Nuclear localization of liver X receptor α and β is differentially regulated

Abstract

Activity of nuclear receptors is regulated by their nuclear localization. Liver X receptors (LXR) α and β are nuclear receptors that regulate transcription of genes for cholesterol metabolism, cholesterol transport, and lipogenesis. While LXR α and β are very similar in structure and exhibit similar ligand binding properties, their physiological roles are quite different. Since the LXRs fall into a class of receptors that move between the nucleus and cytoplasm, experiments were conducted to determine whether LXR α and LXR β show differences in their nuclear localization pattern. To determine the location of each receptor, cell lines stably expressing yellow fluorescent protein (YFP) chimeras with either LXR α or LXR β were examined. Retention in the nucleus of the chimeric proteins in the presence or absence of ligands was assessed using fluorescence microscopy coupled with digitonin permeabilization assays. Surprisingly, differences were found between LXR α and LXR β. Whereas unliganded LXR α was retained in the nucleus, unliganded LXR β was partially exported. Mutations were then introduced into putative nuclear localization sequences (NLS) to determine which sequences are important for nuclear localization and function. Mutation in one such sequence abolished nuclear localization of LXR α, whereas the analogous change in LXR β had a much less dramatic effect. Mutations in analogous putative NLS also differentially affected transcriptional activation by LXR α and LXR β. These data demonstrate for the first time that nuclear retention and localization as well as function of LXR α and LXR β are differentially regulated. © 2006 Wiley-Liss, Inc.