The bile acid receptor (BAR), also known as farnesoid X receptor (FXR) or NR1H4 (nuclear receptor subfamily 1, group H, member 4) is a nuclear receptor that is encoded by the NR1H4 gene in humans.[1][2]
FXR is expressed at high levels in the liver and intestine. Chenodeoxycholic acid and other bile acids are natural ligands for FXR. Similar to other nuclear receptors, when activated, FXR translocates to the cell nucleus, forms a dimer (in this case a heterodimer with RXR) and binds to hormone response elements on DNA, which up- or down-regulates the expression of certain genes.[2]
One of the primary functions of FXR activation is the suppression of cholesterol 7 alpha-hydroxylase (CYP7A1), the rate-limiting enzyme in bile acid synthesis from cholesterol. FXR does not directly bind to the CYP7A1 promoter. Rather, FXR induces expression of small heterodimer partner (SHP), which then functions to inhibit transcription of the CYP7A1 gene. In this way, a 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 triglyceride levels.[3] Studies have also shown the FXR to regulate the expression and activity of epithelial transport proteins involved in fluid homeostasis in the intestine, such as the cystic fibrosis transmembrane conductance regulator (CFTR).[4]
Interactions
Farnesoid X receptor has been shown to interact with:
↑Seol W, Choi HS, Moore DD (Jan 1995). "Isolation of proteins that interact specifically with the retinoid X receptor: two novel orphan receptors". Molecular Endocrinology. 9 (1): 72–85. doi:10.1210/mend.9.1.7760852. PMID7760852.
↑Fiorucci S, Zampella A, Distrutti E (2012). "Development of FXR, PXR and CAR agonists and antagonists for treatment of liver disorders". Current Topics in Medicinal Chemistry. 12 (6): 605–24. doi:10.2174/156802612799436678. PMID22242859.
↑Fiorucci S, Mencarelli A, Distrutti E, Zampella A (May 2012). "Farnesoid X receptor: from medicinal chemistry to clinical applications". Future Medicinal Chemistry. 4 (7): 877–91. doi:10.4155/fmc.12.41. PMID22571613.
↑Vaz B, de Lera ÁR (Nov 2012). "Advances in drug design with RXR modulators". Expert Opinion on Drug Discovery. 7 (11): 1003–16. doi:10.1517/17460441.2012.722992. PMID22954251.
↑Ricketts ML, Boekschoten MV, Kreeft AJ, Hooiveld GJ, Moen CJ, Müller M, Frants RR, Kasanmoentalib S, Post SM, Princen HM, Porter JG, Katan MB, Hofker MH, Moore DD (Jul 2007). "The cholesterol-raising factor from coffee beans, cafestol, as an agonist ligand for the farnesoid and pregnane X receptors". Molecular Endocrinology. 21 (7): 1603–16. doi:10.1210/me.2007-0133. PMID17456796.
Kuipers F, Stroeve JH, Caron S, Staels B (Jun 2007). "Bile acids, farnesoid X receptor, atherosclerosis and metabolic control". Current Opinion in Lipidology. 18 (3): 289–97. doi:10.1097/MOL.0b013e3281338d08. PMID17495603.
Seol W, Choi HS, Moore DD (Jan 1995). "Isolation of proteins that interact specifically with the retinoid X receptor: two novel orphan receptors". Molecular Endocrinology. 9 (1): 72–85. doi:10.1210/mend.9.1.7760852. PMID7760852.
Makishima M, Okamoto AY, Repa JJ, Tu H, Learned RM, Luk A, Hull MV, Lustig KD, Mangelsdorf DJ, Shan B (May 1999). "Identification of a nuclear receptor for bile acids". Science. 284 (5418): 1362–5. doi:10.1126/science.284.5418.1362. PMID10334992.
Parks DJ, Blanchard SG, Bledsoe RK, Chandra G, Consler TG, Kliewer SA, Stimmel JB, Willson TM, Zavacki AM, Moore DD, Lehmann JM (May 1999). "Bile acids: natural ligands for an orphan nuclear receptor". Science. 284 (5418): 1365–8. doi:10.1126/science.284.5418.1365. PMID10334993.
Bramlett KS, Yao S, Burris TP (Dec 2000). "Correlation of farnesoid X receptor coactivator recruitment and cholesterol 7alpha-hydroxylase gene repression by bile acids". Molecular Genetics and Metabolism. 71 (4): 609–15. doi:10.1006/mgme.2000.3106. PMID11136553.
Stegh AH, Barnhart BC, Volkland J, Algeciras-Schimnich A, Ke N, Reed JC, Peter ME (Feb 2002). "Inactivation of caspase-8 on mitochondria of Bcl-xL-expressing MCF7-Fas cells: role for the bifunctional apoptosis regulator protein". The Journal of Biological Chemistry. 277 (6): 4351–60. doi:10.1074/jbc.M108947200. PMID11733517.
Cui J, Heard TS, Yu J, Lo JL, Huang L, Li Y, Schaeffer JM, Wright SD (Jul 2002). "The amino acid residues asparagine 354 and isoleucine 372 of human farnesoid X receptor confer the receptor with high sensitivity to chenodeoxycholate". The Journal of Biological Chemistry. 277 (29): 25963–9. doi:10.1074/jbc.M200824200. PMID12004058.
Huber RM, Murphy K, Miao B, Link JR, Cunningham MR, Rupar MJ, Gunyuzlu PL, Haws TF, Kassam A, Powell F, Hollis GF, Young PR, Mukherjee R, Burn TC (May 2002). "Generation of multiple farnesoid-X-receptor isoforms through the use of alternative promoters". Gene. 290 (1–2): 35–43. doi:10.1016/S0378-1119(02)00557-7. PMID12062799.
Pineda Torra I, Claudel T, Duval C, Kosykh V, Fruchart JC, Staels B (Feb 2003). "Bile acids induce the expression of the human peroxisome proliferator-activated receptor alpha gene via activation of the farnesoid X receptor". Molecular Endocrinology. 17 (2): 259–72. doi:10.1210/me.2002-0120. PMID12554753.
Anisfeld AM, Kast-Woelbern HR, Meyer ME, Jones SA, Zhang Y, Williams KJ, Willson T, Edwards PA (May 2003). "Syndecan-1 expression is regulated in an isoform-specific manner by the farnesoid-X receptor". The Journal of Biological Chemistry. 278 (22): 20420–8. doi:10.1074/jbc.M302505200. PMID12660231.
Pircher PC, Kitto JL, Petrowski ML, Tangirala RK, Bischoff ED, Schulman IG, Westin SK (Jul 2003). "Farnesoid X receptor regulates bile acid-amino acid conjugation". The Journal of Biological Chemistry. 278 (30): 27703–11. doi:10.1074/jbc.M302128200. PMID12754200.
Zhao A, Lew JL, Huang L, Yu J, Zhang T, Hrywna Y, Thompson JR, de Pedro N, Blevins RA, Peláez F, Wright SD, Cui J (Aug 2003). "Human kininogen gene is transactivated by the farnesoid X receptor". The Journal of Biological Chemistry. 278 (31): 28765–70. doi:10.1074/jbc.M304568200. PMID12761213.
Barbier O, Torra IP, Sirvent A, Claudel T, Blanquart C, Duran-Sandoval D, Kuipers F, Kosykh V, Fruchart JC, Staels B (Jun 2003). "FXR induces the UGT2B4 enzyme in hepatocytes: a potential mechanism of negative feedback control of FXR activity". Gastroenterology. 124 (7): 1926–40. doi:10.1016/S0016-5085(03)00388-3. PMID12806625.