Regulation of fibrinolytic activity of neutrophil leukocyte elastase, plasmin, and miniplasmin by plasma protease inhibitors

Synergism of red blood cells and tranexamic acid in the inhibition of fibrinolysis

BACKGROUND

Postpartum hemorrhage (PPH) is the leading cause of maternal death worldwide. The World Maternal Antifibrinolytic trial showed that antifibrinolytic tranexamic acid (TXA) reduces PPH deaths. Maternal anemia increases the risk of PPH. The World Maternal Antifibrinolytic-2 trial is now assessing whether TXA can prevent PPH in women with anemia. Low red blood cell (RBC) counts promote fibrinolysis by altering fibrin structure and plasminogen activation.

OBJECTIVES

We explored interactions between RBCs and TXA in inhibiting fibrinolysis.

METHODS

We used global fibrinolytic assays (ball sedimentation and viscoelasticity) to monitor the lysis of fibrin containing plasminogen and tissue-type plasminogen activator. We applied a fluorogenic kinetic assay to measure plasmin generation in fibrin clots and scanning electron microscopy to study fibrin structure.

RESULTS

According to parallel-line bioassay analysis of the fibrin lysis-time data, the antifibrinolytic potency of 4-128 μM TXA was increased in the presence of 10% to 40% (v/v) RBCs. Global fibrinolysis assays showed that the joint effect of RBCs and TXA was about 15% larger than the sum of their individual effects in the inhibition of fibrinolysis. In plasminogen activation, TXA added the same increment of inhibition to the effect of RBCs at any cell count in the fibrin clot. Regarding fibrin structure, TXA thickened fibrin fibers, which impaired plasminogen activation, whereas RBCs promoted fine fibers that were more resistant to plasmin.

CONCLUSIONS

The antifibrinolytic potency of TXA is enhanced in fibrin formed in the presence of RBCs through inhibition of plasminogen activation and fibrin lysis, which correlates with modifications of fibrin structures.

Evaluation of proteolytic activity and serine proteases distribution in plasma from patients with bladder cancer

Background

Bladder cancer (BC) is an aggressive disease with a poor prognosis. A bladder tumor, like other malignant neoplasms, is characterized by the presence of both cancer cells and stromal cells which secrete cytokines, chemokines, growth factors, and proteolytic enzymes. One such class of proteolytic enzymes are serine proteases, which take part in the tumor microenvironment formation via supporting and contributing to tumor progression. This study aims to evaluate the proteolytic activity and serine protease contribution in plasma from BC patients.

Methods

The research involved patients of Alexandrovsky city clinical hospital aged 52-76 with transitional cell carcinoma of the bladder. All examined patients were divided into five groups: the control group included conditionally healthy donors, while other patients were grouped according to their tumor stage (I, II, III and IV). Plasma plasminogen levels were determined by enzyme-linked immunosorbent assay, and the potential activity was measured by chromogenic plasminogen assay. Serine proteases fractions were obtained by the affinity chromatography method, and enzyme concentration in the selected fractions were determined by the Bradford method. Serine proteases distribution was investigated by electrophoresis in a polyacrylamide gel.

Results

It was determined that the concentration, potential activity of plasminogen, and the total amount of serine proteases in plasma from BC patients were greater than the values of the corresponding indicators in healthy donors. This could be one of the factors contributing to increased proteolysis seen in the process of carcinogenesis. Plasminogen concentration in BC patients with stage IV disease; however, displayed a tendency to be reduced compared to earlier stages, and the potential activity of plasminogen was significantly lower in patients with stages III - IV BC. Futhermore, a tumor stage specific gradual decline in the serine protease plasma content was shown. The results of electrophoretic analysis established a significant diminishment in the percentage of high molecular weight components (under non-reducing conditions) and their complete disappearance (under reducing conditions) in plasma serine protease fractions from BC patients. A decline in the percentage of heavy and light plasmin chains in BC patients was also observed. Additionally, a rise in the degraded forms of plasminogen/plasmin content was seen in BC samples, as well as the presence of fractions corresponding to trypsin and NE (under non-reducing conditions) that were absent in the control samples.

Conclusion

The results indicate significant changes in the proteolytic activity of plasma, from BC patients when compared to healthy controls, which is accompanied by alterations in serine protease distribution caused by tumor microenvironment pecularlities at the different stages of oncopathology.

Plasmin Inhibitor in Health and Diabetes: Role of the Protein as a Therapeutic Target

The vascular obstructive thrombus is composed of a mesh of fibrin fibers with blood cells trapped in these networks. Enhanced fibrin clot formation and/or suppression of fibrinolysis are associated with an increased risk of vascular occlusive events. Inhibitors of coagulation factors and activators of plasminogen have been clinically used to limit fibrin network formation and enhance lysis. While these agents are effective at reducing vascular occlusion, they carry a significant risk of bleeding complications. Fibrin clot lysis, essential for normal hemostasis, is controlled by several factors including the incorporation of antifibrinolytic proteins into the clot. Plasmin inhibitor (PI), a key antifibrinolytic protein, is cross-linked into fibrin networks with higher concentrations of PI documented in fibrin clots and plasma from high vascular risk individuals. This review is focused on exploring PI as a target for the prevention and treatment of vascular occlusive disease. We first discuss the relationship between the PI structure and antifibrinolytic activity, followed by describing the function of the protein in normal physiology and its role in pathological vascular thrombosis. Subsequently, we describe in detail the potential use of PI as a therapeutic target, including the array of methods employed for the modulation of protein activity. Effective and safe inhibition of PI may prove to be an alternative and specific way to reduce vascular thrombotic events while keeping bleeding risk to a minimum. Key Points Plasmin inhibitor (PI) is a key protein that inhibits fibrinolysis and stabilizes the fibrin network.This review is focused on discussing mechanistic pathways for PI action, role of the molecule in disease states, and potential use as a therapeutic target.

Polyphosphate nanoparticles enhance the fibrin stabilization by histones more efficiently than linear polyphosphates

Introduction

Beyond the three-dimensional fibrin network, the mechanical and lytic stability of thrombi is supported by the matrix of neutrophil extracellular traps (NETs) composed of polyanionic DNA meshwork with attached proteins including polycationic histones. Polyphosphates represent another type of polyanions, which in their linear form are known to enhance the fibrin stabilizing effects of DNA and histones. However, in vivo polyphosphates are also present in the form of nanoparticles (PolyP-NP), the interference of which with the fibrin/NET matrix is poorly characterized.

Aims

To compare the effects of linear and nanoparticulate polyphosphates, and their combinations with relevant NET components (DNA, histone H3) on fibrin formation, structure, and lysis in in vitro assays focusing on histone-polyphosphate interactions.

Methods

Transmission electron microscopy and dynamic light scattering for stability of the PolyP-NP preparations. Turbidimetry for kinetics of fibrinogen clotting by thrombin and fibrin dissolution by tissue-type plasminogen activator/plasminogen. Scanning electron microscopy for fibrin structure. Surface plasmon resonance for strength of histone-PolyP interactions.

Results

Both linear PolyP and PolyP-NP accelerated the fibrin formation and slowed down its dissolution and these effects were strongly dependent on the number of individual PolyP particles and not on their size. Addition of DNA did not modify significantly the PolyP-NP effects on fibrin formation and lysis. Both linear and nanoparticulate PolyP counteracted the effect of histone in the acceleration of fibrinogen clotting by thrombin. PolyP-NP, but not linear PolyP enhanced the prolongation of lysis time in fibrin containing histone and caused more pronounced thickening of the fibrin fibers than the linear form. Finally, PolyP-NP bound weaker to histone than the linear form.

Conclusions

The interaction of PolyP with histone was a stronger modulator of fibrin formation and lysis than its interaction with DNA. In addition, the PolyP nanoparticles enhanced the thrombus stabilizing effects of histone more effectively than linear PolyP.

New Insights on Plasmin Long Term Stability and the Mechanism of Its Activity Inhibition Analyzed by Quartz Crystal Microbalance

We used the research quartz crystal microbalance (RQCM) to monitor regulatory effects of plasmin and trypsin in the presence of their inhibitor α₂-antiplasmin. The gold surface of quartz crystals was modified with a β-casein layer that served as a substrate for protease digestion. The addition of plasmin or trypsin as well as their mixtures with α₂-antiplasmin resulted in an increase of resonant frequency, f, and in a decrease of motional resistance, R_m, depending on the molar ratio of protease: antiplasmin. At equimolar concentrations of protease and α₂-antiplasmin (5 nM:5 nM) full inhibition of protease activity took place. Monitoring of plasmin activity on an hourly and daily basis revealed a prominent effect of autolysis and decrease of plasmin activity in freshly activated samples. The degree of inhibition as well as plasmin half-life (t_1/2 = 2.48 ± 0.28 days) connected with its degradation was determined.

Liposomes embedded within fibrin gels facilitate localized macrophage manipulations within nerve

BACKGROUND

Understanding the role of macrophages at discrete spatial locations during nerve regeneration after injury is important. But, methodologies that systemically manipulate macrophages can obscure their roles within discrete spatial locations within nerve.

NEW METHOD

Liposomes were embedded within fibrin gels to construct a delivery system that facilitated macrophage-specific manipulations at a sole spatial region, as macrophages accumulated within the fibrin. Clodronate liposomes were characterized for their toxicity to specific cells composing nerve in vitro, then tested for macrophage-specific depletion in vivo. This delivery system using clodronate liposomes was used to repair a mouse sciatic nerve gap to evaluate its efficacy and effects.

RESULT

Clodronate liposomes showed specific toxicity to macrophages without affecting dorsal root ganglia (DRG)-derived neurons, endothelial cells, or Schwann cells in culture. The delivery system demonstrated sustained release of liposomes for more than 7 days while still retaining liposomes within the fibrin. In vivo, the delivery system demonstrated macrophages were targeted by liposomes, and the use of clodronate liposomes minimized macrophage accumulation within fibrin, while not affecting macrophage accumulation within DRG. Nerve regeneration across the nerve gap repaired using this delivery system was associated with decreased angiogenesis, Schwann cell accumulation, axon growth, and reinnervation of affected muscle.

COMPARISON WITH EXISTING METHODS

This delivery system allowed specific perturbation of macrophages locally in nerve. This method could be applicable across species without the need for genetic manipulations or systemic pharmaceuticals.

CONCLUSION

Liposomes embedded within fibrin gels locally target macrophages at the site of nerve injury, which enables greater precision in conclusions regarding their roles in nerve.

Computational model of tranexamic acid on urokinase mediated fibrinolysis

Understanding the coagulation process is critical to developing treatments for trauma and coagulopathies. Clinical studies on tranexamic acid (TXA) have resulted in mixed reports on its efficacy in improving outcomes in trauma patients. The largest study, CRASH-2, reported that TXA improved outcomes in patients who received treatment prior to 3 hours after the injury, but worsened outcomes in patients who received treatment after 3 hours. No consensus has been reached about the mechanism behind the duality of these results. In this paper we use a computational model for coagulation and fibrinolysis to propose that deficiencies or depletions of key anti-fibrinolytic proteins, specifically antiplasmin, a1-antitrypsin and a2-macroglobulin, can lead to worsened outcomes through urokinase-mediated hyperfibrinolysis.

A new ELISA method for the measurement of total α2-plasmin inhibitor level in human body fluids

The ever-increasing research efforts to develop new antithrombotic therapies have led to the reassessment of the role of alpha-2-plasmin inhibitor (α₂-PI) in pathological conditions. In particular, experimental stroke studies have suggested correlation between increased free α2-PI level and mortality. However there are only a small number of well-characterized and specific assays available for the measurements of free α₂-PI. In plasma α₂-PI undergoes both N- and/or C-terminal cleavages resulting four isoforms with modified susceptibility to FXIII catalyzed cross-linking to fibrin and/or loss of plasmin(ogen) binding. Present paper describes a new sandwich ELISA method for the determination of free total α₂-PI in plasma and other body fluids. A newly generated biotinylated monoclonal antibody recognizes and captures all the four N- and/or C-terminally modified isoforms of α₂-PI while HRPO-labeled polyclonal anti-α₂-PI antibody detects the captured antigen. Performing the 2-step assay in streptavidin-coated microplate can be completed within three hours. The assay is well reproducible, total (within laboratory) imprecision in the normal, pathological and very low ranges were 7.4%, 9.1% and < 19%, respectively. When examining the plasma samples of 197 healthy volunteers, 100 acute ischemic stroke patients and 102 patients with venous thrombosis, strong correlation was observed between total α2-PI antigen levels and α2-PI activity for each group. Using the assay a reference interval of 45-86 mg/L was established for total α₂-PI mass concentration in the plasma. α₂-PI levels were also measured in cerebrospinal fluid samples of 47 individuals the median value and range was 132 (36-379) μg/L. In conclusion, our ELISA enables accurate and fast measurement of total free α₂-PI in human body fluids.

Model development and comparison of low hemorrhage-risk endoluminal patch thrombolytic treatment for ischemic stroke

Clot dissolution drugs delivered into the systemic circulation can dissolve intracranial blood clots in 90 min with 20-50% hemorrhage rate. Immobilizing <5% of the intravenous dosage on an endoluminal patch can reduce the dissolution time to <20 min with negligible hemorrhage risk. The thrombus dissolution behavior in endoluminal patch thrombolytic treatment is modeled and compared with experimental results from a companion study. Analyses showed that the thrombus dissolution time decreases with increasing dosage, but the dissolution time reaches a dosage-independent minimum when uPA dosage on the patch is >800 IU. Model analyses showed that dissolution time in the plateau regime is controlled by diffusion. Further results showed that dissolution time could be reduced in this regime by reducing thrombus thickness. This suggests that a stented endoluminal thrombolytic >800 IU patch that compresses the thrombus to thin the clot thickness can help reduce dissolution time. This ultra-low transition dosage (i.e., 800 IU), compared to 0.6-2.4 million IU in conventional thrombolysis suggests that hemorrhage risk in endoluminal patch thrombolytic treatment is low. The low hemorrhagic-risk endoluminal patch can be considered for use in patients who are ineligible for conventional thrombolytic treatment because of high hemorrhagic treatment risk.

Structure and Function of Trypsin-Loaded Fibrinolytic Liposomes

Protease encapsulation and its targeted release in thrombi may contribute to the reduction of haemorrhagic complications of thrombolysis. We aimed to prepare sterically stabilized trypsin-loaded liposomes (SSL_T) and characterize their structure and fibrinolytic efficiency. Hydrogenated soybean phosphatidylcholine-based SSL_T were prepared and their structure was studied by transmission electron microscopy combined with freeze fracture (FF-TEM), Fourier transform infrared spectroscopy (FT-IR), and small-angle X-ray scattering (SAXS). Fibrinolytic activity was examined at 45, 37, or 24°C on fibrin or plasma clots with turbidimetric and permeation-driven lysis assays. Trypsin was shown to be attached to the inner surface of vesicles (SAXS and FF-TEM) close to the lipid hydrophilic/hydrophobic interface (FT-IR). The thermosensitivity of SSL_T was evidenced by enhanced fibrinolysis at 45°C: time to reduce the maximal turbidity to 20% decreased by 8.6% compared to 37°C and fibrin degradation product concentration in the permeation lysis assay was 2-fold to 5-fold higher than that at 24°C. SSL_T exerted its fibrinolytic action on fibrin clots under both static and dynamic conditions, whereas plasma clot dissolution was observed only in the permeation-driven assay. The improved fibrinolytic efficiency of SSL_T under dynamic conditions suggests that they may serve as a novel therapeutic candidate for dissolution of intravascular thrombi, which are typically exposed to permeation forces.

Global assays of fibrinolysis

Fibrinolysis is an important and integral part of the hemostatic system. Acting as a balance to blood coagulation, the fibrinolytic system protects the body from unwanted thrombus formation and occlusion of blood vessels. As long as blood coagulation and fibrinolysis remain in equilibrium, response to injury, such as vessel damage, is appropriately regulated. However, alterations in this balance may lead to thrombosis or bleeding. A variety of methods have been proposed to assess fibrinolytic activity in blood or its components, but due to the complexity of the system, the design of a "gold standard" assay that reflects overall fibrinolysis has remained an elusive goal. In this review, we describe the most commonly used methods that have been described, such as thromboelastography (TEG and ROTEM), global fibrinolytic capacity in plasma and whole blood, plasma turbidity methods, simultaneous thrombin and plasmin generation assays, euglobulin clot lysis time and fibrin plate methods. All of these assays have strengths and limitations. We suggest that some methods may be preferable for detecting hypofibrinolytic conditions, whereas others may be better for detecting hyperfibrinolytic states.

Natural heterogeneity of α2-antiplasmin: functional and clinical consequences

Human α2-antiplasmin (α2AP, also called α2-plasmin inhibitor) is the main physiological inhibitor of the fibrinolytic enzyme plasmin. α2AP inhibits plasmin on the fibrin clot or in the circulation by forming plasmin-antiplasmin complexes. Severely reduced α2AP levels in hereditary α2AP deficiency may lead to bleeding symptoms, whereas increased α2AP levels have been associated with increased thrombotic risk. α2AP is a very heterogeneous protein. In the circulation, α2AP undergoes both amino terminal (N-terminal) and carboxyl terminal (C-terminal) proteolytic modifications that significantly modify its activities. About 70% of α2AP is cleaved at the N terminus by antiplasmin-cleaving enzyme (or soluble fibroblast activation protein), resulting in a 12-amino-acid residue shorter form. The glutamine residue that serves as a substrate for activated factor XIII becomes more efficient after removal of the N terminus, leading to faster crosslinking of α2AP to fibrin and consequently prolonged clot lysis. In approximately 35% of circulating α2AP, the C terminus is absent. This C terminus contains the binding site for plasmin(ogen), the key component necessary for the rapid and efficient inhibitory mechanism of α2AP. Without its C terminus, α2AP can no longer bind to the lysine binding sites of plasmin(ogen) and is only a kinetically slow plasmin inhibitor. Thus, proteolytic modifications of the N and C termini of α2AP constitute major regulatory mechanisms for the inhibitory function of the protein and may therefore have clinical consequences. This review presents recent findings regarding the main aspects of the natural heterogeneity of α2AP with particular focus on the functional and possible clinical implications.

DNA, histones and neutrophil extracellular traps exert anti-fibrinolytic effects in a plasma environment

In response to various inflammatory stimuli, neutrophils secrete neutrophil extracellular traps (NETs), web-like meshworks of DNA, histones and granular components forming supplementary scaffolds in venous and arterial thrombi. Isolated DNA and histones are known to promote thrombus formation and render fibrin clots more resistant to mechanical forces and tissue-type plasminogen activator (tPA)-induced enzymatic digestion. The present study extends our earlier observations to a physiologically more relevant environment including plasma clots and NET-forming neutrophils. A range of techniques was employed including imaging (scanning electron microscopy (SEM), confocal laser microscopy, and photoscanning of macroscopic lysis fronts), clot permeability measurements, turbidimetric lysis and enzyme inactivation assays. Addition of DNA and histones increased the median fibre diameter of plasma clots formed with 16 nM thrombin from 108 to 121 and 119 nm, respectively, and decreased their permeability constant from 6.4 to 3.1 and 3.7×10(-9) cm(2). Histones effectively protected thrombin from antithrombin-induced inactivation, while DNA inhibited plasminogen activation on the surface of plasma clots and their plasmin-induced resolution by 20 and 40 %, respectively. DNA and histones, as well as NETs secreted by phorbol-myristate-acetate-activated neutrophils, slowed down the tPA-driven lysis of plasma clots and the latter effect could be reversed by the addition of DNase (streptodornase). SEM images taken after complete digestion of fibrin in NET-containing plasma clots evidenced retained NET scaffold that was absent in DNase-treated clots. Our results show that DNA and histones alter the fibrin architecture in plasma clots, while NETs contribute to a decreased lytic susceptibility that can be overcome by DNase.

Microvascular thrombosis, fibrinolysis, ischemic injury, and death after cerebral thromboembolism are affected by levels of circulating α2-antiplasmin

OBJECTIVE

Ischemic stroke is primarily attributable to thrombotic vascular occlusion. Elevated α2-antiplasmin (a2AP) levels correlate with increased stroke risk, but whether a2AP contributes to the pathogenesis of stroke is unknown. We examined how a2AP affects thrombosis, ischemic brain injury, and survival after experimental cerebral thromboembolism.

APPROACH AND RESULTS

We evaluated the effects of a2AP on stroke outcomes in mice with increased, normal, or no circulating a2AP, as well as in mice given an a2AP-inactivating antibody. Higher a2AP levels were correlated with greater ischemic brain injury (rs=0.88, P<0.001), brain swelling (rs=0.82, P<0.001), and reduced middle cerebral artery thrombus dissolution (rs=-0.93, P<0.001). In contrast, a2AP deficiency enhanced thrombus dissolution, increased cerebral blood flow, reduced brain infarction, and decreased brain swelling. By comparison to tissue plasminogen activator (TPA), a2AP inactivation hours after thromboembolism still reduced brain infarction (P<0.001) and hemorrhage (P<0.05). Microvascular thrombosis, a process that enhances brain ischemia, was markedly reduced in a2AP-deficient or a2AP-inactivated mice compared with TPA-treated mice or mice with increased a2AP levels (all P<0.001). Matrix metalloproteinase-9 expression, which contributes to acute brain injury, was profoundly decreased in a2AP-deficient or a2AP-inactivated mice versus TPA-treated mice or mice with increased a2AP levels (all P<0.001). a2AP inactivation markedly reduced stroke mortality versus TPA (P<0.0001).

CONCLUSIONS

a2AP has profound, dose-related effects on ischemic brain injury, swelling, hemorrhage, and survival after cerebral thromboembolism. By comparison to TPA, the protective effects of a2AP deficiency or inactivation seem to be mediated through reductions in microvascular thrombosis and matrix metalloproteinase-9 expression.

Ambivalent roles of carboxypeptidase B in the lytic susceptibility of fibrin

BACKGROUND

Removal of C-terminal lysine residues that are continuously exposed in lysing fibrin is an established anti-fibrinolytic mechanism dependent on the plasma carboxypeptidase TAFIa, which also removes arginines that are exposed at the time of fibrinogen clotting by thrombin.

OBJECTIVE

To evaluate the impact of alterations in fibrin structure mediated by constitutive carboxypeptidase activity on the function of fibrin as a template for tissue plasminogen activator-(tPA) induced plasminogen activation and its susceptibility to digestion by plasmin.

METHODS AND RESULTS

We used the stable carboxypeptidase B (CPB), which shows the same substrate specificity as TAFIa. If 1.5 - 6μM fibrinogen was clotted in the presence of 8U/mL CPB, a denser fibrin network was formed with thinner fibers (the median fiber diameter decreased from 138 - 144nm to 89 - 109nm as established with scanning electron microscopy). If clotting was initiated in the presence of 5 - 10μM arginine, a similar decrease in fiber diameter (82 -95nm) was measured. The fine structure of arginine-treated fibrin enhanced plasminogen activation by tPA, but slowed down lysis monitored using fluorescent tPA and confocal laser microscopy. However, if lysis was initiated with plasmin in CPB-treated fibrin, the rate of dissolution increased to a degree corresponding to doubling of the plasmin concentration.

CONCLUSION

The present data evidence that CPB activity generates fine-mesh fibrin which is more difficult to lyse by tPA, but conversely, CPB and plasmin together can stimulate fibrinolysis, possibly by enhancing plasmin diffusion.

Leukocytes may have 2 opposing effects in intravenous rtPA treatment for ischemic stroke

We hypothesized that leukocytes have 2 opposing effects on patients with ischemic stroke treated with recombinant tissue plasminogen activator (rtPA). Patients with ischemic stroke treated with rtPA were divided into 2 groups using the peripheral leukocyte count: high leukocyte group (HLG) and low leukocyte group (LLG) and were evaluated with the National Institutes of Health stroke scale (NIHSS) during the first 24 hours. We defined significant improvement (SI) as NIHSS improving by more than 50% from the baseline, and deterioration following improvement (DFI) as the achievement of SI within 24 hours but its subsequent loss at 24 hours. Fifty-three patients were enrolled, and the rate of SI within 24 hours was higher in HLG than in LLG (85.2% vs 42.3%, P = .0011). However, the rate of DFI was significantly higher in HLG than in LLG (29.6% vs 7.7%, P = .0413). We found that leukocytes might have not only deleterious but also beneficial effects in intravenous rtPA treatment.

Streptococcus uberis plasminogen activator (SUPA) activates human plasminogen through novel species-specific and fibrin-targeted mechanisms

Bacterial plasminogen (Pg) activators generate plasmin to degrade fibrin blood clots and other proteins that modulate the pathogenesis of infection, yet despite strong homology between mammalian Pgs, the activity of bacterial Pg activators is thought to be restricted to the Pg of their host mammalian species. Thus, we found that Streptococcus uberis Pg activator (SUPA), isolated from a Streptococcus species that infects cows but not humans, robustly activated bovine but not human Pg in purified systems and in plasma. Consistent with this, SUPA formed a higher avidity complex (118-fold) with bovine Pg than with human Pg and non-proteolytically activated bovine but not human Pg. Surprisingly, however, the presence of human fibrin overrides the species-restricted action of SUPA. First, human fibrin enhanced the binding avidity of SUPA for human Pg by 4-8-fold in the presence and absence of chloride ion (a negative regulator). Second, although SUPA did not protect plasmin from inactivation by α(2)-antiplasmin, fibrin did protect human plasmin, which formed a 31-fold higher avidity complex with SUPA than Pg. Third, fibrin significantly enhanced Pg activation by reducing the K(m) (4-fold) and improving the catalytic efficiency of the SUPA complex (6-fold). Taken together, these data suggest that indirect molecular interactions may override the species-restricted activity of bacterial Pg activators; this may affect the pathogenesis of infections or may be exploited to facilitate the design of new blood clot-dissolving drugs.

Modulation of the von Willebrand factor-dependent platelet adhesion through alternative proteolytic pathways

Introduction

Platelet adhesion to collagen under high shear rates depends on the optimal size of the von Willebrand factor (VWF) multimers, which is determined by their limited proteolysis. The present study attempts to identify the role of hemostatic-fibrinolytic enzymes (thrombin, plasmin) and leukocyte-derived proteases (matrix metalloproteinase (MMP)-8, MMP-9, neutrophil elastase) in the cleavage of VWF and to characterize the effect of flow and platelets on this proteolysis and its functional consequences on platelet adhesion.

Methods and results

According to VWF immunoblots, plasmin, neutrophil elastase and thrombin at concentrations of in vivo relevance resulted in extensive degradation of VWF within several minutes. Platelets protected VWF against this proteolysis under static conditions, whereas perfusion of the proteases at 3350 s^-1 shear rate over VWF immobilized on artery cross sections enhanced its degradation and blocked the protective effect of platelets. In parallel with VWF digestion, the examined proteases impaired the VWF-dependent platelet adhesion as reflected in the decreased surface-bound GpIIb/IIIa immunoreactivity following perfusion of collagen-coated surfaces or artery sections with blood and plasmin, neutrophil elastase or thrombin. Within the time frame of minutes no VWF cleavage could be detected under static or flow conditions after exposure to MMP-8 and MMP-9 at concentrations relevant to physiological neutrophil counts.

Conclusion

Our results indicate a shear- and platelet-dependent role for several proteases in the local modulation of the VWF function.

Hindered dissolution of fibrin formed under mechanical stress

BACKGROUND

Recent data indicate that stretching forces cause a dramatic decrease in clot volume accompanied by gross conformational changes of fibrin structure.

OBJECTIVE

The present study attempts to characterize the lytic susceptibility of fibrin exposed to mechanical stress as a model for fibrin structures observed in vivo.

METHODS AND RESULTS

The relevance of stretched fibrin models was substantiated by scanning electron microscopic (SEM) evaluation of human thrombi removed during surgery, where surface fibrin fibers were observed to be oriented in the direction of shear forces, whereas interior fibers formed a random spatial meshwork. These structural variations were modeled in vitro with fibrin exposed to adjustable mechanical stress. After two- and three-fold longitudinal stretching (2 × S, 3 × S) the median fiber diameter and pore area in SEM images of fibrin decreased two- to three-fold. Application of tissue plasminogen activator (tPA) to the surface of model clots, which contained plasminogen, resulted in plasmin generation which was measured in the fluid phase. After 30-min activation 12.6 ± 0.46 pmol mm(-2) plasmin was released from the non-stretched clot (NS), 5.5 ± 1.11 pmol mm(-2) from 2 × S and 2.3 ± 0.36 pmol mm(-2) from 3 × S clot and this hampered plasmin generation was accompanied by decreased release of fibrin degradation products from stretched fibrins. Confocal microscopic images showed that a green fluorescent protein-fusion variant of tPA accumulated in the superficial layer of NS, but not in stretched fibrin.

CONCLUSION

Mechanical stress confers proteolytic resistance to fibrin, which is a result of impaired plasminogen activation coupled to lower plasmin sensitivity of the denser fibrin network.

Platelet protease nexin-1, a serpin that strongly influences fibrinolysis and thrombolysis

BACKGROUND

Protease nexin-1 (PN-1) is a serpin that inhibits plasminogen activators, plasmin, and thrombin. PN-1 is barely detectable in plasma, but we have shown recently that PN-1 is present within the α-granules of platelets.

METHODS AND RESULTS

In this study, the role of platelet PN-1 in fibrinolysis was investigated with the use of human platelets incubated with a blocking antibody and platelets from PN-1-deficient mice. We showed by using fibrin-agar zymography and fibrin matrix that platelet PN-1 inhibited both the generation of plasmin by fibrin-bound tissue plasminogen activator and the activity of fibrin-bound plasmin itself. Rotational thromboelastometry and laser scanning confocal microscopy were used to demonstrate that PN-1 blockade or deficiency resulted in increased clot lysis and in an acceleration of the lysis front. Protease nexin-1 is thus a major determinant of the lysis resistance of platelet-rich clots. Moreover, in an original murine model in which thrombolysis induced by tissue plasminogen activator can be measured directly in situ, we observed that vascular recanalization was significantly increased in PN-1-deficient mice. Surprisingly, general physical health, after tissue plasminogen activator-induced thrombolysis, was much better in PN-1-deficient than in wild-type mice.

CONCLUSIONS

Our results reveal that platelet PN-1 can be considered as a new important regulator of thrombolysis in vivo. Inhibition of PN-1 is thus predicted to promote endogenous and exogenous tissue plasminogen activator-mediated fibrinolysis and may enhance the therapeutic efficacy of thrombolytic agents.

Ensembles of uncertain mathematical models can identify network response to therapeutic interventions

The role of mechanistic modeling and systems biology in molecular medicine remains unclear. In this study, we explored whether uncertain models could be used to understand how a network responds to a therapeutic intervention. As a proof of concept, we modeled and analyzed the response of the human coagulation cascade to recombinant factor VIIa (rFVIIa) and prothrombin (fII) addition in normal and hemophilic plasma. An ensemble of parametrically uncertain human coagulation models was developed (N = 437). Each model described the time evolution of 193 proteins and protein complexes interconnected by 301 interactions under quiescent flow. The 467 unknown model parameters were estimated, using multiobjective optimization, from published in vitro coagulation studies. The model ensemble was validated using published in vitro thrombin measurements and thrombin measurements taken from coronary artery disease patients. Sensitivity analysis was then used to rank-order the importance of model parameters as a function of experimental or physiological conditions. A novel strategy for the systematic comparison of ranks identified a family of fX/FXa and fII/FIIa interactions that became more sensitive with decreasing fVIII/fIX. The fragility of these interactions was preserved following the addition of exogenous rFVIIa and fII. This suggested that exogenous rFVIIa did not alter the qualitative operation of the cascade. Rather, exogenous rFVIIa and fII took advantage of existing fluid and interfacial fX/FXa and fII/FIIa sensitivity to restore normal coagulation in low fVIII/fIX conditions. The proposed rFVIIa mechanism of action was consistent with experimental literature not used in model training. Thus, we demonstrated that an ensemble of uncertain models could unravel key facets of the mechanism of action of a focused intervention. Whereas the current study was limited to coagulation, perhaps the general strategy used could be extended to other molecular networks relevant to human health.

Neutrophil granulocyte-dependent proteolysis enhances platelet adhesion to the arterial wall under high-shear flow

SUMMARY BACKGROUND

Under high shear stress platelets adhere preferentially to the adventitia layer of the arterial vessel wall in a von Willebrand factor (VWF)-dependent manner.

OBJECTIVE

The present study was undertaken in an attempt to characterize the structural background of the relative thromboresistance of the media and the impact of neutrophil leukocyte-derived proteases (matrix metalloproteinases, neutrophil elastase) on platelet adhesion in this layer of the arteries.

METHODS AND RESULTS

Platelet adhesion to cross-sections of the human iliac artery was monitored by indirect immunofluorescent detection of GpIIb/IIIa antigen. Exposure of the vessel wall to activated neutrophils or neutrophil-derived proteases increased platelet adhesion to the media about tenfold over the control level at 3350 s(-1) surface shear rate. In parallel with this enhanced thrombogenicity morphological changes in the media were evidenced by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The fine proteoglycan meshwork seen with Cupromeronic Blue enhancement of the SEM images was removed by the proteolytic treatment and the typical collagen fiber structure was exposed on the AFM images of the media.

CONCLUSION

Through their proteases activated neutrophils degrade proteoglycans, unmask VWF binding sites and thus abolish the thromboresistance of the media in human arteries.

A model for the formation, growth, and lysis of clots in quiescent plasma. A comparison between the effects of antithrombin III deficiency and protein C deficiency

A mathematical model comprised of 23 reaction-diffusion equations is used to simulate the biochemical changes and transport of various reactants involved in coagulation and fibrinolysis in quiescent plasma. The growth and lysis of a thrombus, as portrayed by the model equations, is governed by boundary conditions that include the surface concentration of TF-VIIa, the generation of XIa by contact activation (in vitro), and the secretion of tPA due to endothelial activation. We apply the model to two clinically relevant hypercoagulable states, caused by deficiency of either antithrombin III or protein C. These predictions are compared with published experimental data which validate the utility of the developed model under the special case of static conditions. The incorporation of varying hemodynamic conditions in to the current fluid static model remains to be performed.

PEGylated DX-1000: pharmacokinetics and antineoplastic activity of a specific plasmin inhibitor

Novel inhibitors of the urokinase-mediated plasminogen (plg) activation system are potentially of great clinical benefit as anticancer treatments. Using phage display, we identified DX-1000 a tissue factor pathway inhibitor-derived Kunitz domain protein which is a specific high-affinity inhibitor of plasmin (pln) (K(i) = 99 pM). When tested in vitro, DX-1000 blocks plasmin-mediated pro-matrix metalloproteinase-9 (proMMP-9) activation on cells and dose-dependently inhibits tube formation, while not significantly affecting hemostasis and coagulation. However, this low-molecular weight protein inhibitor ( approximately 7 kDa) exhibits rapid plasma clearance in mice and rabbits, limiting its potential clinical use in chronic diseases. After site-specific PEGylation, DX-1000 retains its activity and exhibits a decreased plasma clearance. This PEGylated derivative is effective in vitro, as well as potent in inhibiting tumor growth of green fluorescent protein (GFP)-labeled MDA-MB-231 cells. 4PEG-DX-1000 treatment causes a significant reduction of urokinase-type plasminogen activator (uPA) and plasminogen expressions, a reduction of tumor proliferation, and vascularization. 4PEG-DX-1000 treatment significantly decreases the level of active mitogen-activated protein kinase (MAPK) in the primary tumors and reduces metastasis incidence. Together, our results demonstrate the potential value of plasmin inhibitors as therapeutic agents for blocking breast cancer growth and metastasis.

Complete inhibition of fibrinolysis by sustained carboxypeptidase B activity: the role and requirement of plasmin inhibitors

BACKGROUND

The antifibrinolytic effect of activated thrombin-activatable fibrinolysis inhibitor (TAFIa) and carboxypeptidase B (CPB) displays threshold behavior. When CPB was used to simulate conditions mimicking continuous TAFIa activity, it affected the lysis of plasma clots differently to clots formed from a minimal fibrinolytic system comprising fibrinogen, plasminogen and alpha(2)-antiplasmin. Whereas CPB saturably prolonged clot lysis in the purified system, the effect of CPB did not appear saturable in plasma clots.

METHODS

To rationalize this difference, we investigated the effects of alpha(2)-antiplasmin, alpha(2)-macroglobulin, antithrombin and aprotinin on CPB-mediated antifibrinolysis.

RESULTS

CPB alone prolonged fibrinolysis in a saturable manner and the efficacy of CPB increased with decreasing tissue-type plasminogen activator (t-PA) concentration. The inhibitors by themselves did not halt fibrinolysis and the potency of each inhibitor in the absence of CPB mirrored their solution-phase plasmin inhibitory potentials: alpha(2)-antiplasmin approximately equal to aprotinin >> alpha(2)-macroglobulin >> antithrombin. With both CPB and inhibitor present, a synergistic effect was observed. The antifibrinolytic sensitivity to CPB was related to the plasmin inhibitory potential of the inhibitor.

CONCLUSIONS

Fibrinolysis could be completely inhibited by alpha(2)-antiplasmin, alpha(2)-macroglobulin and antithrombin, but not aprotinin, in the presence of CPB, and occurred only when the irreversible inhibitor or pool of inhibitors were in excess of plasminogen. Western blot analysis indicated that the CPB-mediated shutdown of fibrinolysis was a result of plasminogen consumption prior to clot lysis. The CPB concentration required for fibrinolytic shutdown was dependent on t-PA concentration and the inhibitory potential of the irreversible inhibitor pool.

Fibrinolysis in a lipid environment: modulation through release of free fatty acids

BACKGROUND

Thrombolysis is conventionally regarded as dissolution of the fibrin matrix of thrombi by plasmin, but the structure of clots in vivo includes additional constituents (proteins, phospholipids) that modulate their solubilization.

OBJECTIVE

We examined the presence of free fatty acids in thrombi and their effects on distinct stages of fibrinolysis (plasminogen activation, plasmin activity).

METHODS AND RESULTS

Using the fluorescent probe acrylodated intestinal fatty acid-binding protein, variable quantities (up to millimolar concentrations) of free fatty acids were demonstrated in surgically removed human thrombi. Oleic acid at relevant concentrations reversibly inhibits more than 90% of the amidolytic activity of plasmin on a synthetic substrate (Spectrozyme PL), but only partially inhibits its fibrinolytic activity measured using turbidimetry. Chromogenic assays detecting the generated plasmin activity show that plasminogen activation by tissue-type plasminogen activator (t-PA) is completely blocked by oleic acid in the fluid phase, but is accelerated on a fibrin matrix. A recombinant derivative of t-PA (reteplase) develops higher fibrin specificity in the presence of oleic acid, because both the inhibition of plasminogen activation in free solution and its enhancement on fibrin template are stronger than with wild-type t-PA.

CONCLUSION

Through the stimulation of plasminogen activation on a fibrin template and the inhibition of plasminogen activators and plasmin in the fluid phase, free fatty acids confine the action of fibrinolytic proteases to the site of clotting, where they partially oppose the thrombolytic barrier function of phospholipids.

The fibrinolysis inhibitor alpha2-antiplasmin in the human cornea

PURPOSE

To determine whether the cornea contains and expresses, at the gene level, the major plasmin inhibitor alpha2-antiplasmin.

METHODS

Corneal sections were immunostained for alpha2-antiplasmin. Extracts of human corneal stroma, epithelium, and endothelium were subjected to immunodot blot and Western blot analysis. Total RNA and alpha2-antiplasmin specific primers were used for RT-PCR. The cDNA was sequenced.

RESULTS

Alpha2-antiplasmin was observed in all three corneal layers by immunolocalization and Western blots. The major alpha2-antiplasmin form observed in most extracts was the 70-kDa form. Total alpha2-antiplasmin was present at 0.119 +/- 0.014 microg/epithelium (n = 10) and 1.45 +/- 0.47 microg/stroma (n = 10). Alpha2-antiplasmin mRNA was detected in epithelial and stromal extracts and cultured human corneal stromal cells. The sequences of the PCR products were identical to that for human alpha2-antiplasmin.

CONCLUSIONS

Alpha2-antiplasmin and its mRNA are present in the cornea and may serve to regulate corneal plasmin activity.

Structural basis of the cofactor function of denatured albumin in plasminogen activation by tissue-type plasminogen activator

Certain denatured proteins function as cofactors in the activation of plasminogen by tissue-type plasminogen activator. The present study approached the structural requirements for the cofactor activity of a model protein (human serum albumin). Heat denaturation of 100-230 microM albumin (80 degrees C and 60-90 min) reproducibly yielded aggregates with radius in the range of 10-150 nm. The major determinant of the cofactor potency was the size of the aggregates. The increase of particle size correlated with the cofactor activity, and there was a minimal requirement for the size of the cofactor (about 10 nm radius). Similar to other proteins, the molecular aggregates with cofactor function contained a significant amount of antiparallel intermolecular beta-sheets. Plasmin pre-digestion increased the cofactor efficiency (related to C-terminal lysine exposure) and did not affect profoundly the structure of the aggregates, suggesting a long-lasting and even a self-augmenting cofactor function of the denatured protein.

Effects of Hemodilution, Blood Loss, and Consumption on Hemostatic Factor Levels During Cardiopulmonary Bypass

OBJECTIVES

The purpose of this study was to determine quantitatively the effects of consumption, hemodilution, and blood loss on coagulation and fibrinolytic factor levels during cardiopulmonary bypass.

DESIGN

A combination of measured levels of prothrombin, antithrombin, fibrinogen, plasminogen, and antiplasmin along with their activation markers F1.2, thrombin-antithrombin complex, fibrinopeptide A, plasmin-antiplasmin complex, and D-dimer were used with a computer model of each patient's vascular and hemostatic systems to estimate the cardiopulmonary bypass-associated loss of each factor because of hemodilution, blood loss, and consumption.

SETTING

University hospital.

PARTICIPANTS

Nine patients undergoing coronary artery bypass graft surgery.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

At baseline, it was estimated that on average 2%, 3%, and 25%, respectively, of the baseline liver secretion of plasminogen, prothrombin,and fibrinogen were consumed by activation of these proteins. During cardiopulmonary bypass, thrombin and plasmin generation were increased, whereas fibrin generation was decreased because of heparin. Compared with baseline, hemodilution during cardiopulmonary bypass resulted in an average 35% +/- 7% decrease in the concentration of coagulation and fibrinolytic proteins, whereas blood loss was responsible for an average 6% +/- 5% decrease in these proteins. Blood loss varied substantially among patients, resulting in <1% to 14% decreases in hemostatic protein levels. On average, consumption because of activation resulted in less than a 1% drop in the concentration of coagulation and fibrinolytic factors during cardiopulmonary bypass.

CONCLUSIONS

Hemodilution is the primary cause of the drop in coagulation and fibrinolytic proteins during routine cardiopulmonary bypass, followed by blood loss, whereas consumption accounts for less than a 1% drop in most patients.

Plasmin generation and D-dimer formation during cardiopulmonary bypass

The purpose of this study was to estimate the in vivo rates of plasmin and D-dimer generation for comparison with the rate of fibrin formation during cardiopulmonary bypass (CPB), a procedure known to induce a hyperfibrinolytic state. Plasmin and D-dimer generation rates were based on measured levels of antiplasmin, plasmin-antiplasmin complex and D-dimer obtained before, during and after CPB from nine males, combined with a computer model of each patient's vascular system that continuously accounted for secretion, clearance, hemodilution, blood loss and transfusion. At baseline the average plasmin and D-dimer generation rates were 0.27 +/- 0.07 and 0.18 +/- 0.07 pmol/s, respectively. Within 5 min of CPB initiation, plasmin generation increased over 100-fold to 36 +/- 40 pmol/s while D-dimer generation increased 200-fold to 37 +/- 39 pmol/s. For the remainder of the CPB, average plasmin and D-dimer generation remained 20-fold to 30-fold above baseline levels. During CPB, the rate of D-dimer generation was similar to the rate of total fibrin formation, indicating that, in the absence of fibrinolytic inhibitors, CPB induces plasmin-mediated removal of fibrin from the vascular system at a rate similar to the rate of fibrin formation.

Understanding the enzymology of fibrinolysis and improving thrombolytic therapy

Cardiovascular disease is responsible for 17 million deaths per year but acute myocardial infarction and stroke can be treated with thrombolytics ("clot busters"), which are plasminogen activators. However, despite many years of study and huge investment from the pharmaceutical industry, clinical trials of new drugs have often been disappointing. Part of the problem may be our incomplete understanding of the regulation of plasminogen activation in vivo. We have developed precise in vitro methods and with the application of computer simulations, we hope to improve our understanding of plasminogen activation to facilitate improvements in thrombolytic therapy.

The kinetics of plasmin inhibition by aprotinin in vivo

INTRODUCTION

The purpose of this study was to estimate, in patients undergoing cardiopulmonary bypass (CPB), the in vivo rates of tissue plasminogen activator (tPA) and plasminogen activator inhibitor 1 (PAI-1) secretion, plasmin generation, fibrin degradation, and plasmin inhibition by aprotinin versus antiplasmin.

MATERIALS AND METHODS

Estimates of in vivo rates were based on measured levels of tPA, PAI-1, antiplasmin, plasmin-antiplasmin complex (PAP), total aprotinin, plasmin-aprotinin complex and D-dimer, combined with a computer model of each patient's vascular system that continuously accounted for secretion, clearance, hemodilution, blood loss and transfusion. Plasmin regulation was studied in nine control patients undergoing CPB without aprotinin versus six patients treated with aprotinin.

RESULTS

In controls, plasmin-antiplasmin levels rose from a baseline of 3.0+/-0.9 to a peak of 8.1+/-2.7 nmol/L after CPB due to an average 44-fold rise in the plasmin generation rate. This rise in plasmin generation during CPB lead to increased fibrin degradation causing D-dimer levels to increase from a baseline of 1.2+/-0.6 to a peak of 9.7+/-4.4 nmol/L due to an average 74-fold rise in the D-dimer generation rate. During CPB in the aprotinin group, plasmin-antiplasmin levels dropped, plasmin-aprotinin complex levels rose, while D-dimer levels remained unchanged from baseline. Compared to controls, the aprotinin group showed similar rates of plasmin generation during CPB, but an 11-fold faster plasmin inhibition rate and a 10-fold lower D-dimer generation rate.

CONCLUSIONS

The rise in plasmin generation and fibrin degradation that occurs during standard CPB is suppressed by the addition of aprotinin, which returns the patient to near baseline fibrin degradation rates during CPB.

Impaired fibrinolytic potential related to elevated ?1-proteinase inhibitor levels in patients with pulmonary thromboembolism

The contribution of neutrophil leukocyte elastase (NE) to in vivo thrombolysis is still an open question. The present study examines the impact of variable levels of alpha1-proteinase inhibitor (alpha1-PI) (the major plasma inhibitor of NE) on fibrinolysis within the setting of thromboembolic diseases. Blood samples were taken from 56 patients with pulmonary thromboembolism prior to treatment. alpha1-PI and alpha1-PI-NE complex were measured in the serum and plasma with immunoturbidimetric and enzyme-linked immunosorbent assay (ELISA) methods, respectively. The fibrinolytic potential [spontaneous, tissue-type plasminogen activator (tPA) induced, and plasmin induced] of the plasma was evaluated in vitro with turbidimetric clot lysis assay. Correlation analysis (Pearson product-moment correlation coefficient, r) of the turbidimetric lysis parameters and the blood levels of alpha1-PI and alpha1-PI-NE complex was carried out. Fibrinolysis is slower in clots prepared from plasma containing elevated levels of alpha1-PI and alpha1-PI-NE complex. The maximal turbidity of the plasma clots shows significant correlation with the alpha1-PI level (r=0.39, p=0.003) and the correlation of the maximal turbidity and the tPA-induced lysis time is also significant (r=0.77, p<0.001). The lysis time correlates with the plasma level of alpha1-PI-NE complex, if fibrinolysis is induced with tPA (r=0.37, p=0.02), but not with plasmin (r=0.19, p=0.4). Our study shows that in pulmonary thromboembolism elevated levels of alpha1-PI are associated with suppressed plasma fibrinolytic potential. This effect can be at least partially explained by the coarse fibrin network structure and retarded plasminogen activator-dependent fibrinolysis.

Phospholipid barrier to fibrinolysis: role for the anionic polar head charge and the gel phase crystalline structure

The massive presence of phospholipids is demonstrated in frozen sections of human arterial thrombi. Purified platelet phospholipids and synthetic phospholipids retard in vitro tissue-type plasminogen activator (tPA)-induced fibrinolysis through effects on plasminogen activation and plasmin function. The inhibition of plasminogen activation on the surface of fibrin correlates with the fraction of anionic phospholipid. The phospholipids decrease the amount of tPA penetrating into the clot by 75% and the depth of the reactive surface layer occupied by the activator by up to 30%, whereas for plasmin both of these parameters decrease by approximately 50%. The phospholipids are not only a diffusion barrier, they also bind the components of the fibrinolytic system. Isothermal titration calorimetry shows binding characterized with dissociation constants in the range 0.35-7.64 microm for plasmin and tPA (lower values with more negative phospholipids). The interactions are endothermic and thermodynamically driven by an increase in entropy, probably caused by the rearrangements in the ordered gel structure of the phospholipids (in line with the stronger inhibition at gel phase temperatures compared with liquid crystalline phase temperatures). These findings show a phospholipid barrier, which should be overcome during lysis of arterial thrombi.

Activated Thrombin-activatable Fibrinolysis Inhibitor Reduces the Ability of High Molecular Weight Fibrin Degradation Products to Protect Plasmin from Antiplasmin

Activated thrombin-activable fibrinolysis inhibitor (TAFIa) is a carboxypeptidase B-like plasma enzyme that can slow clot lysis by removing lysine residues exposed on fibrin as it is cleaved by plasmin. Previously, it was shown that fibrin treated with TAFIa is less able to promote plasminogen activation by tissue-type plasminogen activator. In this study, the effect of TAFIa modification of a fibrin surface on the rate of plasmin inhibition by antiplasmin was studied using high molecular weight fibrin degradation products (HMw-FDPs) as a soluble model for intact plasmin-modified fibrin. To quantify the inhibition, a novel end point assay was employed where plasmin, antiplasmin, and cofactors were mixed in the presence of a chromogenic substrate and the end point in the substrate hydrolysis reaction was used to measure the second order rate constant of inhibition. When HMw-FDPs were titrated in the presence of plasmin and antiplasmin, the rate constant for inhibition decreased by 16-fold at saturation (9.6 x 10(6) m(-1) s(-1) to 0.59 x 10(6) m(-1) s(-1)). When HMw-FDPs were pretreated with TAFIa, nearly two-thirds of the protective effect was lost. When 730 nm HMw-FDPs were treated for 20 min with TAFIa, the rate constant for plasmin inhibition was increased 3-fold from 1.9 x 10(6) m(-1) s(-1) to 6.2 x 10(6) m(-1) s(-1). Therefore, a novel mechanism was identified whereby TAFIa can modulate plasmin levels by increasing the susceptibility of plasmin to inhibition by antiplasmin.

A study of the protection of plasmin from antiplasmin inhibition within an intact fibrin clot during the course of clot lysis

Previous work using soluble fibrin surrogates or very dilute fibrin indicate that inhibition of plasmin by antiplasmin is attenuated by fibrin surrogates; however, this phenomenon has not been quantified within intact fibrin clots. Therefore, a novel system was designed to measure plasmin inhibition by antiplasmin in real time within an intact clot during fibrinolysis. This was accomplished by including the plasmin substrate S2251 and a recombinant fluorescent derivative of plasminogen (S741C-fluorescein) into clots formed from purified components. Steady state plasmin levels were estimated from the rates of S2251 hydrolysis, the rates of plasminogen activation were estimated by fluorescence decrease over time, and residual antiplasmin was deduced from residual fluorescence. From these measurements, the second order rate constant could be inferred at any time during fibrinolysis. Immediately after clot formation, the rate constant for inhibition decreased 3-fold from 9.6 x 10(6) m(-1) s(-1) measured in a soluble buffer system to 3.2 x 10(6) m(-1) s(-1) in an intact fibrin clot. As the clot continued to lyse, the rate constant for inhibition continued to decrease by 38-fold at maximum. To determine whether this protection was the result of plasmin exposure of carboxyl-terminal lysine residues, clots were formed in the presence of activated thrombin-activatable fibrinolysis inhibitor (TAFIa). In the presence of TAFIa, the initial protective effect associated with clot formation occurred; however, the secondary protective effect associated with lysine residue exposure was delayed in a TAFIa concentration-dependent manner. This latter effect represents another mechanism whereby TAFIa attenuates fibrinolysis.

Myosin: a noncovalent stabilizer of fibrin in the process of clot dissolution

Myosin modulates the fibrinolytic process as a cofactor of the tissue plasminogen activator and as a substrate of plasmin. We report now that myosin is present in arterial thrombi and it forms reversible noncovalent complexes with fibrinogen and fibrin with equilibrium dissociation constants in the micromolar range (1.70 and 0.94 microM, respectively). Competition studies using a peptide inhibitor of fibrin polymerization (glycl-prolyl-arginyl-proline [GPRP]) indicate that myosin interacts with domains common in fibrinogen and fibrin and this interaction is independent of the GPRP-binding polymerization site in the fibrinogen molecule. An association rate constant of 1.81 x 10(2) M(-1) x s(-1) and a dissociation rate constant of 3.07 x 10(-4) s(-1) are determined for the fibrinogen-myosin interaction. Surface plasmon resonance studies indicate that fibrin serves as a matrix core for myosin aggregation. The fibrin clots equilibrated with myosin are stabilized against dissolution initiated by plasminogen and tissue-type plasminogen activator (tPA) or urokinase (at fibrin monomer-myosin molar ratio as high as 30) and by plasmin under static and flow conditions (at fibrin monomer-myosin molar ratio lower than 15). Myosin exerts similar effects on the tPA-induced dissolution of blood plasma clots. Covalent modification involving factor XIIIa does not contribute to this stabilizing effect; myosin is not covalently attached to the clot by the time of complete cross-linking of fibrin. Thus, our in vitro data suggest that myosin detected in arterial thrombi binds to the polymerized fibrin, in the bound form its tPA-cofactor properties are masked, and the myosin fibrin clot is relatively resistant to plasmin.

Influence of soluble fibrin on reaction kinetics of plasmin type 1 and type 2 with alpha2-antiplasmin

This study investigates, by slow binding kinetics methods, reaction kinetics of both plasmin types 1 and 2 with alpha -antiplasmin in the presence of increasing concentrations of either epsilon-amino-caproic acid (EACA) or soluble fibrin. All curves of plasmin-alpha -antiplasmin interaction followed the same pattern, indicating reversible slow binding inhibition with an initial loose complex and a following tight complex. Without soluble fibrin or EACA, differences between plasmin types 1 and 2 could be seen in the initial loose complex formation. The presence of increasing concentrations of EACA slowed down the first step of the reaction (without any effect on the second step), resulting in increasing values for K. Plasmin type 1 demonstrated a steep slope of K at an EACA concentration of 1 mmol/l. In plasmin type 2, the increase of K started at higher EACA concentrations. The value for K at a high EACA concentration (100 mmol/l) was the same for both plasmin types. In contrast to EACA, increasing concentrations of soluble fibrin slowed down both reaction steps. At high concentrations of soluble fibrin, the inhibitory effect of alpha -antiplasmin was almost completely abolished. Our data demonstrate that the effect of soluble fibrin and the lysine analogue EACA on plasmin-antiplasmin reactions are different and that the use of lysine analogues does not mimic fibrin in laboratory analyses of plasmin inhibition. In addition, our data indicate theoretical differences between plasmin type 1 and plasmin type 2, when used for local thrombolytic therapy.(2) (2) (i initial) (i initial) (i initial) (i initial) (2)

A Model Incorporating Some of the Mechanical and Biochemical Factors Underlying Clot Formation and Dissolution in Flowing Blood

Multiple interacting mechanisms control the formation and dissolution of clots to maintain blood in a state of delicate balance. In addition to a myriad of biochemical reactions, rheological factors also play a crucial role in modulating the response of blood to external stimuli. To date, a comprehensive model for clot formation and dissolution, that takes into account the biochemical, medical and rheological factors, has not been put into place, the existing models emphasizing either one or the other of the factors. In this paper, after discussing the various biochemical, physiologic and rheological factors at some length, we develop a model for clot formation and dissolution that incorporates many of the relevant crucial factors that have a bearing on the problem. The model, though just a first step towards understanding a complex phenomenon, goes further than previous models in integrating the biochemical, physiologic and rheological factors that come into play.

Gut barrier permeability, reticuloendothelial system function and protease inhibitor levels following intestinal ischaemia and reperfusion--effects of pretreatment with N-acetyl-L-cysteine and indomethacin

BACKGROUND

Pathophysiological mechanisms and ways to intervene on intestinal barrier dysfunction following small intestinal ischaemia and prolonged reperfusion are still not fully clarified.

AIMS

To evaluate the effect of oxygen free radical and prostaglandin inhibition on intestinal barrier injury following intestinal ischaemia/reperfusion.

METHODS

Endothelial and epithelial barrier permeability was evaluated by clearance of radiolabelled albumin. Parameters included 125I-Escherichia coli uptake rate index, host reticuloendothelial system function and organ distribution, as well as protease inhibitor and proenzyme activities in rats subjected to small intestinal ischaemia for 40 minutes followed by 12 hours reperfusion (ischaemia/reperfusion), pretreated with N-acetyl-L-cysteine or indomethacin.

RESULTS

Following ischaemia/reperfusion, endothelial and epithelial permeability increased, reticuloendothelial system activation occurred and plasma protease inhibitors were consumed. N-acetyl-L-cysteine pretreatment resulted in improved endothelial and epithelial barrier integrity, a decrease in protease inhibitor consumption and less pronounced reticuloendothelial system activation. Pretreatment with indomethacin was not effective.

CONCLUSION

Oxygen free radicals seem to play an important role in the development of intestinal barrier impairment following ischaemia/reperfusion. N-acetyl-L-cystine may be a potential agent for preventing ischaemia/reperfusion damage.

Linear mixed irreversible inhibition of the autocatalytic activation of zymogens. Kinetic analysis checked by simulated progress curves

Autocatalytic zymogen activation is a phenomenon of great importance for understanding some fundamental physiological processes involved in the enzyme regulation of gastrointestinal-tract enzymes, blood coagulation, fibrinolysis and the complement system. Examples of such processes are the activation of prekallikrein, trypsinogen and pepsinogen, all of which are controlled by natural proteinase inhibitors. This work studies the kinetics of a general autocatalytic zymogen activation process overlapped by two two-step irreversible inhibitions, i.e. a linear mixed irreversible inhibition. The kinetic equations for the whole course of the reaction are derived for this mechanism. In addition, we determine the corresponding kinetics for a number of particular cases of the general model analyzed, i.e. for reversible and irreversible non-competitive, competitive and uncompetitive inhibition systems which are considered particular cases of the general mechanism studied. The kinetic behavior of the system is related to a parameter, a dimensionless quantity, which shows whether the inhibition or the activation route prevails, in a similar way to that which we have previously carried out for other mechanisms. Finally, based on the kinetic equations obtained, a procedure for discriminating between the different mechanisms considered is suggested. The results of this contribution can be directly applied to most physiological autocatalytic zymogen activations in the presence of an inhibitor, allowing their complete kinetic characterization and suggesting procedures for varying the relative weight of the catalytic and inhibition routes or for changing the predominant route.

Leukocyte proteinases in wound healing: roles in physiologic and pathologic processes

Leukocytes express a number of proteinases which play critical roles in physiologic processes during wound healing. However, if the activity of these proteinases is uncontrolled, they can contribute to devastating tissue injury that can affect most organ systems. Until recently, little was known about the mechanisms by which leukocytes retain the activity of their proteinases within the extracellular space which contains highly effective proteinase inhibitors. Studies of the cell biology of leukocyte proteinases have begun to identify the mechanisms by which proteinases can circumvent the effects of physiologic proteinase inhibitors. Herein, we will review the cell biology of leukocyte proteinases, and we will discuss the mechanisms by which leukocyte proteinases can contribute to physiologic processes occurring during wound healing, as well as their roles in pathologic processes.

Wound fluid from venous leg ulcers degrades plasminogen and reduces plasmin generation by keratinocytes

Plasminogen, the pro-enzyme of plasmin, aids various processes essential for normal, acute wound healing, such as fibrinolysis and cell migration. We have investigated if plasminogen is available to perform these functions in chronic wounds such as venous leg ulcers. We report that plasminogen is degraded by fluid from venous leg ulcers to a number of fragments, including kringle domains 1-3, an angiostatin-related protein. The enzyme responsible was inhibited by the serine protease inhibitor phenyl-methylsulfonyl fluoride, but was not inhibited by alpha1-anti-trypsin, an inhibitor of neutrophil elastase, by alpha2-anti-plasmin, an inhibitor of plasmin, or by the matrix metalloprotease inhibitor 1,10 phenanthroline. Plasminogen degraded by wound fluid was a weaker substrate than intact plasminogen for plasmin generation by the keratinocyte cell line HaCaT. These results suggest that serine protease activity in leg ulcer fluid degrades plasminogen and support the hypothesis that keratinocyte migration may be impaired in leg ulcers because of a reduced availability of intact plasminogen for plasmin generation.