Daisuke Hibi

Site-Specific In Vivo Mutagenicity in the Kidney of gpt Delta Rats Given a Carcinogenic Dose of Ochratoxin A

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

Ochratoxin A induces DNA double-strand breaks and large deletion mutations in the carcinogenic target site of gpt delta rats

Ochratoxin A (OTA) is a carcinogen targeting proximal tubules at the renal outer medulla (ROM) in rodents. We previously reported that OTA increased mutant frequencies of the red/gam gene (Spi(-)), primarily deletion mutations. In the present study, Spi(-) assays and mutation spectrum analyses in the Spi(-) mutants were performed using additional samples collected in our previous study. Spi(-) assay results were similar to those in our previous study, revealing large (>1kb) deletion mutations in the red/gam gene. To clarify the molecular progression from DNA damage to gene mutations, in vivo comet assays and analysis of DNA damage/repair-related mRNA and/or protein expression was performed using the ROM of gpt delta rats treated with OTA at 70, 210 or 630 µg/kg/day by gavage for 4 weeks. Western blotting and immunohistochemical staining demonstrated that OTA increased γ-H2AX expression specifically at the carcinogenic target site. In view of the results of comet assays, we suspected that OTA was capable of inducing double-strand breaks (DSBs) at the target sites. mRNA and/or protein expression levels of homologous recombination (HR) repair-related genes (Rad51, Rad18 and Brip1), but not nonhomologous end joining-related genes, were increased in response to OTA in a dose-dependent manner. Moreover, dramatic increases in the expression of genes involved in G2/M arrest (Chek1 and Wee1) and S/G2 phase (Ccna2 and Cdk1) were observed, suggesting that DSBs induced by OTA were repaired predominantly by HR repair, possibly due to OTA-specific cell cycle regulation, consequently producing large deletion mutations at the carcinogenic target site.

Experimental intranasal infection of equine herpesvirus 9 (EHV-9) in suckling hamsters: Kinetics of viral transmission and inflammation in the nasal cavity and brain

Equine herpesvirus 9 (EHV-9), the newest member of the equine herpesvirus family, is a highly neurotropic herpesvirus that induces encephalitis in a variety of animals. To access transmission of EHV-9 in the nasal cavity and brain, a suckling hamster model was developed so that precise sagittal sections of nasal and cranial cavities including the brain could be processed, which proved useful in detecting viral transmission as well as extension of pathological lesions. Suckling hamsters were inoculated intranasally with EHV-9, and were sacrificed at 6, 12, 18, 24, 36, 48, and 60 h post inoculation (PI). Sagittal sections of the entire head, including nasal and cranial cavities including the brain, were made to assess viral kinetics and identify the progress of the neuropathological lesions. At 12 to 24 h PI the virus attached to and propagated in the olfactory epithelium, and infected adjacent epithelial cells. At 48 h PI, immunohistochemistry for EHV-9 viral antigen showed that virus had extended from the site of infection into the olfactory bulb and olfactory nerve. These results indicate that EHV-9 rapidly invades the brain via the olfactory route after experimental intranasal infection.

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.

Effects of p53 knockout on ochratoxin A-induced genotoxicity in p53-deficient gpt delta mice

Ochratoxin A (OTA) is a mycotoxin produced by fungal species and is carcinogenic targeting the S3 segment of the renal proximal tubules in rodents. We previously reported that exposure of gpt delta rats to OTA induced both mutations in the red/gam gene (Spi(-)), suggesting large deletion mutations, and fluctuations in genes transcribed by p53 in the kidneys, which were associated with DNA double-strand break (DSB) repair, particularly homologous recombination (HR) repair. In the present study, to investigate the effects of p53 knockout on OTA-induced mutagenicity, apoptosis, and karyomegaly in renal tubular cells, p53-proficient and p53-deficient gpt delta mice were given 1 and 5mg/kg of OTA for 4 weeks. Significant increases in Spi(-) mutant frequencies (MFs) were observed in the kidneys of p53-deficient gpt delta mice given 5 mg/kg of OTA, but not in the kidneys of p53-proficient gpt delta mice given the same dose. There were no changes in gpt MFs in both genotypes of mice treated with OTA. Western blotting analysis demonstrated that p53 protein levels in the kidneys of p53-proficient mice given OTA were significantly increased compared with the control. Incidences of apoptosis and karyomegaly in not only the outer stripe of outer medulla but also the cortex were significantly higher in p53-deficient at 5mg/kg than in p53-proficient gpt delta mice at same dose, which had no change in the cortex, the inner stripe of outer stripe, and the inner medulla. Given that p53 regulates HR repair in DSBs, these results suggest that OTA may promote large deletion mutations in the process of HR repair for DSBs. Additionally, the lower incidence of karyomegaly and apoptosis found in the p53-proficient gpt delta mice suggests that these phenomena may arise from OTA-induced DNA damage.

Possible involvement of sulfotransferase 1A1 in estragole-induced DNA modification and carcinogenesis in the livers of female mice.

Estragole (ES), a natural organic compound, is frequently used as a flavoring in food even though it is a hepatocarcinogen in mice. Although formation of ES-specific DNA adducts following conversion from ES to the nucleophilic metabolite by sulfotransferase 1A1 (SULT1A1) has been reported, the modes of action underlying ES-induced hepatocarcinogenesis remain uncertain because conventional genotoxicity tests for ES yield negative results. In the present study, taking notice of the fact that there is a sex difference in SULT1A1 activity in the mouse liver, we assessed the frequency of micronuclei in polychromatic erythrocytes and the mutant frequency (MF) of reporter genes in female gpt delta mice treated with ES at doses of 0 (corn oil), 37.5, 75, 150 or 300mg/kg body weight (bw) by gavage for 13 weeks. Results were compared with those obtained in males. Since one female was found dead at week one, the highest dose was reduced to 250mg/kg bw in females from week two. As reported previously in C57BL/6 mice, the mRNA levels of Sult1a1 in female gpt delta mice were significantly higher than those in the males. The levels of ES-specific DNA adducts in the females were higher than those in the males at all doses except the highest dose. In addition, MFs of the gpt gene were significantly increased from doses of 75mg/kg bw of females, but the increment was observed only at the highest dose in males. There were no changes in the micronucleus test among the groups. Thus, the overall data suggest that specific DNA modifications by the SULT1A1-mediated carbocation formation and the resultant genotoxicity are key events in the early stage of ES-induced hepatocarcinogenesis of mice.

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

Previous reports have proposed that reactive oxygen species resulting from induction of cytochrome P450 (CYP) isozymes might be involved in the modes of action of hepatocarcinogens with CYP‐inducible potency. In the present study, we investigated 8‐hydroxydeoxyguanosine (8‐OHdG) levels, in vivo mutagenicity and glutathione S‐transferase placental form (GST‐P)‐positive foci in the livers of gpt delta rats treated with piperonyl butoxide (PBO) or phenobarbital (PhB) for 4 and 13 weeks. Significant elevations in Cyp 1A1 and Cyp 1A2 mRNA levels after PBO treatment, and in Cyp 2B1 mRNA levels after PBO or PhB treatment, appeared together with remarkable hepatomegaly through the experimental period. Time‐dependent and statistically significant increases in 8‐OHdG levels were observed in the PBO treatment group along with significant increases in proliferating cell nuclear antigen (PCNA)‐positive hepatocytes at 4 weeks, while no increase in 8‐OHdG levels was found in PhB‐treated rats. No changes in mutant frequencies of gpt and red/gam (Spi‐) genes in liver DNA from PBO‐ or PhB‐treated rats were observed at 4 or 13 weeks. A 13‐week exposure to either PBO or PhB did not affect the number and area of GST‐P‐positive hepatocytes. CYP 1A1 and 1A2 induction may be responsible for elevated levels of 8‐OHdG in PBO‐treated rats. However, neither GC:TA transversions nor deletion mutations, typically regarded as 8‐OHdG‐related mutations, were observed in any of the treated rats. We conclude that reactive oxygen species, possibly produced through CYP catalytic pathways, likely induced genomic DNA damage but did not give rise to permanent gene mutation. (Cancer Sci 2010; 101: 2525–2530)

Comprehensive toxicity study of safrole using a medium-term animal model with gpt delta rats.

In order to investigate a medium-term animal model using reporter gene transgenic rodents in which general toxicity, genotoxicity and carcinogenicity are evaluated, F344 gpt delta rats were given a diet containing 0.1% and 0.5% (a carcinogenic dose) safrole for 13 weeks. Serum biochemistry and histopathological examinations revealed overt hepatotoxicity of safrole, in line with previous reports. In the current study, safrole treatment possibly resulted in renal toxicity in male rats. In the in vivo mutation assays, an increase or a tendency to increase of the gpt mutant frequencies (MFs) was observed in both sexes at the carcinogenic dose. The number and area of foci of glutathione S-transferase placental form (GST-P) positive hepatocytes, ratio of proliferating cell nuclear antigen (PCNA)-positive hepatocytes and 8-hydroxydeoxyguanosine (8-OHdG) levels in liver DNA were significantly increased in both sexes of the 0.5% group. The overall data suggested that the present model might be a promising candidate for investigating comprehensive toxicities of the agents. In addition, data demonstrating the base modification and cell proliferation due to exposure to safrole could contribute to understanding safrole-induced hepatocarcinogenesis, which imply expanding in application of this model.

In vivo genotoxicity of methyleugenol in gpt delta transgenic rats following medium-term exposure

Methyleugenol (MEG), which is commonly used as a fragrance and flavoring agent, has been shown to induce hepatocellular tumors in rodents. However, the role of genotoxicity as a possible mechanism of action is not fully understood even though the DNA-reactive metabolite of MEG has been identified. In this study, a gpt delta transgenic rat model was used to clarify whether genotoxic mechanisms are involved in MEG-induced hepatocarcinogenesis following medium-term exposure. F344 gpt delta rats were subjected to repeated oral administration of MEG at dosages of 0, 10, 30, or 100mg/kg (a carcinogenic dose) for 13 weeks. The relative weight of the liver of the male and female rats that were administered 100mg/kg MEG and the absolute weight of the liver of the male rats that were administered 100mg/kg MEG were significantly increased. In addition, the number and area of glutathione S-transferase placental form (GST-P) positive foci and proliferating cell nuclear antigen (PCNA) positive cell ratios in the hepatocytes were significantly increased in the male and female rats that were administered 100mg/kg MEG compared with the control animals. In the in vivo mutation assays, a significant increase in the gpt and Spi(-) mutant frequencies was observed in both sexes at the carcinogenic dose. These results suggest the possible participation of genotoxic mechanisms in MEG-induced hepatocarcinogenesis.

Oxidative DNA damage and in vivo mutagenicity caused by reactive oxygen species generated in the livers of p53‐proficient or ‐deficient gpt delta mice treated with non‐genotoxic hepatocarcinogens

Oxidative stress is thought to participate in chemical carcinogenesis and may trigger gene mutations. To accurately assess the carcinogenesis risk posed to humans by chemical exposure, it is important to understand the pathways by which reactive oxygen species (ROS) are generated and the effects of the resulting oxidative stress. In the present study, p53‐proficient and ‐deficient gpt delta mice were given pentachlorophenol (PCP), phenobarbital (PhB) or piperonyl butoxide (PBO), which are classified as non‐genotoxic hepatocarcinogens in rodents, at the respective carcinogenic doses for 13 weeks. Exposure to PCP or PBO, but not PhB, invoked significant increases in liver DNA 8‐hydroxydeoxyguanosine (8‐OHdG) levels. Treatment with PCP significantly increased mRNA levels of the gene encoding NAD(P):quinone oxidoreductase 1 (NQO1) in the liver, suggesting that redox cycling of the PCP metabolite tetrachlorohydroquinone gave rise to ROS. Exposure to PhB or PBO significantly elevated CYP 2B10 mRNA levels while NQO1 levels were also significantly increased in PBO‐treated mice. Therefore, in addition to involvement of the CYP catalytic pathway in the ROS‐generated system of PBO, catechol derivatives produced from the opening of the PBO functional group methylenedioxy ring probably resulted in ROS generation. However, PCP, PBO and PhB failed to increase gpt and red/gam gene mutations in the liver independently of p53. Overall, the action of oxidative stress by ROS derived from the metabolism of these carcinogens might be limited to cancer‐promoting activity, which supports the previous classification of these carcinogens as non‐genotoxic. Copyright