Pyran-2-one derivatives from Croton crassifolius as potent apoptosis inducers in HepG2 cells via p53-mediated Ras/Raf/ERK pathway.

Chemical investigation of the roots of Croton crassifolius led to the isolation of five pyran-2-one derivatives, including two brand new compounds (1-2), one new natural product (3) and two known compounds (4-5). Their structures and absolute configurations were established by spectroscopic analyses as well as comparison between the calculated optical rotation (OR) values with the experimental data. Interestingly, the new compound 1 showed an unusual negative chemical shift at H-11. It is well known that negative chemical shift values of H NMR spectrum are extremely rare in natural products. Such a negative chemical shift of H NMR spectrum was reproduced by density functional theory (DFT) calculations and explained by the shielding effect from the pyran-2-one ring over the hydrogen atom in the 3D conformations. Then, MTT assay was applied to evaluate the cytotoxicity of the isolated compounds (1-5) against two liver cancer cell lines (HepG2 and MHCC97H). The results suggested that compound 1 displayed the highest cytotoxicity with an IC value of 9.8 μM against HepG2 cells. Moreover, there was no obvious cytotoxicity of compounds 1-5 on normal liver cell line LO2. Furthermore, the mechanism of apoptosis induction in compound 1-treated HepG2 cells was investigated. The results showed that compound 1 could induce apoptosis via p53-mediated Ras/Raf/ERK suppression in HepG2 cells.