Evidence that the beta-isoform of the human glucocorticoid receptor does not act as a physiologically significant repressor

Alternative splicing of the human glucocorticoid receptor (hGR) primary transcript generates two receptor isoforms, hGR alpha and hGR beta, with different carboxyl termini diverging at amino acid 727, By reverse transcriptase-polymerase chain reactions it was previously demonstrated that the hGR beta message had a widespread tissue distribution, To demonstrate the presence of hGR beta as protein we produced specific rabbit antisera to hGR beta, as well as a hGR beta-specific mouse monoclonal IgM antibody, by peptide immunizations, By SDS-polyacrylamide gel electrophoresis and Western immunoblotting we showed that hGR beta is endogenously expressed at the protein level in HeLa cells and human lymphatic leukemia cells. Using an antibody directed against an epitope shared by both isoforms we showed a relatively lower expression of the hGR beta form, We also showed that hGR beta bound to hsp90 by immunoprecipitation of in vitro translated hGR beta in reticulocyte lysate with hsp90-specific antibodies, a coprecipitation occurring also in the presence of dexamethasone. We could not demonstrate that hGR beta inhibited the effects of dexamethasone-activated hGR alpha on a glucocorticoid-responsive reporter gene. In conclusion, low hGR beta expression levels and hGR beta-hsp90 interaction maintained in the presence of ligand and lack of inhibition of hormone-activated hGR alpha effects challenge the concept of the hGR beta isoform as a proposed dominant negative inhibitor of hGR alpha activity.