Abstract P5-03-04: Effect of aging on the function and transformation of murine mammary stem cells

Epidemiological studies have shown that the risk of getting breast cancer progressively increases with age. A woman is 100 times more susceptible to develop breast cancer in her 60s than in her 20s. A better understanding of altered cellular and molecular mechanisms leading to the development of sporadic breast cancers as a result of aging are urgently needed for its prevention and treatment in growing population of older women. Recent research implicated that adult mammary stem cells (MaSCs) might be responsible for the initiation and progression of certain types of breast cancer. But no studies have been reported on how lifelong exposure of MaSCs to endogenous and environmental stresses during aging compromises their self-renewal and differentiation function and predispose them to neoplastic transformation either spontaneously or after carcinogen exposure. In our study, we have investigated the effect of progressive aging and carcinogen exposure on MaSC number and function in a murine model. The FACS isolated MaSC enriched basal cells, characterized by their immunophenotype (Lin − CD49f high CD24 med ) were utilized to evaluate MaSC frequency and function during aging by in vitro mammosphere formation and 3D-ECM sphere differentiation assay as well as by in vivo cleared mammary fat pad transplantation (IVT). The results of our study showed that the percentage of MaSCs analyzed by both FACS profile and in vitro assays, increased steadily with increasing age observed at 2, 7, 17 and 25 month-old C57BL/6 mice. Subsequent IVT using mammosphers or 3D structures formed by young (4 months) and old MaSCs (28 months) derived from C57BL/6 mice showed similar in vivo functional mammary gland regenerative capacity indicating similar self-renewal/differentiation ability between young and old MaSCs. However, we found that the regenerated glands from old MaSCs had significantly higher number of spontaneous pre-neoplastic lesions (52.0%) than those from young MaSCs (18.3%). Significantly, MaSC frequency was also increased by the in vivo exposure of a carcinogen, N-Methyl-N-Nitrosourea (MNU, 25 mg/kg body weight, once a week for three consecutive weeks), in old mice with no effect on total mammary cell number. Furthermore, the old MaSCs expanded by MNU treatment were still capable of in vivo regeneration with similar success rate as that of normal old MaSCs. But, we observed increased frequency of hyperplasia and dysplasia in the regenerated glands initiated by MNU treated old MaSC. Our results indicate that MaSCs might be the precursors of preneoplastic lesions and old MaSCs appear to be predisposed to premalignant transformation, which is enhanced by the exposure to a DNA damaging agent. Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P5-03-04.