HS3ST1 genotype regulates antithrombin's inflammomodulatory tone and associates with atherosclerosis

The HS3ST1 gene controls endothelial cell production of HS^AT+ - a form of heparan sulfate containing a specific pentasaccharide motif that binds the anticoagulant protein antithrombin (AT). HS^AT+ has long been thought to act as an endogenous anticoagulant; however, coagulation was normal in Hs3st1^-/- mice that have greatly reduced HS^AT+ (HajMohammadi et al., 2003). This finding indicates that HS^AT+ is not essential for AT's anticoagulant activity. To determine if HS^AT+ is involved in AT's poorly understood inflammomodulatory activities, Hs3st1^-/- and Hs3st1^+/+ mice were subjected to a model of acute septic shock. Compared with Hs3st1^+/+ mice, Hs3st1^-/- mice were more susceptible to LPS-induced death due to an increased sensitivity to TNF. For Hs3st1^+/+ mice, AT treatment reduced LPS-lethality, reduced leukocyte firm adhesion to endothelial cells, and dilated isolated coronary arterioles. Conversely, for Hs3st1^-/- mice, AT induced the opposite effects. Thus, in the context of acute inflammation, HS^AT+ selectively mediates AT's anti-inflammatory activity; in the absence of HS^AT+, AT's pro-inflammatory effects predominate. To explore if the anti-inflammatory action of HS^AT+ also protects against a chronic vascular-inflammatory disease, atherosclerosis, we conducted a human candidate-gene association study on >2000 coronary catheterization patients. Bioinformatic analysis of the HS3ST1 gene identified an intronic SNP, rs16881446, in a putative transcriptional regulatory region. The rs16881446^G/G genotype independently associated with the severity of coronary artery disease and atherosclerotic cardiovascular events. In primary endothelial cells, the rs16881446^G allele associated with reduced HS3ST1 expression. Together with the mouse data, this leads us to conclude that the HS3ST1 gene is required for AT's anti-inflammatory activity that appears to protect against acute and chronic inflammatory disorders.

Engineering D-helix of antithrombin in alpha-1-proteinase inhibitor confers antiinflammatory properties on the chimeric serpin

Antithrombin (AT) is a heparin-binding serpin in plasma which regulates the proteolytic activity of procoagulant proteases of the clotting cascade. In addition to being an anticoagulant, AT also exhibits antiinflammatory activities when it binds to cell surface heparan sulfate proteoglycans (HSPGs) on the endothelium via its basic residues of D-helix to elicit intracellular signalling responses. By contrast to AT, α1-proteinase inhibitor (α1-PI) is a non-heparin-binding serpin that exhibits very slow reactivity with coagulation proteases and possesses no HSPG-dependent antiinflammatory properties. To determine whether the antiinflammatory signaling specificity of AT can be transferred to α1-PI, we replaced the D-helix of human α1-PI with the corresponding sequence of human AT and expressed the chimeric serpin α1-PI/D-helix) in a bacterial expression system. High molecular weight heparin bound to α1-PI/D-helix and accelerated the inhibition of thrombin by the serpin mutant by a template mechanism reminiscent of the cofactor effect of heparin on inhibition of thrombin by AT. Like AT, α1-PI/D-helix exhibited antiinflammatory properties in both cellular and animal models. Thus, α1-PI/D-helix inhibited the barrier-disruptive effect of proinflammatory cytokines and inhibited the activation of nuclear factor-κB transcription factor in lipopolysaccharide-stimulated endothelial cells by a concentration-dependent manner. Furthermore, the chimeric serpin reduced lipopolysaccharide-mediated lethality, elicited a vascular protective effect and inhibited infiltration of activated leukocytes to the peritoneal cavity of mice in an HMGB1-mediated inflammatory model. These results suggest that grafting the D-helix of AT to α1-PI confers antiinflammatory properties on the serpin and that the chimeric serpin may have therapeutic utility for treating inflammatory disorders.

Protective role of antithrombin in mouse models of liver injury

BACKGROUND & AIMS

Coagulopathy caused by an imbalance of hemostatic factors is associated with the pathophysiology of liver disease. We have investigated the role of antithrombin (AT), a key anticoagulant serpin, in the onset of liver disease.

METHODS

Liver injury was induced by CCl(4) injection and bile duct ligation (BDL) in wild type (WT) and AT-deficient (AT(+/-)) mice. Twenty-four hours after CCl(4) treatment, aspartate-transaminase, alanine-transaminase, liver lesion size, leukocyte infiltration, and apoptosis were reduced in WT animals compared to AT(+/-) mice.

RESULTS

Administration of exogenous AT in AT(+/-) animals did not restore the values observed in WT mice, suggesting that intrahepatic AT might also offer protection against CCl(4). In the BDL model, increased liver injury was also evident in AT(+/-) compared to WT mice. An 85 kDa covalent complex involving AT was identified in immunoblottings of liver lysates from CCl(4)-treated animals. This complex was also present in anoikis hepatocytes and H(2)O(2)-treated HepG2 cells, suggesting a role for AT in apoptosis. Expression of recombinant WT-AT by HEK-EBNA cells increased cell survival while expression of AT mutants, ΔR393 and R47C, did not modify viability. Finally, plasma anti-FXa activity was attenuated by liver injury, with AT(+/-) animals showing a greater reduction than WT mice.

CONCLUSIONS

Our study reveals a protective role of AT against liver injury due to its recognized anticoagulant and anti-inflammatory action. AT may also act via a previously unrecognized antiapoptotic effect. The clinical implications of AT deficiency in patients with liver disease should be further addressed.

Inflammation and coagulation

In the pathogenesis of sepsis, inflammation and coagulation play a pivotal role. Increasing evidence points to an extensive cross-talk between these two systems, whereby inflammation leads to activation of coagulation, and coagulation also considerably affects inflammatory activity. Molecular pathways that contribute to inflammation-induced activation of coagulation have been precisely identified. Pro-inflammatory cytokines and other mediators are capable of activating the coagulation system and down-regulating important physiologic anticoagulant pathways. Activation of the coagulation system and ensuing thrombin generation is dependent on expression of tissue factor and the simultaneous down-regulation of endothelial-bound anticoagulant mechanisms and endogenous fibrinolysis. Conversely, activated coagulation proteases may affect specific cellular receptors on inflammatory cells and endothelial cells and thereby modulate the inflammatory response.

Tissue factor: a mediator of inflammatory cell recruitment, tissue injury, and thrombus formation in experimental colitis

There is growing evidence for an interplay between inflammatory and coagulation pathways in acute and chronic inflammatory diseases. However, it remains unclear whether components of the coagulation pathway, such as tissue factor (TF), contribute to intestinal inflammation, and whether targeting TF will blunt the inflammatory cell recruitment, tissue injury, and enhanced thrombus formation that occur in experimental colitis. Mice were fed 3% dextran sodium sulfate (DSS) to induce colonic inflammation, with some mice receiving a mouse TF-blocking antibody (muTF-Ab). The adhesion of leukocytes and platelets in colonic venules, light/dye-induced thrombus formation in cremaster muscle microvessels, as well as disease activity index, thrombin-antithrombin (TAT) complexes in plasma, and histopathologic changes in the colonic mucosa were monitored in untreated and muTF-Ab-treated colitic mice. In untreated mice, DSS elicited the recruitment of adherent leukocytes and platelets in colonic venules, caused gross and histologic injury, increased plasma TAT complexes, and enhanced thrombus formation in muscle arterioles. muTF-Ab prevented elevation in TAT complexes, reduced blood cell recruitment and tissue injury, and blunted thrombus formation in DSS colitic mice. These findings implicate TF in intestinal inflammation and support an interaction between inflammation and coagulation in experimental colitis.

Latent and polymeric antithrombin: clearance and potential thrombotic risk

Antithrombin, the most potent anticoagulant in vivo, displays a significant conformational flexibility. The native five-stranded anticoagulant form transforms under different conditions or mutations to inactive six-stranded conformations: latent or polymer. However, the function, potential deleterious effects, and clearance of these forms are not completely known. The dimerization of latent antithrombin with a native molecule has been suggested to have thrombotic potential. We have assessed the potential thrombogenicity of high amounts of latent and polymeric antithrombin by experiments performed in mice and human plasma. Moreover, we have analyzed the clearance of (125)I-labeled native, latent, polymer, and thrombin-complexed antithrombins in rat, as well as the clearance of latent antithrombin from plasma of patients treated with commercial concentrates. Our results show that high plasma levels of latent or polymeric antithrombin do not interfere with the anticoagulant function of native antithrombin. Moreover, we confirm that all monomeric forms of antithrombin have similar turnover. Finally, we show that polymers have the longest half-life of all conformers, being in circulation for prolonged periods of time. In conclusion, our data support that latent and polymeric antithrombin would not likely have a thrombotic effect, thus dispelling doubts about the potential harmful effect of latent antithrombin present in commercial concentrates for therapeutic use. Moreover, the suggested antiangiogenic role of latent antithrombin, together with its stability in plasma and its negligible thrombogenicity raises the possibility of its use as a new antiangiogenic drug.

Shape-shifting serpins – advantages of a mobile mechanism

Serpins use an extraordinary mechanism of protease inhibition that depends on a rapid and marked conformational change and causes destruction of the covalently linked protease. Serpins thus provide stoichiometric, irreversible inhibition, and their dependence on conformational change is exploited for signalling and clearance. The regulatory advantages provided by structural mobility are best illustrated by the heparin activation mechanisms of the plasma serpins antithrombin and heparin cofactor II. This mechanistic complexity, however, renders serpins highly susceptible to disease-causing mutations. Recent crystal structures reveal the intricate conformational rearrangements involved in protease inhibition, activity modulation and the unique molecular pathology of the remarkable shape-shifting serpins.

Allosteric activation of antithrombin is independent of charge neutralization or reversal in the heparin binding site

We investigate the hypothesis that heparin activates antithrombin (AT) by relieving electrostatic strain within helix D. Mutation of residues K125 and R129 to either Ala or Glu abrogated heparin binding, but did not activate AT towards inhibition of factors IXa or Xa. However, substitution of residues C-terminal to helix D (R132 and K133) to Ala had minimal effect on heparin affinity but resulted in appreciable activation. We conclude that charge neutralization or reversal in the heparin binding site does not drive the activating conformational change of AT, and that the role of helix D elongation is to stabilize the activated state.

Antithrombin Abnormalities and Perinatal Management

Antithrombin (AT) is an important regulator of the coagulation cascade because of its ability to efficiently inhibit proteases such as Factor (F) Xa and thrombin. Type I hereditary AT deficiency is characterized by a quantitative deficiency in the antigen and activity of AT to about 50% of normal. Type II hereditary AT deficiency is characterized by a normal antigenic level of AT, with a low level of activity due to a dysfunctional protein. Impaired synthesis, consumptive coagulopathy including pregnancy-induced AT deficiency in multiple pregnancies, and urinary protein loss are associated with acquired AT deficiencies. Inherited thrombophilias are the leading cause of maternal thromboembolism and are associated with increased risk of second- and third-trimester fetal loss, abruptions, severe intrauterine growth restriction, and early-onset severe preeclampsia. Among thrombophilias, AT deficiency has long been associated with a significant thrombotic tendency throughout gestation and the puerperium. Treatment for this disorder includes antithrombotic therapy with unfractionated heparin or low molecular weight heparin, followed by an oral vitamin K antagonist, such as warfarin. Some patients with very low AT levels may be resistant to heparin therapy and may require increased doses of heparin or AT concentrates. In addition, an acquired decrease of AT plasma levels is a common finding in patients with preeclampsia. It is suggested that the administration of AT concentrates improves uteroplacental circulation and influence the pathophysiology of preeclampsia. Furthermore, it has been demonstrated that hereditary AT deficiency is associated with fetal loss. In women with a severe thrombotic tendency and recurrent fetal loss, thromboprophylaxis may offer more benefits.

The Factor IXa Heparin-Binding Exosite Is a Cofactor Interactive Site: Mechanism for Antithrombin-Independent Inhibition of Intrinsic Tenase by Heparin

Therapeutic heparin concentrations selectively inhibit the intrinsic tenase complex in an antithrombin-independent manner. To define the molecular target and mechanism for this inhibition, recombinant human factor IXa with alanine substituted for solvent-exposed basic residues (H92, R170, R233, K241) in the protease domain was characterized with regard to enzymatic activity, heparin affinity, and inhibition by low molecular weight heparin (LMWH). These mutations only had modest effects on chromogenic substrate hydrolysis and the kinetics of factor X activation by factor IXa. Likewise, factor IXa H92A and K241A showed factor IXa-factor VIIIa affinity similar to factor IXa wild type (WT). In contrast, factor IXa R170A demonstrated a 4-fold increase in apparent factor IXa-factor VIIIa affinity and dramatically increased coagulant activity relative to factor IXa WT. Factor IXa R233A demonstrated a 2.5-fold decrease in cofactor affinity and reduced ability to stabilize cofactor half-life relative to wild type, suggesting that interaction with the factor VIIIa A2 domain was disrupted. Markedly (R233A) or moderately (H92A, R170A, K241A) reduced binding to immobilized LMWH was observed for the mutant proteases. Solution competition demonstrated that the EC(50) for LMWH was increased less than 2-fold for factor IXa H92A and K241A but over 3.5-fold for factor IXa R170A, indicating that relative heparin affinity was WT > H92A/K241A > R170A >> R233A. Kinetic analysis of intrinsic tenase inhibition demonstrated that relative affinity for LMWH was WT > K241A > H92A > R170A >> R233A, correlating with heparin affinity. Thus, LMWH inhibits intrinsic tenase by interacting with the heparin-binding exosite in the factor IXa protease domain, which disrupts interaction with the factor VIIIa A2 domain.

Enoxaparin

Enoxaparin (Clexane), Lovenox) is a low molecular weight heparin (LMWH) that has been widely used in the prevention of venous thromboembolism (VTE) in surgical patients. More recently, with the recognition of the high incidence of VTE in acutely ill medical (nonsurgical) patients, enoxaparin has been evaluated for thromboprophylaxis in this patient population. Subcutaneous enoxaparin 40 mg once daily has shown efficacy in the short-term thromboprophylaxis of VTE in nonsurgical patients with severely restricted mobility due to acute illness in well controlled clinical trials. The drug is at least similar in efficacy to unfractionated heparin (UFH) and its pharmacological profile allows once-daily administration, in contrast to the twice- or three-times-daily administration required with UFH. The tolerability profile of enoxaparin is also similar to that of UFH, except that the incidences of local haematomas and increased liver enzymes are lower with enoxaparin. The optimal duration of prophylaxis in nonsurgical patients is currently being evaluated and the results of extended prophylaxis with enoxaparin evaluated in the EXCLAIM (EXtended CLinical prophylaxis in Acutely Ill Medical patients) trial are awaited with interest. Currently, short-term enoxaparin appears to provide a cost-effective treatment alternative to UFH for VTE prophylaxis in nonsurgical patients.

The Effect of Dilution on Plasma Coagulation Kinetics Determined by Thrombelastography Is Dependent on Antithrombin Activity and Mode of Activation

Hemodilution-associated hypercoagulability has been the focus of several investigations because significant morbidity and mortality have been associated with perioperative thrombophilia. Because most investigations implicate imbalances in procoagulant/anticoagulant activity as the etiology of hemodilution-associated hypercoagulability, we determined the effects of dilution on coagulation kinetics and clot strength with thrombelastography (TEG(R)). Control plasma (+/-celite activation) and antithrombin (AT)-deficient (<10% activity) plasma were diluted 0%, 10%, 20%, and 30% with saline. TEG(R) variables measured included time to clot initiation (reaction time, R), speed of clot propagation (angle, alpha), and clot strength (amplitude, A; or shear elastic modulus, G). Dilution of control plasma (10%-30%) resulted in a significant (P < 0.05) 16% decrease in R values, no change in alpha values, and decrease in A and G values. AT-deficient plasma had significantly smaller R values compared with control, and dilution did not change R values in AT-deficient plasma. Celite activation eliminated dilution-associated changes in R values in control plasma but resulted in linear decreases (R(2) = 0.88-0.96, P < 0.0001) in alpha, A, and G in response to dilution. Thus, our data indirectly support the concept that decreases in AT activity cause dilution-mediated hypercoagulability in plasma. Finally, celite activation permits quantification of dilution with TEG.

Antithrombin Reduces Compression-Induced Spinal Cord Injury in Rats

Antithrombin (AT), a natural anticoagulant, has been shown to exert anti-inflammatory activity by promoting the endothelial production of prostaglandin I2 (PGI2), thereby reducing tissue injury. To examine whether AT prevents post-traumatic spinal cord injury (SCI), a pathologic condition in which activated neutrophils are critically involved, we tested the effect of AT on SCI induced by compression trauma in rats. Intravenous administration of AT, either before or after the induction of SCI, significantly reduced SCI-related motor disturbances in these animals. AT also significantly inhibited both intramedullary hemorrhage and the decrease in the number of motor neurons following SCI, and inhibited the accumulation of neutrophils in the damaged segment of the spinal cord by inhibiting the increase in transcription of tumor necrosis factor-alpha (TNF-alpha). AT significantly enhanced the increase in the tissue level of 6-keto-PGF1alpha, a stable metabolite of PGI2, at the injured segment of the cord. These therapeutic effects of AT may not depend on its anticoagulant effect. AT did not show any effects in animals pretreated with indomethacin, a potent inhibitor of prostaglandin synthesis, and iloprost, a stable PGI2 analog, produced effects similar to those of AT. Furthermore, intravenously administered AT accumulated selectively at the injured segment of the spinal cord, where thrombin generation might be increased. These findings suggest that AT may reduce the effects of compression trauma-induced SCI by inhibiting neutrophil activation as a consequence of the AT-mediated inhibition of TNF-alpha production. Such therapeutic effects of AT might be mediated by its promoting the endothelial release of PGI2. These findings strongly suggest AT as a potential agent for treating SCI in the clinical setting.

Antithrombin reduces the ischemia/reperfusion-induced spinal cord injury in rats by attenuating inflammatory responses

Antithrombin (AT) reveals its antiinflammatory activity by promoting endothelial release of prostacyclin (PGI(2)) in vivo. Since neuroinflammation is critically involved in the development of ischemia/reperfusion (I/R)-induced spinal cord injury (SCI), it is possible that AT reduces the I/R-induced SCI by attenuating the inflammatory responses. We examined this possibility using rat model of I/R-induced SCI in the present study. AT significantly reduced the mortality and motor disturbances by inhibiting reduction of the number of motor neurons in animals subjected to SCI. Microinfarctions of the spinal cord seen after reperfusion were markedly reduced by AT. AT significantly enhanced the I/R-induced increases in spinal cord tissue levels of 6-keto-PGFIalpha, a stable metabolite of PGI2. AT significantly inhibited the I/R-induced increases in spinal cord tissue levels of TNF-alpha, rat interleukin-8 and myeloperoxidase. In contrast,Trp(49) -modified AT did not show any protective effects. Pretreatment with indomethacin significantly reversed the protective effects of AT. An inactive derivative of factor Xa, which selectively inhibits thrombin generation, has been shown to fail to reduce SCI. Taken together, these observations strongly suggested that AT might reduce I/R-induced SCI mainly by the antiinflammatory effect through promotion of endothelial production of PGI(2). These findings also suggested that AT might be a potential neuroprotective agent.

Infection/inflammation and hemostasis

The association between coagulation and inflammation is a two-way process. Inflammation promotes coagulation, but key hemostatic proteins can promote inflammation. This two-way process creates a vicious cycle on a downward spiral to vascular injury, organ dysfunction, and death that are often the outcome of severe sepsis. Improved understanding of the basic mechanisms underlying the interaction between infection, inflammation, and coagulation has led to the development of several potential new agents for the treatment of severe sepsis. Among these agents, recombinant activated protein C has been shown to improve outcome, which is an exciting development in the search for effective treatments for patients with severe sepsis.

[Coagulation inhibitors in severe sepsis: state of the art]

OBJECTIVE

To present and discuss the rationale and the results of clinical trials using supplementation with physiologic anticoagulants (Tissue Factor Pathway Inhibitor (TFPI), AntiThrombin (AT), and Protein C (PC) in patients with severe sepsis.

RATIONALE

An early activation of the coagulation cascade occurs in severe sepsis. TFPI, AT, and PC are major inhibitors of the coagulation cascade, and additionally modulate inflammatory and vascular reactions. They are consumed or inhibited in the sepsis pathologic process. Therapeutic supplementation with these inhibitors could improve the sepsis-induced organ failures and mortality.

CLINICAL RESULTS

Randomized controlled studies were recently completed. No effect on the mortality rate could be documented after treatment with recombinant TFPI. AT concentrates neither improve mortality, but a biological interaction with heparin therapy could have biased the study results. Treatment with recombinant activated PC (alpha-drotrecogin) was associated with a significant reduction in the mortality rate of severely ill patients and received recently the approval from FDA and EC authorities in this indication. An increase in the rate of hemorrhagic adverse effects has been observed with these compounds, justifying a strict observance of contraindications and of patients selection. PROSPECTIVE: Additional studies are needed to give confirmation of the positive effects of activated PC supplementation in less severely ill patients, children and specific clinical situations. The effects of new anticoagulant compounds are currently evaluated in preclinical studies.

[The Choay domain -- the structure responsible for the anticoagulant action of heparins]

We describe the common structural basis for the anticoagulant action of the many different heparins available to the clinician. From different types of heparin we prepared fractions of virtually single molecular weight. We determined the molar concentration of material (HAM) containing the antithrombin (AT) binding pentasaccharide (A-domain), the specific catalytic activity in thrombin- and factor Xa inactivation and the capacity to inhibit thrombin generation (TG). We also calculated the molar concentration of A-domain with 12 sugar units at its non-reducing end, i.e. the structure that carries anti-thrombin activity (Choay- or C-domain). The anti-thrombin activity and the effects on TG are determined by the concentration of C-domain and independent of the source material or Mr. High Mr fractions (> 15,000) are less active, probably through interaction with non-AT plasma proteins. Anti-factor Xa activity is not indicative of anticoagulant potency but is a sensitive indicator of Mr and therefore predicts favourable pharmacokinetic properties: long half-life in the circulation and high bioavailability.

The nontoxic mushroom Auricularia auricula contains a polysaccharide with anticoagulant activity mediated by antithrombin

An acidic polysaccharide with anticoagulant activity was isolated from the edible mushroom Auricularia auricula using water, alkali or acid extracts. The alkali extract showed the highest anticoagulant activity and was thereby further purified using gel filtration chromatography. Specific anticoagulant activity of the purified polysaccharide was 2 IU/mg and its average mass was approximately 160 kDa. The polysaccharide from this species of mushroom contains mainly mannose, glucose, glucuronic acid and xylose but no sulfate esters. Its anticoagulant activity was due to catalysis of thrombin inhibition by antithrombin but not by heparin cofactor II. Inhibition of Factor Xa by antithrombin was not catalyzed by the polysaccharide. The glucuronic acid residues were essential for the anticoagulant action of the mushroom polysaccharide since the activity disappeared after reduction of its carboxyl groups. In ex vivo tests using rats orally fed with the polysaccharide, we observed an inhibitory effect on platelet aggregation as observed with aspirin, a well-known antiplatelet agent. The polysaccharides from these mushrooms may constitute a new source of compounds with action on coagulation, platelet aggregation and, perhaps, on thrombosis.

Heparin activates antithrombin anticoagulant function by generating new interaction sites (exosites) for blood clotting proteinases

The plasma protein, antithrombin, and its polysaccharide activator, heparin, are essential anticoagulant regulators of blood clotting proteinases that are critical for maintaining hemostasis. Heparin activates antithrombin both by inducing conformational changes in the protein that specifically enhances factor Xa binding and by providing a surface to promote thrombin or factor Xa binding alongside antithrombin in a ternary bridging complex. Although x-ray structures of antithrombin, free and complexed with heparin, have suggested that exposure of a reactive proteinase binding loop is a key feature of conformational activation, mutagenesis of reactive loop residues indicates that the function of this structural change is not to present an optimal loop sequence to target clotting proteinases. Rather, the reactive loop sequence provides only the minimal requirements for recognition by either thrombin or factor Xa, and heparin activation enhances antithrombin recognition by these proteinases through the presentation of exosites outside of the reactive loop. These and other findings suggest that the reactive loop sequence of antithrombin was designed not for optimal recognition by procoagulant proteinases but rather to prevent recognition by the anticoagulant proteinase, activated protein C, thus ensuring that antithrombin functions as an effective anticoagulant.

Antithrombin deficiency: issues in laboratory diagnosis

OBJECTIVE

To review the current understanding of the pathophysiology of antithrombin deficiency and its role in congenital thrombophilia. Recommendations for diagnostic testing of antithrombin function and concentration, derived from the medical literature and consensus opinions of recognized experts in the field, are included. These recommendations specify whom, how, and when to test.

DATA SOURCES

Review of the published medical literature.

DATA EXTRACTION AND SYNTHESIS

A summary of the medical literature and proposed testing recommendations were prepared and presented at the College of American Pathologists Conference XXXVI: Diagnostic Issues in Thrombophilia. After discussion at the conference, consensus recommendations presented in this article were accepted after a two-thirds majority vote by the participants.

CONCLUSIONS

Antithrombin deficiency is an infrequent genetic abnormality that may be a significant contributing cause of thrombophilia. Antithrombin deficiency also may be observed in conjunction with other genetic or acquired risk factors. Assay of antithrombin plasma levels is appropriate in the laboratory evaluation of individuals with thrombophilia, preferably using a functional, amidolytic antithrombin assay. The diagnosis of antithrombin deficiency should be established only after other acquired causes of antithrombin deficiency, such as liver disease, consumptive coagulopathy, or heparin therapy, are excluded. A low antithrombin level should be confirmed with a subsequent assay on a fresh specimen, and family studies may be helpful to establish the diagnosis. Antigenic antithrombin assays may be of benefit in subclassification of the type of antithrombin deficiency and to confirm the decreased antithrombin level in patients with type I deficiency.

The anti-inflammatory actions of antithrombin--a review

Leukocyte-endothelial cell interaction and microvascular perfusion failure are characteristic deteriorations of the microcirculation in endotoxaemia and are known to play a crucial role in the development of septic multiple organ dysfunction. Recent studies have indicated that antithrombin III treatment is capable of significantly ameliorating these microcirculatory disorders. Endothelial cells have important anticoagulant systems, including the heparan sulfate-antithrombin system. Antithrombin III stimulates prostacyclin generation in endothelial cells by interacting with heparan sulfate of endothelial cells and inhibits cytokine and tissue factor production in endothelial cells and monocytes. Similar mechanisms may be involved in cellular actions of antithrombin III causing desensitization of chemoattractant receptors of leukocytes by activating the heparan sulfate proteoglycan, syndecan-4. Thus, antithrombin III might be among the useful agents for treating coagulation abnormalities associated with sepsis or other inflammation because it inhibits not only coagulation but also downregulation of anticoagulant activities of endothelial cells and affects leukocyte activation.

Antithrombin, heparin, and heparan sulfate

OBJECTIVES

To review the experimental and clinical evidence that antithrombin has multiple mechanisms for both its anticoagulant and anti-inflammatory properties. The interaction between antithrombin and specific polysulfated, acidic oligosaccharide moieties found on heparin and related proteoglycan molecules within the circulation and on endothelial surfaces will also be examined.

DATA SOURCES

Review of the literature relating to antithrombin published during the past 25 yrs.

DATA SUMMARY

Antithrombin is the most abundant endogenous anticoagulant circulating in human plasma. This serine protease inhibitor participates in the regulation of clotting in both physiologic and pathologic states. Reduced antithrombin activity in the early phases of sepsis contributes to a procoagulant state with excess activation of the innate immune response. Antithrombin binds to specific pentasaccharides expressed on heparin, glycosaminoglycans, and related proteoglycans within the circulation and along endothelial surfaces. The functions of neutrophils, monocytes, and endothelial cells are altered as a result of their interaction with antithrombin. These effects are mediated by the enzyme inhibitory action of antithrombin and its ability to function as a ligand for antithrombin receptors on cell surfaces. In addition, antithrombin exerts anti-inflammatory properties by both prostacyclin-dependent and prostacyclin-independent actions; heparin interferes with these anti-inflammatory properties. The role of antithrombin in sepsis, its therapeutic utility in severe sepsis, and its combination with heparin remain the subject of considerable debate. The results of a recent phase 3 clinical trials with high-dose antithrombin in sepsis suggested a beneficial effect in patients who did not concomitantly receive heparin, thereby generating new challenges in the understanding of interactions between antithrombin and heparin or heparin-like proteoglycans.

CONCLUSIONS

Antithrombin has complex interactions with host coagulopathic and systemic inflammatory responses under physiologic conditions and in sepsis. The impact of these interactions in critically ill patients and the therapeutic implications of administration of antithrombin, and various doses and types of heparin in such patients, need further clarification.

Physiological function of tissue factor pathway inhibitor and interaction with heparins

Tissue factor pathway inhibitor (TFPI) is now recognized as a major physiological anticoagulant. Its main role is to modulate factor VIIa/tissue factor catalytic activity. Another important role is to potentiate the effect of heparins. TFPI is released from the vascular endothelium after injection of either unfractionated heparin (UFH) or low-molecular-weight heparins (LMWHs), which may then provide high concentrations of TFPI at sites of tissue damage and ongoing thrombosis. In dilute prothrombin-time-based assays, released TFPI contributes approximately one-third to the anticoagulant effect of heparin, the remaining being accounted for by antithrombin. Released TFPI, but not plasma TFPI, contains the basic carboxy-terminal tail which is important for the anticoagulant effect. UFH and LMWH exert differential effects on intravascular TFPI. UFH, but not LMWH, given in therapeutic doses, is associated with a progressive depletion of TFPI, which is associated with a strong rebound activation of coagulation after cessation of treatment. Such depletion may explain the apparent superior efficacy of LMWH observed in clinical trials.

Role of coagulation inhibitors in inflammation

It is becoming increasingly clear that coagulation augments inflammation and that anticoagulants, particularly natural anticoagulants, can limit the coagulation induced increases in the inflammatory response. The latter control mechanisms appear to involve not only the inhibition of the coagulation proteases, but interactions with the cells that either generate anti-inflammatory substances, such as prostacyclin, or limit cell activation. Recent studies have demonstrated a variety of mechanisms by which coagulation, particularly the generation of thrombin, factor Xa and the tissue factor-factor VIIa complex, can augment acute inflammatory responses. Many of these responses are due to the activation of one or more of the protease activated receptors. Activation of these receptors on endothelium can lead to the expression of adhesion molecules and platelet activating factor, thereby facilitating leukocyte activation. Therefore, anticoagulants that inhibit any of these factors would be expected to dampen the inflammatory response. The three major natural anticoagulant mechanisms seem to exert a further inhibition of these processes by impacting cellular responses. Antithrombin has been shown in vitro to increase prostacyclin responses and activated protein C has been shown to inhibit a variety of cellular responses including endotoxin induced calcium fluxes in monocytes and the nuclear translocation of NFKB, a key step in the generation of the inflammatory response. In some, but not all, in vivo models, these natural anticoagulants have been able to inhibit endotoxin/E. coli-mediated leukocyte activation and to diminish cytokine elaboration (TNF, IL-6 and IL-8). Phase III clinical studies for treatment of patients with severe sepsis have been completed for APC, which was successful (1), and for antithrombin, which was not (2). A phase III trial with tissue factor pathway inhibitor is in progress. In this review, the mechanisms by which the different natural anticoagulants are thought to function will be reviewed.

A new global test for the evaluation of the activated factor II-antithrombin system

We developed a new simple test to evaluate the global function of the activated factor II-antithrombin system. The new test measures the clotting time of plasma samples after the addition of a reagent containing a snake venom (Echis carinatus) that can activate prothrombin, with (Ta) and without (To) heparin. The prolongation of clotting times (Ta - To) is directly related to the function of the activated factor II-antithrombin system. The presence of quantitative or functional defects of the natural inhibitors (antithrombin and heparin co-factor II), or high levels of factor II and/or fibrinogen, can trigger a resistance to the inhibition of activated factor II. This new test was used to examine 134 thrombophilic patients as well as 157 normal subjects as controls. The results obtained confirm that the presence of abnormalities relating to the activated factor II-antithrombin system causes a resistance to activated factor II inhibition even if a significant number of patients was found to have a resistance that could not be accounted for. Since the new test can be easily performed automatically and has a good inter- and intra-assay variation coefficient (CV < 4%) it is useful for evaluating the global function of the activated factor II-antithrombin system in screening thrombophilic patients, alongside the tests already known and used to diagnose these patients.

Antithrombotic and fibrinolytic system of human endothelial cells seeded on PTFE: the effects of surface modification of PTFE by ammonia plasma treatment and ECM protein coatings

The aim of this study was to determine the effects of ECM protein coatings and surface modification of PTFE on the ability of seeded human endothelial cells (EC) to secrete prostacyclin (PGI2), plasminogen inhibitor-1 (PAI-1) and tissue plasminogen activator (t-PA). PTFE surfaces were modified by a novel surface modification technique based on ammonia plasma. Fibronectin, collagen type-1 and gelatin-coated ammonia plasma modified PTFE and unmodified PTFE surfaces were employed and compared in this study. All ammonia plasma modified surfaces showed similar secretions of PGI2 compared to non-modified PTFE surfaces. With the exception of gelatin-coated modified PTFE, seeded EC seeded on all modified PTFE showed lower levels of PAI-1 secretion compared to those seeded on unmodified PTFE. The specific activity of t-PA secreted by EC seeded on ammonia plasma modified and fibronectin coated modified PTFE showed increases of 100 and 30%, respectively, when compared to their unmodified counterparts. Our studies show that EC seeded on modified PTFE have ability to secrete PGI2 that modulates the early phase of thrombus formation. Furthermore, superior t-PA profile, along with lower levels of PAl-1 suggest that ammonia plasma modification and use of appropriate ECM proteins can modulate antithrombotic and fibrinolytic properties of in vitro endothelialized vascular prostheses. Accordingly, these surfaces may be suitable to further develop protocols and other strategies for arterial and venous reconstruction.

Fibrin clot formation and lysis: basic mechanisms

The hemostatic balance, introduced more than 40 years ago, addresses the components and reactions involved in fibrin turnover. Fibrin is placed in the core of this delicate balance. Defects in the mechanisms responsible for fibrin turnover might lead to thrombosis or bleeding, and fibrin consequently is an important substrate in the physiology of hemostasis. This review describes the components and processes involved in fibrin formation and fibrin degradation. Particular emphasis is put on the reactions involved in the conversion of fibrinogen to fibrin, the polymerization of fibrin molecules induced by coagulation factor XIII (FXIII), and the degradation of fibrinogen and fibrin mediated by plasmin and elastase. Furthermore, factors influencing fibrin structure and fibrin breakdown are addressed; in particular polymorphisms in the genes coding for fibrinogen and FXIII, but also the physical and biochemical conditions in which fibrin is formed. The past decades have produced a bulk of biochemical publications reviewing fibrin turnover and fibrin structure, and it has been shown that alterations in fibrin structure are important for the development of various disease conditions, whereas, the architecture of fibrin can be modified by certain drugs and chemical compounds. However, these topics deserve increased attention in clinical settings. Of particular importance might be more detailed clinical studies that review the influence of polymorphisms in the genes coding for the key factors involved in fibrin metabolism on the development of hemostatic diseases, but also the role of elastase-induced fibrin degradation deserves increased attention.

Low preoperative antithrombin activity causes reduced response to heparin in adult but not in infant cardiac-surgical patients

We evaluated the interaction of preoperative antithrombin (AT) activity and intraoperative response to heparin in cardiac surgery. Heparin anticoagulation is essential during cardiopulmonary bypass (CPB). Heparin itself has no anticoagulant properties, however it causes a conformational change of the physiologic plasma inhibitor AT that converts this slow-acting serine protease inhibitor into a fast acting one. Thus, adequate AT activity is a prerequisite for sufficient heparin anticoagulation. AT activity is reduced by long-term heparin therapy. This prospective, observational study investigated 1516 consecutive cardiac-surgical patients (1304 patients >1 yr (Group A) and 212 patients < or = 1 yr (Group I)). AT activity was measured the day before surgery by a chromogenic substrate assay. The celite-activated activated clotting time (ACT) was used to guide intraoperative heparin administration. Heparin sensitivity was calculated and the postoperative blood loss and perioperative blood requirement was recorded. Infant patients had significantly less preoperative AT activity compared with older patients: 84 (33)% vs 97 (17)%, median (interquartile range) (P < 0. 05). The subgroup of patients aged <1 mo (n = 64) demonstrated a preoperative AT activity of 56 (27)% as compared with 90 (23)% in infant patients between one month and one year (n = 148). In adult patients, preoperative AT activity depended predominantly on preoperative heparin treatment: 62% of the patients with an AT activity <80% were pretreated with heparin. Five minutes after heparin but before CPB the ACT was 587 (334) s in Group A patients with AT activity > or = 80%, and 516 (232) in patients with AT activity < or = 80% (P < 0.05). The target ACT of 480 s was achieved in 70% of patients with normal AT activity in Group A compared with only 54% of patients with AT activity <80% (P < 0.05). In Group A patients with decreased AT activity, 18% demonstrated an inadequate ACT response-defined as ACT <400 s-to the first bolus injection of heparin. In Group I, preoperative AT activity did not influence the ACT response (ACT 5 min after heparin: 846 [447] s in patients with AT activity > or = 80% vs 1000 [364] s in patients with decreased AT activity). The heparin sensitivity was 2.4 (1.1) s/unit heparin/kg compared with 1.5 (0.8) s/unit heparin/KG in group A (P < 0.05). These results suggest that preoperative diminished AT activity causes reduced response to heparin in adult but not in infant patients. Infant patients demonstrate a higher heparin sensitivity despite lower preoperative AT activity. Measurement of preoperative AT activity identifies adult patients at risk of reduced sensitivity to heparin.

IMPLICATIONS

In patients less than one year of age, low antithrombin (AT) activity is caused by the immature coagulation system. Despite low AT activity, these young patients demonstrate a normal or increased response to heparin anticoagulation before cardiopulmonary bypass (CPB). In contrast, in patients older than one year of age and adult patients decreased preoperative AT activity is mainly caused by preoperative heparin therapy and causes insufficient response to heparin anticoagulation with a standard heparin dosage. Measurement of preoperative AT activity identifies patients at risk of inadequate anticoagulation during CPB.

[Endothelial cells and coagulation abnormalities]

Endothelial cells have two important anticoagulant systems, heparan sulfate-antithrombin system and thrombomodulin-protein C system. Under physiological conditions, these two systems inhibit activation of coagulation on endothelial cells. However, under inflammatory conditions, tumor necrosis factor(TNF)-alpha or other cytokines produced by monocytes reduce the anticoagulant properties of endothelial cell by downregulating expression of heparan sulfate and thrombomodulin on endothelial cells. Antithrombin stimulates prostacyclin generation from endothelial cells by interacting with heparan sulfate of endothelial cells and generated prostacyclin inhibits TNF-alpha production by monocytes. Activated protein C inhibits TNF-alpha production by monocyte dependent of its protease activity. Thus, antithrombin and activated protein C might inhibit the endothelial perturbation induced by cytokines. Antithrombin regulates TNF-alpha induced tissue factor expression on endothelial cells by an unknown mechanism. Thus, antithrombin and activated protein C might be useful agents for treating coagulation abnormalities associated with sepsis or other inflammation because these agents inhibit not only coagulation but also downregulation of anticoagulant activities of endothelial cells.

On the coagulation of platelet-rich plasma. Physiological mechanism and pharmacological consequences

Thrombin formation and blood platelet reactions are intimately linked in haemostasis and in thrombosis. In vivo, procoagulant phospholipids required for the coagulation mechanism are mainly provided by activated platelets, and thrombin is the most potent platelet activator. To study these interactions, an ancient tool of coagulation physiology, the thrombin generation test, was revived and the results obtained were reviewed. The amount of thrombin activity that develops, expressed as the endogenous thrombin potential (the area under the thrombin generation curve), is influenced by the clotting factors (except XII and XIII), the activated protein C system and natural inhibitors on the one hand and by platelet activity on the other. The platelet reactions that we found to be involved are induced by thrombin via glycoprotein (GP) IIb/IIIa activation and by fibrin via interaction with GPIb. von Willebrand factor is crucial in both reactions and therefore an obligatory factor for normal thrombin generation in the presence of platelets. All antithrombotics, be it anticoagulants (e.g. OAC, all heparins or hirudin) or antiplatelet drugs (aspirin, GPIIb/IIIa blockers) diminish thrombin generation.

[Procoagulant nature of fibrin]

The main threat from a beginning thrombus is that it tends to grow, and hence become occlusive and/or embolise. Although the progressive nature of thrombi has been recognised since a long time, the mechanisms behind thrombus growth remain only partially resolved. In order to investigate in what ways thrombi can themselves become foci of further thrombin -and hence fibrin-formation, we studied the effect of fibrin clots on thrombin generation in platelet poor--and platelet rich plasma (PPP and PRP). The thrombin always adsorbed on a natural fibrin clot is not inactivated by plasmatic antithrombins and could be shown to retain its ability to enhance further thrombin formation by activation of clotting factors V and VIII as well as of blood platelets. To our surprise, fibrin clots without any active thrombin adsorbed, because they were obtained by a snake-venom enzyme or because thrombin had been inhibited, retained their capacity to activate blood platelets and make them procoagulant. The activation could be shown to be due to a rearrangement of cell-membrane phospholipids, by which the procoagulant species (phosphatidyl serine and phosphatidyl ethanolamine) became available at the outer cell surface. The platelet membrane receptor involved could be recognised as glycoprotein Ib, interacting with fibrin through the plasma protein von Willebrand factor (vWf). In fact it appeared that vWf is indispensable for the generation of thrombin in PRP, with or without added clot. This assigns a new and hitherto unknown role to vWf. Our results also show that fibrin is far from being the inert end-product of coagulation but is a potent activator of blood platelets and by this action may foster thrombin generation and hence further fibrin production. We surmise this mechanism to be instrumental in the progression of thrombotic processes.

[Natural and synthetic inhibitors of thrombin. I. Natural inhibitors]

Structures and properties of main physiological (heparin, antithrombin III, heparin cofactor II) and nonphysiological (hirudin, thrombin-binding aptamers, cyclotheonamides) natural thrombin inhibitors and its fragments and synthetic analogs (hirugen, hirulogs, hirunorms, pentasaccharides, macrocyclic alpha-keto amides) were reviewed. The molecular mechanisms of interaction of these compounds with thrombin and their anticoagulant activity at preclinical and clinical trials are discussed. The examined of natural thrombin inhibitors and their synthetic fragments and analogs are perspective for prophylaxis and treatment of different thrombo-embolic diseases.

[Natural and synthetic inhibitors of thrombin. II. Synthetic low-molecular-weight inhibitors]

The up-to-date problems, concerning the structure and properties of two types of inhibitors are reviewed. It is particularly considered properties of low-molecular weight thrombin inhibitors that have electrophilic groups capable to react with Ser-195 of thrombin (peptidyl-chloromethyl ketones, aldehydes, ketomethylene derivatives and derivatives of boric and phosphoric acids) and the competitive reversible thrombin inhibitors. The review focuses on methods of modification of the structure in the natural inhibitors and design of new peptidomimetics. The prospects for prophylaxis and treatment of diverse thromboembolic diseases are discussed.

Inactivation of papain by antithrombin due to autolytic digestion: a model of serpin inactivation of cysteine proteinases

Cross-class inhibition of cysteine proteinases by serpins differs from serpin inhibition of serine proteinases primarily in that no stable serpin-cysteine proteinase complex can be demonstrated. This difference in reaction mechanism was elucidated by studies of the inactivation of the cysteine proteinases, papain and cathepsin L, by the serpin antithrombin. The two proteinases were inactivated with second-order rate constants of (1.6+/-0.1)x10(3) and (8.6+/-0. 4)x10(2) M-1.s-1 respectively. An antithrombin to papain inactivation stoichiometry of approximately 3 indicated extensive cleavage of the inhibitor concurrent with enzyme inactivation, a behaviour verified by SDS/PAGE. N-terminal sequence analyses showed cleavage predominantly at the P2-P1 bond, but also at the P2'-P3' bond of antithrombin. The papain band in SDS/PAGE progressively disappeared on reaction of the enzyme with increasing amounts of antithrombin, but no band representing a stable antithrombin-papain complex appeared. SDS/PAGE with 125I-labelled papain showed that the disappearance of papain was caused by cleavage of the enzyme into small fragments. These results suggest a mechanism in which papain attacks a peptide bond in the reactive-bond loop of antithrombin adjacent to that involved in serine proteinase inhibition. The reaction proceeds, similarly to that between serpins and serine proteinases, to form an inactive acyl-intermediate complex, although with the substrate pathway dominating in the papain reaction. In this complex, papain is highly susceptible to proteolysis and is degraded by still active papain, which greatly decreases the lifetime of the complex and results in liberation of fragmented, inactive enzyme. This model may have relevance also for the inactivation of physiologically or pathologically important cysteine proteinases by serpins.

The effect of antithrombin on the systemic inflammatory response in disseminated intravascular coagulation

Sepsis and major trauma are the two most common causes of disseminated intravascular coagulation (DIC) and are characterized by a sudden increase in inflammatory mediators. In general, the outcome of the patient is determined by the degree of the inflammatory response. In severe cases of sepsis and trauma, cascade systems, such as the coagulation, fibrinolytic and complement systems, are activated beyond the capacity of the autoregulatory mechanisms. During DIC, plasma levels of antithrombin (AT)--a serine protease inhibitor that acts mainly on the serine proteases of the coagulation system--decrease due to the formation and subsequent elimination of complexes between AT and activated coagulation factors. The consumption of AT may start a vicious circle by facilitating further intravascular fibrin formation, followed by ischaemic tissue injury and accelerated activation of blood coagulation. Infusion of AT has an anti-inflammatory effect through its ability to counteract microvascular thrombosis. Furthermore, AT induces the release of prostacyclin from the vessel wall by binding to glycosaminoglycans on the surface of endothelial cells. Prostacyclin has a marked anti-inflammatory effect as a result of its inhibitory effect on neutrophils, monocytes and platelets.

Tubular antithrombin at transplantation determines subsequent renal allograft function

BACKGROUND

Antithrombin is found in the microvasculature and tubules of normal and transplanted human kidneys. Although depletion of vascular antithrombin is associated with renal allograft dysfunction, neither the distribution nor clinical significance of tubular antithrombin is known.

METHODS

Changes in tubular antithrombin in biopsy specimens (n=41) obtained from donor kidneys at transplantation were studied immunohistochemically. The relationship between these changes and subsequent graft function was analyzed.

RESULTS

Granular intracellular antithrombin was found only within the proximal tubular epithelium. Allografts with depleted tubular antithrombin at transplantation (n=20) had significantly greater plasma creatinine concentrations at posttransplant days 3 (P < 0.001) and 5 (P < 0.03) than allografts with normal tubular antithrombin (n=21). Indeed, depletion of tubular antithrombin at transplantation correlated with the degree of graft dysfunction at 3 days after transplantation.

CONCLUSIONS

Depleted tubular antithrombin at transplantation is associated with reduced early graft function, and this may identify patients at risk of a complicated follow-up.

[The role of the endothelia in hemostasis]

Unless associated with endothelial lesions or dysfunction, the increased plasma levels of fibrinogen, clotting factors VII and X as well as of fibrin stabilizing factor XIII and of the inhibitors of fibrinolysis, displayed by hyperlipidemic patients, could explain neither the localization nor the mechanisms triggering thrombotic events. Actually, endothelia are provided with both prohemostatic factors (tissue factor, von Willebrand factor, platelet activating factor) and antithrombotic mechanisms (tissue factor pathway inhibitor, proteoglycans activating antithrombin III, thrombodulin activating the protein C system as well as prostacyclin inhibiting platelet aggregation). Endothelia also modulate fibrinolytic activity by producing both activators and inhibitors of plasminogen activation. Evidence was provided that proinflammatory cytokines, anion superoxide and oxidized LDL would cause an upregulation of prohemostatic mechanisms, while down regulating the antithrombotic ones. Attempts made to detect endothelial dysfunction by methods available to the clinical laboratory and author's own observations concerning the behaviour of endothelia-derived plasma von Willebrand factor in various pathological conditions are briefly presented.