Effects of E-Cigarette Flavoring Chemicals on Human Macrophages and Bronchial Epithelial Cells

E-cigarettes utilize a wide range of flavoring chemicals whose respiratory health effects are not well understood. In this study, we used pulmonary-associated cell lines to assess the in vitro cytotoxic effects of thirty flavoring chemicals. Human bronchial epithelial cells (BEAS-2B) and both naïve and activated macrophages (THP-1) were treated with 10, 100, and 1000 µM of flavoring chemicals and analyzed for changes in viability, cell membrane damage, reactive oxygen species (ROS) production, and inflammatory cytokine release. Viability was most greatly affected by decanal, hexanal, nonanal, cinnamaldehyde, eugenol, vanillin, alpha-pinene, eugenol, and limonene. High amounts of ROS were elicited by vanillin, ethyl maltol, and the diketones (2,3-pentanedione, 2,3-heptanedione, and 2,3-hexanedione) from both cell lines. Naïve THP-1 cells produced significant levels of IL-1β, IL-8, and TNF-α when exposed to ethyl maltol and hexanal. Activated THP-1 cells released increased IL-1β and TNF-α when exposed to ethyl maltol, but many flavoring chemicals had an apparent suppressive effect on inflammatory cytokines released by activated macrophages, with varying degrees of accompanying cytotoxicity. The diketones, L-carvone, and linalool, suppressed cytokine release in the absence of cytotoxicity. These findings provide insight into patterns of cytotoxicity and inflammatory cytokine release potentially relevant to the development of pathological changes in the lungs of e-cigarette users.