Discussion
OS is the most prevalent malignant bone tumor. It is highly metastatic, resulting in a very poor survival rate [2]. Approximately 80% of OS patients exhibit subclinical pulmonary micro metastases at the time of diagnosis [18]. The lack of accurate biomarkers has further hindered efforts to improve the clinical outcome of OS. Recently, the dysregulation of ncRNAs in OS has generated significant interest from scientific communities. Being different from the other miRNAs or lncRNAs, circRNAs have emerged as more reliable and promising tumor biomarkers owing to their exceptionally stable structure. Advanced genome sequencing techniques have validated the roles of circRNAs in multiple cancers, including hepatocellular carcinoma [19], gastric cancer [20], colorectal cancer [21] and lung squamous cell carcinoma [22]. However, to date, the expression profiles and roles of circRNAs in OS are not well understood.
Our study provides the first evidence that circ_001722 contributes to the malignant progression of OS. CircRNAs are widely accepted to be an unorthodox RNA species generated by alternative splicing of pre-mRNAs [23]. There are three main classes of circRNAs: exonic circRNAs, exon-intron circRNAs and intronic circRNAs [24]. Herein, we revealed upregulated expression of circ_001722 in OS tissues and cells using high-throughput sequencing and qRT-PCR. Functional analyses further validated the role of circ_001722 in promoting the proliferation and metastasis of OS cells both in vivo and in vitro .
The subcellular distribution of RNAs is intimately tied to their biological functions [25]. Accumulating evidence shows that cytoplasmic circRNAs sponge miRNAs, which represses the translation or induces the degradation of the target mRNAs. Herein, through bioinformatical analysis, we discovered that complementary pairing sites on (82-89 nt) circ_0001722 that can bind to miR-204-5p. Despite this new finding, the involvement of miR-204-5p in the pathogenesis of multiple tumors is not a new phenomenon. Reports on the interactions between miR-204-5p and circRNAs in cancer are scarce. Herein, we found that miR-204-5p expression was inversely correlated with circ_001722. Functional rescue experiments further revealed that the miR-204-5p inhibitor substantially reversed the suppressive effects of circ_001722 depletion on proliferation and metastasis of OS cells, whereas miR-204-5p could abolish the promotive effects of circ_001722 overexpression.
Moreover, we found that RUNX2 is a downstream target of miR-204-5p in OS cells. Functional experiments revealed that circ_001722 upregulated RUNX2 expression by sponging miR-204-5p. But, RUNX2 triggered which pathway activation to accelerate OS progression via mechanisms including suppression of apoptosis and promotion of cell proliferation, migration and invasion, still need to explore. Evidence indicates that RUNX2 is implicated in diverse biological processes, including bone development, tumor invasion and metastasis [26, 27]. Initial findings indicate the carcinogenesis mediated by the circ_001722/miR-204-5p/RUNX2 axis in OS.