Fine particulate matter (PM2.5) aggravates apoptosis of cigarette-inflamed bronchial epithelium in vivo and vitro

Fine particulate matter (PM₂.₅) is an essential risk factor of chronic obstructive pulmonary disease (COPD). Recent studies showed weak association between PM₂.₅ and COPD incidence, but smokers who exposed to higher PM₂.₅ concentration had more opportunity to gain COPD. Cigarette smoking is the most important risk factor of COPD. Thus, we hypothesized: the role of PM₂.₅ played on cigarette-inflamed airways was more significant than normal airways. The study firstly established an animal model of C57BL/6J mice with cigarette smoke exposure and PM₂.₅ orotracheal administration. After calculating pathological scores, mean linear intercept and mean alveolar area, we found PM₂.₅ aggravated pathological injury of cigarette-inflamed lungs, but the injury on normal lungs was not significant. Meanwhile, inflammatory factors as T-bet, IFN-γ and IL-1α were tested using qRT-PCR and ELISA. The results showed PM₂.₅ aggravated inflammation of cigarette-inflamed lungs, but the effect on normal lungs was not significant. The most important pathogenesis of COPD is abnormal apoptosis in airway epithelium, due to oxidative stress following long-term exposure to cigarette smoke. Then, apoptotic responses were detected in lungs. TUNEL analysis demonstrated that PM₂.₅ promoted DNA fragmentation of cigarette-inflamed lungs, but the effect on normal lungs was not significant. Western-blot and immunohistochemistry showed caspase activated significantly in PM₂.₅-cigarette smoke exposed lungs and activated caspase 3 located mainly on bronchial epithelium. Next, human bronchial epithelial cells were cultured treated with cigarette smoke solution (CSS) with or without PM₂.₅. Z-VAD-FMK, a pan-caspase inhibitor, was used to suppress the activation of caspases. After analyzing cell viability, DNA fragmentation, mitochondrial activities and caspase activities, the results clarified that PM₂.₅ aggravated apoptosis in cigarette-inflamed bronchial epithelial cells and the responses could be suppressed by Z-VAD-FMK. Our results gave a new idea about the mechanism of PM₂.₅ on COPD and inferred cigarette-inflamed airways were more vulnerable to PM₂.₅ than normal airways.