A pregnancy early in reproductive life is protective against breast cancer, but the molecular mechanisms that form the basis for this protective effect have not been elucidated. In the October 16 early online edition of Proceedings of the National Academy of Sciences, Lakshmi Sivaraman and colleagues from Baylor College of Medicine, Houston, Texas, showed that the tumor suppressor protein p53 is increased in early pregnancy in rats and mice, possibly explaining why early pregnancy is associated with a reduced risk of breast cancer.

Sivaraman et al. administered estrogen and progesterone to young female rats and mice and found that early exposure to pregnancy levels of these hormones can induce and sustain chronic nuclear p53 expression. These changes in turn dictate the proliferative response to carcinogen challenge, including a block in mammary epithelial cell proliferation. In addition they showed that p53 is not induced by perphenazine and hence...

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