The stimulatory effect of soluble fibrin on plasminogen activation by tissue plasminogen activator as studied by the Coa-set Fibrin Monomer test

Factor V Leiden mutation enhances fibrin formation and dissolution in vivo in a human endotoxemia model

Disseminated intravascular coagulation in sepsis is associated with microvascular thrombosis and organ dysfunction. It was expected that prothrombotic disposition such as factor V Leiden (FVL) mutation would worsen clinical outcome. Astonishingly, clinical trial and animal experimental data indicate that FVL can be associated with improved survival. This study investigated the effect of FVL on the response to endotoxin of the coagulation and fibrinolytic system in humans. Fourteen healthy male subjects without FVL and 15 healthy males with heterozygous FVL received an intravenous bolus dose of endotoxin, 2 ng/kg of body weight. Blood samples were drawn before and 1, 2, 4, 6, and 24 hours after administration of the endotoxin. Injection of endotoxin led to a more pronounced increase in soluble fibrin in patients with FVL than in controls. Patients with FVL displayed a more sustained increase in plasmin-plasmin inhibitor complex after 4, 6, and 24 hours. Patients with FVL mutation also displayed higher levels of D-dimer and fibrinogen-fibrin degradation products in plasma after 24 hours. Patients with FVL generate higher levels of soluble fibrin, which may serve as cofactor in tissue plasminogen activator-induced plasminogen activation, leading to a more sustained activation of fibrinolysis with production of more fibrinogen- and fibrin-degradation products.

Plasminogen activation without changes in tPA and PAI-1 in response to subcutaneous administration of ancrod

Ancrod is a purified fraction of venom from the Malayan pit viper, Calloselasma rhodostoma, currently under investigation for treatment of ischemic stroke. The therapeutic effect is ascribed to a lowering of plasma fibrinogen. Thirty-two healthy volunteers received subcutaneous ancrod at doses of 1.0, 1.5 and 2.0 IU/kg body weight or placebo. Blood samples were drawn before the injection and at various time points until 96 h after the injection. Ancrod leads to the formation of desAA-fibrin, which serves as cofactor in tissue plasminogen activator activity (tPA)-induced plasminogen activation. Unchanged concentrations of prothrombin fragment F1.2 and thrombin-antithrombin complex (TAT) indicate that fibrin formation occurs independent of thrombin. Plasmin generation is independent of an increase in tPA activity or changes in plasminogen activator inhibitor-1 (PAI-1) concentration in plasma. Subcutaneous injection of ancrod leads to a generalized fibrino(geno)lytic response caused solely by providing tPA with soluble fibrin as its cofactor in plasminogen activation. Maximal plasmin activity is present 12 h after subcutaneous injection.

A kinetic analysis of the tissue plasminogen activator and DSPAalpha1 cofactor activities of untreated and TAFIa-treated soluble fibrin degradation products of varying size

The kinetics of tissue plasminogen activator (t-PA) and DSPAalpha1-catalyzed plasminogen activation using untreated and TAFIa-treated fibrin degradation products (FDPs), ranging in weight average molecular weight (M(w)) from 0.48 x 10(6) to 4.94 x 10(6) g/mol, were modeled according to the steady-state template model. The FDPs served as effective cofactors for both activators. The intrinsic catalytic efficiencies of both t-PA (17.4 x 10(5) m(-1) s(-1)) and DSPAalpha1 (6.0 x 10(5) m(-1) s(-1)) were independent of FDP M(w). The intrinsic catalytic efficiency of t-PA was 12-fold higher than that measured under identical conditions with intact fibrin as the cofactor. At sub-saturating levels of cofactor and substrate, rates were strongly dependent on FDP M(w) with DSPAalpha1 but not t-PA. Loss of activity with decreasing FDP M(w) correlated with loss of finger-dependent binding of the activators to the FDPs. TAFIa treatment of the FDPs resulted in 90- and 215-fold decreases in the catalytic efficiencies of t-PA (0.20 x 10(5) m(-)(1) s(-1)) and DSPAalpha1 (0.028 x 10(5) m(-1) s(-1)), yielding cofactors that were still 30- and 50-fold better than fibrinogen with t-PA and DSPAalpha1, respectively. Our results show that for both activators the products released during fibrinolysis are very effective cofactors for plasminogen activation, and both t-PA and DSPAalpha1 cofactor activity are strongly down-regulated by TAFIa.

Analysis of fibrin formation and proteolysis during intravenous administration of ancrod

Ancrod is a purified fraction of venom from the Malayan pit viper, Calloselasma rhodostoma, currently under investigation for treatment of acute ischemic stroke. Treatment with ancrod leads to fibrinogen depletion. The present study investigated the mechanisms leading to the reduction of plasma fibrinogen concentration. Twelve healthy volunteers received an intravenous infusion of 0.17 U/kg body weight of ancrod for 6 hours. Blood samples were drawn and analyzed before and at various time points until 72 hours after start of infusion. Ancrod releases fibrinopeptide A from fibrinogen, leading to the formation of desAA-fibrin monomer. In addition, a considerable proportion of desA-profibrin is formed. Production of desA-profibrin is highest at low concentrations of ancrod, whereas desA-profibrin is rapidly converted to desAA-fibrin at higher concentrations of ancrod. Both desA-profibrin and desAA-fibrin monomers form fibrin complexes. A certain proportion of complexes carries exposed fibrin polymerization sites EA, indicating that the terminal component of the protofibril is a desAA-fibrin monomer unit. Soluble fibrin complexes potentiate tissue-type plasminogen activator-induced plasminogen activation. Significant amounts of plasmin are formed when soluble fibrin in plasma reaches a threshold concentration, leading to the proteolytic degradation of fibrinogen and fibrin. In the present setting, high concentrations of soluble fibrin are detected after 1 hour of ancrod infusion, whereas a rise in fibrinogen and fibrin degradation products, and plasmin-α2–plasmin inhibitor complex levels is first detected after 2 hours of ancrod infusion. Ancrod treatment also results in the appearance of cross-inked fibrin degradation productd-dimer in plasma.

Analysis of fibrin formation and proteolysis during intravenous administration of ancrod

Ancrod is a purified fraction of venom from the Malayan pit viper, Calloselasma rhodostoma, currently under investigation for treatment of acute ischemic stroke. Treatment with ancrod leads to fibrinogen depletion. The present study investigated the mechanisms leading to the reduction of plasma fibrinogen concentration. Twelve healthy volunteers received an intravenous infusion of 0.17 U/kg body weight of ancrod for 6 hours. Blood samples were drawn and analyzed before and at various time points until 72 hours after start of infusion. Ancrod releases fibrinopeptide A from fibrinogen, leading to the formation of desAA-fibrin monomer. In addition, a considerable proportion of desA-profibrin is formed. Production of desA-profibrin is highest at low concentrations of ancrod, whereas desA-profibrin is rapidly converted to desAA-fibrin at higher concentrations of ancrod. Both desA-profibrin and desAA-fibrin monomers form fibrin complexes. A certain proportion of complexes carries exposed fibrin polymerization sites EA, indicating that the terminal component of the protofibril is a desAA-fibrin monomer unit. Soluble fibrin complexes potentiate tissue-type plasminogen activator-induced plasminogen activation. Significant amounts of plasmin are formed when soluble fibrin in plasma reaches a threshold concentration, leading to the proteolytic degradation of fibrinogen and fibrin. In the present setting, high concentrations of soluble fibrin are detected after 1 hour of ancrod infusion, whereas a rise in fibrinogen and fibrin degradation products, and plasmin-α2–plasmin inhibitor complex levels is first detected after 2 hours of ancrod infusion. Ancrod treatment also results in the appearance of cross-inked fibrin degradation productd-dimer in plasma.

Degradation of the alpha-chain of fibrin by human neutrophil elastase reduces the stimulating effect of fibrin on plasminogen activation

The degradation of fibrin by human neutrophil elastase (HNE) and the interference of such degradation on the stimulating effect of fibrin on plasminogen activation by tissue plasminogen activator (t-PA) was studied. By using SDS electrophoresis and Western blotting with subsequent immunostaining with monoclonal antibodies, degradation of the fibrin molecule was monitored. This degradation was related to the stimulating effect on plasminogen activation. Degradation of the alpha-chain was seen to occur before degradation of the beta- and gamma-chains. On the alpha-chain it was found that C-terminal degradation occurred prior to visible degradation of the N-terminal end. This C-terminal degradation was associated with a fall in the stimulation of plasminogen activation, coinciding with a corresponding reduction in the polymerization of fibrin. With further degradation, including N-terminal proteolysis of the alpha-chain, the stimulating effect of fibrin was reduced to that of fibrinogen.

CONCLUSIONS

Our results indicate that HNE degradation of the alpha-chain of fibrin occurs initially from the C-terminal end, affecting the polymerization of fibrin. This impaired polymerization may be important for the observed reduction in the t-PA mediated plasminogen activation.

Thrombin treated plasma employed as a standard for determination of soluble fibrin

The Coa-set Fibrin Monomer test (CFM-test) is a quantitative method for determination of soluble fibrin in plasma. The fibrin standard of the CFM-test is produced by bathroxobin conversion of purified fibrinogen to fibrin. Plasma treated with minute amounts of thrombin may be considered as a more physiological way of preparing fibrin monomers. Therefore, we have employed such thrombin treated plasma (TTP) as an alternative fibrin standard for the CFM-test. The fibrin concentration of the TTP was determined indirectly by quantitation of released fibrinopeptide A. The TTP was stable during freezing, thawing and storage for 3 months at -70 degrees C. The standard curve obtained using TTP as a standard was linear in the range of 0-275 nmol/l fibrin in plasma, but the slope of the line was less steep than the original standard curve. This difference was probably due to the greater plasminogen activating effect of bathroxobin digested fibrinogen compared to soluble fibrin generated in plasma by thrombin, as observed in a previous study. Because of the less steep slope of the alternative standard curve, fibrin levels in plasma samples from 20 healthy volunteers and 25 patients were found to be higher employing TTP as a standard. Preparation of a fibrin standard by incubation of plasma with minute amounts of thrombin will to some extent mimic the process of fibrin generation in vivo. Since we have found a satisfactory stability of such a standard during freezing, thawing and storage, we think the TTP standard might be useful for quantitation of soluble fibrin in plasma.

Does Lp(a) lipoprotein inhibit the fibrinolytic system?

Lp(a) lipoprotein contains a unique apolipoprotein, apolipoprotein (a), that has a striking homology with plasminogen. This homology has brought forward speculations as to an inhibitory effect of Lp(a) lipoproteins on fibrinolysis. The present investigation was undertaken to study the influence of Lp(a) lipoprotein on the fibrinolytic system. In an in vitro model, we have studied the influence of purified Lp(a) lipoprotein on plasminogen activation by tissue plasminogen activator (t-PA) in the presence of soluble fibrin. Increasing concentrations of Lp(a) lipoprotein (0-32 mg/dl) did not inhibit plasminogen activation by t-PA in the presence of thrombin or bathroxobin digested fibrinogen. When purified Lp(a) lipoprotein was added to whole blood, the degree of fibrin degradation obtained following standardized coagulation, as evaluated by the generation of D-dimer, was not reduced. D-dimer levels in plasma and in serum after standardized coagulation, as well as conventional parameters for evaluation of the fibrinolytic system, were determined in 10 individuals with high and 10 individuals with low levels of Lp(a) lipoprotein. No differences in the fibrinolytic parameters were observed between the groups. Thus, we found no evidence that Lp(a) lipoprotein interferes with the fibrinolytic process in the present experiments.