Human Glucocorticoid Receptor β Binds RU-486 and Is TranscriptionallyActive

Abstract

ABSTRACT Human glucocorticoid receptor (hGR) is expressed as two alternately spliced C-terminal isoforms, α and β. In contrast to the canonical hGRα, hGRβ is a nucleus-localized orphan receptor thought not to bind ligand and not to affect gene transcription other than by acting as a dominant negative to hGRα. Here we used confocal microscopy to examine the cellular localization of transiently expressed fluorescent protein-tagged hGRβ in COS-1 and U-2 OS cells. Surprisingly, yellow fluorescent protein (YFP)-hGRβ was predominantly located in the cytoplasm and translocated to the nucleus following application of the glucocorticoid antagonist RU-486. This effect of RU-486 was confirmed with transiently expressed wild-type hGRβ. Confocal microscopy of coexpressed YFP-hGRβ and cyan fluorescent protein-hGRα in COS-1 cells indicated that the receptors move into the nucleus independently. Using a ligand binding assay, we confirmed that hGRβ bound RU-486 but not the hGRα ligand dexamethasone. Examination of the cellular localization of YFP-hGRβ in response to a series of 57 related compounds indicated that RU-486 is thus far the only identified ligand that interacts with hGRβ. The selective interaction of RU-486 with hGRβ was also supported by molecular modeling and computational docking studies. Interestingly, microarray analysis indicates that hGRβ, expressed in the absence of hGRα, can regulate gene expression and furthermore that occupation of hGRβ with the antagonist RU-486 diminishes that capacity despite the lack of helix 12 in the ligand binding domain.