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 (hGRβ) that differs from the wild-type receptor (hGRα) only at the carboxyl terminus. Previously we have shown that hGRβ inhibits the transcriptional activity of hGRα, which is consistent with reports of elevated hGRβ expression in patients with generalized and tissue-specific glucocorticoid resistance. The potential role of hGRβ 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 hGRβ inhibits hGRα-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 hGRβ. hGRβ also inhibits hGRα-mediated repression of an NF-κB-responsive promoter but does not interfere with homologous down-regulation of hGRα. We show that hGRβ can associate with the heat shock protein hsp90 although with lower affinity than hGRα. In addition, hGRβ binds GRE-containing DNA with a greater capacity than hGRα in the absence of glucocorticoids. Glucocorticoid treatment enhances hGRα, but not hGRβ, binding to DNA. Moreover, we demonstrate that hGRα and hGRβ can physically associate with each other in a heterodimer. Finally, we show that the dominant negative activity of hGRβ resides within its unique carboxyl-terminal 15 amino acids. Taken together, our results suggest that formation of transcriptionally impaired hGRα-hGRβ heterodimers is an important component of the mechanism responsible for the dominant negative activity of hGRβ.