SRA coactivation of estrogen receptor-α is phosphorylation-independent, and enhances 4-hydroxytamoxifen agonist activity

The ability of steroid receptor RNA activator (SRA), an AF-1 coactivator, to contribute to differences in estrogen receptor (ER)-α and ERβ transcriptional activity was tested. In transient transfections, SRA expression increased ERα- and ERβ-dependent gene expression. However, when the receptors’ amino-terminal A/B regions were examined as GAL4 DNA binding domain fusions, SRA enhanced the activity of GAL-ABα but not GAL-ABβ. Exogenous SRA also enhanced AF-2 activity for both receptors, indicating that SRA effects are not limited to AF-1. Simultaneously mutating three phosphorylation sites within GAL-ABα domain only modestly reduced SRA coactivation of GAL-ABα, suggesting that phosphorylation does not play a major role in SRA function relative to this domain. SRA enhanced ERα activity stimulated by 4-hydroxytamoxifen, but was unable to convert this mixed antiestrogen to an ERβ agonist. Thus, SRA is an ERα AF-1-specific coactivator that enhances the agonist activity of tamoxifen-bound ERα and may contribute to tamoxifen resistance.