperfusion (P<0.05). The results of this study indicate that marked neutrophil activation and endothelial cell damage occurs after graft reperfusion during orthotopic liver transplantation, and the degree of activation correlates with markers of graft function, which may suggest a role in the etiology of PGD.

Systemic release of thrombomodulin, but not from the cardioplegic, reperfused heart during open heart surgery

Thrombomodulin is a potential marker of endothelial injury. Plasma thrombomodulin was measured in concomitant arterial and coronary sinus samples in 9 patients undergoing elective coronary artery bypass surgery with cardiopulmonary bypass (CPB, 88 +/- 14 min) (mean +/- SD) and cold, crystalloid, antegrade cardioplegia (44 +/- 14 min). Arterial thrombomodulin was 17 +/- 6 ng/ml before surgery, and decreased to 10 +/- 5 ng/ml after heparinization (p < 0.008 compared to initial value). During CPB thrombomodulin increased, with a maximal level of 23 +/- 7 ng/ml (p < 0.008 vs initial value) 40 min after aortic declamping. No difference between arterial and coronary sinus concentrations was detected during reperfusion of the heart. In conclusion, plasma thrombomodulin is decreased by heparin, and increased during CPB. Consequently, thrombomodulin may be used to evaluate endothelial injury during CPB. However, as there is no specific intracoronary release of thrombomodulin during reperfusion, thrombomodulin is not a suitable marker of coronary endothelial injury after cardioplegia.

Plasma levels of thrombomodulin in pulmonary hypertension

BACKGROUND

Long-standing pulmonary hypertension (PH) leads to structural alterations of the pulmonary vasculature and its endothelium, and occlusion of small vessels by microthrombi. In patients with PH, the search for factors inducing or worsening endothelium damage and in situ thrombi is still ongoing. Thrombomodulin (TM), an endothelial cell membrane protein, is a receptor for thrombin and a major anticoagulant proteoglycan.

PURPOSE

To analyze plasma TM levels in patients with different forms of severe PH.

PATIENTS

We prospectively studied 32 consecutive patients with PH referred for heart, lung, or heart-lung transplantation: 11 patients with primary PH (group 1), 11 patients with secondary precapillary PH (Eisenmenger's syndrome, group 2) and 10 patients with secondary postcapillary PH due to congestive heart failure (group 3). Thirty-eight healthy subjects were also studied as a control group.

METHODS

Plasma concentrations of TM were measured by an immunoenzymatic technique that uses two anti-TM monoclonal antibodies that have a strong avidity and react with different epitopes of the molecule.

RESULTS

Thrombomodulin plasma levels decreased in all patients with precapillary PH, and this decrease was highly significant compared with controls (26 +/- 2 versus 44 +/- 2 ng/mL, P = 0.0001). In primary PH, the TM decrease was only significant in males whereas in the Eisenmenger's syndrome TM values were the lowest of all the patients studied, with mean values twice as low as controls (22 +/- 2 versus 44 +/- 2 ng/mL, P = 0.0001). In contrast, in postcapillary PH, studied only in males, TM levels were increased (85 +/- 17 versus 54 +/- 3 ng/mL, P = 0.02). Patients with precapillary PH had more severe disease than patients with postcapillary PH, with higher pulmonary artery pressure and pulmonary vascular resistance (P < 0.001). There was no correlation between TM plasma levels and all hemodynamic variables.

CONCLUSION

We found low levels of plasma TM in patients with precapillary PH but not in postcapillary PH compared with healthy controls. This may be related to the severity of PH and may contribute to the initiation or worsening of in situ thromboses frequently found in pulmonary hypertension. Further studies should analyze whether other markers of endothelial cell damage are correlated with plasma TM levels in patients with precapillary pulmonary hypertension.