Human glucocorticoid receptor β binds RU-486 and is transcriptionally active

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