Tissue factor (TF) is an integral membrane protein essential for hemostasis. During the past several years, a number of studies have suggested that physiologically active TF circulates in blood at concentrations greater than 30 pM either as a component of blood cells and microparticles or as a soluble plasma protein. In our studies using contact pathway-inhibited blood or plasma containing activated platelets, typically no clot is observed for 20 minutes in the absence of exogenous TF. An inhibitory anti-TF antibody also has no effect on the clotting time in the absence of exogenous TF. The addition of TF to whole blood at a concentration as low as 16 to 20 fM results in pronounced acceleration of clot formation. The presence of potential platelet TF activity was evaluated using ionophore-treated platelets and employing functional and immunoassays. No detectable TF activity or antigen was observed on quiescent or ionophore-stimulated platelets. Similarly, no TF antigen was detected on mononuclear cells in nonstimulated whole blood, whereas in lipopolysaccharide (LPS)-stimulated blood a significant fraction of monocytes express TF. Our data indicate that the concentration of physiologically active TF in non-cytokine-stimulated blood from healthy individuals cannot exceed and is probably lower than 20 fM.

Abstract-The 3-hydroxy-3-methylglutaryl (HMG)-coenzyme A (CoA) reductase inhibitors (statins) have been shown to exhibit several vascular protective effects, including antithrombotic properties, that are not related to changes in lipid profile. There is growing evidence that treatment with statins can lead to a significant downregulation of the blood coagulation cascade, most probably as a result of decreased tissue factor expression, which leads to reduced thrombin generation. Accordingly, statin use has been associated with impairment of several coagulant reactions catalyzed by this enzyme. Moreover, evidence indicates that statins, via increased thrombomodulin expression on endothelial cells, may enhance the activity of the protein C anticoagulant pathway. Most of the antithrombotic effects of statins are attributed to the inhibition of isoprenylation of signaling proteins. These novel properties of statins, suggesting that these drugs might act as mild anticoagulants, may explain, at least in part, the therapeutic benefits observed in a wide spectrum of patients with varying cholesterol levels, including subjects with acute coronary events. Key Words: statins Ⅲ blood coagulation Ⅲ thrombin Ⅲ protein C T he 3-hydroxy-3-methylglutaryl-coenzyme A (HMGCoA) reductase inhibitors, the so-called statins, have been proven to be highly effective in the management of hyperlipidemia and the prevention of atherosclerotic vascular disease, especially coronary artery disease (CAD). 1,2 The therapeutic benefits from statin therapy, however, are only poorly correlated with cholesterol lowering, suggesting other mechanisms are at play. Mevalonate, the product of HMGCoA reductase, is the precursor of not only cholesterol but also isoprenoid compounds that permit the attachment of signaling proteins to the cell membrane. 3,4 Abundant experimental and clinical evidence in this rapidly expanding field has resulted in the widely accepted concept of cholesterol-independent pleiotropic effects produced by statins that include alteration of endothelial dysfunction, leading to increased nitric oxide (NO) bioavailability, atherosclerotic plaque stabilization, regulation of angiogenesis, reduction of the inflammatory response, and antithrombotic properties. [5][6][7] Apart from profibrinolytic and antiplatelet effects, reported in several studies, 5,7-9 increasing evidence indicates that the HMG-CoA reductase inhibitors also modulate the blood coagulation cascade at multiple levels, leading to reduced thrombogenicity.The activation of the extrinsic coagulation pathway initiated by an exposure or expression of tissue factor (TF) plays a key role in hemostasis (Figure). The localized process involves multiple components, including blood coagulation factors, platelets, blood cells, the endothelium, and sublayers of a damaged vessel. Platelets provide the catalytic surface for the formation of the procoagulant complexes; the intrinsic factor Xase (FIXa-FVIIIa) and prothrombinase (prothrombin, FXa-FVa). These complexes accelerate thrombin ge...

While platelets are well known to play a central role in hemostasis and thrombosis, there is emerging experimental evidence to suggest that they also mediate tumor cell growth, dissemination, and angiogenesis. An increase in platelet number (thrombocytosis) and activity is seen in patients with a wide spectrum of malignancies, and the former is correlated with a decrease in overall survival and poorer prognosis. Preclinical data suggest that circulating tumor cell partnerships with platelets in the blood facilitate tumor metastases through direct interactions and secreted bioactive proteins. Platelets form aggregates with tumor cells, thereby protecting them from host immune surveillance through physical shielding and induction of "platelet mimicry." There is also laboratory evidence to suggest that activated platelets interact with cancer cells within the tumor microenvironment through paracrine signaling and direct contact, thereby promoting tumor cell growth and survival. For example, platelets release mediators of both tumor angiogenesis and osteoclast resorption. The interplay between platelets and tumor cells is complex and bidirectional with involvement of multiple other components within the tumor microenvironment, including immune cells, endothelial cells, and the extracellular matrix. We review the role of platelets in tumor progression, emphasizing the opportunity these interactions afford to target platelets and platelet function to improve patient outcomes in the cancer prevention and treatment setting.

Thromboelastography is useful for assessment of whole blood coagulation. The objective of this study was to evaluate the possibility of linking the tracing of whole blood clotted in a thromboelastograph TEG with the generation of thrombin assessed by thrombin/antithrombin complex (TAT). Citrated whole blood containing corn trypsin inhibitor from volunteers was clotted in the presence of CaCl 2 and tissue factor. Clotting was monitored with 8 channels of a TEG system. At different time points the whole blood TEG reaction cups were put in a cold quenching solution, centrifuged, and the supernatants were kept at −80°C until assayed for TAT by ELISA. Total Thrombus Generation (TTG) was calculated from the first derivative of the TEG waveform and was compared to thrombin generation measured by TAT. The two vectors of values -the TAT thrombin generation data and the corresponding TEG TTG -were analyzed using Pearson correlation coefficients and linear, non-linear, and natural log (Ln) transformation of TAT values for leastsquares goodness-of-fit curves. The best least-squares fit is an exponential curve. Linearizing using the natural log of the TAT thrombin generation variable produces the same R 2 as for the exponential curve. The prediction equation is y = 8.0465 + 0.0005x (P≤ 0.0001), where y is the TAT thrombin generation in the Ln transformation variable and x is the TEG TTG variable. The high magnitude of R 2 and the high significance of the prediction equation demonstrate the high efficacy of the prediction of TAT thrombin generation using the TEG TTG.

Hemophilia is a bleeding disorder that afflicts about 1 in 5000 males. Treatment relies upon replacement of the deficient factor, and response to treatment both in clinical research and practice is based upon subjective parameters such as pain and joint mobility. Existing laboratory assays quantify the amount of factor in plasma, which is useful diagnostically and prognostically. However, these assays are limited in their ability to fully evaluate the patient’s clot-forming capability. Newer assays, known as global assays, provide a far more detailed view of thrombin generation and clot formation and have been studied in hemophilia for about 10 years. They have the potential to offer a more objective measure of both the hemophilic phenotype as well as the response to treatment. In particular, in patients who develop inhibitors to deficient clotting factors and in whom bypassing agents are required for hemostasis, these assays offer the opportunity to determine the laboratory response to these interventions where traditional coagulation assays cannot. In this article we review the existing literature and discuss several controversial issues surrounding the assays. Last, a vision of future clinical uses of these assays is briefly described.

Aspirin is effective in the prevention of cardiovascular events in high-risk patients. The primary established effect of aspirin on hemostasis is to impair platelet aggregation via inhibition of platelet thromboxane A 2 synthesis, thus reducing thrombus formation on the surface of the damaged arterial wall. Growing evidence also indicates that aspirin exerts additional antithrombotic effects, which appear to some extent unrelated to platelet thromboxane A 2 production. Aspirin can reduce thrombin generation with the subsequent attenuation of thrombin-mediated coagulant reactions such as factor XIII activation. Aspirin also acetylates lysine residues in fibrinogen resulting in increased fibrin clot permeability and enhanced clot lysis as well as directly promoting fibrinolysis with high-dose aspirin. The variable effectiveness of aspirin in terms of clinical outcomes and laboratory findings, which has been termed aspirin resistance, may be related to these additional antithrombotic effects that are altered when associated with common genetic polymorphisms such as the Leu33Pro ␤ 3 -integrin or Val34Leu factor XIII mutations. However, the clinical relevance of these observations is still un- IntroductionAcetylsalicylic acid, or aspirin, was synthesized by Hoffmann in 1898. Initially, this simple chemical compound was used as an antipyretic and anti-inflammatory agent. In the late 1960s aspirin emerged as a potent agent that prolongs the bleeding time and inhibits platelet aggregation. 1 During the last 30 years, many studies have reinforced aspirin's position in the armamentarium of physicians as an antithrombotic wonder drug that has saved the lives of millions of patients with cardiovascular disease. 2 Compelling evidence indicates that therapy with aspirin results in a 25% reduced risk of nonfatal myocardial infarction, nonfatal stroke, or vascular death in high-risk patients, regardless of sex, age, the presence of arterial hypertension, or diabetes. 3 In this article we review a number of the reported effects of aspirin on 3 basic elements of hemostasis: platelet activation and aggregation, the formation of the fibrin network, and the fibrinolytic process (Figure 1). A role of these effects in the phenomenon of aspirin resistance will also be discussed. Platelet activation and aggregation COX-dependent actions of aspirinThe conversion of arachidonic acid to various eicosanoids is regulated by the enzyme cyclooxygenase (COX) of which there are 2 isoforms: COX-1 and COX-2. 4 The COX-1 isoform, present in all tissues, represents the constitutive form of the enzyme, whereas the COX-2 isoform is expressed in inflammatory states in response to oxygen reactive species, endotoxins, cytokines, or growth factors. 5 COX-2 can be found in human atherosclerotic plaques 6 and also in small amounts in newly formed platelets. 7The aspirin-sensitive pathway in platelets initiates the release of arachidonic acid from membrane phospholipids. Aspirin inhibits the COX activity of prostaglandin (PG) G/H synthase (PGHS), by acetyla...

To cite this article: Brummel-Ziedins KE, Vossen CY, Butenas S, Mann KG, Rosendaal FR. Thrombin generation profiles in deep venous thrombosis.J Thromb Haemost 2005; 3: 2497-505.

Summary. Background and objectives: The range of plasma concentrations of hemostatic analytes in the population is wide. In this study these components of blood coagulation phenotype are integrated in an attempt to predict clinical risk. Methods: We modeled tissue factor (TF)-induced thrombin generation in the control population (N ¼ 473) from the Leiden Thrombophilia Study utilizing a numerical simulation model. Hypothetical thrombin generation curves were established by modeling pro-and anticoagulant factor levels for each individual. These curves were evaluated using parameters which describe the initiation, propagation and termination phases of thrombin generation, i.e. time to 10 nM thrombin (approximate clot time), total thrombin and the maximum rates and levels of thrombin generated. Results and conclusions: The time to 10 nM thrombin varied over a 3-fold range (2.9-9.5 min), maximum levels varied over a 4-fold range (200-800 nM), maximum rates varied 4.8-fold (90-435 nM min ) within this control population. Thrombin generation curves, defined by the clotting factor concentrations, were distinguished by sex, age, body mass index (BMI) and oral contraceptive (OC) use. Our results show that the capacity for thrombin generation in response to a TF challenge may represent a method to identify an individual's propensity for developing thrombosis.