Glioma still harbors a high frequency of recurrence and a bleak outcome. Redundant activation of AKT is the inherent characteristics of glioma, which is frequently associated with tumorigenesis and outcomes [19]. Furthermore, emerging evidence has indicated that some aberrant expression of lncRNAs contributes to tumor development, pathogenesis, and survival in glioma [20]. Sequencing of the human genome shows that 70% of the human genome is transcribed into RNA yielding many thousands of ncRNAs, but the overall study of lncRNAs in brain tumors remains lag behind [21]. For example, the roles of few lncRNAs (such as HOTAIR, MALAT1 and CRNDE) have been well studied in gliomas [22–24].
The present study systematically explored LINC00663 expression in clinical glioma specimens and the effects of LINC00663 silencing on cultured glioma cells. qRT-PCR results showed that was upregulated in glioma tissues and cells, and closely related to tumor degree. Further evidence for the adverse impact of LINC00663 expression on glioma progression was found upon analysis of GEO dataset GSE4412, which showed a significant, inverse correlation between LINC00663 levels and survival rates. The results described above strongly prove the oncogenic role of LINC00663. Furthermore, after knocking down LINC00663 expression significantly blocked the proliferation, migration and invasion capacity of glioma cells. Nevertheless, the regulatory mechanisms of LINC00663 are still unknown.
Next, we sought to identify the candidate molecular mechanism by which LINC00663 regulated downstream effectors in glioma. Here, we analyzed the coexpression genes network in CGGA dataset and discovered that AKT-mTOR pathway was affected by LINC00663. It is well known that PTEN is one of the most frequently altered genes in gliomas and loss of its function increases levels of PI(3,4,5)P3 and leads to activation of AKT pathway [25]. Hyper-activation of the AKT pathway is frequently found in glioma and plays a pivotal role in the regulation of tumor cell survival, growth, motility, angiogenesis and metabolism [26]. Multitudes of studies have demonstrated the importance of PI3K in the activation of AKT, while evidence suggests that AKT activation can proceed by the PI3K independent manner [27]. Several lncRNAs (LINK-A, lncRNA OIP5-AS1, MALAT1 and LINC00470) promote the over activation of AKT signaling [28–31]. In this study, Western blot analysis results confirmed AKT activity was apparently suppressed in U87 cells but slightly in U251 cells with LINC00663 knockdown, without affecting mTOR in the AKT pathway, which was worthy of attention in the future. We demonstrated the inhibitory effects of LINC00663 knockdown on the AKT signaling pathway, which is frequently activated in glioma.
A few limitations should be addressed here. Firstly, we did not check the effects of overexpressing LINC00663 on the proliferation, migration and invasion, because the survival of glioma cells with transient LINC00663 overexpression was strongly affected (data not shown). Secondly, an inconsistent conclusion was obtained in different two glioma cell lines, presenting a relationship to p53 gene mutation, further investigations are needed. In addition, it is necessary to perform brain xenograft experiments to confirm the role of LINC00663 on glioma growth in vivo.
In summary, the attenuation of LINC0066 significantly suppressed the proliferation, migration, and invasion of glioma cells, and these effects correlated with reduced AKT activity. LINC00663 can serve as both a growth-promoting lncRNA and a potential therapeutic target to overcome gliomas.