Exposure of plasminogen and a novel plasminogen receptor, Plg-RKT, on activated human and murine platelets

Plasminogen activation rates are enhanced by cell surface binding. We previously demonstrated that exogenous plasminogen binds to phosphatidylserine-exposing and spread platelets. Platelets contain plasminogen in their α-granules, but secretion of plasminogen from platelets has not been studied. Recently, a novel transmembrane lysine-dependent plasminogen receptor, Plg-RKT, has been described on macrophages. Here, we analyzed the pool of plasminogen in platelets and examined whether platelets express Plg-RKT. Plasminogen content of the supernatant of resting and collagen/thrombin-stimulated platelets was similar. Pretreatment with the lysine analog, ε-aminocaproic acid, significantly increased platelet-derived plasminogen (0.33 vs 0.08 nmol/108 platelets) in the stimulated supernatant, indicating a lysine-dependent mechanism of membrane retention. Lysine-dependent, platelet-derived plasminogen retention on thrombin and convulxin activated human platelets was confirmed by flow cytometry. Platelets initiated fibrinolytic activity in fluorescently labeled plasminogen-deficient clots and in turbidimetric clot lysis assays. A 17-kDa band, consistent with Plg-RKT, was detected in the platelet membrane fraction by western blotting. Confocal microscopy of stimulated platelets revealed Plg-RKT colocalized with platelet-derived plasminogen on the activated platelet membrane. Plasminogen exposure was significantly attenuated in thrombin- and convulxin-stimulated platelets from Plg-RKT-/- mice compared with Plg-RKT+/+ littermates. Membrane exposure of Plg-RKT was not dependent on plasminogen, as similar levels of the receptor were detected in plasminogen-/- platelets. These data highlight Plg-RKT as a novel plasminogen receptor in human and murine platelets. We show for the first time that platelet-derived plasminogen is retained on the activated platelet membrane and drives local fibrinolysis by enhancing cell surface-mediated plasminogen activation.

Regulation, Location and Activity of Plasminogen Activator Inhibitor 2 (PAI-2) in Peripheral Blood Monocytes, Macrophages and Foam Cells

Monocytes, macrophages and foam cells are central to atherogenesis. We have examined the potential ability of monocytes, macrophages and foam cells to affect the stability of deposited fibrin, characteristic of the atherosclerotic plaque, by their production of plasminogen activators and their inhibitors. Monocytes respond to thrombin and LPS by up-regulation of PAI-2 synthesis, and PAI-2 is their major product among the plasminogen activators/inhibitors. In contrast, macrophages and foam cells, while they did produce PAI-2, did not respond to thrombin and LPS by an increase in its synthesis. All PAI-2 produced by macrophages and foam cells was accumulated intracellularly, whereas monocytes also secreted PAI-2. Secreted PAI-2 was active as an inhibitor of u-PA, whereas intracellular PAI-2 required detergent treatment to generate activity. Thus monocytes, but not macrophages or foam cells, produce and secrete active PAI-2, thus potentially affecting fibrin stability in the local environment.

Thrombi Formed in a Chandler Loop Mimic Human Arterial Thrombi in Structure and PAI-1 Content and Distribution

We have previously shown that whole blood clots prepared under static conditions are a poor model for human thrombi formed in vivo. The most striking contrast is in the PAI-1 content, some 100 times lower in static clots than in human thrombi. This study aimed to evaluate thrombi formed in an artificial circulation (Chandler loop) using whole blood augmented with platelets. Citrated blood was recalcified and placed in tubing, which was sealed to form a closed loop and circulated. Immunohistochemical staining revealed morphology very similar to human thrombi formed in vivo, comprising dense platelet-rich heads and fibrin-rich tails. Strong positive staining for PAI-1 was observed in fibrin-rich areas of both the head and the tail. The PAI-1 content of the thrombi increased significantly on augmentation of blood with isolated platelets, and reached levels comparable with those in human thrombi. These Chandler thrombi thus provide an appropriate model system for the study of thrombus lysis, in contrast to static blood clots.

Proteins of the Fibrinolytic System in Human Thrombi

The proteins of fibrinolysis have been quantified in human thrombi, to assess the balance between plasminogen activators and their major inhibitor PAI-1. The relative roles of PAI-1 and α2-AP were also examined since we have previously shown that both platelet PAI-1 and plasma α2-AP are important determinants of clot lysis in vitro. Extracts and sections were prepared from human thrombi for quantitative immunoassay and immunohistochemical staining respectively. PAI-1 and α2-AP were present at high concentrations. Levels of t-PA and t-PA-PAI-1 complex were relatively low. Staining confirmed the presence of abundant PAI-1, associated primarily with platelet material within the thrombus and also with fibrin. Staining for α2-AP was also intense and demonstrated strong association with fibrin. The α2-AP concentration was similar to its high plasma concentration, whereas PAI-1 levels were up to 30 times greater than that in circulating blood, suggesting that active recruitment of platelets contributes to the high PAI-1 concentration in thrombi.

Peripheral Blood Monocyte Synthesis of Plasminogen Activator Inhibitor 2 in Response to Native and Modified LDL

Fibrin deposition is a characteristic feature of the atherosclerotic plaque. The balance of fibrinolytic activity is modulated by plasminogen activators (PAs) and plasminogen activator inhibitors (PAIs). We examined expression of components of the fibrinolytic system by peripheral blood monocytes following stimulation by native LDL and LDL modified by acetylation, copper oxidation or minimal modification. Monocytes responded to LDL stimulation by increased production of PAI-2, with no corresponding increase in u-PA. PAI-1 was detected but did not change relative to untreated control; u-PA was undetectable in all samples. Native LDL consistently upregulated PAI-2; this stimulation was not inhibited by inclusion of antioxidants. Acetylated, copper oxidized and minimally modified LDLs increased production of PAI-2, but the ability to stimulate PAI-2 synthesis varied between preparations of modified LDL. Increased levels of PAI-2 in a local environment such as the artery wall may promote fibrin persistence.

The Roles of α2-Antiplasmin and Plasminogen Activator Inhibitor 1 (PAI-1) in the Inhibition of Clot Lysis

The relative importance of the two major inhibitors of fibrinolysis, α2-antiplasmin (α2-AP) and plasminogen activator inhibitor (PAI-1), were investigated using a simple microtitre plate system to study fibrin clot lysis in vitro. Cross-linked fibrin clots contained plasminogen and tissue plasminogen activator (t-PA) at concentrations close to physiological. Purified α2-AP and PAI-1 caused dose-dependent inhibition. All the inhibition due to normal plasma, either platelet-rich or poor, was neutralised only by antibodies to α2-AP. Isolated platelets, at a final concentration similar to that in blood, 2.5 × 108/ml, markedly inhibited clot lysis. This inhibition was neutralised only by antibodies to PAI-1. At the normal circulating ratio of plasma to platelets, α2-AP was the dominant inhibitor. When the platelet:plasma ratio was raised some 20-fold, platelet PAI-1 provided a significant contribution. High local concentrations of PAI-1 do occur in thrombi in vivo, indicating a role for PAI-1, complementary to that of α2-AP, in such situations.

Effect of Physiological Concentrations of Vasopressin on Components of the Fibrinolytic System

High physiological concentrations of plasma vasopressin (aVP) when achieved by infusion cause an increase in plasma factor VIII coagulant activity and shortening of the euglobulin clot lysis time (ECLT). To investigate the effects of aVP on components of the fibrinolytic pathway and on thrombin generation, t healthy volunteers were infused with saline for 30 min followed by aVP for L hour and blood samples taken every 30 min for measurement of aVR ECtf tissue-type plasminogen activator (t-PA), t-PA inhibition (tPA-I), plasminogen activator inhibitor L (PAI-1 Ag), activated partial thromboplastin time (APTT), fibrinopeptide A (FPA), fibrinopeptide B 15-42 (FPBß 15-42) and crosslinked fibrin breakdown products (XL-FDP). Plasma aVP rose to a median of 75 pg/ml after 90 min and fell to 13.8 pdml 30 min later. The APTT fell from 43.5 to 35 sec (p <0.01) but there was no change in plasma FPA or in XL-FDP. Plasminogen activator activity (106/ECLT2) increased from 25 to 736 units (p <0.01) and t-PA from 200 to 1012 mlU/ml (p <0.01). tPA-I fell from 8.0 to 2.7 IU/ml at 90 min (p <0.05) but PAI-1 Ag remained unchanged. Plasma FPBß 15-42 was 2.4 and 1.2 pmol/ml before infusion with aVP and showed a small rise to 3.5 pmol/ml after 60 min (p <0.05). The results show the effects of aVP on fibrinolysis are mediated by an increase in t-PA. In the absence of thrombin generation the rise in t-PA was not accompanied by changes in XL-FDP.

A Plasminogen Activator Inhibitor (PAI-2) Circulates in Two Molecular Forms During Pregnancy

During normal pregnancy maternal haemostasis alters to protect against bleeding with a rise in plasma clotting factor levels and increased inhibition of fibrinolysis. The latter is due in part to increased levels of the 48 kDa plasminogen activator inhibitor (PAI-1) present in the plasma of non-pregnant individuals. A second plasminogen activator inhibitor (PAI-2) occurs in placenta and in a cultured histiocytic lymphoma cell line. We report here the identification by SDS-PAGE and zymography of PAI-2 in plasma during normal pregnancy. PAI-2 was present in two molecular forms of about 75 and 130 kDa, which were detectable at 12 weeks gestation and which persisted in the maternal circulation for up to 7 days after delivery. These forms of PAI-2 appear to be distinct from purifed PAI-2, which has a molecular mass of 47–60 kDa and which is not normally detectable in this zymographic system, since it is sensitive to denaturants. The novel forms of PAI-2 may represent complexes or aggregates that retain activity after SDS-PAGE.

An Enzyme-Linked Immunosorbent Assay (ELISA) Used to Study the Cellular Secretion of Endothelial Plasminogen Activator Inhibitor (PAI-1)

A two-site sandwich ELISA was developed to measure PAI-1 antigen and utilised a polyclonal antiserum produced against PAI-1 purified from human endothelial cell secretory products. The assay was calibrated against a preparation of pure PAI-1 whose protein concentration had been determined by amino acid analysis and the detection limit was 30 pg PAI-1 ml“3 sample. PAI-1 was detected in primate sera but not in a wide range of nonprimate sera and no cross-reactivity with α2-antiplasmin or antithrombin III was observed.

The ELISA was used to study cellular secretion of PAI-1 which was confirmed as a major secretory protein in human umbilical vein endothelial cells (HUVEC). PAI-1 antigen accumulated in the medium in a linear fashion with time and accounted for ≃10% of total secreted protein. Specific activity of intracellular PAI1 was typically 20-fold greater than that of PAI-1 in 24 h conditioned medium and a t½ for inactivation of secreted PAI-1 of 0.53 h was calculated. Purified endotoxin stimulated the secretion of PAI-1 antigen and raised the intracellular levels in HUVEC cultures showing that the anti-fibrinolytic actions of endotoxin are effected by increasing the rate of synthesis and secretion of PAI-1.

Intraluminal Thrombus Enhances Proteolysis in Abdominal Aortic Aneurysms

This study examined whether intraluminal thrombus in abdominal aortic aneurysms (AAAs) is a source of fibrinolytic activity and proteolysis that could weaken the aneurysm wall. Plasmin, tissue plasminogen activator (tPA), and urokinase plasminogen activator (uPA) activity, plasminogen activator inhibitor 1 (PAI-1), and α2-antiplasmin (α2AP) antigen were measured in the AAA wall and juxtamural and luminal aspects of intraluminal thrombus in 18 patients. The aneurysm wall contained 100-fold higher tPA activity (1.06 ± 0.34 [standard error of measurement] U/mg soluble protein) compared with juxtamural thrombus (JMT) (0.011 ± 0.001 ) and luminal thrombus (LT) (0.01 ± 0.001) ( p < .00001) and over 6-fold higher uPA activity (29.3 ± 3.4 IU/mg compared with the JMT (4.3 ± 2.4, p = .00024) and LT (7.9 ± 1.76, p = .0005). The LT had significantly lower levels of PAI-1 (1.26 ± 0.34 ng/mg) than the AAA wall (2.08 ± 0.51, p = .04) and the JMT (3.94 ± 0.85, p = .007). The levels of α2AP in the wall (19.4 ± 3.1 ng/mg) were lower than in the JMT or LT (43.0 ± 7.9 ng/mg, p = .013, and 47.6 ± 6.0 ng/mg, p = .002, respectively). There was no significant difference, however, in plasmin activity among the AAA wall, JMT, and LT. There were significant amounts of latent gelatinase B (matrix metalloproteinase [MMP]-9) in the AAA, JMT, and LT. Mean levels of activated MMP-9 activity were similar in the AAA, JMT, and LT. Plasmin activation of MMPs at the interface between intraluminal thrombus and the aneurysm wall may enhance proteolysis and accelerate aneurysm expansion.

Ventilator-associated pneumonia is characterized by excessive release of neutrophil proteases in the lung

BACKGROUND

Ventilator-associated pneumonia (VAP) is characterized by neutrophils infiltrating the alveolar space. VAP is associated with high mortality, and accurate diagnosis remains difficult. We hypothesized that proteolytic enzymes from neutrophils would be significantly increased and locally produced inhibitors of human neutrophil elastase (HNE) would be decreased in BAL fluid (BALF) from patients with confirmed VAP. We postulated that in suspected VAP, neutrophil proteases in BALF may help identify "true" VAP.

METHODS

BAL was performed in 55 patients with suspected VAP and in 18 control subjects. Isolation of a pathogen(s) at > 10⁴ colony-forming units/mL of BALF dichotomized patients into VAP (n = 12) and non-VAP (n = 43) groups. Matrix metalloproteinases (MMPs), HNE, inhibitors of HNE, and tissue inhibitors of matrix metalloproteinases (TIMPs) were quantified. Plasminogen activator (PA) activity was estimated by sodium dodecyl sulfate polyacrylamide gel electrophoresis and zymography.

RESULTS

Neutrophil-derived proteases HNE, MMP-8, and MMP-9 were significantly increased in cell-free BALF from patients with VAP as compared with those without VAP (median values: HNE, 2,708 ng/mL vs 294 ng/mL, P < .01; MMP-8, 184 ng/mL vs 5 ng/mL, P < .01; MMP-9, 310 ng/mL vs 11 ng/mL, P < .01). HNE activity was also significantly increased in VAP (0.45 vs 0.01 arbitrary units; P < .05). In contrast, no significant differences were observed for protease inhibitors, TIMPs, or PAs. HNE in BALF, at a cutoff of 670 ng/mL, identified VAP with a sensitivity of 93% and specificity of 79%.

CONCLUSIONS

Neutrophil proteases are significantly elevated in the alveolar space in VAP and may contribute to pathogenesis. Neutrophil proteases appear to have potential in suspected VAP for distinguishing true cases from "non-VAP" cases.

Salt bridges regulate both dimer formation and monomeric flexibility in HdeB and may have a role in periplasmic chaperone function

Escherichia coli and Gram-negative bacteria that live in the human gut must be able to tolerate rapid and large changes in environmental pH. Low pH irreversibly denatures and precipitates many bacterial proteins. While cytoplasmic proteins are well buffered against such swings, periplasmic proteins are not. Instead, it appears that some bacteria utilize chaperone proteins that stabilize periplasmic proteins, preventing their precipitation. Two highly expressed and related proteins, HdeA and HdeB, have been identified as acid-activated chaperones. The structure of HdeA is known and a mechanism for activation has been proposed. In this model, dimeric HdeA dissociates at low pH, and the exposed dimeric interface binds exposed hydrophobic surfaces of acid-denatured proteins, preventing their irreversible aggregation. We now report the structure and biophysical characterization of the HdeB protein. The monomer of HdeB shares a similar structure with HdeA, but its dimeric interface is different in composition and spatial location. We have used fluorescence to study the behavior of HdeB as pH is lowered, and like HdeA, it dissociates to monomers. We have identified one of the key intersubunit interactions that controls pH-induced monomerization. Our analysis identifies a structural interaction within the HdeB monomer that is disrupted as pH is lowered, leading to enhanced structural flexibility.

Measurement of fibrin concentration by fast field-cycling NMR

The relaxation of (1)H nuclei due to their interaction with quadrupolar (14)N nuclei in gel structures is measured using fast field-cycling NMR. This phenomenon called quadrupolar dips has been reported in different (1)H-(14)N bond-rich species. In this study, we have studied quadrupolar dips in fibrin, an insoluble protein that is the core matrix of thrombi. Fibrin was formed by the addition of thrombin to fibrinogen in 0.2% agarose gel. T(1)-dispersion curves were measured using fast field-cycling NMR relaxometry, over the field range of 1.5-3.5 MHz (proton Larmor frequency), and were analyzed using a curve-fitting algorithm. A linear increase of signal amplitude with increasing fibrin concentration was observed. This agrees with the current theory that predicts a linear relationship of signal amplitude with the concentration of contributing (14)N spins in the sample. Interestingly, fibrin formation gave rise to the signal, regardless of crosslinking induced by the transglutaminase factor XIIIa. To investigate the effect of proteins that might be trapped in the thrombi in vivo, the plasma protein albumin was added to the fibrin gel, and an increase in the quadrupolar signal amplitude was observed. This study can potentially be useful for thrombi classification by fast field-cycling MRI techniques.

Fractal structures in stenoses and aneurysms in blood vessels

Recent advances in the field of chaotic advection provide the impetus to revisit the dynamics of particles transported by blood flow in the presence of vessel wall irregularities. The irregularity, being either a narrowing or expansion of the vessel, mimicking stenoses or aneurysms, generates abnormal flow patterns that lead to a peculiar filamentary distribution of advected particles, which, in the blood, would include platelets. Using a simple model, we show how the filamentary distribution depends on the size of the vessel wall irregularity, and how it varies under resting or exercise conditions. The particles transported by blood flow that spend a long time around a disturbance either stick to the vessel wall or reside on fractal filaments. We show that the faster flow associated with exercise creates widespread filaments where particles can get trapped for a longer time, thus allowing for the possible activation of such particles. We argue, based on previous results in the field of active processes in flows, that the non-trivial long-time distribution of transported particles has the potential to have major effects on biochemical processes occurring in blood flow, including the activation and deposition of platelets. One aspect of the generality of our approach is that it also applies to other relevant biological processes, an example being the coexistence of plankton species investigated previously.

Model thrombi formed under flow reveal the role of factor XIII-mediated cross-linking in resistance to fibrinolysis

BACKGROUND

Activated factor XIII (FXIIIa), a transglutaminase, introduces fibrin-fibrin and fibrin-inhibitor cross-links, resulting in more mechanically stable clots. The impact of cross-linking on resistance to fibrinolysis has proved challenging to evaluate quantitatively.

METHODS

We used a whole blood model thrombus system to characterize the role of cross-linking in resistance to fibrinolytic degradation. Model thrombi, which mimic arterial thrombi formed in vivo, were prepared with incorporated fluorescently labeled fibrinogen, in order to allow quantification of fibrinolysis as released fluorescence units per minute.

RESULTS

A site-specific inhibitor of transglutaminases, added to blood from normal donors, yielded model thrombi that lysed more easily, either spontaneously or by plasminogen activators. This was observed both in the cell/platelet-rich head and fibrin-rich tail. Model thrombi from an FXIII-deficient patient lysed more quickly than normal thrombi; replacement therapy with FXIII concentrate normalized lysis. In vitro addition of purified FXIII to the patient's preprophylaxis blood, but not to normal control blood, resulted in more stable thrombi, indicating no further efficacy of supraphysiologic FXIII. However, addition of tissue transglutaminase, which is synthesized by endothelial cells, generated thrombi that were more resistant to fibrinolysis; this may stabilize mural thrombi in vivo.

CONCLUSIONS

Model thrombi formed under flow, even those prepared as plasma 'thrombi', reveal the effect of FXIII on fibrinolysis. Although very low levels of FXIII are known to produce mechanical clot stability, and to achieve γ-dimerization, they appear to be suboptimal in conferring full resistance to fibrinolysis.

Activation of single-chain urokinase-type plasminogen activator by platelet-associated plasminogen: a mechanism for stimulation of fibrinolysis by platelets

BACKGROUND AND OBJECTIVE

Platelets are essential for hemostasis, and they cause resistance to fibrinolysis by tissue-type plasminogen activator. In contrast, platelets enhance fibrinolysis mediated by single-chain urokinase-type plasminogen activator (scu-PA). This study investigated the mechanism behind this profibrinolytic role of platelets.

METHODS AND RESULTS

Platelets enhanced scu-PA activity, but not urokinase-type plasminogen activator (u-PA) activity, in plasma clot lysis and chromogenic assays. We established, using the non-cleavable scu-PA mutant (Lys158-->Glu) and protease inhibitors, that platelets increased activation to u-PA by a serine protease. Activation of scu-PA was platelet-dependent, even in plasma. It occurred in platelet-rich but not in platelet-poor plasma, as assessed by sodium dodecylsulfate polyacrylamide gel electrophoresis and zymography after addition of plasminogen activator inhibitor-1. Candidate proteases that are known to activate scu-PA and are present in platelet preparations were investigated. Factor VII activating protease was detected in platelet preparations by western blotting, but its inhibition by antibodies did not inhibit activation of scu-PA by platelets. Plasmin and plasma kallikrein both mimicked the platelet effect, but were distinguished by their responses to a range of inhibitors. Analysis of platelet-associated protease activity and the time course of scu-PA activation pointed towards plasminogen, and the data were consistent with a mechanism of reciprocal activation. The essential role of plasminogen was revealed using platelets from plasminogen-deficient mice, which could not activate scu-PA. Local plasminogen on platelet membranes was markedly more effective than solution-phase plasminogen in activation of scu-PA.

CONCLUSIONS

Platelets enhance fibrinolysis by scu-PA through reciprocal activation of scu-PA and platelet-associated plasminogen, a system that is potentially important in the lysis of platelet-rich thrombi.

Effect of the small molecule plasminogen activator inhibitor-1 (PAI-1) inhibitor, PAI-749, in clinical models of fibrinolysis

BACKGROUND

The principal inhibitor of fibrinolysis in vivo is plasminogen activator inhibitor-1 (PAI-1). PAI-749 is a small molecule inhibitor of PAI-1 with proven antithrombotic efficacy in several preclinical models.

OBJECTIVE

To assess the effect of PAI-749, by using an established ex vivo clinical model of thrombosis and a range of complementary in vitro human plasma-based and whole blood-based models of fibrinolysis.

METHODS

In a double-blind, randomized, crossover study, ex vivo thrombus formation was assessed using the Badimon chamber in 12 healthy volunteers during extracorporeal administration of tissue-type plasminogen activator (t-PA) in the presence of PAI-749 or control. t-PA-mediated lysis of plasma clots and of whole blood model thrombi were assessed in vitro. The role of vitronectin was examined by assessing lysis of fibrin clots generated from purified plasma proteins.

RESULTS

There was a dose-dependent reduction in ex vivo thrombus formation by t-PA (P < 0.0001). PAI-749 had no effect on in vitro or ex vivo thrombus formation or fibrinolysis in the presence or absence of t-PA. Inhibition of PAI-1 with a blocking antibody enhanced fibrinolysis in vitro (P < 0.05).

CONCLUSIONS

Despite its efficacy in a purified human system and in preclinical models of thrombosis, the current study suggests that PAI-749 does not affect thrombus formation or fibrinolysis in a range of established human plasma and whole blood-based systems.

Chaotic advection in blood flow

In this paper we argue that the effects of irregular chaotic motion of particles transported by blood can play a major role in the development of serious circulatory diseases. Vessel wall irregularities modify the flow field, changing in a nontrivial way the transport and activation of biochemically active particles. We argue that blood particle transport is often chaotic in realistic physiological conditions. We also argue that this chaotic behavior of the flow has crucial consequences for the dynamics of important processes in the blood, such as the activation of platelets which are involved in the thrombus formation.

Bispecific targeting of thrombin activatable fibrinolysis inhibitor and plasminogen activator inhibitor-1 by a heterodimer diabody

BACKGROUND AND OBJECTIVES

Thrombin activatable fibrinolysis inhibitor (TAFI) and plasminogen activator inhibitor-1 (PAI-1) play important roles in fibrinolysis. Both reduce plasmin generation, but they exert their antifibrinolytic effects via different mechanisms. This study reports the cloning and characterization of a heterodimer diabody that inhibits TAFI and PAI-1 simultaneously.

METHODS AND RESULTS

The diabody was derived from two inhibiting monoclonal antibodies, i.e. MA-33H1F7, an anti-PAI-1 antibody that induces non-inhibitory substrate behavior of PAI-1, and MA-T12D11, an anti-TAFI antibody that inhibits activation of TAFI by the thrombin-thrombomodulin complex. A single-chain variable fragment (scFv) was derived from MA-T12D11 that displayed slightly reduced binding and inhibitory properties as compared to MA-T12D11. Characterization of the diabody revealed a similar affinity for TAFI and PAI-1 as that of the parental antibodies. Furthermore, the inhibitory properties of MA-33H1F7 and MA-T12D11 were fully preserved in the diabody format. In platelet-free plasma (PFP) clots, addition of the diabody had a stronger effect in shortening lysis times than either MA-T12D11 or MA-33H1F7. A similar reduction in clot lysis time was observed in platelet-rich plasma (PRP) clots. The same effect on clot lysis times in PFP and PRP was also achieved by the combined addition of MA-T12D11 and MA-33H1F7. The lysis rate of human model thrombi, made from whole blood, was approximately doubled after addition of the diabody. Moreover, this effect was significantly better than after the combined addition of the individual antibodies.

CONCLUSIONS

These observations demonstrate that simultaneous inhibition of TAFI and PAI-1 results in faster lysis of the formed thrombus.

Haemorrhage associated with large abdominal aortic aneurysms

Two patients are described who each had a major haemorrhage after minor surgery and who additionally had large abdominal aortic aneurysms. Neither had depletion of platelets or coagulation factors sufficient to account for bleeding. Both had grossly enhanced fibrinolytic activity with evidence of plasmin generation in the circulation. In one patient insertion of an aortic graft was rapidly followed by resolution of the fibrinolytic abnormalities. Overactive fibrinolysis, rather than coagulation factor consumption may account for some of the bleeding syndromes complicating abdominal aortic aneurysms.

Plasminogen binding by oral streptococci from dental plaque and inflammatory lesions

Plasminogen binding by bacteria is a virulence factor important for the entry and dissemination of bacteria in the body. A wide variety of bacteria bind plasminogen, including both organisms causing disease and components of the normal oral flora. The purpose of this study was to examine the characteristics of plasminogen binding by six clinical isolates of oral streptococci from both dental plaque and inflammatory lesions. All the strains bound plasminogen with approximately the same affinity, and binding was specific and lysine-dependent as evidenced by its inhibition by epsilon-aminocaproic acid. All of the test strains were capable of activating bound plasminogen to plasmin without the addition of a plasminogen activator, and subsequent analysis revealed the presence of streptokinase in all strains. However, the streptococci exhibited fibrinolytic activity only in the presence of plasminogen and this could be inhibited by the addition of epsilon-aminocaproic acid. SDS-PAGE and 2D gel electrophoresis coupled with plasminogen ligand blotting showed that only a subset of the total proteins (2-15) were involved in the binding of plasminogen. Partial identification of the binding proteins revealed that four glycolytic enzymes, enolase, phosphoglycerate kinase, glyceraldehyde-3-phosphate dehydrogenase and phosphoglycerate mutase, were predominant in binding plasminogen. The binding of plasminogen by bacteria from pus did not differ from that of the strains from supragingival plaque. The findings illustrate how apparently innocuous commensal bacteria are capable of utilizing a mechanism that is generally regarded as being of importance to pathogenicity and suggest an additional role of plasminogen binding.

TAFIa, PAI-1 and alpha-antiplasmin: complementary roles in regulating lysis of thrombi and plasma clots

PAI-1 and alpha(2)-antiplasmin (alpha(2)AP) are the principal direct inhibitors of fibrinolytic proteases. Thrombin activatable fibrinolysis inhibitor (TAFI), a plasma procarboxypeptidase activated by thrombin-thrombomodulin to form TAFIa, also regulates fibrinolysis by modulating fibrin. In this study, the relative contributions of PAI-1, alpha(2)AP and TAFIa to inhibition of lysis were assessed. In platelet-poor plasma clots, alpha(2)AP, TAFIa and PAI-1 all inhibited lysis, as shown by the addition of neutralizing antibodies to alpha(2)AP and PAI-1 +/- CPI, a potato carboxypeptidase inhibitor. alpha(2)AP played the largest role in regulating plasma clot lysis, but neutralization of inhibitors in combinations was more effective in shortening lysis times, with a maximal effect when all three inhibitors were neutralized. In platelet-rich clots, a larger contribution of PAI-1 was evident. Tissue plasminogen activator induced lysis of model thrombi, made from whole blood, was approximately doubled on incorporation of CPI, illustrating a substantial contribution of TAFIa to inhibition of thrombus lysis. Similar increases in thrombus lysis were observed on inclusion of neutralizing antibodies to PAI-1 and alpha(2)AP, with alpha(2)AP playing the dominant role. Maximal thrombus lysis occurred upon neutralization of all three inhibitors. These observations suggest that, despite the differences in concentrations and activities of inhibitors, and the different modes of action, the roles of the three are complementary in both plasma clot lysis and thrombus lysis.

The C-terminus of ?2-antiplasmin interacts with endothelial cells

The serpin, alpha(2)-antiplasmin (alpha(2)AP), has an extended C-terminus relative to other inhibitors. This 51-residue region contains an RGD sequence; such sequences constitute a key recognition sequence for cell adhesion, mediated through integrins. In the present study, this sequence was expressed in Escherichia coli and its binding to endothelial cells and whether binding depends on the RGD sequence was investigated. Binding to the surface of human umbilical vein endothelial cells (HUVEC-C) was observed by flow cytometry and immunohistochemistry. Binding studies on immobilised cells showed specific and RGD-dependent binding of the peptides to HUVEC-C. The binding of the wild-type peptide to the HUVEC-C was significantly higher than that of a mutant peptide, in which RGD was replaced by SAA (P < 0.05, n = 4). Similarly, ethylenediaminetetraacetic acid decreased the binding of the wild-type peptide (P < 0.05, n = 4). The binding was competed out by full-length alpha(2)AP, fibronectin and anti-alpha(5)beta(1). This is the first evidence of binding of the C-terminus of alpha(2)AP to endothelial cells via its RGD sequence, with most but not all of the binding being integrin-mediated. We speculate that this interaction with alpha(2)AP may potentially play a role in the control of cellular fibrinolysis by regulating local plasmin activity on cell surfaces.

The influence of anti-endothelial/antiphospholipid antibodies on fibrin formation and lysis on endothelial cells

The prothrombotic mechanisms associated with antiphospholipid antibodies remain incompletely defined. Antibody binding to endothelial cells in vitro is a feature of antiphospholipid antibody-positive sera. We hypothesised that impairment of endothelium-dependent fibrinolysis by antiphospholipid/anti-endothelial antibodies is a contributory factor in the pathogenesis of thrombosis. We also aimed to confirm the displacement of annexin-V from endothelial cells and enhanced fibrin formation. Binding of immunoglobulin (Ig) from antiphospholipid antibody-positive sera to endothelial cells was examined using a cell-based enzyme-linked immunosorbent assay. Effects on fibrin formation and lysis were examined on cultured endothelial cell monolayers. Plasminogen activator inhibitor-1 (PAI-1) was assayed in supernatants. We confirmed antibody binding to endothelial cells. With four of 14 antiphospholipid antibody-positive sera there was some prolongation of fibrin clot lysis time, consistent with impairment of endothelial fibrinolytic activity. Secretion of PAI-1 was significantly correlated with clot lysis time on endothelial cell monolayers incubated with antiphospholipid/anti-endothelial antibody-positive sera, but not with control sera. IgG from antiphospholipid antibody-positive sera had little effect on endothelial cell surface annexin-V expression. We conclude that impaired endothelial fibrinolysis is a potential prothrombotic mechanism in subjects with antiphospholipid antibodies. We were unable to confirm enhanced displacement of annexin-V from endothelium by antiphospholipid antibodies.

Thrombus lysis by uPA, scuPA and tPA is regulated by plasma TAFI

The carboxypeptidase, TAFIa or CPU, is known to prolong plasma clot lysis by tissue plasminogen activator (tPA) and to have a role in thrombus stability in vivo. This current study examined lysis by urokinase (uPA) and single chain urokinase (scuPA) in addition to tPA. Further, we investigated the role of TAFIa in a model thrombus system, in which thrombi are formed under conditions of flow. We show that human thrombi, formed in vivo, and model thrombi both contain TAFI. No effect of thrombus TAFIa was observed in thrombus lysis assays, except when thrombi were bathed in plasma, in which case addition of potato tuber carboxypeptidase inhibitor (CPI) resulted in doubling of the rate of lysis. TAFIa inhibited lysis of model thrombi and plasma clots by uPA, scuPA in addition to lysis by tPA. The effect of TAFIa was more evident at high concentrations of plasminogen activator such as those used in thrombolytic therapy. Addition of plasminogen increased lysis and, in its presence, the enhancement by CPI was smaller. Thus the action of TAFIa could be partially overcome by plasminogen, whether lysis was by tPA, uPA or scuPA. This is consistent with TAFIa exerting its effect primarily through modifying the binding of plasminogen to fibrin and to a lesser extent through modification of the binding of tPA to fibrin.

Candida albicans binds human plasminogen: identification of eight plasminogen-binding proteins

Several microbial pathogens augment their invasive potential by binding and activating human plasminogen to generate the proteolytic enzyme plasmin. Yeast cells and cell wall proteins (CWP) of the human pathogenic fungus Candida albicans bound plasminogen with a K(d) of 70 +/- 11 nM and 112 +/- 20 nM respectively. Bound plasminogen could be activated to plasmin by mammalian plasminogen activators; no C. albicans plasminogen activator was detected. Binding of plasminogen to CWP and whole cells was inhibited by epsilon ACA, indicating that binding was predominantly to lysine residues. Candida albicans mutant strains defective in protein glycosylation did not show altered plasminogen binding, suggesting that binding was not mediated via a surface lectin. Binding was sensitive to digestion by basic carboxypeptidase, implicating C-terminal lysine residues in binding. Proteomic analysis identified eight major plasminogen-binding proteins in isolated CWP. Five of these (phosphoglycerate mutase, alcohol dehydrogenase, thioredoxin peroxidase, catalase, transcription elongation factor) had C-terminal lysine residues and three (glyceraldehyde-3-phosphate dehydrogenase, phosphoglycerate kinase and fructose bisphosphate aldolase) did not. Activation of plasminogen could potentially increase the capacity of this pathogenic fungus for tissue invasion and necrosis. Although surface-bound plasmin(ogen) degraded fibrin, no direct evidence for a role in invasion of endothelial matrix or in penetration and damage of endothelial cells was found.

Localization and identification of thrombin and plasminogen activator activities in model human thrombi by in situ zymography

Human thrombi vary in their susceptibility to lysis and this is clinically important. Several potential contributory factors were examined in this study by using model thrombi, created under flow; these provide a robust, reproducible and easily-manipulated system. Here we identify the plasminogen activators (PA) active in model thrombi of known age and define the cellular and plasma contribution to activity in different areas. The cell-rich head of model thrombi had strong thrombin and PA activity, with coagulant activity also at the tail. Thrombin activity decreased as model thrombi were aged. PA activity in the thrombus head also decreased on ageing of thrombi but activity emerged around the thrombi, including the tail. Activity in the head of fresh model thrombi was primarily due to uPA, with some contribution from tPA. Experiments with thrombi prepared from platelet-rich plasma and added leucocytes showed that uPA activity at the head of fresh thrombi was derived from PMN. Older thrombi had tPA activity around the tail of the thrombus; this activity occurred in the absence of cells. This study highlights the importance of PMN-derived uPA activity in the lysis of fresh thrombi, with activity originating in the leucocyte-rich head. It also shows that thrombi are dynamic structures in which fibrin can be repeatedly laid down and lysed, observations that are relevant to therapeutic lysis and potential rethrombosis.

Polymorphonuclear leucocytes have two opposing roles in fibrinolysis

Polymorphonuclear leucocytes (PMN) are important in the resolution of human thrombi, with u-PA as a key player. We have shown that the u-PA activity of PMN depends on the presence of plasma; the study presented here provides an explanation for that requirement. Here we show that PMN degraded scu-PA and also tcu-PA, t-PA and plasmin, resulting in loss of fibrinolytic activity. Plasma protected against this degradation; alpha1-antitrypsin was identified as a protective factor. Purified human neutrophil elastase mirrored the effects of PMN, again neutralized by plasma inhibitors. These findings illustrate the dual role of PMN in the breakdown of thrombi, in that they contribute both u-PA, which lyses fibrin, and other proteases, including elastase, which can cleave fibrin and plasminogen activators/plasmin. Similarly, plasma can potentiate fibrinolysis by neutralization of PMN elastase, in addition to direct inhibition of fibrinolytic proteases. Our previous studies show that PMN in thrombi are mostly pro-fibrinolytic; the anti-fibrinolytic role defined here may be important in other pathologies where fibrin persists.

The symbiosis of Bacillus subtilis L-forms with Chinese cabbage seedlings inhibits conidial germination of Botrytis cinerea

AIMS

To establish whether germination of Botrytis cinerea was affected by the symbiosis of Bacillus subtilis L-form bacteria with Chinese cabbage.

METHODS AND RESULTS

Germinating seeds of Chinese cabbage were co-cultivated with either L-forms of Bacillus subtilis or 5% (w/v) mannitol by soaking for 3 h. Seeds were then washed in sterile water, sown on a minimal medium and incubated in controlled conditions. L-form symbiosis was detected over a time course by ELISA. Conidial germination of Botrytis cinerea was significantly reduced on cotyledonous leaves of L-form-treated plants compared with controls.

CONCLUSIONS

Symbiosis of B. subtilis L-form bacteria during seed germination of Chinese cabbage inhibits conidial germination in plants on subsequent exposure to Botrytis cinerea.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first account of plant symbiosis with L-form bacteria showing antagonism to a fungal plant pathogen. This has promising implications for the use of this L-form as a biocontrol agent.

Human thrombi contain an abundance of active thrombin

This study assessed the abundance and activity of thrombin in human thrombi. removed at autopsy or during surgery. Arterial and venous thrombus sections showed thrombin activity by in situ zymography, based on conversion of fibrinogen to fibrin. Hirudin or antibodies to thrombin abolished the activity. Thrombin activity in extracts of 40 thrombi was quantified by cleavage of fibrinogen or small peptide substrates: the results correlated well (r = 0.87, p<0.0001) with a median activity of about 4.5 IU/g of thrombus (wet weight). Activity correlated poorly with total prothrombin (median 27 microg/g) and was inversely related to antithrombin, but not to PAI-1. Zymography showed two major active bands, thrombin at 37 kDa, and a 50 kDa form that probably corresponds to meizothrombin desF1. The abundant local thrombin demonstrated here has implications for thrombus lysis and extension: incomplete lysis and exposure of active thrombin may lead to re-occlusion of vessels.

Thrombin upregulates tissue transglutaminase in endothelial cells: a potential role for tissue transglutaminase in stability of atherosclerotic plaque

Atherosclerosis is characterized by thickening of the vessel wall, smooth muscle cell proliferation, macrophage infiltration, and deposition of a fibrin network. Transglutaminases are a family of enzymes catalyzing the formation of stable covalent cross-links between proteins. Here, we show that tissue transglutaminase (tTG) synthesis by human umbilical vein endothelial cells is upregulated by thrombin, the serine protease that causes fibrin formation and many cellular inflammatory effects. Thrombin upregulated tTG 2-fold at the mRNA and protein level. Cellular cross-linking activity was increased to an even greater extent; antibody to tTG neutralized the increased activity. The effect on tTG expression required active thrombin and was mediated mainly through protease-activated receptor-1, a thrombin receptor. Increased tTG antigen and activity were evident in human umbilical vein endothelial cells and extracellular matrix in situ. Thrombin treatment also led to a cellular redistribution of tTG. Normal vessel wall stained positively for tTG in the smooth muscle cells and in the subendothelium. The intensity of staining increased in vessel walls with plaque, where there was a striking increase in tTG in the smooth muscle cells immediately below the plaque. These studies indicate a role for tTG in the stabilization of atherosclerotic plaques and suggest that its local expression can be controlled by thrombin.

Characterization of crosslinking sites in fibrinogen for plasminogen activator inhibitor 2 (PAI-2)

PAI-2 is a serpin that can be crosslinked to fibrin(ogen) via the Gln-Gln-Ile-Gln sequence (residues 83-86). We have characterized the lysine residues in fibrinogen to which PAI-2 is crosslinked by tissue transglutaminase and factor XIIIa. There was no competition with the crosslinking of alpha 2-antiplasmin, another inhibitor of fibrinolysis, which was specific for Lys 303 in the A alpha chain. PAI-2 was crosslinked to several lysine residues, all in the A alpha chain, 148, 176, 183, 230, 413, and 457, but not to Lys 303. The contrast with alpha 2-antiplasmin was clear from studies with truncated fibrinogens and competition by peptides. This was confirmed and extended by mass spectrometry of peptides after protease digestion of crosslinked products, which identified the lysine residues to which the inhibitors were crosslinked. PAI-2 remained active after cross-linking and inhibited fibrin breakdown, even by two-chain t-PA. Thus, a second inhibitor of fibrinolysis, in addition to alpha 2-antiplasmin, is crosslinked to fibrin and protects it from lysis.

Plasminogen activator inhibitor-1 and haemostasis in obesity

The connection between obesity and disordered haemostasis is well established, but incompletely understood. There is a strong link between inhibition of fibrinolysis and obesity, and elevation of the plasma inhibitor, plasminogen activator inhibitor-1 (PAI-1), is regarded as a central factor. Here we explore the increased risk of atherothrombotic disorders in obese subjects, and the evidence for metabolic and genetic causes. There is a clear relationship between plasma PAI-1 and obesity, and adipose tissue synthesises PAI-1, as has been shown in mouse and rat models, and more recently in human material. This tissue also produces several effector molecules that can up regulate PAI-1. These molecules include transforming growth factor beta, tumour necrosis factor alpha, angiotensin II and interleukin 6, all of which up regulate PAI-1 in various cell types. The issue of whether adipose tissue directly contributes to plasma PAI-1, or whether it primarily contributes indirectly, its products stimulating other cells to produce PAI-1 that feeds into the plasma pool, is not yet resolved. Finally, we briefly examine other proteins of haemostasis that are products of adipose tissue. Further studies are needed to define the regulation of these proteins, in adipose tissue itself and in other cells influenced by its products, in order to extend recent insights into the links between obesity and haemostasis.

Polymorphonuclear leucocytes mediate endogenous thrombus lysis via a u-PA-dependent mechanism

Many human thrombi lyse spontaneously without the administration of lytic drugs and cause no clinical symptoms. The mechanisms by which this occurs are incompletely understood. We found that model thrombi prepared from whole human blood in a Chandler loop also exhibited significant spontaneous lysis. Lysis was inhibited by chemical protease inhibitors, consistent with proteolysis resulting primarily from serine proteases, with a small contribution from matrix metalloproteinases. Whole blood was fractionated into platelet-rich plasma and cell populations. Significant spontaneous lysis was observed in platelet-rich thrombi enriched with polymorphonuclear leucocytes (PMNs), whereas mononuclear cells (MCs) and erythrocytes did not contribute to lysis. Incorporation of antibodies to urokinase (u-PA) and its receptor u-PAR neutralized a large proportion of the activity. Incubation of plasma with PMNs generated free u-PA activity, which was also detectable in model thrombi and in vivo human thrombi. Purified neutrophils, free of eosinophils, generated activity identical to PMNs. Smaller contributions to lysis by tissue-type plasminogen activator (t-PA), elastase and cathepsin G were also identified. These findings suggest a major role for circulating PMNs in endogenous thrombus lysis.

Myeloid leukaemic cells can lyse fibrin directly

Purified preparations of circulating leukaemic blast cells from patients with acute myeloid (M1-7) or acute lymphoblastic leukaemia, and the myeloid or lymphoid cells from patients with chronic myeloid or lymphocytic forms of leukaemia, were incorporated into clots prepared from fibrinogen and plasminogen. Patterns of lysis were followed and measured by light transmission in a microtitre plate reader. Mature polymorphonuclear and mononuclear cell fractions from normal individuals were studied concurrently for comparison. Blast cells from the myeloid forms of acute leukaemia (M2-4) and 'myeloid' cell fractions from patients with chronic myeloid leukaemia were capable of lysing plasminogen-containing clots; this activity was neutralized by addition of immunoglobulin against urokinase plasminogen activator (u-PA), but not by anti-tissue plasmogen activator (t-PA). Mature polymorphonuclear and mononuclear cells from normal individuals lacked lytic activity, as did the leukaemic cells from patients with acute lymphoblastic or chronic lymphocytic leukaemia. Lysed blast cells showed the presence of free plasminogen activator on sodium dodecyl sulphate polyacrylamide gel electrophoresis (SDS-PAGE) with overlay zymography, also neutralized by anti-u-PA, whereas normal polymorphonuclear and mononuclear cells did not. These observations suggest that mechanisms underlying some forms of severe bleeding in acute myeloid leukaemias have a critical fibrinolytic component generated by the blast cells themselves.

Effective lysis of model thrombi by a t-PA mutant (A473S) that is resistant to alpha2-antiplasmin

This study used two mutants of tissue-type plasminogen activator (t-PA) with resistance to inhibitors of fibrinolysis to define the contribution of plasminogen activator inhibitor (PAI)-1 and alpha2-antiplasmin (alpha2-AP) to the control of fibrin lysis. Wild-type t-PA was compared with KHRR296-299AAAA, which is resistant to PAI-1, and with A473S, which is resistant to alpha2-AP. We examined these forms of t-PA in model systems that are physiologically relevant. Neutralization of alpha2-AP was essential for lysis of plasma clots, irrespective of their platelet content, by either wild-type t-PA or KHRR296-299AAAA. In marked contrast, A473S lysed plasma clots without neutralization of alpha2-AP. Model thrombi, with structures similar to in vivo thrombi, were lysed slowly by wild-type t-PA; the rate and extent of lysis were enhanced by the addition of antibodies to alpha2-AP or PAI-1. A473S was more effective than wild-type t-PA without the addition of antibodies by virtue of its resistance to alpha2-AP. This resistance was remarkable, in that no complex formed between A473S t-PA and alpha2-AP, even after extended incubation, when 50% of wild-type t-PA could be converted to complex. Comparison of A473S and KHRR296-299AAAA mutants showed their similar effectiveness in lysis of platelet-rich model thrombi. Thus, PAI-1 and alpha2-AP contribute approximately equally to the inhibition of thrombus lysis. This study underlines the functional significance of alpha2-AP as a direct inhibitor of t-PA and further explains the basis of the accepted role of alpha2-AP as a regulator of fibrin persistence and thrombus resistance to lysis.

An ELISA for the detection of Bacillus subtilis L-form bacteria confirms their symbiosis in strawberry

AIMS

To develop an ELISA for the detection of antigens derived from stable Bacillus subtilis L-form bacteria and to detect these in plants injected with L-form bacteria.

METHODS AND RESULTS

A sandwich ELISA was developed and its specificity was investigated using L-forms and cell-walled forms of B. subtilis, different Bacillus species and a range of bacteria isolated from glasshouse-grown strawberry plants. The detection limits of the ELISA were approximately 10(3) viable cells ml(-1) for L-forms compared with 10(7) viable cells ml(-1) for cell-walled forms. Results showed that L-forms survived and moved within strawberry tissues injected with L-form bacteria.

CONCLUSION

An ELISA that selectively detects B. subtilis L-form bacteria was developed and shown to confirm the presence of L-forms in plants.

SIGNIFICANCE AND IMPACT OF THE STUDY

This will be a valuable rapid method to further studies on L-form plant interactions.

Criteria for specific measurement of plasminogen (enzymatic; procedure) in human plasma

There is a lack of well-established criteria for the specific measurement of fibrinolytic variables. On behalf of the SSC the Subcommittee on Fibrinolysis started a process to develop such criteria. This report describes the criteria for the measurement of plasminogen (enzymatic; procedure) in human plasma. Apparently, the most specific methods for determination of plasminogen (enzymatic; procedure) adhere to the principle of streptokinase induced activation of plasminogen and recording of activity using a chromogenic substrate. Incorporation of fibrinogen attenuates the potential effect of elevated fibrinogen or fibrin(ogen) fragments in the plasma sample. The criteria for specific measurement of plasminogen(enzymatic; procedure) are based on this analytical principle. The kinetics and principles of the assay procedure are described, and criteria as well as test methods for criteria are detailed. Guidelines for standardization, quality assurance, analytical sensitivity and establishment of reference intervals are given. The pre-analytical conditions regarding preparation of the patient and the specimen are delineated. Nonstandard abbreviations: SSC, Scientific and Standardization Committee; ISTH, The International Society on Thrombosis and Haemostasis; IFCC, The International Federation of Clinical Chemistry; NCCLS, National Center for Clinical Laboratory Standards; HRG, histidine-rich glycoprotein; Lp(a), lipoprotein (a); SK, streptokinase.

Cross-linking of plasminogen activator inhibitor 2 and alpha 2-antiplasmin to fibrin(ogen)

In this study, we identified lysine residues in the fibrinogen Aalpha chain that serve as substrates during transglutaminase (TG)-mediated cross-linking of plasminogen activator inhibitor 2 (PAI-2). Comparisons were made with alpha(2)-antiplasmin (alpha(2)-AP), which is known to cross-link to lysine 303 of the Aalpha chain. A 30-residue peptide containing Lys-303 specifically competed with fibrinogen for cross-linking to alpha(2)-AP but not for cross-linking to PAI-2. Further evidence that PAI-2 did not cross-link via Lys-303 was the cross-linking of PAI-2 to I-9 and des-alphaC fibrinogens, which lack 100 and 390 amino acids from the C terminus of the Aalpha chain, respectively. PAI-2 or alpha(2)-AP was cross-linked to fibrinogen and digested with trypsin or endopeptidase Glu-C, and the resulting peptides analyzed by mass spectrometry. Peptides detected were consistent with tissue TG (tTG)-mediated cross-linking of PAI-2 to lysines 148, 176, 183, 457 and factor XIIIa-mediated cross-linking of PAI-2 to lysines 148, 230, and 413 in the Aalpha chain. alpha(2)-AP was cross-linked only to lysine 303. Cross-linking of PAI-2 to fibrinogen did not compete with alpha(2)-AP, and the two proteins utilized different lysines in the Aalpha chain. Therefore, PAI-2 and alpha(2)-AP can cross-link simultaneously to the alpha polymers of a fibrin clot and promote resistance to lysis.

Plasminogen activator inhibitor 2 and urokinase-type plasminogen activator in plasma and leucocytes in patients with severe sepsis

Proteins influencing plasminogen activation to plasmin, namely plasminogen activators tissue-type plasminogen activator (t-PA) and urokinase-type plasminogen activator (u-PA) and their principal inhibitors, plasminogen activator inhibitor 1 (PAI-1) and PAI-2, were measured in the plasma, the polymorph and mononuclear cell fractions taken from patients with major sepsis who were entering a general intensive care unit. The purpose of this study was to elucidate the factors favouring the persistence of fibrin in the microvasculature and thus contributing to multiple organ failure. Levels of u-PA antigen in plasma rose in sepsis and u-PA activity, not detectable in normal plasma, appeared. Levels of u-PA antigen in the cell fractions fell concomitantly. t-PA antigen in plasma and in the mononuclear cell fraction rose in sepsis, but t-PA activity was not detectable. Plasma PAI-1 antigen levels were strikingly raised in sepsis, presumably accounting for the complete neutralization of t-PA activity. PAI-2 antigen, not normally detected in plasma, appeared in the plasma of some patients, whereas it disappeared from the cellular fractions. Appearance of PAI-2 in plasma was associated with non-survival of the patient. The observations indicate that all the agents involved in plasminogen activation are released into the plasma in major sepsis. The levels of PAI-1 reached were quantitatively sufficient to suppress all activity of the released t-PA, but the inhibitors did not prevent expression of u-PA activity in the circulation. Circulating active u-PA and PAI-2 in the plasma of patients with severe sepsis may represent material originating from leucocytes. Leucocyte release of these agents within fibrin deposits may influence the persistence of fibrin and thus the development of multiple organ failure.

Regulation of plasminogen activation by TGF-beta in cultured human retinal endothelial cells

BACKGROUND/AIMS

Regulation of plasmin mediated extracellular matrix degradation by vascular endothelial cells is important in the development of angiogenesis. The aim was to determine whether transforming growth factor beta (TGF-beta) affected the regulation of components of the plasminogen system by human retinal endothelial cells, in order to define more clearly the role of TGF-beta in retinal angiogenesis in the context of diabetes mellitus.

METHODS

Human retinal endothelial cells (HREC) were isolated from donor eyes and used between passages 4-8. The cells were cultured in medium supplemented with 2, 5, 15, or 25 mM glucose, plus or minus TGF-beta (1 ng/ml). The concentrations of tissue plasminogen activator (t-PA), urokinase plasminogen activator (u-PA), and plasminogen activator inhibitor type 1 (PAI-1) in cell conditioned medium were determined by ELISA and the level of PAI-1 mRNA was determined using northern hybridisation. Cell associated plasminogen activity was determined using a clot lysis assay and a chromogenic assay.

RESULTS

Under basal conditions (5 mM glucose), HREC produced PAI-1, t-PA, and trace amounts of u-PA. Cell surface plasminogen activation observed by lysis of fibrin or by cleavage of chromogenic substrate, was mediated by t-PA. Glucose at varying concentrations (2-25 mM) had no significant effect on t-PA mediated clot lysis. In contrast, treatment with TGF-beta resulted in increased synthesis of PAI-1 protein and mRNA. The increased expression of the PAI-1 mRNAs by TGF-beta did not occur uniformly, the 2.3 kb mRNA transcript was preferentially increased in comparison with the 3.2 kb mRNA (p<0.05).

CONCLUSIONS

These data demonstrate that TGF-beta increases PAI-1 and decreases cell associated lysis. This is sufficient to decrease the normal lytic potential of HREC.

Circulating tissue-type plasminogen activator and plasminogen activator inhibitor type 1 in proliferative diabetic retinopathy: a pilot study

Several haemostatic abnormalities are associated with proliferative diabetic retinopathy. While abnormalities in plasma fibrinolytic activity have been described in diabetic retinopathy, platelets (a rich source of plasminogen activator inhibitor type 1, PAI-1) have received little attention. As a result, little is known about the fibrinolytic potential of circulating whole blood in diabetic retinopathy. The concentrations of tissue-type plasminogen activator (t-PA) and of its fast-acting inhibitor. PAI-1 were measured in plasma from eight patients with type 1 diabetes complicated by proliferative retinopathy, and from eight patients with type 1 diabetes and background or no retinopathy, matched for age, sex and duration of diabetes. The concentration of PAI-1 in platelets was also measured. The ratio of t-PTA to PAI-1 in plasma was significantly higher in patients with proliferative retinopathy than in those without (0.66 vs. 0.37, p < 0.02). The average quantity of PAI-1 per platelet was significantly lower in the group with proliferative retinopathy (0.33 vs. 0.50 ng/10(6) platelets, p < 0.02). These data suggest that among patients with type 1 diabetes, total circulating fibrinolytic potential is higher in those with proliferative retinopathy.

Fibrinolysis and thrombosis

The fibrinolytic system generates plasmin, which dissolves fibrin in haemostatic plugs and in thrombi. It is often regarded simply as a secondary phenomenon responsive to the generation of thrombi but it is, rather, in dynamic balance with fibrin formation, such that abnormalities in either can lead to thrombosis. This chapter summarizes the fibrinolytic system and its regulation. It considers the components of the system in blood, both in plasma and in circulating cells, with emphasis on protease-inhibitor balance. It goes on to discuss local fibrinolytic potential in thrombi, both venous and arterial, and in the diseased vessel wall, presenting evidence that increased local inhibition of fibrinolysis by PAI-1, PAI-2 and alpha2-antiplasmin is intimately involved in thrombus stability and in the generation of fibrin-rich vessel wall lesions. Finally, it reviews the evidence that defective plasma fibrinolysis has a causal role in venous and arterial thrombosis.

Monocyte plasminogen activator inhibitor 2 (PAI-2) inhibits u-PA-mediated fibrin clot lysis and is cross-linked to fibrin

Plasminogen activator inhibitor 2 (PAI-2) is a major product of activated human monocytes. Here we show that monocytes inhibited u-PA- but not t-PA-mediated fibrinolysis, by secreting PAI-2 into an overlying fibrin clot. Extracts of arterial and venous human thrombi were found to contain active PAI-2. PAI-2 was cross-linked to fibrin in a reaction catalyzed by two major transglutaminases (TG), tissue TG and factor XIII. The activity of PAI-2 was not affected by such cross-linking. Cross-linking of PAI-2 to fibrin was inhibited by Tridegin, a specific inhibitor of TG, and also by EDTA and iodoacetamide. The use of competitive peptides mimicking the loop between helices C and D of PAI-2 identified Gln 83 and 86 as residues important in cross-linking. This study defines a mechanism by which PAI-2 is localized to fibrin, where it acts as an effective inhibitor of u-PA-mediated fibrinolysis.

Secretion of plasminogen activator inhibitor 2 by human peripheral blood monocytes occurs via an endoplasmic reticulum-golgi-independent pathway

Plasminogen activator inhibitor 2 (PAI-2) is a serine protease inhibitor (serpin) that is secreted and accumulated intracellularly by monocytes. We investigated PAI-2 synthesis by isolated human peripheral blood monocytes and found that a 47-kDa nonglycosylated form of PAI-2 was abundant in conditioned medium from monocytes. Secretion of PAI-2 by monocytes was not inhibited by agents that inhibit either ER-Golgi pathway-dependent secretion, brefeldin A, or N-linked glycosylation, tunicamycin. IL-1beta served as a control for a protein that is secreted by an ER-Golgi-independent pathway, and secretion of IL-1beta was not inhibited by brefeldin A. This was in contrast to secretion of TNFalpha, which was dependent on the ER-Golgi pathway. None of the treatments was cytotoxic toward monocytes, as measured by release of the intracellular enzyme lactate dehydrogenase (LDH) into the conditioned medium. Subcellular fractionation revealed that PAI-2 and IL-1beta were colocalized. The mechanism for secretion of PAI-2 was not dependent on calcium or intracellular trafficking via the classical vesicular mechanism(s), distinguishing it from IL-1beta secretion. These studies show that PAI-2 is secreted by primary human monocytes via an ER-Golgi-independent pathway.

Does chronic smoking influence fibrinolytic potential in type 1 diabetes mellitus?

Tissue-type plasminogen activator (t-PA) and plasminogen activator inhibitor (PAI-1) were studied in 18 smokers and 18 closely matched non-smokers, all of whom had Type 1 diabetes mellitus (DM). None of the patients had advanced complications of diabetes. The t-PA and PAI-1 antigen levels were measured in plasma before and after venous occlusion, and were normal in Type 1 diabetes regardless of smoking status. Platelet PAI-1 levels were also measured and were found to be normal both in smokers and non-smokers. In smokers with Type 1 DM, plasma PAI-1 was significantly correlated with triglycerides. The normal fibrinolytic potential found in smokers with diabetes contrasts starkly with the significantly elevated plasma PAI-1 reported in smokers without diabetes. In smokers, triglycerides may effect low levels of PAI-1 release into plasma; this process may be significantly augmented in the presence of smoking-induced insulin resistance. The lack of endogenous insulin release in Type 1 diabetes may obviate the characteristic rise in plasma PAI-1 found in smokers who do not have diabetes.

The distribution of the secreted and intracellular forms of plasminogen activator inhibitor 2 (PAI-2) in human peripheral blood monocytes is modulated by serum

Plasminogen activator inhibitor 2 (PAI-2) is produced by activated monocytes in two forms, intracellular and secreted. We have studied the distribution of these two forms in unstimulated human peripheral blood monocytes and after stimulation by thrombin. Fetal calf serum (FCS) in the culture medium was absolutely necessary for accumulation of intracellular PAI-2; but not for synthesis and secretion. Even at a concentration as low as 0.1%, FCS restored accumulation of intracellular PAI-2. Increasing concentrations of FCS resulted in an increase in the ratio of intracellular to secreted PAI-2. The factor that promoted accumulation of intracellular PAI-2 was not a platelet product. Failure of monocytes to accumulate PAI-2 did not reflect leakage due to cell death, as assessed by LDH in culture supernatants. We propose that accumulation of intracellular PAI-2 is not simply due to poor secretion, but is an active process that is modulated by factor(s) found in serum.

From famine to feast: the role of methylglyoxal production in Escherichia coli

The enzyme methylglyoxal synthase (MGS) was partially purified from Escherichia coli extracts, and the amino-terminal sequence of candidate proteins was determined, based on the native protein being a tetramer of about 69 kDa. Database analysis identified an open reading frame in the E. coli genome, YccG, corresponding to a protein of 16.9 kDa. When amplified and expressed from a controlled promoter, it yielded extracts that contained high levels of MGS activity. MGS expressed from the trc promoter accumulated to approximately 20% of total cell protein, representing approximately 900-fold enhanced expression. This caused no detriment during growth on glucose, and the level of methylglyoxal (MG) in the medium rose to only 0.08 mM. High-level expression of MGS severely compromised growth on xylose, arabinose and glycerol. A mutant lacking MGS was constructed, and it grew normally on a range of carbon sources and on low-phosphate medium. However, the mutant failed to produce MG during growth on xylose in the presence of cAMP, and growth was inhibited.