Abstract Rationale The number of E-cigarette and heated tobacco product users has recently been increasing. Although it was initially claimed that these new tobacco products reduce harm, their harmful effects have begun to be pointed out. These devices generate aerosols by heating propylene glycol (PG) and glycerol (Gly). While the adverse effects of nicotine have been widely reported, PG and Gly have only been shown to be safe when taken orally, and there has been little research on its effects on the lungs. Therefore, we aimed to assess the biological impact of them. Initially, in vitro study, human small airway epithelial cells (SAECs) were exposed to PG or Gly, and cell proliferation and cell viability were evaluated. Exposure of SAECs to PG significantly inhibited proliferation and decreased cell viability in a concentration-dependent manner. Gly elicited similar effects but to a reduced degree as compared to the same concentration of PG. Based on these results, we conducted an in vivo study exposing mice to inhaled PG. Methods Twelve-week-old male C57BL/6J mice were exposed to PG aerosols generated by commercially available E-cigarettes using a small-animal exposure system (4 weeks, 30 minutes/day, 6 puffs/min, 35 mL/puff). Pulmonary function was measured using the flexiVent system, bronchoalveolar lavage fluid (BALF) was analyzed for inflammatory cells and cytokines, and lung tissue was assessed by histology, immunohistochemistry, Western blotting, and quantitative polymerase chain reaction. Results PG-exposed mice exhibited increased mean linear intercept and destruction index, consistent with alveolar destruction and airspace enlargement. Pulmonary function testing revealed increased inspiratory capacity and pulmonary compliance, and decreased elastance. BALF revealed elevated neutrophils, and lymphocytes. Lung tissue demonstrated upregulation of oxidative stress markers (8-hydroxy-2’-deoxyguanosine, 4-hydroxy-2-nonenal), inflammatory mediators (nuclear factor kappa-light-chain-enhancer of activated B cells, interleukin-6, interleukin-1β, cyclooxygenase-2), DNA damage markers (single-stranded DNA, p53, p21), and apoptosis-related proteins (Bcl2-associated X protein/Bcl2, Bcl-2 homologous antagonist/killer, cleaved caspase-9), with increased TdT-mediated dUTP nick end labeling assay positivity. Conclusions PG inhalation exposure in mice induced oxidative stress, inflammation, DNA damage, cell cycle arrest, and lung apoptosis, leading to emphysema. These findings suggest that PG-containing aerosols could potentially cause lung injury, underscoring the need for caution regarding the safety of any new tobacco products containing PG. This abstract is funded by: Japan Society for the Promotion of Science KAKENHI Grant (24K02865, T. Sato; 22K16178, Y. Suzuki), and Institute for Environmental and Gender-Specific Medicine, Juntendo University Graduate School of Medicine (E2310, T. Sato)
Nishioki et al. (Fri,) studied this question.