Tissue factor mediates prolonged procoagulant activity on the luminal surface of balloon-injured aortas in rabbits

Novel blood coagulation molecules: Skeletal muscle myosin and cardiac myosin

Essentials Striated muscle myosins can promote prothrombin activation by FXa or FVa inactivation by APC. Cardiac myosin and skeletal muscle myosin are pro-hemostatic in murine tail cut bleeding models. Infused cardiac myosin exacerbates myocardial injury caused by myocardial ischemia reperfusion. Skeletal muscle myosin isoforms that circulate in human plasma can be grouped into 3 phenotypes. ABSTRACT: Two striated muscle myosins, namely skeletal muscle myosin (SkM) and cardiac myosin (CM), may potentially contribute to physiologic mechanisms for regulation of thrombosis and hemostasis. Thrombin is generated from activation of prothrombin by the prothrombinase (IIase) complex comprising factor Xa, factor Va, and Ca++ ions located on surfaces where these factors are assembled. We discovered that SkM and CM, which are abundant motor proteins in skeletal and cardiac muscles, can provide a surface for thrombin generation by the prothrombinase complex without any apparent requirement for phosphatidylserine or lipids. These myosins can also provide a surface that supports the inactivation of factor Va by activated protein C/protein S, resulting in negative feedback downregulation of thrombin generation. Although the physiologic significance of these reactions remains to be established for humans, substantive insights may be gleaned from murine studies. In mice, exogenously infused SkM and CM can promote hemostasis as they are capable of reducing tail cut bleeding. In a murine myocardial ischemia-reperfusion injury model, exogenously infused CM exacerbates myocardial infarction damage. Studies of human plasmas show that SkM antigen isoforms of different MWs circulate in human plasma, and they can be used to identify three plasma SkM phenotypes. A pilot clinical study showed that one SkM isoform pattern appeared to be linked to isolated pulmonary embolism. These discoveries enable multiple preclinical and clinical studies of SkM and CM, which should provide novel mechanistic insights with potential translational relevance for the roles of CM and SkM in the pathobiology of hemostasis and thrombosis.

LIM-only protein FHL2 attenuates vascular tissue factor activity, inhibits thrombus formation in mice and FHL2 genetic variation associates with human venous thrombosis

Bleeding disorders and thrombotic complications are major causes of morbidity and mortality with many cases being unexplained. Thrombus formation involves aberrant expression and activation of tissue factor (TF) in vascular endothelial and smooth muscle cells. Here, we sought to identify factors that modulate TF gene expression and activity in these vascular cells. The LIM-only protein FHL2 is a scaffolding protein that modulates signal transduction pathways with crucial functions in endothelial and smooth muscle cells. However, the role of FHL2 in TF regulation and thrombosis remains unexplored. Using a murine model of venous thrombosis in mesenteric vessels, we demonstrated that FHL2 deficiency results in exacerbated thrombus formation. Gain- and loss-of-function experiments revealed that FHL2 represses TF expression in endothelial and smooth muscle cells through inhibition of the transcription factors nuclear factor κB and activating protein-1. Furthermore, we observed that FHL2 interacts with the cytoplasmic tail of TF. In line with our in vivo observations, FHL2 decreases TF activity in endothelial and smooth muscle cells whereas FHL2 knockdown or deficiency results in enhanced TF activity. Finally, the FHL2 single nucleotide polymorphism rs4851770 was associated with the risk of venous thrombosis in a large population of venous thrombosis cases and control subjects from 12 studies (INVENT consortium). Altogether, our results highlight functional involvement of FHL2 in TF-mediated coagulation and identify FHL2 as a novel gene associated with venous thrombosis in humans.

Role of the protein C receptor in cancer progression

The hemostatic system plays pleiotropic roles in cancer progression by shaping the tumor microenvironment and metastatic niches through thrombin-dependent fibrin deposition and platelet activation. Expanding experimental evidence implicates coagulation protease receptors expressed by tumor cells as additional players that directly influence tumor biology. Pro-angiogenic G protein-coupled signaling of TF through protease activated receptor 2 and regulation of tumor cell and vascular integrins through ligation by alternative spliced TF are established pathways driving tumor progression. Our recent work shows that the endothelial protein C receptor (EPCR), a stem cell marker in hematopoietic, neuronal and epithelial cells, is also crucial for breast cancer growth in the orthotopic microenvironment of the mammary gland. In aggressive triple-negative breast cancer cells, EPCR expression is a characteristic of cancer stem cell-like populations that have tumor initiating properties in vivo. Blocking antibodies to EPCR attenuate in vivo tumor growth and proliferation specifically of EPCR(+) cells on defined integrin matrices in vitro. We also showed that tumor-associated macrophages are a source for upstream coagulation proteases that can activate TF- and EPCR-dependent cellular responses, suggesting that tumor cells utilize the tumor microenvironment for tumor promoting coagulation protease signaling.

In vitro study of role of trace amount of Cu release from Cu-bearing stainless steel targeting for reduction of in-stent restenosis

A novel 316L type Cu-bearing stainless steel was developed in present work, aiming at reducing the occurrence of the in-stent restenosis after implantations of coronary stents, through trace amount of Cu release from surface of the steel in body fluid. It was found that there was a trace amount of Cu released from the Cu-bearing steel in a simulated body fluid, with no cytotoxicity. All the in vitro experimental results proved that this Cu-bearing steel could not only inhibit the proliferation of vascular smooth muscle cells, reducing the formation of thrombosis, which are the main reasons for happening of the in-stent restenosis, but also promote the proliferation of vascular endothelial cells needed for the revascularization, showing that this novel steel is prospective to be a new material for manufacturing coronary stents with function of reducing the in-stent restenosis.

Delivery of TFPI-2 using ultrasound with a microbubble agent (SonoVue) inhibits intimal hyperplasia after balloon injury in a rabbit carotid artery model

Here we report a new, simple and efficient method by using ultrasound and a microbubble agent (SonoVue) for delivering a gene to balloon-injured carotid arteries for restenosis prophylaxis. The tissue factor pathway inhibitor-2 (TFPI-2) has been shown to inhibit the postinjury intimae hyperplasia in atherosclerotic vessels. New Zealand white rabbits were divided into 4 groups with 14 in each, a treatment control for balloon injury, a gene vehicle control, a gene delivery of TFPI-2 without using ultrasound and a gene delivery of TFPI-2 using ultrasound. After four weeks, the injured artery neointimal proliferation was significantly lower in the TFPI-2 group with ultrasound than the control groups (p < 0.01) according to the measurement of the mean luminal diameters by B-mode ultrasonography. The ratio of intimal/media area and the stenosis rate in the gene delivery facilitated by ultrasound were significantly lower than those of the nonultrasound gene delivering method (p < 0.01).

Role of procurement-related injury in early saphenous vein graft failure after coronary artery bypass surgery

Saphenous vein graft thrombosis after coronary artery bypass graft surgery is a poorly understood problem that lessens the benefits of this procedure. Recent studies highlight the importance of injury sustained at the time of saphenous vein graft procurement in the pathogenesis of acute graft thrombosis. In particular, damage to the graft endothelium that occurs secondary to ex vivo pressure distention, a common practice during vein harvest, leads to a loss of antithrombotic factors and increased activity of subendothelial prothrombotic factors. The prothrombotic potential of damaged grafts is further exacerbated by an ischemic storage interval and subsequent exposure to arterial flow conditions after grafting. A clearer understanding of the mechanisms by which endothelial disruption leads to acute saphenous vein graft thrombosis may result in interventions for improving our procurement techniques, interrupting the downstream effects of the damaged saphenous vein graft and/or discriminating damage that is beyond an acceptable threshold of thrombotic risk.

Thrombin generation in vascular tissue

BACKGROUND

Classically, it is thought that the vast majority of thrombin is generated on the surface of platelets, however, thrombotic events occur in patients despite treatment with potent antiplatelet agents.

METHODS AND RESULTS

In freshly harvested left internal mammary artery (IMA) sections, addition of CaCl2 and platelet-poor plasma (PPP) were sufficient to stimulate a profound burst of thrombin and this effect was inhibited by antitissue factor antibodies. Ultracentrifugation of PPP to remove platelet microparticles had no effect on thrombin generation. Both the extrinsic and factor VIII-dependent pathways were necessary for IMA-supported thrombin generation as PPP derived from individuals deficient in factors V, VII, VIII or X did not support thrombin production. Small amounts of thrombin were generated utilizing factor IX (FIX)-deficient plasma, however, thrombin was not generated by aorta from FIX-deficient mice when FIX-deficient plasma was used. The addition of non-lipidated tissue factor (0.6 pM) and CaCl2 to actively proliferating cultured human aortic smooth muscle cells (SMC) resulted in a pronounced burst of thrombin generation occurring between 3 and 15 min after treatment. In the absence of tissue factor, thrombin was generated but at a slower rate and with a peak value 26% of that observed in the presence of tissue factor.

CONCLUSION

Significant thrombin generation can occur on vascular tissue in the absence of platelets or platelet microparticles and on the surface of non-apoptotic SMC.

Estrogenic regulation of tissue factor and tissue factor pathway inhibitor in platelets

Oral estrogen treatment increases thrombotic risk. Tissue factor (TF), tissue factor pathway inhibitor (TFPI), and platelet interaction with leukocytes are important determinants of thrombogenesis. Therefore, the present study was designed to define and compare platelet TF and TFPI mRNA and adhesion protein expression in platelets derived from animals treated with different types of oral estrogens. Ovariectomized pigs were treated with 17beta-estradiol (2 mg/day), conjugated equine estrogen (CEE; 0.625 mg/day), or raloxifene (60 mg/day) for 4 wk. Compared with intact animals, ovariectomy and treatment differentially affected populations of leukocytes: neutrophils decreased whereas lymphocytes increased significantly 4 wk after ovariectomy and with 17beta-estradiol and CEE treatments; eosinophils increased only with 17beta-estradiol treatment. Content of TF protein increased in platelets from 17beta-estradiol- and raloxifene-treated pigs, whereas TF mRNA was detected only in platelets from 17beta-estradiol- and CEE treated pigs. TFPI mRNA increased in platelets after ovariectomy and estrogen treatment. Only a trace of TFPI protein was detected, but a higher-molecular-mass protein was observed in all treatment groups. Expression of CD40 and CD40 ligand increased with ovariectomy and decreased with 17beta-estradiol and CEE treatments more than with raloxifene. The ratio of activated to basal P-selectin expression decreased with ovariectomy and increased with raloxifene treatments. These results suggest that estrogenic formulations may affect individual thrombotic risk by different mechanisms that regulate TF and platelet-leukocytic interactions. These studies provide the rationale for evaluation of interactions among platelets and TF and TFPI expression on thrombin generation during estrogen treatment in humans.

Tissue factor: (patho)physiology and cellular biology

The transmembrane glycoprotein tissue factor (TF) is the initiator of the coagulation cascade in vivo. When TF is exposed to blood, it forms a high-affinity complex with the coagulation factors factor VII/activated factor VIIa (FVII/VIIa), activating factor IX and factor X, and ultimately leading to the formation of an insoluble fibrin clot. TF plays an essential role in hemostasis by restraining hemorrhage after vessel wall injury. An overview of biological and physiological aspects of TF, covering aspects consequential for thrombosis and hemostasis such as TF cell biology and biochemistry, blood-borne (circulating) TF, TF associated with microparticles, TF encryption-decryption, and regulation of TF activity and expression is presented. However, the emerging role of TF in the pathogenesis of diseases such as sepsis, atherosclerosis, certain cancers and diseases characterized by pathological fibrin deposition such as disseminated intravascular coagulation and thrombosis, has directed attention to the development of novel inhibitors of tissue factor for use as antithrombotic drugs. The main advantage of inhibitors of the TF*FVIIa pathway is that such inhibitors have the potential of inhibiting the coagulation cascade at its earliest stage. Thus, such therapeutics exert minimal disturbance of systemic hemostasis since they act locally at the site of vascular injury.

Thrombin, TNF-alpha, and LPS exert overlapping but nonidentical effects on gene expression in endothelial cells and vascular smooth muscle cells

Thrombin, TNF-alpha, and LPS have each been implicated in endothelial cell and vascular smooth muscle cell (VSMC) activation. We wanted to test the hypothesis that these three agonists display mediator and/or cell type-specific properties. The addition of thrombin to human pulmonary artery endothelial cells resulted in an upregulation of PDGF-A, tissue factor (TF), ICAM-1, and urokinase-type plasminogen activator (u-PA), whereas TNF-alpha and LPS failed to induce PDGF-A. These effects were mimicked by protease-activated receptor-1 activation. In VSMC, thrombin induced expression of TF and PDGF-A but failed to consistently induce ICAM-1 or u-PA expression. In contrast, TNF-alpha and LPS increased expression of all four genes in this cell type. Inhibitor studies in endothelial cells demonstrated a critical role for PKC in mediating thrombin, TNF-alpha, and LPS induction of ICAM-1, TF, and u-PA and for p38 MAPK in mediating thrombin, TNF-alpha, and LPS induction of TF. Taken together, these results suggest that inflammatory mediators engage distinct signaling pathways and expression profiles in endothelial cells and VSMC. The data support the notion that endothelial cell activation is not an all-or-nothing phenomenon but rather is dependent on the nature of the extracellular mediator.

The therapeutic potential of novel anticoagulants

Conventional anticoagulant therapy has been based on indirect inhibition of coagulation factors with heparin and warfarin. These agents display liabilities prompting the development of new anticoagulants over the last two decades. The first to be developed was a series of low molecular weight heparins(LMWHs). Their favourable pharmacokinetic profiles and risk/benefit ratios led to widespread use in Europe and, more recently, approval for their use in the USA. Paralleling the development of LMWHs has been the pursuit of a different strategy focused on direct rather than indirect inhibition of enzymes in the coagulation cascade. In contrast to heparin, LMWHs, or other glycosaminoglycans, direct inhibitors exert their effects independent of either antithrombin III (ATIII) or heparin cofactor II (HCII) and more effectively inhibit clot-bound thrombin or FXa. Highly potent, selective (versus other serine proteases)direct thrombin and FXa inhibitors have been identified and isolated from natural sources, such as leeches, ticks and hookworms. The recombinant forms and analogues of the senatural proteins have been produced using molecular biology techniques, i.e., rHirudin, Hirulogs, recombinant tick anticoagulant peptide (rTAP), recombinant antistasin (rATS) and recombinant nematode anticoagulant peptide-5 (rNAP-5). The design of novel structures or the modification of existing chemicals has led to the synthesis of many non-peptide, low molecular weight inhibitors of thrombin and FXa. Some of them are orally active and may be suitable for long-term clinical use. In addition, considerable progress has been made in developing specific TF/VIIa complex inhibitors. The anticoagulation properties of the new agents are being characterised in experimental studies. Some of them have been advanced to large scale clinical trials and their effectiveness, and sometimes relative ineffectiveness,in arterial and venous thromboembolic disorders has been demonstrated. They are being tested for their potential as new antithrombotic agents that act via direct enzyme inhibition. Thus,the clinician should in future be able to target different thrombotic conditions with proven, specific anticoagulant interventions.

Heparin compromises streptokinase-induced arterial patency in rabbits

INTRODUCTION

Little is known with regard to efficacy of heparin as an adjunct to fibrinolytics under conditions of severe vascular damage. In this study, we compared the effects of unfractionated heparin (UH), low-molecular weight heparin (LMWH), and recombinant desulfohirudin (HIR) in combination with streptokinase (SK) in such settings.

MATERIALS AND METHODS

We used an established rabbit model, in which thrombosis, critical stenosis, and vascular wall damage were introduced to a segment of the abdominal aorta and the effects of the respective therapies were assessed by time to patency (TTP in minutes), cumulative patency (CP (%)), lysis of original clot (CL (%)), and net clot accretion (NCA (%)). Treatments were administered over 90 min at the following doses: SK: 33,000 U/kg, UH: 125-250 U/kg, LMWH: 1.25-2.5 mg/kg, HIR: 0.25-0.55 mg/kg.

RESULTS

Unexpectedly, UH and LMWH had a paradoxical and detrimental effect on SK-mediated recanalization as measured by both TTP and CP. Thus, administration of SK vs. SK+UH or SK+LMWH resulted in TTP values of 43+/-8 min vs. 70+/-11 min (p<0.05) and 67+/-12 min. (p<0.08), respectively. For CP, the corresponding values were 21+/-7%, 0.5+/-0.3% and 9+/-8%. This delay in vessel recanalization occurred despite excessive systemic anticoagulation (activated partial thromboplastin time (aPTT) and thrombin clotting time (TCT) ratios >6 and >34, respectively). Of interest, both heparinoids completely inhibited SK-induced fibrinogen consumption (FC). In contrast, recombinant desulfohirudin (HIR) shortened SK-induced TTP (4.97+/-0.81 min) without preserving fibrinogen.

CONCLUSIONS

Our findings suggest that caution needs to be exercised, when using the combination of SK and heparinoids for the treatment of arterial thrombosis under conditions of severe vascular damage and stenosis.

Insights into the redox control of blood coagulation: role of vascular NADPH oxidase-derived reactive oxygen species in the thrombogenic cycle

Various cardiovascular diseases including thrombosis, atherosclerosis, (pulmonary) hypertension and diabetes, are associated with disturbed coagulation. Alterations in the vessel wall common to many cardiovascular disorders have been shown to initiate the activity of the coagulation system, but also to be the result of an abnormal coagulation system. The primary link between the coagulation and the vascular system appears to be tissue factor (TF), which is induced on the surface of vascular cells and initiates the extrinsic pathway of the blood coagulation cascade, leading to the formation of thrombin. Thrombin can also interact with the vascular wall via specific receptors and can increase vascular TF expression. Such a "thrombogenic cycle" may be essentially involved in the pathogenesis of cardiovascular disorders associated with an abnormal coagulation. Therefore, the identification of the signaling pathways regulating this cycle and each of its relevant connecting links is of fundamental importance for the understanding of these disorders and their putative therapeutic potential. Reactive oxygen species (ROS) and the ROS-generating NADPH oxidases have been shown to play important roles as signaling molecules in the vasculature. In this review, we summarize the data supporting a substantial role of ROS in promoting a thrombogenic cycle in the vascular system.

Activated platelets stimulate tissue factor expression in smooth muscle cells

Tissue factor (TF)-induced activation of the coagulation plays a key role in the pathophysiology of acute coronary syndromes. Because TF represents the initial trigger of the coagulation cascade, its expression in the arterial wall is tightly regulated.

OBJECTIVE

To determine whether and which soluble mediators released during platelet activation may upregulate TF expression in smooth muscle cells (SMCs).

METHODS AND RESULTS

Rabbit SMCs were challenged with collagen-activated platelets and their effects on TF mRNA transcription and protein expression were evaluated at different time points (30 min, 1, 2, 4, 8, 12 and 24 h) by RT-PCR, immunofluorescence and a two-stage colorimetric assay. A progressive increase in TF mRNA, peaking at 2 h, was evident in SMCs stimulated with activated platelets with respect to baseline. The increase in TF mRNA expression was associated with a parallel increase in TF protein, as demonstrated by immunofluorescence and by colorimetric assay. In a different set of experiments, selected platelet-derived soluble mediators were shown to induce TF mRNA expression.

CONCLUSIONS

Activated platelets upregulate TF, via release of several soluble mediators, in a cell population widely expressed in the vessel wall and in atherosclerotic plaques, such as SMCs. This phenomenon might play an important role in sustaining thrombus formation in vivo.

MCP-1 deficiency is associated with reduced intimal hyperplasia after arterial injury

Monocyte chemoattractant protein (MCP)-1 is abundant in smooth muscle cells (SMC) and macrophages of atherosclerotic plaques and in the injured arterial wall. MCP-1 and its receptor, CCR2, are important mediators of macrophage accumulation and atherosclerotic plaque progression. We have recently reported that CCR2(-/-) mice have a approximately 60% decrease in intimal hyperplasia and medial DNA synthesis in response to femoral arterial injury. We have now examined the response to femoral arterial injury in MCP-1(-/-) mice. MCP-1 deficiency was associated with a approximately 30% reduction in intimal hyperplasia at 4 weeks and was not associated with diminished medial DNA synthesis. Despite inducing tissue factor in SMC culture, MCP-1 deficiency was not associated with a decrease in neointimal tissue factor after injury. These data suggest that MCP-1 and CCR2 deficiencies have distinct effects on arterial injury. The effects of MCP-1 on intimal hyperplasia may be mediated largely through SMC migration.

Effect of vascular injury on inhibition of venous thrombosis with ZK-807834, a direct inhibitor of factor Xa

Inhibition of factor Xa with the small molecule inhibitor ZK-807834 (Mr 527 Da, Ki 0.11 nM) attenuates progression of thrombosis, but the ED50 is substantially lower for venous compared with arterial thrombosis in experimental animals. To determine whether this reflects differences in the extent of vascular injury, we compared the dose-response of ZK-807834 for inhibition of venous thrombosis induced with a cotton thread and copper wire device in the presence and absence of balloon catheter-induced injury to the vena cava in rabbits. ZK-807834 administration over 2 h (total dosages of 0.0023-2.3 micro mol kg-1, n = 6/group) resulted in dose-dependent reductions in clot weight compared with vehicle controls, but the ED50 was 0.03 micro mol kg-1 for non-injured veins and 0.42 micro mol kg-1 for injured veins. We conclude that vascular injury invokes a tissue factor-mediated response that increases the dose requirements for inhibition of venous thrombosis with ZK-807834.

Overexpression of glycosyl phosphatidylinositol-anchored tissue factor pathway inhibitor-1 inhibits tissue factor activity

The cellular initiation of coagulation by the tissue factor (TF)-activated factor VII complex is transiently inhibited by endogenous tissue factor pathway inhibitor-1 (TFPI-1), whereas exogenously added TFPI-1 is targeted to a degradation pathway. This study investigates the relevance of glycosyl phosphatidylinositol (GPI) anchoring for the anticoagulant properties of TFPI-1. Experiments were performed with the human cell line ECV304 using liposomal gene transfer. For GPI anchoring of TFPI-1 we used a fusion protein of TFPI-1 and the GPI attachment sequence of decay-accelerating factor (GPI-TFPI-1), and compared it with wild-type TFPI-1. We measured TF and TFPI-1 surface expression by flow cytometry and TF proteolytic activity by a chromogenic assay for activated factor X generation. After transfection of GPI-TFPI-1, surface expression of TFPI-1 increased to 134 +/- 9% of mock transfected cells (mean +/- SEM, P = 0.004), and transfection with wild-type TFPI-1 did not significantly alter TFPI-1 surface expression. After transfection with GPI-TFPI-1, TF activity was reduced by 18 +/- 9% compared with mock transfections (P = 0.003), whereas after transfection with TFPI-1 wild type no significant inhibition was observed. This effect was not due to altered TF expression. GPI anchoring is an essential prerequisite for surface expression of TFPI-1 and inhibition of TF activity. Gene transfer of GPI-anchored TFPI, therefore, may be an efficient tool to inhibit local TF-induced coagulation.

Evolving Strategies for the Prevention and Treatment of Coronary Restenosis

Percutaneous coronary intervention has become an increasingly attractive alternative to medical therapy and surgical revascularization for the treatment of coronary artery disease. Over 1.5 million interventional procedures are performed annually worldwide, with the rate of procedures performed continuing to rise dramatically. Restenosis following percutaneous coronary intervention occurs at rates of 20% to 40% and has remained the Achilles heel of the procedure. Numerous early attempts at the prevention of restenosis with oral pharmacologic agents, such as antithrombotic therapies, lipid lowering agents, vasodilators, and growth factor inhibitors, failed to show benefit in clinical trials. The introduction of intracoronary stents resulted in a 30% reduction in restenosis rates by abolishing the early vessel recoil following angioplasty. However, as more complex lesions underwent percutaneous coronary intervention with stenting, rates of “in-stent” restenosis remained high (20% to 30%). With technologic advances and greater understanding of vascular pathobiology, novel therapeutic strategies, such as local delivery of ionizing radiation, immunosuppressive agents, and gene therapy, have been deployed to prevent coronary restenosis. In addition, a number of mechanical and radiation-based strategies have been used to treat those patients who develop restenosis. This review considers these emerging strategies for the prevention and treatment of restenosis.

Parameters of the tissue factor pathway with coumadin/dipyridamole versus ticlopidine as adjunct antithrombotic-drug regimen in coronary artery stenting

A different rate and timing of subacute stent thrombosis after percutaneous coronary intervention was reported with various peri-interventional antithrombotic regimens. Next to platelet activation, coagulation triggered by tissue factor (TF) may play a key role in this process. Thirty-one consecutive patients with stable and unstable angina undergoing coronary stenting were randomly assigned to adjunct oral anticoagulation/anti-platelet therapy (coumadin, dipyridamole, aspirin and heparin; n = 16) or adjunct anti-platelet therapy with thienopyridin (ticlopidine, aspirin and heparin; n = 15). Antigen levels of plasma TF, total tissue factor pathway inhibitor (TFPI) and TFPI/ activated factor X (TFPI/FXa) complex were determined before and for up to 6 days after intervention by immunoassay. At baseline, significantly higher levels of plasma TF and TFPI/FXa were found in patients with unstable angina [TF, 161 pg/ml (126-191 pg/ml); TFPI/FXa, 7.8 ng/ml (6.1-9.6 ng/ml)] compared with stable angina [TF, 62 pg/ml (46-82 pg/ml), P < 0.0001; TFPI/FXa, 4.5 ng/ml (3-7.6 ng/ml), P= 0.003]. One hour after intervention, an increase of plasma TF and TFPI/FXa was seen in both treatment groups. In unstable angina patients, plasma levels of TF, TFPI and TFPI/FXa were more efficiently reduced by adjunct ticlopidine therapy compared with adjunct coumadin/dipyridamole. These data suggest reduced release of circulating TF by combined anti-platelet therapy with ticlopidine and aspirin after coronary artery stenting, which may-contribute to the lower incidence of subacute stent thrombosis previously observed.

Inhibition of tissue factor limits the growth of venous thrombus in the rabbit

Antibody mediated inhibition of tissue factor (TF) function reduces thrombus size in ex vivo perfusion of human blood over a TF-free surface at venous shear rates suggesting that TF might be involved in the mechanism of deep vein thrombosis. Moreover, TF-bearing monocytes and polymorphonuclear (PMN) leukocytes were identified in human ex vivo formed thrombi and in circulating blood. To understand the role of TF in thrombus growth, we applied a rabbit venous thrombosis model in which a collagen-coated thread was installed within the jugular vein or within a silicon vein shunt. The effect of an inhibitory monoclonal antirabbit TF antibody (AP-1) or Napsagatran, a specific inhibitor of thrombin, was quantified by continuously monitoring 125I-fibrinogen incorporation into the growing thrombi. The antithrombotic effect obtained with the anti-TF antibody was comparable to the effect observed with the thrombin inhibitor napsagatran suggesting that in this animal model the thrombus propagation is highly TF dependent. Immunostaining revealed that TF was mostly associated with leukocytes within the thrombi formed in the jugular vein or in the silicon vein shunt. Ex vivo perfusion experiments over collagen-coated coverslips demonstrated the presence of TF-bearing PMN leukocytes in circulating blood. The results suggest that in rabbits venous thrombus growth is mediated by clot-bound TF and that blocking the TF activity can inhibit thrombus propagation.

Prolonged procoagulant activity on overstretch-injured coronary arteries in pigs

This study was designed to assess the time course and nature of the vascular procoagulant response after 1.5-fold balloon overstretch injury of the coronary arteries in pigs. Arteries were excised for chromogenic assay of bound factor (F)Xa and thrombin at 24 h, 3 days, 1 week, or 2 weeks after injury. FXa at the site of injury remained elevated for 1 week (4.9 +/- 5.9 microg cm(-2), n = 10), compared with non-injured control arteries (0.4 +/- 0.2 microg cm(-2), n = 18, P = 0.00025), while thrombin was increased only at 24 h. Tissue factor protein was abundant in non-injured coronaries (10 +/- 6 ng microg(-1) total protein, n = 9) and levels were unchanged by injury (13 +/- 11 ng microg(-1), n = 6) or 24-h administration of tissue factor pathway inhibitor (16 +/- 6 ng microg(-1), n = 6). Persistent tissue factor-mediated procoagulant activity may explain the need for prolonged anticoagulation to attenuate neointimal formation after balloon-induced coronary injury.

Regulation of tissue factor expression in smooth muscle cells with nitric oxide

OBJECTIVE

This study was undertaken to determine the effect of nitric oxide (NO) on tissue factor (TF) expression in vascular smooth muscle cells.

STUDY DESIGN

Rat aortic smooth muscle cells (RASMCs) were exposed to NO delivered exogenously with the NO donor S-nitroso-N-acetylpenicillamine (SNAP) or produced endogenously after infection with an adenoviral vector carrying human inducible NO synthase (AdiNOS). Functional TF activity was assessed with chromogenic TF assay. TF antigen was determined with immunohistochemistry. Northern blot analysis was used to determine steady- state TF messenger RNA (mRNA). Electrophoretic mobility gel shift assay was performed to determine the nuclear binding activity of nuclear factor kappa-B (NFkappaB). NFkappaB activity was inhibited by either prior transduction of RASMCs with mutant IkappaB or treatment with pyrrolidine dithiocarbamate.

RESULTS

RASMCs exposed to SNAP or infected with AdiNOS exhibited increased functional TF activity and antigen. Regardless of the source of NO, a time-dependent and concentration-dependent increase in TF activity was observed. Steady-state TF mRNA levels were also increased by NO delivered via either method. NFkappaB nuclear binding activity was also increased by NO. Inhibition of NFkappaB activity by either pyrrolidine dithiocarbamate treatment or mutant IkappaB transduction abrogated NO-induced enhancement of TF mRNA and functional activity.

CONCLUSION

In RASMC, NO exposure results in upregulation of TF functional activity, antigen, and mRNA. This effect appears to be mediated by an NFkappaB-dependent pathway.

FXa-induced responses in vascular wall cells are PAR-mediated and inhibited by ZK-807834

During thrombosis, vascular wall cells are exposed to clotting factors, including the procoagulant proteases thrombin and factor Xa (FXa), both known to induce cell signaling. FXa shows dose-dependent induction of intracellular Ca(2+) transients in vascular wall cells that is active-site-dependent, Gla-domain-independent, and enhanced by FXa assembly into the prothrombinase complex. FXa signaling is independent of prothrombin activation as shown by the lack of inhibition by argatroban, hirudin and the sulfated C-terminal peptide of hirudin (Hir(54-65)(SO3(-))). This peptide binds to both proexosite I in prothrombin and exosite I in thrombin. In contrast, signaling is completely blocked by the FXa inhibitor ZK-807834 (CI-1031). No inhibition is observed by peptides which block interaction of FXa with effector cell protease 1 receptor (EPR-1), indicating that this receptor does not mediate signaling in the cells assayed. Receptor desensitization studies with thrombin or peptide agonists (PAR-1 or PAR-2) and experiments with PAR-1-blocking antibodies indicate that signaling by FXa is mediated by both PAR-1 and PAR-2. Potential pathophysiological responses to FXa include increased cell proliferation, increased production of the proinflammatory cytokine IL-6 and increased production of prothrombotic tissue factor. These cellular responses, which may complicate vascular disease, are inhibited by ZK-807834.

Mechanisms of angioplasty and stent restenosis: implications for design of rational therapy

Restenosis after angioplasty or stenting remains the major limitation of both procedures. A vast array of drug therapies has been used to prevent restenosis, but they have proven to be predominantly unsuccessful. Recent trends in drug therapy have attempted to refine the molecular and biological targets of therapy, based on the assumption that a single biological process or molecule is critical to restenosis. In contrast, both stenting and brachytherapy, which are highly nonspecific, can successfully reduce restenosis after angioplasty or stenting, respectively. This review examines the biology of both angioplasty and stent stenosis, focussing on human studies. We also review the landmark human trials that have definitively proven successful therapies, such as stenting and brachytherapy. We suggest that the successful trials of stenting and brachytherapy and the failure of other treatments have highlighted the shortcomings of conventional animal models of arterial intervention, and gaps in our knowledge of human disease. In contrast to arguments advocating gene therapy, these studies suggest that the most likely successful drug therapy will have a wide therapeutic range, targeting as many of the components or biological processes contributing to restenosis as possible.

Local gene transfer of tissue factor pathway inhibitor regulates intimal hyperplasia in atherosclerotic arteries

Tissue factor (TF), the initiator of blood coagulation and thrombosis, is up-regulated after vascular injury and in atherosclerotic states. Systemic administration of recombinant TF pathway inhibitor (TFPI) has been reported to decrease intimal hyperplasia after vascular injury and also to suppress systemic mechanisms of blood coagulation and thrombosis. Here we report that, in heritable hyperlipidemic Watanabe rabbits, adenoviral gene transfer of TFPI to balloon-injured atherosclerotic arteries reduced the extent of intimal hyperplasia by 43% (P < 0.05) compared with a control vector used at identical titer (1 x 10(10) plaque-forming units/ml). Platelet aggregation and coagulation studies performed 7 days after local gene transfer of TFPI failed to show any impairment in systemic hemostasis. At time of sacrifice, 4 weeks after vascular injury, the 10 Ad-TFPI treated carotid arteries were free of thrombi, whereas two control-treated arteries were occluded (P, not significant). These findings suggest that TFPI overexpressed in atherosclerotic arteries can regulate hyperplastic response to injury in the absence of changes in the hemostatic system, establishing a role for local TF regulation as target for gene transfer-based antirestenosis therapies.

Oncostatin M promotes biphasic tissue factor expression in smooth muscle cells: evidence for Erk-1/2 activation

Tissue factor (TF), a transmembrane glycoprotein, initiates the extrinsic coagulation cascade. TF is known to play a major role in mediating thrombosis and thrombotic episodes associated with the progression of atherosclerosis. Macrophages at inflammatory sites, such as atherosclerotic lesions, release numerous cytokines that are capable of modulating TF expression. This study examined the role of oncostatin M (OSM), a macrophage/ T-lymphocyte-restricted cytokine, in the expression of TF in vascular smooth muscle cells (SMCs). It is reported here that OSM stimulated a biphasic and sustained pattern of TF messenger RNA (mRNA). The effect of OSM on TF mRNA expression was regulated at the transcriptional level as determined by nuclear run-offs and transient transfection of a TF promoter-reporter gene construct. OSM-induced TF expression was regulated primarily by the transcription factor NF-kappaB. Activation of NF-kappaB by OSM did not require IkappaB-alpha degradation. Inhibition of MEK activity by U0126 prevented OSM-induced TF expression by suppressing NF-kappaB DNA binding activity as determined by gel-shift analysis. Further, inhibition of Erk-1/2 protein by antisense treatment resulted in suppression of TF mRNA expression, indicating a role for Erk-1/2 in modulating NF-kappaB DNA binding activity. These studies suggest that the induced expression of TF by OSM is primarily through the activation of NF-kappaB and that activation of NF-kappaB is regulated in part by the MEK/Erk-1/2 signal transduction pathway. This study indicates that OSM may play a key role in promoting TF expression in SMCs within atherosclerotic lesions.

Local treatment with recombinant tissue factor pathway inhibitor reduces the development of intimal hyperplasia in experimental vein grafts

BACKGROUND

Tissue factor (TF)-initiated thrombin generation has been implicated in the development of intimal hyperplasia after arterial injury. An increase in intimal TF expression has been shown to precede the development of intimal hyperplasia in vein grafts. This study examines the effects of local treatment with recombinant human tissue factor pathway inhibitor (rTFPI) in experimental vein grafts.

METHODS

Thirty-six male New Zealand white rabbits underwent bypass grafting of the carotid artery by use of the reversed ipsilateral jugular vein and were divided into four groups. Twenty animals had ex vivo incubation with rTFPI treatment (50 microg x mL(-1); n = 10) or placebo vehicle (control; n = 10). Sixteen animals received both ex vivo incubation and in vivo gel treatment with rTFPI (50 microg. mL(-1); n = 8) or without rTFPI (gel-control; n = 8). After operation, vein grafts were harvested at 3 days for immunohistochemical and Western analyses and at 28 days for histomorphologic study.

RESULTS

Western analysis demonstrated a 6.2-fold reduction in the expression of TF protein with rTFPI treatment in comparison to without rTFPI treatment. CD-18 leukocyte staining was diminished, whereas Tie-2 endothelial staining was increased in all rTFPI-treated vein grafts, compared with control and gel-control vein grafts. Intimal thickness was reduced by 21% with ex vivo rTFPI treatment compared with placebo (69 +/- 4 versus 87 +/- 5 microm; P <.05) and by 30% with the addition of rTFPI in vivo compared with gel-control (60 +/- 4 versus 86 +/- 5 microm; P <.01).

CONCLUSION

Local administration of rTFPI exerts early beneficial effects and limits the development of intimal hyperplasia in vein grafts. Therefore blocking TF-mediated pathway may offer new therapeutic options to reduce vein graft failure.

Influence of lipoxin A(4) and other lipoxygenase-derived eicosanoids on tissue factor expression

Lipoxins (LX) are eicosanoids generated via transcellular biosynthetic routes during inflammation, hypersensitivity reaction, and after angioplasty. LXs are modulators of leukocyte trafficking and vascular tone. Their influence on the coagulation cascade has not been determined. In this study, we evaluated the influence of LXs on the expression of tissue factor (TF), a key regulator of coagulation. TF activity was measured in lysates of monocytes, human umbilical vein endothelial cells, and ECV304 cells using a one-stage clotting assay. LXA(4) stimulated TF activity in each cell type. The influence of LXA(4) on TF activity by ECV304 cells was studied further to explore the mechanism of induction of TF expression. LXA(4)-induced TF activity was dose dependent, cycloheximide sensitive, and associated with increased TF mRNA levels. Induction of TF activity was specific for LXA(4) and was not observed with LXB(4), the other major lipoxin generated by mammalian cells. Furthermore, ECV304 cell TF expression was not influenced by 15(R/S)-methyl-LXA(4) or 16-phenoxy-LXA(4), synthetic analogs of LXA(4) that activate the myeloid LXA(4) receptor, and was not modulated by SKF-104353, which blocks LXA(4) bioactivities transduced through the putative shared LXA(4)/LTD(4) receptor. LXA(4)-stimulated TF expression was blunted by pertussis toxin and by GF-109203X, an inhibitor of protein kinase C, and was not associated with degradation of IkappaBalpha. Our results establish that LXA(4) induces TF activity via cell signaling pathways with different structural and receptor requirements from those described for inhibition of leukocyte-endothelial cell interactions. They suggest a role for LXA(4) as a modulator of TF-related vascular events during inflammation and thrombosis.

Role of tissue factor-mediated coagulation in ischemia/ reperfusion-induced injury of Langendorf-perfused rabbit hearts

BACKGROUND

Production of oxygen free radicals, and activation of neutrophils and plasma complement contribute to myocardial reperfusion injury, but the role of coagulation has not been assessed.

OBJECTIVE

To characterize tissue-factor-mediated generation of thrombin and its association with tissue injury during reperfusion from normothermic ischemia of isolated, Langendorf-perfused rabbit hearts.

METHODS

Activation of coagulation was assessed by addition of 12% rabbit plasma and human fibrinogen to Krebs-Henseleit-buffer perfusate with measurement of levels of human fibrinopeptide A (hFPA) in the heart effluent as an index of thrombin-mediated formation of fibrin.

RESULTS

Concentrations of hFPA in the effluent were minimal during non-ischemic perfusion (5 +/- 5 ng/ml, n=6) and during 50 min of ischemia (13 +/- 3 ng/ml, n=6), but increased markedly during the first 20 min of reperfusion (to 41 +/- 29 ng/ml, P=0.03 versus before reperfusion). Addition to the perfusate of 10 microg/ml recombinant human tissue-factor-pathway inhibitor, the physiologic inhibitor of tissue-factor-mediated coagulation, abolished increases in the level of hFPA after reperfusion. However, indexes of myocardial injury manifested during reperfusion, including decrease in recovery of left ventricular pressure developed, increase in left ventricular end-diastolic pressure, and increase in activity of creatine kinase in the heart effluent, were not improved by anticoagulation with recombinant human tissue-factor-pathway inhibitor.

CONCLUSION

Our results do not support the hypothesis that coagulation plays a major role in ischemia/reperfusion injury of Langendorf-perfused rabbit hearts.

Abciximab as an adjunct to high-risk carotid or vertebrobasilar angioplasty: preliminary experience

OBJECTIVE

Abciximab, a platelet glycoprotein IIb/IIIa receptor inhibitor, has been shown to reduce the risk of ischemic events associated with coronary intervention. However, its role in neurointerventional procedures needs to be defined. We prospectively evaluated our initial experience with the use of abciximab in a series of high-risk patients undergoing carotid, basilar, or vertebral artery angioplasty.

METHODS

Patients were given an intravenous abciximab bolus (0.25 mg/kg), followed by infusion (10 microg/min) for a period ranging from 12 to 24 hours, as an adjunct to angioplasty in 20 procedures (19 patients). These patients were considered to be at high risk for thromboembolic events because of recent ischemic symptoms and/or complex lesion morphology. Before, immediately after, and 24 hours after the procedure, each patient was evaluated by a neurologist for the presence of new neurological deficits. Any bleeding or other complications during hospitalization were also recorded. Bleeding was defined as major (hemoglobin decrease >5 g/dl), minor (hemoglobin decrease 3-5 g/dl), or insignificant.

RESULTS

Angioplasty was performed in the internal carotid artery (n = 13), vertebral artery (n = 4), or basilar artery (n = 2). Stents were placed across 13 lesions. In one patient, angioplasty could not be performed owing to technical difficulties; however, abciximab was administered because of extensive lesion manipulation. Intraprocedural heparin was given in 19 procedures (35-86 U/kg intravenously) and partially reversed in 6 procedures. Low-dose intra-arterial thrombolytic agents were administered in seven patients before the lesion was crossed. Two patients experienced transient neurological deficits either during (n = 1) or immediately after (n = 1) the procedure. Another patient had complete occlusion of the right vertebral artery after angioplasty with complete recanalization after 24 hours of abciximab infusion. Major or minor bleeding was not observed in any patient. Insignificant bleeding was observed in eight patients. Thrombocytopenia was observed in one patient who received concomitant administration of intravenous heparin and abciximab infusion.

CONCLUSION

We observed a low frequency of neurological events in high-risk patients undergoing angioplasty with or without stent placement. Abciximab seems to be a relatively safe adjunct for carotid or vertebrobasilar endovascular intervention either alone or in combination with low-dose thrombolytics. Partial reversal of intraprocedural heparin should be considered to reduce the risk of postprocedural bleeding.

Prevention and treatment of thromboembolic and ischemic complications associated with endovascular procedures: Part I--Pathophysiological and pharmacological features

Thromboembolic and ischemic complications frequently occur during and after endovascular procedures, because of associated arterial injury and the thrombogenic characteristics of arterial catheters, contrast agents, and implanted devices such as coils and stents. Platelet adhesion, activation, and aggregation occurring at the site of arterial injury are mediated by local factors, including thromboxane A2 (inhibited by aspirin) and adenosine diphosphate (inhibited by ticlopidine and clopidogrel). Concomitantly, thrombin is formed by serial activation of clotting factors via contact with subendothelial tissue factor. Thrombin cleaves fibrinogen into fibrin. Thrombin activation is indirectly blocked by heparin and its analogs. However, after thrombin is clot-bound (with fibrin), it is relatively protected from heparin and is effectively blocked only by direct thrombin inhibitors (hirudin and its analogs). The final common pathway in clot formation is the binding of fibrinogen to platelets via platelet glycoprotein IIb/IIIa receptors, which is inhibited by antibodies to platelet IIb/IIIa receptors. New treatment modalities, such as the use of direct thrombin inhibitors and antibodies to platelet glycoprotein IIb/IIIa, seem to be more effective for prophylaxis and treatment than conventional anticoagulation and antiplatelet therapies.

Expression and purification of recombinant rabbit factor VII

To facilitate studies of the in vivo role of the extrinsic pathway of coagulation in experimental hemostasis and thrombosis, a full-length cDNA-encoding rabbit factor VII was isolated using polymerase chain reaction-mediated DNA amplification from plaque-purified lambda gt11 phage. Repeated DNA sequencing of both full-length rabbit factor VII cDNA and shorter cDNA fragments verified four changes in the previously reported amino acid sequence of mature rabbit factor VII, now predicted to be 405 amino acids in length. Rabbit factor VII cDNA was transfected into human embryonic kidney 293 cells and a cell line that permanently expressed rabbit recombinant factor VII was established. Rabbit recombinant factor VII was purified from tissue culture media using a combination of barium citrate precipitation, DEAE-sepharose FF chromatography, benzamidine agarose, and affinity chromatography using a sheep antirabbit factor VII polyclonal antibody. The purity and authenticity of rabbit recombinant factor VII was confirmed by polyacrylamide gel electrophoresis and Western blot analysis. Homogeneous rabbit recombinant factor VII was fully active biologically as determined by prothrombin time assay in factor FVII-depleted plasmas, of both human and rabbit origin, using either human or rabbit thromboplastin. Rabbit recombinant factor VII should prove useful for future in vivo investigations of experimental coagulopathies.

Clinical significance of increased plasma concentration of macrophage colony-stimulating factor in patients with angina pectoris

OBJECTIVES

To determine the effect of macrophage colony-stimulating factor (MCSF) on atherogenesis in patients with coronary artery disease (CAD), we assessed the relation between the plasma concentration of MCSF and the incidence of acute coronary events in patients with CAD.

BACKGROUND

Cytokines such as MCSF play a central role in inflammatory and proliferative responses in patients with acute coronary syndromes. However, the effect of MCSF on the clinical course in patients with CAD is still not known.

METHODS

We measured the plasma MCSF concentration in 142 patients with documented CAD (62 +/- 9 years) and followed up for a mean period of 14 +/- 6 months. The study included 97 patients with stable angina (SA), 45 patients with unstable angina (UA) and 22 age-matched control subjects. The predictors of coronary events were analyzed by using a Cox proportional hazards model.

RESULTS

The mean plasma MCSF concentration in patients with UA was significantly higher than that in patients with SA and in control subjects (981 +/- 277 vs. 693 +/- 223 vs. 680 +/- 158 pg/ml, p < 0.001). The mean plasma MCSF concentration in the 20 patients with coronary events was significantly higher than that in patients without coronary events (1,192 +/- 232 vs. 690 +/- 213 pg/ml, p < 0.001). The predictors of unfavorable outcome were an increased MCSF concentration, the presence of CAD and a low ejection fraction.

CONCLUSIONS

These findings suggest that an increased circulating MCSF concentration reflects atherosclerotic progression in patients with CAD and predicts future cardiac events.

A Novel Approach to Arterial Thrombolysis

Achieving early, complete, and sustained reperfusion after acute myocardial infarction does not occur in approximately 50% of patients, even with the most potent established thrombolytic therapy. Bleeding is observed with increased concentrations of thrombolytics as well as with adjunctive antithrombotic and antiplatelet agents. A novel approach to enhance thrombolytic therapy is to inhibit the activated form of thrombin-activatable fibrinolysis inhibitor (TAFI), which attenuates fibrinolysis in clots formed from human plasma. Identification of TAFI in rabbit plasma facilitated the development of a rabbit arterial thrombolysis model to compare the thrombolytic efficacy of tissue-plasminogen activator (tPA) alone or with an inhibitor, isolated from the potato tuber (PTI), of activated TAFI (TAFIa). Efficacy was assessed by determining the time to patency, the time the vessel remained patent, the maximal blood flow achieved during therapy, the percentage of the original thrombus, which lysed, the percentage change in clot weight, the net clot accreted, and the release of radioactive fibrin degradation products into the circulation. The results indicate that coadministration of PTI and tPA significantly improved tPA-induced thrombolysis without adversely affecting blood pressure, activated partial thromboplastin time, thrombin clotting time, fibrinogen, or -2-antiplasmin concentrations. The data indicate that inhibitors of TAFIa may comprise novel and very effective adjuncts to tPA and improve thrombolytic therapy to achieve both clot lysis and vessel patency.

Thrombin generation after the abrupt cessation of intravenous unfractionated heparin among patients with acute coronary syndromes: potential mechanisms for heightened prothrombotic potential

OBJECTIVES

The purpose of this study was to determine the mechanistic basis for thrombin generation and increased prothrombotic potential after the abrupt cessation of intravenous (i.v.) unfractionated heparin among patients with acute coronary syndromes.

BACKGROUND

A "rebound" increase in prothrombotic potential has been observed biochemically and clinically after the abrupt cessation of unfractionated heparin (UFH) among patients with acute coronary syndromes. Although the mechanism is unknown, tissue factor and the extrinsic coagulation cascade, both operative in atherosclerotic vascular disease and arterial thrombosis, are thought to be centrally involved.

METHODS

In a single-center, pilot study, 30 patients with either unstable angina or non-ST segment elevation myocardial infarction who had received a continuous i.v. infusion of UFH for 48 h were randomly assigned to: 1) abrupt cessation, 2) i.v. weaning over 12 h or 3) subcutaneous weaning over 12 h.

RESULTS

Thrombin generation (prothrombin fragment 1.2) was evident within 1 h of UFH cessation, increased progressively (by nearly two-fold) at 24 h (p = 0.002) and correlated inversely with tissue factor pathway inhibitor concentration (r = -0.61). Thrombin generation was greatest among patients randomized to abrupt cessation (1.6-fold increase at 24 h) and least in those with i.v. weaning.

CONCLUSIONS

Thrombin generation after the abrupt cessation of UFH may represent a drug-induced impairment of physiologic vascular thromboresistance in response to locally generated tissue factor. A dosing strategy of abbreviated i.v. weaning attenuates but does not prevent heparin rebound among patients with acute coronary syndromes.

Local delivery of a tissue factor antibody reduces early leukocyte infiltration but fails to limit intimal hyperplasia in experimental vein grafts

BACKGROUND

Tissue Factor-mediated thrombin generation involves the generation of VIIa and Xa and has been implicated in the pathogenesis of intimal hyperplasia. In experimental vein grafts, Tissue Factor protein is increased over the first 3 days and colocalized with CD18-positive leukocytes; this increase in Tissue Factor precedes the development of intimal hyperplasia. This study further evaluates the potential role of Tissue Factor in vein graft intimal hyperplasia by directly inhibiting Tissue Factor protein.

METHODS

New Zealand white rabbits underwent interpositional bypass grafting of the common carotid artery using the external jugular vein. Perioperatively, murine anti-rabbit Tissue Factor antibody (109 microg/ml gel, 12,500x IC50 of Tissue Factor activity) was applied to the adventitial surface of the graft, using a pluronic gel (30% soln.). Tissue Factor antibody treated vein grafts were compared to control and empty gel-treated vein grafts. Vein grafts were examined at 3 days to assess CD18-positive leukocyte infiltration and the presence of residual antibody by Western blotting. At 28 days, intimal and medial dimensions were quantified using videomorphometry.

RESULTS

At day 3, there was marked reduction in CD18-positive leukocytes in the Tissue Factor antibody versus control vein grafts (6.3 +/- 4.7 vs 20.8 +/- 7.4 per 200x field, P < 0.05). At 28 days, intimal hyperplasia was similar for the control (70 +/- 4 microm, mean +/- SEM), gel (73 +/- 4 microm), and Tissue Factor antibody (75 +/- 4 microm) vein grafts. However, medial thickness (76 +/- 4 microm;, P < 0.05) was significantly increased compared to the gel treated vein graft (61 +/- 5 microm).

CONCLUSION

Local delivery of pharmacologic doses of an anti-rabbit Tissue Factor antibody decreased CD18-positive leukocyte infiltration but failed to limit intimal hyperplasia in experimental vein grafts. The results suggest that inhibition of Tissue Factor protein modulates polymorphonuclear leukocyte-endothelial interactions but not in the subsequent development of intimal hyperplasia. It implies that the relationship between the extrinsic coagulation cascade and intimal hyperplasia in vein grafts is complex.

Tissue factor is induced by monocyte chemoattractant protein-1 in human aortic smooth muscle and THP-1 cells

Monocyte chemoattractant protein-1 (MCP-1) is a C-C chemokine thought to play a major role in recruiting monocytes to the atherosclerotic plaque. Tissue factor (TF), the initiator of coagulation, is found in the atherosclerotic plaque, macrophages, and human aortic smooth muscle cells (SMC). The exposure of TF during plaque rupture likely induces acute thrombosis, leading to myocardial infarction and stroke. This report demonstrates that MCP-1 induces the accumulation of TF mRNA and protein in SMC and in THP-1 myelomonocytic leukemia cells. MCP-1 also induces TF activity on the surface of human SMC. The induction of TF by MCP-1 in SMC is inhibited by pertussis toxin, suggesting that the SMC MCP-1 receptor is coupled to a Gi-protein. Chelation of intracellular calcium and inhibition of protein kinase C block the induction of TF by MCP-1, suggesting that in SMC it is mediated by activation of phospholipase C. SMC bind MCP-1 with a Kd similar to that previously reported for macrophages. However, mRNA encoding the macrophage MCP-1 receptors, CCR2A and B, is not present in SMC, indicating that they possess a distinct MCP-1 receptor. These data suggest that in addition to being a chemoattractant, MCP-1 may have a procoagulant function and raise the possibility of an autocrine pathway in which MCP-1, secreted by SMC and macrophages, induces TF activity in these same cells.

Tissue factor expression in human arterial smooth muscle cells. TF is present in three cellular pools after growth factor stimulation

Tissue factor (TF) is a transmembrane glycoprotein that initiates the coagulation cascade. Because of the potential role of TF in mediating arterial thrombosis, we have examined its expression in human aortic and coronary artery smooth muscle cells (SMC). TF mRNA and protein were induced in SMC by a variety of growth agonists. Exposure to PDGF AA or BB for 30 min provided all of the necessary signals for induction of TF mRNA and protein. This result was consistent with nuclear runoff analyses, demonstrating that PDGF-induced TF transcription occurred within 30 min. A newly developed assay involving binding of digoxigenin-labeled FVIIa (DigVIIa) and digoxigenin-labeled Factor X (DigX) was used to localize cellular TF. By light and confocal microscopy, prominent TF staining was seen in the perinuclear cytoplasm beginning 2 h after agonist treatment and persisting for 10-12 h. Surface TF activity, measured on SMC monolayers under flow conditions, increased transiently, peaking 4-6 h after agonist stimulation and returning to baseline within 16 h. Peak surface TF activity was only approximately 20% of total TF activity measured in cell lysates. Surface TF-blocking experiments demonstrated that the remaining TF was found as encrypted surface TF, and also in an intracellular pool. The relatively short-lived surface expression of TF may be critical for limiting the thrombotic potential of intact SMC exposed to growth factor stimulation. In contrast, the encrypted surface and intracellular pools may provide a rich source of TF under conditions associated with SMC damage, such as during atherosclerotic plaque rupture or balloon arterial injury.

Restenosis: is there a pharmacologic fix in the pipeline?

One of the most frustrating aspects of restenosis is that it is the result of advances in medical care (there was no restenosis before the days of balloon angioplasty), yet it seems to be resistant to all that science has to offer. Still we believe there is reason to be optimistic. We are at last beginning to see some promise from clinical trials, and data being generated confirm some of the hypotheses previously generated from animal experiments. Thus the effects seen with the GP IIb/IIIa antibody 7E3 suggest that thrombosis may be as important in its long-term sequelae as it is for acute reocclusion. The jury is still out on whether antiproliferative approaches will be a therapeutic option, but local delivery paradigms using novel formulations delivered by catheter or impregnated in stents may allow the concept to be tested without the risk of systemic toxicity. Plans are also underway for gene therapy trials, although we may have to wait for better vector technology before taking these into the coronary bed. Perhaps we should move away from the "single pill" approach and accept that, like many infections, malignancies, or even heart failure, a multifaceted approach with combination therapy will provide the first glimmer of that brighter tomorrow.

Inhibition of thrombin attenuates stenosis after arterial injury in minipigs

OBJECTIVES

We sought to determine whether brief, profound inhibition of thrombin or prothrombin activation by factor Xa limits neointimal formation and stenosis after arterial injury.

BACKGROUND

Thrombin has been implicated as a mediator of neointimal formation, but adjunctive administration of anticoagulant agents has not proven effective to decrease restenosis in patients undergoing coronary angioplasty.

METHODS

We infused recombinant desulfatohirudin (r-hirudin, bolus of 2 mg/kg body weight followed by 2 mg/kg per h, n = 9), heparin (100 U/kg per h, n = 6) or recombinant tick anticoagulant peptide (rTAP, 1-mg/kg bolus followed by 3 mg/kg per h, n = 5), a specific inhibitor of factor Xa, intravenously, beginning 15 min before and for up to 3 h after repetitive balloon hyperinflations sufficient to disrupt the internal elastic lamina in a carotid artery of minipigs with hypercholesterolemia induced by feeding them an atherogenic diet.

RESULTS

Partial thromboplastin time was increased six- to sevenfold over baseline levels at the end of the infusions of the anticoagulant agents. Lumen stenosis measured histologically 4 weeks after balloon-induced carotid injury was 29 +/- 16% (mean +/- SEM) in r-hirudin-treated, 52 +/- 19% in rTAP-treated and 76 +/- 18% in heparin-treated pigs (p < 0.02 for r-hirudin vs. heparin treatment).

CONCLUSIONS

The marked reduction of stenosis in r-hirudin-treated animals indicates that thrombin plays a major role in neointimal formation after balloon-induced arterial injury. A relatively brief interval of profound, direct inhibition of thrombin may be particularly effective to attenuate restenosis after balloon angioplasty.

Preincubation of Dacron grafts with recombinant tissue factor pathway inhibitor decreases their thrombogenicity in vivo

BACKGROUND

We have previously shown that preincubation of whole blood clots with recombinants tissue factor pathway inhibitor (rTFPI) attenuates clot-associated procoagulant activity assessed ex vivo. This study was undertaken to determine whether a single local application of rTFPI induces similar attenuation of the procoagulant activity on preclotted Dacron grafts implanted in an artery in vivo.

METHODS

Dacron grafts (4 mm x 4 cm long) were preclotted in porcine blood and incubated with either rTFPI (5 mg/ml) or arginine-phosphate buffer for 15 minutes. Grafts were implanted end-to-end in the femoral arteries of 10 pigs, with one rTFPI-treated and one buffer-treated graft implanted in each animal. Animals did not undergo anticoagulation either before or after graft implantation. Radiolabeled porcine fibrinogen was injected intravenously, and the grafts underwent perfusion for 1 hour. A subgroup of animals (n = 7) also had infusion of radiolabeled autologous platelets at the time of administration of radiolabeled fibrinogen.

RESULTS

Fibrin(ogen) deposition was decreased in rTFPI-treated grafts by 36% +/- 7% (mean +/- SEM) compared with buffer-treated grafts (p = 0.001). Platelet deposition was also reduced in the rTFPI-treated grafts by 31% +/- 15%, although the reduction did not reach statistical significance (p = 0.10). The extent of rTFPI-mediated attenuation of fibrin(ogen) versus platelet deposition varied independently among animals.

CONCLUSIONS

Clot-directed anticoagulant effects of rTFPI appear to be useful for substantially decreasing the thrombogenicity of Dacron grafts immediately after their implantation. Chronic studies to determine whether the decreases in thrombogenicity result in improved long-term graft patency appear warranted.