Human glucocorticoid receptor isoform beta: recent understanding of its potential implications in physiology and pathophysiology

Abstract

The human glucocorticoid receptor (GR) gene expresses two splicing isoforms alpha and beta through alternative use of specific exons 9alpha and 9beta. In contrast to the classic receptor GRalpha, which mediates most of the known actions of glucocorticoids, the functions of GRbeta have been largely unexplored. Owing to newly developed methods, for example microarrays and the jellyfish fluorescence proteins, we and others have recently revealed novel functions of GRbeta. Indeed, this enigmatic GR isoform influences positively and negatively the transcriptional activity of large subsets of genes, most of which are not responsive to glucocorticoids, in addition to its well-known dominant negative effect against GRalpha-mediated transcriptional activity. A recent report suggested that the "ligand-binding domain" of GRbeta is active, forming a functional ligand-binding pocket associated with the synthetic compound RU 486. In this review, we discuss the functions of GRbeta, its mechanisms of action, and its pathologic implications.

Fig. 1

Genomic and complementary DNA and protein isoforms of the human GR and distribution of functional domains in its linearized molecule. The human GR gene consists of nine exons. Exon 1 is an untranslated region, exon 2 codes for the N-terminal “immunogenic” domain, exons 3 and 4 for the DNA-binding domain, and exons 5–9 for the hinge region and the ligand-binding domain. The GR gene contains two terminal exons 9 (9α and 9β), which produce the classic GRα (GRα-A) and GRβ (GRβ-A) through alternative splicing of these exons. C-terminal gray colored domains in GRα-A and GRβ-A show their specific portions. GRα N-terminal translational isoforms expressed from a single GRα transcript are shown in the middle panel of the figure. Similar N-terminal translational isoforms may also be produced from the GRβ-specific transcript using the same start sites (modified from Ref. [5]). AF-1 and -2 activation functions 1 and 2, DBD DNA-binding domain, HD hinge region, LBD Ligand-binding domain, NL1 and 2 Nuclear translocation signals 1 and 2, NTD N-terminal domain

Fig. 2

The three-dimensional structure of GRα associated with agonist dexamethasone (left) and antagonist RU 486 (right). Results from crystallographic analysis of the GRα associated with agonist dexamethasone (left) or with antagonist RU 486 (right) are shown [16, 75]. The LBD of GRα consists of twelve α-helices and four β-sheets, among which helices 3, 4, 11, and 12 form the ligand-binding pocket for binding to glucocorticoids. Helix 12 changes its localization dramatically upon binding to ligands, playing a critical role in the formation of a binding surface for the coactivator (LXXLL) motif. Image sources were downloaded from the RCSB Protein Data Bank (http://www.rcsb.org) whereas the images were created using the MacPyMOL software. Yellow bold arrow ligand-binding pocket, white arrow helix 12, white arrowhead the coactivator motif peptide fragment of the transcriptional intermediate factor 2

Fig. 3

Nucleocytoplasmic shuttling and transcriptional regulation of GRα. Upon ligand binding, the activated GRα dissociates from the heat-shock proteins (HSPs) and translocates into the nucleus, where it binds as a homodimer to GREs in the promoter regions of target genes or interacts as a monomer with other transcription factors. GRα glucocorticoid receptor α, GRE glucocorticoid response element, HSPs heat-shock proteins, REs response elements, RNPII RNA polymerase II, TF transcription factor

Fig. 4

Genomic and complementary DNA and protein isoforms of the zebrafish GR. The zebrafish (z) GR gene consists of nine exons. The zGR gene expresses zGRα and zGRβ splicing variants through intron retention [32]. C-terminal gray colored and shaded domains in zGRα and zGRβ show their specific portions. They are, respectively, encoded by exon 9 and the 3′ portion of exon 8, which are also shown in the same labeling. DBD DNA-binding domain, LBD Ligand-binding domain, NTD N-terminal domain, UTR untranslated region

Fig. 5

Hypothetical models for GRβ-mediated modulation of the transcriptional activity of its responsive genes. a Through AF-1 located in the NTD, GRβ may interact with numerous transcriptional cofactors and transcriptional factors, lodge into the transcription intermediate complex formed on the promoter region of GRβ-responsive genes, and modulate their transcriptional activity. GRβ may attract histone deacetylases to the transcription intermediate complex formed on the promoter region of genes regulated by this GR isoform. b GRβ might also bind to hypothetical specific response elements located in the promoter region of responsive genes, directly modulating their transcriptional activity. GRβ glucocorticoid receptor β, HDACs histone deacetylases, REs response elements, RNPII RNA polymerase II, TF transcription factor