Abstract
Fibrosis appears to be an out-of-control wound-healing response that drives a progressive formation of scar tissue in an organ. A key profibrotic cytokine, transforming growth factor beta-1 (TGF-β1), upregulates levels of the extracellular sialidase neuraminidase 3 (NEU3), and NEU3 in turn can activate latent TGF-β1 to release active TGF-β1 from the sequestering latency-associated peptide (LAP). In the mouse bleomycin model of pulmonary fibrosis, NEU3 is both necessary and sufficient for pulmonary fibrosis. In this report, we find that NEU3 protein levels increase both intracellularly and extracellularly in cultures of human lung fibroblasts within 5 min of TGF-β1 exposure. This effect is driven by an increase in translation and is independent of new transcription, supporting a model where TGF-β1 causes a pool of weakly translated NEU3 mRNA to increase translation. By participating in the feedback loop, latent TGF-β1 makes cells more sensitive to TGF-β1. LAP also stimulates NEU3 expression and acts synergistically with TGF-β1 to upregulate NEU3. The positive feedback loop is blocked by NEU3 inhibitors. The RNA helicase DEAD-box helicase 3 (DDX3) mediates NEU3 translation, and the DDX3 inhibitor RK-33 blocks the rapid upregulation of NEU3 by TGF-β1 and LAP. Exposure of cells to TGF-β1 but not LAP induces dephosphorylation of DDX3 within two minutes, suggesting that the mechanisms used by TGF-β1 and LAP to activate DDX3 to increase NEU3 levels may differ. Together, these results suggest that a rapid positive feedback loop involving TGF-β1, LAP, and NEU3 helps drive fibrosis.