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

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.

Improvement and validation of a medium-term gpt delta rat model for predicting chemical carcinogenicity and underlying mode of action

We have developed a new medium-term animal model, "GPG", in which an in vivo mutation assay in partially hepatectomized tissue and a tumor-promoting assay were performed. The tumor-promoting assay measures glutathione S-transferase placental form positive foci induced by diethylnitrosamine (DEN) in the residual tissue. Given that a limitation of the original protocol is the potential interaction between the test chemical and DEN, the present study establishes a modified protocol that includes a test chemical washout period. Using CYP2E1 inhibitor and CYP1A or CYP2B inducers, a period of 2 weeks after cessation of exposure to the chemicals was confirmed to be sufficient to return their enzymatic activities to normal levels. Additionally, to avoid the effects of DEN on the pharmacokinetics of the test chemical, re-exposure to the test chemical started 1 week after DEN injection, in which tumor-promoting activities were clearly detected. Consequently, a modified protocol has been established with 2- and 1-week washout periods before and after DEN injection, respectively. The applicability of the modified protocol was demonstrated using the genotoxic hepatocarcinogen, estragole (ES), the genotoxic renal carcinogen, aristolochic acid (AA), and the non-genotoxic hepatocarcinogens, β-naphthoflavone and barbital. Furthermore, the increase of cell cycle-related parameters in ES-treated livers, but not in AA-treated livers, may indicate that the liver is not the carcinogenic target site of AA despite its genotoxic role. Thus, since various parameters related to carcinogenesis can be evaluated concurrently, the GPG model could be a rapid and reliable assay for the assessment of human cancer hazards.

Suppressive effect of liver tumor-promoting activities in rats subjected to combined administration of phenobarbital and piperonyl butoxide

Phenobarbital (PB) is a cytochrome P450 (CYP) 2B inducer, and piperonyl butoxide (PBO) is a CYP1A/2B inducer. These inducers have liver tumor-promoting effects in rats. In this study, we performed a rat two-stage liver carcinogenesis bioassay to examine the tumor-promoting effect of PB and PBO co-administration. Male rats received an intraperitoneal injection of N-diethylnitrosamine (DEN) for initiation. Two weeks after DEN administration, rats were given PB (60 or 120 ppm in drinking water), PBO (1,250 or 2,500 ppm in diet) or 60 ppm PB+1,250 ppm PBO for 6 weeks. One week after the PB/PBO treatment, all rats were subjected to a two-thirds partial hepatectomy. To evaluate the effect of the combined administration, we used two statistical additive models. In the isoadditive model, the average values of the area of GST-P positive foci in the PB+PBO group were significantly lower than those in the High PB or High PBO groups. In the heteroadditive model, the net values of Cyp1a1 mRNA level and microsomal reactive oxygen species (ROS) production in the PB+PBO group were significantly lower than the sum of those in the Low PB or Low PBO groups. On the contrary, there was no interactive effect in the PCNA-positive hepatocyte ratio, mRNA levels of Cyp2b1/2, Gstm3, Gpx2 and Nqo1, and the level of thiobarbituric acid-reactive substances in the PB+PBO group. These results suggest that PB and PBO co-administration causes suppressive effects in liver tumor-promoting activity in rats resulting from inhibited microsomal ROS production because of suppression of CYP1A induction.

Lack of nrf2 results in progression of proliferative lesions to neoplasms induced by long-term exposure to non-genotoxic hepatocarcinogens involving oxidative stress

To explore the role of oxidative stress in chemical carcinogenesis driven by non-genotoxic mechanisms, nrf2-deficient (nrf2(-/-)) and nrf2-wild-type (nrf2(+/+)) mice were exposed to pentachlorophenol (PCP) at concentrations of 600 or 1200 ppm for 60 weeks, or piperonyl butoxide (PBO) at concentrations of 3000 or 6000 ppm in the diet for 52 weeks, respectively. Additional studies were performed to examine 8-hydroxydeoxyguanosine (8-OHdG) levels in liver DNA and hepatotoxicological parameters in serum following 8 weeks of exposure of each group to PBO at the same doses as in the long-term study. Exposure to 600 ppm PCP caused cholangiofibrosis (CF) only in nrf2(-/-) mice, while 1200 ppm PCP induced CF in both genotypes. Moreover, cholangiocarcinomas were found with significant incidence only in nrf2(-/-) mice treated with 1200 ppm PCP. Short-term exposure to 6000 ppm PBO caused significant elevation of 8-OHdG levels in both genotypes, while exposure to 3000 ppm caused a significant increase in 8-OHdG only in nrf2(-/-) mice. There were no inter-genotype changes in the incidences of regenerative hepatocellular hyperplasia (RHH) following long-term exposure to PBO. However, the incidence and multiplicity of hepatocellular adenomas, especially those observed in RHH, were much higher in nrf2-/- mice treated with 6000 ppm PBO than in nrf2+/+ mice treated with 6000 ppm PBO. Therefore, oxidative stress generated through PCP or PBO metabolism may promote the proliferation and progression of preneoplastic lesions to neoplasms.

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.

Corneal damage and lacrimal gland dysfunction in a smoking rat model

Smoking is a serious public health problem around the world and causes many diseases such as chronic obstructive pulmonary disease, lung cancer, and some eye diseases. Cytochrome P450s (CYPs) are xenobiotic-metabolizing enzymes and are distributed in the corneas, protecting the ocular surface against chemical compounds in the environment. Although CYPs are principally detoxification enzymes, CYP1A1 and CYP2A6 are known to participate in the induction of lung cancer by smoking. We studied the participation of CYPs in corneal dysfunction caused by exposure to mainstream cigarette smoke (MCS) in a smoking rat model. Six-week-old male Sprague-Dawley rats were exposed to MCS. Exposure to MCS caused corneal damage and lacrimal gland dysfunction. Immunohistochemical analysis revealed that CYP1A1 expression was upregulated in the corneal epithelium and ducts of the lacrimal glands, accompanied by an increase in production of reactive oxygen species (ROS). An increase in 8-hydroxy-2'-deoxyguanosine, which is a marker of oxidative DNA damage, was detected only in areas where CYP1A1 was expressed, whereas the level of hexanoyl-lysine adduct, which is an initial marker of oxidative damage of phospholipids, did not increase. Exposure to MCS damaged the corneas and lacrimal glands probably through DNA oxidation by ROS produced by CYP1A1. Although the influence of other components in MCS remains unclear, CYPs, especially CYP1A1, probably participate in corneal damage and lacrimal gland dysfunction induced by smoking.

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.