Plasminogen Activation at Low Temperatures in Plasma Samples Gontaining Therapeutic Concentrations of Tissue-Type Plasm i nogen Activator or Other Thrombolytic Agents

It is known that in vitro plasminogen activation in blood samples taken during thrombolytic therapy with tissue-type plasminogen activator (t-PA) may lead to artefactually low fibrinogen and α2-antiplasmin values. To mimic this phenomenon, pooled normal plasma was supplemented with 2.5 μg/ml t-PA and incubated at various temperatures. The rates of fibrinogen degradation and α2-antiplasmin consumption were most pronounced at 37° C, were less pronounced at 25° C, but surprisingly, did not further decrease at 10° C, 0° C or −8° C. In contrast, when plasma was supplemented with 10° IU/ml urokinase or 30 IU/ml streptokinase, the rates of fibrinogen degradation and α2-antiplasmin consumption gradually decreased with incubation temperature and were negligible at 10° C and lower temperatures. The rate of plasminogen activation also decreased gradually with temperature in mixtures of purified fibrinogen, plasminogeo, α2- antiplasmin and t-PA. These results imply that, in a plasma milieu, additional factors with a stimulatory activity are involved in t-PA-induced plasminogen activation at around 0° C. The abnormally high reaction rate at low temperatures explains in vitro plasminogen activation observed during the processing of t-PA-containing blood samples.

In contrast to the activation of plasminogen by t-PA, the slow inhibition of t-PA (2.5 μg/ml) by proteinase inhibitors in plasma could be minimized to a negligible level by keeping the plasma samples at 0° C. This makes it possible to reliably monitor t-PA activity during thrombolytic therapy

Fibrinolytic Activators and Inhibitors in Terminal Renal Insufficiency and in Anephric Patients

Fibrinolytic factors were measured before and after DDAVP- infusion in 18 patients on chronic, regular haemodialysis, 11 of whom underwent bilateral nephrectomy, and in 7 patients in whom non-functioning kidneys were still present. Baseline fibrinolytic activity was normal or high in all but two cases. Before haemodialysis, the response to DDAVP-infusion was greatly reduced in the majority of patients as compared with healthy controls, irrespective of the baseline level. This was in accordance with mean t-PA-antigen levels which increased only slightly after DDAVP. When DDAVP was given after haemodialysis, previous non-responders showed a normal increase in fibrinolytic activity. The level of free t-PA-inhibitors was normal in most cases as were levels of α2-antiplasmin. and α2-macroglobulin.

On the Composition and Function of the Carbohydrate Moiety of Tissue-Type Plasminogen Activator from Human Melanoma Cells

Two variants (I and II) of tissue-type plasminogen activator (t-PA) from human melanoma cells were separated by Lysine Sepharose chromatography. The carbohydrate compositions of the forms were determined by gas-liquid chromatography. Variant I contained 12.8 g and variant II 7.1 g of carbohydrate per 100 g protein. Both variants contained N-acetylgalactosamine, suggesting O-glycosylation in addition to N-glycosylation. The possible role of N-linked oligosaccharides for the biological activity of t-PA was studied using t-PA secreted by melanoma cells in the presence of tunicamycin, an inhibitor of N-glycosylation. The latter t-PA showed the same plasminogen activating and fibrin binding properties as normally glycosylated t-PA, indicating that N-linked carbohydrate is not involved in the fibrinolytic activity of t-PA.

Masking of Fibrinolytic Response to Stimulation by an Inhibitor of Tissue-Type Plasminogen Activator in Plasma

Patients with hyperlipoproteinaemia or spontaneous thromboembolism, known to be poor responders to DDAVP or to venous occlusion with regard to the rise in fibrinolytic activity of the blood, appeared to show a normal increase in t-PA-antigen after the same procedure. In their plasma a higher than normal level of free, fast-acting t-PA-inhibitor was found as measured by titration with purified t-PA. This free t-PA-inhibitor level only decreased after the test, in contrast to its complete disappearance in normal responders. The same happened in a healthy volunteer who failed to exhibit a rise in fibrinolytic activity after exhaustive exercise.

We suppose that the lack of response of the fibrinolytic activity in these cases is due to a high inhibitor level and not to impaired release of t-PA into the blood.

In contrast, patients with terminal renal insufficiency showed only a slight increase in t-PA-antigen after DDAVP. The level of free fast-acting inhibitor was normal in most cases and did not change appreciably during DDAVP-infusion. In these patients, a true impairment of the release of t-PA appears to exist.

Modulation of Rapid Plasminogen Activator Inhibitor in Plasma by Stanozolol

Fibrinolytic activity in plasma euglobulin fractions can be increased by oral administration of stanozolol. This increase is not caused by increased synthesis or release of tissue-type plasminogen activator. A decreased level of fast acting t-PA inhibition is very probably the cause of the higher activity. These results suggest that this inhibition has a regulatory role on fibrinolysis in vivo.

Purification of Human Tissue-Type Plasminogen Activator in Centigram Quantities from Human Melanoma Cell Culture Fluid and Its Conditioning for Use In Vivo

Human tissue-type plasminogen activator was produced in centigram quantities from 50–60 1 batches of conditioned medium of a human melanoma cell culture. The yields of the procedure remained essentially unchanged during 50 subsequent preparations. The final products were of high purity as assessed by SDS gel electrophoresis.

These materials, after filtration on 0.22 μM Milliporefilters were sterile and free of viruses and pyrogens. Their stability was very good in the fluid form at temperatures up to 56° C. Material obtained by the present procedure has been used to investigate the biological and thrombolytic properties of tissue-type plasminogen activator.

Inhibition of Mannose Receptor-mediated Clearance of Tissue-type Plasminogen Activator (t-PA) by Dextran: a New Explanation for Its Antithrombotic Effect

Dextran is used during surgery as a prophylactic agent to prevent deep venous thrombosis. Recently it has been shown that dextran increases t-PA plasma concentrations in patients. As dextran is a potential ligand for the mannose receptor, we studied whether this glucose-polymer would be able to inhibit mannose receptor-mediated clearance of t-PA.

In this report we show that dextran 40 and dextran 70 were able to inhibit t-PA binding to the isolated human mannose receptor (IC5014 and 4 mg/ml, respectively). Both glucose-polymers inhibited mannose receptor-mediated t-PA degradation by human monocyte-derived macrophages in vitro (IC50 7 and 2 mg/ml, respectively). The α2-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP)-mediated t-PA degradation by the macrophages was not affected by dextran. During and after a 45 min infusion of dextran 70 (Macrodex) in rats, in plasma endogenous t-PA concentrations increased to 162 ± 33% and 122 ± 35% respectively. The plasma clearance of a bolus injection of exogenous t-PA was decreased by 33 ± 9% in the same rats.

We conclude that dextran inhibits mannose receptor-mediated t-PA clearance. The inhibition of t-PA clearance during dextran infusion results in increased endogenous t-PA plasma concentrations. Increased t-PA concentrations present during thrombus formation are known to increase thrombus lysability. Thus the inhibition of t-PA clearance can contribute to the antithrombotic effect of dextran.

Characterization of the Interaction of a Complex of Tissue-type Plasminogen Activator and Plasminogen Activator Inhibitor Type 1 with Rat Liver Cells

The present study was undertaken in order to determine the recognition site for tissue-type plasminogen activator-plasminogen activator inhibitor type 1 [t-PA-PAI-1] complexes in rat liver in vivo and in vitro. After intravenous injection into rats t-PA-PAI-1 complexes were rapidly removed from the plasma and the liver took up 80% of the injected dose. Within the liver parenchymal and endothelial liver cells contributed mainly to the uptake of t-PA-PAI-1, and were responsible for 62% and 24% of the liver uptake, respectively. The interaction of t-PA- PAI-1 with isolated rat parenchymal liver cells was of high affinity (Kd 17 nM). A well-known antagonist of the α2-macroglobulin receptor (α2MR/low-density lipoprotein receptor-related protein (LRP), GST-39kDa protein (GST-39kDaP) efficiently inhibited the binding (IC50 0.7 nM) of t-PA-PAI-1 to rat parenchymal liver cells. The interaction of t-PA-PAI-1 with LRP on rat parenchymal liver cells was not Ca2+-dependent and is most probably mediated by a specific determinant on PAI-1, since an anti-PAI-1 monoclonal antibody inhibited the binding of t-PA-PAI-1, where as free t-PA did not. The binding of t-PA-PAI-1 to rat hepatocytes could not be inhibited by a complex of plasmin and α2-antiplasmin nor by various other ligands of LRP like β-VLDL and lactoferrin. Binding of t-PA-PAI-1 to rat parenchymal liver cells was followed by internalization and subsequent degradation in the lysosomal compartment.

It is concluded that parenchymal and endothelial liver cells mediate the removal of t-PA-PAI-1 complexes from the circulation. LRP on rat parenchymal liver cells is responsible for the uptake and degradation of t-PA-PAI-1 and may therefore be important for the regulation of the t-PA levels in the circulation.

Comparison of the Relative Fibrinogenolytic, Fibrinolytic and Thrombolytic Properties of Tissue Plasminogen Activator and Urokinase in Vitro

The relative fibrinogenolytic, fibrinolytic and thrombolytic properties of human tissue plasminogen activator and human urokinase were compared in purified systems, in whole human plasma and in a system composed of a radioactive human blood clot (¹²⁵I-fibrinogen) hanging in circulating human plasma. The human tissue plasminogen activator was highly purified from the culture fluid of a human melanoma cell line.

In purified systems composed of fibrinogen or fibrin, plasminogen and α₂-antiplasmin as well as in whole plasma, tissue plasminogen activator digested fibrin without degrading fibrinogen significantly. Urokinase did not have this specific fibrinolytic effect.

In the circulating plasma system, the degree of fibrinolysis was proportional to the amount of activator added, tissue plasminogen activator being about 10 times more efficient than urokinase. In addition, tissue plasminogen activator appeared to cause negligible fibrinogen degradation. Tissue plasminogen activator still induced significant thrombolysis at a concentration of 10 IU per ml whereas no effect of urokinase was observed at 20 IU per ml. Infusion of 100 IU (1 (μg) of tissue plasminogen activator per ml resulted in moderate activation of the fibrinolytic system as judged from a decrease of plasminogen and α₂-antiplasmin to 40-50 percent. Nevertheless, extensive fibrinolysis (50 to 80 percent of radioactivity released after 12 hrs) and only very limited fibrinogenolysis were observed. An equivalent amount of urokinase (100 IU per ml) only induced approximately 15 percent lysis in 12 hrs. At higher concentrations of urokinase (260 IU per ml or more) extensive activation of the fibrinolytic system was obtained as evidenced by a depletion of plasminogen, α₂-antiplasmin and fibrinogen. This was associated with extensive fibrinolysis (approximately 60 percent after 12 hrs). It is concluded that human tissue plasminogen activator is a more specific and effective fibrinolytic-thrombolytic agent than human urokinase.

In Vitro Stability of a Tissue-Type Plasminogen Activator Mutant, BM 06.022, in Human Plasma

BM 06.022 is a non-glycosylated mutant of human tissue-type plasminogen activator (t-PA) comprising only the kringle-2 and proteinase domains. The in vivo half-life of BM 06.022 antigen is 4- to 5-fold longer than that of t-PA antigen. The in vitro half-life of the activity of BM 06.022 at therapeutic concentrations in plasma is shorter than that of t-PA. In this study the inactivation of BM 06.022 in plasma was further investigated.

Varying concentrations of BM 06.022 were incubated in plasma for 0-150 min. Activity assays on serial samples showed a dose-dependent decline of BM 06.022 activity with a half-life from 72 min at 0.3 μg/ml to 38 min at 10 μg/ml. SDS-polyacrylamide gel electrophoresis (SDS-PAGE) followed by fibrin autography showed the generation of several BM 06.022-complexes. These complexes could be completely precipitated with antibodies against Cl-inactivator, α2-antiplasmin and α1-antitrypsin.

During the incubation of BM 06.022 in plasma, plasmin was generated dose-dependently as revealed by varying degrees of a2-anti-plasmin consumption and fibrinogen degradation. SDS-PAGE and immunoblotting showed that single-chain BM 06.022 was rapidly (i. e. within 45 min) converted into its two-chain form at concentrations of 5 μg/ml BM 06.022 and higher.

In conclusion, BM 06.022 at therapeutic concentrations in plasma was inactivated by Cl-inactivator, a2-antiplasmin and a j-antitrypsin. The half-life of the activity decreased at increasing BM 06.022 concentrations, probably as a result of the generation of two-chain BM 06.022 which may be inactivated faster than the single-chain form.

Purification and Characterization of the Plasminogen Activator Secreted by a Rat Brain Tumor Cell Line in Culture

The plasminogen activator secreted by a cultured rat brain tumor cell line (RT4-71-1) (Imada M. and Sueoka N., Develop.Biol. 69, 97-107, 1978) was purified by chromatography on zinc chelate-agarose, concanavalin A-agarose and Sephadex G-150 in the presence of 0.01% (vol/vol) Tween 80. Aprotinin was added to the culture medium to a concentration of 20 KIU per ml and to the buffers in the first two chromatographic steps to a concentration of 10 KIU per ml. Approximately 90 μg purified material was obtained from 1 1 of culture medium with a yield of 39% and a purification factor of 200. Sodium dodecylsulfate-polyacryl amide gel electrophoresis in the presence of reducing agents showed one main band with M r of about 60,000, and a minor band with M r about 30,000. Fibrinolytic activity was associated with the main band. The rat brain tumor plasminogen activator bound to a fibrin clot to a similar extent as human tissue plasminogen activator, whereas urokinase did not bind. In quenching experiments of the fibrinolytic activities the purified rat brain tumor plasminogen activator appeared to be immunologically related to the human tissue plasminogen activator but unrelated to urokinase.

Binding and Degradation of Tissue-Type Plasminogen Activator by the Human Hepatoma Cell Line Hep G2

In this study, binding and degradation of tissue-type plasminogen activator (t-PA) by the human hepatoma cell line Hep G2 was investigated. Binding at 4° C was time-dependent and reached a maximum after ca. 2 hours. Scatchard analysis of saturation experiments showed about 170,000 high affinity binding sites for t-PA per cell with an apparent Kd of 90 nM. These binding sites were calcium-dependent. Part of the binding to the hepatoma cells was non-saturable, owing to a large amount of low affinity binding sites which are at least partially located on the extracellular matrix of the cells. Competition with mannose- and galactose-terminated glycoproteins had no effect on total binding of 125I-t-PA. Degradation products of 125I-t-PA were found in the supernatant after a short lag phase and then increased linearly for at least 5 hours at 37° C. Degradation could be inhibited by chloroquine, NH4Cl and NaN3. We conclude that the human hepatoma cell line Hep G2 has a specific binding mechanism for t-PA which is not mediated by known carbohydrate receptor systems. Binding is followed by cellular uptake and degradation in the lysosomes.

Daytime Fluctuations in Blood of Tissue-Type Plasminogen Activator (t-PA) and Its Fast-Acting Inhibitor (PAI-1)

Circadian fluctuation in blood fibrinolytic activity was studiedin 10 volunteers in the day-time period at 09.00, 12.00 and15.0 h. Activity of tissue-type plasminogen activator (t-PA) wasfound to increase from 09.00 to 15.00 h in accordance with knownresults with global assays of blood activity.

Characterization of the Binding of Urokinase-Type Plasminogen Activator to the Asialoglycoprotein Receptor

In order to study the role of the asialoglycoprotein receptor (ASGPr) in the rapid plasma clearance of urokinase-type plasminogen activator (u-PA), a microtiter plate binding assay was developed using ASGPr purified from rat liver extracts. Urinary two-chain u-PA bound to immobilized ASGPr in a saturable manner with an EC50 of 0.2 µM. Binding was inhibited by rabbit antibodies against the ASGPr. In line with the known carbohydrate specificity of the ASGPr, GalNAc proved to be the most effective inhibitor from a series of monosaccharides, followed by Gal and Fuc, whereas GlcNAc was ineffective. The N-linked oligosaccharides of urinary u-PA do not terminate with the common Gal-GlcNAc element, but with a GalNAc-GlcNAc element which is partially sulfated. Sulfated forms of u-PA were separated from non-sulfated forms by using the lectin Wisteria floribunda agglutinin. Only the non-sulfated forms of u-PA (30% of the total) appeared to bind to the ASGPr. From different u-PA preparations used for thrombolytic therapy only urinary u-PA and u-PA produced by kidney cell cultures strongly bound to the ASGPr, whereas (recombinant) u-PA expressed in mouse myeloma cells, Chinese hamster ovary cells or E. coli scarcely bound to the receptor. It is concluded that u-PA bearing non-sulfated GalNAc-GlcNAc elements is specifically recognized by the ASGPr present on liver cells.

Compaction of fibrin clots reveals the antifibrinolytic effect of factor XIII

Essentials Factor XIIIa inhibits fibrinolysis by forming fibrin-fibrin and fibrin-inhibitor cross-links. Conflicting studies about magnitude and mechanisms of inhibition have been reported. Factor XIIIa most strongly inhibits lysis of mechanically compacted or retracted plasma clots. Cross-links of α2-antiplasmin to fibrin prevent the inhibitor from being expelled from the clot.

SUMMARY

Background Although insights into the underlying mechanisms of the effect of factor XIII on fibrinolysis have improved considerably in the last few decades, in particular with the discovery that activated FXIII (FXIIIa) cross-links α2 -antiplasmin to fibrin, the topic remains a matter of debate. Objective To elucidate the mechanisms of the antifibrinolytic effect of FXIII. Methods and Results Platelet-poor plasma clot lysis, induced by the addition of tissue-type plasminogen activator, was measured in the presence or absence of a specific FXIIIa inhibitor. Both in a turbidity assay and in a fluorescence assay, the FXIIIa inhibitor had only a small inhibitory effect: 1.6-fold less tissue-type plasminogen activator was required for 50% clot lysis in the presence of the FXIIIa inhibitor. However, when the plasma clot was compacted by centrifugation, the FXIIIa inhibitor had a strong inhibitory effect, with 7.7-fold less tissue-type plasminogen activator being required for 50% clot lysis in the presence of the FXIIIa inhibitor. In both experiments, the effects of the FXIIIa inhibitor were entirely dependent on the cross-linking of α2 -antiplasmin to fibrin. The FXIIIa inhibitor reduced the amount of α2 -antiplasmin present in the compacted clots from approximately 30% to < 4%. The results were confirmed with experiments in which compaction was achieved by platelet-mediated clot retraction. Conclusions Compaction or retraction of fibrin clots reveals the strong antifibrinolytic effect of FXIII. This is explained by the cross-linking of α2 -antiplasmin to fibrin by FXIIIa, which prevents the plasmin inhibitor from being fully expelled from the clot during compaction/retraction.

Increased N-terminal cleavage of alpha-2-antiplasmin in patients with liver cirrhosis

BACKGROUND

The activity of alpha-2-antiplasmin (α2AP), the main fibrinolytic inhibitor, is modified by N- and C-terminal proteolytic cleavages. C-terminal cleavage converts plasminogen-binding α2AP (PB-α2AP) into a non-plasminogen-binding derivative. N-terminal cleavage by antiplasmin-cleaving enzyme (APCE), a soluble, circulating derivative of fibroblast activation protein (FAP), turns native Met-α2AP into Asn-α2AP, which is more quickly crosslinked into fibrin.

OBJECTIVES

We developed two novel enzyme-linked immunosorbent assays (ELISAs) to determine the N-terminal variation of α2AP to test the hypothesis that liver cirrhosis, characterized by increased expression of FAP/APCE, results in increased N-terminal cleavage of α2AP.

PATIENTS/METHODS

α2AP and FAP/APCE antigen levels were measured in the plasma samples of 75 patients with cirrhosis with different severities and 30 healthy control individuals. The percentage of N-terminal cleavage of α2AP was calculated.

RESULTS

Compared with levels (median [interquartile range]) in control individuals, total PB-α2AP levels and Met-PB-α2AP levels were reduced in cirrhosis patients (27.3 [21.4-41.3] μg mL(-1) vs. 56.2 [49.6-62.8] μg mL(-1) , P < 0.001, and 2.7 [1.7-5.5] μg mL(-1) vs. 12.1 [11.0-15.3] μg mL(-1) , P < 0.001, respectively). Interestingly, the percentage of N-terminal cleavage was increased in the patients (87.8 [85.0-91.6]% vs. 77.2 [72.2-79.8]% in controls, P < 0.001), as well as the plasma FAP/APCE levels (166 [60-550] ng mL(-1) in patients vs. 107 [67-157] ng mL(-1) in controls, P < 0.001). Additionally, all variables significantly correlated with the severity of disease.

CONCLUSIONS

Using our novel ELISAs we found increased N-terminal cleavage of α2AP in liver cirrhosis patients, which correlated with the severity of disease and is likely to have reflected the increased FAP/APCE levels in these patients.

Identification and characterization of α1 -antitrypsin in fibrin clots

BACKGROUND AND OBJECTIVES

Preliminary studies indicated that α1 -antitrypsin (A1AT) is the most abundant protein that is non-covalently bound to fibrin clots prepared from plasma. The aim of this study was to identify and characterize fibrin(ogen)-bound A1AT.

METHODS AND RESULTS

Plasma clots were prepared and extensively washed with saline. Clot-bound A1AT could only be extracted using denaturing agents such as urea, thiourea or SDS, pointing to an apparently strong association. Purified fibrinogen, but still containing A1AT as a contaminant, was gel filtered, which showed that the A1AT was bound to fibrinogen. A specific ELISA detected the presence of A1AT-fibrinogen complexes in both purified fibrinogen and pooled normal plasma. Finally, fibrin(ogen)-Sepharose chromatography indicated that A1AT purified from plasma contained a small fraction of fibrin(ogen)-binding A1AT. To study the inhibitory activity of fibrin(ogen)-bound A1AT, both fibrinogen containing A1AT and washed plasma clots were incubated with increasing amounts of elastase. SDS-PAGE and Western blotting showed under both conditions the generation of the A1AT-elastase complex as well as cleaved A1AT. The inhibitory activity of fibrin(ogen)-bound A1AT was also demonstrated by measuring elastase-induced lysis of fibrin clots.

CONCLUSION

Fibrin clots contain strongly bound A1AT, which is functionally active as a serine protease inhibitor (serpin). This A1AT might play a role in the local regulation of proteases involved in coagulation or fibrinolysis and represent a novel link between the inflammatory and hemostatic systems.

Evidence for an enhanced fibrinolytic capacity in cirrhosis as measured with two different global fibrinolysis tests

BACKGROUND AND OBJECTIVES

It has been known for a long time that cirrhosis is associated with hyperfibrinolysis, which might contribute to an increased risk and severity of bleeding. However, recent papers have questioned the presence of a hyperfibrinolytic state in cirrhotic patients and postulated a rebalanced system owing to concomitant changes in both pro- and anti-fibrinolytic factors. Therefore we re-investigated the fibrinolytic state of cirrhotic patients using two different overall tests including a recently developed test for global fibrinolytic capacity (GFC) using whole blood.

PATIENTS AND METHODS

Blood was collected from 30 healthy controls and 75 patients with cirrhosis of varying severity (34 Child-Pugh A, 28 Child-Pugh B and 13 Child-Pugh C). The plasma clot lysis time (CLT), which is inversely correlated with fibrinolysis, was determined as well as the GFC.

RESULTS

The mean CLT was 74.5 min in the controls and decreased significantly to 66.9 min in Child-Pugh class A patients, 59.3 min in class B patients and 61.0 min in class C patients, and hyperfibrinolysis existed in 40% of the patients. The median GFC was 1.7 μg mL(-1) in the controls and increased significantly to 4.0 μg mL(-1) in Child-Pugh class A patients, 11.1 μg mL(-1) in class B patients and 22.5 μg mL(-1) in class C patients, and hyperfibrinolysis existed in 43% of the patients. Taken together, 60% of the patients showed hyperfibrinolysis in at least one of the two global assays.

CONCLUSION

A rebalanced fibrinolytic system may occur, but hyperfibrinolysis is found in the majority of patients with cirrhosis.

The hypercoagulable state in Cushing's disease is associated with increased levels of procoagulant factors and impaired fibrinolysis, but is not reversible after short-term biochemical remission induced by medical therapy

CONTEXT

Cushing's disease (CD) is accompanied by an increased risk of venous thromboembolism. Surgery is the primary treatment of CD.

OBJECTIVE

The aim of the study was to compare hemostatic parameters between patients with CD and controls and to evaluate the effect of medical treatment of CD on hemostasis.

DESIGN AND SETTING

During 80 d, stepwise medical treatment was applied with the somatostatin analog pasireotide, the dopamine agonist cabergoline, and ketoconazole, which suppresses adrenocortical steroidogenesis, at four university medical centers in The Netherlands.

PATIENTS

Seventeen patients with de novo, residual, or recurrent CD were included.

MAIN OUTCOME MEASURES

We measured urinary free cortisol and parameters of coagulation and fibrinolysis.

RESULTS

Patients with CD had significantly higher body mass index (P < 0.001), shortened activated partial thromboplastin time (P < 0.01), and higher levels of fibrinogen, Factor VIII, and protein S activity (P < 0.05) compared to healthy control subjects. In addition, fibrinolytic capacity was impaired in patients with CD as reflected by prolonged clot lysis time (P < 0.001) and higher levels of plasminogen activator inhibitor type 1, thrombin-activatable fibrinolysis inhibitor, and α2-antiplasmin (P < 0.01). There were no statistically significant differences in von Willebrand factor:antigen, antithrombin, and protein C activity. After 80 d, 15 of 17 patients had normalized urinary free cortisol excretion. Despite biochemical remission, only slight decreases in antithrombin (P < 0.01) and thrombin-activatable fibrinolysis inhibitor (P < 0.05) levels were observed. Other parameters of coagulation and fibrinolysis did not change significantly.

CONCLUSIONS

The hypercoagulable state in patients with CD, which is explained by both increased production of procoagulant factors and impaired fibrinolysis, is not reversible upon short-term biochemical remission after successful medical therapy. This may have implications for the duration of anticoagulant prophylaxis in patients with (cured) CD.

Effect of fibrinolysis on bleeding phenotype in moderate and severe von Willebrand disease

Patients with von Willebrand disease (VWD), the most common inherited bleeding disorder, display large variation in bleeding tendency, which is not completely related to VWF levels. The cause of variability in clinical expression is largely unknown. The effect of plasma fibrinolytic capacity on bleeding tendency in VWD patients has not been investigated. We hypothesized that enhanced fibrinolysis may result in a more severe bleeding phenotype. Therefore, we measured the fibrinolytic potential in patients with moderate or severe VWD to investigate the contribution of fibrinolysis to the bleeding tendency. Fibrinolytic potential was measured as plasma clot lysis time (CLT) with and without addition of potato carboxypeptidase inhibitor (PCI) in 638 patients with moderate or severe VWD who participated in a nationwide multicentre cross-sectional study. Bleeding severity was measured using the Bleeding Score (BS).The CLTs were significantly longer, indicative of hypofibrinolysis, in men compared to women with VWD [106.2 (IQR 95.7-118.1) vs. 101.9 (IQR 92.8-114.0) min]. The CLTs prolonged with increasing age. No association was found between VWF or FVIII levels and CLT, or between VWF or FVIII levels and CLT(+PCI) . No association was observed for BS in a model with 10log-transformed CLT, adjusted for age, gender, VWF:Act and FVIII [b = 6.5 (95%CI -0.3 to 13.4)]. Our study showed that the plasma fibrinolytic potential does not influence bleeding tendency in VWD patients and therefore does not explain the variability in bleeding phenotype in VWD.

The role of thrombin activatable fibrinolysis inhibitor in arterial thrombosis at a young age: the ATTAC study

BACKGROUND AND OBJECTIVES

Thrombin activatable fibrinolysis inhibitor (TAFI) attenuates fibrinolysis and may therefore contribute to the pathophysiology of arterial thrombosis. The aim of the present study was to elucidate the pathogenetic role of TAFI levels and genotypes in young patients with arterial thrombosis.

PATIENTS AND METHODS

In a case-control study, 327 young patients with a recent first-ever event of coronary heart disease (CHD subgroup) or cerebrovascular disease (ischemic stroke subgroup) and 332 healthy young controls were included. TAFI levels [intact TAFI, activation peptide (TAFI-AP) and (in)activated TAFI (TAFIa(i)] and TAFI activity were measured and genetic variations in the TAFI gene (-438G/A, 505G/A and 1040C/T) were determined.

RESULTS

In the total group of patients, TAFIa(i) levels were higher (145.1 +/- 37.5%) than in controls (137.5 +/- 31.3%, P = 0.02). Plasma levels of intact TAFI, TAFI-AP and TAFI activity were similar in patients and controls. In the CHD subgroup (n = 218), intact TAFI levels were higher (109.4 +/- 23.0%) than in controls (102.8 +/- 20.7%, P = 0.02). In 325Ile/Ile homozygotes, lower TAFI levels and a decreased risk of arterial thrombosis were observed (OR 0.58, 95% CI 0.34-0.99) compared with patients with the common 325Thr/Thr genotype. This association was most evident in CHD patients (OR 0.48, 95% CI 0.26-0.90). Haplotype analyses supported a role for the Thr325Ile polymorphism.

CONCLUSIONS

TAFIa(i) levels were higher in patients with cardiovascular disease. Furthermore, the TAFI 325Thr/Ile polymorphism was associated with lower TAFI levels and with the risk of cardiovascular disease in young patients, especially in CHD.

On the usefulness of fibrinolysis variables in the characterization of a risk group for myocardial reinfarction

In a prospective study selected fibrinolysis variables were assessed in plasma samples from 29 consecutive patients recovering a first instance of acute myocardial infarction and the results were correlated with reinfarction during the next four years. Nine patients suffered a reinfarction leaving a group of 20 patients without evidence of relapse. The reinfarction group was characterized by lower tissue plasminogen activator activities in plasma euglobulins (p less than 0.05), significantly higher plasma concentrations of tissue plasminogen activator antigen (p less than 0.002) and a tendency to a higher plasma level of plasminogen activator inhibition capacity. There were no significant differences between the groups in plasma concentrations of plasminogen, histidine-rich glycoprotein, plasminogen kringle-4-binding-protein, and alpha 2-antiplasmin.

New insights into the molecular mechanisms of the fibrinolytic system

Fibrinolysis is regulated by specific molecular interactions between its main components. Activation of plasminogen by tissue-type plasminogen activator (t-PA) is enhanced in the presence of fibrin or at the endothelial cell surface. Urokinase-type plasminogen activator (u-PA) binds to a specific cellular u-PA receptor (u-PAR), resulting in enhanced activation of cell-bound plasminogen. Inhibition of fibrinolysis occurs at the level of plasminogen activation or at the level of plasmin. Assembly of fibrinolytic components at the surface of fibrin results in fibrin degradation. Assembly at the surface of cells provides a mechanism for generation of localized cell-associated proteolytic activity. This review includes novel proteins such a thrombin-activatable fibrinolysis inhibitor (TAFI) and discusses new insights into molecular mechanisms obtained from the rapidly growing knowledge of crystal structures of proteins.

Thrombin-activatable fibrinolysis inhibitor is associated with severity and outcome of severe meningococcal infection in children

BACKGROUND AND OBJECTIVES

In pediatric meningococcal sepsis, an imbalance between coagulation and fibrinolysis and proinflammatory action play major roles. We hypothesized that thrombin activatable fibrinolysis inhibitor (TAFI) and/or TAFI activation markers are involved in the pathogenesis of meningococcal sepsis.

PATIENTS AND METHODS

Children with severe meningococcal sepsis (n = 112) previously included in Rotterdam-based trials participated in this study. Clinical and laboratory parameters and severity scores were assessed. TAFI and TAFI activation markers were determined: TAFI activation peptide (TAFI-AP) and (in)activated TAFI [TAFIa(i)]. The -438G/A, Ala147Thr, and Thr325Ile polymorphisms were genotyped.

RESULTS

TAFI levels were significantly decreased in patients with meningococcal disease at admission compared to the convalescence state. TAFI was decreased in patients with septic shock vs. those with no shock. TAFI-AP levels were increased in patients with disseminated intravascular coagulation (DIC) vs. patients without DIC. TAFI-AP and TAFIa(i) were significantly increased in non-survivors vs. survivors. TAFI-AP levels and the TAFI-AP/TAFI ratio were also strongly correlated to severity scores and laboratory parameters. The TAFI 325Ile/Ile genotype was overrepresented in patients with DIC.

CONCLUSIONS

Activation markers of TAFI were associated with the occurrence of DIC and mortality in meningococcal sepsis patients. A determination of TAFI, TAFI-AP, and TAFIa(i) is required to enable coherent interpretation of the role of TAFI in disease.

Development of a new test for the global fibrinolytic capacity in whole blood

BACKGROUND

The development of global tests for the fibrinolytic capacity in blood is hampered by the low base-line fibrinolytic activity in blood, by the involvement of both plasmatic components and blood cells in the fibrinolytic system and by the loss of fibrinolytic activity as a result of the action of plasminogen activator inhibitor-1 (PAI-1).

OBJECTIVE

To develop a new test for the global fibrinolytic capacity (GFC) of whole blood samples.

METHODS AND RESULTS

Collection of blood in thrombin increased the subsequent generation of fibrin degradation products. This was ascribed to rapid clot formation and concomitant reduction of in vitro neutralization of tissue-type plasminogen activator (tPA) by PAI-1. On the basis of this observation, the following test was designed: blood samples were collected in thrombin with and without aprotinin and clots were incubated for 3 h at 37 degrees C. The GFC was assessed from the difference between the fibrin degradation products in the two sera. The assay was applied to blood samples from patients and healthy subjects. Other hemostasis parameters were determined in plasma samples taken simultaneously. The GFC varied considerably (normal range 0.13-13.6 microg mL(-1)); physical exercise strongly increased the GFC. Statistically significant correlations were found with tPA activity, PAI-1 activity and fibrinogen level. A mixture of antibodies against tPA and urokinase-type plasminogen activator (uPA) completely inhibited the GFC. An inhibitor of activated thrombin-activatable fibrinolysis inhibitor (TAFI) accelerated fibrinolysis 8-fold.

CONCLUSION

The new test represents a global assessment of the main fibrinolytic factors in plasma and potentially those associated with blood cells.

Thrombin-activatable fibrinolysis inhibitor is associated with severity and outcome of severe meningococcal infection in children

BACKGROUND AND OBJECTIVES

In pediatric meningococcal sepsis, an imbalance between coagulation and fibrinolysis and proinflammatory action play major roles. We hypothesized that thrombin activatable fibrinolysis inhibitor (TAFI) and/or TAFI activation markers are involved in the pathogenesis of meningococcal sepsis.

PATIENTS AND METHODS

Children with severe meningococcal sepsis (n = 112) previously included in Rotterdam-based trials participated in this study. Clinical and laboratory parameters and severity scores were assessed. TAFI and TAFI activation markers were determined: TAFI activation peptide (TAFI-AP) and (in)activated TAFI [TAFIa(i)]. The -438G/A, Ala147Thr, and Thr325Ile polymorphisms were genotyped.

RESULTS

TAFI levels were significantly decreased in patients with meningococcal disease at admission compared to the convalescence state. TAFI was decreased in patients with septic shock vs. those with no shock. TAFI-AP levels were increased in patients with disseminated intravascular coagulation (DIC) vs. patients without DIC. TAFI-AP and TAFIa(i) were significantly increased in non-survivors vs. survivors. TAFI-AP levels and the TAFI-AP/TAFI ratio were also strongly correlated to severity scores and laboratory parameters. The TAFI 325Ile/Ile genotype was overrepresented in patients with DIC.

CONCLUSIONS

Activation markers of TAFI were associated with the occurrence of DIC and mortality in meningococcal sepsis patients. A determination of TAFI, TAFI-AP, and TAFIa(i) is required to enable coherent interpretation of the role of TAFI in disease.

Hepatocyte-derived fibrinogen-related protein-1 is associated with the fibrin matrix of a plasma clot

In order to study the multiple functions of fibrinogen and fibrin, we are investigating which proteins bind to the fibrin matrix of a plasma clot by using a proteomic approach. Extracts from washed plasma clots were analysed by 2-D gel electrophoresis. A relatively abundant spot was identified as hepatocyte-derived fibrinogen-related protein-1 (HFREP-1) by MALDI-TOF analysis, molecular mass (34 kDa), iso-electric point (pI 5.5) as well as by Western blot analysis. HFREP-1 in plasma almost completely bound to the fibrin matrix during clot formation. Several purified fibrinogen preparations proved to be contaminated with HFREP-1. It is concluded that HFREP-1 (also named hepassocin), a protein with liver cell growth regulatory properties, occurs in plasma and strongly associates with fibrin and possibly fibrinogen.

High functional levels of thrombin-activatable fibrinolysis inhibitor are associated with an increased risk of first ischemic stroke

BACKGROUND AND OBJECTIVE

Several studies have suggested that thrombin-activatable fibrinolysis inhibitor (TAFI) levels are associated with the risk of arterial thrombosis, but results have been contradictory. We studied functional TAFI levels and TAFI gene polymorphisms in 124 patients with a recent ischemic stroke and 125 age- and sex-matched controls to establish the role of TAFI in ischemic stroke.

METHODS AND RESULTS

Functional TAFI levels, defined as TAFI-related retardation (RT), the difference in clot lysis time (LT) in the absence or presence of a specific activated TAFI inhibitor (potato carboxypeptidase inhibitor [PCI]), were higher in patients than controls (19.5 +/- 4.2 vs. 17.7 +/- 3.7 min, P < 0.005). Clot LTs in the presence of PCI, which were independent of TAFI, were also increased in ischemic stroke patients. This indicates that in these patients fibrinolysis is impaired not only by high TAFI levels, but also by other mechanisms. Individuals with functional TAFI levels in the highest quartile had an increased risk of ischemic stroke compared with the lowest quartile [odds ratio (OR) 4.0, 95% confidence interval (CI): 1.6-9.8]. In an unselected group of 36 of the 125 stroke patients functional TAFI levels were also measured at 3 months, and were persistently high. This indicates that increased functional TAFI levels after stroke are not caused by an acute phase reaction. No difference was found between patients and controls with respect to TAFI genotype distribution.

CONCLUSIONS

Increased functional TAFI levels, resulting in decreased fibrinolysis, are associated with an increased risk of ischemic stroke.

The B domain of coagulation factor VIII interacts with the asialoglycoprotein receptor

BACKGROUND

Coagulation factor VIII (FVIII) is a heavily glycosylated heterodimeric plasma protein that consists of a heavy (domains A1-A2-B) and light chain (domains A3-C1-C2). It has been well established that the clearance of FVIII from the circulation involves mechanisms that are sensitive to the low-density lipoprotein receptor (LDLR) family antagonist receptor-associated protein (RAP), including LDLR-related protein. Because FVIII clearance in the presence of a bolus injection of RAP still occurs fairly efficient, also RAP-independent mechanisms are likely to be involved.

OBJECTIVES

In the present study, we investigated the interaction of FVIII with the endocytic lectin asialoglycoprotein receptor (ASGPR) and the physiological relevance thereof.

METHODS AND RESULTS

Surface plasmon resonance studies demonstrated that FVIII dose-dependently bound to ASGPR with high affinity (Kd approximately 2 nM). FVIII subunits were different in that only the heavy chain displayed high-affinity binding to ASGPR. Studies employing a FVIII variant that lacks the B domain revealed that FVIII-ASGPR complex assembly is driven by structure elements within the B domain of the heavy chain. The FVIII heavy chain-ASGPR interaction required calcium ions and was inhibited by soluble D-galactose. Furthermore, deglycosylation of the FVIII heavy chain by endoglycosidase F completely abrogated the interaction with ASGPR. In clearance experiments in mice, the FVIII mean residence time was prolonged by the ASGPR-antagonist asialo-orosomucoid (ASOR).

CONCLUSIONS

We conclude that asparagine-linked oligosaccharide structures of the FVIII B domain recognize the carbohydrate recognition domains of ASGPR and that an ASOR-sensitive mechanism, most likely ASGPR, contributes to the catabolism of coagulation FVIII in vivo.

A new functional assay of thrombin activatable fibrinolysis inhibitor

New thrombin activatable fibrinolysis inhibitor (TAFI) assays are necessary for studying the role of this fibrinolysis inhibitor in cardiovascular disease. The identification of a functional single nucleotide polymorphism (SNP) (1040C/T) leading to a TAFI-variant with increased stability but lower antigen levels has made the determination of functional activity even more essential. Therefore, we developed a new assay for the functional activity of TAFI in citrated plasma samples. This assay is based on the retardation of plasma clot lysis by TAFIa. TAFI activation was induced simultaneously with fibrin formation and lysis was mediated by rt-PA. The variability of other plasma components was minimized by a 20-fold dilution of the samples in TAFI-depleted plasma. Lysis times (-/+ potato carboxypeptidase inhibitor) and the TAFI-related retardation of clot lysis, the functional parameter of the assay, were determined in a group of 92 healthy volunteers, as well as TAFI antigen levels (electroimmunoassay) and two TAFI SNPs (-438A/G and 1040C/T). TAFI-related retardation was 19.8 +/- 5.6 min (mean +/- SD) and was correlated with the antigen level. The specific antifibrinolytic activity of TAFI was associated with the -438A/G and 1040C/T genotypes. Individuals with the 325Ile-variant had on average a 34% higher TAFI-specific antifibrinolytic activity than individuals with the 325Thr-isoform. The TAFI-related retardation in the two groups of individuals did not differ, as a lower level compensated for the higher specific antifibrinolytic activity of the 325Ile-isoform. This assay provides valuable information about the performance of different TAFI isoforms and constitutes a new method for studying the role of TAFI in cardiovascular disease.

Association between thrombin activatable fibrinolysis inhibitor genotype and levels in plasma: comparison of different assays

Thrombin activatable fibrinolysis inhibitor (TAFI) antigen levels exhibit a large interindividual variability in which genetic control seems to play a major role. However, recent reports have questioned the association between TAFI concentration and genotype, suggesting that variable antibody reactivity towards TAFI isoforms, particularly the Thr325Ile polymorphism (1040C/T), may lead to artefacts in TAFI antigen levels. In order to compare assay outcome we determined plasma TAFI levels in 92 healthy individuals, using an enzyme-linked immunosorbent assay (ELISA) (commercial antibodies), an electroimmunoassay (in-house antibodies) and a commercial chromogenic assay (Actichrome TAFI). Each individual was genotyped for the -438A/G and 1040C/T polymorphisms in the TAFI gene. TAFI levels were significantly associated with genotype in both antigen and chromogenic assays. All assays displayed significant correlations with each other. Linear regression and Bland-Altman agreement analysis in the genotype subgroups showed that neither the genotype nor the concentration affected the relationship between the Actichrome TAFI and the electroimmunoassay. In contrast, the ELISA/Actichrome TAFI and the ELISA/electroimmunoassay relationships were concentration- and genotype-dependent. Our results demonstrate that artefacts may arise when measuring TAFI antigen levels by ELISA. Nevertheless, the electroimmunoassay and the Actichrome TAFI assay support a genotype-related variation of TAFI concentration.

Migration of the activation peptide of thrombin-activatable fibrinolysis inhibitor (TAFI) during SDS-polyacrylamide gel electrophoresis

Thrombin-activatable fibrinolysis inhibitor (TAFI) is a plasma zymogen, which upon activation is capable of delaying fibrinolysis. We investigated the migration and detection of the activation peptide of TAFI during SDS-polyacrylamide gel electrophoresis (SDS-PAGE). Purified TAFI before and after activation by thrombin/thrombomodulin was electrophoresed on 4-20% polyacrylamide gels and stained with Coomassie blue as well as Western blotting. Before activation, Coomassie blue staining resulted in one main band of TAFI. After activation, a sharp band corresponding to TAFIa was observed. No distinct activation peptide was detected, in agreement with the literature. Western blotting using a polyclonal anti-TAFI antibody, on the other hand, showed one additional broad band with an Mr of about 33 000 after TAFI activation. N-terminal sequence analysis confirmed that this band represented the activation peptide of TAFI. In addition, we tested the reactivity of two anti-TAFI monoclonal antibodies (MA-T3D8 and MA-T18A8) towards TAFI before and after activation by Western blotting. Both monoclonal antibodies recognized TAFI. After activation of TAFI, MA-T3D8 reacted with TAFIa, while MA-T18A8 reacted with the activation peptide. We identify the 33 000 band as the activation peptide of TAFI and exemplify the use of this information for the characterization of monoclonal antibodies against TAFI.

Binding and retention of polycationic peptides and dendrimers in the vascular wall

Extracellular matrix (ECM) of tissues, vascular tissue in particular, contains a high concentration of negatively charged glycosaminoglycans (GAGs), which are involved in the regulation of cell motility, cell proliferation and the regulation of enzyme activities. Previously, we have shown that the vascular ECM is capable of binding an extremely high concentration of positively charged molecules, such as polylysine. Vascular ECM can be used therefore as a substrate for binding and retention of drugs delivered intravascularly, if these drugs are endowed with an ability to bind to the vascular ECM. In this study, we evaluated a number of positively charged molecules as potential affinity vehicles for delivery of drugs to the vascular ECM. We labelled the molecules of interest with fluorescence and compared them ex vivo in terms of binding and retention in the de-endothelialised rat carotid artery after intravascular delivery under pressure. High molecular weight polylysine (84 kDa) and polyamidoamine (PAMAM) dendrimers accumulated in the wall of the artery up to a concentration of 10 mg/ml and were not washed away significantly after 4 h of perfusion of the artery. A 24-mer peptide containing a consensus sequence for binding to GAGs (ARRRAARA)(3), 2.7 kDa, was comparable to high molecular weight polylysine and dendrimers in terms of binding and retention. A 14-mer GAG-binding peptide from vitronectin and low molecular weight polylysine, 3 kDa, accumulated in the vascular wall up to about 3 mg/ml and was washed away after 30 min of perfusion. A 10-mer consensus GAG-binding peptide did not bind significantly to the vascular tissue. We conclude that the consensus 24-mer GAG-binding peptide is by far superior to polylysine of a similar molecular weight in terms of binding to vascular tissue, and can provide high accumulation and long-term retention of a low molecular weight compound (fluorescein, as a model molecule) in the vascular wall. Rationally designed GAG-binding peptides can be useful as affinity vehicles for targeting drugs to the vascular ECM.

Characterization of the binding of urokinase-type plasminogen activator to the asialoglycoprotein receptor

In order to study the role of the asialoglycoprotein receptor (ASGPr) in the rapid plasma clearance of urokinase-type plasminogen activator (u-PA), a microtiter plate binding assay was developed using ASGPr purified from rat liver extracts. Urinary two-chain u-PA bound to immobilized ASGPr in a saturable manner with an EC50 of 0.2 microM. Binding was inhibited by rabbit antibodies against the ASGPr. In line with the known carbohydrate specificity of the ASGPr, GalNAc proved to be the most effective inhibitor from a series of monosaccharides, followed by Gal and Fuc, whereas GlcNAc was ineffective. The N-linked oligosaccharides of urinary u-PA do not terminate with the common Gal-GlcNAc element, but with a GalNAc-GlcNAc element which is partially sulfated. Sulfated forms of u-PA were separated from non-sulfated forms by using the lectin Wisteria floribunda agglutinin. Only the non-sulfated forms of u-PA (30% of the total) appeared to bind to the ASGPr. From different u-PA preparations used for thrombolytic therapy only urinary u-PA and u-PA produced by kidney cell cultures strongly bound to the ASGPr, whereas (recombinant) u-PA expressed in mouse myeloma cells, Chinese hamster ovary cells or E. coli scarcely bound to the receptor. It is concluded that u-PA bearing non-sulfated GalNAc-GlcNAc elements is specifically recognized by the ASGPr present on liver cells.

Identification of the epidermal growth factor-like domains of thrombomodulin essential for the acceleration of thrombin-mediated inactivation of single-chain urokinase-type plasminogen activator

Single-chain urokinase-type plasminogen activator (scu-PA) can be cleaved by thrombin into a virtually inactive form called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T), a process accelerated by thrombomodulin, which contains six epidermal growth factor (EGF)-like domains. In this study, we identified the EGF-like domains of thrombomodulin required for the acceleration of the inactivation of scu-PA by thrombin using various forms of thrombomodulin (TM). scu-PA was treated with thrombin in the absence and presence of full-length rabbit TM (containing EGF1-6), recombinant TM comprising all of the extracellular domains including EGF1-6 (TMLEO) and recombinant TM comprising EGF4-6 plus the interconnecting region between EGF3 and EGF4 (TMEi4-6), and the tcu-PA/T generated was quantitated in each case. Rabbit TM accelerated the inactivation of scu-PA approximately 35-fold, while both recombinant forms accelerated it only threefold due to the absence of a critical chondroitin sulfate moiety. Subsequently, TME5-6 was prepared by cyanogen bromide digestion of TMEi4-6. TME5-6 bound to thrombin but did not accelerate the activation of protein C. In contrast, the inactivation of scu-PA by thrombin was accelerated to the same extent as that induced by TMLEO and TMEi4-6. This study demonstrates that, in addition to the chondroitin sulfate moiety, only EGF-like domains 5 and 6 are essential for the acceleration of the inactivation of scu-PA by thrombin. This differs from the domains that are critical for activation of protein C (EGF-like domains i4-6) and thrombin activatable fibrinolysis inhibitor (EGF-like domains 3-6).

Basic principles in thrombolysis: regulatory role of plasminogen

During thrombolytic therapy, patients are treated with a plasminogen activator in order to stimulate the fibrinolytic system by converting the precursor plasminogen into the active enzyme plasmin. The fibrinolytic process can be divided into two phases. In the first phase, plasminogen binds to intact fibrin and initial fibrinolysis takes place. As a result, carboxyterminal lysine residues are generated, which represent new binding sites for plasminogen. In the second phase, plasminogen binds to these sites and fibrinolysis is accelerated because the local plasminogen concentration is strongly enhanced and because this plasminogen has a higher reactivity. For instance, both single-chain urokinase-type plasminogen activator (scu-PA) and staphylokinase have a high preference for this type of plasminogen, which explains their fibrin-selective action. A recently discovered thrombin-activatable fibrinolysis inhibitor (TAFI) eliminates carboxyterminal lysine residues from partially degraded fibrin and, thus, inhibits the second phase of fibrinolysis. These mechanisms show that plasminogen plays an important regulatory role in fibrinolysis and thrombolysis.

Polylysine as a vehicle for extracellular matrix-targeted local drug delivery, providing high accumulation and long-term retention within the vascular wall

We present the first steps in the elaboration of an approach of extracellular matrix-targeted local drug delivery (ECM-LDD), designed to provide a high concentration, ubiquitous distribution, and long-term retention of a drug within the vessel wall after local intravascular delivery. The approach is based on the concept of a bifunctional drug comprising a "therapeutic effector" and an "affinity vehicle," which should bind to an abundant component of the vessel wall. The aim of the present study was to select molecules suitable for the role of affinity vehicles for ECM-LDD and to study their intravascular delivery and retention ex vivo and in an animal model. By use of fluorescence microscopy, the following molecules were selected on the basis of strong binding to cross sections of human vessels: protamine, polylysine, polyarginine, a glycosaminoglycan-binding peptide from vitronectin, and a synthetic dendrimer. With polylysine as a prototypic affinity vehicle, we showed that after intravascular delivery, polylysine was concentrated throughout a luminal layer of the vascular wall to an extremely high concentration of 20 g/L and was retained therein for at least 72 hours of perfusion without noticeable losses. Low molecular weight (fluorescein) and high molecular weight (hirudin) compounds could be chemically conjugated to polylysine and were retained in the vessel wall after intravascular delivery of the conjugates. In conclusion, by use of the ECM-LDD method, an extremely high concentration and long-term retention of locally delivered drug can be reached. Polycationic molecules can be considered as potential affinity vehicles for ECM-LDD.

Acceleration of fibrinolysis by high-frequency ultrasound: the contribution of acoustic streaming and temperature rise

High-frequency ultrasound has been shown to accelerate enzymatic fibrinolysis. One of the supposed mechanisms of this effect is the enhancement of mass transport by acoustic streaming, i.e., ultrasound-induced macroscopic flow around the clot. In this study, which is aimed at further elucidating the mechanisms of the acceleration of fibrinolysis by ultrasound, we investigated whether ultrasound would accelerate fibrinolysis if the flow around the thrombus is already present, as may occur in vivo. The effect of the ultrasound-induced temperature rise was also studied. In a model of a plasma clot submerged in plasma, containing tissue-type plasminogen activator, mild stirring of the outer plasma producing a shear rate of 40 seconds(-1) at the surface of the clot resulted in a two-fold acceleration of lysis. A similar effect was obtained with ultrasound (1 MHz, 2 W/cm(2)). Furthermore, if ultrasound was applied together with stirring, only 30% acceleration by ultrasound was documented, fully attributable to the concomitant temperature rise. In a model with tissue-type plasminogen activator incorporated throughout a plasma clot, the effect of ultrasound (two-fold shortening of lysis time) was fully attributable to the concomitant temperature rise of a few degrees. We concluded that the acceleration of enzymatic plasma clot lysis by high-frequency ultrasound in the models we used can be largely explained by a combination of the effects of heating and acoustic streaming, equivalent to mild stirring. The thermal effects can hardly be utilized in vivo due to the danger of tissue overheat. The therapeutic advantage of transcutaneous high-frequency ultrasound as an adjunct to thrombolytic therapy may appear limited to the situations where there is no flow in the direct environment of the thrombus.

The inactivation of single-chain urokinase-type plasminogen activator by thrombin on cultured human endothelial cells

Single-chain urokinase-type plasminogen activator (scu-PA) is cleaved by thrombin, resulting in an inactive molecule called thrombin-cleaved two-chain urokinase-type plasminogen activator (tcu-PA/T). There is no knowledge about cell-mediated inactivation of scu-PA. We have studied whether scu-PA bound to cultured human umbilical vein endothelial cells (HUVEC) could be inactivated by thrombin. High molecular weight scu-PA was bound to HUVEC and incubated with increasing amounts of thrombin for 30 min at 37 degrees C. Cell-bound urokinase-type plasminogen activator (u-PA) was released and levels of scu-PA, tcu-PA/T and active two-chain u-PA were measured using sensitive bioimmunoassays. Cell-bound scu-PA was efficiently inactivated by thrombin. Fifty percent inactivation of scu-PA occurred at about 0.2 nM thrombin. In the presence of monoclonal anti-urokinase receptor IgG, at least 50% of the binding of scu-PA to HUVEC was inhibited. The relative amount of tcu-PA/T that was generated by thrombin was not affected by the monoclonal antibody. These results indicated that scu-PA bound to HUVEC via the urokinase receptor can be inactivated by thrombin. The efficient inactivation of cell-bound scu-PA suggests that a cofactor for thrombin may be involved, like thrombomodulin or glycosaminoglycans. It is concluded that scu-PA bound to the urokinase receptor on a cell surface can be inactivated by thrombin, which may have profound effects on u-PA-mediated local fibrinolysis and extracellular proteolysis during processes in which thrombin is also involved.

Simultaneous detection of endogenous lectins and their binding capacity at the single-cell level--a technical note

Endogenous lectins are proteins/glycoproteins which selectively recognize distinct saccharide ligands and are different from immunoglobulins and carbohydrate-utilizing enzymes. Expression of these molecules can be detected immunohistochemically using nonblocking monoclonal antibodies (A1D6: anti-galectin-3, MR-15-2-2: anti-175 kD mannose receptor). Alternatively, biotinylated (neo)glycoconjugates which are recognized by a studied lectin can be employed as convenient probes to demonstrate specific binding of sugar epitopes by carbohydrate recognition domains (CRD) of endogenous lectins. In this study, we describe a new procedure for immunocytochemical visualization of the expression of endogenous lectins and glycochemical visualization of the reactivity of carbohydrate recognition domain(s), performed simultaneously at the single-cell level.

Urokinase-mediated fibrinolysis in the synovial fluid of rheumatoid arthritis patients may be affected by the inactivation of single chain urokinase type plasminogen activator by thrombin

BACKGROUND

Excessive fibrin deposition within the inflamed joints of rheumatoid arthritis (RA) patients suggests that local fibrinolysis is inefficient, which seems to be in contrast with the observed increased levels of urokinase type plasminogen activator (u-PA). Thrombin-mediated inactivation of single chain u-PA (scu-PA) into an inactive form called thrombin-cleaved two chain u-PA (tcu-PA/T) may provide a possible explanation for this contradiction.

AIM

To assess the occurrence of tcu-PA/T in the synovial fluid of patients with RA and with osteoarthritis (OA), and in the synovial fluid of controls to find support for thrombin-mediated inactivation of scu-PA in RA.

METHODS

Levels of scu-PA and tcu-PA/T were measured in the synovial fluid of 20 RA patients, nine OA patients and 14 controls using sensitive bioimmunoassays. Total urokinase antigen was quantified by a urokinase ELISA.

RESULTS

tcu-PA/T was found in the synovial fluid of all RA and OA patients. Only in seven of 14 control samples, levels of tcu-PA/T could be measured above the detection limit of the assay (0.2 ng/ml). The concentrations of tcu-PA/T, scu-PA and u-PA:Ag were significantly higher in the synovial fluid of the RA and OA patients as compared with the controls, while the RA patients had significantly higher levels of tcu-PA/T and u-PA:Ag than the OA patients. In RA, tcu-PA/T seemed to account for more than 40% of total urokinase antigen, while the contribution of tcu-PA/T to total urokinase antigen was only minor in OA and the controls (9.0% and 6.6%, respectively).

CONCLUSION

A significant part of the high total urokinase antigen in the synovial fluid of RA patients can be attributed to tcu-PA/T, implying that a large amount of scu-PA is not available for fibrinolysis because of its inactivation by thrombin. Thus, thrombin may promote the inflammation process in RA by inhibiting the fibrinolytic system and preventing the removal of fibrin.

The effect of flow on lysis of plasma clots in a plasma environment

Fibrinolysis initially generates channels in an occluding thombus which results in blood flow through the thrombus. Since the impact of flow along the surface of a thrombus on thrombolysis has not been investigated in detail, we studied in vitro how such a flow affects lysis. Compacted and noncompacted plasma clots were used as model thrombi. With compacted clots, fibrin-specific lysis induced by alteplase in the outer plasma was accelerated about 2-fold by strong flow (arterial shear rate). Non-fibrin-specific lysis induced either by a high concentration of alteplase or by streptokinase was slow, was accompanied by rapid depletion of plasminogen in the outer plasma, and was only slightly accelerated by flow. With noncompacted clots, similar acceleration factors were documented, when mild flow (venous shear rate) was applied. Strong flow further accelerated fibrin-specific lysis, up to 10-fold as compared to lysis without flow, but paradoxically retarded non-fibrin-specific lysis. The data suggest that flow accelerates lysis by enhancing transport of plasminogen from the outer plasma to the surface of the clot. Both opposite effects of the strong flow were mediated by forceful intrusion of the outer plasma into the noncompacted clot due to flow irregularities. In the case of non-fibrin-specific lysis this resulted in the replacement of the plasminogen-containing milieu by plasminogen-depleted outer plasma in certain areas of the clot turning them into virtually unlysable fragments. This flow-enforced "plasminogen steal" may contribute to the relatively high percentage of incomplete thrombolysis (TIMI-2 grade flow) documented in a number of trials for non-fibrin-specific thrombolytic agents. In the case of fibrin-specific lysis, the effect of flow on the speed of fibrinolysis is always beneficial.

Fibrinolytic properties of activated FXII

Activated factor XII (FXIIa), the initiator of the contact activation system, has been shown to activate plasminogen in a purified system. However, the quantitative role of FXIIa as a plasminogen activator in contact activation-dependent fibrinolysis in plasma is still unclear. In this study, the plasminogen activator (PA) activity of FXIIa was examined both in a purified system and in a dextran sulfate euglobulin fraction of plasma by measuring fibrinolysis in a fibrin microtiter plate assay. FXIIa was found to have low PA activity in a purified system. Dextran sulfate potentiated the PA activity of FXIIa about sixfold, but had no effect on the PA activity of smaller fragments of FXIIa, missing the binding domain for negatively charged surfaces. The addition of small amounts of factor XII (FXII) to FXII-deficient plasma induced a large increase in contact activation-dependent PA activity, as measured in a dextran sulfate euglobulin fraction, which may be ascribed to FXII-dependent activation of plasminogen activators like prekallikrein. When more FXII was added, PA activity continued to increase but to a lesser extent. In normal plasma, the addition of FXII also resulted in an increase of contact activation-dependent PA activity. These findings suggested a significant contribution of FXIIa as a direct plasminogen activator. Indeed, at least 20% of contact activation-dependent PA activity could be extracted from a dextran sulfate euglobulin fraction prepared from normal plasma by immunodepletion of FXIIa and therefore be ascribed to direct PA activity of FXIIa. PA activity of endogenous FXIIa immunoadsorped from plasma could only be detected in the presence of dextran sulfate. From these results it is concluded that FXIIa can contribute significantly to fibrinolysis as a plasminogen activator in the presence of a potentiating surface.

Inactivation of single-chain urokinase-type plasminogen activator by thrombin in human subjects

Thrombin cleaves single-chain urokinase-type plasminogen activator (scu-PA) into a virtually inactive two-chain form (tcu-PA/T), a process that may protect a blood clot from early fibrinolysis. It is not known under what circumstances tcu-PA/T can be generated in vivo. We have studied the occurrence of tcu-PA/T in human subjects with a varying degree of hypercoagulability. tcu-PA/T was assessed in the plasma of patients with disseminated intravascular coagulation (DIC), endotoxin-treated volunteers, patients with unstable angina pectoris, and patients selected for hip replacement. Relationships between tcu-PA/T and several markers reflecting thrombin generation were examined. tcu-PA/T was observed only in the plasma of patients with DIC and was associated with all thrombin markers and with scu-PA and urokinase antigen. Prothrombin fragment 1 + 2 and urokinase antigen were independent predictors of tcu-PA/T. The fact that tcu-PA/T could not be detected in the other three groups was explained by a lower extent of thrombin generation, a greater inhibition of thrombin by antithrombin, or less available urokinase antigen in these groups. The contribution of scu-PA to total urokinase antigen was decreased in the patients with DIC because of inactivation by thrombin, which may be an additional explanation for the inadequate fibrinolysis observed in these patients. These findings show that scu-PA can be inactivated in the circulation under severe pathophysiologic circumstances and that the process of inactivation depends not only on the generation of thrombin but also on the control of thrombin activity by its inhibitor antithrombin.