Nuclear factorkappaB (NFkappaB) plays a critical role in cancer development and progression. Thus, the NFkappaB signaling pathway provides important targets for cancer chemoprevention and anticancer chemotherapy. The central steps in NFkappaB activation are phosphorylation and proteasome-dependent degradation of its inhibitory proteins termed IkappaBs. Consequently, the major pharmacological approaches to target NFkappaB include (1) repression of IkappaB kinases (IKKs) and (2) blocking the degradation of IkappaBs by proteasome inhibitors. We quantitatively compared the efficacy of various proteasome inhibitors (MG132, lactacystin and epoxomicin) and IKK inhibitors (BAY 11-7082 and PS1145) to block NFkappaB activity induced by TNFalpha or TPA and to sensitize LNCaP prostate carcinoma cells to apoptosis. Our studies revealed significant differences between these two classes of NFkappaB inhibitors. We found that proteasome inhibitors epoxomicin and MG132 attenuated NFkappaB induction much more effectively than the IKK inhibitors. Furthermore, in contrast to IKK inhibitors, all studied proteasome inhibitors specifically blocked TPA-induced generation de novo of NFkappaB p50 homodimers--(p50/p50). These results suggest that the proteasome plays a dominant role in TPA-induced formation of functional p50 homodimers, while IKK activity is less important for this process. Interestingly, profound attenuation of p50/p50 DNA-binding does not reduce the high potency of proteasome inhibitors to suppress NFkappaB-dependent transcription. Finally, proteasome inhibitors were much more effective in sensitizing LNCaP cells to TNFalpha-induced apoptosis compared to IKK inhibitors at the concentrations when both types of agents similarly attenuated NFkappaB activity. We conclude that this remarkable pro-apoptotic potential of proteasome inhibitors is partially mediated through NFkappaB-independent mechanism.