The effect of aging on the repeated-dose liver micronucleus assay

The effect of aging on the repeated-dose liver micronucleus assay using N-nitrosodipropylamine, quinoline, and carbendazim

The repeated dose liver micronucleus (RDLMN) assay has been sufficiently validated in terms of the numbers and types of chemicals studied. However, it remains unclear whether aging affects assay results. The OECD Test Guideline 407 (Repeated Dose 28-Day Oral Toxicity Study in Rodents) indicates that dosing should begin as soon as feasible after weaning and in any event before 9 weeks of age. Therefore, it is particularly important to determine whether there are age-related differences between 6 and 8 weeks of age at the start of dosing when considering the possibility of integrating this assay into a 4-week repeated dose general toxicity study. We evaluated the impact of the rats' age on the RDLMN assay with three chemicals: N-nitrosodipropylamine, quinoline, and carbendazim. There were no significant age-related differences for the first two chemicals, whereas a markedly higher frequency of micronucleated hepatocytes (MNHEPs) was observed in younger rats for carbendazim. However, regardless of the age of animals, micronucleus induction was detected in all three chemicals. Combined with the previous reports on clofibrate and diethylnitrosamine, we concluded that animals of any age from 6 to 8 weeks could be used in the RDLMN assay.

Hepatocyte proliferation activity in untreated rats, measured by immunohistochemical detection of Ki-67: The effect of age on the repeated-dose liver micronucleus assay

The repeated-dose liver micronucleus (RDLMN) assay is a widely accepted method for detecting genotoxic substances. We investigated the effect of animal age on this assay. Proliferation activity in the liver tissue of untreated rats at age = 3.5, 6, 8, 10, or 12 weeks was measured via immunohistochemical expression of Ki-67 protein. The percentage of Ki-67-positive hepatocytes decreased markedly with age, reaching very low levels after 10 weeks, indicating decline with age of proliferative capacities in the liver. We calculated the area under the curve (AUC) of the approximate curve generated from the percentage of Ki-67-positive cells, to estimate the hepatocyte proliferation activity over the dosing period in the two regimens of the 4-week RDLMN assay: dosing initiated at age = 6 or 8 weeks. Hepatocyte proliferation activity of the former regimen was approximately double that of the latter. We also calculated the AUC for the juvenile-rat method, in which rats are treated for two days at age = 3.5 weeks. The AUC calculated for that method was approximately half of that for the 4-week repeated-dosing regimen initiated at 6 weeks of age. These findings suggest that the 4-week RDLMN assay with dosing initiated at age = 6 weeks could be approximately twice as sensitive as the other two methods.

The effect of aging on the repeated-dose liver micronucleus assay using diethylnitrosamine

Background

The repeated-dose liver micronucleus (RDLMN) assay has been well-developed and applied because of its simplicity and the ease of integration into general toxicity studies which is the preferred method from the 3R’s point of view. In this assay, we observed micronucleated hepatocytes which accumulated during a rather long-term dosing period. When considering integration into general toxicity studies, the effects of age of the animals used in the micronucleus assay becomes a major issue. The effect of age on the micronucleus induction rate has been reported in bone marrow micronucleus assays, and it is considered that the decrease in cell proliferation rate due to aging is the cause of the decrease in sensitivity. A decrease in sensitivity due to aging was also reported in a liver micronucleus assay using clofibrate and the cause is considered to be a decrease in hepatocyte proliferation activity due to aging. However, no actual decrease in hepatocyte proliferation rate due to aging has been reported. In addition, there are no reports, so far, on whether similar effects of aging appear when other substances were administered. To investigate the effects of aging in the RDLMN assay, this study focused on the effects of 14-day repeated administration of DEN, a well-known genotoxic hepatocarcinogen with the hepatocyte toxicity which should cause an elevation of cell proliferation rate as a reflective regeneration.

Results

The liver micronuclei induced by DEN were equivalent between the two age groups (i.e., six and eight weeks of age at the start of dosing). In the histopathological examination for the liver, single cell necrosis, karyomegaly, and increased mitosis were observed in the hepatocytes, and the frequency and severity were increased dose-dependently. Ki-67 immunohistochemical analysis which can detect all cells in the cell cycle other than those in the G0 phase revealed dose-dependent increase of cell proliferation activity, and the difference between ages was not observed.

Conclusion

The effect of aging on the RDLMN assay could not be recognized when DEN was administered for 14 days in rats. Meanwhile, it was supported by the histopathological examination and Ki-67 immunohistochemical analysis that such an effect of aging was masked by the compensatory hepatocyte proliferation which was induced by the hepatocyte toxicity of DEN.