The NF-kappa B p50 precursor, p105, contains an internal I kappa B-like inhibitor that preferentially inhibits p50

IRAK1 deficiency potentiates the efficacy of radiotherapy in repressing cervical cancer development

IRAK1 has been implicated in promoting development of various types of cancers and mediating radioresistance. However, its role in cervical cancer tumorigenesis and radioresistance, as well as the potential underlying mechanisms, remain poorly defined. In this study, we evaluated IRAK1 expression in radiotherapy-treated cervical cancer tissues and found that IRAK1 expression is negatively associated with the efficacy of radiotherapy. Consistently, ionizing radiation (IR)-treated HeLa and SiHa cervical cancer cells express a lower level of IRAK1 than control cells. Depletion of IRAK1 resulted in reduced activation of the NF-κB pathway, decreased cell viability, downregulated colony formation efficiency, cell cycle arrest, increased apoptosis, and impaired migration and invasion in IR-treated cervical cancer cells. Conversely, overexpressing IRAK1 mitigated the anti-cancer effects of IR in cervical cancer cells. Notably, treatment of IRAK1-overexpressing IR-treated HeLa and SiHa cells with the NF-κB pathway inhibitor pyrrolidine dithiocarbamate (PDTC) partially counteracted the effects of excessive IRAK1. Furthermore, our study demonstrated that IRAK1 deficiency enhanced the anti-proliferative role of IR treatment in a xenograft mouse model. These collective observations highlight IRAK1's role in mitigating the anti-cancer effects of radiotherapy, partly through the activation of the NF-κB pathway. SUMMARY: IRAK1 enhances cervical cancer resistance to radiotherapy, with IR treatment reducing IRAK1 expression and increasing cancer cell vulnerability and apoptosis.

Nuclear Factor-Kappa B Regulation of Osteoclastogenesis and Osteoblastogenesis

Maintenance of skeletal integrity requires the coordinated activity of multinucleated bone-resorbing osteoclasts and bone-forming osteoblasts. Osteoclasts form resorption lacunae on bone surfaces in response to cytokines by fusion of precursor cells. Osteoblasts are derived from mesenchymal precursors and lay down new bone in resorption lacunae during bone remodeling. Nuclear factorkappa B (NF-κB) signaling regulates osteoclast and osteoblast formation and is activated in osteoclast precursors in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which can also control osteoblast formation through RANK-RANKL reverse signaling in osteoblast precursors. RANKL and some pro-inflammatory cytokines, including tumor necrosis factor (TNF), activate NF-κB signaling to positively regulate osteoclast formation and functions. However, these cytokines also limit osteoclast and osteoblast formation through NF-κB signaling molecules, including TNF receptor-associated factors (TRAFs). TRAF6 mediates RANKL-induced osteoclast formation through canonical NF-κB signaling. In contrast, TRAF3 limits RANKL- and TNF-induced osteoclast formation, and it restricts transforming growth factor β (TGFβ)-induced inhibition of osteoblast formation in young and adult mice. During aging, neutrophils expressing TGFβ and C-C chemokine receptor type 5 (CCR5) increase in bone marrow of mice in response to increased NF-κB-induced CC motif chemokine ligand 5 (CCL5) expression by mesenchymal progenitor cells and injection of these neutrophils into young mice decreased bone mass. TGFβ causes degradation of TRAF3, resulting in decreased glycogen synthase kinase-3β/β-catenin-mediated osteoblast formation and age-related osteoporosis in mice. The CCR5 inhibitor, maraviroc, prevented accumulation of TGFβ+/CCR5+ neutrophils in bone marrow and increased bone mass by inhibiting bone resorption and increasing bone formation in aged mice. This paper updates current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast and osteoblast formation and activation with a focus on the role of TRAF3 signaling, which can be targeted therapeutically to enhance bone mass.

X-ray Crystallographic Study of Preferred Spacing by the NF-κB p50 Homodimer on κB DNA

Though originally characterized as an inactive or transcriptionally repressive factor, the NF-κB p50 homodimer has become appreciated as a physiologically relevant driver of specific target gene expression. By virtue of its low affinity for cytoplasmic IκB protein inhibitors, p50 accumulates in the nucleus of resting cells, where it is a binding target for the transcriptional co-activator IκBζ. In this study, we employed X-ray crystallography to analyze the structure of the p50 homodimer on κB DNA from the promoters of human interleukin-6 (IL-6) and neutrophil-gelatinase-associated lipocalin (NGAL) genes, both of which respond to IκBζ. The NF-κB p50 homodimer binds 11-bp on IL-6 κB DNA, while, on NGAL κB DNA, the spacing is 12-bp. This begs the question: what DNA binding mode is preferred by NF-κB p50 homodimer? To address this, we engineered a "Test" κB-like DNA containing the core sequence 5'-GGGGAATTCCCC-3' and determined its X-ray crystal structure in complex with p50. This revealed that, when presented with multiple options, NF-κB p50 homodimer prefers to bind 11-bp, which necessarily imposes asymmetry on the complex despite the symmetry inherent in both the protein and its target DNA, and that the p50 dimerization domain can contact DNA via distinct modes.

Lost and Found: The Family of NF-κB Inhibitors Is Larger than Assumed in Salmonid Fish

NF-κB signalling is largely controlled by the family of 'inhibitors of NF-κB' (IκB). The relevant databases indicate that the genome of rainbow trout contains multiple gene copies coding for iκbα (nfkbia), iκbε (nfkbie), iκbδ (nkfbid), iκbζ (nfkbiz), and bcl3, but it lacks iκbβ (nfkbib) and iκbη (ankrd42). Strikingly, three nfkbia paralogs are apparently present in salmonid fish, two of which share a high sequence identity, while the third putative nfkbia gene is significantly less like its two paralogs. This particular nfkbia gene product, iκbα, clusters with the human IκBβ in a phylogenetic analysis, while the other two iκbα proteins from trout associate with their human IκBα counterpart. The transcript concentrations were significantly higher for the structurally more closely related nfkbia paralogs than for the structurally less similar paralog, suggesting that iκbβ probably has not been lost from the salmonid genomes but has been incorrectly designated as iκbα. In the present study, two gene variants coding for iκbα (nfkbia) and iκbε (nfkbie) were prominently expressed in the immune tissues and, particularly, in a cell fraction enriched with granulocytes, monocytes/macrophages, and dendritic cells from the head kidney of rainbow trout. Stimulation of salmonid CHSE-214 cells with zymosan significantly upregulated the iκbα-encoding gene while elevating the copy numbers of the inflammatory markers interleukin-1-beta and interleukin-8. Overexpression of iκbα and iκbε in CHSE-214 cells dose-dependently quenched both the basal and stimulated activity of an NF-κB promoter suggesting their involvement in immune-regulatory processes. This study provides the first functional data on iκbε-versus the well-researched iκbα factor-in a non-mammalian model species.

Common Variable Immunodeficiency: More Pathways than Roads to Rome

Fifty years have elapsed since the term common variable immunodeficiency (CVID) was introduced to accommodate the many and varied antibody deficiencies being identified in patients with suspected inborn errors of immunity (IEIs). Since then, how the term is understood and applied for diagnosis and management has undergone many revisions, though controversy persists on how exactly to define and classify CVID. Many monogenic disorders have been added under its aegis, while investigations into polygenic, epigenetic, and somatic contributions to CVID susceptibility have gained momentum. Expansion of the overall IEI landscape has increasingly revealed genotypic and phenotypic overlap between CVID and various other immunological conditions, while increasingly routine genotyping of CVID patients continues to identify an incredible diversity of pathophysiological mechanisms affecting even single genes. Though many questions remain to be answered, the lessons we have already learned from CVID biology have greatly informed our understanding of adaptive, but also innate, immunity.

Differential recognition of canonical NF-κB dimers by Importin α3

Nuclear translocation of the p50/p65 heterodimer is essential for NF-κB signaling. In unstimulated cells, p50/p65 is retained by the inhibitor IκBα in the cytoplasm that masks the p65-nuclear localization sequence (NLS). Upon activation, p50/p65 is translocated into the nucleus by the adapter importin α3 and the receptor importin β. Here, we describe a bipartite NLS in p50/p65, analogous to nucleoplasmin NLS but exposed in trans. Importin α3 accommodates the p50- and p65-NLSs at the major and minor NLS-binding pockets, respectively. The p50-NLS is the predominant binding determinant, while the p65-NLS induces a conformational change in the Armadillo 7 of importin α3 that stabilizes a helical conformation of the p65-NLS. Neither conformational change was observed for importin α1, which makes fewer bonds with the p50/p65 NLSs, explaining the preference for α3. We propose that importin α3 discriminates between the transcriptionally active p50/p65 heterodimer and p50/p50 and p65/65 homodimers, ensuring fidelity in NF-κB signaling.

Nuclear translocation of the p50/p65 heterodimer is essential for NF-κB signaling. Here, the authors identify a bipartite Nuclear Localization Signal in the NF-κB p50/p65 heterodimer that is recognized with high affinity by importin α3.

Origin of the Functional Distinctiveness of NF-κB/p52

The transcription regulators of the NF-κB family have emerged as a critical factor affecting the function of various adult tissues. The NF-κB family transcription factors are homo- and heterodimers made up of five monomers (p50, p52, RelA, cRel and RelB). The family is distinguished by sequence homology in their DNA binding and dimerization domains, which enables them to bind similar DNA response elements and participate in similar biological programs through transcriptional activation and repression of hundreds of genes. Even though the family members are closely related in terms of sequence and function, they all display distinct activities. In this review, we discuss the sequence characteristics, protein and DNA interactions, and pathogenic involvement of one member of family, NF-κB/p52, relative to that of other members. We pinpoint the small sequence variations within the conserved region that are mostly responsible for its distinct functional properties.

Regulation of NFκB Signalling by Ubiquitination: A Potential Therapeutic Target in Head and Neck Squamous Cell Carcinoma?

Head and neck squamous cell carcinoma (HNSCC) is the sixth most common cancer worldwide, with over 600,000 cases per year. The primary causes for HNSCC include smoking and alcohol consumption, with an increasing number of cases attributed to infection with Human Papillomavirus (HPV). The treatment options for HNSCC currently include surgery, radiotherapy, and/or platinum-based chemotherapeutics. Cetuximab (targeting EGFR) and Pembrolizumab (targeting PD-1) have been approved for advanced stage, recurrent, and/or metastatic HNSCC. Despite these advances, whilst HPV+ HNSCC has a 3-year overall survival (OS) rate of around 80%, the 3-year OS for HPV- HNSCC is still around 55%. Aberrant signal activation of transcription factor NFκB plays an important role in the pathogenesis and therapeutic resistance of HNSCC. As an important mediator of inflammatory signalling and the immune response to pathogens, the NFκB pathway is tightly regulated to prevent chronic inflammation, a key driver of tumorigenesis. Here, we discuss how NFκB signalling is regulated by the ubiquitin pathway and how this pathway is deregulated in HNSCC. Finally, we discuss the current strategies available to target the ubiquitin pathway and how this may offer a potential therapeutic benefit in HNSCC.

Combination of ERK2 inhibitor VX-11e and voreloxin synergistically enhances anti-proliferative and pro-apoptotic effects in leukemia cells

ERK1/2 inhibitors are new promising anticancer drugs. The aim of this study was to investigate the effect of the combination of ERK2 inhibitor VX-11e and voreloxin on MOLM-14, K562, REH and MOLT-4 leukemia cell lines. We found that VX-11e alone and in combination with voreloxin significantly decreased ERK activation in all cell lines tested. To evaluate the interactions of the drugs, cells were treated for 24 h with VX-11e or voreloxin alone and in combination at fixed ratios based on IC₅₀ values. The combinatorial effects of both drugs were synergistic over a wide range of concentrations in MOLM-14, REH and MOLT-4 cell lines. In K562 cells, three effects were found to be additive, one antagonistic and only one synergistic. The results showed that incubation with both VX-11e and voreloxin inhibited the growth of leukemia cells, affected cell cycle and induced apoptosis. Furthermore, the molecular mechanism of these effects might be attributed to an increased expression of p21 and a decreased expression of survivin and NF-κB in all cell lines tested except from K562 cells. In conclusion, combination of VX-11e and voreloxin can exert a synergistic anticancer effect in leukemia cells.

Crocodylian nuclear factor kappa B

We deduced the amino acid (aa) sequence of the nuclear factor kappa B (NFκB) protein from genomic data for the American alligator (Alligator mississippiensis), the estuarine crocodile (Crocodylus porosus), and the Indian gharial (Gavialis gangeticus). A 105kDa protein, NFκB1 exhibits complex post-translational processing, multiple mechanisms of activation, and acts as precursor for a p50, a Rel homology transcription factor which influences the expression of key genes for developmental processes, apoptosis, and immune function. The aa sequences of the crocodylian proteins share very high identity with each other (97.2±0.7%), birds (81.0±1.1%, n=6), mammals (75.3±1.6%, n=4), reptiles (80.3±5.1%, n=2), and less identity with fish (55.5±5.5%, n=4) and one amphibian (66.1±0.8%). The crocodylian protein has a well-conserved Rel homology domain, a nuclear localization signal, and a glycine-rich region which facilitates proteasome-mediated generation of p50. The Rel homology domain contains sequences responsible for dimerization, DNA-binding, and nuclear translocation. In addition, seven ankyrin repeats were located, which putatively allow for inhibition of transcriptional regulation by mediating interaction with Inhibitor kappa B. Other features include a death domain, and conserved serine residues, near the C-terminal end, which act as potential phosphorylation sites for activation of the proteolytic generation of p50. Western blot analysis showed both the 105kDa precursor and the 50kDa mature NFκB were expressed in the alligator liver. Nuclear factor κB exhibited diffuse cytoplasmic distribution in alligator hepatocytes, and almost no cytoplasmic localization in infected animals. In addition, nuclear NFκB exhibited specific binding to the consensus NFκB promoter element.

The Ubiquitination of NF-κB Subunits in the Control of Transcription

Nuclear factor (NF)-κB has evolved as a latent, inducible family of transcription factors fundamental in the control of the inflammatory response. The transcription of hundreds of genes involved in inflammation and immune homeostasis require NF-κB, necessitating the need for its strict control. The inducible ubiquitination and proteasomal degradation of the cytoplasmic inhibitor of κB (IκB) proteins promotes the nuclear translocation and transcriptional activity of NF-κB. More recently, an additional role for ubiquitination in the regulation of NF-κB activity has been identified. In this case, the ubiquitination and degradation of the NF-κB subunits themselves plays a critical role in the termination of NF-κB activity and the associated transcriptional response. While there is still much to discover, a number of NF-κB ubiquitin ligases and deubiquitinases have now been identified which coordinate to regulate the NF-κB transcriptional response. This review will focus the regulation of NF-κB subunits by ubiquitination, the key regulatory components and their impact on NF-κB directed transcription.

Lack of Oestrogenic Inhibition of the Nuclear Factor-κB Pathway in Somatolactotroph Tumour Cells

Activation of nuclear factor (NF)-κB promotes cell proliferation and inhibits apoptosis. We have previously shown that oestrogens sensitise normal anterior pituitary cells to the apoptotic effect of tumour necrosis factor (TNF)-α by inhibiting NF-κB nuclear translocation. In the present study, we examined whether oestrogens also modulate the NF-κB signalling pathway and apoptosis in GH3 cells, a rat somatolactotroph tumour cell line. As determined by Western blotting, 17β-oestradiol (E2 ) (10(-9) m) increased the nuclear concentration of NF-κB/p105, p65 and p50 in GH3 cells. However, E2 did not modify the expression of Bcl-xL, a NF-κB target gene. TNF-α induced apoptosis of GH3 cells incubated in either the presence or absence of E2 . Inhibition of the NF-kB pathway using BAY 11-7082 (BAY) (5 μm) decreased the viability of GH3 cells and increased the percentage of terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL)-positive GH3 cells. BAY also increased TNF-α-induced apoptosis of GH3 cells, an effect that was further increased by an inhibitor of the c-Jun N-terminal protein kinase pathway, SP600125 (10 μm). We also analysed the role of the NF-κB signalling pathway on proliferation and apoptosis of GH3 tumours in vivo. The administration of BAY to nude mice bearing GH3 tumours increased the number of TUNEL-positive cells and decreased the number of proliferating GH3 cells. These findings suggest that GH3 cells lose their oestrogenic inhibitory action on the NF-κB pathway and that the pro-apoptotic effect of TNF-α on these tumour pituitary cells does not require sensitisation by oestrogens as occurs in normal pituitary cells. NF-κB was required for the survival of GH3 cells, suggesting that pharmacological inhibition of the NF-κB pathway could interfere with pituitary tumour progression.

NF-κB-Mediated Regulation of Osteoclastogenesis

Osteoclasts are multinucleated cells formed mainly on bone surfaces in response to cytokines by fusion of bone marrow-derived myeloid lineage precursors that circulate in the blood. Major advances in understanding of the molecular mechanisms regulating osteoclast formation and functions have been made in the past 20 years since the discovery that their formation requires nuclear factor-κB (NF-κB) signaling and that this is activated in response to the essential osteoclastogenic cytokine, receptor activator of NF-κB ligand (RANKL), which also controls osteoclast activation to resorb (degrade) bone. These studies have revealed that RANKL and some pro-inflammatory cytokines, including tumor necrosis factor, activate NF-κB and downstream signaling, including c-Fos and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), and inhibition of repressors of NFATc1 signaling, to positively regulate osteoclast formation and functions. However, these cytokines also activate NF-κB signaling that can limit osteoclast formation through the NF-κB signaling proteins, TRAF3 and p100, and the suppressors of c-Fos/NFATc1 signaling, IRF8, and RBP-J. This paper reviews current understanding of how NF-κB signaling is involved in the positive and negative regulation of cytokine-mediated osteoclast formation and activation.

Controlling for gene expression changes in transcription factor protein networks

The development of affinity purification technologies combined with mass spectrometric analysis of purified protein mixtures has been used both to identify new protein-protein interactions and to define the subunit composition of protein complexes. Transcription factor protein interactions, however, have not been systematically analyzed using these approaches. Here, we investigated whether ectopic expression of an affinity tagged transcription factor as bait in affinity purification mass spectrometry experiments perturbs gene expression in cells, resulting in the false positive identification of bait-associated proteins when typical experimental controls are used. Using quantitative proteomics and RNA sequencing, we determined that the increase in the abundance of a set of proteins caused by overexpression of the transcription factor RelA is not sufficient for these proteins to then co-purify non-specifically and be misidentified as bait-associated proteins. Therefore, typical controls should be sufficient, and a number of different baits can be compared with a common set of controls. This is of practical interest when identifying bait interactors from a large number of different baits. As expected, we found several known RelA interactors enriched in our RelA purifications (NFκB1, NFκB2, Rel, RelB, IκBα, IκBβ, and IκBε). We also found several proteins not previously described in association with RelA, including the small mitochondrial chaperone Tim13. Using a variety of biochemical approaches, we further investigated the nature of the association between Tim13 and NFκB family transcription factors. This work therefore provides a conceptual and experimental framework for analyzing transcription factor protein interactions.

Identification and characterization of a novel IκB-ε-like gene from lamprey (Lampetra japonica) with a role in immune response

Nuclear factor of kappa B (NF-κB) is a stimuli-activated transcription factor, regulates the expression of a diverse array of genes. Inhibitor of kappa B-epsilon (IκB-ε) is an inhibitor of NF-κB, which retains NF-κB in an inactive state in the cytoplasm. Lampreys (Lampetra japonica) belong to the lowest class of vertebrates with little information about its IκBs. We have identified a cDNA sequence IκB-ε-like in the lamprey and the deduced amino acid sequence of IκB-ε-like. It contains a conserved DSGxxS motif and six consecutive ankyrin repeats, which are necessary for signal-induced degradation of the molecule. Phylogenetic analysis indicated it had high sequence homology with IκB-εs from other vertebrates. FACS analysis showed that IκB-ε-like located in cytoplasm of leukocytes. The degradation of IκB-ε-like could be observed in leukocytes of L. japonica stimulated with lipopolysaccharide. These results indicate that IκB-ε proteins are conserved across vertebrates and the NF-κB-like signaling pathway may exist in the oldest agnatha.

Genome-wide association study of treatment refractory schizophrenia in Han Chinese

We report the first genome-wide association study of a joint analysis using 795 Han Chinese individuals with treatment-refractory schizophrenia (TRS) and 806 controls. Three loci showed suggestive significant association with TRS were identified. These loci include: rs10218843 (P = 3.04×10⁻⁷) and rs11265461 (P = 1.94×10⁻⁷) are adjacent to signaling lymphocytic activation molecule family member 1 (SLAMF1); rs4699030 (P = 1.94×10⁻⁶) and rs230529 (P = 1.74×10⁻⁷) are located in the gene nuclear factor of kappa light polypeptide gene enhancer in B-cells 1 (NFKB1); and rs13049286 (P = 3.05×10⁻⁵) and rs3827219 (P = 1.66×10⁻⁵) fall in receptor-interacting serine/threonine-protein kinase 4 (RIPK4). One isolated single nucleotide polymorphism (SNP), rs739617 (P = 3.87×10⁻⁵) was also identified to be associated with TRS. The -94delATTG allele (rs28362691) located in the promoter region of NFKB1 was identified by resequencing and was found to associate with TRS (P = 4.85×10⁻⁶). The promoter assay demonstrated that the -94delATTG allele had a significant lower promoter activity than the -94insATTG allele in the SH-SY5Y cells. This study suggests that rs28362691 in NFKB1 might be involved in the development of TRS.

Nfkb1 inhibits LPS-induced IFN-β and IL-12 p40 production in macrophages by distinct mechanisms

Background

Nfkb1-deficient murine macrophages express higher levels of IFN-β and IL-12 p40 following LPS stimulation than control macrophages, but the molecular basis for this phenomenon has not been completely defined. Nfkb1 encodes several gene products including the NF-κB subunit p50 and its precursor p105. p50 is derived from the N-terminal of 105, and p50 homodimers can exhibit suppressive activity when overexpressed. The C-terminal region of p105 is necessary for LPS-induced ERK activation and it has been suggested that ERK activity inhibits both IFN-β and IL-12 p40 following LPS stimulation. However, the contributions of p50 and the C-terminal domain of p105 in regulating endogenous IFN-β(Ifnb) and IL-12 p40 (Il12b) gene expression in macrophages following LPS stimulation have not been directly compared.

Methodology/Principal Findings

We have used recombinant retroviruses to express p105, p50, and the C-terminal domain of p105 (p105ΔN) in Nfkb1-deficient murine bone marrow-derived macrophages at near endogenous levels. We found that both p50 and p105ΔN inhibited expression of Ifnb, and that inhibition of Ifnb by p105ΔN depended on ERK activation, because a mutant of p105ΔN (p105ΔNS930A) that lacks a key serine necessary to support ERK activation failed to inhibit. In contrast, only p105ΔN but not p50 inhibited Il12b expression. Surprisingly, p105ΔNS930A retained inhibitory activity for Il12b, indicating that ERK activation was not necessary for inhibition. The differential effects of p105ΔNS930A on Ifnb and Il12b expression inversely correlated with the function of one of its binding partners, c-Rel. This raised the possibility that p105ΔNS930A influences gene expression by interfering with the function of c-Rel.

Conclusions

These results demonstrate that Nfkb1 exhibits multiple gene-specific inhibitory functions following TLR stimulation of murine macrophages.

The specificity of innate immune responses is enforced by repression of interferon response elements by NF-κB p50

The specific binding of transcription factors to cognate sequence elements is thought to be critical for the generation of specific gene expression programs. Members of the nuclear factor κB (NF-κB) and interferon (IFN) regulatory factor (IRF) transcription factor families bind to the κB site and the IFN response element (IRE), respectively, of target genes, and they are activated in macrophages after exposure to pathogens. However, how these factors produce pathogen-specific inflammatory and immune responses remains poorly understood. Combining top-down and bottom-up systems biology approaches, we have identified the NF-κB p50 homodimer as a regulator of IRF responses. Unbiased genome-wide expression and biochemical and structural analyses revealed that the p50 homodimer repressed a subset of IFN-inducible genes through a previously uncharacterized subclass of guanine-rich IRE (G-IRE) sequences. Mathematical modeling predicted that the p50 homodimer might enforce the stimulus specificity of composite promoters. Indeed, the production of the antiviral regulator IFN-β was rendered stimulus-specific by the binding of the p50 homodimer to the G-IRE-containing IFNβ enhancer to suppress cytotoxic IFN signaling. Specifically, a deficiency in p50 resulted in the inappropriate production of IFN-β in response to bacterial DNA sensed by Toll-like receptor 9. This role for the NF-κB p50 homodimer in enforcing the specificity of the cellular response to pathogens by binding to a subset of IRE sequences alters our understanding of how the NF-κB and IRF signaling systems cooperate to regulate antimicrobial immunity.

Functions of nuclear factor kappaB in bone

Nuclear factor kappaB (NF-kappaB) is a set of multifunctional transcription factors that regulate expression of genes involved in numerous normal cellular activities. They also are activated in many inflammatory and neoplastic conditions in which their expression may be stimulated by proinflammatory cytokines. NF-kappaB, in turn, regulates the expression of cytokines and so can mediate autocrine self-amplifying cycles of cytokine release and NF-kappaB activation, leading to maintenance of inflammatory reactions beyond the initial stimulus, as seen in rheumatoid arthritis and asthma. Since discovery of the requirement of NF-kappaB for basal and cytokine-induced osteoclast formation in the mid-1990s, much has been learned about the role of NF-kappaB in bone. NF-kappaB has roles in skeletal development, endochondral ossification, osteoclast and osteoblast functions, and common bone diseases. NF-kappaB inhibitors have been developed, but none have made it to clinical trials for the treatment of common bone diseases. Here we review the roles for NF-kappaB in bone and in common bone diseases.

Exclusion of NFAT5 from mitotic chromatin resets its nucleo-cytoplasmic distribution in interphase

Background

The transcription factor NFAT5 is a major inducer of osmoprotective genes and is required to maintain the proliferative capacity of cells exposed to hypertonic stress. In response to hypertonicity, NFAT5 translocates to the nucleus, binds to regulatory regions of osmoprotective genes and activates their transcription. Besides stimulus-specific regulatory mechanisms, the activity of transcription factors in cycling cells is also regulated by the passage through mitosis, when most transcriptional processes are downregulated. It was not known whether mitosis could be a point of control for NFAT5.

Methodology/Principal Findings

Using confocal microscopy we observed that NFAT5 was excluded from chromatin during mitosis in both isotonic and hypertonic conditions. Analysis of NFAT5 deletions showed that exclusion was mediated by the carboxy-terminal domain (CTD). NFAT5 mutants lacking this domain showed constitutive binding to mitotic chromatin independent of tonicity, which caused them to localize in the nucleus and remain bound to chromatin in the subsequent interphase without hypertonic stimulation. We analyzed the contribution of the CTD, DNA binding, and nuclear import and export signals to the subcellular localization of this factor. Our results indicated that cytoplasmic localization of NFAT5 in isotonic conditions required both the exclusion from mitotic DNA and active nuclear export in interphase. Finally, we identified several regions within the CTD of NFAT5, some of them overlapping with transactivation domains, which were separately capable of causing its exclusion from mitotic chromatin.

Conclusions/Significance

Our results reveal a multipart mechanism regulating the subcellular localization of NFAT5. The transactivating module of NFAT5 switches its function from an stimulus-specific activator of transcription in interphase to an stimulus-independent repressor of binding to DNA in mitosis. This mechanism, together with export signals acting in interphase, resets the cytoplasmic localization of NFAT5 and prevents its nuclear accumulation and association with DNA in the absence of hypertonic stress.

The Nfkb1 and Nfkb2 proteins p105 and p100 function as the core of high-molecular-weight heterogeneous complexes

Nfkb1 and Nfkb2 proteins p105 and p100 serve both as NF-kappaB precursors and inhibitors of NF-kappaB dimers. In a biochemical characterization of endogenous cytoplasmic and purified recombinant proteins, we found that p105 and p100 assemble into high-molecular-weight complexes that contribute to the regulation of all NF-kappaB isoforms. Unlike the classical inhibitors IkappaBalpha, -beta, and -epsilon, high-molecular-weight complexes of p105 and p100 proteins bind NF-kappaB subunits in two modes: through direct dimerization of Rel homology domain-containing NF-kappaB polypeptides and through interactions of the p105 and p100 ankyrin repeats with preformed NF-kappaB dimers, thereby mediating the bona fide IkappaB activities, IkappaBgamma and IkappaBdelta. Our biochemical evidence suggests an assembly pathway in which kinetic mechanisms control NF-kappaB dimer formation via processing and assembly of large complexes that contain IkappaB activities.

Regulation and function of NF-kappaB transcription factors in the immune system

The mammalian Rel/NF-kappaB family of transcription factors, including RelA, c-Rel, RelB, NF-kappaB1 (p50 and its precursor p105), and NF-kappaB2 (p52 and its precursor p100), plays a central role in the immune system by regulating several processes ranging from the development and survival of lymphocytes and lymphoid organs to the control of immune responses and malignant transformation. The five members of the NF-kappaB family are normally kept inactive in the cytoplasm by interaction with inhibitors called IkappaBs or the unprocessed forms of NF-kappaB1 and NF-kappaB2. A wide variety of signals emanating from antigen receptors, pattern-recognition receptors, receptors for the members of TNF and IL-1 cytokine families, and others induce differential activation of NF-kappaB heterodimers. Although work over the past two decades has shed significant light on the regulation of NF-kappaB transcription factors and their functions, much progress has been made in the past two years revealing new insights into the regulation and functions of NF-kappaB. This recent progress is covered in this review.

Proteolysis of NF-kappaB1 p105 is essential for T cell antigen receptor-induced proliferation

To investigate the importance of proteolysis of NF-kappaB1 p105 induced by the kinase IKK in activation of the transcription factor NF-kappaB, we generated 'Nfkb1(SSAA/SSAA)' mice, in which the IKK-target serine residues of p105 were substituted with alanine. Nfkb1(SSAA/SSAA) mice had far fewer CD4+ regulatory and memory T cells because of cell-autonomous defects. These T cell subtypes require activation of NF-kappaB by the T cell antigen receptor for their generation, and the Nfkb1(SSAA) mutation resulted in less activation of NF-kappaB in CD4+ T cells and proliferation of CD4+ T cells after stimulation of the T cell antigen receptor. The Nfkb1(SSAA) mutation also blocked the ability of CD4+ T cells to provide help to wild-type B cells during a primary antibody response. IKK-induced p105 proteolysis is therefore essential for optimal T cell antigen receptor-induced activation of NF-kappaB and mature CD4+ T cell function.

Lymphoma cell adhesion-induced expression of B cell-activating factor of the TNF family in bone marrow stromal cells protects non-Hodgkin's B lymphoma cells from apoptosis

This study explores whether lymphoma cell adhesion-induced B cell-activating factor (BAFF) expression in bone marrow stromal cells (BMSCs) protects B lymphoma cells from apoptosis. We first showed protection of lymphoma cells from apoptosis by conditioned medium of a stromal cell-lymphoma cell coculture, either spontaneous or induced by mitoxantrone, implying a role for soluble factor(s) in lymphoma cell survival. Addition of BAFF counteracted mitoxantrone-induced apoptosis and elicited a reduction in spontaneous apoptosis in primary lymphomas, suggesting a role of BAFF in sustaining B-cell survival. Abundant BAFF was detected in the BMSC cell line (HS-5) and primary BMSCs by flow cytometry, RT-PCR and immunoblotting. BAFF levels were 20- to 200-fold higher in BMSCs than in lymphoma cells, and lymphoma cell adhesion to BMSCs augmented BAFF secretion twofold through upregulation of BAFF gene expression. Finally, neutralization of BAFF by TACI-Ig or depletion of BAFF by small hairpin RNA (shRNA) in BMSCs significantly enhanced lymphoma cell response to chemotherapy and overcame stroma-mediated drug resistance, suggesting that lymphoma cells use BMSC-derived BAFF as a survival factor. These findings support the hypothesis that lymphoma cells interact with BMSCs, resulting in stromal niches with high BAFF concentration, and identify BMSC-derived BAFF as a functional determinant for B lymphoma cell survival in the bone marrow environment.

Induction of epidermal growth factor receptor expression by Epstein-Barr virus latent membrane protein 1 C-terminal-activating region 1 is mediated by NF-kappaB p50 homodimer/Bcl-3 complexes

The Epstein-Barr virus (EBV) is associated with the development of numerous malignancies, including the epithelial malignancy nasopharyngeal carcinoma (NPC). The viral oncoprotein latent membrane protein 1 (LMP1) is expressed in almost all EBV-associated malignancies and has profound effects on gene expression. LMP1 acts as a constitutively active tumor necrosis factor receptor and activates multiple forms of the NF-kappaB family of transcription factors. LMP1 has two domains that both activate NF-kappaB. In epithelial cells, LMP1 C-terminal activating region 1 (CTAR1) uniquely activates p50/p50-, p50/p52-, and p65-containing complexes while CTAR2 activates canonical p50/p65 complexes. CTAR1 also uniquely upregulates the epidermal growth factor receptor (EGFR). In NPC, NF-kappaB p50/p50 homodimers and the transactivator Bcl-3 were detected on the EGFR promoter. In this study, the role of NF-kappaB p50 and Bcl-3 in LMP1-mediated upregulation of EGFR was analyzed. In LMP1-CTAR1-expressing cells, chromatin immunoprecipitation detected p50 and Bcl-3 on the NF-kappaB consensus sites within the egfr promoter. Transient overexpression of p50 and Bcl-3 increased EGFR expression, confirming the regulation of EGFR by these factors. Treatment with p105/p50 siRNA effectively reduced p105/p50 levels but unexpectedly increased Bcl-3 expression and levels of p50/Bcl-3 complexes, resulting in increased EGFR expression. These data suggest that induction of p50/p50/Bcl-3 complexes by LMP1 CTAR1 mediates LMP1-induced EGFR upregulation and that formation of the p50/p50/Bcl-3 complex is negatively regulated by the p105 precursor. The distinct forms of NF-kappaB that are induced by LMP1 CTAR1 likely activate distinct cellular genes.

Peripheral blood mononuclear cell NF-kappaB p105 mRNA decreases during asthmatic attacks

BACKGROUND

NF-kappaB is a transcription factor involved in expression of many inflammatory cytokines, chemical transmitters, and adhesion molecules. It has been reported to play a major role in the pathogenesis of asthma. NF-kappaB p50, which is the actual subunit that results from the cleavage of p105, is required for the induction of eosinophilia via IL-5 and chemokines.

METHODS

The subjects were 10 patients with a mean age of 59.3 years (14-82 years). NF-kappaB p105 mRNA in peripheral blood mononuclear cells during the presence or absence of asthmatic attacks was investigated. Total RNA was extracted from peripheral blood mononuclear cells. After cDNA was synthesized using random primers, NF-kappaB p105 mRNA level was measured by real-time polymerase chain reaction.

RESULTS

The NF-kappaB p105 mRNA level in peripheral blood mononuclear cells was lower during asthmatic attacks than in the absence of attacks, showing a significant difference (Wilcoxon's signed rank test: p<0.01).

CONCLUSIONS

A drop in NF-kappaB p105 during an asthma attack could result in increased NF-kappaB activity. There is a possibility that a change in the NF-kappaB p105 mRNA level might indicate some pathogenetic state in bronchial asthma attacks.

Induced fit, folding, and recognition of the NF-kappaB-nuclear localization signals by IkappaBalpha and IkappaBbeta

Protein structure prediction codes based on the associative memory Hamiltonian were used to probe the binding modes between the nuclear localization signal (NLS) polypeptide of NF-kappaB and the inhibitors IkappaBalpha and IkappaBbeta. Experimentally, it is known that the NLS polypeptide is unstructured in the NF-kappaB complex with DNA but it forms an extended helical structure with the NLS (residues 301-304) between the two helices in the NF-kappaB/IkappaBalpha complex. The simulations included the NF-kappaB(p65) and (p50) NLS polypeptides and various mutants alone and in the presence of IkappaBalpha and IkappaBbeta. The simulations predict that the NLS polypeptide by itself binds tightly to IkappaBalpha and IkappaBbeta. In the NF-kappaB (p50/p65) heterodimer, the p50 NLS is predicted to remain free to bind to importin alpha. In the interaction with IkappaBalpha, both p65 NLSs are predicted to be bound. In IkappaBbeta, the NLS polypeptide binds to two binding sites, as seen in the crystal structure, with one site heavily favored for stable binding.

Gene deletion of NF-kappaB p105 enhances neointima formation in a mouse model of carotid artery injury

The role of nuclear factor kappa-B (NF-kappaB) p105 for vascular inflammatory gene expression and neointima formation after arterial injury was studied. Mice carotid arteries were injured by ligation. Vascular NF-kappaB activation was monitored using a NF-kappaB luciferase reporter mouse. Mice with gene deletion of the NF-kappaB p105 subunit (p50 precursor) and the corresponding wild types were assessed for vascular gene expression and neointimal hyperplasia. NF-kappaB was activated in the injured vessel wall in wild type mice, and this was accompanied by increased expression of the proinflammatory genes tumor necrosis factor alpha, interleukin 1 beta, and inducible nitric oxide synthase. In contrast, NF-kappaB p105 knockout mice had reduced expression of the inflammatory genes and enhanced neointima formation four weeks after ligation. Basic fibroblast growth factor (bFGF) gene expression increased after arterial ligation. A higher percentage of bFGF positive cells were found in lesions from NF-kappaB p105 knock out mice. These data indicate that the p105 subunit of NF-kappaB plays an essential role in vascular healing, and defects in NF-kappaB p105 promote neointima hyperplasia.

The 20S proteasome processes NF-kappaB1 p105 into p50 in a translation-independent manner

The NF-kappaB p50 is the N-terminal processed product of the precursor, p105. It has been suggested that p50 is generated not from full-length p105 but cotranslationally from incompletely synthesized molecules by the proteasome. We show that the 20S proteasome endoproteolytically cleaves the fully synthesized p105 and selectively degrades the C-terminus of p105, leading to p50 generation in a ubiquitin-independent manner. As small as 25 residues C-terminus to the site of processing are sufficient to promote processing in vivo. However, any p105 mutant that lacks complete ankyrin repeat domain (ARD) is processed aberrantly, suggesting that native processing must occur from a precursor, which extends beyond the ARD. Remarkably, the mutant p105 that lacks the internal region including the glycine-rich region (GRR) is completely degraded by 20S proteasome in vitro. This suggests that the GRR impedes the complete degradation of the p105 precursor, thus contributing to p50 generation.

High levels of p105 (NFKB1) and p100 (NFKB2) proteins in HPV16-transformed keratinocytes: role of E6 and E7 oncoproteins

We have previously shown that functional components of the NF-kappaB signaling pathway are up-regulated and sequestered in the cytoplasm of human papillomavirus 16 (HPV16)-transformed cell lines leading to a reduced activity of NF-kappaB. In this study, we examined the expression of the NF-kappaB precursors p100 and p105 in keratinocytes transformed or not by HPV16. Western immunoblotting experiments demonstrated high levels of p100 and p105 proteins not only in HPV16+ cervical carcinoma-derived keratinocytes but also in keratinocytes stably transfected by HPV16 E6 or E7 oncogenes. Moreover, p100 and p105 proteins were predominantly cytoplasmic and nuclear in keratinocytes expressing E7 and E6, respectively. A predominantly cytoplasmic localization of E7 protein was also detected in all keratinocytes expressing E7. Our results suggest that HPV16 E6 and E7 proteins modulate the expression and the subcellular localization of p100 and p105 NF-kappaB precursors.

DNA binding of repressor nuclear factor-kappaB p50/p50 depends on phosphorylation of Ser337 by the protein kinase A catalytic subunit

The NF-kappaB p50/p50 homodimer is mainly associated with transcriptional repression. Previously, we demonstrated that phosphorylation of NF-kappaB p50 Ser(337) is critical for DNA binding. Here, we report that p50 Ser(337) is constitutively phosphorylated by the protein kinase A catalytic subunit (PKAc) in three different cell types, which may account for the constant binding of p50/p50 to DNA in unstimulated cells. This was demonstrated first by showing that treatment of cells with PKAc-specific inhibitors blocked p50/p50 DNA binding. Second, phosphorylation of p50 by PKAc was prevented by substitution of Ser(337) to alanine. Third, both p50 and PKAc proteins as well as kinase activity that phosphorylates p50 were found to co-fractionate following gel filtration chromatography. Finally, PKAc and p50 were shown to be able to reciprocally co-immunoprecipitate one another, and their physical association was blocked by a PKA catalytic site inhibitory peptide. This indicates that phosphorylation of p50 Ser(337) involves direct contact with the PKAc catalytic center. In contrast to the dramatic elevation of nuclear p50/p65 heterodimers induced by tumor necrosis factor alpha, DNA binding of p50/p50 homodimers was not greatly altered. Taken together, these findings reveal for the first time that there is a direct interaction between PKAc and p50 that accounts for constitutive phosphorylation of p50 Ser(337) and the existence of DNA bound p50/p50 in the nuclei of most resting cells. This mechanism of DNA binding by p50/p50 following phosphorylation of Ser(337) by PKAc may represent an important means for maintaining stable negative regulation of NF-kappaB gene expression in the absence of extracellular stimulation.

Functions of NF-kappaB1 and NF-kappaB2 in immune cell biology

Two members of the NF-kappaB (nuclear factor kappaB)/Rel transcription factor family, NF-kappaB1 and NF-kappaB2, are produced as precursor proteins, NF-kappaB1 p105 and NF-kappaB2 p100 respectively. These are proteolytically processed by the proteasome to produce the mature transcription factors NF-kappaB1 p50 and NF-kappaB2 p52. p105 and p100 are known to function additionally as IkappaBs (inhibitors of NF-kappaB), which retain associated NF-kappaB subunits in the cytoplasm of unstimulated cells. The present review focuses on the latest advances in research on the function of NF-kappaB1 and NF-kappaB2 in immune cells. NF-kappaB2 p100 processing has recently been shown to be stimulated by a subset of NF-kappaB inducers, including lymphotoxin-beta, B-cell activating factor and CD40 ligand, via a novel signalling pathway. This promotes the nuclear translocation of p52-containing NF-kappaB dimers, which regulate peripheral lymphoid organogenesis and B-lymphocyte differentiation. Increased p100 processing also contributes to the malignant phenotype of certain T- and B-cell lymphomas. NF-kappaB1 has a distinct function from NF-kappaB2, and is important in controlling lymphocyte and macrophage function in immune and inflammatory responses. In contrast with p100, p105 is constitutively processed to p50. However, after stimulation with agonists, such as tumour necrosis factor-alpha and lipopolysaccharide, p105 is completely degraded by the proteasome. This releases associated p50, which translocates into the nucleus to modulate target gene expression. p105 degradation also liberates the p105-associated MAP kinase (mitogen-activated protein kinase) kinase kinase TPL-2 (tumour progression locus-2), which can then activate the ERK (extracellular-signal-regulated kinase)/MAP kinase cascade. Thus, in addition to its role in NF-kappaB activation, p105 functions as a regulator of MAP kinase signalling.

A p105-based inhibitor broadly represses NF-kappa B activities

An IkappaBalpha-based NF-kappaB super repressor (sr) has been used widely for studying genes regulated by NF-kappaB transcription factors. Repression of NF-kappaB by IkappaBalpha(sr) also facilitates tumor necrosis factor alpha-induced apoptosis in the cell. However, IkappaBalpha primarily targets RelA and c-Rel-containing complexes, leaving other NF-kappaB/Rel protein complexes, such as p50 and p52 homodimers, and RelB heterodimers uninhibited. Because these atypical NF-kappaB complexes also contribute to gene regulation and are activated in pathological conditions, broad inhibition of all NF-kappaB species is of significant pharmacological and clinical interests. We have designed, generated, and tested a p105-based NF-kappaB super repressor. We showed that p105(sr), which no longer generates p50 and undergoes signal-induced degradation, effectively inhibits all NF-kappaB activities. In addition, we also demonstrated that p105(sr) significantly enhances tumor necrosis factor alpha-mediated killing of MT1/2 skin papilloma cells where p50 homodimer activity is elevated. Our results suggest that p105(sr) is a broader range and effective NF-kappaB super repressor and can potentially be used in cells where a noncanonical NF-kappaB activity is dominant or multiple NF-kappaB activities are activated.

Casper/c-FLIP is physically and functionally associated with NF-κB1 p105

Casper/c-FLIP is a caspase-8-related molecule critically involved in regulation of death receptor-induced apoptosis. It has been shown that Casper can either promote or antagonize apoptosis and can activate the transcription factor NF-kappaB. The exact functions of Casper are controversial. To further understand how Casper signals, we searched Casper-interacting proteins by yeast two-hybrid screening. This effort identified NF-kappaB1 (p105), an atypical IkappaB molecule and the precursor of NF-kappaB subunit p50. Co-immunoprecipitation experiments indicated that Casper interacted with p105 in 293 cells and this interaction was mediated through the C-terminal IkappaB-like domain (IkappaBgamma). Overexpression of p105 and IkappaBgamma inhibited Casper-induced NF-kappaB activation and potentiated Casper-induced apoptosis. Furthermore, Casper and its C-terminal caspase-like domain inhibited p105 processing into p50. Our findings suggest that p105 is involved in Casper-mediated regulation of apoptosis and NF-kappaB activation.

Stability of the Rel homology domain is critical for generation of NF-kappa B p50 subunit

The NF-kappa B transcription factor p50 and the Rel protein-specific transcription inhibitor p105 are both encoded by the nfkb1 gene. The p50 protein is incorporated within the N-terminal portion of p105 and is a unique product of proteasomal processing. Because proteasome-mediated proteolysis generally results in complete degradation of the substrate, how p50 survives the proteasomal processing remains unknown. Survival of proteasomal processing has also been observed recently for the yeast transcription factors SPT23 and MGA2, but the mechanism is also unclear. Here we show evidence that stability of the Rel homology domain (RHD) within the N-terminal portion of the NF-kappa B 1 protein is required for p50 generation. We demonstrated that proteolysis initiated at an internal location of the NF-kappa B 1 protein, which normally generates p50, degrades the N-terminal portion of the NF-kappa B 1 protein when the RHD is destabilized. Our findings highlight the critical role of the unique structure of the RHD for the survival of p50 during proteosomal processing.

Distinctions between c-Rel and other NF-kappaB proteins in immunity and disease

c-Rel is a proto-oncogene first identified as the cellular counterpart of the v-Rel oncogene derived from the avian reticuloendotheliosis retrovirus (REV-T). It was subsequently discovered that c-Rel belongs to the NF-kappaB/Rel transcription factor family whose members share a common DNA recognition motif and similar signaling pathways. Despite the similarities, however, each NF-kappaB/Rel member possesses unique properties with regard to tissue expression pattern, response to receptor signals and target gene specificity. These differences are fairly evident from the non-redundant phenotypes exhibited by individual NF-kappaB/Rel knockout mice. Hence the work described in this review will compare and contrast the various physiological functions of c-Rel to those of other NF-kappaB members, particularly with respect to the regulation of proliferation, survival and effector functions in multiple hematopoietic and immunological cell types. The study of c-Rel knockout mice in several disease models will also be discussed as they reveal an important role for c-Rel in response to allergens, auto-antigens, allo-antigens and pathogenic infection.

p53 represses cyclin D1 transcription through down regulation of Bcl-3 and inducing increased association of the p52 NF-kappaB subunit with histone deacetylase 1

The p53 and NF-kappaB transcription factor families are important, multifunctional regulators of the cellular response to stress. Here we have investigated the regulatory mechanisms controlling p53-dependent cell cycle arrest and cross talk with NF-kappaB. Upon induction of p53 in H1299 or U-2 OS cells, we observed specific repression of cyclin D1 promoter activity, correlating with a decrease in cyclin D1 protein and mRNA levels. This repression was dependent on the proximal NF-kappaB binding site of the cyclin D1 promoter, which has been shown to bind the p52 NF-kappaB subunit. p53 inhibited the expression of Bcl-3 protein, a member of the IkappaB family that functions as a transcriptional coactivator for p52 NF-kappaB and also reduced p52/Bcl-3 complex levels. Concomitant with this, p53 induced a significant increase in the association of p52 and histone deacetylase 1 (HDAC1). Importantly, p53-mediated suppression of the cyclin D1 promoter was reversed by coexpression of Bcl-3 and inhibition of p52 or deacetylase activity. p53 therefore induces a transcriptional switch in which p52/Bcl-3 activator complexes are replaced by p52/HDAC1 repressor complexes, resulting in active repression of cyclin D1 transcription. These results reveal a unique mechanism by which p53 regulates NF-kappaB function and cell cycle progression.

Nuclear factor-kappa B and mitogen-activated protein kinases mediate nitric oxide-enhanced transcriptional expression of interferon-beta

Mitogen-activated protein (MAP) kinase and nuclear factor-kappaB (NF-kappaB) activation are critical for initiating the transcriptional expression of cytokines, cell adhesion molecules, and other factors in the macrophage immune response. Nitric oxide (NO), an endogenous free radical, is a product of macrophages that mediates inflammatory and cytotoxic processes in the immune system. Here we report the effects of NO on MAP kinase signaling and NF-kappaB activation in lipopolysaccharide (LPS)-stimulated RAW 264.7 macrophages and correlate these effects to the induction target genes, including interferon-beta (IFN-beta) and IkappaB-alpha. LPS alone induced a rapid phosphorylation of the stress-activated MAP kinases: c-Jun N-terminal kinase (JNK) and p38. Simultaneous treatment with LPS and the NO donor, diethylamine NONOate (DEA/NO), enhanced and prolonged JNK and p38 phosphorylation. Similarly, DEA/NO prolonged the LPS-induced degradation of the NF-kappaB inhibitory subunit, IkappaB-alpha, despite an increase in IkappaB-alpha mRNA levels. Whereas DEA/NO alone was sufficient to induce JNK and p38 phosphorylation, it was not sufficient to cause IkappaB-alpha degradation. The enhancement of IkappaB-alpha degradation by DEA/NO correlated with an increase in the nuclear levels of the p50 and p65 subunits and DNA-binding activity determined by electrophoretic mobility shift assay. DEA/NO and an additional NO donor, MAHMA/NO, are further demonstrated to enhance the transcriptional expression of the IFN-beta gene. The results suggest a role for NO in enhancing and propagating inflammatory conditions and the immune response.

p105.Ikappa Bgamma and prototypical Ikappa Bs use a similar mechanism to bind but a different mechanism to regulate the subcellular localization of NF-kappa B

p105, also known as NF-kappaB1, is an atypical IkappaB molecule with a multi-domain organization distinct from other prototypical IkappaBs, like IkappaBalpha and IkappaBbeta. To understand the mechanism by which p105 binds and inhibits NF-kappaB, we have used both p105 and its C-terminal inhibitory segment known as IkappaBgamma for our study. We show here that one IkappaBgamma molecule binds to NF-kappaB dimers wherein at least one NF-kappaB subunit is p50. We suggest that the obligatory p50 subunit in IkappaBgamma.NF-kappaB complexes is equivalent to the N-terminal p50 segment in all p105.NF-kappaB complexes. The nuclear localization signal (NLS) of the obligatory p50 subunit is masked by IkappaBgamma, whereas the NLS of the nonobligatory NF-kappaB subunit is exposed. Thus, the global binding mode of all IkappaB.NF-kappaB complexes seems to be similar where one obligatory (or specific) NF-kappaB subunit makes intimate contact with IkappaB and the nonobligatory (or nonspecific) subunit is bound primarily through its ability to dimerize. In the case of IkappaBalpha and IkappaBbeta, the specific NF-kappaB subunit in the complex is p65. In contrast to IkappaBalpha.NF-kappaB complexes, where the exposed NLS of the nonspecific subunit imports the complex to the nucleus, p105.NF-kappaB and IkappaBgamma.NF-kappaB complexes are cytoplasmic. We show that the death domain of p105 (also of IkappaBgamma) is essential for the cytoplasmic sequestration of NF-kappaB by p105 and IkappaBgamma. However, the death domain does not mask the exposed NLS of the complex. We also demonstrate that the death domain alone is not sufficient for cytoplasmic retention and instead functions only in conjunction with other parts in the three-dimensional scaffold formed by the association of the ankyrin repeat domain (ARD) and NF-kappaB dimer. We speculate that additional cytoplasmic protein(s) may sequester the entire p105.NF-kappaB complex by binding through the death domain and other segments, including the exposed NLS.

IKKalpha, IKKbeta, and NEMO/IKKgamma are each required for the NF-kappa B-mediated inflammatory response program

The IKKbeta and NEMO/IKKgamma subunits of the NF-kappaB-activating signalsome complex are known to be essential for activating NF-kappaB by inflammatory and other stress-like stimuli. However, the IKKalpha subunit is believed to be dispensable for the latter responses and instead functions as an in vivo mediator of other novel NF-kappaB-dependent and -independent functions. In contrast to this generally accepted view of IKKalpha's physiological functions, we demonstrate in mouse embryonic fibroblasts (MEFs) that, akin to IKKbeta and NEMO/IKKgamma, IKKalpha is also a global regulator of tumor necrosis factor alpha- and IL-1-responsive IKK signalsome-dependent target genes including many known NF-kappaB targets such as serum amyloid A3, C3, interleukin (IL)-6, IL-11, IL-1 receptor antagonist, vascular endothelial growth factor, Ptx3, beta(2)-microglobulin, IL-1alpha, Mcp-1 and -3, RANTES (regulated on activation normal T cell expressed and secreted), Fas antigen, Jun-B, c-Fos, macrophage colony-stimulating factor, and granulocyte-macrophage colony-stimulating factor. Only a small number of NF-kappaB-dependent target genes were preferentially dependent on IKKalpha or IKKbeta. Constitutive expression of a trans-dominant IkappaBalpha superrepressor (IkappaBalphaSR) in wild type MEFs confirmed that these signalsome-dependent target genes were also dependent on NF-kappaB. A subset of NF-kappaB target genes were IKK-dependent in the absence of exogenous stimuli, suggesting that the signalsome was also required to regulate basal levels of activated NF-kappaB in established MEFs. Overall, a sizable number of novel NF-kappaB/IKK-dependent genes were identified including Secreted Frizzled, cadherin 13, protocadherin 7, CCAAT/enhancer-binding protein-beta and -delta, osteoprotegerin, FOXC2 and FOXF2, BMP-2, p75 neurotrophin receptor, caspase-11, guanylate-binding proteins 1 and 2, ApoJ/clusterin, interferon (alpha and beta) receptor 2, decorin, osteoglycin, epiregulin, proliferins 2 and 3, stromal cell-derived factor, and cathepsins B, F, and Z. SOCS-3, a negative effector of STAT3 signaling, was found to be an NF-kappaB/IKK-induced gene, suggesting that IKK-mediated NF-kappaB activation can coordinately illicit negative effects on STAT signaling.

Gene deletion of NF-kappa B p50 does not alter the hepatic inflammatory response to ischemia/reperfusion

BACKGROUND/AIMS

Nuclear factor kappa B (NF-kappa B) is a primary regulator of gene expression and is activated during hepatic ischemia/reperfusion injury. The objective of the present study was to determine whether activation of NF-kappa B is causally related to the induction of the acute inflammatory response induced by hepatic ischemia/reperfusion.

METHODS

Wild-type (p50(+/+)) and NF-kappa B p50-deficient (p50(-/-)) mice underwent hepatic ischemia/reperfusion. NF-kappa B activation was determined by electrophoretic mobility shift assay. Hepatic neutrophil accumulation was measured by liver myeloperoxidase content. Hepatocellular injury was assessed by serum level of alanine aminotransferase and liver histology.

RESULTS

In p50(+/+) mice, ischemia/reperfusion induced marked activation of NF-kappa B consisting of p50/p65 heterodimers. In contrast, NF-kappa B activation in livers from p50(-/-) mice was abrogated, but p65 was observed in nuclear extracts. Despite amelioration of NF-kappa B activation there was no significant difference between p50(+/+) and p50(-/-) mice in expression of TNF alpha and MIP-2, liver accumulation of neutrophils or hepatocellular injury.

CONCLUSIONS

Gene deletion of NF-kappa B p50 does not alter the hepatic inflammatory response to ischemia/reperfusion. Despite abrogation of DNA-binding by the NF-kappa B p50/p65 complex, p65 was still observed in nuclear extracts suggesting that there may be functional redundancy amongst members of the Rel protein family in order to preserve the inflammatory response.

Regulation of antibacterial and antifungal innate immunity in fruitflies and humans

Insects have been very successful in adapting to their environment, and the ability of the insect immune system to detect and elicit the appropriate response against various invading pathogens has helped in this success. Unlike the vertebrate immune system, which consists of both innate and adaptive components, insect immunity probably consists entirely of an innate immune response, as no evidence of an adaptive response has been found. The innate immune response is described as either a reaction against "lack of self," or the interaction between host germline-encoded receptors and molecules unique to a particular class of invading organisms. Once the invading organism is recognized, the host immune response can be activated via signaling pathways that lead to the appropriate reaction. This review endeavors to put forth how through genetic, molecular, and biochemical studies of the fruit fly Drosophila melanogaster, as well as other insects, it is now understood that aspects of the insect and vertebrate innate immune system are very similar.

[The Rel/NF-kappa-B transcription factors: complex role in cell regulation]

The transcription factor NF-kappa B has attracted widespread attention among researchers. NF-kappa B displays some original characteristics including rapid regulation, the wide range of genes that it controls and its probable involvement in several diseases. In resting cells, NF-kappa B is kept in an inactive form in the cytoplasm where it is bound to a member of the I kappa B family of inhibitory proteins. NF-kappa B can be activated by exposure of cells to physiological as well as non physiological stimuli. Upon cell activation, the inhibitors are modified through site specific phosphorylations which target them for subsequent ubiquitination and proteolytic degradation by the proteasome. Removal of the inhibitor unmasks the nuclear localization signals on subunits of NF-kappa B. Free NF-kappa B moves to the nucleus where it binds to target DNA elements and activate transcription of genes encoding proteins involved in immune responses, inflammation or cell proliferation. NF-kappa B could be considered as a co-ordinating element in the body's responses to situations of stress, infection or inflammation. A tight regulation of NF-kappa B seems to be crucial since a dysfunction could promote pathogenic processes including AIDS (acquired immunodeficiency syndrome), rheumatoid arthritis and cancer. Additionally, it will be important to understand the exact roles for NF-kappa B in regulating apoptosis. NF-kappa B is now regarded as a good therapeutic target and the development of specific inhibitors should lead in the next future to novel therapeutics.

RelB cellular regulation and transcriptional activity are regulated by p100

RelB mediates the constitutive nuclear pool of NF-kappaB transcriptional activity in myeloid and lymphoid cells, which is believed to be secondary to its weak interaction with the classical NF-kappaB inhibitor proteins, the IkappaBs. In other cell types, RelB is located in the cytosol, thus suggesting that RelB is also regulated by an inhibitory protein(s). In this study, it is demonstrated that RelB is associated in the cytosol with p100 but not with IkappaBalpha, IkappaBbeta, IkappaBepsilon, nor p105. Its cytosolic control is not affected by stimuli that lead to RelA nuclear translocation, and RelB nuclear localization is prevented by p100, but not by p105 or IkappaBalpha. Structure function analysis p100-RelB interactions indicates that p100 amino acids 623-900 are required for effective interaction and repression of nuclear translocation and RelB driven NF-kappaB-dependent transcription. Moreover, this carboxyl-portion of p100 contains a nuclear export signal(s), which is required for effective retrieval of RelB from the nucleus. Finally, overexpression of NF-kappaB-inducing kinase, a kinase that has recently been shown to induce p100 processing, possibly through IKKalpha activation, causes nuclear translocation of RelB protein. Thus, these studies indicate that p100 is a bone fide inhibitor of RelB and that this transcription factor may be regulated by NF-kappaB-inducing kinase and/or IKKalpha.

Mechanisms of ubiquitin-mediated, limited processing of the NF-kappaB1 precursor protein p105

In most cases, target proteins of the ubiquitin system are completely degraded. In several exceptions, such as the first step in the activation of the transcriptional regulator NF-kappaB, the substrate, the precursor protein p105, is processed in a limited manner to yield the active subunit p50. p50 is derived from the N-terminal domain of p105, whereas the C-terminal domain is degraded. The mechanisms involved in this unique process have remained elusive. We have shown that a Gly-rich region (GRR) at the C-terminal domain of p50 is one important processing signal and that it interferes with processing of the ubiquitinated precursor by the 26S proteasome. Also, amino acid residues 441-454 are important for processing under non-stimulated conditions. Lys 441 and 442 serve as ubiquitination targets, whereas residues 446-454 may serve as a ligase recognition motif. Following IkappaB kinase (IKK)-mediated phosphorylation, the C-terminal domain of p105, residues 918-934, recruits the SCF(beta-TrCP) ubiquitin ligase, and ubiquitination by this complex leads to accelerated processing. The two sites appear to be recognized under different physiological conditions by two different ligases, targeting two distinct recognition motifs. We have shown that ubiquitin conjugation and processing of a series of precursors of p105 that lack the C-terminal IKK phosphorylation/TrCP binding domain, is progressively inhibited with increasing number of ankyrin repeats. Inhibition is due to docking of active NF-kappaB subunits to the ankyrin repeat domain in the C-terminal half of p105 (IkappaBgamma). Inhibition is alleviated by phosphorylation of the C-terminal domain that leads to ubiquitin-mediated degradation of the ankyrin repeat domain and release of the anchored subunits. We propose a model that may explain the requirement for two sites: a) a basal site that may be involved in co-translational processing prior to the synthesis of the ankyrin repeat domain; and b) a signal-induced site that is involved in processing/degradation of the complete molecule following cell activation, with rapid release of stored, transcriptionally active subunits.

NF-kappaB and cell-cycle regulation: the cyclin connection

The cyclins are a family of proteins that are centrally involved in cell cycle regulation and which are structurally identified by conserved "cyclin box" regions. They are regulatory subunits of holoenzyme cyclin-dependent kinase (CDK) complexes controlling progression through cell cycle checkpoints by phosphorylating and inactivating target substrates. CDK activity is controlled by cyclin abundance and subcellular location and by the activity of two families of inhibitors, the cyclin-dependent kinase inhibitors (CKI). Many hormones and growth factors influence cell growth through signal transduction pathways that modify the activity of the cyclins. Dysregulated cyclin activity in transformed cells contributes to accelerated cell cycle progression and may arise because of dysregulated activity in pathways that control the abundance of a cyclin or because of loss-of-function mutations in inhibitory proteins.Analysis of transformed cells and cells undergoing mitogen-stimulated growth implicate proteins of the NF-kappaB family in cell cycle regulation, through actions on the CDK/CKI system. The mammalian members of this family are Rel-A (p65), NF-kappaB(1) (p50; p105), NF-kappaB(2) (p52; p100), c-Rel and Rel-B. These proteins are structurally identified by an amino-terminal region of about 300 amino acids, known as the Rel-homology domain. They exist in cytoplasmic complexes with inhibitory proteins of the IkappaB family, and translocate to the nucleus to act as transcription factors when activated. NF-kappaB pathway activation occurs during transformation induced by a number of classical oncogenes, including Bcr/Abl, Ras and Rac, and is necessary for full transforming potential. The avian viral oncogene, v-Rel is an NF-kappaB protein. The best explored link between NF-kappaB activation and cell cycle progression involves cyclin D(1), a cyclin which is expressed relatively early in the cell cycle and which is crucial to commitment to DNA synthesis. This review examines the interactions between NF-kappaB signaling and the CDK/CKI system in cell cycle progression in normal and transformed cells. The growth-promoting actions of NF-kappaB factors are accompanied, in some instances, by inhibition of cellular differentiation and by inhibition of programmed cell death, which involve related response pathways and which contribute to the overall increase in mass of undifferentiated tissue.

17 beta-estradiol inhibits tumor necrosis factor-alpha-induced nuclear factor-kappa B activation by increasing nuclear factor-kappa B p105 level in MCF-7 breast cancer cells

Tumor necrosis factor-alpha (TNF-alpha) exerts many cytological effects on a wide range of cells. TNF-alpha can activate nuclear factor-kappa B (NF-kappa B). Activation of NF-kappa B by TNF-alpha mediates many functions of TNF-alpha. The NF-kappa B inhibitor, I kappa B alpha, negatively regulates the activity of NF-kappa B. In MCF-7 cells (an estrogen and TNF-alpha receptor positive cell line), treatment with 17 beta-estradiol (E(2)) inhibited TNF-alpha-induced NF-kappa B DNA binding activity in the gel retardation assays. But, the level of the I kappa B alpha and the TNF-alpha receptor, TNF-R1, were not obviously affected. The NF-kappa B precursor, NF-kappa B p105, has been shown to be associated with NF-kappa B in the cytoplasm and efficiently blocks its nuclear translocation and activation. Treatment of MCF-7 cells with E(2) increased the level of NF-kappa B p105 protein. The anti-estrogen, 4OH-tamoxifen, treatment inhibited E(2)-induced NF-kappa B p105 expression. Our findings indicate that NF-kappa B p105 plays a role in modulating the functions of TNF-alpha in the estrogen receptor positive breast cancer cells.

IkappaBalpha and IkappaBalpha /NF-kappa B complexes are retained in the cytoplasm through interaction with a novel partner, RasGAP SH3-binding protein 2

IkappaBalpha inhibits the transcriptional activity of NF-kappaB both in the cytoplasm by preventing the nuclear translocation of NF-kappaB and in the nucleus where it dissociates NF-kappaB from DNA and transports it back to the cytoplasm. Cytoplasmic localization of inactive NF-kappaB/IkappaBalpha complexes is controlled by mutual masking of nuclear import sequences of NF-kappaB p65 and IkappaBalpha and active CRM1-mediated nuclear export. Here, we describe an additional mechanism accounting for the cytoplasmic anchoring of IkappaBalpha or NF-kappaB/IkappaBalpha complexes. The N-terminal domain of IkappaBalpha contains a sequence responsible for the cytoplasmic retention of IkappaBalpha that is specifically recognized by G3BP2, a cytoplasmic protein that interacts with both IkappaBalpha and IkappaBalpha/NF-kappaB complexes. G3BP2 is composed of an N-terminal domain homologous to the NTF2 protein, followed by an acidic domain sufficient for the interaction with the IkappaBalpha cytoplasmic retention sequence, a region containing five PXXP motifs and a C-terminal domain containing RNA-binding motifs. Overexpression of G3BP2 directly promotes retention of IkappaBalpha in the cytoplasm, indicating that subcellular distribution of IkappaBalpha and NF-kappaB/IkappaBalpha complexes likely results from a equilibrium between nuclear import, nuclear export, and cytoplasmic retention. The molecular organization of G3BP2 suggests that this putative scaffold protein might connect the NF-kappaB signal transduction cascade with cellular functions such as nuclear transport or RNA metabolism.

Inhibition of mineralocorticoid-mediated transcription by NF-kappaB

Nuclear factor-kappaB (NF-kappaB) is a ubiquitous transcription factor that regulates the expression of multiple inflammatory and immune response genes and plays a critical role in host defense and in chronic inflammatory diseases. The mineralocorticoid receptor (MR) belongs to the steroid/thyroid hormone receptor super-family of ligand-induced transcription factors. We demonstrate a dose-dependent, mutual transcriptional antagonism between NF-kappaB and MR in transient transfection experiments. We also show that the antagonism is limited to the p65 subunit of NF-kappaB heterodimer but not p50. Transient cotransfection experiments with MR deletion constructs reveal the necessity of various N-terminal MR domains for this phenomenon. Inhibition of NF-kappaB by IkappaB relieves the repression of NF-kappaB function by MR. These data suggest that the p65 subunit of NF-kappaB interacts with MR indirectly and transrepresses MR activation.

Effects of glucocorticoids on tumor necrosis factor alpha-dependent activation of nuclear factor kappaB and expression of the intercellular adhesion molecule 1 gene in osteoblast-like ROS17/2.8 cells

Recently, we showed that tumor necrosis factor alpha (TNF-alpha) stimulates expression of the intercellular adhesion molecule 1 (ICAM-1) and interleukin-6 (IL-6) genes through activation of p65-p50 heterodimer nuclear factor KB (NF-kappaB) in rat osteoblast-like ROS17/2.8 cells. In the present study, we investigated effects of a synthetic glucocorticoid, dexamethasone (Dex), on TNF-alpha-dependent activation of NF-kappaB and expression of the ICAM-1 gene. ROS17/2.8 cells were pretreated with Dex for 6 h and then exposed to TNF-alpha. Electrophoretic mobility shift assay (EMSA) revealed that TNF-alpha-dependent activation of NF-kappaB was almost completely suppressed by Dex treatment. Increase in ICAM-1 messenger RNA (mRNA) level by TNF-alpha also was markedly suppressed by Dex. Western blot and immunocytochemical analyses showed that Dex attenuated the TNF-alpha-induced nuclear translocation of p65. Treatment with protein synthesis inhibitor cycloheximide (CHX) reversed the Dex effect, indicating that Dex requires de novo protein synthesis for its action. Northern blot analysis revealed that Dex increased IkappaB-alpha mRNA level synergistically with TNF-alpha, whereas it decreased p65 mRNA level. The p105 and IkappaB-beta mRNA levels were not altered by Dex. Consistent with the mRNA level, Dex increased the amount of IkappaB-alpha protein in the cytoplasm in either the presence or the absence of TNF-alpha. Considering a role of IkappaB to sequester NF-kappaB in the cytoplasm, it was suggested that an increase in IkappaB-alpha protein and the concomitant decrease in p65 synthesis account for the Dex-induced suppression of NF-kappaB activation in osteoblastic cells.