TAK1 is required for survival of mouse fibroblasts treated with TRAIL, and does so by NF-kappaB dependent induction of cFLIPL

Caspase-8-driven apoptotic and pyroptotic crosstalk causes cell death and IL-1β release in X-linked inhibitor of apoptosis (XIAP) deficiency

Genetic lesions in X-linked inhibitor of apoptosis (XIAP) pre-dispose humans to cell death-associated inflammatory diseases, although the underlying mechanisms remain unclear. Here, we report that two patients with XIAP deficiency-associated inflammatory bowel disease display increased inflammatory IL-1β maturation as well as cell death-associated caspase-8 and Gasdermin D (GSDMD) processing in diseased tissue, which is reduced upon patient treatment. Loss of XIAP leads to caspase-8-driven cell death and bioactive IL-1β release that is only abrogated by combined deletion of the apoptotic and pyroptotic cell death machinery. Namely, extrinsic apoptotic caspase-8 promotes pyroptotic GSDMD processing that kills macrophages lacking both inflammasome and apoptosis signalling components (caspase-1, -3, -7, -11 and BID), while caspase-8 can still cause cell death in the absence of both GSDMD and GSDME when caspase-3 and caspase-7 are present. Neither caspase-3 and caspase-7-mediated activation of the pannexin-1 channel, or GSDMD loss, prevented NLRP3 inflammasome assembly and consequent caspase-1 and IL-1β maturation downstream of XIAP inhibition and caspase-8 activation, even though the pannexin-1 channel was required for NLRP3 triggering upon mitochondrial apoptosis. These findings uncouple the mechanisms of cell death and NLRP3 activation resulting from extrinsic and intrinsic apoptosis signalling, reveal how XIAP loss can co-opt dual cell death programs, and uncover strategies for targeting the cell death and inflammatory pathways that result from XIAP deficiency.

The roles of ubiquitination in extrinsic cell death pathways and its implications for therapeutics

Regulation of cell survival and death, including apoptosis and necroptosis, is important for normal development and tissue homeostasis, and disruption of these processes can cause cancer, inflammatory diseases, and degenerative diseases. Ubiquitination is a cellular process that induces proteasomal degradation by covalently attaching ubiquitin to the substrate protein. In addition to proteolytic ubiquitination, nonproteolytic ubiquitination, such as M1-linked and K63-linked ubiquitination, has been shown to be important in recent studies, which have demonstrated its function in cell signaling pathways that regulate inflammation and cell death pathways. In this review, we summarize the TRAIL- and TNF-induced death receptor signaling pathways along with recent advances in this field and illustrate how different types of ubiquitination control cell death and survival. In particular, we provide an overview of the different types of ubiquitination, target residues, and modifying enzymes, including E3 ligases and deubiquitinating enzymes. Given the relevance of these regulatory pathways in human disease, we hope that a better understanding of the regulatory mechanisms of cell death pathways will provide insights into and therapeutic strategies for related diseases.

LUBAC and ABIN-1 Modulate TRAIL-Based NF-κB Induction in Human Embryonic Kidney 293 Cells

Tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) is known to activate the canonical NF-κB pathway similar to TNF. The exact mechanism of the entire signaling cascade is still under investigation. The involvement of linear ubiquitylation as upregulating component has already been shown recently in some cell lines, but not in human embryonic kidney 293 (HEK293) cells. The downregulating function of the ABIN-1 (A20 binding and inhibitor of NF-κB) as linear ubiquitylation antagonist has been shown in combination with some NF-κB-inducing pathways, but not with TRAIL. We performed luciferase and western blot assays using HEK293 cells stimulated with either TRAIL (or TNF as a control) to analyze the involvement of linear ubiquitin chain assembly complex (LUBAC) components and the impact of ABIN-1 and ABIN-1-MAD (truncated form without A20 binding site) on NF-κB signaling. For overexpression experiments, we added plasmids of ABIN-1 and ABIN-1-MAD or LUBAC components HOIP, HOIL-1, or SHARPIN (single and combinations). For downregulation experiments five pairs of either SHARPIN, HOIL-1, or HOIP targeting miRNAs or one miRNA for ABIN-1 were designed and added. ABIN-1 and its truncated form ABIN-1-MAD reduced the NF-κB induction significantly indicating its involvement as antagonist (independent of deubiquitinase A20) of linear ubiquitylation in TRAIL-induced NF-κB signaling. In opposition, knockdown of ABIN-1 using a specific ABIN-1 miRNA led a clear increase of NF-κB signaling. Addition of single LUBAC components or combinations (except for SHARPIN with HOIL-1) resulted in clearly stronger NF-κB inductions. MiRNAs targeting LUBAC components significantly reduced NF-κB activation. Thus, in HEK293 cells linear ubiquitylation by LUBAC critically upregulates and ABIN-1 downregulates TRAIL-induced NF-κB signaling and may be interesting targets for future pathological therapies.

The Autophagy Machinery Controls Cell Death Switching between Apoptosis and Necroptosis

Although autophagy controls cell death and survival, underlying mechanisms are poorly understood, and it is unknown whether autophagy affects only whether or not cells die or also controls other aspects of programmed cell death. MAP3K7 is a tumor suppressor gene associated with poor disease-free survival in prostate cancer. Here, we report that Map3k7 deletion in mouse prostate cells sensitizes to cell death by TRAIL (TNF-related apoptosis-inducing ligand). Surprisingly, this death occurs primarily through necroptosis, not apoptosis, due to assembly of the necrosome in association with the autophagy machinery, mediated by p62/SQSTM1 recruitment of RIPK1. The mechanism of cell death switches to apoptosis if p62-dependent recruitment of the necrosome to the autophagy machinery is blocked. These data show that the autophagy machinery can control the mechanism of programmed cell death by serving as a scaffold rather than by degrading cargo.

The Impact of the Interferon/TNF-Related Apoptosis-Inducing Ligand Signaling Axis on Disease Progression in Respiratory Viral Infection and Beyond

Interferons (IFNs) are well described to be rapidly induced upon pathogen-associated pattern recognition. After binding to their respective IFN receptors and activation of the cellular JAK/signal transducer and activator of transcription signaling cascade, they stimulate the transcription of a plethora of IFN-stimulated genes (ISGs) in infected as well as bystander cells such as the non-infected epithelium and cells of the immune system. ISGs may directly act on the invading pathogen or can either positively or negatively regulate the innate and adaptive immune response. However, IFNs and ISGs do not only play a key role in the limitation of pathogen spread but have also been recently found to provoke an unbalanced, overshooting inflammatory response causing tissue injury and hampering repair processes. A prominent regulator of disease outcome, especially in-but not limited to-respiratory viral infection, is the IFN-dependent mediator TRAIL (TNF-related apoptosis-inducing ligand) produced by several cell types including immune cells such as macrophages or T cells. First described as an apoptosis-inducing agent in transformed cells, it is now also well established to rapidly evoke cellular stress pathways in epithelial cells, finally leading to caspase-dependent or -independent cell death. Hereby, pathogen spread is limited; however in some cases, also the surrounding tissue is severely harmed, thus augmenting disease severity. Interestingly, the lack of a strictly controlled and well balanced IFN/TRAIL signaling response has not only been implicated in viral infection but might furthermore be an important determinant of disease progression in bacterial superinfections and in chronic respiratory illness. Conclusively, the IFN/TRAIL signaling axis is subjected to a complex modulation and might be exploited for the evaluation of new therapeutic concepts aiming at attenuation of tissue injury.

TAK1 regulates caspase 8 activation and necroptotic signaling via multiple cell death checkpoints

Necroptosis has emerged as a new form of programmed cell death implicated in a number of pathological conditions such as ischemic injury, neurodegenerative disease, and viral infection. Recent studies indicate that TGFβ-activated kinase 1 (TAK1) is nodal regulator of necroptotic cell death, although the underlying molecular regulatory mechanisms are not well defined. Here we reported that TAK1 regulates necroptotic signaling as well as caspase 8-mediated apoptotic signaling through both NFκB-dependent and -independent mechanisms. Inhibition of TAK1 promoted TNFα-induced cell death through the induction of RIP1 phosphorylation/activation and necrosome formation. Further, inhibition of TAK1 triggered two caspase 8 activation pathways through the induction of RIP1-FADD-caspase 8 complex as well as FLIP cleavage/degradation. Mechanistically, our data uncovered an essential role for the adaptor protein TNF receptor-associated protein with death domain (TRADD) in caspase 8 activation and necrosome formation triggered by TAK1 inhibition. Moreover, ablation of the deubiqutinase CYLD prevented both apoptotic and necroptotic signaling induced by TAK1 inhibition. Finally, blocking the ubiquitin-proteasome pathway prevented the degradation of key pro-survival signaling proteins and necrosome formation. Thus, we identified new regulatory mechanisms underlying the critical role of TAK1 in cell survival through regulation of multiple cell death checkpoints. Targeting key components of the necroptotic pathway (e.g., TRADD and CYLD) and the ubiquitin-proteasome pathway may represent novel therapeutic strategies for pathological conditions driven by necroptosis.

Marine Drugs Regulating Apoptosis Induced by Tumor Necrosis Factor-Related Apoptosis-Inducing Ligand (TRAIL)

Marine biomass diversity is a tremendous source of potential anticancer compounds. Several natural marine products have been described to restore tumor cell sensitivity to TNF-related apoptosis inducing ligand (TRAIL)-induced cell death. TRAIL is involved during tumor immune surveillance. Its selectivity for cancer cells has attracted much attention in oncology. This review aims at discussing the main mechanisms by which TRAIL signaling is regulated and presenting how marine bioactive compounds have been found, so far, to overcome TRAIL resistance in tumor cells.

The TAK1-NF-κB axis as therapeutic target for AML

Development and maintenance of leukemia can be partially attributed to alterations in (anti)-apoptotic gene expression. Genome-wide transcriptome analyses revealed that 89 apoptosis-associated genes were differentially expressed between patient acute myeloid leukemia (AML) CD34(+) cells and normal bone marrow (NBM) CD34(+) cells. Among these, transforming growth factor-β activated kinase 1 (TAK1) was strongly upregulated in AML CD34(+) cells. Genetic downmodulation or pharmacologic inhibition of TAK1 activity strongly impaired primary AML cell survival and cobblestone formation in stromal cocultures. TAK1 inhibition was mainly due to blockade of the nuclear factor κB (NF-κB) pathway, as TAK1 inhibition resulted in reduced levels of P-IκBα and p65 activity. Overexpression of a constitutive active variant of NF-κB partially rescued TAK1-depleted cells from apoptosis. Importantly, NBM CD34(+) cells were less sensitive to TAK1 inhibition compared with AML CD34(+) cells. Knockdown of TAK1 also severely impaired leukemia development in vivo and prolonged overall survival in a humanized xenograft mouse model. In conclusion, our results indicate that TAK1 is frequently overexpressed in AML CD34(+) cells, and that TAK1 inhibition efficiently targets leukemic stem/progenitor cells in an NF-κB-dependent manner.

Transforming growth factor β-activated kinase 1 signaling pathway critically regulates myocardial survival and remodeling

BACKGROUND

Programmed necrosis (necroptosis) plays an important role in development, tissue homeostasis, and disease pathogenesis. The molecular mechanisms that regulate necroptosis in the heart and its physiological relevance in myocardial remodeling and heart failure remain largely unknown.

METHODS AND RESULTS

Here, we identified an obligate function for TAK1 (transforming growth factor β-activated kinase 1, gene name Map3k7) in regulating necroptotic myocyte death, myocardial remodeling, and heart failure propensity. Cardiac-specific ablation of Map3k7 in mice induced spontaneous apoptosis and necroptosis that led to adverse remodeling and heart failure, and these effects were abolished by ablation of tumor necrosis factor receptor-1. Mechanistically, TAK1 functions as a molecular switch in tumor necrosis factor receptor-1 signaling by regulating the formation of 2 cell death complexes, RIP 1 (receptor-interacting protein 1)-FADD (Fas-associated protein with death domain)-caspase 8 and RIP1-RIP3, a process that is dependent on FADD and caspase 8 as scaffolding molecules. Importantly, inhibition of RIP1 or RIP3 largely blocked necroptotic cell death, adverse remodeling, and heart failure in TAK1-deficient mice.

CONCLUSIONS

These results indicate that TAK1 functions as a key survival factor in the heart by directly antagonizing necroptosis, which is critical for the maintenance of myocardial homeostasis and the prevention of adverse myocardial remodeling.

Gingerol sensitizes TRAIL-induced apoptotic cell death of glioblastoma cells

Glioblastoma multiforme (GBM) is the most lethal and aggressive astrocytoma of primary brain tumors in adults. Although there are many clinical trials to induce the cell death of glioblastoma cells, most glioblastoma cells have been reported to be resistant to TRAIL-induced apoptosis. Here, we showed that gingerol as a major component of ginger can induce TRAIL-mediated apoptosis of glioblastoma. Gingerol increased death receptor (DR) 5 levels in a p53-dependent manner. Furthermore, gingerol decreased the expression level of anti-apoptotic proteins (survivin, c-FLIP, Bcl-2, and XIAP) and increased pro-apoptotic protein, Bax and truncate Bid, by generating reactive oxygen species (ROS). We also found that the sensitizing effects of gingerol in TRAIL-induced cell death were blocked by scavenging ROS or overexpressing anti-apoptotic protein (Bcl-2). Therefore, we showed the functions of gingerol as a sensitizing agent to induce cell death of TRAIL-resistant glioblastoma cells. This study gives rise to the possibility of applying gingerol as an anti-tumor agent that can be used for the purpose of combination treatment with TRAIL in TRAIL-resistant glioblastoma tumor therapy.

The oncogenic properties of EWS/WT1 of desmoplastic small round cell tumors are unmasked by loss of p53 in murine embryonic fibroblasts

Background

Desmoplastic small round cell tumor (DSRCT) is characterized by the presence of a fusion protein EWS/WT1, arising from the t (11;22) (p13;q12) translocation. Here we examine the oncogenic properties of two splice variants of EWS/WT1, EWS/WT1-KTS and EWS/WT1 + KTS.

Methods

We over-expressed both EWS/WT1 variants in murine embryonic fibroblasts (MEFs) of wild-type, p53^+/- and p53^-/- backgrounds and measured effects on cell-proliferation, anchorage-independent growth, clonogenicity after serum withdrawal, and sensitivity to cytotoxic drugs and gamma irradiation in comparison to control cells. We examined gene expression profiles in cells expressing EWS/WT1. Finally we validated our key findings in a small series of DSRCT.

Results

Neither isoform of EWS/WT1 was sufficient to transform wild-type MEFs however the oncogenic potential of both was unmasked by p53 loss. Expression of EWS/WT1 in MEFs lacking at least one allele of p53 enhanced cell-proliferation, clonogenic survival and anchorage-independent growth. EWS/WT1 expression in wild-type MEFs conferred resistance to cell-cycle arrest after irradiation and daunorubicin induced apoptosis. We show DSRCT commonly have nuclear localization of p53, and copy-number amplification of MDM2/MDMX. Expression of either isoform of EWS/WT1 induced characteristic mRNA expression profiles. Gene-set enrichment analysis demonstrated enrichment of WNT pathway signatures in MEFs expressing EWS/WT1 + KTS. Wnt-activation was validated in cell lines with over-expression of EWS/WT1 and in DSRCT.

Conclusion

In conclusion, we show both isoforms of EWS/WT1 have oncogenic potential in MEFs with loss of p53. In addition we provide the first link between EWS/WT1 and Wnt-pathway signaling. These data provide novel insights into the function of the EWS/WT1 fusion protein which characterize DSRCT.

Role of the eIF4E binding protein 4E-BP1 in regulation of the sensitivity of human pancreatic cancer cells to TRAIL and celastrol-induced apoptosis

BACKGROUND INFORMATION

Tumour cells can be induced to undergo apoptosis after treatment with the tumour necrosis factor α-related death-inducing ligand (TRAIL). Although human pancreatic cancer cells show varying degrees of response they can be sensitised to the pro-apoptotic effects of TRAIL in the presence of celastrol, a natural compound extracted from the plant Tripterygium wilfordii Hook F. One important aspect of the cellular response to TRAIL is the control of protein synthesis, a key regulator of which is the eukaryotic initiation factor 4E-binding protein, 4E-BP1.

RESULTS

We examined the effects of celastrol and TRAIL in several pancreatic cancer cell lines. In cells that are normally resistant to TRAIL, synergistic effects of TRAIL plus celastrol on commitment to apoptosis and inhibition of protein synthesis were observed. These were associated with a strong up-regulation and dephosphorylation of 4E-BP1. The enhancement of 4E-BP1 expression, which correlated with a threefold increase in the level of the 4E-BP1 transcript, was blocked by inhibitors of reactive oxygen species and the JNK protein kinase. When the expression of 4E-BP1 was reduced by an inducible micro-RNA, TRAIL-mediated apoptosis was inhibited.

CONCLUSION

These results suggest that 4E-BP1 plays a critical role in the mechanism by which TRAIL and celastrol together cause apoptotic cell death in human pancreatic tumour cells.

c-MYC overexpression overrides TAK1 dependency in efficient tumorigenicity of AKT-transformed cells

Transforming growth factor activated kinase 1 (TAK1) provides prosurvival signals in various types of cells, and emerging evidence indicates that targeting TAK1 is a promising means to eliminate certain types of cancer cells. Here, we show that TAK1 is required for efficient tumorigenicity of AKT-transformed cells. TAK1 inhibition accelerates cell apoptosis of AKT-transformed cells in anchorage-independent cell growth accompanying by the downregulation of Mcl-1 and Bcl-2 expression. On the contrary, the tumorigenicity of c-Myc-transformed cells is not significantly affected by TAK1 inhibition. Moreover, AKT-transformed cells with c-Myc overexpression tolerate TAK1 inhibition in anchorage-independent growth and tumorigenicity in vivo. Together, our results provide evidence that TAK1-dependency in the tumorigenicity of AKT-transformed cells can be alleviated by c-Myc overexpression. These findings suggest that dual-targeting TAK1 and c-Myc might be a rational therapeutic strategy for treatment of certain types of cancer.

Human embryonic and induced pluripotent stem cells express TRAIL receptors and can be sensitized to TRAIL-induced apoptosis

Death ligands and their tumor necrosis factor receptor (TNFR) family receptors are the best-characterized and most efficient inducers of apoptotic signaling in somatic cells. In this study, we analyzed whether these prototypic activators of apoptosis are also expressed and able to be activated in human pluripotent stem cells. We examined human embryonic stem cells (hESC) and human-induced pluripotent stem cells (hiPSC) and found that both cell types express primarily TNF-related apoptosis-inducing ligand (TRAIL) receptors and TNFR1, but very low levels of Fas/CD95. We also found that although hESC and hiPSC contain all the proteins required for efficient induction and progression of extrinsic apoptotic signaling, they are resistant to TRAIL-induced apoptosis. However, both hESC and hiPSC can be sensitized to TRAIL-induced apoptosis by co-treatment with protein synthesis inhibitors such as the anti-leukemia drug homoharringtonine (HHT). HHT treatment led to suppression of cellular FLICE inhibitory protein (cFLIP) and Mcl-1 expression and, in combination with TRAIL, enhanced processing of caspase-8 and full activation of caspase-3. cFLIP likely represents an important regulatory node, as its shRNA-mediated down-regulation significantly sensitized hESC to TRAIL-induced apoptosis. Thus, we provide the first evidence that, irrespective of their origin, human pluripotent stem cells express canonical components of the extrinsic apoptotic system and on stress can activate death receptor-mediated apoptosis.

Non-canonical kinase signaling by the death ligand TRAIL in cancer cells: discord in the death receptor family

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL)-based therapy is currently evaluated in clinical studies as a tumor cell selective pro-apoptotic approach. However, besides activating canonical caspase-dependent apoptosis by binding to TRAIL-specific death receptors, the TRAIL ligand can activate non-canonical cell survival or proliferation pathways in resistant tumor cells through the same death receptors, which is counterproductive for therapy. Even more, recent studies indicate metastases-promoting activity of TRAIL. In this review, the remarkable dichotomy in TRAIL signaling is highlighted. An overview of the currently known mechanisms involved in non-canonical TRAIL signaling and the subsequent activation of various kinases is provided. These kinases include RIP1, IκB/ NF-κB, MAPK p38, JNK, ERK1/2, MAP3K TAK1, PKC, PI3K/Akt and Src. The functional consequences of their activation, often being stimulation of tumor cell survival and in some cases enhancement of their invasive behavior, are discussed. Interestingly, the non-canonical responses triggered by TRAIL in resistant tumor cells resemble that of TRAIL-induced signals in non-transformed cells. Better knowledge of the mechanism underlying the dichotomy in TRAIL receptor signaling may provide markers for selecting patients who will likely benefit from TRAIL-based therapy and could provide a rationalized basis for combination therapies with TRAIL death receptor-targeting drugs.

In mouse embryonic fibroblasts, neither caspase-8 nor cellular FLICE-inhibitory protein (FLIP) is necessary for TNF to activate NF-κB, but caspase-8 is required for TNF to cause cell death, and induction of FLIP by NF-κB is required to prevent it

Binding of TNF to TNF receptor-1 can give a pro-survival signal through activation of p65/RelA NF-κB, but also signals cell death. To determine the roles of FLICE-inhibitory protein (FLIP) and caspase-8 in TNF-induced activation of NF-κB and apoptosis, we used mouse embryonic fibroblasts derived from FLIP and caspase-8 gene-deleted mice, and treated them with TNF and a smac-mimetic compound that causes degradation of cellular inhibitor of apoptosis proteins (cIAPs). In cells treated with smac mimetic, TNF and Fas Ligand caused wild-type and FLIP(-/-) MEFs to die, whereas caspase-8(-/-) MEFs survived, indicating that caspase-8 is necessary for death of MEFs triggered by these ligands when IAPs are degraded. By contrast, neither caspase-8 nor FLIP was required for TNF to activate p65/RelA NF-κB, because IκB was degraded, p65 translocated to the nucleus, and an NF-κB reporter gene activated normally in caspase-8(-/-) or FLIP(-/-) MEFs. Reconstitution of FLIP(-/-) MEFs with the FLIP isoforms FLIP-L, FLIP-R, or FLIP-p43 protected these cells from dying when treated with TNF or FasL, whether or not cIAPs were depleted. These results show that in MEFs, caspase-8 is necessary for TNF- and FasL-induced death, and FLIP is needed to prevent it, but neither caspase-8 nor FLIP is required for TNF to activate NF-κB.

TRADD is critical for resistance to TRAIL-induced cell death through NF-κB activation

One major obstacle in the clinical application of TRAIL as a cancer therapeutic agent is the acquisition of TRAIL resistance. We found that deficiency of TRADD sensitizes cells to TRAIL-induced apoptosis. Enhanced cell death in TRADD(-/-) MEFs is associated with defective NF-κB activation, indicating that the pro-survival function of TRADD in TRAIL signaling is mediated at least in part via NF-κB activation. Moreover, siRNA knock-down of TRADD in cancer cells sensitizes them to TRAIL-induced apoptosis. Thus, TRADD has a survival role in TRAIL signaling and may be one potential target for overcoming TRAIL resistance in cancer therapy.

Phosphorylation and polyubiquitination of transforming growth factor beta-activated kinase 1 are necessary for activation of NF-kappaB by the Kaposi's sarcoma-associated herpesvirus G protein-coupled receptor

Kaposi's sarcoma-associated herpesvirus (KSHV) G protein-coupled receptor (vGPCR) protein has been shown to induce several signaling pathways leading to the modulation of host gene expression. The hijacking of these pathways facilitates the viral life cycle and leads to tumorigenesis. In the present work, we show that transforming growth factor β (TGF-β)-activated kinase 1 (TAK1) is an important player in NF-κB activation induced by vGPCR. We observed that the expression of an inactive TAK1 kinase mutant (TAK1M) reduces vGPCR-induced NF-κB nuclear translocation and transcriptional activity. Consequently, the expression of several NF-κB target genes normally induced by vGPCR was blocked by TAK1M expression, including interleukin 8 (IL-8), Gro1, IκBα, COX-2, cIAP2, and Bcl2 genes. Similar results were obtained after downregulation of TAK1 by small interfering RNA (siRNA) technology. The expression of vGPCR recruited TAK1 to the plasma membrane, and vGPCR interacts with TAK1. vGPCR expression also induced TAK1 phosphorylation and lysine 63-linked polyubiquitination, the two markers of the kinase's activation. Finally, inhibition of TAK1 by celastrol inhibited vGPCR-induced NF-κB activation, indicating this natural compound could be used as a potential therapeutic drug against KSHV malignancies involving vGPCR.

New insights into apoptosis signaling by Apo2L/TRAIL

Apoptosis ligand 2 tumor necrosis factor (TNF)-related apoptosis-inducing ligand (Apo2L/TRAIL) belongs to a small subset of proapoptotic protein ligands in the TNF superfamily. This subset, which also includes Fas ligand and TNF-alpha, can activate the extrinsic apoptotic cell death pathway on binding to cognate death receptors at the cell surface. Over the past 10 years, Apo2L/TRAIL has emerged as a promising candidate for cancer therapy, on the basis of its unique ability to trigger apoptosis in various types of cancer cells without significant toxicity toward normal cells. Herein, we review key advances in understanding the molecular events that control apoptosis signaling by Apo2L/TRAIL, which may aid in the development of cancer therapies based on the extrinsic apoptotic pathway.