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

by Selleckchem | Jul 25 2023

Pulmonary fibrosis is potentiated by a positive feedback loop involving the extracellular sialidase enzyme neuraminidase 3 (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 the 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. A total of 180 of these mRNAs shared 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 2 mRNAs insensitive to TGF-β1 caused TGF-β1 to upregulate their translation. RNA-binding proteins including DEAD box helicase 3, X-linked (DDX3), bind the RNA motif, and TGF-β1 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-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 tissue levels of DDX3, TGF-β1, and NEU3. These results suggest that inhibiting an mRNA-binding protein that mediates TGF-β1 upregulation of translation can reduce pulmonary fibrosis.

Comments:

The report describes a positive feedback loop that contributes to the development of pulmonary fibrosis. The loop involves the extracellular enzyme neuraminidase 3 (NEU3), which is regulated by the cytokine transforming growth factor beta 1 (TGF-β1). The study investigated the mechanism by which TGF-β1 upregulates the translation of specific mRNAs, including NEU3, without significantly affecting their mRNA levels.

In human lung fibroblasts, TGF-β1 was found to increase the levels of certain proteins, including NEU3, by enhancing the translation of their corresponding mRNAs. Interestingly, 180 of these mRNAs shared a common 20-nucleotide motif. The presence of this motif was essential for TGF-β1-mediated upregulation of NEU3 translation, as its deletion from NEU3 mRNA abolished the effect. Conversely, the insertion of this motif into two mRNAs that were insensitive to TGF-β1 caused TGF-β1 to upregulate their translation.

Further investigation revealed that RNA-binding proteins, particularly DEAD box helicase 3, X-linked (DDX3), bound to the identified RNA motif. TGF-β1 was found to regulate the protein levels of these RNA-binding proteins and their binding affinity to the motif. DDX3, in particular, was found to be upregulated in fibrotic lesions of patients with pulmonary fibrosis. Inhibiting DDX3 in fibroblasts resulted in a reduction of TGF-β1-mediated upregulation of NEU3 levels.

To validate these findings in an in vivo model, the researchers used a mouse model of pulmonary fibrosis induced by bleomycin injections. In this model, administration of the DDX3 inhibitor RK-33 improved survival rates, reduced lung inflammation and fibrosis, and decreased the levels of DDX3, TGF-β1, and NEU3 in lung tissue.

Overall, these findings suggest that targeting an mRNA-binding protein involved in TGF-β1-mediated upregulation of translation, such as DDX3, could be a potential therapeutic strategy to mitigate pulmonary fibrosis. Inhibition of DDX3 demonstrated beneficial effects in reducing fibrosis and inflammation in a mouse model, highlighting its potential as a therapeutic target for this debilitating lung condition.