Reactive oxygen species induce phosphorylation of serine 118 and 167 on estrogen receptor alpha

Estrogen receptor alpha (ER alpha) is a well-known target for signaling pathways originating from growth factor receptors. Reactive oxygen species (ROS) can induce activation of extracellular response kinase 1/2 (Erk1/2) and protein kinase B (Akt). Both kinases have been implicated in the phosphorylation of serine 118 and serine 167 on ER alpha, respectively. This activity may lead either to ligand-independent activation of ER alpha or down-regulation of ER alpha and may contribute to development of the resistance to endocrine therapy. Treatment of MCF-7 human breast cancer cells with glucose oxidase (GO, 0.1 un/ml) induced transient phosphorylation of serine 118 and serine 167. The increase in expression of p-ser118-ER alpha was 355 +/- A 98% (mean +/- A SD) and of p-ser167-ER alpha was 632 +/- A 355%. These effects were enhanced in Her2 over-expressing MCF7 cells. ER alpha expression declined to 63 +/- A 20% within the first 90 min of treatment and was below 10% 24 h later. ROS induced phosphorylation of ER alpha resulted in decreased expression of pS2 and progesterone receptor. Activation of Erk1/2 and Akt was transient with highest levels of Erk1/2 being 595 +/- A 143% and p-Akt 311 +/- A 125%. Inhibition of Erk1/2 by U0126 (10 mu M) decreased p-ser118-ER alpha by 51.7 +/- A 8.5% and decreased p-ser167-ER alpha by 41.9 +/- A 16.9% whereas inhibition of Akt by LY294002 (20 mu M) and wortmannin (500 nM) or by siRNA knock-down, had no effect on p-ser167-ER alpha expression. Our data show for the first time that ROS can induce post-translational modifications of ER alpha at serine 118 and serine 167, and may lead to ER alpha down-regulation in human breast cancer cells. Both the phosphorylation and consequent down-regulation of ER alpha may be a mechanism associated with development of endocrine therapy resistance.