The kinetics of iron and transferrin incorporation into rabbit erythroid cells and the nature of stromal-bound iron

Erythroid cell mitochondria receive endosomal iron by a "kiss-and-run" mechanism

In erythroid cells, more than 90% of transferrin-derived iron enters mitochondria where ferrochelatase inserts Fe²⁺ into protoporphyrin IX. However, the path of iron from endosomes to mitochondrial ferrochelatase remains elusive. The prevailing opinion is that, after its export from endosomes, the redox-active metal spreads into the cytosol and mysteriously finds its way into mitochondria through passive diffusion. In contrast, this study supports the hypothesis that the highly efficient transport of iron toward ferrochelatase in erythroid cells requires a direct interaction between transferrin-endosomes and mitochondria (the "kiss-and-run" hypothesis). Using a novel method (flow sub-cytometry), we analyze lysates of reticulocytes after labeling these organelles with different fluorophores. We have identified a double-labeled population definitively representing endosomes interacting with mitochondria, as demonstrated by confocal microscopy. Moreover, we conclude that this endosome-mitochondrion association is reversible, since a "chase" with unlabeled holotransferrin causes a time-dependent decrease in the size of the double-labeled population. Importantly, the dissociation of endosomes from mitochondria does not occur in the absence of holotransferrin. Additionally, mutated recombinant holotransferrin, that cannot release iron, significantly decreases the uptake of ⁵⁹Fe by reticulocytes and diminishes ⁵⁹Fe incorporation into heme. This suggests that endosomes, which are unable to provide iron to mitochondria, cause a "traffic jam" leading to decreased endocytosis of holotransferrin. Altogether, our results suggest that a molecular mechanism exists to coordinate the iron status of endosomal transferrin with its trafficking. Besides its contribution to the field of iron metabolism, this study provides evidence for a new intracellular trafficking pathway of organelles.

Thioredoxin-interacting protein regulates the differentiation of murine erythroid precursors

Thioredoxin-interacting protein (TXNIP) is involved in various cellular processes including redox control, metabolism, differentiation, growth, and apoptosis. With respect to hematopoiesis, TXNIP has been shown to play roles in natural killer cells, dendritic cells, and hematopoietic stem cells. Our study investigates the role of TXNIP in erythropoiesis. We observed a rapid and significant increase of TXNIP transcript and protein levels in mouse erythroleukemia cells treated with dimethyl sulfoxide or hexamethylene bisacetamide, inducers of erythroid differentiation. The upregulation of TXNIP was not abrogated by addition of the antioxidant N-acetylcysteine. The increase of TXNIP expression was confirmed in another model of erythroid differentiation, G1E-ER cells, which undergo differentiation upon activation of the GATA1 transcription factor. In addition, we showed that TXNIP levels are induced following inhibition of p38 or c-Jun N-terminal kinase (JNK) mitogen-activated protein kinases. We also observed an increase in iron uptake and a decrease in transferrin receptor protein upon TXNIP overexpression, suggesting a role in iron homeostasis. In vivo, flow cytometry analysis of cells from Txnip(-/-) mice revealed a new phenotype of impaired terminal erythropoiesis in the spleen, characterized by a partial block between basophilic and late basophilic/polychromatic erythroblasts. Based on our data, TXNIP emerges as a novel regulator of terminal erythroid differentiation.

Nitrogen monoxide inhibits haem synthesis in mouse reticulocytes

AI (anaemia of inflammation) often manifests in patients with chronic immune activation due to cancer, chronic infections, autoimmune disorders, rheumatoid arthritis and other diseases. The pathogenesis of AI is complex and involves cytokine-mediated inhibition of erythropoiesis, insufficient erythropoietin production and diminished sensitivity of erythroid progenitors to this hormone, and retention of iron in haemoglobin-processing macrophages. NO (nitric oxide) is a gaseous molecule produced by activated macrophages that has been identified as having numerous effects on iron metabolism. In the present study, we explore the possibility that NO affects iron metabolism in reticulocytes and our results suggest that NO may also contribute to AI. We treated reticulocytes with the NO donor SNP (sodium nitroprusside). The results indicate that NO inhibits haem synthesis dramatically and rapidly at the level of erythroid-specific 5-aminolaevulinic acid synthase 2, which catalyses the first step of haem synthesis in erythroid cells. We also show that NO leads to the inhibition of iron uptake via the Tf (transferrin)-Tf receptor pathway. In addition, NO also causes an increase in eIF2α (eukaryotic initiation factor 2α) phosphorylation levels and decreases globin translation. The profound impairment of haem synthesis, iron uptake and globin translation in reticulocytes by NO raises the possibility that this gas may also contribute to AI.

The long history of iron in the Universe and in health and disease

BACKGROUND

Not long after the Big Bang, iron began to play a central role in the Universe and soon became mired in the tangle of biochemistry that is the prima essentia of life. Since life's addiction to iron transcends the oxygenation of the Earth's atmosphere, living things must be protected from the potentially dangerous mix of iron and oxygen. The human being possesses grams of this potentially toxic transition metal, which is shuttling through his oxygen-rich humor. Since long before the birth of modern medicine, the blood-vibrant red from a massive abundance of hemoglobin iron-has been a focus for health experts.

SCOPE OF REVIEW

We describe the current understanding of iron metabolism, highlight the many important discoveries that accreted this knowledge, and describe the perils of dysfunctional iron handling.

GENERAL SIGNIFICANCE

Isaac Newton famously penned, "If I have seen further than others, it is by standing upon the shoulders of giants". We hope that this review will inspire future scientists to develop intellectual pursuits by understanding the research and ideas from many remarkable thinkers of the past.

MAJOR CONCLUSIONS

The history of iron research is a long, rich story with early beginnings, and is far from being finished. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Antagonistic roles of the ERK and p38 MAPK signalling pathways in globin expression, haem biosynthesis and iron uptake

Late-stage erythroid cells synthesize large quantities of haemoglobin, a process requiring the co-ordinated regulation of globin and haem synthesis as well as iron uptake. In the present study, we investigated the role of the ERK (extracellular-signal-regulated kinase) and p38 MAPK (mitogen-activated protein kinase) signalling pathways in MEL (mouse erythroleukaemia) cell differentiation. We found that treatment of HMBA (hexamethylene bisacetamide)-induced MEL cells with the ERK pathway inhibitor UO126 results in an increase in intracellular haem and haemoglobin levels. The transcript levels of the genes coding for β(major)-globin, the haem biosynthesis enzyme 5-aminolevulinate synthase 2 and the mitochondrial iron transporter mitoferrin 1 are up-regulated. We also showed enhanced expression of globin and transferrin receptor 1 proteins upon UO126 treatment. With respect to iron uptake, we found that ERK inhibitor treatment led to an increase in both haem-bound and total iron. In contrast, treatment of MEL cells with the p38 MAPK pathway inhibitor SB202190 had the opposite effect, resulting in decreased globin expression, haem synthesis and iron uptake. Reporter assays showed that globin promoter and HS2 enhancer-mediated transcription was under the control of MAPKs, as inhibition of the ERK and p38 MAPK pathways led to increased and decreased gene activity respectively. Our present results suggest that the ERK1/2 and p38α/β MAPKs play antagonistic roles in HMBA-induced globin gene expression and erythroid differentiation. These results provide a novel link between MAPK signalling and the regulation of haem biosynthesis and iron uptake in erythroid cells.

Ferritin does not donate its iron for haem synthesis in macrophages

Iron is essential for all life, yet can be dangerous under certain conditions. Iron storage by the 24-subunit protein ferritin renders excess amounts of the metal non-reactive and, consequentially, ferritin is crucial for life. Although the mechanism detailing the storage of iron in ferritin has been well characterized, little is known about the fate of ferritin-stored iron and whether it can be released and reutilized for metabolic use within a single cell. Virtually nothing is known about the use of ferritin-derived iron in non-erythroid cells. We therefore attempted to answer the question of whether iron from ferritin can be used for haem synthesis in the murine macrophage cell line RAW 264.7 cells. Cells treated with ALA (5-aminolaevulinic acid; a precursor of haem synthesis) show increased haem production as determined by enhanced incorporation of transferrin-bound 59Fe into haem. However, the present study shows that, upon the addition of ALA, 59Fe from ferritin cannot be incorporated into haem. Additionally, little 59Fe is liberated from ferritin when haem synthesis is increased upon addition of ALA. In conclusion, ferritin in cultivated macrophages is not a significant source of iron for the cell's own metabolic functions.

Human ind1, an iron-sulfur cluster assembly factor for respiratory complex I

Respiratory complex I (NADH:ubiquinone oxidoreductase) is a large mitochondrial inner membrane enzyme consisting of 45 subunits and 8 iron-sulfur (Fe/S) clusters. While complex I dysfunction is the most common reason for mitochondrial diseases, the assembly of complex I and its Fe/S cofactors remains elusive. Here, we identify the human mitochondrial P-loop NTPase, designated huInd1, that is critically required for the assembly of complex I. huInd1 can bind an Fe/S cluster via a conserved CXXC motif in a labile fashion. Knockdown of huInd1 in HeLa cells by RNA interference technology led to strong decreases in complex I protein and activity levels, remodeling of respiratory supercomplexes, and alteration of mitochondrial morphology. In addition, huInd1 depletion resulted in massive decreases in several subunits (NDUFS1, NDUFV1, NDUFS3, and NDUFA13) of the peripheral arm of complex I, with the concomitant appearance of a 450-kDa subcomplex representing part of the membrane arm. By a novel radiolabeling technique, the amount of iron associated with complex I was also shown to reflect the dependence of this enzyme on huInd1 for assembly. Together, these data identify huInd1 as a new assembly factor for human respiratory complex I with a possible role in the delivery of one or more Fe/S clusters to complex I subunits.

Heterogeneity of horse transferrin: the role of carbohydrate moiety

Homozygous horse transferrin (Tf O) is highly heterogeneous. In starch gel electrophoresis it gives at least 9 zones. Two main components (2a and 4b) were purified by rivanol and ammonium sulphate precipitation, DEAE-Sephadex chromatography and SP-Sephadex chromatography. Molecular weights of 75 200 and 80 500 for components 2a and 4b, respectively, were determined by sedimentation equilibrium ultracentrifugation. Amino acid compositions of the two components were similar, and there were no differences in the N-terminus (glutamic acid followed by glutamine) and the C-terminus (valine). Differences were found in carbohydrate composition between components 2a and 4b. Component 2a contained 10 moles of sugar components per mole of protein (4 hexoses, 4 hexosamines and 2 sialic acids), while component 4b contained twice the number of both total carbohydrates and individual sugar components. Carbohydrates were identified as mannose and galactose (ratio mannose: galactose approximately equal to 1.5:1), N-acetylglucosamine and N-acetylneuraminic acid. At present it is not clear whether the difference between the two components resides solely in the difference of carbohydrate contents. It is proposed that component 2a has one diantennary glycan, while component 4b has two.

Use of Nramp2-transfected Chinese hamster ovary cells and reticulocytes from mk/mk mice to study iron transport mechanisms

OBJECTIVE

We investigated mechanisms involved in iron (Fe) transport by DMT1 (endosomal Fe(II) exporter, encoded by the Nramp2 gene) using wild-type Chinese hamster ovary (CHO) cells and Nramp2-transfected CHO cells, as well as reticulocytes from normal and mk/mk mice that have a defect in DMT1.

MATERIALS AND METHODS

CHO cells and reticulocytes were incubated with 59Fe bound to various ligands. The radioiron was present in its Fe(II) or Fe(III) forms or bound to transferrin (Tf), and the internalized 59Fe measured under varying experimental conditions. Additionally, 125I-Tf interaction with reticulocytes was investigated and 59Fe incorporation into their heme was determined.

RESULTS

Hyperexpression of DMT1 in CHO cells greatly increases their capacity to acquire ferrous iron. Although CHO-Nramp2 cells showed an increase in Fe(III) uptake as compared to CHO cells, they transported Fe(III) with much lower efficacy than Fe(II). In addition to their defect in Fe uptake, mk/mk reticulocytes also showed a decrease in Tf receptor levels.

CONCLUSIONS

Given that CHO cells acquire iron from Fe(II)-ascorbate with much higher rates than from Fe(III)-Tf, Tf-receptor levels represent the rate-limiting step in their iron uptake. As Fe(III) transport by CHO-Nramp2 cells can be inhibited by the impermeable oxidant K3Fe(CN)6, a membrane ferric reductase is probably needed for reduction of Fe(III) to Fe(II), which is then transported by DMT1. DMT1 is not a limiting factor in Fe acquisition by normal reticulocytes and their heme synthesis.

UCP2 modulates cell proliferation through the MAPK/ERK pathway during erythropoiesis and has no effect on heme biosynthesis

UCP2, an inner membrane mitochondrial protein, has been implicated in bioenergetics and reactive oxygen species (ROS) modulation. High levels of UCP2 mRNA were recently found in erythroid cells where UCP2 is hypothesized to function as a facilitator of heme synthesis and iron metabolism by reducing ROS production. We examined UCP2 protein expression and role in mice erythropoiesis in vivo. UCP2 was mainly expressed at early stages of erythroid maturation when cells are not fully committed in heme synthesis. Iron incorporation into heme was unaltered in reticulocytes from UCP2-deficient mice. Although heme synthesis was not influenced by UCP2 deficiency, mice lacking UCP2 had a delayed recovery from chemically induced hemolytic anemia. Analysis of progenitor cells from bone marrow and fetal liver both in vitro and in vivo revealed that UCP2 deficiency results in a significant decrease in cell proliferation at the erythropoietin-dependent phase of erythropoiesis. This was accompanied by reduction in the phosphorylated form of ERK, a ROS-dependent cytosolic regulator of cell proliferation. Analysis of ROS in UCP2 null erythroid cells revealed altered distribution of ROS, resulting in decreased cytosolic and increased mitochondrial ROS. Restoration of the cytosol oxidative state of erythroid progenitor cells by the pro-oxidant Paraquat reversed the effect of UCP2 deficiency on cell proliferation in in vitro differentiation assays. Together, these results indicate that UCP2 is a regulator of erythropoiesis and suggests that inhibition of UCP2 function may contribute to the development of anemia.

Direct interorganellar transfer of iron from endosome to mitochondrion

Iron is a transition metal whose physicochemical properties make it the focus of vital biologic processes in virtually all living organisms. Among numerous roles, iron is essential for oxygen transport, cellular respiration, and DNA synthesis. Paradoxically, the same characteristics that biochemistry exploits make iron a potentially lethal substance. In the presence of oxygen, ferrous iron (Fe(2+)) will catalyze the production of toxic hydroxyl radicals from hydrogen peroxide. In addition, Fe(3+) is virtually insoluble at physiologic pH. To protect tissues from deleterious effects of Fe, mammalian physiology has evolved specialized mechanisms for extracellular, intercellular, and intracellular iron handling. Here we show that developing erythroid cells, which are taking up vast amounts of Fe, deliver the metal directly from transferrin-containing endosomes to mitochondria (the site of heme biosynthesis), bypassing the oxygen-rich cytosol. Besides describing a new means of intracellular transport, our finding is important for developing therapies for patients with iron loading disorders.

Non-heme induction of heme oxygenase-1 does not alter cellular iron metabolism

The catabolism of heme is carried out by members of the heme oxygenase (HO) family. The products of heme catabolism by HO-1 are ferrous iron, biliverdin (subsequently converted to bilirubin), and carbon monoxide. In addition to its function in the recycling of hemoglobin iron, this microsomal enzyme has been shown to protect cells in various stress models. Implicit in the reports of HO-1 cytoprotection to date are its effects on the cellular handling of heme/iron. However, the limited amount of uncommitted heme in non-erythroid cells brings to question the source of substrate for this enzyme in non-hemolytic circumstances. In the present study, HO-1 was induced by either sodium arsenite (reactive oxygen species producer) or hemin or overexpressed in the murine macrophage-like cell line, RAW 264.7. Both of the inducers elicited an increase in active HO-1; however, only hemin exposure caused an increase in the synthesis rate of the iron storage protein, ferritin. This effect of hemin was the direct result of the liberation of iron from heme by HO. Cells stably overexpressing HO-1, although protected from oxidative stress, did not display elevated basal ferritin synthesis. However, these cells did exhibit an increase in ferritin synthesis, compared with untransfected controls, in response to hemin treatment, suggesting that heme levels, and not HO-1, limit cellular heme catabolism. Our results suggest that the protection of cells from oxidative insult afforded by HO-1 is not due to the catabolism of significant amounts of cellular heme as thought previously.

The anemia of "haemoglobin-deficit" (hbd/hbd) mice is caused by a defect in transferrin cycling

OBJECTIVE

The hemoglobin-deficit mouse mutant (hbd) is characterized by a hypochromic, microcytic anemia that is inherited in an autosomal, recessive manner. The recently identified gene responsible, Sec15l1, is specific to hematopoietic stem cells and is homologous to a gene encoding a member of the exocyst pathway in yeast. However, the defective cellular mechanism underlying the hemoglobin deficiency in hbd/hbd mice has not been functionally identified. Here we investigated the possibility that erroneous transferrin trafficking is responsible for the hbd phenotype.

MATERIALS AND METHODS

Reticulocytes were harvested from hbd/hbd mice and from background- and age-matched controls. Iron and transferrin uptake and iron utilization experiments were performed using 59Fe- or 125I-transferrin to follow the trafficking and utilization of the protein and metal.

RESULTS

Compared to controls, iron and transferrin uptake as well as iron incorporation into heme was compromised in hbd reticulocytes. Importantly, reduced heme synthesis in these cells was restored to normal values by using an iron source that bypasses the transferrin-receptor pathway. We also found that +/+ and hbd reticulocytes take up free, ferrous iron at identical rates, while the rates of Tf internalization and externalization were significantly decreased in the mutant cells. Finally, utilization of endosomal radioiron was likewise deficient in the hbd reticulocytes.

CONCLUSION

Our results indicate that heme biosynthesis, DMT1, and the mitochondrial iron handling machinery are all normal in hemoglobin-deficit mice, while transferrin cycling is deficient. Therefore, the product of Sec15l1 is directly involved in vesicular trafficking, docking, fusing, and/or cargo delivery in erythroid precursors.

Intracellular kinetics of iron in reticulocytes: evidence for endosome involvement in iron targeting to mitochondria

In erythroid cells the vast majority of iron (Fe) released from endosomes must cross both the outer and the inner mitochondrial membranes to reach ferrochelatase that inserts Fe into protoporphyrin IX. In the present study, we developed a method whereby a cohort of 59Fe-transferrin (Tf)-laden endosomal vesicles were generated, from which we could evaluate the transfer of 59Fe into mitochondria. Iron chelators, dipyridyl or salicylaldehyde isonicotinoyl hydrazone (SIH), were able to bind the 59Fe when they were present during a 37 degrees C incubation; however, addition of these agents only during lysis at 4 degrees C chelated virtually no 59Fe. Bafilomycin A1 (which prevents endosome acidification) and succinylacetone (an inhibitor of 5-aminolevulinate dehydratase) prevented endosomal 59Fe incorporation into heme. Importantly, both the myosin light chain kinase inhibitor wortmannin and the calmodulin antagonist, N-(6-aminohexyl)-5-chloro-1-naphthalene-sulfonamide (W-7), caused significant inhibition of 59Fe incorporation from 59Fe-Tf-labeled endosomes into heme, suggesting that myosin is required for Tf-vesicle movement. Our results reaffirm the astonishing efficiency of Tf-derived Fe utilization in hemoglobin (Hb)-producing cells and demonstrate that very little of this Fe is present in a chelatable pool. Collectively, these results are congruent with our hypothesis that a transient endosome-mitochondrion interaction mediates iron transfer between these organelles.

The transcytosis of divalent metal transporter 1 and apo-transferrin during iron uptake in intestinal epithelium

Caco-2 cells grown in bicameral chambers are a model system to study intestinal iron absorption. Caco-2 cells exhibit constitutive transport of iron from the apical (luminal) chamber to the basal (serosal) chamber that is enhanced by apo-transferrin in the basal chamber, with the apo-transferrin undergoing endocytosis to the apical portion of the cell. With the addition of iron to the apical surface, divalent metal transporter 1 (DMT1) on the brush-border membrane (BBM) undergoes endocytosis. These findings suggest that in Caco-2 cells DMT1 and apo-transferrin may cooperate in iron transport through transcytosis. To prove this hypothesis, we determined by confocal microscopy that, after addition of iron to the apical chamber, DMT1 from the BBM and Texas red apo-transferrin from the basal chamber colocalized in a perinuclear compartment. Colocalization was also observed by isolating endosomes from Caco-2 cells after ingestion of ultra-small paramagnetic particles from either the basal or apical chamber. The isolated endosomes contained both transferrin and DMT1 independent of the chamber from which the paramagnetic particles were endocytosed. These findings suggest that iron transport across intestinal epithelia may be mediated by transcytosis.

Activation of macrophage cytostatic effector mechanisms during acute graft-versus-host disease: release of intracellular iron and nitric oxide-mediated cytostasis

During acute graft-versus-host disease (GVHD) the activation of macrophages (Mphi) is mediated by 2 signals, interferon (IFN)-gamma and bacteria-derived lipopolysaccharide (LPS). A cascade of inflammatory responses that includes the release of mediators of tissue injury follows Mphi activation. Among the tissues characteristically targeted during acute GVHD are epithelial tissues of the skin and gastrointestinal tract that normally undergo continuous proliferation and are therefore sensitive to cytostatic processes. We have investigated whether Mphi can mediate cytostatic mechanisms capable of interrupting cell proliferation during acute GVHD. GVHD was induced in nonirradiated C57BL/6XAF(1) (B6AF(1)) mice by the injection of 60 x 10(6) (acute GVHD) or 30 x 10(6) (nonlethal GVHD) C57BL/6 (B6) lymphoid cells. Mphi from animals undergoing acute GVHD could be triggered by normally insignificant quantities of LPS to mediate a cytostatic effect on target cells, resulting in the complete shutdown of cellular proliferation. The same amounts of LPS had no effect on Mphi from normal or syngeneically transplanted animals. Mphi mediated the release of significant quantities of intracellular iron from target cells undergoing cytostasis. Reversal of cytostasis occurred following inhibition of nitric oxide (NO) production by N(G)-monomethyl-L-arginine (NMMA). Production of NO by LPS-triggered Mphi reflected the severity of GVHD. NO release increased significantly during acute GVHD but was only transiently increased during nonlethal GVHD. The results provide evidence that, as a result of activation during acute GVHD, Mphi produce NO and induce the release of iron from target cells, resulting in a potent cytostatic effect that inhibits cellular proliferation. (Blood. 2000;96:1836-1843)

Activation of macrophage cytostatic effector mechanisms during acute graft-versus-host disease: release of intracellular iron and nitric oxide–mediated cytostasis

During acute graft-versus-host disease (GVHD) the activation of macrophages (Mφ) is mediated by 2 signals, interferon (IFN)-γ and bacteria-derived lipopolysaccharide (LPS). A cascade of inflammatory responses that includes the release of mediators of tissue injury follows Mφ activation. Among the tissues characteristically targeted during acute GVHD are epithelial tissues of the skin and gastrointestinal tract that normally undergo continuous proliferation and are therefore sensitive to cytostatic processes. We have investigated whether Mφ can mediate cytostatic mechanisms capable of interrupting cell proliferation during acute GVHD. GVHD was induced in nonirradiated C57BL/6XAF1 (B6AF1) mice by the injection of 60 × 106 (acute GVHD) or 30 × 106 (nonlethal GVHD) C57BL/6 (B6) lymphoid cells. Mφ from animals undergoing acute GVHD could be triggered by normally insignificant quantities of LPS to mediate a cytostatic effect on target cells, resulting in the complete shutdown of cellular proliferation. The same amounts of LPS had no effect on Mφ from normal or syngeneically transplanted animals. Mφ mediated the release of significant quantities of intracellular iron from target cells undergoing cytostasis. Reversal of cytostasis occurred following inhibition of nitric oxide (NO) production by NG-monomethyl-L-arginine (NMMA). Production of NO by LPS-triggered Mφ reflected the severity of GVHD. NO release increased significantly during acute GVHD but was only transiently increased during nonlethal GVHD. The results provide evidence that, as a result of activation during acute GVHD, Mφ produce NO and induce the release of iron from target cells, resulting in a potent cytostatic effect that inhibits cellular proliferation.

The molecular mechanisms of the metabolism and transport of iron in normal and neoplastic cells

Iron uptake by mammalian cells is mediated by the binding of serum Tf to the TfR. Transferrin is then internalized within an endocytotic vesicle by receptor-mediated endocytosis and the Fe released from the protein by a decrease in endosomal pH. Apart from this process, several cell types also have other efficient mechanisms of Fe uptake from Tf that includes a process consistent with non-specific adsorptive pinocytosis and a mechanism that is stimulated by small-Mr Fe complexes. This latter mechanism appears to be initiated by hydroxyl radicals generated by the Fe complexes, and may play a role in Fe overload disease where a significant amount of serum non-Tf-bound Fe exists. Apart from Tf-bound Fe uptake, mammalian cells also possess a number of mechanisms that can transport Fe from small-Mr Fe complexes into the cell. In fact, recent studies have demonstrated that the membrane-bound Tf homologue, MTf, can bind and internalize Fe from 59Fe-citrate. However, the significance of this Fe uptake process and its pathophysiological relevance remain uncertain. Iron derived from Tf or small-Mr complexes is probably transported into mammalian cells in the Fe(II) state. Once Fe passes through the membrane, it then becomes part of the poorly characterized intracellular labile Fe pool. Iron in the labile Fe pool that is not used for immediate requirements is stored within the Fe-storage protein, ferritin. Cellular Fe uptake and storage are coordinately regulated through a feedback control mechanism mediated at the post-transcriptional level by cytoplasmic factors known as IRP1 and IRP2. These proteins bind to stem-loop structures known as IREs on the 3 UTR of the TfR mRNA and 5 UTR of ferritin and erythroid delta-aminolevulinic acid synthase mRNAs. Interestingly, recent work has suggested that the short-lived messenger molecule, NO (or its by-product, peroxynitrite), can affect cellular Fe metabolism via its interaction with IRP1. Moreover, NO can decrease Fe uptake from Tf by a mechanism separate to its effects on IRP1, and NO may also be responsible for activated macrophage-mediated Fe release from target cells. On the other hand, the expression of inducible NOS which produces NO, can be stimulated by Fe chelators and decreased by the addition of Fe salts, suggesting that Fe is involved in the control of NOS expression.

Menadione-induced oxidative stress leads to a rapid down-modulation of transferrin receptor recycling

It has been demonstrated that perturbation of oxidative balance plays an important role in numerous pathological states as well as in physiological modifications leading to aging. In order to evaluate the role of the oxidative state in cells, biochemical and ultrastructural studies were carried out on K562 and HL-60 cell cultures. Particular attention was given to the transferrin receptor, which plays an important role in cellular iron metabolism. In order to evaluate if oxidative stress influences the transferrin receptor regulation process, the free-radical inducer menadione was used. The results obtained seem to indicate that oxidative stress is capable of inducing a rapid and specific down-modulation of the membrane transferrin receptor due to a block of receptor recycling on the cell surface, without affecting ligand-binding affinity. These effects were observed in the early stages of menadione treatment and before any typical signs of subcellular damage, including surface blebbing, a well-known cytopathological marker of menadione-induced injury. The mechanisms underlying such phenomena appear to be related to cytoskeletal protein thiol group oxidation as well as to the perturbation of calcium homeostasis, both induced by menadione. It is thus hypothesized that the data reported here represent a specific example of a general mechanism by which cell surface receptor expression and recycling can be influenced by oxidative balance.

Oxidative stress and transferrin receptor recycling

Perturbation of the oxidative balance in biological systems plays an important role in numerous pathological states as well as in many physiological processes such as receptor activity. In order to evaluate if oxidative stress induced by menadione influences membrane receptor processes, a study was conducted on the transferrin receptor. Consequently, biochemical, biophysical and ultrastructural studies were carried out on different cell lines. The results obtained seem to indicate that oxidative stress is able of inducing a rapid and specific down-modulation of membrane transferrin receptor due to a block of receptor recycling on the cell surface without affecting binding affinity.

Ribosomal protein P2, a novel iron-binding protein

We examined the properties of a new iron-binding protein purified previously from rat liver (T. Furukawa, S. Taketani, H. Kohno, and R. Tokunaga, 1991, Biochem. Biophys. Res. Commun. 181, 409-415). The protein was digested with trypsin and the peptides were analyzed by reverse-phase high-performance liquid chromatography. The partial amino acid sequences of the tryptic peptides coincided with that of rat ribosomal protein P2. Immunoblot analysis and iron-binding assay confirmed that the iron-binding protein and ribosomal protein P2 are identical. Then the iron binding ability of ribosomal protein P2 was examined in rat hepatoma H4IIEC3 cells incubated with radioactive iron. When immunoprecipitation with anti-iron-binding protein serum was performed using cells incubated with 59Fe-citrate, about 4% of the 59Fe radioactivity in cells was associated with the iron-binding protein through 30 to 90 min of incubation. About 1.5% of radioactive iron in cells incubated with 59Fe-transferrin was found in immunoprecipitates with anti-iron-binding protein serum during 1 to 5 h of incubation, and 4 to 7% of the radioactivity was found in immunoprecipitates with a monoclonal antibody against ribosomal P proteins in the same incubation. These results demonstrate that ribosomal proteins P2 binds iron taken up by the cells.

Transferrin-receptor-independent but iron-dependent proliferation of variant Chinese hamster ovary cells

The purpose of this study is to clarify the role of iron, transferrin, an iron-binding protein in vertebrate plasma, and transferrin receptors in cell proliferation. Transferrin, which is indispensable for most cells growing in tissue culture, is frequently referred to as a "growth factor". Proliferating cells express high numbers of transferrin receptors, and the binding of transferrin to their receptors that is needed for cells to initiate and maintain their DNA synthesis is sometimes regarded as analogous to other growth factor-receptor interactions. Although numerous previous experiments strongly indicate that the only function of transferrin in supporting cell proliferation is supplying cells with iron, they did not completely rule out some direct or signaling role transferrin receptors could play in cell proliferation. To address this issue, we exploited transferrin-receptor-deficient mutant Chinese hamster ovary (CHO) cells (McGraw, T. E., Greenfield, L., and Maxfield, F. R., 1987, J. Cell. Biol. 105, 207-214) in which various aspects of iron and transferrin metabolism in relation to their capacity to proliferate were investigated. Variant cells neither specifically bind transferrin nor do their extracts contain any detectable functional transferrin receptors, yet they proliferate and synthesize DNA with rates comparable to those observed with parent CHO cells. Desferrioxamine, an iron chelating agent, inhibits growth and DNA synthesis of both variant and control CHO cells. This inhibition can be fully alleviated, in both cell types, by ferric pyridoxal isonicotinoyl hydrazone, which can supply cells with a utilizable form of iron by a pathway not requiring transferrin and their receptors. Studies of 59Fe uptake and 125I-transferrin binding revealed that parent cells can take up iron by at least three mechanisms: from transferrin by receptor-dependent and -independent (nonspecific, nonsaturable, not requiring acidification) pathways and from inorganic iron salts (initially present in the medium as FeSO4). Although variant CHO cells are unable to acquire transferrin iron via the receptor pathway, two remaining mechanisms provide these cells with sufficient amounts of iron for DNA synthesis and cell proliferation. In conclusion, although transferrin receptors are dispensable in terms of their absolute requirement for proliferating cells, a supply of iron is still needed for their DNA synthesis. Transferrin-receptor-deficient CHO cells may be a useful model for investigating receptor-independent iron uptake from transferrin and nontransferrin iron sources.

Kinetics of iron passage through subcellular compartments of rabbit reticulocytes

The kinetics of the separate processes of Fe2(III)-transferrin binding to the transferrin receptor, transferrin-receptor internalization, iron dissociation from transferrin, iron passage through the membrane, and iron mobilization into the cytoplasm were studied by pulse-chase experiments using rabbit reticulocytes and 59Fe, 125I-labeled rabbit transferrin. The binding of 59Fe-transferrin to transferrin receptors was rapid with an apparent rate constant of 2 x 10(5) M-1 sec-1. The rate of internalization of 59Fe-transferrin was directly measured at 520 +/- 100 molecules of Fe2(III)-transferrin internalized/sec/cell with 250 +/- 43 sec needed to internalize the entire complement of reticulocyte transferrin receptors. Subsequent to Fe2(III)-transferrin internalization the flux of 59Fe was followed through three compartments: internalized transferrin, membrane, and cytosol. A process preceding iron dissociation from transferrin and a reaction involving membrane-associated iron required 17 +/- 2 sec and 34 +/- 5 sec, respectively. Apparent rate constants of 0.0075 +/- 0.002 sec-1 and 0.0343 +/- 0.0118 sec-1 were obtained for iron dissociation from transferrin and iron mobilization into the cytosol, respectively. Iron dissociation from transferrin is the rate-limiting step. An apparent rate constant of 0.0112 +/- 0.0025 sec-1 was obtained for processes involving iron transport through the membrane although at least two reactions are likely to be involved. Based on mechanistic considerations, iron transport through the membrane may be attributed to an iron reduction step followed by a translocation step. These data indicate that the uptake of iron in reticulocytes is a sequential process, with steps after the internalization of Fe2(III)-transferrin that are distinct from the handling of transferrin.

Endocytic vesicles contain a calmodulin-activated Ca2+ pump that mediates the inhibition of acidification by calcium

Endocytic vesicles isolated from rabbit reticulocytes contained an intrinsic Ca2(+)-pump activity that was dependent on ATP, activated by calmodulin and inhibited by vanadate. 45Ca2+ uptake and acidification studies indicated that acidification of the endocytic vesicles inversely correlated with the Ca2(+)-pump activity. Acidification was inhibited by externally added Ca2+ and calmodulin and activated by vanadate and EGTA. It is suggested that intravesicular Ca2+ can act as a modulator of endocytic vesicle acidification.

Ferric cacodylate efficiently stimulates growth of rat renal glomerular epithelial cells in vitro

Rat renal glomerular epithelial cells (SGE1 cell line) can be maintained and grown continuously in serum-free medium supplemented with insulin, iron-saturated transferrin (Tr), selenium, bovine serum albumin (BSA), linoleic acid, and epidermal growth factor (EGF). Of the growth supplements used, Tr is essential for proliferation of the cells. In the present study, we describe the use of a unique iron-chelate complex, ferric cacodylate (Fe-Cac), positively charged molecules in neutral buffer, that could almost replace Tr in serum-free culture. It even stimulated the growth of SGE1 cells more efficiently than ferric chloride (FeCl(3)) and other iron-chelate complexes, such as ferric nitrilotriacetate (Fe-NTA) and ferric citrate (Fe-Cit). The growth-stimulatory activity of Fe-Cac was exerted at iron concentrations of more than 0.01 μg/ml, whereas a 10-fold excess of iron concentration was required with FeCl(3), Fe-NTA and Fe-Cit. We observed that SGE1 cells grew until confluent, then formed hemicysts (domes) in serum-free medium containing Fe-Cac, suggesting that Fe-Cac did not merely permit cell growth but also supported polarization and organization of the cells into a functional epithelial architecture. Moreover, since the stimulatory activity of Fe-Cac was completely abolished by desferrioxamine, a strong iron chelator, it is suggested that iron is crucial for growth of SGE1 cells. When the cells were treated with suramin, an inhibitor of cellular pinocytosis and endocytosis of a large spectrum of ligands including receptor-bound growth factors, growth-stimulatory activity of Tr was inhibited, whereas the activity of Fe-Cac was not affected. These results, taken together, strongly suggest that the growth-stimulatory activity of Fe-Cac is associated with iron delivery into the cells through the cell membrane by diffusion, which is different from Tr receptor-mediated endocytosis. The use of Fe-Cac for investigating iron-regulated cell proliferation is suggested.

Assay and characteristics of the iron binding moiety of reticulocyte endocytic vesicles

A 59Fe assay was designed to detect an Fe(III) binding capacity in NP-40 solubilized proteins from rabbit reticulocyte endocytic vesicles. The iron binding capacity had an apparent molecular weight as determined by gel exclusion chromatography of 450,000 daltons. The iron binding moiety coincided with the major nontransferrin iron-containing material of endocytic vesicles labeled in vivo by incubation of cells with 59Fe, 125I-labeled transferrin. The material solubilized from vesicles with NP-40 exhibited two classes of saturable binding sites, one with an association constant for 59Fe-citrate of 3.63 x 10(9) M-1 and with 6.6 x 10(-12) moles of iron bound per mg protein and the other with a constant of 3.96 x 10(8) M-1 and 1.0 x 10(-12) moles of iron bound per mg protein. These affinities are sufficient to satisfy the solubility characteristics of Fe(III) at pH 5.0. Most of the 59Fe bound both in vivo and in vitro to the iron binding moiety could be displaced with 56Fe and an equivalent amount of 59Fe could subsequently be rebound in vitro. The iron binding assay was adopted to vesicle proteins separated by SDS-polyacrylamide gel electrophoresis with subsequent transfer to nitrocellulose and revealed an iron binding activity of molecular weight approximately 95,000 daltons.

Transferrin receptor and ferritin levels during murine mammary gland development

Various types of proliferating cell are known to express transferrin receptors which are necessary for transferrin-mediated cellular iron uptake. Neither the mechanism nor the physiological role of transferrin receptor induction has been established with certainty; although it may reflect an increased cellular requirement for iron which is essential for ribonucleotide reductase, a key enzyme of DNA synthesis. The aim of this study was to examine murine mammary gland transferrin-receptor levels during gland development. As compared to virgin controls, total mammary gland transferrin receptors expressed on the basis of DNA, increase during pregnancy and lactation by 29- and 45-fold, respectively. However, on the basis of DNA, mammary gland ferritin, measured by radioimmunoassay, decreased by about 75% and 85% during pregnancy and lactation, respectively, indicating that the increased transferrin receptor levels probably do not lead to intracellular iron accumulation. When epithelial cells from mammary glands of pregnant mice were cultured in vitro transferrin receptor expression correlated with cell proliferation. These results suggest that normal mammary growth which occurs mainly in mammary epithelial cells is associated with a significant increase in transferrin receptor. Since transferrin receptor levels remain high during lactation they are not associated solely with tissue growth, but may also function in transporting iron during milk production.

Haem inhibits iron uptake subsequent to endocytosis of transferrin in reticulocytes

Haem controls the rate of haem synthesis in erythroid cells by inhibiting iron incorporation from transferrin. The present results indicate that haem primarily inhibits the release of iron from transferrin subsequent to transferrin endocytosis and that the inhibition of transferrin endocytosis caused by relatively high concentrations of haem is a secondary effect. Low concentrations of haem (10-25 microM) significantly inhibit reticulocyte iron uptake and to a greater extent its incorporation into haem, but do not inhibit either the initial rate of transferrin uptake or its internalization by the cells.

Isolation, characterization and N-terminal amino-acid sequence of rabbit transferrin

Rabbit serum transferrin has been isolated and purified by ion-exchange column and high-performance liquid chromatography. The N-terminal amino-acid sequence of 32 residues was determined by automatic Edman degradation in a liquid phase sequenator. Of the first twelve residues sequenced previously three identifications were corrected. Comparison with the known transferrin sequences shows 15 common amino-acid residues. Comparison to human serum transferrin revealed that 37% of amino-acid residues were exchanged. Cys9 and Cys19 which are supposed to be involved in disulphide bridges, are conserved.

Transport of transferrin-bound iron into rat Sertoli cells and spermatids

Transferrin (Tf), a major secretory protein of Sertoli cells, may transport iron to spermatogenic cells. This was assessed by measuring the uptake of Fe from 59Fe-125I-labelled rat Tf by Sertoli cells and round spermatids in vitro. Uptake of Fe from labelled Tf by Sertoli cells after a 72-h pre-incubation period was linear for 20 h (approximately 18 pmol/10(6) cells/20 h), whereas the uptake of Fe from labelled Tf by round spermatids after a 16-h pre-incubation period reached a plateau by 2 h (approximately 5 pmol/10(6) cells/2 h). The corresponding net uptake of Tf by both cell types was less than 0.1 pmol. High speed supernatants prepared from Sertoli cells and spermatids labelled with 59Fe-125I-Tf were fractionated by gel permeation chromatography. Separate peaks of protein-bound 59Fe and 125I-Tf were observed. Protein bound 59Fe could be precipitated with an antiserum to rat ferritin. It is concluded that iron from exogenous Tf is transported into Sertoli cells and round spermatids in vitro, and is complexed to intracellular ferritin. However, the present results do not exclude the possibility that Sertoli cell Tf may serve purposes other than iron transport.

Kidney glomerular explants in serum-free media: role of individual medium components in cell outgrowth

Guinea pig glomeruli were grown for 22 days in a serum-free medium composed of Waymouth's MB 752/1 supplemented with sodium pyruvate, nonessential amino acids, and antibiotics. To this basic medium was added insulin, transferrin, selenium (Se), tri-iodothyronine, or fibronectin (FN) - either singly, or in various combinations - and sequential quantitative studies of the glomerular outgrowths were performed. Total cells in glomerular outgrowths, mitotic index, and glomerular attachment rate were determined and compared with values for glomerular outgrowths in media containing either no additions or all of the above components. FN was required for whole glomerular attachment, while transferrin plus FN was required for mitosis in glomerular cell outgrowths. Insulin and tri-iodothyronine slightly increased glomerular cell outgrowth by slightly increasing whole glomerular attachment, but had little effect on mitosis in glomerular outgrowths. The effect of Se was complex. Se did not affect whole glomerular attachment or mitosis in the presence of transferrin plus FN. However, in a medium containing transferrin, FN, and 3-amino-1,2,4-triazole (AT) (an inhibitor of catalase and glutathione peroxidase), Se increased total cell number but had little effect on the glomerular attachment rate or the mitotic index. Morphologic analysis of glomeruli early in culture suggested that Se may act by decreasing the amount of or delaying the time of cell death. In all of the media tested, total DNA was relatively constant over the course of 22 days, suggesting the possibility that glomerular cells cultured in a serum-free medium are part of a cell renewal system.

Control of heme synthesis during Friend cell differentiation: role of iron and transferrin

In many types of cells the synthesis of delta-aminolevulinic acid (ALA) limits the rate of heme formation. However, results from our laboratory with reticulocytes suggest that the rate of iron uptake from transferrin (Tf), rather than ALA synthase activity, limits the rate of heme synthesis in erythroid cells. To determine whether changes occur in iron metabolism and the control of heme synthesis during erythroid cell development Friend erythroleukemia cells induced to erythroid differentiation by dimethylsulfoxide (DMSO) were studied. While added ALA stimulated heme synthesis in uninduced Friend cells (suggesting ALA synthase is limiting) it did not do so in induced cells. Therefore the possibility was investigated that, in induced cells, iron uptake from Tf limits and controls heme synthesis. Several aspects of iron metabolism were investigated using the synthetic iron chelator salicylaldehyde isonicotinoyl hydrazone (SIH). Both induced and uninduced Friend cells take up and utilize Fe for heme synthesis directly from Fe-SIH without the involvement of transferrin and transferrin receptors and to a much greater extent than from saturating levels of Fe-Tf (20 microM). Furthermore, in induced Friend cells 100 microM Fe-SIH stimulated 2-14C-glycine incorporation into heme up to 3.6-fold as compared to the incorporation observed with saturating concentrations of Fe-Tf. In contrast, Fe-SIH, even when added in high concentrations, did not stimulate heme synthesis in uninduced Friend cells but was able to do so as early as 24 to 48 h following induction. In addition, contrary to previous results with rabbit reticulocytes, Fe-SIH also stimulated globin synthesis in induced Friend cells above the level seen with saturating concentrations of transferrin. These results indicate that some step(s) in the pathway of iron from extracellular Tf to protoporphyrin, rather than the activity of ALA synthase, limits and controls the overall rate of heme and possibly hemoglobin synthesis in differentiating Friend erythroleukemia cells.

Coated pit formation: a membrane function involved in the regulation of cellular iron uptake

Diferric-transferrin induces a marked increase in the number of coated pits on the reticulocyte membrane. This increase is followed by a decline, often to below the initial number. Since a good correlation was found between the rate of iron uptake and the number of coated pits, but not between the rate of transferrin recycling and the coated pit count, it is likely that coated pit formation is necessary for the removal of iron from transferrin. The decline in the number of transferrin-induced coated pits was observed only when haem synthesis was undisturbed, indicating that the accumulation of intracellular haem inhibits coated pit formation. Based on these results we suggest that haem regulates the rate of iron uptake by inhibiting iron removal rather than receptor recycling.

Construction of transferrin-coated liposomes for in vivo transport of exogenous DNA to bone marrow erythroblasts in rabbits

A method for coating liposomes with transferrin is described. Such liposomes have been used for carrying exogenous DNA to rabbit bone marrow (BM) precursor cells, in vivo. A large amount of the DNA contained in the liposomes was delivered specifically into the erythroblasts of the test animals.

Evidence for a pool of non-recycling transferrin receptors in peripheral sheep reticulocytes

Sheep reticulocytes from phlebotomized animals have a total transferrin binding potential that may exceed by an order of magnitude the surface binding capacity. Steady state uptake of transferrin at 37 degrees C is generally less than 50% of the total transferrin binding capacity. During long-term incubation of the reticulocytes, all transferrin binding ability is lost, the ability to internalize being lost most rapidly. The loss in ability to bind transferrin during long-term incubation is independent of the number of surface transferrin binding sites, since removal of surface receptors with pronase does not affect the rate of loss of the internal pool of receptors during long-term incubation. Moreover, after removing surface receptors with pronase, only a fraction of the original number of receptors is restored to the surface, despite the presence of a large pool of internal receptors. These data suggest that only a fraction of the internal pool of receptors is capable of recycling to the cell surface in sheep reticulocytes.

An extended-X-ray-absorption-fine-structure investigation of diferric transferrins and their iron-binding fragments

Iron K-edge extended-X-ray-absorption-fine-structure (e.x.a.f.s.) spectra were recorded for diferric human and rabbit serum transferrins and for diferric chicken ovotransferrin in aqueous solution; for ovotransferrin e.x.a.f.s. spectra from the N-terminal and C-terminal domain fragments were also measured. The overall spectral profiles closely resemble one another, indicating similar iron-binding sites. The simulation of the diferric ovotransferrin spectrum suggests a first co-ordination shell consisting of six low-Z ligands (nitrogen/oxygen), two ligands at a distance of approx. 0.185 nm (1.85 A) and four ligands at approx. 0.204 nm (2.04 A). The two shorter distances may correspond to Fe-O (tyrosine), whereas the longer distance is consistent with Fe-N (histidine) and Fe-O (water). Detailed analysis of the spectra of the N-terminal and C-terminal fragments indicates a difference in the short ligand distance.

Conservation of vitamin C by uric acid in blood

Urate at physiological concentrations increased the stability of ascorbate in human serum approximately fivefold. These measurements were made by depleting human serum of urate with uricase. In model experiments (using phosphate buffer instead of serum), urate protected against iron-induced ascorbate oxidation and, to a lesser extent, pH-induced ascorbate oxidation. In both human serum and in phosphate buffer, urate exerted its protective effect without itself undergoing measurable oxidation as determined by spectrophotometric and high pressure liquid chromatography (HPLC) techniques. These experiments suggest an important physiological role for urate in preserving ascorbate in blood and other biological fluids.

Transferrin receptors and iron utilization in DMSO-inducible and -uninducible Friend erythroleukemia cells

Dimethylsulfoxide (DMSO) induces hemoglobin synthesis and erythroid differentiation of Friend erythroleukemia cells in vitro. Induction is accompanied by increased transferrin-binding activity which is necessary for the cellular acquisition of iron from transferrin for hemoglobin synthesis. There are Friend cell variants in which hemoglobin synthesis is not induced by DMSO unless exogenous hemin is also present. In this study we have compared the inducibility of transferrin receptors and iron incorporation in DMSO-inducible (745) and -uninducible (M-18 and TG-13) Friend cell lines. Cellular transferrin-binding sites were estimated by Scatchard analysis of data obtained from specific binding of [125I]transferrin by the cells. Our results show that unlike 745, DMSO treatment of the variant cell lines M-18 and TG-13 does not result in increased transferrin-binding activity. The number of transferrin-binding sites and the rate of iron uptake is similar in uninduced 745 and DMSO-treated M-18 and TG-13 cells. Although exposure of M-18 cells to DMSO and hemin induces hemoglobinization, this treatment does not cause induction of transferrin receptors. These results indicate that the primary defect in M-18 cells may be the uninducibility of transferrin receptors. We have also shown that exposure of 745 cells to hemin during DMSO treatment prevents the induction of transferrin receptors, suggesting that hemin may control the expression of transferrin receptors in erythroid cells.

Iron is maintained as Fe(II) under aerobic conditions in erythroid cells

It was found that in rabbit erythrocytes loaded with Fe(II) (using the ionophore, A23187) over 90% of this metal is maintained in its ferrous form. In rabbit reticulocytes more than 50% of the transferrin-donated nonheme iron proved to be divalent. These results strongly suggest that under aerobic conditions the intracellular environment of intact erythroid cells is reducing for iron.

Availability, distribution and kinetics of the transferrin receptors of rabbit reticulocytes

The binding of transferrin by reticulocytes was studied at 3 degrees C and 37 degrees C. It was demonstrated that 50% of the total number of receptors are in an exposed position on the outer side of the plasma membrane and 50% of them are in a masked position. All of the receptors can take either of the positions. The transition between these positions is temperature dependent and can be observed as the progressive phase of the transferrin binding. It is suggested that the transferrin receptors are in dimers, one of the subunits being in the exposed position the other in the masked position. The subunits change position simultaneously. This movement of the receptors is independent of the presence of transferrin. In the light of these results a new model for transferrin-cell cycle is proposed.

A lipophilic iron chelator can replace transferrin as a stimulator of cell proliferation and differentiation

Of the different growth supplements used in chemically defined media, only transferrin is required for differentiation of tubules in the embryonic mouse metanephros. Since transferrin is an iron-carrying protein, we asked whether iron is crucial for tubulogenesis. Differentiation of metanephric tubules both in whole embryonic kidneys and in a transfilter system was studied. The tissues were grown in chemically defined media containing transferrin, apotransferrin, the metal-chelator complex ferric pyridoxal isonicotinoyl hydrazone (FePIH), and excesses of ferric ion. Although we found that apotransferrin was not as effective as iron-loaded transferrin in promoting proliferation in the differentiating kidneys, excess ferric ion at up to 100 microM, five times the normal serum concentration, could not promote differentiation or proliferation. However, iron coupled to the nonphysiological, lipophilic iron chelator, pyridoxal isonicotinoyl hydrazone, to form FePIH, could sustain levels of cell proliferation and tubulogenesis similar to those attained by transferrin. Thus, the role of transferrin in cell proliferation during tubulogenesis is solely to provide iron. Since FePIH apparently bypasses the receptor-mediated route of iron intake, the use of FePIH as a tool for investigating cell proliferation and its regulation is suggested.

Inhibition of transferring binding and iron uptake of hematopoietic cell lines by phorbol esters

Phorbol esters inhibit cell growth and the binding of transferrin to receptors on K 562, HL 60 and U 937 human leukemic cell lines. Exposure of these cells to 12-0-tetradecanoyl phorbol-13-acetate (TPA) at 37 degrees C results in a 40% reduction of the specific binding of 125I-transferrin, which is apparent within 15 min. Half-maximal inhibition occurs at about 1 nM. Other tumor promoting phorbol esters also inhibit 125I-transferrin binding in a dose-dependent manner which parallels their known promoting activity in vivo. TPA reduces the number of transferrin receptors, and does not alter the degradation or the internalization of transferrin. In addition, TPA inhibits iron uptake by these cell lines. These effects are specific, since phorbol esters do not affect either cell growth or the binding of transferrin to Friend erythroleukemia cells and Raji cell line. On the basis of these findings it is suggested that the inhibition of transferrin binding may represent one of the mechanisms by which phorbol esters affect the growth and the differentiation of hematopoietic cell lines.

Internalization of iron-transferrin complex by murine L1210 leukemia cells and rat reticulocytes demonstrated by a minibead probe

To determine if the cellular uptake of iron is associated with internalization of iron-transferrin (TF) complex by the cell, we synthesized a visual probe in which TF is covalently bound to amide-modified latex minibead, submicrometer in size (0.345 micron). Incubation of the probe with L1210 leukemia cells and rat reticulocytes led to the binding of the probe to the cell surface visualized and semiquantified by scanning and transmission electron microscopy. The binding was inhibited by preincubation with nonderivatized iron-TF complex. Internalization of the probe occurred through clathrin-coated pits and vesicles. Minibeads derivatized by nontransport proteins or glycine as well as nonderivatized minibeads did not appreciably bind to the cells and were not internalized. Ethylamine, an inhibitor of receptor-mediated endocytosis abolished the internalization but not the binding of the probe which, then, accumulated on the cell surface. These findings provide direct evidence for internalization of TF during the iron uptake.

Studies on the partition of transferrin-donated iron in rabbit reticulocytes. II. Distribution and kinetics of non-haem iron in cytosol

Reticulocytes with high cytosolic non-haem radioiron content were prepared by adding metabolic inhibitors. The cells were then reincubated without the inhibitors for various periods of time. Cytosol was prepared from the labelled cells and fractionated on a Sephadex G-100 column on which four main non-haem radioiron containing peaks could be distinguished. During reincubation the 59Fe content of these cytosol fractions changed in different ways. The high molecular weight fraction showed little metabolic activity, indicating that this fraction is not utilized for haem synthesis when iron is in excess in the cytosol. The second non-haem radioiron containing peak eluted with the haemoglobin moiety. It was clearly distinct from transferrin and could be separated from haemoglobin. This non-haem iron binding substance proved to be rather stable: 1 mM EDTA could not remove a significant amount of iron from it. During reincubation its radioiron content decreased almost linearly with time. Two further peaks eluted in the low molecular weight region. Their kinetics during reincubation suggest that these peaks represent those non-haem iron components of the cytosol which can reversibly move from the cytosol into the stroma compartment.

The preparation and partial characterization of N-terminal and C-terminal iron-binding fragments from rabbit serum transferrin

Two iron-binding fragments of Mr 36 000 and 33 000 corresponding to the N-terminal domain of rabbit serum transferrin were prepared. One iron-binding fragment of Mr 39 000 corresponding to the C-terminal domain was prepared. The N-terminal amino acid sequence of rabbit serum transferrin is: Val-Thr-Glu-Lys-Thr-Val-Asn-Trp-?-Ala-Val-Ser. One glycan unit is presented in rabbit serum transferrin and it is located in the C-terminal domain.

Iron utilization in rabbit reticulocytes. A study using succinylacetone as an inhibitor or heme synthesis

We have investigated the effect of succinylacetone (4,6-dioxoheptanoic acid) on hemoglobin synthesis and iron metabolism in reticulocytes. Succinylacetone, 0.1 and 1 mM, inhibited [2-14C]glycine incorporation into heme by 91.2 and 96.4%, respectively, and into globin by 85 and 90.2%, respectively. 60 microMM hemin completely prevented the inhibition of globin synthesis by succinylacetone, indicating that succinylacetone inhibits specifically the synthesis of heme. Added porphobilinogen, but not delta-aminolevulinic acid, partly overcame the inhibition of 59Fe incorporation into heme caused by succinylacetone suggesting that the drug inhibits delta-aminolevulinic acid dehydratase in reticulocytes. Succinylacetone, 10 microM 0.1 and 1 mM, inhibited 59Fe incorporation into heme by 50, 90 and 93%, respectively, but stimulated reticulocyte 59Fe uptake by about 25-30%. In succinylacetone-treated cells 59Fe accumulates in a fraction containing plasma membranes and mitochondria as well as cytosol ferritin and an unidentified low molecular weight fraction obtained by Sephacryl S-200 chromatography. Reincubation of washed succinylacetone- and 59Fe-transferrin-pretreated reticulocytes results in the transfer of 59Fe from the particulate fraction (plasma membrane plus mitochondria) into hemoglobin and this process is considerably stimulated by added protoporphyrin. Although the nature of the iron accumulated in the membrane-mitochondria fraction in succinylacetone-treated cells is unknown some of it is utilizable for hemoglobin synthesis, while cytosolic ferritin iron would appear to be mostly unavailable for incorporation into heme.