A Functional Serine 118 Phosphorylation Site in Estrogen Receptor-α Is Required for Down-Regulation of Gene Expression by 17β-Estradiol and 4-Hydroxytamoxifen

To evaluate the contribution of ERK1/2 phosphorylation of estrogen receptor (ER)-α to activation and repression of endogenous genes, we produced stably transfected lines of HeLa cells with functional ERK1/2 pathways that express similar levels of wild-type human ERα and ERα mutated to inactivate the well-known MAPK site at serine 118 (ERαS118A). We compared effects of the S118A mutation on 17β-estradiol (E2)-mediated transactivation, which is heavily dependent on activation function (AF) 2 of ERα and on 4-hydroxytamoxifen (OHT)-mediated transactivation, which is heavily dependent on AF1, which includes S118. To examine whether S118 was the key ERK/MAPK phosphorylation site in ERα action, we compared the effects of the S118A mutant and the ERK inhibitor U0126 on expression of endogenous genes. In several estrogen response element-containing genes, the S118A mutation strongly reduced induction by E2, and U0126 did not further reduce expression. Expression of another group of estrogen response element-containing genes was largely unaffected by the S118A mutation. The S118A mutation had variable effects on genes induced by ER tethering or binding near specificity protein-1 and activator protein-1 sites. For five mRNAs whose expression is strongly down-regulated by E2 and partially or completely down-regulated by OHT, the S118A mutation reduced or abolished down-regulation by E2 and nearly abolished down-regulation by OHT. In contrast, for Sma and mothers against decapentaplegic-3-related, which is down-regulated by E2 and not OHT, the S118A mutation had little effect. These data suggest that there may be distinct groups of genes down-regulated by ERα and suggest a novel role for ERK phosphorylation at serine 118 in AF1 in regulating expression of the set of genes down-regulated by OHT.