Taurochenodeoxycholic Acid Increases cAMP Content via Specially Interacting with Bile Acid Receptor TGR5

Taurochenodeoxycholic acid (TCDCA) is one of the main components of bile acids (BAs). TCDCA has been reported as a signaling molecule, exerting anti-inflammatory and immunomodulatory functions. However, it is not well known whether those effects are mediated by TGR5. This study aimed to elucidate the interaction between TCDCA and TGR5. To achieve this aim, first, the TGR5 eukaryotic vector was constructed. The expression level of TGR5 in 293T cells was determined by immunofluorescence, real-time quantitative PCR (RT-PCR, qPCR), and Western blot. The luciferase assay, fluorescence microscopy, and enzyme-linked immunosorbent assay (ELISA) were recruited to check the interaction of TCDCA with TGR5. TCDCA treatment in 293T cells resulted in TGR5 internalization coupled with a significant increase in cAMP luciferase expression. Our results demonstrated that TCDCA was able to bind to the TGR5 receptor and activate it. These results provide an excellent potential therapeutic target for TCDCA research. Moreover, these findings also provide theoretical evidence for further TCDCA research.

Bile Acid Receptors in Cholangiocyte

Bile acids are known to play a role in helping the digestion of lipid and maintenance of the bile flow. However, since the first bile acid receptor was discovered in 1999, it has been found that various bile acid receptors are present. Bile acid receptors are involved in bile acid physiology, energy metabolism, and inflammatory response. G-protein bile acid-activated receptor 1 (GPBAR1) and Sphingosine-1-phosphate receptor 2 (S1PR2) are representative bile acid receptors in cholangiocyte. They are involved in proliferation and secretion of cholangiocyte, which seem to protect cholangiocyte from the toxicity of bile acids. GPBAR1 and S1PR2 are also associated with the progression of cholangiocarcinoma.

Chenodeoxycholic Acid Has Non-Thermogenic, Mitodynamic Anti-Obesity Effects in an In Vitro CRISPR/Cas9 Model of Bile Acid Receptor TGR5 Knockdown

Bile acids (BA) have shown promising effects in animal models of obesity. However, the said effects are thought to rely on a thermogenic effect, which is questionably present in humans. A previous work has shown that the BA chenodeoxycholic acid (CDCA) can revert obesity and accelerate metabolism in animal and cell culture models. Thus, the aim of this study was to understand if this obesity reduction is indeed thermogenically-dependent. A CRISPR/Cas9 model of TGR5 (BA receptor) knockdown in 3T3-L1 adipocytes was developed to diminish thermogenic effects. Various parameters were assessed, including mitochondrial bioenergetics by Seahorse flux analysis, oxidative stress and membrane potential by fluorometry, intermediary metabolism by NMR, protein content assessment by Western Blot, gene expression by qPCR, and confocal microscopy evaluation of mitophagy. CDCA was still capable, for the most part, of reversing the harmful effects of cellular obesity, elevating mitophagy and leading to the reduction of harmed mitochondria within the cells, boosting mitochondrial activity, and thus energy consumption. In summary, CDCA has a non-thermogenic, obesity reducing capacity that hinges on a healthy mitochondrial population, explaining at least some of these effects and opening avenues of human treatment for metabolic diseases.

Role of Gut Microbiota in Bile-Acid Metabolism

The role of the gut microbiota in modifying the pathophysiology of various diseases, including neurodegenerative diseases, is increasingly becoming clear. Bile acids have been shown to be endogenous factors that affect gut microbiota, and bile-acid metabolites directly or indirectly affect host physiology and pathophysiology. The development of metagenomic analysis for gut microbiota and systematic bile-acid measurement using LC–MS/MS has triggered a breakthrough for research in this field. Clinically, an inhibitor of the ileal bile-acid transporter (Elobixibat) was used as a therapeutic agent for chronic constipation, which also paved the way for progress in bile-acid signal research. Additionally, this review emphasizes the importance of gut microbiota-bile acid-receptor signals when considering nutritional approaches to promote healthy longevity.