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No Other Way Out for Iron

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Front Page

No Other Way Out for Iron

Discovery Builds New Understanding of Hemochromatosis

In the early 1960s, television viewers were bombarded with ads for a liquid supplement that claimed it could “perk up iron-poor, tired blood.” The Geritol campaign, deemed fraudulent by the Federal Trade Commission, was pulled in 1965. But this wasn’t the only reversal involving iron. By the mid-’60s, the field of iron biology research was slipping into its own version of anemia. After decades of activity, researchers were impatient with the lack of molecular understanding about how proper iron levels are maintained by the body.

The finding by Adriana Donovan (left), Nancy Andrews, and their colleagues that ferroportin is the cell’s only exit for iron sheds light on a longstanding mystery about hemochromatosis and could point to new therapies. (Photo by Graham Ramsay)

“Many people left the field because it was so frustrating,” said Nancy Andrews, the Leland Fikes professor of pediatrics at Children’s Hospital Boston. One of the most baffling mysteries concerned the situation that arises when there is an excess of iron in the blood rather than too little. Normally, most of the body’s iron is found in oxygen-carrying red blood cells. Some is also found in macrophages, a class of immune cell that engulfs aging red blood cells and recycles their iron. In iron-overload disease, or hemochromatosis, tissues that typically exhibit low iron levels, such as liver, pancreas, and heart, become packed with the mineral while macrophages exhibit unusually low levels.

Why was iron building up in the tissues and not in the normally iron-rich macrophages? Adriana Donovan, HMS instructor in pediatrics at Children’s, Andrews, and their colleagues have found an answer to this decades-old conundrum—and also to the question of what causes hemochromatosis in the first place.

It appears the condition may result from an inability to control the way iron is exported from cells. Iron is absorbed from food by intestinal cells and released into the bloodstream through a tightly controlled system of protein exits. In 2000, Donovan discovered a protein that was shown to ferry iron out of zebrafish cells. It now appears that the protein, ferroportin, is the only mechanism mammalian cells have for exporting iron. The finding, reported in the March Cell Metabolism, suggests that hemochromatosis may be the result of too much ferroportin. Indeed, mutations affecting the expression of a protein that regulates ferroportin have been found in hemochromatosis patients.

No way out. The intestinal cells of mice lacking the iron exporter ferroportin are packed with iron (blue). (Image courtesy of Adriana Donovan)

“So the defect in hemochromatosis is that there is too much ferroportin, and it just keeps pouring iron into the bloodstream, uncontrolled,” said Andrews, who is dean for basic sciences and graduate studies at HMS. Iron entering the bloodstream from intestinal cells and macrophages, both of which express ferroportin, could accumulate in tissues such as liver, pancreas, and heart.

One way around the problem would be to beef up the system that controls feroportin. In fact, the researchers believe this could be a promising therapeutic approach to hemochromatosis—a disease that currently affects up to one in 200 Caucasian Americans and can cause liver failure, heart arrhythmias, and diabetes, if untreated.

The Ins and Outs of Iron
Andrews first began addressing the mystery of hemochromatosis—and the larger one of how iron is regulated in the body—in the mid-1990s, just as iron biology was coming out of the doldrums. Buoyed by the discovery of a mutant gene in some hemochromatosis patients, scientists were racing to find out how the defect causes disease. Andrews suspected it might interfere with how the intestines were letting iron into the bloodstream, but she was stymied. It was still not clear what molecules were responsible for absorbing and exporting iron. Three clues would be discovered over the next few years. The first was uncovered when Andrews and colleagues cloned the gene responsible for importing iron into the cells of the intestine (see Focus, Aug. 15, 1997). The second was discovered a few years later when Donovan, working down the hall in the lab of HMS professor of pediatrics Leonard Zon, cloned a gene that was causing mutant zebrafish to die of anemia. The gene, ferroportin, turned out to encode the long-sought iron exporter (see Focus, March 10, 2000).

A third clue was uncovered in 2002, when researchers discovered a protein, hepcidin, that appeared to have profound effects on how iron was absorbed and distributed in the body. Donovan and colleagues began experimenting with hepcidin and ferroportin. They found that hepcidin not only binds ferroportin, but also regulates how much of it appears on the cell surface.

Excess Exits
As it turns out, hepcidin is present in very low levels in people with several inherited forms of hemochromatosis. Indeed, the mutation found in the mid-1990s appears to affect hepcidin expression. Donovan and Andrews wondered if the resulting low hepcidin levels could be creating a situation in which there are too many iron exits on intestinal cells. Macro-phages, which like intestinal cells are studded with ferroportin, would also release recycled iron from red blood cells back into the bloodstream almost as soon as they engulfed them, rather than holding some in reserve, as they usually do.

“So the defect in hemochromatosis is that there is too much ferroportin, and it just keeps pouring iron into the bloodstream, uncontrolled.”

What appeared to be a stumbling block—the lack of iron in macrophages—made perfect sense in light of the researchers’ hypothesis. What they needed now was to show that ferroportin is the primary portal for iron export in both intestinal cells and macrophages. They began by creating a series of mouse knockouts. First they eliminated ferroportin in every cell. The mice died about a week after conception, suggesting that ferroportin is used by the extraembryonic tissue to deliver iron to the embryo. Antibody staining of normal mouse embryos showed ferroportin was present in the surrounding tissue. To see what effect ferroportin was having later, they created mice that would turn ferroportin off only in the embryo, not the surrounding iron-delivering tissue. The mouse pups were born normal, but soon began exhibiting signs of severe anemia and later died. Upon closer inspection of the ferroportin-deprived mice, the researchers found iron trapped in the cells of the intestines and in the macrophages, suggesting there was no other way for the mineral to get out except through ferroportin.

Of course, people with hemochromatosis have the opposite problem—too much ferroportin and, consequently, too much iron escaping into the bloodstream. The current treatment is not for the faint of heart. “Bloodletting,” said Andrews. “Patients go in every couple of weeks if they can tolerate it and give a unit of blood.” To get their iron levels down often requires as many as 60 such visits. Andrews envisions a kinder, gentler approach: “You could imagine having a small molecule that mimics the effect of hepcidin on ferroportin, and this could potentially be a very good treatment for hemochromatosis.”

—Misia Landau

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