MicroRNA-145 Modulates N6-Methyladenosine Levels by Targeting the 3′-Untranslated mRNA Region of the N6-Methyladenosine Binding YTH Domain Family 2 Protein*

N6-Methyladenosine (m6A) is a prevalent modification present in the mRNAs of higher eukaryotes. YTH domain family 2 (YTHDF2), an m6A “reader” protein, can recognize mRNA m6A sites to mediate mRNA degradation. However, the regulatory mechanism of YTHDF2 is poorly understood. To this end, we investigated the post-transcriptional regulation of YTHDF2. Bioinformatics analysis suggested that the microRNA miR-145 might target the 3′-untranslated region (3′-UTR) of YTHDF2 mRNA. The levels of miR-145 were negatively correlated with those of YTHDF2 mRNA in clinical hepatocellular carcinoma (HCC) tissues, and immunohistochemical staining revealed that YTHDF2 was closely associated with malignancy of HCC. Interestingly, miR-145 decreased the luciferase activities of 3′-UTR of YTHDF2 mRNA. Mutation of predicted miR-145 binding sites in the 3′-UTR of YTHDF2 mRNA abolished the miR-145-induced decrease in luciferase activity. Overexpression of miR-145 dose-dependently down-regulated YTHDF2 expression in HCC cells at the levels of both mRNA and protein. Conversely, inhibition of miR-145 resulted in the up-regulation of YTHDF2 in the cells. Dot blot analysis and immunofluorescence staining revealed that the overexpression of miR-145 strongly increased m6A levels relative to those in control HCC cells, and this increase could be blocked by YTHDF2 overexpression. Moreover, miR-145 inhibition strongly decreased m6A levels, which were rescued by treatment with a small interfering RNA-based YTHDF2 knockdown. Thus, we conclude that miR-145 modulates m6A levels by targeting the 3′-UTR of YTHDF2 mRNA in HCC cells.