Evidence That the β-Isoform of the Human Glucocorticoid Receptor Does Not Act as a Physiologically Significant Repressor

Abstract

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

How Do Corticosteroids Work in Asthma?

“…Glucocorticoid receptor ␣ binds corticosteroids, whereas glucocorticoid receptor ␤ is an alternatively spliced form that binds to DNA but is not activated by corticosteroids (see Glossary). Glucocorticoid receptor ␤ has been implicated in steroid resistance in asthma (18), although whether glucocorticoid receptor ␤ has any functional significance has been questioned (19). Glucocorticoid receptors may also be modified by phosphorylation and other modifications, which may alter the response to corticosteroids.…”

Corticosteroid effects on cell signalling

“…GRb exhibits a very low level of expression compared to GRa [22]. The GRb isoform has been implicated in steroid resistance in asthma [23], although whether GRb can have any functional significance has been questioned in view of the very low levels of expression compared to those of GRa [24].…”

Glucocorticoid receptor alpha, beta and gamma expression vs in vitro glucocorticod resistance in childhood leukemia

“…However, this was not always confirmed. 31 Most clinical evidence that GRb is capable of decreasing GC sensitivity of target cells has been obtained in respiratory medicine. Various authors showed that GRb expression in airway cells and peripheral blood mononuclear cells obtained from patients with GC-resistant asthma was higher than in patients with GC-sensitive asthma or normal s ubjects.…”