In vivo mutagenicity and initiation following oxidative DNA lesion in the kidneys of rats given potassium bromate

Possible contribution of 8-hydroxydeoxyguanosine to gene mutations in the kidney DNA of gpt delta rats following potassium bromate treatment

8-Hydroxydeoxyguanosine (8-OHdG) is well known not only as an effective biomarker of oxidative stress but also as a mutagenic DNA modification. Incorporation of dAMP at the opposite site of 8-OHdG induces G>T or A>C transversions. However, in vivo analyses of gene mutations caused by potassium bromate (KBrO3), which can induce 8-OHdG at carcinogenic target sites, showed that G>T was prominent in the small intestines of mice, but not in the kidneys of rats. Because KBrO3 was a much clearer carcinogen in the kidneys of rats, detailed analyses of gene mutations in the kidney DNA of rats treated with KBrO3 could improve our understanding of oxidative stress-mediated carcinogenesis. In the current study, site-specific reporter gene mutation assays were performed in the kidneys of gpt delta rats treated with KBrO3. Groups of 5 gpt delta rats were treated with KBrO3 at concentrations of 0, 125, 250, or 500 ppm in the drinking water for 9 weeks. At necropsy, the kidneys were macroscopically divided into the cortex and medulla. 8-OHdG levels in DNA extracted from the cortex were dramatically elevated at concentrations of 250 ppm and higher compared with those from the medulla. Cortex-specific increases in mutant frequencies in gpt and red/gam genes were found at 500 ppm. Mutation spectrum and sequence analyses of their mutants demonstrated significant elevations in A>T transversions in the gpt gene and single base deletions at guanine or adenine in the gpt or red/gam genes. While A>T transversions and single base deletions of adenine may result from the oxidized modification of adenine, the contribution of 8-OHdG to gene mutations was limited despite possible participation of the 8-OHdG repair process in guanine deletion.

Changes in metabolic landscapes shape divergent but distinct mutational signatures and cytotoxic consequences of redox stress

Mutational signatures discerned in cancer genomes, in aging tissues and in cells exposed to toxic agents, reflect complex processes underlying transformation of cells from normal to dysfunctional. Due to its ubiquitous and chronic nature, redox stress contributions to cellular makeover remain equivocal. The deciphering of a new mutational signature of an environmentally-relevant oxidizing agent, potassium bromate, in yeast single strand DNA uncovered a surprising heterogeneity in the mutational signatures of oxidizing agents. NMR-based analysis of molecular outcomes of redox stress revealed profound dissimilarities in metabolic landscapes following exposure to hydrogen peroxide versus potassium bromate. The predominance of G to T substitutions in the mutational spectra distinguished potassium bromate from hydrogen peroxide and paraquat and mirrored the observed metabolic changes. We attributed these changes to the generation of uncommon oxidizing species in a reaction with thiol-containing antioxidants; a nearly total depletion of intracellular glutathione and a paradoxical augmentation of potassium bromate mutagenicity and toxicity by antioxidants. Our study provides the framework for understanding multidimensional processes triggered by agents collectively known as oxidants. Detection of increased mutational loads associated with potassium bromate-related mutational motifs in human tumors may be clinically relevant as a biomarker of this distinct type of redox stress.

Toxicity, genotoxicity, and carcinogenicity of 2-methylfuran in a 90-day comprehensive toxicity study in gpt delta rats

2-Methylfuran (2-MF) exists naturally in foods and is used as a flavoring agent. Furan, the core structure of 2-MF, possesses hepatocarcinogenicity in rodents. Accumulation of toxicological information on furan derivatives is needed to elucidate their carcinogenic mode of action. In the current study, we examined the comprehensive toxicological studies of 2-MF using gpt delta rats. 2-MF was intragastrically administered to groups of 10 male and 10 female Sprague-Dawley gpt delta rats at a dose of 0, 1.2, 6, or 30 mg/kg/day for 13 weeks. Effects of 2-MF on the hepatobiliary system including an increase in serum alkaline phosphatase were observed in the 6 and 30 mg/kg groups, and cholangiofibrosis was found in the 30 mg/kg group. The no observed adverse effect level was set at 1.2 mg/kg/day for both sexes and 1.14 mg/kg/day was determined as the benchmark dose low. The acceptable daily intake was calculated to be 11.4 μg/kg/day. Increases in the number and areas of glutathione S-transferase placental form-positive foci in the 30 mg/kg group were apparent, suggesting the hepatocarcinogenicity of 2-MF in rats. By contrast, the lack of increase in in vivo mutagenicity in the liver implied that 2-MF hepatocarcinogenesis may not involve genotoxic mechanisms.

New aspects in deriving health-based guidance values for bromate in swimming pool water

Bromate, classified as a EU CLP 1B carcinogen, is a typical by-product of the disinfection of drinking and swimming pool water. The aim of this study was (a) to provide data on the occurrence of bromate in pool water, (b) to re-evaluate the carcinogenic MOA of bromate in the light of existing data, (c) to assess the possible exposure to bromate via swimming pool water and (d) to inform the derivation of cancer risk-related bromate concentrations in swimming pool water. Measurements from monitoring analysis of 229 samples showed bromate concentrations in seawater pools up to 34 mg/L. A comprehensive non-systematic literature search was done and the quality of the studies on genotoxicity and carcinogenicity was assessed by Klimisch criteria (Klimisch et al., Regul Toxicol Pharmacol 25:1-5, 1997) and SciRAP tool (Beronius et al., J Appl Toxicol, 38:1460-1470, 2018) respectively. Benchmark dose (BMD) modeling was performed using the modeling average mode in BMDS 3.1 and PROAST 66.40, 67 and 69 (human cancer BMDL10; EFSA 2017). For exposure assessment, data from a wide range of sources were evaluated for their reliability. Different target groups (infants/toddlers, children and adults) and exposure scenarios (recreational, sport-active swimmers, top athletes) were considered for oral, inhalation and dermal exposure. Exposure was calculated according to the frequency of swimming events and duration in water. For illustration, cancer risk-related bromate concentrations in pool water were calculated for different target groups, taking into account their exposure using the hBMDL10 and a cancer risk of 1 in 100,000. Convincing evidence was obtained from a multitude of studies that bromate induces oxidative DNA damage and acts as a clastogen in vitro and in vivo. Since statistical modeling of the available genotoxicity data is compatible with both linear as well as non-linear dose-response relationships, bromate should be conservatively considered to be a non-threshold carcinogen. BMD modeling with model averaging for renal cancer studies (Kurokawa et al., J Natl. Cancer Inst, 1983 and 1986a; DeAngelo et al., Toxicol Pathol 26:587-594, 1998) resulted in a median hBMDL10 of 0.65 mg bromate/kg body weight (bw) per day. Evaluation of different age and activity groups revealed that top athletes had the highest exposure, followed by sport-active children, sport-active adults, infants and toddlers, children and adults. The predominant route of exposure was oral (73-98%) by swallowing water, followed by the dermal route (2-27%), while the inhalation route was insignificant (< 0.5%). Accepting the same risk level for all population groups resulted in different guidance values due to the large variation in exposure. For example, for an additional risk of 1 in 100,000, the bromate concentrations would range between 0.011 for top athletes, 0.015 for sport-active children and 2.1 mg/L for adults. In conclusion, the present study shows that health risks due to bromate exposure by swimming pool water cannot be excluded and that large differences in risk exist depending on the individual swimming habits and water concentrations.

In vivo mutagenicity and tumor-promoting activity of 1,3-dichloro-2-propanol in the liver and kidneys of gpt delta rats

1,3-Dichloro-2-propanol (1,3-DCP), a food contaminant, exerts carcinogenic effects in multiple organs, including the liver and kidneys, in rats. However, the underlying mechanisms of 1,3-DCP-induced carcinogenesis remain unclear. Here, the in vivo mutagenicity and tumor-promoting activity of 1,3-DCP in the liver and kidneys were evaluated using medium-term gpt delta rat models previously established in our laboratory (GPG and GNP models). Six-week-old male F344 gpt delta rats were treated with 0 or 50 mg/kg body weight/day 1,3-DCP by gavage for 4 weeks. After 2 weeks of cessation, partial hepatectomy or unilateral nephrectomy was performed to collect samples for in vivo mutation assays, followed by single administration of diethylnitrosamine (DEN) for tumor initiation. One week after DEN injection, 1,3-DCP treatment was resumed, and tumor-promoting activity was evaluated in the residual liver or kidneys by histopathological analysis of preneoplastic lesions. gpt mutant frequencies increased in excised liver and kidney tissues following 1,3-DCP treatment. 1,3-DCP did not affect the development of glutathione S-transferase placental form-positive foci in residual liver tissues, but enhanced atypical tubule hyperplasia in residual kidney tissues. Detailed histopathological analyses revealed glomerular injury and increased cell proliferation of renal tubular cells in residual kidney tissues of rats treated with 1,3-DCP. These results suggested possible involvement of genotoxic mechanisms in 1,3-DCP-induced carcinogenesis in the liver and kidneys. In addition, we found that 1,3-DCP exhibited limited tumor-promoting activity in the liver, but enhanced clonal expansion in renal carcinogenesis via proliferation of renal tubular cells following glomerular injury.

Angiotensin II-induced hypertension increases the mutant frequency in rat kidney

Epidemiological studies revealed an increased risk for kidney cancer in hypertensive patients. In many of these patients, the blood pressure regulating renin-angiotensin-aldosterone system (RAAS) is activated. A stimulated RAAS leads to oxidative stress and increases markers of DNA damage, both in vitro and in animal models of hypertension. However, the mutagenic potential of RAAS activation has not been investigated yet. To quantify hypertension-induced mutations, BigBlue^®+/- rats, which carry a transgenic lacI gene for mutation analysis, were treated for 20 weeks with a mean dose of 400 µg angiotensin II/kg × day. Angiotensin II-treated animals showed significantly increased blood pressure and impaired kidney function. Urinary excretion of oxidized nucleobases was raised. Additionally, in the renal cortex, oxidative stress, oxidatively generated DNA lesions and DNA strandbreaks were significantly increased. Further, a significant elevation of the mutant frequency in kidney DNA was detected. Sequencing revealed the presence of GC → T:A transversions in the mutated lacI genes of the angiotensin II-treated animals as a result of unrepaired oxidatively modified DNA bases, while no such transversions were found in the mutated lacI genes from control animals. The results demonstrate that the oxidative stress and DNA damage previously observed in kidney cells in vitro and in vivo after angiotensin II treatment indeed is associated with the accumulation of mutations in rat kidneys, providing further evidence for a cancer-initiating potential of elevated angiotensin II concentrations.

Statistical Approach to Identify Threshold and Point of Departure in Dose-Response Data

The study presents an integrated, rigorous statistical approach to define the likelihood of a threshold and point of departure (POD) based on dose-response data using nested family of bent-hyperbola models. The family includes four models: the full bent-hyperbola model, which allows for transition between two linear regiments with various levels of smoothness; a bent-hyperbola model reduced to a spline model, where the transition is fixed to a knot; a bent-hyperbola model with a restricted negative asymptote slope of zero, named hockey-stick with arc (HS-Arc); and spline model reduced further to a hockey-stick type model (HS), where the first linear segment has a slope of zero. A likelihood-ratio test is used to discriminate between the models and determine if the more flexible versions of the model provide better or significantly better fit than a hockey-stick type model. The full bent-hyperbola model can accommodate both threshold and nonthreshold behavior, can take on concave up and concave down shapes with various levels of curvature, can approximate the biochemically relevant Michaelis-Menten model, and even be reduced to a straight line. Therefore, with the use of this model, the presence or absence of a threshold may even become irrelevant and the best fit of the full bent-hyperbola model be used to characterize the dose-response behavior and risk levels, with no need for mode of action (MOA) information. Point of departure (POD), characterized by exposure level at which some predetermined response is reached, can be defined using the full model or one of the better fitting reduced models.

Role of oxidative stress in the chemical structure-related genotoxicity of nitrofurantoin in Nrf2-deficient gpt delta mice

Despite its antimicrobial activity, nitrofurantoin (NFT) is a renal carcinogen in rats. Oxidative stress induced by reduction of the nitro group of NFT may contribute to its genotoxicity. This is supported by our recent results indicating that the structure of the nitrofuran plays a key role in NFT-induced genotoxicity, and oxidative DNA damage is involved in renal carcinogenesis. Nuclear factor erythroid 2-related factor 2 (NRF2) regulates cellular responses to oxidative stress. To clarify the role of oxidative stress in the chemical structure-related genotoxic mechanism of NFT, we performed reporter gene mutation assays for NFT and 5-nitro-2-furaldehyde (NFA) using Nrf2-proficient and Nrf2-deficient gpt delta mice. NFT administration for 13 weeks resulted in a significant increase in 8-hydroxydeoxyguanosine (8-OHdG; a marker of oxidative stress) and gpt mutant frequency only in the kidneys of Nrf2-/- mice. The mutation spectrum, characterized by increased substitutions at guanine bases, suggested that oxidative stress is involved in NFT-induced genotoxicity. However, NFA did not increase the mutation frequency in the kidneys, despite the increased 8-OHdG in NFA-treated Nrf2-/- mice. Thus, it is unlikely that oxidative stress is involved in the genotoxic mechanism of NFA. These results imply that nitro reduction plays a key role in the genotoxicity of NFT, but the lack of a role of oxidative stress in the genotoxicity of NFA indicates a potential role of side chain interactions in oxidative stress caused by nitro reduction. These findings provide a basis for the development of safe nitrofurans.

Mechanisms of oxidative stress-induced in vivo mutagenicity by potassium bromate and nitrofurantoin

Oxidative stress is well known as a key factor of chemical carcinogenesis. However, the actual role of oxidative stress in carcinogenesis, such as oxidative stress-related in vivo mutagenicity, remains unclear. It has been reported that 8-hydroxydeoxyguanosine (8-OHdG), an oxidized DNA lesion, might contribute to chemical carcinogenesis. Potassium bromate (KBrO3) and nitrofurantoin (NFT) are known as renal carcinogens in rats. Our previous studies showed an increase in mutant frequencies accompanied by an increased level of 8-OHdG in the kidneys of rodents following KBrO3 or NFT exposure. Furthermore, KBrO3 and NFT induced different types of gene mutations. Thus, in the present study, we performed reporter gene mutation assays and 8-OHdG measurements following KBrO3 or NFT exposure using Nrf2-proficient and Nrf2-deficient mice to clarify the relationship between KBrO3- or NFT-induced oxidative stress and subsequent genotoxicity. Administration of 1,500 ppm of KBrO3 in drinking water resulted in an increase in deletion mutations accompanied by an increase in 8-OHdG level, and administration of 2,500 ppm of NFT in diet induced an increase in guanine base substitution mutations without elevation of the 8-OHdG level in Nrf2-deficient mice. These results demonstrated that the formation of 8-OHdG, which resulted from the oxidizing potential of KBrO3, was directly involved in the increase in deletion mutations, although factors related to oxidative stress other than 8-OHdG might be crucial for NFT-induced guanine base substitution mutations. The present study provides new insight into oxidative stress-related in vivo mutagenicity.

Impact of DNA polymerase ζ mutations on genotoxic thresholds of oxidative mutagens

In regulatory genetic toxicology, it is an axiom that there is no threshold for genotoxicity of chemicals, such that genotoxic chemicals may impose carcinogenic risk on humans even at very low doses. This paradigm is counterintuitive, however, because humans possess a number of self-defense mechanisms that may suppress the genotoxicity at these low doses and therefore manifest a practical threshold. DNA polymerase zeta (Pol ζ) is a specialized Pol that plays an important role in DNA synthesis across DNA damage, thereby modulating cell survival and genotoxicity. In this study, we compared the sensitivity of three types of human cells: D2781N, L2618M, and their wild-type (WT) cells, to the low dose effects of genotoxicity of the oxidizing agents, potassium bromate (KBrO₃) and sodium dichromate (Na₂Cr₂O₇). D2781N cells express a variant form of Pol ζ, whose activity is weaker than that of the WT enzyme. L2618M cells express another variant form of Pol ζ, whose fidelity of DNA replication is lower than that of the WT enzyme. D2781N exhibited the highest sensitivity for TK gene mutation and micronucleus (MN) formation and displayed the lowest practical threshold for MN induction by KBrO₃. In contrast, L2618M exhibited the lowest practical threshold for sister-chromatid exchange (SCE) induction by both chemicals. These results suggest that Pol ζ mutations have significant impacts on practical thresholds of genotoxicity; the factors affecting the practical threshold can differ depending on the endpoint of genotoxicity. Roles of the variant forms of Pol ζ in genotoxicity by the oxidizing agents are discussed.

In vivo reporter gene mutation and micronucleus assays in gpt delta mice treated with a flame retardant decabromodiphenyl ether

Polybrominated diphenyl ethers (PBDEs), a class of brominated flame retardants, have been widely used as additive flame retardants. Recently, the use of brominated flame retardants has been restricted or prohibited under various legislative acts because of the persistence, bioaccumulation potential, and toxicity of these compounds. However, there are also additional concerns regarding environmental contamination and human exposure to PBDEs resulting from informal recycling technology. Decabromodiphenyl ether (decaBDE), one type of PBDE, has carcinogenic potential in the livers of rodents. Although one study has shown that decaBDE exerts genotoxic effects, the other in vitro and in vivo studies were negative for such effects. Thus, it remains unknown whether genotoxic mechanisms are involved in decaBDE-induced hepatocarcinogenesis in rodents. In this study, to explore the genotoxicity of decaBDE in mice, particularly in the context of carcinogenesis, we performed micronucleus assays in the bone marrow and reporter gene mutation assays in the liver using gpt delta mice treated with decaBDE at carcinogenic doses for 28days. Our results demonstrated negative results in micronucleus tests and reporter gene mutation assays. Thus, decaBDE did not exert genotoxic effects at carcinogenic target sites and did not show positive results in conventional in vivo genotoxicity tests in mice for 4-week treatment. Overall, comprehensive evaluation using in vivo genotoxicity data in rats and our data indicated that nongenotoxic mechanisms may be responsible for decaBDE-induced hepatocarcinogenesis.

Transgenic rat models for mutagenesis and carcinogenesis

Rats are a standard experimental animal for cancer bioassay and toxicological research for chemicals. Although the genetic analyses were behind mice, rats have been more frequently used for toxicological research than mice. This is partly because they live longer than mice and induce a wider variety of tumors, which are morphologically similar to those in humans. The body mass is larger than mice, which enables to take samples from organs for studies on pharmacokinetics or toxicokinetics. In addition, there are a number of chemicals that exhibit marked species differences in the carcinogenicity. These compounds are carcinogenic in rats but not in mice. Such examples are aflatoxin B1 and tamoxifen, both are carcinogenic to humans. Therefore, negative mutagenic/carcinogenic responses in mice do not guarantee that the chemical is not mutagenic/carcinogenic to rats or perhaps to humans. To facilitate research on in vivo mutagenesis and carcinogenesis, several transgenic rat models have been established. In general, the transgenic rats for mutagenesis are treated with chemicals longer than transgenic mice for more exact examination of the relationship between mutagenesis and carcinogenesis. Transgenic rat models for carcinogenesis are engineered mostly to understand mechanisms underlying chemical carcinogenesis. Here, we review papers dealing with the transgenic rat models for mutagenesis and carcinogenesis, and discuss the future perspective.

Lack of genotoxic mechanisms in early-stage furan-induced hepatocellular tumorigenesis in gpt delta rats

Furan has been used as an intermediate in the chemical-manufacturing industry and has been shown to contaminate various foods. Although furan induces hepatocellular tumors in rodents, equivocal results from in vitro and in vivo mutagenicity tests have caused controversy regarding the involvement of genotoxic mechanisms in furan-induced carcinogenesis. In the present study, to elucidate the possible mechanisms underlying furan-induced hepatocarcinogenesis, a comprehensive medium-term analysis was conducted using gpt delta rats treated with furan at carcinogenic doses for 13 weeks. In the liver, the frequencies of gpt and Spi^- mutants derived mainly from point and deletion mutations, respectively, were not changed, and there were no furan-specific gpt mutations in furan-treated rats. In contrast, the number and area of glutathione S-transferase placental form (GST-P)- positive foci were significantly increased in the high-dose group. Also, the ratio of PCNA-positive hepatocytes was significantly elevated in the same group, as supported by significant increases in cyclin d1 and cyclin e1 mRNA levels. Thus, it is highly probable that cell proliferation, but not genotoxic mechanisms, contribute to the development of GST-P foci in furan-treated rats. Based on the close relationship between GST-P and neoplastic hepatocytes, these data allowed us to hypothesize that cell proliferation following signal transduction other than the mitogen-activated protein kinase (MAPK)/ERK pathway may play a crucial role in early-stage furan-induced hepatocarcinogenesis. Copyright © 2016 John Wiley & Sons, Ltd.

Effects of Nrf2 silencing on oxidative stress-associated intestinal carcinogenesis in mice

To assess the risk of colorectal cancer in humans with inactivation of NRF2, Nrf2‐proficient (Nrf2^+/+) and ‐deficient (Nrf2^−/−) mice were exposed to potassium bromate (KBrO₃) at concentrations of 750 or 1500 ppm for 52 weeks. Neoplastic proliferative lesions were observed in the small intestine and exhibited accumulations of β‐catenin and cyclin D1. The lesions had characteristics similar to those in experimental models of human hereditary colorectal cancer. An additional 13‐week study was performed to examine the role of Nrf2 in the effects of oxidative stress. Significant increase in combined incidences of preneoplastic and neoplastic lesions in Nrf2^−/− mice administered high‐dose KBrO₃. In the short‐term study, although 8‐hydroxydeoxyguanosine (8‐OHdG) levels in the epithelial DNA of Nrf2^−/− mice at the high dose were significantly lower than those of the corresponding Nrf2^+/+ mice, the difference was very small. mRNA levels of Nrf2‐regulated genes were increased in Nrf2^+/+ mice. Overexpression of cyclooxygenase 2 (COX2) and increased numbers of proliferating cell nuclear antigen (PCNA)‐positive cells in the jejunal crypts were observed in Nrf2^−/− mice administered high‐dose KBrO₃. Overall, these data suggested that individuals having single‐nucleotide polymorphisms in NRF2 may have a risk of colorectal cancer to some extent.

Kinetic Modeling Reveals the Roles of Reactive Oxygen Species Scavenging and DNA Repair Processes in Shaping the Dose-Response Curve of KBrO₃-Induced DNA Damage

We have developed a kinetic model to investigate how DNA repair processes and scavengers of reactive oxygen species (ROS) can affect the dose-response shape of prooxidant induced DNA damage. We used as an example chemical KBrO₃ which is activated by glutathione and forms reactive intermediates that directly interact with DNA to form 8-hydroxy-2-deoxyguanosine DNA adducts (8-OH-dG). The single strand breaks (SSB) that can result from failed base excision repair of these adducts were considered as an effect downstream from 8-OH-dG. We previously demonstrated that, in the presence of effective base excision repair, 8-OH-dG can exhibit threshold-like dose-response dependence, while the downstream SSB can still exhibit a linear dose-response. Here we demonstrate that this result holds for a variety of conditions, including low levels of GSH, the presence of additional SSB repair mechanisms, or a scavenger. It has been shown that melatonin, a terminal scavenger, inhibits KBrO₃-caused oxidative damage. Our modeling revealed that sustained exposure to KBrO₃ can lead to fast scavenger exhaustion, in which case the dose-response shapes for both endpoints are not substantially affected. The results are important to consider when forming conclusions on a chemical's toxicity dose dependence based on the dose-response of early genotoxic events.

Genotoxicity of phenacetin in the kidney and liver of Sprague-Dawley gpt delta transgenic rats in 26-week and 52-week repeated-dose studies

Transgenic rat mutation assays can be used to assess genotoxic properties of chemicals in target organs for carcinogenicity. Mutations in transgenes are genetically neutral and accumulate during a treatment period; thus, assays are suitable for assessing the genotoxic risk of chemicals using a repeated-dose treatment paradigm. However, only a limited number of such studies have been conducted. To examine the utility of transgenic rat assays in repeated-dose studies, we fed male and female Sprague-Dawley gpt delta rats with a 0.5% phenacetin-containing diet for 26 and 52 weeks. A long-term feeding of phenacetin is known to induce renal cancer in rats. Phenacetin administration for 52 weeks in males significantly increased gpt (point mutations) mutant frequency (MF) in the kidney, the target organ of carcinogenesis. In the liver, the nontarget organ of carcinogenesis, gpt MFs were significantly elevated in phenacetin treatment groups of both genders during 26- and 52-week treatments. Furthermore, sensitive to P2 interference (Spi(-)deletions) MF increased in the liver of both genders following 52-week treatment. MFs were higher after treatment for 52 weeks than after treatment for 26 weeks. Frequencies of phenacetin-induced mutations were higher in the liver than in the kidney, suggesting that the intensity of genotoxicity does not necessarily correlate with the induction of tumor formation. Results from gpt delta rat assays of repeated-dose treatments are extremely useful to elucidate the relationship between gene mutations and carcinogenesis in the target organ induced by cancer-causing agents.

Epigenetic changes in p21 expression in renal cells after exposure to bromate

This study tested the hypothesis that bromate (KBrO3)-induced renal cell death is mediated by epigenetic mechanisms. Global DNA methylation, as assessed by 5-methylcytosine staining, was not changed in normal rat kidney cells treated with acute cytotoxic doses of KBrO3 (100 and 200 ppm), as compared with controls. However, KBrO3 treatment did increase p38, p53 and histone 2AX (H2AX) phosphorylation, and p21 expression. Treatment of cells with inhibitors of DNA methyltransferase (5-azacytidine or 5-Aza) and histone deacetylase (trichostatin A or TSA) in addition to KBrO3 increased cytotoxicity, as compared with cells exposed to KBrO3 alone. 5-Aza and TSA co-treatment did not alter p38 or p53 phosphorylation, but slightly decreased H2AX phosphorylation and significantly decreased p21 expression. We also assessed epigenetic changes in cells treated under sub-chronic conditions with environmentally relevant concentrations of KBrO3. Under these conditions (0-10ppm KBrO3 for up to 18 days), we detected no increases in cell death or DNA damage. In contrast, slight alterations were detected in the phosphorylation of H2AX, p38, and p53. Sub-chronic low-dose KBrO3 treatment also induced a biphasic response in p21 expression, with lower concentrations increasing expression, but higher concentrations decreasing expression. Methylation-specific PCR demonstrated that sub-chronic KBrO3 treatment altered the methylation of cytosine bases in the p21 gene, as compared with controls, correlating to alterations in p21 protein expression. Collectively, these data show the novel finding that KBrO3-induced renal cell death is altered by inhibitors of epigenetic modifying enzymes and that KBrO3 itself induces epigenetic changes in the p21 gene.

Potassium bromate causes cell lysis and induces oxidative stress in human erythrocytes

In the present study, we have studied the effect of KBrO3 on human erythrocytes under in vitro conditions. Erythrocytes were isolated from the blood of healthy nonsmoking volunteers and incubated with different concentrations of KBrO3 at 37°C for 60 min. This resulted in marked hemolysis in a KBrO3 -concentration dependent manner. Lysates were prepared from KBrO3 -treated and control erythrocytes and assayed for various parameters. KBrO3 treatment caused significant increase in protein oxidation, lipid peroxidation, hydrogen peroxide levels, and decrease in total sulfhydryl content, which indicates induction of oxidative stress in human erythrocytes. Methemoglobin levels and methemoglobin reductase activity were significantly increased while the total antioxidant power of lysates was greatly reduced upon KBrO3 treatment. Intracellular production of reactive oxygen species increased in a dose dependent manner. Exposure of erythrocytes to KBrO3 also caused decrease in the activities of catalase, glutathione peroxidase, thioredoxin reductase, glucose 6-phosphate dehydrogenase and glutathione reductase whereas the activities of Cu-Zn superoxide dismutase and glutathione-S-transferase were increased. These results show that KBrO3 induces oxidative stress in human erythrocytes through the generation of reactive oxygen species and alters the cellular antioxidant defense system.

Mechanisms of chemical carcinogenesis in the kidneys

Chemical carcinogens are substances which induce malignant tumours, increase their incidence or decrease the time taken for tumour formation. Often, exposure to chemical carcinogens results in tissue specific patterns of tumorigenicity. The very same anatomical, biochemical and physiological specialisations which permit the kidney to perform its vital roles in maintaining tissue homeostasis may in fact increase the risk of carcinogen exposure and contribute to the organ specific carcinogenicity observed with numerous kidney carcinogens. This review will address the numerous mechanisms which play a role in the concentration, bioactivation, and uptake of substances from both the urine and blood which significantly increase the risk of cancer in the kidney.

Evaluation of the genotoxicity of tamoxifen in the liver and kidney of F344 gpt delta transgenic rat in 3-week and 13-week repeated dose studies

Transgenic rat gene mutation assays can be used to assess genotoxicity of chemicals in target organs for carcinogenicity. Mutations in transgenes are genetically neutral and accumulate during a treatment period; thus, the assays are suitable for assessment of the genotoxicity risk of chemicals using a repeated-dose treatment paradigm. However, few such studies have been conducted. To examine the utility of the transgenic rat assays in repeated-dose studies, we treated female F344 gpt delta rats with tamoxifen (TAM) at 20 and 40mg/kg, or toremifene (TOR) at 40mg/kg by gavage daily for 3 weeks. We also fed gpt delta rats with TAM at either 250ppm (15.4-17.6mg/kg) or 500ppm (30.0-32.9mg/kg) for 13 weeks. TAM is carcinogenic in the rat liver and TOR is not carcinogenic. TAM administration significantly increased gpt (point mutations) and Spi(-) (deletions) mutant frequencies (MFs) in the liver, the target organ of carcinogenesis; MFs were higher after treatment for 13 weeks than after treatment for 3 weeks. The MFs in the kidney did not increase in any of the TAM treatment groups. TOR had no effect on MFs (gpt and Spi(-)) in either the liver or the kidney. We conclude that the gpt delta rat assay in the repeated-dose treatment paradigm is sensitive enough to detect gene mutations induced by TAM in the target organ for carcinogenesis. Furthermore, the assay can be integrated into a 13-week dose-finding study for a 2-year cancer bioassay.

Association of brominated proteins and changes in protein expression in the rat kidney with subcarcinogenic to carcinogenic doses of bromate

The water disinfection byproduct bromate (BrO3(-)) produces cytotoxic and carcinogenic effects in rat kidneys. Our previous studies demonstrated that BrO3(-) caused sex-dependent differences in renal gene and protein expression in rats and the elimination of brominated organic carbon in their urine. The present study examined changes in renal cell apoptosis and protein expression in male and female F344 rats treated with BrO3(-) and associated these changes with accumulation of 3-bromotyrosine (3-BT)-modified proteins. Rats were treated with 0, 11.5, 46 and 308 mg/L BrO3(-) in drinking water for 28 days and renal sections were prepared and examined for apoptosis (TUNEL-staining), 8-oxo-deoxyguanosine (8-oxoG), 3-BT, osteopontin, Kim-1, clusterin, and p-21 expression. TUNEL-staining in renal proximal tubules increased in a dose-related manner beginning at 11.5mg BrO3(-)/L in female rats and 46 mg/L in males. Increased 8-oxoG staining was observed at doses as low as 46 mg/L. Osteopontin expression also increased in a dose-related manner after treatment with 46 mg/L, in males only. In contrast, Kim-1 expression increased in a dose-related manner in both sexes, although to a greater extent in females at the highest dose. Clusterin and p21 expression also increased in a dose-related manner in both sexes. The expression of 3-BT-modified proteins only increased in male rats, following a pattern previously reported for accumulation of α-2u-globulin. Increases in apoptosis in renal proximal tubules of male and female rats at the lowest doses suggest a common mode of action for renal carcinogenesis for the two sexes that is independent of α-2u-globulin nephropathy.

Dose-response analysis of bromate-induced DNA damage and mutagenicity is consistent with low-dose linear, nonthreshold processes

Mutagenic agents have long been inferred to act through low-dose linear, nonthreshold processes. However, there is debate about this assumption, with various studies interpreting datasets as showing thresholds for DNA damage and mutation. We have applied rigorous statistical analyses to investigate the shape of dose-response relationships for a series of in vitro and in vivo genotoxicity studies using potassium bromate (KBrO(3) ), a water ozonation byproduct that is bioactivated to a reactive species causing oxidative damage to DNA. We analyzed studies of KBrO(3) genotoxicity where no-effect/threshold levels were reported as well as other representative datasets. In all cases, the data were consistent with low-dose linear models. In the majority of cases, the data were fit either by a linear (straight line) model or a model which was linear at low doses and showed a saturation-like downward curvature at high doses. Other datasets with apparent upward curvature were still adequately represented by models that were linear at low dose. Sensitivity analysis of datasets showing upward curvature revealed that both low-dose linear and nonlinear models provide adequate fits. Additionally, a simple biochemical model of selected key processes in bromate-induced DNA damage was developed and illustrated a situation where response for early primary events suggested an apparent threshold while downstream events were linear. Overall, the statistical analyses of DNA damage and mutations induced by KBrO(3) are consistent with a low-dose linear response and do not provide convincing evidence for the presence of a threshold.

Absorption and disposition of bromate in F344 rats

Bromate (BrO(3)(-)) is a ubiquitous by-product of using ozone to disinfect water containing bromide (Br(-)). The reactivity of BrO(3)(-) with biological reductants suggests that its systemic absorption and distribution to target tissues may display non-linear behavior as doses increase. The intent of this study is to determine the extent to which BrO(3)(-) is systemically bioavailable via oral exposure and broadly identify its pathways of degradation. In vitro experiments of BrO(3)(-) degradation in rat blood indicate a rapid initial degradation immediately upon addition that is >98% complete at concentrations up to 66μM in blood. As initial concentrations are increased, progressively lower fractions are lost prior to the first measurement. Secondary to this initial loss, a slower and predictable first order degradation rate was observed (10%/min). Losses during both phases were accompanied by increases in Br(-) concentrations indicating that the loss of BrO(3)(-) was due to its reduction. In vivo experiments were conducted using doses of BrO(3)(-) ranging from 0.077 to 15.3mg/kg, administered intravenously (IV) or orally (gavage) to female F344 rats. The variable nature and uncertain source of background concentrations of BrO(3)(-) limited derivation of terminal half-lives, but the initial half-life was approximately 10min for all dose groups. The area under the curve (AUC) and peak concentrations (C(t=5')) were linearly related to IV dose up to 0.77mg/kg; however, disproportionate increases in the AUC and C(t=5') and a large decrease in the volume of distribution was observed when IV doses of 1.9 and 3.8mg/kg were administered. The average terminal half-life of BrO(3)(-) from oral administration was 37min, but this was influenced by background levels of BrO(3)(-) at lower doses. With oral doses, the AUC and C(max) increased linearly with dose up to 15.3mgBrO(3)(-)/kg. BrO(3)(-) appeared to be 19-25% bioavailable without an obvious dose-dependency between 0.077 and 1.9mg/kg. The urinary elimination of BrO(3)(-) and Br(-) was measured from female F344 rats for four days following administration of single doses of 8.1mgKBrO(3)/kg and for 15 days after a single dose of 5.0mgKBr/kg. BrO(3)(-) elimination was detected over the first 12h, but Br(-) elimination from BrO(3)(-) over the first 48h was 18% lower than expected based on that eliminated from an equimolar dose of Br(-) (15.5±1.6 vs. 18.8±1.2μmol/kg, respectively). The cumulative excretion of Br(-) from KBr vs. KBrO(3) was equivalent 72h after administration. The recovery of unchanged administered BrO(3)(-) in the urine ranged between 6.0 and 11.3% (creatinine corrected) on the 27th day of treatment with concentrations of KBrO(3) of 15, 60, and 400mg/L of drinking water. The recovery of total urinary bromine as Br(-)+BrO(3)(-) ranged between 61 and 88%. An increase in the fraction of the daily BrO(3)(-) dose recovered in the urine was observed at the high dose to both sexes. The deficit in total bromine recovery raises the possibility that some brominated biochemicals may be produced in vivo and more slowly metabolized and eliminated. This was supported by measurements of dose-dependent increases of total organic bromine (TOBr) that was eliminated in the urine. The role these organic by-products play in BrO(3)(-)-induced cancer remains to be established.

Site-specific in vivo mutagenicity in the kidney of gpt delta rats given a carcinogenic dose of ochratoxin A

Ochratoxin A (OTA) can induce renal tumors that originate from the S3 segment of the proximal tubules in rodents, but the results of conventional mutagenicity tests have caused controversy regarding the role of genotoxic mechanisms in the carcinogenesis. Human exposure to OTA from various foods is unavoidable. Therefore, an understanding of OTA-induced renal carcinogenesis is necessary for accurate estimates of the human risk hazard. In the present study, a 13-week exposure of gpt delta rats to OTA at a carcinogenic dose induced karyomegaly and apoptosis at the outer stripe of the outer medulla (OM) of the kidney but failed to affect the reporter gene mutations in DNA extracted from whole kidneys. This site specificity resulting from the kinetics of specific transporters might be responsible for the negative outcome of in vivo mutagenicity. The kidney was then macroscopically divided, based on anatomical characteristics, into the cortex, the OM, and the inner medulla, each of which was histopathologically confirmed. Spi⁻ mutant frequencies (MFs) but not gpt MFs in the OM after a 4-week exposure to OTA were significantly higher than in controls despite the absence of cortical changes. There were also no changes in 8-hydroxydeoxyguanosine levels in kidney DNA. These results strongly suggest the involvement of a genotoxic mechanism, with the exception of oxidative DNA damage in OTA-induced renal carcinogenesis. In addition, the reporter gene mutation assay using DNA from target sites could be a more powerful tool to investigate in vivo genotoxicities.

Oxidative DNA damage and reporter gene mutation in the livers of gpt delta rats given non-genotoxic hepatocarcinogens with cytochrome P450-inducible potency

Previous reports have proposed that reactive oxygen species resulting from induction of cytochrome P450 (CYP) isozymes might be involved in the modes of action of hepatocarcinogens with CYP-inducible potency. In the present study, we investigated 8-hydroxydeoxyguanosine (8-OHdG) levels, in vivo mutagenicity and glutathione S-transferase placental form (GST-P)-positive foci in the livers of gpt delta rats treated with piperonyl butoxide (PBO) or phenobarbital (PhB) for 4 and 13 weeks. Significant elevations in Cyp 1A1 and Cyp 1A2 mRNA levels after PBO treatment, and in Cyp 2B1 mRNA levels after PBO or PhB treatment, appeared together with remarkable hepatomegaly through the experimental period. Time-dependent and statistically significant increases in 8-OHdG levels were observed in the PBO treatment group along with significant increases in proliferating cell nuclear antigen (PCNA)-positive hepatocytes at 4 weeks, while no increase in 8-OHdG levels was found in PhB-treated rats. No changes in mutant frequencies of gpt and red/gam (Spi(-)) genes in liver DNA from PBO- or PhB-treated rats were observed at 4 or 13 weeks. A 13-week exposure to either PBO or PhB did not affect the number and area of GST-P-positive hepatocytes. CYP 1A1 and 1A2 induction may be responsible for elevated levels of 8-OHdG in PBO-treated rats. However, neither GC:TA transversions nor deletion mutations, typically regarded as 8-OHdG-related mutations, were observed in any of the treated rats. We conclude that reactive oxygen species, possibly produced through CYP catalytic pathways, likely induced genomic DNA damage but did not give rise to permanent gene mutation.

Potassium bromate enhances N-ethyl-N-hydroxyethylnitrosamine-induced kidney carcinogenesis only at high doses in Wistar rats: indication of the existence of an enhancement threshold

As susceptibility to carcinogens varies considerably among different strains of experimental animals, evaluation of dose-response relationships for genotoxic carcinogen in different strains is indispensable for risk assessment. Potassium bromate (KBrO(3)) is a genotoxic carcinogen inducing kidney cancers at high doses in male F344 rats, but little is known about its carcinogenic effects in other strains of rats. The purpose of the present study was to determine dose-response relationships for carcinogenic effects of KBrO(3) on N-ethyl-N-hydroxyethylnitrosamine (EHEN)-induced kidney carcinogenesis in male Wistar rats. We found that KBrO(3) showed significant enhancement effects on EHEN-induced kidney carcinogenesis at above 250 ppm but not at doses of 125 ppm and below when evaluated in terms of induction of either preneoplastic lesions or tumors in male Wistar rats. Furthermore, KBrO(3) significantly increased the formation of oxidative DNA damage at doses of 125 and above but not at doses of 30 ppm and below in kidneys. These results demonstrated that low doses of KBrO(3) exert no effects on development of EHEN-initiated kidney lesions and induction of oxidative DNA damage. Taking account of previous similar findings in male F344 rats, it is strongly suggested that a threshold dose exists for enhancement effects of KBrO(3) on kidney carcinogenesis in rats.

Cellular and molecular mechanisms of bromate-induced cytotoxicity in human and rat kidney cells

The mechanisms of bromate (BrO(3)(-))-induced toxicity in Normal Rat Kidney (NRK) and human embryonic kidney 293 (HEK293) cells were investigated. BrO(3)(-) (added as KBrO(3)) induced concentration-dependent decreases in 3-(4, dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide (MTT) staining after 48 h. BrO(3)(-)-induced necrosis based on tandem increases in annexin V and PI staining. Cell cycle analysis demonstrated that BrO(3)(-) also induced G2/M arrest and nuclear fragmentation, prior to alterations in MTT staining or annexin V and PI staining. Immunoblot analysis demonstrated that the G2/M arrest correlated to induction of phosphorylated (p)-p53, p21, cyclin B1 and p-cdc2. Further, BrO(3)(-) induced time-dependent increases in the activity of the mitogen activated protein kinases p38 and ERK1/2. Treatment of cells with the p38 inhibitor SB202190, but not the ERK1/2 inhibitor PD98059, partially reversed BrO(3)(-)-induced G2/M arrest and decreased BrO(3)(-)-induced p-p53, p21 and cyclin B1 expression. In addition, BrO(3)(-) treatment induced reactive oxygen species (ROS) based on increases in CM-H(2)DCFDA fluorescence. The antioxidant ascorbic acid inhibited BrO(3)(-)-induced p38 activation, G2/M arrest, p-p53, p21 and cyclin B1 expression; however, ascorbic acid had no effect on BrO(3)(-)-induced formation of 8-OHdG, a marker of DNA oxidative damage, whose increases preceded cell death by 24h. These data suggest that ROS mediated MAPK activation is involved in the molecular mechanisms of BrO(3)(-)-induced cell cycle arrest, which occurs independently of 8-OH-dG production. The similar mode of action in both NRK and HEK293 cells suggests that the mechanisms of BrO(3)(-)-induced renal cell death are model-independent.

Integration of in vivo genotoxicity and short-term carcinogenicity assays using F344 gpt delta transgenic rats: in vivo mutagenicity of 2,4-diaminotoluene and 2,6-diaminotoluene structural isomers

An important trend in current toxicology is the replacement, reduction, and refinement of the use of experimental animals (the 3R principle). We propose a model in which in vivo genotoxicity and short-term carcinogenicity assays are integrated with F344 gpt delta transgenic rats. Using this model, the genotoxicity of chemicals can be identified in target organs using a shuttle vector λ EG10 that carries reporter genes for mutations; short-term carcinogenicity is determined by the formation of glutathione S-transferase placenta form (GST-P) foci in the liver. To begin validating this system, we examined the genotoxicity and hepatotoxicity of structural isomers of 2,4-diaminotoluene (2,4-DAT) and 2,6-diaminotoluene (2,6-DAT). Although both compounds are genotoxic in the Ames/Salmonella assay, only 2,4-DAT induces tumors in rat livers. Male F344 gpt delta rats were fed diet containing 2,4-DAT at doses of 125, 250, or 500 ppm for 13 weeks or 2,6-DAT at a dose of 500 ppm for the same period. The mutation frequencies of base substitutions, mainly at G:C base pairs, were significantly increased in the livers of 2,4-DAT–treated rats at all three doses. In contrast, virtually no induction of genotoxicity was identified in the kidneys of 2,4-DAT–treated rats or in the livers of 2,6-DAT–treated rats. GST-P–positive foci were detected in the livers of rats treated with 2,4-DAT at a dose of 500 ppm but not in those treated with 2,6-DAT. Integrated genotoxicity and short-term carcinogenicity assays may be useful for early identifying genotoxic and nongenotoxic carcinogens in a reduced number of experimental animals.

Apolipoprotein E/C1 locus variants modify renal cell carcinoma risk

Lipid peroxidation is considered a unifying mechanistic pathway through which known risk factors induce renal cell carcinoma (RCC). We hypothesized that genes selected a priori for their role in lipid peroxidation would modify cancer risk. We genotyped 635 single nucleotide polymorphisms (SNP) in 38candidate genes in 777 Caucasian RCC cases and 1,035 controls enrolled in a large European case-control study. Top candidate SNPs were confirmed among 718 Caucasian cases and 615 controls in a second study in the United States. Two of the three SNPs (rs8106822 and rs405509) that replicated in the U.S. study were within a regulatory region of the APOE promoter. The OR for rs8106822 A>G variant was 1.22(AG) and 1.41(GG) (P(trend) = 0.01) in the European study, 1.05(AG) and 1.51(GG) (P(trend) = 0.03) in the U.S. study, and 1.15(AG) and 1.44(GG) (P(trend) = 0.001) among 1,485 cases and 1,639 controls combined. The rs405509 G>T variant was associated with risk in the European (OR, 0.87(TG); OR, 0.71(TT); P(trend) = 0.02), the U.S. (OR, 0.68(TG); OR, 0.71(TT); P(trend) = 0.02), and both studies combined (OR(TG), 0.79; OR(TT), 0.71; P(trend) = 0.001), as was the G-G haplotype (r(2) = 0.64; P= 4.7 x 10(-4)). This association is biologically plausible as SNP rs405509 was shown to modify protein binding and transcriptional activity of the APOE protein in vitro and is in linkage disequilibrium with key known variants defining the e2, e3, and e4 alleles that modify risk of atherosclerosis, Alzheimer's disease risk, and progression to AIDS. In two large case-control studies, our findings further define a functional region of interest at the APOE locus that increases RCC susceptibility.

Lack of mutagenic and toxic effects of low dose potassium bromate on kidneys in the Big Blue rat

Potassium bromate (KBrO3) has been classified as a genotoxic carcinogen based on positive results in the Ames test, and chromosome aberration and micronucleus tests. The purpose of the present study was to investigate the dose-response relationship for in vivo mutagenic and toxic effects of KBrO3 in the kidneys of Big Blue rats. In experiment 1, male Big Blue rats were divided into 8 groups. KBrO3 was dissolved in tap water and administered to groups 1-8 at concentrations of 0, 0.02, 0.2, 2, 8, 30, 125 and 500 ppm, respectively, for 16 weeks. Experiment 2 was performed to investigate the effects of KBrO3 at the 0.002 ppm dose approximately contained in the tap water on rat kidneys. Ten Big Blue rats were divided into 2 groups and given distilled water and tap water, respectively, for 16 weeks. In experiment 1, treatment with 500 ppm KBrO3 significantly increased the mutant and total mutation frequencies and frequency of GC to TA transversion of the lacI gene in the kidney compared to non-treatment control group, but 125 ppm and lower doses of KBrO3 had no effects. Histopathologically, renal toxic changes were observed in groups administered KBrO3 at 30 ppm or higher in a dose-dependent manner. PCNA positive cell indices in renal tubular cells were significantly increased in the kidney at doses of 125 and 500 ppm, but not at 30 ppm or lower doses, as compared to the control group. Furthermore, 8-hydroxy-2'-deoxyguanosine formation, a marker of oxidative stress, was significantly increased at 500 ppm. In experiment 2, there were no differences in any parameter between the distilled water and tap water groups. These results suggest the existence of no-effect levels for in vivo mutagenic and toxic effects, proliferation stimulus, and oxidative stress of KBrO3 in rat kidneys.

Detection of oxidative DNA damage, cell proliferation and in vivo mutagenicity induced by dicyclanil, a non-genotoxic carcinogen, using gpt delta mice

To ascertain whether measurement of possible contributing factors to carcinogenesis concurrently with the transgenic mutation assay is useful to understand the mode of action underlying tumorigenesis of non-genotoxic carcinogens, male and female gpt delta mice were given dicyclanil (DC), a mouse hepatocarcinogen showing all negative results in various genotoxicity tests, at a carcinogenic dose for 13 weeks. Together with gpt and Spi(-) mutations, thiobarbituric acid-reactive substances (TBARS), 8-hydroxydeoxyguanosine (8-OHdG) and bromodeoxyuridine labeling indices (BrdU-LIs) in the livers were examined. Whereas there were no changes in TBARS levels among the groups, significant increases in 8-OHdG levels and centrilobular hepatocyte hypertrophy were observed in the treated mice of both genders. In contrast, BrdU-LIs and liver weights for the treated females, but not the males were significantly higher than those for the controls. Likewise, the gpt mutant frequencies (MFs) in the treated females were significantly elevated, GC:TA transversion mutations being predominant. No significant alterations were found in the gpt MFs of the males and the Spi(-) MFs of both sexes. The results for the transgenic mutation assays were consistent with DC carcinogenicity in terms of the sex specificity for females. Considering that 8-OHdG induces GC:TA transversion mutations by mispairing with A bases, it is likely that cells with high proliferation rates and a large amounts of 8-OHdG come to harbor mutations at high incidence. This is the first report demonstrating DC-induced genotoxicity, the results implying that examination of carcinogenic parameters concomitantly with reporter gene mutation assays is able to provide crucial information to comprehend the underlying mechanisms of so-called non-genotoxic carcinogenicity.