Aberrant overexpression of ADAR1 promotes gastric cancer progression by activating mTOR/p70S6K signaling

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.

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.

Regulation of the double-stranded RNA response through ADAR1 licenses metaplastic reprogramming in gastric epithelium

Cells recognize both foreign and host-derived double-stranded RNA (dsRNA) via a signaling pathway that is usually studied in the context of viral infection. It has become increasingly clear that the sensing and handling of endogenous dsRNA is also critical for cellular differentiation and development. The adenosine RNA deaminase, ADAR1, has been implicated as a central regulator of the dsRNA response, but how regulation of the dsRNA response might mediate cell fate during injury and whether such signaling is cell intrinsic remain unclear. Here, we show that the ADAR1-mediated response to dsRNA was dramatically induced in 2 distinct injury models of gastric metaplasia. Mouse organoid and in vivo genetic models showed that ADAR1 coordinated a cell-intrinsic, epithelium-autonomous, and interferon signaling–independent dsRNA response. In addition, dsRNA accumulated within a differentiated epithelial population (chief cells) in mouse and human stomachs as these cells reprogrammed to a proliferative, reparative (metaplastic) state. Finally, chief cells required ADAR1 to reenter the cell cycle during metaplasia. Thus, cell-intrinsic ADAR1 signaling is critical for the induction of metaplasia. Because metaplasia increases cancer risk, these findings support roles for ADAR1 and the response to dsRNA in oncogenesis.

Ribosomal Protein S6: A Potential Therapeutic Target against Cancer?

Ribosomal protein S6 (RPS6) is a component of the 40S small ribosomal subunit and participates in the control of mRNA translation. Additionally, phospho (p)-RPS6 has been recognized as a surrogate marker for the activated PI3K/AKT/mTORC1 pathway, which occurs in many cancer types. However, downstream mechanisms regulated by RPS6 or p-RPS remains elusive, and the therapeutic implication of RPS6 is underappreciated despite an approximately half a century history of research on this protein. In addition, substantial evidence from RPS6 knockdown experiments suggests the potential role of RPS6 in maintaining cancer cell proliferation. This motivates us to investigate the current knowledge of RPS6 functions in cancer. In this review article, we reviewed the current information about the transcriptional regulation, upstream regulators, and extra-ribosomal roles of RPS6, with a focus on its involvement in cancer. We also discussed the therapeutic potential of RPS6 in cancer.

Towards a druggable epitranscriptome: Compounds that target RNA modifications in cancer

Epitranscriptomics is an exciting emerging area that studies biochemical modifications of RNA. The field has been opened up by the technical efforts of the last decade to characterize and quantify RNA modifications, and this has led to a map of post-transcriptional RNA marks in normal cell fate and development. However, the scientific interest has been fuelled by the discovery of aberrant epitranscriptomes associated with human diseases, mainly cancer. The challenge is now to see whether epitrancriptomics offers mechanisms that can be effectively targeted by low MW compounds and are thus druggable. In this review, we will describe the principal RNA modifications (with a focus on mRNA), summarize the latest scientific evidence of their dysregulation in cancer and provide an overview of the state-of-the-art drug discovery to target the epitranscriptome. Finally, we will discuss the principal challenges in the field of chemical biology and drug development to increase the potential of targeted-RNA for clinical benefit.

Circular RNA hsa_circ_0004872 inhibits gastric cancer progression via the miR-224/Smad4/ADAR1 successive regulatory circuit

BACKGROUND

Emerging evidence has shown that circular RNAs (circRNAs) play a crucial regulatory role in the occurrence and development of cancer. Exploring the roles and mechanisms of circRNAs in tumorigenesis and progression may help to identify new diagnostic markers and therapeutic targets. In the present study, we investigated the role and regulatory mechanism of hsa_circ_0004872 in gastric cancer (GC).

METHODS

qRT-PCR was used to determine the expression of hsa_circ_0004872 in GC tissues and cells. EdU, CCK-8, transwell and scratch wound healing assays were used to assess the role of hsa_circ_0004872 in GC cell proliferation, invasion and migration, respectively. Subcutaneous and tail vein tumor injections in nude mice were used to assess the role of hsa_circ_0004872 in vivo. RIP assay, biotin-coupled probe pull-down assay, FISH and luciferase reporter assay were performed to confirm the relationship between hsa_circ_0004872 and the identified miRNA. ChIP assay, luciferase reporter assay and western blot were used to determine the direct binding of Smad4 to the promoter of the ADAR1 gene.

RESULTS

In this study, we found that hsa_circ_0004872 was dramatically downregulated in GC tissues compared with adjacent noncancerous tissues. The expression level of hsa_circ_0004872 was associated with tumor size and local lymph node metastasis. Enforced expression of hsa_circ_0004872 inhibited the proliferation, invasion and migration of GC cells, whereas knockdown of hsa_circ_0004872 had the opposite effects. Nude mice experiments showed that ectopic expression of hsa_circ_0004872 dramatically inhibited tumor growth and metastasis in vivo. Moreover, we demonstrated that hsa_circ_0004872 acted as a "molecular sponge" for miR-224 to upregulate the expression of the miR-224 downstream targets p21 and Smad4. Importantly, we found that the RNA-editing enzyme ADAR1 inhibited hsa_circ_0004872 expression and further led to the upregulation of miR-224. Smad4, the downstream target of miR-224, could further affect hsa_circ_0004872 levels by directly binding to the promoter region of ADAR1 to inhibit ADAR1 expression.

CONCLUSIONS

Our findings showed that hsa_circ_0004872 acted as a tumor suppressor in GC by forming a negative regulatory loop consisting of hsa_circ_0004872/miR-224/Smad4/ADAR1. Thus, hsa_circ_0004872 may serve as a potential biomarker and therapeutic target for GC.

ADAR1 Suppresses Interferon Signaling in Gastric Cancer Cells by MicroRNA-302a-Mediated IRF9/STAT1 Regulation

ADAR (adenosine deaminase acting on RNA) catalyzes the deamination of adenosine to generate inosine, through its binding to double-stranded RNA (dsRNA), a phenomenon known as RNA editing. One of the functions of ADAR1 is suppressing the type I interferon (IFN) response, but its mechanism in gastric cancer is not clearly understood. We analyzed changes in RNA editing and IFN signaling in ADAR1-depleted gastric cancer cells, to clarify how ADAR1 regulates IFN signaling. Interestingly, we observed a dramatic increase in the protein level of signal transducer and activator of transcription 1 (STAT1) and interferon regulatory factor 9 (IRF9) upon ADAR1 knockdown, in the absence of type I or type II IFN treatment. However, there were no changes in protein expression or localization of the mitochondrial antiviral signaling protein (MAVS) and interferon alpha and beta-receptor subunit 2 (IFNAR2), the two known mediators of IFN production. Instead, we found that miR-302a-3p binds to the untranslated region (UTR) of IRF9 and regulate its expression. The treatment of ADAR1-depleted AGS cells with an miR-302a mimic successfully restored IRF9 as well as STAT1 protein level. Hence, our results suggest that ADAR1 regulates IFN signaling in gastric cancer through the suppression of STAT1 and IRF9 via miR-302a, which is independent from the RNA editing of known IFN production pathway.

ADAR expression and copy number variation in patients with advanced gastric cancer

Background

Gastric cancer (GC) is a world health problem and it is the third leading cause of cancer deaths worldwide. The current practice for prognosis assessment in GC is based on radiological and pathological criteria and they may not result in an accurate prognosis. The aim of this study is to evaluate expression and copy number variation of the ADAR gene in advanced GC and clarify its correlation with survival and histopathological characteristics.

Methods

Forty two patients with stage III and IV GC were included in this study. ADAR gene expression and copy number variation were measured by real-time PCR and Quantitative multiplex fluorescent-PCR, respectively. Survival analysis performed based on the Kaplan–Meier method and Mantel–Cox test.

Results

ADAR mRNA was significantly overexpressed in the tumor tissues when compared to the adjacent normal tissues (p < 0.01). Also, ADAR expression level in stage IV was higher than stage III. 40% of patients showed amplification in ADAR gene and there was a positive correlation between ADAR copy number and expression. Increased ADAR expression was clearly correlated with poorer survival outcomes and Mantel–Cox test showed statistically significant differences between low and high expression groups (p < 0.0001). ADAR overexpression and amplification were significantly associated with metastasis, size and stage of tumor.

Conclusions

Together, our data indicate that amplification leads to over expression of ADAR and it could be used as a prognostic biomarker for disease progression, especially for the metastatic process in GC.

ADAR1 may be involved in the proliferation of acute myeloid leukemia cells via regulation of the Wnt pathway

Purpose

Acute myeloid leukemia (AML) is the most common type of leukemia and characterized by the malignant growth of leukemic cells. Adenosine deaminases acting on RNA 1 (ADAR1) have been shown to participate in the proliferation of cancer cells and progression of various cancers. However, the role of ADAR1 in AML has not been investigated.

Patients and methods

We compared the expression levels of ADAR1 between samples obtained from different AML patients and controls using quantitative-polymerase chain reaction and Western blotting. We also investigated the functional role and possible mechanisms via silencing the expression of ADAR1 in vitro and in vivo.

Results

We found that the mRNA and protein levels of ADAR1 were significantly higher in AML patients. The mRNA expression of ADAR1 was positively correlated with the ratio of leukemic cells. Additionally, silencing of ADAR1 expression significantly suppressed the proliferation of AML cells and induced G0/1 arrest. For the analysis of the mechanism, the quantitative-polymerase chain reaction and Western blotting results revealed that ADAR1 knockdown resulted in the decreased expression of Wingless-Int (Wnt) effectors including β-catenin, c-Myc, transcription factor 4, and cyclin D2. In the nude mouse model, inhibition of ADAR1 expression reduced the tumorigenic potential and decreased the expression o]f Wnt effectors.

Conclusion

These results demonstrate that ADAR1 may be involved in the regulation of the proliferation of AML cells partially via regulation of the Wnt signaling pathway.

UNBS5162 as a novel naphthalimide holds efficacy in human gastric carcinoma cell behaviors mediated by AKT/ERK signaling pathway

Purpose: Studies have determined that UNBS5162, recognized as a new naphthalimide, holds inhibitory effects in prostate and breast tumors; however, its functional implication on gastric carcinoma is currently undetermined. Based on this, this study designed to assess the functional role of it on human gastric carcinoma and underlying mechanism of action. Methods: Cell counting kit-8 (CCK-8) assay, transwell assay, and flow cytometry were used to assess capabilities of SGC-7901 cell proliferation, invasion/migration, and apoptosis, respectively. Moreover, western blot was performed to determine the relative expression of protein related to autophagy and protein kinase B (AKT)/extracellular regulated protein kinases (ERK) signaling pathway. Results: We found SGC-7901 cells proliferation, invasion, and migration were significantly inhibited after treatment of UNBS5162. Moreover, the expression levels of anti-apoptotic protein Bcl-2 decreased while the expression of pro-apoptotic protein active caspase 3 and Bax increased concurrently after UNBS5162 stimulation. Further, upregulated LC3 II/I and Beclin-1 and downregulated P62 were induced by UNBS5162 addition. Mechanically, the ratios of phosphorylated-(p-)AKT/AKT, p-mammalian target of rapamycin (mTOR)/mTOR, and p-ERK/ERK were hampered by UNBS5162 application. Conclusion: UNBS5162 could restrain gastric carcinoma cell proliferation, invasion, and migration, which maybe induced by enhancement of apoptosis, autophagy manipulated through AKT/ERK signaling pathway.

ADAR1 silencing-induced HUVEC apoptosis is mediated by FGFR2 under hypoxia stress

Background

The adenosine deaminase acting on RNA 1 (ADAR1) specifically deaminates adenosine to inosine in double-stranded RNA (dsRNA). Emerging evidence indicated that under hypoxia condition, such as tumor microenvironment, ADAR1 level was increased. Interestingly, we found FGFR2 was also increased under hypoxia stress. The purpose of this study was to investigate the regulation mechanism of ADAR1 and the potential role of ADAR1–FGFR2 axis in cell proliferation and apoptosis.

Methods

Using human umbilical vein endothelial cells as cellular model, we explored the function of ADAR1 in regulating cell survival.

Results

We found manipulation of FGFR2 activity could override the cellular effect of ADAR1, suggesting FGFR2 could be a potential effector of ADAR1. Moreover, our results revealed that PI3K-Akt pathway was involved in ADAR1–FGFR2 axis-induced cell proliferation.

Conclusion

In summary, this study supported the notion that ADAR1 could play a role in tumor cell proliferation, which was mediated by FGFR2.

Adenosine signaling: Next checkpoint for gastric cancer immunotherapy?

Adenosine (ADO), generated by the ectonucleotidase CD39 and CD73 from ATP, interacts with its specific G protein-coupled receptors, which can impair anti-tumor immune responses inhibiting the infiltration and function of CD8⁺ T cell and natural killer cell. Recent studies have also identified that ADO pathway plays a critical role in tumor immune surveillance, especially for some non-solid cancers. In addition, although immune checkpoint therapy targeting ADO pathway in gastric cancer is still in an early phase, encouraging results have come out from some drugs targeting ADO pathway. Therefore, target ADO signaling may be a new promising strategy to treat gastric cancer. In this review, we summarized recent works on the role of ADO in cancer immunotherapy and also discussed relative mechanisms underlying the function of ADO signaling in cancer immune responses.

Review of recent efforts to discover biomarkers for early detection, monitoring, prognosis, and prediction of treatment responses of patients with gastric cancer

Gastric cancer (GC) is the leading cause of cancer-related death worldwide. Despite recent advances in diagnosis and therapy, the prognosis of patients with GC is poor. Many patients have inoperable disease upon diagnosis or experience recurrent disease after curative gastrectomy. Unfortunately, tumor markers for GC, such as serum carcinoembryonic antigen and carbohydrate antigen 19-9, lack sufficient sensitivity and specificity. Therefore, effective biomarkers are required to detect early GC and to predict tumor recurrence and chemosensitivity. Areas covered: Here we aimed to review recent developments in techniques that improve the detection of aberrant expression of GC-associated molecules, including protein coding genes, microRNAs, long noncoding RNAs, and methylated promoter DNAs. Expert commentary: Detection of genetic and epigenetic alterations in gastric tissue or in the circulation will likely improve the diagnosis and management of GC to achieve significantly improved outcomes.

Mechanisms and implications of ADAR-mediated RNA editing in cancer

Adenosine deaminases acting on RNA (ADARs) are enzymes that catalyze the conversion of adenosine (A) to inosine (I) in double-stranded RNAs. Inosine exhibits similar properties as guanosine. As a result, A-to-I editing has a great impact on edited RNAs, not only affecting the base pairing properties, but also altering codons after translation. A-to-I editing are known to mediate and diversify transcripts. However, the overall biological effect of ADARs are still largely unknown. Aberrant ADAR activity and editing dysregulation are present in a variety of cancers, including hepatocellular carcinoma, chronic myelogenous leukemia, glioblastoma and melanoma. ADAR-mediated A-to-I editing can influence uncontrolled nucleotide changes, resulting in susceptibility of cells to developmental defects and potential carcinogenicity. A deeper understanding of the biological function of ADARs may provide mechanistic insights in the development of new cancer therapy. Here, we discuss recent advances in research on ADAR in detail including the structure and function of ADARs, the biochemistry of ADAR-mediated RNA editing, and the relevance of ADAR proteins in cancer.

Functions of the RNA Editing Enzyme ADAR1 and Their Relevance to Human Diseases

Adenosine deaminases acting on RNA (ADARs) convert adenosine to inosine in double-stranded RNA (dsRNA). Among the three types of mammalian ADARs, ADAR1 has long been recognized as an essential enzyme for normal development. The interferon-inducible ADAR1p150 is involved in immune responses to both exogenous and endogenous triggers, whereas the functions of the constitutively expressed ADAR1p110 are variable. Recent findings that ADAR1 is involved in the recognition of self versus non-self dsRNA provide potential explanations for its links to hematopoiesis, type I interferonopathies, and viral infections. Editing in both coding and noncoding sequences results in diseases ranging from cancers to neurological abnormalities. Furthermore, editing of noncoding sequences, like microRNAs, can regulate protein expression, while editing of Alu sequences can affect translational efficiency and editing of proximal sequences. Novel identifications of long noncoding RNA and retrotransposons as editing targets further expand the effects of A-to-I editing. Besides editing, ADAR1 also interacts with other dsRNA-binding proteins in editing-independent manners. Elucidating the disease-specific patterns of editing and/or ADAR1 expression may be useful in making diagnoses and prognoses. In this review, we relate the mechanisms of ADAR1′s actions to its pathological implications, and suggest possible mechanisms for the unexplained associations between ADAR1 and human diseases.