Covalent binding of [14C]-2,6-dimethylaniline to DNA of rat liver and ethmoid turbinate

Mutagenic properties of dimethylaniline isomers in mice as evaluated by comet, micronucleus and transgenic mutation assays

Background

The carcinogenic potential of dimethylaniline (DMA) isomers in rodents and humans has been previously reported, and there is sufficient evidence for the carcinogenicity of 2,6-DMA in experimental animals. The target organ of carcinogenesis of 2,6-DMA is the nasal cavity. In the current study, six DMA isomers, 2,3-, 2,4-, 2,5-, 2,6-, 3,4- and 3,5-DMA, were evaluated for mutagenic properties.

Results

Male ddY mice (3/group) were treated intragastrically (i.g.) with 200 mg/kg of one of the six DMAs, and a comet assay was performed on samples of bone marrow, kidney, liver and lung at 3 and 24 h after the treatment. Positive responses were observed in the kidney, liver and lungs of mice from all of the DMA treatment groups after 3 h and in the bone marrow of mice treated with either 3,4- or 3,5-DMA after 3 h; however, these effects were diminished at the 24 h time point. The micronucleus induction in the bone marrow was analysed in the same mouse at 24 h after the treatment. No induction of micronucleated polychromatic erythrocytes was observed after treatment with any of the DMAs.Male transgenic Muta™ mice (five/group) were treated i.g. with 2,5-, 2,6- or 3,5-DMA at 100 mg/kg bw weekly for 4 weeks, and the lacZ and the cII mutation frequencies were examined in the nasal cavity, liver and bone marrow at 7 days after the last treatment. Statistically significant increases in the mutation frequencies of the lacZ and/or cII genes were observed in the nasal cavity of 2,5-DMA or 2,6-DMA treated mice. Sequence analysis showed increased incidences of AT to GC and GC to TA mutations in the nasal tissues.

Conclusions

These findings suggest that the carcinogenic activities of DMAs are associated with mutagenic events.

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.

Elevated 4-aminobiphenyl and 2,6-dimethylaniline hemoglobin adducts and increased risk of bladder cancer among lifelong nonsmokers--The Shanghai Bladder Cancer Study

BACKGROUND

4-Aminobiphenyl (ABP) is an established human bladder carcinogen, with tobacco smoke being a major source of human exposure. Other arylamine compounds, including 2,6-dimethylaniline (2,6-DMA), have been implicated as possible human bladder carcinogens. Hemoglobin adducts of 4-ABP and 2,6-DMA are validated biomarkers of exposure to those compounds in humans.

METHODS

The Shanghai Bladder Cancer Study enrolled 581 incident bladder cancer cases and 604 population controls. Each participant was solicited for his/her history of tobacco use and other lifestyle factors and donation of blood and urine specimens. Red blood cell lysates were used to quantify both hemoglobin adducts of 4-ABP and 2,6-DMA. Urine samples were used to quantify total cotinine. ORs and 95% confidence intervals (CI) for bladder cancer were estimated using unconditional logistic regression methods.

RESULTS

Among lifelong nonsmokers, ORs (95% CIs) of bladder cancer for low (below median of positive values) and high versus undetectable levels of 2,6-DMA hemoglobin adducts were 3.87 (1.39-10.75) and 6.90 (3.17-15.02), respectively (Ptrend < 0.001). Similarly, among lifelong nonsmokers, ORs (95% CIs) of bladder cancer for third and fourth versus first/second quartiles of 4-ABP hemoglobin adducts was 1.30 (0.76-2.22) and 2.29 (1.23-4.24), respectively (Ptrend = 0.009). The two associations were independent of each other.

CONCLUSION

Hemoglobin adducts of 4-ABP and 2,6-DMA were significantly and independently associated with increased bladder cancer risk among lifelong nonsmokers in Shanghai, China.

IMPACT

The findings of the present study in China with previous data in Los Angeles, California strongly implicate arylamines as potential causal agents of human bladder cancer.

Monocyclic aromatic amines as potential human carcinogens: old is new again

Alkylanilines are a group of chemicals whose ubiquitous presence in the environment is a result of the multitude of sources from which they originate. Exposure assessments indicate that most individuals experience lifelong exposure to these compounds. Many alkylanilines have biological activity similar to that of the carcinogenic multi-ring aromatic amines. This review provides an overview of human exposure and biological effects. It also describes recent investigations into the biochemical mechanisms of action that lead to the assessment that they are most probably more complex than those of the more extensively investigated multi-ring aromatic amines. Not only is nitrenium ion chemistry implicated in DNA damage by alkylanilines but also reactions involving quinone imines and perhaps reactive oxygen species. Recent results described here indicate that alkylanilines can be potent genotoxins for cultured mammalian cells when activated by exogenous or endogenous phase I and phase II xenobiotic-metabolizing enzymes. The nature of specific DNA damage products responsible for mutagenicity remains to be identified but evidence to date supports mechanisms of activation through obligatory N-hydroxylation as well as subsequent conjugation by sulfation and/or acetylation. A fuller understanding of the mechanisms of alkylaniline genotoxicity is expected to provide important insights into the environmental and genetic origins of one or more human cancers and may reveal a substantial role for this group of compounds as potential human chemical carcinogens.

Identification of adducts formed by reaction of N-acetoxy-3,5-dimethylaniline with DNA

Aromatic amines constitute one of the most extensively studied classes of chemical carcinogens. Although monocyclic aromatic amines are generally regarded as weak carcinogens, a recent epidemiologic study of bladder cancer found that the arylamine 3,5-dimethylaniline (3,5-DMA) may play a significant role in the etiology of this disease in man. Investigations using experimental animals also strongly suggested that DNA adducts-of indeterminate structure-formed by 3,5-DMA might account for its presumptive activity. The present study was undertaken to determine the structures of the major DNA adducts formed in vitro by the known, and possibly carcinogenic, N-hydroxylated metabolite. Calf thymus DNA (ct-DNA) was modified by reaction with N-acetoxy-3,5-dimethylaniline (N-AcO-3,5-DMA). After enzymatic hydrolysis of DNA to individual 2'-deoxyribonucleosides, adduct profiles were determined using HPLC/MS. 3,5-DMA formed four major DNA adducts, one to 2'-deoxyguanosine (dG), two to 2'-deoxyadenosine (dA), and one to 2'-deoxycytidine (dC). Reactions of N-AcO-3,5-DMA with dG, dA, and dC produced the same adducts as reaction with ct-DNA with very similar profiles. Adducts were isolated chromatographically and unambiguously characterized as N-(deoxyguanosin-8-yl)-3,5-dimethylaniline (dG-C8-3,5-DMA), 4-(deoxyadenosin- N(6)-yl)-3,5-dimethylaniline (dA- N(6)-3,5-DMA), N-(deoxyadenosin-8-yl)-3,5-dimethylaniline (dA-C8-3,5-DMA), and N-(deoxycytidin-5-yl)-3,5-dimethylaniline (dC-C5-3,5-DMA) by high-resolution mass spectra (HR-MS) and NMR spectroscopy including (1)H NMR, (13)C NMR, and two-dimensional NMR. This report includes the first detailed description of a dC adduct of an aromatic amine. The present results provide chemical support for a carcinogenic mechanism of action by 3,5-DMA based on N-hydroxylation and the intermediacy of a nitrenium ion in the formation of DNA adducts.

Nasal cytotoxic and carcinogenic activities of systemically distributed organic chemicals

Toxicity and carcinogenicity in the mucosa of the nasal passages in rodents has been produced by a variety of organic chemicals which are systemically distributed. In this review, 14 such chemicals or classes were identified that produced rodent nasal cytotoxicity, but not carcinogenicity, and 11 were identified that produced nasal carcinogenicity. Most chemicals that affect the nasal mucosa were either concentrated in that tissue or readily activated there, or both. All chemicals with effects in the nasal mucosa that were DNA-reactive, were also carcinogenic, if adequately tested. None of the rodent nasal cytotoxins has been identified as a human systemic nasal toxin. This may reflect the lesser biotransformation activity of human nasal mucosa compared to rodent and the much lower levels of human exposures. None of the rodent carcinogens lacking DNA reactivity has been identified as a nasal carcinogen or other cancer hazard to humans. Some DNA-reactive rodent carcinogens that affect the nasal mucosa, as well as other tissues, have been associated with cancer at various sites in humans, but not the nasal cavity. Thus, findings in only the rodent nasal mucosa do not necessarily predict either a toxic or carcinogenic hazard to that tissue in humans.

DNA adduct formation by 2,6-dimethyl-, 3,5-dimethyl-, and 3-ethylaniline in vivo in mice

Aromatic amines such as 2-naphthylamine and 4-aminobiphenyl are established human bladder carcinogens. Experimental evidence for carcinogenicity of monocylic aromatic amines is limited mostly to other organs, but a recent epidemiologic study of bladder cancer found that 2,6-dimethyl- (2,6-DMA), 3,5-dimethyl- (3,5-DMA), and 3-ethylaniline (3-EA) may play a significant role in the etiology of this disease in man. The present work was undertaken to test whether a genotoxic mechanism can account for the presumptive activity of 2,6-DMA, 3,5-DMA, and 3-EA by quantifying the binding of these compounds to DNA in vivo. Each of these three [(14)C]alkylanilines was administered at approximately 100 microg/kg to C57BL/6 mice, which were subsequently sacrificed 2, 4, 8, 16, and 24 h post-dosing. Bladder, colon, kidney, liver, lung, and pancreas were harvested from each animal, and DNA was isolated from each tissue. Adduct levels were determined by quantifying bound isotope using accelerator mass spectrometry. Adducts were detectable in the bladder and liver DNA samples from every animal at every time point at levels that ranged from 3 per 10(9) to 1.5 per 10(7) nucleotides. Adduct levels were highest in animals given 3,5-DMA and lowest in those given 3-EA. Levels in both bladder and liver declined by severalfold over the course of the experiment. Adducts were detected less frequently in the other four tissues. Taken together, the results strongly suggest that these three alkylanilines are metabolized in vivo to electrophilic intermediates that covalently bind to DNA and that adducts are formed in the DNA of bladder, which is a putative target organ for these alkylanilines.

The effect of deuterium and fluorine substitution upon the mutagenicity of N-hydroxy-2,6-dimethylaniline

2,6-Dimethylaniline (2,6-DMA) is an intermediate in the manufacture of several products, including pesticides, dyestuffs, and synthetic resins. It is also present in nanogram amounts in tobacco smoke, and is a major metabolite of the potent anesthetic and antiarrhythmic drug lidocaine, as well as a nasal carcinogen in rats. As with other aromatic amines, 2,6-DMA can undergo metabolic activation through cytochrome p450-mediated N-hydroxylation, followed by O-esterification to a reactive derivative capable of forming DNA adducts. We have recently characterized four DNA adducts resulting from this metabolic pathway. Three of the adducts arose from reaction of the exocyclic heteroatoms of deoxyadenosine and deoxyguanosine with the carbon para to the arylamine nitrogen. The fourth adduct resulted from reaction of the 2,6-DMA nitrogen with the C8 atom of deoxyguanosine. In order to investigate the relative contribution of the exocyclic heteroatom adducts as compared to the C8-deoxyguanosine adduct to the toxicities elicited by 2,6-DMA, we synthesized and compared the mutagenicity of N-hydroxy-2,6-DMA, N-hydroxy-4-deutero-2,6-DMA, 2,6-dimethylnitrosobenzene, 4-deutero-2,6-dimethylnitrosobenzene, and N-hydroxy-4-fluoro-2,6-DMA. In Salmonella typhimurium TA100, the two deuterated compounds and their non-deuterated analogues gave similar mutagenic responses ( approximately 25 revertants/nmol). Likewise in S. typhimurium TA98, a similar mutant frequency ( approximately 0.7 revertants/nmol) was obtained with the four compounds. With N-hydroxy-4-fluoro-2,6-DMA, the mutant frequency was reduced by approximately 90% in S. typhimurium TA100 and approximately 50% in S. typhimurium TA98. The results suggest that multiple adducts contribute to base substitution mutations detected by S. typhimurium TA100 while the C8-deoxyguanosine adduct is primarily responsible for the frameshift mutations detected by S. typhimurium TA98.

Lack of DNA binding in the rat nasal mucosa and other tissues of the nasal toxicants roflumilast, a phosphodiesterase 4 inhibitor, and a metabolite, 4-amino-3,5-dichloropyridine, in contrast to the nasal carcinogen 2,6-dimethylaniline

The phosphodiesterase 4 inhibitor Roflumilast (B9302-107) (RF) and its metabolite 4-amino-3,5-dichloropyridine (ADCP) produced nasal toxicity in preclinical safety studies with rats. The purpose of this study was to assess the possible formation of DNA adducts, by RF and ADCP, in the nasal mucosa, liver and testes of male rats using the 32P-postlabeling assay. For comparison, rats were exposed to the DNA-reactive carcinogens 2,6-dimethylaniline (DMA), also known as 2,6-xylidine, a nasal carcinogen, and the aromatic amine carcinogens 4,4'-methylene-bis(2-chloroaniline) (MOCA), which yields monocyclic DNA adducts, and 2-acetylaminofluorene (2-AAF). In the case of RF, possible sources of DNA adducts include the parent molecule and its ADCP moiety by enzymatic N-hydroxylation and sulfation, reactions typical of carcinogenic aromatic amines. 4-Acetoxylamino-3,5-dichloropyridine (N-acetoxy-ADCP), a chemically activated derivative of ADCP, was prepared and used to modify DNA which was then used to establish the chromatographic conditions with which to reliably detect whether or not such adducts were formed metabolically from RF and ADCP. Similarly, a standard N-hydroxy-DMA was prepared, but the corresponding N-acetoxy derivative was unstable and decomposed during synthesis. Both N-hydroxy-DMA and N-acetoxy-ADCP were mutagenic in the Salmonella typhimurium Ames assay using strain TA100 without an exogenous bioactivation system, with the former being more potent. N-hydroxy-ADCP was essentially inactive in this assay. For the 32P-postlabeling assay, male Wistar rats were exposed to the test substances and carrier control compounds by intragastric instillation at the selected dose levels for 7 days. Subsequently, the nasal mucosa, liver, and testes of the rats exposed to the test or control compounds were extirpated, the DNA extracted and the samples postlabeled. The patterns of adducts formed with the test compounds were compared to those formed in N-acetoxy-ADCP- and N-hydroxy-DMA-adducted DNA, which were assayed by both nuclease P1 and butanol enhancement methods. Based upon the similarity of results from the two enhancement methods, only the former was used for the in vivo studies. No evidence was obtained for the formation of DNA adducts from RF or its metabolites, specifically ADCP, under the conditions of these assays despite the ability to detect adducts from DNA modified chemically with N-acetoxy-ADCP and DNA adducts from the other compounds in their target organs. In the absence of a pattern of compound-related spots, we conclude that RF does not form DNA adducts having the potential to initiate neoplasia in these three tissues.

Immunohistochemical and ultrastructural studies of 2,6-dimethylaniline-induced nasal proliferative lesions in a rat two-stage nasal carcinogenesis model initiated with N-bis(2-hydroxypropyl)nitrosamine

Proliferative lesions induced by 2,6-dimethylaniline (DMA) in a two-stage rat nasal carcinogenesis model were immunohistochemically and ultrastructurally investigated. Male F344 rats received diet containing 3,000 ppm DMA for 52 weeks after initiation with a single subcutaneous injection of 2400 mg/kg of N-bis(2-hydroxypropyl)nitrosamine (DHPN). Histopathologically, proliferation of Bowman's glands, glandular hyperplasias, dysplastic foci, adenomas, and carcinomas were observed in treated rats. These nasal lesions mostly arose in the olfactory mucosa of the nasal cavity. Immunohistochemically, they were positive for cytokeratin and/or collagen type IV antibodies. Ultrastructurally, intracytoplasmic dense secretory granules (200-850 nm in diameter), identical to those in normal Bowman's glands, were observed in all the lesions, providing further support from an origin from these glands. Based on their cellular characterization, growth pattern and/or proliferative activity, two morphological continua were evident, one from dysplastic foci to carcinomas and the other from proliferation of Bowman's glands to glandular hyperplasias and adenomas. These results suggest that dysplastic foci arise from Bowman's glands and progress to carcinomas, while proliferation of Bowman's glands result in glandular hyperplasias and adenomas.

The regulatory status of xylazine for use in food-producing animals in the United States

Xylazine is commonly used in veterinary medicine as a tranquillizer or adjunct to surgical anaesthesia. Although its use is approved in companion animals and certain species of deer, xylazine remains unapproved for use in food-producing animals in the United States. This paper reviews existing toxicological and residue chemistry information on xylazine in food animals, particularly cattle, and discusses the regulatory status of the drug in the US, as well as the conclusions reached by the Joint FAO/WHO Expert Committee on Food Additives in its recent evaluation of xylazine.

Metabolic capacity of nasal tissue interspecies comparisons of xenobiotic-metabolizing enzymes

High levels of xenobiotic-metabolizing enzymes occur in the nasal mucosa of all species studied. In certain species, including rats and rabbits, unique enzymes are present in the nasal mucosa. The function of these enzymes is not well understood, but it is thought that they play a role in protecting the lungs from toxicity of inhalants. The observation that several nasal xenobiotic-metabolizing enzymes accept odorants as substrates may indicate that these enzymes also play a role in the olfactory process. Xenobiotic-metabolizing enzymes were found in the nasal cavity around 15 years ago. Since that time, much has been learned about the nature of the enzymes and the substrates they accept. In the present review, this information is summarized with special attention to species differences in xenobiotic-metabolizing enzymes of the nasal cavity. Such differences may be important in interpreting the results of toxicity assays in animals because rodents are apparently more susceptible to nasal toxicity after exposure to inhalants than are humans.

DNA damage in the nasal passageway: a literature review

The purpose of this review is to provide a compilation of work examining DNA damage in the nasal cavity. There are numerous methods to identify and quantify damage to DNA and the diversity of methods and toxicologic endpoints is illustrated by the range of studies presented here. There are a large number of independent studies measuring endpoints in the upper respiratory tract; however, with regard to toxicant induced DNA damage in the nasal passageway, the effects of two compounds, 4-(N-Methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone (NNK) and formaldehyde (HCHO), appear to have been extensively characterized. The body of work on NNK and formaldehyde have provided insights into molecular mechanisms of DNA damage and repair and induced cell replication and its relationship to nasal cancer. With new technologies and molecular techniques, the sensitivity to enable evaluations of the minute quantities of nasal tissue available in test species and human biopsy impact the study of the nasal-toxicant interactions. As methods used to characterize DNA damage increase in sensitivity, the importance of both exogenous and endogenous sources of DNA damage, steady-state levels of cellular damage, repair, and resulting mutations, low-dose exposure assessments and inter-species extrapolation will become increasingly complex. Additional studies of DNA damage in the nasal passage will undoubtedly challenge future estimations of risk and impact what are perceived to be acceptable levels of exposure to known and predicted carcinogens. The aim of this paper is to provide to the interested scientist literature relevant to the effects of agents on nasal DNA, so that areas of insufficient information can be identified and used to further develop and expand the knowledge base for nasal DNA toxicant interactions.

Biotransformation of butachlor through mercapturic acid pathway in rat tissue homogenates

The metabolism of butachlor was studied in rat liver and kidney homogenates. In vitro incubation of butachlor with liver fractions (S9, microsome, and cytosolic fractions) formed a considerable amount of butachlor glutathione conjugate (BGSC), while the conjugating activity was not efficient for the kidney S9 fraction. There is a sex difference in the distribution of glutathione S-transferase in the liver. It seems that more enzyme activity is detected in the female liver microsome, while this is not the case in its cytosolic fraction. Further biotransformation of BGSC to mercapturate was not observed in the liver S9 fraction. This metabolite was further transformed to butachlor acetyl cysteine conjugate (BACC) in the presence of acetyl CoA, but to butachlor cysteine conjugate (BCC) in the absence of acetyl CoA. These findings demonstrated that butachlor is initially conjugated with GSH to form BGSC by the enzyme glutathione S-transferase in the liver. This metabolite is apparently transported to the kidneys, where it is transformed to the mercapturate.

Nasal cavity enzymes involved in xenobiotic metabolism: effects on the toxicity of inhalants

A decade ago, the ability of nasal tissues to metabolize inhalants was only dimly suspected. Since then, the metabolic capacities of nasal cavity tissues has been extensively investigated in mammals, including man. Aldehyde dehydrogenases, cytochrome P-450-dependent monooxygenases, rhodanese, glutathione transferases, epoxide hydrolases, flavin-containing monooxygenases, and carboxyl esterases have all been reported to occur in substantial amounts in the nasal cavity. The contributions of these enzyme activities to the induction of toxic effects from inhalants such as benzo-a-pyrene, acetaminophen, formaldehyde, cocaine, dimethylnitrosamine, ferrocene, and 3-trifluoromethylpyridine have been the subject of dozens of reports. In addition, the influence of these enzyme activities on olfaction and their contribution to vapor uptake is beginning to receive attention from the research community. Research in the next decade promises to provide answers to the many still unanswered questions posed by the presence of the substantial xenobiotic metabolizing capacity of the nasal cavity.

Haemoglobin adducts formed by aromatic amines in smokers: sources of inter-individual variability

In a previous study we found that aromatic amines, particularly 4-aminobiphenyl, formed haemoglobin adducts at higher concentrations in the blood of smokers compared to non-smokers. We re-analyse here data on haemoglobin adducts of 14 aromatic amines in order to ascertain if the inter-individual variability left unexplained by tobacco smoking could be attributed to differences in individual metabolic patterns. For this purpose we computed residuals from analysis of variance in order to adjust for individual smoking habits (type and amount of tobacco). Residuals were correlated within two clearly distinct groups: one formed by binuclear compounds (4-aminobiphenyl, 3-aminobiphenyl and 2-naphthylamine) and the other formed by all other (i.e. mononuclear) compounds. Within each group, highly statistically significant correlation coefficients were found, whereas compounds belonging to one group were not correlated to compounds in the other group. These results can be interpreted as a suggestion that two different metabolic pathways exist, one for binuclear and one for mononuclear arylamines, and that inter-individual differences in such pathways can explain part of inter-individual variability in adduct levels. This interpretation is consistent with recent animal experiments suggesting that there are different enzyme systems for the two classes of compounds.