Circular RNAs in cell differentiation and development

Non-canonical mRNA translation initiation in cell stress and cancer

The now well described canonical mRNA translation initiation mechanism of m7G 'cap' recognition by cap-binding protein eIF4E and assembly of the canonical pre-initiation complex consisting of scaffolding protein eIF4G and RNA helicase eIF4A has historically been thought to describe all cellular mRNA translation. However, the past decade has seen the discovery of alternative mechanisms to canonical eIF4E mediated mRNA translation initiation. Studies have shown that non-canonical alternate mechanisms of cellular mRNA translation initiation, whether cap-dependent or independent, serve to provide selective translation of mRNAs under cell physiological and pathological stress conditions. These conditions typically involve the global downregulation of canonical eIF4E1/cap-mediated mRNA translation, and selective translational reprogramming of the cell proteome, as occurs in tumor development and malignant progression. Cancer cells must be able to maintain physiological plasticity to acquire a migratory phenotype, invade tissues, metastasize, survive and adapt to severe microenvironmental stress conditions that involve inhibition of canonical mRNA translation initiation. In this review we describe the emerging, important role of non-canonical, alternate mechanisms of mRNA translation initiation in cancer, particularly in adaptation to stresses and the phenotypic cell fate changes involved in malignant progression and metastasis. These alternate translation initiation mechanisms provide new targets for oncology therapeutics development.

Current state of research on copper complexes in the treatment of breast cancer

Breast cancer, a malignancy originating from the epithelium or ductal epithelium of the breast, is not only highly prevalent in women but is also the leading cause of cancer-related deaths in women worldwide. Research has indicated that breast cancer incidence is increasing in younger women, prompting significant interest from scientists actively researching breast cancer treatment. Copper is highly accumulated in breast cancer cells, leading to the development of copper complexes that cause immunogenic cell death, apoptosis, oxidative stress, redox-mediated cell death, and autophagy by regulating the expression of key cell death proteins or assisting in the onset of cell death. However, they have not yet been applied to clinical therapy due to their solubility in physiological buffers and their different and unpredictable mechanisms of action. Herein, we review existing relevant studies, summarize the detailed mechanisms by which they exert anti-breast cancer effects, and propose a potential mechanism by which copper complexes may exert antitumor effects by causing copper death in breast cancer cells. Since copper death in breast cancer is closely related to prognosis and immune infiltration, further copper complex research may provide an opportunity to mitigate the high incidence and mortality rates associated with breast cancer.

Circular RNAs in renal cell carcinoma: from mechanistic to clinical perspective

CircRNAs, a special type of noncoding RNAs characterized by their stable structure and unique abilities to form backsplicing loops, have recently attracted the interest of scientists. These RNAs are abundant throughout the body and play important roles such as microRNA sponges, templates for transcription, and regulation of protein translation and RNA-binding proteins. Renal cancer development is highly correlated with abnormal circRNA expression in vivo. CircRNAs are currently considered promising targets for novel therapeutic approaches as well as possible biomarkers for prognosis and diagnosis of various malignancies. Despite our growing understanding of circRNA, numerous questions remain unanswered. Here, we address the characteristics of circRNAs and their function, focusing in particular on their impact on drug resistance, metabolic processes, metastasis, cell growth, and programmed cell death in renal cancer. In addition, the application of circRNAs as prognostic and diagnostic biomarkers will be discussed.

hsa_circ_0000417 downregulation suppresses androgen receptor expression and apoptotic signals in human foreskin fibroblasts via sponging miR-6756-5p

BACKGROUND

Dysregulated apoptosis of penile mesenchymal cells during male urethragenesis has been previously demonstrated to underly hypospadiac urethral closure failure, and androgen receptor (AR) has been shown to play a central role in regulating penile mesenchyme cell proliferation and survival. However, the regulatory mechanisms upstream and downstream of AR remain poorly understood. Our clinical data and bioinformatics analysis previously indicated that hsa_circ_0000417, a circRNA significantly downregulated in hypospadias preputial specimens, may act as a ceRNA for AR via sequestering hsa_miR-6756-5p, and that the biological functions of hsa_circ_0000417 may significantly involve the PI3K/AKT pathway. In this study, we employed human foreskin fibroblasts (HFF-1) to experimentally validate this putative hsa_circ_0000417/miR-6756-5p/AR axis and its impact on penile mesenchymal cell proliferation and apoptosis.

METHOD AND RESULTS

We showed that hsa_circ_0000417 knockdown significantly promoted proliferation and suppressed apoptosis of HFF-1 cells. Mechanistically, hsa_circ_0000417 functioned as a molecular sponge for miR-6756-5p in HFF-1 cells and relieved the latter's translational repression on AR mRNA, leading to decreased AKT activation and increased expression of pro-apoptotic proteins BAX and cleaved-caspase 9. Conversely, elevated levels of miR-6756-5p resulted in diminished AR expression concomitant with enhanced AKT activation and HFF-1 cell proliferation.

CONCLUSIONS

Collectively, our data describe for the first time a circRNA-mediated post-transcriptional regulatory mechanism of AR and its functional consequences in penile mesenchymal cells in the context of hypospadias. These findings may contribute to advancing our current understanding of the roles of AR and mesenchymal cell fate decisions during penile morphogenesis.

The m6A reader YTHDC1 and the RNA helicase DDX5 control the production of rhabdomyosarcoma-enriched circRNAs

N6-Methyladenosine (m⁶A) is well-known for controlling different processes of linear RNA metabolism. Conversely, its role in the biogenesis and function of circular RNAs (circRNAs) is still poorly understood. Here, we characterize circRNA expression in the pathological context of rhabdomyosarcoma (RMS), observing a global increase when compared to wild-type myoblasts. For a set of circRNAs, such an increase is due to the raised expression of the m⁶A machinery, which we also find to control the proliferation activity of RMS cells. Furthermore, we identify the RNA helicase DDX5 as a mediator of the back-splicing reaction and as a co-factor of the m⁶A regulatory network. DDX5 and the m⁶A reader YTHDC1 are shown to interact and to promote the production of a common subset of circRNAs in RMS. In line with the observation that YTHDC1/DDX5 depletion reduces RMS proliferation, our results provide proteins and RNA candidates for the study of rhabdomyosarcoma tumorigenicity.

Hsa_circ_0000591 drives osteosarcoma glycolysis and progression by sequestering miR-194-5p and elevating HK2 expression

Osteosarcoma (OS) is the most common bone tumour with a high risk of metastatic progression and recurrence after treatment. Circular RNA hsa_circ_0000591 (circ_0000591) plays a compelling role in OS aggressiveness. However, the function and regulatory mechanism of circ_0000591 need to be further elucidated. As a subject of this study, a differential circRNA circ_0000591 was screened by circRNA microarray expression profiling (GSE96964). Expression changes of circ_0000591 were detected using real-time quantitative polymerase chain reaction (RT-qPCR). Effects of circ_0000591 silencing on OS cell viability, proliferation, colony formation, apoptosis, invasion, and glycolysis were determined via functional experiments. The mechanism by which circ_0000591 functions as a molecular sponge for miRNAs was predicted using bioinformatics analysis and validated using dual-luciferase reporter and RNA pull-down assays. Xenograft assay was done to validate the function of circ_0000591. Circ_0000591 was strongly expressed in OS samples and cells. Silencing of circ_0000591 lessened cell viability, repressed cell proliferation, invasion, glycolysis, and promoted cell apoptosis. Importantly, circ_0000591 regulated HK2 expression by serving as a miR-194-5p molecular sponge. MiR-194-5p silencing impaired circ_0000591 downregulation-mediated suppression of OS cell malignancy and glycolysis. HK2 overexpression weakened the inhibiting impacts of miR-194-5p on OS cell malignancy and glycolysis. Also, circ_0000591 silencing decreased xenograft tumour growth in vivo. Circ_0000591 drove OS glycolysis and growth by upregulating HK2 by sequestering miR-194-5p. The study highlighted the tumour-promoting function of circ_0000591 in OS.

Research progress on circular RNA vaccines

Owing to the success of linear mRNA coronavirus disease 2019 (COVID-19) vaccines, biopharmaceutical companies and research teams worldwide have attempted to develop more stable circular RNA (circRNA) vaccines and have achieved some preliminary results. This review aims to summarize key findings and important progress made in circRNA research, the in vivo metabolism and biological functions of circRNAs, and research progress and production process of circRNA vaccines. Further, considerations regarding the quality control of circRNA vaccines are highlighted herein, and the main challenges and problem-solving strategies in circRNA vaccine development and quality control are outlined to provide a reference for circRNA vaccine-related research.

Circular RNA, circular RARS, promotes aerobic glycolysis of non-small-cell lung cancer by binding with LDHA

PURPOSE

Accumulating evidence has highlighted the critical roles of circular RNAs (circRNAs) in non-small-cell lung cancer (NSCLC). This study aims to unveil the roles of circRARS (circular RARS) (hsa_circ_0001551) in NSCLC.

METHODS

Quantitative real-time PCR was used to determine the expression of circRARS in NSCLC tissues and cells. Kaplan-Meier analysis was used to determine the prognostic value of circRARS expression. CCK8, transwell, and wound healing assays were used to assess the proliferation, invasion, and migration abilities of NSCLC cells. RNA pull-down, cell fraction, glucose consumption, lactate production, and lactate dehydrogenase activity assays were conducted to explore the potential mechanisms of circRARS in NSCLC.

RESULTS

circRARS is upregulated in NSCLC tissues and positively correlated with smoking status, lymph node metastasis, and higher tumor stages. NSCLC patients with high expression of circRARS have poor overall survival. Functional assays demonstrated that circRARS accelerated the proliferation, invasion, and migration of NSCLC cells in vitro. The cell fraction suggested that circRARS mainly accumulated in cytoplasm and the RNA pull-down assay showed lactate dehydrogenase (LDHA) could bind with circRARS. Furthermore, circRARS positively regulates LDHA activity and LDHA expression at the transcription level. Moreover, downregulated circRARS decreases glucose consumption and lactate production and compromises aerobic glycolysis in NSCLC cells. Finally, rescue assays showed circRARS could promote NSCLC cell proliferation by regulating LDHA activity.

CONCLUSION

This study shows that circRARS can promote glycolysis and tumor progression in NSCLC by regulating LDHA.

Circ_0075960 targets the miR-202-5p/CTNND1 axis to promote the growth and migration of endometrial carcinoma cells via regulating Wnt/β-catenin signaling activity

BACKGROUND

Endometrial carcinoma (EC) is one of the most common malignant tumors of the female reproductive tract, involving multiple molecular alterations. Circular RNA (circRNA) dysregulation is frequently observed in EC tissues, suggesting the involvement of circRNA in EC development. We aimed to investigate the role of circ_0075960 in EC.

METHODS

Real-time quantitative polymerase chain reaction (RT-qPCR) and western blot assays were applied for expression analysis. CCK-8, EdU, colony formation, flow cytometry and wound healing assays were employed for functional analysis. The predicted binding relationship between miR-202-5p and circ_0075960 or CTNND1 was validated by dual-luciferase reporter experiment. In vivo animal models were constructed in nude mice to verify the role of circ_0075960 in tumor growth.

RESULTS

Circ_0075960 and CTNND1 were upregulated, while miR-202-5p was downregulated in EC. Knockdown of circ_0075960 induced EC cell apoptosis, suppressed cell proliferation and migration, and repressed tumor growth in animal models. MiR-202-5p was targeted by circ_0075960 and it directly bound to CTNND1 3'UTR. The inhibition of circ_0075960 knockdown or miR-202-5p enrichment on EC cell proliferation and migration was reversed by miR-202-5p depletion or CTNND1 overexpression, respectively. Circ_0075960 targeted miR-202-5p to positively regulate CTNND1 expression. Moreover, circ_0075960 knockdown weakened the activity of Wnt/β-catenin signaling via targeting the miR-202-5p/CTNND1 axis.

CONCLUSION

Circ_0075960 targets the miR-202-5p/CTNND1 axis to modulate Wnt/β-catenin signaling activity, thus contributing to the malignant development of EC.

Hsa_circ_0043949 reinforces temozolomide resistance via upregulating oncogene ITGA1 axis in glioblastoma

Temozolomide (TMZ) resistance limits its use in glioblastoma (GBM). Exosomes can carry circular RNAs (circRNAs) to regulate chemoresistance. To date, the role of exosomal hsa_circ_0043949 (circ_0043949) in GBM resistance to TMZ is unclear. Relative expression of circ_0043949 in clinical samples, GBM cell lines, and exosomes was detected by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). The half-maximal inhibitory concentration (IC₅₀) of TMZ, cell proliferation, apoptosis, invasion, and migration were analyzed via MTT, EdU, flow cytometry, transwell, and wound-healing assays. Relative protein levels were evaluated by western blotting. Target relationship was predicted by bioinformatics analysis and validated by dual-luciferase reporter and RNA pull-down assays. Exosomes were isolated by ultracentrifugation and verified by transmission electron microscopy, nanoparticle tracking analysis (NTA), and western blotting. The effect of exosomal circ_0043949 on TMZ resistance was validated by xenograft assay. Higher expression of circ_0043949 was gained in TMZ-resistant GBM samples and cells. Inhibition of circ_0043949 reduced TMZ resistance via decreasing IC₅₀ of TMZ, repressing proliferation, invasion, migration, and inducing apoptosis in TMZ-resistant GBM cells. Circ_0043949 mediated integrinalpha1 (ITGA1) expression via function as a miR-876-3p sponge. Circ_0043949 was also upregulated in TMZ-resistant GBM cells-derived exosomes, and exosomal circ_0043949 increased the resistance of TMZ-resistant GBM cells to TMZ in xenograft models. TMZ-resistant GBM cells-derived exosomal circ_0043949 promoted TMZ resistance via upregulating ITGA1 expression via sequestering miR-876-3p, offering a potential target for the treatment of TMZ resistance in GBM.

Circular RNAs: Emerging players in the pathogenesis of keloid

Circular RNAs (circRNAs) are a new type of non-coding RNAs originating from precursor messenger RNAs. Recent research has confirmed that circRNAs play a significant role in various biological and pathological processes, including cell viability, migration, and apoptosis. Emerging studies have demonstrated that the deregulated circRNA-miRNA-mRNA interaction network plays a key role in the development of many diseases. Increasing evidence has highlighted the role of ncRNAs (mainly miRNAs and lncRNAs) in the pathogenesis of keloids. Recently, several publications also indicated that circRNAs contribute to keloid development. The discovery of circRNAs changed the current understanding of the biology of keloids It is crucial to elucidate a circRNA-miRNA-mRNA network to understand the pathological mechanism of keloids. In the present review, we summarize the aberrant expression of regulatory roles of circRNAs in keloids. We discuss the potential clinical application of circRNAs in the diagnosis and treatment of keloids.

Integrative network analysis of circular RNAs reveals regulatory mechanisms for hepatic specification of human iPSC-derived endoderm

BACKGROUND

Human-induced pluripotent stem cell (hiPSC)-derived functional hepatic endoderm (HE) is supposed to be an alternative option for replacement therapy for end-stage liver disease. However, the high heterogeneity of HE cell populations is still challenging. Hepatic specification of definitive endoderm (DE) is an essential stage for HE induction in vitro. Recent studies have suggested that circular RNAs (circRNAs) determine the fate of stem cells by acting as competing endogenous RNAs (ceRNAs). To date, the relationships between endogenous circRNAs and hepatic specification remain elusive.

METHODS

The identities of DE and HE derived from hiPSCs were determined by qPCR, cell immunofluorescence, and ELISA. Differentially expressed circRNAs (DEcircRNAs) were analysed using the Arraystar Human circRNA Array. qPCR was performed to validate the candidate DEcircRNAs. Intersecting differentially expressed genes (DEGs) of the GSE128060 and GSE66282 data sets and the DEcircRNA-predicted mRNAs were imported into Cytoscape for ceRNA networks. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) were involved in the enrichment analysis. Hepatic markers and Wnt/β-catenin were detected in hsa_circ_004658-overexpressing cells by western blotting. Dual-luciferase reporter assays were used to evaluate the direct binding among hsa_circ_004658, miRNA-1200 and CDX2. DE cells were transfected with miR-1200 mimics, adenovirus containing CDX2, and Wnt/β-catenin was detected by western blotting.

RESULTS

hiPSC-derived DE and HE were obtained at 4 and 9 days after differentiation, as determined by hepatic markers. During hepatic specification, 626 upregulated and 208 downregulated DEcircRNAs were identified. Nine candidate DEcircRNAs were validated by qPCR. In the ceRNA networks, 111 circRNA-miRNA-mRNA pairs were involved, including 90 pairs associated with hsa_circ_004658. In addition, 53 DEGs were identified among the intersecting mRNAs of the GSE128060 and GSE66282 data sets and the hsa_circ_004658-targeted mRNAs. KEGG and GO analyses showed that the DEGs associated with hsa_circ_004658 were mainly enriched in the WNT signalling pathway. Furthermore, hsa_circ_004658 was preliminarily verified to promote hepatic specification, as determined by hepatic markers (AFP, ALB, HNF4A, and CK19) (p < 0.05). This promotive effect may be related to the inhibition of the Wnt/β-catenin signalling pathway (detected by β-catenin, p-β-catenin, and TCF4) when hsa_circ_004658 was overexpressed (p < 0.05). Dual-luciferase reporter assays showed that there were binding sites for miR-1200 in the hsa_circ_004658 sequence, and confirmed the candidate DEG (CDX2) as a miR-1200 target. The level of miR-1200 decreased and the level of CDX2 protein expression increased when hsa_circ_004658 was overexpressed (p < 0.05). In addition, the results showed that CDX2 may suppress the Wnt/β-catenin signalling during hepatic specification (p < 0.05).

CONCLUSIONS

This study analysed the profiles of circRNAs during hepatic specification. We identified the hsa_circ_004658/miR-1200/CDX2 axis and preliminarily verified its effect on the Wnt/β-catenin signalling pathway during hepatic specification. These results provide novel insight into the molecular mechanisms involved in hepatic specification and could improve liver development in the future.

Circular RNA hsa_circ_0057452 facilitates keloid progression by targeting the microRNA-1225-3p/AF4/FMR2 family member 4 axis

The circular RNA, hsa_circ_0057452, is highly expressed in keloids, but its specific mechanism of action remains unknown. The levels of hsa_circ_0057452, microRNA (miR)-1225-3p, and AF4/FMR2 family member 4 (AFF4) in keloid tissues and keloid fibroblasts (KFs) were determined using quantitative reverse transcription-polymerase chain reaction. Changes in KFs viability, proliferation, apoptosis, and migration were investigated using the cell counting kit-8, bromodeoxyuridine, flow cytometry, and Transwell assays. Luciferase, RNA immunoprecipitation, and RNA pull-down assays were performed to identify the binding relationship among hsa_circ_0057452, miR-1225-3p, and AFF4. We found that hsa_circ_0057452 and AFF4 expression levels were upregulated, whereas miR-1225-3p expression levels were downregulated in keloids. Knockdown of hsa_circ_0057452 or AFF4 suppressed the viability, proliferation, and migration of KFs and induced apoptosis, whereas hsa_circ_0057452 overexpression and miR-1225-3p knockdown showed the opposite trend. Furthermore, hsa_circ_0057452 affected the biological behavior of KFs by releasing AFF4 via sponging of miR-1225-3p. Therefore, our results show that hsa_circ_0057452 promotes keloid progression by targeting miR-1225-3p and regulating AFF4 levels.

CircRNAs in Malignant Tumor Radiation: The New Frontier as Radiotherapy Biomarkers

World Health Organization (WHO) data show that of the top 20 factors that threaten human life and health, cancer is at the forefront, and the therapeutic approaches for cancer consist of surgery, radiotherapy, chemotherapy and immunotherapy. For most highly metastatic and recurrent cancer, radiation therapy is an essential modality to mitigate tumor burden and improve patient survival. Despite the great accomplishments that have been made in clinical therapy, an inevitable challenge in effective treatment is radioresistance, the mechanisms of which have not yet been completely elucidated. In addition, radiosensitization methods based on molecular mechanisms and targets, and clinical applications are still inadequate. Evidence indicates that circular RNAs (circRNAs) are important components in altering tumor progression, and in influencing resistance and susceptibility to radiotherapy. This review summarizes the reasons for tumor radiotherapy resistance induced by circRNAs, and clarifies the molecular mechanisms and targets of action. Moreover, we determine the potential value of circRNAs as clinical indicators in radiotherapy, providing a theoretical basis for circRNAs-based strategies for cancer radiotherapy.

Specific expression and functions of circular RNAs

In recent years, circular RNAs (circRNAs), a new class of RNA molecules characterized by their covalently closed circular structure, have become a new research paradigm in RNA biology. Many circRNAs are conserved among eukaryotes, localize in specific subcellular compartments, and play different biological roles. Accumulating evidence shows that circRNAs regulate a diversity of cellular processes by acting as miRNA sponges, anchors for circRNA binding proteins (cRBPs), transcriptional regulators, molecular scaffolds, and sources for translation of small proteins/peptides. The emergence of the biological functions of circRNAs has brought a new perspective to our understanding of cellular physiology and disease pathogenesis. Recent studies have shown that the expression of circRNAs is tissue- and cell type-specific and specifically regulated through development or disease progression, where they exert specific biological functions. However, the mechanisms underlying these remain largely unknown. A deeper understanding of how the specific expression of circRNAs is regulated to exert specific biological functions will enable the use of circRNA as a biomarker in clinical practice and the development of new therapeutic approaches. This review aims to summarize recent developments in circRNA biogenesis, functions, and molecular mechanisms. We also provide some specific circRNAs as examples to show their tissue-specific distribution and evaluate the possibility of applying circRNA technologies in molecular research and therapeutics.

An Update on the Roles of circRNA-ZFR in Human Malignant Tumors

CircRNAs (circular RNAs) are single-stranded RNAs that form covalently closed loops and function as important regulatory elements of the genome through multiple mechanisms. Increasing evidence had indicated that circRNAs, which might serve as either oncogenes or tumor suppressors, played vital roles in the pathophysiology of human diseases, especially in tumorigenesis and progression. CircRNA-ZFR (circular RNA zinc finger RNA binding protein) is a circular RNA that had attracted much attention in recent years. It has been found that circRNA-ZFR was abnormally expressed in a variety of malignant tumors, and its dysregulated expression was closely related to tumor stage, cancer metastasis and patients’ prognosis. Recent studies had shown that aberrantly expressed circRNA-ZFR could regulate the malignant biological behaviors of tumors through various mechanisms; further exploration of circRNA-ZFR expression in tumors and its regulation on malignant biological behaviors such as tumor proliferation, invasion and drug resistance will provide new ideas for clinical tumors diagnosis and treatment.

Circ_0014235 confers Gefitinib resistance and malignant behaviors in non-small cell lung cancer resistant to Gefitinib by governing the miR-146b-5p/YAP/PD-L1 pathway

Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs), such as Gefitinib, have been recommended as the first-line treatment reagent for advanced EGFR-mutant non-small cell lung cancer (NSCLC). However, the mechanisms of drug resistance development are not fully determined. This study aimed to explore the role of circular RNA (circ_0014235) in Gefitinib-resistant NSCLC. The expression of circ_0014235, microRNA-146b-5p (miR-146b-5p) and Yes1 associated transcriptional regulator (YAP) mRNA was detected by quantitative real-time PCR (qPCR). Cell viability was detected by CCK-8 assay. Cell proliferation was assessed by colony formation assay and EdU assay. Cell cycle and cell apoptosis were determined by flow cytometry assay. The expression of marker proteins, YAP protein and programmed death ligand 1 (PD-L1) protein was detected by Western blot. The putative relationship between miR-146b-5p and circ_0014235 or YAP was ensured by dual-luciferase reporter assay and RIP assay. Animal models were established to explore the role of circ_0014235 in vivo. Circ_0014235 was highly expressed in Gefitinib-resistant NSCLC cells. Circ_0014235 downregulation reduced Gefitinib IC50, inhibited cell proliferation and induced cell apoptosis and cell cycle arrest in Gefitinib-resistant NSCLC cells, while these effects were reversed by the inhibition of miR-146b-5p, a target of circ_0014235. In addition, YAP was a target gene of miR-146b-5p, and circ_0014235 relieved miR-146b-5p-mediated inhibition on YAP by targeting miR-146b-5p. MiR-146b-5p restoration-blocked Gefitinib IC50 and cell malignant behaviors were recovered by YAP overexpression. YAP positively regulated PD-L1 expression, and YAP overexpression contributes to Gefitinib IC50 and cell malignant behaviors by upregulating PD-L1. Circ_0014235 confers Gefitinib resistance and malignant behaviors in Gefitinib-resistant NSCLC by governing the miR-146b-5p/YAP/PD-L1 pathway.

Identification of Potential circRNA-microRNA-mRNA Regulatory Network in Skeletal Muscle

Circular RNAs (circRNAs) are a newly discovered family of regulatory RNAs generated through backsplicing. Genome-wide profiling of circRNAs found that circRNAs are ubiquitously expressed and regulate gene expression by acting as a sponge for RNA-binding proteins (RBPs) and microRNAs (miRNAs). To identify circRNAs expressed in mouse skeletal muscle, we performed high-throughput RNA-sequencing of circRNA-enriched gastrocnemius muscle RNA samples, which identified more than 1,200 circRNAs. In addition, we have identified more than 14,000 and 15,000 circRNAs in aging human skeletal muscle tissue and satellite cells, respectively. A subset of abundant circRNAs was analyzed by RT-PCR, Sanger sequencing, and RNase R digestion assays to validate their expression in mouse skeletal muscle tissues. Analysis of the circRNA-miRNA-mRNA regulatory network revealed that conserved circNfix might associate with miR-204-5p, a suppressor of myocyte enhancer factor 2c (Mef2c) expression. To support the hypothesis that circNfix might regulate myogenesis by controlling Mef2c expression, silencing circNfix moderately reduced Mef2c mRNA expression and inhibited C2C12 differentiation. We propose that circNfix promotes MEF2C expression during muscle cell differentiation in part by acting as a sponge for miR-204-5p.

Research Progress of circRNAs in Glioblastoma

Circular RNAs (circRNAs) are a class of single-stranded covalently closed non-coding RNAs without a 5' cap structure or 3' terminal poly (A) tail, which are expressed in a variety of tissues and cells with conserved, stable and specific characteristics. Glioblastoma (GBM) is the most aggressive and lethal tumor in the central nervous system, characterized by high recurrence and mortality rates. The specific expression of circRNAs in GBM has demonstrated their potential to become new biomarkers for the development of GBM. The specific expression of circRNAs in GBM has shown their potential as new biomarkers for GBM cell proliferation, apoptosis, migration and invasion, which provides new ideas for GBM treatment. In this paper, we will review the biological properties and functions of circRNAs and their biological roles and clinical applications in GBM.

From circRNAs to fusion circRNAs in hematological malignancies

Circular RNAs (circRNAs) represent a type of endogenous noncoding RNA generated by back-splicing events. Unlike the majority of RNAs, circRNAs are covalently closed, without a 5' end or a 3' poly(A) tail. A few circRNAs can be associated with polysomes, suggesting a protein-coding potential. CircRNAs are not degraded by RNA exonucleases or ribonuclease R and are enriched in exosomes. Recent developments in experimental methods coupled with evolving bioinformatic approaches have accelerated functional investigation of circRNAs, which exhibit a stable structure, a long half-life, and tumor specificity and can be extracted from body fluids and used as potential biological markers for tumors. Moreover, circRNAs may regulate the occurrence and development of cancers and contribute to drug resistance through a variety of molecular mechanisms. Despite the identification of a growing number of circRNAs, their effects in hematological cancers remain largely unknown. Recent studies indicate that circRNAs could also originate from fusion genes (fusion circRNAs, f-circRNAs) next to chromosomal translocations, which are considered the primary cause of various cancers, notably hematological malignancies. This Review will focus on circRNAs and f-circRNAs in hematological cancers.

Hsa_circ_0002062 Promotes the Proliferation of Pulmonary Artery Smooth Muscle Cells by Regulating the Hsa-miR-942-5p/CDK6 Signaling Pathway

Currently, new strategies for the diagnosis and treatment of hypoxia-induced pulmonary hypertension (HPH) are urgently required. The unique features of circRNAs have unveiled a novel perspective for understanding the biological mechanisms underlying HPH and the possibility for innovative strategies for treatment of HPH. CircRNAs function as competing endogenous RNAs (CeRNA) to sequester miRNAs and regulate the expression of target genes. This study aimed to explore the roles of hsa_circ_0002062 on the biological behaviors of pulmonary artery smooth muscle cells (PASMCs) in hypoxic conditions. A number of in vitro assays, such as RNA-binding protein immunoprecipitation (RIP), RNA pull-down, and dual-luciferase assays were performed to evaluate the interrelationship between hsa_circ_0002062, hsa-miR-942-5P, and CDK6. The potential physiological functions of hsa_circ_0002062, hsa-miR-942-5P, and CDK6 in hypoxic PASMCs were investigated through expression modulation. Our experiments demonstrated that hsa_circ_0002062 functions as a ceRNA, acts as a sponge for hsa-miR-942-5P, and consequently activates CDK6, which further promotes pulmonary vascular remodeling. Therefore, we speculate that hsa_circ_0002062 could serve as a candidate diagnostic biomarker and potential therapeutic target for HPH.

CircVAMP3: A circRNA with a Role in Alveolar Rhabdomyosarcoma Cell Cycle Progression

Circular RNAs (circRNAs), a class of covalently closed RNAs formed by a back-splicing reaction, have been involved in the regulation of diverse oncogenic processes. In this article we describe circVAMP3, a novel circular RNA overexpressed in RH4, a representative cell line of alveolar rhabdomyosarcoma. We demonstrated that circVAMP3 has a differential m⁶A pattern opposed to its linear counterpart, suggesting that the two isoforms can be differently regulated by such RNA modification. Moreover, we show how circVAMP3 depletion in alveolar rhabdomyosarcoma cells can impair cell cycle progression, through the alteration of the AKT-related pathways, pointing to this non-coding RNA as a novel regulator of the alveolar rhabdomyosarcoma progression and as a putative future therapeutic target.

Circ_0001367 inhibits glioma proliferation, migration and invasion by sponging miR-431 and thus regulating NRXN3

Many studies have reported that circular RNAs play a vital role in the malignant progression of human cancers. However, the role and underlying mechanism of circRNAs in the development of gliomas have not been fully clarified. In this study, we found that circ_0001367 was downregulated in glioma tissues and showed a close correlation with glioma patient survival. Functional assays demonstrated that upregulation of circ_0001367 could suppress the proliferation, migration and invasion of glioma cells in vitro and inhibit glioma growth in vivo. Furthermore, bioinformatics analysis, luciferase reporter assay and RNA immunoprecipitation assay indicated that circ_0001367 can serve as a sponge for miR-431 and that miR-431 acts as an oncogene by regulating neurexin 3 (NRXN3). In addition, rescue experiments verified that circ_0001367 could regulate both the expression and function of NRXN3 in a miR-431-dependent manner. In conclusion, circ_0001367 functions as an suppressor in glioma by targeting the miR-431/NRXN3 axis and may be a promising therapeutic target against gliomas.

Advances of circular RNAs in thyroid cancer: An overview

Circular RNA (circRNA) is a general term for a type of single-stranded RNAs, they are primarily generated via exon back-splice process in precursor mRNAs (pre-mRNAs). circRNAs refer to an emerging type of endogeneity-correlated closed molecules of RNA in a covalent manner. They mainly function as microRNA sponges, protein brackets, and regulatory element in transcription and splicing process. Recently, it has also starting been noticed that they serve as extraordinary models involved in polypeptides producing process. Although circRNAs have been extensively studied, their function in thyroid carcinoma is still lacking. Thus, we present the latest advances in circRNA research and summarize their fundamental rules of regulating process as well as the mechanism. More importantly, We mainly review the role and mechanism of circRNA in thyroid cancer, which provides an emerging perspective and theoretically supports the treatment of thyroid cancer.

Biogenesis, cellular effects, and biomarker value of circHIPK3

Competing endogenous RNAs (ceRNAs) can indirectly regulate gene expression by competitively binding to microRNA(miRNA) through miRNA response elements (MREs) to affect miRNA-induced gene regulation, which is of great biological significance. Among them, circular RNA (circRNA) has become a hotspot due to its highest binding capacity. A specific circRNA discussed in this review, circHIPK3, has been studied for its biological characteristics, function, cellular effects and its relationship with tumors and various diseases. Here, we review the recent researches about circHIPK3 in detail and aim to elucidate accurate conclusions from them. These circHIPK3-miRNAs-mRNA pathways will further advance the application of circHIPK3 in diseases development, early diagnosis and gene targeting therapy.

Retrotransposons as Drivers of Mammalian Brain Evolution

Retrotransposons, a large and diverse class of transposable elements that are still active in humans, represent a remarkable force of genomic innovation underlying mammalian evolution. Among the features distinguishing mammals from all other vertebrates, the presence of a neocortex with a peculiar neuronal organization, composition and connectivity is perhaps the one that, by affecting the cognitive abilities of mammals, contributed mostly to their evolutionary success. Among mammals, hominids and especially humans display an extraordinarily expanded cortical volume, an enrichment of the repertoire of neural cell types and more elaborate patterns of neuronal connectivity. Retrotransposon-derived sequences have recently been implicated in multiple layers of gene regulation in the brain, from transcriptional and post-transcriptional control to both local and large-scale three-dimensional chromatin organization. Accordingly, an increasing variety of neurodevelopmental and neurodegenerative conditions are being recognized to be associated with retrotransposon dysregulation. We review here a large body of recent studies lending support to the idea that retrotransposon-dependent evolutionary novelties were crucial for the emergence of mammalian, primate and human peculiarities of brain morphology and function.

Circular RNA CircCOL5A1 Sponges the MiR-7-5p/Epac1 Axis to Promote the Progression of Keloids Through Regulating PI3K/Akt Signaling Pathway

Keloids, as a result of abnormal wound healing in susceptible individuals, are characterized by the hyper-proliferation of fibroblasts and exaggerated deposition of extracellular matrix. Current surgical and therapeutic modalities provide limited satisfactory results. Growing evidence has highlighted the roles of circRNAs in acting as miRNA sponges. However, up to date, the regulatory mechanism of circRNAs in the pathological process of keloids has rarely been reported. In this study, cell proliferation, cell migration, flow cytometry, western blotting, fluorescence in situ hybridization, dual-luciferase activity, and immunohistochemistry assays were applied to explore the roles and mechanisms of the circCOL5A1/miR-7-5p/Epac1 axis in the keloid. The therapeutic potential of circCOL5A1 was investigated by establishing keloid implantation models. The RT-qPCR result revealed that circCOL5A1 expression was obviously higher in keloid tissues and keloid fibroblasts. Subsequent cellular experiments demonstrated that circCOL5A1 knockdown repressed the proliferation, migration, extracellular matrix (ECM) deposition, whereas promoted cell apoptosis, through the PI3K/Akt signaling pathway. Furthermore, RNA-fluorescence in situ hybridization (RNA-FISH) illustrated that both circCOL5A1 and miR-7-5p were located in the cytoplasm. The luciferase reporter gene assay confirmed that exact binding sites were present between circCOL5A1 and miR-7-5p, as well as between miR-7-5p and Epac1. Collectively, the present study revealed that circCOL5A1 functioned as competing endogenous RNA (ceRNA) by adsorbing miR-7-5p to release Epac1, which contributed to pathological hyperplasia of keloids through activating the PI3K/Akt signaling pathway. Our data indicated that circCOL5A1 might serve as a novel promising therapeutic target and represent a new avenue to understand underlying pathogenesis for keloids.

Circ0083429 Regulates Osteoarthritis Progression via the Mir-346/SMAD3 Axis

Osteoarthritis (OA) is a degenerative joint disease. Currently, apart from symptomatic treatment or joint replacement, no other effective treatments for OA exist. The mechanisms underlying OA remain elusive and require further research. Circular RNAs (circRNAs) are known to be involved in many diseases; however, their function in OA is not yet fully understood. Here, we identified a novel circRNA, Circ0083429. The role of Circ0083429 in OA was confirmed via western blot (WB), quantitative real-time PCR (qRT-PCR), and immunofluorescence (IF) through knockdown and overexpression experiments. The binding of Circ0083429 to downstream miR-346 and its target gene SMAD3 was predicted via bioinformatics analysis and verified using a luciferase reporter assay and RNA pulldown experiments. Finally, the function of Circ0083429 was evaluated in mouse OA models. In our study, we found that Circ0083429 regulates the homeostasis of the extracellular matrix (ECM) in human chondrocytes. Mechanistically, Circ0083429 affects OA by regulating the mRNA level of SMAD3 through the sponging of microRNA (miRNA)-346. Injecting adeno-associated virus Circ0083429 into the intra-junction of the mouse knee alleviated OA. In conclusion, Circ0083429 regulates the ECM via the regulation of the downstream miRNA-346/SMAD3 in human chondrocytes, which provides a new therapeutic strategy for OA.