Functional complementation between FADD and RIP1 in embryos and lymphocytes

Staurosporine induces necroptotic cell death under caspase-compromised conditions in U937 cells

For a long time necrosis was thought to be an uncontrolled process but evidences recently have revealed that necrosis can also occur in a regulated manner. Necroptosis, a type of programmed necrosis is defined as a death receptor-initiated process under caspase-compromised conditions. The process requires the kinase activity of receptor-interacting protein kinase 1 and 3 (RIPK1 and RIPK3) and mixed lineage kinase domain-like protein (MLKL), as a substrate of RIPK3. The further downstream events remain elusive. We applied known inhibitors to characterize the contributing enzymes in necroptosis and their effect on cell viability and different cellular functions were detected mainly by flow cytometry. Here we report that staurosporine, the classical inducer of intrinsic apoptotic pathway can induce necroptosis under caspase-compromised conditions in U937 cell line. This process could be hampered at least partially by the RIPK1 inhibitor necrotstin-1 and by the heat shock protein 90 kDa inhibitor geldanamycin. Moreover both the staurosporine-triggered and the classical death ligand-induced necroptotic pathway can be effectively arrested by a lysosomal enzyme inhibitor CA-074-OMe and the recently discovered MLKL inhibitor necrosulfonamide. We also confirmed that the enzymatic role of poly(ADP-ribose)polymerase (PARP) is dispensable in necroptosis but it contributes to membrane disruption in secondary necrosis. In conclusion, we identified a novel way of necroptosis induction that can facilitate our understanding of the molecular mechanisms of necroptosis. Our results shed light on alternative application of staurosporine, as a possible anticancer therapeutic agent. Furthermore, we showed that the CA-074-OMe has a target in the signaling pathway leading to necroptosis. Finally, we could differentiate necroptotic and secondary necrotic processes based on participation of PARP enzyme.

Autophagy suppresses RIP kinase-dependent necrosis enabling survival to mTOR inhibition

mTOR inhibitors are used clinically to treat renal cancer but are not curative. Here we show that autophagy is a resistance mechanism of human renal cell carcinoma (RCC) cell lines to mTOR inhibitors. RCC cell lines have high basal autophagy that is required for survival to mTOR inhibition. In RCC4 cells, inhibition of mTOR with CCI-779 stimulates autophagy and eliminates RIP kinases (RIPKs) and this is blocked by autophagy inhibition, which induces RIPK- and ROS-dependent necroptosis in vitro and suppresses xenograft growth. Autophagy of mitochondria is required for cell survival since mTOR inhibition turns off Nrf2 antioxidant defense. Thus, coordinate mTOR and autophagy inhibition leads to an imbalance between ROS production and defense, causing necroptosis that may enhance cancer treatment efficacy.

The RIP1/RIP3 necrosome forms a functional amyloid signaling complex required for programmed necrosis

RIP1 and RIP3 kinases are central players in TNF-induced programmed necrosis. Here, we report that the RIP homotypic interaction motifs (RHIMs) of RIP1 and RIP3 mediate the assembly of heterodimeric filamentous structures. The fibrils exhibit classical characteristics of β-amyloids, as shown by Thioflavin T (ThT) and Congo red (CR) binding, circular dichroism, infrared spectroscopy, X-ray diffraction, and solid-state NMR. Structured amyloid cores are mapped in RIP1 and RIP3 that are flanked by regions of mobility. The endogenous RIP1/RIP3 complex isolated from necrotic cells binds ThT, is ultrastable, and has a fibrillar core structure, whereas necrosis is partially inhibited by ThT, CR, and another amyloid dye, HBX. Mutations in the RHIMs of RIP1 and RIP3 that are defective in the interaction compromise cluster formation, kinase activation, and programmed necrosis in vivo. The current study provides insight into the structural changes that occur when RIP kinases are triggered to execute different signaling outcomes and expands the realm of amyloids to complex formation and signaling.

Lys48-linked TAK1 polyubiquitination at lysine-72 downregulates TNFα-induced NF-κB activation via mediating TAK1 degradation

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular processes. TAK1, a member of the MAPKKK family, is essential for TNFα-induced NF-κB activation. Phosphorylation and Lys(63)-linked polyubiquitination (polyUb) of TAK1 are critical for its activation. However, whether TAK1 is regulated by polyubiquitination-mediated protein degradation after its activation remains unknown. Here we report that TNFα induces TAK1 Lys(48) linked polyubiquitination and degradation at the later time course. Furthermore, we provide direct evidence that TAK1 is modified by Lys(48)-linked polyubiquitination at lysine-72 by mass spectrometry. A K72R point mutation on TAK1 abolishes TAK1 Lys(48)-linked polyubiquitination and enhances TAK1/TAB1 co-overexpression-induced NF-κB activation. As expected, TAK1 K72R mutation inhibits TNFα-induced Lys(48)-linked TAK1 polyubiquitination and degradation. TAK1 K72R mutant prolongs TNFα-induced NF-κB activation and enhances TNFα-induced IL-6 gene expression. Our findings demonstrate that TNFα induces Lys(48)-linked polyubiquitination of TAK1 at lysine-72 and this polyubiquitination-mediated TAK1 degradation plays a critical role in the downregulation of TNFα-induced NF-κB activation.

Novel Ser/Thr protein phosphatases in cell death regulation

Cell death is regulated by a myriad of intracellular molecular pathways, with many involving protein phosphorylation and dephosphorylation. In this review, we will focus on Ser/Thr phosphatases-mediated regulation in cell apoptosis as well as on their potential roles in cell necrosis. The emerging functional importance of Ser/Thr protein phosphatases in cell death regulation adds new dimension to the signaling mechanisms of cellular function, physiology, and diseases.

Regulation of protein kinase C inactivation by Fas-associated protein with death domain

Protein kinase C (PKC) plays important roles in diverse cellular processes. PKC has been implicated in regulating Fas-associated protein with death domain (FADD), an important adaptor protein involved in regulating death receptor-mediated apoptosis. FADD also plays an important role in non-apoptosis processes. The functional interaction of PKC and FADD in non-apoptotic processes has not been examined. In this study, we show that FADD is involved in maintaining the phosphorylation of the turn motif and hydrophobic motif in the activated conventional PKC (cPKC). A phosphoryl-mimicking mutation (S191D) in FADD (FADD-D) abolished the function of FADD in the facilitation of the turn motif and hydrophobic motif dephosphorylation of cPKC, suggesting that phosphorylation of Ser-191 negatively regulates FADD. We show that FADD interacts with PP2A, which is a major phosphatase involved in dephosphorylation of activated cPKC and FADD deficiency abolished PP2A mediated dephosphorylation of cPKC. We show that FADD deficiency leads to increased stability and activity of cPKC, which, in turn, promotes cytoskeleton reorganization, cell motility, and chemotaxis. Collectively, these results reveal a novel function of FADD in a non-apoptotic process by modulating cPKC dephosphorylation, stability, and signaling termination.

Killing a cancer: what are the alternatives?

Evading programmed cell death is one of the hallmarks of cancer. Conversely, inducing cell death by pharmacological means is the basis of almost every non-invasive cancer therapy. Research over the past decade has greatly increased our understanding of non-apoptotic programmed cell death events, such as lysosomal-mediated cell death, necroptosis and cell death with autophagy. It is becoming clear that an intricate effector network connects many of these classical and non-classical death pathways. In this Review, we discuss converging and diverging features of these pathways, as well as attempts to exploit this newly gained knowledge pharmacologically to provide therapeutics for cancer.

RIP1-mediated regulation of lymphocyte survival and death responses

RIP1 is an adaptor serine/threonine kinase associated with the signaling complex of death receptors (DRs) including Fas, TNFR1, and TRAIL-Rs which can initiate apoptosis. While DRs are dispensable throughout development, RIP1 deletion results in perinatal lethality. The developmental defect caused by absence of RIP1 remains unexplained. In previous studies, RIP1-deficient hematopoietic progenitors failed to reconstitute the T cell compartment and our recent data indicate a new role for RIP1 in TCR-induced activation of the pro-survival NF-κB pathway. Here, we show that RIP1 is also critical for B cell development. In addition, RIP1(-/-) B cells stimulated through LPS/TLR4 are impaired in NF-κB activation but have no major defect in the Akt pathway. Recently, RIP1 has also emerged as a critical player in necrosis-like death, necroptosis, in various cell lines. We have demonstrated that RIP1 deficiency can reverse the embryonic and T cell proliferation defects in mice lacking FADD, a caspase adaptor protein, which indicates a potential role for RIP1 in mediating in vivo necroptosis. We provide an overview and discussion of the accumulating data revealing insights into the diverse functions of RIP1 in survival and death signaling in lymphocytes.

Non-apoptotic functions of apoptosis-regulatory proteins

During the past two decades, apoptotic cell death has been the subject of an intense wave of investigation, leading to the discovery of multiple gene products that govern both its induction and execution. In parallel, it has progressively become evident that most, if not all, proteins that had initially been discovered for their essential role in apoptosis also mediate a wide range of non-apoptotic functions. On the one hand, apoptotic regulators and executioners are involved in non-lethal physiological processes as diverse as cell cycle progression, differentiation, metabolism, autophagy and inflammation. On the other hand, pro-apoptotic proteins can control other modalities of programmed cell death, in particular regulated necrosis. In this review, we summarize the unconventional roles of the apoptotic core machinery from a functional perspective and discuss their pathophysiological implications.

Smac mimetic bypasses apoptosis resistance in FADD- or caspase-8-deficient cells by priming for tumor necrosis factor α-induced necroptosis

Searching for new strategies to bypass apoptosis resistance, we investigated the potential of the Smac mimetic BV6 in Jurkat leukemia cells deficient in key molecules of the death receptor pathway. Here, we demonstrate for the first time that Smac mimetic primes apoptosis-resistant, FADD- or caspase-8-deficient leukemia cells for TNFα-induced necroptosis in a synergistic manner. In contrast to TNFα, Smac mimetic significantly enhances CD95-induced apoptosis in wild-type but not in FADD-deficient cells. Interestingly, Smac mimetic- and TNFα-mediated cell death occurs without characteristic features of apoptosis (i.e., caspase activation, DNA fragmentation) in FADD-deficient cells. By comparison, Smac mimetic and TNFα trigger activation of caspase-8, -9, and -3 and DNA fragmentation in wild-type cells. Consistently, the caspase inhibitor zVAD.fmk fails to block Smac mimetic- and TNFα-triggered cell death in FADD- or caspase-8-deficient cells, while it confers protection in wild-type cells. By comparison, necrostatin-1, an RIP1 kinase inhibitor, abolishes Smac mimetic- and TNFα-induced cell death in FADD- or caspase-8-deficient. Thus, Smac mimetic enhances TNFα-induced cell death in leukemia cells via two distinct pathways in a context-dependent manner: it primes apoptosis-resistant cells lacking FADD or caspase-8 to TNFα-induced, RIP1-dependent and caspase-independent necroptosis, whereas it sensitizes apoptosis-proficient cells to TNFα-mediated, caspase-dependent apoptosis. These findings have important implications for the therapeutic exploitation of necroptosis as an alternative cell death program to overcome apoptosis resistance.

NF-κB, the first quarter-century: remarkable progress and outstanding questions

The ability to sense and adjust to the environment is crucial to life. For multicellular organisms, the ability to respond to external changes is essential not only for survival but also for normal development and physiology. Although signaling events can directly modify cellular function, typically signaling acts to alter transcriptional responses to generate both transient and sustained changes. Rapid, but transient, changes in gene expression are mediated by inducible transcription factors such as NF-κB. For the past 25 years, NF-κB has served as a paradigm for inducible transcription factors and has provided numerous insights into how signaling events influence gene expression and physiology. Since its discovery as a regulator of expression of the κ light chain gene in B cells, research on NF-κB continues to yield new insights into fundamental cellular processes. Advances in understanding the mechanisms that regulate NF-κB have been accompanied by progress in elucidating the biological significance of this transcription factor in various physiological processes. NF-κB likely plays the most prominent role in the development and function of the immune system and, not surprisingly, when dysregulated, contributes to the pathophysiology of inflammatory disease. As our appreciation of the fundamental role of inflammation in disease pathogenesis has increased, so too has the importance of NF-κB as a key regulatory molecule gained progressively greater significance. However, despite the tremendous progress that has been made in understanding the regulation of NF-κB, there is much that remains to be understood. In this review, we highlight both the progress that has been made and the fundamental questions that remain unanswered after 25 years of study.

Two programmed cell death systems in Escherichia coli: an apoptotic-like death is inhibited by the mazEF-mediated death pathway

A newly discovered apoptotic-like death is inhibited by the previously described mazEF-mediated death pathway, revealing two programmed cell death systems in Escherichia coli.

In eukaryotes, the classical form of programmed cell death (PCD) is apoptosis, which has as its specific characteristics DNA fragmentation and membrane depolarization. In Escherichia coli a different PCD system has been reported. It is mediated by the toxin–antitoxin system module mazEF. The E. coli mazEF module is one of the most thoroughly studied toxin–antitoxin systems. mazF encodes a stable toxin, MazF, and mazE encodes a labile antitoxin, MazE, which prevents the lethal effect of MazF. mazEF-mediated cell death is a population phenomenon requiring the quorum-sensing pentapeptide NNWNN designated Extracellular Death Factor (EDF). mazEF is triggered by several stressful conditions, including severe damage to the DNA. Here, using confocal microscopy and FACS analysis, we show that under conditions of severe DNA damage, the triggered mazEF-mediated cell death pathway leads to the inhibition of a second cell death pathway. The latter is an apoptotic-like death (ALD); ALD is mediated by recA and lexA. The mazEF-mediated pathway reduces recA mRNA levels. Based on these results, we offer a molecular model for the maintenance of an altruistic characteristic in cell populations. In our model, the ALD pathway is inhibited by the altruistic EDF-mazEF-mediated death pathway.

The enteric bacterium Escherichia coli, like most other bacteria, carries on its chromosome a pair of genes, mazE and mazF (mazEF): mazF specifies a toxin, and mazE specifies an antitoxin. Previously, we have shown that E. coli mazEF is responsible for bacterial programmed cell death in response to stressors such as DNA damage. Here, we report that extensive DNA damage can induce a second mode of cell death, which we call apoptotic-like death (ALD). ALD is like apoptosis—a mode of cell death that has previously been recorded only in eukaryotes. During ALD, the cell membrane is depolarized, and the DNA is fragmented and can be detected using the classical TUNEL assay. The MazEF death pathway, however, shows neither of those features, yet also kills the cell. We show that ALD is mediated by two proteins, RecA and LexA, which are noteworthy because LexA is an inhibitor of the SOS response (which is a global response to DNA damage in which the cell cycle is arrested and DNA repair is induced). This defines ALD as a form of SOS response. Furthermore, MazEF and its downstream components cause reduction of recA mRNA levels, which could explain how the MazEF pathway inhibits the ALD pathway. We conclude that the E. coli ALD pathway is a back-up system for the traditional mazEF cell death pathway. Should one of the components of the mazEF pathway be inactivated, bacterial cell death would occur through ALD. These findings also have implications for the mechanisms of “altruistic” cell death among bacterial populations.

RIP kinase-dependent necrosis drives lethal systemic inflammatory response syndrome

Engagement of tumor necrosis factor receptor 1 signals two diametrically opposed pathways: survival-inflammation and cell death. An additional switch decides, depending on the cellular context, between caspase-dependent apoptosis and RIP kinase (RIPK)-mediated necrosis, also termed necroptosis. We explored the contribution of both cell death pathways in TNF-induced systemic inflammatory response syndrome (SIRS). Deletion of apoptotic executioner caspases (caspase-3 or -7) or inflammatory caspase-1 had no impact on lethal SIRS. However, deletion of RIPK3 conferred complete protection against lethal SIRS and reduced the amounts of circulating damage-associated molecular patterns. Pretreatment with the RIPK1 kinase inhibitor, necrostatin-1, provided a similar effect. These results suggest that RIPK1-RIPK3-mediated cellular damage by necrosis drives mortality during TNF-induced SIRS. RIPK3 deficiency also protected against cecal ligation and puncture, underscoring the clinical relevance of RIPK kinase inhibition in sepsis and identifying components of the necroptotic pathway that are potential therapeutic targets for treatment of SIRS and sepsis.

Necroptosis turns TNF lethal

In this issue of Immunity, Duprez et al. (2011) demonstrate that necroptosis, a form of regulated necrosis that is mediated by the kinases RIPK1 and RIPK3, underlies the lethal effects of tumor necrosis factor in vivo.

Intermediate domain of receptor-interacting protein kinase 1 (RIPK1) determines switch between necroptosis and RIPK1 kinase-dependent apoptosis

Receptor-interacting protein kinase 1 (RIPK1) is an important component of the tumor necrosis factor receptor 1 (TNFR1) signaling pathway. Depending on the cell type and conditions, RIPK1 mediates MAPK and NF-κB activation as well as cell death. Using a mutant form of RIPK1 (RIPK1ΔID) lacking the intermediate domain (ID), we confirm the requirement of this domain for activation of these signaling events. Moreover, expression of RIPK1ΔID resulted in enhanced recruitment of caspase-8 to the TNFR1 complex II component Fas-associated death domain (FADD), which allowed a shift from TNF-induced necroptosis to apoptosis in L929 cells. Addition of the RIPK1 kinase inhibitor necrostatin-1 strongly reduced recruitment of RIPK1 and caspase-8 to FADD and subsequent apoptosis, indicating a role for RIPK1 kinase activity in apoptotic complex formation. Our study shows that RIPK1 has an anti-apoptotic function residing in its ID and demonstrates a cellular system as an elegant genetic model for RIPK1 kinase-dependent apoptosis that, in contrast to the Smac mimetic model, does not rely on depletion of cellular inhibitor of apoptosis protein 1 and 2 (cIAP1/2).

Pax2 regulates a fadd-dependent molecular switch that drives tissue fusion during eye development

Tissue fusion is an essential morphogenetic mechanism in development, playing a fundamental role in developing neural tube, palate and the optic fissure. Disruption of genes associated with the tissue fusion can lead to congenital malformations, such as spina bifida, cleft lip/palate and ocular coloboma. For instance, the Pax2 transcription factor is required for optic fissure closure, although the mechanism of Pax2 action leading to tissue fusion remains elusive. This lack of information defining how transcription factors drive tissue morphogenesis at the cellular level is hampering new treatments options. Through loss- and gain-of-function analysis, we now establish that pax2 in combination with vax2 directly regulate the fas-associated death domain (fadd) gene. In the presence of fadd, cell proliferation is restricted in the developing eye through a caspase-dependent pathway. However, the loss of fadd results in a proliferation defect and concomitant activation of the necroptosis pathway through RIP1/RIP3 activity, leading to an abnormal open fissure. Inhibition of RIP1 with the small molecule drug necrostatin-1 rescues the pax2 eye fusion defect, thereby overcoming the underlying genetic defect. Thus, fadd has an essential physiological function in protecting the developing optic fissure neuroepithelium from RIP3-dependent necroptosis. This study demonstrates the molecular hierarchies that regulate a cellular switch between proliferation and the apoptotic and necroptotic cell death pathways, which in combination drive tissue morphogenesis. Furthermore, our data suggest that future therapeutic strategies may be based on small molecule drugs that can bypass the gene defects causing common congenital tissue fusion defects.

Autophagy is a survival force via suppression of necrotic cell death

Macroautophagy or autophagy is a self-digesting mechanism that the cellular contents are engulfed by autophagosomes and delivered to lysosomes for degradation. Although it has been well established that autophagy is an important protective mechanism for cells under stress such as starvation via provision of nutrients and removal of protein aggregates and damaged mitochondria, there is a very complex relation between autophagy and cell death. At present, the molecular cross-talk between autophagy and apoptosis has been well discussed, while the relationship between autophagy and programmed necrotic cell death is less understood. In this review we focus on the role of autophagy in necrotic cell death by detailed discussion on two important forms of necrotic cell death: (i) necroptosis and (ii) poly-(ADP-ribose) polymerase (PARP)-mediated cell death. It is believed that one important aspect of the pro-survival function of autophagy is achieved via its ability to block various forms of necrotic cell death.

IAPs limit activation of RIP kinases by TNF receptor 1 during development

Inhibitor of apoptosis (IAP) proteins cIAP1, cIAP2, and XIAP (X-linked IAP) regulate apoptosis and cytokine receptor signalling, but their overlapping functions make it difficult to distinguish their individual roles. To do so, we deleted the genes for IAPs separately and in combination. While lack of any one of the IAPs produced no overt phenotype in mice, deletion of cIap1 with cIap2 or Xiap resulted in mid-embryonic lethality. In contrast, Xiap(-/-)cIap2(-/-) mice were viable. The death of cIap2(-/-)cIap1(-/-) double mutants was rescued to birth by deletion of tumour necrosis factor (TNF) receptor 1, but not TNFR2 genes. Remarkably, hemizygosity for receptor-interacting protein kinase 1 (Ripk1) allowed Xiap(-/-)cIap1(-/-) double mutants to survive past birth, and prolonged cIap2(-/-)cIap1(-/-) embryonic survival. Similarly, deletion of Ripk3 was able to rescue the mid-gestation defect of cIap2(-/-)cIap1(-/-) embryos, as these embryos survived to E15.5. cIAPs are therefore required during development to limit activity of RIP kinases in the TNF receptor 1 signalling pathway.

TNF and ubiquitin at the crossroads of gene activation, cell death, inflammation, and cancer

Tumor necrosis factor (TNF) is crucial for innate immunity, but deregulated TNF signaling also plays an eminent role in the pathogenesis of many chronic inflammatory diseases and cancer-related inflammation. The signals that mediate both the beneficial and the harmful effects of TNF are initiated when TNF binds to its receptors on the surface of target cells. TNF receptor 1 (TNFR1) is ubiquitously expressed, whereas TNFR2 is mainly expressed on lymphocytes and endothelial cells. This review focuses on the molecular and physiological consequences of the interaction of TNF with TNFR1. The different outcomes of TNF signaling originate at the apical signaling complex that forms when TNF binds to TNFR1, the TNFR1 signaling complex (TNF-RSC). By integrating recently gained insight on the functional importance of the presence of different types of ubiquitination in the TNF-RSC, including linear ubiquitin linkages generated by the linear ubiquitin chain assembly complex (LUBAC), with the equally recent elucidation of the mode in which ubiquitin-binding domains interact with specific di-ubiquitin linkages, this review develops a new concept for the way the concerted action of different ubiquitination events enables the TNF-RSC to generate its signaling output in a spatio-temporally controlled manner. Finally, it will be explained how these new findings and the emerging concept of differential ubiquitination governing the TNF-RSC may impact future research on the molecular mechanism of TNF signaling and the function of this cytokine in normal physiology, chronic inflammation, and cancer.

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.

Rip1 (receptor-interacting protein kinase 1) mediates necroptosis and contributes to renal ischemia/reperfusion injury

Loss of kidney function in renal ischemia/reperfusion injury is due to programmed cell death, but the contribution of necroptosis, a newly discovered form of programmed necrosis, has not been evaluated. Here, we identified the presence of death receptor-mediated but caspase-independent cell death in murine tubular cells and characterized it as necroptosis by the addition of necrostatin-1, a highly specific receptor-interacting protein kinase 1 inhibitor. The detection of receptor-interacting protein kinase 1 and 3 in whole-kidney lysates and freshly isolated murine proximal tubules led us to investigate the contribution of necroptosis in a mouse model of renal ischemia/reperfusion injury. Treatment with necrostatin-1 reduced organ damage and renal failure, even when administered after reperfusion, resulting in a significant survival benefit in a model of lethal renal ischemia/reperfusion injury. Unexpectedly, specific blockade of apoptosis by zVAD, a pan-caspase inhibitor, did not prevent the organ damage or the increase in urea and creatinine in vivo in renal ischemia/reperfusion injury. Thus, necroptosis is present and has functional relevance in the pathophysiological course of ischemic kidney injury and shows the predominance of necroptosis over apoptosis in this setting. Necrostatin-1 may have therapeutic potential to prevent and treat renal ischemia/reperfusion injury.

Exploiting inhibitor of apoptosis proteins as therapeutic targets in hematological malignancies

Resistance to apoptosis is one of the hallmarks of human cancers and contributes to the insensitivity of many cancers to commonly used treatment approaches. Inhibitor of apoptosis (IAP) proteins, a family of anti-apoptotic proteins, have an important role in evasion of apoptosis, as they can both block apoptosis-signaling pathways and promote survival. High expression of IAP proteins is observed in multiple cancers, including hematological malignancies, and has been associated with unfavorable prognosis and poor patients' outcome. Therefore, IAP proteins are currently considered as promising molecular targets for therapy. Indeed, drug-discovery approaches over the last decade aiming at neutralizing IAP proteins have resulted in the generation of small-molecule inhibitors or antisense oligonucleotides that demonstrated in vitro and in vivo antitumor activities in preclinical studies. As some of these strategies have already entered the stage of clinical evaluation, for example, in leukemia, an update on this promising molecular-targeted strategy to interfere with apoptotic pathways is of broad interest.

Many stimuli pull the necrotic trigger, an overview

The lab of Jürg Tschopp was the first to report on the crucial role of receptor-interacting protein kinase 1 (RIPK1) in caspase-independent cell death. Because of this pioneer finding, regulated necrosis and in particular RIPK1/RIPK3 kinase-mediated necrosis, referred to as necroptosis, has become an intensively studied form of regulated cell death. Although necrosis was identified initially as a backup cell death program when apoptosis is blocked, it is now recognized as a cellular defense mechanism against viral infections and as being critically involved in ischemia-reperfusion damage. The observation that RIPK3 ablation rescues embryonic lethality in mice deficient in caspase-8 or Fas-associated-protein-via-a-death-domain demonstrates the crucial role of this apoptotic platform in the negative control of necroptosis during development. Here, we review and discuss commonalities and differences of the increasing list of inducers of regulated necrosis ranging from cytokines, pathogen-associated molecular patterns, to several forms of physicochemical cellular stress. Since the discovery of the crucial role of RIPK1 and RIPK3 in necroptosis, these kinases have become potential therapeutic targets. The availability of new pharmacological inhibitors and transgenic models will allow us to further document the important role of this form of cell death in degenerative, inflammatory and infectious diseases.

Viral infection and the evolution of caspase 8-regulated apoptotic and necrotic death pathways

Pathogens specifically target both the caspase 8-dependent apoptotic cell death pathway and the necrotic cell death pathway that is dependent on receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3). The fundamental co-regulation of these two cell death pathways emerged when the midgestational death of mice deficient in FAS-associated death domain protein (FADD) or caspase 8 was reversed by elimination of RIP1 or RIP3, indicating a far more entwined relationship than previously appreciated. Thus, mammals require caspase 8 activity during embryogenesis to suppress the kinases RIP1 and RIP3 as part of the dialogue between two distinct cell death processes that together fulfil reinforcing roles in the host defence against intracellular pathogens such as herpesviruses.

Dual face apoptotic machinery: from initiator of apoptosis to guardian of necroptosis

In this issue of Immunity, Bonnet et al. (2011) show that skin-specific ablation of the adaptor protein FADD sensitizes keratinocytes to RIPK3-dependent necrotic cell death, which leads to severe skin inflammation.

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.

The role of TNF and Fas dependent signaling in animal models of inflammatory liver injury and liver cancer

Tumor Necrosis Factor (TNF) alpha is a pleiotropic cytokine triggering either pro-inflammatory effects via NF-κB related pathways or apoptosis through activation of caspase-8. The related death ligands Fas and TRAIL use homologous receptors and similar signaling cascades but predominantly induce apoptosis. Here, we summarize our experimental approaches to analyze the mechanisms and consequences of TNF and Fas signaling with the ultimate aim to define molecular targets for the treatment of inflammatory liver disease and liver cancer. By using conditional knockout technology in mice we genetically dissected the I-kappa B kinase (IKK) complex consisting of IKK1/IKKα, IKK2/IKKβ and IKKγ/NEMO. We demonstrated that IKK2/IKKβ, but not IKKγ/NEMO might be a promising target for the prevention of liver injury after ischemia and reperfusion or treating steatohepatitis. Genetic inactivation of IKKγ/NEMO defined a new animal model of spontaneous hepatitis and hepatocarcinogenesis involving constitutive activation of caspase-8 and basal apoptosis. We further show that caspase-8 is not only regulated by post-translational modifications as suggested earlier, but also by complex transcriptional regulation. Targeted stimulation of the caspase-8 promoter by interferons alpha and gamma, cytotoxic drugs or p53 can substantially sensitize hepatoma cells for apoptosis, whereas hepatocellular carcinoma frequently present an inactive caspase-8 gene promoter. In conclusion, our work demonstrates that therapeutic intervention in the TNF-NF-κB-caspase-8 network is technically feasible and could be of potential benefit in inflammatory liver disease.

TNF-induced necroptosis in L929 cells is tightly regulated by multiple TNFR1 complex I and II members

TNF receptor 1 signaling induces NF-κB activation and necroptosis in L929 cells. We previously reported that cellular inhibitor of apoptosis protein-mediated receptor-interacting protein 1 (RIP1) ubiquitination acts as a cytoprotective mechanism, whereas knockdown of cylindromatosis, a RIP1-deubiquitinating enzyme, protects against tumor necrosis factor (TNF)-induced necroptosis. We report here that RIP1 is a crucial mediator of canonical NF-κB activation in L929 cells, therefore questioning the relative cytoprotective contribution of RIP1 ubiquitination versus canonical NF-κB activation. We found that attenuated NF-κB activation has no impact on TNF-induced necroptosis. However, we identified A20 and linear ubiquitin chain assembly complex as negative regulators of necroptosis. Unexpectedly, and in contrast to RIP3, we also found that knockdown of RIP1 did not block TNF cytotoxicity. Cell death typing revealed that RIP1-depleted cells switch from necroptotic to apoptotic death, indicating that RIP1 can also suppress apoptosis in L929 cells. Inversely, we observed that Fas-associated protein via a death domain, cellular FLICE inhibitory protein and caspase-8, which are all involved in the initiation of apoptosis, counteract necroptosis induction. Finally, we also report RIP1-independent but RIP3-mediated necroptosis in the context of TNF signaling in particular conditions.

Programmed necrosis: backup to and competitor with apoptosis in the immune system

Programmed cell death is essential for the development and maintenance of the immune system and its responses to exogenous and endogenous stimuli. Studies have demonstrated that in addition to caspase-dependent apoptosis, necrosis dependent on the kinases RIP1 and RIP3 (also called necroptosis) is a major programmed cell-death pathway in development and immunity. These two programmed cell-death pathways may suppress each other, and necroptosis also serves as an alternative when caspase-dependent apoptosis is inhibited or absent. Here we summarize recent advancements that have identified the molecular mechanisms that underlie necroptosis and explore the mechanisms that regulate the interplay between apoptosis and necroptosis.

Caspase 8 inhibits programmed necrosis by processing CYLD

Caspase 8 initiates apoptosis downstream of TNF death receptors by undergoing autocleavage and processing the executioner caspase 3 (ref. 1). However, the dominant function of caspase 8 is to transmit a pro-survival signal that suppresses programmed necrosis (or necroptosis) mediated by RIPK1 and RIPK3 (refs 2-6) during embryogenesis and haematopoiesis(7-9). Suppression of necrotic cell death by caspase 8 requires its catalytic activity but not the autocleavage essential for apoptosis(10); however, the key substrate processed by caspase 8 to block necrosis has been elusive. A key substrate must meet three criteria: it must be essential for programmed necrosis; it must be cleaved by caspase 8 in situations where caspase 8 is blocking necrosis; and mutation of the caspase 8 processing site on the substrate should convert a pro-survival response to necrotic death without the need for caspase 8 inhibition. We now identify CYLD as a substrate for caspase 8 that satisfies these criteria. Following TNF stimulation, caspase 8 cleaves CYLD to generate a survival signal. In contrast, loss of caspase 8 prevented CYLD degradation, resulting in necrotic death. A CYLD substitution mutation at Asp 215 that cannot be cleaved by caspase 8 switches cell survival to necrotic cell death in response to TNF.

It cuts both ways: reconciling the dual roles of caspase 8 in cell death and survival

Caspase 8 can initiate apoptosis, but it also has non-apoptotic roles; for example, it is required for embryonic development and immune cell proliferation. Recent work has indicated that the requirement for caspase 8 in development and immune cell proliferation is defined by suppression of receptor-interacting protein kinase 3 (RIPK3), a kinase that triggers an alternative form of cell death called programmed necrosis. Interestingly, these recent findings can be reconciled with earlier work on the non-apoptotic roles of caspase 8.

The adaptor protein FADD protects epidermal keratinocytes from necroptosis in vivo and prevents skin inflammation

Epidermal keratinocytes provide an essential structural and immunological barrier forming the first line of defense against potentially pathogenic microorganisms. Mechanisms regulating barrier integrity and innate immune responses in the epidermis are important for the maintenance of skin immune homeostasis and the pathogenesis of inflammatory skin diseases. Here, we show that epidermal keratinocyte-restricted deficiency of the adaptor protein FADD (FADD(E-KO)) induced severe inflammatory skin lesions in mice. The development of skin inflammation in FADD(E-KO) mice was triggered by RIP kinase 3 (RIP3)-mediated programmed necrosis (termed necroptosis) of FADD-deficient keratinocytes, which was partly dependent on the deubiquitinating enzyme CYLD and tumor necrosis factor (TNF)-TNF receptor 1 signaling. Collectively, our findings provide an in vivo experimental paradigm that regulation of necroptosis in keratinocytes is important for the maintenance of immune homeostasis and the prevention of chronic inflammation in the skin.

Death receptor signalling in central nervous system inflammation and demyelination

Death receptors (DRs) are members of the tumor necrosis factor receptor (TNF-R) superfamily that are characterised by the presence of a conserved intracellular death domain and are able to trigger a signalling pathway leading to apoptosis. Strong evidence suggests that DRs contribute to the pathology of tissue destructive diseases, including multiple sclerosis (MS), the most common inflammatory demyelinating disease of the central nervous system (CNS). Here, we review the evidence supporting a role for DRs in MS pathology and its implications for the development of therapeutic strategies for MS and other demyelinating pathologies of the CNS.

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.

'Necrosome'-induced inflammation: must cells die for it?

Necrosis, a form of death characterized by rupture of the cell membrane, is closely interlinked with inflammation. Cellular components released during necrotic death can trigger inflammation. Conversely, inflammation often yields tissue damage and, as a consequence, cell death. Which occurs first--necrosis or inflammation--in specific in vivo situations is currently difficult to tell. A way out of this 'chicken-and-egg' conundrum may be found via the recent finding that both necrotic cell death and inflammation can be initiated by a distinct set of signaling proteins, the 'necrosome', that includes receptor-interacting protein (RIP)1, RIP3 and caspase-8. Further clarifying the function of these signaling proteins should make it possible to establish when they induce inflammation directly and when inflammation is caused by necrotic 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.

Deciphering the rules of programmed cell death to improve therapy of cancer and other diseases

Apoptosis, the major form of programmed cell death in metazoan organisms, plays critical roles in normal development, tissue homeostasis and immunity, and its disturbed regulation contributes to many pathological states, including cancer, autoimmunity, infection and degenerative disorders. In vertebrates, it can be triggered either by engagement of 'death receptors' of the tumour necrosis factor receptor family on the cell surface or by diverse intracellular signals that act upon the Bcl-2 protein family, which controls the integrity of the mitochondrial outer membrane through the complex interactions of family members. Both pathways lead to cellular demolition by dedicated proteases termed caspases. This review discusses the groundbreaking experiments from many laboratories that have clarified cell death regulation and galvanised efforts to translate this knowledge into novel therapeutic strategies for the treatment of malignant and perhaps certain autoimmune and infectious diseases.

RIP1-dependent and independent effects of necrostatin-1 in necrosis and T cell activation

BACKGROUND

Programmed necrosis/necroptosis is an emerging form of cell death that plays important roles in mammalian development and the immune system. The pro-necrotic kinases in the receptor interacting protein (RIP) family are crucial mediators of programmed necrosis. Recent advances in necrosis research have been greatly aided by the identification of chemical inhibitors that block programmed necrosis. Necrostatin-1 (Nec-1) and its derivatives were previously shown to target the pro-necrotic kinase RIP1/RIPK1. The protective effect conferred by Nec-1 and its derivatives in many experimental model systems was often attributed to the inhibition of RIP1 function.

METHODOLOGY/PRINCIPAL FINDINGS

We compared the effect of Nec-1 and siRNA-mediated silencing of RIP1 in the murine fibrosarcoma cell line L929. Treatment of L929 cells with the pan-caspase inhibitor zVAD-fmk or exogenous TNF induces necrosis. Strikingly, we found that siRNA-mediated silencing of RIP1 inhibited zVAD-fmk induced necrosis, but not TNF-induced necrosis. TNF-induced cell death in RIP1 knocked down L929 cells was inhibited by Nec-1, but not the caspase inhibitor zVAD-fmk. We found that PKA-C§ expression, but not Jnk or Erk activation, was moderately inhibited by Nec-1. Moreover, we found that Nec-1 inhibits proximal T cell receptor signaling independent of RIP1, leading to inhibition of T cell proliferation.

CONCLUSIONS/SIGNIFICANCE

Our results reveal that besides RIP1, Nec-1 also targets other factors crucial for necrosis induction in L929 cells. In addition, high doses of Nec-1 inhibit other signal transduction pathways such as that for T cell receptor activation. These results highlight the importance to independently validate results obtained using Nec-1 with other approaches such as siRNA-mediated gene silencing. We propose that some of the previous published results obtained using Nec-1 should be re-evaluated in light of our findings.

cIAPs block Ripoptosome formation, a RIP1/caspase-8 containing intracellular cell death complex differentially regulated by cFLIP isoforms

The intracellular regulation of cell death pathways by cIAPs has been enigmatic. Here we show that loss of cIAPs promotes the spontaneous formation of an intracellular platform that activates either apoptosis or necroptosis. This 2 MDa intracellular complex that we designate "Ripoptosome" is necessary but not sufficient for cell death. It contains RIP1, FADD, caspase-8, caspase-10, and caspase inhibitor cFLIP isoforms. cFLIP(L) prevents Ripoptosome formation, whereas, intriguingly, cFLIP(S) promotes Ripoptosome assembly. When cIAPs are absent, caspase activity is the "rheostat" that is controlled by cFLIP isoforms in the Ripoptosome and decides if cell death occurs by RIP3-dependent necroptosis or caspase-dependent apoptosis. RIP1 is the core component of the complex. As exemplified by our studies for TLR3 activation, our data argue that the Ripoptosome critically influences the outcome of membrane-bound receptor triggering. The differential quality of cell death mediated by the Ripoptosome may cause important pathophysiological consequences during inflammatory responses.

FADD prevents RIP3-mediated epithelial cell necrosis and chronic intestinal inflammation

Intestinal immune homeostasis depends on a tightly regulated cross talk between commensal bacteria, mucosal immune cells and intestinal epithelial cells (IECs). Epithelial barrier disruption is considered to be a potential cause of inflammatory bowel disease; however, the mechanisms regulating intestinal epithelial integrity are poorly understood. Here we show that mice with IEC-specific knockout of FADD (FADD(IEC-KO)), an adaptor protein required for death-receptor-induced apoptosis, spontaneously developed epithelial cell necrosis, loss of Paneth cells, enteritis and severe erosive colitis. Genetic deficiency in RIP3, a critical regulator of programmed necrosis, prevented the development of spontaneous pathology in both the small intestine and colon of FADD(IEC-KO) mice, demonstrating that intestinal inflammation is triggered by RIP3-dependent death of FADD-deficient IECs. Epithelial-specific inhibition of CYLD, a deubiquitinase that regulates cellular necrosis, prevented colitis development in FADD(IEC-KO) but not in NEMO(IEC-KO) mice, showing that different mechanisms mediated death of colonic epithelial cells in these two models. In FADD(IEC-KO) mice, TNF deficiency ameliorated colon inflammation, whereas MYD88 deficiency and also elimination of the microbiota prevented colon inflammation, indicating that bacteria-mediated Toll-like-receptor signalling drives colitis by inducing the expression of TNF and other cytokines. However, neither CYLD, TNF or MYD88 deficiency nor elimination of the microbiota could prevent Paneth cell loss and enteritis in FADD(IEC-KO) mice, showing that different mechanisms drive RIP3-dependent necrosis of FADD-deficient IECs in the small and large bowel. Therefore, by inhibiting RIP3-mediated IEC necrosis, FADD preserves epithelial barrier integrity and antibacterial defence, maintains homeostasis and prevents chronic intestinal inflammation. Collectively, these results show that mechanisms preventing RIP3-mediated epithelial cell death are critical for the maintenance of intestinal homeostasis and indicate that programmed necrosis of IECs might be implicated in the pathogenesis of inflammatory bowel disease, in which Paneth cell and barrier defects are thought to contribute to intestinal inflammation.

Pancreatic β-cell death in response to pro-inflammatory cytokines is distinct from genuine apoptosis

A reduction in functional β-cell mass leads to both major forms of diabetes; pro-inflammatory cytokines, such as interleukin-1beta (IL-1β) and gamma-interferon (γ-IFN), activate signaling pathways that direct pancreatic β-cell death and dysfunction. However, the molecular mechanism of β-cell death in this context is not well understood. In this report, we tested the hypothesis that individual cellular death pathways display characteristic phenotypes that allow them to be distinguished by the precise biochemical and metabolic responses that occur during stimulus-specific initiation. Using 832/13 and INS-1E rat insulinoma cells and isolated rat islets, we provide evidence that apoptosis is unlikely to be the primary pathway underlying β-cell death in response to IL-1β+γ-IFN. This conclusion was reached via the experimental results of several different interdisciplinary strategies, which included: 1) tandem mass spectrometry to delineate the metabolic differences between IL-1β+γ-IFN exposure versus apoptotic induction by camptothecin and 2) pharmacological and molecular interference with either NF-κB activity or apoptosome formation. These approaches provided clear distinctions in cell death pathways initiated by pro-inflammatory cytokines and bona fide inducers of apoptosis. Collectively, the results reported herein demonstrate that pancreatic β-cells undergo apoptosis in response to camptothecin or staurosporine, but not pro-inflammatory cytokines.

Discrimination between primary necrosis and apoptosis by necrostatin-1 in Annexin V-positive/propidium iodide-negative cells

Apoptotic cell death eventually results in secondary necrotic cell death, whereas caspase-independent primary necrotic cell death has been reported and its mechanism involving RIP1 and RIP3 has been recently elucidated. Dual staining with fluorescent Annexin V and propidium iodide (PI) has been used to discriminate apoptotic and necrotic cell death, in which Annexin V-positive/PI-negative staining is regarded as apoptosis and PI-positive staining as necrosis. Here we demonstrate that primary necrotic cells unexpectedly show Annexin V-positive/PI-negative staining before they become PI-positive, and that primary necrotic and apoptotic Annexin V-positive/PI-negative cells can be discriminated by necrostatin-1, an inhibitor of primary necrosis by inhibition of RIP1.

Proliferative versus apoptotic functions of caspase-8 Hetero or homo: the caspase-8 dimer controls cell fate

Caspase-8, the initiator of extrinsically-triggered apoptosis, also has important functions in cellular activation and differentiation downstream of a variety of cell surface receptors. It has become increasingly clear that the heterodimer of caspase-8 with the long isoform of cellular FLIP (FLIP(L)) fulfills these pro-survival functions of caspase-8. FLIP(L), a catalytically defective caspase-8 paralog, can interact with caspase-8 to activate its catalytic function. The caspase-8/FLIP(L) heterodimer has a restricted substrate repertoire and does not induce apoptosis. In essence, caspase-8 heterodimerized with FLIP(L) prevents the receptor interacting kinases RIPK1 and -3 from executing the form of cell death known as necroptosis. This review discusses the latest insights in caspase-8 homo- versus heterodimerization and the implication this has for cellular death or survival. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

CARD-mediated autoinhibition of cIAP1's E3 ligase activity suppresses cell proliferation and migration

E3 ligases mediate the covalent attachment of ubiquitin to target proteins thereby enabling ubiquitin-dependent signaling. Unraveling how E3 ligases are regulated is important because miscontrolled ubiquitylation can lead to disease. Cellular inhibitor of apoptosis (cIAP) proteins are E3 ligases that modulate diverse biological processes such as cell survival, proliferation, and migration. Here, we have solved the structure of the caspase recruitment domain (CARD) of cIAP1 and identified that it is required for cIAP1 autoregulation. We demonstrate that the CARD inhibits activation of cIAP1's E3 activity by preventing RING dimerization, E2 binding, and E2 activation. Moreover, we show that the CARD is required to suppress cell proliferation and migration. Further, CARD-mediated autoregulation is also necessary to maximally suppress caspase-8-dependent apoptosis and vascular tree degeneration in vivo. Taken together, our data reveal mechanisms by which the E3 ligase activity of cIAP1 is controlled, and how its deregulation impacts on cell proliferation, migration and cell survival.