The Emerging Role of the Interactions between Circular RNAs and RNA-binding Proteins in Common Human Cancers

RNA binding proteins as mediators of pathological cardiac remodeling

RNA binding proteins (RBPs) play a central in the post-transcriptional regulation of gene expression, which can account for up to 50% of all variations in protein expression within a cell. Following their binding to target RNAs, RBPs most typically confer changes in gene expression through modulation of alternative spicing, RNA stabilization/degradation, or ribosome loading/translation rate. All of these post-transcriptional regulatory processes have been shown to play a functional role in pathological cardiac remodeling, and a growing body of evidence is beginning to identify the mechanistic contribution of individual RBPs and their cardiac RNA targets. This review highlights the mechanisms of RBP-dependent post-transcriptional gene regulation in cardiomyocytes and fibroblasts and our current understanding of how RNA binding proteins functionally contribute to pathological cardiac remodeling.

Emerging functions and significance of circCDYL in human disorders

Circular RNAs (circRNAs) are a group of non-coding transcripts in which a loop structure is shaped via a back splicing procedure. They have central roles in the regulation of gene expression. hsa_circ_0008285, alternatively named as circCDYL, is a circular RNA originated from the exon 4 of CDYL gene. It is produced by a back-splice incident and is mainly located in the cytoplasm. It has no internal ribosome entry site, open reading frame and intronic sequences. CircCDYL dysregulation has been reported in the malignant conditions including multiple myeloma, mantle cell lymphoma, breast cancer, non-small cell lung cancer, Wilms tumor, bladder cancer, colon cancer, and hepatocellular carcinoma. It also has an emerging role in the pathophysiology of non-malignant conditions including myocardial infarction, gestational diabetes mellitus, membranous nephropathy, and abdominal aortic aneurysm. In the current study, we summarize the emerging roles of circCDYL in malignant and non-malignant conditions.

Circular RNAs in the human brain are tailored to neuron identity and neuropsychiatric disease

Little is known about circular RNAs (circRNAs) in specific brain cells and human neuropsychiatric disease. Here, we systematically identify over 11,039 circRNAs expressed in vulnerable dopamine and pyramidal neurons laser-captured from 190 human brains and non-neuronal cells using ultra-deep, total RNA sequencing. 1526 and 3308 circRNAs are custom-tailored to the cell identity of dopamine and pyramidal neurons and enriched in synapse pathways. 29% of Parkinson's and 12% of Alzheimer's disease-associated genes produced validated circRNAs. circDNAJC6, which is transcribed from a juvenile-onset Parkinson's gene, is already dysregulated during prodromal, onset stages of common Parkinson's disease neuropathology. Globally, addiction-associated genes preferentially produce circRNAs in dopamine neurons, autism-associated genes in pyramidal neurons, and cancers in non-neuronal cells. This study shows that circular RNAs in the human brain are tailored to neuron identity and implicate circRNA-regulated synaptic specialization in neuropsychiatric diseases.

CircRNAs regulate the crosstalk between inflammation and tumorigenesis: The bilateral association and molecular mechanisms

Inflammation, a hallmark of cancer, has been associated with tumor progression, transition into malignant phenotype and efficacy of the chemotherapeutic agents in cancer. Chronic inflammation provides a favorable environment for tumorigenesis by inducing immunosuppression, whereas acute inflammation prompts tumor suppression by generating anti-tumor immune responses. Inflammatory factors derived from interstitial cells or tumor cells can stimulate cell proliferation and survival by modulating oncogenes and/or tumor suppressors. Recently, a new class of RNAs, i.e., circular RNAs (circRNAs), has been implicated in inflammatory diseases. Although there are reports on circRNAs imparting functions in inflammatory insults, whether these circularized transcripts hold the potential to regulate inflammation-induced cancer or tumor-related inflammation, and modulate the interactions between tumor microenvironment (TME) and the inflammatory stromal/immune cells, awaits further elucidation. Contextually, the current review describes the molecular association between inflammation and cancer, and spotlights the regulatory mechanisms by which circRNAs can moderate TME in response to inflammatory signals/triggers. We also present comprehensive information about the immune cell(s)-specific expression and functions of the circRNAs in TME, modulation of inflammatory signaling pathways to drive tumorigenesis, and their plausible roles in inflammasomes and tumor development. Moreover, the therapeutic potential of these circRNAs in harnessing inflammatory responses in cancer is also discussed.

Circular RNAs in the human brain are tailored to neuron identity and neuropsychiatric disease

Little is known about circular RNAs (circRNAs) in specific brain cells and human neuropsychiatric disease. Here, we systematically identified over 11,039 circRNAs expressed in vulnerable dopamine and pyramidal neurons laser-captured from 190 human brains and non-neuronal cells using ultra-deep, total RNA sequencing. 1,526 and 3,308 circRNAs were custom-tailored to the cell identity of dopamine and pyramidal neurons and enriched in synapse pathways. 88% of Parkinson's and 80% of Alzheimer's disease-associated genes produced circRNAs. circDNAJC6, produced from a juvenile-onset Parkinson's gene, was already dysregulated during prodromal, onset stages of common Parkinson's disease neuropathology. Globally, addiction-associated genes preferentially produced circRNAs in dopamine neurons, autism-associated genes in pyramidal neurons, and cancers in non-neuronal cells. This study shows that circular RNAs in the human brain are tailored to neuron identity and implicate circRNA- regulated synaptic specialization in neuropsychiatric diseases.

Hypoxia-responsive circular RNA circAAGAB reduces breast cancer malignancy by activating p38 MAPK and sponging miR-378 h

BACKGROUND

Breast cancer is a prevalent disease in women, with high prevalence worldwide. The hypoxic microenvironment of solid tumors develops during the progress of carcinogenesis and leads to greater malignancy and treatment resistance. Recently, accumulating evidence indicates that non-coding RNAs, such as circular RNAs (circRNAs), play a pivotal role in altering cellular functions. However, the underlying mechanisms of circRNAs in breast cancer are still unclear. Therefore, the purpose of this study was to investigate the role of a tumor-suppressive circRNA, circAAGAB, in breast cancer by assuming down-regulation of circAAGAB under hypoxia and the properties of a tumor suppressor.

METHODS

Firstly, circAAGAB was identified from expression profiling by next generation sequencing. Next, the stability of circAAGAB increased by interacting with the RNA binding protein FUS. Moreover, cellular and nuclear fractionation showed that most circAAGAB resided in the cytoplasm and that it up-regulated KIAA1522, NKX3-1, and JADE3 by sponging miR-378 h. Lastly, the functions of circAAGAB were explored by identifying its down-stream genes using Affymetrix microarrays and validated by in vitro assays.

RESULTS

The results showed that circAAGAB reduced cell colony formation, cell migration, and signaling through p38 MAPK pathway, as well as increased radiosensitivity.

CONCLUSION

These findings suggest that the oxygen-responsive circAAGAB acts as a tumor suppressor in breast cancer, and may contribute to the development of a more specific therapeutic regimen for breast cancer.

Emerging landscape of circFNDC3B and its role in human malignancies

In recent years, more attention has been paid to expanding the abundance of Circular RNAs (circRNAs), while the circRNAs that have been found to have significant functions have not been studied in different diseases. CircFNDC3B is one of the most researched circRNAs generated from fibronectin type III domain-containing protein 3B (FNDC3B) gene. Accumulating researches have reported the multiple functions of circFNDC3B in different cancer types and other non-neoplastic diseases, and predicted that circFNDC3B might be a potential biomarker. Notably, circFNDC3B can play roles in different diseases by binding to various microRNAs (miRNAs), binding to RNA-binding proteins (RBPs), or encoding functional peptides. This paper systematically summarizes the biogenesis and function of circRNAs, reviews and discusses the roles and molecular mechanisms of circFNDC3B and its target genes in different cancers and non-neoplastic diseases, which will do favor to broaden our comprehension of the function of circRNAs and facilitate subsequent research on circFNDC3B.

The malignancy of chordomas is enhanced via a circTLK1/miR-16-5p/Smad3 positive feedback axis

CircRNAs play crucial roles in various malignancies via an increasing number of reported regulatory mechanisms, including the classic sponging mechanism between circRNAs and micro RNAs (miRNAs). We performed bioinformatic analyses and identified circTLK1 as a regulator of malignant chordoma progression. Moreover, we observed that circTLK1 showed high expression in chordoma cells and tissues, while circTLK1 interference suppressed chordoma cell proliferation and invasion. In addition, circTLK1 directly interacted with miR-16-5p, which has previously been shown to repress chordoma, and circTLK1 knockdown suppressed Smad3 expression. Chromatin immunoprecipitation sequencing further demonstrated that Smad3 acts as a positive regulator by interacting with TLK1, thereby mediating the circTLK1/miR-16-5p/Smad3 positive feedback axis. Taken together, our findings suggested that the disruption of the circTLK1/miR-16-5p/Smad3 positive feedback pathway, particularly via the Smad3 inhibitor SIS3, could be a promising therapeutic strategy.

Modulating epigenetic modifications for cancer therapy (Review)

Cancer is a global public health concern. Alterations in epigenetic processes are among the earliest genomic aberrations occurring during cancer development and are closely related to progression. Unlike genetic mutations, aberrations in epigenetic processes are reversible, which opens the possibility for novel pharmacological treatments. Non‑coding RNAs (ncRNAs) represent an essential epigenetic mechanism, and emerging evidence links ncRNAs to carcinogenesis. Epigenetic drugs (epidrugs) are a group of promising target therapies for cancer treatment acting as coadjuvants to reverse drug resistance in cancer. The present review describes central epigenetic aberrations during malignant transformation and explains how epidrugs target DNA methylation, histone modifications and ncRNAs. Furthermore, clinical trials focused on evaluating the effect of these epidrugs alone or in combination with other anticancer therapies and other ncRNA‑based therapies are discussed. The use of epidrugs promises to be an effective tool for reversing drug resistance in some patients with cancer.

Circular RNAs and tuberculosis infection

Tuberculosis (TB) is a deadly infectious disease caused by Mycobacterium tuberculosis (Mtb) that affects the immune system chronically. Therefore, effective control and treatment of tuberculosis requires rapid and accurate diagnostic strategies. Tuberculosis has always been a global burden on health, social and economic systems due to the lack of standard curative and diagnostic (bio)markers. Accordingly, the management and monitoring of patients with active TB at the primary care level may be possible through new, rapid and cost-effective non-sputum-based diagnostic procedures. Biomarkers can help diagnose various diseases, including circular RNA (circRNA), which has recently been introduced as an endogenous, abundant and stable RNA in the cytoplasm with unique tissue specificity. There are frequent reports of circRNA involvement in many pathological and physiological processes in human beings. Recent studies have highlighted the presence of circRNAs in serum and their role as promising biomarkers in the diagnosis of the disease, potentially due to the continuous, stable, closed covalent circular structures and lack of easy degradation by nucleases. The purpose of this review article is to scrutinize the behavior of circulating plasma RNAs in relation to the pathogenesis and diagnosis of tuberculosis.

Circular RNA hsa_circ_0062682 Binds to YBX1 and Promotes Oncogenesis in Hepatocellular Carcinoma

Simple Summary

Circular RNA (circRNA) have a role in carcinogenesis in different cancers, also in hepatocellular carcinoma (HCC). The transcriptome analyses of HCC tumours identified an upregulated circRNA hsa_circ_0062682. We show that this circRNA affects several aspects of oncogenesis, which are cell proliferation, migration, and invasion. Using transcriptome analyses we identified modulated signalling pathways and transcription factors, confirming the observed phenotype in cells. We identified Y-box-binding protein 1 (YBX1), a known oncogene and RNA-binding protein, as a binding partner, which was in line with transcriptome analyses. We also identified a cell-specific response to sorafenib after circRNA modulation, which is in line with a heterogeneous molecular pathology of HCC subtypes.

Abstract

Circular RNAs (circRNAs) have been shown to play an important role in the pathogenesis of hepatocellular carcinoma (HCC). By implementing available transcriptomic analyses of HCC patients, we identified an upregulated circRNA hsa_circ_0062682. Stable perturbations of hsa_circ_0062682 in Huh-7 and SNU-449 cell lines influenced colony formation, migration, cell proliferation, sorafenib sensitivity, and additionally induced morphological changes in cell lines, indicating an important role of hsa_circ_0062682 in oncogenesis. Pathway enrichment analysis and gene set enrichment analysis of the transcriptome data from hsa_circ_0062682 knockdown explained the observed phenotypes and exposed transcription factors E2F1, Sp1, HIF-1α, and NFκB1 as potential downstream targets. Biotinylated oligonucleotide pulldown combined with proteomic analyses identified protein interaction partners of which YBX1, a known oncogene, was confirmed by RNA immunoprecipitation. Furthermore, we discovered a complex cell-type-specific phenotype in response to the oncogenic potential of hsa_circ_0062682. This finding is in line with different classes of HCC tumours, and more studies are needed to shed a light on the molecular complexity of liver cancer.

CircMYH9 increases KPNA2 mRNA stability to promote hepatocellular carcinoma progression in an EIF4A3-dependent manner

Hepatocellular carcinoma (HCC) is the most commonly diagnosed cancer worldwide with a high incidence of recurrence and metastasis; however, the molecular mechanisms underlying HCC development remain to be fully understood. In this study, we identified circMYH9 as an important regulator of HCC. Overexpression of circMYH9 induced, while knockdown of circMYH9 inhibited, the proliferation, migration, and invasion of HCC cells. Mechanistically, circMYH9 bound to eukaryotic translation initiation factor 4A3 (EIF4A3) and increased karyopherin subunit alpha 2 (KPNA2) mRNA stability. circMYH9 knockdown in HCC cells reduced the stability of KPNA2 mRNA. Importantly, circMYH9 regulation of HCC required the activity of KPNA2. In support with this, circMYH9 level was positively correlated with the expression of KPNA2 in HCC patient samples. Taken together, our study was the first to uncover the oncogenic role of circMYH9 in HCC and further elucidated the functional mechanism of circMYH9 by interacting with EIF4A3 to increase KPNA2 mRNA stability. Our findings might provide a novel potential target for the diagnose and treatment of HCC.

Circular RNAs Modulate Cancer Hallmark and Molecular Pathways to Support Cancer Progression and Metastasis

Simple Summary

Circular RNAs (circRNA) are a type of RNA molecule of circular shape that are now being extensively studied due to the important roles they play in different biological processes. In addition, they were also shown to be implicated in disease such as cancer. Cancer is a complex process which is often defined by a combination of specific processes called cancer hallmarks. In this review, we summarize the literature on circRNAs in cancer and classify them as being implicated in specific cancer hallmarks.

Abstract

Circular RNAs (circRNAs) are noncoding products of backsplicing of pre-mRNAs which have been established to possess potent biological functions. Dysregulated circRNA expression has been linked to diseases including different types of cancer. Cancer progression is known to result from the dysregulation of several molecular mechanisms responsible for the maintenance of cellular and tissue homeostasis. The dysregulation of these processes is defined as cancer hallmarks, and the molecular pathways implicated in them are regarded as the targets of therapeutic interference. In this review, we summarize the literature on the investigation of circRNAs implicated in cancer hallmark molecular signaling. First, we present general information on the properties of circRNAs, such as their biogenesis and degradation mechanisms, as well as their basic molecular functions. Subsequently, we summarize the roles of circRNAs in the framework of each cancer hallmark and finally discuss the potential as therapeutic targets.

Crosstalk between N6-methyladenosine modification and circular RNAs: current understanding and future directions

N⁶-methyladenosine (m⁶A) is a prevalent internal modification in eukaryotic RNAs regulated by the so-called "writers", "erasers", and "readers". m⁶A has been demonstrated to exert critical molecular functions in modulating RNA maturation, localization, translation and metabolism, thus playing an essential role in cellular, developmental, and disease processes. Circular RNAs (circRNAs) are a class of non-coding RNAs with covalently closed single-stranded structures generated by back-splicing. CircRNAs also participate in physiological and pathological processes through unique mechanisms. Despite their discovery several years ago, m⁶A and circRNAs has drawn increased research interest due to advances in molecular biology techniques these years. Recently, several scholars have investigated the crosstalk between m⁶A and circRNAs. In this review, we provide an overview of the current knowledge of m⁶A and circRNAs, as well as summarize the crosstalk between these molecules based on existing research. In addition, we present some suggestions for future research perspectives.