Proliferation of Estrogen Receptor-α-Positive Mammary Epithelial Cells Is Restrained by Transforming Growth Factor-β1 in Adult Mice

Transforming growth factor (TGF)-β1 is a potent inhibitor of mammary epithelial proliferation. In human breast, estrogen receptor (ER)-α cells rarely co-localize with markers of proliferation, but their increased frequency correlates with breast cancer risk. To determine whether TGF-β1 is necessary for the quiescence of ER-α-positive populations, we examined mouse mammary epithelial glands at estrus. Approximately 35% of epithelial cells showed TGF-β1 activation, which co-localized with nuclear receptor-phosphorylated Smad 2/3, indicating that TGF-β signaling is autocrine. Nuclear Smad co-localized with nuclear ER-α. To test whether TGF-β inhibits proliferation, we examined genetically engineered mice with different levels of TGF-β1. ER-α co-localization with markers of proliferation (ie, Ki-67 or bromodeoxyuridine) at estrus was significantly increased in the mammary glands of Tgfβ1 C57/bl/129SV heterozygote mice. This relationship was maintained after pregnancy but was absent at puberty. Conversely, mammary epithelial expression of constitutively active TGF-β1 via the MMTV promoter suppressed proliferation of ER-α-positive cells. Thus, TGF-β1 activation functionally restrains ER-α-positive cells from proliferating in adult mammary gland. Accordingly, we propose that TGF-β1 dysregulation may promote proliferation of ER-α-positive cells associated with breast cancer risk in humans.