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.