Farnesoid X receptors (FXRs) are nuclear hormone receptors expressed in high amounts in body tissues that participate in bilirubin metabolism including the liver, intestines, and kidneys. Bile acids (BAs) are the natural ligands of the FXRs. FXRs regulate the expression of the gene encoding for cholesterol 7 alpha-hydroxylase, which is the rate-limiting enzyme in BA synthesis. In addition, FXRs play a critical role in carbohydrate and lipid metabolism and regulation of insulin sensitivity. FXRs also modulate live growth and regeneration during liver injury.
An overview of FXR
The farnesoid X receptor (FXR) is a member of the nuclear receptor super family and is the ligand-activated transcription factor. With a typical nuclear receptor structure, it has a highly conserved DNA binding region (DBD), a carboxyl terminal ligand binding area (LBD), an amino-terminated ligand-independent transcriptional activation function (AF-1), a carboxyl-terminated ligand-dependent activation function area (AF-2) and others. FXR, like other members of the nuclear receptor family, has a FXR endogenous ligand made of bile acid in the DBD region, which is not only an important substance in lipid digestion and absorption, but also a kind of signaling molecule transferring information through the FXR-mediated signaling pathways, participating in the regulation of metabolism in vivo. And FXR not only plays an important role in regulating bile acid and lipid metabolism, but also is related to many metabolic diseases.
Major types of FXR
It has been proved that mouse FXR has four subtypes, including FXRα1, FXRα2, FXRβ1 and FXRβ2. And the FXR transcription of all four subtypes is originated from the same gene.
Inhibition of FXR
Guggul is a natural FXR effective antagonist and it can lower cholesterol levels in mice with a high cholesterol diet.
FXR and diseases
The illuminating of the function of the FXR provides a new direction for the development of new drugs for lowering cholesterol, and efficiently treating cardiovascular diseases. In principle, stimulating the synthesis of bile acid ca increases the excretion of bile acid in the gallbladder, and reduces the bile acid and cholesterol in the intestinal heavy absorption, and thereby reduces cholesterol. In addition, drugs targeted at FXR are expected to be used to treat certain liver diseases, such as cholestasis, cholelithiasis, cirrhosis and others. And FXR agonist and antagonist can affect hypercholesterolemia by interfering with the synthesis, transport and absorption of bile acids. Besides, it may be related to high triglyceride and other diseases.