In the present study, we assessed the effects of pharmacological treatment with PI3K inhibitor LY294002 on an OVA-LPS-induced murine model of neutrophilic airway inflammation. Applying histological, biochemical, and molecular analyses, we found that LY294002 promoted neutrophilic airway inflammatory resolution. Moreover, LY294002 also reduced the plasma concentrations of IL-6 and IL-17. The anti-inflammatory effects in airways were correlated with the downregulation of NLRP3 inflammasome. Our findings suggested that LY294002 as a potential pharmacological target for neutrophilic airway inflammation.
Severe asthma is characterized by neutrophilic inflammation, either in the presence or absence of Th2-induced eosinophilic inflammation [2]. Studies have demonstrated that neutrophil numbers in sputum, BALF and bronchial biopsies were higher in patients with neutrophilic asthma than those of healthy patients or patients with eosinophilic asthma [6, 7]. In the present study, we used OVA together with LPS to sensitize mice and then challenged with OVA resulting in the presence of clinical symptoms including dysphoria, scratching, quick breathing, hair tarnishing, and muscle convulsions, as well as audible-stridor. Moreover, neutrophil numbers and IL-17 levels in BALF were significantly increased in these challenged mice compared to that in control mice, suggesting that the neutrophilic airway inflammation mouse model was established as previously reported [17, 19].
LY294002, a relatively specific PI3K inhibitor, can inhibit the kinase activity by competing with the ATP binding [20]. Evidence has suggested that blockade of PI3K signaling pathway may be a potential therapeutic strategy for asthma and rhinovirus-induced exacerbations [21]. In an OVA-induced murine asthma model, treatment of LY294002 significantly reduced allergic airway inflammation [10]. Moreover, the recent study showed that LY294002 also significantly inhibited IL-25-induced lung tissue eosinophilia, mucus production, collagen deposition, smooth muscle hypertrophy and angiogenesis [11]. An inspiring study reported that treatment of LY294002 restored histone deacetylase 2 and steroid sensitivity in a mouse model of severe, steroid-insensitive allergic airway disease [22]. A recent study showed that inhalation of the prodrug PI3K inhibitor CL27c improves lung function in murine models of glucocorticoid-resistant neutrophilic asthma and fibrosis [23]. In the present study, we further confirmed that inhibition of PI3K by LY294002 attenuated clinical symptoms of neutrophilic airway inflammatory mice. Moreover, in line with these previous studies, LY294002 also significantly reduced inflammatory cells infiltration, especially neutrophils, and mucus production in the airways of our experimental mice. These novel findings further suggest that PI3K signaling pathway may be involved in regulation of Th17-derived neutrophilic airway inflammation [24].
It has been reported that Th17 is the major player in the pathogenesis of mouse and human neutrophilic asthma, in which IL-17 derived from Th17 cells further promotes the recruitment of neutrophils by stimulating the production of neutrophil-attracting cytokines or chemokines from airway epithelial cells [4, 5]. Several studies link PI3K activity with inflammation in allergic airway diseases, and one indicated that rhinovirus infection induced PI3K-dependent neutrophilic airway inflammation [25]. Furthermore, in an OVA-induced asthma model, administration of IC87114, a selective PI3K δ inhibitor, not only attenuated allergic airway inflammation but also significantly reduced the increase in IL-17 protein and mRNA expression [12, 13]. Pharmacologic and genetic blockade of PI3K function restored steroid sensitivity in experimental chronic obstructive pulmonary disease (COPD) [26, 27]. Expectedly, we found that administration of LY294002 significantly decreased the inflammatory cells infiltration in the airways and BALF, and the production of cytokines in BALF. Studies have showed that there existed imbalance between IL-17 and IL-10 in patients with chronic persistent asthma and it was involved in the development of asthma [28]. Liu, et al [29] found that after ligustrazine treatment, neutrophil numbers and IL-10 levels were significantly increased, whereas that of IL-17 were significantly decreased in BALF. In line with our study, we also found that neutrophil numbers and IL-17 levels were significantly reduced, but IL-10 levels in BALF were significantly increased in BALF in LY294002 treated mice. Due to increased blood levels of IL-17 and IL-6 were confirmed in patients with severe asthma [3, 8], we further measured the plasma levels of IL-17 and IL-6 in inflammatory mice by ELISA. Similarly, we observed significantly elevated plasma levels of IL-17 and IL-6 in neutrophilic airway inflammatory mice compared to that in control mice, which was in line with previous studies [30]. Moreover, plasma levels of IL-17 and IL-6 were significantly reduced in LY294002 treated mice relative to that in untreated mice, further suggesting that the anti-inflammatory effects of LY294002 on neutrophilic airway inflammation.
Recent studies have shown that the NLRP3 inflammasome was involved in regulation of the neutrophilic airway inflammation in patients with severe asthma [16, 31]. To examine whether the anti-inflammatory role of LY294002 was correlated with the expression of NLRP3 inflammasome, we measured the expression levels of related proteins in challenged mice. In line with previous studies, the significantly increased expression levels of p-Akt, ASC, NLRP3, caspase-1 and IL-1β suggested that the activated PI3K activity and NLRP3 inflammasome in OVA-LPS-induced neutrophilic airway inflammatory mice. Mature IL-1β derived from NLRP3 inflammasome can promote the differential Th17 cells, maintain the production of Th17-associated cytokines, and facilitate allergic inflammation [32, 33]. Studies found that the increased NLRP3, caspase-1 and IL-1β responses drove steroid-resistant neutrophilic inflammation [31]. Moreover, use of specific NLRP3 inflammasome inhibitors reduced HDM-induced AHR and attenuated steroid-resistant asthma in a mouse model [34]. Expectedly, treatment of LY294002 significantly decreased the expression levels of p-Akt, ASC, NLRP3, caspase-1, and IL-1β suggesting that the activated PI3K activity and NLRP3 inflammasome were inhibited by this treatment.