Parity-induced mouse mammary epithelial cells are pluripotent, self-renewing and sensitive to TGF-β1 expression

A parity-induced mammary population, marked by beta-galactosidase expression conditionally activated through cre-lox recombinase originates in WAP-Cre/Rosa-lox-STOP- lox-LacZ (WAP-Cre/Rosa-LacZ) female mice during pregnancy, lactation and involution. During subsequent pregnancies, these parity-induced mammary epithelial cells (PI-MEC) proliferated to produce new secretory acini composed of secretory luminal cells and myoepithelium. In serial transplantation assays, PI-MEC were able to self-renew over several transplant generations and to contribute significantly to the resulting mammary outgrowths. In limiting dilution transplantation, they proliferated to produce both luminal and myoepithelial cells, comprised both lobule-limited and duct-limited epithelial outgrowths, and differentiated into all the cellular subtypes recognized in murine mammary epithelium. TGF-beta1 expression from the whey acidic protein promoter (WAP) in triply transgenic females did not prevent the appearance of PI-MEC after pregnancy despite the absence of full lactation or their ability to proliferate and produce progeny with diverse cellular fates in situ upon subsequent pregnancies. However, in transplants from triple transgenic parous females, the WAP-TGF-beta1-positive PI-MEC did not contribute to the newly recapitulated mammary outgrowths, suggesting that they were incapable of expansive cellular proliferation (self-renewal). This result is consistent with our earlier publication that WAP-TGF-beta1 expression in mammary epithelium induces premature stem cell senescence in mammary transplants and decreases mammary cancer risk in mouse mammary tumor virus (MMTV)-infected females even after multiple pregnancies.