Tests for genotoxicity and mutagenicity of furan and its metabolite cis-2-butene-1,4-dial in L5178Y tk+/− mouse lymphoma cells

Appearance of sex-determining region Y-box 9 (SOX9)- and glutathione S-transferase placental form (GST-P)-positive hepatocytes as possible carcinogenic events in the early stage of furan-induced hepatocarcinogenesis

Furan, the basic skeleton of various flavoring agents, induces cholangiocellular tumors with higher incidences in the caudate lobe and hepatocellular tumors without the lobe specificity in rats, but the mechanism is unclear. We investigated the lobe distribution of possible carcinogenic events. Furan caused proliferation/infiltration of oval and inflammatory cells prominently in the caudate lobe as early as 4 weeks and cholangiofibrosis in this lobe at 8 weeks. In vivo mutagenicity assays using DNA extracted from the caudate or left lateral lobe of male gpt delta rats, the reporter gene-transgenic rats, treated with 8 mg/kg furan for 4 or 8 weeks showed negative outcomes. The distribution of glutathione S-transferase placental form (GST-P)-positive or sex-determining region Y-box 9 (SOX9)-positive hepatocytes was examined. Significant increases in the number of GST-P-positive hepatocytes were observed in all lobes of furan-treated rats at 8 weeks. By contrast, SOX9-positive hepatocytes, liver injury-inducible progenitor cells, were also found in all lobes of treated rats, the incidences of which were by far the highest in the caudate lobe. In addition, some of these hepatocytes also co-expressed delta like 1 homolog (DLK1), a hepatoblast marker, particularly in areas with a predominant presence of inflammatory cells. Overall, furan induced liver injury, leading to the appearance of SOX9-positive hepatocytes, some of which were subjected to dedifferentiation in the inflammatory microenvironment of a cholangiocarcinoma-prone lobe. Thus, the appearance of SOX9-positive hepatocytes together with GST-P-positive hepatocytes could be initial events in furan-induced hepatocarcinogenesis via non-genotoxic mechanisms.

Adverse outcome pathway exploration of furan-induced liver fibrosis in rats: Genotoxicity pathway or oxidative stress pathway through CYP2E1 activation?

Furan is a widespread endogenous contaminant in heat-processed foods that can accumulate rapidly in the food chain and has been widely detected in foods, such as wheat, bread, coffee, canned meat products, and baby food. Dietary exposure to this chemical may bring health risk. Furan is classified as a possible category 2B human carcinogen by the International Agency for Research on Cancer, with the liver as its primary target organ. Hepatic fibrosis is the most important nontumoral harmful effect of furan and also an important event in the carcinogenesis of furan. Although the specific mechanism of furan-induced liver fibrosis is still unclear, it may involve oxidative stress and genetic toxicity, in which the activation of cytochrome P450 2E1 (CYP2E1) may be the key event. Thus, we conducted a study using an integrating multi-endpoint genotoxicity platform in 120-day in vivo subchronic toxicity test in rats. Results showed that the rats with activated CYP2E1 exhibited DNA double-strand breaks in D4, gene mutations in D60, and increased expression of reactive oxygen species and nuclear factor erythroid 2-related factor 2 in D120. Necrosis, apoptosis, hepatic stellate cell activation, and fibrosis also occurred in the liver, suggesting that furan can independently affect liver fibrosis through oxidative stress and genotoxicity pathways. Point of Departure (PoD) was obtained by benchmark-dose (BMD) method to establish health-based guidance values. The human equivalent dose of PoD derived from BMDL05 was 2.26 μg/kg bw/d. The findings laid a foundation for the safety evaluation and risk assessment of furan and provided data for the further construction and improvement of the adverse outcome pathway network in liver fibrosis.

Early microRNA responses in rodent liver mediated by furan exposure establish dose thresholds for later adverse outcomes

To reduce reliance on long-term in vivo studies, short-term data linking early molecular-based measurements to later adverse health effects is needed. Although transcriptional-based benchmark dose (BMDT) modeling has been used to estimate potencies and stratify chemicals based on potential to induce later-life effects, dose-responsive epigenetic alterations have not been routinely considered. Here, we evaluated the utility of microRNA (miRNA) profiling in mouse liver and blood, as well as in mouse primary hepatocytes in vitro, to indicate mechanisms of liver perturbation due to short-term exposure of the known rodent liver hepatotoxicant and carcinogen, furan. Benchmark dose modeling of miRNA measurements (BMDmiR) were compared to the referent transcriptional (BMDT) and apical (BMDA) estimates. These analyses indicate a robust dose response for 34 miRNAs to furan and involvement of p53-linked pathways in furan-mediated hepatotoxicity, supporting mRNA and apical measurements. Liver-sourced miRNAs were also altered in the blood and primary hepatocytes. Overall, these results indicate mechanistic involvement of miRNA in furan carcinogenicity and provide evidence of their potential utility as accessible biomarkers of exposure and disease.

Review on Furan as a Food Processing Contaminant: Identifying Research Progress and Technical Challenges for Future Research

A wide range of food processing contaminants (FPCs) are usually formed while thermal processing of food products. Furan is a highly volatile compound among FPCs and could be formed in a variety of thermally processed foods. Therefore, identification of possible reasons of furan occurrence in different thermally processed foods, identification of the most consequential sources of furan exposure, factors impacting its formation, and its detection by specific analytical approaches are necessary to indicate gaps and challenges for future research findings. Furthermore, controlling furan formation in processed foods on a factory scale is also challenging, and research advancements are still ongoing in this context. Meanwhile, understanding adverse effects of furan on human health on a molecular level is necessary to gain insights into human risk assessment.

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.

Glycidamide and cis-2-butene-1,4-dial (BDA) as potential carcinogens and promoters of liver cancer - An in vitro study

Acrylamide and furan are environmental and food contaminants that are metabolized by cytochrome P450 2E1 (CYP2E1), giving rise to glycidamide and cis-2-butene-1,4-dial (BDA) metabolites, respectively. Both glycidamide and BDA are electrophilic species that react with nucleophilic groups, being able to introduce mutations in DNA and perform epigenetic remodeling. However, whereas these carcinogens are primarily metabolized in the liver, the carcinogenic potential of acrylamide and furan in this organ is still controversial, based on findings from experimental animal studies. With the ultimate goal of providing further insights into this issue, we explored in vitro, using a hepatocyte cell line and a hepatocellular carcinoma cell line, the putative effect of these metabolites as carcinogens and cancer promoters. Molecular alterations were investigated in cells that survive glycidamide and BDA toxicity. We observed that those cells express CD133 stemness marker, present a high proliferative capacity and display an adjusted expression profile of genes encoding enzymes involved in oxidative stress control, such as GCL-C, GSTP1, GSTA3 and CAT. These molecular changes seem to be underlined, at least in part, by epigenetic remodeling involving histone deacetylases (HDACs). Although more studies are needed, here we present more insights towards the carcinogenic capacity of glycidamide and BDA and also point out their effect in favoring hepatocellular carcinoma progression.

Oxidative Stress, Mutations and Chromosomal Aberrations Induced by In Vitro and In Vivo Exposure to Furan

Furan is a volatile compound that is formed in foods during thermal processing. It is classified as a possible human carcinogen by international authorities based on sufficient evidence of carcinogenicity from studies in experimental animals. Although a vast number of studies both in vitro and in vivo have been performed to investigate furan genotoxicity, the results are inconsistent, and its carcinogenic mode of action remains to be clarified. Here, we address the mutagenic and clastogenic activity of furan and its prime reactive metabolite cis-2 butene-1,4-dial (BDA) in mammalian cells in culture and in mouse animal models in a search for DNA lesions responsible of these effects. To this aim, Fanconi anemia-derived human cell lines defective in the repair of DNA inter-strand crosslinks (ICLs) and Ogg1-/- mice defective in the removal of 8-hydroxyguanine from DNA, were used. We show that both furan and BDA present a weak (if any) mutagenic activity but are clear inducers of clastogenic damage. ICLs are strongly indicated as key lesions for chromosomal damage whereas oxidized base lesions are unlikely to play a critical role.

Comet assay: a versatile but complex tool in genotoxicity testing

The comet assay is a versatile method for measuring DNA strand breaks in individual cells. It can also be applied to cells isolated from treated animals. In this review, we highlight advantages and limitations of this in vivo comet assay in a regulatory context. Modified versions of the standard protocol detect oxidized DNA bases and may be used to reveal sites of DNA base loss, DNA interstrand crosslinks, and the extent of DNA damage induced indirectly by reactive oxygen species elicited by chemical-induced oxidative stress. The assay is, however, at best semi-quantitative, and we discuss possible approaches to improving DNA damage quantitation and highlight the necessity of optimizing protocol standardization to enhance the comparability of results between laboratories. As a genotoxicity test in vivo, the in vivo comet assay has the advantage over the better established micronucleus erythrocyte test that it can be applied to any organ, including those that are specific targets of chemical carcinogens or those that are the first sites of contact of ingested or inhaled mutagens. We illustrate this by examples of its use in risk assessment for the food contaminants ochratoxin and furan. We suggest that improved quantitation is required to reveal the full potential of the comet assay and enhance its role in the battery of in vivo approaches to characterize the mechanisms of toxicity and carcinogenicity of chemicals and to aid the determination of safe human exposure limits.

Furan Induced Characteristic Glutathione S-Transferase Placental Form-Positive Foci in Terms of Cell Kinetics and Gene Expression

Glutathione S-transferase placental form-positive (GST-P⁺) foci are markers of preneoplastic lesions in rat hepatocarcinogenesis. Our previous studies using reporter gene transgenic rats showed that furan, a hepatocarcinogen in rodents, rapidly induces the formation of GST-P⁺ foci after short exposure without reporter gene mutation. We hypothesized that GST-P⁺ foci induced by furan may have biological characteristics different from those induced by diethylnitrosamine (DEN), a genotoxic hepatocarcinogen. Accordingly, we compared the cell kinetics of GST-P⁺ foci after cessation of DEN treatment and performed comprehensive gene expression in DEN- or furan-induced GST-P⁺ foci. The number and area of DEN-induced GST-P⁺ foci were increased after cessation of treatment, whereas furan decreased these parameters. Size distribution analysis showed that large furan-induced GST-P⁺ foci disappeared after cessation of treatment. Hierarchical cluster analysis showed that all samples from GST-P⁺ foci induced by furan were separated from those induced by DEN. SOX9 expression was upregulated in furan-induced GST-P⁺ foci and was detected by immunohistochemistry in large furan-induced GST-P⁺ foci. Our results indicated that large furan-induced GST-P⁺ foci were quite different from DEN-induced GST-P⁺ foci at the molecular and cellular levels. And one of the properties of disappearing large GST-P⁺ foci were characterized by inclusion of hepatocytes expressing SOX9.

Exploration of the cellular effects of the high-dose, long-term exposure to coffee roasting product furan and its by-product cis-2-butene-1,4-dial on human and rat hepatocytes

Coffee is the most popular hot beverage and caffeine is the most used psychoactive drug in the world. Roasting of coffee beans leads to the generation of minute quantities of undesirable compounds, such as furan. It is now thought that the toxicity of furan derives from its processing by CYP450 family of detoxifying enzymes, leading to the formation of cis-2-butene-1,4-dial (BDA). BDA has known cytotoxicity capacities, binding to proteins, nucleic acids, and glutathione (GSH). BDA also appears to mediate furan's toxic effects, since the inhibition of CYP450 family impedes the aforementioned toxicological effects of furan. There are some studies performed on furan's toxicity, but very few on BDA. Furthermore, the doses used in these studies appear to be fairly high when compared with the expected dosage one could be exposed to in a standard day. As such, to understand if furan and BDA could have toxic effects using more realistic doses and longer time frames, human and rat hepatocytes were exposed to furan or BDA for up to 96 h, and several biochemical parameters were assessed. We report here that human hepatocytes were more sensitive than rat's, in particular to furan, for we show a decrease in MTT reduction, ATP levels and increase in carbonyl formation and 8-OHdG accumulation in the longer time points. BDA was mostly ineffective, which we attribute to a low import rate into the cells. In conclusion, we show that there is potential for harm from furan in high doses, which should be carefully addressed.

Risks for public health related to the presence of furan and methylfurans in food

The European Commission asked EFSA for a scientific evaluation on the risk to human health of the presence of furan and methylfurans (2-methylfuran, 3-methylfuran and 2,5-dimethylfuran) in food. They are formed in foods during thermal processing and can co-occur. Furans are produced from several precursors such as ascorbic acid, amino acids, carbohydrates, unsaturated fatty acids and carotenoids, and are found in a variety of foods including coffee and canned and jarred foods. Regarding furan occurrence, 17,056 analytical results were used in the evaluation. No occurrence data were received on methylfurans. The highest exposures to furan were estimated for infants, mainly from ready-to-eat meals. Grains and grain-based products contribute most for toddlers, other children and adolescents. In adults, elderly and very elderly, coffee is the main contributor to dietary exposure. Furan is absorbed from the gastrointestinal tract and is found in highest amounts in the liver. It has a short half-life and is metabolised by cytochrome P450 2E1 (CYP2E1) to the reactive metabolite, cis-but-2-ene-1,4-dialdehyde (BDA). BDA can bind covalently to amino acids, proteins and DNA. Furan is hepatotoxic in rats and mice with cholangiofibrosis in rats and hepatocellular adenomas/carcinomas in mice being the most prominent effects. There is limited evidence of chromosomal damage in vivo and a lack of understanding of the underlying mechanism. Clear evidence for indirect mechanisms involved in carcinogenesis include oxidative stress, gene expression alterations, epigenetic changes, inflammation and increased cell proliferation. The CONTAM Panel used a margin of exposure (MOE) approach for the risk characterisation using as a reference point a benchmark dose lower confidence limit for a benchmark response of 10% of 0.064 mg/kg body weight (bw) per day for the incidence of cholangiofibrosis in the rat. The calculated MOEs indicate a health concern. This conclusion was supported by the calculated MOEs for the neoplastic effects.

Low dose assessment of the carcinogenicity of furan in male F344/N Nctr rats in a 2-year gavage study

Furan is a volatile organic chemical that is a contaminant in many common foods. Furan is hepatocarcinogenic in mice and rats; however, the risk to humans from dietary exposure to furan cannot be estimated accurately because the lowest tested dose of furan in a 2-year bioassay in rats gave nearly a 100% incidence of cholangiocarcinoma. To provide bioassay data that can be used in preparing risk assessments, the carcinogenicity of furan was determined in male F344/N Nctr rats administered 0, 0.02, 0.044, 0.092, 0.2, 0.44, 0.92, and 2 mg furan/kg body weight (BW) by gavage 5 days/week for 2 years. Exposure to furan was associated with the development of malignant mesothelioma on membranes surrounding the epididymis and on the testicular tunics, with the increase being significant at 2 mg furan/kg BW. There was also a dose-related increase in the incidence of mononuclear cell leukemia, with the increase in incidence being significant at 0.092, 0.2, 0.92, and 2 mg furan/kg BW. Dose-related non-neoplastic liver lesions included cholangiofibrosis, mixed cell foci, basophilic foci, biliary tract hyperplasia, oval cell hyperplasia, regenerative hyperplasia, and cytoplasmic vacuolization. The most sensitive non-neoplastic lesion was cholangiofibrosis, the frequency of which increased significantly at 0.2 mg furan/kg BW.

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.

Application of the TGx-28.65 transcriptomic biomarker to classify genotoxic and non-genotoxic chemicals in human TK6 cells in the presence of rat liver S9

In vitro transcriptional signatures that predict toxicities can facilitate chemical screening. We previously developed a transcriptomic biomarker (known as TGx-28.65) for classifying agents as genotoxic (DNA damaging) and non-genotoxic in human lymphoblastoid TK6 cells. Because TK6 cells do not express cytochrome P450s, we confirmed accurate classification by the biomarker in cells co-exposed to 1% 5,6 benzoflavone/phenobarbital-induced rat liver S9 for metabolic activation. However, chemicals may require different types of S9 for activation. Here we investigated the response of TK6 cells to higher percentages of Aroclor-, benzoflavone/phenobarbital-, or ethanol-induced rat liver S9 to expand TGx-28.65 biomarker applicability. Transcriptional profiles were derived 3 to 4 hr following a 4 hr co-exposure of TK6 cells to test chemicals and S9. Preliminary studies established that 10% Aroclor- and 5% ethanol-induced S9 alone did not induce the TGx-28.65 biomarker genes. Seven genotoxic and two non-genotoxic chemicals (and concurrent solvent and positive controls) were then tested with one of the S9s (selected based on cell survival and micronucleus induction). Relative survival and micronucleus frequency was assessed by flow cytometry in cells 20 hr post-exposure. Genotoxic/non-genotoxic chemicals were accurately classified using the different S9s. One technical replicate of cells co-treated with dexamethasone and 10% Aroclor-induced S9 was falsely classified as genotoxic, suggesting caution in using high S9 concentrations. Even low concentrations of genotoxic chemicals (those not causing cytotoxicity) were correctly classified, demonstrating that TGx-28.65 is a sensitive biomarker of genotoxicity. A meta-analysis of datasets from 13 chemicals supports that different S9s can be used in TK6 cells, without impairing classification using the TGx-28.65 biomarker.

Mutagenicity of furan in female Big Blue B6C3F1 mice

Furan is an abundant food and environmental contaminant that is a potent liver carcinogen in rodent models. To determine if furan is genotoxic in vivo, female B6C3F1 Big Blue transgenic mice were treated with 15 mg/kg bw furan by gavage 5 days a week for 6 weeks, or once weekly for 3 weeks. Liver cII transgene mutation-frequency and mutation spectra were determined. Furan did not increase the mutation frequency under either treatment condition. In the 6-week treatment regimen, there was a change in the cII transgene mutation-spectrum, with the fraction of GC to AT transitions significantly reduced. The only other significant change was an increase in GC to CG transversions; these represented a minor contribution to the overall mutation spectrum. A much larger furan-dependent shift was observed in the 3-week study. There was a significant increase in transversion mutations, predominantly GC to TA transversions as well as smaller non-significant changes in GC to CG and AT to TA transversions. To determine if these mutations were caused by cis-2-butene-1,4-dial (BDA), a reactive metabolite of furan, the mutagenic activity and the mutation spectrum of BDA was determined in vitro, in Big Blue mouse embryonic fibroblasts. This compound did not increase the cII gene mutation-frequency but caused a substantial increase in AT to CG transversions. This increase, however, lost statistical significance when adjusted for multiple comparisons. Together, these findings suggest that BDA may not be directly responsible for the in-vivo effects of furan on mutational spectra. Histopathological analysis of livers from furan-treated mice revealed that furan induced multifocal, hepatocellular necrosis admixed with reactive leukocytes and pigment-laden Kupffer cells, enhanced oval-cell hyperplasia, and increased hepatocyte mitoses, some of which were atypical. An indirect mechanism of genotoxicity is proposed in which chronic toxicity followed by inflammation and secondary cell proliferation triggers cancer development in furan-exposed rodents.

Comparative metabolism of furan in rodent and human cryopreserved hepatocytes

Furan is a liver toxicant and carcinogen in rodents. Although humans are most likely exposed to furan through a variety of sources, the effect of furan exposure on human health is still unknown. In rodents, furan requires metabolism to exert its toxic effects. The initial product of the cytochrome P450 2E1-catalyzed oxidation is a reactive α,β-unsaturated dialdehyde, cis-2-butene-1,4-dial (BDA). BDA is toxic and mutagenic and consequently is considered responsible for the toxic effects of furan. The urinary metabolites of furan in rats are derived from the reaction of BDA with cellular nucleophiles, and precursors to these metabolites are detected in furan-exposed hepatocytes. Many of these precursors are 2-(S-glutathionyl)butanedial-amine cross-links in which the amines are amino acids and polyamines. Because these metabolites are derived from the reaction of BDA with cellular nucleophiles, their levels are a measure of the internal dose of this reactive metabolite. To compare the ability of human hepatocytes to convert furan to the same metabolites as rodent hepatocytes, furan was incubated with cryopreserved human and rodent hepatocytes. A semiquantitative liquid chromatography with tandem mass spectrometry assay was developed for a number of the previously characterized furan metabolites. Qualitative and semiquantitative analysis of the metabolites demonstrated that furan is metabolized in a similar manner in all three species. These results indicate that humans may be susceptible to the toxic effects of furan.

Bromodeoxyuridine (BrdU) treatment to measure hepatocellular proliferation does not mask furan-induced gene expression changes in mouse liver

Bromodeoxyuridine (BrdU) is a synthetic nucleoside used to detect cellular proliferation. BrdU incorporates in the place of thymine but pairs with guanine, thereby increasing the risk of transition mutations in dividing cells. Given its mutagenicity, standard practice is to use a second cohort of animals for parallel toxicogenomics studies; however, the impact of BrdU on global gene expression is unknown. To test this, we performed a case study to determine whether the molecular mode of action of furan, a liver carcinogen, could be detected in BrdU-treated samples. We measure global hepatic gene expression using Agilent DNA microarrays in female B6C3F1 mice that were sub-chronically exposed to 0, 1, 4, or 8mg/kg bodyweight (bw) per day furan either in the presence (+BrdU) or absence (-BrdU) of BrdU. Exposure to 0.02% BrdU in drinking water for five days resulted in minimal gene expression changes. A comparison of +BrdU versus -BrdU control mice revealed only 11 probes with fold change≥1.5 and false discovery rate (FDR) corrected p≤0.05. The same comparison in the high dose group yielded only 3 differentially expressed probes. Differentially expressed gene lists generated for furan-treated versus control mice and were compared for the -BrdU and +BrdU groups. The high dose of furan had 452 shared probes and 27 and 90 unique probes for -BrdU and +BrdU groups, respectively. These differences did not impact hierarchical clustering. Further, they did not impair detection of the previously reported furan mode of action, which was well represented in the BrdU-treated samples. Taken together, we demonstrate that BrdU treatment does not mask important furan-induced transcriptional changes. We suggest that BrdU-treated mice could be used for toxicogenomic analysis, which would generally halve the number of rodents required for toxicogenomics studies. However, we also recommend that this type of case study be repeated for other chemicals before the use of BrdU-treated animals in omics studies becomes common practice.

Dose- and time-dependent epigenetic changes in the livers of Fisher 344 rats exposed to furan

The presence of furan in common cooked foods along with evidence from experimental studies that lifetime exposure to furan causes liver tumors in rats and mice has caused concern to regulatory public health agencies worldwide; however, the mechanisms of the furan-induced hepatocarcinogenicity remain unclear. The goal of the present study was to investigate whether or not long-term exposure to furan causes epigenetic alterations in rat liver. Treating of male Fisher 344 rats by gavage 5 days per week with 0, 0.92, 2.0, or 4.4 mg furan/kg body weight (bw)/day resulted in dose- and time-dependent epigenetic changes consisting of alterations in DNA methylation and histone lysine methylation and acetylation, altered expression of chromatin modifying genes, and gene-specific methylation. Specifically, exposure to furan at doses 0.92, 2.0, or 4.4 mg furan/kg bw/day caused global DNA demethylation after 360 days of treatment. There was also a sustained decrease in the levels of histone H3 lysine 9 and H4 lysine 20 trimethylation after 180 and 360 days of furan exposure, and a marked reduction of histone H3 lysine 9 and H3 lysine 56 acetylation after 360 days at 4.4 mg/kg bw/day. These histone modification changes were accompanied by a reduced expression of Suv39h1, Prdm2, and Suv4-20h2 histone methyltransferases and Ep300 and Kat2a histone acetyltransferases. Additionally, furan at 2.0 and 4.4 mg/kg bw/day induced hypermethylation-dependent down-regulation of the Rassf1a gene in the livers after 180 and 360 days. These findings indicate possible involvement of dose- and time-dependent epigenetic modifications in the furan hepatotoxicity and carcinogenicity.

ACB-PCR measurement of spontaneous and furan-induced H-ras codon 61 CAA to CTA and CAA to AAA mutation in B6C3F1 mouse liver

Furan is a rodent liver carcinogen, but the mode of action for furan hepatocarcinogenicity is unclear. H-ras codon 61 mutations have been detected in spontaneous liver tumors of B6C3F1 mice, and the fraction of liver tumors carrying H-ras codon 61 CAA to AAA mutation increased in furan-treated mice. Allele-specific competitive blocker PCR (ACB-PCR) has been used previously to quantify early, carcinogen-induced increases in tumor-associated mutations. The present pilot study investigated whether furan drives clonal expansion of pre-existing H-ras mutant cells in B6C3F1 mouse liver. H-ras codon 61 CAA to CTA and CAA to AAA mutations were measured in DNA isolated from liver tissue of female mice treated with 0, 1, 2, 4, or 8 mg furan/kg body weight, five days per week for three weeks, using five mice per treatment group. Spontaneous levels of mutation were low, with two of five control mice having an H-ras codon 61 CTA or AAA mutant fraction (MF) greater than 10(-5) . Several furan-treated mice had H-ras codon 61 AAA or CTA MFs greater than those measured in control mice and lower bound estimates of induced MF were calculated. However, no statistically-significant differences were observed between treatment groups. Therefore, while sustained exposure to furan is carcinogenic, at the early stage of carcinogenesis examined in this study (three weeks), there was not a significant expansion of H-ras mutant cells.

Reactive metabolites in the biotransformation of molecules containing a furan ring

Many xenobiotics containing a furan ring are toxic and/or carcinogenic. The harmful effects of these compounds require furan ring oxidation. This reaction generates an electrophilic intermediate. Depending on the furan ring substituents, the intermediate is either an epoxide or a cis-enedione with more ring substitution favoring epoxide formation. Either intermediate reacts with cellular nucleophiles such as protein or DNA to trigger toxicities. The reactivity of the metabolite determines which cellular nucleophiles are targeted. The toxicity of a particular furan is also influenced by the presence of competing metabolic pathways or efficient detoxification routes. GSH plays an important role in modulating the harmful effects of this class of compound by reacting with the reactive metabolite. However, this may not represent a detoxification step in all cases.

In-vitro toxicological and proteomic analysis of furan fatty acids which are oxidative metabolites of conjugated linoleic acids

Furan fatty acids (furan-FA) are oxidative products of conjugated linoleic acids (CLA) and may therefore be ingested when CLA-containing food or food-additives are consumed. Due to the presence of a furan ring structure the question arises whether furan-FA may have toxic properties on enterocytes and liver cells. Here we show that furan-FA neither have toxic effects in human colon cancer cell line Caco-2 nor in human hepatoma cell line HepG2 at concentrations that could be relevant for humans. At concentrations up to 100 μM, all tested furan-FA isomers showed no pronounced cytotoxicity and did not affect cellular proliferation or apoptosis up to concentrations of 500 μM. In addition, furan-FA was neither genotoxic in the micronucleus test using Chinese hamster lung fibroblasts (V79) nor in the Ames test independent of the presence or absence of rat liver homogenate for enzymatic activation of the furan ring structure. A proteomic approach revealed that 48 proteins were differentially expressed when Caco-2 cells were incubated with up to 1 mM of 10,13-epoxy-10,12-octadecadienoic acid (10,12-furan-FA). Three of the 30 proteins that could be identified by MALDI-TOF analysis were upregulated and were associated with lipid droplet biogenesis. The remaining 27 proteins were downregulated and were considered to be associated with general cellular processes such as DNA replication and transcription, protein biosynthesis and protein processing, lipid and energy metabolism. From the proteomic data we conclude that furan-FA is predominantly stored in lipid droplets thereby downregulating cellular metabolic activity and driving the cells into a state of rest.

Furan carcinogenicity: DNA binding and genotoxicity of furan in rats in vivo

SCOPE

Furan is a potent hepatotoxicant and liver carcinogen in rodents. However, short-term tests for genotoxicity of furan are inconclusive. The aim of this study was to assess the potential of furan to covalently bind to DNA, and to assess furan genotoxicity in rats in vivo.

MATERIALS AND METHODS

Accelerator mass spectrometry was used to determine the (14) C-content in DNA following administration of [3,4-(14) C]-furan (0.1 and 2.0 mg/kg bw) to F344 rats. DNA damage, micronuclei, chromosomal aberrations, and sister chromatid exchanges were analyzed in F344 rats treated with furan for up to 28 days.

CONCLUSION

The (14) C-content in liver DNA was significantly increased in a dose-dependent manner, with mean concentrations of 7.9 ± 3.5 amol (14) C/μg DNA and 153.3 ± 100.2 amol (14) C/μg DNA, corresponding to 16.5 ± 7.4 and 325.2 ± 212.7 adducts/10(9) nucleotides at 0.1 and 2.0 mg/kg bw, respectively. There was no evidence for genotoxicity of furan in peripheral blood and bone marrow cells. However, a dose-related increase in the incidence of chromosomal aberrations in rat splenocytes and some indication of DNA damage in liver were observed. Collectively, results from this study indicate that furan may operate-at least in part-by a genotoxic mode of action.

Furan in heat-treated foods: formation, exposure, toxicity, and aspects of risk assessment

Furan is formed in a variety of heat-treated foods through thermal degradation of natural food constituents. Relatively high levels of furan contamination are found in ground roasted coffee, instant coffee, and processed baby foods. European exposure estimates suggest that mean dietary exposure to furan may be as high as 1.23 and 1.01 μg/kg bw/day for adults and 3- to 12-month-old infants, respectively. Furan is a potent hepatotoxin and hepatocarcinogen in rodents, causing hepatocellular adenomas and carcinomas in rats and mice, and high incidences of cholangiocarcinomas in rats at doses ≥ 2 mg/kg bw. There is therefore a relatively low margin of exposure between estimated human exposure and doses that cause a high tumor incidence in rodents. Since a genotoxic mode of action cannot be excluded for furan-induced tumor formation, the present exposures may indicate a risk to human health and need for mitigation. This review summarizes the current knowledge on mechanisms of furan formation in food, human dietary exposure to furan, and furan toxicity, and highlights the need to establish the risk resulting from the genotoxic and carcinogenic properties of furan at doses lower than 2 mg/kg bw.

Trapping of cis-2-butene-1,4-dial to measure furan metabolism in human liver microsomes by cytochrome P450 enzymes

Furan is a liver toxicant and carcinogen in rodents. It is classified as a possible human carcinogen, but the human health effects of furan exposure remain unknown. The oxidation of furan by cytochrome P450 (P450) enzymes is necessary for furan toxicity. The product of this reaction is the reactive α,β-unsaturated dialdehyde, cis-2-butene-1,4-dial (BDA). To determine whether human liver microsomes metabolize furan to BDA, a liquid chromatography/tandem mass spectrometry method was developed to detect and quantify BDA by trapping this reactive metabolite with N-acetyl-l-cysteine (NAC) and N-acetyl-l-lysine (NAL). Reaction of NAC and NAL with BDA generates N-acetyl-S-[1-(5-acetylamino-5-carboxypentyl)-1H-pyrrol-3-yl]-l-cysteine (NAC-BDA-NAL). Formation of NAC-BDA-NAL was quantified in 21 different human liver microsomal preparations. The levels of metabolism were comparable to that observed in F-344 rat and B6C3F1 mouse liver microsomes, two species known to be sensitive to furan-induced toxicity. Studies with recombinant human liver P450s indicated that CYP2E1 is the most active human liver furan oxidase. The activity of CYP2E1 as measured by p-nitrophenol hydroxylase activity was correlated to the extent of NAC-BDA-NAL formation in human liver microsomes. The formation of NAC-BDA-NAL was blocked by CYP2E1 inhibitors but not other P450 inhibitors. These results suggest that humans are capable of oxidizing furan to its toxic metabolite, BDA, at rates comparable to those of species sensitive to furan exposure. Therefore, humans may be susceptible to furan's toxic effects.

A review of the electrophilic reaction chemistry involved in covalent DNA binding

The need to assess the ability of a chemical to act as a mutagen or a genotoxic carcinogen (collectively termed genotoxicity) is one of the primary requirements in regulatory toxicology. Several pieces of legislation have led to an increased interest in the use of in silico methods, specifically the formation of chemical categories for the assessment of toxicological endpoints. A key step in the development of chemical categories for genotoxicity is defining the organic chemistry associated with the formation of a covalent bond between DNA and an exogenous chemical. This organic chemistry is typically defined as structural alerts. To this end, this article has reviewed the literature defining the structural alerts associated with covalent DNA binding. Importantly, this review article also details the mechanistic organic chemistry associated with each of the structural alerts. This information is extremely important in terms of meeting regulatory requirements for the acceptance of the chemical category approach. The structural alerts and associated mechanistic chemistry have been incorporated into the Organisation for Economic Co-operation and Development (OECD) (Q)SAR Application Toolbox.

Low doses of the carcinogen furan alter cell cycle and apoptosis gene expression in rat liver independent of DNA methylation

BACKGROUND

Evidence of potent rodent carcinogenicity via an unclear mechanism suggests that furan in various foods [leading to an intake of up to 3.5 microg/kg body weight (bw)/day] may present a potential risk to human health.

OBJECTIVES

We tested the hypothesis that altered expression of genes related to cell cycle control, apoptosis, and DNA damage may contribute to the carcinogenicity of furan in rodents. In addition, we investigated the reversibility of such changes and the potential role of epigenetic mechanisms in response to furan doses that approach the maximum estimated dietary intake in humans.

METHODS

The mRNA expression profiles of genes related to cell cycle, apoptosis, and DNA damage in rat liver treated with furan concentrations of 0.1 and 2 mg/kg bw were measured by quantitative polymerase chain reaction (PCR) arrays. We assessed epigenetic changes by analysis of global and gene-specific DNA methylation [methylation-specific PCR, combined bisulfite restriction analysis (COBRA), and methylated DNA immunoprecipitation chip] and microRNA (miRNA) analyses.

RESULTS

The expression profiles of apoptosis-related and cell-cycle-related genes were unchanged after 5 days of treatment, although we observed a statistically significant change in the expression of genes related to cell cycle control and apoptosis, but not DNA damage, after 4 weeks of treatment. These changes were reversed after an off-dose period of 2 weeks. None of the gene expression changes was associated with a change in DNA methylation, although we detected minor changes in the miRNA expression profile (5 miRNA alterations out of 349 measured) that may have contributed to modification of gene expression in some cases.

CONCLUSION

Nongenotoxic changes in gene expression may contribute to the carcinogenicity of furan in rodents. These findings highlight the need for a more comprehensive risk assessment of furan exposure in humans.

Hepatobiliary toxicity of furan: identification of furan metabolites in bile of male f344/n rats

Furan, which occurs in a wide variety of heat-treated foods, is a potent hepatotoxicant and liver carcinogen in rodents. In a 2-year bioassay, furan caused hepatocellular adenomas and carcinomas in mice and rats but also high incidences of bile duct tumors in rats. Furan is bioactivated by cytochrome P450 enzymes to cis-2-butene-1,4-dial, an α,β-unsaturated dialdehyde, which readily reacts with tissue nucleophiles. The objective of this study was to structurally characterize furan metabolites excreted with bile to better understand the potential role of reactive furan intermediates in the biliary toxicity of furan. Bile duct-cannulated F344/N rats (n = 3) were administered a single oral dose of 5 mg/kg b.wt. [(12)C(4)]furan or stable isotope-labeled [3,4-(13)C]furan, and bile samples collected at 30-min intervals for 4 h were analyzed by liquid chromatography-tandem mass spectrometry. A total of eight furan metabolites derived from reaction of cis-2-butene-1,4-dial with GSH and/or amino acids and subsequent enzymatic degradation were detected in bile. The main metabolite was a cyclic monoglutathione conjugate of cis-2-butene-1,4-dial, which was previously detected in urine of furan-treated rats. Furthermore, a N-acetylcysteine-N-acetyllysine conjugate, previously observed in rat urine, and a cysteinylglycine-glutathione conjugate were identified as major metabolites. These data suggest that degraded protein adducts are in vivo metabolites of furan, consistent with the hypothesis that cytotoxicity mediated through binding of cis-2-butene-1,4-dial to critical target proteins is likely to play a key role in furan toxicity and carcinogenicity.

Toxic and genotoxic effects of oral administration of furan in mouse liver

In this study, the effects induced in mouse liver by repeated oral exposure to furan were investigated. To this aim, the compound was given for 28 days by daily gavage to male B6C3F1 mice at 2, 4, 8 and 15 mg/kg body weight (b.w.)/day. Twenty-four hours after last administration, animals were sacrificed, liver was excised and the following parameters were evaluated: histological alterations, apoptosis, cell proliferation, polyploidy, overall DNA methylation, gene expression and DNA damage by the immunofluorescence detection of foci of phosphorylated histone H2AX (gamma-H2AX) and by alkaline comet assays, using both standard and modified protocols for the detection of DNA cross links. Liver DNA damage by comet assays was also evaluated in mice receiving furan as a single acute oral dose (15, 100 or 250 mg/kg b.w.). Microscopic analysis of liver sections indicated that repeated oral administration of furan was moderately toxic, producing mild histological alterations with necrotic figures, apoptosis and limited regenerative cell proliferation. The flow cytometric analysis of DNA content in single-cell suspensions of liver cells showed a statistically significant increase in polyploid (8N) cells at the highest dose. No treatment-related changes in overall DNA methylation, gamma-H2AX foci, DNA strand breaks and cross links were observed at the end of the 4-week exposure period. However, several genes involved in DNA damage response, beyond stress and liver toxicity, were over-expressed in mice treated with the highest furan dose (15 mg/kg b.w./day). Acute administration of furan induced evident liver toxicity at the highest dose (250 mg/kg b.w.), which was associated with a significant increase of DNA damage in the alkaline comet assay and with a distinct decrease in gamma-ray-induced DNA migration. Overall, the results obtained suggest that the contribution of genotoxicity to the mechanism of furan carcinogenicity in mouse liver should not be dismissed.

Evidence of Oxidative Stress and Associated DNA Damage, Increased Proliferative Drive, and Altered Gene Expression in Rat Liver Produced by the Cholangiocarcinogenic Agent Furan

Furan is a potent cholangiocarcinogen in rat by an as yet undefined mechanism. The risk to man remains unclear. Using a time-course stop study design, we have investigated the potential of furan to induce oxidative stress and DNA damage associated with inflammatory and regenerative responses in rat liver. Furan was administered via oral gavage (30 mg/kg b.w. 5 daily doses per week), and livers were analyzed at time points between eight hr and three months. A one-month recovery group previously treated for three months was also included. There was a marked association between CYP2E1 expression and DNA oxidation (8-oxo-dG) in areas of centrilobular hepatocyte necrosis seen after a single dose. After one-month recovery from three-month treatment, 8-oxo-dG was still observed in areas of furan-induced cholangiofibrosis. Furan-induced changes in the expression of various genes associated with oxidative stress, DNA damage, and cell cycle control were identified during treatment and recovery. We propose that furan-induced cholangiocarcinomas emerge from areas of cholangiofibrosis as a result of a combination of chronic, persistent indirect damage to DNA through oxygen radicals coupled with persistent proliferative signals, including loss of connexin 32, that act to convert this DNA damage to fixed mutations.

Assessment of in vivo genotoxicity of the rodent carcinogen furan: evaluation of DNA damage and induction of micronuclei in mouse splenocytes

In recent years, several surveys have highlighted the presence of the rodent carcinogen furan in a variety of food items. Even though the evidence of carcinogenicity of furan is unequivocal, the underlying mechanism has not been fully elucidated. In particular, the role of genotoxicity in furan carcinogenicity is still not clear, even though this information is considered pivotal for the assessment of the risk posed by the presence of low doses of furan in food. In this work, the genotoxic potential of furan in vivo has been investigated in mice, under exposure conditions similar to those associated with cancer onset in the National Toxicology Program long-term bioassay. To this aim, male B6C3F1 mice were treated by gavage for 4 weeks with 2, 4, 8 and 15 mg furan/kg b.w./day. Spleen was selected as the target organ for genotoxicity assessment, in view of the capability of quiescent splenocytes to accumulate DNA damage induced by repeat dose exposure. The induction of primary DNA damage in splenocytes was evaluated by alkaline single-cell gel electrophoresis (comet assay) and by the immunofluorescence detection of foci of phosphorylated histone H2AX (gamma-H2AX). The presence of cross-links was probed in a modified comet assay, in which cells were irradiated in vitro with gamma-rays before electrophoresis. Chromosome damage was quantitated through the detection of micronuclei in mitogen-stimulated splenocytes using the cytokinesis-block method. Micronucleus induction was also assessed with a modified protocol, using the repair inhibitor 1-beta-arabinofuranosyl-cytosine to convert single-strand breaks in micronuclei. The results obtained show a significant (P < 0.01) increase of gamma-H2AX foci in mitogen-stimulated splenocytes of mice treated with 8 and 15 mg furan/kg b.w. and a statistically significant (P < 0.001) increases of micronuclei in binucleated splenocytes cultured in vitro. Conversely, no effect of in vivo exposure to furan was observed when freshly isolated quiescent splenocytes were analysed by immunofluorescence and in comet assays, both with standard and radiation-modified protocols. These results indicate that the in vivo exposure to furan gives rise to pre-mutagenic DNA damage in resting splenocytes, which remains undetectable until it is converted in frank lesions during the S-phase upon mitogen stimulation. The resulting DNA strand breaks are visualized by the increase in gamma-H2AX foci and may originate micronuclei at the subsequent mitosis.

Biological significance of DNA adducts: comparison of increments over background for various biomarkers of genotoxicity in L5178Y tk(+/-) mouse lymphoma cells treated with hydrogen peroxide and cumene hydroperoxide

DNA is affected by background damage of the order of one lesion per one hundred thousand nucleotides, with depurination and oxidative damage accounting for a major part. This damage contributes to spontaneous mutation and cancer. DNA adducts can be measured with high sensitivity, with limits of detection lower than one adduct per one billion nucleotides. Minute exposures to an exogenous DNA-reactive agent may therefore result in measurable adduct formation, although, as an increment over total DNA damage, a small increment in mutation cannot be measured and would be considered negligible. Here, we investigated whether this discrepancy also holds for adducts that are present as background induced by oxidative stress. L5178Y tk(+/-) mouse lymphoma cells were incubated for 4h with hydrogen peroxide (0, 0.8, 4, 20, 100, 500muM) or cumene hydroperoxide (0, 0.37, 1.1, 3.3, 10muM). Five endpoints of genotoxicity were measured in parallel from aliquots of three replicates of large batches of cells: Two DNA adducts, 8-oxo-7,8-dihydro-2'-deoxyguanosine (8-oxodGuo) and 1,N(6)-etheno-2'-deoxyadenosine (varepsilondAdo) measured by LC-MS/MS, as well as strand breaks assessed with the comet assay and in vitro micronucleus test, and gene mutation as assessed using the thymidine kinase gene mutation assay. Background measures of 8-oxodGuo and varepsilondAdo were 500-1000 and 50-90 adducts per 10(9) nucleotides. Upon treatment, neither hydrogen peroxide nor cumene hydroperoxide significantly increased the DNA adduct levels above control. In contrast, dose-related increases above background were observed with both oxidants in the comet assay, the micronucleus test and the gene mutation assay. Differences in sensitivity of the assays were quantified by estimating the concentration of oxidant that resulted in a doubling of the background measure. We conclude that the increase in DNA breakage and mutation induced by hydrogen peroxide and cumene hydroperoxide observed in our in vitro experimental set-up was no direct consequence of the measured DNA adducts. In comparison with data obtained with the methylating agent methyl methanesulfonate we further conclude that the assumption of DNA adducts being oversensitive biomarkers is adduct-specific.

Statistical model to estimate a threshold dose and its confidence limits for the analysis of sublinear dose-response relationships, exemplified for mutagenicity data

Strongly sublinear dose-response relationships (slope increasing with dose) raise the question about a putative threshold dose below which no biologically relevant effect would be expected. A mathematical threshold with a break in the curve at the threshold dose is generally rejected for consequences of genotoxicity such as mutation, because proportionality between low dose and the rate of DNA-adduct formation is a reasonable hypothesis. In view of an increasing database for distinct deviation from linearity for mutagenicity, we offer a statistical model to analyze continuous response data and estimate a threshold dose together with its confidence limits, thereby taking data quality and degree of sublinearity into account. The simplest mathematical threshold model is a hockey stick defined by a low-dose part with slope zero at background level a to a theoretical break point at threshold dose td, followed by a linear increase above td with slope b. The function is y (dose d)=a+bx(d-td)x1([d>td]). Using the free statistics software package "R", we make a procedure available to estimate the parameters a, b, and td. Confidence intervals are calculated for all parameters at a significance level that can be defined by the user. If the lower limit of the confidence interval for td is >0, linearity is rejected. The procedure is illustrated by two examples. A small data set with three replicates per dose group, indicating a threshold for the induction of thymidine kinase mutants in L5178Y tk(+/-) mouse lymphoma cells treated with methyl methanesulfonate, did not achieve significance. On the other hand, the large data set reported in this issue (Gocke et al.) on lacZ mutants in bone marrow cells of transgenic mice treated with ethyl methanesulfonate strongly favoured the hockey stick model. The question of a theoretically expected linear dose-related increase below the threshold dose is addressed by linear regression of the data below the break point and estimation of an upper limit of the slope. The question of biological relevance of the resulting slope is discussed against the normal variation of background measures in the control group.