Deficiency of T2K leads to apoptotic liver degeneration and impaired NF-kappaB-dependent gene transcription

Arenavirus nucleoproteins prevent activation of nuclear factor kappa B

Arenaviruses include several causative agents of hemorrhagic fever (HF) disease in humans that are associated with high morbidity and significant mortality. Morbidity and lethality associated with HF arenaviruses are believed to involve the dysregulation of the host innate immune and inflammatory responses that leads to impaired development of protective and efficient immunity. The molecular mechanisms underlying this dysregulation are not completely understood, but it is suggested that viral infection leads to disruption of early host defenses and contributes to arenavirus pathogenesis in humans. We demonstrate in the accompanying paper that the prototype member in the family, lymphocytic choriomeningitis virus (LCMV), disables the host innate defense by interfering with type I interferon (IFN-I) production through inhibition of the interferon regulatory factor 3 (IRF3) activation pathway and that the viral nucleoprotein (NP) alone is responsible for this inhibitory effect (C. Pythoud, W. W. Rodrigo, G. Pasqual, S. Rothenberger, L. Martínez-Sobrido, J. C. de la Torre, and S. Kunz, J. Virol. 86:7728-7738, 2012). In this report, we show that LCMV-NP, as well as NPs encoded by representative members of both Old World (OW) and New World (NW) arenaviruses, also inhibits the nuclear translocation and transcriptional activity of the nuclear factor kappa B (NF-κB). Similar to the situation previously reported for IRF3, Tacaribe virus NP (TCRV-NP) does not inhibit NF-κB nuclear translocation and transcriptional activity to levels comparable to those seen with other members in the family. Altogether, our findings demonstrate that arenavirus infection inhibits NF-κB-dependent innate immune and inflammatory responses, possibly playing a key role in the pathogenesis and virulence of arenavirus.

NF-κB as a target for pancreatic cancer therapy

INTRODUCTION

Pancreatic cancer is the fourth leading cause of adult cancer mortality in the USA. It represents one of the greatest challenges in cancer treatment. The NF-κB transcriptional factors are constitutively activated in the majority of pancreatic cancers and are involved in the regulation of numerous aspects of tumor development and progression. NF-κB and the signaling cascades that regulate its activity have thus become attractive targets for novel therapeutic approaches for pancreatic cancer.

AREAS COVERED

This review describes and discusses the most important advances in the comprehension of the complex molecular biology of NF-κB, as well as the development of novel NF-κB-targeting strategies for the treatment of pancreatic cancer.

EXPERT OPINION

Although the inhibition of NF-κB, especially when combined with more classic chemotherapeutic drugs, could be a promising therapeutic strategy, direct targeting NF-κB still faces important challenges. In the future, targeting nonredundant cytosolic mediators of the activation of NF-κB - such as TNF receptor associated factor family member-associated NF-κB activator -binding kinase 1 (TBK1) and TGF-beta activated kinase 1 (TAK1) - could represent a better approach to inhibit key processes in pancreatic tumor cells and make a difference for this devastating disease.

Cellular and molecular biology of optineurin

Optineurin is a gene linked to glaucoma, amyotrophic lateral sclerosis, other neurodegenerative diseases, and Paget's disease of bone. This review describes the characteristics of optineurin and summarizes the cellular and molecular biology investigations conducted so far on optineurin. Data from a number of laboratories indicate that optineurin is a cytosolic protein containing 577 amino acid residues. Interacting with proteins such as myosin VI, Rab8, huntingtin, transferrin receptor, and TANK-binding kinase 1, optineurin is involved in basic cellular functions including protein trafficking, maintenance of the Golgi apparatus, as well as NF-κB pathway, antiviral, and antibacteria signaling. Mutation or alteration of homeostasis of optineurin (such as overexpression or knockdown) results in adverse consequences in the cells, leading to the development of neurodegenerative diseases including glaucoma.

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

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

Innate immune recognition of an AT-rich stem-loop DNA motif in the Plasmodium falciparum genome

Although Toll-like receptor 9 (TLR9) has been implicated in cytokine and type I interferon (IFN) production during malaria in humans and mice, the high AT content of the Plasmodium falciparum genome prompted us to examine the possibility that malarial DNA triggered TLR9-independent pathways. Over 6000 ATTTTTAC ("AT-rich") motifs are present in the genome of P. falciparum, which we show here potently induce type I IFNs. Parasite DNA, parasitized erythrocytes and oligonucleotides containing the AT-rich motif induce type I IFNs via a pathway that did not involve the previously described sensors TLR9, DAI, RNA polymerase-III or IFI16/p204. Rather, AT-rich DNA sensing involved an unknown receptor that coupled to the STING, TBK1 and IRF3-IRF7 signaling pathway. Mice lacking IRF3, IRF7, the kinase TBK1 or the type I IFN receptor were resistant to otherwise lethal cerebral malaria. Collectively, these observations implicate AT-rich DNA sensing via STING, TBK1 and IRF3-IRF7 in P. falciparum malaria.

DNA released from dying host cells mediates aluminum adjuvant activity

Aluminum-based adjuvants (aluminum salts or alum) are widely used in human vaccination, although their mechanisms of action are poorly understood. Here we report that, in mice, alum causes cell death and the subsequent release of host cell DNA, which acts as a potent endogenous immunostimulatory signal mediating alum adjuvant activity. Furthermore, we propose that host DNA signaling differentially regulates IgE and IgG1 production after alum-adjuvanted immunization. We suggest that, on the one hand, host DNA induces primary B cell responses, including IgG1 production, through interferon response factor 3 (Irf3)-independent mechanisms. On the other hand, we suggest that host DNA also stimulates 'canonical' T helper type 2 (T(H)2) responses, associated with IgE isotype switching and peripheral effector responses, through Irf3-dependent mechanisms. The finding that host DNA released from dying cells acts as a damage-associated molecular pattern that mediates alum adjuvant activity may increase our understanding of the mechanisms of action of current vaccines and help in the design of new adjuvants.

Synoviocyte innate immune responses: TANK-binding kinase-1 as a potential therapeutic target in rheumatoid arthritis

OBJECTIVES

Innate immune responses in the rheumatoid synovium contribute to inflammation and joint destruction in RA. Two IκB kinase (IKK)-related kinases, TNF receptor associated factor (TRAF) family member-associated nuclear factor κ-light-chain enhancer of activated B cells (NF-κB) activator (TANK)-binding kinase 1 (TBK1) and IKKε, potentially regulate synovitis by activating IFN response genes. These kinases induce the expression of inflammatory mediators such as C-X-C motif ligand 10 (CXCL10)/IFN-γ-induced protein 10 kDa (IP-10) in fibroblast-like synoviocytes (FLS). Since IP-10 is a promising therapeutic target in RA, we evaluated whether blocking TBK1 might be an effective way to modulate IP-10 expression.

METHODS

Wild-type (WT) and IKKε(-/-) FLS were transfected with TBK1 or control small interfering RNA (siRNA) and stimulated with polyinosinic acid : polycytidylic acid [poly(I:C)]. Gene expression was assayed using quantitative PCR. Cytokine production in culture supernatants was measured by Luminex multiplex analysis. IFN-regulatory factor (IRF3) dimerization was determined by native PAGE. IFN-β and IP-10 promoter activity was measured using luciferase reporter constructs.

RESULTS

Initial studies showed that siRNA markedly decreased TBK1 expression in cultured FLS. Poly(I:C)-induced IRF7 gene expression was inhibited in the absence of TBK1, but not IKKε. IRF3 gene expression was similar to WT cells in TBK1 or IKKε-deficient FLS. IRF3 dimerization required both TBK1 and IKKε. Surprisingly, IRF3-mediated gene and protein expression of IFN-β and IP-10 was dependent on TBK1, not IKKε. Promoter constructs showed that TBK1 decreased IP-10 gene transcription and IP-10 mRNA stability was unaffected by TBK1 deficiency.

CONCLUSION

Based on the selective regulation of IP-10 in FLS, TBK1 appears to be the optimal IKK-related kinase to target in RA.

Streptococcus pneumoniae DNA initiates type I interferon signaling in the respiratory tract

The mucosal epithelium is the initial target for respiratory pathogens of all types. While type I interferon (IFN) signaling is traditionally associated with antiviral immunity, we demonstrate that the extracellular bacterial pathogen Streptococcus pneumoniae activates the type I IFN cascade in airway epithelial and dendritic cells. This response is dependent upon the pore-forming toxin pneumolysin. Pneumococcal DNA activates IFN-β expression through a DAI/STING/TBK1/IRF3 cascade. Tlr4^−/−, Myd88^−/−, Trif^−/−, and Nod2^−/− mutant mice had no impairment of type I IFN signaling. Induction of type I IFN signaling contributes to the eradication of pneumococcal carriage, as IFN-α/β receptor null mice had significantly increased nasal colonization with S. pneumoniae compared with that of wild-type mice. These studies suggest that the type I IFN cascade is a central component of the mucosal response to airway bacterial pathogens and is responsive to bacterial pathogen-associated molecular patterns that are capable of accessing intracellular receptors.

The bacterium Streptococcus pneumoniae is a leading cause of bacterial pneumonia, leading to upwards of one million deaths a year worldwide and significant economic burden. Although it is known that antibody is critical for efficient phagocytosis, it is not known how this pathogen is sensed by the mucosal epithelium. We demonstrate that this extracellular pathogen activates mucosal signaling typically activated by viral pathogens via the pneumolysin pore to activate intracellular receptors and the type I interferon (IFN) cascade. Mice lacking the receptor to type I IFNs have a reduced ability to clear S. pneumoniae, suggesting that the type I IFN cascade is central to the mucosal clearance of this important pathogen.

Cloning, expression analysis and promoter structure of TBK1 (TANK-binding kinase 1) in Atlantic cod (Gadus morhua L.)

TBK1, also termed NAK or T2K, is a ubiquitous member of the IκB kinase (IKK) family that is required for innate and adaptive immune responses. We have identified and characterized the full-length TBK1 cDNA in Atlantic cod. The cod TBK1 gene consists of 2190 bp open reading frame encoding a polypeptide of 729 amino acids. According to a BLAST search, the cloned TBK1 gene has a high degree of sequence similarity (80.7-92%) to the various members of the TBK1 family, indicating that it is conserved during evolution. RT-PCR showed that the largest quantity of TBK1 transcripts was found in spleen, followed by the liver, gill, head kidney, gut, pyloric caeca, while the expression of TBK1 mRNA in muscle and skin was low. Both PMA, poly I:C and β-glucan promoted expression of TBK1 transcripts in vivo. Furthermore, we determined an 875 bp sequence upstream of the transcriptional start site (TSS) and found a number of sequence motifs that matched known transcription factor-binding sites. Activities of the presumptive regulatory regions of this gene were assessed by transfecting different 5'-deletion constructs in CHSE-214 cells. After the expression experiments, the results showed that the basal promoters and positive transcriptional regulator activities of cod TBK1 gene were dependent by sequences located from -875 to -425 bp and from -245 to +28 bp upstream of TSS. This study provides further insights into the transcriptional regulation of cod TBK1.

Rebamipide suppresses TLR-TBK1 signaling pathway resulting in regulating IRF3/7 and IFN-α/β reduction

TANK-binding kinase 1 (TBK1) regulates the interferon regulatory factor (IRF) 3 and IRF7 activation pathways by double strand RNA (dsRNA) via Toll-like receptor (TLR) 3 and by lipopolysaccharide (LPS) via TLR4. Rebamipide is useful for treating inflammatory bowel disease (IBD). Although IBD is associated with nuclear factor-κB (NF-κB), any association with the TBK1-IRF pathway remains unknown. How rebamipide affects the TBK1-IRF pathway is also unclear. We analyzed the relationship between IBD (particularly ulcerative colitis; UC) and the TLR-TBK1-IRF3/7 pathway using human colon tissue, a murine model of colitis and human colonic epithelial cells. Inflamed colonic mucosa over-expressed TKB1, NAP1, IRF3, and IRF7 mRNA compared with normal mucosa. TBK1 was mainly expressed in inflammatory epithelial cells of UC patients. The expression of TBK1, IRF3, IRF7, IFN-α and IFN-β mRNA was suppressed in mice given oral dextran sulfate-sodium (DSS) and daily rectal rebamipide compared with mice given only DSS. Rebamipide reduced the expression of TBK1, IRF3 and IRF7 mRNA induced by LPS/dsRNA, but not of NF-κB mRNA in colonic epithelial cells. Rebamipide might suppress the TLR-TBK1 pathway, resulting in IRF3/7-induction of IFN-α/β and inflammatory factors. TBK1 is important in the induction of inflammation in patients with UC. If rebamipide represses the TLR-TBK1 pathway, then rectal administration should suppress inflammation of the colonic mucosa in patients with UC.

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.

Emerging roles for the non-canonical IKKs in cancer

The IκB Kinase (IKK)-related kinases TBK1 and IKKɛ have essential roles as regulators of innate immunity by modulating interferon and NF-κB signaling. Recent work has also implicated these non-canonical IKKs in malignant transformation. IKKɛ is amplified in ∼30% of breast cancers and transforms cells through the activation of NF-κB. TBK1 participates in RalB-mediated inflammatory responses and cell survival, and is essential for the survival of non-small cell lung cancers driven by oncogenic KRAS. The delineation of target substrates and downstream activities for TBK1 and IKKɛ has begun to define their role(s) in promoting tumorigenesis. In this review, we will highlight the mechanisms by which IKKɛ and TBK1 orchestrate pathways involved in inflammation and cancer.

TANK-binding kinase 1 attenuates PTAP-dependent retroviral budding through targeting endosomal sorting complex required for transport-I

Retroviruses need to bud from producer cells to spread infection. To facilitate its budding, some virus hijacks the multivesicular body (MVB) pathway that is normally used to cargo and degrade ubiquitylated cellular proteins, through interaction between the late domain of Gag polyproteins and the components of MVB machinery. In this study, we demonstrated that TANK-binding kinase 1 (TBK1) directly interacted with VPS37C, a subunit of endosomal sorting complex required for transport-I (ESCRT-I) in the MVB pathway, without affecting the ultrastructure or general function of MVB. Interestingly, overexpression of TBK1 attenuated, whereas short hairpin RNA interference of TBK1 enhanced HIV-1 pseudovirus release from Vero cells in type I IFN (IFN-I)-independent manner. Down-regulation of TBK1 by short hairpin RNA in TZM-bl cells also enhanced live HIV-1 NL4-3 or JR-CSF virus budding without involvement of IFN-I induction. Furthermore, infection of TBK1-deficient mouse embryonic fibroblast cells with a chimeric murine leukemia virus/p6, whose PPPY motif was replaced by PTAP motif of HIV-1, showed that lack of TBK1 significantly enhanced PTAP-dependent, but not PPPY-dependent retrovirus budding. Finally, phosphorylation of VPS37C by TBK1 might regulate the viral budding efficiency, because overexpression of the kinase-inactive mutant of TBK1 (TBK1-K38A) in Vero cells accelerated HIV-1 pseudovirus budding. Therefore, through tethering to VPS37C of the ESCRT-I complex, TBK1 controlled the speed of PTAP-dependent retroviral budding through phosphorylation of VPS37C, which would serve as a novel mechanism of host cell defense independent of IFN-I signaling.

TRAF2 phosphorylation promotes NF-κB-dependent gene expression and inhibits oxidative stress-induced cell death

TRAF2 regulates JNK and IKK activation in response to TNF-α stimulation. This study found that TNF-α and oxidative stress induce TRAF2 phosphorylation and that this phosphorylation inhibits apoptosis by promoting the prolonged phase of IKK activation while inhibiting the prolonged phase of JNK activation.

Tumor necrosis factor α (TNF-α) receptor–associated factor 2 (TRAF2) regulates activation of the c-Jun N-terminal kinase (JNK)/c-Jun and the inhibitor of κB kinase (IKK)/nuclear factor κB (NF-κB) signaling cascades in response to TNF-α stimulation. Gene knockout studies have revealed that TRAF2 inhibits TNF-α–induced cell death but promotes oxidative stress–induced apoptosis. Here we report that TNF-α and oxidative stress both induce TRAF2 phosphorylation at serines 11 and 55 and that this dual phosphorylation promotes the prolonged phase of IKK activation while inhibiting the prolonged phase of JNK activation. Prolonged IKK activation trigged by TNF-α plays an essential role in efficient expression of a subset of NF-κB target genes but has no substantial role in TNF-α–induced cell death. On the other hand, TRAF2 phosphorylation in response to oxidative stress significantly promotes cell survival by inducing prolonged IKK activation and by inhibiting the prolonged phase of JNK activation. Notably, stable expression of phospho-null mutant TRAF2 in cancer cells leads to an increase in the basal and inducible JNK activation and B-cell lymphoma 2 (Bcl-2) phosphorylation. In addition, exposure of cells expressing phospho-null mutant TRAF2 to sublethal oxidative stress results in a rapid degradation of Bcl-2 and cellular inhibitor of apoptosis 1 as well as significantly increased cell death. These results suggest that TRAF2 phosphorylation is essential for cell survival under conditions of oxidative stress.

A single NFκB system for both canonical and non-canonical signaling

Two distinct nuclear factor κB (NFκB) signaling pathways have been described; the canonical pathway that mediates inflammatory responses, and the non-canonical pathway that is involved in immune cell differentiation and maturation and secondary lymphoid organogenesis. The former is dependent on the IκB kinase adaptor molecule NEMO, the latter is independent of it. Here, we review the molecular mechanisms of regulation in each signaling axis and attempt to relate the apparent regulatory logic to the physiological function. Further, we review the recent evidence for extensive cross-regulation between these two signaling axes and summarize them in a wiring diagram. These observations suggest that NEMO-dependent and -independent signaling should be viewed within the context of a single NFκB signaling system, which mediates signaling from both inflammatory and organogenic stimuli in an integrated manner. As in other regulatory biological systems, a systems approach including mathematical models that include quantitative and kinetic information will be necessary to characterize the network properties that mediate physiological function, and that may break down to cause or contribute to pathology.

Progranulin A-mediated MET signaling is essential for liver morphogenesis in zebrafish

The mechanism that regulates embryonic liver morphogenesis remains elusive. Progranulin (PGRN) is postulated to play a critical role in regulating pathological liver growth. Nevertheless, the exact regulatory mechanism of PGRN in relation to its functional role in embryonic liver development remains to be elucidated. In our study, the knockdown of progranulin A (GrnA), an orthologue of mammalian PGRN, using antisense morpholinos resulted in impaired liver morphogenesis in zebrafish (Danio rerio). The vital role of GrnA in hepatic outgrowth and not in liver bud formation was further confirmed using whole-mount in situ hybridization markers. In addition, a GrnA deficiency was also found to be associated with the deregulation of MET-related genes in the neonatal liver using a microarray analysis. In contrast, the decrease in liver size that was observed in grnA morphants was avoided when ectopic MET expression was produced by co-injecting met mRNA and grnA morpholinos. This phenomenon suggests that GrnA might play a role in liver growth regulation via MET signaling. Furthermore, our study has shown that GrnA positively modulates hepatic MET expression both in vivo and in vitro. Therefore, our data have indicated that GrnA plays a vital role in embryonic liver morphogenesis in zebrafish. As a result, a novel link between PGRN and MET signaling is proposed.

Osmoprotective transcription factor NFAT5/TonEBP modulates nuclear factor-kappaB activity

Tonicity responsive binding protein (TonEBP) is a transcription factor that plays a key role in osmoprotection. Here, we demonstrate enhanced activity of prosurvival NF-κB—at the onset of hypertonic challenge that depends on p38 kinase—and Akt-dependent formation of p65-TonEBP complexes that bind to elements of NF-κB-responsive genes.

Tonicity-responsive binding-protein (TonEBP or NFAT5) is a widely expressed transcription factor whose activity is regulated by extracellular tonicity. TonEBP plays a key role in osmoprotection by binding to osmotic response element/TonE elements of genes that counteract the deleterious effects of cell shrinkage. Here, we show that in addition to this “classical” stimulation, TonEBP protects cells against hypertonicity by enhancing nuclear factor-κB (NF-κB) activity. We show that hypertonicity enhances NF-κB stimulation by lipopolysaccharide but not tumor necrosis factor-α, and we demonstrate overlapping protein kinase B (Akt)-dependent signal transduction pathways elicited by hypertonicity and transforming growth factor-α. Activation of p38 kinase by hypertonicity and downstream activation of Akt play key roles in TonEBP activity, IκBα degradation, and p65 nuclear translocation. TonEBP affects neither of these latter events and is itself insensitive to NF-κB signaling. Rather, we reveal a tonicity-dependent interaction between TonEBP and p65 and show that NF-κB activity is considerably enhanced after binding of NF-κB-TonEBP complexes to κB elements of NF-κB–responsive genes. We demonstrate the key roles of TonEBP and Akt in renal collecting duct epithelial cells and in macrophages. These findings reveal a novel role for TonEBP and Akt in NF-κB activation on the onset of hypertonic challenge.

Inhibition of the interferon antiviral response by hepatitis C virus

Hepatitis C virus (HCV) causes acute and chronic hepatitis by targeting the liver hepatocyte for infection and destruction. The standard treatment for chronic HCV infection is pegylated interferon plus ribavirin. Unfortunately, the sustained response rate and associated toxicity with this treatment are far from ideal; more effective and less toxic treatment regimens are needed. With more than 170 million people infected worldwide, there is an unmet medical need for new effective treatments. Recent advances in the understanding of the signaling pathways leading to the host antiviral response to HCV, the mechanisms used by HCV to evade the immune response, the development of cell culture models of HCV infection and the development of small molecule inhibitors of HCV have generated optimism that novel therapeutic approaches to control HCV will soon be available.

Molecular delineation of gamma-ray-induced NF-kappaB activation and pro-inflammatory genes in SMP30 knockout mice

Exposure to gamma radiation causes a wide variety of biological damages and alterations, including oxidative stress. Among the key cellular components that are exquisitely sensitive to oxidative stress is the transcription factor nuclear factor (NF)-kappaB, which plays a central role in the activation of various pro-inflammatory genes. Recently, senescence marker protein 30 (SMP30), which has been used as an aging marker, was shown to have an antioxidant property. In the current study, using SMP30 knockout (SMP30(-/-)) mice that are vitamin C-deficient, we explored the effect of radiation on the activation of NF-kappaB and several key pro-inflammatory genes. Six groups of mice were studied. Group 1 mice were not irradiated and were supplemented with vitamin C (2.5 mg/kg/day). Group 2 mice were irradiated and were not supplemented with vitamin C. Group 3, 4 and 5 mice were irradiated with 1, 3 and 5 Gy of gamma radiation ((60)Co), respectively, without vitamin C supplementation. The wild-type mice (SMP30(+/+)) in group 6 were not irradiated or supplemented. At 24 h after irradiation, mice were killed humanely and the kidneys were removed analysis. The results showed that gamma radiation induced oxidative stress with corresponding NF-kappaB activation; this activated NF-kappaB led to the up-regulation of several major pro-inflammatory mediators such as COX-2, iNOS, VCAM1, ICAM1 and E-selectin in irradiated groups with no vitamin C supplementation. Our data provide molecular insights into mechanisms through which gamma radiation enhances oxidative stress-induced inflammation by showing the activation of NF-kappaB signaling pathway in vitamin C-deficient SMP30(-/-) mice. In addition, our present study produced evidence that gamma radiation exerts its deleterious action by activating the inflammatory process that are known to be a major risk factor for many chronic diseases. Furthermore, our data revealed vitamin C may play an important protective role in attenuating the adverse gamma-radiation-induced adverse effects by suppressing adverse oxidative effects and pro-inflammatory mediators.

NF-kappaB signaling: a tale of two pathways in skeletal myogenesis

NF-kappaB is a ubiquitiously expressed transcription factor that plays vital roles in innate immunity and other processes involving cellular survival, proliferation, and differentiation. Activation of NF-kappaB is controlled by an IkappaB kinase (IKK) complex that can direct either canonical (classical) NF-kappaB signaling by degrading the IkappaB inhibitor and releasing p65/p50 dimers to the nucleus, or causes p100 processing and nuclear translocation of RelB/p52 via a noncanonical (alternative) pathway. Under physiological conditions, NF-kappaB activity is transiently regulated, whereas constitutive activation of this transcription factor typically in the classical pathway is associated with a multitude of disease conditions, including those related to skeletal muscle. How NF-kappaB functions in muscle diseases is currently under intense investigation. Insight into this role of NF-kappaB may be gained by understanding at a more basic level how this transcription factor contributes to skeletal muscle cell differentiation. Recent data from knockout mice support that the classical NF-kappaB pathway functions as an inhibitor of skeletal myogenesis and muscle regeneration acting through multiple mechanisms. In contrast, alternative NF-kappaB signaling does not appear to be required for myofiber conversion, but instead functions in myotube homeostasis by regulating mitochondrial biogenesis. Additional knowledge of these signaling pathways in skeletal myogenesis should aid in the development of specific inhibitors that may be useful in treatments of muscle disorders.

Dendritic cells treated with resveratrol during differentiation from monocytes gain substantial tolerogenic properties upon activation

Resveratrol is a polyphenol that acts on multiple molecular targets important for cell differentiation and activation. Dendritic cells (DCs) are a functionally diverse cell type and represent the most potent antigen-presenting cells of the immune system. In this study, we investigated resveratrol-induced effects on DCs during their differentiation and maturation. Our results show that resveratrol induces DC-associated tolerance, particularly when applied during DC differentiation. Costimulatory molecules CD40, CD80 and CD86 were down-regulated, as was the expression of major histocompatibility complex (MHC) class II molecules. Surface expression of inhibitory immunoglobulin-like transcript 3 (ILT3) and ILT4 molecules was induced, while human leucocyte antigen (HLA)-G expression was not affected. Resveratrol-treated DCs lost the ability to produce interleukin (IL)-12p70 after activation, but had an increased ability to produce IL-10. Such DCs were poor stimulators of allogeneic T cells and had lowered ability to induce CD4(+) T-cell migration. Furthermore, treated cells were able to generate allogeneic IL-10-secreting T cells, but were not competent in inducing FoxP3 expression These tolerogenic effects are probably associated with the effect of resveratrol on multiple molecular targets through which it interferes with DC differentiation and nuclear factor (NF)-kappaB translocation. Our data provide new insights into the molecular and functional mechanisms of the tolerogenic effects that resveratrol exerts on DCs.

The IKK Kinases: Operators of Antiviral Signaling

The ability of a cell to combat an intracellular pathogen requires a mechanism to recognize the threat and elicit a transcriptional response against it. In the context of virus infection, the cell must take measures to inhibit viral replication, meanwhile, convey warning signals to neighboring cells of the imminent threat. This immune response is predominantly mediated by the production of cytokines, notably, interferon beta (IFNβ). IFNβ signaling results in the transcriptional induction of over one hundred antiviral gene products whose timely expression renders infected cells more capable of inhibiting virus replication, while providing the uninfected cells with the reinforcements to generate a less permissive cellular environment. Induction of IFNβ and many aspects of the antiviral response pivot on the function of the IKK and IKK-related kinases. Despite sharing high levels of homology and some degree of functional redundancy, the classic IKK kinases: IKKα and IKKβ, and the IKK-related kinases: TBK1 and IKKɛ, perform distinct roles in regulating the host antiviral defense. These kinases serve as molecular operators in their cooperative ability to integrate incoming cellular cues and act on a range of essential antiviral transcription factors to reshape the cellular transcriptome during infection.

Induction of IFN-beta and the innate antiviral response in myeloid cells occurs through an IPS-1-dependent signal that does not require IRF-3 and IRF-7

Interferon regulatory factors (IRF)-3 and IRF-7 are master transcriptional factors that regulate type I IFN gene (IFN-α/β) induction and innate immune defenses after virus infection. Prior studies in mice with single deletions of the IRF-3 or IRF-7 genes showed increased vulnerability to West Nile virus (WNV) infection. Whereas mice and cells lacking IRF-7 showed reduced IFN-α levels after WNV infection, those lacking IRF-3 or IRF-7 had relatively normal IFN-b production. Here, we generated IRF-3^−/−× IRF-7^−/− double knockout (DKO) mice, analyzed WNV pathogenesis, IFN responses, and signaling of innate defenses. Compared to wild type mice, the DKO mice exhibited a blunted but not abrogated systemic IFN response and sustained uncontrolled WNV replication leading to rapid mortality. Ex vivo analysis showed complete ablation of the IFN-α response in DKO fibroblasts, macrophages, dendritic cells, and cortical neurons and a substantial decrease of the IFN-β response in DKO fibroblasts and cortical neurons. In contrast, the IFN-β response was minimally diminished in DKO macrophages and dendritic cells. However, pharmacological inhibition of NF-κB and ATF-2/c-Jun, the two other known components of the IFN-β enhanceosome, strongly reduced IFN-β gene transcription in the DKO dendritic cells. Finally, a genetic deficiency of IPS-1, an adaptor involved in RIG-I- and MDA5-mediated antiviral signaling, completely abolished the IFN-β response after WNV infection. Overall, our experiments suggest that, unlike fibroblasts and cortical neurons, IFN-β gene regulation after WNV infection in myeloid cells is IPS-1-dependent but does not require full occupancy of the IFN-β enhanceosome by canonical constituent transcriptional factors.

West Nile virus (WNV) is a mosquito-transmitted virus that infects birds, horses, and humans and has become an emerging infectious disease threat in the Western Hemisphere. In humans, WNV can invade into the brain and spinal cord and destroy neurons, causing severe neurological disease, particularly in the immunocompromised and elderly. A better understanding of how the immune system controls WNV infection is critical for developing new treatments and vaccines. In this study, using a mouse model of WNV infection, we evaluate the combined role of two key transcription factors, interferon-regulatory factor-3 (IRF-3) and IRF-7, that orchestrate antiviral and interferon (IFN) responses after infection. Mice that lack both IRF-3 and IRF-7 were highly vulnerable to lethal infection and cells lacking IRF-3 and IRF-7 had a markedly attenuated IFN-α response. Surprisingly, macrophages and dendritic cells lacking IRF-3 and IRF-7 showed a relatively normal IFN-β response. Furthermore, a genetic deficiency of IPS-1, a protein that signals downstream of the RIG-I- and MDA5 cytoplasmic viral RNA sensors, completely abolished IFN-β production. Our experiments suggest that in specific cell types infected with WNV, IFN-β can be induced through an IPS-1-dependent transcriptional signal that does not require the master transcriptional regulators IRF-3 and IRF-7.

TBK1-targeted suppression of TRIF-dependent signaling pathway of Toll-like receptors by 6-shogaol, an active component of ginger

Toll-like receptors (TLRs) are primary sensors that detect a wide variety of microbial components involving induction of innate immune responses. After recognition of microbial components, TLRs trigger the activation of myeloid differential factor 88 (MyD88) and Toll-interleukin-1 (IL-1) receptor domain-containing adapter inducing interferon-beta (TRIF)-dependent downstream signaling pathways. 6-Shoagol, an active ingredient of ginger, inhibits the MyD88-dependent signaling pathway by inhibiting inhibitor-kappaB kinase activity. Inhibitor-kappaB kinase is a key kinase in nuclear factor kappaB (NF-kappaB) activation. However, it is not known whether 6-shogaol inhibits the TRIF-dependent signaling pathway. Our goal was to identify the molecular target of 6-shogaol in the TRIF-dependent pathway of TLRs. 6-Shogaol inhibited the activation of interferon-regulatory factor 3 (IRF3) induced by lipopolysaccharide (LPS) and by polyriboinosinic polyribocytidylic acid (poly[I:C]), overexpression of TRIF, TANK-binding kinase1 (TBK1), and IRF3. Furthermore, 6-shogaol inhibited TBK1 activity in vitro. Together, these results suggest that 6-shogaol inhibits the TRIF-dependent signaling pathway of TLRs by targeting TBK1, and, they imply that 6-shogaol can modulate TLR-derived immune/inflammatory target gene expression induced by microbial infection.

Liver development: lessons from knockout mice and mutant fish

The liver is an organ with vital functions, including the processing and storage of nutrients, maintenance of serum composition, detoxification and bile production. Over the last 10 years, there have been major advances in our understanding of the molecular and cellular mechanisms underlying liver development. These advances have been achieved through the use of knockout mice as well as through forward-genetics studies employing mutant fish. The examination of many such murine and piscine mutants with defects in liver formation and/or function have pinpointed numerous factors crucial for hepatic cell differentiation and growth. In addition, these studies have permitted the identification of several important liver-specific markers that allow the contributions of variouscell types to hepatogenesis to be monitored. This review summarizes our current state of knowledge of the shared molecular mechanisms that underlie liver development in species as diverse as fish and mice. A better molecular understanding of liver formation may provide new insights into both normal liver biology and liver disease.

Glycogen synthase kinase-3 inhibition disrupts nuclear factor-kappaB activity in pancreatic cancer, but fails to sensitize to gemcitabine chemotherapy

Background

Aberrant activation NF-kappaB has been proposed as a mechanism of drug resistance in pancreatic cancer. Recently, inhibition of glycogen synthase kinase-3 has been shown to exert anti-tumor effects on pancreatic cancer cells by suppressing NF-kappaB. Consequently, we investigated whether inhibition of GSK-3 sensitizes pancreatic cancer cells to the chemotherapeutic agent gemcitabine.

Methods

GSK-3 inhibition was achieved using the pharmacological agent AR-A014418 or siRNA against GSK-3 alpha and beta isoforms. Cytotoxicity was measured using a Sulphorhodamine B assay and clonogenic survival following exposure of six different pancreatic cancer cell lines to a range of doses of either gemcitabine, AR-A014418 or both for 24, 48 and 72 h. We measured protein expression levels by immunoblotting. Basal and TNF-alpha induced activity of NF-kappaB was assessed using a luciferase reporter assay in the presence or absence of GSK-3 inhibition.

Results

GSK-3 inhibition reduced both basal and TNF-alpha induced NF-kappaB luciferase activity. Knockdown of GSK-3 beta reduced nuclear factor kappa B luciferase activity to a greater extent than GSK-3 alpha, and the greatest effect was seen with dual knockdown of both GSK-3 isoforms. GSK-3 inhibition also resulted in reduction of the NF-kappaB target proteins XIAP, Bcl-X_L, and cyclin D1, associated with growth inhibition and decreased clonogenic survival. In all cell lines, treatment with either AR-A014418, or gemcitabine led to growth inhibition in a dose- and time-dependent manner. However, with the exception of PANC-1 where drug synergy occurred with some dose schedules, the inhibitory effect of combined drug treatment was additive, sub-additive, or even antagonistic.

Conclusion

GSK-3 inhibition has anticancer effects against pancreatic cancer cells with a range of genetic backgrounds associated with disruption of NF-kappaB, but does not significantly sensitize these cells to the standard chemotherapy agent gemcitabine. This lack of synergy might be context or cell line dependent, but could also be explained on the basis that although NF-kappaB is an important mediator of pancreatic cancer cell survival, it plays a minor role in gemcitabine resistance. Further work is needed to understand the mechanisms of this effect, including the potential for rational combination of GSK3 inhibitors with other targeted agents for the treatment of pancreatic cancer.

Use of the pharmacological inhibitor BX795 to study the regulation and physiological roles of TBK1 and IkappaB kinase epsilon: a distinct upstream kinase mediates Ser-172 phosphorylation and activation

TANK-binding kinase 1 (TBK1) and IkappaB kinase epsilon (IKKepsilon) regulate the production of Type 1 interferons during bacterial and viral infection, but the lack of useful pharmacological inhibitors has hampered progress in identifying additional physiological roles of these protein kinases and how they are regulated. Here we demonstrate that BX795, a potent and relatively specific inhibitor of TBK1 and IKKepsilon, blocked the phosphorylation, nuclear translocation, and transcriptional activity of interferon regulatory factor 3 and, hence, the production of interferon-beta in macrophages stimulated with poly(I:C) or lipopolysaccharide (LPS). In contrast, BX795 had no effect on the canonical NFkappaB signaling pathway. Although BX795 blocked the autophosphorylation of overexpressed TBK1 and IKKepsilon at Ser-172 and, hence, the autoactivation of these protein kinases, it did not inhibit the phosphorylation of endogenous TBK1 and IKKepsilon at Ser-172 in response to LPS, poly(I:C), interleukin-1alpha (IL-1alpha), or tumor necrosis factor alpha and actually enhanced the LPS, poly(I:C), and IL-1alpha-stimulated phosphorylation of this residue. These results demonstrate that the phosphorylation of Ser-172 and the activation of TBK1 and IKKepsilon are catalyzed by a distinct protein kinase(s) in vivo and that TBK1 and IKKepsilon control a feedback loop that limits their activation by LPS, poly(I:C) and IL-1alpha (but not tumor necrosis factor alpha) to prevent the hyperactivation of these enzymes.

The IKK-related kinases: from innate immunity to oncogenesis

Over the past four years, the field of the innate immune response has been highly influenced by the discovery of the IkappaB kinase (IKK)-related kinases, TANK Binding Kinase 1 (TBK1) and IKKi, which regulate the activity of interferon regulatory factor (IRF)-3/IRF-7 and NF-kappaB transcription factors. More recently, additional essential components of the signaling pathways that activate these IKK homologues have been discovered. These include the RNA helicases RIGi and MDA5, and the downstream mitochondrial effector known as CARDIF/MAVS/VISA/IPS-1. In addition to their essential functions in controlling the innate immune response, recent studies have highlighted a role of these kinases in cell proliferation and oncogenesis. The canonical IKKs are well recognized to be a bridge linking chronic inflammation to cancer. New findings now suggest that the IKK-related kinases TBK1 and IKKi also participate in signaling pathways that impact on cell transformation and tumor progression. This review will therefore summarize and discuss the role of TBK1 and IKKi in cellular transformation and oncogenesis by focusing on their regulation and substrate specificity.

TBK1 does not play a role in the control of in vitro Burkholderia pseudomallei growth

Burkholderia pseudomallei, the causative agent of melioidosis, is an important intracellular pathogen in tropical regions. TANK-binding kinase (TBK1), part of the pathway that induces transcription of Type I interferon genes, has been demonstrated to play an important role in controlling intracellular bacterial infections. To investigate the role of tbk1 in protecting against B. pseudomallei we developed tbk1-deficient cell lines by using shRNA for transient knockdown of the tbk1 gene in HeLa and RAW 264.7 cells. In tbk1-deficient RAW cells, the replication of invasive and non-invasive Escherichia coli was significantly increased at 48 h after infection compared with wild-type cells. The result was confirmed using Brucella melitensis in tbk1-deficient HeLa cells, which demonstrated a >1.5-2.0 log higher bacterial count at 6-48 h after infection compared to wild-type cells. By contrast, the growth of Burkholderia pseudomallei expressing either typical (A2) or atypical (G207) lipopolysaccharide was not significantly different between the tbk1-deficient and control cells. These results suggest that the tbk1 gene and its activation may be able to control invasive E. coli, non-invasive E. coli and B. melitensis growth but may not be able to control B. pseudomallei infection. The role of the tbk1 gene in proinflammatory cytokine induction and bacterial intracellular infection needs further investigation to identify mechanistic differences among the life cycles of various intracellular bacteria.

TRAF2 phosphorylation modulates tumor necrosis factor alpha-induced gene expression and cell resistance to apoptosis

TRAF2 is an adaptor protein that regulates the activation of the c-Jun N-terminal kinase (JNK) and IkappaB kinase (IKK) signaling cascades in response to tumor necrosis factor alpha (TNF-alpha) stimulation. Although the downstream events in TNF-alpha signaling are better understood, the membrane-proximal events are still elusive. Here, we demonstrate that TNF-alpha and cellular stresses induce TRAF2 phosphorylation at serine 11 and that this phosphorylation is required for the expression of a subset of NF-kappaB target genes. Although TRAF2 phosphorylation had a minimal effect on the TNF-alpha-induced rapid and transient IKK activation, it was essential for secondary and prolonged IKK activation. Consistent with this, TRAF2 phosphorylation is not required for its recruitment to the TNFR1 complex in response to TNF-alpha stimulation but is required for its association with a cytoplasmic complex containing RIP1 and IKK. In addition, TRAF2 phosphorylation was essential for the full TNF-alpha-induced activation of JNK. Notably, TRAF2 phosphorylation increased both basal and inducible c-Jun and NF-kappaB activities and rendered cells resistant to stress-induced apoptosis. Moreover, TRAF2 was found to be constitutively phosphorylated in some lymphomas. These results unveil a new, finely tuned mechanism for TNF-alpha-induced IKK activation modulated by TRAF2 phosphorylation and suggest that TRAF2 phosphorylation contributes to elevated levels of basal NF-kappaB activity in certain human cancers.

NF-kappaB signaling, liver disease and hepatoprotective agents

The NF-kappaB signaling pathway has particular relevance to several liver diseases including hepatitis (liver infection by Helicobacter, viral hepatitis induced by HBV and HCV), liver fibrosis and cirrhosis and hepatocellular carcinoma. Furthermore, the NF-kappaB signaling pathway is a potential target for development of hepatoprotective agents. Several types of drugs including: selective estrogen receptor modulators (SERMs), antioxidants, proteasome inhibitors, IKK inhibitors and nucleic acid-based decoys have been shown to interfere with NF-kappaB activity at different levels and may be useful for the treatment of liver diseases. However, NF-kappaB also plays an important hepatoprotective function that needs to be taken into consideration during development of new therapeutic regimens.

Advances in targeting IKK and IKK-related kinases for cancer therapy

IkappaB kinases (IKK) and IKK-related kinases play critical roles in regulating the immune response through nuclear factor-kappaB and IFN regulatory factor-dependent signaling transduction cascades. Recently, these kinases have been implicated in the pathogenesis of many human diseases, including cancer. In fact, dysregulation of IKK activities promotes tumor survival, proliferation, migration, metastasis, and angiogenesis-common characteristics of many types of human cancers. Because of their oncogenic effects in human cancer development, targeting IKK and IKK-related kinases is becoming an increasingly popular avenue for the development of novel therapeutic interventions for cancer. This review will briefly cover the recent discovery of the downstream substrates of IKK and IKK-related kinases, and outline the strategies used for targeting IKK as a therapeutic intervention for cancer.

Regulation of IkappaB kinase-related kinases and antiviral responses by tumor suppressor CYLD

The IkappaB kinase (IKK)-related kinases, IKKepsilon and TBK1, participate in the induction of type I interferons (IFNs) during viral infections. Deregulated activation of IKKepsilon and TBK1 also contributes to the abnormal cell survival and transformation. However, how these kinases are negatively regulated remains unclear. We show here that the tumor suppressor CYLD has an essential role in preventing aberrant activation of IKKepsilon/TBK1. CYLD deficiency causes constitutive activation of IKKepsilon/TBK1, which is associated with hyper-induction of IFNs in virus-infected cells. We further show that CYLD targets a cytoplasmic RNA sensor, RIG-I, and inhibits the ubiquitination of this IKKepsilon/TBK1 stimulator. Consistent with the requirement of ubiquitination in RIG-I function, CYLD potently inhibits RIG-I-mediated activation of the IFN-beta promoter. These findings establish CYLD as a key negative regulator of IKKepsilon/TBK1 and suggest a role for CYLD in the control of RIG-I ubiquitination.

Enhanced binding of TBK1 by an optineurin mutant that causes a familial form of primary open angle glaucoma

TANK-binding kinase 1 (TBK1) was identified as a binding partner for Optineurin (OPTN) in two-hybrid screens, an interaction confirmed by overexpression/immunoprecipitation experiments in HEK293 cells and by coimmunoprecipitation of endogenous OPTN and TBK1 from cell extracts. A TBK1 binding site was located between residues 1-127 of OPTN, residues 78-121 displaying striking homology to the TBK1-binding domain of TANK. The OPTN-binding domain was localised to residues 601-729 of TBK1, while TBK1[1-688] which cannot bind to TANK, did not interact with OPTN. The OPTN[E50K] mutant associated with Primary Open Angle Glaucoma (POAG) displayed strikingly enhanced binding to TBK1, suggesting that this interaction may contribute to familial POAG caused by this mutation.

Are the IKKs and IKK-related kinases TBK1 and IKK-epsilon similarly activated?

The IkappaB kinases (IKKs) IKK-alpha and IKK-beta, and the IKK-related kinases TBK1 and IKK-epsilon, have essential roles in innate immunity through signal-induced activation of NF-kappaB, IRF3 and IRF7, respectively. Although the signaling events within these pathways have been extensively studied, the mechanisms of IKK and IKK-related complex assembly and activation remain poorly defined. Recent data provide insight into the requirement for scaffold proteins in complex assembly; NF-kappaB essential modulator coordinates some IKK complexes, whereas TANK, NF-kappaB-activating kinase-associated protein 1 (NAP1) or similar to NAP1 TBK1 adaptor (SINTBAD) assemble TBK1 and IKK-epsilon complexes. The different scaffold proteins undergo similar post-translational modifications, including phosphorylation and non-degradative polyubiquitylation. Moreover, increasing evidence indicates that distinct scaffold proteins assemble IKK, and potentially TBK1 and IKK-epsilon subcomplexes, in a stimulus-specific manner, which might be a mechanism to achieve specificity.

Shared principles in NF-kappaB signaling

The transcription factor NF-kappaB has served as a standard for inducible transcription factors for more than 20 years. The numerous stimuli that activate NF-kappaB, and the large number of genes regulated by NF-kappaB, ensure that this transcription factor is still the subject of intense research. Here, we attempt to synthesize some of the basic principles that have emerged from studies of NF-kappaB, and we aim to generate a more unified view of NF-kappaB regulation.

Selenium suppresses the activation of transcription factor NF-kappa B and IRF3 induced by TLR3 or TLR4 agonists

Toll-like receptors (TLRs) play an important role in recognition of microbial components and induce innate immune responses by recognizing invading microbial pathogens leading to the activation of the adaptive immune responses. The microbial components trigger the activation of two downstream signaling pathways of TLRs; MyD88- and TRIF-dependent pathways leading to the expression of pro-inflammatory cytokines and type I interferons (IFNs). The MyD88- and TRIF-dependent pathways lead to the activation of NF-kappa B and IRF3 through the activation of IKK-beta and TBK1, respectively. Selenium is an essential trace element nutrient possessing anticarcinogenic properties. Here, we attempted to identify the molecular targets of selenium in TLR signaling pathways. Selenium inhibited NF-kappaB activation induced by poly[I:C] (TLR3 agonist), LPS (TLR4 agonist) or overexpression of MyD88 or IKK-beta which is the key kinase of MyD88-dependent signaling pathway. Selenium inhibited IRF3 activation induced by poly[I:C], LPS or the overexpression of TRIF or TBK1. Selenium also suppressed the expression of COX-2 and iNOS and the endogenous IFN beta mRNA induced by poly[I:C] or LPS. Therefore, our results suggest that selenium can modulate both MyD88- and TRIF-dependent signaling pathways of TLRs leading to decreased inflammatory gene expression.

Signaling to NF-kappaB by Toll-like receptors

Innate immunity is the first line of defense against invading pathogens. A family of Toll-like receptors (TLRs) acts as primary sensors that detect a wide variety of microbial components and elicit innate immune responses. All TLR signaling pathways culminate in activation of the transcription factor nuclear factor-kappaB (NF-kappaB), which controls the expression of an array of inflammatory cytokine genes. NF-kappaB activation requires the phosphorylation and degradation of inhibitory kappaB (IkappaB) proteins, which is triggered by two kinases, IkappaB kinase alpha (IKKalpha) and IKKbeta. In addition, several TLRs activate alternative pathways involving the IKK-related kinases TBK1 [TRAF family member-associated NF-kappaB activator (TANK) binding kinase-1] and IKKi, which elicit antiviral innate immune responses. Here, we review recent progress in our understanding of the role of NF-kappaB in TLR signaling pathways and discuss potential implications for molecular medicine.

Convergence of the NF-κB and IRF pathways in the regulation of the innate antiviral response

The type I interferon (IFN) alpha and beta promoters have been a leading paradigm of virus-activated transcriptional regulation for more than two decades, and have contributed substantially to our understanding of virus-inducible gene regulation, the coordinated activities of NF-kappaB and IRF transcription factors, the temporal and spatial recruitment of co-activators to the enhanceosome, and signaling pathways that trigger the innate antiviral response. In 2003, the ISICR Milstein Award was presented to John Hiscott of McGill University and Tom Maniatis of Harvard University for their ongoing research describing the mechanisms of regulation of type 1 interferon genes and specifically for the identification of key signaling kinases involved in phosphorylation of the transcription factors IRF-3 and IRF-7. The specific roles played by IRFs and the IKK-related kinases TBK1 and IKKvarepsilon are now recognized within the broader framework of TLR and RIG-I signaling pathways. This review summarizes the unique features of the IKK-related kinases and offers a summary of recent advances in the regulation of the early host response to virus infection.

Physiological roles and mechanisms of signaling by TRAF2 and TRAF5

RAF2 and TRAF5 are closely related members of the TRAF family of proteins. They are important signal transducers for a wide range of TNF receptor superfamily members, including TNFR1, TNFR2, CD40 and other lymphocyte costimulatory receptors, RANK/TRANCE-R, EDAR, LTbetaR, LMP-1 and IRE1. TRAF2 andTRAF5 therefore regulate diverse physiological roles, ranging from T and B cell signaling and inflammatory responses to organogenesis and cell survival. The major pathways mediated by TRAF2 and TRAF5 are the classical and alternative pathways of NF-kappaB activation, and MAPK and JNK activation. TRAF2 is heavily regulated by ubiquitin signals, and many of the signaling functions of TRAF2 are mediated through its RING domain and likely its own role as an E3 ubiquitin ligase.

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.

The chemotherapeutic agent DMXAA potently and specifically activates the TBK1-IRF-3 signaling axis

Vascular disrupting agents (VDAs) represent a novel approach to the treatment of cancer, resulting in the collapse of tumor vasculature and tumor death. 5,6-dimethylxanthenone-4-acetic acid (DMXAA) is a VDA currently in advanced phase II clinical trials, yet its precise mechanism of action is unknown despite extensive preclinical and clinical investigations. Our data demonstrate that DMXAA is a novel and specific activator of the TANK-binding kinase 1 (TBK1)–interferon (IFN) regulatory factor 3 (IRF-3) signaling pathway. DMXAA treatment of primary mouse macrophages resulted in robust IRF-3 activation and ∼750-fold increase in IFN-β mRNA, and in contrast to the potent Toll-like receptor 4 (TLR4) agonist lipopolysaccharide (LPS), signaling was independent of mitogen-activated protein kinase (MAPK) activation and elicited minimal nuclear factor κB–dependent gene expression. DMXAA-induced signaling was critically dependent on the IRF-3 kinase, TBK1, and IRF-3 but was myeloid differentiation factor 88–, Toll–interleukin 1 receptor domain–containing adaptor inducing IFN-β–, IFN promoter-stimulator 1–, and inhibitor of κB kinase–independent, thus excluding all known TLRs and cytosolic helicase receptors. DMXAA pretreatment of mouse macrophages induced a state of tolerance to LPS and vice versa. In contrast to LPS stimulation, DMXAA-induced IRF-3 dimerization and IFN-β expression were inhibited by salicylic acid. These findings detail a novel pathway for TBK1-mediated IRF-3 activation and provide new insights into the mechanism of this new class of chemotherapeutic drugs.

Liver development and regeneration: from laboratory study to clinical therapy

The liver has an unusual capacity to regenerate after a loss of mass and function caused by surgical resection or toxic liver injury. Over the last 10 years there have been major advances in our understanding of the molecular and cellular mechanisms underlying liver development and regeneration. The numerous factors crucial to these phenomena have been identified mainly by using knockout mice. Forward-genetics studies using zebrafish and medaka have also generated many mutants with liver disorders or defects in liver formation. Our goal is to translate knowledge gained from laboratory work and animal models into novel therapies for human liver diseases. Exciting progress has been achieved using human partial liver transplantation and autologous cell therapy.

Suppression of age-related inflammatory NF-κB activation by cinnamaldehyde

Redox sensitive, pro-inflammatory nuclear transcription factor NF-kappaB plays a key role in both inflammation and aging processes. In a redox state disrupted by oxidative stress, pro-inflammatory genes are upregulated by the activation of NF-kappaB through diverse kinases. Thus, the search and characterization of new substances that modulate NF-kappaB are of recent research interest. Cinnamaldehyde (CNA) is the major component of cinnamon bark oil, which has been widely used as a flavoring agent in foodstuffs such as beverages and ice cream. In the present study, CNA was examined for its molecular modulation of inflammatory NF-kappaB activation via the redox-related NIK/IKK and MAPK pathways through the reduction of oxidative stress. Results show that age-related NF-kappaB activation upregulated NF-kappaB targeting genes, inflammatory iNOS, and COX-2, all of which were inhibited effectively by CNA. Our study further shows that CNA inhibited the activation of NF-kappaB via three signal transduction pathways, NIK/IKK, ERK, and p38 MAPK. Our results indicate that CNA's antioxidative effect and the restoration of redox balance were responsible for its anti-inflammatory action. Thus, the significance of the current study is the new information revealing the anti-inflammatory properties of CNA and the role it plays in the regulation of age-related alterations in signal transduction pathways.

The interferon inducing pathways and the hepatitis C virus

The innate immune response is triggered by a variety of pathogens, including viruses, and requires rapid induction of type I interferons (IFN), such as IFNβ and IFNα. IFN induction occurs when specific pathogen motifs bind to specific cellular receptors. In non-professional immune, virally-infected cells, IFN induction is essentially initiated after the binding of dsRNA structures to TLR3 receptors or to intracytosolic RNA helicases, such as RIG-I /MDA5. This leads to the recruitment of specific adaptors, such as TRIF for TLR3 and the mitochondrial-associated IPS-1/VISA/MAVS/CARDIF adapter protein for the RNA helicases, and the ultimate recruitment of kinases, such as MAPKs, the canonical IKK complex and the TBK1/IKKε kinases, which activate the transcription factors ATF-2/c-jun, NF-κB and IRF3, respectively. The coordinated action of these transcription factors leads to induction of IFN and of pro-inflammatory cytokines and to the establishment of the innate immune response. HCV can cleave both the adapters TRIF and IPS-1/VISA/MAVS/CARDIF through the action of its NS3/4A protease. This provokes abrogation of the induction of the IFN and cytokine pathways and favours viral propagation and presumably HCV chronic infection.

Triggering the innate antiviral response through IRF-3 activation

Rapid induction of type I interferon (IFN) expression is a central event in the establishment of the innate immune response against viral infection and requires the activation of multiple transcriptional proteins following engagement and signaling through Toll-like receptor-dependent and -independent pathways. The transcription factor interferon regulatory factor-3 (IRF-3) contributes to a first line of defense against viral infection by inducing the production of IFN-beta that in turn amplifies the IFN response and the development of antiviral activity. In murine knock-out models, the absence of IRF-3 and the closely related IRF-7 ablates IFN production and increases viral pathogenesis, thus supporting a pivotal role for IRF-3/IRF-7 in the development of the host antiviral response.