Expanding the Paradigm for Estrogen Receptor Binding and Transcriptional Activation

Abstract

Estrogen receptor (ER) binds to a spectrum of functional estrogen response elements (ERE) within the human genome, including ERE half-sites (HERE), inverted and direct repeats. This has been confounding, because ER has been reported to bind weakly, if at all, to these sites in vitro. We show that ER binds strongly to these nonconventional EREs, and the binding is enhanced by the presence of high-mobility group protein B1 (HMGB1). Collectively, these and previous findings reinforce the notion of the plasticity of strong ER/ERE interactions, consistent with their broader range of observed binding specificity. In addition, transient transfection studies using luciferase reporter gene assays show that these EREs drive luciferase activity, and HMGB1 enhances transcriptional activity. Furthermore, HMGB1 gene expression knockdown results in a precipitous drop in luciferase activity, suggesting a prominent role for HMGB1 in activation of estrogen/ER-responsive genes. Therefore, these data advocate that the minimal target site for ER is a cHERE (consensus HERE) that occurs in many different contexts and that HMGB1 enhances both the binding affinity and transcriptional activity. This challenges the current paradigm for ER binding affinity and functional activity and suggests that the paradigm requires significant reevaluation and modification. These findings also suggest a possible mechanism for a cross talk between genes regulated by ER and class II nuclear receptors.