Abstract S5-6: Steroid Hormone Regulation of Mammary Stem Cell Function
Authors: Lindeman GJ , Asselin-Labat M-L , Vaillant F , Sheridan JM , Pal B , Wu D , Simpson ER , Yasuda H , Smyth GK , Martin JT , Visvader JE. 
Published: April 2011

Background: The ovarian hormones estrogen and progesterone profoundly influence breast cancer risk, underpinning the benefit of endocrine therapies in the treatment of breast cancer. Modulation of their effects through ovarian ablation or chemoprevention strategies also significantly decreases breast cancer incidence. Conversely, there is an increased risk of breast cancer associated with pregnancy in the short-term. The cellular mechanisms underlying these observations, however, are poorly defined. We and others recently isolated mammary epithelial populations enriched for mammary stem cells (MaSCs), committed luminal progenitor and mature luminal cells from both mouse and human mammary glands. Unexpectedly, MaSCs exhibited a receptor-negative phenotype for ERα , PR and ErbB2. Given the central important of estrogen and progesterone signaling to mammary gland development and cancer, we sought to determine whether these hormones could indirectly modulate MaSC function. Methods and Results: We utilized mouse models to directly study the effects of steroid hormones on the in vivo repopulating ability of MaSCs. Ovariectomy markedly diminished MaSC number and the extent of ductal outgrowth in vivo. The relative contribution of estrogen and progesterone to the regulation of MaSC activity was next examined using hormone pellets or antagonists. MaSC activity increased in animals treated with both estrogen and progesterone. Remarkably, even three weeks of treatment with the aromatase inhibitor letrozole was sufficient to reduce the MaSC pool. The outgrowth potential of these cells was again affected, suggesting that MaSCs retain a ‘memory’ of estrogen deprivation, perhaps through perturbation of their cycling status. Indeed, cell cycle analysis revealed an increase in the percentage of MaSC-enriched cells in G0/G1 in ovariectomized glands compared to controls. This was accompanied by a profound reduction in the expression of cell cycle genes including Cyclin D1.

We further evaluated the effect of the hormonal environment on MaSC function during pregnancy, where progesterone (and prolactin) have prominent roles. Pregnancy led to a transient 11-fold increase in MaSC numbers. This was accompanied by marked elevation in the expression of the progesterone target gene RANK ligand in luminal cells, together with its receptor RANK in the MaSC-containing population. To determine whether MaSC activity is in part mediated through paracrine signals from RANK ligand, inhibitors of RANK signaling were evaluated. Treatment of virgin or pregnant mice with an anti-RANK ligand monoclonal antibody in vivo significantly impaired the clonogenic activity of the MaSC-enriched but not luminal subpopulation.

Discussion: Despite lacking the steroid hormone receptors ERα and PR, MaSCs appear to be exquisitely sensitive to hormone signaling, presumably via paracrine signaling that includes the RANK signaling pathway. The augmented MaSC pool during pregnancy suggests a cellular basis for the short-term increase in breast cancer incidence following pregnancy. Our findings further indicate that breast cancer chemoprevention may in part be achieved through suppression of MaSC function. We speculate that inhibitors of RANK and other stem cell signaling pathways could represent potential chemoprevention agents.

Citation Information: Cancer Res 2010;70(24 Suppl):Abstract nr S5-6.