Project Summary/Abstract The objective of this project is to characterize a new therapeutic target in order to improve breast cancer therapy by preventing breast cancer from spreading and restoring the effectiveness of hormone therapy. While estrogen receptor (ER) is the most successful therapeutic target in breast cancer, up to one-third of breast cancers lose ER expression and thus do not respond to hormone therapy. The mechanisms for ER loss in the majority of ER- negative breast cancers remain to be investigated. On the other hand, despite most breast cancers are diagnosed during relatively early stage, nearly 30% of them will eventually develop metastasis after treatment. We have now identified 14-3-3tau as a key driver that promotes breast cancer metastasis and ER loss in vivo. We have established a 14-3-3tau xenograft model which recapitulates metastasis and loss of estrogen receptor expression as seen in patients with high levels of 14-3-3tau in their breast tumors. We also developed a new in vitro 3D breast cancer spheroid model of ER loss. This proposal will investigate how 14-3-3tau promotes the evolution of breast cancer from ER-positive to ER-negative and endocrine resistance, and use the established 3D spheroid culture and animal models to identify the drugs capable of blocking these adverse effects. Some small molecule inhibitors for the proposed pathways have been available in clinics or been tested in clinical trials for other conditions. Thus, if confirmed, it would be quite feasible to test them in patients with tumors harboring high levels of 14-3-3tau, which are found in over 60% of breast cancer. Through the examination of 14-3-3tau expression in the breast tumor samples, we might be able to identify the patients who are at risk of developing metastasis and losing response to endocrine therapy. These patients may benefit from treatment with these inhibitors targeting the downstream effectors of 14-3-3tau to prevent endocrine therapy resistance and metastasis. Some of these inhibitors have already been approved for other diseases or are available in clinical trials. Thus, the potential impact of this proposal in providing a novel therapeutic strategy to prevent breast cancer metastasis and to reverse endocrine therapy resistance in breast cancer is very significant.