Rat Pig-a mutation assay responds to the genotoxic carcinogen ethyl carbamate but not the non-genotoxic carcinogen methyl carbamate

Formation of hepatocyte cytoplasmic inclusions and their contribution to methylcarbamate-induced hepatocarcinogenesis in F344 rats

Methylcarbamate (MC), a reaction product between dimethyl dicarbonate and ammonia or ammonium ion, is a potent hepatocarcinogen in F344 rats. Various genotoxicity tests have shown negative results for MC. Although previous studies have described the effects of MC on the liver, including the formation of characteristic basophilic cytoplasmic inclusions (CIs) in hepatocytes, the toxicological significance of CIs and their involvement in hepatocarcinogenesis remain unclear. In the current study, to elucidate the mechanisms of MC hepatocarcinogenesis, we examined hepatotoxicity and genotoxicity after 4 weeks of administration of MC using gpt delta rats with an F344 genetic background as a reporter gene transgenic animal model. Histopathologically, single-cell necrosis, karyomegaly, and the formation of CIs positive for Feulgen staining were observed in hepatocytes at the carcinogenic dose, demonstrating the hepatotoxicity of MC. CIs were also detected as large micronuclei in liver micronucleus tests but not in the bone marrow, suggesting that MC could cause chromosomal instability specifically in the livers of rats. Reporter gene mutation assays demonstrated that MC did not induce mutagenicity even in the liver. Immunofluorescence analyses revealed that CIs exhibited loss of nuclear envelope integrity, increased heterochromatinization, and accumulation of DNA damage. An increase in liver STING protein levels suggested an effect on the cyclic GMP-AMP synthase/stimulator of interferon genes innate immune pathway. Overall, these data demonstrated the possible occurrence of chromothripsis-like chromosomal rearrangements via CIs. Thus, the formation of CIs could be a crucial event in the early stage of MC-induced hepatocarcinogenesis in F344 rats.

Novel insights into the potential mechanisms underlying carbendazim-induced hepatorenal toxicity in rats

Carbendazim (CBZ) is a common environmental pollutant that can contaminate food and water and severely damage human health. Some studies revealed the adverse effect of CBZ on different organs, but its detailed toxicity mechanism has not been elucidated yet. Thus, the present study aims to clarify the mechanisms of CBZ-induced hepatorenal toxicity in rats. Therefore, we partitioned 40 male Wistar rats into four groups (n = 10): a negative control group and three treatment groups, which received 100, 300, and 600 mg/kg of CBZ. All rats received the treatment daily by oral gavage. We collected blood and organ samples (liver and kidney) at 14 and 28 days postdosing. CBZ caused extensive pathological alterations in both the liver and kidneys, such as cellular degeneration and necrosis accompanied by severe inflammatory reactions in a dose- and time-dependent manner. All the CBZ-treated groups displayed strong tumor necrosis factor-α and nuclear factor-κB (NF-κB) immunopositivity. Additionally, CBZ dose-dependently elevated the alanine aminotransferase, aspartate aminotransferase, alkaline phosphatase, urea, and creatinine serum levels and reduced the serum albumin levels. Furthermore, CBZ-induced apoptosis, as indicated by the observed Bax gene upregulation and Bcl-2 gene downregulation in both organs. All these changes may be related to oxidative stress, as indicated by the increase in malondialdehyde levels and the decrease in total antioxidant capacity. Our results demonstrate that CBZ-induced dose- and time-dependent hepatorenal damage through oxidative stress, which activated both the NF-κB signaling pathway and Bcl-based programmed cell death.

Evaluation of the Liver and Blood Micronucleus, and Comet Assay Endpoints in a 14-Day Repeated Dose Study with Methyl Carbamate and 1, 3-Propane Sultone

The repeated-dose liver micronucleus (RDLMN) assay is a novel method for detecting genotoxic chemicals. Two carcinogens methyl carbamate (MC) and 1, 3-propane sultone (PS) were evaluated for the liver micronucleus in a 14-day repeated-dose study with Sprague Dawley rats. Additionally, micronucleated reticulocytes (MN-RET) in peripheral blood and DNA damage (alkaline comet assay) in the liver were also assessed in the same animals. Ten groups of 5 male Sprague Dawley rats were treated once daily with MC (300, 600, or 1200 mg/kg/day), PS (37.5, 75, or 150 mg/kg/day), negative control, or 3 positive controls by oral gavage for 15 days. Blood samples were collected at 3 hours after the last administration for determining MN-RET frequencies (%MN-RET), and the livers were sampled for determining the frequency of micronuclei and DNA damage. MC was negative in the comet assay, liver micronucleus assay, and reticulocyte micronucleus assay, while PS was positive in all three assays. These results are consistent with the previous genotoxic findings of MC and PS. Therefore, the liver micronucleus assay can be effectively integrated into repeated dose studies in animals. Moreover, integration of multiple genotoxicity endpoints into one study can reduce the number of animals, boost the experimental efficiency, and provides a comprehensive evaluation of the genotoxic potential of chemicals.

In vitro and in vivo assessment of the genotoxic effects of ceric ammonium nitrate and 1,3-propane sultone

A variety of methods have been developed for accurate and systematic evaluation of chemical genotoxicity. Ceric ammonium nitrate (CAN) and 1,3-propane sultone (1,3-PS) have been extensively applied in industrial fields. Although 1,3-PS, but not CAN, has been reported as a potent carcinogen, systematic assessment of the genotoxic properties of these chemicals has not been conducted. The purpose of this study was to establish a decision tree for evaluating genotoxicity based on the good laboratory practices (GLP) system using 1,3-PS and CAN as test chemicals. In vitro studies were performed including the bacterial reverse mutation assay, chromosomal aberration assay, and micronucleus assay. We conducted in vivo studies using a combined micronucleus and alkaline comet (MN-CMT) assay and the Pig-a gene mutation assay, which is a promising method for detecting gene mutations in vivo. CAN showed negative responses in all in vitro genotoxicity assays and the in vivo combined MN-CMT assay. Meanwhile, 1,3-PS had positive results in all in vitro and in vivo genotoxicity assays. In this study, we confirmed the genotoxicity of 1,3-PS and CAN using both in vitro and in vivo assays. We propose a decision tree for evaluating chemical-induced genotoxicity.

In vivo genotoxicity testing strategies: Report from the 7th International workshop on genotoxicity testing (IWGT)

The working group reached complete or majority agreement on many issues. Results from TGR and in vivo comet assays for 91 chemicals showed they have similar ability to detect in vivo genotoxicity per se with bacterial mutagens and Ames-positive carcinogens. TGR and comet assay results were not significantly different when compared with IARC Group 1, 2 A, and unclassified carcinogens. There were significantly more comet assay positive responses for Group 2B chemicals, and for IARC classified and unclassified carcinogens combined, which may be expected since mutation is a sub-set of genotoxicity. A liver comet assay combined with the bone marrow/blood micronucleus (MNviv) test would detect in vivo genotoxins that do not exhibit tissue-specific or site-of-contact effects, and is appropriate for routine in vivo genotoxicity testing. Generally for orally administered substances, a comet assay at only one site-of-contact GI tract tissue (stomach or duodenum/jejunum) is required. In MNviv tests, evidence of target tissue exposure can be obtained in a number of different ways, as recommended by ICH S2(R1) and EFSA (Hardy et al., 2017). Except for special cases the i.p. route is inappropriate for in vivo testing; for risk evaluations more weight should be given to data from a physiologically relevant administration route. The liver MN test is sufficiently validated for the development of an OECD guideline. However, the impact of dosing animals >6 weeks of age needs to be evaluated. The GI tract MN test shows promise but needs more validation for an OECD guideline. The Pig-a assay detects systemically available mutagens and is a valuable follow-up to in vitro positive results. A new freeze-thaw protocol provides more flexibility. Mutant reticulocyte and erythrocyte frequencies should both be determined. Preliminary data are available for the Pig-a assay in male rat germ cells which require validation including germ cell DNA mutation origin.

Evaluation of in vitro and in vivo genotoxic and antigenotoxic effects of Rhus coriaria

Rhus coriaria has been important in the treatment of many diseases in traditional use. In this content, the genotoxic, antigenotoxic, and oxidative stress effects of methanol extract of R. coriaria (RCE) were investigated in this study. Two hundred fifty, 500, or 750 µg/mL concentrations of RCE were not found to have DNA damaging effect on pET22-b(+) plasmid and were unable to induce micronuclei in human lymphocytes (24 or 48 h treatment period). However, it did not inhibit the genotoxic effect of mitomycin-c (0.25 µg/mL). Cytotoxic effects of RCE were investigated using mitotic index (MI) and nuclear division index (NDI). Five hundred, 1000, and 2000 mg/kg concentrations of RCE did not induce chromosome aberrations in rat bone marrow cells for 12 or 24 h treatment period. In addition, 2000 mg/kg concentration of RCE showed an antigenotoxic effect by decreasing to genotoxic effect of 400 mg/kg urethane at 12 and 24 h treatment periods. RCE showed cytotoxic effects by significantly decreasing NDI. Moreover, RCE increased cytotoxic effect of Mitomycin C (MMC). However, RCE did not induce cytotoxicity in rat bone marrow cells. The highest concentration of RCE reduced total oxidant level in 12 h treatment. Interestingly, the lowest total oxidant level was found in rats blood treated with the lowest concentration RCE and urethane together. Thousand and 2000 mg/kg concentrations of RCE decreased total antioxidant levels of rat blood at 24 h treatment period. Our results showed that RCE possess cytotoxic effect in short-term treatments in vitro. However, it does not demonstrate genotoxic or cytotoxic effects in vivo.

Extrapolation of in vitro structural alerts for mutagenicity to the in vivo endpoint

As part of the hazard and risk assessment of chemicals in man, it is important to assess the ability of a chemical to induce mutations in vivo. Because of the commonalities in the molecular initiating event, mutagenicity in vitro can correlate well to the in vivo endpoint for certain compound classes; however, the difficulty lies in identifying when this correlation holds true. In silico alerts for in vitro mutagenicity may therefore be used as the basis for alerts for mutagenicity in vivo where an expert assessment is carried out to establish the relevance of the correlation. Taking this into account, a data set of publicly available transgenic rodent gene mutation assay data, provided by the National Institute of Health Sciences of Japan, was processed in the expert system Derek Nexus against the in vitro mutagenicity endpoint. The resulting predictivity was expertly reviewed to assess the validity of the observed correlations in activity and mechanism of action between the two endpoints to identify suitable in vitro alerts for extension to the in vivo endpoint. In total, 20 alerts were extended to predict in vivo mutagenicity, which has significantly improved the coverage of this endpoint in Derek Nexus against the data set provided. Updating the Derek Nexus knowledge base in this way led to an increase in sensitivity for this data set against this endpoint from 9% to 66% while maintaining a good specificity of 89%.

Assessment of the Pig-a, micronucleus, and comet assay endpoints in rats treated by acute or repeated dosing protocols with procarbazine hydrochloride and ethyl carbamate

The utility and sensitivity of the newly developed flow cytometric Pig-a gene mutation assay have become a great concern recently. In this study, we have examined the feasibility of integrating the Pig-a assay as well as micronucleus and Comet endpoints into acute and subchronic general toxicology studies. Male Sprague-Dawley rats were treated for 3 or 28 consecutive days by oral gavage with procarbazine hydrochloride (PCZ) or ethyl carbamate (EC) up to the maximum tolerated dose. The induction of CD59-negative reticulocytes and erythrocytes, micronucleated reticulocytes in peripheral blood, micronucleated polychromatic erythrocytes in bone marrow, and Comet responses in peripheral blood, liver, kidney, and lung were evaluated at one, two, or more timepoints. Both PCZ and EC produced positive responses at most analyzed timepoints in all tissue types, both with the 3-day and 28-day treatment regimens. Furthermore, comparison of the magnitude of the genotoxicity responses indicated that the micronucleus and Comet endpoints generally produced greater responses with the higher dose, short-term treatments in the 3-day study, while the Pig-a assay responded better to the cumulative effects of the lower dose, but repeated subchronic dosing in the 28-day study. Collectively, these results indicate that integration of several in vivo genotoxicity endpoints into a single routine toxicology study is feasible and that the Pig-a assay may be particularly suitable for integration into subchronic dose studies based on its ability to accumulate the mutations that result from repeated treatments. This characteristic may be especially important for assaying lower doses of relatively weak genotoxicants. Environ. Mol. Mutagen. 60:56-71, 2019. © 2018 Wiley Periodicals, Inc.

A statistical approach for analyzing data from the in vivo Pig-a gene mutation assay

The in vivo Pig-a gene mutation assay serves to evaluate the genotoxic potential of chemicals. In the rat blood-based assay, the lack of CD59 on the surface of erythrocytes is quantified via fluorophore-labeled antibodies in conjunction with flow cytometric analysis to determine the frequency of Pig-a mutant phenotype cells. The assay has achieved regulatory relevance as it is suggested as an in vivo follow-up test for Ames mutagens in the recent ICH M7 [25] step 4 document. However, very little work exists regarding suitable statistical approaches for analyzing Pig-a data. In the current report, we present a statistical strategy based on a two factor model involving 'treatment' and 'time' incl. their interaction and a baseline covariate for log proportions to compare treatment and vehicle data per time point as well as in time. In doing so, multiple contrast tests allow us to discover time-related changes within and between treatment groups in addition to multiple treatment comparisons to a control group per single time point. We compare our proposed strategy with the results of classical Dunnett and Wilcoxon-Mann-Whitney tests using two data sets describing the mode of action of Chlorambucil and Glycidyl methacrylate both analyzed in a 28-day treatment schedule.

In vivo pig-a and micronucleus study of the prototypical aneugen vinblastine sulfate

The Pig-a assay is being used in regulatory studies to evaluate the potential of agents to induce somatic cell gene mutations and an OECD test guideline is under development. A working group involved with establishing the guideline recently noted that representative aneugenic agents had not been evaluated, and to help fill this data gap Pig-a mutant phenotype and micronucleated reticulocyte frequencies were measured in an integrated study design to assess the mutagenic and cytogenetic damage responses to vinblastine sulfate exposure. Male Sprague Dawley rats were treated for twenty-eight consecutive days with vinblastine dose levels from 0.0156 to 0.125 mg/kg/day. Micronucleated reticulocyte frequencies in peripheral blood were determined at Days 4 and 29, and mutant cell frequencies were determined at Days -4, 15, 29, and 46. Vinblastine affected reticulocyte frequencies, with reductions noted during the treatment phase and increases observed following cessation of treatment. Micronucleated reticulocyte frequencies were significantly elevated at Day 4 in the high dose group. Although a statistically significant increase in mutant reticulocyte frequencies were found for one dose group at a single time point (Day 46), it was not deemed biologically relevant because there was no analogous finding in mutant RBCs, it occurred at the lowest dose tested, and only 1 rat exceeded an upper bound tolerance interval established with historical negative control rats. Therefore, whereas micronucleus induction reflects vinblastine's well-established aneugenic effect on hematopoietic cells, the lack of a Pig-a response indicates that this tubulin-binding agent does not cause appreciable mutagenicity in this same cell type. Environ. Mol. Mutagen. 59:30-37, 2018. © 2017 Wiley Periodicals, Inc.

Discovery of carbamate degrading enzymes by functional metagenomics

Bioremediation of pollutants is a major concern worldwide, leading to the research of new processes to break down and recycle xenobiotics and environment contaminating polymers. Among them, carbamates have a very broad spectrum of uses, such as toxinogenic pesticides or elastomers. In this study, we mined the bovine rumen microbiome for carbamate degrading enzymes. We isolated 26 hit clones exhibiting esterase activity, and were able to degrade at least one of the targeted polyurethane and pesticide carbamate compounds. The most active clone was deeply characterized. In addition to Impranil, this clone was active on Tween 20, pNP-acetate, butyrate and palmitate, and on the insecticide fenobucarb. Sequencing and sub-cloning of the best target revealed a novel carboxyl-ester hydrolase belonging to the lipolytic family IV, named CE_Ubrb. This study highlights the potential of highly diverse microbiota such as the ruminal one for the discovery of promiscuous enzymes, whose versatility could be exploited for industrial uses.

Detection of Pig-a gene mutants in rat peripheral blood following a single urethane treatment: A comparison of the RBC Pig-a and PIGRET assays

The rat red blood cell (RBC) Pig-a assay has been recommended by an expert working group of the International Workshop on Genotoxicity Testing as a potential new method to evaluate in vivo gene mutations in regulatory genotoxicity risk assessments. In a collaborative study in Japan, an improved Pig-a assay using reticulocytes (PIGRET assay) with magnetic enrichment of CD71-positive cells was evaluated, and it was revealed that this assay could detect the mutagenicity of chemicals earlier than the RBC Pig-a assay could. To verify further the suitability of the PIGRET assay for an in vivo short-term genotoxicity screening test, a joint research study was conducted by the Japanese Environmental Mutagen Society, and 24 compounds were evaluated. One of the compounds evaluated in this study was urethane, a multi-organ rodent carcinogen. Urethane (250, 500, and 1000mg/kg body weight) was orally administered once to 8-week-old male Crl:CD (SD) rats. Blood samples were collected at 1, 2, and 4 weeks after the administration and processed for the RBC Pig-a and PIGRET assays. In the PIGRET assay, the Pig-a mutant frequency (MF) significantly increased at both 2 and 4 weeks after the treatment of 1000mg/kg of urethane. However, in the RBC Pig-a assay, a significant increase in the Pig-a MF was observed only at 1 week after the treatment with 500mg/kg, but the MF value was within our historical control range; therefore, it was judged to be negative. These results suggest that the PIGRET assay might be useful for evaluating the in vivo mutagenicity more clearly than the RBC Pig-a assay after a single treatment of test compounds.

From immunotoxicity to carcinogenicity: the effects of carbamate pesticides on the immune system

The immune system can be the target of many chemicals, with potentially severe adverse effects on the host's health. In the literature, carbamate (CM) pesticides have been implicated in the increasing prevalence of diseases associated with alterations of the immune response, such as hypersensitivity reactions, some autoimmune diseases and cancers. CMs may initiate, facilitate, or exacerbate pathological immune processes, resulting in immunotoxicity by induction of mutations in genes coding for immunoregulatory factors and modifying immune tolerance. In the present study, direct immunotoxicity, endocrine disruption and inhibition of esterases activities have been introduced as the main mechanisms of CMs-induced immune dysregulation. Moreover, the evidence on the relationship between CM pesticide exposure, dysregulation of the immune system and predisposition to different types of cancers, allergies, autoimmune and infectious diseases is criticized. In addition, in this review, we will discuss the relationship between immunotoxicity and cancer, and the advances made toward understanding the basis of cancer immune evasion.

CD48-deficient T-lymphocytes from DMBA-treated rats have de novo mutations in the endogenous Pig-a gene

A major question concerning the scientific and regulatory acceptance of the rodent red blood cell-based Pig-a gene mutation assay is the extent to which mutants identified by their phenotype in the assay are caused by mutations in the Pig-a gene. In this study, we identified T-lymphocytes deficient for the glycosylphosphatidylinositol-anchored surface marker, CD48, in control and 7,12-dimethylbenz[a]anthracene (DMBA)-treated rats using a flow cytometric assay and determined the spectra of mutations in the endogenous Pig-a gene in these cells. CD48-deficient T-cells were seeded by sorting at one cell per well into 96-well plates, expanded into clones, and exons of their genomic Pig-a were sequenced. The majority (78%) of CD48-deficient T-cell clones from DMBA-treated rats had mutations in the Pig-a gene. The spectrum of DMBA-induced Pig-a mutations was dominated by mutations at A:T, with the mutated A being on the nontranscribed strand and A → T transversion being the most frequent change. The spectrum of Pig-a mutations in DMBA-treated rats was different from the spectrum of Pig-a mutations in N-ethyl-N-nitrosourea (ENU)-treated rats, but similar to the spectrum of DMBA mutations for another endogenous X-linked gene, Hprt. Only 15% of CD48-deficient mutants from control animals contained Pig-a mutations; T-cell biology may be responsible for a relatively large fraction of false Pig-a mutant lymphocytes in control animals. Among the verified mutants from control rats, the most common were frameshifts and deletions. The differences in the spectra of spontaneous, DMBA-, and ENU-induced Pig-a mutations suggest that the flow cytometric Pig-a assay detects de novo mutation in the endogenous Pig-a gene.

Integration of Pig-a, micronucleus, chromosome aberration and comet assay endpoints in a 28-day rodent toxicity study with urethane

As part of the international Pig-a validation trials, we examined the induction of Pig-a mutant reticulocytes and red blood cells (RET(CD59-) and RBC(CD59-), respectively) in peripheral blood of male Sprague Dawley(®) rats treated with urethane (25, 100 and 250mg/kg/day) or saline by oral gavage for 29 days. Additional endpoints integrated into this study were: micronucleated reticulocytes (MN-RET) in peripheral blood; chromosome aberrations (CAb) and DNA damage (%tail intensity via the comet assay) in peripheral blood lymphocytes (PBL); micronucleated polychromatic erythrocytes (MN-PCE) in bone marrow; and DNA damage (comet) in various organs at termination (the 29th dose was added for the comet endpoint at sacrifice). Ethyl methanesulfonate (EMS; 200mg/kg/day on Days 3, 4, 13, 14, 15, 27, 28 and 29) was evaluated as the concurrent positive control (PC). All animals survived to termination and none exhibited overt toxicity, but there were significant differences in body weight and body weight gain in the 250-mg/kg/day urethane group, as compared with the saline control animals. Statistically significant, dose-dependent increases were observed for urethane for: RET(CD59-) and RBC(CD59-) (on Days 15 and 29); MN-RET (on Days 4, 15 and 29); and MN-PCE (on Day 29). The comet assay yielded positive results in PBL (Day 15) and liver (Day 29), but negative results for PBL (Days 4 and 29) and brain, kidney and lung (Day 29). No significant increases in PBL CAb were observed at any sample time. Except for PBL CAb (likely due to excessive cytotoxicity), EMS-induced significant increases in all endpoints/tissues. These results compare favorably with earlier in vivo observations and demonstrate the utility and sensitivity of the Pig-a in vivo gene mutation assay, and its ability to be easily integrated, along with other standard genotoxicity endpoints, into 28-day rodent toxicity studies.

Induction of Pig-a mutant erythrocytes in male and female rats exposed to 1,3-propane sultone, ethyl carbamate, or thiotepa

Validation of the Pig-a gene mutation assay has been based mainly on studies in male rodents. To determine if the mutagen-induced responses of the X-linked Pig-a gene differ in females compared to males, groups of five male and female Sprague Dawley rats were exposed to the mutagens 1,3-propane sultone (80mg/kg/day), ethyl carbamate (600mg/kg/day), or thiotepa (7.5mg/kg/day) for three consecutive days (study days 1-3). Pig-a mutant phenotype reticulocyte (RET(CD59-)) and mutant phenotype erythrocyte (RBC(CD59-)) frequencies were determined on study days -4, 15, 30 and 46 using immunomagnetic separation in conjunction with flow cytometric analysis (In Vivo MutaFlow(®)). While the percentage of reticulocytes (%RET) was markedly higher for pre-treatment blood samples from males compared to females (6.6% vs. 3.5%), this sex effect was slight or nonexistent at later time points. Treatment-related effects to %RET were generally modest owing to the 12-day interval between last exposure and blood sampling. Mean RET(CD59-) and RBC(CD59-) frequencies were consistently low in vehicle control animals of both sexes, with 77% of samples exhibiting mutant cell frequencies ≤1×10(-6) over study days 15-46. Treatment with each mutagen caused significant increases to mean RET(CD59-) and RBC(CD59-) frequencies. Whereas genotoxicant-induced RET(CD59-) values were maximal on day 15, induced RBC(CD59-) frequencies were highest at the last sampling time. Sex did not affect 1,3-propane sultone- or thiotepa-induced mutant cell frequencies. While ethyl carbamate-exposed females exhibited higher mean mutant cell frequencies compared to like-treated males, statistical significance was achieved only for RBC(CD59-) at one time point (7.6±1.0×10(-6) compared to 4.7±0.6×10(-6) on day 30). Thus, while some quantitative differences were evident, there were no qualitative differences in how males and females responded to three diverse mutagens. These data support the use of both sexes for Pig-a gene mutation studies.