An Intestinal Microbiota-Farnesoid X Receptor Axis Modulates Metabolic Disease

Abstract

The gut microbiota is associated with metabolic diseases including obesity, insulin resistance, and nonalcoholic fatty liver disease, as shown by correlative studies and by transplant of microbiota from obese humans and mice into germ-free mice. Modification of the microbiota by treatment of high-fat diet (HFD)-fed mice with tempol or antibiotics resulted in decreased adverse metabolic phenotypes. This was owing to lower levels of the genera Lactobacillus and decreased bile salt hydrolase (BSH) activity. The decreased BSH resulted in increased levels of tauro-β-muricholic acid (MCA), a substrate of BSH and a potent farnesoid X receptor (FXR) antagonist. Mice lacking expression of FXR in the intestine were resistant to HFD-induced obesity, insulin resistance, and nonalcoholic fatty liver disease, thus confirming that intestinal FXR is involved in the potentiation of metabolic disease. A potent intestinal FXR antagonist, glycine-β-MCA (Gly-MCA), which is resistant to BSH, was developed, which, when administered to HFD-treated mice, mimics the effect of the altered microbiota on HFD-induced metabolic disease. Gly-MCA had similar effects on genetically obese leptin-deficient mice. The decrease in adverse metabolic phenotype by tempol, antibiotics, and Gly-MCA was caused by decreased serum ceramides. Mice lacking FXR in the intestine also have lower serum ceramide levels, and are resistant to HFD-induced metabolic disease, and this was reversed by injection of C16:0 ceramide. In mouse ileum, because of the presence of endogenous FXR agonists produced in the liver, FXR target genes involved in ceramide synthesis are activated and when Gly-MCA is administered they are repressed, which likely accounts for the decrease in serum ceramides. These studies show that ceramides produced in the ileum under control of FXR influence metabolic diseases.

Keywords: Bile Acids; Ceramides; Farnesoid X Receptor; Metabolic Disease.

Figure 1

Levels of pantothenic acid in the urine of mice that were treated with tempol. Pantohthenic acid is converted to 4′phosphopantothenate, which is a precursor for synthesis of coenzyme A (CoA) and is required for the β-oxidation of fatty acids in mitochondria.

Figure 2

Bile acids are synthesized in the liver and intestine to produced FXR agonist and antagonist. In the liver CDCA is synthesized and then conjugated with taurine to produce TCDCA, or hydroxylated to produce α-MCA which can also be conjugated with taurine to produce T-α-MCA (not shown). UDCA, an isomer of CDCA, is made and hydroxylated to produce β-MCA that is then conjugated with taurine to yield T-β-MCA. TCDCA and T-β-MCA and some ot their free unconjugated forms are exported to the intestine. In the intestine T-β-MCA, an FXR antagonist, and can by deconjugated to taurine and β-MCA by bacterial BSH. CDCA resulting from TCDCA deconjugation, is a potent FXR agonist. The concentrations of CDCA, T-β-MCA and other bile acids in the lower small intestine, likely determine the extent of FXR signaling in ileal epithelial cells.

Figure 3

The role of gut bacterial and intestinal FXR in tempol-induced weight loss. Cholesterol is converted to T-β-MCA that is secreted into to intestine. In the lower small intestine of mice it is hydrolyzed to β-MCA and taurine by BSH. When mice are administered tempol, the levels of Lactobacillus and BSH activity are decreased resulting in increased T-β-MCA that inhibits FXR in the ilium epithelial cells. This results in lower obesity and insulin resistance.

Figure 4

The role FXR signaling and ceramides in the modulation of metabolic disease. In obese mice, a bile acid agonist produced in the liver constitutively stimulates FXR in the ileum resulting in increased production of ceramides. Ceramides then cause lipid toxicity in the liver resulting in increased ER stress and elevated fatty acid synthesis due to increased SREBP-1 signaling. Ceramides also impair adipose function through increase ER stress, decreasing the ration of beige to white adipocytes. In the presence of the FXR antagonist Gly-MCA, ceramide levels are lowered resulting in decreased hepatic lipid synthesis and steatosis and increased adipose beiging.

Figure 5

Potential mechanism by which ceramides increase NAFLD. Ceramides enter the cell and incorporate into the ER and mitochondrial membranes causing increased ER stress and mitochondrial permeability, respectively. Mitochondrial damage can give rise to increased reactive oxygen species and cause apoptosis. Through a mechanism that is still unclear, ceramides can activate SREBP-1c leading to increased fatty acid synthesis and NAFLD.