Progesterone-Inducible Cytokeratin 5-Positive Cells in Luminal Breast Cancer Exhibit Progenitor Properties

Abstract

Progestins play a deleterious role in the onset of breast cancer, yet their influence on existing breast cancer and tumor progression is not well understood. In luminal estrogen receptor (ER)- and progesterone receptor (PR)-positive breast cancer, progestins induce a fraction of cells to express cytokeratin 5 (CK5), a marker of basal epithelial and progenitor cells in the normal breast. CK5+ cells lose expression of ER and PR and are relatively quiescent, increasing their resistance to endocrine and chemotherapy compared to intratumoral CK5-ER+PR+ cells. Characterization of live CK5+ cells has been hampered by a lack of means for their direct isolation. Here, we describe optical (GFP) and bioluminescent (luciferase) reporter models to quantitate and isolate CK5+ cells in luminal breast cancer cell lines utilizing the human KRT5 gene promoter and a viral vector approach. Using this system, we confirmed that the induction of GFP+/CK5+ cells is specific to progestins, is dependent on PR, can be blocked by antiprogestins, and does not occur with other steroid hormones. Progestin-induced, fluorescence-activated cell sorting-isolated CK5+ cells had lower ER and PR mRNA, were slower cycling, and were relatively more invasive and sphere forming than their CK5- counterparts in vitro. Repeated progestin treatment and selection of GFP+ cells enriched for a persistent population of CK5+ cells, suggesting that this transition can be semi-permanent. These data support that in PR+ breast cancers, progestins induce a subpopulation of CK5+ER-PR- cells with enhanced progenitor properties and have implications for treatment resistance and recurrence in luminal breast cancer. © 2012 Springer Science+Business Media New York.