Cellular and molecular effects of early pregnancy on mammary epithelial cell subpopulations in mice and their potential relevance for breast cancer protection

The breast cancer protective effect of an early pregnancy is well established in both humans and rodents, but the underlying mechanism is unclear. Since breast cancers are thought to originate from distinct mammary epithelial cell subtypes, we studied the effect of early parity on the gene expression and the functional properties of specific mouse mammary epithelial cell subpopulations. The latter were isolated by fluorescence-activated cell sorting (FACS) from parous and from age-matched virgin control mice. The isolated cell subpopulations were investigated further by unbiased genomic and bioinformatic methods, as well as by in vitro colony formation and by in vivo mammary gland reconstitution assays. The results of the transcriptome analysis showed an upregulation of differentiation genes and a pronounced decrease in the Wnt/Notch signaling ratio in the basal stem/progenitor cell subpopulation of parous mice. This was associated with a parity-induced downregulation of carcinogenic pathways and a reduction in the in vitro and in vivo proliferation potential. As a possible mechanism for reduced Wnt signaling in basal stem/progenitor cells, we found a more than threefold decrease in the expression of the secreted Wnt ligand Wnt4 in isolated total mammary cells from parous mice, which corresponded to a similar reduction in progesterone receptor positive and Wnt4-secreting cells in intact mammary epithelia. Notably, recombinant Wnt4 partially rescued the parity-induced in vitro proliferation defect of basal stem/progenitor cells, strongly suggesting a causal relation between decreased Wnt4 secretion and parity-induced molecular and functional changes of basal stem/progenitor cells in mice. In conclusion, the study shows that early parity induces differentiation, downregulates the Wnt/Notch signaling ratio and decreases the in vitro and in vivo proliferation potential of basal mammary stem/progenitor cells in mice. Thereby, the study not only delineates the cellular and molecular effects of early parity, but it also paves the way for future studies examining whether inhibitors of Wnt signaling can be used to mimic the parity-induced protective effect against breast cancer.