Recent studies indicate that natural isothiocyanates, such as sulforaphane (SFN) and phenethyl isothiocyanate (PEITC) possess strong antitumor activities in vitro and in vivo. The nuclear factor kappa B (NF-kappaB) is believed to play an important role in cancer chemoprevention due to its involvement in tumor cell growth, proliferation, angiogenesis, invasion, apoptosis, and survival. In this study, we investigated the effects and the molecular mechanisms of SFN and PEITC on NF-kappaB transcriptional activation and NF-kappaB-regulated gene expression in human prostate cancer PC-3 C4 cells. Treatment with SFN (20 and 30 microM) and PEITC (5 and 7.5 microM) significantly inhibited NF-kappaB transcriptional activity, nuclear transloction of p65, and gene expression of NF-kappaB-regulated VEGF, cylcin D1, and Bcl-X(L) in PC-3 C4 cells. To further elucidate the mechanism, we utilized the dominant-negative mutant of inhibitor of NF-kappaB alpha (IkappaBalpha) (SR-IkappaBalpha). Analogous to treatments with SFN and PEITC, SR-IkappaBalpha also strongly inhibited NF-kappaB transcriptional activity as well as VEGF, cylcin D1, and Bcl-X(L) expression. Furthermore, SFN and PEITC also inhibited the basal and UVC-induced phosphorylation of IkappaBalpha and blocked UVC-induced IkappaBalpha degradation in PC-3 C4 cells. In examining the upstream signaling, we found that the dominant-negative mutant of IKKbeta (dnIKKbeta) possessed inhibitory effects similar to SFN and PEITC on NF-kappaB, VEGF, cylcin D1, Bcl-X(L) as well as IkappaBalpha phosphorylation. In addition, treatment with SFN and PEITC potently inhibited phosphorylation of both IKKbeta and IKKalpha and significantly inhibited the in vitro phosphorylation of IkappaBalpha mediated by IKKbeta. Taken together, these results suggest that the inhibition of SFN and PEITC on NF-kappaB transcriptional activation as well as NF-kappaB-regulated VEGF, cyclin D1, and Bcl-X(L) gene expression is mainly mediated through the inhibition of IKK phosphorylation, particularly IKKbeta, and the inhibition of IkappaBalpha phosphorylation and degradation, as well as the decrease of nuclear translocation of p65 in PC-3 cells.