Overexpression of active Akt3 induces differential binding of coregulator proteins to the estrogen receptor as a possible mechanism of tamoxifen resistance

5197 Tamoxifen (TAM) is an effective anti-estrogen for treatment of women with hormone-dependent breast cancer but acquired drug resistance limits its therapeutic benefit. We have previously reported that expression of active Akt3 in MCF-7 breast cancer cells results in estrogen-independent tumors that are actually stimulated to grow after TAM treatment. We hypothesize that this TAM resistance may be attributed to binding of different coregulator proteins and/or different binding affinity of these proteins to the estrogen receptor in MCF-7 cells overexpressing active Akt3 as compared to parental MCF-7 cells. We have immunoprecipitated the estrogen receptor along with bound coregulator proteins in both cells lines after TAM, estradiol, or vehicle treatment. After 2-D gel electrophoresis separation of these immunoprecipitated proteins and comparing them using PDQuest 2-D analysis software (Bio-Rad), we identified protein “spots” that were statistically different under the treatment conditions between the two cell lines. The isolated protein “spots” were subjected to MALDI-TOF mass spectrometry. By searching protein databases through the MASCOT website for protein identification, we have identified estrogen receptor coregulator proteins that may play a potential role in tamoxifen resistance. Current studies are focused on addressing the role of differential protein binding as a possible mechanism of tamoxifen resistance in Akt3 overexpressing breast cancer cells.