Up-regulation of PPAR-γ involved in the therapeutic effect of icariin on cigarette smoke-induced inflammation

Icariin (ICA) might be a potential anti-inflammatory medication in a variety of diseases including COPD, and previous studies showed that ICA could attenuate cigarette smoke (CS)-induced inflammation by inhibiting nuclear factor (NF)-κB. Peroxisome proliferator-activated receptor (PPAR) γ, a nuclear hormone receptor, has been reported to play a critical role in the inflammatory process in COPD. Whether PPAR-γ is involved in the anti-inflammatory effect of icariin on COPD has scarcely been explored. This study aimed at investigating the role of ICA in PPAR-γ expression in the CS-induced model, and then elucidating the therapeutic effects of ICA on COPD based on the PPARγ–NF–κB signaling pathway. The Beas-2B cells and H292 cells were induced with cigarette smoke extract (CSE) for 8 h after treatment with ICA for 16 h. The PPARγ expression and NF-κB pathway-related indicators were detected by western blotting, cellular immunofluorescence, and Real-time PCR. The PPARγ knock down or T0070907-treated Beas-2B cells were constructed to further investigate the relationship between the inhibition of NF-κB by ICA and PPARγ. A COPD model was established by CS exposure for 6 months, and ICA (40 mg/kg) was administrated by gastric perfusion. Then, the pulmonary function, lung histology, inflammatory cytokine levels, and protein expressions were detected. We found ICA up-regulated PPARγ protein expression in both Beas-2B cells and H292 cells, and it improved CSE-induced PPARγ down regulation and NF-κB activation. Furthermore, the inhibition of NF-κB pathway by ICA was partially dependent on PPARγ in the PPARγ knock down or T0070907-treated Beas-2B cells, suggesting that ICA attenuated CSE-induced inflammatory responses were associated with modulating the PPARγ–NF–κB pathway. Moreover, ICA showed similar effects on PPARγ and NF-κB expressions in the COPD model, and it effectively ameliorated the pulmonary function and lung inflammatory infiltration in the COPD rat model. Conclusively, the therapeutic effect of ICA on COPD was indirectly achieved by reducing airway inflammation, which was partially associated with modulating the PPARγ–NF–κB signaling pathway.