Aging is the critical risk factor for many forms of cancer. We used the mammary gland as a model system to study the impact of age on human epithelia in which age is the greatest risk factor for cancer. Dysfunctional progenitor and luminal cells with acquired basal cell properties accumulate during aging for reasons that are not understood. We evaluated the hypothesis that a novel effect of aging is a defective stem cell regulation by the microenvironment. We identified Axl as a previously unappreciated mammary stem cell marker involved in breast epithelial homeostasis and breast cancer progression. We showed that Axl is a marker of a subpopulation of cKit epithelial progenitors. These progenitors are tightly regulated by the microenvironment, specifically the mechanical properties of the niche. We observed that mechano-response mechanisms in cKit progenitors are altered with age. Thus, tissue-level phenotypes of aging in breast may arise in part due to alterations in the Hippo mechano-signal transduction pathway that lead to reduced efficiency of YAP/TAZ activation. Finally, using mass cytometry, we described the first high-dimensional phenotypic heterogeneity in normal human mammary epithelial cells. Computational analysis using unsupervised population partitioning identified clusters of a specific subset of luminal cells that acquired a more basal phenotype and accumulate with age. Moreover, distinct age-related phenotypic signatures were detectable in cKit-progenitor cells, considered the cell-of-origin for breast cancers. Here, we propose a model where reciprocal interactions between the microenvironment and breast epithelial progenitors are skewed during the aging process, leading to a decrease in breast tissue integrity, and increase in phenotypic divergence and susceptibility to tumorigenesis
