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

Abstract Age is the number one risk fact for breast cancer with only 5% of all breast cancers occurring in women under 40 years old and age specific incidence of invasive breast cancer increases from less than 1.5% at age 40, to about 3% at age 50 and over 4% by age 70 in American women. Recent research implicated that adult mammary stem cells (MaSCs) might be responsible for the initiation and progression of certain types of breast cancer. Our preliminary study showed that aging is associated with a significant increase of MaSC frequency, but with a functional decline of self-renewal and differentiation as well as increased neoplastic transformation potential. These findings indicate that aged MaSCs might be the precursors of preneoplastic lesions and serve as the cell of origin for malignant transformation in breast tissue. Therefore, intervention of MaSC aging process could be an effective method for cancer prevention. The drug of rapamycin has been shown to extend life span and ameliorate age-related pathologies (e.g., cancer) in murine models, and a recent study suggested that rapamycin's anti-aging effect may partially act through enhancing the function of tissue-specific stem cells. It is unknown whether rapamycin treatment will also enhance the function of aged MaSCs and decrease their transformation potential. In this study, we fed C57BL/6 mice with microencapsulated rapamycin-containing food (14 mg/kg, food designed to deliver ∼2.24 mg of rapamycin per kg body weight/day to achieve about 4 ng/ml of rapamycin per kg body weight/day) or control diet with empty capsules for 2 year (starting at 2-month old) or 5-10 days (starting at 25.5-month old) and then isolated primary mammary cells at 26-month old for MaSC quantification using an in vitro mammosphere formation and 3D-ECM sphere differentiation assay as well as by the in vivo cleared mammary fat pad transplantation assay. Our findings indicate that short-term (5-10 days) or long-term (> 2 year) rapamycin treatment reversed phenotypic changes associated with aged MaSC, which was mainly characterized by decreased luminal-to-basal cell ratio and increased MaSC frequency. Histological analysis of regenerated glands of aged MaSCs derived from control and rapamycin-treated mice showed a significant decrease of early neoplastic transformation potential in rapamycin-treated group. Subsequent in vivo serial transplantation and mating experiments revealed that rapamycin treatment reverted aged MaSCs more resemble to young MaSCs in self-renewal/differentiation function during regeneration and improved lobulo-alveolar differentiation function for lactation. In conclusion, our findings suggest that rapamycin can rejuvenate the function of aged MaSCs as well as reduce their incidence of preneoplastic transformation. Citation Information: Cancer Res 2013;73(24 Suppl): Abstract nr P5-03-03.