Colon-specific genotoxicity of heterocyclic amines detected by the modified alkaline single cell gel electrophoresis assay of multiple mouse organs

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.

Genotoxicity of three food processing contaminants in transgenic mice expressing human sulfotransferases 1A1 and 1A2 as assessed by the in vivo alkaline single cell gel electrophoresis assay

The food processing contaminants 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP), 5-hydroxymethylfurfural (HMF) and 2,5 dimethylfuran (DMF) are potentially both mutagenic and carcinogenic in vitro and/or in vivo, although data on DMF is lacking. The PHIP metabolite N-hydroxy-PhIP and HMF are bioactivated by sulfotransferases (SULTs). The substrate specificity and tissue distribution of SULTs differs between species. A single oral dose of PhIP, HMF or DMF was administered to wild-type (wt) mice and mice expressing human SULT1A1/1A2 (hSULT mice). DNA damage was studied using the in vivo alkaline single cell gel electrophoresis (SCGE) assay. No effects were detected in wt mice. In the hSULT mice, PhIP and HMF exposure increased the levels of DNA damage in the liver and kidney, respectively. DMF was not found to be genotoxic. The observation of increased DNA damage in hSULT mice compared with wt mice supports the role of human SULTs in the bioactivation of N-hydroxy-PhIP and HMF in vivo.

Investigation of the genotoxic effects of 2-amino-9H-pyrido[2,3-b]indole in different organs of rodents and in human derived cells

Aim of the present study was the investigation of the genotoxicity of amino-alpha-carboline (AalphaC) in human derived cells and of its organ-specific effects in laboratory rodents. This heterocyclic amine (HA) is contained in fried meat and fish in higher concentrations than most other cooked food mutagens. In the present experiments, AalphaC caused dose-dependent induction of micronuclei in the human derived hepatoma cell line HepG2 at concentrations > or =50 microM. In contrast, no significant effects were seen in Hep3B, another human hepatoma cell line, which may be explained by the concurrent lower activity of sulfotransferase (SULT), an enzyme playing a key role in the activation of AalphaC. A positive result was also obtained in the single cell gel electrophoresis (SCGE) assay in peripheral human lymphocytes, but the effect was only significant at the highest concentration (1000 microM). In Fischer F344 rats and ICR mice, the liver was the main target organ for the formation of DNA adducts (at > or =50 mg/kg bw), and in lungs and colon substantially lower levels were detected. Identical organ specificity as in the DNA adduct measurements was seen in SCGE assays with rats, whereas in mice the most pronounced induction of DNA migration was observed in the colon. Comparison of our results with data from earlier experiments indicate that the genotoxic potency of AalphaC is equal to that of other HAs, which are contained in human foods in much smaller amounts. Therefore, our findings can be taken as an indication that the human health risk caused by exposure to AalphaC is higher than that of other HAs that are formed during the cooking of meat and fish.

Genotoxicity of fecal water in a free-living Irish population

The alkaline single-cell gel electrophoresis (comet) assay was used to investigate the genotoxicity of fecal water (FW) isolated from 47 Irish subjects using Caco-2 colonocytes as target cells. Two methods of comet assay analysis were compared to determine the extent of DNA damage and to categorize the samples as having no, low-to-moderate, or high genotoxicity. FW was isolated from stool samples by centrifugation and tested for its ability to induce DNA damage in Caco-2 cells. DNA damage was assessed using the comet assay by measuring the extent of DNA migration from the nucleus (microns, tail length) or by classifying the nuclei into five different categories depending on their morphology. Data collected from the two methods were used to categorize the FW samples on the basis of their genotoxic activity. Both methods showed good agreement. There was an approximately 50:50 split, with half the samples having some level of genotoxic activity and half having no genotoxicity. About one-third of the samples were considered to be highly genotoxic. There was a trend for low pH of the FW to be associated with increased DNA damage, but this was not significant. The results presented in this report show a relatively high incidence of genotoxic FW in samples derived from a free-living Irish population. Our data demonstrate the suitability of classifying nuclei on the basis of their morphology as a means of determining DNA damage. This procedure is very rapid and, therefore, advantageous in analyzing a large number of slides in the absence of an image analysis system.

The comet assay with multiple mouse organs: comparison of comet assay results and carcinogenicity with 208 chemicals selected from the IARC monographs and U.S. NTP Carcinogenicity Database

The comet assay is a microgel electrophoresis technique for detecting DNA damage at the level of the single cell. When this technique is applied to detect genotoxicity in experimental animals, the most important advantage is that DNA lesions can be measured in any organ, regardless of the extent of mitotic activity. The purpose of this article is to summarize the in vivo genotoxicity in eight organs of the mouse of 208 chemicals selected from International Agency for Research on Cancer (IARC) Groups 1, 2A, 2B, 3, and 4, and from the U.S. National Toxicology Program (NTP) Carcinogenicity Database, and to discuss the utility of the comet assay in genetic toxicology. Alkylating agents, amides, aromatic amines, azo compounds, cyclic nitro compounds, hydrazines, halides having reactive halogens, and polycyclic aromatic hydrocarbons were chemicals showing high positive effects in this assay. The responses detected reflected the ability of this assay to detect the fragmentation of DNA molecules produced by DNA single strand breaks induced chemically and those derived from alkali-labile sites developed from alkylated bases and bulky base adducts. The mouse or rat organs exhibiting increased levels of DNA damage were not necessarily the target organs for carcinogenicity. It was rare, in contrast, for the target organs not to show DNA damage. Therefore, organ-specific genotoxicity was necessary but not sufficient for the prediction of organ-specific carcinogenicity. It would be expected that DNA crosslinkers would be difficult to detect by this assay, because of the resulting inhibition of DNA unwinding. The proportion of 10 DNA crosslinkers that was positive, however, was high in the gastrointestinal mucosa, stomach, and colon, but less than 50% in the liver and lung. It was interesting that the genotoxicity of DNA crosslinkers could be detected in the gastrointestinal organs even though the agents were administered intraperitoneally. Chemical carcinogens can be classified as genotoxic (Ames test-positive) and putative nongenotoxic (Ames test-negative) carcinogens. The Ames test is generally used as a first screening method to assess chemical genotoxicity and has provided extensive information on DNA reactivity. Out of 208 chemicals studied, 117 are Ames test-positive rodent carcinogens, 43 are Ames test-negative rodent carcinogens, and 30 are rodent noncarcinogens (which include both Ames test-positive and negative noncarcinogens). High positive response ratio (110/117) for rodent genotoxic carcinogens and a high negative response ratio (6/30) for rodent noncarcinogens were shown in the comet assay. For Ames test-negative rodent carcinogens, less than 50% were positive in the comet assay, suggesting that the assay, which detects DNA lesions, is not suitable for identifying nongenotoxic carcinogens. In the safety evaluation of chemicals, it is important to demonstrate that Ames test-positive agents are not genotoxic in vivo. This assay had a high positive response ratio for rodent genotoxic carcinogens and a high negative response ratio for rodent genotoxic noncarcinogens, suggesting that the comet assay can be used to evaluate the in vivo genotoxicity of in vitro genotoxic chemicals. For chemicals whose in vivo genotoxicity has been tested in multiple organs by the comet assay, published data are summarized with unpublished data and compared with relevant genotoxicity and carcinogenicity data. Because it is clear that no single test is capable of detecting all relevant genotoxic agents, the usual approach should be to carry out a battery of in vitro and in vivo tests for genotoxicity. The conventional micronucleus test in the hematopoietic system is a simple method to assess in vivo clastogenicity of chemicals. Its performance is related to whether a chemical reaches the hematopoietic system. Among 208 chemicals studied (including 165 rodent carcinogens), 54 rodents carcinogens do not induce micronuclei in mouse hematopoietic system despite the positive finding with one or two in vitro tests. Forty-nine of 54 rodent carcinogens that do not induce micronuclei were positive in the comet assay, suggesting that the comet assay can be used as a further in vivo test apart from the cytogenetic assays in hematopoietic cells. In this review, we provide one recommendation for the in vivo comet assay protocol based on our own data.

Toxicity of Trp-P-2 to cultured human and rat keratinocytes

Keratinocytes cultured from human and rat epidermis exhibited strongly divergent sensitivities to toxicity from the heterocyclic amine food mutagen Trp-P-2. To find a biochemical basis for this difference, the cultured cells were compared in their expression of phase 1 and 2 biotransformation activities, mutagenic activation and macromolecular adducts. The human and early passage rat cells expressed similar levels of ethoxyresorufin O-deethylase and N-acetyl transferase activities, their microsomes were similarly active in inducing bacterial mutagenesis when incubated with Trp-P-2, and the keratinocytes accumulated similar levels of DNA adducts over a 4-day treatment period. However, the human cells expressed an order of magnitude higher cytosolic glutathione S-transferase activity than the rat cells, likely providing enhanced protection. Late passage rat epidermal cells were insensitive to Trp-P-2 toxicity, attributable to their rapid loss of measured cytochrome P450 activity. Rat esophageal and fore-stomach epithelial cells resembled late passage rat epidermal cells in their lack of sensitivity to Trp-P-2 toxicity and lack of P450 activity. Human esophageal epithelial cells expressed substantial P450 activity but, in contrast to human epidermal cells, were sensitive to Trp-P-2 toxicity. Thus keratinocytes provide a valuable system in which to examine the basis for species- and tissue-specific differences in toxicity from this carcinogenic heterocyclic amine.

Evaluation of fecal mutagenicity and colorectal cancer risk

Colorectal cancer is one of the most common internal malignancies in Western society. The cause of this disease appears to be multifactorial and involves genetic as well as environmental aspects. The human colon is continuously exposed to a complex mixture of compounds, which is either of direct dietary origin or the result of digestive, microbial and excretory processes. In order to establish the mutagenic burden of the colorectal mucosa, analysis of specific compounds in feces is usually preferred. Alternatively, the mutagenic potency of fecal extracts has been determined, but the interpretation of these more integrative measurements is hampered by methodological shortcomings. In this review, we focus on exposure of the large bowel to five different classes of fecal mutagens that have previously been related to colorectal cancer risk. These include heterocyclic aromatic amines (HCA) and polycyclic aromatic hydrocarbons (PAH), two exogenous factors that are predominantly ingested as pyrolysis products present in food and (partially) excreted in the feces. Additionally, we discuss N-nitroso-compounds, fecapentaenes and bile acids, all fecal constituents (mainly) of endogenous origin. The mutagenic and carcinogenic potency of the above mentioned compounds as well as their presence in feces, proposed mode of action and potential role in the initiation and promotion of human colorectal cancer are discussed. The combined results from in vitro and in vivo research unequivocally demonstrate that these classes of compounds comprise potent mutagens that induce many different forms of genetic damage and that particularly bile acids and fecapentaenes may also affect the carcinogenic process by epigenetic mechanisms. Large inter-individual differences in levels of exposures have been reported, including those in a range where considerable genetic damage can be expected based on evidence from animal studies. Particularly, however, exposure profiles of PAH and N-nitroso compounds (NOC) have to be more accurately established to come to a risk evaluation. Moreover, lack of human studies and inconsistency between epidemiological data make it impossible to describe colorectal cancer risk as a result of specific exposures in quantitative terms, or even to indicate the relative importance of the mutagens discussed. Particularly, the polymorphisms of genes involved in the metabolism of heterocyclic amines are important determinants of carcinogenic risk. However, the present knowledge of gene-environment interactions with regard to colorectal cancer risk is rather limited. We expect that the introduction of DNA chip technology in colorectal cancer epidemiology will offer new opportunities to identify combinations of exposures and genetic polymorphisms that relate to increased cancer risk. This knowledge will enable us to improve epidemiological study design and statistical power in future research.

The alkaline single cell electrophoresis assay with eight mouse organs: results with 22 mono-functional alkylating agents (including 9 dialkyl N-nitrosoamines) and 10 DNA crosslinkers

The genotoxicity of 22 mono-functional alkylating agents (including 9 dialkyl N-nitrosoamines) and 10 DNA crosslinkers selected from IARC (International Agency for Research on Cancer) groups 1, 2A, and 2B was evaluated in eight mouse organs with the alkaline single cell gel electrophoresis (SCGE) (comet) assay. Groups of four mice were treated once intraperitoneally at the dose at which micronucleus tests had been conducted, and the stomach, colon, liver, kidney, bladder, lung, brain, and bone marrow were sampled 3, 8, and/or 24 h later. All chemicals were positive in the SCGE assay in at least one organ. Of the 22 mono-functional alkylating agents, over 50% were positive in all organs except the brain and bone marrow. The two subsets of mono-functional alkylating agents differed in their bone marrow genotoxicity: only 1 of the 9 dialkyl N-nitrosoamines was positive in bone marrow as opposed to 8 of the 13 other alkylating agents, reflecting the fact that dialkyl N-nitrosoamines are poor micronucleus inducers in hematopoietic cells. The two groups of mono-functional alkylating agents also differ in hepatic carcinogenicity in spite of the fact that they are similar in hepatic genotoxicity. While dialkyl N-nitrosoamines produce tumors primarily in mouse liver, only one (styrene-7,8-oxide) out of 10 of the other type of mono-functional alkylating agents is a mouse hepatic carcinogen. Taking into consideration our previous results showing high concordance between hepatic genotoxicity and carcinogenicity for aromatic amines and azo compounds, a possible explanation for the discrepancy might be that chemicals that require metabolic activation show high concordance between genotoxicity and carcinogenicity in the liver. A high percent of the 10 DNA crosslinkers were positive in the SCGE assay in the gastrointestinal mucosa, but less than 50% were positive in the liver and lung. In this study, we allowed 10 min alkali-unwinding to obtain low and stable control values. Considering that DNA crosslinking lesions can be detected as lowering of not only positive but also negative control values, low control values by short alkali-treatment might make it difficult to detect DNA crosslinking lesions. In conclusion, although both mono-functional alkylating agents and DNA crosslinkers are genotoxic in mouse multiple organs, the genotoxicity of DNA crosslinkers can be detected in the gastrointestinal organs even though they were given intraperitoneally followed by the short alkali-treatment.

Search for compounds that inhibit the genotoxic and carcinogenic effects of heterocyclic aromatic amines

Over the last 30 years approximately 160 reports have been published on dietary compounds that protect from the mutagenic and carcinogenic effects of heterocyclic aromatic amines (HAAs). In the first section of this review, the current state of knowledge is briefly summarized. Based on the evaluation of the available data, various protective mechanisms are described, and the use of different methodologies for the detection of protective effects is critically discussed. In most antimutagenicity studies (>70%) bacterial indicators (predominantly Salmonella strain TA98) were used, and about 600 individual compounds and complex mixtures have been identified that attenuate the effects of HAAs. The most frequently used in vivo method to detect protective effects are adduct measurements; anticarcinogenic dietary factors were identified by aberrant crypt foci assays and liver foci tests with rats. The mechanisms of protection include inactivation of HAAs and their metabolites by direct binding, inhibition of enzymes involved in the metabolic activation of the amines, induction of detoxifying enzymes, and interaction with DNA repair processes. The detection spectrum of conventional in vitro mutagenicity assays with metabolically incompetent indicator cells is limited. These procedures reflect only simple mechanisms such as direct binding of the HAAs to pyrroles and fibers. It has been shown that these compounds are also effective in rodents. More complex mechanisms, namely, interactions with metabolic activation reactions are not adequately represented in in vitro assays with exogenous enzyme homogenates, and false-negative as well as false-positive results may be obtained. More appropriate approaches for the detection of protective effects are recently developed test systems with metabolically competent cells such as the human Hep G2 line or primary hepatocytes. SCGE tests and DNA adduct measurements with laboratory rodents enable the detection of antigenotoxic effects in different organs, including those that are targets for tumor induction by the amines. Medium term assays based on aberrant crypt foci in colon and liver foci tests have been used to prove that certain compounds that prevented DNA damage by HAAs also reduced their carcinogenic effects. These experiments are costly and time consuming and, due to the weak induction capacity of the amines, only pronounced anticarcinogenic effects can be detected. Over the years, a large bulk of data on HAA protective compounds has accumulated, but only for a few (e.g., fibers, pyrroles, constituents of teas, and lactic acid bacteria) is there sufficient evidence to support the assumption that they are protective in humans as well.

Detection of nivalenol genotoxicity in cultured cells and multiple mouse organs by the alkaline single-cell gel electrophoresis assay

We tested the genotoxicity of nivalenol (NIV), a potent toxic trichothecene from Fusarium nivale, in cultured CHO cells and in several mouse organs and tissues (liver, kidney, thymus, bone marrow and mucosa of stomach, jejunum, and colon) using the alkaline single-cell gel electrophoresis (SCG, or Comet) assay. NIV at 50 and 100 micrograms/ml damaged the nuclear DNA of CHO cells in the absence of S9 mix, showing that NIV was a direct mutagen. In an in vivo study, mice were sacrificed 2, 4, and 8 h after either oral (20 mg/kg) or intraperitoneal (3.7 mg/kg) administration of NIV. DNA damage was measured by the SCG assay as modified by us. After oral dosing, DNA damage appeared in the kidney and bone marrow at 2 h (returning to almost control level within the following 2 h), and in the stomach, jejunum, and colon at 2, 4, and 8 h, respectively. Liver and thymus DNA were not damaged. After intraperitoneal injection, no DNA damage appeared in any of the organs or tissues tested except for the colon, where extensive DNA damage was observed, as in the oral study, at 8 h. For histopathological examination, mice were sacrificed 2, 4, and 8 h after oral (20 mg/kg) administration of NIV. No necrotic changes were detected in any of the organs where NIV yielded statistically significant DNA damage. To measure the effect of NIV on transport activity in mice, 10 ml/kg (same volume as NIV treatments) of 1% brilliant blue FCF (BB) was administered orally. Thirty minutes later, the BB reached the colon, and simultaneous oral administration of NIV (20 mg/kg, dissolved in 10 ml BB solution) did not affect the dye transport rate. Thus, the strong yet delayed damage to colon DNA may follow from a systemic absorption rather than a topical effect. As a direct mutagen, NIV showed organ specific genotoxicity in mice in time and intensity.