Regulation of RIPK1 activation by TAK1-mediated phosphorylation dictates apoptosis and necroptosis

Withdrawal notice to: ATR-mediated phosphorylation of RIPK1 inhibits DNA damage-induced necroptosis

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Specific signaling pathways mediated programmed cell death in tumor microenvironment and target therapies

Increasing evidence has shown that programmed cell death (PCD) plays a crucial role in tumorigenesis and cancer progression. The components of PCD are complex and include various mechanisms such as apoptosis, necroptosis, alkaliptosis, oxeiptosis, and anoikis, all of which are interrelated in their functions and regulatory pathways. Given the significance of these processes, it is essential to conduct a comprehensive study on PCD to elucidate its multifaceted nature. Key signaling pathways, particularly the caspase signaling pathway, the RIPK1/RIPK3/MLKL pathway, and the mTOR signaling pathway, are pivotal in regulating PCD and influencing tumor progression. In this review, we briefly describe the generation mechanisms of different PCD components and focus on the regulatory mechanisms of these three major signaling pathways within the context of global PCD. Furthermore, we discuss various tumor therapeutic compounds that target different signaling axes of these pathways, which may provide novel strategies for effective tumor therapy and help improve patient outcomes in cancer treatment.

Protein kinases in neurodegenerative diseases: current understandings and implications for drug discovery

Neurodegenerative diseases (e.g., Alzheimer's, Parkinson's, Huntington's disease, and Amyotrophic Lateral Sclerosis) are major health threats for the aging population and their prevalences continue to rise with the increasing of life expectancy. Although progress has been made, there is still a lack of effective cures to date, and an in-depth understanding of the molecular and cellular mechanisms of these neurodegenerative diseases is imperative for drug development. Protein phosphorylation, regulated by protein kinases and protein phosphatases, participates in most cellular events, whereas aberrant phosphorylation manifests as a main cause of diseases. As evidenced by pharmacological and pathological studies, protein kinases are proven to be promising therapeutic targets for various diseases, such as cancers, central nervous system disorders, and cardiovascular diseases. The mechanisms of protein phosphatases in pathophysiology have been extensively reviewed, but a systematic summary of the role of protein kinases in the nervous system is lacking. Here, we focus on the involvement of protein kinases in neurodegenerative diseases, by summarizing the current knowledge on the major kinases and related regulatory signal transduction pathways implicated in diseases. We further discuss the role and complexity of kinase-kinase networks in the pathogenesis of neurodegenerative diseases, illustrate the advances of clinical applications of protein kinase inhibitors or novel kinase-targeted therapeutic strategies (such as antisense oligonucleotides and gene therapy) for effective prevention and early intervention.

Understanding TAK1 deficiency in microglia: Dual mechanisms for photoreceptor protection in a mouse model of retinitis pigmentosa

Retinitis pigmentosa (RP) is a group of inherited retinal diseases characterized by the progressive loss of photoreceptors. Neuroinflammation has been implicated in the pathophysiology of RP and its progression. Previous studies have suggested that the transforming growth factor-beta-activated kinase 1 (TAK1) plays a pivotal role in regulating acute and chronic neuroinflammation. However, the functional role of TAK1 in neuroinflammation remains unclear in RP. Here, we observed TAK1 upregulation in activated microglia of the rd10 mouse model of RP. To create the conditional deletion of TAK1 in microglia, we backcrossed Cx3cr1CreER/CreER mice and Tak1fl/fl mice onto rd10 background. We found that both heterozygous (rd10;Cx3cr1CreER/+;Tak1fl/+) and homozygous (rd10;Cx3cr1CreER/+;Tak1fl/fl) deletion of microglial TAK1 slowed down photoreceptor degeneration but with distinct mechanisms. The heterozygous TAK1 deficiency resulted in a reduction in the activation and proliferation of microglia and the release of proinflammatory cytokines by inhibiting STAT3 signaling. In contrast, the homozygous TAK1 deficiency induced apoptosis in microglia via the TNF/RIPK1/CASP3 signaling pathway, contributing to the reduction of microglia-mediated neurotoxicity and subsequent preservation of photoreceptors in RP. Overall, our findings highlight the crucial role of TAK1 in the survival and activation of microglia. We propose that targeting microglial TAK1, considering its expression levels and subsequent signal transduction, could offer a promising personalized therapeutic strategy for individuals with RP regardless of underlying genetic causes.

Exploring necroptosis: mechanistic analysis and antitumor potential of nanomaterials

Necroptosis, a non-apoptotic mode of programmed cell death, is characterized by the disintegration of the plasma membrane, ultimately leading to cell perforation and rupture. Recent studies have disclosed the mechanism of necroptosis and its intimate link with nanomaterials. Nanomedicine represents a novel approach in the development of therapeutic agents utilizing nanomaterials to treat a range of cancers with high efficacy. This article provides an overview of the primary mechanism behind necroptosis, the current research progress in nanomaterials, their potential use in various diseases-notably cancer, safety precautions, and prospects. The goal is to aid in the development of nanomaterials for cancer treatment.

The Role of Post-Translational Modifications in Necroptosis

Necroptosis, a distinct form of regulated necrosis implicated in various human pathologies, is orchestrated through sophisticated signaling pathways. During this process, cells undergoing necroptosis exhibit characteristic necrotic morphology and provoke substantial inflammatory responses. Post-translational modifications (PTMs)-chemical alterations occurring after protein synthesis that critically regulate protein functionality-constitute essential regulatory components within these complex signaling cascades. This intricate crosstalk between necroptotic pathways and PTM networks presents promising therapeutic opportunities. Our comprehensive review systematically analyzes the molecular mechanisms underlying necroptosis, with particular emphasis on the regulatory roles of PTMs in signal transduction. Through systematic evaluation of key modifications including ubiquitination, phosphorylation, glycosylation, methylation, acetylation, disulfide bond formation, caspase cleavage, nitrosylation, and SUMOylation, we examine potential therapeutic applications targeting necroptosis in disease pathogenesis. Furthermore, we synthesize current pharmacological strategies for manipulating PTM-regulated necroptosis, offering novel perspectives on clinical target development and therapeutic intervention.

Recent advances in the role of gasotransmitters in necroptosis

Necroptosis is a finely regulated programmed cell death process involving complex molecular mechanisms and signal transduction networks. Among them, receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein are the key molecules regulating this process. In recent years, gasotransmitters such as nitric oxide, carbon monoxide and hydrogen sulfide have been suggested to play a regulatory role in necroptosis. This paper reviews the evidence that these gasotransmitters are involved in the regulation of necroptosis by influencing the production of reactive oxygen species, regulating the modification of S subunits of RIPK1 and RIPK3, regulating inflammatory mediators, and signal transduction. In addition, this review explores the potential therapeutic applications of these gasotransmitters in pathological conditions such as cardiovascular disease and ischemia-reperfusion injury. Although some studies have revealed the important role of gasotransmitters in necroptosis, the specific mechanism of action is still not fully understood. Future research is needed to further elucidate the molecular mechanisms of gasotransmitters in precisely regulating necroptosis, which will help develop new therapeutic strategies to prevent and treat related diseases.

Genetic Regulation of Cell Death: Insights from Autoinflammatory Diseases

Metazoans have evolved innate antimicrobial defenses that promote cellular survival and proliferation. Countering the inevitable molecular mechanisms by which microbes sabotage these pathways, multicellular organisms rely on an alternative, perhaps more ancient, strategy that is the immune equivalent of suicide bombing: Infection triggers cell death programs that summon localized or even systemic inflammation. The study of human genetics has now unveiled a level of complexity that refutes the naive view that cell death is merely a blunt instrument or an evolutionary afterthought. To the contrary, findings from patients with rare diseases teach us that cell death-induced inflammation is a sophisticated, tightly choreographed process. We herein review the emerging body of evidence describing a group of illnesses-inborn errors of cell death, which define many of the molecular building blocks and regulatory elements controlling cell death-induced inflammation in humans-and provide a possible road map to countering this process across the spectrum of rare and common illnesses.

From mechanisms to medicine: Ferroptosis as a Therapeutic target in liver disorders

In recent 10 years, ferroptosis has become a hot research direction in the scientific research community as a new way of cell death. Iron toxicity accumulation and lipotoxicity are unique features. Several studies have found that ferroptosis is involved in the regulation of the hepatic microenvironment and various hepatic metabolisms, thereby mediating the progression of related liver diseases. For example, NRF2 and FSP1, as important regulatory proteins of ferroptosis, are involved in the development of liver tumors and liver failure. In this manuscript, we present the mechanisms involved in ferroptosis, the concern of ferroptosis with the liver microenvironment and the progression of ferroptosis in various liver diseases. In addition, we summarize recent clinical advances in targeted ferroptosis therapy for related diseases. We expect that this manuscript can provide a new perspective for clinical treatment of related diseases.

TAK1 at the crossroads of multiple regulated cell death pathways: from molecular mechanisms to human diseases

Regulated cell death (RCD), the form of cell death that can be genetically controlled by multiple signaling pathways, plays an important role in organogenesis, tissue remodeling, and maintenance of organism homeostasis and is closely associated with various human diseases. Transforming growth factor‐beta‐activated kinase 1 (TAK1) is a member of the serine/threonine protein kinase family, which can respond to different internal and external stimuli and participate in inflammatory and immune responses. Emerging evidence suggests that TAK1 is an important regulator at the crossroad of multiple RCD pathways, including apoptosis, necroptosis, pyroptosis, and PANoptosis. The regulation of TAK1 affects disease progression through multiple signaling pathways, and therapeutic strategies targeting TAK1 have been proposed for inflammatory diseases, central nervous system diseases, and cancers. In this review, we provide an overview of the downstream signaling pathways regulated by TAK1 and its binding proteins. Their critical regulatory roles in different forms of cell death are also summarized. In addition, we discuss the potential of targeting TAK1 in the treatment of human diseases, with a specific focus on neurological disorders and cancer.

Cooperation of TRADD- and RIPK1-dependent cell death pathways in maintaining intestinal homeostasis

Dysfunctional NF-κB signaling is critically involved in inflammatory bowel disease (IBD). We investigated the mechanism by which RIPK1 and TRADD, two key mediators of NF-κB signaling, in mediating intestinal pathology using TAK1 IEC deficient model. We show that phosphorylation of TRADD by TAK1 modulates RIPK1-dependent apoptosis. TRADD and RIPK1 act cooperatively to mediate cell death regulated by TNF and TLR signaling. We demonstrate the pathological evolution from RIPK1-dependent ileitis to RIPK1- and TRADD-co-dependent colitis in TAK1 IEC deficient condition. Combined RIPK1 inhibition and TRADD knockout completely protect against intestinal pathology and lethality in TAK1 IEC KO mice. Furthermore, we identify distinctive microbiota dysbiosis biomarkers for RIPK1-dependent ileitis and TRADD-dependent colitis. These findings reveal the cooperation between RIPK1 and TRADD in mediating cell death and inflammation in IBD with NF-κB deficiency and suggest the possibility of combined inhibition of RIPK1 kinase and TRADD as a new therapeutic strategy for IBD.

A decision point between transdifferentiation and programmed cell death priming controls KRAS-dependent pancreatic cancer development

KRAS-dependent acinar-to-ductal metaplasia (ADM) is a fundamental step in the development of pancreatic ductal adenocarcinoma (PDAC), but the involvement of cell death pathways remains unclear. Here, we show that key regulators of programmed cell death (PCD) become upregulated during KRAS-driven ADM, thereby priming transdifferentiated cells to death. Using transgenic mice and primary cell and organoid cultures, we show that transforming growth factor (TGF)-β-activated kinase 1 (TAK1), a kinase regulating cell survival and inflammatory pathways, prevents the elimination of transdifferentiated cells through receptor-interacting protein kinase 1 (RIPK1)-mediated apoptosis and necroptosis, enabling PDAC development. Accordingly, pharmacological inhibition of TAK1 induces PCD in patient-derived PDAC organoids. Importantly, cell death induction via TAK1 inhibition does not appear to elicit an overt injury-associated inflammatory response. Collectively, these findings suggest that TAK1 supports cellular plasticity by suppressing spontaneous PCD activation during ADM, representing a promising pharmacological target for the prevention and treatment of PDAC.

TAB2 deficiency induces dilated cardiomyopathy by promoting mitochondrial calcium overload in human iPSC-derived cardiomyocytes

BACKGROUND

TGF-β-activated kinase 1 binding protein 2 (TAB2) is an intermediary protein that links Tumor necrosis factor receptor 1 (TNFR1) and other receptor signals to the TGF-β-activated kinase 1 (TAK1) signaling complex. TAB2 frameshift mutations have been linked to dilated cardiomyopathy (DCM), while the exact mechanism needs further investigation.

METHODS

In this study, we generated a TAB2 compound heterozygous knockout cell line in induced pluripotent stem cells (iPSCs) derived from a healthy individual using CRISPR/Cas9 technology. IPSCs are not species-dependent, are readily accessible, and raise fewer ethical concerns.

RESULTS

TAB2 disruption had no impact on the cardiac differentiation of iPSCs and led to confirmed TAB2 deficiency in human iPSC-derived cardiomyocytes (hiPSC-CMs). TAB2-deficient hiPSC-CMs were found to develop phenotypic features of DCM, such as distorted sarcomeric ultrastructure, decreased contractility and energy production, and mitochondrial damage at day 30 post differentiation. Paradoxically, TAB2 knockout cell lines showed abnormal calcium handling after 40 days, later than reduced contractility, suggesting that the main cause of impaired contractility was abnormal energy production due to mitochondrial damage. As early as day 25, TAB2 knockout cardiomyocytes showed significant mitochondrial calcium overload, which can lead to mitochondrial damage. Furthermore, TAB2 knockout activated receptor-interacting protein kinase 1 (RIPK1), leading to an increase in mitochondrial calcium uniporter (MCU) expression, thereby augmenting the uptake of mitochondrial calcium ions. Finally, the application of the RIPK1 inhibitor Nec-1s prevents the progression of these phenotypes.

CONCLUSIONS

In summary, TAB2 abatement cardiomyocytes mimic dilated cardiomyopathy in vitro. This finding emphasizes the importance of using a human model to study the underlying mechanisms of this specific disease. More importantly, the discovery of a unique pathogenic pathway introduces a new notion for the future management of dilated cardiomyopathy.

TRIF-TAK1 signaling suppresses caspase-8/3-mediated GSDMD/E activation and pyroptosis in influenza A virus-infected airway epithelial cells

Pyroptosis plays an important role in attracting innate immune cells to eliminate infected niches. Our study focuses on how influenza A virus (IAV) infection triggers pyroptosis in respiratory epithelial cells. Here, we report that IAV infection induces pyroptosis in a human and murine airway epithelial cell line. Mechanistically, IAV infection activates caspase-8 and caspase-3, which cleave and activate gasdermin (GSDM) D and GSDME, respectively. Z-nucleic acid-binding protein 1 (ZBP1) and receptor-interacting protein kinase (RIPK) 1 activity but not RIPK3 are required for caspase-8/3 and GSDMD/E activation and pyroptosis. GSDMD/E, ZBP1, and RIPK1 knockout all block IAV-induced pyroptosis but enhance virus replication. Transforming growth factor β-activated kinase 1 (TAK1) activation via the adaptor protein TRIF suppresses RIPK1, caspase-8/3, and GSDMD/E activation and pyroptosis. The TAK1 inhibitor 5Z-oxzeneonal (5Z) enhances IAV-induced caspase-8/3 and GSDMD/E cleavage in the lung tissues of IAV-infected mice. Our study unveils a previously unrecognized mechanism of regulation of IAV-induced pyroptosis in respiratory epithelial cells.

Targeting caspase-8: a new strategy for combating hepatocellular carcinoma

Hepatocellular carcinoma (HCC) represents the most prevalent form of primary liver cancer and has a high mortality rate. Caspase-8 plays a pivotal role in an array of cellular signaling pathways and is essential for the governance of programmed cell death mechanisms, inflammatory responses, and the dynamics of the tumor microenvironment. Dysregulation of caspase-8 is intricately linked to the complex biological underpinnings of HCC. In this manuscript, we provide a comprehensive review of the regulatory roles of caspase-8 in apoptosis, necroptosis, pyroptosis, and PANoptosis, as well as its impact on inflammatory reactions and the intricate interplay with critical immune cells within the tumor microenvironment, such as tumor-associated macrophages, T cells, natural killer cells, and dendritic cells. Furthermore, we emphasize how caspase-8 plays pivotal roles in the development, progression, and drug resistance observed in HCC, and explore the potential of targeting caspase-8 as a promising strategy for HCC treatment.

Spermidine mediates acetylhypusination of RIPK1 to suppress diabetes onset and progression

It has been established that N-acetyltransferase (murine NAT1 (mNAT1) and human NAT2 (hNAT2)) mediates insulin sensitivity in type 2 diabetes. Here we show that mNAT1 deficiency leads to a decrease in cellular spermidine-a natural polyamine exhibiting health-protective and anti-ageing effects-but understanding of its mechanism is limited. We identify that mNAT1 and hNAT2 modulate a type of post-translational modification involving acetylated spermidine, which we name acetylhypusination, on receptor-interacting serine/threonine-protein kinase 1 (RIPK1)-a key regulator of inflammation and cell death. Spermidine supplementation decreases RIPK1-mediated cell death and diabetic phenotypes induced by NAT1 deficiency in vivo. Furthermore, insulin resistance and diabetic kidney disease mediated by vascular pathology in NAT1-deficient mice can be blocked by inhibiting RIPK1. Finally, we demonstrate a decrease in spermidine and activation of RIPK1 in the vascular tissues of human patients with diabetes. Our study suggests a role for vascular pathology in diabetes onset and progression and identifies the inhibition of RIPK1 kinase as a potential therapeutic approach for the treatment of type 2 diabetes.

Palmitoylation licenses RIPK1 kinase activity and cytotoxicity in the TNF pathway

Tumor necrosis factor (TNF)-induced receptor-interacting serine/threonine protein kinase 1 (RIPK1)-mediated cell death, including apoptosis and necroptosis, is increasingly recognized as a major driver of inflammatory diseases. Cell death checkpoints normally suppress RIPK1 kinase to safeguard the organism from its detrimental consequences. However, the mechanisms licensing RIPK1 kinase activity when a protective checkpoint is disabled remain unclear. Here, we identified S-palmitoylation as a licensing modification for RIPK1 kinase. TNF induces RIPK1 palmitoylation, mediated by DHHC5 and dependent on K63-linked ubiquitination of RIPK1, which enhances RIPK1 kinase activity by promoting the homo-interaction of its kinase domain and promotes cell death upon cell death checkpoint blockade. Furthermore, DHHC5 is amplified by fatty acid in the livers of mice with metabolic dysfunction-associated steatohepatitis, contributing to increased RIPK1 cytotoxicity observed in this condition. Our findings reveal that ubiquitination-dependent palmitoylation licenses RIPK1 kinase activity to induce downstream cell death signaling and suggest RIPK1 palmitoylation as a feasible target for inflammatory diseases.

TBK1-associated adapters TANK and AZI2 protect mice against TNF-induced cell death and severe autoinflammatory diseases

The cytokine TNF can trigger highly proinflammatory RIPK1-dependent cell death. Here, we show that the two adapter proteins, TANK and AZI2, suppress TNF-induced cell death by regulating the activation of TBK1 kinase. Mice lacking either TANK or AZI2 do not show an overt phenotype. Conversely, animals deficient in both adapters are born in a sub-Mendelian ratio and suffer from severe multi-organ inflammation, excessive antibody production, male sterility, and early mortality, which can be rescued by TNFR1 deficiency and significantly improved by expressing a kinase-dead form of RIPK1. Mechanistically, TANK and AZI2 both recruit TBK1 to the TNF receptor signaling complex, but with distinct kinetics due to interaction with different complex components. While TANK binds directly to the adapter NEMO, AZI2 is recruited later via deubiquitinase A20. In summary, our data show that TANK and AZI2 cooperatively sustain TBK1 activity during different stages of TNF receptor assembly to protect against autoinflammation.

PANoptosis in autoimmune diseases interplay between apoptosis, necrosis, and pyroptosis

PANoptosis is a newly identified inflammatory programmed cell death (PCD) that involves the interplay of apoptosis, necrosis, and pyroptosis. However, its overall biological effects cannot be attributed to any one type of PCD alone. PANoptosis is regulated by a signaling cascade triggered by the recognition of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) by various sensors. This triggers the assembly of the PANoptosome, which integrates key components from other PCD pathways via adapters and ultimately activates downstream execution molecules, resulting in cell death with necrotic, apoptotic, and pyroptotic features. Autoimmune diseases are characterized by reduced immune tolerance to self-antigens, leading to abnormal immune responses, often accompanied by systemic chronic inflammation. Consequently, PANoptosis, as a unique innate immune-inflammatory PCD pathway, has significant pathophysiological relevance to inflammation and autoimmunity. However, most previous research on PANoptosis has focused on tumors and infectious diseases, leaving its activation and role in autoimmune diseases unclear. This review briefly outlines the characteristics of PANoptosis and summarizes several newly identified PANoptosome complexes, their activation mechanisms, and key components. We also explored the dual role of PANoptosis in diseases and potential therapeutic approaches targeting PANoptosis. Additionally, we review the existing evidence for PANoptosis in several autoimmune diseases and explore the potential regulatory mechanisms involved.

Bibliometric and visualized analysis on global trends and hotspots of TAK1 in regulated cell death: 1999 to 2024

BACKGROUND

Regulated cell death (RCD) is a genetically controlled form of cell death that plays an important role in organogenesis, tissue remodeling, and pathogenesis of cancers. Transforming growth factor-beta-activation kinase 1 (TAK1) is a member of the serine/threonine protein kinase family, which can respond to internal and external stimuli and participate in inflammatory responses through multiple signaling pathways and cellular processes. In the last two decades, the regulatory roles of TAK1 at the crossroads of multiple RCD pathways, including apoptosis, necroptosis, pyroptosis, and PANoptosis were revealed by 801 articles retrieved from the Web of Science Core Collection database. To analyze global research trends and hotspots concerning the role of TAK1 in RCD, the bibliometric and visualized analysis were applied in the current study.

METHODS

The data for this bibliometrics study were retrieved from the Web of Science Core Collection database. The search formula was (TS=(Apoptosis) OR TS=(pyroptosis) OR TS=(Necroptosis) OR TS=(PANoptosis) OR TS=(Autophagy) OR TS=(Ferroptosis) OR TS=(cuproptosis)) AND ((TS=(TAK1)) OR TS=(MAP3K7)). The co-occurrence and co-cited analysis on basic bibliometric parameters were conducted by VOSviewer. The dual-map overlay of journals, citation bursts, keyword timelines, and keyword bursts were analyzed by CiteSpace.

RESULTS

A total of 801 articles from 46 countries have been included in the analysis. The number of publications demonstrates a consistent increase from 1999 to 2024. The primary research institutions driving this field are Osaka University Notably, the Journal of Biological Chemistry stands out as the most popular journal in this domain. These publications collectively involve contributions from 4663 authors, with Jun Tsuji emerging as a prolific author. Jun Tsuji also gains the highest co-citation frequency. Emerging research hotspots are encapsulated by keywords, including apoptosis, NF-κB, inflammation, autophagy, and TNFα.

CONCLUSION

This is the first bibliometric and visualized study to analyze the global trends and hotspots of TAK1 in RCD. Based on the analysis of 801 articles, the results provide a retrospective and comprehensive visualized view of the research hotspots and frontiers of TAK1 at the crossroads of multiple RCD signaling pathways and propose ideas for guiding their future investigations in molecular mechanisms and therapeutic strategies in this field.

Adaptor protein Abelson interactor 1 in homeostasis and disease

Dysregulation of Abelson interactor 1 (ABI1) is associated with various states of disease including developmental defects, pathogen infections, and cancer. ABI1 is an adaptor protein predominantly known to regulate actin cytoskeleton organization processes such as those involved in cell adhesion, migration, and shape determination. Linked to cytoskeleton via vasodilator-stimulated phosphoprotein (VASP), Wiskott-Aldrich syndrome protein family (WAVE), and neural-Wiskott-Aldrich syndrome protein (N-WASP)-associated protein complexes, ABI1 coordinates regulation of various cytoplasmic protein signaling complexes dysregulated in disease states. The roles of ABI1 beyond actin cytoskeleton regulation are much less understood. This comprehensive, protein-centric review describes molecular roles of ABI1 as an adaptor molecule in the context of its dysregulation and associated disease outcomes to better understand disease state-specific protein signaling and affected interconnected biological processes.

EGFR inhibits TNF-α-mediated pathway by phosphorylating TNFR1 at tyrosine 360 and 401

Tumour necrosis factor receptor 1 (TNFR1) induces the nuclear factor kappa-B (NF-κB) signalling pathway and regulated cell death processes when TNF-α ligates with it. Although mechanisms regulating the downstream pathways of TNFR1 have been elucidated, the direct regulation of TNFR1 itself is not well known. In this study, we showed that the kinase domain of the epidermal growth factor receptor (EGFR) regulates NF-κB signalling and TNF-α-induced cell death by directly phosphorylating TNFR1 at Tyr 360 and 401 in its death domain. In contrast, EGFR inhibition by EGFR inhibitors, such as erlotinib and gefitinib, prevented their interaction. Once TNFR1 is phosphorylated, its death domain induces the suppression of the NF-κB pathways, complex II-mediated apoptosis, or necrosome-dependent necroptosis. Physiologically, in mouse models, EGF treatment mitigates TNF-α-dependent necroptotic skin inflammation induced by treatment with IAP and caspase inhibitors. Our study revealed a novel role for EGFR in directly regulating TNF-α-related pathways.

Activation of the NLRP1B inflammasome by caspase-8

Cleavage of the innate immune receptor NLRP1B by various microbial proteases causes the proteasomal degradation of its N-terminal fragment and the subsequent release of a C-terminal fragment that forms an inflammasome. We reported previously that metabolic stress caused by intracellular bacteria triggers NLRP1B activation, but the mechanism by which this occurs was not elucidated. Here we demonstrate that TLR4 signaling in metabolically stressed macrophages promotes the formation of a TRIF/RIPK1/caspase-8 complex. Caspase-8 activity, induced downstream of this TLR4 pathway or through a distinct TNF receptor pathway, causes cleavage and activation of NLRP1B, which facilitates the maturation of both pro-caspase-1 and pro-caspase-8. Thus, our findings indicate that caspase-8 and NLRP1B generate a positive feedback loop that amplifies cell death processes and promotes a pro-inflammatory response through caspase-1. The ability of NLRP1B to detect caspase-8 activity suggests that this pattern recognition receptor may play a role in the defense against a variety of pathogens that induce apoptosis.

The romantic history of signaling pathway discovery in cell death: an updated review

Cell death is a fundamental physiological process in all living organisms. Processes such as embryonic development, organ formation, tissue growth, organismal immunity, and drug response are accompanied by cell death. In recent years with the development of electron microscopy as well as biological techniques, especially the discovery of novel death modes such as ferroptosis, cuprotosis, alkaliptosis, oxeiptosis, and disulfidptosis, researchers have been promoted to have a deeper understanding of cell death modes. In this systematic review, we examined the current understanding of modes of cell death, including the recently discovered novel death modes. Our analysis highlights the common and unique pathways of these death modes, as well as their impact on surrounding cells and the organism as a whole. Our aim was to provide a comprehensive overview of the current state of research on cell death, with a focus on identifying gaps in our knowledge and opportunities for future investigation. We also presented a new insight for macroscopic intracellular survival patterns, namely that intracellular molecular homeostasis is central to the balance of different cell death modes, and this viewpoint can be well justified by the signaling crosstalk of different death modes. These concepts can facilitate the future research about cell death in clinical diagnosis, drug development, and therapeutic modalities.

Targeting necroptosis: a promising avenue for respiratory disease treatment

Respiratory diseases are a growing concern in public health because of their potential to endanger the global community. Cell death contributes critically to the pathophysiology of respiratory diseases. Recent evidence indicates that necroptosis, a unique form of programmed cell death (PCD), plays a vital role in the molecular mechanisms underlying respiratory diseases, distinguishing it from apoptosis and conventional necrosis. Necroptosis is a type of inflammatory cell death governed by receptor-interacting serine/threonine protein kinase 1 (RIPK1), RIPK3, and mixed-lineage kinase domain-like protein (MLKL), resulting in the release of intracellular contents and inflammatory factors capable of initiating an inflammatory response in adjacent tissues. These necroinflammatory conditions can result in significant organ dysfunction and long-lasting tissue damage within the lungs. Despite evidence linking necroptosis to various respiratory diseases, there are currently no specific alternative treatments that target this mechanism. This review provides a comprehensive overview of the most recent advancements in understanding the significance and mechanisms of necroptosis. Specifically, this review emphasizes the intricate association between necroptosis and respiratory diseases, highlighting the potential use of necroptosis as an innovative therapeutic approach for treating these conditions.

TBK1 and IKKε protect target cells from IFNγ-mediated T cell killing via an inflammatory apoptotic mechanism

Cytotoxic T cells produce interferon gamma (IFNγ), which plays a critical role in anti-microbial and anti-tumor responses. However, it is not clear whether T cell-derived IFNγ directly kills infected and tumor target cells, and how this may be regulated. Here, we report that target cell expression of the kinases TBK1 and IKKε regulate IFNγ cytotoxicity by suppressing the ability of T cell-derived IFNγ to kill target cells. In tumor targets lacking TBK1 and IKKε, IFNγ induces expression of TNFR1 and the Z-nucleic acid sensor, ZBP1, to trigger RIPK1-dependent apoptosis, largely in a target cell-autonomous manner. Unexpectedly, IFNγ, which is not known to signal to NFκB, induces hyperactivation of NFκB in TBK1 and IKKε double-deficient cells. TBK1 and IKKε suppress IKKα/β activity and in their absence, IFNγ induces elevated NFκB-dependent expression of inflammatory chemokines and cytokines. Apoptosis is thought to be non-inflammatory, but our observations demonstrate that IFNγ can induce an inflammatory form of apoptosis, and this is suppressed by TBK1 and IKKε. The two kinases provide a critical connection between innate and adaptive immunological responses by regulating three key responses: (1) phosphorylation of IRF3/7 to induce type I IFN; (2) inhibition of RIPK1-dependent death; and (3) inhibition of NFκB-dependent inflammation. We propose that these kinases evolved these functions such that their inhibition by pathogens attempting to block type I IFN expression would enable IFNγ to trigger apoptosis accompanied by an alternative inflammatory response. Our findings show that loss of TBK1 and IKKε in target cells sensitizes them to inflammatory apoptosis induced by T cell-derived IFNγ.

METTL3 restricts RIPK1-dependent cell death via the ATF3-cFLIP axis in the intestinal epithelium

Intestinal epithelial cells (IECs) are pivotal for maintaining intestinal homeostasis through self-renewal, proliferation, differentiation, and regulated cell death. While apoptosis and necroptosis are recognized as distinct pathways, their intricate interplay remains elusive. In this study, we report that Mettl3-mediated m6A modification maintains intestinal homeostasis by impeding epithelial cell death. Mettl3 knockout induces both apoptosis and necroptosis in IECs. Targeting different modes of cell death with specific inhibitors unveils that RIPK1 kinase activity is critical for the cell death triggered by Mettl3 knockout. Mechanistically, this occurs via the m6A-mediated transcriptional regulation of Atf3, a transcription factor that directly binds to Cflar, the gene encoding the anti-cell death protein cFLIP. cFLIP inhibits RIPK1 activity, thereby suppressing downstream apoptotic and necroptotic signaling. Together, these findings delineate the essential role of the METTL3-ATF3-cFLIP axis in homeostatic regulation of the intestinal epithelium by blocking RIPK1 activity.

Yersinia infection induces glucose depletion and AMPK-dependent inhibition of pyroptosis in mice

Nutritional status and pyroptosis are important for host defence against infections. However, the molecular link that integrates nutrient sensing into pyroptosis during microbial infection is unclear. Here, using metabolic profiling, we found that Yersinia pseudotuberculosis infection results in a significant decrease in intracellular glucose levels in macrophages. This leads to activation of the glucose and energy sensor AMPK, which phosphorylates the essential kinase RIPK1 at S321 during caspase-8-mediated pyroptosis. This phosphorylation inhibits RIPK1 activation and thereby restrains pyroptosis. Boosting the AMPK-RIPK1 cascade by glucose deprivation, AMPK agonists, or RIPK1-S321E knockin suppresses pyroptosis, leading to increased susceptibility to Y. pseudotuberculosis infection in mice. Ablation of AMPK in macrophages or glucose supplementation in mice is protective against infection. Thus, we reveal a molecular link between glucose sensing and pyroptosis, and unveil a mechanism by which Y. pseudotuberculosis reduces glucose levels to impact host AMPK activation and limit host pyroptosis to facilitate infection.

Blockade of IKK signaling induces RIPK1-independent apoptosis in human macrophages

Regulated cell death in response to microbial infection plays an important role in immune defense and is triggered by pathogen disruption of essential cellular pathways. Gram-negative bacterial pathogens in the Yersinia genus disrupt NF-κB signaling via translocated effectors injected by a type III secretion system, thereby preventing induction of cytokine production and antimicrobial defense. In murine models of infection, Yersinia blockade of NF-κB signaling triggers cell-extrinsic apoptosis through Receptor Interacting Serine-Threonine Protein Kinase 1 (RIPK1) and caspase-8, which is required for bacterial clearance and host survival. Unexpectedly, we find that human macrophages undergo apoptosis independently of RIPK1 in response to Yersinia or chemical blockade of IKKβ. Instead, IKK blockade led to decreased cFLIP expression, and overexpression of cFLIP contributed to protection from IKK blockade-induced apoptosis in human macrophages. We found that IKK blockade also induces RIPK1 kinase activity-independent apoptosis in human T cells and human pancreatic cells. Altogether, our data indicate that, in contrast to murine cells, blockade of IKK activity in human cells triggers a distinct apoptosis pathway that is independent of RIPK1 kinase activity. These findings have implications for the contribution of RIPK1 to cell death in human cells and the efficacy of RIPK1 inhibition in human diseases.

ChRIPK1 caused necroptosis signaling pathway deficiency in Crassostrea hongkongensis

RIPK1/TAK1 are important for programmed cell death, including liver death, necroptosis and apoptosis. However, there have been few published reports on the functions of RIPK1/TAK1 in invertebrates. In this study, full-length ChRIPK1 and ChTAK1 were cloned from C. hongkongensis through the rapid amplification of cDNA ends (RACE) technology. ChRIPK1 has almost no homology with human RIPK1 and lacks a kinase domain at the N-terminus but has a DD and RHIM domain. ChTAK1 is conserved throughout evolution. qRT‒PCR was used to analyze the mRNA expression patterns of ChRIPK1 in different tissues, developmental stages, and V. coralliilyticus-infected individuals, and both were highly expressed in the mantle and gills, while ChRIPK1 was upregulated in hemocytes and gills after V. coralliilyticus or S. aureus infection, which indicates that ChRIPK1 is involved in immune regulation. Fluorescence assays revealed that ChRIPK1 localized to the cytoplasm of HEK293T cells in a punctiform manner, but the colocalization of ChRIPK1 with ChTAK1 abolished the punctiform morphology. In the dual-luciferase reporter assay, both ChRIPK1 and ChRIPK1-RIHM activated the NF-κB signaling pathway in HEK293T cells, and ChTAK1 activated ChRIPK1 in the NF-κB signaling pathway. The apoptosis rate of the hemocytes was not affected by the necroptosis inhibitor Nec-1 but was significantly decreased, and ChRIPK1 expression was knocked down in the hemocytes of C. hongkongensis. These findings indicated that ChRIPK1 induces apoptosis but not necroptosis in oysters. This study provides a theoretical basis for further research on the molecular mechanism by which invertebrates regulate the programmed cell death of hemocytes in oysters.

The TRIF-RIPK1-Caspase-8 signalling in the regulation of TLR4-driven gene expression

The inflammatory response is tightly regulated to eliminate invading pathogens and avoid excessive production of inflammatory mediators and tissue damage. Caspase-8 is a cysteine protease that is involved in programmed cell death. Here we show the TRIF-RIPK1-Caspase-8 is required for LPS-induced CYLD degradation in macrophages. TRIF functions in the upstream of RIPK1. The homotypic interaction motif of TRIF and the death domain of RIPK1 are essential for Caspase-8 activation. Caspase-8 cleaves CYLD and the D235A mutant is resistant to the protease activity of Caspase-8. TRIF and RIPK1 serve as substrates of Capase-8 in vitro. cFLIP interacts with Caspase-8 to modulate its protease activity on CYLD and cell death. Deficiency in TRIF, Caspase-8 or CYLD can lead to a decrease or increase in the expression of genes encoding inflammatory cytokines. Together, the TRIF-Caspase-8 and CYLD play opposite roles in the regulation of TLR4 signalling.

Influenza virus infection activates TAK1 to suppress RIPK3-independent apoptosis and RIPK1-dependent necroptosis

Many DNA viruses develop various strategies to inhibit cell death to facilitate their replication. However, whether influenza A virus (IAV), a fast-replicating RNA virus, attenuates cell death remains unknown. Here, we report that IAV infection induces TAK1 phosphorylation in a murine alveolar epithelial cell line (LET1) and a murine fibroblastoma cell line (L929). The TAK1-specific inhibitor 5Z-7-Oxzeneonal (5Z) and TAK1 knockout significantly enhance IAV-induced apoptosis, as evidenced by increased PARP, caspase-8, and caspase-3 cleavage. TAK1 inhibition also increases necroptosis as evidenced by increased RIPK1S166, RIPK3T231/S232, and MLKLS345 phosphorylation. Mechanistically, TAK1 activates IKK, which phosphorylates RIPK1S25 and inhibits its activation. TAK1 also activates p38 and its downstream kinase MK2, which phosphorylates RIPK1S321 but does not affect RIPK1 activation. Further investigation revealed that the RIPK1 inhibitor Nec-1 and RIPK1 knockout abrogate IAV-induced apoptosis and necroptosis; re-expression of wild-type but not kinase-dead (KD)-RIPK1 restores IAV-induced cell death. ZBP1 knockout abrogates IAV-induced cell death, whereas RIPK3 knockout inhibits IAV-induced necroptosis but not apoptosis. 5Z treatment enhances IAV-induced cell death and slightly reduces the inflammatory response in the lungs of H1N1 virus-infected mice and prolongs the survival of IAV-infected mice. Our study provides evidence that IAV activates TAK1 to suppress RIPK1-dependent apoptosis and necroptosis, and that RIPK3 is required for IAV-induced necroptosis but not apoptosis in epithelial cells.

Regulation of RIPK1 Phosphorylation: Implications for Inflammation, Cell Death, and Therapeutic Interventions

Receptor-interacting protein kinase 1 (RIPK1) plays a crucial role in controlling inflammation and cell death. Its function is tightly controlled through post-translational modifications, enabling its dynamic switch between promoting cell survival and triggering cell death. Phosphorylation of RIPK1 at various sites serves as a critical mechanism for regulating its activity, exerting either activating or inhibitory effects. Perturbations in RIPK1 phosphorylation status have profound implications for the development of severe inflammatory diseases in humans. This review explores the intricate regulation of RIPK1 phosphorylation and dephosphorylation and highlights the potential of targeting RIPK1 phosphorylation as a promising therapeutic strategy for mitigating human diseases.

The protein phosphatase PP6 promotes RIPK1-dependent PANoptosis

BACKGROUND

The innate immune system serves as the first line of host defense. Transforming growth factor-β-activated kinase 1 (TAK1) is a key regulator of innate immunity, cell survival, and cellular homeostasis. Because of its importance in immunity, several pathogens have evolved to carry TAK1 inhibitors. In response, hosts have evolved to sense TAK1 inhibition and induce robust lytic cell death, PANoptosis, mediated by the RIPK1-PANoptosome. PANoptosis is a unique innate immune inflammatory lytic cell death pathway initiated by an innate immune sensor and driven by caspases and RIPKs. While PANoptosis can be beneficial to clear pathogens, excess activation is linked to pathology. Therefore, understanding the molecular mechanisms regulating TAK1 inhibitor (TAK1i)-induced PANoptosis is central to our understanding of RIPK1 in health and disease.

RESULTS

In this study, by analyzing results from a cell death-based CRISPR screen, we identified protein phosphatase 6 (PP6) holoenzyme components as regulators of TAK1i-induced PANoptosis. Loss of the PP6 enzymatic component, PPP6C, significantly reduced TAK1i-induced PANoptosis. Additionally, the PP6 regulatory subunits PPP6R1, PPP6R2, and PPP6R3 had redundant roles in regulating TAK1i-induced PANoptosis, and their combined depletion was required to block TAK1i-induced cell death. Mechanistically, PPP6C and its regulatory subunits promoted the pro-death S166 auto-phosphorylation of RIPK1 and led to a reduction in the pro-survival S321 phosphorylation.

CONCLUSIONS

Overall, our findings demonstrate a key requirement for the phosphatase PP6 complex in the activation of TAK1i-induced, RIPK1-dependent PANoptosis, suggesting this complex could be therapeutically targeted in inflammatory conditions.

N-glycosylation by Mgat5 imposes a targetable constraint on immune-mediated tumor clearance

The regulated glycosylation of the proteome has widespread effects on biological processes that cancer cells can exploit. Expression of N-acetylglucosaminyltransferase V (encoded by Mgat5 or GnT-V), which catalyzes the addition of β1,6-linked N-acetylglucosamine to form complex N-glycans, has been linked to tumor growth and metastasis across tumor types. Using a panel of murine pancreatic ductal adenocarcinoma (PDAC) clonal cell lines that recapitulate the immune heterogeneity of PDAC, we found that Mgat5 is required for tumor growth in vivo but not in vitro. Loss of Mgat5 results in tumor clearance that is dependent on T cells and dendritic cells, with NK cells playing an early role. Analysis of extrinsic cell death pathways revealed Mgat5-deficient cells have increased sensitivity to cell death mediated by the TNF superfamily, a property that was shared with other non-PDAC Mgat5-deficient cell lines. Finally, Mgat5 knockout in an immunotherapy-resistant PDAC line significantly decreased tumor growth and increased survival upon immune checkpoint blockade. These findings demonstrate a role for N-glycosylation in regulating the sensitivity of cancer cells to T cell killing through classical cell death pathways.

Emerging role of immunogenic cell death in cancer immunotherapy

Cancer immunotherapy, such as immune checkpoint blockade (ICB), has emerged as a groundbreaking approach for effective cancer treatment. Despite its considerable potential, clinical studies have indicated that the current response rate to cancer immunotherapy is suboptimal, primarily attributed to low immunogenicity in certain types of malignant tumors. Immunogenic cell death (ICD) represents a form of regulated cell death (RCD) capable of enhancing tumor immunogenicity and activating tumor-specific innate and adaptive immune responses in immunocompetent hosts. Therefore, gaining a deeper understanding of ICD and its evolution is crucial for developing more effective cancer therapeutic strategies. This review focuses exclusively on both historical and recent discoveries related to ICD modes and their mechanistic insights, particularly within the context of cancer immunotherapy. Our recent findings are also highlighted, revealing a mode of ICD induction facilitated by atypical interferon (IFN)-stimulated genes (ISGs), including polo-like kinase 2 (PLK2), during hyperactive type I IFN signaling. The review concludes by discussing the therapeutic potential of ICD, with special attention to its relevance in both preclinical and clinical settings within the field of cancer immunotherapy.

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.

A guide to cell death pathways

Regulated cell death mediated by dedicated molecular machines, known as programmed cell death, plays important roles in health and disease. Apoptosis, necroptosis and pyroptosis are three such programmed cell death modalities. The caspase family of cysteine proteases serve as key regulators of programmed cell death. During apoptosis, a cascade of caspase activation mediates signal transduction and cellular destruction, whereas pyroptosis occurs when activated caspases cleave gasdermins, which can then form pores in the plasma membrane. Necroptosis, a form of caspase-independent programmed necrosis mediated by RIPK3 and MLKL, is inhibited by caspase-8-mediated cleavage of RIPK1. Disruption of cellular homeostatic mechanisms that are essential for cell survival, such as normal ionic and redox balance and lysosomal flux, can also induce cell death without invoking programmed cell death mechanisms. Excitotoxicity, ferroptosis and lysosomal cell death are examples of such cell death modes. In this Review, we provide an overview of the major cell death mechanisms, highlighting the latest insights into their complex regulation and execution, and their relevance to human diseases.

Mitochondrial Dysfunction by FADDosome Promotes Gastric Mucosal Injury in Portal Hypertensive Gastropathy

Mucosal epithelial death is an essential pathological characteristic of portal hypertensive gastropathy (PHG). FADDosome can regulate mucosal homeostasis by controlling mitochondrial status and cell death. However, it remains ill-defined whether and how the FADDosome is involved in the epithelial death of PHG. The FADDosome formation, mitochondrial dysfunction, glycolysis process and NLRP3 inflammasome activation in PHG from both human sections and mouse models were investigated. NLRP3 wild-type (NLRP3-WT) and NLRP3 knockout (NLRP3-KO) littermate models, critical element inhibitors and cell experiments were utilized. The mechanism underlying FADDosome-regulated mitochondrial dysfunction and epithelial death in PHG was explored. Here, we found that FADD recruited caspase-8 and receptor-interacting serine/threonine-protein kinase 1 (RIPK1) to form the FADDosome to promote Drp1-dependent mitochondrial fission and dysfunction in PHG. Also, FADDosome modulated NOX2 signaling to strengthen Drp1-dependent mitochondrial fission and alter glycolysis as well as enhance mitochondrial reactive oxygen species (mtROS) production. Moreover, due to the dysfunction of electron transport chain (ETC) and alteration of antioxidant enzymes activity, this altered glycolysis also contributed to mtROS production. Subsequently, the enhanced mtROS production induced NLRP3 inflammasome activation to result in the epithelial pyroptosis and mucosal injury in PHG. Thus, the FADDosome-regulated pathways may provide a potential therapeutic target for PHG.

TAK1 inhibition leads to RIPK1-dependent apoptosis in immune-activated cancers

Poor survival and lack of treatment response in glioblastoma (GBM) is attributed to the persistence of glioma stem cells (GSCs). To identify novel therapeutic approaches, we performed CRISPR/Cas9 knockout screens and discovered TGFβ activated kinase (TAK1) as a selective survival factor in a significant fraction of GSCs. Loss of TAK1 kinase activity results in RIPK1-dependent apoptosis via Caspase-8/FADD complex activation, dependent on autocrine TNFα ligand production and constitutive TNFR signaling. We identify a transcriptional signature associated with immune activation and the mesenchymal GBM subtype to be a characteristic of cancer cells sensitive to TAK1 perturbation and employ this signature to accurately predict sensitivity to the TAK1 kinase inhibitor HS-276. In addition, exposure to pro-inflammatory cytokines IFNγ and TNFα can sensitize resistant GSCs to TAK1 inhibition. Our findings reveal dependency on TAK1 kinase activity as a novel vulnerability in immune-activated cancers, including mesenchymal GBMs that can be exploited therapeutically.

Eupatolide, isolated from Liriodendron tulipifera, sensitizes TNF-mediated dual modes of apoptosis and necroptosis by disrupting RIPK1 ubiquitination

Ubiquitination of RIPK1 plays an essential role in the recruitment of the IKK complex, an upstream component of pro-survival NF-κB. It also limits TNF-induced programmed cell death by inhibiting the spatial transition from TNFR1-associated complex-I to RIPK1-dependent death-inducing complex-II or necrosome. Thus, the targeted disruption of RIPK1 ubiquitination, which induces RIPK1-dependent cell death, has proven to be a useful strategy for improving the therapeutic efficacy of TNF. In this study, we found that eupatolide, isolated from Liriodendron tulipifera, is a potent activator of the cytotoxic potential of RIPK1 by disrupting the ubiquitination of RIPK1 upon TNFR1 ligation. Analysis of events upstream of NF-κB signaling revealed that eupatolide inhibited IKKβ-mediated NF-κB activation while having no effect on IKKα-mediated non-canonical NF-κB activation. Pretreatment with eupatolide drastically interfered with RIPK1 recruitment to the TNFR1 complex-I by disrupting RIPK1 ubiquitination. Moreover, eupatolide was sufficient to upregulate the activation of RIPK1, facilitating the TNF-mediated dual modes of apoptosis and necroptosis. Thus, we propose a novel mechanism by which eupatolide activates the cytotoxic potential of RIPK1 at the TNFR1 level and provides a promising anti-cancer therapeutic approach to overcome TNF resistance.

PARP5A and RNF146 phase separation restrains RIPK1-dependent necroptosis

Phase separation is a vital mechanism that mediates the formation of biomolecular condensates and their functions. Necroptosis is a lytic form of programmed cell death mediated by RIPK1, RIPK3, and MLKL downstream of TNFR1 and has been implicated in mediating many human diseases. However, whether necroptosis is regulated by phase separation is not yet known. Here, we show that upon the induction of necroptosis and recruitment by the adaptor protein TAX1BP1, PARP5A and its binding partner RNF146 form liquid-like condensates by multivalent interactions to perform poly ADP-ribosylation (PARylation) and PARylation-dependent ubiquitination (PARdU) of activated RIPK1 in mouse embryonic fibroblasts. We show that PARdU predominantly occurs on the K376 residue of mouse RIPK1, which promotes proteasomal degradation of kinase-activated RIPK1 to restrain necroptosis. Our data demonstrate that PARdU on K376 of mouse RIPK1 provides an alternative cell death checkpoint mediated by phase separation-dependent control of necroptosis by PARP5A and RNF146.

RIPK1 activation in Mecp2-deficient microglia promotes inflammation and glutamate release in RTT

Rett syndrome (RTT) is a devastating neurodevelopmental disorder primarily caused by mutations in the methyl-CpG binding protein 2 (Mecp2) gene. Here, we found that inhibition of Receptor-Interacting Serine/Threonine-Protein Kinase 1 (RIPK1) kinase ameliorated progression of motor dysfunction after onset and prolonged the survival of Mecp2-null mice. Microglia were activated early in myeloid Mecp2-deficient mice, which was inhibited upon inactivation of RIPK1 kinase. RIPK1 inhibition in Mecp2-deficient microglia reduced oxidative stress, cytokines production and induction of SLC7A11, SLC38A1, and GLS, which mediate the release of glutamate. Mecp2-deficient microglia release high levels of glutamate to impair glutamate-mediated excitatory neurotransmission and promote increased levels of GluA1 and GluA2/3 proteins in vivo, which was reduced upon RIPK1 inhibition. Thus, activation of RIPK1 kinase in Mecp2-deficient microglia may be involved both in the onset and progression of RTT.

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.

Emerging Microglial Therapies and Targets in Clinical Trial

Modulation of microglia function for treatment of neurodegenerative and neuropsychiatric disorders is an emerging field of neuroscience drug development. This is largely attributed to human genetic association studies combined with biological evidence indicating that the innate immune system acts as a causal contributor superimposed on the reactive component of neuronal loss in neurological dysfunction. The identification of disease risk gene variants that encode immune-modulatory proteins in microglia provides tools to evaluate how microglia cellular function or dysfunction affect neuronal health. The development of clinical stage therapeutic compounds that modify myeloid cell function enables us to investigate how modulating microglia function could become a transformational approach to mitigate neurological disorders. Improving our ability to boost microglia-promoting homeostatic and reparative functions hopefully will translate into achieving a better outcome for patients affected by neurological diseases. In this chapter, we aim to provide an overview of the microglial emerging therapies and targets being studied in current clinical trials.

Advances in the regulatory mechanisms of mTOR in necroptosis

The mammalian target of rapamycin (mTOR), an evolutionarily highly conserved serine/threonine protein kinase, plays a prominent role in controlling gene expression, metabolism, and cell death. Programmed cell death (PCD) is indispensable for maintaining homeostasis by removing senescent, defective, or malignant cells. Necroptosis, a type of PCD, relies on the interplay between receptor-interacting serine-threonine kinases (RIPKs) and the membrane perforation by mixed lineage kinase domain-like protein (MLKL), which is distinguished from apoptosis. With the development of necroptosis-regulating mechanisms, the importance of mTOR in the complex network of intersecting signaling pathways that govern the process has become more evident. mTOR is directly responsible for the regulation of RIPKs. Autophagy is an indirect mechanism by which mTOR regulates the removal and interaction of RIPKs. Another necroptosis trigger is reactive oxygen species (ROS) produced by oxidative stress; mTOR regulates necroptosis by exploiting ROS. Considering the intricacy of the signal network, it is reasonable to assume that mTOR exerts a bifacial effect on necroptosis. However, additional research is necessary to elucidate the underlying mechanisms. In this review, we summarized the mechanisms underlying mTOR activation and necroptosis and highlighted the signaling pathway through which mTOR regulates necroptosis. The development of therapeutic targets for various diseases has been greatly advanced by the expanding knowledge of how mTOR regulates necroptosis.

Inhibitor of nuclear factor kappa B kinase subunit epsilon regulates murine acetaminophen toxicity via RIPK1/JNK

Drug-induced liver injury (DILI) still poses a major clinical challenge and is a leading cause of acute liver failure. Inhibitor of nuclear factor kappa B kinase subunit epsilon (IKBKE) is essential for inflammation and metabolic disorders. However, it is unclear how IKBKE regulates cellular damage in acetaminophen (APAP)-induced acute liver injury. Here, we found that the deficiency of IKBKE markedly aggravated APAP-induced acute liver injury by targeting RIPK1. We showed that APAP-treated IKBKE-deficient mice exhibited severer liver injury, worse mitochondrial integrity, and enhanced glutathione depletion than wild-type mice. IKBKE deficiency may directly upregulate the expression of total RIPK1 and the cleaved RIPK1, resulting in sustained JNK activation and increased translocation of RIPK1/JNK to mitochondria. Moreover, deficiency of IKBKE enhanced the expression of pro-inflammatory factors and inflammatory cell infiltration in the liver, especially neutrophils and monocytes. Inhibition of RIPK1 activity by necrostatin-1 significantly reduced APAP-induced liver damage. Thus, we have revealed a negative regulatory function of IKBKE, which acts as an RIPK1/JNK regulator to mediate APAP-induced hepatotoxicity. Targeting IKBKE/RIPK1 may serve as a potential therapeutic strategy for acute or chronic liver injury.

Molecular and functional characteristics of receptor-interacting protein kinase 1 (RIPK1) and its therapeutic potential in Alzheimer's disease

Inflammation and cell death processes positively control the organ homeostasis of an organism. Receptor-interacting protein kinase 1 (RIPK1), a member of the RIPK family, is a crucial regulator of cell death and inflammation, and control homeostasis at the cellular and tissue level. Necroptosis, a programmed form of necrosis-mediated cell death and tumor necrosis factor (TNF)-induced necrotic cell death, is mostly regulated by RIPK1 kinase activity. Thus, RIPK1 has recently emerged as an upstream kinase that controls multiple cellular pathways and participates in regulating inflammation and cell death. All the major cell types in the central nervous system (CNS) have been found to express RIPK1. Selective inhibition of RIPK1 has been shown to prevent neuronal cell death, which could ultimately lead to a significant reduction of neurodegeneration and neuroinflammation. In addition, the kinase structure of RIPK1 is highly conducive to the development of specific pharmacological small-molecule inhibitors. These factors have led to the emergence of RIPK1 as an important therapeutic target for Alzheimer's disease (AD).

Housing conditions affect enterocyte death mode and turnover rate in mouse small intestine

Small intestinal enterocytes are continuously renewed. Shedding/death of enterocytes involves receptor-interacting protein kinase 1 (RIPK1)-dependent (but RIPK3-independent) necrotic death, but the regulatory mechanism of the processes is not fully understood. Here, we show that mouse housing conditions, such as the type of bedding material and the presence or absence of a Shepherd Shack, affect enterocyte turnover rate and determine whether enterocyte shedding/death is RIPK1-independent or -dependent. Mice housed with ALPHA-dri (αDri, hard paper chip) bedding material without a Shepherd Shack had a higher, largely RIPK1-dependent enterocyte turnover rate and higher blood corticosterone levels, suggesting the involvement of minor stress, whereas mice housed with αDri plus a Shepherd Shack or with Soft Chip had a lower, RIPK1-independent turnover rate and lower blood corticosterone levels. Corticosterone administration to a small intestine culture derived from mice housed with αDri plus a Shepherd Shack or with Soft Chip increased enterocyte shedding/death and turnover. By using kinase inhibitors and knockout mice, we showed that the switch from RIPK1-independent to RIPK1-dependent enterocyte shedding/death and turnover involves suppression of TANK-binding kinase 1. Our results demonstrate that housing conditions may cause minor stress, which alters the mode of enterocyte shedding/death and enterocyte turnover rate in mice.

Regulated necrosis pathways: a potential target for ischemic stroke

Globally, ischemic stroke causes millions of deaths per year. The outcomes of ischemic stroke are largely determined by the amount of ischemia-related and reperfusion-related neuronal death in the infarct region. In the infarct region, cell injuries follow either the regulated pathway involving precise signaling cascades, such as apoptosis and autophagy, or the nonregulated pathway, which is uncontrolled by any molecularly defined effector mechanisms such as necrosis. However, numerous studies have recently found that a certain type of necrosis can be regulated and potentially modified by drugs and is nonapoptotic; this type of necrosis is referred to as regulated necrosis. Depending on the signaling pathway, various elements of regulated necrosis contribute to the development of ischemic stroke, such as necroptosis, pyroptosis, ferroptosis, pathanatos, mitochondrial permeability transition pore-mediated necrosis and oncosis. In this review, we aim to summarize the underlying molecular mechanisms of regulated necrosis in ischemic stroke and explore the crosstalk and interplay among the diverse types of regulated necrosis. We believe that targeting these regulated necrosis pathways both pharmacologically and genetically in ischemia-induced neuronal death and protection could be an efficient strategy to increase neuronal survival and regeneration in ischemic stroke.