Effects of intermittent exposure to aflatoxin B1 on DNA and RNA adduct formation in rat liver: dose-response and temporal patterns.

We studied the effects of intermittent exposure to aflatoxin B1 (AFB1) on hepatic DNA and RNA adduct formation. Fisher-344 male rats were fed 0.01, 0.04, 0.4, or 1.6 ppm of AFB1 intermittently for 8, 12, 16, and 20 weeks, alternating with 4 weeks of dosing and 4 weeks of rest. Other groups of rats were fed 1.6 ppm of AFB1 continuously for 4, 8, 12, and 16 weeks. Control rats received AFB1-free NIH-31 meal diet. AFB1-DNA and -RNA adducts were measured by HPLC with fluorescence detection. The data are presented as total DNA or RNA adducts. The DNA and RNA adduct levels increased or decreased depending on the cycles of dosing and rest. Rats removed from treatment 1 month after 1 or 2 dosing cycles (8 and 16 weeks of intermittent exposure) showed approximately a twofold decrease in DNA adduct levels and a two- to elevenfold decrease in RNA adduct levels compared with rats euthanized immediately after the last dosing cycle (12 and 20 weeks of intermittent exposure). Our data indicate that DNA and RNA adducts increased linearly, from 0.01 ppm to 1.6 ppm of AFB1 after 12 and 20 weeks of intermittent treatment. A linear dose response was also apparent for DNA but not for RNA adducts after 8 and 16 weeks of treatment. As biomarkers of exposure, AFB1-RNA adducts were three to nine times more sensitive than AFB1-DNA adducts but showed greater variability. These results suggest that binding of AFB1 to hepatic DNA is a linear function of the dose, regardless of the way this is administered. The dose-response relationship for RNA adducts depends on the length of the no-dosing cycles and on the turnover rate of RNA.

Immunotoxicity of aflatoxin B1 in rats: effects on lymphocytes and the inflammatory response in a chronic intermittent dosing study.

We investigated the effects of aflatoxin B1 (AFB1) on isolated splenic lymphocytes and the histo-morphologic changes in the spleens and liver of Fisher-344 male rats. Weaned animals were fed chow diets that contained 0, 0.01, 0.04, 0.4, or 1.6 ppm AFB1, using an intermittent dosing regimen (4 weeks on and 4 weeks off AFB1), for 40 weeks. An additional group of animals was fed the 1.6 ppm AFB1 diet continuously. The intermittent dosing regimen was designed to evaluate effects of cumulative dose and exposure for risk assessment comparisons. The percentages of T and B cells were affected as shown by flow cytometric analysis after the dosing cycles. The observed changes appeared to reverse or compensate to some extent after the off cycles. Lymphocytes were stimulated in culture for analysis of the production of IL-2, IL-1, and IL-6. Significantly increased production of IL-1 and IL-6 was seen in the second dosing cycle (12 weeks) and the second "off" cycle (16 weeks) at the higher doses. Inflammatory infiltrates were seen in the liver after eight weeks of continuous and intermittent dosing and were increased in size and number at 12 weeks in both 1.6 ppm dose groups correlating with the peak production of Il-1 and IL-6. We concluded that AFB1 effects on the immune system can be either stimulatory or suppressive dependent on a critical exposure window of dose and time. Immune cells in spleen such as T-lymphocytes and macrophages, both important mediators of inflammatory responses to tissue damage, were affected differently in the continuous and intermittent exposures to AFB1.

Current and future application of genetic toxicity assays: the role and value of in vitro mammalian assays.

With the advent of new technologies (e.g., genomics, automated analyses, and in vivo monitoring), new regulations (e.g., the reduction of animal tests by the European REACH), and new approaches to toxicology (e.g., Toxicity Testing in the 21st Century, National Research Council), the field of regulatory genetic toxicology is undergoing a serious re-examination. Within this context, Toxicological Sciences has published a series of articles in its Forum Section on the theme, "Genetic Toxicity Assessment: Employing the Best Science for Human Safety Evaluation" (beginning with Goodman et al.). As a contribution to the Forum discussions, we present current methods for evaluating mutagenic/genotoxic risk using standard genotoxicity test batteries, and suggest ways to address and incorporate new technologies. We recognize that the occurrence of positive results in relation to cancer prediction has led to criticism of in vitro mammalian cell genetic toxicity assays. We address criticism of test results related to weak positives, associated only with considerable toxicity, only seen at high concentrations, not accompanied by positive results in the other tests of standard test batteries, and/or not correlating well with rodent carcinogenicity tests. We suggest that the problems pointed out by others with these assays already have been resolved, to a large extent, by international groups working to update assay protocols, and by changes in data interpretation at regulatory agencies. New guidances at the U.S. Environmental Protection Agency and the U.S. Food and Drug Administration improve data evaluation and help refocus risk assessment. We discuss the results of international groups working together to integrate new technologies and evaluate new tests, including human monitoring. We suggest that strategies for identifying human health risks should naturally change to integrate new technologies; however, changes should be made only when justified by strong scientific evidence of improvement in the risk assessment paradigm.