Abstract A09: In utero exposure to estrogen promotes mammary tumor risk in MMTV-erbB-2 transgenic mice through induction of ER-erbB-2 crosstalk

The mammary gland during in utero stage is in a largely undifferentiated state, which is particularly vulnerable to environmental factors. Increasing evidence indicates that intrauterine hormonal and nutritional alterations may significantly modify breast cancer risk later in life. A well-known case is the “DES Daughter” story. However, the underlying mechanisms are largely unknown and more prenatal risk factors remain to be identified. Therefore, development of sensitive and reliable model systems to identify those factors with mechanistic insight is of great significance in breast cancer prevention. Previously, most studies on in utero exposure-modulated breast cancer risk used carcinogenic models. While this system is valuable in providing proof of concept, due to challenges in elucidating molecular pathways induced by in utero exposures and by carcinogens, molecular mechanistic studies in these models are compromised. In this study, we aimed to establish an animal model for studying in utero exposure to environmental hormones-induced mammary tumor risk using MMTV-erbB-2 transgenic mice. erbB-2 is an oncogene that is amplified in ~25-30% of human breast cancer. The MMTV-erbB-2 mice have a defined genetic background and tumor development patterns, which is of advantage in mechanistic studies. When pregnant MMTV-erbB-2 mice were injected with 20 ng/mouse/day of Estradiol (E2) between day 13 and day 19 of gestation, mammary tumor development of the offspring was significantly accelerated, as compared to control. The average tumor latencies of the control and the E2 group were 36 and 32 weeks respectively. In utero exposure to E2 also induced earlier vaginal opening and prolonged length of estrous stages. Whole mount analysis indicated that in utero exposure to E2 increased mammary epithelial density at 6 and 10 weeks of age. Importantly, we also found that in utero exposure to E2 induced a concurrent upregulation of signaling in estrogen receptor α (ER α) and receptor tyrosine kinase pathways in the mammary tissues, as indicated by increased expression of ER target genes (c-myc, cyclin D1, EGFR, and Bcl-2) and phosphorylation of EGFR, erbB-2, Akt1 and Erk1/2. Use of gefitinib also, a small molecular inhibitor of EGFR, abrogated the upregulation of both pathways in mammary tissues with in utero exposure to E2, suggesting a role of EGFR in in utero exposure to E2 induced crosstalk between the two pathways. Our data indicates that in utero exposure to E2 induces a systematic pro-estrogenic effect and enhanced signaling and crosstalk between ER and EGFR/erbB-2 pathways. Deregulation of the two critical pathways in the premalignant tissues is associated with mammary tumor risk later in life. This will lead to further mechanistic studies, such as in utero exposure to hormones, induced epigenetic regulation, and reprogramming. This model system may represent a model for studying gene-environment interaction in breast carcinogenesis and can be used to study the effects of in utero exposure to other environmental and dietary factors modulated breast cancer risk. Citation Format: Zhikun Ma, Amanda Blackwelder, Xiaohe Yang. In utero exposure to estrogen promotes mammary tumor risk in MMTV-erbB-2 transgenic mice through induction of ER-erbB-2 crosstalk. [abstract]. In: Proceedings of the AACR Special Conference: The Translational Impact of Model Organisms in Cancer; Nov 5-8, 2013; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2014;12(11 Suppl):Abstract nr A09.