Iron regulatory proteins and the molecular control of mammalian iron metabolism

Conditional derepression of ferritin synthesis in cells expressing a constitutive IRP1 mutant

Iron regulatory protein 1 (IRP1), a major posttranscriptional regulator of cellular iron and energy metabolism, is controlled by an iron-sulfur cluster switch. Cysteine-437 is critical for coordinating the cluster, and its replacement yields mutants that do not respond to iron perturbations and constitutively bind to cognate mRNA iron-responsive elements (IREs). The expression of IRP1(C437S) in cells has been associated with aberrations in iron homeostasis and toxicity. We have established clones of human lung (H1299) and breast (MCF7) cancer cells that express high levels of IRP1(C437S) in a tetracycline-inducible manner. As expected, IRP1(C437S) stabilizes transferrin receptor mRNA and inhibits translation of ferritin mRNA in both cell types by binding to their respective IREs. However, H1299 transfectants grown at high densities are able to overcome the IRP1(C437S)-mediated inhibition in ferritin synthesis. The mechanism involves neither alteration in ferritin mRNA levels nor utilization of alternative transcription start sites to eliminate the IRE or relocate it in less inhibitory downstream positions. The derepression of ferritin mRNA translation occurs under conditions where global protein synthesis appears to be impaired, as judged by a significant enrichment in the expression of the underphosphorylated form of the translational regulator 4E-BP1. Collectively, these data document an example where ferritin mRNA translation evades control of the IRE-IRP system. The physiological implications of this response are reflected in protection against iron-mediated toxicity, oxidative stress, and apoptosis.

An oxidative stress-mediated positive-feedback iron uptake loop in neuronal cells

Intracellular reactive iron is a source of free radicals and a possible cause of cell damage. In this study, we analyzed the changes in iron homeostasis generated by iron accumulation in neuroblastoma (N2A) cells and hippocampal neurons. Increasing concentrations of iron in the culture medium elicited increasing amounts of intracellular iron and of the reactive iron pool. The cells had both IRP1 and IRP2 activities, being IRP1 activity quantitatively predominant. When iron in the culture medium increased from 1 to 40 microm, IRP2 activity decreased to nil. In contrast, IRP1 activity decreased when iron increased up to 20 microm, and then, unexpectedly, increased. IRP1 activity at iron concentrations above 20 microm was functional as it correlated with increased (55) Fe uptake. The increase in IRP1 activity was mediated by oxidative-stress as it was largely abolished by N-acetyl-L-cysteine. Culturing cells with iron resulted in proteins and DNA modifications. In summary, iron uptake by N2A cells and hippocampus neurons did not shut off at high iron concentrations in the culture media. As a consequence, iron accumulated and generated oxidative damage. This behavior is probably a consequence of the paradoxical activation of IRP1 at high iron concentrations, a condition that may underlie some processes associated with neuronal degeneration and death.

Developmental changes in the expression of iron regulatory proteins and iron transport proteins in the perinatal rat brain

The perinatal brain requires a tightly regulated iron transport system. Iron regulatory proteins (IRPs) 1 and 2 are cytosolic proteins that regulate the stability of mRNA for the two major cellular iron transporters, transferrin receptor (TfR) and divalent metal transporter-1 (DMT-1). We studied the localization of IRPs, their change in expression during perinatal development, and their relationship to TfR and DMT-1 in rat brain between postnatal days (PND) 5 and 15. Twelve-micron frozen coronal sections of fixed brain tissue were obtained from iron-sufficient Sprague-Dawley rat pups on PND 5, 10, and 15, and were visualized at 20 to 1,000x light microscopy for diaminobenzidine activity after incubation with specific primary IRP-1, IRP-2, DMT-1, and TfR antibodies and a universal biotinylated secondary and tertiary antibody system. IRP and transport protein expression increased in parallel over time. IRP1, IRP2, and DMT-1 were partially expressed in the choroid plexus epithelial cells at PND 5 and 10, and fully expressed at PND 15. The cerebral blood vessels and ependymal cells strongly expressed IRP1, IRP2, and DMT-1 as early as PND 5. Substantive TfR staining was not seen in the choroid plexus or ependyma until PND 15. Glial and neuronal expression of IRP1, IRP2, DMT-1, and TfR in cortex, hippocampal subareas and striatum increased over time, but showed variability in cell number and intensity of expression based on brain region, cell type, and age. These developmental changes in IRP and transporter expression suggest potentially different time periods of brain structure vulnerability to iron deficiency or iron overload.

Proteomic analysis and molecular characterization of tissue ferritin light chain in hepatocellular carcinoma

To investigate a molecular basis for iron depletion in human hepatocellular carcinoma (HCC), 19 cases of HCC were analyzed by two-dimensional electrophoresis (2DE) and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Results were compared with those of paired adjacent nontumorous liver tissues. Comparative analysis of the respective spot patterns in 2DE showed that tissue ferritin light chain (T-FLC), an iron-storage protein, was either severely suppressed or reduced to undetectable levels in HCC, which was further supported by Western blot and immunohistochemical analysis. In contrast, transferrin receptor (TfR) was shown to be overexpressed in the same HCC samples. Interestingly, the relative levels of messenger RNA (mRNA) expression of T-FLC in HCC, which were measured by a real-time quantitative reverse-transcription polymerase chain reaction (PCR), exhibited almost the same levels as those in normal tissues, suggesting that the translational or posttranslational modification of T-FLC may be the cause of T-FLC suppression in HCC. Furthermore, with PCR-based loss of heterozygosity analysis, only 1 of 19 HCCs showed chromosomal deletions at 19q13.3-q13.4 where T-FLC is located, indicating that the suppression of T-FLC is unlikely due to structural genomic changes with HCC. In conclusion, both proteomic and genomic evidence support not only a basis for the suppression of T-FLC in HCC but also provide a new clue to the unresolved question of iron depletion during hepatocarcinogenesis.

Cloning and molecular characterization of two mosquito iron regulatory proteins

Iron regulatory proteins (IRPs) control the synthesis of various proteins at the translational level by binding to iron responsive elements (IREs) in the mRNAs. Iron, infection, and stress can alter IRP/IRE binding activity. Insect messenger RNAs for ferritin and succinate dehydrogenase subunit b have IREs that are active translational control sites. We have cloned and sequenced cDNAs encoding proteins from the IRP1 family for the mosquitoes, Aedes aegypti and Anopheles gambiae. Both deduced amino acid sequences show substantial similarity to human IRP1 and Drosophila IRP1A and IRP1B, and all of the residues thought to be involved in aconitase activity and iron-sulfur cluster formation are conserved. Recombinant A. aegypti IRP1 binds to transcripts of the IREs of mosquito or human ferritin subunit mRNAs. No significant change in A. gambiae IRP1 messenger RNA could be detected during the various developmental stages of the life cycle, following iron loading by blood feeding, or after bacterial or parasitic infections. These data suggest that there is no change in gene transcription. Furthermore, bacterial challenge of A. gambiae cells did not change IRP1 protein levels. In contrast, IRP1 binding activity for the IRE was elevated following immune induction. These data show that changes in IRP1/IRE binding activity occur as part of the insect immune response.

Iron treatment downregulates DMT1 and IREG1 mRNA expression in Caco-2 cells

Iron deficiency is the most common nutritional disorder worldwide, whereas pathologic elevations of body iron stores can occur under certain circumstances due to genetic abnormalities or in association with other diseases. The intestine is the exclusive locus of homeostatic regulation of body iron stores, which is accomplished by changes in iron absorption efficiency by largely unknown molecular mechanisms in response to alterations in body iron stores. Recently, a number of novel genes involved in iron metabolism, such as the iron uptake transporter DMT1/DCT1/Nramp2 and the iron export transporter IREG1/ferroportin1/MTP1, have been identified, providing important insights about molecular aspects of intestinal iron absorption and its regulation. The aim of this study was to investigate the effects of iron treatment on DMT1 and IREG1 mRNA expression in Caco-2 cells, a human intestinal cell line. Exposure of the cells to iron (200 micromol/L ferric nitrilotriacetic acid for 72 h) significantly decreased transferrin receptor mRNA (80%), DMT1 mRNA (57%) and IREG1 mRNA (52%). These observations are consistent with the notion of parallel regulation of these iron-responsive genes in vivo to protect the enterocyte from iron toxicity and mediate a decreased efficiency of intestinal iron absorption to prevent iron overload.

Iron deficiency decreases mitochondrial aconitase abundance and citrate concentration without affecting tricarboxylic acid cycle capacity in rat liver

Mitochondrial aconitase (m-acon) is the tricarboxylic acid (TCA) cycle enzyme that converts citrate to isocitrate. m-Acon mRNA is a potential target for regulation by iron regulatory proteins (IRPs), suggesting a link between dietary iron intake, m-acon synthesis, and energy metabolism. Our previous studies indicate that m-acon is one of a limited number of proteins that is down-regulated in iron-deficient liver. Here we use isolated hepatocytes to study the relationships among decreased m-acon abundance, TCA cycle function and cellular citrate concentration in iron deficiency. Rats were fed an iron-deficient (ID) (2 mg Fe/kg diet) diet, or they were pair-fed (PF) or freely fed (C) a control diet (50 mg Fe/kg diet) for up to 21 d. Hepatocyte total IRP activity was greater by d 2 in the ID group than in the C and PF groups and by d 10, the difference was maximal. Liver IRP activity was inversely correlated with m-acon abundance (r = -0.93, P < 0.0001). However, the decrease in m-acon abundance did not affect the ability of hepatocytes to oxidize 2-[(14)C]pyruvate or 1-[(14)C]acetate, indicating that TCA cycle capacity was not affected. Interestingly, by d 21, total liver citrate concentration was 40% lower in ID than in PF rats, suggesting enhanced utilization of citrate. However, the decrease in citrate concentration was not reflected in a change in liver total lipid concentration. Taken together, our results indicate that the iron-dependent regulation of m-acon in liver does not alter TCA cycle capacity but suggest that IRP-mediated changes in m-acon expression may modulate citrate use in other aspects of intermediary or iron metabolism.

Uroporphyria in mice: thresholds for hepatic CYP1A2 and iron

In mice treated with 5-aminolevulinic acid (ALA) and polyhalogenated aromatic compounds, the levels of both hepatic cytochrome P450 (CYP)1A2 and iron-which can be quite different among inbred strains-are critical in causing experimental uroporphyria. Here we investigate the development of uroporphyria as a function of CYP1A2 and iron levels in the liver of mice having a common C57BL/6 genetic background. We compared Cyp1a2(-/-) knockout mice, Cyp1a2(+/-) heterozygotes, Cyp1a2(+/+) wild type, and Cyp1a2(+/+) mice pretreated with a low dose of 3,3',4,4',5-pentachlorobiphenyl (PCB126) (4 microg/kg). Cyp1a2(+/-) mice contain about 60% of the hepatic CYP1A2 content of Cyp1a2(+/+) mice, and the PCB126-pretreated Cyp1a2(+/+) mice have about twice the wild-type levels of CYP1A2. ALA- and iron-treated Cyp1a2(+/+) mice are known to accumulate hepatic uroporphyrin; this accumulation was increased 7-fold by pretreatment with the low dose of PCB126. ALA- and iron-treated Cyp1a2(+/-) heterozygote mice accumulated no uroporphyrin in 4 weeks, but by 8 weeks accumulated significant amounts of uroporphyrin. As previously reported, the ALA- and iron-treated Cyp1a2(-/-) knockout mouse has no CYP1A2 and exhibits no detectable uroporphyrin accumulation. Iron dose-response curves in ALA- and PCB126-treated Cyp1a2(+/+) mice showed that hepatic iron levels greater than 850 microg/g liver were required to produce significant uroporphyrin accumulation in the liver. Other measures of hepatic effects of iron (iron-response element-binding protein [IRP]-iron response element [IRE] binding activity and accumulation of protoporphyrin from ALA) decreased when the level of iron was considerably lower than 850 microg/g liver. At low iron doses, accumulation of iron was principally in Kupffer cells, whereas at the higher doses (required to stimulate uroporphyrin accumulation), more iron was found in parenchymal cells. We conclude that small changes in hepatic CYP1A2 levels can dramatically affect uroporphyria in C57BL/6 mice, providing the animals have been sufficiently loaded with iron; these data might be clinically relevant to acquired (sporadic) porphyria cutanea tarda, because humans show greater than 60-fold genetic differences in hepatic basal CYP1A2.

Effects of copper and ceruloplasmin on iron transport in the Caco 2 cell intestinal model

Previous studies have implicated copper proteins, including ceruloplasmin, in intestinal iron transport. Polarized Caco2 cells with tight junctions were used to examine the possibilities that (a) ceruloplasmin promotes iron absorption by enhancing release at the basolateral cell surface and (b) copper deficiency reduces intestinal iron transport. Iron uptake and overall transport were followed for 90 min with 1 &mgr;M 59Fe(II) applied to the apical surface of Caco2 cell monolayers. Apotransferrin (38 &mgr;M) was in the basolateral chamber. Induction of iron deficiency with desferrioxamine (100 &mgr;M; 18 h) markedly increased uptake and overall transport of iron. Uptake increased from about 20% to about 65% of dose, and overall 59Fe transport from <1% to 60% of dose. On the basis of actual iron released into the basal chamber (measured with bathophenanthroline), transport increased 8-fold. Desferrioxamine pretreatment reduced cellular Fe by 55%. The addition of freshly isolated, enzymatically active human ceruloplasmin to the basolateral chamber during absorption had no effect on uptake or transport of iron by the cells. Unexpectedly, pretreatment with three different chelators of copper (18 h), which reduced cellular levels about 40%, more than doubled iron uptake and raised overall transport to 20%. This was so, whether or not cells were also made iron deficient with desferrioxamine. Acute addition of 1 &mgr;M Cu(II) to the apical chamber had no significant effect upon iron uptake, retention, or transport in iron deficient or normal cells, in the presence of absence of ascorbate. We conclude that intestinal absorption of Fe(II) is unlikely to depend upon plasma ceruloplasmin, and that cuproproteins involved in this form of iron transport must be binding copper tightly.

Detection of a [3Fe-4S] cluster intermediate of cytosolic aconitase in yeast expressing iron regulatory protein 1. Insights into the mechanism of Fe-S cluster cycling

Interconversion of iron regulatory protein 1 (IRP1) with cytosolic aconitase (c-aconitase) occurs via assembly/disassembly of a [4Fe-4S] cluster. Recent evidence implicates oxidants in cluster disassembly. We investigated H(2)O(2)-initiated Fe-S cluster disassembly in c-aconitase expressed in Saccharomyces cerevisiae. A signal for [3Fe-4S] c-aconitase was detected by whole-cell EPR of aerobically grown, aco1 yeast expressing wild-type IRP1 or a S138A-IRP1 mutant (IRP1(S138A)), providing the first direct evidence of a 3Fe intermediate in vivo. Exposure of yeast to H(2)O(2) increased this 3Fe c-aconitase signal up to 5-fold, coincident with inhibition of c-aconitase activity. Untreated yeast expressing IRP1(S138D) or IRP1(S138E), which mimic phosphorylated IRP1, failed to give a 3Fe signal. H(2)O(2) produced a weak 3Fe signal in yeast expressing IRP1(S138D). Yeast expressing IRP1(S138D) or IRP1(S138E) were the most sensitive to inhibition of aconitase-dependent growth by H(2)O(2) and were more responsive to changes in media iron status. Ferricyanide oxidation of anaerobically reconstituted c-aconitase yielded a strong 3Fe EPR signal with wild-type and S138A c-aconitases. Only a weak 3Fe signal was obtained with S138D c-aconitase, and no signal was obtained with S138E c-aconitase. This, paired with loss of c-aconitase activity, strongly argues that the Fe-S clusters of these phosphomimetic c-aconitase mutants undergo more complete disassembly upon oxidation. Our results demonstrate that 3Fe c-aconitase is an intermediate in H(2)O(2)-initiated Fe-S cluster disassembly in vivo and suggest that cluster assembly/disassembly in IRP1 is a dynamic process in aerobically growing yeast. Further, our results support the view that phosphorylation of IRP1 can modulate its response to iron through effects on Fe-S cluster stability and turnover.

Increased IRP1 and IRP2 RNA binding activity accompanies a reduction of the labile iron pool in HFE-expressing cells

Iron regulatory proteins (IRPs), the cytosolic proteins involved in the maintenance of cellular iron homeostasis, bind to stem loop structures found in the mRNA of key proteins involved iron uptake, storage, and metabolism and regulate the expression of these proteins in response to changes in cellular iron needs. We have shown previously that HFE-expressing fWTHFE/tTA HeLa cells have slightly increased transferrin receptor levels and dramatically reduced ferritin levels when compared to the same clonal cell line without HFE (Gross et al., 1998, J Biol Chem 273:22068-22074). While HFE does not alter transferrin receptor trafficking or non-transferrin mediated iron uptake, it does specifically reduce (55)Fe uptake from transferrin (Roy et al., 1999, J Biol Chem 274:9022-9028). In this report, we show that IRP RNA binding activity is increased by up to 5-fold in HFE-expressing cells through the activation of both IRP isoforms. Calcein measurements show a 45% decrease in the intracellular labile iron pool in HFE-expressing cells, which is in keeping with the IRP activation. These results all point to the direct effect of the interaction of HFE with transferrin receptor in lowering the intracellular labile iron pool and establishing a new set point for iron regulation within the cell.

Iron stress in plants

Although iron is an essential nutrient for plants, its accumulation within cells can be toxic. Plants, therefore, respond to both iron deficiency and iron excess by inducing expression of different gene sets. Here, we review recent advances in the understanding of iron homeostasis in plants gained through functional genomic approaches

Iron prevents ferritin turnover in hepatic cells

It has long been assumed that iron regulates the turnover of ferritin, but evidence for or against this idea has been lacking. This issue was addressed using rat hepatoma cells with characteristics of hepatocytes subjected to a continuous influx of iron. Iron-pretreated cells were pulsed with [(35)S]Met for 60 min or with (59)Fe overnight and harvested up to 30 h thereafter, during which they were/were not cultured with ferric ammonium citrate (FAC; 180 microm). Radioactivity in ferritin/ferritin subunits of cell heat supernatants was determined by autoradiography of rockets obtained by immunoelectrophoresis or after precipitation with ferritin antibody and SDS-PAGE. Both methods gave similar results. During the +FAC chase, the concentration of ferritin in the cells increased linearly with time. Without FAC, the half-life of (35)S-ferritin was 19-20 h; with FAC there was no turnover. Without FAC, the iron in ferritin had an apparent half-life of 20 h; in the presence of FAC there was no loss of (59)Fe. Without FAC, concentrations of ferritin iron and protein also decreased in parallel. We conclude that a continuous influx of excess iron can completely inhibit the degradation of ferritin protein and that the iron and protein portions of ferritin molecules may be coordinately degraded.

Characterization of the iron transporter DMT1 (NRAMP2/DCT1) in red blood cells of normal and anemic mk/mk mice

Divalent metal transporter 1 (DMT1) is the major transferrin-independent iron uptake system at the apical pole of intestinal cells, but it may also transport iron across the membrane of acidified endosomes in peripheral tissues. Iron transport and expression of the 2 isoforms of DMT1 was studied in erythroid cells that consume large quantities of iron for biosynthesis of hemoglobin. In mk/mk mice that express a loss-of-function mutant variant of DMT1, reticulocytes have a decreased cellular iron uptake and iron incorporation into heme. Interestingly, iron release from transferrin inside the endosome is normal in mk/mk reticulocytes, suggesting a subsequent defect in Fe(++) transport across the endosomal membrane. Studies by immunoblotting using membrane fractions from peripheral blood or spleen from normal mice where reticulocytosis was induced by erythropoietin (EPO) or phenylhydrazine (PHZ) treatment suggest that DMT1 is coexpressed with transferrin receptor (TfR) in erythroid cells. Coexpression of DMT1 and TfR in reticulocytes was also detected by double immunofluorescence and confocal microscopy. Experiments with isoform-specific anti-DMT1 antiserum strongly suggest that it is the non-iron-response element containing isoform II of DMT1 that is predominantly expressed by the erythroid cells. As opposed to wild-type reticulocytes, mk/mk reticulocytes express little if any DMT1, despite robust expression of TfR, suggesting a possible effect of the mutation on stability and targeting of DMT1 isoform II in these cells. Together, these results provide further evidence that DMT1 plays a central role in iron acquisition via the transferrin cycle in erythroid cells.

The intracellular location of iron regulatory proteins is altered as a function of iron status in cell cultures and rat brain

Iron regulatory proteins (IRPs) are proteins involved in the regulation of intracellular iron homeostasis that bind to specific mRNA structures termed iron responsive elements (IREs). Because the target mRNAs for the IRPs are both cytosolic and membrane associated, we hypothesize that movement of IRPs between the cytosolic and the membrane associated subcellular fractions occurs in response to intracellular iron changes. We tested this hypothesis in a cell culture model, using mouse fibroblast cells (NIH 3T3) and macrophage cells (J774), and in a rat model of early iron deficiency and excess. This presented the first opportunity to examine IRP binding activity in rat brain during states of dietary iron deficiency and excess. Binding activity for IRPs was demonstrated in both membrane and cytosolic fractions in the cell lines and the rat brain homogenates. Although IRP binding activity is predominantly located in the cytosol (90%), there was increased IRP/IRE binding activity in both cytosolic and membrane fractions when the cells were treated with deferoxamine, and decreased binding activity after treatment with iron. In the rat study, brain cortex, hippocampus and striatum homogenates had more IRP binding activity in iron-deficient rats and less in iron-supplemented rats in a region- and time-specific manner. The intracellular distribution of IRPs also changed between the cytosolic and membrane fractions of the brain homogenates in conjunction with changes in iron. These in vivo studies are consistent with the cell culture analyses showing intracellular redistribution of IRPs as a function of iron status. The results of these experiments extend our understanding of cytoplasmic mRNA binding protein activity and raise questions regarding the mechanism by which mRNA binding proteins can locate their target mRNAs within cells. The elucidation of this mechanism will have a significant impact on our understanding of eukaryotic gene regulation.

An IRP-like protein from Plasmodium falciparum binds to a mammalian iron-responsive element

This study cloned and sequenced the complementary DNA (cDNA) encoding of a putative malarial iron responsive element-binding protein (PfIRPa) and confirmed its identity to the previously identified iron-regulatory protein (IRP)-like cDNA from Plasmodium falciparum. Sequence alignment showed that the plasmodial sequence has 47% identity with human IRP1. Hemoglobin-free lysates obtained from erythrocyte-stage P falciparum contain a protein that binds a consensus mammalian iron-responsive element (IRE), indicating that a protein(s) with iron-regulatory activity was present in the lysates. IRE-binding activity was found to be iron regulated in the electrophoretic mobility shift assays. Western blot analysis showed a 2-fold increase in the level of PfIRPa in the desferrioxamine-treated cultures versus control or iron-supplemented cells. Malarial IRP was detected by anti-PfIRPa antibody in the IRE-protein complex from P falciparum lysates. Immunofluorescence studies confirmed the presence of PfIRPa in the infected red blood cells. These findings demonstrate that erythrocyte P falciparum contains an iron-regulated IRP that binds a mammalian consensus IRE sequence, raising the possibility that the malaria parasite expresses transcripts that contain IREs and are iron-dependently regulated.

A mutation, in the iron-responsive element of H ferritin mRNA, causing autosomal dominant iron overload

Ferritin, which is composed of H and L subunits, plays an important role in iron storage and in the control of intracellular iron distribution. Synthesis of both ferritin subunits is controlled by a common cytosolic protein, iron regulatory protein (IRP), which binds to the iron-responsive element (IRE) in the 5'-UTR of the H- and L-ferritin mRNAs. In the present study, we have identified a single point mutation (A49U) in the IRE motif of H-ferritin mRNA, in four of seven members of a Japanese family affected by dominantly inherited iron overload. Gel-shift mobility assay and Scatchard-plot analysis revealed that a mutated IRE probe had a higher binding affinity to IRP than did the wild-type probe. When mutated H subunit was overexpressed in COS-1 cells, suppression of H-subunit synthesis and of the increment of radiolabeled iron uptake were observed. These data suggest that the A49U mutation in the IRE of H-subunit is responsible for tissue iron deposition and is a novel cause of hereditary iron overload, most likely related to impairment of the ferroxidase activity generated by H subunit.

"Opening" the ferritin pore for iron release by mutation of conserved amino acids at interhelix and loop sites

Ferritin concentrates, stores, and detoxifies iron in most organisms. The iron is a solid, ferric oxide mineral (< or =4500 Fe) inside the protein shell. Eight pores are formed by subunit trimers of the 24 subunit protein. A role for the protein in controlling reduction and dissolution of the iron mineral was suggested in preliminary experiments [Takagi et al. (1998) J. Biol. Chem. 273, 18685-18688] with a proline/leucine substitution near the pore. Localized pore disorder in frog L134P crystals coincided with enhanced iron exit, triggered by reduction. In this report, nine additional substitutions of conserved amino acids near L134 were studied for effects on iron release. Alterations of a conserved hydrophobic pair, a conserved ion pair, and a loop at the ferritin pores all increased iron exit (3-30-fold). Protein assembly was unchanged, except for a slight decrease in volume (measured by gel filtration); ferroxidase activity was still in the millisecond range, but a small decrease indicates slight alteration of the channel from the pore to the oxidation site. The sensitivity of reductive iron exit rates to changes in conserved residues near the ferritin pores, associated with localized unfolding, suggests that the structure around the ferritin pores is a target for regulated protein unfolding and iron release.

Modulation of cellular iron metabolism by hydrogen peroxide. Effects of H2O2 on the expression and function of iron-responsive element-containing mRNAs in B6 fibroblasts

Cellular iron uptake and storage are coordinately controlled by binding of iron-regulatory proteins (IRP), IRP1 and IRP2, to iron-responsive elements (IREs) within the mRNAs encoding transferrin receptor (TfR) and ferritin. Under conditions of iron starvation, both IRP1 and IRP2 bind with high affinity to cognate IREs, thus stabilizing TfR and inhibiting translation of ferritin mRNAs. The IRE/IRP regulatory system receives additional input by oxidative stress in the form of H(2)O(2) that leads to rapid activation of IRP1. Here we show that treating murine B6 fibroblasts with a pulse of 100 microm H(2)O(2) for 1 h is sufficient to alter critical parameters of iron homeostasis in a time-dependent manner. First, this stimulus inhibits ferritin synthesis for at least 8 h, leading to a significant (50%) reduction of cellular ferritin content. Second, treatment with H(2)O(2) induces a approximately 4-fold increase in TfR mRNA levels within 2-6 h, and subsequent accumulation of newly synthesized protein after 4 h. This is associated with a profound increase in the cell surface expression of TfR, enhanced binding to fluorescein-tagged transferrin, and stimulation of transferrin-mediated iron uptake into cells. Under these conditions, no significant alterations are observed in the levels of mitochondrial aconitase and the Divalent Metal Transporter DMT1, although both are encoded by two as yet lesser characterized IRE-containing mRNAs. Finally, H(2)O(2)-treated cells display an increased capacity to sequester (59)Fe in ferritin, despite a reduction in the ferritin pool, which results in a rearrangement of (59)Fe intracellular distribution. Our data suggest that H(2)O(2) regulates cellular iron acquisition and intracellular iron distribution by both IRP1-dependent and -independent mechanisms.

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.

Overexpression of H ferritin and up-regulation of iron regulatory protein genes during differentiation of 3T3-L1 pre-adipocytes

The role of iron-dependent oxidative metabolism in protecting the oxidable substrates contained in mature adipocytes is still unclear. Because differentiation increases ferritin formation in several cell types, thereby leading to an accumulation of H-rich isoferritins, we investigated whether differentiation affects iron metabolism in 3T3-L1 pre-adipocytes. To this aim, we evaluated the expression of the genes coding for the H and L ferritin subunits and for cytoplasmic iron regulatory protein (IRP) during the differentiation of 3T3-L1 cells in adipocytes induced by the addition of isobutylmethylxanthine, insulin, and dexamethasone. Differentiation enhanced ferritin formation and caused overexpression of the H subunit, thus altering the H/L subunit ratio. Northern blot analysis showed increased levels of H subunit mRNA. A gel retardation assay of cytoplasmic extract from differentiated cells, using an iron-responsive element as a probe, revealed enhanced an RNA binding capacity of IRP1, which correlated with the increase of IRP1 mRNA. The observed correlation between differentiation and iron metabolism in adipocytes suggests that an accumulation of H-rich isoferritin may limit the toxicity of iron in adipose tissue, thus exerting an antioxidant function.