Ethyl maltol enhances copper mediated cytotoxicity in lung epithelial cells

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Ethyl maltol (EM) is a flavoring agent commonly used in foods that falls under the generally recognized as safe category. It is added to many commercial e-cigarette vaping fluids and has been detected in the aerosols. Considering that EM facilitates heavy metal transport across plasma membranes, and that heavy metals have been detected in aerosols generated from e-cigarettes, this study examines whether EM enhances heavy metal mediated toxicity. A decrease in viability was observed in the Calu-6 and A549 lung epithelial cell lines co-exposed to EM and copper (Cu) but no decrease was observed after co-exposure to EM with iron (Fe). Interestingly, co-exposure to EM and Fe decreased viability of the HEK293 and IMR-90 fibroblast cell lines but co-exposure to EM and Cu did not. Increases in the apoptotic markers Annexin V staining and fragmented nuclei were observed in Calu-6 cells co-exposed to EM and Cu. Co-exposure to EM and Cu in Calu-6 cells resulted in DNA damage as indicated by activation of ATM and expression of γH2A.x foci. Finally, co-exposure to EM and Cu caused oxidative stress as indicated by increases in the generation of reactive oxygen species and the expression of ferritin light chain mRNA and hemeoxygenase-1 mRNA and protein. These data show that co-exposure to EM and Cu, at concentrations that are not toxic for either chemical individually, induce apoptosis and evoke oxidative stress and DNA damage in lung epithelial cells. We suggest that there is a greater risk of lung damage in users of c-cigarette who vape with vaping fluid containing EM.