Cell death following the loss of ADAR1 mediated A-to-I RNA editing is not effected by the intrinsic apoptosis pathway

ZBP1 activation triggers hematopoietic stem and progenitor cell death resulting in bone marrow failure in mice

Human bone marrow failure (BMF) syndromes result from the loss of hematopoietic stem and progenitor cells (HSPC), and this loss has been attributed to cell death; however, the cell death triggers, and mechanisms remain unknown. During BMF, tumor necrosis factor-α (TNFα) and interferon-γ (IFNγ) increase. These ligands are known to induce necroptosis, an inflammatory form of cell death mediated by RIPK1, RIPK3, and MLKL. We previously discovered that mice with a hematopoietic RIPK1 deficiency (Ripk1HEM KO) exhibit inflammation, HSPC loss, and BMF, which is partially ameliorated by a RIPK3 deficiency; however, whether RIPK3 exerts its effects through its function in mediating necroptosis or other forms of cell death remains unclear. Here, we demonstrate that similar to a RIPK3 deficiency, an MLKL deficiency significantly extends survival and like Ripk3 deficiency partially restores hematopoiesis in Ripk1HEM KO mice revealing that both necroptosis and apoptosis contribute to BMF in these mice. Using mouse models, we show that the nucleic acid sensor Z-DNA binding protein 1 (ZBP1) is up-regulated in mouse RIPK1-deficient bone marrow cells and that ZBP1's function in endogenous nucleic acid sensing is necessary for HSPC death and contributes to BMF. We also provide evidence that IFNγ mediates HSPC death in Ripk1HEM KO mice, as ablation of IFNγ but not TNFα receptor signaling significantly extends survival of these mice. Together, these data suggest that RIPK1 maintains hematopoietic homeostasis by preventing ZBP1 activation and induction of HSPC death.

ADAR1 exacerbates ischemic brain injury via astrocyte-mediated neuron apoptosis

Astrocytes affect stroke outcomes by acquiring functionally dominant phenotypes. Understanding molecular mechanisms dictating astrocyte functional status after brain ischemia/reperfusion may reveal new therapeutic strategies. Adenosine deaminase acting on RNA (ADAR1), an RNA editing enzyme, is not normally expressed in astrocytes, but highly induced in astrocytes in ischemic stroke lesions. The expression of ADAR1 steeply increased from day 1 to day 7 after middle cerebral artery occlusion (MCAO) for 1 h followed by reperfusion. ADAR1 deficiency markedly ameliorated the volume of the cerebral infarction and neurological deficits as shown by the rotarod and cylinder tests, which was due to the reduction of the numbers of activated astrocytes and microglia. Surprisingly, ADAR1 was mainly expressed in astrocytes while only marginally in microglia. In primary cultured astrocytes, ADAR1 promoted astrocyte proliferation via phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Furthermore, ADAR1 deficiency inhibited brain cell apoptosis in mice with MCAO as well as in activated astrocyte-conditioned medium-induced neurons in vitro. It appeared that ADAR1 induces neuron apoptosis by secretion of IL-1β, IL-6 and TNF-α from astrocytes through the production of reactive oxygen species. These results indicated that ADAR1 is a novel regulator promoting the proliferation of the activated astrocytes following ischemic stroke, which produce various inflammatory cytokines, leading to neuron apoptosis and worsened ischemic stroke outcome.

Biological roles of A-to-I editing: implications in innate immunity, cell death, and cancer immunotherapy

Adenosine-to-inosine (A-to-I) editing, a key RNA modification widely found in eukaryotes, is catalyzed by adenosine deaminases acting on RNA (ADARs). Such RNA editing destabilizes endogenous dsRNAs, which are subsequently recognized by the sensors of innate immune and other proteins as autologous dsRNAs. This prevents the activation of innate immunity and type I interferon-mediated responses, thereby reducing the downstream cell death induced by the activation of the innate immune sensing system. ADARs-mediated editing can also occur in mRNAs and non-coding RNAs (ncRNAs) in different species. In mRNAs, A-to-I editing may lead to missense mutations and the selective splicing of coding regions. Meanwhile, in ncRNAs, A-to-I editing may affect targeting and disrupt ncRNAs maturation, leading to anomalous cell proliferation, invasion, and responses to immunotherapy. This review highlights the biological functions of A-to-I editing, its role in regulating innate immunity and cell death, and its potential molecular significance in tumorigenesis and cancer targeted therapy and immunotherapy.

ADAR1 and ZBP1 in innate immunity, cell death, and disease

ADAR1 and ZBP1 are the only two mammalian proteins that contain Zα domains, which are thought to bind to nucleic acids in the Z-conformation. These two molecules are crucial in regulating diverse biological processes. While ADAR1-mediated RNA editing supports host survival and development, ZBP1-mediated immune responses provide host defense against infection and disease. Recent studies have expanded our understanding of the functions of ADAR1 and ZBP1 beyond their classical roles and established their fundamental regulation of innate immune responses, including NLRP3 inflammasome activation, inflammation, and cell death. Their roles in these processes have physiological impacts across development, infectious and inflammatory diseases, and cancer. In this review, we discuss the functions of ADAR1 and ZBP1 in regulating innate immune responses in development and disease.

Recent Advances in Adenosine-to-Inosine RNA Editing in Cancer

RNA epigenetics, or epitranscriptome, is a growing group of RNA modifications historically classified into two categories: RNA editing and RNA modification. RNA editing is usually understood as post-transcriptional RNA processing (except capping, splicing and polyadenylation) that changes the RNA nucleotide sequence encoded by the genome. This processing can be achieved through the insertion or deletion of nucleotides or deamination of nucleobases, generating either standard nucleotides such as uridine (U) or the rare nucleotide inosine (I). Adenosine-to-inosine (A-to-I) RNA editing is the most prevalent type of RNA modification in mammals and is catalyzed by adenosine deaminase acting on the RNA (ADAR) family of enzymes that recognize double-stranded RNAs (dsRNAs). Inosine mimics guanosine (G) in base pairing with cytidine (C), thereby A-to-I RNA editing alters dsRNA secondary structure. Inosine is also recognized as guanosine by the splicing and translation machineries, resulting in mRNA alternative splicing and protein recoding. Therefore, A-to-I RNA editing is an important mechanism that causes and regulates "RNA mutations" in both normal physiology and diseases including cancer. In this chapter, we reviewed current paradigms and developments in the field of A-to-I RNA editing in the context of cancer.

ADAR1-dependent editing regulates human β cell transcriptome diversity during inflammation

INTRODUCTION

Enterovirus infection has long been suspected as a possible trigger for type 1 diabetes. Upon infection, viral double-stranded RNA (dsRNA) is recognized by membrane and cytosolic sensors that orchestrate type I interferon signaling and the recruitment of innate immune cells to the pancreatic islets. In this context, adenosine deaminase acting on RNA 1 (ADAR1) editing plays an important role in dampening the immune response by inducing adenosine mispairing, destabilizing the RNA duplexes and thus preventing excessive immune activation.

METHODS

Using high-throughput RNA sequencing data from human islets and EndoC-βH1 cells exposed to IFNα or IFNγ/IL1β, we evaluated the role of ADAR1 in human pancreatic β cells and determined the impact of the type 1 diabetes pathophysiological environment on ADAR1-dependent RNA editing.

RESULTS

We show that both IFNα and IFNγ/IL1β stimulation promote ADAR1 expression and increase the A-to-I RNA editing of Alu-Containing mRNAs in EndoC-βH1 cells as well as in primary human islets.

DISCUSSION

We demonstrate that ADAR1 overexpression inhibits type I interferon response signaling, while ADAR1 silencing potentiates IFNα effects. In addition, ADAR1 overexpression triggers the generation of alternatively spliced mRNAs, highlighting a novel role for ADAR1 as a regulator of the β cell transcriptome under inflammatory conditions.

RNA modifications act as regulators of cell death

Currently, more than one hundred types of RNA modifications have been found, and many of these modifications are reversible and dynamically regulated. RNA modifications can regulate RNA stability and translation and are thus involved in multiple biological activities. Recently, RNA modifications have been shown to have important roles in the regulation of cell death. Cell death is a critical process that maintains tissue homoeostasis and is regulated by multiple pathways in response to specific stimuli. In this review, we summarize the current understanding of the roles of RNA modifications in cell death mediation and discuss the prospects of such research.Abbreviations: m6A, N6-Methyladenosine; m6Am, N6,2'-O-Dimethyladenosine; m1A, N1-Methyladenosine; m5C, 5-Methylcytosine; hm5C, 5-Hydroxymethylcytosine; Ψ, pseudouridine; A-to-I, adenosine-to- inosine; hnRNPs, heterogeneous nuclear ribonucleoproteins; MOMP, mitochondrial outer membrane permeabilization; DD, death domain; DISC, death-inducing signalling complex; DED, death effector domain; FADD, FAS-associated protein with the death domain; TRADD, TNF receptor-associated protein with death domain; CMA, chaperone- mediated autophagy; PE, phosphatidylethanolamine; AD, alzheimer's disease; AML, acute myeloid leukaemia; miR, microRNA; 6-OHDA, 6-hydroxydopamine hydrochloride; R-2HG, R-2-hydroxyglutarate; IRES, internal ribosome entry site; BMSCs, bone-derived mesenchymal stem cells; NPCs, nucleus pulposus cells; HsCG, human chorionic gonadotropin; snoRNAs, small nucleolar RNAs; ER, endoplasmic reticulum; lncRNAs, long noncoding RNAs; TNM, tumour-node-metastasis.

Double-stranded RNA-specific adenosine deaminase-knockdown inhibits the proliferation and induces apoptosis of DU145 and PC3 cells by promoting the phosphorylation of H2A.X variant histone

Double-stranded RNA-specific adenosine deaminase (ADAR1) is a member of the adenosine deaminases acting on RNA family that catalyze the adenosine-to-inosine editing of double-stranded RNA substrates. Several studies have reported that ADAR1 is closely associated with numerous malignancies. However, the functional roles of ADAR1 in prostate cancer (PCa) have not been fully elucidated. Thus, the present study aimed to investigate the effects of ADAR1 on PCa. The results demonstrated that ADAR1 was highly expressed in PCa tissues compared with normal tissues. Furthermore, the protein expression level of ADAR1 was significantly increased in castration-resistant PCa (CRPCa) tissues and CRPCa cell lines. Thus, these findings indicated that ADAR1 may act as a tumor promoter for PCa development. Next, the potential effects of ADAR1-knockdown on the proliferation of DU145 and PC3 cells were investigated. ADAR1 was knocked down via small interfering RNA transfection, which was found to exert antitumor effects on DU145 and PC3 cells at 24 and 48 h post transfection. Furthermore, a significant positive association was observed between ADAR1-knockdown and the apoptosis of DU145 and PC3 cells, which increased the phosphorylation of H2A.X variant histone. The results of the present study indicated a positive association between ADAR1 expression and PCa, which may promote the development of CRPCa. Moreover, ADAR1-knockdown may serve as a tumor suppressor and represent a potential target for the treatment of PCa.

MicroRNAs in Skeletal Muscle and Hints on Their Potential Role in Muscle Wasting During Cancer Cachexia

Cancer-associated cachexia is a heterogeneous, multifactorial syndrome characterized by systemic inflammation, unintentional weight loss, and profound alteration in body composition. The main feature of cancer cachexia is represented by the loss of skeletal muscle tissue, which may or may not be accompanied by significant adipose tissue wasting. Such phenotypic alteration occurs as the result of concomitant increased myofibril breakdown and reduced muscle protein synthesis, actively contributing to fatigue, worsening of quality of life, and refractoriness to chemotherapy. According to the classical view, this condition is primarily triggered by interactions between specific tumor-induced pro-inflammatory cytokines and their cognate receptors expressed on the myocyte membrane. This causes a shift in gene expression of muscle cells, eventually leading to a pronounced catabolic condition and cell death. More recent studies, however, have shown the involvement of regulatory non-coding RNAs in the outbreak of cancer cachexia. In particular, the role exerted by microRNAs is being widely addressed, and several mechanistic studies are in progress. In this review, we discuss the most recent findings concerning the role of microRNAs in triggering or exacerbating muscle wasting in cancer cachexia, while mentioning about possible roles played by long non-coding RNAs and ADAR-mediated miRNA modifications.

The p150 Isoform of ADAR1 Blocks Sustained RLR signaling and Apoptosis during Influenza Virus Infection

Signaling through retinoic acid inducible gene I (RIG-I) like receptors (RLRs) is tightly regulated, with activation occurring upon sensing of viral nucleic acids, and suppression mediated by negative regulators. Under homeostatic conditions aberrant activation of melanoma differentiation-associated protein-5 (MDA5) is prevented through editing of endogenous dsRNA by RNA editing enzyme Adenosine Deaminase Acting on RNA (ADAR1). In addition, ADAR1 is postulated to play pro-viral and antiviral roles during viral infections that are dependent or independent of RNA editing activity. Here, we investigated the importance of ADAR1 isoforms in modulating influenza A virus (IAV) replication and revealed the opposing roles for ADAR1 isoforms, with the nuclear p110 isoform restricting versus the cytoplasmic p150 isoform promoting IAV replication. Importantly, we demonstrate that p150 is critical for preventing sustained RIG-I signaling, as p150 deficient cells showed increased IFN-β expression and apoptosis during IAV infection, independent of RNA editing activity. Taken together, the p150 isoform of ADAR1 is important for preventing sustained RIG-I induced IFN-β expression and apoptosis during viral infection.

What do editors do? Understanding the physiological functions of A-to-I RNA editing by adenosine deaminase acting on RNAs

Adenosine-to-inosine (A-to-I) editing is a post-transcriptional modification of RNA which changes its sequence, coding potential and secondary structure. Catalysed by the adenosine deaminase acting on RNA (ADAR) proteins, ADAR1 and ADAR2, A-to-I editing occurs at approximately 50 000-150 000 sites in mice and into the millions of sites in humans. The vast majority of A-to-I editing occurs in repetitive elements, accounting for the discrepancy in total numbers of sites between species. The species-conserved primary role of editing by ADAR1 in mammals is to suppress innate immune activation by unedited cell-derived endogenous RNA. In the absence of editing, inverted paired sequences, such as Alu elements, are thought to form stable double-stranded RNA (dsRNA) structures which trigger activation of dsRNA sensors, such as MDA5. A small subset of editing sites are within coding sequences and are evolutionarily conserved across metazoans. Editing by ADAR2 has been demonstrated to be physiologically important for recoding of neurotransmitter receptors in the brain. Furthermore, changes in RNA editing are associated with various pathological states, from the severe autoimmune disease Aicardi-Goutières syndrome, to various neurodevelopmental and psychiatric conditions and cancer. However, does detection of an editing site imply functional importance? Genetic studies in humans and genetically modified mouse models together with evolutionary genomics have begun to clarify the roles of A-to-I editing in vivo. Furthermore, recent developments suggest there may be the potential for distinct functions of editing during pathological conditions such as cancer.