Achyranthoside H methyl ester, a novel oleanolic acid saponin derivative from Achyranthes fauriei roots, induces apoptosis in human breast cancer MCF-7 and MDA-MB-453 cells via a caspase activation pathway

Achyranthoside H methyl ester (AH-Me) is an oleanolic acid saponin derivative isolated from the roots of Achyranthes fauriei through diazomethane treatment. AH-Me exhibited significant cytotoxicity against human breast cancer MCF-7 and MDA-MB-453 cells, with respective ID50 values of 4.0 and 6.5 mu M in the MTT assay. AH-Me is a unique saponin containing three methoxycarbonyl groups in the sugar moiety linked to the C-3 position of oleanolic acid. The demethylation of these methoxycarbonyl groups by alkaline hydrolysis caused a marked reduction of the cytotoxicity of AH-Me, suggesting that the methoxycarbonyl groups of AH-Me are key groups for the acquisition of cytotoxicity against human cancer cells. The staining of cancer cells with 4',6'-diamidino-2-phenylindole ( DAPI) showed that the population of cells with altered nuclear morphology, for example chromatin condensation and fragmentation, increased markedly after AH-Me treatment. Exposure of MCF-7 and MDA-MB-453 cells to AH-Me resulted in a dose-dependent and time-dependent increase in the sub-G1 population, and in the cleavage of poly-ADP-ribose polymerase (PARP) followed by the formation of an 89 kD peptide. Pretreatment of the cells with the pan-caspase inhibitor z-VAD-fmk abolished the cleavage of PARP by AH-Me treatment and suppressed the antiproliferative effect of AH-Me on tumor cell growth. These results together led to the suggestion that AH-Me induces apoptosis via the caspase activation pathway in human breast cancer cells, and apoptosis is the major mode of the cytotoxic effect triggered by AH-Me.