Inhibitory effects of pregnancy and lactation against N-methyl-N-nitrosourea-induced mammary carcinomas in female Lewis rats

The role of parity in protection against Lewis rat mammary carcinogenesis was investigated. Pregnancy and lactation prior to N-methyl-N-nitrosourea (MNU) exposure and pregnancy and lactation following MNU administration significantly reduced both the incidence of mammary carcinoma and the average number of mammary carcinomas per rat, and significantly prolonged the latency when compared with respective age-matched virgins (AMVs). Lactation showed an additive effect on the reduction of mammary cancer. Pregnancy suppressed the number of estrogen receptor(ER)- and progesterone receptor (PgR)-positive cells and lowered the cell proliferation rate in the non-tumoral mammary glands. Since the majority of the mammary carcinomas were hormone-dependent in both the parous and AMVs, pregnancy and lactation appear to decrease the ER and/or PgR-positive cells presumed to be the progenitor of hormone-dependent carcinomas, and they lowered the cell turnover necessary for tumor promotion in parous rats, resulting in a lower mammary: carcinoma yield.