Abstract P3-05-05: PTHrP affects breast cancer initiation and progression through Stat5 signaling pathway

PTHrP affects mammary development and breast cancer. PTHrP is expressed in basal myoepithelial cells until late pregnancy, when it also becomes expressed in alveolar epithelial cells. During lactation, PTHrP is secreted into milk and into the circulation, and regulates systemic calcium and bone metabolism. Secretion of PTHrP by breast cancers stimulates bone resorption, promoting the skeletal metastases and/or development of hypercalcemia. Less is known about the contribution of PTHrP to development and/or progression of primary breast cancers. Recent GWAS reports have identified the PTHrP gene as a breast cancer susceptibility locus. Using data from the Cancer Genome Atlas (TCGA) Project and a Yale tissue microarray (YTMA49) we have found that increased breast tumor PTHrP expression predicts a more aggressive phenotype and increased mortality, signifying the clinical relevance of high tumor PTHrP levels. In an attempt to define how PTHrP affects breast cancer, we assessed the effects of PTHrP overexpression on the development of mammary tumors in mice. We developed a tetracyline-regulated, MMTV-driven transgenic model of PTHrP overexpression targeted to mammary epithelial cells (MMTV-rtTA;tetO-hPTHrP). Surprisingly, we found that PTHrP overexpression in luminal epithelial cells causes alveolar hyperplasia, secretory differentiation and milk production in virgin mice, associated with a lower number of luminal progenitor cells and basal stem cells upon FACS analysis. Additionally, Signal Transducer and Activator of Transcription 5 (Stat5) is highly activated in the setting of PTHrP-induced alveolar hyperplasisa. This activation is functionally relevant as evidenced by higher levels of β-Casein mRNA and protein, the transcription of which is regulated by activated Stat5 (pStat5). This effect of PTHrP was reversible upon withdrawal of doxycycline (Dox). Addition of Dox to primary mammary epithelial cell cultures from these mice recapitulated the findings. Knocking out PTHrP receptor (PTH1R) in these mice did not affect this phenotype, suggesting that PTHrP acts in a receptor-independent manner. Overexpression of PTHrP in MMTV-PyMT mice dramatically accelerated formation of mammary tumors reducing both latency of tumor formation and survival of the mice. It caused a higher rate of proliferation and a lower rate of apoptosis in vivo. All MMTV-rtTA;tetO-hPTHrP;MMTV-PyMT mice developed palpable tumors in all mammary glands within 3 weeks of age, became hypercalcemic and died before 4.5 weeks of age. These mice had higher levels of pStat5 and βCasein. Tumor cells cultured ex vivo and cell lines established from these tumors overexpressed PTHrP when stimulated with Dox. PTHrP overexpression, and not exogenous PTHrP, caused higher levels of proliferation, pStat5, and β-Casein. This PTHrP-induced activity of Stat5 was blocked by treatment of these cells with a pharmacological inhibitor of Jak2, a tyrosine kinase upstream of Stat5. Additionally, T47D cells transfected with PTHrP showed higher levels of pStat5, suggesting that PTHrP could activate Stat5 signaling regardless of the cell type. We show that PTHrP overexpression results in Stat5 activation and increased proliferation in breast cancer cells in a cell-autonomous fashion and independent of its receptor. Citation Format: Takyar FM, Boras-Granic K, Kim W, Dann P, Wysolmerski JJ. PTHrP affects breast cancer initiation and progression through Stat5 signaling pathway [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P3-05-05.