Evidence that glucocorticoid response elements in the 5′-noncoding region of the hamster β2-adrenergic receptor gene are obligate for glucocoticoid regulation of receptor mRNA levels

Abstract

Glucocorticoids and thyroid hormones regulate the expression of G-protein-linked receptors, typified by the β-adrenergic receptor. The influence of glucocorticoids on the steady-state levels of receptor and receptor mRNA were examined in Chinese hamster ovary (CHO) cells in culture. Incubation of wild-type CHO cells with dexamethasone, a potent glucocorticoid, elevated both receptor number (as measured by radioligand binding) and receptor mRNA levels (as measured by DNA-excess solution hybridization). Both responses were time- and dose-dependent. Stable transfectant CHO clones harboring the hamster β 2- adrenergic receptor cDNA under the control of the SV40 early promoter were investigated for their ability to respond to glucocorticoid stimulation. The cDNA employed in these studies displays glucocorticoid-response elements (GREs) in the coding and 3′- noncoding regions, but lacks GREs identified in the 5′-noncoding regions of the gene. Transfectant clones expressing 200 (X13), 600 (32E), or 1580 (32A) fmol of receptor/10 6 cells, as compared to wild-type CHO clones expressing 15 fmol/10 6 cells, displayed elevated steady-state levels of receptor mRNA. Incubating the transfectant CHO clones with glucocorticoids, however, failed to enhance the level of receptor or receptor mRNA derived from the expression vector employed in the transfection. These data demonstrate for wild-type CHO cells that glucocorticoids stimulate an increase in receptor mRNA and receptor expression, but that in CHO clones stably transfected with a cDNA lacking specifically the GREs in the 5′-noncoding region of the gene, the glucocorticoid-stimulated increase in β-adrenergic receptor mRNA is not evident.