Necroptosis signalling is tuned by phosphorylation of MLKL residues outside the pseudokinase domain activation loop

The importance of murine phospho-MLKL-S345 in situ detection for necroptosis assessment in vivo

Necroptosis is a caspase-independent modality of cell death implicated in many inflammatory pathologies. The execution of this pathway requires the formation of a cytosolic platform that comprises RIPK1 and RIPK3 which, in turn, mediates the phosphorylation of the pseudokinase MLKL (S345 in mouse). The activation of this executioner is followed by its oligomerisation and accumulation at the plasma-membrane where it leads to cell death via plasma-membrane destabilisation and consequent permeabilisation. While the biochemical and cellular characterisation of these events have been amply investigated, the study of necroptosis involvement in vivo in animal models is currently limited to the use of Mlkl-/- or Ripk3-/- mice. Yet, even in many of the models in which the involvement of necroptosis in disease aetiology has been genetically demonstrated, the fundamental in vivo characterisation regarding the question as to which tissue(s) and specific cell type(s) therein is/are affected by the pathogenic necroptotic death are missing. Here, we describe and validate an immunohistochemistry and immunofluorescence-based method to reliably detect the phosphorylation of mouse MLKL at serine 345 (pMLKL-S345). We first validate the method using tissues derived from mice in which Caspase-8 (Casp8) or FADD are specifically deleted from keratinocytes, or intestinal epithelial cells, respectively. We next demonstrate the presence of necroptotic activation in the lungs of SARS-CoV-infected mice and in the skin and spleen of mice bearing a Sharpin inactivating mutation. Finally, we exclude necroptosis occurrence in the intestines of mice subjected to TNF-induced septic shock. Importantly, by directly comparing the staining of pMLKL-345 with that of cleaved Caspase-3 staining in some of these models, we identify spatio-temporal and functional differences between necroptosis and apoptosis supporting a role of RIPK3 in inflammation independently of MLKL versus the role of RIPK3 in activation of necroptosis.

Protein phosphorylation and kinases: Potential therapeutic targets in necroptosis

Necroptosis is a pivotal contributor to the pathogenesis of various human diseases, including those affecting the nervous system, cardiovascular system, pulmonary system, and kidneys. Extensive investigations have elucidated the mechanisms and physiological ramifications of necroptosis. Among these, protein phosphorylation emerges as a paramount regulatory process, facilitating the activation or inhibition of specific proteins through the addition of phosphate groups to their corresponding amino acid residues. Currently, the targeting of kinases has gained recognition as a firmly established and efficacious therapeutic approach for diverse diseases, notably cancer. In this comprehensive review, we elucidate the intricate role of phosphorylation in governing key molecular players in the necroptotic pathway. Moreover, we provide an in-depth analysis of recent advancements in the development of kinase inhibitors aimed at modulating necroptosis. Lastly, we deliberate on the prospects and challenges associated with the utilization of kinase inhibitors to modulate necroptotic processes.

Immunogenic cell death in cancer: targeting necroptosis to induce antitumour immunity

Most metastatic cancers remain incurable due to the emergence of apoptosis-resistant clones, fuelled by intratumour heterogeneity and tumour evolution. To improve treatment, therapies should not only kill cancer cells but also activate the immune system against the tumour to eliminate any residual cancer cells that survive treatment. While current cancer therapies rely heavily on apoptosis - a largely immunologically silent form of cell death - there is growing interest in harnessing immunogenic forms of cell death such as necroptosis. Unlike apoptosis, necroptosis generates second messengers that act on immune cells in the tumour microenvironment, alerting them of danger. This lytic form of cell death optimizes the provision of antigens and adjuvanticity for immune cells, potentially boosting anticancer treatment approaches by combining cellular suicide and immune response approaches. In this Review, we discuss the mechanisms of necroptosis and how it activates antigen-presenting cells, drives cross-priming of CD8+ T cells and induces antitumour immune responses. We also examine the opportunities and potential drawbacks of such strategies for exposing cancer cells to immunological attacks.

Mediators of necroptosis: from cell death to metabolic regulation

Necroptosis, a programmed cell death mechanism distinct from apoptosis, has garnered attention for its role in various pathological conditions. While initially recognized for its involvement in cell death, recent research has revealed that key necroptotic mediators, including receptor-interacting protein kinases (RIPKs) and mixed lineage kinase domain-like protein (MLKL), possess additional functions that go beyond inducing cell demise. These functions encompass influencing critical aspects of metabolic regulation, such as energy metabolism, glucose homeostasis, and lipid metabolism. Dysregulated necroptosis has been implicated in metabolic diseases, including obesity, diabetes, metabolic dysfunction-associated steatotic liver disease (MASLD) and alcohol-associated liver disease (ALD), contributing to chronic inflammation and tissue damage. This review provides insight into the multifaceted role of necroptosis, encompassing both cell death and these extra-necroptotic functions, in the context of metabolic diseases. Understanding this intricate interplay is crucial for developing targeted therapeutic strategies in diseases that currently lack effective treatments.

PROTACs Targeting MLKL Protect Cells from Necroptosis

Mixed Lineage Kinase domain-Like pseudokinase (MLKL) is implicated in a broad range of diseases due to its role as the ultimate effector of necroptosis and has therefore emerged as an attractive drug target. Here, we describe the development of PROteolysis TArgeting Chimeras (PROTACs) as a novel approach to knock down MLKL through chemical means. A series of candidate degraders were synthesized from a high-affinity pyrazole carboxamide-based MLKL ligand leading to the identification of a PROTAC molecule that effectively degraded MLKL and completely abrogated cell death in a TSZ model of necroptosis. By leveraging the innate ability of these PROTACs to degrade MLKL in a dose-dependent manner, the quantitative relationship between MLKL levels and necroptosis was interrogated. This work demonstrates the feasibility of targeting MLKL using a PROTAC approach and provides a powerful tool to further our understanding of the role of MLKL within the necroptotic pathway.

Skp2-mediated MLKL degradation confers cisplatin-resistant in non-small cell lung cancer cells

Non-small cell lung cancer (NSCLC) is the most prevalent type of cancer and the leading cause of cancer-related death. Chemotherapeutic resistance is a major obstacle in treating NSCLC patients. Here, we discovered that the E3 ligase Skp2 is overexpressed, accompanied by the downregulation of necroptosis-related regulator MLKL in human NSCLC tissues and cell lines. Knockdown of Skp2 inhibited viability, anchorage-independent growth, and in vivo tumor development of NSCLC cells. We also found that the Skp2 protein is negatively correlated with MLKL in NSCLC tissues. Moreover, Skp2 is increased and accompanied by an upregulation of MLKL ubiquitination and degradation in cisplatin-resistant NSCLC cells. Accordingly, inhibition of Skp2 partially restores MLKL and sensitizes NSCLC cells to cisplatin in vitro and in vivo. Mechanistically, Skp2 interacts and promotes ubiquitination-mediated degradation of MLKL in cisplatin-resistant NSCLC cells. Our results provide evidence of an Skp2-dependent mechanism regulating MLKL degradation and cisplatin resistance, suggesting that targeting Skp2-ubiquitinated MLKL degradation may overcome NSCLC chemoresistance.

Advances in mechanism and regulation of PANoptosis: Prospects in disease treatment

PANoptosis, a new research hotspot at the moment, is a cell death pattern in which pyroptosis, apoptosis, and necroptosis all occur in the same cell population. In essence, PANoptosis is a highly coordinated and dynamically balanced programmed inflammatory cell death pathway that combines the main features of pyroptosis, apoptosis, and necroptosis. Many variables, such as infection, injury, or self-defect, may be involved in the occurrence of PANoptosis, with the assembly and activation of the PANoptosome being the most critical. PANoptosis has been linked to the development of multiple systemic diseases in the human body, including infectious diseases, cancer, neurodegenerative diseases, and inflammatory diseases. Therefore, it is necessary to clarify the process of occurrence, the regulatory mechanism of PANoptosis, and its relation to diseases. In this paper, we summarized the differences and relations between PANoptosis and the three types of programmed cell death, and emphatically expounded molecular mechanism and regulatory patterns of PANoptosis, with the expectation of facilitating the application of PANoptosis regulation in disease treatment.

MLKL deficiency protects against low-grade, sterile inflammation in aged mice

MLKL and RIPK3 are the core signaling proteins of the inflammatory cell death pathway, necroptosis, which is a known mediator and modifier of human disease. Necroptosis has been implicated in the progression of disease in almost every physiological system and recent reports suggest a role for necroptosis in aging. Here, we present the first comprehensive analysis of age-related histopathological and immunological phenotypes in a cohort of Mlkl^-/- and Ripk3^-/- mice on a congenic C57BL/6 J genetic background. We show that genetic deletion of Mlkl in female mice interrupts immune system aging, specifically delaying the age-related reduction of circulating lymphocytes. -Seventeen-month-old Mlkl^-/- female mice were also protected against age-related chronic sterile inflammation in connective tissue and skeletal muscle relative to wild-type littermate controls, exhibiting a reduced number of immune cell infiltrates in these sites and fewer regenerating myocytes. These observations implicate MLKL in age-related sterile inflammation, suggesting a possible application for long-term anti-necroptotic therapy in humans.

Resveratrol ameliorates myocardial ischemia/reperfusion induced necroptosis through inhibition of the Hippo pathway

Myocardial ischemia-reperfusion (I/R) injury is a major cause of poor hemodynamic reconstitution outcomes after myocardial infarction or circulatory arrest. Currently, the search for effective therapeutic agents and tools is a focus of research in the field of myocardial I/R injury. Resveratrol (Res) has been extensively studied in recent years because of its good cardiovascular therapeutic effects, but its specific mechanism of action has not been fully elucidated. Therefore, the aim of this study was to investigate the mechanism of interaction between myocardial I/R injury and Res in vitro and in vivo. In our in vivo study, we used PI/TUNEL staining and western blotting to detect relevant necroptotic key molecules such as RIP1, RIP3 and p-MLKL/MLKL to observe myocardial necroptosis. The extent of myocardial injury was determined using hematoxylin and eosin (HE) staining and 2,3,5-triphenyltetrazolium chloride (TTC) staining as well as serum levels of CK-MB and LDH and echocardiography. In the in vitro study, cellular injury was assessed by CCK-8 and cell supernatant LDH levels. In addition, we used small interfering RNA (siRNA) transfection to knock down YAP, a key effector molecule of the Hippo pathway, to validate the molecular mechanism of action by which Res exerts myocardial protection. The localization of YAP in H9c2 cardiomyocytes was examined using immunofluorescence. Our data demonstrated that Res could ameliorate myocardial I/R-induced necroptosis by modulating the Hippo pathway, and that the beneficial effect of Res might be associated with nuclear translocation of the transcriptional regulator YAP.

Surviving death: emerging concepts of RIPK3 and MLKL ubiquitination in the regulation of necroptosis

Lytic forms of programmed cell death, like necroptosis, are characterised by cell rupture and the release of cellular contents, often provoking inflammatory responses. In the recent years, necroptosis has been shown to play important roles in human diseases like cancer, infections and ischaemia/reperfusion injury. Coordinated interactions between RIPK1, RIPK3 and MLKL lead to the formation of a dedicated death complex called the necrosome that triggers MLKL-mediated membrane rupture and necroptotic cell death. Necroptotic cell death is tightly controlled by post-translational modifications, among which especially phosphorylation has been characterised in great detail. Although selective ubiquitination is relatively well-explored in the early initiation stages of necroptosis, the mechanisms and functional consequences of RIPK3 and MLKL ubiquitination for necrosome function and necroptosis are only starting to emerge. This review provides an overview on how site-specific ubiquitination of RIPK3 and MLKL regulates, fine-tunes and reverses the execution of necroptotic cell death.

Necroptosis at a glance

Necroptosis, or programmed necrosis, is an inflammatory form of cell death with important functions in host defense against pathogens and tissue homeostasis. The four cytosolic receptor-interacting protein kinase homotypic interaction motif (RHIM)-containing adaptor proteins RIPK1, RIPK3, TRIF (also known as TICAM1) and ZBP1 mediate necroptosis induction in response to infection and cytokine or innate immune receptor activation. Activation of the RHIM adaptors leads to phosphorylation, oligomerization and membrane targeting of the necroptosis effector protein mixed lineage kinase domain-like (MLKL). Active MLKL induces lesions on the plasma membrane, leading to the release of pro-inflammatory damage-associated molecular patterns (DAMPs). Thus, activities of the RHIM adaptors and MLKL are tightly regulated by posttranslational modifications to prevent inadvertent release of immunogenic contents. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of the regulatory mechanisms of necroptosis and its biological functions in tissue homeostasis, pathogen infection and other inflammatory diseases.

Membrane permeabilization is mediated by distinct epitopes in mouse and human orthologs of the necroptosis effector, MLKL

Necroptosis is a lytic programmed cell death pathway with origins in innate immunity that is frequently dysregulated in inflammatory diseases. The terminal effector of the pathway, MLKL, is licensed to kill following phosphorylation of its pseudokinase domain by the upstream regulator, RIPK3 kinase. Phosphorylation provokes the unleashing of MLKL's N-terminal four-helix bundle (4HB or HeLo) domain, which binds and permeabilizes the plasma membrane to cause cell death. The precise mechanism by which the 4HB domain permeabilizes membranes, and how the mechanism differs between species, remains unclear. Here, we identify the membrane binding epitope of mouse MLKL using NMR spectroscopy. Using liposome permeabilization and cell death assays, we validate K69 in the α3 helix, W108 in the α4 helix, and R137/Q138 in the first brace helix as crucial residues for necroptotic signaling. This epitope differs from the phospholipid binding site reported for human MLKL, which comprises basic residues primarily located in the α1 and α2 helices. In further contrast to human and plant MLKL orthologs, in which the α3-α4 loop forms a helix, this loop is unstructured in mouse MLKL in solution. Together, these findings illustrate the versatility of the 4HB domain fold, whose lytic function can be mediated by distinct epitopes in different orthologs.

Necroptosis Plays a Crucial Role in Vascular Injury during DVT and Is Enhanced by IL-17B

Background. This study investigated whether vascular endothelial necroptosis is involved in deep vein thrombosis (DVT) and how IL-17B facilitates necroptosis signaling. Methods. The DVT mouse model was induced by ligation of the IVC. The cross-sectional area of thrombus increases and the thrombus occupied the entire venous lumen at 48 h after ligation. Meanwhile, the increased expression of p-RIP3/RIP3 was most pronounced at 48 h after ligation, and the p-MLKL/MLKL peaked at 72 h. Results. Based on Illumina sequencing and KEGG pathway analyses, the activated RIP3/MLKL is associated with increased IL-17B. With thrombus formation, IL-17B was upregulated and enhanced the expression of RIP3 and MLKL in the IVC wall, as well as their phosphorylation levels (all $P<0.05$ , the comparison group consisted of the control group, DVT group, DVT/IL-17B group, and DVT/anti-IL-17B group). The p-RIP3/RIP3 and p-MLKL/MLKL ratios were reduced by anti-IL-17B. Similarly, the weight and cross-sectional area of the thrombi were increased by IL-17B and decreased by the IL-17B antibody. IL-17B had a smaller effect on thrombosis in knockout mice compared with WT mice. In vitro, the IL-17B protein expression and the level of RIP3 and MLKL phosphorylation increased high in the OGD cells, accompanied by increased expression of IL-6 and TNF-α. IL-17B enhanced the expression of IL-6 and TNF-α but had little effect on the IL-6 and TNF-α after transfected with siRIP3 or siMLKL. Similarly, the plasma IL-17B, IL-6, and TNF-α were significantly increased after thrombosis in WT mice, and enhanced by IL-17B. But IL-17B did not increase the plasma IL-6 and TNF-α in knockout mice. Conclusions. In conclusion, those results suggest that vascular endothelial necroptosis plays a crucial role in vascular injury and IL-17B could enhance the necroptosis pathway.

Targeting protein kinases benefits cancer immunotherapy

Small-molecule kinase inhibitors have been well established and successfully developed in the last decades for cancer target therapies. However, intrinsic or acquired drug resistance is becoming the major barrier for their clinical application. With the development of immunotherapies, in particular the discovery of immune checkpoint inhibitors (ICIs), the combination of ICIs with other therapies have recently been extensively explored, among which combination of ICIs with kinase inhibitors achieves promising clinical outcome in a plethora of cancer types. Here we comprehensively summarize the potent roles of protein kinases in modulating immune checkpoints both in tumor and immune cells, and reshaping tumor immune microenvironments by evoking innate immune response and neoantigen generation or presentation. Moreover, the clinical trial and approval of combined administration of kinase inhibitors with ICIs are collected, highlighting the precise strategies to benefit cancer immune therapies.

Co-expression of recombinant RIPK3:MLKL complexes using the baculovirus-insect cell system

Pseudokinase domains are found throughout the kingdoms of life and serve myriad roles in cell signaling. These domains, which resemble protein kinases but are catalytically-deficient, have been described principally as protein interaction domains. Broadly, pseudokinases have been reported to function as: allosteric regulators of conventional enzymes; scaffolds to nucleate assembly and/or localization of signaling complexes; molecular switches; or competitors of signaling complex assembly. From detailed structural and biochemical studies of individual pseudokinases, a picture of how they mediate protein interactions is beginning to emerge. Many such studies have relied on recombinant protein production in insect cells, where endogenous chaperones and modifying enzymes favor bona fide folding of pseudokinases. Here, we describe methods for co-expression of pseudokinases and their interactors in insect cells, as exemplified by the MLKL pseudokinase, which is the terminal effector in the necroptosis cell death pathway, and its upstream regulator kinase RIPK3. These methods are broadly applicable to co-expression of other pseudokinases with their interaction partners from bacmids using the baculovirus-insect cell expression system.

CRISPR deletions in cell lines for reconstitution studies of pseudokinase function

The non-catalytic cousins of protein kinases, the pseudokinases, have grown to prominence as indispensable signaling entities over the past decade, despite their lack of catalytic activity. Because their importance has only been fully embraced recently, many of the 10% of the human kinome categorized as pseudokinases are yet to be attributed biological functions. The advent of CRISPR-Cas9 editing to genetically delete pseudokinases in a cell line of interest has proven invaluable to dissecting many functions and remains the method of choice for gene knockout. Here, using the terminal effector pseudokinase in the necroptosis cell death pathway, MLKL, as an exemplar, we describe a method for genetic knockout of pseudokinases in cultured cells. This method does not retain the CRISPR guide sequence in the edited cells, which eliminates possible interference in subsequent reconstitution studies where mutant forms of the pseudokinase can be reintroduced into cells exogenously for detailed mechanistic characterization.

Regulation of Inflammatory Cell Death by Phosphorylation

Cell death is a necessary event in multi-cellular organisms to maintain homeostasis by eliminating unrequired or damaged cells. Currently, there are many forms of cell death, and several of them, such as necroptosis, pyroptosis and ferroptosis, even apoptosis trigger an inflammatory response by releasing damage-associated molecular patterns (DAMPs), which are involved in the pathogenesis of a variety of human inflammatory diseases, including autoimmunity disease, diabetes, Alzheimer’s disease and cancer. Therefore, the occurrence of inflammatory cell death must be strictly regulated. Recently, increasing studies suggest that phosphorylation plays a critical role in inflammatory cell death. In this review, we will summarize current knowledge of the regulatory role of phosphorylation in inflammatory cell death and also discuss the promising treatment strategy for inflammatory diseases by targeting related protein kinases that mediate phosphorylation or phosphatases that mediate dephosphorylation.

A proteomic perspective on TNF-mediated signalling and cell death

The tumour necrosis factor (TNF) is the most potent inducer of cell death amongst cytokines. It is crucial for processes including homeostasis, the development of the immune system and fighting infections. However, high levels of TNF due to genetic disorders or persistent infections can contribute to autoinflammatory and autoimmune diseases or life-threatening conditions like sepsis. These diseases generally display increased levels of cell death, which, downstream of the TNF receptor, can either be caspase-dependent (apoptosis) or caspase-independent (necroptosis). Significant efforts have been invested in unravelling and manipulating signalling mechanisms regulating these two different types of cell death. Here I discuss how modern proteomic approaches like phosphoproteomics and secretomics provide a novel perspective on this central cytokine and its effect on inflammation and cell survival.

Roles of receptor-interacting protein kinase 1 in SH-SY5Y cells with beta amyloid-induced neurotoxicity

Alzheimer's disease (AD), the major cause of dementia, affects the elderly population worldwide. Previous studies have shown that depletion of receptor‐interacting protein kinase 1 (RIPK1) expression reverted the AD phenotype in murine AD models. Necroptosis, executed by mixed lineage kinase domain‐like (MLKL) protein and activated by RIPK1 and RIPK3, has been shown to be involved in AD. However, the role of RIPK1 in beta‐amyloid (Aβ)‐induced necroptosis is not yet fully understood. In this study, we explored the role of RIPK1 in the SH‐SY5Y human neuroblastoma cells treated with Aβ 1–40 or Aβ 1–42. We showed that Aβ‐induced neuronal cell death was independent of apoptosis and autophagy pathways. Further analyses depicted that activation of RIPK1/MLKL‐dependant necroptosis pathway was observed in vitro. We demonstrated that inhibition of RIPK1 expression rescued the cells from Aβ‐induced neuronal cell death and ectopic expression of RIPK1 was found to enhance the stability of the endogenous APP. In summary, our findings demonstrated that Aβ can potentially drive necroptosis in an RIPK1‐MLKL‐dependent manner, proposing that RIPK1 plays an important role in the pathogenesis of AD.

Apoptosis, Pyroptosis, and Necroptosis-Oh My! The Many Ways a Cell Can Die

Cell death is an essential process in all living organisms and occurs through different mechanisms. The three main types of programmed cell death are apoptosis, pyroptosis, and necroptosis, and each of these pathways employs complex molecular and cellular mechanisms. Although there are mechanisms and outcomes specific to each pathway, they share common components and features. In this review, we discuss recent discoveries in these three best understood modes of cell death, highlighting their singularities, and examining the intriguing notion that common players shape different individual pathways in this highly interconnected and coordinated cell death system. Understanding the similarities and differences of these cell death processes is crucial to enable targeted strategies to manipulate these pathways for therapeutic benefit.

FTY720 induces neutrophil extracellular traps via a NADPH oxidase-independent pathway

FTY720 is an immunosuppressive agent which has been approved to treat multiple sclerosis (MS). The main object of the present study is to investigate whether FTY720 has the potential to induce the formation of neutrophil extracellular traps (NETs) in vitro. Using Sytox Green assay and fluorescence microscopy, our results showed that FTY720 trigged the NET formation. In contrast to classic NET formation induced by Phorbol 12-myristate 13-acetate (PMA), FTY720-induced NETs were detected earlier and independent of NADPH oxidase (NOX) activity. Pharmacological inhibitor experiments indicated that autophagy was also required for the NET formation induced by FTY720. Moreover, p38 and AKT inhibitor significantly suppressed the NET formation by FTY720, whereas ERK inhibitor had no effect, suggesting that FTY720-induced NETs depended on the activation of p38 and AKT. We further found that citrullination of histone H3 and peptidylarginine deiminase 4 (PAD4) did not mediated FTY720-induced NET formation. Interestingly, necroptosis signaling activation was involved in the vital NET formation by FTY720, however, plasma membrane rupture resulting from necroptosis was not a major component of NET formation described here. Collectively, these findings indicated that FTY720 could be a potential antibacterial drug to protect host against pathogen infection.

Oligomerization-driven MLKL ubiquitylation antagonizes necroptosis

Mixed lineage kinase domain-like (MLKL) is the executioner in the caspase-independent form of programmed cell death called necroptosis. Receptor-interacting serine/threonine protein kinase 3 (RIPK3) phosphorylates MLKL, triggering MLKL oligomerization, membrane translocation and membrane disruption. MLKL also undergoes ubiquitylation during necroptosis, yet neither the mechanism nor the significance of this event has been demonstrated. Here, we show that necroptosis-specific multi-mono-ubiquitylation of MLKL occurs following its activation and oligomerization. Ubiquitylated MLKL accumulates in a digitonin-insoluble cell fraction comprising organellar and plasma membranes and protein aggregates. Appearance of this ubiquitylated MLKL form can be reduced by expression of a plasma membrane-located deubiquitylating enzyme. Oligomerization-induced MLKL ubiquitylation occurs on at least four separate lysine residues and correlates with its proteasome- and lysosome-dependent turnover. Using a MLKL-DUB fusion strategy, we show that constitutive removal of ubiquitin from MLKL licences MLKL auto-activation independent of necroptosis signalling in mouse and human cells. Therefore, in addition to the role of ubiquitylation in the kinetic regulation of MLKL-induced death following an exogenous necroptotic stimulus, it also contributes to restraining basal levels of activated MLKL to avoid unwanted cell death.

MLKL promotes cellular differentiation in myeloid leukemia by facilitating the release of G-CSF

The blockade of cellular differentiation represents a hallmark of acute myeloid leukemia (AML), which is largely attributed to the dysfunction of lineage-specific transcription factors controlling cellular differentiation. However, alternative mechanisms of cellular differentiation programs in AML remain largely unexplored. Here we report that mixed lineage kinase domain-like protein (MLKL) contributes to the cellular differentiation of transformed hematopoietic progenitor cells in AML. Using gene-targeted mice, we show that MLKL facilitates the release of granulocyte colony-stimulating factor (G-CSF) by controlling membrane permeabilization in leukemic cells. Mlkl^−/− hematopoietic stem and progenitor cells released reduced amounts of G-CSF while retaining their capacity for CSF3 (G-CSF) mRNA expression, G-CSF protein translation, and G-CSF receptor signaling. MLKL associates with early endosomes and controls G-CSF release from intracellular storage by plasma membrane pore formation, whereas cell death remained unaffected by loss of MLKL. Of note, MLKL expression was significantly reduced in AML patients, specifically in those with a poor-risk AML subtype. Our data provide evidence that MLKL controls myeloid differentiation in AML by controlling the release of G-CSF from leukemic progenitor cells.

MLKL in cancer: more than a necroptosis regulator

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.

Necroptosis molecular mechanisms: Recent findings regarding novel necroptosis regulators

Necroptosis is a form of programmed necrosis that is mediated by various cytokines and pattern recognition receptors (PRRs). Cells dying by necroptosis show necrotic phenotypes, including swelling and membrane rupture, and release damage-associated molecular patterns (DAMPs), inflammatory cytokines, and chemokines, thereby mediating extreme inflammatory responses. Studies on gene knockout or necroptosis-specific inhibitor treatment in animal models have provided extensive evidence regarding the important roles of necroptosis in inflammatory diseases. The necroptosis signaling pathway is primarily modulated by activation of receptor-interacting protein kinase 3 (RIPK3), which phosphorylates mixed-lineage kinase domain-like protein (MLKL), mediating MLKL oligomerization. In the necroptosis process, these proteins are fine-tuned by posttranslational regulation via phosphorylation, ubiquitination, glycosylation, and protein-protein interactions. Herein, we review recent findings on the molecular regulatory mechanisms of necroptosis.

The Role of the Key Effector of Necroptotic Cell Death, MLKL, in Mouse Models of Disease

Necroptosis is an inflammatory form of lytic programmed cell death that is thought to have evolved to defend against pathogens. Genetic deletion of the terminal effector protein—MLKL—shows no overt phenotype in the C57BL/6 mouse strain under conventional laboratory housing conditions. Small molecules that inhibit necroptosis by targeting the kinase activity of RIPK1, one of the main upstream conduits to MLKL activation, have shown promise in several murine models of non-infectious disease and in phase II human clinical trials. This has triggered in excess of one billion dollars (USD) in investment into the emerging class of necroptosis blocking drugs, and the potential utility of targeting the terminal effector is being closely scrutinised. Here we review murine models of disease, both genetic deletion and mutation, that investigate the role of MLKL. We summarize a series of examples from several broad disease categories including ischemia reperfusion injury, sterile inflammation, pathogen infection and hematological stress. Elucidating MLKL’s contribution to mouse models of disease is an important first step to identify human indications that stand to benefit most from MLKL-targeted drug therapies.

Primidone blocks RIPK1-driven cell death and inflammation

The receptor-interacting serine/threonine protein kinase 1 (RIPK1) is a key mediator of regulated cell death and inflammation. Recent studies suggest that RIPK1 inhibition would fundamentally improve the therapy of RIPK1-dependent organ damage in stroke, myocardial infarction, kidney failure, and systemic inflammatory response syndrome. Additionally, it could ameliorate or prevent multi-organ failure induced by cytokine release in the context of hyperinflammation, as seen in COVID-19 patients. Therefore, we searched for a RIPK1 inhibitor and present the aromatic antiepileptic and FDA-approved drug primidone (Liskantin®) as a potent inhibitor of RIPK1 activation in vitro and in a murine model of TNFα-induced shock, which mimics the hyperinflammatory state of cytokine release syndrome. Furthermore, we detected for the first time RIPK1 activation in the respiratory tract epithelium of hospitalized patients who tested positive for SARS-CoV-2 infection. Our data provide a strong rationale for evaluating the drug primidone in conditions of hyperinflammation in humans.

Conformational interconversion of MLKL and disengagement from RIPK3 precede cell death by necroptosis

Phosphorylation of the MLKL pseudokinase by the RIPK3 kinase leads to MLKL oligomerization, translocation to, and permeabilization of, the plasma membrane to induce necroptotic cell death. The precise choreography of MLKL activation remains incompletely understood. Here, we report Monobodies, synthetic binding proteins, that bind the pseudokinase domain of MLKL within human cells and their crystal structures in complex with the human MLKL pseudokinase domain. While Monobody-32 constitutively binds the MLKL hinge region, Monobody-27 binds MLKL via an epitope that overlaps the RIPK3 binding site and is only exposed after phosphorylated MLKL disengages from RIPK3 following necroptotic stimulation. The crystal structures identified two distinct conformations of the MLKL pseudokinase domain, supporting the idea that a conformational transition accompanies MLKL disengagement from RIPK3. These studies provide further evidence that MLKL undergoes a large conformational change upon activation, and identify MLKL disengagement from RIPK3 as a key regulatory step in the necroptosis pathway.

A family harboring an MLKL loss of function variant implicates impaired necroptosis in diabetes

Maturity-onset diabetes of the young, MODY, is an autosomal dominant disease with incomplete penetrance. In a family with multiple generations of diabetes and several early onset diabetic siblings, we found the previously reported P33T PDX1 damaging mutation. Interestingly, this substitution was also present in a healthy sibling. In contrast, a second very rare heterozygous damaging mutation in the necroptosis terminal effector, MLKL, was found exclusively in the diabetic family members. Aberrant cell death by necroptosis is a cause of inflammatory diseases and has been widely implicated in human pathologies, but has not yet been attributed functions in diabetes. Here, we report that the MLKL substitution observed in diabetic patients, G316D, results in diminished phosphorylation by its upstream activator, the RIPK3 kinase, and no capacity to reconstitute necroptosis in two distinct MLKL-/- human cell lines. This MLKL mutation may act as a modifier to the P33T PDX1 mutation, and points to a potential role of impairment of necroptosis in diabetes. Our findings highlight the importance of family studies in unraveling MODY's incomplete penetrance, and provide further support for the involvement of dysregulated necroptosis in human disease.

MLKL: Functions beyond serving as the Executioner of Necroptosis

Recently, necroptosis, as a programmed cell death pathway, has drawn much attention as it has been implicated in multiple pathologies, especially in the field of inflammatory diseases. Pseudokinase mixed lineage kinase domain-like protein (MLKL) serves as a terminal-known obligate effector in the process of necroptosis. To date, the majority of research on MLKL has focused on its role in necroptosis, and the prevailing view has been that the sole function of MLKL is to mediate necroptosis. However, increasing evidence indicates that MLKL can serve as a regulator of many diseases via its non-necroptotic functions. These functions of MLKL shed light on its functional complexity and diversity. In this review, we briefly introduce the current state of knowledge regarding the structure of MLKL, necroptosis signaling, as well as cross-linkages among necroptosis and other regulated cell death pathways, and we particularly highlight recent progress related to newly identified functions and inhibitors of MLKL. These discussions promote a better understanding of the role of MLKL in diseases, which will foster efforts to pharmacologically target this molecule in clinical treatments.

The regulation of necroptosis by post-translational modifications

Necroptosis is a caspase-independent, lytic form of programmed cell death whose errant activation has been widely implicated in many pathologies. The pathway relies on the assembly of the apical protein kinases, RIPK1 and RIPK3, into a high molecular weight cytoplasmic complex, termed the necrosome, downstream of death receptor or pathogen detector ligation. The necrosome serves as a platform for RIPK3-mediated phosphorylation of the terminal effector, the MLKL pseudokinase, which induces its oligomerization, translocation to, and perturbation of, the plasma membrane to cause cell death. Over the past 10 years, knowledge of the post-translational modifications that govern RIPK1, RIPK3 and MLKL conformation, activity, interactions, stability and localization has rapidly expanded. Here, we review current knowledge of the functions of phosphorylation, ubiquitylation, GlcNAcylation, proteolytic cleavage, and disulfide bonding in regulating necroptotic signaling. Post-translational modifications serve a broad array of functions in modulating RIPK1 engagement in, or exclusion from, cell death signaling, whereas the bulk of identified RIPK3 and MLKL modifications promote their necroptotic functions. An enhanced understanding of the modifying enzymes that tune RIPK1, RIPK3, and MLKL necroptotic functions will prove valuable in efforts to therapeutically modulate necroptosis.

A toolbox for imaging RIPK1, RIPK3, and MLKL in mouse and human cells

Necroptosis is a lytic, inflammatory cell death pathway that is dysregulated in many human pathologies. The pathway is executed by a core machinery comprising the RIPK1 and RIPK3 kinases, which assemble into necrosomes in the cytoplasm, and the terminal effector pseudokinase, MLKL. RIPK3-mediated phosphorylation of MLKL induces oligomerization and translocation to the plasma membrane where MLKL accumulates as hotspots and perturbs the lipid bilayer to cause death. The precise choreography of events in the pathway, where they occur within cells, and pathway differences between species, are of immense interest. However, they have been poorly characterized due to a dearth of validated antibodies for microscopy studies. Here, we describe a toolbox of antibodies for immunofluorescent detection of the core necroptosis effectors, RIPK1, RIPK3, and MLKL, and their phosphorylated forms, in human and mouse cells. By comparing reactivity with endogenous proteins in wild-type cells and knockout controls in basal and necroptosis-inducing conditions, we characterise the specificity of frequently-used commercial and recently-developed antibodies for detection of necroptosis signaling events. Importantly, our findings demonstrate that not all frequently-used antibodies are suitable for monitoring necroptosis by immunofluorescence microscopy, and methanol- is preferable to paraformaldehyde-fixation for robust detection of specific RIPK1, RIPK3, and MLKL signals.

Location, location, location: A compartmentalized view of TNF-induced necroptotic signaling

Necroptosis is a lytic, proinflammatory cell death pathway, which has been implicated in host defense and, when dysregulated, the pathology of many human diseases. The central mediators of this pathway are the receptor-interacting serine/threonine protein kinases RIPK1 and RIPK3 and the terminal executioner, the pseudokinase mixed lineage kinase domain-like (MLKL). Here, we review the chronology of signaling along the RIPK1-RIPK3-MLKL axis and highlight how the subcellular compartmentalization of signaling events controls the initiation and execution of necroptosis. We propose that a network of modulators surrounds the necroptotic signaling core and that this network, rather than acting universally, tunes necroptosis in a context-, cell type-, and species-dependent manner. Such a high degree of mechanistic flexibility is likely an important property that helps necroptosis operate as a robust, emergency form of cell death.

The necroptotic cell death pathway operates in megakaryocytes, but not in platelet synthesis

Necroptosis is a pro-inflammatory cell death program executed by the terminal effector, mixed lineage kinase domain-like (MLKL). Previous studies suggested a role for the necroptotic machinery in platelets, where loss of MLKL or its upstream regulator, RIPK3 kinase, impacted thrombosis and haemostasis. However, it remains unknown whether necroptosis operates within megakaryocytes, the progenitors of platelets, and whether necroptotic cell death might contribute to or diminish platelet production. Here, we demonstrate that megakaryocytes possess a functional necroptosis signalling cascade. Necroptosis activation leads to phosphorylation of MLKL, loss of viability and cell swelling. Analyses at steady state and post antibody-mediated thrombocytopenia revealed that platelet production was normal in the absence of MLKL, however, platelet activation and haemostasis were impaired with prolonged tail re-bleeding times. We conclude that MLKL plays a role in regulating platelet function and haemostasis and that necroptosis signalling in megakaryocytes is dispensable for platelet production.

Pore formation in regulated cell death

The discovery of alternative signaling pathways that regulate cell death has revealed multiple strategies for promoting cell death with diverse consequences at the tissue and organism level. Despite the divergence in the molecular components involved, membrane permeabilization is a common theme in the execution of regulated cell death. In apoptosis, the permeabilization of the outer mitochondrial membrane by BAX and BAK releases apoptotic factors that initiate the caspase cascade and is considered the point of no return in cell death commitment. Pyroptosis and necroptosis also require the perforation of the plasma membrane at the execution step, which involves Gasdermins in pyroptosis, and MLKL in the case of necroptosis. Although BAX/BAK, Gasdermins and MLKL share certain molecular features like oligomerization, they form pores in different cellular membranes via distinct mechanisms. Here, we compare and contrast how BAX/BAK, Gasdermins, and MLKL alter membrane permeability from a structural and biophysical perspective and discuss the general principles of membrane permeabilization in the execution of regulated cell death.

Detecting Necroptosis in Virus-Infected Cells

Necroptosis has been implicated as a critical cell death pathway in cancers, Alzheimer's and other neurodegenerative diseases, and virus-infected cells. Necroptosis occurs when mixed-lineage kinase domain-like protein (MLKL) punctures the cytoplasmic membrane allowing a rapid influx of water leading to a loss of cellular integrity. As its role in human disease becomes apparent, methods identifying necroptosis will need to be further developed and optimized. Here we describe identification of necroptosis through quantifying cell death with pathway inhibitors and using western blots to identify end points of MLKL activation and protein-protein interactions leading to it.

Viral MLKL Homologs Subvert Necroptotic Cell Death by Sequestering Cellular RIPK3

Necroptotic cell death has been implicated in many human pathologies and is thought to have evolved as an innate immunity mechanism. The pathway relies on two key effectors: the kinase receptor-interacting protein kinase 3 (RIPK3) and the terminal effector, the pseudokinase mixed-lineage kinase-domain-like (MLKL). We identify proteins with high sequence similarity to the pseudokinase domain of MLKL in poxvirus genomes. Expression of these proteins from the BeAn 58058 and Cotia poxviruses, but not swinepox, in human and mouse cells blocks cellular MLKL activation and necroptotic cell death. We show that viral MLKL-like proteins function as dominant-negative mimics of host MLKL, which inhibit necroptosis by sequestering RIPK3 via its kinase domain to thwart MLKL engagement and phosphorylation. These data support an ancestral role for necroptosis in defense against pathogens. Furthermore, mimicry of a cellular pseudokinase by a pathogen adds to the growing repertoire of functions performed by pseudokinases in signal transduction.

The Killer Pseudokinase Mixed Lineage Kinase Domain-Like Protein (MLKL)

Whereas the apoptosis cell death pathway typically enables cells to undergo death in an immunologically silent manner, cell death by necroptosis induces cell lysis and release of cellular constituents known to elicit an immune response. Consequently, the origins of necroptosis likely originated in host defense against pathogens, although recently it has emerged that dysregulation of the pathway underlies many human pathologies. The past decade has seen a rapid advance in our understanding of the molecular mechanisms underlying necroptotic cell death, including the implication of the pseudokinase, mixed lineage kinase domain-like protein (MLKL), as the terminal effector in the pathway. Here, I review our current understanding of how MLKL is activated by the upstream receptor interacting protein kinase (RIPK)3, the proposed mechanism(s) by which MLKL kills cells, and recently described layers of regulation that tune MLKL's killing activity.

MLKL trafficking and accumulation at the plasma membrane control the kinetics and threshold for necroptosis

Mixed lineage kinase domain-like (MLKL) is the terminal protein in the pro-inflammatory necroptotic cell death program. RIPK3-mediated phosphorylation is thought to initiate MLKL oligomerization, membrane translocation and membrane disruption, although the precise choreography of events is incompletely understood. Here, we use single-cell imaging approaches to map the chronology of endogenous human MLKL activation during necroptosis. During the effector phase of necroptosis, we observe that phosphorylated MLKL assembles into higher order species on presumed cytoplasmic necrosomes. Subsequently, MLKL co-traffics with tight junction proteins to the cell periphery via Golgi-microtubule-actin-dependent mechanisms. MLKL and tight junction proteins then steadily co-accumulate at the plasma membrane as heterogeneous micron-sized hotspots. Our studies identify MLKL trafficking and plasma membrane accumulation as crucial necroptosis checkpoints. Furthermore, the accumulation of phosphorylated MLKL at intercellular junctions accelerates necroptosis between neighbouring cells, which may be relevant to inflammatory bowel disease and other necroptosis-mediated enteropathies.

Mixed lineage kinase domain-like (MLKL) is the terminal protein in the pro-inflammatory necroptotic cell death program. Here the authors show that MLKL trafficking and plasma membrane accumulation are crucial necroptosis checkpoints, and that accumulation of phosphorylated MLKL at intercellular junctions promotes necroptosis.

Distinct pseudokinase domain conformations underlie divergent activation mechanisms among vertebrate MLKL orthologues

The MLKL pseudokinase is the terminal effector in the necroptosis cell death pathway. Phosphorylation by its upstream regulator, RIPK3, triggers MLKL's conversion from a dormant cytoplasmic protein into oligomers that translocate to, and permeabilize, the plasma membrane to kill cells. The precise mechanisms underlying these processes are incompletely understood, and were proposed to differ between mouse and human cells. Here, we examine the divergence of activation mechanisms among nine vertebrate MLKL orthologues, revealing remarkable specificity of mouse and human RIPK3 for MLKL orthologues. Pig MLKL can restore necroptotic signaling in human cells; while horse and pig, but not rat, MLKL can reconstitute the mouse pathway. This selectivity can be rationalized from the distinct conformations observed in the crystal structures of horse and rat MLKL pseudokinase domains. These studies identify important differences in necroptotic signaling between species, and suggest that, more broadly, divergent regulatory mechanisms may exist among orthologous pseudoenzymes.

Identification of MLKL membrane translocation as a checkpoint in necroptotic cell death using Monobodies

The necroptosis cell death pathway has been implicated in host defense and in the pathology of inflammatory diseases. While phosphorylation of the necroptotic effector pseudokinase Mixed Lineage Kinase Domain-Like (MLKL) by the upstream protein kinase RIPK3 is a hallmark of pathway activation, the precise checkpoints in necroptosis signaling are still unclear. Here we have developed monobodies, synthetic binding proteins, that bind the N-terminal four-helix bundle (4HB) "killer" domain and neighboring first brace helix of human MLKL with nanomolar affinity. When expressed as genetically encoded reagents in cells, these monobodies potently block necroptotic cell death. However, they did not prevent MLKL recruitment to the "necrosome" and phosphorylation by RIPK3, nor the assembly of MLKL into oligomers, but did block MLKL translocation to membranes where activated MLKL normally disrupts membranes to kill cells. An X-ray crystal structure revealed a monobody-binding site centered on the α4 helix of the MLKL 4HB domain, which mutational analyses showed was crucial for reconstitution of necroptosis signaling. These data implicate the α4 helix of its 4HB domain as a crucial site for recruitment of adaptor proteins that mediate membrane translocation, distinct from known phospholipid binding sites.

FLIP(L): the pseudo-caspase

Possessing structural homology with their active enzyme counterparts but lacking catalytic activity, pseudoenzymes have been identified for all major enzyme groups. Caspases are a family of cysteine‐dependent aspartate‐directed proteases that play essential roles in regulating cell death and inflammation. Here, we discuss the only human pseudo‐caspase, FLIP(L), a paralog of the apoptosis‐initiating caspases, caspase‐8 and caspase‐10. FLIP(L) has been shown to play a key role in regulating the processing and activity of caspase‐8, thereby modulating apoptotic signaling mediated by death receptors (such as TRAIL‐R1/R2), TNF receptor‐1 (TNFR1), and Toll‐like receptors. In this review, these canonical roles of FLIP(L) are discussed. Additionally, a range of nonclassical pseudoenzyme roles are described, in which FLIP(L) functions independently of caspase‐8. These nonclassical pseudoenzyme functions enable FLIP(L) to play key roles in the regulation of a wide range of biological processes beyond its canonical roles as a modulator of cell death.

Molecular mechanisms of necroptosis and relevance for neurodegenerative diseases

Necroptosis is a regulated cell death pathway morphologically similar to necrosis that depends on the kinase activity of receptor interacting protein 3 (RIP3) and the subsequent activation of the pseudokinase mixed lineage kinase domain-like protein (MLKL), being also generally dependent on RIP1 kinase activity. Necroptosis can be recruited during pathological conditions, usually following the activation of death receptors under specific cellular contexts. In this regard, necroptosis has been implicated in the pathogenesis of multiple disorders, including acute and chronic neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, and multiple sclerosis. Here, we summarize the molecular mechanisms regulating the induction of necroptosis and downstream effectors of this form of cell death, besides exploring non-necroptotic roles for necroptosis-related proteins that may impact on alternative cell death pathways and inflammatory mechanisms in disease. Finally, we outline the recent evidence implicating necroptosis in neurodegenerative conditions and the emerging therapeutic perspectives targeting necroptosis in these diseases.

Complex roles of necroptosis in cancer

Necroptosis is a tightly regulated form of necrosis that requires the activation of receptor-interacting protein (RIP) kinases RIPK1 and RIPK3, as well as the RIPK3 substrate mixed lineage kinase domain-like protein (MLKL). Because of membrane rupture, necroptotic cells release damage-associated molecular patterns (DAMPs) that evoke immune responses. Necroptosis is emerging as an important cellular response in the modulation of cancer initiation, progression, and metastasis. Necroptosis of cancer cells is considered to be an immunogenic cell death capable of activating anti-tumor immunity. Necroptosis also participates in the promotion of myeloid cell-induced adaptive immune suppression and thus contributes to oncogenesis. In addition, necroptosis of endothelial cells and tumor cells is conducive to tumor metastasis. In this review, we summarize the current knowledge of the complex role of necroptosis in cancer and discuss the potential of targeting necroptosis components for cancer therapies.

Molecular Insights into the Mechanism of Necroptosis: The Necrosome As a Potential Therapeutic Target

Necroptosis, or regulated necrosis, is an important type of programmed cell death in addition to apoptosis. Necroptosis induction leads to cell membrane disruption, inflammation and vascularization. It plays important roles in various pathological processes, including neurodegeneration, inflammatory diseases, multiple cancers, and kidney injury. The molecular regulation of necroptotic pathway has been intensively studied in recent years. Necroptosis can be triggered by multiple stimuli and this pathway is regulated through activation of receptor-interacting protein kinase 1 (RIPK1), RIPK3 and pseudokinase mixed lineage kinase domain-like (MLKL). A better understanding of the mechanism of regulation of necroptosis will further aid to the development of novel drugs for necroptosis-associated human diseases. In this review, we focus on new insights in the regulatory machinery of necroptosis. We further discuss the role of necroptosis in different pathologies, its potential as a therapeutic target and the current status of clinical development of drugs interfering in the necroptotic pathway.

Programmed necrosis and its role in management of breast cancer

Breast cancer is one of the major causes of cancer related deaths in women worldwide. A major factor responsible for treatment failure in breast cancer is the development of resistance to commonly used chemotherapeutic drugs leading to disease relapse. Several studies have shown dysregulation of molecular machinery of apoptosis, the major programmed cell death pathway in breast malignancies. Thus, there is an unmet need to search for an alternative cell death pathway which can work when apoptosis is compromised. Necroptosis or programmed necrosis is a relatively recently described entity which has attracted attention in this context. Classically, even in physiological conditions necroptosis is found to act if apoptosis is not functional due to some reason. Recently, more and more studies are being conducted in different malignancies to explore the possibility and utility of inducing cell death by necroptosis. The present review describes the key molecular players involved in necroptotic pathway and their status in breast cancer. In addition, the research done to utilize this pathway for treatment of breast cancer has also been highlighted.

The anticonvulsive Phenhydan® suppresses extrinsic cell death

Different forms of regulated cell death-like apoptosis and necroptosis contribute to the pathophysiology of clinical conditions including ischemia-reperfusion injury, myocardial infarction, sepsis, and multiple sclerosis. In particular, the kinase activity of the receptor-interacting serine/threonine protein kinase 1 (RIPK1) is crucial for cell fate in inflammation and cell death. However, despite its involvement in pathological conditions, no pharmacologic inhibitor of RIPK1-mediated cell death is currently in clinical use. Herein, we screened a collection of clinical compounds to assess their ability to modulate RIPK1-mediated cell death. Our small-scale screen identified the anti-epilepsy drug Phenhydan® as a potent inhibitor of death receptor-induced necroptosis and apoptosis. Accordingly, Phenhydan® blocked activation of necrosome formation/activation as well as death receptor-induced NF-κB signaling by influencing the membrane function of cells, such as lipid raft formation, thus exerting an inhibitory effect on pathophysiologic cell death processes. By targeting death receptor signaling, the already FDA-approved Phenhydan® may provide new therapeutic strategies for inflammation-driven diseases caused by aberrant cell death.