Assessment of DNA adducts and the frequency of 6-thioguanine resistant T-lymphocytes in F344 rats fed 2,4-toluenediamine or implanted with a toluenediisocyanate-containing polyester polyurethane foam

Evaluation of p-phenylenediamine, o-phenylphenol sodium salt, and 2,4-diaminotoluene in the rat comet assay as part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiated international validation study of in vivo rat alkaline comet assay

As part of the Japanese Center for the Validation of Alternative Methods (JaCVAM)-initiated international validation study of in vivo rat alkaline comet assay (comet assay), p-phenylenediamine dihydrochloride (PPD), o-phenylphenol sodium salt (OPP), and 2,4-diaminotoluene (2,4-DAT), were analyzed in this laboratory as coded test chemicals. Male Sprague-Dawley rats (7-9 weeks of age) were given three oral doses of the test compounds, 24 and 21 h apart and liver and stomach were sampled 3h after the final dose administration. Under the conditions of the test, no increases in DNA damage were observed in liver and stomach with PPD and OPP up to 100 and 1000 mg/kg/day, respectively. 2,4-DAT, a known genotoxic carcinogen, induced a weak but reproducible, dose-related and statistically significant increase in DNA damage in liver cells while no increases were observed in stomach cells.

New exposure biomarkers as tools for breast cancer epidemiology, biomonitoring, and prevention: a systematic approach based on animal evidence

BACKGROUND

Exposure to chemicals that cause rodent mammary gland tumors is common, but few studies have evaluated potential breast cancer risks of these chemicals in humans.

OBJECTIVE

The goal of this review was to identify and bring together the needed tools to facilitate the measurement of biomarkers of exposure to potential breast carcinogens in breast cancer studies and biomonitoring.

METHODS

We conducted a structured literature search to identify measurement methods for exposure biomarkers for 102 chemicals that cause rodent mammary tumors. To evaluate concordance, we compared human and animal evidence for agents identified as plausibly linked to breast cancer in major reviews. To facilitate future application of exposure biomarkers, we compiled information about relevant cohort studies.

RESULTS

Exposure biomarkers have been developed for nearly three-quarters of these rodent mammary carcinogens. Analytical methods have been published for 73 of the chemicals. Some of the remaining chemicals could be measured using modified versions of existing methods for related chemicals. In humans, biomarkers of exposure have been measured for 62 chemicals, and for 45 in a nonoccupationally exposed population. The Centers for Disease Control and Prevention has measured 23 in the U.S. population. Seventy-five of the rodent mammary carcinogens fall into 17 groups, based on exposure potential, carcinogenicity, and structural similarity. Carcinogenicity in humans and rodents is generally consistent, although comparisons are limited because few agents have been studied in humans. We identified 44 cohort studies, with a total of > 3.5 million women enrolled, that have recorded breast cancer incidence and stored biological samples.

CONCLUSIONS

Exposure measurement methods and cohort study resources are available to expand biomonitoring and epidemiology related to breast cancer etiology and prevention.

Induction of LacZ mutations in Muta™Mouse can distinguish carcinogenic from non-carcinogenic analogues of diaminotoluenes and nitronaphthalenes

2,4-Diaminotoluene (2,4-DAT) is a liver carcinogen in rats and mice whereas 2,6-DAT is not. Both are genotoxic in vitro. Tests for mutations in transgenic mice, unscheduled DNA synthesis (UDS), DNA damage and enhancement of initiated foci in vivo have shown some discrimination between these two analogues, but only after oral administration. 1- and 2-nitronaphthalene (1- and 2-NNT) are also both genotoxic in vitro, although, unlike 2,4- and 2,6-DAT, they do not require metabolic activation. There is some evidence that 2-NNT may be able to induce liver and bladder tumours, and there is some evidence that 1-NNT is not carcinogenic to rats or mice, but none of the data are convincing. When tested for induction of LacZ mutations in Muta Mouse after topical exposure (human occupational exposure route) at their maximum tolerated doses, 2,4-DAT induced a positive response in liver and a marginal response in kidney, whereas 2,6-DAT was negative. 2-NNT also induced a positive mutagenic response in liver, and a marginal response in bladder, whereas 1-NNT was negative. Neither 2,4- nor 2,6-DAT induced mutations at the site of application (skin) as might be expected for chemicals requiring activation by liver enzymes. 2-NNT, which is a direct-acting mutagen in vitro, gave a marginal response for induced mutation at the site of application, but 1-NNT was negative. This study shows that investigation of induction of LacZ mutations after topical application in vivo can provide useful data to help discriminate potentially carcinogenic from non-carcinogenic chemicals that are mutagenic in vitro. Robust carcinogenicity data are needed to determine whether 2-NNT can induce tumours in the liver and bladder.

Strategies and testing methods for identifying mutagenic risks

The evolution of testing strategies and methods for identification of mutagenic agents is discussed, beginning with the concern over potential health and population effects of chemical mutagens in the late 1940s that led to the development of regulatory guidelines for mutagenicity testing in the 1970s and 1980s. Efforts to achieve international harmonization of mutagenicity testing guidelines are summarized, and current issues and needs in the field are discussed, including the need for quantitative methods of mutagenic risk assessment, dose-response thresholds, indirect mechanisms of mutagenicity, and the predictivity of mutagenicity assays for carcinogenicity in vivo. Speculation is offered about the future of mutagenicity testing, including possible near-term changes in standard test batteries and the longer-term roles of expression profiling of damage-response genes, in vivo mutagenicity testing methods, and models that better account for differences in metabolism between humans and laboratory model systems.

Hprt lymphocyte mutant frequency in relation to DNA adduct formation in rats fed the hepatocarcinogen 2-acetylaminofluorene

The lymphocyte hypoxanthine-guanine phosphoribosyltransferase (Hprt) assay is frequently used as a biomarker for the exposure of both humans and laboratory animals to potentially carcinogenic agents. To obtain information concerning the sensitivity of the rat Hprt lymphocyte assay toward aromatic amine carcinogens, male F344 rats were fed 0.02% 2-acetylaminofluorene (2-AAF) for 1 month and then returned to control diet for 2 months. At 4, 27, 48, 62, and 90 days after the initiation of 2-AAF-feeding, the frequency of mutants in the Hprt gene was determined. In addition, DNA was isolated from liver nuclei, spleen lymphocytes, bone marrow, and thymus, and DNA adducts were analyzed by 32P-postlabeling. 2-AAF feeding resulted in a significant induction of 6-thioguanine-resistant T-lymphocytes and the mutant frequency continued to increase after the 2-AAF feeding was stopped. The same major DNA adduct, N-(deoxyguanosin-8-yl)-2-aminofluorene, was detected in liver, spleen lymphocytes, bone marrow, and thymus. DNA adduct levels were greatest in the tumor target tissue (liver) but occurred in all T-lymphocyte compartments, being highest in spleen lymphocytes. The DNA adduct levels were highest at the end of the 1-month 2-AAF feeding period and decreased rapidly in all tissues. The data indicate that the Hprt lymphocyte mutagenesis assay detects arylamine carcinogens, but with relatively low sensitivity.

Development and utilization of the rat lymphocyte hprt mutation assay

Much of the recent progress in the field of genetic toxicology has come from an increased understanding of the molecular and cellular biology of the mammalian organism. Most prominent has been the ability to detect and quantify somatic mutation and relate the nature of the mutation to the specific type of chemical damage. Building upon the foundation of the human lymphocyte hypoxanthine guanine phosphoribosyl transferase (hprt) system, and later, the mouse hprt system, methods for the detection and quantification of hprt mutations in rat lymphocytes were developed. These methods are described in this report as is the ongoing validation of the assay. Additionally, the characterization of the recovered mutants and a comparison of the mutation spectrum in the rat lymphocyte system to the spectrum in cancer genes, such as H-ras and p53, and the spectrum in transgenic systems, such as lacI, are included. The development of the rat lymphocyte hprt system and validation of the assay at the molecular level, provide an effective and reliable measure of genetic damage in an in vivo system which is readily comparable to measurement of genetic damage in the human.

Lymphocyte mutant frequency in relation to DNA adduct formation in rats treated with tumorigenic doses of the mammary gland carcinogen 7,12-dimethylbenz[a]anthracene

The ability of the rat lymphocyte hprt assay to detect tissue-specific carcinogens was evaluated using 7,12-dimethylbenz[a]anthracene (DMBA) administered under conditions that result in mammary gland tumors. Fifty-day-old female Sprague-Dawley rats were given single doses of 5 and 20 mg/kg DMBA by gavage, and the frequency of 6-thioguanine-resistant (TGr) T-lymphocytes was measured over a period of 21 weeks. A time- and dose-dependent increase in mutant frequency was found, with a maximum frequency found 9-15 weeks after treatment with 20 mg/kg of DMBA. Rats were also dosed with 1, 2.5, 5, 10, 15 and 20 mg/kg of DMBA and assayed for TGr mutant frequency 10 weeks after treatment. A significant linear dose-response was found, with all the DMBA doses resulting in significant increases in mutant frequency. To determine whether or not DMBA-induced mutants in rat lymphocytes reflected the DNA damage in the target tissue, rats were treated with 5 and 20 mg/kg of DMBA and spleen lymphocytes and mammary gland tissue were assayed for DNA adduct formation 1, 3 and 7 days later. A similar pattern of 32P- postlabeled adducts, involving both dG and dA nucleotides, was found in DNA from both the target tissue and the surrogate lymphocytes. Adduct formation was dose responsive in both tissues, with a 2.3- to 4-fold higher concentration in mammary gland as compared with lymphocytes. These results indicate that the rat lymphocyte hprt assay is sensitive to a mammary gland carcinogen and that similar types of DNA adducts are associated with both the lymphocyte mutants and the mammary gland tumors induced by DMBA.