Analysis of nuclear factor-κB (NF-κB) essential modulator (NEMO) binding to linear and lysine-linked ubiquitin chains and its role in the activation of NF-κB

Nuclear factor-κB (NF-κB) essential modulator (NEMO), a component of the inhibitor of κB kinase (IKK) complex, controls NF-κB signaling by binding to ubiquitin chains. Structural studies of NEMO provided a rationale for the specific binding between the UBAN (ubiquitin binding in ABIN and NEMO) domain of NEMO and linear (Met-1-linked) di-ubiquitin chains. Full-length NEMO can also interact with Lys-11-, Lys-48-, and Lys-63-linked ubiquitin chains of varying length in cells. Here, we show that purified full-length NEMO binds preferentially to linear ubiquitin chains in competition with lysine-linked ubiquitin chains of defined length, including long Lys-63-linked deca-ubiquitins. Linear di-ubiquitins were sufficient to activate both the IKK complex in vitro and to trigger maximal NF-κB activation in cells. In TNFα-stimulated cells, NEMO chimeras engineered to bind exclusively to Lys-63-linked ubiquitin chains mediated partial NF-κB activation compared with cells expressing NEMO that binds to linear ubiquitin chains. We propose that NEMO functions as a high affinity receptor for linear ubiquitin chains and a low affinity receptor for long lysine-linked ubiquitin chains. This phenomenon could explain quantitatively distinct NF-κB activation patterns in response to numerous cell stimuli.     

IκB激酶(IKK)结构与功能研究进展

真核细胞核转录因子Rel/NF·κB家族广泛存在于从昆虫至人类细胞中,参与细胞的分化、发育、凋亡、黏附及炎症反应。通常NF·κB与其抑制蛋白IκB结合以无活性的方式滞留于胞浆,当多种外部信号作用于细胞时,活化的NF·κB进入细胞核内发挥其功能。目前,对NF·κB激活的信号传导机制已基本明确,IκB激酶(IκB Kinase,IKK)在其中起重要作用,本文综述了近年来对IKK结构、功能及其调控的最新进展。     

Thioredoxin-mediated Denitrosylation Regulates Cytokine-induced Nuclear Factor κB (NF-κB) Activation

S-nitrosylation of nuclear factor κB (NF-κB) on the p65 subunit of the p50/p65 heterodimer inhibits NF-κB DNA binding activity. We have recently shown that p65 is constitutively S-nitrosylated in the lung and that LPS-induced injury elicits a decrease in SNO-p65 levels concomitant with NF-κB activation in the respiratory epithelium and initiation of the inflammatory response. Here, we demonstrate that TNFα-mediated activation of NF-κB in the respiratory epithelium similarly induces p65 denitrosylation. This process is mediated by the denitrosylase thioredoxin (Trx), which becomes activated upon cytokine-induced degradation of thioredoxin-interacting protein (Txnip). Similarly, inhibition of Trx activity in the lung attenuates LPS-induced SNO-p65 denitrosylation, NF-κB activation, and airway inflammation, supporting a pathophysiological role for this mechanism in lung injury. These data thus link stimulus-coupled activation of NF-κB to a specific, protein-targeted denitrosylation mechanism and further highlight the importance of S-nitrosylation in the regulation of the immune response.     

香鱼(Plecoglossus altivelis)NFκB抑制因子α基因PaIκBα克隆及表达

核转录因子κB抑制因子α(IκBα)是NFκB/IκB信号传导通路的重要成员,参与机体抗细菌感染等多种免疫反应,可通过蛋白质间的相互作用结合核转录因子NFκB,从而调控生物体多种免疫基因表达。该研究采用RACE技术从香鱼中克隆得到核转录因子κB抑制因子PaIκBα基因的cDNA全长序列(1341bp,GenBank Accession No.JN801027),开放阅读框ORF为936bp,编码311个氨基酸,5’非编码区为64bp,3’非编码区为341bp。生物信息学分析表明,香鱼IκBα蛋白的序列中包含5个保守的锚蛋白重复序列,N末端含有信号诱导蛋白,C末端含有PEST序列。同源性比对结果表明,香鱼IκBα蛋白与胡瓜鱼IκBα的同源性最高,为95%;其次是大西洋鲑、虹鳟、尼罗罗非鱼和鳜鱼等,同源性分别为76%、75%、70%和68%。系统进化树分析表明,香鱼IκBα蛋白与胡瓜鱼、虹鳟、尼罗罗非鱼、鳜鱼和大西洋鲑等亲缘关系最近。RT-PCR分析表明,PaIκBα基因在香鱼肝脏、肾脏、脾脏和鳃中表达水平较高,其次是肠、脑和肌肉,在心脏中表达极少。嗜水气单胞菌(Aeromonas hydrophil)感染香鱼后,PaIκBα基因表达增强,感染24h达最大值,表明PaIκBα基因在香鱼受到嗜水气单胞菌刺激的免疫过程中可能发挥着重要作用。     

Nitric oxide modulates sodium vitamin C transporter 2 (SVCT-2) protein expression via protein kinase G (PKG) and nuclear factor-κB (NF-κB)

Ascorbate is an important antioxidant, which also displays important functions in neuronal tissues, including the retina. The retina is responsible for the initial steps of visual processing, which is further refined in cerebral high-order centers. The retina is also a prototypical model for studying physiologic aspects of cells that comprise the nervous system. Of major importance also is the cellular messenger nitric oxide (NO). Previous studies have demonstrated the significance of NO for both survival and proliferation of cultured embryonic retinal cells. Cultured retinal cells express a high-affinity ascorbate transporter, and the release of ascorbate is delicately regulated by ionotropic glutamate receptors. Therefore, we proposed whether there is interplay between the ascorbate transport system and NO signaling pathway in retinal cells. Here we show compelling evidence that ascorbate uptake is tightly controlled by NO and its downstream signaling pathway in culture. NO also modulates the expression of SVCT-2, an effect mediated by cGMP and PKG. Kinetic studies suggest that NO increases the transport capacity for ascorbate, but not the affinity of SVCT-2 for its substrate. Interestingly, NO utilizes the NF-κB pathway, in a PKG-dependent manner, to modulate both SVCT-2 expression and ascorbate uptake. These results demonstrate that NO exerts a fine-tuned control of the availability of ascorbate to cultured retinal cells and strongly reinforces ascorbate as an important bioactive molecule in neuronal tissues.     

Ezrin-Radixin-Moesin-binding Phosphoprotein 50 (EBP50) and Nuclear Factor-κB (NF-κB): A FEED-FORWARD LOOP FOR SYSTEMIC AND VASCULAR INFLAMMATION*

The interaction between vascular cells and macrophages is critical during vascular remodeling. Here we report that the scaffolding protein, ezrin-binding phosphoprotein 50 (EBP50), is a central regulator of macrophage and vascular smooth muscle cells (VSMC) function. EBP50 is up-regulated in intimal VSMC following endoluminal injury and promotes neointima formation. However, the mechanisms underlying these effects are not fully understood. Because of the fundamental role that inflammation plays in vascular diseases, we hypothesized that EBP50 mediates macrophage activation and the response of vessels to inflammation. Indeed, EBP50 expression increased in primary macrophages and VSMC, and in the aorta of mice, upon treatment with LPS or TNFα. This increase was nuclear factor-κB (NF-κB)-dependent. Conversely, activation of NF-κB was impaired in EBP50-null VSMC and macrophages. We found that inflammatory stimuli promote the formation of an EBP50-PKCζ complex at the cell membrane that induces NF-κB signaling. Macrophage activation and vascular inflammation after acute LPS treatment were reduced in EBP50-null cells and mice as compared with WT. Furthermore, macrophage recruitment to vascular lesions was significantly reduced in EBP50 knock-out mice. Thus, EBP50 and NF-κB participate in a feed-forward loop leading to increased macrophage activation and enhanced response of vascular cells to inflammation.     

Liver Kinase B1 Suppresses Lipopolysaccharide-induced Nuclear Factor κB (NF-κB) Activation in Macrophages

Liver kinase B1 (LKB1), a serine/threonine kinase, is a tumor suppressor and metabolic regulator. Recent data suggest that LKB1 is essential in regulating homeostasis of hematopoietic cells and immune responses. However, its role in macrophages and innate immune system remains unclear. Here we report that macrophage LKB1 inhibits pro-inflammatory signaling in response to LPS. LPS-induced pro-inflammatory cytokines and pro-inflammatory enzymes were monitored in bone marrow-derived macrophages isolated from myeloid cell-specific LKB1 knock out mice and their wild type littermate control mice. LPS induced higher levels of pro-inflammatory cytokines and pro-inflammatory enzymes in bone marrow-derived macrophages from LKB1 KO than those from wild type mice. Consistently, LPS induced higher levels of NF-κB activation in LKB1-deficient macrophages than those in wild type. Further, LPS stimulation significantly increased LKB1 phosphorylation at serine 428, which promoted its binding to IκB kinaseβ (IKKβ), resulting in the inhibition of NF-κB. Finally, LPS injection caused higher levels of cytokine release and more severe tissue injury in the lung tissues of LKB1 KO mice than in those of control mice. We conclude that LKB1 inhibits LPS-induced NF-κB activation in macrophages.     

Suppression of NF-κB and NF-κB regulated oxidative stress and neuroinflammation by BAY 11-7082 (IκB phosphorylation inhibitor) in experimental diabetic neuropathy

Inflammation is an emerging patho-mechanism of diabetes and its complications. NF-κB pathway is one of the central machinery initiating and propagating inflammatory responses. The present study envisaged the involvement of NF-κB inflammatory cascade in the pathophysiology of diabetic neuropathy using BAY 11-7082, an IκB phosphorylation inhibitor. Streptozotocin was used to induce diabetes in Sprauge Dawley rats. BAY 11-7082 (1 &; 3 mg/kg) was administered to diabetic rats for 14 days starting from the end of six weeks post diabetic induction. Diabetic rats developed deficits in nerve functions and altered nociceptive parameters and also showed elevated expression of NF-κB (p65), IκB and p-IκB along with increased levels of IL-6 &; TNF-α and inducible enzymes (COX-2 and iNOS). Furthermore, there was an increase in oxidative stress and decrease in Nrf2/HO-1 expression. We observed that BAY 11-7082 alleviated abnormal sensory responses and deficits in nerve functions. BAY 11-7082 also ameliorated the increase in expression of NF-κB, IκB and p-IκB. BAY 11-7082 curbed down the levels of IL-6, TNF-α, COX-2 and iNOS in the sciatic nerve. Lowering of lipid peroxidation and improvement in GSH levels was also seen along with increased expression of Nrf2/HO-1. Thus it can be concluded that NF-κB expression and downstream expression of proinflammatory mediators are prominent features of nerve damage leading to inflammation and oxidative stress and BAY 11-7082 was able to ameliorate experimental diabetic neuropathy by modulating neuroinflammation and improving antioxidant defence.     

NF-κB essential modulator (NEMO) interaction with linear and lys-63 ubiquitin chains contributes to NF-κB activation

The IκB kinase (IKK) complex acts as a gatekeeper of canonical NF-κB signaling in response to upstream stimulation. IKK activation requires sensing of ubiquitin chains by the essential IKK regulatory subunit IKKγ/NEMO. However, it has remained enigmatic whether NEMO binding to Lys-63-linked or linear ubiquitin chains is critical for triggering IKK activation. We show here that the NEMO C terminus, comprising the ubiquitin binding region and a zinc finger, has a high preference for binding to linear ubiquitin chains. However, immobilization of NEMO, which may be reminiscent of cellular oligomerization, facilitates the interaction with Lys-63 ubiquitin chains. Moreover, selective mutations in NEMO that abolish association with linear ubiquitin but do not affect binding to Lys-63 ubiquitin are only partially compromising NF-κB signaling in response to TNFα stimulation in fibroblasts and T cells. In line with this, TNFα-triggered expression of NF-κB target genes and induction of apoptosis was partially compromised by NEMO mutations that selectively impair the binding to linear ubiquitin chains. Thus, in vivo NEMO interaction with linear and Lys-63 ubiquitin chains is required for optimal IKK activation, suggesting that both type of chains are cooperating in triggering canonical NF-κB signaling.     

Molecular characterization and expression analysis of an inhibitor of NF-κB (IκB) from Asiatic hard clam Meretrix meretrix

Inhibitor of NF-κB (IκB) is an important member of Rel/NF-κB signaling pathway, which is an important mediator of immune responses in innate immune system. In this study, the IκB cDNA of hard clam Meretrix meretrix (designated as Mm-IκB) was cloned and characterized. The full-length cDNA of Mm-IκB was of 2098 bp, containing a 5' untranslated region (UTR) of 123 bp, a 3' UTR of 810 bp with a poly (A) tail, and an open reading frame (ORF) of 1164 bp encoding a polypeptide of 387 amino acids. The high similarity of Mm-IκB with other IκBs from invertebrates indicated that Mm-IκB should be a member of IκB family. Similar to most IκBs, Mm-IκB possessed all conserved features critical for the fundamental structure and function of IκBs, such as five ankyrin repeats and a conserved degradation motif (DS(44)RYSS(48)). Two PEST domains and a phosphorylation site motif (S(367)EEE(370)) at the C-terminus of Mm-IκB were identified. By quantitative real-time RT-PCR analysis, mRNA level of Mm-IκB was found to be most abundantly expressed in the tissues of mantle, gill and hepatopancreas, weakly expressed in muscle, foot and haemocyte. The Mm-IκB gene expression was significantly up-regulated at 24 h in haemocyte and at 12 h in gill after Vibrio anguillarum challenge, respectively. The results suggested the involvement of Mm-IκB in response against bacterial infection and further highlighted its functional importance in the immune system of M. meretrix.