Halobenzoquinone-Induced Alteration of Gene Expression Associated with Oxidative Stress Signaling Pathways

Comprehensive Toxicological Assessment of Halobenzoquinones in Drinking Water at Environmentally Relevant Concentration

Halobenzoquinones (HBQs), an emerging unregulated category of disinfection byproduct (DBP) in drinking water, have aroused an increasing concern over their potential health risks. However, the chronic toxicity of HBQs at environmentally relevant concentrations remains largely unknown. Here, the occurrence and concentrations of 13 HBQs in drinking water from a northern megacity in China were examined using ultrahigh performance liquid chromatography coupled with triple-quadrupole tandem mass spectrometry (UHPLC-MS/MS). Four HBQs, including 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), 2,6-dibromo-1,4-benzoquinone (2,6-DBBQ), 2,3,6-trichloro-1,4-benzoquinone (TriCBQ), and 2,5-dibromo-1,4-benzoquinone (2,5-DBBQ), were detected beyond 50% occurrence frequency and at median concentrations from 4 to 50 ng/L. The chronic toxicity of these four HBQs to normal human colon and liver cells (FHC and THLE-2) was investigated at these concentrations. After 90 days of exposure, 2,5-DBBQ and 2,6-DCBQ induced the highest levels of oxidative stress and deoxyribonucleic acid (DNA) damage in colon and liver cells, respectively. Moreover, 2,5-DBBQ and 2,6-DCBQ were also found to induce epithelial-mesenchymal transition (EMT) in normal human liver cells via the extracellular signal regulated kinase (ERK) signaling pathway. Importantly, heating to 100 °C (boiling) was found to efficiently reduce the levels of these four HBQs in drinking water. These results suggested that environmentally relevant concentrations of HBQs could induce cytotoxicity and genotoxicity in normal human cells, and boiling is a highly efficient way of detoxification for HBQs.

The alternation of halobenzoquinone disinfection byproduct on toxicogenomics of DNA damage and repair in uroepithelial cells

Halobenzoquinones (HBQs) were recently discovered as an emerging class of drinking water disinfection byproducts with carcinogenic concern. However, the molecular mechanism underlying HBQs-induced DNA damage is not clear. In this study, we integrated in vitro genotoxicity, computational toxicology, and the quantitative toxicogenomic analysis of HBQs on DNA damage/repair pathways in human bladder epithelial cells SV-HUC-1. The results showed that HBQs could induce cytotoxicity with the descending order as 2,6-DIBQ > 2,6-DCBQ ≈ 2,6-DBBQ. Also, HBQs can increase DNA damage in SV-HUC-1 cells and thus generate genotoxicity. However, there is no significant difference in genotoxicity among the three HBQs. The results of molecular docking and molecular dynamics simulation further confirmed that HBQs had high binding fractions and stability to DNA. Toxicogenomic analysis indicated that HBQs interfered with DNA repair pathways, mainly affecting base excision repair, nucleotide excision repair and homologous recombination repair. These results have provided new insights into the underlying molecular mechanisms of HBQs-induced DNA damage, and contributed to the understanding of the relationship between exposure to DBPs and risks of developing bladder cancer.

Redox-directed identification of toxic transformation products during ozonation of aromatics

The toxicity assessment of transformation products (TPs) formed in oxidative water treatment is crucial but challenging because of their low concentration, structural diversity, and mixture complexity. Here, this study developed a novel redox-directed approach for identification of toxic TPs without the individual toxicity and concentration information. This approach based on sodium borohydride reduction comprised an integrated process of toxicological evaluation, fluorescence excitation-emission matrix characterization, high-resolution mass spectrometry detection, followed by ecological toxicity assessment of identified TPs. The redox-directed identification of primary causative toxicants was experimentally tested for the increased nonspecific toxicity observations in the ozonated effluents of model aromatics. Reduction reaction caused a remarkable decrease in toxicity and increase in fluorescence intensity, obtaining a good linear relation between them. More than ten monomeric or dimeric p-benzoquinone (p-BQ) TPs were identified in the ozonated effluents. The occurrence of the p-BQ TPs was further verified through parallel sodium sulfite reduction and actual wastewater ozonation experiments. In vitro bioassays of luminescent bacteria, as well as in silico genotoxicity and cytotoxicity predictions, indicate that the toxicity of p-BQ TPs is significantly higher than that of their precursors and other TPs. These together demonstrated that the identified p-BQ TPs are primary toxicity contributors. The redox-directed approach facilitated the revelation of primary toxicity contribution, illustrating emerging p-BQs are a concern for aquatic ecosystem safety in the oxidative treatment of aromatics-contaminated wastewater.

Polystyrene may alter the cooperation mechanism of gut microbiota and immune system through co-exposure with DCBQ

The toxicity of Polystyrene (PS) may be higher through co-exposure with other pollutants. Human can simultaneously face the challenges from the various pollutants. Nevertheless, little research has been done on the combined effects of PS and 2,6-dichloro-p-benzoquinone (DCBQ) disinfection byproduct. Considering the potential risk of PS and DCBQ, we aimed to illustrate the effects of PS in combination with DCBQ on the immune responses of mice. We found that cotreatment of DCBQ and PS may inhibit the activity of spleen CD4+ T cells and interfere with the normal function of the immune system. Further research found that DCBQ + PS resulted in increasing amount of the inflammatory cells in intestine via histopathological evaluation. The reason might be that DCBQ + PS has changed the composition of intestinal flora, abnormally activated intestinal macrophage, and inhibited the expression of immune-related genes, thus leading to intestinal immune disorders and triggering intestinal inflammation. In summary, PS may alter the cooperation mechanism of gut microbiota and immune system through co-exposure with DCBQ. Current results suggested that more attention should be paid to the combined toxic effects of environmental contaminants.

Oxidative stress as a key event in 2,6-dichloro-1,4-benzoquinone-induced neurodevelopmental toxicity

2,6-dichloro-1,4-benzoquinone (DCBQ) has been identified as an emerging disinfection byproducts (DBPs) in drinking water and has the potential to induce neurodevelopmental toxicity. However, there is rarely a comprehensive toxicological evaluation of the neurodevelopmental toxicity of DCBQ. Here, neural differentiating SH-SY5Y cells were used as an in vitro model. Our results have found that DCBQ has decreased cell viability and neural differentiation, generated higher level of reactive oxygen species (ROS), increased the percentage of apoptosis and lowered the level of mitochondrial membrane potential, suggesting the neurodevelopmental toxicity of DCBQ. In addition, antioxidant N-acetyl-L-cysteine (NAC) could significantly attenuate these DCBQ-induced neurotoxic effects, supporting our hypothesis that the neurodevelopmental toxicity may be related with oxidative stress induced by DCBQ. We further demonstrated that DCBQ-induced neurodevelopmental toxicity could promote the mitochondrial apoptosis pathway and inhibit the prosurvival PI3K/AKT/mTOR pathway through inducing ROS, which ultimately inhibited cell proliferation and induced apoptosis in neural differentiating SH-SY5Y cells. These findings have provided novel insights into the risk of neurodevelopmental toxic effects associated with DCBQ exposure, emphasizing the importance of assessing the potential neurodevelopmental toxicity of DBPs.

Investigation of the nephrotoxicity of 2,6-dichloro-1,4-benzoquinone disinfection by-product in mice through a 28-day toxicity test

In recent years, 2,6-dichloro-1,4-benzoquinone (DCBQ) has become an emerging water disinfection by-product and widely distributed in disinfected water. Although kidney is a potential target of DCBQ, a systematic study of the in vivo nephrotoxicity of DCBQ is rare. In this study, a 28-day oral toxicity test was used to assess the nephrotoxic effects of DCBQ on mice. And the potential mechanisms of nephrotoxicity induced by DCBQ were explored through inflammation, oxidative stress, apoptosis and gut microbiota. The results showed that the kidney indexes of mice were not altered in DCBQ-exposed group in comparison with the control group. The histopathological investigation revealed that DCBQ caused swollen of renal tube, destruction of the renal structure, and infiltration of inflammatory cell in kidney. DCBQ has induced oxidative damage in kidney, as the observation of the increase of the renal superoxide dismutase (SOD) and catalase (CAT) activity. Also, DCBQ has triggered the inflammatory response in kidney through the increased expression of IL-1β, NF-κB and iNOS. Moreover, DCBQ has activated the apoptosis pathway, as indicated by the increased mRNA expression of Caspase-3 and Caspase-9. We eventually found an association between gut microbiota and nephrotoxic variables, demonstrating the importance of gut-kidney axis in DCBQ toxicity. Our results suggested that exposure to DCBQ in disinfected water might be a risk factor for kidney and provided novel insights into the underlying mechanisms of DCBQ-induced kidney injury, contributing to better interpretation of the health impact of the environmentally emerging contaminant DCBQ.

Methoxylated Modification of Glutathione-Mediated Metabolism of Halobenzoquinones In Vivo and In Vitro

Xenobiotics were generally detoxified in organisms through interaction with endogenous molecules, which may also generate metabolites of increased toxicity. Halobenzoquinones (HBQs), a group of highly toxic emerging disinfection byproducts (DBPs), can be metabolized by reacting with glutathione (GSH) to form various glutathionylated conjugates (SG-HBQs). In this study, the cytotoxicity of HBQs in CHO-K1 cells showed a wavy curve as a function of increased GSH dosage, which was inconsistent with the commonly recognized progressive detoxification curve. We hypothesized that the formation and cytotoxicity of GSH-mediated HBQ metabolites contribute to the unusual wave-shaped cytotoxicity curve. Results showed that glutathionyl-methoxyl HBQs (SG-MeO-HBQs) were identified to be the primary metabolites significantly correlated with the unusual cytotoxicity variation of HBQs. The formation pathway was initiated by stepwise metabolism via hydroxylation and glutathionylation to produce detoxified hydroxyl HBQs (OH-HBQs) and SG-HBQs, followed by methylation to generate SG-MeO-HBQs of potentiated toxicity. To further verify the occurrence of the aforementioned metabolism in vivo, SG-HBQs and SG-MeO-HBQs were detected in the liver, kidney, spleen, testis, bladder, and feces of HBQ-exposed mice, with the highest concentration quantified in the liver. The present study supported that the co-occurrence of metabolism can be antagonistic, which enhanced our understanding of the toxicity and metabolic mechanism of HBQs.

Molecular insights of 2,6-dichlorobenzoquinone-induced cytotoxicity in zebrafish embryo: Activation of ROS-mediated cell cycle arrest and apoptosis

2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), as an emerging disinfection by-product, has been frequently detected in waters, posing potential health risk on public health. Although some studies have pointed out that 2,6-DCBQ exposure can induce cytotoxicity, limited information is available for underlying mechanism for 2,6-DCBQ-induced cytotoxicity. To explore this mechanism, we assessed the levels of reactive oxygen species (ROS), acridine orange (AO) staining, and the mRNA transcriptions of genes (Chk2, Cdk2, Ccna, Ccnb and Ccne) involved in cell-cycle and genes (p53, bax, bcl-2 and caspase 3) involved in apoptosis in zebrafish embryo, after exposed to different concentrations (10, 30, 60, 90 and 120 μg/L) of 2,6-DCBQ for 72 h. Our results indicated that 2,6-DCBQ exposure induced ROS generation and cell apoptosis, and disturbed the mRNA transcription of genes related to cell cycle and apoptosis in zebrafish embryo. Moreover, we also found that 30 ~ 60 μg/L 2,6-DCBQ is the important transition from cell-cycle arrest to cell apoptosis. These results provided novel insight into 2,6-DCBQ-induced cytotoxicity.

Emerging Disinfection Byproduct 2,6-Dichlorobenzoquinone-Induced Cardiovascular Developmental Toxicity of Embryonic Zebrafish and Larvae: Imaging and Transcriptome Analysis

Epidemiological studies have observed the potential association of water disinfection byproduct (DBP) exposure with cardiac defects. Aromatic DBPs represent a significant portion of total DBPs, but their effects on cardiovascular development are unclear. In this study, we examined the effects of an aromatic DBP, 2,6-dichlorobenzoquinone (DCBQ), on the cardiovascular development of zebrafish embryos. After exposure to 2, 4, and 8 μM DCBQ, morphological images of growing zebrafish embryos clearly showed cardiovascular malformation. Fluorescent images of transgenic zebrafish strains with fluorescently labeled heart and blood vessels show that DCBQ exposure resulted in deformed atrium-ventricle looping, degenerated abdomen and trunk vessels, pericardial edema, and decreased blood flow. Furthermore, the expression of the marker gene myl7 (essential for the differentiation and motility of cardiomyocytes) was inhibited in a dose-dependent manner by DCBQ exposure. Finally, transcriptome analysis found that in the 4 μM DCBQ exposure group, the numbers of differentially expressed genes (DEGs) were 113 (50 upregulated and 63 downregulated) at 24 hpf, 2123 (762 upregulated and 1361 downregulated) at 48 hpf, and 61 (11 upregulated and 50 downregulated) at 120 hpf; in the 8 μM DCBQ exposure group, the number of DEGs was 1407 (647 upregulated and 760 downregulated) at 120 hpf. The FoxO signaling pathway was significantly altered. The in vivo results demonstrate the effects of 2,6-DCBQ (0-8 μM) on cardiovascular development, contributing to the understanding of the developmental toxicity of aromatic DBP halobenzoquinones (HBQs).

Quantitative structure-toxicity relationships of halobenzoquinone isomers on DNA reactivity and genotoxicity

Halobenzoquinones (HBQs) are an emerging class of drinking water disinfection byproducts that have been predicted as bladder carcinogens. However, data on the genotoxicity of HBQs are still scarce. This study performed a quantitative structure-toxicity relationship (QSTR) analysis of HBQ isomers on DNA reactivity and genotoxicity. The interaction of HBQs with calf thymus DNA (ct-DNA) was studied using multi-spectroscopic and molecular docking techniques. UV-Vis absorption spectra observed a significant hyperchromic effect with the increase of HBQ concentration. The fluorescence intensity of both probe-ct-DNA decreased with the increasing concentration of HBQs, indicating that the interaction mode between each HBQs and DNA was quite complicated, and there were both minor groove binding and intercalation binding. Molecular docking showed that HBQs interacted with DNA predominantly via hydrogen bond at guanine-rich areas in the minor groove of DNA. The genotoxicity of HBQs on human hepatocytes (L-02) was evaluated by micronucleus test, and the results showed that HBQs could cause significant chromosomal damage. The rank order of HBQ isomers on DNA reactivity and genotoxicity was 2,5-HBQs > their corresponding 2,6-HBQs. QSTR analysis found that dipole moment is the key structural descriptor influencing both DNA reactivity and genotoxicity of HBQ isomers. This study suggested that HBQs have caused genotoxicity which was influenced by their isomeric effects, warranting a comprehensive understanding of the genotoxic and carcinogenic risks associated with HBQs exposure.

Neurotoxicity and transcriptome changes in embryonic zebrafish induced by halobenzoquinone exposure

Halobenzoquinones (HBQs) are emerging disinfection byproducts (DBPs) with a widespread presence in drinking water that exhibit much higher cytotoxicity than regulated DBPs. However, the developmental neurotoxicity of HBQs has not been studied in vivo. In this work, we studied the neurotoxicity of HBQs on zebrafish embryos, after exposure to varying concentrations (0-8 µmol/L) of three HBQs, 2,5-dichloro-1,4-benzoquinone (2,5-DCBQ), 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), and 2,5-dibromo-1,4-benzoquinone (2,5-DBBQ) for 4 to 120 hr post fertilization (hpf). HBQ exposure significantly decreased the locomotor activity of larvae, accompanied by significant reduction of neurotransmitters (dopamine and γ-aminobutyric acid) and acetylcholinesterase activity. Furthermore, the expression of genes involved in neuronal morphogenesis (gfap, α1-tubulin, mbp, and syn-2α) were downregulated by 4.4-, 5.2-, 3.0-, and 4.5-fold in the 5 µmol/L 2,5-DCBQ group and 2.0-, 1.6-, 2.1-, and 2.3-fold in the 5 µmol/L 2,5-DBBQ group, respectively. Transcriptomic analysis revealed that HBQ exposure affected the signaling pathways of neural development. This study demonstrates the significant neurotoxicity of HBQs in embryonic zebrafish and provides molecular evidence for understanding the potential mechanisms of HBQ neurotoxicity.

Quantitative identification of halo-methyl-benzoquinones as disinfection byproducts in drinking water using a pseudo-targeted LC-MS/MS method

Halobenzoquinones (HBQs) as disinfection byproducts (DBPs) in drinking water is prioritized for research due to their prevalent occurrence and high toxicity. However, only fifteen HBQs can be identified among a high diversity using targeted LC-MS/MS analysis in previous studies due to the lack of chemical standards. In this study, we developed a pseudo-targeted LC-MS/MS method for detecting and quantifying diverse HBQs. Distinct fragment characteristics of HBQs was observed according to the halogen substituent effects, and extended to the development of a multiple-reaction-monitoring (MRM) method for the quantification of the 46 HBQs that were observed in simulated drinking water using non-targeted analysis. The fragmentation mechanism was supported by the changes of Gibbs free energy (ΔG), and a linear relationship between the ΔG and the ionization efficiency of analytes was developed accordingly for quantification of these 46 HBQs, 30 of which were lack of chemical standards. It is noted that 29 of the 30 newly-identified HBQs were halo-methyl-benzoquinones (HMBQs), which were predicted to be carcinogens related with drinking-water bladder cancer risk and be more toxic than non-methyl HBQs. Using the new method, twelve HMBQs were detected in actual drinking water samples with concentrations up to 100.4 ng/L, 3 times higher than that reported previously. The cytotoxicity in CHO cells of HMBQs was over 1-fold higher than that of non-methyl-HBQs. Therefore, HMBQs are an essential, highly toxic group of HBQs in drinking water, which deserve particular monitoring and control.

Effects of physical exercise on muscle metabolism and meat quality characteristics of Mongolian sheep

The objective of this study was to investigate the effects of exercise training on muscle metabolism, fatty acid composition, carcass traits, and meat quality characteristics of Mongolian sheep. Fourteen Mongolian sheep were randomly divided into two groups (7 sheep in each) and placed in two adjacent livestock pens. One group of sheep was kept in the pen (Control [C] group) and the other group of sheep (Training [T] group) were driven away in a field to walk twice a day. The results showed a reduction in pH measured 45 min post mortem, L*, a*, and b* value, intramuscular fat, and carcass length, and an increase in the ultimate pH value and shear force in the meat of T group in comparison with that of C group (p < .050). Also, exercise training moderately affected the fatty acid composition of LT muscle. Compared with C group, the concentrations of myristoleic acid (C14:1) and stearic acid (C18:0) were increased (p < .050), while the concentrations of C20:3 n‐6, neurolic acid (C24:1), and n‐3 polyunsaturated fatty acid (PUFA) were decreased in T group (p < .050). Transcriptome analysis highlighted 621 genes differentially expressed in two groups, including 385 were up‐regulated (e.g., GLUT4 and PGC‐1α) and 236 were down‐regulated (e.g., PLIN1 and ACSL3) in T with respect to C group. Besides, considering these genes, a number of enrichment pathways related to muscle metabolic processes, involving carbohydrate metabolism, lipid metabolism, oxidation reduction process, and muscle tissue development, were highlighted. In conclusion, these results contributed to a better understanding of the possible biological and molecular processes underlying the effects of exercise training on muscle metabolism and meat quality in Mongolian sheep, and provide useful information for contributing to understand the phenotypic and functional differences in meat quality of sheep.

Immune dysfunction induced by 2,6-dichloro-1,4-benzoquinone, an emerging water disinfection byproduct, due to the defects of host-microbiome interactions

2,6-dichloro-1,4-benzoquinone (DCBQ), as an emerging water disinfection byproducts (DBPs), has posed potential risks via the digestion system. However, little is known about the toxicity of DCBQ on the gut microbiome, which plays a critical role on human health. This study has comprehensively investigated the impact of DCBQ on the intestinal microbiome, metabolic functions, and immunity after the mice orally exposure to DCBQ at the concentration of 31.25, 62.5 and 125 mg/kg body weight for 28 days. Our results indicated that DCBQ exposure has perturbed the balance between T helper (Th) 1 mediated pro-inflammatory response and Th2 mediated anti-inflammatory response in mice, especially inducing the activation of immune system toward a Th2 response. DCBQ group has induced gut microbiota dysbiosis, and at phylum level, Proteobacteria was relatively less abundant compared with that in the control group. Furthermore, DCBQ exposure has dramatically perturbed metabolites profiles which were involved in 28 metabolic pathways, such as amino acids biosynthesis and metabolism, lipid metabolism. In particular, the altered gut microbiota showed strong correlations with both the altered metabolites and the altered immunological variables after DCBQ exposure. This study provides evidence on the adverse effects and mechanisms of water disinfection byproduct DCBQ through the interaction of immune-microbiome-metabolome, highlighting the importance to assess DBPs-associated risks.

Identification, Occurrence, and Cytotoxicity of Haloanilines: A New Class of Aromatic Nitrogenous Disinfection Byproducts in Chloraminated and Chlorinated Drinking Water

Identifying disinfection byproducts (DBPs) with high health risk is an unresolved challenge. In this study, six members of a new class of aromatic nitrogenous DBPs─2-chloroaniline, 2-bromoaniline, 2,4-dichloroaniline, 2-chloro-4-bromoaniline, 4-chloro-3-nitroaniline, and 2-chloro-4-nitroaniline─are reported as DBPs in drinking water for the first time. Haloanilines completely degraded within 1 h in the presence of chlorine (1 mg/L), while about 20% remained in the presence of chloramine (1 mg/L) after 120 h. Haloanilines showed high stability in the absence of disinfectants, with <30% degradation at pH 5-9 over 120 h. Eight haloanilines were determined in chloraminated finished water and tap water at total concentrations of up to 443 ng/L. The most abundant was 2-bromoaniline, with a median concentration of 104 ng/L. The cytotoxicity of eight haloanilines and regulated trichloromethane and dichloroacetic acid (DCAA) was evaluated using Hep G2 cell assay. The EC₅₀ values of eight haloanilines were 1-2 orders of magnitude lower than those of the regulated DBPs. The lowest toxic concentration of 2-chloro-4-nitroaniline was 1 μM, 500 times lower than that of DCAA. The formation and control of haloanilines in drinking water warrant further investigation.

Molecular composition of halobenzoquinone precursors in natural organic matter in source water

Halobenzoquinones (HBQs) are emerging disinfection byproducts generated during the reaction of chlorine disinfectant with natural organic matter (NOM) in source water. In this study, the correlations between molecular weight and HBQs generation of river NOM was evaluated. The compositional and functional characteristics of primary HBQs precursors were revealed by using Orbitrap mass spectrometry combined with molecular tagging. The NOM fraction larger than 50 kDa resulted in approximately 9 times more HBQs (50.9 ± 2.7 ng/mgC) than low molecular weight fractions. Significant correlations were found between the yields of HBQs and lignin-like and highly oxygen compounds in NOM, suggesting their critical roles in HBQs formation. Derivatizating the aldehydes, ketones, hydroxyl and carboxyl groups in NOM could reduce HBQs yields by 90.7%-100%. Unraveling the molecular characteristics of HBQs precursors in NOM would greatly benefit the prediction of HBQs yields of different source water, and develop more efficient disinfection byproduct control strategies.

Toxic effects of substituted p-benzoquinones and hydroquinones in in vitro bioassays are altered by reactions with the cell assay medium

Substituted para-benzoquinones and hydroquinones are ubiquitous transformation products that arise during oxidative water treatment of phenolic precursors, for example through ozonation or chlorination. The benzoquinone structural motive is associated with mutagenicity and carcinogenicity, and also with induction of the oxidative stress response through the Nrf2 pathway. For either endpoint, toxicological data for differently substituted compounds are scarce. In this study, oxidative stress response, as indicated by the AREc32 in vitro bioassay, was induced by differently substituted para-benzoquinones, but also by the corresponding hydroquinones. Bioassays that indicate defense against genotoxicity (p53RE-bla) and DNA repair activity (UmuC) were not activated by these compounds. Stability tests conducted under incubation conditions, but in the absence of cell lines, showed that tested para-benzoquinones reacted rapidly with constituents of the incubation medium. Compounds were abated already in phosphate buffer, but even faster in biological media, with reactions attributed to amino- and thiol-groups of peptides, proteins, and free amino acids. The products of these reactions were often the corresponding substituted hydroquinones. Conversely, differently substituted hydroquinones were quantitatively oxidized to p-benzoquinones over the course of the incubation. The observed induction of the oxidative stress response was attributed to hydroquinones that are presumably oxidized to benzoquinones inside the cells. Despite the instability of the tested compounds in the incubation medium, the AREc32 in vitro bioassay could be used as an unspecific sum parameter to detect para-benzoquinones and hydroquinones in oxidatively treated waters.

Analytical discovery of water disinfection byproducts of toxicological relevance: highlighting halobenzoquinones

Analytical advancement enables discoveries in water research, and challenges in the identification and determination of a wide range of trace level toxic compounds in water drives the development of new analytical platforms and tools. The identification of toxic disinfection byproducts (DBPs) in disinfected drinking water is an excellent example. Water disinfection is necessary to protect the public from waterborne disease. However, an unintentional consequence is the formation of DBPs resulting from reactions of disinfectants with natural organic matter in source water. To date, regulated DBPs do not account for the increased bladder cancer risk estimated in epidemiological studies. The majority of halogenated DBPs remain unidentified and the discovery of unknown DBPs of toxicological relevance continues to be a major focus of current water research. This review will highlight halobenzoquinones as a class of DBPs that serves as an example of analytical development and toxicological studies. We will feature recent trends and gaps in analytical technologies for identification of unknown DBPs and bioassays for evaluation of the toxicological effects of specific DBPs and their mixtures.

Mechanistic Study on Oxidative DNA Damage and Modifications by Haloquinoid Carcinogenic Intermediates and Disinfection Byproducts

Haloquinones (XQs) are a group of carcinogenic intermediates of the haloaromatic environmental pollutants and newly identified chlorination disinfection byproducts (DBPs) in drinking water. The highly reactive hydroxyl radicals/alkoxyl radicals and quinone enoxy/ketoxy radicals were found to arise in XQs and H₂O₂ or organic hydroperoxides system, independent of transition-metal ions. However, it was not clear whether these haloquinoid carcinogens and hydroperoxides can cause oxidative DNA damage and modifications, and if so, what are the underlying molecular mechanisms. We found that 8-oxodeoxyguanosine (8-oxodG), DNA strand breaks, and three methyl oxidation products could arise when DNA was treated with tetrachloro-1,4-benzoquinone and H₂O₂ via a metal-independent and intercalation-enhanced oxidation mechanism. Similar effects were observed with other XQs, which are generally more efficient than the typical Fenton system. We further extended our studies from isolated DNA to genomic DNA in living cells. We also found that potent oxidation of DNA to the more mutagenic imidazolone dIz could be induced by XQs and organic hydroperoxides such as t-butylhydroperoxide or the physiologically relevant hydroperoxide 13S-hydroperoxy-9Z,11E-octadecadienoic acid via an unprecedented quinone-enoxy radical-mediated mechanism. These findings should provide new perspectives to explain the potential genotoxicity, mutagenesis, and carcinogenicity for the ubiquitous haloquinoid carcinogenic intermediates and DBPs.

Cytotoxicity of hexabromocyclododecane, 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane and 1,2,5,6-tetrabromocyclooctane in human SH-SY5Y neuroblastoma cells

With the listing of the of cycloaliphatic brominated flame retardants (CBFR) hexabromocyclododecane (HBCD) as a persistent organic pollutant (POP) by the Stockholm Convention, much attention has been paid to the environmental behaviors and biological effects of HBCD, as well as its potential alternatives, such as 1,2-dibromo-4-(1,2-dibromoethyl) cyclohexane (TBECH) and 1,2,5,6-tetrabromocyclooctane (TBCO). In this study, the neurotoxicity of HBCD, TBECH, and TBCO in human SH-SY5Y cells were compared. The results showed that HBCD, TBECH, and TBCO induced cytotoxicity, including dose-dependent cell viability decreases, cell membrane permeability increases, cytoskeleton development damage, and apoptosis induction, with the cytotoxicity in the order of HBCD > TBCO > TBECH. The expression levels of apoptotic proteins (caspase-3, Bax, caspase-9, Bcl-2, and cytochrome c (Cyt c)) followed the same order, which indicated that mitochondrial apoptotic pathway may be one of the mechanisms responsible for their neurotoxicity. In order to study the mechanisms of cytotoxicity, CBFRs-induced reactive oxygen species (ROS) and the intracellular calcium levels were determined. The ROS levels were significantly elevated for three CBFRs treatment, suggesting that oxidative stress contributes to their cytotoxicity. The intracellular calcium concentrations were significantly enhanced for HBCD and TBCO treatment, but not for TBECH, indicating that in addition to ROS, cytotoxicity of HBCD and TBCO may follow Ca²⁺-mediated apoptotic pathway. This study first compared the neurotoxicity of different CBFRs, providing valuable information for their risk assessment.

The occurrence, characteristics, transformation and control of aromatic disinfection by-products: A review

With the development of analytical technology, more emerging disinfection by-products (DBPs) have been identified and detected. Among them, aromatic DBPs, especially heterocyclic DBPs, possess relatively high toxicity compared with regulated DBPs, which has been proved by bioassays. Thus, the occurrence of aromatic DBPs is of great concern. This article provides a comprehensive review and summary of the characteristics, occurrence, transformation pathways and control of aromatic DBPs. Aromatic DBPs are frequently detected in drinking water, wastewater and swimming pool water, among which swimming pool water illustrates highest concentration. Considering the relatively high concentration and toxicity, halophenylacetonitriles (HPANs) and halonitrophenols (HNPs) are more likely to be toxicity driver among frequently detected phenyl DBPs. Aromatic DBPs can be viewed as important intermediate products of dissolved organic matter (DOM) during chlor(am)ination. High molecular weight DOM could convert to aromatic DBPs via direct or indirect pathways, and they can further decompose into regulated aliphatic DBPs such as trihalomethanes (THMs) and haloacetic acids (HAAs) by ring opening and side chain cleavage. Even though no single DBPs control strategy is efficient to all aromatic DBPs, the decrease of overall toxicity may be achieved by several methods including absorption, solar radiation and boiling. By systematically considering aromatic DBPs and aliphatic DBPs, a better trade-off can be made to reduce health risk induced by DBPs.

Comparative cytotoxicity of halogenated aromatic DBPs and implications of the corresponding developed QSAR model to toxicity mechanisms of those DBPs: Binding interactions between aromatic DBPs and catalase play an important role

Halogenated aromatic disinfection byproducts (DBPs) are a new group of emerging DBPs identified recently. They have been detected in disinfected drinking water, wastewater effluents, recreational water and oil/gas produced water, at concentrations of ng/L to μg/L in general. Previously studies have demonstrated that most of them can induce developmental toxicity and growth inhibition in aquatic organisms based on in vivo bioassays. In this study, to further understand the adverse effects of aromatic DBPs to human health, the comparative cytotoxicity of 15 halogenated aromatic DBPs belonging to four subgroups (i.e., halophenols, halonitrophenols, halohydroxybenzaldehydes and halohydroxybenzoic acids) was evaluated with mammalian Chinese Hamster Ovary cells. The results indicated that the selected aromatic DBPs exhibited an in vitro toxicity rank order of halonitrophenols > halophenols > halohydroxybenzaldehydes > halohydroxybenzoic acids. The potential toxicity mechanisms involved with the antioxidant system were investigated by using molecular docking analysis between key antioxidant enzymes (i.e., catalase, superoxide dismutase, and glutathione S-transferase) and aromatic DBPs. Based on the observed cytotoxicity data and screening the candidate descriptors (including binding energies between the aromatic DBPs and key antioxidant enzymes as well as physical-chemical/quantum-chemical/topological descriptors), a QSAR model was developed as log (LC₅₀) ^-1 = - 1.050E_CAT + 0.300E_HOMO - 0.238E_LUMO- 0.164, indicating the importance of the interactions of aromatic DBPs towards catalase and the electrophilic/nucleophilic reactivity of aromatic DBPs in the toxicity mechanisms. In addition, the occurrence of the aromatic DBPs in tap water and finished water was studied in a mega city Shenzhen located in South China. Results showed that halogenated aromatic DBPs commonly existed in Shenzhen drinking water at ng/L levels, and three nitrogenous aromatic DBPs were detected in real drinking water for the first time. The major toxicity drivers among the target aromatic DBPs were identified through the integration of the measured concentrations and observed cytotoxicity; notably, DBPs with the highest concentrations may not contribute the highest proportions of overall toxicity.

Global Transcriptional Analysis of Nontransformed Human Intestinal Epithelial Cells (FHs 74 Int) after Exposure to Selected Drinking Water Disinfection By-Products

BACKGROUND

Drinking water disinfection inadvertently leads to the formation of numerous disinfection by-products (DBPs), some of which are cytotoxic, mutagenic, genotoxic, teratogenic, and potential carcinogens both in vitro and in vivo.

OBJECTIVES

We investigated alterations to global gene expression (GE) in nontransformed human small intestine epithelial cells (FHs 74 Int) after exposure to six brominated and two chlorinated DBPs: bromoacetic acid (BAA), bromoacetonitrile (BAN), 2,6-dibromo-p-benzoquinone (DBBQ), bromoacetamide (BAM), tribromoacetaldehyde (TBAL), bromate (BrO3-), trichloroacetic acid (TCAA), and trichloroacetaldehyde (TCAL).

METHODS

Using whole-genome cDNA microarray technology (Illumina), we examined GE in nontransformed human cells after 4h exposure to DBPs at predetermined equipotent concentrations, identified significant changes in gene expression (p≤0.01), and investigated the relevance of these genes to specific toxicity pathways via gene and pathway enrichment analysis.

RESULTS

Genes related to activation of oxidative stress-responsive pathways exhibited fewer alterations than expected based on prior work, whereas all DBPs induced notable effects on transcription of genes related to immunity and inflammation.

DISCUSSION

Our results suggest that alterations to genes associated with immune and inflammatory pathways play an important role in the potential adverse health effects of exposure to DBPs. The interrelationship between these pathways and the production of reactive oxygen species (ROS) may explain the common occurrence of oxidative stress in other studies exploring DBP toxicity. Finally, transcriptional changes and shared induction of toxicity pathways observed for all DBPs caution of additive effects of mixtures and suggest further assessment of adverse health effects of mixtures is warranted. https://doi.org/10.1289/EHP4945.

Occurrence, Formation, and Oxidative Stress of Emerging Disinfection Byproducts, Halobenzoquinones, in Tea

Halobenzoquinones (HBQs) are frequently detected disinfection byproducts (DBPs) in drinking water with high toxicity and relevance to public health. In this study, we characterized the occurrence, formation, and oxidative stress of the HBQs in tea. 2,6-DCBQ and TetraC-1,2-BQ were identified in all prepared teas at total concentrations of 1.3-2.0 ng/L. 2,6-DCBQ originated from drinking water DBPs, while TetraC-1,2-BQ originated from tea leaves or were generated during tea polyphenol chlorination. HBQs in tea induced the formation of reactive oxygen species and semiquinone radicals, and the oxidative stress could be depleted by tea polyphenols, e.g., (-)-epigallocatechin gallate (EGCG). High-resolution mass spectrometry analysis indicated that the HBQs combined with EGCG and formed adducts at a ratio of 1:1 or 2:1 with the binding sites on the A ring and B ring of EGCG. The viability of HepG2 cells exposed to 50 μM 2,6-DCBQ was increased from 20.0% to 65.2% when 50 μM of EGCG was added. These results demonstrated that various HBQs can occur in tea due to the HBQ DBPs in drinking water, the leachate from tea leaves, and the chlorination of tea polyphenols; furthermore, the oxidative stress and cellular toxicity induced by HBQs in tea could be decreased by tea polyphenols. This is the first study to report HBQs in tea, elucidate the sources of HBQs, and assess relevant health risks.

Halobenzoquinone-Induced Developmental Toxicity, Oxidative Stress, and Apoptosis in Zebrafish Embryos

The developmental toxicity of water disinfection byproducts remains unclear. Here we report the study of halobenzoquinone (HBQ)-induced in vivo developmental toxicity and oxidative stress using zebrafish embryos as a model. Embryos were exposed to 0.5-10 μM of individual HBQs and 0.5-5 mM haloacetic acids for up to 120 h postfertilization (hpf). LC₅₀ values of the HBQs at 24 hpf were 4.6-9.8 μM, while those of three haloacetic acids were up to 200 times higher at 1900-2600 μM. HBQ exposure resulted in significant developmental malformations in larvae, including failed inflation of the gas bladder, heart malformations, and curved spines. An increase in reactive oxygen species was observed, together with a decrease in superoxide dismutase activity and glutathione content. Additionally, the antioxidant N-acetyl-l-cysteine significantly mitigated all HBQ-induced effects, supporting that oxidative stress contributes to HBQ toxicity. Further experiments examined HBQ-induced effects on DNA and genes. HBQ exposure increased 8-hydroxydeoxyguanosine levels, DNA fragmentation, and apoptosis in larvae, with apoptosis induction related to changes in the gene expression of p53 and mdm2. These results suggest that HBQs are acutely toxic, causing oxidative damage and developmental toxicity to zebrafish larvae.