Formation of hepatocyte cytoplasmic inclusions and their contribution to methylcarbamate-induced hepatocarcinogenesis in F344 rats

Methylcarbamate (MC), a reaction product between dimethyl dicarbonate and ammonia or ammonium ion, is a potent hepatocarcinogen in F344 rats. Various genotoxicity tests have shown negative results for MC. Although previous studies have described the effects of MC on the liver, including the formation of characteristic basophilic cytoplasmic inclusions (CIs) in hepatocytes, the toxicological significance of CIs and their involvement in hepatocarcinogenesis remain unclear. In the current study, to elucidate the mechanisms of MC hepatocarcinogenesis, we examined hepatotoxicity and genotoxicity after 4 weeks of administration of MC using gpt delta rats with an F344 genetic background as a reporter gene transgenic animal model. Histopathologically, single-cell necrosis, karyomegaly, and the formation of CIs positive for Feulgen staining were observed in hepatocytes at the carcinogenic dose, demonstrating the hepatotoxicity of MC. CIs were also detected as large micronuclei in liver micronucleus tests but not in the bone marrow, suggesting that MC could cause chromosomal instability specifically in the livers of rats. Reporter gene mutation assays demonstrated that MC did not induce mutagenicity even in the liver. Immunofluorescence analyses revealed that CIs exhibited loss of nuclear envelope integrity, increased heterochromatinization, and accumulation of DNA damage. An increase in liver STING protein levels suggested an effect on the cyclic GMP-AMP synthase/stimulator of interferon genes innate immune pathway. Overall, these data demonstrated the possible occurrence of chromothripsis-like chromosomal rearrangements via CIs. Thus, the formation of CIs could be a crucial event in the early stage of MC-induced hepatocarcinogenesis in F344 rats.

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

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

Site-specific genotoxicity of rubiadin: localization and histopathological changes in the kidneys of rats

Rubiadin (Rub) is a genotoxic component of madder color (MC) that is extracted from the root of Rubia tinctorum L. MC induces renal tumors and preneoplastic lesions that are found in the proximal tubule of the outer stripe of the outer medulla (OSOM), suggesting that the renal carcinogenicity of MC is site specific. To clarify the involvement of Rub in renal carcinogenesis of MC, we examined the distribution of Rub in the kidney of male gpt delta rats that were treated with Rub for 28 days. We used desorption electrospray ionization quadrupole time-of-flight mass spectrometry imaging (DESI-Q-TOF-MSI), along with the histopathological analysis, immunohistochemical staining, and reporter gene mutation assays of the kidney. DESI-Q-TOF-MSI revealed that Rub and its metabolites, lucidin and Rub-sulfation, were specifically distributed in the OSOM. Histopathologically, karyomegaly characterized by enlarged nuclear and microvesicular vacuolar degeneration occurred in proximal tubule epithelial cells in the OSOM. The ɤ-H2AX- and p21-positive cells were also found in the OSOM rather than the cortex. Although dose-dependent increases in gpt and Spi- mutant frequencies were observed in both the medulla and cortex, the mutant frequencies in the medulla were significantly higher. The mutation spectra of gpt mutants showed that A:T-T:A transversion was predominant in Rub-induced gene mutations, consistent with those of MC. Overall, the data showed that the distribution of Rub and its metabolites resulted in site-specific histopathological changes, DNA damage, and gene mutations, suggesting that the distribution of genotoxic components and metabolites is responsible for the site-specific renal carcinogenesis of MC.

A 13-week comprehensive toxicity study with adductome analysis demonstrates the toxicity, genotoxicity, and carcinogenicity of the natural flavoring agent elemicin

Elemicin, an alkenylbenzene flavoring, exists naturally in foods, herbs, and spices. Some alkenylbenzenes are hepatotoxic and hepatocarcinogenic in rodents. However, few studies have examined the toxicology of elemicin. In the current study, we comprehensively evaluated the general toxicity, genotoxicity, and carcinogenicity of elemicin using gpt delta rats and DNA adductome analysis. Groups of 10 male F344 gpt delta rats were treated with elemicin by gavage at a dose of 0, 25, 100, or 400 mg/kg bw/day for 13 weeks. Liver weights were significantly increased with histopathological changes in groups receiving 100 mg/kg bw/day or more. Significant increases in serum hepatotoxic parameters were observed in the 400 mg/kg bw/day group. Based on the observed changes in liver weights, 18.6 mg/kg bw was identified as the low benchmark dose. Significant increases in the number and area of glutathione S-transferase placental form-positive foci and gpt mutant frequencies were apparent only in the 400 mg/kg/day group, although elemicin-specific DNA adducts were detected from the lowest dose, suggesting that elemicin exhibited hepatocarcinogenicity in rats only at higher doses. Because elemicin showed no mutagenicity at lower doses, there was an adequate safety margin between the acceptable daily intake and the estimated daily intake of elemicin.

Comprehensive analysis of the general toxicity, genotoxicity, and carcinogenicity of 3-acetyl-2,5-dimethylfuran in male gpt delta rats

The safety of flavoring agents has been evaluated according to classification by chemical structure and using a decision tree approach. The genotoxic potential found in some flavoring agents has highlighted the importance of efficient toxicity studies. We performed a comprehensive toxicity analysis using reporter gene transgenic rats to assess the safety of 3-acetyl-2,5-dimethylfuran (ADF), a flavoring agent exhibiting genotoxic potential in silico and in vitro assays. Male F344 gpt delta rats were given 0, 30, or 300 mg/kg body weight/day ADF by gavage for 13 weeks. In serum biochemistry analyses, triglyceride, total cholesterol, phospholipid, and total protein levels and albumin/globulin ratios were significantly altered in the 30 and 300 mg/kg groups. Histopathologically, nasal cavity toxicity and hepatocellular hypertrophy were observed in the 300 mg/kg group. In the livers of 300 mg/kg group, a significant increase in gpt mutant frequencies were observed along with ADF-specific DNA adduct formation. The number and area of glutathione S-transferase placental form-positive foci were significantly increased in the same group. Thus, ADF affected nasal cavity, liver, and lipid metabolism and showed genotoxicity and possible carcinogenicity in the liver. Overall, our comprehensive toxicity study using gpt delta rats provided insights into the safety evaluation of ADF.

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.

A 90-day subchronic toxicity study of Myrrh in F344 rats

Myrrh is a flavoring agent and food additive. Here, we performed a subchronic toxicity study of Myrrh in male and female F344 rats by feeding at 5,000, 15,000 and 50,000 ppm for 90 days. No deaths or clinical signs were observed. Suppression of body weight gain was observed from the early phase of administration in both males and females in the 50,000 ppm group. Because there were no obvious changes in food intake in any of the Myrrh groups compared with the control group, suppression of body weight gain was considered an adverse effect of Myrrh. Hematology and serum biochemistry parameters with significant changes observed in the Myrrh groups were considered to have no toxicological significance. We observed a significant increase in relative kidney weight in male rats treated with 50,000 ppm Myrrh; this effect was considered to be related to the appearance of hyaline droplets in the epithelium of the proximal tubules histopathologically observed in this group. Immunohistochemical staining with anti-α_2u-globulin antibodies suggested that these hyaline droplets were caused by factors other than α_2u-globulin deposition. Thus, the no-observed-adverse-effect level of Myrrh was determined to be 15,000 ppm (males: 0.85 g/kg/day, females: 0.95 g/kg/day).

Chromosome aberrations induced by the non-mutagenic carcinogen acetamide involve in rat hepatocarcinogenesis through micronucleus formation in hepatocytes

Chromosome aberrations (CAs), i.e. changes in chromosome number or structure, are known to cause chromosome rearrangements and subsequently tumorigenesis. However, the involvement of CAs in chemical-induced carcinogenesis is unclear. In the current study, we aimed to clarify the possible involvement of CAs in chemical carcinogenesis using a rat model with the non-mutagenic hepatocarcinogen acetamide. In an in vivo micronucleus (MN) test, acetamide was revealed to induce CAs specifically in rat liver at carcinogenic doses. Acetamide also induced centromere-positive large MN (LMN) in hepatocytes. Immunohistochemical and electron microscopic analyses of the LMN, which can be histopathologically detected as basophilic cytoplasmic inclusion, revealed abnormal expression of nuclear envelope proteins, increased heterochromatinization, and massive DNA damage. These molecular pathological features in LMN progressed with acetamide exposure in a time-dependent manner, implying that LMN formation can lead to chromosome rearrangements. Overall, these data suggested that CAs induced by acetamide play a pivotal role in acetamide-induced hepatocarcinogenesis in rats and that CAs can cause chemical carcinogenesis in animals via MN formation.

Lack of In Vivo Mutagenicity of Acetamide in a 13-Week Comprehensive Toxicity Study Using F344 gpt Delta Rats

Acetamide, a food contaminant, has been shown to induce hepatocellular tumors in rats. However, the mode of action underlying acetamide-induced hepatocarcinogenesis remains unclear. In the current study, we aimed to examine the possible involvement of in vivo mutagenicity in hepatocarcinogenesis of acetamide and evaluate its toxicological profile using a comprehensive medium-term toxicity study in gpt delta rats. Six-week-old male F344 gpt delta rats were given a basal diet containing 0%, 0.625%, 1.25%, or 2.5% acetamide for 13 weeks. In general toxicologic assessment, hepatotoxic parameters in serum, such as aspartate aminotransferase and alanine aminotransferase were significantly changed at the 1.25% group and higher. Histopathological examination of the liver revealed that various changes related to hepatic injury were observed at the 1.25% group and higher. Interestingly, Feulgen-positive cytoplasmic inclusion was frequently observed in hepatocytes in these groups. In the hematopoietic system, red blood cell parameters in plasma, such as mean corpuscular volume and mean corpuscular hemoglobin were significantly changed at the 1.25% group and higher, and decrease of erythroblast in the spleen was observed histopathologically in the 2.5% group. Thus, the no-observed-adverse-effect level of acetamide in this study was 0.625% (equivalent to 394 mg/kg body weight/day). In vivo mutation assays showed that acetamide induced no changes in gpt and red/gam gene mutant frequencies, even at the carcinogenic target site. In contrast, Ki67-positive hepatocytes were increased significantly at carcinogenic doses. Therefore, these results suggested that cell proliferation activity, but not mutagenicity, played crucial roles in acetamide-induced hepatocarcinogenesis in rats.

A 90-day subchronic toxicity study of 5-methyl-2-phenyl-2-hexenal in F344 rats

5-Methyl-2-phenyl-2-hexenal (MPH) has been used as a flavoring agent. In the present study, we performed a subchronic toxicity study in male and female F344 rats with oral administration of MPH by gavage at 0, 8, 24 and 70 mg/kg body weight (BW)/day for 90 days. No mortality or clinical signs were observed during the experimental period. Body weight and food consumption for all treated groups of both sexes were essentially the same as for the respective control groups. Hematologic examination demonstrated significant decreases in monocyte counts for females given 24 and 70 mg/kg BW/day. However, these changes were not substantial and no related histopathological changes were observed, suggesting that these changes were not toxicologically significant. Among organ weights, the absolute and/or relative weights of testes and liver were significantly increased in the 70 mg/kg BW/day groups of males and females, respectively, but no related histopathological changes were observed, suggesting that these changes did not reflect adverse effects. In addition, no treatment-related histopathological changes were observed for any of the tissues examined. Based on the overall data, the no-observed-adverse-effect level (NOAEL) for MPH was determined to be 70 mg/kg BW/day, the highest dose tested, in both male and female rats.

The role of DNA polymerase ζ in benzo[a]pyrene-induced mutagenesis in the mouse lung

DNA polymerase zeta (Polζ) is a heterotetramer composed of the catalytic subunit Rev3l, Rev7 and two subunits of Polδ (PolD2/Pol31 and PolD3/Pol32), and this polymerase exerts translesion DNA synthesis (TLS) in yeast. Because Rev3l knockout results in embryonic lethality in mice, the functions of Polζ need further investigation in vivo. Then, we noted the two facts that substitution of leucine 979 of yeast Rev3l with methionine reduces Polζ replication fidelity and that reporter gene transgenic rodents are able to provide the detailed mutation status. Here, we established gpt delta mouse knocked in the constructed gene encoding methionine instead of leucine at residue 2610 of Rev3l (Rev3l L2610M gpt delta mice), to clarify the role of Polζ in TLS of chemical-induced bulky DNA adducts in vivo. Eight-week-old gpt delta mice and Rev3l L2610M gpt delta mice were treated with benzo[a]pyrene (BaP) at 0, 40, 80, or 160 mg/kg via single intraperitoneal injection. At necropsy 31 days after treatment, lungs were collected for reporter gene mutation assays. Although the gpt mutant frequency was significantly increased by BaP in both mouse genotypes, it was three times higher in Rev3l L2610M gpt delta than gpt delta mice after treatment with 160 mg/kg BaP. The frequencies of G:C base substitutions and characteristic complex mutations were significantly increased in Rev3l L2610M gpt delta mice compared with gpt delta mice. The BaP dose-response relationship suggested that Polζ plays a central role in TLS when protective mechanisms against BaP mutagenesis, such as error-free TLS, are saturated. Overall, Polζ may incorporate incorrect nucleotides at the sites opposite to BaP-modified guanines and extend short DNA sequences from the resultant terminal mismatches only when DNA is heavily damaged.

Background data of 2-year-old male and female F344 gpt delta rats

Although gpt delta rats, as reporter gene-transgenic rats, were originally developed for in vivo mutation assays, they have also been used to evaluate chemical carcinogenesis and comprehensive toxicity. Therefore, it is necessary to accumulate background data on carcinogenicity and general toxicity in gpt delta rats. Here, we investigated the background data of 110-week-old male and female F344 gpt delta rats and wild-type rats. There was no effect of reporter gene transfection on animal survival rates and body weights during the experiment. The relative weight of male gpt delta rat adrenals was significantly higher than that of wild-type rats, possibly due to the higher incidence of pheochromocytoma. There were no intergenotype differences in the incidence of nonneoplastic lesions in both sexes, including chronic progressive nephropathy and focus of cellular alteration in the liver, which had a higher incidence in both genotypes. Additionally, the significantly higher incidence of adrenal pheochromocytoma in male gpt delta rats than that in wild-type rats was likely incidental because of the lack of differences in the incidences of preneoplastic (male and female) and neoplastic (female) adrenal lesions in both genotypes. Other neoplastic lesions in both sexes showed no intergenotype differences in incidence rates, although large granular lymphocytic leukemia in the spleen and Leydig cell tumors in the testes of males showed higher incidence rates. Overall, there were no effects of reporter gene transfection on the spectrum of spontaneous lesions in F344 gpt delta rats, thus supporting their applicability in evaluating chemical toxicity and carcinogenicity.

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.

Mechanisms Underlying Exacerbation of Osmotic Nephrosis Caused by Pre-existing Kidney Injury

Osmotic nephrosis, a disease caused by intravenous infusion of various fluids such as hypertonic sucrose and isotonic polysaccharide-based plasma volume expanders, exhibits specific histopathological features, including vacuolated and swollen proximal tubules, ie, "clear tubules". Pre-existing kidney injury exacerbates this condition, resulting in major clinical problems. However, the underlying mechanisms are unclear. Animal models often yield results that are directly translatable to humans. Therefore, in this study, we performed detailed histopathological analyses of the formation of clear tubules in rats treated with gentamicin or ischemia/reperfusion (IR) operation followed by dextran administration. The results showed that clear tubules may originate from regenerative tubules. Additionally, we classified regenerative tubules into 3 categories based on their development, with a particular focus on the middle and late stages. Comprehensive microarray and real-time polymerase chain reaction analyses of mRNA extracted from regenerative tubules at each stage using laser microdissection revealed that regenerative tubules in the middle stage showed an imbalance between dextran absorption and metabolism, resulting in accumulation of dextran, particularly in the cytoplasm of the tubules. Overall, our findings demonstrated that clear tubules originated from regenerated tubules and that tubules at the middle stage became clear tubules because of an imbalance during their development. This could explain why osmotic nephrosis is exacerbated in the presence of kidney lesions.

DNA modifications that do not cause gene mutations confer the potential for mutagenicity by combined treatment with food chemicals

Cell proliferation plays a key role in fixing mutations induced by DNA damage. We clarified whether this phenomenon occurred after combined treatment with chemicals in food. The effects of antibiotic flumequine (FL), a residue of veterinary medicinal products in foodstuffs, on mutagenicity in the liver were examined in mice treated with estragole (ES), a natural food flavouring compound. Gpt delta mice were orally administered 10 or 100 mg/kg/day ES and simultaneously fed a diet containing 0.4% FL for 4 weeks. Proliferating cell nuclear antigen-positive cells and cell cycle-related genes were additively increased in the livers of combined treatment groups as compared with high-dose ES or FL groups. Mutant frequencies (MFs) in gpt after cotreatment with low-dose ES and FL were significantly increased, although treatment with ES alone increased MFs only in the high-dose group. Sult1a1 mRNA levels were unchanged after FL treatment. Liquid chromatography with tandem-mass spectrometry analysis showed that FL did not affect the amount of ES-specific DNA adducts in the livers, indicating that FL treatment did not influence metabolic pathways of ES. Thus, enhancement of the mutagenic potential of a chemical by chemical-induced cell proliferation may occur as a result of the combined effects of chemicals in food.

Molecular Pathological Differences in Global Gene Expression between Two Sustained Proliferative Lesions, Nodular Regenerative Hepatocellular Hyperplasia and Hepatocellular Adenoma, in Mice

Long-term exposure to piperonyl butoxide (PBO) induces multiple nodular masses along with hepatocellular tumors in the liver of mice. The histopathological features of the nodules led to our diagnosis of nodular regenerative hepatocellular hyperplasia (NRH). However, because of the lack of data on the biological characteristics of NRH, whether this lesion is truly nonneoplastic remains unknown. In this study, the molecular characteristics of NRH were compared with those of hepatocellular adenoma (HCA) by global gene expression analysis. Six-week-old male ICR mice were fed a diet containing 6,000 ppm PBO for 43 weeks to induce NRH and HCA development. Complementary DNA microarray analysis was performed using messenger RNA extracted from NRH and HCA frozen sections collected by laser microdissection. Hierarchical cluster analysis showed that all NRH samples clustered together but were separate from the HCA cluster. Pathway analysis revealed activation of the cell cycle and Delta-Notch signaling in both lesions, but the latter was more upregulated in HCA. Downregulation of cytochrome p450 enzymes was observed in NRH, but not in HCA. These results imply that NRH differs from HCA in terms of not only morphological but also molecular characteristics.

Effects of inhibition of hepatic sulfotransferase activity on renal genotoxicity induced by lucidin-3-O-primeveroside

Sulfotransferase 1A (SULT1A) expression is lower in the liver of humans than that of rodents. Therefore, species differences should be taken into consideration when assessing the risk of rodent hepatocarcinogens metabolically activated by SULT1A in humans. Although some renal carcinogens require SULT1A-mediated activation, it is unclear how SULT1A activity in the liver affects renal carcinogens. To explore the effects of SULT1A activity in the liver on genotoxicity induced by SULT1A-activated renal carcinogens, B6C3F₁ mice or gpt delta mice of the same strain background were given lucidin-3-O-primeveroside (LuP), a hepatic and renal carcinogen of rodents, for 4 or 13 weeks, respectively, and pentachlorophenol (PCP) as a liver-specific SULT inhibitor, was given from 1 week before LuP treatment to the end of the experiment. A 4 week exposure of LuP induced lucidin-specific DNA adduct formation. The suppression of Sult1a expression was observed only in the liver but not in the kidneys of PCP-treated mice, but co-administration of PCP suppressed LuP-induced DNA adduct formation in both organs. Thirteen-week exposure of LuP increased mutation frequencies and cotreatment with PCP suppressed these increases in both organs. Given that intact levels of SULT activity in the liver were much higher than in the kidneys of rodents, SULT1A may predominantly activate LuP in the liver, consequently leading to genotoxicity not only in the liver but also in the kidney. Thus, species differences should be considered in human risk assessment of renal carcinogens activated by SULT1A as in the case of the corresponding liver carcinogens.

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

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

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

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

Methods for the analysis of organophosphorus flame retardants-Comparison of GC-EI-MS, GC-NCI-MS, LC-ESI-MS/MS, and LC-APCI-MS/MS

Organophosphorus flame retardants (PFRs) are extensively used as alternatives to banned polybrominated diphenyl ethers (PBDEs) and hexabromocyclododecane (HBCD). In this study, we analyzed 14 PFRs by means of four mass-spectrometry-based methods: gas chromatography combined with electron-impact mass spectrometry (GC-EI-MS) or negative-chemical-ionization mass spectrometry (GC-NCI-MS) and liquid chromatography combined with tandem mass spectrometry using electrospray ionization (LC-ESI-MS/MS) or atmospheric pressure chemical ionization (LC-APCI-MS/MS). The limits of quantification (LOQs) for LC-ESI-MS/MS and LC-APCI-MS/MS (0.81-970 pg) were 1-2 orders of magnitude lower than the LOQs for GC-EI-MS and GC-NCI-MS (2.3-3900 pg). LC-APCI-MS/MS showed the lowest LOQs (mean = 41 pg; median = 3.4 pg) for all but two of the PFRs targeted in this study. For LC-APCI-MS/MS, the lowest LOQ was observed for tributyl phosphate (TBP) (0.81 pg), and the highest was observed for tris(butoxyethyl) phosphate (TBOEP) (36 pg). The results of this study indicate that LC-APCI-MS/MS is the optimum analytical method for the target PFRs, at least in terms of LOQ.

Simultaneous determination of brominated and phosphate flame retardants in flame-retarded polyester curtains by a novel extraction method

The use of novel brominated flame retardants (BFRs) and phosphate-based flame retardants (PFRs) has increased as substitutes for hexabromocyclododecane (HBCD) in many consumer products. To facilitate collection of data on chemicals used as flame retardants in textiles and fabrics, we developed an analytical method using liquid chromatography interfaced with tandem mass spectrometry (LC-MS/MS). We compared two extraction methods, one involving ultrasonic extraction (traditional method) using dichloromethane, toluene or acetone and the other encompassing complete dissolution of textile with 25% 1,1,1,3,3,3-hexafluoro-2-propanol/chloroform. The dissolution method extracted up to 204 times more BFRs and PFRs than the traditional ultrasonic extraction. Tris(2,3-dibromopropyl) isocyanurate (TDBP-TAZTO), triphenylphosphine oxide (TPhPO), tris(1,3-dichloro-2-propyl) phosphate (TDCPP), tricresyl phosphate (TCsP), and triphenyl phosphate (TPhP) were found in 40 flame-retarded curtain samples purchased from Japanese market in 2014. TDBP-TAZTO was detected in polyester curtains for the first time. Some of the flame-retarded curtain samples did not contain any of the known target analytes, which suggested the presence of other unknown flame retardants in those fabrics.

Phosphorylation of protein phosphatase 2A facilitated an early stage of chemical carcinogenesis

Protein phosphatase 2A (PP2A) is a serine-threonine phosphatase that regulates cell signaling pathways. Its inactivation is correlated with tumor malignancy, possibly due to the effects on cell differentiation and malignant cell transformation. Therefore, it has been noted that PP2A could be a promising target for cancer therapy. In our previous study of the hepatocarcinogen estragole (ES), cell proliferation may be required to convert ES-specific DNA adducts to mutations. To explore the trigger for cell proliferation, gpt delta rats were administered ES by gavage at doses of 3, 30 and 300mg/kg/day for 4weeks. ES-induced cell proliferation and gene mutations were observed at only the high dose whereas ES-specific DNA adducts were detected in a dose-dependent manner. Western blot analyses revealed activation of the Akt and ERK pathways without activation of upstream regulators, such as c-Raf, PKC and, PI3K. Phosphorylation of the PP2A C subunit at Tyr307 was found along with phosphorylation of Src. The overall data might imply that PP2A inactivation is responsible for cell cycle progression through activation of the Akt and ERK pathways at high doses of ES. Based on γ-H2AX immunohistochemistry and Western blot analysis for Rad51 protein, the resultant mutation spectra showed large deletion mutations that might result from double strand breaks of DNA. Thus, it is likely that inactivation of PP2A resulted in acceleration and exacerbation of gene mutations. We conclude that PP2A might contribute to an early stage of chemical carcinogenesis, suggesting that PP2A could be a molecular target of primary cancer prevention.

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

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

Anti-Inflammatory Effects of Capsaicin and Piperine on Helicobacter pylori-Induced Chronic Gastritis in Mongolian Gerbils

BACKGROUND

Spices have been used for thousands of years, and recent studies suggest that certain spices confer beneficial effects on gastric disorders. The purpose of this study was to evaluate possible chemopreventive effects of spice-derived compounds on Helicobacter pylori (H. pylori)-induced gastritis.

METHODS

We examined the inhibitory effects of curcumin, capsaicin, and piperine on H. pylori in vitro by determining the colony-forming units and real-time RT-PCR in H. pylori stimulated AGS gastric cancer cells. For in vivo analysis, 6-week-old SPF male Mongolian gerbils were infected with H. pylori, fed diets containing 5000 ppm curcumin, 100 ppm capsaicin, or 100 ppm piperine, and sacrificed after 13 weeks.

RESULTS

All three compounds inhibited in vitro proliferation of H. pylori, with curcumin being the most effective. Infiltration of neutrophils and mononuclear cells was suppressed by piperine both in the antrum and corpus of H. pylori-infected gerbils. Capsaicin also decreased neutrophils in the antrum and corpus and mononuclear cell infiltration and heterotopic proliferative glands in the corpus. mRNA expression of Tnf-α and formation of phospho-IκB-α in the antrum were reduced by both capsaicin and piperine. In addition, piperine suppressed expression of Il-1β, Ifn-γ, Il-6, and iNos, while H. pylori UreA and other virulence factors were not significantly attenuated by any compounds.

CONCLUSION

These results suggest that capsaicin and piperine have anti-inflammatory effects on H. pylori-induced gastritis in gerbils independent of direct antibacterial effects and may thus have potential for use in the chemoprevention of H. pylori-associated gastric carcinogenesis.

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

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

Role of p53 in the progression from ochratoxin A-induced DNA damage to gene mutations in the kidneys of mice

Carcinogenic doses of ochratoxin A (OTA) cause increases of mutant frequencies (MFs) of the red/gam gene (Spi(-)) in the kidneys of p53-deficient gpt delta mice, but not in p53-proficient mice. Here, we investigated the role of p53 in the progression from OTA-induced DNA damage to gene mutations. To this end, p53-proficient and -deficient mice were administered 5 mg/kg OTA for 3 days or 4 weeks by gavage. After 3 days of administration, comet assays were performed and there were no differences in the degrees of OTA-induced DNA damage between p53-proficient and -deficient mice. However, the frequencies of γ-H2AX-positive tubular epithelial cells in p53-deficient mice were significantly higher than those in p53-proficient mice, implying that p53 inhibited the progression from DNA damage to DNA double-strand breaks (DSBs). Evaluation of global gene expression and relevant mRNA/protein expression levels demonstrated that OTA increased the expression of Cdkn1a, which encodes the p21 protein, in p53-proficient mice, but not in p53-deficient mice. Moreover, in p53-deficient mice, mRNA levels of cell cycle progression and DSB repair (homologous recombination repair [HR])-related genes were significantly increased. Thus, G1/S arrest due to activation of the p53/p21 pathway may contribute to the prevention of DSBs in p53-proficient mice. In addition, single base deletions/insertions/substitutions were predominant, possibly due to HR. Overall, these results suggested that OTA induced DSBs at the carcinogenic target site in mice and that p53/p21-mediated cell cycle control prevented an increase in the formation of DSBs, leading to gene mutations.

No effect of high fat diet-induced obesity on spontaneous reporter gene mutations in gpt delta mice

A large number of epidemiological studies have demonstrated that obesity is a risk factor for several human cancers. Several animal studies using rodents with diet-induced or genetic obesity have also demonstrated that obesity can promote tumor development. However, the effects of obesity on the early stages of carcinogenesis, and especially on the spontaneous occurrence of somatic gene mutations, remain unclear. To investigate the effects of obesity on the rate of spontaneous gene mutations, we performed reporter gene mutation assays in liver, kidney, and colon, organs in which obesity appears to be associated with cancer development on the basis of epidemiological or animal studies, in mice with high fat diet (HFD)-induced obesity. Six-week-old male and female C57BL/6 gpt delta mice were fed HFD or standard diet (STD) for 13 or 26 weeks. At the end of the experiments, reporter gene mutation assays of liver, kidney, and colon were performed. Final body weights and serum leptin levels of male and female mice fed HFD for 13 or 26 weeks were significantly increased compared with corresponding STD-fed groups. Reporter gene mutation assays of liver, kidney, and colon revealed that there were no significant differences in gpt or Spi- mutant frequencies between STD- and HFD-fed mice in either the 13-week or 26-week groups. These results indicate that HFD treatment and consequent obesity does not appear to influence the spontaneous occurrence of somatic gene mutations.

In vivo genotoxicity of Ginkgo biloba extract in gpt delta mice and constitutive androstane receptor knockout mice

The National Toxicology Program study of Ginkgo biloba extract (GBE), a herbal supplement, reported concerns regarding genotoxicity and clear evidence of hepatocarcinogenicity and liver hypertrophy in mice. To clarify the genotoxicity of GBE in vivo, we performed reporter gene mutation assay using gpt delta mice. We also used a combined liver comet assay and bone marrow micronucleus assay using C3H-derived constitutive androstane receptor knockout (CARKO) and wild-type mice. No remarkable increases in gpt or Spi(-) mutation frequencies were observed in DNA extracted from the livers of gpt delta mice that had been exposed to GBE up to 2000 mg/kg bw/day. In the comet and micronucleus assays, no statistically significant increases in positive cells were observed at doses up to 2000 mg/kg bw/day of GBE in either mouse genotype. The present study provides clear evidence that GBE is not genotoxic in vivo. Our results indicate that GBE-induced hepatocarcinogenesis in mice occurs through a non-genotoxic mode of action.

Gene expression analysis of a Helicobacter pylori-infected and high-salt diet-treated mouse gastric tumor model: identification of CD177 as a novel prognostic factor in patients with gastric cancer

BACKGROUND

Helicobacter pylori (H. pylori) infection and excessive salt intake are known as important risk factors for stomach cancer in humans. However, interactions of these two factors with gene expression profiles during gastric carcinogenesis remain unclear. In the present study, we investigated the global gene expression associated with stomach carcinogenesis and prognosis of human gastric cancer using a mouse model.

METHODS

To find candidate genes involved in stomach carcinogenesis, we firstly constructed a carcinogen-induced mouse gastric tumor model combined with H. pylori infection and high-salt diet. C57BL/6J mice were given N-methyl-N-nitrosourea in their drinking water and sacrificed after 40 weeks. Animals of a combination group were inoculated with H. pylori and fed a high-salt diet. Gene expression profiles in glandular stomach of the mice were investigated by oligonucleotide microarray. Second, we examined an availability of the candidate gene as prognostic factor for human patients. Immunohistochemical analysis of CD177, one of the up-regulated genes, was performed in human advanced gastric cancer specimens to evaluate the association with prognosis.

RESULTS

The multiplicity of gastric tumor in carcinogen-treated mice was significantly increased by combination of H. pylori infection and high-salt diet. In the microarray analysis, 35 and 31 more than two-fold up-regulated and down-regulated genes, respectively, were detected in the H. pylori-infection and high-salt diet combined group compared with the other groups. Quantitative RT-PCR confirmed significant over-expression of two candidate genes including Cd177 and Reg3g. On immunohistochemical analysis of CD177 in human advanced gastric cancer specimens, over-expression was evident in 33 (60.0%) of 55 cases, significantly correlating with a favorable prognosis (P = 0.0294). Multivariate analysis including clinicopathological factors as covariates revealed high expression of CD177 to be an independent prognostic factor for overall survival.

CONCLUSIONS

These results suggest that our mouse model combined with H. pylori infection and high-salt diet is useful for gene expression profiling in gastric carcinogenesis, providing evidence that CD177 is a novel prognostic factor for stomach cancer. This is the first report showing a prognostic correlation between CD177 expression and solid tumor behavior.

Development of a Medium-term Animal Model Using gpt Delta Rats to Evaluate Chemical Carcinogenicity and Genotoxicity

In this study, the potential for development of an animal model (GPG46) capable of rapidly detecting chemical carcinogenicity and the underlying mechanisms of action were examined in gpt delta rats using a reporter gene assay to detect mutations and a medium-term rat liver bioassay to detect tumor promotion. The tentative protocol for the GPG46 model was developed based on the results of dose-response exposure to diethylnitrosamine (DEN) and treatment with phenobarbital over time following DEN administration. Briefly, gpt delta rats were exposed to various chemicals for 4 weeks, followed by a partial hepatectomy (PH) to collect samples for an in vivo mutation assay. The mutant frequencies (MFs) of the reporter genes were examined as an indication of tumor initiation. A single intraperitoneal (ip) injection of 10 mg/kg DEN was administered to rats 18 h after the PH to initiate hepatocytes. Tumor-promoting activity was evaluated based on the development of glutathione S-transferase placental form (GST-P)-positive foci at week 10. The genotoxic carcinogens 2-acetylaminofluorene (2-AAF), 2-amino-3-methylimidazo [4,5-f] quinolone (IQ) and safrole (SF), the non-genotoxic carcinogens piperonyl butoxide (PBO) and phenytoin (PHE), the non-carcinogen acetaminophen (APAP) and the genotoxic non-hepatocarcinogen aristolochic acid (AA) were tested to validate the GPG46 model. The validation results indicate that the GPG46 model could be a powerful tool in understanding chemical carcinogenesis and provide valuable information regarding human risk hazards.

The effects of long-term exposure to ozokerite mainly consisting of an aliphatic series of hydrocarbons using F344 rats

Combined chronic toxicity and carcinogenicity studies of ozokerite (OZK), a natural wax substance used as a food additive for a gum base, were performed in male and female F344 rats. Dietary concentrations of 0%, 0.05%, 0.1% and 0.2% OZK were applied in a 52-week chronic toxicity study and 0%, 0.1% and 0.2% in a 104-week carcinogenicity study. In the chronic toxicity study, treatment with OZK caused a xenobiotic reaction against absorbed OZK, including formation of histiocytosis and granulomas with crystalline material in many organs in all of the treated males and females. Particularly in the liver, granulomatous inflammation was accompanied by hepatocellular vacuolation and changes in the serum biochemical parameters indicative of hepatic disorder. The number and area of glutathione S-transferase placental form (GST-P) positive foci were increased in all of the treated groups of both sexes, suggesting the proliferative effect of OZK. In the carcinogenicity study, the incidence of hepatocellular adenoma and the total tumor incidence in the liver of all of the treated males were significantly increased compared with the controls. In conclusion, long-term exposure to OZK caused systemic chronic inflammation due to a foreign body response. OZK was weakly carcinogenic in the liver of male F344 rats.

Undifferentiated Sarcoma of the Salivary Gland in a Mongolian Gerbil (Meriones unguiculatus)

A subcutaneous mass was found in the lower ventral neck region of a 55-week-old male Mongolian gerbil (Meriones unguiculatus). Histopathologically, the mass involved salivary glands and featured diffuse proliferation of pleomorphic neoplastic cells with large necrotic foci. The lesion was well demarcated from the surrounding tissue, although invasive growth to fibrous septa was occasionally observed. The neoplastic cells were mainly arranged in irregular sheets with severe cellular atypia, round to oval nuclei and varying amounts of eosinophilic cytoplasm. Mitotic figures and multinucleated giant cells were frequent. Immunohistochemical analysis revealed that the neoplastic cells were strongly positive for vimentin and S-100 and negative for NSE, cytokeratin, α-SMA, c-kit, factor VIII, CD34, α-1-antitrypsin, lysozyme and MSR-A. Based on the results, the mass was diagnosed as an undifferentiated sarcoma of the salivary gland. To the best of our knowledge, this is the first report of such a tumor in Mongolian gerbils.

Loss of adiponectin promotes intestinal carcinogenesis in Min and wild-type mice

BACKGROUND & AIMS

Metabolic syndrome- and obesity-associated cancers, including colon cancer, are common in Western countries. Visceral fat accumulation and decreased levels of plasma adiponectin (APN) have been associated with development of human colorectal adenoma. We investigated the function of APN in intestinal carcinogenesis.

METHODS

APN+/+, APN+/-, or APN-/- mice (C57BL/6J) were given injections of azoxymethane (AOM), which led to development of intestinal tumors; these strains of mice were also crossed with Min mice to assess polyp formation. Adipocytokine levels and phosphorylation/activation of AMP-activated protein kinase (AMPK) were evaluated to investigate the mechanisms of APN in tumor growth.

RESULTS

The total number of polyps in the intestines of male APN+/-Min and APN-/-Min mice increased 2.4- and 3.2-fold, respectively, by the age of 9 weeks and 3.2- and 3.4-fold, respectively, by 12 weeks, compared with those of APN+/+Min mice. Similar results were obtained from female mice. AOM induced colon tumor formation in 40% of APN+/+, 50% of APN+/-, and 71% of APN-/- (P<.05) mice, respectively; mean values for tumor multiplicity of each genotype were 0.5, 0.6, and 1.1 (P<.05), respectively. Phosphorylation of AMPK decreased in intestinal epithelial cells of APN-/- mice compared with APN+/+ mice. Among serum adipocytokines, plasminogen activator inhibitor-1 levels increased in APN-/-Min mice and APN-/- mice that received injections of AOM. Activation of AMPK suppressed expression of plasminogen activator inhibitor-1 in Min mice.

CONCLUSIONS

Mice with disruptions in APN develop more intestinal tumors and have decreased activation (phosphorylation) of AMPK and increased levels of plasminogen activator inhibitor-1, compared with wild-type mice. APN and its receptor might be developed as targets for cancer chemopreventive agents.

Induction of glandular stomach cancers in Helicobacter pylori-infected Mongolian gerbils by 1-nitrosoindole-3-acetonitrile

Helicobacter pylori (H. pylori) infection and high intake of various traditional salt-preserved foods are regarded as risk factors for human gastric cancer. We previously reported that Chinese cabbage contains indole compounds, such as indole-3-acetonitrile, a mutagen precursor. 1-Nitrosoindole-3-acetonitrile (NIAN), formed by the treatment of indole-3-acetonitrile with nitrite under acidic conditions, shows direct-acting mutagenicity. In the present study, NIAN administration by gavage to Mongolian gerbils (MGs) at the dose of 100 mg/kg two times a week resulted in three adduct spots (1.6 adducts/10(8) nucleotides in total), detected in DNA samples from the glandular stomach by (32) P-postlabeling methods. Treatment with six consecutive doses of 100 mg/kg of NIAN, two times a week for 3 weeks, induced well-and moderately-differentiated glandular stomach adenocarcinomas in the MGs at the incidence of 31% under H. pylori infection at 54-104 weeks. Such lesions were not induced in MGs given broth alone, broth + NIAN or infection with H. pylori alone. Thus, endogenous carcinogens formed from nitrosation of indole compounds could be critical risk factors for human gastric cancer development under the influence of H. pylori infection.