DNA-damaging ability of isoprene and isoprene mono-epoxide (EPOX I) in human cells evaluated with the comet assay

Isoprene is produced in combustion processes and is widely used as an industrial chemical. It is a natural product emitted by plants and endogenously produced by humans and other mammals. Therefore, exposure to isoprene from both endogenous and exogenous sources is unavoidable and occurs during the entire human life. Based on evaluations of the International Agency for Research on Cancer (IARC), isoprene has been classified in Group 2B (possibly carcinogenic to humans). In the present work, we have demonstrated, by use of the single-cell gel electrophoresis assay (SCGE or comet assay), that isoprene is able to induce DNA damage in peripheral blood mononuclear cells (PBMCs) in the presence of metabolic activation. In addition, treatment of cells with the main isoprene mono-epoxide (EPOX I) induced time- and dose- dependent DNA damage in both PBMCs and human leukaemia cells (HL60). The metabolic activation system, represented by rat liver post-mitochondrial fractions (S9), was obtained from rats that had been treated - or not - with inducing agents such as phenobarbital and ethanol. The inclusion of S9 fractions (4mg protein/mL) from non-induced or phenobarbital-induced rats resulted in a statistically significant enhancement of isoprene genotoxicity. A different pattern was obtained by the addition of ethanol-induced S9, which appeared highly genotoxic by itself even in the absence of isoprene. Reducing the concentration of ethanol-induced S9 to 0.25mg protein/mL resulted in a considerable enhancement of isoprene genotoxicity. In the absence of clear epidemiological evidence of the carcinogenicity of isoprene in humans, the results of this study seem to be particularly important since they add new findings to support the classification of this chemical as possibly carcinogenic to humans.