Iron transfer from ferritin to transferrin. Effect of serum factors

The transfer of iron from 59Fe-labelled human spleen ferritin to human apotransferrin occurs in the absence of either reducing or chelating agents. The reaction is first order with respect to ferritin and zero order to apotransferrin. The transfer is enhanced by low-Mr substances from human serum such as ascorbate, citrate, bicarbonate and lactate. A mixture of the four molecules at their normal physiological concentrations can increase the iron exchange to the same extent as that observed with an ultrafiltrate of serum. A pathway of intracellular iron mobilization is considered.     

Iron and transferrin distribution in reticulocytes incubated with heme synthesis inhibitors

A high level of non-heme iron (either labelled or unlabelled) in mitochondria, ferritin and low-molecular-weight pool of reticulocytes was induced by preincubation with isonicotinic acid hydrazide or penicillamine together with either ⁵⁹Fe- or ⁵⁶Fe-labelled transferrin. Addition of apotransferrin during reincubation of ⁵⁹Fe-labelled reticulocytes was accompanied by the transfer of ⁵⁹Fe from low-molecular-weight pool to transferrin, which was found in the reticulocyte cytosol both free and bound to a carrier. Similarly, when cells were reincubated with ¹²⁵I-labelled transferrin, more ¹²⁵I-labelled radioactivity was found, in both free and carrier-bound transferrin peaks, in reticulocytes with a high level of low-molecular-weight cold iron than in control ones. These results suggest that transferrin enters reticulocytes takes up iron from low-molecular-weight pool.     

Erythropoietin effects on iron metabolism in rat bone marrow cells

This paper describes a study of the incorporation of ^{5 9}Fe from ^{5 9}Fe-labelled rat transferrin into rat bone marrow cells in culture. ^{5 9}Fe was found in both stroma and cytoplasm of marrow cells, and the cytoplasmic ^{5 9}Fe separated by polyacrylamide gel electrophoresis, into ferritin, haemoglobin and a low molecular weight fraction.The incorporation of ^{5 9}Fe into all three cytoplasmic fractions, but not into the stroma, increased progressively with time. Erythropoietin stimulated the increase of ^{5 9}Fe in ferritin within 1 h, the earliest time examined, and more than 3 h later in the stroma and haemoglobin.A proportion of the ⁵⁹Fe incorporated into the stroma and low molecular weight iron fractions during a 1 h incubation with ⁵⁹Fe-labelled transferrin was mobilised into ferritin and haemoglobin during a subsequent 4-h “cold-chase”. Erythropoietin, when present during the “cold-chase”, did not influence these ⁵⁹Fe fluxes. The erythropoietin stimulation of ⁵⁹Fe incorporation into ferritin, one of the earliest erythropoietin effects to be recorded, was therefore considered to be due to an increase of ⁵⁹Fe uptake by the hormone-responsive cells rather than a direct effect on ferritin synthesis.20-h cultures containing erythropoietin when incubated with ⁵⁹Fe-labelled transferrin for 4 h, showed dose-related erythropoietin stimulation of ⁵⁹Fe incorporation into haemoglobin only.In the presence of 10 mM isonicotinic acid hydrazide, ⁵⁹Fe incorporation into haemoglobin was inhibited, as in reticulocytes (Ponka, P. and Neuwirt, J. (1969) Blood 33, 690–707), while that into the stroma, ferritin and low molecular weight iron fractions, was stimulated; there were no reproducible effects of erythropoietin.     

An experimental study on the pathway of iron transfer from macrophages to erythrocytes in rat liver

In order to reveal the pathway of iron release from macrophages, a 59Fe-labelled ferric hydroxide-potassium polyvinyl sulfate complex (Fe-PVS) was injected intravenously into anemic rats and the level of radioactivity in the liver, spleen, bone marrow, blood plasma and red blood cells (RBC) was estimated at various time intervals after the injection. Histochemical observation of ferric iron and ferritin in the liver was also made on anemic rats treated using unlabelled Fe-PVS. Fe-PVS injection promoted the recovery of anemia causing a rapid increase in the RBC number, with activated erythropoiesis occurring in the spleen and bone marrow. Soon after the injection, most of the radio iron was found in the liver with a small amount in the circulating erythrocytes, bone marrow and spleen. The iron level in the liver decreased gradually with a rapid increase in the iron level of the erythrocytes which reached a very high level 6 days after the 59Fe-PVS injection. Histochemical observations showed a heavy deposition of ferritin in the Kupffer cells 3 days after Fe-PVS injection. This deposition was minimized after 6 days with an increase in the level of ferritin in the parenchymal cells in the central area of acini. The level of radioferritin estimated biochemically in the nonparenchymal cell fractions of the liver revealed that the level dropped by about one third approximately 3.5 days after the Fe-PVS injection, showing the stimulated ferritin release at this stage. Results indicate that Kupffer cells in the liver play an important role in ferritin synthesis from the phagocytized iron compounds and that the iron is supplied for erythroid cell proliferation.     

Mobilization of ferritin iron in erythroblasts by chelating agents

Intracellular ferritin in newt (Triturus cristatus) erythroblasts was accessible to the chelating effects of EDTA and pyridoxal phosphate. EDTA (0.5-1 mM) promoted release of radioactive iron from ferritin of pulse-labelled erythroblasts during chase incubation, but its continuous presence was not necessary for ferritin iron mobilization. Brief exposure to EDTA was sufficient to release 60-70% of ferritin 59Fe content during ensuing chase in EDTA-free medium. EDTA also suppressed cellular iron uptake and utilization for heme synthesis, but these activities were restored upon its removal. Pyridoxal-5'-phosphate (0.5-5 mM) also stimulated loss of radioactive iron from ferritin; however, ferritin iron release by pyridoxal phosphate required its continued presence. Unlike EDTA, pyridoxal phosphate did not interfere with iron uptake or its utilization for heme synthesis. Chelator-mobilized ferritin iron accumulated initially in the hemolysate as a low-molecular-weight component and appeared to be eventually released into the medium. No radioactive ferritin was found in the medium of chelator-treated cells, indicating that secretion or loss of ferritin was not responsible for decreasing cellular ferritin 59Fe content. Moreover, there was no transfer of radioactive iron between the low-molecular-weight component released into the medium and plasma transferrin. These results indicate that chelator-released ferritin iron is not available for cellular utilization in heme synthesis and that ferritin iron released by this process is not an alternative or complementary iron source for heme synthesis. Correlation of these data with effects of succinylacetone inhibition of heme synthesis and with previous studies indicates that the main role of erythroid cell ferritin is absorption and storage of excess iron not used for heme synthesis.     

Ferritin is not a required intermediate for iron utilization in heme synthesis

Pulse-chase analysis of newt (Triturus cristatus) erythroblasts has shown that ferritin is not a primary source of iron for heme synthesis. During chase incubation with and without non-radioactive plasma iron in the medium, no transfer of 59Fe from ferritin to hemoglobin was detected although the integrity of heme synthesis was maintained. In puromycin-inhibited cells where iron uptake was drastically curtailed, heme synthesis continued to occur, though at reduced levels; incorporation of 59Fe from the plasma appeared initially in heme and hemoglobin without any prior labelling of ferritin. These results indicate that ferritin is neither an obligatory iron intermediate in heme synthesis nor a cytosolic transport molecule involved in mobilization of iron from the transferrin-receptor complex. The most likely role for erythroid ferritin is storage of excess iron.     

Iron overload of the bone marrow by trimethylhexanoyl-ferrocene in rats.

Iron-deficient female Wistar rats were fed a diet which contained 0.5% 3,5,5-trimethylhexanoyl (TMH)-ferrocene over a 57-week period. The state of iron deficiency was characterized by means of the absence of stainable iron in the bone marrow. After the first days on the iron-enriched diet, ferritin-containing siderosomes were found, in numerous erythroblasts up to orthochromatic normoblasts and in reticulocytes, i.e. the dispensed iron was used for haemoglobin synthesis. After 1 week the first macrophages showed a positive Perls' Prussian blue reaction. In the cytoplasm they stored the iron in the form of free ferritin molecules and lysosomally as aggregated ferritin and/or haemosiderin. The iron loading of the macrophages increased in both of the storage qualities proportionally with duration of the feeding period and reached a maximum after 38 weeks. Final stages showed extremely iron-loaded macrophages with high concentrations of free ferritin molecules and large siderosomes, partially flowing together to still greater units. Iron deposits within endothelial cells of bone marrow sinusoids can be observed for the first time after 4 weeks. In these cells the iron is stored as ferritin in siderosomes of relatively small and uniform size; free ferritin molecules in the cytosol were of only slight concentration. The TMH-ferrocene model of iron overload shows in the bone marrow: (1) an unimpeded utilization of the iron component for erythropoiesis, (2) development of excessive iron overload of the bone marrow in macrophages and endothelial cells of sinusoids and (3) a pattern of distribution of iron as seen in secondary haemochromatosis.     

The relationship between iron release, ferritin synthesis and intracellular iron distribution in mouse peritoneal macrophages. Evidence for a reduced level of metabolically available iron in elicited macrophages

The rate of iron release from thioglycollate-elicited mouse peritoneal macrophages pulsed with 59Fe-labelled transferrin-antitransferrin immune complexes was lower than that from resident or Corynebacterium parvum-activated macrophages. Anaerobic conditions increased the rate of iron release by thioglycollate-elicited macrophages but had no effect on resident or C. parvum-activated macrophages. Thioglycollate-elicited macrophages also contained less ferritin and were deficient in their ability to synthesis ferritin. Incubation of these cells in medium containing 100 microM iron caused some increase in ferritin synthesis, but the response to iron was much less pronounced than that by resident or C. parvum-activated macrophages. In the thioglycollate-elicited macrophages, relatively less iron was incorporated into ferritin, and more into other soluble macromolecules and insoluble haemosiderin-like compounds than in the other types of macrophages. It is proposed that thioglycollate-elicited macrophages tend to divert iron to a relatively inert intracellular pool, and that this could account for their reduced ability to release iron. Such a mechanism might help to explain the reduced release of iron by liver and spleen macrophages occurring during inflammation.     

Iron mobilization from isolated hepatocytes

It is not known which message and mechanism triggers the cell to mobilize iron from ferritin. In this paper we present the results of incubation experiments with 59Fe-labelled hepatocytes. Anemic serum gives a significant higher rate of iron mobilization than normal serum. The involvement of apo-transferrin is ruled out because it did not increase iron mobilization. Citrate increased iron mobilization which is not the result of an increase in NADH/NAD+-ratio because addition of ethanol did not stimulate iron mobilization. Desferrioxamine is used clinically in iron overloaded patients and it is known that iron removal is a very slow process. Although desferrioxamine can mobilize iron from ferritin in hepatocytes, a considerable amount remains inside the cell as a low molecular weight fraction. This fraction represents chelator bound iron and is slowly released into the circulation.     

Uptake of ferritin and iron bound to ferritin by rat hepatocytes: modulation by apotransferrin, iron chelators and chloroquine

Rat liver ferritin is an effective donor of iron to rat hepatocytes. Uptake of iron from ferritin by the cells is partially inhibited by including apotransferrin in the culture medium, but not by inclusion of diferric transferrin. This inhibition is dependent on the concentration of apotransferrin, with a 30% depression in iron incorporation in the cells detected at apotransferrin concentrations above 40 micrograms/ml. However, apotransferrin does not interfere with uptake of 125I-labeled ferritin, suggesting that apotransferrin decreases retention of iron taken up from ferritin by hepatocytes by sequestering a portion of released iron before it has entered the metabolic pathway of the cells. The iron chelators desferrioxamine (100 microM), citrate (10 mM) and diethylenetriaminepentaacetate (100 microM) reduce iron uptake by the cells by 35, 25 and 8%, respectively. In contrast, 1 mM ascorbate increases iron accumulation by 20%. At a subtoxic concentration of 100 microM, chloroquine depresses ferritin and iron uptake by hepatocytes by more than 50% after 3 h incubation. Chloroquine presumably acts by retarding lysosomal degradation of ferritin and recycling of ferritin receptors.     

The nature of iron released by resident and stimulated mouse peritoneal macrophages

Mouse peritoneal macrophages were allowed to ingest ⁵⁹Fe, ¹²⁵I-labelled transferrin-antitransferrin immune complexes, and the release of ⁵⁹Fe and degraded transferrin was studied. Some iron was released as ferritin, but a major portion was bound by bovine transferrin present in the culture medium, which contained fetal calf serum. If the medium was saturated with iron prior to incubation with the cells, little of the released iron was then bound by transferrin but appeared either as a high molecular weight fraction or, if nitrilotriacetate was present in the medium, some also appeared as a low molecular weight fraction. The release of non-ferritin iron was biphasic, the early, rapid phase being more prolonged with resident cells than with stimulated cells. The rate of release in the late phase did not differ significantly between resident and stimulated cells. Incubation at 0°C completely suppressed the release of degraded transferrin, but iron release continued at about 30% of the rate seen in control cultures at 37°C. A model for the intracellular handling of ingested iron is proposed to take account of the different release patterns of resident and stimulated macrophages.     

Iron uptake studies on erythroid cells

Iron uptake from ⁵⁵Fe-labelled transferrin, ferric citrate and the two fungal sideramines, ferricrocin and fusigen was studied using four erythroid cell cultures: Friend virus-transformed erythroleukemic cells (mouse), transformed bone marrow cells, Detroit-98 (human), reticulocytes (bovine), bone marrow cells (rabbit). The present comparative study reveals pronounced differences in iron uptake behaviour. Compared to transferrin, ferric citrate and the sideramines are preferred in transformed erythroid cells. In reticulocytes transferrin and ferric citrate showed a better uptake as compared to the two sideramines. Primary bone marrow cells showed nearly equal iron uptake rates using transferrin or ferricrocin.     

Preferential utilization in vitro of iron bound to diferric transferrin by rabbit reticulocytes.

59Fe uptake by rabbit reticulocytes from human transferrin-bound iron was studied by using transferrin solutions (35, 50, 65, 80 and 100% saturated with iron) whose only common characteristic was their content of diferric transferrin. During the early incubation period, 59Fe uptake from each preparation by reticulocytes was identical despite wide variations in amounts of total transferrin, total iron, monoferric transferrin and apotransferrin in solution. During the later phase of incubation, rate of uptake declined and was proportional to each solution's monoferric transferrin content. Uptake was also studied in a comparative experiment which used two identical, partially saturated transferrin preparations, one uniformly 59Fe-labelled and the other tracer-labelled with [59Fe]diferric transferrin. In both experiments, iron uptake by reticulocytes corresponded to utilization of a ferric ion from diferric transferrin before utilization of iron from monoferric transferrin.     

A study of ferritin iron formation in the rabbit alveolar macrophage

In order to investigate the intracellular pathway of iron to ferritin, rabbit alveolar macrophages were incubated with ⁵⁹FCl₃, homogenized by sonification, and a soluble cell fraction separated from the stroma by centrifugation at 23 000 g. The soluble fraction was examined by gel filtration using Sephadex. Two peaks were identified in the eluate at 254 nm; peak I contained a group of proteins, including ferritin, and most of the eluted radioactivity. The ⁵⁹Fe in this peak was confined to ferritin; no other ⁵⁹Fe-binding protein was identified. Peak II contained a small amount of ⁵⁹Fe. Chase experiments with ‘cold’ iron showed that peak I ⁵⁹Fe was derived from ⁵⁹Fe associated with the cell stroma. A protein carrier for ⁵⁹Fe between the stroma and ferritin was not identified in the eluate of the soluble fraction. Rather it appeared that iron moved from the stroma through the cytoplasm to ferritin in a low molecular weight form.     

Iron absorption by CaCo 2 cells cultivated in serum-free medium as in vitro model of the human intestinal epithelial barrier

A cell culture system consisting of confluent monolayer of human enterocyte-like CaCo 2 cells, cultivated in a serum-free nutritive medium, on microporous synthetic membranes has been used as an in vitro model of the intestinal epithelial barrier. The uptake of ⁵⁵ferric citrate, as well as the transepithelial passage from the apical to the basolateral pole, have been studied. CaCo 2 cells accumulate iron in a time- and concentration-dependent process, largely specific from the apical pole. When ⁵⁵ferric citrate is added at the apical pole, radioiron appears at the basal pole and the clearance rate is ～four times higher than in the opposite direction; the amounts of ⁵⁵Fe increase with the concentration in iron citrate and the duration of incubation. At least two concurrent mechanisms could be involved in iron absorption across monolayers of CaCo 2 cells. A first route would correspond to a paracellular passage of the metal from the apical to the basal pole. The second route would involve a selective intake of iron at the apical pole and could require a reduction of ferric iron, prior to the entry. Iron accumulated by the cells would, for a minor part, be stored within ferritin, whereas the major part would be excreted at the basolateral pole, either as low molecular weight material of undetermined chemical composition but from which iron is easily mobilized by apotransferrin or associated with neosynthesized apotransferrin. Vesicular transport and protein synthesis seem to be required. © 1994 Wiley-Liss, Inc.     

LOSS OF IRON FROM MOUSE PERITONEAL MACROPHAGES IN VITRO AFTER UPTAKE OF [55FE]FERRITIN AND [55FE]FERRITIN RABBIT ANTIFERRITIN COMPLEXES

Mouse peritoneal macrophages in culture for 24 h were exposed to horse [⁵⁵Fe]ferritin and rabbit antihorse [⁵⁵Fe]ferritin antibody complex and the amount of ⁵⁵Fe in the medium was assayed up to 2 days after the pulse uptake. Cell survival was assayed by photographing the same areas of the tissue culture Petri dish on successive days and by counting cell numbers per unit area. In experiments in which quantitative assay for cell death is negligible, about 10–20% of the iron ingested by pinocytosis or phagocytosis is released to iron-free medium containing either freshly dialyzed or deironized newborn calf serum (10%). Over the 2-day postpulse period, iron loss is linear. This loss of iron to the medium is significantly reduced by adding iron-saturated newborn calf serum in the postpulse recovery period. A significant portion of the iron released to the medium is bound to transferrin. When human serum is used in the tissue culture system, similar quantities (10–25%) of the ingested iron are lost to the medium 2 days after the pulse.     

Nestin Positive Bone Marrow Derived Cells Responded to Injury Mobilize into Peripheral Circulation and Participate in Skin Defect Healing

Exogenously infused mesenchymal stem cells (MSCs) are thought to migrate to injury site through peripheral blood stream and participate in tissue repair. However, whether and how endogenous bone marrow MSCs mobilized to circulating and targeted to tissue injury has raised some controversy, and related studies were restricted by the difficulty of MSCs identifying in vivo. Nestin, a kind of intermediate filament protein initially identified in neuroepithelial stem cells, was recently reported as a credible criteria for MSCs in bone marrow. In this study, we used a green fluorescent protein (GFP) labeled bone marrow replacement model to trace the nestin positive bone marrow derived cells (BMDCs) of skin defected-mice. We found that after skin injured, numbers of nestin⁺ cells in peripheral blood and bone marrow both increased. A remarkable concentration of nestin⁺ BMDCs around skin wound was detected, while few of these cells could be observed in uninjured skin or other organs. This recruitment effect could not be promoted by granulocyte colony-stimulating factor (G-CSF), suggests a different mobilization mechanism from ones G-CSF takes effect on hematopoietic cells. Our results proposed nestin⁺ BMDCs as mobilized candidates in skin injury repair, which provide a new insight of endogenous MSCs therapy.     

2-Hydroxypyridine-N-oxides: effective new chelators in iron mobilisation

The 2-hydroxypyridine-N-oxide derivatives, 2-hydroxypyridine-N-oxide, 2,4-dihydroxypyridine-N-oxide, 2-hydroxy-4-methoxypyridine-N-oxide and 2-hydroxy-4-(2'-methoxyethoxy)pyridine-N-oxide have been shown to remove iron from human transferrin and horse spleen ferritin at pH 7.4 at levels higher than those caused by desferrioxamine. Their reactions with transferrin were mainly biphasic and took 2-5 h to reach completion but iron mobilisation from ferritin was slower and their reactions continued after 40 h of incubation. The intraperitoneal and intragastric administration of 2,4-dihydroxypyridine-N-oxide to two iron-loaded 59Fe-labelled mice caused an increase in 59Fe excretion which is comparable to that caused by desferrioxamine intraperitoneally. These results increase the prospects for the use of these chelators as probes for studying iron metabolism and in the treatment of iron overload and other diseases of iron imbalance.     

Superoxide-mediated release of iron from ferritin by some flavoenzymes

NADH-lipoamide dehydrogenase mobilized iron from ferritin under aerobic conditions. Superoxide dismutase strongly inhibited this mobilization, indicating that the superoxide radical is generated by the enzymatic reaction and release iron from ferritin. Addition of lipoamide as an electron acceptor to NADH-lipoamide dehydrogenase increased the release of iron from ferritin and this release was partially inhibited by superoxide dismutase. Similarly, addition of menadione (2-methyl-1, 4-naphthoquinone) as an electron acceptor to xanthine-xanthine oxidase promoted the release of iron from ferritin and this release was strongly inhibited by superoxide dismutase. These results suggest that dihydrolipoamide and semiquinone of menadione can react with oxygen to form the superoxide radical that mediates release of iron from ferritin.     

KINETICS OF ERYTHROPOIETIN STIMULATED PROLIFERATION OF ERYTHROID PROGENITORS IN THE SPLEEN

The effect of RBC transfusion and erythropoietin (EPO) on the proliferation of immature erythrocyte progenitors was studied in the spleens of RBC transfused, lethally irradiated mice injected with bone marrow. Transfusion decreased expansion of the progenitors and slowed their proliferation: the mean cycle time as measured by per cent labelled mitosis (PLM) on the third day after injection of bone marrow was 10.7 hr in transfused as compared to 5.6 hr in non-transfused mice. One injection of five units of erythropoietin on day 2 decreased the mean cycle time to 7.3 hr in transfused mice and increased expansion of the progenitor cells. The effects of erythropoietin on cell proliferation were prompt: a significant increase of incorporation of ³H-TdR into DNA occurred within 2 hr of injection. Erythroblasts were absent from the spleens of transfused, irradiated bone marrow injected mice; however, erythroblasts appeared by 72 hr and 48 hr following EPO injection either 2 days or 5 days after transplantation respectively. Increased uptake of radioactive iron in spleen after erythropoietin injection preceded the appearance of erythroblasts by 2 and 1 days when erythropoietin was injected either 2 or 5 days after marrow transplantation respectively. The increase in cellular proliferation induced by erythropoietin in transfused irradiated mice injected with bone marrow equivalent to 0.35 femoral shaft was manifested as an increase of the total DNA content in the spleen by 119 μg (11.9 × 10⁶ cells) within 48 hr of injection. The cellular increment produced by EPO injection on day 5 to mice given 0.05 femoral shaft consisted mainly of undifferentiated mononuclear cells, most of which were labelled, with erythroblasts comprising only one quarter of the increment. Erythropoietin inactivated by mild acid hydrolysis failed to increase cellular proliferation.