Circular RNA - circCLIP2 is predominantly expressed in GSC niche and enhances glioblastoma aggressiveness via EMT and ECM signaling

Glioblastoma (GBM) is the most common and malignant type of brain tumor in adults and no effective therapies exist to combat this disease. Surgical resection, radio- and temozolomide-based-chemotherapy are insufficient mainly due to inter- and intra-tumoral heterogeneity of GBM tumors, therefore personalized approaches are required. Additionally, diverse tumor microenvironment with a leading role of glioblastoma stem cells (GSCs) in hypoxic core underlie the aggressiveness and recurrence of GBM. However, the molecules and pathways which directly promote tumor aggressiveness together with the driving process of epithelial-to-mesenchymal transition (EMT) and the components of extracellular matrix (ECM), remain only partially covered with prior studies focused mainly on coding oncogenes and tumor suppressor genes. Over the past years, a special attention has been paid to non-coding, circular RNAs (circRNAs) differentially expressed in various cancers, including GBM. Despite high abundance and stability, the exact biological roles of most individual circRNAs have not been fully revealed, so far. Besides anti-cancer targets, circRNAs have been considered as essential biomarkers and stratifying criterion of malignancy. Thus, studies on functions of differentially expressed circRNAs in GBM are an emerging field to be explored. In herein study, the biological role of circRNA CLIP2 (circCLIP2) was found highly overexpressed in primary tumor tissues. Upon the knock-down of circCLIP2 in 2D and 3D in vitro models, functional tests performed clearly indicate a significant contribution of circCLIP2 to aggressive potential of GBM as showed by decreased rates of proliferation, migration and invasion. Additional studies indicated predominant expression of circCLIP2 in GSC fractions additionally elevated by hypoxic conditions. The increased rates of proliferation and migration as well as molecular analyses were also confirmed in in vivo settings where xenograft model was applied. The expression of EMT marker Snail2 and ECM-degrading enzyme metalloproteinase 9 (MMP9) were significantly downregulated in the tumors formed from circCLIP2 silenced cells. Taken together, these results indicate a strong contribution of circCLIP2 in the aggressive phenotype of GBM which is due to a modulation of GSC proliferative potential as well as migration and invasion via EMT together with the modulator of ECM composition.