Cholestane glycosides from Ornithogalum saundersiae bulbs and their apoptosis-inducing activity via a mitochondria-independent pathway

Ornithogalum saundersiae (Liliaceae) is native to South Africa and cultivated as an excellent garden plant in the world. We have previously reported a total of 29 cholestane glycosides isolated from the MeOH extract of O. saundersiae bulbs and their cytotoxic activity against malignant tumor cells. Among the cholestane glycosides, OSW-1 showed exceptionally potent cytotoxic activity against various tumor cells. In this presentation, we wish to report further cytotoxic cholestane glycosides isolated from the MeOH extract of O. saundersiae bulbs.

The concentrated MeOH extract of O. saundersiae bulbs was passed through a Diaion HP-20 column eluted with 20% MeOH, EtOH and EtOAc, successively. The EtOH eluate fraction was subjected to column chromatography on silica gel and ODS silica gel, as well as preparative HPLC to give 40 cholestane glycosides including 20 new naturally occurring compounds. The structures of the new compounds were determined by 1D and 2D NMR spectroscopic analysis and the results of hydrolysis.

Cholestane glycosides 1-40 exhibited cytotoxic activity against HL-60 human promyelocytic leukemia cells with IC₅₀ values ranging from 0.0019 nM to 7.72µM. HL-60 cells treated with (22S)-3β,22-dihydroxycholest-5,24-dien-16β-yl α-L-rhamnopyranoside (17), which had an IC₅₀ value of 0.16µM, displayed the typical characteristics of apoptosis, such as the nuclear chromatin condensation, DNA laddering, accumulation of sub-G1 cells, and activation of caspase-3. The loss of the mitochondria membrane potential and release of cytochrome c to the cytosol were not observed in the HL-60 cells treated with 17, suggesting that 17 may induce apoptosis in HL-60 cells via a mitochondria-independent pathway. Similar experiments were conducted for several other compounds.