Farnesoid X receptor

The bile acid receptor (BAR), also known as farnesoid X receptor (FXR) or NR1H4 (nuclear receptor subfamily 1, group H, member 4) is a is_associated_with::nuclear receptor that is encoded by the NR1H4 gene in humans.

Function
FXR is expressed at high levels in the liver and intestine. is_associated_with::Chenodeoxycholic acid and other is_associated_with::bile acids are natural ligands for FXR. Similar to other nuclear receptors, when activated, FXR translocates to the is_associated_with::cell nucleus, forms a dimer (in this case a heterodimer with RXR) and binds to is_associated_with::hormone response elements on DNA, which up- or down-regulates the expression of certain is_associated_with::genes.

One of the primary functions of FXR activation is the suppression of is_associated_with::cholesterol 7 alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in bile acid synthesis from is_associated_with::cholesterol. FXR does not directly bind to the CYP7A1 promoter. Rather, FXR induces expression of is_associated_with::small heterodimer partner (SHP), which then functions to inhibit transcription of the CYP7A1 gene. In this way a is_associated_with::negative feedback pathway is established in which synthesis of bile acids is inhibited when cellular levels are already high. FXR has also been found to be important in regulation of hepatic is_associated_with::triglyceride levels. Studies have also shown the FXR to regulate the expression and activity of epithelial transport proteins involved in fluid homestasis in the intestine, such as the cystic fibrosis conductance transmembrane regulator (CFTR)

Interactions
Farnesoid X receptor has been shown to interact with:
 * Peroxisome proliferator-activated receptor gamma coactivator 1-alpha and
 * is_associated_with::Retinoid X receptor alpha.

Ligands
A number of ligands for FXR are known, of both natural and synthetic origin.


 * Agonists
 * is_associated_with::Cafestol
 * is_associated_with::Chenodeoxycholic acid
 * is_associated_with::Obeticholic acid
 * is_associated_with::Fexaramine


 * Antagonists
 * is_associated_with::Guggulsterone