Bile Acids - Classification, Synthesis and Metabolism, Detection Methods

Bile acid is the main organic component of bile, which is synthesized in the liver and enters the intestine, where it is repeatedly used in the body through the hepatic-intestinal cycle to maximize its effects and meet the body's bile acid requirements. Previously, it was thought that its role was to promote the absorption of lipids in the intestine and to participate in cholesterol metabolism. However, in recent years, it has been found that it can act as a ligand to bind with farnesyl derivative X receptor and G protein-coupled bile acid receptor to mediate a series of signal transduction pathways, thus playing a role in glycolipid metabolic diseases, intestinal flora, and immune system.

Classification of Bile Acids

• Classified by structure

Free bile acids: bile acids, deoxycholic acid, goose deoxycholic acid, stone bile acids

Conjugated bile acids: glycocholic acid, taurocholic acid, glycocholic goose deoxycholic acid, taurocholic goose deoxycholic acid

• Classified by source

Primary bile acids: bile acids, goose deoxycholic acid and combinations with glycine or taurine

Secondary bile acids: deoxycholic acid and rock bile acid

Synthesis and Metabolism of Bile Acids

The dynamic balance of hepatocyte cholesterol depends to a large extent on the conversion of cholesterol into bile acids. Bile acids are directly synthesized by hepatocytes from cholesterol by two pathways (classical and alternative) called primary bile acids, including cholie acid (CA) and chenodeoxycholie acid (CDCA), under the action of cytochrome (CYP450).

After the primary bile acids enter the intestine with bile, the bile acids are transformed into deoxycholie acid (DCA) by 7α-dehydroxylation of intestinal flora, and goose deoxycholie acid is transformed into lthoholie acid (LCA), which is called secondary bile acids. The above bile acids are combined with glycine or taurine in hepatocytes and are called conjugated bile acids, such as glycocholic acid, glycocholic goose deoxycholic acid, taurocholic acid and taurocholic goose deoxycholic acid.

Conjugated bile acids are the main form secreted into bile by the liver. Under the action of intestinal bacteria, the conjugated bile acids can be hydrolyzed to remove glycine or taurine to become free bile acids.

At the ileal terminus, most of the unbound BAs are reabsorbed by sodium-dependent BA transporter protein (ASBT) into the enterocytes and enter the portal circulation via the basolateral BA transporter proteins OSTα, OSTβ, and MRP2. They are then taken up by NTCP and OATP1 into the hepatocytes. Bile acids that have been hydrolyzed in the liver to remove taurine or glycine are re-formed as conjugated bile acids, which are then secreted into the bile. This process is the enterohepatic cycle of bile acids.

Bile Acid Metabolism and Disease

The synthesis, secretion, reabsorption and processing of bile acids are closely related to the liver, bile and intestine. Therefore, liver, biliary or intestinal diseases must affect bile acid metabolism, and abnormal bile acid metabolism inevitably affects organ function and cholesterol metabolism level.

• Diabetes

Diabetes mellitus is a chronic disease. Hyocholic acid (HCA), the most abundant class of bile acids in pigs, has been found to promote intestinal endocrine L-cells through a unique signaling mechanism that activates G protein-coupled bile acid receptor 5 (TGR5) and inhibits farnesol X receptor (FXR) signaling to endocrine cells to produce and secrete GLP-1, thereby effectively regulating glucose homeostasis. Analysis of clinical samples also showed that serum HCA was negatively correlated with diabetes and blood glucose levels. These findings are of great value for studying the role and mechanism of bile acids in regulating blood glucose and developing novel drugs for diabetes.

Mechanism of HCA in regulating blood glucose

• Inflammation regulation

In the normal population, bile acids protect intestinal epithelial cells and provide resistance to pathogenic bacteria, suggesting that these bile acids have the potential to regulate intestinal immune cells. Jun Huh's group from the Department of Immunology at Harvard Medical School screened nearly 30 compounds, including primary and secondary bile acids, for bile acids that modulate T-cell function. They used wild-type C57BL/6 mice (B6Jax mice) to isolate initial CD4+ T cells and co-cultured them with bile acids in the differentiated state of TH17 and Treg cells, and found that two lithophan derivatives significantly affected TH17 and Treg cell differentiation. This study suggests that bile acid metabolites can modulate host immunity by directly regulating TH17 and Treg cell homeostasis.

Lithotrizoic acid derivatives affect Th17 and Treg cell differentiation

• Immunity to Infection

The highly contagious norovirus causes diarrhea and vomiting and spreads rapidly in densely populated spaces. The virus kills about 200,000 people each year, yet there is still no treatment for this enterovirus. A new study led by scientists at Washington University School of Medicine in St. Louis suggests that the severity of norovirus infection can be suppressed or increased depending on where in the gut the virus is located. They found that normal gut bacteria exacerbated the severity of viral infections in the lower part of the small intestine, but also blocked or inhibited viral infections in the upper part of the small intestine. Intestinal microbes may have completely opposite effects on norovirus infection, depending on the specific location of the infection in the gut. Further, the authors found that this difference in response was driven by bile acids, and that bile acids in the upper small intestine (but not in the lower) stimulated the immune system to respond to infection. The complexity of the interactions between gut microbes and bile acids could explain some of the variability in norovirus infection.

Small intestinal regional effect model of bile acids on norovirus infection in mice

Bile Acid Analysis

The Creative Proteomics targeted metabolomics platform enables absolute quantification of multiple bile acids. UPLC-Triple Quadrupole Mass Spectrometry (UPLC-TQMS) is used for the separation and spectral peak area acquisition of bile acids in complex samples. A standard curve is constructed using full-spectrum bile acid standards to calculate the absolute concentration of each bile acid in the sample.

Advantages of bile acid analysis platform:

• High specificity and accuracy: precise characterization and absolute quantification using MRM technology
• Advanced platform: 5500 Q-trap mass spectrometry with wide linear range
• Strict quality control: double quality control of internal standard + external standard
• High acquisition rate for reliable quantification of more analytes
• Excellent durability for reproducible detection of low levels of analytes in complex matrices

References

1. Zheng, X., et al. (2021). Hyocholic acid species improve glucose homeostasis through a distinct TGR5 and FXR signaling mechanism. Cell metabolism, 33(4), 791-803.
2. Fu, T., et al. (2019). FXR regulates intestinal cancer stem cell proliferation. Cell, 176(5), 1098-1112.
3. Qi, X., et al. (2019). Gut microbiota–bile acid–interleukin-22 axis orchestrates polycystic ovary syndrome. Nature medicine, 25(8), 1225-1233.
4. Hang, S., et al. (2019). Bile acid metabolites control TH17 and Treg cell differentiation. Nature, 576(7785), 143-148.
5. Grau, K. R., et al. (2020). The intestinal regionalization of acute norovirus infection is regulated by the microbiota via bile acid-mediated priming of type III interferon. Nature Microbiology, 5(1), 84-92.