Edited by Alex TokerCytoplasmic localization of proline, glutamic acid, leucinerich protein 1 (PELP1) is observed in ϳ40% of women with invasive breast cancer. In mouse models, PELP1 overexpression in the mammary gland leads to premalignant lesions and eventually mammary tumors. In preliminary clinical studies, cytoplasmic localization of PELP1 was seen in 36% of women at high risk of developing breast cancer. Here, we investigated whether cytoplasmic PELP1 signaling promotes breast cancer initiation in models of immortalized human mammary epithelial cells

Progress in breast cancer prevention is limited by an inability to reliably predict which women will develop breast cancer and which high-risk women will respond to chemoprevention therapies. Thus, there is a critical need to determine the molecular mechanisms that promote breast cancer initiation in order to 1) identify biological markers of disease susceptibility and 2) to develop novel targeted chemoprevention agents. A promising target for preventing breast cancer initiation is proline, glutamic acid, and leucine rich protein 1 (PELP1). PELP1 was first identified as an estrogen receptor (ER) co-activator in ER+ breast cancer cell lines. Subsequent studies found that PELP1 functions in ER+ and ER- breast cancer cell lines and is overexpressed in 80% of invasive breast cancers. Interestingly, while PELP1 is primarily localized to the nucleus in the normal breast epithelial cells, in about 40% of invasive breast tumors a significant amount of PELP1 is localized in the cytoplasm. Interestingly, in a mammary gland specific transgenic mouse model, PELP1-cyto expression induced mammary gland hyperplasia. We therefore analyzed breast needle aspirates from asymptomatic high-risk women, and found cytoplasmic PELP1 in 4/11 (36%) samples. These findings suggest that altered localization of PELP1 may be an early event in breast cancer initiation. The objective of this research project is to identify the molecular mechanisms associated with PELP1-induced breast cancer initiation. We performed global gene expression analyses of our in vitro human mammary epithelial cell (HMEC) models to identify potential downstream pathways regulated by PELP1-cyto. Interestingly, genes regulated by PELP1-cyto differ greatly from those regulated by nuclear PELP1. Our results showed that PELP1-cyto induces expression of cell survival genes, and inflammatory cytokines and chemokines, and activates the NF-κB signaling pathway. Thus, we hypothesize that cytoplasmic PELP1 induces activation of inflammatory cytokines and chemokines via NF-κB, which promotes tumor initiation and a pro-tumorigenic microenvironment. In support of our hypothesis we have found that PELP1-cyto expression promotes phosphorylation of the non-canonical IKK, TBK1, and the NF-κB subunit p65. Additionally, we found that conditioned medium from PELP1-cyto cells, induced expression of IL-1β, TNFα, and IL-8 in THP-1 cells that had been differentiated into macrophages, suggesting that PELP1-cyto HMECs release cytokines that promote the activation of macrophages. The goal of future studies is to validate these findings in vitro and in vivo and identify the cytokines and/or chemokines induced by PELP1-cyto that promote breast cancer initiation and activation of tumor associated macrophages. Citation Format: Julie H Ostrander, Brian J Girard, Todd Knutson, Bethanie Kuker, Victoria Seewaldt. Cytoplasmic PELP1 promotes breast cancer initiation via NF-κB signaling [abstract]. In: Proceedings of the Thirty-Seventh Annual CTRC-AACR San Antonio Breast Cancer Symposium: 2014 Dec 9-13; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2015;75(9 Suppl):Abstract nr P4-05-03.