Interaction of Glucocorticoid Receptor (GR) with Estrogen Receptor (ER) α and Activator Protein 1 (AP1) in Dexamethasone-mediated Interference of ERα Activity

The role of glucocorticoids in the inhibition of estrogen (17-beta-estradiol (E2))-regulated estrogen receptor (ER)-positive breast cancer cell proliferation is well established. We and others have seen that synthetic glucocorticoid dexamethasone (Dex) antagonizes E2-stimulated endogenous ER alpha target gene expression. However, how glucocorticoids negatively regulate the ER alpha signaling pathway is still poorly understood. ChIP studies using ER alpha- and glucocorticoid receptor (GR)-positive MCF-7 cells revealed that GR occupies several ER alpha-binding regions (EBRs) in cells treated with E2 and Dex simultaneously. Interestingly, there was little or no GR loading to these regions when cells were treated with E2 or Dex alone. The E2+Dex-dependent GR recruitment is associated with the displacement of ER alpha and steroid receptor coactivator-3 from the target EBRs leading to the repression of ER alpha-mediated transcriptional activation. The recruitment of GR to EBRs requires assistance from ER alpha and FOXA1 and is facilitated by AP1 binding within the EBRs. The GR binding to EBRs is mediated via direct protein-protein interaction between the GR DNA-binding domain and ER alpha. Limited mutational analyses indicate that arginine 488 located within the C-terminal zinc finger domain of the GR DNA-binding domain plays a critical role in stabilizing this interaction. Together, the results of this study unravel a novel mechanism involved in glucocorticoid inhibition of ER alpha transcriptional activity and E2-mediated cell proliferation and thus establish a foundation for future exploitation of the GR signaling pathway in the treatment of ER-positive breast cancer.