Aging is the number one risk factor for breast
cancer. Recent research implicated that mammary stem
cells (MaSCs) and progenitors might initiate certain types of breast cancer. In this study, MaSC-enriched basal
cells were utilized for the evaluation of MaSC frequency and function during
aging by in vitro mammosphere formation and 3D-ECM sphere differentiation assays
and by in vivo cleared mammary fat pad transplantation (IVT) as we reported
recently (Dong et al., Stem Cell Res.
10:396-404, 2013). We found that the basal-to-luminal
cell ratios and the frequency of MaSCs analyzed with the in vitro assays increased steadily with increasing age in various
strains of mice. The old (25-32 months) mammary gland had significantly higher
frequency of atypical ductal hyperplasia (ADH) and ductal carcinoma in situ
(DCIS) lesions in comparison with the young (2-4 months) mammary gland.
Subsequent IVT using mammospheres or 3D-ECM structures formed by young or old MaSCs showed that the regenerated glands from
old MaSCs had significantly higher number of spontaneous ADH/DCIS lesions than
those from young MaSCs. These findings indicate that aged MaSCs can serve as
the cell of origin for early neoplastic transformation in breast tissue.
Subsequent whole genome RNA-seq revealed age-associated differential expression
of genes involved in immune, inflammatory, and wounding responses in both
mammosphere-forming cells and stromal cells suggesting that these may be the
main cellular processes contributing to the dysfunctional MaSC phenotypes. Consistently,
treatment of old mice with rapamycin, an anti-inflammation drug, showed that both
5-10 days or 2 year treatment reversed phenotypic changes associated with aged mammary
gland. Histological analysis of regenerated glands by aged MaSCs from control and
rapamycin-treated mice showed significant decreases of early neoplastic lesions
in rapamycin-treated group. In
conclusion, our findings suggest that aging causes MaSC to form early
neoplastic lesions, which can be inhibited by rapamycin treatment.