THE NONLIGAND BINDING BETA-ISOFORM OF THE HUMAN GLUCOCORTICOID RECEPTOR (HGR-BETA) - TISSUE-LEVELS, MECHANISM OF ACTION, AND POTENTIAL PHYSIOLOGICAL-ROLE

Background: Alternative splicing of the transcripts of the human gluco corticoid receptor gene results in two mutually exclusive products, th e classic, ligand-binding glucocorticoid receptor (hGR alpha), and a d ominant negative non-ligand-binding isoform, hGR beta. Materials and M ethods: 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 a nd fractionated HeLa cells, using specific quantitative Western blots and/or immunocytochemistry. We studied the potential interactions of h GR beta with heat shock protein (hsp) 90 and/or hGR alpha using cross immunoadsorption/precipitation procedures followed by Western blots. R esults: For the first time, we demonstrated the natural existence of t he 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 a mong different tissues, and was approximately 0.2 in HeLa cells. In th e latter, both isoforms were distributed in the cytoplasm and nucleus in the absence of the hormonal ligand, and translocated into the nucle us after addition of dexamethasone. The cytosolic and nuclear hGR alph a-to-hGR beta ratio remained the same before and after dexamethasone e xposure, suggesting that upon activation the two isoforms translocated into the nucleus in equal proportions. hGR alpha-and hGR beta-specifi c antibodies cross-adsorbed and precipitated cytosolic and nuclear glu cocorticoid hGR alpha and hGR beta, respectively, as well as hsp90, su ggesting that hGR alpha: and hGR beta are in complex with hsp90 and/or each other. Conclusions: The hGR beta protein is widely expressed thr oughout the human body and present mostly in the cytoplasm of human ce lls, in complex with hsp90 and other proteins. In the presence of gluc ocorticoid, hGR beta probably heterodimerizes with ligand-bound hGR al pha and translocates into the nucleus to act as a dominant negative in hibitor of the classic receptor.