Abstract 1807: Progestin enrichment of the cancer stem cell-like pool provides novel therapeutic targets for hormone-dependent human breast cancer

Clinical trials in post-menopausal women show that combined hormone replacement therapy (HRT), containing both estrogen (E) and a progestin (P) such as medroxyprogesterone acetate (MPA), is associated with an increased risk of breast cancer compared with E alone (JNCI, 2000, 92:328). We previously showed that both natural and synthetic P accelerate tumor development in vivo, a process that is blocked by anti-progestins (Can Res 2007; 67:9929). Furthermore, P also increases lymph node metastasis (Menopause 2010; 17:1040), leading us to hypothesize that P-induced tumor growth and metastasis may be mediated by an enrichment of the pool of cancer stem cell (CSC)-like cells. This increases the proliferative and metastatic potential of already existing tumors. CSCs are a small and highly tumorigenic cell population present in most tumors, which have self-renewal properties. In tumors of the breast, the CSC subpopulation has a phenotypic signature that is CD24 −/low , CD44 high and aldehyde dehydrogenase-positive (ALDH + ) (Cell Stem Cell, 2007; 1:555). Using flow cytometry to measure CD44 density under a variety of experimental conditions we sought to determine whether P influences expression of CSC markers in P-responsive human breast tumors that grow in vivo. Following exposure of T47-D cells for 24 h with physiological levels (10 nM) of natural and synthetic P, CD44 protein expression was increased considerably. Compared with controls, induction of CD44 increased 10-fold in response to MPA (Control 5.9%, MPA 60.7%). Alternative steroid hormones failed to induce CD44 in tumor cells. This phenotypic shift was blocked by the anti-progestin RU-486, indicating that the process is mediated via progesterone receptors (PR). Cells treated with P formed mammospheres more easily than their non-treated counterparts, and a subset of the cell population with induced CD44 high also demonstrated high ALDH enzyme activity. RT-PCR analysis showed that MPA specifically induced CD44v3 and CD44v6 transcripts. Importantly, a variety of synthetic P similarly induced CD44 protein expression. Based on our observations we contend that exposure of breast cancer cells to synthetic P, such as MPA, may lead to an enrichment of the CSC-like pool, supporting the development of P-accelerated tumors in vivo. These findings also suggest that a novel means by which to combat P-dependent tumor growth might be through blockade of CD44 functions in the pool of stem cell-like cells. This could be achieved by immunotherapeutic means, using tissue selective antiprogestins, or through a combination approach involving both immunotherapy against CD44 and small molecule targeting of PR. Supported in part by COR award from the College of Veterinary Medicine, and by generous gifts from donors of Ellis Fischel Cancer Center, University of Missouri. Citation Format: Sandy Goyette, Benford Mafuvadze, Matthew T. Cook, Yayun Liang, Salman M. Hyder. Progestin enrichment of the cancer stem cell-like pool provides novel therapeutic targets for hormone-dependent human breast cancer. [abstract]. In: Proceedings of the 107th Annual Meeting of the American Association for Cancer Research; 2016 Apr 16-20; New Orleans, LA. Philadelphia (PA): AACR; Cancer Res 2016;76(14 Suppl):Abstract nr 1807.