Mutational analysis of TRAF6 reveals a conserved functional role of the RING dimerization interface and a potentially necessary but insufficient role of RING-dependent TRAF6 polyubiquitination towards NF-κB activation

Enhanced NF-κB activation via HIV-1 Tat-TRAF6 cross-talk

The Tat proteins of HIV-1 and simian immunodeficiency virus (SIV) are essential for activating viral transcription. In addition, Tat stimulates nuclear factor κB (NF-κB) signaling pathways to regulate viral gene expression although its molecular mechanism is unclear. Here, we report that Tat directly activates NF-κB through the interaction with TRAF6, which is an essential upstream signaling molecule of the canonical NF-κB pathway. This interaction increases TRAF6 oligomerization and auto-ubiquitination, as well as the synthesis of K63-linked polyubiquitin chains to further activate the NF-κB pathway and HIV-1 transcription. Moreover, ectopic expression of TRAF6 significantly activates HIV-1 transcription, whereas TRAF6 knockdown inhibits transcription. Furthermore, Tat-mediated activation of NF-κB through TRAF6 is conserved among HIV-1, HIV-2, and SIV isolates. Our study uncovers yet another mechanism by which HIV-1 subverts host transcriptional pathways to enhance its own transcription.

A novel tumor necrosis factor receptor-associated factor 6 (TRAF6) gene from Macrobrachiumrosenbergii involved in antibacterial defense against Aeromonas hydrophila

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is an adapter protein that triggers downstream cascades mediated by both TNFR and the interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. TRAF6 is involved in various biological processes, including innate and adaptive immunity. In the present study, a homolog of TRAF6 from Macrobrachium rosenbergii (MrTRAF6) was identified and characterized. The full-length cDNA of MrTRAF6 consisted of 2,114 nucleotides with an open reading frame (ORF) of 1,695 nucleotides encoding a 564-amino acid protein that contained a conserved TRAF family motif including two RING-type zinc fingers and a C-terminal meprin and TRAF homology (MATH) domain. The putative amino sequence of MrTRAF6 shared 45.5-97.3% identity with TRAF6s from other crustacean species with the highest identity to Macrobrachium nipponense TRAF6. Phylogenetic analysis revealed that MrTRAF6 was closely related to TRAF6 of invertebrates and clustered with crustaceans. According to gene expression analysis, the MrTRAF6 transcript demonstrated broad expression in all tissues tested, with the highest expression level in gill and the lowest in muscle tissues. Upon immune challenge with Aeromonas hydrophila, significant upregulation of MrTRAF6 expression was found in the gill, hepatopancreas, hemocyte, and muscle. Furthermore, an RNA interference assay showed that silencing MrTRAF6 by dsRNA could reduce the expression of mannose-binding lectin (MBL) and crustin, but no significant change was detected in anti-lipopolysaccharide factor 5 (ALF5) levels. In addition, the cumulative mortality rate of MrTRAF6-silenced M. rosenbergii was significantly increased after A. hydrophila infection. These findings indicated that MrTRAF6 is involved in antibacterial activity and plays a critical role in the innate immune response of M. rosenbergii.

In vitro and in vivo cancer cell apoptosis triggered by competitive binding of Cinchona alkaloids to the RING domain of TRAF6

TRAF6 is highly expressed in many tumors and plays an important role in the immune system. The aim of this study is to confirm anti-tumor activities of all naturally occurring Cinchona alkaloids that have been screened using computational docking program, and to validate the accuracy and specificity of the RING domain of TRAF6 as a potential anti-tumor target, and to explore their effect on the immune system. Results reported herein would demonstrate that Cinchona alkaloids could induce apoptosis in HeLa cells, inhibit the ubiquitination and phosphorylation of both AKT and TAK1, and up-regulate the ratio of Bax/Bcl-2. In addition, these compounds could induce apoptosis in vivo, and increase the secretion of TNF-α, IFN-γ, and IgG, while not significantly impacting the ratio of CD4+T/CD8+T. These investigations suggest that the RING domain of TRAF6 could serve as a de novo biological target for therapeutic treatment in cancers.

TRAF6 Plays a Proviral Role in Tick-Borne Flavivirus Infection through Interaction with the NS3 Protease

Tick-borne flaviviruses (TBFVs) can cause life-threatening encephalitis and hemorrhagic fever. To identify virus-host interactions that may be exploited as therapeutic targets, we analyzed the TBFV polyprotein in silico for antiviral protein-binding motifs. We obtained two putative tumor necrosis factor receptor-associated factor 6 (TRAF6)-binding motifs (TBMs) within the protease domain of the viral nonstructural 3 (NS3) protein. Here, we show that TBFV NS3 interacted with TRAF6 during infection and that TRAF6 supports TBFV replication. The proviral role of TRAF6 was not seen with mosquito-borne flaviviruses, consistent with the lack of conserved TBMs. Mutation of the second TBM within NS3 disrupted TRAF6 binding, coincident with reduced abundance of mature, autocatalytically derived form of the NS3 protease and significant virus attenuation in vitro. Our studies reveal insights into how flaviviruses exploit innate immunity for the purpose of viral replication and identify a potential target for therapeutic design.

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Langat virus (LGTV) NS3 protease interacts with TRAF6 during infection

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Tick-borne, unlike mosquito-borne, flaviviruses use TRAF6 for optimal replication

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E117A mutation of LGTV NS3 reduces TRAF6 binding and mature protease abundance

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LGTV with a mutated TRAF6-binding motif is attenuated in vitro

Identification of tumor necrosis factor receptor-associated factor 6 in the pearl mussel Hyriopsis cumingii and its involvement in innate immunity and pearl sac formation

Tumor necrosis factor receptor-associated factor 6 (TRAF6) acts as a central intracellular signal adapter molecule that mediates the tumor necrosis factor receptor superfamily and the interleukin-1 receptor/Toll-like receptor family in vertebrates and invertebrates. In the present study, HcTRAF6, a molluscan homologue of TRAF6 from Hyriopsis cumingii, has been cloned and identified. The entire open reading frame of HcTRAF6 was found to comprise a 1965-bp region that encodes a predicted protein of 654 amino acids, which contains conserved characteristic domains including a RING domain, two TRAF-type zinc finger domains, a typical coiled coil and the MATH domain. Phylogenetic analysis revealed that HcTRAF6 was aggregated closely with CsTRAF6 from Cyclina sinensis in the invertebrate cluster of mollusks. Further, qRT-PCR analysis showed that HcTRAF6 mRNA was extensively distributed in mussel tissues with a high expression in gills. After immune stimulation with Aeromonas hydrophila and lipopolysaccharides, the transcription of HcTRAF6 was obviously induced in the gills and hemocytes. In addition, significant fluctuation in HcTRAF6 expression was observed in the pearl sac, gills and hemocytes after mantle implantation. These findings confirmed its role in the alloimmune response. Dual-luciferase reporter assay showed that over-expression of HcTRAF6 could enhance the activity of the NF-κB reporter in a dose-dependent manner. Further, the RNA interference showed that the up-regulation of antimicrobial peptides in anti-bacterial infection was strongly suppressed in HcTRAF6-silenced mussels and that depletion of HcTRAF inhibited the elimination of A. hydrophila. All these findings together prove that HcTRAF6 functions as an efficient regulator in innate immune mechanisms against invading pathogens and the alloimmune mechanism after mantle implantation in H. cumingii.

Molecular basis of hypohidrotic ectodermal dysplasia: an update

Recent advances in understanding the molecular events underlying hypohidrotic ectodermal dysplasia (HED) caused by mutations of the genes encoding proteins of the tumor necrosis factor α (TNFα)-related signaling pathway have been presented. These proteins are involved in signal transduction from ectoderm to mesenchyme during development of the fetus and are indispensable for the differentiation of ectoderm-derived structures such as eccrine sweat glands, teeth, hair, skin, and/or nails. Novel data were reviewed and discussed on the structure and functions of the components of TNFα-related signaling pathway, the consequences of mutations of the genes encoding these proteins, and the prospect for further investigations, which might elucidate the origin of HED.

TRAF6 is a novel regulator of Notch signaling in Drosophila melanogaster

Notch signaling pathway unravels a fundamental cellular communication system that plays an elemental role in development. It is evident from different studies that the outcome of Notch signaling depends on signal strength, timing, cell type, and cellular context. Since Notch signaling affects a spectrum of cellular activity at various developmental stages by reorganizing itself in more than one way to produce different intensities in the signaling output, it is important to understand the context dependent complexity of Notch signaling and different routes of its regulation. We identified, TRAF6 (Drosophila homolog of mammalian TRAF6) as an interacting partner of Notch intracellular domain (Notch-ICD). TRAF6 genetically interacts with Notch pathway components in trans-heterozygous combinations. Immunocytochemical analysis shows that TRAF6 co-localizes with Notch in Drosophila third instar larval tissues. Our genetic interaction data suggests that the loss-of-function of TRAF6 leads to the rescue of previously identified Kurtz-Deltex mediated wing notching phenotype and enhances Notch protein survival. Co-expression of TRAF6 and Deltex results in depletion of Notch in the larval wing discs and down-regulates Notch targets, Wingless and Cut. Taken together, our results suggest that TRAF6 may function as a negative regulator of Notch signaling.

The PP4R1 subunit of protein phosphatase PP4 targets TRAF2 and TRAF6 to mediate inhibition of NF-κB activation

TRAFs constitute a family of proteins that have been implicated in signal transduction by immunomodulatory cellular receptors and viral proteins. TRAF2 and TRAF6 have an E3-ubiquitin ligase activity, which is dependent on the integrity of their RING finger domain and it has been associated with their ability to activate the NF-κB and AP1 signaling pathways. A yeast two-hybrid screen with TRAF2 as bait, identified the regulatory subunit PP4R1 of protein phosphatase PP4 as a TRAF2-interacting protein. The interaction of TRAF2 with PP4R1 depended on the integrity of the RING finger domain of TRAF2. PP4R1 could interact also with the TRAF2-related factor TRAF6 in a RING domain-dependent manner. Exogenous expression of PP4R1 inhibited NF-κB activation by TRAF2, TRAF6, TNF and the Epstein-Barr virus oncoprotein LMP1. In addition, expression of PP4R1 downregulated IL8 induction by LMP1, whereas downregulation of PP4R1 by RNA interference enhanced the induction of IL8 by LMP1 and TNF. PP4R1 could mediate the dephosphorylation of TRAF2 Ser11, which has been previously implicated in TRAF2-mediated activation of NF-κB. Finally, PP4R1 could inhibit TRAF6 polyubiquitination, suggesting an interference with the E3 ubiquitin ligase activity of TRAF6. Taken together, our data identify a novel mechanism of NF-κB pathway inhibition which is mediated by PP4R1-dependent targeting of specific TRAF molecules.

New insights into the role of sequestosome 1/p62 mutant proteins in the pathogenesis of Paget's disease of bone

Paget's disease of bone (PDB) is characterized by focal areas of aberrant and excessive bone turnover, specifically increased bone resorption and disorganized bone formation. Germline mutations in the sequestosome 1/p62 (SQSTM1/p62) gene are common in PDB patients, with most mutations affecting the ubiquitin-associated domain of the protein. In vitro, osteoclast precursor cells expressing PDB-mutant SQSTM1/p62 protein are associated with increases in nuclear factor κB activation, osteoclast differentiation, and bone resorption. Although the precise mechanisms by which SQSTM1/p62 mutations contribute to disease pathogenesis and progression are not well defined, it is apparent that as well as affecting nuclear factor κB signaling, SQSTM1/p62 is a master regulator of ubiquitinated protein turnover via autophagy and the ubiquitin-proteasome system. Additional roles for SQSTM1/p62 in the oxidative stress-induced Keap1/Nrf2 pathway and in caspase-mediated apoptosis that were recently reported are potentially relevant to the pathogenesis of PDB. Thus, SQSTM1/p62 may serve as a molecular link or switch between autophagy, apoptosis, and cell survival signaling. The purpose of this review is to outline recent advances in understanding of the multiple pathophysiological roles of SQSTM1/p62 protein, with particular emphasis on their relationship to PDB, including challenges associated with translating SQSTM1/p62 research into clinical diagnosis and treatment.

HIV-1 Nef interacts with HCV Core, recruits TRAF2, TRAF5 and TRAF6, and stimulates HIV-1 replication in macrophages

Tumor necrosis factor receptor-associated factor (TRAF) signaling plays a central role in many biological activities, such as the regulation of immune and inflammatory responses and control of apoptosis, which are key events in the pathogenesis of the human immunodeficiency virus (HIV)-1 and the hepatitis C virus (HCV) infections. Here we show that TRAF2, TRAF5 and TRAF6 interact with the HIV-1 Nef protein, an immunomodulatory viral protein expressed and released by cells infected by the virus. We also found that TRAF2 and TRAF5 interact with the HCV Core protein. Interestingly, we observed that HIV-1 Nef interacts with HCV Core. The activation of TRAF (2, 5, 6) - mediated by HIV-1 Nef and HCV Core - enhanced the activation of the nuclear factor-kappa B (NF-κB) and increased HIV-1 replication in monocyte- derived macrophages (MDMs). The knockdown of TRAF2, TRAF5 and TRAF6 resulted in decreased NF-κB activation and reduced HIV-1 replication in MDMs. Our results reveal a mechanism by which the activation of the TRAF pathway by HIV-1 Nef and HCV Core favors the replication of HIV-1 in macrophages and could be a critical factor for optimal replication of HIV-1 in macrophages of HIV-HCV-coinfected patients.

Functional studies for the TRAF6 mutation associated with hypohidrotic ectodermal dysplasia

BACKGROUND

Hypohidrotic ectodermal dysplasia (HED) is a rare condition characterized by hypotrichosis, hypohidrosis and hypodontia. A de novo heterozygous mutation in the tumour necrosis factor receptor-associated factor 6 gene (TRAF6) was recently identified in a patient with HED, while functional consequences resulting from the mutation remained unknown.

OBJECTIVES

To determine the mechanism by which the TRAF6 mutation results in HED.

METHODS

We performed coimmunoprecipitation (co-IP) studies to determine whether the mutation would affect the interaction of TRAF6 with transforming growth factor β-activated kinase 1 (TAK1), TAK1-binding protein 2 (TAB 2) and ectodysplasin-A receptor-associated death domain protein (EDARADD). We then performed co-IP and glutathione S-transferase-pulldown assays to determine the TRAF6 binding sequences in EDARADD. In addition, we analysed the effect of the mutant TRAF6 protein on the affinity between wild-type TRAF6 and EDARADD, as well as on EDARADD-mediated nuclear factor (NF)-κB activation.

RESULTS

The mutant TRAF6 protein was capable of forming a complex with TAK1 and TAB 2 in a similar way to wild-type TRAF6. However, the mutant TRAF6 protein completely lost the affinity to EDARADD, while the wild-type TRAF6 bound to the N-terminal domain of EDARADD. Furthermore, the mutant TRAF6 inhibited the interaction between the wild-type TRAF6 and EDARADD, and also potentially reduced the EDARADD-mediated NF-κB activity.

CONCLUSIONS

We conclude that the mutant TRAF6 protein shows a dominant negative effect against the wild-type TRAF6 protein, which is predicted to affect the EDARADD-mediated activation of NF-κB during the development of ectoderm-derived organs, and to lead to the HED phenotype.

The ubiquitin ligase TRAF6 negatively regulates the JAK-STAT signaling pathway by binding to STAT3 and mediating its ubiquitination

STAT3 is a key transcription factor that mediates various cellular and organismal processes, such as cell growth, apoptosis, immune response and cancer. However, the molecular mechanisms of STAT3 regulation remain poorly understood. Here, we identified TRAF6 as a new STAT3 interactor. TRAF6 augmented the ubiquitination of STAT3 and deactivated its transcriptional activity induced by IFNα stimulation or overexpressed with JAK2. Both the RING domain and the TRAF-type zinc finger domain of TRAF6 were indispensable for STAT3 deactivation. Accordingly, TRAF6 also down-regulated the expression of two known STAT3 target genes, CRP and ACT. Therefore, we showed that TRAF6 is a new regulator of JAK/STAT signaling and provide a new mechanistic explanation for the crosstalk between the NF-κB and the JAK-STAT pathways.

Effect of TRAF6 on the biological behavior of human lung adenocarcinoma cell

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a unique adaptor protein of the tumor necrosis factor receptor-associated factor family that mediates both tumor necrosis factor receptor and interleukin-1 receptor/Toll-like receptor signaling. A recent study showed that TRAF6 played an important role in tumorigenesis and invasion through activation of nuclear factor kappa B (NF-κB). However, the biological role of TRAF6 remains unknown in lung cancer up to now. To address the expression of TRAF6 in lung cancer cells, four lung cancer cell lines (A549, HCC827, NCI-H292, and 95-D) and human bronchial epithelial cells were used to detect the expression of TRAF6 protein by western blotting. Results indicated that TRAF6 displayed an upregulation in human lung cancer cell lines. To investigate the effects of TRAF6 on the biological behavior of human lung adenocarcinoma cell, we generated human lung adenocarcinoma A549 cell line in which TRAF6 was depleted. The results showed that downregulation of TRAF6 could decrease cell viability, suppress cell proliferation and invasion, and promote cell apoptosis. At the same time, we explored the effects of TRAF6 on the expression of the following proteins: phosphor-NF-κB (p-p65), cyclin D1, caspase-3, and matrix metalloproteinase 9 (MMP9). Downregulation of TRAF6 could decrease the expression of p-p65, cyclin D1, and MMP9 and increase the expression of caspase-3. All these results suggested that TRAF6 might be involved in the potentiation of growth, proliferation, and invasion of A549 cell line, as well as the inhibition of A549 cell apoptosis by the activation of NF-κB. To make a long story short, the overexpression of TRAF6 might be related to the tumorigenesis and invasion of lung cancer.

A rare heterozygous TRAF6 variant is associated with hypohidrotic ectodermal dysplasia

BACKGROUND

Mutations in the genes encoding components of the tumour necrosis factor (TNF)-α-like pathway cause hypohidrotic ectodermal dysplasia (HED). It has been postulated that the TNF receptor-associated factor 6 (TRAF6) is also involved in this pathway.

OBJECTIVES

To investigate mutations in the TRAF6 gene in an individual with HED.

METHODS

Genetic analysis was performed on TRAF6 in a patient with HED, her parents, her sister and 150 ethnically matched, healthy individuals.

RESULTS

In the patient, sequencing analysis of one DNA strand revealed a deletion of eight nucleotides (c.1074-1081delCAATTTG) in the 5' fragment of the last exon of TRAF6, while no deletion was detected in the other DNA strand indicating a heterozygous mutation. No such sequence abnormality was detected in the patient's parents and her sister.

CONCLUSION

This is the first report of a heterozygous TRAF6 sequence variant associated with symptoms typical of HED.

NF-κB: where did it come from and why?

The vast majority of research on nuclear factor κB (NF-κB) signaling in the past 25 years has focused on its roles in normal and disease-related processes in vertebrates, especially mice and humans. Recent genome and transcriptome sequencing efforts have shown that homologs of NF-κB transcription factors, inhibitor of NF-κB (IκB) proteins, and IκB kinases are present in a variety of invertebrates, including several in phyla simpler than Arthropoda, the phylum containing insects such Drosophila. Moreover, many invertebrates also contain genes encoding homologs of upstream signaling proteins in the Toll-like receptor signaling pathway, which is well-known for its downstream activation of NF-κB for innate immunity. This review describes what we now know or can infer and speculate about the evolution of the core elements of NF-κB signaling as well as the biological processes controlled by NF-κB in invertebrates. Further research on NF-κB in invertebrates is likely to uncover information about the evolutionary origins of this key human signaling pathway and may have relevance to our management of the responses of ecologically and economically important organisms to environmental and adaptive pressures.

Leishmania donovani exploits host deubiquitinating enzyme A20, a negative regulator of TLR signaling, to subvert host immune response

TLRs, which form an interface between mammalian host and microbe, play a key role in pathogen recognition and initiation of proinflammatory response thus stimulating antimicrobial activity and host survival. However, certain intracellular pathogens such as Leishmania can successfully manipulate the TLR signaling, thus hijacking the defensive strategies of the host. Despite the presence of lipophosphoglycan, a TLR2 ligand capable of eliciting host-defensive cytokine response, on the surface of Leishmania, the strategies adopted by the parasite to silence the TLR2-mediated proinflammatory response is not understood. In this study, we showed that Leishmania donovani modulates the TLR2-mediated pathway in macrophages through inhibition of the IKK-NF-κB cascade and suppression of IL-12 and TNF-α production. This may be due to impairment of the association of TRAF6 with the TAK-TAB complex, thus inhibiting the recruitment of TRAF6 in TLR2 signaling. L. donovani infection drastically reduced Lys 63-linked ubiquitination of TRAF6, and the deubiquitinating enzyme A20 was found to be significantly upregulated in infected macrophages. Small interfering RNA-mediated silencing of A20 restored the Lys 63-linked ubiquitination of TRAF6 as well as IL-12 and TNF-α levels with a concomitant decrease in IL-10 and TGF-β synthesis in infected macrophages. Knockdown of A20 led to lower parasite survival within macrophages. Moreover, in vivo silencing of A20 by short hairpin RNA in BALB/c mice led to increased NF-κB DNA binding and host-protective proinflammatory cytokine response resulting in effective parasite clearance. These results suggest that L. donovani might exploit host A20 to inhibit the TLR2-mediated proinflammatory gene expression, thus escaping the immune responses of the host.