Hormonal prevention of breast cancer: Mimicking the protective effect of pregnancy

Full term pregnancy early in life is the most effective natural protection against breast cancer in women. Rats treated with chemical carcinogen are s imilarly protected by a previous pregnancy from mammary carcinogenesis. Pro liferation and differentiation of the mammary gland does not explain this p henomenon, as shown by the relative ineffectiveness of perphenazine, a pote nt mitogenic and differentiating agent. Here, we show that short term treat ment of nulliparous rats with pregnancy levels of estradiol 17 beta and pro gesterone has high efficacy in protecting them from chemical carcinogen ind uced mammary canters. Because the mammary gland is exposed to the highest p hysiological concentrations of estradiol and progesterone during full term pregnancy, it is these elevated levels of hormones that likely induce prote ction from mammary cancer. Thus, it appears possible to mimic the protectiv e effects of pregnancy against breast cancer in nulliparous rats by short t erm specific hormonal intervention.