Soy compounds thwart estrogen.

Although soybeans have gained renown as a source of the isoflavones genistein and daidzein, which can mimic the activity of the hormone estrogen, those same compounds occasionally have the opposite effect and block estrogen's activity. Now, a team of New Orleans researchers reports that a different family of soy isoflavones blocks estrogen even more consistently.

These unusual isoflavones, known as glyceollins, might lead researchers to improved drugs that starve breast cancers of the estrogen that many depend upon, notes research leader Stephen M. Boue of the Agriculture Department's Southern Regional Research Center.

When soybeans are infected or otherwise stressed, the plants make three glyceollins with natural pesticidal properties. In their experiments, Boue and his team infected tissue from soybean seeds with a fungus. Within 3 days, the concentration of glyceollins in the bean tissue spiked to as much as 1,000 parts per million.

When added to human breast cancer cells in a lab dish, the glyceollins inhibited the estrogen-sensitive cells' growth. The glyceollins also dramatically reduced the ability of estrogen to turn on the cellular receptors that it usually activates.

Estrogen receptors come in two types: alpha and beta. Though glyceollins inhibited estrogen's binding to both, the compounds proved far more effective at suppressing activity of the alpha receptors--the ones that play a central role in the growth of most breast cancers. In contrast, Boue notes, daidzein and genistein have their biggest effect on the beta receptors.--J.R.