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Necroptosis is a form of regulated cell death, which is induced by ligand binding to TNF family death domain receptors, pattern recognizing receptors and virus sensors. The common feature of these receptor systems is the implication of proteins, which contain a receptor interaction protein kinase (RIPK) homology interaction motif (RHIM) mediating recruitment and activation of receptor-interacting protein kinase 3 (RIPK3), which ultimately activates the necroptosis executioner mixed lineage kinase domain-like (MLKL). In case of the TNF family members, the initiator is the survival- and cell death-regulating RIPK1 kinase, in the case of Toll-like receptor 3/4 (TLR3/4), a RHIM-containing adaptor, called TRIF, while in the case of Z-DNA-binding protein ZBP1/DAI, the cytosolic viral sensor itself contains a RHIM domain. In this review, we discuss the different protein complexes that serve as nucleation platforms for necroptosis and the mechanism of execution of necroptosis. Transgenic models (knockout, kinase-dead knock-in) and pharmacologic inhibition indicate that RIPK1, RIPK3 or MLKL are implicated in many inflammatory, degenerative and infectious diseases. However, the conclusion of necroptosis being solely involved in the etiology of diseases is blurred by the pleiotropic roles of RIPK1 and RIPK3 in other cellular processes such as apoptosis and inflammasome activation.

Cells may die from accidental cell death (ACD) or regulated cell death (RCD). ACD is a biologically uncontrolled process, whereas RCD involves tightly structured signaling cascades and molecularly defined effector mechanisms. A growing number of novel non-apoptotic forms of RCD have been identified and are increasingly being implicated in various human pathologies. Here, we critically review the current state of the art regarding non-apoptotic types of RCD, including necroptosis, pyroptosis, ferroptosis, entotic cell death, netotic cell death, parthanatos, lysosome-dependent cell death, autophagy-dependent cell death, alkaliptosis and oxeiptosis. The in-depth comprehension of each of these lethal subroutines and their intercellular consequences may uncover novel therapeutic targets for the avoidance of pathogenic cell loss.

Immunogenic cell death significantly contributes to the success of anti-cancer therapies, but immunogenicity of different cell death modalities widely varies. Ferroptosis, a form of cell death that is characterized by iron accumulation and lipid peroxidation, has not yet been fully evaluated from this perspective. Here we present an inducible model of ferroptosis, distinguishing three phases in the process—‘initial’ associated with lipid peroxidation, ‘intermediate’ correlated with ATP release and ‘terminal’ recognized by HMGB1 release and loss of plasma membrane integrity—that serves as tool to study immune cell responses to ferroptotic cancer cells. Co-culturing ferroptotic cancer cells with dendritic cells (DC), reveals that ‘initial’ ferroptotic cells decrease maturation of DC, are poorly engulfed, and dampen antigen cross-presentation. DC loaded with ferroptotic, in contrast to necroptotic, cancer cells fail to protect against tumor growth. Adding ferroptotic cancer cells to immunogenic apoptotic cells dramatically reduces their prophylactic vaccination potential. Our study thus shows that ferroptosis negatively impacts antigen presenting cells and hence the adaptive immune response, which might hinder therapeutic applications of ferroptosis induction. Inducing ferroptosis of tumour cells is a promising therapeutic approach in cancer. Authors show here that on the other hand, cells dying via ferroptosis are less immunogenic than necroptotic cells, they inhibit maturation and antigen cross-presentation of dendritic cells, hence lessen the anti-tumour immune response.

Mixed lineage kinase domain-like protein (MLKL) emerged as executioner of necroptosis, a RIPK3-dependent form of regulated necrosis. Cell death evasion is one of the hallmarks of cancer. Besides apoptosis, some cancers suppress necroptosis-associated mechanisms by for example epigenetic silencing of RIPK3 expression. Conversely, necroptosis-elicited inflammation by cancer cells can fuel tumor growth. Recently, necroptosis-independent functions of MLKL were unraveled in receptor internalization, ligand-receptor degradation, endosomal trafficking, extracellular vesicle formation, autophagy, nuclear functions, axon repair, neutrophil extracellular trap (NET) formation, and inflammasome regulation. Little is known about the precise role of MLKL in cancer and whether some of these functions are involved in cancer development and metastasis. Here, we discuss current knowledge and controversies on MLKL, its structure, necroptosis-independent functions, expression, mutations, and its potential role as a pro- or anti-cancerous factor. Analysis of MLKL expression patterns reveals that MLKL is upregulated by type I/II interferon, conditions of inflammation, and tissue injury. Overall, MLKL may affect cancer development and metastasis through necroptosis-dependent and -independent functions.

Cell death research was revitalized by the understanding that necrosis can occur in a regulated and genetically controlled manner. Although necroptosis is the most recognized form of regulated necrosis, other examples of this process have emerged. Understanding how these pathways are interconnected should enable regulated necrosis to be therapeutically targeted. Cell death research was revitalized by the understanding that necrosis can occur in a highly regulated and genetically controlled manner. Although RIPK1 (receptor-interacting protein kinase 1)- and RIPK3–MLKL (mixed lineage kinase domain-like)-mediated necroptosis is the most understood form of regulated necrosis, other examples of this process are emerging, including cell death mechanisms known as parthanatos, oxytosis, ferroptosis, NETosis, pyronecrosis and pyroptosis. Elucidating how these pathways of regulated necrosis are interconnected at the molecular level should enable this process to be therapeutically targeted.

Key Points Although for a long time necrosis was considered to be a purely accidental cell death subroutine, multiple lines of evidence now show that necrotic cell death can be regulated, both in its occurrence and in its course. The term 'necroptosis' was introduced by Yuan's research group in 2005 to indicate 'programmed' (as opposed to 'accidental') necrosis. The best characterized signal transduction cascade leading to necroptosis is initiated by ligand-bound tumour necrosis factor (TNF) receptor 1 (TNFR1), which allows for the assembly of a cytoplasmic supramolecular complex — TNFR complex I — that includes (among other proteins) TNFR-associated death domain (TRADD), cellular inhibitor of apoptosis 1 (cIAP1), cIAP2 and receptor-interacting protein kinase 1 (RIP1; also known as RIPK1). In complex I, RIP1 can be ubiquitylated by cIAPs and deubiquitylated by cylindromatosis (CYLD) and A20 (also known as TNFAIP3). Whereas ubiquitylated RIP1 promotes the activation of the nuclear factor κB (NF-κB) system, deubiquitylated RIP1 functions as a cell death-inducing kinase. On TNFR1 internalization, the so-called TNFR complex II is formed, which contains TRADD, FAS-associated protein with a death domain (FADD) and caspase 8. Normally, caspase 8 becomes activated in complex II, thereby igniting a pro-apoptotic caspase cascade. When caspase activation is prevented, however, RIP1 physically and functionally interacts with RIP3 (also known as RIPK3), thereby generating a necroptosis-inducing complex known as the necrosome. Necroptosis can also be ignited by pathogen recognition receptors, including Toll-like receptors, NOD-like receptors and retinoic acid-inducible gene I-like receptors, as well as in response to DNA damage, presumably by a poly(ADP-ribose) polymerase-1 (PARP1)-dependent signalling pathway. Although the underlying molecular mechanisms remain obscure, reactive oxygen species (ROS), bioenergetic metabolic cascades and the release of cytotoxic factors from lysosomes and mitochondria all contribute to the execution of necroptosis. Regulated necrosis has been seen in multiple, evolutionarily distant model organisms, including yeast, nematodes, fruit flies, rodents, primates and human cells, corroborating the notion that necroptosis may represent a phylogenetically conserved mechanism for programmed cell death. Numerous in vivo studies indicate that the inhibition of necroptosis (by genetic means or by RIP1-targeting agents called necrostatins) can confer consistent cytoprotection, suggesting that necroptosis may constitute a promising target for drug development. Although necrosis was regarded as an uncontrolled mode of cell death, evidence now shows that it can be highly regulated. The initiation of programmed necrosis (necroptosis) by death receptors requires receptor-interacting protein 1 (RIP1) and RIP3, and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. For a long time, apoptosis was considered the sole form of programmed cell death during development, homeostasis and disease, whereas necrosis was regarded as an unregulated and uncontrollable process. Evidence now reveals that necrosis can also occur in a regulated manner. The initiation of programmed necrosis, 'necroptosis', by death receptors (such as tumour necrosis factor receptor 1) requires the kinase activity of receptor-interacting protein 1 (RIP1; also known as RIPK1) and RIP3 (also known as RIPK3), and its execution involves the active disintegration of mitochondrial, lysosomal and plasma membranes. Necroptosis participates in the pathogenesis of diseases, including ischaemic injury, neurodegeneration and viral infection, thereby representing an attractive target for the avoidance of unwarranted cell death.

Key Points Recent research has identified a series of previously unrecognized regulated cell death modalities beyond apoptosis, including necroptosis, parthanatos, ferroptosis and oxytosis. Several genetic approaches in model systems have implicated these forms of regulated cell death in diverse pathologically relevant conditions. Targeted and phenotypic screening approaches have led to the identification of small molecules that can modulate these pathways. Some of these regulated forms of cell death appear to be intertwined with the innate immune response, thus possibly affecting treatment outcomes with cell-death inducers or inhibitors. Strategies aiming to interfere with multiple cell death pathways might represent the optimal treatment paradigm for translational settings. Owing to their recent discovery, small-molecule modulators of regulated cell death are still in their infancy and therefore require further chemical optimization to reach clinical testing. Forms of cell death besides apoptosis and necrosis are becoming increasingly well understood, and are relevant to many disease contexts. Here, Conrad et al . describe the mechanisms underlying regulated forms of necrosis — including necroptosis, ferroptosis, parthanatos and cyclophilin D-mediated necrosis — and efforts to induce or prevent them in disease. The discovery of regulated cell death presents tantalizing possibilities for gaining control over the life–death decisions made by cells in disease. Although apoptosis has been the focus of drug discovery for many years, recent research has identified regulatory mechanisms and signalling pathways for previously unrecognized, regulated necrotic cell death routines. Distinct critical nodes have been characterized for some of these alternative cell death routines, whereas other cell death routines are just beginning to be unravelled. In this Review, we describe forms of regulated necrotic cell death, including necroptosis, the emerging cell death modality of ferroptosis (and the related oxytosis) and the less well comprehended parthanatos and cyclophilin D-mediated necrosis. We focus on small molecules, proteins and pathways that can induce and inhibit these non-apoptotic forms of cell death, and discuss strategies for translating this understanding into new therapeutics for certain disease contexts.

In the last few years many new cell death modalities have been described. To classify different types of cell death, the term ‘regulated cell death’ was introduced to discriminate it from ‘accidental cell death’. Regulated cell death involves the activation of genetically encoded molecular machinery that couples the presence of some signal to cell death. These forms of cell death, like apoptosis, necroptosis and pyroptosis have important physiological roles in development, tissue repair, and immunity. Accidental cell death occurs in response to physical or chemical insults and occurs independently of molecular signalling pathways. Ferroptosis, an emerging and recently (re)discovered type of regulated cell death occurs through Fe(II)-dependent lipid peroxidation when the reduction capacity of a cell is insufficient. Ferroptosis is coined after the requirement for free ferrous iron. Here, we will consider the extent to which ferroptosis is similar to other regulated cell deaths and explore emerging ideas about the physiological role of ferroptosis.

Lipid peroxidation (LPO) drives ferroptosis execution. However, LPO has been shown to contribute also to other modes of regulated cell death (RCD). To clarify the role of LPO in different modes of RCD, we studied in a comprehensive approach the differential involvement of reactive oxygen species (ROS), phospholipid peroxidation products, and lipid ROS flux in the major prototype modes of RCD viz. apoptosis, necroptosis, ferroptosis, and pyroptosis. LC-MS oxidative lipidomics revealed robust peroxidation of three classes of phospholipids during ferroptosis with quantitative predominance of phosphatidylethanolamine species. Incomparably lower amounts of phospholipid peroxidation products were found in any of the other modes of RCD. Nonetheless, a strong increase in lipid ROS levels was detected in non-canonical pyroptosis, but only during cell membrane rupture. In contrast to ferroptosis, lipid ROS apparently was not involved in non-canonical pyroptosis execution nor in the release of IL-1β and IL-18, while clear dependency on CASP11 and GSDMD was observed. Our data demonstrate that ferroptosis is the only mode of RCD that depends on excessive phospholipid peroxidation for its cytotoxicity. In addition, our results also highlight the importance of performing kinetics and using different methods to monitor the occurrence of LPO. This should open the discussion on the implication of particular LPO events in relation to different modes of RCD.

Autophagy is a lysosomal degradation pathway that degrades damaged or superfluous cell components into basic biomolecules, which are then recycled back into the cytosol. In this respect, autophagy drives a flow of biomolecules in a continuous degradation-regeneration cycle. Autophagy is generally considered a pro-survival mechanism pro- tecting cells under stress or poor nutrient conditions. Current research clearly shows that autophagy fulfills numer- ous functions in vital biological processes. It is implicated in development, differentiation, innate and adaptive immu- nity, ageing and cell death. In addition, accumulating evidence demonstrates interesting links between autophagy and several human diseases and tumor development. Therefore, autophagy seems to be an important player in the life and death of cells and organisms. Despite the mounting knowledge about autophagy, the mechanisms through which the autophagic machinery regulates these diverse processes are not entirely understood. In this review, we give a comprehensive overview of the autophagic signaling pathway, its role in general cellular processes and its connection to cell death. In addition, we present a brief overview of the possible contribution of defective autophagic signaling to disease.