Linking oxidative events to inflammatory and adaptive gene expression induced by exposure to an organic particulate matter component

Live cell imaging of oxidative stress in human airway epithelial cells exposed to isoprene hydroxyhydroperoxide

Exposure to respirable air particulate matter (PM2.5) in ambient air is associated with morbidity and premature deaths. A major source of PM2.5 is the photooxidation of volatile plant-produced organic compounds such as isoprene. Photochemical oxidation of isoprene leads to the formation of hydroperoxides, environmental oxidants that lead to inflammatory (IL-8) and adaptive (HMOX1) gene expression in human airway epithelial cells (HAEC). To examine the mechanism through which these oxidants alter intracellular redox balance, we used live-cell imaging to monitor the effects of isoprene hydroxyhydroperoxides (ISOPOOH) in HAEC expressing roGFP2, a sensor of the glutathione redox potential (EGSH). Non-cytotoxic exposure of HAEC to ISOPOOH resulted in a rapid and robust increase in EGSH that was independent of the generation of intracellular or extracellular hydrogen peroxide. Our results point to oxidation of GSH through the redox relay initiated by glutathione peroxidase 4, directly by ISOPOOH or indirectly by ISOPOOH-generated lipid hydroperoxides. We did not find evidence for involvement of peroxiredoxin 6. Supplementation of HAEC with polyunsaturated fatty acids enhanced ISOPOOH-induced glutathione oxidation, providing additional evidence that ISOPOOH initiates lipid peroxidation of cellular membranes. These findings demonstrate that ISOPOOH is a potent environmental airborne hydroperoxide with the potential to contribute to oxidative burden of human airway posed by inhalation of secondary organic aerosols.

Airway epithelial Paraoxonase-2 in obese asthma

Background

Obesity in asthmatics has been associated with higher airway oxidative stress in which dysfunctional mitochondria are a potential contributing source of excess free radicals. Paraoxonase 2 (PON2) plays an important role in reducing mitochondrial-derived oxidative stress and could, therefore, have therapeutic potential in these patients.

Objectives

We used primary human bronchial epithelial cells (HBECs) from asthmatics and healthy controls to evaluate: a) protein levels of Paraoxonase 2 and b) to test the potential protective effect of quercetin supplementation in cells under oxidative stress conditions.

Results

Compared to lean controls, obese asthmatics had significantly lower PON2 airway epithelial levels (respectively, 1.08 vs. 0.47 relative units normalized by GAPDH) (p-value < 0.006). Treating HBECs in vitro for 24 hrs. with 25μM quercetin significantly increased PON2 protein levels: 15.5 treated cells vs. 9.8 untreated cells (relative units normalized by GAPDH) (p value = 0.004). Notably, compared to untreated cells, quercetin supplementation reduces mitochondrial superoxide and hydrogen peroxide production on HBECs cells exposed to different oxidative stress triggers such as 1–2 Naphthoquinone (1–2 NQ) and hydrogen peroxide, suggesting that PON2 might play a protective role ameliorating oxidative injury on human airway epithelium.

Conclusion

Compared to lean controls, obese asthmatics have significantly reduced PON2 levels in airway epithelial cells. Treatment with quercetin in vitro increased PON2 protein levels and prevented oxidative stress from different types of stimuli. Hence, quercetin supplementation may be a potential therapeutic strategy to prevent obesity-mediated airway oxidative stress in obese asthmatics.

Exposure Effects Beyond the Epithelial Barrier: Transepithelial Induction of Oxidative Stress by Diesel Exhaust Particulates in Lung Fibroblasts in an Organotypic Human Airway Model

In vitro bronchial epithelial monoculture models have been pivotal in defining the adverse effects of inhaled toxicant exposures; however, they are only representative of one cellular compartment and may not accurately reflect the effects of exposures on other cell types. Lung fibroblasts exist immediately beneath the bronchial epithelial barrier and play a central role in lung structure and function, as well as disease development and progression. We tested the hypothesis that in vitro exposure of a human bronchial epithelial cell barrier to the model oxidant diesel exhaust particulates caused transepithelial oxidative stress in the underlying lung fibroblasts using a human bronchial epithelial cell and lung fibroblast coculture model. We observed that diesel exhaust particulates caused transepithelial oxidative stress in underlying lung fibroblasts as indicated by intracellular accumulation of the reactive oxygen species hydrogen peroxide, oxidation of the cellular antioxidant glutathione, activation of NRF2, and induction of oxidative stress-responsive genes. Further, targeted antioxidant treatment of lung fibroblasts partially mitigated the oxidative stress response gene expression in adjacent human bronchial epithelial cells during diesel exhaust particulate exposure. This indicates that exposure-induced oxidative stress in the airway extends beyond the bronchial epithelial barrier and that lung fibroblasts are both a target and a mediator of the adverse effects of inhaled chemical exposures despite being separated from the inhaled material by an epithelial barrier. These findings illustrate the value of coculture models and suggest that transepithelial exposure effects should be considered in inhalation toxicology research and testing.

Airborne Aerosols and Human Health: Leapfrogging from Mass Concentration to Oxidative Potential

The mass concentration of atmospheric particulate matter (PM) has been systematically used in epidemiological studies as an indicator of exposure to air pollutants, connecting PM concentrations with a wide variety of human health effects. However, these effects can be hardly explained by using one single parameter, especially because PM is formed by a complex mixture of chemicals. Current research has shown that many of these adverse health effects can be derived from the oxidative stress caused by the deposition of PM in the lungs. The oxidative potential (OP) of the PM, related to the presence of transition metals and organic compounds that can induce the production of reactive oxygen and nitrogen species (ROS/RNS), could be a parameter to evaluate these effects. Therefore, estimating the OP of atmospheric PM would allow us to evaluate and integrate the toxic potential of PM into a unique parameter, which is related to emission sources, size distribution and/or chemical composition. However, the association between PM and particle-induced toxicity is still largely unknown. In this commentary article, we analyze how this new paradigm could help to deal with some unanswered questions related to the impact of atmospheric PM over human health.

Toxicity of iron nanoparticles towards primary cultures of human bronchial epithelial cells

Air pollution is a public health issue and the toxicity of ambient particulate matter (PM) is well-recognized. Although it does not mostly contribute to the total mass of PM, increasing evidence indicates that the ultrafine fraction has generally a greater toxicity than the others do. A better knowledge of the underlying mechanisms involved in the pathological disorders related to nanoparticles (NPs) remains essential. Hence, the goal of this study was to determine better whether the exposure to a relatively low dose of well-characterized iron-rich NPs (Fe-NPs) might alter some critical toxicological endpoints in a relevant primary culture model of human bronchial epithelial cells (HBECs). We sought to use Fe-NPs representative of those frequently found in the industrial smokes of metallurgical industries. After having noticed the effective internalization of Fe-NPs, oxidative, inflammatory, DNA repair, and apoptotic endpoints were investigated within HBECs, mainly through transcriptional screening. Taken together, these results revealed that, despite it only produced relatively low levels of reactive oxygen species without any significant oxidative damage, low-dose Fe-NPs quickly significantly deregulated the transcription of some target genes closely involved in the proinflammatory response. Although this inflammatory process seemed to stay under control over time in case of this acute scenario of exposure, the future study of its evolution after a scenario of repeated exposure could be very interesting to evaluate the toxicity of Fe-NPs better.

Dithiothreitol-based oxidative potential for airborne particulate matter: an estimation of the associated uncertainty

Oxidative stress is considered as one of the main mechanisms by which airborne particles produce adverse health effects. Several methods to estimate the oxidative potential (OP) of particulate matter (PM) have been proposed. Among them, the dithiothreitol (DTT) assay has gained popularity due to its simplicity and overall low implementation cost. Usually, the estimations of OP^DTT are based on n-replicates of a set of samples and their associated standard deviation. However, interlaboratory comparisons of OP^DTT can be difficult and lead to misinterpretations. This work presents an estimation of the total uncertainty for the OP^DTT measurement of PM₁₀ and PM_2.5 samples collected in Santiago (Chile), based on recommendations by the Joint Committee for Guides in Metrology and Eurachem. The expanded uncertainty expressed as a percentage of the mass-normalized OP^DTT measurements was 18.0% and 16.3% for PM₁₀ and PM_2.5 samples respectively. The dominating contributor to the total uncertainty was identified (i.e., DTT consumption rate, related to the regression and repeatability of experimental data), while the volumetric operations (i.e., pipettes) were also important. The results showed that, although the OP measured following the DTT assay has been successfully used to estimate the potential health impacts of airborne PM, uncertainty estimations must be considered before interpreting the results.

Molecular mechanism and health effects of 1,2-Naphtoquinone

Extensive literature regarding the health side effects of ambient pollutants (AP) are available, such as diesel exhaust particles (DEPs), but limited studies are available on their electrophilic contaminant 1,2-Naphthoquinone (1,2-NQ), enzymatically derived from naphthalene. This review summarizes relevant toxicologic and biological properties of 1,2-NQ as an environmental pollutant or to a lesser degree as a backbone in drug development to treat infectious diseases. It presents evidence of 1,2-NQ-mediated genotoxicity, neurogenic inflammation, and cytotoxicity due to several mechanistic properties, including the production of reactive oxygen species (ROS), that promote cell damage, carcinogenesis, and cell death. Many signal transduction pathways act as a vulnerable target for 1,2-NQ, including kappaB kinase b (IKKbeta) and protein tyrosine phosphatase 1B (PTP1B). Antioxidant molecules act in defense against ROS/RNS-mediated 1,2-NQ responses to injury. Nonetheless, its inhibitory effects at PTP1B, altering the insulin signaling pathway, represents a new therapeutic target to treat diabetes type 2. Questions exist whether exposure to 1,2-NQ may promote arylation of the Keap1 factor, a negative regulator of Nrf2, as well as acting on the sepiapterin reductase activity, an NADPH-dependent enzyme which catalyzes the formation of critical cofactors in aromatic amino acid metabolism and nitric oxide biosynthesis. Exposure to 1,2-NQ is linked to neurologic, behavioral, and developmental disturbances as well as increased susceptibility to asthma. Limited new knowledge exists on molecular modeling of quinones molecules as antitumoral and anti-microorganism agents. Altogether, these studies suggest that 1,2-NQ and its intermediate compounds can initiate a number of pathological pathways as AP in living organisms but it can be used to better understand molecular pathways.

Investigation of inflammation inducing substances in PM2.5 particles by an elimination method using thermal decomposition

The substances associated with PM2.5-induced inflammatory response were investigated using an elimination method. PM2.5 were heated at temperatures of 120, 250, and 360°C. The results demonstrated microbial substances such as LPS and b-glucan, and chemicals including BaP, 1,2-NQ, and 9,10-PQ were reduced drastically in PM2.5 heated at 120°C. On the other hand, DBA, 7,12-BAQ, and BaP-1,6-Q were not noticeably reduced. Most of these substances had disappeared in PM2.5 heated at 250°C and 360°C. Metals (eg, Fe, Cu, Cr, Ni) in PM2.5 exhibited a slight thermo-dependent increase. RAW264.7 macrophages with or without NAC were exposed to unheated PM2.5, oxidative stress-related and unrelated inflammatory responses were induced. PM2.5-induced lung inflammation in mice is caused mainly by thermo-sensitive substances (LPS, b-glucan, BaP, 1,2-NQ, 9,10-PQ, etc.). Also, a slight involvement of thermo-resistant substances (DBA, 7,12-BAQ, BaP-1,6-Q, etc.) and transition metals was observed. The thermal decomposition method could assist to evaluate the PM2.5-induded lung inflammation.

Long chain lipid hydroperoxides increase the glutathione redox potential through glutathione peroxidase 4

BACKGROUND

Peroxidation of PUFAs by a variety of endogenous and xenobiotic electrophiles is a recognized pathophysiological process that can lead to adverse health effects. Although secondary products generated from peroxidized PUFAs have been relatively well studied, the role of primary lipid hydroperoxides in mediating early intracellular oxidative events is not well understood.

METHODS

Live cell imaging was used to monitor changes in glutathione (GSH) oxidation in HAEC expressing the fluorogenic sensor roGFP during exposure to 9-hydroperoxy-10E,12Z-octadecadienoic acid (9-HpODE), a biologically important long chain lipid hydroperoxide, and its secondary product 9-hydroxy-10E,12Z-octadecadienoic acid (9-HODE). The role of hydrogen peroxide (H₂O₂) was examined by direct measurement and through catalase interventions. shRNA-mediated knockdown of glutathione peroxidase 4 (GPx4) was utilized to determine its involvement in the relay through which 9-HpODE initiates the oxidation of GSH.

RESULTS

Exposure to 9-HpODE caused a dose-dependent increase in GSH oxidation in HAEC that was independent of intracellular or extracellular H₂O₂ production and was exacerbated by NADPH depletion. GPx4 was involved in the initiation of GSH oxidation in HAEC by 9-HpODE, but not that induced by exposure to H₂O₂ or the low molecular weight alkyl tert-butyl hydroperoxide (TBH).

CONCLUSIONS

Long chain lipid hydroperoxides can directly alter cytosolic E_GSH independent of secondary lipid oxidation products or H₂O₂ production. NADPH has a protective role against 9-HpODE induced E_GSH changes. GPx4 is involved specifically in the reduction of long-chain lipid hydroperoxides, leading to GSH oxidation.

SIGNIFICANCE

These results reveal a previously unrecognized consequence of lipid peroxidation, which may provide insight into disease states involving lipid peroxidation in their pathogenesis.

Imaging Approaches to Assessments of Toxicological Oxidative Stress Using Genetically-encoded Fluorogenic Sensors

While oxidative stress is a commonly cited toxicological mechanism, conventional methods to study it suffer from a number of shortcomings, including destruction of the sample, introduction of potential artifacts, and a lack of specificity for the reactive species involved. Thus, there is a current need in the field of toxicology for non-destructive, sensitive, and specific methods that can be used to observe and quantify intracellular redox perturbations, more commonly referred to as oxidative stress. Here, we present a method for the use of two genetically-encoded fluorogenic sensors, roGFP2 and HyPer, to be used in live-cell imaging studies to observe xenobiotic-induced oxidative responses. roGFP2 equilibrates with the glutathione redox potential (EGSH), while HyPer directly detects hydrogen peroxide (H2O2). Both sensors can be expressed into various cell types via transfection or transduction, and can be targeted to specific cellular compartments. Most importantly, live-cell microscopy using these sensors offers high spatial and temporal resolution that is not possible using conventional methods. Changes in the fluorescence intensity monitored at 510 nm serves as the readout for both genetically-encoded fluorogenic sensors when sequentially excited by 404 nm and 488 nm light. This property makes both sensors ratiometric, eliminating common microscopy artifacts and correcting for differences in sensor expression between cells. This methodology can be applied across a variety of fluorometric platforms capable of exciting and collecting emissions at the prescribed wavelengths, making it suitable for use with confocal imaging systems, conventional wide-field microscopy, and plate readers. Both genetically-encoded fluorogenic sensors have been used in a variety of cell types and toxicological studies to monitor cellular EGSH and H2O2 generation in real-time. Outlined here is a standardized method that is widely adaptable across cell types and fluorometric platforms for the application of roGFP2 and HyPer in live-cell toxicological assessments of oxidative stress.

Investigating mitochondrial dysfunction in human lung cells exposed to redox-active PM components

Exposure to ambient particulate matter (PM) causes cardiopulmonary morbidity and mortality through mechanisms that involve oxidative stress. 1,2-naphthoquinone (1,2-NQ) is a ubiquitous component of PM and a potent redox-active electrophile. We previously reported that 1,2-NQ increases mitochondrial H₂O₂ production through an unidentified mechanism. We sought to characterize the effects of 1,2-NQ exposure on mitochondrial respiration as a source of H₂O₂ in human airway epithelial cells. We measured the effects of acute exposure to 1,2-NQ on oxygen consumption rate (OCR) in the human bronchial epithelial cell line BEAS-2B and mitochondrial preparations using extracellular flux analysis. Complex-specific assays and NADPH depletion by glucose deprivation distinguished between mitochondrial and non-mitochondrial oxygen utilization. 1,2-NQ exposure of BEAS cells caused a rapid, marked dose-dependent increase in OCR that was independent of mitochondrial respiration, exceeded the OCR observed after mitochondrial uncoupling, and remained sensitive to NADPH depletion, implicating extra-mitochondrial redox cycling processes. Similar effects were observed with the environmentally relevant redox-cycling quinones 1,4-naphthoquinone and 9,10-phenanthrenequinone, but not with quinones that do not redox cycle, such as 1,4-benzoquinone. In mitochondrial preparations, 1,2-NQ caused a decrease in Complex I-linked substrate oxidation, suggesting impairment of pyruvate utilization or transport, a novel mechanism of mitochondrial inhibition by an environmental exposure. This study also highlights the methodological utility and challenges in the use of extracellular flux analysis to elucidate the mechanisms of action of redox-active electrophiles present in ambient air.

Mutagenicity and oxidative damage induced by an organic extract of the particulate emissions from a simulation of the deepwater horizon surface oil burns

Emissions from oil fires associated with the "Deepwater Horizon" explosion and oil discharge that began on April 20, 2010 in the Gulf of Mexico were analyzed chemically to only a limited extent at the time but were shown to induce oxidative damage in vitro and in mice. To extend this work, we burned oil floating on sea water and performed extensive chemical analyses of the emissions (Gullett et al., Marine Pollut Bull, in press, ). Here, we examine the ability of a dichloromethane extract of the particulate material with an aerodynamic size ≤ 2.5 µm (PM2.5 ) from those emissions to induce oxidative damage in human lung cells in vitro and mutagenicity in 6 strains of Salmonella. The extract had a percentage of extractable organic material (EOM) of 7.0% and increased expression of the heme oxygenase (HMOX1) gene in BEAS-2B cells after exposure for 4 hr at 20 µg of EOM/ml. However, the extract did not alter mitochondrial respiration rate as measured by extracellular flux analysis. The extract was most mutagenic in TA100 +S9, indicative of a role for polycyclic aromatic hydrocarbons (PAHs), reflective of the high concentrations of PAHs in the emissions (1 g/kg of oil consumed). The extract had a mutagenicity emission factor of 1.8 ± 0.1 × 105 revertants/megajoulethermal in TA98 +S9, which was greater than that of diesel exhaust and within an order of magnitude of open burning of wood and plastic. Thus, organics from PM2.5 of burning oil can induce oxidative responses in human airway epithelial cells and are highly mutagenic. Environ. Mol. Mutagen. 58:162-171, 2017. © 2017 Wiley Periodicals, Inc.

Exposure to High-Dose Diesel Exhaust Particles Induces Intracellular Oxidative Stress and Causes Endothelial Apoptosis in Cultured In Vitro Capillary Tube Cells

Previous studies suggest a direct correlation between exposure to diesel exhaust particles (DEP) and the onset of vascular permeability, presumably through the disruption of the adherens junctions. This would lead to deleterious effects on vasculature, such as acute myocardial infarction and atherosclerosis. Although the mechanism remains unclear, we demonstrate DEP-induced mitochondrial reactive oxygen species generation, which may be a central cause of the above vascular disorders. In vitro capillary-like HUVEC tube cells are used in this study and show that acute DEP exposure stimulates ATP depletion, followed by depolarization of their actin cytoskeleton, which sequentially inhibits PI3K/Akt activity and induces endothelial apoptosis. These events are accompanied by induction of p53/Mdm2 feedback regulation at 10 µg/mL DEP and produce 20 % cell apoptosis. Nevertheless, 100 µg/mL DEP augments tube cell apoptosis up to 70 % but disrupts the p53 negative regulator Mdm2. Addition of N-acetylcysteine provides substantial protection against the cytotoxic effects of DEP. In summary, exposure to a low dose of DEP actin triggers cytoskeleton depolarization, reduces PI3K/Akt activity, and induces a p53/Mdm2 feedback loop, and a high dose causes apoptosis by depleting Mdm2.

Live-cell imaging approaches for the investigation of xenobiotic-induced oxidant stress

BACKGROUND

Oxidant stress is arguably a universal feature in toxicology. Research studies on the role of oxidant stress induced by xenobiotic exposures have typically relied on the identification of damaged biomolecules using a variety of conventional biochemical and molecular techniques. However, there is increasing evidence that low-level exposure to a variety of toxicants dysregulates cellular physiology by interfering with redox-dependent processes.

SCOPE OF REVIEW

The study of events involved in redox toxicology requires methodology capable of detecting transient modifications at relatively low signal strength. This article reviews the advantages of live-cell imaging for redox toxicology studies.

MAJOR CONCLUSIONS

Toxicological studies with xenobiotics of supra-physiological reactivity require careful consideration when using fluorogenic sensors in order to avoid potential artifacts and false negatives. Fortunately, experiments conducted for the purpose of validating the use of these sensors in toxicological applications often yield unexpected insights into the mechanisms through which xenobiotic exposure induces oxidant stress.

GENERAL SIGNIFICANCE

Live-cell imaging using a new generation of small molecule and genetically encoded fluorophores with excellent sensitivity and specificity affords unprecedented spatiotemporal resolution that is optimal for redox toxicology studies. This article is part of a Special Issue entitled Air Pollution, edited by Wenjun Ding, Andrew J. Ghio and Weidong Wu.

Dissecting Redox Biology Using Fluorescent Protein Sensors

SIGNIFICANCE

Fluorescent protein sensors have revitalized the field of redox biology by revolutionizing the study of redox processes in living cells and organisms.

RECENT ADVANCES

Within one decade, a set of fundamental new insights has been gained, driven by the rapid technical development of in vivo redox sensing. Redox-sensitive yellow and green fluorescent protein variants (rxYFP and roGFPs) have been the central players.

CRITICAL ISSUES

Although widely used as an established standard tool, important questions remain surrounding their meaningful use in vivo. We review the growing range of thiol redox sensor variants and their application in different cells, tissues, and organisms. We highlight five key findings where in vivo sensing has been instrumental in changing our understanding of redox biology, critically assess the interpretation of in vivo redox data, and discuss technical and biological limitations of current redox sensors and sensing approaches.

FUTURE DIRECTIONS

We explore how novel sensor variants may further add to the current momentum toward a novel mechanistic and integrated understanding of redox biology in vivo. Antioxid. Redox Signal. 24, 680-712.

Incomplete lung recovery following sub-acute inhalation of combustion-derived ultrafine particles in mice

Background

Particulate matter (PM) is one of the six criteria pollutant classes for which National Ambient Air Quality Standards have been set by the United States Environmental Protection Agency. Exposures to PM have been correlated with increased cardio-pulmonary morbidity and mortality. Butadiene soot (BDS), generated from the incomplete combustion of 1,3-butadiene (BD), is both a model PM mixture and a real-life example of a petrochemical product of incomplete combustion. There are numerous events, including wildfires, accidents at refineries and tank car explosions that result in sub-acute exposure to high levels of airborne particles, with the people exposed facing serious health problems. These real-life events highlight the need to investigate the health effects induced by short-term exposure to elevated levels of PM, as well as to assess whether, and if so, how well these adverse effects are resolved over time. In the present study, we investigated the extent of recovery of mouse lungs 10 days after inhalation exposures to environmentally-relevant levels of BDS aerosols had ended.

Methods

Female BALB/c mice exposed to either HEPA-filtered air or to BDS (5 mg/m³ in HEPA filtered air, 4 h/day, 21 consecutive days) were sacrificed immediately, or 10 days after the final BDS exposure. Bronchoalveolar lavage fluid (BALF) was collected for cytology and cytokine analysis. Lung proteins and RNA were extracted for protein and gene expression analysis. Lung histopathology evaluation also was performed.

Results

Sub-acute exposures of mice to hydrocarbon-rich ultrafine particles induced: (1) BALF neutrophil elevation; (2) lung mucosal inflammation, and (3) increased BALF IL-1β concentration; with all three outcomes returning to baseline levels 10 days post-exposure. In contrast, (4) lung connective tissue inflammation persisted 10 days post-exposure; (5) we detected time-dependent up-regulation of biotransformation and oxidative stress genes, with incomplete return to baseline levels; and (6) we observed persistent particle alveolar load following 10 days of recovery.

Conclusion

These data show that 10 days after a 21-day exposure to 5 mg/m³ of BDS has ended, incomplete lung recovery promotes a pro-biotransformation, pro-oxidant, and pro-inflammatory milieu, which may be a starting point for potential long-term cardio-pulmonary effects.

Tumour-promoting activity of polycyclic aromatic hydrocarbons and their oxygenated or nitrated derivatives

Various types of polycyclic aromatic compounds (PACs) in diesel exhaust particles are thought to contribute to carcinogenesis in mammals. Although the carcinogenicity, mutagenicity and tumour-initiating activity of these compounds have been evaluated, their tumour-promoting activity is unclear. In the present study, to determine the tumour-inducing activity of PACs, including previously known mutagenic compounds in atmospheric environments, a transformation assay for promoting activity mediated by the release of contact inhibition was conducted for six polycyclic aromatic hydrocarbons (PAHs), seven oxygenated PAHs (oxy-PAHs) and seven nitrated PAHs (nitro-PAHs) using mouse embryonic fibroblast cells transfected with the v-Ha-ras gene (Bhas 42 cells). Of these, two PAHs [benzo[k]fluoranthene (B[k]FA) and benzo[b]fluoranthene (B[b]FA)], one oxy-PAH [6H-benzo[cd]pyren-6-one (BPO)] and two nitro-PAHs (3-nitro-7H-benz[de]anthracen-7-one and 6-nitrochrysene) were found to exhibit particularly powerful tumour-promoting activity (≥10 foci following exposure to <100nM). In addition, clear mRNA expression of CYP1A1, which is associated with aryl hydrocarbon receptor (AhR)-mediated activation, was observed following the exposure of cells to two PAHs (B[k]FA and B[b]FA) and three oxy-PAHs (1,2-naphthoquinone, 11H-benzo[b]fluoren-11-one and BPO). Further, an HO-1 antioxidant response activation was observed following exposure to B[k]FA, B[b]FA and BPO, suggesting that the induction of tumour-promoting activity in these compounds is correlated with the dysfunction of signal transduction via AhR-mediated responses and/or oxidative stress responses.

Protein Sulfenylation: A Novel Readout of Environmental Oxidant Stress

Oxidative stress is a commonly cited mechanism of toxicity of environmental agents. Ubiquitous environmental chemicals such as the diesel exhaust component 1,2-naphthoquinone (1,2-NQ) induce oxidative stress by redox cycling, which generates hydrogen peroxide (H2O2). Cysteinyl thiolate residues on regulatory proteins are subjected to oxidative modification by H2O2 in physiological contexts and are also toxicological targets of oxidant stress induced by environmental contaminants. We investigated whether exposure to environmentally relevant concentrations of 1,2-NQ can induce H2O2-dependent oxidation of cysteinyl thiols in regulatory proteins as a readout of oxidant stress in human airway epithelial cells. BEAS-2B cells were exposed to 0-1000 μM 1,2-NQ for 0-30 min, and levels of H2O2 were measured by ratiometric spectrofluorometry of HyPer. H2O2-dependent protein sulfenylation was measured using immunohistochemistry, immunoblotting, and isotopic mass spectrometry. Catalase overexpression was used to investigate the relationship between H2O2 generation and protein sulfenylation in cells exposed to 1,2-NQ. Multiple experimental approaches showed that exposure to 1,2-NQ at concentrations as low as 3 μM induces H2O2-dependent protein sulfenylation in BEAS-2B cells. Moreover, the time of onset and duration of 1,2-NQ-induced sulfenylation of the regulatory proteins GAPDH and PTP1B showed significant differences. Oxidative modification of regulatory cysteinyl thiols in human lung cells exposed to relevant concentrations of an ambient air contaminant represents a novel marker of oxidative environmental stress.

Protective Effect of Folic Acid on Oxidative DNA Damage: A Randomized, Double-Blind, and Placebo Controlled Clinical Trial

Although previous reports have linked DNA damage with both transmissions across generations as well as our own survival, it is unknown how to reverse the lesion. Based on the data from a Randomized, Double-blind, Placebo Controlled Clinical Trial, this study aimed to assess the efficacy of folic acid supplementation (FAS) on DNA oxidative damage reversal.In this randomized clinical trial (RCT), a total of 450 participants were enrolled and randomly assigned to 3 groups to receive folic acid (FA) 0.4 mg/day (low-FA), 0.8 mg/day (high-FA), or placebo (control) for 8 weeks. The urinary 8-hydroxy-2'-deoxyguanosine (8-OHdG) and creatinine (Cr) concentration at pre- and post-FAS were measured with modified enzyme-linked immunosorbent assay (ELISA) and high-performance liquid chromatography (HPLC), respectively. A multivariate general linear model was applied to assess the individual effects of FAS and the joint effects between FAS and hypercholesterolemia on oxidative DNA damage improvement. This clinical trial was registered with ClinicalTrials.gov, number NCT02235948.Of the 438 subjects that received FA fortification or placebo, the median (first quartile, third quartile) of urinary 8-OHdG/Cr for placebo, low-FA, and high-FA groups were 58.19 (43.90, 82.26), 53.51 (38.97, 72.74), 54.73 (39.58, 76.63) ng/mg at baseline and 57.77 (44.35, 81.33), 51.73 (38.20, 71.30), and 50.65 (37.64, 76.17) ng/mg at the 56th day, respectively. A significant decrease of urinary 8-OHdG was observed after 56 days FA fortification (P < 0.001). Compared with the placebo, after adjusting for some potential confounding factors, including the baseline urinary 8-OHdG/Cr, the urinary 8-OHdG/Cr concentration significantly decreased after 56 days FAS [β (95% confidence interval) = -0.88 (-1.62, -0.14) and P = 0.020 for low-FA; and β (95% confidence interval) = -2.68 (-3.42, -1.94) and P < 0.001 for high-FA] in a dose-response fashion (Ptrend < 0.001). Test of interaction between hypercholesterolemia and FA supplementation on urinary 8-OHdG reduction was significant (P = 0.001).The present study demonstrates that FA fortification is independently linked to the reduction of urinary 8-OHdG/Cr in a dose-related pattern, which suggests that FA is beneficial to protect against oxidative damage to DNA. This effect is apparently stronger in those with hypercholesterolemia. The authors provide a new insight into the prevention and reversal of oxidative DNA damage.

The Transcriptional Effects of PCB118 and PCB153 on the Liver, Adipose Tissue, Muscle and Colon of Mice: Highlighting of Glut4 and Lipin1 as Main Target Genes for PCB Induced Metabolic Disorders

Epidemiological studies have associated environmental exposure to polychlorinated biphenyls (PCBs) with an increased risk of type 2 diabetes; however, little is known about the underlying mechanisms involved in the metabolic side-effects of PCB. Our study evaluated the transcriptional effects of a subchronic exposure (gavage at Day 0 and Day 15 with 10 or 100 μmol/Kg bw) to PCB118 (dioxin-like PCB), PCB153 (non-dioxin-like PCB), or an equimolar mixture of PCB118 and PCB153 on various tissues (liver, visceral adipose tissue, muscle, and colon) in mice. Our results showed that a short-term exposure to PCB118 and/or PCB153 enhanced circulating triglyceride levels but did not affect glycemia. Among the studied tissues, we did not observe any modification of the expression of inflammation-related genes, such as cytokines or chemokines. The main transcriptional effects were observed in visceral adipose and liver tissues. We found a downregulation of lipin1 and glut4 expression in these two target organs. In adipose tissue, we also showed a downregulation of Agpat2, Slc25a1, and Fasn. All of these genes are involved in lipid metabolism and insulin resistance. In muscles, we observed an induction of CnR1 and Foxo3 expression, which may be partly involved in PCB metabolic effects. In summary, our results suggest that lipin1 and glut4, notably in adipose tissue, are the main targeted genes in PCB-induced metabolic disorders, however, further studies are required to fully elucidate the mechanisms involved.

A controlled trial of acute effects of human exposure to traffic particles on pulmonary oxidative stress and heart rate variability

Background

For many individuals, daily commuting activities on roadways account for a substantial proportion of total exposure, as well as peak-level exposures, to traffic-related air pollutants (TRAPS) including ultrafine particles, but the health impacts of these exposures are not well-understood. We sought to determine if exposure to TRAPs particles during commuting causes acute oxidative stress in the respiratory tract or changes in heart rate variability (HRV), a measure of autonomic activity.

Methods

We conducted a randomized, cross-over trial in which twenty-one young adults took two 1.5-hr rides in a passenger vehicle in morning rush-hour traffic. The subjects wore a powered-air-purifying respirator, and were blinded to high-efficiency particulate air (HEPA) filtration during one of the rides. At time points before and after the rides, we measured HRV and markers of oxidative stress in exhaled breath condensate (EBC) including nitrite, the sum of nitrite and nitrate, malondialdehyde, and 8-isoprostane. We used mixed linear models to evaluate the effect of exposure on EBC and HRV outcomes, adjusting for pre-exposure response levels. We used linear models to examine the effects of particle concentrations on EBC outcomes at post-exposure time points.

Results

Mean EBC nitrite and the sum of nitrite and nitrate were increased from baseline at immediately post-exposure comparing unfiltered to filtered rides (2.11 μM vs 1.70 μM, p = 0.02 and 19.1 μM vs 10.0 μM, p = 0.02, respectively). Mean EBC malondialdehyde (MDA) concentrations were about 10% greater following the unfiltered vs. filtered exposures, although this result was not statistically significant. We found no significant associations between exposure to traffic particles and HRV outcomes at any of the time points. At immediately post-exposure, an interquartile range increase in particle number concentration was associated with statistically significant increases in nitrite (99.4%, 95% CI 32.1% to 166.7%) and nitrite + nitrate (75.7%, 95% CI 21.5% to 130.0%).

Conclusions

Increases in markers of oxidative stress in EBC may represent early biological responses to widespread exposures to TRAPs particles that affect passengers in vehicles on heavily trafficked roadways.

Role of H2O2 in the oxidative effects of zinc exposure in human airway epithelial cells

Human exposure to particulate matter (PM) is a global environmental health concern. Zinc (Zn²⁺) is a ubiquitous respiratory toxicant that has been associated with PM health effects. However, the molecular mechanism of Zn²⁺ toxicity is not fully understood. H₂O₂ and Zn²⁺ have been shown to mediate signaling leading to adverse cellular responses in the lung and we have previously demonstrated Zn²⁺ to cause cellular H₂O₂ production. To determine the role of Zn²⁺-induced H₂O₂ production in the human airway epithelial cell response to Zn²⁺ exposure. BEAS-2B cells expressing the redox-sensitive fluorogenic sensors HyPer (H₂O₂) or roGFP2 (E_GSH) in the cytosol or mitochondria were exposed to 50 µM Zn²⁺ for 5 min in the presence of 1 µM of the zinc ionophore pyrithione. Intracellular H₂O₂ levels were modulated using catalase expression either targeted to the cytosol or ectopically to the mitochondria. HO-1 mRNA expression was measured as a downstream marker of response to oxidative stress induced by Zn²⁺ exposure. Both cytosolic catalase overexpression and ectopic catalase expression in mitochondria were effective in ablating Zn²⁺-induced elevations in H₂O₂. Compartment-directed catalase expression blunted Zn²⁺-induced elevations in cytosolic E_GSH and the increased expression of HO-1 mRNA levels. Zn²⁺ leads to multiple oxidative effects that are exerted through H₂O₂-dependent and independent mechanisms.

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We used targeted catalase expression to examine the role of H₂O₂ in Zn²⁺-induced effects.

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Cytosolic or mitochondrial catalase ablated Zn²⁺-induced mitochondrial H₂O₂ production.

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Catalase expression blunted Zn²⁺-induced cytosolic E_GSH and HO-1 mRNA.

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Independently, decreasing GSH_total or increasing E_GSH failed to induce HO-1 mRNA.

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Zn²⁺ causes multiple oxidative effects by H₂O₂-dependent and independent mechanisms.

Emissions from commercial-grade charbroiling meat operations induce oxidative stress and inflammatory responses in human bronchial epithelial cells

Commercial charbroiling emissions are a significant source of ambient particulate matter (PM) in urban settings. The objective of this study was to determine whether organic extract of PM emissions from commercial charbroiling meat operations could induce an inflammatory response in human bronchial epithelial cells and whether this effect was mediated by oxidative stress. PM samples were collected during cooking hamburgers on a commercial-grade under-fired charbroiler and sequentially extracted with water and methanol to obtain the aqueous PM suspension (AqPM) and organic extract (OE). The pro-oxidative and pro-inflammatory effects of OE were assessed using human bronchial epithelial cell line BEAS-2B. While AqPM did not have any effect, OE effectively induced the expression of heme oxygennase-1 and cyclooxygenase-2 in BEAS-2B cells. OE also up-regulated the levels of IL-6, IL-8, and prostaglandin E2. OE-induced cellular inflammatory response could be effectively suppressed by the antioxidant N-acetyl cysteine, nuclear factor (erythroid-derived 2)-like 2 activator sulforaphane and p38 MAPK inhibitor SB203580. In conclusion, organic chemicals emitted from commercial charbroiling meat operations could induce an inflammatory response in human bronchial epithelial cells, which was mediated by oxidative stress and p38 MAPK.

Monitoring intracellular redox changes in ozone-exposed airway epithelial cells

BACKGROUND

The toxicity of many xenobiotic compounds is believed to involve oxidative injury to cells. Direct assessment of mechanistic events involved in xenobiotic-induced oxidative stress is not easily achievable. Development of genetically encoded probes designed for monitoring intracellular redox changes represents a methodological advance with potential applications in toxicological studies.

OBJECTIVE

We tested the utility of redox-sensitive green fluorescent protein (roGFP)-based redox sensors for monitoring real-time intracellular redox changes induced by xenobiotics in toxicological studies.

METHODS

roGFP2, a reporter of the glutathione redox potential (E(GSH)), was used to monitor EGSH in cultured human airway epithelial cells (BEAS-2B cells) undergoing exposure to 0.15-1.0 ppm ozone (O(3)). Cells were imaged in real time using a custom-built O(3) exposure system coupled to a confocal microscope.

RESULTS

O(3) exposure induced a dose- and time-dependent increase of the cytosolic EGSH. Additional experiments confirmed that roGFP2 is not directly oxidized, but properly equilibrates with the glutathione redox couple: Inhibition of endogenous glutaredoxin 1 (Grx1) disrupted roGFP2 responses to O(3), and a Grx1-roGFP2 fusion protein responded more rapidly to O(3) exposure. Selenite-induced up-regulation of GPx (glutathione peroxidase) expression-enhanced roGFP2 responsiveness to O(3), suggesting that (hydro)peroxides are intermediates linking O(3) exposure to glutathione oxidation.

CONCLUSION

Exposure to O(3) induces a profound increase in the cytosolic E(GSH) of airway epithelial cells that is indicative of an oxidant-dependent impairment of glutathione redox homeostasis. These studies demonstrate the utility of using genetically encoded redox reporters in making reliable assessments of cells undergoing exposure to xenobiotics with strong oxidizing properties.

Inflammation-related effects of diesel engine exhaust particles: studies on lung cells in vitro

Diesel exhaust and its particles (DEP) have been under scrutiny for health effects in humans. In the development of these effects inflammation is regarded as a key process. Overall, in vitro studies report similar DEP-induced changes in markers of inflammation, including cytokines and chemokines, as studies in vivo. In vitro studies suggest that soluble extracts of DEP have the greatest impact on the expression and release of proinflammatory markers. Main DEP mediators of effects have still not been identified and are difficult to find, as fuel and engine technology developments lead to continuously altered characteristics of emissions. Involved mechanisms remain somewhat unclear. DEP extracts appear to comprise components that are able to activate various membrane and cytosolic receptors. Through interactions with receptors, ion channels, and phosphorylation enzymes, molecules in the particle extract will trigger various cell signaling pathways that may lead to the release of inflammatory markers directly or indirectly by causing cell death. In vitro studies represent a fast and convenient system which may have implications for technology development. Furthermore, knowledge regarding how particles elicit their effects may contribute to understanding of DEP-induced health effects in vivo, with possible implications for identifying susceptible groups of people and effect biomarkers.

Carbon nanoparticles induce ceramide- and lipid raft-dependent signalling in lung epithelial cells: a target for a preventive strategy against environmentally-induced lung inflammation

Background

Particulate air pollution in lung epithelial cells induces pathogenic endpoints like proliferation, apoptosis, and pro-inflammatory reactions. The activation of the epidermal growth factor receptor (EGFR) is a key event responsible for signalling events involving mitogen activated protein kinases specific for these endpoints. The molecular events leading to receptor activation however are not well understood. These events are relevant for the toxicological evaluation of inhalable particles as well as for potential preventive strategies in situations when particulate air pollution cannot be avoided. The current study therefore had the objective to elucidate membrane-coupled events leading to EGFR activation and the subsequent signalling cascade in lung epithelial cells. Furthermore, we aimed to identify the molecular target of ectoine, a biophysical active substance which we described to prevent carbon nanoparticle-induced lung inflammation.

Methods

Membrane signalling events were investigated in isolated lipid rafts from lung epithelial cells with regard to lipid and protein content of the signalling platforms. Using positive and negative intervention approaches, lipid raft changes, subsequent signalling events, and lung inflammation were investigated in vitro in lung epithelial cells (RLE-6TN) and in vivo in exposed animals.

Results

Carbon nanoparticle treatment specifically led to an accumulation of ceramides in lipid rafts. Detailed analyses demonstrated a causal link of ceramides and subsequent EGFR activation coupled with a loss of the receptor in the lipid raft fractions. In vitro and in vivo investigations demonstrate the relevance of these events for carbon nanoparticle-induced lung inflammation. Moreover, the compatible solute ectoine was able to prevent ceramide-mediated EGFR phosphorylation and subsequent signalling as well as lung inflammation in vivo.

Conclusion

The data identify a so far unknown event in pro-inflammatory signalling and contribute to the understanding of particle cell interaction and therefore to risk identification and risk assessment of inhalable xenobiotics. Moreover, as this cellular reaction can be prevented by the well tolerated substance ectoine, a molecular preventive strategy for susceptible persons against airway inflammation is proposed.

Nitric oxide and superoxide mediate diesel particle effects in cytokine-treated mice and murine lung epithelial cells--implications for susceptibility to traffic-related air pollution

Background

Epidemiologic studies associate childhood exposure to traffic-related air pollution with increased respiratory infections and asthmatic and allergic symptoms. The strongest associations between traffic exposure and negative health impacts are observed in individuals with respiratory inflammation. We hypothesized that interactions between nitric oxide (NO), increased during lung inflammatory responses, and reactive oxygen species (ROS), increased as a consequence of traffic exposure ─ played a key role in the increased susceptibility of these at-risk populations to traffic emissions.

Methods

Diesel exhaust particles (DEP) were used as surrogates for traffic particles. Murine lung epithelial (LA-4) cells and BALB/c mice were treated with a cytokine mixture (cytomix: TNFα, IL-1β, and IFNγ) to induce a generic inflammatory state. Cells were exposed to saline or DEP (25 μg/cm²) and examined for differential effects on redox balance and cytotoxicity. Likewise, mice undergoing nose-only inhalation exposure to air or DEP (2 mg/m³ × 4 h/d × 2 d) were assessed for differential effects on lung inflammation, injury, antioxidant levels, and phagocyte ROS production.

Results

Cytomix treatment significantly increased LA-4 cell NO production though iNOS activation. Cytomix +  DEP-exposed cells incurred the greatest intracellular ROS production, with commensurate cytotoxicity, as these cells were unable to maintain redox balance. By contrast, saline + DEP-exposed cells were able to mount effective antioxidant responses. DEP effects were mediated by: (1) increased ROS including superoxide anion (O₂˙^-), related to increased xanthine dehydrogenase expression and reduced cytosolic superoxide dismutase activity; and (2) increased peroxynitrite generation related to interaction of O₂˙^- with cytokine-induced NO. Effects were partially reduced by superoxide dismutase (SOD) supplementation or by blocking iNOS induction. In mice, cytomix +  DEP-exposure resulted in greater ROS production in lung phagocytes. Phagocyte and epithelial effects were, by and large, prevented by treatment with FeTMPyP, which accelerates peroxynitrite catalysis.

Conclusions

During inflammation, due to interactions of NO and O₂˙^-, DEP-exposure was associated with nitrosative stress in surface epithelial cells and resident lung phagocytes. As these cell types work in concert to provide protection against inhaled pathogens and allergens, dysfunction would predispose to development of respiratory infection and allergy. Results provide a mechanism by which individuals with pre-existing respiratory inflammation are at increased risk for exposure to traffic-dominated urban air pollution.

Glutathione-S-transferase M1 regulation of diesel exhaust particle-induced pro-inflammatory mediator expression in normal human bronchial epithelial cells

Background

Diesel exhaust particles (DEP) contribute substantially to ambient particulate matter (PM) air pollution in urban areas. Inhalation of PM has been associated with increased incidence of lung disease in susceptible populations. We have demonstrated that the glutathione S-transferase M1 (GSTM1) null genotype could aggravate DEP-induced airway inflammation in human subjects. Given the critical role airway epithelial cells play in the pathogenesis of airway inflammation, we established the GSTM1 deficiency condition in primary bronchial epithelial cells from human volunteers with GSTM1 sufficient genotype (GSTM1+) using GSTM1 shRNA to determine whether GSTM1 deficiency could exaggerate DEP-induced expression of interleukin-8 (IL-8) and IL-1β proteins. Furthermore, the mechanisms underlying GSTM1 regulation of DEP-induced IL-8 and IL-1β expression were also investigated.

Methods

IL-8 and IL-1β protein levels were measured using enzyme-linked immunosorbent assay. GSTM1 deficiency in primary human bronchial epithelial cells was achieved using lentiviral GSTM1 shRNA particles and verified using real-time polymerase chain reaction and immunoblotting. Intracellular reactive oxygen species (ROS) production was evaluated using flow cytometry. Phosphorylation of protein kinases was detected using immunoblotting.

Results

Exposure of primary human bronchial epithelial cells (GSTM1+) to 25-100 μg/ml DEP for 24 h significantly increased IL-8 and IL-1β protein expression. Knockdown of GSTM1 in these cells further elevated DEP-induced IL-8 and IL-1β expression, implying that GSTM1 deficiency aggravated DEP-induced pro-inflammatory response. DEP stimulation induced the phosphorylation of extracellular signal-regulated kinase (ERK) and Akt, the downstream kinase of phosphoinositide 3-kinase (PI3K), in GSTM1+ bronchial epithelial cells. Pharmacological inhibition of ERK kinase and PI3K activity blocked DEP-induced IL-8 and IL-1β expression. DEP-induced ERK and Akt phosphorylation could be increased by GSTM1 knockdown. In addition, pretreatment of HBEC with the antioxidant N-acetyl cysteine significantly inhibited DEP-induced ERK and Akt phosphorylation, and subsequent IL-8 and IL-1β expression.

Conclusion

GSTM1 regulates DEP-induced IL-8 and IL-1β expression in primary human bronchial epithelial cells by modulation of ROS, ERK and Akt signaling.

Multi-walled carbon nanotubes induce COX-2 and iNOS expression via MAP kinase-dependent and -independent mechanisms in mouse RAW264.7 macrophages

BACKGROUND

Carbon nanotubes (CNTs) are engineered graphene cylinders with numerous applications in engineering, electronics and medicine. However, CNTs cause inflammation and fibrosis in the rodent lung, suggesting a potential human health risk. We hypothesized that multi-walled CNTs (MWCNTs) induce two key inflammatory enzymes in macrophages, cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS), through activation of extracellular signal-regulated kinases (ERK1,2).

METHODS

RAW264.7 macrophages were exposed to MWCNTs or carbon black nanoparticles (CBNPs) over a range of doses and time course. Uptake and subcellular localization of MWCNTs was visualized by transmission electron microscopy (TEM). Protein levels of COX-2, iNOS, and ERK1,2 (total ERK and phosphorylated ERK) were measured by Western blot analysis. Prostaglandin-E(2) (PGE(2)) and nitric oxide (NO) levels in cell supernatants were measured by ELISA and Greiss assay, respectively.

RESULTS

MWCNTs, but not CBNPs, induced COX-2 and iNOS in a time- and dose-dependent manner. COX-2 and iNOS induction by MWCNTs correlated with increased PGE(2) and NO production, respectively. MWCNTs caused ERK1,2 activation and inhibition of ERK1,2 (U0126) blocked MWCNT induction of COX-2 and PGE2 production, but did not reduce the induction of iNOS. Inhibition of iNOS (L-NAME) did not affect ERK1,2 activation, nor did L-NAME significantly decrease COX-2 induction by MWCNT. Nickel nanoparticles (NiNPs), which are present in MWCNTs as a residual catalyst, also induced COX-2 via ERK-1,2. However, a comparison of COX-2 induction by MWCNTs containing 4.5 and 1.8% Ni did not show a significant difference in ability to induce COX-2, indicating that characteristics of MWCNTs in addition to Ni content contribute to COX-2 induction.

CONCLUSION

This study identifies COX-2 and subsequent PGE(2) production, along with iNOS induction and NO production, as inflammatory mediators involved in the macrophage response to MWCNTs. Furthermore, our work demonstrates that COX-2 induction by MWCNTs in RAW264.7 macrophages is ERK1,2-dependent, while iNOS induction by MWCNTs is ERK1,2-independent. Our data also suggest contributory physicochemical factors other than residual Ni catalyst play a role in COX-2 induction to MWCNT.