Hypochlorous acid stimulation of the mitogen-activated protein kinase pathway enhances cell survival

Ratiometric fluorescent probe with AIE characteristics for hypochlorite detection and biological imaging

It is very important and highly valuable to detect ClO- in samples and living cells with accuracy and speed. In this work, a novel fluorescent probe NA was prepared from 4-bromo-1,8-naphthalic anhydride by acylation reaction and Suzuki coupling reaction and used for the detection of ClO-. Thiomethyl serves as the recognition group for probe NA, while naphthalimide serves as fluorescent chromophore. The probe exhibited an extremely pronounced blue shift from yellow to blue fluorescence within 1 min after the addition of hypochlorite (ClO-). The probe demonstrates high sensitivity to ClO- with a limit of detection (LOD) of 1.22 µM. Also, probe NA demonstrates excellent selectivity and immunity to interference. Additionally, simple fluorescent test strips containing probe NA were prepared in this study, enabling rapid detection of ClO- in water samples. And NA had been effectively used to image endogenous and exogenous ClO-fluorescence in living cells. The results suggest that probe NA has significant potential for portable detection and biological applications.

Dual-Emissive Detection of ATP and Hypochlorite Ions for Monitoring Inflammation-Driven Liver Injury In Vitro and In Vivo

Reactive oxygen species play a pivotal role in liver disease, contributing to severe liver damage and chronic inflammation. In liver injury driven by inflammation, adenosine-5'-triphosphate (ATP) and hypochlorite ion (ClO-) emerge as novel biomarkers, reflecting mitochondrial dysfunction and amplified oxidative stress, respectively. However, the dynamic fluctuations of ATP and ClO- in hepatocytes and mouse livers remain unclear, and multidetection techniques for these biomarkers are yet to be developed. This study presents RATP-NClO, a dual-channel fluorescent bioprobe capable of synchronously detecting ATP and ClO- ions. RATP-NClO exhibits excellent selectivity and sensitivity for ATP and ClO- ions, demonstrating a dual-channel fluorescence response in a murine hepatocyte cell line. Upon intravenous administration, RATP-NClO reveals synchronized ATP depletion and ClO- amplification in the livers of mice with experimental metabolic dysfunction-associated steatohepatitis (MASH). Through a comprehensive analysis of the principal mechanism of the developed bioprobe and the verification of its reliable detection ability in both in vitro and in vivo settings, we propose it as a unique tool for monitoring changes in intracellular ATP and ClO- level. These findings underscore its potential for practical image-based monitoring and functional phenotyping of MASH pathogenesis.

Myeloperoxidase induces monocyte migration and activation after acute myocardial infarction

Introduction

Myocardial infarction (MI) is a significant contributor to morbidity and mortality worldwide. Many individuals who survive the acute event continue to experience heart failure (HF), with inflammatory and healing processes post-MI playing a pivotal role. Polymorphonuclear neutrophils (PMN) and monocytes infiltrate the infarcted area, where PMN release high amounts of the heme enzyme myeloperoxidase (MPO). MPO has numerous inflammatory properties and MPO plasma levels are correlated with prognosis and severity of MI. While studies have focused on MPO inhibition and controlling PMN infiltration into the infarcted tissue, less is known on MPO's role in monocyte function.

Methods and results

Here, we combined human data with mouse and cell studies to examine the role of MPO on monocyte activation and migration. We revealed a correlation between plasma MPO levels and monocyte activation in a patient study. Using a mouse model of MI, we demonstrated that MPO deficiency led to an increase in splenic monocytes and a decrease in cardiac monocytes compared to wildtype mice (WT). In vitro studies further showed that MPO induces monocyte migration, with upregulation of the chemokine receptor CCR2 and upregulation of inflammatory pathways identified as underlying mechanisms.

Conclusion

Taken together, we identify MPO as a pro-inflammatory mediator of splenic monocyte recruitment and activation post-MI and provide mechanistic insight for novel therapeutic strategies after ischemic injury.

Myeloperoxidase is a critical mediator of anthracycline-induced cardiomyopathy

Cardiotoxicity is a major complication of anthracycline therapy that negatively impacts prognosis. Effective pharmacotherapies for prevention of anthracycline-induced cardiomyopathy (AICM) are currently lacking. Increased plasma levels of the neutrophil-derived enzyme myeloperoxidase (MPO) predict occurrence of AICM in humans. We hypothesized that MPO release causally contributes to AICM. Mice intravenously injected with the anthracycline doxorubicin (DOX) exhibited higher neutrophil counts and MPO levels in the circulation and cardiac tissue compared to saline (NaCl)-treated controls. Neutrophil-like HL-60 cells exhibited increased MPO release upon exposition to DOX. DOX induced extensive nitrosative stress in cardiac tissue alongside with increased carbonylation of sarcomeric proteins in wildtype but not in Mpo-/- mice. Accordingly, co-treatment of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) with DOX and MPO aggravated loss of hiPSC-CM-contractility compared to DOX treatment alone. DOX-treated animals exhibited pronounced cardiac apoptosis and inflammation, which was attenuated in MPO-deficient animals. Finally, genetic MPO deficiency and pharmacological MPO inhibition protected mice from the development of AICM. The anticancer efficacy of DOX was unaffected by MPO deficiency. Herein we identify MPO as a critical mediator of AICM. We demonstrate that DOX induces cardiac neutrophil infiltration and release of MPO, which directly impairs cardiac contractility through promoting oxidation of sarcomeric proteins, cardiac inflammation and cardiomyocyte apoptosis. MPO thus emerges as a promising pharmacological target for prevention of AICM.

Protective Effects of Therapeutic Neutrophil Depletion and Myeloperoxidase Inhibition on Left Ventricular Function and Remodeling in Myocardial Infarction

Myocardial infarction (MI) is a leading cause of morbidity and mortality worldwide. Improved survival has led to an increasing incidence of ischemic cardiomyopathy, making it a major reason for hospitalization in the western world. The inflammatory response in the ischemic myocardium determines the extent of structural remodeling and functional deterioration, with neutrophils (PMN) being a key modulator of the propagation and resolution of inflammation. The heme enzyme myeloperoxidase (MPO) is abundantly expressed in PMN and is an important mediator of their inflammatory capacities. Here, we examine the effects of PMN reduction, MPO deficiency and MPO inhibition in two murine models of MI. Reduction in PMN count resulted in less scar formation and improved cardiac function. Similar results were obtained in genetically MPO deficient mice, suggesting that MPO is a critical factor in PMN-mediated cardiac remodeling. To test our findings in a therapeutic approach, we orally administered the MPO inhibitor AZM198 in the context of MI and could demonstrate improved cardiac function and reduced structural remodeling. Therefore, MPO appears to be a favorable pharmacological target for the prevention of long-term morbidity after MI.

Hypochlorous Acid: From Innate Immune Factor and Environmental Toxicant to Chemopreventive Agent Targeting Solar UV-Induced Skin Cancer

A multitude of extrinsic environmental factors (referred to in their entirety as the 'skin exposome') impact structure and function of skin and its corresponding cellular components. The complex (i.e. additive, antagonistic, or synergistic) interactions between multiple extrinsic (exposome) and intrinsic (biological) factors are important determinants of skin health outcomes. Here, we review the role of hypochlorous acid (HOCl) as an emerging component of the skin exposome serving molecular functions as an innate immune factor, environmental toxicant, and topical chemopreventive agent targeting solar UV-induced skin cancer. HOCl [and its corresponding anion (OCl-; hypochlorite)], a weak halogen-based acid and powerful oxidant, serves two seemingly unrelated molecular roles: (i) as an innate immune factor [acting as a myeloperoxidase (MPO)-derived microbicidal factor] and (ii) as a chemical disinfectant used in freshwater processing on a global scale, both in the context of drinking water safety and recreational freshwater use. Physicochemical properties (including redox potential and photon absorptivity) determine chemical reactivity of HOCl towards select biochemical targets [i.e. proteins (e.g. IKK, GRP78, HSA, Keap1/NRF2), lipids, and nucleic acids], essential to its role in innate immunity, antimicrobial disinfection, and therapeutic anti-inflammatory use. Recent studies have explored the interaction between solar UV and HOCl-related environmental co-exposures identifying a heretofore unrecognized photo-chemopreventive activity of topical HOCl and chlorination stress that blocks tumorigenic inflammatory progression in UV-induced high-risk SKH-1 mouse skin, a finding with potential implications for the prevention of human nonmelanoma skin photocarcinogenesis.

Stamp2 Protects From Maladaptive Structural Remodeling and Systolic Dysfunction in Post-Ischemic Hearts by Attenuating Neutrophil Activation

The six-transmembrane protein of prostate 2 (Stamp2) acts as an anti-inflammatory protein in macrophages by protecting from overt inflammatory signaling and Stamp2 deficiency accelerates atherosclerosis in mice. Herein, we describe an unexpected role of Stamp2 in polymorphonuclear neutrophils (PMN) and characterize Stamp2’s protective effects in myocardial ischemic injury. In a murine model of ischemia and reperfusion (I/R), echocardiography and histological analyses revealed a pronounced impairment of cardiac function in hearts of Stamp2-deficient- (Stamp2^-/-) mice as compared to wild-type (WT) animals. This difference was driven by aggravated cardiac fibrosis, as augmented fibroblast-to-myofibroblast transdifferentiation was observed which was mediated by activation of the redox-sensitive p38 mitogen-activated protein kinase (p38 MAPK). Furthermore, we observed increased production of reactive oxygen species (ROS) in Stamp2^-/- hearts after I/R, which is the likely cause for p38 MAPK activation. Although myocardial macrophage numbers were not affected by Stamp2 deficiency after I/R, augmented myocardial infiltration by polymorphonuclear neutrophils (PMN) was observed, which coincided with enhanced myeloperoxidase (MPO) plasma levels. Primary PMN isolated from Stamp2^-/- animals exhibited a proinflammatory phenotype characterized by enhanced nuclear factor (NF)-κB activity and MPO secretion. To prove the critical role of PMN for the observed phenotype after I/R, antibody-mediated PMN depletion was performed in Stamp2^-/- mice which reduced deterioration of LV function and adverse structural remodeling to WT levels. These data indicate a novel role of Stamp2 as an anti-inflammatory regulator of PMN and fibroblast-to-myofibroblast transdifferentiation in myocardial I/R injury.

Halogen-Induced Chemical Injury to the Mammalian Cardiopulmonary Systems

The halogens chlorine (Cl₂) and bromine (Br₂) are highly reactive oxidizing elements with widespread industrial applications and a history of development and use as chemical weapons. When inhaled, depending on the dose and duration of exposure, they cause acute and chronic injury to both the lungs and systemic organs that may result in the development of chronic changes (such as fibrosis) and death from cardiopulmonary failure. A number of conditions, such as viral infections, coexposure to other toxic gases, and pregnancy increase susceptibility to halogens significantly. Herein we review their danger to public health, their mechanisms of action, and the development of pharmacological agents that when administered post-exposure decrease morbidity and mortality.

Role of myeloperoxidase and oxidant formation in the extracellular environment in inflammation-induced tissue damage

The heme peroxidase family generates a battery of oxidants both for synthetic purposes, and in the innate immune defence against pathogens. Myeloperoxidase (MPO) is the most promiscuous family member, generating powerful oxidizing species including hypochlorous acid (HOCl). Whilst HOCl formation is important in pathogen removal, this species is also implicated in host tissue damage and multiple inflammatory diseases. Significant oxidant formation and damage occurs extracellularly as a result of MPO release via phagolysosomal leakage, cell lysis, extracellular trap formation, and inappropriate trafficking. MPO binds strongly to extracellular biomolecules including polyanionic glycosaminoglycans, proteoglycans, proteins, and DNA. This localizes MPO and subsequent damage, at least partly, to specific sites and species, including extracellular matrix (ECM) components and plasma proteins/lipoproteins. Biopolymer-bound MPO retains, or has enhanced, catalytic activity, though evidence is also available for non-catalytic effects. These interactions, particularly at cell surfaces and with the ECM/glycocalyx induce cellular dysfunction and altered gene expression. MPO binds with higher affinity to some damaged ECM components, rationalizing its accumulation at sites of inflammation. MPO-damaged biomolecules and fragments act as chemo-attractants and cell activators, and can modulate gene and protein expression in naïve cells, consistent with an increasing cycle of MPO adhesion, activity, damage, and altered cell function at sites of leukocyte infiltration and activation, with subsequent tissue damage and dysfunction. MPO levels are used clinically both diagnostically and prognostically, and there is increasing interest in strategies to prevent MPO-mediated damage; therapeutic aspects are not discussed as these have been reviewed elsewhere.

The effects of neutrophil-generated hypochlorous acid and other hypohalous acids on host and pathogens

Neutrophils are predominant immune cells that protect the human body against infections by deploying sophisticated antimicrobial strategies including phagocytosis of bacteria and neutrophil extracellular trap (NET) formation. Here, we provide an overview of the mechanisms by which neutrophils kill exogenous pathogens before we focus on one particular weapon in their arsenal: the generation of the oxidizing hypohalous acids HOCl, HOBr and HOSCN during the so-called oxidative burst by the enzyme myeloperoxidase. We look at the effects of these hypohalous acids on biological systems in general and proteins in particular and turn our attention to bacterial strategies to survive HOCl stress. HOCl is a strong inducer of protein aggregation, which bacteria can counteract by chaperone-like holdases that bind unfolding proteins without the need for energy in the form of ATP. These chaperones are activated by HOCl through thiol oxidation (Hsp33) or N-chlorination of basic amino acid side-chains (RidA and CnoX) and contribute to bacterial survival during HOCl stress. However, neutrophil-generated hypohalous acids also affect the host system. Recent studies have shown that plasma proteins act not only as sinks for HOCl, but get actively transformed into modulators of the cellular immune response through N-chlorination. N-chlorinated serum albumin can prevent aggregation of proteins, stimulate immune cells, and act as a pro-survival factor for immune cells in the presence of cytotoxic antigens. Finally, we take a look at the emerging role of HOCl as a potential signaling molecule, particularly its role in neutrophil extracellular trap formation.

Myeloperoxidase: A versatile mediator of endothelial dysfunction and therapeutic target during cardiovascular disease

Myeloperoxidase (MPO) is a prominent mammalian heme peroxidase and a fundamental component of the innate immune response against microbial pathogens. In recent times, MPO has received considerable attention as a key oxidative enzyme capable of impairing the bioactivity of nitric oxide (NO) and promoting endothelial dysfunction; a clinically relevant event that manifests throughout the development of inflammatory cardiovascular disease. Increasing evidence indicates that during cardiovascular disease, MPO is released intravascularly by activated leukocytes resulting in its transport and sequestration within the vascular endothelium. At this site, MPO catalyzes various oxidative reactions that are capable of promoting vascular inflammation and impairing NO bioactivity and endothelial function. In particular, MPO catalyzes the production of the potent oxidant hypochlorous acid (HOCl) and the catalytic consumption of NO via the enzyme's NO oxidase activity. An emerging paradigm is the ability of MPO to also influence endothelial function via non-catalytic, cytokine-like activities. In this review article we discuss the implications of our increasing knowledge of the versatility of MPO's actions as a mediator of cardiovascular disease and endothelial dysfunction for the development of new pharmacological agents capable of effectively combating MPO's pathogenic activities. More specifically, we will (i) discuss the various transport mechanisms by which MPO accumulates into the endothelium of inflamed or diseased arteries, (ii) detail the clinical and basic scientific evidence identifying MPO as a significant cause of endothelial dysfunction and cardiovascular disease, (iii) provide an up-to-date coverage on the different oxidative mechanisms by which MPO can impair endothelial function during cardiovascular disease including an evaluation of the contributions of MPO-catalyzed HOCl production and NO oxidation, and (iv) outline the novel non-enzymatic mechanisms of MPO and their potential contribution to endothelial dysfunction. Finally, we deliver a detailed appraisal of the different pharmacological strategies available for targeting the catalytic and non-catalytic modes-of-action of MPO in order to protect against endothelial dysfunction in cardiovascular disease.

Nitroxides Mitigate Neutrophil-Mediated Damage to the Myocardium after Experimental Myocardial Infarction in Rats

Reperfusion therapy increases survival post-acute myocardial infarction (AMI) while also stimulating secondary oxidant production and immune cell infiltration. Neutrophils accumulate within infarcted myocardium within 24 h post-AMI and release myeloperoxidase (MPO) that catalyses hypochlorous acid (HOCl) production while increasing oxidative stress and inflammation, thereby enhancing ventricular remodelling. Nitroxides inhibit MPO-mediated HOCl production, potentially ameliorating neutrophil-mediated damage. Aim: Assess the cardioprotective ability of nitroxide 4-methoxyTEMPO (4MetT) within the setting of AMI. Methods: Male Wistar rats were separated into 3 groups: SHAM, AMI/R, and AMI/R + 4MetT (15 mg/kg at surgery via oral gavage) and subjected to left descending coronary artery ligation for 30 min to generate an AMI, followed by reperfusion. One cohort of rats were sacrificed at 24 h post-reperfusion and another 28 days post-surgery (with 4MetT (15 mg/kg) administration twice daily). Results: 3-chlorotyrosine, a HOCl-specific damage marker, decreased within the heart of animals in the AMI/R + 4-MetT group 24 h post-AMI, indicating the drug inhibited MPO activity; however, there was no evident difference in either infarct size or myocardial scar size between the groups. Concurrently, MPO, NfκB, TNFα, and the oxidation marker malondialdehyde increased within the hearts, with 4-MetT only demonstrating a trend in decreasing MPO and TNF levels. Notably, 4MetT provided a significant improvement in cardiac function 28 days post-AMI, as assessed by echocardiography, indicating potential for 4-MetT as a treatment option, although the precise mechanism of action of the compound remains unclear.

Redox regulation in host-pathogen interactions: thiol switches and beyond

Our organism is exposed to pathogens on a daily basis. Owing to this age-old interaction, both pathogen and host evolved strategies to cope with these encounters. Here, we focus on the consequences of the direct encounter of cells of the innate immune system with bacteria. First, we will discuss the bacterial strategies to counteract powerful reactive species. Our emphasis lies on the effects of hypochlorous acid (HOCl), arguably the most powerful oxidant produced inside the phagolysosome of professional phagocytes. We will highlight individual examples of proteins in gram-negative bacteria activated by HOCl via thiol-disulfide switches, methionine sulfoxidation, and N-chlorination of basic amino acid side chains. Second, we will discuss the effects of HOCl on proteins of the host. Recent studies have shown that both host and bacteria address failing protein homeostasis by activation of chaperone-like holdases through N-chlorination. After discussing the role of individual proteins in the HOCl-defense, we will turn our attention to the examination of effects on host and pathogen on a systemic level. Recent studies using genetically encoded redox probes and redox proteomics highlight differences in redox homeostasis in host and pathogen and give first hints at potential cellular HOCl signaling beyond thiol-disulfide switch mechanisms.

Neutrophil-Mediated Cardiac Damage After Acute Myocardial Infarction: Significance of Defining a New Target Cell Type for Developing Cardioprotective Drugs

Significance: Acute myocardial infarction (AMI) is a leading cause of death worldwide. Post-AMI survival rates have increased with the introduction of angioplasty as a primary coronary intervention. However, reperfusion after angioplasty represents a clinical paradox, restoring blood flow to the ischemic myocardium while simultaneously inducing ion and metabolic imbalances that stimulate immune cell recruitment and activation, mitochondrial dysfunction and damaging oxidant production. Recent Advances: Preclinical data indicate that these metabolic imbalances contribute to subsequent heart failure through sustaining local recruitment of inflammatory leukocytes and oxidative stress, cardiomyocyte death, and coronary microvascular disturbances, which enhance adverse cardiac remodeling. Both left ventricular dysfunction and heart failure are strongly linked to inflammation and immune cell recruitment to the damaged myocardium. Critical Issues: Overall, therapeutic anti-inflammatory and antioxidant agents identified in preclinical trials have failed in clinical trials. Future Directions: The versatile neutrophil-derived heme enzyme, myeloperoxidase (MPO), is gaining attention as an important oxidative mediator of reperfusion injury, vascular dysfunction, adverse ventricular remodeling, and atrial fibrillation. Accordingly, there is interest in therapeutically targeting neutrophils and MPO activity in the setting of heart failure. Herein, we discuss the role of post-AMI inflammation linked to myocardial damage and heart failure, describe previous trials targeting inflammation and oxidative stress post-AMI, highlight the potential adverse impact of neutrophil and MPO, and detail therapeutic options available to target MPO clinically in AMI patients.

The Role of Myeloperoxidase in Biomolecule Modification, Chronic Inflammation, and Disease

Significance: The release of myeloperoxidase (MPO) by activated leukocytes is critical in innate immune responses. MPO produces hypochlorous acid (HOCl) and other strong oxidants, which kill bacteria and other invading pathogens. However, MPO also drives the development of numerous chronic inflammatory pathologies, including atherosclerosis, neurodegenerative disease, lung disease, arthritis, cancer, and kidney disease, which are globally responsible for significant patient mortality and morbidity. Recent Advances: The development of imaging approaches to precisely identify the localization of MPO and the molecular targets of HOCl in vivo is an important advance, as typically the involvement of MPO in inflammatory disease has been inferred by its presence, together with the detection of biomarkers of HOCl, in biological fluids or diseased tissues. This will provide valuable information in regard to the cell types responsible for releasing MPO in vivo, together with new insight into potential therapeutic opportunities. Critical Issues: Although there is little doubt as to the value of MPO inhibition as a protective strategy to mitigate tissue damage during chronic inflammation in experimental models, the impact of long-term inhibition of MPO as a therapeutic strategy for human disease remains uncertain, in light of the potential effects on innate immunity. Future Directions: The development of more targeted MPO inhibitors or a treatment regimen designed to reduce MPO-associated host tissue damage without compromising pathogen killing by the innate immune system is therefore an important future direction. Similarly, a partial MPO inhibition strategy may be sufficient to maintain adequate bacterial activity while decreasing the propagation of inflammatory pathologies.

Halogen Inhalation-Induced Lung Injury and Acute Respiratory Distress Syndrome

OBJECTIVE

Exposure to halogens, such as chlorine or bromine, results in environmental and occupational hazard to the lung and other organs. Chlorine is highly toxic by inhalation, leading to dyspnea, hypoxemia, airway obstruction, pneumonitis, pulmonary edema, and acute respiratory distress syndrome (ARDS). Although bromine is less reactive and oxidative than chlorine, inhalation also results in bronchospasm, airway hyperresponsiveness, ARDS, and even death. Both halogens have been shown to damage the systemic circulation and result in cardiac injury as well. There is no specific antidote for these injuries since the mechanisms are largely unknown.

DATA SOURCES

This review was based on articles published in PubMed databases up to January, 2018, with the following keywords: "chlorine," "bromine," "lung injury," and "ARDS."

STUDY SELECTION

The original articles and reviews including the topics were the primary references.

RESULTS

Based on animal studies, it is found that inhaled chlorine will form chlorine-derived oxidative products that mediate postexposure toxicity; thus, potential treatments will target the oxidative stress and inflammation induced by chlorine. Antioxidants, cAMP-elevating agents, anti-inflammatory agents, nitric oxide-modulating agents, and high-molecular-weight hyaluronan have shown promising effects in treating acute chlorine injury. Elevated free heme level is involved in acute lung injury caused by bromine inhalation. Hemopexin, a heme-scavenging protein, when administered postexposure, decreases lung injury and improves survival.

CONCLUSIONS

At present, there is an urgent need for additional research to develop specific therapies that target the basic mechanisms by which halogens damage the lungs and systemic organs.

Myeloperoxidase and Neurological Disorder: A Crosstalk

Myeloperoxidase (MPO) is a protein present in azurophilic granules, macrophages, and neutrophils that are released into extracellular fluid (ECF) during inflammation. MPO releases hypochlorous acid (HOCl) and other chlorinated species. It is derived from hydrogen peroxide (H₂O₂) showing response during inflammatory conditions and plays a role in the immune defense against pathogens. MPO may show unwanted effects by indirectly increasing the formation of reactive nitrogen species (RNS), reactive oxygen species (ROS), and tumor necrosis factor alpha (TNF-α) leading to inflammation and oxidative stress. As neuroinflammation is one of the inevitable biological components among most of neurological disorders, MPO and its receptor may be explored as candidates for future clinical interventions. The purpose of this review is to provide an overview of the pathophysiological characteristics of MPO and further explore the possibilities to target it for clinical use. Targeting MPO is promising and may open an avenue to act as a biomarker for diagnosis with defined risk stratification in patients with various neurological disorders.

Myeloperoxidase Mediates Postischemic Arrhythmogenic Ventricular Remodeling

RATIONALE

Ventricular arrhythmias remain the leading cause of death in patients suffering myocardial ischemia. Myeloperoxidase, a heme enzyme released by polymorphonuclear neutrophils, accumulates within ischemic myocardium and has been linked to adverse left ventricular remodeling.

OBJECTIVE

To reveal the role of myeloperoxidase for the development of ventricular arrhythmias.

METHODS AND RESULTS

In different murine models of myocardial ischemia, myeloperoxidase deficiency profoundly decreased vulnerability for ventricular tachycardia on programmed right ventricular and burst stimulation and spontaneously as assessed by ECG telemetry after isoproterenol injection. Experiments using CD11b/CD18 integrin-deficient (CD11b-/-) mice and intravenous myeloperoxidase infusion revealed that neutrophil infiltration is a prerequisite for myocardial myeloperoxidase accumulation. Ventricles from myeloperoxidase-deficient (Mpo-/-) mice showed less pronounced slowing and decreased heterogeneity of electric conduction in the peri-infarct zone than wild-type mice. Expression of the redox-sensitive gap junctional protein Cx43 (Connexin 43) was reduced in the peri-infarct area of wild-type compared with Mpo-/- mice. In isolated wild-type cardiomyocytes, Cx43 protein content decreased on myeloperoxidase/H2O2 incubation. Mapping of induced pluripotent stem cell-derived cardiomyocyte networks and in vivo investigations linked Cx43 breakdown to myeloperoxidase-dependent activation of matrix metalloproteinase 7. Moreover, Mpo-/- mice showed decreased ventricular postischemic fibrosis reflecting reduced accumulation of myofibroblasts. Ex vivo, myeloperoxidase was demonstrated to induce fibroblast-to-myofibroblast transdifferentiation by activation of p38 mitogen-activated protein kinases resulting in upregulated collagen generation. In support of our experimental findings, baseline myeloperoxidase plasma levels were independently associated with a history of ventricular arrhythmias, sudden cardiac death, or implantable cardioverter-defibrillator implantation in a cohort of 2622 stable patients with an ejection fraction >35% undergoing elective diagnostic cardiac evaluation.

CONCLUSIONS

Myeloperoxidase emerges as a crucial mediator of postischemic myocardial remodeling and may evolve as a novel pharmacological target for secondary disease prevention after myocardial ischemia.

Hypochlorous Acid and Taurine-N-Monochloramine in Periodontal Diseases

Chronic periodontitis is a multi-factorial disease involving anaerobic bacteria and the generation of an inflammatory response, including the production of metalloproteinases, pro-inflammatory cytokines, and eicosanoids. Hypochlorous acid (HOCl) and taurine-N-monochloramine (TauCl) are the end-products of the neutrophilic polymorphonuclear leukocyte (PMN) respiratory burst. They act synergistically to modulate the inflammatory response. In the extracellular environment, HOCl and TauCl may directly neutralize interleukin 6 (IL-6) and several metalloproteinases, while HOCl increases the capacity of α2-macroglobulin to bind Tumor Necrosis Factor-alpha, IL-2, and IL-6, and facilitates the release of various growth factors. TauCl inhibits the production of inflammatory mediators, prostaglandins, and nitric oxide. HOCl activates tyrosine kinase signaling cascades, generating an increase in the production of extracellular matrix components, growth factors, and inflammatory mediators. Thus, HOCl and TauCl appear to play a crucial role in the periodontal inflammatory process. Taken together, these findings may offer opportunities for the development of novel host-modulating therapies for the treatment of periodontitis.

Myeloperoxidase: Bridging the gap in neurodegeneration

Neurodegenerative conditions present a group of complex disease pathologies mostly due to unknown aetiology resulting in neuronal death and permanent neurological disability. Any undesirable stress to the brain, disrupts homeostatic balance, through a remarkable convergence of pathophysiological changes and immune dysregulation. The crosstalk between inflammatory and oxidative mechanisms results in the release of neurotoxic mediators apparently spearheaded by myeloperoxidase derived from activated microglia, astrocytes, neurons as well as peripheral inflammatory cells. These isolated entities combinedly have the potential to flare up and contribute significantly to neuropathology and disease progression. Recent, clinicopathological evidence support the association of myeloperoxidase and its cytotoxic product, hypochlorous acid in a plethora of neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, Multiple sclerosis, Stroke, Epilepsy etc. But the biochemical and mechanistic insights into myeloperoxidase mediated neuroinflammation and neuronal death is still an uncharted territory. The current review outlines the emerging recognition of myeloperoxidase in neurodegeneration, which may offer novel therapeutic and diagnostic targets for neurodegenerative disorders.

Chlorine-induced cardiopulmonary injury

Chlorine (Cl2 ) is utilized worldwide for a diverse range of industrial applications, including pulp bleaching, sanitation, and pharmaceutical development. Though Cl2 has widespread use, little is known regarding the mechanisms of toxicity associated with Cl2 exposure, which occurs during industrial accidents or acts of terrorism. Previous instances of Cl2 exposure have led to reported episodes of respiratory distress that result in high morbidity and mortality. Furthermore, studies suggest that acute Cl2 exposure also results in systemic vascular injury and subsequent myocardial contractile dysfunction. Here, we review both lung and cardiac pathology associated with acute Cl2 inhalation and discuss recently published data that suggest that mitochondrial dysfunction underlies the pathogenesis of Cl2 -induced toxicity. Last, we discuss our findings that suggest that upregulation of autophagy protects against Cl2 -induced lung inflammation and can be a potential therapeutic target for ameliorating the toxic effects of Cl2 exposure.

Comparative reactivity of myeloperoxidase-derived oxidants with mammalian cells

Myeloperoxidase is an important heme enzyme released by activated leukocytes that catalyzes the reaction of hydrogen peroxide with halide and pseudo-halide ions to form various hypohalous acids. Hypohalous acids are chemical oxidants that have potent antibacterial, antiviral, and antifungal properties and, as such, play key roles in the human immune system. However, increasing evidence supports an alternative role for myeloperoxidase-derived oxidants in the development of disease. Excessive production of hypohalous acids, particularly during chronic inflammation, leads to the initiation and accumulation of cellular damage that has been implicated in many human pathologies including atherosclerosis, neurodegenerative disease, lung disease, arthritis, inflammatory cancers, and kidney disease. This has sparked a significant interest in developing a greater understanding of the mechanisms involved in myeloperoxidase-derived oxidant-induced mammalian cell damage. This article reviews recent developments in our understanding of the cellular reactivity of hypochlorous acid, hypobromous acid, and hypothiocyanous acid, the major oxidants produced by myeloperoxidase under physiological conditions.

Angiotensin-converting-enzyme inhibition counteracts angiotensin II-mediated endothelial cell dysfunction by modulating the p38/SirT1 axis

OBJECTIVE

Oxidative stress has been linked to endothelial dysfunction and angiotensin II stimulates the reactive oxygen species production contributing to several cardiovascular diseases. We have studied the chain of events induced by angiotensin-converting-enzyme (ACE) activation in vascular umbilical vein endothelial cells (HUVECs) by using an ACE inhibitor such as zofenoprilat.

METHODS

We used specific assay to measure the superoxide anion production, tetrazolium bromide (MTT) assay for cell viability, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL) assay for cell apoptosis, and western blot for protein analysis in the study.

RESULTS

Zofenoprilat counteracts the superoxide anion production and cell apoptosis induced by angiotensin I treatment by blocking the extrinsic caspase cascade, NF-kB and p38 activation. p38 inhibitor SB203580 reverted the angiotensin II oxidant effects while the p38 constitutively activation, by MKK6 transfection, abrogated the zofenoprilat effects. Characterizing the zofenoprilat downstream effector we found that zofenoprilat reverted the SirT-1 downregulation induced by angiotensin II. p38 activation by angiotensin II was strictly correlated with SirT1 protein downregulation; SB203580 significantly prevented SirT1 downregulation induced by angiotensin II while the p38 constitutive activation abolished SIRT1 protein basal levels. p38 directly bound SirT1 sequestering it in the cytoplasm. SirT1 inhibition by sirtinol annulled zofenoprilat action while SirT1 overexpression reverted the cytotoxic effects of angiotensin II. Finally, zofenoprilat negatively controlled angiotensin I receptor protein expression through SirT1.

CONCLUSION

The p38-SirT1 axis is found markedly relevant in modulating the cardiovascular benefit deriving from ACE-inhibitors and might represent a novel target for innovative drugs in cardiovascular prevention.

Advances in the Pathogenesis of Adhesion Development: The Role of Oxidative Stress

Over the past several years, there has been increasing recognition that pathogenesis of adhesion development includes significant contributions of hypoxia induced at the site of surgery, the resulting oxidative stress, and the subsequent free radical production. Mitochondrial dysfunction generated by surgically induced tissue hypoxia and inflammation can lead to the production of reactive oxygen and nitrogen species as well as antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase which when optimal have the potential to abrogate mitochondrial dysfunction and oxidative stress, preventing the cascade of events leading to the development of adhesions in injured peritoneum. There is a significant cross talk between the several processes leading to whether or not adhesions would eventually develop. Several of these processes present avenues for the development of measures that can help in abrogating adhesion formation or reformation after intraabdominal surgery.

Vascular VPO1 expression is related to the endothelial dysfunction in spontaneously hypertensive rats

Reactive oxygen species (ROS) contributes to endothelial dysfunction that is involved in the pathogeneses of hypertension. Vascular peroxidase 1 (VPO1) can utilize ROS to catalyze peroxidative reactions, possibly enhancing endothelial dysfunction. This study is to identify VPO1's involvement in endothelial dysfunction and hypertension. Sixty-four spontaneously hypertensive rats (SHRs) and 64 age-matched, bodyweight controlled normotensive Wistar-Kyoto rats (WKYs) were randomly grouped and studied at the age of 5, 8, 13 and 20 weeks (16 animals, each). Blood pressure and vasodilator responses to acetylcholine in aortic rings were observed. The expressions of VPO1 and endothelial NO synthase (eNOS) in aortas were assessed by quantitative reverse transcription-PCR and western blotting analysis. Plasma concentrations of hydrogen peroxide (H2O2) and NO, NOX activity, hypochlorous acid (HOCl) production, and 3-nitrotyrosine content in aortic homogenates were also determined in this study. Along with the development of hypertension in SHR rats, VPO1 expression was up-regulated together with a significant increase in NOX activity, HOCl production, 3-nitrotyrosine content, and plasma H2O2 level compared with WKYs at 8, 13 and 20 weeks of age. In contrast, blood NO levels were decreased and aortic relaxation to acetylcholine was deteriorated in SHRs. The over-expression of VPO1 during the development of hypertension, accompanied by the endothelial dysfunction, the decreased NO levels, the elevated NOX and ROS activities, indicates a clear connection between VPO1 gene and hypertension. VPO1 may pathogenetically contribute to hypertension via signal pathways involving NOX-H2O2-VPO1-HOCl or JNK/p38 MAPK although further studies are needed to determine the precise mechanisms.

Myeloperoxidase: a leukocyte-derived protagonist of inflammation and cardiovascular disease

SIGNIFICANCE

The heme-enzyme myeloperoxidase (MPO) is one of the major neutrophil bactericidal proteins and is stored in large amounts inside azurophilic granules of neutrophils. Upon cell activation, MPO is released and extracellular MPO has been detected in a wide range of acute and chronic inflammatory conditions. Recent ADVANCES AND CRITICAL ISSUES: Apart from its role during infection, MPO has emerged as a critical modulator of inflammation throughout the last decade and is currently discussed in the initiation and propagation of cardiovascular diseases. MPO-derived oxidants (e.g., hypochlorous acid) interfere with various cell functions and contribute to tissue injury. Recent data also suggest that MPO itself exerts proinflammatory properties independent of its catalytic activity. Despite advances in unraveling the complex action of MPO and MPO-derived oxidants, further research is warranted to determine the precise nature and biological role of MPO in inflammation.

FUTURE DIRECTIONS

The identification of MPO as a central player in inflammation renders this enzyme an attractive prognostic biomarker and a potential target for therapeutic interventions. A better understanding of the (patho-) physiology of MPO is essential for the development of successful treatment strategies in acute and chronic inflammatory diseases.

Vitamin D protects human endothelial cells from H₂O₂ oxidant injury through the Mek/Erk-Sirt1 axis activation

Endothelium homeostasis alterations govern the pathogenesis of cardiovascular diseases. Several studies show that vitamins anti-oxidant proprieties rescue the endothelial functions adversely affected by oxidative stress in several diseases. We investigated the vitamin D anti-oxidant potential in human endothelial cells exposed to H2O2 oxidative stress. Vitamin D protected endothelial cells against H2O2 oxidative stress counteracting the superoxide anion generation, the apoptosis and blocking the extrinsic caspase cascade by positively controlling phospho-active ERKs level. MEKs/ERKs inhibitor U0126 reverted the vitamin D anti-oxidant effects. Characterizing the vitamin D downstream effector, we found that vitamin D up-regulated SirT-1 and reverted the SirT-1 down-regulation induced by H2O2. ERKs activation by vitamin D strictly correlated with SirT-1 protein accumulation since both MEKs/ERKs inhibition and ERK1/2 silencing decreased SIRT-1. SirT-1 inhibition by Sirtinol reverted the vitamin D anti-oxidant effects. Thus, vitamin D significantly reduced the endothelial malfunction and damage caused by oxidative stress, through the activation of MEKs/ERKs/SirT-1 axis.

Fibrosis in Atrial Fibrillation - Role of Reactive Species and MPO

Atrial fibrosis with enhanced turnover and deposition of matrix proteins leads to inhomogeneous atrial electrical conduction and gives rise to electrical reentry circuits resulting in atrial fibrillation. The multifactorial pathogenesis of atrial fibrosis involves resident cardiac cells as well as infiltrating leukocytes, both generating and sequestering matrix metalloproteinases (MMPs), a key enzyme family involved in fibrosis. A growing body of evidence points toward an important role of reactive oxygen species (ROS) in the release and activation of pro-MMPs and the stimulation of pro-fibrotic cascades. Myeloperoxidase (MPO), a bactericidal enzyme released from activated polymorphonuclear neutrophils (PMN) is not only associated with a variety of cardiovascular diseases, but has also been shown to be mechanistically linked to atrial fibrosis and fibrillation. MPO catalyzes the generation of reactive species like hypochlorous acid, which affect intracellular signaling cascades in various cells and advance activation of pro-MMPs and deposition of atrial collagen resulting in atrial arrhythmias. Thus, inflammatory mechanisms effectively promote atrial structural remodeling and importantly contribute to the initiation and perpetuation of atrial fibrillation.

The human lactoferrin-derived peptide hLF1-11 exerts immunomodulatory effects by specific inhibition of myeloperoxidase activity

Because of their ability to eliminate pathogens and to modulate various host immune responses, antimicrobial peptides are considered as candidate agents to fight infections by (antibiotic-resistant) pathogens. We recently reported that hLF1-11 (GRRRRSVQWCA), an antimicrobial peptide derived from the N terminus of human lactoferrin, displays diverse modulatory activities on monocytes, thereby enhancing their actions in innate immune responses. The aim of this study was to identify the cellular target of hLF1-11 that mediates these effects. Results revealed that hLF1-11 binds and subsequently penetrates human monocytes, after which it inhibits the enzymatic activities of myeloperoxidase (MPO). Moreover, a chemical inhibitor of MPO (aminobenzoic acid hydrazide) mimicked the effects of hLF1-11 on the inflammatory response by monocytes and on monocyte-macrophage differentiation. Computer-assisted molecular modeling predicted that hLF1-11 can bind to the edge of and within the crevice of the active site of MPO. Experiments with a set of hLF1-11 peptides with amino acid substitutions identified the stretch of arginines and the cysteine at position 10 as pivotal in these immunomodulatory properties of hLF1-11. We conclude that hLF1-11 may exert its modulatory effects on human monocytes by specific inhibition of MPO activity.

Involvement of vascular peroxidase 1 in angiotensin II-induced vascular smooth muscle cell proliferation

AIMS

Vascular peroxidase 1 (VPO1) is a newly identified haem-containing peroxidase that catalyses the oxidation of a variety of substrates by hydrogen peroxide (H(2)O(2)). Considering the well-defined effects of H(2)O(2) on the vascular remodelling during hypertension, and that VPO1 can utilize H(2)O(2) generated from co-expressed NADPH oxidases to catalyse peroxidative reactions, the aims of this study were to determine the potential role of VPO1 in vascular remodelling during hypertension.

METHODS AND RESULTS

The vascular morphology and the expression of VPO1 in arterial tissues of spontaneously hypertensive rats and Wistar-Kyoto rats were assessed. The VPO1 expression was significantly increased concomitantly with definite vascular remodelling assessed by evaluating the media thickness, lumen diameter, media thickness-to-lumen diameter ratio and mean nuclear area in artery media in spontaneously hypertensive rats. In addition, in cultured rat aortic smooth muscle cells we found that the angiotensin II-mediated cell proliferation was inhibited by knockdown of VPO1 using small hairpin RNA. Moreover, the NADPH oxidase inhibitor, apocynin, and the hydrogen peroxide scavenger, catalase, but not the ERK1/2 inhibitor, PD98059, attenuated angiotensin II-mediated up-regulation of VPO1 and generation of hypochlorous acid.

CONCLUSION

VPO1 is a novel regulator of vascular smooth muscle cell proliferation via NADPH oxidase-H(2)O(2)-VPO1-hypochlorous acid-ERK1/2 pathways, which may contribute to vascular remodelling in hypertension.

Ascorbate and deferoxamine administration after chlorine exposure decrease mortality and lung injury in mice

Chlorine (Cl(2)) gas exposure poses an environmental and occupational hazard that frequently results in acute lung injury. There is no effective treatment. We assessed the efficacy of antioxidants, administered after exposure, in decreasing mortality and lung injury in C57BL/6 mice exposed to 600 ppm of Cl(2) for 45 minutes and returned to room air. Ascorbate and deferoxamine were administered intramuscularly every 12 hours and by nose-only inhalation every 24 hours for 3 days starting after 1 hour after exposure. Control mice were exposed to Cl(2) and treated with vehicle (saline or water). Mortality was reduced fourfold in the treatment group compared with the control group (22 versus 78%; P = 0.007). Surviving animals in the treatment group had significantly lower protein concentrations, cell counts, and epithelial cells in their bronchoalveolar lavage (BAL). Lung tissue ascorbate correlated inversely with BAL protein as well as with the number of neutrophils and epithelial cells. In addition, lipid peroxidation was reduced threefold in the BAL of mice treated with ascorbate and deferoxamine when compared with the control group. Administration of ascorbate and deferoxamine reduces mortality and decreases lung injury through reduction of alveolar-capillary permeability, inflammation, and epithelial sloughing and lipid peroxidation.

Mechanisms and modification of chlorine-induced lung injury in animals

Chlorine (Cl(2)) is a reactive oxidant gas used extensively in industrial processes. Exposure of both humans and animals to high concentrations of Cl(2) results in acute lung injury, which may resolve spontaneously or progress to acute respiratory failure. Injury to airway and alveolar epithelium may result from chemical reactions of Cl(2), from HOCl (the hydrolysis product of Cl(2)), and/or from the various reaction products, such as chloramines, that are formed from the reactions of these chlorinating species with biological molecules. Subsequent reactions may initiate self-propagating reactions and induce the production of inflammatory mediators compounding injury to pulmonary surfactant, ion channels, and components of lung epithelial and airway cells. Low-molecular-weight antioxidants, such as ascorbate, glutathione, and urate, present in the lung epithelial lining fluid and tissue, remove Cl(2) and HOCl and thus decrease injury to critical target biological targets. However, levels of lung antioxidants of animals exposed to Cl(2) in concentrations likely to be encountered in the vicinity of industrial accidents decrease rapidly and irreversibly. Our measurements show that prophylactic administration of a mixture containing ascorbate and desferal N-acetyl-cysteine, a precursor of reduced glutathione, prevents Cl(2)-induced injury to the alveolar epithelium of rats exposed to Cl(2). The clinical challenge is to deliver sufficient quantities of antioxidants noninvasively, after Cl(2) exposure, to decrease morbidity and mortality.

Reactive oxygen species and alpha,beta-unsaturated aldehydes as second messengers in signal transduction

Signaling by H(2)O(2), alpha,beta-unsaturated aldehydes, such as 4-hydroxy-2-nonenal (HNE) and related chemical species, is thought to differ from signaling by other second messengers because the oxidants and other electrophiles can readily undergo nonenzymatic reactions and are therefore classified as "reactive." This brief review will describe how and when the chemistry of signaling is similar or differs from classic second messengers, such as cyclic AMP, or posttranslational signaling, such as farnesylation or ubiquitination. The chemistry of cysteine provides a common factor that underlies signaling by H(2)O(2) and HNE. Nonetheless, as H(2)O(2) and HNE are rapidly metabolized in vivo, spatial considerations are extremely important in their actions. Therefore, the locations of sources of H(2)O(2) and alpha,beta-unsaturated aldehydes, the NADPH oxidases, mitochondria, membrane lipids, and redox cycling toxicants, as well as their targets, are key factors. The activation of the JNK pathway by HNE and endogenously generated H(2)O(2) illustrates these principles.

Chronic ethanol feeding potentiates Fas Jo2-induced hepatotoxicity: role of CYP2E1 and TNF-alpha and activation of JNK and P38 MAP kinase

We have previously shown that treatment of mice with pyrazole or acute ethanol potentiated Fas agonistic Jo2 antibody-induced liver injury by a mechanism involving induction of CYP2E1 and elevated oxidative stress. The current study evaluated whether chronic alcohol feeding potentiates Fas-induced liver injury and whether CYP2E1 plays a role in any enhanced hepatotoxicity. Wild-type and CYP2E1 knockout mice were fed ethanol or isocaloric dextrose for 4 weeks followed by a single treatment with either saline or Jo2. Mice were killed 8 h after the Jo2 challenge. There were three- to five fold increases in transaminases and more extensive eosinophilic necrosis, hemorrhage, and infiltration of inflammatory cells in the central zone of the hepatic lobule in the ethanol-fed mice treated with Jo2 compared to the dextrose/Jo2- or ethanol/saline-treated mice. Liver injury was blunted in ethanol-fed CYP2E1 knockout mice treated with Jo2. The chronic ethanol feeding produced steatosis, elevation of CYP2E1, and oxidative stress in wild-type but not CYP2E1 knockout mice. These changes in wild-type mice fed ethanol were similar after saline or Jo2 treatment. The Jo2 treatment produced activation of JNK and P38 MAP kinase, increased activity of caspase-8 and -3, and lowered hepatic GSH levels in both the dextrose- and the alcohol-fed mice. JNK was activated at early times after Jo2 treatment in the ethanol-fed mice. Serum TNF-alpha levels were strikingly elevated in the wild-type ethanol/Jo2 group, which showed liver injury, compared to all the other groups, which did not show liver injury. Inhibition of JNK or P38 MAPK partially, but not completely, prevented the elevated liver injury in the wild-type ethanol/Jo2 mice. These results show that chronic ethanol feeding enhances Fas-induced liver injury by a mechanism associated with induction of CYP2E1, elevated serum TNF-alpha levels, and activation of MAPK.

15-epi-lipoxin A4 inhibits myeloperoxidase signaling and enhances resolution of acute lung injury

RATIONALE

Apoptosis is essential for removal of neutrophils from inflamed tissues and efficient resolution of inflammation. Myeloperoxidase (MPO), abundantly expressed in neutrophils, not only generates cytotoxic oxidants but also signals through the beta(2) integrin Mac-1 to rescue neutrophils from constitutive apoptosis, thereby prolonging inflammation.

OBJECTIVES

Because aspirin-triggered 15-epi-lipoxin A(4) (15-epi-LXA(4)) modulates Mac-1 expression, we investigated the impact of 15-epi-LXA(4) on MPO suppression of neutrophil apoptosis and MPO-mediated neutrophil-dependent acute lung injury.

METHODS

Human neutrophils were cultured with MPO with or without 15-epi-LXA(4) to investigate development of apoptosis. Acute lung injury was produced by intratracheal injection of carrageenan plus MPO or intraperitoneal injection of live Escherichia coli in mice, and the animals were treated with 15-epi-LXA(4) at the peak of inflammation.

MEASUREMENTS AND MAIN RESULTS

15-Epi-LXA(4) through down-regulation of Mac-1 expression promoted apoptosis of human neutrophils by attenuating MPO-induced activation of extracellular signal-regulated kinase and Akt-mediated phosphorylation of Bad and by reducing expression of the antiapoptotic protein Mcl-1, thereby aggravating mitochondrial dysfunction. The proapoptotic effect of 15-epi-LXA(4) was dominant over MPO-mediated effects even when it was added at 4 hours post MPO. In mice, treatment with 15-epi-LXA(4) accelerated the resolution of established carrageenan plus MPO-evoked as well as E. coli-induced neutrophil-dependent pulmonary inflammation through redirecting neutrophils to caspase-mediated cell death and facilitating their removal by macrophages.

CONCLUSIONS

These results demonstrate that aspirin-triggered 15-epi-LXA(4) enhances resolution of inflammation by overriding the powerful antiapoptosis signal from MPO, thereby demonstrating a hitherto unrecognized mechanism by which aspirin promotes resolution of inflammation.

Thiol chemistry and specificity in redox signaling

Exposure of cells to sublethal oxidative stress results in the modulation of various signaling pathways. Oxidants can activate and inactivate transcription factors, membrane channels, and metabolic enzymes, and regulate calcium-dependent and phosphorylation signaling pathways. Oxidation and reduction of thiol proteins are thought to be the major mechanisms by which reactive oxidants integrate into cellular signal transduction pathways. This review focuses on mechanisms for sensing and transmitting redox signals, from the perspective of their chemical reactivity with specific oxidants. We discuss substrate preferences for different oxidants and how the kinetics of these reactions determines how each oxidant will react in a cell. This kinetic approach helps to identify initial oxidant-sensitive targets and elucidate mechanisms involved in transmission of redox signals. It indicates that only those proteins with very high reactivity, such as peroxiredoxins, are likely to be direct targets for hydrogen peroxide. Other more modestly reactive thiol proteins such as protein tyrosine phosphatases are more likely to become oxidized by an indirect mechanism. The review also examines oxidative changes observed during receptor-mediated signaling, the strengths and limitations of detection methods for reactive oxidant production, and the evidence for hydrogen peroxide acting as the second messenger. We discuss areas where observations in cell systems can be rationalized with the reactivity of specific oxidants and where further work is needed to understand the mechanisms involved.

The pro-inflammatory oxidant hypochlorous acid induces Bax-dependent mitochondrial permeabilisation and cell death through AIF-/EndoG-dependent pathways

At sites of chronic inflammation, such as in the inflamed rheumatoid joint, activated neutrophils release hydrogen peroxide (H(2)O(2)) and the enzyme myeloperoxidase to catalyse the formation of hypochlorous acid (HOCl). 3-chlorotyrosine, a marker of HOCl in vivo, has been observed in synovial fluid proteins from rheumatoid arthritis patients. However the mechanisms of HOCl-induced cytotxicity are unknown. We determined the molecular mechanisms by which HOCl induced cell death in human mesenchymal progenitor cells (MPCs) differentiated into a chondrocytic phenotype as a model of human cartilage cells and show that HOCl induced rapid Bax conformational change, mitochondrial permeability and release of intra-mitochondrial pro-apoptotic proteins which resulted in nuclear translocation of AIF and EndoG. siRNA-mediated knockdown of Bax substantially prevented mitochondrial permeability, release of intra-mitochondrial pro-apoptotic proteins. Cell death was inhibited by siRNA-mediated knockdown of Bax, AIF or EndoG. Although we observed several biochemical markers of apoptosis, caspase activation was not detected either by western blotting, fluorescence activity assays or by using caspase inhibitors to inhibit cell death. This was further supported by findings that (1) in vitro exposure of recombinant human caspases to HOCl caused significant inhibition of caspase activity and (2) the addition of HOCl to staurosporine-treated MPCs inhibited the activity of cellular caspases. Our results show for the first time that HOCl induced Bax-dependent mitochondrial permeability which led to cell death without caspase activity by processes involving AIF/EndoG-dependent pathways. Our study provides a novel insight into the potential mechanisms of cell death in the inflamed human joint.