Sulforaphane (SFN) is a chemopreventive agent present in various cruciferous vegetables, including broccoli. Here, we show that treatment with tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) in combination with subtoxic doses of SFN significantly induces rapid apoptosis in TRAIL-resistant hepatoma cells. Neither TNF-α- nor Fas-mediated apoptosis was sensitized in hepatoma cells by cotreatment with SFN, suggesting that SFN can selectively sensitize cells to TRAIL-induced apoptosis but not to apoptosis mediated by other death receptors. We found that SFN treatment significantly upregulated mRNA and protein levels of DR5, a death receptor of TRAIL. This was accompanied by an increase in the generation of reactive oxygen species. Pretreatment with N-acetylcysteine and overexpression of catalase inhibited SFN-induced upregulation of DR5 and almost completely blocked the cotreatment-induced apoptosis. Furthermore, the SFN-mediated sensitization to TRAIL was efficiently reduced by administration of a blocking antibody or small interfering RNAs for DR5. These results collectively indicate that SFN-induced generation of reactive oxygen species and the subsequent upregulation of DR5 are critical for triggering and amplifying TRAIL-induced apoptotic signaling. We also found that SFN can sensitize both Bcl-xL- and Bcl-2-overexpressing hepatoma cells to TRAIL-induced apoptosis, indicating that treatment with a combination of TRAIL and SFN may be a safe strategy for treating resistant hepatomas.
