A no observed adverse effect level for DNA adduct formation in rat liver with prolonged dosing of the hepatocarcinogen 2-acetylaminofluorene

Abstract

We have previously reported that the DNA-reactive hepatocarcinogen 2-acetylaminofluorene (AAF) produced in rat liver several key effects that were less than linear over a range of repeat doses, and that at low cumulative doses, no-adverse-effect-levels (NOAEL) were observed for several effects including hepatocellular cytotoxicity, enhanced cell proliferation, induction of preneoplastic foci and promotable liver neoplasms, although DNA adducts were still formed at dosages below the lowest NOAEL of 28 mg kg −1 cumulative dose for these other effects. This report details two further dose-effect studies at lower repeat doses than those previously used as well as identification of the specific types of DNA adducts formed.AAF was administered orally to male F344 rats by gavage at repeat dosages which in one experiment (A) ranged from 0.01 to 2.24 mg kg −1 bw per day, 7 days per week for 12 weeks followed by recovery for 4 weeks, and in a second (B) at lower dosages of 0.0026 or 0.026 mg kg −1 bw per day 3 days per week for 16 weeks. Initially the nonacetylated guanine adduct, N-(deoxyguanosine-8-yl)-aminofluorene, predominated. With continued dosing, the pattern of adducts changed such that by 4 weeks more acetylated, N-(deoxyguanosine-N 2 -yl)-AAF and N-(deoxyguanosine-8-yl)-AAF, adducts were present. In experiment A, total adducts reached a maximum by 12 weeks with levels of 6.0 adducts per 10 8 nucleotides at the lowest dosage. In experiment B, the total DNA adducts at the lowest dosage was below the limit of detection at 12 weeks, and at 0.6 in 10 8 nucleotides at 16 weeks, levels within the background range of 1.0-3.1 per 10 8 nucleotides. Thus, the cumulative dose of 0.125 mg kg −1 bw over 16 weeks was a NOAEL for adducts and hence would be predicted to be a threshold for hepatocarcinogenicity. Fig. 7 Dose-response curve for CD1B/CD2B dose levels. Experiment B; values of the DNA adducts level in the control group over time are presented as an average. Paper Toxicology Research 238 | Toxicol. Res., 2015, 4, 233-240This journal is

Effect of 2-acetylaminofluorene and its genotoxic metabolites on DNA adduct formation and DNA damage in 3D reconstructed human skin tissue models

“…One possible explanation is that the N -OH-2-AF-derived DNA adducts are more potent inducers of DNA strand breaks than the N -OH-2-AAF-derived DNA adducts, although dG-C8-AF is assumed to be the major adduct formed after treatment with both N -OH-2-AF and N -OH-2-AAF. However, 32 P-postlabelling does not allow direct structural identification of the DNA adducts formed ( 59 , 60 ) and 2-AAF has been shown to be capable of forming multiple adducts which can be labelled with different efficiency during 32 P-postlabelling ( 66 , 68 ). Differences in the short- and long-term persistence of 2-AAF-induced DNA adducts have been reported ( 64 , 68 , 76 ).…”

Carcinogenicity of multi-walled carbon nanotubes: challenging issue on hazard assessment

“…Moreover, rats exposed to low doses of two other genotoxic hepatocarcinogens contained in food, N -nitrosodiethylamine and 2-amino-3-methylimidazo[4,5- f ]quinoline, also showed the existence of different no-effect levels for different parameters relevant to carcinogenesis 70 , 73) . In addition, Williams et al 74 , 75) found NOAEL for DNA adduct formation in the livers of rats treated with low doses of the genotoxic hepatocarcinogen, 2-acetylaminofluorene. These studies concluded that a threshold, at least a practical threshold, exists for the carcinogenicity of genotoxic carcinogens, including direct mutagens.…”

Mechanisms of DNA-reactive and epigenetic chemical carcinogens: applications to carcinogenicity testing and risk assessment

“…However, current research using rigorous approaches reveals no-effect levels for DNA-reactive carcinogens. [56][57][58] Nevertheless, the features of DNA-reactive carcinogens are indicative of a high degree of hazard to potentially exposed humans (see below) and it is probably for this reason that the majority of carcinogens recognized by the International Agency for Research on Cancer (IARC) 59 as having caused human cancer is of the DNA-reactive type (Table 2).…”

“…The concepts of dose-effect relationships and thresholds are discussed in detail by Calabrese. 174 The most extensive investigations of thresholds have been in rat liver carcinogenesis where no-observed-adverse-effect-levels (NOAEL) have been documented for DNA adduct formation, 57 induced cell proliferation, 56 formation of preneoplastic lesions 56,175,176 and development of promotable neoplasms. 169 The accumulated data base supports the concept of thresholds for effects of DNA-reactive carcinogens.…”