Chronic Exposure to Electronic Cigarettes Induces Lung Oxidative Stress, Inflammation, Fibrosis, and Impaired DNA Repair

Electronic cigarettes (e-cig) have been marketed as a safer alternative to tobacco cigarette smoking (TCS). While there are far less toxic compounds in e-cig vape (ECV) than in tobacco cigarette smoke, detectable levels of toxicants are still present. Multiple in vitro and short-term in vivo studies on e-cig-induced lung cell damage have been reported; however, questions remain regarding the health effects of chronic e-cig use on the lungs. Therefore, studies were performed in a chronic mouse exposure model to investigate the deleterious effects of long-term ECV exposure on the lungs.C57/BL6 male mice were exposed for 2 hours/day, 5 days/week to either air (control group; 25 mice) or ECV generated from e-cig liquid containing 24 mg/ml nicotine (e-cig group; 25 mice) for 16 weeks. At the end of the exposure, animals were euthanized, lungs were lavaged, then the recovered bronchoalveolar lavage fluid (BALF) was prepared to assess the percentages of inflammatory cells in BALF. Fixed lung sections were also prepared and stained with hematoxylin and eosin (H & E) or Masson's trichrome stain to assess the degree of lung inflammation and fibrosis; respectively. Superoxide radical generation was measured in frozen lung sections using DHE. Western blotting (WB) and immunofluorescence (IF) in lung tissue were used to detect the levels of nitrotyrosine, and the expression of the DNA repair enzymes XPC and OGG1.The number of inflammatory cells present in the BALF, including neutrophils, macrophages, and eosinophils, was significantly higher in ECV-exposed mice compared to that with air exposure. In the ECV-exposed mice, H & E stained lung sections showed clear inflammatory infiltrates with lymphoid aggregates of lymphocytes, plasma cells and macrophages that stretch from the perivascular and peribronchiolar areas into adjacent septal adipose tissue. Masson's trichrome stained slides showed increase in connective tissue around airways and bronchioles in e-cig exposed mice, indicating the presence of pulmonary fibrosis. Using the fluorescence-based superoxide probe DHE, significantly increased superoxide generation was observed in the lungs of ECV exposed mice, compared to control. This DHE-derived fluorescence was largely quenched by preincubation with the SOD mimetic MnTBAP, confirming that the observed fluorescence was largely derived from superoxide. WB and IF showed elevated levels of nitrotyrosine. Reduced expression of the DNA damage repair proteins XPC and OGG1 were seen in lung tissue of the ECV exposed mice compared to the controls.Chronic ECV exposure induced lung inflammation, pulmonary fibrosis, and increased superoxide generation. ECV exposure increased oxidative stress and inflammation in the lungs while decreasing the levels of DNA damage repair proteins. These findings suggest that chronic e-cig use can trigger oxidative stress with inflammatory lung damage and impaired DNA repair that together may predispose to neoplastic transformation and cancer onset.