Characterization of non-transferrin-bound iron clearance by rat liver

The (Bio)Chemistry of Non-Transferrin-Bound Iron

In healthy individuals, virtually all blood plasma iron is bound by transferrin. However, in several diseases and clinical conditions, hazardous non-transferrin-bound iron (NTBI) species occur. NTBI represents a potentially toxic iron form, being a direct cause of oxidative stress in the circulating compartment and tissue iron loading. The accumulation of these species can cause cellular damage in several organs, namely, the liver, spleen, and heart. Despite its pathophysiological relevance, the chemical nature of NTBI remains elusive. This has precluded its use as a clinical biochemical marker and the development of targeted therapies. Herein, we make a critical assessment of the current knowledge of NTBI speciation. The currently accepted hypotheses suggest that NTBI is mostly iron bound to citric acid and iron bound to serum albumin, but the chemistry of this system remains fuzzy. We explore the complex chemistry of iron complexation by citric acid and its implications towards NTBI reactivity. Further, the ability of albumin to bind iron is revised and the role of protein post-translational modifications on iron binding is discussed. The characterization of the NTBI species structure may be the starting point for the development of a standardized analytical assay, the better understanding of these species’ reactivity or the identification of NTBI uptake mechanisms by different cell types, and finally, to the development of new therapies.

Clinical Factors Associated with Hepatocellular Iron Deposition in End-stage Liver Disease

Background and Aims: Hepatocellular iron accumulation in patients with chronic liver disease has been linked to adverse outcomes. The objective of this study was to identify clinical factors associated with hemosiderosis. Methods: A total of 103 consecutive liver transplant recipients were identified, in whom liver biopsy had been performed prior to transplantation. Laboratory and clinical data at biopsy and transplant were abstracted from the medical records and hepatocyte iron was graded in the biopsy and explant. The association of change in iron score from biopsy to transplant, with the time interval between these two events, was examined using linear mixed model analysis for repeated measures. Results: Most subjects had advanced fibrosis (F3-F4) at liver biopsy, which was performed on average about 2.5 years before transplant. Over 80% of patients had no or 1+ hepatocyte iron at biopsy; iron increased between biopsy and transplant in about 40%. The only demographic or clinical feature that correlated with increased iron was the presence of a transjugular intrahepatic portosystemic shunt. Increased iron at transplant was associated with higher serum iron and transferrin saturation at biopsy, and with lower hemoglobin level, greater mean corpuscular volume, mean corpuscular hemoglobin and mean corpuscular hemoglobin concentration, higher ferritin and model for end-stage liver disease score at transplant. Conclusions: The development of hemosiderosis in end-stage liver disease is associated with lower hemoglobin levels and alterations in red blood cell indices that are suggestive of hemolysis. These observations suggest that extravascular hemolysis may play a role in the development of secondary iron overload.

Liver Iron Retention Estimated from Utilization of Oral and Intravenous Radioiron in Various Anemias and Hemochromatosis in Humans

Patients with hereditary hemochromatosis and non-transfusion-dependent hereditary anemia develop predominantly liver iron-overload. We present a unique method allowing quantification of liver iron retention in humans during first-pass of ⁵⁹Fe-labeled iron through the portal system, using standard ferrokinetic techniques measuring red cell iron uptake after oral and intravenous ⁵⁹Fe administration. We present data from patients with iron deficiency (ID; N = 47), hereditary hemochromatosis (HH; N = 121) and non-transfusion-dependent hereditary anemia (HA; N = 40). Mean mucosal iron uptake and mucosal iron transfer (±SD) were elevated in patients with HH (59 ± 18%, 80 ± 15% respectively), HA (65 ± 17%, 74 ± 18%) and ID (84 ± 14%, 94 ± 6%) compared to healthy controls (43 ± 19%, 64 ± 18%) (p < 0.05) resulting in increased iron retention after 14 days compared to healthy controls in all groups (p < 0.01). The fraction of retained iron utilized for red cell production was 0.37 ± 0.17 in untreated HA, 0.55 ± 0.20 in untreated HH and 0.99 ± 0.22 in ID (p < 0.01). Interestingly, compared to red blood cell iron utilization after oral iron administration, red blood cell iron utilization was higher after injection of transferrin-bound iron in HA and HH. Liver iron retention was considerably higher in HH and HA compared to ID. We hypothesize that albumin serves as a scavenger of absorbed Fe(II) for delivering albumin-bound Fe(III) to hepatocytes.

Iron-Induced Liver Injury: A Critical Reappraisal

Iron is implicated in the pathogenesis of a number of human liver diseases. Hereditary hemochromatosis is the classical example of a liver disease caused by iron, but iron is commonly believed to contribute to the progression of other forms of chronic liver disease such as hepatitis C infection and nonalcoholic fatty liver disease. In this review, we present data from cell culture experiments, animal models, and clinical studies that address the hepatotoxicity of iron. These data demonstrate that iron overload is only weakly fibrogenic in animal models and rarely causes serious liver damage in humans, calling into question the concept that iron overload is an important cause of hepatotoxicity. In situations where iron is pathogenic, iron-induced liver damage may be potentiated by coexisting inflammation, with the resulting hepatocyte necrosis an important factor driving the fibrogenic response. Based on the foregoing evidence that iron is less hepatotoxic than is generally assumed, claims that assign a causal role to iron in liver injury in either animal models or human liver disease should be carefully evaluated.

Review article: iron disturbances in chronic liver diseases other than haemochromatosis - pathogenic, prognostic, and therapeutic implications

BACKGROUND

Disturbances in iron regulation have been described in diverse chronic liver diseases other than hereditary haemochromatosis, and iron toxicity may worsen liver injury and outcome.

AIMS

To describe manifestations and consequences of iron dysregulation in chronic liver diseases apart from hereditary haemochromatosis and to encourage investigations that clarify pathogenic mechanisms, define risk thresholds for iron toxicity, and direct management METHODS: English abstracts were identified in PubMed by multiple search terms. Full length articles were selected for review, and secondary and tertiary bibliographies were developed.

RESULTS

Hyperferritinemia is present in 4%-65% of patients with non-alcoholic fatty liver disease, autoimmune hepatitis, chronic viral hepatitis, or alcoholic liver disease, and hepatic iron content is increased in 11%-52%. Heterozygosity for the C282Y mutation is present in 17%-48%, but this has not uniformly distinguished patients with adverse outcomes. An inappropriately low serum hepcidin level has characterised most chronic liver diseases with the exception of non-alcoholic fatty liver disease, and the finding has been associated mainly with suppression of transcriptional activity of the hepcidin gene. Iron overload has been associated with oxidative stress, advanced fibrosis and decreased survival, and promising therapies beyond phlebotomy and oral iron chelation have included hepcidin agonists.

CONCLUSIONS

Iron dysregulation is common in chronic liver diseases other than hereditary haemochromatosis, and has been associated with liver toxicity and poor prognosis. Further evaluation of iron overload as a co-morbid factor should identify the key pathogenic disturbances, establish the risk threshold for iron toxicity, and promote molecular interventions.

Non-transferrin-bound iron transporters

Most cells in the body acquire iron via receptor-mediated endocytosis of transferrin, the circulating iron transport protein. When cellular iron levels are sufficient, the uptake of transferrin decreases to limit further iron assimilation and prevent excessive iron accumulation. In iron overload conditions, such as hereditary hemochromatosis and thalassemia major, unregulated iron entry into the plasma overwhelms the carrying capacity of transferrin, resulting in non-transferrin-bound iron (NTBI), a redox-active, potentially toxic form of iron. Plasma NTBI is rapidly cleared from the circulation primarily by the liver and other organs (e.g., pancreas, heart, and pituitary) where it contributes significantly to tissue iron overload and related pathology. While NTBI is usually not detectable in the plasma of healthy individuals, it does appear to be a normal constituent of brain interstitial fluid and therefore likely serves as an important source of iron for most cell types in the CNS. A growing body of literature indicates that NTBI uptake is mediated by non-transferrin-bound iron transporters such as ZIP14, L-type and T-type calcium channels, DMT1, ZIP8, and TRPC6. This review provides an overview of NTBI uptake by various tissues and cells and summarizes the evidence for and against the roles of individual transporters in this process.

ALK3 undergoes ligand-independent homodimerization and BMP-induced heterodimerization with ALK2

The bone morphogenetic protein (BMP) type I receptors ALK2 and ALK3 are essential for expression of hepcidin, a key iron regulatory hormone. In mice, hepatocyte-specific Alk2 deficiency leads to moderate iron overload with periportal liver iron accumulation, while hepatocyte-specific Alk3 deficiency leads to severe iron overload with centrilobular liver iron accumulation and a more marked reduction of basal hepcidin levels. The objective of this study was to investigate whether the two receptors have additive roles in hepcidin regulation. Iron overload in mice with hepatocyte-specific Alk2 and Alk3 (Alk2/3) deficiency was characterized and compared to hepatocyte-specific Alk3 deficient mice. Co-immunoprecipitation studies were performed to detect the formation of ALK2 and ALK3 homodimer and heterodimer complexes in vitro in the presence and absence of ligands. The iron overload phenotype of hepatocyte-specific Alk2/3-deficient mice was more severe than that of hepatocyte-specific Alk3-deficient mice. In vitro co-immunoprecipitation studies in Huh7 cells showed that ALK3 can homodimerize in absence of BMP2 or BMP6. In contrast, ALK2 did not homodimerize in either the presence or absence of BMP ligands. However, ALK2 did form heterodimers with ALK3 in the presence of BMP2 or BMP6. ALK3-ALK3 and ALK2-ALK3 receptor complexes induced hepcidin expression in Huh7 cells. Our data indicate that: (I) ALK2 and ALK3 have additive functions in vivo, as Alk2/3 deficiency leads to a greater degree of iron overload than Alk3 deficiency; (II) ALK3, but not ALK2, undergoes ligand-independent homodimerization; (III) the formation of ALK2-ALK3 heterodimers is ligand-dependent and (IV) both receptor complexes functionally induce hepcidin expression in vitro.

Ferrous glycinate regulates cell energy metabolism by restrictinghypoxia-induced factor-1α expression in human A549 cells

Iron or oxygen regulates the stability of hypoxia inducible factor-1α (HIF-1α). We investigated whether ferrous glycinate would affect HIF-1α accumulation, aerobic glycolysis and mitochondrial energy metabolism in human A549 lung cancer cells. Incubation of A549 cells with ferrous glycinate decreased the protein levels of HIF-1α, which was abrogated by proteosome inhibitor, or prolyl hydroxylase inhibitor. The addition of ferrous glycinate decreased protein levels of glucose transporter-1, hexokinase-2, and lactate dehydrogenase A, and decreased pyruvate dehydrogenase kinase-1 (PDK-1) and pyruvate dehydrogenase (PDH) phosphorylation in A549 cells. Ferrous glycinate also increased the expression of the mitochondrial transcription factor A (TFAM), and the mitochondrial protein, cytochrome c oxidase (COX-IV). Silencing of HIF-1α expression mimicked the effects of ferrous glycinate on PDK-1, PDH, TFAM and COX-IV in A549 cells. Ferrous glycinate increased mitochondrial membrane potential and ATP production in A549 cells. These results suggest that ferrous glycinate may reverse Warburg effect through down regulating HIF-1α in A549 cells.

Kinetics of iron import into developing mouse organs determined by a pup-swapping method

The kinetics of dietary iron import into various organs of mice were evaluated using a novel pup-swapping approach. Newborn pups whose bodies primarily contained (56)Fe or (57)Fe were swapped at birth such that each nursed on milk containing the opposite isotope. A pup from each litter was euthanized weekly over a 7-week period. Blood plasma was obtained, and organs were isolated typically after flushing with Ringer's buffer. (56)Fe and (57)Fe concentrations were determined for organs and plasma; organ volumes were also determined. Mössbauer spectra of equivalent (57)Fe-enriched samples were used to quantify residual blood in organs; this fraction was excluded from later analysis. Rates of import into brain, spleen, heart, and kidneys were highest during the first 2 weeks of life. In contrast, half of iron in the newborn liver exited during that time, and influx peaked later. Two mathematical models were developed to analyze the import kinetics. The only model that simulated the data adequately assumed that an iron-containing species enters the plasma and converts into a second species and that both are independently imported into organs. Consistent with this, liquid chromatography with an on-line ICP-MS detector revealed numerous iron species in plasma besides transferrin. Model fitting required that the first species, assigned to non-transferrin-bound iron, imports faster into organs than the second, assigned to transferrin-bound-iron. Non-transferrin-bound iron rather than transferrin-bound-iron appears to play the dominant role in importing iron into organs during early development of healthy mice.

Iron overload and chelation therapy in myelodysplastic syndromes

Iron overload remains a concern in MDS patients especially those requiring recurrent blood transfusions. The consequence of iron overload may be more relevant in patients with low and intermediate-1 risk MDS who may survive long enough to experience such manifestations. It is a matter of debate whether this overload has time to yield organ damage, but it is quite evident that cellular damage and DNA genotoxic effect are induced. Iron overload may play a critical role in exacerbating pre-existing morbidity or even unmask silent ones. Under these circumstances, iron chelation therapy could play an integral role in the management of these patients. This review entails an in depth analysis of iron overload in MDS patients; its pathophysiology, effect on survival, associated risks and diagnostic options. It also discusses management options in relation to chelation therapy used in MDS patients and the impact it has on survival, hematologic response and organ function.

Sugars increase non-heme iron bioavailability in human epithelial intestinal and liver cells

Previous studies have suggested that sugars enhance iron bioavailability, possibly through either chelation or altering the oxidation state of the metal, however, results have been inconclusive. Sugar intake in the last 20 years has increased dramatically, and iron status disorders are significant public health problems worldwide; therefore understanding the nutritional implications of iron-sugar interactions is particularly relevant. In this study we measured the effects of sugars on non-heme iron bioavailability in human intestinal Caco-2 cells and HepG2 hepatoma cells using ferritin formation as a surrogate marker for iron uptake. The effect of sugars on iron oxidation state was examined by measuring ferrous iron formation in different sugar-iron solutions with a ferrozine-based assay. Fructose significantly increased iron-induced ferritin formation in both Caco-2 and HepG2 cells. In addition, high-fructose corn syrup (HFCS-55) increased Caco-2 cell iron-induced ferritin; these effects were negated by the addition of either tannic acid or phytic acid. Fructose combined with FeCl3 increased ferrozine-chelatable ferrous iron levels by approximately 300%. In conclusion, fructose increases iron bioavailability in human intestinal Caco-2 and HepG2 cells. Given the large amount of simple and rapidly digestible sugars in the modern diet their effects on iron bioavailability may have important patho-physiological consequences. Further studies are warranted to characterize these interactions.

Non-transferrin-bound iron (NTBI) uptake by T lymphocytes: evidence for the selective acquisition of oligomeric ferric citrate species

Iron is an essential nutrient in several biological processes such as oxygen transport, DNA replication and erythropoiesis. Plasma iron normally circulates bound to transferrin. In iron overload disorders, however, iron concentrations exceed transferrin binding capacity and iron appears complexed with low molecular weight molecules, known as non-transferrin-bound iron (NTBI). NTBI is responsible for the toxicity associated with iron-overload pathologies but the mechanisms leading to NTBI uptake are not fully understood. Here we show for the first time that T lymphocytes are able to take up and accumulate NTBI in a manner that resembles that of hepatocytes. Moreover, we show that both hepatocytes and T lymphocytes take up the oligomeric Fe3Cit3 preferentially to other iron-citrate species, suggesting the existence of a selective NTBI carrier. These results provide a tool for the identification of the still elusive ferric-citrate cellular carrier and may also open a new pathway towards the design of more efficient iron chelators for the treatment of iron overload disorders.

Involvement of polyamines in iron(III) transport in human intestinal Caco-2 cell lines

Natural polyamines such as putrescine (Put), spermidine (Spd), and spermine (Spm), which are present in the human diet in large amounts, associated with their active transporter, are assumed to play a role in non-heme iron uptake and iron bioavailability from nutrients. Enterocytes and hepatocytes play pivotal roles in the regulation of body iron homeostasis. In this study, we report the effects of natural polyamines on iron transport in the Caco-2 cell line. In enterocyte-like Caco-2 cells, polyamines did not significantly modulate the transepithelial iron flux across the cell monolayer cultured on permeable membranes. In contrast, Spd, Spm, and to a lesser extent, Put were shown to activate Caco-2 cell iron uptake and to induce an increase in the ferritin level. This iron co-transport in enterocytes, which involved an interaction between iron and polyamine then cell uptake of the polyamine-iron complexes by the polyamine transport system, was more pronounced in proliferating than in differentiated Caco-2 cells. Moreover, it was observed at physiological concentrations of both polyamines and iron. It could thus play a role in the rapid renewal of enterocytes. These data suggest the involvement of polyamines as components of the pool of transferrin-independent iron-chelating vectors. Further investigations are needed to demonstrate their biological relevance in physiological situations.

Review article: the iron overload syndromes

BACKGROUND

Iron overload syndromes encompass a wide range of hereditary and acquired conditions. Major developments in the field of genetics and the discovery of hepcidin as a central regulator of iron homeostasis have greatly increased our understanding of the pathophysiology of iron overload syndromes.

AIM

To review advances in iron regulation and iron overload syndrome with special emphasis on hereditary haemochromatosis, the prototype iron overload syndrome.

METHODS

A PubMed search using words such as 'iron overload', 'hemochromatosis', 'HFE', 'Non-HFE', 'secondary iron overload' was undertaken.

RESULTS

Iron overload is associated with significant morbidity and mortality. Sensitive diagnostic tests and effective therapy are widely available and can prevent complications associated with iron accumulation in end- organs. Therapeutic phlebotomy remains the cornerstone of therapy for removal of excess body iron, but novel therapeutic agents including oral iron chelators have been developed for iron overload associated with anaemia.

CONCLUSIONS

Iron overload disorders are common. Inexpensive screening tests as well as confirmatory diagnostic tests are widely available. Increased awareness of the causes and importance of early diagnosis and knowledge of the appropriate use of genetic testing are encouraged. The availability of novel treatments should increase therapeutic options for patients with iron overload disorders.

A novel N491S mutation in the human SLC11A2 gene impairs protein trafficking and in association with the G212V mutation leads to microcytic anemia and liver iron overload

BACKGROUND

DMT1 is a transmembrane iron transporter involved in iron duodenal absorption and cellular iron uptake. Mutations in the human SLC11A2 gene coding DMT1 lead to microcytic anemia and hepatic iron overload, with unexpectedly low levels of plasma ferritin in the presence of iron stores.

DESIGN AND METHODS

We report a patient with a similar phenotype due to two mutations in the SLC11A2 gene, the known p.Gly212Val (G212V) mutation and a novel one, p.Asn491Ser (N491S). To assess the expression of DMT1 in human liver, we studied the expression of the four DMT1 mRNA isoforms by real-time quantitative PCR in control human liver samples. We also studied the effect of G212V and N491S DMT1 mutations on RNA splicing in blood leukocytes and cellular trafficking of dsRed2-tagged-DMT1 protein in the human hepatic cell line HuH7.

RESULTS

Our results showed that i) only the isoforms 1B-IRE and 1B-nonIRE were significantly expressed in human liver; ii) the G212V mutation did not seem to affect mRNA splicing and the N491S mutation induced a splicing alteration leading to a truncated protein, which seemed quantitatively of low relevance; and iii) the N491S mutation, in contrast to the G212V mutation, led to abnormal protein trafficking.

CONCLUSIONS

Our data confirm the major role of DMT1 in the maintenance of iron homeostasis in humans and demonstrate that the N491S mutation, through its deleterious effect on protein trafficking, contributes together with the G212V mutation to the development of anemia and hepatic iron overload.

Perturbation of hepcidin expression by BMP type I receptor deletion induces iron overload in mice

Bone morphogenetic protein (BMP) signaling induces hepatic expression of the peptide hormone hepcidin. Hepcidin reduces serum iron levels by promoting degradation of the iron exporter ferroportin. A relative deficiency of hepcidin underlies the pathophysiology of many of the genetically distinct iron overload disorders, collectively termed hereditary hemochromatosis. Conversely, chronic inflammatory conditions and neoplastic diseases can induce high hepcidin levels, leading to impaired mobilization of iron stores and the anemia of chronic disease. Two BMP type I receptors, Alk2 (Acvr1) and Alk3 (Bmpr1a), are expressed in murine hepatocytes. We report that liver-specific deletion of either Alk2 or Alk3 causes iron overload in mice. The iron overload phenotype was more marked in Alk3- than in Alk2-deficient mice, and Alk3 deficiency was associated with a nearly complete ablation of basal BMP signaling and hepcidin expression. Both Alk2 and Alk3 were required for induction of hepcidin gene expression by BMP2 in cultured hepatocytes or by iron challenge in vivo. These observations demonstrate that one type I BMP receptor, Alk3, is critically responsible for basal hepcidin expression, whereas 2 type I BMP receptors, Alk2 and Alk3, are required for regulation of hepcidin gene expression in response to iron and BMP signaling.

Non-transferrin bound iron: a key role in iron overload and iron toxicity

BACKGROUND

Besides transferrin iron, which represents the normal form of circulating iron, non-transferrin bound iron (NTBI) has been identified in the plasma of patients with various pathological conditions in which transferrin saturation is significantly elevated.

SCOPE OF THE REVIEW

To show that: i) NTBI is present not only during chronic iron overload disorders (hemochromatosis, transfusional iron overload) but also in miscellaneous diseases which are not primarily iron overloaded conditions; ii) this iron species represents a potentially toxic iron form due to its high propensity to induce reactive oxygen species and is responsible for cellular damage not only at the plasma membrane level but also towards different intracellular organelles; iii) the NTBI concept may be expanded to include intracytosolic iron forms which are not linked to ferritin, the major storage protein which exerts, at the cellular level, the same type of protective effect towards the intracellular environment as transferrin in the plasma.

MAJOR CONCLUSIONS

Plasma NTBI and especially labile plasma iron determinations represent a new important biological tool since elimination of this toxic iron species is a major therapeutic goal.

GENERAL SIGNIFICANCE

The NTBI approach represents an important mechanistic concept for explaining cellular iron excess and toxicity and provides new important biochemical diagnostic tools. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron

Recent studies have shown that overexpression of the transmembrane protein Zrt- and Irt-like protein 14 (Zip14) stimulates the cellular uptake of zinc and nontransferrin-bound iron (NTBI). Here, we directly tested the hypothesis that Zip14 transports free zinc, iron, and other metal ions by using the Xenopus laevis oocyte heterologous expression system, and use of this approach also allowed us to characterize the functional properties of Zip14. Expression of mouse Zip14 in RNA-injected oocytes stimulated the uptake of (55)Fe in the presence of l-ascorbate but not nitrilotriacetic acid, indicating that Zip14 is an iron transporter specific for ferrous ion (Fe(2+)) over ferric ion (Fe(3+)). Zip14-mediated (55)Fe(2+) uptake was saturable (K(0.5) ≈ 2 μM), temperature-dependent (apparent activation energy, E(a) = 15 kcal/mol), pH-sensitive, Ca(2+)-dependent, and inhibited by Co(2+), Mn(2+), and Zn(2+). HCO(3)(-) stimulated (55)Fe(2+) transport. These properties are in close agreement with those of NTBI uptake in the perfused rat liver and in isolated hepatocytes reported in the literature. Zip14 also mediated the uptake of (109)Cd(2+), (54)Mn(2+), and (65)Zn(2+) but not (64)Cu (I or II). (65)Zn(2+) uptake also was saturable (K(0.5) ≈ 2 μM) but, notably, the metal-ion inhibition profile and Ca(2+) dependence of Zn(2+) transport differed from those of Fe(2+) transport, and we propose a model to account for these observations. Our data reveal that Zip14 is a complex, broad-scope metal-ion transporter. Whereas zinc appears to be a preferred substrate under normal conditions, we found that Zip14 is capable of mediating cellular uptake of NTBI characteristic of iron-overload conditions.

Static magnetic field exposure reproduces cellular effects of the Parkinson's disease drug candidate ZM241385

Background

This study was inspired by coalescing evidence that magnetic therapy may be a viable treatment option for certain diseases. This premise is based on the ability of moderate strength fields (i.e., 0.1 to 1 Tesla) to alter the biophysical properties of lipid bilayers and in turn modulate cellular signaling pathways. In particular, previous results from our laboratory (Wang et al., BMC Genomics, 10, 356 (2009)) established that moderate strength static magnetic field (SMF) exposure altered cellular endpoints associated with neuronal function and differentiation. Building on this background, the current paper investigated SMF by focusing on the adenosine A_2A receptor (A_2AR) in the PC12 rat adrenal pheochromocytoma cell line that displays metabolic features of Parkinson's disease (PD).

Methodology and Principal Findings

SMF reproduced several responses elicited by ZM241385, a selective A_2AR antagonist, in PC12 cells including altered calcium flux, increased ATP levels, reduced cAMP levels, reduced nitric oxide production, reduced p44/42 MAPK phosphorylation, inhibited proliferation, and reduced iron uptake. SMF also counteracted several PD-relevant endpoints exacerbated by A_2AR agonist CGS21680 in a manner similar to ZM241385; these include reduction of increased expression of A_2AR, reversal of altered calcium efflux, dampening of increased adenosine production, reduction of enhanced proliferation and associated p44/42 MAPK phosphorylation, and inhibition of neurite outgrowth.

Conclusions and Significance

When measured against multiple endpoints, SMF elicited qualitatively similar responses as ZM241385, a PD drug candidate. Provided that the in vitro results presented in this paper apply in vivo, SMF holds promise as an intriguing non-invasive approach to treat PD and potentially other neurological disorders.

Effects of deferasirox and deferiprone on cellular iron load in the human hepatoma cell line HepaRG

Two oral chelators, CP20 (deferiprone) and ICL670 (deferasirox), have been synthesized for the purpose of treating iron overload diseases, especially thalassemias. Given their antiproliferative effects resulting from the essential role played by iron in cell processes, such compounds might also be useful as anticancer agents. In the present study, we tested the impact of these two iron chelators on iron metabolism, in the HepaRG cell line which allowed us to study proliferating and differentiated hepatocytes. ICL670 uptake was greater than the CP20 uptake. The iron depletion induced by ICL670 in differentiated cells increased soluble transferrin receptor expression, decreased intracellular ferritin expression, inhibited (55)Fe (III) uptake, and reduced the hepatocyte concentration of the labile iron pool. In contrast, CP20 induced an unexpected slight increase in intracellular ferritin, which was amplified by iron-treated chelator exposure. CP20 also promoted Fe(III) uptake in differentiated HepaRG cells, thus leading to an increase of both the labile pool and storage forms of iron evaluated by calcein fluorescence and Perls staining, respectively. In acellular conditions, compared to CP20, iron removing ability from the calcein-Fe(III) complex was 40 times higher for ICL670. On the whole, biological responses of HepaRG cells to ICL670 treatment were characteristic of expected iron depletion. In contrast, the effects of CP20 suggest the potential involvement of this compound in the iron uptake from the external medium into the hepatocytes from the HepaRG cell line, therefore acting like a siderophore in this cell model.

Role of lung iron in determining the bacterial and host struggle in cystic fibrosis

Cystic fibrosis (CF) is the most common lethal genetic disorder in Caucasian populations. It is a multiorgan system disease that affects the lungs, gastrointestinal tract, liver, and pancreas. The majority of morbidity and mortality in CF relates to chronic airway infection with a variety of bacterial species, commencing in very early infancy, which results in lung destruction and ultimately organ failure ( 41 , 43 ). This review focuses on iron homeostasis in the CF lung and its role in determining the success and chronicity of Pseudomonas aeruginosa infection. There have been previous excellent reviews regarding iron metabolism in the lower respiratory tract and mechanisms of P. aeruginosa iron acquisition, and we direct readers to these articles for further background reading ( 31 , 53 , 58 , 77 , 96 ). In this review, we have brought the “two sides of the coin” together to provide a holistic overview of the relationship between host and bacterial iron homeostasis and put this information into the context of current understanding on infection in the CF lung.

Simultaneous multiple element detection by particle beam/hollow cathode-optical emission spectroscopy as a tool for metallomic studies: determinations of metal binding with apo-transferrin

Particle beam/hollow cathode-optical emission spectroscopy (PB/HC-OES) is presented as a tool for the determination of metal ion loading in transferrin (Tf). The elemental specificity of optical emission spectroscopy provides a means of assessing metal ion concentrations as well as the relative amounts of metal per unit protein concentration (up to 2 moles of Fe per mole of protein). The PB/HC-OES method allows for the simultaneous detection of metal content (Fe (I) 371.99, Ni (I) 341.41 nm, Zn (I) 213.86 nm, and Ag (I) 338.28 nm in this case), as well as elemental carbon and sulfur (C (I) 156.14 nm and S (I) 180.73 nm) that are reflective of the protein composition and concentration. Quantification for the metal species is based on calibration functions derived from aqueous solutions, with limits of detection for the entire suite being less than 1.0 μM. Determinations in this manner eliminate much of the ambiguity inherent in UV-VIS absorbance determinations of Tf metal binding. Validation of this method is obtained by analyzing loading response of Fe(3+) into Tf using the PB/HC-OES method and comparing the results with those of the standard UV-VIS absorbance method. Maximum Fe(3+) loading of Tf (based on the number of available binding sites) was determined to be 71.2 ± 4.7% by the PB/HC-OES method and 67.5 ± 2.5% for the UV-VIS absorbance method. Element emission ratios between the dopant metals and the carbon and sulfur protein constituents allow for concentration independent determinations of metal binding into Tf. Loading percentages were determined for Ni(2+), Zn(2+), and Ag(+) into Tf with maximum loading values of 19.5 ± 0.4%, 41.0 ± 4.4%, and 141.2 ± 4.3%, respectively. While of no apparent biological significance, Ag(+) presents an interesting case as a surrogate for Pt(2+), whose binding with Tf has shown to be quite different from the other metals. A different mode from the others is indeed observed, and is consistent with conjecture on the Pt(2+) mechanisms. Competitive binding studies not easily performed using absorbance spectroscopy are easily performed by simultaneous, multielement analysis, reflective of the metals and the protein content. In this work, there is clear competition between and Fe(3+) and Zn(2+) for binding in the C-terminus lobe of Tf, while Ni(2+) binds within the N-terminus lobe. Addition of Ag(+) to this mixture does not affect the other metals' distributions, but reflects binding at other protein sites.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

The regulation of cellular iron metabolism

While iron is an essential trace element required by nearly all living organisms, deficiencies or excesses can lead to pathological conditions such as iron deficiency anemia or hemochromatosis, respectively. A decade has passed since the discovery of the hemochromatosis gene, HFE, and our understanding of hereditary hemochromatosis (HH) and iron metabolism in health and a variety of diseases has progressed considerably. Although HFE-related hemochromatosis is the most widespread, other forms of HH have subsequently been identified. These forms are not attributed to mutations in the HFE gene but rather to mutations in genes involved in the transport, storage, and regulation of iron. This review is an overview of cellular iron metabolism and regulation, describing the function of key proteins involved in these processes, with particular emphasis on the liver's role in iron homeostasis, as it is the main target of iron deposition in pathological iron overload. Current knowledge on their roles in maintaining iron homeostasis and how their dysregulation leads to the pathogenesis of HH are discussed.

Neoplastic transformation of rat liver epithelial cells is enhanced by non-transferrin-bound iron

Background

Iron overload is associated with liver toxicity, cirrhosis, and hepatocellular carcinoma in humans. While most iron circulates in blood as transferrin-bound iron, non-transferrin-bound iron (NTBI) also becomes elevated and contributes to toxicity in the setting of iron overload. The mechanism for iron-related carcinogenesis is not well understood, in part due to a shortage of suitable experimental models. The primary aim of this study was to investigate NTBI-related hepatic carcinogenesis using T51B rat liver epithelial cells, a non-neoplastic cell line previously developed for carcinogenicity and tumor promotion studies.

Methods

T51B cells were loaded with iron by repeated addition of ferric ammonium citrate (FAC) to the culture medium. Iron internalization was documented by chemical assay, ferritin induction, and loss of calcein fluorescence. Proliferative effects were determined by cell count, toxicity was determined by MTT assay, and neoplastic transformation was assessed by measuring colony formation in soft agar. Cyclin levels were measured by western blot.

Results

T51B cells readily internalized NTBI given as FAC. Within 1 week of treatment at 200 μM, there were significant but well-tolerated toxic effects including a decrease in cell proliferation (30% decrease, p < 0.01). FAC alone induced little or no colony formation in soft agar. In contrast, FAC addition to cells previously initiated with N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) resulted in a concentration dependent increase in colony formation. This was first detected at 12 weeks of FAC treatment and increased at longer times. At 16 weeks, colony formation increased more than 10 fold in cells treated with 200 μM FAC (p < 0.001). The iron chelator desferoxamine reduced both iron uptake and colony formation. Cells cultured with 200 μM FAC showed decreased cyclin D1, decreased cyclin A, and increased cyclin B1.

Conclusion

These results establish NTBI as a tumor promoter in T51B rat liver epithelial cells. Changes in cyclin proteins suggest cell cycle disregulation contributes to tumor promotion by NTBI in this liver cell model.

Zip14 (Slc39a14) mediates non-transferrin-bound iron uptake into cells

Zip14 is a member of the SLC39A zinc transporter family, which is involved in zinc uptake by cells. Up-regulation of Zip14 by IL-6 appears to contribute to the hepatic zinc accumulation and hypozincemia of inflammation. At least three members of the SLC39A family transport other trace elements, such as iron and manganese, in addition to zinc. We analyzed the capability of Zip14 to mediate non-transferrin-bound iron (NTBI) uptake by overexpressing mouse Zip14 in HEK 293H cells and Sf9 insect cells. Zip14 was found to localize to the plasma membrane, and its overexpression increased the uptake of both (65)Zn and (59)Fe. Addition of bathophenanthroline sulfonate, a cell-impermeant ferrous iron chelator, inhibited Zip14-mediated iron uptake from ferric citrate, suggesting that iron is taken up by HEK cells as Fe(2+). Iron uptake by HEK and Sf9 cells expressing Zip14 was inhibited by zinc. Suppression of endogenous Zip14 expression by using Zip14 siRNA reduced the uptake of both iron and zinc by AML12 mouse hepatocytes. Zip14 siRNA treatment also decreased metallothionein mRNA levels, suggesting that compensatory mechanisms were not sufficient to restore intracellular zinc. Collectively, these results indicate that Zip14 can mediate the uptake of zinc and NTBI into cells and that it may play a role in zinc and iron metabolism in hepatocytes, where this transporter is abundantly expressed. Because NTBI is commonly found in plasma of patients with hemochromatosis and transfusional iron overload, Zip14-mediated NTBI uptake may contribute to the hepatic iron loading that characterizes these diseases.

Modulation of cell proliferation and polyamine metabolism in rat liver cell cultures by the iron chelator O-trensox

The antiproliferative effects of the iron chelator O-trensox and the ornithine-decarboxylase (ODC) inhibitor alpha-difluoromethylornithine (DFMO) were characterized in the rat hepatoma cell line FAO, the rat liver epithelial cell line (RLEC) and the primary rat hepatocyte cultures stimulated by EGF. We observed that O-trensox and DFMO decreased cell viabilty and DNA replication in the three culture models. The cytostatic effect of O-trensox was correlated to a cytotoxicity, higher than for DFMO, and to a cell cycle arrest in G0/G1 or S phases. Moreover, O-trensox and DFMO decreased the intracellular concentration of spermidine in the three models without changing significantly the spermine level. We concluded that iron, but also polyamine depletion, decrease cell growth. However, the drop in cell proliferation obtained with O-trensox was stronger compared to DFMO effect. Altogether, our data provide insights that, in the three rat liver cell culture models, the cytostatic effect of the iron chelator O-trensox may be the addition of two mechanisms: iron and polyamine depletion.

Iron overload

Iron overload disorders represent a heterogenous group of conditions resulting from inherited and acquired causes. With the discovery of new proteins and genetic defects we have gained greater insight into their causation at the molecular level and the complex mechanisms of normal and disordered iron homeostasis. Here we review the normal mechanisms and regulation of gastrointestinal iron absorption and liver iron transport and their dysregulation in iron overload states. Advances in the understanding of the natural history of iron overload disorders and new methods for clinical detection and management of hereditary hemochromatosis are also reviewed.

Divalent metal-ion transporter DMT1 mediates both H+ -coupled Fe2+ transport and uncoupled fluxes

The H(+) -coupled divalent metal-ion transporter DMT1 serves as both the primary entry point for iron into the body (intestinal brush-border uptake) and the route by which transferrin-associated iron is mobilized from endosomes to cytosol in erythroid precursors and other cells. Elucidating the molecular mechanisms of DMT1 will therefore increase our understanding of iron metabolism and the etiology of iron overload disorders. We expressed wild type and mutant DMT1 in Xenopus oocytes and monitored metal-ion uptake, currents and intracellular pH. DMT1 was activated in the presence of an inwardly directed H(+) electrochemical gradient. At low extracellular pH (pH(o)), H(+) binding preceded binding of Fe(2+) and its simultaneous translocation. However, DMT1 did not behave like a typical ion-coupled transporter at higher pH(o), and at pH(o) 7.4 we observed Fe(2+) transport that was not associated with H(+) influx. His(272) --> Ala substitution uncoupled the Fe(2+) and H(+) fluxes. At low pH(o), H272A mediated H(+) uniport that was inhibited by Fe(2+). Meanwhile H272A-mediated Fe(2+) transport was independent of pH(o). Our data indicate (i) that H(+) coupling in DMT1 serves to increase affinity for Fe(2+) and provide a thermodynamic driving force for Fe(2+) transport and (ii) that His-272 is critical in transducing the effects of H(+) coupling. Notably, our data also indicate that DMT1 can mediate facilitative Fe(2+) transport in the absence of a H(+) gradient. Since plasma membrane expression of DMT1 is upregulated in liver of hemochromatosis patients, this H(+) -uncoupled facilitative Fe(2+) transport via DMT1 can account for the uptake of nontransferrin-bound plasma iron characteristic of iron overload disorders.

Nontransferrin-bound iron uptake by hepatocytes is increased in the Hfe knockout mouse model of hereditary hemochromatosis

Hereditary hemochromatosis (HH) is an iron-overload disorder caused by a C282Y mutation in the HFE gene. In HH, plasma nontransferrin-bound iron (NTBI) levels are increased and NTBI is bound mainly by citrate. The aim of this study was to examine the importance of NTBI in the pathogenesis of hepatic iron loading in Hfe knockout mice. Plasma NTBI levels were increased 2.5-fold in Hfe knockout mice compared with control mice. Total ferric citrate uptake by hepatocytes isolated from Hfe knockout mice (34.1 +/- 2.8 pmol Fe/mg protein/min) increased by 2-fold compared with control mice (17.8 +/- 2.7 pmol Fe/mg protein/min; P <.001; mean +/- SEM; n = 7). Ferrous ion chelators, bathophenanthroline disulfonate, and 2',2-bipyridine inhibited ferric citrate uptake by hepatocytes from both mouse types. Divalent metal ions inhibited ferric citrate uptake by hepatocytes, as did diferric transferrin. Divalent metal transporter 1 (DMT1) mRNA and protein expression was increased approximately 2-fold by hepatocytes from Hfe knockout mice. We conclude that NTBI uptake by hepatocytes from Hfe knockout mice contributed to hepatic iron loading. Ferric ion was reduced to ferrous ion and taken up by hepatocytes by a pathway shared with diferric transferrin. Inhibition of uptake by divalent metals and up-regulation of DMT1 expression suggested that NTBI uptake was mediated by DMT1.

Iron mobilization, cytoprotection, and inhibition of cell proliferation in normal and transformed rat hepatocyte cultures by the hydroxypyridinone CP411, compared to CP20: a biological and physicochemical study

The present study analyzes the iron mobilization, the cytoprotective, and the antiproliferative effects of the lipophilic hydroxypyridinone CP411, in comparison with the hydrophilic chelator CP20 or deferiprone used in the treatment of iron overload. Primary rat hepatocyte cultures and the rat hepatoma cell line Fao were used. Chelator cell uptake was evaluated by mass spectrometry in the two models. This method was also used to investigate the stability of the chelators in an acellular system as well as their scavenging and chelating effects against the hydroxyl radical generated by the Fenton reaction. The iron mobilization and the cytoprotective effects of the chelators were evaluated in primary cultures by measuring respectively 55Fe and lactate dehydrogenase release in the culture medium. The antiproliferative effect of the chelators was studied using the Fao cell line and measuring DNA synthesis by thymidine incorporation and DNA content by flow cytometry. We observed that CP411 entered the hepatocytes and the Fao cells respectively 4 and 13 times more than CP20. CP411 was 2.5 times more effective than CP20 to mobilize iron from preloaded hepatocytes. Pretreatment of the hepatocytes with CP20 or CP411 decreased the toxic effect of iron and CP411 was 1.6 times more effective than CP20. A dose-dependent decrease of DNA synthesis, correlated to an accumulation of cells in S phase, was observed in the Fao cell line in the presence of CP411, while CP20 was without effect. CP411 effect was inhibited by addition of iron simultaneously with the chelator, the addition of Zn or Cu was without effect. The inhibitory effect of CP411 was reversible since, 24hr after removal of the chelator, DNA replication reached the control level. The results show that CP411 is more efficient to protect the hepatocyte from the toxic effect of iron load and to inhibit tumor cell proliferation. Its higher efficiency may result from its better cell uptake since equimolar solutions of the two chelators in an acellular system exhibit the same ability to inhibit the Fenton reaction.

Superoxide-dependent iron uptake: a new role for anion exchange protein 2

Lung cells import iron across the plasma membrane as ferrous (Fe2+) ion by incompletely understood mechanisms. We tested the hypothesis that human bronchial epithelial (HBE) cells import non-transferrin-bound iron (NTBI) using superoxide-dependent ferri-reductase activity involving anion exchange protein 2 (AE2) and extracellular bicarbonate (HCO3-). HBE cells that constitutively express AE2 mRNA by reverse transcriptase-polymerase chain reaction and AE2 protein by Western analysis avidly transported NTBI after exposure to either Fe2+ or Fe3+, but reduction of Fe3+ to Fe2+ was first required. The ability of HBE cells to reduce Fe3+ and transport Fe2+ was inhibited by active extracellular superoxide dismutase (SOD). Similarly, HBE cells that overexpress Cu,Zn SOD after adenoviral infection with AdSOD1 showed diminished iron uptake. The role of AE2 in iron uptake was indicated by three lines of evidence: (i) lack of both iron reduction and iron transport in bicarbonate-free buffer at controlled pH, (ii) failure of HBE cells treated with stilbene AE inhibitors to reduce Fe3+ or transport iron, and (iii) inhibition of iron uptake in HBE cells by inhibition of AE2 protein expression with antisense oligonucleotides. We thus disclose a novel ferri-reductase mechanism of NTBI uptake by human lung cells that employs superoxide exchange for HCO3- by AE2 protein in the plasma membrane.

Molecular mechanisms and regulation of iron transport

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

Iron uptake and Nramp2/DMT1/DCT1 in human bronchial epithelial cells

The capacity of natural resistance-associated macrophage protein-2 [Nramp2; also called divalent metal transporter-1 (DMT1) and divalent cation transporter-1 (DCT1)] to transport iron and its ubiquitous expression make it a likely candidate for transferrin-independent uptake of iron in peripheral tissues. We tested the hypothesis that non-transferrin-bound iron uptake by airway epithelial cells is associated with Nramp2/DMT1/DCT1 and that exposure to iron can increase Nramp2/DMT1/DCT1 mRNA and protein expression and transport of this metal. Exposure of BEAS-2B cells to ferric ammonium citrate (FAC) resulted in a decrease in Fe(3+) concentration in the supernatant that was dependent on time and initial iron concentration. In the presence of internalized calcein, FAC quenched the fluorescent signal, indicating intracellular transport of the metal. The Nramp2/DMT1/DCT1 mRNA isoform without an iron-response element (IRE) increased with exposure of BEAS-2B cells to FAC. RT-PCR demonstrated no change in the mRNA for the isoform with an IRE. Similarly, Western blot analysis for the isoform without an IRE confirmed an increased expression of this protein after FAC exposure, whereas the isoform with an IRE exhibited no change. Finally, immunohistochemistry revealed an increase in the isoform without an IRE in the rat lung epithelium after instillation of FAC. Comparable to mRNA and protein increases, iron transport was elevated after pretreatment of BEAS-2B cells with iron-containing compounds. We conclude that airway epithelial cells increase mRNA and expression of the Nramp2/DMT1/DCT1 without an IRE after exposure to iron. The increase results in an elevated transport of iron and its probable detoxification by these cells.

A novel strategy against ischemia and reperfusion injury: cytoprotection with heme oxygenase system

Much interest has recently been focused on the physiological/pathological role of the heme oxygenase (HO) system, the rate-limiting step in the conversion of heme, in inflammatory events. The HO system may be instrumental in mediating a number of cytoprotective effects, because of its end products, biliverdin, carbon monoxide (CO) and ferrous free iron (Fe2+). As each of the byproducts acts dependently and/or co-operatively with each other, their in vivo effects are complex. In general, the HO system is thought to exert three major functions in ischemia/reperfusion injury: (1) anti-oxidant effects; (2) maintenance of microcirculation; and (3) modulatory effects upon the cell cycle. The anti-oxidant functions depend on heme degradation, oxygen consumption and the production of biliverdin/ferritin via iron accumulation. On the other hand, the production of CO, which has vasodilatory and anti-platelet aggregative properties, can maintain tissue microcirculation. Strikingly, CO may also be instrumental in anti-apoptotic and cell arrest mechanisms. The HO system prevents early injury in the re-perfused organ, and inhibits the function of immune reactive cells, such as neutrophils, macrophages and lymphocytes. The role of the HO system as a novel strategy to mitigate an antigen-independent ischemia/reperfusion injury has been documented in a number of transplantation models.

Nature of nontransferrin-bound iron

Nontransferrin-bound iron is now recognized to exist by many workers. It is present in the serum of patients suffering from a wide range of disease states and may be induced by certain therapeutic treatments. The chemical nature of this iron pool is unknown but almost certainly it is a multicomponent pool including a considerable proportion of protein-bound iron. Methods are required to separate and quantify these different components. The biological properties of the individual isoforms need to be established; it is possible that some forms are relatively nontoxic, while others are highly toxic. This paper reviews what is known about the nature of nontransferrin-bound iron and describes the methods currently available to quantify this important serum component.

A fluorescence-based one-step assay for serum non-transferrin-bound iron

We introduce a method for monitoring non-transferrin-bound iron (NTBI), a labile and potentially toxic form of serum iron associated with imbalanced iron metabolism. The assay employs fluorescein-labeled apotransferrin (Fl-aTf), which undergoes fluorescence quenching upon binding iron. It has the advantages of simplicity, high sensitivity, and detection of those forms of NTBI that persist in sera with low transferrin saturations. Since NTBI is not readily available for detection, it is mobilized by 10 mM oxalate. Endogenous serum apotransferrin, capable of binding oxalate-mobilized NTBI, is blocked by 0.1 mM gallium(III). This metal, like iron, binds to Fl-aTf, but it neither quenches its fluorescence nor interferes with quenching by iron. Serum and reagent containing oxalate, Ga(Cl)(3), and Fl-aTf are mixed in multiwell plates and fluorescence is determined after 1 h in a microplate reader. To compensate for artifactual fluorescence changes caused by serum color, parallel samples are prepared with excess unlabeled apotransferrin, which scavenges all iron in the sample. Sera from eight hemochromatosis patients were tested for NTBI by the present assay and by an established alternative method, with qualitatively similar results. A potential application of the test is for screening large numbers of samples from patients at risk of developing NTBI.

Additive stimulatory effect of extracellular calcium and potassium on non-transferrin ferric iron uptake by HeLa and K562 cells

We studied the effects of Ca(2+) and K(+) on non-transferrin iron uptake from ferric citrate complex by HeLa and K562 cells. Uptake experiments in Na-HEPES buffer (137 mM NaCl, 4 mM KCl) showed that extracellular Ca(2+) stimulated the iron uptake. The rate of iron uptake in 4 mM Ca(2+) was about 3-5 times higher than without Ca(2+). The iron uptake in K-HEPES buffer (68 mM NaCl, 75 mM KCl) with a high K(+) level was transiently stimulated during the first 10 min. The rate of iron uptake for 0.4 mM Ca(2+) was approximately 3 times higher in K-HEPES buffer than in Na-HEPES buffer. The calcium channel blockers verapamil (50 microM) and nifedipine (5 microM) had no effect on the uptake either in control Na-HEPES buffer or after K(+) stimulation in K-HEPES buffer. The sodium channel blocker lidocaine (50 microM) also had no effect on the uptake of iron in Na-HEPES buffer as well as after K(+) stimulation. Furthermore, the iron uptake was not significantly affected when Na(+) in the Na-HEPES and K-HEPES buffers was replaced by isotonic saccharose. We conclude that extracellular calcium per se, and not intracellular calcium or Ca(2+) transport, stimulates ferric iron uptake by both HeLa and K562 cells. A high level of extracellular K(+) also stimulates the uptake, probably via cell membrane depolarization. Na(+) is not involved in these stimulations of iron uptake. The transient K(+) effect and continuous Ca(2+) effect seem to be additive.

Expression of stimulator of Fe transport is not enhanced in Hfe knockout mice

Hfe knockout (-/-) mice recapitulate many of the biochemical abnormalities of hereditary hemochromatosis (HH), but the molecular mechanisms involved in the etiology of iron overload in HH remain poorly understood. It was found previously that livers of patients with HH contained 5-fold higher SFT (stimulator of Fe transport) mRNA levels relative to subjects without HH. Because this observation suggests a possible role for SFT in HH, we investigated SFT mRNA expression in Hfe(-/-) mice. The 4- and 10-wk-old Hfe(-/-) mice do not have elevated levels of hepatic SFT transcripts relative to age-matched Hfe(+/+) mice, despite having 2.2- and 3.3-fold greater hepatic nonheme iron concentrations, respectively. Northern blot analyses of various mouse tissues revealed that SFT is widely expressed. The novel observation that SFT transcripts are abundant in brain prompted a comparison of SFT transcript levels and nonheme iron levels in the brains of Hfe(+/+) and Hfe(-/-) mice. Neither SFT mRNA levels nor nonheme iron levels differed between groups. Further comparisons of Hfe(-/-) and Hfe(+/+) mouse tissues revealed no significant differences in SFT mRNA levels in duodenum, the site of increased iron absorption in HH. Important distinctions between Hfe(-/-) mice and HH patients include not only differences in the relative rate and magnitude of iron loading but also the lack of fibrosis and phlebotomy treatment in the knockout animals.

Iron and gallium increase iron uptake from transferrin by human melanoma cells: further examination of the ferric ammonium citrate-activated iron uptake process

Previously we showed that preincubation of cells with ferric ammonium citrate (FAC) resulted in a marked increase in Fe uptake from both (59)Fe-transferrin (Tf) and (59)Fe-citrate (D.R. Richardson, E. Baker, J. Biol. Chem. 267 (1992) 13972-13979; D.R. Richardson, P. Ponka, Biochim. Biophys. Acta 1269 (1995) 105-114). This Fe uptake process was independent of the transferrin receptor and appeared to be activated by free radicals generated via the iron-catalysed Haber-Weiss reaction. To further understand this process, the present investigation was performed. In these experiments, cells were preincubated for 3 h at 37 degrees C with FAC or metal ion solutions and then labelled for 3 h at 37 degrees C with (59)Fe-Tf. Exposure of cells to FAC resulted in Fe uptake from (59)Fe-citrate that became saturated at an Fe concentration of 2.5 microM, while FAC-activated Fe uptake from Tf was not saturable up to 25 microM. In addition, the extent of FAC-activated Fe uptake from citrate was far greater than that from Tf. These results suggest a mechanism where FAC-activated Fe uptake from citrate may result from direct interaction with the transporter, while Fe uptake from Tf appears indirect and less efficient. Preincubation of cells with FAC at 4 degrees C instead of 37 degrees C prevented its effect at stimulating (59)Fe uptake from (59)Fe-Tf, suggesting that an active process was involved. Previous studies by others have shown that FAC can increase ferrireductase activity that may enhance (59)Fe uptake from (59)Fe-Tf. However, there was no difference in the ability of FAC-treated cells compared to controls to reduce ferricyanide to ferrocyanide, suggesting no change in oxidoreductase activity. To examine if activation of this Fe uptake mechanism could occur by incubation with a range of metal ions, cells were preincubated with either FAC, ferric chloride, ferrous sulphate, ferrous ammonium sulphate, gallium nitrate, copper chloride, zinc chloride, or cobalt chloride. Stimulation of (59)Fe uptake from Tf was shown (in order of potency) with ferric chloride, ferrous sulphate, ferrous ammonium sulphate, and gallium nitrate. The other metal ions examined decreased (59)Fe uptake from Tf. The fact that redox-active Cu(II) ion did not stimulate Fe uptake while redox-inactive Ga(III) did, suggests a mechanism of transporter activation not solely dependent on free radical generation. Indeed, the activation of Fe uptake appears dependent on the presence of the Fe atom itself or a metal ion with atomic similarities to Fe (e.g. Ga).

Vascular release of nonheme iron in perfused rabbit lungs

In this study, we hypothesized that the lung actively releases excess iron into the circulation to regulate iron homeostasis. We measured nonheme iron (NHFe) in the perfusate of control isolated perfused rabbit lungs and lungs with ischemia-reperfusion (I/R) ventilated with normoxic (21% O(2)) or hypoxic (95% N(2)) gas mixtures. Some were perfused with bicarbonate-free (HEPES) buffer or treated with the anion exchange inhibitor DIDS. The control lungs released approximately 0.25 microg/ml of NHFe or 20% of the total lung NHFe into the vascular space that was not complexed with ferritin, transferrin, or lactoferrin or bleomycin reactive. The I/R lungs released a similar amount of NHFe during ischemia and some bleomycin-detectable iron during reperfusion. NHFe release was attenuated by approximately 50% in both control and ischemic lungs by hypoxia and by >90% in control lungs and approximately 60% in ischemic lungs by DIDS and HEPES. Reperfusion injury was not affected by DIDS or HEPES but was attenuated by hypoxia. These results indicate that biologically nonreactive nonheme iron is released rapidly by the lung into the vascular space via mechanisms that are linked to bicarbonate exchange. During prolonged ischemia, redox-active iron is also released into the vascular compartment by other mechanisms and may contribute to lung injury.

Iron transport

Iron homeostasis is maintained by regulating its absorption: Under conditions of deficiency, assimilation is enhanced but iron uptake is otherwise limited to prevent toxicity due to overload. Iron deficiency remains the most important micronutrient deficiency worldwide, but increasing awareness of the genetic basis for iron-loading diseases points to iron overload as a major public health issue as well. Recent identification of mutant alleles causing iron uptake disorders in mice and humans provides new insights into the mechanisms involved in iron transport and its regulation. This article summarizes these discoveries and discusses their impact on our current understanding of iron transport and its regulation.

The importance of non-transferrin bound iron in disorders of iron metabolism

The concept of non-transferrin bound iron (NTBI) was introduced 22 years ago by Hershko et al. (Brit. J. Haematol. 40 (1978) 255). It stemmed from a suspicion that, in iron overloaded patients, the large amounts of excess iron released into the circulation are likely to exceed the serum transferrin (Tf) iron-binding capacity (TIBC), leading to the appearance of various forms of iron not bound to Tf. In accordance with this assumption, NTBI was initially looked for and detected in patients with > or = 100% Tf-saturation. As techniques for its detection became more sophisticated and sensitive, NTBI was also found in conditions where Tf was not fully saturated, leading to a revision of the original view of NTBI as a simple spillover phenomenon. In this review, we will discuss some of the properties of NTBI, methods for its detection, its significance and potential value as an indicator for therapeutic regimens of iron chelation and supplementation.