A medium-term gpt delta rat model as an in vivo system for analysis of renal carcinogenesis and the underlying mode of action

In vivo mutagenicity and tumor-promoting activity of 1,3-dichloro-2-propanol in the liver and kidneys of gpt delta rats

1,3-Dichloro-2-propanol (1,3-DCP), a food contaminant, exerts carcinogenic effects in multiple organs, including the liver and kidneys, in rats. However, the underlying mechanisms of 1,3-DCP-induced carcinogenesis remain unclear. Here, the in vivo mutagenicity and tumor-promoting activity of 1,3-DCP in the liver and kidneys were evaluated using medium-term gpt delta rat models previously established in our laboratory (GPG and GNP models). Six-week-old male F344 gpt delta rats were treated with 0 or 50 mg/kg body weight/day 1,3-DCP by gavage for 4 weeks. After 2 weeks of cessation, partial hepatectomy or unilateral nephrectomy was performed to collect samples for in vivo mutation assays, followed by single administration of diethylnitrosamine (DEN) for tumor initiation. One week after DEN injection, 1,3-DCP treatment was resumed, and tumor-promoting activity was evaluated in the residual liver or kidneys by histopathological analysis of preneoplastic lesions. gpt mutant frequencies increased in excised liver and kidney tissues following 1,3-DCP treatment. 1,3-DCP did not affect the development of glutathione S-transferase placental form-positive foci in residual liver tissues, but enhanced atypical tubule hyperplasia in residual kidney tissues. Detailed histopathological analyses revealed glomerular injury and increased cell proliferation of renal tubular cells in residual kidney tissues of rats treated with 1,3-DCP. These results suggested possible involvement of genotoxic mechanisms in 1,3-DCP-induced carcinogenesis in the liver and kidneys. In addition, we found that 1,3-DCP exhibited limited tumor-promoting activity in the liver, but enhanced clonal expansion in renal carcinogenesis via proliferation of renal tubular cells following glomerular injury.

Comprehensive expression analysis of mRNA and microRNA for the investigation of compensatory mechanisms in the rat kidney after unilateral nephrectomy

Compensation is a physiological response that occurs during chemical exposure to maintain homeostasis. Because compensatory responses are not usually considered adverse effects, it is important to understand compensatory mechanisms for chemical risk assessment. Although the kidney is a major target organ for toxicity, there is controversy over whether hyperplasia or hypertrophy contributes to the compensatory mechanism, and there is limited information to apply for chemical risk assessment. In the present study, compensatory mechanisms of the kidney were investigated in a unilateral nephrectomy (UNx) model using adult male and female F344 rats. In residual kidneys of male and female rats after UNx, 5-bromo-2'-deoxyuridine-labeling indices and mRNA expression of cell cycle-related genes were increased, although there were no fluctuations in mRNA expression of transforming growth factor-β1, which contributes to hypertrophy in renal tubules. Pathway analysis using mRNA expression data from a complementary DNA (cDNA) microarray revealed that canonical pathways related to cell proliferation were mainly activated and that forkhead box M1 (FOXM1) was an upstream regulator of compensatory cell proliferation in residual kidneys of male and female rats. cDNA microarray for microRNAs (miRNAs) demonstrated that nine miRNAs were downregulated in residual kidneys, and mRNA/miRNA integrated analysis indicated that miRNAs were associated with the expression of factors downstream of FOXM1. Overall, these results suggested that FOXM1-mediated hyperplasia rather than hypertrophy contributed to compensatory mechanisms in the kidney and that miRNAs regulated downstream FOXM1 signaling. These results will be beneficial for evaluating nephrotoxicity in chemical risk assessment and for developing new biomarkers to predict nephrotoxicity.

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.

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.

Past, Present and Future Directions of gpt delta Rodent Gene Mutation Assays

Genotoxicity is a critical endpoint of toxicity to regulate environmental chemicals. Genotoxic chemicals are believed to have no thresholds for the action and impose genotoxic risk to humans even at very low doses. Therefore, genotoxic carcinogens, which induce tumors via genotoxic mechanisms, are regulated more strictly than non-genotoxic carcinogens, which induce tumors through non-genotoxic mechanisms such as hormonal effects, cell proliferation and cell toxicity. Although Ames bacterial mutagenicity assay is the gold standard to identify genotoxicity of chemicals, the genotoxicity should be further examined in rodents because Ames positive chemicals are not necessarily genotoxic in vivo. To better evaluate the genotoxicity of chemicals in a whole body system, gene mutation assays with gpt delta transgenic mice and rats have been developed. A feature of the assays is to detect point mutations and deletions by two distinct selection methods, ie, gpt and Spi- assays, respectively. The Spi- assay is unique in that it allows analyses of deletions and complex DNA rearrangements induced by double-strand breaks in DNA. Here, I describe the concept of gpt delta gene mutation assays and the application in food safety research, and discuss future perspectives of genotoxicity assays in vivo.