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

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, ERalpha and ERbeta. We show that SRA potentiates the estrogen-induced transcriptional activity of both ERalpha and ERbeta. We demonstrate that the transcriptional activity of ERalpha 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 ERbeta, denoting the ability of SRA to mediate differential activation of ERalpha and ERbeta. The presence of an intact serine residue at position 118 (S(118)) in ERalpha AF-1 is required for coactivation of ERalpha by SRA. We also show that activation of the mitogen activated protein kinase (MAPK) induces ligand-independent coactivation of ERalpha 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) ERalpha mutant in response to H-Ras(V12), suggesting that phosphorylation of S(118) by MAPK participates in the ligand-independent effect of SRA on ERalpha.