Air pollution, especially fine particulate matters with a diameter less than 2.5μm (PM_2.5), has become a serious public health problem (Heck et al., 2017; Loxham et al., 2019). The mortality, morbidity, and risk factors in China published in the Lancet showed that particulate matter (PM) pollution is the top fourth risk factor for the number of deaths in 2017 (Zhou et al., 2019). Many epidemiological investigations suggested that PM_2.5 is associated with respiratory and other non-communicable diseases, and the increase in PM_2.5 is associated with increased hospital admissions for respiratory diseases (Qiu et al., 2012; Tian et al., 2019). An 18years cohort study indicated that ambient air pollutants was significantly associated with declining lung function and increasing emphysema (Wang et al., 2019a). Additionally, PM_2.5 exposure could lead to lung injury characterized by pulmonary dysfunction, inflammatory cell infiltration, pulmonary edema, and pulmonary fibrosis (Feng et al., 2019; Li et al., 2019). These harmful effects of PM_2.5 are related to oxidative stress and inflammatory responses (Li et al., 2018; Zhang et al., 2019). Further researches showed that PM_2.5 may exacerbate inflammation in murine lung via a Toll-like receptor2 (TLR2)/Toll-like receptor4 (TLR4)/MyD88-signaling pathway (He et al., 2017). The activation of TLR4/Nuclear factor Kappa B (NF-κB) observed in PM_2.5-induced lung epithelial cells could lead to further inflammation infiltration (Gu et al., 2017b). Therefore, it is pivotal to develop therapeutic strategies to prevent the respiratory system injury caused by PM_2.5.

Regular exercise may be able to modulate the immune system to enhance resistance to respiratory infections (Pedersen and Hoffman-Goetz, 2000; Wang et al., 2020). Numerous researchers have indicated the protective effects of exercise against various diseases through its anti-inflammatory and antioxidant capability (De Sousa et al., 2017; Metsios et al., 2020). A related study reported that moderate-intensity aerobic physical training reduces oxidative stress and protects against the development of emphysema induced by cigarette smoke in mice (Toledo et al., 2012). Our previous work also found that aerobic interval training improves the pulmonary function and impedes the lesion progression induced by acute exposure to different PM_2.5 concentrations because of the effective inhibition of oxidative stress and inflammation (Qin et al., 2020b). However, the effects of exercise on lung injury induced by sub-chronic exposure to PM_2.5 are still uncertain, and the underlying mechanism of the protective effects of exercise training remains to be elucidated.

Cyclooxygenase 2 (COX-2) is a well-known inflammatory mediator that can regulate the conversion of arachidonic acid to prostaglandin E₂ (PGE₂; Tsai et al., 2017). COX-2/PGE₂ plays key roles in the pathogenesis of PM_2.5-induced inflammation (Fernando et al., 2019). PM exposure results in airway inflammation through the upregulation of COX-2/PGE₂ (Song et al., 2020). PM exposure stimulates COX-2/PGE₂ inflammatory signaling pathways in human fibroblast-like synoviocytes (Tsai et al., 2017). P38-mitogen-activated protein kinase (MAPK) plays a substantial role in proinflammatory responses and is closely related to the beneficial effect of exercise (Sur et al., 2008). p38-MAPK regulates COX-2 expression (Xu et al., 2008; Yu et al., 2014). Moreover, exercise remarkably inhibits COX-2 activity, which leads to the suppression of pro-inflammatory cytokines (Lee et al., 2015). However, whether p38–COX-2–PGE₂ signaling pathways are involved in the protective mechanism of exercise against the inflammation induced by PM_2.5 is unknown. Thus, the inhibition of p38–COX-2–PGE₂ pathway may provide a preventive approach for the inflammation induced by PM_2.5.

Here, we used a whole-body inhalation enrichment system to conduct PM_2.5 exposure toxicology research and exposed rats to real-time PM_2.5 inhalation for 3weeks. We assessed the pulmonary function, histopathological characteristics, and inflammatory condition of rats to evaluate to whether aerobic interval training plays a protective role against the lung injury induced by PM_2.5. Additionally, we observed the p38–COX-2–PGE₂ signaling pathways involved in the protective effect of exercise on ameliorative inflammation. We hypothesized that exercise is related to the downregulated activation of p38-MAPK, which further inhibited the pathways of the COX-2 product, PGE₂.

The major findings of the present study are that aerobic interval training may be an effective way to protect against PM_2.5-induced lung inflammation in rats, and the p38-COX-2-PGE2 signaling pathways might be involved in the protective effect of exercise on ameliorative inflammation. These findings implied that regular exercise training can effectively improve immune function, especially for the young individuals with high amounts of sedentary behavior. Therefore, developing regular exercise habits is essential for the young individuals to against the injury of smog weather. Our study suggests that exercise training may be as an adjuvant in the prevention of pulmonary disease due to PM_2.5 exposure.

In this study, a whole-body inhalation PM_2.5 enrichment system for rats was used to mimic the actual living environment of humans to the greatest extent possible (Chu et al., 2019; Su et al., 2020). The mean mass concentration of exposure chambers in our study was 237.01±206.41μgm⁻³, which is a relatively moderate pollution levels of air pollution according to the Air Quality Guidelines of China. The mean mass concentration of PM_2.5 represented the ambient air pollution in early winter in northern China. In our animal models, after 3weeks subacute PM_2.5 exposure, the significant airway obstruction, declined of respiratory muscle strength, bronchial mucosal exfoliation, stenosis of the pulmonary vascular lumen, ultrastructure damages (mitochondrial swelling and microvilli shedding) were observed in S+PM_2.5 rats compared with the control. In a previous study, after 89.95μgm⁻³ PM_2.5 exposure for 3weeks, there was increasingly inflammation characterized by alveolar capillary congestion and peribronchiolar neutrophils infiltration in mice (Li et al., 2019). Yang et al. (2018) reported that 2-week (750μgm⁻³, 4h day⁻¹, 5daysweek⁻¹) PM_2.5 inhalation results in lung edema and lesions, oxidative stress, and acute inflammatory responses, which subsequently result in lung tissue damage. The main factors of PM_2.5-induced lung injury are related to the concentration, duration, and composition of PM_2.5 (Ning et al., 2019). Moreover, in previous study, we observed a low tolerance of aging rats (16months vs. 8weeks) to high concentrations of PM_2.5. 2-week exposure duration of whole-body PM_2.5 inhalation led to lung injury, and the degree of lung injury in aging rats were more deleterious than that of present result in young adult rats (Qin et al., 2021). Meanwhile, the PM_2.5 exposure duration in aging rats were less than that of young adult rats (4h/day; 2weeks vs. 6h/day; 3weeks). It is also implied that aging individuals are susceptible to PM_2.5-induced lung injury.

Furthermore, we also analyzed the composition of PM_2.5 inside the exposure chambers. The prevalent toxic heavy metals, including Zn and Cr, and the common PAHs, including phenanthrene, can activate oxidative stress and inflammation in lung tissues (Maret, 2012; Han et al., 2019; Ma et al., 2020). Growing evidence suggests that these toxic elements in PM_2.5 are closely related to the development of lung injury (Ning et al., 2019; Qin et al., 2021). In addition, heavy metals, including Zn, Cu, and Cr, and PAHs, including acenaphthylene and phenanthrene, are associated with diesel and gasoline exhaust emissions (Valavanidis et al., 2006; Hu et al., 2016). Due to the exposure chambers of present study located near several high-speed road and urban trunk road in Tongzhou, Beijing, the primary point sources of ambient PM_2.5 were derived from motor vehicle exhaust, which further implied that traffic air pollution related to lung health. Thus, we should try to avoid rush-hour traffic when smog condition is serious.

Proverbially, physical activity is an effective nonpharmacological treatment for the prevention of chronic diseases and the enhancement of immunity; moreover, exercise type, intensity, and duration are the main factors that impact the intervention effect (MacInnis and Gibala, 2017; Nieman and Pence, 2020). In the present work, we selected aerobic interval training. Aerobic interval training can provide cardiorespiratory fitness and body fat reduction that are similar to or greater than those of traditional moderate-intensity continuous training (Molmen-Hansen et al., 2012; Wewege et al., 2017). Moreover, aerobic interval training has changeable exercise rhythm and challenging intensity, therefore, it has become an attractive approach for adults. In addition, we measured the VO_2max of rats to ensure an accurate initial exercise intensity, which was adjusted every 2weeks to maintain satisfactory exercise effects (Qin et al., 2020a). Our results proved that the 8-week aerobic interval training alleviated the declined in ventilation function and respiratory muscle strength, as well as released tract obstruction; these effects may be related to the improvement of peribronchiolar and vascular wall lesions and inflammatory infiltration after regular exercise training. The findings are directly in line with previous findings (Rietberg et al., 2017). Notably, lamellar bodies and microvilli in type II alveolar cells increased after the 8-week aerobic interval training. Lamellar bodies are the specialized secretory organelles of type II alveolar cell that package phospholipid film and regulate its secretion (Mulugeta et al., 2002). The phospholipid film of dipalmitoyl phosphatidylcholine on the surface of lung alveoli reduces surface tension for optimal gas exchange and builds a hydrophobic protective lining as environmental barrier (Schmitz and Müller, 1991; Menon et al., 2018). One study indicated that lamellar bodies in A549 cells disappear compared with the control when exposed to 100μgPM_2.5 (Peng et al., 2019). A similar change was also observed in our studies. However, research on the direct effect of exercise training on lamellar bodies is rare. Our results showed that the increase in lamellar bodies may be associated with the improvement of pulmonary ventilation and compliance function after the 8-week aerobic exercise training. Our research also provided direct in vivo evidence that the protective effects of aerobic interval exercise mediated the pulmonary dysfunction induced by PM_2.5 exposure.

The lung injury induced by PM_2.5 is linked to inflammatory responses (Feng et al., 2019), including peribronchiolar neutrophil infiltration, increased proinflammatory cytokines, and the activation of relative inflammatory pathway. Zhang et al. (2019) found that PM_2.5 induced Rac1and regulated AKT signaling associated with lung inflammation (Zhang et al., 2019). Gu et al. (2017a) indicated that PM_2.5 promotes the overactivation of the Notch signaling pathway and aggravates the immune disorder of COPD. Gu et al. (2017b) reported that PM_2.5 induced lung epithelial cells by the activation of TLR4/NF-kB leading to inflammation infiltration. Thus, PM_2.5 induced inflammatory responses may be an essential factor of lung injury. Similar results were obtained in present study. In addition, previous studies also investigated the effect of intermittent exercise on immune function. Interval training could modulate autoimmunity by decreasing the polarization of T cells into deleterious Th1 and Th17 cells (Goldberg et al., 2021). Ten weeks of low-volume, high-intensity interval exercise could improve neutrophil and monocyte function and enhance innate immune system in sedentary adults (Bartlett et al., 2017). Animal research also indicated that interval exercise training could reduce the inflammation induced by cisplatin nephrotoxicity and downregulate the TLR4/NF-κB signaling pathway (Leite et al., 2021). In the present study, we verified that aerobic interval training produces similar anti-inflammatory effects for PM_2.5-induced inflammation. Furthermore, p38–COX-2–PGE₂ signaling pathways were assessed to explore the putative mechanisms of protective effect of exercise against the lung injury induced by PM_2.5 exposure. After 3weeks of PM_2.5 exposure, the high expression of p-p38, COX-2, and PGE₂ presented inflammatory response as induced by PM_2.5. However, we found that aerobic interval training could prevent the rise in p-p38 level and then hindered the activation of the COX-2-PGE₂ inflammatory pathway. Consequently, our data indicated that exercise could alleviate the lung injury induced by PM_2.5 possibly through p38–COX-2–PGE₂ pathway. Currently, the detailed mechanisms of the p38–COX-2–PGE₂ pathway involved in inflammation prevention by exercise is still uncertain and needs further verification.

This study has several limitations. First, we only used male rats. The deleterious effects of PM_2.5 and physiological characteristics are different between sexes (Kampa and Castanas, 2008; Wang et al., 2019b). However, we only selected male rats without considering the estrous cycle. Second, only the protective effects of exercise on lung injury induced by subacute PM exposure (3weeks) were observed. Whether the aerobic exercise training has a protective effect on long-term exposure to smog weather will be studied in the future. Lastly, the detailed mechanisms of the p38–COX-2–PGE₂ pathway in preventing inflammation needs additional analysis. The critical regulatory molecules underlying these interactive effects warrant further investigation.

In summary, we showed that 8-week aerobic interval exercise may be an effective way to protect against PM_2.5-induced lung inflammation in rats. Aerobic exercise alleviated the airway obstruction, weakened respiratory muscle strength, bronchial mucosal exfoliation, ultrastructure damage, and inflammatory responses induced by PM_2.5 in exercise-related rats. These benefits of exercise were related with the downregulated activation of p38 and MAPK, which further inhibited COX-2-PGE₂ pathways. In future, the guidelines in different age groups on exercise promotion behavior combined with environmental factors are essential. It requires further researches to elucidate the relevant mechanism of exercise on preventing the injury induced by PM_2.5 exposure, which will offer an effective measure for health promotion and diseases prevention.

The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding author.

The animal study was reviewed and approved by Animal Ethical Committee of the China Institute of Sports Science.

FQ, ZF, and MX contributed equally in the ideas, writing of the manuscript, and drafted the manuscript. FQ and JZ conceived and designed the research and edited and revised the manuscript. MX, ZF, ZW, YD, and FQ performed the experiments. FQ, SC, and ZF analyzed the data. FQ, JZ, and CQ interpreted the experimental results. CQ, SC, and LZ prepared the figures. JZ approved the final version of the manuscript. All authors contributed to the article and approved the submitted version.

This work is supported by the China Postdoctoral Science Foundation (2018T110076), the National Natural Science Foundation of China (31900845 and 11775059), and the Fundamental Research Foundation of the China Institute of Sport Science (20-18).

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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