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AF-1 domain of ER    α     regulates the agonistic and antagonistic actions of tamoxifen

Tamoxifen significantly reduces breast tumor recurrenceby binding to the activation function-2 (AF-2) domain of theER. Acquired resistance to tamoxifen in breast cancer patientsis a serious therapeutic problem. Antiestrogen-resistant breastcancer often shows increased expression of the epidermal growthfactor receptor (EGFR) family members, EGFR and ErbB2. In thisreport we now show that overexpression of EGFR or activatedAKT-2 in MCF-7 cells leads to phosphorylation of Ser167 in theAF-1 domain of ER, enhanced ER-amplified in breast cancer 1(ER:AIB1) interaction in the presence of tamoxifen, and resistanceto tamoxifen. In contrast, transfection of activated MAPK kinase,an immediate upstream activator of MAPK (ERK 1 and 2) into MCF-7cells leads to phosphorylation of Ser118 in the AF-1 domainof ER, inhibition of ER-amplified in breast cancer 1 (ER:AIB1)interaction in the presence of Tam, and maintenance of sensitivityto tamoxifen. Inhibition of AKT by short inhibitory RNA blockedSer167 phosphorylation in ER and restored tamoxifen sensitivity.Doxycyclin-inducible knockdown of AKT-2, reverses’ tamoxifen resistance in EGFR and AKT-2 overexpressing tamoxifen resistant tumors in vivo. However, maximum sensitivity to tamoxifen was observed whenboth AKT and MAPK were inhibited. Taken together, these datademonstrate that different phosphorylation sites in the AF-1domain of ER regulate the agonistic and antagonistic actionsof tamoxifen in human breast cancer cells.

98th AACR Annual Meeting-- Apr 14-18, 2007; Los Angeles, CA