Atrazine treatment potentiates excretion of mutagenic urine in 2,6-dinitrotoluene-treated Fischer 344 rats

Atrazine and breast cancer: a framework assessment of the toxicological and epidemiological evidence

The causal relationship between atrazine exposure and the occurrence of breast cancer in women was evaluated using the framework developed by Adami et al. (2011) wherein biological plausibility and epidemiological evidence were combined to conclude that a causal relationship between atrazine exposure and breast cancer is “unlikely”. Carcinogenicity studies in female Sprague-Dawley (SD) but not Fischer-344 rats indicate that high doses of atrazine caused a decreased latency and an increased incidence of combined adenocarcinoma and fibroadenoma mammary tumors. There were no effects of atrazine on any other tumor type in male or female SD or Fischer-344 rats or in three strains of mice. Seven key events that precede tumor expression in female SD rats were identified. Atrazine induces mammary tumors in aging female SD rats by suppressing the luteinizing hormone surge, thereby supporting a state of persistent estrus and prolonged exposure to endogenous estrogen and prolactin. This endocrine mode of action has low biological plausibility for women because women who undergo reproductive senescence have low rather than elevated levels of estrogen and prolactin. Four alternative modes of action (genotoxicity, estrogenicity, upregulation of aromatase gene expression or delayed mammary gland development) were considered and none could account for the tumor response in SD rats. Epidemiological studies provide no support for a causal relationship between atrazine exposure and breast cancer. This conclusion is consistent with International Agency for Research on Cancer’s classification of atrazine as “unclassifiable as to carcinogenicity” and the United States Environmental Protection Agency's classification of atrazine as “not likely to be carcinogenic.”

Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens

One of the consequences of the low specificity of the in vitro mammalian cell genotoxicity assays reported in our previous paper [D. Kirkland, M. Aardema, L. Henderson, L. Muller, Evaluation of the ability of a battery of three in vitro genotoxicity tests to discriminate rodent carcinogens and non-carcinogens. I. Sensitivity, specificity and relative predictivity, Mutat. Res. 584 (2005) 1-256] is industry and regulatory agencies dealing with a large number of false-positive results during the safety assessment of new chemicals and drugs. Addressing positive results from in vitro genotoxicity assays to determine which are "false" requires extensive resources, including the conduct of additional animal studies. In order to reduce animal usage, and to conserve industry and regulatory agency resources, we thought it was important to raise the question as to whether the protocol requirements for a valid in vitro assay or the criteria for a positive result could be changed in order to increase specificity without a significant loss in sensitivity of these tests. We therefore analysed some results of the mouse lymphoma assay (MLA) and the chromosomal aberration (CA) test obtained for rodent carcinogens and non-carcinogens in more detail. For a number of chemicals that are positive only in either of these mammalian cell tests (i.e. negative in the Ames test) there was no correlation between rodent carcinogenicity and level of toxicity (we could not analyse this for the CA test as insufficient data were available in publications), magnitude of response or lowest effective positive concentration. On the basis of very limited in vitro and in vivo data, we could also find no correlation between the above parameters and formation of DNA adducts. Therefore, a change to the current criteria for required level of toxicity in the MLA, to limit positive calls to certain magnitudes of response, or to certain concentration ranges would not improve the specificity of the tests without significantly reducing the sensitivity. We also investigated a possible correlation between tumour profile (trans-species, trans-sex and multi-site versus single-species, single-sex and single-site) and pattern of genotoxicity results. Carcinogens showing the combination of trans-species, trans-sex and multi-site tumour profile were much more prevalent (70% more) in the group of chemicals giving positive results in all three in vitro assays than amongst those giving all negative results. However, single-species, single-sex, single-site carcinogens were not very prevalent even amongst those chemicals giving three negative results in vitro. Surprisingly, when mixed positive and negative results were compared, multi-site carcinogens were highly prevalent amongst chemicals giving only a single positive result in the battery of three in vitro tests. Finally we extended our relative predictivity (RP) calculations to combinations of positive and negative results in the genotoxicity battery. For two out of three tests positive, the RP for carcinogenicity was no higher than 1.0 and for 2/3 tests negative the RP for non-carcinogenicity was either zero (for Ames+MLA+MN) or 1.7 (for Ames+MLA+CA). Thus, all values were less than a meaningful RP of two, and indicate that it is not possible to predict outcome of the rodent carcinogenicity study when only 2/3 genotoxicity results are in agreement.

The genotoxicity of ambient outdoor air, a review: Salmonella mutagenicity

Mutagens in urban air pollution come from anthropogenic sources (especially combustion sources) and are products of airborne chemical reactions. Bacterial mutation tests have been used for large, multi-site, and/or time series studies, for bioassay-directed fractionation studies, for identifying the presence of specific classes of mutagens, and for doing site- or source-comparisons for relative levels of airborne mutagens. Early research recognized that although carcinogenic PAHs were present in air samples they could not account for the majority of the mutagenic activity detected. The mutagenicity of airborne particulate organics is due to at least 500 identified compounds from varying chemical classes. Bioassay-directed fractionation studies for identifying toxicants are difficult to compare because they do not identify all of the mutagens present, and both the analytical and bioassay protocols vary from study to study. However, these studies show that the majority of mutagenicity is usually associated with moderately polar/highly polar classes of compounds that tend to contain nitroaromatic compounds, aromatic amines, and aromatic ketones. Smog chamber studies have shown that mutagenic aliphatic and aromatic nitrogen-containing compounds are produced in the atmosphere when organic compounds (even non-mutagenic compounds) are exposed to nitrogen oxides and sunlight. Reactions that occur in the atmosphere, therefore, can have a profound effect on the genotoxic burden of ambient air. This review illustrates that the mutagenesis protocol and tester strains should be selected based on the design and purpose of the study and that the correlation with animal cancer bioassay results depends upon chemical class. Future emphasis needs to be placed on volatile and semi-volatile genotoxicants, and on multi-national studies that identify, quantify, and apportion mutagenicity. Initial efforts at replacing the Salmonella assay for ambient air studies with some emerging technology should be initiated.

Lack of genotoxicity of the herbicide atrazine in cultured human lymphocytes

The widely used herbicide atrazine was evaluated for genotoxicity in cultured human peripheral blood lymphocytes. Sister-chromatid exchanges (SCE), chromosome aberrations (CA) and micronuclei (MN) were scored as genetic endpoints. To detect eventual metabolic modification in the genotoxicity of this herbicide, the cultures of SCE and MN demonstration were also treated with S9 microsomal fraction. From our results we can conclude that atrazine was able to exert a weak cytotoxic effect. However, the overall evaluation of the genotoxicity data indicate that this herbicide is not effective in the three assays conducted, irrespective of the presence of metabolic activation, which would mean a general lack of effectiveness of atrazine to induce clastogenic and aneugenic damage in cultured human lymphocytes.