Hiroyasu Konno

Cyclic Dinucleotides Trigger ULK1 (ATG1) Phosphorylation of STING to Prevent Sustained Innate Immune Signaling

Activation of the stimulator of interferon genes (STING) pathway by microbial or self-DNA, as well as cyclic dinucleotides (CDNs), results in the induction of numerous genes that suppress pathogen replication and facilitate adaptive immunity. However, sustained gene transcription is rigidly prevented to avoid lethal STING-dependent proinflammatory disease by mechanisms that remain unknown. We demonstrate here that, after autophagy-dependent STING delivery of TANK-binding kinase 1 (TBK1) to endosomal/lysosomal compartments and activation of transcription factors interferon regulatory factor 3 (IRF3) and NF-κB, STING is subsequently phosphorylated by serine/threonine UNC-51-like kinase (ULK1/ATG1), and IRF3 function is suppressed. ULK1 activation occurred following disassociation from its repressor AMP activated protein kinase (AMPK) and was elicited by CDNs generated by the cGAMP synthase, cGAS. Thus, although CDNs may initially facilitate STING function, they subsequently trigger negative-feedback control of STING activity, thus preventing the persistent transcription of innate immune genes.

STING Recognition of Cytoplasmic DNA Instigates Cellular Defense

How the cell recognizes cytosolic DNA including DNA-based microbes to trigger host-defense-related gene activation remains to be fully resolved. Here, we demonstrate that STING (stimulator of interferon genes), an endoplasmic reticulum translocon-associated transmembrane protein, acts to detect cytoplasmic DNA species. STING homodimers were able to complex with self- (apoptotic, necrotic) or pathogen-related ssDNA and dsDNA and were indispensible for HSV-1-mediated transcriptional activation of a wide array of innate immune and proinflammatory genes in addition to type I IFN. Our data indicate that STING instigates cytoplasmic DNA-mediated cellular defense gene transcription and facilitates adoptive responses that are required for protection of the host. In contrast, chronic STING activation may manifest inflammatory responses and possibly autoimmune disease triggered by self-DNA.

Inflammation-driven carcinogenesis is mediated through STING

Chronic stimulation of innate immune pathways by microbial agents or damaged tissue is known to promote inflammation-driven tumorigenesis by mechanisms that are not well understood. Here we demonstrate that mutagenic 7,12-dimethylbenz(a)anthracene (DMBA), cisplatin and etoposide induce nuclear DNA leakage into the cytosol that intrinsically activates stimulator of interferon genes (STING)-dependent cytokine production. Inflammatory cytokine levels are subsequently augmented in a STING-dependent extrinsic manner by infiltrating phagocytes purging dying cells. Consequently, STING(-/-) mice, or wild-type mice adoptively transferred with STING(-/-) bone marrow, are almost completely resistant to DMBA-induced skin carcinogenesis compared with their wild-type counterparts. Our data establish a role for STING in the control of cancer, shed significant insight into the causes of inflammation-driven carcinogenesis and may provide a basis for therapeutic strategies to help prevent malignant disease.

Deregulation of STING Signaling in Colorectal Carcinoma Constrains DNA Damage Responses and Correlates With Tumorigenesis.

Stimulator of interferon genes (STING) has been shown to be critical for controlling antiviral responses as well as anti-tumor adaptive immunity, but little is known regarding its regulation in human tumors. Here, we report that STING signaling is recurrently suppressed in a wide variety of cancers, including colorectal carcinoma. Loss of STING signaling impeded DNA damage responses accountable for generating key cytokines that facilitate tissue repair and anti-tumor T cell priming, such as type I interferons (IFNs). Correspondingly, defective STING function was also highly predictive of effectual DNA-virus-mediated oncolytic activity. Thus, impaired STING responses may enable damaged cells to evade host immunosurveillance processes, although they provide a critical prognostic measurement that could help predict the outcome of effective oncoviral therapy.

Diverse roles of STING-dependent signaling on the development of cancer

Stimulator of interferon genes (STING) is a cellular sensor that controls cytosolic DNA-activated innate immune signaling. We have previously demonstrated that STING-deficient mice are resistant to carcinogen-induced skin cancer, similar to myeloid differentiation primary response gene 88 (MyD88) deficient mice, since the production of STING-dependent DNA-damage-induced proinflammatory cytokines, that likely require MyD88 signaling to exert their growth-promoting activity, are prevented. In contrast, MyD88-deficient mice are sensitive to colitis-associated cancer (CAC), since selected cytokines generated following DNA-damage also activate repair pathways, which can help prevent tumor development. Here, we demonstrate that STING signaling facilitates wound repair processes and that analogous to MyD88-deficient mice, STING-deficient mice (SKO) are prone to CAC induced by DNA-damaging agents. SKO mice harboring tumors exhibited low levels of tumor-suppressive interleukin-22 binding protein (IL-22BP) compared to normal mice, a cytokine considered critical for preventing colon-related cancer. Our data indicate that STING constitutes a critical component of the host early response to intestinal damage and is essential for invigorating tissue repair pathways that may help prevent tumorigenesis.

Recurrent Loss of STING Signaling in Melanoma Correlates with Susceptibility to Viral Oncolysis.

The innate immunoregulator STING stimulates cytokine production in response to the presence of cytosolic DNA, which can arise following DNA damage. Extrinsic STING signaling is also needed for antigen-presenting cells to stimulate antitumor T-cell immunity. Here, we show that STING signaling is recurrently suppressed in melanoma cells, where this event may enable immune escape after DNA damage. Mechanistically, STING signaling was suppressed most frequently by epigenetic silencing of either STING or the cyclic GMP-AMP synthase, which generates STING-activating cyclic dinucleotides after binding cytosolic DNA species. Loss of STING function rendered melanoma cells unable to produce type I IFN and other immune cytokines after exposure to cytosolic DNA species. Consequently, such cells were highly susceptible to infection with DNA viruses including HSV1, a variant of which is being developed presently as a therapeutic oncolytic virus [talimogene laherparepvec (T-VEC)]. Our findings provide insight into the basis for susceptibility to viral oncolysis by agents such as HSV1. Cancer Res; 76(22); 6747-59. ©2016 AACR.

Activation of STING requires palmitoylation at the Golgi

Stimulator of interferon genes (STING) is essential for the type I interferon response against DNA pathogens. In response to the presence of DNA and/or cyclic dinucleotides, STING translocates from the endoplasmic reticulum to perinuclear compartments. However, the role of this subcellular translocation remains poorly defined. Here we show that palmitoylation of STING at the Golgi is essential for activation of STING. Treatment with palmitoylation inhibitor 2-bromopalmitate (2-BP) suppresses palmitoylation of STING and abolishes the type I interferon response. Mutation of two membrane-proximal Cys residues (Cys88/91) suppresses palmitoylation, and this STING mutant cannot induce STING-dependent host defense genes. STING variants that constitutively induce the type I interferon response were found in patients with autoimmune diseases. The response elicited by these STING variants is effectively inhibited by 2-BP or an introduction of Cys88/91Ser mutation. Our results may lead to new treatments for cytosolic DNA-triggered autoinflammatory diseases.

The STING controlled cytosolic-DNA activated innate immune pathway and microbial disease

The innate immune system is critically important for the primary sensing of invading pathogens. Over the past decade, the cellular sensors important for recognizing microbial entry into the host cell have been largely elucidated. These sensors, some of which are evolutionarily conserved, include the Toll-like receptor (TLR) and RIG-I-like helicase family (RLH) pathway that can recognize bacterial and viral non-self nucleic acid. In addition, a cellular sensor referred to as STING (for stimulator of interferon genes) has been shown to be critical for triggering host defense countermeasures, including stimulation of the adaptive immune response, following the detection of cytosolic DNA species. The STING pathway has now been shown to be critical for activating innate immune gene transcription in response to infection by DNA pathogens such as herpes simplex virus 1 (HSV1) as well as retroviruses. In addition, it is clear that chronic STING activation can also cause autoinflammatory disease manifested by self-DNA. Here we review recent developments in our understanding of STING function, including importance in the control of microbial disease.

Ubiquitination of STING at lysine 224 controls IRF3 activation

The E3 ligase MUL1 regulates STING-dependent innate immune responses to cytosolic DNA. Fine-tuning the STING In eukaryotic cells, cytosolic DNA is often an indicator of viral infection. STING (stimulator of interferon genes), an endoplasmic reticulum–associated protein, is a central node that connects cytosolic DNA sensing with transcription factors (such as interferon regulatory factors) that drive antiviral host responses. STING activation is tightly regulated. Chronic STING activation has been documented in autoinflammatory settings, whereas STING agonists are being considered as a means to activate innate immune responses to cancers. Here, Ni et al. show that MUL1 (mitochondrial E3 ubiquitin protein ligase 1) ubiquitinates STING at lysine 224 to promote STING-dependent production of antiviral cytokines and chemokines. By extending our understanding of STING activation, these studies may lead to new approaches to modulating STING activation in various settings. Cytosolic DNA species derived from invading microbes or leaked from the nuclear or mitochondrial compartments of the cell can trigger the induction of host defense genes by activating the endoplasmic reticulum–associated protein STING (stimulator of interferon genes). Using a mass spectrometry–based approach, we show that after association with cyclic dinucleotides, delivery of Tank-binding kinase 1 to interferon regulatory factors (IRFs), such as IRF3, relies on K63-linked ubiquitination of K224 on STING. Blocking K224 ubiquitination specifically prevented IRF3 but not nuclear factor κB activation, additionally indicating that STING trafficking is not required to stimulate the latter signaling pathway. By carrying out a limited small interfering RNA screen, we have identified MUL1 (mitochondrial E3 ubiquitin protein ligase 1) as an E3 ligase that catalyzes the ubiquitination of STING on K224. These data demonstrate the critical role of K224 ubiquitination in STING function and provide molecular insight into the mechanisms governing host defense responses.

Suppression of STING signaling through epigenetic silencing and missense mutation impedes DNA damage mediated cytokine production

The production of cytokines in response to DNA-damage events may be an important host defense response to help prevent the escape of pre-cancerous cells. The innate immune pathways involved in these events are known to be regulated by cellular molecules such as stimulator of interferon genes (STING), which controls type I interferon and pro-inflammatory cytokine production in response to the presence of microbial DNA or cytosolic DNA that has escaped from the nucleus. STING signaling has been shown to be defective in a variety of cancers, such as colon cancer and melanoma, actions that may enable damaged cells to escape the immunosurveillance system. Here, we report through examination of databases that STING signaling may be commonly suppressed in a greater variety of tumors due to loss-of-function mutation or epigenetic silencing of the STING/cGAS promoter regions. In comparison, RNA activated innate immune pathways controlled by RIG-I/MDA5 were significantly less affected. Examination of reported missense STING variants confirmed that many exhibited a loss-of-function phenotype and could not activate cytokine production following exposure to cytosolic DNA or DNA-damage events. Our data imply that the STING signaling pathway may be recurrently suppressed by a number of mechanisms in a considerable variety of malignant disease and be a requirement for cellular transformation.