Abstract P6-07-04: Rapamycin inhibits the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice

Rapamycin, a well-studied mTOR inhibitor, has been demonstrated to inhibit mammary carcinogenesis at multiple stages, including initiation, invasion, and metastasis, in preclinical animal models. Nevertheless, the cancer preventative potential and underlying mechanisms remain unclear, especially in individual breast cancer subtypes like ErbB2/Her2-positive breast cancers. ErbB2 amplification/overexpression is a particular clinical concern because it occurs in approximately one-third of human breast cancers and is associated with poor prognosis. Therefore, we used MMTV-ErbB2 transgenic mice as our model system to test the efficacy of rapamycin in the prevention of ErbB2-mediated mammary tumor development. Our initial data provided proof of concept regarding the anti-cancer effects of rapamycin in vivo . Indeed, rapamycin (1.5 mg/kg/day for 12 days) significantly reduced the volume and weight of syngeneic 78617 cell-derived mammary tumors in MMTV-ErbB2 mice, despite observed decreases in CD4 + and CD8 + immune cells. Since advanced mammary gland development can serve as an indicator of breast cancer risk, we investigated the effects of rapamycin on mammary gland development in MMTV-ErbB2 mice that were treated with low-dose rapamycin (1 mg/kg/day) between weeks 10 and 20 of age. As such, rapamycin significantly attenuated mammary morphogenesis at 20 weeks of age, as indicated by decreased branching density, ductal elongation, and proliferative index of the premalignant mammary glands. Flow cytometric analysis of isolated primary mammary epithelial cells (MECs) was performed using CD24 and CD49f markers to identify MEC populations. We found that rapamycin has a significant impact on MEC stemness based on changes in luminal (CD24 high CD49f low ), mammary stem cell (MaSC)-enriched (CD24 high CD49f high ), and myoepithelial/basal (CD24 low/high CD49f high ) MEC populations. We also used CD61 and CD49f markers to identify a population enriched with luminal progenitor cells (CD61 high CD49f high ) that was selectively inhibited by rapamycin. Consistent with our flow cytometric analyses, rapamycin inhibited the luminal progenitor cell-enriched population, self-renewal, and anchorage-independent cell growth of primary MECs, as demonstrated by colony-forming cell, mammosphere, and 3D culture assays, respectively. These functional stem cell assays further corroborate that rapamycin suppresses the stemness of primary MECs. Molecular analysis of MECs demonstrated that rapamycin inhibited mTOR signaling, as expected. Importantly, rapamycin also significantly suppressed the receptor tyrosine kinase/ErbB2, estrogen receptor, Wnt/β-catenin, and TGFβ/Smad3 signaling pathways prior to malignant transformation. Collectively, our study provides evidence that rapamycin has potential cancer preventative effects in the mammary glands of MMTV-ErbB2 mice during the premalignant risk window. These rapamycin-induced anti-cancer effects ultimately highlight the promising clinical significance of rapamycin for the prevention and treatment of human ErbB2-overexpressing breast cancers. Citation Format: Yang X, Zhao Q, Parris AB, Howard EW, Zhao M, Guo Z, Xing Y, Ma Z. Rapamycin inhibits the stemness of mammary epithelial cells in the premalignant tissues of MMTV-ErbB2 transgenic mice [abstract]. In: Proceedings of the 2017 San Antonio Breast Cancer Symposium; 2017 Dec 5-9; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2018;78(4 Suppl):Abstract nr P6-07-04.