Short-term exposure to estrogen and progesterone induces partial protection against N-nitroso-N-methylurea-induced mammary tumorigenesis in Wistar-Furth rats

The lifetime protective effect of a full term pregnancy for breast cancer i s a reproducible and consistent finding in human beings and in rodent model s. The duration of pregnancy necessary to confer protection has yielded con tradictory results. As the administration of estrogen and progesterone mimi cs the full-term pregnancy effect on conferring protection, we examined whe ther short-term exposure to estrogen and progesterone confers protection ag ainst N-nitroso-N-methylurea-induced mammary carcinogenesis in Wistar-Furth rats. The results reported herein show that treatment of rats with estroge n or progesterone alone for 21 days does not confer protection, but a 10-da y exposure to the same concentrations of estrogen and progesterone induced a partial protective effect. The significance of these results are discusse d in terms of the contradictory results in the literature and the role of m orphological differentiation in conferring the protective effect. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.