Tissue factor-independent effects of recombinant factor VIIa on hemostasis

The molecular mechanisms responsible for the hemostatic efficacy of recombinant activated factor VII (rFVIIa; NovoSeven (R), Novo Nordisk, Bagsvaerd, Denmark) in platelet-related bleeding disorders remain unclear. The general concept is that rFVIIa locally enhances thrombin generation at the site of injury, where tissue factor (TF) has become exposed. However, a growing amount of evidence shows that rFVIIa is also able to exert its activity in a manner independent of TF. Using an in vitro flow model, we recently showed that TF-independent thrombin generation is responsible for increased platelet deposition onto injured vessels following rFVIIa administration. Furthermore, it has been shown that rFVIIa can restore platelet aggregation in Glanzmann's thrombasthenia (GT) patients via TF-independent thrombin generation. However, the mechanism behind TF-independent thrombin generation remains to be elucidated. It is postulated that, in vivo, both the TF-dependent and TF-independent thrombin generation induced by rFVIIa contribute to the control of hemorrhage in patients with platelet-related bleeding disorders and, perhaps, other causes of hemorrhagic diatheses

The glycoprotein Ib-IX-V complex contributes to tissue factor-independent thrombin generation by recombinant factor VIIa on the activated platelet surface

Several lines of evidence suggest that recombinant factor VIIa (rFVIIa) is able to activate factor X on an activated platelet, in a tissue factor-independent manner. We hypothesized that, besides the anionic surface, a receptor on the activated platelet surface is involved in this process. Here, we showed that, in an ELISA setup, a purified extracellular fragment of GPIb alpha bound to immobilized rFVIIa. Surface plasmon resonance established a affinity constant (K-d) of approximately 20 nM for this interaction. In addition, CHO cells transfected with the GPIb-IX-V complex could adhere to immobilized rFVIIa, whereas wild-type CHO cells could not. Furthermore, platelets stimulated with a combination of collagen and thrombin adhered to immobilized rFVIIa under static conditions. Platelet adhesion was inhibited by treatment with O-sialoglycoprotein endopeptidase, which specifically cleaves GPIb alpha from the platelet surface. In addition, rFVIIa-mediated thrombin generation on the activated platelet surface was inhibited by cleaving GPIb alpha from its surface. In summary, 3 lines of evidence showed that rFVIIa interacts with the GPIb-IX-V complex, and this interaction enhanced tissue factor-independent thrombin generation mediated by rFVIIa on the activated platelet surface. The rFVIIa-GPIb alpha interaction could contribute to cessation of bleeding after administration of rFVIIa to patients with bleeding disorders

Activated factor V is a cofactor for the activation of factor XI by thrombin in plasma

The mechanism by which the intrinsic pathway of coagulation contributes to physiological hemostasis is enigmatic. Thrombin activates factor XI, a key zymogen in this pathway, which leads to increased thrombin generation. As thrombin-dependent activation of factor XI in vitro is relatively inefficient, we hypothesized that a physiological cofactor supports this reaction in a plasma environment. We therefore investigated whether the cofactors of coagulation, activated factor V, activated factor VIII, high-molecular weight kininogen, or protein S, influenced activation of factor XI by thrombin. Only activated factor V stimulated activation of factor XI by thrombin in a purified system. Binding studies demonstrated that factor XI specifically interacts with both factor V and factor Va through multiple binding sites. We further investigated this cofactor function of activated factor V in plasma. Depletion of factor V, or the addition of activated protein C, decreased the activation of the intrinsic pathway by thrombin in plasma. However, activated protein C did not exert this effect in the plasma of a homozygous carrier of the prothrombotic factor V Leiden mutation. In conclusion, we propose a role for (activated) factor V as a cofactor in the activation of factor XI by thrombin. These findings offer insights into the coagulation system in both health and diseas

A heparin-bonded vascular graft generates no systemic effect on markers of hemostasis activation or detectable heparin-induced thrombocytopeniaassociated antibodies in humans

ObjectivesAlmost a third of patients who undergo peripheral bypass procedures do not have suitable veins, making the use of prosthetic materials necessary. Prosthetic materials can cause platelet adhesion and activation of the coagulation cascade on the graft. One potential strategy to reduce this thrombogenicity is to covalently bind heparin to the endoluminal surface of grafts. This human in vivo study examined systemic effects of the endoluminal heparin and addressed whether graft implantation results in (1) a measurable reduction of systemic markers of hemostasis activation compared with control grafts and (2) antibody formation against heparin, potentially responsible for heparin-induced thrombocytopenia (HIT).MethodsThe study included 20 patients undergoing femoropopliteal bypass grafting, of whom 10 received a standard Gore-Tex Thin Walled Stretch Vascular Graft (W. L. Gore & Associates, Flagstaff, Ariz) and 10 received a heparin-bonded expanded polytetrafluoroethylene (ePTFE) graft (Gore-Tex Propaten Vascular Graft). Blood samples were drawn before and directly after the operation and at days 1, 3, 5, and week 6 after surgery. Established markers of in vivo activation of platelets and blood coagulation (prothrombin fragment 1+2, fibrinopeptide A, soluble glycoprotein V, thrombin-antithrombin complexes, and D-dimers) were measured using standard commercially available techniques. Antiplatelet factor 4/heparin antibody titers were measured using a commercially available enzyme-linked immunosorbent assay, and platelet counts were determined.ResultsNo statistical differences were observed in any of the markers of in vivo activation of platelets and blood coagulation between patients receiving Propaten or control ePTFE. Moreover, no antibodies against heparin could be demonstrated up to 6 weeks after implantation.ConclusionsNo measurable effect of heparin immobilization on systemic markers of hemostasis was found using a heparin-bonded ePTFE graft in vivo. Also, no antibodies against heparin could be detected up to 6 weeks after implantation

Heparin immobilization reduces thrombogenicity of small-caliber expanded polytetrafluoroethylene grafts

ObjectiveThe patency of small-diameter expanded polytetrafluoroethylene (ePTFE) grafts for vascular reconstruction is impaired by acute thrombotic occlusion. Prosthetic materials are thrombogenic and cause platelet adhesion and activation of the coagulation cascade. Heparin is a potent anticoagulant drug widely used to prevent and treat thrombosis. A new ePTFE graft with long-term bonding of heparin is now commercially available in several European countries, but a basic analysis of its mechanism of action in humans has never been performed. This study was performed to evaluate the thrombogenicity of heparin-bonded ePTFE grafts compared with standard ePTFE in a newly developed human ex vivo model.MethodsNonanticoagulated blood was drawn from antecubital veins of 10 healthy donors with a 19-gauge needle. The proximal end of a 60-cm ePTFE vascular graft with a diameter of 3 mm was connected to the needle while the distal end was connected to a syringe, which was placed in a syringe pump. Every volunteer served as his or her own control by using a heparin-bonded ePTFE graft on one arm and a standard ePTFE graft on the other arm. The perfusions were performed over 6 minutes with a flow rate of 20 mL/min, corresponding to a shear rate of 74/s. Serial samples were taken at the distal end of the graft for determination of prothrombin fragment 1 + 2, fibrinopeptide A, and P-selectin expression on perfused platelets. Fibrin deposition and platelet deposition were studied by using scanning electronic microscopy.ResultsFibrinopeptide A production over time was significantly reduced on the heparin-bonded ePTFE grafts compared with standard ePTFE grafts (P < .05). There was no increase in the production of prothrombin fragment 1 + 2 or P selectin over time on either type of graft. Scanning electronic microscopy scanning showed platelet deposition and fibrin formation on standard ePTFE grafts, whereas no platelets or fibrin were observed on heparin-bonded ePTFE grafts.ConclusionsHeparin immobilization substantially reduces the thrombogenicity of small-diameter ePTFE in a newly developed human ex vivo model. In this study, we provide evidence that the mechanism of action of the heparin bonding is due not only to anticoagulant but also to antiplatelet effects. Heparin bonding may be an important improvement of ePTFE, resulting in better patency rates for vascular reconstructions.Clinical RelevanceHeparin immobilization reduces the thrombogenicity of small-caliber expanded polytetrafluoroethylene (ePTFE) grafts. The patency of small-diameter ePTFE grafts for vascular reconstruction is impaired by acute thrombotic occlusion. Prosthetic materials are thrombogenic and cause platelet adhesion and activation of the coagulation cascade. Heparin is a potent anticoagulant drug widely used to prevent and treat thrombosis. A new ePTFE graft with long-term bonding of heparin is now commercially available, but a basic analysis of its mechanism of action in humans has never been performed. This study was performed to evaluate the thrombogenicity of heparin-bonded ePTFE grafts compared with standard ePTFE in a newly developed human ex vivo model. We demonstrated that heparin immobilization reduces thrombogenicity on small-caliber ePTFE grafts. Heparin-bonded ePTFE grafts might therefore result in better patency rates for vascular reconstructions with vascular grafts

Endocannabinoids Control Platelet Activation and Limit Aggregate Formation under Flow

<div><p>Background</p><p>The endocannabinoid system has previously been implicated in the regulation of neurons and inflammatory cells. Additionally, it has been reported that endocannabinoid receptors are present on circulating platelets, but there has been conflicting evidence on their contribution to platelet function.</p><p>Objectives</p><p>Our aim was to examine the role of endocannabinoids in platelet function <i>in vitro</i> and <i>in vivo</i>.</p><p>Methods and Results</p><p>We studied the effects of the well-characterized endogenous endocannabinoid anandamide on platelet aggregation in suspension, α-granule release, calcium mobilization, Syk phosphorylation, as well as platelet spreading and aggregate formation under flow. Anandamide inhibits platelet aggregation and α-granule release by collagen, collagen-derived peptide CRP-XL, ADP, arachidonic acid and thromboxane A2 analogue U46619. However, activation via thrombin receptor PAR-1 stays largely unaffected. Calcium mobilization is significantly impaired when platelets are stimulated with collagen or CRP-XL, but remains normal in the presence of the other agonists. In line with this finding, we found that anandamide prevents collagen-induced Syk phosphorylation. Furthermore, anandamide-treated platelets exhibit reduced spreading on immobilized fibrinogen, have a decreased capacity for binding fibrinogen in solution and show perturbed platelet aggregate formation under flow over collagen. Finally, we investigated the influence of <i>Cannabis sativa</i> consumption by human volunteers on platelet activation. Similar to our <i>in vitro</i> findings with anandamide, <i>ex vivo</i> collagen-induced platelet aggregation and aggregate formation on immobilized collagen under flow were impaired in whole blood of donors that had consumed <i>Cannabis sativa</i>.</p><p>Conclusions</p><p>Endocannabinoid receptor agonists reduce platelet activation and aggregate formation both <i>in vitro</i> and <i>ex vivo</i> after <i>Cannabis sativa</i> consumption. Further elucidation of this novel regulatory mechanism for platelet function may prove beneficial in the search for new antithrombotic therapies.</p></div

<i>Cannabis sativa</i> consumption limits platelet aggregate formation under flow and reduces platelet responsiveness to collagen.

<p>Whole blood from <i>Cannabis sativa</i> consumers (n = 4; “Consumers”) or healthy control donors (“Controls”) was perfused for 5 minutes over immobilized collagen at a shear rate of 1600 s⁻¹ (A). In further experiments, collagen-induced platelet aggregation was investigated in platelet-rich plasma from these <i>Cannabis sativa</i> consumers and controls (B). Representative aggregation curves are shown in panel C. Finally, collagen-induced platelet aggregation was studied in platelet-rich plasma of three self-reported <i>Cannabis sativa</i> consumers on two separate instances: once after 10 days of daily consumption of <i>Cannabis sativa</i> (“Use”), as well as after a period of 10 days without consumption (“Withdrawal”).</p

Anandamide inhibits platelet α-granule secretion and limits platelet aggregate formation.

<p>Platelet α-granule release was studied by flow cytometric analysis of platelet P-selectin externalization. Washed platelets from healthy human donors were pre-exposed to a concentration series of anandamide for 40 min at 37°C and subsequently stimulated with 2.5 µg/mL CRP-XL (A), 2.5 µM U46619 (B) or 5 µM TRAP (C). Data were normalized to the agonist-induced P-selectin expression of vehicle-pretreated control platelets. Reconstituted blood, which was preincubated with anandamide, was perfused over a collagen-coated surface at a shear rate of 1600 s⁻¹ and snapshots were taken after 5 minutes (D). Subsequently, cover slips were rinsed with buffer for 1 minute to investigate aggregate stability (E). The left panel shows vehicle control, the right panel indicates a perfusion in the presence of 50 µM anandamide. Data are shown as mean ± SD and represent 3 or more individual experiments.</p

Anandamide reduces glycoprotein IIb/IIIa activation and inhibits platelet spreading.

<p>Washed platelets from healthy human donors were exposed to a concentration series (0–50 µM) of anandamide for 40 minutes at 37°C and subsequently stimulated with 2.5 µg/mL CRP-XL (A), 2.5 µM U46619 (B) or 5 µM TRAP (C). Platelet GPIIb/IIIa activation was studied by flow cytometry. Data were normalized to the agonist-induced GPIIb/IIIa activation of vehicle-pretreated control platelets. Washed platelets were preincubated with 50 µM anandamide or vehicle for 40 minutes at 37°C and perfused over immobilized fibrinogen- at a shear rate of 25 s⁻¹. Subsequently, differential interference contrast microscopy images were taken (D; representative images after 15 minutes of perfusion, scale bars represent 10 µm). Quantification of the spreading behaviour of individual platelets on immobilized fibrinogen (E; spreading was quantified for 3 individual platelets per separate experiment). Quantification of the amount of adhering platelets per field (F). Data represent mean and standard deviation (SD) of 3 or more individual experiments.</p

The inhibitory effect of anandamide on platelet activation is not dependent on platelet preactivation.

<p>Washed platelets were pretreated with 25 µM FAAH-inhibitor URB597. Subsequently, platelet aggregation was induced by various agonists after a suboptimal exposure to anandamide (10 µM, preincubated for 40 minutes). 0.5 µg/mL collagen (A), 0.1 µg/mL CRP-XL (B), 10 µM AA (C), 10 µM ADP (D), 1 µM U46619 (E) or 1 µM TRAP (F). In further experiments, washed platelets were pretreated with 10 µM indomethacin, exposed to anandamide (10 µM, preincubated for 40 minutes) and stimulated with collagen (0.5 µg/mL: G).</p