Iron normally enters the body through the gastrointestinal tract, mostly through the enterocytes of the duodenum. The amount of iron absorbed is normally tightly regulated according to body needs. Active erythropoiesis and/or iron deficiency increase absorption; iron overload and systemic inflammation decrease absorption. Nevertheless, the amount of iron absorbed increases with the administered dose even though the percentage absorbed decreases (Fig. 42-1). Accidental or deliberate ingestion of large doses of medicinal iron can therefore cause iron intoxication.
The relationship between oral iron dosage and amount of iron absorbed in humans. When the logarithm of the dose is plotted against the logarithm of the amount of iron absorbed, a rectilinear relationship is observed. Thus, at all levels, the greater the dose of iron, the more is absorbed, although the percent of the dose that is absorbed progressively declines. (Reproduced with permission from Mackenzie B, Garrick MD: Iron Imports. II. Iron uptake at the apical membrane in the intestine. Am J Physiol Gastrointest Liver Physiol 289(6):G981–G986, 2005.)
MECHANISM OF TRANSPORT ACROSS THE INTESTINAL MUCOSA
Understanding the mechanism of iron absorption has been made more difficult by the fact that the pathways for the uptake of inorganic iron and of heme by enterocytes are different but seem to merge within the intestinal cell where heme is converted to inorganic iron. How much heme (if any) is exported intact by enterocytes and bound by plasma heme-binding protein hemopexin is not clear, but hemopexin knockout mice show minor retention of iron in duodenal enterocytes without any effect on systemic iron homeostasis,21 arguing against a major contribution from this mechanism, at least in mice. Efforts to identify the apical heme import mechanism in enterocytes have not yet been definitive.22
Following the reduction of ferric iron to ferrous iron, in part by duodenal cytochrome b (dcytb) reductase,23 ferrous iron is transported into the intestinal villus cell by the divalent metal transporter (DMT)-1.24,25 How iron transits within the enterocytes is not yet known. Basolateral export of ferrous iron is mediated by ferroportin26,27,28 in association with hephaestin29 and plasma ceruloplasmin30 to oxidize iron to the ferric state. Ferric iron is taken up by plasma apotransferrin. Figure 42–2 illustrates some of the steps that are thought to regulate iron transport across the mucosal cell.
Schematic of iron uptake from the intestine and transfer to the plasma by an intestinal villus cell. Nonheme dietary iron includes Fe(II) and Fe(III) salts and organic complexes. Fe3+ is reduced to Fe2+ by ascorbic acid and apical membrane ferrireductases that include duodenal cytochrome b (dcytb). The acid microclimate at the brush-border provides an H+ electrochemical potential gradient to drive transport of Fe2+ via the divalent metal-ion transporter (DMT-1) into the enterocyte. DMT-1 may also contribute to the absorption of other nutritionally important metal ions (e.g., Mn2+). Heme can be taken up by endocytosis, and Fe2+ is liberated within the endosome/lysosome, but the molecular identity of proteins involved, including heme carrier protein 1 (HCP1), is yet to be elucidated. Basolateral export of Fe2 may be mediated by ferroportin in association with hephaestin. Fe2Tf, diferric transferrin; HO, heme oxygenase. (Data from Smith MD, Pannacciulli IM: Absorption of inorganic iron from graded doses: its significance in relation to iron absorption tests and mucosal block theory. Br J Haematol 4(4):428–434, 1958.)
Role of the Monocyte–Macrophage System
In humans, the destruction and production of erythrocytes generates most of the iron flux in and out of plasma (20 to 25 mg/day recycled in adults compared to 1 to 2 mg/day absorbed). Iron from other cell types is likely also recycled, but this source contributes little to iron flux and has not been studied. Destruction of aged erythrocytes and hemoglobin degradation occur within macrophages (Chap. 32). This proceeds at a rate sufficient to release approximately 20 percent of the hemoglobin iron from the cell to the plasma compartment within a few hours. Approximately 80 percent of this iron is rapidly reincorporated into hemoglobin. Thus, 20 to 70 percent of the hemoglobin iron of nonviable erythrocytes reappears in circulating red cells in 12 days. The remainder of the iron enters the storage pool as ferritin or hemosiderin and then turns over very slowly. In normal subjects, approximately 40 percent of this iron remains in storage after 140 days. When there is an increased iron demand for hemoglobin synthesis, however, storage iron may be mobilized more rapidly.31 Conversely, in the presence of infection or another inflammatory process (e.g., ulcerative colitis or malignancy), iron is more slowly reused in hemoglobin synthesis and is associated with anemia (Chap. 37).32,33
As human erythrocytes age during their average 120-day life span, they shrink, stiffen, and their membranes accumulate markers of senescence.34 These changes eventually trigger phagocytosis by splenic or hepatic sinusoidal macrophages. Macrophages also take up the products of intravascular hemolysis, including hemoglobin (bound by haptoglobin) and heme (bound by hemopexin), using specific endocytic receptors for the complexes.35 The vesicles involved in phagocytosis and endocytosis must fuse with lysosomes to digest cellular materials or protein complexes and to free heme from hemoglobin. The membrane complex of nicotinamide adenine dinucleotide phosphate (NADPH) cytochrome c reductase, heme oxygenase 1, and biliverdin reductase releases ferrous iron from heme and simultaneously protects erythrophagocytosing macrophages from heme-induced toxicity.36 The subcellular location of the conversion of heme to iron is not known with certainty. Heme oxygenase 1 is mostly located in the endoplasmic reticulum in erythrophagocytic macrophages37 with the catalytic face in the cytosol, and little, if any, heme oxygenase in the phagosomal membrane. Moreover, the phagosomal membrane is enriched in the heme transporter HRG1,38 and macrophage heme has a signaling role in inducing various proteins involved in macrophage iron metabolism, indicating that it may leave the phagosome, and the heme oxygenase-1–mediated release of iron may occur in the cytoplasm. However, the ferrous iron transporter Nramp1, and perhaps DMT-1, may also participate in subcellular iron transport.39 Ultimately, depending on systemic iron requirements, the released ferrous iron is either exported to plasma via ferroportin40 or trapped in macrophage cytoplasmic ferritin. By a mechanism potentially important at the low oxygen tensions found in some tissues, plasma ceruloplasmin41,42,43 catalyzes the conversion of ferrous to ferric iron, the form of iron loaded to plasma transferrin for systemic distribution.
SYSTEMIC IRON HOMEOSTASIS
The mechanism by which body iron content is regulated by the modulation of iron absorption has been a subject of intense interest for the past 65 years. It has now become clear that intestinal iron absorption, plasma iron concentrations, and tissue distribution of iron are subject to endocrine regulation similar to that of other simple nutrients, for example, glucose or calcium, albeit in a somewhat more complex fashion.
Hepcidin, a 25-amino-acid peptide hormone with 4 disulfide bonds,44,45,46,47 is produced predominantly by hepatocytes and plays a central role in systemic iron homeostasis. Hepcidin regulates plasma iron concentrations by controlling the absorption of iron by the intestinal epithelial enterocytes and its release from iron-recycling macrophages and hepatocytes involved in iron storage. The structural similarity of hepcidin and a class of antimicrobial peptides termed defensins suggests that the hormone evolved from the latter to modulate iron homeostasis as a mechanism of body defense against microorganisms. Overexpression of hepcidin results in marked iron-deficiency anemia in mice48 and a refractory anemia resembling the anemia of chronic inflammation in humans,49 and injection of synthetic hepcidin rapidly lowers plasma iron concentrations.50 As many microorganisms are dependent on plasma iron for survival in the circulation, hepcidin can exert host defense. In fact, patients with iron overload and high plasma iron levels are susceptible to such infections, such as with Yersinia enterocolitica (Chap. 43).
Hepcidin exerts its iron-regulatory effect by binding to ferroportin, a transmembrane iron-export protein expressed on enterocytes, macrophages, and hepatocytes. Once hepcidin has bound to ferroportin, the ferroportin is internalized and undergoes proteolysis.40,51 With membrane ferroportin depleted, iron cannot be exported from the enterocyte, the macrophage or the hepatocyte into the plasma (Fig. 42–3). This results in decreased iron absorption from the gastrointestinal tract and a fall in the plasma iron concentration. Hepcidin production is stimulated by inflammatory cytokines such as interleukin (IL)-6,52,53 and the overproduction of hepcidin is one of the factors in the pathogenesis of the anemia of chronic inflammation (Chap. 37).
Regulation of iron flows into plasma by hepcidin. Ferroportin is the only known transporter that exports iron from cells to plasma (and extracellular fluid). Hepcidin induces ferroportin endocytosis and proteolysis and thereby controls the transfer of iron to plasma from all its major sources: iron-absorbing duodenal enterocytes, iron-storing hepatocytes, and iron-recycling macrophages.
The regulation of hepcidin production seems to be entirely transcriptional. In humans and laboratory rodents, hepcidin mRNA and plasma hepcidin levels increase in parallel with iron-loading and inflammatory stimuli,44,54,55 and are decreased by erythropoietic activity56 and iron deficiency.57
Regulation of Hepcidin by Iron
Both elevated plasma iron concentrations and increased liver stores are sensed in the intact organism and regulate hepcidin transcription,58,59 but the relevant mechanisms are only partially understood. For reasons that are not understood, involving perhaps the complex interactions of hepatocytes with other liver cells, isolated hepatocytes do not show consistently increased hepcidin synthesis after iron treatment, although small effects were observed when the cells were freshly harvested from mice.60 Important clues are provided by hereditary disorders in which hepcidin transcription is dysregulated. As indicated in Table 42–3, impairment of the function of several genes is associated with iron overload in humans and in experimental animals. In addition to genes that encode the hormone hepcidin itself and its receptor, ferroportin, or encode proteins primarily involved in iron transport, there are a number of genes whose products are likely to function in iron sensing, signal transduction and transcriptional regulation. These include human hemochromatosis protein (HFE), transferrin receptor-2, bone morphogenetic proteins (BMPs), BMP receptor and its signaling pathway, and hemojuvelin, all of which encode proteins that normally stimulate hepcidin transcription to prevent iron overload. In the best-supported model, hepcidin transcription is regulated in an iron-dependent manner by the BMP pathway. Complexes of HFE, transferrin receptor-1, and transferrin receptor-2 may be involved in sensing the concentration of iron-transferrin and interact in as yet unknown manner with the BMP receptor to stimulate the transcription of hepcidin.61,62,63,64 Hemojuvelin, whose autosomal recessive mutations cause a very severe form of hereditary hemochromatosis, serves as a coreceptor for the BMPs.65,66 A soluble fragment of hemojuvelin acts as an inhibitor of the interaction of BMP with the receptor, but it is not clear whether it has a physiologic regulatory role.67,68 Regulation of hepcidin transcription itself is complex, involving the formation of a complex of liver-specific and response-specific transcription factors bound to a distal BMP-RE2/bZIP/HNF4α/COUP region and to the proximal BMP-RE1/STAT region of the hepcidin promoter, possibly by physical association of the two regions.69 A pathway that inhibits the transcription of hepcidin exists as well. Tmprss6 (also called matriptase 2) is a membrane serine protease that inhibits hepcidin transcription, likely by proteolysis of hemojuvelin.70,71 This function was discovered when random mutagenesis in mice produced an iron-deficient animal with mutagenized Tmprss6.72 Subsequently, humans with mutations of the Tmprss6 orthologue were shown to manifest iron-refractory iron-deficiency anemia that does not respond to oral iron and only partially to parenteral iron therapy.49
Table 42–3.Proteins That Play a Role in Iron Homeostasis in Humans or in Animal Models ||Download (.pdf) Table 42–3. Proteins That Play a Role in Iron Homeostasis in Humans or in Animal Models
|Proteins That Affect Iron Homeostasis ||Effect of Deficiency or Mutation ||References to Human Data ||References to Murine Data ||Comments |
|HFE ||Parenchymal Fe increased ||94 ||95, 96 ||Most patients with hereditary hemochromatosis are homozygous for the 845 A→G (C282Y) mutation of this gene. In signaling pathway to hepcidin |
|Ferroportin (SLC40A1, SLC11A3) ||Macrophage Fe increased (loss of function) ||97 ||98 ||Autosomal dominant, hepcidin receptor, cellular iron exporter |
|Parenchymal Fe increased (resistance to hepcidin) ||99, 100 ||101 ||Autosomal dominant |
|β2-Microglobulin ||Parenchymal Fe increased ||Unknown ||102, 103 ||Facilitates transport of HFE to membrane |
|Transferrin ||Parenchymal Fe increased ||104–106 ||107, 108 ||Plasma iron transporter, holotransferrin concentrations regulate hepcidin |
|Transferrin receptor-1 ||Lethal; increased CNS Fe ||Unknown ||109 ||Mediates cellular iron uptake, essential for erythropoiesis, may be involved in signaling for hepcidin regulation |
|Transferrin receptor-2 ||Parenchymal Fe increased ||84, 110 ||111 ||Signaling for hepcidin regulation |
|Hephaestin ||Fe deficiency ||Unknown ||29 ||Sex-linked gene; deletion of exons is cause of sla mouse |
|Ceruloplasmin ||Fe increased ||112 ||42 ||Brain iron accumulation and neurologic disease |
|Ferritin H chain ||Fe increased ||113 ||Unknown ||Dominant IRE mutation |
|Duodenal cytochrome b (dcytb) ||Unknown ||Unknown ||23 ||Mild iron restriction under erythropoietic stress |
|Nramp1 (SLC11A1) ||Alters iron distribution in macrophages ||Unknown ||39 ||Deficiency increases susceptibility to infection in mice |
|Nramp2 (DMT-1) ||Hypochromic microcytic anemia and hepatic siderosis in people; Fe deficiency in rodents ||114, 115 ||116, 117 ||Anemia is ameliorated by erythropoietin therapy in humans; same naturally occurring mutations found in the mk mouse and the Belgrade rat |
|Hepcidin ||Parenchymal Fe Increased ||118 ||46, 119 ||The hormone-regulating iron absorption, plasma iron concentration, and systemic distribution |
|Hemojuvelin ||Parenchymal Fe increased ||65 ||120, 121 ||Signaling for hepcidin regulation |
|Tmprss6 ||Fe deficiency ||49 ||70, 72 ||Signaling for hepcidin regulation, membrane protease, cleaves hemojuvelin |
|BMP6 ||Parenchymal Fe increased ||Unknown ||122, 123 ||Necessary for iron regulation in mice |
|BMP receptor subunit ||Parenchymal Fe increased ||Unknown ||124 ||Necessary for iron regulation in mice |
|SMAD4 in the liver ||Parenchymal Fe increased ||Unknown ||125 ||In signaling pathway for hepcidin regulation |
|Neogenin ||Parenchymal Fe increased ||Unknown ||126, 127 ||Necessary for hepcidin regulation |
Regulation of Hepcidin by Erythropoiesis
Intestinal iron absorption is increased severalfold after hemorrhage or erythropoietin administration, and is chronically increased in patients with ineffective erythropoiesis but not in aplastic anemia.73 These observations led to the hypothesis that the marrow generates an “erythroid regulator”73 that modulates intestinal iron absorption. Later studies in mouse models56 provided evidence that the erythroid regulator is a marrow-derived suppressor of hepcidin. Erythroferrone is an erythropoietin-induced erythroblast-secreted glycoprotein that acts on hepatocytes to suppress their hepcidin production and is required for rapid suppression of hepcidin after hemorrhage or erythropoietin administration.74 It also contributes to hepcidin suppression and iron overload in murine models of β-thalassemia intermedia. Growth differentiation factor 15 (GDF15), a member of the BMP family, may also contribute to pathologic hepcidin suppression in anemias with ineffective erythropoiesis.75
Regulation of Hepcidin by Inflammation
Within hours after the onset of systemic infection, plasma iron concentration decreases. The response is thought to contribute to host defense, particularly against microbes with high dependence on environmental iron.76 This response, hypoferremia of inflammation, is also triggered by noninfectious causes of acute and chronic inflammation. Hypoferremia of inflammation is mediated by cytokine-induced increase in plasma hepcidin concentrations54 causing hepcidin-induced sequestration of iron in macrophages. The main human cytokine responsible for hepcidin induction is IL-652,53 acting via the JAK2-STAT3 pathway,77,78,79 but other cytokines including activin B may also contribute.80 Chronic inflammation impairs iron supply to erythropoiesis and combines with other effects of inflammation to cause anemia of inflammation (anemia of chronic disease, see Chap. 37).