The Antiestrogens 4-Hydroxytamoxifen and Fulvestrant Are Inhibitors of Oncogenic Factor Y-box Binding Protein-1 Expression in Breast Cancer Cells

Purpose: The cold-shock protein Y-box binding protein-1 (YB-1) is associated with more frequent relapse and higher aggressiveness in breast cancer and, notably, is a client protein of estrogen receptor alpha (ER alpha). Thus, the authors hypothesized that endocrine therapy using the antiestrogens 4-hydroxytamoxifen (4-OHT) and fulvestrant (FUL) may affect YB-1 expression. Materials and Methods: YB-1 localization in the breast cancer cell line MCF-7 was determined by fluorescence microscopy and GST-ER alpha pull-down assay. Modulation of YB-1 expression in the presence of 4-OHT and FUL was determined by quantitative RT-PCR as well as by Western blot analysis. Results: YB-1 is primary localized in the perinuclear cytoplasm of MCF-7 cells. YB-1 binds directly to recombinant GST-ER alpha fusion protein as shown by pull-down assay. Incubation experiments with 4-OHT and FUL demonstrated a strong time-and dose-dependent suppression of YB-1 expression by these antiestrogens. Inhibitory effects were assessed on the level of YB-1 mRNA as well as on the level of YB-1 protein. Conclusion: The data presented here suggest that 4-OHT and FUL therapy targets both proliferative ER alpha as well as pro-oncogenic YB-1. Thus, 4-OHT's and FUL's anticancer efficacy may play a more global role in breast cancer progression control than originally thought.