In Vitro Assessment of Translocation and Toxicological Effects of Nicotine and Ethyl Maltol from e-Cigarettes Using Air-Liquid Interface-Cultured Bronchial Epithelial Cells

Introduction: The use of e-cigarettes is increasing rapidly. As only scant information is available on the toxicological hazard of inhaling flavoring compounds in e-liquids, we assessed effects of the widely used flavoring ethyl maltol on bronchial epithelial cells. We investigated whether ethyl maltol and/or nicotine could induce inflammatory effects and would have the potential to become systemically available. Methods: Bronchial epithelial cells (Calu-3) were cultured at the air-liquid interface and exposed to e-cigarette aerosol with/without nicotine and with/without ethyl maltol using a smoking machine for one or two 30-minute periods per day for 2 consecutive days. Similar experiments were carried out using fully differentiated human primary bronchial epithelial cells (MucilAir) from different donors. Results: Exposure to neither of the four e-cigarette aerosols affected cell viability or membrane integrity. Both nicotine and ethyl maltol rapidly translocated to the basolateral side in both cell models. Exposure to e-liquids affected the production of MCP-1, IL-8, IP-10, IL-17A, and IL-6. For Calu-3 cells, these effects were mostly related to nicotine, and for MucilAir cells, these effects mostly related to ethyl maltol. Exposure of MucilAir cells to e-liquids with nicotine and ethyl maltol resulted in an increased mRNA expression of IL-8, and exposure of MucilAir cells to e-liquid with nicotine led to a decreased expression of MCP-1. Conclusion: Our data show that both cell models can be used to assess translocation of inhaled compounds using a human relevant exposure method. Inhalation of nicotine and ethyl maltol may cause local effects in the respiratory tract and could lead to systemic exposure.