Let-7a Functions as a Tumor Suppressor in Ewing's Sarcoma Cell Lines Partly by Targeting Cyclin-Dependent Kinase 6

MicroRNAs play an important role in the development and progression of Ewing's sarcoma (ES). Especially, the expression of let-7a has been reported to be significantly downregulated in various cancers, and can affect the initiation and maintenance of tumor progression. However, the relative effects of let-7a on ES cells and relative mechanisms are largely unknown. In this study, we identified the underexpression of let-7a in human ES cells comparing with the human mesenchymal stem cells. Then, we sought to compensate for its loss through exogenous transfection with let-7a mimic into ES cell lines A673 and SK-ES-1. Restored let-7a expression inhibited cell proliferation, migration, as well as invasion; arrested cell cycle progression; and induced cell apoptosis of both cell lines. Moreover, bioinformatic prediction suggested that cyclin-dependent kinase 6 (CDK6), which is overexpressed and functions as an oncoprotein in ES cells, is a putative target gene of let-7a. Using mRNA and protein expression analysis and luciferase assays, we further identified the target role of CDK6. Finally, we found that restored CDK6 expression in ES cells that had been treated with let-7a mimic before could partly dampen let-7a-mediated tumor suppression. Taken together, our results showed that let-7a acted as a tumor suppressor in ES by targeting CDK6, and it may provide novel diagnostic and therapeutic options for human Ewing sarcoma clinical operation in future.