Potent mitogenic activity of 4-acetylaminofluorene to the rat liver

Restriction site mutation (RSM) analysis of 2-acetylaminofluorene (2-AAF)-induced mouse liver mutations and comparison with the measurement of in vivo micronucleus induction in the bone marrows of (2-AAF)-treated mice

We report here the successful application of the restriction site mutation (RSM) assay in detecting 2-acetylaminofluorene (2-AAF)-induced mouse liver mutations. A total of seven 2-AAF-induced liver mutations were detected out of a total of 304 analyses performed on 2-AAF-treated liver tissue. No mutations were detected in the 190 RSM analyses performed on untreated liver tissue. The 2-AAF-induced point mutations comprised 60% GC-->TA transversions, 30% GC-->AT transitions, 10% GC-->CG transversions, and 1 insertional event was also detected. All seven mutations were detected in intron 6 of the mouse p53 gene, with no mutations detectable in exons 4 or 5, supporting our previous data on the greater mutability of intron regions. In addition to the RSM analysis, we also report the application of the in vivo bone marrow micronucleus assay in detecting the clastogenicity of 2-AAF. We detected a small, but statistically significant, increase in the number of micronuclei induced by 2-AAF, but only after 2,000 cells were scored. This also confirms previous data showing that 2-AAF is a weak clastogen. Finally, we attempted to compare the sensitivity of the two assays to 2-AAF-induced genotoxicity, as had been previously undertaken with ENU. Both assays detected genotoxicity in their respective tissues; however, different endpoints were analysed. The RSM assay appears to be more adaptable than the micronucleus assay, due to its tissue and organism independence and has the potential to provide more molecular information on genotoxicity. Teratogenesis Carcinog. Mutagen. 20:107-117, 2000.

Mitogenic short-term effects on hepatocytes and adrenocortical cells: phenobarbital and reserpine compared to carcinogenic and non-carcinogenic fluorene derivatives

The adrenal cortex has a low physiologic cell renewal and shows only a moderate cell replication even after contralateral adrenalectomy. Although rather unsusceptible to the malignancy-inducing action of carcinogens, a single oral dose of various tumorigenic xenobiotics induced an additive mitotic response of adrenocortical cells studied after 48 h. Presently we report on three different response patterns in rats. First, a selective mitostimulation of the zona glomerulosa occured after reserpine associated with a loss of body weight, thymus and liver weight. These are unspecific stress effects and occur also after exogenous ACTH. Second, hepatomitogenic and liver-enlarging congeners, e.g. fluorene (FEN), fluorenone (FON) and 4-benzoyl-FON, but also the genotoxic 2-acetylaminofluorene (2-AAF) and 2,4,7-trinitro-FON induced a selective mitotic response of the zona fasciculata (ZF). After the lowest effective dose of FEN or FON the afore-mentioned effects occured simultaneously, but were absent in the high dose group (only studied with fluorene). The 2-benzyl and 2-benzoyl-substituted derivatives were ineffective at all. Third, a bizonal response was found only after phenobarbital (PB) or the lowest effective FEN dose. The preventive action of a low PB dose on the 2-AAF-induced ZF response indicates a modified metabolism. We conclude that the rapid mitotic ZF response is an endogenously mediated net effect of interactions between metabolic and various adaptive mechanisms. The latter are reported to be activated in a stressor-dependent manner and converge in the adrenals. In this way the early mitotic ZF response could reflect indirectly 'specific' proliferation-prone properties of xenobiotics.

Alterations in epidermal growth factor binding to hepatic membranes following dietary exposure of rats to known hepatocarcinogens

Administration of the chemical carcinogen 2-acetylaminofluorene (2-AAF) has previously been shown to lower hepatic epidermal growth factor (EGF) binding levels during chemically induced hepatocarcinogenesis. To further characterize the specificity of this response, EGF binding levels for liver microsomes were determined after a 3-week administration of subacute doses of 2-AAF and five other known hepatocarcinogens: 3'-methyl-4-dimethylaminoazobenzene (3'Me-DAB), 2-AAF, aflatoxin B1 (AFB1), thioacetamide (TA), ethionine, benzidine (Benz), as well as four non-hepatocarcinogens: fluorene, p-aminoazobenzene, 4-acetylaminofluorene (4-AAF), and 3-methylcholanthrene. Five of six of the hepatocarcinogens tested (3'Me-DAB, 2-AAF, TA, AFB1 and Benz) caused significant lowering of EGF binding levels, and one of the four non-hepatocarcinogens (4-AAF) caused significant lowering of EGF binding levels. Paired feeding studies indicated that the decreases in EGF binding levels were not a result of differences in net diet consumption. These findings show that decreases in EGF binding capacity are caused by a diverse group of known hepatocarcinogenic compounds at an early stage in the carcinogenesis process.

An improved method of mouse liver micronucleus analysis: an application to age-related genetic alteration and polyploidy study

The performance of a micronucleus test in liver cells in vivo requires two laborious procedures: stimulation of hepatocytes to division and dissociation of liver tissue into a single-cell suspension. We propose the method of inhalation treatment of mice with carbon tetrachloride to induce cell proliferation and alkaline dissociation of previously fixed tissue. The micronucleus incidence and ploidy classes in terms of cytophotometric DNA content were determined in liver of mice of three age groups (around 2.5, 5.0 and 7.0 months old) after CCl4 treatment or partial hepatectomy. The data obtained show that both methods give the same results. The fraction of micronucleated hepatocytes was 0.69% at the age of 2.5 months; it increased to 8.5% and then to 13.5% at 5.0 and 7.0 months respectively. Simultaneously, the ploidy classes changed both with the aging of the animal and after induced liver regeneration. The percentage distribution of micronucleated cells by ploidy class showed that cells carrying micronuclei were the higher ploidies rather than the population in general. Since polyploid cells contain multiple molecular targets for genetic damage, the micronucleation index per genome unit was estimated. Then the real rate of accumulation of both intrinsic endogenous (and probably the exogenously induced) preclastogenic genetic alterations in hepatocytes during the adulthood of mice was evaluated to be 0.03% per diploid genome per day. This seems to be the first description of the phenomenon of liver cell aging in terms of micronuclear aberrations.

Genetic toxicity of 2-acetylaminofluorene, 2-aminofluorene and some of their metabolites and model metabolites

2-Acetylaminofluorene and 2-aminofluorene are among the most intensively studied of all chemical mutagens and carcinogens. Fundamental research findings concerning the metabolism of 2-acetylaminofluorene to electrophilic derivatives, the interaction of these derivatives with DNA, and the carcinogenic and mutagenic responses that are associated with the resulting DNA damage have formed the foundation upon which much of genetic toxicity testing is based. The parent compounds and their proximate and ultimate mutagenic and carcinogenic derivatives have been evaluated in a variety of prokaryotic and eukaryotic assays for mutagenesis and DNA damage. The reactive derivatives are active in virtually all systems, while 2-acetylaminofluorene and 2-aminofluorene are active in most systems that provide adequate metabolic activation. Knowledge of the structures of the DNA adducts formed by 2-acetylaminofluorene and 2-aminofluorene, the effects of the adducts on DNA conformation and synthesis, adduct distribution in tissues, cells and DNA, and adduct repair have been used to develop hypotheses to understand the genotoxic and carcinogenic effects of these compounds. Molecular analysis of mutations produced in cell-free, bacterial, in vitro mammalian, and intact animal systems have recently been used to extend these hypotheses.

Recommendations for the performance of UDS tests in vitro and in vivo

The Working Group (WG) dealt with the harmonization of routine methodologies of tests for unscheduled DNA synthesis (UDS) both in vitro and in vivo. In contrast to the existing guidelines from OECD, EPA and EC on in vitro UDS tests (there is no Japanese UDS guideline), the Working Group recommends that in general in vitro UDS tests should be performed with primary hepatocytes. For routine applications any other cell types would need special justification. Hepatocytes from male rats are preferable, unless there are contra-indications on the basis of e.g. toxicokinetic data. According to the OECD, EPA and EC guidelines, UDS may be analysed by means of autoradiography (AR) or liquid scintillation counting (LSC). The WG recommends use of AR. LSC is less suitable due to the problem of differentiation between UDS activity and replicative DNA synthesis, and the disadvantage that cells cannot be analysed individually. Since a specific cell type was recommended by the WG, methodological aspects could be described in more detail than in the present guidelines. For in vitro tests, it was agreed that the initial viability of freshly isolated hepatocytes should be at least 70%. With regard to the need for confirmatory experiments in the event of a clear-cut negative result, the majority view was that confirmation by a second (normally not identical) experiment is still needed; this is in line with the present OECD and EC guidelines. Evaluation of results from UDS tests should be based primarily on net nuclear grain (NNG) values, although it is recognised that nuclear and cytoplasmic grains result from different biological processes. Since grain counts are influenced by a number of methodological parameters, no global threshold NNG value can be recommended for discrimination of positive and negative UDS results. For in vitro assays, the criteria for positive findings go beyond those of the present guidelines and two alternative approaches are given which are based on (1) dose-dependent increases in NNG values and (2) reproducibility, dose-effect relationship and cytotoxicity. At present there is no official guideline on the performance of in vivo UDS tests. Some fundamental recommendations given for in vitro methodology also apply to the in vivo assay. For routine testing with the in vivo UDS test, again the general use of hepatocytes from male rats is recommended. However, concerning the requirement to use one or two sexes, consistency with other in vivo genotoxicity assays (e.g. the micronucleus assay) would be preferable. As for the in vitro methodology, AR is preferred rather than LSC.(ABSTRACT TRUNCATED AT 400 WORDS)

DNA repair is less efficient in the nuclear matrix than in non-matrix nuclear fractions in the liver of rats treated with 2-aminofluorene

The amount of DNA adducts and radioactive thymidine incorporation into DNA fractions attached and not attached to the nuclear matrix in the liver of rats treated with the carcinogen 2-aminofluorene (2-AF) were compared. The rate of [3H]thymidine incorporation was directly proportional to the amount of adducts in total hepatic DNA. Within the first 10 h after the carcinogen treatment, the level of adducts in the nuclear matrix DNA was higher than in the whole nuclei. The rate of [3H]thymidine incorporation into the nuclear matrix DNA was 5-30% lower than into DNA in whole nuclei at any time after 2-AF injection. We suggest that in rat liver cells, the 2-AF-induced DNA repair does not occur in close contact with the nuclear matrix.

An in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes as an early prediction assay for nongenotoxic hepatocarcinogens screening of 22 known positives and 25 noncarcinogens

To evaluate the applicability of an in vivo-in vitro replicative DNA synthesis (RDS) test using rat hepatocytes, we conducted the RDS test with 22 nongenotoxic (Ames-negative) hepatocarcinogens and 25 noncarcinogens under our standardized conditions and judgement criteria. Compared to controls (RDS incidence of under 1.0%), the RDS test gave positive results for 18 hepatocarcinogens (positive sensitivity: 82%), and negative results for 20 noncarcinogens (negative specificity: 80%), and thus the overall concordance was 81%. These findings strongly suggest that the RDS test is an extremely useful method for early detection of nongenotoxic hepatocarcinogens.

Comparison of micronucleus formation in mouse bone marrow and spleen

The frequencies of micronucleated erythrocytes were compared in bone marrow and spleen of mice killed 24 and 48 h after a single i.p. dose of one directly acting carcinogen, N-nitroso-N-ethylurea (NEU, 100 mg/kg), and two indirectly acting ones, N-nitrosodimethylamine (NDMA, 50 mg/kg) and 7,12-dimethylbenz[a]anthracene (7,12-DMBA, 50 mg/kg). The treated/control ratio of the incidence of micronucleated polychromatic erythrocytes (MnPCEs) was similar in the two tissues for NDMA at 24 h (sampling at 48 h was precluded by toxicity) and for 7,12-DMBA at 48 h, while it was higher in the bone marrow than in the spleen for NEU at both 24 and 48 h and for 7,12-DMBA at 24 h. Concerning micronucleated normochromatic erythrocytes (MnNCEs), their frequency in both tissues was always lower than that of MnPCEs; however, while in bone marrow a marked increase in their incidence was induced by NEU and 7,12-DMBA, any response was absent in spleen, thus suggesting that this organ does not sequester micronucleated erythrocytes. These results already indicate that the spleen is not a useful alternative to the bone marrow in the micronucleus assay. Moreover, counting of MnPCEs in the spleen is made more difficult and prone to error by the low frequency of PCEs, and by their greater toxicity-induced reduction. This last effect was found to be enhanced by the use of old mice.

Cytoskeleton modifications induced by phenobarbital, 2-acetylaminofluorene and 4-acetylaminofluorene in normal and initiated/selected hepatocytes: relation with the "resistant" phenotype

Initiated/selected (ISH) and normal (NH) rat hepatocytes were used to study cytoskeleton modifications induced by three liver acting chemicals: 2-AAF, a liver complete carcinogen; PB, a liver tumor promoter; and 4-AAF, a non-carcinogen analogue of 2-AAF. Cytoskeleton alterations were visualized by disappearance of F-actin fibers and tubulin depolymerization. The three drugs induced actin fragmentation in normal hepatocytes; a net loss of actin protein was observed with PB. They also induced varied tubulin depolymerization. The principal difference between chemicals is that 2-AAF led to non-reversible effects, in comparison with PB and 4-AAF which induced reversible damages on cytoskeleton. By contrast to normal hepatocytes, the cytoskeleton of ISH obtained from rats subjected to the "resistant" hepatocyte protocol was much less susceptible to the effect of the three chemicals. Moreover, we observed a lack of LDH release in the culture medium and a very rapid inducibility of GST activity after exposure of ISH to drugs. The moderate effect of the three chemicals on actin and tubulin in ISH could thus be explained by the "resistant" metabolic profile of these cells.

Relative sensitivity of 32P-postlabelling of DNA and the autoradiographic UDS assay in the liver of rats exposed to 2-acetylaminofluorene (2AAF)

Groups of male Alderley Park rats were dosed concomitantly with 2-acetylaminofluorene (2AAF) by gavage at doses between 0.01 mg/kg and 40 mg/kg, and livers sampled 2-72 h later. The liver of one group of animals was perfused to yield hepatocytes which were assayed in vitro for unscheduled DNA synthesis (UDS) via incorporation of tritiated thymidine and autoradiography. DNA was extracted from the livers of the other group and DNA adduct levels determined using the 32P-postlabelling technique. The major C-8 2-aminofluorene/guanosine adduct and 3 minor adducts were quantitated, enabling the relative sensitivity of the 2 techniques to be compared. A dose- and time-related UDS response was observed, which, at the most sensitive time-point (12 h) enabled DNA repair to be discerned at a dose level of 0.1-1 mg/kg of 2AAF, a response classified as formally positive at 5 mg/kg 2AAF. Only the C-8 adduct, as determined by 32P-postlabelling, was discernible at 0.01 mg/kg of 2AAF, although other adducts were visible on autoradiograms at higher dose levels. It is concluded that as part of a well-defined dose response, UDS can be discerned with confidence for doses of 2AAF between approximately 0.1 and 5 mg/kg, and DNA adducts for doses of 2AAF between approximately 0.01 and 1 mg/kg. Discernible UDS for 2AAF in the rat liver is apparent at approximately 13 DNA (total) adducts/10(8) nucleotides, or approximately 8 DNA (C-8) adducts/10(8) nucleotides. The presumed C-8 2-acetylaminofluorene/guanosine adduct, prepared by reaction of 2-acetoxy-2-acetylaminofluorene (2AAAF) with DNA, was a significant but unreliable marker of 2AAF/DNA adducts in the rat liver in vivo. DNA repair did not appear to remove DNA adducts selectively, and adducts remained in DNA when discernible DNA repair had ceased.

Lack of DNA single-strand breaks in rat liver cells exposed to 4-acetylaminofluorene, in vivo and in vitro

The induction of primary DNA damage by the non-carcinogen 4-AAF was reinvestigated in liver cells by comparison with the carcinogen 2-AAF. DNA alkaline elution showed the appearance of single-strand breaks in total liver DNA of rats 4 h after gavage with 200 mg/kg of 4-AAF. The decrease in hepatocyte viability and yield observed in these livers after collagenase perfusion indicated a cytotoxic effect of 4-AAF treatment. Viable hepatocytes isolated from 4-AAF-treated rats as well as hepatocytes from normal rats treated with 4-AAF in vitro did not present DNA single-strand breaks.

In vivo micronucleus test using mouse hepatocytes

The bone-marrow micronucleus (BMM) test is highly specific for clastogenic effects but its sensitivity is determined to a great extent by the substances tested, particularly by their metabolism. Some compounds, such as unstable mutagens or those which generate short-lived metabolites, are not detected in this test because the metabolites produced in the liver do not reach the bone marrow. In an attempt to provide qualitative and quantitative assessments of chromosomal mutations produced in vivo by genotoxic agents not detected in the mouse BMM test, a mouse-liver micronucleus test, adapted from Tates model, was developed. The animals were treated twice, with an interval of 24 h between treatments, and then subjected to partial hepatectomy (PH) 24 h after the second treatment in order to induce mitotic stimulation. The incidence of micronucleated hepatocytes was determined 96 h after PH. The test was evaluated with 5 procarcinogens, each with a complex metabolic pattern: dimethylnitrosamine (DMN), diethylnitrosamine (DEN), 1,1-dimethylhydrazine (1,1-DMH), 4-aminophenol (4-APOL), 4-aminobiphenyl (4-ABPYL) and one direct unstable mutagen, beta-propiolactone (BPL). All these compounds are negative in the mouse BMM test but caused a major increase in the incidence of micronuclei in mouse hepatocytes. This test is simple and can be readily compared with the BMM test. Furthermore, it offers a better assessment of the impact of a compound at the chromosomal level in a metabolically competent cell and can therefore be used for the evaluation of the genotoxic activity of compounds with complex metabolic pathways.

Genetic toxicology of 1,1,2-trichloroethylene

1,1,2-Trichloroethylene (TCE) is a widely used halogenated solvent, produced in hundreds of millions of kg each year for industrial purposes. Occupational and environmental exposure of human populations to TCE has been reported in industrialized areas. Long-term carcinogenicity studies in rodents demonstrate that exposure to high doses of TCE results in the induction of liver and lung tumors in the mouse, and tumors of the kidney and the testis in the rat. An indirect mechanism, based on the stimulation of liver peroxisome proliferation by TCE metabolites, was proposed to explain species differences in TCE hepatocarcinogenicity. Mutagenicity studies indicate that TCE is weakly active both in vitro, where liver microsomes produce electrophilic TCE metabolites, and also in vivo in mouse bone marrow, where high rates of micronuclei, but no structural chromosome aberrations, are found. Among TCE metabolites, trichloroacetic acid was reported to be carcinogenic to mouse liver. Furthermore, both trichloroacetic acid and chloral hydrate were found to be genotoxic in vivo, inducing structural and numerical chromosome abnormalities, respectively.

The rat-liver carcinogen N-nitrosomorpholine initiates unscheduled DNA synthesis and induces micronuclei in the rat liver in vivo

Alkylation of DNA is generally accepted as the primary event in the carcinogenicity of nitrosamines. However, the cyclic nitrosamine N-nitrosomorpholine (NMOR), a potent rat hepatocarcinogen, has been reported as binding at very low levels to the liver DNA of treated rats. This led us to investigate the activity of NMOR in two in vivo rat-liver genotoxicity assays--for the induction of unscheduled DNA synthesis (UDS) and the production of micronucleated hepatocytes in the liver micronucleus assay (LMN). Rats treated with oral doses of NMOR (10-200 mg/kg) gave a positive liver UDS response either 2.5 h or 12 h after dosing. Similarly, treatment with oral doses of NMOR (10 or 100 mg/kg) followed by mitogenic stimulation with 4-acetylaminofluorene (4AAF) resulted in high incidences of micronucleated hepatocytes in the LMN assay. These data confirm that the genotoxicity reported for NMOR in vitro can be reproduced in vivo and that NMOR interacts with liver DNA of treated rats. Earlier reports of only very weak binding of radiolabelled NMOR to rat liver DNA in vivo are discussed within the context of these data.

Host-mediated bacterial mutagenesis and enterohepatic circulation of benzidine-derived mutagenic metabolites in rodents

1. Administration of benzidine (100 mg/kg, i.p.) to bile duct-cannulated rats led to a sustained excretion of metabolites in bile which, following glucuronide hydrolysis, were mutagenic to Salmonella typhimurium strain TA98. 2. When the biliary metabolites were re-infused into the duodena of a further group of rats, enterohepatic circulation of mutagens was indicated by extensive re-excretion of biliary mutagens in the recipients. 3. Furthermore, in mouse host-mediated mutagenicity assays, both i.p. injection of benzidine (100 mg/kg) and intracaecal administration of rat biliary metabolites of benzidine produced a mutagenic response in Salmonella typhimurium strain TA98 cells isolated from the liver. 4. The results indicate that enterohepatic circulation adds to the biological persistence of reactive metabolites of benzidine and may contribute to the carcinogenicity of this aromatic amine.

Induction of sister-chromatid exchanges by procarcinogens in metabolically competent Chinese hamster epithelial liver cells

An epithelial cell strain has been established from the livers of male Chinese hamsters (CHEL cells). These cells, which proliferate in culture and retain their metabolic enzymatic activities during several subcultures, were used in a sister-chromatid exchange assay to evaluate the effectiveness of polycyclic aromatic hydrocarbons (PAHs), aflatoxin B1 (AFB1) and cyclophosphamide (CP). The results obtained demonstrate that CHEL cells are metabolically competent to activate different classes of procarcinogens into biologically active metabolites. Moreover, they showed a selective capacity to discriminate chemical carcinogens from noncarcinogens. Thus, the CHEL cell system appears to be a promising alternative to the short-term tests that include cell-free rodent liver homogenate to evaluate new promutagens and/or procarcinogens.

A non-invasive micronucleus assay in the rat liver

The potent rat-liver mitogen 4-acetylaminofluorene (4AAF) is shown here to provide an effective replacement for the surgical procedure of 2/3 partial hepatectomy (2/3PH) in the in vivo rat-liver micronucleus assay described by Tates and his colleagues. This protocol modification enables the assay to be conducted on a routine basis. Control observations for both 2/3PH and 4AAF-treated rats are presented, together with evidence indicating 4AAF itself to be without activity in the assay, irrespective of the mitogenic stimulus. The activities of the rat carcinogens DMN, 2AAF, DMH and 6BT, and of the non-carcinogens 4AAF and 4N are demonstrated. Recommendations for the conduct of the modified assay are made.