NF-B is a transcription factor whose nuclear residence is controlled by IB family members. In the NF-B-IB autoregulatory loop, activated (nuclear) Rel A⅐NF-B1 induces the resynthesis of IB␣ recapturing nuclear Rel A back into the cytoplasm within 1 h of stimulation. In contrast, NF-B1 subunits redistribute more slowly into the cytoplasm (from 6 to 12 h). Here we examine the role of inducible cytoplasmic BCL-3 expression in terminating nuclear NF-B1. Although BCL-3 is a nuclear protein in B lymphocytes, surprisingly, BCL-3 is primarily a cytoplasmic protein in HepG2 cells. Cytoplasmic BCL-3 abundance is induced 6 -12 h after tumor necrosis factor-␣ stimulation where it complexes with NF-B1 homodimers. Moreover, BCL-3 mRNA and protein expression are induced by NF-B-activating agents. Two observations are interpreted to indicate that bcl-3 is transactivated by NF-B/Rel A: 1) expression of a dominant negative NF-B inhibitor blocks tumor necrosis factor-␣-induced BCL-3 expression and 2) expression of constitutively active Rel A is sufficient to induce BCL-3 expression. In gene transfer studies, we identify two high affinity NF-B-binding sites, B1 (located at ؊872 to ؊861 nucleotides) and B2 (؊106 to ؊96 nucleotides), and although both bind with high affinity to Rel A, only B2 is required for NF-B-dependent induction of the native BCL-3 promoter. Down-regulation of BCL-3 induction results in prolonged, enhanced NF-B1 binding and increased NF-B-dependent transcription. Together, these data suggest the presence of an NF-B-BCL-3 autoregulatory loop important in terminating NF-B1 action and that individual NF-B isoforms are actively terminated through coordinate induction of inhibitory IB molecules to restore cellular homeostasis.

NF-B is an inducible transcription factor mediating innate immune responses whose activity is controlled by the multiprotein IB kinase (IKK) "signalsome". The core IKK consists of two catalytic serine kinases, IKK␣ and IKK␤, and a noncatalytic subunit, IKK␥. IKK␥ is required for IKK activity by mediating kinase oligomerization and serving to couple the core catalytic subunits to upstream mitogen-activated protein 3-kinase cascades. We have discovered an alternatively spliced IKK␥ mRNA isoform, encoding an in-frame deletion of exon 5, termed IKK␥-⌬. Using a specific reverse transcription-PCR assay, we find that IKK␥-⌬ is widely expressed in cultured human cells and normal human tissues. Because IKK␥-⌬ protein is lacking a critical coiled-coil domain important in protein-protein interactions, we sought to determine its signaling properties by examining its ability to self associate, couple to activators of the canonical pathway, and mediate human T-cell leukemia virus type 1 (HTLV-1) Tax Nuclear factor-B (NF-B) is an inducible transcription factor that controls the expression of inducible inflammatory and antiapoptotic genes (49, 51). NF-B responds to a diverse series of inflammatory activators, including UV light, doublestranded RNA, cytokines, vasoactive peptides, and viral oncogenes through several distinct intracellular pathways (reviewed in reference 36). Because of its central role as an integrator of stress and inflammatory stimuli, the pathways controlling NF-B have been intensively investigated.Currently, it is thought that NF-B activation is controlled by two distinct pathways, termed the canonical (23) and noncanonical pathways (9, 43). The canonical pathway controls nuclear translocation of the prototypical NF-B complex, composed of 65-kDa Rel A-50-kDa NF-B1 heterodimers. Under normal conditions, the Rel A · NF-B1 complex is sequestered and inactivated in the cytoplasm by the IB inhibitors, proteins which inactivate Rel A DNA binding and nuclear translocation by masking its nuclear localization sequence (reviewed in reference 1). NF-B activators induce IB phosphorylation on serine residues 32 and 36 on its NH 2 -terminal regulatory domain (reviewed in reference 23). Phospho-IB is then specifically bound by the Skp1-Cullin-F-Box-type E3 ubiquitin ligase, E3RS, initiating IB ubiquitination and proteolysis through the proteasome (4, 23, 24) and calpain pathways (15). Nuclear translocated NF-B binds high-affinity chromatin sites and activates the expression of a diverse gene network (50) by inducing assembly of active promoters (2) and recruiting coactivators to target gene promoters (44).The cytoplasmic IB kinase (IKK), also known as the "signalsome," is the rate-limiting kinase responsible for inducible IB␣ phosphorylation (23) and is composed of core catalytic kinases and scaffolding proteins. The catalytic core contains the ubiquitous helix-loop-helix proteins IKK␣ and IKK␤ (33,54), associated with the noncatalytic regulatory protein, IKK␥, in a precise stoichiometric relationship of 2 catalytic subuni...