Circ-GLIS3 expression was upregulated in A172/TMZ-R cells and cell-derived exsomes
A172/TMZ-R and SVG cells were identified through microscopic examination (Fig. 1A, B). To determine whether cell-derived exosomes were successfully purified, the exosome morphology was verified under transmission electron microscopy (Fig. 1C, D). CircRNA sequencing was performed in A172/TMZ-R and SVG cell lines. Differential expression was revealed through boxplots, scatter plots, volcano plots and hierarchical clustering (Fig. 1E). Then, primer screening and RT-PCR were conducted to localize candidate circRNAs. According to the relative circRNA expression, we found that circ-GLIS3 was significantly upregulated in A172/TMZ-R cells (both in cells and exsomes).
CircRNA-GLIS3 knockdown inhibited the progression of TMZ resistant glioma cells
To examine the biological function of circ-GLIS3 in TMZ resistant glioma, it was silenced in A172/TMZ-R and overexpressed in U251/TMZ-R cells (Fig. 2A and B). CCK-8 and clone formation assays illustrated that silencing of circ-GLIS3 could suppress A172/TMZ-R cell proliferation, whereas its overexpression could promote U251/TMZ-R cell proliferation (Fig. 2A, B, C, D). Transwell assays revealed that silencing of circ-GLIS3 inhibited A172/TMZ-R cell invasion abilities, while its overexpression promoted the invasion abilities in U251/TMZ-R cells (Fig. 2E, F). These results suggested that circ-GLIS3 may act as a promoter in TMZ-resistant glioma cell progression.
Circ-GLIS3 upregulated MED31 expression by sponging miR-548m in TMZ resistant glioma cells
We assumed that circ-GLIS3 could act as a miRNA sponge to regulate glioma phenotype. Through bioinformatics tool screening, we found that among all candidate miRNAs, miR-548m may act as the downstream target of circ-GLIS3 (https://circinteractome. nia.nih.gov/). The junction sites are UACCUUUG and AUGGAAAC. The downstream target of miR-548m was predicted by TargetScan database, MED31 (Chromosome 17: 6,643, 311-6,651,634) was selected as the downstream target, and the potential junction sites of miR-548m were UACCUUU and AUGGAAA. RT-PCR revealed that miR-548m was markedly decreased while MED31 mRNA was markedly increased in A172/TMZ-R and U251/TMZ-R cells, accompanied with circ-GLIS3 reduction (Fig. 3A, B).
Moreover, in our study, we transfected sh-circ-GLIS3 into A172/TMZ-R cells. The results indicated that circ-GLIS3 knockdown obviously repressed MED31 mRNA expression in TMZ resistant cells, whereas inh-miR548m and MED31 mRNA could rescue this effect. Meanwhile we transfected circ-GLIS3 mimics into U251/TMZ-R cells. The results indicated that circ-GLIS3 overexpression obviously promoted MED31 mRNA expression in TMZ resistant glioma cells, whereas miR548m mimics and MED31 inhibitor could achieve the opposite effect. Overall, these data prove that circ-GLIS3 is targeted by miR-548m as a miRNA sponge to regulate MED31 expression.
At the same time we explored the subcellular location of circ-GLIS3 to investigate its role in glioma progression. The results revealed that circ-GLIS3 was located mostly in the cytoplasm of A172/TMZ-R cells (Fig. 3C), which indicated that circ-GLIS3 could regulate glioma pathological processes in posttranscriptional procedures.
Circ-GLIS3 promoted TMZ resistant glioma cell progression through miR-548m/MED31 axis
In EdU cell proliferation assay, proliferating cell ratio statistically decreased in si-circ-GLIS3 group, in clone formation assay, the results illustrated that silencing of circ-GLIS3 obviously reduced colony-formation ability in A172/TMZ-R and U251/TMZ-R cells (Fig. 4A, B, C, D). In Transwell assay, the number of invasive cells was significantly reduced in si-circ-GLIS3 group (Fig. 4E, F). The scratch assay indicated that circ-GLIS3 knockdown could inhibit cell migration ability (Fig. 4G, H), these results indicated that circ-GLIS3 could promote TMZ resistant glioma cell’s proliferation, invasion and migration abilities.
To further verify the regulatory relationship among circ-GLIS3, miR-548m and MED31, we tested the proliferation, invasion and migration abilities of A172/TMZ-R and U251/TMZ-R cells after transfection with inh-miR-548m and MED31 mRNA. The results indicated that the downregulation of miR-548m and upregulation of MED31 could rescue the si-circ-GLIS3-inhibited proliferation, invasion and migration abilities (Fig. 4). Through these cell function assays, we concluded that circ-GLIS3 could promote glioma cell proliferation, invasion and migration through miR-548m/ MED31 axis.
Circ-GLIS3 inhibited cell cycle arrest and cell apoptosis in TMZ resistant glioma cells through miR-548m/MED31 axis
We proceeded flow cytometry to investigate the effects of circ-GLIS3/miR-548m/MED31 signaling pathway on TMZ resistant glioma cell cycle and apoptosis. As shown in Fig. 5A and 5B, the apoptosis ratio was higher in si-circ-GLIS3 group, Moreover the cells in si-circ-GLIS3 group were arrested in G0/G1 phase, meanwhile circ-GLIS3 cells co-transfected with miR-548m inhibitor or MED31 mRNA could reverse this effect. These results proved that circ-GLIS3 may inhibit cell cycle arrest and cell apoptosis in TMZ resistant glioma cells by binding to miR-548m and upregulating MED31 expression.
Downregulation of circ-GLIS3 suppressed TMZ resistant tumor growth in vivo
Nude mouse xenograft tumors were generated to verify the effect of circ-GLIS3 on TMZ resistant glioma proliferation in vivo. In tumor-bearing mice injected with circ-GLIS3-deficient A172/TMZ-R cells, the volume and weight of tumors were markedly smaller than those injected with normal A172/TMZ-R cells (Fig. 6A, C, D). The timing measurements represented a large slowdown in tumor growth (Fig. 6B). HE staining showed that apoptotic cells in subcutaneous tumor sections were obviously increased in the si-circ-GLIS3 group (Fig. 6E). All these results proved that circ-GLIS3 could promote TMZ resistant glioma cell growth in vivo.