Possible involvement of oxidative stress in dicyclanil-induced hepatocarcinogenesis in mice

Insecticides that Interfere with Insect Growth and Development

The insecticides discussed in this chapter target structures and physiological systems unique to insects and similar creatures and are consequently of low mammalian toxicity. This is despite one group, the ecdysone agonists, targeting an insect steroid hormonal system. Curiously, a remarkably large number of these compounds, notwithstanding their structural diversity, have adverse effects on the mammalian haematological system. As these compounds target insect development, they are not ‘knock-down’ insecticides and are not very effective against established adult insect infestation.

Veterinary Pesticides

Veterinary pesticides are used to treat a range of parasitic conditions in companion and farm animals. These products are based on a number of different compounds with different modes of action and different spectra of toxicity. The older agents include the synthetic pyrethroids and organophosphorus compounds, while the newer examples include, for example, representatives of the insect growth promoters, the neonicotinoids, and the oxadiazones. For many of these compounds, toxicity is associated with their pharmacological activity or mode of action. Thus the synthetic pyrethroids and the organophosphorus compounds exert neurotoxic effects. For others, toxicity may be associated with mechanisms that are independent of their mode of action. When used according to the manufacturer's instructions, these products are generally safe and efficacious. However, accidental contamination and misuse can lead to toxicity in operators and treated animals. These compounds are important in the treatment of parasitic disease in animals and their regulation and uses are based on favourable risk-benefit outcomes.

Evaluation of in vivo genotoxic potential of fenofibrate in rats subjected to two-week repeated oral administration

Fenofibrate (FF), a peroxisome proliferator-activated receptor-alpha agonist, has been used as one of the hypolipidemic drugs in man and induces oxidative stress and promotes hepatocarcinogenesis in the liver of rodents. This chemical belongs to a class of non-genotoxic carcinogens, but DNA damage secondary to oxidative stress resulting from reactive oxygen species (ROS) generation is suspected in rodents given this chemical. To examine whether FF has genotoxic potential, partially hepatectomized F344 male rats were treated orally with 0, 1,000 or 2,000 mg/kg of FF for 2 weeks, followed by diet containing 0.15% 2 acetyl aminofluorene (2 AAF) for enhancement the tumor-promoting effect for 10 days and a single oral dose of carbon tetrachloride (CCl4) as the first experiment (liver initiation assay). As the second experiment, the in vivo liver comet assay was performed in hepatectomized rats, and the expression of some DNA repair genes was examined. In the liver initiation assay, the number and area of glutathione S-transferase placental form (GST-P)-positive single cells and foci did not increase in the FF treated groups. In the comet assay, positive results were obtained after 3 h of the last treatment of FF, and the expression of some DNA repair genes such as Apex1, Ogg1 and Mlh1 were upregulated in rats given the high dose of FF at 3 h after the treatment but not in 24 h after the treatment. The results of the present study suggest that FF causes some DNA damage in livers of rats, but is not a strong genotoxic substance leading to a DNA mutation since such DNA damage was repaired by the increased activity of some DNA repair genes.

Suppressive effect of Siraitia grosvenorii extract on dicyclanil-promoted hepatocellular proliferative lesions in male mice

Dicyclanil (DC) generates reactive oxygen species (ROS) due to Cyp1a1 induction, and DNA damage caused by oxidative stress is probably involved in hepatocarcinogenesis in mice. To clarify the modifying effect of the Siraitia grosvenorii extract (SGE), which has antioxidative properties, we employed a 2-stage liver carcinogenesis model in partially hepatectomized male ICR mice. Mice maintained on diet containing DC at a concentration of 1,500 ppm for 9 weeks after a single intraperitoneal injection of diethylnitrosamine (DEN) at a dose of 30 mg/kg and they were given water containing 2,500 ppm of SGE for 11 weeks including 2 weeks as pre-administration on DC. SGE inhibited the induction of gamma-glutamyltranspeptidase-positive hepatocytes, lipid peroxidation, and gene expression of Cyp1a1, all of which were caused by DC. To examine whether SGE indirectly inhibits Cyp1a1 expression induced by inhibition of aryl hydrocarbon receptor (Ahr)-mediated signal transduction caused by DC, mice with high (C57BL/6J mice) and low affinities (DBA/2J mice) to Ahr were given DC-containing diet and/or SGE-containing tap water for 2 weeks. Cyp1a1 gene expression was significantly lower in C57BL/6J mice administered DC + SGE than in C57BL/6J mice administered DC alone; there was no difference in the Cyp1a1 expression between DBA/2J mice administered DC + SGE and DC alone. These results suggest that SGE suppresses the induction of Cyp1a1, leading to inhibition of ROS generation and consequently inhibited hepatocarcinogenesis, probably due to suppression of Ahr activity.

Concurrent administration of ascorbic acid enhances liver tumor-promoting activity of kojic acid in rats

We previously found that administration of ascorbic acid (AA) enhances the liver tumor-promoting activity of kojic acid (KA) in mice. To examine the reproducibility of these results in rats and the underlying mechanism of this effect, we employed a two-stage liver carcinogenesis model using male F344 rats. Two weeks after initiation with diethylnitrosamine (DEN), the animals received a diet containing 2% KA and drinking water with or without 5,000 ppm AA for a period of 7 weeks. A DEN-alone group was also established as a control. One week after the commencement of the administration, the animals were subjected to two-thirds partial hepatectomy. At the end of the experiment, the livers were analyzed immunohistochemically, and the mRNA expression level and extent of lipid peroxidation were measured. AA treatment enhanced the KA-induced tumor-promoting activity in terms of the number and area of liver cell foci that were positive for glutathione-S-transferase placental form. AA coadministration increased the number of hepatocytes positive for proliferating cell nuclear antigen and inversely decreased the number of TUNEL-positive cells. However, the increased level of thiobarbituric acid reactive substances resulting from KA treatment was suppressed by coadministration of AA. Gene expression analyses using low-density microarrays and real-time RT-PCR showed that coadministration of AA resulted in upregulation of genes related to cell proliferation and downregulation of those involved in apoptosis and/or cell cycle arrest. These results indicate that the concerted effects of AA on cell proliferation and apoptosis/cell cycle arrest probably through its antioxidant activity are involved in this enhancement.

Enhancement of hepatocellular proliferative activity of kojic acid in mice by a simultaneous administration of ascorbic acid

To examine the tumor modification activity of kojic acid (KA) by sodium ascorbic acid (AA), 5-week-old male ICR mice were administered intraperitoneally with N-diethylnitrosamine (DEN) as an initiation treatment. Two weeks after the initiation treatment, animals were fed basal diet containing 0 (Group 1: DEN alone) or 3% KA (Group 3: DEN+KA), drinking water containing 5,000 ppm AA (Group 2: DEN+AA) or 3% KA and 5,000 ppm AA (Group 4: DEN+KA+AA) for 6 weeks. One week after the administration of KA and/or AA, all mice were subjected to two-thirds partial hepatectomy. At the end of the experimental period, all surviving mice were sacrificed and removed the liver. The liver weights of the Groups 3 and 4 were significantly increased, and the number of proliferating cell nuclear antigen positive hepatocytes and the gene expressions of Ccnc, Ccnd1, Ercc and Cyp7a1 were significantly increased in the Group 4, as compared to the Group 1. These results of the present study suggest that AA enhances the hepatocellular proliferative activity of KA in mice.

Detection of oxidative DNA damage, cell proliferation and in vivo mutagenicity induced by dicyclanil, a non-genotoxic carcinogen, using gpt delta mice

To ascertain whether measurement of possible contributing factors to carcinogenesis concurrently with the transgenic mutation assay is useful to understand the mode of action underlying tumorigenesis of non-genotoxic carcinogens, male and female gpt delta mice were given dicyclanil (DC), a mouse hepatocarcinogen showing all negative results in various genotoxicity tests, at a carcinogenic dose for 13 weeks. Together with gpt and Spi(-) mutations, thiobarbituric acid-reactive substances (TBARS), 8-hydroxydeoxyguanosine (8-OHdG) and bromodeoxyuridine labeling indices (BrdU-LIs) in the livers were examined. Whereas there were no changes in TBARS levels among the groups, significant increases in 8-OHdG levels and centrilobular hepatocyte hypertrophy were observed in the treated mice of both genders. In contrast, BrdU-LIs and liver weights for the treated females, but not the males were significantly higher than those for the controls. Likewise, the gpt mutant frequencies (MFs) in the treated females were significantly elevated, GC:TA transversion mutations being predominant. No significant alterations were found in the gpt MFs of the males and the Spi(-) MFs of both sexes. The results for the transgenic mutation assays were consistent with DC carcinogenicity in terms of the sex specificity for females. Considering that 8-OHdG induces GC:TA transversion mutations by mispairing with A bases, it is likely that cells with high proliferation rates and a large amounts of 8-OHdG come to harbor mutations at high incidence. This is the first report demonstrating DC-induced genotoxicity, the results implying that examination of carcinogenic parameters concomitantly with reporter gene mutation assays is able to provide crucial information to comprehend the underlying mechanisms of so-called non-genotoxic carcinogenicity.

Molecular pathological analysis for determining the possible mechanism of piperonyl butoxide-induced hepatocarcinogenesis in mice

Piperonyl butoxide (PBO), alpha-[2-(2-butoxyethoxy)ethoxy]-4,5-methylene-dioxy-2-propyltoluene, is widely used as a synergist for pyrethrins. In order to clarify the possible mechanism of non-genotoxic hepatocarcinogenesis induced by PBO, molecular pathological analyses consisting of low-density microarray analysis and real-time reverse transcriptase (RT)-PCR were performed in male ICR mice fed a basal powdered diet containing 6000 or 0 ppm PBO for 1, 4, or 8 weeks. The animals were sacrificed at weeks 1, 4, and 8, and the livers were histopathologically examined and analyzed for gene expression using the microarray at weeks 1 and 4 followed by real-time RT-PCR at each time point. Reactive oxygen species (ROS) products were also measured using liver microsomes. At each time point, the hepatocytes of PBO-treated mice showed centrilobular hypertrophy and increased lipofuscin deposition in Schmorl staining. The ROS products were significantly increased in the liver microsomes of PBO-treated mice. In the microarray analysis, the expression of oxidative and metabolic stress-related genes--cytochrome P450 (Cyp) 1A1, Cyp2A5 (week 1 only), Cyp2B9, Cyp2B10, and NADPH-cytochrome P450 oxidoreductase (Por) was over-expressed in mice given PBO at weeks 1 and 4. Fluctuations of these genes were confirmed by real-time RT-PCR in PBO-treated mice at each time point. In additional real-time RT-PCR, the expression of Cyclin D1 gene, key regulator of cell-cycle progression, and Xrcc5 gene, DNA damage repair-related gene, was significantly increased at each time point and at week 8, respectively. These results suggest the possibility that PBO has the potential to generate ROS via the metabolic pathway and to induce oxidative stress, including oxidative DNA damage, resulting in the induction of hepatocellular tumors in mice.