microRNA-17 regulates the expression of ATG7 and modulates the autophagy process, improving the sensitivity to temozolomide and low-dose ionizing radiation treatments in human glioblastoma cells

ATG7 is a key autophagy-promoting gene that plays a critical role in the regulation of cell death and survival of various cell types. We report here that microRNAs (miRNAs), a class of endogenous 22-24 nucleotide noncoding RNA molecules able to affect stability and translation of mRNA, may represent a novel mechanism for regulating ATG7 expression and therefore autophagy. We demonstrated that ATG7 is a potential target for miR-17, and this miRNA could negatively regulate ATG7 expression, resulting in a modulation of the autophagic status in T98G glioblastoma cells. Treatment of these tumor cells with the miR-17 mimic decreased, and with the antagomir increased, the expression of ATG7 protein. Dual luciferase reporter assay confirmed that a specific miR-17 binding sequence in the 3'-UTR of ATG7 contributed to the modulation of the expression of the gene by miR-17. Interestingly, our results showed that anti-miR-17 administration activated autophagy through autophagosome formation, as resulted by LC3B and ATG7 protein expression increase, and by the analysis of GFP-LC3 positive autophagosome vesicles in living cells. Furthermore, the autophagy activation by anti-miR-17 resulted in a decrease of the threshold resistance at temozolomide doses in T98G cells, while miR-17 modulation in U373-MG glioblastoma cells resulted in a sensitization to low ionizing radiation doses. Our study of the role of miR-17 in regulating ATG7 expression and autophagy reveals a novel function for this miRNA sequence in a critical cellular event with significant impacts in cancer development, progression and treatment.