Structural requirements of ligands for the oxysterol liver X receptors LXRalpha and LXR beta

LXR alpha and -beta are nuclear receptors that regulate the metabolism of s everal important lipids, including cholesterol and bile acids. Previously, we have proposed that LXRs regulate these pathways through their interactio n with specific, naturally occurring oxysterols, including 22(R)-hydroxycho lesterol, 24(S)-hydroxycholesterol, and 24(S),25-epoxycholesterol. Using a ligand binding assay that incorporates scintillation proximity technology t o circumvent many of the problems associated with assaying extremely hydrop hobic ligands, we now demonstrate that these oxysterols bind directly to LX Rs at concentrations that occur in vivo. To characterize further the struct ural determinants required for potent LXR ligands, we synthesized and teste d a series of related compounds for binding to LXRs and activation of trans cription. These studies revealed that position-specific monooxidation of th e sterol side chain is requisite for LXR high-affinity binding and activati on. Enhanced binding and activation can also be achieved through the use of 24-oxo ligands that act as hydrogen bond accepters in the side chain. In a ddition, introduction of an oxygen on the sterol B-ring results in a ligand with LXR alpha-subtype selectivity. These results support the hypothesis t hat naturally occurring oxysterols are physiological ligands for LXRs and s how that a rational, structure-based approach can be used to design potent LXR ligands for pharmacologic use.