Eupatolide inhibits lipopolysaccharide-induced COX-2 and iNOS expression in RAW264.7 cells by inducing proteasomal degradation of TRAF6

Abstract

Inula britannica is a traditional medicinal plant used to treat bronchitis, digestive disorders, and inflammation in Eastern Asia. Here, we identified eupatolide, a sesquiterpene lactone from I. britannica, as an inhibitor of cyclooxygenase-2 (COX-2) and inducible nitric oxide synthase (iNOS) expression. Eupatolide inhibited the production of nitric oxide (NO) and prostaglandin E2 (PGE2) as well as iNOS and COX-2 protein expression in lipopolysaccharide (LPS)-stimulated RAW264.7 cells. Eupatolide dose-dependently decreased the mRNA levels and the promoter activities of COX-2 and iNOS in LPS-stimulated RAW264.7 cells. Moreover, eupatolide significantly suppressed the LPS-induced expression of nuclear factor-κappa B (NF-κB) and activator protein-1 (AP-1) reporter genes. Pretreatment of eupatolide inhibited LPS-induced phosphorylation and degradation of IκBα, and phosphorylation of RelA/p65 on Ser-536 as well as the activation of mitogen-activated protein kinases (MAPKs) and Akt in LPS-stimulated RAW264.7 cells. Eupatolide induced proteasomal degradation of tumor necrosis factor receptor-associated factor-6 (TRAF6), and subsequently inhibited LPS-induced TRAF6 polyubiquitination. These results suggest that eupatolide blocks LPS-induced COX-2 and iNOS expression at the transcriptional level through inhibiting the signaling pathways such as NF-κB and MAPKs via proteasomal degradation of TRAF6. Taken together, eupatolide may be a novel anti-inflammatory agent that induces proteasomal degradation of TRAF6, and a valuable compound for modulating inflammatory conditions.