Novel acylated steroidal glycosides from Caralluma tuberculata induce caspase-dependent apoptosis in cancer cells.

AIM OF THE STUDY: Pregnane glycosides are potent cytotoxic agents which may represent new leads in the development of anti-tumour drugs, particularly in the treatment of breast cancer, because of the structural similarity to estrogenic agonists. Caralluma species are natural sources of a wide variety of pregnane glycosides. The aim of the study was to isolate, using an activity-guided fractionation approach, novel pregnane glycosides for testing on breast cancer and other tumour lines. MATERIALS AND METHODS: The effect of crude extracts, specific organic fractions and isolated compounds from Caralluma tuberculata was tested on the growth and viability of MCF-7 estrogen-dependent, and MDA-MB-468 estrogen-independent breast cancer cells, Caco-2 human colonic cells, HUVECs and U937 cells. Neutral red uptake and MTT assays were used. Apoptosis was detected by Western blot of poly-(ADP ribose) polymerase (PARP) as were other markers of nuclear fragmentation (DNA ladder assay, staining of cells with nuclear dye DAPI). The involvement of caspases was investigated using the pan-caspase inhibitor Z-VAD-FMK. RESULTS: The ethyl acetate fraction of Caralluma tuberculata was found to be the most potent anti-proliferative fraction against all three cancer cell lines. Two novel steroidal glycosides were isolated from the active fraction after a series of chromatographic experiments. The structure of the isolated compounds was elucidated solely based on 2D-NMR (HMBC, HETCOR, DQF-COSY) and MS spectral analysis as compound 1: 12-O-benzoyl-20-O-acetyl-3ß,12ß,14ß,20ß-tetrahydroxy-pregnan-3-ylO-ß-D-glucopyranosyl-(1→4)-ß-D-glucopyranosyl-(1 → 4)-3-methoxy-ß-D-ribopyranoside, and as compound 2: 7-O-acetyl-12-O-benzoyl-3ß,7ß,12ß,14ß-tetrahydroxy-17ß-(3-methylbutyl-O-acetyl-1-yl)-androstan-3-ylO-ß-D-glucopyranosyl-(1 → 4)-6-deoxy-ß-D-allopyranosyl-(1 → 4)-ß-D-cymaropyranosyl-(1 → 4)-ß-D-cymapyranosyl-(1→ 4)-ß-D-cymaropyranoside. Compound 1 (pregnane glycoside) and compound 2 (androstan glycoside) induced apoptosis at <25 µM after 48 h as assessed by cell shrinkage, PARP cleavage, DNA fragmentation, and reversal with the caspase inhibitor. CONCLUSIONS: Two novel steroid glycosides isolated from Caralluma tuberculata possess moderate, micromolar cytotoxic activity on breast cancer and other cells in vitro, which may indicate a source of activity in vivo of interest to future drug design.