Nurr1 attenuates hepatic stellate cell activation by inhibiting EMT and cell cycle progression via interaction with Smad3.

BACKGROUND

Epithelial–mesenchymal transition (EMT) plays a vital role in hepatic fibrosis, as hepatic stellate cells (HSCs) can undergo EMT during liver fibrogenesis. Our previous work indicated that overexpression of the orphan nuclear receptor Nurr1 (NR4A2) can inhibit HSC activation and ameliorate liver fibrosis. However, the underlying mechanism by which Nurr1 affects EMT in hepatic fibrosis remains unclear. METHODS AND

RESULTS

HSC-T6 cells were transfected with either a Nurr1 overexpression plasmid or a small interfering RNA targeting Nurr1 to modulate Nurr1 levels. Transforming growth factor β1 (TGF-β1) was used to activate HSC-T6 cells. In vitro analyses showed that Nurr1 overexpression decreased α-smooth muscle actin (α-SMA) levels and increased E-cadherin (E-CAD) levels, whereas Nurr1 knockdown had the opposite effect. Consistently, Nurr1 overexpression reduced mRNA expression of EMT-related genes (α-SMA, Snail, Slug, and Smad3), while Nurr1 silencing elevated their expression. Flow cytometry revealed that Nurr1 overexpression increased the proportion of cells in S phase and reduced the G₂/M population, whereas Nurr1 knockdown also increased S-phase cells but led to accumulation in G₂/M phase. Chromatin immunoprecipitation (ChIP) assays demonstrated a significant enrichment of the Smad3 promoter in Nurr1-immunoprecipitated chromatin, indicating that Nurr1 directly binds to the Smad3 promoter.

CONCLUSIONS

Nurr1 suppresses HSC activation by downregulating EMT gene expression and modulating the cell cycle. Our findings suggest that Nurr1 inhibits the fibrogenic activation of HSCs, at least in part through transcriptional repression of EMT drivers (such as Smad3) and induction of S-phase cell cycle arrest.