Abstract C13: Direct binding of ERα to YB-1 suppresses HER2 expression in human breast cancer
Authors: Shibata Tomohiro , Watari Kosuke , Izumi Hiroto , Kawahara Akihiko , Hattori Satoshi , Fukumitsu Chihiro , Murakami Yuichi , Takahashi Ryuji , Toh Uhi , Ito Ken-ichi , Tanaka Maki , Kage Masayoshi , Kuwano Michihiko , Ono Mayumi 
Published: May 2016

[Background] ERα and HER2 are two major biomarkers of therapeutic efficacy in breast cancer patients. Y-box binding protein YB-1 is an oncoprotein involved in breast cancer and a transcription factor for HER2/ErbB2. However, regulatory mechanisms of ERα and HER2 expression by YB-1 are largely unknown. In our present study, we examined how nuclear YB-1 regulates expression of HER2 and ERα in breast cancer cells.

[Materials and methods] We established YB-1/Tet-On system in which nuclear YB-1 expression was induced by doxycycline, and examined how YB-1 could modulate expression of HER2 and ERα in breast cancer cells. Furthermore, we established the ERα/Tet-On system, in which ERα expression was markedly induced after treatment with doxycycline. To assess whether the YB-1-HER2-ERα correlation was affected by menopause, we analyzed biopsy samples of 116 premenopausal and 114 postmenopausal patients, who had not received any therapeutic drugs, by IHC.

[Results] We first examined whether YB-1 regulates expression of ERα and HER2 by using various human breast cancer cell lines, and observed following findings. (1) Nuclear YB-1 overexpression increased HER2 expression and decreases ERα expression, while HER2 expression was suppressed by estradiol and enhanced by anti-estrogen drugs. (2) ERα binding to YB-1 suppressed YB-1 binding to the HER2 promoter region. Furthermore, binding of YB-1 to ERα was enhanced by estradiol and suppressed by anti-estrogen drugs. (3) YB-1 induced proteasomal degradation of ERα when both interact directly.

We have next examined whether expression of HER2, nuclear YB-1, and ERα are significantly associated by IHC analysis of biopsy samples of breast cancers. In breast cancers of postmenopausal, but not premenopausal patients, nuclear YB-1 expression was positively correlated with HER2 expression and negatively correlated with ERα expression.

[Conclusions] In our study, we presented a new finding that YB-1 promoted proteasomal degradation of ERα by direct interaction and that YB-1-induced HER2 expression was suppressed by ERα. Furthermore, in breast tumors of postmenopausal patients, nuclear YB-1 expression was positively correlated with HER2 expression and negatively correlated with ERα expression. Therefore, this study could contribute to further development of optimized endocrine- and HER2- targeted therapeutics against breast cancer.

Citation Format: Tomohiro Shibata, Kosuke Watari, Hiroto Izumi, Akihiko Kawahara, Satoshi Hattori, Chihiro Fukumitsu, Yuichi Murakami, Ryuji Takahashi, Uhi Toh, Ken-ichi Ito, Maki Tanaka, Masayoshi Kage, Michihiko Kuwano, Mayumi Ono. Direct binding of ERα to YB-1 suppresses HER2 expression in human breast cancer. [abstract]. In: Proceedings of the AACR-NCI-EORTC International Conference: Molecular Targets and Cancer Therapeutics; 2015 Nov 5-9; Boston, MA. Philadelphia (PA): AACR; Mol Cancer Ther 2015;14(12 Suppl 2):Abstract nr C13.