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Homo sapiens 

Liver X Receptor Beta

Kelly Heilman '12 and Crystal Piras '12


I. Introduction

Liver X receptors are ligand activated transcription factors that are part of the protein family on nuclear receptors.  Nuclear receptors regulate transcription  intracellularly mainly through recruitment of coactivators. Liver X receptors are activated by oxysterol ligands, also called oxygenated cholesterol metabolites.  These LXR's are involved in the regulation of cholesterol, and therefore are of interest in treating cardiovascular diseases such as atherosclerosis.  More specifically, LXRα and LXR β regulate lipogenesis in the liver through LXR target genes including ChREBP (carbohydrate response element binding protein) and SREBP1c (sterol regulatory element binding protein 1c).  These proteins are involved in production of LDL cholesterol and Glucose metabolism in the liver.  Most of the current research on these receptors is done on the pretext of developing LXR ligands that may treat or prevent cardiovascular disease without causing unwanted hepatic side effects.  Several known LXR ligands include the endogenous ligand 24(S),25-epoxycholesterol (eCH) and synthetic agonists T1317 and GW3965 (both non-sterol). 


II. General Structure

Typical of nuclear receptors, liver X receptor β has three layers of alpha helices.  The asymmetric unit of this crystal consists of an LXR homodimer complexed with its coactivator SRC-1. Each LXR monomer is characterized by 10 total helices and the AF2 Helix.  The AF2 helix interacts with the ligand binding pocket as well as the transcription cofactor SRC-1 and is a domain common to Nuclear Receptors.   Helices 7, 9, and 10 make up the dimer interface.   Other characteristic features of the LXR β include a long helix 1, which is made up of about 18 amino acid residues.   LXR β has a large ligand binding pocket that is 830 Å3.  

III. Ligand Binding Pocket

LXR β is a ligand activated transcription factor that binds a variety of endogenous and ectopic ligands.   Each ligand is thought to differentially regulate transcription of different genes.  For this reason, current work is investigating ligands that would preferentially prevent genes associated with cardiovascular disease from being transcribed.  The ligand binding domain is a large cavity made up of three layers of alpha helices: Helices 3, 5, 6, 7, 11, and 12. The LBD is completely  lined with hydrophobic residues except for His-435 located in helix 11. The hydrophobic residues of helix 3 are Phe-268, Phe-271, Thr-272, Leu-274, Ala-275. Of helix 5 Ile-309, Met-312, Leu-313, and Thr-316 line the LBD. Helix 6 contributes to the LBD but it does not contribute any amino acid residues to the lining of the cavity. Only two resides in helix 7 line the LBD: Phe-349  and Ile-353 The hydrophobic residues of helix 11 are Val-439, and Leu-442. Of helix 12, Leu-453 and Trp-457 line the LBD. Ile-327 is located in a small three stranded beta sheet and it also contributes hydrophobicity to the ligand binding domain. The entrance to the LBD is located between helix five and the loop between β strands S1 and S2.   The AF2 helix helps stabilize the LBD.  This helix assists in binding through hydrophobic contacts between Trp-457 and His-435 of helix 11 and is essential for the activation of the receptor in LXR beta.  


IV. Ligand Binding eCH

An endogenous oxysterol ligand, 24(S),25-epoxycholesterol, called eCH, is shown bound in the ligand binding domain.  eCH consists of a four ring steroid core, a phenolic hydroxyl, a sterol chain, and an epoxide oxygen. click here to view ligands   The main interaction crucial to ligand binding occurs between the epoxide oxygen and His-435.   This interaction involves a hydrogen bond between the epoxide oxygen and the imidazole ring of His-435.   A similar interaction also occurs between His-435 and the acidic carbonyl groups of synthetic ligands that also can bind LXR.  Another interaction occurs between the A-ring phenolic hydroxyl group and the Glu-281 located on helix 3.

V. Ligand Activation: Histadine-Tryptophan Switch

When the ligand is bound to His-435, this residue is oriented in a unique interaction with the active site. In this instance, His-435 orthogonally interacting with Trp-457 on the inner surface of the AF2 helix, in which the planar face of Trp-457 is oriented towards the edge of His-435.   This face-to-edge orientation puts the AF-2 helix in the "activated" conformation so that AF-2 can interact with the SRC-1 cofactor.  In this orientation, histadine exhibits rotational freedom that allows it to "swing" across the face of the tryptophan and interact with the strongly negative pi electron cloud of the benzene residue or the weakly negative electron cloud associated with the five membered ring residue.   This interaction is conserved in ligand activation by all other known LXR ligands.  The activation of LXR by different ligands is partially due to the ability of His-435 to act as a hydrogen donor and an acceptor.    Once in the activated conformation, the AF-2 helix facilitates TIF-2 binding.  TIF-2 is a coactivator transcription initiation factor. The binding of the cofactor facilitates transcription. 

VI. References

Hoerer S, Schmid A, Heckel A, Budzinski RM, Nar H. (2003). Crystal structure of the human liver X receptor beta ligand-binding domain in complex with a synthetic agonist. Journal of Molecular Biology, 334, 853-861.

Ishii, S. (2004). Carbohydrate response element binding protein directly promotes lipogenic enzyme gene transcription Proceedings of the National Academy of Sciences, 101(44), 15597-15602.

Kliewer, S. A. (1997). Activation of the nuclear receptor LXR by oxysterols defines a new hormone response pathway Journal of Biological Chemistry, 272(6), 3137 - 3140.

Liver X receptor ß (NR1H2) | Retrieved 12/6/2010, 2010, from

Williams, S., Bledsoe, R.K., Collins, J.L., Boggs, S., Lambert, M.H., Miller, A.B., Moore, J., McKee, D.D., Moore, L., Nichols, J., Parks, D., Watson, M., Wisely, B., Willson, T.M. (2003). X-ray crystal structure of the liver X receptor β ligand binding domain. The Journal of Biological Chemistry, 278(July 18), 27138-27143.

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