Neutral Ceramidase Mediates Nonalcoholic Steatohepatitis by Regulating Monounsaturated Fatty Acids and Gut IgA+ B Cells

Background and Aims Nonalcoholic steatohepatitis (NASH) is associated with obesity and an increased risk for liver cirrhosis and cancer. Neutral ceramidase (NcDase), which is highly expressed in the intestinal brush border of the small intestine, plays a critical role in digesting dietary sphingolipids (ceramide) to regulate the balance of sphingosine and free fatty acids. It remains unresolved whether obesity‐associated alteration of NcDase contributes to the manifestation of NASH. Here, we revealed that NcDase deficiency in murine models of NASH prevents hepatic inflammation and fibrosis but not steatosis. Approach and Results NcDase −/− mice display reduced stearoyl‐CoA desaturase (SCD) 1 expression with a compositional decrease of monounsaturated fatty acids (MUFAs) under the different dietary conditions. We further found that NcDase is a functional regulator of intestinal B cells and influences the abundance and quality of the secretory IgA response toward commensal bacteria. Analysis of composition of the gut microbiota found that Clostridiales colonization was increased in NcDase −/− mice. The colonization of germ‐free mice with gut microbiota from NcDase −/− mice resulted in a greater decrease in the expression of SCD1 and the level of MUFAs in the liver relative to gut microbiota from wild‐type littermates, which are associated with the alternation of IgA‐bound bacteria, including increase of Ruminococcaceae and reduction of Desulfovibrio . Mechanistically, NcDase is a crucial link that controls the expression of SCD1 and MUFA‐mediated activation of the Wnt/β‐catenin. Very importantly, our experiments further demonstrated that Wnt3a stimulation can enhance the activity of NcDase in hepatocytes. Conclusions Thus, the NcDase‐SCD1‐Wnt feedback loop promotes the diet‐induced steatohepatitis and fibrosis through the regulation of intestinal IgA + immune cells.