Abstract
The human glycoprotein hormone alpha-subunit (alphaGSU) gene is transcriptionally regulated by glucocorticoids in a cell type-specific fashion. In direct contrast to repression of alphaGSU by glucocorticoids in placenta, glucocorticoid receptor (GR) modulation in the pituitary is little understood. We show that glucocorticoids stimulate the alphaGSU promoter in immortalized pituitary gonadotrope-derived LbetaT2 cells, whereas estrogens, androgens, and progestins have no significant effect. Moreover, GR acts in a dose-dependent manner at physiological concentrations of glucocorticoids. Transient transfection of GR with dexamethasone (Dex) treatment further stimulates the alphaGSU promoter, but this induction is severely diminished using a receptor mutated in the DNA-binding domain. Truncation and cis mutations demonstrate that glucocorticoid response element 2 (GRE2) and cAMP-response element 2 (CRE2) within -168 bp of the human alphaGSU promoter are critical for induction. Moreover, dominant-negative CRE-binding protein markedly inhibits basal but also Dex induction of alphaGSU promoter activity. Additionally, GR specifically binds to GRE2 in the human alphaGSU promoter in vitro and to the 5' region of the endogenous mouse alphaGSU gene in vivo. Furthermore, overexpression of the homeobox factor, Distal-less 3 that regulates this gene in placental cells through a site partially overlapping GRE2, blocks Dex induction of alphaGSU in gonadotrope cells, indicating that placenta-specific expression of Dlx3 may interfere with GR, resulting in repression in placental cells vs. induction in gonadotrope cells. These results demonstrate the stimulatory role played by glucocorticoids in alphaGSU gene expression in the pituitary gonadotrope, in contrast to repression in placental cells, and highlight the tissue-specific nature of steroid hormone action.