Identification of a cell protein (FIP-3) as a modulator of NF-kappaB activity and as a target of an adenovirus inhibitor of tumor necrosis factor alpha-induced apoptosis

An Old Acquaintance: Could Adenoviruses Be Our Next Pandemic Threat?

Human adenoviruses (HAdV) are one of the most important pathogens detected in acute respiratory diseases in pediatrics and immunocompromised patients. In 1953, Wallace Rowe described it for the first time in oropharyngeal lymphatic tissue. To date, more than 110 types of HAdV have been described, with different cellular tropisms. They can cause respiratory and gastrointestinal symptoms, even urinary tract inflammation, although most infections are asymptomatic. However, there is a population at risk that can develop serious and even lethal conditions. These viruses have a double-stranded DNA genome, 25-48 kbp, 90 nm in diameter, without a mantle, are stable in the environment, and resistant to fat-soluble detergents. Currently the diagnosis is made with lateral flow immunochromatography or molecular biology through a polymerase chain reaction. This review aimed to highlight the HAdV variability and the pandemic potential that a HAdV3 and 7 recombinant could have considering the aggressive outbreaks produced in health facilities. Herein, we described the characteristics of HAdV, from the infection to treatment, vaccine development, and the evaluation of the social determinants of health associated with HAdV, suggesting the necessary measures for future sanitary control to prevent disasters such as the SARS-CoV-2 pandemic, with an emphasis on the use of recombinant AdV vaccines to control other potential pandemics.

Dysregulation of Tweak and Fn14 in skeletal muscle of spinal muscular atrophy mice

BACKGROUND

Spinal muscular atrophy (SMA) is a childhood neuromuscular disorder caused by depletion of the survival motor neuron (SMN) protein. SMA is characterized by the selective death of spinal cord motor neurons, leading to progressive muscle wasting. Loss of skeletal muscle in SMA is a combination of denervation-induced muscle atrophy and intrinsic muscle pathologies. Elucidation of the pathways involved is essential to identify the key molecules that contribute to and sustain muscle pathology. The tumor necrosis factor-like weak inducer of apoptosis (TWEAK)/TNF receptor superfamily member fibroblast growth factor-inducible 14 (Fn14) pathway has been shown to play a critical role in the regulation of denervation-induced muscle atrophy as well as muscle proliferation, differentiation, and metabolism in adults. However, it is not clear whether this pathway would be important in highly dynamic and developing muscle.

METHODS

We thus investigated the potential role of the TWEAK/Fn14 pathway in SMA muscle pathology, using the severe Taiwanese Smn^-/-; SMN2 and the less severe Smn^2B/- SMA mice, which undergo a progressive neuromuscular decline in the first three post-natal weeks. We also used experimental models of denervation and muscle injury in pre-weaned wild-type (WT) animals and siRNA-mediated knockdown in C2C12 muscle cells to conduct additional mechanistic investigations.

RESULTS

Here, we report significantly dysregulated expression of Tweak, Fn14, and previously proposed downstream effectors during disease progression in skeletal muscle of the two SMA mouse models. In addition, siRNA-mediated Smn knockdown in C2C12 myoblasts suggests a genetic interaction between Smn and the TWEAK/Fn14 pathway. Further analyses of SMA, Tweak^-/-, and Fn14^-/- mice revealed dysregulated myopathy, myogenesis, and glucose metabolism pathways as a common skeletal muscle feature, providing further evidence in support of a relationship between the TWEAK/Fn14 pathway and Smn. Finally, administration of the TWEAK/Fn14 agonist Fc-TWEAK improved disease phenotypes in the two SMA mouse models.

CONCLUSIONS

Our study provides mechanistic insights into potential molecular players that contribute to muscle pathology in SMA and into likely differential responses of the TWEAK/Fn14 pathway in developing muscle.

Pathogenesis and management of adenoviral keratoconjunctivitis

Human adenovirus (HAdV) is a ubiquitous virus that infects the mucosa of the eye. It is the most common cause of infectious conjunctivitis worldwide, affecting people of all ages and demographics. Pharyngoconjunctival fever outbreak is due to HAdV types 3, 4, and 7, whereas outbreaks of epidemic keratoconjunctivitis are usually caused by HAdV types 8, 19, 37, and 54. Primary cellular receptors, such as CAR, CD46, and sialic acid interact with fiber-knob protein to mediate adenoviral attachment to the host cell, whereas adenoviral penton base–integrin interaction mediates internalization of adenovirus. Type 1 immunoresponse to adenoviral ocular infection involves both innate immunity mediated by natural killer cells and type 1 interferon, as well as adaptive immunity mediated mainly by CD8 T cells. The resulting ocular manifestations are widely variable, with pharyngoconjunctival fever being the most common, manifesting clinically with fever, pharyngitis, and follicular conjunctivitis. Epidemic keratoconjunctivitis, however, is the severest form, with additional involvement of the cornea leading to development of subepithelial infiltrates. Because there is currently no US Food and Drug Administration-approved treatment for adenoviral ocular infection, current management is palliative. The presence of sight-threatening complications following ocular adenoviral infection warrants the necessity for developing antiadenoviral therapy with enhanced therapeutic index. Future trends that focus on adenoviral pathogenesis, including adenoviral protein, which utilize host receptors to promote infection, could be potential therapeutic targets, yielding shorter active disease duration and reduced disease burden.

Role of IκB kinase β in regulating the remodeling of the CARMA1-Bcl10-MALT1 complex

The current work investigates the notion that inducible clustering of signaling mediators of the IKK pathway is important for platelet activation. Thus, while the CARMA1, Bcl10, and MALT1 (CBM) complex is essential for triggering IKK/NF-κB activation upon platelet stimulation, the signals that elicit its formation and downstream effector activation remain elusive. We demonstrate herein that IKKβ is involved in membrane fusion, and serves as a critical protein kinase required for initial formation and the regulation of the CARMA1/MALT1/Bcl10/CBM complex in platelets. We also show that IKKβ regulates these processes via modulation of phosphorylation of Bcl10 and IKKγ polyubiquitination. Collectively, our data demonstrate that IKKβ regulates membrane fusion and the remodeling of the CBM complex formation.

Alternative RNA splicing in the endothelium mediated in part by Rbfox2 regulates the arterial response to low flow

Low and disturbed blood flow drives the progression of arterial diseases including atherosclerosis and aneurysms. The endothelial response to flow and its interactions with recruited platelets and leukocytes determine disease progression. Here, we report widespread changes in alternative splicing of pre-mRNA in the flow-activated murine arterial endothelium in vivo. Alternative splicing was suppressed by depletion of platelets and macrophages recruited to the arterial endothelium under low and disturbed flow. Binding motifs for the Rbfox-family are enriched adjacent to many of the regulated exons. Endothelial deletion of Rbfox2, the only family member expressed in arterial endothelium, suppresses a subset of the changes in transcription and RNA splicing induced by low flow. Our data reveal an alternative splicing program activated by Rbfox2 in the endothelium on recruitment of platelets and macrophages and demonstrate its relevance in transcriptional responses during flow-driven vascular inflammation.

Incontinentia pigmenti with secondary Raynaud's phenomenon: A case report and review of the literature

Purpose

To describe a patient with incontinentia pigmenti (IP) and Raynaud's phenomenon (RP).

Observations

A 5 year-old girl with history of IP was noted to have RP. Visual acuity was unaffected in both eyes, and fundus examination demonstrated regressed peripheral neovascularization. Photos of the patient's hands demonstrated pale discoloration associated with exposure to cold.

Conclusions and importance

IP, known to affect small cerebral and retinal blood vessels, can also affect the small blood vessels in the extremities, resulting in secondary RP.

Inducible turnover of optineurin regulates T cell activation

Optineurin (Optn) is an adaptor protein with homology to NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase (IKK) complex. Dysregulation of Optn has been linked to neurodegenerative, autoimmune and bone diseases. Optn shares a high degree of homology with NEMO, but is not part of the same high-molecular weight complex containing IKKα and IKKβ. Despite its homology with NEMO and the fact that it has been the subject of extensive study in several cell types, there are no published studies addressing the role of Optn during T cell activation. Here we demonstrate that ectopic expression of Optn down-regulates TCR-induced NF-κB activation and TNF-α production, in a manner dependent on ubiquitin-binding. Conversely, knock-down of Optn enhances NF-κB activation and the production of TNF-α. Consistent with a negative regulatory role for this protein, we observed transient loss of Optn after TCR stimulation in both cell lines and in primary murine T cells. The acute loss of Optn appears to be due to both protein degradation and exocytosis, the latter via activation-induced exosomes. This study therefore provides novel information regarding the role of Optn during TCR activation, suggesting the possible importance of Optn during inflammation and/or autoimmune diseases.

NEMO regulates a cell death switch in TNF signaling by inhibiting recruitment of RIPK3 to the cell death-inducing complex II

Incontinentia Pigmenti (IP) is a rare X-linked disease characterized by early male lethality and multiple abnormalities in heterozygous females. IP is caused by NF-κB essential modulator (NEMO) mutations. The current mechanistic model suggests that NEMO functions as a crucial component mediating the recruitment of the IκB-kinase (IKK) complex to tumor necrosis factor receptor 1 (TNF-R1), thus allowing activation of the pro-survival NF-κB response. However, recent studies have suggested that gene activation and cell death inhibition are two independent activities of NEMO. Here we describe that cells expressing the IP-associated NEMO-A323P mutant had completely abrogated TNF-induced NF-κB activation, but retained partial antiapoptotic activity and exhibited high sensitivity to death by necroptosis. We found that robust caspase activation in NEMO-deficient cells is concomitant with RIPK3 recruitment to the apoptosis-mediating complex. In contrast, cells expressing the ubiquitin-binding mutant NEMO-A323P did not recruit RIPK3 to complex II, an event that prevented caspase activation. Hence NEMO, independently from NF-κB activation, represents per se a key component in the structural and functional dynamics of the different TNF-R1-induced complexes. Alteration of this process may result in differing cellular outcomes and, consequently, also pathological effects in IP patients with different NEMO mutations.

Quantification of cellular NEMO content and its impact on NF-κB activation by genotoxic stress

NF-κB essential modulator, NEMO, plays a key role in canonical NF-κB signaling induced by a variety of stimuli, including cytokines and genotoxic agents. To dissect the different biochemical and functional roles of NEMO in NF-κB signaling, various mutant forms of NEMO have been previously analyzed. However, transient or stable overexpression of wild-type NEMO can significantly inhibit NF-κB activation, thereby confounding the analysis of NEMO mutant phenotypes. What levels of NEMO overexpression lead to such an artifact and what levels are tolerated with no significant impact on NEMO function in NF-κB activation are currently unknown. Here we purified full-length recombinant human NEMO protein and used it as a standard to quantify the average number of NEMO molecules per cell in a 1.3E2 NEMO-deficient murine pre-B cell clone stably reconstituted with full-length human NEMO (C5). We determined that the C5 cell clone has an average of 4 x 10⁵ molecules of NEMO per cell. Stable reconstitution of 1.3E2 cells with different numbers of NEMO molecules per cell has demonstrated that a 10-fold range of NEMO expression (0.6–6x10⁵ molecules per cell) yields statistically equivalent NF-κB activation in response to the DNA damaging agent etoposide. Using the C5 cell line, we also quantified the number of NEMO molecules per cell in several commonly employed human cell lines. These results establish baseline numbers of endogenous NEMO per cell and highlight surprisingly normal functionality of NEMO in the DNA damage pathway over a wide range of expression levels that can provide a guideline for future NEMO reconstitution studies.

A method for the quantitative analysis of stimulation-induced nuclear translocation of the p65 subunit of NF-κB from patient-derived dermal fibroblasts

Developmental and immune-mediated disease has been linked to genetic mutation of key signaling components involved in NF-κB activation that leads to impaired activation or regulation of the canonical IKK complex. We identify patients with suspected or known defects of the NF-κB signaling pathway through clinical phenotyping and genetic sequencing. To help understand how mutations cause disease, we quantitate the kinetics and dose-response of NF-κB activation signaling events in their cells. Following activation of the canonical IKK complex, phosphorylation of the inhibitor of NF-κB proteins (IκB) leads to their degradation and the subsequent translocation of NF-κB family members from the cell cytoplasm to the nucleus. Here, we provide a method to obtain patient-derived dermal fibroblasts and quantitatively assess the integrity of the signal transduction pathway from receptor activation to nuclear p65 translocation.

STAT1 interaction with E3-14.7K in monocytes affects the efficacy of oncolytic adenovirus

Oncolytic viruses based on adenovirus type 5 (Ad5) have been developed as a new class of therapeutic agents for cancers that are resistant to conventional therapies. Clinical experience shows that these agents are safe, but virotherapy alone has not achieved long-term cure in cancer patients. The vast majority of oncolytic adenoviruses used in clinical trials to date have deletion of the E3B genes. It has been demonstrated that the antitumor potency of the E3B-deleted mutant (dl309) is inferior to adenovirus with E3B genes intact. Tumors treated with dl309 show markedly greater macrophage infiltration than E3B-intact adenovirus. However, the functional mechanisms for this were not previously known. Here, we demonstrate that deletion of E3B genes increases production of chemokines by monocytes after adenovirus infection and increases monocyte migration. The E3B 14,700-Da protein (E3B-14.7K) inhibits STAT1 function by preventing its phosphorylation and nuclear translocation. The STAT1 inhibitor, fludarabine, rescues the effect of E3B-14.7K deletion by downregulating target chemokine expression in human and murine monocytes and results in an enhanced antitumor efficacy with dl309 in vivo. These findings have important implications for clinical use of E3B-deleted oncolytic adenovirus and other E3B-deleted adenovirus vector-based therapy.

Cytokine patterns associated with a serotype 8 fowl adenovirus infection

This study examined cytokine gene expression patterns associated with fowl adenovirus (FAdV) infection. The selected cytokine mRNA was quantified by quantitative real-time reverse transcription-PCR in spleen, liver, and cecal tonsil during the course of infection of chickens with a serotype 8 FAdV (FAdV-8). Compared to uninfected chickens, infected birds had higher mRNA expression of interleukin (IL)-18 and IL-10 in spleen and liver, respectively. Interferon gamma (IFN-γ) mRNA expressed in spleen and liver of infected chickens was significantly upregulated, while the expression of IL-8 mRNA in spleen and liver of infected chickens was significantly downregulated. There was no significant difference between infected and uninfected groups in terms of cytokine gene expression in cecal tonsil. These results indicate that these four cytokines might play an important role in driving the immune responses following FAdV-8 infection.

Gene Expression Analysis of Toll-Like Receptor Pathways in Heterophils from Genetic Chicken Lines that Differ in Their Susceptibility to Salmonella enteritidis

Previously conducted studies using two chicken lines (A and B) show that line A birds have increased resistance to a number of bacterial and protozoan challenges and that heterophils isolated from line A birds are functionally more responsive. Furthermore, when stimulated with Toll-like receptor (TLR) agonists, heterophils from line A expressed a totally different cytokine and chemokine mRNA expression pattern than heterophils from line B. A large-scale gene expression profile using an Agilent 44K microarray on heterophils isolated from line A and line B also revealed significantly differential expression in many immune-related genes following Salmonella enteritidis (SE) stimulation, which included genes involved in the TLR pathway. Therefore, we hypothesize the differences between the lines result from distinctive TLR pathway signaling cascades that mediate heterophil function and, thus, innate immune responsiveness to SE. Using quantitative RT-PCR on mRNA from heterophils isolated from control and SE-stimulated heterophils of each line, we profiled the expression of all chicken homologous genes identified in a reference TLR pathway. Several differentially expressed genes found were involved in the TLR-induced My88-dependent pathway, showing higher gene expression in line A than line B heterophils following SE stimulation. These genes included the TLR genes TLR4, TLR15, TLR21, MD-2, the adaptor proteins Toll-interleukin 1 receptor domain-containing adaptor protein (TIRAP), Tumor necrosis factor-receptor associated factor 3 (TRAF3), the IκB kinases transforming growth factor-β-activating kinase 1 (TAK1), IKKε and IKKα, the transcription factors NFkB2 and interferon regulatory factor 7, phosphatidylinositol-3 kinase (PI-3K), and the mitogen-activated protein kinase p38. These results indicate that higher expression of TLR signaling activation of both MyD88-dependent and TRIF-dependent pathways are more beneficial to avian heterophil-mediated innate immunity and a complicated regulation of downstream adaptors is involved in stronger induction of a TLR-mediated innate response in the resistant line A. These findings identify new targets for genetic selection of chickens to increase resistance to bacterial infections.

RIP1-mediated regulation of lymphocyte survival and death responses

RIP1 is an adaptor serine/threonine kinase associated with the signaling complex of death receptors (DRs) including Fas, TNFR1, and TRAIL-Rs which can initiate apoptosis. While DRs are dispensable throughout development, RIP1 deletion results in perinatal lethality. The developmental defect caused by absence of RIP1 remains unexplained. In previous studies, RIP1-deficient hematopoietic progenitors failed to reconstitute the T cell compartment and our recent data indicate a new role for RIP1 in TCR-induced activation of the pro-survival NF-κB pathway. Here, we show that RIP1 is also critical for B cell development. In addition, RIP1(-/-) B cells stimulated through LPS/TLR4 are impaired in NF-κB activation but have no major defect in the Akt pathway. Recently, RIP1 has also emerged as a critical player in necrosis-like death, necroptosis, in various cell lines. We have demonstrated that RIP1 deficiency can reverse the embryonic and T cell proliferation defects in mice lacking FADD, a caspase adaptor protein, which indicates a potential role for RIP1 in mediating in vivo necroptosis. We provide an overview and discussion of the accumulating data revealing insights into the diverse functions of RIP1 in survival and death signaling in lymphocytes.

NEMO differentially regulates TCR and TNF-α induced NF-κB pathways and has an inhibitory role in TCR-induced NF-κB activation

NF-κB essential modulator (NEMO), the regulatory subunit of the IκB kinase (IKK) complex, is an essential adaptor both for inflammation stimuli and TCR-induced NF-κB activation. However, the exact mechanism of its function has not been fully understood. Here, we report that knockdown of NEMO by RNA interference in Jurkat E6.1 cells enhanced TCR-induced NF-κB report gene activity and IL-2 production by promotion of IκBα degradation and p65 nuclear translocation, whereas inhibited TNF-α and LPS-induced IκBα degradation without influencing the phosphorylation of MAPKs. In human primary T and Jurkat E6.1 cells, both CD3/CD28 and PMA/Ionomycin induced NF-κB activation showed a para-curve correlation with the dosage of small interfering RNA targeting NEMO (siNEMO): the NF-κB report gene activity was increased along with ascending doses of transfected siNEMO and reached the highest activity when knockdown about 70% of NEMO, then turned to decline and gradually be blocked once almost thoroughly knockdown of NEMO. Meanwhile, TNF-α induced NF-κB was always inhibited no matter how much NEMO was knockdown. Subcellular fractionation results suggested that upon CD3/CD28 costimulation, NEMO and IKKβ may not cotranslocate to cytoskeleton fraction as a conventional NEMO/IKK complex with a static stoichiometric ratio, instead the ratio of NEMO: IKKβ continuously shift from high to low. Depletion of NEMO accelerated TCR-induced cytoskeleton translocation of IKKβ. Altogether, this study suggests that NEMO may function as a rheostat exerting a negative action on TCR-induced NF-κB activation and differentially regulates TNF-α and TCR-induced NF-κB pathways.

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.

The cytomegaloviral protein pUL138 acts as potentiator of tumor necrosis factor (TNF) receptor 1 surface density to enhance ULb'-encoded modulation of TNF-α signaling

Human cytomegalovirus is a ubiquitous herpesvirus that establishes lifelong latent infection. Changes in immune homeostasis induce the reactivation of lytic infection, which is mostly inapparent in healthy individuals but often causes overt disease in immunocompromised hosts. Based on discrepant tumor necrosis factor receptor 1 surface disposition between human cytomegalovirus AD169 variants differing in the ULb' region, we identified the latency-associated gene product pUL138, which also is expressed during productive infection, as a selective potentiator of tumor necrosis factor receptor 1, one of the key receptors of innate immunity. Ectopically expressed pUL138 coprecipitated with tumor necrosis factor receptor 1, extended the protein half-life, and enhanced its signaling responses, thus leading to tumor necrosis factor receptor 1 hyperresponsiveness. Conversely, the targeted deletion of UL138 from the human cytomegaloviral genome strongly reduced tumor necrosis factor receptor 1 surface densities of infected cells. Remarkably, the comparison of UL138 deficiency to ULb' deficiency revealed the presence of further positive modulators of tumor necrosis factor alpha signal transduction encoded within the human cytomegalovirus ULb' region, identifying this region as a hub for multilayered tumor necrosis factor alpha signaling regulation.

The zinc finger domain of IKKγ (NEMO) protein in health and disease

Inhibitor of κB kinase (IKK) gamma (IKKγ), also known as nuclear factor κB (NF-κB) essential modulator (NEMO), is a component of the IKK complex that is essential for the activation of the NF-κB pathway. The NF-κB pathway plays a major role in the regulation of the expression of genes that are involved in immune response, inflammation, cell adhesion, cell survival and development. As part of the IKK complex, IKKγ plays a regulatory role by linking the complex to upstream signalling molecules. IKKγ contains two coiled-coil regions, a leucine zipper domain and a highly conserved zinc finger domain. Mutations affecting IKKγ have been associated with X-linked hypohidrotic ectodermal dysplasia with immune deficiency (HED-ID), with the majority of these mutations affecting the C-terminal region of the protein where the zinc finger is located. The zinc finger of IKKγ is needed for NF-κB activation in a cell- and stimulus-specific manner. The major mechanism by which the zinc finger plays this role appears to be the recognition of polyubiquitinated upstream signalling intermediates. This assertion reinforces the current notion that ubiquitination plays a major role in mediating protein–protein interactions in the NF-κB signalling pathway. Because the zinc finger domain of IKKγ is very likely involved in mediating interactions with ubiquitinated proteins, investigations that look for upstream activators or inhibitors of the IKK complex that bind to and interact with the zinc finger of IKKγ are required to gain a better insight into the exact roles of this domain and into the pathogenesis of HED-ID.

The critical protein interactions and structures that elicit growth deregulation in cancer and viral replication

One of the greatest challenges in biomedicine is to define the critical targets and network interactions that are subverted to elicit growth deregulation in human cells. Understanding and developing rational treatments for cancer requires a definition of the key molecular targets and how they interact to elicit the complex growth deregulation phenotype. Viral proteins provide discerning and powerful probes to understand both how cells work and how they can be manipulated using a minimal number of components. The small DNA viruses have evolved to target inherent weaknesses in cellular protein interaction networks to hijack the cellular DNA and protein replication machinery. In the battle to escape the inevitability of senescence and programmed cell death, cancers have converged on similar mechanisms, through the acquisition and selection of somatic mutations that drive unchecked cellular replication in tumors. Understanding the dynamic mechanisms through which a minimal number of viral proteins promote host cells to undergo unscheduled and pathological replication is a powerful strategy to identify critical targets that are also disrupted in cancer. Viruses can therefore be used as tools to probe the system-wide protein-protein interactions and structures that drive growth deregulation in human cells. Ultimately this can provide a path for developing system context-dependent therapeutics. This review will describe ongoing experimental approaches using viruses to study pathways deregulated in cancer, with a particular focus on viral cellular protein-protein interactions and structures.

A structural guide to proteins of the NF-kappaB signaling module

The prosurvival transcription factor NF-kappaB specifically binds promoter DNA to activate target gene expression. NF-kappaB is regulated through interactions with IkappaB inhibitor proteins. Active proteolysis of these IkappaB proteins is, in turn, under the control of the IkappaB kinase complex (IKK). Together, these three molecules form the NF-kappaB signaling module. Studies aimed at characterizing the molecular mechanisms of NF-kappaB, IkappaB, and IKK in terms of their three-dimensional structures have lead to a greater understanding of this vital transcription factor system.

NF-κB inducing kinase: a key regulator in the immune system and in cancer

NF-κB inducing kinase (NIK) is a kinase that activates the canonical and non-canonical NF-κB pathways to control transcriptional expression of certain proteins such as cytokines, chemokines and NF-κB signaling molecules. Many advances have been made in understanding the molecular mechanisms by which the stability of NIK is regulated to affect downstream signaling. Genetic mouse models suggest that NIK plays an essential role in the regulation of the immune system as well as in the bone microenvironment. Increasing evidence links NIK to the tumorigenesis of hematological cancers, such as multiple myeloma, and solid tumors, such as pancreatic carcinoma and melanoma. Understanding the mechanism by which NIK is de-regulated will potentially provide therapeutic options for certain diseases such as autoimmunity and cancer.

Proteins that bind to IKKgamma (NEMO) and down-regulate the activation of NF-kappaB

Inhibitor of kappaB kinase (IKK) gamma (IKKgamma), also referred to as nuclear factor kappaB (NF-kappaB) essential modulator (NEMO), is an important component of the IKK complex. Following the exposure of cells to NF-kappaB-inducing stimuli, the IKK complex catalyzes the phosphorylation of inhibitor of kappaB (IkappaB) proteins, which is a critical step that leads to the activation of NF-kappaB via the canonical pathway. The exact functions of IKKgamma as part of the IKK complex have not been fully elucidated. A number of proteins have been identified as directly interacting with IKKgamma and modulating the activity of the IKK complex. This mini review covers eight proteins that have been reported to bind to IKKgamma and lead to the suppression of the activities of the IKK complex and hence result in the down-regulation of the activation of NF-kappaB. The reported mechanisms by which these interactions suppress the activation of the IKK complex include the deubiquitination of IKKgamma and competition with upstream activators for binding to IKKgamma.

A mechanism for the suppression of interleukin-1-induced nuclear factor κB activation by protein phosphatase 2Cη-2

IL-1 (interleukin-1) is a pro-inflammatory cytokine that has a variety of effects during the process of inflammation. Stimulating cells with IL-1 initiates a signalling cascade that includes the activation of NF-κB (nuclear factor κB), and subsequently induces a variety of inflammatory genes. Although the molecular mechanism for the IL-1-induced activation of NF-κB has been well documented, much less is known about the mechanism by which protein phosphatases down-regulate this pathway. Here we show that mouse PP2Cη-2 (protein serine/threonine phosphatase 2Cη-2), a novel member of the protein serine/threonine phosphatase 2C family, inhibits the IL-1–NF-κB signalling pathway. Ectopic expression of PP2Cη-2 in human embryonic kidney HEK293IL-1RI cells inhibited the IL-1-induced activation of NF-κB. TAK1 (transforming-growth-factor-β-activated kinase 1) mediates the IL-1 signalling pathway to NF-κB, and we observed that the TAK1-induced activation of NF-κB was suppressed by PP2Cη-2 expression. Expression of IKKβ [IκB (inhibitory κB) kinase β], which lies downstream of TAK1, activates NF-κB, and this activation was also readily reversed by PP2Cη-2 co-expression. Additionally, PP2Cη-2 knockdown with small interfering RNA further stimulated the IL-1-enhanced phosphorylation of IKKβ and destabilization of IκBα in HeLa cells. PP2Cη-2 knockdown also increased the IL-1-induced expression of IL-6 mRNA. Furthermore, IKKβ was readily dephosphorylated by PP2Cη-2 in vitro. These results suggest that PP2Cη-2 inhibits the IL-1–NF-κB signalling pathway by selectively dephosphorylating IKKβ.

NEMO/IKKgamma regulates an early NF-kappaB-independent cell-death checkpoint during TNF signaling

TNF receptor 1 (TNFR1) ligation can result in cell survival or cell death. What determines which of the two opposing responses is triggered is not fully understood. The current model suggests that it is the activation of the NF-kappaB pathway and its induction of prosurvival genes, or the lack thereof, which determines the outcome. NF-kappaB essential modifier (NEMO)/IkappaB kinase-gamma (IKKgamma)-deficient cells are highly sensitive to apoptosis, and as NEMO is essential for NF-kappaB activation, it has been assumed that this is due to the lack of NF-kappaB. This study demonstrates that this assumption was incorrect and that NEMO has another antiapoptotic function that is independent of its role in the NF-kappaB pathway. NEMO prevents receptor interacting protein-1 (RIP1) from engaging CASPASE-8 before NF-kappaB-mediated induction of antiapoptotic genes. Without NEMO, RIP1 associates with CASPASE-8 resulting in rapid tumor necrosis factor (TNF)-induced apoptosis. These results suggest that there are two cell-death checkpoints following TNF stimulation: an early transcription-independent checkpoint whereby NEMO restrains RIP1 from activating the caspase cascade, followed by a later checkpoint dependent on NF-kappaB-mediated transcription of prosurvival genes.

Regulation of I(kappa)B kinase complex by phosphorylation of (gamma)-binding domain of I(kappa)B kinase (beta) by Polo-like kinase 1

IkappaB kinase (IKK) complex is a key regulator of NF-kappaB pathways. Signal-induced interaction of the IKKgamma (NEMO) subunit with the C-terminal IKKgamma/NEMO-binding domain (gammaBD) of IKKbeta is an essential interaction for IKK regulation. Underlying regulatory mechanism(s) of this interaction are not known. Phosphorylation of gammaBD has been suggested to play a regulatory role for IKK activation. However, a kinase that phosphorylates gammaBD has not been identified. In this study, we used a C-terminal fragment of IKKbeta as substrate and purified Polo-like kinase 1 (Plk1) from HeLa cell extracts by standard chromatography as a gammaBD kinase. Plk1 phosphorylates serines 733, 740, and 750 in the gammaBD of IKKbeta in vitro. Phosphorylating gammaBD with Plk1 decreased its affinity for IKKgamma in pulldown assay. We generated phosphoantibodies against serine 740 and showed that gammaBD is phosphorylated in vivo. Expressing a constitutively active Plk1 in mammalian cells reduced tumor necrosis factor (TNF)-induced IKK activation, resulting in decreased phosphorylation of endogenous IkappaBalpha and reduced NF-kappaB activation. To activate endogenous Plk1, cells were treated with nocodazole, which reduced TNF-induced IKK activation, and increased the phosphorylation of gammaBD. Knocking down Plk1 in mammalian cells restored TNF-induced IKK activation in nocodazole-treated cells. Activation of Plk1 inhibited TNF-induced expression of cyclin D1. In cells in which Plk1 was knocked down, TNFalpha increased expression of cyclin D1 and the proportion of cells in the S phase of the cell cycle. Taken together, this study shows that phosphorylation regulates the interaction of gammaBD of IKKbeta with IKKgamma and therefore plays a critical role for IKK activation. Moreover, we identify Plk1 as a gammaBD kinase, which negatively regulates TNF-induced IKK activation and cyclin D1 expression, thereby affecting cell cycle regulation. Untimely activation of cyclin D1 by TNFalpha can provide a potential mechanism for an involvement of TNFalpha in inflammation-induced cancer.

Signal processing by its coil zipper domain activates IKK gamma

NF-kappaB activation occurs upon degradation of its inhibitor I-kappaB and requires prior phosphorylation of the inhibitor by I-kappaB kinase (IKK). Activity of IKK is governed by its noncatalytic subunit IKKgamma. Signaling defects due to missense mutations in IKKgamma have been correlated to its inability to either become ubiquitylated or bind ubiquitin noncovalently. Because the relative contribution of these events to signaling had remained unknown, we have studied mutations in the coil-zipper (CoZi) domain of IKKgamma that either impair signaling or cause constitutive NF-kappaB activity. Certain signaling-deficient alleles neither bound ubiquitin nor were they ubiquitylated by TRAF6. Introducing an activating mutation into those signaling-impaired alleles restored their ubiquitylation and created mutants constitutively activating NF-kappaB without repairing the ubiquitin-binding defect. Constitutive activity therefore arises downstream of ubiquitin binding but upstream of ubiquitylation. Such constitutive activity reveals a signal-processing function for IKKgamma beyond that of a mere ubiquitin-binding adaptor. We propose that this signal processing may involve homophilic CoZi interactions as suggested by the enhanced affinity of CoZi domains from constitutively active IKKgamma.

Mutations in the zinc finger domain of IKK gamma block the activation of NF-kappa B and the induction of IL-2 in stimulated T lymphocytes

Mutations in the zinc finger of I kappa B kinase gamma (IKK gamma) are associated with hypohidrotic ectodermal dysplasia-immune deficiency (HED-ID) in which the major immune deficit is the inability to switch Ab heavy chain class. However, the pathophysiologic role of the mutations has not been fully delineated. Since help from activated Th cells is essential in Ab class switching, we sought to examine how these mutations affect T cell activation. Using a human T cell line that was null for IKK gamma, we generated cells stably expressing two of the reported mutations, namely, D406V and C417R. Cells expressing either mutation failed to induce IL-2 following stimulation with PMA/ionomycin while the induction of IL-2 was restored in cells reconstituted with the wild type IKK gamma. The lack of IL-2 upregulation correlated with the lack of NF-kappaB activation as evidenced by the inability to induce I kappa B alpha degradation, NF-kappaB binding to DNA and the expression of a reporter gene. However, both mutations did not prevent the incorporation of IKK gamma into the IKK complex and, interestingly, the induced phosphorylation of I kappa B alpha at S32 and S36 and its subsequent ubiquitination were not affected. The suppression of IL-2 induction was solely due to the inhibition of NF-kappaB activation as the mutations did not impair the activation of AP-1 and NFAT. Our data indicated that the failure of T cells to undergo activation in response to TCR stimuli may play a role in the pathophysiology of HED-ID and also showed that IKK gamma has a role in the post-ubiquitination processing of I kappa B alpha.

Inhibition of Jak1-dependent signal transduction in airway epithelial cells infected with adenovirus

Adenoviral evolution has generated mechanisms to resist host cell defense systems, but the biochemical basis for evasion of multiple antiviral pathways in the airway by adenoviruses is incompletely understood. We hypothesized that adenoviruses modulate airway epithelial responses to type I interferons by altering the levels and activation of specific Janus family kinase-signal transducer and activator of transcription (JAK-STAT) signaling components. In this study, specific effects of adenovirus type 5 (AdV) on selected JAK-STAT signal transduction pathways were identified in human tracheobronchial epithelial cells, with focus on type I interferon-dependent signaling and gene expression. We found that wild-type AdV infection inhibited IFN-alpha-induced expression of antiviral proteins in epithelial cells by blocking phosphorylation of the Stat1 and Stat2 transcription factors that are required for activation of type I interferon-dependent genes. These effects correlated with AdV-induced down-regulation of expression of the receptor-associated tyrosine kinase Jak1 through a decrease in Jak1 mRNA levels. Phosphorylation of Stat3 in response to IL-6 and oncostatin M was also lost in AdV-infected cells, indicating loss of epithelial cell responses to other cytokines that depend on Jak1. In contrast, IL-4- and IL-13-dependent phosphorylation of Stat6 was not affected during AdV infection, indicating that the virus modulates specific signaling pathways, as these Stat6-activating pathways can function independent of Jak1. Taken together, the results indicate that AdV down-regulates host epithelial cell Jak1 to assure inhibition of the antiviral effects of multiple mediators to subvert airway defense responses and establish a productive infection.

SINTBAD, a novel component of innate antiviral immunity, shares a TBK1-binding domain with NAP1 and TANK

The expression of antiviral genes during infection is controlled by inducible transcription factors such as IRF3 (interferon regulatory factor). Activation of IRF3 requires its phosphorylation by TBK1 (TANK-binding kinase) or IKKi (inhibitor of nuclear factor kappaB kinase, inducible). We have identified a new and essential component of this pathway, the adaptor protein SINTBAD (similar to NAP1 TBK1 adaptor). SINTBAD constitutively binds TBK1 and IKKi but not related kinases. Upon infection with Sendai virus, SINTBAD is essential for the efficient induction of IRF-dependent transcription, as are two further TBK1 adaptors, TANK and NAP1. We identified a conserved TBK1/IKKi-binding domain (TBD) in the three adaptors, predicted to form an alpha-helix with residues essential for kinase binding clustering on one side. Isolated TBDs compete with adaptor binding to TBK1 and prevent poly(I:C)-induced IRF-dependent transcription. Our results suggest that efficient signal transduction upon viral infection requires SINTBAD, TANK and NAP1 because they link TBK1 and IKKi to virus-activated signalling cascades.

IkappaB kinase complexes: gateways to NF-kappaB activation and transcription

Transcription factors of the NF-kappaB family regulate hundreds of genes in the context of multiple important physiological and pathological processes. NF-kappaB activation depends on phosphorylation-induced proteolysis of inhibitory IkappaB molecules and NF-kappaB precursors by the ubiquitin-proteasome system. Most of the diverse signaling pathways that activate NF-kappaB converge on IkappaB kinases (IKK), which are essential for signal transmission. Many important details of the composition, regulation and biological function of IKK have been revealed in the last years. This review summarizes current aspects of structure and function of the regular stoichiometric components, the regulatory transient protein interactions of IKK and the mechanisms that contribute to its activation, deactivation and homeostasis. Both phosphorylation and ubiquitinatin (destructive as well as non-destructive) are crucial post-translational events in these processes. In addition to controlling induced IkappaB degradation in the cytoplasm and processing of the NF-kappaB precursor p100, nuclear IKK components have been found to act directly at the chromatin level of induced genes and to mediate responses to DNA damage. Finally, IKK is engaged in cross talk with other pathways and confers functions independently of NF-kappaB.

Regulation and function of IKK and IKK-related kinases

Members of the nuclear factor kappa B (NF-kappaB) family of dimeric transcription factors (TFs) regulate expression of a large number of genes involved in immune responses, inflammation, cell survival, and cancer. NF-kappaB TFs are rapidly activated in response to various stimuli, including cytokines, infectious agents, and radiation-induced DNA double-strand breaks. In nonstimulated cells, some NF-kappaB TFs are bound to inhibitory IkappaB proteins and are thereby sequestered in the cytoplasm. Activation leads to phosphorylation of IkappaB proteins and their subsequent recognition by ubiquitinating enzymes. The resulting proteasomal degradation of IkappaB proteins liberates IkappaB-bound NF-kappaB TFs, which translocate to the nucleus to drive expression of target genes. Two protein kinases with a high degree of sequence similarity, IKKalpha and IKKbeta, mediate phosphorylation of IkappaB proteins and represent a convergence point for most signal transduction pathways leading to NF-kappaB activation. Most of the IKKalpha and IKKbeta molecules in the cell are part of IKK complexes that also contain a regulatory subunit called IKKgamma or NEMO. Despite extensive sequence similarity, IKKalpha and IKKbeta have largely distinct functions, due to their different substrate specificities and modes of regulation. IKKbeta (and IKKgamma) are essential for rapid NF-kappaB activation by proinflammatory signaling cascades, such as those triggered by tumor necrosis factor alpha (TNFalpha) or lipopolysaccharide (LPS). In contrast, IKKalpha functions in the activation of a specific form of NF-kappaB in response to a subset of TNF family members and may also serve to attenuate IKKbeta-driven NF-kappaB activation. Moreover, IKKalpha is involved in keratinocyte differentiation, but this function is independent of its kinase activity. Several years ago, two protein kinases, one called IKKepsilon or IKK-i and one variously named TBK1 (TANK-binding kinase), NAK (NF-kappaB-activated kinase), or T2K (TRAF2-associated kinase), were identified that exhibit structural similarity to IKKalpha and IKKbeta. These protein kinases are important for the activation of interferon response factor 3 (IRF3) and IRF7, TFs that play key roles in the induction of type I interferon (IFN-I). Together, the IKKs and IKK-related kinases are instrumental for activation of the host defense system. This Review focuses on the functions of IKK and IKK-related kinases and the molecular mechanisms that regulate their activities.

Inhibition of TNF receptor 1 internalization by adenovirus 14.7K as a novel immune escape mechanism

The adenoviral protein E3-14.7K (14.7K) is an inhibitor of TNF-induced apoptosis, but the molecular mechanism underlying this protective effect has not yet been explained exhaustively. TNF-mediated apoptosis is initiated by ligand-induced recruitment of TNF receptor-associated death domain (TRADD), Fas-associated death domain (FADD), and caspase-8 to the death domain of TNF receptor 1 (TNFR1), thereby establishing the death-inducing signaling complex (DISC). Here we report that adenovirus 14.7K protein inhibits ligand-induced TNFR1 internalization. Analysis of purified magnetically labeled TNFR1 complexes from murine and human cells stably transduced with 14.7K revealed that prevention of TNFR1 internalization resulted in inhibition of DISC formation. In contrast, 14.7K did not affect TNF-induced NF-kappaB activation via recruitment of receptor-interacting protein 1 (RIP-1) and TNF receptor-associated factor 2 (TRAF-2). Inhibition of endocytosis by 14.7K was effected by failure of coordinated temporal and spatial assembly of essential components of the endocytic machinery such as Rab5 and dynamin 2 at the site of the activated TNFR1. Furthermore, we found that the same TNF defense mechanisms were instrumental in protecting wild-type adenovirus-infected human cells expressing 14.7K. This study describes a new molecular mechanism implemented by a virus to escape immunosurveillance by selectively targeting TNFR1 endocytosis to prevent TNF-induced DISC formation.

A dominant function of IKK/NF-kappaB signaling in global lipopolysaccharide-induced gene expression

Porphyromonas gingivalis is an etiologic pathogen of periodontitis that is one of the most common inflammatory diseases. Recently, we found that P. gingivalis LPS activated the transcription factor nuclear factor-kappaB (NF-kappaB) through the IkappaB kinase complex (IKK). NF-kappaB is a transcription factor that controls inflammation and host responses. In this study, we examined the role of IKK/NF-kappaBin P. gingivalis LPS-induced gene expression on a genome-wide basis using a combination of microarray and biochemical approaches. A total of 88 early response genes were found to be induced by P. gingivalis LPS in a human THP.1 monocytic cell lines. Interestingly, the induction of most of these genes was abolished or attenuated under the inactivation of IKK/NF-kappaB. Among those IKK/NF-kappaB-dependent genes, 20 genes were NF-kappaB-inducible genes reported previously, and 59 genes represented putative novel NF-kappaB target genes. Using transcription factor binding analysis, we found that most of these putative NF-kappaB target genes contained one or multiple NF-kappaB-binding sites. Also, some transcription factor-binding motifs were overrepresented in the promoter of both known and putative NF-kappaB-dependent genes, indicating that these genes may be regulated in a similar fashion. Furthermore, we found that several transcription factors associated with metabolic and inflammatory responses, including nuclear receptors, activator of protein-1, and early growth responses, were induced by P. gingivalis LPS through IKK/NF-kappaB, indicating that IKK/NF-kappaB may utilize these transcription factors to mediate secondary responses. Taken together, our results demonstrate that IKK/NF-kappaB signaling plays a dominant role in P. gingivalis LPS-induced early response gene expression, suggesting that IKK/NF-kappaB is a therapeutic target for periodontitis.

Adenovirus RIDalphabeta complex inhibits lipopolysaccharide signaling without altering TLR4 cell surface expression

The transmembrane heterotrimer complex 10.4K/14.5K, also known as RID (for "receptor internalization and degradation"), is encoded by the adenovirus E3 region, and it down-regulates the cell surface expression of several unrelated receptors. We recently showed that RID expression correlates with down-regulation of the cell surface expression of the tumor necrosis factor (TNF) receptor 1 in several human cells. This observation provided the first mechanistic explanation for the inhibition of TNF alpha-induced chemokines by RID. Here we analyze the immunoregulatory activities of RID on lipopolysaccharide (LPS) and interleukin-1 beta (IL-1beta)-mediated responses. Although both signaling pathways are strongly inhibited by RID, the chemokines up-regulated by IL-1beta stimulation are only marginally inhibited. In addition, RID inhibits signaling induced by LPS without affecting the expression of the LPS receptor Toll-like receptor 4, demonstrating that RID need not target degradation of the receptor to alter signal transduction. Taken together, our data demonstrate the inhibitory effect of RID on two additional cell surface receptor-mediated signaling pathways involved in inflammatory processes. The data suggest that RID has intracellular targets that impair signal transduction and chemokine expression without evidence of receptor down-regulation.

Extending the nuclear roles of IkappaB kinase subunits

The transcription factor NF-kappaB plays a key role in a wide variety of cellular processes such as innate and adaptive immunity, cellular proliferation, apoptosis and development. In unstimulated cells, NF-kappaB is sequestered in the cytoplasm through its tight association with inhibitory proteins called IkappaBs, comprising notably IkappaBalpha. A key step in NF-kappaB activation is the phosphorylation of IkappaBalpha by the so-called IkappaB kinase (IKK) complex, which targets the inhibitory protein for proteasomal degradation and allows the freed NF-kappaB to enter the nucleus where it can transactivate its target genes. The IKK complex is composed of two catalytic subunits called IKKalpha and IKKbeta, and a regulatory subunit called NEMO/IKKgamma. Despite their key role in mediating IkappaBalpha phosphorylation in the cytoplasm, recent works have provided evidence that IKK subunits also translocate into the nucleus to regulate NF-kappaB-dependent and -independent gene expression, paving the way of a novel and exciting field of research. In this review, we will describe the current knowledge in that research area.

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.

The N-terminus of PKR is responsible for the activation of the NF-kappaB signaling pathway by interacting with the IKK complex

The interferon-induced double-stranded RNA (dsRNA)-activated protein kinase (PKR) has been shown to activate NF-kappaB independently of its kinase function after interaction with the IKK complex. In order to investigate the mechanism of NF-kappaB activation by PKR, we identified the domain of PKR responsible for stimulating the NF-kappaB pathway in PKR-deficient fibroblasts using an NF-kappaB dependent reporter assay. The N-terminal 1-265 AA of PKR activates NF-kappaB, whereas the 1-180 AA N-terminus restricted to the two dsRNA Binding Domains (DRBD), the third basic domain alone (AA 181-265), or the C-terminus of PKR (AA 266-550) were unable to stimulate the expression of the NF-kappaB dependent reporter gene. Using confocal microscopy, we confirmed that PKR full length as well as PKR N-terminus colocalized with IKKbeta. By GST-pulldown analysis, using different PKR domains, we then revealed the specific ability of the PKR N-terminus 1-265 to bind to and activate IKK and showed that this activation requires the integrity of the IKK complex. This activation is not only due to DRBDs since the DRBD fragment 1-180 failed to inhibit PKR 1-265 induced NF-kappaB activation. Our results therefore demonstrate that the ability of PKR to mediate NF-kappaB activation resides in its full N-terminus, and requires both DRBDs and the third basic domain.

Dissecting role of regulatory factors in NF-kappaB pathway with siRNA

NF-kappaB, a family of related transcription factors, has been a focus of intense scientific research during the past decade. Multiple stimuli, both extracellular and intracellular, lead to its activation. The NF-kappaB pathway regulates expression of a diverse array of genes involved in different biological processes. Various pathological states are characterized by the dysregulated NF-kappaB pathway. Recently, NF-kappaB activation has been connected with multiple aspects of oncogenesis and serves as an important mechanism to regulate cell survival in response to chemotherapy by activating different genes that inhibit apoptosis. Several methods of inhibiting NF-kappaB activation, such as antisense oligonucleotides, proteosome inhibitors and RNA interference (RNAi) are currently under investigation. RNAi represents a powerful tool to better define the role of specific genes in different signal transduction pathways and has recently been used to define the function of genes that regulate the NF-kappaB pathway. This review discusses the emerging role of RNAi to dissect the function of regulatory factors in the NF-kappaB pathway and its potential use as a targeted therapy.

The RIP kinases: crucial integrators of cellular stress

Since the discovery of the first member ten years ago, the receptor-interacting protein (RIP) family kinases have emerged as essential sensors of cellular stress. The different members integrate both extracellular stress signals transmitted by various cell-surface receptors and signals emanating from intracellular stress. The cascades of events initiated by activated RIPs are complex. Not only are pro-survival, inflammatory and immune responses triggered by RIP kinases via the activation of transcription factors such as NF-kappaB and AP-1, but opposing, death-inducing programs can also be initiated by the RIP kinases. Hence, RIP kinases are crucial regulators of cell survival and cell death.

The Crohn's disease protein, NOD2, requires RIP2 in order to induce ubiquitinylation of a novel site on NEMO

BACKGROUND

Crohn's disease is an autoimmune inflammatory disorder of the gastrointestinal tract and is characterized clinically by dysregulation of both pro-inflammatory and anti-inflammatory cytokine signaling networks. The function of the Crohn's disease protein, NOD2, highlights the biphasic nature of the pathology of Crohn's disease. NOD2 can both strongly activate and negatively attenuate NF-kB signaling. The biochemical mechanism for this dual function of NOD2 is unknown.

RESULTS

We demonstrate that NOD2 activation leads to ubiquitinylation of NEMO, a key component of the NF-kB signaling complex. This ubiquitinylation is agonist dependant, and it does not regulate proteosomal destruction of NEMO. We show the NOD2-dependent ubiquitinylation of NEMO is dependent on the scaffolding protein kinase RIP2. Crohn's disease-associated polymorphisms of NOD2 show a decreased ability to bind RIP2, and this decreased ability to bind RIP2 correlates with a decreased ability to ubiquitinylate NEMO. We map the site of NEMO ubiquitinylation to a novel NEMO ubiquitinylation site (Lysine 285) and show that this ubiquityinylation occurs in vivo. Lastly, we show functionally that RIP2-induced ubiquitinylation of NEMO is at least in part responsible for RIP2-mediated NF-kB activation.

CONCLUSIONS

These data suggest that this novel mode of regulation of the NF-kB signaling pathway could be a factor underlying the pathogenesis of Crohn's disease.

Activity of hypoxia-inducible factor 2alpha is regulated by association with the NF-kappaB essential modulator

The hypoxia-inducible factors 1alpha (HIF-1alpha) and 2alpha (HIF-2alpha) are key regulators of the transcriptional response to low oxygen and are closely related in domain architecture, DNA binding, and activation mechanisms. Despite these similarities, targeted disruption of the HIF-alpha genes in mice results in distinctly different phenotypes demonstrating nonredundancy of function, although the underlying mechanisms remain unclear. Here we report on the novel and specific interaction of HIF-2alpha, but not HIF-1alpha, with the NF-kappaB essential modulator (NEMO) using immunoprecipitation, mammalian two-hybrid, and in vitro protein interaction assays. Reporter gene assays demonstrate that this interaction specifically enhances normoxic HIF-2alpha transcriptional activity, independently of the HIF-2alpha transactivation domain, consistent with a model by which NEMO aids CBP/p300 recruitment to HIF-2alpha. In contrast, HIF-2alpha overexpression does not alter NF-kappaB signaling, suggesting that the functional consequence of the HIF-2alpha/NEMO interaction is limited to the HIF pathway. The specificity of NEMO for HIF-2alpha represents one of the few known differential protein-protein interactions between the HIF-alpha proteins, which has important implications for the activity of HIF-2alpha and is also the first postulated NF-kappaB-independent role for NEMO.

Inhibition of tumor necrosis factor (TNF) signal transduction by the adenovirus group C RID complex involves downregulation of surface levels of TNF receptor 1

Adenoviruses employ multiple genes to inhibit the host antiviral responses. There is increasing evidence that these immunoregulatory genes may function either during lytic or latent infection. Adenovirus early transcription region 3 (E3) encodes at least seven proteins, five of which block the acquired or innate immune response. Previous findings from this laboratory demonstrated that the E3 proteins 10.4K and 14.5K, which form a complex in the plasma membrane, inhibit tumor necrosis factor (TNF)-induced activation of NF-kappaB and the synthesis of chemokines. To determine the mechanism of inhibition of these pathways by the adenovirus E3 10.4K/14.5K proteins, we have examined the effects of this viral complex on the inhibition of AP-1 and NF-kappaB activation by TNF and found a reduction in assembly of the TNF receptor 1 (TNFR1) signaling complex at the plasma membrane accompanied by downregulation of surface levels of TNFR1.

L-gamma-Glutamyl-L-cysteinyl-glycine (glutathione; GSH) and GSH-related enzymes in the regulation of pro- and anti-inflammatory cytokines: a signaling transcriptional scenario for redox(y) immunologic sensor(s)?

Of the antioxidant/prooxidant mechanisms mediating the regulation of inflammatory mediators, particularly cytokines, oxidative stress-related pathways remain a cornerstone. It is conspicuous that there is a strong association between free radical accumulation (ROS/RNS; oxidative stress) and the evolution of inflammation and inflammatory-related responses. The scenario that upholds a consensus on the aforementioned is still evolving to unravel, from an immunologic perspective, the molecular mechanisms associated with ROS/RNS-dependent inflammation. Cytokines are keynote players when it comes to defining an intimate relationship among reduction-oxidation (redox) signals, oxidative stress and inflammation. How close we are to identifying the molecular basis of this intricate association should be weighed against the involvement of specific signaling molecules and, potentially, transcription factors. L-gamma-Glutamyl-L-cysteinyl-glycine, or glutathione (GSH), an antioxidant thiol, has shaped, and still is refining, the face of oxidative signaling in terms of regulating the milieu of inflammatory mediators, ostensibly via the modulation (expression/repression) of oxygen- and redox-responsive transcription factors, hence termed redox(y)-sensitive cofactors. When it comes to the arena of oxygen sensing, oxidative stress and inflammation, nuclear factor-kappaB (NF-kappaB) and hypoxia-inducible factor-1alpha (HIF-1alpha) are key players that determine antioxidant/prooxidant responses with oxidative challenge. It is the theme therein to underlie current understanding of the molecular association hanging between oxidative stress and the evolution of inflammation, walked through an elaborate discussion on the role of transcription factors and cofactors. Would that classify glutathione and other redox signaling cofactors as potential anti-inflammatory molecules emphatically remains of particular interest, especially in the light of identifying upstream and downstream molecular pathways for conceiving therapeutic, alleviating strategy for oxidant-mediated, inflammatory-related disease conditions.

Signaling to NF-kappaB

The transcription factor NF-kappaB has been the focus of intense investigation for nearly two decades. Over this period, considerable progress has been made in determining the function and regulation of NF-kappaB, although there are nuances in this important signaling pathway that still remain to be understood. The challenge now is to reconcile the regulatory complexity in this pathway with the complexity of responses in which NF-kappaB family members play important roles. In this review, we provide an overview of established NF-kappaB signaling pathways with focus on the current state of research into the mechanisms that regulate IKK activation and NF-kappaB transcriptional activity.