Mutual regulation of the transcriptional activator NF-kappa B and its inhibitor, I kappa B-alpha.

Abstract

The NF-#cB transcription factor complex is sequestered in the cytoplasm by the inhibitory protein I#cB-a (MAD-3). Various cellular stimuli relieve this inhibition by mechanisms largely unknown, leading to NF-KB nuclear localization and transactivation of its target genes. It is demonstrated here with human T lymphocytes and monocytes that different stimuli, including tumor necrosis factor a and phorbol 12-myristate 13-acetate, cause rapid degradation ofIOcB-a, with concomitant activation of NF-cB, followed by a dramatic increase in IKB-a mRNA and protein synthesis. Transfection studies reveal that the IKB-a mRNA and the encoded protein are potently induced by NF-cB and by homodimers of p65 and of c-Rel. We propose a model in which NF-.cB and IKB-a mutually regulate each other in a cycle: saturating amounts of the inhibitory IKB-a protein are destroyed upon stimulation, allowing rapid activation of NF-KB. Subsequently, IcB-a mRNA and protein levels are qulickly induced by the activated NF-cB. This resurgence of IKB-a protein acts to restore an equilibrium in which NF-KB is again inhibited.NF-KB is a dimeric transcription factor that binds and regulates gene expression through decameric cis-acting KB DNA motifs (reviewed in refs. 1 and 2). Although a p50/p65 heterodimer has traditionally been referred to as NF-KB and remains the prototypical and most abundant form, it has been recognized recently that several distinct but closely related homo-and heterodimeric factors are responsible for KB site-dependent DNA binding activity and regulation. The various dimeric factors are composed of members of the family of Rel-related polypeptides. One subclass of this family, distinguished by its proteolytic processing from precursor forms and lack of recognized activation domains, includes p50 (NFKB1) (3-6) and pSOB (NFKB2, pS2) (7-10), whereas the second subclass contains recognized activation domains and includes p65 (RelA) (11-13), RelB (14,15),18) Activation of the NF-KB transcription factor and various related forms can be initiated by a variety of agents, including tumor necrosis factor a (TNF-a) and phorbol 12-myristate 13-acetate (PMA) (1, 2). Activation proceeds through a post-translational event in which preformed cytoplasmic NF-KB is released from a cytoplasmic inhibitory protein, (20)(21)(22)(23). IKB-a inhibits transactivation of the p50/p65 heterodimer, by binding to the p65 component, blocking the dimer's translocation to the nucleus (20,21,23). IKB-a also inhibits complexes containing c-Rel or RelB (24,25). IKB-a blocks binding in vitro of various NF-KB dimers to KB binding sites in DNA (11,12,15,22,26,27). Because the latter effect requires nuclear IKB-a, its relevance, in vivo, is unknown. Although lKB-a is generally a cytoplasmic protein (21, 23), it and its chicken homolog (pp4O) have also been detected in the nucleus (refs. 28-30 and K.B., G.F., and U.S., unpublished results). In addition to the wellcharacterized and cloned IKB-a and its chicken and rat homologs (24, 31), another biochemically d...

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