Sudachinoid- and Ichangensin-Type Limonoids from Citrus junos Downregulate Pro-Inflammatory Cytokines

Limonoids, a dominant group of phytochemicals in the Rutaceae family, are known to exhibit several pharmacological activities. To identify natural products having efficacy against inflammatory bowel disease (IBD), we isolated 13 limonoids including a new compound, methyl sudachinoid A, from the seeds of Citrus junos and investigated their anti-inflammatory effects by assessing the expression of pro-inflammatory cytokines in lipopolysaccharide-stimulated RAW 264.7 mouse macrophages and HT-29 human colon epithelial cells. Our findings revealed that limonoids significantly downregulated the pro-inflammatory cytokines, such as interleukin (IL)-1β, IL-6, IL-8, tumor necrosis factor-α, and nuclear transcription factor κB. In particular, sudachinoid-type compounds, methyl sudachinoid A and sudachinoid B, and ichangensin-type compound, 1-O-methyichangensin downregulated the expression of pro-inflammatory cytokines more potently than other limonoids, nomilin and limonin, which have been previously reported to exhibit anti-inflammatory activities in other cells; nomilin and limonin were therefore employed as positive controls in this study. Herein, we reveal that the anti-inflammatory activities of limonoids including a new compound methyl sudachinoid A from C. junos were mediated via the downregulation of pro-inflammatory cytokines and these limonoids can be employed as potential therapeutic phytochemicals for IBD.