SMURF1-Interference Effects Autophagy and Pulmonary Fibrosis through SMAD7 and the TGF-β1/SMAD Pathway

SMAD-specific E3 ubiquitin proten ligase 1 (SMURF1) is involved in transforming growth factor (TGF)-β1/Smad pathway-mediated tissue fibrosis. However, its role in pulmonary fibrosis and the related molecular mechanisms are still unclear. This study aims to investigate whether SMURF1 inhibits autophagy and promotes pulmonary fibrosis via SMAD family member 7 (SMAD7) and TGF-β1/SMAD signal pathway. MRC-5 cells were treated with TGF-β1 followed by MURF1- interference. The rate of cell migration was assessed using the cell scratch test. Autophagosomes were analyzed using a transmission electron microscope. mRNA levels of SMURF1, SMAD7, TGF-β1, phosphorylated (p)-SMAD1, p-SMAD3, α-smooth muscle actin (α-SMA), matrix metallopeptidase 7 (MMP7), microtubule-associated protein light chain 3 (LC3 ) and Beclin1 were evaluated by quantitative real-time PCR (qPCR), Western blotting, and immunofluorescence. The interaction between SMURF1 and SMAD7 was investigated in a co-immunoprecipitation (Co-IP) experiment. We found that after TGF-β1 treatment, the mRNA levels of SMURF1, α-SMA, MMP7, and p-Smad1/3 were increased, and the levels of Beclin1 and LC3 were decreased. Apart from these, cell autophagy was decreased, while the migration ability was increased. After SMURF1-interference, SMURF1, α-SMA and MMP7 mRNA levels were significantly decreased, p-SMAD1 was slightly reduced, and p-Smad3 was not changed. As for Beclin1 and LC3, their transcription increased, cell autophagy increased, and migratory ability decreased. The interaction between SMURF1 and Smad7 was confirmed by Co-IP. In conclusion, SMURF1 may inhibit autophagy and promote lung fibrosis by downregulating SMAD7 and activating the TGF-β1/SMAD pathway. These results may serve as a basis for the development of new therapeutic targets in the pulmonary fibrosis clinic.