In vivo genotoxicity of 2-amino-3,8-dimethylimidazo[4, 5-f]quinoxaline in lacI transgenic (Big Blue) mice

Mutagenicity of carcinogenic heterocyclic amines in Salmonella typhimurium YG strains and transgenic rodents including gpt delta

Chemical carcinogens to humans have been usually identified by epidemiological studies on the relationships between occupational or environmental exposure to the agents and specific cancer induction. In contrast, carcinogenic heterocyclic amines were identified under the principle that mutagens in bacterial in the Ames test are possible human carcinogens. In the 1970s to 1990s, more than 10 heterocyclic amines were isolated from pyrolysates of amino acids, proteins, meat or fish as mutagens in the Ames test, and they were demonstrated as carcinogens in rodents. In the 1980s and 1990s, we have developed derivatives of the Ames tester strains that overexpressed acetyltransferase of Salmonella typhimurium. These strains such as Salmonella typhimurium YG1024 exhibited a high sensitivity to the mutagenicity of the carcinogenic heterocyclic amines. Because of the high sensitivity, YG1024 and other YG strains were used for various purposes, e.g., identification of novel heterocyclic amines, mechanisms of metabolic activation, comparison of mutagenic potencies of various heterocyclic amines, and the co-mutagenic effects. In the 1990s and 2000s, we developed transgenic mice and rats for the detection of mutagenicity of chemicals in vivo. The transgenics were generated by the introduction of reporter genes for mutations into fertilized eggs of mice and rats. We named the transgenics as gpt delta because the gpt gene of Escherichia coli was used for detection of point mutations such as base substitutions and frameshifts and the red/gam genes of λ phage were employed to detect deletion mutations. The transgenic rodents gpt delta and other transgenics with lacI or lacZ as reporter genes have been utilized for characterization of mutagenicity of heterocyclic amines in vivo. In this review, we summarized the in vitro mutagenicity of heterocyclic amines in Salmonella typhimurium YG strains and the in vivo mutagenicity in transgenic rodents. We discussed the relationships between in vitro and in vivo mutagenicity of the heterocyclic amines and their relations to the carcinogenicity.

Cytochrome P450-mediated metabolism and DNA binding of 2-amino-1,7-dimethylimidazo[4,5-g]quinoxaline and its carcinogenic isomer 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline in mice

2-Amino-1,7-dimethylimidazo[4,5-g]quinoxaline (MeIgQx) is a recently discovered heterocyclic aromatic amine (HAA) that is formed during the cooking of meats. MeIgQx is an isomer of 2-amino-3,8-dimethylmidazo[4,5-f]quinoxaline (MeIQx), a rodent carcinogen and possible human carcinogen that also occurs in cooked meats. MeIgQx is a bacterial mutagen, but knowledge about its metabolism and carcinogenic potential is lacking. Metabolism studies on MeIgQx and MeIQx were conducted with human and mouse liver microsomes, and recombinant human P450s. DNA binding studies were also investigated in mice to ascertain the genotoxic potential of MeIgQx in comparison to MeIQx. Both HAAs underwent comparable rates of N-oxidation to form genotoxic N-hydroxylated metabolites with mouse liver microsomes (0.2-0.3 nmol/min/mg protein). The rate of N-oxidation of MeIQx was 4-fold greater than the rate of N-oxidation of MeIgQx with human liver microsomes (1.7 vs 0.4 nmol/min/mg protein). The rate of N-oxidation, by recombinant human P450 1A2, was comparable for both substrates (6 pmol/min/pmol P450 1A2). MeIgQx also underwent N-oxidation by human P450s 1A1 and 1B1 at appreciable rates, whereas MeIQx was poorly metabolized by these P450s. The potential of MeIgQx and MeIQx to form DNA adducts was assessed in female C57BL/6 mice given [(14)C]-MeIgQx (10 μCi, 9.68 mg/kg body wt) or [(14)C]-MeIQx (10 μCi, 2.13 mg/kg body wt). DNA adduct formation in the liver, pancreas, and colorectum was measured by accelerator mass spectrometry at 4, 24, or 48 h post-treatment. Variable levels of adducts were detected in all organs. The adduct levels were similar for both HAAs, when adjusted for dose, and ranged from 1 to 600 adducts per 10(7) nucleotides per mg/kg dose. Thus, MeIgQx undergoes metabolic activation and binds to DNA at levels that are comparable to MeIQx. Given the high amounts of MeIgQx formed in cooked meats, further investigations are warranted to assess the carcinogenic potential of this HAA.

Metabolism of Maillard reaction products by the human gut microbiota--implications for health

The human colonic microbiota imparts metabolic versatility on the colon, interacts at many levels in healthy intestinal and systemic metabolism, and plays protective roles in chronic disease and acute infection. Colonic bacterial metabolism is largely dependant on dietary residues from the upper gut. Carbohydrates, resistant to digestion, drive colonic bacterial fermentation and the resulting end products are considered beneficial. Many colonic species ferment proteins but the end products are not always beneficial and include toxic compounds, such as amines and phenols. Most components of a typical Western diet are heat processed. The Maillard reaction, involving food protein and sugar, is a complex network of reactions occurring during thermal processing. The resultant modified protein resists digestion in the small intestine but is available for colonic bacterial fermentation. Little is known about the fate of the modified protein but some Maillard reaction products (MRP) are biologically active by, e. g. altering bacterial population levels within the colon or, upon absorption, interacting with human disease mechanisms by induction of inflammatory responses. This review presents current understanding of the interactions between MRP and intestinal bacteria. Recent scientific advances offering the possibility of elucidating the consequences of microbe-MRP interactions within the gut are discussed.

Detailed review of transgenic rodent mutation assays

Induced chromosomal and gene mutations play a role in carcinogenesis and may be involved in the production of birth defects and other disease conditions. While it is widely accepted that in vivo mutation assays are more relevant to the human condition than are in vitro assays, our ability to evaluate mutagenesis in vivo in a broad range of tissues has historically been quite limited. The development of transgenic rodent (TGR) mutation models has given us the ability to detect, quantify, and sequence mutations in a range of somatic and germ cells. This document provides a comprehensive review of the TGR mutation assay literature and assesses the potential use of these assays in a regulatory context. The information is arranged as follows. (1) TGR mutagenicity models and their use for the analysis of gene and chromosomal mutation are fully described. (2) The principles underlying current OECD tests for the assessment of genotoxicity in vitro and in vivo, and also nontransgenic assays available for assessment of gene mutation, are described. (3) All available information pertaining to the conduct of TGR assays and important parameters of assay performance have been tabulated and analyzed. (4) The performance of TGR assays, both in isolation and as part of a battery of in vitro and in vivo short-term genotoxicity tests, in predicting carcinogenicity is described. (5) Recommendations are made regarding the experimental parameters for TGR assays, and the use of TGR assays in a regulatory context.

Lack of elevated liver carcinogenicity of aminophenylnorharman in p53-deficient mice

The hepatocarcinogenic potential of 9-(4'-aminophenyl)-9H-pyrido[3,4-b]indole (aminophenylnorharman, APNH) was investigated using male and female p53 deficient mice. Incidence of oval cell hyperplasia was 2/14 (14.3%), 14/23 (60.9%), and 2/10 (20%) in p53 nullizygous (-/-), heterozygous (+/-), and wild type (+/+) mice, respectively, exposed to 30 ppm APNH for 15 weeks, while hepatocellular anisonucleosis was observed only in APNH-treated p53 (-/-) mice. At 40 weeks, hepatocellular carcinomas had developed in 16/46 (34.8%) and 10/27 (37.0%) of female p53 (+/-) and (+/+) mice in contrast to only 1/45 (2.2%) and 2/12 (16.7%) in their male counterparts, respectively, without any detectable p53 gene mutations. Dose-dependent APNH-DNA adduct formation and transcriptional induction of CYP 1A1, but not CYP 1A2, was revealed with 7-day APNH treatment using female C57BL/6J mice. These results suggested hepatocarcinogenicity of APNH in mice could be linked to the liver microenvironment including hormonal milieu but independent of p53 expression and p53 gene mutations.

Low dose genotoxicity of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) in gpt delta transgenic mice

Although humans are chronically exposed to most environmental chemicals at low doses, genotoxicity assays with rodents are usually performed at high doses with short treatment period. To investigate the dose-response of genotoxicity at lower doses, gpt delta transgenic mice were fed a diet containing 300, 30 or 3 parts per million (ppm) of 2-amino-3,8-dimethylimidazo[4,5-f]quinoxaline (MeIQx) for 12 weeks and the gpt mutations in the liver were analyzed. In addition, the mice were continuously fed a diet containing MeIQx at a dose of 300 ppm for 78 weeks to examine the effect of a long-term treatment. In the mice treated for 12 weeks, the gpt mutant frequencies (MFs) were 8.6-, 2.3- and 1.2-fold higher than the control level at the doses of 300, 30 and 3 ppm, respectively. G:C to T:A transversion was the most predominant type of mutations and the fold increases in the specific MF of G:C to T:A were 58.2, 4.4 and 1.7 above the control at the three doses, respectively. The increases in the whole gpt and specific MFs at 3 ppm were not statistically significant. In the mice treated with 300 ppm of MeIQx for 78 weeks, the gpt MF was about 20 times higher than that of the untreated mice fed a control diet for 78 weeks, which was about two times higher than that of the untreated mice at 12 weeks. These results suggest that no obvious genotoxic effects can be detectable at the dose of MeIQx at 3 ppm in the liver and a longer treatment substantially enhances the genotoxicity. Factors constituting the practical threshold dose are discussed.

Regional mutagenicity of heterocyclic amines in the intestine: mutation analysis of the cII gene in lambda/lacZ transgenic mice

Transgenic mouse assays have revealed that the mouse intestine, despite its resistance to carcinogenesis, is sensitive to the mutagenicity of some heterocyclic amines (HCAs). Little is known, however, about the level and localization of that sensitivity. We assessed the mutagenicity of four orally administered (20 mg/kg per day for 5 days) HCAs-2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) hydrochloride, 2-amino-3-methylimidazo[4,5-f]quinoline (IQ), 2-amino-3,4-dimethylimidazo[4,5-f]quinoline (MeIQ), and 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) acetate-in the intestine of male MutaMice. Two weeks after the last administration, we isolated epithelium from the small intestine, cecum, and colon and analyzed lacZ and cII transgene mutations. PhIP increased the lacZ mutant frequency (MF) in all the samples, and in the small intestine, cII and lacZ MFs were comparable. In the cII gene, G:C to T:A and G:C to C:G transversions were characteristic PhIP-induced mutations (which has also been reported for the rat colon, where PhIP is carcinogenic). In the small intestine, PhIP increased the cII MF to four-fold that of the control, but IQ, MeIQ, and Trp-P-2 did not have a significant mutagenic effect. In the cecum, cII MFs induced by IQ and MeIQ were 1.9 and 2.7 times those in the control, respectively. The MF induced by MeIQ in the colon was 3.1 times the control value. Mutagenic potency was in the order PhIP>MeIQ>IQ; Trp-P-2 did not significantly increase the MF in any tissue. The cecum was the most susceptible organ to HCA mutagenicity.

Use of transgenic and mutant animal models in the study of heterocyclic amine-induced mutagenesis and carcinogenesis

Heterocyclic amines (HCAs) are potent mutagens generated during the cooking of meat and fish, and several of these compounds produce tumors in conventional experimental animals. During the past 5 years or so, HCAs have been tested in a number of novel in vivo murine models, including the following: lacZ, lacI, cII, c-myc/lacZ, rpsL, and gptDelta. transgenics, XPA-/-, XPC-/-, Msh2+/-, Msh2-/- and p53+/- knock-outs, Apc mutant mice (ApcDelta716, Apc1638N, Apcmin), and A33DeltaNbeta-cat knock-in mice. Several of these models have provided insights into the mutation spectra induced in vivo by HCAs in target and non-target organs for tumorigenesis, as well as demonstrating enhanced susceptibility to HCA-induced tumors and preneoplastic lesions. This review describes several of the more recent reports in which novel animal models were used to examine HCA-induced mutagenesis and carcinogenesis in vivo, including a number of studies which assessed the inhibitory activities of chemopreventive agents such as 1,2-dithiole-3-thione, conjugated linoleic acids, tea, curcumin, chlorophyllin-chitosan, and sulindac.

Mutagenicity of aristolochic acid in the lambda/lacZ transgenic mouse (MutaMouse)

Aristolochic acid (AA) is found in a plant that causes urothelial carcinomas in patients with Chinese herb nephropathy (CHN). To evaluate the in vivo mutagenicity of AA, we analysed the mutant frequency (MF) in the lacZ and cII gene of 10 organs of the lambda/lacZ transgenic mouse (MutaMouse) after intragastric treatment with AA (15mg/kg per week x 4). Simultaneously, the clastogenicity of AA was evaluated by the peripheral blood micronucleus assay. The nature of the mutations induced by AA was revealed by the sequence analysis of the cII gene, which is also a phenotypically selectable marker in the lambda transgene. MFs in the target organs-forestomach, kidney, and bladder of AA-treated mice were significantly higher than those of control mice (forestomach 33- and 15-fold; kidney 10- and 9-fold; bladder 16- and 31-fold, for the lacZ and cII, respectively). The MFs in non-target organs, except the colon, showed only slight increases. Sequence analysis of cII mutants in target organs revealed that AA induced mainly A:T to T:A transversions whereas G:C to A:T transitions at CpG sites predominated among spontaneous mutations. These results suggested that AA, which is activated by cytochrome P450 and peroxidase to form cyclic nitrenium ions that bind to deoxyadenine, caused the A to T transversions in the target organs of mice.

Dinitropyrenes induce gene mutations in multiple organs of the lambda/lacZ transgenic mouse (Muta Mouse)

Dinitropyrenes (DNPs), 1,3-, 1,6- and 1,8-dinitropyrene, are carcinogenic compounds found in diesel engine exhaust. DNPs are strongly mutagenic in the bacterial mutation assay (Ames test), mainly inducing frameshift type mutations. To assess mutagenicity of DNPs in vivo is important in evaluating their possible involvement in diesel exhaust-induced carcinogenesis in human. For this purpose, we used the lambda/lacZ transgenic mouse (Muta Mouse) to examine induction of mutations in multiple organs. A commercially available mixture of DNPs (1,3-, 1,6-, 1,8-, and unidentified isomer (s) with a content of 20.2, 30.4, 35.2, and 14.2%, respectively) was injected intragastrically at 200 and 400mg/kg once each week for 4 weeks. Seven days after the final treatment, liver, lung, colon, stomach, and bone marrow were collected for mutation analysis. The target transgene was recovered by the lambda packaging method and mutation of lacZ gene was analyzed by a positive selection with galE(-) E. coli. In order to determine the sequence alterations by DNPs, the mutagenicity of the lambda cII gene was also examined by the positive selection with hfl(-) E. coli. Since cII gene (294bp) is much smaller than the lacZ (3024bp), it facilitated the sequence analysis. Strongest increases in mutant frequencies (MFs) were observed in colon for both lacZ (7.5x10(-5) to 43.3x10(-5)) and cII (2.7x10(-5) to 22.5x10(-5)) gene. Three-four-fold increases were observed in stomach for both genes. A statistically significant increase in MFs was also evident in liver and lung for the lacZ gene, and in lung and bone marrow for the cII gene. The sequence alterations of the cII gene recovered from 37 mutants in the colon were compared with 50 mutants from untreated mice. Base substitution mutations predominated for both untreated (91%) and DNP-treated (84%) groups. The DNPs treatment increased the incidence of G:C to T:A transversion (2-43%) and decreased G:C to A:T transitions (70-22%). The G:C to T:A transversions, characteristic to DNPs treatment, is probably caused by the guanine-C8 adduct, which is known as a major DNA-adduct induced by DNPs, through an incorporation of adenine opposite the adduct ("A"-rule). The present study showed a relevant use of the cII gene as an additional target for mutagenesis in the Muta Mouse and revealed a mutagenic specificity of DNPs in vivo.

Mutation spectrum of o-aminoazotoluene in the cII gene of lambda/lacZ transgenic mice (MutaMouse)

The o-aminoazotoluene (AAT) has been evaluated as a possible human carcinogen by the International Agency for Research on Cancer. In rodents, it is carcinogenic mainly in the liver, and also in lung following long term administration. We previously examined in lambda/lacZ transgenic mice for the induction of lacZ mutations in liver, lung, urinary bladder, colon, kidney, bone marrow, and testis. AAT induced gene mutations strongly in the liver and colon. In the present report, we reveal the molecular nature of mutations induced by AAT in the lambda cII gene (the cII gene, a phenotypically selectable marker in the lambda transgene, has 294bp, which makes it easier to sequence than the original target, the 3kb lacZ gene). The cII mutant frequency in liver and colon was five and nine times higher, respectively, in AAT-treated mice than in control mice. Sequence analysis revealed that AAT induced G:C to T:A transversions, whereas spontaneous mutations consisted primarily of G:C to A:T transitions at CpG sites.

Strategies and testing methods for identifying mutagenic risks

The evolution of testing strategies and methods for identification of mutagenic agents is discussed, beginning with the concern over potential health and population effects of chemical mutagens in the late 1940s that led to the development of regulatory guidelines for mutagenicity testing in the 1970s and 1980s. Efforts to achieve international harmonization of mutagenicity testing guidelines are summarized, and current issues and needs in the field are discussed, including the need for quantitative methods of mutagenic risk assessment, dose-response thresholds, indirect mechanisms of mutagenicity, and the predictivity of mutagenicity assays for carcinogenicity in vivo. Speculation is offered about the future of mutagenicity testing, including possible near-term changes in standard test batteries and the longer-term roles of expression profiling of damage-response genes, in vivo mutagenicity testing methods, and models that better account for differences in metabolism between humans and laboratory model systems.