Glioma is the most common brain malignancy in adults. Although the incidence of glioma is relatively low, the degree of malignancy is high, and the degree of malignancy increases following with the grade of glioma [Alexander and Cloughesy 2017]. Unfortunately, molecular mechanisms underlying of glioma remain incompletely clarified, and effective therapeutic strategies are limited. Extensive evidence suggests that miRNAs contribute to the progression of various tumors, including lung cancer [Dimitrova et al. 2016], breast cancer [Dvinge et al. 2013], gastric cancer [Ueda et al. 2010] and glioma [Sumazin et al. 2011]. miR-450 has been reported that affects the biological behavior of HepG2 cells by targeting DNMT3a [Wang et al. 2019]. Liu et al. reported that upregulation of miR-450 inhibits the progression of lung cancer in vitro and in vivo by targeting IRF2 [Liu et al. 2016]. These results suggested that miR-450 is a functional miRNA in tumors. However, the role and molecular mechanisms of miR-450a in glioma are less reported.
TCGA database is the largest and most comprehensive public database of tumors over the world. We analyzed miR-450a expression of approximately 500 case glioma patients and clinical information, and found that miR-450a expression was higher in glioma tissues than normal brain tissues. Additionally, miR-450a had the highest expression in GBM patients. However, only 5 case GBM patients had miRNA expression profile data in the recently updated TCGA database glioma data, and then we combined the analysis with the CGGA database. Consistently, similar results are also showed in the CGGA database, and these results indicated that miR-450a expression was strongly linked to glioma grade. Furthermore, glioma patients with higher miR-450a expression obtained poorer prognosis. Thus, we deemed that miR-450a may play an important role in glioma.
To investigate the effect of miR-450a on glioma cells, we downregulated the miR-450a expression in U87 and U251 cells by transfected miR-450a inhibitor. Subsequently, we performed a series of assay, including CCK8, cell cycle assay, colony formation assay, transwell assay and wound healing assay, to explore the effect of miR-450a on the malignant phenotype of glioma cells. These results demonstrated that miR-450a promotes glioma cells proliferation, invasion and migration. Animals experiment also confirmed that miR-450a contribute to glioma growth. Furthermore, we explored the miR-450a underlying molecular mechanism to promote malignant phenotype of glioma. We predicted the target genes of miR-450a using miRDB, miRTarBase, miRWalk and TargetScan databases, and obtained 18 intersection genes. Then, we analyzed the correlation between these 18 candidate target genes and miR-450a expression in TCGA database, and found that PPM1L had the strongest correlation with miR-450a. Luciferase report assay also verified that PPM1L mRNA 3’UTR exists complementarity sequence with miR-450a. PPM1L expression and prognosis analysis in glioma samples from TCGA and CGGA databases indicated that PPM1L may act as a glioma suppress gene. Then, we co-transfected with miR-450a inhibitor and PPM1L shRNA to investigate the malignant phenotype of glioma cells again, and revealed that PPM1L downregulation effectively reversed the tumor-promoting effect of miR-450a. Therefore, our data verified that downregulation miR-450a inhibits glioma cells growth, invasion and migration by targeting PPM1L. But, Liu et al. and Wang et al. reported that miR-450 acts as a tumor suppressor in lung cancer and liver cancer [Wang et al. 2019; Liu et al. 2016]. We deem that miR-450a may play a dual regulatory role in tumors.
PPM1L is a member of the serine/threonine phosphatase family and involves in immune regulation after myocardial infarction [Wang et al. 2019]. However, rarely reported role of PPM1L in tumors. Although PPM1L expression and prognosis data from online databases indicated that PPM1L is downregulated in glioma, the functional role of PPM1L in glioma remains unclear. In the future, we will continue to explore the function of PPM1L in gliomas.
In conclusion, our data revealed miR-450a plays an essential role on promoting glioma cells proliferation, invasion and migration by directly targeting PPM1L, indicating miR-450a/PPM1L axis may as potential molecular biomarker for glioma patients. Thus, miR-450a/PPM1L may act as a novel diagnostic or treatment target in glioma.