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Background & aimsIncreased hepatocyte death contributes to the pathology of acute and chronic liver diseases. However, the role of hepatocyte pyroptosis and extracellular inflammasome release in liver disease is unknown.MethodsWe used primary mouse and human hepatocytes, hepatocyte-specific leucine 351 to proline Nlrp3KICreA mice, and GsdmdKO mice to investigate pyroptotic cell death in hepatocytes and its impact on liver inflammation and damage. Extracellular NOD-, LRR-, and pyrin domain-containing protein 3 (NLRP3) inflammasomes were isolated from mutant NLRP3-YFP HEK cells and internalisation was studied in LX2 and primary human hepatic stellate cells. We also examined a cohort of 154 adult patients with biopsy-proven non-alcoholic fatty liver disease (Sir Charles Gairdner Hospital, Nedlands, Western Australia).ResultsWe demonstrated that primary mouse and human hepatocytes can undergo pyroptosis upon NLRP3 inflammasome activation with subsequent release of NLRP3 inflammasome proteins that amplify and perpetuate inflammasome-driven fibrogenesis. Pyroptosis was inhibited by blocking caspase-1 and gasdermin D activation. The activated form of caspase-1 was detected in the livers and in serum from patients with non-alcoholic steatohepatitis and correlated with disease severity. Nlrp3KICreA mice showed spontaneous liver fibrosis under normal chow diet, and increased sensitivity to liver damage and inflammation after treatment with low dose lipopolysaccharide. Mechanistically, hepatic stellate cells engulfed extracellular NLRP3 inflammasome particles leading to increased IL-1β secretion and α-smooth muscle actin expression. This effect was abrogated when cells were pre-treated with the endocytosis inhibitor cytochalasin B.ConclusionsThese results identify hepatocyte pyroptosis and release of inflammasome components as a novel mechanism to propagate liver injury and liver fibrosis development.Lay summaryOur findings identify a novel mechanism of inflammation in the liver. Experiments in cell cultures, mice, and human samples show that a specific form of cell death, called pyroptosis, leads to the release of complex inflammatory particles, the NLRP3 inflammasome, from inside hepatocytes into the extracellular space. From there they are taken up by other cells and thereby mediate inflammatory and pro-fibrogenic stress signals. The discovery of this mechanism may lead to novel treatments for chronic liver diseases in the future.

The NLRP3 inflammasome plays an important role in liver fibrosis 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 (HSC) can directly regulate their activation and contribute to liver fibrosis. Primary HSC isolated from WT, Nlrp3(−/−), or Nlrp3(L351PneoR) knock-in crossed to inducible (estrogen receptor Cre - CreT) mice were incubated with LPS and 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 blot, FACS, ELISA, immunofluorescence and immunohistochemistry. Stimulated WT HSC displayed increased levels of NLRP3 inflammasome-induced ROS production and Cathepsin B activity, accompanied by an upregulation of mRNA and protein levels of fibrotic makers, an effect abrogated in Nlrp3(−/−) HSC. Nlrp3(L351P) CreT HSC also showed elevated mRNA and protein expression of fibrotic markers 24h after inflammasome activation induced with 4OH-tamoxifen. Protein and mRNA expression levels of fibrotic markers were also found to be increased in isolated HSC 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 αSMA 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 HSC directly triggering liver fibrosis.

IL-6 is a known downstream target of IL-1β and is consistently increased in serum from patients with NLRP3 inflammasome-mediated conditions. Therefore, IL-6 could be a therapeutic target in the treatment of IL-1β-provoked inflammation. IL-6 was increased in serum with accompanying neutrophilia in tissues of an inducible mouse model of Muckle-Wells syndrome. However, an IL-6-null background failed to provide phenotypic rescue and did not significantly impact inflammatory cytokine levels. In a second model of IL-1β-driven inflammation, NLRP3 activation by monosodium urate crystals similarly increased IL-6. Consistent with our Muckle-Wells syndrome model, ablation of IL-6 did not impact an acute neutrophilic response in this in vivo evaluation of gouty arthritis. Taken together, our results indicate that IL-6 is a reliable marker of inflammation, with no direct role in inflammasome-mediated disease.