Recent studies suggest a direct contribution of
nicotine, the addictive component of tobacco and tobacco
smoke, to human
carcinogenesis. To assess the genotoxicity of
nicotine, the
DNA-damaging effect on human lymphocytes and target cells from lymphatic tissue of the palatine tonsils from 10 healthy patients was tested with the alkaline single-cell
microgel electrophoresis (Comet) assay. The degree of
DNA migration, a measure of possible
DNA single strand breaks,
alkali labile sites, and incomplete excision repair sites, was expressed as the Olive tail moment, the percentage of
DNA in the tail, and the tail length. One hour exposure to
nicotine at 0.125, 0.25, 0.5, 1, 2, and 4 mM induced a statistically significant dose-dependent increase of
DNA migration up to 3.8-fold and 3.2-fold in tonsillar cells and lymphocytes, respectively. The lowest concentration eliciting significant DNA damage was 0.5 mM
nicotine. The genotoxic effect was confirmed in a second series of experiments using
nicotine of high purity from two different suppliers. There were no significant differences between the two series, excluding artifacts from the source of
nicotine. Finally, DNA damage by
nicotine was compared in cells incubated in medium strictly adjusted to neutral pH, with non-adjusted medium becoming alkaline with increasing
nicotine concentrations. Again no differences in
DNA migration were observed. The data indicate that
nicotine expresses significant direct genotoxic effects in human target cells in vitro. However, no differences in DNA damage were observed in cells from smokers and nonsmokers incubated without
nicotine. The lack of higher DNA damage in smokers compared to nonsmokers could be a question of
nicotine dose, rapid DNA repair, or interactions with other
smoke constituents. These results require further investigations on the contribution of
nicotine to tobacco
carcinogenesis.