Sulforaphane [SFN; 1-isothiocyanato-4-(methylsulfinyl)-butane; CH3-SO-(CH2)4-N=C=S)], a phytochemical abundant in cruciferous vegetables, has been shown to offer significant protection against chemically induced cancers by inducing expression of phase 2 drug metabolizing enzymes such as glutathione transferases. Evidence is also mounting to indicate that SFN can inhibit growth of human cancer cells in culture by inducing apoptotic cell death. However, the sequence of events leading to SFN-induced apoptosis is poorly defined. Here, we report that SFN suppressed proliferation of PC-3 human prostate cancer cells by inducing apoptosis that was characterized by appearance of cells with sub-G0/G1 DNA content, formation of cytoplasmic histone associated DNA fragments, and cleavage of poly(ADP-ribose) polymerase (PARP). SFN-induced apoptosis was associated with up-regulation of Bax, down-regulation of Bcl-2, and cleavage of caspase-3, -9 and -8. SFN-induced apoptosis, and cleavage of procaspase-3 and PARP were blocked upon pretreatment of cells with pan caspase inhibitor z-VADfmk, and specific inhibitors of caspase-9 (z-LEHDfmk) and caspase-8 (z-IETDfmk) suggesting involvement of both caspase-9 and caspase-8 pathways in SFN-induced cell death. Oral administration of SFN (5.6 μmol, three times/week) significantly inhibited growth of PC-3 xenografts in nude mice in two independent experiments. For example, ten days after starting therapy, the average tumor volumes in control and SFN treated mice were 170 ± 13 and 80 ± 14 mm3, respectively, indicating a >50% reduction in tumor volume due to SFN administration. Similarly, 20 days after tumor cell implantation, the average tumor volume in SFN treated mice (165 ± 21 mm3) was about 71% lower than that of control mice. Growth inhibitory effect of SFN was also evident in terms of wet tumor weight. Average body weight of the control and SFN treated mice did not differ significantly throughout the treatment protocol. Consistent with the results in cultured PC-3 cells, TUNEL assay revealed a significantly higher count of apoptotic bodies in tumor sections from SFN treated mice compared with controls (about 3.3-fold higher compared with control, P < 0.05). To the best of our knowledge, the present study is the first published report to document in vivo anticancer activity of SFN in a tumor xenograft model. In conclusion, the results of the present study strongly argue for a systematic pre-clinical and clinical evaluation of SFN for its activity against human prostate cancer. This investigation was supported by USPHS grant CA101753, awarded by the National Cancer Institute.
[Proc Amer Assoc Cancer Res, Volume 45, 2004]