Evidence that the β-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, hGRα and hGRβ, with different carboxyl termini diverging at amino acid 727. By reverse transcriptase- polymerase chain reactions it was previously demonstrated that the hGRβ message had a widespread tissue distribution. To demonstrate the presence of hGRβ as protein we produced specific rabbit antisera to hGRβ, as well as a hGRβ-specific mouse monoclonal IgM antibody, by peptide immunizations. By SDS-polyacrylamide gel electropheresis and Western immunoblotting we showed that hGRβ 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 hGRβ form. We also showed that hGRβ bound to hsp90 by immunoprecipitation of in vitro translated hGRβ in reticulocyte lysate with hsp90-specific antibodies, a coprecipitation occurring also in the presence of dexamethasone. We could not demonstrate that hGRβ inhibited the effects of dexamethasone-activated hGRα on a glucocorticoid-responsive reporter gene. In conclusion, low hGRβ expression levels and hGRβ-hsp90 interaction maintained in the presence of ligand and lack of inhibition of hormone-activated hGRα effects challenge the concept of the hGRβ isoform as a proposed dominant negative inhibitor of hGRα activity.