TRAF6 regulates cell fate decisions by inducing caspase 8-dependent apoptosis and the activation of NF-kappaB

Pachymic acid activates TP53INP2/TRAF6/caspase-8 pathway to promote apoptosis in renal cell carcinoma cells

While pachymic acid (PA), a key component of Poria cocos (Schw.), has demonstrated anti-tumor effects in lung, breast, and pancreatic cancers, its impact on renal cell carcinoma (RCC) is unclear. This study evaluated the effect of PA on proliferation, migration, and apoptosis in human renal cancer A498 and ACHN cells as well as in cancer xenograft mice using wound scratch test, Western blotting, and co-immunoprecipitation assays. In a dose- and time-dependent manner, PA exhibited significant inhibition of RCC cell proliferation, migration, and invasion, accompanied by the induction of apoptosis. Additionally, PA upregulated the expression of tumor protein p53-inducible nuclear protein 2 (TP53INP2) and tumor necrosis factor receptor-associated factor 6 (TRAF6), which were downregulated in renal papillary and chromophobe carcinoma, resulting in inhibited tumor growth in mice. PA treatment elevated cleaved-caspase 3 and 8, and PARP levels, and facilitated TP53INP2 and TRAF6 binding to caspase 8, promoting its ubiquitination. Molecular docking revealed interactions between PA and TP53INP2, TRAF6. In summary, PA inhibits RCC development by upregulating TP53INP2 and promoting TRAF6-induced caspase 8 ubiquitination, activating apoptotic pathways.

MAP kinase ERK5 modulates cancer cell sensitivity to extrinsic apoptosis induced by death-receptor agonists

Death receptor ligand TRAIL is a promising cancer therapy due to its ability to selectively trigger extrinsic apoptosis in cancer cells. However, TRAIL-based therapies in humans have shown limitations, mainly due inherent or acquired resistance of tumor cells. To address this issue, current efforts are focussed on dissecting the intracellular signaling pathways involved in resistance to TRAIL, to identify strategies that sensitize cancer cells to TRAIL-induced cytotoxicity. In this work, we describe the oncogenic MEK5-ERK5 pathway as a critical regulator of cancer cell resistance to the apoptosis induced by death receptor ligands. Using 2D and 3D cell cultures and transcriptomic analyses, we show that ERK5 controls the proteostasis of TP53INP2, a protein necessary for full activation of caspase-8 in response to TNFα, FasL or TRAIL. Mechanistically, ERK5 phosphorylates and induces ubiquitylation and proteasomal degradation of TP53INP2, resulting in cancer cell resistance to TRAIL. Concordantly, ERK5 inhibition or genetic deletion, by stabilizing TP53INP2, sensitizes cancer cells to the apoptosis induced by recombinant TRAIL and TRAIL/FasL expressed by Natural Killer cells. The MEK5-ERK5 pathway regulates cancer cell proliferation and survival, and ERK5 inhibitors have shown anticancer activity in preclinical models of solid tumors. Using endometrial cancer patient-derived xenograft organoids, we propose ERK5 inhibition as an effective strategy to sensitize cancer cells to TRAIL-based therapies.

Long noncoding RNA SNHG16 regulates TLR4-mediated autophagy and NETosis formation in alveolar hemorrhage associated with systemic lupus erythematosus

BACKGROUND

Dysregulated long noncoding RNA (lncRNA) expression with increased apoptosis has been demonstrated in systemic lupus erythematosus (SLE) patients with alveolar hemorrhage (AH). SNHG16, a lncRNA, can enhance pulmonary inflammation by sponging microRNAs, and upregulate toll-like receptor 4 (TLR4) expression via stabilizing its mRNAs. TRAF6, a TLR4 downstream signal transducer, can induce autophagy and NETosis formation. In this study, we investigated whether SNHG16 could regulate TLR4-mediated autophagy and NETosis formation in SLE-associated AH.

METHODS

Expression of SNHG16, TLR4 and TRAF6 and cell death processes were examined in lung tissues and peripheral blood (PB) leukocytes from AH patients associated with SLE and other autoimmune diseases, and in the lungs and spleen from a pristane-induced C57BL/6 mouse AH model. SNHG16-overexpressed or -silenced alveolar and myelocytic cells were stimulated with lipopolysaccharide (LPS), a TLR4 agonist, for analyzing autophagy and NETosis, respectively. Pristane-injected mice received the intra-pulmonary delivery of lentivirus (LV)-SNHG16 for overexpression and prophylactic/therapeutic infusion of short hairpin RNA (shRNA) targeting SNHG16 to evaluate the effects on AH. Renal SNHG16 expression was also examined in lupus nephritis (LN) patients and a pristane-induced BALB/c mouse LN model.

RESULTS

Up-regulated SNHG16, TLR4 and TRAF6 expression with increased autophagy and NETosis was demonstrated in the SLE-AH lungs. In such patients, up-regulated SNHG16, TLR4 and TRAF6 expression was found in PB mononuclear cells with increased autophagy and in PB neutrophils with increased NETosis. There were up-regulated TLR4 expression and increased LPS-induced autophagy and NETosis in SNHG16-overexpressed cells, while down-regulated TLR4 expression and decreased LPS-induced autophagy and NETosis in SNHG16-silenced cells. Pristane-injected lung tissues had up-regulated SNHG16, TLR4/TRAF6 levels and increased in situ autophagy and NETosis formation. Intra-pulmonary LV-SNHG16 delivery enhanced AH through up-regulating TLR4/TRAF6 expression with increased cell death processes, while intra-pulmonary prophylactic and early therapeutic sh-SNHG16 delivery suppressed AH by down-regulating TLR4/TRAF6 expression with reduced such processes. In addition, there was decreased renal SNHG16 expression in LN patients and mice.

CONCLUSIONS

Our results demonstrate that lncRNA SNHG16 regulates TLR4-mediated autophagy and NETosis formation in the human and mouse AH lungs, and provide a therapeutic potential of intra-pulmonary delivery of shRNA targeting SNHG16 in this SLE-related lethal manifestation.

IKK-mediated TRAF6 and RIPK1 interaction stifles cell death complex assembly leading to the suppression of TNF-α-induced cell death

Tumor necrosis factor α (TNF-α) is a pro-inflammatory cytokine capable of inducing extrinsic apoptosis and necroptosis. Tumor necrosis factor receptor-associated factor 6 (TRAF6), an E3 ligase, is a member of the TRAF family of proteins, which mediates inflammatory signals by activating nuclear factor kappa B (NFкB) and mitogen-activated protein kinase (MAPK). Although the functions of TRAF6 have been identified, its role in TNF-α-induced cell death remains poorly understood. Here, we report that TRAF6 is a negative modulator of TNF-α-induced cell death but does not affect TNF-α-induced NFκB activation. TRAF6 deficiency accelerates both TNF-α-induced apoptosis and necroptosis; however, the acceleration can be reversed by reconstituting TRAF6 or TRAF6C70A, suggesting that E3 ligase activity is not required for this activity. Mechanistically, TRAF6 directly interacts with RIPK1 during TNF-α-induced cell death signaling, which prevents RIPK1 from interacting with components of the cell death complex such as itself, FADD or RIPK3. These processes suppress the assembly of the death complex. Notably, IKK was required for TRAF6 to interact with RIPK1. In vivo, Traf6-/- embryos exhibited higher levels of cell death in the liver but could be rescued by the simultaneous knockout of Tnf. Finally, TRAF6 knockdown xenografts were highly sensitive to necroptotic stimuli. We concluded that TRAF6 suppresses TNF-α-induced cell death in coordination with IKK complexes in vivo and in vitro by suppressing the assembly of cell death complex.

Transcriptomics Reveal Molecular Differences in Equine Oocytes Vitrified before and after In Vitro Maturation

In the last decade, in vitro embryo production in horses has become an established clinical practice, but blastocyst rates from vitrified equine oocytes remain low. Cryopreservation impairs the oocyte developmental potential, which may be reflected in the messenger RNA (mRNA) profile. Therefore, this study aimed to compare the transcriptome profiles of metaphase II equine oocytes vitrified before and after in vitro maturation. To do so, three groups were analyzed with RNA sequencing: (1) fresh in vitro matured oocytes as a control (FR), (2) oocytes vitrified after in vitro maturation (VMAT), and (3) oocytes vitrified immature, warmed, and in vitro matured (VIM). In comparison with fresh oocytes, VIM resulted in 46 differentially expressed (DE) genes (14 upregulated and 32 downregulated), while VMAT showed 36 DE genes (18 in each category). A comparison of VIM vs. VMAT resulted in 44 DE genes (20 upregulated and 24 downregulated). Pathway analyses highlighted cytoskeleton, spindle formation, and calcium and cation ion transport and homeostasis as the main affected pathways in vitrified oocytes. The vitrification of in vitro matured oocytes presented subtle advantages in terms of the mRNA profile over the vitrification of immature oocytes. Therefore, this study provides a new perspective for understanding the impact of vitrification on equine oocytes and can be the basis for further improvements in the efficiency of equine oocyte vitrification.

Molecular characterization and expression analyses of five genes involved in the MyD88-dependent pathway of yellow catfish (Pelteobagrus fulvidraco) responding to challenge of Aeromonas hydrophila

MyD88-dependent pathway mediated by Toll-like receptor is one of the vital ways activating immune responses. In order to identify the role of MyD88-dependent signaling pathway in yellow catfish, the Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 (p105) (Pf: abbreviation of Pelteobagrus fulvidraco) were cloned and characterized respectively. The Pf_MyD88, Pf_IRAK4, Pf_IRAK1 and Pf_TRAF6 were all highly conserved among species and showed the highest homology to that of Pangasianodon hypophthalmus. Pf_NFκB1 showed the highest homology to that of Ictalurus punetaus. All of the five genes showed similar expression patterns in various tissues, with the highest expression level in the liver. These genes also showed similar expression levels in different embryonic development stages, except Pf_IRAK4. The higher expression level was detected from fertilized eggs to 1 day post hatching (dph), lower expression from 3 dph to 30 dph. After stimulation of inactivated Aeromonas hydrophila, the mRNA expressions of Pf_MyD88, Pf_IRAK4, Pf_IRAK1, Pf_TRAF6 and Pf_NFκB1 were significantly increased at 24 h in the liver, spleen, head kidney and trunk kidney, suggesting that all the five genes were involved in the innate immune response of yellow catfish. These results showed that MyD88-dependent signaling pathway plays important roles for disease defensing in the innate immune response. Meanwhile, inactivated A. hydrophila can cause strong innate immune response, which provides theoretical bases for the application of inactivated vaccines in defense against bacterial diseases of teleost.

The dependency of autophagy and ubiquitin proteasome system during skeletal muscle atrophy

Among the four proteolytic systems in the cell, autophagy and the ubiquitin-proteasome system (UPS) are the main proteolytic events that allow for the removal of cell debris and proteins to maintain cellular homeostasis. Previous studies have revealed that these systems perform their functions independently of each other. However, recent studies indicate the existence of regulatory interactions between these proteolytic systems via ubiquitinated tags and a reciprocal regulation mechanism with several crosstalk points. UPS plays an important role in the elimination of short-lived/soluble misfolded proteins, whereas autophagy eliminates defective organelles and persistent insoluble protein aggregates. Both of these systems seem to act independently; however, disruption of one pathway affects the activity of the other pathway and contributes to different pathological conditions. This review summarizes the recent findings on direct and indirect dependencies of autophagy and UPS and their execution at the molecular level along with the important drug targets in skeletal muscle atrophy.

Induction of IRAK-M in melanoma induces caspase-3 dependent apoptosis by reducing TRAF6 and calpastatin levels

Melanoma represents the most serious type of skin cancer. Although recent years have seen advances using targeted and immunotherapies, most patients remain at high risk for tumor recurrence. Here we show that IRAK-M, a negative regulator of MyD88 signaling, is deficient or low in melanoma and expression levels correlate with patient survival. Inducing IRAK-M expression using genetic approaches or epigenetic modifiers initiates apoptosis by prompting its interaction with TRAF6 via IRAK-M's C-terminal domain. This complex recruits and degrades calpastatin which stimulates calpain activity and triggers caspase-3-dependent but caspase-8,-9-independent apoptosis. Using a drug screen, we identified compounds that induced IRAK-M expression. Administration of IRAK-M-inducing drugs reduced tumor growth in mice but was ineffective against IRAK-M knock-down tumors. These results uncover a previously uncharacterized apoptosis pathway, emphasize IRAK-M as a potential therapeutic target and suggest that the anticancer activity of certain drugs could do so through their ability to induce IRAK-M expression.

Gas6/Axl signaling attenuates alveolar inflammation in ischemia-reperfusion-induced acute lung injury by up-regulating SOCS3-mediated pathway

Background

Axl is a cell surface receptor tyrosine kinase, and activation of the Axl attenuates inflammation induced by various stimuli. Growth arrest-specific 6 (Gas6) has high affinity for Axl receptor. The role of Gas6/Axl signaling in ischemia-reperfusion-induced acute lung injury (IR-ALI) has not been explored previously. We hypothesized that Gas6/Axl signaling regulates IR-induced alveolar inflammation via a pathway mediated by suppressor of cytokine signaling 3 (SOCS3).

Methods

IR-ALI was induced by producing 30 min of ischemia followed by 90 min of reperfusion in situ in an isolated and perfused rat lung model. The rats were randomly allotted to a control group and IR groups, which were treated with three different doses of Gas6. Mouse alveolar epithelium MLE-12 cells were cultured in control and hypoxia-reoxygenation (HR) conditions with or without Gas6 and Axl inhibitor R428 pretreatment.

Results

We found that Gas6 attenuated IR-induced lung edema, the production of proinflammatory cytokines in perfusates, and the severity of ALI ex vivo. IR down-regulated SOCS3 expression and up-regulated NF-κB, and Gas6 restored this process. In the model of MLE-12 cells with HR, Gas6 suppressed the activation of TRAF6 and NF-κB by up-regulating SOCS3. Axl expression of alveolar epithelium was suppressed in IR-ALI but Gas6 restored phosphorylation of Axl. The anti-inflammatory effect of Gas6 was antagonized by R428, which highlighted that phosphorylation of Axl mediated the protective role of Gas6 in IR-ALI.

Conclusions

Gas6 up-regulates phosphorylation of Axl on alveolar epithelium in IR-ALI. The Gas6/Axl signaling activates the SOCS3-mediated pathway and attenuates IR-related inflammation and injury.

Regulation of death receptor signaling by the autophagy protein TP53INP2

TP53INP2 positively regulates autophagy by binding to Atg8 proteins. Here, we uncover a novel role of TP53INP2 in death-receptor signaling. TP53INP2 sensitizes cells to apoptosis induced by death receptor ligands. In keeping with this, TP53INP2 deficiency in cultured cells or mouse livers protects against death receptor-induced apoptosis. TP53INP2 binds caspase-8 and the ubiquitin ligase TRAF6, thereby promoting the ubiquitination and activation of caspase-8 by TRAF6. We have defined a TRAF6-interacting motif (TIM) and a ubiquitin-interacting motif in TP53INP2, enabling it to function as a scaffold bridging already ubiquitinated caspase-8 to TRAF6 for further polyubiquitination of caspase-8. Mutations of key TIM residues in TP53INP2 abrogate its interaction with TRAF6 and caspase-8, and subsequently reduce levels of death receptor-induced apoptosis. A screen of cancer cell lines showed that those with higher protein levels of TP53INP2 are more prone to TRAIL-induced apoptosis, making TP53INP2 a potential predictive marker of cancer cell responsiveness to TRAIL treatment. These findings uncover a novel mechanism for the regulation of caspase-8 ubiquitination and reveal TP53INP2 as an important regulator of the death receptor pathway.

Inhibition of NKCC1 Modulates Alveolar Fluid Clearance and Inflammation in Ischemia-Reperfusion Lung Injury via TRAF6-Mediated Pathways

Background: The expression of Na-K-2Cl cotransporter 1 (NKCC1) in the alveolar epithelium is responsible for fluid homeostasis in acute lung injury (ALI). Increasing evidence suggests that NKCC1 is associated with inflammation in ALI. We hypothesized that inhibiting NKCC1 would attenuate ALI after ischemia-reperfusion (IR) by modulating pathways that are mediated by tumor necrosis-associated factor 6 (TRAF6).

Methods: IR-ALI was induced by producing 30 min of ischemia followed by 90 min of reperfusion in situ in an isolated and perfused rat lung model. The rats were randomly allotted into four groups comprising two control groups and two IR groups with and without bumetanide. Alveolar fluid clearance (AFC) was measured for each group. Mouse alveolar MLE-12 cells were cultured in control and hypoxia-reoxygenation (HR) conditions with or without bumetanide. Flow cytometry and transwell monolayer permeability assay were carried out for each group.

Results: Bumetanide attenuated the activation of p-NKCC1 and lung edema after IR. In the HR model, bumetanide decreased the cellular volume and increased the transwell permeability. In contrast, bumetanide increased the expression of epithelial sodium channel (ENaC) via p38 mitogen-activated protein kinase (p38 MAPK), which attenuated the reduction of AFC after IR. Bumetanide also modulated lung inflammation via nuclear factor-κB (NF-κB). TRAF6, which is upstream of p38 MAPK and NF-κB, was attenuated by bumetanide after IR and HR.

Conclusions: Inhibition of NKCC1 by bumetanide reciprocally modulated epithelial p38 MAPK and NF-κB via TRAF6 in IR-ALI. This interaction attenuated the reduction of AFC via upregulating ENaC expression and reduced lung inflammation.

Functional diversity of miR-146a-5p and TRAF6 in normal and oral cancer cells

Numerous studies implicate miR-146a as pleiotropic regulator of carcinogenesis; however, its roles in carcinogenesis are not fully understood. A clue from expression analyses of miR-146a-5p in all 13 oral squamous cell carcinoma (OSCC) cell lines examined and in OSCC tissues, whole blood and whole saliva of OSCC patients in vivo revealed that miR‑146a-5p expression was highly upregulated. Particularly, we widened the view of its upregulation in saliva, implicating that high miR-146a-5p expression is not only correlated closely to the development of human oral cancer, but also to a possible candidate as a diagnostic marker of OSCC. Indeed, further examination showed that exogenous miR-146a-5p expression showed pleiotropic effects on cell proliferation and apoptosis which were partially based on the contextual responses of activation of JNK, downstream of TRAF6 that was targeted by miR-146a-5p in normal human keratinocytes and OSCC cell lines. TRAF6 suppression by a TRAF6-specific siRNA resulted in contradictory consequences on cellular processes in normal and OSCC cells. Notably, TRAF6 downregulation by both miR-146a-5p and TRAF6-specific siRNA deactivated JNK in SCC-9, but not in normal human keratinocytes. In support of the proliferation-promoting effect of miR-146a-5p, silencing of endogenous miR-146a-5p significantly reduced proliferation of SCC-9. Together, these results suggest that miR-146a-5p affects proliferation and apoptosis in a cellular context-dependent manner and selectively disarms the TRAF6-mediated branch of the TGF-β signaling in OSCC cell lines by sparing Smad4 involvement.

Double-stranded RNA induces chicken T-cell lymphoma apoptosis by TRIF and NF-κB

Toll-like receptor-3 (TLR3), a member of the pathogen recognition receptor family, has been reported to activate immune response and to exhibit pro-apoptotic activity against some tumor cells. However it is unclear whether TLR3 has same function against chicken lymphoma. In this paper we investigated the effect of TLR3 activation on a Marek’s disease lymphoma-derived chicken cell line, MDCC-MSB1. The TLR3 agonist poly (I:C) activated TLR3 pathway and inhibited tumor cells proliferation through caspase-dependent apoptosis. Using pharmacological approaches, we found that an interferon-independent mechanism involving Toll-IL-1-receptor domain-containing adapter-inducing IFN-α (TRIF) and nuclear factor κB (NF-κB) causes the apoptosis of MDCC-MSB1 cells. This is the first report about the function of TLR3 in chicken T-cell lymphoma, especially in signal pathway. The mechanisms underlying TLR3-mediated apoptosis may contribute to the development of new drug to treat lymphomas and oncovirus infections.

TRAF6 is a critical factor in fish immune response to virus infection

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is one of the key adaptor molecule in Toll-like receptor signal transduction that triggers downstream cascades involved in innate immunity. In our previous study, the molecular characteristics of EtTRAF6 (TRAF6 from Epinephelus tauvina), the tissue distributions, expression patterns after challenging with bacterial and viral pathogens were investigated. Here we identified EtTRAF6 as an important regulator of virus-triggered signaling pathway. Overexpression of EtTRAF6-ORF and truncated forms of EtTRAF6, including EtTRAF6-C (delete the MATH domain), EtTRAF6-N (delete the RING domain) and EtTRAF6-MATH, inhibited IFN-β activity strongly in grouper spleen (GS) cells. Overexpression of EtTRAF6 repressed virus-induced production of type I IFNs. When EtTRAF6 cotransfected with EcIRF3 or EcIRF7, EtTRAF6 inhibited IRF-induced activation of IFN-β. Over-expressed EtTRAF6 inhibited the transcription of SGIV genes significantly in GS cells. Although TRAF6 has a role in apoptosis regulation, it is not known if EtTRAF6 has any role in apoptosis regulation. Strikingly, when over-expressed in fathead minnow (FHM) cells, EtTRAF6 protected them from cell death induced by SGIV. Therefore, these results suggest that TRAF6 may play a critical role in their response to SGIV infection, through regulation of a cell death pathway that is common to fish and humans.

Mycobacterium indicus pranii (MIP) mediated host protective intracellular mechanisms against tuberculosis infection: Involvement of TLR-4 mediated signaling

Mycobacterium tuberculosis infection inflicts the disease Tuberculosis (TB), which is fatal if left untreated. During M. tuberculosis infection, the pathogen modulates TLR-4 receptor down-stream signaling, indicating the possible involvement of TLR-4 in the regulation of the host immune response. Mycobacterium indicus pranii (MIP) possesses immuno-modulatory properties which induces the pro-inflammatory responses via induction of TLR-4-mediated signaling. Here, we observed the immunomodulatory properties of MIP against tuberculosis infection. We have studied the detailed signaling mechanisms employed by MIP in order to restore the host immune response against the in vitro tuberculosis infection. We observed that in infected macrophages MIP treatment significantly increased the TLR-4 expression as well as activation of its downstream signaling, facilitating the activation of P38 MAP kinase. MIP treatment was able to activate NF-κB via involvement of TLR-4 signaling leading to the enhanced pro-inflammatory cytokine and NO generation in the infected macrophages and generation of protective immune response. Therefore, we may suggest that, TLR4 may represent a novel therapeutic target for the activation of the innate immune response during Tuberculosis infection.

Socs1 and Socs3 degrades Traf6 via polyubiquitination in LPS-induced acute necrotizing pancreatitis

Mechanisms involved in inflammatory development during acute pancreatitis (AP) are largely vague, especially in the transformation of acute edematous pancreatitis (AEP) into acute necrotizing pancreatitis (ANP). This current study aims to investigate the functions of Traf6 in different AP models in vitro and in vivo, and to identify the possible regulatory mechanism in the progression of inflammation from mild to severe. Our data revealed that the level of Traf6 expression was significantly increased in the mild AP induced by caerulein, and the upregulation of Traf6 played a protective role in acinar cells against caerulein-induced apoptosis. In contrast, only Traf6 protein but not mRNA was downregulated in the severe ANP induced by combination treatment of caerulein and LPS. Mechanistic studies showed that LPS upregulated the levels of Socs1 and Socs3 expressions in acinar cells, Socs1 and Socs3 interacted Traf6 directly and degraded Traf6 protein via polyubiquitination, thereby counteracted the protective function of Traf6. In vivo study further showed that combination treatment of caerulein and LPS failed to induce an ANP model in the TLR4 knockout mice, and the level of Traf6 expression in the pancreatic tissues remained the same as that from the acute edematous pancreatitis (AEP) mouse. Taken together, our study reveals that Traf6 functioned as a protective factor in the progression of AP, and LPS-induced Socs1 and Socs3 exacerbate mild AP to severe AP, which provides evidence for developing a new therapeutic target to combat AP.

Molecular genomic- and transcriptional-aspects of a teleost TRAF6 homolog: Possible involvement in immune responses of Oplegnathus fasciatus against pathogens

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) is a crucial docking molecule for TNFR superfamily and Interleukin-1 receptor/Toll-like receptor (IL-1R/TLR) superfamily. As an adaptor protein in pathogen-induced signaling cascades, TRAF6 modulates both adaptive- and innate-immunity. In order to understand the immune responses of teleost TRAF6, Oplegnathus fasciatus TRAF6-like gene (OfTRAF6) was identified and characterized. Genomic length of OfTRAF6 (4 kb), obtained by means of a genomic BAC library, spanned seven exons which represented a putative coding sequence of 1716 bp and encoded 571 amino acids (aa) with an estimated molecular weight of 64 kDa. This putative protein demonstrated the classical tetra-domain architecture composed of a zinc finger RING-type profile, two zinc finger TRAF-type profiles, a coiled-coil region and a MATH domain. While the sequence similarity with human TRAF6 was 66.5%, OfTRAF6 shared a higher overall similarity with teleost homologs (∼75-92%). Phylogeny of TRAF-family was examined and TRAF6-subfamily appeared to be the precursor of other subfamilies. In addition, the clustering pattern confirmed that OfTRAF6 is a novel member of TRAF6subfamily. Based on comparative genomic analysis, we found that vertebrate TRAF6 exhibits two distinct structures in teleost and tetrapod lineages. An intron-loss event has probably occurred in TRAF6 gene during the evolution of tetrapods from teleosts. Inspection of putative OfTRAF6 promoter revealed the presence of several immune responsive transcription factor binding sites. Real-time qPCR assay detected OfTRAF6 transcripts in eleven juvenile fish tissues with higher levels in peripheral blood cells followed by liver. Putative role of OfTRAF6 in response to flagellin, LPS, poly I:C, pathogenic bacteria (Edwardsiella tarda and Streptococcus iniae) and rock bream iridovirus (RBIV) was profiled in different tissues and OfTRAF6 revealed up-regulated transcript levels. Altogether, these findings implicate that OfTRAF6 is not only involved in flagellin-induced signaling cascade, but also contributes to the antibacterial- and antiviral-responses.

Multi-parametric analysis of cell death pathways using live-cell microscopy

Programmed cell death is a complex process with new forms being discovered with regularity. Each pathway has a distinct and overlapping biochemical and physiological change occurring as the cell prepares for death. Detection of these changes can be facilitated by the availability of various fluorescent probes and advances in microscope systems. By analyzing these probes over time using fluorescence microscopy, the changes that occur in each cell en route to death can be analyzed on a cell-by-cell basis. While the timing of events varies considerably from cell to cell, it has been found that the sequence of events is highly conserved. Transient events, which would be difficult to detect using population averaging techniques, are readily detected when cells are analyzed individually in time lapse. The protocols in this unit describe using probes for real-time imaging of one of the apoptotic cell death pathways using various inducers, as well as the associated hardware necessary for imaging so that the imaging itself is not affecting cell viability.

Phylogenetic Analysis of Brassica rapa MATH-Domain Proteins

The MATH (meprin and TRAF-C homology) domain is a fold of seven anti-parallel β-helices involved in protein-protein interaction. Here, we report the identification and characterization of 90 MATH-domain proteins from the Brassica rapa genome. By sequence analysis together with MATH-domain proteins from other species, the B. rapa MATH-domain proteins can be grouped into 6 classes. Class-I protein has one or several MATH domains without any other recognizable domain; Class-II protein contains a MATH domain together with a conserved BTB (Broad Complex, Tramtrack, and Bric-a-Brac ) domain; Class-III protein belongs to the MATH/Filament domain family; Class-IV protein contains a MATH domain frequently combined with some other domains; Class-V protein has a relative long sequence but contains only one MATH domain; Class-VI protein is characterized by the presence of Peptidase and UBQ (Ubiquitinylation) domains together with one MATH domain. As part of our study regarding seed development of B. rapa, six genes are screened by SSH (Suppression Subtractive Hybridization) and their expression levels are analyzed in combination with seed developmental stages, and expression patterns suggested that Bra001786, Bra03578 and Bra036572 may be seed development specific genes, while Bra001787, Bra020541 and Bra040904 may be involved in seed and flower organ development. This study provides the first characterization of the MATH domain proteins in B. rapa.

Inactivation of DNA-dependent protein kinase promotes heat-induced apoptosis independently of heat-shock protein induction in human cancer cell lines

The inhibition of DNA damage response pathway seems to be an attractive strategy for cancer therapy. It was previously reported that in rodent cells exposed to heat stress, cell growth was promoted by the activity of DNA-dependent protein kinase (DNA-PK), an enzyme involved in DNA non-homologous end joining (NHEJ) required for double-strand break repair. The absence of a functioning DNA-PK was associated with down regulation of heat shock protein 70 (HSP70). The objective of this study is thus to investigate the role of DNA-PK inhibition in heat-induced apoptosis in human cell lines. The inhibitors of phosphorylation of the DNA-PK catalytic subunit (DNA-PKcs) at Ser2056, such as NU7026 and NU7441, were utilized. Furthermore, knock down of DNA-PKcs was carried out using small interfering RNA (siDNA-PKcs). For heat exposure, cells were placed in water bath at 44°C for 60 min. Apoptosis was evaluated after 24 h incubation flow cytometrically. Proteins were extracted after 24 h and analyzed for HSP70 and HSP40 expression by Western blotting. Total RNA was extracted 6 h after treatment and analyzed using a GeneChip® microarray system to identify and select the up-regulated genes (≥1.5 fold). The results showed an enhancement in heat-induced apoptosis in absence of functioning DNA-PKcs. Interestingly, the expression levels of HSP70 and HSP40 were elevated in the absence of DNA-PKcs under heat stress. The results of genetic network analysis showed that HSPs and JUN genes were up-regulated independently of DNA-PKcs in exposed parent and knock out cells. In the presence of functioning DNA-PKcs, there was an observed up-regulation of anti-apoptotic genes, such as NR1D1, whereas in the absence of DNA-PKcs the pro-apoptotic genes, such as EGR2, were preferentially up-regulated. From these findings, we concluded that in human cells, the inactivation of DNA-PKcs can promote heat-induced apoptosis independently of heat-shock proteins.

TNF receptor-associated factor 6 in advanced non-small cell lung cancer: clinical and prognostic implications

PURPOSE

Tumor necrosis factor (TNF) receptor-associated factor 6 (TRAF6) represents a central point of convergence for the signal transduction by the TNFR and the IL-lR/TLR superfamilies. We conducted this retrospective clinical study focusing on TRAF6 expression associated with overall survival and chemotherapeutical sensitivity in a large population with advanced non-small cell lung cancer (NSCLC).

METHODS

A total of 324 patients with stage III and IV NSCLC were retrospectively enrolled. Immunohistochemistry was performed to evaluate the expression of TRAF6, apoptosis-related proteins Bcl-2, Bax, Fas, and FasL, as well as the density of CD8(+) and FOXP3(+) tumor infiltrating lymphocytes (TILs) in tumor microenvironment.

RESULTS

A total of 193 carcinomas (59.6 %) were identified as high expression of TRAF6. TRAF6 expression was not significantly related with histology and clinic stage. No obvious correlation of TRAF6 expression with apoptosis-related protein and TILs density was identified. TRAF6 status was correlated inversely with response to chemotherapy in overall patients (response rates 24.9 and 32.8 %, for patients with high-TRAF6 and low-TRAF6 tumors, respectively, P = 0.039). However, multivariate logistic regression analysis could not identify TRAF6 status as an independent predictor for the response to chemotherapy in overall cohort (95 % CI: 0.91-3.32, P = 0.065). The overall survival was not significantly associated with TRAF6 expression (P = 0.616).

CONCLUSIONS

Our results provide new insight for the biological properties and clinical relevance of the TRAF6 in NSCLC. TRAF6 is a promise target for therapeutic strategies against cancer.

CD40-mediated cell death requires TRAF6 recruitment

CD40 has an important role in T cell-B cell interaction which rescues B lymphocytes from undergoing apoptosis. However, various studies have demonstrated that CD40 can also play a direct role in the induction of specific cell death and thus in the inhibition of tumour cell proliferation. Our previous studies showed that CD40-mediated cell death was independent of caspases and required no de novo protein synthesis. Knowing that CD40 signaling is mediated by its association with several intracellular effectors, including members of TNFR-associated factors (TRAFs) family, the goal of the present study is to investigate the mechanisms involved in the induction of cell death by CD40. Our data reveals that CD40-mediated cell death required lysosomal membrane permeabilization and the subsequent cathepsin B release. In addition, CD40 homodimer formation, a phenomenon known to be necessary for some CD40-mediated signals, was shown to negatively regulate cell death induced by CD40. Moreover, using HEK293 cells ectopically expressing CD40 deficient in TRAF binding, we showed that CD40-mediated apoptosis occurred in the absence of TRAF2 and TRAF3 association, but was significantly reduced when CD40 was deficient in its TRAF6 binding. Therefore, by outlining the role of lysosomal pathways and intracellular effectors, namely TRAF6 in CD40-mediated cell death, our study identifies new targets for anti-cancer therapy.

Characterization of apoptosis and proliferation in esophageal carcinoma EC109 cells following siRNA-induced down-regulation of TRAF6

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is an activator of the NF-κB transcription factor. NF-κB is involved in a variety of inflammatory, anti-apoptotic, and gene regulatory pathways and was recently found to be over-expressed in esophageal cancer cells. Here we investigated the function of TRAF6 in the esophageal cancer cell line EC109. siRNA targeting TRAF6 was introduced into EC109 cells and TRAF6 mRNA and protein levels were subsequently examined via RT-PCR and western blotting. Rates of apoptosis and cell proliferation were also measured using flow cytometry, ethynyl deoxyuridine (EdU), and CCK-8 (Cell Counting Kit-8) assays. The real-time PCR array was applied to profile the expression of TRAF6 related genes. TRAF6-siRNA reduced TRAF6 mRNA and protein expressions. NF-κB p65 protein expression was decreased in TRAF6-targeting siRNA-transfected cells compared to cells of the negative control. TRAF6-siRNA also significantly inhibited proliferation and enhanced apoptosis of EC109 cells. These studies suggested that TRAF6 was required for NF-κB activation in EC109 cells and it may be a good molecular target for suppressing the survival and proliferation of esophageal cancer cells.

Competition between TRAF2 and TRAF6 regulates NF-kappaB activation in human B lymphocytes

OBJECTIVE

To investigate the role of TNF receptor-associated factor 2 (TRAF-2) and TRAF6 in CD40-induced nuclear factor-kappaB (NF-kappaB) signaling pathway and whether CD40 signaling requires TRAF2.

METHODS

Human B cell lines were transfected with plasmids expressing wild type TRAF2 or dominant negative TRAF2, TRAF2-shRNA, or TRAF6-shRNA. The activation of NF-kappaB was detected by Western blot, kinase assay, transfactor enzyme-linked immunosorbent assay (ELISA), and fluorescence resonance energy transfer (FRET). Analysis of the role of TRAF-2 and TRAF-6 in CD40-mediated NF-kappaB activity was examined following stimulation with recombinant CD154.

RESULTS

TRAF2 induced activity of IkappaB-kinases (IKKalpha, IKKi/epsilon), phosphorylation of IkappaBalpha, as well as nuclear translocation and phosphorylation of p65/RelA. In contrast, TRAF6 strongly induced NF-kappaB activation and nuclear translocation of p65 as well as p50 and c-Rel. Engagement of CD154-induced nuclear translocation of p65 was inhibited by a TRAF6-shRNA, but conversely was enhanced by a TRAF2-shRNA. Examination of direct interactions between CD40 and TRAFs by FRET documented that both TRAF2 and TRAF6 directly interacted with CD40. However, the two TRAFs competed for CD40 binding.

CONCLUSIONS

These results indicate that TRAF2 can signal in human B cells, but it is not essential for CD40-mediated NF-kappaB activation. Moreover, TRAF2 can compete with TRAF6 for CD40 binding, and thereby limit the capacity of CD40 engagement to induce NF-kappaB activation.

Development of a nonintegrating Rev-dependent lentiviral vector carrying diphtheria toxin A chain and human TRAF6 to target HIV reservoirs

Persistence of HIV despite highly active antiretroviral therapy (HAART) is a lasting challenge to virus eradication. To develop a strategy complementary to HAART, we constructed a series of Rev-dependent lentiviral vectors carrying diphtheria toxin A chain (DT-A) and its attenuated mutants, as well as human TRAF6. Expression of these suicide genes following delivery through viral particles is dependent on Rev, which exists only in infected cells. Among these toxins, DT-A has been known to trigger cell death with as little as a single molecule, whereas two of the attenuated mutants in this study, DT-A(176) and DT-A(ΔN), were well-tolerated by cells at low levels. TRAF6 induced apoptosis only with persistent overexpression. Thus, these suicide genes, which induce cell death at different expression levels, offer a balance between efficacy and safety. To minimize possible mutagenesis introduced by retroviral integration in non-target cells, we further developed a non-integrating Rev-dependent (NIRD) lentiviral vector to deliver these genes. In addition, we constructed a DT-A-resistant human cell line by introducing a human elongation factor 2 (EF-2) mutant into HEK293T cells. This allowed us to manufacture the first high-titer NIRD lentiviral particles carrying DT-A to target HIV-positive cells.

TRAF5 is a downstream target of MAVS in antiviral innate immune signaling

The recognition of nucleic acids by the innate immune system during viral infection results in the production of type I interferons and the activation of antiviral immune responses. The RNA helicases RIG-I and MDA-5 recognize distinct types of cytosolic RNA species and signal through the mitochondrial protein MAVS to stimulate the phosphorylation and activation of the transcription factors IRF3 and IRF7, thereby inducing type I interferon expression. Alternatively, the activation of NF-κB leads to proinflammatory cytokine production. The function of MAVS is dependent on both its C-terminal transmembrane (TM) domain and N-terminal caspase recruitment domain (CARD). The TM domain mediates MAVS dimerization in response to viral RNA, allowing the CARD to bind to and activate the downstream effector TRAF3. Notably, dimerization of the MAVS CARD alone is sufficient to activate IRF3, IRF7, and NF-κB. However, TRAF3-deficient cells display only a partial reduction in interferon production in response to RNA virus infection and are not defective in NF-κB activation. Here we find that the related ubiquitin ligase TRAF5 is a downstream target of MAVS that mediates both IRF3 and NF-κB activation. The TM domain of MAVS allows it to dimerize and thereby associate with TRAF5 and induce its ubiquitination in a CARD-dependent manner. Also, NEMO is recruited to the dimerized MAVS CARD domain in a TRAF3 and TRAF5-dependent manner. Thus, our findings reveal a possible function for TRAF5 in mediating the activation of IRF3 and NF-κB downstream of MAVS through the recruitment of NEMO. TRAF5 may be a key molecule in the innate response against viral infection.

Severe acute respiratory syndrome coronavirus M protein inhibits type I interferon production by impeding the formation of TRAF3.TANK.TBK1/IKKepsilon complex

Severe acute respiratory syndrome (SARS) coronavirus is highly pathogenic in humans and evades innate immunity at multiple levels. It has evolved various strategies to counteract the production and action of type I interferons, which mobilize the front-line defense against viral infection. In this study we demonstrate that SARS coronavirus M protein inhibits gene transcription of type I interferons. M protein potently antagonizes the activation of interferon-stimulated response element-dependent transcription by double-stranded RNA, RIG-I, MDA5, TBK1, IKKϵ, and virus-induced signaling adaptor (VISA) but has no influence on the transcriptional activity of this element when IRF3 or IRF7 is overexpressed. M protein physically associates with RIG-I, TBK1, IKKϵ, and TRAF3 and likely sequesters some of them in membrane-associated cytoplasmic compartments. Consequently, the expression of M protein prevents the formation of TRAF3·TANK·TBK1/IKKϵ complex and thereby inhibits TBK1/IKKϵ-dependent activation of IRF3/IRF7 transcription factors. Taken together, our findings reveal a new mechanism by which SARS coronavirus circumvents the production of type I interferons.

Dynamics of the RING domain from human TRAF6 by 15N NMR spectroscopy: implications for biological function

Activation of transcription factor NF-kappaB requires Lys63-linked polyubiquitination of the E3 ubiquitin ligase TRAF6 via protein-protein interactions mediated by a RING domain. In this study, intra- and intermolecular chemical exchange processes of the TRAF6 RING domain were analyzed by (15)N NMR spectroscopy. Micro- to millisecond time scale motions were assessed through R 1, R 2, NOE, and cross-correlated relaxation measurements, and the kinetics of these motions were quantified with relaxation dispersion. The relaxation experiments indicate that the protein core is rigid, consistent with the functional requirement that RING domains form a binding scaffold for E2 ubiquitin conjugation enzymes. Chemical exchange is observed at the C-terminal end of the main alpha-helix of the RING domain. The C-terminal end of the main alpha-helix from the RING domain is involved in E2-E3 interactions, and modulation of slow motions for this region of the helix may be a general mechanism by which these interactions achieve ubiquitin transfer. Chemical shift mapping indicates that the TRAF6 RING domain does not self-associate in solution. Numerous RING domains are homo- or heterodimeric, and this is thought to be a functional necessity for recruitment of substrates for ubiquitination, or recruitment of multiple E2 enzymes for efficient substrate ubiquitination. However, lack of self-association for the RING domain from TRAF6, and the observation that the intact protein is a trimer, suggests that close association of RING domains within a homodimeric scaffold may not be a fundamental requirement for biological function.

Caspase-4 interacts with TNF receptor-associated factor 6 and mediates lipopolysaccharide-induced NF-kappaB-dependent production of IL-8 and CC chemokine ligand 4 (macrophage-inflammatory protein-1 )

Human caspase-4 does not have a corresponding mouse ortholog. Caspase-4 falls within the class of "inflammatory caspases," being homologous with human caspases 1 and 5 and mouse caspases 1, 11, and 12. To address the function of caspase-4, we generated caspase-4-deficient human THP1 monocytic cell lines which exhibited substantially reduced LPS-induced secretion of several chemokines and cytokines, including IL-8 (CXCL8), CCL4 (macrophage-inflammatory protein-1beta), CCL20 (macrophage-inflammatory protein-3alpha), and IL-1beta. The LPS-induced expression of the mRNAs encoding these cytokines was correspondingly reduced in the caspase-4-deficient clones. Because a specific NF-kappaB inhibitor blocked LPS-induced IL-8 and CCL4 mRNA expression as well as IL-8 and CCL4 secretion in THP1 cells, we investigated the role of caspase-4 in NF-kappaB signaling. LPS-induced NF-kappaB nuclear translocation and activation were inhibited in all caspase-4-deficient clones. LPS stimulation led to the interaction of endogenous caspase-4 and TNFR-associated factor 6 (TRAF6) via a TRAF6-binding motif (PPESGE), which we identified in caspase-4. Mutation of this site in caspase-4 resulted in the loss of the TRAF6-caspase-4 interaction. Similar TRAF6-binding motifs are known to be functionally important for TRAF6 interactions with other molecules including caspase-8, and for mediating NF-kappaB activation in various immune and nonimmune cell types. Our data suggest that the TRAF6-caspase-4 interaction, triggered by LPS, leads to NF-kappaB-dependent transcriptional up-regulation and secretion of important cytokines and chemokines in innate immune signaling in human monocytic cells.

Structure, interactions, and dynamics of the RING domain from human TRAF6

A key step in the signaling cascade responsible for activation of the transcription factor NF-kappaB involves Lys63-linked polyubiquitination of TRAF6. Covalent attachment of ubiquitin (Ub) to TRAF6, and subsequent poly(Ub) chain synthesis, is catalyzed by the hUev1a-hUbc13 heterodimer. hUbc13 is a catalytically competent E2 enzyme, and hUev1a is an E2-like protein that binds substrate Ub. The hUev1a-hUbc13 heterodimer is targeted to TRAF6 through interactions between hUbc13 and the N-terminal RING domain from TRAF6. Nuclear magnetic resonance (NMR) spectroscopy was used to determine the solution state structure of the RING domain from human TRAF6, and the interaction between hUbc13 and TRAF6 was characterized using NMR chemical shift mapping. The main-chain dynamics of the RING domain from TRAF6 were studied using (15)N NMR relaxation. Analysis of the main-chain dynamics data indicates that residues within the alpha-helix and beta-sheet of the RING domain are as rigid as regions of canonical secondary structure in larger proteins, consistent with the biological role of RING-domain E3 proteins, which requires that the E3 contain a recognition site for recruitment of E2 ubiquitin conjugation enzymes.

TNF-receptor associated factor 6-deficient fibroblast is sensitive to the TNF-α-induced cell death: Involvement of reactive oxygen species

Tumor necrosis factor receptor (TNFR)-associated factor 6 (TRAF6) has mainly been involved in signaling from CD40 and IL-1 receptor family. While TNF-alpha exerts various biological effects including cell death, the role of TRAF6 in the TNF-alpha signaling remains to be unclear. Here, we demonstrated that murine embryonic fibroblasts (MEFs) derived from TRAF6 knockout (TRAF6KO) mice have increased sensitivity to actinomycin D plus TNF-alpha-induced cell death compared with wild-type MEF. Reactive oxygen species (ROS) were accumulated more in TRAF6KO MEF than in wild-type MEF. An antioxidant, butylated hydroxyanisole (BHA) completely inhibited TNF-alpha-induced cell death and DNA fragmentation. Thus, the TNF-alpha-induced cell death in TRAF6KO MEF was ROS-dependent. Reconstitution of full-length TRAF6 but not N-terminal-deleted TRAF6 constructs in TRAF6KO MEF reversed TNF-alpha-induced cell death, ROS accumulation, and DNA fragmentation completely. Thus, we concluded that resistance against TNF-alpha-induced cell death is rendered by TRAF6, which regulates ROS accumulation.