The Unliganded Glucocorticoid Receptor Positively Regulates the Tumor Suppressor Gene <i>BRCA1</i> through GABP Beta

Abstract

Loss of BRCA1 tumor suppressor function is a critical event in breast tumorigenesis. We have previously identified the stress hormone hydrocortisone as a negative regulator of BRCA1 expression in nonmalignant mammary cells. Here, we have identified a direct role for the unliganded glucocorticoid receptor (GR) in BRCA1 upregulation in the absence of hydrocortisone. The positive regulatory effect of GR is lost upon the addition of hydrocortisone. We have shown that GR interacts with the BRCA1 promoter only in the absence of hydrocortisone, and that this interaction is mediated through the β-subunit of the ets transcription factor GA-binding protein (GABP) at the RIBS promoter element. GR and GABPβ interact in both coimmunoprecipitation and mammalian two-hybrid assays, and this interaction involves the N-terminal to central regions of both proteins. This work presents the first evidence of a ligand-independent role for GR as a positive regulator of gene expression, and loss of GR from the BRCA1 promoter in response to stress hormones leads to decreased BRCA1 expression. Because low levels of BRCA1 have been implicated in the development of sporadic breast cancer, this may represent a novel mechanism through which prolonged stress signaling increases breast cancer risk.