Radiobiology of sphere-forming rat mammary epithelial cells under nonadherent culture

The mammary gland is highly susceptible to radiation carcinogenesis. It is vital to delineate the effect of radiation on tissue stem and progenitor cells, from which cancer cells arise, to understand the carcinogenic mechanisms of radiation. Herein we established a technique for the formation of ‘mammospheres’ of rat mammary epithelial cells (RMECs) using the nonadherent culturing method, which was previously developed for culturing human mammary progenitor cells, and examined the biological and radiobiological characteristics of mammosphere-forming cells. We found that mammospheres mainly consisted of proliferating cells that were double positive for cytokeratin (CK)14 and CK18 (markers for myoepithelial and luminal epithelial cells, respectively). Under differentiating conditions in adherent culture, mammospheres generated both CK14 and CK18 single-positive cells, implying that mammospheres consist of bipotent progenitor cells. Orthotropic transplantation assays showed that mammospheres contained lower stem cell activity than primary RMECs, suggesting that mammospheres are not enriched in stem cells. When primary RMECs were irradiated with up to 8 Gy in suspension and subjected to mammosphere formation, the efficiency of mammosphere formation, bromodeoxyuridine incorporation of cells therein, CK14/CK18 expression in mammospheres and their generation of single-positive cells in adherent culture were unaffected. These results suggest that mammosphere-forming cells of RMECs are resistant to radiation-induced proliferative death and perturbation of differentiation.第15回幹細胞シンポジウ