Received for publication October 27, 2005.
Revised December 17, 2005.
Accepted for publication December 20, 2005.

Oxysterol 22(R)-hydroxycholesterol induces the expression of the bile salt export pump through nuclear receptor FXR but not LXR

Abstract

Oxysterols are intermediates in the synthesis of bile acids and steroid hormones from cholesterol and function as ligands for liver x receptor (LXR). Bile salt export pump (BSEP) is responsible for canalicular secretion of bile acids and is tightly regulated by its substrates bile acids through nuclear receptor farsenoid X receptor (FXR). In a microarray study, BSEP was markedly induced not only by chenodeoxycholic acid (CDCA) but also by oxysterol 22(R)-hydroxycholeterol (22(R)-OHC). We hypothesized that the expression of BSEP was induced by oxysterols through activation of LXR. To test the hypothesis, human primary hepatocytes or hepatoma cells were treated with oxysterol 22(R)-OHC and the expression of BSEP was determined. The level of BSEP mRNA was increased as much as 5 folds upon oxysterol induction. In contrast to our hypothesis, the oxysterol-induced upregulation of BSEP is mediated through FXR but not LXR. BSEP promoter activity was markedly induced by 22(R)-OHC in the presence of FXR but not LXRs. Mutation of the FXR element IR1 in the BSEP promoter significantly reduced its ability to response to oxysterol induction. To determine whether 22(R)-OHC and CDCA bind to similar structural features of FXR, site-directed mutagenesis was performed in the FXR ligand binding domain (LBD). Mutation of residues R331 and I352 abolishes activation mediated by CDCA and 22(R)-OHC. In contrast, substitution of residues L340 and R351 differentiates CDCA and 22(R)-OHC mediated activation. In conclusion, oxysterol 22(R)-OHC functions as FXR ligand to induce BSEP expression and differs in the binding with FXR from CDCA.

Key words: BSEP, Bile acids, Cholestasis, FXR, LXR, Oxysterol

This article has been cited by other articles:

				X. Song, R. Kaimal, B. Yan, and R. Deng Liver receptor homolog 1 transcriptionally regulates human bile salt export pump expression J. Lipid Res., May 1, 2008; 49(5): 973 - 984. [Abstract] [Full Text] [PDF]

				S.-Y. Cai, L. Xiong, C. G. Wray, N. Ballatori, and J. L. Boyer The farnesoid X receptor FXR{alpha}/NR1H4 acquired ligand specificity for bile salts late in vertebrate evolution Am J Physiol Regulatory Integrative Comp Physiol, September 1, 2007; 293(3): R1400 - R1409. [Abstract] [Full Text] [PDF]

				C. J. Delvecchio, P. Bilan, K. Radford, J. Stephen, B. L. Trigatti, G. Cox, K. Parameswaran, and J. P. Capone Liver X Receptor Stimulates Cholesterol Efflux and Inhibits Expression of Proinflammatory Mediators in Human Airway Smooth Muscle Cells Mol. Endocrinol., June 1, 2007; 21(6): 1324 - 1334. [Abstract] [Full Text] [PDF]

				R. Deng, D. Yang, A. Radke, J. Yang, and B. Yan The Hypolipidemic Agent Guggulsterone Regulates the Expression of Human Bile Salt Export Pump: Dominance of Transactivation over Farsenoid X Receptor-Mediated Antagonism J. Pharmacol. Exp. Ther., March 1, 2007; 320(3): 1153 - 1162. [Abstract] [Full Text] [PDF]

				D. D. Moore, S. Kato, W. Xie, D. J. Mangelsdorf, D. R. Schmidt, R. Xiao, and S. A. Kliewer International Union of Pharmacology. LXII. The NR1H and NR1I Receptors: Constitutive Androstane Receptor, Pregnene X Receptor, Farnesoid X Receptor {alpha}, Farnesoid X Receptor beta, Liver X Receptor {alpha}, Liver X Receptor beta, and Vitamin D Receptor Pharmacol. Rev., December 1, 2006; 58(4): 742 - 759. [Abstract] [Full Text] [PDF]

				S. Caron, B. Cariou, and B. Staels FXR: More than a Bile Acid Receptor? Endocrinology, September 1, 2006; 147(9): 4022 - 4024. [Full Text] [PDF]