Role of NLRP3 inflammasome activation during cholestatic liver injury

Accumulation of toxic bile salts in hepatocytes leads to hepatic toxicity and injury by cell death. Apoptotic and pyroptotic cell death is a consequence of caspase-8 or caspase-1 activation and is mediated by the NLRP3 inflammasome. The activation of the NLRP3 inflammasome results in liver inflammation, fibrosis and hepatocyte pyroptosis in mice. Thus we investigate what the role of the NLRP3 inflammasome activation is during cholestatic liver injury.

To evaluate the role of the NLRP3 inflammasome we used the bile duct ligation (BDL) model (as established by Weiskirchen et al.) in WT and NLRP3-/-. After performing BDL we waited 2 days (acute model) or 28 d (chronic model) and sacrificed the mice. Inflammation, fibrosis and cell death were evaluated with qPCR, IHC, IF and western blot.

After 2 d of BDL we can see that NLRP3-/- mice have elevated serum transaminases compared to WT mice. In addition, hepatic inflammation, as shown by mRNA levels of TNFα, IL-1b, MCP-1 and influx of Ly6G+Cd11b+ cells, was observed. This increased liver injury can be explained by increased necroptosis in NLRP3-/- mice, as shown by an increase in mitochondrial ROS and RIPK3. After 28 d however, NLRP3-/- mice have reduced serum transaminases, less inflammation and reduced fibrosis. TUNEL and caspase-3 staining also show decreased cell death in NLRP3-/- mice. Furthermore, caspase-3 staining shows areas of pyroptotic cell death in WT mice after 28 d of BDL. This can be explained due to the inability of NLRP3-/- mice to activate caspase-1 and release IL-1b. Due to the increased RIP3-mediated necroptosis in NLRP3-/- mice expansion of progenitor/oval cells is triggered which could explain the reduced liver injury.

In conclusion, in the acute phase NLRP3-/- mice have more liver injury through mitochondrial ROS and RIPK1 – 3 activation, necroptosis and thus increased inflammatory response. Progenitor/oval cell activation mediates tissue repair in chronic phase. In the chronic phase NLRP3-/- mice have less liver injury due to the lack of pyroptotic cell death which leads to less inflammation and fibrosis.