OTULIN protects the liver against cell death, inflammation, fibrosis, and cancer

OTULIN-related conditions: Report of a new case and review of the literature using GenIA

OTULIN encodes an eponymous linear deubiquitinase (DUB) essential for controlling inflammation as a negative regulator of the canonical NF-κB signaling pathway via the regulation of M1-Ub dynamics. Biallelic loss-of-function (LOF) mutations in OTULIN cause an autosomal recessive condition named Otulin-Related Autoinflammatory Syndrome (ORAS), also known as Otulipenia or AutoInflammation, Panniculitis, and Dermatosis Syndrome (AIPDS). Monoallelic OTULIN LOF, also known as OTULIN Haploinsufficiency (OHI) or Immunodeficiency 107 (IMD107), has been linked to an incompletely penetrant, dominantly inherited susceptibility to invasive Staphylococcal infections. At the same time, a recent novel ORAS-like inflammatory syndrome was described in association with a heterozygous missense mutation that appears to exert dominant negative (DN) effects. In this manuscript, we report the identification of a novel homozygous missense mutation, c.595 T > A; p.(Trp199Arg), in a Moroccan infant with an ORAS phenotype and provide experimental evidence for its pathogenicity. We go on to systematically review the literature for OTULIN-associated conditions by using the GenIA database (www.geniadb.net) to collect, extract and harmonize all clinical, laboratory and functional data for published patients and variants. Our comprehensive synthesis of genotypic, phenotypic, and mechanistic data enables a more in-depth view of the diverse mechanisms and pathways by which the OTULIN pathogenic variants may lead to human immune disease. This review may help variant classification activities and inform future variant evaluation, as well as the development of diagnostic and management guidelines. It also identifies current knowledge gaps and raises additional questions warranting future investigation.

Dominant negative OTULIN-related autoinflammatory syndrome

OTU deubiquitinase with linear linkage specificity (OTULIN) regulates inflammation and cell death by deubiquitinating linear ubiquitin chains generated by the linear ubiquitin chain assembly complex (LUBAC). Biallelic loss-of-function mutations causes OTULIN-related autoinflammatory syndrome (ORAS), while OTULIN haploinsuffiency has not been associated with spontaneous inflammation. However, herein, we identify two patients with the heterozygous mutation p.Cys129Ser in OTULIN. Consistent with ORAS, we observed accumulation of linear ubiquitin chains, increased sensitivity to TNF-induced death, and dysregulation of inflammatory signaling in patient cells. While the C129S mutation did not affect OTULIN protein stability or binding capacity to LUBAC and linear ubiquitin chains, it did ablate OTULIN deubiquitinase activity. Loss of activity facilitated the accumulation of autoubiquitin chains on LUBAC. Altered ubiquitination of LUBAC inhibits its recruitment to the TNF receptor signaling complex, promoting TNF-induced cell death and disease pathology. By reporting the first dominant negative mutation driving ORAS, this study expands our clinical understanding of OTULIN-associated pathology.

OTULIN deficiency: focus on innate immune system impairment

OTULIN deficiency is a complex disease characterized by a wide range of clinical manifestations, including skin rash, joint welling, lipodystrophy to pulmonary abscess, and sepsis shock. This disease is mechanistically linked to mutations in the OTULIN gene, resulting in an immune disorder that compromises the body's ability to effectively combat pathogens and foreign stimuli. The OTULIN gene is responsible for encoding a deubiquitinating enzyme crucial for hydrolyzing Met1-poly Ub chains, and its dysfunction leads to dysregulated immune responses. Patients with OTULIN deficiency often exhibit an increase in monocytes, including neutrophils and macrophages, along with inflammatory clinical features. The onset of symptoms typically occurs at an early age. However, individuals with OTULIN haploinsufficiency are particularly susceptible to life-threatening staphylococcal infections. Currently, the most effective treatment for patients with OTULIN biallelic mutations involves the use of TNF-blocking agents, which target the dysregulated immune response. In conclusion, OTULIN deficiency presents a complex clinical picture with diverse manifestations, attributed to mutations in the OTULIN gene. Understanding the underlying mechanisms is crucial for developing targeted therapeutic interventions to address this challenging condition. Further research into the pathophysiology of OTULIN deficiency is essential for improving clinical management and outcomes for affected individuals.

Systematic HOIP interactome profiling reveals critical roles of linear ubiquitination in tissue homeostasis

Linear ubiquitination catalyzed by HOIL-1-interacting protein (HOIP), the key component of the linear ubiquitination assembly complex, plays fundamental roles in tissue homeostasis by executing domain-specific regulatory functions. However, a proteome-wide analysis of the domain-specific interactome of HOIP across tissues is lacking. Here, we present a comprehensive mass spectrometry-based interactome profiling of four HOIP domains in nine mouse tissues. The interaction dataset provides a high-quality HOIP interactome resource with an average of approximately 90 interactors for each bait per tissue. HOIP tissue interactome presents a systematic understanding of linear ubiquitination functions in each tissue and also shows associations of tissue functions to genetic diseases. HOIP domain interactome characterizes a set of previously undefined linear ubiquitinated substrates and elucidates the cross-talk among HOIP domains in physiological and pathological processes. Moreover, we show that linear ubiquitination of Integrin-linked protein kinase (ILK) decreases focal adhesion formation and promotes the detachment of Shigella flexneri-infected cells. Meanwhile, Hoip deficiency decreases the linear ubiquitination of Smad ubiquitination regulatory factor 1 (SMURF1) and enhances its E3 activity, finally causing a reduced bone mass phenotype in mice. Overall, our work expands the knowledge of HOIP-interacting proteins and provides a platform for further discovery of linear ubiquitination functions in tissue homeostasis.

OTULIN Can Improve Spinal Cord Injury by the NF-κB and Wnt/β-Catenin Signaling Pathways

Spinal cord injury (SCI) is a significant health concern, as it presently has no effective treatment in the clinical setting. Inflammation is a key player in the pathophysiological process of SCI, with a number of studies evidencing that the inhibition of the NF-κB signaling pathway may impede the inflammatory response and improve SCI. OTULIN, as a de-ubiquitination enzyme, the most notable is its anti-inflammatory effect. OTULIN can inhibit the NF-κB signaling pathway to suppress the inflammatory reaction via de-ubiquitination. In addition, OTULIN may promote vascular regeneration through the Wnt/β-catenin pathway in the wake of SCI. In this review, we analyze the structure and physiological function of OTULIN, along with both NF-κB and Wnt/β-catenin signaling pathways. Furthermore, we examine the significant role of OTULIN in SCI through its impairment of the NF-κB signaling pathway, which could open the possibility of it being a novel interventional target for the condition.

OTULIN-related conditions: Report of a new case and review of the literature using GenIA

OTULIN encodes an eponymous linear deubiquitinase (DUB), which through the regulation of M1-Ub dynamics, is essential for controlling inflammation as a negative regulator of the canonical NF-κB signaling pathway. Biallelic loss-of-function (LOF) mutations in OTULIN cause an autosomal recessive condition named Otulin-Related Autoinflammatory Syndrome (ORAS), also known as Otulipenia or AutoInflammation, Panniculitis, and Dermatosis Syndrome (AIPDS). Monoallelic OTULIN LOF, also known as OTULIN Haploinsufficiency (OHI) or Immunodeficiency 107 (IMD107), has been linked to an incompletely penetrant, dominantly inherited susceptibility to invasive Staphylococcal infections. At the same time, a recent novel ORAS-like inflammatory syndrome was described in association with a heterozygous missense mutation that appears to exert dominant negative effects. In this manuscript, we report the identification of a novel homozygous missense mutation, c.595T>A; p.(Trp199Arg), in a Moroccan infant with an ORAS phenotype. We go on to systematically review the literature for OTULIN-related human disease phenotypes by using the GenIA database to collect, extract and harmonize all clinical, laboratory and functional data for published patients and variants. Our comprehensive synthesis of genotypic, phenotypic, and mechanistic data enables a more in-depth view of the diverse mechanisms and pathways by which the OTULIN pathogenic variants may lead to human immune disease. This review may help variant classification activities and the drafting of diagnostic and management guidelines; but it also identifies outstanding knowledge gaps and raises additional questions for future investigation.

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.

A Novel Scoring Model of Deubiquitination Patterns Predicts Prognosis and Immunotherapeutic Response in Hepatocellular Carcinoma

Aberrant expression of deubiquitinases (DUBs) is significantly associated with tumorigenesis. However, the precise impact of deubiquitination on the tumour microenvironment (TME) and immunotherapy in hepatocellular carcinoma (HCC) remains unclear. In this study, we comprehensively characterized the transcriptional and genetic alterations of 26 overall survival (OS)-related DUBs in HCC. The consensus clustering algorithm was used to identify patients with distinct deubiquitination patterns. We then established a DUBscore model using the principal component analysis (PCA) algorithm to quantify the deubiquitination patterns of individual HCC patients. Finally, we performed weighted gene coexpression network analysis (WGCNA) to identify the key DUBs. Consequently, three distinct deubiquitination patterns were identified, each showing significant differences in the characteristics of the TME, immune response, and clinical prognosis. Further analysis revealed that the DUBscore was an independent prognostic factor and could predict the response to immunotherapy for patients with HCC. Ultimately, BRCC3 was identified as a key DUB based on the DUBscore, which was significantly overexpressed in tumour tissues, as confirmed by qRT‒PCR and immunohistochemistry (IHC). We analysed the distribution and expression of BRCC3 in various types of immune cells using single-cell RNA sequencing (scRNA-seq). In conclusion, our study revealed the crucial role of deubiquitination patterns in shaping TME complexity and diversity. A more personalized and effective antitumour immunotherapy strategy can be developed by utilizing the DUBscore model to identify deubiquitination patterns in individual HCC patients. Our findings also highlight that BRCC3 may serve as a potential therapeutic target in HCC and a predictive marker for immunotherapeutic response.

Deubiquitinases in cancer

Ubiquitination is an essential regulator of most, if not all, signalling pathways, and defects in cellular signalling are central to cancer initiation, progression and, eventually, metastasis. The attachment of ubiquitin signals by E3 ubiquitin ligases is directly opposed by the action of approximately 100 deubiquitinating enzymes (DUBs) in humans. Together, DUBs and E3 ligases coordinate ubiquitin signalling by providing selectivity for different substrates and/or ubiquitin signals. The balance between ubiquitination and deubiquitination is exquisitely controlled to ensure properly coordinated proteostasis and response to cellular stimuli and stressors. Not surprisingly, then, DUBs have been associated with all hallmarks of cancer. These relationships are often complex and multifaceted, highlighted by the implication of multiple DUBs in certain hallmarks and by the impact of individual DUBs on multiple cancer-associated pathways, sometimes with contrasting cancer-promoting and cancer-inhibiting activities, depending on context and tumour type. Although it is still understudied, the ever-growing knowledge of DUB function in cancer physiology will eventually identify DUBs that warrant specific inhibition or activation, both of which are now feasible. An integrated appreciation of the physiological consequences of DUB modulation in relevant cancer models will eventually lead to the identification of patient populations that will most likely benefit from DUB-targeted therapies.

Mechanisms underlying linear ubiquitination and implications in tumorigenesis and drug discovery

Linear ubiquitination is a distinct type of ubiquitination that involves attaching a head-to-tail polyubiquitin chain to a substrate protein. Early studies found that linear ubiquitin chains are essential for the TNFα- and IL-1-mediated NF-κB signaling pathways. However, recent studies have discovered at least sixteen linear ubiquitination substrates, which exhibit a broader activity than expected and mediate many other signaling pathways beyond NF-κB signaling. Dysregulation of linear ubiquitination in these pathways has been linked to many types of cancers, such as lymphoma, liver cancer, and breast cancer. Since the discovery of linear ubiquitin, extensive effort has been made to delineate the molecular mechanisms of how dysregulation of linear ubiquitination causes tumorigenesis and cancer development. In this review, we highlight newly discovered linear ubiquitination-mediated signaling pathways, recent advances in the role of linear ubiquitin in different types of cancers, and the development of linear ubiquitin inhibitors. Video Abstract.

Myeloid OTULIN deficiency couples RIPK3-dependent cell death to Nlrp3 inflammasome activation and IL-1β secretion

Loss-of-function mutations in the deubiquitinase OTULIN result in an inflammatory pathology termed "OTULIN-related autoinflammatory syndrome" (ORAS). Genetic mouse models revealed essential roles for OTULIN in inflammatory and cell death signaling, but the mechanisms by which OTULIN deficiency connects cell death to inflammation remain unclear. Here, we identify OTULIN deficiency as a cellular condition that licenses RIPK3-mediated cell death in murine macrophages, leading to Nlrp3 inflammasome activation and subsequent IL-1β secretion. OTULIN deficiency uncoupled Nlrp3 inflammasome activation from gasdermin D-mediated pyroptosis, instead allowing RIPK3-dependent cell death to act as an Nlrp3 inflammasome activator and mechanism for IL-1β release. Accordingly, elevated serum IL-1β levels in myeloid-specific OTULIN-deficient mice were diminished by deleting either Ripk3 or Nlrp3. These findings identify OTULIN as an inhibitor of RIPK3-mediated IL-1β release in mice.

In an experimental myocardial infarction model, L-arginine pre-intervention may exert cardioprotective effects by regulating OTULIN levels and mitochondrial dynamics

The experimental myocardial infarction (MI) model originating from isoproterenol (ISO) is frequently preferred in research due to its similarity to MI-induced damage in humans. Beneficial effects of L-arginine (L-Arg), a semi-essential amino acid, in cardiovascular diseases have been shown in many studies. This study was carried out to determine whether L-Arg pre-intervention has protective effects on heart tissue in the experimental MI model. The 28 rats used in the study were randomly divided into 4 equal groups: control, L-Arg, ISO, and L-Arg+ISO. Upon completion of all applications, cardiac markers in serum and biochemical, histopathological, and immunohistochemical examinations in cardiac tissues were performed. Cardiac markers, histopathological changes, oxidative stress, inflammation, and apoptosis were increased in the experimental MI model. In addition, administration of ISO deregulated OTULIN levels and mitochondrial dynamics in heart tissue. However, L-Arg pre-intervention showed a significant protective effect against changes in ISO-induced MI. L-Arg supplementation with cardioprotective effect may reduce the risks of possible pathophysiological processes in MI.

OTULIN protects the intestinal epithelium from apoptosis during inflammation and infection

The intestinal epithelium is a single cell layer that is constantly renewed and acts as a physical barrier that separates intestinal microbiota from underlying tissues. In inflammatory bowel disease (IBD) in humans, as well as in experimental mouse models of IBD, this barrier is impaired, causing microbial infiltration and inflammation. Deficiency in OTU deubiquitinase with linear linkage specificity (OTULIN) causes OTULIN-related autoinflammatory syndrome (ORAS), a severe inflammatory pathology affecting multiple organs including the intestine. We show that mice with intestinal epithelial cell (IEC)-specific OTULIN deficiency exhibit increased susceptibility to experimental colitis and are highly sensitive to TNF toxicity, due to excessive apoptosis of OTULIN deficient IECs. OTULIN deficiency also increases intestinal pathology in mice genetically engineered to secrete excess TNF, confirming that chronic exposure to TNF promotes epithelial cell death and inflammation in OTULIN deficient mice. Mechanistically we demonstrate that upon TNF stimulation, OTULIN deficiency impairs TNF receptor complex I formation and LUBAC recruitment, and promotes the formation of the cytosolic complex II inducing epithelial cell death. Finally, we show that OTULIN deficiency in IECs increases susceptibility to Salmonella infection, further confirming the importance of OTULIN for intestinal barrier integrity. Together, these results identify OTULIN as a major anti-apoptotic protein in the intestinal epithelium and provide mechanistic insights into how OTULIN deficiency drives gastrointestinal inflammation in ORAS patients.

Role of linear ubiquitination in inflammatory responses and tissue homeostasis

Polyubiquitination is a post-translational modification involved in a wide range of immunological events, including inflammatory responses, immune cell differentiation, and development of inflammatory diseases. The versatile functions of polyubiquitination are based on different types of ubiquitin linkage, which enable various UBD (ubiquitin binding domain)-containing adaptor proteins to associate and induce distinct biological outputs. A unique and atypical type of polyubiquitin chain comprising a conjugation between the N-terminal methionine of the proximal ubiquitin moiety and the C-terminal glycine of the distal ubiquitin moiety, referred to as a linear or M1-linked ubiquitin chain, has been studied exclusively within the field of immunology because it is distinct from other polyubiquitin forms: linear ubiquitin chains are generated predominantly by various inflammatory stimulants, including tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β), and act as a critical modulator of transient and optimal signal transduction. Moreover, accumulating evidence suggests that linear ubiquitin chains are of physiological significance. Dysregulation of linear ubiquitination triggers chronic inflammation and immunodeficiency via downregulation of linear ubiquitin-dependent nuclear factor-kappa B (NF-κB) signaling and by triggering TNF-α-induced cell death, suggesting that linear ubiquitination is a homeostatic regulator of tissue-specific functions. In this review, we focus on our current understating of the molecular and cellular mechanisms by which linear ubiquitin chains control inflammatory environments. Furthermore, we review the role of linear ubiquitination on T cell development, differentiation, and function, thereby providing insight into its direct association with maintaining the immune system.

RBCK1 promotes hepatocellular carcinoma metastasis and growth by stabilizing RNF31

RNF31 (HOIP), RBCK1 (HOIL-1L), and SHARPIN are subunits of the linear ubiquitin chain assembly complex. Their function and specific molecular mechanisms in hepatocellular carcinoma (HCC) have not been reported previously. Here, we investigated the role of RNF31 and RBCK1 in HCC. We showed that RNF31 and RBCK1 were overexpressed in HCC and that upregulation of RNF31 and RBCK1 indicated poor clinical outcomes in patients with HCC. RNF31 overexpression was significantly associated with more satellite foci and vascular invasion in patients with HCC. Additionally, RBCK1 expression correlated positively with RNF31 expression in HCC tissues. Functionally, RBCK1 and RNF31 promote the metastasis and growth of HCC cells. Moreover, the RNF31 inhibitor gliotoxin inhibited the malignant behavior of HCC cells. Mechanistically, RBCK1 interacted with RNF31 and repressed its ubiquitination and proteasomal degradation. In summary, the present study revealed an oncogenic role and regulatory relationship between RBCK1 and RNF31 in facilitating proliferation and metastasis in HCC, suggesting that they are potential prognostic markers and therapeutic targets for HCC.

HOIP modulates the stability of GPx4 by linear ubiquitination

LUBAC-mediated linear ubiquitination plays a pivotal role in regulation of cell death and inflammatory pathways. Genetic deficiency in LUBAC components leads to severe immune dysfunction or embryonic lethality. LUBAC has been extensively studied for its role in mediating TNF signaling. However, Tnfr1 knockout is not able to fully rescue the embryonic lethality of LUBAC deficiency, suggesting that LUBAC may modify additional key cellular substrates in promoting cell survival. GPx4 is an important selenoprotein involved in regulating cellular redox homeostasis in defense against lipid peroxidation-mediated cell death known as ferroptosis. Here we demonstrate that LUBAC deficiency sensitizes to ferroptosis by promoting GPx4 degradation and downstream lipid peroxidation. LUBAC binds and stabilizes GPx4 by modulating its linear ubiquitination both in normal condition and under oxidative stress. Our findings identify GPx4 as a key substrate of LUBAC and a previously unrecognized role of LUBAC-mediated linear ubiquitination in regulating cellular redox status and cell death.

LncRNA Airn maintains LSEC differentiation to alleviate liver fibrosis via the KLF2-eNOS-sGC pathway

BACKGROUND

Long noncoding RNAs (lncRNAs) have emerged as important regulators in a variety of human diseases. The dysregulation of liver sinusoidal endothelial cell (LSEC) phenotype is a critical early event in the fibrotic process. However, the biological function of lncRNAs in LSEC still remains unclear.

METHODS

The expression level of lncRNA Airn was evaluated in both human fibrotic livers and serums, as well as mouse fibrotic livers. Gain- and loss-of-function experiments were performed to detect the effect of Airn on LSEC differentiation and hepatic stellate cell (HSC) activation in liver fibrosis. Furthermore, RIP, RNA pull-down-immunoblotting, and ChIP experiments were performed to explore the underlying mechanisms of Airn.

RESULTS

We have identified Airn was significantly upregulated in liver tissues and LSEC of carbon tetrachloride (CCl₄)-induced liver fibrosis mouse model. Moreover, the expression of AIRN in fibrotic human liver tissues and serums was remarkably increased compared with healthy controls. In vivo studies showed that Airn deficiency aggravated CCl₄- and bile duct ligation (BDL)-induced liver fibrosis, while Airn over-expression by AAV8 alleviated CCl₄-induced liver fibrosis. Furthermore, we revealed that Airn maintained LSEC differentiation in vivo and in vitro. Additionally, Airn inhibited HSC activation indirectly by regulating LSEC differentiation and promoted hepatocyte (HC) proliferation by increasing paracrine secretion of Wnt2a and HGF from LSEC. Mechanistically, Airn interacted with EZH2 to maintain LSEC differentiation through KLF2-eNOS-sGC pathway, thereby maintaining HSC quiescence and promoting HC proliferation.

CONCLUSIONS

Our work identified that Airn is beneficial to liver fibrosis by maintaining LSEC differentiation and might be a serum biomarker for liver fibrogenesis.

Deubiquitinases in Cancers: Aspects of Proliferation, Metastasis, and Apoptosis

Simple Summary

This review summarizes the current DUBs findings that correlate with the most common cancers in the world (liver, breast, prostate, colorectal, pancreatic, and lung cancers). The DUBs were further classified by their biological functions in terms of proliferation, metastasis, and apoptosis. The work provides an updated of the current findings, and could be used as a quick guide for researchers to identify target DUBs in cancers.

Abstract

Deubiquitinases (DUBs) deconjugate ubiquitin (UBQ) from ubiquitylated substrates to regulate its activity and stability. They are involved in several cellular functions. In addition to the general biological regulation of normal cells, studies have demonstrated their critical roles in various cancers. In this review, we evaluated and grouped the biological roles of DUBs, including proliferation, metastasis, and apoptosis, in the most common cancers in the world (liver, breast, prostate, colorectal, pancreatic, and lung cancers). The current findings in these cancers are summarized, and the relevant mechanisms and relationship between DUBs and cancers are discussed. In addition to highlighting the importance of DUBs in cancer biology, this study also provides updated information on the roles of DUBs in different types of cancers.

Cell Death-Related Ubiquitin Modifications in Inflammatory Syndromes: From Mice to Men

Aberrant cell death can cause inflammation and inflammation-related diseases. While the link between cell death and inflammation has been widely established in mouse models, evidence supporting a role for cell death in the onset of inflammatory and autoimmune diseases in patients is still missing. In this review, we discuss how the lessons learnt from mouse models can help shed new light on the initiating or contributing events leading to immune-mediated disorders. In addition, we discuss how multiomic approaches can provide new insight on the soluble factors released by dying cells that might contribute to the development of such diseases.

LUBAC

Yuri Shibata and David Komander discuss the composition, regulation and functions of the linear ubiquitin chain assembly complex (LUBAC).

Deubiquitinases in cell death and inflammation

Apoptosis, pyroptosis, and necroptosis are distinct forms of programmed cell death that eliminate infected, damaged, or obsolete cells. Many proteins that regulate or are a part of the cell death machinery undergo ubiquitination, a post-translational modification made by ubiquitin ligases that modulates protein abundance, localization, and/or activity. For example, some ubiquitin chains target proteins for degradation, while others function as scaffolds for the assembly of signaling complexes. Deubiquitinases (DUBs) are the proteases that counteract ubiquitin ligases by cleaving ubiquitin from their protein substrates. Here, we review the DUBs that have been found to suppress or promote apoptosis, pyroptosis, or necroptosis.

Roles of ubiquitination in the crosstalk between tumors and the tumor microenvironment (Review)

The interaction between a tumor and the tumor microenvironment (TME) plays a key role in tumorigenesis and tumor progression. Ubiquitination, a crucial post-translational modification for regulating protein degradation and turnover, plays a role in regulating the crosstalk between a tumor and the TME. Thus, identifying the roles of ubiquitination in the process may assist researchers to investigate the mechanisms underlying tumorigenesis and tumor progression. In the present review article, new insights into the substrates for ubiquitination that are involved in the regulation of hypoxic environments, angiogenesis, chronic inflammation-mediated tumor formation, and the function of cancer-associated fibroblasts and infiltrating immune cells (tumor-associated macrophages, T-cells, myeloid-derived suppressor cells, dendritic cells, and natural killer cells) are summarized. In addition, the potential targets of the ubiquitination proteasome system within the TME for cancer therapy and their therapeutic effects are reviewed and discussed.

RIP1 post-translational modifications

Receptor interacting protein 1 (RIP1) kinase is a critical regulator of inflammation and cell death signaling, and plays a crucial role in maintaining immune responses and proper tissue homeostasis. Mounting evidence argues for the importance of RIP1 post-translational modifications in control of its function. Ubiquitination by E3 ligases, such as inhibitors of apoptosis (IAP) proteins and LUBAC, as well as the reversal of these modifications by deubiquitinating enzymes, such as A20 and CYLD, can greatly influence RIP1 mediated signaling. In addition, cleavage by caspase-8, RIP1 autophosphorylation, and phosphorylation by a number of signaling kinases can greatly impact cellular fate. Disruption of the tightly regulated RIP1 modifications can lead to signaling disbalance in TNF and/or TLR controlled and other inflammatory pathways, and result in severe human pathologies. This review will focus on RIP1 and its many modifications with an emphasis on ubiquitination, phosphorylation, and cleavage, and their functional impact on the RIP1's role in signaling pathways.

Linear ubiquitination modification of NR6A1 by LUBAC inhibits RIPK3 kinase activity and attenuates apoptosis of vascular smooth muscle cells

Nuclear receptor subfamily 6 group A member 1 (NR6A1) is involved in promoting the apoptotic process of vascular smooth muscle cells (VSMCs) which is a critical process involved in atherosclerosis, but the action mechanism remains to be determined. Therefore, we studied the underlying mechanisms by which NR6A1 accelerated VSMC apoptosis in atherosclerosis. An atherosclerosis model has been established in apolipoprotein E-deficient rats with a high-fat diet for 12 weeks, which was characterized by pathological aortic plaques, increased lipid deposition and collagen content in aortic tissues, and high cholesterol and triglycerides levels in the serum. NR6A1 was experimentally shown to increase at protein level rather than messenger RNA level in atherosclerotic rats. Immunofluorescence exhibited the main location of NR6A1 in the cell nucleus of rat aortic tissues. By performing ectopic expression experiments, NR6A1 was demonstrated to suppress the viability and expedite the apoptosis of VSMCs, corresponding to augmented caspase-3, caspase-8, and caspase-9 activities. It was further unraveled that NR6A1 could activate receptor-interacting serine/threonine-protein kinase 3 (RIPK3) by inducing its phosphorylation. Conversely, RIPK3 inhibitor GSK872 undermined the proapoptotic effect of NR6A1 on VSMCs. The co-immunoprecipitation assay identified that linear ubiquitin chain assembly complex (LUBAC) can be pulled down by NR6A1. Furthermore. LUBAC inhibited the expression of NR6A1 by promoting its linear ubiquitination, thereby dephosphorylating RIPK3 and consequently inhibiting the VSMC apoptosis. Overall, LUBAC-induced linear ubiquitination of NR6A1 can potentially arrest the apoptosis of VSMCs in atherosclerosis by downregulating RIPK3 and attenuating caspase activity. This finding suggests promising athero-protective targets by limiting VSMC apoptosis.

Mecheliolide elicits ROS-mediated ERS driven immunogenic cell death in hepatocellular carcinoma

The nonnegligible reason for the poor prognosis of hepatocellular carcinoma (HCC) is resistance to conventional chemotherapy. Immunogenic cell death (ICD) is a rare immunostimulatory form of cell death that can reengage the tumor-specific immune system. ICD can improve the clinical outcomes of chemotherapeutics by promoting a long-term cancer immunity. The discovery of potential ICD inducers is emerging as a promising direction. In the present study, micheliolide (MCL), a natural guaianolide sesquiterpene lactone, was screened out by the virtual screening strategies, identified as an inhibitor of thioredoxin reductase (TrxR) and was evaluated to have high potential to induce ICD. Here, we showed that MCL induced ICD-associated DAMPs (damage-associated molecular patterns, such as CRT exposure, ATP secretion and HMGB1 release). MCL significantly triggered the regression of established tumors in an immunocompetent mouse vaccine model, and induced ICD (DCs maturation, the stimulation of CD4⁺, and CD8⁺ T-cells responses) in vivo. Mechanistically, we found that the magnitude of ICD-associated effects induced upon exposure of HCC cells to MCL was dependent on the generation of reactive oxygen species (ROS)-mediated endoplasmic reticulum stress (ERS). In addition, the suppression of ROS normalized MCL-induced ERS, in contrast, the downregulation of TrxR synergized with the ERS driven by MCL. We also systematically detected the H₂O₂ generation using Hyper7 sensors in HCC cells exposed to MCL. Notably, MCL inhibited the development of HCC organoids. Collectively, our results reveal a potential association between the TrxR inhibitors and ICD, presenting valuable insights into the MCL-activated ICD in HCC cells.

Compound heterozygous variants in OTULIN are associated with fulminant atypical late-onset ORAS

Autoinflammatory diseases are a heterogenous group of disorders defined by fever and systemic inflammation suggesting involvement of genes regulating innate immune responses. Patients with homozygous loss-of-function variants in the OTU-deubiquitinase OTULIN suffer from neonatal-onset OTULIN-related autoinflammatory syndrome (ORAS) characterized by fever, panniculitis, diarrhea, and arthritis. Here, we describe an atypical form of ORAS with distinct clinical manifestation of the disease caused by two new compound heterozygous variants (c.258G>A (p.M86I)/c.500G>C (p.W167S)) in the OTULIN gene in a 7-year-old affected by a life-threatening autoinflammatory episode with sterile abscess formation. On the molecular level, we find binding of OTULIN to linear ubiquitin to be compromised by both variants; however, protein stability and catalytic activity is most affected by OTULIN variant p.W167S. These molecular changes together lead to increased levels of linear ubiquitin linkages in patient-derived cells triggering the disease. Our data indicate that the spectrum of ORAS patients is more diverse than previously thought and, thus, supposedly asymptomatic individuals might also be affected. Based on our results, we propose to subdivide the ORAS into classical and atypical entities.

Novel Genetic Discoveries in Primary Immunodeficiency Disorders

The field of Immunology is one that has undergone great expansion in recent years. With the advent of new diagnostic modalities including a variety of genetic tests (discussed elsewhere in this journal), the ability to diagnose a patient with a primary immunodeficiency disorder (PIDD) has become a more streamlined process. With increased availability of genetic testing for those with suspected or known PIDD, there has been a significant increase in the number of genes associated with this group of disorders. This is of great importance as a misdiagnosis of these rare diseases can lead to a delay in what can be critical treatment options. At times, those options can include life-saving medications or procedures. Presentation of patients with PIDD can vary greatly based on the specific genetic defect and the part(s) of the immune system that is affected by the variation. PIDD disorders lead to varying levels of increased risk of infection ranging from a mild increase such as with selective IgA deficiency to a profound risk with severe combined immunodeficiency. These diseases can also cause a variety of other clinical findings including autoimmunity and gastrointestinal disease.

MicroRNA-137-3p Improves Nonalcoholic Fatty Liver Disease through Activating AMPKα

Nonalcoholic fatty liver disease (NAFLD) is one of the most common chronic liver diseases worldwide and can develop to nonalcoholic steatohepatitis and later hepatic cirrhosis with a high prevalence to hepatocellular carcinoma. Oxidative stress and chronic hepatic inflammation are implicated in the pathogenesis of NAFLD. MicroRNA-137-3p (miR-137-3p) are associated with oxidative stress and inflammation; however, its role and mechanism in NAFLD remain unclear. Mice were fed with a high-fat diet (HFD) for 24 weeks to establish the NAFLD model. To overexpress or suppress hepatic miR-137-3p expression, mice were intraperitoneally injected with the agomir, antagomir, or respective controls of miR-137-3p at a dose of 100 mg/kg weekly for 6 consecutive weeks before the mice were sacrificed. To validate the involvement of AMP-activated protein kinase alpha (AMPKα) or cAMP-specific phosphodiesterase 4D (PDE4D), HFD mice were intraperitoneally injected with 20 mg/kg compound C or 0.5 mg/kg rolipram every other day for 8 consecutive weeks before the mice were sacrificed. Hepatic miR-137-3p expression was significantly decreased in mice upon HFD stimulation. miR-137-3p agomir alleviated, while miR-137-3p antagomir facilitated HFD-induced oxidative stress, inflammation, and hepatic dysfunction in mice. Mechanistically, we revealed that miR-137-3p is directly bound to the 3'-untranslated region of PDE4D and subsequently increased hepatic cAMP level and protein kinase A activity, thereby activating the downstream AMPKα pathway. In summary, miR-137-3p improves NAFLD through activating AMPKα and it is a promising therapeutic candidate to treat NAFLD.

Dual Role of p73 in Cancer Microenvironment and DNA Damage Response

Understanding the mechanisms that regulate cancer progression is pivotal for the development of new therapies. Although p53 is mutated in half of human cancers, its family member p73 is not. At the same time, isoforms of p73 are often overexpressed in cancers and p73 can overtake many p53 functions to kill abnormal cells. According to the latest studies, while p73 represses epithelial–mesenchymal transition and metastasis, it can also promote tumour growth by modulating crosstalk between cancer and immune cells in the tumor microenvironment, M2 macrophage polarisation, Th2 T-cell differentiation, and angiogenesis. Thus, p73 likely plays a dual role as a tumor suppressor by regulating apoptosis in response to genotoxic stress or as an oncoprotein by promoting the immunosuppressive environment and immune cell differentiation.

Redressing the interactions between stem cells and immune system in tissue regeneration

Skeletal muscle has an extraordinary regenerative capacity reflecting the rapid activation and effective differentiation of muscle stem cells (MuSCs). In the course of muscle regeneration, MuSCs are reprogrammed by immune cells. In turn, MuSCs confer immune cells anti-inflammatory properties to resolve inflammation and facilitate tissue repair. Indeed, MuSCs can exert therapeutic effects on various degenerative and inflammatory disorders based on their immunoregulatory ability, including effects primed by interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α). At the molecular level, the tryptophan metabolites, kynurenine or kynurenic acid, produced by indoleamine 2,3-dioxygenase (IDO), augment the expression of TNF-stimulated gene 6 (TSG6) through the activation of the aryl hydrocarbon receptor (AHR). In addition, insulin growth factor 2 (IGF2) produced by MuSCs can endow maturing macrophages oxidative phosphorylation (OXPHOS)-dependent anti-inflammatory functions. Herein, we summarize the current understanding of the immunomodulatory characteristics of MuSCs and the issues related to their potential applications in pathological conditions, including COVID-19.

Emerging roles of the HECT-type E3 ubiquitin ligases in hematological malignancies

Ubiquitination-mediated proteolysis or regulation of proteins, ultimately executed by E3 ubiquitin ligases, control a wide array of cellular processes, including transcription, cell cycle, autophagy and apoptotic cell death. HECT-type E3 ubiquitin ligases can be distinguished from other subfamilies of E3 ubiquitin ligases because they have a C-terminal HECT domain that directly catalyzes the covalent attachment of ubiquitin to their substrate proteins. Deregulation of HECT-type E3-mediated ubiquitination plays a prominent role in cancer development and chemoresistance. Several members of this subfamily are indeed frequently deregulated in human cancers as a result of genetic mutations and altered expression or activity. HECT-type E3s contribute to tumorigenesis by regulating the ubiquitination rate of substrates that function as either tumour suppressors or oncogenes. While the pathological roles of the HECT family members in solid tumors are quite well established, their contribution to the pathogenesis of hematological malignancies has only recently emerged. This review aims to provide a comprehensive overview of the involvement of the HECT-type E3s in leukemogenesis.

OTULIN maintains skin homeostasis by controlling keratinocyte death and stem cell identity

OTULIN is a deubiquitinase that specifically cleaves linear ubiquitin chains. Here we demonstrate that the ablation of Otulin selectively in keratinocytes causes inflammatory skin lesions that develop into verrucous carcinomas. Genetic deletion of Tnfr1, knockin expression of kinase-inactive Ripk1 or keratinocyte-specific deletion of Fadd and Mlkl completely rescues mice with OTULIN deficiency from dermatitis and tumorigenesis, thereby identifying keratinocyte cell death as the driving force for inflammation. Single-cell RNA-sequencing comparing non-lesional and lesional skin reveals changes in epidermal stem cell identity in OTULIN-deficient keratinocytes prior to substantial immune cell infiltration. Keratinocytes lacking OTULIN display a type-1 interferon and IL-1β response signature, and genetic or pharmacologic inhibition of these cytokines partially inhibits skin inflammation. Finally, expression of a hypomorphic mutant Otulin allele, previously shown to cause OTULIN-related autoinflammatory syndrome in humans, induces a similar inflammatory phenotype, thus supporting the importance of OTULIN for restraining skin inflammation and maintaining immune homeostasis.

OTULIN inhibits RIPK1-mediated keratinocyte necroptosis to prevent skin inflammation in mice

Linear ubiquitination regulates inflammatory and cell death signalling. Deficiency of the linear ubiquitin chain-specific deubiquitinase, OTULIN, causes OTULIN-related autoinflammatory syndrome (ORAS), a systemic inflammatory pathology affecting multiple organs including the skin. Here we show that mice with epidermis-specific OTULIN deficiency (OTULINE-KO) develop inflammatory skin lesions that are driven by TNFR1 signalling in keratinocytes and require RIPK1 kinase activity. OTULINE-KO mice lacking RIPK3 or MLKL have only very mild skin inflammation, implicating necroptosis as an important etiological mediator. Moreover, combined loss of RIPK3 and FADD fully prevents skin lesion development, showing that apoptosis also contributes to skin inflammation in a redundant function with necroptosis. Finally, MyD88 deficiency suppresses skin lesion development in OTULINE-KO mice, suggesting that toll-like receptor and/or IL-1 signalling are involved in mediating skin inflammation. Thus, OTULIN maintains homeostasis and prevents inflammation in the skin by inhibiting TNFR1-mediated, RIPK1 kinase activity-dependent keratinocyte death and primarily necroptosis.

Regulation of CYLD activity and specificity by phosphorylation and ubiquitin-binding CAP-Gly domains

Non-degradative ubiquitin chains and phosphorylation events govern signaling responses by innate immune receptors. The deubiquitinase CYLD in complex with SPATA2 is recruited to receptor signaling complexes by the ubiquitin ligase LUBAC and regulates Met1- and Lys63-linked polyubiquitin and receptor signaling outcomes. Here, we investigate the molecular determinants of CYLD activity. We reveal that two CAP-Gly domains in CYLD are ubiquitin-binding domains and demonstrate a requirement of CAP-Gly3 for CYLD activity and regulation of immune receptor signaling. Moreover, we identify a phosphorylation switch outside of the catalytic USP domain, which activates CYLD toward Lys63-linked polyubiquitin. The phosphorylated residue Ser568 is a novel tumor necrosis factor (TNF)-regulated phosphorylation site in CYLD and works in concert with Ser418 to enable CYLD-mediated deubiquitination and immune receptor signaling. We propose that phosphorylated CYLD, together with SPATA2 and LUBAC, functions as a ubiquitin-editing complex that balances Lys63- and Met1-linked polyubiquitin at receptor signaling complexes to promote LUBAC signaling.

MicroRNA-23a-5p Is Involved in the Regulation of Lipopolysaccharide-Induced Acute Lung Injury by Targeting HSP20/ASK1

Inflammation and oxidative stress contribute to the progression of acute lung injury (ALI). MicroRNA-23a-5p (miR-23a-5p) has been reported to regulate inflammation and oxidative stress; however, its role in ALI is still poorly elucidated. Mice were intravenously treated with the miR-23a-5p antagomir, agomir, or the negative controls for 3 consecutive days and then received a single intratracheal injection of lipopolysaccharide (LPS, 5 mg/kg) to induce ALI. Pulmonary function, bronchoalveolar lavage fluids (BALFs), arterial blood gas, and molecular biomarkers associated with inflammation and oxidative stress were analyzed. In addition, murine peritoneal macrophages were isolated and treated with LPS to verify the role of miR-23a-5p in vitro. We detected an elevation of miR-23a-5p expression in the lungs from ALI mice. The miR-23a-5p antagomir was prevented, whereas the miR-23a-5p agomir aggravated inflammation, oxidative stress, lung tissue injury, and pulmonary dysfunction in LPS-treated mice. Besides, the miR-23a-5p antagomir also reduced the productions of proinflammatory cytokines and free radicals in LPS-treated primary macrophages, which were further augmented in cells following the miR-23a-5p agomir treatment. Additional findings demonstrated that the miR-23a-5p agomir exacerbated LPS-induced ALI via activating apoptosis signal-regulating kinase 1 (ASK1), and that pharmacological or genetic inhibition of ASK1 significantly repressed the deleterious effects of the miR-23a-5p agomir. Moreover, we proved that the miR-23a-5p agomir activated ASK1 via directly reducing heat shock protein 20 (HSP20) expression. miR-23a-5p is involved in the regulation of LPS-induced inflammation, oxidative stress, lung tissue injury, and pulmonary dysfunction by targeting HSP20/ASK1, and it is a valuable therapeutic candidate for the treatment of ALI.

MicroRNA-214-5p aggravates sepsis-related acute kidney injury in mice

Acute kidney injury (AKI) is a devastating comorbidity in sepsis and correlates with a very poor prognosis and increased mortality. Currently, we use lipopolysaccharide (LPS) to establish sepsis-related AKI and try to demonstrate the pathophysiological role of microRNA-214-5p (miR-214-5p) in this process. Mice were intravenously injected with the miR-214-5p agomir, antagomir or negative controls for three consecutive days and then received a single intraperitoneal injection of LPS (10 mg/kg) for 24 h to induce AKI. Besides, the Boston University mouse proximal tubular cell lines were stimulated with LPS (10 μg/ml) for 8 h to investigate the role of miR-214-5p in vitro. To inhibit adenosine monophosphate-activated protein kinase (AMPK), compound C (CpC) was used in vivo. For glucagon-like peptide-1 receptor (GLP-1R) silence, cells were transfected with the small interfering RNA against GLP-1R. miR-214-5p level was upregulated in LPS-treated kidneys and proximal tubular cell lines. The miR-214-5p antagomir reduced LPS-induced renal inflammation and oxidative stress, thereby preventing renal damage and dysfunction. In contrast, the miR-214-5p agomir aggravated LPS-induced inflammation, oxidative stress and AKI in vivo and in vitro. Mechanistically, we found that the miR-214-5p antagomir prevented septic AKI via activating AMPK and that CpC treatment completely abrogated its renoprotective effect in mice. Further detection showed that miR-214-5p directly bound to the 3'-untranslational region of GLP-1R to inhibit GLP-1R/AMPK axis. Our data identify miR-214-5p as a promising therapeutic candidate to treat sepsis-related AKI.

OTULIN allies with LUBAC to govern angiogenesis by editing ALK1 linear polyubiquitin

OTULIN coordinates with LUBAC to edit linear polyubiquitin chains in embryonic development, autoimmunity, and inflammatory diseases. However, the mechanism by which angiogenesis, especially that of endothelial cells (ECs), is regulated by linear ubiquitination remains unclear. Here, we reveal that constitutive or EC-specific deletion of Otulin resulted in arteriovenous malformations and embryonic lethality. LUBAC conjugates linear ubiquitin chains onto Activin receptor-like kinase 1 (ALK1), which is responsible for angiogenesis defects, inhibiting ALK1 enzyme activity and Smad1/5 activation. Conversely, OTULIN deubiquitinates ALK1 to promote Smad1/5 activation. Consistently, embryonic survival of Otulin-deficient mice was prolonged by BMP9 pretreatment or EC-specific ALK1Q200D (constitutively active) knockin. Moreover, mutant ALK1 from type 2 hereditary hemorrhagic telangiectasia (HHT2) patients exhibited excessive linear ubiquitination and increased HOIP binding. As such, a HOIP inhibitor restricted the excessive angiogenesis of ECs derived from ALK1G309S-expressing HHT2 patients. These results show that OTULIN and LUBAC govern ALK1 activity to balance EC angiogenesis.

Low level of plasminogen increases risk for mortality in COVID-19 patients

The pathophysiology of coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), and especially of its complications is still not fully understood. In fact, a very high number of patients with COVID-19 die because of thromboembolic causes. A role of plasminogen, as precursor of fibrinolysis, has been hypothesized. In this study, we aimed to investigate the association between plasminogen levels and COVID-19-related outcomes in a population of 55 infected Caucasian patients (mean age: 69.8 ± 14.3, 41.8% female). Low levels of plasminogen were significantly associated with inflammatory markers (CRP, PCT, and IL-6), markers of coagulation (D-dimer, INR, and APTT), and markers of organ dysfunctions (high fasting blood glucose and decrease in the glomerular filtration rate). A multidimensional analysis model, including the correlation of the expression of coagulation with inflammatory parameters, indicated that plasminogen tended to cluster together with IL-6, hence suggesting a common pathway of activation during disease's complication. Moreover, low levels of plasminogen strongly correlated with mortality in COVID-19 patients even after multiple adjustments for presence of confounding. These data suggest that plasminogen may play a pivotal role in controlling the complex mechanisms beyond the COVID-19 complications, and may be useful both as biomarker for prognosis and for therapeutic target against this extremely aggressive infection.

OTULIN in NF-κB signaling, cell death, and disease

Tight control of inflammatory signaling pathways is an absolute requirement to avoid chronic inflammation and disease. One of the proteins responsible for such control is OTU deubiquitinase with linear linkage specificity (OTULIN), the only mammalian deubiquitinating enzyme (DUB) exclusively hydrolyzing linear ubiquitin chains from proteins modified by the linear ubiquitin chain assembly complex (LUBAC) described thus far. Recent findings show that loss-of-function mutations in OTULIN underlie a severe early-onset human autoinflammatory disease and severe pathology in experimental mouse models. Here, we review the molecular and cellular mechanisms by which OTULIN controls inflammation and discuss the involvement of OTULIN in inflammatory disease development. We also highlight several newly identified roles for OTULIN, including a ubiquitin-independent function.

Deubiquitinases: Modulators of Different Types of Regulated Cell Death

The mechanisms and physiological implications of regulated cell death (RCD) have been extensively studied. Among the regulatory mechanisms of RCD, ubiquitination and deubiquitination enable post-translational regulation of signaling by modulating substrate degradation and signal transduction. Deubiquitinases (DUBs) are involved in diverse molecular pathways of RCD. Some DUBs modulate multiple modalities of RCD by regulating various substrates and are powerful regulators of cell fate. However, the therapeutic targeting of DUB is limited, as the physiological consequences of modulating DUBs cannot be predicted. In this review, the mechanisms of DUBs that regulate multiple types of RCD are summarized. This comprehensive summary aims to improve our understanding of the complex DUB/RCD regulatory axis comprising various molecular mechanisms for diverse physiological processes. Additionally, this review will enable the understanding of the advantages of therapeutic targeting of DUBs and developing strategies to overcome the side effects associated with the therapeutic applications of DUB modulators.

OTULIN is a new target of EA treatment in the alleviation of brain injury and glial cell activation via suppression of the NF-κB signalling pathway in acute ischaemic stroke rats

OBJECTIVE

Ovarian tumour domain deubiquitinase with linear linkage specificity (OTULIN) is a potent negative regulator of the nuclear factor-κB (NF-κB) signalling pathway, and it plays a strong neuroprotective role following acute ischemic stroke. Electroacupuncture (EA) is an effective adjuvant treatment for reducing brain injury and neuroinflammation via the inhibition of NF-κB p65 nuclear translocation, but the underlying mechanism is not clear. The present study investigated whether OTULIN was necessary for EA to mitigate brain injury and glial cell activation in a transient middle cerebral artery occlusion (tMCAO) model in rats.

METHODS

An acute ischaemic stroke model was established via tMCAO surgery in Sprague-Dawley (SD) rats. EA was performed once daily at "Baihui (GV 20)", "Hegu (LI 4)", and "Taichong (LR 3)" acupoints. The effect of EA on the spatiotemporal expression of OTULIN in the ischaemic penumbra of the cerebral cortex was detected within 7 days after reperfusion. The effects of OTULIN gene silencing on EA neurological deficits, cerebral infarct volume, neuronal damage, the activation of microglia and astrocytes, the contents of tumour necrosis factor alpha (TNF-α), interleukin-1 beta (IL-1β) and interleukin-6 (IL-6), and the expression of p-IκBa, IκBa and nucleus/cytoplasm NF-κB p65 protein were assessed.

RESULTS

EA treatment increased endogenous OTULIN expression, which peaked at 48 h. Enhanced OTULIN was primarily located in neurons, but a small amount of OTULIN was detected in microglia. OTULIN silencing obviously reversed EA neuroprotection, which was demonstrated by worsened neurobehavioural performance, cerebral infarct volume and neuronal injury. The inhibitory effect of EA on the NF-κB pathway was also attenuated by enhanced IκBα phosphorylation and NF-κB p65 nuclear translocation. EA partially inhibited the transformation of microglia and astrocytes from resting states to activated states and reduced the secretion of TNF-α, IL-1β and IL-6. However, these preventive effects were reversed after the silencing of OTULIN expression.

CONCLUSIONS

OTULIN provides a new potential therapeutic target for EA to alleviate acute ischaemic stroke-induced brain injury and the activation of glial cells, which are related to suppression of the NF-κB signalling pathway.

Molecular Mechanisms of DUBs Regulation in Signaling and Disease

The large family of deubiquitinating enzymes (DUBs) are involved in the regulation of a plethora of processes carried out inside the cell by protein ubiquitination. Ubiquitination is a basic pathway responsible for the correct protein homeostasis in the cell, which could regulate the fate of proteins through the ubiquitin–proteasome system (UPS). In this review we will focus on recent advances on the molecular mechanisms and specificities found for some types of DUBs enzymes, highlighting illustrative examples in which the regulatory mechanism for DUBs has been understood in depth at the molecular level by structural biology. DUB proteases are responsible for cleavage and regulation of the multiple types of ubiquitin linkages that can be synthesized inside the cell, known as the ubiquitin-code, which are tightly connected to specific substrate functions. We will display some strategies carried out by members of different DUB families to provide specificity on the cleavage of particular ubiquitin linkages. Finally, we will also discuss recent progress made for the development of drug compounds targeting DUB proteases, which are usually correlated to the progress of many pathologies such as cancer and neurodegenerative diseases.

Met1-linked ubiquitin signalling in health and disease: inflammation, immunity, cancer, and beyond

Post-translational modification of proteins with ubiquitin (ubiquitination) provides a rapid and versatile mechanism for regulating cellular signalling systems. Met1-linked (or 'linear') ubiquitin chains have emerged as a key regulatory signal that controls cell death, immune signalling, and other vital cellular functions. The molecular machinery that assembles, senses, and disassembles Met1-linked ubiquitin chains is highly specific. In recent years, the thorough biochemical and genetic characterisation of the enzymes and proteins of the Met1-linked ubiquitin signalling machinery has paved the way for substantial advances in our understanding of how Met1-linked ubiquitin chains control cell signalling and biology. Here, we review current knowledge and recent insights into the role of Met1-linked ubiquitin chains in cell signalling with an emphasis on their role in disease biology. Met1-linked ubiquitin has potent regulatory functions in immune signalling, NF-κB transcription factor activation, and cell death. Importantly, mounting evidence shows that dysregulation of Met1-linked ubiquitin signalling is associated with multiple human diseases, including immune disorders, cancer, and neurodegeneration. We discuss the latest evidence on the cellular function of Met1-linked ubiquitin in the context of its associated diseases and highlight new emerging roles of Met1-linked ubiquitin chains in cell signalling, including regulation of protein quality control and metabolism.

The Impact of Acute or Chronic Alcohol Intake on the NF-κB Signaling Pathway in Alcohol-Related Liver Disease

Ethanol misuse is frequently associated with a multitude of profound medical conditions, contributing to health-, individual- and social-related damage. A particularly dangerous threat from this classification is coined as alcoholic liver disease (ALD), a liver condition caused by prolonged alcohol overconsumption, involving several pathological stages induced by alcohol metabolic byproducts and sustained cellular intoxication. Molecular, pathological mechanisms of ALD principally root in the innate immunity system and are especially associated with enhanced functionality of the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway. NF-κB is an interesting and convoluted DNA transcription regulator, promoting both anti-inflammatory and pro-inflammatory gene expression. Thus, the abundancy of studies in recent years underlines the importance of NF-κB in inflammatory responses and the mechanistic stimulation of inner molecular motifs within the factor components. Hereby, in the following review, we would like to put emphasis on the correlation between the NF-κB inflammation signaling pathway and ALD progression. We will provide the reader with the current knowledge regarding the chronic and acute alcohol consumption patterns, the molecular mechanisms of ALD development, the involvement of the NF-κB pathway and its enzymatic regulators. Therefore, we review various experimental in vitro and in vivo studies regarding the research on ALD, including the recent active compound treatments and the genetic modification approach. Furthermore, our investigation covers a few human studies.

Ubiquitin-dependent and -independent functions of OTULIN in cell fate control and beyond

Ubiquitination, and its control by deubiquitinating enzymes (DUBs), mediates protein stability, function, signaling and cell fate. The ovarian tumor (OTU) family DUB OTULIN (FAM105B) exclusively cleaves linear (Met1-linked) poly-ubiquitin chains and plays important roles in auto-immunity, inflammation and infection. OTULIN regulates Met1-linked ubiquitination downstream of tumor necrosis factor receptor 1 (TNFR1), toll-like receptor (TLR) and nucleotide-binding and oligomerization domain-containing protein 2 (NOD2) receptor activation and interacts with the Met1 ubiquitin-specific linear ubiquitin chain assembly complex (LUBAC) E3 ligase. However, despite extensive research efforts, the receptor and cytosolic roles of OTULIN and the distributions of multiple Met1 ubiquitin-associated E3-DUB complexes in the regulation of cell fate still remain controversial and unclear. Apart from that, novel ubiquitin-independent OTULIN functions have emerged highlighting an even more complex role of OTULIN in cellular homeostasis. For example, OTULIN interferes with endosome-to-plasma membrane trafficking and the OTULIN-related pseudo-DUB OTULINL (FAM105A) resides at the endoplasmic reticulum (ER). Here, we discuss how OTULIN contributes to cell fate control and highlight novel ubiquitin-dependent and -independent functions.

The Balance of TNF Mediated Pathways Regulates Inflammatory Cell Death Signaling in Healthy and Diseased Tissues

Tumor necrosis factor alpha (TNF; TNFα) is a critical regulator of immune responses in healthy organisms and in disease. TNF is involved in the development and proper functioning of the immune system by mediating cell survival and cell death inducing signaling. TNF stimulated signaling pathways are tightly regulated by a series of phosphorylation and ubiquitination events, which enable timely association of TNF receptors-associated intracellular signaling complexes. Disruption of these signaling events can disturb the balance and the composition of signaling complexes, potentially resulting in severe inflammatory diseases.