Redundant and receptor-specific activities of TRADD, RIPK1 and FADD in death receptor signaling

18‑α‑glycyrrhetinic acid induces apoptosis in gingival fibroblasts exposed to phenytoin

Phenytoin (PHT)-induced gingival overgrowth is caused by the increased proliferation and reduced apoptosis of gingival fibroblasts in inflammatory gingiva. Licorice has long been used as a component of therapeutic preparations. It inhibits cell proliferation, induces cell apoptosis and has anti-inflammatory effects. 18-α-glycyrrhetinic acid (18α-GA), the active compound in licorice, promotes apoptosis in various types of cells. The present study determined whether 18α-GA affects apoptosis in gingival fibroblasts exposed to PHT. The present study aimed to establish a basis for the therapeutic application of 18α-GA to treat the gingival overgrowth induced by PHT. Human gingival fibroblasts from healthy donors were cultured to semi-confluence and then stimulated in serum-free DMEM containing PHT with or without 18α-GA for subsequent experiments. Apoptotic cells were detected by ELISA. Analysis of the distribution of cell cycle phases and the apoptotic cell population was performed by flow cytometry. The expression levels of mRNAs and proteins of apoptotic regulators were measured using reverse transcription-quantitative PCR and western blotting, respectively. Caspase (CASP) activities were assessed by an ELISA. Treatment with 18α-GA markedly increased the number of apoptotic cells, reduced BCL2 mRNA expression, increased CASP2 and receptor (TNFRSF)-interacting serine-threonine kinase 1 (RIPK1) domain containing adaptor with death domain, Fas (TNFRSF6)-associated via death domain, RIPK1, tumor necrosis factor receptor superfamily; member 1A, TNF receptor-associated factor 2, CASP2, CASP3 and CASP9 mRNA expression, and also upregulated the protein expression levels and activities of caspase-2, caspase-3 and caspase-9. These results demonstrated that 18α-GA induced apoptosis through the activation of the Fas and TNF pathways in the death receptor signaling pathway in gingival fibroblasts treated with PHT. 18α-GA exhibited therapeutic potential for the treatment of PHT-induced gingival overgrowth.

MLKL regulates radiation-induced death in breast cancer cells: an interplay between apoptotic and necroptotic signals

Resistance to caspase-dependent apoptosis is often responsible for treatments failure in cancer. Necroptosis is a type of programmed necrosis that occurs under caspase-deficient conditions that could overcome apoptosis resistance. Our purpose was to investigate the interrelationship between apoptotic and necroptotic death pathways and their influence on the response of breast cancer cells to radiotherapy in vitro. Human BC cell lines MCF-7 and MDA-MB-231 were treated with ionizing radiation, and then several markers of apoptosis, necroptosis, and survival were assessed in the presence and absence of necroptosis inhibition. MLKL knockdown was achieved by siRNA transfection. Our main findings emphasize the role of necroptosis in cellular response to radiation represented in the dose- and time-dependent elevated expression of necroptotic markers RIPK1, RIPK3, and MLKL. Knockdown of necroptotic marker MLKL by siRNA led to a significant elevation in MDA-MB-231 and MCF-7 survival with a dose modifying factor (DMF) of 1.23 and 1.61, respectively. Apoptotic markers Caspase 8 and TRADD showed transitory or delayed upregulation, indicating that apoptosis was not the main mechanism by which cells respond to radiation exposure. Apoptotic markers also showed a significant elevation following MLKL knockdown, suggesting its role either as a secondary or death alternative pathway. The result of our study emphasizes the critical role of the necroptotic pathway in regulating breast cancer cells responses to radiotherapy and suggests a promising utilization of its key modulator, MLKL, as a treatment strategy to improve the response to radiotherapy.

Molecular signatures in prion disease: altered death receptor pathways in a mouse model

BACKGROUND

Prion diseases are transmissible and fatal neurodegenerative diseases characterized by accumulation of misfolded prion protein isoform (PrPSc), astrocytosis, microgliosis, spongiosis, and neurodegeneration. Elevated levels of cell membrane associated PrPSc protein and inflammatory cytokines hint towards the activation of death receptor (DR) pathway/s in prion diseases. Activation of DRs regulate, either cell survival or apoptosis, autophagy and necroptosis based on the adaptors they interact. Very little is known about the DR pathways activation in prion disease. DR3 and DR5 that are expressed in normal mouse brain were never studied in prion disease, so also their ligands and any DR adaptors. This research gap is notable and investigated in the present study.

METHODS

C57BL/6J mice were infected with Rocky Mountain Laboratory scrapie mouse prion strain. The progression of prion disease was examined by observing morphological and behavioural abnormalities. The levels of PrP isoforms and GFAP were measured as the marker of PrPSc accumulation and astrocytosis respectively using antibody-based techniques that detect proteins on blot and brain section. The levels of DRs, their glycosylation and ectodomain shedding, and associated factors warrant their examination at protein level, hence western blot analysis was employed in this study.

RESULTS

Prion-infected mice developed motor deficits and neuropathology like PrPSc accumulation and astrocytosis similar to other prion diseases. Results from this research show higher expression of all DR ligands, TNFR1, Fas and p75NTR but decreased levels DR3 and DR5. The levels of DR adaptor proteins like TRADD and TRAF2 (primarily regulate pro-survival pathways) are reduced. FADD, which primarily regulate cell death, its level remains unchanged. RIPK1, which regulate pro-survival, apoptosis and necroptosis, its expression and proteolysis (inhibits necroptosis but activates apoptosis) are increased.

CONCLUSIONS

The findings from the present study provide evidence towards the involvement of DR3, DR5, DR6, TL1A, TRAIL, TRADD, TRAF2, FADD and RIPK1 for the first time in prion diseases. The knowledge obtained from this research discuss the possible impacts of these 16 differentially expressed DR factors on our understanding towards the multifaceted neuropathology of prion diseases and towards future explorations into potential targeted therapeutic interventions for prion disease specific neuropathology.

TNF-Related Apoptosis-Inducing Ligand: Non-Apoptotic Signalling

TNF-related apoptosis-inducing ligand (TRAIL or Apo2 or TNFSF10) belongs to the TNF superfamily. When bound to its agonistic receptors, TRAIL can induce apoptosis in tumour cells, while sparing healthy cells. Over the last three decades, this tumour selectivity has prompted many studies aiming at evaluating the anti-tumoral potential of TRAIL or its derivatives. Although most of these attempts have failed, so far, novel formulations are still being evaluated. However, emerging evidence indicates that TRAIL can also trigger a non-canonical signal transduction pathway that is likely to be detrimental for its use in oncology. Likewise, an increasing number of studies suggest that in some circumstances TRAIL can induce, via Death receptor 5 (DR5), tumour cell motility, potentially leading to and contributing to tumour metastasis. While the pro-apoptotic signal transduction machinery of TRAIL is well known from a mechanistic point of view, that of the non-canonical pathway is less understood. In this study, we the current state of knowledge of TRAIL non-canonical signalling.

RIP3 in Necroptosis: Underlying Contributions to Traumatic Brain Injury

Traumatic brain injury (TBI) is a global public safety issue that poses a threat to death, characterized by high fatality rates, severe injuries and low recovery rates. There is growing evidence that necroptosis regulates the pathophysiological processes of a variety of diseases, particularly those affecting the central nervous system. Thus, moderate necroptosis inhibition may be helpful in the management of TBI. Receptor-interacting protein kinase (RIP) 3 is a key mediator in the necroptosis, and its absence helps restore the microenvironment at the injured site and improve cognitive impairment after TBI. In this report, we review different domains of RIP3, multiple analyses of necroptosis, and associations between necroptosis and TBI, RIP3, RIP1, and mixed lineage kinase domain-like. Next, we elucidate the potential involvement of RIP3 in TBI and highlight how RIP3 deficiency enhances neuronal function.

Cell type-specific molecular mechanisms and implications of necroptosis in inflammatory respiratory diseases

Necroptosis is generally considered as an inflammatory cell death form. The core regulators of necroptotic signaling are receptor-interacting serine-threonine protein kinases 1 (RIPK1) and RIPK3, and the executioner, mixed lineage kinase domain-like pseudokinase (MLKL). Evidence demonstrates that necroptosis contributes profoundly to inflammatory respiratory diseases that are common public health problem. Necroptosis occurs in nearly all pulmonary cell types in the settings of inflammatory respiratory diseases. The influence of necroptosis on cells varies depending upon the type of cells, tissues, organs, etc., which is an important factor to consider. Thus, in this review, we briefly summarize the current state of knowledge regarding the biology of necroptosis, and focus on the key molecular mechanisms that define the necroptosis status of specific cell types in inflammatory respiratory diseases. We also discuss the clinical potential of small molecular inhibitors of necroptosis in treating inflammatory respiratory diseases, and describe the pathological processes that engage cross talk between necroptosis and other cell death pathways in the context of respiratory inflammation. The rapid advancement of single-cell technologies will help understand the key mechanisms underlying cell type-specific necroptosis that are critical to effectively treat pathogenic lung infections and inflammatory respiratory diseases.

Role of Necroptosis, a Regulated Cell Death, in Seizure and Epilepsy

Epilepsy is a chronic neurological disease that is characterized by spontaneous and recurrent seizures. Regulated cell death is a controlled process and has been shown to be involved in neurodegenerative diseases. Necroptosis is a type of regulated cell death, and its association with epilepsy has been documented. Necroptosis signaling can be divided into two pathways: canonical and non-canonical pathways. Inhibition of caspase-8, dimerization of receptor-interacting protein kinase 1 (RIP1) and RIP3, activation of mixed-lineage kinase domain-like protein (MLKL), movement of MLKL to the plasma membrane, and cell rupture occurred in these pathways. Through literature review, it has been revealed that there is a relationship between seizure, neuroinflammation, and oxidative stress. The seizure activity triggers the activation of various pathways within the central nervous system, including TNF-α/matrix metalloproteases, Neogenin and Calpain/ Jun N-terminal Kinase 1, which result in distinct responses in the brain. These responses involve the activation of neurons and astrocytes, consequently leading to an increase in the expression levels of proteins and genes such as RIP1, RIP3, and MLKL in a time-dependent manner in regions such as the hippocampus (CA1, CA3, dentate gyrus, and hilus), piriform cortex, and amygdala. Furthermore, the imbalance in calcium ions, depletion of adenosine triphosphate, and elevation of extracellular glutamate and potassium within these pathways lead to the progression of necroptosis, a reduction in seizure threshold, and increased susceptibility to epilepsy. Therefore, it is plausible that therapeutic targeting of these pathways could potentially provide a promising approach for managing seizures and epilepsy.

Molecular and functional characteristics of receptor-interacting protein kinase 1 (RIPK1) and its therapeutic potential in Alzheimer's disease

Inflammation and cell death processes positively control the organ homeostasis of an organism. Receptor-interacting protein kinase 1 (RIPK1), a member of the RIPK family, is a crucial regulator of cell death and inflammation, and control homeostasis at the cellular and tissue level. Necroptosis, a programmed form of necrosis-mediated cell death and tumor necrosis factor (TNF)-induced necrotic cell death, is mostly regulated by RIPK1 kinase activity. Thus, RIPK1 has recently emerged as an upstream kinase that controls multiple cellular pathways and participates in regulating inflammation and cell death. All the major cell types in the central nervous system (CNS) have been found to express RIPK1. Selective inhibition of RIPK1 has been shown to prevent neuronal cell death, which could ultimately lead to a significant reduction of neurodegeneration and neuroinflammation. In addition, the kinase structure of RIPK1 is highly conducive to the development of specific pharmacological small-molecule inhibitors. These factors have led to the emergence of RIPK1 as an important therapeutic target for Alzheimer's disease (AD).

Fn14 and TNFR2 as regulators of cytotoxic TNFR1 signaling

Tumor necrosis factor (TNF) receptor 1 (TNFR1), TNFR2 and fibroblast growth factor-inducible 14 (Fn14) belong to the TNF receptor superfamily (TNFRSF). From a structural point of view, TNFR1 is a prototypic death domain (DD)-containing receptor. In contrast to other prominent death receptors, such as CD95/Fas and the two TRAIL death receptors DR4 and DR5, however, liganded TNFR1 does not instruct the formation of a plasma membrane-associated death inducing signaling complex converting procaspase-8 into highly active mature heterotetrameric caspase-8 molecules. Instead, liganded TNFR1 recruits the DD-containing cytoplasmic signaling proteins TRADD and RIPK1 and empowers these proteins to trigger cell death signaling by cytosolic complexes after their release from the TNFR1 signaling complex. The activity and quality (apoptosis versus necroptosis) of TNF-induced cell death signaling is controlled by caspase-8, the caspase-8 regulatory FLIP proteins, TRAF2, RIPK1 and the RIPK1-ubiquitinating E3 ligases cIAP1 and cIAP2. TNFR2 and Fn14 efficiently recruit TRAF2 along with the TRAF2 binding partners cIAP1 and cIAP2 and can thereby limit the availability of these molecules for other TRAF2/cIAP1/2-utilizing proteins including TNFR1. Accordingly, at the cellular level engagement of TNFR2 or Fn14 inhibits TNFR1-induced RIPK1-mediated effects reaching from activation of the classical NFκB pathway to induction of apoptosis and necroptosis. In this review, we summarize the effects of TNFR2- and Fn14-mediated depletion of TRAF2 and the cIAP1/2 on TNFR1 signaling at the molecular level and discuss the consequences this has in vivo.

Functions of the RIP kinase family members in the skin

The receptor interacting protein kinases (RIPK) are a family of serine/threonine kinases that are involved in the integration of various stress signals. In response to several extracellular and/or intracellular stimuli, RIP kinases engage signaling cascades leading to the activation of NF-κB and mitogen-activated protein kinases, cell death, inflammation, differentiation and Wnt signaling and can have kinase-dependent and kinase-independent functions. Although it was previously suggested that seven RIPKs are part of the RIPK family, phylogenetic analysis indicates that there are only five genuine RIPKs. RIPK1 and RIPK3 are mainly involved in controlling and executing necroptosis in keratinocytes, while RIPK4 controls proliferation and differentiation of keratinocytes and thereby can act as a tumor suppressor in skin. Therefore, in this review we summarize and discuss the functions of RIPKs in skin homeostasis as well as the signaling pathways involved.

Regulation of anoikis by extrinsic death receptor pathways

Metastatic cancer cells can develop anoikis resistance in the absence of substrate attachment and survive to fight tumors. Anoikis is mediated by endogenous mitochondria-dependent and exogenous death receptor pathways, and studies have shown that caspase-8-dependent external pathways appear to be more important than the activity of the intrinsic pathways. This paper reviews the regulation of anoikis by external pathways mediated by death receptors. Different death receptors bind to different ligands to activate downstream caspases. The possible mechanisms of Fas-associated death domain (FADD) recruitment by Fas and TNF receptor 1 associated-death domain (TRADD) recruitment by tumor necrosis factor receptor 1 (TNFR1), and DR4- and DR5-associated FADD to induce downstream caspase activation and regulate anoikis were reviewed. This review highlights the possible mechanism of the death receptor pathway mediation of anoikis and provides new insights and research directions for studying tumor metastasis mechanisms. Video Abstract.

Antibody-based soluble and membrane-bound TWEAK mimicking agonists with FcγR-independent activity

Fibroblast growth factor (FGF)-inducible 14 (Fn14) activates the classical and alternative NFκB (nuclear factor 'kappa-light-chain-enhancer' of activated B-cells) signaling pathway but also enhances tumor necrosis factor (TNF)-induced cell death. Fn14 expression is upregulated in non-hematopoietic cells during tissue injury and is also often highly expressed in solid cancers. In view of the latter, there were and are considerable preclinical efforts to target Fn14 for tumor therapy, either by exploiting Fn14 as a target for antibodies with cytotoxic activity (e.g. antibody-dependent cellular cytotoxicity (ADCC)-inducing IgG variants, antibody drug conjugates) or by blocking antibodies with the aim to interfere with protumoral Fn14 activities. Noteworthy, there are yet no attempts to target Fn14 with agonistic Fc effector function silenced antibodies to unleash the proinflammatory and cell death-enhancing activities of this receptor for tumor therapy. This is certainly not at least due to the fact that anti-Fn14 antibodies only act as effective agonists when they are presented bound to Fcγ receptors (FcγR). Thus, there are so far no antibodies that robustly and selectively engage Fn14 signaling without triggering unwanted FcγR-mediated activities. In this study, we investigated a panel of variants of the anti-Fn14 antibody 18D1 of different valencies and domain architectures with respect to their inherent FcγR-independent ability to trigger Fn14-associated signaling pathways. In contrast to conventional 18D1, the majority of 18D1 antibody variants with four or more Fn14 binding sites displayed a strong ability to trigger the alternative NFκB pathway and to enhance TNF-induced cell death and therefore resemble in their activity soluble (TNF)-like weak inducer of apoptosis (TWEAK), one form of the natural occurring ligand of Fn14. Noteworthy, activation of the classical NFκB pathway, which naturally is predominately triggered by membrane-bound TWEAK but not soluble TWEAK, was preferentially observed with a subset of constructs containing Fn14 binding sites at opposing sites of the IgG scaffold, e.g. IgG1-scFv fusion proteins. A superior ability of IgG1-scFv fusion proteins to trigger classical NFκB signaling was also observed with the anti-Fn14 antibody PDL192 suggesting that we identified generic structures for Fn14 antibody variants mimicking soluble and membrane-bound TWEAK.

Involvement of AKT/PI3K Pathway in Sanguinarine's Induced Apoptosis and Cell Cycle Arrest in Triple-negative Breast Cancer Cells

BACKGROUND/AIM

Chemotherapy resistance in triple-negative breast cancer (TNBC) cells is well documented. Therefore, it is necessary to develop safer and more effective therapeutic agents to enhance the outcomes of chemotherapeutic agents. The natural alkaloid sanguinarine (SANG) has demonstrated therapeutic synergy when coupled with chemotherapeutic agents. SANG can also induce cell cycle arrest and trigger apoptosis in various cancer cells.

MATERIALS AND METHODS

In this study, we investigated the molecular mechanism underlying SANG activity in MDA-MB-231 and MDA-MB-468 cells as two genetically different models of TNBC. We employed various assays including Alamar Blue to measure the effect of SANG on cell viability and proliferation rate, flow cytometry analysis to study the potential of the compound to induce apoptosis and cell cycle arrest, quantitative qRT PCR apoptosis array to measure the expression of different genes mediating apoptosis, and the western system was used to analyze the impact of the compound on AKT protein expression.

RESULTS

SANG lowered cell viability and disrupted cell cycle progression in both cell lines. Furthermore, S-phase cell cycle arrest-mediated apoptosis was found to be the primary contributor to cell growth inhibition in MDA-MB-231 cells. SANG-treated TNBC cells showed significantly up-regulated mRNA expression of 18 genes associated with apoptosis, including eight TNF receptor superfamily (TNFRSF), three members of the BCL2 family, and two members of the caspase (CASP) family in MDA-MB-468 cells. In MDA-MB-231 cells, two members of the TNF superfamily and four members of the BCL2 family were affected. The western study data showed the inhibition of AKT protein expression in both cell lines concurrent with up-regulated BCL2L11 gene. Our results point to the AKT/PI3K signaling pathway as one of the key mechanisms behind SANG-induced cell cycle arrest and death.

CONCLUSION

SANG shows anticancer properties and apoptosis-related gene expression changes in the two TNBC cell lines and suggests AKT/PI3K pathway implication in apoptosis induction and cell cycle arrest. Thus, we propose SANG's potential as a solitary or supplementary treatment agent against TNBC.

The Many Faces of MLKL, the Executor of Necroptosis

Necroptosis is a recently discovered form of regulated cell death characterized by the disruption of plasma membrane integrity and the release of intracellular content. Mixed lineage kinase domain-like (MLKL) protein is the main player of this cell death pathway as it mediates the final step of plasma membrane permeabilization. Despite the significant progress in our knowledge of the necroptotic pathway and MLKL biology, the precise mechanism of how MLKL functions remain unclear. To understand in what way MLKL executes necroptosis, it is crucial to decipher how the molecular machinery of regulated cell death is activated in response to different stimuli or stressors. It is also indispensable to unveiling the structural elements of MLKL and the cellular players that are required for its regulation. In this review, we discuss the key steps that lead to MLKL activation, possible models that explain how it becomes the death executor in necroptosis, and its emerging alternative functions. We also summarize the current knowledge about the role of MLKL in human disease and provide an overview of existing strategies aimed at developing new inhibitors that target MLKL for necroptosis intervention.

The indispensability of macrophage adaptor proteins in chronic inflammatory diseases

Adaptor proteins represent key signalling molecules involved in regulating immune responses. The host's innate immune system recognizes pathogens via various surface and intracellular receptors. Adaptor molecules are centrally involved in different receptor-mediated signalling pathways, acting as bridges between the receptors and other molecules. The presence of adaptors in major signalling pathways involved in the pathogenesis of various chronic inflammatory diseases has drawn attention toward the role of these proteins in such diseases. In this review, we summarize the importance and roles of different adaptor molecules in macrophage-mediated signalling in various chronic disease states. We highlight the mechanistic roles of adaptors and how they are involved in protein-protein interactions (PPI) via different domains to carry out signalling. Hence, we also provide insights into how targeting these adaptor proteins can be a good therapeutic strategy against various chronic inflammatory diseases.

A biochemical necroptosis model explains cell-type-specific responses to cell death cues

Necroptosis is a form of regulated cell death associated with degenerative disorders, autoimmune and inflammatory diseases, and cancer. To better understand the biochemical mechanisms regulating necroptosis, we constructed a detailed computational model of tumor necrosis factor-induced necroptosis based on known molecular interactions from the literature. Intracellular protein levels, used as model inputs, were quantified using label-free mass spectrometry, and the model was calibrated using Bayesian parameter inference to experimental protein time course data from a well-established necroptosis-executing cell line. The calibrated model reproduced the dynamics of phosphorylated mixed lineage kinase domain-like protein, an established necroptosis reporter. A subsequent dynamical systems analysis identified four distinct modes of necroptosis signal execution, distinguished by rate constant values and the roles of the RIP1 deubiquitinating enzymes A20 and CYLD. In one case, A20 and CYLD both contribute to RIP1 deubiquitination, in another RIP1 deubiquitination is driven exclusively by CYLD, and in two modes either A20 or CYLD acts as the driver with the other enzyme, counterintuitively, inhibiting necroptosis. We also performed sensitivity analyses of initial protein concentrations and rate constants to identify potential targets for modulating necroptosis sensitivity within each mode. We conclude by associating numerous contrasting and, in some cases, counterintuitive experimental results reported in the literature with one or more of the model-predicted modes of necroptosis execution. In all, we demonstrate that a consensus pathway model of tumor necrosis factor-induced necroptosis can provide insights into unresolved controversies regarding the molecular mechanisms driving necroptosis execution in numerous cell types under different experimental conditions.

Bioluminescent RIPoptosome Assay for FADD/RIPK1 Interaction Based on Split Luciferase Assay in a Human Neuroblastoma Cell Line SH-SY5Y

Different programed cell death (PCD) modalities involve protein-protein interactions in large complexes. Tumor necrosis factor α (TNFα) stimulated assembly of receptor-interacting protein kinase 1 (RIPK1)/Fas-associated death domain (FADD) interaction forms Ripoptosome complex that may cause either apoptosis or necroptosis. The present study addresses the interaction of RIPK1 and FADD in TNFα signaling by fusion of C-terminal (CLuc) and N-terminal (NLuc) luciferase fragments to RIPK1-CLuc (R1C) or FADD-NLuc (FN) in a caspase 8 negative neuroblastic SH-SY5Y cell line, respectively. In addition, based on our findings, an RIPK1 mutant (R1C K612R) had less interaction with FN, resulting in increasing cell viability. Moreover, presence of a caspase inhibitor (zVAD.fmk) increases luciferase activity compared to Smac mimetic BV6 (B), TNFα -induced (T) and non-induced cell. Furthermore, etoposide decreased luciferase activity, but dexamethasone was not effective in SH-SY5Y. This reporter assay might be used to evaluate basic aspects of this interaction as well as for screening of necroptosis and apoptosis targeting drugs with potential therapeutic application.

Conjugation of the Fn14 Ligand to a SMAC Mimetic Selectively Suppresses Experimental Squamous Cell Carcinoma in Mice

The mimetic of SMAC induced cell death in cancers by depleting the inhibitor of apoptosis proteins. Recent studies showed that Fn14 is overexpressed in the cells of squamous cell carcinoma (SCC), providing a promising candidate target for selective antitumor therapy. In this study, we conjugated a small-molecule SMAC mimetic MV1 to the ligand of Fn14, TWEAK. Our results showed that TWEAK‒MV1 conjugate retained adequate binding specificity to Fn14-positive SCC cells in vitro and accumulated selectively in tumor tissue of cutaneous SCC xenografts mice after intraperitoneal administration. This conjugation compound exhibited remarkable effectiveness in suppressing tumor growth and extending overall survival without causing significant side effects in SCC xenograft mice. Moreover, TWEAK‒MV1 conjugate greatly enhanced both apoptotic and necroptotic cell death both in vitro and in vivo, accompanied by a cellular inhibitor of apoptosis proteins degradation as well as activation of receptor-interacting protein kinase. Taken together, our preclinical data suggested that the designed conjugation compound of TWEAK and MV1 might provide a potential therapeutic strategy for cutaneous SCC with improved antitumor efficacy and negligible toxicity.

TRAIL receptors promote constitutive and inducible IL-8 secretion in non-small cell lung carcinoma

Interleukin-8 (IL-8/CXCL8) is a pro-angiogenic and pro-inflammatory chemokine that plays a role in cancer development. Non-small cell lung carcinoma (NSCLC) produces high amounts of IL-8, which is associated with poor prognosis and resistance to chemo-radio and immunotherapy. However, the signaling pathways that lead to IL-8 production in NSCLC are unresolved. Here, we show that expression and release of IL-8 are regulated autonomously by TRAIL death receptors in several squamous and adenocarcinoma NSCLC cell lines. NSCLC constitutively secrete IL-8, which could be further enhanced by glucose withdrawal or by treatment with TRAIL or TNFα. In A549 cells, constitutive and inducible IL-8 production was dependent on NF-κB and MEK/ERK MAP Kinases. DR4 and DR5, known regulators of these signaling pathways, participated in constitutive and glucose deprivation-induced IL-8 secretion. These receptors were mainly located intracellularly. While DR4 signaled through the NF-κB pathway, DR4 and DR5 both regulated the ERK-MAPK and Akt pathways. FADD, caspase-8, RIPK1, and TRADD also regulated IL-8. Analysis of mRNA expression data from patients indicated that IL-8 transcripts correlated with TRAIL, DR4, and DR5 expression levels. Furthermore, TRAIL receptor expression levels also correlated with markers of angiogenesis and neutrophil infiltration in lung squamous carcinoma and adenocarcinoma. Collectively, these data suggest that TRAIL receptor signaling contributes to a pro-tumorigenic inflammatory signature associated with NSCLC.

Targeting fibroblast growth factor (FGF)-inducible 14 (Fn14) for tumor therapy

Fibroblast growth factor-inducible 14 (Fn14) is a member of the tumor necrosis factor (TNF) receptor superfamily (TNFRSF) and is activated by its ligand TNF-like weak inducer of apoptosis (TWEAK). The latter occurs as a homotrimeric molecule in a soluble and a membrane-bound form. Soluble TWEAK (sTWEAK) activates the weakly inflammatory alternative NF-κB pathway and sensitizes for TNF-induced cell death while membrane TWEAK (memTWEAK) triggers additionally robust activation of the classical NF-κB pathway and various MAP kinase cascades. Fn14 expression is limited in adult organisms but becomes strongly induced in non-hematopoietic cells by a variety of growth factors, cytokines and physical stressors (e.g., hypoxia, irradiation). Since all these Fn14-inducing factors are frequently also present in the tumor microenvironment, Fn14 is regularly found to be expressed by non-hematopoietic cells of the tumor microenvironment and most solid tumor cells. In general, there are three possibilities how the tumor-Fn14 linkage could be taken into consideration for tumor therapy. First, by exploitation of the cancer associated expression of Fn14 to direct cytotoxic activities (antibody-dependent cell-mediated cytotoxicity (ADCC), cytotoxic payloads, CAR T-cells) to the tumor, second by blockade of potential protumoral activities of the TWEAK/Fn14 system, and third, by stimulation of Fn14 which not only triggers proinflammtory activities but also sensitizes cells for apoptotic and necroptotic cell death. Based on a brief description of the biology of the TWEAK/Fn14 system and Fn14 signaling, we discuss the features of the most relevant Fn14-targeting biologicals and review the preclinical data obtained with these reagents. In particular, we address problems and limitations which became evident in the preclinical studies with Fn14-targeting biologicals and debate possibilities how they could be overcome.

Roles of RIPK3 in necroptosis, cell signaling, and disease

Receptor-interacting protein kinase-3 (RIPK3, or RIP3) is an essential protein in the "programmed" and "regulated" cell death pathway called necroptosis. Necroptosis is activated by the death receptor ligands and pattern recognition receptors of the innate immune system, and the findings of many reports have suggested that necroptosis is highly significant in health and human disease. This significance is largely because necroptosis is distinguished from other modes of cell death, especially apoptosis, in that it is highly proinflammatory given that cell membrane integrity is lost, triggering the activation of the immune system and inflammation. Here, we discuss the roles of RIPK3 in cell signaling, along with its role in necroptosis and various pathways that trigger RIPK3 activation and cell death. Lastly, we consider pathological situations in which RIPK3/necroptosis may play a role.

Molecular cloning, functional characterization of duck TRADD and its effect on infection with duck Tembusu virus

Tumor necrosis factor receptor 1 (TNFR1) associated death domain protein (TRADD) is a pivotal adaptor in TNF signaling pathway and plays an important role in apoptosis and immune regulation. The function of TRADD has been investigated extensively in mammals, however, the role of TRADD in ducks remains obscure. To reveal the function of duck TRADD (duTRADD) in the apoptosis and innate immune response, the TRADD homologue of mallard (Anas platyrhynchos) has been cloned and the function of duTRADD is investigated in this study. We conducted sequence analysis of the duTRADD, the open reading frame (ORF) region of duTRADD gene was 1065 bp, encoding 354 amino acids (aa), which shares similar functional domain to its mammalian counterpart. Tissue distribution profile of duTRADD in 7-day-old ducklings showed that the expression level of the gene was the highest in heart, followed by liver and brain. Accordingly, duck Tembusu virus (DTMUV) has been shown to decrease duTRADD expression, while overexpression of duTRADD inhibited DTMUV replication in a dose-dependent manner. Furthermore, duTRADD activated the transcriptional activity of caspase-3/8/9, the flow cytometry showed that duTRADD significantly induced apoptosis. However, duTRADD showed hardly any effect on the transcriptional activity of IFN-α/β and its downstream interferon-stimulated genes (ISGs). The current data support the conclusion that duTRADD is a novel pro-apoptotic protein with a critical role in defense against DTMUV invasion. These results lay the theoretical foundation for the development of new anti-DTMUV strategies.

TRAIL protects the immature lung from hyperoxic injury

The hyperoxia-induced pro-inflammatory response and tissue damage constitute pivotal steps leading to bronchopulmonary dysplasia (BPD) in the immature lung. The pro-inflammatory cytokines are considered attractive candidates for a directed intervention but the complex interplay between inflammatory and developmental signaling pathways requires a comprehensive evaluation before introduction into clinical trials as studied here for the death inducing ligand TRAIL. At birth and during prolonged exposure to oxygen and mechanical ventilation, levels of TRAIL were lower in tracheal aspirates of preterm infants <29 weeks of gestation which developed moderate/severe BPD. These findings were reproduced in the newborn mouse model of hyperoxic injury. The loss of TRAIL was associated with increased inflammation, apoptosis induction and more pronounced lung structural simplification after hyperoxia exposure for 7 days while activation of NFκB signaling during exposure to hyperoxia was abrogated. Pretreatment with recombinant TRAIL rescued the developmental distortions in precision cut lung slices of both wildtype and TRAIL^-/- mice exposed to hyperoxia. Of importance, TRAIL preserved alveolar type II cells, mesenchymal progenitor cells and vascular endothelial cells. In the situation of TRAIL depletion, our data ascribe oxygen toxicity a more injurious impact on structural lung development. These data are not surprising taking into account the diverse functions of TRAIL and its stimulatory effects on NFκB signaling as central driver of survival and development. TRAIL exerts a protective role in the immature lung as observed for the death inducing ligand TNF-α before.

Extracellular vesicles of Fusobacterium nucleatum compromise intestinal barrier through targeting RIPK1-mediated cell death pathway

Microbial factors that mediate microbes-host interaction in ulcerative colitis (UC), a chronic disease seriously affecting human health, are not fully known. The emerging oncobacterium Fusobacterium nucleatum (Fn) secretes extracellular vesicles carrying several types of harmful molecules in the intestine which can alter microbes-host interaction, especially the epithelial homeostasis in UC. However, the mechanism is not yet clear. Previously, we isolated EVs by the ultracentrifugation of Fn culture media and characterized them as the potent inducer of pro-inflammatory cytokines. Here, we examined the mechanism in detail. We found that in macrophage/Caco-2 co-cultures, FnEVs significantly promoted epithelial barrier loss and oxidative stress damage, which are related to epithelial necroptosis caused by the activation of receptor-interacting protein kinase 1 (RIPK1) and receptor-interacting protein kinase 3 (RIPK3). Furthermore, FnEVs promoted the migration of RIPK1 and RIPK3 into necrosome in Caco2 cells. Notably, these effects were reversed by TNF-α neutralizing antibody or Necrostatin-1 (Nec-1), a RIPK1 inhibitor. This suggested that FADD-RIPK1-caspase-3 signaling is involved in the process. Moreover, the observed effects were verified in the murine colitis model treated with FnEVs or by adoptive transfer of FnEVs-trained macrophages. In conclusion, we propose that RIPK1-mediated epithelial cell death promotes FnEVs-induced gut barrier disruption in UC and the findings can be used as the basis to further investigate this disease.

SMAC Mimetics as Therapeutic Agents in HIV Infection

Although combination antiretroviral therapy is extremely effective in lowering HIV RNA to undetectable levels in the blood, HIV persists in latently infected CD4⁺ T-cells and persistently infected macrophages. In latently/persistently infected cells, HIV proteins have shown to affect the expression of proteins involved in the apoptosis pathway, notably the inhibitors of apoptosis proteins (IAPs), and thereby influence cell survival. IAPs, which are inhibited by endogenous second mitochondrial-derived activators of caspases (SMAC), can serve as targets for SMAC mimetics, synthetic compounds capable of inducing apoptosis. There is increasing evidence that SMAC mimetics can be used to reverse HIV latency and/or kill cells that are latently/persistently infected with HIV. Here, we review the current state of knowledge of SMAC mimetics as an approach to eliminate HIV infected cells and discuss the potential future use of SMAC mimetics as part of an HIV cure strategy.

Natural Peptides Inducing Cancer Cell Death: Mechanisms and Properties of Specific Candidates for Cancer Therapeutics

Nowadays, cancer has become the second highest leading cause of death, and it is expected to continue to affect the population in forthcoming years. Additionally, treatment options will become less accessible to the public as cases continue to grow and disease mechanisms expand. Hence, specific candidates with confirmed anticancer effects are required to develop new drugs. Among the novel therapeutic options, proteins are considered a relevant source, given that they have bioactive peptides encrypted within their sequences. These bioactive peptides, which are molecules consisting of 2–50 amino acids, have specific activities when administered, producing anticancer effects. Current databases report the effects of peptides. However, uncertainty is found when their molecular mechanisms are investigated. Furthermore, analyses addressing their interaction networks or their directly implicated mechanisms are needed to elucidate their effects on cancer cells entirely. Therefore, relevant peptides considered as candidates for cancer therapeutics with specific sequences and known anticancer mechanisms were accurately reviewed. Likewise, those features which turn certain peptides into candidates and the mechanisms by which peptides mediate tumor cell death were highlighted. This information will make robust the knowledge of these candidate peptides with recognized mechanisms and enhance their non-toxic capacity in relation to healthy cells and further avoid cell resistance.

Natural Peptides Inducing Cancer Cell Death: Mechanisms and Properties of Specific Candidates for Cancer Therapeutics

Nowadays, cancer has become the second highest leading cause of death, and it is expected to continue to affect the population in forthcoming years. Additionally, treatment options will become less accessible to the public as cases continue to grow and disease mechanisms expand. Hence, specific candidates with confirmed anticancer effects are required to develop new drugs. Among the novel therapeutic options, proteins are considered a relevant source, given that they have bioactive peptides encrypted within their sequences. These bioactive peptides, which are molecules consisting of 2–50 amino acids, have specific activities when administered, producing anticancer effects. Current databases report the effects of peptides. However, uncertainty is found when their molecular mechanisms are investigated. Furthermore, analyses addressing their interaction networks or their directly implicated mechanisms are needed to elucidate their effects on cancer cells entirely. Therefore, relevant peptides considered as candidates for cancer therapeutics with specific sequences and known anticancer mechanisms were accurately reviewed. Likewise, those features which turn certain peptides into candidates and the mechanisms by which peptides mediate tumor cell death were highlighted. This information will make robust the knowledge of these candidate peptides with recognized mechanisms and enhance their non-toxic capacity in relation to healthy cells and further avoid cell resistance.

A strategy of screening and binding analysis of bioactive components from traditional Chinese medicine based on surface plasmon resonance biosensor

Elucidating the active components of traditional Chinese medicine (TCM) is essential for understanding the mechanisms of TCM and promote its rational use as well as TCM-derived drug development. Recent studies have shown that surface plasmon resonance (SPR) technology is promising in this field. In the present study, we propose an SPR-based integrated strategy to screen and analyze the major active components of TCM. We used Radix Paeoniae Alba (RPA) as an example to identify the compounds that can account for its anti-inflammatory mechanism via tumor necrosis factor receptor type 1 (TNF-R1). First, RPA extraction was analyzed using an SPR-based screening system, and the potential active ingredients were collected, enriched, and identified as paeoniflorin and paeonol. Next, the affinity constants of paeoniflorin and paeonol were determined as 4.9 and 11.8 μM, respectively. Then, SPR-based competition assays and molecular docking were performed to show that the two compounds could compete with tumor necrosis factor-α (TNF-α) while binding to the subdomain 1 site of TNF-R1. Finally, in biological assays, the two compounds suppressed cytotoxicity and apoptosis induced by TNF-α in the L929 cell line. These findings prove that SPR technology is a useful tool for determining the active ingredients of TCM at the molecular level and can be used in various aspects of drug development. The SPR-based integrated strategy is reliable and feasible in TCM studies and will shed light on the elucidation of the pharmacological mechanism of TCM and facilitate its modernization.

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A surface plasmon resonance-based integrated strategy was established to analyze traditional Chinese medicine.

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Surface plasmon resonance technology can be used for ligand screening, affinity detection, and binding site confirmation.

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Paeoniflorin and paeonol were identified as TNF-R1-bound ingredients in RPA.

RIPK1 and TRADD Regulate TNF-Induced Signaling and Ripoptosome Formation

TNF is a proinflammatory cytokine that is critical for the coordination of tissue homeostasis. RIPK1 and TRADD are the main participants in the transduction of TNF signaling. However, data on the cell fate-controlling functions of both molecules are quite controversial. Here, we address the functions of RIPK1 and TRADD in TNF signaling by generating RIPK1- or TRADD-deficient human cell lines. We demonstrate that RIPK1 is relevant for TNF-induced apoptosis and necroptosis in conditions with depleted IAPs. In addition, TRADD is dispensable for necroptosis but required for apoptosis. We reveal a new possible function of TRADD as a negative regulator of NIK stabilization and subsequent ripoptosome formation. Furthermore, we show that RIPK1 and TRADD do not appear to be essential for the activation of MAPK signaling. Moreover, partially repressing NF-κB activation in both RIPK1 and TRADD KO cells does not result in sensitization to TNF alone due to the absence of NIK stabilization. Importantly, we demonstrate that RIPK1 is essential for preventing TRADD from undergoing TNF-induced ubiquitination and degradation. Taken together, our findings provide further insights into the specific functions of RIPK1 and TRADD in the regulation of TNF-dependent signaling, which controls the balance between cell death and survival.

Mechanism of interaction between autophagy and apoptosis in cancer

The mechanisms of two programmed cell death pathways, autophagy, and apoptosis, are extensively focused areas of research in the context of cancer. Both the catabolic pathways play a significant role in maintaining cellular as well as organismal homeostasis. Autophagy facilitates this by degradation and elimination of misfolded proteins and damaged organelles, while apoptosis induces canonical cell death in response to various stimuli. Ideally, both autophagy and apoptosis have a role in tumor suppression, as autophagy helps in eliminating the tumor cells, and apoptosis prevents their survival. However, as cancer proceeds, autophagy exhibits a dual role by enhancing cancer cell survival in response to stress conditions like hypoxia, thereby promoting chemoresistance to the tumor cells. Thus, any inadequacy in either of their levels can lead to tumor progression. A complex array of biomarkers is involved in maintaining coordination between the two by acting as either positive or negative regulators of one or both of these pathways of cell death. The resulting crosstalk between the two and its role in influencing the survival or death of malignant cells makes it quintessential, among other challenges facing chemotherapeutic treatment of cancer. In view of this, the present review aims to highlight some of the factors involved in maintaining their diaphony and stresses the importance of inhibition of cytoprotective autophagy and deletion of the intermediate pathways involved to facilitate tumor cell death. This will pave the way for future prospects in designing drug combinations facilitating the synergistic effect of autophagy and apoptosis in achieving cancer cell death.

Anti-Fn14 Antibody-Conjugated Nanoparticles Display Membrane TWEAK-Like Agonism

Conventional bivalent IgG antibodies targeting a subgroup of receptors of the TNF superfamily (TNFSF) including fibroblast growth factor-inducible 14 (anti-Fn14) typically display no or only very limited agonistic activity on their own and can only trigger receptor signaling by crosslinking or when bound to Fcγ receptors (FcγR). Both result in proximity of multiple antibody-bound TNFRSF receptor (TNFR) molecules, which enables engagement of TNFR-associated signaling pathways. Here, we have linked anti-Fn14 antibodies to gold nanoparticles to mimic the “activating” effect of plasma membrane-presented FcγR-anchored anti-Fn14 antibodies. We functionalized gold nanoparticles with poly-ethylene glycol (PEG) linkers and then coupled antibodies to the PEG surface of the nanoparticles. We found that Fn14 binding of the anti-Fn14 antibodies PDL192 and 5B6 is preserved upon attachment to the nanoparticles. More importantly, the gold nanoparticle-presented anti-Fn14 antibody molecules displayed strong agonistic activity. Our results suggest that conjugation of monoclonal anti-TNFR antibodies to gold nanoparticles can be exploited to uncover their latent agonism, e.g., for immunotherapeutic applications.

Membrane lymphotoxin-α2β is a novel tumor necrosis factor (TNF) receptor 2 (TNFR2) agonist

In the early 1990s, it has been described that LTα and LTβ form LTα₂β and LTαβ₂ heterotrimers, which bind to TNFR1 and LTβR, respectively. Afterwards, the LTαβ₂–LTβR system has been intensively studied while the LTα₂β–TNFR1 interaction has been ignored to date, presumably due to the fact that at the time of identification of the LTα₂β–TNFR1 interaction one knew already two ligands for TNFR1, namely TNF and LTα. Here, we show that LTα₂β interacts not only with TNFR1 but also with TNFR2. We furthermore demonstrate that membrane-bound LTα₂β (memLTα₂β), despite its asymmetric structure, stimulates TNFR1 and TNFR2 signaling. Not surprising in view of its ability to interact with TNFR2, LTα₂β is inhibited by Etanercept, which is approved for the treatment of rheumatoid arthritis and also inhibits TNF and LTα.

Regulation of distinct caspase-8 functions in retinal ganglion cells and astroglia in experimental glaucoma

Retinal ganglion cells (RGCs) expanding from the retina to the brain are primary victims of neurodegeneration in glaucoma, a leading cause of blindness; however, the neighboring astroglia survive the glaucoma-related stress and promote neuroinflammation. In light of diverse functions of caspase-8 in apoptosis, cell survival, and inflammation, this study investigated the importance of caspase-8 in different fates of glaucomatous RGCs and astroglia using two experimental approaches in parallel. In the first approach, cell type-specific responses of RGCs and astroglia to a caspase-8 cleavage-inhibiting pharmacological treatment were studied in rat eyes with or without experimentally induced glaucoma. The second approach utilized an experimental model of glaucoma in mice in which astroglial caspase-8 was conditionally deleted by cre/lox. Findings of these experiments revealed cell type-specific distinct processes that regulate caspase-8 functions in experimental glaucoma, which are involved in inducing the apoptosis of RGCs and promoting the survival and inflammatory responses of astroglia. Deletion of caspase-8 in astroglia protected RGCs against glia-driven inflammatory injury, while the inhibition of caspase-8 cleavage inhibited apoptosis in RGCs themselves. Various caspase-8 functions impacting both RGC apoptosis and astroglia-driven neuroinflammation may suggest the multi-target potential of caspase-8 regulation to provide neuroprotection and immunomodulation in glaucoma.

Reactive Oxygen Species: Drivers of Physiological and Pathological Processes

Since the Great Oxidation Event, about 2.4 billion years ago, the Earth is immersed in an oxidizing atmosphere. Thus, it has been proposed that excess oxygen, originally a waste product of photosynthetic cyanobacteria, induced oxidative stress and the production of reactive oxygen species (ROS), which have since acted as fundamental drivers of biologic evolution and eukaryogenesis. Indeed, throughout an organism’s lifespan, ROS affect directly (as mutagens) or indirectly (as messengers and regulators) all structural and functional components of cells, and many aspects of cell biology. Whether left unchecked by protective antioxidant systems, excess ROS not only cause genomic mutations but also induce irreversible oxidative modification of proteins (protein oxidation and peroxidation), lipids and glycans (advanced lipoxidation and glycation end products), impairing their function and promoting disease or cell death. Conversely, low-level local ROS play an important role both as redox-signaling molecules in a wide spectrum of pathways involved in the maintenance of cellular homeostasis (MAPK/ERK, PTK/PTP, PI3K-AKT-mTOR), and regulating key transcription factors (NFκB/IκB, Nrf2/KEAP1, AP-1, p53, HIF-1). Consequently, ROS can shape a variety of cellular functions, including proliferation, differentiation, migration and apoptosis. In this review, we will give a brief overview of the relevance of ROS in both physiological and pathological processes, particularly inflammation and aging. In-depth knowledge of the molecular mechanisms of ROS actuation and their influence under steady-state and stressful conditions will pave the way for the development of novel therapeutic interventions. This will mitigate the harmful outcomes of ROS in the onset and progression of a variety of chronic inflammatory and age-related diseases.

The multifaceted role of TRAIL signaling in cancer and immunity

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is a member of the TNF superfamily that can lead to the induction of apoptosis in tumor or infected cells. However, activation of TRAIL signaling may also trigger nonapoptotic pathways in cancer and in nontransformed cells, that is, immune cells. Here, we review the current knowledge on noncanonical TRAIL signaling. The biological outcomes of TRAIL signaling in immune and malignant cells are presented and explained, with a focus on the role of TRAIL for natural killer (NK) cell function. Furthermore, we highlight the technical difficulties in dissecting the precise molecular mechanisms involved in the switch between apoptotic and nonapoptotic TRAIL signaling. Finally, we discuss the consequences thereof for a therapeutic manipulation of TRAIL in cancer and possible approaches to bypass these difficulties.

Functional roles in cell signaling of adaptor protein TRADD from a structural perspective

TRADD participates in various receptor signaling pathways and plays vital roles in many biological activities, including cell survival and apoptosis, in different cellular contexts. TRADD has two distinct functional domains, a TRAF-binding domain at the N-terminus and a death domain (DD) at the C-terminus. The TRAF binding domain of TRADD folds into an α-β plait topology and is mainly responsible for binding TRAF2, while the TRADD-DD can interact with a variety of DD-containing proteins, including receptors and intracellular signaling molecules. After activation of specific receptors such as TNFR1 and DR3, TRADD can bind to the receptor through DD-DD interaction, creating a membrane-proximal platform for the recruitment of downstream molecules to propagate cellular signals. In this review, we highlight recent advances in the studies of the structural mechanism of TRADD adaptor functions for NF-κB activation and apoptosis induction. We also provide suggestions for future structure research related to TRADD-mediated signaling pathways.

Arg-GlcNAcylation on TRADD by NleB and SseK1 Is Crucial for Bacterial Pathogenesis

Death receptor signaling is critical for cell death, inflammation, and immune homeostasis. Hijacking death receptors and their corresponding adaptors through type III secretion system (T3SS) effectors has been evolved to be a bacterial evasion strategy. NleB from enteropathogenic Escherichia coli (EPEC) and SseK1/2/3 from Salmonella enterica serovar Typhimurium (S. Typhimurium) can modify some death domain (DD) proteins through arginine-GlcNAcylation. Here, we performed a substrate screen on 12 host DD proteins with conserved arginine during EPEC and Salmonella infection. NleB from EPEC hijacked death receptor signaling through tumor necrosis factor receptor 1 (TNFR1)-associated death domain protein (TRADD), FAS-associated death domain protein (FADD), and receptor-interacting serine/threonine-protein kinase 1 (RIPK1), whereas SseK1 and SseK3 disturbed TNF signaling through the modification of TRADD Arg235/Arg245 and TNFR1 Arg376, respectively. Furthermore, mouse infection studies showed that SseK1 but not SseK3 rescued the bacterial colonization deficiency contributed by the deletion of NleBc (Citrobacter NleB), indicating that TRADD was the in vivo substrate. The result provides an insight into the mechanism by which attaching and effacing (A/E) pathogen manipulate TRADD-mediated signaling and evade host immune defense through T3SS effectors.

Auto Arginine-GlcNAcylation Is Crucial for Bacterial Pathogens in Regulating Host Cell Death

Many Gram-negative bacterial pathogens utilize the type III secretion system (T3SS) to inject virulence factors, named effectors, into host cells. These T3SS effectors manipulate host cellular signaling pathways to facilitate bacterial pathogenesis. Death receptor signaling plays an important role in eukaryotic cell death pathways. NleB from enteropathogenic Escherichia coli (EPEC) and SseK1/3 from Salmonella enterica serovar Typhimurium (S. Typhimurium) are T3SS effectors. They are defined as a family of arginine GlcNAc transferase to modify a conserved arginine residue in the death domain (DD) of the death receptor TNFR and their corresponding adaptors to hijack death receptor signaling. Here we identified that these enzymes, NleB, SseK1, and SseK3 could catalyze auto-GlcNAcylation. Residues, including Arg13/53/159/293 in NleB, Arg30/158/339 in SseK1, and Arg153/184/305/335 in SseK3 were identified as the auto-GlcNAcylation sites by mass spectrometry. Mutation of the auto-modification sites of NleB, SseK1, and SseK3 abolished or attenuated the capability of enzyme activity toward their death domain targets during infection. Loss of this ability led to the increased susceptibility of the cells to TNF- or TRAIL-induced cell death during bacterial infection. Overall, our study reveals that the auto-GlcNAcylation of NleB, SseK1, and SseK3 is crucial for their biological activity during infection.

Stress Management: Death Receptor Signalling and Cross-Talks with the Unfolded Protein Response in Cancer

Throughout tumour progression, tumour cells are exposed to various intense cellular stress conditions owing to intrinsic and extrinsic cues, to which some cells are remarkably able to adapt. Death Receptor (DR) signalling and the Unfolded Protein Response (UPR) are two stress responses that both regulate a plethora of outcomes, ranging from proliferation, differentiation, migration, cytokine production to the induction of cell death. Both signallings are major modulators of physiological tissue homeostasis and their dysregulation is involved in tumorigenesis and the metastastic process. The molecular determinants of the control between the different cellular outcomes induced by DR signalling and the UPR in tumour cells and their stroma and their consequences on tumorigenesis are starting to be unravelled. Herein, I summarize the main steps of DR signalling in relation to its cellular and pathophysiological roles in cancer. I then highlight how the UPR and DR signalling control common cellular outcomes and also cross-talk, providing potential opportunities to further understand the development of malignancies.

Die Another Way: Interplay between Influenza A Virus, Inflammation and Cell Death

Influenza A virus (IAV) is a major concern to human health due to the ongoing global threat of a pandemic. Inflammatory and cell death signalling pathways play important roles in host defence against IAV infection. However, severe IAV infections in humans are characterised by excessive inflammation and tissue damage, often leading to fatal disease. While the molecular mechanisms involved in the induction of inflammation during IAV infection have been well studied, the pathways involved in IAV-induced cell death and their impact on immunopathology have not been fully elucidated. There is increasing evidence of significant crosstalk between cell death and inflammatory pathways and a greater understanding of their role in host defence and disease may facilitate the design of new treatments for IAV infection.

Future Therapeutic Directions for Smac-Mimetics

It is well accepted that the ability of cancer cells to circumvent the cell death program that untransformed cells are subject to helps promote tumor growth. Strategies designed to reinstate the cell death program in cancer cells have therefore been investigated for decades. Overexpression of members of the Inhibitor of APoptosis (IAP) protein family is one possible mechanism hindering the death of cancer cells. To promote cell death, drugs that mimic natural IAP antagonists, such as second mitochondria-derived activator of caspases (Smac/DIABLO) were developed. Smac-Mimetics (SMs) have entered clinical trials for hematological and solid cancers, unfortunately with variable and limited results so far. This review explores the use of SMs for the treatment of cancer, their potential to synergize with up-coming treatments and, finally, discusses the challenges and optimism facing this strategy.

TRAF2 Controls Death Receptor-Induced Caspase-8 Processing and Facilitates Proinflammatory Signaling

Tumor necrosis factor (TNF) receptor associated factor-2 (TRAF2) knockout (KO) cells were generated to investigate the role of TRAF2 in signaling by TNFR1 and the CD95-type death receptors (DRs) TRAILR1/2 and CD95. To prevent negative selection effects arising from the increased cell death sensitivity of TRAF2-deficient cells, cell lines were used for the generation of the TRAF2 KO variants that were protected from DR-induced apoptosis downstream of caspase-8 activation. As already described in the literature, TRAF2 KO cells displayed enhanced constitutive alternative NFκB signaling and reduced TNFR1-induced activation of the classical NFκB pathway. There was furthermore a significant but only partial reduction in CD95-type DR-induced upregulation of the proinflammatory NFκB-regulated cytokine interleukin-8 (IL8), which could be reversed by reexpression of TRAF2. In contrast, expression of the TRAF2-related TRAF1 protein failed to functionally restore TRAF2 deficiency. TRAF2 deficiency resulted furthermore in enhanced procaspase-8 processing by DRs, but this surprisingly came along with a reduction in net caspase-8 activity. In sum, our data argue for (i) a non-obligate promoting function of TRAF2 in proinflammatory DR signaling and (ii) a yet unrecognized stabilizing effect of TRAF2 on caspase-8 activity.

TNFR1 and TNFR2 in the Control of the Life and Death Balance of Macrophages

Macrophages stand in the first line of defense against a variety of pathogens but are also involved in the maintenance of tissue homeostasis. To fulfill their functions macrophages sense a broad range of pathogen- and damage-associated molecular patterns (PAMPs/DAMPs) by plasma membrane and intracellular pattern recognition receptors (PRRs). Intriguingly, the overwhelming majority of PPRs trigger the production of the pleiotropic cytokine tumor necrosis factor-alpha (TNF). TNF affects almost any type of cell including macrophages themselves. TNF promotes the inflammatory activity of macrophages but also controls macrophage survival and death. TNF exerts its activities by stimulation of two different types of receptors, TNF receptor-1 (TNFR1) and TNFR2, which are both expressed by macrophages. The two TNF receptor types trigger distinct and common signaling pathways that can work in an interconnected manner. Based on a brief general description of major TNF receptor-associated signaling pathways, we focus in this review on research of recent years that revealed insights into the molecular mechanisms how the TNFR1-TNFR2 signaling network controls the life and death balance of macrophages. In particular, we discuss how the TNFR1-TNFR2 signaling network is integrated into PRR signaling.

c-FLIP, a Novel Biomarker for Cancer Prognosis, Immunosuppression, Alzheimer's Disease, Chronic Obstructive Pulmonary Disease (COPD), and a Rationale Therapeutic Target

Dysregulation of c-FLIP (cellular FADD-like IL-1β-converting enzyme inhibitory protein) has been shown in several diseases including cancer, Alzheimer's disease, and chronic obstructive pulmonary disease (COPD). c-FLIP is a critical anti-cell death protein often overexpressed in tumors and hematological malignancies and its increased expression is often associated with a poor prognosis. c-FLIP frequently exists as long (c-FLIPL) and short (c-FLIPS) isoforms, regulates its anti-cell death functions through binding to FADD (FAS associated death domain protein), an adaptor protein known to activate caspases-8 and -10 and links c-FLIP to several cell death regulating complexes including the death-inducing signaling complex (DISC) formed by various death receptors. c-FLIP also plays a critical role in necroptosis and autophagy. Furthermore, c-FLIP is able to activate several pathways involved in cytoprotection, proliferation, and survival of cancer cells through various critical signaling proteins. Additionally, c-FLIP can inhibit cell death induced by several chemotherapeutics, anti-cancer small molecule inhibitors, and ionizing radiation. Moreover, c-FLIP plays major roles in aiding the survival of immunosuppressive tumor-promoting immune cells and functions in inflammation, Alzheimer's disease (AD), and chronic obstructive pulmonary disease (COPD). Therefore, c-FLIP can serve as a versatile biomarker for cancer prognosis, a diagnostic marker for several diseases, and an effective therapeutic target. In this article, we review the functions of c-FLIP as an anti-apoptotic protein and negative prognostic factor in human cancers, and its roles in resistance to anticancer drugs, necroptosis and autophagy, immunosuppression, Alzheimer's disease, and COPD.