Increased circular RNA hsa_circ_0012673 acts as a sponge of miR-22 to promote lung adenocarcinoma proliferation

Circular RNAs in tuberculosis and lung cancer

This review signifies the role of circular RNAs (circRNAs) in tuberculosis (TB) and lung cancer (LC), focusing on pathogenesis, diagnosis, and treatment. CircRNAs, a newly discovered type of non-coding RNA, have emerged as key regulators of gene expression and promising biomarkers in various bodily fluids due to their stability. The current review discusses circRNA biogenesis, highlighting their RNase-R resistance due to their loop forming structure, making them effective biomarkers. It details their roles in gene regulation, including splicing, transcription control, and miRNA interactions, and their impact on cellular processes and diseases. For LC, the review identifies circRNA dysregulation affecting cell growth, motility, and survival, and their potential as therapeutic targets and biomarkers. In TB, it addresses circRNAs' influence on host anti-TB immune responses, proposing their use as early diagnostic markers. The paper also explores the interplay between TB and LC, emphasizing circRNAs as dual biosignatures, and the necessity for differential diagnosis. It concludes that no single circRNA biomarker is universally applicable for both TB and LC. Ultimately, the review highlights the pivotal role of circRNAs in TB and LC, encouraging further research in biomarker identification and therapeutic development concomitant for both diseases.

Extracellular Vesicles in Lung Cancer: Implementation in Diagnosis and Therapeutic Perspectives

Lung cancer represents the leading cause of cancer-related mortality worldwide, with around 1.8 million deaths in 2020. For this reason, there is an enormous interest in finding early diagnostic tools and novel therapeutic approaches, one of which is extracellular vesicles (EVs). EVs are nanoscale membranous particles that can carry proteins, lipids, and nucleic acids (DNA and RNA), mediating various biological processes, especially in cell-cell communication. As such, they represent an interesting biomarker for diagnostic analysis that can be performed easily by liquid biopsy. Moreover, their growing dataset shows promising results as drug delivery cargo. The aim of our work is to summarize the recent advances in and possible implications of EVs for early diagnosis and innovative therapies for lung cancer.

CircSSBP2 acts as a MiR-2400 sponge to promote intramuscular preadipocyte proliferation by regulating NDRG1

Intramuscular fat (IMF) is a critical factor in beef quality. IMF is mainly distributed between muscle fibres and its accumulation can affect the marbling and meat quality of beef. IMF formation and deposition is a complex process and in recent years a group of non-coding RNAs (ncRNAs), known as circRNAs, have been discovered to play an important role in regulating intramuscular fat deposition. CircRNAs form a covalent loop structure after reverse splicing of precursor mRNAs. They can act by adsorbing miRNAs, thereby reducing their repressive effects on downstream target genes. Based on high-throughput sequencing of circRNAs in intramuscular fat of Qinchuan and Japanese black cattle, we identified a novel circSSBP2 that is differentially expressed between the two species and associated with adipogenesis. We show that circSSBP2 knockdown promotes bovine intramuscular preadipocyte proliferation, whereas overexpression inhibits bovine intramuscular preadipocyte proliferation. We also show that circSSBP2 can act as a molecular sponge for miR-2400 and that miR-2400 overexpression promotes bovine intramuscular preadipocyte proliferation. Furthermore, N-myc downstream-regulated gene 1 (NDRG1) was identified as a direct target gene of miR-2400, and NDRG1 interference promoted the proliferation of bovine intramuscular preadipocytes. In conclusion, our results suggest that circSSBP2 inhibits the proliferation of bovine intramuscular preadipocytes by regulating the miR-2400/NDRG1 axis.

The functional significance of circRNA/miRNA/mRNA interactions as a regulatory network in lung cancer biology

Lung cancer, the leading cause of cancer-related deaths, presents significant challenges to patients due to its poor prognosis. Recent research has increasingly implicated circular RNAs in the development and progression of lung cancer. These circular RNAs have been found to impact various aspects of tumor behavior, including proliferation, metastasis, cell cycle regulation, apoptosis, cancer stem cells, therapy response, and the tumor microenvironment. One of the key mechanisms by which circular RNAs exert their influence is through their ability to act as miRNA sponges, sequestering microRNAs and preventing them from targeting other RNA molecules. Accumulating evidence suggests that circular RNAs can function as competing endogenous RNAs, affecting the expression of target mRNAs by sequestering microRNAs. Dysregulation of competing endogenous RNAs networks involving circular RNAs, microRNAs, and mRNAs leads to the aberrant expression of oncogenes and tumor suppressors involved in lung cancer pathogenesis. Understanding the dynamic interplay and molecular mechanisms among circular RNAs, microRNAs, and mRNAs holds great promise for advancing early diagnosis, personalized therapeutic interventions, and improved patient outcomes in lung cancer. Therefore, this study aims to provide an in-depth exploration of the executive roles of circular RNAs/microRNAs/ mRNAs interactions in lung cancer pathogenesis and their potential utility for diagnosing lung cancer, predicting patient prognosis, and guiding targeted therapies. By offering a comprehensive overview of the dysregulation of the axes as driving factors in lung cancer, we aim to pave the way for their translation into clinical practice in the future.

Silencing of circ_0088036 inhibits growth and invasion of lung adenocarcinoma through miR-203/SP1 axis

Lung cancer is the leading cause of cancer-related deaths worldwide. Circular RNA (circRNA) circ_0088036 is a recently discovered circRNA known for its roles in rheumatoid arthritis. The study aimed to study the function of circ_0088036 in lung adenocarcinoma (LUAD). Circ_0088036 expressions were analyzed in the Gene Expression Omnibus (GEO) database. The relationship between circ_0088036 expressions and clinicopathological data of LUAD was assessed. The messenger RNA and protein levels were analyzed by quantitative real-time polymerase chain reaction and Western blot. Cell viability, apoptosis, and invasion were tested by Cell Counting Kit-8, flow cytometry, and transwell assay. The direct interaction between microRNA-203 (miR-203) and circ_0088036 or specificity protein 1 (SP1) was confirmed by dual-luciferase reporter assay, RNA pull-down, and RNA immunoprecipitation assays. Circ_0088036 was overexpressed in LUAD from the analysis of the GEO database. The poor prognosis was found in the patients with high expressions of circ_0088036. The level of Circ_0088036 was increased in LUAD tissues and cells. In terms of function, the deletion of circ_0088036 inhibited LUAD tumorigenesis in vitro by repressing cell growth, invasion, and epithelial-mesenchymal transition (EMT). In mechanism, circ_0088036 could competitively sponge miR-203, thereby affecting the expressions of the target gene SP1. In addition, lessening of miR-203 and enlarging of SP1 could eliminate the anticancer effect of short hairpin RNA-circ_0088036 on LUAD cells. Besides, the knockout of circ_0088036 hindered the growth of xenografted tumors in vivo. Circ_0088036 promoted the LUAD cell growth, invasion, and EMT via modulating the miR-203/SP1 axis in LUAD.

DCDA: CircRNA-Disease Association Prediction with Feed-Forward Neural Network and Deep Autoencoder

Circular RNA is a single-stranded RNA with a closed-loop structure. In recent years, academic research has revealed that circular RNAs play critical roles in biological processes and are related to human diseases. The discovery of potential circRNAs as disease biomarkers and drug targets is crucial since it can help diagnose diseases in the early stages and be used to treat people. However, in conventional experimental methods, conducting experiments to detect associations between circular RNAs and diseases is time-consuming and costly. To overcome this problem, various computational methodologies are proposed to extract essential features for both circular RNAs and diseases and predict the associations. Studies showed that computational methods successfully predicted performance and made it possible to detect possible highly related circular RNAs for diseases. This study proposes a deep learning-based circRNA-disease association predictor methodology called DCDA, which uses multiple data sources to create circRNA and disease features and reveal hidden feature codings of a circular RNA-disease pair with a deep autoencoder, then predict the relation score of the pair by a deep neural network. Fivefold cross-validation results on the benchmark dataset showed that our model outperforms state-of-the-art prediction methods in the literature with the AUC score of 0.9794.

Non-coding RNAs as potential therapeutic targets for receptor tyrosine kinase signaling in solid tumors: current status and future directions

This review article presents an in-depth analysis of the current state of research on receptor tyrosine kinase regulatory non-coding RNAs (RTK-RNAs) in solid tumors. RTK-RNAs belong to a class of non-coding RNAs (nc-RNAs) responsible for regulating the expression and activity of receptor tyrosine kinases (RTKs), which play a critical role in cancer development and progression. The article explores the molecular mechanisms through which RTK-RNAs modulate RTK signaling pathways and highlights recent advancements in the field. This include the identification of potential new RTK-RNAs and development of therapeutic strategies targeting RTK-RNAs. While the review discusses promising results from a variety of studies, encompassing in vitro, in vivo, and clinical investigations, it is important to acknowledge the challenges and limitations associated with targeting RTK-RNAs for therapeutic applications. Further studies involving various cancer cell lines, animal models, and ultimately, patients are necessary to validate the efficacy of targeting RTK-RNAs. The specificity of ncRNAs in targeting cellular pathways grants them tremendous potential, but careful consideration is required to minimize off-target effects, the article additionally discusses the potential clinical applications of RTK-RNAs as biomarkers for cancer diagnosis, prognosis, and treatment. In essence, by providing a comprehensive overview of the current understanding of RTK-RNAs in solid tumors, this review emphasizes their potential as therapeutic targets for cancer while acknowledging the associated challenges and limitations.

Circular RNA hsa_circ_0012673 Promotes Breast Cancer Progression via miR-576-3p/SOX4 Axis

Circular RNAs (circRNAs) have been reported to exert critical roles in human cancers. In this study, we investigated the role and molecular mechanism of hsa_circ_0012673 in breast cancer. Herein, we found that the expression of hsa_circ_0012673 was upregulated in breast cancer tissues and cell lines. Knockdown of hsa_circ_0012673 using RNA interference technique suppressed the proliferation, migration, and invasion of breast cancer cells. Mechanistically, hsa_circ_0012673 sponged miR-576-3p to stabilize SRY-box transcription factor 4 (SOX4), and thereby facilitating breast cancer cell proliferation, migration and invasion. Collectively, our study identified the oncogenic properties of hsa_circ_0012673/miR-576-3p/SOX4 axis in breast cancer, providing potential and exploitable diagnostic and therapeutic molecules for breast cancer.

Landscape of circular RNAs in different types of lung cancer and an emerging role in therapeutic resistance (Review)

Lung cancer is one of the most common malignant tumor types and the leading cause of cancer‑associated death worldwide. Different types of lung cancer exhibit differences in terms of pathophysiology and pathogenesis, and also treatment and prognosis. Accumulating evidence has indicated that circular RNAs (circRNAs) are abnormally expressed among different types of lung cancer and confer important biological functions in progression and prognosis. However, studies comparing different circRNAs in lung cancer subtypes are scarce. Furthermore, circRNAs have an important role in drug resistance and are related to clinicopathological features in lung cancer. Summaries of the association of circRNAs with drug resistance are also scarce in the literature. The present study outlined the biological functions of circRNAs and focused on discriminating differential circRNA patterns and mechanisms in three different types of lung cancer. The emerging roles of circRNAs in the resistance to chemotherapy, targeted therapy, radiotherapy and immunotherapy were also highlighted. Understanding these aspects of circRNAs sheds light on novel physiological and pathophysiological processes of lung cancer and suggests the application of circRNAs as biomarkers for diagnosis and prognosis, as well as therapeutic resistance.

Circ-GSK3B up-regulates GSK3B to suppress the progression of lung adenocarcinoma

GSK3B is the mRNA form of glycogen synthase kinase 3 beta (GSK-3β), which is a critical repressor of Wnt/β-catenin signaling pathway and generally inhibited in cancer cells. Plenty of researches have disclosed that circular RNAs, namely circRNAs exert important functions in the progression of various human malignancies including lung adenocarcinoma (LUAD). Therefore, we attempted to explore whether there existed certain circRNAs that could mediate LUAD development by regulating GSK3B expression and Wnt/β-catenin pathway. In the present research, circ-GSK3B (hsa_circ_0066903) was found to be significantly down-regulated in LUAD tissues and cells and it suppressed the proliferation, migration and stemness of LUAD cells. Furthermore, it was discovered that circ-GSK3B competitively sponged miR-3681-3p and miR-3909 to elevate GSK3B expression. Circ-GSK3B could impair the binding ability of FKBP51 to GSK-3β to inhibit the phosphorylation of GSK-3β^S9, resulting in the inactivation of Wnt/β-catenin signaling. In addition, the regulatory effect of circ-GSK3B on LUAD tumorigenesis and cell progression was testified through in vitro and in vivo rescue experiments. In conclusion, circ-GSK3B suppressed LUAD development through up-regulating and activating GSK3B.

Circular RNA (circ)_0129047 upregulates bone morphogenetic protein receptor type 2 expression to inhibit lung adenocarcinoma progression by sponging microRNA (miR)-1206

Circular RNAs (circRNAs) play significant roles in the tumorigenesis and progression of various cancers, including lung adenocarcinoma (LAC). However, their underlying biological functions in LAC remain unclear. Here, we investigated the tumor suppressor role of the newly identified circRNA, circ_0129047, in LAC tumorigenesis and progression. The expression levels of circ_0129047, microRNA (miR)-1206, and bone morphogenetic protein receptor type 2 (BMPR2) mRNA in LAC cells and tissues were monitored using reverse transcription-quantitative polymerase chain reaction. Dual-luciferase reporter, RNA immunoprecipitation, and RNA pull-down assays were used to confirm the targeting relationships among circ_0129047, miR-1206, and BMPR2 mRNA. Functional experiments for A549 and PC9 cells were performed using cell counting kit-8, bromodeoxyuridine enzyme-linked immunosorbent, caspase-3 activity, cell adhesion, wound healing, and transwell assays. Circ_0129047 expression levels were reduced in LAC cells and tissues. Mechanistically, we discovered that circ_0129047 could sponge miR-1206, and miR-1206 could directly target BMPR2. In addition, circ_0129047 or BMPR2 knockdown facilitated the viability, proliferation, adhesion, migration, and invasion, while inhibiting the apoptosis of LAC cells. Furthermore, the inhibitory effects of circ_0129047 or BMPR2 overexpression on the malignant phenotype of LAC cells could be reversed by the overexpression of miR-1206. In conclusion, circ _0129047 was found to play a tumor suppressive role in LAC progression; it upregulated BMPR2 expression to inhibit LAC progression by sponging miR-1206.

Abbreviations: non-small cell lung cancer (NSCLC); small cell lung cancer (SCLC); lung adenocarcinoma (LAC); Circular RNA (circRNA); MicroRNA (miRNA); bone morphogenetic protein (BMP); squamous cell lung cancer (SCC); RNA immunoprecipitation (RIP)

CircRNAs in lung adenocarcinoma: diagnosis and therapy

Lung adenocarcinoma (LUAD) is the common histological subtype of non-small-cell lung carcinoma (NSCLC). Circular RNAs (circRNAs) represent a new class of non-coding RNAs (ncRNAs) involved in the development of cancer. Accumulating evidence indicated that a large number of circular RNAs were found to be involved in many biological processes, including tumor initiation, proliferation and progression. These circRNAs present great potentials as new biomarkers and vital targets for disease diagnosis and prognosis. In this review, we mainly focus on the differentially expressed circRNAs and their functions in the pathogenesis of LUAD, which makes it possible for the utility of circRNAs as novel biomarkers for early diagnosis and therapy. Especially, it is helpful to develop circRNAs as crucial therapeutic targets, thus providing a promising biomedical application in the field of cancer gene therapy.

Construction of competitive endogenous RNA network related to circular RNA and prognostic nomogram model in lung adenocarcinoma

Early researches have revealed that circular RNA (circRNA) had the potential of biomarkers and could affect tumor progression through regulatory networks. However, few research focused on the function of circRNA in lung adenocarcinoma and the regulation mechanism of competitive endogenous RNA. In present study, through differential expression analysis, 10 circRNAs, 98 miRNAs(microRNA) and 2497 mRNAs were screened. Based on the 10 circRNAs and related databases, a competitive endogenous RNA regulatory network (ceRNA network) containing 7 circRNAs, 13 miRNAs and 147 mRNAs was constructed. KEGG and GO analysis suggested that 147 mRNAs were obviously enriched in biological pathway related to LUAD. By constructing a PPI network, 12 hub genes were identified by MCODE. The result of survival analysis showed that 10 hub genes (BIRC5, MKI67, CENPF, RRM2, BUB1, MELK, CEP55, CDK1, NEK2, TOP2A) were significantly related to the survival of LUAD. We randomly divided 483 clinical data into two parts: train set and validation set. The train set was used for Cox regression analysis, 3 prognostic factors (stage, T, CDK1) were screened. The nomogram model was constructed based on stage, T and CDK1. The model was evaluated by ROC curve, calibration chart, Kaplan-Meier (KM) curve and validation set data. The results indicated that the model has good accuracy. Our study elucidated the regulatory mechanism of circRNA in lung adenocarcinoma, and the nomogram model also provided insight for the clinical analysis of lung adenocarcinoma.

A genome-wide circular RNA transcriptome in rat

Circular RNAs (circRNAs) are a novel class of noncoding RNAs that back-splice from 5ʹ donor site and 3ʹ acceptor sites to form a circular structure. A number of circRNAs have been discovered in model organisms including human, mouse, Drosophila, among other organisms. There are a few candidate-based studies on circRNAs in rat, a well-studied model organism as well. A number of pipelines have been published to identify the back splice junctions for the discovery of circRNAs but studies comparing these tools have suggested that a combination of tools would be a better approach to identify high-confidence circRNAs. The availability of a recent dataset of transcriptomes encompassing 11 tissues, 4 developmental stages, and 2 genders motivated us to explore the landscape of circRNAs in the organism in this context. In order to understand the difference among different pipelines, we employed five different combinations of tools to identify circular RNAs from the dataset. We compared the results of the different combination of tools/pipelines with respect to alignment, total number of circRNAs identified and read-coverage. In addition, we identified tissue-specific, development-stage specific and gender-specific circRNAs and further independently validated 16 circRNA junctions out of 24 selected candidates in 5 tissue samples and estimated the quantitative expression of five circRNA candidates using real-time polymerase chain reaction and our analysis suggests three candidates as tissue-enriched. This study is one of the most comprehensive studies which provides a map of circRNAs transcriptome as well as to understand the difference among different computational pipelines in rat.

Construction and comprehensive analysis of a ceRNA network to reveal potential prognostic biomarkers for lung adenocarcinoma

Background

More and more studies have proven that circular RNAs (circRNAs) play vital roles in cancer development via sponging miRNAs. However, the expression pattern of competing endogenous RNA (ceRNA) in lung adenocarcinoma (LUAD) remains largely unclear. The current study explored functional roles and the regulatory mechanisms of circRNA as ceRNAs in LUAD and their potential impact on LUAD patient prognosis.

Methods

In this study, we systematically screened differential expression circRNAs (DEcircRNAs), miRNAs (DEmiRNAs) and mRNAs (DEGs) associated with LUAD. Then, DEcircRNAs, DEmiRNAs and DEGs were selected to construct a circRNA–miRNA–mRNA prognosis-related regulatory network based on interaction information from the ENCORI database. Subsequently, the gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis were performed on the genes in the network to predict the potential underlying mechanisms and functions of circRNAs in LUAD. In addition, Kaplan–Meier survival analysis was performed to evaluate clinical outcomes of LUAD patients, and drug sensitivity analysis was used to screen potential biomarkers for drug treatment of patients with LUAD.

Results

As a result, 10 circRNAs were aberrantly expressed in LUAD tissues. The ceRNA network was built, which included 3 DEcircRNAs, 6 DEmiRNAs and 157 DEGs. The DEGs in the ceRNA network of hsa_circ_0049271 enriched in biological processes of cell proliferation and the Jak-STAT signaling pathway. We also detected 7 mRNAs in the ceRNA network of hsa_circ_0049271 that were significantly associated with the overall survival of LUAD patients (P < 0.05). Importantly, four genes (PDGFB, CCND2, CTF1, IL7R) identified were strongly associated with STAT3 activation and drugs sensitivity in GDSC.

Conclusions

In summary, a ceRNA network of hsa_circ_0049271 was successfully constructed, which including one circRNA, two miRNAs, and seven mRNAs. Seven mRNAs (PDGFB, TNFRSF19, CCND2, CTF1, IL11RA, IL7R and MAOA) were remarkably associated with the prognosis of LUAD patients. Among seven mRNA species, four genes (PDGFB, CCND2, CTF1, and IL7R) could be considered as drug targets in LUAD. Our research will provide new insights into the prognosis-related ceRNA network in LUAD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-021-08462-8.

Circular RNAs in Lung Cancer: Recent Advances and Future Perspectives

Globally, lung cancer is the most commonly diagnosed cancer and carries with it the greatest mortality rate, with 5-year survival rates varying from 4–17% depending on stage and geographical differences. For decades, researchers have studied disease mechanisms, occurrence rates and disease development, however, the mechanisms underlying disease progression are not yet fully elucidated, thus an increased understanding of disease pathogenesis is key to developing new strategies towards specific disease diagnoses and targeted treatments. Circular RNAs (circRNAs) are a class of non-coding RNA widely expressed in eukaryotic cells, and participate in various biological processes implicated in human disease. Recent studies have indicated that circRNAs both positively and negatively regulate lung cancer cell proliferation, migration, invasion and apoptosis. Additionally, circRNAs could be promising biomarkers and targets for lung cancer therapies. This review systematically highlights recent advances in circRNA regulatory roles in lung cancer, and sheds light on their use as potential biomarkers and treatment targets for this disease.

CircRNAs: role in human diseases and potential use as biomarkers

Circular RNAs (circRNAs) are a class of endogenous RNAs characterized by a covalent loop structure. In comparison to other types of RNAs, the abundance of circRNAs is relatively low but due to the circular configuration, their stability is very high. In addition, circRNAs display high degree of tissue specificity. The sponging activity of circRNAs toward microRNAs is the best-described mode of action of circRNAs. However, the ability of circRNAs to bind with specific proteins, as well as to encode short proteins, propose alternative functions. This review introduces the biogenesis of circRNAs and summarizes the roles played by circRNAs in human diseases. These include examples of their functional roles in several organ-specific cancers, such as head and neck and breast and lung cancers. In addition, we review potential functions of circRNAs in diabetes, cardiovascular, and neurodegenerative diseases. Recently, a growing number of studies have demonstrated involvement of circRNAs in a wide spectrum of signaling molecular pathways, but at the same time many different and controversial views on circRNAs role and function are emerging. We conclude by offering cellular homeostasis generated by networks comprising circular RNAs, other non-coding RNAs and RNA-binding proteins. Accordingly, it is predictable that circRNAs, due to their highly stable nature and remarkable tissue specificity, will emerge as reliable biomarkers of disease course and treatment efficacy.

CircKRT1 drives tumor progression and immune evasion in oral squamous cell carcinoma by sponging miR-495-3p to regulate PDL1 expression

Regulatory functions of circRNAs by targeting the micro RNA (miRNA)/mRNA axis have been increasingly found in oral squamous cell carcinoma (OSCC). CircRNA keratin 1 (CircKRT1) and miR-495-3p were dysregulated in OSCC. Programmed death ligand 1 (PDL1) was an important immunotherapeutic molecule in OSCC. Our objective was to explore whether circKRT1 could regulate cancer progression and immune evasion in OSCC by affecting the miR-495-3p/PDL1 axis. RNA expression was examined by quantitative real-time polymerase chain reaction. All protein levels were detected by western blot. OSCC cell growth was assessed by CCK-8 and colony formation assays. Cell migratory and invasive abilities were evaluated by transwell assay. CD8⁺ T-cell cytotoxicity was determined via lactate dehydrogenase assay. CD8⁺ T-cell percentage and apoptosis were analyzed by flow cytometry. Target screening was performed by Veen Diagram and RNA pull-down assay. Target binding was verified using dual-luciferase reporter and RNA immunoprecipitation assays. A xenograft in mice was conducted for in vivo experiment. CircKRT1 and PDL1 were highly expressed in OSCC tissues and cells. CircKRT1 knockdown repressed OSCC cell growth, migration, invasion, epithelial-mesenchymal transition, and CD8⁺ T-cell apoptosis, but enhanced CD8⁺ T cytotoxicity and percentage. The inhibitory effects of circKRT1 downregulation on OSCC progression and immune evasion were related to PDL1 expression inhibition. CircKRT1 sponged miR-495-3p and miR-495-3p targeted PDL1. OSCC progression and immune evasion were regulated by circKRT1 via the miR-495-3p/PDL1 axis. CircKRT1 also facilitated OSCC progression in vivo by regulating miR-495-3p and PDL1. This study clarified that circKRT1 worked as a miR-495-3p sponge to regulate PDL1, consequently affecting cancer progression and immune evasion in OSCC.

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.

CircRNAs as promising biomarker in diagnostic and prognostic of lung cancer: An updated meta-analysis

BACKGROUND

As a class of endogenous non-coding RNAs with closed-loop structure, circular RNAs (circRNAs) are receiving more and more attention. CircRNAs have been reported to be widely expressed in various human cancers and are implicated in tumorigenesis and progression. The present study aimed to systematically evaluate the clinicopathological, diagnostic and prognostic values of circRNAs in lung cancer.

METHODS

We searched literature from PubMed, Web of science, Cochrane Library, EMBASE and Ovid online databases up to May 29, 2020. Statistical analyses were undertaken based on Stata 11.0, Meta-DiSc 1.4, and RevMan 5.3 software.

RESULTS

Finally, a total of 63 eligible articles were included in our meta-analysis, including 18 studies for diagnosis, 22 studies for prognosis and 57 studies for clinicopathological features. In terms of diagnostic values, circRNAs could discriminate between lung cancer patients and the normal individuals with a relatively high pooled area under the curve (AUC) of 0.83 (95%CI, 0.80-0.86). For the prognostic values, we found that elevated expression of oncogenic circRNAs could predict poor survival outcomes based on multivariate analysis (HR = 2.430, 95%CI = 2.003-2.948, P < 0.001 for OS; HR = 2.228, 95%CI = 1.289-3.853, P = 0.004 for DFS) while tumor-suppressor circRNAs was correlated with better OS in univariate analysis (HR = 0.627, 95%CI = 0.519-0.757, P < 0.001). The pooled results suggested that elevated expression of carcinogenic circRNAs was associated with tumor size (OR = 1.676, 95%CI = 1.209-2.323, P = 0.002), smoking statue (OR = 1.260, 95%CI = 1.062-1.494, P = 0.008), TNM stage (OR = 2.345, 95%CI = 1.617-3.399, P < 0.001), differentiation grade (OR = 1.843, 95%CI = 1.228-2.765, P = 0.003), and lymphatic metastasis (OR = 2.097, 95%CI = 1.482-2.967, P < 0.001). Moreover, the expression of tumor-suppressor circRNAs was related to the improved clinicopathological features (lymphatic metastasis: OR = 0.536, 95%CI = 0.311-0.926, P = 0.025).

CONCLUSIONS

Our meta-analysis demonstrated that circRNAs could be used as feasible and important biomarkers for diagnosis, prognosis and clinicopathological features in lung cancer.

Circular RNA Signature in Lung Adenocarcinoma: A MiOncoCirc Database-Based Study and Literature Review

Circular RNAs (circRNAs) are a class of endogenous non-coding RNAs (ncRNAs) with a structure of covalently closed continuous loops, which can regulate gene expression by acting as a microRNA sponge or through other mechanisms. Recent studies have identified that the expression of candidate circRNAs are dysregulated in various tumors and hence are considered as promising diagnostic or therapeutic targets across cancer types. However, the expression and function of circRNAs in lung adenocarcinoma (LUAD) remains unclear. In this article, we investigated the expression of circRNAs in LUAD via MiOncoCirc, which is the first and comprehensive database characterizing circRNAs across >2,000 cancer samples using an exome capture RNA sequencing. We identified seven abnormally expressed circRNAs in LUAD, including circCDR1-AS, circHIPK3, circFNDC3B, circPCMTD1, circRHOBTB3, circFAM13B, and circMAN1A2, as well as conducted a literature review about the function and features of these circRNAs. Previous studies have demonstrated that circCDR1-AS, circMAN1A2, and circHIPK3 were upregulated and significantly correlated with a poor survival, or promoted the tumor progression in lung cancer, whereas other circRNAs have not been fully explored. Besides, we reviewed all the publications regarding circRNAs and LUAD, and noticed that the dysregulation of these circRNAs impacts the development of LUAD through a variety of regulatory mechanisms. In conclusion, the underlying mechanisms of aberrant expression and functions of circRNAs in LUAD are worthy of being further investigated.

Circular RNA hsa_circ_0030018 acts as a sponge of miR-599 to aggravate esophageal carcinoma progression by regulating ENAH expression

Esophageal carcinoma (EC) bears one of the most rapid-growing incidences in cancers, which also has the highest mortality rate worldwide. Multiple studies have authenticated that circular RNAs (circRNAs) significantly work on the progression of cancers. circRNA hsa_circ_0030018 was also verified to exert functions on the development of glioma previously. Nevertheless, the biological function of hsa_circ_0030018 in EC has not been well elucidated yet. In the present study, the results displayed the expression of hsa_circ_0030018 was dramatically increased in EC cells. Inhibition of has_circ_0030018 suppressed cell proliferation, migration, and epithelial-mesenchymal transition (EMT) process in EC. Based on molecular mechanism assays, has_circ_0030018 served as a sponge of miR-599. Enabled homolog (ENAH), which exhibited high expression in EC cells, was confirmed to be a downstream target gene of miR-599. Additionally, has_circ_0030018 positively regulated ENAH expression while miR-599 negatively regulated ENAH expression. Finally, by employing rescue assays, ENAH deficiency partially counteracted the promoting function of miR-599 silence on cell proliferation, migration, and EMT process in EC cotransfected with sh- has_circ_0030018#1 cells. In conclusion, hsa_circ_0030018 acted as a sponge of miR-599 to aggravate EC progression by regulating ENAH expression. Therefore, hsa_circ_0030018 might serve as a promising biomarker and therapeutic target for EC.

Circular RNAs: Promising Molecular Biomarkers of Human Aging-Related Diseases via Functioning as an miRNA Sponge

Circular RNAs (circRNAs) are a new class of noncoding single-stranded RNAs that differ from linear microRNAs (miRNAs), since they form covalently closed loop structures without free 3′ poly(A) tails or 5′ caps. circRNAs are the competitive endogenous RNAs (ceRNAs) by binding to miRNA through miRNA response elements (MREs) (i.e., “miRNA sponge”), thereby reducing the quantity of miRNA available to target mRNA, subsequently promoting mRNA stability or protein expression, which involves the initiation and progress of human diseases. Owing to these features of abundance, stability, conservative property, and tissue and stage specificity, widely distributing in the extracellular space and in various bodily fluids, circRNAs can be considered as potential biomarkers for various diseases. Here, we reviewed the promising circRNAs being disease biomarkers, focused on their regulatory function by acting as miRNA sponges, and described their roles in cancer, cardiovascular or neurodegenerative diseases, osteoarthritis, rheumatoid arthritis, diabetes, and other human aging-related diseases, which provide a new direction for pathogenesis, diagnosis, and treatment of human aging-related diseases.

circRNA-miRNA-mRNA regulatory network in human lung cancer: an update

Circular RNAs, as hopeful diagnosis markers and therapeutic molecules, have been studied, probed and applied into several diseases, such as cardiovascular diseases, systemic lupus erythematosus, leukemia, pulmonary tuberculosis, and cancer especially. Recently, mounting evidence has supported that circRNAs play a key role in the tumorigenesis, progress, invasion and metastasis in lung cancer. Its special structure—3′–5′ covalent loop—allow it to execute several special functions in both normal eukaryotic cells and cancer cells. Our review summaries the latest studies on characteristics and biogenesis of circRNAs, and highlight the regulatory functions about miRNA sponge of lung-cancer-related circRNAs. In addition, the interaction of the circRNA-miRNA-mRNA regulatory network will also be elaborated in detail in this review. Therefore, this review can provide a new idea or strategy for further development and application in clinical setting in terms of early-diagnosis and better treatment.

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.

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.

CircTIMELESS regulates the proliferation and invasion of lung squamous cell carcinoma cells via the miR-136-5p/ROCK1 axis

Numerous studies demonstrate that circular RNAs (circRNAs) are critical regulators of the occurrence and progression of tumors. However, research on the involvement of circRNAs in lung squamous cell carcinoma (LUSC) is limited. In our study, circTIMELESS (also named hsa_circ_0000408 in the Human circRNA Database) was upregulated in both LUSC tissues and LUSC cells, and circTIMELESS expression was positively associated with the TNM stage. Moreover, circTIMELESS silencing markedly suppressed invasion in vitro and disrupted proliferation in vitro as well as in vivo. Additional investigations have shown that circTIMELESS functions as a miR-136-5p "sponge" and regulates miR-136-5p expression. Furthermore, the impact of miR-136-5p upregulation was consistent with the results of circTIMELESS silencing, both of which inhibited the proliferation and invasion of LUSC cells. Additional results showed that Rho-associated coiled-coil containing protein kinase 1 (ROCK1) is targeted by miR-136-5p. The results of recovery experiments showed that ROCK1 overexpression partly rescued the impact of circTIMELESS silencing and miR-136-5p upregulation on proliferation and invasion. Consequently, our findings confirmed that circTIMELESS exists in LUSC and acts as a tumor promoter through the miR-136-5p/ROCK1 axis. Based on these findings, circTIMELESS may be potentially utilized as a therapeutic target for LUSC.

Circular RNA in Lung Cancer Research: Biogenesis, Functions, and Roles

Lung cancer is one of the most common and deadly malignancies worldwide, in spite of advances in targeted therapy in recent years. An effective strategy for lung cancer prevention remains a major problem. Advances in next-generation sequencing have helped in understanding the RNA and identifying novel circular RNAs (circRNAs) that may have a broad impact on the early diagnosis and treatment of lung cancer. The circRNAs, exhibiting spatiotemporal-specific expression, are stable and conserved and present diverse biological functions in the normal and diseased states, including cancer. In this review, we summarize the recent advances in elucidating the functional role of circRNAs in lung cancer pathogenesis and discuss their potential mechanisms.

Non-coding RNA and lung cancer progression

Lung cancer (LC) is a major killer disease globally. This situation is further supported by yearly increase in new LC cases and its poor 5-year survival which is less than 15%. Although a large percentage of LC cases have been attributed to smoking, a considerable amount of nonsmokers also develops this disease, thereby suggesting a genetic and/or epigenetic undertone to LC development. Several growth-related genes such as epidermal growth factor receptor (EGFR) and vascular endothelial growth factor (VEGF) as well as tumor suppressor genes such as p53 have been implicated in LC pathogenesis and progression. Likewise, the genome only contains approximately 1% of coding regions. Hence, noncoding portion of the genome such as noncoding RNAs (ncRNAs) has been studied and discovered to play a cogent role in LC pathogenesis. More precisely, microRNAs (miRNAs) and long ncRNAs (lncRNAs) have been studied for decades. Posttranscriptional gene modulation function of miRNAs is well established and characterized. Likewise, the antagonizing interaction between lncRNAs and miRNAs had also been proven to further control gene expression during healthy and disease conditions like LC. More recently, renewed attention toward circular RNAs [circular RNAs (circRNAs)] study showed that circRNAs can also sponge miRNAs to modulate gene expressions too. Hence, miRNAs, lncRNAs, and circRNAs seem to function within a circuit to optimally determine which gene is needed to be upregulated or downregulated in biological system. Therefore, this review will discuss important ncRNAs, namely miRNA, lncRNA, and circRNA in LC progression. Paracrine effect of exosomal ncRNA will be also reviewed. In addition, the prospect of these ncRNAs in enhancing better LC treatment will be highlighted as well.

hsa_circ_0062389 promotes the progression of non-small cell lung cancer by sponging miR-103a-3p to mediate CCNE1 expression

Recently, increasing evidence showed that circular RNAs (circRNAs) play critical roles in tumor progression. However, the roles of hsa_circ_0062389 in non-small cell lung cancer (NSCLC) development remain unclear. In the present study, hsa_circ_0062389 expression was significantly increased in NSCLC tissues and cell lines. High hsa_circ_0062389 expression was associated with advanced TNM stage and lymph-node metastasis. Function assays showed that hsa_circ_0062389 suppression reduced NSCLC cells proliferation and arrested cell cycle in G0/G1 phase. In mechanism, hsa_circ_0062389 directly interacted with miR-103a-3p in NSCLC, and CCNE1 acted as a target of miR-103a-3p. Furthermore, rescue assays showed that miR-103a-3p suppression or CCNE1 overexpression abolished the effects of hsa_circ_0062389 suppression on lung cancer cells progression. Therefore, our results showed that the hsa_circ_0062389/miR-103a-3p/CCNE1 axis might contribute to the tumorigenesis of NSCLC, which provided a new strategy for cancer treatment.

Circular RNAs: pivotal molecular regulators and novel diagnostic and prognostic biomarkers in non-small cell lung cancer

PURPOSE

Circular RNAs (circRNAs), a large class of non-coding RNAs with covalently closed-loop structures, are abundant, stable, conserved, and have tissue and developmental-stage specificities. The biological functions of circRNAs are varied. Moreover, circRNAs participate in various pathological processes, especially in multiple cancers. Lung cancer is the most frequent malignant tumor worldwide. Many studies have suggested that circRNAs are pivotal in non-small cell lung cancer. This article aims to provide a retrospective review of the latest research on the functions of circRNAs in non-small cell lung cancer. In particular, we focus our discussion on the role of circRNAs in cell-cycle regulation and the epithelial-mesenchymal transition, and also discuss the known regulatory molecular mechanisms of circRNAs in non-small cell lung cancer.

METHODS

We reviewed the literature on circRNAs and non-small cell lung cancer from PubMed databases. Specifically, we focused on the roles and mechanisms of circRNAs in regulating the cell cycle and the epithelial-mesenchymal transition.

RESULTS

Dysregulation of circRNAs is closely correlated with proliferation, migration, and invasion of non-small cell lung cancer, especially in terms of modulating cell-cycle regulation and the epithelial-mesenchymal transition.

CONCLUSION

Taken together, circRNAs have potential as biomarkers for the diagnosis, prognosis, and treatment of non-small cell lung cancer.

Circular RNA expression profile in peripheral whole blood of lung adenocarcinoma by high: Throughput sequencing

BACKGROUND

Lung adenocarcinoma (LA) is a most common form of non-small cell lung cancer (NSCLC). To date, there are still no effective early diagnosis methods for patients to be cured in time. Noncoding RNA plays an important role in oncogenesis and tumor development. The expression profile of circular RNA (circRNA) in peripheral whole blood (PWB) of LA has not been systematically investigated. In this study, we identified the differentially expressed (DE) circRNAs in PWB of LA by high-throughput sequencing.

METHODS

Five paired LA and normal participants PWB samples were chosen to investigate the expression profile of circRNAs by high-throughput sequencing. Twenty LA and 10 normal controls PWB samples were subjected to reverse-transcription polymerase chain reaction (RT-PCR) for validation of circRNAs expression profile. Gene Ontology (GO) functional analysis, Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis, and circRNA-miRNA network analysis was also performed to predict the function of circRNAs in PWB.

RESULTS

A total of 10566 circRNAs were identified and annotated, most of the circRNAs were exonic (78.14%). Statistical analysis revealed 4390 DE circRNAs, in which were 3009 upregulated circRNAs and1381downregulated circRNAs in LA. RT-PCR results showed that circRNA expression in LA was higher than that in controls. GO functional analysis, KEGG pathway analysis, and circRNA-miRNA network analysis all showed that circRNAs correlated with tumor development and progression to a certain degree. The current study is the first to systematically characterize and annotate circRNA expression in PWB of LA. Some host genes of the DE circRNAs were involved in tumor signaling pathway and had complicated correlations with tumor related miRNAs, indicating that circRNAs might involve in development and progression of LA.

CONCLUSIONS

Our study revealed that circRNAs were abnormally expressed in PWB of LA, which might offer potential targets for the early diagnosis of the disease and new genetic insights into LA.

Circular RNAs in gynecological disease: promising biomarkers and diagnostic targets

Circular RNAs (circRNAs) are a category of RNA molecules with covalently closed circles lacking both a 5′ cap and a 3′ tail. In recent years, circRNAs have attracted much attention and become a research hotspot of the RNA field following miRNAs and lncRNAs. CircRNAs exhibit tissue specificity, structural stability, and evolutionary conservation. Although the biological effects of circRNAs are still underestimated, many studies have shown that circRNAs have functions including regulation of transcription, translation into proteins and miRNA sponges. In this review, we briefly described the biogenesis and function of circRNAs and present circular transcripts in gynecological disease.

Circular RNAs and RNA Splice Variants as Biomarkers for Prognosis and Therapeutic Response in the Liquid Biopsies of Lung Cancer Patients

Lung cancer, including non-small cell lung carcinoma (NSCLC), is the most frequently diagnosed cancer. It is also the leading cause of cancer-related mortality worldwide because of its late diagnosis and its resistance to therapies. Therefore, the identification of biomarkers for early diagnosis, prognosis, and monitoring of therapeutic response is urgently needed. Liquid biopsies, especially blood, are considered as promising tools to detect and quantify circulating cancer biomarkers. Cell-free circulating tumor DNA has been extensively studied. Recently, the possibility to detect and quantify RNAs in tumor biopsies, notably circulating cell-free RNAs, has gained great attention. RNA alternative splicing contributes to the proteome diversity through the biogenesis of several mRNA splice variants from the same pre-mRNA. Circular RNA (circRNA) is a new class of RNAs resulting from pre-mRNA back splicing. Owing to the development of high-throughput transcriptomic analyses, numerous RNA splice variants and, more recently, circRNAs have been identified and found to be differentially expressed in tumor patients compared to healthy controls. The contribution of some of these RNA splice variants and circRNAs to tumor progression, dissemination, or drug response has been clearly demonstrated in preclinical models. In this review, we discuss the potential of circRNAs and mRNA splice variants as candidate biomarkers for the prognosis and the therapeutic response of NSCLC in liquid biopsies.

Hsa_circ_0000467 promotes cancer progression and serves as a diagnostic and prognostic biomarker for gastric cancer

BACKGROUND

Emerging evidence indicates that dysregulation of circular RNAs (circRNAs) is implicated in the development of malignancies. However, the diagnostic value and functional role of circRNAs in gastric cancer (GC) remain largely elusive.

METHODS

Quantitative reverse transcription-polymerase chain reaction (qRT-PCR) was performed to detect the expression of hsa_circ_0000467 in tissues, cell lines, and plasma. A receiver operating characteristic (ROC) curve was constructed to estimate the diagnostic value of hsa_circ_0000467. The association between the expression level of hsa_circ_0000467 and the clinicopathological features was analyzed. Moreover, cell functional assays were performed.

RESULTS

Hsa_circ_0000467 was significantly upregulated in GC tissue compared to adjacent nontumor tissue (n = 51, P < 0.05). Similar results were detected in the HGC-27, MGC-803, AGS, NUGC-3, GES-1 cell lines (n = 15, P < 0.001), and in the plasma samples from GC patients (n = 20, P < 0.05). The area under the ROC curve of hsa_circ_0000467 was 0.790, which is superior to commonly used biomarkers including CEA and CA-724. We found that the expression levels of hsa_circ_0000467 in the same patient were significantly lower after surgery (n = 20, P < 0.05). Moreover, the hsa_circ_0000467 expression level is closely associated with TNM stage. Additionally, Cox multivariate analysis showed that hsa_circ_0000467 is a novel independent prognostic factor. Furthermore, in vitro experiments demonstrated that knockdown of hsa_circ_0000467 markedly inhibited the proliferation, migration, and invasion of GC cells. Moreover, hsa_circ_0000467 silencing increased tumor apoptosis in vitro.

CONCLUSION

Hsa_circ_0000467 can act as a novel noninvasive biomarker for the diagnosis and prognosis of GC and may be a potential therapeutic target for GC.

Insights into the biogenesis and potential functions of exonic circular RNA

Circular RNAs (circRNAs) exhibit unique properties due to their covalently closed nature. Models of circRNAs synthesis and function are emerging but much remains undefined about this surprisingly prevalent class of RNA. Here, we identified exonic circRNAs from human and mouse RNA-sequencing datasets, documenting multiple new examples. Addressing function, we found that many circRNAs co-sediment with ribosomes, indicative of their translation potential. By contrast, circRNAs with potential to act as microRNA sponges were scarce, with some support for a collective sponge function by groups of circRNAs. Addressing circRNA biogenesis, we delineated several features commonly associated with circRNA occurrence. CircRNA-producing genes tend to be longer and to contain more exons than average. Back-splice acceptor exons are strongly enriched at ordinal position 2 within genes, and circRNAs typically have a short exon span with two exons being the most prevalent. The flanking introns either side of circRNA loci are exceptionally long. Of note also, single-exon circRNAs derive from unusually long exons while multi-exon circRNAs are mostly generated from exons of regular length. These findings independently validate and extend similar observations made in a number of prior studies. Furthermore, we analysed high-resolution RNA polymerase II occupancy data from two separate human cell lines to reveal distinctive transcription dynamics at circRNA-producing genes. Specifically, RNA polymerase II traverses the introns of these genes at above average speed concomitant with an accentuated slow-down at exons. Collectively, these features indicate how a perturbed balance between transcription and linear splicing creates important preconditions for circRNA production. We speculate that these preconditions need to be in place so that looping interactions between flanking introns can promote back-splicing to raise circRNA production to appreciable levels.

Circular RNA circASS1 is downregulated in breast cancer cells MDA-MB-231 and suppressed invasion and migration

Aim: The study aimed to investigate the role of circular RNA circASS1 in breast cancer cells. Materials & methods: Circular RNAs microarray expression profile were analyzed in MCF-7, MDA-MB-231, and qRT-PCR and western blotting were used to quantify expression of circASS1 and its parental gene ASS1. Wound healing, migration and invasion assay were performed. Luciferase assay system was used to detect harbored miRNA. Results: CircASS1 in MDA-MB-231 is downregulated comparing to MCF-7, and overexpression of circASS1 could suppress invasion and migration. While silence, it could promote invasion and migration. MiR-4443 functioning as a tumor promoter gene could be captured by circASS1. ASS1 is upregulated in loss-of-function experiments, while downregulated in gain-of-function experiments. Conclusion: CircASS1 suppresses invasion and migration capacity of breast cancer cells and harbored miR-4443.

Mechanisms of action of vitamin D in colon cancer

Colorectal cancer (CRC) is the neoplasia that is most frequently associated with vitamin D deficiency in epidemiological and observational studies in terms of incidence and mortality. Many mechanistic studies show that the active vitamin D metabolite (1α,25-dihydroxyvitamin D₃ or calcitriol) inhibits proliferation and promotes epithelial differentiation of human colon carcinoma cell lines that express vitamin D receptor (VDR) via the regulation of a high number of genes. A key action underlining this effect is the multilevel inhibition of the Wnt/β-catenin signaling pathway, whose abnormal activation in colon epithelial cells initiates and promotes CRC. Recently, our group has shown that calcitriol modulates gene expression and inhibits protumoral properties of patient-derived colon cancer-associated fibroblasts (CAFs). Accordingly, high VDR expression in tumor stromal fibroblasts is associated with longer survival of CRC patients. Moreover, many types of immune cells express VDR and are regulated by calcitriol, which probably contributes to its action against CRC. Given the role attributed to the intestinal microbiota in CRC and the finding that it is altered by vitamin D deficiency, an indirect antitumoral effect of calcitriol is also plausible at this level. In summary, calcitriol has an array of potential protective effects against CRC by acting on carcinoma cells, CAFs, immune cells and probably also the gut microbiota.

CircRNAs and cancer: Biomarkers and master regulators

Circular RNAs (circRNAs) are a novel class of regulatory RNAs that despite being relatively abundant have only recently begun to be explored. There are many thousands of genes that appear capable of producing circRNAs, however the function of all but a handful remain to be determined. What is emerging about these highly conserved molecules is that they play important roles in biology and cancer biology in particular. The most explored function of circRNAs is as master regulators of gene expression that act to sequester or ´sponge´ other gene expression regulators, in particular miRNAs. They have also been demonstrated to function via direct modulation of transcription, and by interfering with splicing mechanisms. Although generally expressed in low abundance when compared to their linear counterparts, they are often expressed in a tissue- and developmental stage- specific manner. Coupled with their remarkable resistance to RNAse activity due to a covalent closed cyclic structure, circRNAs show great promise as novel biomarkers of cancer and other diseases. In this review we consider the current state of knowledge regarding these molecules, their synthesis, function, and association with cancer. We will also review some of the challenges that remain to be resolved if this emerging class of RNAs are really to become useful in the clinic.

Research progress of circular RNAs in lung cancer

Lung cancer is one of the most common cancers and the leading cause of cancer-related death worldwide. Despite encouraging results achieved with targeted therapy in recent years, the early diagnosis and treatment of lung cancer remains a major problem. Circular RNA (circRNA), a type of RNA with covalently closed continuous loop structures, has structural stability and certain tissue specificity. Recent studies have found that circRNAs have an important role in tumor development and are expected to be revealed as new targets for tumor prediction and treatment. Research on the biological functions and regulation mechanisms of circRNAs in lung cancer is in its infancy but is gathering momentum. In this review, we discuss the properties, biogenesis, biological function, and research progress of circRNAs in lung cancer to provide a theoretical foundation and new directions for studies on circRNAs in lung cancer.

Progress in research on the role of circular RNAs in lung cancer

Background

Circular RNA (circRNA), as a covalently closed circular RNA molecule, is widely present, which is recognized as a competing endogenous RNA. A large number of differentially expressed circRNAs have been identified and are recognized as potential biomarkers for the diagnosis of tumors.

Main body

CircRNAs play an important role in the regulation of cell signaling pathways. The main biological functions of circRNAs include acting as miRNA sponges, regulating the transcription of the parental genes, and acting as adapters to regulate the interactions between proteins and encoding proteins. Compared with normal tissues, there are differentially expressed circRNAs in lung cancer tissue, and the expression levels of circRNAs are correlated with clinicopathological features of lung cancer. Their roles in pathway regulation are described, and the diagnostic and prognostic values are further evaluated.

Conclusion

In lung cancer, circRNAs participate in the proliferation, migration, and invasion, acting as a competitive endogenous RNA. Differentially expressed circRNAs may serve as non-invasive diagnostic markers for lung cancers. Further investigation of the roles of circRNAs in the pathogenesis and regulatory pathways is conducive to the development of novel approaches for the diagnosis and accurate treatment of lung cancers.

Circ-U2AF1 promotes human glioma via derepressing neuro-oncological ventral antigen 2 by sponging hsa-miR-7-5p

The prognosis for human glioma, a malignant tumor of the central nervous system, is poor due to its rapid growth, genetic heterogeneity, and inadequate understanding of its underlying molecular mechanisms. Circular RNAs composed of exonic sequences, represent an understudied form of noncoding RNAs (ncRNAs) that was discovered more than a decade ago, function as microRNA sponges. We aimed to assess the relationship between circ-U2AF1 (CircRNA ID: hsa_circ_0061868) and hsa-mir-7-5p and examine their effects on proliferation, apoptosis, and the metastatic phenotype of glioma cells regulated by neuro-oncological ventral antigen 2 (NOVA2). We found that the expression levels of circ-U2AF1 and NOVA2 were upregulated, while hsa-miR-7-5p was downregulated in human glioma tissues and glioma cell lines. Our data and bioinformatic analysis indicated the association of these molecules with glioma grade, a positive correlation between circ-U2AF1 and NOVA2 expression levels and a negative correlation of hsa-miR-7-5p with both circ-U2AF1 and NOVA2, respectively. In addition, silencing of circ-U2AF1 expression resulted in increased hsa-miR-7-5p expression and decreased NOVA2 expression both in vitro and in vivo. Luciferase assay confirmed hsa-miR-7-5p as a direct target of circ-U2AF1 and NOVA2 as a direct target of hsa-miR-7-5p. Functionally, silencing of circ-U2AF1 inhibits glioma development by repressing NOVA2 via upregulating hsa-miR-7-5p both in vitro and in vivo. Thus, we assumed that circ-U2AF1 promotes glioma malignancy via derepressing NOVA2 by sponging hsa-miR-7-5p. Taken together, we suggest that circ-U2AF1 can be a prognostic biomarker and the circ-U2AF1/hsa-miR-7-5p/NOVA2 regulatory pathway may be a novel therapeutic target for treating gliomas.

Emerging roles of circRNA_NEK6 targeting miR-370-3p in the proliferation and invasion of thyroid cancer via Wnt signaling pathway

OBJECTIVE

To identify the significantly altered circRNAs and mRNAs in thyroid cancer, investigate their target miRNAs and determine their biological functions.

METHODS

The differentially expressed circRNAs, mRNAs and pathways in thyroid cancer were identified by microarray analysis and gene set enrichment analysis (GSEA). The correlative circRNAs and mRNAs were found out through Pearson correlative analysis. The common target miRNAs of circNEK6 and FZD8 related to thyroid cancer was screened out through Targetscan, miRanda and HMDD analysis. The mRNA and protein expressions in thyroid cancer tissues and cells were detected by qRT-PCR and western blot. CircRNA was confirmed by the RNase R digestion and nucleic acid electrophoresis. The target relationships were verified by the dual luciferase reporter assay. Cell viability, invasion and apoptosis were determined by MTT assay, Transwell assay and flow cytometry, respectively.

RESULTS

CircNEK6 and FZD8 were significantly up-regulated in thyroid cancer, with strong correlations. The Wnt signaling pathway was activated in thyroid cancer. MiR-370-3p was the common target miRNA of circNEK6 and FZD8, and it was down-regulated in thyroid cancer. Overexpression of circNEK6 and FZD8 could promote the growth and invasion of thyroid cancer cells, while up-regulation of miR-370-3p could suppress thyroid cancer progression and inhibit the Wnt signaling pathway. MiR-370-3p's effect on thyroid cancer cells could be rescued by circNEK6 or FZD8.

CONCLUSION

CircNEK6 promoted the progression of thyroid cancer through up-regulating FZD8 and activating Wnt signaling pathway by targeting miR-370-3p.