Genotoxicity of atmospheric pollutants: focus on polycyclic aromatic hydrocarbons
Thierry Douki, Jean-Luc Ravanat
Polycyclic aromatic hydrocarbons (PAHs) are toxic compounds present in food, urban pollution and produced by some industries. In the air, PAHs are mostly due to automobile exhausts and urban heating. The PAHs family represents more than 100 molecules, some of them being carcinogenic, especially in lung, bladder and skin. The most carcinogenic PAH is benzo[a]pyrene (B[a]P) which, after metabolization into its diol epoxide (BPDE), generates adducts in DNA. Another proposed genotoxic pathway is the induction of oxidative stress.
Different types of PAH-induced DNA damage and the tools to quantify them
We used HPLC-tandem mass spectrometry to quantify DNA adducts and the Comet assay (calibrated with respect to the effect of gamma radiation) for the measurement of oxidative lesions (strand breaks and 8-oxoGua). We thus showed that oxidative stress is poorly involved in human cells lines (HepG2 hepatocytes, A549 lung cells, T24 bladder cells) exposed to B[a]P. Surprising observations were made for the formation of BPDE adducts. While an expected dose-dependence was observed in HepG2, the formation of BPDE adducts was found to reach a maximum at sub-micromolar concentrations in A549 and then decrease for larger concentrations. These differences in genotoxic properties from one cell type to the other were explained by differences in metabolic properties. Metabolism, or more precisely lack of, also explained the absence of detectable adducts in T24. These observations emphasis the need for proper design of in vitro assays.
Formation of DNA strand breaks and BPDE adducts in HepG21 hepatocytes exposed to atmospheric extracts. In all samples, the B[a]P concentration was 40 nM.
Another level of complexity was added when the effects of mixtures was investigated. Synergistic or inhibitory effects were observed on the formation of BPDE adduct when cells were incubated with B[a]P in the presence of other PAHs. Impact on the induction of metabolism was also observed. The situation was found to be even more complex with extracts from atmospheric samples. When applying an amount of sample corresponding to a constant concentration of B[a]P, the level of BPDE adducts and oxidative damage drastically varied depending on the origin of the extract. As a general trend, samples with large fractions of PAHs (metallurgic industries using carbon-based electrodes, urban traffic in winter, …) led to little oxidative damage and potentialized the formation of BPDE adducts with respect to pure B[a]P. In contrast, extracts with a complex composition such as urban atmosphere in summer led to the inhibition of BPDE adducts and high levels of oxidative lesions. Interestingly, these results are not correlated with the toxicity predicted by the “Toxicity Equivalent Factors” approach. These data emphasize the difficulty for assessing risks associated with exposure to mixtures.
Collaboration Prof. Anne Maitre CHU Grenoble, Financial support from région Rhône-Alpes and Ademe).
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Last update : 09/08 2014 (1064)