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.