Iron Metabolism Overview: Proteins, Pathways, and General Pathology

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Iron metabolism overview: proteins, pathways and general pathology
Paul J. Schmidt, PhD
Department of Pathology, Boston Children’s Hospital and Harvard Medical
School, Boston, MA
[email protected]
Introduction
Although essential for most life on Earth, iron is highly toxic when present in
excess. As a result, iron uptake, distribution, and storage must be closely
regulated. Dysregulated iron metabolism may lead to tissue damage and is the
cause of a number of human pathological disorders.
The average adult contains approximately 4 grams of iron. Almost two-thirds of
this total amount is sequestered within hemoglobin of red blood cells (RBCs). It is
estimated that almost 200 billion RBC’s need to be replaced each day and nearly
25mg of iron is required to support this erythropoiesis. Reticuloendothelial
macrophages phagocytose senescent RBC’s, harvesting the heme and returning
20-25mg of iron into circulation each day. A small amount of iron enters the body
daily from the diet (1-2mg) and helps to compensate for the loss of iron through
bleeding or epithelial cell sloughing. There is no known means to regulate
organismal iron excretion; therefore, iron uptake is closely modulated to maintain
whole body iron balance. Insufficient iron leads to restricted erythropoiesis and
anemia while elevated iron uptake causes increased tissue loading and damage.
General iron homeostasis, and the pathophysiology of iron-related human
diseases,1-5 has been reviewed. Furthermore, a large number of animal models
have been generated to interrogate iron transport and regulation. 6
Cellular iron storage
One method for preventing iron-mediated cellular damage is to assure that the
metal is securely sequestered within the cell. Ferritin was the first iron protein
characterized and crystallized7 and all mammalian cells are known to produce
the protein. This iron storage protein is composed of light and heavy (L-ferritin
and H-ferritin) polypeptide chains. The folded protein forms a cage-like structure
that can hold an estimated 4500 iron atoms. Ferritin acts as an iron reservoir,
isolating excess iron in a safe storage site, and releasing the metal when needed
for cellular functions.8
Intracellular Iron Regulatory Proteins
In order to maintain appropriate cellular iron regulation, cells sense intracellular
iron levels and regulate the expression of iron binding and transport proteins. Iron
regulatory proteins (IRP’s) bind to untranslated mRNA sequences termed iron
regulatory elements (IRE’s) and modulate transcription of iron transport or
storage genes.9 The function of IRP’s is iron regulated. Under iron replete
conditions IRP1 contains a 4Fe-4S cluster and is unable to bind IRE’s, while
IRP2 undergoes proteosomal degradation.10,11 IRE’s found in the 5’ end of
mRNA encoding such proteins as ferritin and ferroportin (FPN1) are bound by
IRP’s under conditions of iron scarcity. This binding prevents translation of the
mRNA’s by blocking ribosomal assembly. Conversely, the binding of IRP’s to five
tandem IRE’s in the 3’ untranslated regions of transferrin receptor 1 (TFR1) or
divalent metal transporter (DMT1) greatly stabilizes the mRNA and allows for
elevated protein expression and increased iron transport.
Dietary Uptake and Distribution of Systemic Iron
The uptake of iron from the diet is strictly regulated to assure sufficient metal for
RBC production and prevent elevated tissue loading. Non-heme dietary iron
enters the body through enterocytes located in the first section of the large
intestine. Duodenal enterocytes reduce iron to the ferrous state at the apical
membrane and then transport it into the cell through divalent metal transporter 1
(DMT1).12-14 The basolateral transporter ferroportin15-17 transfers iron out of the
enterocyte where it is oxidized to the ferric state and binds transferrin (TF) in the
plasma. Holo-transferrin is the primary ligand for TFR1 and endocytosis of the
complex is the major entryway for iron into most cells. The STEAP family of
ferrireductases18 is thought to be required for appropriate intracellular
mobilization of iron taken up through the transferrin cycle.
Hepcidin is the Master Regulator of Iron Metabolism
Hepcidin is an iron-regulated, liver-expressed protein with antimicrobial
properties.19-21 The serum concentration of hepcidin is also modulated by
inflammation through Interleukin-6 (IL-6) activation of the STAT3 signaling
pathway.22,23 Importantly, hepcidin binds to ferroportin, the only known iron
exporter. This interaction leads to internalization and degradation of the
transporter in duodenal enterocytes and reticulodendothelial macrophages.24
Hepcidin expression is transcriptionally regulated by iron. As organismal iron
increases, serum hepcidin becomes elevated and decreases the amount of
ferroportin on cell surfaces. This leads to the sequestration of iron in
macrophages and diminished dietary iron uptake by duodenal enterocytes. The
converse it also true. As iron levels fall, hepcidin expression decreases and more
iron is released from the recycling compartment and enters the body from the
diet. Finch first postulated that the stores regulator of systemic iron metabolism
exists and is essential for the maintenance of appropriate iron balance.25 If the
stores regulator fails due to disruptions in the iron regulatory apparatus hepcidin
is inappropriately modulated, leading to elevated iron deposition and resulting in
hereditary hemochromatosis (HH).
It is also known that hepcidin expression is inhibited by anemia, hypoxia26 and in
situations of ineffective erythropoiesis. Certain anemias, such as -thalassemia
intermedia, are characterized by ineffective erythropoiesis. In this disease
hepcidin production is suppressed even in the presence of elevated iron
loading.27 Based on this and other observations it appears that the postulated
erythroid regulator25 is a more powerful hepcidin modulator than the stores
regulator. The erythroid regulator is hypothesized to relay a signal from the site of
erythropoiesis (bone marrow) to the liver and diminish hepcidin expression.
Erythropoiesis itself downregulates hepcidin expression and bone marrow
derived factors are required for this process.28,29 Growth differentiation factor 15
(GDF-15)30 has been postulated to be the erythroid regulator; however, recent
work suggests that GDF-15 may not be the mediator.31 Most recently,
erythroferrone (ERFE), a hormone mediating hepcidin suppression during early
stages of stress erythropoiesis, was discovered.32 Characterization of the
erythroid regulator, and its role in hepcidin regulation, is ongoing.
Regulators Of Hepcidin Expression
Hemojuvelin (HJV) plays a central role in the transcriptional regulation of
hepcidin. Loss of functional HJV leads to diminished hepcidin expression,
elevated iron uptake and disease.33-35 HJV is a member of the repulsive
guidance molecule family (RGM) and a bone morphogenetic protein (BMP) coreceptor.36 TMPRSS6, also known as Matriptase-2, is a liver serine protease.
Mutations in TMPRSS6 causes elevated hepcidin levels leading to iron
deficiency anemia.37-40 Current thought suggests that TMPRSS6 may function to
regulate HJV at the post-translational level by cleaving HJV from the cell
membrane and generating a soluble form (sHJV) of the protein.41,42
The HFE protein is an atypical major histocompatibility (MHC) class I-like
molecule that binds to TFR143. HFE shares a binding site on TFR1 with its
primary ligand TF.44,45 Mutations in HFE are the most common cause of
hereditary hemochromatosis.46 Furthermore, TFR2 is a homolog of TFR1
expressed predominantly in the liver.47 Loss of functional HFE or TFR2 results in
diminished hepcidin expression and excess iron loading. It has been
hypothesized that HFE and TFR2 act as sensors for the serum concentration of
TF-Fe2. In this model, HFE in the liver is normally sequestered by TFR1 under
conditions of low circulating iron; however, HFE can be displaced from TFR1
through an increase in TF saturation.48 It is uncertain how this mechanism
regulates hepcidin expression; however, work in cell culture demonstrated an
HFE/TFR2 interaction.49,50 Based on experiments with over-expressd proteins in
cell culture51,52 it had been hypothesized that HFE, TFR2 and HJV may form a
stable complex that functions to regulate hepcidin expression through HJV.
Further work is needed to clarify this iron sensing pathway.
Conclusion:
The understanding of systemic iron regulation, transport and storage has greatly
increased over the past 25 years. A significant number of iron transport and
regulatory proteins have been characterized and mutations in many of these
genes are known to result in dysregulated iron metabolism and disease.
Translational work is underway seeking to harness this knowledge and produce
therapeutic strategies applicable to human disease conditions caused by
inappropriate hepcidin regulation.
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