Suppression of inducible nitric oxide synthase by (−)-isoeleutherin from the bulbs of Eleutherine americana through the regulation of NF-κB activity

Eleutherine americana Merr. et Heyne (Iridaceae) has been used as a folk medicine for the treatment of coronary disorders and wound-healing. In our previous phytochemical study, several pyranonaphthoquinoids, including (−)-isoeleutherin, were isolated from the bulbs of E. americana. Because inhibitors of inducible nitric oxide synthase (iNOS) might be useful as anti-inflammatory agents, we investigated the potential of pyranonaphthoquinoids to inhibit iNOS activity. We found that (−)-isoeleutherin inhibits lipopolysaccharide (LPS)-stimulated induction of nitric oxide (NO) in a dose-dependent manner (IC50 = 7.4 μM). Using western blots and reverse transcription-polymerase chain reaction, we showed that (−)-isoeleutherin suppresses the expression of iNOS protein and mRNA. In addition, (−)-isoeleutherin inhibited the expression of various cytokines such as interleukin-1β and interferon-β. Activation of the transcriptional activity of NF-κB by LPS was also inhibited by treatment with (−)-isoeleutherin, suggesting that (−)-isoeleutherin-mediated suppression of iNOS expression is associated with the regulation of transcription factor NF-κB. These findings suggest that (−)-isoeleutherin might be a novel anti-inflammatory agent, and that it may work by inhibiting NFkB activation in macrophages.