Anti-proliferative tirucallane triterpenoids from gum resin of Boswellia sacra

Abstract

Eight new tirucallane triterpenoids (1-2, 5-10) along with two known compounds (3-4) were isolated from the gum resin of Boswellia sacra. Their structures were elucidated by extensive physicochemical and spectroscopic analysis, as well as computational calculations, and single crystal X-ray diffraction. Spirosacraoic acid A (1) and B (2), possess an unusual 6/5/6/5 rearranged spirocyclic carbon skeleton. All the isolates were evaluated for their anti-proliferative activity against two tumor cell lines (HepG2 and HCT-116 cells). Compound 10 displayed remarkable inhibitory activity against HepG2 cells in a dose-dependent manner with the IC₅₀ value of 28.01 μM. High content analysis (HCA) showed that 10 induces apoptosis in HepG2 cells. The western blotting results revealed that 10 could up-regulate the ratio of the expression of Bax/BCL-2, and promote the caspase 3 activation and PARP cleavage. Mechanically, molecular modeling studies demonstrated that 10 could dock into EGFR active site. Meanwhile 10 significantly decreased the protein expression of p-EGFR. Furthermore, inhibition of EGFR by addition of EGFR siRNA enhanced the growth inhibitory effects of 10 on HepG2 cells, indicating that the anti-tumor effect of 10 on HepG2 cells was mediated by inhibition of EGFR.

Keywords: Anti-proliferative; Boswellia sacra; Cell apoptosis; EGFR; Olibanum; Tirucallane triterpenoids.