Carcinogenesis induced by a single administration of 1,2-diethylhydrazine in female rats of various ages

Attenuation of DNA polymerase beta-dependent base excision repair and increased DMS-induced mutagenicity in aged mice

The biological mechanisms responsible for aging remain poorly understood. We propose that increases in DNA damage and mutations that occur with age result from a reduced ability to repair DNA damage. To test this hypothesis, we have measured the ability to repair DNA damage in vitro by the base excision repair (BER) pathway in tissues of young (4-month-old) and old (24-month-old) C57BL/6 mice. We find in all tissues tested (brain, liver, spleen and testes), the ability to repair damage is significantly reduced (50-75%; P<0.01) with age, and that the reduction in repair capacity seen with age correlates with decreased levels of DNA polymerase beta (beta-pol) enzymatic activity, protein and mRNA. To determine the biological relevance of this age-related decline in BER, we measured spontaneous and chemically induced lacI mutation frequency in young and old animals. In line with previous findings, we observed a three-fold increase in spontaneous mutation frequency in aged animals. Interestingly, lacI mutation frequency in response to dimethyl sulfate (DMS) does not significantly increase in young animals whereas identical exposure in aged animals results in a five-fold increase in mutation frequency. Because DMS induces DNA damage processed by the BER pathway, it is suggested that the increased mutagenicity of DMS with age is related to the decline in BER capacity that occurs with age. The inability of the BER pathway to repair damages that accumulate with age may provide a mechanistic explanation for the well-established phenotype of DNA damage accumulation with age.

Genotoxic activation of hydrazine, two dialkylhydrazines, thiourea and ethylene thiourea in the somatic w/w + assay of Drosophila melanogaster

Genotoxic activation of hydrazine (HZ), two symmetrical dialkylhydrazines, namely, 1,2dimethylhydrazine and 1,2-diethylhydrazine (SDMH and SDEH), thiourea (TU) and ethylene thiourea (ETU) has been evaluated by means of the w/w + somatic assay of Drosophila. Both low bioactivation insecticide-susceptible (IS) and high biotransformation insecticide-resistant (IR) strains were used. The combined application of insecticide-susceptible and insecticide-resistant strains should, in principle, detect somatic cell recombinagens in the Drosophila melanogaster in vivo w/w + assay. The IS strain was more susceptible to toxicity induced by the test chemicals than the IR stocks. Its performance in the biotransformation of the chemicals tested was rather poor. TU was inactive in all strains. With the active compounds, spot frequencies increased approximately linearly with dose for each spot type. SDEH gave a strong positive result in all three female genotypes exposed. HZ, ETU and SDMH were overall weakly positive in the IR strain Haag-79 (HG-R). Interestingly, ETU was clearly positive in the IR Hikone-R (HK-R) strain. A comparison of the recombinagenic potencies between the active and the weakly positive compounds, and among strains, showed pronounced genotype-dependent differences between the low and the high bioactivation strains. The ability of Drosophila to express several procarcinogens in relation to insecticide-resistance after activation catalyzed by CYP450 enzymes is discussed.

Effect of aging and interval between primary and secondary treatment in carcinogenesis induced by neonatal exposure to 5-bromodeoxyuridine and subsequent administration of N-nitrosomethylurea in rats

LIO rats were exposed to s.c. injections (3.2 mg) of a synthetic analogue of thymidine, 5-bromo-2'-deoxyuridine (BrdUrd) on the 1st, 3rd, 7th and 21st days of life and at the age of 3 or 15 months they were i.v. injected with N-nitrosomethylurea (NMU) at a single dose of 10 or 50 mg/kg or with solvent. It was shown that early neonatal exposure to BrdUrd was followed by the increase in the incidence of tumor development and by the decrease of their latency. The carcinogenic effect of NMU alone correlated with the dose of the carcinogen in 3-month-old rats total and malignant tumors and tumors of some localization was decreased in the elder ones, but survival of tumor-bearing rats was decreased in the elder group as compared to the younger one. These data suggests the age-related decrease in both the carcinogenic effect of NMU and in the number of events which are necessary for a tumor development. The exposure to BrdUrd was followed by the increase in the susceptibility of rats to subsequent carcinogenic effect of NMU injected at the doses of 10 or 50 mg/kg into 3- and 15-month-old rats, mostly to the tissues being target to NMU. Our data have demonstrated that the exposure to BrdUrd in the early life was followed by the irreversible initiating effect which persists over a long time in a several tissues.