Structurally diverse meroterpenoids from Arnebia euchroma and their anti-inflammatory effects through NF-κB pathway

Meroterpenoids in the traditional Chinese medicine Arnebia euchroma are thought to be key components in its anti-inflammatory activity. In the present study, 17 meroterpenoids including four types of structural skeletons (1-17), together with a monoterpenoid (18), were isolated from the roots of A. euchroma. HRESIMS, 1D and 2D NMR, electronic circular dichroism, and quantum computing-assisted methods were employed to determine the structures of four previously undescribed compounds (1-3, and 14). Zicaomeroterin B (2) feature with a novel benzo[b]oxepin moiety was confirmed by X-ray analysis based on the crystalline mate method. Meroterpenoids 2, 5, 6, 13, and 17 exhibited significant inhibition against the production of NO, IL-6, and TNF-α in LPS-stimulated macrophages without obvious cytotoxic effects. Furthermore, compounds 2, 5, and 6 significantly inhibited the phosphorylation activation of NF-κB p65 and its nuclear translocation in luciferase reporter test, immunoblotting, and immunofluorescence imaging, which in turn inhibited the NF-κB pathway and exerted anti-inflammatory effects. These findings suggested that meroterpenoids 2, 5, and 6 in A. euchroma are the potential lead compounds for anti-inflammatory agents based on NF-κB signaling pathway.