Bax inhibitor-1 protects from nonalcoholic steatohepatitis by limiting inositol-requiring enzyme 1 alpha signaling in mice

Endoplasmic reticulum (ER) stress is activated in nonalcoholic fatty liver disease (NAFLD), raising the possibility that ER stress-dependent metabolic dysfunction, inflammation, and cell death underlie the transition from steatosis to steatohepatitis (nonalcoholic steatohepatitis; NASH). B-cell lymphoma 2 (BCL2)-associated X protein (Bax) inhibitor-1 (BI-1), a negative regulator of the ER stress sensor, inositol-requiring enzyme 1 alpha (IRE1α), has yet to be explored in NAFLD as a hepatoprotective agent. We hypothesized that the genetic ablation of BI-1 would render the liver vulnerable to NASH because of unrestrained IRE1α signaling. ER stress was induced in wild-type and BI-1^-/- mice acutely by tunicamycin (TM) injection (1 mg/kg) or chronically by high-fat diet (HFD) feeding to determine NAFLD phenotype. Livers of TM-treated BI-1^-/- mice showed IRE1α-dependent NOD-like receptor family, pyrin domain containing 3 (NLRP3) inflammasome activation, hepatocyte death, fibrosis, and dysregulated lipid homeostasis that led to liver failure within a week. The analysis of human NAFLD liver biopsies revealed BI-1 down-regulation parallel to the up-regulation of IRE1α endoribonuclease (RNase) signaling. In HFD-fed BI-1^-/- mice that presented NASH and type 2 diabetes, exaggerated hepatic IRE1α, X-box binding protein 1 (XBP1), and C/EBP homologous protein (CHOP) expression was linked to activated NLRP3 inflammasome and caspase-1/-11. Rises in interleukin (IL)-1β, IL-6, monocyte chemoattractant protein 1 (MCP1), chemokine (C-X-C motif) ligand 1 (CXCL1), and alanine transaminase (ALT)/aspartate transaminase (AST) levels revealed significant inflammation and injury, respectively. Pharmacological inhibition of IRE1α RNase activity with the small molecules, STF-083010 or 4μ8c, was evaluated in HFD-induced NAFLD. In BI-1^-/- mice, either treatment effectively counteracted IRE1α RNase activity, improving glucose tolerance and rescuing from NASH. The hepatocyte-specific role of IRE1α RNase activity in mediating NLRP3 inflammasome activation and cell death was confirmed in primary mouse hepatocytes by IRE1α axis knockdown or its inhibition with STF-083010 or 4μ8c.

Conclusion: Targeting IRE1α-dependent NLRP3 inflammasome signaling with pharmacological agents or by BI-1 may represent a tangible therapeutic strategy for NASH. (Hepatology 2018).