Prevention of vascular graft occlusion and thrombus-associated thrombin generation by inhibition of factor XI

The protease thrombin is required for normal hemostasis and pathologic thrombogenesis. Since the mechanism of coagulation factor XI (FXI)-dependent thrombus growth remains unclear, we investigated the contribution of FXI to thrombus formation in a primate thrombosis model. Pretreatment of baboons with a novel anti-human FXI monoclonal antibody (aXIMab; 2 mg/kg) inhibited plasma FXI by at least 99% for 10 days, and suppressed thrombin-antithrombin (TAT) complex and beta-thromboglobulin (betaTG) formation measured immediately downstream from thrombi forming within collagen-coated vascular grafts. FXI inhibition with aXIMab limited platelet and fibrin deposition in 4-mm diameter grafts without an apparent increase in D-dimer release from thrombi, and prevented the occlusion of 2-mm diameter grafts without affecting template bleeding times. In comparison, pretreatment with aspirin (32 mg/kg) prolonged bleeding times but failed to prevent graft occlusion, supporting the concept that FXI blockade may offer therapeutic advantages over other antithrombotic agents in terms of bleeding complications. In whole blood, aXIMab prevented fibrin formation in a collagen-coated flow chamber, independent of factor XII and factor VII. These data suggest that endogenous FXI contributes to arterial thrombus propagation through a striking amplification of thrombin generation at the thrombus luminal surface.

Aptamers influence the hemostatic system by activating the intrinsic coagulation pathway in an in vitro Chandler-Loop model

One class of oligonucleotides with a high potential for use in medical applications is short nucleic acids, widely known as aptamers. Although several aptamers are already being used clinically, there are very few studies dealing with the impact aptamers have on the hemostatic system. In this study, we have performed a comprehensive evaluation of the hemostatic system including coagulation, platelets, complement, and inflammatory activation by using different aptamer concentrations and fresh human whole blood in a well-established flow model. We found that single-stranded aptamers did not have a negative influence on platelets, complement, or inflammation but were able to activate factor XII, kallikrein, and prothrombin in a concentration-dependent manner. Consequently, the influence of aptamers on the coagulation system should be taken into consideration before the use of any aptamer-based drugs in patients.

Hydroxyethyl starch induced acquired von Willebrand's disease

Hydroxyethyl starch (HES) (Hespan, DuPont) is a widely used synthetic volume expander which in standard doses of up to 1.5l in 24 h has no significant effect on coagulation (Munsch et al. 1988). However, the data sheet states that in large volumes HES may alter the coagulation mechanism. We now report a case of HES induced acquired von Willebrand's disease (vWD) in which severe bleeding occurred.

Phenotypic consequences of genetic variation at hemizygous alleles: Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency

PURPOSE

We tested the hypothesis that Sotos syndrome (SoS) due to the common deletion is a contiguous gene syndrome incorporating plasma coagulation factor twelve (FXII) deficiency. The relationship between FXII activity and the genotype at a functional polymorphism of the FXII gene was investigated.

METHODS

A total of 21 patients including those with the common deletion, smaller deletions, and point mutations, and four control individuals were analyzed. We examined FXII activity in patients and controls, and analyzed their FXII 46C/T genotype using direct DNA sequencing.

RESULTS

Among 10 common deletion patients, seven patients had lower FXII activity with the 46T allele of the FXII gene, whereas three patients had normal FXII activity with the 46C allele. Two patients with smaller deletions, whose FXII gene is not deleted had low FXII activity, but one patient with a smaller deletion had normal FXII. Four point mutation patients and controls all had FXII activities within the normal range.

CONCLUSION

FXII activity in SoS patients with the common deletion is predominantly determined by the functional polymorphism of the remaining hemizygous FXII allele. Thus, Sotos syndrome is a contiguous gene syndrome incorporating coagulation factor twelve (FXII) deficiency.

Role of zinc in hemostasis: a review

Zinc is a multi-functional element that is found in almost 300 enzymes where it performs catalytic, co-catalytic, and/or structural functions. In 1982, Gordon et al. (Am J Clin Ntr 35:849-857, 1982) found that a low zinc diet caused poor platelet aggregation and increased bleeding tendency in adult males. This fact drew interest to the role of zinc in blood clotting. It has been shown that hyperzincemia predisposes to increased coagulability, and hypozincemia to poor platelet aggregation and increased bleeding time. The blood clotting disturbances can be regressed by appropriate zinc intake management. Considering the importance of zinc as an essential element, its participation in regulation of the equilibrium between pro- and anti-thrombotic factors originating in platelets and endothelium prompted further investigations.

The plasma kallikrein-kinin system: its evolution from contact activation

The plasma kallikrein-kinin system consists of the proteins factor XII (FXII), prekallikrein (PK), and high molecular weight kininogen. It was first recognized as a surface-activated coagulation system that is activated when blood or plasma interacts with artificial surfaces. Although surface-activated contact activation occurs in vivo in the case of tissue destruction or a developing thrombus, the physiologic basis for the activation and function of this system has not been delineated. New investigations indicate that there is a proteolytic pathway on cells for PK activation independent of FXII. This pathway for PK with subsequent FXII activation indicates physiologic activities. These activities include blood pressure regulation and modulation of thrombosis risk independently of hemostasis. Furthermore, they include regulation of endothelial cell proliferation, angiogenesis and apoptosis through a cellular-based, outside-in signaling system. The present characterizations of this system, which incorrectly had been thought to initiate coagulation, represent an evolution of understanding in this field.

The dual role of the contact system in bacterial infectious disease

Hemostasis is a sensitive and tightly regulated process, involving the vascular endothelium and blood cells as well as factors of the coagulation and fibrinolytic cascades. Over the last four decades evidence has accumulated that during infection, inflammatory mediators from the microbe and/or host are capable to modulate the equilibrium between the procoagulant and anticoagulant status of the host. Dependent on the mode of activation, these changes can cause either local or systemic inflammatory reactions that may be beneficial or deleterious to the human host. The present review aims to present the state of the art with respect to the role of the contact system (also known as the intrinsic pathway of coagulation or the kallikrein/kinin system) in innate immunity and systemic inflammatory reactions.

Competitive-protein adsorption in contact activation of blood factor XII

Contact activation of blood factor XII (FXII, Hageman factor) is moderated by the protein composition of the fluid phase in which FXII is dissolved. Solution yield of FXIIa arising from FXII contact with hydrophilic activating particles (fully water-wettable glass) suspended in a protein cocktail is shown to be significantly greater than that obtained under corresponding activation conditions in buffer solutions containing only FXII. By contrast, solution yield of FXIIa arising from FXII contact with hydrophobic particles (silanized glass) suspended in protein cocktail is sharply lower than that obtained in buffer. This confirms that contact activation is not specific to anionic hydrophilic surfaces as proposed by the accepted biochemistry of surface activation. Rather, contact activation in the presence of proteins unrelated to the plasma coagulation cascade leads to an apparent specificity for hydrophilic surfaces that is actually due to a relative diminution of activation at hydrophobic surfaces and an enhancement at hydrophilic surfaces. Furthermore, the rate of FXIIa accumulation in whole-plasma and buffer solution is found to decrease with time in the continuous presence of activating surfaces, leading to a steady-state FXIIa yield dependent on the initial FXII solution concentration for both hydrophilic and hydrophobic procoagulant particles suspended in either plasma, protein cocktail, or buffer. These results strongly suggest that activation competes with an autoinhibition reaction in which FXIIa itself inhibits FXII-->FXIIa. Experimental results are modeled using a reaction scheme invoking FXII activation and autoinhibition linked to protein adsorption to procoagulant surfaces, where FXII activation is presumed to proceed by either autoactivation (FXII-->surface-->FXIIa) and autohydrolysis (FXII-->FXIIa-->2FXIIa) in buffer solution or autoactivation and reciprocal activation (kallikrein-mediated hydrolysis) in plasma. FXII adsorption competition with other proteins in the fluid phase is proposed to affect the balance of activation and autoinhibition, leading to the observed moderation of FXIIa yield.

Identification and characterization of plasma kallikrein-kinin system inhibitors from salivary glands of the blood-sucking insectTriatoma infestans

Two plasma kallikrein-kinin system inhibitors in the salivary glands of the kissing bug Triatoma infestans, designated triafestin-1 and triafestin-2, have been identified and characterized. Reconstitution experiments showed that triafestin-1 and triafestin-2 inhibit the activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII and prekallikrein, and subsequent release of bradykinin. Binding analyses showed that triafestin-1 and triafestin-2 specifically interact with factor XII and high molecular weight kininogen in a Zn2+-dependent manner, suggesting that they specifically recognize Zn2+-induced conformational changes in factor XII and high molecular weight kininogen. Triafestin-1 and triafestin-2 also inhibit factor XII and high molecular weight kininogen binding to negatively charged surfaces. Furthermore, they interact with both the N-terminus of factor XII and domain D5 of high molecular weight kininogen, which are the binding domains for biological activating surfaces. These results suggest that triafestin-1 and triafestin-2 inhibit activation of the kallikrein-kinin system by interfering with the association of factor XII and high molecular weight kininogen with biological activating surfaces, resulting in the inhibition of bradykinin release in an animal host during insect blood-feeding.

Evidence of a U-shaped association between factor XII activity and overall survival

INTRODUCTION

The clinical relevance of decreased coagulation factor XII (FXII) plasma activity as a risk factor for both venous and arterial thrombosis is still discussed controversially. The current study evaluated the predictive value of FXII levels for all-cause mortality in a large Viennese patient cohort.

PATIENTS AND METHODS

Individuals, whose FXII activity levels were determined for suspected coagulation disorders or thrombophilia screening between 1991-2003 were included in this study (n = 8936, 51% male, 49% female, median age 43 years). Death/survival was determined by record linkage with the Austrian Death Registry. The median observation period was 5 years covering a total of 46 400 person years; the death rate was 17.1%. For Cox regression analysis, FXII plasma activity was divided into 11 categories of 10% steps with the category of > 100% FXII serving as a reference category.

RESULTS

With decreasing FXII plasma activity, hazard ratios for all-cause mortality gradually increased linearly from 1.0 in the > 100% category to 1.5 (95% CI: 1.2-1.9) in the 80-90% category to 4.7 (95% CI: 3.4-6.5) in the 10-20% category. Similar results were obtained, when only vascular mortality or death as a result of ischemic heart disease was considered. No significant increase in all-cause mortality (HR: 1.4, 95%CI 0.7-2.8) was observed in the small group of FXII-deficient subjects [0-10% category (n = 58)].

CONCLUSIONS

This study first demonstrates a strong and almost linear association of FXII plasma activity between 90% and 10% with all-cause mortality in a large Viennese patient cohort. Interestingly, mortality rates are not increased when FXII activity is below 10%, resulting in a U-shaped survival curve.

Identification and characterization of a new kallikrein-kinin system inhibitor from the salivary glands of the malaria vector mosquito Anopheles stephensi

A new kallikrein-kinin system inhibitor, designated anophensin, was identified in the salivary glands of the malaria vector mosquito, Anopheles stephensi. In vitro reconstitution experiments showed that anophensin inhibits activation of the kallikrein-kinin system by inhibiting the reciprocal activation of factor XII (FXII) and prekallikrein (PK), and subsequent release of bradykinin. Additionally, anophensin inhibits activation of the kallikrein-kinin system on cultured human umbilical vein endothelial cells (HUVECs). Direct binding assays show that this inhibitory effect is due to Zn(2+)-dependent specific binding of anophensin to both FXII and high molecular weight kininogen (HK). Furthermore, anophensin interacts with both the N-terminus of FXII and domain D5 of HK, which are the binding domains for biological activating surfaces. These results suggest that anophensin inhibits activation of the kallikrein-kinin system by interfering with the association of FXII and HK with biological activating surfaces, resulting in the inhibition of bradykinin release in a host animal during insect blood-feeding.

Heparin modulation of human plasma kallikrein on different substrates and inhibitors

The interplay of different proteases and glycosaminoglycans is able to modulate the activity of the enzymes and to affect their structures. Human plasma kallikrein (huPK) is a proteolytic enzyme involved in intrinsic blood clotting, the kallikrein-kinin system and fibrinolysis. We investigated the effect of heparin on the action, inhibition and secondary structure of huPK. The catalytic efficiency for the hydrolysis of substrates by huPK was determined by Michaelis-Menten kinetic plots: 5.12x10(4) M-1 s-1 for acetyl-Phe-Arg-p-nitroanilide, 1.40x10(5) M-1 s-1 for H-D-Pro-Phe-Arg-p-nitroanilide, 2.25x10(4) M-1 s-1 for Abz-Gly-Phe-Ser-Pro-Phe-Arg-Ser-Ser-Arg-Gln-EDDnp, 4.24x10(2)M-1 s-1 for factor XII and 5.58x10(2) M-1 s-1 for plasminogen. Heparin reduced the hydrolysis of synthetic substrates (by 2.0-fold), but enhanced factor XII and plasminogen hydrolysis (7.7- and 1.4-fold, respectively). The second-order rate constants for inhibition of huPK by antithrombin and C1-inhibitor were 2.40x10(2) M-1 s-1 and 1.70x10(4) M-1 s-1, respectively. Heparin improved the inhibition of huPK by these inhibitors (3.4- and 1.4-fold). Despite the fact that huPK was able to bind to a heparin-Sepharose matrix, its secondary structure was not modified by heparin, as monitored by circular dichroism. These actions may have a function in the control or maintenance of some pathophysiological processes in which huPK participates.

Increased activity of coagulation factor XII (Hageman factor) causes hereditary angioedema type III

Hereditary angioedema (HAE) is characterized clinically by recurrent acute skin swelling, abdominal pain, and potentially life-threatening laryngeal edema. Three forms of HAE have been described. The classic forms, HAE types I and II, occur as a consequence of mutations in the C1-inhibitor gene. In contrast to HAE types I and II, HAE type III has been observed exclusively in women, where it appears to be correlated with conditions of high estrogen levels--for example, pregnancy or the use of oral contraceptives. A recent report proposed two missense mutations (c.1032C-->A and c.1032C-->G) in F12, the gene encoding human coagulation factor XII (FXII, or Hageman factor) as a possible cause of HAE type III. Here, we report the occurrence of the c.1032C-->A (p.Thr328Lys) mutation in an HAE type III-affected family of French origin. Investigation of the F12 gene in a large German family did not reveal a coding mutation. Haplotype analysis with use of microsatellite markers is compatible with locus heterogeneity in HAE type III. To shed more light on the pathogenic relevance of the HAE type III-associated p.Thr328Lys mutation, we compared FXII activity and plasma levels in patients carrying the mutation with that of healthy control individuals. Our data strongly suggest that p.Thr328Lys is a gain-of-function mutation that markedly increases FXII amidolytic activity but that does not alter FXII plasma levels. We conclude that enhanced FXII enzymatic plasma activity in female mutation carriers leads to enhanced kinin production, which results in angioedema. Transcription of F12 is positively regulated by estrogens, which may explain why only women are affected with HAE type III. The results of our study represent an important step toward an understanding of the molecular processes involved in HAE type III and provide diagnostic and possibly new therapeutic opportunities.

Procoagulant activity of surface-immobilized Hageman factor

Procoagulant activity of surface-immobilized coagulation factor XIIa (activated Hageman factor) is reported. Activity of FXIIa immobilized onto the surfaces of three silanized-glass procoagulants spanning a wide range of wettability was assayed in normal and FXII-deficient plasmas. Previously published mathematical models were used to characterize the procoagulant activity of protein-immobilized materials and soluble enzymes. Results show that FXIIa activity is unrelated to underlying procoagulant surface chemistry and is similar to soluble FXIIa activity. The uninfluential role of the surface on FXIIa suggests that the solid surface activates FXII in biomaterial-induced blood coagulation but is not otherwise involved in FXIIa activity as described by the classical mechanism.

Probing single-stranded DNA and its biomolecular interactions through direct catalytic activation of factor XII, a protease of the blood coagulation cascade

Triggered self-activation of factor XII, a blood coagulation protease, was utilized for the amplified visual detection of ss-DNA targets in a non-sequence specific way. Factor XII holds potential as a low-affinity and therefore non-interfering probe for DNA secondary structure and for the screening of protein binding to ss-DNA. The observation that ss-DNA also accelerates coagulation of human blood plasma is relevant to the emerging field of aptamer therapeutics.

Investigation of tissue factor and other hemostatic profiles in experimental hypothyroidism

The influence of thyroid failure on hemostasis has been studied and is still not well understood. These patients have high risk for cardiovascular diseases because of the lipid metabolism and procoagulant agents. But the influence of thyroid failure on hemostasis is controversial. Tissue factor (TF) has an important role in the thromboembolic state. Recent experiments have demonstrated that TF-dependent activation of the coagulation cascade plays an important role in the pathophysiology of intravascular thrombus formation. The purpose of the present study was to investigate the contributions of TF, factor VII:C (FVII:C), factor XII:C (FXII:C), and fibrinogen in experimental hypothyroidism. TF was obtained from the thyroid gland and lung tissue of 10 rats following experimental hypothyroidism induced for 30 d and compared with similar tissue from 10 control rats. Significantly increased TF activities were found in hypothyroid rats. By contrast, FVII:C level was significantly decreased when compared with the control group. In this respect it is interesting to note that a hypercoagulable state due to increased thromboplastic activity may occur. Based on those results, elevated tissue factor activities (TFa) of the patients with low thyroid dysfunction may have another risk factor for cardiovascular diseases.

Autoactivation of blood factor XII at hydrophilic and hydrophobic surfaces

Contact activation of blood factor XII (FXII, Hageman factor) in neat-buffer solution is shown not to be specific for anionic hydrophilic procoagulants as proposed by the accepted biochemistry of surface activation. Rather, FXII activation in the presence of plasma proteins leads to an apparent specificity for hydrophilic surfaces that is actually due to a relative diminution of the FXII-->FXIIa reaction at hydrophobic surfaces. FXII activation in neat-buffer solution was effectively instantaneous upon contact with either hydrophilic (fully water-wettable clean glass) or hydrophobic (poorly water-wettable silanized glass) procoagulant particles, with greater FXIIa yield obtained by activation with hydrophobic procoagulants. In sharp contrast, both activation rate and yield was found to be significantly attenuated at hydrophobic surfaces in the presence of plasma proteins. Putative FXIIa produced by surface activation with both hydrophilic and hydrophobic procoagulants was shown to hydrolyze blood factor XI (FXI) to the activated form FXIa (FXIFXIIa-->FXIa) that causes FXI-deficient plasma to rapidly coagulate.

Demonstration of a threshold response in a proteolytic feedback system: control of the autoactivation of factor XII

Mathematical analysis of positive feedback loops in proteolytic systems has previously suggested that when the active enzymes are subject to inhibitory control these systems will exhibit threshold behavior. This is demonstrated in the present study, for the autolytic activation of factor XII in the presence of a contact activator and an irreversible inhibitor of factor XIIa. The threshold between the two system states - complete factor XII activation, or complete stability - is dependent on the kinetic balance between the catalytic rate of autoactivation and rate of enzyme (factor XIIa) inhibition. Activation of the system can be brought about by either increasing the catalytic rate (in this study, by using more potent contact-activation conditions), or by lowering the enzyme inhibition rate. Previous mathematical work predicted complete stability in a positive-feedback system that is below threshold, and this has been experimentally confirmed.

Targeting coagulation factor XII provides protection from pathological thrombosis in cerebral ischemia without interfering with hemostasis

Formation of fibrin is critical for limiting blood loss at a site of blood vessel injury (hemostasis), but may also contribute to vascular thrombosis. Hereditary deficiency of factor XII (FXII), the protease that triggers the intrinsic pathway of coagulation in vitro, is not associated with spontaneous or excessive injury-related bleeding, indicating FXII is not required for hemostasis. We demonstrate that deficiency or inhibition of FXII protects mice from ischemic brain injury. After transient middle cerebral artery occlusion, the volume of infarcted brain in FXII-deficient and FXII inhibitor–treated mice was substantially less than in wild-type controls, without an increase in infarct-associated hemorrhage. Targeting FXII reduced fibrin formation in ischemic vessels, and reconstitution of FXII-deficient mice with human FXII restored fibrin deposition. Mice deficient in the FXII substrate factor XI were similarly protected from vessel-occluding fibrin formation, suggesting that FXII contributes to pathologic clotting through the intrinsic pathway. These data demonstrate that some processes involved in pathologic thrombus formation are distinct from those required for normal hemostasis. As FXII appears to be instrumental in pathologic fibrin formation but dispensable for hemostasis, FXII inhibition may offer a selective and safe strategy for preventing stroke and other thromboembolic diseases.

Are hemostasis and thrombosis two sides of the same coin?

Factor XII (FXII), a clotting enzyme that can initiate coagulation in vitro, has long been considered dispensable for normal blood clotting in vivo because hereditary deficiencies in FXII are not associated with spontaneous or excessive bleeding. However, new studies show that mice lacking FXII are protected against arterial thrombosis (obstructive clot formation) and stroke. Thus, FXII could be a unique drug target that could be blocked to prevent thrombosis without the side effect of increased bleeding.

Factor XII (Hageman factor) is a missing link between stress and hypercoagulability and plays an important role in the pathophysiology of ischemic stroke

A new hypothesis is presented on the function of factor XII, which is postulated to be a "missing link" between acute stress and transient hypercoagulability. The implications of this idea are developed to show how chronic stress, which involves activation of hypertension and migraine as well as hypercoagulability, can cause of cerebrovascular disease. "Acute stress" is defined as "the normal short-term physiological response to the perception of major threats or demands". "Chronic stress" is "the abnormal ongoing physiological response to the continuing perception of unresolvable major threats or demands". The factor XII hypothesis is as follows: Acute stress includes release of epinephrine by the adrenal medulla. Epinephrine activates platelets by binding to alpha-2A adrenergic receptors. Activated platelets convert pre-bound factor XII to its active form, which then initiates the intrinsic coagulation cascade. This can be called the "activated platelet initiation pathway" for coagulation. Neither tissue factor nor pre-formed thrombin is required. Thrombosis proceeds to completion, but only a minute amount of thrombin is formed, and the process normally stops at this point. In people who lapse into a state of chronic stress, essential hypertension, which is also a manifestation of stress, synergizes with hypercoagulability: there is both a baseline rise in blood pressure and systemic platelet activation as well as superimposed labile rises of both. Upregulation of these two stress parameters is atherogenic: epinephrine-activated platelets stimulating thrombin formation interact with endothelial cells activated by angiotensin II to cause, first, smooth muscle cell proliferation, which is a histological hallmark of atherosclerosis, and, lastly, a symptomatic thrombotic occlusion-the stroke. The migraine symptoms which often accompany this process are a marker of chronic stress and ongoing pathophysiologic damage. Therapeutic predictions are made regarding novel ways of blocking stress-induced hypercoagulability and hypertension. Hypercoagulability could be targeted by monoclonal antibodies directed against the platelet-specific alpha-2 adrenergic receptor or the (putative) platelet receptor for Factor XII; hypertension could be treated with monoclonal antibodies directed against the beta-adrenergic receptor in the juxtaglomerular apparatus or by surgical denervation of the kidneys, either of which would decrease the renin release which helps drive the hypertension.

Role of hepatocyte nuclear factor 4alpha in control of blood coagulation factor gene expression

Hepatocyte nuclear factor 4alpha (HNF4alpha) plays an important role in the maintenance of many liver-specific functions. Liver-specific HNF4alpha-null mice were used to determine whether hepatic HNF4alpha regulates blood coagulation in vivo. These mice exhibited reduced expression of hepatic coagulation factors V, IX, XI, XII, and XIIIB and a prolonged activated partial thromboplastin time but not prothrombin time. Promoter analysis of the mouse FXII and FXIIIB genes was performed to determine whether HNF4alpha directly regulates the genes encoding these coagulation factors. Sequence analysis revealed the presence of one and two HNF4alpha binding sites in the mouse FXII and FXIIIB genes, respectively. Using transient transfection and electrophoretic mobility shift analyses with the mouse FXII and FXIIIB promoters, it was established that the high levels of promoter activity were dependent on HNF4alpha binding sites and the expression of HNF4alpha. In conclusion, HNF4alpha has a critical role in blood coagulation homeostasis by directing transcription of the FXII and XIIIB genes.