Localization of iron transport and regulatory proteins in human cells

Interaction between iron and dihydromyricetin extracted from vine tea

In this research, the interaction between dihydromyricetin (DMY) obtained from vine tea and iron ions (Fe (II) and Fe (III)) was investigated at pH 3.0, 5.0, and 7.0 with UV absorption and fluorescence quenching spectroscopy. The effects of DMY on the stability and solubility of iron ion were also studied. The results showed the presence of iron ions changed the UV absorption spectra of DMY at the experimental pH values. And the fluorescence spectra showed that iron ion had enhanced fluorescence effect on DMY. In addition, DMY was capable of protecting Fe (II) from being oxidized and improving the solubility of Fe (III).

Increased DMT-1 expression in placentas of women living in high-Cd-contaminated areas of Thailand

Cadmium (Cd) is a toxic heavy metal and contamination was reported in soil and rice in several areas of Thailand. Humans are normally exposed to environmental Cd, leading to gradual Cd accumulation in their bodies, including the placenta. DMT-1 is a divalent metal transporter which is found in placental tissue and plays a vital role in the transportation of Fe²⁺ and Cd²⁺. This study investigated DMT-1 protein and mRNA expressions in full term human placentas comparing those from high-Cd-contaminated areas (high-Cd group) and low-Cd-contaminated areas (low-Cd group), n = 6 per group. The maternal blood Cd (B-Cd) and placental Cd (P-Cd) of the high-Cd group was significantly raised in comparison with those in the low-Cd group. DMT-1 in the fetal portion of the placentas was localized in the apical and basal portions of the cytoplasm of the syncytiotrophoblastic cells, the endothelium of fetal capillaries which is functional structure of the placental barrier, and was also found in the cytoplasm of Hofbauer cells. Moreover, DMT-1 localization in the maternal portion was also detected in most decidual cells. In addition, the DMT-1 protein and mRNA expressions in the high-Cd group were significantly higher than those in the low-Cd group. Therefore, we suggest that pregnant women, who are exposed to environmental Cd, show an increased level of Cd in their maternal blood and this Cd can accumulate in the placenta. Intracellular Cd may induce DMT-1 mRNA transcription which further translates into DMT-1 protein, which can then function as a reciprocal Cd transporter in placental tissue.

HFE gene: Structure, function, mutations, and associated iron abnormalities

The hemochromatosis gene HFE was discovered in 1996, more than a century after clinical and pathologic manifestations of hemochromatosis were reported. Linked to the major histocompatibility complex (MHC) on chromosome 6p, HFE encodes the MHC class I-like protein HFE that binds beta-2 microglobulin. HFE influences iron absorption by modulating the expression of hepcidin, the main controller of iron metabolism. Common HFE mutations account for ~90% of hemochromatosis phenotypes in whites of western European descent. We review HFE mapping and cloning, structure, promoters and controllers, and coding region mutations, HFE protein structure, cell and tissue expression and function, mouse Hfe knockouts and knockins, and HFE mutations in other mammals with iron overload. We describe the pertinence of HFE and HFE to mechanisms of iron homeostasis, the origin and fixation of HFE polymorphisms in European and other populations, and the genetic and biochemical basis of HFE hemochromatosis and iron overload.

Role of hyaluronic acid, its degrading enzymes, degradation products, and ferritin in the assessment of fibrosis stage in Egyptian patients with chronic hepatitis C

BACKGROUND/AIMS

Liver biopsy is considered a gold standard for fibrosis staging, but it has a high risk of morbidity. Therefore, there is an interest in developing noninvasive markers for the prediction of liver fibrosis stages.

METHODS

Hyaluronic acid, ferritin, N-acetyl-β-D-glucosaminidase, β-glucuronidase, glucosamine, aspartate transaminase, and alanine transaminase were assayed in 210 individuals with chronic hepatitis C infection. Statistical analysis was carried out by logistic regression and receiver-operating characteristic curves.

RESULTS

The best linear combination of only significant blood markers was used for the determination of the fibrosis discriminant score; score=[1.64 (numerical constant)-0.002×hyaluronic acid (pg/l)-2.68×β-glucuronidase (µmol/ml/min)-0.026×glucosamine (µg/dl)-0.001×ferritin-0.033 (ng/ml)×aspartate transaminase/alanine transaminase]. The selected fibrosis discriminant score function correctly classified 81% of patients with severe liver fibrosis at a discriminant cut-off score=0.55 (i.e. less than 0.55 indicated mild liver fibrosis and greater than 0.55 indicated severe liver fibrosis), with a sensitivity of 100% and a specificity of 73%.

CONCLUSION

A simple fibrosis index can be useful to select hepatitis C virus-infected patients with a very low risk of significant fibrosis in whom the protocol of liver biopsies may be avoided.

H(+)-coupled divalent metal-ion transporter-1: functional properties, physiological roles and therapeutics

Divalent metal-ion transporter-1 (DMT1) is a widely expressed, iron-preferring membrane transport protein. Animal models establish that DMT1 plays indispensable roles in intestinal nonheme-iron absorption and iron acquisition by erythroid precursor cells. Rare mutations in human DMT1 result in severe microcytic-hypochromic anemia. When we express DMT1 in RNA-injected Xenopus oocytes, we observe rheogenic Fe(2+) transport that is driven by the proton electrochemical potential gradient. In that same preparation, DMT1 also transports cadmium and manganese but not copper. Whether manganese metabolism relies upon DMT1 remains unclear but DMT1 contributes to the effects of overexposure to cadmium and manganese in some tissues. There exist at least four DMT1 isoforms that arise from variant transcription of the SLC11A2 gene. Whereas these isoforms display identical functional properties, N- and C-terminal variations contain cues that direct the cell-specific targeting of DMT1 isoforms to discrete subcellular compartments (plasma membrane, endosomes, and lysosomes). An iron-responsive element (IRE) in the mRNA 3'-untranslated region permits the regulation of some isoforms by iron status, and additional mechanisms by which DMT1 is regulated are emerging. Natural-resistance-associated macrophage protein-1 (NRAMP1)-the only other member of the mammalian SLC11 gene family-contributes to antimicrobial function by extruding from the phagolysosome divalent metal ions (e.g. Mn(2+)) that may be essential cofactors for bacteria-derived enzymes or required for bacterial growth. The principal or only intestinal nonheme-iron transporter, DMT1 is a validated therapeutic target in hereditary hemochromatosis (HHC) and other iron-overload disorders.

Iron transport machinery of human cells: players and their interactions

Organisms, like cells, maintain tight control of iron. In humans as well as other mammals, control is achieved through the regulation of iron uptake into the body rather than through the excretion of iron. The mechanisms by which humans and mice regulate both iron uptake and the distribution of iron within the body and cells are reviewed. Special emphasis is given to the iron transporters involved in this process.

Emerging roles of pathogens in Alzheimer disease

Chronic spirochetal infection can cause slowly progressive dementia, cortical atrophy and amyloid deposition in the atrophic form of general paresis. There is a significant association between Alzheimer disease (AD) and various types of spirochete (including the periodontal pathogen Treponemas and Borrelia burgdorferi), and other pathogens such as Chlamydophyla pneumoniae and herpes simplex virus type-1 (HSV-1). Exposure of mammalian neuronal and glial cells and organotypic cultures to spirochetes reproduces the biological and pathological hallmarks of AD. Senile-plaque-like beta amyloid (Aβ) deposits are also observed in mice following inhalation of C. pneumoniae in vivo, and Aβ accumulation and phosphorylation of tau is induced in neurons by HSV-1 in vitro and in vivo. Specific bacterial ligands, and bacterial and viral DNA and RNA all increase the expression of proinflammatory molecules, which activates the innate and adaptive immune systems. Evasion of pathogens from destruction by the host immune reactions leads to persistent infection, chronic inflammation, neuronal destruction and Aβ deposition. Aβ has been shown to be a pore-forming antimicrobial peptide, indicating that Aβ accumulation might be a response to infection. Global attention and action is needed to support this emerging field of research because dementia might be prevented by combined antibiotic, antiviral and anti-inflammatory therapy.

Differential HFE gene expression is regulated by alternative splicing in human tissues

BACKGROUND

The pathophysiology of HFE-derived Hereditary Hemochromatosis and the function of HFE protein in iron homeostasis remain uncertain. Also, the role of alternative splicing in HFE gene expression regulation and the possible function of the corresponding protein isoforms are still unknown. The aim of this study was to gain insights into the physiological significance of these alternative HFE variants.

METHODOLOGY/PRINCIPAL FINDINGS

Alternatively spliced HFE transcripts in diverse human tissues were identified by RT-PCR, cloning and sequencing. Total HFE transcripts, as well as two alternative splicing transcripts were quantified using a real-time PCR methodology. Intracellular localization, trafficking and protein association of GFP-tagged HFE protein variants were analysed in transiently transfected HepG2 cells by immunoprecipitation and immunofluorescence assays. Alternatively spliced HFE transcripts present both level- and tissue-specificity. Concerning the exon 2 skipping and intron 4 inclusion transcripts, the liver presents the lowest relative level, while duodenum presents one of the highest amounts. The protein resulting from exon 2 skipping transcript is unable to associate with β2M and TfR1 and reveals an ER retention. Conversely, the intron 4 inclusion transcript gives rise to a truncated, soluble protein (sHFE) that is mostly secreted by cells to the medium in association with β2M.

CONCLUSIONS/SIGNIFICANCE

HFE gene post-transcriptional regulation is clearly affected by a tissue-dependent alternative splicing mechanism. Among the corresponding proteins, a sHFE isoform stands out, which upon being secreted into the bloodstream, may act in remote tissues. It could be either an agonist or antagonist of the full length HFE, through hepcidin expression regulation in the liver or by controlling dietary iron absorption in the duodenum.

Iron uptake by Caco-2 cells following in vitro digestion: effects of heat treatments of pork meat and pH of the digests

The present in vitro studies report on iron uptake by Caco-2 cells from pepsin and pepsin+pancreatin-digested pork meat proteins at pH values between 4.6 and 7 mimicking conditions in the duodenum and the proximal jejunum, respectively. Heat treatment of the pork meat resulted in increased iron uptake from pepsin-digested samples to Caco-2 cells at pH 4.6. The major enhancing effects on iron uptake by Caco-2 cells were observed after pepsin digestion in the pH range 4.6-6.0, whereas the pepsin+pancreatin-digested samples resulted in negligible iron uptake in Caco-2 cells at pH 7. Thus, the results emphasize the importance of separating pepsin-digested and pepsin+pancreatin-digested proteins during in vitro studies on iron availability. Furthermore, the present results showed the pH dependency of iron uptake anticipated. The enhancing effect of ascorbic acid was verified by increased iron uptake from pepsin-digested pork meat samples at pH 4.6, while no effect of ascorbic acid was observed at pH 7 in pepsin+pancreatin-digested samples.

Hemoglobin binding activity and hemoglobin-binding protein of Prevotella nigrescens

Prevotella nigrescens, lacking siderophores was found to bind to the hemoproteins. The binding was observed also in the envelope which was prepared by sonication of the cell. The binding occurred in the pH-dependent manner; the binding was observed below neutral pHs of the incubation mixtures but only slightly observed in the neutral and alkaline pHs. Furthermore, hemoglobin bound to the envelope was dissociated at high pHs buffers. Maximum amounts of hemoglobin bound to 1 mg envelope was 51.2 μg. Kd for the reaction at pH 5.0 was 2.1 × 10^-10M (210 pM). From the dot blot assay, hemoglobin could bind to a protein solubilized from the envelope by a detergent, referred to as hemoglobin-binding protein (HbBP), then it was purified by the sequential procedures of ion exchange chromatography, affinity chromatography and isoelectric focusing. Molecular weight and isoelectric point of the HbBP were 46 kDa and 6.1, respectively.

Transplantation of a liver with the C282Y mutation into a recipient heterozygous for H63D results in iron overload

Hemochromatosis is a common hereditary disease associated with progressive iron overload eventually leading to parenchymal damage of the liver, heart, pancreas, and other organs. Liver transplantation has been the single most important therapy to extend long-term survival in patients with a variety of acute and chronic liver diseases. We report a case of inadvertent transplantation of a hemochromatotic liver into a nonhemochromatotic recipient, resulting in rapid iron overload. Neither the recipient nor the donor had iron overload at the time of transplantation, but the donor liver was subsequently found to be homozygous for C282Y mutation. The report includes 8 years follow-up, serial biopsies, and molecular studies. Iron overload in our patient transplanted with a C282Y homozygous liver provides an "in vivo" model for the pathophysiology of hemochromatosis and further supports liver playing a primary role in the maintenance of iron hemostasis rather intestine being the sole regulatory site.

Passively released heme from hemoglobin and myoglobin is a potential source of nutrient iron for Bordetella bronchiseptica

Colonization by Bordetella bronchiseptica results in a variety of inflammatory respiratory infections, including canine kennel cough, porcine atrophic rhinitis, and a whooping cough-like disease in humans. For successful colonization, B. bronchiseptica must acquire iron (Fe) from the infected host. A vast amount of Fe within the host is sequestered within heme, a metalloporphyrin which is coordinately bound in hemoglobin and myoglobin. Utilization of hemoglobin and myoglobin as sources of nutrient Fe by B. bronchiseptica requires expression of BhuR, an outer membrane protein. We hypothesize that hemin is acquired by B. bronchiseptica in a BhuR-dependent manner after spontaneous loss of the metalloporphyrin from hemoglobin and/or myoglobin. Sequestration experiments demonstrated that direct contact with hemoglobin or myoglobin was not required to support growth of B. bronchiseptica in an Fe-limiting environment. Mutant myoglobins, each exhibiting a different affinity for heme, were employed to demonstrate that the rate of growth of B. bronchiseptica was directly correlated with the rate at which heme was lost from the hemoprotein. Finally, Escherichia coli cells expressing recombinant BhuR had the capacity to remove hemin from solution. Collectively, these experiments provided strong experimental support for the model that BhuR is a hemin receptor and B. bronchiseptica likely acquires heme during infection after passive loss of the metalloporphyrin from hemoglobin and/or myoglobin. These results also suggest that spontaneous hemin loss by hemoglobin and myoglobin may be a common mechanism by which many pathogenic bacteria acquire heme and heme-bound Fe.

Overexpression of HFE in HepG2 cells reveals differences in intracellular distribution and co-localization of wt- and mutated forms

Liver is the primary target organ of Hereditary Hemochromatosis Type I, with the HFE mutations C282Y and H63D recognized as markers of this iron-overload disease. Hepatocytes are also the main site of synthesis of HFE. However, most early studies of overexpression of HFE were done in non-hepatic, non-HFE-expressing, cell lines. Here we report the setting up of a stable transfection model of wt- and mutant-HFE (H63D and C282Y) proteins in a hepatic cell line (HepG2), the analysis of its intracellular distribution and the effect of diferric transferrin on HFE localization. The C282Y mutant is retained in the ER, whereas HFE-wt and H63D co-localize with TfR1 exclusively in early recycling endosomes. Holotransferrin induces a re-localization of wt- and H63D-HFE, from early recycling endosomes to the cytoplasmic membrane. In conclusion our results establish the HepG2 cell line as a valuable model for the study of HFE.

The Hereditary Hemochromatosis protein HFE and its chaperone β2-microglobulin localise predominantly to the endosomal-recycling compartment

Hereditary Hemochromatosis is an iron overload disease most frequently associated with mutations in the HFE gene. While clinical studies of the disease have received extensive attention by various groups, the localisation, trafficking and function of the HFE protein, and its chaperone beta2-microglobulin (beta2M), require further investigation. In this study, we present data on the cellular localisation of HFE and its clinically relevant mutants in HuTu 80 cells. We find by confocal microscopy that HFE localises to the endosomal-recycling compartment (ERC), with minimal localisation to sorting or late endosomes. Interestingly, we also demonstrate that beta2M localises to the ERC where it co-localises with HFE. We find that exogenous expression of HFE results in enhanced beta2M cellular levels and that beta2M is necessary for cell surface expression of HFE. Finally, we have analysed the functional effects of exogenous expression of HFE and beta2M on transferrin binding to the cell surface. In summary, our study sheds light on the localisation and functional effects of the HFE and its chaperone protein beta2M.

Localisation of proteins of iron metabolism in the human placenta and liver

Two anatomical sites that are important in human iron metabolism are the liver and placenta. Liver macrophages recycle iron from erythrocytes, and the placenta transfers iron from the mother to the fetus. The cellular distribution of proteins involved in iron transport in these two sites was studied. Transferrin receptor-1 (TfR1) and Ferroportin (FPN) expression was found on the placental syncytiotrophoblast (STB) and were polarised such that TfR1 was on the apical maternal-facing membrane and FPN was on the basal fetal-facing membrane, consistent with unidirectional iron transport from mother to fetus. Ferritin was strongly expressed in the stroma, suggesting that fetal tissue can store and accumulate iron. HFE was on some parts of the basal STB and, where present, HFE clearly colocalised with FPN but not TfR1. In the stroma, both HFE and FPN were present on CD68+ Hofbauer macrophage cells. In liver, the location of HFE is controversial. Using four mouse monoclonals and two polyclonal sera we showed that the pattern of HFE expression mirrored the distribution of CD68+ macrophage Kupffer cells. FPN was also most strongly expressed by CD68+ Kupffer cells. These findings contribute to understanding how iron is transported and stored in the human placenta and liver.

Ontogenic changes in lactoferrin receptor and DMT1 in mouse small intestine: implications for iron absorption during early life

It has been proposed that lactoferrin receptor (LfR) may be involved in intestinal iron transport during early life. However, it is known that iron homeostasis is regulated by divalent metal transporter 1 (DMT1; Nramp2/DCT1) in the adult small intestine. To address the hypothesis that LfR may play a role as an alternative iron transport pathway during early life, we used immunohistochemistry (IHC) to examine the localization of mouse LfR (mLfR) and DMT1. In addition to studying the localization and abundance of LfR and DMT1 on the apical membrane, intestinal brush-border membrane vesicles (BBMV) were isolated during the first 3 postnatal weeks (postnatal day (PD) 0, 5, 10, and 20). We found that mLfR is expressed in fetal mice as early as gestational days (GD) 13.5, 15.5, and 18.5. A 34 kD band for mLfR was detected at PD 0 through PD 20 in total intestine homogenate. However, mLfR protein did not appear in the BBMV preparations until PD 5 and was highly expressed at PD 10. By IHC, DMT1 protein was minimally observed at PD 0 and PD 5, but by PD 10 DMT1 was predominantly localized in the apical membrane of the maturing intestine. BBMV fractionation revealed 50-120 kD protein bands for DMT1. In these BBMV preparations, the apical-membrane-associated 120 kD band for DMT1 increased in abundance with age. However, in the corresponding total homogenates, only the deglycosylated form of DMT1 (50 kD) was identified. These results indicate that DMT1 is mislocalized during late gestation, minimally expressed during early life, and predominantly expressed in its deglycosylated form until PD 20. The immunolocalization and abundant protein expression of mLfR suggest that accrual of iron from Lf may be the principal iron uptake pathway at this age. In conclusion, our findings support the notion that until the development-dependent expression of DMT1 in the intestine is induced, mLfR may serve as an alternative iron uptake pathway.

Extra-mitochondrial localisation of frataxin and its association with IscU1 during enterocyte-like differentiation of the human colon adenocarcinoma cell line Caco-2

Friedreich's ataxia is a recessive neurodegenerative disease due to insufficient expression of the mitochondrial protein frataxin. Although it has been shown that frataxin is involved in the control of intracellular iron metabolism, by interfering with the mitochondrial biosynthesis of proteins with iron/sulphur (Fe/S) clusters its role has not been well established. We studied frataxin protein and mRNA expression and localisation during cellular differentiation. We used the human colon adenocarcinoma cell line Caco-2, as it is considered a good model for intestinal epithelial differentiation and the study of intestinal iron metabolism. Here we report that the protein, but not the mRNA frataxin levels, increase during the enterocyte-like differentiation of Caco-2 cells, as well as in in-vivo-differentiated enterocytes at the upper half of the crypt-villus axis. Furthermore, subcellular fractionation and double immunostaining, followed by confocal analysis, reveal that frataxin localisation changes during Caco-2 cell differentiation. In particular, we found an extramitochondrial localisation of frataxin in differentiated cells. Finally, we demonstrate a physical interaction between extramitochondrial frataxin and IscU1, a cytoplasmic isoform of the human Fe/S cluster assembly machinery. Based on our data, we postulate that frataxin could be involved in the biosynthesis of iron-sulphur proteins not only within the mitochondria, but also in the extramitochondrial compartment. These findings might be of relevance for the understanding of both the pathogenesis of Friedreich's ataxia and the basic mechanism of Fe/S cluster biosynthesis.

Molecular and ionic mimicry and the transport of toxic metals

Despite many scientific advances, human exposure to, and intoxication by, toxic metal species continues to occur. Surprisingly, little is understood about the mechanisms by which certain metals and metal-containing species gain entry into target cells. Since there do not appear to be transporters designed specifically for the entry of most toxic metal species into mammalian cells, it has been postulated that some of these metals gain entry into target cells, through the mechanisms of ionic and/or molecular mimicry, at the site of transporters of essential elements and/or molecules. The primary purpose of this review is to discuss the transport of selective toxic metals in target organs and provide evidence supporting a role of ionic and/or molecular mimicry. In the context of this review, molecular mimicry refers to the ability of a metal ion to bond to an endogenous organic molecule to form an organic metal species that acts as a functional or structural mimic of essential molecules at the sites of transporters of those molecules. Ionic mimicry refers to the ability of a cationic form of a toxic metal to mimic an essential element or cationic species of an element at the site of a transporter of that element. Molecular and ionic mimics can also be sub-classified as structural or functional mimics. This review will present the established and putative roles of molecular and ionic mimicry in the transport of mercury, cadmium, lead, arsenic, selenium, and selected oxyanions in target organs and tissues.

The time-dependent transport of chromium in adult rats from the bloodstream to the urine

While chromium was proposed to be an essential trace element over 40 years ago and if essential should possess a specific transport and distribution mechanism, the details of its transport from the bloodstream to the urine have not been elucidated. However, chromium is known to be maintained in the bloodstream bound to transferrin and to be excreted in the urine bound to the oligopeptide chromodulin or a similar chromodulin-like species. Injection of (51)Cr-labeled transferrin into the bloodstream resulted in a rapid and insulin-sensitive movement of chromium into the tissues as Cr transferrin; greater than 50% of the Cr is transported to the tissues within 30 min. Tissue levels of Cr are maximal 30 min after injection; decreases in tissue Cr with time are mirrored by increases in urine Cr. Approximately 50% of the (51)Cr appears in the urine within 360 min of injection in the absence of added insulin; insulin treatment concurrent with injection of (51)Cr-labeled transferrin results in approximately 80% of the label appearing in the urine within 180 min. The removal of (51)Cr from the blood is faster than the appearance of (51)Cr in the urine; the lag in time indicates that the Cr transferrin in the blood and Cr in the urine are not in direct equilibrium and that intermediates in the transport of Cr must be involved. This establishes a clear pathway of transport of Cr starting from transport by transferrin from the bloodstream into the tissues, followed by release and processing in the tissues to form chromodulin, excretion into the bloodstream, rapid clearance of chromodulin or a similar species into the urine, and ultimately excretion as this species. Insulin stimulates the processing of Cr in the tissues.

Manganese toxicity upon overexposure

Manganese (Mn) is a required element and a metabolic byproduct of the contrast agent mangafodipir trisodium (MnDPDP). The Mn released from MnDPDP is initially sequestered by the liver for first-pass elimination, which allows an enhanced contrast for diagnostic imaging. The administration of intravenous Mn impacts its homeostatic balance in the human body and can lead to toxicity. Human Mn deficiency has been reported in patients on parenteral nutrition and in micronutrient studies. Mn toxicity has been reported through occupational (e.g. welder) and dietary overexposure and is evidenced primarily in the central nervous system, although lung, cardiac, liver, reproductive and fetal toxicity have been noted. Mn neurotoxicity results from an accumulation of the metal in brain tissue and results in a progressive disorder of the extrapyramidal system which is similar to Parkinson's disease. In order for Mn to distribute from blood into brain tissue, it must cross either the blood-brain barrier (BBB) or the blood-cerebrospinal fluid barrier (BCB). Brain import, with no evidence of export, would lead to brain Mn accumulation and neurotoxicity. The mechanism for the neurodegenerative damage specific to select brain regions is not clearly understood. Disturbances in iron homeostasis and the valence state of Mn have been implicated as key factors in contributing to Mn toxicity. Chelation therapy with EDTA and supplementation with levodopa are the current treatment options, which are mildly and transiently efficacious. In conclusion, repeated administration of Mn, or compounds that readily release Mn, may increase the risk of Mn-induced toxicity.

Transcriptome variations in human CaCo-2 cells: a model for enterocyte differentiation and its link to iron absorption

Complete clinical expression of the HFE1 hemochromatosis is very likely modulated by genes linked to duodenal iron absorption, whose level is conditioned by unknown processes taking place during enterocyte differentiation. We carried out a transcriptomic study on CaCo-2 cells used as a model of enterocyte differentiation in vitro. Of the 720 genes on the microarrays, 80, 50, and 56 were significantly down-regulated up-regulated, and invariant during differentiation. With regard to iron metabolism, we showed that HEPH, SLC11A2, SLC11A3, and TF are significantly up-regulated, while ATP7B and SLC39A1 (and SFT) are down-regulated and ACO1, dCYTb, FECH, and FTH1 show constant expression. Ontological annotations highlight the decrease in the expression of cell cycle and DNA metabolism associated genes as well as transcription, protein metabolism, signal transduction, and nucleocytoplasmic transport associated genes, whereas there are increases in the expression of genes linked to cell adhesion, lipid and xenobiotic metabolism, iron transport and homeostasis, and immune response.

Tranferrins receptor association and endosomal localization of soluble HFE are not sufficient for regulation of cellular iron homeostasis

Iron uptake and storage are tightly regulated to guarantee sufficient iron for essential cellular processes and to prevent the production of damaging free radicals. A non-classical class I MHC molecule, the hemochromatosis factor HFE, has been shown to regulate iron metabolism, potentially via its direct interaction with the transferrin receptor (TfR). In this study, we demonstrate that a soluble beta2microglobulin-HFE monochain (sHFE) folds with beta2microglobulin (beta2m) and associates with the TfR, indicating that the transmembrane and cytoplasmic domains are not necessary for assembly and trafficking through the ER-Golgi network. We also demonstrate human TfR-specific uptake and accumulation of extracellular sHFE by treated cells. The sHFE localized to the endosomal compartment albeit we observed variation in the time taken for endosomal trafficking between different cell types. The sHFE monochain was effective in reducing Tf uptake into cells, however this did not correlate to any changes in TfR or ferritin synthesis, in contrast to the HFE-induced increase and decrease of TfR and ferritin, respectively. These findings of incongruent sHFE activity suggest that either variation in affinity binding of sHFE to TfR prevents efficient modulation of iron-regulated proteins or that HFE has multiple functions some of which may be independent of TfR but dependent on interactions within the endosomal compartment for effective modulation of iron metabolism.

Iron metabolism in the reticuloendothelial system

Comprised mainly of monocytes and tissue macrophages, the reticuloendothelial system (RES) plays two major roles in iron metabolism: it recycles iron from senescent red blood cells and it serves as a large storage depot for excess iron. Although iron recycling by the RES represents the largest pathway of iron efflux in the body, the precise mechanisms involved have remained elusive. However, studies characterizing the function and regulation of Nramp1, DMT1, HFE, FPN1, CD163, and hepcidin are rapidly expanding our knowledge of the molecular aspects of RE iron handling. This review summarizes fundamental physiological and biochemical aspects of iron metabolism in the RES and focuses on how recent studies have advanced our understanding of these areas. Also discussed are novel insights into the molecular mechanisms contributing to the abnormal RE iron metabolism characteristic of hereditary hemochromatosis and the anemia of chronic disease.

Structure and liver cell expression pattern of the HFE gene in the rat

BACKGROUND/AIMS

Very little is known about the HFE gene in the rat. The aim of the present study was to determine: (1) the structure of the rat HFE gene; and (2) the tissue expression of the HFE mRNA in the rat, with special emphasis on the liver.

METHODS

Cloning of the rat HFE gene was performed using library screening and PCR. Exon-intron borders were assigned by DNA sequencing. Parenchymal and non-parenchymal liver cells were isolated by fractionation of normal rat liver. HFE mRNA levels were determined by Northern blot (tissues) and real-time PCR (isolated liver cells).

RESULTS

The rat HFE gene contained six exons and five introns. The HFE gene is expressed in multiple tissues in the rat, including bone marrow, with the highest expression in the liver. We observed HFE transcripts in several categories of isolated rat liver cells. Unexpectedly, expression also occurred in rat hepatocytes.

CONCLUSIONS

The exon-intron pattern of the HFE gene is strongly conserved between rat and mouse. The pattern of tissue expression of the HFE gene is rather similar in humans and rodents. The finding of HFE gene expression in rat hepatocytes raises interesting questions regarding its role in the hepatocyte iron metabolism.

Erythroblast iron metabolism in sideroblastic and sideropenic states

Iron appears to exert self-regulatory control over erythroblast iron uptake, iron storage and its incorporation into haem. It does this via iron regulatory proteins (IRPs) which bind reversibly to the iron responsive elements (IREs) on the mRNA of transferrin receptor (TfR), erythroid 5-aminolaevulinic acid synthase (ALA-S2) and ferritin. Iron deficiency leads to the binding of IRP to IRE. This binding inhibits the translation of mRNA for ALA-S2 and ferritin but stabilizes mRNA for TfR expression. Sideroblastic erythropoiesis is highly ineffective and characterized by mitochondrial iron loading. The study of X-linked sideroblastic anaemia has shown that the entry of iron into the mitochondria is poorly controlled and able to occur when protoporphyrin production is reduced, as is seen with the ALA-S2 mutations, or when it is increased as has been seen with ABC7 transporter mutations. Sideropenia characterises both iron deficiency anaemia (IDA) and the anaemia of chronic disease (ACD). Erythroblasts in ACD seem doubly equipped to protect their iron supply with their ability to increase the efficiency of transferrin-iron uptake as well as to activate the IRP/IRE system to increase surface TfR production. This increase in efficiency restricts the need to increase surface TfR production and maintains serum soluble TfR (sTfR) values within the normal range in iron replete ACD. The coexistence of iron deficiency with chronic disease, however, is associated with an increase in both the efficiency and number and a highly significant rise in sTfR values.

Co-localization of the mammalian hemochromatosis gene product (HFE) and a newly identified transferrin receptor (TfR2) in intestinal tissue and cells

Mutations in the HFE gene and a newly identified second transferrin receptor gene, TfR2, cause hemochromatosis. The cognate proteins, HFE and TfR2, are therefore of key importance in human iron homeostasis. HFE is expressed in small intestinal crypt cells where transferrin-iron entry may determine subsequent iron absorption by mature enterocytes, but the physiological function of TfR2 is unknown. Using specific peptide antisera, we examined the duodenal localization of HFE and TfR2 in humans and mice, with and without HFE deficiency, by confocal microscopy. We also investigated potential interactions of these proteins in human intestinal cells in situ. Duodenal expression of HFE and TfR2 (but not TfR1) in wild-type mice and humans was restricted to crypt cells, in which they co-localized. HFE deficiency disrupted this interaction, altering the cellular distribution of TfR2 in human crypts. In human Caco-2 cells, HFE and TfR2 co-localized to a distinct CD63-negative vesicular compartment showing marked signal enhancement on exposure to iron-saturated transferrin ligand, indicating that HFE preferentially interacts with TfR2 in a specialized early endosomal transport pathway for transferrin-iron. This interaction occurs specifically in small intestinal crypt cells that differentiate to become iron-absorbing enterocytes. Our immunohistochemical findings provide evidence for a novel mechanism for the regulation of iron balance in mammals.

Cadmium transport by human Nramp 2 expressed in Xenopus laevis oocytes

Using the Xenopus oocyte expression system, human Nramp2, a human intestinal iron transporter, was shown to work as a cadmium transporter. An 1824-bp human Nramp2 cDNA was constructed by PCR cloning from reverse transcription products of human kidney mRNA. When the pH of the extracellular solution was 6.0, human Nramp2 transported (109)Cd(2+). Substitution of external Cl(-) with NO3- had no effect on human Nramp2-dependent cadmium uptake. The concentration-dependent Cd(2+) transport of human Nramp2 indicated Michaelis-Menten type transport with an average K(m) value of 1.04 +/- 0.13 microM and an average V(max) of 14.7 +/- 1.9 pmol/oocyte/h (n = 3). Cd(2+) transport via human Nramp2 was inhibited significantly by Cd(2+), Fe(2+), Pb(2+), Mn(2+), Cu(2+), and Ni(2+), while it was not inhibited by Hg(2+) and Zn(2+). Transport of 0.1 microM Cd(2+) by human Nramp2 was inhibited by metallothionein (IC50 = 0.14 microM). Therefore, human Nramp2 is suggested to function as a pH-dependent cadmium absorption transporter on the luminal membrane of human intestinal cells.

Alternative splicing regulates the subcellular localization of divalent metal transporter 1 isoforms

Divalent metal transporter 1 (DMT1) is responsible for dietary-iron absorption from apical plasma membrane in the duodenum and iron acquisition from the transferrin cycle endosomes in peripheral tissues. Two isoforms of the DMT1 transcript generated by alternative splicing of the 3' exons have been identified in mouse, rat, and human. These isoforms can be distinguished by the different C-terminal amino acid sequences and by the presence (DMT1A) or absence (DMT1B) of an iron response element located in the 3' untranslated region of the mRNA. However, it has been still unknown whether the structural differences between the two DMT1 isoforms is functionally important. Here, we report that each DMT1 isoform exhibits a differential cell type-specific expression patterns and distinct subcellular localizations. DMT1A is predominantly expressed by epithelial cell lines, whereas DMT1B is expressed by the blood cell lines. In HEp-2 cells, GFP-tagged DMT1A is localized in late endosomes and lysosomes, whereas GFP-tagged DMT1B is localized in early endosomes. Using site-directed mutagenesis, a Y(555)XLXX sequence in the cytoplasmic tail of DMT1B has been identified as an important signal sequence for the early endosomal-targeting of DMT1B. In polarized MDCK cells, GFP-tagged DMT1A and DMT1B are localized in the apical plasma membrane and their respective specific endosomes. Disruption of the N-glycosylation sites in each of the DMT1 isoforms affects their polarized distribution into the apical plasma membrane but not their correct endosomal localization. Our data indicate that the cell type-specific expression patterns and the distinct subcellular localizations of two DMT1 isoforms may be involved in the different iron acquisition steps from the subcellular membranes in various cell types.

Previously uncharacterized isoforms of divalent metal transporter (DMT)-1: implications for regulation and cellular function

Divalent metal transporter 1 (DMT1) mediates apical iron uptake into duodenal enterocytes and also transfers iron from the endosome into the cytosol after cellular uptake via the transferrin receptor. Hence, mutations in DMT1 cause systemic iron deficiency and anemia. DMT1 mRNA levels are increased in the duodenum of iron-deficient animals. This regulation has been observed for DMT1 mRNA harboring an iron-responsive element (IRE) in its 3' UTR, but not for a processing variant lacking a 3'UTR IRE, suggesting that the IRE regulates the expression of DMT1 mRNA in response to iron levels. Here, we show that iron regulation of DMT1 involves the expression of a previously unrecognized upstream 5' exon (exon 1A) of the human and murine DMT1 gene. The expression of this previously uncharacterized 5' exon is tissue-specific and particularly prevalent in the duodenum and kidney. It adds an in-frame AUG translation initiation codon extending the DMT1 ORF by a conserved sequence of 29-31 amino acids. In combination with the IRE- and non-IRE variants in the 3'UTR, our results reveal the existence of four DMT1 mRNA isoforms predicting the synthesis of four different DMT1 proteins. We show that two regulatory regions, the 5' promoter/exon 1A region and the IRE-containing terminal exon participate in iron regulation of DMT1 expression, which operate in a tissue-specific way. These results uncover an unexpected complexity of DMT1 expression and regulation, with implications for understanding the physiology, cell biology, and pathophysiology of mammalian iron metabolism.

Molecular aspects of iron absorption: Insights into the role of HFE in hemochromatosis

Hereditary hemochromatosis is the most common genetic disorder occurring in persons of northern European descent, and the clinical hallmark of the disease is the gradual accumulation of iron in internal organs, especially the liver, heart, and pancreas, which ultimately leads to organ failure. HFE, the gene that is defective in the majority of cases, was identified in 1996 and, although the exact role that HFE plays in the uptake and utilization of iron is not yet clear, important aspects of HFE function are emerging. Identification and studies of new proteins involved in the absorption of iron in the gut and in somatic cells has led to a clearer picture of how humans absorb iron from the diet and regulate this absorption to meet metabolic needs and to balance body iron stores. This review focuses on the molecular aspects of iron absorption and the role that HFE may play in these processes.

Intestinal iron uptake determined by divalent metal transporter is enhanced in HFE-deficient mice with hemochromatosis

BACKGROUND & AIMS

Overexpression of duodenal divalent metal transporter (DMT1) messenger RNA occurs in hemochromatosis and HFE-knockout mice, suggesting that DMT1 mediates enhanced absorption of iron; however, increased expression of functional DMT1 protein has yet to be substantiated. We examined the role of DMT1 and the mucosal iron uptake defect in HFE-knockout mice.

METHODS

Unidirectional iron uptake of 59Fe by small intestinal mucosa in vitro was compared between matched pairs of HFE-knockout and wild-type mice. DMT1-specific antibodies were used to block iron transport and to quantify duodenal protein expression.

RESULTS

Ferrous iron uptake at 3.5-450 micromol/L was greatly enhanced in HFE-knockouts compared with wild-type, the apparent V(max) for Fe2+ transport being doubled (P < 0.01). Supplied as Fe3+, uptake was only enhanced in HFE-knockouts at < or =18 micromol/L, when the iron was almost completely converted to Fe2+ by mucosal ferrireductases. DMT1 antibody reduced the apparent Vmax for mucosal Fe2+ transport in HFE-knockouts to below wild-type control values (P < 0.02); immunoreactive mucosal DMT1 protein was increased nearly 2-fold in HFE-knockouts (P < 0.01).

CONCLUSIONS

Disruption of the HFE gene up-regulates functional DMT1 transporters and enhances uptake of ferrous iron by this mechanism; DMT1 also mediates increased uptake after reduction of ferric iron presented at physiological concentrations.