Myeloperoxidase-dependent lipid peroxidation promotes the oxidative modification of cytosolic proteins in phagocytic neutrophils

Post-translational modifications on the metal-sequestering protein calprotectin

Human calprotectin (CP, S100A8/S100A9 oligomer) is an abundant neutrophil protein that contributes to innate immunity by sequestering nutrient metal ions in the extracellular space. This process starves invading microbial pathogens of essential metal nutrients, which can inhibit growth and colonization. Over the past decade, fundamental and clinical studies have revealed that the S100A8 and S100A9 subunits of CP exhibit a variety of post-translational modifications (PTMs). This review summarizes PTMs on the CP subunits that have been detected and highlights two recent studies that evaluated the structural and functional consequences of methionine and cysteine oxidation on CP. Collectively, these investigations indicate that the molecular speciation of extracellular CP is complex and composed of multiple proteoforms. Moreover, PTMs may impact biological function and the lifetime of the protein. It is therefore important that post-translationally modified CP species receive consideration and integration into the current working model for how CP functions in nutritional immunity.

Inside the phagosome: A bacterial perspective

The neutrophil phagosome is one of the most hostile environments that bacteria must face and overcome if they are to succeed as pathogens. Targeting bacterial defense mechanisms should lead to new therapies that assist neutrophils to kill pathogens, but this has not yet come to fruition. One of the limiting factors in this effort has been our incomplete knowledge of the complex biochemistry that occurs within the rapidly changing environment of the phagosome. The same compartmentalization that protects host tissue also limits our ability to measure events within the phagosome. In this review, we highlight the limitations in our knowledge, and how the contribution of bacteria to the phagosomal environment is often ignored. There appears to be significant heterogeneity among phagosomes, and it is important to determine whether survivors have more efficient defenses or whether they are ingested into less threatening environments than other bacteria. As part of these efforts, we discuss how monitoring or recovering bacteria from phagosomes can provide insight into the conditions they have faced. We also encourage the use of unbiased screening approaches to identify bacterial genes that are essential for survival inside neutrophil phagosomes.

Selenocompounds and Sepsis: Redox Bypass Hypothesis for Early Diagnosis and Treatment: Part A-Early Acute Phase of Sepsis: An Extraordinary Redox Situation (Leukocyte/Endothelium Interaction Leading to Endothelial Damage)

Significance: Sepsis is a health disaster. In sepsis, an initial, beneficial local immune response against infection evolves rapidly into a generalized, dysregulated response or a state of chaos, leading to multiple organ failure. Use of life-sustaining supportive therapies creates an unnatural condition, enabling the complex cascades of the sepsis response to develop in patients who would otherwise die. Multiple attempts to control sepsis at an early stage have been unsuccessful. Recent Advances: Major events in early sepsis include activation and binding of leukocytes and endothelial cells in the microcirculation, damage of the endothelial surface layer (ESL), and a decrease in the plasma concentration of the antioxidant enzyme, selenoprotein-P. These events induce an increase in intracellular redox potential and lymphocyte apoptosis, whereas apoptosis is delayed in monocytes and neutrophils. They also induce endothelial mitochondrial and cell damage. Critical Issues: Neutrophil production increases dramatically, and aggressive immature forms are released. Leukocyte cross talk with other leukocytes and with damaged endothelial cells amplifies the inflammatory response. The release of large quantities of reactive oxygen, halogen, and nitrogen species as a result of the leukocyte respiratory burst, endothelial mitochondrial damage, and ischemia/reperfusion processes, along with the marked decrease in selenoprotein-P concentrations, leads to peroxynitrite damage of the ESL, reducing flow and damaging the endothelial barrier. Future Directions: Endothelial barrier damage by activated leukocytes is a time-sensitive event in sepsis, occurring within hours and representing the first step toward organ failure and death. Reducing or stopping this event is necessary before irreversible damage occurs.

Regulation of the epigenetic landscape by immune cell oxidants

Excessive production of microbicidal oxidants by neutrophils can damage host tissue. The short-term response of cells to oxidative stress is well understood, but the mechanisms behind long-term consequences require further clarification. Epigenetic pathways mediate cellular adaptation, and are therefore a potential target of oxidative stress. Indeed, there is evidence that many proteins and metabolites involved in epigenetic pathways are redox sensitive. In this review we provide an overview of the epigenetic landscape and discuss the potential for redox regulation. Using this information, we highlight specific examples where neutrophil oxidants react with epigenetic pathway components. We also use published data from redox proteomics to map out known intersections between oxidative stress and epigenetics that may signpost helpful directions for future investigation. Finally, we discuss the role neutrophils play in adaptive pathologies with a focus on tumour initiation and progression. We hope this information will stimulate further discourse on the emerging field of redox epigenomics.

6-Gingerol, an active constituent of ginger, attenuates lipopolysaccharide-induced oxidation, inflammation, cognitive deficits, neuroplasticity, and amyloidogenesis in rat

This study examined the protective effect of 6-Gingerol (6G) against lipopolysaccharide (LPS)-induced cognitive impairments, oxidative stress, neuroplasticity, amyloidogenesis, and inflammation. Male rats were allocated into six groups in this manner; Group I placed on normal saline only. Group II was treated for 7 days with LPS alone intraperitoneally at 250 µg/kg body weight (bw). Group III received 6G alone at 50 mg/kg bw orally for 14 days. Groups IV and V received 6G at 20 and 50 mg/kg bw for 7 days, respectively, and LPS for another 7 days to induce neurotoxicity. Group VI received 5 mg/kg bw of donepezil for 7 days and LPS for 7 days. Pretreatment with 20 and 50 mg/kg bw of 6G protected against LPS-mediated learning and memory function, and also locomotor and motor deficits. Besides, 20 and 50 mg/kg bw 6G mitigated LPS-induced alteration in markers of oxidative stress. Furthermore, induction of amyloidogenesis associated with disruption of histoarchitecture and high expression of interleukin 1β, inducible nitric oxide synthase, amyloid precursor protein (APP), β-secretase 1, and brain-derived neurotrophic factor by LPS was mitigated by the two doses of 6G in the rat hippocampus and cerebral cortex region of the brain. 6G pretreatment at the two doses mitigated LPS-mediated histopathological changes in the hippocampus and cerebral cortex of rats. Overall, our results demonstrate that the protective effect of 6G is mediated through the reversal of neurobehavioral deficit, oxidative stress, inflammation, and amyloidogenesis, thus making 6G a possible chemoprophylactic agent against brain injury as a result of LPS exposure. PRACTICAL APPLICATIONS: In the search for a holistic prevention of inflammation-associated neurodegeneration, nutraceuticals are becoming prominent. Hence, this study presents 6G, an active constituent of ginger, as a chemoprotective, antioxidant, and anti-inflammatory agent, which is able to ameliorate cognitive impairments, oxidative stress, neuroplasticity, amyloidogenesis, and inflammation in LPS-induced rat model of neuroinflammation.

Myeloperoxidase-Derived 2-Chlorohexadecanal Is Generated in Mouse Heart during Endotoxemia and Induces Modification of Distinct Cardiomyocyte Protein Subsets In Vitro

Sepsis is a major cause of mortality in critically ill patients and associated with cardiac dysfunction, a complication linked to immunological and metabolic aberrations. Cardiac neutrophil infiltration and subsequent release of myeloperoxidase (MPO) leads to the formation of the oxidant hypochlorous acid (HOCl) that is able to chemically modify plasmalogens (ether-phospholipids) abundantly present in the heart. This reaction gives rise to the formation of reactive lipid species including aldehydes and chlorinated fatty acids. During the present study, we tested whether endotoxemia increases MPO-dependent lipid oxidation/modification in the mouse heart. In hearts of lipopolysaccharide-injected mice, we observed significantly higher infiltration of MPO-positive cells, increased fatty acid content, and formation of 2-chlorohexadecanal (2-ClHDA), an MPO-derived plasmalogen modification product. Using murine HL-1 cardiomyocytes as in vitro model, we show that exogenously added HOCl attacks the cellular plasmalogen pool and gives rise to the formation of 2-ClHDA. Addition of 2-ClHDA to HL-1 cardiomyocytes resulted in conversion to 2-chlorohexadecanoic acid and 2-chlorohexadecanol, indicating fatty aldehyde dehydrogenase-mediated redox metabolism. However, a recovery of only 40% indicated the formation of non-extractable (protein) adducts. To identify protein targets, we used a clickable alkynyl analog, 2-chlorohexadec-15-yn-1-al (2-ClHDyA). After Huisgen 1,3-dipolar cycloaddition of 5-tetramethylrhodamine azide (N₃-TAMRA) and two dimensional-gel electrophoresis (2D-GE), we were able to identify 51 proteins that form adducts with 2-ClHDyA. Gene ontology enrichment analyses revealed an overrepresentation of heat shock and chaperone, energy metabolism, and cytoskeletal proteins as major targets. Our observations in a murine endotoxemia model demonstrate formation of HOCl-modified lipids in the heart, while pathway analysis in vitro revealed that the chlorinated aldehyde targets specific protein subsets, which are central to cardiac function.

The Imbalance among Oxidative Biomarkers and Antioxidant Defense Systems in Thromboangiitis Obliterans (Winiwarter-Buerger Disease)

(1) Background: Thromboangiitis obliterans or Winiwarter-Buerger disease (WBD), is an inflammatory, thrombotic occlusive, peripheral vascular disease, usually occurring in young smokers. The pathophysiological mechanisms underlying the disease are not clearly understood. The aim of this study is to investigate the imbalance between oxidants and antioxidants occurring in these patients. (2) Patients and Methods: In this cross-sectional study, 22 male patients with WBD and 20 healthy male smoking habit matched control group were included. To evaluate the possible sources of oxidative stress, the antioxidant biomarkers, and the markers of lipid peroxidation and protein oxidation, serum samples were analyzed for total oxidative status (TOS), total antioxidant capacity (TAC), myeloperoxidase (MPO), coenzyme Q10 (CoQ10), superoxide dismutase (SOD), glutathione reductase (GR), malondialdehyde (MDA), and protein carbonyl (PC) activity and/or content. (3) Results: The circulating levels of TOS, TAC, and CoQ10 were significantly higher in WBD patients, with respect to healthy smokers as controls. No significant difference was found among the serum level of PC, total cholesterol, MPO, and GR activity in WBD patients and healthy smoker controls. The activity of SOD and the mean serum level of MDA were significantly lower in WBD patients, with respect to healthy smoker controls. (4) Conclusion: Considerably high levels of oxidative stress were detected in WBD patients, which were greater than the antioxidant capacity. The low level of MDA may be associated with the enzymatic degradation of lipid peroxidation products. High levels of CoQ10 and low levels of SOD may be related to a harmful oxidative cooperation, leading to the vasoconstriction of WBD, representing a promising tool to discern possible different clinical risks of this poorly understood peripheral occlusive disease.

The Transcription Factor ArcA Modulates Salmonella's Metabolism in Response to Neutrophil Hypochlorous Acid-Mediated Stress

Salmonella Typhimurium, a bacterial pathogen with high metabolic plasticity, can adapt to different environmental conditions; these traits enhance its virulence by enabling bacterial survival. Neutrophils play important roles in the innate immune response, including the production of microbicidal reactive oxygen species (ROS). In addition, the myeloperoxidase in neutrophils catalyzes the formation of hypochlorous acid (HOCl), a highly toxic molecule that reacts with essential biomolecules, causing oxidative damage including lipid peroxidation and protein carbonylation. The bacterial response regulator ArcA regulates adaptive responses to oxygen levels and influences the survival of Salmonella inside phagocytic cells. Here, we demonstrate by whole transcriptomic analyses that ArcA regulates genes related to various metabolic pathways, enabling bacterial survival during HOCl-stress in vitro. Also, inside neutrophils, ArcA controls the transcription of several metabolic pathways by downregulating the expression of genes related to fatty acid degradation, lysine degradation, and arginine, proline, pyruvate, and propanoate metabolism. ArcA also upregulates genes encoding components of the oxidative pathway. These results underscore the importance of ArcA in ATP generation inside the neutrophil phagosome and its participation in bacterial metabolic adaptations during HOCl stress.

Peroxiredoxin expression and redox status in neutrophils and HL-60 cells

Peroxiredoxins (Prxs) are thiol peroxidases with a key role in antioxidant defense and redox signaling. They could be important in neutrophils for handling the large amount of oxidants that these cells produce. We investigated the redox state of Prx1 and Prx2 in HL-60 promyelocytic cells differentiated to neutrophil-like cells (dHL-60) and in human neutrophils. HL-60 cell differentiation with dimethyl sulfoxide caused a large decrease in expression of both Prxs, and all-trans retinoic acid also decreased Prx1 expression. Prx1 was mostly reduced in dHL-60 cells. NADPH oxidase activation by phorbol myristate acetate (PMA) or ingestion of Staphylococcus aureus induced rapid oxidation to disulfide-linked dimers, and eventually hyperoxidation. The NADPH oxidase inhibitor, diphenyleneiodonium, prevented Prx1 dimerization in stimulated dHL-60 cells, and decreased the extent of oxidation under resting conditions. In contrast, Prx1 and Prx2 were present in neutrophils from human blood as disulfides, and PMA or S. aureus caused no further oxidation. They remained oxidized on incubation with diphenyleneiodonium in media. Although this suggests that Prx redox cycling could be deficient in neutrophils, thioredoxin expression and thioredoxin reductase activity were similar in neutrophils and dHL-60 cells. Additionally, neutrophil thioredoxin was initially reduced and underwent oxidation after PMA activation. Thus, although the Prxs respond to oxidant generation in dHL-60 cells, in neutrophils they appear "locked" as disulfides. On this basis we propose that neutrophil Prxs are inefficient antioxidants and contribute little to peroxide removal during the oxidative burst, and speculate that they might be involved in other cell processes.

Oxidative cross-linking of calprotectin occurs in vivo, altering its structure and susceptibility to proteolysis

Calprotectin, the major neutrophil protein, is a critical alarmin that modulates inflammation and plays a role in host immunity by strongly binding trace metals essential for bacterial growth. It has two cysteine residues favourably positioned to act as a redox switch. Whether their oxidation occurs in vivo and affects the function of calprotectin has received little attention. Here we show that in saliva from healthy adults, and in lavage fluid from the lungs of patients with respiratory diseases, a substantial proportion of calprotectin was cross-linked via disulfide bonds between the cysteine residues on its S100A8 and S100A9 subunits. Stimulated human neutrophils released calprotectin and subsequently cross-linked it by myeloperoxidase-dependent production of hypochlorous acid. The myeloperoxidase-derived oxidants hypochlorous acid, taurine chloramine, hypobromous acid, and hypothiocyanous acid, all at 10 μM, cross-linked calprotectin (5 μM) via reversible disulfide bonds. Hypochlorous acid generated A9-A9 and A8-A9 cross links. Hydrogen peroxide (10 μM) did not cross-link the protein. Purified neutrophil calprotectin existed as a non-covalent heterodimer of A8/A9 which was converted to a heterotetramer - (A8/A9)₂ - with excess calcium ions. Low level oxidation of calprotectin with hypochlorous acid produced substantial proportions of high order oligomers, whether oxidation occurred before or after addition of calcium ions. At high levels of oxidation the heterodimer could not form tetramers with calcium ions, but prior addition of calcium ions afforded some protection for the heterotetramer. Oxidation and formation of the A8-A9 disulfide cross link enhanced calprotectin's susceptibility to proteolysis by neutrophil proteases. We propose that reversible disulfide cross-linking of calprotectin occurs during inflammation and affects its structure and function. Its increased susceptibility to proteolysis will ultimately result in a loss of function.

Modulation of Innate Immunity by G-CSF and Inflammatory Response by LBPK95A Improves the Outcome of Sepsis in a Rat Model

Introduction

Sepsis is the primary cause of death from infection. We wanted to improve the outcome of sepsis by stimulating innate immunity in combination with modulating the severity of inflammatory responses in rats.

Method

Sepsis was induced by the injection of feces suspension (control). A 5-day course of G-CSF treatment was given before the septic insult (G-CSF). The inflammatory response was decreased using various doses of the LPS-blocking peptide LBPK95A (5 mg/kg = 100% Combi group, 0.5 mg/kg = 10% Combi group, and 0.05 mg/kg = 1% Combi group). Survival rates were observed. Bacterial clearance, neutrophil infiltration, tissue damage, and the induction of hepatic and systemic inflammatory responses were determined 2 h and 12 h after the septic insult.

Results

High-dose LBPK95A (100% Combi) reduced the survival rate to 10%, whereas low-dose LBPK95A (10% and 1% Combi) increased the survival rates to 50% and 80%, respectively. The survival rates inversely correlated with multiorgan damage as indicated by the serum levels of ALT and urea. G-CSF treatment increased the white blood cell counts, hepatic neutrophil infiltration, and bacterial clearance in the liver, lung, and blood. The blockade of the LPS-LBP interaction decreased neutrophil infiltration, led to increased white blood cell count, and decreased hepatic neutrophil infiltration, irrespective of dose. However, bacterial clearance improved in the 1% and 10% Combi groups but worsened in the 100% Combi group. G-CSF increased TNF-α and IL-6 levels. Irrespective of dose, the blockade of the LPS-LBP interaction was associated with low systemic cytokine levels and delayed increases in hepatic TNF-α and IL-6 mRNA expression. The delayed increase in cytokines was associated with the phosphorylation of STAT3 and AKT.

Conclusion

Our results revealed that increasing innate immunity by G-CSF pretreatment and decreasing inflammatory responses using LBPK95A improved the survival rates in a rat sepsis model and could be a novel strategy to treat sepsis.

Diphenyl diselenide abrogates brain oxidative injury and neurobehavioural deficits associated with pesticide chlorpyrifos exposure in rats

Exposure to pesticide chlorpyrifos (CPF) is associated with neurodevelopmental toxicity both in humans and animals. Diphenyl diselenide (DPDS) is a simple synthetic organoselenium well reported to possess antioxidant, anti-inflammatory and neuroprotective effects. However, there is paucity of information on the beneficial effects of DPDS on CPF-mediated brain injury and neurobehavioural deficits. The present study investigated the neuroprotective mechanism of DPDS in rats sub-chronically treated with CPF alone at 5 mg/kg body weight or orally co-treated with DPDS at 2.5 and 5 mg/kg body weight for 35 consecutive days. Endpoint analyses using video-tracking software in a novel environment revealed that co-treatment with DPDS significantly (p < 0.05) protected against CPF-mediated locomotor and motor deficits precisely the decrease in maximum speed, total distance travelled, body rotation, absolute turn angle, forelimb grip strength as well as the increase in negative geotaxis and incidence of fecal pellets. The enhancement in the neurobehavioral activities of rats co-treated with DPDS was verified by track plot analyses. Besides, DPDS assuaged CPF-induced decrease in acetylcholinesterase and antioxidant enzymes activities and the increase in myeloperoxidase activity and lipid peroxidation level in the mid-brain, cerebral cortex and cerebellum of the rats. Histologically, DPDS co-treatment abrogated CPF-mediated neuronal degeneration in the cerebral cortex, dentate gyrus and cornu ammonis3 in the treated rats. In conclusion, the neuroprotective mechanisms of DPDS is related to the prevention of oxidative stress, enhancement of redox status and acetylcholinesterase activity in brain regions of the rats. DPDS may be a promising chemotherapeutic agent against brain injury resulting from CPF exposure.

The dual role of Reactive Oxygen Species in autoimmune and inflammatory diseases: evidence from preclinical models

Reactive oxygen species (ROS) are created in cells during oxidative phosphorylation by the respiratory chain in the mitochondria or by the family of NADPH oxidase (NOX) complexes. The first discovered and most studied of these complexes, NOX2, mediates the oxidative burst in phagocytes. ROS generated by NOX2 are dreadful weapons: while being essential to kill ingested pathogens they can also cause degenerative changes on tissue if production and release are not balanced by sufficient detoxification. In the last fifteen years evidence has been accumulating that ROS are also integral signaling molecules and are important for regulating autoimmunity and immune-mediated inflammatory diseases. It seems that an accurate redox balance is necessary to sustain an immune state that both prevents the development of overt autoimmunity (the bright side of ROS) and minimizes collateral tissue damage (the dark side of ROS). Herein, we review studies from rodent models of arthritis, lupus, and neurodegenerative diseases that show that low NOX2-derived ROS production is linked to disease and elaborate on the underlying cellular and molecular mechanisms and the translation of these results to disease in humans.

Influence of Dental Restorations on Oxidative Stress in Gingival Crevicular Fluid

Biocompatibility of dental materials (DM) can be evaluated by gingival crevicular fluid (GCF) oxidative stress (OS) status. The goal of the study was to ascertain influence of dental caries degree, teeth position, and type and amount of applied DM on GCF OS profile. For this purpose, we tested six DMs that were sealed in one session: amalgam (Amg), composites: Tetric EvoCeram and Beautifil (BF), phosphate cement-zinc phosphate and polycarboxylate cements-zinc polycarboxylate cements, and glass ionomer cement (GIC). The study included 88 dental outpatients. Follow-up was scheduled at 7th and 30th day. Oxidative stress parameters (malondialdehyde (MDA) and glutathione (GSH) levels and total superoxide dismutase (tSOD) activity) were measured before (0th day) and after the treatment (7th and 30th day) in GCF. Control teeth were mirror-positioned healthy teeth. The DM accomplished the following effects (listed in descending order): increase of GSH in GCF was realized by ZPoC > BF > GIC > Amg; tSOD activity increase by ZPoC > BF > Amg; and MDA decrease by ZPoC > ZPhC > Amg > TEC. Dental caries provokes insignificant rise of OS in GCF. ZPoC and ZPhC showed the highest antioxidant effect, contrary to GIC. Restorations with antioxidant properties may reduce gum diseases initiated by caries lesion, what is of great clinical relevance in dentistry.

Endotoxin promotes neutrophil hierarchical chemotaxis via the p38-membrane receptor pathway

Neutrophils are the most abundant leukocytes in peripheral blood and play critical a role in bacterial infection, tumor immunity and wound repair. Clarifying the process of neutrophil chemotaxis to target sites of immune activity has been a focus of increased interest within the past decade. In bacterial infectious foci, neutrophils migrate toward the bacterial-derived chemoattractant N-formyl-Met-Leu-Phe (fMLP) and ignore other intermediary chemoattractants to arrive at the area of infection. Using an under agarose chemotaxis assay, we observed that the bacterial fMLP-induced neutrophil chemotaxis signal overrode interleukin 8 (IL-8)- and leukotriene B4 (LTB4)-induced chemotaxis signals. Moreover, in the presence of bacterial lipopolysaccharide (LPS), the fMLP-induced hierarchical chemotaxis signal was enhanced. Further studies revealed that LPS increased the membrane expression of the fMLP receptor, formyl peptide receptor 1 (FPR1). However, expression levels of the membrane receptors for IL-8 and LTB4 were decreased by LPS administration. A human Phospho-mitogen-activated protein kinase (MAPK) proteome array showed that the p38 pathway was significantly activated by LPS stimulation. Moreover, p38 was responsible for the altered expression of neutrophil membrane chemoattractant receptors. Inhibition of neutrophil p38 restored LPS-improved hierarchical chemotaxis. Taken together, these data indicate that endotoxin promotes neutrophil hierarchical chemotaxis via the p38-membrane receptor pathway.

Expression, Purification, and Antimicrobial Activity of S100A12

Calgranulin proteins are important mediators of innate immunity and are members of the S100 class of the EF-hand family of calcium binding proteins. Some S100 proteins have the capacity to bind transition metals with high affinity and effectively sequester them away from invading microbial pathogens in a process that is termed "nutritional immunity". S100A12 (EN-RAGE) binds both zinc and copper and is highly abundant in innate immune cells such as macrophages and neutrophils. We report a refined method for the expression, enrichment and purification of S100A12 in its active, metal-binding configuration. Utilization of this protein in bacterial growth and viability analyses reveals that S100A12 has antimicrobial activity against the bacterial pathogen, Helicobacter pylori. The antimicrobial activity is predicated on the zinc-binding activity of S100A12, which chelates nutrient zinc, thereby starving H. pylori which requires zinc for growth and proliferation.

Neutrophil-generated oxidative stress and protein damage in Staphylococcus aureus

Staphylococcus aureus is a ubiquitous, versatile and dangerous pathogen. It colonizes over 30% of the human population, and is one of the leading causes of death by an infectious agent. During S. aureus colonization and invasion, leukocytes are recruited to the site of infection. To combat S. aureus, leukocytes generate an arsenal of reactive species including superoxide, hydrogen peroxide, nitric oxide and hypohalous acids that modify and inactivate cellular macromolecules, resulting in growth defects or death. When S. aureus colonization cannot be cleared by the immune system, antibiotic treatment is necessary and can be effective. Yet, this organism quickly gains resistance to each new antibiotic it encounters. Therefore, it is in the interest of human health to acquire a deeper understanding of how S. aureus evades killing by the immune system. Advances in this field will have implications for the design of future S. aureus treatments that complement and assist the host immune response. In that regard, this review focuses on how S. aureus avoids host-generated oxidative stress, and discusses the mechanisms used by S. aureus to survive oxidative damage including antioxidants, direct repair of damaged proteins, sensing oxidant stress and transcriptional changes. This review will elucidate areas for studies to identify and validate future antimicrobial targets.

Pleiotropic effects of 4-hydroxynonenal on oxidative burst and phagocytosis in neutrophils

Metabolic control of cellular function is significant in the context of inflammation-induced metabolic dysregulation in immune cells. Generation of reactive oxygen species (ROS) such as hydrogen peroxide and superoxide are one of the critical events that modulate the immune response in neutrophils. When activated, neutrophil NADPH oxidases consume large quantities of oxygen to rapidly generate ROS, a process that is referred to as the oxidative burst. These ROS are required for the efficient removal of phagocytized cellular debris and pathogens. In chronic inflammatory diseases, neutrophils are exposed to increased levels of oxidants and pro-inflammatory cytokines that can further prime oxidative burst responses and generate lipid oxidation products such as 4-hydroxynonenal (4-HNE). In this study we hypothesized that since 4-HNE can target glycolysis then this could modify the oxidative burst. To address this the oxidative burst was determined in freshly isolated healthy subject neutrophils using 13-phorbol myristate acetate (PMA) and the extracellular flux analyzer. Neutrophils pretreated with 4-HNE exhibited a significant decrease in the oxidative burst response and phagocytosis. Mass spectrometric analysis of alkyne-HNE treated neutrophils followed by click chemistry detected modification of a number of cytoskeletal, metabolic, redox and signaling proteins that are critical for the NADPH oxidase mediated oxidative burst. These modifications were confirmed using a candidate immunoblot approach for critical proteins of the active NADPH oxidase enzyme complex (Nox2 gp91phox subunit and Rac1 of the NADPH oxidase) and glyceraldehyde phosphate dehydrogenase, a critical enzyme in the metabolic regulation of oxidative burst. Taken together, these data suggest that 4-HNE-induces a pleiotropic mechanism to inhibit neutrophil function. These mechanisms may contribute to the immune dysregulation associated with chronic pathological conditions where 4-HNE is generated.

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Phagocytosis and glycolysis are inhibited in neutrophils by 4-hydroxynonenal.

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Click chemistry with alkyne-HNE identifies over 100 potential protein targets.

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Rac1, NOX2 and GAPDH are modified by 4-HNE.

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The 4-HNE-dependent inhibition of neutrophil function is mediated by a pleiotropic mechanism.

Reactive Oxygen Species and Neutrophil Function

Neutrophils are essential for killing bacteria and other microorganisms, and they also have a significant role in regulating the inflammatory response. Stimulated neutrophils activate their NADPH oxidase (NOX2) to generate large amounts of superoxide, which acts as a precursor of hydrogen peroxide and other reactive oxygen species that are generated by their heme enzyme myeloperoxidase. When neutrophils engulf bacteria they enclose them in small vesicles (phagosomes) into which superoxide is released by activated NOX2 on the internalized neutrophil membrane. The superoxide dismutates to hydrogen peroxide, which is used by myeloperoxidase to generate other oxidants, including the highly microbicidal species hypochlorous acid. NOX activation occurs at other sites in the cell, where it is considered to have a regulatory function. Neutrophils also release oxidants, which can modify extracellular targets and affect the function of neighboring cells. We discuss the identity and chemical properties of the specific oxidants produced by neutrophils in different situations, and what is known about oxidative mechanisms of microbial killing, inflammatory tissue damage, and signaling.

Impact of simultaneous stimulation of 5-lipoxygenase and myeloperoxidase in human neutrophils

Human neutrophil 5-lipoxygenase (5-LOX) oxidizes arachidonic acid (AA) to 5S-hydro(pero)xy-6E,8Z,11Z, 14Z-eicosatetraenoic acid (5-H(p)ETE) and leukotriene (LT)A4, which is further converted to the chemoattractant LTB4. These cells contain also the heme enzyme myeloperoxidase (MPO) producing several potent oxidants such as hypochlorous acid (HOCl). Previously, it was shown that MPO-metabolites influence 5-LOX product formation. Here, we addressed the question, whether a simultaneous activation of MPO and 5-LOX in neutrophils results in comparable changes of 5-LOX activity. Human neutrophils were stimulated with H2O2 or phorbol 12-myristate 13-acetate (PMA) for MPO activation and subsequently treated with calcium ionophore A23187 inducing 5-LOX product formation on endogenous AA. Special attention was drawn to neutrophil vitality, formation of MPO-derived metabolites and redox status. The pre-stimulation with H2O2 resulted in a concentration-dependent increase in the ratio of 5-HETE to the sum of LTB4+6-trans-LTB4 in consequence of MPO activation. Thereby no impairment of cell vitality and only a slightly reduction of total glutathione level was observed. An influence of MPO on 5-LOX product formation could be suggested using an MPO inhibitor. In contrast, the pre-stimulation with PMA resulted in different changes of 5-LOX product formation leading to a reduced amount of 5-HETE unaffected by MPO inhibition. Furthermore, impaired cell vitality and diminished redox status was detected after PMA stimulation. Nevertheless, a MPO-induced diminution of LTB4 was obvious. Further work is necessary to define the type of 5-LOX modification and investigate the effect of physiological MPO activators.

Assessment of electrophile damage in a human brain endothelial cell line utilizing a clickable alkyne analog of 2-chlorohexadecanal

Peripheral leukocytes aggravate brain damage by releasing cytotoxic mediators that compromise blood-brain barrier function. One of the oxidants released by activated leukocytes is hypochlorous acid (HOCl) that is formed via the myeloperoxidase-H2O2-chloride system. The reaction of HOCl with the endogenous plasmalogen pool of brain endothelial cells results in the generation of 2-chlorohexadecanal (2-ClHDA), a toxic, lipid-derived electrophile that induces blood-brain barrier dysfunction in vivo. Here, we synthesized an alkynyl-analog of 2-ClHDA, 2-chlorohexadec-15-yn-1-al (2-ClHDyA) to identify potential protein targets in the human brain endothelial cell line hCMEC/D3. Similar to 2-ClHDA, 2-ClHDyA administration reduced cell viability/metabolic activity, induced processing of pro-caspase-3 and PARP, and led to endothelial barrier dysfunction at low micromolar concentrations. Protein-2-ClHDyA adducts were fluorescently labeled with tetramethylrhodamine azide (N3-TAMRA) by 1,3-dipolar cycloaddition in situ, which unveiled a preferential accumulation of 2-ClHDyA adducts in mitochondria, the Golgi, endoplasmic reticulum, and endosomes. Thirty-three proteins that are subject to 2-ClHDyA-modification in hCMEC/D3 cells were identified by mass spectrometry. Identified proteins include cytoskeletal components that are central to tight junction patterning, metabolic enzymes, induction of the oxidative stress response, and electrophile damage to the caveolar/endosomal Rab machinery. A subset of the targets was validated by a combination of N3-TAMRA click chemistry and specific antibodies by fluorescence microscopy. This novel alkyne analog is a valuable chemical tool to identify cellular organelles and protein targets of 2-ClHDA-mediated damage in settings where myeloperoxidase-derived oxidants may play a disease-propagating role.

Oxidation of calprotectin by hypochlorous acid prevents chelation of essential metal ions and allows bacterial growth: Relevance to infections in cystic fibrosis

Calprotectin provides nutritional immunity by sequestering manganese and zinc ions. It is abundant in the lungs of patients with cystic fibrosis but fails to prevent their recurrent infections. Calprotectin is a major protein of neutrophils and composed of two monomers, S100A8 and S100A9. We show that the ability of calprotectin to limit growth of Staphylococcus aureus and Pseudomonas aeruginosa is exquisitely sensitive to oxidation by hypochlorous acid. The N-terminal cysteine residue on S100A9 was highly susceptible to oxidation which resulted in cross-linking of the protein monomers. The N-terminal methionine of S100A8 was also readily oxidized by hypochlorous acid, forming both the methionine sulfoxide and the unique product dehydromethionine. Isolated human neutrophils formed these modifications on calprotectin when their myeloperoxidase generated hypochlorous acid. Up to 90% of the N-terminal amine on S100A8 in bronchoalveolar lavage fluid from young children with cystic fibrosis was oxidized. Oxidized calprotectin was higher in children with cystic fibrosis compared to disease controls, and further elevated in those patients with infections. Our data suggest that oxidative stress associated with inflammation in cystic fibrosis will stop metal sequestration by calprotectin. Consequently, strategies aimed at blocking extracellular myeloperoxidase activity should enable calprotectin to provide nutritional immunity within the airways.

The Human Antimicrobial Protein Calgranulin C Participates in Control of Helicobacter pylori Growth and Regulation of Virulence

During infectious processes, antimicrobial proteins are produced by both epithelial cells and innate immune cells. Some of these antimicrobial molecules function by targeting transition metals and sequestering these metals in a process referred to as "nutritional immunity." This chelation strategy ultimately starves invading pathogens, limiting their growth within the vertebrate host. Recent evidence suggests that these metal-binding antimicrobial molecules have the capacity to affect bacterial virulence, including toxin secretion systems. Our previous work showed that the S100A8/S100A9 heterodimer (calprotectin, or calgranulin A/B) binds zinc and represses the elaboration of the H. pylori cag type IV secretion system (T4SS). However, there are several other S100 proteins that are produced in response to infection. We hypothesized that the zinc-binding protein S100A12 (calgranulin C) is induced in response to H. pylori infection and also plays a role in controlling H. pylori growth and virulence. To test this, we analyzed gastric biopsy specimens from H. pylori-positive and -negative patients for S100A12 expression. These assays showed that S100A12 is induced in response to H. pylori infection and inhibits bacterial growth and viability in vitro by binding nutrient zinc. Furthermore, the data establish that the zinc-binding activity of the S100A12 protein represses the activity of the cag T4SS, as evidenced by the gastric cell "hummingbird" phenotype, interleukin 8 (IL-8) secretion, and CagA translocation assays. In addition, high-resolution field emission gun scanning electron microscopy (FEG-SEM) was used to demonstrate that S100A12 represses biogenesis of the cag T4SS. Together with our previous work, these data reveal that multiple S100 proteins can repress the elaboration of an oncogenic bacterial surface organelle.