Intestinal ferritin H is required for an accurate control of iron absorption

Knockout mouse models of iron homeostasis

Murine models have made valuable contributions to our understanding of iron metabolism. Investigation of mice with inherited forms of anemia has led to the discovery of novel proteins involved in iron homeostasis. A growing number of murine models are being developed to investigate mitochondrial iron metabolism. Mouse strains are available for the major forms of hereditary hemochromatosis. Findings in murine models support the concept that the pathogenesis of nearly all forms of hereditary hemochromatosis involves inappropriately low expression of hepcidin. The availability of mice with floxed iron-related genes allows the study of the in vivo consequences of cell-selective deletion of these genes.

Regulation of cellular iron metabolism

Iron is an essential but potentially hazardous biometal. Mammalian cells require sufficient amounts of iron to satisfy metabolic needs or to accomplish specialized functions. Iron is delivered to tissues by circulating transferrin, a transporter that captures iron released into the plasma mainly from intestinal enterocytes or reticuloendothelial macrophages. The binding of iron-laden transferrin to the cell-surface transferrin receptor 1 results in endocytosis and uptake of the metal cargo. Internalized iron is transported to mitochondria for the synthesis of haem or iron–sulfur clusters, which are integral parts of several metalloproteins, and excess iron is stored and detoxified in cytosolic ferritin. Iron metabolism is controlled at different levels and by diverse mechanisms. The present review summarizes basic concepts of iron transport, use and storage and focuses on the IRE (iron-responsive element)/IRP (iron-regulatory protein) system, a well known post-transcriptional regulatory circuit that not only maintains iron homoeostasis in various cell types, but also contributes to systemic iron balance.

A possible role for secreted ferritin in tissue iron distribution

Ferritin is known as a well-conserved iron detoxification and storage protein that is found in the cytosol of many prokaryotic and eukaryotic organisms. In insects and worms, ferritin has evolved into a classically secreted protein that transports iron systemically. Mammalian ferritins are found intracellularly in the cytosol, as well as in the nucleus, the endo-lysosomal compartment and the mitochondria. Extracellular ferritin is found in fluids such as serum and synovial and cerebrospinal fluids. We recently characterized the biophysical properties, secretion mechanism and cellular origin of mouse serum ferritin, which is actively secreted by a non-classical pathway involving lysosomal processing. Here, we review the data to support a hypothesis that intracellular and extracellular ferritin may play a role in intra- and intercellular redistribution of iron.

Unraveling mechanisms regulating systemic iron homeostasis

Systemic iron balance must be tightly regulated to prevent the deleterious effects of iron deficiency and iron overload. Hepcidin, a circulating hormone that is synthesized by the liver, has emerged as a key regulator of systemic iron homeostasis. Hepcidin inhibits the absorption of dietary iron from the intestine and the release of iron derived from red blood cells from macrophages. Therefore, variation in hepcidin levels modifies the total amount of iron stored in the body and the availability of iron for erythropoiesis. The production of hepcidin by the liver is modulated by multiple physiological stimuli, including iron loading, inflammation, and erythropoietic activity. Investigation of the functions of the gene products mutated in inherited iron disorders using tissue-culture systems and animal models has provided valuable insights into the mechanisms by which these hepcidin responses are mediated. This review focuses on recent advances in our understanding of the molecular mechanisms underlying the regulation of systemic iron homeostasis.

Ferrit(in)ing out new mechanisms in iron homeostasis

Exquisite control of intestinal iron absorption prevents iron deficiency and toxic iron overload. Absorption is modulated by a circulating hormone, hepcidin, which inactivates the iron transporter ferroportin. In this issue of Cell Metabolism, Vanoaica and colleagues show that absorption is also regulated within the intestinal epithelium, through production of the iron-sequestering protein H-ferritin.