Interaction of Hemojuvelin with Neogenin Results in Iron Accumulation in Human Embryonic Kidney 293 Cells

Regulation of iron acquisition and iron distribution in mammals

Both cellular iron deficiency and excess have adverse consequences. To maintain iron homeostasis, complex mechanisms have evolved to regulate cellular and extracellular iron concentrations. Extracellular iron concentrations are controlled by a peptide hormone hepcidin, which inhibits the supply of iron into plasma. Hepcidin acts by binding to and inducing the degradation of the cellular iron exporter, ferroportin, found in sites of major iron flows: duodenal enterocytes involved in iron absorption, macrophages that recycle iron from senescent erythrocytes, and hepatocytes that store iron. Hepcidin synthesis is in turn controlled by iron concentrations, hypoxia, anemia and inflammatory cytokines. The molecular mechanisms that regulate hepcidin production are only beginning to be understood, but its dysregulation is involved in the pathogenesis of a spectrum of iron disorders. Deficiency of hepcidin is the unifying cause of hereditary hemochromatoses, and excessive cytokine-stimulated hepcidin production causes hypoferremia and contributes to anemia of inflammation.

Bone morphogenetic protein signaling by hemojuvelin regulates hepcidin expression

Hepcidin is a key regulator of systemic iron homeostasis. Hepcidin deficiency induces iron overload, whereas hepcidin excess induces anemia. Mutations in the gene encoding hemojuvelin (HFE2, also known as HJV) cause severe iron overload and correlate with low hepcidin levels, suggesting that hemojuvelin positively regulates hepcidin expression. Hemojuvelin is a member of the repulsive guidance molecule (RGM) family, which also includes the bone morphogenetic protein (BMP) coreceptors RGMA and DRAGON (RGMB). Here, we report that hemojuvelin is a BMP coreceptor and that hemojuvelin mutants associated with hemochromatosis have impaired BMP signaling ability. Furthermore, BMP upregulates hepatocyte hepcidin expression, a process enhanced by hemojuvelin and blunted in Hfe2-/- hepatocytes. Our data suggest a mechanism by which HFE2 mutations cause hemochromatosis: hemojuvelin dysfunction decreases BMP signaling, thereby lowering hepcidin expression.

Hereditary hemochromatosis: genetic complexity and new diagnostic approaches

Since the discovery of the hemochromatosis gene (HFE) in 1996, several novel gene defects have been detected, explaining the mechanism and diversity of iron-overload diseases. At least 4 main types of hereditary hemochromatosis (HH) have been identified. Surprisingly, genes involved in HH encode for proteins that all affect pathways centered around liver hepcidin synthesis and its interaction with ferroportin, an iron exporter in enterocytes and macrophages. Hepcidin concentrations in urine negatively correlate with the severity of HH. Cytokine-mediated increases in hepcidin appear to be an important causative factor in anemia of inflammation, which is characterized by sequestration of iron in the macrophage system. For clinicians, the challenge is now to diagnose HH before irreversible damage develops and, at the same time, to distinguish progressive iron overload from increasingly common diseases with only moderately increased body iron stores, such as the metabolic syndrome. Understanding the molecular regulation of iron homeostasis may be helpful in designing innovative and reliable DNA and protein tests for diagnosis. Subsequently, evidence-based diagnostic strategies must be developed, using both conventional and innovative laboratory tests, to differentiate between the various causes of distortions of iron metabolism. This review describes new insights in mechanisms of iron overload, which are needed to understand new developments in diagnostic medicine.

Understanding iron homeostasis through genetic analysis of hemochromatosis and related disorders

Genetic analysis of hemochromatosis has led to the discovery of a number of genes whose mutations disrupt iron homeostasis and lead to iron overload. The introduction of molecular tests into clinical practice has provided a tool for early diagnosis of these conditions. It has become clear that hemochromatosis includes a spectrum of disorders that range from simple biochemical abnormalities to chronic asymptomatic tissue damage in midlife to serious life-threatening diseases in young subjects. Molecular studies have identified the systemic loop that controls iron homeostasis and is centered on the hepcidin-ferroportin interaction. The complexity of this regulatory pathway accounts for the genetic heterogeneity of hemochromatosis and related disorders and raises the possibility that genes encoding components of the pathway may be modifiers of the main genotype. Molecular diagnosis has improved the classification of the genetic conditions leading to iron overload and identified novel entities, characterized by both iron loading and variable degrees of anemia. Despite the progress in the diagnosis, classification, and mechanisms of iron overload disorders, the treatment of affected patients continues to rely on regular phlebotomy. Understanding the molecular circuitry of iron control may lead to the identification of potential therapeutic targets for novel treatment strategies to be used in association with or as an alternative to phlebotomy.