title: Impact of glucocorticoid receptor density on ligand-independent dimerization, cooperative ligand-binding and basal priming of transactivation: a cell culture model.
abstract: Glucocorticoid receptor (GR) levels vary between tissues and individuals and are altered by physiological and pharmacological effectors. However, the effects and implications of differences in GR concentration have not been fully elucidated. Using three statistically different GR concentrations in transiently transfected COS-1 cells, we demonstrate, using co-immunoprecipitation (CoIP) and fluorescent resonance energy transfer (FRET), that high levels of wild type GR (wtGR), but not of dimerization deficient GR (GRdim), display ligand-independent dimerization. Whole-cell saturation ligand-binding experiments furthermore establish that positive cooperative ligand-binding, with a concomitant increased ligand-binding affinity, is facilitated by ligand-independent dimerization at high concentrations of wtGR, but not GRdim. The down-stream consequences of ligand-independent dimerization at high concentrations of wtGR, but not GRdim, are shown to include basal priming of the system as witnessed by ligand-independent transactivation of both a GRE-containing promoter-reporter and the endogenous glucocorticoid (GC)-responsive gene, GILZ, as well as ligand-independent loading of GR onto the GILZ promoter. Pursuant to the basal priming of the system, addition of ligand results in a significantly greater modulation of transactivation potency than would be expected solely from the increase in ligand-binding affinity. Thus ligand-independent dimerization of the GR at high concentrations primes the system, through ligand-independent DNA loading and transactivation, which together with positive cooperative ligand-binding increases the potency of GR agonists and shifts the bio-character of partial GR agonists. Clearly GR-levels are a major factor in determining the sensitivity to GCs and a critical factor regulating transcriptional programs.
authors.affiliation: Department of Biochemistry, University of Stellenbosch, Matieland, Stellenbosch, Republic of South Africa.
authors.forename: Steven
authors.initials: S
authors.lastname: Robertson
authors.forename: Johann M
authors.initials: JM
authors.lastname: Rohwer
authors.forename: Janet P
authors.initials: JP
authors.lastname: Hapgood
authors.forename: Ann
authors.initials: A
authors.lastname: Louw
mesh_terms: D000818:Animals; D001483:Base Sequence; D019556:COS Cells; D002522:Chlorocebus aethiops; D017931:DNA Primers; D019281:Dimerization; D031541:Fluorescence Resonance Energy Transfer; D017930:Genes, Reporter; D008024:Ligands; D008954:Models, Biological; D011401:Promoter Regions, Genetic; D011485:Protein Binding; D060888:Real-Time Polymerase Chain Reaction; D011965:Receptors, Glucocorticoid / Q000378:metabolism*; D015533:Transcriptional Activation
chemical_list: D017931:DNA Primers; D008024:Ligands; D011965:Receptors, Glucocorticoid