The Dominant Negative Activity of the Human Glucocorticoid Receptor β Isoform

Abstract

Alternative splicing of the human glucocorticoid receptor gene generates a nonhormone binding splice variant (hGR␤) that differs from the wild-type receptor (hGR␣) only at the carboxyl terminus. Previously we have shown that hGR␤ inhibits the transcriptional activity of hGR␣, which is consistent with reports of elevated hGR␤ expression in patients with generalized and tissue-specific glucocorticoid resistance. The potential role of hGR␤ in the regulation of target cell sensitivity to glucocorticoids prompted us to further evaluate its dominant negative activity in other model systems and to investigate its mode of action. We demonstrate in multiple cell types that hGR␤ inhibits hGR␣-mediated activation of the mouse mammary tumor virus promoter. In contrast, the ability of the progesterone and androgen receptors to activate this promoter is only weakly affected by hGR␤. hGR␤ also inhibits hGR␣-mediated repression of an NF-B-responsive promoter but does not interfere with homologous down-regulation of hGR␣. We show that hGR␤ can associate with the heat shock protein hsp90 although with lower affinity than hGR␣. In addition, hGR␤ binds GRE-containing DNA with a greater capacity than hGR␣ in the absence of glucocorticoids. Glucocorticoid treatment enhances hGR␣, but not hGR␤, binding to DNA. Moreover, we demonstrate that hGR␣ and hGR␤ can physically associate with each other in a heterodimer. Finally, we show that the dominant negative activity of hGR␤ resides within its unique carboxyl-terminal 15 amino acids. Taken together, our results suggest that formation of transcriptionally impaired hGR␣-hGR␤ heterodimers is an important component of the mechanism responsible for the dominant negative activity of hGR␤.

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

“…43 The relatively low GRb mRNA levels that were found in this study (median 85-to 200-fold lower than GRa) indicate that it is unlikely that the GRb is capable of inhibiting GRa-mediated cell lysis in childhood leukemia. In accordance with this, we were not able to demonstrate a significant correlation between GRb mRNA expression or GRa/GRb mRNA ratios and in vitro GC resistance.…”

“…35,[41][42][43][44][45][46] In this study, we focused on the expression levels of the GR splice variants GRa, GRb, and GRg in relation to in vitro GC resistance in childhood leukemia.…”

Nontranscriptional actions of the glucocorticoid receptor

“…GR has the prototypical modular structure of nuclear receptors: an N-terminal transcriptional activation function 1 domain, a central DNA-binding domain (DBD), and a C-terminal ligand-binding domain (LBD) harboring a ligand-dependent transcriptional activation function-2 domain [6,26]. Alternative mRNA splicing results in a second GR isoform, GRβ, that is defective in steroid binding and acts as a dominant negative inhibitor of GRα in vitro [3,27,28]. However, a clear functional role for GRβ has yet to be established.…”

Expression of glucocorticoid receptor mRNAs in glucocorticoid-resistant nasal polyps

“…hGRα functions as ligand-dependent transcription factor that regulates expression of several inflammation-related target genes. hGRα is expressed in almost all tissues and cells, and in the absence of GC it is mainly located in the cytoplasm of cells as part of a large multiprotein complex (Oakley et al, 1999). This complex consists of the receptor, two molecules of heat shock protein hsp90, and several additional factors (Pratt et al, 1993;Smith et al, 1993).…”

“…hGRα belongs to the superfamily of large steroid-nuclear receptor that also includes receptors for mineralocorticoids, thyroid hormone, retinoic acid, and vitamin D (Oakley et al, 1999). hGRα functions as ligand-dependent transcription factor that regulates expression of several inflammation-related target genes.…”