Unravelling the complexities of the NF-κB signalling pathway using mouse knockout and transgenic models

A systematic survey of the response of a model NF-κB signalling pathway to TNFα stimulation

White's lab established that strong, continuous stimulation with tumour necrosis factor-α (TNFα) can induce sustained oscillations in the subcellular localisation of the transcription factor nuclear factor κB (NF-κB). But the intensity of the TNFα signal varies substantially, from picomolar in the blood plasma of healthy organisms to nanomolar in diseased states. We report on a systematic survey using computational bifurcation theory to explore the relationship between the intensity of TNFα stimulation and the existence of sustained NF-κB oscillations. Using a deterministic model developed by Ashall et al. in 2009, we find that the system's responses to TNFα are characterised by a supercritical Hopf bifurcation point: above a critical intensity of TNFα the system exhibits sustained oscillations in NF-kB localisation. For TNFα below this critical value, damped oscillations are observed. This picture depends, however, on the values of the model's other parameters. When the values of certain reaction rates are altered the response of the signalling pathway to TNFα stimulation changes: in addition to the sustained oscillations induced by high-dose stimulation, a second oscillatory regime appears at much lower doses. Finally, we define scores to quantify the sensitivity of the dynamics of the system to variation in its parameters and use these scores to establish that the qualitative dynamics are most sensitive to the details of NF-κB mediated gene transcription.

NF-κB, the first quarter-century: remarkable progress and outstanding questions

The ability to sense and adjust to the environment is crucial to life. For multicellular organisms, the ability to respond to external changes is essential not only for survival but also for normal development and physiology. Although signaling events can directly modify cellular function, typically signaling acts to alter transcriptional responses to generate both transient and sustained changes. Rapid, but transient, changes in gene expression are mediated by inducible transcription factors such as NF-κB. For the past 25 years, NF-κB has served as a paradigm for inducible transcription factors and has provided numerous insights into how signaling events influence gene expression and physiology. Since its discovery as a regulator of expression of the κ light chain gene in B cells, research on NF-κB continues to yield new insights into fundamental cellular processes. Advances in understanding the mechanisms that regulate NF-κB have been accompanied by progress in elucidating the biological significance of this transcription factor in various physiological processes. NF-κB likely plays the most prominent role in the development and function of the immune system and, not surprisingly, when dysregulated, contributes to the pathophysiology of inflammatory disease. As our appreciation of the fundamental role of inflammation in disease pathogenesis has increased, so too has the importance of NF-κB as a key regulatory molecule gained progressively greater significance. However, despite the tremendous progress that has been made in understanding the regulation of NF-κB, there is much that remains to be understood. In this review, we highlight both the progress that has been made and the fundamental questions that remain unanswered after 25 years of study.

Combined loss of cRel/p50 subunits of NF-κB leads to impaired innate host response in sepsis

NF-κB, which comprises homo- and hetero-dimers of the five members of the Rel family, plays a crucial role in immunity to infection. The cRel and p50 subunits have been implicated in the development and function of the immune cells, but their in vivo importance remains poorly explored in sepsis. We aimed to study the impact of the combined loss of these two subunits on the innate response to infection in a cecal ligation and puncture model of sepsis. We have explored the possible defects in host defense, including pathogen clearance, bacterial phagocytosis and cytokine plasma release. We also performed gene profiling of cRel(-/-)p50(-/-) and wild-type LPS-stimulated peritoneal macrophages. Deficiency of cRel and p50 led to enhanced mortality to sepsis that was associated with defective macrophages phagocytosis, decreased bacterial clearance and moderate cytokine response. Transcription profile analysis revealed a common inflammatory response but a significant down-regulated transcription of genes encoding for pathogen recognition receptors and antimicrobial molecules, supporting the in vivo findings in mice. In conclusion, the cRel and p50 subunits of NF-κB play an important combined role in the innate response and are crucial for survival and pathogen clearance in polymicrobial sepsis.

Signaling by the tumor necrosis factor receptor superfamily in B-cell biology and disease

Members of the tumor necrosis factor receptor superfamily (TNFRSF) participate prominently in B-cell maturation and function. In particular, B-cell activating factor belonging to the TNF family receptor (BAFF-R), B-cell maturation antigen (BCMA), and transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI) play critical roles in promoting B-cell survival at distinct stages of development by engaging a proliferation-inducing ligand (APRIL) and/or BAFF. CD40 is also essential for directing the humoral response to T-cell-dependent antigens. Signaling by the TNFRSF is mediated primarily, albeit not exclusively, via the TNFR-associated factor (TRAF) proteins and activation of the canonical and/or non-canonical nuclear factor-κB (NF-κB) pathways. Dysregulated signaling by TNFRSF members can promote B-cell survival and proliferation, causing autoimmunity and neoplasia. In this review, we present a current understanding of the functions of and distinctions between APRIL/BAFF signaling by their respective receptors expressed on particular B-cell subsets. These findings are compared and contrasted with CD40 signaling, which employs similar signaling conduits to achieve distinct cellular outcomes in the context of the germinal center response. We also underscore how new findings and conceptual insights into TNFRSF signaling are facilitating the understanding of B-cell malignancies and autoimmune diseases.

Effect of Helicobacter pylori cdrA on interleukin-8 secretions and nuclear factor kappa B activation

AIM: To investigate genetic diversity of Helicobacter pylori (H. pylori) cell division-related gene A (cdrA) and its effect on the host response.

METHODS: Inactivation of H. pylori cdrA, which is involved in cell division and morphological elongation, has a role in chronic persistent infections. Genetic property of H. pylori cdrA was evaluated using polymerase chain reaction and sequencing in 128 (77 American and 51 Japanese) clinical isolates obtained from 48 and 51 patients, respectively. Enzyme-linked immunosorbent assay was performed to measure interleukin-8 (IL-8) secretion with gastric biopsy specimens obtained from American patients colonized with cdrA-positive or -negative strains and AGS cells co-cultured with wild-type HPK5 (cdrA-positive) or its derivative HPKT510 (cdrA-disruptant). Furthermore, the cytotoxin-associated gene A (cagA) status (translocation and phosphorylation) and kinetics of transcription factors [nuclear factor-kappa B (NF-κB) and inhibition kappa B] were investigated in AGS cells co-cultured with HPK5, HPKT510 and its derivative HPK5CA (cagA-disruptant) by western blotting analysis with immunoprecipitation.

RESULTS: Genetic diversity of the H. pylori cdrA gene demonstrated that the cdrA status segregated into two categories including four allele types, cdrA-positive (allele types;Iand II) and cdrA-negative (allele types; III and IV) categories, respectively. Almost all Japanese isolates were cdrA-positive (I: 7.8% and II: 90.2%), whereas 16.9% of American isolates were cdrA-positive (II) and 83.1% were cdrA-negative (III: 37.7% and IV: 45.5%), indicating extended diversity of cdrA in individual American isolates. Comparison of each isolate from different regions (antrum and corpus) in the stomach of 29 Americans revealed that cdrA status was identical in both isolates from different regions in 17 cases. However, 12 cases had a different cdrA allele and 6 of them exhibited a different cdrA category between two regions in the stomach. Furthermore, in 5 of the 6 cases possessing a different cdrA category, cdrA-negative isolate existed in the corpus, suggesting that cdrA-negative strain is more adaptable to colonization in the corpus. IL-8 secretions from AGS revealed that IL-8 levels induced by a cdrA-disrupted HPKT510 was significantly lower (P < 0.01) compared to wild-type HPK5: corresponding to 50%-60% of those of wild-type HPK5. These data coincided with in vivo data that an average value of IL-8 in biopsy specimens from cdrA-positive and cdrA-negative groups was 215.6 and 135.9 pg/mL, respectively. Western blotting analysis documented that HPKT510 had no effect on CagA translocation and phosphorylation, however, nuclear accumulation of NF-κB was lower by HPKT510 compared to HPK5.

CONCLUSION: Colonization by a cdrA-negative or cdrA-dysfunctional strain resulted in decreased IL-8 production and repression of NF-κB, and hence, attenuate the host immunity leading to persistent infection.

Nuclear factor-κB in immunity and inflammation: the Treg and Th17 connection

Although nuclear factor-kB (NF-kB) is generally considered to be a pro-inflammatory transcription factor, recent studies indicate that it also plays a critical role in the development of an anti-inflammatory T cell subset called regulatory T (Treg) cells. Two NF-kB proteins, c-Rel and p65, drive the development of Treg cells by promoting the formation of a Foxp3-specific enhanceosome. Consequently, c-Rel-deficient mice have marked reductions in Treg cells, and c-Rel-deficient T cells are compromised in Treg cell differentiation. However, with the exception of Foxp3, most NF-kB target genes in immune cells are pro-inflammatory. These include several Th17-related cytokine genes and the retinoid-related orphan receptor-g (Rorg or Rorc) that specifies Th17 differentiation and lineage-specific function. T cells deficient in c-Rel or p65 are significantly compromised in Th17 differentiation, and c-Rel -deficient mice are defective in Th17 responses. Thus, NF-kB is required for the development of both anti-inflammatory Treg and pro-inflammatory Th17 cells.

The NF-κB subunit c-Rel stimulates cardiac hypertrophy and fibrosis

Cardiac remodeling and hypertrophy are the pathological consequences of cardiovascular disease and are correlated with its associated mortality. Activity of the transcription factor NF-κB is increased in the diseased heart; however, our present understanding of how the individual subunits contribute to cardiovascular disease is limited. We assign a new role for the c-Rel subunit as a stimulator of cardiac hypertrophy and fibrosis. We discovered that c-Rel-deficient mice have smaller hearts at birth, as well as during adulthood, and are protected from developing cardiac hypertrophy and fibrosis after chronic angiotensin infusion. Results of both gene expression and cross-linked chromatin immunoprecipitation assay analyses identified transcriptional activators of hypertrophy, myocyte enhancer family, Gata4, and Tbx proteins as Rel gene targets. We suggest that the p50 subunit could limit the prohypertrophic actions of c-Rel in the normal heart, because p50 overexpression in H9c2 cells repressed c-Rel levels and the absence of cardiac p50 was associated with increases in both c-Rel levels and cardiac hypertrophy. We report for the first time that c-Rel is highly expressed and confined to the nuclei of diseased adult human hearts but is restricted to the cytoplasm of normal cardiac tissues. We conclude that c-Rel-dependent signaling is critical for both cardiac remodeling and hypertrophy. Targeting its activities could offer a novel therapeutic strategy to limit the effects of cardiac disease.

TANK-binding kinase 1 (TBK1) controls cell survival through PAI-2/serpinB2 and transglutaminase 2

The decision between survival and death in cells exposed to TNF relies on a highly regulated equilibrium between proapoptotic and antiapoptotic factors. The TNF-activated antiapoptotic response depends on several transcription factors, including NF-κB and its RelA/p65 subunit, that are activated through phosphorylation-mediated degradation of IκB inhibitors, a process controlled by the IκB kinase complex. Genetic studies in mice have identified the IκB kinase-related kinase TANK-binding kinase 1 (TBK1; also called NAK or T2K) as an additional regulatory molecule that promotes survival downstream of TNF, but the mechanism through which TBK1 exerts its survival function has remained elusive. Here we show that TBK1 triggers an antiapoptotic response by controlling a specific RelA/p65 phosphorylation event. TBK1-induced RelA phosphorylation results in inducible expression of plasminogen activator inhibitor-2 (PAI-2), a member of the serpin family with known antiapoptotic activity. PAI-2 limits caspase-3 activation through stabilization of transglutaminase 2 (TG2), which cross-links and inactivates procaspase-3. Importantly, Tg2(-/-) mice were found to be more susceptible to apoptotic cell death in two models of TNF-dependent acute liver injury. Our results establish PAI-2 and TG2 as downstream mediators in the antiapoptotic response triggered upon TBK1 activation.

Acute myeloid leukemia developing in patients with autoimmune diseases

Therapy-related acute myeloid leukemia is an unfortunate complication of cancer treatment, particularly for patients with highly curable primary malignancies and favorable life expectancy. The risk of developing therapy-related acute myeloid leukemia also applies to patients with non-malignant conditions, such as autoimmune diseases treated with cytotoxic and/or immunosuppressive agents. There is considerable evidence to suggest that there is an increased occurrence of hematologic malignancies in patients with autoimmune diseases compared to the general population, with a further increase in risk after exposure to cytotoxic therapies. Unfortunately, studies have failed to reveal a clear correlation between leukemia development and exposure to individual agents used for the treatment of autoimmune diseases. Given the dismal outcome of secondary acute myeloid leukemia and the wide range of available agents for treatment of autoimmune diseases, an increased awareness of this risk and further investigation into the pathogenetic mechanisms of acute leukemia in autoimmune disease patients are warranted. This article will review the data available on the development of acute myeloid leukemia in patients with autoimmune diseases. Possible leukemogeneic mechanisms in these patients, as well as evidence supporting the association of their primary immunosuppressive status and their exposure to specific therapies, will also be reviewed. This review also supports the idea that it may be misleading to label leukemias that develop in patients with autoimmune diseases who are exposed to cytotoxic agents as 'therapy-related leukemias'. A better understanding of the molecular defects in autoimmune disease patients who develop acute leukemia will lead to a better understanding of the association between these two diseases entities.

Bcl-XL, but not Bcl-2, can protect human B-lymphoma cell lines from parthenolide-induced apoptosis

In this report, we investigated the effects of the natural product parthenolide on human B-lymphoma cell lines. We show that parthenolide inhibited NF-κB transcription factor c-Rel (REL). In addition, the sensitivity of several human B-lymphoma cell lines to parthenolide-induced apoptosis inversely correlated with their levels of anti-apoptosis protein Bcl-X(L). Furthermore, ectopic expression of Bcl-X(L) (but not Bcl-2) in two B-lymphoma cell lines decreased their sensitivity to parthenolide-induced apoptosis. Finally, over-expression of a transforming mutant of REL, which increased expression of endogenous Bcl-X(L), decreased the sensitivity of BJAB B-lymphoma cells to parthenolide-induced apoptosis. These results demonstrate that the NF-κB target gene products Bcl-X(L) and Bcl-2 can play different roles in protecting B-lymphoma cells from chemical-induced apoptosis.

The NF-κB1 transcription factor prevents the intrathymic development of CD8 T cells with memory properties

The role of specific members of the NF-κB family of transcription factors in CD8 T-cell selection and development is largely unknown. Here, we show that mice lacking NF-κB1 develop a unique population of conventional CD8 single-positive (SP) thymocytes with memory T cell-like properties that populate peripheral immune organs. Development of this memory-like population is not due to PLZF(+) thymocytes and instead coincides with changes in CD8 T-cell selection. These include a reduction in the efficiency of negative selection and a dependence on MHC class Ia or Ib expressed by haematopoietic cells. These findings indicate that NF-κB1 regulates multiple events in the thymus that collectively inhibit the excess development of CD8(+) thymocytes with memory cell characteristics.

Principles of dimer-specific gene regulation revealed by a comprehensive characterization of NF-κB family DNA binding

The unique DNA-binding properties of distinct NF-κB dimers influence the selective regulation of NF-κB target genes. To more thoroughly investigate these dimer-specific differences, we combined protein-binding microarrays and surface plasmon resonance to evaluate DNA sites recognized by eight different NF-κB dimers. We observed three distinct binding-specificity classes and clarified mechanisms by which dimers might regulate distinct sets of genes. We identified many new nontraditional NF-κB binding site (κB site) sequences and highlight the plasticity of NF-κB dimers in recognizing κB sites with a single consensus half-site. This study provides a database that can be used in efforts to identify NF-κB target sites and uncover gene regulatory circuitry.

c-Rel controls multiple discrete steps in the thymic development of Foxp3+ CD4 regulatory T cells

The development of natural Foxp3⁺ CD4 regulatory T cells (nTregs) proceeds via two steps that involve the initial antigen dependent generation of CD25⁺GITR^hiFoxp3⁻CD4⁺ nTreg precursors followed by the cytokine induction of Foxp3. Using mutant mouse models that lack c-Rel, the critical NF-κB transcription factor required for nTreg differentiation, we establish that c-Rel regulates both of these developmental steps. c-Rel controls the generation of nTreg precursors via a haplo-insufficient mechanism, indicating that this step is highly sensitive to c-Rel levels. However, maintenance of c-Rel in an inactive state in nTreg precursors demonstrates that it is not required for a constitutive function in these cells. While the subsequent IL-2 induction of Foxp3 in nTreg precursors requires c-Rel, this developmental transition does not coincide with the nuclear expression of c-Rel. Collectively, our results support a model of nTreg differentiation in which c-Rel generates a permissive state for foxp3 transcription during the development of nTreg precursors that influences the subsequent IL-2 dependent induction of Foxp3 without a need for c-Rel reactivation.

The Th17 immune response is controlled by the Rel-RORγ-RORγ T transcriptional axis

Transcription factors c-Rel and RelA/p65 bind and activate two Rorg promoters to drive Th17 differentiation.

The Th17 cells use the retinoid-related orphan receptor-γ (Rorg or Rorc) to specify their differentiation and lineage-specific function. However, how Rorg is switched on during Th17 differentiation is unknown. We report here that c-Rel and RelA/p65 transcription factors drive Th17 differentiation by binding to and activating two distinct Rorg promoters that control RORγT and RORγ expression, respectively. Similar to RORγT, RORγ is selectively expressed in Th17 cells and is effective in specifying the Th17 phenotype. T cells deficient in c-Rel or RelA are significantly compromised in Th17 differentiation, and c-Rel–deficient mice are defective in Th17 responses. Thus, Th17 immunity is controlled by a Rel–RORγ–RORγT axis, and strategies targeting Rel/NF-κB can be effective for controlling Th17 cell–mediated diseases.

The NF-κB transcription factor c-Rel is required for Th17 effector cell development in experimental autoimmune encephalomyelitis

Experimental autoimmune encephalomyelitis (EAE) is a T cell-mediated autoimmune disease involving effector Th subsets such as Th1 and Th17. In this study, we demonstrate that mice lacking the NF-κB transcription factor family member c-Rel (rel(-/-)), which are known to be resistant to EAE, show impaired Th17 development. Mixed bone marrow chimeras and EAE adoptive transfer experiments show that the deficiency of effector Th17 cells in rel(-/-) mice is T cell intrinsic. Consistent with this finding, c-Rel was activated in response to TCR signaling in the early stages of Th17 development and controlled the expression of Rorc, which encodes the Th17 transcription factor retinoic acid-related orphan receptor γt. CD28, but not IL-2, repression of Th17 development was dependent on c-Rel, implicating a dual role for c-Rel in modulating Th17 development. Adoptive transfer experiments also suggested that c-Rel control of regulatory T cell differentiation and homeostasis influences EAE development and severity by influencing the balance between Th17 and regulatory T cells. Collectively, our findings indicate that in addition to promoting Th1 differentiation, c-Rel regulates the development and severity of EAE via multiple mechanisms that impact on the generation of Th17 cells.

Inhibitor of kappa B epsilon (IκBε) is a non-redundant regulator of c-Rel-dependent gene expression in murine T and B cells

Inhibitors of kappa B (IκBs) -α, -β and -ε effect selective regulation of specific nuclear factor of kappa B (NF-κB) dimers according to cell lineage, differentiation state or stimulus, in a manner that is not yet precisely defined. Lymphocyte antigen receptor ligation leads to degradation of all three IκBs but activation only of subsets of NF-κB-dependent genes, including those regulated by c-Rel, such as anti-apoptotic CD40 and BAFF-R on B cells, and interleukin-2 (IL-2) in T cells. We report that pre-culture of a mouse T cell line with tumour necrosis factor-α (TNF) inhibits IL-2 gene expression at the level of transcription through suppressive effects on NF-κB, AP-1 and NFAT transcription factor expression and function. Selective upregulation of IκBε and suppressed nuclear translocation of c-Rel were very marked in TNF-treated, compared to control cells, whether activated via T cell receptor (TCR) pathway or TNF receptor. IκBε associated with newly synthesised c-Rel in activated cells and, in contrast to IκBα and -β, showed enhanced association with p65/c-Rel in TNF-treated cells relative to controls. Studies in IκBε-deficient mice revealed that basal nuclear expression and nuclear translocation of c-Rel at early time-points of receptor ligation were higher in IκBε-/- T and B cells, compared to wild-type. IκBε-/- mice exhibited increased lymph node cellularity and enhanced basal thymidine incorporation by lymphoid cells ex vivo. IκBε-/- T cell blasts were primed for IL-2 expression, relative to wild-type. IκBε-/- splenic B cells showed enhanced survival ex vivo, compared to wild-type, and survival correlated with basal expression of CD40 and induced expression of CD40 and BAFF-R. Enhanced basal nuclear translocation of c-Rel, and upregulation of BAFF-R and CD40 occurred despite increased IκBα expression in IκBε-/- B cells. The data imply that regulation of these c-Rel-dependent lymphoid responses is a non-redundant function of IκBε.

Nuclear JAK2: form and function in cancer

The conventional view of Janus kinase 2 (JAK2) is a nonreceptor tyrosine kinase which transmits information to the nucleus via the signal transducer and activator of transcriptions (STATs) without leaving the cytoplasm. However, accumulating data suggest that JAK2 may signal by exporting from cytoplasm to nucleus, where it guides the transcriptional machinery independent of STATs protein. Recent studies demonstrated that JAK2 is a crucial component of signaling pathways operating in the nucleus. Especially the latest landmark discovery confirmed that JAK2 goes into the nucleus and directly interacts with nucleoproteins, such as histone H3 at tyrosine 41 (H3Y41), nuclear factor 1-C2 (NF1-C2) and SWI/SNF-related helicases/ATPases (RUSH)-1α, indicating that JAK2 has a fresh nuclear function. Nuclear JAK2 is linked to a variety of cellular functions, such as cell cycle progression, apoptosis and genetic instability. The balance between these functions is an essential factor in determining whether a cell remains benign or becomes malignant. The aim of this review is intended to summarize the state of our knowledge on nuclear localization of JAK2 and nuclear JAK2 pathways, and to highlight the emerging roles for nuclear JAK2 in carcinogenesis.

Myeloid IκBα deficiency promotes atherogenesis by enhancing leukocyte recruitment to the plaques

Activation of the transcription factor NF-κB appears to be involved in different stages of atherogenesis. In this paper we investigate the role of NF-κB inhibitor IκBα in atherosclerosis. Myeloid-specific deletion of IκBα results in larger and more advanced lesions in LDL-R-deficient mice without affecting the compositional phenotype of the plaques or systemic inflammatory markers in the plasma. We show that IκBα-deleted macrophages display enhanced adhesion to an in vitro endothelial cell layer, coinciding with an increased expression of the chemokine CCL5. Also, in vivo we found that IκBα^del mice had more leukocytes adhering to the luminal side of the endothelial cell layers that cover the atherosclerotic plaques. Moreover, we introduce ER-MP58 in this paper as a new immunohistochemical tool for quantifying newly recruited myeloid cells in the atherosclerotic lesion. This staining confirms that in IκBα^del mice more leukocytes are attracted to the plaques. In conclusion, we show that IκBα deletion in myeloid cells promotes atherogenesis, probably through an induced leukocyte recruitment to plaques.

RelA and RelB transcription factors in distinct thymocyte populations control lymphotoxin-dependent interleukin-17 production in γδ T cells

The NF-κB transcription factor regulates numerous immune responses but its contribution to interleukin-17 (IL-17) production by T cells is largely unknown. Here, we report that IL-17, but not interferon-γ (IFN-γ), production by γδ T cells required the NF-κB family members RelA and RelB as well as the lymphotoxin-β-receptor (LTβR). In contrast, LTβR-NF-κB signaling was not involved in the differentiation of conventional αβ Th17 cells. Impaired IL-17 production in RelA- or RelB-deficient T cells resulted in a diminished innate immune response to Escherichia coli infection. RelA controlled the expression of LT ligands in accessory thymocytes whereas RelB, acting downstream of LTβR, was required for the expression of RORγt and RORα4 transcription factors and the differentiation of thymic precursors into γδT17 cells. Thus, RelA and RelB within different thymocyte subpopulations cooperate in the regulation of IL-17 production by γδ T cells and contribute to the host's ability to fight bacterial infections.

SHARPIN is a component of the NF-κB-activating linear ubiquitin chain assembly complex

Cpdm (chronic proliferative dermatitis) mice develop chronic dermatitis and an immunodeficiency with increased serum IgM, symptoms that resemble those of patients with X-linked hyper-IgM syndrome and hypohydrotic ectodermal dysplasia (XHM-ED), which is caused by mutations in NEMO (NF-κB essential modulator; also known as IKBKG). Spontaneous null mutations in the Sharpin (SHANK-associated RH domain interacting protein in postsynaptic density) gene are responsible for the cpdm phenotype in mice. SHARPIN shows significant similarity to HOIL-1L (also known as RBCK1), a component of linear ubiquitin chain assembly complex (LUBAC), which induces NF-κB activation through conjugation of linear polyubiquitin chains to NEMO. Here, we identify SHARPIN as an additional component of LUBAC. SHARPIN-containing complexes can linearly ubiquitinate NEMO and activated NF-κB. Thus, we re-define LUBAC as a complex containing SHARPIN, HOIL-1L, and HOIP (also known as RNF31). Deletion of SHARPIN drastically reduced the amount of LUBAC, which resulted in attenuated TNF-α- and CD40-mediated activation of NF-κB in mouse embryonic fibroblasts (MEFs) or B cells from cpdm mice. Considering the pleomorphic phenotype of cpdm mice, these results confirm the predicted role of LUBAC-mediated linear polyubiquitination in NF-κB activation induced by various stimuli, and strongly suggest the involvement of LUBAC-induced NF-κB activation in various disorders.

NF-kappaB dysregulation in microRNA-146a-deficient mice drives the development of myeloid malignancies

MicroRNA miR-146a has been implicated as a negative feedback regulator of NF-κB activation. Knockout of the miR-146a gene in C57BL/6 mice leads to histologically and immunophenotypically defined myeloid sarcomas and some lymphomas. The sarcomas are transplantable to immunologically compromised hosts, showing that they are true malignancies. The animals also exhibit chronic myeloproliferation in their bone marrow. Spleen and marrow cells show increased transcription of NF-κB-regulated genes and tumors have higher nuclear p65. Genetic ablation of NF-κB p50 suppresses the myeloproliferation, showing that dysregulation of NF-κB is responsible for the myeloproliferative disease.

PU.1 and C/EBP(alpha) synergistically program distinct response to NF-kappaB activation through establishing monocyte specific enhancers

Unraveling the complexity of transcriptional programs coded by different cell types has been one of the central goals of cell biology. By using genome-wide location analysis, we examined how two different cell types generate different responses to the NF-κB signaling pathway. We showed that, after TNF-α treatment, the NF-κB p65 subunit binds to distinct genome locations and subsequently induces different subsets of genes in human monocytic THP-1 cells versus HeLa cells. Interestingly, the differential p65 binding in two cell types correlates with preexisting cell type-specific enhancers before TNF-α stimulation, marked by histone modifications. We also found that two transcription factors, PU.1 and C/EBPα, appear to synergistically mediate enhancer creation and affect NF-κB target selection in THP-1 cells. In HeLa cells, coexpression of PU.1 and C/EBPα conferred TNF-α responsiveness to a subset of THP-1-specific NF-κB target genes. These results suggest that the diversity of transcriptional programs in mammalian cells arises, at least in part, from preexisting enhancers that are established by cell-specific transcription factors.

Advances in osteoclast biology: old findings and new insights from mouse models

The maintenance of adequate bone mass is dependent upon the controlled and timely removal of old, damaged bone. This complex process is performed by the highly specialized, multinucleated osteoclast. Over the past 15 years, a detailed picture has emerged describing the origins, differentiation pathways and activation stages that contribute to normal osteoclast function. This information has primarily been obtained by the development and skeletal analysis of genetically modified mouse models. Mice harboring mutations in specific genetic loci exhibit bone defects as a direct result of aberrations in normal osteoclast recruitment, formation or function. These findings include the identification of the RANK-RANKL-OPG system as a primary mediator of osteoclastogenesis, the characterization of ion transport and cellular attachment mechanisms and the recognition that matrix-degrading enzymes are essential components of resorptive activity. This Review focuses on the principal observations in osteoclast biology derived from genetic mouse models, and highlights emerging concepts that describe how the osteoclast is thought to contribute to the maintenance of adequate bone mass and integrity throughout life.

Critical role of cRel subunit of NF-κB in sepsis survival

NF-κB is a critical regulator of gene expression during severe infections. NF-κB comprises homo- and heterodimers of proteins from the Rel family. Among them, p50 and p65 have been clearly implicated in the pathophysiology of sepsis. In contrast, the role of cRel in sepsis is still controversial and has been poorly studied in single-pathogen infections. We aimed to investigate the consequences of cRel deficiency in a cecal ligation and puncture (CLP) model of sepsis. We have approached the underlying mechanisms of host defense by analyzing bacterial clearance, systemic inflammation, and the distribution of spleen dendritic cell subsets. Moreover, by using a genome-wide technology, we have also analyzed the CLP-induced modifications in gene expression profiles both in wild-type (wt) and in rel(-/-) mice. The absence of cRel enhances mortality due to polymicrobial sepsis. Despite normal pathogen clearance, cRel deficiency leads to an altered systemic inflammatory response associated with a sustained loss of the spleen lymphoid dendritic cells. Furthermore, a whole-blood microarray study reveals that the differential outcome between wt and rel(-/-) mice during sepsis is preceded by remarkable changes in the expression of hundreds of genes involved in aspects of host-pathogen interaction, such as host survival and lipid metabolism. In conclusion, cRel is a key NF-κB member required for host antimicrobial defenses and a regulatory transcription subunit that controls the inflammatory and immune responses in severe infection.

IkappaB kinase 2 determines oligodendrocyte loss by non-cell-autonomous activation of NF-kappaB in the central nervous system

The IκB kinase complex induces nuclear factor kappa B activation and has recently been recognized as a key player of autoimmunity in the central nervous system. Notably, IκB kinase/nuclear factor kappa B signalling regulates peripheral myelin formation by Schwann cells, however, its role in myelin formation in the central nervous system during health and disease is largely unknown. Surprisingly, we found that brain-specific IκB kinase 2 expression is dispensable for proper myelin assembly and repair in the central nervous system, but instead plays a fundamental role for the loss of myelin in the cuprizone model. During toxic demyelination, inhibition of nuclear factor kappa B activation by conditional ablation of IκB kinase 2 resulted in strong preservation of central nervous system myelin, reduced expression of proinflammatory mediators and a significantly attenuated glial response. Importantly, IκB kinase 2 depletion in astrocytes, but not in oligodendrocytes, was sufficient to protect mice from myelin loss. Our results reveal a crucial role of glial cell-specific IκB kinase 2/nuclear factor kappa B signalling for oligodendrocyte damage during toxic demyelination. Thus, therapies targeting IκB kinase 2 function in non-neuronal cells may represent a promising strategy for the treatment of distinct demyelinating central nervous system diseases.

Maximal adamantyl-substituted retinoid-related molecule-induced apoptosis requires NF-κB noncanonical and canonical pathway activation

NF-κB transcription factors have a critical role in regulating cell survival and apoptosis. We have previously shown that 4-(3-Cl-(1-adamantyl)-4-hydroxyphenyl)-3-chlorocinnamic acid (3-Cl-AHPC), an adamantyl-substituted retinoid molecule, induced apoptosis and required NF-κB activation in prostate and breast carcinoma cells. Here, we show that 3-Cl-AHPC activated both IκB kinase (IKK)α and IKKβ with subsequent activation of the canonical and noncanonical NF-κB pathways in the human breast carcinoma and leukemia cell lines. 3-Cl-AHPC-mediated activation of the NF-κB canonical pathway occurred within 6 h, whereas maximal activation of the NF-κB noncanonical pathway required 48 h. Knockout of IKKα or IKKβ expression in mouse embryonic fibroblast cells and knockdown of IKKα or IKKβ in MDA-MB-468 cells resulted in the inhibition of 3-Cl-AHPC-mediated apoptosis, indicating that activation of canonical and noncanonical pathways are required for maximal 3-Cl-AHPC-mediated apoptosis. 3-Cl-AHPC activation of the noncanonical pathway was preceded by caspase-mediated decrease in the E3-ligase c-IAP1 with subsequent stabilization of NF-κB-inducing kinase (NIK) expression, increased binding of NIK by TRAF3, activation of IKKα, and the resultant increased levels of RelB and p52. Increased expression of c-IAP1 blocked 3-Cl-AHPC-mediated stabilization of NIK levels and 3-Cl-AHPC-mediated apoptosis. Cdc37 expression was required for activation of IKKα and IKKβ by 3-Cl-AHPC. These findings suggest that NF-κB pathways have an important role in 3-Cl-AHPC-mediated apoptosis.

Phosphorylation and polyubiquitination of transforming growth factor beta-activated kinase 1 are necessary for activation of NF-kappaB by the Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) G protein-coupled receptor (vGPCR) protein has been shown to induce several signaling pathways leading to the modulation of host gene expression. The hijacking of these pathways facilitates the viral life cycle and leads to tumorigenesis. In the present work, we show that transforming growth factor β (TGF-β)-activated kinase 1 (TAK1) is an important player in NF-κB activation induced by vGPCR. We observed that the expression of an inactive TAK1 kinase mutant (TAK1M) reduces vGPCR-induced NF-κB nuclear translocation and transcriptional activity. Consequently, the expression of several NF-κB target genes normally induced by vGPCR was blocked by TAK1M expression, including interleukin 8 (IL-8), Gro1, IκBα, COX-2, cIAP2, and Bcl2 genes. Similar results were obtained after downregulation of TAK1 by small interfering RNA (siRNA) technology. The expression of vGPCR recruited TAK1 to the plasma membrane, and vGPCR interacts with TAK1. vGPCR expression also induced TAK1 phosphorylation and lysine 63-linked polyubiquitination, the two markers of the kinase's activation. Finally, inhibition of TAK1 by celastrol inhibited vGPCR-induced NF-κB activation, indicating this natural compound could be used as a potential therapeutic drug against KSHV malignancies involving vGPCR.

The regulatory logic of the NF-kappaB signaling system

NF-kappaB refers to multiple dimers of Rel homology domain (RHD) containing polypeptides, which are controlled by a stimulus-responsive signaling system that mediates the physiological responses to inflammatory intercellular cytokines, pathogen exposure, and developmental signals. The NF-kappaB signaling system operates on transient or short timescales, relevant to inflammation and immune responses, and on longer-term timescales relevant to cell differentiation and organ formation. Here, we summarize our current understanding of the kinetic mechanisms that allow for NF-kappaB regulation at these different timescales. We distinguish between the regulation of NF-kappaB dimer formation and the regulation of NF-kappaB activity. Given the number of regulators and reactions involved, the NF-kappaB signaling system is capable of integrating a multitude of signals to tune NF-kappaB activity, signal dose responsiveness, and dynamic control. We discuss the prevailing mechanisms that mediate signaling cross talk.

Inhibition of NF-κB signaling retards eosinophilic dermatitis in SHARPIN-deficient mice

The NF-κB pathway performs pivotal roles in diverse physiological processes such as immunity, inflammation, proliferation, and apoptosis. NF-κB is kept inactive in the cytoplasm through association with inhibitors (IκB), and translocates to the nucleus to activate its target genes after the IκBs are phosphorylated and degraded. Here, we demonstrate that loss of function of SHANK-associated RH domain interacting protein (SHARPIN) leads to activation of NF-κB signaling in skin, resulting in the development of an idiopathic hypereosinophilic syndrome (IHES) with eosinophilic dermatitis in C57BL/KaLawRij-Sharpin(cpdm)/RijSunJ mice, and clonal expansion of B-1 B cells and CD3(+)CD4(-)CD8(-) T cells. Transcription profiling in skin revealed constitutive activation of classical NF-κB pathways, predominantly by overexpressed members of IL1 family. Compound-null mutants for both the IL1 receptor accessory protein (Il1rap(tm1Roml)) and SHARPIN (Sharpin(cpdm)) resulted in mice having decreased skin disease severity. Inhibition of IκBA degradation by the proteasome inhibitor bortezomib alleviated the dermatitis in Sharpin(cpdm) mice. These results indicate that absence of SHARPIN causes IHES with eosinophilic dermatitis by NF-κB activation, and bortezomib may be an effective treatment for skin problems of IHES.

Regulation of the IL-21 gene by the NF-κB transcription factor c-Rel

IL-21 is a member of the common gamma-chain-dependent cytokine family and is a key modulator of lymphocyte development, proliferation, and differentiation. IL-21 is highly expressed in activated CD4(+) T cells and plays a critical role in the expansion and differentiation of the Th cell subsets, Th17 and follicular helper T (T(FH)) cells. Because of its potent activity in both myeloid and lymphoid cell immune responses, it has been implicated in a number of autoimmune diseases and has also been used as a therapeutic agent in the treatment of some cancers. In this study, we demonstrate that c-Rel, a member of the NF-kappaB family of transcription factors, is required for IL-21 gene expression in T lymphocytes. IL-21 mRNA and protein levels are reduced in the CD4(+) cells of rel(-/-) mice when compared with rel(+/+) mice in both in vitro and in vivo models. A c-Rel binding site identified in the proximal promoter of il21 is confirmed to bind c-Rel in vitro and in vivo and to regulate expression from the il21 promoter in T cells. Downstream of IL-21 expression, Th17, T(FH), and germinal center B cell development are also impaired in rel(-/-) mice. The administration of IL-21 protein rescued the development of T(FH) cells but not germinal center B cells. Taken together, c-Rel plays an important role in the expression of IL-21 in T cells and subsequently in IL-21-dependent T(FH) cell development.

Inhibitor of growth-4 promotes IkappaB promoter activation to suppress NF-kappaB signaling and innate immunity

Ing4 is a member of the inhibitor of growth (ING) family of chromatin-modifying proteins. Biochemical experiments indicate that Ing4 is a subunit of the HB01-JADE-hEAF6 histone acetyltransferase complex responsible for most nucleosomal histone H4 acetylation in eukaryotes, and transfection studies suggest that Ing4 may regulate a wide variety of cellular processes, including DNA repair, apoptosis, cell-cycle regulation, metastasis, angiogenesis, and tumor suppression. However, in vivo evidence for a physiological role for Ing4 in cell-growth regulation is lacking. We have generated Ing4-deficient mice to explore the role of Ing4 in development, tumorigenesis, and in NF-kappaB signaling. Ing4-null mice develop normally and are viable. Although mice deficient for Ing4 fail to form spontaneous tumors, they are hypersensitive to LPS treatment and display elevated cytokine responses. Macrophages isolated from Ing4-null mice have increased levels of nuclear p65/RelA protein, resulting in increased RelA binding to NF-kappaB target promoters and up-regulation of cytokine gene expression. However, increased promoter occupancy by RelA in LPS-stimulated, Ing4-null cells does not always correlate with increased NF-kappaB target-gene expression, as RelA activation of a subset of cytokine promoters also requires Ing4 for proper histone H4 acetylation. Furthermore, activation of the IkappaB alpha promoter by RelA is also Ing4-dependent, and LPS-stimulated, Ing4-null cells have reduced levels of IkappaB alpha promoter H4 acetylation and IkappaB gene expression. Thus, Ing4 negatively regulates the cytokine-mediated inflammatory response in mice by facilitating NF-kappaB activation of IkappaB promoters, thereby suppressing nuclear RelA levels and the activation of select NF-kappaB target cytokines.

Defective regulation of CXCR2 facilitates neutrophil release from bone marrow causing spontaneous inflammation in severely NF-kappa B-deficient mice

NF-kappaB is a major regulator of innate and adaptive immunity. Neutrophilic granulocytes (neutrophils) constitutively express RelA/p65 (Rela), c-Rel (Crel), and p50 (Nfkappab1) but not p52 (Nfkappab2) subunits. In this paper, we describe Crel(-/-)Nfkappab1(-/-)Rela(+/-) mice that have the most severe genetic neutrophil NF-kappaB deficiency compatible with life, Rela(-/-) mice being embryonic lethal. Crel(-/-)Nfkappab1(-/-)Rela(+/-) mice developed spontaneous dermal and intestinal inflammation associated with chronic neutrophilia, elevated CXCL1, and G-CSF. The bone marrow contained fewer nucleated cells and was enriched in myeloid progenitor cells. Neutrophilia was preserved when Crel(-/-)Nfkappab1(-/-)Rela(+/-) bone marrow was transferred into wild-type mice, but mixed bone marrow chimeras receiving wild-type and Crel(-/-)Nfkappab1(-/-)Rela(+/-) bone marrow showed normal circulating neutrophil numbers, excluding an intrinsic proliferation advantage. In mixed bone marrow chimeras, Crel(-/-)Nfkappab1(-/-)Rela(+/-) neutrophils were preferentially mobilized from the bone marrow in response to CXCL1 injection, LPS-induced lung inflammation, and thioglycollate-induced peritonitis. Crel(-/-)Nfkappab1(-/-)Rela(+/-) neutrophils expressed higher levels of the CXCL1 receptor CXCR2 both under resting and stimulated conditions and failed to downregulate CXCR2 during inflammation. Treatment with an anti-CXCR2 Ab abolished preferential mobilization of Crel(-/-)Nfkappab1(-/-)Rela(+/-) neutrophils in peritonitis in mixed chimeric mice and neutrophilia in Crel(-/-)Nfkappab1(-/-)Rela(+/-) mice. We conclude that severe NF-kappaB deficiency facilitates neutrophil mobilization, which causes elevated numbers of preactivated neutrophils in blood and tissues, leading to spontaneous inflammation. These neutrophil effects may limit the usefulness of global NF-kappaB inhibitors for the treatment of inflammatory diseases.

Mycobacterium tuberculosis upregulates microglial matrix metalloproteinase-1 and -3 expression and secretion via NF-kappaB- and Activator Protein-1-dependent monocyte networks

Inflammatory tissue destruction is central to pathology in CNS tuberculosis (TB). We hypothesized that microglial-derived matrix metalloproteinases (MMPs) have a key role in driving such damage. Analysis of all of the MMPs demonstrated that conditioned medium from Mycobacterium tuberculosis-infected human monocytes (CoMTb) stimulated greater MMP-1, -3, and -9 gene expression in human microglial cells than direct infection. In patients with CNS TB, MMP-1/-3 immunoreactivity was demonstrated in the center of brain granulomas. Concurrently, CoMTb decreased expression of the inhibitors, tissue inhibitor of metalloproteinase-2, -3, and -4. MMP-1/-3 secretion was significantly inhibited by dexamethasone, which reduces mortality in CNS TB. Surface-enhanced laser desorption ionization time-of-flight analysis of CoMTb showed that TNF-alpha and IL-1beta are necessary but not sufficient for upregulating MMP-1 secretion and act synergistically to drive MMP-3 secretion. Chemical inhibition and promoter-reporter analyses showed that NF-kappaB and AP-1 c-Jun/FosB heterodimers regulate CoMTb-induced MMP-1/-3 secretion. Furthermore, NF-kappaB p65 and AP-1 c-Jun subunits were upregulated in biopsy granulomas from patients with cerebral TB. In summary, functionally unopposed, network-dependent microglial MMP-1/-3 gene expression and secretion regulated by NF-kappaB and AP-1 subunits were demonstrated in vitro and, for the first time, in CNS TB patients. Dexamethasone suppression of MMP-1/-3 gene expression provides a novel mechanism explaining the benefit of steroid therapy in these patients.

Interleukin-23 production in dendritic cells is negatively regulated by protein phosphatase 2A

IL-12 and IL-23 are produced by activated antigen-presenting cells but the two induce distinct immune responses by promoting Th1 and Th17 cell differentiation, respectively. IL-23 is a heterodimeric cytokine consisting of two subunits: p40 that is shared with IL-12 and p19 unique to IL-23. In this study, we showed that the production of IL-23 but not IL-12 was negatively regulated by protein phosphatase 2A (PP2A) in dendritic cells (DC). PP2A inhibits IL-23 production by suppressing the expression of the IL-23p19 gene. Treating DC with okadaic acid that inhibits the PP2A activity or knocking down the catalytic subunit of PP2A with siRNA enhanced IL-23 but not IL-12 production. Unlike PP2A, MAP kinase phosphatase-1 or CYLD did not show an effect on IL-23 production supporting the specificity of PP2A. PP2A-mediated inhibition requires a newly made protein that is likely responsible for bringing PP2A and IKKbeta together upon LPS stimulation, which then results in the termination of IKK phosphorylation. Thus, our results uncovered an important role of the protein phosphatase in the regulation of IL-23 production and identified PP2A as a previously uncharacterized inhibitor of IL-23p19 expression in DC.

c-Rel but not NF-kappaB1 is important for T regulatory cell development

Regulatory T (Treg) cells are crucial for maintaining peripheral tolerance and controlling T-cell responses. The generation of Treg in the thymus requires TCR triggering and CD28 costimulation. Engagement of these receptors induces a number of signalling pathways, including the activation of NF-kappaB via PKCtheta and the Bcl-10/CARMA1/MALT complex. Previous studies have shown that PKCtheta, Bcl-10 and CARMA1 are important for Treg development. It is unclear, however, whether different members of the NF-kappaB family contribute to Treg development or homeostasis. In this study, we show that Treg numbers are reduced in the absence of c-Rel but not NF-kappaB1 (p50). Furthermore, using mixed bone marrow chimeras from WT and KO animals, we demonstrate that the requirement for PKCtheta, Bcl-10 and c-Rel is T-cell intrinsic, and cannot be rescued by the presence of WT cells. Therefore, c-Rel and NF-kappaB1 have differential roles in Treg development.

Activation of noncanonical NF-kappaB signaling by the oncoprotein Tio

NF-kappaB transcription factors are key regulators of cellular proliferation and frequently contribute to oncogenesis. The herpesviral oncoprotein Tio, which promotes growth transformation of human T cells in a recombinant herpesvirus saimiri background, potently induces canonical NF-kappaB signaling through membrane recruitment of the ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6). Here, we show that, in addition to Tio-TRAF6 interaction, the Tio-induced canonical NF-kappaB signal requires the presence of the regulatory subunit of the inhibitor of kappaB kinase (IKK) complex, NF-kappaB essential modulator (NEMO), and the activity of its key kinase, IKKbeta, to up-regulate expression of endogenous cellular inhibitor of apoptosis 2 (cIAP2) and interleukin 8 (IL-8) proteins. Dependent on TRAF6 and NEMO, Tio enhances the expression of the noncanonical NF-kappaB proteins, p100 and RelB. Independent of TRAF6 and NEMO, Tio mediates stabilization of the noncanonical kinase, NF-kappaB-inducing kinase (NIK). Concomitantly, Tio induces efficient processing of the p100 precursor molecule to its active form, p52, as well as DNA binding of nuclear p52 and RelB. In human T cells transformed by infection with a Tio-recombinant virus, sustained expression of p100, RelB, and cIAP2 depends on IKKbeta activity, yet processing to p52 remains largely unaffected by IKKbeta inhibition. However, long term inhibition of IKKbeta disrupts the continuous growth of the transformed cells and induces cell death. Hence, the Tio oncoprotein triggers noncanonical NF-kappaB signaling through NEMO-dependent up-regulation of p100 precursor and RelB, as well as through NEMO-independent generation of p52 effector.

Up regulation of GW112 Gene by NF kappaB promotes an antiapoptotic property in gastric cancer cells

To clarify the regulatory mechanism of GW112 gene expression, 5'-flanking region of the human GW112 gene was isolated and characterized in the present study. 5'-RACE analysis showed a single transcription start site, which is located 142 nucleotides upstream of the translation initiation site. Transient transfection studies with serial deletion constructs and close examination of the sequences identified a putative NF kappaB binding sequence between -442 and -430, which could be responsible for efficient expression of the GW112 gene. Indeed, GW112 gene was found to be regulated by NF kappaB signals including overexpressed p65 and I kappaB alpha, IKK inhibitor, and proteasome inhibitor. Binding of NF kappaB to its putative site was confirmed by EMSA and ChIP assays. These results suggest that NF kappaB is an essential regulatory factor for GW112 transcription. Based on this finding, we next confirmed that inhibition of GW112 expression could induce apoptosis in the presence of cytotoxic agent in gastric cancer cells. Furthermore, knocking-down or overexpression of GW112 gene in gastric cancer cells demonstrated that GW112 has an antiapoptotic property against the cytotoxic agents-induced apoptosis. Taken together, these results suggest that GW112 could be an important mediator in NF kappaB-dependent tumorigenesis of digestive tract tissues.

The c-Rel subunit of nuclear factor-kappaB regulates murine liver inflammation, wound-healing, and hepatocyte proliferation

In this study, we determined the role of the nuclear factor-kappaB (NF-kappaB) subunit c-Rel in liver injury and regeneration. In response to toxic injury of the liver, c-Rel null (c-rel(-/-)) mice displayed a defect in the neutrophilic inflammatory response, associated with impaired induction of RANTES (Regulated upon Activation, Normal T-cell Expressed, and Secreted; also known as CCL5). The subsequent fibrogenic/wound-healing response to both chronic carbon tetrachloride and bile duct ligation induced injury was also impaired and this was associated with deficiencies in the expression of fibrogenic genes, collagen I and alpha-smooth muscle actin, by hepatic stellate cells. We additionally report that c-Rel is required for the normal proliferative regeneration of hepatocytes in response to toxic injury and partial hepatectomy. Absence of c-Rel was associated with blunted and delayed induction of forkhead box M1 (FoxM1) and its downstream targets cyclin B1 and Cdc25C. Furthermore, isolated c-rel(-/-) hepatocytes expressed reduced levels of FoxM1 and a reduced rate of basal and epidermal growth factor-induced DNA synthesis. Chromatin immunoprecipitation revealed that c-Rel binding to the FoxM1 promoter is induced in the regenerating liver.

CONCLUSION

c-Rel has multiple functions in the control of liver homeostasis and regeneration and is a transcriptional regulator of FoxM1 and compensatory hepatocyte proliferation.

NF-kappaB subtypes regulate CCCTC binding factor affecting corneal epithelial cell fate

CCCTC binding factor (CTCF) controls DNA imprinting, insulates important gene expression, and mediates growth factor- and stress-induced cell fate. However, regulatory mechanisms involved in intracellular CTCF activity are largely unknown. In this study, we show that epidermal growth factor (EGF)-induced increase and UV stress-induced decrease in CTCF activities mediate human corneal epithelial cell proliferation and apoptosis, respectively. CTCF is regulated by activation of different NF-kappaB subtypes via stimulation by EGF and UV stress. EGF-induced formation of a p65/p50 heterodimer activated CTCF transcription to promote cellular proliferation. This was accomplished by the heterodimer binding to a kappaB site in the promoter region of CTCF gene. In contrast, UV stress induced formation of a p50/p50 homodimer, which suppressed CTCF expression leading to apoptosis. Thus, CTCF by itself plays a central role in mediating the dichotomous effects of growth factor- and stress-stimulated NF-kappaB activation on cell survival and death. These results suggest that CTCF is a downstream component of the NF-kappaB pathway involved in the core transcriptional network of cell fate.

Roles of the NF-kappaB pathway in lymphocyte development and function

This article focuses on the functions of NF-kappaB that vitally impact lymphocytes and thus adaptive immunity. NF-kappaB has long been known to be essential for many of the responses of mature lymphocytes to invading pathogens. In addition, NF-kappaB has important functions in shaping the immune system so it is able to generate adaptive responses to pathogens. In both contexts, NF-kappaB executes critical cell-autonomous functions within lymphocytes as well as within supportive cells, such as antigen-presenting cells or epithelial cells. It is these aspects of NF-kappaB's physiologic impact that we address in this article.

c-Rel is required for the development of thymic Foxp3+ CD4 regulatory T cells

During thymopoiesis, a unique program of gene expression promotes the development of CD4 regulatory T (T reg) cells. Although Foxp3 maintains a pattern of gene expression necessary for T reg cell function, other transcription factors are emerging as important determinants of T reg cell development. We show that the NF-κB transcription factor c-Rel is highly expressed in thymic T reg cells and that in c-rel^−/− mice, thymic T reg cell numbers are markedly reduced as a result of a T cell–intrinsic defect that is manifest during thymocyte development. Although c-Rel is not essential for TGF-β conversion of peripheral CD4⁺CD25⁻ T cells into CD4⁺Foxp3⁺ cells, it is required for optimal homeostatic expansion of peripheral T reg cells. Despite a lower number of peripheral T reg cells in c-rel^−/− mice, the residual peripheral c-rel^−/− T reg cells express normal levels of Foxp3, display a pattern of cell surface markers and gene expression similar to those of wild-type T reg cells, and effectively suppress effector T cell function in culture and in vivo. Collectively, our results indicate that c-Rel is important for both the thymic development and peripheral homeostatic proliferation of T reg cells.

B cell signaling and fate decision

Antigen receptors on the surface of B lymphocytes trigger adaptive immune responses after encountering their cognate antigens but also control a series of antigen-independent checkpoints during B cell development. These physiological processes are regulated by the expression and function of cell surface receptors, intracellular signaling molecules, and transcription factors. The function of these proteins can be altered by a dynamic array of post-translational modifications, using two interconnected mechanisms. These modifications can directly induce an altered conformational state in the protein target of the modification itself. In addition, they can create new binding sites for other protein partners, thereby contributing to where and when such multiple protein assemblies are activated within cells. As a new type of post-transcriptional regulator, microRNAs have emerged to influence the development and function of B cells by affecting the expression of target mRNAs.

Selectivity of the NF-{kappa}B response

NF-kappaB is activated by many stimuli and NF-kappaB binding sites have been identified in a wide variety of genes. Yet, NF-kappaB-dependent gene expression must be stimulus- and cell-type-specific. In others words, the cellular response to different NF-kappaB activating stimuli, such as TNFalpha, IL-1, and LPS, must be different; and the response of different cell types, such as lymphocytes, fibroblasts, or epithelial cells, to the same NF-kappaB-inducing stimulus must also be different. Finally, kinetics of gene expression must be accounted for, so that all NF-kappaB-dependent genes are not activated simultaneously even if cell type and stimulus are constant. Here, we explore the mechanistic framework in which such regulatory aspects of NF-kappaB-dependent gene expression have been analyzed because they are likely to form the basis for physiological responses.

The pleiotropy of dioxin toxicity--xenobiotic misappropriation of the aryl hydrocarbon receptor's alternative physiological roles

The aryl hydrocarbon receptor is a signal regulated transcription factor that has best been characterised as regulating the xenobiotic response to a variety of planar aromatic hydrocarbons. There is compelling evidence that it mediates most, if not all, of the toxic effects of dioxin (2,3,7,8-tetrachlorodibenzo-p-dioxin). Dioxin exposure results in a wide variety of toxic outcomes including severe wasting syndrome, chloracne, thymic involution, severe immune suppression, reduced fertility, hepatotoxicity, teratogenicity, tumour promotion and death. The pleiotropy of toxic outcomes implies the disruption of a wide range of normal physiological functions. The aryl hydrocarbon receptor has developmentally restricted expression as well as developmental defects in gene-targeted mice. It has a wide range of target genes that do not fit into the classical xenobiotic metabolising gene battery and has recently been shown to interact with NF-kappa B and the estrogen receptor. There is also evidence for its activation in the absence of exogenous ligand, all of which point to various roles outside xenobiotic metabolism. Ligands so far identified display differential activation potential with respect to receptor activity. This article addresses activities of the aryl hydrocarbon receptor that are outside the xenobiotic response. Known physiological roles are discussed as well as how their disruption contributes to the pleiotropic toxicity of TCDD.

Targeting NF-kappa B with a natural triterpenoid alleviates skin inflammation in a mouse model of psoriasis

Psoriasis vulgaris is a common chronic inflammatory skin disease involving cytokines and an activated cellular immune system. At variance to skin from patients with atopic dermatitis or from healthy subjects, human psoriatic skin lesions exhibit strong activation of transcription factor NF-kappaB that is mainly confined to dermal macrophages, whereas only a few dendritic cells but no CD3+ lymphocytes show activated NF-kappaB. Since NF-kappaB signaling is required for the induction and/or function of many cytokines and aberrant cytokine expression has been proposed as an underlying cause of psoriasis, we investigated whether NF-kappaB targeting would affect the course of the disease in the CD18 hypomorphic (CD18(hypo)) mouse model of psoriasis. When mice with severe psoriasiform lesions were treated systemically or locally with the IkappaB kinase inhibitor acetyl-11-keto-beta-boswellic acid (AKbetaBA), NF-kappaB signaling and the subsequent NF-kappaB-dependent cytokine production as shown by the TNF-alpha production of macrophages were profoundly suppressed. Additionally, application of the compound counteracted the intradermal MCP-1, IL-12, and IL-23 expression in previously lesional skin areas, led to resolution of the abundant immune cell infiltrates, and significantly reduced the increased proliferation of the keratinocytes. Overall, the AKbetaBA treatment was accompanied by a profound improvement of the psoriasis disease activity score in the CD18(hypo) mice with reconstitution of a nearly normal phenotype within the chosen observation period. Our data demonstrate that NF-kappaB signaling is pivotal for the pathogenesis in the CD18(hypo) mouse model of psoriasis. Therefore, targeting NF-kappaB might provide an effective strategy for the treatment of psoriasis.

Nuclear factor kappaB inhibitors alleviate and the proteasome inhibitor PS-341 exacerbates radiation toxicity in zebrafish embryos

Inflammatory changes are a major component of the normal tissue response to ionizing radiation, and increased nuclear factor kappaB (NF-kappaB) activity is an important mediator of inflammatory responses. Here, we used zebrafish embryos to assess the capacity of two different classes of pharmacologic agents known to target NF-kappaB to modify radiation toxicity in the vertebrate organism. These were proteasome inhibitors, including lactacystin, MG132, and PS-341 (Bortezomib/VELCADE), and direct inhibitors of NF-kappaB activity, including ethyl pyruvate (EP) and the synthetic triterpenoid CDDO-TFEA (RTA401), among others. The proteasome inhibitors either did not significantly affect radiation sensitivity of zebrafish embryos (MG132, lactacystin) or rendered zebrafish embryos more sensitive to the lethal effects of ionizing radiation (PS-341). Radiosensitization by PS-341 was reduced in fish with impaired p53 expression or function but not associated with enhanced expression of select p53 target genes. In contrast, the direct NF-kappaB inhibitors EP and CDDO-TFEA significantly improved overall survival of lethally irradiated zebrafish embryos. In addition, direct NF-kappaB inhibition reduced radiation-induced apoptosis in the central nervous system, abrogated aberrations in body axis development, restored metabolization and secretion of a reporter lipid through the gastrointestinal system, and improved renal clearance compromised by radiation. In contrast to amifostine, EP and CDDO-TFEA not only protected against but also mitigated radiation toxicity when given 1 to 2 hours postexposure. Finally, four additional IkappaB kinase inhibitors with distinct mechanisms of action similarly improved overall survival of lethally irradiated zebrafish embryos. In conclusion, inhibitors of canonical pathways to NF-kappaB activation may be useful in alleviating radiation toxicity in patients.