In vivomutagenicity of vinyl carbamate and ethyl carbamate in lung and small intestine of F1(Big Blue® × A/J) transgenic mice

Impact of sampling time on the detection of mutations in rapidly proliferating tissues using transgenic rodent gene mutation models: A review

The OECD Test Guideline 488 (TG 488) for the Transgenic Rodent Gene Mutation Assay has undergone several revisions to update the recommended design for studying mutations in somatic tissues and male germ cells. The recently revised TG recommends a single sampling time of 28 days following 28 days of exposure (i.e., 28 + 28 days) for all tissues, irrespective of proliferation rates. An alternative design (i.e., 28 + 3 days) is appropriate when germ cell data is not required, nor considered. While the 28 + 28 days design is clearly preferable for slowly proliferating somatic tissues and germ cells, there is still uncertainty about the impact of extending the sampling time to 28 days for rapidly somatic tissues. Here, we searched the available literature for evidence supporting the applicability and utility of the 28 + 28 days design for rapidly proliferating tissues. A total of 79 tests were identified. When directly comparing results from both designs in the same study, there was no evidence that the 28 + 28 days regimen resulted in a qualitatively different outcome from the 28 + 3 days design. Studies with a diverse range of agents that employed only a 28 + 28 days protocol provide further evidence that this design is appropriate for rapidly proliferating tissues. Benchmark dose analyses demonstrate high quantitative concordance between the 28 + 3 and 28 + 28 days designs for rapidly proliferating tissues. Accordingly, our review confirms that the 28 + 28 days design is appropriate to assess mutagenicity in both slowly and rapidly proliferating somatic tissues, and germ cells, and provides further support for the recommended design in the recently adopted TG 488.

Lysosomal Reacidification Ameliorates Vinyl Carbamate-Induced Toxicity and Disruption on Lysosomal pH

Ethyl carbamate (EC) is a carcinogen toxicant, commonly found in fermented foods and beverages. The carcinogenic and toxic possibility of EC is thought to be related to its metabolite vinyl carbamate (VC). However, we found interesting mechanisms underlying VC-induced toxicity in this study, which were greatly different from EC. We first conducted a simple synthesis procedure for VC and found that VC possessed higher toxicity but failed to regulate levels of reactive oxygen species, glutathione, and autophagy. Notably, VC treatment resulted in upregulation of lysosomal pH, which was responsible for its cytotoxicity. Cyclic adenosine monophosphate (cAMP) pretreatment could enhance restoration of lysosomal acidity and ameliorate VC-induced damage. Inhibition of protein kinase A and cystic fibrosis transmembrane conductance regulator can block cAMP-induced cytoprotection. Together, our results provided the evidence for novel mechanisms of toxicity and possible protection method under VC exposure, which might give new perspectives on the study of EC-induced toxicity.

Capturing the primordial Kras mutation initiating urethane carcinogenesis

The environmental carcinogen urethane exhibits a profound specificity for pulmonary tumors driven by an oncogenic Q₆₁L/R mutation in the gene Kras. Similarly, the frequency, isoform, position, and substitution of oncogenic RAS mutations are often unique to human cancers. To elucidate the principles underlying this RAS mutation tropism of urethane, we adapted an error-corrected, high-throughput sequencing approach to detect mutations in murine Ras genes at great sensitivity. This analysis not only captured the initiating Kras mutation days after urethane exposure, but revealed that the sequence specificity of urethane mutagenesis, coupled with transcription and isoform locus, to be major influences on the extreme tropism of this carcinogen.

Why the carcinogen urethane causes only lung tumours driven by a specific oncogenic mutation in just one Ras gene in mice is unclear. Here, the authors capture mutations immediately after urethane exposure and show that the sequence specificity of mutagenesis, transcriptional status, and Ras genetic loci may all contribute to this specificity.

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%.

Correlation of In Vivo Versus In Vitro Benchmark Doses (BMDs) Derived From Micronucleus Test Data: A Proof of Concept Study

In this study, we explored the applicability of using in vitro micronucleus (MN) data from human lymphoblastoid TK6 cells to derive in vivo genotoxicity potency information. Nineteen chemicals covering a broad spectrum of genotoxic modes of action were tested in an in vitro MN test using TK6 cells using the same study protocol. Several of these chemicals were considered to need metabolic activation, and these were administered in the presence of S9. The Benchmark dose (BMD) approach was applied using the dose-response modeling program PROAST to estimate the genotoxic potency from the in vitro data. The resulting in vitro BMDs were compared with previously derived BMDs from in vivo MN and carcinogenicity studies. A proportional correlation was observed between the BMDs from the in vitro MN and the BMDs from the in vivo MN assays. Further, a clear correlation was found between the BMDs from in vitro MN and the associated BMDs for malignant tumors. Although these results are based on only 19 compounds, they show that genotoxicity potencies estimated from in vitro tests may result in useful information regarding in vivo genotoxic potency, as well as expected cancer potency. Extension of the number of compounds and further investigation of metabolic activation (S9) and of other toxicokinetic factors would be needed to validate our initial conclusions. However, this initial work suggests that this approach could be used for in vitro to in vivo extrapolations which would support the reduction of animals used in research (3Rs: replacement, reduction, and refinement).

Acrylamide-induced carcinogenicity in mouse lung involves mutagenicity: cII gene mutations in the lung of big blue mice exposed to acrylamide and glycidamide for up to 4 weeks

Potential health risks for humans from exposure to acrylamide (AA) and its epoxide metabolite glycidamide (GA) have garnered much attention lately because substantial amounts of AA are present in a variety of fried and baked starchy foods. AA is tumorigenic in rodents, and a large number of in vitro and in vivo studies indicate that AA is genotoxic. A recent cancer bioassay on AA demonstrated that the lung was one of the target organs for tumor induction in mice; however, the mutagenicity of AA in this tissue is unclear. Therefore, to investigate whether or not gene mutation is involved in the etiology of AA- or GA-induced mouse lung carcinogenicity, we screened for cII mutant frequency (MF) in lungs from male and female Big Blue (BB) mice administered 0, 1.4, and 7.0 mM AA or GA in drinking water for up to 4 weeks (19-111 mg/kg bw/days). Both doses of AA and GA produced significant increases in cII MFs, with the high doses producing responses 2.7-5.6-fold higher than the corresponding controls (P ≤ 0.05; control MFs = 17.2 ± 2.2 and 15.8 ± 3.5 × 10(-6) in males and females, respectively). Molecular analysis of the mutants from high doses indicated that AA and GA produced similar mutation spectra and that these spectra were significantly different from the spectra in control mice (P ≤ 0.01). The predominant types of mutations in the lung cII gene from AA- and GA-treated mice were A:T → T:A, and G:C → C:G transversions, and -1/+1 frameshifts at a homopolymeric run of Gs. The MFs and types of mutations induced by AA and GA in the lung are consistent with AA exerting its genotoxicity via metabolism to GA. These results suggest that AA is a mutagenic carcinogen in mouse lungs and therefore further studies on its potential health risk to humans are warranted. Environ. Mol. Mutagen. 56:446-456, 2015. © 2015 Wiley Periodicals, Inc.

Nitrogen regulation involved in the accumulation of urea in Saccharomyces cerevisiae

Rice wine is a popular traditional alcoholic drink with a long history in China. However, the presence of the potential carcinogen ethyl carbamate (EC) raises a series of food safety concerns. Although the metabolic pathway of urea (the major precusor of EC) has been characterized in Saccharomyces cerevisiae, the regulation of urea accumulation remains unclear, making the efficient elimination of urea difficult. To demonstrate the regulatory mechanisms governing urea accumulation, three key nitrogen sources that can inhibit urea utilization for a commercial S. cerevisiae strain were identified. In addition, regulators of nitrogen catabolite repression (NCR) and target of rapamycin (TOR) pathways were identified as being involved in urea accumulation by real-time quantitative PCR. Based on these results, preferred nitrogen sources were found to repress urea utilization by converting them to glutamine or glutamate. Moreover, the results indicated that the manner of urea metabolism regulation was different for two positive regulators involved in NCR; Gln3p can be retained in the cytoplasm by glutamine, while Gat1p can be retained by glutamine and glutamate. Furthermore, this was confirmed by fluorescence location detection. These new findings provide new targets for eliminating EC and other harmful nitrogen-containing compounds in fermented foods.

Mechanisms of lung tumorigenesis by ethyl carbamate and vinyl carbamate

Vinyl carbamate (VC) and ethyl carbamate (EC) induce the formation of lung tumors. The mechanism involves a two-step oxidation of EC to VC and VC to an epoxide, both of which are mediated mainly by CYP2E1. Interaction of the epoxide with DNA leads to the formation of DNA adducts, including 1,N(6)ethenodeoxyadenosine and 1,N(4)-ethenodeoxycytidine. The production of DNA adducts correlated with capacities for the bioactivation of VC, which are higher in the lungs of A/J than in C57BL/6 mice. Importantly, CYP2E1 is higher in the lungs of A/J than in C57BL/6 mice. Studies using F(1) (Big Blue x A/J) transgenic mice revealed the formation of mutations in the lambda cII gene after treatment with VC. Mutations induced by VC were mainly A:T-->G:C transitions and A:T-->T:A transversions, while mutations induced by EC were mainly G:C-->A:T transitions. An EC dose that was 17-fold higher than that for VC was required to produce a similar level of mutant frequency in the lung. Pretreatment of mice with the CYP2E1 inhibitor, diallyl sulfone, significantly inhibited the mutant frequencies induced by VC. Mutations in the endogeneous Kras2 gene were found in codon 61 of exon 2 and were identified as A:T transversions and A-->G transitions in the second base and A-->T transversions in the third base. These mutations were reduced by treatment of mice with diallyl sulfone before VC and coincided with a reduction in the number of lung tumors with Kras2 mutations. These findings affirmed that the metabolism of EC and VC is a prerequisite for, or at least substantially contributes to, initiation of the cascade of events leading to lung tumor formation.

Inhibition of vinyl carbamate-induced lung tumors and Kras2 mutations by the garlic derivative diallyl sulfone

Vinyl carbamate (VC) is derived from ethyl carbamate (EC), a chemical found in alcoholic beverages and fermented foods. The objectives of this study were to characterize the formation of lung tumors induced by VC in F1 (Big BluexA/J) mice, and to identify the mutations formed in the Kras2 gene. In addition, we have tested the hypothesis that pretreatment with diallyl sulfone (DASO2) inhibits the adverse effects of VC. Mice were treated with VC (60 mg/kg, i.p.) or DASO2 (50 mg/kg, p.o.) 2 h prior to VC (DASO2/VC). Lung tumor multiplicity was significantly lower (21%) in mice treated with DASO2/VC than with VC. Lung tumors induced by VC are manifested as solid or papillary tumors, with the latter being regarded as a more malignant phenotype as they demonstrate no growth restrictions. Solid (42%) and papillary tumors (58%) were found in similar proportions in VC-treated mice. The number of papillary tumors was significantly decreased (44.5%) in mice treated with DASO2/VC, while there was a proportional increase (44.5%) in the number of solid tumors. The number of tumors with mutations in the first and second exon of Kras2 was significantly lower after treatment with DASO2/VC (7%) than after treatment with VC (61%). The mutations were mainly found in codon 61, and were identified as A-->T transversions (31%) and A-->G transitions (25%) in the second base, and A-->T transversions (12%) in the third base. All of these mutations were significantly reduced by DASO2 pretreatment. The number of tumors containing Kras2 mutations was highest (38%) in the large papillary tumors. Hence, mice treated with DASO2/VC had decreased frequencies of Kras2 mutations and reduced numbers of small and large papillary tumors, suggesting that activation of the Kras2 gene may be implicated in lung tumor formation and progression.

Inhibition of vinyl carbamate-induced mutagenicity and clastogenicity by the garlic constituent diallyl sulfone in F1 (Big Blue® × A/J) transgenic mice

Vinyl carbamate (VC) is a metabolite of ethyl carbamate (EC), a naturally occurring compound found in fermented foods and alcoholic beverages. CYP2E1 mediates the sequential oxidation of EC to VC and subsequently to the vinyl carbamate epoxide, which is believed to be the ultimate carcinogenic species. Here, we have tested the hypothesis that bioactivation of VC by CYP2E1 plays a central role in the development of its mutagenicity and clastogenicity, and further that inhibition of CYP2E1 by diallyl sulfone (DASO(2)) leads to diminution in their incidences. DASO(2) is a garlic constituent that is oxidized by CYP2E1, leading to inactivation of this P450. F(1) (Big Blue x A/J) transgenic mice harboring the lambda cII gene were used for in vivo identification and quantitation of mutations in the lung and small intestine. Mice were pre-treated with DASO(2) (12.5-200 mg/kg, p.o.), treated 2 h later with VC (60 mg/kg, i.p.) and were killed 4 weeks later. Our results showed that pre-treatment of mice with DASO(2) at doses of 50-200 mg/kg significantly decreased the VC-induced mutant frequencies (MFs) by 50-70%. In the small intestine, pre-treatment with 200 mg/kg of DASO(2) decreased the MF by approximately 40%. Clastogenicity, as assessed by the frequency of micronucleated reticulocytes, was significantly decreased (33-44%) by pre-treatment with DASO(2) (50-200 mg/kg). These results demonstrated that bioactivation of VC by CYP2E1 plays a valid role in the development of mutagenicity and clastogenicity, and further that inhibition of this pathway by DASO(2) produces a protective effect.