Apoptotic caspases suppress mtDNA-induced STING-mediated type I IFN production

Salidroside prevents skin carcinogenesis induced by DMBA/TPA in a mouse model through suppression of inflammation and promotion of apoptosis

Salidroside (SR) is a main component of Rhodiola rosea L. and exhibits a variety of pharmacologic properties. The present study was carried out to explore the potential effect of SR against skin cancer induced by 7,12-dimethylbenz(a)anthracene (DMBA) and 12-O-tetradecanoylphorbol-13‑acetate (TPA) in female Institute for Cancer Research (ICR) mice and to reveal the underlying molecular targets regulated by SR. The mice were randomly divided into 4 groups: control, DMBA/TPA, DMBA/TPA+SR (20 mg/kg) and DMBA/TPA+SR (40 mg/kg). SR was administered to mice five times a week after DMBA treatments. In our study, we found that SR dose-dependently ameliorated skin cancer incidence and the multiplicity in the animal models by reducing the release of inflammation-related cytokines, including tumor necrosis factor α (TNF-α), interleukin-1β (IL-1β), interleukin-18 (IL-18), interleukin-6 (IL-6), cyclooxygenase 2 (COX2) and transforming growth factor β-1 (TGF-β1). Suppression of the nuclear factor (NF)-κB signaling pathway by SR was effective to prevent skin carcinogenesis. Furthermore, TUNEL analysis indicated that compared to the DMBA/TPA group, enhanced apoptosis was observed in the DMBA/TPA+SR group. In addition, p53 expression levels were increased by SR in the DMBA/TPA-induced mice. Therefore, SR was effective for inducing apoptosis during skin cancer progression triggered by DMBA/TPA. Consistently, p21, p53 upregulated modulator of apoptosis (PUMA), Bax and caspase-3 were highly induced by SR to enhance the apoptotic response for preventing skin cancer. Moreover, in vitro, we found that SR dramatically reduced the inflammatory response, while enhancing the aoptotic response by blocking NF-κB and activating caspase-3 pathways, respectively. In addition, flow cytometric analysis further confirmed the induction of apoptosis by SR in DMBA-treated cells in vitro. Taken together, the in vivo and in vitro studies illustrated that SR might be a promising compound to reduce skin cancer risk.

Mitochondrial DNA and TLR9 drive muscle inflammation upon Opa1 deficiency

Opa1 participates in inner mitochondrial membrane fusion and cristae morphogenesis. Here, we show that muscle-specific Opa1 ablation causes reduced muscle fiber size, dysfunctional mitochondria, enhanced Fgf21, and muscle inflammation characterized by NF-κB activation, and enhanced expression of pro-inflammatory genes. Chronic sodium salicylate treatment ameliorated muscle alterations and reduced the muscle expression of Fgf21. Muscle inflammation was an early event during the progression of the disease and occurred before macrophage infiltration, indicating that it is a primary response to Opa1 deficiency. Moreover, Opa1 repression in muscle cells also resulted in NF-κB activation and inflammation in the absence of necrosis and/or apoptosis, thereby revealing that the activation is a cell-autonomous process and independent of cell death. The effects of Opa1 deficiency on the expression NF-κB target genes and inflammation were absent upon mitochondrial DNA depletion. Under Opa1 deficiency, blockage or repression of TLR9 prevented NF-κB activation and inflammation. Taken together, our results reveal that Opa1 deficiency in muscle causes initial mitochondrial alterations that lead to TLR9 activation, and inflammation, which contributes to enhanced Fgf21 expression and to growth impairment.

Systemic lupus erythematosus and systemic sclerosis: All roads lead to platelets

Systemic lupus erythematosus (SLE) and systemic sclerosis (SSc) are two phenotypically distincts inflammatory systemic diseases. However, SLE and SSc share pathogenic features such as interferon signature, loss of tolerance against self-nuclear antigens and increased tissue damage such as fibrosis. Recently, platelets have emerged as a major actor in immunity including auto-immune diseases. Both SLE and SSc are characterized by strong platelet system activation, which is likely to be both the witness and culprit in their pathogenesis. Platelet activation pathways are multiple and sometimes redundant. They include immune complexes, Toll-like receptors activation, antiphospholipid antibodies and ischemia-reperfusion associated with Raynaud phenomenon. Once activated, platelet promote immune dysregulation by priming interferon production by immune cells, providing CD40L supporting B lymphocyte functions and providing a source of autoantigens. Platelets are actively implicated in SLE and SSc end-organ damage such as cardiovascular and renal disease and in the promotion of tissue fibrosis. Finally, after understanding the main pathogenic implications of platelet activation in both diseases, we discuss potential therapeutics targeting platelets.

The cGAS-cGAMP-STING pathway connects DNA damage to inflammation, senescence, and cancer

The cGAS–cGAMP–STING pathway mediates immune and inflammatory responses to cytosolic DNA. This review summarizes recent findings on how genomic instability leads to cGAS activation and how this pathway critically connects DNA damage to autoinflammatory diseases, cellular senescence, and cancer.

Detection of microbial DNA is an evolutionarily conserved mechanism that alerts the host immune system to mount a defense response to microbial infections. However, this detection mechanism also poses a challenge to the host as to how to distinguish foreign DNA from abundant self-DNA. Cyclic guanosine monophosphate (GMP)–adenosine monophosphate (AMP) synthase (cGAS) is a DNA sensor that triggers innate immune responses through production of the second messenger cyclic GMP-AMP (cGAMP), which binds and activates the adaptor protein STING. However, cGAS can be activated by double-stranded DNA irrespective of the sequence, including self-DNA. Although how cGAS is normally kept inactive in cells is still not well understood, recent research has provided strong evidence that genomic DNA damage leads to cGAS activation to stimulate inflammatory responses. This review summarizes recent findings on how genomic instability and DNA damage trigger cGAS activation and how cGAS serves as a link from DNA damage to inflammation, cellular senescence, and cancer.

Combination Cancer Therapy with Immune Checkpoint Blockade: Mechanisms and Strategies

The success of immune checkpoint blockade in patients with a wide variety of malignancies has changed the treatment paradigm in oncology. However, combination therapies with immune checkpoint blockade will be needed to overcome resistance and broaden the clinical utility of immunotherapy. Here we discuss a framework for rationally designing combination therapy strategies based on enhancing major discriminatory functions of the immune system that are corrupted by cancer-namely, antigenicity, adjuvanticity, and homeostatic feedback inhibition. We review recent advances on how conventional genotoxic cancer therapies, molecularly targeted therapies, epigenetic agents, and immune checkpoint inhibitors can restore these discriminatory functions. Potential barriers that can impede response despite combination therapy are also discussed.

Apoptotic Caspases: Multiple or Mistaken Identities?

The mitochondrial caspase cascade was originally thought to be required for apoptotic death driven by Bak/Bax-mediated intrinsic apoptosis. It has also been ascribed several 'non-apoptotic' functions, including differentiation, proliferation, and cellular reprogramming. Recent work has demonstrated that, during apoptosis, the caspase cascade suppresses damage-associated molecular pattern (DAMP)-initiated production of cytokines such as type I interferon by the dying cell. The caspase cascade is not required for death to occur; instead, it shapes the immunogenic properties of the apoptotic cell. This raises questions about the role of apoptotic caspases in regulating DAMP signaling more generally, puts a new perspective on their non-apoptotic functions, and suggests that pharmacological caspase inhibitors might find new applications as antiviral or anticancer agents.

STING dependent sensing - Does HIV actually care?

Sensing of DNA is essential for the innate immune system to detect threats, like viruses, intracellular bacteria or cellular DNA damage. At the centre of this conserved mammalian mechanism stands the adaptor protein STING. STING is highly regulated and is part of a complex signalling network. This network depends on the sensors cGAS and IFI16 to detect misplaced DNA in the cytoplasm as well as on the kinase TBK1 and the transcription factor IRF3. The DNA sensing machinery has been implicated in many diseases, among others HIV. Here we present a comprehensive review of current status on the STING pathway with all its components and regulations related to HIV pathogenesis. By this, we try to answer the question if STING-mediated DNA sensing plays a role in HIV infections.

Mitochondrial DNA as an inflammatory mediator in cardiovascular diseases

Mitochondria play a central role in multiple cellular functions, including energy production, calcium homeostasis, and cell death. Currently, growing evidence indicates the vital roles of mitochondria in triggering and maintaining inflammation. Chronic inflammation without microbial infection — termed sterile inflammation — is strongly involved in the development of heart failure. Sterile inflammation is triggered by the activation of pattern recognition receptors (PRRs) that sense endogenous ligands called damage-associated molecular patterns (DAMPs). Mitochondria release multiple DAMPs including mitochondrial DNA, peptides, and lipids, which induce inflammation via the stimulation of multiple PRRs. Among the mitochondrial DAMPs, mitochondrial DNA (mtDNA) is currently highlighted as the DAMP that mediates the activation of multiple PRRs, including Toll-like receptor 9, Nod-like receptors, and cyclic GMP–AMP synthetase/stimulator of interferon gene pathways. These PRR signalling pathways, in turn, lead to the activation of nuclear factor-κB and interferon regulatory factor, which enhances the transcriptional activity of inflammatory cytokines and interferons, and induces the recruitment of inflammatory cells. As the heart is an organ comprising abundant mitochondria for its ATP consumption (needed to maintain constant cyclic contraction and relaxation), the generation of massive amounts of mitochondrial radical oxygen species and mitochondrial DAMPs are predicted to occur and promote cardiac inflammation. Here, we will focus on the role of mtDNA in cardiac inflammation and review the mechanism and pathological significance of mtDNA-induced inflammatory responses in cardiac diseases.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

Mitochondrial metabolism and cancer

Glycolysis has long been considered as the major metabolic process for energy production and anabolic growth in cancer cells. Although such a view has been instrumental for the development of powerful imaging tools that are still used in the clinics, it is now clear that mitochondria play a key role in oncogenesis. Besides exerting central bioenergetic functions, mitochondria provide indeed building blocks for tumor anabolism, control redox and calcium homeostasis, participate in transcriptional regulation, and govern cell death. Thus, mitochondria constitute promising targets for the development of novel anticancer agents. However, tumors arise, progress, and respond to therapy in the context of an intimate crosstalk with the host immune system, and many immunological functions rely on intact mitochondrial metabolism. Here, we review the cancer cell-intrinsic and cell-extrinsic mechanisms through which mitochondria influence all steps of oncogenesis, with a focus on the therapeutic potential of targeting mitochondrial metabolism for cancer therapy.

On taking the STING out of immune activation

Nearly a decade ago, an endoplasmic reticulum (ER) adaptor protein called stimulator of interferon genes (STING) was found to be critical in the induction of type I IFN production in response to DNA virus infection. STING functions by sensing cytoplasmic DNA and activates key transcription factors, including IFN regulatory factor (IRF)-3 and IRF7, to initiate type I IFN expression. Type I IFNs are vital in immunity against viral infections and can influence cancer cell proliferation, migration, and apoptosis. Several studies have shown that STING activation results in potent antitumor activity by generating strong tumor-specific cytotoxic T-cell responses. Moreover, compared with wild-type, STING-knockout mice show greater susceptibility to viral infections. In this review, we discuss the importance of STING signaling during the induction of immune responses, especially those associated with type I IFN in viral infections and tumor immunity. Furthermore, we highlight recent data that unravel how the STING signaling pathway can be negatively regulated.

An updated view on the functions of caspases in inflammation and immunity

The binary classification of mammalian caspases as either apoptotic or inflammatory is now obsolete. Emerging data indicate that all mammalian caspases are intricately involved in the regulation of inflammation and immunity. They participate in embryonic and adult tissue homeostasis, control leukocyte differentiation, activation and effector functions, and mediate innate and adaptive immunity signaling. Caspases also promote host resistance by regulating anti-oxidant defense and pathogen clearance through regulation of phagosomal maturation, actin dynamics and phagosome-lysosome fusion. Beyond apoptosis, they regulate inflammatory cell death, eliciting rapid pyroptosis of infected cells, while inhibiting necroptosis-mediated tissue destruction and chronic inflammation. In this review, we describe the cellular and molecular mechanisms underlying non-apoptotic functions of caspases in inflammation and immunity and provide an updated view of their functions as central regulators of tissue homeostasis and host defense.

Negative regulation of type I IFN signaling

Type I IFNs (α, β, and others) are a family of cytokines that are produced in physiological conditions as well as in response to the activation of pattern recognition receptors. They are critically important in controlling the host innate and adaptive immune response to viral and some bacterial infections, cancer, and other inflammatory stimuli. However, dysregulation of type I IFN production or response can contribute to immune pathologies termed "interferonopathies", pointing to the importance of balanced activating signals with tightly regulated mechanisms of tuning this signaling. Here, we summarize the recent advances of how type I IFN production and response are controlled at multiple levels of the type I IFN signaling cascade.

Mitochondrial DNA damage and subsequent activation of Z-DNA binding protein 1 links oxidative stress to inflammation in epithelial cells

This report identifies mitochondrial DNA (mtDNA) as a target and active mediator that links low-level oxidative stress to inflammatory response in pulmonary epithelial cells. Extrusion of mtDNA into the bronchoalveolar lavage fluid occurs as an early event in mice subjected to cigarette smoke injury, concomitantly with the depletion of mtDNA in the lung tissue. In cultured lung epithelial cells, prolonged, low-level oxidative stress damages the mtDNA, without any detectable damage to the nuclear DNA. In turn, cellular depletion of the mtDNA occurs, together with a transient remodeling of cellular bioenergetics and morphology - all without any detectable impairment in overall cell viability. Damaged mtDNA first enters the cytoplasm, where it binds to Z-DNA binding protein 1 (ZBP1) and triggers inflammation via the TANK-binding kinase 1 /interferon regulatory factor 3 signaling pathway. Fragments of the mtDNA are subsequently released into the extracellular space via exosomes. MtDNA-containing exosomes are capable of inducing an inflammatory response in naïve (non-oxidatively stressed) epithelial cells. In vivo, administration of isolated mtDNA into the in lungs of naïve mice induces the production of pro-inflammatory mediators, without histopathologic evidence of tissue injury. We propose that mtDNA-specific damage, and subsequent activation of the ZBP1 pathway, is a mechanism that links prolonged, low-level oxidative stress to autocrine and paracrine inflammation during the early stages of inflammatory lung disease.

The Aging Mitochondria

Mitochondrial dysfunction is a central event in many pathologies and contributes as well to age-related processes. However, distinguishing between primary mitochondrial dysfunction driving aging and a secondary mitochondrial impairment resulting from other cell alterations remains challenging. Indeed, even though mitochondria undeniably play a crucial role in aging pathways at the cellular and organismal level, the original hypothesis in which mitochondrial dysfunction and production of free radicals represent the main driving force of cell degeneration has been strongly challenged. In this review, we will first describe mitochondrial dysfunctions observed in aged tissue, and how these features have been linked to mitochondrial reactive oxygen species (ROS)–mediated cell damage and mitochondrial DNA (mtDNA) mutations. We will also discuss the clues that led to consider mitochondria as the starting point in the aging process, and how recent research has showed that the mitochondria aging axis represents instead a more complex and multifactorial signaling pathway. New working hypothesis will be also presented in which mitochondria are considered at the center of a complex web of cell dysfunctions that eventually leads to cell senescence and death.

Lymphocytes eject interferogenic mitochondrial DNA webs in response to CpG and non-CpG oligodeoxynucleotides of class C

Circulating mitochondrial DNA (mtDNA) is receiving increasing attention as a danger-associated molecular pattern in conditions such as autoimmunity, cancer, and trauma. We report here that human lymphocytes [B cells, T cells, natural killer (NK) cells], monocytes, and neutrophils derived from healthy blood donors, as well as B cells from chronic lymphocytic leukemia patients, rapidly eject mtDNA as web filament structures upon recognition of CpG and non-CpG oligodeoxynucleotides of class C. The release was quenched by ZnCl₂, independent of cell death (apoptosis, necrosis, necroptosis, autophagy), and continued in the presence of TLR9 signaling inhibitors. B-cell mtDNA webs were distinct from neutrophil extracellular traps concerning structure, reactive oxygen species (ROS) dependence, and were devoid of antibacterial proteins. mtDNA webs acted as rapid (within minutes) messengers, priming antiviral type I IFN production. In summary, our findings point at a previously unrecognized role for lymphocytes in antimicrobial defense, utilizing mtDNA webs as signals in synergy with cytokines and natural antibodies, and cast light on the interplay between mitochondria and the immune system.

Apoptosis in megakaryocytes and platelets: the life and death of a lineage

Despite their profoundly different cellular composition, size, and function, megakaryocytes and platelets both depend on restraint of the intrinsic (or "mitochondrial") apoptosis pathway by BCL-2 family prosurvival proteins for their development and viability. Activation of the pathway contributes to the clearance of megakaryocytes following platelet shedding and constrains platelet lifespan in the circulation. Important questions remain as to how apoptosis is initiated in these cells at steady state and in response to pathophysiological insults.

Therapeutic opportunities based on caspase modulation

Caspases are a family of proteolytic enzymes that play a critical role in the regulation of programmed cell death via apoptosis. Activation of caspases is frequently impaired in human cancers, contributing to cancer formation, progression and therapy resistance. A better understanding of the molecular mechanisms regulating caspase activation in cancer cells is therefore highly important. Thus, targeted modulation of caspase activation and apoptosis represents a promising approach for the development of new therapeutic options to elucidate cancer cell death.

Cytosolic sensing of immuno-stimulatory DNA, the enemy within

In the cytoplasm, DNA is sensed as a universal danger signal by the innate immune system. Cyclic GMP-AMP synthase (cGAS) is a cytosolic DNA sensor/enzyme that catalyzes formation of 2'-5'-cGAMP, an atypical cyclic di-nucleotide second messenger that binds and activates the Stimulator of Interferon Genes (STING), resulting in recruitment of Tank Binding Kinase 1 (TBK1), activation of the transcription factor Interferon Regulatory Factor 3 (IRF3), and trans-activation of innate immune response genes, including type I Interferon cytokines (IFN-I). Activation of the pro-inflammatory cGAS-STING-IRF3 response is triggered by direct recognition of the DNA genomes of bacteria and viruses, but also during RNA virus infection, neoplastic transformation, tumor immunotherapy and systemic auto-inflammatory diseases. In these circumstances, the source of immuno-stimulatory DNA has often represented a fundamental yet poorly understood aspect of the response. This review focuses on recent findings related to cGAS activation by an array of self-derived DNA substrates, including endogenous retroviral elements, mitochondrial DNA (mtDNA) and micronuclei generated as a result of genotoxic stress and DNA damage. These findings emphasize the role of the cGAS axis as a cell-intrinsic innate immune response to a wide variety of genomic insults.

Autophagy induced during apoptosis degrades mitochondria and inhibits type I interferon secretion

Cells undergoing Bax/Bak-mediated apoptosis exhibit signs of autophagy, but how it is activated and its significance is unknown. By directly activating Bax/Bak with BH3-only proteins or BH3 mimetic compounds, we demonstrate that mitochondrial damage correlated with a rapid increase in intracellular [AMP]/[ATP], phosphorylation of 5' AMP-activated protein kinase (AMPK), and activation of unc-51 like autophagy activating kinase 1 (ULK1). Consequently, autophagic flux was triggered early in the apoptotic pathway, as activation of the apoptosome and caspases were not necessary for its induction. Bax/Bak-triggered autophagy resulted in the clearance of damaged mitochondria in an ATG5/7-dependent manner that did not require Parkin. Importantly, Bax/Bak-mediated autophagy inhibited the secretion of the pro-inflammatory cytokine interferon-β (IFN-β) produced in response to mitochondrial damage, but not another cytokine interleukin-6 (IL-6). These findings show that Bax/Bak stimulated autophagy is essential for ensuring immunological silence during apoptosis.

ASK family in infection and inflammatory disease

Living organisms are continuously exposed to pathogens such as viruses and bacteria. Soon after a limited number of germline-encoded receptors, called pathogen recognition receptors, sense pathogen-associated molecular patterns, hosts trigger innate immune responses, including production of type Ⅰ interferons, proinflammatory cytokines, and cellular apoptosis, to limit propagation of invading pathogens. Importantly, these host responses are also activated during inflammatory diseases, irrespective of pathogen infection, and often play a causal role in pathogenesis and progression of these diseases, thereby implying an intimate link between immune responses and inflammatory disease. The apoptosis signal-regulating kinase (ASK) family belongs to the larger MAP3K family that controls various stress responses. Here, I summarize the critical roles of members of the ASK family during infection and inflammatory disease, and discuss the relationship between these two noxious conditions.

CD95/Fas Increases Stemness in Cancer Cells by Inducing a STAT1-Dependent Type I Interferon Response

Stimulation of CD95/Fas drives and maintains cancer stem cells (CSCs). We now report that this involves activation of signal transducer and activator of transcription 1 (STAT1) and induction of STAT1-regulated genes and that this process is inhibited by active caspases. STAT1 is enriched in CSCs in cancer cell lines, patient-derived human breast cancer, and CD95^high-expressing glioblastoma neurospheres. CD95 stimulation of cancer cells induced secretion of type I interferons (IFNs) that bind to type I IFN receptors, resulting in activation of Janus-activated kinases, activation of STAT1, and induction of a number of STAT1-regulated genes that are part of a gene signature recently linked to therapy resistance in five primary human cancers. Consequently, we identified type I IFNs as drivers of cancer stemness. Knockdown or knockout of STAT1 resulted in a strongly reduced ability of CD95L or type I IFN to increase cancer stemness. This identifies STAT1 as a key regulator of the CSC-inducing activity of CD95.

Bcl-xL/Bak interaction and regulation by miRNA let-7b in the intrinsic apoptotic pathway of stored platelets

Bcl-2 family proteins play key roles in the intrinsic apoptosis pathway in platelets, with both pro- and antiapoptotic protein expressions regulating survival during ex vivo storage. We detected a significant decrease in antiapoptotic Bcl-x_L and increase in proapoptotic Bak expression on the third day of storage and as a result the ratio of Bak:Bcl-x_L also decreased. Moreover, we identified an interaction between Bcl-x_L and Bak. These shifts corresponded with activation of the apoptotic pathway, suggesting these proteins might play an important role in platelet survival. We then performed bioinformatic analysis to gain insight into protein expression regulation during storage. This identified a potential binding site of the microRNA (miRNA) let-7b in the 3'-UTR of the Bcl-x_L gene, which we confirmed by a dual-luciferase reporter assay. We also determined that let-7b was upregulated during platelet storage, and let-7b transfection influenced Bcl-x_L and Bak protein, but not mRNA, expression. Together, these data suggest that only posttranscriptional mechanisms are available for regulating gene expression in anucleate platelets.

Huaier Restrains Proliferative and Migratory Potential of Hepatocellular Carcinoma Cells Partially Through Decreased Yes-Associated Protein 1

In China, Trametes robiniophila Murr (Huaier), a traditional Chinese herbal medicine, has been widely used in adjuvant therapies of hepatocellular carcinoma (HCC). However, the molecular mechanisms have not been fully understood. The aims of this study are to investigate the functions and mechanisms of Huaier on inhibiting proliferation and migration of HCC cells. Firstly, cell counting kit-8 (CCK-8) and colony formation shown Huaier inhibited proliferation of HCC Bel-7404, Bel-7402 and SMMC-7721 cells in a dose-dependent manner, and this inhibition might be due to Huaier decreased the expressions of the proliferating cell nuclear antigen (PCNA), the nuclear proliferation related antigen (Ki-67) and CyclinD1 detected by western blotting analysis. Notably, we also found Huaier treatment did not cause any cytotoxicity to normal human hepatocyte L-02 cells. Next, we found Huaier dose-dependently decreased Bcl-2 expression and increased Bax expression in Bel-7404 cells. The activities of cleaved caspase substrates had also been enhanced after Huaier treatment, suggesting Huaier treatment could induce HCC cell apoptosis. Then, the inhibitory effects of Huaier on migration of Bel-7404, Bel-7402 and SMMC-7721 cells via inhibiting Epithelial mesenchymal transition (EMT) had also been proved. Moreover, we confirmed yes-associated protein 1 (YAP1) was up-regulated in HCC cells and tissues, and overexpression of YAP1 promoted HCC cell proliferation and migration. Then, western blot and immunefluorescence shown Huaier had the inhibitory effects on YAP1 in HCC cells. On the other hand, human liver cancer tissue microarray (TMA) shown YAP1 expression was closely to clinic. Our study also confirmed Huaier had the inhibitory effects on YAP1 in xenograft mice models, it could be because Huaier treatment translocated YAP1 from nucleus to cytoplasm, and further promoted phosphorylation of YAP1 to be degraded by ubiquitination. Hence, we conclude that Huaier may restrain the proliferation and migration of HCC cells via down-regulation of YAP1. In summary, our study reveals the potential mechanisms of Huaier on inhibiting proliferation and migration of HCC cells. Importantly, for the first time, we found that Huaier can inhibit YAP1 expression in this anti-tumor process. We believe this finding is beneficial for the clinical applications of Huaier and the targeted therapies for HCC.

"Toll-free" pathways for production of type I interferons

Pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs) are recognized by different cellular pathogen recognition receptors (PRRs), which are expressed on cell membrane or in the cytoplasm of cells of the innate immune system. Nucleic acids derived from pathogens or from certain cellular conditions represent a large category of PAMPs/DAMPs that trigger production of type I interferons (IFN-I) in addition to pro-inflammatory cytokines, by specifically binding to intracellular Toll-like receptors or cytosolic receptors. These cytosolic receptors, which are not related to TLRs and we call them “Toll-free” receptors, include the RNA-sensing RIG-I like receptors (RLRs), the DNA-sensing HIN200 family, and cGAS, amongst others. Viruses have evolved myriad strategies to evoke both host cellular and viral factors to evade IFN-I-mediated innate immune responses, to facilitate their infection, replication, and establishment of latency. This review outlines these “Toll-free” innate immune pathways and recent updates on their regulation, with focus on cellular and viral factors with enzyme activities.

Viewing BCL2 and cell death control from an evolutionary perspective

The last 30 years of studying BCL2 have brought cell death research into the molecular era, and revealed its relevance to human pathophysiology. Most, if not all metazoans use an evolutionarily conserved process for cellular self destruction that is controlled and implemented by proteins related to BCL2. We propose the anti-apoptotic BCL2-like and pro-apoptotic BH3-only members of the family arose through duplication and modification of genes for the pro-apoptotic multi-BH domain family members, such as BAX and BAK1. In that way, a cell suicide process that initially evolved as a mechanism for defense against intracellular parasites was then also used in multicellular organisms for morphogenesis and to maintain the correct number of cells in adults by balancing cell production by mitosis.

DsbA-L prevents obesity-induced inflammation and insulin resistance by suppressing the mtDNA release-activated cGAS-cGAMP-STING pathway

Chronic inflammation in adipose tissue plays a key role in obesity-induced insulin resistance. However, the mechanisms underlying obesity-induced inflammation remain elusive. Here we show that obesity promotes mtDNA release into the cytosol, where it triggers inflammatory responses by activating the DNA-sensing cGAS-cGAMP-STING pathway. Fat-specific knockout of disulfide-bond A oxidoreductase-like protein (DsbA-L), a chaperone-like protein originally identified in the mitochondrial matrix, impaired mitochondrial function and promoted mtDNA release, leading to activation of the cGAS-cGAMP-STING pathway and inflammatory responses. Conversely, fat-specific overexpression of DsbA-L protected mice against high-fat diet-induced activation of the cGAS-cGAMP-STING pathway and inflammation. Taken together, we identify DsbA-L as a key molecule that maintains mitochondrial integrity. DsbA-L deficiency promotes inflammation and insulin resistance by activating the cGAS-cGAMP-STING pathway. Our study also reveals that, in addition to its well-characterized roles in innate immune surveillance, the cGAS-cGAMP-STING pathway plays an important role in mediating obesity-induced metabolic dysfunction.

Cutting Edge in IFN Regulation: Inflammatory Caspases Cleave cGAS

Caspases have important functions beyond their established role in driving inflammation and apoptosis. In this issue of Immunity, Wang et al. (2017) demonstrate that inflammasome-triggered caspases cleave and inactivate the DNA sensor cGAS, thus restricting the type I interferon response to cytosolic DNA.

Apoptotic Cells Induced Signaling for Immune Homeostasis in Macrophages and Dendritic Cells

Inefficient and abnormal clearance of apoptotic cells (efferocytosis) contributes to systemic autoimmune disease in humans and mice, and inefficient chromosomal DNA degradation by DNAse II leads to systemic polyarthritis and a cytokine storm. By contrast, efficient clearance allows immune homeostasis, generally leads to a non-inflammatory state for both macrophages and dendritic cells (DCs), and contributes to maintenance of peripheral tolerance. As many as 3 × 10⁸ cells undergo apoptosis every hour in our bodies, and one of the primary “eat me” signals expressed by apoptotic cells is phosphatidylserine (PtdSer). Apoptotic cells themselves are major contributors to the “anti-inflammatory” nature of the engulfment process, some by secreting thrombospondin-1 (TSP-1) or adenosine monophosphate and possibly other immune modulating “calm-down” signals that interact with macrophages and DCs. Apoptotic cells also produce “find me” and “tolerate me” signals to attract and immune modulate macrophages and DCs that express specific receptors for some of these signals. Neither macrophages nor DCs are uniform, and each cell type may variably express membrane proteins that function as receptors for PtdSer or for opsonins like complement or opsonins that bind to PtdSer, such as protein S and growth arrest-specific 6. Macrophages and DCs also express scavenger receptors, CD36, and integrins that function via bridging molecules such as TSP-1 or milk fat globule-EGF factor 8 protein and that differentially engage in various multi-ligand interactions between apoptotic cells and phagocytes. In this review, we describe the anti-inflammatory and pro-homeostatic nature of apoptotic cell interaction with the immune system. We do not review some forms of immunogenic cell death. We summarize the known apoptotic cell signaling events in macrophages and DCs that are related to toll-like receptors, nuclear factor kappa B, inflammasome, the lipid-activated nuclear receptors, Tyro3, Axl, and Mertk receptors, as well as induction of signal transducer and activator of transcription 1 and suppressor of cytokine signaling that lead to immune system silencing and DC tolerance. These properties of apoptotic cells are the mechanisms that enable their successful use as therapeutic modalities in mice and humans in various autoimmune diseases, organ transplantation, graft-versus-host disease, and sepsis.

Innate immunity and tolerance toward mitochondria

Mitochondria are intracellular organelles that originate from a bacterial symbiont, and they retain multiple features of this bacterial ancestry. The immune system evolved to detect the presence of invading pathogens, including bacteria, to eliminate them by a diversity of antimicrobial mechanisms and to mount long-term protective immunity. Due to their bacterial ancestry, mitochondria are sensed by the innate immune system, and trigger inflammatory responses comparable to those induced by pathogenic bacteria. In both cases, innate sensing mechanisms involve Toll-Like Receptors, Formyl Peptide Receptors, inflammasomes or the cGAS/STING pathway. Stressed mitochondria release mitochondrial molecules, such as cardiolipin and mitochondrial DNA, which are sensed as cellular damage potentially caused by infections. Recent research has identified several conditions in which mitochondrial stress-induced immunity is essential to effective antimicrobial defenses. But, in pathological conditions, the abnormal activation of the innate immune system by damaged mitochondria results in auto-inflammatory or autoimmune diseases. To prevent undesirable mitochondria-targeted responses, immune tolerance toward mitochondria must be established, involving regulation of mitophagy and mitochondrial permeability, as well as activation of specific nucleases and pro-apoptotic caspases. Overall, recent findings identify mitochondria as central in the induction of innate immunity, and provide new insights as to how immune responses to these multi-functional organelles might be exploited therapeutically in various disease states.

Cyclic Dinucleotides in the Scope of the Mammalian Immune System

First discovered in prokaryotes and more recently in eukaryotes, cyclic dinucleotides (CDNs) constitute a unique branch of second messenger signaling systems. Within prokaryotes CDNs regulate a wide array of different biological processes, whereas in the vertebrate system CDN signaling is largely dedicated to activation of the innate immune system. In this book chapter we summarize the occurrence and signaling pathways of these small-molecule second messengers, most importantly in the scope of the mammalian immune system. In this regard, our main focus is the role of the cGAS-STING axis in the context of microbial infection and sterile inflammation and its implications for therapeutic applications.

Oxysterol Restraint of Cholesterol Synthesis Prevents AIM2 Inflammasome Activation

Type I interferon restrains interleukin-1β (IL-1β)-driven inflammation in macrophages by upregulating cholesterol-25-hydroxylase (Ch25h) and repressing SREBP transcription factors. However, the molecular links between lipid metabolism and IL-1β production remain obscure. Here, we demonstrate that production of 25-hydroxycholesterol (25-HC) by macrophages is required to prevent inflammasome activation by the DNA sensor protein absent in melanoma 2 (AIM2). We find that in response to bacterial infection or lipopolysaccharide (LPS) stimulation, macrophages upregulate Ch25h to maintain repression of SREBP2 activation and cholesterol synthesis. Increasing macrophage cholesterol content is sufficient to trigger IL-1β release in a crystal-independent but AIM2-dependent manner. Ch25h deficiency results in cholesterol-dependent reduced mitochondrial respiratory capacity and release of mitochondrial DNA into the cytosol. AIM2 deficiency rescues the increased inflammasome activity observed in Ch25h^-/-. Therefore, activated macrophages utilize 25-HC in an anti-inflammatory circuit that maintains mitochondrial integrity and prevents spurious AIM2 inflammasome activation.

Linarin sensitizes tumor necrosis factor-related apoptosis (TRAIL)-induced ligand-triggered apoptosis in human glioma cells and in xenograft nude mice

Tumor necrosis factor-related apoptosis-induced ligand (TRAIL) is reported as a promising anti-cancer therapeutic agent. Nevertheless, a variety of cancer cells, including human malignant glioma cells, are resistant to TRAIL treatment, indicating that it is necessary to find effective strategies to overcome the TRAIL resistance. Linarin (LIN), a natural flavonoid compound in Flos Chrysanthemi Indici (FCI), has been exhibited to exert various pharmacological activities, including anti-cancer. Here in our study, we found that non-cytotoxic doses of LIN (5μM) dramatically potentiated TRAIL (80ng/ml)-induced cytotoxicity (52.36±1.58%) and apoptosis (68.50±1.23%) using 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) and flow cytometry assays, respectively, in human glioma cells of U87MG. Apoptosis was evidenced by enhanced cleavage of Caspase-8/-9/-3 and poly (ADP-ribose) polymerase (PARP), and reduced anti-apoptotic proteins, including B-cell leukemia/lymphoma 2 (Bcl-2), mantle cell lymphoma (Mcl)-1, and Survivin. Moreover, both intrinsic and extrinsic apoptosis pathways were included in apoptosis induced by LIN and TRAIL co-treatment, along with high release of Cyto-c into cytoplasm and enhancement of fas-associated protein with death domain (FADD), death-inducing signaling complex (DISC), death receptor 4 (DR) 4 and DR5, respectively. Reactive oxygen species (ROS) generation, up to 39.86±2.32%, was also highly triggered by TRAIL and LIN combinational treatment, which was accompanied with high phosphorylation of c-Jun-N-terminal kinase (JNK). In vivo, TRAIL and LIN double treatment significantly reduced the tumor growth using xenograft tumor model through inducing apoptosis. We demonstrated that combining LIN with TRAIL treatments might be effective against TRAIL-resistant glioma cells through inducing apoptosis regulated by ROS generation.

Topoisomerase 1 Inhibition Promotes Cyclic GMP-AMP Synthase-Dependent Antiviral Responses

Inflammatory responses, while essential for pathogen clearance, can also be deleterious to the host. Chemical inhibition of topoisomerase 1 (Top1) by low-dose camptothecin (CPT) can suppress transcriptional induction of antiviral and inflammatory genes and protect animals from excessive and damaging inflammatory responses. We describe the unexpected finding that minor DNA damage from topoisomerase 1 inhibition with low-dose CPT can trigger a strong antiviral immune response through cyclic GMP-AMP synthase (cGAS) detection of cytoplasmic DNA. This argues against CPT having only anti-inflammatory activity. Furthermore, expression of the simian virus 40 (SV40) large T antigen was paramount to the proinflammatory antiviral activity of CPT, as it potentiated cytoplasmic DNA leakage and subsequent cGAS recruitment in human and mouse cell lines. This work suggests that the capacity of Top1 inhibitors to blunt inflammatory responses can be counteracted by viral oncogenes and that this should be taken into account for their therapeutic development.

Recent studies suggest that low-dose DNA-damaging compounds traditionally used in cancer therapy can have opposite effects on antiviral responses, either suppressing (with the example of CPT) or potentiating (with the example of doxorubicin) them. Our work demonstrates that the minor DNA damage promoted by low-dose CPT can also trigger strong antiviral responses, dependent on the presence of viral oncogenes. Taken together, these results call for caution in the therapeutic use of low-dose chemotherapy agents to modulate antiviral responses in humans.

Transient apoptosis inhibition in donor stem cells improves hematopoietic stem cell transplantation

During hematopoietic stem cell transplantation, a substantial number of donor cells are lost because of apoptotic cell death. Transplantation-associated apoptosis is mediated mainly by the proapoptotic BCL-2 family proteins BIM and BMF, and their proapoptotic function is conserved between mouse and human stem and progenitor cells. Permanent inhibition of apoptosis in donor cells caused by the loss of these BH3-only proteins improves transplantation outcome, but recipients might be exposed to increased risk of lymphomagenesis or autoimmunity. Here, we address whether transient inhibition of apoptosis can serve as a safe but efficient alternative to improve the outcome of stem cell transplantation. We show that transient apoptosis inhibition by short-term overexpression of prosurvival BCL-XL, known to block BIM and BMF, is not only sufficient to increase the viability of hematopoietic stem and progenitor cells during engraftment but also improves transplantation outcome without signs of adverse pathologies. Hence, this strategy represents a promising and novel therapeutic approach, particularly under conditions of limited donor stem cell availability.

Small molecule inhibition of cGAS reduces interferon expression in primary macrophages from autoimmune mice

Cyclic GMP-AMP synthase is essential for innate immunity against infection and cellular damage, serving as a sensor of DNA from pathogens or mislocalized self-DNA. Upon binding double-stranded DNA, cyclic GMP-AMP synthase synthesizes a cyclic dinucleotide that initiates an inflammatory cellular response. Mouse studies that recapitulate causative mutations in the autoimmune disease Aicardi-Goutières syndrome demonstrate that ablating the cyclic GMP-AMP synthase gene abolishes the deleterious phenotype. Here, we report the discovery of a class of cyclic GMP-AMP synthase inhibitors identified by a high-throughput screen. These compounds possess defined structure-activity relationships and we present crystal structures of cyclic GMP-AMP synthase, double-stranded DNA, and inhibitors within the enzymatic active site. We find that a chemically improved member, RU.521, is active and selective in cellular assays of cyclic GMP-AMP synthase-mediated signaling and reduces constitutive expression of interferon in macrophages from a mouse model of Aicardi-Goutières syndrome. RU.521 will be useful toward understanding the biological roles of cyclic GMP-AMP synthase and can serve as a molecular scaffold for development of future autoimmune therapies.Upon DNA binding cyclic GMP-AMP synthase (cGAS) produces a cyclic dinucleotide, which leads to the upregulation of inflammatory genes. Here the authors develop small molecule cGAS inhibitors, functionally characterize them and present the inhibitor and DNA bound cGAS crystal structures, which will facilitate drug development.

miR-25/93 mediates hypoxia-induced immunosuppression by repressing cGAS

The mechanisms by which hypoxic tumors evade immunological pressure and anti-tumor immunity remain elusive. Here, we report that two hypoxia-responsive microRNAs, miR25 and miR93, are important for establishing an immunosuppressive tumor microenvironment by down-regulating expression of the DNA-sensor cGAS. Mechanistically, miR25/93 targets NCOA3, an epigenetic factor that maintains basal levels of cGAS expression, leading to repression of cGAS upon hypoxia. This allows hypoxic tumor cells to escape immunological responses induced by damage-associated molecular pattern molecules (DAMPs), specifically the release of mtDNA. Moreover, restoring cGAS expression results in an anti-tumor immune response. Clinically, decreased levels of cGAS are associated with poor prognosis for patients with breast cancer harboring high levels of miR25/93. Together, these data suggest that inactivation of the cGAS pathway plays a critical role in tumor progression, and reveals a direct link between hypoxia-responsive miRNAs and adaptive immune responses to the hypoxic tumor microenvironment, thus unveiling potential new therapeutic strategies.

cGAS senses long and HMGB/TFAM-bound U-turn DNA by forming protein-DNA ladders

Cytosolic DNA arising from intracellular pathogens triggers a powerful innate immune response. It is sensed by cyclic GMP-AMP synthase (cGAS), which elicits the production of type I interferons by generating the second messenger 2'3'-cyclic-GMP-AMP (cGAMP). Endogenous nuclear or mitochondrial DNA can also be sensed by cGAS under certain conditions, resulting in sterile inflammation. The cGAS dimer binds two DNA ligands shorter than 20 base pairs side-by-side, but 20-base-pair DNA fails to activate cGAS in vivo and is a poor activator in vitro. Here we show that cGAS is activated in a strongly DNA length-dependent manner both in vitro and in human cells. We also show that cGAS dimers form ladder-like networks with DNA, leading to cooperative sensing of DNA length: assembly of the pioneering cGAS dimer between two DNA molecules is ineffective; but, once formed, it prearranges the flanking DNA to promote binding of subsequent cGAS dimers. Remarkably, bacterial and mitochondrial nucleoid proteins HU and mitochondrial transcription factor A (TFAM), as well as high-mobility group box 1 protein (HMGB1), can strongly stimulate long DNA sensing by cGAS. U-turns and bends in DNA induced by these proteins pre-structure DNA to nucleate cGAS dimers. Our results suggest a nucleation-cooperativity-based mechanism for sensitive detection of mitochondrial DNA and pathogen genomes, and identify HMGB/TFAM proteins as DNA-structuring host factors. They provide an explanation for the peculiar cGAS dimer structure and suggest that cGAS preferentially binds incomplete nucleoid-like structures or bent DNA.

cGAS Conducts Micronuclei DNA Surveillance

DNA damage elicits a potent proinflammatory immune response. A collection of four papers now reveals that micronuclear DNA is a new cell intrinsic immunostimulatory molecule, and that accumulation of the immune sensor cyclic GMP-AMP synthase (cGAS) in micronuclei leads to a cell-cycle-dependent proinflammatory response following DNA damage.

Mitochondrial permeabilization engages NF-κB-dependent anti-tumour activity under caspase deficiency

Apoptosis represents a key anti-cancer therapeutic effector mechanism. During apoptosis, mitochondrial outer membrane permeabilization (MOMP) typically kills cells even in the absence of caspase activity. Caspase activity can also have a variety of unwanted consequences that include DNA damage. We therefore investigated whether MOMP-induced caspase-independent cell death (CICD) might be a better way to kill cancer cells. We find that cells undergoing CICD display potent pro-inflammatory effects relative to apoptosis. Underlying this, MOMP was found to stimulate NF-κB activity through the downregulation of inhibitor of apoptosis proteins. Strikingly, engagement of CICD displays potent anti-tumorigenic effects, often promoting complete tumour regression in a manner dependent on intact immunity. Our data demonstrate that by activating NF-κB, MOMP can exert additional signalling functions besides triggering cell death. Moreover, they support a rationale for engaging caspase-independent cell death in cell-killing anti-cancer therapies.