The mRNA-binding protein DDX3 mediates TGF-?1 upregulation of translation and promotes pulmonary fibrosis

Pulmonary fibrosis is potentiated by a positive feedback loop involving the extracellular sialidase enzyme neuraminidase 3 (NEU3) causing release of active TGF-beta 1 and TGF-beta 1 upregulating NEU3 by increasing translation without affecting mRNA levels. In this report, we elucidate the TGF-beta 1 upregulation of the translation mechanism. In human lung fibroblasts, TGF-beta 1 increased levels of proteins, including NEU3, by increasing translation of the encoding mRNAs without significantly affecting levels of these mRNAs. A total of 180 of these mRNAs shared a common 20-nucleotide motif. Deletion of this motif from NEU3 mRNA eliminated the TGF-beta 1 upregulation of NEU3 translation, while insertion of this motif in 2 mRNAs insensitive to TGF-beta 1 caused TGF-beta 1 to upregulate their translation. RNA-binding proteins including DEAD box helicase 3, X-linked (DDX3), bind the RNA motif, and TGF-beta 1 regulates their protein levels and/or binding to the motif. We found that DDX3 was upregulated in the fibrotic lesions in patients with pulmonary fibrosis, and inhibiting DDX3 in fibroblasts reduced TGF-beta 1 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 tissue levels of DDX3, TGF-beta 1, and NEU3. These results suggest that inhibiting an mRNA-binding protein that mediates TGF-beta 1 upregulation of translation can reduce pulmonary fibrosis.