Eucalyptol, limonene and pinene enteric capsules attenuate airway inflammation and obstruction in lipopolysaccharide-induced chronic bronchitis rat model via TLR4 signaling inhibition

Chronic bronchitis (CB), a type of chronic obstructive pulmonary disease (COPD), poses a significant global health burden owing to its high morbidity and mortality rates. Eucalyptol, limonene and pinene enteric capsules (ELPs) are clinically used as expectorants to treat various respiratory diseases, including CB, but their acting mechanisms remain unclear. In this study, we investigated the anti-CB effects of ELP in a rat model of lipopolysaccharide (LPS)-induced CB. The molecular mechanisms underlying its inhibitory effects on airway inflammation were further explored in LPS-stimulated Beas-2B cells. ELP was characterized using gas chromatography. The production of inflammatory mediators in bronchoalveolar lavage fluid (BALF) was determined using an enzyme-linked immunosorbent assay. The expression of MUC5AC, MUC5B, and p-p65 in the lung tissue was measured using immunohistochemical staining. The gene expression of inflammatory mediators was determined using qRT-PCR. The expression levels of the target proteins were detected by western blotting. Nuclear localization of p65 was determined using an immunofluorescence assay. Compared to the CB model rats, ELP-treated rats showed reduced airway resistance, inflammation, and goblet cell hyperplasia. In BALF, ELP decreased the levels of inflammatory mediators, including TNF-α, IL-6, MIP-1α, and CCL5. ELP also suppressed LPS-induced elevation of MUC5AC, MUC5B, and p-p65 in the lung tissue. The metabolic pathway changes caused by LPS challenge were improved by ELP treatment. In LPS-exposed Beas-2B cells, ELP treatment inhibited the expression of TNFA, IL6, CCL5, MCP1, and MIP2A and decreased the phospho-levels of toll-like receptor 4 (TLR4) signaling-related proteins, including p-p38, p-JNK, p-ERK, p-TBK1, p-IKKα/β, p-IκB, p-p65, and p-c-Jun. ELP also hindered the nuclear translocation of p65, c-Jun, and IRF3. This study showed that ELP has a potential therapeutic effect in LPS-induced CB rat model, possibly by suppressing TLR4 signaling. These results justify the clinical use of ELP for the treatment of pulmonary inflammatory diseases.