Redox homeostasis maintained by GPX4 facilitates STING activation.

Stimulator-of-interferon genes (STING) is vital for sensing cytosolic DNA and initiating innate immune responses against microbial infection and tumors. Redox homeostasis is the balance of oxidative and reducing reactions present in all living systems. Yet, how the intracellular redox state controls STING activation is unclear. Here, we show that cellular redox homeostasis maintained by glutathione peroxidase 4 (GPX4) is required for STING activation. GPX4 deficiency enhanced cellular lipid peroxidation and thus specifically inhibited the cGAS-STING pathway. Concordantly, GPX4 deficiency inhibited herpes simplex virus-1 (HSV-1)-induced innate antiviral immune responses and promoted HSV-1 replication in vivo. Mechanistically, GPX4 inactivation increased production of lipid peroxidation, which led to STING carbonylation at C88 and inhibited its trafficking from the endoplasmic reticulum (ER) to the Golgi complex. Thus, cellular stress-induced lipid peroxidation specifically attenuates the STING DNA-sensing pathway, suggesting that GPX4 facilitates STING activation by maintaining redox homeostasis of lipids.

The UAF1-USP1 Deubiquitinase Complex Stabilizes cGAS and Facilitates Antiviral Responses.

Cyclic guanosine monophosphate-adenosine monophosphate synthase (cGAS) detects cytoplasmic microbial DNA and self-DNA from genomic instability, initiates innate immunity, and plays fundamental roles in defense against viruses and the development of various diseases. The cellular cGAS level determines the magnitude of the response to DNA. However, the underlying mechanisms of the control of cGAS stability, especially its feedback regulation during viral infection, remain largely unknown. In this study, we show that viral infection induces the expression of the UAF1-USP1 deubiquitinase complex in primary peritoneal macrophages (PMs) of C57BL/6J mice. UAF1-USP interacts with cGAS, selectively cleaves its K48-linked polyubiquitination, and thus stabilizes its protein expression in PMs and HEK293T cells. Concordantly, the UAF1-USP1 deubiquitinase complex enhances cGAS-dependent type I IFN responses in PMs. Uaf1 deficiency and ML323 (a specific inhibitor of UAF1-USP1 deubiquitinase complex) attenuates cGAS-triggered antiviral responses and facilitates viral replication both in vitro and in vivo. Thus, our study uncovers a positive feedback mechanism of cGAS-dependent antiviral responses and suggests the UAF1-USP1 complex as a potential target for the treatment of diseases caused by aberrant cGAS activation.

Mint3 potentiates TLR3/4- and RIG-I-induced IFN-ß expression and antiviral immune responses.

Type I IFNs (IFN-α/ß) play crucial roles in the elimination of invading viruses. Multiple immune cells including macrophages recognize viral infection through a variety of pattern recognition receptors, such as Toll-like receptors (TLRs) and retinoic acid-inducible gene-I (RIG-I)-like receptors, and initiate type I IFN secretion and subsequent antiviral immune responses. However, the mechanisms by which host immune cells can produce adequate amounts of type I IFNs and then eliminate viruses effectively remain to be further elucidated. In the present study, we show that munc18-1-interacting protein 3 (Mint3) expression can be markedly induced during viral infection in macrophages. Mint3 enhances TLR3/4- and RIG-I-induced IRF3 activation and IFN-ß production by promoting K63-linked polyubiquitination of TNF receptor-associated factor 3 (TRAF3). Consistently, Mint3 deficiency greatly attenuated antiviral immune responses and increased viral replication. Therefore, we have identified Mint3 as a physiological positive regulator of TLR3/4 and RIG-I-induced IFN-ß production and have outlined a feedback mechanism for the control of antiviral immune responses.

MLN4924, a First-in-Class NEDD8-Activating Enzyme Inhibitor, Attenuates IFN-ß Production.

Neddylation is a posttranslational protein modification that conjugates ubiquitin-like protein neural precursor cell-expressed developmentally downregulated protein 8 (NEDD8) to target proteins and regulates diverse cellular processes. MLN4924, a novel NEDD8 activating enzyme inhibitor, which has emerged as a promising anticancer drug, has a multifaceted function by inhibiting the process of neddylation. However, the potential roles of MLN4924 and neddylation in IFN-ß production remain unknown. In this study, we show that MLN4924 inhibits TLR3/4- and retinoic acid-inducible gene-I-induced IFN-ß expression in different cells, whereas NEDD8 knockdown had no effects on IFN-ß expression. The ability of the MLN4924 to inhibit IFN-ß production was confirmed in vivo, as mice treated with MLN4924 exhibited decreased levels of IFN-ß upon LPS or polyinosinic-polycytidylic acid stimulation. Furthermore, we show that MLN4924 inhibits IFN regulatory factor 3 (IRF3) transcriptional activation and prevents IRF3 binding to IFN-ß promoter. Our findings suggest that MLN4924 inhibits TLR3/4- and retinoic acid-inducible gene-I-induced IFN-ß expression by preventing IRF3 binding to the IFN-ß promoter, with a neddylation-independent manner. Therefore, our results provide new insight into the mechanism of MLN4924 and may have significant implications for the treatment of MLN4924.

TOB1 attenuates IRF3-directed antiviral responses by recruiting HDAC8 to specifically suppress IFN-ß expression.

Interferon regulatory factor 3 (IRF3) is a key transcription factor required for the secretion of type I interferons (IFN-α/ß) and initiation of antiviral immune response. However, the negative feedback regulator of IRF3-directed antiviral response remains unknown. In this study, we demonstrated that viral infection induced the interaction of the transducer of ERBB2.1 (TOB1) with IRF3, which bound to the promoter region of Ifnb1 in macrophages. TOB1 inhibited Ifnb1 transcription by disrupting IRF3 binding and recruiting histone deacetylase 8 (HDAC8) to the Ifnb1 promoter region. Consequently, TOB1 attenuated IRF3-directed IFN-ß expression in virus-infected macrophages. Tob1 deficiency enhanced antiviral response and suppressed viral replication in vivo. Thus, we identified TOB1 as a feedback inhibitor of host antiviral innate immune response and revealed a mechanism underlying viral immune escape.

RNF39 mediates K48-linked ubiquitination of DDX3X and inhibits RLR-dependent antiviral immunity.

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs) are major cytosolic RNA sensors and play crucial roles in initiating antiviral innate immunity. Furthermore, RLRs have been implicated in multiple autoimmune disorders. Thus, RLR activation should be tightly controlled to avoid detrimental effects. "DEAD-box RNA helicase 3, X-linked" (DDX3X) is a key adaptor in RLR signaling, but its regulatory mechanisms remain unknown. Here, we show that the E3 ubiquitin ligase RNF39 inhibits RLR pathways through mediating K48-linked ubiquitination and proteasomal degradation of DDX3X. Concordantly, Rnf39 deficiency enhances RNA virus-triggered innate immune responses and attenuates viral replication. Thus, our results uncover a previously unknown mechanism for the control of DDX3X activity and suggest RNF39 as a priming intervention target for diseases caused by aberrant RLR activation.

UAF1 deubiquitinase complexes facilitate NLRP3 inflammasome activation by promoting NLRP3 expression.

NOD-like receptor protein 3 (NLRP3) detects microbial infections or endogenous danger signals and activates the NLRP3 inflammasome, which has important functions in host defense and contributes to the pathogenesis of inflammatory diseases, and thereby needs to be tightly controlled. Deubiquitination of NLRP3 is considered a key step in NLRP3 inflammasome activation. However, the mechanisms by which deubiquitination controls NLRP3 inflammasome activation are unclear. Here, we show that the UAF1/USP1 deubiquitinase complex selectively removes K48-linked polyubiquitination of NLRP3 and suppresses its ubiquitination-mediated degradation, enhancing cellular NLRP3 levels, which are indispensable for subsequent NLRP3 inflammasome assembly and activation. In addition, the UAF1/USP12 and UAF1/USP46 complexes promote NF-κB activation, enhance the transcription of NLRP3 and proinflammatory cytokines (including pro-IL-1ß, TNF, and IL-6) by inhibiting ubiquitination-mediated degradation of p65. Consequently, Uaf1 deficiency attenuates NLRP3 inflammasome activation and IL-1ß secretion both in vitro and in vivo. Our study reveals that the UAF1 deubiquitinase complexes enhance NLRP3 and pro-IL-1ß expression by targeting NLRP3 and p65 and licensing NLRP3 inflammasome activation.

First small-molecule PROTACs for G protein-coupled receptors: inducing α 1A-adrenergic receptor degradation.

Proteolysis targeting chimeras (PROTACs) are dual-functional hybrid molecules that can selectively recruit an E3 ubiquitin ligase to a target protein to direct the protein into the ubiquitin-proteasome system (UPS), thereby selectively reducing the target protein level by the ubiquitin-proteasome pathway. Nowadays, small-molecule PROTACs are gaining popularity as tools to degrade pathogenic protein. Herein, we present the first small-molecule PROTACs that can induce the α 1A-adrenergic receptor (α 1A-AR) degradation, which is also the first small-molecule PROTACs for G protein-coupled receptors (GPCRs) to our knowledge. These degradation inducers were developed through conjugation of known α 1-adrenergic receptors (α 1-ARs) inhibitor prazosin and cereblon (CRBN) ligand pomalidomide through the different linkers. The representative compound 9c is proved to inhibit the proliferation of PC-3 cells and result in tumor growth regression, which highlighted the potential of our study as a new therapeutic strategy for prostate cancer.

Huaier aqueous extract protects against dextran sulfate sodium-induced experimental colitis in mice by inhibiting NLRP3 inflammasome activation.

The use of Trametes robiniophila Murr. (Huaier) as a complementary therapy for cancer has recently become increasingly common in China. However, whether Huaier can regulate host immune responses, especially innate immunity, remains largely unknown. The NLRP3 inflammasome is a multimeric complex consisting of NLRP3, ASC and caspase-1. NLRP3 inflammasomes respond to a variety of endogenous (damage-associated molecular patterns) and exogenous (pathogen-associated molecular patterns) stimuli, and play crucial roles in host defense against pathogens and multiple diseases such as ulcerative colitis (UC). In this study, we investigated the anti-inflammatory effect of Huaier in dextran sulfate sodium (DSS)-induced murine colitis and revealed the underlying mechanisms by targeting NLRP3 inflammasomes. In C57BL/6 mice, oral administration of Huaier attenuated DSS-induced colon shortening and colonic pathological damage. Furthermore, we analyzed the effect of Huaier on NLRP3 inflammasome activation in macrophages. Huaier inhibited NLRP3 inflammasome activation-induced IL-1ß secretion and caspase-1 cleavage. Moreover, Huaier decreased NLRP3 protein expression via promoting NLRP3 degradation through the autophagy lysosome pathway. Therefore, our findings demonstrate a novel function for Huaier in the regulation of NLRP3 inflammasome activation and suggest a potential role for Huaier in NLRP3 inflammasome-associated diseases.

USP1-UAF1 deubiquitinase complex stabilizes TBK1 and enhances antiviral responses.

Optimal activation of TANK-binding kinase 1 (TBK1) is crucial for initiation of innate antiviral immunity and maintenance of immune homeostasis. Although several E3 ubiquitin ligases have been reported to regulate TBK1 activation by mediating its polyubiquitination, the functions of deubiquitinase on TBK1 activity remain largely unclear. Here, we identified a deubiquitinase complex, which is formed by ubiquitin specific peptidase 1 (USP1) and USP1-associated factor 1 (UAF1), as a viral infection-induced physiological enhancer of TBK1 expression. USP1-UAF1 complex enhanced TLR3/4 and RIG-I-induced IFN regulatory factor 3 (IRF3) activation and subsequent IFN-ß secretion. Mechanistically, USP1 and UAF1 bound to TBK1, removed its K48-linked polyubiquitination, and then reversed the degradation process of TBK1. Furthermore, we found that ML323, a specific USP1-UAF1 inhibitor, attenuated IFN-ß expression and enhanced viral replication both in vitro and in vivo. Therefore, our results outline a novel mechanism for the control of TBK1 activity and suggest USP1-UAF1 complex as a potential target for the prevention of viral diseases.

The E3 Ubiquitin Ligase TRIM40 Attenuates Antiviral Immune Responses by Targeting MDA5 and RIG-I.

Retinoic acid-inducible gene-I (RIG-I)-like receptors (RLRs), including melanoma differentiation-associated gene 5 (MDA5) and RIG-I, are crucial for host recognition of non-self RNAs, especially viral RNA. Thus, the expression and activation of RLRs play fundamental roles in eliminating the invading RNA viruses and maintaining immune homeostasis. However, how RLR expression is tightly regulated remains to be further investigated. In this study, we identified a major histocompatibility complex (MHC)-encoded gene, tripartite interaction motif 40 (TRIM40), as a suppressor of RLR signaling by directly targeting MDA5 and RIG-I. TRIM40 binds to MDA5 and RIG-I and promotes their K27- and K48-linked polyubiquitination via its E3 ligase activity, leading to their proteasomal degradation. TRIM40 deficiency enhances RLR-triggered signaling. Consequently, TRIM40 deficiency greatly enhances antiviral immune responses and decreases viral replication in vivo. Thus, we demonstrate that TRIM40 limits RLR-triggered innate activation, suggesting TRIM40 as a potential therapeutic target for the control of viral infection.

The E3 ubiquitin ligase TRIM31 attenuates NLRP3 inflammasome activation by promoting proteasomal degradation of NLRP3.

The NLRP3 inflammasome has a fundamental role in host defence against microbial pathogens and its deregulation may cause diverse inflammatory diseases. NLRP3 protein expression is a rate-limiting step for inflammasome activation, thus its expression must be tightly controlled to maintain immune homeostasis and avoid detrimental effects. However, how NLRP3 expression is regulated remains largely unknown. In this study, we identify E3 ubiquitin ligase TRIM31 as a feedback suppressor of NLRP3 inflammasome. TRIM31 directly binds to NLRP3, promotes K48-linked polyubiquitination and proteasomal degradation of NLRP3. Consequently, TRIM31 deficiency enhances NLRP3 inflammasome activation and aggravates alum-induced peritonitis in vivo. Furthermore, TRIM31 deficiency attenuates the severity of dextran sodium sulfate (DSS)-induced colitis, an inflammatory bowel diseases model in which NLRP3 possesses protective roles. Thus, our research describes a mechanism by which TRIM31 limits NLRP3 inflammasome activity under physiological conditions and suggests TRIM31 as a potential therapeutic target for the intervention of NLRP3 inflammasome related diseases.