Toll-like receptor 2 (TLR2) is suggested to initiate the activation of NLRP3 inflammasome, and considered to be involved in asthma. The findings that melatonin modulates TLRs-mediated immune responses, together with the suppressing effect of TLRs on endogenous melatonin synthesis, support the possibility that a feedback loop exists between TLRs system and endogenous melatonin synthesis. To determine whether TLR2-melatonin feedback loop exists in allergic airway disease and regulates NLRP3 inflammasome activity, wild-type (WT) and TLR2 −/− mice were challenged with OVA to establish allergic airway disease model. Following OVA challenge, WT mice exhibited increased-expression of TLR2, activation of NLRP3 inflammasome and marked airway inflammation, which were all effectively inhibited in the TLR2 −/− mice, indicating that TLR2-NLRP3 mediated airway inflammation. Meanwhile, melatonin biosynthesis was reduced in OVA-challenged WT mice, while such reduction was notably rescued by TLR2 deficiency, suggesting that TLR2-NLRP3-mediated allergic airway inflammation was associated with decreased endogenous melatonin biosynthesis. Furthermore, addition of melatonin to OVA-challenged WT mice pronouncedly ameliorated airway inflammation, decreased TLR2 expression and NLRP3 inflammasome activation, further implying that melatonin in turn inhibited airway inflammation via suppressing TLR2-NLRP3 signal. Most interestingly, although melatonin receptor antagonist luzindole significantly reduced the protein expressions of ASMT, AANAT and subsequent level of melatonin in OVA-challenged TLR2 −/− mice, it exhibited null effect on leukocytes infiltration, Th2-cytokines production and NLRP3 activity. These results indicate that a TLR2-melatonin feedback loop regulates NLRP3 inflammasome activity in allergic airway inflammation, and melatonin may be a promising therapeutic medicine for airway inflammatory diseases such as asthma.

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Abscisic acid (ABA), a well-known natural phytohormone reportedly exerts antiinflammatory and anti-oxidative properties in diabetes and colitis. However, the efficacy of ABA against allergic airway inflammation and the underlying mechanism remain unknown. Herein, an OVA-induced murine allergic airway inflammation model was established and treated with ABA in the presence or absence of PPAR-γ antagonist GW9662. The results showed that ABA effectively stunted the development of airway inflammation, and concordantly downregulated OVAinduced activation of NLRP3 inflammasome, suppressed oxidative stress and decreased the expression of mitochondrial fusion/fission markers including Optic Atrophy 1 (OPA1), Mitofusion 2 (Mfn2), dynamin-related protein 1 (DRP1) and Fission 1 (Fis1). Moreover, ABA treatment further increased OVA-induced expression of PPAR-γ, while GW9662 abrogated the inhibitory effect of ABA on allergic airway inflammation as well as on the activation of NLRP3 inflammasome and oxidative stress. Consistently, ABA inhibited the activation of NLRP3 inflammasome, suppressed oxidative stress and mitochondrial fusion/fission in LPS-stimulated Raw264.7 cells via PPAR-γ. Collectively, ABA ameliorates OVA-induced allergic airway inflammation in a PPAR-γ dependent manner, and such effect of ABA may be associated with its inhibitory effect on NLRP3 inflammasome and oxidative stress. Our results suggest the potential of ABA or ABA-rich food in protecting against asthma.

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