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Tumor fighters. Estrogen and progesterone trigger extra cancer-fighting proteins in rat cells (C and D), compared to those free of extra hormone (A and B).

Pregnancy Awakens a Tumor Suppressor

Women who have a baby before they're 35 tend to have a lower risk of breast cancer. That's been clear for years, now researchers have found finally found out why. Pregnancy's cancer-fighting effects may be due to a famous tumor inhibitor, p53.

The p53 protein helps keep cancer in check by ordering a cellular mop-up in response to DNA damage. One strategy is to stop a cell from dividing until it self-destructs or its DNA can be repaired. In rats, researchers have observed that treatment with pregnancy hormones estrogen and progesterone protects them against breast cancer. Pregnancy hormones also cause a spike in p53 levels. However, the rats are protected from breast cancer long after hormone levels return to normal. Molecular biologist Bert O'Malley and his colleagues at Baylor College of Medicine in Houston, Texas, wanted to pin down why.

When O'Malley's team exposed female rats to estrogen and progesterone to mimic pregnancy, the animals maintained high levels of p53 in their breast cells long after the hormone treatment ended. Pregnancy induced the same effect: For 35 days after the rats had given birth, p53 levels were significantly higher than in virgin rats. When the rats were later exposed to a cancer-causing chemical, the hormone-treated rats showed high levels of p53 and reduced cell proliferation compared with nontreated, virgin control rats. The researchers report their results in the 16 October issue of the early edition of the Proceedings of the National Academy of Sciences.

O'Malley's group has shown for the first time that the extra protection afforded by p53 lasts far beyond the initial hormonal burst, says D. Joseph Jerry, a cancer researcher at the University of Amherst, Massachusetts. He speculates that this new understanding may lead to preventative treatments that target p53--and bypass the side-effects associated with hormones. Jerry cautions, though, that other molecular players may play a central role in keeping cancer at bay.

Related sites

More on p53
Bracing p53 for the War on Cancer (ScienceNOW, 23 December 1999)