Punica granatum L. (Punicaceae) is a popular fruit all over the world. Owning to its enriched polyphenols, P. granatum has been widely used in treating inflammation-related diseases, such as cardiovascular diseases and cancer. Twenty polyphenols, containing nine unreported ones, named punicagranins A-I (1-9), along with eleven known isolates (10-20), were obtained from the peels. Their detailed structures were elucidated based on UV, IR, NMR, MS, optical rotation, ECD analyses and chemical evidence. The potential anti-inflammatory activities of all polyphenols were examined on a lipopolysaccharide (LPS)-induced inflammatory macrophages model, which indicated that enhancing nitric oxide (NO) production in response to inflammation stimulated in RAW 264.7 cells was controlled by compounds 1, 3, 5-8, 10, 11, 14 and 16-20 in a concentration-dependent manner. The investigation of structure-activity relationships for tannins 6-8 and 12-20 suggested that HHDP, flavogallonyl and/or gallagyl were key groups for NO production inhibitory activity. Western blotting indicated that compounds 6-8 could down-regulate the phosphorylation levels of proteins p38 MAPK, IKKα/β, IκBα and NF-κB p65 as well as inhibit the levels of inflammation-related cytokines and mediators, such as IL-6, TNF-α, iNOS and COX-2, at the concentration of 30 μM. In conclusion, polyphenols are proposed to be the potential anti-inflammatory active ingredients in P. granatum peels, and their molecular mechanism is likely related to the regulation of the p38 MAPK and NF-κB signaling pathways.
Keywords: NF-κB signaling pathway; Punica granatum L.; RAW 264.7 cells; anti-inflammation; p38 MAPK; polyphenols.