A novel TLR2-triggered signalling crosstalk synergistically intensifies TNF-mediated IL-6 induction.

Toll-like receptors (TLR) recognize pathogens and trigger the production of vigorous pro-inflammatory cytokines [such as tumour necrosis factor (TNF)] that induce systemic damages associated with sepsis and chronic inflammation. Cooperation between signals of TLR and TNF receptor has been demonstrated through the participation of TNF receptor 1 (TNFR) adaptors in endotoxin tolerance. Here, we identify a TLR2-mediated synergy, through a MyD88-independent crosstalk, which enhances subsequent TNF-mediated nuclear factor-kappa B activation and interleukin-6 induction. Membrane-associated adaptor MAL conduces the link between TNF receptor-associated factor 6 (TRAF6) and TNFR-associated death domain, leading to a distinctive K63-ubiquitinylated TRAF6 recruitment into TNFR complex. In summary, our results reveal a novel route of TLR signal that synergistically amplifies TNF-mediated responses, indicating an innovative target for inflammation manipulation.

Phospho-SXXE/D motif mediated TNF receptor 1-TRADD death domain complex formation for T cell activation and migration.

In TNF-treated cells, TNFR1, TNFR-associated death domain protein (TRADD), Fas-associated death domain protein, and receptor-interacting protein kinase proteins form the signaling complex via modular interaction within their C-terminal death domains. In this paper, we report that the death domain SXXE/D motifs (i.e., S381DHE motif of TNFR1-death domain as well as S215LKD and S296LAE motifs of TRADD-death domain) are phosphorylated, and this is required for stable TNFR1-TRADD complex formation and subsequent activation of NF-κB. Phospho-S215LKD and phospho-S296LAE motifs are also critical to TRADD for recruiting Fas-associated death domain protein and receptor-interacting protein kinase. IκB kinase ß plays a critical role in TNFR1 phosphorylation of S381, which leads to subsequent T cell migration and accumulation. Consistently, we observed in inflammatory bowel disease specimens that TNFR1 was constitutively phosphorylated on S381 in those inflammatory T cells, which had accumulated in high numbers in the inflamed mucosa. Therefore, SXXE/D motifs found in the cytoplasmic domains of many TNFR family members and their adaptor proteins may serve to function as a specific interaction module for the α-helical death domain signal transduction.

TNFR1-induced activation of the classical NF-κB pathway.

The molecular mechanisms underlying activation of the IκB kinase (IKK) complex are presumably best understood in the context of tumor necrosis factor (TNF) receptor-1 (TNFR1) signaling. In fact, it seems that most, if not all, proteins relevant for this process have been identified and extensive biochemical and genetic data are available for the role of these factors in TNF-induced IKK activation. There is evidence that protein modification-independent assembly of a core TNFR1 signaling complex containing TNFR1-associated death domain, receptor interacting kinase 1, TNF receptor-associated factor 2 and cellular inhibitor of apoptosis protein 1 and 2 starts a chain of nondegrading ubiquitination events that culminate in the recruitment and activation of IKK complex-stimulating kinases and the IKK complex itself. Here, we sum up the known details of TNFR1-induced IKK activation, address arising contradictions and discuss possible explanations resolving the apparent discrepancies.

The role of TRADD in TRAIL-induced apoptosis and signaling.

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily. TRAIL is promising for anticancer therapy because it induces apoptosis in cancer cells with little or no toxicity to normal cells; hence, TRAIL-receptor agonists are currently undergoing clinical trials for cancer treatment. However, many molecular signaling mechanisms in TRAIL signaling are not completely characterized. The functions of adaptor proteins, including TNF-receptor-associated death domain protein (TRADD) and receptor-interacting protein-1 (RIP1) in TRAIL signaling have been controversial. We demonstrate that while wild-type mouse embryonic fibroblasts (MEFs) are completely resistant to TRAIL-induced apoptosis, MEFs derived from Tradd(-/-) mice are hypersensitive to TRAIL (IC(50)~0.5 nM rmTRAIL, 24 h), an effect also seen in primary keratinocytes treated with TRAIL/CHX. Restoration of TRADD in Tradd(-/-) MEFs restores TRAIL resistance, indicating that TRADD plays a survival role in TRAIL signaling. We show that TRADD is recruited to the TRAIL-receptor complex, and RIP1 recruitment is mediated by TRADD. While early activation of the MAP kinase ERK is deficient in Tradd(-/-) cells, the main mechanism for enhanced TRAIL sensitivity is likely due to increased recruitment of FADD to the receptor complex, indicating that TRADD may limit FADD binding within the receptor complex and also mediate RIP1-dependent nonapoptotic signaling events, thus reducing caspase activation and subsequent apoptosis. These novel findings have potential implications for cancer therapy using TRAIL-receptor agonists.

Emerging role for TNF-α in erectile dysfunction.

INTRODUCTION: A role for cytokines in the pathophysiology of erectile dysfunction (ED) has emerged. Cytokines induce genes that synthesize other peptides in the cytokine family and several mediators, such as prostanoids, leukotrienes, nitric oxide, bradykinin, reactive oxygen species, and platelet-activating factor, all of which can affect vascular function. Consistent with the fact that the cavernosal tissue is a complex extension of the vasculature, risk factors that affect the vasculature have been shown to affect cavernosal function as well. Accordingly, the penile tissue has been recognized as an early sentinel for atherosclerosis that underlies coronary artery disease and cardiovascular diseases (CVD). AIM: To review the literature pertaining to the role of tumor necrosis factor-alpha (TNF-α) in ED. METHODS: PubMed search for pertinent publications on the role of cytokines, particularly TNF-α, in CVD and ED. MAIN OUTCOME MEASURES: Clinical and experimental evidence demonstrates that TNF-α may play a role in ED. RESULTS: TNF-α has been shown to play an important role in CVD, mainly due to its direct effects on the vasculature. In addition, high levels of TNF-α were demonstrated in patients with ED. In this review, we present a short description of the physiology of erection and the cytokine network. We focus on vascular actions of TNF-α that support a role for this cytokine as a potential candidate in the pathophysiology of ED, particularly in the context of CVD. A brief overview of its discovery, mechanisms of synthesis, receptors, and its main actions on the systemic and penile vasculature is also presented. CONCLUSIONS: Considering that ED results from a systemic arterial defect not only confined to the penile vasculature, implication of TNF-α in the pathophysiology of ED offers a humoral linking between CVD and ED.

Death receptor signal transducers: nodes of coordination in immune signaling networks.

Death receptors (DRs) are members of the tumor necrosis factor receptor superfamily that possess a cytoplasmic death domain (DD). DRs regulate important operational and homeostatic aspects of the immune system. They transmit signals through apical protein complexes, which are nucleated by the DD adaptors FADD and TRADD, to control cellular outcomes that range from apoptosis to gene activation. FADD and TRADD also nucleate several distal signaling complexes, which mediate cross-talk between distinct DR signaling pathways. Moreover, together with other DR signal transducers, FADD and TRADD participate in functional complexes assembled by certain non-DR immune cell receptors, such as pattern-recognition receptors. Thus, DR signal transducers may provide important nodes of coordination in immune signaling networks.

Beyond tumor necrosis factor receptor: TRADD signaling in toll-like receptors.

Tumor necrosis factor receptor 1-associated death domain protein (TRADD) is the core adaptor recruited to TNF receptor 1 (TNFR1) upon TNFalpha stimulation. In cells from TRADD-deficient mice, TNFalpha-mediated apoptosis and TNFalpha-stimulated NF-kappaB, JNK, and ERK activation are defective. TRADD is also important for germinal center formation, DR3-mediated costimulation of T cells, and TNFalpha-mediated inflammatory responses in vivo. TRADD deficiency does not enhance IFNgamma-induced signaling. Importantly, TRADD has a novel role in TLR3 and TLR4 signaling. TRADD participates in the TLR4 complex formed upon LPS stimulation, and TRADD-deficient macrophages show impaired cytokine production in response to TLR ligands in vitro. Thus, TRADD is a multifunctional protein crucial both for TNFR1 signaling and other signaling pathways relevant to immune responses.

A New TRADDition in intracellular antiviral signaling.

To effectively defeat viral infection, a mammalian host must detect the presence of viruses and activate a robust antiviral response, which is mediated by cytokines known as type I interferons (IFNs). Much excitement has been generated by the discovery of the retinoic acid-induced gene 1-like helicase (RLH) pathway, which detects RNA-containing viruses in the cytoplasm and activates the type I IFN response. A study now describes a role for the tumor necrosis factor receptor (TNFR)-associated death domain (TRADD) in the antiviral response and sheds new light on how the RLH signaling complex is organized upon viral infection.

Tumor necrosis factor receptor-associated death domain mediated neuronal death contributes to the glial activation and subsequent neuroinflammation in Japanese encephalitis.

While a number of studies have documented the importance of microglia in central nervous system (CNS) response to injury, infection and in disease state, little is known regarding how the neuronal death initiates the cascades of secondary neuroinflammation. We have exploited an experimental model of Japanese encephalitis to better understand how neuronal death following viral infection initiates microglial activation following Japanese encephalitis virus infection. We have earlier shown that the altered expression of tumor necrosis factor receptor-1 (TNFR-1) and TNFR associated death domain (TRADD) following Japanese encephalitis virus infection regulates the downstream apoptotic cascades. Here we have reported that silencing TRADD expression with small-interfering RNA reduced neuronal apoptosis and subsequent microglial and astroglial activation and release of various pro-inflammatory mediators. Our findings suggest that the engagement of TNFR-1 and TRADD following Japanese encephalitis virus infection plays a crucial role in glial activation also and influences the outcome of viral pathogenesis.