Denbinobin, a phenanthraquinone derivative isolated from the stems of Ephemerantha lonchophylla, induced apoptosis in human colon cancer HCT-116 cells in a concentration- and time-dependent manner as measured by MTT assay and FACS analysis, respectively. The denbinobin-induced apoptotic reaction was shown by nuclear breakage and DNA fragmentation using DAPI and TUNEL staining. Interestingly, denbinobin-induced apoptosis in HCT-116 cells was not dependent on caspase activation based on the results showing that addition of a pan-caspase inhibitor z-VAD-fmk did not suppress apoptotic cell death. Moreover, denbinobin-induced apoptosis was not accompanied by processing of procaspase-3, 9, and 8, or of PARP. However, denbinobin induced a reduction in mitochondrial membrane potential (ΔΨm). Since the loss of mitochondrial membrane potential was observed in denbinobin-treated HCT-116 cells, we speculate that alterations in the Bcl-2 family proteins and the release of mitochondrial apoptogenic factors might be involved. The results showed that denbinobin upregulated Bax and downregulated Bcl-2 and Bcl-xL expression. Moreover, denbinobin induced AIF and endonuclease G released from mitochondria which then translocated to the nucleus. Taken together, our findings suggest a possible pathway for denbinobin-induced apoptosis in human colon cancer HCT-116 cells in a caspase-independent, mitochondria-mediated pathway. These results indicate that denbinobin, an extract from traditional Chinese herbal medicine, may represent a new source of anti-cancer agents.

[Proc Amer Assoc Cancer Res, Volume 46, 2005]