USP18 recruits USP20 to promote innate antiviral response through deubiquitinating STING/MITA

USP18 deubiquitinates and stabilizes SOX9 to promote the stemness and malignant progression of glioblastoma

Glioblastoma (GBM), the most common and aggressive primary brain tumour, is associated with poor prognosis, primarily due to its stem-like subpopulation, glioblastoma stem cells (GSCs). The deubiquitinase (DUB) family has attracted an increasing amount of attention due to its roles in GSC biology and tumour aggressiveness. In this study, we focused on ubiquitin-specific peptidase 18 (USP18), a member of the DUB family whose role in GBM is poorly understood. Through integrated bioinformatics analyses and experimental investigations using patient-derived samples, cell models, and animal models, we elucidated the role of USP18 in enhancing GSC stemness and promoting malignant behaviours. Our findings revealed that USP18 expression is significantly elevated in GBM and is correlated with a poor prognosis. Mechanistically, USP18 interacts with SRY-box transcription factor 9 (SOX9), stabilising its protein levels by cleaving K48-linked polyubiquitin chains. Additionally, we identified YY1 as a transcriptional regulator of USP18, increasing its expression in GBM cells. These findings reveal that USP18 is a potential therapeutic target and highlight the novel YY1/USP18/SOX9 signalling axis implicated in GBM progression.

DNA-RNA hybrids in inflammation: sources, immune response, and therapeutic implications

Cytoplasmic DNA-RNA hybrids are emerging as important immunogenic nucleic acids, that were previously underappreciated. DNA-RNA hybrids, formed during cellular processes like transcription and replication, or by exogenous pathogens, are recognized by pattern recognition receptors (PRRs), including cGAS, DDX41, and TLR9, which trigger immune responses. Post-translational modifications (PTMs) including ubiquitination, phosphorylation, acetylation, and palmitoylation regulate the activity of PRRs and downstream signaling molecules, fine-tuning the immune response. Targeting enzymes involved in DNA-RNA hybrid metabolism and PTMs regulation offers therapeutic potential for inflammatory diseases. Herein, we discuss the sources, immune response, and therapeutic implications of DNA-RNA hybrids in inflammation, highlighting the significance of DNA-RNA hybrids as potential targets for the treatment of inflammation.

STING and metabolism-related diseases: Roles, mechanisms, and applications

The stimulator of interferon genes (STING) pathway plays a critical role in innate immunity, acting as a central mediator that links cytosolic DNA sensing to inflammatory signaling. STING not only responds to cellular metabolic states but also actively regulates key metabolic processes, including glycolysis, lipid metabolism, and redox balance. This bidirectional interaction underscores the existence of a dynamic feedback mechanism between STING signaling and metabolic pathways, which is essential for maintaining cellular homeostasis. This review provides a comprehensive analysis, beginning with an in-depth overview of the classical STING signaling pathway, followed by a detailed examination of its reciprocal regulation of various metabolic pathways. Additionally, it explores the role and mechanisms of STING signaling in metabolic disorders, including obesity, diabetes, and atherosclerosis. By integrating these insights into the mutual regulation between STING and its metabolism, novel therapeutic strategies targeting this pathway in metabolic diseases have been proposed.

GAMT facilitates tumor progression via inhibiting p53 in clear cell renal cell carcinoma

BACKGROUND

Clear cell renal cell carcinoma (ccRCC) is the most common type of RCC. Even though the targeted drugs for the treatment of ccRCC have a certain therapeutic effect, due to the problem of drug resistance, the search for new targets for targeted therapy of ccRCC remains urgent. GAMT is an enzyme involved in creatine metabolism. However, the precise biological roles and molecular mechanisms of GAMT in ccRCC are not fully understood.

RESULTS

Here, we found that GAMT was upregulated in ccRCC cells and tissues and associated with poor prognosis. Further, GAMT has pro-oncogenic abilities in promoting ccRCC development and progression. Intriguingly, GAMT exerted biological functions independent of its role in catalyzing creatine synthesis. Mechanistically, GAMT overexpression contributes to the development and progression of ccRCC by inhibiting tumor suppressor p53. Finally, we identified fisetin as a novel GAMT inhibitor and validated its role in suppressing ccRCC progression and sensitizing ccRCC cells to targeted drug axitinib via in vivo and in vitro assays.

CONCLUSIONS

This study reveals that GAMT has pro-oncogenic abilities in promoting ccRCC development and progression. GAMT exerted its non-enzymatic functions possibly by regulating the expression of p53. Fisetin, the novel GAMT inhibitor identified herein, may serve as a new antitumor drug for ccRCC treatment.

HnRNPC triggers the degradation of MITA to suppress the interferon-mediated antiviral response

Heterogeneous nuclear ribonucleoproteins (hnRNPs) are a group of 34-120 kDa nuclear proteins that have recently been reported to participate in virus replication. The hnRNP family contains approximately 20 members, including hnRNP A1, hnRNP A2, hnRNP A2B1, hnRNPC, hnRNPD and hnRNPK. HnRNPC plays important roles in RNA biology, including expression, stability, mRNA splicing, nonspecific sequence export and 3'-end processing; however, the mechanisms underlying hnRNPC regulatory roles are not fully understood. Here, we found that zebrafish hnRNPC promoted spring viraemia of carp virus (SVCV) replication by increasing the stability of SVCV phosphoprotein while inhibiting the K48-linked ubiquitination of virus phosphoprotein, thereby suppressing the type I interferon (IFN) response. Mechanistically, hnRNPC could interact with the mediator of IFN regulatory factor 3 activation (MITA) to activate K48-linked ubiquitination for MITA degradation through the C-terminal domain of hnRNPC. We also showed that human hnRNPC could interact with MITA and that the overexpression of human hnRNPC decreased MITA protein in HEK293 cells, suggesting that the negative regulatory effects of hnRNPC on the type I IFN response are evolutionarily conserved. Collectively, our data indicate that hnRNPC promotes virus replication by suppressing IFN production activated by MITA and increasing the availability of viral proteins. Our work reveals an evolutionarily conserved mechanism that controls the IFN-mediated antiviral response by a member of the hnRNP family in vertebrates.

KRAS4B oncogenic mutants promote non-small cell lung cancer progression via the interaction of deubiquitinase USP25 with RNF31

Kirsten rat sarcoma viral oncogene homolog (KRAS) oncogenic mutations are genetic drivers in various cancers, including non-small cell lung cancer (NSCLC). However, the regulatory mechanisms underlying the progression of NSCLC driven by oncogenic KRAS mutants are incompletely understood. Here, we show that ubiquitin specific peptidase 25 (USP25) impedes ring finger protein 31 (RNF31)-mediated linear ubiquitination of KRAS oncogenic mutants (KRASmuts) independently of its deubiquitinase activity, which facilitates the plasma membrane (PM) localization and the downstream oncogenic signaling of KRASmuts. Importantly, knockout (KO) of USP25 effectively suppresses tumor growth and RAS signaling in KRASmuts-driven autochthonous NSCLC mouse models and xenograft models, which is restored by additional deletion or inhibition of RNF31. Notably, knockin of USP25C178A in KRasG12D-driven NSCLC models fails to inhibit cancer progression and reconstitution of USP25C178A into USP25 KO A549 cells restores tumor growth. These findings identify previously uncharacterized roles of USP25 and RNF31 in oncogenic KRAS-driven NSCLC progression and provide potential therapeutic targets for KRASmuts-related cancers.

A subset of neutrophils activates anti-tumor immunity and inhibits non-small-cell lung cancer progression

Neutrophils in the tumor microenvironment (TME) are heterogeneous populations associated with cancer prognosis and immunotherapy. However, the plasticity and function of heterogeneous neutrophils in the TME of non-small-cell lung cancer (NSCLC) remain unclear. Here, we show that neutrophils produce high levels of interleukin (IL)-8, which induce the differentiation of CD74highSiglecFlow neutrophils and suppress the generation of CD74lowSiglecFhigh neutrophils in the TME of IL-8-humanized NSCLC mice. The CD74highSiglecFlow neutrophils boost anti-tumor T cell responses via antigen cross-presentation. Deleting CD74 in IL-8-humanized neutrophils impairs T cell activation and exacerbates NSCLC progression, whereas a CD74 agonist enhances T cell activation and the efficacy of anti-programmed cell death 1 (PD-1) or osimertinib therapies. Additionally, the CD74highCD63low neutrophils in the TME and peripheral blood of advanced NSCLC patients phenocopy the CD74highSiglecFlow neutrophils in the TME of NSCLC mice and correlate well with the responsiveness to anti-PD-1 plus chemotherapies. These findings demonstrate an IL-8-CD74high neutrophil axis that promotes anti-tumor immunity in NSCLC.

PBLD enhances antiviral innate immunity by promoting the p53-USP4-MAVS signaling axis

Phenazine biosynthesis-like domain-containing protein (PBLD) has been reported to be involved in the development of many cancers. However, whether PBLD regulates innate immune responses and viral replication is unclear. In this study, although it was found that the activity of PBLD extends to other PRRs, we focused on the RLR pathway activated via the p53-USP4-MAVS axis in response to virus infections. We found that PBLD deubiquitinates and stabilizes MAVS through ubiquitin-specific protease 4 (USP4) to promote antiviral innate immunity. Mechanistically, PBLD activates the transcription of USP4 via the upregulation of p53. USP4, which is a MAVS-interacting protein, substantially stabilizes the MAVS protein by deconjugating K48-linked ubiquitination chains from the MAVS protein at Lys461 during RNA virus infection. Most intriguingly, RNA virus-infected primary macrophages (peritoneal macrophages, PMs, and bone marrow-derived macrophages, BMDMs) and internal organ cells (lung and liver) from PBLD-deficient mice suppress the IFN-I response and promote viral replication. Notably, PBLD-deficient mice are more susceptible to RNA virus infection than their wild-type littermates. Our findings highlight a unique function of PBLD in antiviral innate immunity through the p53-USP4-MAVS signaling, providing a preliminary basis for research on PBLD as a target molecule for treating RNA virus infection.

The role of the cGAS-STING signaling pathway in viral infections, inflammatory and autoimmune diseases

Pattern recognition receptors are an essential part of the immune system, which detect pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) and help shape both innate and adaptive immune responses. When dsDNA is present, cyclic GMP-AMP Synthase (cGAS) produces a second messenger called cyclic GMP-AMP (cGAMP), which then triggers an adaptor protein called STING, and eventually activates the expression of type I interferon (IFN) and pro-inflammatory cytokines in immune cells. The cGAS-STING signaling pathway has been receiving a lot of attention lately as a key immune-surveillance mediator. In this review, we summarize the present circumstances of the cGAS-STING signaling pathway in viral infections and inflammatory diseases, as well as autoimmune diseases. Modulation of the cGAS-STING signaling pathway provides potential strategies for treating viral infections, inflammatory diseases, and autoimmune diseases.

The deubiquitinase BAP1 and E3 ligase UBE3C sequentially target IRF3 to activate and resolve the antiviral innate immune response

Ubiquitination is essential for the proteasomal turnover of IRF3, the central factor mediating the antiviral innate immune response. However, the spatiotemporal regulation of IRF3 ubiquitination for the precise activation and timely resolution of innate immunity remains unclear. Here, we identified BRCA1-associated protein-1 (BAP1) and ubiquitin-protein ligase E3C (UBE3C) as the key deubiquitinase and ubiquitinase for temporal control of IRF3 stability during viral infection. In the early stage, BAP1 dominates and removes K48-linked ubiquitination of IRF3 in the nucleus, preventing its proteasomal degradation and facilitating efficient interferon (IFN)-β production. In the late stage, E3 ligase UBE3C, induced by IFN-β, specifically mediates IRF3 ubiquitination and promotes its proteasomal degradation. Overall, the sequential interactions with BAP1 and UBE3C govern IRF3 stability during innate response, ensuring effective viral clearance and inflammation resolution. Our findings provide insights into the temporal control of innate signaling and suggest potential interventions in viral infection.

Polydatin attenuates diabetic renal inflammatory fibrosis via the inhibition of STING pathway

Diabetic nephropathy (DN) is a complication of diabetes and is mainly characterized by renal fibrosis, which could be attributed to chronic kidney inflammation. Stimulator of interferon genes (STING), a linker between immunity and metabolism, could ameliorate various metabolic and inflammatory diseases. However, the regulatory role of STING in DN remains largely unexplored. In this study, knockdown of STING decreased extracellular matrix (ECM), pro-inflammatory, and fibrotic factors in high glucose (HG)-induced glomerular mesangial cells (GMCs), whereas overexpression of STING triggered the inflammatory fibrosis process, suggesting that STING was a potential target for DN. Polydatin (PD) is a glucoside of resveratrol and has been reported to ameliorate DN by inhibiting inflammatory responses. Nevertheless, whether PD improved DN via STING remains unclear. Here, transcriptomic profiling implied that the STING/NF-κB pathway might be an important target for PD. We further found that PD decreased the protein expression of STING, and subsequently suppressed the activation of downstream targets including TBK1 phosphorylation and NF-κB nuclear translocation, and eventually inhibited the production of ECM, pro-inflammatory and fibrotic factors in HG-induced GMCs. Notably, results of molecular docking, molecular dynamic simulations, surface plasmon resonance, cellular thermal shift assay and Co-immunoprecipitation assay indicated that PD directly bound to STING and restored the declined proteasome-mediated degradation of STING induced by HG. In diabetic mice, PD also inhibited the STING pathway and improved the pathological changes of renal inflammatory fibrosis. Our study elucidated the regulatory role of STING in DN, and the novel mechanism of PD treating DN via inhibiting STING expression.

USP20 deubiquitinates and stabilizes the reticulophagy receptor RETREG1/FAM134B to drive reticulophagy

The endoplasmic reticulum (ER) serves as a hub for various cellular processes, and maintaining ER homeostasis is essential for cell function. Reticulophagy is a selective process that removes impaired ER subdomains through autophagy-mediatedlysosomal degradation. While the involvement of ubiquitination in autophagy regulation is well-established, its role in reticulophagy remains unclear. In this study, we screened deubiquitinating enzymes (DUBs) involved in reticulophagy and identified USP20 (ubiquitin specific peptidase 20) as a key regulator of reticulophagy under starvation conditions. USP20 specifically cleaves K48- and K63-linked ubiquitin chains on the reticulophagy receptor RETREG1/FAM134B (reticulophagy regulator 1), thereby stabilizing the substrate and promoting reticulophagy. Remarkably, despite lacking a transmembrane domain, USP20 is recruited to the ER through its interaction with VAPs (VAMP associated proteins). VAPs facilitate the recruitment of early autophagy proteins, including WIPI2 (WD repeat domain, phosphoinositide interacting 2), to specific ER subdomains, where USP20 and RETREG1 are enriched. The recruitment of WIPI2 and other proteins in this process plays a crucial role in facilitating RETREG1-mediated reticulophagy in response to nutrient deprivation. These findings highlight the critical role of USP20 in maintaining ER homeostasis by deubiquitinating and stabilizing RETREG1 at distinct ER subdomains, where USP20 further recruits VAPs and promotes efficient reticulophagy.Abbreviations: ACTB actin beta; ADRB2 adrenoceptor beta 2; AMFR/gp78 autocrine motility factor receptor; ATG autophagy related; ATL3 atlastin GTPase 3; BafA1 bafilomycin A1; BECN1 beclin 1; CALCOCO1 calcium binding and coiled-coil domain 1; CCPG1 cell cycle progression 1; DAPI 4',6-diamidino-2-phenylindole; DTT dithiothreitol; DUB deubiquitinating enzyme; EBSS Earle's Balanced Salt Solution; FFAT two phenylalanines (FF) in an acidic tract; GABARAP GABA type A receptor-associated protein; GFP green fluorescent protein; HMGCR 3-hydroxy-3-methylglutaryl-CoA reductase; IL1B interleukin 1 beta; LIR LC3-interacting region; MAP1LC3/LC3 microtubule associated protein 1 light chain 3; PIK3C3/Vps34 phosphatidylinositol 3-kinase catalytic subunit type 3; RB1CC1/FIP200 RB1 inducible coiled-coil 1; RETREG1/FAM134B reticulophagy regulator 1; RFP red fluorescent protein; RHD reticulon homology domain; RIPK1 receptor interacting serine/threonine kinase 1; RTN3L reticulon 3 long isoform; SEC61B SEC61 translocon subunit beta; SEC62 SEC62 homolog, preprotein translocation factor; SIM super-resolution structured illumination microscopy; SNAI2 snail family transcriptional repressor 2; SQSTM1/p62 sequestosome 1; STING1/MITA stimulator of interferon response cGAMP interactor 1; STX17 syntaxin 17; TEX264 testis expressed 264, ER-phagy receptor; TNF tumor necrosis factor; UB ubiquitin; ULK1 unc-51 like autophagy activating kinase 1; USP20 ubiquitin specific peptidase 20; USP33 ubiquitin specific peptidase 33; VAMP8 vesicle associated membrane protein 8; VAPs VAMP associated proteins; VMP1 vacuole membrane protein 1; WIPI2 WD repeat domain, phosphoinositide interacting 2; ZFYVE1/DFCP1 zinc finger FYVE-type containing 1.

USP11 Exacerbates Radiation-Induced Pneumonitis by Activating Endothelial Cell Inflammatory Response via OTUD5-STING Signaling

PURPOSE

Radiation-induced pneumonitis (RIP) seriously limits the application of radiation therapy in the treatment of thoracic tumors, and its etiology and pathogenesis remain elusive. This study aimed to elucidate the role of ubiquitin-specific peptidase 11 (USP11) in the progression of RIP and the associated underlying mechanisms.

METHODS AND MATERIALS

Changes in cytokines and infiltrated immune cells were detected by enzyme-linked immunosorbent assays and immunohistochemistry after exposure to 20 Gy x-ray with whole-thorax irradiation. The effects of USP11 expression on endothelial cell proliferation and apoptosis were analyzed by costaining of CD31/Ki67 and CD31/caspase-3 in vivo, and the production of cytokines and reactive oxygen species was confirmed by reverse-transcription polymerase chain reaction and flow cytometry in vitro. Comprehensive proteome and ubiquitinome analyses were used for USP11 substrate screening after radiation. Results were verified by Western blotting and coimmunoprecipitation experiments. Recombinant adeno-associated virus lung vectors expressing OTUD5 were used for localized overexpression of OTUD5 in mouse pulmonary tissue, and immunohistochemistry was conducted to analyze cytokine expression.

RESULTS

The progression of RIP was significantly alleviated by reduced expression of proinflammatory cytokines in both Usp11-knockout (Usp11-/-) mice and in mice treated with the USP11 inhibitor mitoxantrone. Likewise, the absence of USP11 resulted in decreased permeability of pulmonary vessels and neutrophils and macrophage infiltration. The proliferation rates of endothelial cells were prominently increased in the Usp11-/- lung, whereas apoptosis in Usp11-/- lungs decreased after irradiation compared with that observed in Usp11+/+ lungs. Conversely, USP11 overexpression increased proinflammatory cytokine expression and reactive oxygen species production in endothelial cells after radiation. Comprehensive proteome and ubiquitinome analyses indicated that USP11 overexpression upregulates the expression of several deubiquitinating enzymes, including USP22, USP33, and OTUD5. We demonstrate that USP11 deubiquitinates OTUD5 and implicates the OTUD5-STING signaling pathway in the progression of the inflammatory response in endothelial cells.

CONCLUSIONS

USP11 exacerbates RIP by triggering an inflammatory response in endothelial cells both in vitro and in vivo, and the OTUD5-STING pathway is involved in the USP11-dependent promotion of RIP. This study provides experimental support for the development of precision intervention strategies targeting USP11 to mitigate RIP.

The CRL3KCTD10 ubiquitin ligase-USP18 axis coordinately regulates cystine uptake and ferroptosis by modulating SLC7A11

SLC7A11 is a cystine transporter and ferroptosis inhibitor. How the stability of SLC7A11 is coordinately regulated in response to environmental cystine by which E3 ligase and deubiquitylase (DUB) remains elusive. Here, we report that neddylation inhibitor MLN4924 increases cystine uptake by causing SLC7A11 accumulation, via inactivating Cullin-RING ligase-3 (CRL-3). We identified KCTD10 as the substrate-recognizing subunit of CRL-3 for SLC7A11 ubiquitylation, and USP18 as SLC7A11 deubiquitylase. Upon cystine deprivation, the protein levels of KCTD10 or USP18 are decreased or increased, respectively, contributing to SLC7A11 accumulation. By destabilizing or stabilizing SLC7A11, KCTD10, or USP18 inversely regulates the cystine uptake and ferroptosis. Biologically, MLN4924 combination with SLC7A11 inhibitor Imidazole Ketone Erastin (IKE) enhanced suppression of tumor growth. In human breast tumor tissues, SLC7A11 levels were negatively or positively correlated with KCTD10 or USP18, respectively. Collectively, our study defines how SLC7A11 and ferroptosis is coordinately regulated by the CRL3KCTD10/E3-USP18/DUB axis, and provides a sound rationale of drug combination to enhance anticancer efficacy.

The roles of nuclear orphan receptor NR2F6 in anti-viral innate immunity

Proper transcription regulation by key transcription factors, such as IRF3, is critical for anti-viral defense. Dynamics of enhancer activity play important roles in many biological processes, and epigenomic analysis is used to determine the involved enhancers and transcription factors. To determine new transcription factors in anti-DNA-virus response, we have performed H3K27ac ChIP-Seq and identified three transcription factors, NR2F6, MEF2D and MAFF, in promoting HSV-1 replication. NR2F6 promotes HSV-1 replication and gene expression in vitro and in vivo, but not dependent on cGAS/STING pathway. NR2F6 binds to the promoter of MAP3K5 and activates AP-1/c-Jun pathway, which is critical for DNA virus replication. On the other hand, NR2F6 is transcriptionally repressed by c-Jun and forms a negative feedback loop. Meanwhile, cGAS/STING innate immunity signaling represses NR2F6 through STAT3. Taken together, we have identified new transcription factors and revealed the underlying mechanisms involved in the network between DNA viruses and host cells.

USP18 Antagonizes Pyroptosis by Facilitating Selective Autophagic Degradation of Gasdermin D

As a key executioner of pyroptosis, Gasdermin D (GSDMD) plays a crucial role in host defense and emerges as an essential therapeutic target in the treatment of inflammatory diseases. So far, the understanding of the mechanisms that regulate the protein level of GSDMD to prevent detrimental effects and maintain homeostasis is currently limited. Here, we unveil that ubiquitin-specific peptidase 18 (USP18) works as a negative regulator of pyroptosis by targeting GSDMD for degradation and preventing excessive innate immune responses. Mechanically, USP18 recruits E3 ubiquitin ligase mind bomb homolog 2 (MIB2) to catalyze ubiquitination on GSDMD at lysine (K) 168, which acts as a recognition signal for the selective autophagic degradation of GSDMD. We further confirm the alleviating effect of USP18 on LPS-triggered inflammation in vivo. Collectively, our study demonstrates the role of USP18 in regulating GSDMD-mediated pyroptosis and reveals a previously unknown mechanism by which GSDMD protein level is rigorously controlled by selective autophagy.

Viral infection and spread are inhibited by the polyubiquitination and downregulation of TRPV2 channel by the interferon-stimulated gene TRIM21

Interferon (IFN) contributes to the host's antiviral response by inducing IFN-stimulated genes (ISGs). However, their functional targets and the mechanism of action remain elusive. Here, we report that one such ISG, TRIM21, interacts with and degrades the TRPV2 channel in myeloid cells, reducing its expression and providing host protection against viral infections. Moreover, viral infection upregulates TRIM21 in paracrine and autocrine manners, downregulating TRPV2 in neighboring cells to prevent viral spread to uninfected cells. Consistently, the Trim21-/- mice are more susceptible to HSV-1 and VSV infection than the Trim21+/+ littermates, in which viral susceptibility is rescued by inhibition or deletion of TRPV2. Mechanistically, TRIM21 catalyzes the K48-linked ubiquitination of TRPV2 at Lys295. TRPV2K295R is resistant to viral-infection-induced TRIM21-dependent ubiquitination and degradation, promoting viral infection more profoundly than wild-type TRPV2 when reconstituted into Lyz2-Cre;Trpv2fl/fl myeloid cells. These findings characterize targeting the TRIM21-TRPV2 axis as a conducive strategy to control viral spread to bystander cells.

cGAS-STING, an important signaling pathway in diseases and their therapy

Since cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway was discovered in 2013, great progress has been made to elucidate the origin, function, and regulating mechanism of cGAS-STING signaling pathway in the past decade. Meanwhile, the triggering and transduction mechanisms have been continuously illuminated. cGAS-STING plays a key role in human diseases, particularly DNA-triggered inflammatory diseases, making it a potentially effective therapeutic target for inflammation-related diseases. Here, we aim to summarize the ancient origin of the cGAS-STING defense mechanism, as well as the triggers, transduction, and regulating mechanisms of the cGAS-STING. We will also focus on the important roles of cGAS-STING signal under pathological conditions, such as infections, cancers, autoimmune diseases, neurological diseases, and visceral inflammations, and review the progress in drug development targeting cGAS-STING signaling pathway. The main directions and potential obstacles in the regulating mechanism research and therapeutic drug development of the cGAS-STING signaling pathway for inflammatory diseases and cancers will be discussed. These research advancements expand our understanding of cGAS-STING, provide a theoretical basis for further exploration of the roles of cGAS-STING in diseases, and open up new strategies for targeting cGAS-STING as a promising therapeutic intervention in multiple diseases.

Deubiquitinase OTUD6a drives cardiac inflammation and hypertrophy by deubiquitination of STING

BACKGROUND

Cardiac hypertrophy is a crucial pathological characteristic of hypertensive heart disease and subsequent heart failure. Deubiquitinating enzymes (DUBs) have been found to be involved in the regulation of myocardial hypertrophy. OTU Domain-Containing Protein 6a (OTUD6a) is a recently identified DUB. To date, the potential role of OTUD6a in myocardial hypertrophy has not yet been revealed.

METHODS AND RESULTS

We examined the up-regulated level of OTUD6a in mouse or human hypertrophic heart tissues. Then, transverse aortic constriction (TAC)- or angiotensin II (Ang II)- induced ventricular hypertrophy and dysfunction were significantly attenuated in OTUD6a gene knockout mice (OTUD6a-/-). In mechanism, we identified that the Stimulator of Interferon Genes (STING) is a direct substrate protein of OTUD6a via immunoprecipitation assay and mass spectrometry. OTUD6a maintains STING stability via clearing the K48-linked ubiquitin in cardiomyocytes. Subsequently, OTUD6a regulates the STING-downstream NF-κB signaling activation and inflammatory gene expression both in vivo and in vitro. Inhibition of STING blocked OTUD6a overexpression-induced inflammatory and hypertrophic responses in cardiomyocytes.

CONCLUSION

This finding extends our understanding of the detrimental role of OTUD6a in myocardial hypertrophy and identifies STING as a deubiquinating substrate of OTUD6a, indicating that targeting OTUD6a could be a potential strategy for the treatment of cardiac hypertrophy.

HERC5-catalyzed ISGylation potentiates cGAS-mediated innate immunity

The cytosolic DNA sensor cyclic GMP-AMP synthase (cGAS) is essential to elicit type I interferon cascade response; thus, the activity of cGAS must be strictly regulated to boost the antiviral innate immunity. Here, we report that cGAS is responsible for the DNA-induced ISG15 conjugation system. The E3 HERC5 catalyzes the ISGylation of cytoplasmic cGAS at lysine 21, 187, 219, and 458, whereas Ubl carboxy-terminal hydrolase 18 removes the ISGylation of cGAS. The interaction of cGAS and HERC5 depends on the cGAS C-terminal domain and the RRC1-4 and RRC1-5 domains of HERC5. Mechanically, HERC5-catalyzed ISGylation promotes DNA-induced cGAS oligomerization and enhances cGAS enzymatic activity. Deficiency of ISGylation attenuates the downstream inflammatory gene expression induced by the cGAS-STING axis and the antiviral ability in mouse and human cells. Mice deficient in Isg15 or Herc6 are more vulnerable to herpes simplex virus 1 infection. Collectively, our study shows a positive feedback regulation of the cGAS-mediated innate immune pathway by ISGylation.

The ISG15-Protease USP18 Is a Pleiotropic Enhancer of HIV-1 Replication

The innate immune response to viruses is formed in part by interferon (IFN)-induced restriction factors, including ISG15, p21, and SAMHD1. IFN production can be blocked by the ISG15-specific protease USP18. HIV-1 has evolved to circumvent host immune surveillance. This mechanism might involve USP18. In our recent studies, we demonstrate that HIV-1 infection induces USP18, which dramatically enhances HIV-1 replication by abrogating the antiviral function of p21. USP18 downregulates p21 by accumulating misfolded dominant negative p53, which inactivates wild-type p53 transactivation, leading to the upregulation of key enzymes involved in de novo dNTP biosynthesis pathways and inactivated SAMHD1. Despite the USP18-mediated increase in HIV-1 DNA in infected cells, it is intriguing to note that the cGAS-STING-mediated sensing of the viral DNA is abrogated. Indeed, the expression of USP18 or knockout of ISG15 inhibits the sensing of HIV-1. We demonstrate that STING is ISGylated at residues K224, K236, K289, K347, K338, and K370. The inhibition of STING K289-linked ISGylation suppresses its oligomerization and IFN induction. We propose that human USP18 is a novel factor that potentially contributes in multiple ways to HIV-1 replication.

EFHD2 cooperates with E3 ubiquitin ligase Smurf1 to facilitate virus infection by promoting the degradation of TRAF6 in teleost fish

The ubiquitin-proteasome system is one of the most important protein stability regulation systems. It can precisely regulate host immune responses by targeting signaling proteins. TRAF6 is a crucial E3 ubiquitin ligase in host antiviral signaling pathway. Here, we discovered that EF-hand domain-containing protein D2 (EFHD2) collaborated with the E3 ubiquitin ligase Smurf1 to potentiate the degradation of TRAF6, hence facilitating RNA virus Siniperca chuatsi rhabdovirus infection. The mechanism analysis revealed that EFHD2 interacted with Smurf1 and enhanced its protein stability by impairing K48-linked polyubiquitination of Smurf1, thereby promoting Smurf1-catalyzed degradation of TRAF6. This study initially demonstrated a novel mechanism by which viruses utilize host EFHD2 to achieve immune escape and provided a new perspective on the exploration of mammalian innate immunity.IMPORTANCEViruses induce host cells to activate several antiviral signaling pathways. TNF receptor-associated factor 6 (TRAF6) plays an essential role in these pathways. Numerous studies have been done on the mechanisms of TRAF6-mediated resistance to viral invasion. However, little is known about the strategies that viruses employ to antagonize TRAF6-mediated antiviral signaling pathway. Here, we discovered that EFHD2 functions as a host factor to promote viral replication. Mechanistically, EFHD2 potentiates Smurf1 to catalyze the ubiquitin-proteasomal degradation of TRAF6 by promoting the deubiquitination and stability of Smurf1, which in turn inhibits the production of proinflammatory cytokines and interferons. Our study also provides a new perspective on mammalian resistance to viral invasion.

Non-canonical regulation of the reactivation of an oncogenic herpesvirus by the OTUD4-USP7 deubiquitinases

Deubiquitinases (DUBs) remove ubiquitin from substrates and play crucial roles in diverse biological processes. However, our understanding of deubiquitination in viral replication remains limited. Employing an oncogenic human herpesvirus Kaposi's sarcoma-associated herpesvirus (KSHV) to probe the role of protein deubiquitination, we found that Ovarian tumor family deubiquitinase 4 (OTUD4) promotes KSHV reactivation. OTUD4 interacts with the replication and transcription activator (K-RTA), a key transcription factor that controls KSHV reactivation, and enhances K-RTA stability by promoting its deubiquitination. Notably, the DUB activity of OTUD4 is not required for K-RTA stabilization; instead, OTUD4 functions as an adaptor protein to recruit another DUB, USP7, to deubiquitinate K-RTA and facilitate KSHV lytic reactivation. Our study has revealed a novel mechanism whereby KSHV hijacks OTUD4-USP7 deubiquitinases to promote lytic reactivation, which could be potentially harnessed for the development of new antiviral therapies.

MARCH5 promotes STING pathway activation by suppressing polymer formation of oxidized STING

Stimulator of interferon genes (STING) is a core DNA sensing adaptor in innate immune signaling. STING activity is regulated by a variety of post-translational modifications (PTMs), including phosphorylation, ubiquitination, sumoylation, palmitoylation, and oxidation, as well as the balance between active and inactive polymer formation. It remains unclear, though, how different PTMs and higher order structures cooperate to regulate STING activity. Here, we report that the mitochondrial ubiquitin ligase MARCH5 (Membrane Associated Ring-CH-type Finger 5, also known as MITOL) ubiquitinates STING and enhances its activation. A long-term MARCH5 deficiency, in contrast, leads to the production of reactive oxygen species, which then facilitate the formation of inactive STING polymers by oxidizing mouse STING cysteine 205. We show that MARCH5-mediated ubiquitination of STING prevents the oxidation-induced STING polymer formation. Our findings highlight that MARCH5 balances STING ubiquitination and polymer formation and its control of STING activation is contingent on oxidative conditions.

Mitochondrial DNA-triggered innate immune response: mechanisms and diseases

Various cellular stress conditions trigger mitochondrial DNA (mtDNA) release from mitochondria into the cytosol. The released mtDNA is sensed by the cGAS-MITA/STING pathway, resulting in the induced expression of type I interferon and other effector genes. These processes contribute to the innate immune response to viral infection and other stress factors. The deregulation of these processes causes autoimmune diseases, inflammatory metabolic disorders and cancer. Therefore, the cGAS-MITA/STING pathway is a potential target for intervention in infectious, inflammatory and autoimmune diseases as well as cancer. In this review, we focus on the mechanisms underlying the mtDNA-triggered activation of the cGAS-MITA/STING pathway, the effects of the pathway under various physiological and pathological conditions, and advances in the development of drugs that target cGAS and MITA/STING.

Deubiquitylating Enzymes in Cancer and Immunity

Deubiquitylating enzymes (DUBs) maintain relative homeostasis of the cellular ubiquitome by removing the post-translational modification ubiquitin moiety from substrates. Numerous DUBs have been demonstrated specificity for cleaving a certain type of ubiquitin linkage or positions within ubiquitin chains. Moreover, several DUBs perform functions through specific protein-protein interactions in a catalytically independent manner, which further expands the versatility and complexity of DUBs' functions. Dysregulation of DUBs disrupts the dynamic equilibrium of ubiquitome and causes various diseases, especially cancer and immune disorders. This review summarizes the Janus-faced roles of DUBs in cancer including proteasomal degradation, DNA repair, apoptosis, and tumor metastasis, as well as in immunity involving innate immune receptor signaling and inflammatory and autoimmune disorders. The prospects and challenges for the clinical development of DUB inhibitors are further discussed. The review provides a comprehensive understanding of the multi-faced roles of DUBs in cancer and immunity.

ATR-dependent ubiquitin-specific protease 20 phosphorylation confers oxaliplatin and ferroptosis resistance

Oxaliplatin (OXA) resistance is a major clinic challenge in hepatocellular carcinoma (HCC). Ferroptosis is a kind of iron-dependent cell death. Triggering ferroptosis is considered to restore sensitivity to chemotherapy. In the present study, we found that USP20 was overexpressed in OXA-resistant HCC cells. High expression of USP20 in HCC was associated with poor prognosis. USP20 contributes OXA resistance and suppress ferroptosis in HCC. Pharmacological inhibition or knockdown of USP20 triggered ferroptosis and increased the sensitivity of HCC cells to OXA both in vitro and in vivo. Coimmunoprecipitation results revealed that the UCH domain of USP20 interacted with the N terminal of SLC7A11. USP20 stabilized SLC7A11 via removing K48-linked polyubiquitination of SLC7A11 protein at K30 and K37. Most importantly, DNA damage-induced ATR activation was required for Ser132 and Ser368 phosphorylation of USP20. USP20 phosphorylation at Ser132 and Ser368 enhanced its stability and thus conferred OXA and ferroptosis resistance of HCC cells. Our study reveals a previously undiscovered association between OXA and ferroptosis and provides new insight into mechanisms regarding how DNA damage therapies always lead to therapeutic resistance. Therefore, targeting USP20 may mitigate the development of drug resistance and promote ferroptosis of HCC in patients receiving chemotherapy.

SIX4 Controls Anti-PD-1 Efficacy by Regulating STING Expression

The cGAS/STING cytosolic DNA-sensing pathway plays a significant role in antitumor immunity. Expression of STING is tightly regulated and commonly reduced or defective in many types of cancer. We have identified SIX4 as a significant regulator of STING expression in colon cancer cells. We showed that knockout of SIX4 decreased STING expression at the mRNA and protein levels while ectopic expression of SIX4 increased STING expression. Depletion of SIX4 led to attenuated STING activation and downstream signaling. Reexpression of SIX4 or ectopic expression of STING in SIX4 knockout cells reversed the effect. Ectopic expression of SIX4 enhanced DMXAA and cGAMP-induced STING activation and downstream signaling. Importantly, decrease of SIX4 expression substantially decreased tumor infiltration of CD8+ T cells and reduced the efficacy of PD-1 antibodies to diminish tumor growth in immune competent mice in vivo. Finally, analysis of The Cancer Genome Atlas colon cancer dataset indicated that tumors with high SIX4 expression were significantly enriched in the Inflammatory Response pathway. SIX4 expression also correlated with expression of multiple IFN-stimulated genes, inflammatory cytokines, and CD8A. Taken together, our results implicate that SIX4 is a principal regulator of STING expression in colon cancer cells, providing an additional mechanism and genetic marker to predict effective immune checkpoint blockade therapy responses.

SIGNIFICANCE

Our studies demonstrate that SIX4 is an important regulator of STING expression, providing a genetic marker or a therapeutic target to predict or enhance immune checkpoint blockade therapy responses in colon cancer.

USP18 enhances dengue virus replication by regulating mitochondrial DNA release

Dengue virus (DENV) infection remains a challenging health threat worldwide. Ubiquitin-specific protease 18 (USP18), which preserves the anti-interferon (IFN) effect, is an ideal target through which DENV mediates its own immune evasion. However, much of the function and mechanism of USP18 in regulating DENV replication remains incompletely understood. In addition, whether USP18 regulates DENV replication merely by causing IFN hyporesponsiveness is not clear. In the present study, by using several different approaches to block IFN signaling, including IFN neutralizing antibodies (Abs), anti-IFN receptor Abs, Janus kinase inhibitors and IFN alpha and beta receptor subunit 1 (IFNAR1)knockout cells, we showed that USP18 may regulate DENV replication in IFN-associated and IFN-unassociated manners. Localized in mitochondria, USP18 regulated the release of mitochondrial DNA (mtDNA) to the cytosol to affect viral replication, and mechanisms such as mitochondrial reactive oxygen species (mtROS) production, changes in mitochondrial membrane potential, mobilization of calcium into mitochondria, 8-oxoguanine DNA glycosylase 1 (OGG1) expression, oxidation and fragmentation of mtDNA, and opening of the mitochondrial permeability transition pore (mPTP) were involved in USP18-regulated mtDNA release to the cytosol. We therefore identify mitochondrial machineries that are regulated by USP18 to affect DENV replication and its association with IFN effects.

Type I/type III IFN and related factors regulate JEV infection and BBB endothelial integrity

BACKGROUND

Japanese encephalitis virus (JEV) remains a predominant cause of Japanese encephalitis (JE) globally. Its infection is usually accompanied by disrupted blood‒brain barrier (BBB) integrity and central nervous system (CNS) inflammation in a poorly understood pathogenesis. Productive JEV infection in brain microvascular endothelial cells (BMECs) is considered the initial event of the virus in penetrating the BBB. Type I/III IFN and related factors have been described as negative regulators in CNS inflammation, whereas their role in JE remains ambiguous.

METHODS

RNA-sequencing profiling (RNA-seq), real-time quantitative PCR, enzyme-linked immunosorbent assay, and Western blotting analysis were performed to analyze the gene and protein expression changes between mock- and JEV-infected hBMECs. Bioinformatic tools were used to cluster altered signaling pathway members during JEV infection. The shRNA-mediated immune factor-knockdown hBMECs and the in vitro transwell BBB model were utilized to explore the interrelation between immune factors, as well as between immune factors and BBB endothelial integrity.

RESULTS

RNA-Seq data of JEV-infected hBMECs identified 417, 1256, and 2748 differentially expressed genes (DEGs) at 12, 36, and 72 h post-infection (hpi), respectively. The altered genes clustered into distinct pathways in gene ontology (GO) terms and KEGG pathway enrichment analysis, including host antiviral immune defense and endothelial cell leakage. Further investigation revealed that pattern-recognition receptors (PRRs, including TLR3, RIG-I, and MDA5) sensed JEV and initiated IRF/IFN signaling. IFNs triggered the expression of interferon-induced proteins with tetratricopeptide repeats (IFITs) via the JAK/STAT pathway. Distinct PRRs exert different functions in barrier homeostasis, while treatment with IFN (IFN-β and IFN-λ1) in hBMECs stabilizes the endothelial barrier by alleviating exogenous destruction. Despite the complex interrelationship, IFITs are considered nonessential in the IFN-mediated maintenance of hBMEC barrier integrity.

CONCLUSIONS

This research provided the first comprehensive description of the molecular mechanisms of host‒pathogen interplay in hBMECs responding to JEV invasion, in which type I/III IFN and related factors strongly correlated with regulating the hBMEC barrier and restricting JEV infection. This might help with developing an attractive therapeutic strategy in JE.

Tumor-promoting action of ubiquitin protease 43 in gastric cancer progression through deubiquitination and stabilization of stress-inducible phosphoprotein 1

Gastric cancer (GC) is the 5th most common cancer over the world. Ubiquitin protease 43 (UBP43) is a multifunctional protein with deubiquitinase activities. Abnormal expression of UBP43 has been reported in numerous types of malignancies. Bioinformatic analysis was performed to identify the differentially expressed genes (Fold change ≥2 or ≤ -2 and p < 0.01) in GC from the datasets downloaded from Gene Expression Omnibus and Gene Expression Profiling Interactive Analysis databases, which showed that UBP43 and stress-inducible phosphoprotein 1 (STIP1) were up-regulated in both datasets. Online databases displayed the binding of UBP43 to STIP1 and the positive correlation between the two proteins. This study aims to explore: the role of UBP43 in cell proliferation and apoptosis in GC; the relationship between UBP43 and STIP1; and whether UBP43 exerts its function via STIP1 in GC. Knockdown/overexpression stable GC cell lines were generated by transducing lentivirus carrying coding sequence/short hairpin RNA of UBP43 and puromycin selection. GC patients with higher expressions of UBP43 had poor prognosis. Loss-/gain-of-function experiments revealed that pro-proliferative and anti-apoptotic abilities of UBP43 in GC cells and xenografts. UBP43 could interact with STIP1, inhibit its ubiquitination, and promote its protein stability, thereby enhancing STIP1 expression. Moreover, STIP1 knockdown reversed the pro-proliferative ability of UBP43 in GC cells. Our study uncovers that the pro-proliferative role of UBP43 in GC development is STIP1-dependent and indicates that UBP43 may act as a potent therapeutic target in GC treatment.

USP18 enhances the resistance of BRAF-mutated melanoma cells to vemurafenib by stabilizing cGAS expression to induce cell autophagy

This study aims to discern the possible molecular mechanism of the effect of ubiquitin-specific peptidase 18 (USP18) on the resistance to BRAF inhibitor vemurafenib in BRAF V600E mutant melanoma by regulating cyclic GMP-AMP synthase (cGAS). The cancer tissues of BRAF V600E mutant melanoma patients before and after vemurafenib treatment were collected, in which the protein expression of USP18 and cGAS was determined. A BRAF V600E mutant human melanoma cell line (A2058R) resistant to vemurafenib was constructed with its viability, apoptosis, and autophagy detected following overexpression and depletion assays of USP18 and cGAS. Xenografted tumors were transplanted into nude mice for in vivo validation. Bioinformatics analysis showed that the expression of cGAS was positively correlated with USP18 in melanoma, and USP18 was highly expressed in melanoma. The expression of cGAS and USP18 was up-regulated in cancer tissues of vemurafenib-resistant patients with BRAF V600E mutant melanoma. Knockdown of cGAS inhibited the resistance to vemurafenib in A2058R cells and the protective autophagy induced by vemurafenib in vitro. USP18 could deubiquitinate cGAS to promote its protein stability. In vivo experimentations confirmed that USP18 promoted vemurafenib-induced protective autophagy by stabilizing cGAS protein, which promoted resistance to vemurafenib in BRAF V600E mutant melanoma cells. Collectively, USP18 stabilizes cGAS protein expression through deubiquitination and induces autophagy of melanoma cells, thereby promoting the resistance to vemurafenib in BRAF V600E mutant melanoma.

Monogenic lupus: Tracing the therapeutic implications from single gene mutations

Monogenic lupus, a distinctive variant of systemic lupus erythematosus (SLE), is characterized by early onset, family-centric clustering, and heightened disease severity. So far, over thirty genetic variations have been identified as single-gene etiology of SLE and lupus-like phenotypes. The critical role of these gene mutations in disrupting various immune pathways is increasingly recognized. In particular, single gene mutation-driven dysfunction within the innate immunity, notably deficiencies in the complement system, impedes the degradation of free nucleic acid and immune complexes, thereby promoting activation of innate immune cells. The accumulation of these components in various tissues and organs creates a pro-inflammatory microenvironment, characterized by a surge in pro-inflammatory cytokines, chemokines, reactive oxygen species, and type I interferons. Concurrently, single gene mutation-associated defects in the adaptive immune system give rise to the emergence of autoreactive T cells, hyperactivated B cells and plasma cells. The ensuing spectrum of cytokines and autoimmune antibodies drives systemic disease manifestations, primarily including kidney, skin and central nervous system-related phenotypes. This review provides a thorough overview of the single gene mutations and potential consequent immune dysregulations in monogenic lupus, elucidating the pathogenic mechanisms of monogenic lupus. Furthermore, it discusses the recent advances made in the therapeutic interventions for monogenic lupus.

The battle between the innate immune cGAS-STING signaling pathway and human herpesvirus infection

The incidence of human herpesvirus (HHVs) is gradually increasing and has affected a wide range of population. HHVs can result in serious consequences such as tumors, neonatal malformations, sexually transmitted diseases, as well as pose an immense threat to the human health. The cGAS-STING pathway is one of the innate immune pattern-recognition receptors discovered recently. This article discusses the role of the cGAS-STING pathway in human diseases, especially in human herpesvirus infections, as well as highlights how these viruses act on this pathway to evade the host immunity. Moreover, the author provides a comprehensive overview of modulators of the cGAS-STING pathway. By focusing on the small molecule compounds based on the cGAS-STING pathway, novel targets and concepts have been proposed for the development of antiviral drugs and vaccines, while also providing a reference for the investigation of disease models related to the cGAS-STING pathway. HHV is a double-stranded DNA virus that can trigger the activation of intracellular DNA sensor cGAS, after which the host cells initiate a cascade of reactions that culminate in the secretion of type I interferon to restrict the viral replication. Meanwhile, the viral protein can interact with various molecules in the cGAS-STING pathway. Viruses can evade immune surveillance and maintain their replication by inhibiting the enzyme activity of cGAS and reducing the phosphorylation levels of STING, TBK1 and IRF3 and suppressing the interferon gene activation. Activators and inhibitors of the cGAS-STING pathway have yielded numerous promising research findings in vitro and in vivo pertaining to cGAS/STING-related disease models. However, there remains a dearth of small molecule modulators that have been successfully translated into clinical applications, which serves as a hurdle to be overcome in the future.

Role of IL-27 in HSV-1-Induced Herpetic Stromal Keratitis

Herpetic stromal keratitis (HSK) is a painful and vision-impairing disease caused by recurrent HSV-1 infection of the cornea. The virus replication in the corneal epithelium and associated inflammation play a dominant role in HSK progression. Current HSK treatments targeting inflammation or virus replication are partially effective and promote HSV-1 latency, and long-term use can cause side effects. Thus, understanding molecular and cellular events that control HSV-1 replication and inflammation is crucial for developing novel HSK therapies. In this study, we report that ocular HSV-1 infection induces the expression of IL-27, a pleiotropic immunoregulatory cytokine. Our data indicate that HSV-1 infection stimulates IL-27 production by macrophages. Using a primary corneal HSV-1 infection mouse model and IL-27 receptor knockout mice, we show that IL-27 plays a critical role in controlling HSV-1 shedding from the cornea, the optimum induction of effector CD4+ T cell responses, and limiting HSK progression. Using in vitro bone marrow-derived macrophages, we show that IL-27 plays an antiviral role by regulating macrophage-mediated HSV-1 killing, IFN-β production, and IFN-stimulated gene expression after HSV-1 infection. Furthermore, we report that IL-27 is critical for macrophage survival, Ag uptake, and the expression of costimulatory molecules involved in the optimum induction of effector T cell responses. Our results indicate that IL-27 promotes endogenous antiviral and anti-inflammatory responses and represents a promising target for suppressing HSK progression.

ROP16 of Toxoplasma gondii Inhibits Innate Immunity by Triggering cGAS-STING Pathway Inactivity through the Polyubiquitination of STING

cGAS-STING signaling is a major pathway in inducing type Ⅰ IFN, which plays a crucial role in the defense against T. gondii infection. In contrast, T. gondii develops multiple strategies to counteract the host defense, causing serious diseases in a wide range of hosts. Here, we demonstrate that T. gondii rhoptry protein 16 (ROP16) dampens type I interferon signaling via the inhibition of the cGAS (cyclic GMP-AMP synthase) pathway through the polyubiquitination of STING. Mechanistically, ROP16 interacts with STING through the SignalP domain and inhibits the K63-linked ubiquitination of STING in an NLS (nuclear localization signal)-domain-dependent manner. Consequently, knocking out the ROP16 in PRU tachyzoites promotes the STING-mediated production of type I IFNs and limits the replication of T. gondii. Together, these findings describe a distinct pathway where T. gondii exploits the ubiquitination of STING to evade host anti-parasite immunity, revealing new insights into the interaction between the host and parasites.

USP12 promotes antiviral responses by deubiquitinating and stabilizing IFI16

Deubiquitinating enzymes (DUBs) regulate antiviral immune response through targeting DNA sensor signaling pathway members. As one of the DNA sensors, interferon (IFN)-γ inducible protein 16 (IFI16) play a major role in response to virus infections through activating the canonical STING/TBK-1/IRF3 signaling pathway. Only a few studies discuss the function of DUBs in IFI16-mediated antiviral response. Ubiquitin-specific protease 12 (USP12), which is one of the major members of the USP family, participates in various biological functions. However, whether USP12 regulates the nucleic acid sensor to modulate antiviral immune responses has not yet been elucidated. In this study, we found that knockout or knockdown of USP12 impaired the HSV-1-induced expressions of IFN-β, CCL-5, IL-6, and downstream interferon-stimulated genes (ISGs). Moreover, USP12 deficiency increased HSV-1 replication and host susceptibility to HSV-1 infection. Mechanistically, USP12 inhibited the proteasome-dependent degradation of IFI16 through its deubiquitinase activity, thereby maintaining IFI16 stability and promoting IFI16-STING-IRF3- and p65-mediated antiviral signaling. Overall, our findings demonstrate an essential role of USP12 in DNA-sensing signaling and contribute to the understanding of deubiquitination-mediated regulation of innate antiviral responses.

Alphavirus infection triggers antiviral RNAi immunity in mammals

RNA interference (RNAi) is a well-established antiviral immunity. However, for mammalian somatic cells, antiviral RNAi becomes evident only when viral suppressors of RNAi (VSRs) are disabled by mutations or VSR-targeting drugs, thereby limiting its scope as a mammalian immunity. We find that a wild-type alphavirus, Semliki Forest virus (SFV), triggers the Dicer-dependent production of virus-derived small interfering RNAs (vsiRNAs) in both mammalian somatic cells and adult mice. These SFV-vsiRNAs are located at a particular region within the 5' terminus of the SFV genome, Argonaute loaded, and active in conferring effective anti-SFV activity. Sindbis virus, another alphavirus, also induces vsiRNA production in mammalian somatic cells. Moreover, treatment with enoxacin, an RNAi enhancer, inhibits SFV replication dependent on RNAi response in vitro and in vivo and protects mice from SFV-induced neuropathogenesis and lethality. These findings show that alphaviruses trigger the production of active vsiRNA in mammalian somatic cells, highlighting the functional importance and therapeutic potential of antiviral RNAi in mammals.

The Many Ways to Deal with STING

The stimulator of interferon genes (STING) is an adaptor protein involved in the activation of IFN-β and many other genes associated with the immune response activation in vertebrates. STING induction has gained attention from different angles such as the potential to trigger an early immune response against different signs of infection and cell damage, or to be used as an adjuvant in cancer immune treatments. Pharmacological control of aberrant STING activation can be used to mitigate the pathology of some autoimmune diseases. The STING structure has a well-defined ligand binding site that can harbor natural ligands such as specific purine cyclic di-nucleotides (CDN). In addition to a canonical stimulation by CDNs, other non-canonical stimuli have also been described, whose exact mechanism has not been well defined. Understanding the molecular insights underlying the activation of STING is important to realize the different angles that need to be considered when designing new STING-binding molecules as therapeutic drugs since STING acts as a versatile platform for immune modulators. This review analyzes the different determinants of STING regulation from the structural, molecular, and cell biology points of view.

The SARS-CoV-2 papain-like protease suppresses type I interferon responses by deubiquitinating STING

The SARS-CoV-2 papain-like protease (PLpro), which has deubiquitinating activity, suppresses the type I interferon (IFN-I) antiviral response. We investigated the mechanism by which PLpro antagonizes cellular antiviral responses. In HEK392T cells, PLpro removed K63-linked polyubiquitin chains from Lys289 of the stimulator of interferon genes (STING). PLpro-mediated deubiquitination of STING disrupted the STING-IKKε-IRF3 complex that induces the production of IFN-β and IFN-stimulated cytokines and chemokines. In human airway cells infected with SARS-CoV-2, the combined treatment with the STING agonist diABZi and the PLpro inhibitor GRL0617 resulted in the synergistic inhibition of SARS-CoV-2 replication and increased IFN-I responses. The PLpros of seven human coronaviruses (SARS-CoV-2, SARS-CoV, MERS-CoV, HCoV-229E, HCoV-HKU1, HCoV-OC43, and HCoV-NL63) and four SARS-CoV-2 variants of concern (α, β, γ, and δ) all bound to STING and suppressed STING-stimulated IFN-I responses in HEK293T cells. These findings reveal how SARS-CoV-2 PLpro inhibits IFN-I signaling through STING deubiquitination and a general mechanism used by seven human coronaviral PLpros to dysregulate STING and to facilitate viral innate immune evasion. We also identified simultaneous pharmacological STING activation and PLpro inhibition as a potentially effective strategy for antiviral therapy against SARS-CoV-2.

Unlocking STING as a Therapeutic Antiviral Strategy

Invading pathogens have developed weapons that subvert physiological conditions to weaken the host and permit the spread of infection. Cells, on their side, have thus developed countermeasures to maintain cellular physiology and counteract pathogenesis. The cyclic GMP-AMP (cGAMP) synthase (cGAS) is a pattern recognition receptor that recognizes viral DNA present in the cytosol, activating the stimulator of interferon genes (STING) protein and leading to the production of type I interferons (IFN-I). Given its role in innate immunity activation, STING is considered an interesting and innovative target for the development of broad-spectrum antivirals. In this review, we discuss the function of STING; its modulation by the cellular stimuli; the molecular mechanisms developed by viruses, through which they escape this defense system; and the therapeutical strategies that have been developed to date to inhibit viral replication restoring STING functionality.

USP18 promotes endometrial receptivity via the JAK/STAT1 and the ISGylation pathway

Interferon-tau (IFNT), a pregnancy recognition signal in ruminants, promotes the establishment of endometrial receptivity by inducing the expression of interferon-stimulated genes (ISGs) via the Janus kinase/signal transducer and activator of transcription (JAK/STATs) signaling pathway. However, the precise mechanisms remain largely unknown. Ubiquitin-specific protease 18 (USP18) acts specifically on the ISGylation modification system to exert deubiquitination and participates in the regulation of the type I IFN signaling pathway. The purpose of this study was to determine the role and mechanism of USP18 on endometrial receptivity in goat. USP18 was mainly localized in the uterine luminal and glandular epithelium, and its expression levels were significantly increased from days 5-18 of early pregnancy. Progesterone (P₄), estradiol (E₂), and IFNT significantly stimulated USP18 expression in goat endometrial epithelial cells (gEECs) cultured in vitro. Meanwhile, the markers of endometrial receptivity HOXA11, ITGB1, ITGB3, and ITGB5 were significantly upregulated after USP18 overexpression in gEECs. However, USP18 interference significantly inhibited the expression of HOXA10, ITGB1, ITGB3, and ITGB5 in gEECs. In addition, both the phosphorylation levels of STAT1 and the expression of ISGylation-modified proteins were significantly increased after USP18 silencing in gEECs. Furthermore, pretreatment with the STAT1 inhibitor Fludara markedly restored the effect of USP18 interference in gEECs. In summary, USP18 may play an important role in promoting goat endometrial receptivity by regulating the JAK/STAT1 pathway and ISGylation.

Single High-Dose Irradiation-Induced iRhom2 Upregulation Promotes Macrophage Antitumor Activity Through cGAS/STING Signaling

PURPOSE

The clinical application of stereotactic body radiation therapy (SBRT) allows a high dose of radiation to be safely delivered to extracranial targets within the body; however, a high dose per fraction (hypofractionation) has opened the radiation oncology field to new questions on a variety of dose-fractionation schedules, especially the immunomodulatory effects of radiation therapy, which can change after various dose-fractionation schedules. We investigated the immunomodulatory effects of different fractionation schedules.

METHODS AND MATERIALS

We established a subcutaneous tumor model in wild-type C57BL/6J mice and STING (stimulator of interferon genes)-deficient mice. We then compared the tumor control efficacy of 3 different fractionation schedules: 2 Gy × 8, 4.5 Gy × 3, and 10 Gy × 1, which are similar biologically effective doses.

RESULTS

We found the fractionation schedule of 10 Gy × 1 had a significantly higher antitumor effect, suggesting that a single high dose induced enhanced antitumor immunity compared with conventional fractionation (2 Gy × 8) and moderate hypofractionation (4.5 Gy × 3). However, in STING-deficient mice, differential tumor control was not observed among the 3 dose-fractionation schedules, suggesting that cGAS (cyclic GMP-AMP synthase)/STING signaling is involved in the antitumor immune effects of single high-dose schedules. Mechanistically, we found that conventional fractionation induced apoptosis; by comparison, a single high dose was more attuned to induced necroptosis, leading to the release of intracellular irradiation-induced double-stranded DNA (dsDNA) due to the loss of plasma membrane integrity, which then activated the dsDNA sensing signaling cGAS/STING in the recruited macrophage. Furthermore, iRhom2, a member of the conserved family of inhibitory rhomboid-like pseudoproteases, was upregulated in infiltrated macrophages in the single high-dose irradiation microenvironment. Therefore, iRhom2 positively regulates STING and directly promotes tumor necrosis factor α secretion. This exacerbates necroptosis of irradiated tumor cells, leading to continuous dsDNA release and enhancement of cGAS/STING signaling antitumor immunity in a positive feedback loop.

CONCLUSIONS

iRhom2 amplifies antitumor signaling in a positive feedback loop mediated by cGAS/STING signaling and tumor necrosis factor-driven necroptosis after single high-dose radiation.

Current understanding of the cGAS-STING signaling pathway: Structure, regulatory mechanisms, and related diseases

The innate immune system protects the host from external pathogens and internal damage in various ways. The cGAS-STING signaling pathway, comprised of cyclic GMP-AMP synthase (cGAS), stimulator of interferon genes (STING), and downstream signaling adaptors, plays an essential role in protective immune defense against microbial DNA and internal damaged-associated DNA and is responsible for various immune-related diseases. After binding with DNA, cytosolic cGAS undergoes conformational change and DNA-linked liquid-liquid phase separation to produce 2'3'-cGAMP for the activation of endoplasmic reticulum (ER)-localized STING. However, further studies revealed that cGAS is predominantly expressed in the nucleus and strictly tethered to chromatin to prevent binding with nuclear DNA, and functions differently from cytosolic-localized cGAS. Detailed delineation of this pathway, including its structure, signaling, and regulatory mechanisms, is of great significance to fully understand the diversity of cGAS-STING activation and signaling and will be of benefit for the treatment of inflammatory diseases and cancer. Here, we review recent progress on the above-mentioned perspectives of the cGAS-STING signaling pathway and discuss new avenues for further study.

Multifaceted functions of STING in human health and disease: from molecular mechanism to targeted strategy

Since the discovery of Stimulator of Interferon Genes (STING) as an important pivot for cytosolic DNA sensation and interferon (IFN) induction, intensive efforts have been endeavored to clarify the molecular mechanism of its activation, its physiological function as a ubiquitously expressed protein, and to explore its potential as a therapeutic target in a wide range of immune-related diseases. With its orthodox ligand 2'3'-cyclic GMP-AMP (2'3'-cGAMP) and the upstream sensor 2'3'-cGAMP synthase (cGAS) to be found, STING acquires its central functionality in the best-studied signaling cascade, namely the cGAS-STING-IFN pathway. However, recently updated research through structural research, genetic screening, and biochemical assay greatly extends the current knowledge of STING biology. A second ligand pocket was recently discovered in the transmembrane domain for a synthetic agonist. On its downstream outputs, accumulating studies sketch primordial and multifaceted roles of STING beyond its cytokine-inducing function, such as autophagy, cell death, metabolic modulation, endoplasmic reticulum (ER) stress, and RNA virus restriction. Furthermore, with the expansion of the STING interactome, the details of STING trafficking also get clearer. After retrospecting the brief history of viral interference and the milestone events since the discovery of STING, we present a vivid panorama of STING biology taking into account the details of the biochemical assay and structural information, especially its versatile outputs and functions beyond IFN induction. We also summarize the roles of STING in the pathogenesis of various diseases and highlight the development of small-molecular compounds targeting STING for disease treatment in combination with the latest research. Finally, we discuss the open questions imperative to answer.

The cGAS-STING pathway: Post-translational modifications and functional implications in diseases

Recent studies have illustrated the functional significance of DNA recognition in the activation of innate immune responses among a variety of diseases. The cyclic GMP-AMP synthase-stimulator of interferon genes (cGAS-STING) pathway has been found to be modulated by post-translational modifications and can regulate the immune response via type I IFNs. Accumulating evidence indicates a pivotal role of cGAS-STING signaling, being protective or pathogenic, in the development of diseases. Thus, a comprehensive understanding of the post-translational modifications of cGAS-STING pathway and their role in disease development will provide insights in predicting individual disease outcomes and developing appropriate therapies. In this review, we will discuss the regulation of the cGAS-STING pathway and its implications in disease pathologies, as well as pharmacologic strategies to target the cGAS-STING pathway for therapeutic intervention.

Blockade of USP14 potentiates type I interferon signaling and radiation-induced antitumor immunity via preventing IRF3 deubiquitination

PURPOSE

The adaptive immune responses induced by radiotherapy has been demonstrated to largely rely on STING-dependent type I interferons (IFNs) production. However, irradiated tumor cells often fail to induce dendritic cells (DCs) to produce type I IFNs. Hence, we aim to uncover the limitation of STING-mediated innate immune sensing following radiation, and identify efficient reagents capable to rescue the failure of type I IFNs induction for facilitating radiotherapy.

METHODS

A targeted cell-based phenotypic screening was performed to search for active molecules that could elevate the production of type I IFNs. USP14 knockout or inhibition was assayed for IFN production and the activation of STING signaling in vitro. The mechanisms of USP14 were investigated by western blot and co-immunoprecipitation in vitro. Additionally, combinational treatments with PT33 and radiation in vivo and in vitro models were performed to evaluate type I IFNs responses to radiation.

RESULTS

PT33 was identified as an enhancer of STING agonist elicited type I IFNs production to generate an elevated and durable STING activation profile in vitro. Mechanistically, USP14 inhibition or deletion impairs the deubiquitylation of K63-linked IRF3. Furthermore, blockade of USP14 with PT33 enhances DC sensing of irradiated-tumor cells in vitro, and synergizes with radiation to promote systemic antitumor immunity in vivo.

CONCLUSION

Our findings reveal that USP14 is one of the major IFN production suppressors and impairs the activation of IRF3 by removing the K63-linked ubiquitination of IRF3. Therefore, blockage of USP14 results in the gain of STING signaling activation and radiation-induced adaptive immune responses.

Viruses utilize ubiquitination systems to escape TLR/RLR-mediated innate immunity

When the viruses invade the body, they will be recognized by the host pattern recognition receptors (PRRs) such as Toll like receptor (TLR) or retinoic acid-induced gene-I like receptor (RLR), thus causing the activation of downstream antiviral signals to resist the virus invasion. The cross action between ubiquitination and proteins in these signal cascades enhances the antiviral signal. On the contrary, more and more viruses have also been found to use the ubiquitination system to inhibit TLR/RLR mediated innate immunity. Therefore, this review summarizes how the ubiquitination system plays a regulatory role in TLR/RLR mediated innate immunity, and how viruses use the ubiquitination system to complete immune escape.