Anti-Inflammatory Effect of Citrus Unshiu Peel in LPS-Stimulated RAW 264.7 Macrophage Cells

Citrus Unshiu peel (CUP) has been traditionally used in East Asia as a drug for the treatment of vomiting and dyspepsia. However, its effects on inflammation remain unknown. In this study, we investigated the effects of CUP on the production of pro-inflammatory mediators in lipopolysaccharide (LPS)-stimulated RAW 264.7 cells. The research focused on determining whether CUP could inhibit the expression of inducible nitric oxide synthase (iNOS), cyclooxygenase (COX)-2 and the activation of nuclear factor (NF)-κB, mitogen-activated protein kinases (MAPKs), as well as the secretion of nitric oxide (NO), prostaglandin (PG) E(2), tumor necrosis factor-α (TNF-α) and interleukin (IL)-6 in LPS-stimulated RAW 264.7 cells. We found that CUP represses LPS-induced iNOS and COX-2 gene expression as well as NO, PGE(2), TNF-α and IL-6 production. Additionally, CUP inhibited the LPS-induced phosphorylation of extracellular signal-regulated kinase (ERK), p38, and c-Jun NH(2)-terminal kinase (JNK) MAPK, and suppressed IκBα degradation and nuclear translocation of NF-κB. Collectively, our results indicate that CUP inhibits the production of various inflammatory mediators via blockade of MAPK phosphorylation pursuant to the inhibition of IκBα degradation and the nuclear translocation of NF-κB. These findings are the first to clarify the mechanism underlying the anti-inflammatory effect exerted by CUP in RAW 264.7 macrophage cells stimulated by inflammatory agents.