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

Abstract

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

Increased Glucocorticoid Receptor β Alters Steroid Response in Glucocorticoid-insensitive Asthma

“…The ability of GCR␤ to antagonize the function of GCR␣ suggests its importance in the regulation of cell sensitivity to GCs. It has been reported that GCR␤ competes with GCR␣ in the nucleus for coactivators, and that GCR␣-GCR␤ heterodimers are transcriptionally inactive (41).…”

ATF2 impairs glucocorticoid receptor–mediated transactivation in human CD8+ T cells

“…20,21 GCR␤ differs from GCR␣ in its carboxyl terminus, where the last 50 amino acids of GCR␣ are replaced by a nonhomologous, 15 amino acid sequence. As a result of this difference, GCR␤ may compete with GCR␣ for binding to glucocorticoid response element (GRE) sites 22 or compete for the transcriptional coactivator molecules, 23 inhibiting steroid responses. 24,25 The expression level of GCR␤ and GCR␣ in different cell types is the factor that determines variations in cellular responses to steroids.…”

Novel insights into the molecular mechanisms underlying generalized glucocorticoid resistance and hypersensitivity syndromes

“…13,[16][17][18] Indeed, hGRβ does not bind natural or synthetic glucocorticoids; it resides in the nucleus of certain cell types, such as epithelial cells and neutrophils and exerts inhibitory effects on the transcriptional activity of the hGRα through well-studied molecular mechanisms. [19][20][21] However, given that studies from several groups have demonstrated that hGRβ can directly induce or repress a number of target genes, [22][23][24][25] while Chatzopoulou and collaborators recently revealed that GRβ does not have transcriptional activity in zebrafish, 26 it is evident that the specific functional role of GRβ remains to be elucidated. New evidence suggests that hGRβ has an important role in insulin signaling and might be involved in gluconeogenesis and inflammation in mouse liver.…”