Liver plays crucial role in iron metabolism

Dec 7 2005

A new study in the December Cell Metabolism reveals an unexpected connection between a tumor suppressor gene in the liver and the normally careful control over the amount of iron absorbed from the diet.

The surprising finding demonstrates a critical role for the liver in iron metabolism. The discovery also suggests a new avenue for the treatment of hereditary hemochromatosis, an iron-overload disease that is one of the most common genetic disorders among Caucasians, according to researchers.

Chu-Xia Deng, from the National Institute of Diabetes and Digestive and Kidney Diseases, and his colleagues report that mice lacking the SMAD4 gene in the liver only suffer from a toxic buildup of iron, particularly in their liver, kidneys, and pancreas--symptoms similar to those exhibited by humans with hemochromatosis. In other respects, the animals appeared remarkably normal, the researchers found.

"Unexpectedly, the liver-specific knockout of SMAD4 does not have a major impact on liver development; instead it results in a dramatic accumulation of iron in the liver of mutant mice," Deng said. "In addition, several other organs with intact SMAD4, including pancreas, kidney, eye, and brain, also exhibit accumulation of iron starting from 2 months of age.

"Our work not only creates a new animal model for the study of hemochromatosis but also clearly indicates that the liver is a physiological center for regulation of iron homeostasis," he added.

Iron is an essential nutrient found in many foods, particularly in red meat and iron-fortified bread and cereal. In the body, iron becomes part of hemoglobin, a molecule in the blood that transports oxygen from the lungs to all body tissues. Iron deficiency results in anemia, whereas iron overload leads to organ and tissue damage.

Symptoms of hemochromatosis, the most common iron-overload disease, can include bronzed skin, enlarged liver, diabetes, and abnormalities of the pancreas and joints. Frequent removal of blood in the same manner applied to blood donors can lower iron levels and alleviate the symptoms. Without treatment, the disease can lead to organ failure.

In the current study, the researchers set out to explore SMAD4's role in liver development and maintenance, by creating mice with normal levels of the gene in all part of the body except the liver. A member of a critical cell signaling pathway known as the transforming growth factor ß (TGF-ß) superfamily, SMAD 4 plays many central roles in development.

Earlier studies had also linked abnormalities in SMAD4 to multiple forms of cancer. For example, mutations in the tumor suppressor gene are frequently detected in pancreatic cancer, colon cancer, and a common form of lung cancer called adenocarcinoma, Deng said.

"Yet we didn't see cancer in the mice and saw only minor defects in the liver," Deng said. "Still, the mice were sick with apparent problems in their pancreas and other organs. Initially, we had no information to suggest that the gene might play a role in iron balance."

The mice gradually lost weight, and more than half developed other signs of illness, including rough-looking fur, loss of muscle mass, and slower movement, by 10 months of age, the researchers reported. Many of the animals died at this stage, they said. Autopsy revealed that the animals had developed a dark red pancreas and brownish pigment deposition in multiple organs, including the liver and parts of the kidneys.

After extensive analysis, the team detected a significant overload of iron in tissue samples taken from the mutant mice. After reaching 2 months of age, the animals accumulated iron in the organs that had appeared abnormal, they reported.

To find out why the liver defect caused widespread iron accumulation, the team examined the activity of other genes previously found to play a role in iron metabolism. Mice lacking SMAD4 in the liver showed an approximately 100-fold decline in expression of hepcidin, an essential gene that limits iron absorption.

Hepcidin secreted from the liver into the bloodstream is known to curb iron intake in the intestine and prevent the release of iron from cells in the blood, the researchers said.

In the absence of hepcidin, other genes responsible for iron absorption in the intestine rose, the group found. Additional examination revealed that SMAD4 normally plays a direct role in increasing hepcidin activity.

A lack of hepcidin has been associated with iron overload, whereas excess hepcidin results in iron-deficiency anemia in mice, the researchers said. Furthermore, they added, people with two mutant copies of the gene suffer from severe juvenile hemochromatosis.

The findings, therefore, have potential significance for the treatment of iron overload, Deng said.

"Several genes cause hemochromatosis, but in all cases, hepcidin levels are down," he said. "Our findings suggest that drugs that boost the TGF- ß signaling pathway, either through SMAD4 or other members of the pathway, hold promise for treatment."