The Human Glucocorticoid Receptor (hGR) β Isoform Suppresses the Transcriptional Activity of hGRα by Interfering with Formation of Active Coactivator Complexes

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Abstract
The human glucocorticoid receptor (hGR) β, a splicing variant of the classic receptor hGRα, functions as a dominant-negative inhibitor of hGRα. We explored the mechanism(s) underlying this effect of hGRβ by evaluating the interactions of this isoform with known steroid receptor coactivators. We found that hGRβ suppressed the transcriptional activity of both activation function (AF)-1 and AF-2 of hGRα, indicating that hGRβ may exert its dominant-negative effect by affecting the function of coactivators that are attracted to these transactivation domains. hGRβ bound to one of the p160 coactivators, the glucocorticoid receptor-interacting protein 1 (GRIP1) via its preserved AF-1 but not via its defective AF-2 in vitro. In a chromatin immunoprecipitation assay, hGRβ prevented coprecipitation of GRIP1 with hGRα tethered to glucocorticoid response elements of the endogenous tyrosine aminotransferase promoter, whereas deletion of the AF-1 of hGRβ abolished this effect. In further experiments, overexpression of GRIP1 attenuated the suppressive effect of hGRβ on hGRα-mediated transactivation of the mouse mammary tumor virus promoter. Competition for binding to glucocorticoid response elements or heterodimerization with hGRα via the D loop dimerization interface occurred, but they were not necessary for the suppressive effect of hGRβ on the transcriptional activity of hGRα. Our findings suggest that hGRβ suppresses the transcriptional activity of hGRα by competing with hGRα for binding to GRIP1, and possibly other p160 coactivators, through its preserved AF-1. These findings suggest that participation of hGRβ in the formation of a coactivator complex renders this complex ineffective.

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