ABSTRACT
The farnesoid X receptor (FXR) plays an important role in mediating the dialog between the host and gut microbiota, particularly through modulation of enterohepatic circulation of bile acids. Mounting evidence suggests that genetic ablation of Fxr in the gut or gut-restricted chemical antagonism of the FXR promotes beneficial health effects, including the prevention of nonalcoholic fatty liver disease in rodent models. However, questions remain unanswered, including whether modulation of FXR activity plays a role in shaping the gut microbiota community structure and function and what metabolic pathways of the gut microbiota contribute in an FXR-dependent manner to the host phenotype. In this report, new insights are gained into the metabolic contribution of the gut microbiota to the metabolic phenotypes, including establishing a link between FXR antagonism, bacterial bile salt hydrolase activity, and fermentation. Multiple approaches, including unique mouse models as well as metabolomics and genome-scale metabolic models, were employed to confirm these results. The gut microbiota modulates obesity and associated metabolic phenotypes in part through intestinal farnesoid X receptor (FXR) signaling. Glycine-β-muricholic acid (Gly-MCA), an intestinal FXR antagonist, has been reported to prevent or reverse high-fat diet (HFD)-induced and genetic obesity, insulin resistance, and fatty liver; however, the mechanism by which these phenotypes are improved is not fully understood. The current study investigated the influence of FXR activity on the gut microbiota community structure and function and its impact on hepatic lipid metabolism. Predictions about the metabolic contribution of the gut microbiota to the host were made using 16S rRNA-based PICRUSt (phylogenetic investigation of communities by reconstruction of unobserved states), then validated using 1H nuclear magnetic resonance-based metabolomics, and results were summarized by using genome-scale metabolic models. Oral Gly-MCA administration altered the gut microbial community structure, notably reducing the ratio of Firmicutes to Bacteroidetes and its PICRUSt-predicted metabolic function, including reduced production of short-chain fatty acids (substrates for hepatic gluconeogenesis and de novo lipogenesis) in the ceca of HFD-fed mice. Metabolic improvement was intestinal FXR dependent, as revealed by the lack of changes in HFD-fed intestine-specific Fxr-null (Fxr
ΔIE) mice treated with Gly-MCA. Integrative analyses based on genome-scale metabolic models demonstrated an important link between Lactobacillus and Clostridia bile salt hydrolase activity and bacterial fermentation. Hepatic metabolite levels after Gly-MCA treatment correlated with altered levels of gut bacterial species. In conclusion, modulation of the gut microbiota by inhibition of intestinal FXR signaling alters host liver lipid metabolism and improves obesity-related metabolic dysfunction. IMPORTANCE The farnesoid X receptor (FXR) plays an important role in mediating the dialog between the host and gut microbiota, particularly through modulation of enterohepatic circulation of bile acids. Mounting evidence suggests that genetic ablation of Fxr in the gut or gut-restricted chemical antagonism of the FXR promotes beneficial health effects, including the prevention of nonalcoholic fatty liver disease in rodent models. However, questions remain unanswered, including whether modulation of FXR activity plays a role in shaping the gut microbiota community structure and function and what metabolic pathways of the gut microbiota contribute in an FXR-dependent manner to the host phenotype. In this report, new insights are gained into the metabolic contribution of the gut microbiota to the metabolic phenotypes, including establishing a link between FXR antagonism, bacterial bile salt hydrolase activity, and fermentation. Multiple approaches, including unique mouse models as well as metabolomics and genome-scale metabolic models, were employed to confirm these results.
Farnesoid X Receptor Signaling Shapes the Gut Microbiota and Controls Hepatic Lipid Metabolism