TRAF2 deficiency results in hyperactivity of certain TNFR1 signals and impairment of CD40-mediated responses

[Immune Regulation by TNF Receptor-associated Factor 5]

The tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family of molecules are intracellular adaptors that regulate cellular signaling through members of the TNFR and Toll-like receptor superfamily. Mammals have seven TRAF molecules numbered sequentially from TRAF1 to TRAF7. Although TRAF5 was identified as a potential regulator of TNFR superfamily members, the in vivo function of TRAF5 has not yet been fully elucidated. We identified an unconventional role of TRAF5 in interleukin-6 (IL-6) receptor signaling involving CD4+ T cells. Moreover, TRAF5 binds to the signal-transducing glycoprotein 130 (gp130) receptor for IL-6 and inhibits the activity of the janus kinase (JAK)-signal transducer and activator of transcription (STAT) signaling pathway. In addition, Traf5-deficient CD4+ T cells exhibit significantly enhanced IL-6-driven differentiation of T helper 17 (Th17) cells, which exacerbates neuroinflammation in experimental autoimmune encephalomyelitis. Furthermore, TRAF5 demonstrates a similar activity to gp130 for IL-27, another cytokine of the IL-6 family. Additionally, Traf5-deficient CD4+ T cells display significantly increased IL-27-mediated differentiation of Th1 cells, which increases footpad swelling in delayed-type hypersensitivity response. Thus, TRAF5 functions as a negative regulator of gp130 in CD4+ T cells. This review aimed to explain how TRAF5 controls the differentiation of CD4+ T cells and discuss how the expression of TRAF5 in T cells and other cell types can influence the development and progression of autoimmune and inflammatory diseases.

The Role of Tumor Necrosis Factor Associated Factors (TRAFs) in Vascular Inflammation and Atherosclerosis

TNF receptor associated factors (TRAFs) represent a family of cytoplasmic signaling adaptor proteins that regulate, bundle, and transduce inflammatory signals downstream of TNF- (TNF-Rs), interleukin (IL)-1-, Toll-like- (TLRs), and IL-17 receptors. TRAFs play a pivotal role in regulating cell survival and immune cell function and are fundamental regulators of acute and chronic inflammation. Lately, the inhibition of inflammation by anti-cytokine therapy has emerged as novel treatment strategy in patients with atherosclerosis. Likewise, growing evidence from preclinical experiments proposes TRAFs as potent modulators of inflammation in atherosclerosis and vascular inflammation. Yet, TRAFs show a highly complex interplay between different TRAF-family members with partially opposing and overlapping functions that are determined by the level of cellular expression, concomitant signaling events, and the context of the disease. Therefore, inhibition of specific TRAFs may be beneficial in one condition and harmful in others. Here, we carefully discuss the cellular expression and signaling events of TRAFs and evaluate their role in vascular inflammation and atherosclerosis. We also highlight metabolic effects of TRAFs and discuss the development of TRAF-based therapeutics in the future.

The immunological significance of tumor necrosis factor receptor-associated factors (TRAFs)

The tumor necrosis factor receptor (TNFR)-associated factor (TRAF) family of molecules are intracellular signaling adaptors and control diverse signaling pathways mediated not only by the TNFR superfamily and the Toll-like receptor/interleukin-1 receptor superfamily but also by unconventional cytokine receptors such as IL-6 and IL-17 receptors. There are seven family members, TRAF1 to TRAF7, in mammals. Exaggerated immune responses induced through TRAF signaling downstream of these receptors often lead to inflammatory and autoimmune diseases including rheumatoid arthritis, inflammatory bowel disease, psoriasis and autoinflammatory syndromes, and thus those signals are major targets for therapeutic intervention. For this reason, it has been very important to understand signaling mechanisms regulated by TRAFs that greatly impact on life/death decisions and the activation, differentiation and survival of cells of the innate and adaptive immune systems. Accumulating evidence suggests that dysregulated cellular expression and/or signaling of TRAFs causes overproduction of proinflammatory cytokines, which facilitates aberrant activation of immune cells. In this review, I will explain the structural and functional aspects that are responsible for the cellular activity and disease outcomes of TRAFs, and summarize the findings of recent studies on TRAFs in terms of how individual TRAF family molecules regulates biological and disease processes in the body in both positive and negative ways. This review also discusses how TRAF mutations contribute to human disease.

Deletion of SREBF1, a Functional Bone-Muscle Pleiotropic Gene, Alters Bone Density and Lipid Signaling in Zebrafish

Through a genome-wide analysis of bone mineral density (BMD) and muscle mass, identification of a signaling pattern on 17p11.2 recognized the presence of sterol regulatory element-binding factor 1 (SREBF1), a gene responsible for the regulation of lipid homeostasis. In conjunction with lipid-based metabolic functions, SREBF1 also codes for the protein, SREBP-1, a transcription factor known for its role in adipocyte differentiation. We conducted a quantitative correlational study. We established a zebrafish (ZF) SREBF1 knockout (KO) model and used a targeted customized lipidomics approach to analyze the extent of SREBF1 capabilities. For lipidomics profiling, we isolated the dorsal muscles of wild type (WT) and KO fishes, and we performed liquid chromatography-tandem mass spectrometry screening assays of these samples. In our analysis, we profiled 48 lipid mediators (LMs) derived from various essential polyunsaturated fatty acids to determine potential targets regulated by SREBF1, and we found that the levels of 11,12 epoxyeicosatrienoic acid (11,12-EET) were negatively associated with the number of SREBF1 alleles (P = 0.006 for a linear model). We also compared gene expression between KO and WT ZF by genome-wide RNA-sequencing. Significantly enriched pathways included fatty acid elongation, linoleic acid metabolism, arachidonic acid metabolism, adipocytokine signaling, and DNA replication. We discovered trends indicating that BMD in adult fish was significantly lower in the KO than in the WT population (P < 0.03). These studies reinforce the importance of lipidomics investigation by detailing how the KO of SREBF1 affects both BMD and lipid-signaling mediators, thus confirming the importance of SREBF1 for musculoskeletal homeostasis.

TRAF family molecules in T cells: Multiple receptors and functions

The TNFR superfamily of receptors, the major focus of the recent TNFR Superfamily Conference held in June 2019, employ the TNFR-associated factor (TRAF) family of adaptor proteins in key aspects of their signaling pathways. Although many early studies investigated TRAF functions via exogenous overexpression in nonhematopoietic cell lines, it has subsequently become clear that whereas TRAFs share some overlap in function, each also plays unique biologic roles, that can be highly context dependent. This brief review summarizes the current state of knowledge of functions of each of the TRAF molecules that mediate important functions in T lymphocytes: TRAFs 1, 2, 3, 5, and 6. Due to our current appreciation of the contextual nature of TRAF function, our focus is upon findings made specifically in T lymphocytes. Key T cell functions for each TRAF are detailed, as well as future knowledge gaps of interest and importance.

Signaling control of antibody isotype switching

Class switch recombination (CSR) generates isotype-switched antibodies with distinct effector functions essential for mediating effective humoral immunity. CSR is catalyzed by activation-induced deaminase (AID) that initiates DNA lesions in the evolutionarily conserved switch (S) regions at the immunoglobulin heavy chain (Igh) locus. AID-initiated DNA lesions are subsequently converted into DNA double stranded breaks (DSBs) in the S regions of Igh locus, repaired by non-homologous end-joining to effect CSR in mammalian B lymphocytes. While molecular mechanisms of CSR are well characterized, it remains less well understood how upstream signaling pathways regulate AID expression and CSR. B lymphocytes express multiple receptors including the B cell antigen receptor (BCR) and co-receptors (e.g., CD40). These receptors may share common signaling pathways or may use distinct signaling elements to regulate CSR. Here, we discuss how signals emanating from different receptors positively or negatively regulate AID expression and CSR.

TRAF2 Deficiency in B Cells Impairs CD40-Induced Isotype Switching That Can Be Rescued by Restoring NF-κB1 Activation

Effective humoral immunity requires class switch recombination (CSR) catalyzed by activation-induced cytidine deaminase (AID). In response to T cell-dependent (TD) Ags, CSR can be induced by CD40 signaling in B cells. TNFR-associated factors 2 and 3 (TRAF2/TRAF3) function as adaptors of the CD40 signaling pathway. B cell-intrinsic TRAF2 or TRAF3 (B-TRAF2 or B-TRAF3) knockout mice were previously reported to have indistinguishable phenotypes in gene expression, B cell survival and development, and enlarged peripheral lymphoid organs. However, it remains unknown whether deficiency of B-TRAF2 or B-TRAF3 differentially affects TD humoral immune responses and CD40-induced CSR. In this article, we show that B-TRAF2 is essential for optimal isotype switching induced by in vivo TD Ag immunization or by engaging CD40 in vitro. Our data clarify the controversial role of B-TRAF3 and confirm its dispensability in CD40-induced CSR. Mechanistically, CD40-induced AID expression was markedly impaired by B-TRAF2, but not B-TRAF3, deficiency. Moreover, B-TRAF2 deficiency causes defective activation of the NF-κB1 complex in a CD40-autonomous manner, and restoring CD40-induced NF-κB1 activation in TRAF2-deficient B cells rescues AID expression and CSR. We conclude that TRAF2 is essential but TRAF3 is dispensable for TD humoral immunity and CD40-induced CSR. Our studies provide significant biological bases for optimizing treatment of B cell-associated immune disorders by targeting CD40 signaling.

TRAF2 Cooperates with Focal Adhesion Signaling to Regulate Cancer Cell Susceptibility to Anoikis

TRAF2, a RING finger adaptor protein, plays an important function in tumor necrosis factor (TNF)- and TNF-like weak inducer of apoptosis (TWEAK)-dependent signaling, in particular during inflammatory and immune responses. We identified a functional interaction of TRAF2 with focal adhesion (FA) signaling involving the focal adhesion kinase (FAK) in the regulation of cell susceptibility to anoikis. Comparison of TRAF2-proficient (TRAF2^+/+) versus TRAF2-deficient (TRAF2^-/-), and FAK-proficient (FAK^+/+) versus FAK-deficient (FAK^-/-) mouse embryonic fibroblasts and their matched reconstituted cells demonstrated that TRAF2 interacts physically with the N-terminal portion of FAK and colocalizes to cell membrane protrusions. This interaction was found to be critical for promoting resistance to cell anoikis. Similar results were confirmed in the human breast cancer cell line MDA-MB-231, where TRAF2 and FAK downregulation promoted cell susceptibility to anoikis. In human breast cancer tissues, genomic analysis of The Cancer Genome Atlas database revealed coamplification of TRAF2 and FAK in breast cancer tissues with a predictive value for shorter survival, further supporting a potential role of TRAF2-FAK cooperative signaling in cancer progression.

Role of TRAFs in Signaling Pathways Controlling T Follicular Helper Cell Differentiation and T Cell-Dependent Antibody Responses

Follicular helper T (T_FH) cells represent a highly specialized CD4⁺ T cell subpopulation that supports the generation of germinal centers (GC) and provides B cells with critical signals promoting antibody class switching, generation of high affinity antibodies, and memory formation. T_FH cells are characterized by the expression of the chemokine receptor CXCR5, the transcription factor Bcl-6, costimulatory molecules ICOS, and PD-1, and the production of cytokine IL-21. The acquisition of a T_FH phenotype is a complex and multistep process that involves signals received through engagement of the TCR along with a multitude of costimulatory molecules and cytokines receptors. Members of the Tumor necrosis factor Receptor Associated Factors (TRAF) represent one of the major classes of signaling mediators involved in the differentiation and functions of T_FH cells. TRAF molecules are the canonical adaptor molecules that physically interact with members of the Tumor Necrosis Factor Receptor Superfamily (TNFRSF) and actively modulate their downstream signaling cascades through their adaptor function and/or E3 ubiquitin ligase activity. OX-40, GITR, and 4-1BB are the TRAF-dependent TNFRSF members that have been implicated in the differentiation and functions of T_FH cells. On the other hand, emerging data demonstrate that TRAF proteins also participate in signaling from the TCR and CD28, which deliver critical signals leading to the differentiation of T_FH cells. More intriguingly, we recently showed that the cytoplasmic tail of ICOS contains a conserved TANK-binding kinase 1 (TBK1)-binding motif that is shared with TBK1-binding TRAF proteins. The presence of this TRAF-mimicking signaling module downstream of ICOS is required to mediate the maturation step during T_FH differentiation. In addition, JAK-STAT pathways emanating from IL-2, IL-6, IL-21, and IL-27 cytokine receptors affect T_FH development, and crosstalk between TRAF-mediated pathways and the JAK-STAT pathways can contribute to generate integrated signals required to drive and sustain T_FH differentiation. In this review, we will introduce the molecular interactions and the major signaling pathways controlling the differentiation of T_FH cells. In each case, we will highlight the contributions of TRAF proteins to these signaling pathways. Finally, we will discuss the role of individual TRAF proteins in the regulation of T cell-dependent humoral responses.

Genome-wide CRISPR-Cas9 viability screen reveals genes involved in TNF-α-induced apoptosis of human umbilical vein endothelial cells

Tumor necrosis factor α (TNF-α), a pivotal cytokine in sepsis, protects the host against pathogens by promoting an inflammatory response while simultaneously inducing apoptosis of the vascular endothelium. Unfortunately, inhibitors targeting certain components of the TNF-α signaling pathway to reduce cellular apoptosis have failed to translate into clinical applications, partly due to the adverse effects of excessive immunosuppression. In an attempt to discover potential targets in the TNF-α signaling pathway to modulate moderate inflammation and apoptosis during the development of sepsis, we performed a pooled genome-wide CRISPR/Cas9 knockout screen in human umbilical vein endothelial cells (HUVECs). Tumor necrosis factor receptor superfamily member 1A (TNFRSF1A), B-cell lymphoma 2 (BCL2), Bcl2-associated death promoter (BAD), and NLR family member X1 (NLRX1) deficiencies were identified as the effective genetic suppressors of TNF-α cytotoxicity on a list of candidate regulators. CRISPR-mediated NLRX1 knockout conferred cellular resistance to challenge with TNF-α, and NLRX1 could be induced to colocalize with mitochondria following TNF-α stimulation. Thus, our work demonstrates the advantage of genome-scale screening with Cas9 and validates NLRX1 as a potential modulator of TNF-α-induced vascular endothelial apoptosis during sepsis.

TRAF2 of black carp upregulates MAVS-mediated antiviral signaling during innate immune response

Tumor necrosis factor receptor-associated factor 2 (TRAF2) is a crucial component of the tumor necrosis factor (TNF) mediated signaling of higher vertebrates. To elucidate its function in teleost fish, TRAF2 homologue of black carp (Mylopharyngodon piceus) has been cloned and characterized in this study. The open reading frame (ORF) of black carp TRAF2 (bcTRAF2) consists of 1611 nucleotides and bcTRAF2 contains 536 amino acids. bcTRAF2 protein migrated around 65 KDa in immunoblot analysis of both EPC and HEK293T cells. bcTRAF2 was identified as a cytosolic protein and suggested to be associated with vesicles scattering in the cytoplasm. NF-κB transcription instead of IFN transcription was activated by bcTRAF2 in reporter assay. It was interesting that bcMAVS-mediated IFN production was up-regulated by bcTRAF2 in a dose dependent manner in reporter assay. Accordingly, EPC cells transfected with both bcMAVS and bcTRAF2 showed enhanced antiviral activity comparing EPC cells only expressing bcMAVS. When co-expressed with bcMAVS, bcTRAF2 was redistributed in the cytoplasm and its subcellular location overlapped with the subcellular location of bcMAVS, which suggested the association between these two molecules. Taken together, the data generated in this paper supported the conclusion that bcTRAF2 was recruited into host innate immune response and positively regulated bcMAVS-mediated antiviral signaling.

TRAF2 exerts opposing effects on basal and TNFα-induced activation of the classic IKK complex in hematopoietic cells in mice

The role of TRAF2 and TRAF5 in TNFα-induced NF-κB activation has become complicated owing to the accumulation of conflicting data. Here, we report that 7-day-old TRAF2-knockout (KO) and TRAF2 TRAF5 double KO (TRAF2/5-DKO) mice exhibit enhanced canonical IκB kinase (IKK) and caspase-8 activation in spleen and liver, and that subsequent knockout of TNFα suppresses the basal activity of caspase-8, but not of IKK. In primary TRAF2 KO and TRAF2/5-DKO cells, TNFα-induced immediate IKK activation is impaired, whereas delayed IKK activation occurs normally; as such, owing to elevated basal and TNFα-induced delayed IKK activation, TNFα stimulation leads to significantly increased induction of a subset of NF-κB-dependent genes in these cells. In line with this, both TRAF2 KO and TRAF2/5-DKO mice succumb to a sublethal dose of TNFα owing to increased expression of NF-κB target genes, diarrhea and bradypnea. Notably, depletion of IAP1 and IAP2 (also known as BIRC2 and BIRC3, respectively) also results in elevated basal IKK activation that is independent of autocrine TNFα production and that impairs TNFα-induced immediate IKK activation. These data reveal that TRAF2, IAP1 and IAP2, but not TRAF5, cooperatively regulate basal and TNFα-induced immediate IKK activation.

Conditional-ready mouse embryonic stem cell derived macrophages enable the study of essential genes in macrophage function

The ability to differentiate genetically modified mouse embryonic stem (ES) cells into functional macrophages provides a potentially attractive resource to study host-pathogen interactions without the need for animal experimentation. This is particularly useful in instances where the gene of interest is essential and a knockout mouse is not available. Here we differentiated mouse ES cells into macrophages in vitro and showed, through a combination of flow cytometry, microscopic imaging, and RNA-Seq, that ES cell-derived macrophages responded to S. Typhimurium, in a comparable manner to mouse bone marrow derived macrophages. We constructed a homozygous mutant mouse ES cell line in the Traf2 gene that is known to play a role in tumour necrosis factor-α signalling but has not been studied for its role in infections or response to Toll-like receptor agonists. Interestingly, traf2-deficient macrophages produced reduced levels of inflammatory cytokines in response to lipopolysaccharide (LPS) or flagellin stimulation and exhibited increased susceptibility to S. Typhimurium infection.

Tumor necrosis factor receptor-associated factor 2 mediates mitochondrial autophagy

BACKGROUND

Tumor necrosis factor (TNF) signaling protects against ischemia/reperfusion-induced cardiomyocyte death, in vitro, ex vivo, and in vivo. TNF-receptor-associated factor 2 (TRAF2), an E3 ubiquitin ligase, coordinates cytoprotective signaling downstream of both TNF receptors, via unclear mechanisms. Noting that TRAF2 is recruited to mitochondria, and that autophagic removal of ubiquitin-tagged damaged mitochondria is cytoprotective, we tested the hypothesis that TRAF2 mediates mitochondrial autophagy.

METHODS AND RESULTS

TRAF2 localizes to the mitochondria in neonatal rat cardiac myocytes, and TNF treatment transcriptionally upregulates TRAF2 abundance in the mitochondrial subfraction. TRAF2 colocalizes with ubiquitin, p62 adaptor protein, and mitochondria within LC3-bound autophagosomes; and exogenous TRAF2 enhances autophagic removal of mitochondria. TRAF2 knockdown with adenoviral shRNA transduction induces accumulation of depolarized mitochondria in resting neonatal rat cardiac myocytes, as well as in those treated with TNF or uncoupling agent carbonyl cyanide m-chlorophenyl hydrazone, suggesting an essential role for TRAF2 in homeostatic and stress-induced mitochondrial autophagy. TRAF2 also colocalizes and interacts with PARKIN, a previously described E3 ubiquitin ligase and mitophagy effector, on depolarized mitochondria in neonatal rat cardiac myocytes. Exogenous expression of TRAF2, but not its E3 ligase-deficient mutants, is sufficient to partially restore mitophagy in the setting of PARKIN knockdown, suggesting redundancy in their ubiquitin ligase roles. TRAF2 abundance increases in the mitochondrial subfraction of ischemia/reperfusion-modeled hearts; and exogenous TRAF2, but not its E3 ligase-deficient mutants, reduces depolarized mitochondria and rescues cell death in neonatal rat cardiac myocytes subjected to hypoxia/reoxygenation.

CONCLUSIONS

Taken together, these data indicate an essential role for TRAF2 in concert with PARKIN as a mitophagy effector, which contributes to TRAF2-induced cytoprotective signaling.

Regulation of NF-κB by TNF family cytokines

The NF-κB family of inducible transcription factors is activated in response to a variety of stimuli. Amongst the best-characterized inducers of NF-κB are members of the TNF family of cytokines. Research on NF-κB and TNF have been tightly intertwined for more than 25 years. Perhaps the most compelling examples of the interconnectedness of NF-κB and the TNF have come from analysis of knock-out mice that are unable to activate NF-κB. Such mice die embryonically, however, deletion of TNF or TNFR1 can rescue the lethality thereby illustrating the important role of NF-κB as the key regulator of transcriptional responses to TNF. The physiological connections between NF-κB and TNF cytokines are numerous and best explored in articles focusing on a single TNF family member. Instead, in this review, we explore general mechanisms of TNF cytokine signaling, with a focus on the upstream signaling events leading to activation of the so-called canonical and noncanonical NF-κB pathways by TNFR1 and CD40, respectively.

TNFR1 signaling kinetics: spatiotemporal control of three phases of IKK activation by posttranslational modification

TNFα is a pleotropic cytokine that plays a central role in the inflammatory response by activating the NF-κB signaling pathway, and is targeted in a range of chronic inflammatory diseases, underscoring the therapeutic importance of understanding its underlying molecular mechanisms. Although K63-linked ubiquitination of RIP1 by TRAF2/5 and cIAP1/2 was thought to serve as a scaffold to activate the NF-κB pathway, the recent accumulation of conflicting results has challenged the necessity of these proteins in NF-κB activation. In addition, several serine/threonine kinases have been implicated in TNFα-induced IKK activation; however, the targeted disruption of these kinases had no effect on transient IKK activation. The recent discovery of RIP1-dependent and -independent activation of the early and delayed phases of IKK and TRAF2 phosphorylation-dependent activation of the prolonged phase of IKK offers a reconciliatory model for the interpretation of contradictory results in the field. Notably, the TNFα-induced inflammatory response is not exclusively controlled by the NF-κB pathway but is subject to regulatory crosstalk between NF-κB and other context-dependent pathways. Thus further elucidation of these spatiotemporally-coordinated signaling mechanisms has the potential to provide novel molecular targets and therapeutic strategies for NF-κB intervention.

cIAP1/2 negatively regulate RANKL-induced osteoclastogenesis through the inhibition of NFATc1 expression

Receptor activator of nuclear factor κB (RANK) is a member of the tumor necrosis factor receptor superfamily (TNFRSF) and triggers osteoclastogenesis by inducing the expression of NFATc1 through the activation of the NF-κB and MAPK pathways. Cellular inhibitors of apoptosis proteins 1 and 2 (cIAP1/2), which are ubiquitin E3 ligases, are involved in the activation of the NF-κB and MAPK pathways by various members of the TNFRSF. However, the involvement of cIAP1/2 in RANK signaling has remained largely unknown. In this study, we reveal the involvement of cIAP1/2 in RANK ligand (RANKL)-induced osteoclastogenesis. The over-expression of cIAP1 or cIAP2 in the mouse monocytic cell line Raw264.7 resulted in the significant suppression of RANKL-induced NFATc1 mRNA expression and osteoclastogenesis, whereas the activation of the NF-κB and MAPK pathways was barely changed by these over-expressions. The depletion of endogenous cIAP1/2 by their specific inhibitor MV1 or their siRNA-mediated knockdown resulted in enhanced RANKL-induced NFATc1 expression and osteoclastogenesis without affecting the activation of the NF-κB and MAPK pathways. In combination, these results indicate that cIAP1/2 negatively regulate osteoclastogenesis by inhibiting NFATc1 mRNA expression in a manner that is distinct from the previously identified functions of cIAP1/2.

ER stress activates NF-κB by integrating functions of basal IKK activity, IRE1 and PERK

NF-κB, a transcription factor, becomes activated during the Unfolded Protein Response (UPR), an endoplasmic reticulum (ER) stress response pathway. NF-κB is normally held inactive by its inhibitor, IκBα. Multiple cellular pathways activate IKK (IκBα Kinase) which phosphorylate IκBα leading to its degradation and NF-κB activation. Here, we find that IKK is required for maximum activation of NF-κB in response to ER stress. However, unlike canonical NFκB activation, IKK activity does not increase during ER stress, but rather the level of basal IKK activity is critical for determining the extent of NF-κB activation. Furthermore, a key UPR initiator, IRE1, acts to maintain IKK basal activity through IRE1's kinase, but not RNase, activity. Inputs from IRE1 and IKK, in combination with translation repression by PERK, another UPR initiator, lead to maximal NF-κB activation during the UPR. These interdependencies have a significant impact in cancer cells with elevated IKK/NF-κB activity such as renal cell carcinoma cells (786-0). Inhibition of IKK by an IKK inhibitor, which significantly decreases NF-κB activity, is overridden by UPR induction, arguing for the importance of considering UPR activation in cancer treatment.

A novel dual signaling axis for NSP 5a3a induced apoptosis in head and neck carcinoma

NSP 5a3a is a novel structural protein found to be over-expressed in certain cancer cell lines in-vitro such as Hela, Saos-2, and MCF-7 while barely detectable levels in normal body tissues except for Testis. This particular isoform has been known to interact with cyto- nuclear proteins B23, known to be involved in multi-faceted cellular processes such as cell division, apoptosis, ribosome biogenesis, and rRNA processing, as well as with hnRNP-L, known to be involved with RNA metabolism and rRNA processing. A previous preliminary investigation of NSP 5a3a as a potential target in Head and Neck Carcinoma revealed a novel p73 dependent mechanism through which NSP 5a3a induced apoptosis in Head and Neck cell lines when over-expressed in-vitro. Our present investigation further elucidated a novel dual axis signaling point by which NSP 5a3a induces apoptosis in Head and Neck cell line HN30 through p73-DAXX and TRAF2-TRADD. Interestingly, this novel mechanism appears independent of canonical caspases involved in the intrinsic mitochondrial pathway as well as those in the death receptor pathway thru TRAF2 and TRADD.

A novel role for RIP1 kinase in mediating TNFα production

Receptor-interacting protein 1 (RIP1) is a Ser/Thr kinase with both kinase-dependent and kinase-independent roles in death receptor signaling. The kinase activity of RIP1 is required for necroptosis, a caspase-independent pathway of programmed cell death. In some cell types, the inhibition of caspases leads to autocrine production of TNFα, which then activates necroptosis. Here, we describe a novel role for RIP1 kinase in regulating TNFα production after caspase inhibition. Caspase inhibitors activate RIP1 kinase and another protein, EDD, to mediate JNK signaling, which stimulates Sp1-dependent transcription of TNFα. This pathway is independent of nuclear factor κB and also occurs after Smac mimetic/IAP antagonist treatment or the loss of TNF receptor-associated factor 2 (Traf2). These findings implicate cIAP1/2 and Traf2 as negative regulators of this RIP1 kinase-dependent TNFα production pathway and suggest a novel role for RIP1 kinase in mediating TNFα production under certain conditions.

Aberrant accumulation of interleukin-10-secreting neutrophils in TRAF2-deficient mice

Highly coordinated expression of inflammatory and anti-inflammatory cytokines is crucial for maintaining homeostasis of the gut that is constantly exposed to large amounts of commensal bacteria. We have previously reported that tumor necrosis factor (TNF) receptor-associated factor (Traf)2(-/-) mice spontaneously develop severe colitis and that the development of colitis largely depends on TNFα-dependent apoptosis of colonic epithelial cells. However, the detailed molecular mechanisms underlying the immunological disorders of Traf2(-/-) mice are not fully understood. Here we show that interleukin (IL)-10-secreting neutrophils accumulated in peripheral blood and bone marrow (BM) cells from Traf2(-/-) mice compared with those from wild-type mice. Treatment of Traf2(-/-) mice with neutralizing antibody against TNFα or crossing Traf2(-/-) mice with Tnfr1(-/-) mice reduced the percentages of IL-10-secreting neutrophils, suggesting that the development of IL-10-secreting neutrophils largely depended on TNFα signals. Moreover, stimulation of BM cells from wild-type mice with lipopolysaccharide and Pam3CS(K)4, a ligand for Toll-like receptor 4 and 2, respectively, induced differentiation of BM cells into IL-10-secreting neutrophils. These results suggest that the development of IL-10-secreting neutrophils is not restricted to Traf2(-/-) mice, but could be generalized to wild-type mice under certain conditions such as inflammation. Finally, combined treatment of Traf2(-/-) mice with neutralizing antibodies against TNFα and IL-10, but not each antibody alone, substantially ameliorated colitis and prolonged survival. Together, abrogation of immunosuppressive conditions mediated by IL-10-secreting neutrophils might be an alternative strategy to treat chronic inflammatory diseases at least under certain conditions.

Epstein-Barr virus LMP2A reduces hyperactivation induced by LMP1 to restore normal B cell phenotype in transgenic mice

Epstein-Barr virus (EBV) latently infects most of the human population and is strongly associated with lymphoproliferative disorders. EBV encodes several latency proteins affecting B cell proliferation and survival, including latent membrane protein 2A (LMP2A) and the EBV oncoprotein LMP1. LMP1 and LMP2A signaling mimics CD40 and BCR signaling, respectively, and has been proposed to alter B cell functions including the ability of latently-infected B cells to access and transit the germinal center. In addition, several studies suggested a role for LMP2A modulation of LMP1 signaling in cell lines by alteration of TRAFs, signaling molecules used by LMP1. In this study, we investigated whether LMP1 and LMP2A co-expression in a transgenic mouse model alters B cell maturation and the response to antigen, and whether LMP2A modulates LMP1 function. Naïve LMP1/2A mice had similar lymphocyte populations and antibody production by flow cytometry and ELISA compared to controls. In the response to antigen, LMP2A expression in LMP1/2A animals rescued the impairment in germinal center generation promoted by LMP1. LMP1/2A animals produced high-affinity, class-switched antibody and plasma cells at levels similar to controls. In vitro, LMP1 upregulated activation markers and promoted B cell hyperproliferation, and co-expression of LMP2A restored a wild-type phenotype. By RT-PCR and immunoblot, LMP1 B cells demonstrated TRAF2 levels four-fold higher than non-transgenic controls, and co-expression of LMP2A restored TRAF2 levels to wild-type levels. No difference in TRAF3 levels was detected. While modulation of other TRAF family members remains to be assessed, normalization of the LMP1-induced B cell phenotype through LMP2A modulation of TRAF2 may be a pathway by which LMP2A controls B cell function. These findings identify an advance in the understanding of how Epstein-Barr virus can access the germinal center in vivo, a site critical for both the genesis of immunological memory and of virus-associated tumors.

As a ubiquitous human pathogen, Epstein-Barr virus (EBV) infection is associated with several human B cell diseases characterized by inappropriate B cell activation and function, including infectious mononucleosis and certain cancers. EBV latent membrane protein 1 (LMP1) and 2A (LMP2A) hijack cell signaling pathways to alter B cell activation and function, and are detected in EBV-associated diseases. Defining the effect on B cell function when LMP1 and LMP2A are expressed together in the same cell is critical to understanding how EBV subverts normal B cell behavior before disease develops. Using transgenic mice, we have demonstrated that LMP2A dampens cellular proliferation and activation induced by LMP1, which may be due to the LMP2A-associated decrease in the levels of TRAF2, a signaling protein used by LMP1. LMP2A also allows B cells carrying LMP1 to enter the germinal center during an immune response, a site that gives rise to EBV-associated tumors in humans. In sum, this study highlights the biological outcomes of LMP1 and LMP2A co-expression in B cells and contributes to the knowledge of how EBV subverts normal B cell behavior before disease develops.

Hepatic TRAF2 regulates glucose metabolism through enhancing glucagon responses

Obesity is associated with intrahepatic inflammation that promotes insulin resistance and type 2 diabetes. Tumor necrosis factor receptor-associated factor (TRAF)2 is a key adaptor molecule that is known to mediate proinflammatory cytokine signaling in immune cells; however, its metabolic function remains unclear. We examined the role of hepatic TRAF2 in the regulation of insulin sensitivity and glucose metabolism. TRAF2 was deleted specifically in hepatocytes using the Cre/loxP system. The mutant mice were fed a high-fat diet (HFD) to induce insulin resistance and hyperglycemia. Hepatic glucose production (HGP) was examined using pyruvate tolerance tests, (2)H nuclear magnetic resonance spectroscopy, and in vitro HGP assays. The expression of gluconeogenic genes was measured by quantitative real-time PCR. Insulin sensitivity was analyzed using insulin tolerance tests and insulin-stimulated phosphorylation of insulin receptors and Akt. Glucagon action was examined using glucagon tolerance tests and glucagon-stimulated HGP, cAMP-responsive element-binding (CREB) phosphorylation, and expression of gluconeogenic genes in the liver and primary hepatocytes. Hepatocyte-specific TRAF2 knockout (HKO) mice exhibited normal body weight, blood glucose levels, and insulin sensitivity. Under HFD conditions, blood glucose levels were significantly lower (by >30%) in HKO than in control mice. Both insulin signaling and the hypoglycemic response to insulin were similar between HKO and control mice. In contrast, glucagon signaling and the hyperglycemic response to glucagon were severely impaired in HKO mice. In addition, TRAF2 overexpression significantly increased the ability of glucagon or a cAMP analog to stimulate CREB phosphorylation, gluconeogenic gene expression, and HGP in primary hepatocytes. These results suggest that the hepatic TRAF2 cell autonomously promotes hepatic gluconeogenesis by enhancing the hyperglycemic response to glucagon and other factors that increase cAMP levels, thus contributing to hyperglycemia in obesity.

IAPs limit activation of RIP kinases by TNF receptor 1 during development

Inhibitor of apoptosis (IAP) proteins cIAP1, cIAP2, and XIAP (X-linked IAP) regulate apoptosis and cytokine receptor signalling, but their overlapping functions make it difficult to distinguish their individual roles. To do so, we deleted the genes for IAPs separately and in combination. While lack of any one of the IAPs produced no overt phenotype in mice, deletion of cIap1 with cIap2 or Xiap resulted in mid-embryonic lethality. In contrast, Xiap(-/-)cIap2(-/-) mice were viable. The death of cIap2(-/-)cIap1(-/-) double mutants was rescued to birth by deletion of tumour necrosis factor (TNF) receptor 1, but not TNFR2 genes. Remarkably, hemizygosity for receptor-interacting protein kinase 1 (Ripk1) allowed Xiap(-/-)cIap1(-/-) double mutants to survive past birth, and prolonged cIap2(-/-)cIap1(-/-) embryonic survival. Similarly, deletion of Ripk3 was able to rescue the mid-gestation defect of cIap2(-/-)cIap1(-/-) embryos, as these embryos survived to E15.5. cIAPs are therefore required during development to limit activity of RIP kinases in the TNF receptor 1 signalling pathway.

Historical perspectives on tumor necrosis factor and its superfamily: 25 years later, a golden journey

Although activity that induced tumor regression was observed and termed tumor necrosis factor (TNF) as early as the 1960s, the true identity of TNF was not clear until 1984, when Aggarwal and coworkers reported, for the first time, the isolation of 2 cytotoxic factors: one, derived from macrophages (molecular mass 17 kDa), was named TNF, and the second, derived from lymphocytes (20 kDa), was named lymphotoxin. Because the 2 cytotoxic factors exhibited 50% amino acid sequence homology and bound to the same receptor, they came to be called TNF-α and TNF-β. Identification of the protein sequences led to cloning of their cDNA. Based on sequence homology to TNF-α, now a total of 19 members of the TNF superfamily have been identified, along with 29 interacting receptors, and several molecules that interact with the cytoplasmic domain of these receptors. The roles of the TNF superfamily in inflammation, apoptosis, proliferation, invasion, angiogenesis, metastasis, and morphogenesis have been documented. Their roles in immunologic, cardiovascular, neurologic, pulmonary, and metabolic diseases are becoming apparent. TNF superfamily members are active targets for drug development, as indicated by the recent approval and expanding market of TNF blockers used to treat rheumatoid arthritis, psoriasis, Crohns disease, and osteoporosis, with a total market of more than US $20 billion. As we learn more about this family, more therapeutics will probably emerge. In this review, we summarize the initial discovery of TNF-α, and the insights gained regarding the roles of this molecule and its related family members in normal physiology and disease.

Crucial role for TNF receptor-associated factor 2 (TRAF2) in regulating NFκB2 signaling that contributes to autoimmunity

TNF receptor-associated factor 2 (TRAF2) is a key intracellular signaling mediator that acts downstream of not only TNFα but also various members of the TNFα superfamily. Here, we report that, despite their lack of TNFα signaling, TRAF2(-/-)TNFα(-/-) mice develop an inflammatory disorder characterized by autoantibody accumulation and organ infiltration by T cells with the phenotypes of activated, effector, and memory cells. RAG1(-/-) mice reconstituted with TRAF2(-/-)TNFα(-/-) bone marrow cells showed increased numbers of hyperactive T cells and rapidly developed progressive and eventually lethal inflammation. No inflammation was observed in RAG1(-/-) mice reconstituted with TRAF2(-/-)TNFα(-/-)T-cell receptor β(-/-) or TRAF2(-/-)TNFα(-/-)NFκB-induced kinase(+/-) bone marrow cells. The pathogenic TRAF2(-/-)TNFα(-/-) T cells showed constitutive NFκB2p52 activation and produced elevated levels of T-helper 1 and T-helper 17 cytokines. Our results suggest that a regulatory circuit consisting of TRAF2-NFκB-induced kinase-NFκB2p52 is essential for the proper control of effector T-cell polarization and that loss of T-cell TRAF2 function induces constitutive NFκB2p52 activity that drives fatal autoimmune inflammation independently of TNFα signaling. The involvement of this regulatory circuit in controlling autoimmune responses highlights the delicate balance required to avoid paradoxical adverse events when implementing new targeted anti-inflammatory therapies.

The regulation of TNF signalling: what a tangled web we weave

In the past 2 years there has been an explosion of information regarding molecules that regulate TNF-R1 signalling, and even reviews published in 2010 are out of date. TNF-R1 activation of NF-κB is a text book example of a signal transduction pathway regulated by ubiquitin and many of the concepts concerning the different roles of ubiquitin chains were first outlined in TNF-R1 signalling. What was once a very simple pathway with clearly defined roles for ubiquitin in regulating TNF-R1 signalling has, however, now become so complicated that we have 'an embarrassment of riches'. The less polite might claim our pathways of TNF-R1 signalling look as complicated as a web constructed by a drug-addled spider. This review will pick apart only one small strand of the web, and will address the role of ubiquitin in the activation of NF-κB by TNF with a focus on interpreting in vivo results. Nevertheless some of the concepts, for example the role of linear ubiquitin chains in regulating signalling, may be applicable to the family in general.

HIV-1 Nef induces proinflammatory state in macrophages through its acidic cluster domain: involvement of TNF alpha receptor associated factor 2

Background

HIV-1 Nef is a virulence factor that plays multiple roles during HIV replication. Recently, it has been described that Nef intersects the CD40 signalling in macrophages, leading to modification in the pattern of secreted factors that appear able to recruit, activate and render T lymphocytes susceptible to HIV infection. The engagement of CD40 by CD40L induces the activation of different signalling cascades that require the recruitment of specific tumor necrosis factor receptor-associated factors (i.e. TRAFs). We hypothesized that TRAFs might be involved in the rapid activation of NF-κB, MAPKs and IRF-3 that were previously described in Nef-treated macrophages to induce the synthesis and secretion of proinflammatory cytokines, chemokines and IFNβ to activate STAT1, -2 and -3.

Methodology/Principal Findings

Searching for possible TRAF binding sites on Nef, we found a TRAF2 consensus binding site in the AQEEEE sequence encompassing the conserved four-glutamate acidic cluster. Here we show that all the signalling effects we observed in Nef treated macrophages depend on the integrity of the acidic cluster. In addition, Nef was able to interact in vitro with TRAF2, but not TRAF6, and this interaction involved the acidic cluster. Finally silencing experiments in THP-1 monocytic cells indicate that both TRAF2 and, surprisingly, TRAF6 are required for the Nef-induced tyrosine phosphorylation of STAT1 and STAT2.

Conclusions

Results reported here revealed TRAF2 as a new possible cellular interactor of Nef and highlighted that in monocytes/macrophages this viral protein is able to manipulate both the TRAF/NF-κB and TRAF/IRF-3 signalling axes, thereby inducing the synthesis of proinflammatory cytokines and chemokines as well as IFNβ.

Tumor necrosis factor receptor-associated factor (TRAF) 2 controls homeostasis of the colon to prevent spontaneous development of murine inflammatory bowel disease

Fine-tuning of host cell responses to commensal bacteria plays a crucial role in maintaining homeostasis of the gut. Here, we show that tumor necrosis factor receptor-associated factor (Traf)2(-/-) mice spontaneously developed severe colitis and succumbed within 3 weeks after birth. Histological analysis revealed that apoptosis of colonic epithelial cells was enhanced, and B cells diffusely infiltrated into the submucosal layer of the colon of Traf2(-/-) mice. Expression of proinflammatory cytokines, including Tnfa, Il17a, and Ifng, was up-regulated, whereas expression of antimicrobial peptides was down-regulated in the colon of Traf2(-/-) mice. Moreover, a number of IL-17-producing helper T cells were increased in the colonic lamina propria of Traf2(-/-) mice. These cellular alterations resulted in drastic changes in the colonic microbiota of Traf2(-/-) mice compared with Traf2(+/+) mice. Treatment of Traf2(-/-) mice with antibiotics ameliorated colitis along with down-regulation of proinflammatory cytokines and prolonged survival, suggesting that the altered colonic microbiota might contribute to exacerbation of colitis. Finally, deletion of Tnfr1, but not Il17a, dramatically ameliorated colitis in Traf2(-/-) mice by preventing apoptosis of colonic epithelial cells, down-regulation of proinflammatory cytokines, and restoration of wild-type commensal bacteria. Together, TRAF2 plays a crucial role in controlling homeostasis of the colon.

The kinase NIK as a therapeutic target in multiple myeloma

INTRODUCTION

Multiple myeloma (MM) is a neoplasm derived from B lymphocytes and often results in uncontrolled clonal expansion of antibody-secreting cells. While current treatments are able to prolong survival, MM remains incurable. Excessive NF-κB activity in MM contributes to tumor progression and survival.

AREAS COVERED

The contribution of NF-κB-inducing kinase (NIK) to alternative NF-κB signaling, where it is the key kinase, and classical NF-κB signaling. Modulation of NIK by natural and chemical factors and current and potential therapies for MM that target NIK.

EXPERT OPINION

Mutations affecting the activation of NIK have been identified in MM samples and cell lines, suggesting that NIK may be an important target for therapy of MM. NIK contributes to activation of both NF-κB pathways in MM, giving us the opportunity to limit two pathways contributing to oncogenic survival with a single therapeutic. Many of the mutations identified in MM cells result in the same outcome, hyperactive NIK, thus a single therapeutic may be effective in many patients even though they carry differing mutations. As NIK appears only to activate classical NF-κB when overexpressed, and in normal cells NIK levels are usually low, it is possible that therapeutics designed to limit the amount of NIK may not produce serious side effects in healthy cells.

Targeting TNF for Treatment of Cancer and Autoimmunity

Tumor necrosis factor-alpha (TNF-alpha) was first isolated two decades ago as a macrophageproduced protein that can effectively kill tumor cells. TNF-alpha is also an essential component of the immune system and is required for hematopoiesis, for protection from bacterial infection and for immune cell-mediated cytotoxicity. Extensive research, however, has revealed that TNF-alpha is one of the major players in tumor initiation, proliferation, invasion, angiogenesis and metastasis. The proinflammatory activities link TNF-alpha with a wide variety of autoimmune diseases, including psoriasis, inflammatory bowel disease, rheumatoid arthritis, systemic sclerosis, systemic lupus erythematosus, multiple sclerosis, diabetes and ankylosing spondylitis. Systemic inhibitors of TNF such as etanercept (Enbrel) (a soluble TNF receptor) and infliximab (Remicade) and adalimumab (Humira) (anti-TNF antibodies) have been approved for the treatment inflammatory bowel disease, psoriasis and rheumatoid arthritis. These drugs, however, exhibit severe side effects and are expensive. Hence orally active blockers of TNF-alpha that are safe, efficacious and inexpensive are urgently needed. Numerous products from fruits, vegetable and traditional medicinal plants have been described which can suppress TNF expression and TNF signaling but their clinical potential is yet uncertain.

TACI induces cIAP1-mediated ubiquitination of NIK by TRAF2 and TANK to limit non-canonical NF-kappaB signaling

B-cell-activating factor of the TNF family (BAFF) is a critical factor for B-cell survival and maturation through non-canonical nuclear factor kappaB (NF-kappaB) pathway, a NF-kappaB inducing kinase (NIK)-dependent pathway for the processing of NF-kappaB2 p100 to generate p52. While BAFF acts primarily through BAFF receptor (BAFF-R), the transmembrane activator and CAML interactor (TACI), the other receptor for BAFF, is thought to serve as a negative regulator for B-cell responses. However, how TACI regulates NF-kappaB2 activity is largely unknown. In this study, we showed that constitutive activation of TACI signaling suppressed BAFF-R-mediated NF-kappaB2 p100 processing with the up-regulation of cellular inhibitors of apoptosis 1 (cIAP1) and TNF receptor associated factor (TRAF)-associated NF-kappaB activator (TANK). The ubiquitination of NIK by cIAP1 was inhibited by the expression of TRAF2 with physical binding to cIAP1. TANK deficiency by small interfering RNA (siRNA) impaired TACI-dependent inhibition of NF-kappaB2 p100 processing. TANK also inhibited TRAF2-mediated cIAP1 inactivation. Moreover, the recruitment of TRAF2 to TACI induced the ubiquitination of NIK. Taken together, the regulation of NIK by TACI through the interaction of TANK/TRAF2/cIAP1 plays a pivotal role in the suppression of non-canonical NF-kappaB signaling.

Regulation of TNFRSF and innate immune signalling complexes by TRAFs and cIAPs

There have been a number of recent discoveries relating to the functions of inhibitors of apoptosis (IAPs) and TNF receptor-associated factors (TRAFs) in regulating signalling from TNF receptor superfamily (TNFRSF) members and some tantalizing glimpses into a wider area of influence, that of innate immune signalling. Discoveries relating to the function of these ubiquitin E3 ligases in regulating signalling from the eponymous member of the family, TNF-R1, are dealt with superbly in a separate review by Wertz and Dixit and so we will confine our discussion to the subset of the TNFRSF that does not contain a death domain (DD). In line with the available data we will divide the review into two parts, the first is restricted to the role of TRAFs 2 and 3 and cIAPs in regulating TNFRSF signalling, whereas the second will be more speculative, asking what role IAPs and TRAFs have in innate immune signalling.

TRAF2 must bind to cellular inhibitors of apoptosis for tumor necrosis factor (tnf) to efficiently activate nf-{kappa}b and to prevent tnf-induced apoptosis

Tumor necrosis factor (TNF) receptor-associated factor-2 (TRAF2) binds to cIAP1 and cIAP2 (cIAP1/2) and recruits them to the cytoplasmic domain of several members of the TNF receptor (TNFR) superfamily, including the TNF-TNFR1 ligand-receptor complex. Here, we define a cIAP1/2-interacting motif (CIM) within the TRAF-N domain of TRAF2, and we use TRAF2 CIM mutants to determine the role of TRAF2 and cIAP1/2 individually, and the TRAF2-cIAP1/2 interaction, in TNFR1-dependent signaling. We show that both the TRAF2 RING domain and the TRAF2 CIM are required to regulate NF-kappaB-inducing kinase stability and suppress constitutive noncanonical NF-kappaB activation. Conversely, following TNFR1 stimulation, cells bearing a CIM-mutated TRAF2 showed reduced canonical NF-kappaB activation and TNF-induced RIPK1 ubiquitylation. Remarkably, the RING domain of TRAF2 was dispensable for these functions. However, like the TRAF2 CIM, the RING domain of TRAF2 was required for protection against TNF-induced apoptosis. These results show that TRAF2 has anti-apoptotic signaling roles in addition to promoting NF-kappaB signaling and that efficient activation of NF-kappaB by TNFR1 requires the recruitment of cIAP1/2 by TRAF2.

TRAF-mediated TNFR-family signaling

The tumor necrosis factor (TNF) superfamily consists of a wide variety of cell-bound and secreted proteins that regulate numerous cellular processes. In particular, TNF-family proteins regulate the proliferation and death of tumor cells, as well as activated immune cells. This overview discusses the mammalian TNF receptor-associated factors (TRAFs), of which TRAF1, 2, 3, 5, and 6 have been shown to interact directly or indirectly with members of the TNF receptor superfamily. Structural features of TRAF proteins are described along with a discussion of TRAF-interacting proteins and the signaling pathways activated by the TRAF proteins. Finally, we examine the phenotypes observed in TRAF-knockout mice.

no poles encodes a predicted E3 ubiquitin ligase required for early embryonic development of Drosophila

In a screen for cell-cycle regulators, we identified a Drosophila maternal effect-lethal mutant that we named ;no poles' (nopo). Embryos from nopo females undergo mitotic arrest with barrel-shaped, acentrosomal spindles during the rapid S-M cycles of syncytial embryogenesis. We identified CG5140, which encodes a candidate RING domain-containing E3 ubiquitin ligase, as the nopo gene. A conserved residue in the RING domain is altered in our EMS-mutagenized allele of nopo, suggesting that E3 ligase activity is crucial for NOPO function. We show that mutation of a DNA checkpoint kinase, CHK2, suppresses the spindle and developmental defects of nopo-derived embryos, revealing that activation of a DNA checkpoint operational in early embryos contributes significantly to the nopo phenotype. CHK2-mediated mitotic arrest has been previously shown to occur in response to mitotic entry with DNA damage or incompletely replicated DNA. Syncytial embryos lacking NOPO exhibit a shorter interphase during cycle 11, suggesting that they may enter mitosis prior to the completion of DNA replication. We show that Bendless (BEN), an E2 ubiquitin-conjugating enzyme, interacts with NOPO in a yeast two-hybrid assay; furthermore, ben-derived embryos arrest with a nopo-like phenotype during syncytial divisions. These data support our model that an E2-E3 ubiquitination complex consisting of BEN-UEV1A (E2 heterodimer) and NOPO (E3 ligase) is required for the preservation of genomic integrity during early embryogenesis.

TRAF2 suppresses basal IKK activity in resting cells and TNFalpha can activate IKK in TRAF2 and TRAF5 double knockout cells

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) and TRAF5 are adapter proteins involved in TNFalpha-induced activation of the c-Jun N-terminal kinase and nuclear factor kappaB (NF-kappaB) pathways. Currently, TNFalpha-induced NF-kappaB activation is believed to be impaired in TRAF2 and TRAF5 double knockout (T2/5 DKO) cells. Here, we report instead that T2/5 DKO cells exhibit high basal IkappaB kinase (IKK) activity and elevated expression of NF-kappaB-dependent genes in unstimulated conditions. Although TNFalpha-induced receptor-interacting protein 1 ubiquitination is indeed impaired in T2/5 DKO cells, TNFalpha stimulation further increases IKK activity in these cells, resulting in significantly elevated expression of NF-kappaB target genes to a level higher than that in wild-type cells. Inhibition of NIK in T2/5 DKO cells attenuates basal IKK activity and restores robust TNFalpha-induced IKK activation to a level comparable with that seen in wild-type cells. This suggests that TNFalpha can activate IKK in the absence of TRAF2 and TRAF5 expression and receptor-interacting protein 1 ubiquitination. In addition, both the basal and TNFalpha-induced expression of anti-apoptotic proteins are normal in T2/5 DKO cells, yet these DKO cells remain sensitive to TNFalpha-induced cell death, due to the impaired recruitment of anti-apoptotic proteins to the TNFR1 complex in the absence of TRAF2. Thus, our data demonstrate that TRAF2 negatively regulates basal IKK activity in resting cells and inhibits TNFalpha-induced cell death by recruiting anti-apoptotic proteins to the TNFR1 complex rather than by activating the NF-kappaB pathway.

HSP70 interacts with TRAF2 and differentially regulates TNFalpha signalling in human colon cancer cells

Members of tumour necrosis factor (TNF) family usually trigger both survival and apoptotic signals in various cell types. Heat shock proteins (HSPs) are conserved proteins implicated in protection of cells from stress stimuli. However, the mechanisms of HSPs in TNFα-induced signalling pathway have not been fully elucidated. We report here that HSP70 over-expression in human colon cancer cells can inhibit TNFα-induced NFκB activation but promote TNFα-induced activation of c-Jun N-terminal kinase (JNK) through interaction with TNF receptor (TNFR)-associated factor 2 (TRAF2). We provide evidence that HSP70 over-expression can sequester TRAF2 in detergent-soluble fractions possibly through interacting with TRAF2, leading to reduced recruitment of receptor-interacting protein (RIP1) and IκBα kinase (IKK) signalosome to the TNFR1–TRADD complex and inhibited NFκB activation after TNFα stimuli. In addition, we found that HSP70–TRAF2 interaction can promote TNFα-induced JNK activation. Therefore, our study suggests that HSP70 may differentially regulate TNFα-induced activation of NFκB and JNK through interaction with TRAF2, contributing to the pro-apoptotic roles of HSP70 in TNFα-induced apoptosis of human colon cancer cells.

cIAP1-dependent TRAF2 degradation regulates the differentiation of monocytes into macrophages and their response to CD40 ligand

Peripheral blood monocytes are plastic cells that migrate to tissues and differentiate into various cell types, including macrophages, dendritic cells, and osteoclasts. We have described the migration of cellular inhibitor of apoptosis protein 1 (cIAP1), a member of the IAP family of proteins, from the nucleus to the Golgi apparatus in monocytes undergoing differentiation into macrophages. Here we show that, once in the cytoplasm, cIAP1 is involved in the degradation of the adaptor protein tumor necrosis factor receptor-associated factor 2 (TRAF2) by the proteosomal machinery. Inhibition of cIAP1 prevents the decrease in TRAF2 expression that characterizes macrophage formation. We demonstrate that TRAF2 is initially required for macrophage differentiation as its silencing prevents Ikappa-Balpha degradation, nuclear factor-kappaB (NF-kappaB) p65 nuclear translocation, and the differentiation process. Then, we show that cIAP1-mediated degradation of TRAF2 allows the differentiation process to progress. This degradation is required for the macrophages to be fully functional as TRAF2 overexpression in differentiated cells decreases the c-Jun N-terminal kinase-mediated synthesis and the secretion of proinflammatory cytokines, such as interleukin-8 and monocyte chemoattractant protein 1 (MCP-1) in response to CD40 ligand. We conclude that TRAF2 expression and subsequent degradation are required for the differentiation of monocytes into fully functional macrophages.

A functional map of NFkappaB signaling identifies novel modulators and multiple system controls

Using cell-based genomic screens and functional assays, positive and negative modulators of NFκB signaling were identified and mapped onto the known NFκB signaling cascade.

Background

The network of signaling pathways that leads to activation of the NFκB transcription factors is a branched structure with different inputs and cross-coupling with other signaling pathways. How these signals are integrated to produce specific, yet diverse responses is not clearly understood. To identify the components and structural features of the NFκB network, a series of cell-based, genomic screens was performed using a library of approximately 14,500 full-length genes.

Results

A total of 154 positive and 88 negative modulators of NFκB signaling were identified. Using a series of dominant-negative constructs and functional assays, these modulators were mapped to the known NFκB signaling cascade. Most of the positive modulators acted upstream of the IκB kinase complex, supporting previous observations that the IκB kinases represent the primary point of convergence in the network. A number of negative modulators were localized downstream of the IκB kinase β (IKBKB) subunit, suggesting that they form an additional layer of negative control within the system. The expression of the modulators at the RNA level was distributed disproportionately across tissues, providing flexibility in network structure, and the number of positive and negative modulators present in a given tissue was highly correlated, suggesting that positive and negative regulation is balanced at the tissue level.

Conclusion

The relative locations of the modulators are consistent with an hourglass structure for the NFκB network that is characteristic of robust systems. The tissue distribution of the modulators and downstream location of the negative modulators serve as layers of control within the system that allow differential responses to different stimuli.

TRAF6 deficiency promotes TNF-induced cell death through inactivation of GSK3beta

TNF receptor-associated factor 6 (TRAF6) plays a key role in the regulation of innate immune responses by mediating signals from both TNF receptors (TNFRs) and interleukin-1 receptors (IL-1Rs)/Toll-like receptors (TLRs). Here, we define a new role for TRAF6 in antagonizing cell death during TNF signaling. In TRAF6-deficient 3T3 (T6(-/-) 3T3) cells, TNF stimulation leads to the accumulation of reactive oxygen species (ROS), which in turn results in prolonged c-Jun N-terminal kinase (JNK) activation and accelerated cell death. Furthermore, TNF-induced p65/RelA phosphorylation as well as transcriptional activity of nuclear factor-kappaB (NF-kappaB) was significantly downregulated in T6(-/-) 3T3 cells. Interestingly, TRAF6 deficiency leads to constitutive phosphorylation and inactivation of glycogen synthase kinase 3beta (GSK3beta). Restoration of GSK3beta activity through exogenous expression of a GSK3beta constitutive active form rescued cell death in TRAF6-null 3T3 cells. These data suggest a role for TRAF6 in the maintenance of cell survival by regulating GSK3beta activity in TNF signaling.

Roles of TRAF6 in CD40 signaling

CD40 provides signals crucial to the activation of antigen-presenting cells during humoral and cell-mediated immune responses. A complex cohort of proteins interacts with the cytoplasmic domain of CD40 and mediates signaling. One member of this cohort is TNF receptor associated factor six (TRAF6). TRAF6 contributes to the CD40-mediated activation of NF-kappaB, stress-activated protein kinases, and perhaps other signaling molecules. TRAF6 may have roles as an adapter molecule, an activator of mitogen-activated protein kinases, and as a repressor of certain signaling circuits. Establishing the significance and interplay of these roles will lead to a more complete understanding of mechanisms important to the CD40-mediated activation of the immune system and will reveal novel targets for the development of therapeutic agents.

TRAF proteins in CD40 signaling

The tumor necrosis factor receptor (TNFR) superfamily molecule CD40 is expressed by a wide variety of cell types following activation signals, and constitutively on B lymphocytes, macrophages, and dendritic cells. CD40 signals to cells stimulate kinase activation, gene expression, production of a antibody and a variety of cytokines, expression or upregulation of surface molecules, and protection or promotion of apoptosis. Initial steps in CD40-mediated signal cascades involve the interactions of CD40 with various members of the TNFR-associated factor (TRAF) family of cytoplasmic proteins. This review summarizes current understanding of the nature of these interactions, and how they induce and regulate CD40 functions.