Antioxidants improve antibacterial function in hyperoxia-exposed macrophages

GAT107-mediated α7 nicotinic acetylcholine receptor signaling attenuates inflammatory lung injury and mortality in a mouse model of ventilator-associated pneumonia by alleviating macrophage mitochondrial oxidative stress via reducing MnSOD-S-glutathionylation

Supraphysiological concentrations of oxygen (hyperoxia) can compromise host defense and increase susceptibility to bacterial and viral infections, causing ventilator-associated pneumonia (VAP). Compromised host defense and inflammatory lung injury are mediated, in part, by high extracellular concentrations of HMGB1, which can be decreased by GTS-21, a partial agonist of α7 nicotinic acetylcholine receptor (α7nAChR). Here, we report that a novel α7nAChR agonistic positive allosteric modulator (ago-PAM), GAT107, at 3.3 mg/kg, i.p., significantly decreased animal mortality and markers of inflammatory injury in mice exposed to hyperoxia and subsequently infected with Pseudomonas aeruginosa. The incubation of macrophages with 3.3 μM of GAT107 significantly decreased hyperoxia-induced extracellular HMGB1 accumulation and HMGB1-induced macrophage phagocytic dysfunction. Hyperoxia-compromised macrophage function was correlated with impaired mitochondrial membrane integrity, increased superoxide levels, and decreased manganese superoxide dismutase (MnSOD) activity. This compromised MnSOD activity is due to a significant increase in its level of glutathionylation. The incubation of hyperoxic macrophages with 3.3 μM of GAT107 significantly decreases the levels of glutathionylated MnSOD, and restores MnSOD activity and mitochondrial membrane integrity. Thus, GAT107 restored hyperoxia-compromised phagocytic functions by decreasing HMGB1 release, most likely via a mitochondrial-directed pathway. Overall, our results suggest that GAT107 may be a potential treatment to decrease acute inflammatory lung injury by increasing host defense in patients with VAP.

The Positive Allosteric Modulation of alpha7-Nicotinic Cholinergic Receptors by GAT107 Increases Bacterial Lung Clearance in Hyperoxic Mice by Decreasing Oxidative Stress in Macrophages

Supplemental oxygen therapy with supraphysiological concentrations of oxygen (hyperoxia; >21% O2) is a life-saving intervention for patients experiencing respiratory distress. However, prolonged exposure to hyperoxia can compromise bacterial clearance processes, due to oxidative stress-mediated impairment of macrophages, contributing to the increased susceptibility to pulmonary infections. This study reports that the activation of the α7 nicotinic acetylcholine receptor (α7nAChR) with the delete allosteric agonistic-positive allosteric modulator, GAT107, decreases the bacterial burden in mouse lungs by improving hyperoxia-induced lung redox imbalance. The incubation of RAW 264.7 cells with GAT107 (3.3 µM) rescues hyperoxia-compromised phagocytic functions in cultured macrophages, RAW 264.7 cells, and primary bone marrow-derived macrophages. Similarly, GAT107 (3.3 µM) also attenuated oxidative stress in hyperoxia-exposed macrophages, which prevents oxidation and hyper-polymerization of phagosome filamentous actin (F-actin) from oxidation. Furthermore, GAT107 (3.3 µM) increases the (1) activity of superoxide dismutase 1; (2) activation of Nrf2 and (3) the expression of heme oxygenase-1 (HO-1) in macrophages exposed to hyperoxia. Overall, these data suggest that the novel α7nAChR compound, GAT107, could be used to improve host defense functions in patients, such as those with COVID-19, who are exposed to prolonged periods of hyperoxia.

The nitric oxide donor, (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate/D-NO), increases survival by attenuating hyperoxia-compromised innate immunity in bacterial clearance in a mouse model of ventilator-associated pneumonia

Mechanical ventilation (MV) with supraphysiological levels of oxygen (hyperoxia) is a life-saving therapy for the management of patients with respiratory distress. However, a significant number of patients on MV develop ventilator-associated pneumonia (VAP). Previously, we have reported that prolonged exposure to hyperoxia impairs the capacity of macrophages to phagocytize Pseudomonas aeruginosa (PA), which can contribute to the compromised innate immunity in VAP. In this study, we show that the high mortality rate in mice subjected to hyperoxia and PA infection was accompanied by a significant decrease in the airway levels of nitric oxide (NO). Decreased NO levels were found to be, in part, due to a significant reduction in NO release by macrophages upon exposure to PA lipopolysaccharide (LPS). Based on these findings, we postulated that NO supplementation should restore hyperoxia-compromised innate immunity and decrease mortality by increasing the clearance of PA under hyperoxic conditions. To test this hypothesis, cultured macrophages were exposed to hyperoxia (95% O₂) in the presence or absence of the NO donor, (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2-diolate (DETA-NONOate/D-NO). Interestingly, D-NO (up to 37.5 µM) significantly attenuated hyperoxia-compromised macrophage migratory, phagocytic, and bactericidal function. To determine whether the administration of exogenous NO enhances the host defense in bacteria clearance, C57BL/6 mice were exposed to hyperoxia (99% O₂) and intranasally inoculated with PA in the presence or absence of D-NO. D-NO (300 µM-800 µM) significantly increased the survival of mice inoculated with PA under hyperoxic conditions, and significantly decreased bacterial loads in the lung and attenuated lung injury. These results suggest the NO donor, D-NO, can improve the clinical outcomes in VAP by augmenting the innate immunity in bacterial clearance. Thus, provided these results can be extrapolated to humans, NO supplementation may represent a potential therapeutic strategy for preventing and treating patients with VAP.

Impact of hyperoxemia on mortality in critically ill patients with ventilator-associated pneumonia

Background

The objective of this study was to assess the impact of hyperoxemia on mortality in critically ill patients with ventilator-associated pneumonia (VAP).

Methods

This observational study was performed in a 50-bed mixed intensive care unit (ICU) during a 1-year period. Quantitative microbiological confirmation was required for VAP diagnosis. Hyperoxemia was defined as peripheral capillary oxygen saturation (SpO₂) ≥98%. SpO₂ was hourly collected in all study patients during the whole period of mechanical ventilation. The primary objective was to assess the influence of hyperoxemia on ICU mortality.

Results

Ninety-three patients with VAP were all included in this study. ICU-mortality rate was 32% (30 of 93 patients). The mean percentage of time spent with hyperoxemia in survivors and nonsurvivors at ICU admission, before, after or at the time of VAP diagnosis was not significantly different. Multivariate analysis identified age, and sequential organ dysfunction assessment at the day of VAP occurrence as independent risk factors for ICU mortality [odds ratio (OR) =1.04 (95% CI, 1.01-1.08) per year, P=0.019; 1.19 (95% CI, 1.06-1.34) per point, P=0.003; respectively]. The time spent with hyperoxemia before VAP occurrence was not significantly associated with mechanical ventilation free days, or ICU length of stay.

Conclusions

Hyperoxemia at ICU admission, or during ICU stay, had no significant impact on ICU mortality in critically ill patients with VAP.

Reactive Oxygen Species, Biomarkers of Microvascular Maturation and Alveolarization, and Antioxidants in Oxidative Lung Injury

The lungs of extremely low gestational age neonates (ELGANs) are deficient in pulmonary surfactant and are incapable of efficient gas exchange necessary for successful transition from a hypoxic intrauterine environment to ambient air. To improve gas exchange and survival, ELGANs often receive supplemental oxygen with mechanical ventilation which disrupts normal lung developmental processes, including microvascular maturation and alveolarization. Factors that regulate these developmental processes include vascular endothelial growth factor and matrix metalloproteinases, both of which are influenced by generation of oxygen byproducts, or reactive oxygen species (ROS). ELGANs are also deficient in antioxidants necessary to scavenge excessive ROS. Thus, the accumulation of ROS in the preterm lungs exposed to prolonged hyperoxia, results in inflammation and development of bronchopulmonary dysplasia (BPD), a form of chronic lung disease (CLD). Despite advances in neonatal care, BPD/CLD remains a major cause of neonatal morbidity and mortality. The underlying mechanisms are not completely understood, and the benefits of current therapeutic interventions are limited. The association between ROS and biomarkers of microvascular maturation and alveolarization, as well as antioxidant therapies in the setting of hyperoxia-induced neonatal lung injury are reviewed in this article.

Relationship between hyperoxemia and ventilator associated pneumonia

Previous studies suggest a relationship between hyperoxemia and ventilator-associated pneumonia (VAP). Hyperoxemia is responsible for denitrogenation phenomena, and inhibition of surfactant production, promoting atelectasis in mechanically ventilated patients. Further, hyperoxemia impairs the efficacy of alveolar macrophages to migrate, phagocyte and kill bacteria. Oxygen can also cause pulmonary-specific toxic effect called hyperoxic acute lung injury leading to longer duration of mechanical ventilation. All these hyperoxic effects are well-known risk factors for VAP. A recent retrospective large single center study identified hyperoxemia as an independent risk factor for VAP. However, two recent randomized controlled trials evaluated the impact of conservative oxygen strategy versus a liberal strategy, but did not confirm the role of hyperoxemia in lower respiratory tract infection occurrence. In this review, we discuss animal and human studies suggesting a relationship between these two common conditions in mechanically ventilated patients and potential interventions that should be evaluated. Further large prospective studies in carefully selected groups of patients are required to confirm the potential role of hyperoxemia in VAP pathogenesis and to evaluate the impact of a conservative oxygen strategy vs. a conventional strategy on the incidence of VAP.

Ascorbic Acid Attenuates Hyperoxia-Compromised Host Defense against Pulmonary Bacterial Infection

Supraphysiological concentrations of oxygen (hyperoxia) can compromise host defense and increase susceptibility to bacterial infections, causing ventilator-associated pneumonia. The phagocytic activity of macrophages is impaired by hyperoxia-induced increases in the levels of reactive oxygen species (ROS) and extracellular high-mobility group box protein B1 (HMGB1). Ascorbic acid (AA), an essential nutrient and antioxidant, has been shown to be beneficial in various animal models of ROS-mediated diseases. The aim of this study was to determine whether AA could attenuate hyperoxia-compromised host defense and improve macrophage functions against bacterial infections. C57BL/6 male mice were exposed to hyperoxia (≥98% O₂, 48 h), followed by intratracheal inoculation with Pseudomonas aeruginosa, and simultaneous intraperitoneal administration of AA. AA (50 mg/kg) significantly improved bacterial clearance in the lungs and airways, and significantly reduced HMGB1 accumulation in the airways. The incubation of RAW 264.7 cells (a macrophage-like cell line) with AA (0-1,000 μM) before hyperoxic exposure (95% O₂) stabilized the phagocytic activity of macrophages in a concentration-dependent manner. The AA-enhanced macrophage function was associated with significantly decreased production of intracellular ROS and accumulation of extracellular HMGB1. These data suggest that AA supplementation can prevent or attenuate the development of ventilator-associated pneumonia in patients receiving oxygen support.

The compromise of macrophage functions by hyperoxia is attenuated by ethacrynic acid via inhibition of NF-κB-mediated release of high-mobility group box-1

The prolonged exposure to hyperoxia can compromise macrophage functions and contribute to the development of ventilator-associated pneumonia. High levels of extracellular high-mobility group box-1 (HMGB1) in the airways of mice exposed to hyperoxia can directly cause macrophage dysfunction. Hence, inhibition of the release of nuclear HMGB1 into the extracellular milieu may help to maintain macrophage functions under hyperoxic conditions. The present study investigates whether ethacrynic acid (EA) affects hyperoxia-induced HMGB1 release from macrophages and improves their functions. Macrophage-like RAW 264.7 cells and bone marrow-derived macrophages were exposed to different concentrations of EA for 24 hours in the presence of 95% O2. EA significantly decreased the accumulation of extracellular HMGB1 in cultured media. Importantly, the phagocytic activity and migration capability of macrophages were significantly enhanced in EA-treated cells. Interestingly, hyperoxia-induced NF-κB activation was also inhibited in these cells. To determine whether NF-κB plays a role in hyperoxia-induced HMGB1 release, BAY 11-7082, an inhibitor of NF-κB activation, was used. Similar to EA, BAY 11-7082 significantly inhibited the accumulation of extracellular HMGB1 and improved hyperoxia-compromised macrophage migration and phagocytic activity. Furthermore, 24-hour hyperoxic exposure of macrophages caused hyperacetylation of HMGB1 and its subsequent cytoplasmic translocation and release, which were inhibited by EA and BAY 11-7082. Together, these results suggest that EA enhances hyperoxia-compromised macrophage functions by inhibiting HMGB1 hyperacetylation and its release from macrophages, possibly through attenuation of the NF-κB activation. Therefore, the activation of NF-κB could be one of the underlying mechanisms that mediate hyperoxia-compromised macrophage functions.

Antibacterial, antioxidant and cytotoxic studies of total saponin, alkaloid and sterols contents of decoction of Joshanda

The current study was aimed to assess antibacterial, antioxidant and cytotoxic activity of total saponin, alkaloid and sterol contents of Joshanda decoction followed by its constituent’s analysis via thin layer chromatography (TLC). Saponins and alkaloids showed prominent antibacterial activity against Staphylococcus aureus, Bacillus subtilis, Bacillus cereus and Klebsiella pneumoniae whereas sterols only against S. aureus. Saponin and alkaloid contents of 97 and 108 µg/ml, respectively, showed prominent free radical scavenging activity against 1,1-diphenyl-2-picrylhidrazyl, with mild cytotoxicity in brine shrimp cytotoxic test. Under ultraviolet light at 254 nm, TLC of total saponins showed eight different compounds, total sterols comprising three while total alkaloids two compounds of various polarities. It is concluded that the various contents of Joshanda decoction possess outstanding susceptibility against bacteria implicating primarily upper respiratory tract infections augmented by strong antioxidant activity.

High Mobility Group Box-1 mediates hyperoxia-induced impairment of Pseudomonas aeruginosa clearance and inflammatory lung injury in mice

Mechanical ventilation with supraphysiological concentrations of oxygen (hyperoxia) is routinely used to treat patients with respiratory distress. However, a significant number of patients on ventilators exhibit enhanced susceptibility to infections and develop ventilator-associated pneumonia (VAP). Pseudomonas aeruginosa (PA) is one of the most common species of bacteria found in these patients. Previously, we demonstrated that prolonged exposure to hyperoxia can compromise the ability of alveolar macrophages (AMs), an essential part of the innate immunity, to phagocytose PA. This study sought to investigate the potential molecular mechanisms underlying hyperoxia-compromised innate immunity against bacterial infection in a murine model of PA pneumonia. Here, we show that exposure to hyperoxia (≥ 99% O2) led to a significant elevation in concentrations of airway high mobility group box-1 (HMGB1) and increased mortality in C57BL/6 mice infected with PA. Treatment of these mice with a neutralizing anti-HMGB1 monoclonal antibody (mAb) resulted in a reduction in bacterial counts, injury, and numbers of neutrophils in the lungs, and an increase in leukocyte phagocytic activity compared with mice receiving control mAb. This improved phagocytic function was associated with reduced concentrations of airway HMGB1. The correlation between phagocytic activity and concentrations of extracellular HMGB1 was also observed in cultured macrophages. These results indicate a pathogenic role for HMGB1 in hyperoxia-induced impairment with regard to a host's ability to clear bacteria and inflammatory lung injury. Thus, HMGB1 may provide a novel molecular target for improving hyperoxia-compromised innate immunity in patients with VAP.

Hydrogen peroxide enhances phagocytosis of Pseudomonas aeruginosa in hyperoxia

Mechanical ventilation with hyperoxia is a necessary treatment for patients with respiratory distress. However, patients on mechanical ventilation have increased susceptibility to infection. Studies including ours have shown that reactive oxygen species (ROS), generated by exposure to prolonged hyperoxia, can cause a decrease in the phagocytic activity of alveolar macrophages. Hydrogen peroxide (H₂O₂) is a form of ROS generated under hyperoxic conditions. In this study, we examined whether treatment with H₂O₂ directly affects macrophage phagocytic ability in RAW 264.7 cells that were exposed to either 21% O₂ (room air) or 95% O₂ (hyperoxia). Moderate concentrations (ranging from 10 to 250 μM) of H₂O₂ significantly enhanced macrophage phagocytic activity and restored hyperoxia-suppressed phagocytosis through attenuation of hyperoxia-induced disorganization of actin cytoskeleton and actin oxidation. These results indicate that H₂O₂ at low-moderate concentrations can be beneficial to host immune responses by improving macrophage phagocytic activity.

Oxygen toxicity and reactive oxygen species: the devil is in the details

Reactive oxygen species (ROS) serve as cell signaling molecules for normal biologic processes. However, the generation of ROS can also provoke damage to multiple cellular organelles and processes, which can ultimately disrupt normal physiology. An imbalance between the production of ROS and the antioxidant defenses that protect cells has been implicated in the pathogenesis of a variety of diseases, such as cancer, asthma, pulmonary hypertension, and retinopathy. The nature of the injury will ultimately depend on specific molecular interactions, cellular locations, and timing of the insult. This review will outline the origins of endogenous and exogenously generated ROS. The molecular, cellular, pathologic, and physiologic targets will then be discussed with a particular emphasis on aspects relevant to child development. Finally, antioxidant defenses that scavenge ROS and mitigate associated toxicities will be presented, with a discussion of potential therapeutic approaches for the prevention and/or treatment of human diseases using enzymatic and nonenzymatic antioxidants.