Callicarpnoids A–C, structurally intriguing ent-Clerodane diterpenoid dimers with cytotoxicity against MCF-7 and HCT-116 cell lines from Callicarpa arborea Roxb.

[Display omitted] • Three unique ent -clerodane diterpenoid dimers were isolated from Callicarpa arborea. • The absolute configuration of 1 was deduced by single-crystal X-ray diffraction. • The absolute configuration of 2 and 3 was confirmed by NMR calculations with DP4 + analysis and ECD calculations. • The compound 2 and 3 showed potent cytotoxicity against MCF-7 and HCT-116 cell lines. Callicarpnoids A–C (1 – 3), three new ent -clerodane diterpenoid dimers formed via a [4 + 2] hetero Diels-Alder cycloaddition, appeared as a third example of this type of dimers, were isolated from the stems of Callicarpa arborea Roxb.. Their structures were elucidated by comprehensive spectroscopic analysis, and the absolute configurations were confirmed by single-crystal X-ray diffraction and electronic circular dichroism (ECD) calculations, as well as DP4 + analysis. Cytotoxicity test in two cell lines indicated that compounds 2 and 3 had significant cytotoxic effect against breast cancer cell (MCF-7) and colorectal cancer cell (HCT-116) with IC 50 ranging from 5.2 to 7.2 μ M, comparable to those of the positive control. Furthermore, the western blot analysis revealed that the protein expression levels of Bax were increased following compounds 2 and 3 treatment, whereas the expression levels of caspase 8, caspase 3, caspase 9 and Bcl 2 were decreased in a dose-dependent manner, indicating that compounds 2 and 3 may induce apoptosis via both intrinsic and extrinsic pathways in MCF-7 and HCT-116 cells. [ABSTRACT FROM AUTHOR]