# Latency-Associated Peptide Rapidly Upregulates Neuraminidase 3 in a Profibrotic Translation-Based Positive Feedback Loop

## TL;DR

TGF-β1 rapidly increases NEU3 levels through translation, creating a feedback loop that worsens fibrosis, and this can be blocked by inhibiting DDX3.

## Contribution

Discovery of a rapid, translation-based positive feedback loop involving TGF-β1, LAP, and NEU3 in fibrosis.

## Key findings

- TGF-β1 increases NEU3 levels via translation, not transcription.
- LAP synergizes with TGF-β1 to upregulate NEU3, forming a feedback loop.
- DDX3 inhibition with RK-33 blocks NEU3 upregulation by TGF-β1 and LAP.

## Abstract

What are the main findings?
TGF-β1, a key driver of fibrosis, rapidly increases levels of the profibrotic sialidase NEU3 via translational upregulation, independent of new transcription.After activation by NEU3, the TGF-β1 sequestering protein LAP also upregulates NEU3, and works synergistically with TGF-β1, contributing to a TGF-β1 → NEU3 → TGF-β1 positive feedback loop. LAP upregulation of NEU3 is blocked by inhibiting the RNA binding protein DDX3 with RK-33.

TGF-β1, a key driver of fibrosis, rapidly increases levels of the profibrotic sialidase NEU3 via translational upregulation, independent of new transcription.

After activation by NEU3, the TGF-β1 sequestering protein LAP also upregulates NEU3, and works synergistically with TGF-β1, contributing to a TGF-β1 → NEU3 → TGF-β1 positive feedback loop. LAP upregulation of NEU3 is blocked by inhibiting the RNA binding protein DDX3 with RK-33.

What is the implication of the main finding?
LAP is a potent amplifier of profibrotic signaling.DDX3 inhibition is a potential strategy to inhibit the LAP → NEU3 → TGF-β1 pathway.

LAP is a potent amplifier of profibrotic signaling.

DDX3 inhibition is a potential strategy to inhibit the LAP → NEU3 → TGF-β1 pathway.

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.