Decreased BRCA1 protein expression alert the response of MCF7 cells to tamoxifen

B44

Approximately 60% of breast cancers are estrogen receptor-positive and tamoxifen, which targets the estrogen receptor (ER), is one of the most effective treatments. However, de novo and acquired tamoxifen resistance remain important clinical problems. The circumvention of tamoxifen resistance using rational therapeutic approaches requires an understanding of the underlying molecular mechanisms of the cellular response to tamoxifen. Decreased protein expression of the tumor suppressor BRCA1 has been observed in sporadic breast cancer and is associated with poor prognosis and a more aggressive phenotype. BRCA1 has also been shown to directly inhibit ER activity. ER activity plays a major role in mammary epithelial cell proliferation during carcinogenesis so it is not surprising that decreased BRCA1 protein is associated with poor prognosis. Furthermore, tamoxifen was shown to promote mammary cancer development in a mouse model of BRCA1-mutation-related breast cancer. In order to reveal the relationship between decreased BRCA1 protein levels and tamoxifen response, MCF7 cells stably expressing shRNA targeted against the BRCA1 gene were established using recombinant retrovirus, resulting in a 38% reduction of BRCA1 protein levels compared to cells expressing scrambled control shRNA. BRCA1 knockdown was accompanied by increased cell proliferation and decreased response to tamoxifen as determined by MTT assay. To asses the molecular mechanisms of these effects, we used mass spectrometry to determine the global changes in protein expression. For these studies, iTRAQ isobaric tags were used the peptides were separated with two-dimensional liquid chromatography. Tandem mass spectrometry was performed on an ABI 4800 MALDI ToF/ToF. Changes in levels of expression of proteins involved in processes such as metabolism, differentiation, cell death and biosynthesis were identified. An understanding of the mechanisms of tamoxifen resistance is the first step toward developing new therapeutic strategies for women with this phenotype. Identification of global protein expression patterns will provide valuable information toward understanding the mechanisms underlying tamoxifen response and can be used immediately to develop a clinical strategy for treating tamoxifen resistant tumors. A greater understanding of the cellular and molecular mechanisms of tamoxifen will lead to individualized treatment against breast cancer.