Profiles of differentially expressed circRNAs in esophageal and breast cancer

The critical role of circular RNAs in drug resistance in gastrointestinal cancers

Nowadays, drug resistance (DR) in gastrointestinal (GI) cancers, as the main reason for cancer-related mortality worldwide, has become a serious problem in the management of patients. Several mechanisms have been proposed for resistance to anticancer drugs, including altered transport and metabolism of drugs, mutation of drug targets, altered DNA repair system, inhibited apoptosis and autophagy, cancer stem cells, tumor heterogeneity, and epithelial-mesenchymal transition. Compelling evidence has revealed that genetic and epigenetic factors are strongly linked to DR. Non-coding RNA (ncRNA) interferences are the most crucial epigenetic alterations explored so far, and among these ncRNAs, circular RNAs (circRNAs) are the most emerging members known to have unique properties. Due to the absence of 5' and 3' ends in these novel RNAs, the two ends are covalently bonded together and are generated from pre-mRNA in a process known as back-splicing, which makes them more stable than other RNAs. As far as the unique structure and function of circRNAs is concerned, they are implicated in proliferation, migration, invasion, angiogenesis, metastasis, and DR. A clear understanding of the molecular mechanisms responsible for circRNAs-mediated DR in the GI cancers will open a new window to the management of GI cancers. Hence, in the present review, we will describe briefly the biogenesis, multiple features, and different biological functions of circRNAs. Then, we will summarize current mechanisms of DR, and finally, discuss molecular mechanisms through which circRNAs regulate DR development in esophageal cancer, pancreatic cancer, gastric cancer, colorectal cancer, and hepatocellular carcinoma.

Circ_0,007,331 Promotes the PTX Resistance and Progression of Breast Cancer via miR-200b-3p/ANLN

INTRODUCTION

The objective of our study was to explore the expression pattern of circular ribonucleic acid (RNA)_0,007,331 (circ_0,007,331) in breast cancer (BC) and its functional association with cellular paclitaxel (PTX) resistance and proliferation, migration, invasion and apoptosis.

METHODS

Real-time quantitative polymerase chain reaction was applied to measure RNA expression. The PTX resistance of BC cells was analyzed by cell counting kit-8 assay. Flow cytometry was applied to assess cell cycle progression and cell apoptosis. Transwell assays were utilized to analyze cell migration and invasion abilities. Protein expression was determined by Western blot assay. The target relationship between microRNA-200b-3p (miR-200b-3p) and circ_0,007,331 or Anillin (ANLN) was verified by dual-luciferase reporter assay and RNA-pull down assay. The in vivo role of circ_0,007,331 was analyzed using xenograft tumor model.

RESULTS

Circ_0,007,331 expression was elevated in PTX-resistant BC cell lines relative to parental BC cell lines. Circ_0,007,331 contributed to the PTX resistance, proliferation, migration, invasion and suppressed the apoptosis of BC cells. Circ_0,007,331 interacted with miR-200b-3p in BC cells. Circ_0,007,331 silencing-mediated effects in BC cells were largely overturned by the knockdown of miR-200b-3p. ANLN was a target of miR-200b-3p in BC cells. Circ_0,007,331 silencing reduced ANLN expression partly through upregulating miR-200b-3p in BC cells. miR-200b-3p overexpression-induced effects in BC cells were largely counteracted by the accumulation of ANLN. Circ_0,007,331 silencing aggravated PTX-mediated inhibitory effect on tumor growth in vivo.

CONCLUSIONS

Circ_0,007,331 contributed to the PTX resistance, proliferation and motility and inhibited the apoptosis of BC cells through mediating miR-200b-3p/ANLN signaling.

circFAM120B functions as a tumor suppressor in esophageal squamous cell carcinoma via the miR-661/PPM1L axis and the PKR/p38 MAPK/EMT pathway

Extensive changes of circRNA expression underscore their essential contributions to multiple hallmarks of cancers; however, their functions and mechanisms of action in esophageal squamous cell carcinoma (ESCC) remain undetermined. Here, we adopted a three-stage approach by first screening for significantly differentially expressed circRNAs in ESCC and performing an external validation study, followed by the functional analyses. The properties of circRNAs were evaluated using Sanger sequencing, RNase R digestion, actinomycin D treatment, subcellular localization analysis, and fluorescence in situ hybridization. Target transcripts were predicted using online tools and verified by dual-luciferase, RNA immunoprecipitation, qRT-PCR, and western blot. Biotin-labeled RNA-protein pull-down, mass spectrometry, and RNA immunoprecipitation were employed to identify proteins interacting with circRNAs. Gain- and loss-of-function experiments were performed to uncover the roles of circRNAs, their target genes, and binding proteins in the proliferation, metastasis, and invasion. We observed that circFAM120B (hsa_circ_0001666) was frequently downregulated in cancer tissues and patient plasma, and its expression level was related to overall survival in ESCC patients. Overexpression of circFAM120B inhibited the proliferation, metastasis, and invasion of ESCC while silencing it enhanced malignant phenotypes. Mechanistically, circFAM120B was predominantly located in the cytoplasm, guarantying its sponging for miR-661 to restore the expression of PPM1L, a tumor suppressor. We observed that circFAM120B could reduce the stability of RNA-dependent protein kinase (PKR) by promoting its ubiquitination-dependent degradation and subsequently regulating the p38 MAPK signaling pathway, resulting in the repression of EMTs in ESCC cells. Our findings suggest that circFAM120B is a promising biomarker of ESCC, which acts as a tumor suppressor via the circFAM120B/miR-661/PPM1L axis and PKR/p38 MAPK/EMT pathway, supporting its significance as a candidate therapeutic target.

circFIG 4 drives the carcinogenesis and metastasis of esophagus cancer via the miR-493-5p/E2F3 axis

Background

Esophageal cancer (EC) is a highly malignant tumor of the digestive tract. Circular RNAs (circRNAs) have been verified to play a regulatory role in the occurrence and progression of different cancers, including EC. This research aimed to investigate the role and molecular mechanism of circFIG 4 in EC progression.

Methods

The analyses of circFIG 4, miR‐493‐5p, and neuro‐oncological ventral antigen 2 levels were administrated by quantitative real‐time polymerase chain reaction. The characteristics of circFIG 4 were determined by Ribonuclease R assay and Actinomycin D assay. Cell proliferation was assessed via colony formation assay and 5‐ethynyl‐2′‐deoxyuridine incorporation assay. Cell cycle distribution and apoptosis were evaluated by flow cytometry. Western blot was performed to assess protein expression. The targeted interaction among circFIG 4, miR‐493‐5p, and E2F transcription factor 3 (E2F3) were validated using dual‐luciferase reporter or RNA immunoprecipitation assays.

Results

circFIG 4 was overtly upregulated in EC and was relatively stable in EC cells. circFIG 4 knockdown impeded proliferation, migration, and invasion and expedited apoptosis in EC cells. circFIG 4 served as a miR‐493‐5p sponge to act in the development of EC. Furthermore, circFIG 4 modulated EC progression via targeting miR‐493‐5p and miR‐493‐5p suppressed EC progression via targeting E2F3. circFIG 4 modulated E2F3 expression through acting as a sponge of miR‐493‐5p. Moreover, circFIG 4 knockdown inhibited EC tumorigenesis by targeting miR‐493‐5p/E2F3 axis tumor growth in vivo.

Conclusion

circFIG 4 silence mitigated EC malignant progression at least partly by mediating the miR‐493‐5p/E2F3 pathway, highlighting new biomarkers and therapeutic targets for EC treatment.

CircSOD2: A Novel Regulator for Smooth Muscle Proliferation and Neointima Formation

OBJECTIVE

Vascular smooth muscle cell (SMC) proliferation contributes to neointima formation following vascular injury. Circular RNA-a novel type of noncoding RNA with closed-loop structure-exhibits cell- and tissue-specific expression patterns. However, the role of circular RNA in SMC proliferation and neointima formation is largely unknown. The objective of this study is to investigate the role and mechanism of circSOD2 in SMC proliferation and neointima formation. Approach and Results: Circular RNA profiling of human aortic SMCs revealed that PDGF (platelet-derived growth factor)-BB up- and downregulated numerous circular RNAs. Among them, circSOD2, derived from back-splicing event of SOD2 (superoxide dismutase 2), was significantly enriched. Knockdown of circSOD2 by short hairpin RNA blocked PDGF-BB-induced SMC proliferation. Inversely, circSOD2 ectopic expression promoted SMC proliferation. Mechanistically, circSOD2 acted as a sponge for miR-206, leading to upregulation of NOTCH3 (notch receptor 3) and NOTCH3 signaling, which regulates cyclin D1 and CDK (cyclin-dependent kinase) 4/6. In vivo studies showed that circSOD2 was induced in neointima SMCs in balloon-injured rat carotid arteries. Importantly, knockdown of circSOD2 attenuated injury-induced neointima formation along with decreased neointimal SMC proliferation.

CONCLUSIONS

CircSOD2 is a novel regulator mediating SMC proliferation and neointima formation following vascular injury. Therefore, circSOD2 could be a potential therapeutic target for inhibiting the development of proliferative vascular diseases.

EIF4A3-mediated hsa_circ_0088088 promotes the carcinogenesis of breast cancer by sponging miR-135-5p

Circular RNAs have participated in oncology progress. Nevertheless, the potential mechanisms are not completely understood. We intended to inspect the functions of hsa_circ_0088088 on breast malignancy, together with the possible mechanism(s). hsa_circ_0088088 expression in breast malignancy was studied using quantitative real-time reverse transcription-polymerase chain reaction (qRT-PCR). The overall survival was studied by the Kaplan-Meier curve. The biological functions of hsa_circ_0088088 aberrant expression on cell growth and metastasis were evaluated in MDA-MB-231 cells. Bioinformatics analysis, RNA immunoprecipitation (RIP), and qRT-PCR were accomplished to confirm the possible regulatory effects of eukaryotic initiation factor 4A3 (EIF4A3) on the biogenesis of hsa_circ_0088088. Furthermore, a dual-luciferase reporter assay, qRT-PCR, and RNA pull-down assay were used to confirm the association between the hsa_circ_0088088 and miR-135-5p in MDA-MB-231 cells. hsa_circ_0088088 was upregulated in the tumor tissues and cells, and higher expression presented an unfavorable prognosis. hsa_circ_0088088 overexpression promoted cell growth and metastasis in MDA-MB-231 cells. EIF4A3 was found to positively regulate hsa_circ_0088088. Furthermore, we confirmed that hsa_circ_0088088 sponges miR-135-5p and directly targets miR-135-5p with respect to the cell growth and metastasis in MDA-MB-231 cells. Our data suggest that EIF4A3-induced hsa_circ_0088088 stimulates the carcinogenic effects of breast tumors by sponging miR-135-5p.

The biogenesis, function and clinical significance of circular RNAs in breast cancer

Circular RNAs (circRNAs) are noncoding RNAs that form covalently closed loop structures. CircRNAs are dysregulated in cancer and play key roles in tumorigenesis, diagnosis, and tumor therapy. CircRNAs function as competing endogenous RNAs or microRNA sponges that regulate transcription and splicing, binding to proteins, and translation. CircRNAs may serve as novel biomarkers for cancer diagnosis, and they show potential as therapeutic targets in cancers including breast cancer (BC). In women, BC is the most common malignant tumor worldwide and the second leading cause of cancer death. Although evidence indicates that circRNAs play a critical role in BC, the mechanisms regulating the function of circRNAs in BC remain poorly understood. Here, we provide literature review aiming to clarify the role of circRNAs in BC and summarize the latest research. We provide a systematic overview of the biogenesis and biological functions of circRNAs, elaborate on the functional roles of circRNAs in BC, and highlight the value of circRNAs as diagnostic and therapeutic targets in BC.

Effect of circular RNA, mmu_circ_0000296, on neuronal apoptosis in chronic cerebral ischaemia via the miR-194-5p/Runx3/Sirt1 axis

Chronic cerebral ischaemia (CCI) is a common pathological disorder, which is associated with various diseases, such as cerebral arteriosclerosis and vascular dementia, resulting in neurological dysfunction. As a type of non-coding RNA, circular RNA is involved in regulating the occurrence and development of diseases, such as ischaemic brain injury. Here, we found that HT22 cells and hippocampus treated with CCI had low expression of circ_0000296, Runx3, Sirt1, but high expression of miR-194-5p. Overexpression of circ_0000296, Runx3, Sirt1, and silenced miR-194-5p significantly inhibited neuronal apoptosis induced by CCI. This study demonstrated that circ_0000296 specifically bound to miR-194-5p; miR-194-5p bound to the 3'UTR region of Runx3 mRNA; Runx3 directly bound to the promoter region of Sirt1, enhancing its transcriptional activity. Overexpression of circ_0000296 by miR-194-5p reduced the negative regulatory effect of miR-194-5p on Runx3, promoted the transcriptional effect of Runx3 on Sirt1, and inhibited neuronal apoptosis induced by CCI. mmu_circ_0000296 plays an important role in regulating neuronal apoptosis induced by CCI through miR-194-5p/Runx3/Sirt1 pathway.

Circular RNAs: Their Role in the Pathogenesis and Orchestration of Breast Cancer

As one of the most frequently occurring malignancies in women, breast cancer (BC) is still an enormous threat to women all over the world. The high mortality rates in BC patients are associated with BC recurrence, metastatic progression to distant organs, and therapeutic resistance. Circular RNAs (circRNAs), belonging to the non-coding RNAs (ncRNAs), are connected end to end to form covalently closed single-chain circular molecules. CircRNAs are widely found in different species and a variety of human cells, with the features of diversity, evolutionary conservation, stability, and specificity. CircRNAs are emerging important participators in multiple diseases, including cardiovascular disease, inflammation, and cancer. Recent studies have shown that circRNAs are involved in BC progress by regulating gene expression at the transcriptional or post-transcriptional level via binding to miRNAs then inhibiting their function, suggesting that circRNAs may be potential targets for early diagnosis, treatment, and prognosis of BC. Herein, in this article, we have reviewed and summarized the current studies about the biogenesis, features, and functions of circRNAs. More importantly, we emphatically elucidate the pivotal functions and mechanisms of circRNAs in BC growth, metastasis, diagnosis, and drug resistance. Deciphering the complex networks, especially the circRNA-miRNA target gene axis, will endow huge potentials in developing therapeutic strategies for combating BC.

Circular RNA hsa_circ_0000700 promotes cell proliferation and migration in Esophageal Squamous Cell Carcinoma by sponging miR-1229

Accumulating evidence has demonstrated that circular RNAs (circRNAs) are involved in the pathogenesis of cancer, including that of esophageal squamous cell carcinoma (ESCC). The current study aimed to investigate the role of hsa_circ_0000700 in ESCC. hsa_circ_0000700, miR-1229, and related functional gene expression was measured by RT-qPCR. To characterize the functions of hsa_circ_0000700 and miR-1229, ESCC cells were infected with hsa_circ_0000700-specific siRNA, miR-1229 mimics, and an inhibitor alone or in combination. Cell Counting Kit-8 (CCK8), colony formation, EdU, flow cytometry, and Transwell assays were employed to evaluate cell proliferation, apoptosis, and migration. Luciferase reporter and RNA immunoprecipitation assays were used to confirm the targeting relationship between hsa_circ_0000700 and miR-1229. Finally, a competing endogenous RNAs (ceRNA) network was built for hsa_circ_0000700, and miR-1229 targets were analyzed by bioinformatics. circ_0000700 expression was significantly upregulated in ESCC cell lines. Actinomycin D and RNase R treatment confirmed that circ_0000700 was more stable than its linear CDH9 mRNA form. Moreover, a cytoplasmic and nuclear fractionation assay suggested that circ_0000700 was mainly distributed in the cytoplasm of ECA-109 and TE-1 cells. In vitro, the proliferative and migratory capacities of ECA-109 and TE-1 cells were inhibited by knocking down circ_0000700 expression. Additionally, miR-1229 silencing reversed the circ_0000700-specific siRNA-induced attenuation of malignant phenotypes. Mechanistically, circ_0000700 was identified as a sponge of miR-1229 and could activate PRRG4, REEP5, and PSMB5 indirectly to promote ESCC progression. In summary, our results suggest that hsa_circ_0000700 functions as an oncogenic factor by sponging miR-1229 in ESCC.

Circ-UBR1 facilitates proliferation, metastasis, and inhibits apoptosis in breast cancer by regulating the miR-1299/CCND1 axis

AIMS

Circular RNA (circRNA) is abnormally expressed in cancers and has been linked to cancer progression, including breast cancer (BC). However, the role and mechanism of circ-UBR1 in BC progression remains to be further studied.

MATERIALS AND METHODS

Quantitative real-time PCR (qRT-PCR) was conducted to analyze the expression of circ-UBR1, miR-1299 and Cyclin D1 (CCND1). Cell counting kit 8 (CCK8) assay was used to measure cell viability. Cell apoptosis and cell cycle distribution were analyzed by flow cytometry. Then, the migration and invasion of cells were determined by transwell assay. Moreover, BC tumor xenograft model was built to evaluate the function of circ-UBR1 silencing on BC tumor volume and weight. Dual-luciferase reporter assay and RNA immunoprecipitation (RIP) assay were applied to illuminate the interaction between miR-1299 and circ-UBR1 or CCND1. In addition, relative CCND1 protein expression was assessed using western blot (WB) analysis.

KEY FINDINGS

Our results revealed that circ-UBR1 was upregulated in BC, and its silencing could inhibit BC cell proliferation, metastasis, and promote apoptosis in vitro, as well as restrain BC tumor growth in vivo. Meanwhile, we found that circ-UBR1 could sponge miR-1299, and miR-1299 inhibitor could reverse the effect of circ-UBR1 knockdown on BC cell progression. Furthermore, CCND1 was a target of miR-1299, and CCND1 overexpression could reverse the effect of miR-1299 mimic on BC cell progression. Also, the downregulation of circ-UBR1 could inhibit CCND1 expression, while this effect could be inverted by miR-1299 inhibitor.

SIGNIFICANCE

Our data indicated that circ-UBR1 might play a pro-cancer role in BC progression by regulating the miR-1299/CCND1 axis.

circCUL2 regulates gastric cancer malignant transformation and cisplatin resistance by modulating autophagy activation via miR-142-3p/ROCK2

Background

Circular RNAs (circRNAs) are a class of noncoding RNAs (ncRNAs) and can modulate gene expression by binding to miRNAs; further, circRNAs have been shown to participate in several pathological processes. However, the expression and biological function of circCUL2 in gastric cancer (GC) remains largely unknown.

Methods

circRNA microarrays and quantitative real-time PCR (qRT-PCR) were used to identify differentially expressed circRNAs in GC tissues and cell lines. circCUL2 knockdown and overexpression were performed to indicate the functional role of circCUL2 in vitro and in vivo. The expression and regulation of circCUL2, miR-142-3p and ROCK2 were evaluated using fluorescence in situ hybridization (FISH), dual-luciferase assays, RNA pull-down assays, RNA immunoprecipitation (RIP) and rescue experiments. Furthermore, the regulation of cisplatin sensitivity and autophagy by circCUL2/miR-142-3p/ROCK2 was demonstrated by cellular apoptosis assays, western blot, immunofluorescence and transmission electron microscopy analyses.

Results

The level of circCUL2, which is stable and cytoplasmically localized, was significantly reduced in GC tissues and cells. Overexpressed circCUL2 inhibited malignant transformation in vitro and tumorigenicity in vivo. In the AGS and SGC-7901 cell lines, circCUL2 sponged miR-142-3p to regulate ROCK2, thus modulating tumor progression. Furthermore, in the AGS/DDP and SGC-7901/DDP cell lines, circCUL2 regulated cisplatin sensitivity through miR-142-3p/ROCK2-mediated autophagy activation.

Conclusion

circCUL2 may function as a tumor suppressor and regulator of cisplatin sensitivity through miR-142-3p/ROCK2-mediated autophagy activation, which could be a key mechanism and therapeutic target for GC.

Supplementary information

Supplementary information accompanies this paper at 10.1186/s12943-020-01270-x.

The Potential of Circular RNAs as Cancer Biomarkers

Circular RNA (circRNA) is a covalently closed RNA structure that has several proposed functions related to cancer development. Recently, cancer-specific and tissue-specific circRNAs have been identified by high-throughput sequencing and are curated in publicly available databases. CircRNAs have features that are ideal properties of biomarkers, including conservation, abundance, and stability in plasma, saliva, and urine. Many circRNAs with predictive and prognostic significance in cancer have been described, and functional mechanisms for some circRNAs have been suggested. CircRNA also has great potential as a noninvasive biomarker for early cancer detection, although further investigation is necessary before clinical application is feasible.See all articles in this CEBP Focus section, "NCI Early Detection Research Network: Making Cancer Detection Possible."

Construction of a circRNA-Related ceRNA Prognostic Regulatory Network in Breast Cancer

Purpose

Accumulating evidence has indicated that circRNAs are closely involved in tumorigenesis and progression of human cancers. However, the molecular mechanism underlying function of circRNAs in breast cancer has not been thoroughly elucidated. Currently, we aimed to characterize the circRNA-related competing endogenous RNA (ceRNA) regulatory network in breast cancer and to construct prognostic model.

Materials and Methods

First, we constructed circRNA expression profiles for paired breast cancer in a Chinese population using a human circRNA microarray. Expression profiles of circRNAs, miRNAs, and mRNAs were retrieved from our circRNA dataset, the Gene Expression Omnibus (GEO) and The Cancer Genome Atlas (TCGA) databases. We applied the limma and edgeR packages to identify differentially expressed RNAs. Weighted gene correlation network analysis (WGCNA) was used to identify critical modules of mRNAs. Next, a ceRNA network was established based on circRNA-miRNA and miRNA-mRNA intersections. Both Cox regression analysis and ROC curve analysis were performed to generate prognostic model. Additionally, we performed Gene Set Enrichment Analysis (GSEA) on prognostic signatures.

Results

Total of 59 circRNAs, 98 miRNAs and 3966 mRNAs were identified as differentially expressed RNAs. We first identified 38 miRNA-mRNA pairs and 38 circRNA-miRNA pairs to construct the circRNA-miRNA-mRNA regulatory network and then generated a prognostic model based on 7 signatures (MMD, SLC29A4, CREB5, FOS, ANKRD29, MYOCD, and PIGR), and patients with high-risk scores presented poor prognosis. Several cancer-related pathways were enriched, including the TGF-β pathway, the focal adhesion pathway, and the JAK-STAT signaling pathway, and 20 prognostic ceRNA regulatory networks were subsequently identified.

Conclusion

In all, we screened a series of dysregulated circRNAs, miRNAs, and mRNAs, and constructed circRNA-related ceRNA network in breast cancer. Our findings may help to deepen the understanding of circRNA-related regulatory mechanisms. Moreover, we generated a prognostic model that provided new insight into postoperative management for breast cancer.

Comprehensive Analysis of Differentially Expressed circRNAs Reveals a Colorectal Cancer-Related ceRNA Network

The morbidity and mortality of colorectal cancer (CRC) remained to be very high worldwide. Recently, circRNAs had been revealed to have a crucial role in cancer prognosis and progression. Numerous researches have shown that RNA sequencing technology and in silico method were widely used to identify pathogenic mechanisms and uncover promising targets for diagnosis and therapy. In this study, these methods were analyzed to obtain differentially expressed circRNAs (DECs). We identified upregulated 316 circRNAs and reduced 76 circRNAs in CRC samples, in comparison with those in normal tissues. In addition, a competitive endogenous network of circRNA-miRNA-mRNA was established to predict the mechanisms of circRNAs. Bioinformatics analysis revealed that these circRNAs participated in metabolism regulation and cell cycle progression. Of note, we observed the hub genes and miRNAs in this ceRNA network were associated with the survival time in CRC. We think this study could provide potential prognostic biomarkers and targets for CRC.

Knockdown of circRAD18 Mitigates Breast Cancer Progression through the Regulation of miR-613/HK2 Axis

Background

Breast cancer (BC) remains the most prevalent malignancy and the leading cause of cancer death. Circular RNAs (circRNAs) have been discovered to serve as crucial regulators in BC. In the current work, we aimed to study the impact of circRAD18 (hsa_circ_0002453) on BC progression and mechanism governing it.

Materials and Methods

The expression levels of circRAD18, miR-613 and hexokinase 2 (HK2) mRNA were determined by quantitative real-time polymerase chain reaction (qRT-PCR). CircRAD18 identification was performed using RNase R digestion and actinomycin D assay. Cell viability, colony formation, apoptosis, migration, invasion and glycolysis were measured by Cell Counting Kit-8 assay, colony formation assay, flow cytometry, transwell analysis and extracellular acidification rate detection assay, respectively. Western blot was used to assess the levels of E-Cadherin, Vimentin, N-Cadherin and HK2 protein. The targeted interplay between miR-613 and circRAD18 or HK2 was detected by dual-luciferase reporter assay. Xenograft model assay was performed to observe the role of circRAD18 in vivo.

Results

CircRAD18 was highly expressed in BC tissues and cells. CircRAD18 depletion hindered BC cell malignant behaviors, as evidenced by the inhibition in cell viability, colony formation, migration, invasion, epithelial to mesenchymal transition and glycolysis, as well as the promotion in cell apoptosis. CircRAD18 directly interacted with miR-613, and miR-613 mediated the repressive effect of circRAD18 knockdown on BC cell malignant behaviors. Moreover, HK2 was a direct target of miR-613, and circRAD18 positively regulated HK2 expression via sponging miR-613. Additionally, circRAD18 knockdown repressed tumor growth in vivo by miR-613.

Conclusion

Our current work suggested that circRAD18 silencing suppressed BC cell malignant behaviors in vitro and tumor growth in vivo at least partly via the regulation of the miR-613/HK2 axis, highlighting that circRAD18 might be a promising therapeutic target for BC treatment.

CircRNAs: potency of protein translation and feasibility of novel biomarkers and therapeutic targets for head and neck cancers

Circular RNAs (circRNAs), a new star noncoding RNA (ncRNA), show stability, conservation, abundance, and tissue and stage specificity. They act as key regulators of biological processes. They target the mRNAs of many other different genes or signaling pathways, and closely link associated genes into regulatory networks. Growing evidence has demonstrated that circRNAs may play an important role in the carcinogenesis, progression and chemoradiation resistance of many cancers including head and neck cancers (HNC). CircRNA, like other ncRNA, such as miRNA, lncRNA, usually is considered to be non-protein coding transcript. However, recent studies indicated that abnormal translation of circRNAs may be involved in human diseases. In this review, we collected the origin, classification, characteristics, function of circRNAs, exosmal circRNAs, and then synthesize current study results to highlight aberration of circRNAs in various types of HNC, and try to clarify the molecular mechanisms of circRNAs affecting the pathogenesis and progression of HNC, as well as pay particular attention to provide a new avenue to the diagnosis and treatment strategy for HNC.

The Regulatory Functions of Circular RNAs in Digestive System Cancers

Circular ribonucleic acids (circRNAs), which are a type of covalently closed circular RNA, are receiving increasing attention. An increasing amount of evidence suggests that circRNAs are involved in the biogenesis and development of multiple diseases such as digestive system cancers. Dysregulated circRNAs have been found to act as oncogenes or tumour suppressors in digestive system cancers. Moreover, circRNAs are related to ageing and a wide variety of processes in tumour cells, such as cell apoptosis, invasion, migration, and proliferation. Moreover, circRNAs can perform a remarkable multitude of biological functions, such as regulating splicing or transcription, binding RNA-binding proteins to enable function, acting as microRNA (miRNA) sponges, and undergoing translated into proteins. However, in digestive system cancers, circRNAs function mainly as miRNA sponges. Herein, we summarise the latest research progress on biological functions of circRNAs in digestive system cancers. This review serves as a synopsis of potential therapeutic targets and biological markers for digestive system cancer.

Biological Roles and Mechanisms of Circular RNA in Human Cancers

Circular RNA (circRNA) is an intriguing class of RNA with covalently closed-loop structure and is highly stable and conservative. As new members of the ncRNAs, the function, mechanism, potential diagnostic biomarker, and therapeutic target have raised increased attention. Most circRNAs are presented with characteristics of abundance, stability, conservatism, and often exhibiting tissue/developmental-stage-specific manner. Over 30,000 circRNAs have been identified with their unique structures to maintain stability more easily than linear RNAs. An increased numbers of circRNAs are dysregulated and involved in several biological processes of malignance, such as tumorigenesis, growth, invasion, metastasis, apoptosis, and vascularization. Emerging evidence suggests that circRNAs play important roles by acting as miRNA sponge or protein scaffolding, autophagy regulators, and interacting with RNA-binding protein (RBP), which may potentially serve as a novel promising biomarker for prevention, diagnosis and therapeutic target for treatment of human cancer with great significance either in scientific research or clinic arena. This review introduces concept, major features of circRNAs, and mainly describes the major biological functions and clinical relevance of circRNAs, as well as expressions and regulatory mechanisms in various types of human cancer, including pathogenesis, mode of action, potential target, signaling regulatory pathways, drug resistance, and therapeutic biomarkers. All of which provide evidence for the potential utilities of circRNAs in the diagnosis and treatment of cancer.

Circular RNAs and gastrointestinal cancers: Epigenetic regulators with a prognostic and therapeutic role

Both environmental and genetic factors are involved in the initiation and development of gastrointestinal cancer. Covalent closed circular RNAs (circRNAs) are produced by a mechanism called "back-splicing" from mRNAs. They are highly stable and show cell and tissue specific expression patterns. Although some functions such as "microRNA sponge" and "RNA binding protein sponge" have been reported for a small number of circRNAs, the function of thousands of other circRNAs is still unknown. Dysregulation of circRNAs has been reported in many GI cancers and are involved in metastasis and invasion. CircRNAs have been reported to be useful as prognostic markers and targets for developing new treatments. We first describe the properties and biogenesis of circRNAs. We then summarize recent reports about circRNA functions, expression status, and their potential to be used as biomarkers in GI cancers including, gastric cancer, colorectal cancer, esophageal cancer, hepatocellular carcinoma, gallbladder cancer and pancreatic cancer.

Characterization of a five-microRNA signature as a prognostic biomarker for esophageal squamous cell carcinoma

This study aims to identify a miRNAs signature for predicting overall survival (OS) in esophageal squamous cell carcinoma (ESCC) patients. MiRNA expression profiles and corresponding clinical information of 119 ESCC patients were obtained from NCBI GEO and used as the training set. Differentially expressed miRNAs (DEmiRNAs) were screened between early-stage and late-stage samples. Cox regression analysis, recursive feature elimination (RFE)-support vector machine (SVM) algorithm, and LASSO Cox regression model were used to identify prognostic miRNAs and consequently build a prognostic scoring model. Moreover, promising target genes of these prognostic miRNAs were predicted followed by construction of miRNA-target gene networks. Functional relevance of predicted target genes of these prognostic miRNAs in ESCC was analyzed by performing function enrichment analyses. There were 46 DEmiRNAs between early-stage and late-stage samples in the training set. A risk score model based on five miRNAs was built. The five-miRNA risk score could classify the training set into a high-risk group and a low-risk group with significantly different OS time. Risk stratification ability of the five-miRNA risk score was successfully validated on an independent set from the Cancer Genome Atlas (TCGA). Various biological processes and pathways were identified to be related to these miRNAs, such as Wnt signaling pathway, inflammatory mediator regulation of TRP channels pathway, and estrogen signaling pathway. The present study suggests a pathological stage-related five-miRNA signature that may have clinical implications in predicting prognosis of ESCC patients.

Circ-ASH2L promotes tumor progression by sponging miR-34a to regulate Notch1 in pancreatic ductal adenocarcinoma

BACKGROUND

Circular RNAs (circRNAs) have recently been shown to play important roles in different tumors. However, their detailed roles and regulatory mechanisms in pancreatic ductal adenocarcinoma (PDAC) are not well understood. This study aimed to identify enriched circRNAs and detect their functions and mechanisms in PDAC cells and tissues.

METHODS

circRNA-ASH2L (circ-ASH2L) was identified by circRNA microarray studies based on previous studies, and further detected in PDAC cells and samples by qRT-PCR. The functions of circ-ASH2L were identified by transwell, EdU, cell cycle or Tube formation assays. The regulatory mechanisms of circ-ASH2L were explored by WB, RIP, FISH, dual-luciferase assays, RNA pulldown or other assays.

RESULTS

We identified a circRNA (circ-ASH2L) based on our previous studies, detected its expression in different malignant cells and found that circ-ASH2L was highly expressed in pancreatic cells or tumor tissues and correlated with tumor malignancy. Further studies revealed that circ-ASH2L promoted tumor invasion, proliferation and angiogenesis by regulating miR-34a, thus regulate Notch 1 expression. Circ-ASH2L served as a miRNA sponge for miR-34a and promoted tumor progression in vivo. Finally, we analyzed circ-ASH2L expression in clinical tissues and found that high circ-ASH2L expression was correlated with lymphatic invasion and TNM stage and was an independent risk factor for pancreatic patient survival.

CONCLUSIONS

circ-ASH2L play an important role in tumor invasion, and high circ-ASH2L may be a useful marker of PDAC diagnosis or progression.

CircLMNB1 promotes colorectal cancer by regulating cell proliferation, apoptosis and epithelial-mesenchymal transition

Objective

The aberrant expression of circular RNAs (circRNAs) is a frequent occurrence in various cancers. However, the functions and roles played by most circRNAs in colorectal cancer (CRC) remain largely unknown.

Materials and methods

Levels of circLMNB1 expression were evaluated by qRT-PCR and FISH assays. The influence of circLMNB1 knockdown on LoVo and HCT116 cell proliferation, cycling, apoptosis, migration, and invasion were assessed by the CCK-8, assay, Edu assay, flow cytometry, Hoechst staining, and the Transwell assay, respectively. The relative levels of EMT- and apoptosis-related proteins were determined by Western blotting.

Results

CircLMNB1 expression was significantly upregulated in CRC tissues and cells. Knockdown of circLMNB1 by siRNA in LoVo cells suppressed cell proliferation, migration and invasion, and facilitated cell cycle arrest and apoptosis In addition, we proved that knockdown of circLMNB1 upregulated E-cadherin, Bax and caspase-3 expression, and downregulated MMP2, MMP-9, and N-cadherin expression in LoVo cells. Further results showed that overexpression of circLMNB1 enhanced the malignant characteristics of HCT116 cells.

Conclusion

Our findings revealed that blocking of circLMNB1 could inhibit CRC development, and help to explain the underlying mechanism by which circLMNB1 knockdown inhibits the metastasis of CRC. Finally, this study suggests circLMNB1 as a novel biomarker for CRC.

Roles of circular RNA in breast cancer: present and future

Breast cancer is one of the most common cancers with the highest morbidity and mortality among women despite the treatment approaches have advanced including surgery, endocrine therapy and targeted therapy. Novel biomarkers are warranted to be discovered for the early detection, treatment and prognosis for breast cancer. CircRNA is a class of covalently closed single-stranded circular RNA molecules without free 5' or 3' end which makes them well expressed and more stable than their linear counterparts. In this review, we mainly discuss the oncogenic or anti-oncogenic roles of circRNAs can be utilized in the treatment and prognosis of breast cancer. A large number of circRNAs have shown great potential to function in carcinogenesis, metastasis or chemoresistance of breast cancer through transcriptional regulation of RNAs including miRNA and mRNA, in addition to their promise as stable biomarkers that can be used for monitoring breast cancer progression. However, the translation phenomenon of circRNAs in breast cancer and the diagnostic value of circRNAs in breast cancer requires further investigation for which the detection of circRNAs in plasma exosomes could be worthy of a try. Above all, engineered exosomes preloaded with engineered anti-oncogenic circRNAs are likely to provide a novel direction in the personal medicine of breast cancer.

Hsa_circ_0067997 promotes the progression of gastric cancer by inhibition of miR-515-5p and activation of X chromosome-linked inhibitor of apoptosis (XIAP)

Circular RNAs (circRNAs) have been revealed to play vital roles in modulating gene expression and participate in several pathological responses including gastric cancer (GC). However, the larger numbers of the circRNAs in GC progression remain undetermined. In the present study, four GC related circRNAs expression profiles from the Gene Expression Omnibus (GEO) database were enrolled. We identified hsa_ circRNA_00067997 (circ_0067997) as an oncogene in GC. qRT-PCR was used to validate the expression of circ_0067997 in GC tissues and cell lines. The results revealed that circ_0067997 was upregulated in GC, and high circ_0067997 expression was associated with the poor overall survival rate of GC patients. Knockdown of circ_0067997 significantly reduced cell viability, inhibited colony formation, and attenuated invasive ability, whereas overexpression of circ_0067997 exhibited opposed effects. Circ_0067997 was identified to be a sponge for miR-515-5p directly. Moreover, XIAP was demonstrated to be targeted and regulated by miR-515-5p. In conclusion, circ_0067997 was identified to be an oncogene in GC by regulating miR-515-5p/XIAP axis.

Emerging Epigenetic Regulation of Circular RNAs in Human Cancer

Circular RNAs (circRNAs) are novel members of the noncoding RNA family. Their characteristic covalent closed-loop structure endows circRNAs that are much more stable than the corresponding linear transcript. circRNAs are ubiquitous in eukaryotic cells, and their functions are diverse and include adsorbing microRNAs (miRNAs; acting as miRNA sponges), regulating transcription, interacting with RNA-binding proteins, and translating and deriving pseudogenes. Moreover, circRNAs are associated with the occurrence and progression of a variety of cancers, acting as new biomarkers for early diagnosis to evaluate curative effects and patient prognosis. Here, this paper briefly describes the characteristics and functions of circRNAs, and it further concludes the relationship between circRNAs and human cancer.

circSMARCA5 Functions as a Diagnostic and Prognostic Biomarker for Gastric Cancer

Background

Circular RNAs have been implicated in various malignancies and can function as potential biomarkers for cancers. Reportedly, circSMARCA5 was downregulated in hepatocellular carcinoma and glioblastoma multiforme, but upregulated in prostate cancer. The functional roles and clinical significance of circSMARCA5 still remain unknown in the context of gastric cancer (GC).

Methods

Expression levels of circSMARCA5 were detected by qRT-PCR. Clinical data including patient basic information, clinicopathological features, and survival data were obtained. The Kaplan-Meier methods, multivariate Cox regression models, and the receiver operating characteristic curve were used to assess the clinical significance of circSMARCA5 in GC. Cell proliferation assays and transwell assays were performed to elucidate the functional roles of circSMARCA5 in GC.

Results

The circSMARCA5 level was decreased in GC tissues and cell lines. The low expression level of circSMARCA5 was correlated to poorer overall survival and disease-free survival. Low circSMARCA5 expression was revealed as an independent unfavorable predictive factor for GC. The results indicated that circSMARCA5 had a moderate ability for discrimination between GC patients and controls with an area under the curve of 0.806. Upregulation of circSMARCA5 dampened the proliferation, migration, and invasion of GC cells, whereas circSMARCA5 knockdown promoted GC progression.

Discussion

Our results demonstrated that circSMARCA5 was decreased and exerted tumor-suppressive effects in GC. circSMARCA5 can function as a potential biomarker for GC prognosis and diagnosis.

Upregulated circ RNA hsa_circ_0000337 promotes cell proliferation, migration, and invasion of esophageal squamous cell carcinoma

Background

As a new class of endogenous ncRNAs, circRNAs have been recently verified to be involved in the carcinogenesis and progression of human cancers. In the current study, we attempted to explore the potential function of a candidate circRNA (hsa_circ_0000337) in esophageal squamous cell carcinoma (ESCC).

Patients and methods

The altered expression of hsa_circ_0000337 was validated in clinical samples from 48 patients with ESCC. The human esophageal carcinoma cell lines KYSE-150 and TE-1, and the normal human esophageal epithelial cell line (HET-1A) were applied for functional analysis of hsa_circ_0000337. Cell proliferation was measured using the Cell Counting Kit-8 assay and the colony formation assay. Cell invasion and migration were detected by Transwell and wound healing assays, respectively. We further performed bioinformatic analysis and luciferase reporter assays to explore the role of hsa_circ_0000337 as a miRNA sponge.

Results

hsa_circ_0000337 was significantly upregulated in ESCC tissues compared to adjacent normal-appearing tissues (P<0.0001). In our in vitro experiment, the expression of hsa_circ_0000337 was higher in TE-1 compared to the normal human esophageal epithelial cell line HET-1A (P<0.001), but was not significantly different in KYSE-150 (P>0.05). Knockdown of hsa_circ_0000337 significantly inhibited cell proliferation, migration, and invasion in TE-1 and KYSE-150 cell lines. Bioinformatics predicted and luciferase reporter assay verified that hsa_circ_0000337 could bind to miR-670-5p, a ncRNA involved in carcinogenesis. It is estimated that 21 genes are regulated by miR-670-5p.

Conclusion

hsa_circ_0000337 was found to be an upregulated circRNA that is related to ESCC and promotes the progression of disease by regulating cell proliferation, migration, and invasion. These findings suggest that this circRNA could be a promising diagnostic biomarker and potential therapeutic target.

Inhibition of hsa_circ_0001313 (circCCDC66) induction enhances the radio-sensitivity of colon cancer cells via tumor suppressor miR-338-3p: Effects of cicr_0001313 on colon cancer radio-sensitivity

Circular RNA_0001313 (circ_0001313), also known as circCCDC66, is a novel circRNA that recently found to be upregulated in colon cancer tissues and promote colon cancer progression. However, the role of circ_0001313 in regulating radio-sensitivity of colon cancer and its molecular mechanism remain undetermined. Here we found circ_0001313 was significantly upregulated and miR-338-3p was downregulated in radio-resistant colon cancer tissues compared to radio-sensitive tissues. Radiation treatment in colon cells triggered a remarkable upregulation of circ_0001313 and a downregulation of miR-338-3p. Knockdown of circ_0001313 reduced cell viability, colony formation rate and increased caspase-3 activity in colon cancer cells under irradiation. Moreover, circ_0001313 act as a sponge for miR-338-3p in colon cancer cells. Furthermore, miR-338-3p could reverse the effects of circ_0001313 knockdown on cell viability, colony formation, and caspase-3 activity. These findings revealed that knockdown of circ_0001313 could induce radio-sensitivity of colon cancer cells by negatively regulating miR-338-3p.