Sulforaphane [1-isothiocyanato-4-(methylsulfinyl)-butane; CH3-SO-(CH2)4-N=C=S], which is a constituent of many edible cruciferous vegetables including broccoli, has been shown to inhibit chemically induced cancer in animal models. Evidence is accumulating to indicate that SFN can suppress growth of cancer cells in culture as well as in vivo by causing apoptosis. However, the signaling mechanism by which the cell death is initiated remains elusive. Using PC-3 and DU145 human prostate cancer cells as a model, we demonstrate that apoptosis induction by SFN is initiated by generation of reactive oxygen species (ROS). Exposure of PC-3 cells to SFN resulted in a dose- and time-dependent increase in ROS generation that was accompanied by disruption of mitochondrial membrane potential, release of cytochrome c to the cytosol, and apoptosis induction. All these effects were fully blocked upon pretreatment of PC-3 cell with N-acetylcysteine, a known antioxidant. Consistent with these results, adenovirus-mediated transduction of catalase, a H2O2 scavenger, offered near complete protection against SFN-induced ROS generation and cell death. The ROS generation as well as apoptosis induction by SFN was partially but statistically significantly attenuated upon pretreatment with cyclosporin A, an inhibitor of the mitochondrial membrane permeability transition, suggesting involvement of mitochondria in SFN-induced ROS generation. On the other hand, ectopic expression of Bcl-2, whose protein level is reduced upon treatment with SFN in different cellular systems including PC-3 cells, failed to confer significant protection against SFN-induced apoptosis as judged by analysis of cytoplasmic histone associated DNA fragmentation, appearance of sub-diploid cells and cleavage of poly(ADP-ribose)polymerase. In conclusion, the results of the present study clearly indicate that SFN-induced cell death in human prostate cancer cells is initiated by ROS generation, whereas Bcl-2 overexpression fails to confer significant protection against cell death in our model (supported by NCI grant CA101753).
[Proc Amer Assoc Cancer Res, Volume 46, 2005]