Science 2002 May 2; [epub ahead of print]
A Natural Product That Lowers Cholesterol as an Antagonist Ligand for the FXR.
Urizar NL, Liverman AB, Dodds DT, Silva FV, Ordentlich P, Yan Y, Gonzalez FJ, Heyman RA, Mangelsdorf DJ, Moore DD.
Department of Molecular and Cellular Biology, Baylor College of Medicine, 1 Baylor Plaza, Houston, TX 77030, USA., Howard Hughes Medical Institute, University of Texas - Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75390, USA., X-Ceptor Therapeutics, Inc., 4757 Nexus Center Drive, San Diego, CA 92121, USA., Laboratory of Metabolism, Center for Cancer Research, National Cancer Institute, Building 37, Room 3E24, Bethesda, MD 20892, USA.
Extracts of the resin of the guggul tree (Commiphora mukul) lower LDL (low-density lipoprotein) cholesterol levels in humans. The plant sterol guggulsterone (4,17(20)-pregnadiene-3,16-dione) is the active agent in this extract. We show that guggulsterone is a highly efficacious antagonist of the farnesoid X receptor (FXR), a nuclear hormone receptor that is activated by bile acids. Guggulsterone treatment decreases hepatic cholesterol in wild type mice fed a high cholesterol diet, but is not effective in FXR null mice. Thus, we propose that inhibition of FXR activation is the basis for the hypolipidemic activity of guggulsterone. Other natural products with specific biologic effects may modulate the activity of FXR or other relatively promiscuous nuclear hormone receptors.
J Biol Chem 2002 May 9; [epub ahead of print]
The amino acid residues Asn354 and Ile372 of human FXR confer the receptor With high sensitivity to chenodeoxycholate.
Cui J, Heard TS, Yu J, Lo JL, Huang L, Li Y, Schaeffer JM, Wright SD.
Atherosclerosis & Endocrinology, Merck Research Laboratories, Rahway, NJ 07065.
The critical steps in bile acid metabolism have remarkable differences between human and mice. It is known that human cholesterol 7-hydroxylase, the enzyme catalyzing the rate-limiting step of bile acid synthesis, is more sensitive to bile acid suppression. In addition, hepatic bile acid export in humans is more dependent on the bile salt export pump (BSEP). To explore the molecular basis for these species differences, we analyzed the function of the ligand-binding domain (LBD) of human and murine FXR, a nuclear receptor for bile acids. We observed a strong inter-species difference in bile acid-mediated FXR function: in the coactivator association assay, chenodeoxycholate (CDCA) activated human FXR-LBD with 10-fold higher affinity and 3-fold higher maximum response than murine FXR-LBD. Consistently, in HepG2 human FXR-LBD more robustly increased reporter expression in the presence of CDCA. The basis for these differences was investigated by preparing chimeric receptors and by site-directed mutagenesis. Remarkably, the double replacements of Lys366 and Val384 in murine FXR (corresponding to Asn354 and Ile372 in human FXR) with Asn366 and Ile384 explained the difference in both potency and maximum activation: compared to the wild-type murine FXR-LBD, the double mutant gained 8-fold affinity and more than 250% maximum response to CDCA in vitro. This mutant also increased reporter expression to a comparable extent as did human FXR-LBD in HepG2. These results demonstrate that Asn354 and Ile372 are critically important for FXR function, and that murine FXR can be humanized by substituting with the two corresponding residues of human FXR. Consistent with the difference in FXR-LBD transactivation, CDCA induced endogenous expression of human BSEP by 10- to 12-fold and murine BSEP by 2- to 3-fold in primary hepatocytes. This study not only provides the identification of critical residues for FXR function but may also explain the species difference in bile acids/cholesterol metabolisms.
(just parked, not implying that I know anything about significance. here's the earlier work of Makishima, Shan et al.)
Science 1999 May 21;284(5418):1362-5
Identification of a nuclear receptor for bile acids.
Makishima M, Okamoto AY, Repa JJ, Tu H, Learned RM, Luk A, Hull MV, Lustig KD, Mangelsdorf DJ, Shan B.
Howard Hughes Medical Institute and Department of Pharmacology, University of Texas Southwestern Medical Center, 5323 Harry Hines Boulevard, Dallas, TX 75235-9050, USA.
Bile acids are essential for the solubilization and transport of dietary lipids and are the major products of cholesterol catabolism. Results presented here show that bile acids are physiological ligands for the farnesoid X receptor (FXR), an orphan nuclear receptor. When bound to bile acids, FXR repressed transcription of the gene encoding cholesterol 7alpha-hydroxylase, which is the rate-limiting enzyme in bile acid synthesis, and activated the gene encoding intestinal bile acid-binding protein, which is a candidate bile acid transporter. These results demonstrate a mechanism by which bile acids transcriptionally regulate their biosynthesis and enterohepatic transport. |
