By identifying a key molecule involved in transporting toxic compounds out of the bloodstream and into mother's milk, researchers may have uncovered a new way to make breast-feeding safer and healthier.

Even with a wide variety of formulas available on the market, it is generally recognized that for a healthy infant there is no substitute for mother's milk. Not only does breast-feeding provide essential nutrients, but it has also proved to have a primary role in bolstering the infant's immune system. Unfortunately, breast milk can also provide a vehicle for considerably nastier compounds; research has found that breast milk can become a concentrated reservoir for many environmental toxins and carcinogens.

One such molecule is PhIP, a carcinogen that can be absorbed from cigarette smoke or well-done meat and that is known to accumulate markedly in breast milk. Alfred Schinkel and his colleagues at the Netherlands Cancer Institute (Amsterdam, the Netherlands) and Cardiff University (Cardiff, UK) were interested in identifying the mechanism by which PhIP and other compounds are deposited in milk. As they describe in a recent article from Nature Medicine (online 30 January, doi:10.1038/nm1186), they chose to focus on a drug transporter protein called ABCG2, which has previously been linked to drug resistance in breast tumors and is involved in the active elimination from the body of drugs and other toxic compounds in such organs as the intestine and kidney.

Schinkel's team found that, unlike other drug transporters, ABCG2 expression in the mammary glands increases markedly after the onset of lactation in mice, cows, and humans. They followed up by tracking the accumulation of PhIP in intravenously injected mice and found that, although PhIP was collecting in the milk of wild-type animals (a roughly 12-fold elevation relative to plasma levels), there was no such increase in mice in which Abcg2 had been knocked out. Similar experiments with the drug topotecan showed that an inhibitor specific for Abcg2 also blocked this transport process, with treated mice showing considerably reduced drug levels in their breast milk.

Although the authors express some puzzlement about the selective pressure that would favor a transporter that actively moves potentially toxic compounds into the mammary tissue, they nonetheless find their identification of this protein to be an encouraging development. Identifying chemicals that are ABCG2 substrates—and helping new mothers avoid them—could help make breast-feeding even healthier for infants than it was before.