NFkappaB activation by Fas is mediated through FADD, caspase-8, and RIP and is inhibited by FLIP

Catalase inhibition can modulate the ability of peripheral blood T cells to undergo apoptosis in Crohn's disease

Crohn's disease (CD) is a chronic relapsing inflammatory disorder in which defective apoptosis of mucosal T cells is postulated to produce sustained inflammation and reactive oxygen species accumulation. Whether CD T cells are intrinsically resistant to apoptosis or whether this resistance is acquired at the intestinal site needs to be clarified, as the cellular mechanisms modulate the impaired apoptosis in these cells. Here, we analysed peripheral blood T cells from patients naïve to specific CD treatment at the onset and from healthy controls. Non-activated freshly purified lymphocytes were cultured and submitted to in vitro protocols for activation (CD3/CD28 antibodies) and apoptosis (Fas antibody). Cells were analysed by flow cytometry. Caspases (3, 8, and 9) and catalase activity were measured; protein levels of bax, Bcl-2, and NF-kB were detected by western blotting, and cytokines by Luminex-based assays. The results showed that CD4 T cells from CD patients are less prone to apoptosis before they can migrate to the intestinal mucosa. Caspase-9, FasR, sIL-2Rα, IL-17A, IFNγ, IL-6, TNF-α, and IL-10 were shown to be significantly different in CD but not for the rest of the analysed biological elements. Catalase activity was significantly reduced in CD T cells, which was confirmed in ex vivo experiments in which catalase inhibition in T cells from healthy controls triggered apoptosis inhibition in a dose-dependent manner. In conclusion, apoptosis inhibition of CD T cells is a feature of these cells before they can migrate to the intestinal mucosa. Noteworthy, the impaired apoptosis of T cells can be directly influenced by catalase inhibition.

Interleukin-10 induces TNF-driven apoptosis and ROS production in salivary gland cancer cells

Treatment resistance after chemo-/immunotherapy occurs in patients with head and neck squamous cell cancers (HNSCs), including salivary gland cancers (SGCs). Interleukin-10 (IL-10), a cytokine with pro- and anti-cancer effects, has an unclear impact on HNSC/SGC cells. We show that HNSC patients exhibiting high expression of IL-10 and its receptor IL-10Rα experience have prolonged overall survival. Immunoreactive IL-10 was low in ductal cells of human SGC biopsies. Human (A253) and murine WR21-SGC cells expressed IL-10Rβ, but only A253 cells expressed IL-10 and IL-10Rα. The addition of recombinant IL-10 impaired SGC cell proliferation and induced apoptosis in vitro. N-acetylcysteine restored IL-10-induced reactive oxygen species (ROS) production but did not prevent IL-10-mediated viability loss. Mechanistically, recIL-10 delayed cell cycle progression from G0/G1 to the S phase with cyclin D downregulation and upregulation of NF-kB. IL-10 increased tumor necrosis factor-α (TNF-α) in A253 and WR21 and FasL in WR21 cells. Neutralizing antibodies against TNF-α and NF-kB inhibition restored SGC proliferation after IL-10 treatment, emphasizing the critical role of TNF-α and NF-kB in IL-10-mediated anti-tumor effects. These findings underscore the potential of IL-10 to impede SGC cell growth through apoptosis induction, unraveling potential therapeutic targets for intervention in salivary gland carcinomas.

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.

Membrane-bound CD95 ligand modulates CD19-mediated B cell receptor signaling and EBV activation

Post-transplant lymphoproliferative disorders (PTLDs) are associated with Epstein-Barr virus (EBV) infection in transplant recipients. Most of lymphoblastoid cell lines (LCLs) derived from EBV-immortalized B cells or PTLDs are sensitive to CD95-mediated apoptosis and cytotoxic T cell (CTL) killing. CD95 ligand (CD95L) exists as a transmembrane ligand (mCD95L) or a soluble form (sCD95L). Using recombinant mCD95L and sCD95L, we observed that sCD95L does not affect LCLs. While high expression of mCD95L in CTLs promotes apoptosis of LCLs, low expression induces clathrin-dependent CD19 internalization, caspase-dependent CD19 cleavage, and proteasomal/lysosomal-dependent CD19 degradation. The CD95L/CD95-mediated CD19 degradation impairs B cell receptor (BCR) signaling and inhibits BCR-mediated EBV activation. Interestingly, although inhibition of the caspase activity restores CD19 expression and CD19-mediated BCR activation, it fails to rescue BCR-mediated EBV lytic gene expression. EBV-specific CTLs engineered to overexpress mCD95L exhibit a stronger killing activity against LCLs. This study highlights that engineering EBV-specific CTLs to express a higher level of mCD95L could represent an attractive therapeutic approach to improve T cell immunotherapy for PTLDs.

Control of Cell Death in Health and Disease

Apoptosis, necroptosis, and pyroptosis are genetically programmed cell death mechanisms that eliminate obsolete, damaged, infected, and self-reactive cells. Apoptosis fragments cells in a manner that limits immune cell activation, whereas the lytic death programs of necroptosis and pyroptosis release proinflammatory intracellular contents. Apoptosis fine-tunes tissue architecture during mammalian development, promotes tissue homeostasis, and is crucial for averting cancer and autoimmunity. All three cell death mechanisms are deployed to thwart the spread of pathogens. Disabling regulators of cell death signaling in mice has revealed how excessive cell death can fuel acute or chronic inflammation. Here we review strategies for modulating cell death in the context of disease. For example, BCL-2 inhibitor venetoclax, an inducer of apoptosis, is approved for the treatment of certain hematologic malignancies. By contrast, inhibition of RIPK1, NLRP3, GSDMD, or NINJ1 to limit proinflammatory cell death and/or the release of large proinflammatory molecules from dying cells may benefit patients with inflammatory diseases.

Regulated cell death in glioma: promising targets for natural small-molecule compounds

Gliomas are prevalent malignant tumors in adults, which can be categorized as either localized or diffuse gliomas. Glioblastoma is the most aggressive and deadliest form of glioma. Currently, there is no complete cure, and the median survival time is less than one year. The main mechanism of regulated cell death involves organisms coordinating the elimination of damaged cells at risk of tumor transformation or cells hijacked by microorganisms for pathogen replication. This process includes apoptosis, necroptosis, autophagy, ferroptosis, pyroptosis, necrosis, parthanayosis, entosis, lysosome-dependent death, NETosis, oxiptosis, alkaliptosis, and disulfidaptosis. The main goal of clinical oncology is to develop therapies that promote the effective elimination of cancer cells by regulating cell death are the main goal of clinical oncology. Recently, scientists have utilized pertinent regulatory factors and natural small-molecule compounds to induce regulated cell death for the treatment of gliomas. By analyzing the PubMed and Web of Science databases, this paper reviews the research progress on the regulation of cell death and the role of natural small-molecule compounds in glioma. The aim is to provide help for the treatment of glioblastoma.

Novel PI(3)K-p85α/p110δ-ITK-LAT-PLC-γ2 and Fyn-ADAP-Carma1-TAK1 Pathways Define Reverse Signaling via FasL

The role of FasL in initiating death signals through Fas is well characterized. However, the reverse signaling pathway downstream of FasL in effector lymphocytes is poorly understood. Here, we identify that FasL functions as an independent activation receptor in NK cells. Activation via FasL results in the production of LFN-γ, GM-CSF, RANTES, MIP-1α, and MIP1-β. Proximal signaling of FasL requires Lck and Fyn. Upon activation, FasL facilitates the phosphorylation of PI(3)K-p85α/p55α subunits. A catalytically inactive PI(3)K-p110δD910A mutation significantly impairs the cytokine and chemokine production by FasL. Activation of ITK and LAT downstream of FasL plays a central role in recruiting and phosphorylating PLC-γ2. Importantly, Fyn-mediated recruitment of ADAP links FasL to the Carmal/ Bcl10/Tak1 signalosome. Lack of Carma1, CARD domain of Carma1, or Tak1 significantly reduces FasL-mediated cytokine and chemokine production. These findings, for the first time, provide a detailed molecular blueprint that defines FasL-mediated reverse signaling.

cFLIPL acts as a suppressor of TRAIL- and Fas-initiated inflammation by inhibiting assembly of caspase-8/FADD/RIPK1 NF-κB-activating complexes

TRAIL and FasL are potent inducers of apoptosis but can also promote inflammation through assembly of cytoplasmic caspase-8/FADD/RIPK1 (FADDosome) complexes, wherein caspase-8 acts as a scaffold to drive FADD/RIPK1-mediated nuclear factor κB (NF-κB) activation. cFLIP is also recruited to FADDosomes and restricts caspase-8 activity and apoptosis, but whether cFLIP also regulates death receptor-initiated inflammation is unclear. Here, we show that silencing or deletion of cFLIP leads to robustly enhanced Fas-, TRAIL-, or TLR3-induced inflammatory cytokine production, which can be uncoupled from the effects of cFLIP on caspase-8 activation and apoptosis. Mechanistically, cFLIPL suppresses Fas- or TRAIL-initiated NF-κB activation through inhibiting the assembly of caspase-8/FADD/RIPK1 FADDosome complexes, due to the low affinity of cFLIPL for FADD. Consequently, increased cFLIPL occupancy of FADDosomes diminishes recruitment of FADD/RIPK1 to caspase-8, thereby suppressing NF-κB activation and inflammatory cytokine production downstream. Thus, cFLIP acts as a dual suppressor of apoptosis and inflammation via distinct modes of action.

Single-cell functional genomics reveals determinants of sensitivity and resistance to natural killer cells in blood cancers

Cancer cells can evade natural killer (NK) cell activity, thereby limiting anti-tumor immunity. To reveal genetic determinants of susceptibility to NK cell activity, we examined interacting NK cells and blood cancer cells using single-cell and genome-scale functional genomics screens. Interaction of NK and cancer cells induced distinct activation and type I interferon (IFN) states in both cell types depending on the cancer cell lineage and molecular phenotype, ranging from more sensitive myeloid to less sensitive B-lymphoid cancers. CRISPR screens in cancer cells uncovered genes regulating sensitivity and resistance to NK cell-mediated killing, including adhesion-related glycoproteins, protein fucosylation genes, and transcriptional regulators, in addition to confirming the importance of antigen presentation and death receptor signaling pathways. CRISPR screens with a single-cell transcriptomic readout provided insight into underlying mechanisms, including regulation of IFN-γ signaling in cancer cells and NK cell activation states. Our findings highlight the diversity of mechanisms influencing NK cell susceptibility across different cancers and provide a resource for NK cell-based therapies.

Killer instincts: natural killer cells as multifactorial cancer immunotherapy

Natural killer (NK) cells integrate heterogeneous signals for activation and inhibition using germline-encoded receptors. These receptors are stochastically co-expressed, and their concurrent engagement and signaling can adjust the sensitivity of individual cells to putative targets. Against cancers, which mutate and evolve under therapeutic and immunologic pressure, the diversity for recognition provided by NK cells may be key to comprehensive cancer control. NK cells are already being trialled as adoptive cell therapy and targets for immunotherapeutic agents. However, strategies to leverage their naturally occurring diversity and agility have not yet been developed. In this review, we discuss the receptors and signaling pathways through which signals for activation or inhibition are generated in NK cells, focusing on their roles in cancer and potential as targets for immunotherapies. Finally, we consider the impacts of receptor co-expression and the potential to engage multiple pathways of NK cell reactivity to maximize the scope and strength of antitumor activities.

How does caspases regulation play role in cell decisions? apoptosis and beyond

Caspases are a family of cysteine proteases, and the key factors behind the cellular events which occur during apoptosis and inflammation. However, increasing evidence shows the non-conventional pro-survival action of apoptotic caspases in crucial processes. These cellular events include cell proliferation, differentiation, and migration, which may appear in the form of metastasis, and chemotherapy resistance in cancerous situations. Therefore, there should be a precise and strict control of caspases activity, perhaps through maintaining the threshold below the required levels for apoptosis. Thus, understanding the regulators of caspase activities that render apoptotic caspases as non-apoptotic is of paramount importance both mechanistically and clinically. Furthermore, the functions of apoptotic caspases are affected by numerous post-translational modifications. In the present mini-review, we highlight the various mechanisms that directly impact caspases with respect to their anti- or non-apoptotic functions. In this regard, post-translational modifications (PTMs), isoforms, subcellular localization, transient activity, substrate availability, substrate selection, and interaction-mediated regulations are discussed.

The role of caspase-8 in inflammatory signalling and pyroptotic cell death

The programmed cell death machinery exhibits surprising flexibility, capable of crosstalk and non-apoptotic roles. Much of this complexity arises from the diverse functions of caspase-8, a cysteine-aspartic acid protease typically associated with activating caspase-3 and - 7 to induce apoptosis. However, recent research has revealed that caspase-8 also plays a role in regulating the lytic gasdermin cell death machinery, contributing to pyroptosis and immune responses in contexts such as infection, autoinflammation, and T-cell signalling. In mice, loss of caspase-8 results in embryonic lethality from unrestrained necroptotic killing, while in humans caspase-8 deficiency can lead to an autoimmune lymphoproliferative syndrome, immunodeficiency, inflammatory bowel disease or, when it can't cleave its substrate RIPK1, early onset periodic fevers. This review focuses on non-canonical caspase-8 signalling that drives immune responses, including its regulation of inflammatory gene transcription, activation within inflammasome complexes, and roles in pyroptotic cell death. Ultimately, a deeper understanding of caspase-8 function will aid in determining whether, and when, targeting caspase-8 pathways could be therapeutically beneficial in human diseases.

The complex web of FasL: cell type-specific roles in affecting and controlling acute graft-vs-host disease

FasL has divergent roles in both causing graft-vs-host disease and preventing this condition, which depends on the immune cell type that expresses it.

The dual role of the CD95 and CD95L signaling pathway in glioblastoma

Binding of CD95, a cell surface death receptor, to its homologous ligand CD95L, transduces a cascade of downstream signals leading to apoptosis crucial for immune homeostasis and immune surveillance. Although CD95 and CD95L binding classically induces programmed cell death, most tumor cells show resistance to CD95L-induced apoptosis. In some cancers, such as glioblastoma, CD95-CD95L binding can exhibit paradoxical functions that promote tumor growth by inducing inflammation, regulating immune cell homeostasis, and/or promoting cell survival, proliferation, migration, and maintenance of the stemness of cancer cells. In this review, potential mechanisms such as the expression of apoptotic inhibitor proteins, decreased activity of downstream elements, production of nonapoptotic soluble CD95L, and non-apoptotic signals that replace apoptotic signals in cancer cells are summarized. CD95L is also expressed by other types of cells, such as endothelial cells, polymorphonuclear myeloid-derived suppressor cells, cancer-associated fibroblasts, and tumor-associated microglia, and macrophages, which are educated by the tumor microenvironment and can induce apoptosis of tumor-infiltrating lymphocytes, which recognize and kill cancer cells. The dual role of the CD95-CD95L system makes targeted therapy strategies against CD95 or CD95L in glioblastoma difficult and controversial. In this review, we also discuss the current status and perspective of clinical trials on glioblastoma based on the CD95-CD95L signaling pathway.

CD95 gene deletion may reduce clonogenic growth and invasiveness of human glioblastoma cells in a CD95 ligand-independent manner

CD95 (Fas/APO-1) is a multifunctional cell surface receptor with antithetic roles. First described to mediate cell death, interactions of CD95 with its natural ligand, CD95L, have also been described to induce tumor-promoting signaling leading to proliferation, invasion and stem cell maintenance, mainly in cancer cells that are resistant to CD95-mediated apoptosis. While activation of CD95-mediated apoptosis in cancer cells may not be clinically practicable due to toxicity, inhibition of tumor-promoting CD95 signaling holds therapeutic potential. In the present study, we characterized CD95 and CD95L expression in human glioma-initiating cells (GIC), a glioblastoma cell population with stem cell features, and investigated the consequences of CRISPR-Cas9-mediated CD95 or CD95L gene deletion. In vitro, GIC expressed CD95 but not CD95L and were sensitive to CD95-mediated apoptosis. Upon genetic deletion of CD95, GIC acquired resistance to CD95L-induced apoptosis but exhibited inferior clonogenic growth, sphere-forming capacity, and invasiveness compared with control cells, suggesting the existence of CD95L-independent constitutive CD95 signaling with tumor-promoting properties in GIC. In vivo, GIC expressed CD95 and a non-canonical form of CD95L lacking the CD95-binding region. CD95 genetic deletion did not prolong survival in immunocompromised GIC-bearing mice. Altogether, these data indicate that canonical CD95L may not be expressed in human GIC and suggest the existence of a CD95L-independent CD95-signaling pathway that maintains some malignancy traits of GIC. The lack of altered survival of tumor-bearing mice after genetic deletion of CD95 suggests that CD95 signaling is not essential to maintain the growth of human GIC xenografted into the brains of nude mice. The ligand-independent tumor-promoting role of constitutive CD95 in our GIC models in vitro highlights the complexity and challenges associated with targeting CD95 with therapeutic intent.

A TNFR2-Specific TNF Fusion Protein With Improved In Vivo Activity

Tumor necrosis factor (TNF) receptor-2 (TNFR2) has attracted considerable interest as a target for immunotherapy. Indeed, using oligomeric fusion proteins of single chain-encoded TNFR2-specific TNF mutants (scTNF80), expansion of regulatory T cells and therapeutic activity could be demonstrated in various autoinflammatory diseases, including graft-versus-host disease (GvHD), experimental autoimmune encephalomyelitis (EAE) and collagen-induced arthritis (CIA). With the aim to improve the in vivo availability of TNFR2-specific TNF fusion proteins, we used here the neonatal Fc receptor (FcRn)-interacting IgG1 molecule as an oligomerizing building block and generated a new TNFR2 agonist with improved serum retention and superior in vivo activity.

Methods

Single-chain encoded murine TNF80 trimers (sc(mu)TNF80) were fused to the C-terminus of an in mice irrelevant IgG1 molecule carrying the N297A mutation which avoids/minimizes interaction with Fcγ-receptors (FcγRs). The fusion protein obtained (irrIgG1(N297A)-sc(mu)TNF80), termed NewSTAR2 (New selective TNF-based agonist of TNF receptor 2), was analyzed with respect to activity, productivity, serum retention and in vitro and in vivo activity. STAR2 (TNC-sc(mu)TNF80 or selective TNF-based agonist of TNF receptor 2), a well-established highly active nonameric TNFR2-specific variant, served as benchmark. NewSTAR2 was assessed in various in vitro and in vivo systems.

Results

STAR2 (TNC-sc(mu)TNF80) and NewSTAR2 (irrIgG1(N297A)-sc(mu)TNF80) revealed comparable in vitro activity. The novel domain architecture of NewSTAR2 significantly improved serum retention compared to STAR2, which correlated with efficient binding to FcRn. A single injection of NewSTAR2 enhanced regulatory T cell (Treg) suppressive activity and increased Treg numbers by > 300% in vivo 5 days after treatment. Treg numbers remained as high as 200% for about 10 days. Furthermore, a single in vivo treatment with NewSTAR2 upregulated the adenosine-regulating ectoenzyme CD39 and other activation markers on Tregs. TNFR2-stimulated Tregs proved to be more suppressive than unstimulated Tregs, reducing conventional T cell (Tcon) proliferation and expression of activation markers in vitro. Finally, singular preemptive NewSTAR2 administration five days before allogeneic hematopoietic cell transplantation (allo-HCT) protected mice from acute GvHD.

Conclusions

NewSTAR2 represents a next generation ligand-based TNFR2 agonist, which is efficiently produced, exhibits improved pharmacokinetic properties and high serum retention with superior in vivo activity exerting powerful protective effects against acute GvHD.

Novel NFkB mutation in a case of lymphoproliferative disorder case report

BACKGROUND

Lymphoproliferative disorders include a heterogeneous list of disorders that commonly involve dysregulation of lymphocyte proliferation resulting in lymphadenopathy and bone marrow infiltration. These disorders have various presentations, most notably autoimmune manifestations, organomegaly, lymphadenopathy, dysgammaglobulinemia, and increased risk of chronic infections.

CASE PRESENTATION

A young boy presented with symptoms overlapping different lymphoproliferative disorders including episodes of chronic respiratory tract infections, dysgammaglobulinemia, lymphadenopathy associated with splenomegaly as well as skin rashes. Genetic studies revealed multiple heterozygous variants including a novel mutation in NFκB1 gene.

CONCLUSION

This novel mutation can reveal new aspects in the pathogenesis of lymphoproliferative disorders and propose new treatments for them.

Therapeutic approaches targeting CD95L/CD95 signaling in cancer and autoimmune diseases

Cell death plays a pivotal role in the maintenance of tissue homeostasis. Key players in the controlled induction of cell death are the Death Receptors (DR). CD95 is a prototypic DR activated by its cognate ligand CD95L triggering programmed cell death. As a consequence, alterations in the CD95/CD95L pathway have been involved in several disease conditions ranging from autoimmune diseases to inflammation and cancer. CD95L-induced cell death has multiple roles in the immune response since it constitutes one of the mechanisms by which cytotoxic lymphocytes kill their targets, but it is also involved in the process of turning off the immune response. Furthermore, beyond the canonical pro-death signals, CD95L, which can be membrane-bound or soluble, also induces non-apoptotic signaling that contributes to its tumor-promoting and pro-inflammatory roles. The intent of this review is to describe the role of CD95/CD95L in the pathophysiology of cancers, autoimmune diseases and chronic inflammation and to discuss recently patented and emerging therapeutic strategies that exploit/block the CD95/CD95L system in these diseases.

Interferon-γ primes macrophages for pathogen ligand-induced killing via a caspase-8 and mitochondrial cell death pathway

Cell death plays an important role during pathogen infections. Here, we report that interferon-γ (IFNγ) sensitizes macrophages to Toll-like receptor (TLR)-induced death that requires macrophage-intrinsic death ligands and caspase-8 enzymatic activity, which trigger the mitochondrial apoptotic effectors, BAX and BAK. The pro-apoptotic caspase-8 substrate BID was dispensable for BAX and BAK activation. Instead, caspase-8 reduced pro-survival BCL-2 transcription and increased inducible nitric oxide synthase (iNOS), thus facilitating BAX and BAK signaling. IFNγ-primed, TLR-induced macrophage killing required iNOS, which licensed apoptotic caspase-8 activity and reduced the BAX and BAK inhibitors, A1 and MCL-1. The deletion of iNOS or caspase-8 limited SARS-CoV-2-induced disease in mice, while caspase-8 caused lethality independent of iNOS in a model of hemophagocytic lymphohistiocytosis. These findings reveal that iNOS selectively licenses programmed cell death, which may explain how nitric oxide impacts disease severity in SARS-CoV-2 infection and other iNOS-associated inflammatory conditions.

The role of Ubiquitination in Apoptosis and Necroptosis

Cell death pathways have evolved to maintain tissue homoeostasis and eliminate potentially harmful cells from within an organism, such as cells with damaged DNA that could lead to cancer. Apoptosis, known to eliminate cells in a predominantly non-inflammatory manner, is controlled by two main branches, the intrinsic and extrinsic apoptotic pathways. While the intrinsic pathway is regulated by the Bcl-2 family members, the extrinsic pathway is controlled by the Death receptors, members of the tumour necrosis factor (TNF) receptor superfamily. Death receptors can also activate a pro-inflammatory type of cell death, necroptosis, when Caspase-8 is inhibited. Apoptotic pathways are known to be tightly regulated by post-translational modifications, especially by ubiquitination. This review discusses research on ubiquitination-mediated regulation of apoptotic signalling. Additionally, the emerging importance of ubiquitination in regulating necroptosis is discussed.

Non-apoptotic function of caspase-8 confers prostate cancer enzalutamide resistance via NF-κB activation

Caspase-8 is a unique member of caspases with a dual role in cell death and survival. Caspase-8 expression is often lost in some tumors, but increased in others, indicating a potential pro-survival function in cancer. By analyzing transcriptome of enzalutamide-resistant prostate cancer cells, we found that resistance was conferred by a mild caspase-8 upregulation that in turn led to NF-κB activation and the subsequent upregulation of the downstream IL-8. Mechanistically, we found that the pro-survival and enzalutamide-resistance-promoting features of caspase-8 were independent of its proteolytic activity, using a catalytically-inactive caspase-8 mutant. We further demonstrated that caspase-8 pro-apoptotic function was inhibited via cFLIP binding. Moreover, high caspase-8 expression was correlated with a worse prognosis in prostate cancer patients. Collectively, our work demonstrates that enzalutamide-resistance is mediated by caspase-8 upregulation and the consequent increase in NF-κB/IL-8 mediated survival signaling, highlighting caspase-8 and NF-κB as potential therapeutic targets to overcome enzalutamide-resistance in CRPC.

Caspase-8 mutations associated with head and neck cancer differentially retain functional properties related to TRAIL-induced apoptosis and cytokine induction

The cysteine protease, caspase-8, undergoes dimerization, processing, and activation following stimulation of cells with death ligands such as TRAIL, and mediates TRAIL induction of the extrinsic apoptosis pathway. In addition, caspase-8 mediates TRAIL-induced activation of NF-κB and upregulation of immunosuppressive chemokines/cytokines, via a mechanism independent of caspase-8 catalytic activity. The gene encoding procaspase-8 is mutated in 10% of human head and neck squamous cell carcinomas (HNSCCs). Despite a paucity of experimental evidence, HNSCC-associated caspase-8 mutations are commonly assumed to be loss of function. To investigate their functional properties and phenotypic effects, 18 HNSCC-associated caspase-8 mutants were expressed in doxycycline-inducible fashion in cell line models wherein the endogenous wild-type caspase-8 was deleted. We observed that 5/8 mutants in the amino-terminal prodomain, but 0/10 mutants in the carboxyl-terminal catalytic region, retained an ability to mediate TRAIL-induced apoptosis. Caspase-8 proteins with mutations in the prodomain were defective in dimerization, whereas all ten of the catalytic region mutants efficiently dimerized, revealing an inverse relationship between dimerization and apoptosis induction for the mutant proteins. Roughly half (3/8) of the prodomain mutants and 9/10 of the catalytic region mutants retained the ability to mediate TRAIL induction of immunosuppressive CXCL1, IL-6, or IL-8. Doxycycline-induced expression of wild-type caspase-8 or a representative mutant led to an increased percentage of T and NKT cells in syngeneic HNSCC xenograft tumors. These findings demonstrate that HNSCC-associated caspase-8 mutants retain properties that may influence TRAIL-mediated apoptosis and cytokine induction, as well as the composition of the tumor microenvironment.

Induction of interferon-β and interferon signaling by TRAIL and Smac mimetics via caspase-8 in breast cancer cells

Breast cancer prognosis is frequently good but a substantial number of patients suffer from relapse. The death receptor ligand TRAIL can in combination with Smac mimetics induce apoptosis in some luminal-like ER-positive breast cancer cell lines, such as CAMA-1, but not in MCF-7 cells. Here we show that TRAIL and the Smac mimetic LCL161 induce non-canonical NF-κB and IFN signaling in ER-positive MCF-7 cells and in CAMA-1 breast cancer cells when apoptosis is blocked by caspase inhibition. Levels of p52 are increased and STAT1 gets phosphorylated. STAT1 phosphorylation is induced by TRAIL alone in MCF-7 cells and is independent of non-canonical NF-κB since downregulation of NIK has no effect. The phosphorylation of STAT1 is a rather late event, appearing after 24 hours of TRAIL stimulation. It is preceded by an increase in IFNB1 mRNA levels and can be blocked by siRNA targeting the type I IFN receptor IFNAR1 and by inhibition of Janus kinases by Ruxolitinib. Moreover, downregulation of caspase-8, but not inhibition of caspase activity, blocks TRAIL-mediated STAT1 phosphorylation and induction of IFN-related genes. The data suggest that TRAIL-induced IFNB1 expression in MCF-7 cells is dependent on a non-apoptotic role of caspase-8 and leads to autocrine interferon-β signaling.

Cryo-EM structural analysis of FADD:Caspase-8 complexes defines the catalytic dimer architecture for co-ordinated control of cell fate

Regulated cell death is essential in development and cellular homeostasis. Multi-protein platforms, including the Death-Inducing Signaling Complex (DISC), co-ordinate cell fate via a core FADD:Caspase-8 complex and its regulatory partners, such as the cell death inhibitor c-FLIP. Here, using electron microscopy, we visualize full-length procaspase-8 in complex with FADD. Our structural analysis now reveals how the FADD-nucleated tandem death effector domain (tDED) helical filament is required to orientate the procaspase-8 catalytic domains, enabling their activation via anti-parallel dimerization. Strikingly, recruitment of c-FLIP_S into this complex inhibits Caspase-8 activity by altering tDED triple helix architecture, resulting in steric hindrance of the canonical tDED Type I binding site. This prevents both Caspase-8 catalytic domain assembly and tDED helical filament elongation. Our findings reveal how the plasticity, composition and architecture of the core FADD:Caspase-8 complex critically defines life/death decisions not only via the DISC, but across multiple key signaling platforms including TNF complex II, the ripoptosome, and RIPK1/RIPK3 necrosome.

The core FADD:Caspase-8 complex and its regulatory partners, such as the cell death inhibitor c-FLIP, coordinate cell fate. Here authors present the structure of full-length procaspase-8 in a complex with FADD and reveal how recruitment of c-FLIP_S into this complex inhibits Caspase-8 activity.

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.

Molecular mechanisms of FasL-mediated 'reverse-signaling'

Effector lymphocytes, including NK and T cells, express FasL. Expression of Fas, the receptor for FasL in tumor cells, renders them susceptible to NK and T cell-mediated killing. The functional relevance of FasL in initiating death signals in tumor cells is well-characterized. However, the cytoplasmic interacting partners and the potential signaling pathways downstream of FasL are far from fully defined. FasL possesses an 81 amino acid long cytoplasmic tail with multiple unique recruitment motifs. We predict multiple interdependent signaling complexes form the core of the 'reverse signaling' downstream of FasL. A direct interaction between the proline-rich domain of FasL and the SH3 domain of PI(3)K-p85α initiates the first pathway. This cascade helps FasL to link to PLC-γ2 via PIP₃ or the Akt-dependent activation of mTOR complexes. Independently, a GRB2/GADs-binding PXXP cytoplasmic motif of FasL can initiate a Ras-GTP-dependent PAK1→C-Raf→MEK1/2→ERK1/2 activation. FasL can recruit Fyn via the proline-rich domain leading to the recruitment of ADAP. Through its ability to directly interact with Carma1 and TAK1, ADAP initiates the formation of the Carma1/Bcl10/Malt1-based CBM signalosome that is primarily responsible for inflammatory cytokine production. Here, we explore the conserved cytoplasmic domains of FasL, the potential signaling molecules that interact, and the functional downstream consequences within the effector lymphocytes to define the FasL-mediated 'reverse signaling'.

TNF-alpha-induced microglia activation requires miR-342: impact on NF-kB signaling and neurotoxicity

Growing evidences suggest that sustained neuroinflammation, caused by microglia overactivation, is implicated in the development and aggravation of several neurological and psychiatric disorders. In some pathological conditions, microglia produce increased levels of cytotoxic and inflammatory mediators, such as tumor necrosis factor alpha (TNF-α), which can reactivate microglia in a positive feedback mechanism. However, specific molecular mediators that can be effectively targeted to control TNF-α-mediated microglia overactivation, are yet to be uncovered. In this context, we aim to identify novel TNF-α-mediated micro(mi)RNAs and to dissect their roles in microglia activation, as well as to explore their impact on the cellular communication with neurons. A miRNA microarray, followed by RT-qPCR validation, was performed on TNF-α-stimulated primary rat microglia. Gain- and loss-of-function in vitro assays and proteomic analysis were used to dissect the role of miR-342 in microglia activation. Co-cultures of microglia with hippocampal neurons, using a microfluidic system, were performed to understand the impact on neurotoxicity. Stimulation of primary rat microglia with TNF-α led to an upregulation of Nos2, Tnf, and Il1b mRNAs. In addition, ph-NF-kB p65 levels were also increased. miRNA microarray analysis followed by RT-qPCR validation revealed that TNF-α stimulation induced the upregulation of miR-342. Interestingly, miR-342 overexpression in N9 microglia was sufficient to activate the NF-kB pathway by inhibiting BAG-1, leading to increased secretion of TNF-α and IL-1β. Conversely, miR-342 inhibition led to a strong decrease in the levels of these cytokines after TNF-α activation. In fact, both TNF-α-stimulated and miR-342-overexpressing microglia drastically affected neuron viability. Remarkably, increased levels of nitrites were detected in the supernatants of these co-cultures. Globally, our findings show that miR-342 is a crucial mediator of TNF-α-mediated microglia activation and a potential target to tackle microglia-driven neuroinflammation.

The Role of Caspase-2 in Regulating Cell Fate

Caspase-2 is the most evolutionarily conserved member of the mammalian caspase family and has been implicated in both apoptotic and non-apoptotic signaling pathways, including tumor suppression, cell cycle regulation, and DNA repair. A myriad of signaling molecules is associated with the tight regulation of caspase-2 to mediate multiple cellular processes far beyond apoptotic cell death. This review provides a comprehensive overview of the literature pertaining to possible sophisticated molecular mechanisms underlying the multifaceted process of caspase-2 activation and to highlight its interplay between factors that promote or suppress apoptosis in a complicated regulatory network that determines the fate of a cell from its birth and throughout its life.

CD95 Structure, Aggregation and Cell Signaling

CD95 is a pre-ligand-associated transmembrane (TM) receptor. The interaction with its ligand CD95L brings to a next level its aggregation and triggers different signaling pathways, leading to cell motility, differentiation or cell death. This diversity of biological responses associated with a unique receptor devoid of enzymatic property raises the question of whether different ligands exist, or whether the fine-tuned control of CD95 aggregation and conformation, its distribution within certain plasma membrane sub-domains or the pattern of post-translational modifications account for this such broad-range of cell signaling. Herein, we review how the different domains of CD95 and their post-translational modifications or the different forms of CD95L can participate in the receptor aggregation and induction of cell signaling. Understanding how CD95 response goes from cell death to cell proliferation, differentiation and motility is a prerequisite to reveal novel therapeutic options to treat chronic inflammatory disorders and cancers.

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.

Knock down of Fas-Associated Protein with Death Domain (FADD) Sensitizes Osteosarcoma to TNFα-induced Cell Death

Fas-associated protein with death domain (FADD) was first identified for its role in linking death receptors to the apoptotic signaling pathway with subsequent cell death. Later studies reported non-apoptotic functions for FADD in normal cells and cancer cells. Non-apoptotic functions for FADD in osteosarcoma (OS) have not been reported. In this study, FADD protein expression was knocked down in human CCHOSD, LM7, and SaOS2 OS cell lines followed by assessment of sensitivity to TNFα- or TRAIL-induced cell death. Knock down of FADD significantly increased TNFα-induced cell death in LM7 and SaOS2 cell lines. The mode of TNFα-induced cell death was apoptosis and not necroptosis. Inhibition of nuclear factor kappa B (NFκB) in wildtype cells increased TNFα-induced cell death to similar levels observed in FADD knockdown cells, suggesting a role for FADD in NFκB pro-survival cell signaling. In addition, knock down of FADD increased SMAC mimetic-mediated TNFα-induced cell death in all cell lines studied. The results of this study indicate that FADD has a pro-survival function in OS following TNFα treatment that involves NFκB signaling. The results also indicate that the pro-survival function of FADD is associated with XIAP activity.

Functional characterisation of Holothuria leucospilota Fas-associated death domain in the innate immune-related signalling pathways

In this study, the functions of Holothuria leucospilota Fas-associated death domain (HLFADD) in the innate immune-related signalling pathways were investigated. The results showed that over-expression of HLFADD in HEK293T cells could activate the transcription factors NF-κB and activator protein-1 (AP-1), and induce the secretion of downstream pro-inflammatory cytokines IL-6, IL-8 and IL-18, suggesting the involvement of the sea cucumber FADD in activating the NF-κB and c-Jun NH₂-terminal kinase-dependent pathways. On the other hand, HLFADD could down-regulate the activations of NF-κB and AP-1 that induced by over-expression of H. leucospilota myeloid differentiation factor 88 (HLMyD88), which is supposed to be mediated through its interaction with HLMyD88 to keep the MyD88-dependent TLR signalling at a proper magnitude. The interaction of HLFADD and HLMyD88 were further supported by a co-immunoprecipitation assay. Moreover, HLFADD could activate transcription factor IFN regulatory factor-3 and induced the secretion of downstream IFN-α and IFN-β, indicating that the sea cucumber FADD may also activate the antiviral IFN signalling pathway. In summary, our study may give new insights on the functions of sea cucumber FADD in the innate immune-related signalling pathways.

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.

The anti-cancer drug gefitinib accelerates Fas-mediated apoptosis by enhancing caspase-8 activation in cancer cells

Fas/CD95 plays a pivotal role in T cell-mediated cytotoxicity. Accumulating evidence has suggested that resistance to Fas-mediated apoptosis contributes to the escape of cancer cells from immune destruction, and allows to undergo proliferation and outgrowth of cancer cells. In this study, we found that the anti-cancer drug gefitinib, a tyrosine kinase inhibitor of epidermal growth factor receptor (EGFR), has an ability to enhance Fas-mediated cytotoxicity. In the presence of nontoxic concentrations of gefitinib, Fas-induced activation of caspase-8 and subsequent apoptosis was dramatically promoted, suggesting that gefitinib increases the sensitivity to Fas-mediated apoptosis. Interestingly, the effects of gefitinib were observed in EGFR or p53 knockout (KO) cells. These observations indicate that both EGFR and p53 are dispensable for the enhancement. On the other hand, gefitinib clearly downregulated heat shock protein 70 (HSP70) as previously reported. Considering that HSP70 contributes to protection of cells against Fas-mediated apoptosis, gefitinib may increase the sensitivity to Fas-mediated apoptosis by downregulating HSP70. Thus, our findings reveal novel properties of gefitinib, which may provide insight into the alternative therapeutic approaches of gefitinib for Fas-resistant tumors.

CD95/Fas and metastatic disease: What does not kill you makes you stronger

CD95 (also known as Fas) is the prototype of death receptors; however, evidence suggests that this receptor mainly implements non-apoptotic signaling pathways such as NF-κB, MAPK, and PI3K that are involved in cell migration, differentiation, survival, and cytokine secretion. At least two different forms of CD95 L exist. The multi-aggregated transmembrane ligand (m-CD95 L) is cleaved by metalloproteases to release a homotrimeric soluble ligand (s-CD95 L). Unlike m-CD95 L, the interaction between s-CD95 L and its receptor CD95 fails to trigger apoptosis, but instead promotes calcium-dependent cell migration, which contributes to the accumulation of inflammatory Th17 cells in damaged organs of lupus patients and favors cancer cell invasiveness. Novel inhibitors targeting the pro-inflammatory roles of CD95/CD95 L may provide attractive therapeutic options for patients with chronic inflammatory disorders or cancer. This review discusses the roles of the CD95/CD95 L pair in cell migration and metastasis.

c-FLIP and CD95 signaling are essential for survival of renal cell carcinoma

Clear cell renal cell carcinoma (ccRCC) is the most-prominent tumor type of kidney cancers. Resistance of renal cell carcinoma (RCC) against tumor therapy is often owing to apoptosis resistance, e.g., by overexpression of anti-apoptotic proteins. However, little is known about the role of the apoptosis inhibitor c-FLIP and its potential impact on death receptor-induced apoptosis in ccRCC cells. In this study, we demonstrate that c-FLIP is crucial for resistance against CD95L-induced apoptosis in four ccRCC cell lines. Strikingly, downregulation of c-FLIP expression by short hairpin RNA (shRNA)interference led to spontaneous caspase activation and apoptotic cell death. Of note, knockdown of all c-FLIP splice variants was required to induce apoptosis. Stimulation of ccRCC cells with CD95L induced NF-κB and MAP kinase survival pathways as revealed by phosphorylation of RelA/p65 and Erk1/2. Interestingly, CD95L surface expression was high in all cell lines analyzed, and CD95 but not TNF-R1 clustered at cell contact sites. Downstream of CD95, inhibition of the NF-κB pathway led to spontaneous cell death. Surprisingly, knockdown experiments revealed that c-FLIP inhibits NF-κB activation in the context of CD95 signaling. Thus, c-FLIP inhibits apoptosis and dampens NF-κB downstream of CD95 but allows NF-κB activation to a level sufficient for ccRCC cell survival. In summary, we demonstrate a complex CD95-FLIP-NF-κB-signaling circuit, in which CD95-CD95L interactions mediate a paracrine survival signal in ccRCC cells with c-FLIP and NF-κB both being required for inhibiting cell death and ensuring survival. Our findings might lead to novel therapeutic approaches of RCC by circumventing apoptosis resistance.

Depletion of the Receptor-Interacting Protein Kinase 3 (RIP3) Decreases Photoreceptor Cell Death During the Early Stages of Ocular Murine Cytomegalovirus Infection

Purpose

The purpose of this study was to determine if the receptor-interacting protein kinase 3 (RIP3) plays a significant role in innate immune responses and death of bystander retinal neurons during murine cytomegalovirus (MCMV) retinal infection, by comparing the innate immune response and cell death in RIP3-depleted mice (Rip3^−/−) and Rip3^+/+ control mice.

Methods

Rip3^−/− and Rip3^+/+ mice were immunosuppressed (IS) and inoculated with MCMV via the supraciliary route. Virus-injected and mock-injected control eyes were removed at days 4, 7, and 10 post infection (p.i.) and markers of innate immunity and cell death were analyzed.

Results

Compared to Rip3^+/+ mice, significantly more MCMV was recovered and more MCMV-infected RPE cells were observed in injected eyes of Rip3^−/− mice at days 4 and 7 p.i. In contrast, fewer TUNEL-stained photoreceptors were observed in Rip3^−/− eyes than in Rip3^+/+ eyes at these times. Electron microscopy showed that significantly more apoptotic photoreceptor cells were present in Rip3^+/+ mice than in Rip3^−/− mice. Immunohistochemistry showed that the majority of TUNEL-stained photoreceptors died via mitochondrial flavoprotein apoptosis-inducing factor (AIF)-mediated, caspase 3–independent apoptosis. The majority of RIP3-expressing cells in infected eyes were RPE cells, microglia/macrophages, and glia, whereas retinal neurons contained much lower amounts of RIP3. Western blots showed significantly higher levels of activated nuclear factor–κB and caspase 1 were present in Rip3^+/+ eyes compared to Rip3^−/− eyes.

Conclusions

Our results suggest that RIP3 enhances innate immune responses against ocular MCMV infection via activation of the inflammasome and nuclear factor–κB, which also leads to inflammation and death of bystander cells by multiple pathways including apoptosis and necroptosis.

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

We evaluated redundant and receptor-specific activities of TRADD, RIPK1, and FADD in RIPK3-expressing HeLa cells lacking expression of these proteins or any combination of two of these factors. We confirmed the opposing role of FADD in TNF- and TRAIL-induced necroptosis and observed an anti-necroptotic function of TRADD. RIPK1 and TRADD act in a redundant manner in TNF- but not TRAIL-induced apoptosis. Complementary, FADD proved to be sufficient for TRAIL- but not for TNF-induced apoptosis. TRADD and RIPK1, however, redundantly mediated proinflammatory signaling in response to TNF and TRAIL. FADD deficiency sensitized more efficiently for TNFR1-mediated necroptosis than caspase-8 deficiency pointing to a caspase-8 independent inhibitory activity of FADD on TNF-induced necroptosis. Based on these characteristics, we propose a model in which the death receptor-specific activities of TRADD, RIPK1, and FADD are traced back to their hierarchically different position in TNFR1- and TRAIL death receptor signaling.

Osteocalcin triggers Fas/FasL-mediated necroptosis in adipocytes via activation of p300

The uncarboxylated form of osteocalcin (GluOC) regulates glucose and lipid metabolism in mice. We previously showed that low-dose (≤10 ng/ml) GluOC induces the expression of adiponectin and peroxisome proliferator-activated receptor γ (PPARγ) via a cAMP-PKA-ERK-CREB signaling pathway in 3T3-L1 adipocytes. We also noticed that high-dose (≥20 ng/ml) GluOC inhibits the expression of adiponectin and PPARγ in these cells. We have here explored the mechanism underlying these effects of high-dose GluOC. High-dose GluOC triggered morphological changes in 3T3-L1 adipocytes suggestive of the induction of cell death. It activated the putative GluOC receptor GPRC6A and thereby induced the production of cAMP and activation of protein kinase A (PKA), similar to signaling by low-dose GluOC with the exception that the catalytic subunit of PKA also entered the nucleus. Cytosolic PKA induced phosphorylation of cAMP response element-binding protein (CREB) at serine-133 via extracellular signal-regulated kinase (ERK). Nuclear PKA appeared to mediate the inhibitory phosphorylation of salt-inducible kinase 2 (SIK2) at serine-358 and thereby to alleviate the inhibitory phosphorylation of the CREB co-activator p300 at serine-89. The activation of CREB and p300 resulted in increased expression of the transcription factor FoxO1 and consequent upregulation of Fas ligand (FasL) at the plasma membrane. The interaction of FasL with Fas on neighboring adipocytes triggered the phosphorylation at threonine-357/serine-358 and homotrimerization of mixed-lineage kinase domain-like protein (MLKL), a key regulator of necroptosis, as well as Ca²⁺ influx via transient receptor potential melastatin 7 (TRPM7), the generation of reactive oxygen species and lipid peroxides, and dephosphorylation of dynamin-related protein 1 (DRP1) at serine-637, resulting in mitochondrial fragmentation. Together, our results indicate that high-dose GluOC triggers necroptosis through upregulation of FasL at the plasma membrane in a manner dependent of activation of CREB-p300, followed by the activation of Fas signaling in neighboring adipocytes.

FLIP as a therapeutic target in cancer

One of the classic hallmarks of cancer is disruption of cell death signalling. Inhibition of cell death promotes tumour growth and metastasis, causes resistance to chemo- and radiotherapies as well as targeted agents, and is frequently due to overexpression of antiapoptotic proteins rather than loss of pro-apoptotic effectors. FLIP is a major apoptosis-regulatory protein frequently overexpressed in solid and haematological cancers, in which its high expression is often correlated with poor prognosis. FLIP, which is expressed as long (FLIP(L)) and short (FLIP(S)) splice forms, achieves its cell death regulatory functions by binding to FADD, a critical adaptor protein which links FLIP to the apical caspase in the extrinsic apoptotic pathway, caspase-8, in a number of cell death regulating complexes, such as the death-inducing signalling complexes (DISCs) formed by death receptors. FLIP also plays a key role (together with caspase-8) in regulating another form of cell death termed programmed necrosis or 'necroptosis', as well as in other key cellular processes that impact cell survival, including autophagy. In addition, FLIP impacts activation of the intrinsic mitochondrial-mediated apoptotic pathway by regulating caspase-8-mediated activation of the pro-apoptotic Bcl-2 family member Bid. It has been demonstrated that FLIP can not only inhibit death receptor-mediated apoptosis, but also cell death induced by a range of clinically relevant chemotherapeutic and targeted agents as well as ionizing radiation. More recently, key roles for FLIP in promoting the survival of immunosuppressive tumour-promoting immune cells have been discovered. Thus, FLIP is of significant interest as an anticancer therapeutic target. In this article, we review FLIP's biology and potential ways of targeting this important tumour and immune cell death regulator.

Beyond Cell Death: New Functions for TNF Family Cytokines in Autoimmunity and Tumor Immunotherapy

Originally discovered as an inducer of apoptosis, the TNF-family receptor Fas (CD95, APO-1, TNFRSF6) has more recently been found to have functions beyond cell death, including T cell co-stimulation and promoting terminal differentiation of CD4⁺ and CD8⁺ T cells. Other TNF family members also discovered as apoptosis inducers, such as TRAIL (APO-2L, TNFSF10), can promote inflammation through caspase-8. Surprisingly, non-apoptotic signaling through Fas can protect from the autoimmunity seen in Fas deficiency independently from the cell death inducing functions of the receptor. Non-apoptotic Fas signaling can induce tumor cell growth and migration, and impair the efficacy of T cell adoptive immunotherapy. Blocking of non-apoptotic functions of these receptors may be a novel strategy to regulate autoimmunity and inflammation, and enhance antitumor immunity.

Cisplatin-induced renal inflammation is ameliorated by cilastatin nephroprotection

Background

Cisplatin is a potent chemotherapeutic drug whose nephrotoxic effect is a major complication and a dose-limiting factor for antitumoral therapy. There is much evidence that inflammation contributes to the pathogenesis of cisplatin-induced nephrotoxicity. We found that cilastatin, a renal dehydropeptidase-I inhibitor, has protective effects in vitro and in vivo against cisplatin-induced renal damage by inhibiting apoptosis and oxidation. Here, we investigated the potential use of cilastatin to protect against cisplatin-induced kidney injury and inflammation in rats.

Methods

Male Wistar rats were divided into four groups: control, cilastatin-control, cisplatin and cilastatin-cisplatin. Nephrotoxicity was assessed 5 days after administration of cisplatin based on blood urea nitrogen, creatinine, glomerular filtration rate (GFR), kidney injury molecule (KIM)-1 and renal morphology. Inflammation was measured using the electrophoretic mobility shift assay, immunohistochemical studies and evaluation of inflammatory mediators.

Results

Compared with the control rats, cisplatin-administered rats were affected by significant proximal tubule damage, decreased GFR, increased production of inflammatory mediators and elevations in urea, creatinine and tissue KIM-1 levels. Cilastatin prevented these changes in renal function and ameliorated histological damage in cisplatin-administered animals. Cilastatin also reduced pro-inflammatory cytokine levels, activation of nuclear factor-κB and CD68-positive cell concentrations.

Conclusions

Cilastatin reduces cisplatin-induced nephrotoxicity, which is associated with decreased inflammation in vivo. Although the exact role of decreased inflammation in nephroprotection has not been fully elucidated, treatment with cilastatin could be a novel strategy for the prevention of cisplatin-induced acute kidney injury.

Caspase-8: regulating life and death

Roles for cell death in development, homeostasis, and the control of infections and cancer have long been recognized. Although excessive cell damage results in passive necrosis, cells can be triggered to engage molecular programs that result in cell death. Such triggers include cellular stress, oncogenic signals that engage tumor suppressor mechanisms, pathogen insults, and immune mechanisms. The best-known forms of programmed cell death are apoptosis and a recently recognized regulated necrosis termed necroptosis. Of the two best understood pathways of apoptosis, the extrinsic and intrinsic (mitochondrial) pathways, the former is induced by the ligation of death receptors, a subset of the TNF receptor (TNFR) superfamily. Ligation of these death receptors can also induce necroptosis. The extrinsic apoptosis and necroptosis pathways regulate each other and their balance determines whether cells live. Integral in the regulation and initiation of death receptor-mediated activation of programmed cell death is the aspartate-specific cysteine protease (caspase)-8. This review describes the role of caspase-8 in the initiation of extrinsic apoptosis execution and the mechanism by which caspase-8 inhibits necroptosis. The importance of caspase-8 in the development and homeostasis and the way that dysfunctional caspase-8 may contribute to the development of malignancies in mice and humans are also explored.

Caspase-10 Negatively Regulates Caspase-8-Mediated Cell Death, Switching the Response to CD95L in Favor of NF-κB Activation and Cell Survival

Formation of the death-inducing signaling complex (DISC) initiates extrinsic apoptosis. Caspase-8 and its regulator cFLIP control death signaling by binding to death-receptor-bound FADD. By elucidating the function of the caspase-8 homolog, caspase-10, we discover that caspase-10 negatively regulates caspase-8-mediated cell death. Significantly, we reveal that caspase-10 reduces DISC association and activation of caspase-8. Furthermore, we extend our co-operative/hierarchical binding model of caspase-8/cFLIP and show that caspase-10 does not compete with caspase-8 for binding to FADD. Utilizing caspase-8-knockout cells, we demonstrate that caspase-8 is required upstream of both cFLIP and caspase-10 and that DISC formation critically depends on the scaffold function of caspase-8. We establish that caspase-10 rewires DISC signaling to NF-κB activation/cell survival and demonstrate that the catalytic activity of caspase-10, and caspase-8, is redundant in gene induction. Thus, our data are consistent with a model in which both caspase-10 and cFLIP coordinately regulate CD95L-mediated signaling for death or survival.

Signaling by cell surface death receptors: Alterations in head and neck cancer

Cell surface death receptors are members of the tumor necrosis factor receptor (TNFR) superfamily and mediate signals leading to the induction of apoptosis or necroptosis, as well as NF-κB-mediated cell survival. These biochemical processes play key roles in cell growth, development, tissue homeostasis, and immune responses. The downstream signaling complexes activated by different death receptors can differ significantly and are subject to multiple, distinct regulatory mechanisms. Dysregulation of signaling by the TNFR superfamily contributes to a variety of pathologic conditions, including defective immune responses and cancer. Caspase-8 signaling is important for mediating death receptor signals leading to either apoptosis or NF-κB activation. By contrast, inactivation of caspase-8 or loss of caspase-8 expression shifts death receptor signaling to the necroptosis pathway. Notably, the gene encoding caspase-8 is mutated in roughly ten percent of head and neck cancers. These findings support the hypothesis that alterations in the biochemical pathways mediated by death receptors have important consequences for the development of head and neck, and possibly other, cancers.

Paving TRAIL's Path with Ubiquitin

Despite its name, signalling induced by the tumour necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is versatile. Besides eliciting cell death by both apoptosis and necroptosis, TRAIL can also induce migration, proliferation, and cytokine production in cancerous and non-cancerous cells. Unravelling the mechanisms regulating the intricate balance between these different outputs could therefore facilitate our understanding of the role of TRAIL in tissue homeostasis, immunity, and cancer. Ubiquitination and its reversal, deubiquitination, are crucial modulators of immune receptor signalling. This review discusses recent progress on the orchestration of TRAIL signalling outcomes by ubiquitination of various components of the signalling complexes, our understanding of the molecular switches that decide between cell death and gene activation, and what remains to be discovered.

Association of Increased F4/80high Macrophages With Suppression of Serum-Transfer Arthritis in Mice With Reduced FLIP in Myeloid Cells

OBJECTIVE

Macrophages are critical in the pathogenesis of rheumatoid arthritis (RA). We recently demonstrated that FLIP is necessary for the differentiation and/or survival of macrophages. We also showed that FLIP is highly expressed in RA synovial macrophages. This study was undertaken to determine if a reduction in FLIP in mouse macrophages reduces synovial tissue macrophages and ameliorates serum-transfer arthritis.

METHODS

Mice with Flip deleted in myeloid cells (Flip^f/f LysM^c/+ mice) and littermate controls were used. Arthritis was induced by intraperitoneal injection of K/BxN serum. Disease severity was evaluated by clinical score and change in ankle thickness, and joints were examined by histology and immunohistochemistry. Cells were isolated from the ankles and bone marrow of the mice and examined by flow cytometry, real-time quantitative reverse transcriptase-polymerase chain reaction, or Western blotting.

RESULTS

In contrast to expectations, Flip^f/f LysM^c/+ mice developed more severe arthritis early in the clinical course, but peak arthritis was attenuated and the resolution phase more complete than in control mice. Prior to the induction of serum-transfer arthritis, the number of tissue-resident macrophages was reduced. On day 9 after arthritis induction, the number of F4/80^high macrophages in the joints of the Flip^f/f LysM^c/+ mice was not decreased, but increased. FLIP was reduced in the F4/80^high macrophages in the ankles of the Flip^f/f LysM^c/+ mice, while F4/80^high macrophages expressed an antiinflammatory phenotype in both the Flip^f/f LysM^c/+ and control mice.

CONCLUSION

Our observations suggest that reducing FLIP in macrophages by increasing the number of antiinflammatory macrophages may be an effective therapeutic approach to suppress inflammation, depending on the disease stage.

The molluscum contagiosum virus death effector domain containing protein MC160 RxDL motifs are not required for its known viral immune evasion functions

The molluscum contagiosum virus (MCV) uses a variety of immune evasion strategies to antagonize host immune responses. Two MCV proteins, MC159 and MC160, contain tandem death effector domains (DEDs). They are reported to inhibit innate immune signaling events such as NF-κB and IRF3 activation, and apoptosis. The RxDL motif of MC159 is required for inhibition of both apoptosis and NF-κB activation. However, the role of the conserved RxDL motif in the MC160 DEDs remained unknown. To answer this question, we performed alanine mutations to neutralize the arginine and aspartate residues present in the MC160 RxDL in both DED1 and DED2. These mutations were further modeled against the structure of the MC159 protein. Surprisingly, the RxDL motif was not required for MC160's ability to inhibit MAVS-induced IFNβ activation. Further, unlike previous results with the MC159 protein, mutations within the RxDL motif of MC160 had no effect on the ability of MC160 to dampen TNF-α-induced NF-κB activation. Molecular modeling predictions revealed no overall changes to the structure in the MC160 protein when the amino acids of both RxDL motifs were mutated to alanine (DED1 = R67A D69A; DED2 = R160A D162A). Taken together, our results demonstrate that the RxDL motifs present in the MC160 DEDs are not required for known functions of the viral protein.

Bonny light crude oil-induced alteration in levels of testicular stress proteins is accompanied by apoptosis in rats after treatment withdrawal

BACKGROUND

The folkloric use of Bonny light crude oil (BLCO) in the treatment of gastrointestinal disorders and as an anti-poison is a generally acceptable practice in the Niger Delta area of Nigeria. The testicular dysfunction induced by BLCO exposure is of public concern with a view to its folkloric usage. The present study investigated the effects of BLCO exposure and withdrawal on the levels of testicular stress proteins and apoptosis-related proteins in rats.

METHODS

Adult male Wistar rats were exposed to 800 mg/kg body weight of BLCO for 7 days. One-half of the rats in each group were sacrificed on day 8, while the remaining one-half stayed an additional 45 days without treatment.

RESULTS

Western blot analysis showed that administration of BLCO resulted in a significant increase in the levels of stress proteins and apoptosis-related proteins by 50% and above relative to control, except cytosolic nuclear factor-κB (NF-κB), which decreased significantly relative to control. This was followed by a concomitant increase in the expression of caspase-3, FasL, and NF-κB by immunofluorescence staining within the testicular germ cells. Apoptosis showed a significant increase in TUNEL-positive cells. Following withdrawal of treatment, BLCO-mediated alteration in stress proteins and induction of apoptosis persisted relative to control.

CONCLUSIONS

Collectively, BLCO induced irreversible alteration in testicular stress proteins and apoptosis in rats within the time course of investigation. These findings highlight the potential long-term adverse effects of BLCO on individuals unduly exposed to BLCO.

Cell death-independent activities of the death receptors CD95, TRAILR1, and TRAILR2

Since their identification more than 20 years ago, the death receptors CD95, TRAILR1, and TRAILR2 have been intensively studied with respect to their cell death-inducing activities. These receptors, however, can also trigger a variety of cell death-independent cellular responses reaching from the activation of proinflammatory gene transcription programs over the stimulation of proliferation and differentiation to induction of cell migration. The cell death-inducing signaling mechanisms of CD95 and the TRAIL death receptors are well understood. In contrast, despite the increasing recognition of the biological and pathophysiological relevance of the cell death-independent activities of CD95, TRAILR1, and TRAILR2, the corresponding signaling mechanisms are less understood and give no fully coherent picture. This review is focused on the cell death-independent activities of CD95 and the TRAIL death receptors and addresses mainly three questions: (a) how are these receptors linked to noncell death pathways at the molecular level, (b) which factors determine the balance of cell death and cell death-independent activities of CD95 and the TRAIL death receptors at the cellular level, and (c) what are the consequences of the cell death-independent functions of these receptors for their role in cancer and inflammatory diseases.