Transimination of quinone imines: a mechanism for embedding exogenous redox activity into the nucleosome

Haloquinone Chloroimides as Toxic Disinfection Byproducts Identified in Drinking Water

Haloquinone chloroimides (HQCs) are suspected to be highly toxic contaminants, and their production during drinking water disinfection is predicted. However, HQC disinfection byproducts (DBPs) have not been reported in drinking water to date because of analytical limitations. In this study, we developed an analytical method to detect five HQCs, including 2,6-dichloroquinone-4-chloroimide (2,6-DCQC), 2,6-dibromoquinone-4-chloroimide (2,6-DBQC), 2-chloroquinone-4-chloroimide (2-CQC), 3-chloroquinone-4-chloroimide (3-CQC), and 2,6-dichloroquinone-3-methyl-chloroimide (2,6-DCMQC). This method combined a derivatization reaction of HQCs with phenol in alkaline solutions to produce halogenated indophenols, a solid-phase extraction pretreatment using hydrophilic-lipophilic balanced (HLB) cartridges, and a multiple reaction monitoring (MRM) method for quantification. The method was demonstrated to be sensitive and accurate with recoveries of 71-85% and limits of detection of 0.1-0.2 ng/L for the five tested HQCs. Using this method, five tested HQCs were identified in drinking water samples from nine water treatment plants and water distribution systems as new DBPs at concentrations of up to 23.1 ng/L. The cytotoxicity of the five tested HQCs in HepG2 cells was higher than or comparable to that of 2,6-dichloro-1,4-benzoquinone (2,6-DCBQ), an emerging DBP that was hundreds to thousands of times more toxic than regulated DBPs. This study presents the first analytical method for HQC DBPs in drinking water and the first set of occurrence and cytotoxicity data of HQC DBPs.

Dihydroceramide Desaturase Functions as an Inducer and Rectifier of Apoptosis: Effect of Retinol Derivatives, Antioxidants and Phenolic Compounds

Dihydroceramide desaturase (Degs1) catalyses the introduction of a 4,5-trans double bond into dihydroceramide to form ceramide. We show here that Degs1 is polyubiquitinated in response to retinol derivatives, phenolic compounds or anti-oxidants in HEK293T cells. The functional predominance of native versus polyubiquitinated forms of Degs1 appears to govern cytotoxicity. Therefore, 4-HPR or celecoxib appear to stimulate the de novo ceramide pathway (with the exception of C24:0 ceramide), using native Degs1, and thereby promote PARP cleavage and LC3B-I/II processing (autophagy/apoptosis). The ubiquitin-proteasomal degradation of Degs1 is positively linked to cell survival via XBP-1s and results in a concomitant increase in dihydroceramides and a decrease in C24:0 ceramide levels. However, in the case of 4-HPR or celecoxib, the native form of Degs1 functionally predominates, such that the apoptotic programme is sustained. In contrast, 4-HPA or AM404 do not produce apoptotic ceramide, using native Degs1, but do promote a rectifier function to induce ubiquitin-proteasomal degradation of Degs1 and are not cytotoxic. Therefore, Degs1 appears to function both as an 'inducer' and 'rectifier' of apoptosis in response to chemical cellular stress, the dynamic balance for which is dependent on the nature of chemical stress, thereby determining cytotoxicity. The de novo synthesis of ceramide or the ubiquitin-proteasomal degradation of Degs1 in response to anti-oxidants, retinol derivatives and phenolic compounds appear to involve sensors, and for rectifier function, this might be Degs1 itself.

In Vitro and In Vivo Analysis of the Effects of 3,5-DMA and Its Metabolites in Neural Oxidative Stress and Neurodevelopmental Toxicity

3,5-Dimethylaniline (3,5-DMA), a monocyclic aromatic amine, is widely present in a spectrum of sources including tobacco, dyes, combustion products, and suspended particulates. 3,5-DMA and its metabolites form superoxides, resulting in apoptosis or oncogenesis. Data of a direct effect of 3,5-DMA on the nervous system, especially the developing brain, are lacking. Therefore, we investigated the effects of 3,5-DMA and its metabolites on fetal neurite growth and brain development using in vitro cell cultures of primary cortical neurons to observe whether these compounds caused neuronal cytotoxicity and affected neurite structural development. With increasing concentrations of 3,5-DMA (10, 50, 100, 500, 1000 μM) and its major metabolite 5-dimethylaminophenol (3,5-DMAP) (10, 50, 100, 500, 1000 μM), reactive oxygen species (ROS), cytotoxicity, and DNA damage increased significantly in the cells and dendritic arborization decreased. The addition of 5 mM N-acetylcysteine, an ROS scavenger, reduced ROS in the cells and alleviated the neuronal damage. In vivo studies in Sprague Dawley pregnant rats suggested that exposure to 3,5-DMA (10, 30, 60, 100 mg/kg/day) subcutaneously from GD15 to GD17 led to fetal cerebral cortex thinning. BrdU labeling showed that 3,5-DMA reduced the number and generation of cortical cells. To detect the laminar position of newly generated neurons, cortex layer markers such as Satb2, Ctip2, and Tbr1 were used. 3,5-DMA perturbed the cortical layer distribution in developing fetal rats. In summary, this is the first study to provide evidence for 3,5-DMA and its metabolites causing anomalies of the fetal central nervous system development through ROS production.

Anti-cancer effects of 3,5-dimethylaminophenol in A549 lung cancer cells

Exposure to 3,5-dimethylaminophenol (3,5-DMAP), the metabolite of the 3-5-dimethylaniline, was shown to cause high levels of oxidative stress in different cells. The aim of the present work was to observe whether this metabolite can lead to cytotoxicity, oxidative stress, DNA damage and cell cycle changes in non-small cell lung cancer A549 cells. 3,5-DMAP caused a dose-dependent increase in cytotoxicity, generation of superoxide (O₂^-.), inductions in the enzyme activities orchestrating cellular antioxidant balance, increases in lipid peroxidation as well as DNA damage. However, 3,5-DMAP showed significantly lower cytotoxicity towards human lung fibroblast (HLF) cells. 3,5-DMAP also led to molecular events, like inducing apoptotic markers (ie. p53, Bad, Bax and cytochrome c); decreasing anti-apoptotic proteins (Bcl-2) and alterations in cell cycle. Our findings indicate that the cytotoxicity caused by this particular alkylaniline metabolite led to initiation of caspase 3-mediated apoptosis. Furthermore, 3,5-DMAP attenuated carcinogenic properties like migration capacity of A549 cells and eventually inhibited growth of A549 cells in an in vivo mouse model. Tumor sections showed that 3,5-DMAP down-regulated c-Myc expression but up-regulated p53 and cytochrome c, all of which might result in tumor growth arrest. Co-treatment with N-acetylcysteine provided reductions in cytotoxicity and positively modulated genetic events induced by 3,5-DMAP in A549 cells. In conclusion, our findings demonstrate 3,5-DMAP may be a potential anti-cancer drug in cancer, due to its self redox cycling properties.

RP-UHPLC-UV-ESI-MS/MS analysis of LPMO generated C4-oxidized gluco-oligosaccharides after non-reductive labeling with 2-aminobenzamide

Lytic polysaccharide monooxygenases (LPMOs) are able to cleave recalcitrant polysaccharides, such as cellulose, by oxidizing the C1 and/or C4 atoms. The analysis of the resulting products requires a variety of analytical techniques. Up to now, these techniques mainly focused on the identification of non-oxidized and C1-oxidized oligosaccharides. The analysis of C4-oxidized gluco-oligosaccharides is mostly performed by using high pressure anion exchange chromatography (HPAEC). However, the alkaline conditions used during HPAEC analysis lead to tautomerization of C4-oxidized gluco-oligosaccharides, which limits the use of this technique. Here, we describe the use of reverse phase-ultra high performance liquid chromatography (RP-UHPLC) in combination with non-reductive 2-aminobenzamide (2-AB) labeling. Non-reductive 2-AB labeling enabled separation of C4-oxidized gluco-oligosaccharides from their non-oxidized counterparts. Moreover, RP-UHPLC does not require buffered mobile phases, which reduce mass spectrometry (MS) sensitivity. The latter is seen as an advantage over other techniques such as hydrophilic interaction liquid chromatography and porous graphitized carbon coupled to MS. RP-UHPLC coupled to UV detection and mass spectrometry allowed the identification of both labeled non-oxidized and C4-oxidized oligosaccharides. Non-reductive labeling kept the ketone at the C4-position of LPMO oxidized oligosaccharides intact, while selective reducing agents such as sodium triacetoxyborohydride (STAB) reduced this ketone group. Our results show that RP-UHPLC-UV-ESI-MS in combination with non-reductively 2-AB labeling is a suitable technique for the separation and identification of LPMO-generated C4-oxidized gluco-oligosaccharides.

Hydroxyphenylation of Histone Lysines: Post-translational Modification by Quinone Imines

Monocyclic aromatic amines are widespread environmental contaminants with multiple sources such as combustion products, pharmaceuticals, and pesticides. Their phenolic metabolites are converted intracellularly to electrophilic quinone imines upon autoxidation and can embed in the cellular matrix through a transimination reaction that leaves a redox-active residue as a substituent of lysine side-chain amino groups. To demonstrate the occurrence of this process within the cellular nucleus, Chinese hamster ovary AA8 cells were treated with the para-phenol of 3,5-dimethylamine, after which the histone proteins were isolated, derivatized, and subjected to tryptic digestion. The resulting peptides were analyzed by tandem mass spectrometry to determine which lysines were modified. Nine residues in histones H2A, H2B, and H4 were identified; these were located in histone tails, close to where DNA makes contact with the nuclear core particle, elsewhere on the protein surface, and deep within the core. Kinetics of disappearance of the modified lysines in cultured cells was determined using isotope-dilution mass spectrometry. AA8 cells were also transfected with the genetically encoded hydrogen peroxide biosensor HyPer in constructs that lead to expression of HyPer in different cellular compartments. Challenging the resulting cells with the dimethylaminophenol resulted in sustained fluorescence emission in each of the compartments, demonstrating ongoing production of H2O2. The kinetics of modified lysine loss determined by mass spectrometry was consistent with persistence of HyPer fluorescence emission. We conclude that the para-phenol of 3,5-dimethylamine can become stably integrated into the histone proteins, which are minimally repaired, if at all, and function as a persistent source of intracellular H2O2.

Protective effects of ascorbic acid against the genetic and epigenetic alterations induced by 3,5-dimethylaminophenol in AA8 cells

Exposure to monocyclic aromatic alkylanilines (MAAs), namely 2,6-dimethylaniline (2,6-DMA), 3,5-dimethylaniline (3,5-DMA) and 3-ethylaniline (3-EA), was significantly and independently associated with bladder cancer incidence. 3,5-DMAP (3,5-dimethylaminophenol), a metabolite of 3,5-DMA, was shown to induce an imbalance in cytotoxicity cellular antioxidant/oxidant status, and DNA damage in mammalian cell lines. This study was designed to evaluate the protective effect of ascorbic acid (Asc) against the cytotoxicity, reactive oxygen species (ROS) production, genotoxicity and epigenetic changes induced by 3,5-DMAP in AA8 Chinese Hamster Ovary (CHO) cells. In different cellular fractions, 3,5-DMAP caused alterations in the enzyme activities orchestrating a cellular antioxidant balance, decreases in reduced glutathione levels and a cellular redox ratio as well as increases in lipid peroxidation and protein oxidation. We also suggest that the cellular stress caused by this particular alkylaniline leads to both genetic (Aprt mutagenesis) and epigenetic changes in histones 3 and 4 (H3 and H4). This may further cause molecular events triggering different pathological conditions and eventually cancer. In both cytoplasm and nucleus, Asc provided increases in 3,5-DMAP-reduced glutathione levels and cellular redox ratio and decreases in the lipid peroxidation and protein oxidation. Asc was also found to be protective against the genotoxic and epigenetic effects initiated by 3,5-DMAP. In addition, Asc supplied protection against the cell cycle (G1 phase) arrest induced by this particular alkylaniline metabolite.

Intracellular generation of ROS by 3,5-dimethylaminophenol: persistence, cellular response, and impact of molecular toxicity

Epidemiological studies have demonstrated extensive human exposure to the monocyclic aromatic amines, particularly to 3,5-dimethylaniline, and found an association between exposure to these compounds and risk for bladder cancer. Little is known about molecular mechanisms that might lead to the observed risk. We previously suggested that the hydroxylated 3,5-dimethylaniline metabolite, 3,5-dimethylaminophenol (3,5-DMAP), played a central role in effecting genetic change through the generation of reactive oxygen species (ROS) in a redox cycle with 3,5-dimethylquinoneimine. Experiments here characterize ROS generation by 3,5-DMAP exposure in nucleotide repair-proficient and -deficient Chinese hamster ovary cells as a function of time. Besides, various cellular responses discussed herein indicate that ROS production is the principal cause of cytotoxicity. Fluorescence microscopy of cells exposed to 3,5-DMAP confirmed that ROS production occurs in the nuclear compartment, as suggested by a previous study demonstrating covalent linkage between 3,5-DMAP and histones. 3,5-DMAP was also compared with 3,5-dimethylhydroquinone to determine whether substitution of one of the phenolic hydroxyl groups by an amino group had a significant effect on some of the investigated parameters. The comparatively much longer duration of observable ROS produced by 3,5-DMAP (7 vs. 1 day) provides further evidence that 3,5-DMAP becomes embedded in the cellular matrix in a form capable of continued redox cycling. 3,5-DMAP also induced dose-dependent increase of H2O2 and ·OH, which were determined as the major free radicals contributing to the cytotoxicity and apoptosis mediated via caspase-3 activation. Overall, this study provides insight into the progression of alkylaniline-induced toxicity.