Phosphorylation of the Mutant K303R Estrogen Receptor α at Serine 305 Impacts Aromatase Inhibitor Sensitivity

We previously identified a lysine to arginine transition at residue 303 (K303R) in ERα in invasive breast cancers, which confers resistance to the aromatase inhibitor (AI) anastrozole (Ana) when expressed in MCF-7 breast cancer cells. Here we show that AI resistance arises through an enhanced cross-talk of the IGF-1R/IRS-1/Akt pathway with ERα, and the serine (S) residue 305 adjacent to the K303R mutation plays a key role in mediating this cross-talk. The ERα S305 residue is an important site that modifies response to tamoxifen; thus, we questioned whether this site could also influence AI response. We generated stable transfectants expressing wild-type (WT), K303R ERα, or a double K303R/S305A mutant receptor, and found that the AI-resistant phenotype associated with expression of the K303R mutation was dependent on activation of S305 within the receptor. Ana significantly reduced growth in K303R/S305A-expressing cells. Preventing S305 phosphorylation with a blocking peptide inhibited IGF-1R/IRS-1/Akt activation, and also restored AI sensitivity. Our data suggest that the K303R mutation and the S305 ERα residue may be a novel determinant of aromatase inhibitor response in breast cancer, and blockade of S305 phosphorylation represents a new therapeutic strategy for treating tumors resistant to hormone therapy.