KINETICS OF MAMMARY EPITHELIAL-CELL PROLIFERATION IN PITUITARY ISOGRAFTED BALB C MICE/

Recently, we have published that treatment of pituitary isografted BAL B/c mice with a single injection of N-methyl-N-nitrosourea (MNU) leads to the rapid development of mammary tumors in over 90% of the animals (Guzman et al., Cancer Res., 52, 5732-5737). In the present study, we characterized the changes in proliferative activity and lobulo-alveol ar differentiation of MECs at different time intervals after isografti ng animals with pituitary glands. Virgin BALB/c mice 1, 3, 5 or 8 week s after pituitary isografting were either pulse-labeled for 2 h or con tinuously infused with bromodeoxyuridine (BrdU) and the percentage of BrdU-labeled MECs was assessed. The S-phase duration (T(s)) of NIECs w as evaluated by double labeling with [H-3]thymidine and BrdU. The popu lation potential doubling time (T(p)) was calculated from the values o f BrdU-LI and T(s). Three stages of proliferation and differentiation of MECs in pituitary isografted virgin BALB/c mice were observed: (i) A sharp increase in the percentage of proliferating MECs of the termin al ducts and ductal branchings in the first 1-2 weeks, (ii) Developmen t of lobulo-alveolar structures from the terminal ductal and alveolar buds, between weeks 3 and 5 with the highest BrdU-LI in week 3 and (ii i) Multiplication of the alveolar structures and decrease in the BrdU- LI between weeks 5 and 8. The BrdU-LIs of alveolar cells 5 weeks after isografting the animals were significantly higher than those of the d uctal cells. The continuous administration of BrdU for 3, 5 or 7 days by using osmotic pumps revealed zones in the ducts where almost all ME Cs were labeled as well as zones lacking proliferate activity. When th e BrdU administration was extended for 10-14 days, almost all (>95%) d uctal and lobular epithelial cells were labeled. A small percentage (< 5%), of ductal and lobulo-alveolar MECs cells, remained unlabeled even after 14 days infusion of BrdU. The T(s) and T(p) values were shorter in pituitary isografted animals than in controls, but no significant difference was found for either values between the ductal and alveolar cells in either isografted or control mice. Changes in proliferation kinetics of mouse MECs in pituitary isografted animals correlated with the circulating concentrations of prolactin, progesterone and 17beta- estradiol, but not with corticosterone, growth hormone or thyroxin. We propose that these time dependent differences in the structural compo sition and proliferative activity of MECs in pituitary isografted anim als can be used as a model system for evaluation of the role of cell p roliferation and differentiation in mammary carcinogenesis. In a paral lel study, we used this model system to induce mammary tumors by a sin gle injection of MNU in mice isografted with pituitaries for 1, 3, 5 o r 8 weeks. We observed that MNU induced mammary carcinogenesis in pitu itary isografted animals required elevated proliferative activity of N ECs at the time of carcinogen administration (Swanson et al., submitte d).