Studies on the mechanism of acetamide hepatocarcinogenicity

Involvement of nuclear atrophy of binucleated hepatocytes in the large micronucleus formation induced by rat hepatocarcinogen acetamide

Acetamide is a hepatocarcinogen in rats. We previously revealed that acetamide induces characteristic large micronuclei in rat liver, suggesting the possible involvement of chromosome aberrations in acetamide-induced hepatocarcinogenesis. To elucidate the mechanism of large micronuclei formation, in this study we examined time-dependent changes in rat hepatocytes after administration of acetamide. Male 6-week-old F344 rats were gavaged with a single-dose administration of acetamide. A liver micronucleus test showed large micronuclei formation 48 and 72 h after acetamide administration. Histopathological analysis showed binucleated hepatocytes with a unilateral atrophic nucleus beginning 6 h after acetamide administration, and the number reached a maximum at 24 h. At 48 h, the number of binucleated hepatocytes with an atrophic nucleus decreased, and apoptotic hepatocytes and large micronucleated hepatocytes appeared. The changes in the frequency of these abnormal binucleated hepatocytes demonstrated a transition from atrophic nuclei to large micronuclei. Immunohistopathological examinations of binucleated hepatocytes showed loss of nuclear lamina, accumulation of barrier-to-autointegration factor (BAF) and chromatin condensation with heterochromatinization at the atrophic site of nuclei. Results of a BrdU-labeling assay were negative. The abnormal expression of BAF in morphologically normal nuclei suggested that nuclear envelope aberration in hepatocytes was an initial event of the nuclear atrophy. In addition, lack of involvement of cell division in the nuclear atrophy and large micronucleus formation was also demonstrated by BrdU-labeling assay. Overall, our data suggest that large micronuclei induced by acetamide are formed in binucleated hepatocytes through nuclear atrophy.

Untargeted Metabolite Profiling of Adipose Tissue in Rats Exposed to Mepiquat

Mepiquat (Mep) is a contaminant produced by Maillard reaction with reducing sugar, free lysine and an alkylating agent under typical roasting conditions, particularly in the range of 200-240 °C. It has been reported that exposure to Mep is harmful to rats. However, its metabolic mechanism is still not clear. In this study, untargeted metabolomics was used to reveal the effect of Mep on the metabolic profile of adipose tissue in Sprague-Dawley rats. Twenty-six differential metabolites were screened out. Eight major perturbed metabolic pathways were found, which were linoleic acid metabolism, Phenylalanine, tyrosine, and tryptophan biosynthesis, phenylalanine metabolism, arachidonic acid metabolism, Glycine, serine, and threonine metabolism, glycerolipid metabolism, Alanine, aspartate, and glutamate metabolism, and glyoxylate and dicarboxylic acid metabolism. This study lays a solid foundation for clarifying the toxic mechanism of Mep.

Mechanism of reactive oxygen species generation and oxidative DNA damage induced by acrylohydroxamic acid, a putative metabolite of acrylamide

Acrylamide is formed during the heating of food and is also found in cigarette smoke. It is classified by the International Agency for Research on Cancer as a probable human carcinogen (Group 2A). Glycidamide, an epoxide metabolite of acrylamide, is implicated in the mechanism of acrylamide carcinogenicity. Acrylamide causes oxidative DNA damage in target organs. We sought to clarify the mechanism of acrylamide-induced oxidative DNA damage by investigating site-specific DNA damage and reactive oxygen species (ROS) generation by a putative metabolite of acrylamide, acrylohydroxamic acid (AA). Our results, using ³²P-5'-end-labeled DNA fragments, indicated that, although AA alone did not damage DNA, AA treated with amidase induced DNA damage in the presence of Cu(II). DNA cleavage occurred preferentially at T and C, and particularly at T in 5'-TG-3' sequences, and the DNA cleavage pattern was similar to that of hydroxylamine. The DNA damage was inhibited by methional, catalase, and Cu(I)-chelator bathocuproine, suggesting that H₂O₂ and Cu(I) are involved in the mechanism of DNA damage induced by AA treated with amidase. In addition, amidase-treated AA increased 8-oxo-7,8-dihydro-2'-deoxyguanosine formation in calf thymus DNA, an indicator of oxidative DNA damage, in a dose-dependent manner. In conclusion, hydroxylamine, possibly produced from AA treated with amidase, was autoxidized via the Cu(II)/Cu(I) redox cycle and H₂O₂ generation, suggesting that oxidative DNA damage induced by ROS plays an important role in acrylamide-related carcinogenesis.

A toxicogenomic approach for the risk assessment of the food contaminant acetamide

Acetamide (CAS 60-35-5) is detected in common foods. Chronic rodent bioassays led to its classification as a group 2B possible human carcinogen due to the induction of liver tumors in rats. We used a toxicogenomics approach in Wistar rats gavaged daily for 7 or 28 days at doses of 300 to 1500 mg/kg/day (mkd) to determine a point of departure (POD) and investigate its mode of action (MoA). Ki67 labeling was increased at doses ≥750 mkd up to 3.3-fold representing the most sensitive apical endpoint. Differential gene expression analysis by RNA-Seq identified 1110 and 1814 differentially expressed genes in male and female rats, respectively, following 28 days of treatment. Down-regulated genes were associated with lipid metabolism while up-regulated genes included cell signaling, immune response, and cell cycle functions. Benchmark dose (BMD) modeling of the Ki67 labeling index determined the BMD10 lower confidence limit (BMDL10) as 190 mkd. Transcriptional BMD modeling revealed excellent concordance between transcriptional POD and apical endpoints. Collectively, these results indicate that acetamide is most likely acting through a mitogenic MoA, though specific key initiating molecular events could not be elucidated. A POD value of 190 mkd determined for cell proliferation is suggested for risk assessment purposes.

The food contaminant acetamide is not an in vivo clastogen, aneugen, or mutagen in rodent hematopoietic tissue

Acetamide (CAS 60-35-5) is classified by IARC as a Group 2B, possible human carcinogen, based on the induction of hepatocellular carcinomas in rats following chronic exposure to high doses. Recently, acetamide was found to be present in a variety of human foods, warranting further investigation. The regulatory body JECFA has previously noted conflicting reports on acetamide's ability to induce micronuclei (MN) in mice in vivo. To better understand the potential in vivo genotoxicity of acetamide, we performed acute MN studies in rats and mice, and a subchronic study in rats, the target species for liver cancer. In the acute exposure, animals were gavaged with water vehicle control, 250, 1000, or 2000 mg/kg acetamide, or the positive control (1 mg/kg mitomycin C). In the subchronic assay, bone marrow of rats gavaged at 1000 mg/kg/day (limit dose) for 28 days was evaluated. Both acute and subchronic exposures showed no change in the ratio of polychromatic to total erythrocytes (P/E) at any dose, nor was there any increase in the incidence of micronucleated polychromatic erythrocytes (MN-PCE). Potential mutagenicity of acetamide was evaluated in male rats gavaged with vehicle control or 1500 mg/kg/day acetamide using the in vivoPig-a gene mutation assay. There was no increase in mutant red blood cells or reticulocytes in acetamide-treated animals. In both acute and sub-chronic studies, elevated blood plasma acetamide in treated animals provided evidence of systemic exposure. We conclude based on this study that acetamide is not clastogenic, aneugenic, or mutagenic in vivo in rodent hematopoietic tissue warranting a formal regulatory re-evaluation.

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In vivo micronucleus tests with acetamide in mice and rats.

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Acetamide blood plasma levels demonstrated evidence of exposure.

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Acetamide does not induce micronuclei in rats and mice.

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Acetamide does not increase mutations in the rat Pig-a gene mutation assay.

Automatic knowledge extraction from chemical structures: the case of mutagenicity prediction

This work proposes a new structure-activity relationship (SAR) approach to mine molecular fragments that act as structural alerts for biological activity. The entire process is designed to fit with human reasoning, not only to make the predictions more reliable but also to permit clear control by the user in order to meet customized requirements. This approach has been tested on the mutagenicity endpoint, showing marked prediction skills and, more interestingly, bringing to the surface much of the knowledge already collected in the literature as well as new evidence.

Mutagenicity of cytochrome P450 2E1 substrates in the Ames test with the metabolic competent S. typhimurium strain YG7108pin3ERb5

Poor metabolic competence of in vitro systems was proposed to be one of their major shortcomings accounting for false negative results in genotoxicity testing. For several "low molecular weight cancer suspects" this was specifically attributed to the lack of cytochrome P450 2E1 (CYP2E1) in conventional in vitro metabolising systems. One promising attempt to overcome this problem is the transfection of "methyltransferase-deficient"S. typhimurium strains with the plasmid pin3ERb5. This plasmid contains DNA encoding for a complete electron transport chain, comprising P450 reductase, cytochrome b5 and cytochrome P450 2E1. In order to answer the question if CYP2E1 substrates that yield negative or inconclusive results in the Ames test can be activated by metabolic competent bacterial strains, we used YG7108pin3ERb5 to investigate the following compounds: acetamide, acrylamide, acrylonitrile, allyl chloride, ethyl acrylate, ethyl carbamate, methyl-methacrylate, vinyl acetate, N-nitrosopyrrolidine, trichloroethylene and tetrachloroethylene. N-Nitrosodiethylamine served as a positive control. In addition to these known or proposed CYP2E1 substrates, we investigated the polycyclic aromatic hydrocarbon benzo[alpha]pyrene and the heterocyclic aromatic amines 2-aminofluorene and 2-aminoanthracene.

RESULTS

The extensive metabolic competence of the transformed strain is underlined by results showing strong mutagenicity between 10 and 500 micro g N-nitrosopyrrolidine per plate. Unexpectedly, 2-aminoanthracene was mutagenic at a concentration range between 25 and 250 micro g per plate using YG7108pin3ERb5. Moreover, we demonstrate for the first time a clear response of sufficiently characterised allyl chloride in the Ames test at a reasonably low concentration range between 300 and 1500 micro g per plate. We achieved similar results in the parent strain YG7108 with conventional metabolic activation. Without metabolic activation less pronounced mutagenicity occurred, suggesting a contribution of a direct alkylating effect. Propylene oxide is usually contained in allyl chloride as stabilizer at amounts up to 0.09%. Though YG7108 revealed to be very sensitive towards propylene oxide, allyl chloride dissolved in water was not mutagenic, showing that no water soluble compounds contribute to its mutagenicity. None of the remaining compounds showed mutagenic effects using YG7108pin3ERb5.

CONCLUSION

YG7108pin3ERb5 and its parent strain YG7108 are sensitive for compounds which are negative in conventional tester strains including N-nitrosodiethylamine, N-nitrosopyrrolidine, propylene oxide and allyl chloride.

Application of Fourier transform infrared spectroscopy for monitoring hydrolysis and synthesis reactions catalyzed by a recombinant amidase

This study demonstrates the use of Fourier transform infrared (FTIR) spectroscopy for monitoring both synthesis and hydrolysis reactions catalyzed by a recombinant amidase (EC 3.5.1.4) from Pseudomonas aeruginosa. The kinetics of hydrolysis of acetamide, propionamide, butyramide, acrylamide, benzamide, phenylalaninamide, alaninamide, glycinamide, and leucinamide were determined. This revealed that very short-chain substrates displayed higher amidase activity than did branched side-chain or aromatic substrates. In addition, on reducing the polarity and increasing the substrates' bulkiness, a reduction of the amidase affinity for the substrates took place. Using FTIR spectroscopy it was possible to monitor and quantify the synthesis of several hydroxamic acid derivatives and ester hydrolysis products. These products may occur simultaneously in a reaction catalyzed by the amidase. The substrates used for the study of such reactions were ethyl acetate and glycine ethyl ester. Hydroxylamine was the nucleophile substrate used for the synthesis of acetohydroxamate compounds. Results presented in this article demonstrate the usefulness of FTIR spectroscopy as an important tool for understanding the enzyme structure-activity relationship because it provides a simple and rapid real-time assay for the detection and quantification of amidase hydrolysis and synthesis reactions in situ.

Genotoxicity test system based on p53R2 gene expression in human cells: examination with 80 chemicals

p53R2, which encodes a subunit of ribonucleotide reductase, is activated by DNA damage induced by gamma-ray and ultraviolet irradiation, and also by genotoxic chemicals such as adriamycin. For the purpose of constructing an easy-operating genotoxicity test system using human cell lines, we developed a p53R2-dependent luciferase reporter gene assay, and demonstrated dose-dependent luminescence caused by adriamycin in two human cell lines that express wild-type p53, MCF-7 and HepG2. The performance of this assay system was evaluated with 80 chemicals including those known in the Ames test as genotoxic or non-genotoxic. When the luciferase activity of cells treated with the test sample was over 200% to that of control cells in a dose-dependent increasing manner, the sample was judged positive as a genotoxic chemical. Forty of 43 Ames-positive chemicals induced luciferase activity in this assay system. Eight Ames-negative chemicals also induced luciferase activity. These eight chemicals are genotoxic in other in vitro test systems using mammalian cells. It is suggested that this assay system can be applied to rapid screening of chemicals for potential human genotoxicity.

Application of toxicological risk assessment principles to the chemical constituents of cigarette smoke

OBJECTIVE

To provide a hazard prioritisation for reported chemical constituents of cigarette smoke using toxicological risk assessment principles and assumptions. The purpose is to inform prevention efforts using harm reduction.

DATA SOURCES

International Agency for Research on Cancer Monographs; California and US Environmental Protection Agency cancer potency factors (CPFs) and reference exposure levels; scientific journals and government reports from the USA, Canada, and New Zealand.

STUDY SELECTION

This was an inclusive review of studies reporting yields of cigarette smoke constituents using standard ISO methods.

DATA EXTRACTION

Where possible, the midpoint of reported ranges of yields was used.

DATA SYNTHESIS

Data on 158 compounds in cigarette smoke were found. Of these, 45 were known or suspected human carcinogens. Cancer potency factors were available for 40 of these compounds and reference exposure levels (RELs) for non-cancer effects were found for 17. A cancer risk index (CRI) was calculated by multiplying yield levels with CPFs. A non-cancer risk index (NCRI) was calculated by dividing yield levels with RELs. Gas phase constituents dominate both CRI and NCRI for cigarette smoke. The contribution of 1,3-butadiene (BDE) to CRI was more than twice that of the next highest contributing carcinogen (acrylonitrile) using potencies from the State of California EPA. Using those potencies from the USEPA, BDE ranked third behind arsenic and acetaldehyde. A comparison of CRI estimates with estimates of smoking related cancer deaths in the USA showed that the CRI underestimates the observed cancer rates by about fivefold using ISO yields in the exposure estimate.

CONCLUSIONS

The application of toxicological risk assessment methods to cigarette smoke provides a plausible and objective framework for the prioritisation of carcinogens and other toxicant hazards in cigarette smoke. However, this framework does not enable the prediction of actual cancer risk for a number of reasons that are discussed. Further, the lack of toxicology data on cardiovascular end points for specific chemicals makes the use of this framework less useful for cardiovascular toxicity. The bases for these priorities need to be constantly re-evaluated as new toxicology information emerges.

Mechanism of metal-mediated DNA damage induced by a metabolite of carcinogenic acetamide

Acetamide is carcinogenic in rats and mice. To clarify the mechanism of carcinogenesis by acetamide, we investigated DNA damage by and acetamide metabolite, acetohydroxamic acid (AHA), using 32P-5'-end-labeled DNA fragments. AHA treated with amidase induced DNA damage in the presence of Cu(II) and displayed a similar DNA cleavage pattern of hydroxylamine. DNA damage was inhibited by both catalase and bathocuproine, suggesting that H2O2 and Cu(I) are involved. Carboxy-PTIO, a specific scavenger of nitric oxide (NO), partially inhibited DNA damage. The amount of 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodG) by amidase-treated AHA was similar to that by hydroxylamine. ESR spectrometry revealed that amidase-treated AHA as well as hydroxylamine generated NO in the presence of Cu(II). From these results, it has been suggested that AHA might be converted into hydroxylamine by amidase. These results suggest that metal-mediated DNA damage mediated by amidase-catalyzed hydroxylamine generation plays an important role in the carcinogenicity of acetamide.

Measuring enzymatic activity of a recombinant amidase using Fourier transform infrared spectroscopy

A method based on Fourier transform infrared spectroscopy (FT-IR) has been developed for assaying the Pseudomonas aeruginosa native amidase (E.C. 3.5.1.4), overproduced in an Escherichia coli strain. The kinetic of acetamide hydrolysis by the enzyme, in aqueous media, was monitored by measuring the intensity of the acetamide amide I band maximum at 1635 cm(-1) as a function of time. A value of 0.5mM(-1) cm(-1) was obtained for the extinction coefficient (epsilon) of acetamide at this frequency. The rate of the hydrolysis was found to be linear with the concentration of the enzyme up to 90 microM. The Michaelis-Menten kinetics parameters V and K(m) were determined as 30.7 U/mg and 4mM, respectively. These results were similar to those obtained using high-performance liquid chromatography analysis of the same hydrolytic reaction catalyzed by amidase either in water or in buffer. This suggests that the precision of the FT-IR method is suitable for the kinetic studies of amidase with the additional advantage of being able to perform a real-time measurement of the enzymatic activity.

Use of a cell transformation assay with established cell lines, and a metabolic cooperation assay with V79 cells for the detection of tumour promoters: a review

Extensive studies on the safety evaluation of chemicals have indicated that a considerable number of non-genotoxic chemicals are carcinogenic. Tumour promoters are likely to be among these non-genotoxic carcinogens, and their detection is considered to be an important approach to the prevention of cancer. In this review, the results are summarised for in vitro transformation assays involving established cell lines, and for an assay for inhibition of gap junctional intercellular communication for the detection of tumour promoters, which involves V79 cells. Although the number of chemicals examined is still too small to permit a full evaluation of the correlation between in vitro cell transformation and in vivo carcinogenicity, it is clear that the sensitivity of the focus formation assay is very high. In the case of the metabolic cooperation assay, the sensitivity appears to be rather poor, but the assay can be considered to be useful because of its simple procedure and its considerable database. These in vitro assays for tumour promoters are recommended as useful tools for the detection of non-genotoxic carcinogens.

Chemicals with carcinogenic activity in the rodent liver; mechanistic evaluation of human risk

A wide variety of chemicals, both naturally occurring and synthetic, have exhibited carcinogenic activity in rodent liver. Some are clearly DNA reactive whereas others produce only epigenetic effects. Hepatocarcinogens are categorized according to these properties and the characteristics of examples of both types are reviewed. DNA-reactive rodent hepatocarcinogens represent human cancer risk even at non-toxic exposures, whereas epigenetic agents pose either no risk because their effects are specific to rodents, or a risk only at high exposures at which they produce the same cellular effects in humans that are the basis for their carcinogenic activity in rodents.

Activities of the rodent carcinogens thioacetamide and acetamide in the mouse bone marrow micronucleus assay

Thioacetamide is confirmed as active in the mouse bone marrow micronucleus assay, following single oral administration to both sexes of two different strains of mice (C57BL/6, BALB/c) in five separate experiments. Acetamide is shown to be consistently and clearly negative as a micronucleus-inducing agent in mouse bone marrow in four repeated assays using different sexes of two strains of mice (C57BL/6, CBA). The present findings support the adequacy of the limited micronucleus test protocol (male animals, two sampling times) for the efficient detection of genotoxic rodent carcinogens in this assay.

Influence of sample preparation on cellular glutathione recovery from adherent cells in culture

During the last decade, the unbound glutathione content of cultured adherent cells has become a very important biological marker for many pharmacological and toxicological in vitro studies with regard to the protective role of the tripeptide in its reduced form (GSH). However, the literature does not provide extensive information on the influence of sample preparation on cellular GSH and thiol analyses. Using the fibroblast-like V79 cell line as model, we undertook a comparative study of the efficiency of different procedures reported in the literature with respect to GSH recovery. Depending on the preanalytical step, up to 10-fold discrepancies could be observed in the recovery of intracellular GSH. Different parameters that must be controlled in order to maximize GSH recovery are discussed. The optimal strategy consisted in rapid perchloric acid deproteinization performed directly in the dish, which was extremely valuable for preparing GSH samples from adherent cells, and especially from cells expressing elevated gamma-glutamyl transferase activity.