The ubiquitin-modifying enzyme A20 restricts ubiquitination of the kinase RIPK3 and protects cells from necroptosis

Enhancement of reactive oxygen species metabolism by antioxidant enzyme Cu/Zn-SOD can block betanodavirus-induced necroptosis and suppress viral replication in fish cells

Very little is known about oxidative stress as a modulator of signaling between the host and virus in viral nervous necrosis (VNN) within the aquaculture field. In the present study, we examined whether blocking ROS signaling using mitochondrial Cu/Zn-SOD could affect host cell death and the viral replication of RGNNV during infection in fish cells. Upon the overexpression of Cu/Zn-SOD in fish GF-1 cells, superoxide generation in RGNNV infection was reduced 0.6-fold, which correlated to host cell viability in the middle-late stages. Regarding the regulation of reactive oxygen species (ROS) signaling by superoxide, Cu/Zn-SOD overexpression can enhance superoxide's metabolism to hydrogen peroxide, which suppresses the RIPK3-mediated cell death signals at 48 hpf. On the other hand, ROS-mediated signal suppression can enhance Bcl-2 family Bcl-2/Bcl-xL expression in the early and middle replication stages. Furthermore, the enhancement of superoxide metabolism can reduce the virus' replication ability and expression of the non-structural genes B1 and B2, as well as the genome replication gene Protein A and the major capsid protein protein α, which were correlated with the viral load dropping by two log viral titers at 48 and 72 hpf. Taken together, these data suggest that ROS signals trigger host stress responses related to cell death/necroptosis in RGNNV infection. Then, ROS-mediated stress signals can modulate anti-cell death signals through the Bcl-2/Bcl-xL pathway. In conclusion, an ROS-mediated stress response is required for viral expression and replication cycles, providing new insights into controlling RNA viruses.

Melatonin and necroptosis: therapeutic aspects based on cellular mechanisms

Necroptosis is a regulated form of programmed cell death. It is integral to the development of various diseases, such as neurological disorders, lung injuries, liver fibrosis, and testicular toxicity. This process is orchestrated by essential molecular pathways that involve receptor-interacting protein kinases (RIP1/RIP3) and mixed lineage kinase domain-like protein (MLKL), which lead to membrane disruption, the release of damage-associated molecular patterns (DAMPs), and subsequent inflammation. Melatonin, recognized for its potent antioxidant and anti-inflammatory properties, has emerged as a promising therapeutic agent that can inhibit necroptosis through various mechanisms. In neurological diseases such as intracerebral hemorrhage, traumatic brain injury, and cerebral ischemia-reperfusion, melatonin suppresses necroptosis by downregulating RIP1/RIP3/MLKL signaling, upregulating A20 (TNFAIP3), and inhibiting the HMGB1/TLR4/NF-κB pathway. In lung disorders, melatonin attenuates cadmium- and LPS-induced necroptosis by modulating the Ang-II axis, reducing TNF-α-mediated signaling, and acting through MT1/MT2 receptors. In liver fibrosis models, melatonin inhibits RIP1-RIP3 necrosome formation, reduces oxidative stress, and decreases DAMPs-mediated immune responses. Similarly, in testicular toxicity induced by tetrabromobisphenol A (TBBPA), melatonin suppresses necroptosis by inhibiting the several pathways and reducing ROS overproduction. These findings highlight melatonin's ability to modulate necroptosis across diverse tissues, offering protection against inflammation, oxidative stress, and cell death. By targeting necroptosis-related pathways, melatonin presents a versatile therapeutic strategy for treating diseases characterized by excessive cell death and inflammation, including stroke, myocardial infarction, pancreatitis, and autoimmune disorders. This review underscores the potential of melatonin as a novel intervention for necroptosis-related pathologies, emphasizing its role in restoring cellular homeostasis and mitigating tissue damage.

Research progress of deubiquitinating enzymes in cerebral ischemia-reperfusion injury

Cerebral ischemia-reperfusion injury (CIRI) is a critical pathological process driving neurological deterioration following ischemic stroke, involving multifaceted mechanisms such as inflammatory cascades, oxidative stress, and programmed cell death (PCD). Deubiquitinases (DUBs), as key regulators of the ubiquitin-proteasome system, dynamically modulate protein stability, signal transduction, and subcellular localization through editing the ubiquitin code, exhibiting dual roles in CIRI-both as drivers of pathogenesis and potential therapeutic targets. This review systematically elucidates the core regulatory mechanisms of DUBs in CIRI: (i) suppression of neuroinflammation via modulation of NLRP6/NF-κB pathways; (ii) mitigation of oxidative stress through the KEAP1-NRF2 axis and mitochondrial quality control; and (iii) neuroprotection by intercepting necroptosis, ferroptosis, and other PCD pathways. We further reveal that CIRI disrupts DUBs functionality through a tripartite mechanism-transcriptional dysregulation, catalytic inactivation, and subcellular mislocalization-transforming DUBs from guardians of homeostasis into mediators of injury. Consequently, DUBs-targeted strategies, including small-molecule inhibitors (e.g., IU1, Vialinin A), genetic editing approaches (e.g., BRCC3 silencing, A20 overexpression), and exosome-based delivery systems (e.g., the KLF3-AS1/miR-206/USP22 axis), demonstrate significant neuroprotective potential. However, challenges persist, such as substrate specificity, ubiquitin chain-type dependency, and barriers to clinical translation. Future research must integrate multi-omics technologies, develop brain-targeted delivery platforms, and explore synergistic effects of DUBs modulation with existing therapies to advance precision medicine in stroke treatment.

Orientia tsutsugamushi Modulates RIPK3 Cellular Levels but Does Not Inhibit Necroptosis

Scrub typhus is an emerging chigger-borne disease caused by the obligate intracellular bacterium Orientia tsutsugamushi. Necroptosis is a form of programmed cell death (PCD) mediated by RIPK3 (serine/threonine kinase receptor interacting protein 3) and its downstream effector MLKL (mixed-lineage kinase domain-like). While O. tsutsugamushi modulates apoptosis, another form of PCD, its interplay with necroptosis is unknown. Much of Orientia pathobiology is linked to its ankyrin repeat (AR)-containing effectors (Anks). Two of these, Ank1 and Ank6, share similarities with the cowpox AR protein, vIRD (viral inducer of RIPK3 degradation) that prevents necroptosis. Here, we show that Ank1 and Ank6 reduce RIPK3 cellular levels although not as robustly as and mechanistically distinct from vIRD. Orientia infection lowers RIPK3 amounts and does not elicit necroptosis in endothelial cells. In HeLa cells ectopically expressing RIPK3, Orientia fails to inhibit RIPK3 and MLKL phosphorylation as well as cell death. MLKL colocalization with Orientia or Listeria monocytogenes, another intracytoplasmic pathogen, was not observed. Thus, O. tsutsugamushi reduces cellular levels of RIPK3 and does not elicit necroptosis but cannot inhibit this PCD pathway once it is induced. This study is a first step toward understanding how the relationship between Orientia and necroptosis contributes to scrub typhus pathogenesis.

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.

An Anti-CD147 Antibody-Drug Conjugate Mehozumab-DM1 is Efficacious Against Hepatocellular Carcinoma in Cynomolgus Monkey

Effective treatment strategies are urgently needed for hepatocellular carcinoma (HCC) patients due to frequent therapeutic resistance and recurrence. Antibody-drug conjugate (ADC) is a specific antibody-drug conjugated with small molecular compounds, which has potent killing activity against cancer cells. However, few ADC candidates for HCC are undergoing clinical evaluation. CD147 is a tumor-associated antigen that is highly expressed on the surface of tumor cells. Here CD147 is found significantly upregulated in tumor tissues of HCC. Mehozumab-DM1, a humanized anti-CD147 monoclonal antibody conjugated with Mertansine (DM1) is developed. Mehozumab-DM1 is effectively internalized by cancer cells and demonstrated potent antitumor efficacy in HCC cells. In vivo evaluation of Mehozumab-DM1 is conducted in a CRISPR-mediated PTEN and TP53 mutation cynomolgus monkey liver cancer model, which is poorly responsive to sorafenib treatment. Mehozumab-DM1 demonstrated potent tumor inhibitory efficacy at doses of 0.2 and 1.0 mg kg-1 treatment groups in cynomolgus monkey. No treatment-related adverse reactions or body weight loss are observed. Interestingly, Mehozumab-DM1 treatment induced RIPK-dependent tumor cell necroptosis through inhibiting IκB kinase/NF-κB pathway. In conclusion, Mehozumab-DM1 potently inhibits hepatoma through effective internalization to release payload and inducing cell necroptosis to enhance the bystander effect, which is a promising treatment for refractory HCC.

Deubiquitination of RIPK3 by OTUB2 potentiates neuronal necroptosis after ischemic stroke

As a common and severe cerebrovascular disease, ischemic stroke casts a significant shadow over global health. Unfortunately, the mechanisms regulating neuronal death in the affected areas remain largely unclear. Here, we found that deletion of the deubiquitinating enzyme Otubain-2 (OTUB2) significantly alleviated ischemia-induced cerebral infarction and neurological deficits, accompanied by a reduction in neuronal loss, glial activation, and neuroinflammation. OTUB2 was predominantly expressed in neurons and its deletion decreased receptor-interacting protein kinase 3 (RIPK3)-mediated neuronal necroptosis. Moreover, OTUB2 increased RIPK3 protein abundance by inhibiting the proteasomal degradation of RIPK3. Mechanistically, OTUB2 removed K48-linked polyubiquitin chains from RIPK3 through its active site C51. Importantly, pharmacological inhibition of OTUB2 alleviated ischemic brain injury in mice and reduced oxygen-glucose deprivation-induced neuronal death in human brain organoids. These results demonstrate that OTUB2 critically regulates ischemic stroke injury by potentiating neuronal necroptosis, suggesting that OTUB2 inhibition may become a potential therapeutic approach for treating ischemic stroke.

Integrated computational analysis identifies therapeutic targets with dual action in cancer cells and T cells

Many cancer drugs that target cancer cell pathways also impair the immune system. We developed a computational target discovery platform to enable examination of both cancer and immune cells so as to identify pathways that restrain tumor progression and potentiate anti-tumor immunity. Immune-related CRISPR screen analyzer of functional targets (ICRAFT) integrates immune-related CRISPR screen datasets, single-cell RNA sequencing (scRNA-seq) data, and pre-treatment RNA-seq data from clinical trials, enabling a systems-level approach to therapeutic target discovery. Using ICRAFT, we identified numerous targets that enhance both cancer cell susceptibility to immune attack and T cell activation, including tumor necrosis factor (TNF) alpha-induced protein 3 (TNFAIP3), protein tyrosine phosphatase non-receptor type 2 (PTPN2), and suppressor of cytokine signaling 1 (SOCS1). In cancer cells, Tnfaip3 (A20) deletion activated the TNF-nuclear factor kappa-B (NF-κB) pathway, promoting chemokine expression and T cell recruitment to the tumor. T cell-mediated elimination of Tnaifp3-null cancer cells was primarily driven by TNF-induced apoptosis. Inactivation of Tnfaip3 in T cells enhanced anti-tumor efficacy. By integrating diverse functional genomics and clinical datasets, ICRAFT provides an interactive resource toward a deeper understanding of anti-tumor immunity and immuno-oncology drug development.

TFAP2A-regulated CRNDE enhances colon cancer progression and chemoresistance via RIPK3 interaction

Colon cancer (CC) is a common malignancy with rising incidence worldwide. Despite advances in treatment strategies, many patients still face a poor prognosis due to the development of drug resistance. Long non-coding RNAs (lncRNAs) have emerged as important regulators of various biological processes and have been implicated in cancer progression. Among them, colorectal neoplasia differentially expressed (CRNDE) has drawn attention for its potential roles in different cancers. However, its specific functions in CC remain unclear. In this study, we identified CRNDE as highly expressed in CC, contributing to tumor progression and drug resistance. Mechanically, CRNDE is regulated by the transcription factor TFAP2A. Additionally, CRNDE inhibits pyroptosis, a form of programmed cell death, by promoting the ubiquitin-mediated degradation of RIPK3, thereby reducing the sensitivity of CC cells to 5-fluorouracil (5-FU). Our findings suggest that the TFAP2A/CRNDE/RIPK3 axis plays critical roles in colon cancer progression and chemoresistance, highlighting potential therapeutic targets for improving treatment outcomes.

NF-κB regulated expression of A20 controls IKK dependent repression of RIPK1 induced cell death in activated T cells

IKK signalling is essential for survival of thymocytes by repressing RIPK1 induced cell death rather than its canonical function of activating NF-κB. The role of IKK signalling in activated T cells is unclear. To investigate this, we analysed activation of IKK2 deficient T cells. While TCR triggering was normal, proliferation and expansion was profoundly impaired. This was not due to defective cell cycle progression, rather dividing T cells became sensitised to TNF induced cell death, since inhibition of RIPK1 kinase activity rescued cell survival. Gene expression analysis of activated IKK2 deficient T cells revealed defective expression of Tnfaip3, that encodes A20, a negative regulator of NF-κB. To test whether A20 expression was required to protect IKK2 deficient T cells from cell death, we generated mice with T cells lacking both A20 and IKK2. Doing this resulted in near complete loss of peripheral T cells, in contrast to mice lacking one or other gene. Strikingly, this phenotype was completely reversed by inactivation of RIPK1 kinase activity in vivo. Together, our data show that IKK signalling in activated T cells protects against RIPK1 dependent death, both by direct phosphorylation of RIPK1 and through NF-κB mediated induction of A20, that we identify for the first time as a key modulator of RIPK1 activity in T cells.

Protein shapeshifting in necroptotic cell death signaling

Necroptosis is a mode of programmed cell death executed by the mixed lineage kinase domain-like (MLKL) pseudokinase following its activation by the upstream receptor-interacting protein kinase-3 (RIPK3), subsequent to activation of death, Toll-like, and pathogen receptors. The pathway originates in innate immunity, although interest has surged in therapeutically targeting necroptosis owing to its dysregulation in inflammatory diseases. Here, we explore how protein conformation and higher order assembly of the pathway effectors - Z-DNA-binding protein-1 (ZBP1), RIPK1, RIPK3, and MLKL - can be modulated by post-translational modifications, such as phosphorylation, ubiquitylation, and lipidation, and intermolecular interactions to tune activities and modulate necroptotic signaling flux. As molecular level knowledge of cell death signaling grows, we anticipate targeting the conformations of key necrosomal effector proteins will emerge as new avenues for drug development.

Differential expression of the ubiquitin-editing enzyme A20 in gastric biopsies indicates the severity of disease

A20, an ubiquitin-editing enzyme, plays a pivotal role in regulating cell signaling and immune responses. Dysregulated A20 expression has been associated with various pathological conditions, including inflammatory diseases and malignancies, where its expression levels often correlate with differing prognoses in solid tumors. This study aimed to explore the expression and cellular localization of A20 in both nonpathological and diseased human gastric tissues to gain deeper insights into its involvement in gastric pathologies. We analyzed paraffin-embedded gastric tissue samples from 326 patients. A20 expression was assessed using immunohistochemistry (IHC) with results categorized according to the Remmele and Stegner immunoreactive score (IRS). The study compared A20 expression across a spectrum of gastric pathologies, including Helicobacter pylori (HP) gastritis, autoimmune gastritis (A-gastritis), reactive gastropathy (C-gastritis), Ex-HP-gastritis, adenomas, and adenocarcinomas, with nonpathological gastric mucosa serving as a baseline. Our findings demonstrate a significant increase in A20 expression in HP-gastritis (p = 0.019), A-gastritis (p = 0.001), adenomas (p < 0.001), and adenocarcinomas (p < 0.001). Conversely, no significant differences in A20 expression were observed in C-gastritis or Ex-HP-gastritis cases.

A20 negatively regulates necroptosis-induced microglia/macrophages polarization and mediates cerebral ischemic tolerance via inhibiting the ubiquitination of RIP3

Neuronal necroptosis appears to be suppressed by the deubiquitinating enzyme A20 and is capable to regulate the polarization of microglia/macrophages after cerebral ischemia. We have demonstrated that hypoxic preconditioning (HPC) can alleviate receptor interacting protein 3 (RIP3)-induced necroptosis in CA1 after transient global cerebral ischemia (tGCI). However, it is still unclear whether HPC serves to regulate the phenotypic polarization of microglia/macrophages after cerebral ischemia by mitigating neuronal necroptosis. We hence aim to elucidate the underlying mechanism(s) by which the ubiquitination of RIP3-dependent necroptosis regulated by A20 affects microglia/macrophages phenotype after cerebral ischemic tolerance. We found that microglia/macrophages in CA1 of rats underwent M1 and M2 phenotypic polarization in response to tGCI. Notably, the treatment with HPC, as well as inhibitors of necroptosis, including Nec-1 and mixed lineage kinase domain-like (MLKL) siRNA, attenuated neuroinflammation associated with M1 polarization of microglia/macrophages induced by tGCI. Mechanistically, HPC was revealed to upregulate A20 and in turn enhance the interaction between A20 and RIP3, thereby reducing K63-linked polyubiquitination of RIP3 in CA1 after tGCI. Consequently, RIP3-dependent necroptosis and the M1 polarization of microglia/macrophages were blocked either by HPC or via overexpression of A20 in neurons, which ultimately mitigated cerebral injury in CA1 after tGCI. These data support that A20 serves as a crucial mediator of microglia/macrophages polarization by suppressing neuronal necroptosis in a RIP3 ubiquitination-dependent manner after tGCI. Also, a novel mechanism by which HPC functions in cerebral ischemic tolerance is elucidated.

Necroptosis-based glioblastoma prognostic subtypes: implications for TME remodeling and therapy response

BACKGROUND

Glioblastoma (GBM) is an aggressive primary brain tumor with a high recurrence rate and poor prognosis. Necroptosis, a pathological hallmark of GBM, is poorly understood in terms of its role in prognosis, tumor microenvironment (TME) alteration, and immunotherapy.

METHODS & RESULTS

We assessed the expression of 55 necroptosis-related genes in GBM and normal brain tissues. We identified necroptosis-stratified clusters using Uni-Cox and Least Absolute Shrinkage and Selection Operator (LASSO) regression to establish the 10-gene Glioblastoma Necroptosis Index (GNI). GNI demonstrated significant prognostic efficacy in the TCGA dataset (n = 160) and internal validation dataset (n = 345) and in external validation cohorts (n = 591). The GNI-high subgroup displayed a mesenchymal phenotype, lacking the IDH1 mutation, and MGMT methylation. This subgroup was characterized by significant enrichment in inflammatory and humoral immune pathways with prominent cell adhesion molecules (CD44 and ICAM1), inflammatory cytokines (TGFB1, IL1B, and IL10), and chemokines (CX3CL1, CXCL9, and CCL5). The TME in this subgroup showed elevated infiltration of M0 macrophages, neutrophils, mast cells, and regulatory T cells. GNI-related genes appeared to limit macrophage polarization, as confirmed by immunohistochemistry and flow cytometry. The top 30% high-risk score subset exhibited increased CD8 T cell infiltration and enhanced cytolytic activity. GNI showed promise in predicting responses to immunotherapy and targeted treatment.

CONCLUSIONS

Our study highlights the role of necroptosis-related genes in glioblastoma (GBM) and their effects on the tumor microenvironment and patient prognosis. TheGNI demonstrates potential as a prognostic marker and provides insights into immune characteristics and treatment responsiveness.

Unraveling the immunomodulatory impact of hydroxychloroquine on peripheral T cells using single-cell RNA sequencing

Hydroxychloroquine (HCQ) is widely used in the treatment of a variety of autoimmune diseases. However, the mechanisms responsible for the immunomodulatory properties of HCQ in T cells remain unclear. Here we used single-cell RNA-sequencing to examine the effect of HCQ on T cells following in vitro stimulation. HCQ treatment led to a reduction in effector CD4+ T cells and upregulation of inhibitory genes including CTLA4 and TNFAIP3 in effector and naive CD4+ T cells, respectively. HCQ induced a significant expansion of effector CD8+ T cells, and significantly upregulated key cytotoxicity genes including GZMA, GZMB, GZMH, KLRD1, NKG7, and PRF1, as well as IFNG expression. Furthermore, HCQ treatment led to a reduction in the CD38+ CD8+ T cell subset, which is characterized by defective cytotoxicity and thought to both play a pathogenic role and increase susceptibility to infections in autoimmunity. We analyzed single-cell RNA-sequencing data in effector CD8+ T cells from lupus patients with or without HCQ treatment and confirmed upregulation of key cytotoxicity genes in patients receiving HCQ. In conclusion, this work provides additional insights into the immunomodulatory effects of HCQ and indicates that HCQ improves T cell cytotoxicity, which could explain a previously suggested protective effect of HCQ against infections in patients with autoimmune diseases.

Chemotherapy resistance in acute myeloid leukemia is mediated by A20 suppression of spontaneous necroptosis

Acute myeloid leukemia (AML) is a deadly hematopoietic malignancy. Although many patients achieve complete remission with standard induction therapy, a combination of cytarabine and anthracycline, ~40% of patients have induction failure. These refractory patients pose a treatment challenge, as they do not respond to salvage therapy or allogeneic stem cell transplant. Herein, we show that AML patients who experience induction failure have elevated expression of the NF-κB target gene tumor necrosis factor alpha-induced protein-3 (TNFAIP3/A20) and impaired necroptotic cell death. A20High AML are resistant to anthracyclines, while A20Low AML are sensitive. Loss of A20 in AML restores sensitivity to anthracycline treatment by inducing necroptosis. Moreover, A20 prevents necroptosis in AML by targeting the necroptosis effector RIPK1, and anthracycline-induced necroptosis is abrogated in A20High AML. These findings suggest that NF-κB-driven A20 overexpression plays a role in failed chemotherapy induction and highlights the potential of targeting an alternative cell death pathway in AML.

Caspase-8 in inflammatory diseases: a potential therapeutic target

Caspase-8, a renowned cysteine-aspartic protease within its enzyme family, initially garnered attention for its regulatory role in extrinsic apoptosis. With advancing research, a growing body of evidence has substantiated its involvement in other cell death processes, such as pyroptosis and necroptosis, as well as its modulatory effects on inflammasomes and proinflammatory cytokines. PANoptosis, an emerging concept of cell death, encompasses pyroptosis, apoptosis, and necroptosis, providing insight into the often overlapping cellular mortality observed during disease progression. The activation or deficiency of caspase-8 enzymatic activity is closely linked to PANoptosis, positioning caspase-8 as a key regulator of cell survival or death across various physiological and pathological processes. Aberrant expression of caspase-8 is closely associated with the development and progression of a range of inflammatory diseases, including immune system disorders, neurodegenerative diseases (NDDs), sepsis, and cancer. This paper delves into the regulatory role and impact of caspase-8 in these conditions, aiming to elucidate potential therapeutic strategies for the future intervention.

Deubiquitination of RIPK2 by OTUB2 augments NOD2 signalling and protective effects in intestinal inflammation

BACKGROUND

Inflammatory bowel disease (IBD) is a chronic inflammatory disorder of the gastrointestinal tract, but the molecular mechanisms underlying IBD are incompletely understood. In this study, we explored the role and regulating mechanism of otubain 2 (OTUB2), a deubiquitinating enzyme, in IBD.

METHODS

To study the function of OTUB2 in IBD, we generated Otub2-/- mice and treated them with dextran sulfate sodium (DSS) to induce experimental colitis. Bone marrow transplantation was performed to identify the cell populations that were affected by OTUB2 in colitis. The molecular mechanism of OTUB2 in signal transduction was studied by various biochemical methods.

RESULTS

OTUB2 was highly expressed in colon-infiltrating macrophages in both humans with IBD and mice with DSS-induced experimental colitis. Colitis was significantly aggravated in Otub2-/- mice and bone marrow chimeric mice receiving Otub2-/- bone marrow. OTUB2-deficiency impaired the production of cytokines and chemokines in macrophages in response to the NOD2 agonist muramyl dipeptide (MDP). Upon MDP stimulation, OTUB2 promoted NOD2 signaling by stabilizing RIPK2. Mechanistically, OTUB2 inhibited the proteasomal degradation of RIPK2 by removing K48-linked polyubiquitination on RIPK2, which was mediated by the active C51 residue in OTUB2. In mice, OTUB2 ablation abolished the protective effects of MDP administration in colitis.

CONCLUSION

This study identified OTUB2 as a novel regulator of intestinal inflammation.

Necroptosis and autoimmunity

Autoimmunity is a normal physiological state that requires immunological homeostasis and surveillance, whereas necroptosis is a type of inflammatory cell death. When necroptosis occurs, various immune system cells must perform their appropriate duties to preserve immunological homeostasis, whether the consequence is expanding or limiting the inflammatory response and the pathological condition is cleared or progresses to the autoimmune disease stage. This article discusses necroptosis based on RIP homotypic interaction motif (RHIM) interaction under various physiological and pathological situations, with the RIPK1-RIPK3-MLKL necrosome serving as the regulatory core. In addition, the cell biology of necroptosis involved in autoimmunity and its application in autoimmune diseases were also reviewed.

Recognition of necroptosis: From molecular mechanisms to detection methods

Necroptosis is a crucial modality of programmed cell death characterized by distinct morphological and biochemical hallmarks, including cell membrane rupture, organelle swelling, cytoplasmic and nuclear disintegration, cellular contents leakage, and release of damage-associated molecular patterns (DAMPs), accompanied by the inflammatory responses. Studies have shown that necroptosis is involved in the etiology and evolution of a variety of pathologies including organ damage, inflammation disorders, and cancer. Despite its significance, the field of necroptosis research grapples with the challenge of non-standardized detection methodologies. In this review, we introduce the fundamental concepts and molecular mechanisms of necroptosis and critically appraise the principles, merits, and inherent limitations of current detection technologies. This endeavor seeks to establish a methodological framework for necroptosis detection, thereby propelling deeper insights into the research of cell necroptosis.

4-1BB-encoding CAR causes cell death via sequestration of the ubiquitin-modifying enzyme A20

CD28 and 4-1BB costimulatory endodomains included in chimeric antigen receptor (CAR) molecules play a critical role in promoting sustained antitumor activity of CAR-T cells. However, the molecular events associated with the ectopic and constitutive display of either CD28 or 4-1BB in CAR-T cells have been only partially explored. In the current study, we demonstrated that 4-1BB incorporated within the CAR leads to cell cluster formation and cell death in the forms of both apoptosis and necroptosis in the absence of CAR tonic signaling. Mechanistic studies illustrate that 4-1BB sequesters A20 to the cell membrane in a TRAF-dependent manner causing A20 functional deficiency that in turn leads to NF-κB hyperactivity, cell aggregation via ICAM-1 overexpression, and cell death including necroptosis via RIPK1/RIPK3/MLKL pathway. Genetic modulations obtained by either overexpressing A20 or releasing A20 from 4-1BB by deleting the TRAF-binding motifs of 4-1BB rescue cell cluster formation and cell death and enhance the antitumor ability of 4-1BB-costimulated CAR-T cells.

Genetic Mutations Associated With TNFAIP3 (A20) Haploinsufficiency and Their Impact on Inflammatory Diseases

TNF-α-induced protein 3 (TNFAIP3), commonly referred to as A20, is an integral part of the ubiquitin-editing complex that significantly influences immune regulation, apoptosis, and the initiation of diverse immune responses. The A20 protein is characterized by an N-terminal ovarian tumor (OTU) domain and a series of seven zinc finger (ZNF) domains. Mutations in the TNFAIP3 gene are implicated in various immune-related diseases, such as Behçet's disease, polyarticular juvenile idiopathic arthritis, autoimmune thyroiditis, autoimmune hepatitis, and rheumatoid arthritis. These mutations can lead to a spectrum of symptoms, including, but not limited to, recurrent fever, ulcers, rashes, musculoskeletal and gastrointestinal dysfunctions, cardiovascular issues, and respiratory infections. The majority of these mutations are either nonsense (STOP codon) or frameshift mutations, which are typically associated with immune dysfunctions. Nonetheless, missense mutations have also been identified as contributors to these conditions. These genetic alterations may interfere with several biological pathways, notably abnormal NF-κB signaling and dysregulated ubiquitination. Currently, there is no definitive treatment for A20 haploinsufficiency; however, therapeutic strategies can alleviate the symptoms in patients. This review delves into the mutations reported in the TNFAIP3 gene, the clinical progression in affected individuals, potential disease mechanisms, and a brief overview of the available pharmacological interventions for A20 haploinsufficiency. Mandatory genetic testing of the TNFAIP3 gene should be performed in patients diagnosed with autoinflammatory disorders to better understand the genetic underpinnings and guide treatment decisions.

A20 in hepatic stellate cells suppresses chronic hepatitis by inhibiting DCLK1-JNK pathway-dependent chemokines

Hepatic stellate cells (HSCs) are responsible for liver fibrosis accompanied by its activation into myofibroblasts and the abundant production of extracellular matrix. However, the HSC contribution to progression of liver inflammation has been less known. We aimed to elucidate the mechanism in HSCs underlying the inflammatory response and the function of tumor necrosis factor α-related protein A20 (TNFAIP3). We established A20 conditional knockout (KO) mice crossing Twist2-Cre and A20 floxed mice. Using these mice, the effect of A20 was analyzed in mouse liver and HSCs. The human HSC line LX-2 was also used to examine the role and underlying molecular mechanism of A20. In this KO model, A20 was deficient in >80% of HSCs. Spontaneous inflammation with mild fibrosis was found in the liver of the mouse model without any exogenous agents, suggesting that A20 in HSCs suppresses chronic hepatitis. Comprehensive RNA sequence analysis revealed that A20-deficient HSCs exhibited an inflammatory phenotype and abnormally expressed chemokines. A20 suppressed JNK pathway activation in HSCs. Loss of A20 function in LX-2 cells also induced excessive chemokine expression, mimicking A20-deficient HSCs. A20 overexpression suppressed chemokine expression in LX-2. In addition, we identified DCLK1 in the genes regulated by A20. DCLK1 activated the JNK pathway and upregulates chemokine expression. DCLK1 inhibition significantly decreased chemokine induction by A20-silencing, suggesting that A20 controlled chemokine expression in HSCs via the DCLK1-JNK pathway. In conclusion, A20 suppresses chemokine induction dependent on the DCLK1-JNK signaling pathway. These findings demonstrate the therapeutic potential of A20 and the DCLK1-JNK pathway for the regulation of inflammation in chronic hepatitis.

The Role of Necroptosis in Cerebral Ischemic Stroke

Cerebral ischemia, also known as ischemic stroke, accounts for nearly 85% of all strokes and is the leading cause of disability worldwide. Due to disrupted blood supply to the brain, cerebral ischemic injury is trigged by a series of complex pathophysiological events including excitotoxicity, oxidative stress, inflammation, and cell death. Currently, there are few treatments for cerebral ischemia owing to an incomplete understanding of the molecular and cellular mechanisms. Accumulated evidence indicates that various types of programmed cell death contribute to cerebral ischemic injury, including apoptosis, ferroptosis, pyroptosis and necroptosis. Among these, necroptosis is morphologically similar to necrosis and is mediated by receptor-interacting serine/threonine protein kinase-1 and -3 and mixed lineage kinase domain-like protein. Necroptosis inhibitors have been shown to exert inhibitory effects on cerebral ischemic injury and neuroinflammation. In this review, we will discuss the current research progress regarding necroptosis in cerebral ischemia as well as the application of necroptosis inhibitors for potential therapeutic intervention in ischemic stroke.

Unraveling the intriguing interplay: Exploring the role of lncRNAs in caspase-independent cell death

Cell death plays a crucial role in various physiological and pathological processes. Until recently, programmed cell death was mainly attributed to caspase-dependent apoptosis. However, emerging evidence suggests that caspase-independent cell death (CICD) mechanisms also contribute significantly to cellular demise. We and others have reported and functionally characterized numerous long noncoding RNAs (lncRNAs) that modulate caspase-dependent apoptotic pathways potentially in a pathway-dependent manner. However, the interplay between lncRNAs and CICD pathways has not been comprehensively documented. One major reason for this is that most CICD pathways have been recently discovered with some being partially characterized at the molecular level. In this review, we discuss the emerging evidence that implicates specific lncRNAs in the regulation and execution of CICD. We summarize the diverse mechanisms through which lncRNAs modulate different forms of CICD, including ferroptosis, necroptosis, cuproptosis, and others. Furthermore, we highlight the intricate regulatory networks involving lncRNAs, protein-coding genes, and signaling pathways that orchestrate CICD in health and disease. Understanding the molecular mechanisms and functional implications of lncRNAs in CICD may unravel novel therapeutic targets and diagnostic tools for various diseases, paving the way for innovative strategies in disease management and personalized medicine. This article is categorized under: RNA in Disease and Development > RNA in Disease.

RIPK3 cleavage is dispensable for necroptosis inhibition but restricts NLRP3 inflammasome activation

Caspase-8 activity is required to inhibit necroptosis during embryogenesis in mice. In vitro studies have suggested that caspase-8 directly cleaves RIPK1, CYLD and the key necroptotic effector kinase RIPK3 to repress necroptosis. However, recent studies have shown that mice expressing uncleavable RIPK1 die during embryogenesis due to excessive apoptosis, while uncleavable CYLD mice are viable. Therefore, these results raise important questions about the role of RIPK3 cleavage. To evaluate the physiological significance of RIPK3 cleavage, we generated Ripk3D333A/D333A mice harbouring a point mutation in the conserved caspase-8 cleavage site. These mice are viable, demonstrating that RIPK3 cleavage is not essential for blocking necroptosis during development. Furthermore, unlike RIPK1 cleavage-resistant cells, Ripk3D333A/D333A cells were not significantly more sensitive to necroptotic stimuli. Instead, we found that the cleavage of RIPK3 by caspase-8 restricts NLRP3 inflammasome activation-dependent pyroptosis and IL-1β secretion when Inhibitors of APoptosis (IAP) are limited. These results demonstrate that caspase-8 does not inhibit necroptosis by directly cleaving RIPK3 and further underscore a role for RIPK3 in regulating the NLRP3 inflammasome.

Mechanism study of ubiquitination in T cell development and autoimmune disease

T cells play critical role in multiple immune processes including antigen response, tumor immunity, inflammation, self-tolerance maintenance and autoimmune diseases et. Fetal liver or bone marrow-derived thymus-seeding progenitors (TSPs) settle in thymus and undergo T cell-lineage commitment, proliferation, T cell receptor (TCR) rearrangement, and thymic selections driven by microenvironment composed of thymic epithelial cells (TEC), dendritic cells (DC), macrophage and B cells, thus generating T cells with diverse TCR repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg with the help of TEC and DC, serving for immune tolerance. The sequential proliferation, cell fate decision, and selection during T cell development and self-tolerance establishment are tightly regulated to ensure the proper immune response without autoimmune reaction. There are remarkable progresses in understanding of the regulatory mechanisms regarding ubiquitination in T cell development and the establishment of self-tolerance in the past few years, which holds great potential for further therapeutic interventions in immune-related diseases.

Cellular heterogeneity in TNF/TNFR1 signalling: live cell imaging of cell fate decisions in single cells

Cellular responses to TNF are inherently heterogeneous within an isogenic cell population and across different cell types. TNF promotes cell survival by activating pro-inflammatory NF-κB and MAPK signalling pathways but may also trigger apoptosis and necroptosis. Following TNF stimulation, the fate of individual cells is governed by the balance of pro-survival and pro-apoptotic signalling pathways. To elucidate the molecular mechanisms driving heterogenous responses to TNF, quantifying TNF/TNFR1 signalling at the single-cell level is crucial. Fluorescence live-cell imaging techniques offer real-time, dynamic insights into molecular processes in single cells, allowing for detection of rapid and transient changes, as well as identification of subpopulations, that are likely to be missed with traditional endpoint assays. Whilst fluorescence live-cell imaging has been employed extensively to investigate TNF-induced inflammation and TNF-induced cell death, it has been underutilised in studying the role of TNF/TNFR1 signalling pathway crosstalk in guiding cell-fate decisions in single cells. Here, we outline the various opportunities for pathway crosstalk during TNF/TNFR1 signalling and how these interactions may govern heterogenous responses to TNF. We also advocate for the use of live-cell imaging techniques to elucidate the molecular processes driving cell-to-cell variability in single cells. Understanding and overcoming cellular heterogeneity in response to TNF and modulators of the TNF/TNFR1 signalling pathway could lead to the development of targeted therapies for various diseases associated with aberrant TNF/TNFR1 signalling, such as rheumatoid arthritis, metabolic syndrome, and cancer.

The Complexity of Being A20: From Biological Functions to Genetic Associations

A20, encoded by TNFAIP3, is a critical negative regulator of immune activation. A20 is a ubiquitin editing enzyme with multiple domains, each of which mediates or stabilizes a key ubiquitin modification. A20 targets diverse proteins that are involved in pleiotropic immunologic pathways. The complexity of A20-mediated immunomodulation is illustrated by the varied effects of A20 deletion in different cell types and disease models. Clinically, the importance of A20 is highlighted by its extensive associations with human disease. A20 germline variants are associated with a wide range of inflammatory diseases, while somatic mutations promote development of B cell lymphomas. More recently, the discovery of A20 haploinsufficiency (HA20) has provided real world evidence for the role of A20 in immune cell function. Originally described as an autosomal dominant form of Behcet's disease, HA20 is now considered a complex inborn error of immunity with a broad spectrum of immunologic and clinical phenotypes.

Novel diagnostic biomarkers related to necroptosis and immune infiltration landscape in acute myocardial infarction

Background

Acute myocardial infarction (AMI) can occur suddenly, which may induce deadly outcomes, and the population suffering from AMI presents a younger trend. Necroptosis, the new cell necrosis type, is associated with the pathogenic mechanisms of diverse cardiovascular diseases (CVDs). Its diagnostic value and molecular mechanisms in AMI are still unclear. Objective: This study focused on determining key necroptosis-related genes as well as immune infiltration in AMI.

Methods

We first examined the GSE66360 dataset for identifying necroptosis-related differentially expressed genes (NRDEGs). Thereafter, GO and functional annotation were performed, then a PPI network was built. In addition, "CIBERSORT" in R was applied in comparing different immune infiltration degrees in AMI compared with control groups. The receiver operating characteristic (ROC) curve was plotted to evaluate whether hub NRDEGs could be used in AMI diagnosis. Associations of immune cells with candidate NRDEGs biomarkers were examined by Spearman analysis. Finally, hub NRDEGs were validated by cell qPCR assays and another two datasets.

Results

A total of 15 NRDEGs were identified and multiple enrichment terms associated with necroptosis were discovered through GO and KEGG analysis. Upon module analysis, 10 hub NRDEGs were filtered out, and the top six hub NRDEGs were identified after ROC analysis. These top six NRDEGs might have a certain effect on modulating immune infiltrating cells, especially for mast cells activated, NK cells activated and neutrophils. Finally, two AMI datasets and qPCR assay came to identical findings.

Conclusion

Our results offer the reliable molecular biomarkers and new perspectives for necroptosis in AMI, which lay a certain foundation for developing novel anti-AMI therapeutic targets.

Interferon autoantibodies as signals of a sick thymus

Type I interferons (IFN-I) are key immune messenger molecules that play an important role in viral defense. They act as a bridge between microbe sensing, immune function magnitude, and adaptive immunity to fight infections, and they must therefore be tightly regulated. It has become increasingly evident that thymic irregularities and mutations in immune genes affecting thymic tolerance can lead to the production of IFN-I autoantibodies (autoAbs). Whether these biomarkers affect the immune system or tissue integrity of the host is still controversial, but new data show that IFN-I autoAbs may increase susceptibility to severe disease caused by certain viruses, including SARS-CoV-2, herpes zoster, and varicella pneumonia. In this article, we will elaborate on disorders that have been identified with IFN-I autoAbs, discuss models of how tolerance to IFN-Is is lost, and explain the consequences for the host.

IE1 of Human Cytomegalovirus Inhibits Necroptotic Cell Death via Direct and Indirect Modulation of the Necrosome Complex

Programmed necrosis is an integral part of intrinsic immunity, serving to combat invading pathogens and restricting viral dissemination. The orchestration of necroptosis relies on a precise interplay within the necrosome complex, which consists of RIPK1, RIPK3 and MLKL. Human cytomegalovirus (HCMV) has been found to counteract the execution of necroptosis during infection. In this study, we identify the immediate-early 1 (IE1) protein as a key antagonist of necroptosis during HCMV infection. Infection data obtained in a necroptosis-sensitive cell culture system revealed a robust regulation of post-translational modifications (PTMs) of the necrosome complex as well as the importance of IE1 expression for an effective counteraction of necroptosis. Interaction analyses unveiled an association of IE1 and RIPK3, which occurs in an RHIM-domain independent manner. We propose that this interaction manipulates the PTMs of RIPK3 by promoting its ubiquitination. Furthermore, IE1 was found to exert an indirect activity by modulating the levels of MLKL via antagonizing its interferon-mediated upregulation. Overall, we claim that IE1 performs a broad modulation of innate immune signaling to impede the execution of necroptotic cell death, thereby generating a favorable environment for efficient viral replication.

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.

N-arachidonoylphenolamine alleviates ischaemia/reperfusion-induced cardiomyocyte necroptosis by restoring proteasomal activity

Necroptosis and apoptosis contribute to the pathogenesis of myocardial ischaemia/reperfusion (I/R) injury and subsequent heart failure. N-arachidonoylphenolamine (AM404) is a paracetamol lipid metabolite that has pleiotropic activity to modulate the endocannabinoid system. However, the protective role of AM404 in modulating I/R-mediated myocardial damage and the underlying mechanism remain largely unknown. A murine I/R model was generated by occlusion of the left anterior descending artery. AM404 (20 mg/kg) was injected intraperitoneally into mice at 2 and 24 h before the I/R operation. Our data revealed that AM404 administration to mice greatly ameliorated I/R-triggered impairment of myocardial performance and reduced infarct area, myocyte apoptosis, oxidative stress and inflammatory response accompanied by the reduction of receptor interacting protein kinase (RIPK)1/3- mixed lineage kinase domain-like (MLKL)-mediated necroptosis and upregulation of the immunosubunits (β2i and β5i). In contrast, administration of epoxomicin (a proteasome inhibitor) dramatically abolished AM404-dependent protection against myocardial I/R damage. Mechanistically, AM404 treatment increases β5i expression, which interacts with Pellino-1 (Peli1), an E3 ligase, to form a complex with RIPK1/3, thereby promoting their degradation, which leads to inhibition of cardiomyocyte necroptosis in the I/R heart. In conclusion, these findings demonstrate that AM404 could prevent cardiac I/R damage and may be a promising drug for the treatment of ischaemic heart disease.

The Function of Ubiquitination in T-Cell Development

Thymus is an important primary lymphoid organ for T cell development. After T-lineage commitment, the early thymic progenitors (ETPs) develop into CD4-CD8- (DN), CD4+CD8+ (DP) and further CD4+ SP or CD8+ SP T cells. Under the help of thymic epithelial cells (TEC), dendritic cell (DC), macrophage, and B cells, ETPs undergo proliferation, T cell receptor (TCR) rearrangement, β-selection, positive selection, and negative selection, and thus leading to the generation of T cells that are diverse repertoire immunocompetent but not self-reactive. Additionally, some self-reactive thymocytes give rise to Treg under the help of TEC and DC. The regulation of T cell development is complicated. As a post-translational modification, ubiquitination regulates signal transduction in diverse biological processes. Ubiquitination functions in T cell development through regulating key signal pathway or maturation and function of related cells. In this review, the regulation of T cell development by ubiquitination is summarized and discussed.

Candidate Genetic and Molecular Drivers of Dysregulated Adaptive Immune Responses After Traumatic Brain Injury

Abstract Neuroinflammation is a significant and modifiable cause of secondary injury after traumatic brain injury (TBI), driven by both central and peripheral immune responses. A substantial proportion of outcome after TBI is genetically mediated, with an estimated heritability effect of around 26%, but because of the comparatively small datasets currently available, the individual drivers of this genetic effect have not been well delineated. A hypothesis-driven approach to analyzing genome-wide association study (GWAS) datasets reduces the burden of multiplicity testing and allows variants with a high prior biological probability of effect to be identified where sample size is insufficient to withstand data-driven approaches. Adaptive immune responses show substantial genetically mediated heterogeneity and are well established as a genetic source of risk for numerous disease states; importantly, HLA class II has been specifically identified as a locus of interest in the largest TBI GWAS study to date, highlighting the importance of genetic variance in adaptive immune responses after TBI. In this review article we identify and discuss adaptive immune system genes that are known to confer strong risk effects for human disease, with the dual intentions of drawing attention to this area of immunobiology, which, despite its importance to the field, remains under-investigated in TBI and presenting high-yield testable hypotheses for application to TBI GWAS datasets.

Receptor-interacting protein 1 and 3 kinase activity are required for high-fat diet induced liver injury in mice

Background

The RIP1-RIP3-MLKL-mediated cell death pathway is associated with progression of non-alcohol-associated fatty liver/steatohepatitis (NAFL/NASH). Previous work identified a critical role for MLKL, the key effector regulating necroptosis, but not RIP3, in mediating high fat diet-induced liver injury in mice. RIP1 and RIP3 have active N-terminus kinase domains essential for activation of MLKL and subsequent necroptosis. However, little is known regarding domain-specific roles of RIP1/RIP3 kinase in liver diseases. Here, we hypothesized that RIP1/RIP3 kinase activity are required for the development of high fat diet-induced liver injury.

Methods

Rip1K45A/K45A and Rip3K51A/K51A kinase-dead mice on a C57BL/6J background and their littermate controls (WT) were allowed free access to a diet high in fat, fructose and cholesterol (FFC diet) or chow diet.

Results

Both Rip1K45A/K45A and Rip3K51A/K51A mice were protected against FFC diet-induced steatosis, hepatocyte injury and expression of hepatic inflammatory cytokines and chemokines. FFC diet increased phosphorylation and oligomerization of MLKL and hepatocyte death in livers of WT, but not in Rip3K51A/K51A, mice. Consistent with in vivo data, RIP3 kinase deficiency in primary hepatocytes prevented palmitic acid-induced translocation of MLKL to the cell surface and cytotoxicity. Additionally, loss of Rip1 or Rip3 kinase suppressed FFC diet-mediated formation of crown-like structures (indicators of dead adipocytes) and expression of mRNA for inflammatory response genes in epididymal adipose tissue. Moreover, FFC diet increased expression of multiple adipokines, including leptin and plasminogen activator inhibitor 1, in WT mice, which was abrogated by Rip3 kinase deficiency.

Discussion

The current data indicate that both RIP1 and RIP3 kinase activity contribute to FFC diet-induced liver injury. This effect of RIP1 and RIP3 kinase deficiency on injury is consistent with the protection of Mlkl-/- mice from high fat diet-induced liver injury, but not the reported lack of protection in Rip3-/- mice. Taken together with previous reports, our data suggest that other domains of RIP3 likely counteract the effect of RIP3 kinase in response to high fat diets.

The mechanism of linear ubiquitination in regulating cell death and correlative diseases

Linear ubiquitination is a specific post-translational modification in which ubiquitin is linked through M1 residue to form multiple types of polyubiquitin chains on substrates in order to regulate cellular processes. LUBAC comprised by HOIP, HOIL-1L, and SHARPIN as a sole E3 ligase catalyzes the generation of linear ubiquitin chains, and it is simultaneously adjusted by deubiquitinases such as OTULIN and CYLD. Several studies have shown that gene mutation of linear ubiquitination in mice accompanied by different modalities of cell death would develop relative diseases. Cell death is a fundamental physiological process and responsible for embryonic development, organ maintenance, and immunity response. Therefore, it is worth speculating that linear ubiquitin mediated signaling pathway would participate in different diseases. The relative literature search was done from core collection of electronic databases such as Web of Science, PubMed, and Google Scholar using keywords about main regulators of linear ubiquitination pathway. Here, we summarize the regulatory mechanism of linear ubiquitination on cellular signaling pathway in cells with apoptosis, necroptosis, autophagy, pyroptosis, and ferroptosis. Intervening generation of linear ubiquitin chains in relative signaling pathway to regulate cell death might provide novel therapeutic insights for various human diseases.

Ubiquitination and cell-autonomous immunity

Cell-autonomous immunity is the first line of defense by which cells recognize and contribute to eliminating invasive pathogens. It is composed of immune signaling networks that sense microbial pathogens, promote pathogen restriction, and stimulate their elimination, including host cell death. Ubiquitination is a pivotal orchestrator of these pathways, by changing the activity of signal transducers and effector proteins in an efficient way. In this review, we will focus on how ubiquitin connects the pathways that sense pathogens to the cellular responses to invaders and shed light on how ubiquitination impacts the microenvironment around the infected cell, stimulating the appropriate immune response. Finally, we discuss therapeutic options directed at favoring cell-autonomous immune responses to infection.

Functions of the RIP kinase family members in the skin

The receptor interacting protein kinases (RIPK) are a family of serine/threonine kinases that are involved in the integration of various stress signals. In response to several extracellular and/or intracellular stimuli, RIP kinases engage signaling cascades leading to the activation of NF-κB and mitogen-activated protein kinases, cell death, inflammation, differentiation and Wnt signaling and can have kinase-dependent and kinase-independent functions. Although it was previously suggested that seven RIPKs are part of the RIPK family, phylogenetic analysis indicates that there are only five genuine RIPKs. RIPK1 and RIPK3 are mainly involved in controlling and executing necroptosis in keratinocytes, while RIPK4 controls proliferation and differentiation of keratinocytes and thereby can act as a tumor suppressor in skin. Therefore, in this review we summarize and discuss the functions of RIPKs in skin homeostasis as well as the signaling pathways involved.

Breaking bad: necroptosis in the pathogenesis of gastrointestinal diseases

A delicate balance between programmed cell death and proliferation of intestinal epithelial cells (IEC) exists in the gut to maintain homeostasis. Homeostatic cell death programs such as anoikis and apoptosis ensure the replacement of dead epithelia without overt immune activation. In infectious and chronic inflammatory diseases of the gut, this balance is invariably disturbed by increased levels of pathologic cell death. Pathological forms of cell death such as necroptosis trigger immune activation barrier dysfunction, and perpetuation of inflammation. A leaky and inflamed gut can thus become a cause of persistent low-grade inflammation and cell death in other organs of the gastrointestinal (GI) tract, such as the liver and the pancreas. In this review, we focus on the advances in the molecular and cellular understanding of programmed necrosis (necroptosis) in tissues of the GI tract. In this review, we will first introduce the reader to the basic molecular aspects of the necroptosis machinery and discuss the pathways leading to necroptosis in the GI system. We then highlight the clinical significance of the preclinical findings and finally evaluate the different therapeutic approaches that attempt to target necroptosis against various GI diseases. Finally, we review the recent advances in understanding the biological functions of the molecules involved in necroptosis and the potential side effects that may occur due to their systemic inhibition. This review is intended to introduce the reader to the core concepts of pathological necroptotic cell death, the signaling pathways involved, its immuno-pathological implications, and its relevance to GI diseases. Further advances in our ability to control the extent of pathological necroptosis will provide better therapeutic opportunities against currently intractable GI and other diseases.

RIPK3 promoter hypermethylation in hepatocytes protects from bile acid-induced inflammation and necroptosis

Necroptosis facilitates cell death in a controlled manner and is employed by many cell types following injury. It plays a significant role in various liver diseases, albeit the cell-type-specific regulation of necroptosis in the liver and especially hepatocytes, has not yet been conceptualized. We demonstrate that DNA methylation suppresses RIPK3 expression in human hepatocytes and HepG2 cells. In diseases leading to cholestasis, the RIPK3 expression is induced in mice and humans in a cell-type-specific manner. Overexpression of RIPK3 in HepG2 cells leads to RIPK3 activation by phosphorylation and cell death, further modulated by different bile acids. Additionally, bile acids and RIPK3 activation further facilitate JNK phosphorylation, IL-8 expression, and its release. This suggests that hepatocytes suppress RIPK3 expression to protect themselves from necroptosis and cytokine release induced by bile acid and RIPK3. In chronic liver diseases associated with cholestasis, induction of RIPK3 expression may be an early event signaling danger and repair through releasing IL-8.

PRMT1 reverts the immune escape of necroptotic colon cancer through RIP3 methylation

Necroptosis plays a double-edged sword role in necroptotic cancer cell death and tumor immune escape. How cancer orchestrates necroptosis with immune escape and tumor progression remains largely unclear. We found that RIP3, the central activator of necroptosis, was methylated by PRMT1 methyltransferase at the amino acid of RIP3 R486 in human and the conserved amino acid R479 in mouse. The methylation of RIP3 by PRMT1 inhibited the interaction of RIP3 with RIP1 to suppress RIP1-RIP3 necrosome complex, thereby blocking RIP3 phosphorylation and necroptosis activation. Moreover, the methylation-deficiency RIP3 mutant promoted necroptosis, immune escape and colon cancer progression due to increasing tumor infiltrated myeloid-derived immune suppressor cells (MDSC), while PRMT1 reverted the immune escape of RIP3 necroptotic colon cancer. Importantly, we generated a RIP3 R486 di-methylation specific antibody (RIP3ADMA). Clinical patient samples analysis revealed that the protein levels of PRMT1 and RIP3ADMA were positively correlated in cancer tissues and both of them predicted the longer patient survival. Our study provides insights into the molecular mechanism of PRMT1-mediated RIP3 methylation in the regulation of necroptosis and colon cancer immunity, as well as reveals PRMT1 and RIP3ADMA as the valuable prognosis markers of colon cancer.

The Interplay Between Autophagy and Regulated Necrosis

Significance: Autophagy is critical to cellular homeostasis. Emergence of the concept of regulated necrosis, such as necroptosis, ferroptosis, pyroptosis, and mitochondrial membrane-permeability transition (MPT)-derived necrosis, has revolutionized the research into necrosis. Both altered autophagy and regulated necrosis contribute to major human diseases. Recent studies reveal an intricate interplay between autophagy and regulated necrosis. Understanding the interplay at the molecular level will provide new insights into the pathophysiology of related diseases. Recent Advances: Among the three forms of autophagy, macroautophagy is better studied for its crosstalk with regulated necrosis. Macroautophagy seemingly can either antagonize or promote regulated necrosis, depending upon the form of regulated necrosis, the type of cells or stimuli, and other cellular contexts. This review will critically analyze recent advances in the molecular mechanisms governing the intricate dialogues between macroautophagy and main forms of regulated necrosis. Critical Issues: The dual roles of autophagy, either pro-survival or pro-death characteristics, intricate the mechanistic relationship between autophagy and regulated necrosis at molecular level in various pathological conditions. Meanwhile, key components of regulated necrosis are also involved in the regulation of autophagy, which further complicates the interrelationship. Future Directions: Resolving the controversies over causation between altered autophagy and a specific form of regulated necrosis requires approaches that are more definitive, where rigorous evaluation of autophagic flux and the development of more reliable and specific methods to quantify each form of necrosis will be essential. The relationship between chaperone-mediated autophagy or microautophagy and regulated necrosis remains largely unstudied. Antioxid. Redox Signal. 38, 550-580.

A20 interacts with mTORC2 to inhibit the mTORC2/Akt/Rac1 signaling axis in hepatocellular carcinoma cells

A20 acts as a tumor suppressor in hepatocellular carcinoma, especially inhibiting metastasis of the malignant cells. However, the mechanisms whereby A20 plays the inhibitory roles are not understood completely. Rac1 signaling is essential for cell migration in hepatocellular carcinoma metastasis. Nevertheless, it is not known whether and how A20 inhibits Rac1 signaling to suppress the migration of hepatocellular carcinoma cell. Thereby, we analyzed the relationship between A20 and Rac1 activation, as well as the activity of Akt and mTORC2, two signaling components upstream of Rac1, using gain and loss of function experiments. We found that the overexpression of A20 repressed, while the knockdown or knockout of A20 promoted, the activation of Rac1, Akt and mTORC2 in hepatocellular carcinoma cells. Moreover, the inhibitory effect of A20 on the mTORC2/Akt/Rac1 signaling axis was due to the interaction between A20 and mTORC2 complex. The binding of A20 to mTORC2 was mediated by the ZnF7 domain of A20 and M1 ubiquitin chain in the mTORC2 complex. Furthermore, A20 inhibited metastasis of hepatocellular carcinoma cells via restraining mTORC2 in a hepatocellular carcinoma xenograft mouse model. These findings revealed the relationship between A20 and mTORC2, and explained the molecular mechanisms of A20 in inhibition of hepatocellular carcinoma metastasis.

Dimethyl fumarate inhibits necroptosis and alleviates systemic inflammatory response syndrome by blocking the RIPK1-RIPK3-MLKL axis

Necroptosis has been implicated in various inflammatory diseases including tumor-necrosis factor-α (TNF-α)-induced systemic inflammatory response syndrome (SIRS). Dimethyl fumarate (DMF), a first-line drug for treating relapsing-remitting multiple sclerosis (RRMS), has been shown to be effective against various inflammatory diseases. However, it is still unclear whether DMF can inhibit necroptosis and confer protection against SIRS. In this study, we found that DMF significantly inhibited necroptotic cell death in macrophages induced by different necroptotic stimulations. Both the autophosphorylation of receptor-interacting serine/threonine kinase 1 (RIPK1) and RIPK3 and the downstream phosphorylation and oligomerization of MLKL were robustly suppressed by DMF. Accompanying the suppression of necroptotic signaling, DMF blocked the mitochondrial reverse electron transport (RET) induced by necroptotic stimulation, which was associated with its electrophilic property. Several well-known anti-RET reagents also markedly inhibited the activation of the RIPK1-RIPK3-MLKL axis accompanied by decreased necrotic cell death, indicating a critical role of RET in necroptotic signaling. DMF and other anti-RET reagents suppressed the ubiquitination of RIPK1 and RIPK3, and they attenuated the formation of necrosome. Moreover, oral administration of DMF significantly alleviated the severity of TNF-α-induced SIRS in mice. Consistent with this, DMF mitigated TNF-α-induced cecal, uterine, and lung damage accompanied by diminished RIPK3-MLKL signaling. Collectively, DMF represents a new necroptosis inhibitor that suppresses the RIPK1-RIPK3-MLKL axis through blocking mitochondrial RET. Our study highlights DMF's potential therapeutic applications for treating SIRS-associated diseases.

The double-edged functions of necroptosis

Necroptosis refers to a regulated form of cell death induced by a variety of stimuli. Although it has been implicated in the pathogenesis of many diseases, there is evidence to support that necroptosis is not purely a detrimental process. We propose that necroptosis is a "double-edged sword" in terms of physiology and pathology. On the one hand, necroptosis can trigger an uncontrolled inflammatory cascade response, resulting in severe tissue injury, disease chronicity, and even tumor progression. On the other hand, necroptosis functions as a host defense mechanism, exerting antipathogenic and antitumor effects through its powerful pro-inflammatory properties. Moreover, necroptosis plays an important role during both development and regeneration. Misestimation of the multifaceted features of necroptosis may influence the development of therapeutic approaches targeting necroptosis. In this review, we summarize current knowledge of the pathways involved in necroptosis as well as five important steps that determine its occurrence. The dual role of necroptosis in a variety of physiological and pathological conditions is also highlighted. Future studies and the development of therapeutic strategies targeting necroptosis should fully consider the complicated properties of this type of regulated cell death.

Innate and adaptive immune abnormalities underlying autoimmune diseases: the genetic connections

With the exception of an extremely small number of cases caused by single gene mutations, most autoimmune diseases result from the complex interplay between environmental and genetic factors. In a nutshell, etiology of the common autoimmune disorders is unknown in spite of progress elucidating certain effector cells and molecules responsible for pathologies associated with inflammatory and tissue damage. In recent years, population genetics approaches have greatly enriched our knowledge regarding genetic susceptibility of autoimmunity, providing us with a window of opportunities to comprehensively re-examine autoimmunity-associated genes and possible pathways. In this review, we aim to discuss etiology and pathogenesis of common autoimmune disorders from the perspective of human genetics. An overview of the genetic basis of autoimmunity is followed by 3 chapters detailing susceptibility genes involved in innate immunity, adaptive immunity and inflammatory cell death processes respectively. With such attempts, we hope to expand the scope of thinking and bring attention to lesser appreciated molecules and pathways as important contributors of autoimmunity beyond the 'usual suspects' of a limited subset of validated therapeutic targets.

MLKL post-translational modifications: road signs to infection, inflammation and unknown destinations

Necroptosis is a caspase-independent modality of cell death that requires the activation of the executioner MLKL. In the last ten years the field gained a substantial amount of evidence regarding its involvement in host response to pathogens, TNF-induced inflammatory diseases as well as pathogen recognition receptors (PRR)-induced inflammation. However, there are still a lot of questions that remain unanswered. While it is clear that there are specific events needed to drive MLKL activation, substantial differences between human and mouse MLKL not only highlight different evolutionary pressure, but also provide potential insights on alternative modalities of activation. While in TNF-induced necroptosis it is clear the involvement of the RIPK3 mediated phosphorylation, it still remains to be understood how certain inflammatory in vivo phenotypes are not equally rescued by either RIPK3 or MLKL loss. Moreover, the plethora of different reported phosphorylation events on MLKL, even in cells that do not express RIPK3, suggest indeed that there is more to MLKL than RIPK3-mediated activation, not only in the execution of necroptosis but perhaps in other inflammatory conditions that include IFN response. The recent discovery of MLKL ubiquitination has highlighted a new checkpoint in the regulation of MLKL activation and the somewhat conflicting evidence reported certainly require some untangling. In this review we will highlight the recent findings on MLKL activation and involvement to pathogen response with a specific focus on MLKL post-translational modifications, in particular ubiquitination. This review will highlight the outstanding main questions that have risen from the last ten years of research, trying at the same time to propose potential avenues of research.