The mRNA binding protein DDX3 mediates TGF-β1 upregulation of translation and promotes pulmonary fibrosis

Pulmonary fibrosis is potentiated by a positive feedback loop involving the extracellular sialidase enzyme NEU3 causing release of active TGF-β1, and TGF-β1 upregulating NEU3 by increasing translation without affecting mRNA levels. In this report, we elucidate the TGF-β1 upregulation of translation mechanism. In human lung fibroblasts, TGF-β1 increased levels of proteins, including NEU3, by increasing translation of the encoding mRNAs without significantly affecting levels of these mRNAs. 180 of these mRNAs share a common 20 nucleotide motif. Deletion of this motif from NEU3 mRNA eliminated the TGF-β1 upregulation of NEU3 translation, while insertion of this motif in two mRNAs insensitive to TGF-β1 caused TGF-β1 to upregulate their translation. RNA-binding proteins including DDX3 bind the RNA motif, and TGF-β1 regulates their protein levels and/or binding to the motif. We found that DDX3 is upregulated in the fibrotic lesions in pulmonary fibrosis patients, and inhibiting DDX3 in fibroblasts reduced TGF-β1 upregulation of NEU3 levels. In the mouse bleomycin model of pulmonary fibrosis, injections of the DDX3 inhibitor RK-33 potentiated survival and reduced lung inflammation, fibrosis, and lung tissue levels of DDX3, TGF-β1, and NEU3. These results suggest that inhibiting a mRNA-binding protein that mediates TGF-β1 upregulation of translation can reduce pulmonary fibrosis.