Ligand-independent coactivation of ERα AF-1 by steroid receptor RNA activator (SRA) via MAPK activation

Abstract

Nuclear receptor coactivators are factors that enhance the transcriptional activity of the receptor. Coactivators usually work in ligand-independent and/or dependent manners by interacting with activation function-1 (AF-1) and AF-2 of the receptor, respectively. The recently characterized steroid receptor RNA activator (SRA) was cloned as an AF-1-dependent coactivator and shown to enhance the transcriptional activity of selected steroid receptors. In this work, we describe the effect of SRA on the activity of the two estrogen receptor (ER) isoforms, ERα and ERβ. We show that SRA potentiates the estrogen-induced transcriptional activity of both ERα and ERβ. We demonstrate that the transcriptional activity of ERα can be enhanced by SRA in a ligand-independent manner through the AF-1 domain. However, this AF-1-dependent effect of SRA is not observed on ERβ, denoting the ability of SRA to mediate differential activation of ERα and ERβ. The presence of an intact serine residue at position 118 (S 118) in ERα AF-1 is required for coactivation of ERα by SRA. We also show that activation of the mitogen activated protein kinase (MAPK) induces ligand-independent coactivation of ERα by SRA, a mechanism that is independent of the AF-2. Finally, SRA is unable to rescue the loss of activity of the S 118 ERα mutant in response to H–Ras V12, suggesting that phosphorylation of S 118 by MAPK participates in the ligand-independent effect of SRA on ERα.