Repression of ESR1 through actions of estrogen receptor alpha and Sin3A at the proximal promoter.

Downregulation of receptors by their ligands is a fundamental process by which cells control sensitivity to stimuli. For steroid hormones, this involves lipophilic ligands binding to intracellular receptors to induce a decline in receptor number. Regulation of estrogen (E2) receptor alpha (ERα) by E2 is one example. The E2-induced decline in ERα is, in part, mediated through direct regulation of the protein. It is well documented that decreases in ERα protein levels in response to E2 occur via the ubiquitin-proteasome pathway (1, 42). The mRNA levels of ERα also decrease, but the mechanism responsible for E2-induced repression of the ERα gene, ESR1, is not established (5, 49, 52). ERα is a ligand-activated transcription factor that mediates the effects of E2 by regulating gene expression. Activation by ERα has been studied in detail, but little is understood about how E2-bound ERα represses transcription. E2-induced repression is, however, of significant biological importance. Microarray analyses of E2-treated breast cancer cell lines show that the number of repressed genes is greater than or near the number of activated genes (10, 19, 29, 32). Yet, there are limited reports investigating E2-induced repression, and no generalized mechanism has emerged (6, 13, 22, 25, 43, 47, 59, 60, 71, 74). Antagonist-induced repression by selective ER modulators involves conformational changes that prevent coactivator binding to ERα (55). Such a conformational blockade does not occur with agonist binding and thus cannot account for E2-induced gene repression. Many repressive complexes exist to restrict gene expression in response to cellular signals. One example is the Sin3 complex, which was identified in yeast but is conserved in mammals (41, 58). The Sin3 core complex consists of the Sin3A scaffolding protein; histone deacetylase 1 (HDAC1) and HDAC2; RbAp46 and RbAp48, which stabilize the complex to nucleosomes; and SAP18 and SAP30, which stabilize the interaction between Sin3A and the HDACs (23, 73). The specificity and function of the core complex can be expanded by adding extra catalytic modules onto the Sin3A platform. These include histone methylation, DNA methylation, chromatin remodeling, and monosaccharide transferase ability (reviewed in reference 57). Sin3A lacks intrinsic DNA binding, so it must be targeted to promoters via interaction with other DNA-binding or adaptor proteins. Interactions have been found for Sin3 and Mad, NRSF, p53, MeCP2, and many others (3, 38, 40, 51). Repression of ESR1 is a crucial brake on the E2 signaling pathway. High levels of ERα in breast cancer cells leads to activation of E2-regulated genes in the absence of ligand (18, 28). Further evidence shows that mice with upregulated ERα expression develop ductal hyperplasia, lobular hyperplasia, and ductal carcinoma in situ, demonstrating the consequences of unregulated ERα levels at all stages of breast cancer development (20). It is also proposed that ESR1 is amplified in subsets of breast cancers and in precancerous breast diseases (26). This evidence suggests that failure of E2 to limit ERα levels could contribute to the uncontrolled cellular proliferation that occurs in cancer. In this report, a new model of E2-induced gene repression is identified on the basis of analysis of ESR1. The findings show that repression is accomplished by the effects of ERα and Sin3A at the proximal promoter of ESR1 that dominate over activating factors in distal and proximal regions. These data add to the limited knowledge base of E2-induced repression mechanisms.