The dominant negative activity of the human glucocorticoid receptor beta isoform - Specificity and mechanisms of action

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