The non-ligand binding β-isoform of the human glucocorticoid receptor (hGRβ): Tissue levels, mechanism of action, and potential physiologic role

Abstract

Background: Alternative splicing of the transcripts of the human glucocorticoid receptor gene results in two mutually exclusive products, the classic, ligand-binding glucocorticoid receptor (hGRα), and a dominant negative non-ligand-binding isoform, hGRβ. Materials and Methods: We examined the existence of and quantified both hGRα and hGRβ isoforms in a panel of human tissues, as well as in intact and fractionated HeLa cells, using specific quantitative Western blots and/or immunocytochemistry. We studied the potential interactions of hGRβ with heat shock protein (hsp) 90 and/or hGRα using cross-immunoadsorption/precipitation procedures followed by Western blots. Results: For the first time, we demonstrated the natural existence of the hGRβ protein, which was widely expressed in human tissues. The ratio of immunoreactive hGRα to hGRβ varied from 0.2 to 1.0 among different tissues, and was approximately 0.2 in HeLa cells. In the latter, both isoforms were distributed in the cytoplasm and nucleus in the absence of the hormonal ligand, and translocated into the nucleus after addition of dexamethasone. The cytosolic and nuclear hGRα-to-hGRβ ratio remained the same before and after dexamethasone exposure, suggesting that upon activation the two isoforms translocated into the nucleus in equal proportions. hGRα- and hGRβ-specific antibodies cross-adsorbed and precipitated cytosolic and nuclear glucocorticoid hGRα and hGRβ, respectively, as well as hsp90, suggesting that hGRα and hGRβ are in complex with hsp90 and/or each other. Conclusions: The hGRβ protein is widely expressed throughout the human body and present mostly in the cytoplasm of human cells, in complex with hsp90 and other proteins. In the presence of glucocorticoid, hGRβ probably heterodimerizes with ligand-bound hGRα and translocates into the nucleus to act as a dominant negative inhibitor of the classic receptor.