Identification, in vitro evaluation and modeling studies of the constituents from the roots of Arnebia euchroma for antitumor activity and STAT3 inhibition.

Phytochemical investigation of the roots of Arnebia euchroma led to the isolation of a new dimeric naphthoquinone, shikometabolin H (1), a new meroterpeniod, epoxyarnebinol (5) and a new ring A-seco hopane triterpenoid, 2, 3-secodiplopterol dioic acid (7), together with 18 known compounds. The structures of the new compounds were elucidated by extensive spectroscopic analyses, including UV, IR, HRESIMS, 1D and 2D NMR. The absolute configuration of 1 was assigned by comparison of the experimental and calculated ECD data, whereas that of 5 was achieved by the comparison between experimental and calculated OR values. Compound 7 was found to be the first ring A-seco hopane triterpenoid ever reported. Data from MTT assays showed that compounds 1, 2, 4, 8, 11-14 and 19 possessed promising anti-proliferative activities in human melanoma cells A375 and A2058, and human colorectal adenocarcinoma cells HCT116 and SW620. In addition, molecular docking study was carried out to predict the possible binding modes of these active compounds with STAT3. Immunoblotting results further confirmed the inhibitory effects of compounds 1, 4, 8, 11, 14 and 19 on STAT3, which was indicated by the downregulated protein levels of phosphorylated and/or total STAT3 in A2058 and HCT116 cells.