A dominant interfering mutant of FADD/MORT1 enhances deletion of autoreactive thymocytes and inhibits proliferation of mature T lymphocytes

Prospects and hazards of silica nanoparticles: Biological impacts and implicated mechanisms

With the thrive of nanotechnology, silica nanoparticles (SiNPs) have been extensively adopted in the agriculture, food, cosmetic, and even biomedical industries. Due to the mass production and use, SiNPs inevitably entered the environment, resulting in ecological toxicity and even posing a threat to human health. Although considerable investigations have been conducted to assess the toxicity of SiNPs, the correlation between SiNPs exposure and consequent health risks remains ambiguous. Since the biological impacts of SiNPs can differ from their design and application, the toxicity assessment for SiNPs may be extremely difficult. This review discussed the application of SiNPs in different fields, especially their biomedical use, and documented their potential release pathways into the environment. Meanwhile, the current process of assessing SiNPs-related toxicity on various model organisms and cell lines was also detailed, thus estimating the health threats posed by SiNPs exposure. Finally, the potential toxic mechanisms of SiNPs were also elaborated based on results obtained from both in vivo and in vitro trials. This review generally summarizes the biological effects of SiNPs, which will build up a comprehensive perspective of the application and toxicity of SiNPs.

Apoptotic cell death in disease-Current understanding of the NCCD 2023

Apoptosis is a form of regulated cell death (RCD) that involves proteases of the caspase family. Pharmacological and genetic strategies that experimentally inhibit or delay apoptosis in mammalian systems have elucidated the key contribution of this process not only to (post-)embryonic development and adult tissue homeostasis, but also to the etiology of multiple human disorders. Consistent with this notion, while defects in the molecular machinery for apoptotic cell death impair organismal development and promote oncogenesis, the unwarranted activation of apoptosis promotes cell loss and tissue damage in the context of various neurological, cardiovascular, renal, hepatic, infectious, neoplastic and inflammatory conditions. Here, the Nomenclature Committee on Cell Death (NCCD) gathered to critically summarize an abundant pre-clinical literature mechanistically linking the core apoptotic apparatus to organismal homeostasis in the context of disease.

A second FADD mediates coelomocyte apoptosis response to Vibrio splendidus infection in sea cucumber Apostichopus japonicus

Fas-associated protein with death domain (FADD) is a pivotal adaptor protein that functions in mediating cell death, cell cycle regulation, and particular in innate immunity by the main death receptors. In this study, a second FADD gene in sea cucumber Apostichopus japonicus (termed AjFADD-2) was cloned and its potential function in the innate responses was analyzed. The full-length cDNA of AjFADD-2 consists of 2405 bp and contains a 47 bp 5'-untranslated region (UTR), a 1629 bp 3'-UTR, and a 729 bp ORF encoding 242 amino acids. AjFADD-2 possesses two conserved domains of intracellular N-terminal death effector domain and an extracellular C-terminal death domain, which is different from the first cloned FADD gene in A. japonicus that only possesses the death domain. AjFADD-2 was examined in all sampled six tissues and was significantly induced in V. splendidus-challenged sea cucumbers and LPS-exposed coelomocytes. Subcellular localization detection showed that AjFADD-2 was primarily observed in the coelomocyte cytoplasm, and transferred to the nucleus post V. splendidus challenge. Consistently, AjFADD-2 knockdown significantly inhibited apoptosis in V. splendidus-challenged sea cucumbers and LPS-exposed coelomocytes. Taken together, our results provided evidence that AjFADD functioned as a positive regulator of coelomocytes apoptosis in response to pathogen V. splendidus challenge.

Programmed Cell Death of Endothelial Cells in Myocardial Infarction and Its Potential Therapeutic Strategy

Cardiovascular disease, especially coronary artery disease and stroke, kills around one-third of the world’s population, and myocardial infarction, a primary symptom of coronary heart disease, is a major worldwide health problem. Cardiovascular disease research has historically focused on promoting angiogenesis following myocardial damage. Myocardial vascular repair is crucial for improving myocardial infarction prognosis. Endothelial cells, the largest population of nonmyocytes within myocardial tissue, play an important role in angiogenesis. In recent years, different types of programmed cell death such as apoptosis, necroptosis, pyroptosis, ferroptosis, and autophagy have been described and found to be linked with cardiovascular diseases such as myocardial infarction, heart failure, and myocarditis. This will have important implications for reforming the treatment strategy of cardiovascular diseases. Different types of cell death of endothelial cells in myocardial infarction have been proposed, the roles and mechanisms of endothelial cell death in myocardial infarction are summarized in this review, and endothelial cell death inhibition as a therapeutic technique for treating myocardial infarction might be advantageous to human health.

The genetic landscape of the FAS pathway deficiencies

Dysfunction of the FAS-FASLG pathway causes a lymphoproliferative disorder with autoimmunity called Autoimmune lymphoproliferative syndrome (ALPS) mainly caused by FAS mutations. The goal of this review is to describe the genetic bases of the autoimmune lymphoproliferative syndrome and to underline their genetic complexity with the contribution of both germline and somatic events accounting for the variable clinical penetrance of the FAS mutations. Starting from the cohort of patients studied in the French cohort (>165 cases), we also reviewed the literature cases in order to depict a full description of the mutations affecting the FAS-FASLG pathway involved in the outcome of this rare non-malignant and non-infectious pediatric lymphoproliferative disease. We also discussed the variable clinical penetrance associated with mutations affecting the extracellular domain of the protein. Such non-penetrant germline mutations are frequently associated with an additional somatic event impacting the second allele of FAS. Moreover, the uncomplete clinical penetrance associated with mutations affecting the intracellular domain of FAS, in patient lacking additional FAS somatic event, suggested a potential digenic inheritance with a FAS mutation accompanied by a genetic modifier possibly impacting another player of the lymphocytes homeostasis (affecting the survival, activation or apoptosis of the peripheral leukocytes).

Examination of Fas-Induced Apoptosis of Murine Thymocytes in Thymic Tissue Slices Reveals That Fas Is Dispensable for Negative Selection

The death receptor Fas can induce cell death through the extrinsic pathway of apoptosis in a variety of cells, including developing thymocytes. Although Fas-induced cell death has been researched and modeled extensively, most of the studies have been done in vitro because of the lethality of Fas triggering in vivo. Thus, little is known about the time line of this type of cell death in vivo, specifically, how does the presence of macrophages and pro-survival cytokines affect apoptosis progression. In addition, although the sequence and timing of events during intrinsic pathway activation in thymocytes in situ have been described, no corresponding data for the extrinsic pathway are available. To address this gap in our knowledge, we established a novel system to study Fas-induced thymocyte cell death using tissue explants. We found that within 1 h of Fas ligation, caspase 3 was activated, within 2 h phosphatidylserine was externalized to serve as an "eat-me" signal, and at the same time, we observed signs of cell loss, likely due to efferocytosis. Both caspase 3 activation and phosphatidylserine exposure were critical for cell loss. Although Fas ligand (FasL) was delivered simultaneously to all cells, we observed significant variation in the entry into the cell death pathway. This model also allowed us to revisit the role of Fas in negative selection, and we ruled out an essential part for it in the deletion of autoreactive thymocytes. Our work provides a timeline for the apoptosis-associated events following Fas triggering in situ and confirms the lack of involvement of Fas in the negative selection of thymocytes.

Emerging connectivity of programmed cell death pathways and its physiological implications

The removal of functionally dispensable, infected or potentially neoplastic cells is driven by programmed cell death (PCD) pathways, highlighting their important roles in homeostasis, host defence against pathogens, cancer and a range of other pathologies. Several types of PCD pathways have been described, including apoptosis, necroptosis and pyroptosis; they employ distinct molecular and cellular processes and differ in their outcomes, such as the capacity to trigger inflammatory responses. Recent genetic and biochemical studies have revealed remarkable flexibility in the use of these PCD pathways and indicate a considerable degree of plasticity in their molecular regulation; for example, despite having a primary role in inducing pyroptosis, inflammatory caspases can also induce apoptosis, and conversely, apoptotic stimuli can trigger pyroptosis. Intriguingly, this flexibility is most pronounced in cellular responses to infection, while apoptosis is the dominant cell death process through which organisms prevent the development of cancer. In this Review, we summarize the mechanisms of the different types of PCD and describe the physiological and pathological processes that engage crosstalk between these pathways, focusing on infections and cancer. We discuss the intriguing notion that the different types of PCD could be seen as a single, coordinated cell death system, in which the individual pathways are highly interconnected and can flexibly compensate for one another.

A missense mutation in the MLKL brace region promotes lethal neonatal inflammation and hematopoietic dysfunction

MLKL is the essential effector of necroptosis, a form of programmed lytic cell death. We have isolated a mouse strain with a single missense mutation, Mlkl^D139V, that alters the two-helix 'brace' that connects the killer four-helix bundle and regulatory pseudokinase domains. This confers constitutive, RIPK3 independent killing activity to MLKL. Homozygous mutant mice develop lethal postnatal inflammation of the salivary glands and mediastinum. The normal embryonic development of Mlkl^D139V homozygotes until birth, and the absence of any overt phenotype in heterozygotes provides important in vivo precedent for the capacity of cells to clear activated MLKL. These observations offer an important insight into the potential disease-modulating roles of three common human MLKL polymorphisms that encode amino acid substitutions within or adjacent to the brace region. Compound heterozygosity of these variants is found at up to 12-fold the expected frequency in patients that suffer from a pediatric autoinflammatory disease, chronic recurrent multifocal osteomyelitis (CRMO).

Modulation of CD95-mediated signaling by post-translational modifications: towards understanding CD95 signaling networks

CD95 is a member of the death receptor family and is well-known to promote apoptosis. However, accumulating evidence indicates that in some context CD95 has not only the potential to induce apoptosis but also can trigger non-apoptotic signal leading to cell survival, proliferation, cancer growth and metastasis. Despite extensive investigations focused on alterations in the expression level of CD95 and associated signal molecules, very few studies, however, have investigated the effects of post-translational modifications such as glycosylation, phosphorylation, palmitoylation, nitrosylation and glutathionylation on CD95 function. Post-translational modifications of CD95 in mammalian systems are likely to play a more prominent role than anticipated in CD95 induced cell death. In this review we will focus on the alterations in CD95-mediated signaling caused by post-translational modifications of CD95.

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.

Caspase-8 restricts antiviral CD8 T cell hyperaccumulation

The magnitude of CD8 T cell responses against viruses is checked by the balance of proliferation and death. Caspase-8 (CASP8) has the potential to influence response characteristics through initiation of apoptosis, suppression of necroptosis, and modulation of cell death-independent signal transduction. Mice deficient in CASP8 and RIPK3 (Casp8 ^-/- Ripk3 ^-/- ) mount enhanced peak CD8 T cell levels against the natural mouse pathogen murine cytomegalovirus (MCMV) or the human pathogen herpes simplex virus-1 compared with littermate control RIPK3-deficient or WT C57BL/6 mice, suggesting an impact of CASP8 on the magnitude of antiviral CD8 T cell expansion and not on contraction. The higher peak response to MCMV in Casp8 ^-/- Ripk3 ^-/- mice resulted from accumulation of greater numbers of terminally differentiated KLRG1^hi effector CD8 T cell subsets. Antiviral Casp8 ^-/- Ripk3 ^-/- T cells exhibited enhanced proliferation when splenocytes were transferred into WT recipient mice. Thus, cell-autonomous CASP8 normally restricts CD8 T cell proliferation following T cell receptor activation in response to foreign antigen. Memory inflation is a hallmark quality of the T cell response to cytomegalovirus infection. Surprisingly, MCMV-specific memory inflation was not sustained long-term in Casp8 ^-/- Ripk3 ^-/- mice even though these mice retained immunity to secondary challenge. In addition, the accumulation of abnormal B220⁺CD3⁺ T cells in these viable CASP8-deficient mice was reduced by chronic MCMV infection. Combined, these data brings to light the cell death-independent role of CASP8 during CD8 T cell expansion in mice lacking the confounding impact of RIPK3-mediated necroptosis.

Programmed Cell Death in Immune Defense: Knowledge and Presumptions

Cell-culture studies are our main source of knowledge of the various forms of programmed cell death. Yet genetic perturbations of death-protein function in animal models are almost the only source of our knowledge of the physiological roles of these programs. Shortcomings in the state of knowledge acquired by these two experimental approaches are exemplified in this Perspective by reference to research on the contribution of apoptosis to lymphocyte development, a subject on which there is already much knowledge, and on the role of necroptosis in inflammation, about which information is just beginning to emerge. To address these shortcomings, there is need to find ways to verify the notions obtained through the current experimental approaches by directly monitoring death programs within specific cells in vivo.

Dominant Negative FADD/MORT1 Inhibits the Development of Intestinal Intraepithelial Lymphocytes With a Marked Defect on CD8αα+TCRγδ+ T Cells

Intestinal intraepithelial lymphocytes (IELs) play a critical role in mucosal immune system, which differ from thymus-derived cells and develop locally in gut. Although the development of IELs has been studied in some detail, the molecular cues controlling their local development remain unclear. Here, we demonstrate that FADD, a classic adaptor protein required for death-receptor-induced apoptosis, is a critical regulator of the intestinal IEL development. The mice with a dominant negative mutant of FADD (FADD-DN) display an abnormal development of intestinal IELs with a marked reduction in the numbers of CD8αα⁺TCRγδ⁺ T cells. As a precursor for CD8αα⁺ development, lamina propria lymphocytes in lin-negative expression (lin^- LPLs) were analyzed and the massive accumulation of IL-7R^-lin^- LPLs was observed in FADD-DN mice. As IL-7R is one of Notch1-target genes, we further observed that the level of Notch1 expression was lower in Lin^- LPLs from FADD-DN mice compared with normal mice. The downregulation of Notch1 expression induced by FADD-DN overexpression was also confirmed in Jurkat T cells. Considering that IL-7 and its receptor IL7-R play a differentiation inducing role in the development of intestinal IELs, the influence of FADD via its DD domain on Notch1 expression might be a possible molecular signal involved in the early IELs development. In addition, loss of γδ T-IELs in FADD-DN mice aggravates DSS-induced colitis, suggesting that FADD is a relevant contribution to the field of mucosal immunology and intestinal homeostasis.

T Cells and Regulated Cell Death: Kill or Be Killed

Cell death plays two major complementary roles in T cell biology: mediating the removal of cells that are targeted by T cells and the removal of T cells themselves. T cells serve as major actors in the adaptive immune response and function by selectively killing cells which are infected or dysfunctional. This feature is highly involved during homeostatic maintenance, and is relied upon and modulated in the context of cancer immunotherapy. The vital recognition and elimination of both autoreactive T cells and cells which are unable to recognize threats is a highly selective and regulated process. Moreover, detection of potential threats will result in the activation and expansion of T cells, which on resolution of the immune response will need to be eliminated. The culling of these T cells can be executed via a multitude of cell death pathways which are used in context-specific manners. Failure of these processes may result in an accumulation of misdirected or dysfunctional T cells, leading to complications such as autoimmunity or cancer. This review will focus on the role of cell death regulation in the maintenance of T cell homeostasis, as well as T cell-mediated elimination of infected or dysfunctional cells, and will summarize and discuss the current knowledge of the cellular mechanisms which are implicated in these processes.

Molecular cloning and characterization of FADD from the orange-spotted grouper (Epinephelus coioides)

Fas-associated protein with death domain (FADD) is the key adaptor protein that transmits apoptotic signals mediated by the main death receptors. Besides being an essential instrument in cell death, FADD is also implicated in proliferation, cell cycle progression, tumor development, inflammation, innate immunity, and autophagy. In the present study, a FADD homologue (EcFADD) from the orange-spotted grouper (Epinephelus coioides) was cloned and its possible role in fish immunity was analyzed. The full length cDNA of EcFADD contains 808 base pairs (bp), including a 573 bp open reading frame that encodes a 190 amino acid protein with a predicted molecular mass of 21.81 kDa. Quantitative real-time polymerase chain reaction analysis indicated that EcFADD was distributed in all examined tissues. The expression of EcFADD in the spleen of E. coioides was differentially up-regulated when challenged with Singapore grouper iridovirus (SGIV) or polyinosine-polycytidylic acid(poly[I:C]). EcFADD was abundantly distributed in both the cytoplasm and nucleus in grouper spleen (GS) and fathead minnow (FHM) epithelial cells. Over-expression of EcFADD inhibited SGIV infection and replication and SGIV-induced apoptosis. To achieve antiviral and anti-apoptosis activities, FADD promoted the activation of interferon-stimulated response element (ISRE) and type I interferon (IFN) genes in the antiviral IFN signaling pathway and inhibited activation of apoptosis-related transcription factors p53. Our results not only characterize FADD but also reveal new immune functions and the molecular mechanisms by which FADD responds to virus infection and virus-induced apoptosis.

Necroptosis in tumorigenesis, activation of anti-tumor immunity, and cancer therapy

While the mechanisms underlying apoptosis and autophagy have been well characterized over recent decades, another regulated cell death event, necroptosis, remains poorly understood. Elucidating the signaling networks involved in the regulation of necroptosis may allow this form of regulated cell death to be exploited for diagnosis and treatment of cancer, and will contribute to the understanding of the complex tumor microenvironment. In this review, we have summarized the mechanisms and regulation of necroptosis, the converging and diverging features of necroptosis in tumorigenesis, activation of anti-tumor immunity, and cancer therapy, as well as attempts to exploit this newly gained knowledge to provide therapeutics for cancer.

A protective role for FADD dominant negative (FADD-DN) mutant in trinitrochlorobenzene (TNCB)-induced murine contact hypersensitivity reactions

BACKGROUND

Fas-associated protein with death domain (FADD) is a classic adaptor protein in apoptosis. Increasing evidence has shown that FADD is also implicated in T-cell development, activation and proliferation. The role of FADD in inflammatory disorders remains largely unexplored.

AIM

To assess the role of FADD in inflammatory disorders.

METHODS

We established an experimental model of contact hypersensitivity (CHS) by using 2,4,6-trinitrochlorobenzene (TNCB) on transgenic mice expressing a dominant negative mutant of FADD (FADD-DN), RESULTS: CHS responses were clearly attenuated in FADD-DN mice compared with control mice. In the retroauricular lymph nodes, the ratio of CD8+ T cells was also decreased.

CONCLUSION

FADD-DN appears to play a protective role in TNCB-induced CHS reactions.

Viewing BCL2 and cell death control from an evolutionary perspective

The last 30 years of studying BCL2 have brought cell death research into the molecular era, and revealed its relevance to human pathophysiology. Most, if not all metazoans use an evolutionarily conserved process for cellular self destruction that is controlled and implemented by proteins related to BCL2. We propose the anti-apoptotic BCL2-like and pro-apoptotic BH3-only members of the family arose through duplication and modification of genes for the pro-apoptotic multi-BH domain family members, such as BAX and BAK1. In that way, a cell suicide process that initially evolved as a mechanism for defense against intracellular parasites was then also used in multicellular organisms for morphogenesis and to maintain the correct number of cells in adults by balancing cell production by mitosis.

On How Fas Apoptosis-Independent Pathways Drive T Cell Hyperproliferation and Lymphadenopathy in lpr Mice

Fas induces massive apoptosis in T cells after repeated in vitro T cell receptor (TCR) stimulation and is critical for lymphocyte homeostasis in Fas-deficient (lpr) mice. Although the in vitro Fas apoptotic mechanism has been defined, there is a large conceptual gap between this in vitro phenomenon and the pathway that leads to in vivo development of lymphadenopathy and autoimmunity. A striking abnormality in lpr mice is the excessive proliferation of CD4⁺ and CD8⁺ T cells, and more so of the double-negative TCR⁺CD4⁻CD8⁻B220⁺ T cells. The basis of lpr T cell hyperproliferation remains elusive, as it cannot be explained by Fas-deficient apoptosis. T cell-directed p21 overexpression reduces hyperactivation/hyperproliferation of all lpr T cell subtypes and lymphadenopathy in lpr mice. p21 controls expansion of repeatedly stimulated T cells without affecting apoptosis. These results confirm a direct link between hyperactivation/hyperproliferation, autoreactivity, and lymphadenopathy in lpr mice and, with earlier studies, suggest that Fas apoptosis-independent pathways control lpr T cell hyperproliferation. lpr T cell hyperproliferation could be an indirect result of the defective apoptosis of repeatedly stimulated lpr T cells. Nonetheless, in this perspective, we argue for an alternative setting, in which lack of Fas would directly cause lpr T cell hyperactivation/hyperproliferation in vivo. We propose that Fas/Fas ligand (FasL) acts as an activation inhibitor of recurrently stimulated T cells, and that its disruption causes overexpansion of T cells in lpr mice. Research to define the underlying mechanism of this Fas/FasL effect could resolve the phenotype of lpr mice and lead to therapeutics for related human syndromes.

Fas-670A>G polymorphism is not associated with an increased risk of acute myeloid leukemia development

The association between the increased risk of acute myeloid leukemia (AML) and Fas promoter polymorphisms has been reported previously; however, the results are inconclusive. The present study performed one case-control study to investigate the association, and a total of 98 AML patients and 2,014 healthy controls were genotyped. The data showed that the distribution of Fas-670AA, GA and GG genotypes among the AML patients were not significantly different from those of the healthy controls, all P>0.05. Following this a sub-study was conducted to analyze individuals who neither smoked nor drank. The results demonstrated that there was still no significant association between the Fas-670 polymorphism and risk of AML development, all P>0.05. Furthermore, in order to address a more accurate estimation of the association, a meta-analysis was conducted. Data were systematically collected from the Pubmed, EMBASE and the Wanfang Library. A total of 3 studies were included in this meta-analysis, which contained 1,144 AML cases and 3,806 controls. No significant association was detected between the Fas-670A>G polymorphism and AML risk [GA+GG vs. AA: odds ratio (OR) 0.93; 95% confidence interval (CI), 0.79–1.09; GG vs. AA: OR, 1.01; 95% CI, 0.82–1.24; GA vs. AA: OR, 1.12; 95% CI, 0.94–1.32; GG vs. AA+GA: OR, 0.94; 95% CI, 0.79–1.12; G vs. A: OR, 1.01; 95% CI, 0.91–1.12; all P>0.05). The analysis clearly indicated that there was no significant connection between the Fas-670A>G polymorphism and the increased risk of AML.

Necroptosis and Inflammation

Necroptosis is a regulated form of necrosis, with the dying cell rupturing and releasing intracellular components that can trigger an innate immune response. Toll-like receptor 3 and 4 agonists, tumor necrosis factor, certain viral infections, or the T cell receptor can trigger necroptosis if the activity of the protease caspase-8 is compromised. Necroptosis signaling is modulated by the kinase RIPK1 and requires the kinase RIPK3 and the pseudokinase MLKL. Either RIPK3 deficiency or RIPK1 inhibition confers resistance in various animal disease models, suggesting that inflammation caused by necroptosis contributes to tissue damage and that inhibitors of these kinases could have therapeutic potential. Recent studies have revealed unexpected complexity in the regulation of cell death programs by RIPK1 and RIPK3 with the possibility that necroptosis is but one mechanism by which these kinases promote inflammation.

A novel function of RIP1 in postnatal development and immune homeostasis by protecting against RIP3-dependent necroptosis and FADD-mediated apoptosis

RIP1 is an adaptor kinase originally identified as being able to associate with TNFR1 and Fas, and is later shown to be involved in signaling induced by TLRs. Major signaling pathways regulated by RIP1 include necroptosis, apoptosis, and pro-survival/inflammation NF-κB activation. Previous studies show that RIP1 deficiency has no effect on mouse embryogenesis, but blocks postnatal development. This phenotype could not readily be explained, since mice lacking TNFR1, Fas, or TLRs show no apparent developmental defect. Certain types of RIP1-deficient cells are hypersensitive to TNF-induced apoptosis. However, in our previous study, deletion of the apoptotic adaptor protein, FADD, provides marginal improvement of postnatal development of rip1^−/− mice. Remarkably, the current data shows that haploid insufficiency of RIP3, a known mediator of necroptosis, allowed survival of rip1^−/−fadd^−/− mice beyond weaning age, although the resulting rip1^−/−fadd^−/− rip3^+/− mice were significant smaller in size and weight. Moreover, complete absence of RIP3 further improved postnatal development of the resulting rip1^−/−fadd^−/−rip3^−/− mice, which display normal size and weight. In such triple knockout (TKO) mice, lymphocytes underwent normal development, but progressively accumulated as mice age. This lymphoproliferative (lpr) disease in TKO mice is, however, less severe than that of fadd^−/−rip3^−/− double knockout mice. In total, the data show that the postnatal developmental defect in rip1^−/− mice is due in part to FADD-mediated apoptosis as well as RIP3-dependent necroptosis. Moreover, the function of RIP1 contributes to development of lpr diseases.

The large conductance calcium-activated potassium channel affects extrinsic and intrinsic mechanisms of apoptosis

The large-conductance calcium-activated K(+) or BK channel underlies electrical signals in a number of different cell types. Studies show that BK activity can also serve to regulate cellular homeostasis by protecting cells from apoptosis resulting from events such as ischemia. Recent coimmunoprecipitation studies, combined with mass spectrometry, suggest putative protein partners that interact with BK to regulate intrinsic and extrinsic apoptotic pathways. This study tests two of those partners to determine the effects on these two signaling pathways. Through reciprocal coimmunoprecipitation (coIP) experiments, we show that BK interacts with p53 and fas-associated protein with death domain (FADD) in mouse brain and when overexpressed in a heterologous expression system, such as HEK293 cells. Moreover, coIP experiments with N- and C-terminal fragments reveal that FADD interacts with the C-terminus of BK, whereas p53 interacts with either the N- or the C-terminus. Immunolocalization studies show that BK colocalizes with p53 and FADD in the mitochondrion and plasmalemma, respectively. HEK cells that stably express BK are more resistant to apoptosis when p53 or FADD is overexpressed or when their intrinsic and extrinsic pathways are stimulated via mitomycin C or tumor necrosis factor-related apoptosis-inducing ligand (TRAIL), respectively. Moreover, when stimulating with TRAIL, caspase-8 activation decreases in BK-expressing cells. These data suggest that BK is part of a larger complex of proteins that protects against apoptosis by interacting with proapoptotic proteins, such as p53 and FADD.

Autoreactive T cells induce necrosis and not BCL-2-regulated or death receptor-mediated apoptosis or RIPK3-dependent necroptosis of transplanted islets in a mouse model of type 1 diabetes

AIMS/HYPOTHESIS

Type 1 diabetes results from T cell-mediated destruction of pancreatic beta cells. The mechanisms of beta cell destruction in vivo, however, remain unclear. We aimed to test the relative roles of the main cell death pathways: apoptosis, necrosis and necroptosis, in beta cell death in the development of CD4(+) T cell-mediated autoimmune diabetes.

METHODS

We altered expression levels of critical cell death proteins in mouse islets and tested their ability to survive CD4(+) T cell-mediated attack using an in vivo graft model.

RESULTS

Loss of the B cell leukaemia/lymphoma 2 (BCL-2) homology domain 3-only proteins BIM, PUMA or BID did not protect beta cells from this death. Overexpression of the anti-apoptotic protein BCL-2 or combined deficiency of the pro-apoptotic multi-BCL2 homology domain proteins BAX and BAK also failed to prevent beta cell destruction. Furthermore, loss of function of the death receptor Fas or its essential downstream signalling molecule Fas-associated death domain (FADD) in islets was also not protective. Using electron microscopy we observed that dying beta cells showed features of necrosis. However, islets deficient in receptor-interacting serine/threonine protein kinase 3 (RIPK3), a critical initiator of necroptosis, were still normally susceptible to CD4(+) T cell-mediated destruction. Remarkably, simultaneous inhibition of apoptosis and necroptosis by combining loss of RIPK3 and overexpression of BCL-2 in islets did not protect them against immune attack either.

CONCLUSIONS/INTERPRETATION

Collectively, our data indicate that beta cells die by necrosis in autoimmune diabetes and that the programmed cell death pathways apoptosis and necroptosis are both dispensable for this process.

Necroptotic signaling in adaptive and innate immunity

The vertebrate immune system is highly dependent on cell death for efficient responsiveness to microbial pathogens and oncogenically transformed cells. Cell death pathways are vital to the function of many immune cell types during innate, humoral and cellular immune responses. In addition, cell death regulation is imperative for proper adaptive immune self-tolerance and homeostasis. While apoptosis has been found to be involved in several of these roles in immunity, recent data demonstrate that alternative cell death pathways are required. Here, we describe the involvement of a programmed form of cellular necrosis called "necroptosis" in immunity. We consider the signaling pathways that promote necroptosis downstream of death receptors, type I transmembrane proteins of the tumor necrosis factor (TNF) receptor family. The involvement of necroptotic signaling through a "RIPoptosome" assembled in response to innate immune stimuli or genotoxic stress is described. We also characterize the induction of necroptosis following antigenic stimulation in T cells lacking caspase-8 or FADD function. While necroptotic signaling remains poorly understood, it is clear that this pathway is an essential component to effective vertebrate immunity.

FADD regulates thymocyte development at the β-selection checkpoint by modulating Notch signaling

Non-apoptotic functions of Fas-associated protein with death domain (FADD) have been implicated in T lineage lymphocytes, but the nature of FADD-dependent non-apoptotic mechanism in early T-cell development has not been completely elucidated. In this study, we show that tissue-specific deletion of FADD in immature (CD44^–CD25⁺) thymocytes results in severe perturbation of αβ lineage development. Meanwhile, loss of FADD signaling at a later (CD44^–CD25^–) developmental stage does not affect subsequent T-cell development. Collectively, our work presents that FADD deficiency induces failed survival in double-negative 4 (DN4) cells, while pre-T-cell receptor (TCR) signal remains intact. In addition, Notch signaling is positive regulated on DN4 and double-positive thymocytes in T-cell-specific FADD-knockout mice, which express higher levels of a subset of Notch-target genes, including Hes1, Deltex1 and CD25. Moreover, a transcriptional repressor of Notch1, NKAP is downregulated coupled with the loss of FADD in thymocytes and is found to associate with FADD. These data suggest that as a death receptor, FADD is also required for cell survival in β-selection as a regulator of Notch1 expression.

Functions of caspase 8: the identified and the mysterious

Initially discovered as an initiator protease in apoptosis mediated by death receptors, caspase-8 is now known to have an apparently confounding opposing effect in securing cell survival. It is required to allow mouse embryo survival, and the survival of hematopoietic cells during their development and activation. Classic models in which caspase-8 is depleted or inhibited frequently result in inhibition of apoptosis, and conversion to death through a necrotic pathway. This bewildering switch is now known to be driven by activation of a pathway dependent on protein kinases of the RIP family, which engage a pathway known as necroptosis. If caspase-8 does not control this pathway, necrotic death results. The pro-apoptotic and pro-survival functions of caspase-8 are regulated by a specific interaction with the pseudo-caspase cFLIP, and it is thought that the heterocomplex between these two partners alters the substrate specificity of caspase-8 in favor of inactivating components of the RIP kinase pathway. The description of how caspase-8 and cFLIP coordinate the switch between apoptosis and survival is just beginning. The mechanism is not known, the differential targets are not known, and the reason of why an apoptotic initiator has been co-opted as a critical survival factor is only guessed at. Elucidating these unknowns will be important in understanding mechanisms and possible therapeutic targets in autoimmune, inflammatory, and metastatic diseases.

Structural studies of death receptors

This chapter describes reports of the structural characterization of death ligands and death receptors (DRs) from the tumor necrosis factor (TNF) and TNF receptor families. The review discusses the interactions of these proteins with agonist ligands, inhibitors, and downstream signaling molecules. Though historically labeled as being implicated in programmed cell death, the function of these proteins extends to nonapoptotic pathways. The review highlights, from a structural biology perspective, the complexity of DR signaling and the ongoing challenge to discern the precise mechanisms that occur at the point of DR activation, including how the degree to which the receptors are induced to cluster may be related to the nature of the impact upon the cell. The potential for posttranslational modification and receptor internalization to play roles in DR signaling is briefly discussed.

RIP3: a molecular switch for necrosis and inflammation

The receptor-interacting protein kinase 3 (RIP3/RIPK3) has emerged as a critical regulator of programmed necrosis/necroptosis, an inflammatory form of cell death with important functions in pathogen-induced and sterile inflammation. RIP3 activation is tightly regulated by phosphorylation, ubiquitination, and caspase-mediated cleavage. These post-translational modifications coordinately regulate the assembly of a macromolecular signaling complex termed the necrosome. Recently, several reports indicate that RIP3 can promote inflammation independent of its pronecrotic activity. Here, we review our current understanding of the mechanisms that drive RIP3-dependent necrosis and its role in different inflammatory diseases.

Control of life-or-death decisions by RIP1 kinase

RIP1 kinase, a multifunctional protein that contains an N-terminal Ser/Thr kinase and a C-terminal death domain, has emerged as a key regulatory molecule involved in regulating both cell death and cell survival. When the proinflammatory cytokine TNFα stimulates its receptor, TNFR1, RIP1 regulates whether the cell lives by activating NF-κB or dies by apoptosis or necroptosis, two distinct pathways of programmed cell death that may be activated to eliminate unwanted cells. The kinase domain of RIP1 is involved in regulating necroptosis, and the death domain regulates RIP1 recruitment to the intracellular domain of TNFR1. The intermediate domain of RIP1 activates NF-κB and also interacts with RIP3 kinase, a downstream mediator of RIP1 in the execution of necroptosis. This review focuses on the functional roles of RIP1 in regulating multiple cellular mechanisms, the dynamic regulation of RIP1, and the physiological and pathological roles of RIP1 kinase in human health and disease.

Structural determinants of DISC function: new insights into death receptor-mediated apoptosis signalling

Death receptors are members of the tumour necrosis factor (TNF) receptor superfamily characterised by an ~80 amino acid long alpha-helical fold, termed the death domain (DD). Death receptors diversified during early vertebrate evolution indicating that the DD fold has plasticity and specificity that can be easily adjusted to attain additional functions. Eight members of the death receptor family have been identified in humans, which can be divided into four structurally homologous groups or clades, namely: the p75(NTR) clade (consisting of ectodysplasin A receptor, death receptor 6 (DR6) and p75 neurotrophin (NTR) receptor); the tumour necrosis factor receptor 1 clade (TNFR1 and DR3), the CD95 clade (CD95/FAS) and the TNF-related apoptosis-inducing ligand receptor (TRAILR) clade (TRAILR1 and TRAILR2). Receptors in the same clade participate in similar processes indicating that structural diversification enabled functional specialisation. On the surface of nearly all human cells multiple death receptors are expressed, enabling the cell to respond to a plethora of external signals. Activation of different death receptors converges on the activation of three main signal transduction pathways: nuclear factor-κB-mediated differentiation or inflammation, mitogen-associated protein kinase-mediated stress response and caspase-mediated apoptosis. While the ability to induce cell death is true for nearly all DRs, the FAS and TRAILR clades have specialised in inducing cell death. Here we summarise recent discoveries about the molecular regulation and structural requirements of apoptosis induction by death receptors and discuss how this information can be used to better explain the biological functions, similarities and distinguishing features of death receptors.

The roles of FADD in extrinsic apoptosis and necroptosis

Fas-associated protein with death domain (FADD), an adaptor that bridges death receptor signaling to the caspase cascade, is indispensible for the induction of extrinsic apoptotic cell death. Interest in the non-apoptotic function of FADD has greatly increased due to evidence that FADD-deficient mice or dominant-negative FADD transgenic mice result in embryonic lethality and an immune defect without showing apoptotic features. Numerous studies have suggested that FADD regulates cell cycle progression, proliferation, and autophagy, affecting these phenomena. Recently, programmed necrosis, also called necroptosis, was shown to be a key mechanism that induces embryonic lethality and an immune defect. Supporting these findings, FADD was shown to be involved in various necroptosis models. In this review, we summarize the mechanism of extrinsic apoptosis and necroptosis, and discuss the in vivo and in vitro roles of FADD in necroptosis induced by various stimuli.

Programmed necrosis and autophagy in immune function

It has long been known that apoptosis is vital to the generation and maintenance of proper adaptive immune function. An example is the essential requirement for apoptotic signaling during the generation of self-tolerant lymphocytes: the apoptotic death of B and T cells with overt autoreactivity is essential to central tolerance. More recently, the contributions of additional processes including cellular autophagy and programmed necrosis have been implicated in controlling both innate and adaptive immune functions. Evidence has been provided to demonstrate that the death of cells following ligation of death receptors (DRs), a subfamily of cell surface molecules related to tumor necrosis factor receptor 1, is not exclusively the domain of caspase-dependent apoptosis. In cells lacking the capacity to activate caspase-8 following DR ligation, cell death instead occurs via programmed necrosis, or as it has been recently termed, 'necroptosis'. This death process depends on RIP1 and RIP3, serine/threonine kinases that are recruited by DRs, and likely by other cellular signals including DNA damage and antigen receptor ligation. The generation of RIP1/RIP3 containing 'necrosomes' activates downstream necroptotic signaling that ultimately targets cellular energetic metabolism. Also related to cellular metabolic regulation, cellular autophagy has also been found to play unique and important roles in immunity. In this review, we describe the roles of necroptosis and autophagy in innate and adaptive immunity and speculate on the intriguing interplay between these two cellular processes.

Type I interferons induce apoptosis by balancing cFLIP and caspase-8 independent of death ligands

Interferons induce a pleiotropy of responses through binding the same cell surface receptor. Here we investigated the molecular mechanism driving interferon-induced apoptosis. Using a nonbiased small interfering RNA (siRNA) screen, we show that silencing genes whose products are directly engaged in the initiation of interferon signaling completely abrogate the interferon antiproliferative response. Apoptosis-related genes such as the caspase-8, cFLIP, and DR5 genes specifically interfere with interferon-induced apoptosis, which we found to be independent of the activity of death ligands. The one gene for which silencing resulted in the strongest proapoptotic effect upon interferon signaling is the cFLIP gene, where silencing shortened the time of initiation of apoptosis from days to hours and increased dramatically the population of apoptotic cells. Thus, cFLIP serves as a regulator for interferon-induced apoptosis. A shift over time in the balance between cFLIP and caspase-8 results in downstream caspase activation and apoptosis. While gamma interferon (IFN-γ) also causes caspase-8 upregulation, we suggest that it follows a different path to apoptosis.

Protease activity of procaspase-8 is essential for cell survival by inhibiting both apoptotic and nonapoptotic cell death dependent on receptor-interacting protein kinase 1 (RIP1) and RIP3

Caspase-8 has an important role as an initiator caspase during death receptor-mediated apoptosis. Moreover, it has been reported to contribute to the regulation of cell fate in various types of cells including T-cells. In this report, we show that caspase-8 has an essential role in cell survival in mouse T-lymphoma-derived L5178Y cells. The knockdown of caspase-8 expression decreased the growth rate and increased cell death, both of which were induced by the absence of protease activity of procaspase-8. The cell death was associated with reactive oxygen species (ROS) accumulation, caspase activation, and autophagosome formation. The cell death was inhibited completely by treatment with ROS scavengers, but only partly by treatment with caspase inhibitors, expression of Bcl-xL, and knockdown of caspase-3 or Atg-7 which completely inhibits apoptosis or autophagosome formation, respectively, indicating that apoptosis and autophagy-associated cell death are induced simultaneously by the knockdown of caspase-8 expression. Further analysis indicated that RIP1 and RIP3 regulate this multiple cell death, because the cell death as well as ROS production was completely inhibited by not only treatment with the RIP1 inhibitor necrostatin-1, but also by knockdown of RIP3. Thus, in the absence of protease activity of procaspase-8, RIP1 and RIP3 simultaneously induce not only nonapoptotic cell death conceivably including autophagic cell death and necroptosis but also apoptosis through ROS production in mouse T-lymphoma cells.

FADD protein release mirrors the development and aggressiveness of human non-small cell lung cancer

BACKGROUND

The need to unfold the underlying mechanisms of lung cancer aggressiveness, the deadliest cancer in the world, is of prime importance. Because Fas-associated death domain protein (FADD) is the key adaptor molecule transmitting the apoptotic signal delivered by death receptors, we studied the presence and correlation of intra- and extracellular FADD protein with development and aggressiveness of non-small cell lung cancer (NSCLC).

METHODS

Fifty NSCLC patients were enrolled in this prospective study. Intracellular FADD was detected in patients' tissue by immunohistochemistry. Tumours and distant non-tumoural lung biopsies were cultured through trans-well membrane in order to analyse extracellular FADD. Correlation between different clinical/histological parameters with level/localisation of FADD protein has been investigated.

RESULTS

Fas-associated death domain protein could be specifically downregulated in tumoural cells and FADD loss correlated with the presence of extracellular FADD. Indeed, human NSCLC released FADD protein, and tumoural samples released significantly more FADD than non-tumoural (NT) tissue (P=0.000003). The release of FADD by both tumoural and NT tissue increased significantly with the cancer stage, and was correlated with both early and late steps of the metastasis process.

CONCLUSION

The release of FADD by human NSCLC could be a new marker of poor prognosis as it correlates positively with both tumour progression and aggressiveness.

A death effector domain chain DISC model reveals a crucial role for caspase-8 chain assembly in mediating apoptotic cell death

Formation of the death-inducing signaling complex (DISC) is a critical step in death receptor-mediated apoptosis, yet the mechanisms underlying assembly of this key multiprotein complex remain unclear. Using quantitative mass spectrometry, we have delineated the stoichiometry of the native TRAIL DISC. While current models suggest that core DISC components are present at a ratio of 1:1, our data indicate that FADD is substoichiometric relative to TRAIL-Rs or DED-only proteins; strikingly, there is up to 9-fold more caspase-8 than FADD in the DISC. Using structural modeling, we propose an alternative DISC model in which procaspase-8 molecules interact sequentially, via their DED domains, to form a caspase-activating chain. Mutating key interacting residues in procaspase-8 DED2 abrogates DED chain formation in cells and disrupts TRAIL/CD95 DISC-mediated procaspase-8 activation in a functional DISC reconstitution model. This provides direct experimental evidence for a DISC model in which DED chain assembly drives caspase-8 dimerization/activation, thereby triggering cell death.

Caspase-8 inactivation in T cells increases necroptosis and suppresses autoimmunity in Bim-/- mice

In the absence of the pro-apoptotic factor Bim, caspase-8 plays an important role in restraining autoimmunity by inducing cell death in T cells.

Dysregulation of either the extrinsic or intrinsic apoptotic pathway can lead to various diseases including immune disorders and cancer. In addition to its role in the extrinsic apoptotic pathway, caspase-8 plays nonapoptotic functions and is essential for T cell homeostasis. The pro-apoptotic BH3-only Bcl-2 family member Bim is important for the intrinsic apoptotic pathway and its inactivation leads to autoimmunity that is further exacerbated by loss of function of the death receptor Fas. We report that inactivation of caspase-8 in T cells of Bim^−/− mice restrained their autoimmunity and extended their life span. We show that, similar to caspase-8^−/− T cells, Bim^−/− T cells that also lack caspase-8 displayed elevated levels of necroptosis and that inhibition of this cell death process fully rescued the survival and proliferation of these cells. Collectively, our data demonstrate that inactivation of caspase-8 suppresses the survival and proliferative capacity of Bim^−/− T cells and restrains autoimmunity in Bim^−/− mice.

RIPK-dependent necrosis and its regulation by caspases: a mystery in five acts

Caspase-8, FADD, and FLIP orchestrate apoptosis in response to death receptor ligation. Mysteriously however, these proteins are also required for normal embryonic development and immune cell proliferation, an observation that has led to their implication in several nonapoptotic processes. While many scenarios have been proposed, recent genetic and biochemical evidence points to unregulated signaling by the receptor-interacting protein kinases-1 (RIPK1) and RIPK3 as the lethal defect in caspase-8-, FADD-, and FLIP-deficient animals and tissues. The RIPKs are known killers, being responsible for a nonapoptotic form of cell death with features similar to necrosis. However, the mechanism by which caspase-8, FADD, and FLIP prevent runaway RIPK activation is unknown, and the signals that trigger these events during development and immune cell activation remain at large. In this review, we will lay out the evidence as it now stands, reinterpreting earlier observations in light of new clues and considering where the investigation might lead.

Apoptosis and the airway epithelium

The airway epithelium functions as a barrier and front line of host defense in the lung. Apoptosis or programmed cell death can be elicited in the epithelium as a response to viral infection, exposure to allergen or to environmental toxins, or to drugs. While apoptosis can be induced via activation of death receptors on the cell surface or by disruption of mitochondrial polarity, epithelial cells compared to inflammatory cells are more resistant to apoptotic stimuli. This paper focuses on the response of airway epithelium to apoptosis in the normal state, apoptosis as a potential regulator of the number and types of epithelial cells in the airway, and the contribution of epithelial cell apoptosis in important airways diseases.

Ripped to death

An old puzzle in the field of cell death was solved recently: the mysterious embryonic lethality of animals deficient in caspase-8 or Fas-associated death domain (FADD), proteins involved in a pathway of apoptosis. This lethality is caused by a failure to develop the yolk sac vasculature rather than a lack of apoptosis. Remarkably, development is rescued by ablation of either of two receptor interacting serine-threonine kinases (RIPKs). Despite being well known cell killers, caspase-8 and FADD act together to block RIPK-mediated necrosis. To manifest this newly elucidated pro-survival function, FADD and caspase-8 depend on FLIP(Long), a catalytically inactive caspase-8 homolog. In this review, the mechanism by which RIPK necrotic death is inhibited by this trio is discussed, as well as how RIPKs might themselves mediate cell death.

Complementary roles of Fas-associated death domain (FADD) and receptor interacting protein kinase-3 (RIPK3) in T-cell homeostasis and antiviral immunity

Caspase-8 (casp8) is required for extrinsic apoptosis, and mice deficient in casp8 fail to develop and die in utero while ultimately failing to maintain the proliferation of T cells, B cells, and a host of other cell types. Paradoxically, these failures are not caused by a defect in apoptosis, but by a presumed proliferative function of this protease. Indeed, following mitogenic stimulation, T cells lacking casp8 or its adaptor protein FADD (Fas-associated death domain protein) develop a hyperautophagic morphology, and die a programmed necrosis-like death process termed necroptosis. Recent studies have demonstrated that receptor-interacting protein kinases (RIPKs) RIPK1 and RIPK3 together facilitate TNF-induced necroptosis, but the precise role of RIPKs in the demise of T cells lacking FADD or casp8 activity is unknown. Here we demonstrate that RIPK3 and FADD have opposing and complementary roles in promoting T-cell clonal expansion and homeostasis. We show that the defective proliferation of T cells bearing an interfering form of FADD (FADDdd) is rescued by crossing with RIPK3(-/-) mice, although such rescue ultimately leads to lymphadenopathy. Enhanced recovery of these double-mutant T cells following stimulation demonstrates that FADD, casp8, and RIPK3 are all essential for clonal expansion, contraction, and antiviral responses. Finally, we demonstrate that caspase-mediated cleavage of RIPK1-containing necrosis inducing complexes (necrosomes) is sufficient to prevent necroptosis in the face of death receptor signaling. These studies highlight the "two-faced" nature of casp8 activity, promoting clonal expansion in some situations and apoptotic demise in others.