Circular RNAs (circRNAs) are a novel class of non-coding RNA that assumes a covalently closed continuous conformation. CircRNAs were previously thought to be the byproducts of splicing errors caused by low abundance and the technological limitations. With the recent development of high-throughput sequencing technology, numerous circRNAs have been discovered in many species. Recent studies have revealed that circRNAs are stable and widely expressed, and often exhibit cell type-specific or tissue-specific expression. Most circRNAs can be generated from exons, introns, or both. Remarkably, emerging evidence indicates that some circRNAs can serve as microRNA (miRNA) sponges, regulate transcription or splicing, and can interact with RNA binding proteins (RBPs). Moreover, circRNAs have been reported to play essential roles in myriad life processes, such as aging, insulin secretion, tissue development, atherosclerotic vascular disease risk, cardiac hypertrophy and cancer. Although circRNAs are ancient molecules, they represent a newly appreciated form of noncoding RNA and as such have great potential implications in clinical and research fields. Here, we review the current understanding of circRNA classification, function and significance in physiological and pathological processes. We believe that future research will increase our understanding of the regulation and function of these novel molecules.

Background/Aims: Circular RNAs (circRNAs) are a special novel type of a stable, diverse and conserved noncoding RNA in mammalian cells. Particularly in cancer, circRNAs have been reported to be widely involved in the physiological/pathological process of life. However, it is unclear whether circRNAs are specifically involved in pancreatic ductal adenocarcinoma (PDAC). Methods: We investigated the expression profile of circRNAs in six PDAC cancer samples and paired adjacent normal tissues using microarray. A high-throughput circRNA microarray was used to identify dysregulated circular RNAs in six PDAC patients. Bioinformatic analyses were applied to study these differentially expressed circRNAs. Furthermore, quantitative reverse transcription polymerase chain reaction (qRT-PCR) was performed to confirm these results. Results: We revealed and confirmed that a number of circRNAs were dysregulated, which suggests a potential role in pancreatic cancer. Conclusions: this study demonstrates that clusters of circRNAs are aberrantly expressed in PDAC compared with normal samples and provides new potential targets for the future treatment of PDAC and novel insights into PDAC biology.

Hepatocellular carcinoma (HCC) is one of the most common malignant tumours and it carries a poor prognosis due to a high rate of recurrence or metastasis after surgery. Bmi-1 plays a significant role in the growth and metastasis of many solid tumours. However, the exact mechanisms underlying Bmi-1-mediated cell invasion and metastasis, especially in HCC, are not yet known. In the present study, we sought to evaluate the expression of Bmi-1 in HCC samples and its relationship with clinicopathological characteristics and prognostic value, we also investigated related mechanisms underlying Bmi-1-mediated cell invasion in HCC. Our results showed that Bmi-1 is upregulated in HCC tissues compared to matched non-cancer liver tissues; and its expression is positively associated with tumour size, metastasis, venous invasion and AJCC TNM stage, respectively; multivariate analysis showed that high expression of Bmi-1 was an independent prognostic factor for overall survival. In addition, the shRNA-mediated inhibition of Bmi-1 reduced the invasiveness of two HCC cell lines in vitro by upregulating phosphatase and the tensin homolog deleted on chromosome 10 (PTEN) expression, inhibiting the phosphatidylinositol 3-kinase (PI3K)/Akt signalling pathway and downregulating the expression and activities of matrix metalloproteinase (MMP)-2 and MMP-9 and vascular endothelial growth factor (VEGF). These data demonstrate that Bmi-1 plays a vital role in HCC invasion and that Bmi-1 is a potential therapeutic target for HCC.

Pancreatic ductal adenocarcinoma (PDAC) remains a common and deadly cancer. Despite numerous efforts, no reliable biomarker is available for daily clinical practice. Circular RNAs (circRNAs) are an abundant, stable and conserved class of RNA molecules that exhibit tissue/developmental-stage-specific expression (Salzman et al., 2012; Jeck et al., 2013; Memczak et al., 2013). CircRNAs play a crucial role in disease, especially in cancer, and provide new potential diagnostic and therapeutic targets for disease (Hansen et al., 2013; Qu et al., 2015).This research was designed to explore the expression profile of circRNAs in PDAC to serve as new diagnosis and treatment strategies for PDAC. Microarray and sample annotation data were deposited in Gene Expression Omnibus (GEO) under accession number GSE69362.