Regulation of Ikappa B kinase (IKK)gamma /NEMO function by IKKbeta -mediated phosphorylation

Necrotizing Enterocolitis: Clinical Features, Histopathological Characteristics, and Genetic Associations

Necrotizing enterocolitis (NEC) is an inflammatory bowel necrosis seen in premature infants. Although the etiopathogenesis of NEC is unclear, genetic factors may alter a patient's susceptibility, clinical course, and outcomes. This review draws from existing studies focused on individual genes and others based on microarray-based high-throughput discovery techniques. We have included evidence from our own studies and from an extensive literature search in the databases PubMed, EMBASE, and Scopus. To avoid bias in the identification of studies, keywords were short-listed a priori from anecdotal experience and PubMed's Medical Subject Heading (MeSH) thesaurus.

Inflammatory osteolysis is regulated by site-specific ISGylation of the scaffold protein NEMO

Inflammatory osteolysis is governed by exacerbated osteoclastogenesis. Ample evidence points to central role of NF-κB in such pathologic responses, yet the precise mechanisms underpinning specificity of these responses remain unclear. We propose that motifs of the scaffold protein IKKγ/NEMO partly facilitate such functions. As proof-of-principle, we used site-specific mutagenesis to examine the role of NEMO in mediating RANKL-induced signaling in mouse bone marrow macrophages, known as osteoclast precursors. We identified lysine (K)270 as a target regulating RANKL signaling as K270A substitution results in exuberant osteoclastogenesis in vitro and murine inflammatory osteolysis in vivo. Mechanistically, we discovered that K270A mutation disrupts autophagy, stabilizes NEMO, and elevates inflammatory burden. Specifically, K270A directly or indirectly hinders binding of NEMO to ISG15, a ubiquitin-like protein, which we show targets the modified proteins to autophagy-mediated lysosomal degradation. Taken together, our findings suggest that NEMO serves as a toolkit to fine-tune specific signals in physiologic and pathologic conditions.

The human skeleton contains over 200 bones that together act as an internal framework for the body. Over our lifetime, the body constantly removes older bone tissue from the skeleton and replaces it with new bone tissue. This “bone remodeling” also controls how bones are repaired after being damaged by injuries, disease or normal wear and tear.

Cells known as osteoclasts are responsible for breaking down old bone tissue and participate in repairing damaged bone. A cellular pathway known as NF-kB signaling stimulates other cells called “bone marrow macrophages” to become osteoclasts. A certain level of NF-kB signaling is required to maintain a healthy skeleton. However, under certain inflammatory conditions, the level of NF-kB signaling becomes too high causing hyperactive osteoclasts to accumulate and inflict severe bone breakdown. This abnormal osteoclast activity leads to eroded and fragile bones and joints, as is the case in diseases such as rheumatoid arthritis and osteoporosis.

Previous studies have shown that a protein called NEMO is a core component of the NF-kB signal pathway, but the precise role of NEMO in the diseased response remained unclear. Adapala, Swarnkar, Arra et al. have now used site-directed mutagenesis approach to study the role of NEMO in bone marrow macrophages in mice. The experiments showed that one specific site within the NEMO protein, referred to as lysine 270, is crucial for its role in controlling osteoclasts and the breakdown of bone tissue. Mutating NEMO at lysine 270 led to uncontrolled NF-kB signaling in the bone marrow macrophages. Further experiments showed that lysine 270 served as a sensor to allow NEMO to bind another protein called ISG15, which in turn helped to decrease NF-kB signaling and slow down the erosion of the bone.

These findings suggest that site-specific targeting of NEMO, rather than inhibiting the whole NF-kB pathway, may help to reduce the symptoms of bone disease while maintaining the beneficial roles of this essential pathway. However, additional research is required to identify NEMO sites responsible for controlling the inflammatory component.

NEMO Links Nuclear Factor-κB to Human Diseases

The nuclear factor (NF)-κB essential modulator (NEMO) is a key regulator in NF-κB-mediated signaling. By transmitting extracellular or intracellular signals, NEMO can control NF-κB-regulated genes. NEMO dysfunction is associated with inherited diseases such as incontinentia pigmenti (IP), ectodermal dysplasia, anhidrotic, with immunodeficiency (EDA-ID), and some cancers. We focus on molecular studies, human case reports, and mouse models emphasizing the significance of NEMO molecular interactions and modifications in health and diseases. This knowledge opens new opportunities to engineer suitable drugs that may putatively target precise NEMO functions attributable to various diseases, while leaving other functions intact, and eliminating cytotoxicity. Indeed, with the advent of novel gene editing tools such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein (Cas)9, treating some inherited diseases may in the long run, become a reality.

Kenny mediates selective autophagic degradation of the IKK complex to control innate immune responses

Selective autophagy is a catabolic process with which cellular material is specifically targeted for degradation by lysosomes. The function of selective autophagic degradation of self-components in the regulation of innate immunity is still unclear. Here we show that Drosophila Kenny, the homolog of mammalian IKKγ, is a selective autophagy receptor that mediates the degradation of the IκB kinase complex. Selective autophagic degradation of the IκB kinase complex prevents constitutive activation of the immune deficiency pathway in response to commensal microbiota. We show that autophagy-deficient flies have a systemic innate immune response that promotes a hyperplasia phenotype in the midgut. Remarkably, human IKKγ does not interact with mammalian Atg8-family proteins. Using a mathematical model, we suggest mechanisms by which pathogen selection might have driven the loss of LIR motif functionality during evolution. Our results suggest that there may have been an autophagy-related switch during the evolution of the IKKγ proteins in metazoans.

Kenny mediates selective autophagic degradation of the IKK complex to control innate immune responses

Selective autophagy is a catabolic process with which cellular material is specifically targeted for degradation by lysosomes. The function of selective autophagic degradation of self-components in the regulation of innate immunity is still unclear. Here we show that Drosophila Kenny, the homolog of mammalian IKKγ, is a selective autophagy receptor that mediates the degradation of the IκB kinase complex. Selective autophagic degradation of the IκB kinase complex prevents constitutive activation of the immune deficiency pathway in response to commensal microbiota. We show that autophagy-deficient flies have a systemic innate immune response that promotes a hyperplasia phenotype in the midgut. Remarkably, human IKKγ does not interact with mammalian Atg8-family proteins. Using a mathematical model, we suggest mechanisms by which pathogen selection might have driven the loss of LIR motif functionality during evolution. Our results suggest that there may have been an autophagy-related switch during the evolution of the IKKγ proteins in metazoans.

Selective autophagy describes the selective degradation of cellular components upon stress or nutrient deficiency, but whether it modulates innate immunity is unclear. Here the authors show that Drosophila Kenny may be an evolution-selected autophagy receptor for the down-regulation of innate NF-κB activation

NF-κB and the Transcriptional Control of Inflammation

The NF-κB transcription factor was discovered 30 years ago and has since emerged as the master regulator of inflammation and immune homeostasis. It achieves this status by means of the large number of important pro- and antiinflammatory factors under its transcriptional control. NF-κB has a central role in inflammatory diseases such as rheumatoid arthritis, inflammatory bowel disease, and autoimmunity, as well as diseases comprising a significant inflammatory component such as cancer and atherosclerosis. Here, we provide an overview of the studies that form the basis of our understanding of the role of NF-κB subunits and their regulators in controlling inflammation. We also describe the emerging importance of posttranslational modifications of NF-κB in the regulation of inflammation, and highlight the future challenges faced by researchers who aim to target NF-κB transcriptional activity for therapeutic benefit in treating chronic inflammatory diseases.

Ac-SDKP suppresses TNF-α-induced ICAM-1 expression in endothelial cells via inhibition of IκB kinase and NF-κB activation

N-acetyl-seryl-aspartyl-lysyl-proline (Ac-SDKP) is a naturally occurring tetrapeptide that prevents inflammation and fibrosis in hypertension and other cardiovascular diseases. We previously showed that, in angiotensin II-induced hypertension, Ac-SDKP decreased the activation of nuclear transcription factor NF-κB, whereas, in experimental autoimmune myocarditis and hypertension animal models, it also reduced the expression of endothelial leukocyte adhesion molecule ICAM-1. However, the mechanisms by which Ac-SDKP downregulated ICAM-1 expression are still unclear. TNF-α is a proinflammatory cytokine that induces ICAM-1 expression in various cell types via TNF receptor 1 and activation of the classical NF-κB pathway. We hypothesized that in endothelial cells Ac-SDKP suppresses TNF-α-induced ICAM-1 expression by decreasing IKK phosphorylation that as a consequence leads to a decrease of IκB phosphorylation and NF-κB activation. To test this hypothesis, human coronary artery endothelial cells were treated with Ac-SDKP and then stimulated with TNF-α. We found that TNF-α-induced ICAM-1 expression was significantly decreased by Ac-SDKP in a dose-dependent manner. Ac-SDKP also decreased TNF-α-induced NF-κB translocation from cytosol to nucleus, as assessed by electrophoretic mobility shift assay, which correlated with a decrease in IκB phosphorylation. In addition, we found that Ac-SDKP decreased TNF-α-induced IKK phosphorylation and IKK-β expression. However, Ac-SDKP had no effect on TNF-α-induced phosphorylation of p38 MAP kinase or ERK. Thus we conclude that Ac-SDKP inhibition of TNF-α activation of canonical, i.e., IKK-β-dependent, NF-κB pathway and subsequent decrease in ICAM-1 expression is achieved via inhibition of IKK-β.

A novel pVHL-independent but NEMO-driven pathway in renal cancer promotes HIF stabilization

Activation of hypoxia-inducible factor (HIF) is due to loss of von Hippel-Lindau protein (pVHL) function in most clear cell renal cell carcinomas (ccRCCs). Here we describe a novel pVHL-independent mechanism of HIF regulation and identify nuclear factor (NF)-κB essential modulator (NEMO) as a hitherto unknown oncogenic factor influencing human ccRCC progression. Over 60% of human ccRCCs (n=157) have negative or weak NEMO protein expression by immunohistochemistry. Moderate/strong NEMO protein expression is more frequent in VHL wild-type ccRCCs. We show that NEMO stabilizes HIFα via direct interaction and independently of NF-κB signaling in vitro. NEMO prolongs tumor cell survival via regulation of apoptosis and activation of epithelial-to-mesenchymal transition, facilitating tumor metastasis. Our findings suggest that NEMO-driven HIF activation is involved in progression of ccRCC. Therefore, NEMO may represent a clinically relevant link between NF-κB and the VHL/HIF pathways. Targeting NEMO with specific inhibitors in patients with metastatic ccRCC could be a novel treatment approach in patients with ccRCC expressing functional pVHL.

Protein kinase networks that limit TLR signalling

TLRs (Toll-like receptors) detect invading micro-organisms which triggers the production of pro-inflammatory mediators needed to combat infection. Although these signalling networks are required to protect the host against invading pathogens, dysregulation of TLR pathways contributes to the development of chronic inflammatory diseases and autoimmune disorders. Molecular mechanisms have therefore evolved to restrict the strength of TLR signalling. In the present review, I highlight recent advances in our understanding of the protein kinase networks required to suppress the innate immune response by negatively regulating TLR signalling and/or promoting the secretion of anti-inflammatory cytokines. I present my discoveries on the key roles of the IKK (inhibitor of nuclear factor κB kinase)-related kinases and the SIKs (salt-inducible kinases) in limiting innate immunity within the greater context of the field.

MEKK3 and TAK1 synergize to activate IKK complex in Helicobacter pylori infection

Helicobacter pylori colonises the gastric epithelial cells of half of the world's population and represents a risk factor for gastric adenocarcinoma. In gastric epithelial cells H. pylori induces the immediate early response transcription factor nuclear factor of kappa light polypeptide gene enhancer in B-cells (NF-κB) and the innate immune response. We show that H. pylori induces in a type IV secretion system-dependent (T4SS) and cytotoxin associated gene A protein (CagA)-independent manner a transient activation of the inhibitor of NF-κB (IκBα) kinase (IKK)-complex. IKKα and IKKβ expression stabilises the regulatory IKK complex subunit NF-κB essential modulator (NEMO). We provide evidence for an intimate mutual control of the IKK complex by mitogen-activated protein kinase kinase kinase 3 (MEKK3) and transforming growth factor β activated kinase 1 (TAK1). TAK1 interacts transiently with the E3 ubiquitin ligase tumor necrosis factor receptor-associated factor 6 (TRAF6). Protein modifications in the TAK1 molecule, e.g. TAK1 autophosphorylation and K63-linked ubiquitinylation, administer NF-κB signalling including transient recruitment of the IKK-complex. Overall, our data uncover H. pylori-induced interactions and protein modifications of the IKK complex, and its upstream regulatory factors involved in NF-κB activation.

Molecular control of the NEMO family of ubiquitin-binding proteins

Research over the past decade has revealed how NF-κB essential modulator (NEMO; also known as IKKγ) regulates the IKKα-IKKβ signalling axis in the innate immune system. The discovery that NEMO is a polyubiquitin-binding protein and that the IKK complex is modulated by other protein kinases that are themselves controlled by polyubiquitin chains has provided a deeper molecular understanding of the non-degradative roles of ubiquitylation. New mechanistic insights of NEMO and related polyubiquitin-binding proteins have become a paradigm for how the interplay between phosphorylation and ubiquitylation controls cell signalling networks in health and disease.

Novel Phosphorylations of IKKγ/NEMO

Central to NF-κB signaling pathways is IKKγ/NEMO, a regulatory subunit of the cytoplasmic IκB kinase (IKK) complex, which undergoes various posttranslational modifications, specifically phosphorylation, to regulate its function. Furthermore, Kaposi’s sarcoma-associated herpesvirus (KSHV) FADD-like interleukin-1β (IL-1β) converting enzyme (FLICE) inhibitory protein (vFLIP) activates the NF-κB signaling pathway by directly interacting with IKKγ/NEMO. However, the exact functions of IKKγ/NEMO phosphorylation and its KvFLIP interaction in NF-κB activation remain elusive. Here, we report two novel phosphorylation sites of IKKγ/NEMO and their negative effect on the IKKγ/NEMO-mediated NF-κB signaling pathway. First, the Src family protein tyrosine kinases (SF-PTKs), including Src, Fyn, Lyn, and Fgr, interact with and phosphorylate tyrosine residue 374 (Y374) of IKKγ/NEMO. Mutation of the Y374 residue to phenylalanine (Y374F) specifically abolished SF-PTK-mediated tyrosine phosphorylation, leading to increased tumor necrosis factor alpha (TNF-α)-induced NF-κB activity. Moreover, our mass spectrometry analysis found that the serine 377 residue (S377) of IKKγ/NEMO underwent robust phosphorylation upon KvFLIP expression. Replacement of the IKKγ/NEMO S377 residue by alanine (S377A) or glutamic acid (S377E) resulted in a significant increase or decrease of NF-κB activity and TNF-α-mediated IL-6 cytokine production, respectively. Our study thus demonstrates that the Y374 or S377 residue located at the C-terminal proline-rich domain of human IKKγ/NEMO undergoes phosphorylation upon TNF-α treatment or KvFLIP expression, respectively, resulting in the suppression of IKKγ/NEMO activity to induce NF-κB activation. This study suggests the potential phosphorylation-mediated feedback negative regulation of IKKγ/NEMO activity in the NF-κB signaling pathway.

Since unchecked regulation of NF-κB has been linked to uncontrolled proliferation and cell death, the downregulation of the NF-κB signaling pathway is as important as its activation. Specifically, the phosphorylation-mediated modification of IKKγ/NEMO is a critical regulatory mechanism of NF-κB activity. Here, we report two novel phosphorylations of IKKγ/NEMO and their negative effects on the NF-κB signaling pathway. First, the Src family protein tyrosine kinase interacts with and phosphorylates tyrosine residue 374 of IKKγ/NEMO, suppressing tumor necrosis factor alpha (TNF-α)-induced NF-κB activity. Additionally, Kaposi’s sarcoma-associated herpesvirus (KSHV) FADD-like interleukin-1β (IL-1β) converting enzyme (FLICE) inhibitory protein (KvFLIP) expression induces a robust phosphorylation of the serine 377 residue of IKKγ/NEMO, resulting in a significant decrease of NF-κB activity. Our study thus demonstrates that the Y374 or S377 residue of IKKγ/NEMO undergoes phosphorylation upon TNF-α treatment or KvFLIP expression, respectively, resulting in the suppression of IKKγ/NEMO activity to induce NF-κB activation. This also suggests the potential phosphorylation-mediated feedback negative regulation of IKKγ/NEMO activity in the NF-κB signaling pathway.

HOIL-1L interacting protein (HOIP) is essential for CD40 signaling

CD40 is a cell surface receptor important in the activation of antigen-presenting cells during immune responses. In macrophages and dendritic cells, engagement of CD40 by its ligand CD154 provides signals critical for anti-microbial and T cell-mediated immune responses, respectively. In B cells, CD40 signaling has a major role in regulating cell proliferation, antibody production, and memory B cell development. CD40 engagement results in the formation of a receptor-associated complex that mediates activation of NF-κB, stress-activated protein kinases, and other signaling molecules. However, the mechanisms that link CD40 to these signaling events have been only partially characterized. Known components of the CD40 signaling complex include members of the TNF receptor-associated factor (TRAF) family of proteins. We previously showed that the TRAF family member TRAF2 mediates recruitment of HOIL-1L-interacting protein (HOIP) to the cytoplasmic domain of CD40, suggesting that HOIP has a role in the CD40 signaling pathway. To determine the role of HOIP in CD40 signaling, we used somatic cell gene targeting to generate mouse B cell lines deficient in HOIP. We found that the CD40-induced upregulation of CD80 and activation of germline immunoglobulin epsilon transcription were defective in HOIP-deficient cells. We also found that the CD40-mediated activation of NF-κB and c-Jun kinase was impaired. Recruitment of IκB kinase proteins to the CD40 signaling complex was undetectable in HOIP-deficient cells, potentially explaining the defect in NF-κB activation. Restoration of HOIP expression reversed the defects in cellular activation and signaling. These results reveal HOIP as a key component of the CD40 signaling pathway.

Norisoboldine inhibits the production of pro-inflammatory cytokines in lipopolysaccharide-stimulated RAW 264.7 cells by down-regulating the activation of MAPKs but not NF-κB

Norisoboldine is the main isoquinoline alkaloid occurring in Radix Linderae, the dry roots of Lindera aggregata (Lauraceae family). It has been previously implicated to be able to ameliorate the synovial inflammation and abnormal immune conditions in collagen-induced arthritis of mice. To get insight to the potential anti-inflammatory mechanisms of this alkaloid compound, the present study was undertaken to explore the effects of norisoboldine on the production of pro-inflammatory cytokines from macrophages stimulated by lipopolysaccharide. In vitro, norisoboldine substantially reduced the production of nitric oxide (NO), tumor necrosis factor (TNF)-α as well as interleukin (IL)-1β from RAW264.7 macrophage cells in a concentration-dependent manner, whereas it only slightly reduced the production of interleukin-6 (IL-6) at both protein and transcription levels. Of note, the preventive effects of norisoboldine on the release of pro-inflammatory cytokines were correlated with the inhibitory action on the phosphorylations of mitogen-activated protein (MAP) kinases including p38, extracellular signal-regulated kinase (ERK) and c-jun NH(2)-terminal kinase (JNK), but not on the activation and translocation of nuclear factor-κB (NF-κB). It can be therefore concluded that norisoboldine inhibits the macrophage activation and the resultant production of pro-inflammatory cytokines via down-regulating the activation of MAPKs signaling pathways rather than NF-κB.

Novel cross-talk within the IKK family controls innate immunity

Members of the IKK {IκB [inhibitor of NF-κB (nuclear factor κB)] kinase} family play a central role in innate immunity by inducing NF-κB- and IRF [IFN (interferon) regulatory factor]-dependent gene transcription programmes required for the production of pro-inflammatory cytokines and IFNs. However, the molecular mechanisms that activate these protein kinases and their complement of physiological substrates remain poorly defined. Using MRT67307, a novel inhibitor of IKKϵ/TBK1 (TANK {TRAF [TNF (tumour-necrosis-factor)-receptor-associated factor]-associated NF-κB activator}-binding kinase 1) and BI605906, a novel inhibitor of IKKβ, we demonstrate that two different signalling pathways participate in the activation of the IKK-related protein kinases by ligands that activate the IL-1 (interleukin-1), TLR (Toll-like receptor) 3 and TLR4 receptors. One signalling pathway is mediated by the canonical IKKs, which directly phosphorylate and activate IKKϵ and TBK1, whereas the second pathway appears to culminate in the autocatalytic activation of the IKK-related kinases. In contrast, the TNFα-induced activation of the IKK-related kinases is mediated solely by the canonical IKKs. In turn, the IKK-related kinases phosphorylate the catalytic subunits of the canonical IKKs and their regulatory subunit NEMO (NF-κB essential modulator), which is associated with reduced IKKα/β activity and NF-κB-dependent gene transcription. We also show that the canonical IKKs and the IKK-related kinases not only have unique physiological substrates, such as IκBα, p105, RelA (IKKα and IKKβ) and IRF3 (IKKϵ and TBK1), but also have several substrates in common, including the catalytic and regulatory (NEMO and TANK) subunits of the IKKs themselves. Taken together, our studies reveal that the canonical IKKs and the IKK-related kinases regulate each other by an intricate network involving phosphorylation of their catalytic and regulatory (NEMO and TANK) subunits to balance their activities during innate immunity.

Regulation of lipopolysaccharide-induced inflammatory response by heat shock protein 27 in THP-1 cells

HSP27 is a member of the small HSP family which has been linked to different signaling pathways regulating critical cellular functions. But the role of HSP27 in LPS-induced inflammatory signaling pathways is still unclear. In the present study, both overexpression and RNA interference experiments indicated that HSP27 increased LPS-induced expression of iNOS and COX-2 and release of NO/PGE2 through enhancing NF-kappaB but not MAPK activation. The effects of HSP27 on LPS-induced iNOS/COX-2 expression and relative signaling cascade were closely related with the phosphorylation of HSP27. Further studies have shown that HSP27-regulated LPS-induced activation of NF-kappaB by interacting with TRAF6 and increasing the association of TRAF6-IKKgamma. This could be a probable mechanism by which HSP27 modulates LPS-induce inflammatory signaling pathways. Thus, HSP27 may play a potential role in regulating inflammatory responses in immunologic system.

The NF-kappaB-independent functions of IKK subunits in immunity and cancer

The I kappaB kinase (IKK) complex is involved in transcriptional activation by phosphorylating the inhibitory molecule I kappaB alpha, a modification that triggers its subsequent degradation, enabling activation of nuclear factor kappa B (NF-kappaB). Importantly, recent reports indicate that multiple cytoplasmic and nuclear proteins distinct from the NF-kappaB and I kappaB proteins are phosphorylated by the catalytic subunits of the IKK complex, IKK alpha or IKK beta. Here, I describe how IKK subunits can have crucial roles in allergy, inflammation and immunity by targeting proteins such as SNAP23 and IRF7, but also in cancer by phosphorylating key molecules such as p53, TSC1 and FOXO3a through NF-kappaB-independent pathways. Thus, these recent findings considerably widen the biological roles of these kinases and suggest that a full understanding of the biological roles of IKK alpha and IKK beta requires an exhaustive characterization of their substrates.

Total alkaloids from Radix Linderae prevent the production of inflammatory mediators in lipopolysaccharide-stimulated RAW 264.7 cells by suppressing NF-kappaB and MAPKs activation

Radix Linderae, the dry roots of Lindera aggregata (Sims) Kosterm (L. strychnifolia Vill), has been long-term used in traditional Chinese medicine for treating various diseases, and alkaloids are believed to be the main active components. Previously, we reported that the total alkaloids from Radix Linderae (TARL) could effectively alleviate inflammation and protect joints from destruction in mouse collagen-induced arthritis, an animal model of human rheumatoid arthritis (RA). To get insight into the underlying mechanisms of TARL, the present study was performed to investigate the effects of TARL on the activation of macrophages and resultant production of inflammatory mediators. In vitro, TARL concentration-dependently prevented the production of nitric oxide, interleukin-1beta (IL-1beta) and tumor necrosis factor-alpha (TNF-alpha), as well as the expressions of iNOS, IL-1beta and TNF-alpha mRNA in RAW 264.7 cells stimulated by lipopolysaccharide (LPS). However, it showed little effect on the production of interleukin-6 (IL-6) and the expression of IL-6 mRNA. Signal transduction studies showed that TARL significantly down-regulated the phosphorylation of extracellular signal-regulated kinase (ERK) and p38 MAP kinase rather than c-jun NH(2)-terminal kinase (JNK). Additionally, TARL prominently decreased LPS-induced activation of IKKalpha and phosphorylation of p65 on serine 276, but had little impact on the phosphorylation and degradation of IkappaBalpha. In summary, our results demonstrate that TARL exhibits inhibitory effects on the production of inflammatory mediators from macrophages via blocking NF-kappaB and MAPKs signaling pathways. The findings provide a plausible explanation for the therapeutic efficiency of TARL on the inflammation and joint destruction in RA.

Solution structure of NEMO zinc finger and impact of an anhidrotic ectodermal dysplasia with immunodeficiency-related point mutation

The regulatory NEMO (NF-kappaB essential modulator) protein has a crucial role in the canonical NF-kappaB signaling pathway notably involved in immune and inflammatory responses, apoptosis and oncogenesis. The regulatory domain is located in the C-terminal half of NEMO and contains a classical CCHC-type zinc finger (ZF). We have investigated the structural and functional effects of a cysteine to phenylalanine point mutation (C417F) in the ZF motif, identified in patients with anhidrotic ectodermal dysplasia with immunodeficiency. The solution structures of the wild type and mutant ZF were determined by NMR. Remarkably, the mutant adopts a global betabetaalpha fold similar to that of the wild type and retains thermodynamic stability, i.e., the ability to bind zinc with a native-like affinity, although the last zinc-chelating residue is missing. However, the mutation induces enhanced dynamics in the motif and leads to an important loss of stability. A detailed analysis of the wild type solution structure and experimental evidences led to the identification of two possible protein-binding surfaces that are largely destabilized in the mutant. This is sufficient to alter NEMO function, since functional complementation assays using NEMO-deficient pre-B and T lymphocytes show that full-length C417F pathogenic NEMO leads to a partial to strong defect in LPS, IL-1beta and TNF-alpha-induced NF-kappaB activation, respectively, as compared to wild type NEMO. Altogether, these results shed light onto the role of NEMO ZF as a protein-binding motif and show that a precise structural integrity of the ZF should be preserved to lead to a functional protein-recognition motif triggering full NF-kappaB activation.

IkappaB kinase beta phosphorylates the K63 deubiquitinase A20 to cause feedback inhibition of the NF-kappaB pathway

Misregulation of NF-kappaB signaling leads to infectious, inflammatory, or autoimmune disorders. IkappaB kinase beta (IKKbeta) is an essential activator of NF-kappaB and is known to phosphorylate the NF-kappaB inhibitor, IkappaBalpha, allowing it to undergo ubiquitin-mediated proteasomal degradation. However, beyond IkappaBalpha, few additional IKKbeta substrates have been identified. Here we utilize a peptide library and bioinformatic approach to predict likely substrates of IKKbeta. This approach predicted Ser381 of the K63 deubiquitinase A20 as a likely site of IKKbeta phosphorylation. While A20 is a known negative regulator of innate immune signaling pathways, the mechanisms regulating the activity of A20 are poorly understood. We show that IKKbeta phosphorylates A20 in vitro and in vivo at serine 381, and we further show that this phosphorylation event increases the ability of A20 to inhibit the NF-kappaB signaling pathway. Phosphorylation of A20 by IKKbeta thus represents part of a novel feedback loop that regulates the duration of NF-kappaB signaling following activation of innate immune signaling pathways.

Post-translational modifications regulating the activity and function of the nuclear factor kappa B pathway

The diverse cellular and biological functions of the nuclear factor kappa B (NF-kappaB) pathway, together with the catastrophic consequences of its aberrant regulation, demand specific and highly regulated control of its activity. As described in this review, regulation of the NF-kappaB pathway is brought about through multiple post-translational modifications that control the activity of the core components of NF-kappaB signaling: the IkappaB kinase (IKK) complex, the IkappaB proteins and the NF-kappaB subunits themselves. These regulatory modifications, which include phosphorylation, ubiquitination, acetylation, sumoylation and nitrosylation, can vary, depending on the nature of the NF-kappaB-inducing stimulus. Moreover, they frequently have distinct, sometimes antagonistic, functional consequences and the same modification can have different effects depending on the context. Given the important role of NF-kappaB in human health and disease, understanding these pathways will not only provide valuable insights into mechanism and function, but could also lead to new drug targets and the development of diagnostic and prognostic biomarkers for many pathological conditions.

Dimerization of the I kappa B kinase-binding domain of NEMO is required for tumor necrosis factor alpha-induced NF-kappa B activity

Previous studies have demonstrated that peptides corresponding to a six-amino-acid NEMO-binding domain from the C terminus of IkappaB kinase alpha (IKKalpha) and IKKbeta can disrupt the IKK complex and block NF-kappaB activation. We have now mapped and characterized the corresponding amino-terminal IKK-binding domain (IBD) of NEMO. Peptides corresponding to the IBD were efficiently recruited to the IKK complex but displayed only a weak inhibitory potential on cytokine-induced NF-kappaB activity. This is most likely due to the formation of sodium dodecyl sulfate- and urea-resistant NEMO dimers through a dimerization domain at the amino terminus of NEMO that overlaps with the region responsible for binding to IKKs. Mutational analysis revealed different alpha-helical subdomains within an amino-terminal coiled-coil region are important for NEMO dimerization and IKKbeta binding. Furthermore, NEMO dimerization is required for the tumor necrosis factor alpha-induced NF-kappaB activation, even when interaction with the IKKs is unaffected. Hence, our data provide novel insights into the role of the amino terminus of NEMO for the architecture of the IKK complex and its activation.

Posttranslational modifications of NEMO and its partners in NF-kappaB signaling

NEMO, the regulatory subunit of the IkappaB kinase (IKK) complex that controls the activation of the transcription factor NF-kappaB, is required for IKK function in most situations, but its exact mode of action has remained elusive until recently. A series of publications now provides information about how posttranscriptional modifications of NEMO, such as ubiquitination, sumoylation or phosphorylation, regulate its function in the IKK complex. These modifications might also regulate a cytosolic pool of free NEMO that controls the activation of NF-kappaB induced by genotoxic stress. Together with a better identification of the modifications controlling partners of NEMO, a clearer picture of how IKK becomes activated upon cell stimulation is starting to emerge, providing new clues for how the NF-kappaB pathway could be modulated for therapeutic purposes.

TNFalpha- and IKKbeta-mediated TANK/I-TRAF phosphorylation: implications for interaction with NEMO/IKKgamma and NF-kappaB activation

Pro-inflammatory cytokines trigger signalling cascades leading to NF-kappaB (nuclear factor-kappaB)-dependent gene expression through IKK [IkappaB (inhibitory kappaB) kinase]-dependent phosphorylation and subsequent degradation of the IkappaB proteins and via induced phosphorylation of p65. These signalling pathways rely on sequentially activated kinases which are assembled by essential and non-enzymatic scaffold proteins into functional complexes. Here, we show that the pro-inflammatory cytokine TNFalpha (tumour necrosis factor alpha) promotes TANK [TRAF (TNF receptor-associated factor) family member associated NF-kappaB activator] recruitment to the IKK complex via a newly characterized C-terminal zinc finger. Moreover, we show that TANK is phosphorylated by IKKbeta upon TNFalpha stimulation and that this modification negatively regulates TANK binding to NEMO (NF-kappaB essential modulator). Interestingly, reduced TANK expression by RNA interference attenuates TNFalpha-mediated induction of a subset of NF-kappaB target genes through decreased p65 transactivation potential. Therefore the scaffold protein TANK is required for the cellular response to TNFalpha by connecting upstream signalling molecules to the IKKs and p65, and its subsequent IKKbeta-mediated phosphorylation may be a mechanism to terminate the TANK-dependent wave of NF-kappaB activation.

An alternative splice product of IkappaB kinase (IKKgamma), IKKgamma-delta, differentially mediates cytokine and human T-cell leukemia virus type 1 tax-induced NF-kappaB activation

NF-kappaB is an inducible transcription factor mediating innate immune responses whose activity is controlled by the multiprotein IkappaB kinase (IKK) "signalsome". The core IKK consists of two catalytic serine kinases, IKKalpha and IKKbeta, and a noncatalytic subunit, IKKgamma. IKKgamma is required for IKK activity by mediating kinase oligomerization and serving to couple the core catalytic subunits to upstream mitogen-activated protein 3-kinase cascades. We have discovered an alternatively spliced IKKgamma mRNA isoform, encoding an in-frame deletion of exon 5, termed IKKgamma-delta. Using a specific reverse transcription-PCR assay, we find that IKKgamma-delta is widely expressed in cultured human cells and normal human tissues. Because IKKgamma-Delta protein is lacking a critical coiled-coil domain important in protein-protein interactions, we sought to determine its signaling properties by examining its ability to self associate, couple to activators of the canonical pathway, and mediate human T-cell leukemia virus type 1 (HTLV-1) Tax-induced NF-kappaB activity. Coimmunoprecipitation and confocal colocalization assays indicate IKKgamma-delta has strong homo- and heterotypic association with wild-type (WT) IKKgamma and, like IKKgamma WT, associates with the IKKbeta kinase. Similarly, IKKgamma-delta mediates IKK kinase activity and downstream NF-kappaB-dependent transcription in response to tumor necrosis factor (TNF) and the NF-kappaB-inducing kinase-IKKalpha signaling pathway. Surprisingly, however, in contrast to IKKgamma WT, IKKgamma-delta is not able to mediate HTLV-1 Tax-induced NF-kappaB-dependent transcription, even though IKKgamma-delta binds and colocalizes with Tax. These observations suggest that IKKgamma-delta is a functionally distinct alternatively spliced mRNA product differentially mediating TNF-induced, but not Tax-induced, signals converging on the IKK signalsome. Differing levels of IKKgamma-delta expression, therefore, may affect signal transduction cascades coupling to IKK.

A Point Mutation in NEMO Associated with Anhidrotic Ectodermal Dysplasia with Immunodeficiency Pathology Results in Destabilization of the Oligomer and Reduces Lipopolysaccharide- and Tumor Necrosis Factor-mediated NF-κB Activation

The NEMO (NF-kappaB essential modulator) protein plays a crucial role in the canonical NF-kappaB pathway as the regulatory component of the IKK (IkappaB kinase) complex. The human disease anhidrotic ectodermal dysplasia with immunodeficiency (EDA-ID) has been recently linked to mutations in NEMO. We investigated the effect of an alanine to glycine substitution found in the NEMO polypeptide of an EDA-ID patient. This pathogenic mutation is located within the minimal oligomerization domain of the protein, which is required for the IKK activation in response to diverse stimuli. The mutation does not dramatically change the native-like state of the trimer, but temperature-induced unfolding studied by circular dichroism showed that it leads to an important loss in the oligomer stability. Furthermore, fluorescence studies showed that the tyrosine located in the adjacent zinc finger domain, which is possibly required for NEMO ubiquitination, exhibits an alteration in its spectral properties. This is probably due to a conformational change of this domain, providing evidence for a close interaction between the oligomerization domain and the zinc finger. In addition, functional complementation assays using NEMO-deficient pre-B and T lymphocytes showed that the pathogenic mutation reduced TNF-alpha and LPS-induced NF-kappaB activation by altering the assembly of the IKK complex. Altogether, our findings provide understanding as to how a single point mutation in NEMO leads to the observed EDA-ID phenotype in relation to the NEMO-dependent mechanism of IKK activation.

N-Acetyl-L-cysteine and pyrrolidine dithiocarbamate inhibited nuclear factor-kappaB activation in alveolar macrophages by different mechanisms

AIM

To study the effects of N-acetyl-L-cysteine (NAC) and pyrrolidine dithiocarbamate (PDTC) on the phosphorylation of IkappaB kinase (IKK) beta, IKK alpha, and IkB alpha in alveolar macrophages (AM), and to explore the pharmacological mechanisms of NAC and PDTC as inhibitors of NF-kappaB activation.

METHODS

AM were collected from bronchoalveolar lavage fluid from the patients with chronic obstructive pulmonary disease. The AM were incubated for 1.5 h with NAC and PDTC, and then stimulated for 90 min by either tumor necrosis factor (TNF)- alpha or interleukin (IL)-1. Western blotting was used to detect the protein phosphorylation levels of IKKbeta, IKK alpha, and IkappaB alpha. NF-kappaB activity was analyzed by using an electrophoretic mobility shift assay.

RESULTS

NAC inhibited the phosphorylation of IKKbeta, IKK alpha, and IkappaB alpha induced by TNF-a, but had no effect on the phosphorylation of IKKbeta, IKK alpha and IkappaB alpha induced by IL-1. PDTC did not inhibit the phosphorylation of IkappaB alpha induced by TNF- alpha or IL-1. Similarly, NAC inhibited the activation of NF-kB induced by TNF- alpha, but had no effect on the activation of NF-kappaB induced by IL-1. PDTC significantly inhibited the activation of NF-kappa B induced by TNF- alpha and IL-1. The electrophoretic mobility shift assay also showed that PDTC and NAC do not directly inhibit NF-kappa B DNA binding activity in vitro.

CONCLUSION

PDTC prevents the degradation of IkappaB alpha via the ubiquitylation-proteasome proteolytic pathway. NAC can inhibit the processes upstream of IKK activation induced by TNF- alpha, which results in the decline of NF-kappaB activity.

Binding of Manumycin A Inhibits IκB Kinase β Activity

IkappaB kinase (IKK) catalytic subunits play a key role in cytokinemediated nuclear factor (NF)-kappaB signaling, and a loss of NF-kappaB function appears to inhibit inflammation and oncogenesis. Manumycin A is a potent and selective farnesyltransferase inhibitor with antitumor activity. We found that manumycin A caused a rapid and potent inhibition of IKK activity induced by tumor necrosis factor alpha in a number of cell types. Most unexpectedly, other classes of farnesyltransferase inhibitors had no inhibitory effect. To identify the molecular mechanisms of manumycin A action, cultured human HepG2 hepatoma cells were transiently transfected with various IKKalpha and IKKbeta constructs, and a striking difference in manumycin A sensitivity was observed. Furthermore, cells expressing wild-type IKKbeta and IKKbeta mutated in the activation loop at Cys-179 exhibited covalent homotypic dimerization of IKKbeta in response to manumycin A, whereas substitution of Cys-662 and -716 conferred protection against dimer formation. Direct inhibition of IKK activity and formation of stable IKKbeta dimers were observed in the presence of manumycin A that could be blocked by dithiothreitol. IKK interaction with the adaptor protein IKKgamma/NEMO was disrupted in manumycin A-treated cells. Most importantly, administration of manumycin A to mice xenografted with murine B16F10 tumors caused potent IKK-suppressive effects. Thus, manumycin A with its epoxyquinoid moieties plays an important regulatory function in IKK signaling through pathways distinct from its role as a protein farnesylation inhibitor.

Site-specific Monoubiquitination of IκB Kinase IKKβ Regulates Its Phosphorylation and Persistent Activation

Transcription factor NF-kappaB governs the expression of multiple genes involved in cell growth, immunity, and inflammation. Nuclear translocation of NF-kappaB is regulated from the cytoplasm by IkappaB kinase-beta (IKKbeta), which earmarks inhibitors of NF-kappaB for polyubiquination and proteasome-mediated degradation. Activation of IKKbeta is contingent upon signal-induced phosphorylation of its T loop at Ser-177/Ser-181. T loop phosphorylation also renders IKKbeta a substrate for monoubiquitination in cells exposed to chronic activating cues, such as the Tax oncoprotein or sustained signaling through proinflammatory cytokine receptors. Here we provide evidence that the T loop-proximal residue Lys-163 in IKKbeta serves as a major site for signal-induced monoubiquitination with significant regulatory potential. Conservative replacement of Lys-163 with Arg yielded a monoubiquitination-defective mutant of IKKbeta that retains kinase activity in Tax-expressing cells but is impaired for activation mediated by chronic signaling from the type 1 receptor for tumor necrosis factor-alpha. Phosphopeptide mapping experiments revealed that the Lys-163 --> Arg mutation also interferes with proper in vivo but not in vitro phosphorylation of cytokine-responsive serine residues located in the distal C-terminal region of IKKbeta. Taken together, these data indicate that chronic phosphorylation of IKKbeta at Ser-177/Ser-181 leads to monoubiquitin attachment at nearby Lys-163, which in turn modulates the phosphorylation status of IKKbeta at select C-terminal serines. This mechanism for post-translational cross-talk may play an important role in the control of IKKbeta signaling during chronic inflammation.

Convergent actions of I kappa B kinase beta and protein kinase C delta modulate mRNA stability through phosphorylation of 14-3-3 beta complexed with tristetraprolin

Regulation of gene expression at the level of mRNA stability is a major topic of research; however, knowledge about the regulatory mechanisms affecting the binding and function of AU-rich element (ARE)-binding proteins (AUBPs) in response to extracellular signals is minimal. The beta1,4-galactosyltransferase 1 (beta4GalT1) gene enabled us to study the mechanisms involved in binding of tristetraprolin (TTP) as the stability of its mRNA is regulated solely through one ARE bound by TTP in resting human umbilical vein endothelial cells. Here, we provide evidence that the complex formation of TTP with 14-3-3beta is required to bind beta4GalT1 mRNA and promote its decay. Furthermore, upon tumor necrosis factor alpha stimulation, the activation of both Ikappabeta kinase and protein kinase Cdelta is involved in the phosphorylation of 14-3-3beta on two serine residues, paralleled by release of binding of TTP and 14-3-3beta from beta4GalT1 mRNA, nuclear sequestration of TTP, and beta4GalT1 mRNA stabilization. Thus, a key mechanism regulating mRNA binding and function of the destabilizing AUBP TTP involves the phosphorylation status of 14-3-3beta.

The flavonoid luteolin prevents lipopolysaccharide-induced NF-kappaB signalling and gene expression by blocking IkappaB kinase activity in intestinal epithelial cells and bone-marrow derived dendritic cells

The nuclear factor (NF)-kappaB transcriptional system is a major effector pathway involved in inflammation and innate immune responses. The flavonoid luteolin is found in various herbal extracts and has shown anti-inflammatory properties. However, the mechanism of action and impact of luteolin on innate immunity is still unknown. We report that luteolin significantly blocks lipopolysaccharide (LPS)-induced IkappaB phosphorylation/degradation, NF-kappaB transcriptional activity and intercellular adhesion molecule-1 (ICAM-1) gene expression in rat IEC-18 cells. Using chromatin immunoprecipitation, we demonstrate that LPS-induced RelA recruitment to the ICAM-1 gene promoter is significantly reduced in luteolin-treated cells. Moreover, in vitro kinase assays show that luteolin directly inhibits LPS-induced IkappaB kinase (IKK) activity in IEC-18 cells. Using bone-marrow derived dendritic cells (BMDCs) isolated from interleukin (IL)-10(-/-) mice or from recently engineered transgenic mice expressing the enhanced green fluorescent protein (EGFP) under the transcriptional control of NF-kappaB cis-elements (cis-NF-kappaB(EGFP)), we found that luteolin blocks LPS-induced IkappaB phosphorylation and IKK activity, and decreases EGFP, IL-12 and tumour necrosis factor-alpha gene expression. Moreover, intraperitoneal administration of luteolin significantly inhibited LPS-induced EGFP expression in both peripheral blood mononuclear cells and splenocytes isolated from cis-NF-kappaB(EGFP) mice. These results indicate that luteolin blocks LPS-induced NF-kappaB signalling and proinflammatory gene expression in intestinal epithelial cells and dendritic cells. Modulation of innate immunity by natural plant products may represent an attractive strategy to prevent intestinal inflammation associated with dysregulated innate immune responses.

Tumor necrosis factor alpha sensitizes malignant cells to chemotherapeutic drugs via the mitochondrial apoptosis pathway independently of caspase-8 and NF-kappaB

The Hodgkin cell line HD-MyZ is resistant to apoptosis induced by tumor necrosis factor alpha (TNFalpha). In the present work, we show that pretreatment with TNFalpha sensitized the cells to apoptosis induced by antineoplastic agents and ceramide. TNFalpha pretreatment resulted in enhanced cleavage and activity of caspase-3 upon addition of etoposide, epirubicin or ceramide. No caspase-8 activation was detectable, although caspase-8 could be activated in cell-free extracts. Inhibition of caspase-8 by z-IETD-fmk did not block the sensitizing effect of TNFalpha. Furthermore, exogenous ceramide, a mediator of TNFalpha signaling, could not substitute for TNFalpha in sensitization to drug-induced apoptosis. In contrast, we observed mitochondrial changes following cotreatment of cells with TNFalpha and drugs. Mitochondrial permeability transition, cytochrome c release and subsequent processing of caspase-9 preceded the onset of apoptosis, and were enhanced by TNFalpha pretreatment. Interestingly, although transcription factor NF-kappaB protected HD-MyZ cells from drug-induced apoptosis, TNFalpha-mediated sensitization was independent of NF-kappaB, since overexpressing a dominant-negative IkappaB mutant did not alter the TNFalpha effect. Sensitization for drug-induced apoptosis by TNFalpha was abrogated by Bcl-x(L). Thus, the sensitizing effect of TNFalpha is mediated by the mitochondrial pathway and involves processing of caspase-2, -3 and -9, but appears to be independent of caspase-8 processing, Bid cleavage and NF-kappaB signaling. Therefore, sensitization by TNFalpha is mediated at least in part through different pathways, as reported for TRAIL. There, sensitization occurs through a FADD/caspase-8-dependent mechanism. Regarding TNFalpha, the sensitizing effect was also observed in myeloid leukemia cells. Therefore, TNFalpha or alternate molecules activating its pathways might be useful as sensitizers for chemotherapy in hematological malignancies.

Reduced levels of the adenomatous polyposis coli (APC) protein are associated with ceramide-induced apoptosis of colon cancer cells

PURPOSE

Mutations of the adenomatous polyposis coli (APC) and p53 genes are commonly found in colorectal cancers. We therefore analyzed the relative roles of APC and p53 in the induction of apoptosis of colon cancer cells by comparing the effects of the natural chemopreventive agent, C(2)-ceramide, on different human colon cancer cell lines with and without wild-type p53 and APC genes.

METHODS

We studied the effect of C(2)-ceramide and C(2)-dihydroceramide on proliferation and/or apoptosis of colon cancer cell lines in vitro and determined the role of p53 and APC proteins in these processes. The protein and mRNA levels in colon cancer cell lines with and without treatments were determined by Western and Northern blot analysis, respectively. The cell cycle and apoptosis profiles were determined by FACS analysis and PARP-1 cleavage.

RESULTS

Our findings indicate that C(2)-ceramide can induce apoptosis independently of the p53/p21(Waf-1/Cip-1) pathway. In addition, the C(2)-ceramide induction of apoptosis showed a correlation with a reduction in the levels of the APC protein and mRNA. Moreover, the C(2)-ceramide-induced apoptosis was blocked by pre-treatment with ZnCl(2), which stabilizes APC protein levels.

CONCLUSIONS

These results suggest that C(2)-ceramide treatment reduces the levels of APC protein and that the reduction in the levels of this protein plays a key role in the ability of C(2)-ceramide to induce apoptosis of colon cancer cells.

The IkappaB kinase (IKK) inhibitor, NEMO-binding domain peptide, blocks osteoclastogenesis and bone erosion in inflammatory arthritis

Activation of NF-kappaB leads to expression of ample genes that regulate inflammatory and osteoclastogenic responses. The process is facilitated by induction of IkappaB kinase (IKK) complex that phosphorylates IkappaB and leads to its dissociation from the NF-kappaB complex, thus permitting activation of NF-kappaB. The IKK complex contains primarily IKKalpha, IKKbeta, and the regulatory kinase IKKgamma, also known as NEMO. NEMO regulates the IKK complex activity through its binding to carboxyl-terminal region of IKKalpha and IKKbeta, termed NEMO-binding domain (NBD). In this regard, a cell-permeable NBD peptide has been shown to block association of NEMO with the IKK complex and inhibit activation of NF-kappaB. Given the pivotal role of cytokine-induced NF-kappaB in osteoclastogenesis and inflammatory bone loss, we deduced that cell-permeable TAT-NBD peptide may hinder osteoclastogenesis and bone erosion in inflammatory arthritis. Using NBD peptides, we show that wild type, but not mutant, NBD blocks IKK activation and reduces cytokine-induced promoter and DNA binding activities of NF-kappaB and inhibits cytokine-induced osteoclast formation by osteoclast precursors. Consistent with the key role of NF-kappaB in osteoinflammatory responses in vivo, wild type TAT-NBD administered into mice prior to induction of inflammatory arthritis efficiently block in vivo osteoclastogenesis, inhibits focal bone erosion, and ameliorates inflammatory responses in the joints of arthritic mice. The mutant NBD peptide fails to exert these functions. These results provide strong evidence that IKKs are potent regulators of cytokine-induced osteoclastogenesis and inflammatory arthritis. More importantly, blockade of NEMO assembly with the IKK complex is a viable strategy to avert inflammatory osteolysis.

The zinc finger mutation C417R of I-kappa B kinase gamma impairs lipopolysaccharide- and TNF-mediated NF-kappa B activation through inhibiting phosphorylation of the I-kappa B kinase beta activation loop

The activation of the I-kappaB kinase (IKK) complex by TNF or LPS stimulates phosphorylation and degradation of I-kappaBalpha, leading to the nuclear translocation of NF-kappaB. The IKK complex is mainly composed of two catalytic subunits, IKKalpha and IKKbeta, and a chaperon subunit IKKgamma. Although IKKgamma does not have catalytic activity, it is essential for IKK activation induced by multiple stimuli. Importantly, the key residue cysteine 417 at the zinc finger domain of IKKgamma has been found to be mutated to arginine (IKKgammaC417R) in a human genetic disorder called the anhydrotic ectodermal dysplasia with immunodeficiency. To understand the underlying mechanisms of immunodeficiency, we examined whether the IKKgammaC417R mutant modified IKK activation and NF-kappaB transcription stimulated by LPS or TNF in human monocytes. We found that overexpression of IKKgammaC417R severely impaired LPS- and TNF-induced I-kappaBalpha phosphorylation and degradation in a dominant-negative fashion. Also, LPS- and TNF-induced NF-kappaB transcription was inhibited by IKKgammaC417R. The reconstitution of IKKgamma, but not IKKgammaC417R, in IKKgamma-deficient cells restored NF-kappaB signaling, indicating the zinc finger structure of IKKgamma plays a key role in IKK activation. Moreover, C417R mutation in IKKgamma abolished both LPS- and TNF-induced phosphorylation of the activation loop of IKKbeta. Collectively, our results indicated that the zinc finger structure of IKKgamma plays a key role in LPS- and TNF-induced NF-kappaB activation. The anhydrotic ectodermal dysplasia with immunodeficiency patients' immunodeficiency may be associated with NF-kappaB defect in response to bacterial stimulation.

The trimerization domain of NEMO is composed of the interacting C-terminal CC2 and LZ coiled-coil subdomains

NEMO (NF-kappaB essential modulator) plays a key role in the canonical NF-kappaB pathway as the scaffold/regulatory component of the IkappaB kinase (IKK) complex. The self-association of NEMO involves the C-terminal halves of the polypeptide chains containing two putative coiled-coil motifs (a CC2 and a LZ leucine zipper), a proline-rich region, and a ZF zinc finger motif. Using purified truncation mutants, we showed that the minimal oligomerization domain of NEMO is the CC2-LZ segment and that both CC2 and LZ subdomains are necessary to restore the LPS-dependent activation of the NF-kappaB pathway in a NEMO-deficient cell line. We confirmed the association of the oligomerization domain in a trimer and investigated the specific role of CC2 and LZ subdomains in the building of the oligomer. Whereas a recombinant CC2-LZ polypeptide self-associated into a trimer with an association constant close to that of the wild-type protein, the isolated CC2 and LZ peptides, respectively, formed trimers and dimers with weaker association constants. Upon mixing, isolated CC2 and LZ peptides associated to form a stable hetero-hexamer as shown by gel filtration and fluorescence anisotropy experiments. We propose a structural model for the organization of the oligomerization domain of activated NEMO in which three C-terminal domains associate into a pseudo-hexamer forming a six-helix bundle. This model is discussed in relation to the mechanism of activation of the IKK complex by upstream activators.

Segregation of NF-kappaB activation through NEMO/IKKgamma by Tax and TNFalpha: implications for stimulus-specific interruption of oncogenic signaling

Nuclear factor-kappaB essential modulator (NEMO), also called IKKgamma, has been proposed as a 'universal' adaptor of the I-kappaB kinase (IKK) complex for stimuli such as proinflammatory cytokines, microbes, and the HTLV-I Tax oncoprotein. Currently, it remains unclear whether the many signals that activate NF-kappaB through NEMO converge identically or differently. We have adopted two approaches to answer this question. First, we generated and targeted intracellularly three NEMO-specific monoclonal antibodies (mAbs). These mAbs produced two distinct intracellular NF-kappaB inhibition profiles segregating TNFalpha from Tax activation. Second, using NEMO knockout mouse fibroblasts and 10 NEMO mutants, we found that different regions function in trans either to complement or to inhibit dominantly TNFalpha, IL-1beta, or Tax activation of NF-kappaB. For instance, NEMO (1-245 amino acids) supported Tax-mediated NF-kappaB activation, but did not serve TNFalpha- or IL-1beta signaling. Altogether, our findings indicate that while NEMO 'universally' adapts numerous NF-kappaB activators, it may do so through separable domains. We provide the first evidence that selective targeting of NEMO can abrogate oncogenic Tax signaling without affecting signals used for normal cellular metabolism.

Nuclear Role of IκB Kinase-γ/NF-κB Essential Modulator (IKKγ/NEMO) in NF-κB-dependent Gene Expression

The I kappa B kinase (IKK) complex, which is composed of the two kinases IKK alpha and IKK beta and the regulatory subunit IKK gamma/nuclear factor-kappa B (NF-kappa B) essential modulator (NEMO), is important in the cytokine-induced activation of the NF-kappa B pathway. In addition to modulation of IKK activity, the NF-kappa B pathway is also regulated by other processes, including the nucleocytoplasmic shuttling of various components of this pathway and the post-translational modification of factors bound to NF-kappa B-dependent promoters. In this study, we explored the role of the nucleocytoplasmic shuttling of components of the IKK complex in the regulation of the NF-kappa B pathway. IKK gamma/NEMO was demonstrated to shuttle between the cytoplasm and the nucleus and to interact with the nuclear coactivator cAMP-responsive element-binding protein-binding protein (CBP). Using both in vitro and in vivo analysis, we demonstrated that IKK gamma/NEMO competed with p65 and IKK alpha for binding to the N terminus of CBP, inhibiting CBP-dependent transcriptional activation. These results indicate that, in addition to the key role of IKK gamma/NEMO in regulating cytokine-induced IKK activity, its ability to shuttle between the cytoplasm and the nucleus and to bind to CBP can lead to transcriptional repression of the NF-kappa B pathway.

Protein Phosphatase 2Cβ Association with the IκB Kinase Complex Is Involved in Regulating NF-κB Activity

The NF-kappaB pathway is important in the control of the immune and inflammatory response. One of the critical events in the activation of this pathway is the stimulation of the IkappaB kinases (IKKs) by cytokines such as tumor necrosis factor-alpha and interleukin-1. Although the mechanisms that modulate IKK activation have been studied in detail, much less is known about the processes that down-regulate its activity following cytokine treatment. In this study, we utilized biochemical fractionation and mass spectrometry to demonstrate that protein phosphatase 2Cbeta (PP2Cbeta) can associate with the IKK complex. PP2Cbeta association with the IKK complex led to the dephosphorylation of IKKbeta and decreased its kinase activity. The binding of PP2Cbeta to IKKbeta was decreased at early times post-tumor necrosis factor-alpha treatment and was restored at later times following treatment with this cytokine. Experiments utilizing siRNA directed against PP2Cbeta demonstrated an in vivo role for this phosphatase in decreasing IKK activity at late times following cytokine treatment. These studies are consistent with the ability of PP2Cbeta to down-regulate cytokine-induced NF-kappaB activation by altering IKK activity.

Roles for homotypic interactions and transautophosphorylation in IkappaB kinase beta IKKbeta) activation [corrected]

The nuclear factor kappaB (NF-kappaB)/Rel family of transcription factors participates in a wide range of biological activities including inflammation, immunity, and apoptosis. NF-kappaB is kept inactive in the cytoplasm in unstimulated cells by virtue of the masking of its nuclear localization sequence by bound IkappaB protein. Cellular stimuli trigger the destruction of IkappaB proteins and the liberation of NF-kappaB to enter the nucleus and activate gene expression. A multisubunit IkappaB kinase complex (IKK) phosphorylates IkappaB proteins and mediates the activation of NF-kappaB by proinflammatory stimuli such as tumor necrosis factor alpha. Phosphorylation of IkappaB proteins triggers their polyubiquitination and their subsequent recognition and degradation by the proteasome. The IKK complex contains two catalytic subunits, IKKalpha and IKKbeta, and a noncatalytic subunit, NF-kappaB essential modifier/IKKgamma. IKK activation depends upon the phosphorylation of residues in the activation loop of IKKbeta and the subsequent activation of IKKbeta kinase activity. However, the events contributing to IKKbeta phosphorylation are not well understood. Here, we present evidence that the activation of IKKbeta depends on its ability to form homotypic interactions and to transautophosphorylate. We find that an intact leucine zipper in IKKbeta is necessary for homotypic interactions, kinase activation, and phosphorylation on its activation loop. Enforced oligomerization of an IKKbeta mutant defective in forming homotypic interactions restores kinase activation. Homotypic interactions allow IKKbeta molecules to transautophosphorylate one another on their activation loops. Finally, the oligomerization of IKKbeta is stimulated by tumor necrosis factor alpha in cultured cells. Our findings support a model whereby ligand-induced homotypic interactions between IKKbeta molecules result in IKKbeta phosphorylation and consequently IKK activation.

Tumor necrosis factor-alpha induces nuclear factor-kappaB-dependent TRPC1 expression in endothelial cells

We investigated the role of tumor necrosis factor-alpha (TNF-alpha) in activating the store-operated Ca2+ channels in endothelial cells via the expression of transient receptor potential channel (TRPC) isoforms. We observed that TNF-alpha exposure of human umbilical vein endothelial cells resulted in TRPC1 mRNA and protein expression, whereas it had no effect on TRPC3, TRPC4, or TRPC5 expression. The TRPC1 expression was associated with increased Ca2+ influx after intracellular Ca2+ store depletion with either thrombin or thapsigargin. We cloned the 5'-regulatory region of the human TRPC1 (hTRPC1) gene which contained a TATA box and CCAAT sequence close to the transcription initiation site. We also identified four nuclear factor-kappaB (NF-kappaB)-binding sites in the 5'-regulatory region. To address the contribution of NF-kappaB in the mechanism of TRPC1 expression, we determined the effects of TNF-alpha on expression of the reporter luciferase after transfection of hTRPC1 promoter-luciferase (hTRPC1-Pro-Luc) construct in the human dermal microvascular endothelial cell line. Reporter activity increased >4-fold at 4 h after TNF-alpha challenge. TNF-alpha-induced increase in reporter activity was markedly reduced by co-expression of either kinase-defective IKKbeta kinase mutant or non-phosphorylatable IkappaB mutant. Treatment with NEMO-binding domain peptide, which prevents NF-kappaB activation by selectively inhibiting IKKgamma interaction with IKK complex, also blocked the TNF-alpha-induced TRPC1 expression. Thus, TNF-alpha induces TRPC1 expression through an NF-kappaB-dependent pathway in endothelial cells, which can trigger augmented Ca2+ entry following Ca2+ store depletion. The augmented Ca2+ entry secondary to TRPC1 expression may be an important mechanism of endothelial injury induced by TNF-alpha.

A nucleosomal function for IkappaB kinase-alpha in NF-kappaB-dependent gene expression

NF-kappaB is a principal transcriptional regulator of diverse cytokine-mediated processes and is tightly controlled by the IkappaB kinase complex (IKK-alpha/beta/gamma). IKK-beta and IKK-gamma are critical for cytokine-induced NF-kappaB function, whereas IKK-alpha is thought to be involved in other regulatory pathways. However, recent data suggest a role for IKK-alpha in NF-kappaB-dependent gene expression in response to cytokine treatment. Here we demonstrate nuclear accumulation of IKK-alpha after cytokine exposure, suggesting a nuclear function for this protein. Consistent with this, chromatin immunoprecipitation (ChIP) assays reveal that IKK-alpha was recruited to the promoter regions of NF-kappaB-regulated genes on stimulation with tumour-necrosis factor-alpha. Notably, NF-kappaB-regulated gene expression is suppressed by the loss of IKK-alpha and this correlates with a complete loss of gene-specific phosphorylation of histone H3 on serine 10, a modification previously associated with positive gene expression. Furthermore, we show that IKK-alpha can directly phosphorylate histone H3 in vitro, suggesting a new substrate for this kinase. We propose that IKK-alpha is an essential regulator of NF-kappaB-dependent gene expression through control of promoter-associated histone phosphorylation after cytokine exposure. These findings provide additional insight into the role of the IKK complex in NF-kappaB-regulated gene expression.

IKK beta plays an essential role in the phosphorylation of RelA/p65 on serine 536 induced by lipopolysaccharide

Activation of the I kappa B kinase (IKK) complex by LPS induces phosphorylation and degradation of I kappa B alpha, leading to the nuclear translocation of NF-kappa B. Although it is essential for NF-kappa B activation, emerging evidence has indicated that the nuclear translocation of NF-kappa B is not sufficient to activate NF-kappa B-dependent transcription. Here, we reported that LPS induced the phosphorylation of the p65 trans-activation domain on serine 536 in monocytes/macrophages. Using mouse embryonic fibroblasts lacking either IKK alpha or IKK beta, we found that IKK beta played an essential role in LPS-induced p65 phosphorylation on serine 536, while IKK alpha was partially required for the p65 phosphorylation. The LPS-induced p65 phosphorylation on serine 536 was independent of the phosphatidylinositol 3'-kinase/Akt signaling pathway. Furthermore, we found that the phosphorylation on serine 536 increased the p65 transcription activity. In summary, our results demonstrate that IKK beta plays an essential role in the LPS-induced p65 phosphorylation on serine 536, which may represent a mechanism to regulate the NF-kappa B transcription activity by LPS.

In vivo identification of inducible phosphoacceptors in the IKKgamma/NEMO subunit of human IkappaB kinase

Transcription factor NF-kappaB plays a pivotal regulatory role in the genetic programs for cell cycle progression and inflammation. Nuclear translocation of NF-kappaB is controlled by an inducible protein kinase called IKK, which earmarks cytoplasmic inhibitors of NF-kappaB for proteolytic destruction. IKK contains two structurally related catalytic subunits termed IKKalpha and IKKbeta as well as a noncatalytic subunit called IKKgamma/NEMO. Mutations in the X-linked gene encoding IKKgamma can interfere with NF-kappaB signaling and lead to immunodeficiency disease. Although its precise mechanism of action remains unknown, IKKgamma is phosphorylated in concert with the induction of NF-kappaB by the viral oncoprotein Tax and the proinflammatory cytokine tumor necrosis factor alpha (TNF). We now demonstrate that TNF-induced phosphorylation of IKKgamma is blocked in cells deficient for IKKbeta but not IKKalpha. Phosphopeptide-mapping experiments with metabolically radiolabeled cells indicate that IKKbeta phosphorylates human IKKgamma at Ser-31, Ser-43, and Ser-376 following the enforced expression of either the Tax oncoprotein or the type 1 TNF receptor. Inducible phosphorylation of IKKgamma is attenuated following the deletion of its COOH-terminal zinc finger domain (amino acids 397-419), a frequent target for mutations that occur in IKKgamma-associated immunodeficiencies. As such, IKKbeta-mediated phosphorylation of IKKgamma at these specific serine targets may facilitate proper regulation of NF-kappaB signaling in the immune system.

Serine phosphorylation of insulin receptor substrate 1 by inhibitor kappa B kinase complex

Insulin resistance contributes importantly to the pathophysiology of type 2 diabetes mellitus. One mechanism mediating insulin resistance may involve the phosphorylation of serine residues in insulin receptor substrate-1 (IRS-1), leading to impairment in the ability of IRS-1 to activate downstream phosphatidylinositol 3-kinase-dependent pathways. Insulin-resistant states and serine phosphorylation of IRS-1 are associated with the activation of the inhibitor kappaB kinase (IKK) complex. However, the precise molecular mechanisms by which IKK may contribute to the development of insulin resistance are not well understood. In this study, using phosphospecific antibodies against rat IRS-1 phosphorylated at Ser(307) (equivalent to Ser(312) in human IRS-1), we observed serine phosphorylation of IRS-1 in response to TNF-alpha or calyculin A treatment that paralleled surrogate markers for IKK activation. The phosphorylation of human IRS-1 at Ser(312) in response to tumor necrosis factor-alpha was significantly reduced in cells pretreated with the IKK inhibitor 15 deoxy-prostaglandin J(2) as well as in cells derived from IKK knock-out mice. We observed interactions between endogenous IRS-1 and IKK in intact cells using a co-immunoprecipitation approach. Moreover, this interaction between IRS-1 and IKK in the basal state was reduced upon IKK activation and increased serine phosphorylation of IRS-1. Data from in vitro kinase assays using recombinant IRS-1 as a substrate were consistent with the ability of IRS-1 to function as a direct substrate for IKK with multiple serine phosphorylation sites in addition to Ser(312). Taken together, our data suggest that IRS-1 is a novel direct substrate for IKK and that phosphorylation of IRS-1 at Ser(312) (and other sites) by IKK may contribute to the insulin resistance mediated by activation of inflammatory pathways.