Circ_0007031 enhances tumor progression and promotes 5-fluorouracil resistance in colorectal cancer through regulating miR-133b/ABCC5 axis

Examining the evidence for mutual modulation between m6A modification and circular RNAs: current knowledge and future prospects

The resistance of cancer cells to treatment significantly impedes the success of therapy, leading to the recurrence of various types of cancers. Understanding the specific mechanisms of therapy resistance may offer novel approaches for alleviating drug resistance in cancer. Recent research has shown a reciprocal relationship between circular RNAs (circRNAs) and N6-methyladenosine (m6A) modification, and their interaction can affect the resistance and sensitivity of cancer therapy. This review aims to summarize the latest developments in the m6A modification of circRNAs and their importance in regulating therapy resistance in cancer. Furthermore, we explore their mutual interaction and exact mechanisms and provide insights into potential future approaches for reversing cancer resistance.

The function and mechanism of circRNAs in 5-fluorouracil resistance in tumors: Biological mechanisms and future potential

5-Fluorouracil (5-FU) is a well-known chemotherapy drug extensively used in the treatment of breast cancer. It works by inhibiting cancer cell proliferation and inducing cell death through direct incorporation into DNA and RNA via thymidylate synthase (TS). Circular RNAs (circRNAs), a novel family of endogenous non-coding RNAs (ncRNAs) with limited protein-coding potential, contribute to 5-FU resistance. Their identification and targeting are crucial for enhancing chemosensitivity. CircRNAs can regulate tumor formation and invasion by adhering to microRNAs (miRNAs) and interacting with RNA-binding proteins, regulating transcription and translation. MiRNAs can influence enzymes responsible for 5-FU metabolism in cancer cells, affecting their sensitivity or resistance to the drug. In the context of 5-FU resistance, circRNAs can target miRNAs and regulate biological processes such as cell proliferation, cell death, glucose metabolism, hypoxia, epithelial-to-mesenchymal transition (EMT), and drug efflux. This review focuses on the function of circRNAs in 5-FU resistance, discussing the underlying molecular pathways and biological mechanisms. It also presents recent circRNA/miRNA-targeted cancer therapeutic strategies for future clinical application.

Roles of circRNAs in the progression of colorectal cancer: novel strategies for detection and therapy

Endogenous noncoding RNAs with a covalently closed loop are known as circular RNAs (circRNAs). Recently, published works have revealed that circRNAs, which act as microRNA sponges, are critical for the biological behavior of several kinds of malignancies, including tumor cell proliferation, apoptosis, invasion, and metastasis. Additionally, there is a significant correlation between circRNAs and tumor resistance, stage, prognosis, and size. At present, colorectal cancer (CRC) is one of the most serious malignant tumors for human health. CircRNAs could represent potential targets to use in the prevention, diagnosis, and therapy of CRC, according to many studies. To fully comprehend the role of circRNAs in the incidence and progression of CRC, this review outlines the regulatory role and mechanisms of circRNAs in CRC and assesses their potential relevance as diagnostic and treatment possibilities for CRC. Our goal is to offer meaningful biological information for clinical evaluation and decision-making process for CRC treatment.

Utilizing non-coding RNA-mediated regulation of ATP binding cassette (ABC) transporters to overcome multidrug resistance to cancer chemotherapy

Multidrug resistance (MDR) is one of the primary factors that produces treatment failure in patients receiving cancer chemotherapy. MDR is a complex multifactorial phenomenon, characterized by a decrease or abrogation of the efficacy of a wide spectrum of anticancer drugs that are structurally and mechanistically distinct. The overexpression of the ATP-binding cassette (ABC) transporters, notably ABCG2 and ABCB1, are one of the primary mediators of MDR in cancer cells, which promotes the efflux of certain chemotherapeutic drugs from cancer cells, thereby decreasing or abolishing their therapeutic efficacy. A number of studies have suggested that non-coding RNAs (ncRNAs), particularly microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs), play a pivotal role in mediating the upregulation of ABC transporters in certain MDR cancer cells. This review will provide updated information about the induction of ABC transporters due to the aberrant regulation of ncRNAs in cancer cells. We will also discuss the measurement and biological profile of circulating ncRNAs in various body fluids as potential biomarkers for predicting the response of cancer patients to chemotherapy. Sequence variations, such as alternative polyadenylation of mRNA and single nucleotide polymorphism (SNPs) at miRNA target sites, which may indicate the interaction of miRNA-mediated gene regulation with genetic variations to modulate the MDR phenotype, will be reviewed. Finally, we will highlight novel strategies that could be used to modulate ncRNAs and circumvent ABC transporter-mediated MDR.

Circular RNA circSEMA5A facilitates colorectal cancer development by regulating microRNA-195-5p to target CCNE1 axis

Colorectal cancer (CRC) is one of the most prevalent malignancies with a high mortality rate worldwide. Circular RNAs (circRNAs) have lately emerged as key molecules involved in cancer development and metastasis. CircSEMA5 is reported to be oncogenic in some cancers, yet its role in the pathogenesis of CRC remains unknown. Herein, we attempted to investigate the functional role and molecular mechanism of circSEMA5A underlying CRC progression. RT-qPCR and RNase R digestion assays were used to evaluate circSEMA5A expression characteristics in CRC cells. Loss-of-function assays were performed to clarify circSEMA5A role in CRC biological processes. Bioinformatics and mechanism experiments were conducted to assess the association of circSEMA5A or CCNE1 with miR-195-5p in CRC cells. Rescue assays were conducted to explore the regulatory function of circSEMA5A-miR-195-5p-CCNE1 in CRC cellular processes. Through bioinformatics and functional screening, we found that circSEMA5A was highly expressed in CRC cells and was mainly localized in the nucleus. CircSEMA5A promoted CRC proliferative, migratory, and invasive capabilities in cultured cells and facilitated the tumorigenic process in xenografts; however, circSEMA5A silencing repressed tumor metastasis in CRC cells. Mechanistically, circSEMA5A was competitively bound with miR-195-5p to upregulate CCNE1 expression. Moreover, the impact of circSEMA5A knockdown on CRC cell proliferative, migratory, and invasive capabilities was countervailed by miR-195-5p inhibitor or CCNE1 overexpression. To summarize, circSEMA5A is a novel circRNA that serves as an oncogene in CRC progression. CircSEMA5A facilitates CRC cell malignancy and tumor growth through sponging miR-195-5p to upregulate CCNE1, thus providing a new direction for CRC diagnosis and targeted therapy.

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

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

Novel strategies to reverse chemoresistance in colorectal cancer

Colorectal cancer (CRC) is a common gastrointestinal malignancy with high morbidity and fatality. Chemotherapy, as traditional therapy for CRC, has exerted well antitumor effect and greatly improved the survival of CRC patients. Nevertheless, chemoresistance is one of the major problems during chemotherapy for CRC and significantly limits the efficacy of the treatment and influences the prognosis of patients. To overcome chemoresistance in CRC, many strategies are being investigated. Here, we review the common and novel measures to combat the resistance, including drug repurposing (nonsteroidal anti-inflammatory drugs, metformin, dichloroacetate, enalapril, ivermectin, bazedoxifene, melatonin, and S-adenosylmethionine), gene therapy (ribozymes, RNAi, CRISPR/Cas9, epigenetic therapy, antisense oligonucleotides, and noncoding RNAs), protein inhibitor (EFGR inhibitor, S1PR2 inhibitor, and DNA methyltransferase inhibitor), natural herbal compounds (polyphenols, terpenoids, quinones, alkaloids, and sterols), new drug delivery system (nanocarriers, liposomes, exosomes, and hydrogels), and combination therapy. These common or novel strategies for the reversal of chemoresistance promise to improve the treatment of CRC.

Biological functions, mechanisms, and clinical significance of circular RNA in colorectal cancer

Colorectal cancer (CRC) is a leading cause of cancer-related death worldwide due to the lack of effective diagnosis and prognosis biomarkers and therapeutic targets, resulting in poor patient survival rates. Circular RNA (circRNA) is a type of endogenous non-coding RNA (ncRNA) with a closed-loop structure that plays a crucial role in physiological processes and pathological diseases. Recent studies indicate that circRNAs are involved in the diagnosis, prognosis, drug resistance, and development of tumors, particularly in CRC. Therefore, circRNA could be a potential new target for improving CRC diagnosis, prognosis, and treatment. This review focuses on the origin and biological functions of circRNA, summarizes recent research on circRNA's role in CRC, and discusses the potential use of circRNAs as clinical biomarkers for cancer diagnosis and prognosis, as well as therapeutic targets for CRC treatment.

Systematic analysis of circRNA biomarkers for diagnosis, prognosis and therapy in colorectal cancer

As the third most common cancer and the second leading cause of cancer death worldwide, colorectal cancer (CRC) poses a serious threat to people's health. In recent years, circRNA has been widely reported as a new biomarker in CRC, but a comprehensive summary and analysis is lacking. This study aims to evaluate the diagnostic, therapeutic and prognostic significance of circRNAs in CRC by systematically analysing their expression patterns, biological functions and clinical significance in CRC. The literature on circRNA in CRC was searched in the PubMed database and included for analysis after screening according to strict inclusion and exclusion criteria. The UALCAN online tool was used to obtain host gene expression data. The miRTargetLink 2.0 was used to predict target genes for miRNAs action in CRC patients. Cytoscape was used to construct circRNA-miRNA-mRNA interaction networks. From the 236 included papers, we identified 217 circRNAs and their associated 108 host genes and 145 miRNAs. Among the 145 miRNAs, 27 miRNAs had no corresponding target genes. After prediction of target genes and differential analysis, a total of 25 target genes were obtained and a circRNA-miRNA-mRNA interaction network was constructed. Among the 217 circRNAs, 74 were associated with diagnosis, 160 with treatment and 51 with prognosis. And 154 of them function as oncogenes while 58 as tumour suppressor genes. In addition, these circRNAs include 32 exosomal circRNAs, which have unique advantages as biomarkers. In total, we summarize and analyze the expression patterns, biological functions and clinical significance of circRNAs in CRC. In addition, we constructed some new circRNA-miRNA-mRNA regulatory axes based on the miRNAs sponged by circRNAs.

Circ_0082182 upregulates the NFIB level via sponging miR-326 to promote oxaliplatin resistance and malignant progression of colorectal cancer cells

Circular RNAs (circRNAs) are key regulators in tumor metastasis and drug resistance. This study was designed to investigate circ_0082182 function and mechanism in oxaliplatin (OXA) resistance and cancer progression of colorectal cancer (CRC). The circ_0082182, microRNA-326 (miR-326), and nuclear factor I B (NFIB) levels were quantified by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). Cell sensitization was analyzed by Cell Counting Kit-8 assay. The proliferation ability was determined via EdU assay, and apoptosis was measured by flow cytometry. Transwell assay and wound healing assay were performed to assess cell invasion and migration. The protein level was examined through Western blot. The binding interaction was conducted via dual-luciferase reporter assay. Xenograft tumor assay was used to explore the circ_0082182 function in vivo. The circ_0082182 level was upregulated in OXA-resistant CRC samples and cells. Downregulation of circ_0082182 suppressed OXA resistance, proliferation, invasion, and migration but promoted apoptosis of OXA-resistant CRC cells. Circ_0082182 acted as a sponge for miR-326. The regulatory role of circ_0082182 was ascribed to the miR-326 sponging function. MiR-326 directly targeted NFIB to impede OXA resistance and cancer progression in CRC cells. NFIB level was regulated by circ_0082182 via sponging miR-326. Circ_0082182 promoted tumor growth in OXA-resistant xenograft tumor model through mediating the miR-326/NFIB axis. These data suggested that circ_0082182 elevated the NFIB expression to regulate OXA resistance and CRC progression by absorbing miR-326.

Mechanism underlying circRNA dysregulation in the TME of digestive system cancer

Circular RNAs (circRNAs) are a new series of noncoding RNAs (ncRNAs) that have been reported to be expressed in eukaryotic cells and have a variety of biological functions in the regulation of cancer pathogenesis and progression. The TME, as a microscopic ecological environment, consists of a variety of cells, including tumor cells, immune cells and other normal cells, ECM and a large number of signaling molecules. The crosstalk between circRNAs and the TME plays a complicated role in affecting the malignant behaviors of digestive system cancers. Herein, we summarize the mechanisms underlying aberrant circRNA expression in the TME of the digestive system cancers, including immune surveillance, angiogenesis, EMT, and ECM remodelling. The regulation of the TME by circRNA is expected to be a new therapeutic method.

Non-coding RNA in cancer drug resistance: Underlying mechanisms and clinical applications

Cancer is one of the most frequently diagnosed malignant diseases worldwide, posing a serious, long-term threat to patients’ health and life. Systemic chemotherapy remains the first-line therapeutic approach for recurrent or metastatic cancer patients after surgery, with the potential to effectively extend patient survival. However, the development of drug resistance seriously limits the clinical efficiency of chemotherapy and ultimately results in treatment failure and patient death. A large number of studies have shown that non-coding RNAs (ncRNAs), particularly microRNAs, long non-coding RNAs, and circular RNAs, are widely involved in the regulation of cancer drug resistance. Their dysregulation contributes to the development of cancer drug resistance by modulating the expression of specific target genes involved in cellular apoptosis, autophagy, drug efflux, epithelial-to-mesenchymal transition (EMT), and cancer stem cells (CSCs). Moreover, some ncRNAs also possess great potential as efficient, specific biomarkers in diagnosis and prognosis as well as therapeutic targets in cancer patients. In this review, we summarize the recent findings on the emerging role and underlying mechanisms of ncRNAs involved in cancer drug resistance and focus on their clinical applications as biomarkers and therapeutic targets in cancer treatment. This information will be of great benefit to early diagnosis and prognostic assessments of cancer as well as the development of ncRNA-based therapeutic strategies for cancer patients.

Mutual regulation between N6-methyladenosine (m6A) modification and circular RNAs in cancer: impacts on therapeutic resistance

The resistance of tumor cells to therapy severely impairs the efficacy of treatment, leading to recurrence and metastasis of various cancers. Clarifying the underlying mechanisms of therapeutic resistance may provide new strategies for overcoming cancer resistance. N6-methyladenosine (m6A) is the most prevalent RNA modification in eukaryotes, and is involved in the regulation of RNA splicing, translation, transport, degradation, stability and processing, thus affecting several physiological processes and cancer progression. As a novel type of multifunctional non-coding RNAs (ncRNAs), circular RNAs (circRNAs) have been demonstrated to play vital roles in anticancer therapy. Currently, accumulating studies have revealed the mutual regulation of m6A modification and circRNAs, and their interaction can further influence the sensitivity of cancer treatment. In this review, we mainly summarized the recent advances of m6A modification and circRNAs in the modulation of cancer therapeutic resistance, as well as their interplay and potential mechanisms, providing promising insights and future directions in reversal of therapeutic resistance in cancer.

Prediction of Pathogenic Factors in Dysbiotic Gut Microbiomes of Colorectal Cancer Patients Using Reverse Microbiomics

Background

Gut microbiome plays a crucial role in the formation and progression of colorectal cancer (CRC). To better identify the underlying gene-level pathogenic mechanisms of microbiome-associated CRC, we applied our newly developed Reverse Microbiomics (RM) to predict potential pathogenic factors using the data of microbiomes in CRC patients.

Results

Our literature search first identified 40 bacterial species enriched and 23 species depleted in the guts of CRC patients. These bacteria were systematically modeled and analyzed using the NCBI Taxonomy ontology. Ten species, including 6 enriched species (e.g., Bacteroides fragilis, Fusobacterium nucleatum and Streptococcus equinus) and 4 depleted species (e.g., Bacteroides uniformis and Streptococcus thermophilus) were chosen for follow-up comparative genomics analysis. Vaxign was used to comparatively analyze 47 genome sequences of these ten species. In total 18 autoantigens were predicted to contribute to CRC formation, six of which were reported with experimental evidence to be correlated with drug resistance and/or cell invasiveness of CRC. Interestingly, four human homology proteins (EDK89078.1, EDK87700.1, EDK89777.1, and EDK89145.1) are conserved among all enriched strains. Furthermore, we predicted 76 potential virulence factors without homology to human proteins, including two riboflavin synthase proteins, three ATP-binding cassettes (ABC) transporter protein family proteins, and 12 outer membrane proteins (OMPs). Riboflavin synthase is present in all the enriched strains but not in depleted species. The critical role of riboflavin synthase in CRC development was further identified from its hub role in our STRING-based protein-protein interaction (PPI) network analysis and from the finding of the riboflavin metabolism as the most significantly enriched pathway in our KEGG pathway analysis. A novel model of the CRC pathogenesis involving riboflavin synthase and other related proteins including TpiA and GrxC was further proposed.

Conclusions

The RM strategy was used to predict 18 autoantigens and 76 potential virulence factors from CRC-associated microbiome data. In addition to many of these autoantigens and virulence factors experimentally verified as reported in the literature, our study predicted many new pathogenetic factors and developed a new model of CRC pathogenesis involving the riboflavin synthase from the enriched colorectal bacteria and other associated proteins.

Advances in the Study of CircRNAs in Tumor Drug Resistance

Recent studies have revealed that circRNAs can affect tumor DNA damage and repair, apoptosis, proliferation, and invasion and influence the transport of intratumor substances by acting as miRNA sponges and transcriptional regulators and binding to proteins in a variety of ways. However, research on the role of circRNAs in cancer radiotherapy and chemoresistance is still in its early stages. Chemotherapy is a common approach to oncology treatment, but the development of tumor resistance limits the overall clinical efficacy of chemotherapy for cancer patients. The current study suggests that circRNAs have a facilitative or inhibitory effect on the development of resistance to conventional chemotherapy in a variety of tumors, suggesting that circRNAs may serve as a new direction for the study of antitumor drug resistance. In this review, we will briefly discuss the biological features of circRNAs and summarize the recent progression of the involvement of circRNAs in the development and pathogenesis of cancer chemoresistance.

Non-Coding RNAs Regulate the Resistance to Anti-EGFR Therapy in Colorectal Cancer

Colorectal cancer (CRC) is the third prevalent cancer worldwide, the morbidity and mortality of which have been increasing in recent years. As molecular targeting agents, anti-epidermal growth factor receptor (EGFR) monoclonal antibodies (McAbs) have significantly increased the progression-free survival (PFS) and overall survival (OS) of metastatic CRC (mCRC) patients. Nevertheless, most patients are eventually resistant to anti-EGFR McAbs. With the intensive study of the mechanism of anti-EGFR drug resistance, a variety of biomarkers and pathways have been found to participate in CRC resistance to anti-EGFR therapy. More and more studies have implicated non-coding RNAs (ncRNAs) primarily including microRNAs (miRNAs), long non-coding RNAs (lncRNAs), and circular RNAs (circRNAs), are widely involved in tumorigenesis and tumor progression. They function as essential regulators controlling the expression and function of oncogenes. Increasing data have shown ncRNAs affect the resistance of molecular targeted drugs in CRC including anti-EGFR McAbs. In this paper, we have reviewed the advance in mechanisms of ncRNAs in regulating anti-EGFR McAbs therapy resistance in CRC. It provides insight into exploring ncRNAs as new molecular targets and prognostic markers for CRC.

The Emerging Roles of Circular RNAs in the Chemoresistance of Gastrointestinal Cancer

Gastrointestinal (GI) cancer represents a major global health problem due to its aggressive characteristics and poor prognosis. Despite the progress achieved in the development of treatment regimens, the clinical outcomes and therapeutic responses of patients with GI cancer remain unsatisfactory. Chemoresistance arising throughout the clinical intervention is undoubtedly a critical barrier for the successful treatment of GI cancer. However, the precise mechanisms associated with chemoresistance in GI cancer remain unclear. In the past decade, accumulating evidence has indicated that circular RNAs (circRNAs) play a key role in regulating cancer progression and chemoresistance. Notably, circRNAs function as molecular sponges that sequester microRNAs (miRNAs) and/or proteins, and thus indirectly control the expression of specific genes, which eventually promote or suppress drug resistance in GI cancer. Therefore, circRNAs may represent potential therapeutic targets for overcoming drug resistance in patients with GI cancer. This review comprehensively summarizes the regulatory roles of circRNAs in the development of chemoresistance in different GI cancers, including colorectal cancer, gastric cancer and esophageal cancer, as well as deciphers the underlying mechanisms and key molecules involved. Increasing knowledge of the important functions of circRNAs underlying drug resistance will provide new opportunities for developing efficacious therapeutic strategies against GI cancer.

Circular RNA protein tyrosine kinase 2 (circPTK2) promotes colorectal cancer proliferation, migration, invasion and chemoresistance

The dysregulated circular RNAs (circRNAs) are linked to progression and chemoresistance in colorectal cancer (CRC). However, the role of circRNA protein tyrosine kinase 2 (circPTK2) in CRC progression and chemoresistance is uncertain. The circPTK2, microRNA (miR)-136-5p, m6A 'reader' protein YTH domain family protein 1 (YTHDF1), β-catenin and cyclin D1 abundances were examined via quantitative reverse transcription PCR or Western blotting. The progression was investigated by cell counting kit-8 (CCK-8), colony formation, transwell and xenograft analysis. The resistance to 5-fluorouracil (5-FU) and oxaliplatin was analyzed via detecting cell viability and apoptosis using CCK-8 analysis and flow cytometry. The binding relationship was examined through dual-luciferase reporter, RNA immunoprecipitation and pull-down analysis. In our study, circPTK2 abundance was enhanced in CRC and associated with liver metastasis, clinical stage and chemoresistance. CircPTK2 knockdown constrained cell proliferation, migration, invasion, resistance to 5-FU and oxaliplatin, and the Wnt/β-catenin signaling. MiR-136-5p was bound with circPTK2 and downregulated in CRC. MiR-136-5p knockdown attenuated the influence of circPTK2 silence on CRC progression and chemoresistance. YTHDF1 was targeted via miR-136-5p and upregulated in CRC samples and cells. MiR-136-5p targeted YTHDF1 to restrain CRC progression and chemoresistance. In addition, we confirmed that circPTK2 silence reduced xenograft tumor growth. In conclusion, circPTK2 interference suppressed CRC proliferation, migration, invasion and chemoresistance via regulating miR-136-5p and YTHDF1.Abbreviations: circRNAs: circular RNAs; CRC: colorectal cancer; circPTK2: circRNA protein tyrosine kinase 2; miR: microRNA; YTHDF1: YTH domain family protein 1; CCK-8: cell counting kit-8; 5-FU: 5-fluorouracil; RIP: RNA immunoprecipitation.

Research progress of circRNAs in chemotherapy resistance of digestive system neoplasms

Circular RNAs (circRNAs) are a novel class of noncoding RNA molecules with a unique closed continuous loop structure. CircRNAs are abundant in eukaryotic cells, have unique stability and tissue specificity, and can play a biological regulatory role at various levels, such as transcriptional and posttranscriptional levels. Accumulating evidence indicates that circRNAs play critical roles in tumor genesis, development, and chemotherapy. Chemotherapy is a primary type of intervention for most cancers, but its therapeutic efficacy is usually retarded by intrinsic and acquired resistance. CircRNAs regulate tumor chemoresistance through various molecular mechanisms, such as affecting apoptosis, promoting drug transportation, promoting DNA repair, promoting epithelial-mesenchymal transformation, regulating the characteristics of tumor stem cells, and affecting autophagy. This review summarizes the recent progress and mechanisms of circRNAs in cancer cell resistance to chemotherapy.

Roles of circular RNAs in colorectal cancer

Colorectal cancer (CRC) is one of the most common types of malignant cancer worldwide and poses a significant burden on both the individual and healthcare systems. Despite advances in treatment options, advanced-stage CRC has a high mortality rate due to its heterogeneity, metastatic potential and/or delay in diagnosis. In recent years, an increasing number of studies have indicated that circular RNAs (circRNAs) serve important roles in several types of cancer, including CRC. Recent studies have revealed that circRNAs are aberrantly expressed in CRC tissues and function as oncogenic or tumor suppressive regulators of CRC carcinogenesis and development. Numerous circRNAs have been associated with the clinicopathological features of patients with CRC and have been considered as potential biomarkers for the diagnosis and prognosis of CRC, as well as targets for treatment. However, a deeper understanding of their potential function is required. In the present review, the current body of knowledge on the biogenesis and functions of CRC-associated circRNAs, and their potential value in clinical applications, such as in CRC diagnosis, prognosis and treatment, is discussed and summarized.

Circ_0007031 Silencing Inhibits Cell Proliferation and Induces Cell Apoptosis via Downregulating MELK at a miR-485-3p-Dependent Way in Colorectal Cancer

Colorectal cancer (CRC) is a malignant cancer with an increasing incidence. Circular RNA (circRNA) is recently found to participate in the regulation of CRC progression. However, the role of circ_0007031 in CRC malignant progression remains elusive. 50 CRC patients were implicated in this study. Quantitative real-time polymerase chain reaction (qRT-PCR) was performed to detect the RNA expression of circ_0007031, microRNA-485-3p (miR-485-3p) and maternal embryonic leucine zipper kinase (MELK). Western blot analysis was conducted to determine protein expression. Cell viability and proliferation were demonstrated by cell counting kit-8 and 5-Ethynyl-29-deoxyuridine (EdU) assays, respectively. Cell cycle and apoptosis were investigated by flow cytometry analysis. The interaction among circ_0007031, miR-485-3p and MELK was predicted by online databases, and identified by dual-luciferase reporter assay. Mouse model assay was conducted to reveal the effect of circ_0007031 on tumor formation in vivo. Circ_0007031 and MELK expression were obviously increased, while miR-485-3p expression was decreased in CRC tissues and cells compared with normal colorectal tissues or cells. Circ_0007031 knockdown repressed proliferation, whereas induced cell arrest at G0/G1 phase and apoptosis. On the opposite, circ_0007031 overexpression promoted cell proliferation and induced cell arrest at S phase. Additionally, miR-485-3p inhibitors attenuated circ_0007031 silencing-mediated CRC cell malignancy. MiR-485-3p was unveiled to regulate CRC cell processes via targeting MELK. Circ_0007031 controlled MELK expression via interacting with miR-485-3p. Furthermore, circ_0007031 contributed to tumor formation in vivo. Circ_0007031 knockdown repressed CRC malignant progression by reducing MELK expression through associating with miR-485-3p, suggesting that circ_0007031 was a potential target for the therapy of CRC.

Oncogenic Functions and Clinical Significance of Circular RNAs in Colorectal Cancer

Colorectal cancer (CRC) is ranked as the second most commonly diagnosed disease in females and the third in males worldwide. Therefore, the finding of new more reliable biomarkers for early diagnosis, for prediction of metastasis, and resistance to conventional therapies is an important challenge in overcoming the disease. The current review presents circular RNAs (circRNAs) with their unique features as potential prognostic and diagnostic biomarkers in CRC. The review highlights the mechanism of action and the role of circRNAs with oncogenic functions in the CRC as well as the association between their expression and clinicopathological characteristics of CRC patients. The comprehension of the role of oncogenic circRNAs in CRC pathogenesis is growing rapidly and the next step is using them as suitable new drug targets in the personalized treatment of CRC patients.

The Mechanistic Roles of ncRNAs in Promoting and Supporting Chemoresistance of Colorectal Cancer

Colorectal Cancer (CRC) is one of the most common gastrointestinal malignancies which has quite a high mortality rate. Despite the advances made in CRC treatment, effective therapy is still quite challenging, particularly due to resistance arising throughout the treatment regimen. Several studies have been carried out to identify CRC chemoresistance mechanisms, with research showing different signalling pathways, certain ATP binding cassette (ABC) transporters and epithelial mesenchymal transition (EMT), among others to be responsible for the failure of CRC chemotherapies. In the last decade, it has become increasingly evident that certain non-coding RNA (ncRNA) families are involved in chemoresistance. Research investigations have demonstrated that dysregulation of microRNAs (miRNAs), long non-coding RNAs (lncRNAs) and circular RNAs (circRNAs) contribute towards promoting resistance in CRC via different mechanisms. Considering the currently available data on this phenomenon, a better understanding of how these ncRNAs participate in chemoresistance can lead to suitable solutions to overcome this problem in CRC. This review will first focus on discussing the different mechanisms of CRC resistance identified so far. The focus will then shift onto the roles of miRNAs, lncRNAs and circRNAs in promoting 5-fluorouracil (5-FU), oxaliplatin (OXA), cisplatin and doxorubicin (DOX) resistance in CRC, specifically using ncRNAs which have been recently identified and validated under in vivo or in vitro conditions.

Circular RNAs: new biomarkers of chemoresistance in cancer

Chemotherapeutics are validated conventional treatments for patients with advanced cancer. However, with continual application of chemotherapeutics, chemoresistance, which is often predictive of poor prognosis, has gradually become a concern in recent years. Circular RNAs (circRNAs), a class of endogenous noncoding RNAs (ncRNAs) with a closed-loop structure, have been reported to be notable targets and markers for the prognosis, diagnosis, and treatment of many diseases, particularly cancer. Although dozens of studies have shown that circRNAs play major roles in drug-resistance activity in tumors, the mechanisms by which circRNAs affect chemoresistance have yet to be explored. In this review, we describe the detailed mechanisms of circRNAs and chemotherapeutics in various cancers and summarize potential therapeutic targets for drug-resistant tumors.