Hydrogen peroxide promotes aging-related platelet hyperactivation and thrombosis

Blood hydrogen peroxide break-down activity in healthy subjects and in patients at risk of cardiovascular events

BACKGROUND AND AIMS

Antioxidant status has been shown to be associated with cardiovascular events (CVEs). The aim of the study was to develop an assay measuring serum hydrogen peroxide (H₂O₂) break-down activity (HBA) of healthy subjects (HS) and to validate it in a cohort of patients with atrial fibrillation (AF).

METHODS

We developed the HBA assay in 121 HS and validated it in 842 AF patients. The occurrence of CVEs was registered and correlated with HBA in AF during a median follow-up of 30.6 months (3226 patient-years). A combined endpoint of CVEs included fatal/non-fatal ischemic stroke and myocardial infarction, cardiovascular death and transient ischemic attack.

RESULTS

In HS, median HBA was 61.2% [IQR: 52.9-69.4]. AF patients disclosed lower HBA than 30 HS balanced for age and sex (48.6% [IQR: 24.7-65.1] vs. 59.4% [IQR: 49.2-66.2], p < 0.001). During a mean follow-up of 30.6 months (3226 patient-years), 168 CVEs occurred (5.2%/year). A multivariable Cox's proportional hazards regression analysis showed that age group 3 (71-80 years, HR:5.419, p = 0.020), age group 4 (>80 years, HR:9.783, p = 0.002), diabetes (HR:1.464, p = 0.049), previous cardiac events (HR:1.887, p = 0.001) and HBA (below median, HR:2.313, p < 0.001) predicted CVEs.

CONCLUSIONS

We developed an easy assay to measure serum HBA, which was associated with CVEs in AF patients. This assay may represent an additional useful tool for cardiovascular risk stratification and should be validated in other high-risk populations.

Antioxidant activity from extra virgin olive oil via inhibition of hydrogen peroxide-mediated NADPH-oxidase 2 activation

OBJECTIVES

Extra virgin olive oil (EVOO) supplementation is associated with a significant reduction in cardiovascular disease but the underlying mechanism is still unclear.

METHODS

In platelets that were taken from healthy subjects (n = 5), agonist-induced hydrogen peroxide (H₂O₂) production and NADPH oxidase 2 (NOX2) activation in the presence of or without catalase, which catabolizes H₂O₂, were investigated. Platelet H₂O₂ production, NOX2 activation, EVOO vitamin E, and total polyphenols as well as EVOO's ability to scavenge H₂O₂ were also measured.

RESULTS

Platelet NOX2 activation and H₂O₂ production were significantly inhibited in catalase-treated platelets and platelets that were incubated with five different EVOOs. The EVOO content of vitamin E was 53 to 223 mg/kg and total polyphenols 145 to 392 mg/L Gallic acid equivalent. EVOOs quenched in vitro H₂O₂ by 39 to 62%, which is an effect that is significantly correlated with vitamin E and total polyphenol concentrations (R = 0.688; P <0.001 and R = 0.541; P <0.001, respectively).

CONCLUSIONS

This in vitro study provides the first evidence that EVOO downregulates platelet H₂O₂ and in turn NOX2 activity via H₂O₂ scavenging.

Molecularly Engineered Theranostic Nanoparticles for Thrombosed Vessels: H2O2-Activatable Contrast-Enhanced Photoacoustic Imaging and Antithrombotic Therapy

A thrombus (blood clot), composed mainly of activated platelets and fibrin, obstructs arteries or veins, leading to various life-threatening diseases. Inspired by the distinctive physicochemical characteristics of thrombi such as abundant fibrin and an elevated level of hydrogen peroxide (H₂O₂), we developed thrombus-specific theranostic (T-FBM) nanoparticles that could provide H₂O₂-triggered photoacoustic signal amplification and serve as an antithrombotic nanomedicine. T-FBM nanoparticles were designed to target fibrin-rich thrombi and be activated by H₂O₂ to generate CO₂ bubbles to amplify the photoacoustic signal. In the phantom studies, T-FBM nanoparticles showed significant amplification of ultrasound/photoacoustic signals in a H₂O₂-triggered manner. T-FBM nanoparticles also exerted H₂O₂-activatable antioxidant, anti-inflammatory, and antiplatelet activities on endothelial cells. In mouse models of carotid arterial injury, T-FBM nanoparticles significantly enhanced the photoacoustic contrast specifically in thrombosed vessels and significantly suppressed thrombus formation. We anticipate that T-FBM nanoparticles hold great translational potential as nanotheranostics for H₂O₂-associated cardiovascular diseases.

Age-related increase of thromboxane B2 and risk of cardiovascular disease in atrial fibrillation

Aging is strictly associated with an increased incidence of cardiovascular events (CVEs) in the general population. Mechanisms underlying the risk of CVEs are still unclear. Platelet activation contributes to the onset of cardiovascular complications. The incidence of atrial fibrillation (AF) increases with age, and the natural history of AF is often complicated by CVEs. We prospectively investigated the relationship between age, urinary thromboxane (Tx) B₂, which reflects platelet activation, and CVEs in 833 AF patients. Median TxB₂ level was 120 [66-200] ng/mg of urinary creatinine. At multivariable linear regression analysis, age (B: 0.097, p=0.005) and previous MI/CHD (B: 0.069, p=0.047) were associated with log-TxB₂ levels. When we divided our population into age classes (i.e. < 60, 60-69, 70-79, ≥ 80 years), we found a significant difference in TxB₂ levels across classes (p=0.005), with a significant elevation at 74.6 years. During a mean follow-up of 40.9 months, 128 CVEs occurred; the rate of CVEs significantly increased with age classes (Log-rank test, p < 0.001). TxB₂ levels were higher in patients with, compared to those without, CVEs in patients aged 70-79 (p < 0.001) and ≥ 80 (p = 0.020) years. In conclusion, TxB₂ levels enhance by increasing age, suggesting that platelet activation contributes to CVEs in elderly patients with AF.

Reactive Oxygen Species Generated by CD45-Cells Distinct from Leukocyte Population in Platelet Concentrates Is Correlated with the Expression and Release of Platelet Activation Markers during Storage

Background

Platelet stimulation with agonists is accompanied by the generation of reactive oxygen species (ROS) which promotes further platelet activation and aggregation. Considering different cell populations in platelet concentrates (PCs), this study investigates the correlation of ROS generation with the expression and release of platelet activation markers during storage.

Methods

Samples obtained from 6 PCs were subjected to flow cytometry and ELISA to evaluate the expression and shedding of platelet P-selectin or CD40L during storage. Intracellular ROS were detected in either CD45- or CD45+ population by flow cytometry using dihydrorhodamine 123, while ROS production was analyzed in both P-selectin+ or P-selectin- and CD40L+ or CD40L- populations. To further evaluate the correlation between ROS generation and release function, TRAP-stimulated platelets were also subjected to flow cytometry analysis.

Results

ROS detected in the CD45-population (leukocyte-free platelets) was significantly increased by fMLP and PMA. P-selectin- or CD40L- platelet did not show significant amount of ROS. Total ROS generation was significantly increased during platelet storage (day 0 vs. day 5; p = 0.0002) while this increasing pattern was directly correlated with the expression of P-selectin (r = 0.72; p = 0.0001) and CD40L (r = 0.69; p = 0.0001). ROS generations were significantly correlated with ectodomain shedding of these pro-inflammatory molecules.

Conclusion

Our data confirmed increasing levels of intracellular ROS generation in both platelets (CD45-) and platelet-leukocyte aggregates (CD45+) during PC storage. The amount of detected ROS is directly correlated with platelet activation and release in each population while platelet-leukocyte aggregates generate higher levels of ROS than single platelets.

Sirt3 deficiency does not affect venous thrombosis or NETosis despite mild elevation of intracellular ROS in platelets and neutrophils in mice

Inflammation is a common denominator in chronic diseases of aging. Yet, how inflammation fuels these diseases remains unknown. Neutrophils are the primary leukocytes involved in the early phase of innate immunity and inflammation. As part of their anti-microbial defense, neutrophils form extracellular traps (NETs) by releasing decondensed chromatin lined with cytotoxic proteins. NETs have been shown to induce tissue injury and thrombosis. Here, we demonstrated that Sirt3, a nicotinamide adenine dinucleotide (NAD+)-dependent protein deacetylase, an enzyme linked to human longevity, was expressed in mouse neutrophils and platelets. Using Sirt3-/- mice as a model of accelerated aging, we investigated the effects of Sirt3 deficiency on NETosis and platelet function, aiming to detect enhancement of thrombosis. More mitochondrial reactive oxygen species (ROS) were generated in neutrophils and platelets of Sirt3-/- mice compared to WT, when stimulated with a low concentration of phorbol 12-myristate 13-acetate (PMA) and a high concentration of thrombin, respectively. There were no differences in in vitro NETosis, with or without stimulation. Platelet aggregation was mildly augmented in Sirt3-/- mice compared to WT mice, when stimulated with a low concentration of collagen. The effect of Sirt3 deficiency on platelet and neutrophil activation in vivo was examined by the venous thrombosis model of inferior vena cava stenosis. Elevation of plasma DNA concentration was observed after stenosis in both genotypes, but no difference was shown between the two genotypes. The systemic response to thrombosis was enhanced in Sirt3-/- mice with significantly elevated neutrophil count and reduced platelet count. However, no differences were observed in incidence of thrombus formation, thrombus weight and thrombin-antithrombin complex generation between WT and Sirt3-/- mice. We conclude that Sirt3 does not considerably impact NET formation, platelet function, or venous thrombosis in healthy young mice.

Procoagulant platelets: generation, function, and therapeutic targeting in thrombosis

Current understanding of how platelets localize coagulation to wound sites has come mainly from studies of a subpopulation of activated platelets. In this review, we summarize data from the last 4 decades that have described these platelets with a range of descriptive titles and attributes. We identify striking overlaps in the reported characteristics of these platelets, which imply a single subpopulation of versatile platelets and thus suggest that their commonality requires unification of their description. We therefore propose the term procoagulant platelet as the unifying terminology. We discuss the agonist requirements and molecular drivers for the dramatic morphological transformation platelets undergo when becoming procoagulant. Finally, we provide perspectives on the biomarker potential of procoagulant platelets for thrombotic events as well as on the possible clinical benefits of inhibitors of carbonic anhydrase enzymes and the water channel Aquaporin-1 for targeting this subpopulation of platelets as antiprocoagulant antithrombotics.

Atherothrombosis and Oxidative Stress: Mechanisms and Management in Elderly

SIGNIFICANCE

The incidence of cardiovascular events (CVEs) increases with age, representing the main cause of death in an elderly population. Aging is associated with overproduction of reactive oxygen species (ROS), which may affect clotting and platelet activation, and impair endothelial function, thus predisposing elderly patients to thrombotic complications. Recent Advances: There is increasing evidence to suggest that aging is associated with an imbalance between oxidative stress and antioxidant status. Thus, upregulation of ROS-producing enzymes such as nicotinamide adenine dinucleotide phosphate (NADPH) oxidase and myeloperoxidase, along with downregulation of antioxidant enzymes, such as superoxide dismutase and glutathione peroxidase, occurs during aging. This imbalance may predispose to thrombosis by enhancing platelet and clotting activation and eliciting endothelial dysfunction. Recently, gut-derived products, such as trimethylamine N-oxide (TMAO) and lipopolysaccharide, are emerging as novel atherosclerotic risk factors, and gut microbiota composition has been shown to change by aging, and may concur with the increased cardiovascular risk in the elderly.

CRITICAL ISSUES

Antioxidant treatment is ineffective in patients at risk or with cardiovascular disease. Further, anti-thrombotic treatment seems to work less in the elderly population.

FUTURE DIRECTIONS

Interventional trials with antioxidants targeting enzymes implicated in aging-related atherothrombosis are warranted to explore whether modulation of redox status is effective in lowering CVEs in the elderly. Antioxid. Redox Signal. 27, 1083-1124.

Review manuscript: Mechanisms of platelet activation by the pneumococcus and the role of platelets in community-acquired pneumonia

There is increasing recognition of the involvement of platelets in orchestrating inflammatory responses, driving the activation of neutrophils, monocytes and vascular endothelium, which, if poorly controlled, may lead to microvascular dysfunction. Importantly, hyperreactivity of platelets has been implicated in the pathogenesis of myocardial injury and the associated particularly high prevalence of acute cardiovascular events in patients with severe community-acquired pneumonia (CAP), of which Streptococcus pneumoniae (pneumococcus) is the most commonly encountered aetiologic agent. In this context, it is noteworthy that a number of studies have documented various mechanisms by which the pneumococcus may directly promote platelet aggregation and activation. The major contributors to platelet activation include several different types of pneumococcal adhesin, the pore-forming toxin, pneumolysin, and possibly pathogen-derived hydrogen peroxide, which collectively represent a major focus of the current review. This is followed by an overview of the limited experimental studies together with a larger series of clinical studies mainly focused on all-cause CAP, which have provided evidence in support of associations between alterations in circulating platelet counts, most commonly thrombocytopenia, and a poor clinical outcome. The final section of the review covers, albeit briefly, systemic biomarkers of platelet activation which may have prognostic potential.

Superoxide Dismutase 2 is dispensable for platelet function

Increased intracellular reactive oxygen species (ROS) promote platelet activation. The sources of platelet-derived ROS are diverse and whether or not mitochondrial derived ROS, modulates platelet function is incompletely understood. Studies of platelets from patients with sickle cell disease, and diabetes suggest a correlation between mitochondrial ROS and platelet dysfunction. Therefore, we generated mice with a platelet specific knockout of superoxide dismutase 2 (SOD2-KO) to determine if increased mitochondrial ROS increases platelet activation. SOD2-KO platelets demonstrated decreased SOD2 activity and increased mitochondrial ROS, however total platelet ROS was unchanged. Mitochondrial function and content were maintained in non-stimulated platelets. However SOD2-KO platelets demonstrated decreased mitochondrial function following thrombin stimulation. In vitro platelet activation and spreading was normal and in vivo, deletion of SOD2 did not change tail-bleeding or arterial thrombosis indices. In pathophysiological models mediated by platelet-dependent immune mechanisms such as sepsis and autoimmune inflammatory arthritis, SOD2-KO mice were phenotypically identical to wildtype controls. These data demonstrate that increased mitochondrial ROS does not result in platelet dysfunction.

Platelet oxidative stress as a novel target of cardiovascular risk in frail older people

The average lifespan of humans and the percentage of people entering the 65 and older age group are growing rapidly. Within this age group, cardiovascular diseases (CVD) increase steeply and are the most common cause of death. During aging, experimental and clinical studies support the pivotal role played by reactive oxidant species in the mechanism of platelet activation. Frailty has been implicated as a causative and prognostic factor in patients with CVD. Oxidative stress is increased in frail older people, and may lead to accelerated aging and higher incidence of oxidative diseases such as CVD. The present article aims to highlight the relative contribution of platelet oxidative stress as a key target of frailty in elderly people with CVD.

Xanthohumol isolated from Humulus lupulus prevents thrombosis without increased bleeding risk by inhibiting platelet activation and mtDNA release

AIM

As the global population has reached 7 billion and the baby boom generation reaches old age, thrombosis has become the major contributor to the global disease burden. It has been reported that, in moderate doses, beer may protect against thrombosis. Xanthohumol (XN), an antioxidant, is found at high concentrations in hop cones (Humulus lupulus L.) and is a common ingredient of beer. Here, the aim of the present work was to investigate the effects of XN on antithrombotic and antiplatelet activities, and study its mechanism.

APPROACH AND RESULTS

Using ferric chloride-induced carotid artery injury, inferior vena cava ligation model, and platelet function tests, we demonstrated that XN uniquely prevents both venous and arterial thrombosis by inhibiting platelet activation. Interestingly, in tail bleeding time studies, XN did not increase bleeding risk, which is recognized as a major limitation of current antithrombotic therapies. We also demonstrated that XN induces Sirt1 expression and thereby decreases reactive oxygen species (ROS) overload, prevents mitochondrial dysfunction, and reduces activated platelet-induced mitochondrial hyperpolarization, respiratory disorders, and associated membrane damage at low concentrations. In mitochondrial function assays designed to detect amounts of extracellular mitochondrial DNA (mtDNA), we found that XN prevents mtDNA release, which induces platelet activation in a DC-SIGN-dependent manner.

CONCLUSIONS

XN exemplifies a promising new class of antiplatelet agents that are highly effective at inhibiting platelet activation by decreasing ROS accumulation and platelet mtDNA release without incurring a bleeding risk. This study has also provided novel insights into mechanisms of thrombotic diseases with possible therapeutic implications.

Fibrin-Targeted and H2O2-Responsive Nanoparticles as a Theranostics for Thrombosed Vessels

A thrombus (blood clot) is formed in injured vessels to maintain the integrity of vasculature. However, obstruction of blood vessels by thrombosis slows blood flow, leading to death of tissues fed by the artery and is the main culprit of various life-threatening cardiovascular diseases. Herein, we report a rationally designed nanomedicine that could specifically image obstructed vessels and inhibit thrombus formation. On the basis of the physicochemical and biological characteristics of thrombi such as an abundance of fibrin and an elevated level of hydrogen peroxide (H₂O₂), we developed a fibrin-targeted imaging and antithrombotic nanomedicine, termed FTIAN, as a theranostic system for obstructive thrombosis. FTIAN inhibited the generation of H₂O₂ and suppressed the expression of tumor necrosis factor-alpha (TNF-α) and soluble CD40 ligand (sCD40L) in activated platelets, demonstrating its intrinsic antioxidant, anti-inflammatory, and antiplatelet activity. In a mouse model of ferric chloride (FeCl₃)-induced carotid thrombosis, FTIAN specifically targeted the obstructive thrombus and significantly enhanced the fluorescence/photoacoustic signal. When loaded with the antiplatelet drug tirofiban, FTIAN remarkably suppressed thrombus formation. Given its thrombus-specific imaging along with excellent therapeutic activities, FTIAN offers tremendous translational potential as a nanotheranostic agent for obstructive thrombosis.

Platelet granule release is associated with reactive oxygen species generation during platelet storage: A direct link between platelet pro-inflammatory and oxidation states

BACKGROUND

Upon platelet stimulation with agonists, reactive oxygen species (ROS) generation enhances platelet activation and granule release. Whether ROS generation during platelet storage could be directly correlated with the expression of proinflammatory molecules and granule release has been investigated in this study.

MATERIAL AND METHOD

PRP-platelet concentrates were subjected to flowcytometry analysis to assess the expression of platelet activation marker, P-selectin and CD40L during storage. Intracellular ROS generation was also detected in platelet by flowcytometry using dihydrorhodamine (DHR) 123. Through the dual staining, ROS production was analyzed in either P-selectin positive or negative populations.

RESULTS

ROS formation in platelet population was significantly increased by either TRAP (a potent agonist that induces granule release) or PMA (a classic inducer of ROS generation), while the effects of each agonists on P-selectin expression and ROS generation in platelets were comparable. Platelet storage was also associated with the increasing levels of ROS (day 0 vs. day 5; p<0.001) while this increasing pattern was directly correlated with the either expressed P-selectin or CD40L. In addition, in 5 day-stored platelets, samples with ROS levels above 40% showed significantly higher levels of P-selectin and CD40L expression. P-selectin negative population of platelet did not show significant amount of ROS.

CONCLUSION

Our data demonstrated decreased levels of important platelet pro-inflammatory molecules in stored platelets with lower levels of intraplatelet ROS. However, whether quenching of ROS generation during platelet storage can attenuate adverse transfusion reactions raised by platelet pro-inflammatory status is required to be further studied.

The Pathophysiological Role of NOX2 in Hypertension and Organ Damage

NADPH oxidases (NOXs) represent one of the major sources of reactive oxygen species in the vascular district. Reactive oxygen species are responsible for vascular damage that leads to several cardiovascular pathological conditions. Among NOX isoforms, NOX2 is widely expressed in many cells types, such as cardiomyocytes, endothelial cells, and vascular smooth muscle cells, confirming its pivotal role in vascular pathophysiology. Studies in mice models with systemic deletion of NOX2, as well as in transgenic mice overexpressing NOX2, have demonstrated the undeniable involvement of NOX2 in the development of hypertension, atherosclerosis, diabetes mellitus, cardiac hypertrophy, platelet aggregation, and aging. Of note, the inhibition of NOX2 has been found to be protective for cardiovascular homeostasis. Here, we review the evidence demonstrating that the modulation of NOX2 activity is able to improve vascular physiology, suggesting that NOX2 may be a potential target for therapeutic applications.

COPD and stroke: are systemic inflammation and oxidative stress the missing links?

Chronic obstructive pulmonary disease (COPD) is characterized by progressive airflow limitation and loss of lung function, and is currently the third largest cause of death in the world. It is now well established that cardiovascular-related comorbidities such as stroke contribute to morbidity and mortality in COPD. The mechanisms linking COPD and stroke remain to be fully defined but are likely to be interconnected. The association between COPD and stroke may be largely dependent on shared risk factors such as aging and smoking, or the association of COPD with traditional stroke risk factors. In addition, we propose that COPD-related systemic inflammation and oxidative stress may play important roles by promoting cerebral vascular dysfunction and platelet hyperactivity. In this review, we briefly discuss the pathogenesis of COPD, acute exacerbations of COPD (AECOPD) and cardiovascular comorbidities associated with COPD, in particular stroke. We also highlight and discuss the potential mechanisms underpinning the link between COPD and stroke, with a particular focus on the roles of systemic inflammation and oxidative stress.

Utility of microfluidic devices to study the platelet-endothelium interface

The integration of biomaterials and understanding of vascular biology has led to the development of perfusable endothelialized flow models, which have been used as valuable tools to study the platelet-endothelium interface under shear. In these models, the parameters of geometry, compliance, biorheology, and cellular complexity are varied to recapitulate the physical biology of platelet recruitment and activation under physiologically relevant conditions of blood flow. In this review, we summarize the mechanistic insights learned from perfusable microvessel models and discuss the potential utility as well as challenges of endothelialized microfluidic devices to study platelet function in the bloodstream in vitro.

Platelet CD36 promotes thrombosis by activating redox sensor ERK5 in hyperlipidemic conditions

Atherothrombosis is a process mediated by dysregulated platelet activation that can cause life-threatening complications and is the leading cause of death by cardiovascular disease. Platelet reactivity in hyperlipidemic conditions is enhanced when platelet scavenger receptor CD36 recognizes oxidized lipids in oxidized low-density lipoprotein (oxLDL) particles, a process that induces an overt prothrombotic phenotype. The mechanisms by which CD36 promotes platelet activation and thrombosis remain incompletely defined. In this study, we identify a mechanism for CD36 to promote thrombosis by increasing activation of MAPK extracellular signal-regulated kinase 5 (ERK5), a protein kinase known to be exquisitely sensitive to redox stress, through a signaling pathway requiring Src kinases, NADPH oxidase, superoxide radical anion, and hydrogen peroxide. Pharmacologic inhibitors of ERK5 blunted platelet activation and aggregation in response to oxLDL and targeted genetic deletion of ERK5 in murine platelets prevented oxLDL-induced platelet deposition on immobilized collagen in response to arterial shear. Importantly, in vivo thrombosis experiments after bone marrow transplantation from platelet-specific ERK5 null mice into hyperlipidemic apolipoprotein E null mice showed decreased platelet accumulation and increased thrombosis times compared with mice transplanted with ERK5 expressing control bone marrows. These findings suggest that atherogenic conditions critically regulate platelet CD36 signaling by increasing superoxide radical anion and hydrogen peroxide through a mechanism that promotes activation of MAPK ERK5.

Redox Proteomics and Platelet Activation: Understanding the Redox Proteome to Improve Platelet Quality for Transfusion

Blood banks use pathogen inactivation (PI) technologies to increase the safety of platelet concentrates (PCs). The characteristics of PI-treated PCs slightly differ from those of untreated PCs, but the underlying reasons are not well understood. One possible cause is the generation of oxidative stress during the PI process. This is of great interest since reactive oxygen species (ROS) act as second messengers in platelet functions. Furthermore, there are links between protein oxidation and phosphorylation, another mechanism that is critical for cell regulation. Current research efforts focus on understanding the underlying mechanisms and identifying new target proteins. Proteomics technologies represent powerful tools for investigating signaling pathways involving ROS and post-translational modifications such as phosphorylation, while quantitative techniques enable the comparison of the platelet resting state versus the stimulated state. In particular, redox cysteine is a key player in platelet activation upon stimulation by different agonists. This review highlights the experiments that have provided insights into the roles of ROS in platelet function and the implications for platelet transfusion, and potentially in diseases such as inflammation and platelet hyperactivity. The review also describes the implication of redox mechanism in platelet storage considerations.

Targeting factor VIII expression to platelets for hemophilia A gene therapy does not induce an apparent thrombotic risk in mice

Essentials Platelet-Factor (F) VIII gene therapy is a promising treatment in hemophilia A. This study aims to evaluate if platelet-FVIII expression would increase the risk for thrombosis. Targeting FVIII expression to platelets does not induce or elevate thrombosis risk. Platelets expressing FVIII are neither hyper-activated nor hyper-responsive.

SUMMARY

Background Targeting factor (F) VIII expression to platelets is a promising gene therapy approach for hemophilia A, and is successful even in the presence of inhibitors. It is well known that platelets play important roles not only in hemostasis, but also in thrombosis and inflammation. Objective To evaluate whether platelet-FVIII expression might increase thrombotic risk and thereby compromise the safety of this approach. Methods In this study, platelet-FVIII-expressing transgenic mice were examined either in steady-state conditions or under prothrombotic conditions induced by inflammation or the FV Leiden mutation. Native whole blood thrombin generation assay, rotational thromboelastometry analysis and ferric chloride-induced vessel injury were used to evaluate the hemostatic properties. Various parameters associated with thrombosis risk, including D-dimer, thrombin-antithrombin complexes, fibrinogen, tissue fibrin deposition, platelet activation status and activatability, and platelet-leukocyte aggregates, were assessed. Results We generated a new line of transgenic mice that expressed 30-fold higher levels of platelet-expressed FVIII than are therapeutically required to restore hemostasis in hemophilic mice. Under both steady-state conditions and prothrombotic conditions induced by lipopolysaccharide-mediated inflammation or the FV Leiden mutation, supratherapeutic levels of platelet-expressed FVIII did not appear to be thrombogenic. Furthermore, FVIII-expressing platelets were neither hyperactivated nor hyperactivatable upon agonist activation. Conclusion We conclude that, in mice, more than 30-fold higher levels of platelet-expressed FVIII than are required for therapeutic efficacy in hemophilia A are not associated with a thrombotic predilection.

NADPH Oxidase-2 and Atherothrombosis: Insight From Chronic Granulomatous Disease

The phagocytic cell enzyme NADPH oxidase-2 (Nox2) is critical for killing micro-organisms via production of reactive oxygen species and thus is a key element of the innate immune system. Nox2 is also detectable in endothelial cells and platelets where it has vasoconstrictive and aggregating properties, respectively. Patients with X-linked chronic granulomatous disease with hereditary Nox2 deficiency not only have impaired bacterial killing but, in association with loss of Nox2 function, also have enhanced carotid artery dilation, impaired platelet-related thrombosis, and reduced carotid atherosclerotic burden. Experimental studies corroborated these reports in chronic granulomatous disease by demonstrating (1) Nox2 is upregulated in atherosclerotic plaque, and this upregulation significantly correlates with oxidative stress and (2) pharmacological inhibition of Nox2 is associated with a delayed atherosclerotic progression in animal models. Furthermore, the role of Nox2 in platelet-associated thrombosis was substantiated by experiments showing impaired platelet activation in animals treated with a Nox2 inhibitor or impaired platelet aggregation along with reduced platelet-related thrombosis in the mouse knockout model of Nox2. Interestingly, in chronic granulomatous disease patients and in the mouse knockout model of Nox2, no defects of primary hemostasis were detected. This review analyses experimental and clinical data suggesting Nox2 is a potential target for counteracting the atherothrombotic process.

Metformin Uniquely Prevents Thrombosis by Inhibiting Platelet Activation and mtDNA Release

Thrombosis and its complications are the leading cause of death in patients with diabetes. Metformin, a first-line therapy for type 2 diabetes, is the only drug demonstrated to reduce cardiovascular complications in diabetic patients. However, whether metformin can effectively prevent thrombosis and its potential mechanism of action is unknown. Here we show, metformin prevents both venous and arterial thrombosis with no significant prolonged bleeding time by inhibiting platelet activation and extracellular mitochondrial DNA (mtDNA) release. Specifically, metformin inhibits mitochondrial complex I and thereby protects mitochondrial function, reduces activated platelet-induced mitochondrial hyperpolarization, reactive oxygen species overload and associated membrane damage. In mitochondrial function assays designed to detect amounts of extracellular mtDNA, we found that metformin prevents mtDNA release. This study also demonstrated that mtDNA induces platelet activation through a DC-SIGN dependent pathway. Metformin exemplifies a promising new class of antiplatelet agents that are highly effective at inhibiting platelet activation by decreasing the release of free mtDNA, which induces platelet activation in a DC-SIGN-dependent manner. This study has established a novel therapeutic strategy and molecular target for thrombotic diseases, especially for thrombotic complications of diabetes mellitus.

Aging-Related Decline of Glutathione Peroxidase 3 and Risk of Cardiovascular Events in Patients With Atrial Fibrillation

BACKGROUND

Experimental studies demonstrated that glutathione peroxidase 3 (GPx3), an antioxidant enzyme that catabolizes hydrogen peroxide, protects against thrombosis. Little is known about its role in cardiovascular disease.

METHODS AND RESULTS

A prospective cohort study was conducted in 909 atrial fibrillation patients. Serum activities of GPx3, superoxide dismutase (SOD), and catalase were measured at baseline to assess the risk of cardiovascular events during a mean follow-up of 43.4 months (3291 person-years). Serum Nox2 and urinary excretion of 11-deydro-thromboxane B2 were also measured. During follow-up 160 cardiovascular events occurred (4.9%/year). Significantly lower values of GPx3 (P<0.001) and SOD (P=0.037) were detected in patients with, compared to those without, cardiovascular events. A lower survival rate was observed in patients with GPx3 (P<0.001) and SOD (P=0.010) activities below the median, as compared to those above. In a fully adjusted Cox regression model, GPx3 was the only antioxidant enzyme predictor of cardiovascular events (hazard ratio 0.647, 95% confidence interval 0.524-0.798, P<0.001). GPx3 was inversely associated with urinary 11-dehydro-thromboxane B2 (B -0.337, P<0.001) and serum Nox2 (B: -0.423, P<0.001). GPx3 activity progressively decreased with decades of age (P<0.001), with a progressive reduction in people aged ≥70 years.

CONCLUSIONS

This study provides evidence that a low antioxidant status, as depicted by reduced levels of GPx3, increases the risk of cardiovascular events in patients with atrial fibrillation. The age-related decline of GPx3 may represent a mechanism for the enhanced cardiovascular risk in the elderly population.

Platelets are versatile cells: New discoveries in hemostasis, thrombosis, immune responses, tumor metastasis and beyond

Platelets are small anucleate blood cells generated from megakaryocytes in the bone marrow and cleared in the reticuloendothelial system. At the site of vascular injury, platelet adhesion, activation and aggregation constitute the first wave of hemostasis. Blood coagulation, which is initiated by the intrinsic or extrinsic coagulation cascades, is the second wave of hemostasis. Activated platelets can also provide negatively-charged surfaces that harbor coagulation factors and markedly potentiate cell-based thrombin generation. Recently, deposition of plasma fibronectin, and likely other plasma proteins, onto the injured vessel wall has been identified as a new "protein wave of hemostasis" that may occur even earlier than the first wave of hemostasis, platelet accumulation. Although no experimental evidence currently exists, it is conceivable that platelets may also contribute to this protein wave of hemostasis by releasing their granule fibronectin and other proteins that may facilitate fibronectin self- and non-self-assembly on the vessel wall. Thus, platelets may contribute to all three waves of hemostasis and are central players in this critical physiological process to prevent bleeding. Low platelet counts in blood caused by enhanced platelet clearance and/or impaired platelet production are usually associated with hemorrhage. Auto- and allo-immune thrombocytopenias such as idiopathic thrombocytopenic purpura and fetal and neonatal alloimmune thrombocytopenia may cause life-threatening bleeding such as intracranial hemorrhage. When triggered under pathological conditions such as rupture of an atherosclerotic plaque, excessive platelet activation and aggregation may result in thrombosis and vessel occlusion. This may lead to myocardial infarction or ischemic stroke, the major causes of mortality and morbidity worldwide. Platelets are also involved in deep vein thrombosis and thromboembolism, another leading cause of mortality. Although fibrinogen has been documented for more than half a century as essential for platelet aggregation, recent studies demonstrated that fibrinogen-independent platelet aggregation occurs in both gene deficient animals and human patients under physiological and pathological conditions (non-anti-coagulated blood). This indicates that other unidentified platelet ligands may play important roles in thrombosis and might be novel antithrombotic targets. In addition to their critical roles in hemostasis and thrombosis, emerging evidence indicates that platelets are versatile cells involved in many other pathophysiological processes such as innate and adaptive immune responses, atherosclerosis, angiogenesis, lymphatic vessel development, liver regeneration and tumor metastasis. This review summarizes the current knowledge of platelet biology, highlights recent advances in the understanding of platelet production and clearance, molecular and cellular events of thrombosis and hemostasis, and introduces the emerging roles of platelets in the immune system, vascular biology and tumorigenesis. The clinical implications of these basic science and translational research findings will also be discussed.

mTORC1 promotes aging-related venous thrombosis in mice via elevation of platelet volume and activation

Aging is associated with an increased incidence of venous thromboembolism (VTE), resulting in significant morbidity and mortality in the elderly. Platelet hyperactivation is linked to aging-related VTE. However, the mechanisms through which aging enhances platelet activation and susceptibility to VTE are poorly understood. In this study, we demonstrated that mechanistic target of rapamycin complex 1 (mTORC1) signaling is essential for aging-related platelet hyperactivation and VTE. mTORC1 was hyperactivated in platelets and megakaryocytes (MKs) from aged mice, accompanied by elevated mean platelet volume (MPV) and platelet activation. Inhibition of mTORC1 with rapamycin led to a significant reduction in susceptibility to experimental deep vein thrombosis (DVT) in aged mice (P < .01). To ascertain the specific role of platelet mTORC1 activation in DVT, we generated mice with conditional ablation of the mTORC1-specific component gene Raptor in MKs and platelets (Raptor knockout). These mice developed markedly smaller and lighter thrombi, compared with wild-type littermates (P < .01) in experimental DVT. Mechanistically, increased reactive oxygen species (ROS) production with aging induced activation of mTORC1 in MKs and platelets, which, in turn, enhanced bone marrow MK size, MPV, and platelet activation to promote aging-related VTE. ROS scavenger administration induced a significant decrease (P < .05) in MK size, MPV, and platelet activation in aged mice. Our findings collectively demonstrate that mTORC1 contributes to enhanced venous thrombotic susceptibility in aged mice via elevation of platelet size and activation.

Platelet function and ageing

There are clear age-related changes in platelet count and function, driven by changes in hematopoietic tissue, the composition of the blood and vascular health. Platelet count remains relatively stable during middle age (25–60 years old) but falls in older people. The effect of age on platelet function is slightly less clear. The longstanding view is that platelet reactivity increases with age in an almost linear fashion. There are, however, serious limitations to the data supporting this dogma. We can conclude that platelet function increases during middle age, but little evidence exists on the changes in platelet responsiveness in old age (>75 years old). This change in platelet function is driven by differential mRNA and microRNA expression, an increase in oxidative stress and changes in platelet receptors. These age-related changes in platelets are particularly pertinent given that thrombotic disease and use of anti-platelet drugs is much more prevalent in the elderly population, yet the majority of platelet research is carried out in young to middle-aged (20–50 years old) human volunteers and young mice (2–6 months old). We know relatively little about exactly how platelets from people over 75 years old differ from those of middle-aged subjects, and we know even less about the mechanisms that drive these changes. Addressing these gaps in our knowledge will provide substantial understanding in how cell signalling changes during ageing and will enable the development of more precise anti-platelet therapies.

Role of oxidative stress on platelet hyperreactivity during aging

Thrombotic events are common causes of morbidity and mortality in the elderly. Age-accelerated vascular injury is commonly considered to result from increased oxidative stress. There is abundant evidence that oxidative stress regulate several components of thrombotic processes, including platelet activation. Thus oxidative stress can trigger platelet hyperreactivity by decreasing nitric oxide bioavailability. Therefore oxidative stress measurement may help in the early identification of asymptomatic subjects at risk of thrombosis. In addition, oxidative stress inhibitors and platelet-derived nitric oxide may represent a novel anti-aggregation/-activation approach. In this article the relative contribution of oxidative stress and platelet activation in aging is explored.

Endothelial cell regulation of pulmonary vascular tone, inflammation, and coagulation

The pulmonary endothelium represents a heterogeneous cell monolayer covering the luminal surface of the entire lung vasculature. As such, this cell layer lies at a critical interface between the blood, airways, and lung parenchyma, and must act as a selective barrier between these diverse compartments. Lung endothelial cells are able to produce and secrete mediators, display surface receptor, and cellular adhesion molecules, and metabolize circulating hormones to influence vasomotor tone, both local and systemic inflammation, and coagulation functions. In this review, we will explore the role of the pulmonary endothelium in each of these systems, highlighting key regulatory functions of the pulmonary endothelial cell, as well as novel aspects of the pulmonary endothelium in contrast to the systemic cell type. The interactions between pulmonary endothelial cells and both leukocytes and platelets will be discussed in detail, and wherever possible, elements of endothelial control over physiological and pathophysiological processes will be examined.

2-Arachidonoylglycerol enhances platelet formation from human megakaryoblasts

Platelets modulate vascular system integrity, and their loss is critical in haematological pathologies and after chemotherapy. Therefore, identification of molecules enhancing platelet production would be useful to counteract thrombocytopenia. We have previously shown that 2-arachidonoylglycerol (2-AG) acts as a true agonist of platelets, as well as it commits erythroid precursors toward the megakaryocytic lineage. Against this background, we sought to further interrogate the role of 2-AG in megakaryocyte/platelet physiology by investigating terminal differentiation, and subsequent thrombopoiesis. To this end, we used MEG-01 cells, a human megakaryoblastic cell line able to produce in vitro platelet-like particles. 2-AG increased the number of cells showing ruffled surface and enhanced surface expression of specific megakaryocyte/platelet surface antigens, typical hallmarks of terminal megakaryocytic differentiation and platelet production. Changes in cytoskeleton modeling also occurred in differentiated megakaryocytes and blebbing platelets. 2-AG acted by binding to CB1 and CB2 receptors, because specific antagonists reverted its effect. Platelets were split off from megakaryocytes and were functional: they contained the platelet-specific surface markers CD61 and CD49, whose levels increased following stimulation with a natural agonist like collagen. Given the importance of 2-AG for driving megakaryopoiesis and thrombopoiesis, not surprisingly we found that its hydrolytic enzymes were tightly controlled by classical inducers of megakaryocyte differentiation. In conclusion 2-AG, by triggering megakaryocyte maturation and platelet release, may have clinical efficacy to counteract thrombocytopenia-related diseases.

Primary and secondary haemostasis changes related to aging

Life expectancy has increased in many countries as a result the world's population is aging. The projections indicate that the proportion of the elderly in a few decades will increase significantly. Aging carries with it a series of physiological changes; one of them is an imbalance in the hemostatic system. Thus the levels or activity of various proteins involved, such as most coagulation factors, natural anticoagulants and the fibrinolytic system are altered so that the hemostatic balance leans toward thrombosis. Also, platelet activity suggests a state of abnormal activation (P-selectin, beta thromboglobulin and platelet factor). In this review we will systematically examine the alterations in the hemostatic components that occur during aging. Therefore, understanding these hemostatic changes could contribute to developing strategies for the proper management of health in old age.

Evaluation of extracellular adenine nucleotides hydrolysis in platelets and biomarkers of oxidative stress in Down syndrome individuals

PURPOSE

Down syndrome (DS) is caused by the triplication of chromosome 21. Studies have demonstrated platelets abnormalities and oxidative stress in DS subjects. The enzymes NTPDase, 5'-nucleotidase and adenosine deaminase (ADA) represent an important therapeutic target since they interfere in the extracellular nucleotide pool altering platelet functions. In this study, we evaluated the ectonucleotidases activities and oxidative stress parameters in samples of DS and healthy individuals.

METHODS AND RESULTS

The population consisted of 28 subjects with DS and 28 healthy subjects as a control group. Blood was obtained from each subject and used for platelet and serum preparation. NTPDase activity using ATP as substrate was increased in platelets of DS patients in relation to the control group; however, no alterations were observed in the ADP hydrolysis. A decrease in the 5'-nucleotidase activity and an increase in the ADA activity was observed in platelet of DS subjects when compared to healthy individuals (P<0.05). The lipid peroxidation and total thiol content was decreased in serum of DS individuals. Furthermore, superoxide dismutase and catalase activities were increased in whole blood of this group (P<0.05).

CONCLUSION

Alterations in the ectonucleotidase activities in platelets as well as changes in the oxidative stress parameters may contribute to the clinical features of DS.

Clinical Application of NOX Activity and Other Oxidative Biomarkers in Cardiovascular Disease: A Critical Review

SIGNIFICANCE

The oxidative stress theory of atherosclerosis is based on the assumption that the production of reactive oxidant species (ROS) by blood, as well as resident cells of the artery wall, elicits the formation of oxidized low-density lipoproteins (ox-LDL), which, in turn, promotes a series of inflammatory responses, ultimately leading to atherosclerotic plaque. This theory prompted the development of new laboratory methodologies that aimed at assessing the relationship between oxidative stress and clinical progression of human atherosclerosis.

CRITICAL ISSUES

Markers assessing the oxidation of phospholipid and protein components of LDL were among the first to be developed. Clinical trials included cross-sectional as well as retrospective and prospective studies that, however, provided equivocal results. Thus, clear evidence that oxidative biomarkers add more to the risk stratification by common atherosclerotic risk factors is still lacking.

RECENT ADVANCES

More recently, the analysis of oxidative stress focused on enzymatic pathways generating ROS, such as NADPH oxidase and myeloperoxidase (MPO). Experimental and clinical studies suggest that both enzymes may be implicated in promoting atherosclerotic disease. Novel laboratory methodologies have been, therefore, developed to study NADPH oxidase and MPO in patients with stable atherosclerosis as well as in patients with acute coronary and cerebro-vascular syndromes.

FUTURE DIRECTIONS

This review will analyze the strengths and weaknesses of the current methodology to study these enzymes in human atherosclerosis with particular regard to their clinical application in several settings of cardiovascular disease. Clinical methodology and results of previous studies with regard to markers of LDL oxidation have also been reviewed as a useful background for the future development of clinical trials.

Evaluation of the antiaggregant activity of ascorbyl phenolic esters with antioxidant properties

Beneficial effects of the antioxidant L-ascorbic acid (Asc) in human health are well known. Its particular role in hemostasis deserves further consideration, since it has been described a dose-dependent effect of Asc in platelet activity. Contrary, it has been demonstrated that phenolic compounds have inhibitory effects on platelet aggregation stimulated by the physiological agonist thrombin (Thr). Here, we have evaluated the actions of three synthetic phenolic esters of Asc: L-ascorbyl 6-protocatechuate (Prot Asc), L-ascorbyl 6-gallate (Gal Asc), and L-ascorbyl 6-caffeate (Caf Asc). All these Asc derivatives exhibited greater radical scavenging activity than Asc, and in experiments using human platelets from healthy subjects, they do not evoke changes in platelet viability upon their administration. Nevertheless, these compounds altered platelet calcium homeostasis in response to Thr, although Prot Asc induced a smaller effect than Gal Asc, Caf Asc, and Asc. As a consequence, platelet aggregation was also impaired by these compounds, reporting Prot Asc and Caf Asc a weaker antiaggregant action than Gal Asc and Asc. Treatments with Gal Asc and Caf Asc altered in larger extent the phosphorylation pattern of pp60(Src) and mammalian target of rapamycin (mTOR) evoked by stimulating human platelets with Thr. Summarizing, Prot Asc is the ascorbyl phenolic ester with the strongest antioxidant properties and weakest antiaggregant actions, and its use as antioxidant may be safer than the rest of derivatives in order to prevent thrombotic alteration in patients that need treatment with antioxidant therapies.

Deficiency of superoxide dismutase impairs protein C activation and enhances susceptibility to experimental thrombosis

OBJECTIVE

Clinical evidence suggests an association between oxidative stress and vascular disease, and in vitro studies have demonstrated that reactive oxygen species can have prothrombotic effects on vascular and blood cells. It remains unclear, however, whether elevated levels of reactive oxygen species accelerate susceptibility to experimental thrombosis in vivo.

APPROACH AND RESULTS

Using a murine model with genetic deficiency in superoxide dismutase-1 (SOD1), we measured susceptibility to carotid artery thrombosis in response to photochemical injury. We found that SOD1-deficient (Sod1(-/-)) mice formed stable arterial occlusions significantly faster than wild-type (Sod1(+/+)) mice (P<0.05). Sod1(-/-) mice also developed significantly larger venous thrombi than Sod1(+/+) mice after inferior vena cava ligation (P<0.05). Activation of protein C by thrombin in lung was diminished in Sod1(-/-) mice (P<0.05 versus Sod1(+/+) mice), and generation of activated protein C in response to infusion of thrombin in vivo was decreased in Sod1(-/-) mice (P<0.05 versus Sod1(+/+) mice). SOD1 deficiency had no effect on the expression of thrombomodulin, endothelial protein C receptor, or tissue factor in lung or levels of protein C in plasma. Exposure of human thrombomodulin to superoxide in vitro caused oxidation of multiple methionine residues, including critical methionine 388, and a 40% decrease in thrombomodulin-dependent activation of protein C (P<0.05). SOD and catalase protected against superoxide-induced methionine oxidation and restored protein C activation in vitro (P<0.05).

CONCLUSIONS

SOD prevents thrombomodulin methionine oxidation, promotes protein C activation, and protects against arterial and venous thrombosis in mice.

The role of TGFβ1 and LRG1 in cardiac remodelling and heart failure

Heart failure is a life-threatening condition that carries a considerable emotional and socio-economic burden. As a result of the global increase in the ageing population, sedentary life-style, increased prevalence of risk factors, and improved survival from cardiovascular events, the incidence of heart failure will continue to rise. Despite the advances in current cardiovascular therapies, many patients are not suitable for or may not benefit from conventional treatments. Thus, more effective therapies are required. Transforming growth factor (TGF) β family of cytokines is involved in heart development and dys-regulated TGFβ signalling is commonly associated with fibrosis, aberrant angiogenesis and accelerated progression into heart failure. Therefore, a potential therapeutic pathway is to modulate TGFβ signalling; however, broad blockage of TGFβ signalling may cause unwanted side effects due to its pivotal role in tissue homeostasis. We found that leucine-rich α-2 glycoprotein 1 (LRG1) promotes blood vessel formation via regulating the context-dependent endothelial TGFβ signalling. This review will focus on the interaction between LRG1 and TGFβ signalling, their involvement in the pathogenesis of heart failure, and the potential for LRG1 to function as a novel therapeutic target.

The influence of body mass index, age and sex on inflammatory disease risk in semi-captive Chimpanzees

Obesity and ageing are emerging issues in the management of captive primates, including Chimpanzees, Pan troglodytes. Studies on humans show that obesity and old age can independently increase the risk of inflammatory-associated diseases indicated by elevated levels of pro-inflammatory cells and proteins in the blood of older or obese compared to levels in younger or non-obese individuals. In humans, sex can influence the outcomes of these risks. Health management of these problems in chimpanzee populations requires an understanding of similarities and differences of factors influencing inflammatory disease risks in humans and in chimpanzees. We examined the relationship between age, sex and Body Mass Index (BMI) with hematological biomarkers of inflammatory disease risk established for humans which include the neutrophil to lymphocyte ratio (NLR), and neutrophil, white blood cell (WBC), platelet microparticle and platelet counts. We found that higher values of NLR, neutrophil count and platelet microparticle count were associated with higher BMI values and older age indicating increased inflammation risk in these groups; a similar pattern to humans. There was a strong sex by age interaction on inflammation risk, with older males more at risk than older females. In contrast to human studies, total WBC count was not influenced by BMI, but like humans, WBC and platelet counts were lower in older individuals compared to younger individuals. Our findings are similar to those of humans and suggest that further insight on managing chimpanzees can be gained from extensive studies of ageing and obesity in humans. We suggest that managing BMI should be an integral part of health management in captive chimpanzee populations in order to partially reduce the risk of diseases associated with inflammation. These results also highlight parallels in inflammation risk between humans and chimpanzees and have implications for understanding the evolution of inflammation related diseases in apes.

Different degrees of NADPH oxidase 2 regulation and in vivo platelet activation: lesson from chronic granulomatous disease

Background

In vitro study showed that NADPH oxidase (NOx), the most important enzyme producing reactive oxygen species (ROS), plays a role in the process of platelet activation. However, it is unclear if changes in its activity affect platelet activation in vivo.

Methods and Results

In vivo and ex vivo experiments assessing platelet activation were investigated in healthy subjects, obese patients, and subjects with different low rates of NOx2 activity, namely X‐linked chronic granulomatous disease (X‐CGD) patients and X‐CGD carriers. We included 27 X‐CGD patients, 31 women carriers of hereditary deficiency of NOx2, 31 obese women, and 62 healthy subjects matched for sex and age. Plasma levels of soluble sCD40 L (sCD40L) and soluble P (sP)‐selectin, 2 markers of in vivo platelet activation, were reduced in X‐CGD patients (sCD40L=−55%; sP‐selectin=−51%, P<0.001) and in X‐CGD carriers (sCD40L=−41%; sP‐selectin=−57%, P<0.001) compared with respective controls. Conversely, obese women, who disclosed NOx2 upregulation, had significantly higher plasma levels of sCD40L (+47%, P<0.001) and sP‐selectin (+70%, P<0.001) compared with controls. Ex vivo study showed platelet isoprostane downexpression and enhanced platelet NO generation in both X‐CGD patients and X‐CGD carriers compared with controls; opposite findings were observed in obese patients. Correlation analysis showed that platelet NOx2 regulation was directly associated with plasma levels of sCD40L (R=0.336, P<0.001) and sP‐selectin (R=0.441; P<0.001).

Conclusions

The study provides the first evidence that in vivo platelet activation is significantly and directly associated with NOx2 activity. Platelet NOx2 may be a novel target for platelet activation inhibition.

P66Shc mediates increased platelet activation and aggregation in hypercholesterolemia

BACKGROUND AND HYPOTHESIS

Hypercholesterolemia leads to a prothrombotic phenotype. Platelet hyperactivity associated with hypercholesterolemia has been attributed, in part, to oxidative stress. P66Shc is a well-known determinant of cellular and organismal oxidative stress. However, its role in platelet biology is not known. We hypothesized that p66Shc mediates platelet hyperactivation and hyperaggregation in hypercholesterolemia.

METHODS AND RESULTS

P66Shc was expressed in both human and mouse platelets, as determined by qRT-PCR and immunoblotting. Mouse platelet p66Shc expression was upregulated by hypercholesterolemia induced by high-fat diet feeding. Compared to wild-type mice, high-fat diet-induced p66Shc expression in platelets was suppressed in transgenic mice expressing a short hairpin RNA targeting p66Shc (p66ShcRNAi). High-fat diet feeding of wild-type mice amplified surface P-selectin expression on platelets stimulated by the thrombin receptor agonist protease-activated receptor-4 (PAR4), and increased aggregation of platelets induced by thrombin. These exaggerated platelet responses induced by high-fat diet feeding were significantly blunted in p66ShcRNAi mice. Finally, thrombin-stimulated platelet reactive oxygen species were suppressed in p66ShcRNAi mice.

CONCLUSIONS

Hypercholesterolemia stimulates p66Shc expression in platelets, promoting platelet oxidative stress, hyperreactivity and hyperaggregation via p66Shc.

Role of Endothelial Cell–Derived Angptl2 in Vascular Inflammation Leading to Endothelial Dysfunction and Atherosclerosis Progression

Objective—

Cardiovascular disease (CVD), the most common morbidity resulting from atherosclerosis, remains a frequent cause of death. Efforts to develop effective therapeutic strategies have focused on vascular inflammation as a critical pathology driving atherosclerosis progression. Nonetheless, molecular mechanisms underlying this activity remain unclear. Here, we ask whether angiopoietin-like protein 2 (Angptl2), a proinflammatory protein, contributes to vascular inflammation that promotes atherosclerosis progression.

Approach and Results—

Histological analysis revealed abundant Angptl2 expression in endothelial cells and macrophages infiltrating atheromatous plaques in patients with cardiovascular disease. Angptl2 knockout in apolipoprotein E–deficient mice ( ApoE −/− / Angptl2 −/− ) attenuated atherosclerosis progression by decreasing the number of macrophages infiltrating atheromatous plaques, reducing vascular inflammation. Bone marrow transplantation experiments showed that Angptl2 deficiency in endothelial cells attenuated atherosclerosis development. Conversely, ApoE −/− mice crossed with transgenic mice expressing Angptl2 driven by the Tie2 promoter ( ApoE −/− /Tie2- Angptl2 Tg), which drives Angptl2 expression in endothelial cells but not monocytes/macrophages, showed accelerated plaque formation and vascular inflammation because of increased numbers of infiltrated macrophages in atheromatous plaques. Tie2- Angptl2 Tg mice alone did not develop plaques but exhibited endothelium-dependent vasodilatory dysfunction, likely because of decreased production of endothelial cell–derived nitric oxide. Conversely, Angptl2 −/− mice exhibited less severe endothelial dysfunction than did wild-type mice when fed a high-fat diet. In vitro, Angptl2 activated proinflammatory nuclear factor-κB signaling in endothelial cells and increased monocyte/macrophage chemotaxis.

Conclusions—

Endothelial cell–derived Angptl2 accelerates vascular inflammation by activating proinflammatory signaling in endothelial cells and increasing macrophage infiltration, leading to endothelial dysfunction and atherosclerosis progression.

Platelet bioenergetic screen in sickle cell patients reveals mitochondrial complex V inhibition, which contributes to platelet activation

Bioenergetic dysfunction, although central to the pathogenesis of numerous diseases, remains uncharacterized in many patient populations because of the invasiveness of obtaining tissue for mitochondrial studies. Although platelets are an accessible source of mitochondria, the role of bioenergetics in regulating platelet function remains unclear. Herein, we validate extracellular flux analysis in human platelets and use this technique to screen for mitochondrial dysfunction in sickle cell disease (SCD) patients, a population with aberrant platelet activation of an unknown mechanism and in which mitochondrial function has never been assessed. We identify a bioenergetic alteration in SCD patients characterized by deficient complex V activity, leading to decreased mitochondrial respiration, membrane hyperpolarization, and augmented oxidant production compared with healthy subjects. This dysfunction correlates with platelet activation and hemolysis in vivo and can be recapitulated in vitro by exposing healthy platelets to hemoglobin or a complex V inhibitor. Further, reproduction of this dysfunction in vitro activates healthy platelets, an effect prevented by attenuation of mitochondrial hyperpolarization or by scavenging mitochondrial oxidants. These data identify bioenergetic dysfunction in SCD patients for the first time and establish mitochondrial hyperpolarization and oxidant generation as potential pathogenic mechanism in SCD as well as a modulator of healthy platelet function.

Statins as regulators of redox signaling in platelets

SIGNIFICANCE

Reactive oxidant species (ROS) are highly reactive molecules produced by several cell lines including platelets and serve as second messenger for intracellular signaling. In recent years it became evident that ROS are also implicated in the thrombotic process. Statins are lipid lowering molecules which reduce serum cholesterol and retard atherosclerotic complication and its clinical sequelae. However there is evidence that statins may exert an antiplatelet effects by interfering with redox signaling.

RECENT ADVANCES

Experimental and clinical studies provided evidence that intra-platelet ROS formation is implicated in the process of thrombosis, as impaired ROS neutralization is associated with serious thrombotic complication and eventually death. Recent studies demonstrated that statins possess antiplatelet activity via inhibition of platelet NADPH oxidase-derived ROS formation. This effect results in down-regulation of isoprostanes, which are pro-aggregating molecules, and up-regulation of nitric oxide, which is a platelet inhibitor; such changes occurred immediately after statin's administration and were independent from lipid lowering property.

CRITICAL ISSUES

Experimental and clinical studies documented that statins possess an antithrombotic effects which may account for thrombotic-related vascular outcomes. This has been evidenced in clinical settings such as percutaneous coronary intervention, myocardial infarction and venous thrombosis. It is still unclear, however, if the statin's antithrombotic effect is dose-related.

FUTURE DIRECTIONS

Future studies should be addressed to analyze if the antiplatelet effect of statins may preferentially occur at high dosage of statins. Furthermore, the antiplatelet effects of statins could turn useful in clinical settings where the clinical efficacy of aspirin and other antiplatelet drugs are still uncertain.

Effects of downregulation of microRNA-181a on H2O2-induced H9c2 cell apoptosis via the mitochondrial apoptotic pathway

Glutathione peroxidase-1 (GPx1) is a pivotal intracellular antioxidant enzyme that enzymatically reduces hydrogen peroxide to water to limit its harmful effects. This study aims to identify a microRNA (miRNA) that targets GPx1 to maintain redox homeostasis. Dual luciferase assays combined with mutational analysis and immunoblotting were used to validate the bioinformatically predicted miRNAs. We sought to select miRNAs that were responsive to oxidative stress induced by hydrogen peroxide (H₂O₂) in the H9c2 rat cardiomyocyte cell line. Quantitative real-time PCR (qPCR) demonstrated that the expression of miR-181a in H₂O₂-treated H9c2 cells was markedly upregulated. The downregulation of miR-181a significantly inhibited H₂O₂-induced cellular apoptosis, ROS production, the increase in malondialdehyde (MDA) levels, the disruption of mitochondrial structure, and the activation of key signaling proteins in the mitochondrial apoptotic pathway. Our results suggest that miR-181a plays an important role in regulating the mitochondrial apoptotic pathway in cardiomyocytes challenged with oxidative stress. MiR-181a may represent a potential therapeutic target for the treatment of oxidative stress-associated cardiovascular diseases.

Platelet NOX, a novel target for anti-thrombotic treatment

There is a growing body of evidence to suggest that reactive oxidant species (ROS) including O2-, OH- or H2O2 act as second messengers to activate platelets via 1) calcium mobilisation, 2) nitric oxide (NO) inactivation, and 3) interaction with arachidonic to give formation of isoprostanes. Among the enzymes generating ROS formation NOX2, the catalytic core of NADPH oxidase (NOX), plays a prominent role as shown by the almost absent ROS production by platelets taken from patients with hereditary deficiency of NOX2. Experimental and clinical studies provided evidence that NOX2 is implicated in platelet activation. Thus, impaired platelet activation has been detected in patients with NOX2 hereditary deficiency. Similarly, normal platelets added with NOX2 specific inhibitors disclosed impaired platelet activation along with ROS down-regulation. Accordingly, animals prone to atherosclerosis treated with apocynin, a NOX inhibitor, showed reduced platelet adhesion and atherosclerotic plaque. Furthermore, a significant association between NOX2 up-regulation and platelet activation has been detected in patients at athero-thrombotic risk, but a cause-effect relationship needs to be established. These findings may represent a rationale to plan interventional trials with NOX inhibitors to establish if blocking NOX2 or other NOX isoforms may represent a novel anti-platelet approach.