The non-ligand binding beta-isoform of the human glucocorticoid receptor (hGR beta): tissue levels, mechanism of action, and potential physiologic role.
BACKGROUND
Alternative splicing of the transcripts of the human glucocorticoid receptor gene results in two mutually exclusive products, the classic, ligand-binding glucocorticoid receptor (hGR alpha), and a dominant negative non-ligand-binding isoform, hGR beta.


METHODS
We examined the existence of and quantified both hGR alpha and hGR beta 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 hGR beta with heat shock protein (hsp) 90 and/or hGR alpha using cross-immunoadsorption/precipitation procedures followed by Western blots.


RESULTS
For the first time, we demonstrated the natural existence of the hGR beta protein, which was widely expressed in human tissues. The ratio of immunoreactive hGR alpha to hGR beta 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 hGR alpha-to-hGR beta ratio remained the same before and after dexamethasone exposure, suggesting that upon activation the two isoforms translocated into the nucleus in equal proportions. hGR alpha- and hGR beta-specific antibodies cross-adsorbed and precipitated cytosolic and nuclear glucocorticoid hGR alpha and hGR beta, respectively, as well as hsp90, suggesting that hGR alpha and hGR beta are in complex with hsp90 and/or each other.


CONCLUSIONS
The hGR beta 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, hGR beta probably heterodimerizes with ligand-bound hGR alpha and translocates into the nucleus to act as a dominant negative inhibitor of the classic receptor.
de Castro
M
M
Developmental Endocrinology Branch, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20892, USA.
Elliot
S
S
Kino
T
T
Bamberger
C
C
Karl
M
M
Webster
E
E
Chrousos
G P
GP
D017398:Alternative Splicing; D001692:Biological Transport; D001704:Biopolymers; D015153:Blotting, Western; D002467:Cell Nucleus / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D018841:HSP90 Heat-Shock Proteins / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D007150:Immunohistochemistry; D008024:Ligands; D011485:Protein Binding; D011965:Receptors, Glucocorticoid / Q000037:antagonists & inhibitors / Q000235:genetics / Q000378:metabolism*
D001704:Biopolymers; D018841:HSP90 Heat-Shock Proteins; D008024:Ligands; D011965:Receptors, Glucocorticoid