ARMH3-mediated recruitment of PI4KB directs Golgi-to-endosome trafficking and activation of the antiviral effector STING

Epigenetic regulation of cGAS and STING expression in cancer

Although cancer immunotherapy has become a successful therapeutic strategy in a certain range of solid cancer and hematological malignancies, this efficacy of immunotherapy is impeded by limited success rates due to an immunologically "cold" state. The cGAS-STING signaling pathway is an evolutionarily conserved system which can find cytoplasmic DNA to regulate the innate immune and adaptive immune response. Beyond the host defense and autoimmune disorders, recent advances have now expanded the roles of cGAS-STING that is precise activated and tight regulated to improve anticancer immunity. Mounting evidence now has shown the crucial role of epigenetic regulation in mediating the expression of key genes associated with the cGAS-STING signaling pathway. In this review, we highlight the structure and cellular localization of cGAS and STING as well as intracellular cascade reaction of cGAS-STING signal transduction. We further summarize recent findings of epigenetic regulatory mechanisms that control the expression of cGAS and STING in cancer. The review aims to offer theoretical basis and reference for targeting the epigenetic mechanisms that control cGAS and STING gene expression to promote the development of more effective combination therapeutic regimens to enhance the efficacy of cancer immunotherapy in clinical practice and cancer clinical and cancer research workers.

The journey of STING: Guiding immune signaling through membrane trafficking

Stimulator of Interferon Genes (STING) serves as a pivotal mediator in the innate immune signaling pathway, transducing signals from various DNA receptors and playing a crucial role in natural immune processes. During cellular quiescence, STING protein resides in the endoplasmic reticulum (ER), and its activation typically occurs through the cGAS-STING signaling pathway. Upon activation, STING protein is transported to the Golgi apparatus, thereby initiating downstream signaling cascades. Vesicular transport serves as the primary mechanism for STING protein trafficking between the ER and Golgi apparatus, with COPII mediating anterograde transport from the ER to Golgi apparatus, while COPI is responsible for retrograde transport. Numerous factors influence these transport processes, thereby exerting either promoting or inhibitory effects on STING protein expression. Upon reaching the Golgi apparatus, to prevent over-activation, STING protein is transported to post-Golgi compartments for degradation. In addition to the conventional lysosomal degradation pathway, ESCRT has also been identified as one of the degradation pathways for STING protein. This review summarizes the recent findings on the membrane trafficking pathways of STING, highlighting their contributions to the regulation of cytokine production, the activation of immune cells, and the coordination of immune signaling pathways.

A Bioactive Injectable Hydrogel Regulates Tumor Metastasis and Wound Healing for Melanoma via NIR-Light Triggered Hyperthermia

Surgical resection remains the mainstream treatment for malignant melanoma. However, challenges in wound healing and residual tumor metastasis pose significant hurdles, resulting in high recurrence rates in patients. Herein, a bioactive injectable hydrogel (BG-Mngel) formed by crosslinking sodium alginate (SA) with manganese-doped bioactive glass (BG-Mn) is developed as a versatile platform for anti-tumor immunotherapy and postoperative wound healing for melanoma. The incorporation of Mn2+ within bioactive glass (BG) can activate the cGAS-STING immune pathway to elicit robust immune response for cancer immunotherapy. Furthermore, doping Mn2+ in BG endows system with excellent photothermal properties, hence facilitating STING activation and reversing the tumor immune-suppressive microenvironment. BG exhibits favorable angiogenic capacity and tissue regenerative potential, and Mn2+ promotes cell migration in vitro. When combining BG-Mngel with anti-PD-1 antibody (α-PD-1) for the treatment of malignant melanoma, it shows enhanced anti-tumor immune response and long-term immune memory response. Remarkably, BG-Mngel can upregulate the expression of genes related to blood vessel formation and promote skin tissue regeneration when treating full-thickness wounds. Overall, BG-MnGel serves as an effective adjuvant therapy to regulate tumor metastasis and wound healing for malignant melanoma.

Duck STING mediates antiviral autophagy directing the interferon signaling pathway to inhibit duck plague virus infection

Migratory birds are important vectors for virus transmission, how migratory birds recognize viruses and viruses are sustained in birds is still enigmatic. As an animal model for waterfowl among migratory birds, studying and dissecting the antiviral immunity and viral evasion in duck cells may pave a path to deciphering these puzzles. Here, we studied the mechanism of antiviral autophagy mediated by duck STING in DEF cells. The results collaborated that duck STING could significantly enhance LC3B-II/I turnover, LC3B-EGFP puncta formation, and mCherry/EGFP ratio, indicating that duck STING could induce autophagy. The autophagy induced by duck STING is not affected by shRNA knockdown of ATG5 expression, deletion of the C-terminal tail of STING, or TBK1 inhibitor BX795 treatment, indicating that duck STING activated non-classical selective autophagy is independent of interaction with TBK1, TBK1 phosphorylation, and interferon (IFN) signaling. The STING R235A mutant and Sar1A/B kinase mutant abolished duck STING induced autophagy, suggesting binding with cGAMP and COPII complex mediated transport are the critical prerequisite. Duck STING interacted with LC3B through LIR motifs to induce autophagy, the LIR 4/7 motif mutants of duck STING abolished the interaction with LC3B, and neither activated autophagy nor IFN expression, indicating that duck STING associates with LC3B directed autophagy and dictated innate immunity activation. Finally, we found that duck STING mediated autophagy significantly inhibited duck plague virus (DPV) infection via ubiquitously degraded viral proteins. Our study may shed light on one scenario about the control and evasion of diseases transmitted by migratory birds.

AMPK targets PDZD8 to trigger carbon source shift from glucose to glutamine

The shift of carbon utilization from primarily glucose to other nutrients is a fundamental metabolic adaptation to cope with decreased blood glucose levels and the consequent decline in glucose oxidation. AMP-activated protein kinase (AMPK) plays crucial roles in this metabolic adaptation. However, the underlying mechanism is not fully understood. Here, we show that PDZ domain containing 8 (PDZD8), which we identify as a new substrate of AMPK activated in low glucose, is required for the low glucose-promoted glutaminolysis. AMPK phosphorylates PDZD8 at threonine 527 (T527) and promotes the interaction of PDZD8 with and activation of glutaminase 1 (GLS1), a rate-limiting enzyme of glutaminolysis. In vivo, the AMPK-PDZD8-GLS1 axis is required for the enhancement of glutaminolysis as tested in the skeletal muscle tissues, which occurs earlier than the increase in fatty acid utilization during fasting. The enhanced glutaminolysis is also observed in macrophages in low glucose or under acute lipopolysaccharide (LPS) treatment. Consistent with a requirement of heightened glutaminolysis, the PDZD8-T527A mutation dampens the secretion of pro-inflammatory cytokines in macrophages in mice treated with LPS. Together, we have revealed an AMPK-PDZD8-GLS1 axis that promotes glutaminolysis ahead of increased fatty acid utilization under glucose shortage.

Inhibition of O-GlcNAc transferase activates type I interferon-dependent antitumor immunity by bridging cGAS-STING pathway

The O-GlcNAc transferase (OGT) is an essential enzyme that mediates protein O-GlcNAcylation, a unique form of posttranslational modification of many nuclear and cytosolic proteins. Recent studies observed increased OGT and O-GlcNAcylation levels in a broad range of human cancer tissues compared to adjacent normal tissues, indicating a universal effect of OGT in promoting tumorigenesis. Here, we show that OGT is essential for tumor growth in immunocompetent hosts by repressing the cyclic GMP-AMP synthase (cGAS)-dependent DNA sensing pathway. We found that deletion of OGT (Ogt -/- ) caused a marked reduction in tumor growth in both syngeneic tumor models and a genetic colorectal cancer (CRC) model induced by mutation of the Apc gene (Apc min ). Pharmacological inhibition or genetic deletion of OGT induced a robust genomic instability (GIN), leading to cGAS-dependent production of the type I interferon (IFN-I) and IFN-stimulated genes (ISGs). As a result, deletion of Cgas or Sting from Ogt -/- cancer cells restored tumor growth, and this correlated with impaired CD8+ T cell-mediated antitumor immunity. Mechanistically, we found that OGT-dependent cleavage of host cell factor C1 (HCF-1) is required for the avoidance of GIN and IFN-I production in tumors. In summary, our results identify OGT-mediated genomic stability and activate cGAS-STING pathway as an important tumor cell-intrinsic mechanism to repress antitumor immunity.

The multiple faces of cGAS-STING in antitumor immunity: prospects and challenges

As a key sensor of double-stranded DNA (dsDNA), cyclic GMP-AMP synthase (cGAS) detects cytosolic dsDNA and initiates the synthesis of 2'3' cyclic GMP-AMP (cGAMP) that activates the stimulator of interferon genes (STING). This finally promotes the production of type I interferons (IFN-I) that is crucial for bridging innate and adaptive immunity. Recent evidence show that several antitumor therapies, including radiotherapy (RT), chemotherapy, targeted therapies and immunotherapies, activate the cGAS-STING pathway to provoke the antitumor immunity. In the last decade, the development of STING agonists has been a major focus in both basic research and the pharmaceutical industry. However, up to now, none of STING agonists have been approved for clinical use. Considering the broad expression of STING in whole body and the direct lethal effect of STING agonists on immune cells in the draining lymph node (dLN), research on the optimal way to activate STING in tumor microenvironment (TME) appears to be a promising direction. Moreover, besides enhancing IFN-I signaling, the cGAS-STING pathway also plays roles in senescence, autophagy, apoptosis, mitotic arrest, and DNA repair, contributing to tumor development and metastasis. In this review, we summarize the recent advances on cGAS-STING pathway's response to antitumor therapies and the strategies involving this pathway for tumor treatment.

Cholesterol-binding motifs in STING that control endoplasmic reticulum retention mediate anti-tumoral activity of cholesterol-lowering compounds

The cGAS-STING pathway plays a crucial role in anti-tumoral responses by activating inflammation and reprogramming the tumour microenvironment. Upon activation, STING traffics from the endoplasmic reticulum (ER) to Golgi, allowing signalling complex assembly and induction of interferon and inflammatory cytokines. Here we report that cGAMP stimulation leads to a transient decline in ER cholesterol levels, mediated by Sterol O-Acyltransferase 1-dependent cholesterol esterification. This facilitates ER membrane curvature and STING trafficking to Golgi. Notably, we identify two cholesterol-binding motifs in STING and confirm their contribution to ER-retention of STING. Consequently, depletion of intracellular cholesterol levels enhances STING pathway activation upon cGAMP stimulation. In a preclinical tumour model, intratumorally administered cholesterol depletion therapy potentiated STING-dependent anti-tumoral responses, which, in combination with anti-PD-1 antibodies, promoted tumour remission. Collectively, we demonstrate that ER cholesterol sets a threshold for STING signalling through cholesterol-binding motifs in STING and we propose that this could be exploited for cancer immunotherapy.

The Janus-faced functions of Apolipoproteins L in membrane dynamics

The functions of human Apolipoproteins L (APOLs) are poorly understood, but involve diverse activities like lysis of bloodstream trypanosomes and intracellular bacteria, modulation of viral infection and induction of apoptosis, autophagy, and chronic kidney disease. Based on recent work, I propose that the basic function of APOLs is the control of membrane dynamics, at least in the Golgi and mitochondrion. Together with neuronal calcium sensor-1 (NCS1) and calneuron-1 (CALN1), APOL3 controls the activity of phosphatidylinositol-4-kinase-IIIB (PI4KB), involved in both Golgi and mitochondrion membrane fission. Whereas secreted APOL1 induces African trypanosome lysis through membrane permeabilization of the parasite mitochondrion, intracellular APOL1 conditions non-muscular myosin-2A (NM2A)-mediated transfer of PI4KB and APOL3 from the Golgi to the mitochondrion under conditions interfering with PI4KB-APOL3 interaction, such as APOL1 C-terminal variant expression or virus-induced inflammatory signalling. APOL3 controls mitophagy through complementary interactions with the membrane fission factor PI4KB and the membrane fusion factor vesicle-associated membrane protein-8 (VAMP8). In mice, the basic APOL1 and APOL3 activities could be exerted by mAPOL9 and mAPOL8, respectively. Perspectives regarding the mechanism and treatment of APOL1-related kidney disease are discussed, as well as speculations on additional APOLs functions, such as APOL6 involvement in adipocyte membrane dynamics through interaction with myosin-10 (MYH10).

The activation of the adaptor protein STING depends on its interactions with the phospholipid PI4P

Activation of the endoplasmic reticulum (ER)-resident adaptor protein STING, a component of a cytosolic DNA-sensing pathway, induces the transcription of genes encoding type I interferons (IFNs) and other proinflammatory factors. Because STING is activated at the Golgi apparatus, control of the localization and activation of STING is important in stimulating antiviral and antitumor immune responses. Through a genome-wide CRISPR interference screen, we found that STING activation required the Golgi-resident protein ACBD3, which promotes the generation of phosphatidylinositol 4-phosphate (PI4P) at the trans-Golgi network, as well as other PI4P-associated proteins. Appropriate localization and activation of STING at the Golgi apparatus required ACBD3 and the PI4P-generating kinase PI4KB. In contrast, STING activation was enhanced when the lipid-shuttling protein OSBP, which removes PI4P from the Golgi apparatus, was inhibited by the US Food and Drug Administration-approved antifungal itraconazole. The increase in the abundance of STING-activating phospholipids at the trans-Golgi network resulted in the increased production of IFN-β and other cytokines in THP-1 cells. Furthermore, a mutant STING that could not bind to PI4P failed to traffic from the ER to the Golgi apparatus in response to a STING agonist, whereas forced relocalization of STING to PI4P-enriched areas elicited STING activation in the absence of stimulation with a STING agonist. Thus, PI4P is critical for STING activation, and manipulating PI4P abundance may therapeutically modulate STING-dependent immune responses.

DGARM/C10orf76/ARMH3 for Ceramide Transfer Zone at the Endoplasmic Reticulum-Distal Golgi Contacts

Phosphatidylinositol 4-monophosphate (PtdIns(4)P) is one of the key membrane components which mark the membrane contact sites. In the mammalian Golgi complex, PtdIns(4)P is produced at various subregions via specific mechanisms for each site. Particularly, PtdIns(4)P pools generated at the distal Golgi regions are pivotal for the determination of membrane contacts between the endoplasmic reticulum (ER) and Golgi, at which inter-organelle lipid transport takes place. In this short review, we will focus on C10orf76 (or ARMH3), which we propose to rename as DGARM after a distal Golgi armadillo repeat protein, for its function in generating a PtdIns(4)P pool crucial for ER-to-distal Golgi ceramide transport. We further discuss from the viewpoint of the evolutionary conservation of DGARM.

Metabolic Enzymes in Viral Infection and Host Innate Immunity

Metabolic enzymes are central players for cell metabolism and cell proliferation. These enzymes perform distinct functions in various cellular processes, such as cell metabolism and immune defense. Because viral infections inevitably trigger host immune activation, viruses have evolved diverse strategies to blunt or exploit the host immune response to enable viral replication. Meanwhile, viruses hijack key cellular metabolic enzymes to reprogram metabolism, which generates the necessary biomolecules for viral replication. An emerging theme arising from the metabolic studies of viral infection is that metabolic enzymes are key players of immune response and, conversely, immune components regulate cellular metabolism, revealing unexpected communication between these two fundamental processes that are otherwise disjointed. This review aims to summarize our present comprehension of the involvement of metabolic enzymes in viral infections and host immunity and to provide insights for potential antiviral therapy targeting metabolic enzymes.

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.

Interferon induction by STING requires its translocation to the late endosomes

To combat microbial infections, mammalian cells use a variety of innate immune response pathways to induce synthesis of anti-microbial proteins. The cGAS/STING pathway recognizes cytoplasmic viral or cellular DNA to elicit signals that lead to type I interferon and other cytokine synthesis. cGAMP, synthesized by DNA-activated cGAS, activates the ER-associated protein, STING, which oligomerizes and translocates to other intracellular membrane compartments to trigger different branches of signaling. We have reported that, in the ER, EGFR-mediated phosphorylation of Tyr245 of STING is required for its transit to the late endosomes, where it recruits and activates the transcription factor IRF3 required for IFN induction. In the current study, we inquired whether STING Tyr245 phosphorylation per se or STING's location in the late endosomes was critical for its ability to recruit IRF3 and induce IFN. Using pharmacological inhibitors or genetic ablation of proteins that are essential for specific steps of STING trafficking, we demonstrated that the presence of STING in the late endosomal membranes, even without Tyr245 phosphorylation, was sufficient for IRF3-mediated IFN induction.

Membrane traffic governs the STING inflammatory signalling

The cGAS-STING innate immune pathway has recently emerged as a critical driver of inflammation in a variety of settings, such as virus infection, cellular stress and tissue damage. The pathway detects microbial and host-derived double-stranded DNA (dsDNA) in the cytosol, and triggers the production of the type I interferons through the activation of IRF3. The detailed mechanistic and biochemical understanding of the pathway has enabled the development of pharmacological agents for the treatment of chronic inflammation and cancer. STING is an endoplasmic reticulum (ER)-localized transmembrane protein. Upon emergence of cytosolic dsDNA, STING exits the ER and migrates sequentially to the Golgi, recycling endosomes and lysosomes. Importantly, the intracellular translocation of STING is essential for the activation and inactivation of the STING signalling. In this review, I summarize the recent insights into the regulators of the membrane traffic of STING and STING-associated autoinflammatory diseases.

Mitochondrial DNA and the STING pathway are required for hepatic stellate cell activation

BACKGROUND AND AIMS

TGF-β induces multiple structural and functional changes in quiescent HSCs, including an increase in proliferation, mitochondrial mass, and matrix deposition. HSC transdifferentiation requires significant bioenergetic capacity, and it is not known how TGF-β-mediated transcriptional upregulation is coordinated with the bioenergetic capacity of HSCs.

APPROACH AND RESULTS

Mitochondria are key bioenergetic organelles, and here, we report that TGF-β induces release of mitochondrial DNA (mtDNA) from healthy HSCs through voltage-dependent anion channels (VDACs), with the formation of an mtDNA-CAP on the external mitochondrial membrane. This stimulates organization of cytosolic cyclic GMP-AMP synthase (cGAS) onto the mtDNA-CAP and subsequent activation of the cGAS-STING-IRF3 pathway. TGF-β is unable to induce conversion of HSCs from a quiescent to a transdifferentiated phenotype in the absence of mtDNA, VDAC, or stimulator of interferon genes (STING). Transdifferentiation by TGF-β is blocked by a STING inhibitor, which also reduces liver fibrosis prophylactically and therapeutically.

CONCLUSIONS

We have identified a pathway that requires the presence of functional mitochondria for TGF-β to mediate HSC transcriptional regulation and transdifferentiation and therefore provides a key link between bioenergetic capacity of HSCs and signals for transcriptional upregulation of genes of anabolic pathways.

High-Intensity Interval Training Ameliorates High-Fat Diet-Induced Metabolic Disorders via the Cyclic GMP-AMP Synthase-Stimulator of Interferon Gene Signaling Pathway

Metabolic diseases are growing in prevalence worldwide. Although the pathogenesis of metabolic diseases remains ambiguous, the correlation between cyclic GMP-AMP synthase (cGAS)-stimulator of interferon gene (STING) and metabolic diseases has been identified recently. Exercise is an effective intervention protecting against metabolic diseases, however, the role of the cGAS-STING signaling pathway in this process is unclear, and the effect and mechanism of different exercise intensities on metabolic disorders are still unknown. Thus, we explored the association between exercise to ameliorate HFD-induced metabolic disorders and the cGAS-STING signaling pathway and compared the effects of high-intensity interval training (HIIT) and moderate-intensity continuous training (MICT). Male C57BL/6 mice (6-8 weeks old) were fed HFD for 8 weeks to establish a metabolic disease model and were subjected to 8-week MICT or HIIT training. Glucose tolerance tests (GTT) and insulin tolerance tests (ITT) were used to assess glucose metabolism. Serum triglyceride (TG) and total cholesterol (TC) were measured to evaluate lipid metabolism. Oil red staining was used to observe the lipid droplets in the gastrocnemius muscle. An enzyme-linked immunosorbent assay was used to detect the serum inflammatory factors IL-6 and IFN-β. The protein expression of the cGAS-STING signaling pathway was detected by the WesTM automatic protein expression analysis system. We reported that HFD induced metabolic disorders with obesity, abnormal glucolipid metabolism, and significant inflammatory responses. Both HIIT and MICT ameliorated the above adverse reactions, but MICT was superior to HIIT in improving glucolipid disorders. Additionally, HIIT significantly increased the expression of STING protein, as well as the phosphorylation of TBKI and the ratio of p-IRF3/IRF3. MICT only increased the expression of STING protein. Our findings suggest that HIIT may alleviate HFD-induced metabolic disorder phenotype through the cGAS-STING signaling pathway. However, the improvement of MICT on metabolic disorder phenotype is less associated with the cGAS-STING pathway, which needs to be further explored.

Mechanistic insights into the role of herpes simplex virus 1 in Alzheimer's disease

Alzheimer's Disease (AD) is an aging-associated neurodegenerative disorder, threatening millions of people worldwide. The onset and progression of AD can be accelerated by environmental risk factors, such as bacterial and viral infections. Human herpesviruses are ubiquitous infectious agents that underpin numerous inflammatory disorders including neurodegenerative diseases. Published studies concerning human herpesviruses in AD imply an active role HSV-1 in the pathogenesis of AD. This review will summarize the current understanding of HSV-1 infection in AD and highlight some barriers to advance this emerging field.

cGAS-STING signaling in cell death: Mechanisms of action and implications in pathologies

Cyclic GMP-AMP synthase (cGAS) monitors dsDNA in the cytosol in response to pathogenic invasion or tissue injury, initiating cGAS-STING signaling cascades that regulate various cellular physiologies, including IFN /cytokine production, autophagy, protein synthesis, metabolism, senescence, and distinct types of cell death. cGAS-STING signaling is crucial for host defense and tissue homeostasis; however, its dysfunction frequently leads to infectious, autoimmune, inflammatory, degenerative, and cancerous diseases. Our knowledge regarding the relationships between cGAS-STING signaling and cell death is rapidly evolving, highlighting their essential roles in pathogenesis and disease progression. Nevertheless, the direct control of cell death by cGAS-STING signaling, rather than IFN/NF-κB-mediated transcriptional regulation, remains relatively unexplored. This review examines the mechanistic interplays between cGAS-STING cascades and apoptosis, necroptosis, pyroptosis, ferroptosis, and autophagic/lysosomal cell death. We will also discuss their pathological implications in human diseases, particularly in autoimmunity, cancer, and organ injury scenarios. We hope that this summary will stimulate discussion for further exploration of the complex life-or-death responses to cellular damage mediated by cGAS-STING signaling.

STeerING PI4P for innate immune activation

STING transverses the endoplasmic reticulum (ER), Golgi, and endosomal compartments before its degradation within the lysosomes. In this issue of Immunity, Fang et al. demonstrate that the enrichment of cholesterol and sphingomyelin in the trans-Golgi network and endosomes mediated by the ARMH3-PI4KB-PI4P pathway plays a pivotal role in STING activation under cGAS-dependent and -independent conditions.