Hepcidin expression inversely correlates with the expression of duodenal iron transporters and iron absorption in rats

Mechanisms of Disease: the role of hepcidin in iron homeostasis—implications for hemochromatosis and other disorders

The defensin-like circulatory peptide hepcidin is the iron-regulatory hormone that links innate immunity and iron metabolism. In response to inflammatory stimuli, the liver produces hepcidin: this antimicrobial peptide then limits the iron that is vital to invading pathogens, by decreasing iron release/transfer from enterocytes and macrophages and causing secondary hypoferremia. This may lead, however, to reduced iron availability for erythropoiesis and therefore to anemia (and anemia of chronic disease). When iron is scarce, the rate at which it is released into the bloodstream must be enhanced: indeed, iron starvation and hypoxia readily abrogate hepcidin expression. Conversely, if excess iron enters the circulation, hepcidin transcription is turned on and iron release from the intestine and macrophages abrogated. Circumstantial evidence indicates that the effect of circulatory iron on hepcidin requires functional HFE and hemojuvelin, two proteins of unknown function that have recently been linked to human hereditary hemochromatosis. In this disease it is likely that inadequate levels of circulating hepcidin lead to the uncontrolled release of iron from the intestine and macrophages, followed by tissue iron overload and organ damage. Given its role as the iron-regulatory hormone, the modulation of hepcidin activity using agonists or antagonists might offer new treatment opportunities in different human iron-dependent disorders.

Pathogenesis of hereditary hemochromatosis

Hereditary hemochromatosis comprises several inherited disorders of iron homeostasis characterized by increased gastrointestinal iron absorpstion and resultant tissue iron deposition. The identification of HFE and other genes involved in iron metabolism has greatly expanded our understanding of hereditary hemochromatosis. Two major hypotheses have been proposed to explain the pathogenesis of HFE-related hereditary hemochromatosis: the hepcidin hypothesis and the duodenal crypt cell programming hypothesis.

Tumour necrosis factor alpha regulates iron transport and transporter expression in human intestinal epithelial cells

TNFalpha has dramatic effects on iron metabolism contributing to the generation of hypoferraemia in the anaemia of chronic disease. Interestingly, TNFalpha is also synthesised and released within the intestinal mucosa, suggesting that this pro-inflammatory cytokine may play a role in regulating dietary iron absorption. To investigate this possibility, we stimulated intestinal Caco-2 cells with TNFalpha (10 ng/ml). In TNFalpha-treated cells, apical iron uptake was significantly decreased and this was accompanied by a reduction in divalent metal transporter protein and mRNA expression. Our data suggest that TNFalpha could regulate dietary iron absorption and that the apical transport machinery is the target for these actions.

Tissue-specific changes in iron metabolism genes in mice following phenylhydrazine-induced haemolysis

Iron metabolism in animals is altered by haemolytic anaemia induced by phenylhydrazine (PHZ). In common with a number of other modulators of iron metabolism, the mode and the mechanisms of this response are yet to be determined. However, recent studies have shown increased expression of the ferrous transporter DMT1 in the duodenum and other tissues of mice administered PHZ. We examined the expression of the ferric reductase Dcytb, DMT1 and some other genes involved in Fe metabolism in tissues of mice dosed with PHZ. The expression of iron-related genes in the duodenum, liver, and spleen of the mice were evaluated using Northern blot analyses, RT-PCR and immunocytochemistry. Dcytb, and DMT1 mRNA and protein increased markedly in the duodenum of mice given PHZ. The efflux protein Ireg1 also increased in the duodenum of the treated mice. These changes correlated with a decrease in hepatic hepcidin expression. Dcytb, DMT1, Ireg1 and transferrin receptor 1 mRNA expression in the spleen and liver of mice treated with PHZ responded to the enhanced iron demand associated with the resulting stimulation of erythropoiesis. Enhanced iron absorption observed in PHZ-treated animals is facilitated by the up-regulation of the genes involved in iron transport and recycling. The probable association of the erythroid and the store regulators of iron homeostasis and absorption in the mice is discussed.

Iron in ferritin or in salts (ferrous sulfate) is equally bioavailable in nonanemic women

BACKGROUND

Recent studies in humans suggest that ferritin iron in soybeans has high bioavailability. However, direct evidence for this is lacking because the soybeans were intrinsically labeled; thus, iron bound to other ligands, such as phytate, was also labeled.

OBJECTIVE

The objectives of the study were to evaluate the absorption of iron from extrinsically labeled, purified ferritin (horse spleen) reconstituted with either high-phosphate iron mineral (plant-type) or low-phosphate iron mineral (animal-type) and to compare it with iron absorption from ferrous sulfate.

DESIGN

Nonanemic, healthy young women were fed a standard breakfast meal supplemented with (59)Fe-labeled ferritin or ferrous sulfate, in randomized order. Fifteen subjects received ferritin with the low-phosphate iron mineral, and 15 subjects received ferritin with the high-phosphate iron mineral. Iron absorption was measured in a whole-body counter after 14 and 28 d and by red blood cell incorporation after 28 d.

RESULTS

There was no significant difference in iron absorption between ferritin and ferrous sulfate: low-phosphate iron mineral ferritin (x +/- SD: 21.4 +/- 14.7%) compared with ferrous sulfate (21.9 +/- 14.6%), or high-phosphate iron mineral ferritin (22.2 +/- 19.2%) compared with ferrous sulfate (16.7 +/- 7.1%). Results obtained by using whole-body retention of iron and red blood cell incorporation differed with the type of iron, which suggests that pathways for iron uptake and utilization differed for the 2 forms.

CONCLUSIONS

Iron is equally well absorbed from ferritin and ferrous sulfate independent of the phosphate content of the ferritin iron mineral. Thus, dietary ferritin iron is likely to be a good source of iron.

Diferric transferrin regulates transferrin receptor 2 protein stability

Transferrin receptor 2 (TfR2) is a type 2 transmembrane protein expressed in hepatocytes that binds iron-bound transferrin (Tf). Mutations in TfR2 cause one form of hereditary hemochromatosis, a disease in which excessive absorption of dietary iron can lead to liver cirrhosis, diabetes, arthritis, and heart failure. The function of TfR2 in iron homeostasis is unknown. We have studied the regulation of TfR2 in HepG2 cells. Western blot analysis shows that TfR2 increases in a time- and dose-dependent manner after diferric Tf is added to the culture medium. In cells exposed to diferric Tf, the amount of TfR2 returns to control levels within 8 hours after the removal of diferric Tf from the medium. However, TfR2 does not increase when non-Tf-bound iron (FeNTA) or apo Tf is added to the medium. The response to diferric Tf appears to be hepatocyte specific. Real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis shows that TfR2 mRNA levels do not change in cells exposed to diferric Tf. Rather, the increase in TfR2 is attributed to an increase in the half-life of TfR2 protein in cells exposed to diferric Tf. Our results support a role for TfR2 in monitoring iron levels by sensing changes in the concentration of diferric Tf.

The effects of inhibition of haem biosynthesis by griseofulvin on intestinal iron absorption

The relationship between haem biosynthesis and intestinal iron absorption in mice was investigated by ascertaining the effect of the haem synthesis inhibitor, griseofulvin, on duodenal iron absorption using both in vivo and in vitro measurements. Urinary 5-aminolaevulinic acid levels were increased within 24 hr of feeding mice with griseofulvin diet (2.5% w/w), with more marked increases seen after 3-7 days. Urinary porphobilinogen levels also showed a similar trend. In vivo intestinal iron absorption was significantly reduced (P<0.05) in experimental mice, mainly due to reduction in the transfer of 59Fe from the enterocytes to the portal circulation. In vitro studies using isolated duodenal fragments also exhibited marked decreases in both iron uptake and Fe (III) reduction. Changes in mucosal Divalent Metal Transporter 1 (DMT-1), Dcytb and Ireg1 (iron regulated protein 1) mRNA levels paralleled the changes in iron absorption. The reduction in iron absorption after griseofulvin treatment was normalised when mice were simultaneously injected with haem-arginate. These data support the hypothesis that intermediates in haem biosynthesis, particularly 5-aminolaevulinic acid, regulate intestinal iron absorption.

Lessons learned from genetic and nutritional iron deficiencies

A recent study of mice with sex-linked anemia compared differences between genetic and nutritional iron deficiencies. Comparison of these models helps to illuminate how the body regulates dietary iron absorption at the molecular level.

Inhibition of iron transport across human intestinal epithelial cells by hepcidin

We investigated the effects of the iron regulatory peptide hepcidin on iron transport by the human intestinal epithelial Caco-2 cell line. Caco-2 cells were exposed to hepcidin for 24 hours prior to the measurement of both iron transport and transporter protein and mRNA expression. Incubation with hepcidin significantly decreased apical iron uptake by Caco-2 cells. This was accompanied by a decrease in both the protein and the mRNA expression of the iron-response element containing variant of the divalent metal transporter (DMT1[+IRE]). In contrast, iron efflux and iron-regulated gene1 (IREG1) expression were unaffected by hepcidin. Hepcidin interacts directly with a model intestinal epithelium. The primary effect of this regulatory peptide is to modulate the apical membrane uptake machinery, thereby controlling the amount of iron absorbed from the diet.

Adaptive changes of duodenal iron transport proteins in celiac disease

Iron deficiency is a manifestation of celiac disease (CD) usually attributed to a decreased absorptive surface, although no data on the regulation of iron transport under these conditions are currently available. Our aim was to evaluate divalent metal transporter 1 (DMT1), duodenal cytochrome b (Dcytb), ferroportin 1 (FP1), hephaestin, and transferrin receptor 1 (TfR1) expression, as well as iron regulatory protein (IRP) activity in duodenal biopsies from control, anemic, and CD patients. We studied 10 subjects with dyspepsia, 6 with iron-deficiency anemia, and 25 with CD. mRNA levels were determined by real-time PCR, protein expression by Western blotting or immunohistochemistry, and IRP activity by gel shift assay. Our results showed that DMT1, FP1, hephaestin, and TfR1 mRNA levels were significantly increased in CD patients with reduced body iron stores compared with controls, similar to what was observed in anemic patients. Protein expression paralleled the mRNAs changes. DMT1 protein expression was localized in differentiated enterocytes at the villi tips in controls, whereas with iron deficiency it was observed throughout the villi. FP1 expression was localized on the basolateral membrane of enterocytes and increased with low iron stores. TfR1 was localized in the crypts in controls but also in the villi with iron deficiency. These changes were paralleled by IRP activity, which increased in all iron-deficient subjects. We conclude that duodenal DMT1, FP1, hephaestin, and TfR1 expression and IRP activity, thus the iron absorption capacity, are upregulated in CD patients as a consequence of iron deficiency, whereas the increased enterocyte proliferation observed in CD has no effect on iron uptake regulation.

Divalent Metal Transporter 1 in Lead and Cadmium Transport

The effect of exposure to cadmium (Cd) and lead (Pb) on human health has been recognized for many years and recent information suggests that minimal exposure levels are themselves too high. Common scenarios for Pb exposure include occupational, residential, and/or behavioral (hand-to-mouth activity) settings. The main source of Cd exposure for nonsmokers is dietary, through plants or animals that accumulate the metal. Specific cellular importers for Pb and Cd are unlikely as these metals are nonessential and toxic. Accordingly, in the intestine, the operational mechanism is assumed to be inadvertent uptake through pathways intended for essential nutrients such as iron. Results from experimental and epidemiological studies indicated that diets low in iron (Fe) result in increased absorption of Pb and Cd, suggesting common molecular mechanisms of Cd and Pb transport. Indeed, recent mechanistic studies found that the intestinal transporter for nonheme iron, divalent metal transporter 1 (DMT1), mediates the transport of Pb and Cd. DMT1 is regulated, in part, by dietary iron, and chemical species of Cd and Pb that are transported by DMT1 would be made available through digestion and are also found in plasma. Accordingly, the involvement of DMT1 in metal uptake offers a mechanistic explanation for why an iron-deficient diet is a risk factor for Pb and Cd poisoning. It also suggests that diets rich in iron-containing food could be protective against heavy metal poisoning.

Hepcidin regulation of ferroportin 1 expression in the liver and intestine of the rat

Hepcidin has been implicated as the iron stores regulator: a hepatic signaling molecule that regulates intestinal iron absorption by undefined mechanisms. The possibility that hepcidin regulates the expression of ferroportin 1 (FPT1), the basolateral iron transporter, was examined in rats after administration of LPS, an iron chelator, or His-tagged recombinant hepcidin (His-rHepc). In the liver, LPS stimulated a biphasic increase of hepcidin mRNA with peaks of mRNA at 6 and 36 h. Concurrently, hepatic FPT1 mRNA expression decreased to minimal level at 6 h and then increased with a peak at 24-36 h. LPS also induced biphasic changes in intestinal FPT1 mRNA expression, with decreased levels at 6 h and increased expression at 48 h. Whereas the initial decrease of FPT1 coincides with an LPS-induced decrease in serum iron, both intestinal and hepatic FPT1 expression recovered, whereas serum iron concentration continued to decrease for at least 24 h. Dietary iron ingestion increased intestinal ferritin protein production but did not reduce intestinal FPT1 mRNA expression. The iron chelator pyrrolidinedithiocarbamate (PDTC) stimulated hepatic hepcidin without suppressing intestinal FPT1 expression. In PDTC-treated rats, LPS stimulated no additional hepatic hepcidin expression but did increase intestinal FPT1 expression. Administration of HisrHepc induced significant reduction of intestinal FPT1 expression. Taken together, these data suggest that hepcidin mediates LPS-induced downregulation of intestinal FPT1 expression and that the hepcidin signaling pathway involves a PDTC-sensitive step.

Nutrient absorption

Our understanding of nutrient absorption continues to grow, from the development of unique animal models and from studies in which cutting-edge molecular and cellular biologic approaches have been used to analyze the structure and function of relevant molecules. Studies of the molecular genetics of inherited disorders have also provided many new insights into these processes. A major advance in lipid absorption has been the cloning and characterization of several intestinal acyl CoA:monoacylglycerol acyltransferases; these may provide new targets for antiobesity drug therapy. Studies of intestinal cholesterol absorption and reverse cholesterol transport have encouraged the development of novel potential treatments for hyperlipidemia. Observations in genetically modified mice and in humans with mutations in glucose transporter 2 suggest the importance of a separate microsomal membrane transport pathway for glucose transport. The study of iron metabolism has advanced greatly with the identification of the hemochromatosis gene and the continued examination of the genetic regulation of iron absorptive pathways. Several human thiamine transporters have been identified, and their specific roles in different tissues are being explored.

Increased duodenal expression of divalent metal transporter 1 and iron-regulated gene 1 in cirrhosis

Hepatic hemosiderosis and increased iron absorption are common findings in cirrhosis. It has been proposed that a positive relation exists between intestinal iron absorption and the development of hepatic hemosiderosis. The current study investigated the duodenal expression of the iron transport molecules divalent metal transporter 1 (DMT1 [IRE]), iron-regulated gene 1 (Ireg1 [ferroportin]), hephaestin, and duodenal cytochrome b (Dyctb) in 46 patients with cirrhosis and 20 control subjects. Total RNA samples were extracted from duodenal biopsy samples and the expression of the iron transport genes was assessed by ribonuclease protection assays. Expression of DMT1 and Ireg1 was increased 1.5 to 3-fold in subjects with cirrhosis compared with iron-replete control subjects. The presence of cirrhosis per se and serum ferritin (SF) concentration were independent factors that influenced the expression of DMT1. However, only SF concentration was independently associated with Ireg1 expression. In cirrhosis, the expression of DMT1 and Ireg1 was not related to the severity of liver disease or cirrhosis type. There was no correlation between the duodenal expression of DMT1 and Ireg1 and the degree of hepatic siderosis. In conclusion, the presence of cirrhosis is an independent factor associated with increased expression of DMT1 but not Ireg1. The mechanism by which cirrhosis mediates this change in DMT1 expression has yet to be determined. Increased expression of DMT1 may play an important role in the pathogenesis of cirrhosis-associated hepatic iron overload.

Effect of hepcidin on intestinal iron absorption in mice

The effect of the putative iron regulatory peptide hepcidin on iron absorption was investigated in mice. Hepcidin peptide was synthesized and injected into mice for up to 3 days, and in vivo iron absorption was measured with tied-off segments of duodenum. Liver hepcidin expression was measured by reverse transcriptase-polymerase chain reaction. Hepcidin significantly reduced mucosal iron uptake and transfer to the carcass at doses of at least 10 microg/mouse per day, the reduction in transfer to the carcass being proportional to the reduction in iron uptake. Synthetic hepcidin injections down-regulated endogenous liver hepcidin expression excluding the possibility that synthetic hepcidin was functioning by a secondary induction of endogenous hepcidin. The effect of hepcidin was significant at least 24 hours after injection of hepcidin. Liver iron stores and hemoglobin levels were unaffected by hepcidin injection. Similar effects of hepcidin on iron absorption were seen in iron-deficient and Hfe knockout mice. Hepcidin inhibited the uptake step of duodenal iron absorption but did not affect the proportion of iron transferred to the circulation. The effect was independent of iron status of mice and did not require Hfe gene product. The data support a key role for hepcidin in the regulation of intestinal iron uptake.

Hepcidin, the recently identified peptide that appears to regulate iron absorption

A newly identified iron regulator, hepcidin, appears to communicate body iron status and demand for erythropoiesis to the intestine, and in turn, modulates intestinal iron absorption. Hepcidin was first purified from human blood and urine as an antimicrobial peptide and was found to be predominantly expressed in the liver. A lack of hepcidin expression has been associated with iron overload and overexpression of hepcidin results in iron-deficiency anemia in mice. In addition, hepcidin levels decrease in mice fed a low iron diet and increase in mice fed a high iron diet. These observations support the role of hepcidin as a signal that limits intestinal iron absorption. Hepcidin expression is also affected by hypoxia and inflammation and is decreased in hemochromatosis patients. Thus, the relationship between body iron status and hepcidin is altered in hemochromatosis patients. In addition, hepcidin is decreased in HFE knockout mice, which demonstrates characteristics of iron overload as in hemochromatosis patients. Hence, HFE is suggested to act as a regulator of hepcidin expression. Transcription factors, such as C/EBPalpha, are also suggested to be involved in the regulation of hepcidin gene expression. However, much remains to be investigated in the regulation of hepcidin by iron, hypoxia and inflammation.

The role of the iron responsive element in the control of ferroportin1/IREG1/MTP1 gene expression

BACKGROUND/AIMS

MTP1/Ferroportin1/IREG1, the product of the SLC40A1 gene, is a main iron export protein in mammals. However, the way this gene is regulated by iron is still unclear. The aim of this study was to investigate the functional role of genomic SLC40A1 elements in response to iron.

METHODS

Vectors containing either reverse similar 2.6 kb 5' flanking region or deletion constructs, including one devoid of an iron responsive element (SLC40A1-DeltaIRE-Luc), were analyzed by luciferase reporter gene in transfected HepG2, CaCO2 and U937 cells. Expression of iron genes and activity of the iron regulatory protein were also studied.

RESULTS

Iron increased and desferrioxamine decreased luciferase activity in all the cell types using both the full-length construct and the promoter deletion constructs, in the absence of changes in SLC40A1 or luciferase mRNA levels. To test the role of the SLC40A1 5' untranslated region, we first demonstrated that wild type and not SLC40A1-DeltaIRE-Luc could bind iron regulatory protein. Then, in cells transfected with SLC40A1-DeltaIRE-Luc, we found that, in spite of iron regulatory protein activation, the response to iron manipulation was lost.

CONCLUSIONS

We demonstrate that the iron responsive element in the SLC40A1 gene is functional and that it controls gene expression through the cytoplasmic iron regulatory protein system.

SLC11 family of H+-coupled metal-ion transporters NRAMP1 and DMT1

NRAMP1 (natural resistance-associated macrophage protein-1) and DMT1 (divalent metal-ion transporter-1) make up the SLC11 gene family of metal-ion transporters that are energized by the H(+) electrochemical gradient. Long known to confer resistance to bacterial infection, NRAMP1 functions at the phagolysosomal membrane of macrophages and neutrophils. NRAMP1 most likely contributes to macrophage antimicrobial function by extruding essential metal ions (including Mn(2+)) from the phagolysosome via H(+)/metal-ion cotransport. An alternative hypothesis in the literature proposes that NRAMP1 concentrate Fe(2+) within the phagolysosome by means of H(+)/Fe(2+) antiport, resulting in the generation of toxic free radicals. DMT1 is expressed widely and accepts as substrates a broad range of transition metal ions, among which Fe(2+) is transported with high affinity ( K(0.5) approximately 2 microM). DMT1 accounts both for the intestinal absorption of free Fe(2+) and for transferrin-associated endosomal Fe(2+) transport in erythroid precursors and many other cell types. DMT1 is up-regulated dramatically in the intestine by dietary iron restriction and, despite high serum iron levels, is not appropriately down-regulated in hereditary hemochromatosis.

Dietary iron regulates hepatic hepcidin 1 and 2 mRNAs in mice

Recently discovered peptide-hepcidin (Hepc) may be a central player in the communication of iron body stores to the intestinal absorptive cells and thus involved in the maintenance of iron homeostasis. The aim of this study was to determine the effects of the level of dietary iron on Hepc gene expression in the liver. OF1 male mice were fed for 3 weeks either control diet (35 mg iron/kg diet), low-iron diet (1 mg iron/kg diet), or high-iron diet (500 mg iron/kg diet), and Hepc 1 and 2 mRNA abundance in the liver was assessed by reverse transcriptase-polymerase chain reaction (RT-PCR). Results clearly showed that Hepc gene expression is upregulated by high dietary iron and downregulated when the dietary iron level is low. Both Hepc 1 and Hepc 2 expression responds coordinately to dietary iron. This work provides additional evidence of the key role of Hepc in the regulation of iron homeostasis.

Duodenal cytochrome b and hephaestin expression in patients with iron deficiency and hemochromatosis

BACKGROUND & AIMS

An increased duodenal expression of the iron transporters, divalent-metal-transporter-1, and ferroportin is observed in patients with iron deficiency or hereditary hemochromatosis. Two oxidoreductases, termed duodenal cytochrome b and hephaestin, are proposed to co-operate with divalent-metal-transporter-1 and FPN1, respectively, to transfer iron from the duodenal lumen to the circulation.

METHODS

In the present study, we investigated the mRNA and protein expression of Dcytb and hephaestin in duodenal biopsies from patients with iron deficiency, HFE, and non-HFE-associated hemochromatosis and in control subjects by means of real-time polymerase chain reaction, Western blot, and immunofluorescence.

RESULTS

In iron deficiency a coordinated upregulation of the iron transporters divalent-metal-transporter-1 and ferroportin and of duodenal-cytochrome b and hephaestin was found, whereas in patients with HFE and non-HFE-associated hemochromatosis duodenal-cytochrome b and hephaestin protein and mRNA expression were not significantly different from control subjects. However, HFE but not non-HFE hemochromatosis patients presented with an increased duodenal ferric reductase activity. Spearman rank correlations showed that Dcytb, hephaestin, FPN1, and DMT1 mRNA expression are positively related to each other independently of the underlying disease, which ensures an efficient transepithelial transport of absorbed iron.

CONCLUSIONS

Our data show that duodenal-cytochrome b activity in iron deficiency is stimulated via enhanced protein expression, whereas in HFE hemochromatosis it is up-regulated post-translationally. This points to different kinetics of intestinal iron uptake between iron deficiency and HFE hemochromatosis and also indicates that duodenal iron accumulation in HFE and non-HFE hemochromatosis is pathophysiologically different.

Systemic regulation of Hephaestin and Ireg1 revealed in studies of genetic and nutritional iron deficiency

Hephaestin is a membrane-bound multicopper ferroxidase necessary for iron egress from intestinal enterocytes into the circulation. Mice with sex-linked anemia (sla) have a mutant form of Hephaestin and a defect in intestinal basolateral iron transport, which results in iron deficiency and anemia. Ireg1 (SLC11A3, also known as Ferroportin1 or Mtp1) is the putative intestinal basolateral iron transporter. We compared iron levels and expression of genes involved in iron uptake and storage in sla mice and C57BL/6J mice fed iron-deficient, iron-overload, or control diets. Both iron-deficient wild-type mice and sla mice showed increased expression of Heph and Ireg1 mRNA, compared to controls, whereas only iron-deficient wild-type mice had increased expression of the brush border transporter Dmt1. Unlike iron-deficient mice, sla mouse enterocytes accumulated nonheme iron and ferritin. These results indicate that Dmt1 can be modulated by the enterocyte iron level, whereas Hephaestin and Ireg1 expression respond to systemic rather than local signals of iron status. Thus, the basolateral transport step appears to be the primary site at which the small intestine responds to alterations in body iron requirements.

Neonatal hemochromatosis

Neonatal hemochromatosis is a rare gestational condition in which iron accumulates in the fetal tissues in a distribution like that seen in hereditary hemochromatosis. Extensive liver damage is the dominant clinical feature, with late fetal loss or early neonatal death. NH recurs within sibships at a rate higher than that predicted for simple Mendelian autosomal-recessive inheritance, possibly suggesting the role of a maternal factor. Immunomodulation during pregnancy at risk appears to lessen the severity of disease.

Effect of alcohol on iron storage diseases of the liver

The consumption of excess alcohol in patients with liver iron storage diseases, in particular the iron-overload disease hereditary haemochromatosis (HH), has important clinical consequences. HH, a common genetic disorder amongst people of European descent, results in a slow, progressive accumulation of excess hepatic iron. If left untreated, the condition may lead to fibrosis, cirrhosis and primary hepatocellular carcinoma. The consumption of excess alcohol remains an important cause of hepatic cirrhosis and alcohol consumption itself may lead to altered iron homeostasis. Both alcohol and iron independently have been shown to result in increased oxidative stress causing lipid peroxidation and tissue damage. Therefore, the added effects of both toxins may exacerbate the pathogenesis of disease and impose an increased risk of cirrhosis. This review discusses the concomitant effects of alcohol and iron on the pathogenesis of liver disease. We also discuss the implications of co-existent alcohol and iron in end-stage liver disease.

Hepcidin, a key regulator of iron metabolism and mediator of anemia of inflammation

Human hepcidin, a 25-amino acid peptide made by hepatocytes, may be a new mediator of innate immunity and the long-sought iron-regulatory hormone. The synthesis of hepcidin is greatly stimulated by inflammation or by iron overload. Evidence from transgenic mouse models indicates that hepcidin is the predominant negative regulator of iron absorption in the small intestine, iron transport across the placenta, and iron release from macrophages. The key role of hepcidin is confirmed by the presence of nonsense mutations in the hepcidin gene, homozygous in the affected members, in 2 families with severe juvenile hemochromatosis. Recent evidence shows that deficient hepcidin response to iron loading may contribute to iron overload even in the much milder common form of hemochromatosis, from mutations in the HFE gene. In anemia of inflammation, hepcidin production is increased up to 100-fold and this may account for the defining feature of this condition, sequestration of iron in macrophages. The discovery of hepcidin and its role in iron metabolism could lead to new therapies for hemochromatosis and anemia of inflammation.

Expression of hepcidin in hereditary hemochromatosis: evidence for a regulation in response to the serum transferrin saturation and to non-transferrin-bound iron

Experimental data suggest the antimicrobial peptide hepcidin as a central regulator in iron homeostasis. In this study, we characterized the expression of human hepcidin in experimental and clinical iron overload conditions, including hereditary hemochromatosis. Using quantitative reverse transcriptase-polymerase chain reaction (RT-PCR), we determined expression of hepcidin and the most relevant iron-related genes in liver biopsies from patients with hemochromatosis and iron-stain-negative control subjects. Regulation of hepcidin mRNA expression in response to transferrin-bound iron, non-transferrin-bound iron, and deferoxamine was analyzed in HepG2 cells. Hepcidin expression correlated significantly with serum ferritin levels in controls, whereas no significant up-regulation was observed in patients with hemochromatosis despite iron-overload conditions and high serum ferritin levels. However, patients with hemochromatosis showed an inverse correlation between hepcidin transcript levels and the serum transferrin saturation. Moreover, we found a significant correlation between hepatic transcript levels of hepcidin and transferrin receptor-2 irrespective of the iron status. In vitro data indicated that hepcidin expression is down-regulated in response to non-transferrin-bound iron. In conclusion, the presented data suggest a close relationship between the transferrin saturation and hepatic hepcidin expression in hereditary hemochromatosis. Although the causality is not yet clear, this interaction might result from a down-regulation of hepcidin expression in response to significant levels of non-transferrin-bound iron.

Hepatobiliary pathology

Insights provided by molecular biology, immunohistochemistry, and transmission electron microscopy have increased our understanding of the pathogenesis and histopathology of hepatitis C virus (HCV) infection, nonalcoholic steatohepatitis (NASH), and bile ductular proliferative reactions in a number of liver diseases. Human and chimpanzee liver infected with HCV showed viral-like particles (50 to 60 nm in diameter) as well as aggregates of short tubules that represent viral envelope material. Interactions of HCV core protein with apolipoproteins have a role in the pathogenesis of HCV-related steatosis. Pathologists should be aware of the spectrum of liver pathology described with the use of highly active antiretroviral therapy (HAART) agents for the human immunodeficiency virus infection, which includes microvesicular steatosis and more severe hepatic injury with confluent necrosis. Proliferation of bile ductular structures is influenced by specific molecules and proteins (eg, the mucin-associated trefoil proteins and estrogens). The interplay between Notch receptors and Jagged 1 protein, as expressed by many cells of the liver (including bile duct epithelium) varies in primary sclerosing cholangitis (PSC) and primary biliary cirrhosis (PBC). Cholangiocarcinoma does not appear to be a long-term complication of small duct PSC. The fatty liver diseases, both alcoholic and nonalcoholic, are characterized by production of reactive oxygen species that have detrimental effects such as opening mitochondrial permeability transition pores with resultant release of cytochrome c into the cytosol. Hepatocellular carcinoma is now a recognized late complication of NASH. The derivation of hepatic stem cells, the roles of HFE protein and other hepatic and intestinal transport proteins in hemochromatosis, and the histopathologic interpretive challenge of centrilobular lesions in posttransplant liver biopsies are among other recent studies considered in this review.

The orchestration of body iron intake: how and where do enterocytes receive their cues?

Our understanding of how iron transverses the intestinal epithelium has improved greatly in recent years, although the mechanism by which body iron demands regulate this process remains poorly understood. By critically examining the earlier literature in this field and considering it in combination with recent advances we have formulated a model explaining how iron absorption could be regulated by body iron requirements. In particular, this analysis suggests that signals to alter absorption exert a direct effect on mature enterocytes rather than influencing the intestinal crypt cells. We propose that the liver plays a central role in the maintenance of iron homeostasis by regulating the expression of hepcidin in response to changes in the ratio of diferric transferrin in the circulation to the level of transferrin receptor 1. Such changes are detected by transferrin receptor 2 and the HFE/transferrin receptor 1 complex. Circulating hepcidin then directly influences the expression of Ireg1 in the mature villus enterocytes of the duodenum, thereby regulating iron absorption in response to body iron requirements. In this manner, the body can rapidly and appropriately respond to changes in iron demands by adjusting the release of iron from the duodenal enterocytes and, possibly, the macrophages of the reticuloendothelial system. This model can explain the regulation of iron absorption under normal conditions and also the inappropriate absorption seen in pathological states such as hemochromatosis and thalassemia.

Iron transport: emerging roles in health and disease

In the theater of cellular life, iron plays an ambiguous and yet undoubted lead role. Iron is a ubiquitous core element of the earth and plays a central role in countless biochemical pathways. It is integral to the catalysis of the redox reactions of oxidative phosphorylation in the respiratory chain, and it provides a specific binding site for oxygen in the heme binding moiety of hemoglobin, which allows oxygen transport in the blood. Its biological utility depends upon its ability to readily accept or donate electrons, interconverting between its ferric (Fe3+) and ferrous (Fe2+) forms. In contrast to these beneficial features, free iron can assume a dangerous aspect catalyzing the formation of highly reactive compounds such as cytotoxic hydroxyl radicals that cause damage to the macromolecular components of cells, including DNA and proteins, and thereby cellular destruction. The handling of iron in the body must therefore be very carefully regulated. Most environmental iron is in the Fe3+ state, which is almost insoluble at neutral pH. To overcome the virtual insolubility and potential toxicity of iron, a myriad of specialized transport systems and associated proteins have evolved to mediate regulated acquisition, transport, and storage of iron in a soluble, biologically useful, non-toxic form. We are gradually beginning to understand how these proteins individually and in concert serve to maintain cellular and whole body homeostasis of this crucial yet potentially harmful metal ion. Furthermore, studies are increasingly implicating iron and its associated transport in specific pathologies of many organs. Investigation of the transport proteins and their functions is beginning to unravel the detailed mechanisms underlying the diseases associated with iron deficiency, iron overload, and other dysfunctions of iron metabolism.

Iron misregulation in the brain: a primary cause of neurodegenerative disorders

High iron concentrations in the brains of patients and the discovery of mutations in the genes associated with iron metabolism in the brain suggest that iron misregulation in the brain plays a part in neuronal death in some neurodegenerative disorders, such as Alzheimer's, Parkinson's, and Huntington's diseases and Hallervorden-Spatz syndrome. Iron misregulation in the brain may have genetic and non-genetic causes. The disrupted expression or function of proteins involved in iron metabolism increases the concentration of iron in the brain. Disturbances can happen at any of several stages in iron metabolism (including uptake and release, storage, intracellular metabolism, and regulation). Increased brain iron triggers a cascade of deleterious events that lead to neurodegeneration. An understanding of the process of iron regulation in the brain, the proteins important in this process, and the effects of iron misregulation could help to treat or prevent neurodegenerative disorders.

Molecular mechanisms and regulation of iron transport

Iron homeostasis is primarily maintained through regulation of its transport. This review summarizes recent discoveries in the field of iron transport that have shed light on the molecular mechanisms of dietary iron uptake, pathways for iron efflux to and between peripheral tissues, proteins implicated in organellar transport of iron (particularly the mitochondrion), and novel regulators that have been proposed to control iron assimilation. The transport of both transferrin-bound and nontransferrin-bound iron to peripheral tissues is discussed. Finally, the regulation of iron transport is also considered at the molecular level, with posttranscriptional, transcriptional, and posttranslational control mechanisms being reviewed.

Disrupted hepcidin regulation in HFE-associated haemochromatosis and the liver as a regulator of body iron homoeostasis

BACKGROUND

The mechanisms responsible for disturbed iron homoeostasis in hereditary haemochromatosis are poorly understood. However, results of some studies indicate a link between hepcidin, a liver-derived peptide, and intestinal iron absorption, suggesting that this molecule could play a part in hepatic iron overload. To investigate this possible association, we studied the hepatic expression of the gene for hepcidin (HAMP) and a gene important in iron transport (IREG1) in patients with haemochromatosis, in normal controls, and in Hfe-knockout mice.

METHODS

We extracted total RNA from the liver tissue of 27 patients with HFE-associated haemochromatosis, seven transplant donors (controls), and Hfe-knockout mice. HAMP and IREG1 mRNA concentrations were examined by ribonuclease protection assays and expressed relative to the housekeeping gene GAPD.

FINDINGS

There was a significant decrease in HAMP expression in untreated patients compared with controls (5.4-fold, 95% CI 3.3-7.5; p<0.0001) despite significantly increased iron loading. Similarly, we noted a decrease in Hamp expression in iron-loaded Hfe-knockout mice. Hepatic IREG1 expression was greatly upregulated in patients with haemochromatosis (1.8-fold, 95% CI 1.5-2.2; p=0.002). There was a significant correlation between hepatic iron concentration and expression of HAMP (r=0.59, p=0.02) and IREG1 (r=0.67, p=0.007) in untreated patients.

INTERPRETATION

Lack of HAMP upregulation in HFE-associated haemochromatosis despite significant hepatic iron loading indicates that HFE plays an important part in the regulation of hepcidin expression in response to iron overload. Our results imply that the liver is important in the pathophysiology of HFE-associated haemochromatosis. Furthermore, the increase in hepatic IREG1 expression in haemochromatosis suggests that IREG1 could function to facilitate the removal of excess iron from the liver.

Hepcidin, a new iron regulatory peptide

Maintaining normal iron homeostasis is essential for the organism, as both iron deficiency and iron excess are associated with cellular dysfunction. Recently, several lines of evidence have suggested that hepcidin, a peptide mainly produced by the liver, plays a major role in the control of body iron homeostasis. The subject of this paper is to summarize the advances toward the understanding of function and regulation of hepcidin in iron metabolism and to provide new data on the regulation of hepcidin gene expression by erythropoietin, the major regulator of mammalian erythropoiesis.

Molecular and functional roles of duodenal cytochrome B (Dcytb) in iron metabolism

Dcytb has been identified as the mammalian transplasma ferric reductase that catalyzes the reduction of ferric to ferrous iron in the process of iron absorption. Its mRNA and protein levels are up-regulated by several independent stimulators of iron absorption. Furthermore, its cDNA encodes putative binding sites for heme and ascorbic acid. Using Northern and Western blots, RT-PCR and confocal microscopy, we studied the expression and localisation of Dcytb in cell lines and tissues of CD1 mice. Dcytb expression and function were modulated by iron. Dcytb and DMT1, both predominantly localised in the apical region of the duodenum were up-regulated in iron deficiency. Dcytb, the iron regulated ferric reductase may also utilize cytoplasmic ascorbate as electron donor for transmembrane reduction of iron. Dcytb expression was found in other tissues apart from the duodenum and its regulation and functions at these other sites are of interest in iron metabolism.

Transferrin receptor 1 (TfR1) and putative stimulator of Fe transport (SFT) expression in iron deficiency and overload: an overview

Transferrin Receptor 1 (TfR1) and putative Stimulator of Fe Transport (SFT) represent two different proteins involved in iron metabolism in mammalian cells. The expression of TfR1 in the duodenum of subjects with normal body iron stores has been mainly localized in the basolateral portion of the cytoplasm of crypt cells, supporting the idea that this molecule may be involved in the sensing of body iron stores. In iron deficiency anemia TfR1 expression demonstrated an inverse relationship with body iron stores as assessed by immunohistochemistry with anti-TfR1 antibodies. In iron overload, TfR1 expression in the duodenum differed according to the presence or absence of the C282Y mutation in the HFE gene, being increased in HFE-related hemochromatosis and similar to controls in non-HFE-related iron overload. SFT is characterized by its ability to increase iron transport both through the transferrin dependent and independent uptake, and could thus affect iron absorption in the intestine. Immunohistochemistry using anti-SFT antibodies which recognize a putative stimulator of Fe transport of approximately 80 KDa revealed a localization of this protein in the apical part of the cytoplasm of enterocytes localized at the tip of the villi. The expression of the protein recognized by these antibodies was increased in iron deficiency, as well as in patients carrying the C282Y HFE mutation. Thus, the increased expression of both proteins only in patients with HFE-related hemochromatosis suggests that other factors should be involved in determining non-HFE-related iron overload.

The ceruloplasmin homolog hephaestin and the control of intestinal iron absorption

Hephaestin is the gene affected in the sex-linked anemic (sla) mouse. These animals have a defect in the export of iron from intestinal enterocytes into the circulation and this implicates hephaestin in the basolateral transfer step of iron absorption. Hephaestin is homologous to the plasma copper-containing protein ceruloplasmin, and all residues involved in copper binding and disulfide bond formation in ceruloplasmin are conserved in hephaestin. Unlike ceruloplasmin, hephaestin is an integral membrane protein with a single trans-membrane domain. It is highly expressed throughout the small intestine, to a lesser extent in the colon, and at low levels in several other tissues. Surprisingly, most hephaestin appears to be located intracellularly in a perinuclear distribution. Like ceruloplasmin, hephaestin has a ferroxidase activity which is predicted to underlie its biological function. In addition, its expression is stimulated under iron deficient conditions. Analysis of the sla mouse has supported our model for the regulation of intestinal iron absorption whereby changes in systemic iron requirements alter the levels of basolateral transport components with subsequent regulation of brush border transport.

Decreased liver hepcidin expression in the Hfe knockout mouse

Hepcidin is a circulating antimicrobial peptide which has been proposed to regulate the uptake of dietary iron and its storage in reticuloendothelial macrophages. Transgenic mice lacking hepcidin expression demonstrate abnormalities of iron homeostasis similar to Hfe knockout mice and to patients with HFE-associated hereditary hemochromatosis (HH). To identify any association between liver hepcidin expression and the iron homeostasis abnormalities observed in HH, we compared liver hepcidin mRNA content in wild type and Hfe knockout mice. Because the iron homeostasis abnormalities in the Hfe knockout mice are greatest early in life, we analyzed mice at different ages. At four weeks of age, Hfe knockout mice had significantly decreased liver hepcidin mRNA expression compared to wild type mice. The decreased hepcidin expression was associated with hepatic iron deposition, elevated transferrin saturations, and decreased splenic iron concentrations. At 10 weeks of age, despite marked hepatic iron loading, Hfe knockout mice demonstrated liver hepcidin mRNA expression similar to that observed in wild type mice. Placing 8 week-old wild type and Hfe knockout mice on a 2% carbonyl iron diet for 2 weeks led to a similar degree of hepatic iron loading in each group. However, while the wild type mice demonstrated a mean five-fold increase in liver hepcidin mRNA, no change was observed in the Hfe knockout mice. The lack of an increase in liver hepcidin expression in these iron-loaded Hfe knockout mice was associated with sparing of iron deposition into the spleen. These data indicate that the normal relationship between body iron stores and liver hepcidin mRNA levels is altered in Hfe knockout mice, such that liver hepcidin expression is relatively decreased. We speculate that decreased hepcidin expression relative to body iron stores contributes to the iron homeostasis abnormalities characteristic of HH.