Molecular determinants of LXRalpha agonism.
Liver X receptors (LXRs) are nuclear receptors that participate in the regulation of cholesterol, bile acid, and glucose metabolism. Despite the identification of the natural oxysterol and nonsteroidal ligands for LXRalpha, little is known about the structure of the LXRalpha ligand-binding domain (LBD). We constructed a three-dimensional (3D) homology model of the LBD of LXRalpha based on the crystal structure of the retinoic acid receptor gamma (RARgamma) and all-trans retinoic acid complex. We combined molecular modeling and classical structure-function techniques to define the interactions between the LBD and three structurally diverse ligands, 22(R)-hydroxycholesterol (22RHC), N-(2,2,2-trifluoro-ethyl)-N-[4-(2,2,2-trifluoro-1-hydroxy-1-trifluoromethyl-ethyl)-phenyl]-benzenesulfonamide (T0901317) and (3-[3-[(2-chloro-3-trifluoromethyl-benzyl)-(2,2-diphenyl-ethyl)-amino]-propoxy]-phenyl)-acetic acid (GW3965). Sixteen individual amino acid point mutations were made in the predicted ligand-binding cavity of the LBD, and each of these mutant receptors was assessed for their ability to be activated by these three ligands. The majority of individual mutations resulted in lack of activation by all three ligands. Two residues were identified that resulted in a significant increase in basal activity while retaining responsiveness to the ligands. Interestingly, a number of residues were identified that appear to be selective in their response to a particular ligand, indicating that these three ligands recognize distinct structural components within the ligand-binding cavity. These data, together with our docking study, enable us to identify the amino acids that coordinate the interaction of both steroidal and non-steroidal ligands in the ligand-binding pocket of LXRalpha.