The human fibrinolytic system

Antifibrinolytic Therapy and Perioperative Considerations

Fibrinolysis is a physiologic component of hemostasis that functions to limit clot formation. However, after trauma or surgery, excessive fibrinolysis may contribute to coagulopathy, bleeding, and inflammatory responses. Antifibrinolytic agents are increasingly used to reduce bleeding, allogeneic blood administration, and adverse clinical outcomes. Tranexamic acid is the agent most extensively studied and used in most countries. This review will explore the role of fibrinolysis as a pathologic mechanism, review the different pharmacologic agents used to inhibit fibrinolysis, and focus on the role of tranexamic acid as a therapeutic agent to reduce bleeding in patients after surgery and trauma.

Distinct mechanisms account for acquired von Willebrand syndrome in plasma cell dyscrasias

Acquired von Willebrand syndrome (AVWS) is a rare bleeding disorder that may cause life-threatening hemorrhages in patients with plasma cell dyscrasias (PCDs). Early diagnosis and treatment require a thorough understanding of its underlying pathophysiology. Two patients with IgG MGUS presented with dramatically decreased plasma von Willebrand factor (VWF) and a severe type-1 pattern on multimer analysis. A prompt response to intravenous immunoglobulins (IVIG), but not to VWF/FVIII, was consistent with accelerated immunologic clearance of plasma VWF. Another IgG MGUS patient showed a type-2 pattern and a less pronounced response to IVIG, suggesting that additional mechanism(s) contributed to AVWS evolution. In a patient with Waldenström's macroglobulinemia and severe depletion of plasma VWF, multimer analysis indicated association of the IgM paraprotein with VWF before, but not after plasmapheresis, resulting in destruction of the agarose gel and a characteristically distorted band structure of VWF multimers. A type-2 pattern with highly abnormal VWF triplets and laboratory evidence of excessive fibrinolytic activity suggested that plasmin-mediated VWF degradation contributed to AVWS in a patient with multiple myeloma (MM) and AL amyloidosis. Finally, in a patient with IgG MM, maximally prolonged PFA-100® closure times and a specific defect in ristocetin-induced platelet agglutination, both of which resolved after remission induction, indicated interference of the paraprotein with VWF binding to platelet GPIb. Importantly, in none of the six patients, circulating autoantibodies to VWF were detected by a specific in-house ELISA. In summary, when evaluating PCD patients with severe bleeding symptoms, AVWS due to various pathogenic mechanisms should be considered.

Antithrombotic treatment for acute extracranial carotid artery dissections: a meta-analysis

INTRODUCTION

Carotid artery dissection is a leading cause of stroke in younger patients, with an associated prevalence of 2.6-3.0 per 100,000 population. This meta-analysis aims to determine whether in patients managed medically, treatment with anticoagulants or antiplatelet agents was associated with a better outcome with respect to mortality, ischaemic stroke, and major bleeding episodes.

PATIENTS AND METHODS

A comprehensive search strategy was employed of the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE (January 1966 to March 2015), and EMBASE (January 1980 to March 2015) databases. Primary outcomes were death (all causes) or disability. Secondary outcomes were ischaemic stroke, symptomatic intracranial haemorrhage, and major extracranial haemorrhage during the reported follow-up period.

RESULTS

No completed randomized trials were found. Comparing antiplatelets with anticoagulants across 38 studies (1,398 patients), there were no significant differences in the odds of death (effects size, ES, -0.007, p = .871), nor in the death and disability comparison or across any secondary outcomes.

CONCLUSION

There were no randomised trials comparing either anticoagulants or antiplatelets with control, thus there is no level 1 evidence to support their routine use for the treatment of carotid artery dissection. Also, there were no randomised trials that directly compared anticoagulants with antiplatelet drugs, and the reported non-randomised studies did not show any evidence of a significant difference between the two.

Postmenopausal oral estrogen therapy affects hemostatic factors, but does not account for reduction in the progression of subclinical atherosclerosis

BACKGROUND

Hemostatic factors influenced by postmenopausal hormone therapy may contribute to atherosclerosis. The Estrogen in the Prevention of Atherosclerosis Trial (EPAT), a 2-year, randomized, double-blind, placebo-controlled trial, demonstrated reduced subclinical atherosclerosis progression measured by change in common carotid artery intima-media thickness (CIMT) with unopposed oral 17beta-estradiol.

OBJECTIVES

To assess the effect of postmenopausal hormone therapy on the levels of several hemostatic factors, and the relationship between these factors and the progression of subclinical atherosclerosis.

PATIENTS AND METHODS

We measured tissue plasminogen activator (t-PA) antigen, factor (F) VII, D-dimer and albumin longitudinally, and plasminogen activator inhibitor type 1 (PAI-1) and fibrinogen at trial-end, in 186 postmenopausal women.

RESULTS

Estradiol vs. placebo was associated with greater FVII and lower t-PA, albumin, PAI-1 and fibrinogen (all P < or = 0.001), with no estradiol effect on D-dimer (P = 0.42). Only mean on-trial t-PA was positively associated with the absolute level of CIMT on-trial (r = 0.29, P < 0.0001), but this was attenuated with age and body mass index adjustment. No longitudinally measured hemostatic factor was associated with CIMT progression. However, higher CIMT during the trial was significantly related to increases in t-PA.

CONCLUSIONS

These results confirm previous findings regarding estrogen's effect on hemostatic factors and show that albumin is negatively associated with estrogen therapy. These hemostatic factors did not account for the reduction of CIMT progression with 17beta-estradiol seen in EPAT. Atherosclerosis itself may affect levels of hemostatic factors (reverse causality), with subsequent involvement in atherosclerosis-associated thrombosis.

A study of cutaneous lesions caused by Leishmania mexicana in plasminogen-deficient mice

The role of plasminogen, the zymogenic form of the serine protease plasmin, was investigated in the infection of Leishmania mexicana in plasminogen-deficient (plg(-/-)) and Plg wild-type (plg(+/+)) mice. Differences in the lesion size were observed between male plg(+/+) and plg(-/-) mice. However, these differences were not observed in female mice. In both genders, examination of the lesion tissues at 8 weeks post-infection showed differences in the immunoreactivity pattern with anti-Leishmania antibodies. The parasites were limited to isolated foci in the plg(-/-) mice lesion, in contrast to the scattered pattern observed in plg(+/+) mice. These results support the hypothesis that the interaction of the parasite with the host plasminogen-plasmin system might contribute to the virulence of L. mexicana.

Exercise and Training Effects on Blood Haemostasis in Health and Disease

In recent years, the dysfunction of the haemostatic system in relation to the clinical complications from arterioscleroses and cardiovascular diseases has become more recognised. Blood coagulation and fibrinolysis comprise two important physiological systems, which are regulated by a balance between activators and inhibitors. Activation of blood coagulation is associated with accelerated clot formation, whereas activation of blood fibrinolysis enhances the breakdown of the blood clot. Available evidence suggests that strenuous exercise induces activation of blood coagulation with simultaneous enhancement of blood fibrinolysis. Although the responses of blood coagulation and fibrinolysis appear to be related to the exercise intensity and its duration, recent reports suggest that moderate exercise intensity is followed by activation of blood fibrinolysis without concomitant hyper-coagulability, while very intense exercise is associated with concurrent activation of blood coagulation and fibrinolysis. Similar to blood coagulation and fibrinolysis, systemic platelet-related thrombogenic factors have been shown to be involved in the initiation and progression of atherogenesis and plaque growth. Although exercise effects on platelet aggregation and function in healthy individuals have been examined, the results reported have been conflicting. However, for patients with coronary heart disease, the balance of evidence available would strongly suggest that platelet aggregation and functions are increased with exercise. Few studies are available concerning the influence of training on blood coagulation and fibrinolysis and the exact effects of exercise training on the equilibrium between blood coagulation and fibrinolysis is not as yet known. Although the effects of physical training on platelets have been briefly investigated, available meagre evidence suggests that exercise training is associated with favourable effects on platelet aggregation and activation in both men and women.

Failure of peripheral arterial thrombolysis due to elevated plasminogen activator inhibitor type 1

To reduce the risk of intracerebral hemorrhage during thrombolytic therapy, a lower dose of tissue plasminogen activator (t-PA) or urokinase is used for acute peripheral arterial thrombi versus coronary thrombi. We hypothesized that elevated plasminogen activator inhibitor-1 (PAI-1) activity could neutralize infused t-PA or urokinase, resulting in lysis failure. Active PAI-1, active t-PA and total t-PA antigen were measured in 20 patients receiving t-PA, and active PAI-1 was measured in four patients receiving urokinase for acute peripheral arterial thrombosis. The 18 patients that successfully lysed their thrombi all had low active PAI-1 levels (10 +/- 19 pmol/l) during infusion of thrombolytic therapy, while six patients that failed to lyse their thrombi had high active PAI-1 levels (1533 +/- 1384 pmol/l, P = 0.00007) during infusion. Active t-PA levels during t-PA infusion were higher in the group that lysed their thrombi (536 +/- 423 pmol/l versus 42 +/- 45 pmol/l, P = 0.04) even though total t-PA levels were lower (1240 +/- 493 pmol/l versus 1956 +/- 709 pmol/l, P = 0.03). In the patients that failed to lysed their thrombi, > 95% of infused t-PA was neutralized by PAI-1. We conclude that elevated PAI-1 during acute peripheral arterial thrombolysis is associated with an increased risk of lysis failure due to reduced levels of circulating active t-PA or urokinase.

Role of the plasminogen system in basal adhesion formation and carbon dioxide pneumoperitoneum-enhanced adhesion formation after laparoscopic surgery in transgenic mice

OBJECTIVE

To evaluate the role of plasminogen activator inhibitor-1 (PAI-1), urokinase plasminogen activator (uPA), and tissue-type plasminogen activator (tPA) in adhesion formation after laparoscopic surgery.

DESIGN

Prospective, randomized study.

SETTING

Academic research center.

ANIMAL(S)

Seventy female wild-type and transgenic knockout mice for PAI-1 (PAI-1(-/-)), uPA (uPA(-/-)) or tPA (tPA(-/-)).

INTERVENTION(S)

Standardized lesions to induce peritoneal adhesions were performed during laparoscopy. To evaluate basal adhesions and pneumoperitoneum-enhanced adhesions, the pneumoperitoneum was maintained for 10 minutes or 60 minutes, respectively. Peritoneal biopsy samples were obtained during and after 60 minutes of carbon dioxide pneumoperitoneum.

MAIN OUTCOME MEASURE(S)

Adhesions were blindly scored after 7 days. Concentrations of PAI-1 and tPA were measured by using enzyme-linked immunosorbent assay.

RESULT(S)

In PAI-1, uPA, and tPA wild-type mice, pneumoperitoneum enhanced adhesions. Compared with wild-type mice, basal adhesions were fewer in PAI-1(-/-) mice and more in uPA(-/-) and tPA(-/-) mice. Pneumoperitoneum did not enhance adhesions in these transgenic mice. PAI-1 concentration increased after 60 minutes of pneumoperitoneum whereas tPA concentration did not change.

CONCLUSION(S)

Impaired fibrinolysis increases basal adhesions. The absence of pneumoperitoneum-enhanced adhesions in PAI-1(-/-), uPA(-/-), and tPA(-/-) mice and the increase in PAI-1 expression indicate that PAI-1 up-regulation by carbon dioxide pneumoperitoneum is a mechanism of pneumoperitoneum-enhanced adhesion formation.

Current management of intraventricular hemorrhage

BACKGROUND

Intraventricular hemorrhage (IVH) continues to present a challenge to neurosurgeons, often being accompanied by significant morbidity and mortality. The purpose of this paper is to present a review of the recent literature concerning the treatment of patients with IVH, and describe our current management scheme for this disorder.

METHODS

A literature search was conducted to identify key articles pertaining to the pathophysiology and treatment of IVH, focusing on the more recent articles. The bibliographies of selected papers were also screened for additional useful publications.

RESULTS

Management of IVH is primarily directed at controlling intracranial pressure through an external ventricular drain (EVD), but this catheter often becomes occluded by coagulated blood. The fibrinolytic system of the cerebrospinal fluid is limited, and blood may remain in the ventricles for months after a hemorrhage. IVH has a poor prognosis, partly because of the continuing mass effect of blood clots on the ventricular walls. Therefore, investigators have administered fibrinolytic agents directly into the ventricles of patients with IVH. Clinical studies of fibrinolytic therapy for IVH have found a 30 to 35% reduction in mortality with treatment, but have not yet clearly documented an improved neurologic outcome for the survivors.

CONCLUSIONS

Fibrinolytic therapy may be life saving in severe cases of IVH. While many issues need to be resolved, our current practice is to administer intraventricular tissue plasminogen activator (t-PA or alteplase) if hemorrhage involves > or =30% of the volume of one of the lateral ventricles and/or the 3(rd) or 4(th) ventricle. We currently give t-PA after ruling out or treating a possible source of further bleeding, such as an unsecured aneurysm.

Fibrinolytic therapy in intraventricular hemorrhage

OBJECTIVE

To review the literature concerning intraventricular administration of fibrinolytic agents to treat patients with intraventricular hemorrhage (IVH).

DATA SOURCES

An extensive literature search (MEDLINE, EMBASE, Conference Proceedings) was conducted to identify articles in English published between 1966 and May 2000 pertaining to the pathophysiology of IVH and its treatment by intraventricular administration of recombinant tissue plasminogen activator (alteplase) or urokinase (u-PA). The bibliographies of selected identified articles were also screened for publications not found in the computerized search.

STUDY SELECTION

All pertinent publications were reviewed and considered. Those describing the intraventricular administration of fibrinolytic agents to patients with IVH were included.

DATA SYNTHESIS

IVH has a poor prognosis, partly due to the mass effect of blood clots on the ventricular walls. The cerebrospinal fluid has a limited fibrinolytic system. Therefore, clots may remain in the ventricles for months after a hemorrhage. The management of IVH is primarily directed at controlling intracranial pressure through an external ventricular drain, but this catheter often becomes occluded by coagulated blood. To overcome this problem, and to dissolve the residual blood clot, investigators have administered alteplase or u-PA directly into the ventricles of patients with IVH. Complications of this therapy include infection and possible rebleeding. Clinical studies of fibrinolytic therapy for IVH have found a 30-35% reduction in mortality with treatment, but to date, have not clearly documented improved neurologic outcome of the survivors.

CONCLUSIONS

Fibrinolytic therapy with alteplase or u-PA may be life-saving in severe cases of IVH. Yet many technical issues remain to be resolved, such as the optimal dose, frequency, method, timing, and duration of administration of the agent. Additional randomized, double-blind, placebo-controlled studies need to be performed so that the true value of this therapy can be assessed.

Interaction of Leishmania mexicana promastigotes with the plasminogen-plasmin system

The binding of human plasminogen and plasmin to the promastigote form of Leishmania mexicana was investigated. L. mexicana was capable to bind both molecules, the binding being inhibited by epsilon-aminocaproic acid. Scatchard plot analysis revealed a dissociation constant (Kd) value of 2.4+/-0.8 microM and 0.9+/-0.1 x 10(4) binding sites per cell for plasminogen and a Kd value of 1.2+/-0.4 microM and 1.6+/-0.2 x 10(5) binding sites per cell for plasmin. C-terminal lysine residues are involved in plasminogen binding to cells, since carboxypeptidase B treatment reduced this binding by 34%. Ligand blotting analysis showed a group of proteins, with molecular masses between 105 and 115 kDa, capable to interact with plasminogen. Zymogram analysis showed that the protease activity acquired by L. mexicana, due to the interaction with either plasminogen or plasmin, comprises an important fraction of the total protease activity at pH 7.7. Plasminogen activation by tissue-type plasminogen activator (t-PA) was enhanced by the presence of L. mexicana promastigotes. These results raise the question whether the interaction of L. mexicana with components of the fibrinolytic system is involved in the virulence of the parasite.

Measurement of Different Forms of Tissue Plasminogen Activator in Plasma

Background: We evaluated assays to measure both total tissue plasminogen activator (tPA) and the three principle forms of tPA in plasma: active tPA, tPA complexed with plasminogen activator inhibitor type 1 (PAI-1), and tPA complexed with C1-inhibitor.

Methods: Active tPA was measured by use of an indirect amidolytic assay and immunofunctional assays. tPA/PAI-1, tPA/C1-inhibitor, and total tPA antigen were measured by use of microtiter plates coated with anti-tPA antibodies and, respectively, anti-PAI-1, anti-C1-inhibitor, and anti-tPA antibodies conjugated to peroxidase.

Results: The immunofunctional tPA assay detected 1 U/L (0.001 U/mL) tPA and recovered 108% ± 12% of active tPA added to samples containing high (mean, 60 000 IU/L) PAI-1 activities vs a detection limit of 10 U/L (0.01 U/mL) and 13% ± 25% recovery for the indirect amidolytic tPA activity assay. For measurement of tPA/PAI-1 complex, polyclonal anti-PAI-1 conjugates recovered 112% ± 20% of the expected tPA/PAI-1 vs recovery of only 38% ± 16% when monoclonal anti-PAI-1 conjugates were used. Of three methods tested, two total tPA antigen assays correlated well (r2 = 0.85) and showed recoveries near 100%, whereas the third method showed lower correlations, higher intercepts, and falsely high recovery. A single anti-tPA capture antibody that performed the best in the individual assay evaluations was used to measure the different forms of tPA in 22 samples with a range of tPA and PAI-1 values. The sum of the molar concentrations of active tPA, tPA/PAI-1, and tPA/C1-inhibitor using the optimized methods was equal to 94% ± 7% of measured total tPA.

Conclusion: Optimized assays based on a single anti-tPA capture antibody can be used to accurately measure the major forms of tPA in plasma.

Structure of the complex of the antistasin-type inhibitor bdellastasin with trypsin and modelling of the bdellastasin-microplasmin system

The serine proteinase plasmin is, together with tissue-type plasminogen activator (tPA) and urokinase-type plasminogen activator (uPA), involved in the dissolution of blood clots in a fibrin-dependent manner. Moreover, plasmin plays a key role in a variety of other activation cascades such as the activation of metalloproteinases, and has also been implicated in wound healing, pathogen invasion, cancer invasion and metastasis. The leech-derived (Hirudo medicinalis) antistasin-type inhibitor bdellastasin represents a specific inhibitor of trypsin and plasmin and thus offers a unique opportunity to evaluate the concept of plasmin inhibition. The complexes formed between bdellastasin and bovine as well as porcine beta-trypsin have been crystallised in a monoclinic and a tetragonal crystal form, containing six molecules and one molecule per asymmetric unit, respectively. Both structures have been solved and refined to 3.3 A and 2.8 A resolution. Bdellastasin turns out to have an antistasin-like fold exhibiting a bis-domainal structure like the tissue kallikrein inhibitor hirustasin. The interaction between bdellastasin and trypsin is restricted to the C-terminal subdomain of bdellastasin, particularly to its primary binding loop, comprising residues Asp30-Glu38. The reactive site of bdellastasin differs from other antistasin-type inhibitors of trypsin-like proteinases, exhibiting a lysine residue instead of an arginine residue at P1. A model of the bdellastasin-microplasmin complex has been created based on the X-ray structures. Our modelling studies indicate that both trypsin and microplasmin recognise bdellastasin by interactions which are characteristic for canonically binding proteinase inhibitors. On the basis of our three-dimensional structures, and in comparison with the tissue-kallikrein-bound and free hirustasin and the antistasin structures, we postulate that the binding of the inhibitors toward trypsin and plasmin is accompanied by a switch of the primary binding loop segment P5-P3. Moreover, in the factor Xa inhibitor antistasin, the core of the molecule would prevent an equivalent rotation of the P3 residue, making exosite interactions of antistasin with factor Xa imperative. Furthermore, Arg32 of antistasin would clash with Arg175 of plasmin, thus impairing a favourable antistasin-plasmin interaction and explaining its specificity.

Reverse biochemistry: use of macromolecular protease inhibitors to dissect complex biological processes and identify a membrane-type serine protease in epithelial cancer and normal tissue

Serine proteases of the chymotrypsin fold are of great interest because they provide detailed understanding of their enzymatic properties and their proposed role in a number of physiological and pathological processes. We have been developing the macromolecular inhibitor ecotin to be a "fold-specific" inhibitor that is selective for members of the chymotrypsin-fold class of proteases. Inhibition of protease activity through the use of wild-type and engineered ecotins results in inhibition of rat prostate differentiation and retardation of the growth of human PC-3 prostatic cancer tumors. In an effort to identify the proteases that may be involved in these processes, reverse transcription-PCR with PC-3 poly(A)+ mRNA was performed by using degenerate oligonucleotide primers. These primers were designed by using conserved protein sequences unique to chymotrypsin-fold serine proteases. Five proteases were identified: urokinase-type plasminogen activator, factor XII, protein C, trypsinogen IV, and a protease that we refer to as membrane-type serine protease 1 (MT-SP1). The cloning and characterization of the MT-SP1 cDNA shows that it encodes a mosaic protein that contains a transmembrane signal anchor, two CUB domains, four LDLR repeats, and a serine protease domain. Northern blotting shows broad expression of MT-SP1 in a variety of epithelial tissues with high levels of expression in the human gastrointestinal tract and the prostate. A His-tagged fusion of the MT-SP1 protease domain was expressed in Escherichia coli, purified, and autoactivated. Ecotin and variant ecotins are subnanomolar inhibitors of the MT-SP1 activated protease domain, suggesting a possible role for MT-SP1 in prostate differentiation and the growth of prostatic carcinomas.

The crystal structure of the novel snake venom plasminogen activator TSV-PA: a prototype structure for snake venom serine proteinases

BACKGROUND

Trimeresurus stejnejeri venom plasminogen activator (TSV-PA) is a snake venom serine proteinase that specifically activates plasminogen. Snake venom serine proteinases form a subfamily of trypsin-like proteinases that are characterised by a high substrate specificity and resistance to inhibition. Many of these venom enzymes specifically interfere with haemostatic mechanisms and display a long circulating half-life. For these reasons several of them have commercial applications and are potentially attractive pharmacological tools.

RESULTS

The crystal structure of TSV-PA has been determined to 2.5 A resolution and refined to an R factor of 17.8 (R free, 24.4). The enzyme, showing the overall polypeptide fold of trypsin-like serine proteinases, displays unique structural elements such as the presence of a phenylalanine at position 193, a C-terminal tail clamped via a disulphide bridge to the 99-loop, and a structurally conserved Asp97 residue. The presence of a cis proline at position 218 is in agreement with evolutionary relationships to glandular kallikrein.

CONCLUSIONS

We postulate that Phe 193 accounts for the high substrate specificity of TSV-PA and renders it incapable of forming a stable complex with bovine pancreatic trypsin inhibitor and other extended substrates and inhibitors. Mutational studies previously showed that Asp97 is crucial for the plasminogenolytic activity of TSV-PA, here we identify the conservation of Asp97 in both types of mammalian plasminogen activator - tissue-type (tPA) and urokinase-type (uPA). It seems likely that Asp97 of tPA and uPA will have a similar role in plasminogen recognition. The C-terminal extension of TSV-PA is conserved among snake venom serine proteinases, although its function is unknown. The three-dimensional structure presented here is the first of a snake venom serine proteinase and provides an excellent template for modelling other homologous family members.