Abstract EBP50 (Ezrin-radixin-moesin-binding phosphoprotein-50, also known as NHERF, NHERF1) has been reported to interact with epidermal growth factor receptor (EGFR) and enhance EGFR activity and its downstream ERK signaling in a cervical cancer cell line HeLa. However, our previous results demonstrated that EBP50 could suppress breast cancer cell proliferation, suggesting its inhibitory effect on EGFR signaling in breast cancer cell. So we detected the effect of EBP50 expression on EGF-induced cell proliferation and the activation of EGFR signaling in breast cancer cell lines, MDA-MB-231 and MCF-7. In MDA-MB-231 cells which expressed low level of EBP50, EBP50 over-expression inhibited EGF-induced cell proliferation. In MCF-7 cells which expressed high level of EBP50, EBP50 knockdown promoted EGF-induced cell proliferation. EBP50 was reported to interact with EGFR via its carboxyl terminal regulatory domain, which is adjacent to the autophosphorylation domain of the receptor, so it is possible that the association of EBP50 and EGFR mask the phosphorylation site of EGFR so as to prevent EGFR activation. To test this hypothesis, we detected the phosphorylation status of EGFR in the presence or absence of the EBP50 expression. Over-expression of EBP50 in MDA-MB-231 inhibits EGF-stimulated EGFR phosphorylation, and knockdown of EBP50 in MCF-7 cells led to the enhanced EGF-stimulated EGFR phosphorylation. Meanwhile, the total expression levels of EGFR were not affected by the different expression level of EBP50 during EGF stimulation. In the case of EBP50 expression on the EGFR downstream signaling, EBP50 over-expression inhibited EGF-stimulated ERK activity in MDA-MB-231 cells, and EBP50 knockdown resulted in the enhanced EGF-stimulated ERK activity in MCF-7 cells. These data are consistent with results of MDA-MB-231 and MCF-7 cell lines pretreated with EGFR tyrosine kinase inhibitor-AG1478, that EGF-stimulated ERK activity was inhibited when EGFR phosphorylation was blocked. Taken together, our results demonstrated that EBP50 can suppress EGF-induced proliferation of breast cancer cells by retarding EGFR phosphorylation and blocking EGFR downstream signaling in breast cancer cells. These results provide further insights into the molecular mechanism by which EBP50 regulates the development and progression of breast cancer. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 1935. doi:10.1158/1538-7445.AM2011-1935