A Review: The Significance of Toll-Like Receptors 2 and 4, and NF-κB Signaling in Endothelial Cells during Atherosclerosis

Atherosclerosis (AS) is a chronic inflammatory vascular disease that begins with endothelial activation followed by a series of inflammatory responses, plaque formation, and finally rupture. An early event in endothelial dysfunction is activation of the nuclear factor-κB (NF-κB) signaling axis. Toll-like receptors (TLRs) in endothelial cells (ECs) play an essential role in recognizing pathogen-associated molecular patterns (PAMPs), damage-associated molecular patterns (DAMPs), and lifestyle-associated molecular patterns (LAMPs). Activation of the canonical NF-κB pathway stimulates the expression of cytokines, chemokines, and an array of additional genes which activate and amplify AS-associated inflammatory responses. In this review, we discuss the involvement of TLR2/4 and NF-κB signaling in ECs during AS initiation, as well as regulation of the inflammatory response during AS by noncoding RNAs, especially microRNA (miRNA) and circular RNA (circRNA).

CircPRKD3/miR-6783-3p responds to mechanical force to facilitate the osteogenesis of stretched periodontal ligament stem cells

BACKGROUND

The mechanotransduction mechanisms by which cells regulate tissue remodeling are not fully deciphered. Circular RNAs (circRNAs) are crucial to various physiological processes, including cell cycle, differentiation, and polarization. However, the effects of mechanical force on circRNAs and the role of circRNAs in the mechanobiology of differentiation and remodeling in stretched periodontal ligament stem cells (PDLSCs) remain unclear. This article aims to explore the osteogenic function of mechanically sensitive circular RNA protein kinase D3 (circPRKD3) and elucidate its underlying mechanotransduction mechanism.

MATERIALS AND METHODS

PDLSCs were elongated with 8% stretch at 0.5 Hz for 24 h using the Flexcell® FX-6000™ Tension System. CircPRKD3 was knockdown or overexpressed with lentiviral constructs or plasmids. The downstream molecules of circPRKD3 were predicted by bioinformatics analysis. The osteogenic effect of related molecules was evaluated by quantitative real-time PCR (qRT-PCR) and western blot.

RESULTS

Mechanical force enhanced the osteogenesis of PDLSCs and increased the expression of circPRKD3. Knockdown of circPRKD3 hindered PDLSCs from osteogenesis under mechanical force, while overexpression of circPRKD3 promoted the early osteogenesis process of PDLSCs. With bioinformatics analysis and multiple software predictions, we identified hsa-miR-6783-3p could act as the sponge of circPRKD3 to indirectly regulate osteogenic differentiation of mechanically stimulated PDLSCs.

CONCLUSIONS

Our results first suggested that both circPRKD3 and hsa-miR-6783-3p could enhance osteogenesis of stretched PDLSCs. Furthermore, hsa-miR-6783-3p could sponge circPRKD3 to indirectly regulate RUNX2 during the periodontal tissue remodeling process in orthodontic treatment.

[Research Progress in the Regulatory Role of circRNA-miRNA Network in Bone Remodeling]

The dynamic balance between bone formation and bone resorption is a critical process of bone remodeling. The imbalance of bone formation and bone resorption is closely associated with the occurrence and development of various bone-related diseases. Under both physiological and pathological conditions, non-coding RNAs (ncRNAs) play a crucial regulatory role in protein expression through either inhibiting mRNAs translation or promoting mRNAs degradation. Circular RNAs (circRNAs) are a type of non-linear ncRNAs that can resist the degradation of RNA exonucleases. There is accumulating evidence suggesting that circRNAs and microRNAs (miRNAs) serve as critical regulators of bone remodeling through their direct or indirect regulation of the expression of osteogenesis-related genes. Additionally, recent studies have revealed the involvement of the circRNAs-miRNAs regulatory network in the process by which mesenchymal stem cells (MSCs) differentiate towards the osteoblasts (OB) lineage and the process by which bone marrow-derived macrophages (BMDM) differentiate towards osteoclasts (OC). The circRNA-miRNA network plays an important regulatory role in the osteoblastic-osteoclastic balance of bone remodeling. Therefore, a thorough understanding of the circRNA-miRNA regulatory mechanisms will contribute to a better understanding of the regulatory mechanisms of the balance between osteoblastic and osteoclastic activities in the process of bone remodeling and the diagnosis and treatment of related diseases. Herein, we reviewed the functions of circRNA and microRNA. We also reviewed their roles in and the mechanisms of the circRNA-miRNA regulatory network in the process of bone remodeling. This review provides references and ideas for further research on the regulation of bone remodeling and the prevention and treatment of bone-related diseases.

Hsa_circ_0001326 inhibited the proliferation, migration, and invasion of trophoblast cells via miR-145-5p/TGFB2 axis

PROBLEM

Preeclampsia (PE) is an obstetric disease involving multiple systems, which account for maternal and fetal complications and increased mortality. Circular RNAs (circRNAs) were recently deemed to associate with the pathogenesis of PE. This study aims to clarify the correlation between circRNA hsa_circ_0001326 and PE and explore its biological function in PE.

METHOD OF STUDY

The expression of hsa_circ_0001326 in PE placentas was detected by real-time quantitative PCR (qRT-PCR). After overexpressing or inhibiting hsa_circ_0001326 in trophoblast cells, the cell growth, migration, and invasion were evaluated by Cell Counting Kit-8 (CCK-8) and transwell assays. Western blot assay was applied to detect the epithelial-mesenchymal transition (EMT) proteins, E-cadherin and Vimentin. Furthermore, a dual-luciferase reporter assay was applied to verify the binding sites of hsa_circ_0001326, miR-145-5p, and transforming growth factor beta 2 (TGFB2).

RESULTS

Hsa_circ_0001326 was found to be higher expressed in PE placentas than in normal placentas. Furthermore, hsa_circ_0001326 played a negative regulating role in trophoblast cell viability, migration, and invasion. Overexpression of hsa_circ_0001326 inhibited the viability, migration, and invasion of trophoblast cells, while inhibition of hsa_circ_0001326 showed opposite effects. Mechanistically, hsa_circ_0001326 sponged miR-145-5p to elevate TGFB2 expression in trophoblast cells.

CONCLUSION

This study provided evidence that the up-regulated hsa_circ_0001326 in PE restrained trophoblast cells proliferation, migration, and invasion by sponging miR-145-5p to elevate TGFB2 expression. Our results might provide a novel insight into the role of hsa_circ_0001326 in the pathogenesis of PE.

Circ_FURIN knockdown assuages Testosterone-induced human ovarian granulosa-like tumor cell disorders by sponging miR-423-5p to reduce MTM1 expression in polycystic ovary syndrome

BACKGROUND

Polycystic ovary syndrome (PCOS) is a common endocrine disorder among reproductive-age women. The mechanism by which circular RNA (circRNA) drives PCOS development remains unclear. Thus, the study is designed to explore the role of a novel circRNA, circ_FURIN, in the PCOS cell model and the underlying mechanism.

METHODS

PCOS cell model was established by treating human ovarian granulosa-like tumor cells (KGN) with Testosterone (TTR). RNA expressions of circ_FURIN, microRNA-423-5p (miR-423-5p) and myotubularin 1 (MTM1) were detected by quantitative real-time polymerase chain reaction (qRT-PCR). Protein expression was checked by Western blot. Cell proliferation was investigated by a 5-Ethynyl-29-deoxyuridine assay, 3-(4,5-Dimethylthazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and flow cytometry analysis for cell cycle. Apoptotic cells were quantified by flow cytometry analysis for cell apoptosis. The interplay between miR-423-5p and circ_FURIN or MTM1 was identified by dual-luciferase reporter and RNA pull-down assays.

RESULTS

Circ_FURIN and MTM1 expressions were significantly upregulated, whereas miR-423-5p was downregulated in the ovarian cortex tissues of PCOS patients and TTR-treated KGN cells compared with controls. Circ_FURIN depletion relieved TTR-induced proliferation inhibition and apoptosis promotion. Besides, knockdown of miR-423-5p, a target miRNA of circ_FURIN, rescued circ_FURIN knockdown-mediated effects under TTR treatment. MiR-423-5p remitted TTR-induced cell disorders by binding to MTM1. Moreover, circ_FURIN modulated MTM1 expression through miR-423-5p.

CONCLUSION

Circ_FURIN silencing protected against TTR-induced dysfunction by the miR-423-5p/MTM1 pathway in human ovarian granulosa-like tumor cells.

Hsa_circ_0023990 Promotes Tumor Growth and Glycolysis in Dedifferentiated TC via Targeting miR-485-5p/FOXM1 Axis

BACKGROUND

During the transformation to dedifferentiated thyroid cancer (TC) types, the ability of papillary thyroid carcinomas (PTCs) to concentrate radioactive iodine might be lost, raising difficulty for the current therapy. circRNAs were proved to be implicated in the progression of various cancers. In this study, we aimed to investigate the functional role and mechanism of hsa_circ_0023990 in dedifferentiated TC.

METHODS

The expression pattern of genes were detected using quantitative PCR or western blot assays. Cell proliferation was determined by CCK8, colony formation, EdU, and cell-cycle assays. Glycolysis was assessed using glucose uptake and lactate production assays. Luciferase reporter assay was performed to examine the interactions between miR-485-5p and hsa_circ_0023990 or FOXM1. Xenograft assay was allowed for observation of tumor growth in vivo.

RESULTS

Hsa_circ_0023990 and FOXM1 were upregulated in dedifferentiated TC tissues and cell lines. The higher level of hsa_circ_0023900 could stimulate the proliferation and glycolysis of dedifferentiated TC cells via positively regulating FOXM1. Mechanistically, miR-485-5p was demonstrated to interact with hsa_circ_0023990 and FOXM1 and involved in the regulation of has_circ_0023990 and FOXM1 in TC biological processes.

CONCLUSION

Our results discovered the functional network of hsa_circ_0023990 in dedifferentiated TC development by facilitating cell proliferation and glycolysis via miR-485-5p/FOXM1 axis, implying that hsa_circ_0023990 might be a potential therapeutic target for the dedifferentiated TC treatment.

Circular RNA screening identifies circMYLK4 as a regulator of fast/slow myofibers in porcine skeletal muscles

The type of myofiber is related to the quality of meat. The slow oxidized myofiber helps to increase the tenderness and juiciness of muscle. Numerous studies have shown that circRNA plays a key role in skeletal muscle development. However, the role of circRNA in porcine skeletal myofiber types is unclear. In this study, we performed high-throughput RNA sequencing to study the differential expression of circRNA in the longissimus dorsi and the soleus muscle. A total of 40,757 circRNAs were identified, of which 181 were significantly different. Interestingly, some circRNAs were involved in metabolism pathways, AMPK, FoxO, and PI3K-Akt signaling pathways. Besides, we focused on a novel circRNA-circMYLK4. By injecting circMYLK4-AAV into piglets, we found that circMYLK4 significantly increased the mRNA and protein levels of the slow muscle marker genes. In summary, our study laid an essential foundation for further research of circRNA in myofiber type conversion and higher meat quality.

Biological and clinical implications of metastasis-associated circular RNAs in oesophageal squamous cell carcinoma

Oesophageal squamous cell carcinoma (OSCC) is a prevalent malignancy with high morbidity and mortality as a result of early metastasis and poor prognosis. Metastasis is a multistep process, involving various signalling pathways. Circular RNAs (circRNAs) are a class of covalently closed noncoding RNAs, the aberrant expression of which is reported to be involved in several biological events, including cell transformation, proliferation, migration, invasion, apoptosis and metastasis. Several studies have reported interactions between circRNAs and metastasis-associated signalling pathways. The abundance, stability and highly specific expression of candidate circRNAs make them potential biomarkers and therapeutic targets in OSCC. In this review article, we comprehensively describe metastasis-related circRNAs and their interactions with epithelial-mesenchymal transition-associated molecules. We also describe the molecular mechanisms and clinical relevance of circRNAs in OSCC progression and metastasis.

Hsa_circ_0004058 inhibits apoptosis of SRA01/04 cells by promoting autophagy via miR-186/ATG7 axis

Senile cataract is a common age-related disease in ophthalmology. Hsa_circ_0004058 has been reported to be down-regulated in the lens epithelial cells of senile cataract patients, suggesting that hsa_circ_0004058 is associated with senile cataract. However, the underlying mechanism is still unknown. This study attempted to determine the functional role of hsa_circ_0004058 in senile cataract. We treated human lens epithelial cells (SRA01/04) with H2O2 as senile cataract model, and found that cell viability and autophagy of SRA01/04 cells were severely decreased by H2O2 treatment. Hsa_circ_0004058 was notably down-regulated in H2O2-treated SRA01/04 cells. Moreover, hsa_circ_0004058 overexpression reduced apoptotic cells and the expression of Cleaved-caspase-3 and Bax, and enhanced Bcl-2 expression in H2O2-treated SRA01/04 cells. However, hsa_circ_0004058 silencing caused the opposite results. Hsa_circ_0004058 up-regulation accelerated the expression of autophagy-related proteins LC3-II/LC3-I and Beclin-1 in H2O2-treated SRA01/04 cells, which was partly abolished by 3-Methyladenine (autophagy inhibitor). Additionally, hsa_circ_0004058 functioned as a competing endogenous RNA to competitive binding miR-186, and thus accelerated the expression of its down-stream target, ATG7. Hsa_circ_0004058 promoted autophagy of SRA01/04 cells by regulating miR-186/ATG7 axis. In conclusion, these data demonstrates that hsa_circ_0004058 inhibits apoptosis of SRA01/04 cells by promoting autophagy, which attributes to regulate miR-186/ATG7 axis. Thus, hsa_circ_0004058 may be a potential target for senile cataract treatment.

Circular RNA_0001495 increases Robo1 expression by sponging microRNA-527 to promote the proliferation, migration and invasion of bladder cancer cells

Bladder cancer (BCa) is a heterogeneous disease that poses great threats on public health. Increasing studies have identified the vital functions of circular RNAs (circRNAs) in BCa treatment. Hence, this current study set out to explore the modulatory role of circ_0001495 in BCa development. First, the expression of circ_0001495 was determined by reverse transcription quantitative polymerase chain reaction. Cell biological processes were then analyzed after altering the circ_0001495 expression in T24 cells. Next, interactions among circ_0001495, microRNA-527 (miR-527) and roundabout guidance receptor 1 (Robo1) were investigated by dual luciferase reporter gene assay, RNA pull down assay and FISH assay. Lastly, xenograft tumors in nude mice were established to explore the effect of circ_0001495 in vivo. It was found that circ_0001495 was highly expressed in BCa tissues and cells, and was further correlated with poor prognosis in BCa patients. In addition, circ_0001495 inhibited the activity of miR-527 by acting as a sponge to sponge miR-527, which further elevated the Robo1 expression. Lastly, circ_0001495 was found to promote the proliferation, migration and invasion of BCa cells in vitro through the miR-527/Robo1 axis and promote the growth and metastasis of BCa tumors in vivo. Altogether, findings in our study highlight the promoting role of circ_0001495 in the progression of BCa by increasing Robo1 via sponging miR-527, representing a promising target for BCa management.

A novel circular RNA, hsa_circ_0030998 suppresses lung cancer tumorigenesis and Taxol resistance by sponging miR-558

Circular RNAs (circRNAs) are single-stranded RNAs which form a covalently closed continuous loop. Although originally shown to be non-protein-coding, some circRNAs can give rise to micropeptides. circRNAs have also been shown to play essential regulatory roles in a variety of developmental and disease processes. In a previous study, hsa_circ_0030998 was identified as a circRNA downregulated in lung cancer, but its potential implications and mechanisms in lung cancer were not addressed. Here, we showed that overexpressing circ_0030998 decreased proliferation, migration, and invasion of lung cancer cells, while also dampening resistance to Taxol, a classical antitumor drug. Depleting circ_0030998 reversed these phenotypic effects. A high circ_0030998 expression was correlated with a high survival rate in lung cancer patients. Additionally, we found circ_0030998 could downregulate miR-558 expression, serving as a microRNA sponge. In conclusion, our data support that hsa_circ_0030998 can slow down the progression of lung cancer by targeting miR-558 and suppress malignant phenotypes such as proliferation, migration, and invasion progression of lung cancer cells. Therefore, we highlight that circ_0030998 could be a novel tumor suppressor of lung cancer.

Circular RNA circ-ARFIP2 regulates proliferation, migration and invasion in human vascular smooth muscle cells via miR-338-3p-dependent modulation of KDR

Dysfunction of vascular smooth muscle cells (VSMCs) plays a critical role in the pathogenesis of intracranial aneurysm (IA). Circular RNAs (circRNAs) have been implicated in the pathogenesis of IA by reducing microRNA (miRNA) activity. In this paper, we investigated the precise roles of circRNA ADP ribosylation factor interacting protein 2 (circ-ARFIP2, circ_0021001) in VSMC dysfunction. The levels of circ-ARFIP2, miR-338-3p and kinase insert domain receptor (KDR) were detected by quantitative real-time polymerase chain reaction (qRT-PCR) or western blot. Ribonuclease (RNase) R and subcellular fractionation assays were used to assess the stability and localization of circ-ARFIP2, respectively. Cell viability was detected by Cell Counting Kit-8 (CCK-8) assay, and cell invasion was measured by transwell assay. Cell proliferation was gauged by 5-Ethynyl-2'-Deoxyuridine (EdU) assay. Cell migration was evaluated by transwell and wound-healing assays. Targeted correlations among circ-ARFIP2, miR-338-3p and KDR were validated by dual-luciferase reporter and RNA immunoprecipitation (RIP) assays. Circ-ARFIP2 and KDR were underexpressed and miR-338-3p was overexpressed in the arterial wall tissues of IA patients. Overexpression of circ-ARFIP2 in human umbilical artery smooth muscle cells (HUASMCs) showed a significant promotion in cell proliferation, migration and invasion. Mechanistically, circ-ARFIP2 targeted miR-338-3p, and circ-ARFIP2 regulated cell behaviors by miR-338-3p. KDR was a direct and functional target of miR-338-3p. Moreover, KDR was a downstream effector of circ-ARFIP2 function. Circ-ARFIP2 regulated KDR expression by targeting miR-338-3p. Our present findings demonstrated that the increased level of circ-ARFIP2 enhanced HUASMC proliferation, migration and invasion at least in part by the miR-338-3p/KDR axis.

hsa_circ_0005721 triggers proliferation, migration and invasion of osteosarcoma by upregulating the linear transcript TEP1

PURPOSE

This study aimed to illustrate the biological role of hsa_circ_0005721 in the development of osteosarcoma and the molecular mechanism.

METHODS

hsa_circ_0005721 levels in 30 pairs of osteosarcoma and non-tumor tissues were detected by quantitative real-time polymerase chain reaction (qRT-PCR). Functional experiments were conducted to assess the influence of hsa_circ_0005721 on proliferative, metastatic and apoptotic rates of osteosarcoma cells. The downstream target of hsa_circ_0005721 and their co-regulatory mechanism in malignant development of osteosarcoma were analyzed by dual-luciferase reporter assay and rescue experiments, respectively.

RESULTS

hsa_circ_0005721 was upregulated in osteosarcoma tissues and cell lines. Knockdown of hsa_circ_0005721 suppressed proliferative and metastatic rates of U-2OS and Saos-2 cells, and stimulated apoptosis. Serving as a ceRNA, hsa_circ_0005721 upregulated the linear transcript TEP1 by competitively binding miR-16-5p, thus exerting its biological functions in regulating osteosarcoma development.

CONCLUSIONS

This study for the first time identified the upregulated hsa_circ_0005721 in osteosarcoma, which triggers the malignant development of osteosarcoma by upregulating the linear transcript TEP1.

Circ_0006948 drives the malignant development of bladder cancer via activating the epithelial-mesenchymal transition

PURPOSE

To detect the expression characteristic of circ_0006948 in bladder cancer (BC), and to analyze its relationship with pathological parameters and prognosis in BC patients. In addition, molecular mechanisms of circ_0006948 on driving the malignant progression of BC by activating epithelial-mesenchymal transition (EMT) was explored.

METHODS

Circ_0006948 levels in 72 BC and paracancerous tissues were detected, and their relationship with pathological parameters and prognosis in BC patients was analyzed by chi-square test. After establishing circ_0006948 knockdown model in 253j and T24 cells, phenotype changes were assessed by cell counting kit-8 (CCK-8), transwell and wound healing assay. Regulatory effects of circ_0006948 on EMT-associated gene expressions in BC cells were determined by Western blot. Finally, the interaction between circ_0006948 and N-cadherin was evaluated by rescue experiments.

RESULTS

Circ_0006948 was upregulated in BC tissues and cell lines. High level of circ_0006948 indicated advanced tumor stage, high rates of lymph node metastasis and distant metastasis, and poor prognosis in BC. Knockdown of circ_0006948 reduced proliferative and metastatic abilities in BC cells. The key protein in the EMT signaling E-cadherin was upregulated by knockdown of circ_0006948 in BC cells, while N-cadherin, Vimentin, β-catenin and MMP-9 were downregulated. The interaction between circ_0006948 and N-cadherin was identified, and they were co-responsible for the malignant development of BC.

CONCLUSIONS

Circ_0006948 is upregulated in BC samples, and it is closely linked to tumor stage, metastasis and prognosis in BC patients. It drives proliferative and metastatic abilities in BC cells by activating EMT.

Circ_0067835 regulates allergic inflammatory response in type-2 innate lymphoid cells in allergic rhinitis (AR) via miR-155/GATA3

Allergic rhinitis (AR) is a familiar respiratory allergic inflammatory disease with higher incidence. The pathogenesis of AR is particularly complex. Therefore, a lot of work is acquired to excavate deep mechanisms, thereby providing effective strategies for AR diagnose and treatment. AR mice model was induced by recombinant murine IL-33 (0.05 µg/µl) on days 1, 3, and 5. The lentiviral vectors carrying si-circ_0067835, miR-155 mimic, si-NC or miR-NC were injected into AR mice. Thus, mice were divided into control, AR, AR + si-NC, AR + si-circ_0067835, AR + si-circ_0067835 + miR-NC, and AR + si-circ_0067835 + miR-155 mimic groups. qRT-PCR experiment was used to measure the expression of circ_0067835 and miR-155. Behavioral test result was quantified to assess AR mice model. Hematoxylin and eosin (HE) staining was performed to analyze histopathological changes. Helper T cell 2 (Th2) cytokines (IL-4, IL-5, IL-9 and IL-13) and percentage of type-2 innate lymphoid cells (ILC2s) in nasal mucosa tissues in AR mice model were evaluated needing western blot, ELISA, and flow cytometry. Besides, the targeting relationship between circ_0067835 and miR-155, or between miR-155 and GATA3, was investigated via luciferase report assay. Circ_0067835 expression levels were raised in the nasal mucosa tissues of AR mice. Inhibiting circ_0067835 could reduce Type2 cytokines and ILC2s levels in AR mice model. Furthermore, circ_0067835 targeted and positively regulated miR-155 expression, and GATA3 was a downstream target of miR-155 and adjusted by circ_0067835/miR-155 axis. In addition, silencing circ_0067835 inhibited cytokines and ILC2s levels by down-regulating miR-155. Circ_0067835 effectively inhibited AR response in ILC2s through participation of miR-155/GATA3 axis.

Gastric cancer derived exosomes mediate the delivery of circRNA to promote angiogenesis by targeting miR-29a/VEGF axis in endothelial cells

Accumulating evidence has been found that circular RNA (circRNA) plays a critical role in the initiation and development of various diseases by modulating gene expression in the cytoplasm. However, the role of circ_0044366 (termed circ29) in gastric cancer (GC) has yet to be elusive. We detected that exosomal circ29 was confirmed to be highly expressed in GC and can significantly impair the proliferation, migration, tube formation of HUVEC by exosomal communication. Interestingly, this effect could be blocked by the effect of miR-29a. In brief, we confirmed that circ29, as a sponge of miR-29a, plays a responsible role in the occurrence and development of GC by regulating the VEGF pathway. Therefore, it may be used as a potential target for the treatment of GC.

Hsa_circ_0054633 mediates apoptosis and insulin secretion in human pancreatic β cells through miR-409-3p/caspase-8 axis

BACKGROUND

CircRNAs are reported to be aberrantly expressed and perform biological functions in diverse processes. This study aimed to investigate the potential involvement of hsa_circ_0054633 in high glucose (HG)‑induced diabetic model and its potential mechanism.

METHODS

The expression of hsa_circ_0054633, miR-409-3p and caspase-8 was detected by real-time PCR and western blotting. Cell viability, apoptosis and the protein levels of apoptosis-related factors were revealed by CCK-8 colorimetry, flow cytometry and western blotting, respectively. Insulin secretion was determined by enzyme-linked immunosorbent assay (ELISA) and the measurement of insulin-related transcription factors. The target association between miR-409-3p and hsa_circ_0054633 or caspase-8 was confirmed by dual-luciferase reporter assays and biotin-based pulldown assay.

RESULTS

Hsa_circ_0054633 was highly expressed and the expression of miR-409-3p was downregulated in serum of DM patients and HG-treated human pancreatic β cell line NES2Y. Further investigation indicated that hsa_circ_0054633 suppression promoted cell proliferation, inhibited apoptosis and facilitated insulin secretion in HG-treated NES2Y cells. Mechanical analysis revealed that hsa_circ_0054633 regulated caspase-8 expression via sponging miR-409-3p. Rescue experiments demonstrated that miR-409-3p knockdown or caspase-8 overexpression reversed the effects of hsa_circ_0054633 in HG-stimulated NES2Y cells.

CONCLUSION

Inhibition of hsa_circ_0054633 protected against HG-induced NES2Y cell apoptosis and impairment of insulin secretion by regulating miR-409-3p/caspase-8 axis.

The novel circular RNA circ-PGAP3 retards cervical cancer growth by regulating the miR-769-5p/p53 axis

Cervical cancer (CC) is still an intractable disease that seriously affects women's health. Elucidating its pathogenesis will bring new targets for clinical treatment. Circular RNA (circRNA) is an endogenous RNA that has recently been reported to be closely related to cancer progression and development. In the current study, by performing in silico analysis and qRT-PCR assay, we found a circRNA derived from PGAP3, referred as circ-PGAP3 (hsa_circ_0106800, chr17:37843549-37844086), which was significantly downregulated in CC tissues. Low circ-PGAP3 was closely linked to poor prognosis. And overexpression of circ-PGAP3 significantly reduced CC cell proliferation in vitro and tumor growth in vivo. In terms of mechanism, circ-PGAP3 was transcriptionally elevated by p53, a well-recognized tumor suppressor, and circ-PGAP3 was located in the cytoplasm where sponged miR-769-5p to increase the levels of p53 and its downstream targets. Importantly, the regulatory feedback loop of circ-PGAP3/p53 was also confirmed in vivo. Overall, our data clearly expounded the tumor-inhibiting role of circ-PGAP3 in CC, circ-PGAP3 repressed CC tumorigenesis via regulating the miR-769-5p/p53 axis. Therefore, restoration of circ-PGAP3 may be a promising therapeutic target for this thorny disease.

Database Resources for Functional Circular RNAs

Circular RNA (or circRNA) is a type of single-stranded covalently closed circular RNA molecule and play important roles in diverse biological pathways. A comprehensive functionally annotated circRNA database will help to understand the circRNAs and their functions. CircFunBase is such a web-accessible database that aims to provide a high-quality functional circRNA resource including experimentally validated and computationally predicted functions. CircFunBase provides visualized circRNA-miRNA interaction networks. In addition, a genome browser is provided to visualize the genome context of circRNA. In this chapter, we illustrate examples of searching for circRNA and getting detailed information of circRNA. Moreover, other circRNA related databases are outlined.

Circular RNA circ-PTEN elevates PTEN inhibiting the proliferation of non-small cell lung cancer cells

Phosphatase and tensin homolog (PTEN) is a well-known tumor suppressor in various cancer types, including non-small cell lung cancer (NSCLC). Circular RNA (circRNA) has recently been proven to be strongly linked with cancer progression. Here, we aimed to investigate the biological relevance and clinical significance of circRNA derived from PTEN in NSCLC. We found that circ-PTEN (hsa_circ_0094342) was significantly decreased in NSCLC tissues and serum, which was attributed to the upregulation of RNA-binding protein DHX9. Low circ-PTEN was linked with malignant clinical features and poor outcome. Exogenous expression of circ-PTEN markedly inhibited NSCLC cell proliferation in vitro as well as retarded tumor growth in vivo. Circ-PTEN increased the expression of its host gene PTEN via acting as a sponge for miR-155 and miR-330-3p, leading to the inactivation of the carcinogenic PI3K/AKT signaling pathway. The xenograft tumor model also indicated the existence of circ-PTEN/miR-155/miR-330-3p/PTEN regulatory axis in vivo. Our data for the first time demonstrate that circ-PTEN functions as a tumor-inhibiting circRNA in NSCLC through post-transcriptionally regulating PTEN, hinting a promising diagnostic/prognostic biomarker as well as therapeutic target for NSCLC patients.

Whole-genome characterization of chronological age-associated changes in methylome and circular RNAs in moso bamboo (Phyllostachys edulis) from vegetative to floral growth

In mammals, DNA methylation is associated with aging. However, age-related DNA methylation changes during phase transitions largely remain unstudied in plants. Moso bamboo (Phyllostachys edulis) requires a very long time to transition from the vegetative to the floral phase. To comprehensively investigate the association of DNA methylation with aging, we present here single-base-resolution DNA methylation profiles using both high-throughput bisulfite sequencing and single-molecule nanopore-based DNA sequencing, covering the long period of vegetative growth and transition to flowering in moso bamboo. We discovered that CHH methylation gradually accumulates from vegetative to reproductive growth in a time-dependent fashion. Differentially methylated regions, correlating with chronological aging, occurred preferentially at both transcription start sites and transcription termination sites. Genes with CG methylation changes showed an enrichment of Gene Ontology (GO) categories in 'vegetative to reproductive phase transition of meristem'. Combining methylation data with mRNA sequencing revealed that DNA methylation in promoters, introns and exons may have different roles in regulating gene expression. Finally, circular RNA (circRNA) sequencing revealed that the flanking introns of circRNAs are hypermethylated and enriched in long terminal repeat (LTR) retrotransposons. Together, the observations in this study provide insights into the dynamic DNA methylation and circRNA landscapes, correlating with chronological age, which paves the way to study further the impact of epigenetic factors on flowering in moso bamboo.

Circular RNA circANKS1B acts as a sponge for miR-152-3p and promotes prostate cancer progression by upregulating TGF-α expression

BACKGROUND

A growing number of studies indicate that circular RNAs (circRNAs) play critical roles in human diseases, and show great potential as biomarkers and therapeutic targets. This study aimed to investigate the expression and function of circANKS1B in prostate cancer (PC).

METHODS

The expression of circANKS1B and miR-152-3p was analyzed by real-time quantitative reverse-transcription polymerase chain reaction (qRT-PCR). Cell migration and invasion were measured using a transwell assay. The interaction between circANKS1B and miR-152-3p was confirmed by a dual-luciferase reporter gene assay. Rescue experiments were conducted to determine whether circANKS1B regulated the invasion of PC cells via the circANKS1B-miR-152-3p-TGF-α pathway.

RESULTS

The expression of circANKS1B was markedly upregulated in both PC cells and tissues. Moreover, high circANKS1B expression was associated with poor prognosis in PC patients. Dual-luciferase reporter assay indicated that circANKS1B directly bound to miR-152-3p. Furthermore, circANKS1B negatively regulated miR-152-3p expression. Knockdown of circANKS1B markedly suppressed cell migration and invasion and TGF-α expression in PC cells, whereas the effects of circANKS1B silencing were reversed by miR-152-3p deficiency. In addition, the impact of miR-152-3p silencing on invasion of circANKS1B-deficient PC cells was also abrogated by TGF-α deficiency. Overall, circANKS1B acts as a sponge for miR-152-3p to promote PC progression by upregulating TGF-α expression.

CONCLUSION

Our findings reveal that circANKS1B may be a potential prognostic biomarker and therapeutic target for PC.

Circular RNAs as Novel Regulators of β-Cell Functions under Physiological and Pathological Conditions

Circular RNAs (circRNAs) constitute a large class of non-coding RNAs characterized by a covalently closed circular structure. They originate during mRNA maturation through a modification of the splicing process and, according to the included sequences, are classified as Exonic, Intronic, or Exonic-Intronic. CircRNAs can act by sequestering microRNAs, by regulating the activity of specific proteins, and/or by being translated in functional peptides. There is emerging evidence indicating that dysregulation of circRNA expression is associated with pathological conditions, including cancer, neurological disorders, cardiovascular diseases, and diabetes. The aim of this review is to provide a comprehensive and updated view of the most abundant circRNAs expressed in pancreatic islet cells, some of which originating from key genes controlling the differentiation and the activity of insulin-secreting cells or from diabetes susceptibility genes. We will particularly focus on the role of a group of circRNAs that contribute to the regulation of β-cell functions and that display altered expression in the islets of rodent diabetes models and of type 2 diabetic patients. We will also provide an outlook of the unanswered questions regarding circRNA biology and discuss the potential role of circRNAs as biomarkers for β-cell demise and diabetes development.

Circ_0061012 contributes to IL-22-induced proliferation, migration and invasion in keratinocytes through miR-194-5p/GAB1 axis in psoriasis

Psoriasis is a chronic inflammation-associated skin disorder featured by excessive proliferation and abnormal differentiation of keratinocytes. Here, we intended to investigate the role of circular RNA 0061012 (circ_0061012) in psoriasis progression. The expression of circ_0061012, SLMO2-ATP5E readthrough (SLMO2-ATP5E) messenger RNA (mRNA), microRNA-194-5p (miR-194-5p) and GRB2 associated binding protein 1 (GAB1) mRNA was determined by quantitative real-time polymerase chain reaction (qRT-PCR). Cell proliferation and metastasis were analyzed by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay and transwell assays. Western blot assay was used to measure the protein levels of Ki67, matrix metallopeptidase 9 (MMP9) and GAB1. Dual-luciferase reporter assay and RNA immune co-precipitation (RIP) assay were used to verify the interaction between miR-194-5p and circ_0061012 or GAB1. Circ_0061012 abundance was significantly enhanced in lesional skin samples from psoriasis patients than that in normal skin specimens from healthy volunteers. Interleukin-22 (IL-22) treatment increased the expression of circ_0061012 in a dose-dependent manner. Circ_0061012 silencing alleviated IL-22-induced promoting effects in the proliferation, migration and invasion of HaCaT cells. Circ_0061012 interacted with miR-194-5p, and miR-194-5p knockdown counteracted circ_0061012 silencing-mediated influences in IL-22-induced HaCaT cells. GAB1 was a target of miR-194-5p in HaCaT cells, and miR-194-5p hampered proliferation and metastasis which were induced by IL-22 partly through targeting GAB1. Circ_0061012 elevated the expression of GAB1 through sponging miR-194-5p in HaCaT cells. Circ_0061012 accelerated IL-22-induced proliferation and metastasis in HaCaT cells through enhancing GAB1 expression via sponging miR-194-5p in psoriasis.

Circular RNA circ_0081001 knockdown enhances methotrexate sensitivity in osteosarcoma cells by regulating miR-494-3p/TGM2 axis

BACKGROUND

Circular RNAs (circRNAs) have been shown to participate in the chemoresistance and tumorigenesis of multiple cancers. The purpose of this research was to investigate the function of circ_0081001 in methotrexate (MTX) resistance of osteosarcoma (OS) and its potential molecular mechanism.

METHODS

The expression of circ_0081001, cytochrome P450 family 51 subfamily A member 1 (CYP51A1), and miR-494-3p was detected by qRT-PCR. Cell viability, apoptosis, migration, and invasion were evaluated by Cell Counting Kit-8 (CCK-8) assay, flow cytometry, and transwell assay, respectively. Western blot (WB) assay was used to measure the protein levels of cleaved-caspase3 (cleaved-casp3), E-cadherin, N-cadherin, and transglutaminase-2 (TGM2). The interaction between miR-494-3p and circ_0081001 or TGM2 was predicted by bioinformatics analysis and verified using the dual-luciferase reporter assay. The mice xenograft model was established to investigate the roles of circ_0081001 in MTX resistance of OS in vivo.

RESULTS

Circ_0081001 and TGM2 were upregulated, and miR-494-3p was downregulated in MTX-resistant OS tissues and cells. Moreover, circ_0081001 interference enhanced cell sensitivity to MTX through promoting apoptosis and inhibiting cell viability and metastasis in vitro. Furthermore, circ_0081001 was identified as a molecular sponge of miR-494-3p to upregulate TGM2 level. In addition, circ_0081001 knockdown inhibited MTX resistance via upregulating miR-494-3p and downregulating TGM2. Besides, circ_0081001 downregulation improved MTX sensitivity of OS in vivo.

CONCLUSION

Knockdown of circ_0081001 enhanced MTX sensitivity of OS cells through downregulating TGM2 by sponging miR-494-3p, elucidating a novel regulatory mechanism for chemoresistance of OS and providing a potential circRNA-targeted therapy for OS.

Circular RNAs as important players in human gastric cancer

As one of the most prevalent gastrointestinal diseases, gastric cancer (GC) is the second leading cause of cancer-related deaths worldwide. Since GC has no clinical manifestations in the early stage of the disease, most patients are detected in the later phases of disease and have an unfortunately lower chance of recovery. Circular RNAs (circRNAs), a novel category of non-coding RNAs (ncRNAs), are mainly engaged in the regulation of gene expression at the transcriptional and post-transcriptional levels. Numerous evidences have revealed that circRNAs play key roles in GC as they are involved in cell proliferation, growth, and apoptosis via modulating the expression of some target genes, miRNAs, and proteins. Many studies have addressed the impact of circRNA dysregulation on GC initiation, progression, and invasion via binding to miRNAs or RNA binding proteins. Moreover, changes in circRNA expression are associated with pathological and clinical features of GC highlighting their potentials as diagnostic or prognostic biomarkers in GC. In the current study, the recent findings on the significance of circRNAs in the development and progression of GC are reviewed. We focus on the implications of circRNAs as potential diagnostic or prognostic biomarkers in this malignancy.

Biogenesis and functions of circular RNAs and their role in diseases of the female reproductive system

A member of the newly discovered RNA family, circular RNA (circRNA) is considered as the intermediate product of by-product splicing or abnormal RNA splicing. With the development of RNA sequencing, circRNA has recently drawn research interest. CircRNA exhibits stability, species conservatism, and tissue cell specificity. It acts as a miRNA sponge in the circRNA-microRNA (miRNA-mRNA axis, which can regulate gene transcription and protein translation. Studies have confirmed that circRNA is ubiquitous in eukaryotic cells, which play an important role in the regulation of human gene expression and participate in the occurrence and development of various human diseases. CircRNA may be closely related to the occurrence and development of female reproductive system diseases. By analyzing the biological functions and mechanism of circRNA, we find that circRNA has certain development prospects as biomarkers of the female reproductive system diseases. The production and degradation of circRNA, biological functions, and their association with the occurrence of diseases of female reproductive system are reviewed in this article.

Emerging Role for Linear and Circular Spermine Oxidase RNAs in Skeletal Muscle Physiopathology

Skeletal muscle atrophy is a pathological condition so far without effective treatment and poorly understood at a molecular level. Emerging evidence suggest a key role for circular RNAs (circRNA) during myogenesis and their deregulation has been reported to be associated with muscle diseases. Spermine oxidase (SMOX), a polyamine catabolic enzyme plays a critical role in muscle differentiation and the existence of a circRNA arising from SMOX gene has been recently identified. In this study, we evaluated the expression profile of circular and linear SMOX in both C2C12 differentiation and dexamethasone-induced myotubes atrophy. To validate our findings in vivo their expression levels were also tested in two murine models of amyotrophic lateral sclerosis: SOD1G93A and hFUS+/+, characterized by progressive muscle atrophy. During C2C12 differentiation, linear and circular SMOX show the same trend of expression. Interestingly, in atrophy circSMOX levels significantly increased compared to the physiological state, in both in vitro and in vivo models. Our study demonstrates that SMOX represents a new player in muscle physiopathology and provides a scientific basis for further investigation on circSMOX RNA as a possible new therapeutic target for the treatment of muscle atrophy.

Hsa_circ_0081534 increases the proliferation and invasion of nasopharyngeal carcinoma cells through regulating the miR-508-5p/FN1 axis

Accumulating lines of evidence indicate that circular RNAs (circRNAs) are involved in the pathogenesis of human cancers, including nasopharyngeal carcinoma (NPC). However, the influences of hsa_circ_0081534 upon the pathogenesis and dynamics of NPC are undescribed. In this study, we identified a circRNA hsa_circ_0081534 was significantly upregulated in NPC tissues and cell lines. Inhibition of hsa_circ_0081534 induced a decrease in NPC cells proliferation and invasion in vitro, and repressed tumor growth in vivo. In mechanism, hsa_circ_0081534 promoted NPC progression by sponging miR-508-5p. Fibronectin 1 (FN1) is a target gene of miR-508-5p. In addition, rescue assays showed that FN1 overexpression (or miR-508-5p inhibitors) abolished the roles of hsa_circ_0081534 inhibition on NPC cells proliferation and invasion. Therefore, hsa_circ_0081534 promoted the proliferation, and invasion of NPC cells via regulating the miR-508-5p/FN1 axis. Our findings suggested that hsa_circ_0081534 could be a novel therapeutic target for the treatment of NPC patients.

Knockdown of hsa_circ_0037658 inhibits the progression of osteoarthritis via inducing autophagy

Osteoarthritis (OA) is a chronic musculoskeletal degeneration disease that can result in chronic pain and functional disability. Circular RNAs (CirRNAs) are known to be involved in OA. It was reported that hsa_circ_0037658 was notably upregulated in OA tissues; however, the biological role of hsa_circ_0037658 in OA remains unclear. To investigate the function of hsa_circ_0037658 in OA, CHON-001 cells were treated with IL-1β. The effect of hsa_circ_0037658 knockdown on cell growth was tested by CCK-8 and immunofluorescence staining. In addition, the correlation between hsa_circ_0037658 and autophagy was explored by LC3 staining and western blot. The results indicated that hsa_circ_0037658 was significantly upregulated in IL-1β-treated CHON-001 cells. The silencing of hsa_circ_0037658 could protect CHON-001 cell injury against IL-1β. Moreover, hsa_circ_0037658 shRNA reversed IL-1β-induced cell growth inhibition via inducing cell autophagy. Furthermore, knockdown of hsa_circ_0037658 notably alleviated the symptom of OA in vivo. To sum up, knockdown of hsa_circ_0037658 suppressed the progression of OA via inducing autophagy. Thus, hsa_circ_0037658 might serve as a potential target for the treatment of OA.

Non-Coding RNAs in the Brain-Heart Axis: The Case of Parkinson's Disease

Parkinson's disease (PD) is a complex and heterogeneous disorder involving multiple genetic and environmental influences. Although a wide range of PD risk factors and clinical markers for the symptomatic motor stage of the disease have been identified, there are still no reliable biomarkers available for the early pre-motor phase of PD and for predicting disease progression. High-throughput RNA-based biomarker profiling and modeling may provide a means to exploit the joint information content from a multitude of markers to derive diagnostic and prognostic signatures. In the field of PD biomarker research, currently, no clinically validated RNA-based biomarker models are available, but previous studies reported several significantly disease-associated changes in RNA abundances and activities in multiple human tissues and body fluids. Here, we review the current knowledge of the regulation and function of non-coding RNAs in PD, focusing on microRNAs, long non-coding RNAs, and circular RNAs. Since there is growing evidence for functional interactions between the heart and the brain, we discuss the benefits of studying the role of non-coding RNAs in organ interactions when deciphering the complex regulatory networks involved in PD progression. We finally review important concepts of harmonization and curation of high throughput datasets, and we discuss the potential of systems biomedicine to derive and evaluate RNA biomarker signatures from high-throughput expression data.

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

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

CircRNA_100876 sponges miR-136 to promote proliferation and metastasis of gastric cancer by upregulating MIEN1 expression

Previous reports have revealed that circRNA_100876 was extremely important in the progression of triple-negative breast cancer. Nevertheless, the mechanism towards the role of circRNA_100876 in Gastric cancer (GC) remains unknown. Here, we determined circRNA_100876 expression by quantitative real-time PCR (qRT-PCR) in twenty pairs of GC tissues and adjacent tissues. Our data indicated that the expression of circRNA_100876 was raised in GC tissues. In vitro, functional experiments confirmed that cell proliferation, invasion along with migration was promoted by circRNA_100876 in GC tissues. Simultaneously, relative luciferase assay uncovered that circRNA_100876 functioned as a sponge for miR-136, followed by retarding miR-136-induced inhibited effects on the corresponding target, MIEN1. Moreover, we revealed that the expression of MIEN1 was up-regulated and correlated to much worse prognosis of GC. Collectively, our data identified that the promotion of GC growth and metastasis induced by circRNA_100876 interacted with miR-136 and MIEN1, indicating an emerging announcement for uncovering the potential mechanism of GC progression.

Role of circular RNAs in brain development and CNS diseases

In mammals, many classes of noncoding RNAs (ncRNAs) are expressed at a much higher level in the brain than in other organs. Recent studies have identified a new class of ncRNAs called circular RNAs (circRNAs), which are produced by back-splicing and fusion of either exons, introns, or both exon-intron into covalently closed loops. The circRNAs are also highly enriched in the brain and increase continuously from the embryonic to the adult stage. Although the functional significance and mechanism of action of circRNAs are still being actively explored, they are thought to regulate the transcription of their host genes and sequestration of miRNAs and RNA binding proteins. Some circRNAs are also shown to have translation potential to form peptides. The expression and abundance of circRNAs seem to be spatiotemporally maintained in a normal brain. Altered expression of circRNAs is also thought to mediate several disorders, including brain-tumor growth, and acute and chronic neurodegenerative disorders by affecting mechanisms such as angiogenesis, neuronal plasticity, autophagy, apoptosis, and inflammation. This review discusses the involvement of various circRNAs in brain development and CNS diseases. A better understanding of the circRNA function will help to develop novel therapeutic strategies to treat CNS complications.

Hsa_circ_0008225 inhibits tumorigenesis of glioma via sponging miR-890 and promoting ZMYND11 expression

BACKGROUND

Circular RNAs (circRNAs) play an important role in the tumorigenesis of glioma. Our study indicated that low hsa_circ_0008225 expression was associated with poor overall survival in patients with glioma. However, the relevant mechanism of hsa_circ_0008225 in glioma tumorigenesis remains unclear.

METHODS

Two datasets (GSE86202 and GSE92322) were downloaded from the Gene Expression Omnibus (GEO) database. The differentially expressed circRNAs (DEcircRNAs) between glioma tissues and matched normal tissues were screened using R language.

RESULTS

A total of 79 overlapping DEcircRNAs were identified by comparison of glioma and matched normal tissues. In addition, low hsa_circ_0008225 expression was associated with poor overall survival in patients with glioma. Overexpression of hsa_circ_0008225 markedly inhibited the proliferation, migration and invasion of SHG44 cells via inducing apoptosis. Mechanically, overexpression of hsa_circ_0008225 increased the expression of miR-890 targeted gene ZMYND11 via acting as a competitive 'sponge' of miR-890.

CONCLUSION

Our results suggested that hsa_circ_0008225 functions as a tumor inhibitor in glioma by sponging miR-890 and then promoting the function of ZMYND11. Therefore, hsa_circ_0008225 could be a potential prognostic biomarker for the treatment of glioma.

Circular RNA-protein interactions: functions, mechanisms, and identification

Circular RNAs (circRNAs) are covalently closed, endogenous RNAs with no 5' end caps or 3' poly(A) tails. These RNAs are expressed in tissue-specific, cell-specific, and developmental stage-specific patterns. The biogenesis of circRNAs is now known to be regulated by multiple specific factors; however, circRNAs were previously thought to be insignificant byproducts of splicing errors. Recent studies have demonstrated their activity as microRNA (miRNA) sponges as well as protein sponges, decoys, scaffolds, and recruiters, and some circRNAs even act as translation templates in multiple pathophysiological processes. CircRNAs bind and sequester specific proteins to appropriate subcellular positions, and they participate in modulating certain protein-protein and protein-RNA interactions. Conversely, several proteins play an indispensable role in the life cycle of circRNAs from biogenesis to degradation. However, the exact mechanisms of these interactions between proteins and circRNAs remain unknown. Here, we review the current knowledge regarding circRNA-protein interactions and the methods used to identify and characterize these interactions. We also summarize new insights into the potential mechanisms underlying these interactions.

Functions of circular RNAs and their potential applications in gastric cancer

Introduction: In recent years, circular RNAs (circRNAs) have emerged in the field of RNA research and their biological functions are now being gradually identified. circRNAs are divided into three categories: exonic circular RNAs (ecircRNAs), exon-intron circular RNAs (EIciRNAs), and intronic circular RNAs (ciRNAs). The circular structure of circRNAs confers unique biological characteristics upon them, such as enhanced stability over linear RNAs.Areas covered: circRNAs function to competitively bind with microRNAs (miRNAs) and proteins, participate in protein coding, regulate transcription, and form pseudogenes after reverse transcription. In gastric cancer, the circRNA-miRNA-mRNA axis is the most studied mechanisms underlying gastric cancer occurrence and development. Some specific and sensitive circRNAs, such as hsa_circ_102958, hsa_circ_0000520, and hsa_circ_0001017 may have potential diagnostic potential in early-stage gastric cancer. Abnormal expression of some circRNAs, including circ-LMTK2, circ-PSMC3, and circ-DLST are associated with the development of gastric cancer. Other circRNAs, such as hsa_circ_0001368, circ-ZFR, and circ-ERBB2, may also play important roles in gastric cancer treatment.Expert opinion: Exploring the roles of circRNAs in gastric cancer occurrence and development will help us to elucidate the functions of circRNAs and develop potential tools for early diagnosis and effective treatment of gastric cancer.

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.

CircularRNA as novel biomarkers in liver diseases

The liver is an essential organ that is primarily responsible for digestion and eliminating toxic substances from the body. After the industrial revolution, Western diet and lifestyle changes have increased the incidence of several liver diseases, including non-alcoholic fatty liver disease (NAFLD), non-alcoholic steatohepatitis (NASH), liver cirrhosis, and hepatocellular carcinoma (HCC). NAFLD and NASH are mostly asymptomatic at early stages, and the disease progression from NAFLD to life-threatening HCC remains not fully understood. Circular RNA (circRNA) is consist of a circular structure, and the circRNA-microRNA(miRNA)-mRNA axes have been shown to be involved in several cellular events, including apoptosis, vascularization, metastasis, etc. The highly stable structure of circRNAs has enabled themselves to be used as putative biomarkers of several diseases. Here, we conducted a literature review and discussed the identified roles of circRNAs in NAFLD, NASH, liver cirrhosis, and HCC. For example, deficiency of circRNA_0046366 and circRNA_0046367 has been shown as the characteristics of NAFLD, and restoration of these circRNAs ameliorates the oxidative stress, lipotoxicity, and disease severity in NAFLD. Silencing of circ_0071410 was shown to alleviate hepatic stellate activation, the key step of liver cirrhosis. CDR1 and circ_0067934 can facilitate the invasion and metastasis of HCC, while circMTO1 negatively regulates the progression of HCC. Although several research works have been conducted, the whole picture of circRNA-related underlying mechanisms is unclear. Future works using high-throughput bioinformatic approaches will be needed to delineate the role of circRNAs in liver diseases and to further develop novel diagnostics and therapeutics.

Advances on circRNAs Contribute to Carcinogenesis and Progression in Papillary Thyroid Carcinoma

In view of the highly increased prevalence of papillary thyroid carcinoma (PTC) year by year, it is of great importance to explore new molecular targets for anticancer strategies. Emerging evidence indicates that circular RNAs (circRNAs), characterized by a closed-loop structure and high stability, play important roles in tumorigenesis and development of human cancer by regulating multiple complex biological processes, such as cellular proliferation, metastasis, and metabolism. A comprehensive understanding of the roles of circRNAs will facilitate the development of promising future therapeutic strategies for treating cancers, including PTC. In this paper, we review the profile of circRNA in PTC, its regulatory roles, and the pathological mechanism as well as their related clinical significance. In addition, challenges of this specific field are discussed.

Hsa_circ_0001546 acts as a miRNA-421 sponge to inhibit the chemoresistance of gastric cancer cells via ATM/Chk2/p53-dependent pathway

Circular RNAs (circRNAs) are a new class of noncoding RNAs, play a crucial role in tumor initiation and development. Hsa_circ_0001546 is a novel circular RNA that was downregulated in gastric cancer (GC) tissues, however its function and mechanism in GC has not been studied. Our study verified that circ_0001546 was decreased in GC and correlated with the poor prognosis. Next, Pull-down assay and dual-luciferase reporter assay verified that miR-421 was a target of circ_0001546 while ATM (Ataxia telangiectasia mutated) was target by miR-421. Overexpression of circ_0001546 inhibited the proliferation and chemoresistance of HGC-27 cells, and increased the expression of ATM. In addition, circ_0001546 overexpression reversed the effect of miR-421 overexpression. What is more, circ_0001546 inhibits the chemoresistance of HGC-27 cells to L-OPH (Oxaliplatin) may through the activation of the ATM/checkpoint kinase 2 (Chk2)/p53-dependent signaling pathway. In summary, our study proved that circ_0001546 sponges miR-421 to upregulate the expression level of ATM and inhibit the proliferation and chemoresistance through the activation of the ATM/Chk2/p53-dependent pathway.

Circular RNA Regulation of Myogenesis

Circular RNA (circRNA) is a novel class of non-coding RNA generated by pre-mRNA back splicing, which is characterized by a closed-loop structure. Although circRNAs were firstly reported decades ago, their regulatory roles have not been discovered until recently. In this review, we discussed the putative biogenesis pathways and regulatory functions of circRNAs. Recent studies showed that circRNAs are abundant in skeletal muscle tissue, and their expression levels are regulated during muscle development and aging. We, thus, characterized the expression profile of circRNAs in skeletal muscle and discussed regulatory functions and mechanism-of-action of specific circRNAs in myogenesis. The future investigation into the roles of circRNAs in both physiological and pathological conditions may provide novel insights in skeletal muscle development and provide new therapeutic strategies for muscular diseases.

Adipose circular RNAs exhibit dynamic regulation in obesity and functional role in adipogenesis

Non-coding RNAs are emerging as novel regulators in adipocyte differentiation and function. Circular RNAs (circRNAs) are a new class of non-coding transcripts generated across all eukaryotic tissues, but their function in adipose biology remains unknown. Here we perform deep sequencing of visceral and subcutaneous fat to discover thousands of adipose circRNAs, many of which are species conserved, tissue specific and dynamically regulated during adipogenesis and obesity. We identify circTshz2-1 and circArhgap5-2 as indispensable regulators of adipogenesis in vitro. To characterize the function of circRNAs in vivo, we deliver adenoviral shRNA targeting circArhgap5-2 into mouse inguinal tissue and show that the expression of this circRNA is essential in maintaining the global adipocyte transcriptional programme involved in lipid biosynthesis and metabolism. We also demonstrate that the pro-adipogenic function of circArhgap5-2 is conserved in human adipocytes. Our results provide important evidence that circRNAs serve as important regulators in adipocyte differentiation and metabolism.

circOMA1-Mediated miR-145-5p Suppresses Tumor Growth of Nonfunctioning Pituitary Adenomas by Targeting TPT1

CONTEXT

Nonfunctioning pituitary adenomas (NFPAs) are the major cause of hypopituitarism and infertility. However, the pathogenesis of NFPAs remains largely unknown. Previous studies have demonstrated the crucial role of miRNAs in the progression of pituitary adenomas. Increasing evidence has indicated that circular RNAs (circRNAs) might mediate miRNA transcriptional activity, providing new insights to study the pathogenesis of NFPAs.

OBJECTIVES

To explore the regulation and activity of the circRNA-miRNA-mRNA axis in the tumorigenesis of NFPAs.

DESIGN

The function of miR-145-5p in NFPAs was investigated invitro and invivo. The mechanical details were explored and potential targets of miR-145-5p were identified. Finally, miR-145-5p-associated circRNAs were functionally recognized and confirmed.

RESULTS

miR-145-5p was markedly decreased in NFPA samples and correlated negatively with NFPA invasiveness. Overexpression of miR-145-5p suppressed NFPA cell proliferation and invasiveness and promoted apoptosis. Further results confirmed that translationally controlled tumor protein (TPT1) is a target of miR-145-5p and mediated the effect of miR-145-5p. TPT1 and its downstream factors Mcl-1 and Bcl-xL were downregulated, and Bax was upregulated by miR-145-5p. Moreover, circOMA1 (hsa_circRNA_0002316) was demonstrated to sponge miR-145-5p, whose suppression on NFPA cells was abrogated by circOMA1 overexpression. circOMA1 silencing exhibited a similar inhibitory effect with miR-145-5p overexpression by downregulating TPT1. We found that circOMA1 could further upregulate Mcl-1 and Bcl-xL and downregulate Bax.

CONCLUSIONS

circOMA1 promotes NFPA progression by acting as the sponge of tumor suppressor miR-145-5p to regulate the TPT1 signaling pathway, revealing a therapeutic target in preventing the tumorigenesis of NFPAs.