Inhibition of NFkappaB in cutaneous T-cell lymphoma HUT-78 cells by nuclear translocation of IkappaBalpha induced by proteasome inhibitors

MG132 and bortezomib are proteasome inhibitors known to suppress the proteasome-mediated degradation of proteins. In this study, I have shown that proteasome inhibition leads to nuclear translocation and accumulation of IkappaBalpha which in-turn leads to specific inhibition of NFkappaB DNA binding activity and induction of apoptosis in cutaneous T-cell lymphoma HuT-78 cells. In addition, I have demonstrated that the proteasome inhibition induced nuclear IkappaBalpha leads to transcriptional inhibition of NFkappaB-regulated anti-apoptotic genes cIAP1 and cIAP2, but not Bc1-2. IkappaBalpha is predominantly localized in the cytoplasm, while NFkappaB proteins are mainly present in the nucleus of untreated Hut-78 cells. MG132 and bortezomib are potent, selective and reversible proteasome inhibitors that have been shown to suppress degradation of proteins by inhibiting the 26S proteasome complex. Nuclear IkappaBalpha results in the inhibition of NFkappaB DNA binding activity in vitro , and prevents NFkappaB p65 and NFkappaB p50 recruitment to its regulated anti-apoptotic cIAPI and cIAP2 promoters in vivo, thus inducing apoptosis. Interestingly, NFkappaB p65 is not recruited to Bcl-2 promoter in Hut-78 cells, in contrast to NFkappaB p50. Although IkappaBalpha translocates to the nucleus in a dose and time dependent manner upon proteasome inhibition, the total cellular levels of IkappaBalpha remained constant in Hut-78 cells. Moreover, I have shown that this nuclear translocation of IkappaBalpha is irreversible in cutaneous T-cell lymphoma Hut-78 cells. Nuclear IkappaBalpha binds to NFkappaB p65 and dissociates it from the promoters of cIAP1 and cIAP2, thus inhibiting transcription of NFkappaB-dependent anti-apoptotic genes. This new mechanism could provide a valuable insight into developing novel therapies for cancer and leukemia characterized by high levels of NFkappaB activity.