Bile acids activate farnesoid X receptor (FXR) and the G-protein-coupled receptor, TGR5, and also several cell-signaling
pathways to regulate bile acid synthesis and lipid metabolism. Pharmacological activation of either FXR or TGR5 receptor has been shown to improve lipid, glucose, and energy homeostasis, glucose tolerance, and insulin sensitivity.[2, 3] Paradoxically, loss of FXR in obese and diabetic mice reduced body weight and improved peripheral insulin PD0325901 sensitivity, and decreasing bile acid pool size with the specific FXR agonist, GW4064, caused increased susceptibility to diet-induced obesity, fatty liver, and hypertriglyceridemia. It is likely that activation of different bile acid signaling in different mouse models might have different effects on hepatic metabolism, diabetes, and obesity. In Cyp7a1 transgenic (Cyp7a1-tg)
mice, both CYP7A1 enzyme activity and bile acid pool size are doubled, biliary cholesterol and bile acid secretion are stimulated, and serum cholesterol is decreased, whereas serum triglyceride levels remain the same. find more These metabolic changes caused by increased CYP7A1 expression result in significantly improved lipid homeostasis and protection against hepatic steatosis, insulin resistance (IR), and obesity. Therefore, further study is necessary to understand the participation of bile acid synthesis in the regulation of metabolic homeostasis, nonalcoholic fatty liver disease (NAFLD), and diabetes. Bile acid metabolism is closely linked to whole-body cholesterol homeostasis; bile acid synthesis and bile-acid–facilitated biliary cholesterol secretion are the only significant pathways for cholesterol elimination from the body. Furthermore, the liver acquires cholesterol through dietary absorption,
receptor-mediated uptake, and MCE de novo synthesis. Intracellular cholesterol/oxysterols play an important role in the regulation of cholesterol synthesis through the transcriptional factor, sterol response element-binding protein 2 (SREBP2). Upon increased intracellular cholesterol levels, SREBP2 precursor (125 kDa) forms a complex with insulin-induced gene (INSIG) and SREBP cleavage-activating protein (SCAP), which is retained in the endoplasmic reticulum (ER) membrane. When cholesterol levels decrease, SCAP escorts SREBP2 precursor to the Golgi, where two steroid-sensitive proteases (S1P and S2P) cleave an N-terminal fragment (68 kDa), subsequently translocating into the nuclei to activate its target genes, including low-density lipoprotein receptor (LDLR) and key genes involved in de novo cholesterol synthesis. microRNAs (miRs) are small noncoding RNAs that, after base pairing with complementary sequences of target messenger RNAs (mRNAs), promote mRNA degradation or inhibit protein synthesis. miR-33a, encoded by intron 16 of the SREBP2 gene, has recently been shown to regulate cellular cholesterol homeostasis, biliary bile acid secretion, and fatty acid oxidation.