Epidemiologic studies suggest that diet rich in plant-derived foods plays an important role in the prevention of prostate cancer. Curcumin, the yellow pigment in the spice turmeric, has been shown to exhibit chemopreventive and growth inhibitory activities against multiple tumor cell lines. We have shown previously that curcumin and tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)/Apo2L interact to induce cytotoxicity in the LNCaP prostate cancer cell line. In this study, we investigated the mechanism by which curcumin augments TRAIL-induced cytotoxicity in LNCaP cells. Subtoxic concentrations of the curcumin-TRAIL combination induced strong apoptotic response in LNCaP cells as demonstrated by the binding of Annexin V-FITC and cleavage of procaspase-3. Furthermore, LNCaP cells express constitutively active nuclear factor-kappaB (NF-kappaB), which is inhibited by curcumin. Because NF-kappaB has been shown to mediate resistance to TRAIL-induced apoptosis in tumor cells, we investigated whether there is a relationship between NF-kappaB activation and resistance to TRAIL in LNCaP prostate cancer cells. Pretreatment with curcumin inhibited the activation of NF-kappaB and sensitized LNCaP cells to TRAIL. A similar increase in the sensitivity of LNCaP cells to TRAIL-induced apoptosis was observed following inhibition of NF-kappaB by dominant negative mutant IkappaBalpha, an inhibitor of NF-kappaB. Finally, curcumin was found to inhibit NF-kappaB by blocking phosphorylation of IkappaBalpha. We conclude that NF-kappaB mediates resistance of LNCaP cells to TRAIL and that curcumin enhances the sensitivity of these tumor cells to TRAIL by inhibiting NF-kappaB activation by blocking phosphorylation of IkappaBalpha and its degradation.