Functional and proliferative effects of repeated low-dose oral administration of furan in rat liver

A comprehensive review of furan in foods: From dietary exposures and in vivo metabolism to mitigation measures

Furan is a thermal food processing contaminant that is ubiquitous in various food products such as coffee, canned and jarred foods, and cereals. A comprehensive summary of research progress on furan is presented in this review, including discussion of (i) formation pathways, (ii) occurrence and dietary exposures, (iii) analytical techniques, (iv) toxicities, (v) metabolism and metabolites, (vi) risk assessment, (vii) potential biomarkers, and (viii) mitigation measures. Dietary exposure to furan varies among different countries and age groups. Furan acts through various toxicological pathways mediated by its primary metabolite, cis-2-butene-1,4-dial (BDA). BDA can readily react with glutathione, amino acids, biogenic amines, or nucleotides to form corresponding metabolites, some of which have been proposed as potential biomarkers of exposure to furan. Present risk assessment of furan mainly employed the margin of exposure approach. Given the widespread occurrence of furan in foods and its harmful health effects, mitigating furan levels in foods or exploring potential dietary supplements to protect against furan toxicity is necessary for the benefit of food safety and public health.

Proteomic Analysis of Subchronic Furan Exposure in the Liver of Male Fischer F344 Rats

Furan is a volatile compound formed during the thermal processing of foods. Chronic exposure has been shown to cause cholangiocarcinoma and hepatocellular tumors in rodent models. We conducted a 90 day subchronic study in Fisher 344 rats exposed to various doses by gavage to determine the NOAEL. Previous reports have outlined changes in the liver using gross necropsy examination, histopathology, clinical biochemistry, hematology, immunohistochemistry, and toxicogenomics. The data revealed that males were more sensitive than females. The focus of this study was to evaluate the toxicoproteomic changes by 2-dimensional differential in gel electrophoresis followed by mass spectrometry analysis. To compliment previous studies, protein expression changes were evaluated of male animals after 90 days of exposure to doses of 0, 0.03, 0.5, and 8.0 mg/kg bw/d. Significant statistical treatment-related changes compared to the controls identified 45 protein spots containing 38 unique proteins. Proteins identified are implicated in metabolism, redox regulation, protein folding/proteolysis as well as structural and transport proteins. At lower doses, multiple cytoprotective pathways are activated to maintain a homeostasis but ultimately the loss of protein function and impairment of several pathways could lead to adverse health effects at higher doses of furan administration.

Biomonitoring of heat-induced food contaminants: Quantitative analysis of furan dependent glutathione- and lysine-adducts in rat urine as putative biomarkers of exposure

Furan is a liver toxicant and carcinogen that occurs in heat-processed foods. Due to its volatility, analysis of furan in food does not provide reliable estimates of exposure. Biomarker-based approaches offer the opportunity to more accurately assess human exposure, but a correlation between concentrations of potential biomarkers of furan exposure and external dose has not been established. Bioactivation of furan and subsequent reaction of cis-2-butene-1,4-dial (BDA) with cellular nucleophiles gives rise to a range of metabolites that may serve as biomarkers of furan exposure. In this study, N-[4-carboxy-4-(3-mercapto-1H-pyrrol-1-yl)-1-oxobutyl]-L-cysteinylglycine cyclic sulfide, a mono-glutathione adduct of BDA (GSH-BDA), and R-2-acetylamino-6-(2,5-dihydro-2-oxo-1H-pyrrol-1-yl)-1-hexanoic acid, an adduct of BDA with N^α-acetyl-L-lysine (NAcLys-BDA), were synthesized and analysed by LC-MS/MS in urine of rats treated with furan at 0, 0.1, 0.5 and 2.0 mg/kg bw for 5 and 28 days. GSH-BDA and NAcLys-BDA were both excreted in a dose-related manner. 24 h excretion rates ranged between 0.6 and 1.1% of the administered dose for GSH-BDA, and 1.4-2.1% for NAcLys-BDA. In contrast to GSH-BDA, NAcLys-BDA was also present in urine of controls, suggesting either endogenous formation or background exposure. Overall, the close correlation between urinary furan metabolites and external dose provides experimental support for biomarker-based approaches to monitor human exposure to furan.

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.

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.

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.

Functional and cellular consequences of covalent target protein modification by furan in rat liver

Furan hepatotoxicity is thought to be linked to covalent binding of its reactive metabolite, cis-2-butene-1,4-dial, to hepatic proteins critical for cell homeostasis and survival. We previously identified 61 putative furan target proteins, which participate in various cellular processes including carbohydrate metabolism, fatty acid β-oxidation, adenosine triphosphate (ATP) synthesis, protein folding and maintenance of redox homeostasis. To further investigate the biological significance of target protein modification, this study was designed to determine the impact of furan on the activity of key target enzymes involved in glycolysis, β-oxidation, ATP synthesis, and redox regulation in rat liver, and to link these functional changes to alterations in cellular processes. While cis-2-butene-1,4-dial inhibited thioredoxin 1 (Txn1) in a cell-free assay, in livers of rats treated with a single high dose of furan Txn1 activity was markedly increased due to rapid up-regulation of Txn1 mRNA expression. Significant inhibition of glyceraldehyde-3-phosphate dehydrogenase and metabolic changes consistent with blocked glycolytic breakdown of glucose were observed in rat liver in response to a single high dose of furan. In contrast, furan treatment resulted in increased activity of enoyl-CoA hydratase and enhanced production of ketone bodies, indicative of increased utilization of fatty acids as energy source. Consistent with changes in TCA cycle metabolites, furan treatment resulted in a reduction of succinate dehydrogenase activity, supporting mitochondrial dysfunction as a critical event in furan toxicity. No significant changes in target protein function were observed following repeated administration of furan at lower dose (0.1 and 0.5mg/kg bw for 4 weeks) closer to estimated human exposure to furan via food. Although the relative contribution of furan mediated alterations in metabolic pathways and antioxidant defense to the overall toxic response to furan, including considerations of dose and time, remains to be established, our work contributes to mapping biological processes and toxicity pathways modulated by reactive electrophiles.

Toxicogenomic assessment of liver responses following subchronic exposure to furan in Fischer F344 rats

Furan is a widely used industrial chemical and a contaminant in heated foods. Chronic furan exposure causes cholangiocarcinoma and hepatocellular tumors in rats at doses of 2 mg/kg bw/day or greater, with gender differences in frequency and severity. The hepatic transcriptional alterations induced by low doses of furan (doses below those previously tested for induction of liver tumors) and the potential mechanisms underlying gender differences are largely unexplored. We used DNA microarrays to examine the global hepatic mRNA and microRNA transcriptional profiles of male and female rats exposed to 0, 0.03, 0.12, 0.5 or 2 mg/kg bw/day furan over 90 days. Marked gender differences in gene expression responses to furan were observed, with many more altered genes in exposed males than females, confirming the increased sensitivity of males even at the low doses. Pathway analysis supported that key events in furan-induced liver tumors in males include gene expression changes related to oxidative stress, apoptosis and inflammatory response, while pathway changes in females were consistent with primarily adaptive responses. Pathway benchmark doses (BMDs) were estimated and compared to relevant apical endpoints. Transcriptional pathway BMDs could only be examined in males. These median BMDs ranged from 0.08 to 1.43 mg/kg bw/day and approximated those derived from traditional histopathology. MiR-34a (a P53 target) was the only microRNA significantly increased at the 2 mg/kg bw/day, providing evidence to support the importance of apoptosis and cell proliferation in furan hepatotoxicity. Overall, this study demonstrates the use of transcriptional profiling to discern mode of action and mechanisms involved in gender differences.

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.

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.