Inducible NF-κB Activation Is Permitted by Simultaneous Degradation of Nuclear IκBα

Signal-induced phosphorylation and ubiquitination of IκBα targets this inhibitor of NF-κB for proteasome-mediated degradation, thus permitting the release of active NF-κB. Upon cell stimulation, NF-κB activation results in neotranscription and neosynthesis of its own inhibitor, IκBα. As reported earlier, the neosynthesized inhibitor is then accumulated in the nucleus, where it rapidly binds to and terminates the function of nuclear NF-κB upon withdrawal of the stimulus. The present work was aimed at understanding how NF-κB activity is preserved while stimuli persist, despite intense, simultaneous IκBα neosynthesis, which would be expected to end NF-κB activity. We here show that incoming IκBα in the nucleus represents a target for resident nuclear proteasome complexes. Signal-induced, proteasome-dependent degradation of phosphorylated and ubiquitinated IκBα occurs in the nucleus, thus permitting the onset and persistence of NF-κB activity as long as stimulation is maintained. Our results suggest that intranuclear proteolysis of IκBα is necessarily required to avoid self-termination of NF-κB activity during cell activation. Signal-induced phosphorylation and ubiquitination of IκBα targets this inhibitor of NF-κB for proteasome-mediated degradation, thus permitting the release of active NF-κB. Upon cell stimulation, NF-κB activation results in neotranscription and neosynthesis of its own inhibitor, IκBα. As reported earlier, the neosynthesized inhibitor is then accumulated in the nucleus, where it rapidly binds to and terminates the function of nuclear NF-κB upon withdrawal of the stimulus. The present work was aimed at understanding how NF-κB activity is preserved while stimuli persist, despite intense, simultaneous IκBα neosynthesis, which would be expected to end NF-κB activity. We here show that incoming IκBα in the nucleus represents a target for resident nuclear proteasome complexes. Signal-induced, proteasome-dependent degradation of phosphorylated and ubiquitinated IκBα occurs in the nucleus, thus permitting the onset and persistence of NF-κB activity as long as stimulation is maintained. Our results suggest that intranuclear proteolysis of IκBα is necessarily required to avoid self-termination of NF-κB activity during cell activation. nuclear export sequence fetal calf serum carbobenzoxyl-leucin-leucin-leucinal interleukin tumor necrosis factor leptomycin B Inhibitory IκB proteins tightly control the biological activity of Rel/NF-κB transcription factors through their association with homo- or heterodimers of this family. Members of the family share a highly conserved NH2-terminal sequence termed the Rel homology domain, which is required for DNA binding, dimerization, nuclear localization, and interaction with the IκB molecules. In response to an inflammatory stimulus, cytokine, or viral infection, IκB proteins are rapidly degraded by the 26 S multicatalytic proteasome. Degradation of IκBα, the most intensively characterized inhibitor, requires phosphorylation on serine residues 32–36 (1.Alkalay I. Yaron A. Hatzubai A. Orian A. Ciechanover A. Benneriah Y. Proc. Natl. Acad. Sci. U. S. A. 1995; 92: 10599-10603Crossref PubMed Scopus (388) Google Scholar, 2.Brockman J.A. Scherer D.C. McKinsey T.A. Hall S.M. Qi X.X. Lee W.Y. Ballard D.W. Mol. Cell. Biol. 1995; 15: 2809-2818Crossref PubMed Google Scholar, 3.Brown K. Park S. Kanno T. Franzoso G. Siebenlist U. Proc. Natl. Acad. Sci. U. S. A. 1993; 90: 2532-2536Crossref PubMed Scopus (575) Google Scholar, 4.Traenckner E.B.M. Pahl H.L. Henkel T. Schmidt K.N. Wilk S. Baeuerle P.A. EMBO J. 1995; 14: 2876-2883Crossref PubMed Scopus (930) Google Scholar, 5.DiDonato J. Mercurio F. Rosette C. Wu-Li J. Suyang H. Ghosh S. Karin M. Mol. Cell. 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The of IκBα and NF-κB proteins expression in stimulation with the that within the TNFα a of the of IκBα by the localization of to the cell nucleus is and the accumulation of an increase in the nuclear of which the of NF-κB in the cell nucleus of of the of IκBα by newly protein becomes detectable TNFα by the onset of the a in the of IκBα was that and while was in the cell nucleus, the neosynthesized IκBα showed a localization to the nucleus induced IκBα phosphorylation or ubiquitination of the inhibitor in NF-κB·IκBα complex S. May M.J. Kopp E.B. Annu. Rev. Immunol. 1998; 16: 225-260Crossref PubMed Scopus (4584) Google Scholar). it is that IκBα to the cell nucleus from a of the This and the reported activity of IκB kinase which in TNFα E. M. M. Karin M. Cell. 1997; Full Text Full Text PDF PubMed Scopus Google Scholar) with the IκB kinase for IκBα as a when to NF-κB E. Chen Y. Karin M. 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Upon with TNFα the nuclear of IκBα could be by the of an nuclear import of newly protein and the accumulation by As a the NF-κB-dependent transcription in by TNFα could be or However, of NF-κB-DNA binding activity and NF-κB-dependent gene expression that the activity of NF-κB is by the simultaneous accumulation of IκBα by A and Indeed, to TNFα induced NF-κB activation and NF-κB-dependent expression of the gene of expression of the gene the control of an promoter by In NF-κB-dependent transcription is and in despite nuclear import of IκBα. this that the of IκBα accumulated by to was degraded in the cell nucleus as a consequence of stimulation by of protein the to the of IκBα in the nuclear Indeed, show that IκBα accumulated in from by from to is to We that the to degradation is only the of the by or to phosphorylation of the IκBα protein and the activation of NF-κB However, NF-κB-DNA binding and accumulation in the of activated despite of in the TNFα the of IκBα accumulated in to was with and a could be at and ensuring specific of IκBα the of the nuclear of the protein was by cell signaling, which induced degradation of IκBα. of nuclear IκBα is to be a physiological mechanism and termination of NF-κB-dependent transcription while cell Our are to from a that IκBα accumulated upon in the cell nucleus represents a to cell degradation M.S. Thompson J. Hay R.T. Dargemont C. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). The is to the of by the to specific inhibition of export in the to IκBα Indeed, a of by be when are to for long of M.S. Thompson J. Hay R.T. Dargemont C. J. Biol. Chem. 1999; 274: Full Text Full Text PDF PubMed Scopus Google Scholar). a during of this (29.Johnson C. Van Antwerp D. Hope T.J. EMBO J. 1999; 18: 6682-6693Crossref PubMed Google Scholar) reported that at in the degradation of IκBα. on that also that the of nuclear IκBα can be by cell (29.Johnson C. Van Antwerp D. Hope T.J. EMBO J. 1999; 18: 6682-6693Crossref PubMed Google Scholar). The in this is that nuclear IκBα, to be detectable upon extinction of the activation stimulus, is in fact the nucleus in to stimulation is degraded in the nuclear as long as stimulation Our the of two the transcriptional activity of NF-κB the nucleus of nuclear NF-κB to active as long as stimulation is and it results from proteasome-mediated degradation of nuclear IκBα, thus the termination of this The when NF-κB activity is results from retrograde transport of NF-κB proteins to the cytoplasm by nuclear IκBα is when stimulation is thus the nucleus from then NF-κB molecules. two would thus to the and the of NF-κB-dependent gene the to cell activation or or