NLR Family Pyrin Domain-Containing 3 Inflammasome Activation in Hepatic Stellate Cells Induces Liver Fibrosis in Mice

The NLR family pyrin domain-containing 3 (NLRP3) inflammasome plays an important role in liver fibrosis (LF) development. However, the mechanisms involved in NLRP3-induced fibrosis are unclear. Our aim was to test the hypothesis that the NLRP3 inflammasome in hepatic stellate cells (HSCs) can directly regulate their activation and contribute to LF. Primary HSCs isolated from wild-type (WT), Nlrp3(-/-), or Nlrp3(L351PneoR) knock-in crossed to inducible (estrogen receptor Cre-CreT) mice were incubated with lipopolysaccharide (LPS) and adenosine triphosphate (ATP), or 4OH-tamoxifen, respectively. HSC-specific Nlrp3(L351P) knock-in mice were generated by crossing transgenic mice expressing lecithin retinol acyltransferase (Lrat)-driven Cre and maintained on standard rodent chow for 6 months. Mice were then sacrificed; liver tissue and serum were harvested. Nlrp3 inflammasome activation along with HSC phenotype and fibrosis were assessed by RT-PCR, western blotting, fluorescence-activated cell sorting (FACS), enzyme-linked immunosorbent assay, immunofluorescence (IF), and immunohistochemistry (IHC). Stimulated WT HSCs displayed increased levels of NLRP3 inflammasome-induced reactive oxygen species (ROS) production and cathepsin B activity, accompanied by an up-regulation of mRNA and protein levels of fibrotic makers, an effect abrogated in Nlrp3(-/-) HSCs. Nlrp3(L351P) CreT HSCs also showed elevated mRNA and protein expression of fibrotic markers 24 hours after inflammasome activation induced with 4-hydroxytamoxifen (4OHT). Protein and mRNA expression levels of fibrotic markers were also found to be increased in isolated HSCs and whole liver tissue from Nlrp3(L351P) Lrat Cre mice compared to WT. Liver sections from 24-week-old Nlrp(L351P) Lrat Cre mice showed fibrotic changes with increased alpha smooth muscle actin (alpha SMA) and desmin-positive cells and collagen deposition, independent of inflammatory infiltrates; these changes were also observed after LPS challenge in 8-week-old Nlrp(L351P) Lrat Cre mice. Conclusion: Our results highlight a direct role for the NLRP3 inflammasome in the activation of HSCs directly triggering LF.