Abstract 5071: Palbociclib inhibits the stemness of mammary epithelial cells in premalignant tissues of MMTV-erbB2 transgenic mice

Field: Biochemistry, Genetics and Molecular Biology (Life Sciences)

Palbociclib is the first CDK4/6-Cyclin D inhibitor approved by the FDA for ER+/Her2(erbB2)-breast cancer treatment. With the success of the drug in hormone-positive breast cancer, it is encouraging to explore more options of Palbociclib as an anticancer agent. erbB2 is a receptor tyrosine kinase (RTK) frequently overexpressed in breast cancer. The molecular hallmarks of erbB2+ breast cancer include the upregulation of cyclin D. The aim of this study was to explore the potential of Palbociclib as a preventive agent using MMTV-erbB2 transgenic mice, with a focus on the stemness of mammary epithelial cells (MECs) in the premalignant tissues. We first tested the in vivo effect of Palbociclib on tumor cells on MMTV-erbB2 mammary tumors using syngeneic tumor graft models. The animals bearing grafted tumors were treated with saline (control), low (75 mg/kg/day) and high (150 mg/kg/day) Palbociclib via oral gavage every 3 days for 29 days. We found that Palbociclib significantly inhibited tumor growth in a concentration dependent manner. To test the effect of Palbociclib on MEC stemness in premalignant mammary tissues, the animals at 9 wks of age were treated with saline, low and high (0, 75, 150 mg/kg/day) doses of Palbociclib every 3 days for 4 wks. We found the efficiency of both primary and secondary mammosphere formation of MECs from drug treated mice was significantly inhibited, suggesting the inhibition of mammary stemness in the premalignant tissues. Consistently, results from 3D cultures showed that colony numbers of the cells from drug treated mice were also lower than the control. Using CD24 and CD49f as markers, flow cytometry was performed to analyze the effect of Palbociclib on MEC subpopulations, including luminal, basal, and stromal populations. We showed that the percentage of luminal cells and putative mammary reconstitution units (MRUs) in drug treated mice was significantly inhibited. We also found that the percentage of CD61+/CD49+ cells, which are enriched with luminal progenitor cells and the origin of tumor initiation cells of this model, in Palbociclib treated tissues were also inhibited. The data suggest that Palbociclib induces MEC reprogramming in the premalignant tissues. Moreover, we found that the protein levels of pRb, E2F1, cyclin D1, c-myc, and cdc2 in cell cycle regulation, and pAKT, pERK, pmTOR, and p4EBP1 in the RTK pathway, pER, and DVL2 and LRP6 in the Wnt pathway were significantly downregulated in Palbociclib treated tissues, which was more evident in the high dose group. Taken together, our data indicate that Palbociclib inhibits MEC proliferation and stemness in the premalignant mammary tissues. Downregulation of cell cycle regulators and the signaling in ER, RTK, and Wnt pathway plays a critical role in this process. Our data support further investigation of Palbociclib in the prevention of ER+/ErbB2+ breast cancer.Citation Format: Amanda B. Parris, Yongxuan Liu, Zhikun Ma, Erin W. Howard, Xiaoshan Feng, Xiaohe Yang. Palbociclib inhibits the stemness of mammary epithelial cells in premalignant tissues of MMTV-erbB2 transgenic mice [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 5071.