Comprehensive Circular RNA Profiling Reveals That hsa_circ_0005075, a New Circular RNA Biomarker, Is Involved in Hepatocellular Crcinoma Development

Role of Virally Encoded Circular RNAs in the Pathogenicity of Human Oncogenic Viruses

Human oncogenic viruses are a group of important pathogens that etiologically contribute to at least 12% of total cancer cases in the world. As an emerging class of non-linear regulatory RNA molecules, circular RNAs (circRNAs) have gained increasing attention as a crucial player in the regulation of signaling pathways involved in viral infection and oncogenesis. With the assistance of current circRNA enrichment and detection technologies, numerous novel virally-encoded circRNAs (vcircRNAs) have been identified in the human oncogenic viruses, initiating an exciting new era of vcircRNA research. In this review, we discuss the current understanding of the roles of vcircRNAs in the respective viral infection cycles and in virus-associated pathogenesis.

Circular RNA hsa_circ_0003288 induces EMT and invasion by regulating hsa_circ_0003288/miR-145/PD-L1 axis in hepatocellular carcinoma

Background

Epithelial-mesenchymal transition (EMT) has been associated with wound healing, tumorigenesis, and metastasis. Circular RNAs (circRNAs) are functional non-coding RNAs involved in multiple human cancers. However, whether and how circRNAs contribute to the EMT in hepatocellular carcinomas (HCC) remains to be deciphered. In this study, we investigated the regulation and function of hsa_circ_0003288 on programmed death-1 ligand 1 (PD-L1) during EMT and HCC invasiveness.

Methods

Hsa_circ_0003288 expression was measured by real-time quantitative reverse transcriptase PCR (qRT-PCR). Luciferase reporter assays, RNA pull-down assay and fluorescence in situ hybridization (FISH) were used to determine the correlation between hsa_circ_0003288 and miR-145 and between miR-145 and PD-L1. Furthermore, ectopic overexpression and siRNA-mediated downregulation of hsa_circ_0003288, transwell assays, and in vivo studies were used to determine the function of hsa_circ_0003288 on the EMT and invasiveness of L02 and HCC cells.

Results

miR-145 directly targeted the PD-L1 3′-untranslated region (UTR) region, and hsa_circ_0003288 acted as a miR-145 sponge to regulate PD-L1 expression. Overexpression of hsa_circ_0003288 increased PD-L1 levels and promoted EMT, migration, and invasiveness of L02 cells. These observations were reversed after knockdown of hsa_circ_0003288 in HepG2 and Huh7 cells. Overexpression of PD-L1 rescued EMT, migration, and invasiveness of HepG2 and Huh7 cells after knockdown of hsa_circ_0003288. Furthermore, hsa_circ_0003288 knockdown reduced EMT in in vivo studies. Hsa_circ_0003288/PD-L1 axis was found to mediate the metastatic phenotypes via the PI3K/Akt pathway in HCC. Additionally, expression levels of hsa_circ_0003288 were increased and positively correlated with PD-L1 expression in HCC tissues.

Conclusion

Our findings demonstrated that hsa_circ_0003288 promoted EMT and invasion of HCC via the hsa_circ_0003288/miR-145/PD-L1 axis through the PI3K/Akt pathway. Targeting hsa_circ_0003288 may be a therapeutic strategy for the treatment of HCC.

Development of a two-in-one integrated assay for the analysis of circRNA-microRNA interactions

The competitive endogenous RNA hypothesis is a new mechanism of RNA dialogue, in which circRNA-miRNA interaction (cmRRI) is found to be widely involved in the regulation of gene expression in tumors and other diseases. It is urgent but challenging to develop a convenient and efficient method to study the interaction between target circRNA and the candidate miRNAs. In this work, a biosensing method that allows directly analyzing cmRRI has been developed, so as to reveal the RNA dialogue strategy. The sensing system uses a bifunctional magnetic bead for the capture of target circRNA/miRNA complex as well as the signal amplification. Based on the nature of circRNA as a miRNA sponge, only if the target circRNAs and its regulatory miRNAs coexist as a complex, can the rolling circle amplification reaction be initiated to give a fluorescent signal as the output. Compared with traditional methods where the circRNA and its regulatory miRNAs have been separately analyzed, our design allows the integrated profiling of specific cmRRI by correlation characterization of two correlative RNAs, which represents a function-oriented method. The presented method also shows the analysis of the potential binding affinity of candidate miRNAs to target circRNAs. Furthermore, we have verified the ability of the sensor to directly detect cmRRI in biological samples, which reveals the promising applicability of this method for biomedical and clinical researches in the future.

Advances in the Identification of Circular RNAs and Research Into circRNAs in Human Diseases

Circular RNAs (circRNAs) are a class of endogenous non-coding RNAs (ncRNAs) with a closed-loop structure that are mainly produced by variable processing of precursor mRNAs (pre-mRNAs). They are widely present in all eukaryotes and are very stable. Currently, circRNA studies have become a hotspot in RNA research. It has been reported that circRNAs constitute a significant proportion of transcript expression, and some are significantly more abundantly expressed than other transcripts. CircRNAs have regulatory roles in gene expression and critical biological functions in the development of organisms, such as acting as microRNA sponges or as endogenous RNAs and biomarkers. As such, they may have useful functions in the diagnosis and treatment of diseases. CircRNAs have been found to play an important role in the development of several diseases, including atherosclerosis, neurological disorders, diabetes, and cancer. In this paper, we review the status of circRNA research, describe circRNA-related databases and the identification of circRNAs, discuss the role of circRNAs in human diseases such as colon cancer, atherosclerosis, and gastric cancer, and identify remaining research questions related to circRNAs.

Dysregulation of miR-23b-5p promotes cell proliferation via targeting FOXM1 in hepatocellular carcinoma

Growing evidence demonstrates that MicroRNAs (miRNAs) play an essential role in contributing to tumor development and progression. However, the underlying role and mechanisms of miR-23b-5p in hepatocellular carcinoma (HCC) formation remain unclear. Our study showed that miR-23b-5p was downregulated in the HCC tissues and cell lines, and lower expression of miR-23b-5p was associated with more severe tumor size and poorer survival. Gain- or loss-of-function assays demonstrated that miR-23b-5p induced G0/G1 cell cycle arrest and inhibited cell proliferation both in vitro and in vivo. qRT-PCR, western blot and luciferase assays verified that Mammalian transcription factor Forkhead Box M1 (FOXM1), upregulated in HCC specimens, was negatively correlated with miR-23b-5p expression and acted as a direct downstream target of miR-23b-5p. In addition, miR-23b-5p could regulate cyclin D1 and c-MYC expression by directly targeting FOXM1. Further study revealed that restoration of FOXM1 neutralized the cell cycle arrest and cell proliferation inhibition caused by miR-23b-5p. Taken together, our findings suggest that miR-23b-5p acted as a tumor suppressor role in HCC progression by targeting FOXM1 and may serve as a potential novel biomarker for HCC diagnosis and prognosis.

The Role of circRNAs in Human Papillomavirus (HPV)-Associated Cancers

Circular RNAs (circRNAs) are a new class of "non-coding RNAs" that originate from non-sequential back-splicing of exons and/or introns of precursor messenger RNAs (pre-mRNAs). These molecules are generally produced at low levels in a cell-type-specific manner in mammalian tissues, but due to their circular conformation they are unaffected by the cell mRNA decay machinery. circRNAs can sponge multiple microRNAs or RNA-binding proteins and play a crucial role in the regulation of gene expression and protein translation. Many circRNAs have been shown to be aberrantly expressed in several cancer types, and to sustain specific oncogenic processes. Particularly, in virus-associated malignancies such as human papillomavirus (HPV)-associated anogenital carcinoma and oropharyngeal and oral cancers, circRNAs have been shown to be involved in tumorigenesis and cancer progression, as well as in drug resistance, and some are useful diagnostic and prognostic markers. HPV-derived circRNAs, encompassing the HPV E7 oncogene, have been shown to be expressed and to serve as transcript for synthesis of the E7 oncoprotein, thus reinforcing the virus oncogenic activity in HPV-associated cancers. In this review, we summarize research advances in the biogenesis of cell and viral circRNAs, their features and functions in the pathophysiology of HPV-associated tumors, and their importance as diagnostic, prognostic, and therapeutic targets in anogenital and oropharyngeal and oral cancers.

Non-coding RNAs: Emerging from the discovery to therapeutic applications

The knowledge about non-coding RNAs (ncRNAs) is rapidly increasing with new data continuously emerging, regarding their diverse types, applications, and roles. Particular attention has been given to ncRNA with regulatory functions, which may have a critical role both in biological and pathological conditions. As a result of the diversity of ncRNAs and their ubiquitous involvement in several biologic processes, ncRNA started to be considered in the biomedical field, with immense potential to be exploited either as biomarkers or as therapeutic agents in certain pathologies. Indeed, ncRNA-based therapeutics have been proposed in many disorders and some even reached clinical trials. However, to prepare an RNA product suitable for pharmacological applications, certain criteria must be fulfilled, and it has to be guaranteed RNA purity, stability, and bioactivity. So, in this review, the different types of ncRNAs are identified and characterized, by describing their biogenesis, functions, and applications. A perspective on the main challenges and innovative approaches for the future and broad therapeutic application of RNA is also presented.

The Prognostic and Predictive Significance of circRNA CDR1as in Tumor Progression

Cerebellar degeneration-related protein 1 antisense (CDR1as) is an important member of the circRNAs family, also known as cirs-7. Its main function in vivo is to act as a mir-7 sponge. Accumulated studies show that CDR1as is closely related to various diseases, especially cancer. Our analysis show that CDR1as expression in human cancer is significantly associated with poor overall survival (hazard ratio [HR] = 2.50, 95% confidence interval [CI] = 2.06–3.04; p < 0.00001) and that high CDR1as expression is associated with the tumor node metastasis stage (odds ratio [OR] = 2.13, 95% CI = 1.63–2.78; p < 0.00001), and distant metastasis (OR = 3.50, 95% CI = 1.90–6.64; p < 0.00001). Furthermore, the results reveal the prognostic significance of CDR1as in neoplasms of the digestive system (HR = 1.69, 95% CI = 2.14–2.71; p < 0.001), colorectal cancer (HR = 1.34, 95% CI = 1.96–2.85; p < 0.001), and non-small cell lung cancer (HR = 2.40, 95% CI = 3.42–4.83; p = 0.008). In this study, we summarize in detail the latest research findings and demonstrate the function and regulatory mechanism of CDR1as in various cancer processes, and its potential as a biomarker for cancer prevention and prognosis.

Recent Update on the Role of Circular RNAs in Hepatocellular Carcinoma

After being overlooked for decades, circular RNAs (circRNAs) have recently generated considerable interest. circRNAs play a role in a variety of normal and pathological biological processes, including hepatocarcinogenesis. Many circRNAs contribute to hepatocarcinogenesis through sponging of microRNAs (miRs) and disruption of cellular signaling pathways that play a part in control of cell proliferation, metastasis and apoptosis. In most cases, overexpressed circRNAs sequester miRs to cause de-repressed translation of mRNAs that encode oncogenic proteins. Conversely, low expression of circRNAs has also been described in hepatocellular carcinoma (HCC) and is associated with inhibited production of tumor suppressor proteins. Other functions of circRNAs that contribute to hepatocarcinogenesis include translation of truncated proteins and acting as adapters to regulate influence of transcription factors on target gene expression. circRNAs also affect hepatocyte transformation indirectly. For example, the molecules regulate immune surveillance of cancerous cells and influence the liver fibrosis that commonly precedes HCC. Marked over- or under-expression of circRNA expression in HCC, with correlating plasma concentrations, has diagnostic utility and assays of these RNAs are being developed as biomarkers of HCC. Although knowledge in the field has recently surged, the myriad of described effects suggests that not all may be vital to hepatocarcinogenesis. Nevertheless, investigation of the role of circRNAs is providing valuable insights that are likely to contribute to improved management of a serious and highly aggressive cancer.

Identification of Circular RNA hsa_circ_0001599 as a Novel Biomarker for Large-Artery Atherosclerotic Stroke

Circular RNAs (circRNAs) are a recently discovered noncoding RNA isoform capable of regulating neurological disease incidence. The present study was designed to characterize the circRNA expression profiles present in large-artery atherosclerosis (LAA)-type acute ischemic stroke patients and to detect biomarkers suitable for LAA-stroke detection. Using a RNA-seq-based approach, we characterized circRNA expression profiles in five LAA-stroke patients and four controls. We confirmed the differential expression of target circRNAs through quantitative real-time polymerase chain reaction (qRT-PCR), and used Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway enrichment analyses to explore their functional roles. The diagnostic value of specific circRNAs was evaluated through a receiver operating characteristic (ROC) curve analysis. We identified 182 upregulated and 176 downregulated circRNAs in LAA-stroke patients and confirmed the differential expression of six circRNAs through qRT-PCR. These differentially expressed circRNAs are primarily associated with chromatin modification, autophagy, platelet activation, and neural precursor cell proliferation. The hsa_circRNA_0001599 expression levels were positively correlated with the National Institutes of Health Stroke Scale scores and infarct volumes, with an ROC analysis of hsa_circRNA_0001599 in LAA-stroke, yielding an area under the curve of 0.805 (95% confidence interval: 0.748-0.862; p < 0.001), consistent with sensitivity and specificity values of 64.41% and 89.93%, respectively, for the diagnosis of LAA-stroke. A transcriptome-wide survey of differential circRNA expression in LAA-stroke patients revealed hsa_circRNA_0001599 as a putative circRNA biomarker of LAA-stroke diagnosis.

Circular RNA hsa_circ_0004277 Stimulates Malignant Phenotype of Hepatocellular Carcinoma and Epithelial-Mesenchymal Transition of Peripheral Cells

Accumulating evidence shows that exosomal circRNAs reflect the physiological status of donor cells, and various cell reactions are induced after exosomal circRNAs are captured by recipient cells. In this study, qRT-PCR was performed to detect circ-0004277 expression in hepatocellular carcinoma (HCC) cell lines, tissues, and plasma exosomes. The effects of circ-0004277 on the proliferation and migration of HCC cells were assessed by cell counting, 5-ethynyl-2'-deoxyuridine assays, Transwell migration assays, and tumor formation in nude mice. We found that circ-0004277 was significantly upregulated in HCC cells, tissues, and plasma exosomes compared to that in normal controls. Overexpression of circ-0004277 enhanced the proliferation, migration, and epithelial-mesenchymal transition (EMT) of HCC cells in vivo and in vitro. Furthermore, exosomes from HCC cells enhanced circ-0004277 expression in surrounding normal cells and stimulated EMT progression. ZO-1, a tight junction adapter protein, was downregulated in HCC tissues. In conclusion, our findings suggest that circ-0004277 promotes the malignant phenotype of HCC cells via inhibition of ZO-1 and promotion of EMT progression. In addition, exosomal circ-0004277 from HCC cells stimulates EMT of peripheral cells through cellular communication to further promote the invasion of HCC into normal surrounding tissues.

Expression profile of circular RNAs in cystic echinococcosis pericystic tissue

Objective

The aim of the present study was to investigate the expression profiles of circular RNAs (circRNAs) in the pericystic tissue of patients with cystic echinococcosis (CE).

Patients and methods

CircRNA expression profiles were obtained by circRNA microarray of four matched pairs of pericystic tissues affected by CE and adjacent normal liver tissues. qRT‐PCR was used to validate the differential expression of some circRNAs identified by the microarray analysis. The potential functions of the differentially expressed circRNAs in the CE pericystic tissues were predicted by bioinformatic analysis.

Results

Compared with the adjacent normal liver tissues, 177 circRNAs were upregulated and 166 circRNAs were downregulated in CE pericystic tissues based on a ≥2.0‐fold change. The top 10 upregulated circRNAs were hsa_circRNA_001654,hsa_circRNA_103361,hsa_circRNA_001490,hsa_circRNA_104310,hsa_circRNA_100395,hsa_circRNA_102485,hsa_circRNA_001459,hsa_circRNA_104193,hsa_circRNA_400043, and hsa_circRNA_006773; The top 10 downregulated circRNAs were hsa_circRNA_400633,hsa_circRNA_404974,hsa_circRNA_068482 ,hsa_circRNA_100974,hsa_circRNA_049637,hsa_circRNA_404798,hsa_circRNA_400064,hsa_circRNA_004045,hsa_circRNA_101379, and hsa_circRNA_016771;The circRNA‐seq results for 15 selected differentially expressed circRNAs were validated by qRT‐PCR. The qRT‐PCR analysis showed that hsa_circRNA_006773, hsa_circRNA_049637, hsa_circRNA_104349, and hsa_circRNA_406281 were differentially expressed in CE pericystic tissues when compared with their expression in the adjacent normal liver tissues. Interestingly, 319 miRNAs and 52 mRNAs were predicted to be adsorbed by these four differentially expressed circRNAs. Gene Ontology analysis revealed that these circRNAs may be involved in the response to organic cyclic compounds and endogenous stimuli and in cellular organismal processes.

Conclusion

Differential expression of circRNAs may be associated with the development and progression of CE, and these circRNAs might be useful as biomarkers for prognosis prediction and as treatment targets.

The integrated analysis of circRNAs, miRNAs and mRNAs revealed the potential role in the TGF-β and TNF-α signaling pathways of OPLL

Ossification of the posterior longitudinal ligament (OPLL), one of spinal disease causing myelopathy, is characterized by the ectopic ossification and narrowing of the spinal cord. However, the pathogenesis of OPLL is largely unclear. In this study, transcriptome expression profiles (circRNAs, lncRNAs, and mRNAs) were identified via high-throughput sequencing using peripheral blood mononuclear cells (PBMCs) from OPLL and non-OPLL patients. We found that 1150 mRNAs, 331 circRNAs, and 1429 lncRNAs were significantly differentially expressed in the PBMCs of OPLL patients. GO and KEGG enrichment analyses revealed that most mRNAs were associated with inflammation. The co-expression networks indicated that circRNAs and lncRNAs could regulate the mRNAs through influencing the inflammation of OPLL. The circRNA-miRNA-mRNA integrated network showed that circRNA-regulated mRNAs associated with TGF-β and TNF-α signaling pathways. These analyses indicate that circRNAs, lncRNAs, and mRNAs from PBMCs might contribute to inflammation in OPLL.

Circular RNAs: Promising biomarkers for cancer diagnosis and prognosis

Circular RNAs (circRNAs), a group of non-coding RNA characterized by the presence of covalent bonds linking 3' and 5' ends, act as miRNA sponges to participate in the tumorigenesis. Being stable, conserved and cell- or tissue-specific, circRNAs have shown their potentials as molecular markers for cancer. Convenient and noninvasive approaches may be developed based on the roles of circRNAs to diagnose or predict the prognosis of tumors. Although most of the potential mechanisms are not entirely clear, circRNAs have shown a universal and critical role in regulating cellular processes of cancers. This review summarized the classification, formation, characteristics, detection, and biological functions of circRNAs. We proposed the possibility of using circRNAs as biomarkers for cancer diagnosis, treatment and prognosis.

Predicting article citations using data of 100 top-cited publications in the journal Medicine since 2011: A bibliometric analysis

BACKGROUND

Publications regarding the 100 top-cited articles in a given discipline are common, but studies reporting the association between article topics and their citations are lacking. Whether or not reviews and original articles have a higher impact factor than case reports is a point for verification in this study. In addition, article topics that can be used for predicting citations have not been analyzed. Thus, this study aims to METHODS:: We searched PubMed Central and downloaded 100 top-cited abstracts in the journal Medicine (Baltimore) since 2011. Four article types and 7 topic categories (denoted by MeSH terms) were extracted from abstracts. Contributors to these 100 top-cited articles were analyzed. Social network analysis and Sankey diagram analysis were performed to identify influential article types and topic categories. MeSH terms were applied to predict the number of article citations. We then examined the prediction power with the correlation coefficients between MeSH weights and article citations.

RESULTS

The citation counts for the 100 articles ranged from 24 to 127, with an average of 39.1 citations. The most frequent article types were journal articles (82%) and comparative studies (10%), and the most frequent topics were epidemiology (48%) and blood and immunology (36%). The most productive countries were the United States (24%) and China (23%). The most cited article (PDID = 27258521) with a count of 135 was written by Dr Shang from Shandong Provincial Hospital Affiliated to Shandong University (China) in 2016. MeSH terms were evident in the prediction power of the number of article citations (correlation coefficients  = 0.49, t = 5.62).

CONCLUSION

The breakthrough was made by developing dashboards showing the overall concept of the 100 top-cited articles using the Sankey diagram. MeSH terms can be used for predicting article citations. Analyzing the 100 top-cited articles could help future academic pursuits and applications in other academic disciplines.

Circular RNAs: Emerging Role in Cancer Diagnostics and Therapeutics

Circular RNAs (circRNAs) are rapidly coming to the fore as major regulators of gene expression and cellular functions. They elicit their influence via a plethora of diverse molecular mechanisms. It is not surprising that aberrant circRNA expression is common in cancers and they have been implicated in multiple aspects of cancer pathophysiology such as apoptosis, invasion, migration, and proliferation. We summarize the emerging role of circRNAs as biomarkers and therapeutic targets in cancer.

Circ_0000215 Increases the Expression of CXCR2 and Promoted the Progression of Glioma Cells by Sponging miR-495-3p

BACKGROUND

In recent years, accumulating studies have found that circular RNA (circRNA) exerts a great effect on tumor progression. Circ_0000215, a novel circRNA, remains largely unknown in terms of its effect and mechanism in glioma.

METHOD

Quantitative real-time polymerase chain reaction (qRT-PCR) was carried out to detect the expressions of circ_0000215, miR-495-3p and CXCR2 in human glial cell line HEB and glioma cell lines (A172, U251, U87, SHG-44, LN-18), human glioma tissues and adjacent healthy tissues. Gain- and loss-assays of circ_0000215 were conducted. Cell proliferation ability was detected via the CCK8 assay, and cell invasion ability was examined by Transwell assay. CXCR2 expression was evaluated via RT-PCR and Western blot. Moreover, bioinformatics was applied to analyze the targeting molecules of circ_0000215 and CXCR2. Verification of the relationship between these molecules were supported through the dual-luciferase reporter gene and RNA immunocoprecipitation (RIP) assay.

RESULTS

Circ_0000215 and CXCR2 were remarkably upregulated in glioma tissues and cells. Overexpression of circ_0000215 notably promoted the proliferation, invasion and epithelial-mesenchymal transition (EMT) but inhibited apoptosis of glioma cells, while knocking down circ_0000215 had the opposite effects. Additionally, miR-495-3p, a sponge RNA of circ_0000215, inhibited the growth, invasion and EMT of glioma cells. Mechanistically, miR-495-3p targeted CXCR2 and negatively regulated CXCR2/PI3K/Akt pathway. However, the effects of miR-495-3p were all dampened by overexpression of circ_0000215.

CONCLUSION

These data demonstrated that circ_0000215 functions as a competitive endogenous RNA by sponging miR-495-3p, thus accelerating glioma progression through CXCR2 axis.

Circular RNAs in gastrointestinal cancer: Current knowledge, biomarkers and targeted therapy (Review)

Circular RNAs (circRNAs) are a type of endogenous non‑coding RNAs that are connected at the 3' and 5' ends by exon or intron cyclization, which forms a covalently closed loop. They are stable, well conserved, exhibit specific expression in mammalian cells and can function as microRNA (miRNA or miR) sponges to regulate the target genes of miRNAs, which influences biological processes. Such as tumor proliferation, invasion, metastasis, apoptosis and tumor stage. circRNAs represent promising candidates for clinical diagnosis and treatment. In the present review, the biogenesis, classification and functions of circRNAs in tumors are briefly summarized and discussed. In addition, the participation of circRNAs in signal transduction pathways regulating gastrointestinal cancer cellular functions is highlighted.

Epigenetic Associations between lncRNA/circRNA and miRNA in Hepatocellular Carcinoma

Simple Summary

Non-coding RNAs such as microRNAs, long non-coding RNAs, and circular RNAs contribute to the development and progression of hepatocellular carcinoma through epigenetic association. Long non-coding RNAs and circular RNAs act as competing endogenous RNAs that contain binding sites for miRNAs and thus compete with the miRNAs, which results in promotion of miRNA target gene expression, thereby leading to proliferation and metastasis of hepatocellular carcinoma. Competing endogenous RNAs have the potential to become diagnostic biomarkers and therapeutic targets for treatment of hepatocellular carcinoma.

Abstract

The three major members of non-coding RNAs (ncRNAs), named microRNAs (miRNAs), long non-coding RNAs (lncRNAs), and circular RNAs (circRNAs), play an important role in hepatocellular carcinoma (HCC) development. Recently, the competing endogenous RNA (ceRNA) regulation model described lncRNA/circRNA as a sponge for miRNAs to indirectly regulate miRNA downstream target genes. Accumulating evidence has indicated that ceRNA regulatory networks are associated with biological processes in HCC, including cancer cell growth, epithelial to mesenchymal transition (EMT), metastasis, and chemoresistance. In this review, we summarize recent discoveries, which are specific ceRNA regulatory networks (lncRNA/circRNA-miRNA-mRNA) in HCC and discuss their clinical significance.

Dual Effects of Non-Coding RNAs (ncRNAs) in Cancer Stem Cell Biology

The identification of cancer stem cells (CSCs) as initiators of carcinogenesis has revolutionized the era of cancer research and our perception for the disease treatment options. Additional CSC features, including self-renewal and migratory and invasive capabilities, have further justified these cells as putative diagnostic, prognostic, and therapeutic targets. Given the CSC plasticity, the identification of CSC-related biomarkers has been a serious burden in CSC characterization and therapeutic targeting. Over the past decades, a compelling amount of evidence has demonstrated critical regulatory functions of non-coding RNAs (ncRNAs) on the exclusive features of CSCs. We now know that ncRNAs may interfere with signaling pathways, vital for CSC phenotype maintenance, such as Notch, Wnt, and Hedgehog. Here, we discuss the multifaceted contribution of microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), as representative ncRNA classes, in sustaining the CSC-like traits, as well as the underlying molecular mechanisms of their action in various CSC types. We further discuss the use of CSC-related ncRNAs as putative biomarkers of high diagnostic, prognostic, and therapeutic value.

Identification of circular RNA hsa_circ_0044556 and its effect on the progression of colorectal cancer

Background

Circular RNAs (circRNAs) are a novel class of noncoding RNAs. Increasing evidence indicates that circRNAs play an important role in the occurrence and development of tumors. However, the role of circRNA hsa_circ_0044556 in the progression of colorectal cancer (CRC) remains unclear.

Methods

First, we searched for differentially expressed circRNAs using a circRNA microarray in paired CRC and adjacent normal tissues. The circRNA hsa_circ_0044556 was screened out from the existing CRC circRNA microarray in the Gene Expression Omnibus database and our microarray. The clinical significance of hsa_circ_0044556 expression level in CRC patients was then investigated. Finally, the functions of the targets of this circRNA were determined in CRC cell lines.

Results

Hsa_circ_0044556 was highly expressed in CRC patients and was positively correlated with tumor stage and lymph node metastasis. In CRC cell lines, the proliferation, migration, and invasion of cancer cells were inhibited by knocking down hsa_circ_0044556 expression.

Conclusion

Hsa_circ_0044556 promoted the progression of CRC. It is possible that hsa_circ_0044556 will become a novel biomarker or therapeutic target for CRC.

Blocking circ_0000520 Suppressed Breast Cancer Cell Growth, Migration and Invasion Partially via miR-1296/SP1 Axis Both in vitro and in vivo

Background

Breast cancer (BCa) is an overwhelming malignant tumor mainly in women globally. Circular RNAs (circRNAs) are a special type of noncoding RNAs involved in competing endogenous RNA (ceRNA) network, a classic molecular mechanism of the tumorigenesis of human cancers, including BCa. Here, we intended to explore the role and mechanism of hsa_circ_0000520 (circ_0000520) in BCa cells.

Methods

Expression of circ_0000520, miRNA-1296-5p (miR-1296) and specificity protein 1 (SP1) was measured by real time-quantitative PCR and Western blotting. Cell growth was measured by cell counting kit-8, colony formation assay and flow cytometry method. Cell migration and invasion were assessed by transwell assays and Western blotting. Tumor growth was determined by xenograft models. The direct interaction among circ_0000520, miR-1296 and SP1 was confirmed by dual-luciferase reporter assay and RNA pull-down assay.

Results

circ_0000520 was upregulated in BCa tumors and cell lines (T47D, MCF7, MDA-MB-231, BT549, and SKBR3), and circ_0000520 high expression was associated with poor overall survival. Blocking circ_0000520 suppressed cell viability, colony formation, migration and invasion, but promoted cell cycle arrest and apoptosis rate in MDA-MB-231 and MCF7 cells. circ_0000520 could directly regulate miR-1296 expression, and SP1 was a novel target for miR-1296. Moreover, the anti-tumor role of circ_0000520 silencing was abrogated by miR-1296 downregulation or SP1 restoration. Notably, tumor growth of MDA-MB-231 cells in mice was restrained by circ_0000520 deletion.

Conclusion

circ_0000520 knockdown could suppress cell growth, migration and invasion both in vitro and in vivo through regulating miR-1296/SP1 pathway.

Current Status of Research on the Role of Circular RNAs in Hepatocellular Carcinoma and Clinical Implications

The latest statistics show that rates of morbidity and mortality for hepatocellular carcinoma are gradually increasing over time. Accumulating evidence indicates that circular RNAs (circRNAs) participate in the regulation of gene transcription and translation and exert a crucial role in endogenous RNA network. circRNAs are implicated in the pathogenesis of numerous tumors including hepatocellular carcinoma (HCC), gastric carcinoma and bladder cancer. Of note, the effect of circRNAs in HCC has drawn increasing public attention. Previous studies revealed that the function of circRNAs mainly consists of sponges of miRNA and RNA-binding proteins, alternative splicing of pre-mRNAs, transcriptional and translational regulators, and potential to encode proteins. In addition, recent research data indicate that the expression level of circRNAs is closely correlated with metastasis, invasion, and occurrence of HCC in patients. These findings imply that circRNAs may be useful as biomarkers for diagnosis and prediction of prognosis of HCC. In this review, we have systemically summarized current viewpoints regarding the role of circRNAs expression in HCC to provide an important reference illustrating the underlying mechanism of HCC.

[Research progress on circular RNA in oral squamous cell carcinoma]

Circular RNA, a non-coding RNA that forms a covalently closed continuous loop, exists widely in eukaryotic cells. The biogenesis and biological function of this type of RNA indicate that it can play a crucial role in diseases such as tumors, neural system diseases, and cardiovascular diseases; moreover, this RNA may have great potential use as a biomarker in these diseases. Oral squamous cell carcinoma (OSCC) is a common malignancy in oral surgery that is difficult to cure, metastasizes easily, and has poor prognosis. In this review, we summarize the loop-forming mechanisms and functions of circular RNA and describe the progress of current research in the development of oral cancer.

hsa_circ_0000520 influences herceptin resistance in gastric cancer cells through PI3K-Akt signaling pathway

Background

To investigate whether hsa_circ_0000520 affects Herceptin resistance in gastric cancer by regulating the PI3K‐AKT signaling.

Methods

The expression of hsa_circ_0000520 was detected by qRT‐PCR in gastric cancer tissues and cell lines. A Herceptin‐resistant gastric cancer cell was established. PcDNA and pcDNA‐hsa_circ_0000520 were transfected into NCI‐N87R cells and treated with Herceptin at a concentration of 10 μg/mL for 24 hours. MTT tested cell proliferation, and apoptosis was measured by flow cytometry. IGF‐1 treatment was used to activate PI3K‐Akt signaling. The expression levels of related proteins were detected.

Results

The expression of hsa_circ_0000520 was reduced in gastric cancer tissues and cell lines, and hsa_circ_0000520 in NCI‐N87R cells was significantly lower than that of NCI‐N87 cells. Compared with the CON group, the cell viability of the Herceptin group was significantly reduced, the apoptosis rate was significantly increased, the level of Bax protein was significantly increased, and the levels of Bcl‐2, p‐PI3K, and p‐Akt protein were significantly reduced. Compared with the Herceptin + pcDNA group, the cell viability of the Herceptin + hsa_circ_0000520 group was significantly reduced, the apoptosis rate was significantly increased, the level of Bax protein was significantly increased, and the levels of p‐PI3K and p‐Akt proteins were significantly reduced. After IGF‐1 treatment, the cell viability was significantly increased, the apoptosis rate was significantly reduced, the level of Bax protein was significantly reduced, and the level of Bcl‐2 protein was significantly increased.

Conclusion

Hsa_circ_0000520 overexpression may reverse the Herceptin resistance of gastric cancer cells by inhibiting the PI3K‐Akt signaling pathway.

CircRNA FGFR3 induces epithelial-mesenchymal transition of ovarian cancer by regulating miR-29a-3p/E2F1 axis

Circular RNAs (circRNAs) are a class of non-coding RNAs that regulate gene expression after transcription. However, the specific function of circRNAs in ovarian cancer remains undetermined. Previous studies have demonstrated abnormal expression of circFGFR3 in several cancers. The present study was designed to reveal the roles of circFGFR3 in ovarian cancer (OC). CircFGFR3 expression in OC tissues and cells was detected by RT-qPCR. The effects of CircFGFR3 on OC cells were evaluated by transwell assay and CCK-8 assay. Finally, the underlying mechanism was further revealed by luciferase reporter assay and western blotting. Our results showed that circFGFR3 expression was higher in OC cells and tissues than in normal ovarian cells and adjacent normal tissues; in addition, in OC patients, a high level of CircFGFR3 was related to lower survival rates and higher recurrence rates than a low level of circFGFR3. CircFGFR3 overexpression promotes OC progression by inducing epithelial-mesenchymal transition (EMT) in vitro. Mechanistically, circFGFR3 upregulates E2F1 expression by sponging miR-29a-3p, and the overexpression of E2F1 or the suppression of miR-29a-3p induces OC cell EMT. Therefore, circFGFR3 serves as a promoter of OC by inducing OC cell EMT via the miR-29a-3p/E2F1 axis and circFGFR3 may be a prognostic biomarker for OC patients.

The emerging role and significance of circular RNAs in viral infections and antiviral immune responses: possible implication as theranostic agents

Circular RNAs (circRNAs) are ubiquitously expressed, covalently closed rings, produced by pre-mRNA splicing in a reversed order during post-transcriptional processing. Circularity endows 3'-5'-linked circRNAs with stability and resistance to exonucleolytic degradation which raises the question whether circRNAs may be relevant as potential therapeutic targets or agents. High stability in biological systems is the most remarkable property and a major criterion for why circRNAs could be exploited for a range of RNA-centred medical applications. Even though various biological roles and regulatory functions of circRNAs have been reported, their in-depth study is challenging because of their circular structure and sequence-overlap with linear mRNA counterparts. Moreover, little is known about their role in viral infections and in antiviral immune responses. We believe that an in-depth and detailed understanding of circRNA mediated viral protein regulations will increase our knowledge of the biology of these novel molecules. In this review, we aimed to provide a comprehensive basis and overview on the biogenesis, significance and regulatory roles of circRNAs in the context of antiviral immune responses and viral infections including hepatitis C virus infection, hepatitis B virus infection, hepatitis delta virus infection, influenza A virus infection, Epstein-Barr virus infection, kaposi's sarcoma herpesvirus infection, human cytomegalovirus infection, herpes simplex virus infection, human immunodeficiency virus infection, porcine epidemic diarrhoea virus infection, ORF virus infection, avian leukosis virus infection, simian vacuolating virus 40 infection, transmissible gastroenteritis coronavirus infection, and bovine viral diarrhoea virus infection. We have also discussed the critical regulatory role of circRNAs in provoking antiviral immunity, providing evidence for implications as therapeutic agents and as diagnostic markers.

MiR-145 in cancer therapy resistance and sensitivity: A comprehensive review

MircoRNA (miRNA) are a group of small, non–coding, regulatory RNA with an average length of approximately 22 nucleotides, which mostly modulate gene expression post–transcriptionally through complementary binding to the 3ʹ‐untranslated region (3ʹ‐UTR) of multiple target genes. Emerging evidence has shown that miRNA are frequently dysregulated in a variety of human malignancies. Among them, microRNA‐145 (miR‐145) has been increasingly identified as a critical suppressor of carcinogenesis and therapeutic resistance. Resistance to tumor therapy is a challenge in cancer treatment due to the daunting range of resistance mechanisms. We reviewed the status quo of recent advancements in the knowledge of the functional role of miR‐145 in therapeutic resistance and the tumor microenvironment. It may serve as an innovative biomarker for therapeutic response and cancer prognosis.

The Expression, Functions and Mechanisms of Circular RNAs in Gynecological Cancers

Circular RNAs (circRNAs) are covalently closed, endogenous non-coding RNAs and certain circRNAs are linked to human tumors. Owing to their circular form, circRNAs are protected from degradation by exonucleases, and therefore, they are more stable than linear RNAs. Many circRNAs have been shown to sponge microRNAs, interact with RNA-binding proteins, regulate gene transcription, and be translated into proteins. Mounting evidence suggests that circRNAs are dysregulated in cancer tissues and can mediate various signaling pathways, thus affecting tumorigenesis, metastasis, and remodeling of the tumor microenvironment. First, we review the characteristics, biogenesis, and biological functions of circRNAs, and describe various mechanistic models of circRNAs. Then, we provide a systematic overview of the functional roles of circRNAs in gynecological cancers. Finally, we describe the potential future applications of circRNAs as biomarkers for prognostic stratification and as therapeutic targets in gynecological cancers. Although the function of most circRNAs remains elusive, some individual circRNAs have biologically relevant functions in cervical cancer, ovarian cancer, and endometrial cancer. Certain circRNAs have the potential to serve as biomarkers and therapeutic targets in gynecological cancers.

Circular RNA circSATB2 promotes progression of non-small cell lung cancer cells

BACKGROUND

Lung cancer has high morbidity and mortality worldwide with non-small cell lung cancer (NSCLC) accounting for 85% of the cases. Therapies for lung cancer have relatively poor outcomes and further improvements are required. Circular RNAs have been reported to participate in the occurrence and progression of cancer. Information on the functions and mechanism of circRNAs in lung cancer is limited and needs more exploration.

METHODS

We detected expression of genes and proteins by qPCR and western blot. Function of circSATB2 was investigated using RNA interference and overexpression assays. Location of circSATB2 was assessed by fluorescence in situ hybridization (FISH). Interaction of circSATB2, miR-326 and FSCN1 was confirmed by dual-luciferase reporter assay.

RESULTS

Data from the investigation showed that circSATB2 was highly expressed in NSCLC cells and tissues. circSATB2 positively regulated fascin homolog 1, actin-bundling protein 1 (FSCN1) expression via miR-326 in lung cancer cells. Furthermore, circSATB2 can be transferred by exosomes and promote the proliferation, migration and invasion of NSCLC cells, as well as induce abnormal proliferation in normal human bronchial epithelial cells. Also, circSATB2 was highly expressed in serumal exosomes from lung cancer patients with high sensitivity and specificity for clinical detection and was related to lung cancer metastasis.

CONCLUSIONS

circSATB2 participated in the progression of NSCLC and was differentially expressed in lung cancer tissue and serumal exosomes. circSATB2 may be potential biomarker for the diagnosis of NSCLC.

Circular RNA circ-CSPP1 regulates CCNE2 to facilitate hepatocellular carcinoma cell growth via sponging miR-577

OBJECTIVE

Circ-centro-some/spindle pole-associated protein (CSPP1) has been confirmed to be characterized in diverse human malignancies and its ectopic expression may regulate tumor progression and development. However, in hepatocellular carcinoma (HCC), its biological role, clinical significance and molecular mechanism are still unclear.

METHODS

Circ-CSPP1 expression and its prognostic values in HCC tissues were detected by qRT-PCR or in situ hybridization (ISH), and enriched by using Rnase R. The functional experiments (Circ-CSPP1 was overexpressed or knocked down) were performed in HCC cells. The HCC cell growth was analyzed by CCK-8 assay, transwell, wound healing and colony formation assays. The interation between circ-CSPP1 and miR-577/miR-577 and cyclin E2 (CCNE2) were determined by dual luciferase assay or RNA binding protein immunoprecipitation (RIP) assay. The RNA fluorescence in situ hybridization (FISH) assay was used to detect the subcellular distribution. Finally, an in vivo nude mouse tumor model was constructed.

RESULTS

In HCC patients and cells, circ-CSPP1 was aberrantly expressed, and its upregulation predicted poor prognosis, and closely correlated with tumor size and TNM stage. Circ-CSPP1 resisted RnaseR digestion, indicating it is a circular RNA structure. Moreover, overexpression of circ-CSPP1 promoted HCC cell viability, colony formation, migration, and invasion in vitro. Knockdown of circ-CSPP1 showed contrary results. Circ-CSPP1 acts as a miR-577 sponge and positively regulated the target of miR-577, CCNE2. Besides, miR-577 inhibitor rescued the suppressive effects of circ-CSPP1 knockdown on HCC cell growth, whereas was completely reversed by silencing of CCNE2. Finally, the in vivo experiments confirmed that circ-CSPP1 knockdown regulated xenograft tumor volume and downregulated CCNE2, p-Rb, E2F1 and c-myc expression.

CONCLUSION

These findings revealed that circ-CSPP1 contributed to HCC progression by positively regulating CCNE2 via miR-577, thus established its potential as new a prognostic and therapeutic marker for HCC patients.

Circular RNA expression and association with the clinicopathological characteristics in papillary thyroid carcinoma

Papillary thyroid carcinoma (PTC) is the most common type of thyroid cancer. Circular RNAs (circRNAs) are a novel class of RNAs, with higher stability and tissue specificity, which may be of value as novel clinical markers. High-throughput RNA sequencing was used to profile the expression of circRNAs in 5 pairs of cancer and normal tissues, and reverse transcription-quantitative PCR (RT-qPCR) analysis was employed to verify the results of the RNA sequencing in 45 cases of PTC. The dysregulated circRNA expression and clinicopathological characteristics were assessed and the potential roles of circRNAs in the cellular miRNA and mRNA network were predicted using bioinformatics analysis. The results demonstrated that, compared with normal tissues, a total of 53 circRNAs were dysregulated in tumour tissues, and 8 circRNAs were validated at the mRNA level (P<0.001 and P<0.01). Among those, the expression of chr5:161330882-161336769- (P=0.015), chr9:22046750-22097364+ (P=0.041) and chr8:18765448-18804898- (P=0.036) were obviously associated with the BRAF^V600E mutation, chr12:129699809-129700698- was associated with capsular invasion (P=0.025) and chr5:38523418-38530666- was associated with pT stage (P=0.037) and lymph node metastasis (P=0.002). Therefore, some dysregulated circRNAs were found to be associated with BRAF^V600E mutation, capsular invasion, advanced pT stage and lymph node metastasis of PTC, indicating that circRNAs may be involved in tumourigenesis and cancer progression, and they may be putative biomarkers for the diagnosis and evaluation of progression of PTC.

Deep Insights in Circular RNAs: from biogenesis to therapeutics

Abstract

Circular RNAs (circRNAs) have emerged as a universal novel class of eukaryotic non-coding RNA (ncRNA) molecules and are becoming a new research hotspot in RNA biology. They form a covalent loop without 5′ cap and 3′ tail, unlike their linear counterparts. Endogenous circRNAs in mammalian cells are abundantly conserved and discovered so far. In the biogenesis of circRNAs exonic, intronic, reverse complementary sequences or RNA-binding proteins (RBPs) play a very important role. Interestingly, the majority of them are highly conserved, stable, resistant to RNase R and show developmental-stage/tissue-specific expression. CircRNAs play multifunctional roles as microRNA (miRNA) sponges, regulators of transcription and post-transcription, parental gene expression and translation of proteins in various diseased conditions. Growing evidence shows that circRNAs play an important role in neurological disorders, atherosclerotic vascular disease, and cancer and potentially serve as diagnostic or predictive biomarkers due to its abundance in various biological samples. Here, we review the biogenesis, properties, functions, and impact of circRNAs on various diseases.

Graphical Abstract

Comprehensive profiling of circRNAs and the tumor suppressor function of circHIPK3 in clear cell renal carcinoma

Increasing evidence has indicated that circular RNAs (circRNAs) play a key role in the development and progression of diverse cancers, but their role in clear cell renal carcinoma (CCRCC) tumorigenesis is not well understood. In this study, we firstly performed comprehensive circRNA-seq from CCRCC tissues and pair-matched adjacent normal tissues. In total, 1184 circRNAs were dysregulated in human CCRCC tissues compared with those in adjacent normal tissues. We randomly selected four circRNAs, including circHIPK3 (circBase ID: hsa_circ_0000284), to test the circRNA-seq data in another 40 CCRCC tissues by quantitative real-time PCR (qRT-PCR). Furthermore, we found that circHIPK3 was downregulated in CCRCC tissues and cell lines. Overexpression of circHIPK3 effectively suppressed CCRCC cell invasion and migration in vitro, and inhibited CCRCC cell proliferation in vitro and in vivo. Moreover, bioinformatic analysis and luciferase reporter assay showed that circHIPK3 targeted miR-637 in CCRCC cells. Hence, CircHIPK3 may represent a tumor suppressor and target miR-637 in clear cell renal carcinoma.

CircPSMC3 Suppresses Migration and Invasion of Non-Small Cell Lung Cancer Cells via miR-182-5p/NME2 Axis

Background

CircPSMC3 has been reported to play important roles in the occurrence and development of cancer. However, the role of circPSMC3 in NSCLC (non-small cell lung cancer) and the underlying mechanisms remain unclear.

Material/Methods

The expression of circPSMC3 in NSCLC tissues was measured through qRT-PCR (quantitative real-time polymerase chain reaction). The effect of circPSMC3 on the invasion and migration of NSCLC cell line H1299 was determined through transwell invasion assay and wound healing assay. Dual-luciferase reporter assay was performed for exploring the regulatory interaction between circPSMC3, miR-182-5p, and NME2.

Results

Compared with adjacent normal tissues, the expression of circPSMC3 in NSCLC tissues was decreased. Lower circPSMC3 expression was closely associated with lymph node metastasis and higher TNM stage in NSCLC patients. Biological function analysis suggested that circPSMC3 inhibits the invasion and migration of H1299 cells through upregulating the expression of NME2. Mechanistically, circPSMC3 sponges miR-182-5p to suppress the invasion and migration of NSCLC cells via upregulating NME2 expression.

Conclusions

CircPSMC3 inhibits the invasion and migration of NSCLC cells through the miR-182-5p/NME2 signaling pathway.

Circular RNA HIPK3 contributes to hyperglycemia and insulin homeostasis by sponging miR-192-5p and upregulating transcription factor forkhead box O1

It has been shown that circular RNAs, a class of non-coding RNA molecules, play an important role in the regulation of glucose and lipid homeostasis. In the present study, we sought to investigate the function of circular RNA HIPK3 (circHIPK3) in diabetes-associated metabolic disorders, including hyperglycemia and insulin resistance. Results show that oleate stimulated circHIPK3 increase, and that circHIPK3 enhanced the stimulatory effect of oleate on adipose deposition, triglyceride (TG) content, and cellular glucose content in HepG2 cells. MiR-192-5p was the potential target of circHIPK3, since circHIPK3 significantly decreased miR-192-5p mRNA level, whereas anti-circHIPK3 significantly increased miR-192-5p mRNA level. Further study shows that transcription factor forkhead box O1 (FOXO1) was a downstream regulator of miR-192-5p, since miR-192-5p significantly decreased FOXO1 expression, whereas circHIPK3 significantly increased FOXO1 expression. Notably, the inhibitory effect of miR-192-5p was significantly reversed by circHIPK3. In vivo study shows that anti-miR-192-5p significantly increased blood glucose content, which was significantly inhibited by FOXO1 shRNA. MiR-192-5p significantly decreased adipose deposition and TG content in HepG2 cells, which was significantly reversed by the co-treatment with circHIPK3. Forskolin/dexamethasone (FSK/DEX) significantly increased cellular glucose, mRNA level of phosphoenolpyruvate carboxykinase (PEPCK) and glucose 6-phosphatase (G6Pase), and this stimulatory effect of FSK/DEX was significantly inhibited by miR-192-5p. In the presence of circHIPK3, however, the inhibitory effect of miR-192-5p was totally lost. In summary, the present study demonstrated that circHIPK3 contributes to hyperglycemia and insulin resistance by sponging miR-192-5p and up-regulating FOXO1.

Diagnostic and prognostic value of circular RNAs in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is the sixth most common malignant tumour, which has posed a heavy health and financial burden worldwide. Due to limited symptoms at the early stage and the limitation in current biomarkers, HCC patients are usually diagnosed at the advanced stage with a pessimistic overall survival rate. Circular RNAs (circRNAs) are a subclass of single‐stranded RNAs characterized by a covalently closed loop structure without 3’‐ or 5’‐end. With advances in high‐throughput sequencing technology and bioinformatics, accumulating studies have demonstrated the promotor or suppressor roles of circRNAs in the carcinogenesis, progression, and metastasis of HCC. Moreover, circRNAs are characteristic of higher abundance, stability and conservation compared with linear RNAs. Therefore, circRNAs have emerged as one of the most promising diagnostic and prognostic biomarkers for HCC with reliable accuracy, sensitivity and specificity. In this review, we briefly introduce the characteristics of circRNAs and summarize the roles of circRNAs in the biological procedures of HCC. Furthermore, we provide an overview on the potential diagnostic and prognostic value of circRNAs as biomarkers for patients with HCC. Finally, we discuss future perspectives of circRNAs in cancer research.

Plasma exo-hsa_circRNA_0056616: A potential biomarker for lymph node metastasis in lung adenocarcinoma

Background: To investigate the relationship between CXCR4-related circular RNAs (circRNAs) in exosomes and lymph node metastasis of lung adenocarcinoma. Methods: Totally 41 lung adenocarcinoma tissues (21 with lymph node metastasis and 20 without) were collected. Expression of CXCR4 protein was detected by western blotting analysis. A stable PC9/CXCR4-shRNA and PC14/CXCR4-shRNA knockdown lung adenocarcinoma cell lines were established and subjected to functional assays (cell proliferation, colony formation, migration and invasion) for phenotype changes. Exo-hsa-circRNAs (has-circRNAs in exosomes) were detected in vivo and in vitro. The diagnostic value of differentially expressed exo-has-circRNAs was evaluated. Results: Expression levels of CXCR4 were higher in patients with lymph node metastasis than in those without (P = 0.001). Silencing CXCR4 expression in PC9 and PC14 cell lines with short hairpin RNA could effectively abolish colony formation frequency, proliferation rate, migration rate, and the number of invasive cells (all P < 0.001). Exo_circRNA_0056616 was detected in both PC-9/CXCR4-shRNA cells and lung adenocarcinoma plasma at significantly higher levels than in the corresponding control (P < 0.001). When a receiver operating characteristic (ROC) curve for plasma exo-hsa_circRNA_0056616 levels and diagnosis of lymph node metastasis of lung adenocarcinoma was generated, a cutoff value of 0.394 was identified with an area under the curve of 0.812 (95% confidence interval 0.720-0.903), a sensitivity of 0.792, and specificity of 0.810. Conclusions: Taken together, our findings suggested that CXCR4 was higher in the lung adenocarcinoma tissues with lymph node metastasis. Higher plasma levels of exo-hsa_circRNA_0056616 in these patients also suggest that this circRNA represents a potential biomarker for lymph node metastasis predictor in lung adenocarcinoma.

CircRNA_101951 promotes migration and invasion of colorectal cancer cells by regulating the KIF3A-mediated EMT pathway

Colorectal cancer (CRC) is one of the most lethal tumor types worldwide. Circular RNAs (circRNAs), which are covalent closed loops of RNA, perform vital roles for the proliferation and metastasis of a variety of tumor types. In the present study, the expression, function and molecular mechanisms of action of a novel circRNA, circRNA_101951, were examined in CRC. The expression levels of circRNA_101951 in CRC tissue and cell lines were examined using reverse transcription-quantitative (RT-qPCR). Cell proliferation, the clone formation ability, cell apoptosis, the cell cycle and the cell migratory and invasive abilities were examined using MTT assays, colony formation assays, flow cytometric assays, and cell migration and invasion assays, respectively. The effects of circRNA_101951 on Kinesin II family member 3A (KIF3A) related gene expression were examined using RT-qPCR and western blot assays. The results indicated that circRNA_101951 was increased in CRC tissues and cell lines. The downregulation of circRNA_101951 inhibited cell proliferation and colony formation as well as cell migration and invasion of CRC cell lines. In addition, the downregulation of circRNA_101951 blocked the KIF3A-mediated epithelial-mesenchymal transition (EMT) pathway, which was detected by examining the expression levels of KIF3A and EMT related proteins. In conclusion, the current data revealed that circRNA_101951 may act as a potential biomarker for patients with CRC, and provided a novel insight demonstrating that the suppression of circRNA_101951 may be a potential therapeutic strategy for CRC.

Silencing of circRACGAP1 sensitizes gastric cancer cells to apatinib via modulating autophagy by targeting miR-3657 and ATG7

The positive results of the apatinib phase III trial have cast new light on treatment for patients with advanced gastric cancer (GC). However, in terms of safety, apatinib toxicities may lead to a dose modification or treatment interruption. Therefore, proper intervention is urgently needed to help patients benefit from apatinib treatment. In this study, we found that apatinib promoted autophagy activation via upregulation of ATG7 expression and autophagy inhibition enhanced apatinib-induced apoptosis. With microRNA and circular RNA-sequencing analyses of GC xenograft models, we demonstrated that circRACGAP1 functioned as an endogenous sponge for miR-3657 to inhibit its activity and further upregulate ATG7 expression. Silencing of circRACGAP1 inhibited apatinib-induced autophagy, which was rescued by miR-3657. Moreover, knockdown of circRACGAP1 sensitized GC cells to apatinib via autophagy inhibition in vitro and in vivo. These findings provided the first evidence that the circRACGAP1-miR-3657-ATG7 axis mediates a novel regulatory pathway critical for the regulation of apatinib sensitivity in GC. Thus, specific blockage of circRACGAP1 may be a potential therapeutic strategy to reduce the toxicities of apatinib and enhance its therapeutic effect in human GC.

Microarray Analysis of circRNA Expression Pattern in Corneal Neovascularization

PURPOSE

To identify differentially expressed circular RNAs (circRNAs) in corneal neovascularization.

METHODS

We established an alkali burn-induced corneal neovascularization model and performed circRNA expression profiling to identify differentially expressed circRNAs between avascular corneas and vascularized corneas. Gene ontology enrichment and Kyoto Encyclopedia of Genes and Genomes analyses of the host genes of dysregulated circRNAs were performed to determine the related biological modules and pathological pathways. Real-time polymerase chain reactions were performed to detect the expression pattern of circRNAs in the clinical samples. In vitro experiments were performed to determine the role of circRNAs in vascular endothelial angiogenic effects.

RESULTS

Two hundred twenty-nine circRNAs were differentially expressed between avascular corneas and vascularized corneas. The host genes of dysregulated circRNAs were targeted to cell cycle (biologic process), cytoplasm (cellular component), and protein binding (molecular function). Rap1 signaling was identified as the most enriched signaling pathway. Clinical studies showed that the human ortholog of cZFP609 and cKifap3 was dysregulated in the vascularized human corneas. cKifap3 silencing facilitated vascular endothelial angiogenic effects by regulating endothelial cell proliferation, migration, and tube formation.

CONCLUSIONS

This study suggests that circRNAs are involved in the pathogenesis of corneal neovascularization. cZFP609 and cKifap3 may serve as promising targets for the treatment of corneal neovascularization.

Circular RNA: functions, applications and prospects

Following improvement and integration of novel genome sequencing techniques, a new stage light has been shone upon circular RNAs (circRNA) in regard to their structure and functionality. In comparison to their linear counterparts, circRNA possess a multitude of remarkable functions such as microRNA (miRNA) sponging, RNA-binding protein (RBP) regulation and translational capabilities whose research have recently gained traction. Comprehensive computational modelling and analysis have revealed the relationship between the sequence composition of circRNAs and their biogenesis and structural formation as well as spatial identification. Due to fluctuations in expressional activity in tissue-specific environments and stimulations by tumour cells, circRNAs have sparked considerable interest in being employed as plausible biomarkers in disease control and treatment as consequence of their impressive specificity and biocompatibility. Additionally, an increasing number of studies have proposed them as viable solutions to be just as competent as presently used disease markers and medicine, if not better. In this review, I briefly summarized the characteristics, biogenesis and function of circRNA and introduced the potential applications and prospects of circRNA. With vigorous research being carried out regarding their still unclear diversified roles and precise molecular structure, circRNAs are bound to become the new revolutionary perspective on cellular regulation, protein signalling and disease pathogenesis.

Cisplatin resistance-associated circRNA_101237 serves as a prognostic biomarker in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) is a leading cause of cancer-associated mortality worldwide. Despite clinical advances, the survival rate of patients with HCC remains low, as most patients are diagnosed with HCC when they are already at the advanced stage. Certain circular RNAs (circRNAs) are closely associated with the development of liver cancer. In the present study, a circRNA array was performed to screen differentially expressed circRNAs in HCC tissues. The further analysis focused on the newly identified circRNA_101237, the host gene of which, cyclin-dependent kinase 8, is located at chr13:26974589-26975761. CircRNA_101237 was determined to be upregulated in tumor tissue and serum of patients with HCC as compared with that in paracancerous tissues and the serum of healthy controls, respectively. In addition, the expression of circRNA_101237 was associated with tumor size, lymph node metastasis, distant metastasis and TNM stage. Univariate and multivariate analysis indicated that serum circRNA_101237 levels were an independent predictor of survival prognosis in patients with HCC. The overall survival of patients with high expression of circRNA_101237 was reduced compared with that of patients with low expression of circRNA_101237. Of note, cisplatin induced the expression of circRNA_101237 in HCC cells in a dose- and time-dependent manner in vitro, and the levels of circRNA_101237 in the serum of patients with cisplatin-resistant HCC and in cisplatin-resistant Huh7 cells were increased. The present study provided novel insight into the use of circRNA_101237 as a diagnostic biomarker for HCC and a potential therapeutic target.

Comprehensive profiling of circular RNA expressions reveals potential diagnostic and prognostic biomarkers in multiple myeloma

Background

This study aimed to explore the heterogeneity of circRNA expression pattern via microarray, and further evaluate the potential of 10 specific circRNAs as diagnostic and prognostic biomarkers in multiple myeloma (MM).

Methods

In exploration stage (stage I), circRNA expression profiles were detected by the microarray in bone marrow plasma cells from 4 MM patients and 4 healthy controls (HCs), and bioinformatic analyses were performed. In validation stage (stage II), top 10 upregulated and top 10 downregulated circRNAs identified in stage I were detected in 60 MM patients and 30 HCs for further validation; the diagnostic and prognostic values of these circRNAs in MM patients were analyzed.

Results

In stage I, 122 upregulated and 260 downregulated circRNAs were identified in MM patients compared with HCs. GO, KEGG and pathway enrichment analyses revealed that these circRNAs were implicated in neoplastic pathways such as MAPK and VEGF signaling pathways. In stage II, circ-PTK2, circ-RNF217, circ-RERE, circ-NAGPA and circ-KCNQ5 were validated to be upregulated and circ-AFF2, circ-WWC3, circ-DNAJC5, circ-KLHL2, circ-IQGAP1 and circ-AL137655 were validated to be downregulated in MM compared with controls. Circ-PTK2 and circ-RNF217 were correlated with poor treatment response and survival, while circ-AFF2 predicted good treatment response and survival in MM patients.

Conclusions

This study provides valuable reference for profound understanding about circRNA expression patterns in MM, and validates that circ-PTK2, circ-RNF217 and circ-AFF2 might serve as potential prognostic biomarkers in MM.

circRASSF2 Acts as ceRNA and Promotes Papillary Thyroid Carcinoma Progression through miR-1178/TLR4 Signaling Pathway

Circular RNAs (circRNAs) are a class of non-coding RNAs broadly expressed in cells of various species. However, the molecular mechanisms that link circRNAs with progression of papillary thyroid carcinoma (PTC) are not well understood. In the present study, we attempted to provide novel basis for targeted therapy for PTC from the aspect of circRNA-miRNA-mRNA interaction. We investigated the expression of circRNAs in five paired PTC tissues and normal tissues by microarray analysis. The circRNA microarray assay followed by qRT-PCR was used to verify the differential expression of hsa_circ_0059354, which is located on chromosome 20 and derived from RASSF2, and thus we named it circRASSF2. The qRT-PCR analysis was to investigate the expression pattern of circRASSF2 in PTC tissues and cell lines. Then the effects of circRASSF2 on cell proliferation and apoptosis were assessed in PTC in vitro. Furthermore, bioinformatics online programs predicted and luciferase reporter assays were used to validate the association of circRASSF2 and miR-1178 in PTC cells. In this study, circRASSF2 was observed to be upregulated in PTC tissues and cell lines. Knockdown of circRASSF2 inhibited cell proliferation and promoted cell apoptosis in PTC cells. Bioinformatics analysis predicted that there is a circRASSF2/miR-1178/TLR4 axis in PTC. A dual-luciferase reporter system validated the direct interaction of circRASSF2, miR-1178, and TLR4. Furthermore, circRASSF2 facilitates PTC progression in vivo. Importantly, we demonstrated that circRASSF2 was upregulated in serum exosomes from PTC patients. In summary, our study demonstrates that circRASSF2 modulates PTC progression through the miR-1178/TLR4 pathway. Our findings indicate that circRASSF2 may serve as a promising therapeutic target for the treatment of PTC patients.

Circ-TCF4.85 silencing inhibits cancer progression through microRNA-486-5p-targeted inhibition of ABCF2 in hepatocellular carcinoma

The current study aimed to explore the role of the circular RNA circ‐TCF4.85 and its downstream target microRNA‐486‐5p (miR‐486‐5p) in hepatocellular carcinoma (HCC) development. Circ‐TCF4.85 was detected to be highly expressed in HCC tissues. Next, we found that silencing of circ‐TCF4.85 repressed HCC cell proliferation, invasion, and migration, while enhancing apoptosis. In addition, biotin‐coupled probe pull‐down and miRNA capture assays, as well as fluorescence in situ hybridization, confirmed that circ‐TCF4.85 could bind to miR‐486‐5p. In rescue experiments, miR‐486‐5p had the potential to eliminate the tumor‐suppressive effects of circ‐TCF4.85 knockdown in HCC. Moreover, miR‐486‐5p was shown to target ABCF2 gene, which was positively regulated by circ‐TCF4.85. Finally, nude mice subcutaneously injected with si‐circ‐TCF4.85‐transfected HCC cells presented with inhibited xenograft tumor formation in vivo. Taken together, our results reveal that silencing of circ‐TCF4.85 suppresses HCC progression via miR‐486‐5p‐targeted inhibition of ABCF2.

Circular RNA cSMARCA5 regulates the progression of cervical cancer by acting as a microRNA‑432 sponge

Circular RNAs (circRNAs) have been shown to be involved in the development of cancer. The aim of the present study was to investigate the role of circRNA SMARCA5 (cSMARCA5) in human cervical cancer. In the present study, cSMARCA5 expression was upregulated in cervical cancer tissues and cell lines. Furthermore, the proliferation rate of cells transduced with viral plasmids expressing small interfering RNA targeting cSMARCA5 was downregulated. Bioinformatics analysis predicted that microRNA (miR)‑432 targeted cSMARCA5, and miR‑432 was able to interact with epidermal growth factor receptor (EGFR) by binding to its 3'‑untranslated region. The expression levels of EGFR, ERK1 and ERK2 were increased in cervical cancer tissues. Furthermore, correlation analysis revealed that cSMARCA5 levels were positively correlated with ERK1 and ERK2 levels. In conclusion, the present findings suggested that cSMARCA5 may play an important role in the progression of cervical cancer via the ERK signaling pathway by modulating miR‑432.

Circular RNA Expression Profiles and the Pro-tumorigenic Function of CircRNA_10156 in Hepatitis B Virus-Related Liver Cancer

Liver cancer is one of the most common malignant tumors in the world. Circular RNAs (circRNAs) perform important functions in cancer progression and are regarded as prospective biomarkers for cancer diagnosis and therapy. Here, we used the high-throughput RNA sequencing technology in conjunction with bioinformatics tools to profile circRNA expression in patients with HBV-related liver cancer. A total of 13,124 circRNAs were identified in HBV-related liver cancer, approximately 86.25% of which were sense-overlapping circRNAs. Moreover, 2,996 circRNAs exhibited different expression patterns between liver cancer tissues and matched pericancerous tissues. Function annotation indicated that these aberrantly expressed circRNAs were primarily engaged in cellular processes and cancer-associated pathways. Notably, the circRNA-miRNA interaction networks showed that 6,020 circRNAs were predicted to target 1,654 miRNAs. Quantitative RT-PCR (qRT-PCR) assay indicated that ten randomly selected circRNAs displayed consistent expression patterns with the sequencing results. We further predicted that circRNA_10156 might work as a molecular sponge of miR-149-3p, which served an important function in tumor development. Consequently, our results demonstrated that depletion of circRNA_10156 upregulated miR-149-3p, reduced Akt1 expression, and suppressed liver cancer cell proliferation. The present study will facilitate the elucidation of biological functions of circRNAs in the progression of HBV-related liver cancer providing prospective biomarkers and therapeutic targets for this disease. Our findings also reveal that circRNA_10156 might represent a promising therapeutic target for liver cancer management.