Persistence of furan-induced epigenetic aberrations in the livers of F344 rats

Furan promotes cytotoxic effects through DNA damage and cell apoptosis in Leydig cells

Furan is an organic chemical that can cause adverse effects on human health and is formed as a result of the thermal decomposition of many food components during cooking, storage, and processing techniques. Studies have shown that exposure to furan causes nephrotoxicity, hepatotoxicity, immunotoxicity, and reproductive toxicity. According to our current knowledge of the literature, the genotoxic mode of action of furan is highly controversial. The genotoxic effects of furan on the male reproductive system, however, have not been studied. In this study, the TM3 Leydig cell line was treated with 750, 1500, and 3000 μM concentrations of furan for 24 h. Following the completion of the exposure period, the cytotoxicity of furan in TM3 Leydig cells was assessed using a cell viability assay and a spectrophotometric measurement of lactate dehydrogenase (LDH) enzyme levels. The double fluorescence staining method was used to demonstrate furan-induced apoptosis, and DNA damage was shown using the micronucleus, comet, and chromosomal aberration assays. The result indicated that furan administration of Leydig cells resulted in an increase in structural chromosomal aberration, comet, and micronucleus formation, reduced cell viability, increased LDH activity, and a higher incidence of apoptotic cells. These findings revealed that furan induces DNA damage in TM3 Leydig cells, causing genotoxicity and DNA damage-induced cytotoxicity.

Food-Borne Chemical Carcinogens and the Evidence for Human Cancer Risk

Commonly consumed foods and beverages can contain chemicals with reported carcinogenic activity in rodent models. Moreover, exposures to some of these substances have been associated with increased cancer risks in humans. Food-borne carcinogens span a range of chemical classes and can arise from natural or anthropogenic sources, as well as form endogenously. Important considerations include the mechanism(s) of action (MoA), their relevance to human biology, and the level of exposure in diet. The MoAs of carcinogens have been classified as either DNA-reactive (genotoxic), involving covalent reaction with nuclear DNA, or epigenetic, involving molecular and cellular effects other than DNA reactivity. Carcinogens are generally present in food at low levels, resulting in low daily intakes, although there are some exceptions. Carcinogens of the DNA-reactive type produce effects at lower dosages than epigenetic carcinogens. Several food-related DNA-reactive carcinogens, including aflatoxins, aristolochic acid, benzene, benzo[a]pyrene and ethylene oxide, are recognized by the International Agency for Research on Cancer (IARC) as causes of human cancer. Of the epigenetic type, the only carcinogen considered to be associated with increased cancer in humans, although not from low-level food exposure, is dioxin (TCDD). Thus, DNA-reactive carcinogens in food represent a much greater risk than epigenetic carcinogens.

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.

Formation of furan in baby food products: Identification and technical challenges

Furan is generally produced during thermal processing of various foods including baked, fried, and roasted food items such as cereal products, coffee, canned, and jarred prepared foods as well as in baby foods. Furan is a toxic and carcinogenic compound to humans and may be a vital hazard to infants and babies. Furan could be formed in foods through thermal degradation of carbohydrates, dissociation of amino acids, and oxidation of polyunsaturated fatty acids. The detection of furan in food products is difficult due to its high volatility and low molecular weight. Headspace solid-phase microextraction coupled with gas chromatography/mass spectrometer (GC/MS) is generally used for analysis of furan in food samples. The risk assessment of furan can be characterized using margin of exposure approach (MOE). Conventional strategies including cooking in open vessels, reheating of commercially processed foods with stirring, and physical removal using vacuum treatment have remained unsuccessful for the removal of furan due to the complex production mechanisms and possible precursors of furan. The innovative food-processing technologies such as high-pressure processing (HPP), high-pressure thermal sterilization (HPTS), and Ohmic heating have been adapted for the reduction of furan levels in baby foods. But in recent years, only HPP has gained interest due to successful reduction of furan because of its nonthermal mechanism. HPP-treated baby food products are commercially available from different food companies. This review summarizes the mechanism involved in the formation of furan in foods, its toxicity, and identification in infant foods and presents a solution for limiting its formation, occurrence, and retention using novel strategies.

Protective effect of Spirulina platensis against physiological, ultrastructural and cell proliferation damage induced by furan in kidney and liver of rat

The modulatory role of the Spirulina platensis (SP) against furan-induced (FU) hepatic and renal damage was assessed in this study. For achieving this, sixty rats were distributed into six groups: control group, SP-administered group (300 mg/kg b.wt orally for 28 days), a FU-intoxicated group (16 mg/kg b.wt, orally, daily for 28 days), protective co-treated group SP/F (administered SP 300 mg/kg b.wt, one week before, and concurrently with FU intoxication), therapeutic co-treated group FU/SP (administered SP 300 mg/kg b.wt, one week after FU intoxication for 28 days) and protective/therapeutic co-treated group SP/FU/SP (administered SP one week before and after, concurrently with FU intoxication). Subsequently, the biochemical responses and the histology of hepatic and renal tissues in treated rats were assessed. The results indicated that FU intoxication induced a significant hepato- and nephropathy represented by the elevation in the values of tissue injury biomarkers and reduction in protein levels. Histologically, a wide range of morphological, cytotoxic, inflammatory, and vascular alterations as well as downregulation in the immunoexpression of the proliferating cell nuclear antigen (PCNA) and the proliferation-associated nuclear antigen (Ki-67) were induced by FU intoxication. Oral SP administration, particularly in the protective/therapeutic co-treated group, markedly supressed the serum levels of the tissue injury biomarkers, diminished the inflammatory response, restored the cytotoxic alterations, upregulated the immunoexpression of PCNA and Ki-67, and restored the perturbed morphology of the hepatic and renal tissues. In conclusion, the obtained data demonstrated that SP co-administration elicits both protective and therapeutic potential against the FU-induced hepato- and nephropathy.

Genotoxic and Epigenotoxic Alterations in the Lung and Liver of Mice Induced by Acrylamide: A 28 Day Drinking Water Study

Acrylamide has been classified as a "Group 2A carcinogen" (probably carcinogenic to humans) by the International Agency for Research on Cancer. The carcinogenicity of acrylamide is attributed to its well-recognized genotoxicity. In the present study, we investigated the effect of acrylamide on epigenetic alterations in mice. Female B6C3F1 mice received acrylamide in drinking water for 28 days, at doses previously used in a 2 year cancer bioassay (0, 0.0875, 0.175, 0.35, and 0.70 mM), and the genotoxic and epigenetic effects were investigated in lungs, a target organ for acrylamide carcinogenicity, and livers, a nontarget organ. Acrylamide exposure resulted in a dose-dependent formation of N7-(2-carbamoyl-2-hydroxyethyl)guanine and N3-(2-carbamoyl-2-hydroxyethyl)adenine in liver and lung DNA. In contrast, the profiles of global epigenetic alterations differed between the two tissues. In the lungs, acrylamide exposure resulted in a decrease of histone H4 lysine 20 trimethylation (H4K20me3), a common epigenetic feature of human cancer, while in the livers, there was increased acetylation of histone H3 lysine 27 (H3K27ac), a gene transcription activating mark. Treatment with 0.70 mM acrylamide also resulted in substantial alterations in the DNA methylation and whole transcriptome in the lungs and livers; however, there were substantial differences in the trends of DNA methylation and gene expression changes between the two tissues. Analysis of differentially expressed genes showed a marked up-regulation of genes and activation of the gene transcription regulation pathway in livers, but not lungs. This corresponded to increased histone H3K27ac and DNA hypomethylation in livers, in contrast to hypermethylation and transcription silencing in lungs. Our results demonstrate that acrylamide induced global epigenetic alterations independent of its genotoxic effects, suggesting that epigenetic events may determine the organ-specific carcinogenicity of acrylamide. Additionally this study provides strong support for the importance of epigenetic alterations, in addition to genotoxic events, in the mechanism of carcinogenesis induced by genotoxic chemical carcinogens.

Mechanisms of DNA-reactive and epigenetic chemical carcinogens: applications to carcinogenicity testing and risk assessment

Chemicals with carcinogenic activity in either animals or humans produce increases in neoplasia through diverse mechanisms. One mechanism is reaction with nuclear DNA. Other mechanisms consist of epigenetic effects involving either modifications of regulatory macromolecules or perturbation of cellular regulatory processes. The basis for distinguishing between carcinogens that have either DNA reactivity or an epigenetic activity as their primary mechanism of action is detailed in this review. In addition, important applications of information on these mechanisms of action to carcinogenicity testing and human risk assessment are discussed.

Differential susceptibility of kidneys and livers to proliferative processes and transcriptional level of the genes encoding desmin, vimentin, connexin 43, and nestin in rats exposed to furan

In this study, we aimed to assess the differential toxic impact, induced by furan exposure, on the liver and kidney tissues by estimating reactive oxygen species (ROS) level, total antioxidant capacity (TAC), oxidative damage, and the tissue injury markers in a male rat model. To explain such impacts, 20 rats were assigned into two groups: a control group, where rats were administered corn oil as a vehicle, and a furan-administered group, where furan was orally administered to rats at a dose of 16 mg/kg b wt/day (five days per week over eight weeks). The transcriptional levels of intermediate filament proteins (desmin, vimentin, nestin, and connexin 43) were assessed by using quantitative real-time polymerase chain reaction (PCR), and the cell proliferation markers (proliferating cell nuclear antigen [PCNA] and proliferation-associated nuclear antigen [Ki-67]) were recognized by immunohistochemical analysis. Furthermore, the ultrastructural changes of liver and kidney were monitored using electron microscopy. Our findings showed that furan exposure could induce hepatic and renal damage to different extents. Furan can increase the ROS content, oxidative damage indices, and liver tissue injury indices but not kidney injury indices. Furthermore, it decreases the TAC in the serum of exposed rats. In addition, furan exposure was associated with changes in the mRNA expression pattern of intermediate filament proteins in both kidney and liver tissues. Moreover, furan enhances the expression of PCNA and Ki-67 in the liver tissues but not in the kidney tissues. The ultrastructure evaluation revealed the incidence of glomerular podocyte degeneration and hepatocyte injury. These results conclusively demonstrate that the deleterious effects of furan are caused by promoting fibrosis and hepatocyte proliferation in liver tissues and triggering podocyte injury in the kidney tissues.

Effect of aflatoxin B1, benzo[a]pyrene, and methapyrilene on transcriptomic and epigenetic alterations in human liver HepaRG cells

The increasing number of man-made chemicals in the environment that may pose a carcinogenic risk highlights the need for developing reliable time- and cost-effective approaches for carcinogen detection and identification. To address this issue, we investigated the utility of high-throughput microarray gene expression and next-generation genome-wide DNA methylation sequencing for the in vitro identification of genotoxic and non-genotoxic carcinogens. Terminally differentiated and metabolically competent human liver HepaRG cells were treated at minimally cytotoxic concentrations of (i) the genotoxic human liver carcinogen aflatoxin B₁ (AFB1) and its structural non-carcinogenic analog aflatoxin B₂ (AFB2); (ii) the genotoxic human lung carcinogen benzo[a]pyrene (B[a]P) and its non-carcinogenic isomer benzo[e]pyrene (B[e]P); and (iii) the non-genotoxic liver carcinogen methapyrilene for 72 h and transcriptomic and DNA methylation profiles were examined. Treatment of HepaRG cells with the liver carcinogens AFB1 and methapyrilene generated distinct gene-expression profiles, whereas B[a]P had only a slight effect on gene expression. In contrast to transcriptomic alterations, treatment of HepaRG cells with the carcinogenic and non-carcinogenic chemicals resulted in profound changes in the DNA methylation footprint; however, the correlation between gene-specific DNA methylation and gene expression changes was minimal. Among the carcinogen-altered genes, transferrin (TF) emerged as sensitive marker for an initial screening of chemicals for their potential liver carcinogenicity. Potential liver carcinogens (i.e., chemicals causing altered TF gene expression) could then be subjected to gene-expression analyses to differentiate genotoxic from non-genotoxic liver carcinogens. This approach may substantially enhance the identification and assessment of potential liver carcinogens.

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.

New insights into the molecular mechanisms of chemical carcinogenesis: In vivo adduction of histone H2B by a reactive metabolite of the chemical carcinogen furan

Furan is a rodent hepatocarcinogen ubiquitously found in the environment and heat-processed foods. Furan undergoes cytochrome P450 2E1-catalyzed bioactivation to cis-2-butene-1,4-dial (BDA), which has been shown to form an electrophilic conjugate (GSH-BDA) with glutathione. Both BDA and GSH-BDA yield covalent adducts with lysine residues in proteins. Dose- and time-dependent epigenetic histone alterations have been observed in furan-treated rats. While the covalent modification of histones by chemical carcinogens has long been proposed, histone-carcinogen adducts have eluded detection in vivo. In this study, we investigated if the covalent modification of histones by furan may occur in vivo prior to epigenetic histone alterations. Using a "bottom-up" methodology, involving the analysis of tryptic peptides by liquid chromatography - high resolution mass spectrometry, we obtained evidence for a cross-link between GSH-BDA and lysine 107 of histone H2B isolated from the livers of male F344 rats treated with tumorigenic doses of furan. This cross-link was detected at the shortest treatment period (90 days) in the lowest dose group (0.92mg/kg body weight/day), prior to the identification of epigenetic changes, and occurred at a lysine residue that is a target for epigenetic modifications and crucial for nucleosome stability. Our results represent the first unequivocal proof of the occurrence of carcinogen-modified histones in vivo and suggest that such modification happens at the initial stages of furan-induced carcinogenesis. This type of alteration may be general in scope, opening new insights into the mechanisms of chemical carcinogenesis/toxicity and new opportunities for the development of early compound-specific biomarkers of exposure.

Furan-induced transcriptomic and gene-specific DNA methylation changes in the livers of Fischer 344 rats in a 2-year carcinogenicity study

Furan is a significant food contaminant and a potent hepatotoxicant and rodent liver carcinogen. The carcinogenic effect of furan has been attributed to genotoxic and non-genotoxic, including epigenetic, changes in the liver; however, the mechanisms of the furan-induced liver tumorigenicity are still unclear. The goal of the present study was to investigate the role of transcriptomic and epigenetic events in the development of hepatic lesions in Fischer (F344) rats induced by furan treatment in a classic 2-year rodent tumorigenicity bioassay. High-throughput whole-genome transcriptomic analysis demonstrated distinct alterations in gene expression in liver lesions induced in male F344 rats treated with 0.92 or 2.0 mg furan/kg body weight (bw)/day for 104 weeks. Compared to normal liver tissue, 1336 and 1541 genes were found to be differentially expressed in liver lesions in rats treated with 0.92 and 2.0 mg furan/kg bw/day, respectively, among which 1001 transcripts were differentially expressed at both doses. Pairing transcriptomic and next-generation bisulfite sequencing analyses of the common differentially expressed genes identified 42 CpG island-containing genes in which the methylation level was correlated inversely with gene expression. Forty-eight percent of these genes (20 genes, including Areg, Jag1, and Foxe1) that exhibited the most significant methylation and gene expression changes were involved in key pathways associated with different aspects of liver pathology. Our findings illustrate that gene-specific DNA methylation changes have functional consequences and may be an important component of furan hepatotoxicity and hepatocarcinogenicity.

Protective role of garlic oil against oxidative damage induced by furan exposure from weaning through adulthood in adult rat testis

Furan is produced in a wide variety of heat-treated foods via thermal degradation. Furan contamination is found to be relatively high in processed baby foods, cereal products, fruits juices, and canned vegetables. Several studies have demonstrated that furan is a potent hepatotoxin and hepatocarcinogen in rodents. However, few studies have investigated the toxic effects of furan in the testis. In addition, the exact mechanism(s) by which furan exerts toxicity in the testis has not been fully elucidated. In this study, we investigated the potential of furan exposure from weaning through adulthood to induce oxidative stress in adult rat testis, as well as the potential of garlic oil (GO) to ameliorate the induced toxicity. Our results reveal that furan administration significantly reduced serum testosterone levels and increased the levels of malondialdehyde (MDA); furthermore, furan administration decreased significantly the enzymatic activity of testicular antioxidants, including glutathione (GSH), superoxide dismutase (SOD), and catalase (CAT) and induced histopathological alterations in the testis. GO co-administration ameliorated the reduction in testosterone levels and dramatically attenuated the furan-induced oxidative and histopathological changes. In addition, Go significantly down-regulated the increased caspase-3 and cytochrome P450 2E1 (CYP2E1) expression in the furan-treated testis. To the best of our knowledge, this study is the first to demonstrate the furan-induced oxidative changes in the adult rat testis and the protective role of GO to ameliorate these changes through its antioxidant effects and its ability to inhibit CYP2E1 production.

Epigenetic alterations induced by genotoxic occupational and environmental human chemical carcinogens: A systematic literature review

Accumulating evidence suggests that epigenetic alterations play an important role in chemically-induced carcinogenesis. Although the epigenome and genome may be equally important in carcinogenicity, the genotoxicity of chemical agents and exposure-related transcriptomic responses have been more thoroughly studied and characterized. To better understand the evidence for epigenetic alterations of human carcinogens, and the potential association with genotoxic endpoints, we conducted a systematic review of published studies of genotoxic carcinogens that reported epigenetic endpoints. Specifically, we searched for publications reporting epigenetic effects for the 28 agents and occupations included in Monograph Volume 100F of the International Agency for the Research on Cancer (IARC) that were classified as "carcinogenic to humans" (Group 1) with strong evidence of genotoxic mechanisms of carcinogenesis. We identified a total of 158 studies that evaluated epigenetic alterations for 12 of these 28 carcinogenic agents and occupations (1,3-butadiene, 4-aminobiphenyl, aflatoxins, benzene, benzidine, benzo[a]pyrene, coke production, formaldehyde, occupational exposure as a painter, sulfur mustard, and vinyl chloride). Aberrant DNA methylation was most commonly studied, followed by altered expression of non-coding RNAs and histone changes (totaling 85, 59 and 25 studies, respectively). For 3 carcinogens (aflatoxins, benzene and benzo[a]pyrene), 10 or more studies reported epigenetic effects. However, epigenetic studies were sparse for the remaining 9 carcinogens; for 4 agents, only 1 or 2 published reports were identified. While further research is needed to better identify carcinogenesis-associated epigenetic perturbations for many potential carcinogens, published reports on specific epigenetic endpoints can be systematically identified and increasingly incorporated in cancer hazard assessments.

Evaluation of serum and liver toxicokinetics for furan and liver DNA adduct formation in male Fischer 344 rats

Furan is a food processing contaminant found in many common cooked foods that induces liver toxicity and liver cancer in animal models treated with sufficient doses. The metabolism of furan occurs primarily in the liver where CYP 2E1 produces a highly reactive bis-electrophile, cis-2-butene-1,4-dial (BDA). BDA reacts with nucleophilic groups in amino acids and DNA in vitro to form covalent adducts. Evidence for BDA-nucleoside adduct formation in vivo is limited but important for assessing the carcinogenic hazard of dietary furan. This study used controlled dosing with furan in Fischer 344 rats to measure serum and liver toxicokinetics and the possible formation of BDA-nucleoside adducts in vivo. After gavage exposure, furan concentrations in the liver were consistently higher than those in whole blood (∼6-fold), which is consistent with portal vein delivery of a lipophilic compound into the liver. Formation of BDA-2'-deoxycytidine in furan-treated rat liver DNA was not observed using LC/MS/MS after single doses as high as 9.2 mg/kg bw or repeated dosing for up to 360 days above a consistent background level (1-2 adducts per 10(8) nucleotides). This absence of BDA-nucleoside adduct formation is consistent with the general lack of evidence for genotoxicity of furan in vivo.