Effects of NLRP6 on the proliferation and activation of human hepatic stellate cells

Nod-like receptor pyrin domain-containing proteins (NLRPs) are known to take part in the pathogenesis of chronic liver diseases, including liver fibrosis. However, no known direct role of NLRP6, a member of NLRPs, has been reported in liver fibrosis. Here, we found that NLRP6 expression was decreased in fibrotic and cirrhotic livers. In a human hepatic stellate cell line, LX-2, overexpression of NLRP6 suppressed cell proliferation, hydroxyproline accumulation, as well as the expression of type I and type III collagens (Col-I and Col-III), α-smooth muscle actin (α-SMA) and matrix metalloproteinases (MMP2 and MMP9), whereas NLRP6 knockdown displayed reverse effects. Furthermore, NLRP6 significantly suppressed the phosphorylation of Smad2/3 (p-Smad2/3) and enhanced the expression of protein phosphatase magnesium dependent 1 A (PPM1A), the only phosphatase for Smad2/3. NLRP6 overexpression abrogated TGF-β1-stimulated hydroxyproline accumulation and p-Smad2/3. Co-immunoprecipitation assay demonstrated that NLRP6 was able to form a complex with PPM1A. NLRP6 overexpression did not change the level of p-Smad2/3 in LX-2 cells with PPM1A knockdown. These data indicated that PPM1A was required for the inhibitory effects of NLRP6 on TGF-β1/Smad2/3 signaling. In conclusion, our results suggest that NLRP6 exerts anti-fibrotic effects in LX-2 cells via regulating PPM1A/Smad2/3 and that NLRP6 may be an effective target in the treatment of liver fibrosis.