Kinetics of mammary epithelial cell proliferation in pituitary isografted BALB/c mice

Recently, we have published that treatment of pituitary isograftedBALB/c mice with a single injection of N-methyl-N-nitrosourea(MNU) leads to the rapid development of mammary tumors in over90% of the animals (Guzman et al., Cancer Res., 52, 5732–5737).In the present study, we characterized the changes in proliferativeactivity and lobulo—alveolar differentiation of MECs atdifferent time intervals after isografting animals with pituitaryglands. Virgin BALB/c mice 1, 3, 5 or 8 weeks after pituitaryisografting were either pulse-labeled for 2 h or continuouslyinfused with bromodeoxyuridine (BrdU) and the percentage ofBrdU-labeled MECs was assessed. The S-phase duration (T_s) ofMECs was evaluated by double labeling with [³H]thymidine andBrdU. The population potential doubling time (T_p) was calculatedfrom the values of BrdU-LI and T_s. Three stages of proliferationand differentiation of MECs in pituitary isografted virgin BALB/cmice were observed: (i) A sharp increase in the percentage ofproliferating MECs of the terminal ducts and ductal branchingsin the first 1–2 weeks, (ii) Development of lobulo—alveolarstructures from the terminal ductal and alveolar buds, betweenweeks 3 and 5 with the highest BrdU-LI in week 3 and (iii) Multiplicationof the alveolar structures and decrease in the BrdU-LI betweenweeks 5 and 8. The BrdU-LIs of alveolar cells 5 weeks afterisografting the animals were significantly higher than thoseof the ductal cells. The continuous administration of BrdU for3, 5 or 7 days by using osmotic pumps revealed zones in theducts where almost all MECs were labeled as well as zones lackingproliferate activity. When the BrdU administration was extendedfor 10–14 days, almost all (>95%) ductal and lobularepithelial cells were labeled. A small percentage (<5%),of ductal and lobulo-alveolar MECs cells, remained unlabeledeven after 14 days infusion of BrdU. The T_s and T_p values wereshorter in pituitary isografted animals than in controls, butno significant difference was found for either values betweenthe ductal and alveolar cells in either isografted or controlmice. Changes in proliferation kinetics of mouse MECs in pituitaryisografted animals correlated with the circulating concentrationsof prolactin, progesterone and 17ß-estradiol, butnot with corticosterone, growth hormone or thyroxin. We proposethat these time dependent differences in the structural compositionand proliferative activity of MECs in pituitary isografted animalscan be used as a model system for evaluation of the role ofcell proliferation and differentiation in mammary carcinogenesis.In a parallel study, we used this model system to induce mammarytumors by a single injection of MNU in mice isografted withpituitaries for 1, 3, 5 or 8 weeks. We observed that MNU inducedmammary carcinogenesis in pituitary isografted animals requiredelevated proliferative activity of MECs at the time of carcinogenadministration (Swanson et al., submitted).