objective: Endoplasmic reticulum to nucleus signaling 1 (ERN1, also called IRE1A) is a sensor of the unfolded protein response that is activated in livers of patients with nonalcoholic steatohepatitis (NASH). Hepatocytes release ceramide-enriched inflammatory extracellular vesicles (EVs) following activation of IRE1A. We studied the effects of inhibiting IRE1A on release of inflammatory EVs in mice with diet-induced steatohepatitis.
methods: C57BL/6J mice and mice with hepatocyte-specific disruption of Ire1a (IRE1α) mice were fed a diet high in fat, fructose, and cholesterol (FFC) to induce development of steatohepatitis or a standard chow diet (controls). Some mice were given intraperitoneal injections of the... More
objective: Endoplasmic reticulum to nucleus signaling 1 (ERN1, also called IRE1A) is a sensor of the unfolded protein response that is activated in livers of patients with nonalcoholic steatohepatitis (NASH). Hepatocytes release ceramide-enriched inflammatory extracellular vesicles (EVs) following activation of IRE1A. We studied the effects of inhibiting IRE1A on release of inflammatory EVs in mice with diet-induced steatohepatitis.
methods: C57BL/6J mice and mice with hepatocyte-specific disruption of Ire1a (IRE1α) mice were fed a diet high in fat, fructose, and cholesterol (FFC) to induce development of steatohepatitis or a standard chow diet (controls). Some mice were given intraperitoneal injections of the IRE1A inhibitor 4μ8C. Mouse liver and primary hepatocytes were transduced with adenovirus or adeno-associated virus that expressed IRE1A. Livers were collected from mice and analyzed by quantitative PCR and chromatin immunoprecipitation assays; plasma samples were analyzed by ELISA. EVs were derived from hepatocytes and injected intravenously into mice. Plasma EVs were characterized by nanoparticle-tracking analysis, electron microscopy, immunoblots, and nanoscale flow cytometry; we used a membrane-tagged reporter mouse to detect hepatocyte-derived EVs. Plasma and liver tissues from patients with NASH and without NASH (controls) were analyzed for EV concentration and by RNAscope and gene expression analyses.
results: Disruption of Ire1a in hepatocytes or inhibition of IRE1A reduced release of EVs and liver injury, inflammation, and accumulation of macrophages in mice on the FFC diet. Activation of IRE1A, in livers of mice, stimulated release of hepatocyte-derived EVs, and also from cultured primary hepatocytes. Mice given intravenous injections of IRE1A-stimulated, hepatocyte-derived EVs accumulated monocyte-derived macrophages in liver. IRE1A-stimulated EVs were enriched in ceramides. Chromatin immunoprecipitation showed that IRE1A activated X-box binding protein 1 (XBP1) to increase transcription of serine palmitoyltransferase genes, which encode the rate-limiting enzyme for ceramide biosynthesis. Administration of a pharmacological inhibitor of serine palmitoyltransferase to mice reduced the release of EVs. Levels of XBP1 and serine palmitoyltransferase were increased in liver tissues, and numbers of EVs were increased in plasma, from patients with NASH compared with controls and correlated with the histologic features of inflammation.
conclusions: In mouse hepatocytes, activated IRE1A promotes transcription of serine palmitoyltransferase genes via XBP1, resulting in ceramide biosynthesis and release of EVs. The EVs recruit monocyte-derived macrophages to liver, resulting in inflammation and injury in mice with diet-induced steatohepatitis. Levels of XBP1, serine palmitoyltransferase, and EVs are all increased in liver tissues from patients with NASH. Strategies to block this pathway might be developed to reduce liver inflammation in patients with NASH.
