Reactions of activated factor X-phosphatide mixtures in vitro and in vivo

Quantification of hypercoagulable state after blunt trauma: microparticle and thrombin generation are increased relative to injury severity, while standard markers are not

BACKGROUND

Major trauma is an independent risk factor for developing venous thromboembolism. While increases in thrombin generation and/or procoagulant microparticles have been detected in other patient groups at greater risk for venous thromboembolism, such as cancer or coronary artery disease, this association has yet to be documented in trauma patients. This pilot study was designed to characterize and quantify thrombin generation and plasma microparticles in individuals early after traumatic injury.

METHODS

Blood was collected in the trauma bay from 52 blunt injured patients (cases) and 19 uninjured outpatients (controls) and processed to platelet poor plasma to allow for (1) isolation of microparticles for identification and quantification by flow cytometry, and (2) in vitro thrombin generation as measured by calibrated automatic thrombography. Data collected are expressed as either mean ± standard deviation or median with interquartile range.

RESULTS

Among the cases, which included 39 men and 13 women (age, 40 ± 17 years), the injury severity score was 13 ± 11, the international normalized ratio was 1.0 ± 0.1, the thromboplastin time was 25 ± 3 seconds, and platelet count was 238 ± 62 (thousands). The numbers of total (cell type not specified) procoagulant microparticles, as measured by Annexin V staining, were increased compared to nontrauma controls (541 ± 139/μL and 155 ± 148/μL, respectively; P < .001). There was no significant difference in the amount of thrombin generated in trauma patients compared to controls; however, peak thrombin was correlated to injury severity (Spearman correlation coefficient R, 0.35; P = .02).

CONCLUSION

Patients with blunt trauma have greater numbers of circulating procoagulant microparticles and increased in vitro thrombin generation. Future studies to characterize the cell-specific profiles of microparticles and changes in thrombin generation kinetics after traumatic injury will determine whether microparticles contribute to the hypercoagulable state observed after injury.

Procoagulant activity, but not number, of microparticles increases with age and in individuals after a single venous thromboembolism

The Calibrated Automated Thrombogram (CAT), a plate-based assay that measures thrombin generation and inhibition in plasma samples, is modified to measure the procoagulant activity of phospholipid associated with plasma microparticles (MP). The assay uses a tissue factor trigger without addition of 4 μM exogenous phospholipid (PL) used in the standard CAT. Calibrated with 4:1 posphatidylcholine- phosphatidylserine (PCPS) liposomes, the assay defines a median of 40 nM procoagulant phospholipid (PPL) equivalents in plasma containing MPs from 50 normal donors, with a range from 20 - 200 nM. Like the standard CAT, the modified assay detected no difference in plasma from 36 individuals with a history of a single episode of venous thromboembolism. However the male cases had double the PPL activity, as measured by rate of thrombin generation, of females; and there was a significant correlation among cases of increased thrombin generation with age. In contrast, there were no gender disparities or age correlations among control plasmas. The findings suggest that procoagulant activity of plasma microparticles, facilitated by a simplified, one-stage plate-based assay, offer a promising avenue of investigation of mechanisms and management of venous thromboembolic disorders.

Thromsbospondin-1 binds to the heavy chain of elastase activated coagulation factor V (FVaHNE) and enhances thrombin generation on the surface of a promyelocytic cell line

INTRODUCTION

Thrombospondin 1 (TSP1) has the ability to bind to HL-60 cells and to reversibly inhibit human neutrophil elastase (HNE). Human factor V (FV) can be cleaved by HNE thereby providing FV with cofactor activity (FVa(HNE)). Experiments were performed to evaluate the ability of HNE expressed on the surface of HL-60 cells to generate FVa(HNE) to support thrombin generation, and to determine the effect of TSP1 on this reaction.

RESULTS

Western blot analysis showed TSP1 forming a complex with FVa(HNE) within a region corresponding to the heavy chain of FV. Enzymatic reactions were performed to determine the role of TSP1-HNE-FVa(HNE) on the surface of HL-60 cells, namely the assembly of the prothrombinase complex. Thrombin generation was measured by the chromogenic substrate S2238. Exposure of factor V to HL-60 cells prior to the addition of prothrombin and activated factor X provided FV with cofactor activity. HL-60 cells were found capable of synthesizing factor V with cofactor activity, but HL-60 cells failed to synthesize and/or to provide factor X with enzymatic activity. The ability of HL-60 cells to synthesize FV and TSP1 was demonstrated. The addition of exogenous TSP1 enhanced both the rate and amount of thrombin generated on the HL-60 cell surface.

CONCLUSION

Despite the ability of TSP1 to reversibly inhibit HNE in a purified system, TSP1 expression favors the reactions leading to thrombin generation on the HL-60 cell surface. These observations are relevant to clinical conditions where there is a prothrombotic state such as malignant tumors.

Stroma free human platelet lysates potentiate the in vivo thrombogenicity of factor Xa by the provision of coagulant-active phospholipid

A stroma free platelet lysate supernatant (PLS) was prepared by repeatedly freezing and thawing a human platelet suspension separated from platelet rich plasma by gel filtration. The material was potently thrombogenic in a stasis model in rabbits, but only when combined with a purified preparation of factor Xa. The dose of factor Xa used was not thrombogenic when given alone. Initial evaluation suggested the presence of a factor V activator when PLS shortened the clotting time of normal but not a V deficient plasma. Subsequent evaluation, in a system employing purified components of the prothrombinase complex and a marker of thrombin generation, dansyl-arginine-N-(3-ethyl-1,5-pentanediyl)amide (DAPA), failed to confirm the presence of a factor V activator. Further evaluation in this system demonstrated that the procoagulant activity related to the content of coagulant-active phospholipid. Thin layer chromatography confirmed the presence of phospholipids known to have coagulant activity. These components were isolated by a lipid extraction procedure and the phospholipid replacing activity in the in vitro assay with DAPA was quantitatively retained. The extracted material was non-thrombogenic in vivo but augmented the thrombogenicity of a purified preparation of Xa.

A phospholipase A2 with anticoagulant activity. II. Inhibition of the phospholiped activity in coagulation

An anticoagulant factor with phospholipase A2 activity has been isolated from Vipera berus venom. Phospholipase activity was studied on platelet phospholipid and on brain cephalin. The venom factor showed a potent anticoagulant activity: 1 mug impaired the clotting of 1 ml of citrated recalcified platelet-poor plasma. The anticoagulant inhibited clotting by antagonism to phospholipid. The antagonism constant (Kan = 6.8-10(-9) M) demonstrated the high affinity of the inhibitor for phospholipid. As with other phospholipases A2, the venom factor was thermoresistant but very sensitive to photo-oxidation. Both activities (anticoagulant activity and phospholipase activity) were not markedly dissociated by either denaturation or neutralization processes. Slightly different curves of photo-oxidative inactivation of both activities suggested the presence, on the molecule, of two very close sites responsible for phospholipase and anticoagulant activities. The inhibitor effect on coagulation was independent of the hydrolysis process. In fact, lysoderivatives and fatty acids, resulting from complete hydrolysis with the venom factor, were as active as the native phospholipids. Moreover phospholipase A2 from other viperidae venom, which did not have anticoagulant activity, produced similarly active lysoderivatives. This showed that the cleavage of the beta-acyl bond does not interfere with the activity of phospholipid. A possible mechanism of clotting inhibition by the venom factor was proposed. Owing to its high affinity for phospholipid, the inhibitor would complex phospholipid at its protein binding site impairing the normal arrangement of coagulation protein factors and, consequently, their activation. The positive charges of the inhibitor (pI = 9.2) could bind with phosphoryl or carboxyl groups of phospholipid, making them unavailable for protein binding. The complex formation involves a loss of dissociating capacity of the enzyme towards its substrate. This required an additional interaction of the inhibitor with a coagulation protein factor. The inhibitor could be removed from the complex by specific antibodies, permitting recovery of normal phospholipid-protein interaction. The role of calcium in the complex has not yet been elucidated. This venom factor affords a useful tool for investigating the phospholipid-clotting protein interaction.

Procoagulant activity of platelets in recalcified plasma

High-spun platelet-poor plasma (HSPPP) and platelet-rich plasma (PRP), prepared with minimal contact activation and pH shifts, were recalcified with little dilution in the presence of phospholipid prepared from chloroform-extracted, acetone-dried brain tissue. The clotting times of HSPPP and PRP were the same whether or not the platelets in PRP were first incubated or aggregated with ADP or with collagen. This suggests that platelet procoagulant activity in normal recalcified plasma derives only from provision of platelet phospholipid.