Regulation of myofibroblast differentiation by miR-424 during epithelial-to-mesenchymal transition

Idiopathic pulmonary fibrosis (IPF) is one of the most common and severe interstitial lung diseases. Epithelial-to-mesenchymal transition (EMT) is a process whereby epithelial cells undergo transition to a mesenchymal phenotype. This process has been shown to contribute to IPF. MicroRNAs (miRNAs) are small non-coding RNAs of 18-24 nucleotides in length which regulate gene expression. Several studies have implicated miRNAs in EMT; however, specific miRNAs that regulate EMT in IPF have not yet been identified. In this study, we identified 6 up-regulated and 3 down-regulated miRNAs in a human lung epithelial cell EMT model using miRNA microarray and real-time PCR. Overexpression of one of these up-regulated miRNAs, miR-424, increased the expression of α-smooth muscle actin, an indicator of myofibroblast differentiation, but had no effects on the epithelial or mesenchymal cell markers. miR-424 enhanced the activity of the TGF-β signaling pathway, as demonstrated by a luciferase reporter assay. Further experiments showed that miR-424 decreased the protein expression of Smurf2, a negative regulator of TGF-β signaling, indicating that miR-424 exerts a forward regulatory loop in the TGF-β signaling pathway. Our results suggest that miR-424 regulates the myofibroblast differentiation during EMT by potentiating the TGF-β signaling pathway, likely through Smurf2.