ADAR1-mediated regulation of melanoma invasion

ADAR1-regulated miR-142-3p/RIG-I axis suppresses antitumor immunity in nasopharyngeal carcinoma

Following the initial treatment of nasopharyngeal carcinoma (NPC), tumor progression often portends an adverse prognosis for these patients. MicroRNAs (miRNAs) have emerged as critical regulators of tumor immunity, yet their intricate mechanisms in NPC remain elusive. Through comprehensive miRNA sequencing, tumor tissue microarrays and tissue samples analysis, we identified miR-142-3p as a significantly upregulated miRNA that is strongly associated with poor prognosis in recurrent NPC patients. To elucidate the underlying molecular mechanism, we employed RNA sequencing, coupled with cellular and tissue assays, to identify the downstream targets and associated signaling pathways of miR-142-3p. Our findings revealed two potential targets, CFL2 and WASL, which are directly targeted by miR-142-3p. Functionally, overexpressing CFL2 or WASL significantly reversed the malignant phenotypes induced by miR-142-3p both in vitro and in vivo. Furthermore, signaling pathway analysis revealed that miR-142-3p repressed the RIG-I-mediated immune defense response in NPC by inhibiting the nuclear translocation of IRF3, IRF7 and p65. Moreover, we discovered that ADAR1 physically interacted with Dicer and promoted the formation of mature miR-142-3p in a dose-dependent manner. Collectively, ADAR1-mediated miR-142-3p processing promotes tumor progression and suppresses antitumor immunity, indicating that miR-142-3p may serve as a promising prognostic biomarker and therapeutic target for NPC patients.

ADAR1 enhances tumor proliferation and radioresistance in non-small cell lung cancer by interacting with Rad18

PURPOSE

Posttranslational modification significantly contributes to the transcriptional diversity of tumors. Adenosine deaminase acting on RNA 1 (ADAR1) and its mediated adenosine-to-inosine (A-to-I) editing have been reported to influence tumorigenesis across various cancer types. Nevertheless, the relationship between ADAR1 and radioresistence remains to be elucidated.

METHODS

The protein expression was detected by immunohistochemistry and Western Blot, while the mRNA expression was measured by RT-qPCR. The tumor growth was evaluated by CCK8, colony formation assays, EdU assay, and in-vivo mouse model. γ-H2AX foci formation, neutral comet tailing assay, and clonogenic cell survival assay were performed to determine the DNA damage and radiosensitivity. RNA-seq was conducted to identify the main downstream effector. The interaction between ADAR1 and Rad18 was examined by immunofluorescence and co-immunoprecipitation.

RESULTS

We reported that ADAR1 was upregulated and correlated with poor prognosis in non-small cell lung cancer (NSCLC). In addition, we demonstrated that silencing ADAR1 significantly impaired tumor growth and improved tumor sensitivity to radiotherapy in vitro and in vivo. Mechanistically, we found that Rad18, which has been established as a versatile modulator of DNA repair, was the major downstream effector of ADAR1. ADAR1 not only regulated Rad18 mRNA expression by E2F3 but also colocalized and interacted with Rad18. Finally, our rescue experiments demonstrated that ADAR1's protumorigenic functions were partially dependent on Rad18.

CONCLUSION

Our results revealed the role of ADAR1 in cooperation with Rad18 in modulating oncogenesis and radioresistance in NSCLC for the first time, and suggested the therapeutic potential of targeting ADAR1 in overcoming radioresistance.

Identification and characterization of ADAR1 mutations and changes in gene expression in human cancers

ADAR1 (Adenosine deaminase action on RNA1) is involved in post-transcriptional RNA editing. ADAR1 mutations have been identified in many cancers but its role in tumor formation is still not well understood. Here we used available cancer genomes deposited on CSOMIC and cBioPortal to identify and characterize mutations and changes in ADAR1 expression in cancer cells. We identify several high frequency substitutions including one at R767 which is located in one of the dsRNA interacting domains. In silico protein structure analysis suggest the R767 mutations affect the protein stability and are likely to destabilize interaction with dsRNA. Gene expression analysis shows that in samples with under-expressed ADAR1, there is a statistically significant increase in expression of BLCAP (Bladder Cancer Associated Protein). Although BLCAP was initially identified in bladder cancers, more recent evidence shows that it is a tumor suppressor and BLCAP mutations have been detected in many cancer cells. Epistatic analysis using the cBioPortal mutual exclusivity calculator for the TCGA pan-cancer data shows that co-mutations between ADAR1 and other genes regulated by it are likely in cancer cells except for PTEN, AKT1 and BLCAP. This suggests that when ADAR1 function is impaired, PTEN, AKT1 and BLCAP become essential for survival of cancer cells. We also identified several samples with high mutation burden between ADAR1 and other genes regulated primarily in endometrial cancers. Finally, we show that the deaminase domain is highly conserved in metazoans and mutations within conserved residues do occur in human cancers suggesting that destabilization of the enzyme function is contributing to cancer development.

Adenosine-to-Inosine RNA editing in cancer: molecular mechanisms and downstream targets

Adenosine-to-Inosine (A-to-I), one of the most prevalent RNA modifications, has recently garnered significant attention. The A-to-I modification actively contributes to biological and pathological processes by affecting the structure and function of various RNA molecules, including double stranded RNA, transfer RNA, microRNA, and viral RNA. Increasing evidence suggests that A-to-I plays a crucial role in the development of human disease, particularly in cancer, and aberrant A-to-I levels are closely associated with tumorigenesis and progression through regulation of the expression of multiple oncogenes and tumor suppressor genes. Currently, the underlying molecular mechanisms of A-to-I modification in cancer are not comprehensively understood. Here, we review the latest advances regarding the A-to-I editing pathways implicated in cancer, describing their biological functions and their connections to the disease.

Cancer metastasis under the magnifying glass of epigenetics and epitranscriptomics

Most of the cancer-associated mortality and morbidity can be attributed to metastasis. The role of epigenetic and epitranscriptomic alterations in cancer origin and progression has been extensively demonstrated during the last years. Both regulations share similar mechanisms driven by DNA or RNA modifiers, namely writers, readers, and erasers; enzymes responsible of respectively introducing, recognizing, or removing the epigenetic or epitranscriptomic modifications. Epigenetic regulation is achieved by DNA methylation, histone modifications, non-coding RNAs, chromatin accessibility, and enhancer reprogramming. In parallel, regulation at RNA level, named epitranscriptomic, is driven by a wide diversity of chemical modifications in mostly all RNA molecules. These two-layer regulatory mechanisms are finely controlled in normal tissue, and dysregulations are associated with every hallmark of human cancer. In this review, we provide an overview of the current state of knowledge regarding epigenetic and epitranscriptomic alterations governing tumor metastasis, and compare pathways regulated at DNA or RNA levels to shed light on a possible epi-crosstalk in cancer metastasis. A deeper understanding on these mechanisms could have important clinical implications for the prevention of advanced malignancies and the management of the disseminated diseases. Additionally, as these epi-alterations can potentially be reversed by small molecules or inhibitors against epi-modifiers, novel therapeutic alternatives could be envisioned.

Dissecting multifunctional roles of forkhead box transcription factor D1 in cancers

As a member of the forkhead box (FOX) family of transcription factors (TF), FOXD1 has recently been implicated as a crucial regulator in a variety of human cancers. Accumulating evidence has established dysregulated and aberrant FOXD1 signaling as a prominent feature in cancer development and progression. However, there is a lack of systematic review on this topic. Here, we summarized the present understanding of FOXD1 functions in cancer biology and reviewed the downstream targets and upstream regulatory mechanisms of FOXD1 as well as the related signaling pathways within the context of current reports. We highlighted the functional features of FOXD1 in cancers to identify the future research consideration of this multifunctional transcription factor and potential therapeutic strategies targeting its oncogenic activity.

Zα domain proteins mediate the immune response

The Zα domain has a compact α/β architecture containing a three-helix bundle flanked on one side by a twisted antiparallel β sheet. This domain displays a specific affinity for double-stranded nucleic acids that adopt a left-handed helical conformation. Currently, only three Zα-domain proteins have been identified in eukaryotes, specifically ADAR1, ZBP1, and PKZ. ADAR1 is a double-stranded RNA (dsRNA) binding protein that catalyzes the conversion of adenosine residues to inosine, resulting in changes in RNA structure, function, and expression. In addition to its editing function, ADAR1 has been shown to play a role in antiviral defense, gene regulation, and cellular differentiation. Dysregulation of ADAR1 expression and activity has been associated with various disease states, including cancer, autoimmune disorders, and neurological disorders. As a sensing molecule, ZBP1 exhibits the ability to recognize nucleic acids with a left-handed conformation. ZBP1 harbors a RIP homotypic interaction motif (RHIM), composed of a highly charged surface region and a leucine-rich hydrophobic core, enabling the formation of homotypic interactions between proteins with similar structure. Upon activation, ZBP1 initiates a downstream signaling cascade leading to programmed cell death, a process mediated by RIPK3 via the RHIM motif. PKZ was identified in fish, and contains two Zα domains at the N-terminus. PKZ is essential for normal growth and development and may contribute to the regulation of immune system function in fish. Interestingly, some pathogenic microorganisms also encode Zα domain proteins, such as, Vaccinia virus and Cyprinid Herpesvirus. Zα domain proteins derived from pathogenic microorganisms have been demonstrated to be pivotal contributors in impeding the host immune response and promoting virus replication and spread. This review focuses on the mammalian Zα domain proteins: ADAR1 and ZBP1, and thoroughly elucidates their functions in the immune response.

miR-30e-5p-mediated FOXD1 promotes cell proliferation by blocking cellular senescence and apoptosis through p21/CDK2/Rb signaling in head and neck carcinoma

Forkhead box D1 (FOXD1) belongs to the FOX protein family, which has been found to function as a oncogene in multiple cancer types, but its role in head and neck squamous cell carcinoma (HNSCC) requires further investigation. Our research aimed to investigate the function of FOXD1 in HNSCC. Bioinformatics analysis indicated that mRNA level of FOXD1 was highly expressed in HNSCC tissues, and over-expressed FOXD1 was related to poor prognosis. Moreover, FOXD1 knockdown increased the ratio of senescent cells but decreased the proliferation ability, while FOXD1 overexpression obtained the opposite results. In vitro experiments revealed that FOXD1 bound to the p21 promoter and inhibited its transcription, which blocked the cyclin dependent kinase 2 (CDK2)/retinoblastoma (Rb) signaling pathway, thus preventing senescence and accelerating proliferation of tumor cells. CDK2 inhibitor could reverse the process to some extent. Further research has shown that miR-3oe-5p serves as a tumor suppressant by repressing the translation of FOXD1 through combining with the 3'-untranslated region (UTR). Thus, FOXD1 resists cellular senescence and facilitates HNSCC cell proliferation by affecting the expression of p21/CDK2/Rb signaling, suggesting that FOXD1 may be a potential curative target for HNSCC.

RNA Editing Enzyme ADAR1 Suppresses the Mobility of Cancer Cells via ARPIN

Deamination of adenine or cytosine in RNA, called RNA editing, is a constitutively active and common modification. The primary role of RNA editing is tagging RNA right after its synthesis so that the endogenous RNA is recognized as self and distinguished from exogenous RNA, such as viral RNA. In addition to this primary function, the direct or indirect effects on gene expression can be utilized in cancer where a high level of RNA editing activity persists. This report identified actin-related protein 2/3 complex inhibitor (ARPIN) as a target of ADAR1 in breast cancer cells. Our comparative RNA sequencing analysis in MCF7 cells revealed that the expression of ARPIN was decreased upon ADAR1 depletion with altered editing on its 3'UTR. However, the expression changes of ARPIN were not dependent on 3'UTR editing but relied on three microRNAs acting on ARPIN. As a result, we found that the migration and invasion of cancer cells were profoundly increased by ADAR1 depletion, and this cellular phenotype was reversed by the exogenous ARPIN expression. Altogether, our data suggest that ADAR1 suppresses breast cancer cell mobility via the upregulation of ARPIN.

ADAR1 Non-Editing Function in Macrophage Activation and Abdominal Aortic Aneurysm

BACKGROUND

Macrophage activation plays a critical role in abdominal aortic aneurysm (AAA) development. However, molecular mechanisms controlling macrophage activation and vascular inflammation in AAA remain largely unknown. The objective of the study was to identify novel mechanisms underlying adenosine deaminase acting on RNA (ADAR1) function in macrophage activation and AAA formation.

METHODS

Aortic transplantation was conducted to determine the importance of nonvascular ADAR1 in AAA development/dissection. Ang II (Angiotensin II) infusion of ApoE-/- mouse model combined with macrophage-specific knockout of ADAR1 was used to study ADAR1 macrophage-specific role in AAA formation/dissection. The relevance of macrophage ADAR1 to human AAA was examined using human aneurysm specimens. Moreover, a novel humanized AAA model was established to test the role of human macrophages in aneurysm formation in human arteries.

RESULTS

Allograft transplantation of wild-type abdominal aortas to ADAR1+/- recipient mice significantly attenuated AAA formation, suggesting that nonvascular ADAR1 is essential for AAA development. ADAR1 deficiency in hematopoietic cells decreased the prevalence and severity of AAA while inhibited macrophage infiltration and aorta wall inflammation. ADAR1 deletion blocked the classic macrophage activation, diminished NF-κB (nuclear factor kappa B) signaling, and enhanced the expression of a number of anti-inflammatory microRNAs. Mechanistically, ADAR1 interacted with Drosha to promote its degradation, which attenuated Drosha-DGCR8 (DiGeorge syndrome critical region 8) interaction, and consequently inhibited pri- to pre-microRNA processing of microRNAs targeting IKKβ, resulting in an increased IKKβ (inhibitor of nuclear factor kappa-B) expression and enhanced NF-κB signaling. Significantly, ADAR1 was induced in macrophages and interacted with Drosha in human AAA lesions. Reconstitution of ADAR1-deficient, but not the wild type, human monocytes to immunodeficient mice blocked the aneurysm formation in transplanted human arteries.

CONCLUSIONS

Macrophage ADAR1 promotes aneurysm formation in both mouse and human arteries through a novel mechanism, that is, Drosha protein degradation, which inhibits the processing of microRNAs targeting NF-kB signaling and thus elicits macrophage-mediated vascular inflammation in AAA.

The A-to-I editing of KPC1 promotes intrahepatic cholangiocarcinoma by attenuating proteasomal processing of NF-κB1 p105 to p50

BACKGROUND

Aberrant RNA editing of adenosine-to-inosine (A-to-I) has been linked to multiple human cancers, but its role in intrahepatic cholangiocarcinoma (iCCA) remains unknown. We conducted an exome-wide investigation to search for dysregulated RNA editing that drive iCCA pathogenesis.

METHODS

An integrative whole-exome and transcriptome sequencing analysis was performed to elucidate the RNA editing landscape in iCCAs. Putative RNA editing sites were validated by Sanger sequencing. In vitro and in vivo experiments were used to assess the effects of an exemplary target gene Kip1 ubiquitination-promoting complex 1 (KPC1) and its editing on iCCA cells growth and metastasis. Crosstalk between KPC1 RNA editing and NF-κB signaling was analyzed by molecular methods.

RESULTS

Through integrative omics analyses, we revealed an adenosine deaminases acting on RNA 1A (ADAR1)-mediated over-editing pattern in iCCAs. ADAR1 is frequently amplified and overexpressed in iCCAs and plays oncogenic roles. Notably, we identified a novel ADAR1-mediated A-to-I editing of KPC1 transcript, which results in substitution of methionine with valine at residue 8 (p.M8V). KPC1 p.M8V editing confers loss-of-function phenotypes through blunting the tumor-suppressive role of wild-type KPC1. Mechanistically, KPC1 p.M8V weakens the affinity of KPC1 to its substrate NF-κB1 p105, thereby reducing the ubiquitinating and proteasomal processing of p105 to p50, which in turn enhances the activity of oncogenic NF-κB signaling.

CONCLUSIONS

Our findings established that amplification-driven ADAR1 overexpression results in overediting of KPC1 p.M8V in iCCAs, leading to progression via activation of the NF-κB signaling pathway, and suggested ADAR1-KPC1-NF-κB axis as a potential therapeutic target for iCCA.

ADAR1 promotes systemic sclerosis via modulating classic macrophage activation

Introduction

As a multisystem autoimmune disorder disease, systemic sclerosis (SSc) is characterized by inflammation and fibrosis in the skin and other internal organs. However, mechanisms underlying the inflammatory response that drives the development of SSc remain largely unknown.

Methods

ADAR1 heterozygous knockout (AD1+/-) mice and myeloid-specific ADAR1 knockout mice were used to determine the function of ADAR1 in SSc. Histopathological analyses and western blot confirmed the role of ADAR1 in bleomycin-induced increased skin and lung fibrosis.

Results

In this study, we discover that adenosine deaminase acting on RNA (ADAR1), a deaminase converting adenosine to inosine (i.e., RNA editing) in RNA, is abundantly expressed in macrophages in the early stage of bleomycin-induced SSc. Importantly, ADAR1 is essential for SSc formation and indispensable for classical macrophage activation because ADAR1 deficiency in macrophages significantly ameliorates skin and lung sclerosis and inhibits the expression of inflammation mediator inducible NO synthase (iNOS) and IL-1β in macrophages. Mechanistically, deletion of ADAR1 blocks macrophage activation through diminishing NF-κB signaling.

Discussion

Our studies reveal that ADAR1 promotes macrophage activation in the onset of SSc. Thus, targeting ADAR1 could be a potential novel therapeutic strategy for treating sclerosis formation.

Unifying Different Cancer Theories in a Unique Tumour Model: Chronic Inflammation and Deaminases as Meeting Points

The increase in cancer incidences shows that there is a need to better understand tumour heterogeneity to achieve efficient treatments. Interestingly, there are several common features among almost all types of cancers, with chronic inflammation induction and deaminase dysfunctions singled out. Deaminases are a family of enzymes with nucleotide-editing capacity, which are classified into two main groups: DNA-based and RNA-based. Remarkably, a close relationship between inflammation and the dysregulation of these molecules has been widely documented, which may explain the characteristic intratumor heterogeneity, both at DNA and transcriptional levels. Indeed, heterogeneity in cancer makes it difficult to establish a unique tumour progression model. Currently, there are three main cancer models—stochastic, hierarchic, and dynamic—although there is no consensus on which one better resembles cancer biology because they are usually overly simplified. Here, to accurately explain tumour progression, we propose interactions among chronic inflammation, deaminases dysregulation, intratumor genetic heterogeneity, cancer phenotypic plasticity, and even the previously proposed appearance of cancer stem-like cell populations in the edges of advanced solid tumour masses (instead of being the cells of origin of primary malignancies). The new tumour development model proposed in this study does not contradict previously accepted models and it may open up a window to interesting therapeutic approaches.

ADAR1-Mediated RNA Editing and Its Role in Cancer

It is well known that the stability of RNA, the interaction between RNA and protein, and the correct translation of protein are significant forces that drive the transition from normal cell to malignant tumor. Adenosine deaminase acting on RNA 1 (ADAR1) is an RNA editing enzyme that catalyzes the deamination of adenosine to inosine (A-to-I), which is one dynamic modification that in a combinatorial manner can give rise to a very diverse transcriptome. ADAR1-mediated RNA editing is essential for survival in mammals and its dysregulation results in aberrant editing of its substrates that may affect the phenotypic changes in cancer. This overediting phenomenon occurs in many cancers, such as liver, lung, breast, and esophageal cancers, and promotes tumor progression in most cases. In addition to its editing role, ADAR1 can also play an editing-independent role, although current research on this mechanism is relatively shallowly explored in tumors. In this review, we summarize the nature of ADAR1, mechanisms of ADAR1 editing-dependent and editing-independent and implications for tumorigenesis and prognosis, and pay special attention to effects of ADAR1 on cancers by regulating non-coding RNA formation and function.

Adenosine-deaminase-acting-on-RNA-1 facilitates T-cell migration toward human melanoma cells

The effect of tumor/T-cell interactions on subsequent immune infiltration is undefined. Here we report that pre-exposure of melanoma cells to cognate T cells enhanced the chemotaxis of new T cells in vitro. The effect was HLA class I-restricted and IFNγ-dependent, as it was abolished by β2M-knockdown, MHC-blocking antibodies, JAK1 inhibitors, JAK1-silencing and IFNgR1-blocking antibodies. RNA-sequencing of 73 melanoma metastases showed a significant correlation between the interferon-inducible p150 isoform of adenosine-deaminase-acting-on-RNA-1 (ADAR1) enzyme and immune infiltration. Consistent with this, co-cultures of cognate melanoma/T-cell pairs led to IFNγ-dependent induction of ADAR1-p150 in the melanoma cells, as visualized in situ using dynamic cell blocks, in ovo using fertilized chick eggs, and in vitro with Western blots. ADAR1 staining and RNA-sequencing in patient-derived biopsies following immunotherapy showed a rise in ADAR1-p150 expression concurrently with CD8+ cell infiltration and clinical response. Silencing ADAR1-p150 abolished the IFNγ-driven enhanced T-cell migration, confirming its mechanistic role. Silencing and overexpression of the constitutive isoform of ADAR1, ADAR1-p110, decreased and increased T-cell migration, respectively. Chemokine arrays showed that ADAR1 controls the secretion of multiple chemokines from melanoma cells, probably through microRNA-mediated regulation. Chemokine receptor blockade eliminated the IFNγ-driven T-cell chemotaxis. We propose that the constitutive ADAR1 downregulation observed in melanoma contributes to immune exclusion, whereas antigen-specific T cells induce ADAR1-p150 by releasing IFNγ, which can drive T-cell infiltration.

Glycan Epitope and Integrin Expression Dynamics Characterize Neural Crest Epithelial-to-Mesenchymal Transition (EMT) in Human Pluripotent Stem Cell Differentiation

The neural crest gives rise to progeny as diverse as peripheral neurons, myelinating cells, cranial muscle, bone and cartilage tissues, and melanocytes. Neural crest derivation encompasses complex morphological change, including epithelial-to-mesenchymal transition (EMT) and migration to the eventual target locations throughout the body. Neural crest cultures derived from stem cells provide an attractive source for developmental studies in human model systems, of immediate biomedical relevance for neurocristopathies, neural cancer biology and regenerative medicine, if only appropriate markers for lineage and cell type definition and quality control criteria were available. Implementing a defined, scalable protocol to generate neural crest cells from embryonic stem cells, we identify stage-defining cluster-of-differentiation (CD) surface markers during human neural crest development in vitro. Acquisition of increasingly mesenchymal phenotype was characterized by absence of neuroepithelial stemness markers (CD15, CD133, CD49f) and by decrease of CD57 and CD24. Increased per-cell-expression of CD29, CD44 and CD73 correlated with established EMT markers as determined by immunofluorescence and immunoblot analysis. The further development towards migratory neural crest was associated with decreased CD24, CD49f (ITGA6) and CD57 (HNK1) versus an enhanced CD49d (ITGA4), CD49e (ITGA5) and CD51/CD61 (ITGAV/ITGB3) expression. Notably, a shift from CD57 to CD51/CD61 was identified as a sensitive surrogate surface indicator of EMT in neural crest in vitro development. The reported changes in glycan epitope and integrin surface expression may prove useful for elucidating neural crest stemness, EMT progression and malignancies.

ADAR1 and its implications in cancer development and treatment

The family of adenosine deaminases acting on RNA (ADARs) regulates global gene expression output by catalyzing adenosine-to-inosine (A-to-I) editing of double-stranded RNA (dsRNA) and through interacting with RNA and other proteins. ADARs play important roles in development and disease, including an increasing connection to cancer progression. ADAR1 has demonstrated a largely pro-oncogenic role in a growing list of cancer types, and its function in cancer has been attributed to diverse mechanisms. Here, we review existing literature on ADAR1 biology and function, its roles in human disease including cancer, and summarize known cancer-associated phenotypes and mechanisms. Lastly, we discuss implications and outstanding questions in the field, including strategies for targeting ADAR1 in cancer.

The E3 ubiquitin ligase SMURF2 stabilizes RNA editase ADAR1p110 and promotes its adenosine-to-inosine (A-to-I) editing function

Epitranscriptomic changes in RNA catalyzed by the RNA-editing enzyme ADAR1 play an essential role in the regulation of diverse molecular and cellular processes, both under physiological conditions and in disease states, including cancer. Yet, despite a growing body of evidence pointing to ADAR1 as a potential therapeutic target, the mechanisms regulating its cellular abundance and activity, particularly of its constitutively expressed and ubiquitous form, ADAR1p110, are poorly understood. Here, we report the HECT-type E3 ubiquitin ligase SMURF2 as a pivotal regulator of ADAR1p110. We show that SMURF2, which is primarily known to promote the ubiquitin-mediated degradation of its protein substrates, protects ADAR1p110 from proteolysis and promotes its A-to-I editase activity in human and mouse cells and tissues. ADAR1p110's interactome analysis performed in human cells also showed a positive influence of SMURF2 on the stability and function of ADAR1p110. Mechanistically, we found that SMURF2 directly binds, ubiquitinates and stabilizes ADAR1p110 in an E3 ubiquitin ligase-dependent manner, through ADAR1p110 ubiquitination at lysine-744 (K744). Mutation of this residue to arginine (K744R), which is also associated with several human disorders, including dyschromatosis symmetrica hereditaria (DSH) and some types of cancer, abolished SMURF2-mediated protection of ADAR1p110 from both proteasomal and lysosomal degradation and inactivated ADAR1p110-mediated RNA editing. Our findings reveal a novel mechanism underlying the regulation of ADAR1 in mammalian cells and suggest SMURF2 as a key cellular factor influencing the protein abundance, interactions and functions of ADAR1p110.

Targeting GPCRs and Their Signaling as a Therapeutic Option in Melanoma

Simple Summary

Sixteen G-protein-coupled receptors (GPCRs) have been involved in melanogenesis or melanomagenesis. Here, we review these GPCRs, their associated signaling, and therapies.

Abstract

G-protein-coupled receptors (GPCRs) serve prominent roles in melanocyte lineage physiology, with an impact at all stages of development, as well as on mature melanocyte functions. GPCR ligands are present in the skin and regulate melanocyte homeostasis, including pigmentation. The role of GPCRs in the regulation of pigmentation and, consequently, protection against external aggression, such as ultraviolet radiation, has long been established. However, evidence of new functions of GPCRs directly in melanomagenesis has been highlighted in recent years. GPCRs are coupled, through their intracellular domains, to heterotrimeric G-proteins, which induce cellular signaling through various pathways. Such signaling modulates numerous essential cellular processes that occur during melanomagenesis, including proliferation and migration. GPCR-associated signaling in melanoma can be activated by the binding of paracrine factors to their receptors or directly by activating mutations. In this review, we present melanoma-associated alterations of GPCRs and their downstream signaling and discuss the various preclinical models used to evaluate new therapeutic approaches against GPCR activity in melanoma. Recent striking advances in our understanding of the structure, function, and regulation of GPCRs will undoubtedly broaden melanoma treatment options in the future.

Midlife brain metastases in the United States: Is male at risk?

Background

Population‐based estimates of the impact of gender throughout the whole course of brain metastases (BMs) at the time of diagnosis of systemic malignancies are insufficient. We aimed to discover the influence of gender on the presence of BMs in newly diagnosed malignancies and the survival of those patients on a population‐based level.

Methods

Midlife patients (40 years ≤ age ≤60 years) with newly diagnosed malignancies and BMs at the time of diagnosis were abstracted from the Surveillance, Epidemiology, and End Results (SEER) database of the National Cancer Institute. Clinical variables adjusted patient data. The LASSO regression was performed to exclude the possibility of collinearity. Univariable and multivariable logistic regression analyses were applied to find independent predictors for the presence of BMs, while univariable and multivariable Cox proportional hazard regression analyses were used to determine prognosticators of survival. K‐M curves were used to perform the survival analysis.

Result

276,327 population‐based samples met inclusion criteria between 2014 and 2016, and 5747 (2.08%) patients were diagnosed with BMs at the time of diagnosis of systematic malignancies. Among all midlife patients with cancer, 44.02% (121,634) were male, while 51.68% (2970) were male among patients with BMs at the time of diagnosis. The most frequent tumor type was breast cancer (23.11%), and lung cancer had the highest incidence proportion of BMs among the entire cohort (19.34%). The multivariable logistic regression model suggested that female (vs. male, odds ratio [OR] 1.07, 95% CI: 1.01–1.14, p < 0.001) was associated with a higher risk of the presence of BMs at the time of diagnosis. Moreover, in the multivariable Cox model for all‐cause mortality in individuals with BMs at diagnosis, female (vs. male, hazard ratio [HR], 0.86, 95% CI, 0.80–0.92, p < 0.001) was shown to have a lower risk of decreased all‐cause mortality.

Conclusion

The middle‐aged females were at increased risk of developing BMs, while the middle‐aged males with BMs were at higher risk of having poorer survival.

Identification of Therapeutic Targets and Prognostic Biomarkers Among Integrin Subunits in the Skin Cutaneous Melanoma Microenvironment

The roles of different integrin alpha/beta (ITGA/ITGB) subunits in skin cutaneous melanoma (SKCM) and their underlying mechanisms of action remain unclear. Oncomine, UALCAN, GEPIA, STRING, GeneMANIA, cBioPortal, TIMER, TRRUST, and Webgestalt analysis tools were used. The expression levels of ITGA3, ITGA4, ITGA6, ITGA10, ITGB1, ITGB2, ITGB3, ITGB4, and ITGB7 were significantly increased in SKCM tissues. The expression levels of ITGA1, ITGA4, ITGA5, ITGA8, ITGA9, ITGA10, ITGB1, ITGB2, ITGB3, ITGB5, ITGB6 and ITGB7 were closely associated with SKCM metastasis. The expression levels of ITGA1, ITGA4, ITGB1, ITGB2, ITGB6, and ITGB7 were closely associated with the pathological stage of SKCM. The expression levels of ITGA6 and ITGB7 were closely associated with disease-free survival time in SKCM, and the expression levels of ITGA6, ITGA10, ITGB2, ITGB3, ITGB6, ITGB7, and ITGB8 were markedly associated with overall survival in SKCM. We also found significant correlations between the expression of integrin subunits and the infiltration of six types of immune cells (B cells, CD8+ T cells, CD4+T cells, macrophages, neutrophils, and dendritic cells). Finally, Gene Ontology (GO) enrichment analysis and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis were performed, and protein-protein interaction (PPI) networks were constructed. We have identified abnormally-expressed genes and gene regulatory networks associated with SKCM, improving understanding of the underlying pathogenesis of SKCM.

8-Chloro-Adenosine Inhibits Proliferation of MDA-MB-231 and SK-BR-3 Breast Cancer Cells by Regulating ADAR1/p53 Signaling Pathway

8-Chloro-adenosine (8-Cl-Ado) has been shown to exhibit its antitumor activity by inducing apoptosis in human lung cancer A549 and H1299 cells or autophagy in chronic lymphocytic leukemia, and MDA-MB-231 and MCF-7 breast cancer cells. Adenosine deaminases acting on RNA 1 (ADAR1) is tightly associated with cancer development and progression. The aim of this study was to investigate the role of ADAR1 in the proliferation of MDA-MB-231 and SK-BR-3 breast cancer cell lines after 8-Cl-Ado exposure and its possible mechanisms. After 8-Cl-Ado exposure, CCK-8 assay was performed to determine the cell proliferation; flow cytometry was used to analyze the cell cycle profiles and apoptosis; and the protein levels of ADAR1, p53, p21, and cyclin D1 were measured by western blotting. The results showed that the cell proliferation was greatly inhibited, G1 cell cycle was arrested, and apoptosis was induced after 8-Cl-Ado exposure. ADAR1 and cyclin D1 protein levels were dramatically decreased, while p53 and p21 levels were increased after 8-Cl-Ado exposure. Moreover, the cell growth inhibition was rescued, apoptosis was reduced, and p53 and p21 protein levels were downregulated, while cyclin D1 was upregulated when cells were transfected with plasmids expressing ADAR1 proteins. More importantly, RNA-binding domain of ADAR1 is critical to the cell growth inhibition of breast cancer cells exposed to 8-Cl-Ado. Together, 8-Cl-Ado inhibits the cell proliferation, induces G1 phase arrest and apoptosis at least by targeting ADAR1/p53/p21 signaling pathway. The findings may provide us with insights into the role of ADAR1 in breast cancer progression and help us better understand the effects of 8-Cl-Ado in the treatment of breast cancer.

Integrin α5 promotes migration and invasion through the FAK/STAT3/AKT signaling pathway in icotinib-resistant non-small cell lung cancer cells

Patients with non-small cell lung cancer (NSCLC) treated with EGFR-tyrosine kinase inhibitors (TKIs) ultimately develop drug resistance and metastasis. Therefore, there is a need to identify the underlying mechanisms of resistance to EGFR-TKIs. In the present study, colony formation and MTT assays were performed to investigate cell viability following treatment with icotinib. Gene Expression Omnibus datasets were used to identify genes associated with resistance. Wound healing and Transwell assays were used to detect cell migration and invasion with icotinib treatment and integrin α5-knockdown. The expression levels of integrin α5 and downstream genes were detected using western blotting. Stable icotinib-resistant (IcoR) cell lines (827/IcoR and PC9/IcoR) were established that showed enhanced malignant properties compared with parental cells (HCC827 and PC9). Furthermore, the resistant cell lines were resistant to icotinib in terms of proliferation, migration and invasion. The enrichment of function and signaling pathways analysis showed that integrin α5-upregulation was associated with the development of icotinib resistance. The knockdown of integrin α5 attenuated the migration and invasion capability of the resistant cells. Moreover, a combination of icotinib and integrin α5 siRNA significantly inhibited migration and partly restored icotinib sensitivity in IcoR cells. The expression levels of phosphorylated (p)-focal adhesion kinase (FAK), p-STAT3 and p-AKT decreased after knockdown of integrin α5, suggesting that FAK/STAT3/AKT signaling had a notable effect on the resistant cells. The present study revealed that the integrin α5/FAK/STAT3/AKT signaling pathway promoted icotinib resistance and malignancy in IcoR NSCLC cells. This signaling pathway may provide promising targets against acquired resistance to EGFR-TKI in patients with NSCLC.

Unbiased Identification of trans Regulators of ADAR and A-to-I RNA Editing

Adenosine-to-inosine RNA editing is catalyzed by adenosine deaminase acting on RNA (ADAR) enzymes that deaminate adenosine to inosine. Although many RNA editing sites are known, few trans regulators have been identified. We perform BioID followed by mass spectrometry to identify trans regulators of ADAR1 and ADAR2 in HeLa and M17 neuroblastoma cells. We identify known and novel ADAR-interacting proteins. Using ENCODE data, we validate and characterize a subset of the novel interactors as global or site-specific RNA editing regulators. Our set of novel trans regulators includes all four members of the DZF-domain-containing family of proteins: ILF3, ILF2, STRBP, and ZFR. We show that these proteins interact with each ADAR and modulate RNA editing levels. We find ILF3 is a broadly influential negative regulator of editing. This work demonstrates the broad roles that RNA binding proteins play in regulating editing levels, and establishes DZF-domain-containing proteins as a group of highly influential RNA editing regulators.

Cell Adhesion Molecules in Plasticity and Metastasis

Prior to metastasis, modern therapeutics and surgical intervention can provide a favorable long-term survival for patients diagnosed with many types of cancers. However, prognosis is poor for patients with metastasized disease. Melanoma is the deadliest form of skin cancer, yet in situ and localized, thin melanomas can be biopsied with little to no postsurgical follow-up. However, patients with metastatic melanoma require significant clinical involvement and have a 5-year survival of only 34% to 52%, largely dependent on the site of colonization. Melanoma metastasis is a multi-step process requiring dynamic changes in cell surface proteins regulating adhesiveness to the extracellular matrix (ECM), stroma, and other cancer cells in varied tumor microenvironments. Here we will highlight recent literature to underscore how cell adhesion molecules (CAM) contribute to melanoma disease progression and metastasis.

Thalidomide targets EGFL6 to inhibit EGFL6/PAX6 axis-driven angiogenesis in small bowel vascular malformation

BACKGROUND

Small bowel vascular malformation disease (SBVM) is the most common cause of obscure gastrointestinal bleeding (OGIB). Several studies suggested that EGFL6 was able to promote the growth of tumor endothelial cells by forming tumor vessels. To date, it remains unclear how EGFL6 promotes pathological angiogenesis in SBVM and whether EGFL6 is a target of thalidomide.

METHODS

We took advantage of SBVM plasma and tissue samples and compared the expression of EGFL6 between SBVM patients and healthy people via ELISA and Immunohistochemistry. We elucidated the underlying function of EGFL6 in SBVM in vitro and by generating a zebrafish model that overexpresses EGFL6, The cycloheximide (CHX)-chase experiment and CoIP assays were conducted to demonstrate that thalidomide can promote the degradation of EGFL6 by targeting CRBN.

RESULTS

The analysis of SBVM plasma and tissue samples revealed that EGFL6 was overexpressed in the patients compared to healthy people. Using in vitro and in vivo assays, we demonstrated that an EMT pathway triggered by the EGFL6/PAX6 axis is involved in the pathogenesis of SBVM. Furthermore, through in vitro and in vivo assays, we elucidated that thalidomide can function as anti-angiogenesis medicine through the regulation of EGFL6 in a proteasome-dependent manner. Finally, we found that CRBN can mediate the effect of thalidomide on EGFL6 expression and that the CRBN protein interacts with EGFL6 via a Lon N-terminal peptide.

CONCLUSION

Our findings revealed a key role for EGFL6 in SBVM pathogenesis and provided a mechanism explaining why thalidomide can cure small bowel bleeding resulting from SBVM.

ADAR1 regulates melanoma cell invasiveness by controlling beta3-integrin via microRNA-30 family members

Melanoma cells utilize multiple mechanisms to exit the primary tumor mass, invade the surroundings and subsequently distant tissues. We have previously reported that the expression of the RNA editing enzyme ADAR1 (adenosine deaminase acting on RNA) is downregulated in metastatic melanoma, which facilitates proliferation and invasion. Here we show that ADAR1 controls melanoma invasiveness by regulating ITGB3 expression via miR-30a and miR-30d. ADAR1 overexpression or knockdown leads to an increase or decrease, respectively, in the expression of both microRNAs. The effect is independent of RNA-editing. Dual luciferase assays show that both microRNAs directly regulate the expression of the ITGB3 integrin. Overexpression of the miR-30a or miR-30d lead to a decrease in ITGB3 and a resultant decreased invasive and metastatic capacities. Neutralization of the endogenous miR-30a or miR-30d leads to the opposite effect. The microRNAs regulate ITGB3 levels probably through a post-transcriptional effect, as both mRNA and protein levels of ITGB3 are affected. These results further expand our knowledge on the ADAR1-ITGB3 network and its central role in acquisition of the invasive phenotype of metastatic melanoma.

Epigenetic deregulation in cancer: Enzyme players and non-coding RNAs

Data obtained from cutting-edge research have shown that deregulated epigenetic marks are critical hallmarks of cancer. Rapidly emerging scientific evidence has helped in developing a proper understanding of the mechanisms leading to control of cellular functions, from changes in chromatin accessibility, transcription and translation, and in post-translational modifications. Firstly, mechanisms of DNA methylation and demethylation are introduced, as well as modifications of DNA and RNA, with particular focus on N6-methyladenosine (m6A), discussing the effects of these modifications in normal cells and in malignancies. Then, chromatin modifying proteins and remodelling complexes are discussed. Many enzymes and accessory proteins in these complexes have been found mutated or have undergone differential splicing, leading to defective protein complexes. Epigenetic mechanisms acting on nucleosomes by polycomb repressive complexes and on chromatin by SWI/SNF complexes on nucleosome assembly/disassembly, as well as main mutated genes linked to cancers, are reviewed. Among enzymes acting on histones and other proteins erasing the reversible modifications are histone deacetylases (HDACs). Sirtuins are of interest since most of these enzymes not only deacylate histones and other proteins, but also post-translationally modify proteins adding a Mono-ADP-ribose (MAR) moiety. MAR can be read by MACRO-domain containing proteins such as histone MacroH2A1, with specific function in chromatin assembly. Finally, recent advances are presented on non-coding RNAs with a scaffold function, prospecting their role in assembly of chromatin modifying complexes, recruiting enzyme players to chromatin regions. Lastly, the imbalance in metabolites production due to mitochondrial dysfunction is presented, with the potential of these metabolites to inhibit enzymes, either writers, readers or erasers of epitranscriptome marks. In the perspectives, studies are overwied on drugs under development aiming to limit excessive enzyme activities and to reactivate chromatin modifying complexes, for therapeutic application. This knowledge may lead to novel drugs and personalised medicine for cancer patients.

Mass spectrometry-based proteomic capture of proteins bound to the MACC1 promoter in colon cancer

MACC1 (metastasis associated in colon cancer 1) is a key driver that induces metastasis in colon cancer. However, the mechanisms by which MACC1 expression is transcriptionally regulated and the factors enriched at the MACC1 promoter remain largely unknown. The binding of proteins to specific DNA sites in the genome is a major determinant of genomic maintenance and the regulation of specific genes. The study herein utilized two methods to study the binding proteins of the MACC1 promoter region in colon cancer. Specifically, we adopted CRISPR-based chromatin affinity purification with mass spectrometry (CRISPR-ChAP-MS) and a biotin-streptavidin pulldown assay coupled with MS to identify the specific proteome bound to the MACC1 promoter in two colon cell lines with different metastatic potential. A total of 24 proteins were identified by CRISPR-ChAP-MS as binding to the MACC1 promoter, among which c-JUN was validated by ChIP-PCR. A total of 739 binding protein candidates were identified by biotin-streptavidin pulldown assays coupled with MS, of which HNF4G and PAX6 were validated and compared for their binding to the same promoter sites in the two cell lines. Our studies suggest distinctive proteomic factors associated with the MACC1 promoter in colon cells with different metastatic potential. The dynamic regulatory factors accumulated at the promoter of MACC1 may provide novel insights into the regulatory mechanisms of MACC1 transcription.

AKT-Dependent Phosphorylation of ADAR1p110 and ADAR2 Represents a New and Important Link Between Cell Signaling and RNA Editing

RNA editing is a process by which nascent RNA transcripts are covalently modified, thus enhancing the complexity of the transcriptome. The most common modifications are deaminations of adenosine to inosine at sites of complex RNA secondary structure, a process that is carried out by the adenosine deaminase acting on double-strand RNA (ADAR) family of RNA editases. Although much has been learned about the ADAR family members since their discovery, very little information on their post-transcriptional regulation has been reported. Similar to most proteins, the ADAR family members are post-translationally modified at multiple sites. We recently reported that members of the AKT kinase family directly phosphorylate ADAR1p110 and ADAR2 on a conserved threonine within the catalytic domain of the protein. Phosphorylation was observed to differentially inhibit the enzymatic activity of the ADAR proteins toward known RNA substrates. The direct downstream involvement of the AKT kinases in multiple major signaling pathways associated with cell survival, growth, glucose metabolism (insulin signaling), and differentiation is well established; thus, the AKT kinases represent a link between ADAR-dependent A-to-I editing and major signal transduction pathways that are necessary for cell maintenance and development.

IL6R-STAT3-ADAR1 (P150) interplay promotes oncogenicity in multiple myeloma with 1q21 amplification

1q21 amplification is an important prognostic marker in multiple myeloma. In this study we identified that IL6R (the interleukin-6 membrane receptor) and ADAR1 (an RNA editing enzyme) are critical genes located within the minimally amplified 1q21 region. Loss of individual genes caused suppression to the oncogenic phenotypes, the magnitude of which was enhanced when both genes were concomitantly lost. Mechanistically, IL6R and ADAR1 collaborated to induce a hyper-activation of the oncogenic STAT3 pathway. High IL6R confers hypersensitivity to interleukin-6 binding, whereas, ADAR1 forms a constitutive feed-forward loop with STAT3 in a P150-isoform-predominant manner. In this respect, ADAR1-P150 acts as a direct transcriptional target for STAT3 and this STAT3-induced-P150 in turn directly interacts with and stabilizes the former protein, leading to a larger pool of proteins acting as oncogenic transcription factors for pro-survival genes. The importance of both IL6R and ADAR1-P150 in STAT3 signaling was further validated when concomitant knockdown of both genes impeded IL6-induced-STAT3 pathway activation. Clinical evaluation of various datasets of myeloma patients showed that low expression of either one or both genes was closely associated with a compromised STAT3 signature, confirming the involvement of IL6R and ADAR1 in the STAT3 pathway and underscoring their essential role in disease pathogenesis. In summary, our findings highlight the complexity of the STAT3 pathway in myeloma, in association with 1q21 amplification. This study therefore reveals a novel perspective on 1q21 abnormalities in myeloma and a potential therapeutic target for this cohort of high-risk patients.

ADAR1 promotes the epithelial-to-mesenchymal transition and stem-like cell phenotype of oral cancer by facilitating oncogenic microRNA maturation

Background

Adenosine deaminases acting on RNA (ADARs) are involved in adenosine-to-inosine (A-to-I) editing and implicated in tumorigenesis and prognosis. Emerging evidence has indicated that ADAR1, an ADAR family member, participates in the regulation of various cancers; however, its biological function in oral squamous cell carcinoma (OSCC) remains unclear. This study aimed to determine the role of ADAR1 in OSCC progression.

Methods

ADAR1 expression in both normal tissues and carcinoma tissues and in OSCC cell lines was examined by real-time PCR and western blotting. Gain-of-function and loss-of-function approaches were used to examine the effect of ADAR1 on the migration, invasion, epithelial-mesenchymal transition (EMT) and stemness of OSCC. Furthermore, the relationship between ADAR1 and Dicer was determined by co-immunoprecipitation, and the expression of OSCC-associated oncogenic miRNAs was evaluated by real-time PCR. For in vivo experiments, a xenograft model where OSCC cells stably expressing ADAR1 were implanted was used to investigate the effect of ADAR1 on tumor growth and progression, and the expression of ADAR1, PCNA, SOX2 and POU5F1 was further detected by immunohistochemistry. The impact of ADAR1 expression on the survival status of OSCC patients was determined by survival analysis.

Results

ADAR1 was overexpressed in OSCC and significantly associated with poor patient survival. There was a positive correlation between ADAR1 and the migration, invasion, EMT and stemness of OSCC. Mechanistically, ADAR1 was physically associated with Dicer, and six OSCC-associated oncogenic miRNAs were increased in OSCC cells with ADAR1 overexpression. In the mouse xenograft model of OSCC, ADAR1 overexpression promoted tumor growth and progression. Moreover, ADAR1 was highly expressed in OSCC patients with low survival rates.

Conclusions

Our findings demonstrated that ADAR1 may play a significant role in OSCC progression via combining with Dicer to regulate oncogenic miRNA maturation and further affect cell migration and invasion.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1300-2) contains supplementary material, which is available to authorized users.

Adenosine deaminase acting on RNA (ADAR1), a suppressor of double-stranded RNA-triggered innate immune responses

Herbert "Herb" Tabor, who celebrated his 100th birthday this past year, served the Journal of Biological Chemistry as a member of the Editorial Board beginning in 1961, as an Associate Editor, and as Editor-in-Chief for 40 years, from 1971 until 2010. Among the many discoveries in biological chemistry during this period was the identification of RNA modification by C6 deamination of adenosine (A) to produce inosine (I) in double-stranded (ds) RNA. This posttranscriptional RNA modification by adenosine deamination, known as A-to-I RNA editing, diversifies the transcriptome and modulates the innate immune interferon response. A-to-I editing is catalyzed by a family of enzymes, adenosine deaminases acting on dsRNA (ADARs). The roles of A-to-I editing are varied and include effects on mRNA translation, pre-mRNA splicing, and micro-RNA silencing. Suppression of dsRNA-triggered induction and action of interferon, the cornerstone of innate immunity, has emerged as a key function of ADAR1 editing of self (cellular) and nonself (viral) dsRNAs. A-to-I modification of RNA is essential for the normal regulation of cellular processes. Dysregulation of A-to-I editing by ADAR1 can have profound consequences, ranging from effects on cell growth and development to autoimmune disorders.

The adaptive potential of RNA editing-mediated miRNA-retargeting in cancer

A-to-I RNA editing is a post-transcriptional mechanism that converts the genomically coded Adenosine (A) into Inosine (I) at the RNA level. This type of RNA editing is the most frequent in humans and is mediated by the ADAR enzymes. RNA editing can alter the genetic code of mRNAs, but also affect the functions of noncoding RNAs such as miRNAs. Recent studies have identified thousands of microRNA editing events in different cancer types. However, the important role played by miRNA-editing in cancer has been reported for just a few microRNAs. Herein, we recapitulate the current studies on cancer-related microRNA editing and discuss their importance in tumor growth and progression. This article is part of a Special Issue entitled: mRNA modifications in gene expression control edited by Dr. Soller Matthias and Dr. Fray Rupert.

ADAR1 Editing and its Role in Cancer

It is well established that somatic mutations and escape of immune disruption are two essential factors in cancer initiation and progression. With an increasing number of second-generation sequencing data, transcriptomic modifications, so called RNA mutations, are emerging as significant forces that drive the transition from normal cell to malignant tumor, as well as providing tumor diversity to escape an immune attack. Editing of adenosine to inosine (A-to-I) in double-stranded RNA, catalyzed by adenosine deaminases acting on RNA (ADARs), is one dynamic modification that in a combinatorial manner can give rise to a very diverse transcriptome. Since the cell interprets inosine as guanosine (G), A-to-I editing can result in non-synonymous codon changes in transcripts as well as yield alternative splicing, but also affect targeting and disrupt maturation of microRNAs. ADAR-mediated RNA editing is essential for survival in mammals, however, its dysregulation causes aberrant editing of its targets that may lead to cancer. ADAR1 is commonly overexpressed, for instance in breast, lung, liver and esophageal cancer as well as in chronic myelogenous leukemia, where it promotes cancer progression. It is well known that ADAR1 regulates type I interferon (IFN) and its induced gene signature, which are known to operate as a significant barrier to tumor formation and progression. Adding to the complexity, ADAR1 expression is also regulated by IFN. In this review, we discussed the regulatory mechanisms of ADAR1 during tumorigenesis through aberrant editing of specific substrates. Additionally, we hypothesized that elevated ADAR1 levels play a role in suppressing an innate immunity response in cancer cells.

The Role of RNA Editing in Cancer Development and Metabolic Disorders

Numerous human diseases arise from alterations of genetic information, most notably DNA mutations. Thought to be merely the intermediate between DNA and protein, changes in RNA sequence were an afterthought until the discovery of RNA editing 30 years ago. RNA editing alters RNA sequence without altering the sequence or integrity of genomic DNA. The most common RNA editing events are A-to-I changes mediated by adenosine deaminase acting on RNA (ADAR), and C-to-U editing mediated by apolipoprotein B mRNA editing enzyme, catalytic polypeptide 1 (APOBEC1). Both A-to-I and C-to-U editing were first identified in the context of embryonic development and physiological homeostasis. The role of RNA editing in human disease has only recently started to be understood. In this review, the impact of RNA editing on the development of cancer and metabolic disorders will be examined. Distinctive functions of each RNA editase that regulate either A-to-I or C-to-U editing will be highlighted in addition to pointing out important regulatory mechanisms governing these processes. The potential of developing novel therapeutic approaches through intervention of RNA editing will be explored. As the role of RNA editing in human disease is elucidated, the clinical utility of RNA editing targeted therapies will be needed. This review aims to serve as a bridge of information between past findings and future directions of RNA editing in the context of cancer and metabolic disease.