Ferroportin/IREG-1/MTP-1/SLC40A1 modulates the uptake of iron at the apical membrane of enterocytes

Monocyte MRI Relaxation Rates Are Regulated by Extracellular Iron and Hepcidin

Many chronic inflammatory conditions are mediated by an increase in the number of monocytes in peripheral circulation, differentiation of monocytes to macrophages, and different macrophage subpopulations during pro- and anti-inflammatory stages of tissue injury. When hepcidin secretion is stimulated during inflammation, the iron export protein ferroportin is targeted for degradation on a limited number of cell types, including monocytes and macrophages. Such changes in monocyte iron metabolism raise the possibility of non-invasively tracking the activity of these immune cells using magnetic resonance imaging (MRI). We hypothesized that hepcidin-mediated changes in monocyte iron regulation influence both cellular iron content and MRI relaxation rates. In response to varying conditions of extracellular iron supplementation, ferroportin protein levels in human THP-1 monocytes decreased two- to eightfold, consistent with paracrine/autocrine regulation of iron export. Following hepcidin treatment, ferroportin protein levels further decreased two- to fourfold. This was accompanied by an approximately twofold increase in total transverse relaxation rate, R₂*, compared to non-supplemented cells. A positive correlation between total cellular iron content and R₂* improved from moderate to strong in the presence of hepcidin. These findings suggest that hepcidin-mediated changes detected in monocytes using MRI could be valuable for in vivo cell tracking of inflammatory responses.

Internalization and Decrease of Duodenal DMT1 Involved in Transient Suppression of Iron Uptake in Short-Acting Mucosal Block

Mucosal block (MB) is induced by the oral administration of excess iron (10 mg) and suppresses intestinal iron absorption for 3-72 h. The inhibition of iron absorption is accompanied by the downregulation of molecules associated with intestinal iron absorption. Recently, we found that a smaller amount of iron (1 mg) also induced a transient suppression of iron uptake without affecting gene expression levels (short-acting mucosal block, SAMB), which is specific to iron-deficient rats. In this study, we investigated how the nonheme iron transporters divalent metal transporter 1 (DMT1) and ferroportin (FPN) are involved in the transient suppression of iron uptake in SAMB. To induce SAMB, a test solution containing 1 mg iron was infused into the duodenum loop in iron-sufficient and iron-deficient rats. Total duodenal DMT1 and DMT1-IRE expression were increased during iron deficiency. After 15 min of 1 mg iron loading, the fluorescence intensity of duodenal DMT1 in iron-deficient rats was decreased and was comparable to that in iron-sufficient rats. Internalized DMT1-IRE as puncta was observed at 15 and 60 min after 1 mg iron loading, and the number of punctas was significantly increased after 60 min compared with control. There was no effect of 1 mg iron loading on the intracellular distribution of duodenal FPN. Our results suggest that the decrease and internalization of DMT1-IRE protein may be related, at least in part, to iron uptake suppression in SAMB.

Hepcidin-mediated Iron Regulation in P19 Cells is Detectable by Magnetic Resonance Imaging

Magnetic resonance imaging can be used to track cellular activities in the body using iron-based contrast agents. However, multiple intrinsic cellular iron handling mechanisms may also influence the detection of magnetic resonance (MR) contrast: a need to differentiate among those mechanisms exists. In hepcidin-mediated inflammation, for example, downregulation of iron export in monocytes and macrophages involves post-translational degradation of ferroportin. We examined the influence of hepcidin endocrine activity on iron regulation and MR transverse relaxation rates in multi-potent P19 cells, which display high iron import and export activities, similar to alternatively-activated macrophages. Iron import and export were examined in cultured P19 cells in the presence and absence of iron-supplemented medium, respectively. Western blots indicated the levels of transferrin receptor, ferroportin and ubiquitin in the presence and absence of extracellular hepcidin. Total cellular iron was measured by inductively-coupled plasma mass spectrometry and correlated to transverse relaxation rates at 3 Tesla using a gelatin phantom. Under varying conditions of iron supplementation, the level of ferroportin in P19 cells responds to hepcidin regulation, consistent with degradation through a ubiquitin-mediated pathway. This response of P19 cells to hepcidin is similar to that of classically-activated macrophages. The correlation between total cellular iron content and MR transverse relaxation rates was different in hepcidin-treated and untreated P19 cells: slope, Pearson correlation coefficient and relaxation rate were all affected. These findings may provide a tool to non-invasively distinguish changes in endogenous iron contrast arising from hepcidin-ferroportin interactions, with potential utility in monitoring of different macrophage phenotypes involved in pro- and anti-inflammatory signaling. In addition, this work demonstrates that transverse relaxivity is not only influenced by the amount of cellular iron but also by its metabolism.

Twenty Years of Ferroportin Disease: A Review or An Update of Published Clinical, Biochemical, Molecular, and Functional Features

Iron overloading disorders linked to mutations in ferroportin have diverse phenotypes in vivo, and the effects of mutations on ferroportin in vitro range from loss of function (LOF) to gain of function (GOF) with hepcidin resistance. We reviewed 359 patients with 60 ferroportin variants. Overall, macrophage iron overload and low/normal transferrin saturation (TSAT) segregated with mutations that caused LOF, while GOF mutations were linked to high TSAT and parenchymal iron accumulation. However, the pathogenicity of individual variants is difficult to establish due to the lack of sufficiently reported data, large inter-assay variability of functional studies, and the uncertainty associated with the performance of available in silico prediction models. Since the phenotypes of hepcidin-resistant GOF variants are indistinguishable from the other types of hereditary hemochromatosis (HH), these variants may be categorized as ferroportin-associated HH, while the entity ferroportin disease may be confined to patients with LOF variants. To further improve the management of ferroportin disease, we advocate for a global registry, with standardized clinical analysis and validation of the functional tests preferably performed in human-derived enterocytic and macrophagic cell lines. Moreover, studies are warranted to unravel the definite structure of ferroportin and the indispensable residues that are essential for functionality.

Tumor-initiating cells of breast and prostate origin show alterations in the expression of genes related to iron metabolism

The importance of iron in the growth and progression of tumors has been widely documented. In this report, we show that tumor-initiating cells (TICs), represented by spheres derived from the MCF7 cell line, exhibit higher intracellular labile iron pool, mitochondrial iron accumulation and are more susceptible to iron chelation. TICs also show activation of the IRP/IRE system, leading to higher iron uptake and decrease in iron storage, suggesting that level of properly assembled cytosolic iron-sulfur clusters (FeS) is reduced. This finding is confirmed by lower enzymatic activity of aconitase and FeS cluster biogenesis enzymes, as well as lower levels of reduced glutathione, implying reduced FeS clusters synthesis/utilization in TICs. Importantly, we have identified specific gene signature related to iron metabolism consisting of genes regulating iron uptake, mitochondrial FeS cluster biogenesis and hypoxic response (ABCB10, ACO1, CYBRD1, EPAS1, GLRX5, HEPH, HFE, IREB2, QSOX1 and TFRC). Principal component analysis based on this signature is able to distinguish TICs from cancer cells in vitro and also Leukemia-initiating cells (LICs) from non-LICs in the mouse model of acute promyelocytic leukemia (APL). Majority of the described changes were also recapitulated in an alternative model represented by MCF7 cells resistant to tamoxifen (TAMR) that exhibit features of TICs. Our findings point to the critical importance of redox balance and iron metabolism-related genes and proteins in the context of cancer and TICs that could be potentially used for cancer diagnostics or therapy.

Targeting Iron Homeostasis in Acute Kidney Injury

Iron is an essential metal involved in several major cellular processes required to maintain life. Because of iron's ability to cause oxidative damage, its transport, metabolism, and storage is strictly controlled in the body, especially in the small intestine, liver, and kidney. Iron plays a major role in acute kidney injury and has been a target for therapeutic intervention. However, the therapies that have been effective in animal models of acute kidney injury have not been successful in human beings. Targeting iron trafficking via ferritin, ferroportin, or hepcidin may offer new insights. This review focuses on the biology of iron, particularly in the kidney, and its implications in acute kidney injury.

Iron Supplementation Attenuates the Inflammatory Status of Anemic Piglets by Regulating Hepcidin

Iron deficiency is common throughout the world and has been linked to immunity impairments. Using piglets to model human infants, we assessed the impact of systemic iron homeostasis on proinflammatory status. Artificially reared piglets were parenterally supplied with iron dextran by intramuscular administration at the age of 3 days. Relative to no iron supplementation (control), iron dextran-treated (FeDex) piglets increased hematological parameters as well as iron levels in serum and tissues from days 21 to 49. High expression of hepcidin was observed in FeDex-treated piglets, which correlated with suppressed expression of ferroportin in duodenum. Lower levels of proinflammatory cytokine (IL-6, TNF-α, IFN-γ, and IL-1β) transcripts were detected in ileum of FeDex-treated piglets, which indicated that iron supplementation could attenuate the increase of inflammatory cytokines caused by iron deficiency. Histopathological analysis of liver and duodenum proved the less inflammatory responses after iron supplementation. Hepcidin was highly stimulated by FeDex supplementation and attenuated the inflammation of anemia, which implied that hepcidin might had antiinflammatory function and is a candidate regulator of the cross-talk between iron regulation and inflammation.

Ferroportin deficiency impairs manganese metabolism in flatiron mice

We examined the physiologic role of ferroportin (Fpn) in manganese (Mn) export using flatiron (ffe/+) mice, a genetic model of Fpn deficiency. Blood (0.0123 vs. 0.0107 mg/kg; P = 0.0003), hepatic (1.06 vs. 0.96 mg/kg; P = 0.0125), and bile Mn levels (79 vs. 38 mg/kg; P = 0.0204) were reduced in ffe/+ mice compared to +/+ controls. Erythrocyte Mn-superoxide dismutase was also reduced at 6 (0.154 vs. 0.096, P = 0.0101), 9 (0.131 vs. 0.089, P = 0.0162), and 16 weeks of age (0.170 vs. 0.090 units/mg protein/min; P < 0.0001). (54)Mn uptake after intragastric gavage was markedly reduced in ffe/+ mice (0.0187 vs. 0.0066% dose; P = 0.0243), while clearance of injected isotope was similar in ffe/+ and +/+ mice. These values were compared to intestinal absorption of (59)Fe, which was significantly reduced in ffe/+ mice (8.751 vs. 3.978% dose; P = 0.0458). The influence of the ffe mutation was examined in dopaminergic SH-SY5Y cells and human embryonic HEK293T cells. While expression of wild-type Fpn reversed Mn-induced cytotoxicity, ffe mutant H32R failed to confer protection. These combined results demonstrate that Fpn plays a central role in Mn transport and that flatiron mice provide an excellent genetic model to explore the role of this exporter in Mn homeostasis. -

Iron regulates the expression of ferroportin 1 in the cultured hFOB 1.19 osteoblast cell line

Iron metabolism is tightly regulated in osteoblasts, and ferroportin 1 (FPN1) is the only identified iron exporter in mammals to date. In the present study, the regulation of FNP1 in human osteoblasts was investigated following various iron treatments. The human osteoblast cell line hFOB 1.19 was treated with ferric ammonium citrate (FAC) or desferrioxamine (DFO) of various concentrations. The intracellular iron ion levels were measured using a confocal laser scanning microscope. In addition, the mRNA and protein expression levels of FPN1 were detected by quantitative polymerase chain reaction, western blot analysis and immunofluorescence. The results demonstrated that increasing iron concentrations via FAC treatment increased the expression of FPN1. By contrast, decreasing the iron concentration by DFO treatment decreased FNP1 expression levels. In addition to demonstrating that the FNP1 expression changed according to the iron concentration, the observations indicated that changes in FPN1 expression may contribute to the maintenance of the intracellular iron balance in osteoblasts.

Moonlighting cell surface GAPDH recruits Apo Transferrin to effect iron egress from mammalian cells

Iron homeostasis is a tightly regulated process with precise control of its influx and egress from cells. Though mechanisms of its import into cells via iron carrier molecules are well characterized, iron export remains poorly understood. The current paradigm envisages unique functions associated with specialized macromolecules for its cellular import (transferrin receptors) or export (ferroportin) Earlier studies have revealed that, iron depleted cells recruit Glyceraldehyde-3-phosphate dehydrogenase (GAPDH), a moonlighting protein to their surface for internalization of the iron carrier holo transferrin (holo Tf). Here we report that under the contrary condition of intracellular iron excess, cells switch the isoform of GAPDH on their surface which now recruits iron free apo transferrin in close association with ferroportin to facilitate efflux of iron. Increased surface GAPDH expression synchronized with increased apo Tf binding and enhanced iron export from cells, a capability lost in GAPDH knockdown cells. These findings were confirmed in vivo utilizing a rodent model of iron overload. Besides identifying for the first time an apo transferrin receptor, our work uncovers two-way switching of multifunctional molecules for managing cellular micronutrient requirements.

Silencing the Menkes copper-transporting ATPase (Atp7a) gene in rat intestinal epithelial (IEC-6) cells increases iron flux via transcriptional induction of ferroportin 1 (Fpn1)

The Menkes copper-transporting ATPase (Atp7a) gene is induced in rat duodenum during iron deficiency, consistent with copper accumulation in the intestinal mucosa and liver. To test the hypothesis that ATP7A influences intestinal iron metabolism, the Atp7a gene was silenced in rat intestinal epithelial (IEC-6) cells using short hairpin RNA (shRNA) technology. Perturbations in intracellular copper homeostasis were noted in knockdown cells, consistent with the dual roles of ATP7A in pumping copper into the trans-Golgi (for cuproenzyme synthesis) and exporting copper from cells. Intracellular iron concentrations were unaffected by Atp7a knockdown. Unexpectedly, however, vectorial iron ((59)Fe) transport increased (∼33%) in knockdown cells grown in bicameral inserts and increased further (∼70%) by iron deprivation (compared with negative control shRNA-transfected cells). Additional experiments were designed to elucidate the molecular mechanism of increased transepithelial iron flux. Enhanced iron uptake by knockdown cells was associated with increased expression of a ferrireductase (duodenal cytochrome b) and activity of a cell-surface ferrireductase. Increased iron efflux from knockdown cells was likely mediated via transcriptional activation of the ferroportin 1 gene (by an unknown mechanism). Moreover, Atp7a knockdown significantly attenuated expression of an iron oxidase [hephaestin (HEPH); by ∼80%] and membrane ferroxidase activity (by ∼50%). Cytosolic ferroxidase activity, however, was retained in knockdown cells (75% of control cells), perhaps compensating for diminished HEPH activity. This investigation has thus documented alterations in iron homeostasis associated with Atp7a knockdown in enterocyte-like cells. Alterations in copper transport, trafficking, or distribution may underlie the increase in transepithelial iron flux noted when ATP7A activity is diminished.

Ferroportin and exocytoplasmic ferroxidase activity are required for brain microvascular endothelial cell iron efflux

The mechanism(s) of iron flux across the brain microvasculature endothelial cells (BMVEC) of the blood-brain barrier remains unknown. Although both hephaestin (Hp) and the ferrous iron permease ferroportin (Fpn) have been identified in BMVEC, their roles in iron efflux have not been examined. Using a human BMVEC line (hBMVEC), we have demonstrated that these proteins are required for iron efflux from these cells. Expression of both Hp and Fpn protein was confirmed in hBMVEC by immunoblot and indirect immunofluorescence; we show that hBMVEC express soluble ceruloplasmin (Cp) transcript as well. Depletion of endogenous Hp and Cp via copper chelation leads to the reduction of hBMVEC Fpn protein levels as well as a complete inhibition of (59)Fe efflux. Both hBMVEC Fpn protein and (59)Fe efflux activity are restored upon incubation with 6.6 nm soluble plasma Cp. These results are independent of the source of cell iron, whether delivered as transferrin- or non-transferrin-bound (59)Fe. Our results demonstrate that iron efflux from hBMVEC Fpn requires the action of an exocytoplasmic ferroxidase, which can be either endogenous Hp or extracellular Cp.

A gene expression atlas of the domestic pig

BACKGROUND

This work describes the first genome-wide analysis of the transcriptional landscape of the pig. A new porcine Affymetrix expression array was designed in order to provide comprehensive coverage of the known pig transcriptome. The new array was used to generate a genome-wide expression atlas of pig tissues derived from 62 tissue/cell types. These data were subjected to network correlation analysis and clustering.

RESULTS

The analysis presented here provides a detailed functional clustering of the pig transcriptome where transcripts are grouped according to their expression pattern, so one can infer the function of an uncharacterized gene from the company it keeps and the locations in which it is expressed. We describe the overall transcriptional signatures present in the tissue atlas, where possible assigning those signatures to specific cell populations or pathways. In particular, we discuss the expression signatures associated with the gastrointestinal tract, an organ that was sampled at 15 sites along its length and whose biology in the pig is similar to human. We identify sets of genes that define specialized cellular compartments and region-specific digestive functions. Finally, we performed a network analysis of the transcription factors expressed in the gastrointestinal tract and demonstrate how they sub-divide into functional groups that may control cellular gastrointestinal development.

CONCLUSIONS

As an important livestock animal with a physiology that is more similar than mouse to man, we provide a major new resource for understanding gene expression with respect to the known physiology of mammalian tissues and cells. The data and analyses are available on the websites http://biogps.org and http://www.macrophages.com/pig-atlas.

Ferroportin-1 is a 'nuclear'-negative acute-phase protein in rat liver: a comparison with other iron-transport proteins

Liver is the central organ of iron metabolism. During acute-phase-response (APR), serum iron concentration rapidly decreases. The current study aimed to compare expression and localization of iron transport protein ferroportin-1 (Fpn-1) and of other iron import proteins after experimental tissue damage induced by injecting turpentine oil in the hind limbs of rats and mice. Serum and spleen iron concentration decreased with an increase in total liver, cytoplasmic and nuclear iron concentration. In liver, mRNA amount of Fpn-1, Fpn-1a, Fpn-1b, HFE, hemojuvelin (HJV) and hephaestin (heph) genes showed a rapid decrease. Hepcidin, divalent metal transporter-1 (DMT-1), transferrin (Tf) and Tf-receptor-1 (TfR1), TfR-2 (TfR2) gene expression was increased. Western blot analysis of liver tissue lysate confirmed the changes observed at mRNA level. In spleen, a rapid decrease in gene expression of Fpn-1, Fpn-1a, Fpn-1b, DMT-1, Tf, TfR1 and TfR2, and an increase in hepcidin was observed. Immunohistochemistry of DMT-1 and TfR2 were mainly detected in the nucleus of rat liver and spleen, whereas TfR1 was clearly localized in the plasma membrane. Fpn-1 was mostly found in the nuclei of liver cells, whereas in spleen, the protein was mainly detected in the cell membrane. Western blot analysis of liver fractions confirmed immunohistochemical results. In livers of wild-type mice, gene expression of Fpn-1, Fpn-1a and Fpn-1b was downregulated, whereas hepcidin gene expression was increased. In contrast, these changes were less pronounced in IL-6ko-mice. Cytokine (IL-6, IL-1b and TNF-a) treatment of rat hepatocytes showed a downregulation of Fpn-1, Fpn-1a and Fpn-1b, and upregulation of hepcidin gene expression. Moreover, western blot analysis of cell lysate of IL-6-treated hepatocytes detected, as expected, an increase of a2-macroglobulin (positive acute-phase protein), whereas albumin (negative acute-phase protein) and Fpn-1 were downregulated. Our results demonstrate that liver behaves as a 'sponge' for iron under acute-phase conditions, and Fpn-1 behaves as a negative acute-phase protein in rat hepatocytes mainly, but not exclusively, because of the effect of IL-6. These changes could explain iron retention in the cytoplasm and in the nucleus of hepatocytes during APR.

Profile of altered brain iron acquisition in restless legs syndrome

Restless legs syndrome is a neurological disorder characterized by an urgency to move the legs during periods of rest. Data from a variety of sources provide a compelling argument that the amount of iron in the brain is lower in individuals with restless legs syndrome compared with neurologically normal individuals. Moreover, a significant percentage of patients with restless legs syndrome are responsive to intravenous iron therapy. The mechanism underlying the decreased iron concentrations in restless legs syndrome brains is unknown. We hypothesize that the source of the brain iron deficit is at the blood-brain interface. Thus we analysed the expression of iron management proteins in the epithelial cells of the choroid plexus and the brain microvasculature in post-mortem tissues. The choroid plexus, obtained at autopsy, from 18 neurologically normal controls and 14 individuals who had primary restless legs syndrome was subjected to histochemical staining for iron and immunostaining for iron management proteins. Iron and heavy chain ferritin staining was reduced in the epithelial cells of choroid plexus in restless legs syndrome. Divalent metal transporter, ferroportin, transferrin and its receptor were upregulated in the choroid plexus in restless legs syndrome. Microvessels were isolated from the motor cortex of 11 restless legs syndrome and 14 control brains obtained at autopsy and quantitative immunoblot analyses was performed. Expression of heavy chain ferritin, transferrin and its receptor in the microvessels from restless legs syndrome was significantly decreased compared with the controls but divalent metal protein 1, ferroportin, prohepcidin, mitochondrial ferritin and light-chain ferritin remained unchanged. The presence of an iron regulatory protein was demonstrated in the brain microvasculature and the activity of this protein is decreased in restless legs syndrome; a finding similar to our earlier report in neuromelanin cells from the substantia nigra of restless legs syndrome brains. This study reveals that there are alterations in the iron management protein profile in restless legs syndrome compared with controls at the site of blood-brain interface suggesting fundamental differences in brain iron acquisition in individuals with restless legs syndrome. Furthermore, the decrease in transferrin receptor expression in the microvasculature in the presence of relative brain iron deficiency reported in restless legs syndrome brains may underlie the problems associated with brain iron acquisition in restless legs syndrome. The consistent finding of loss of iron regulatory protein activity in restless legs syndrome brain tissue further implicates this protein as a factor in the underlying cause of the iron deficiency in the restless legs syndrome brain. The data herein provide evidence for regulation of iron uptake and storage within brain microvessels that challenge the existing paradigm that the blood-brain barrier is merely a transport system.

Influence of garlic or its main active component diallyl disulfide on iron bioavailability and toxicity

Garlic is regularly consumed and is known to have diverse biologic activities, particularly due to its antioxidant properties. In this study, we hypothesized that crude garlic can prevent iron-mediated oxidative stress in a rat model of nutritional iron overload, and we used an in vitro model to confirm the results. For the in vivo studies, rats received a basal diet supplemented with or without carbonyl iron (3%) and were fed distilled water or garlic solution (1g/kg body weight) by gavage for 3 weeks. The presence of both garlic and iron led to a 2-fold increase in plasma iron and a 50% increase in liver iron as compared with iron alone. However, garlic did not offer any protection against iron-induced oxidative stress. Duodenal divalent metal transporter-1 mRNA expression was fully repressed by iron and by the combined treatments but was also reduced by garlic alone. To confirm these data, we tested the effect of diallyl disulfide, one of the active components in garlic, in vitro on polarized Caco-2 cells. A 24-hour treatment decreased iron uptake at the apical side of Caco-2 cells but increased the percentage of iron transfer at the basolateral side. This probably resulted from a modest induction of ferroportin mRNA and protein expression. These results suggest that garlic, when given in the presence of iron, enhances iron absorption by increasing ferroportin expression. The presence of garlic in the diet at the dose studied does not seem to protect against iron-mediated oxidative stress.

Regulatory mechanisms of intestinal iron absorption-uncovering of a fast-response mechanism based on DMT1 and ferroportin endocytosis

Knowledge on the intestinal iron transport process and the regulation of body iron stores has greatly increased during the last decade. The liver, through the sensing of circulating iron, is now recognized as the central organ in this regulation. High iron levels induce the synthesis of hepcidin, which in turn decreases circulating iron by inhibiting its recycling from macrophages and its absorption at the intestine. Another mechanism for the control of iron absorption by the enterocyte is an active Iron Responsive Element (IRE)/Iron Regulatory Protein (IRP) system. The IRE/IRP system regulates the expression of iron uptake and storage proteins thus regulating iron absorption. Similarly, increasing evidence points to the transcriptional regulation of both divalent metal transporter 1 (DMT1) and ferroportin expression. A new mechanism of regulation related to a phenomenon called the mucosal block is starting to be unveiled. The mucosal block describes the ability of an initial dose of ingested iron to block absorption of a second dose given 2-4 h later. Here, we review the mechanisms involved in the expression of DMT1 and ferroportin, and present recent evidence on the molecular components and cellular processes involved in the mucosal block response. Our studies indicate that mucosal block is a fast-response endocytic mechanism destined to decrease intestinal iron absorption during a high ingest of iron.

Iron supply determines apical/basolateral membrane distribution of intestinal iron transporters DMT1 and ferroportin 1

Intestinal iron absorption comprises the coordinated activity of the influx transporter divalent metal transporter 1 (DMT1) and the efflux transporter ferroportin (FPN). In this work, we studied the movement of DMT1 and FPN between cellular compartments as a function of iron supply. In rat duodenum, iron gavage resulted in the relocation of DMT1 to basal domains and the internalization of basolateral FPN. Considerable FPN was also found in apical domains. In Caco-2 cells, the apical-to-basal movement of cyan fluorescent protein-tagged DMT1 was complete 90 min after the addition of iron. Steady-state membrane localization studies in Caco-2 cells revealed that iron status determined the apical/basolateral membrane distribution of DMT1 and FPN. In agreement with the membrane distribution of the transporters, (55)Fe flux experiments revealed inward and outward iron fluxes at both membrane domains. Antisense oligonucleotides targeted to DMT1 or FPN inhibited basolateral iron uptake and apical iron efflux, respectively, indicating the participation of DMT1 and FPN in these fluxes. The fluxes were regulated by the iron supply; increased iron reduced apical uptake and basal efflux and increased basal uptake and apical efflux. These findings suggest a novel mechanism of regulation of intestinal iron absorption based on inward and outward fluxes at both membrane domains, and repositioning of DMT1 and FPN between membrane and intracellular compartments as a function of iron supply. This mechanism should be complementary to those based in the transcriptional or translational regulation of iron transport proteins.

Role of the kidney in iron homeostasis: renal expression of Prohepcidin, Ferroportin, and DMT1 in anemic mice

It is known that renal tissue plays a role in normal iron homeostasis. The current study examines kidney function in iron metabolism under hemolytic anemia studying renal expression of Prohepcidin, Ferroportin (MTP1), and divalent metal transporter 1 (DMT1). The relationship between these proteins and iron pigments was also investigated. Immunohistochemical procedures to study renal expression of Prohepcidin, MTP1, and DMT1 were performed in healthy and anemic mice. Renal tissue iron was determined by Prussian blue iron staining. To assess anemia evolution and erythropoietic recovery, we used conventional tests. In healthy mice, Prohepcidin expression was marked in proximal tubules and inner medulla and absent in outer medulla. Cortical tissue of healthy mice also showed MTP1 immunostaining, mainly in the S2 segment of proximal tubules. Medullar tissue showed MTP1 expression in the inner zone. In addition, S2 segments showed intense DMT1 immunoreactivity with homogeneous DMT1 distribution throughout renal medulla. The main cortical findings in hemolytic anemia were in S2 segments of proximal tubules where we found that decreased Prohepcidin expression coincided with an increment in Ferroportin and DMT1 expression. This expression pattern was concomitant with increased iron in the same tubular zone. However, in medullar tissue both Prohepcidin and MTP1 decreased and DMT1 was detected mainly in larger diameter tubules. Our findings clearly demonstrate that in hemolytic anemia, renal Prohepcidin acts in coordination with renal Ferroportin and DMT1, indicating the key involvement of kidney in iron homeostasis when iron demand is high. Further research is required to learn more about these regulatory mechanisms.

Effect of hemolytic and iron-deficiency anemia on intestinal absorption and tissue accumulation of cadmium

Abnormal iron (Fe) metabolism induces iron-deficiency anemia (FeDA) and also affects body cadmium (Cd) accumulation. However, whether hemolytic anemia also affects Cd metabolism is not known. We compared the intestinal absorption and tissue accumulation of Cd after oral administration of Cd to mice with hemolytic anemia induced by treatment with phenylhydrazine (PHA mice) to that in mice with FeDA. Although the hematocrit decreased significantly in mice with either type of anemia, the Fe concentration decreased in the livers and kidneys of FeDA mice, but increased in those of PHA mice. After an oral administration with various amounts of Cd, hepatic and renal Cd concentrations significantly increased in both FeDA and PHA mice. An intraduodenal injection of Fe raised the hepatic Fe content in FeDA mice to the control level and raised the hepatic Fe content in PHA mice to 2.4 times that in control mice. Intestinal divalent metal transporter 1 (DMT1) expression increased significantly in mice with both types of anemia. These data indicate that, despite the accumulation of hepatic Fe associated with PHA, PHA also significantly increases hepatic and renal Cd accumulation according to an stimulation of intestinal DMT1 expression, as occurs in FeDA mice. This suggests that anemia may be a risk factor for Cd accumulation.

A partially purified putative iron P type-ATPase mediates Fe3+-transport into proteoliposome

We report that two fractions containing proteins from rat hepatocyte nuclei, obtained by nondenaturing gel electrophoresis, were able to bind iron and ATP, and to hydrolyze ATP. Electroelution of these two active fractions followed by SDS-PAGE analysis showed an identical protein pattern, each one containing four proteins in a range of 62-80 kDa. Phosphorylated protein bands were also detected in acid gel and disappeared after treatment with hydroxylamine/acetate or KOH, and upon chasing with cold ATP. A proteoliposome system, made by the incorporation of these partially purified protein fractions into phosphatidylcholine vesicles, carried out Fe(3+)-citrate uptake in a Mg(2+)-ATP-dependent way; Fe(3+) accumulation increased with time reaching a plateau in 30 min. Iron uptake was not supported by AMP-PNP, was partially inhibited by orthovanadate and was not affected by a mix of specific inhibitors of known ATPases. These results support our previous hypothesis that a putative nuclear membrane Fe(3+)-ATPase is involved in nuclear iron homeostasis.

Hemochromatosis: genetics and pathophysiology

A number of genetic disorders can result in the accumulation of excess iron in the body. These causes of hereditary hemochromatosis include defects in genes encoding HFE, transferrin receptor 2, ferroportin, hepcidin, and hemojuvelin. Hepcidin, with its cognate receptor, ferroportin, has emerged as a central regulator of iron homeostasis; all of the known causes of hemochromatosis appear to prevent this system from functioning normally. The most common form of primary hemochromatosis is that caused by C282Y mutation of the HFE gene. This mutation is most prevalent among Northern Europeans. Although the frequency of the homozygous genotype is approximately 5 per 1000, the disease itself is quite rare because the clinical penetrance of the genotype is very low.

Role of L-type Ca2+ channels in iron transport and iron-overload cardiomyopathy

Excessive body iron or iron overload occurs under conditions such as primary (hereditary) hemochromatosis and secondary iron overload (hemosiderosis), which are reaching epidemic levels worldwide. Primary hemochromatosis is the most common genetic disorder with an allele frequency greater than 10% in individuals of European ancestry, while hemosiderosis is less common but associated with a much higher morbidity and mortality. Iron overload leads to iron deposition in many tissues especially the liver, brain, heart and endocrine tissues. Elevated cardiac iron leads to diastolic dysfunction, arrhythmias and dilated cardiomyopathy, and is the primary determinant of survival in patients with secondary iron overload as well as a leading cause of morbidity and mortality in primary hemochromatosis patients. In addition, iron-induced cardiac injury plays a role in acute iron toxicosis (iron poisoning), myocardial ischemia–reperfusion injury, Friedreich ataxia and neurodegenerative diseases. Patients with iron overload also routinely suffer from a range of endocrinopathies, including diabetes mellitus and anterior pituitary dysfunction. Despite clear connections between elevated iron and clinical disease, iron transport remains poorly understood. While low-capacity divalent metal and transferrin-bound transporters are critical under normal physiological conditions, L-type Ca²⁺ channels (LTCC) are high-capacity pathways of ferrous iron (Fe²⁺) uptake into cardiomyocytes especially under iron overload conditions. Fe²⁺ uptake through L-type Ca²⁺ channels may also be crucial in other excitable cells such as pancreatic beta cells, anterior pituitary cells and neurons. Consequently, LTCC blockers represent a potential new therapy to reduce the toxic effects of excess iron.

Iron Imports. V. Transport of iron through the intestinal epithelium

Iron absorption across the brush-border membrane requires divalent metal transporter 1 (DMT1), whereas ferroportin (FPN) and hephaestin are required for exit across the basolateral membrane. However, how iron passes across the enterocyte is poorly understood. Both chaperones and transcytosis have been postulated to account for intracellular iron transport. With iron feeding, DMT1 undergoes endocytosis and FPN translocates from the apical cytosol to the basolateral membrane. The fluorescent metallosensor calcein offered to the basolateral surface of enterocytes is found in endosomes in the apical compartment, and its fluorescence is quenched when iron is offered to the apical surface. These experiments are consistent with vesicular iron transport as a possible pathway for intracellular iron transport.

Vesicular transport and apotransferrin in intestinal iron absorption, as shown in the Caco-2 cell model

The potential roles of vesicular transport and apotransferrin (entering from the blood) in intestinal Fe absorption were investigated using Caco-2 cell monolayers with tight junctions in bicameral chambers as a model. As shown previously, addition of 39 microM apotransferrin (apoTf) to the basolateral fluid during absorption studies markedly stimulated overall transport of 1 microM (59)Fe from the apical to the basal chamber and stimulated its basolateral release from prelabeled cells, implicating endo- and exocytosis. Rates of transport more than doubled. Uptake was also stimulated, but only 20%. Specific inhibitors of aspects of vesicular trafficking were applied to determine their potential effects on uptake, retention, and basolateral (overall) transport of (59)Fe. Nocodazole and 5'-(4-fluorosulfonylbenzoyl)-adenosine each reduced uptake and basolateral transport up to 50%. Brefeldin A inhibited about 10%. Tyrphostin A8 (AG10) reduced uptake 35% but markedly stimulated basolateral efflux, particularly that dependent on apoTf. Cooling of cells to 4 degrees C (which causes depolymerization of microtubules and lowers energy availability) profoundly inhibited uptake and basolateral transfer of Fe (7- to 12-fold). Apical efflux (which was substantial) was not temperature affected. Our results support the involvement of apoTf cycling in intestinal Fe absorption and indicate that as much as half of the iron uses apoTf and non-apoTf-dependent vesicular pathways to cross the basolateral membrane and brush border of enterocytes.

Use of the zebrafish system to study primitive and definitive hematopoiesis

The zebrafish (Danio rerio) has emerged as an ideal organism for the study of hematopoiesis, the process by which all the cellular elements of the blood are formed. These elements, including erythrocytes, granulocytes, monocytes, lymphocytes, and thrombocytes, are formed through complex genetic signaling pathways that are highly conserved throughout phylogeny. Large-scale forward genetic screens have identified numerous blood mutants in zebrafish, helping to elucidate specific signaling pathways important for hematopoietic stem cells (HSCs) and the various committed blood cell lineages. Here we review both primitive and definitive hematopoiesis in zebrafish, discuss various genetic methods available in the zebrafish model for studying hematopoiesis, and describe some of the zebrafish blood mutants identified to date, many of which have known human disease counterparts.

A role for tumour necrosis factor alpha in human small bowel iron transport

Cytokines are integral to the development of anaemia of chronic inflammation. Cytokines modulate hepcidin expression and iron sequestration by the reticuloendothelial system but their direct effects on small bowel iron transport are not well characterized. The aim of the present study was to examine the local effects of TNFalpha (tumour necrosis factor alpha) on small bowel iron transport and on iron transporter expression in the absence of hepcidin. The effects of TNFalpha on iron transport were determined using radiolabelled iron in an established Caco-2 cell model. The effect of TNFalpha on the expression and localization of the enterocyte iron transporters DMT-1 (divalent metal transporter 1), IREG-1 (iron-regulated transporter 1) and ferritin was determined utilizing Caco-2 cells and in a human ex vivo small bowel culture system. TNFalpha mediated an early induction in both iron import and iron export, which were associated with increased DMT-1 and IREG-1 mRNA and protein expression (P<0.05). However, by 24 h, both iron import and iron export were significantly inhibited, coinciding with an induction of ferritin heavy chain (P<0.05) and a decrease in DMT-1 and IREG-1 to baseline levels. In addition, there was a relocalization of IREG-1 away from the basolateral cell border and increased iron deposition in villous enterocytes. In conclusion, TNFalpha has a direct effect on small bowel iron transporter expression and function, leading to an inhibition of iron transport.

Mobilisation of recently absorbed 59Fe in ex vivo perfused rat duodena and the influence of iron status and subsequently absorbed chelators

To investigate the effect of subsequently absorbed metal chelators on recently absorbed 59Fe, duodenal segments from iron-deficient and iron-adequate rats were perfused ex vivo until the 59Fe tissue load had reached a steady state. Subsequently, the segments were perfused with 3 model chelators and their iron complexes: nitrilotriacetic acid (NTA), ethylenediaminetetraacetic acid (EDTA) and citrate. Of these, NTA and EDTA bind iron much tighter than citrate, and Fe-NTA complexes exchange iron within seconds while Fe-EDTA complexes need 48 h to reach equilibrium. Duodenal mucosa-to-serosa transport rates were comparable for all 3 chelators and correlated linearly with luminal concentration. Subsequent perfusion with increasing NTA, Fe-NTA(1:2) and EDTA concentrations mobilised increasing amounts of 59Fe from the duodenum. Mobilised 59Fe moved preferentially back into the luminal perfusate in iron-adequate segments. In iron-deficient segments, 59Fe preferentially continued the absorption process across the basolateral membrane. Fe-EDTA(1:1) hardly mobilised any 59Fe back into the lumen, though basolateral transfer increased at high concentrations. Citrate and Fe-citrate(1:1) mobilised 59Fe only at very high concentrations. This behaviour is in accordance with the rules of complex chemistry: strong, fast reacting ligands like NTA show most impact. Slowly reacting complexes like Fe-EDTA(1:1) have little mobilising impact in spite of strong affinity between EDTA and iron. The low affinity between iron and citrate can be compensated by large concentration. Moreover, iron-deficient segments show stronger re-uptake of mobilised 59Fe from the lumen and a stronger transfer of 59Fe from the tissue across the basolateral membrane. Both are compatible with the more marked expression of divalent metal transporter 1 (DMT-1) and IREG-1 at the brushborder and basolateral membrane of iron-deficient enterocytes. The data suggest that iron ions interact with food ligands during their passage from the apical to the basolateral side of duodenal enterocytes.

Comparative studies of duodenal and macrophage ferroportin proteins

Intestinal epithelial cells and reticuloendothelial macrophages are, respectively, involved in diet iron absorption and heme iron recycling from senescent erythrocytes, two critical processes of iron homeostasis. These cells appear to use the same transporter, ferroportin (Slc40a1), to export iron. The aim of this study was to compare the localization, expression, and regulation of ferroportin in both duodenal and macrophage cells. Using a high-affinity purified polyclonal antibody, we analyzed the localization and expression of ferroportin protein in the spleen, liver, and duodenum isolated from normal mice as well as from well-characterized mouse models of altered iron homeostasis. Ferroportin was found to be predominantly expressed in enterocytes of the duodenum, in splenic macrophages, and in liver Kupffer cells. Interestingly, the protein species detected in these cells migrated differently on SDS-PAGE. These differences in apparent molecular masses were partly explained by posttranslational complex N-linked glycosylations. In addition, in enterocytes, the transporter was mostly expressed at the basolateral membrane, whereas in bone marrow-derived macrophages, ferroportin was found predominantly localized in the intracellular vesicular compartment. However, some microdomains positive for ferroportin were also detected at the plasma membrane of macrophages. Despite these differences, we observed a parallel upregulation of ferroportin expression in tissue macrophages and enterocytes in response to iron-restricted erythropoiesis, suggesting that iron homeostasis is likely maintained through coordinate expression of the iron exporter in both intestinal and phagocytic cells. Our data also confirm a predominant regulation of ferroportin through systemic regulator(s) likely including hepcidin.

Intestinal over-expression of iron transporters induces iron overload in birds in captivity

Hereditary hemochromatosis (HH) is a frequent genetic disease of older subjects of northern European descent. It is characterized by increased iron absorption and severe iron overloading in parenchymal organs. A similar disturbance of iron metabolism occurs in specific animal species in captivity. To address the key features leading to high absorption and thus to iron overload in these animals, we have studied the two iron transport proteins DMT1 and Ireg1 in the best-known susceptible species, the mynah bird. Here, we show that these birds have a high expression of DMT1 in the duodenum and also a strikingly high expression of Ireg1 along the whole small intestine. We believe that the iron accumulation in susceptible species only occurs in captivity because of a genotypic adaptation to their natural environment, where contrary to captivity, dietary iron is hardly available. The Caucasian population carrying mutations leading to iron overload today may have also benefited from the genetic advantage of up-regulating iron transport millennia ago, when dietary iron was scarce.

Augmented internalisation of ferroportin to late endosomes impairs iron uptake by enterocyte-like IEC-6 cells

Absorption of iron occurs by duodenal enterocytes, involving uptake by the divalent metal transporter-1 (DMT1) and release by ferroportin. Ferroportin responds to the hepatocyte-produced 25-amino-acid-peptide hepcidin-25 by undergoing internalisation to late endosomes that impair iron release. Ferroportin is also expressed on the apical membrane of polarised Caco-2 cells, rat intestinal cells and in IEC-6 cells (an intestinal epithelial cell line). A blocking antibody to ferroportin also impairs the uptake, but not the release, of iron. In this study IEC-6 cells were used to study the mechanism of impairment or recovery from impairment produced by the blocking antibody and the fate of DMT1 and ferroportin. Uptake of 1 muM Fe(II) was studied by adding the antibody from time 0 and after adding or removing the antibody once a steady state had been reached. Surface binding, maximum iron transport rate V(max) and transporter affinity (K(m)) were measured after impairment of iron uptake. Ferroportin and DMT1 distribution were assessed by immunofluorescence microscopy. Antibody-mediated impairment, or recovery from impairment, of Fe(II) uptake occurred within minutes. Impairment was lost when the antibody was combined with the immunizing peptide. DMT1 and ferroportin undergo internalisation to late endosomes and, in the presence of the antibody, augmented internalisation of DMT1 and ferroportin caused swelling of late endosomes. Surface binding of Fe(II) and iron transport V(max) were reduced by 50%, indicating that the antibody removed membrane-bound DMT1. The ferroportin antibody induced rapid turnover of membrane ferroportin and DMT1 and its internalisation to late endosomes, resulting in impaired Fe(II) uptake.

Changes in the transcriptional profile of transporters in the intestine along the anterior-posterior and crypt-villus axes

Background

The purpose of this work was to characterize the expression of drug and nutrient carriers along the anterior-posterior and crypt-villus axes of the intestinal epithelium and to study the validity of utilizing whole gut tissue rather than purified epithelial cells to examine regional variations in gene expression.

Results

We have characterized the mRNA expression profiles of 76 % of all currently known transporters along the anterior-posterior axis of the gut. This is the first study to describe the expression profiles of the majority of all known transporters in the intestine. The expression profiles of transporters, as defined according to the Gene Ontology consortium, were measured in whole tissue of the murine duodenum, jejunum, ileum and colon using high-density microarrays. For nine transporters (Abca1, Abcc1, Abcc3, Abcg8, Slc10a2, Slc28a2, Slc2a1, Slc34a2 and Slc5a8), the mRNA profiles were further measured by RT-PCR in laser micro-dissected crypt and villus epithelial cells corresponding to the aforementioned intestinal regions. With respect to differentially regulated transporters, the colon had a distinct expression profile from small intestinal segments. The majority (59 % for p cutoff ≤ 0.05) of transporter mRNA levels were constant across the intestinal sections studied. For the transporter subclass "carrier activity", which contains the majority of known carriers for biologically active compounds, a significant change (p ≤ 0.05) along the anterior-posterior axis was observed.

Conclusion

All nine transporters examined in laser-dissected material demonstrated good replication of the region-specific profiles revealed by microarray. Furthermore, we suggest that the distribution characteristics of Slc5a8 along the intestinal tract render it a suitable candidate carrier for monocarboxylate drugs in the posterior portion of the intestine. Our findings also predict that there is a significant difference in the absorption of carrier-mediated compounds in the different intestinal segments. The most pronounced differences can be expected between the adjoining segments ileum and colon, but the differences between the other adjoining segments are not negligible. Finally, for the examined genes, profiles measured in whole intestinal tissue extracts are representative of epithelial cell-only gene expression.

Endocytic and Transcytotic Processes in Villous Syncytiotrophoblast: Role in Nutrient Transport to the Human Fetus

The supply of nutrients to the developing fetus is a major function of the human hemochorial placenta, a placenta type in which the fetal chorion is in direct contact with the maternal blood. At term, nutrients have to be transported across two cell layers in chorionic villi, the syncytiotrophoblast (STB) and fetal endothelial cells. The STB is a continuous syncytium covering the entire surface of chorionic villi. This polarized epithelium is specialized in exchange processes and membrane trafficking between the apical membrane facing the maternal blood and the basal membrane facing the fetal endothelium. To meet placental and fetal requirements, the STB selectively takes up and transports a variety of nutrients, hormones, growth factors and cytokines and also transfers passive immunity to the fetus by receptor-mediated transcytosis. In this review in vivo and in vitro systems currently used to study STB functions are discussed and the potential mechanisms of transplacental IgG, iron, lipoprotein and glucose transport are presented. As revealed in this article, the placenta is a tissue where intensive cell biological research is required to unravel endocytic trafficking pathways in a highly specialized cell such as the STB.

Differences in the uptake of iron from Fe(II) ascorbate and Fe(III) citrate by IEC-6 cells and the involvement of ferroportin/IREG-1/MTP-1/SLC40A1

Dietary iron is present in the intestine as Fe(II) and Fe(III). Since enterocytes take up Fe(II) by the divalent metal transporter (DMT1), Fe(III) must be reduced. Whether other Fe(III) transport processes are present is unknown. Release of iron from the enterocyte into the plasma involves the iron-regulated transporter-1/metal transporter protein-1 (IREG-1/MTP-1, ferroportin) but ferroportin is also found on the apical membrane. We compared the uptake of iron from Fe(II):ascorbate and Fe(III):citrate using the rat intestinal enterocyte cell line-6 (IEC-6), in the presence of ferrous chelators, a blocking antibody to ferroportin, at different pH and during the over-expression of DMT1. Firstly, surface ferrireduction was absent. Secondly, blocking ferroportin partly and totally reduced Fe(II) and Fe(III) uptake, respectively. Thirdly, optimal Fe(II) uptake occurred at pH 5.5 but Fe(III) uptake was unaffected by pH and, fourthly, over-expression of DMT1 increased uptake of Fe(II) and Fe(III). This indicates that an increased extracellular H+ concentration facilitates DMT1-mediated Fe(II) uptake at the cell membrane. However, since Fe(III) uptake required DMT1, but not cell surface ferrireduction, and was independent of variations in extracellular pH, it appears that Fe(III) is internalised before ferrireduction and transport by DMT1. Ferroportin may function as a modulator of DMT1 activity and play a role in Fe(III) uptake, possibly by affecting the number or affinity of citrate binding sites.