Critical Role of Residues in Helices 9 and 10 of the Liver X Receptor Alpha (LXRα) in Mediating Heterodimer Choice between Heterodimeric Complexes

Abstract

The Liver X Receptor Alpha (LXRα) is a ligand‐activated nuclear transcription factor that is activated upon binding to oxysterols and plays an important role in lipid homeostasis. Heterodimerization with a common heterodimeric partner, the retinoid X receptor (RXR), regulates the transcription of target genes. LXRα‐RXRα activation has been shown to inhibit cardiovascular disease and reduce atherosclerotic lesions in mice. Additionally, LXRα can also form heterodimers with the Peroxisome Proliferator‐Activated Receptor Alpha (PPARα), another ligand‐activated nuclear receptor involved in fatty acid (FA) metabolism. Key residues located in helices 9 and 10 of LXRα may mediate dimerization of LXRα with partner receptors. These residues show a high degree of conservation across species and among nutrient‐sensing nuclear receptors. Little is known about the effects of charge disruption at the LXRα interface on protein‐protein and protein‐ligand associations. Using overlap PCR, we generated LXRα mutants in which the interface had been modified to determine whether heterodimerization selective variants of LXRα can be generated. Fluorescence spectroscopy in cells and in vitro demonstrated that charge reversal at the LXRα dimerization interface has the ability to influence LXRα protein and ligand binding. Using the intrinsic quenching of the aromatic amino acids located in the ligand binding pocket of LXRα, we have demonstrated that LXRα R415A does not bind T‐0901317, but retains its ability to bind 25‐hydroxycholesterol and palmitic acid. Direct protein‐protein binding experiments using the fluorescently labeled proteins suggested that LXRα R415A binding to RXRα is unaffected whereas binding to PPARα is enhanced. Another mutant of LXRα leaves LXRα‐PPARα binding intact but reduces LXRα‐RXRα binding. This indicates that key residues at the interface differentially regulate protein‐protein and protein‐ligand interactions of LXRα. Key contact regions of LXRα may represent a new drug target for modulating LXRα signaling in lipid‐related disorders.Support or Funding InformationThis research was supported by USPHS NIH grant DK77573 and the Emerging Science Seed Grant provided by the Boonshoft School of Medicine and Wright State University.