Sulforaphane-induced cell death in human prostate cancer cells is initiated by reactive oxygen species.

J Biol Chem. 2005 Mar 11; [Epub ahead of print] Related Articles, Links

Singh SV, Srivastava SK, Choi S, Lew KL, Antosiewicz J, Xiao D, Zeng Y, Watkins SC, Johnson CS, Trump DL, Lee YJ, Xiao H, Herman-Antisiewicz A.

Pharmacology and Urology, University of Pittsburgh Cancer Institute, Pittsburgh, PA 15213.

We have shown previously that sulforaphane (SFN), a constituent of many edible cruciferous vegetables including broccoli, suppresses growth of prostate cancer cells in culture as well as in vivo by causing apoptosis but the sequence of events leading to cell death is poorly defined. Using PC-3 and DU145 human prostate cancer cells as a model, we now demonstrate, for the first time, that the initial signal for SFN-induced apoptosis is derived from reactive oxygen species (ROS). Exposure of PC-3 cells to growth suppressive concentrations of SFN resulted in ROS generation, which was accompanied by disruption of mitochondrial membrane potential, cytosolic release of cytochrome c, and apoptosis. All these effects were significantly blocked on pretreatment with N-acetylcysteine and overexpression of catalase. The SFN-induced ROS generation was significantly attenuated on pretreatment with mitochondrial respiratory chain complex I inhibitors including diphenyleneiodonium chloride and Rotenone. SFN treatment also caused a rapid and significant depletion of glutathione (GSH) levels. Collectively, these observations indicated that SFN-induced ROS generation is probably mediated by a non-mitochondrial mechanism involving GSH depletion as well as a mitochondrial component. Ectopic expression of Bcl-xL, but not Bcl-2, in PC-3 cells offered significant protection against the cell death caused by SFN. In addition, SFN treatment resulted in an increase in the level of death-receptor Fas, activation of caspase-8 and cleavage of Bid. Furthermore, SV40 immortalized mouse embryonic fibroblasts (MEFs) derived from Bid knockout mice displayed significant resistance towards SFN-induced apoptosis compared with wild type MEFs. In conclusion, the results of the present study indicate that SFN-induced apoptosis in prostate cancer cells is initiated by ROS generation, and that both intrinsic and extrinsic caspase cascades contribute to the cell death caused by this highly promising cancer chemopreventive agent.