Gut dysbiosis contributes to NASH in a pig model

Gut microbiota contributes to the development of NASH through modulation of gut barrier, inflammation, altered lipid metabolism, bile acid signaling and short-chain fatty acids (SCFA). We describe the impact of a choline-deficient amino acid defined high fat diet (CDAHFD) on the gut microbiota in a male Göttingen Minipig model and on selected pathways involved in the development of NASH. Eight weeks of CDAHFD resulted in a significantly altered colon microbiota mainly driven by the bacterial families Lachnospiraceae and Enterobacteriaceae, being decreased and increased in abundance, respectively. CDAHFD decreased colon content of SCFA and increased colonic pH. In addition, serum imidazole propionate was significantly elevated as a consequence of CDAHFD feeding. Hepatic gene expression analysis indicated upregulation of ras homolog enriched in brain and mechanistic target of rapamycin (mTOR) and downregulation of insulin receptor substrate 1 and insulin receptor substrate 2 in CDAHFD fed minipigs. Furthe consequences of CDAHFD was increased serum cholesterol, increased serum bile acids (BA) and hyperglucagonemia, but not hyperaminoacidemia. In Göttingen Minipigs, CDAHFD introduced a shift in the colon microbiota resulting in a bacterial community with a decreased capacity for SCFA production. Consequently, the pH rises creating an environment with the potential to activate the bacterial enzymes urocanate reductase facilitating production imidazole propionate. We speculate that imidazole propionate activates hepatic mTORC1 resulting in impaired insulin sensitivity in the liver. Moreover, we suggest that imidazole propionate has the ability to activate mTOR in a-cells and contribute to an hyperglucagonemic state through a-cell hyperplasia. Additionally, the changed colon microbiota could be linked to a changed BA profile and hepatic fibrosis, further contributing to the NASH development.

肠道微生物群（gut microbiota）通过调控肠道屏障、炎症反应、脂质代谢、胆汁酸信号通路以及短链脂肪酸（short-chain fatty acids, SCFA），参与非酒精性脂肪性肝炎（NASH）的发生发展。本研究旨在阐明胆碱缺乏氨基酸定义型高脂饮食（choline-deficient amino acid defined high fat diet, CDAHFD）对雄性哥廷根小型猪（Göttingen Minipig）模型肠道微生物群的影响，以及其对非酒精性脂肪性肝炎发生发展相关通路的调控作用。为期8周的CDAHFD干预可显著改变结肠微生物群结构，其变化主要由毛螺菌科（Lachnospiraceae）丰度降低与肠杆菌科（Enterobacteriaceae）丰度升高所驱动。CDAHFD可降低结肠内短链脂肪酸含量，并升高结肠pH值。此外，CDAHFD喂养可显著升高血清咪唑丙酸水平。肝脏基因表达分析结果显示，CDAHFD喂养的小型猪中，脑富集RAS同源蛋白（ras homolog enriched in brain）及雷帕霉素靶蛋白（mechanistic target of rapamycin, mTOR）的表达上调，而胰岛素受体底物1（insulin receptor substrate 1）与胰岛素受体底物2（insulin receptor substrate 2）的表达下调。CDAHFD的其他影响还包括血清胆固醇、血清胆汁酸（bile acids, BA）水平升高以及高胰高血糖素血症，但未出现高氨基酸血症。在哥廷根小型猪模型中，CDAHFD可引起结肠微生物群结构改变，使肠道菌群的短链脂肪酸合成能力下降。随之而来的结肠pH升高可营造出可激活细菌尿刊酸还原酶（urocanate reductase）的环境，进而促进咪唑丙酸的生成。我们推测，咪唑丙酸可激活肝脏mTOR复合物1（mTORC1），进而导致肝脏胰岛素敏感性受损。此外，我们认为咪唑丙酸可激活α细胞（a-cells）内的mTOR，并通过α细胞增生引发高胰高血糖素血症。此外，结肠微生物群的改变可能与胆汁酸谱变化及肝纤维化（hepatic fibrosis）相关，进而进一步促进非酒精性脂肪性肝炎的发生发展。