Cellular responses to cancer chemopreventive agent D,L-sulforaphane in human prostate cancer cells are initiated by mitochondrial reactive oxygen species

Purpose: Present study was undertaken to elucidate the mechanism of cellular responses to D,L-sulforaphane (SFN), a highly promising cancer chemopreventive agent. Methods: Mitochondrial DNA deficient Rho-0 variants of LNCaP and PC-3 cells were generated by culture in the presence of ethidium bromide. Apoptosis was assessed by analysis of cytoplasmic histone-associated DNA fragmentation and activation of caspase-3. Immunoblotting was performed to determine the expression of apoptosis- and cell cycle-regulating proteins. Generation of reactive oxygen species (ROS), mitochondrial membrane potential (MMP), and cell cycle distribution were measured by flow cytometry. Results: The Rho-0 variants of LNCaP and PC-3 cells were significantly more resistant to SFN-induced ROS generation, apoptotic DNA fragmentation, disruption of MMP, cytosolic release of cytochrome c, and G2/M phase cell cycle arrest compared with corresponding wild-type cells. SFN-induced autophagy, which serves to protect against apoptotic cell death in PC-3 and LNCaP cells, was also partially but markedly suppressed in Rho-0 variants compared with wild-type cells. SFN statistically significantly inhibited activities of mitochondrial respiratory chain enzymes in LNCaP and PC-3 cells. Conclusion: These results indicate, for the first time, that mitochondria-derived ROS serve to initiate diverse cellular responses to SFN exposure in human prostate cancer cells.