Abstract 90: Estrogen accelerates the development of estrogen receptor-negative breast cancer.

Proceedings: AACR 104th Annual Meeting 2013; Apr 6-10, 2013; Washington, DC Long-term cumulative exposure to estrogen increases the breast cancer risk in women. Breast tissue must have a normal complement of estrogen receptor (ER) and progesterone receptor (PGR) for the normal development of breast tissue. Clinically, about 22% of patients with breast cancer overexpress the ERBB2 (HER2/neu) receptor tyrosine kinase. However, many HER2/neu-positive mammary tumors do not express ER or PGR by the time they are clinically recognizable. So far, the role of estrogen in the development of ER-negative breast cancers is not clearly defined. Aromatase is a key enzyme for estrogen biosynthesis and its expression pattern between human and mouse is drastically different. We generated transgenic humanized aromatase (Aromhum) mice to mimic the human aromatase distribution pattern in the mouse. Aromhum mice exhibited an increased incidence of lobuloalveolar breast hyperplasia associated with increased estrogen levels locally in mammary glands but not in the circulation. To demonstrate the important role of ER signaling in ER-negative breast tumor development and eventually to test a prevention strategy using aromatase inhibitors in this mouse model, Aromhum mice were crossed with ERBB2 transgenic mice to generate wild-type (WT), Aromhum, ERBB2 and Aromhum/ERBB2 (AE) mice. WT or Aromhum transgenic mice were free of tumors during the 26 week of follow-up. On the other hand, 56% of ERBB2 (only) mice developed mammary tumors, as expected. Interestingly, AE mice had a significantly higher (86%, p<0.05) tumor incidence compared with ERBB2 mice. Thus, it appeared that the human aromatase transgene increased the rate of ERBB2–induced mammary tumor growth. The tumors that arose in either ERBB2 or AE mice were histological adenocarcinomas as was previously reported for the ERBB2 transgenic mice. Levels of ER and PGR in benign mammary epithelial cells were higher in Aromhum and AE mice as compared to WT and ERBB2 mice. We further measured the dynamic change of sex hormone receptor status from normal mammary epithelial cells to fully developed tumors. Different stages of mammary tumors were chosen from nonpalpable microscopic visible early stage tumor to various sizes of late stage tumors. ER and PGR were present in early stage tumor but gradually decreased and eventually were absent in late stage tumors. The estrogen response gene, cyclin D1 was higher in benign and malignant mammary epithelium of Aromhum and AE mice compared with WT mice. Cell proliferation evaluated by Ki67 staining was significantly higher in benign mammary epithelium and tumors of AE mice as compared to those of ERBB2 mice. In conclusion, estrogen expedited the early development of ER-negative mammary tumors caused by a primary genetic hit (HER2/neu) via increasing cell proliferation through ER-mediated signal transduction. This study provides a fundamental mechanism for testing prevention strategies using aromatase inhibitors in this mouse model. Citation Format: Hong Zhao, John S. Coon, Robert T. Chatterton, Dolores Huberts, David Christopher Brooks, Francesco J. DeMayo, Serdar E. Bulun. Estrogen accelerates the development of estrogen receptor-negative breast cancer. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 90. doi:10.1158/1538-7445.AM2013-90