Effect of iron chelators on the transferrin receptor in K562 cells

Delivery of iron to K562 cells by diferric transferrin involves a cycle of binding to surface receptors, internalization into an acidic compartment, transfer of iron to ferritin, and release of apotransferrin from the cell. To evaluate potential feedback effects of iron on this system, we exposed cells to iron chelators and monitored the activity of the transferrin receptor. In the present study, we found that chelation of extracellular iron by the hydrophilic chelators desferrioxamine B, diethylenetriaminepentaacetic acid, or apolactoferrin enhanced the release from the cells of previously internalized 125I-transferrin. Presaturation of these compounds with iron blocked this effect. These chelators did not affect the uptake of iron from transferrin. In contrast, the hydrophobic chelator 2,2-bipyridine, which partitions into cell membranes, completely blocked iron uptake by chelating the iron during its transfer across the membrane. The 2,2-bipyridine did not, however, enhance the release of 125I-transferrin from the cells, indicating that extracellular iron chelation is the key to this effect. Desferrioxamine, unlike the other hydrophilic chelators, can enter the cell and chelate an intracellular pool of iron. This produced a parallel increase in surface and intracellular transferrin receptors, reaching 2-fold at 24 h and 3-fold at 48 h. This increase in receptor number required ongoing protein synthesis and could be blocked by cycloheximide. Diethylenetriaminepentaacetic acid or desferrioxamine presaturated with iron did not induce new transferrin receptors. The new receptors were functionally active and produced an increase in 59Fe uptake from 59Fe-transferrin. We conclude that the transferrin receptor in the K562 cell is regulated in part by chelatable iron: chelation of extracellular iron enhances the release of apotransferrin from the cell, while chelation of an intracellular iron pool results in the biosynthesis of new receptors.     

The release of iron by Sertoli cells in culture

In seminiferous tubules, iron transport from the blood to the abluminal germinal cells must occur through the Sertoli cell cytoplasm. We investigated the release of previously accumulated iron by cultured Sertoli cells. We found that Sertoli cells contain easily releasable and less easily releasable iron pools. Iron is released in a low molecular weight form (molecular weight less than 30,000). A high concentration of this low molecular weight iron in the medium reduces further iron release by Sertoli cells, whereas the addition of more medium or fresh medium increases further iron release. Apotransferrin stimulates the release of iron in a dose-dependent manner by chelating the low molecular weight iron. Rat and human apotransferrin are completely competitive in this respect. Diethylenetriamine penta acetic acid (DTPA), an extracellular iron chelator, and apotransferrin compete for iron binding and stimulation of iron release, indicating that no binding or uptake of the chelator by the cells is required. Desferrioxamine (DFO), an intracellular iron chelator, on the other hand, increases iron release more drastically, and apotransferrin cannot compete with it for iron. The addition of extracellular iron also increases the amount of 59Fe in the medium, probably by reducing the re-uptake of 59Fe. This is also demonstrated with primaquine, which blocks endocytosis and increases the amount of 59Fe in the medium. The presence of germinal cells also stimulates the release of iron by Sertoli cells. When cocultured, the germinal cells internalize iron as it is release by Sertoli cells.     

Uptake of transferrin and iron by cultured rat placental cells

This paper describes a method for the culture of rat placental cells. The method involved separation of the basal layer from the labyrinth and sequential digestion of the cells. The cells were demonstrated not to be fibroblasts and are described in terms of their appearance under the light and electron microscopes. Transferrin and iron uptake by the cells was examined and compared with results achieved using other methods of study. The results showed that transferrin bound to receptors on the cell surface and that the transferrin, once bound, was taken into the cell. Only this internalized transferrin was capable of donating iron to the cells. The iron was accumulated within the cells and did not appear to be released to the incubation medium. The apparent dissociation constant (Ka) for transferrin was found to be 6.96 X 10(6) M-1, a value similar to that described by earlier workers. The placental cells had 3.4 X 10(11) binding sites/microgram DNA, equivalent to approximately 1 X 10(6) sites/cell. From these data, and from the rate of accumulation of iron by the cells, the receptor turnover time was estimated as being between 5 and 10 min.     

Chelator-mediated iron efflux from reticulocytes

The mechanism by which bipyridine and phenanthroline types of iron chelator inhibit iron uptake from transferrin and iron efflux mediated by pyridoxal isonicotinoyl hydrazone was investigated using rabbit reticulocytes with the aim of providing more information on the normal process of iron uptake by developing erythroid cells. It was shown that the chelators block cellular uptake by chelating the iron immediately after release from transferrin while it is still in the membrane fraction of the cells. The iron-chelator is then released from the cells by a process which is very similar to that of transferrin release with respect to kinetics and sensitivity to incubation temperature and the effects of metabolic inhibitors and other chemical reagents. These results are compatible with the conclusion that both transferrin and the iron-chelators in the cells are mainly present in endocytotic vesicles and are released from the cells by exocytosis. The chelators were also shown to block the pyridoxal isonicotinoyl hydrazone-mediated efflux of iron from cells which had taken up iron in the presence of isoniazid, an inhibitor of haem synthesis, by chelating the iron in the cytosol and the mitochondria. In this case, the iron-chelator complexes were not released from the cells. Measurement of the diethyl ether/water partition coefficients of bipyridine and 1,10-phenanthroline and their iron complexes gave much higher values for the free chelators, supporting the concept that the chelators trap the iron intracellularly because of differences in the lipid solubility and, hence, membrane permeability to the free chelators and their iron complexes.     

Iron uptake in reticulocytes. Inhibition mediated by the ionophores monensin and nigerisin

The lipophilic carboxylic ionophores monensin and nigerisin reversibly blocked iron uptake by erythroid cells. At low concentrations of ionophores (0.25-0.5 microM), the disruption of the compartment in which iron is released affected minimally the release of iron from transferrin but effectively inhibited iron uptake. Iron released from transferrin was extruded from the cell synchronously with but not bound to transferrin. The compartment disrupted by the ionophores, and in which iron is released from transferrin, is apparently contiguous to the extracellular medium. Contiguity was assessed by determining the effect of extracellular Na+ and K+ on the activity of the ionophores. The above data fit a model of iron uptake in which iron is released from transferrin in an acidic compartment in immediate contiguity with the cell plasma membrane. Iron is then bound by its membrane acceptor and is translocated to the cytosolic side of the plasma membrane. At submicromolar concentrations, the ionophores monensin and nigerisin produce a small increase in the pH of the acidic compartment. The pH change, which is not sufficient to block the release of iron from transferrin, is enough to block the binding of released iron to its acceptor in the plasma membrane, thus producing inhibition of iron uptake.     

Effect of ATP depletion and temperature on the transferrin-mediated uptake and release of iron by BeWo choriocarcinoma cells.

We have recently described the transferrin-mediated uptake and release of iron by BeWo cells [van der Ende, du Maine, Simmons, Schwartz & Strous (1987) J. Biol. Chem. 262, 8910-8916]. We now extend our studies of the mechanisms responsible for uptake and release of iron by these cells. Following preloading, 59Fe release was maximal (about 12%) after about 4 h. Replacement of the extracellular medium with an equal volume of fresh medium either prior to or following the time at which equilibrium was reached further stimulated 59Fe release. Both the rate and maximum amount of iron release decreased if longer loading times were used. Preincubation of BeWo cells for 15 min with 10 mM-sodium cyanide and 50 mM-2-deoxyglucose prior to the determination of 59Fe release did not alter the amount released into medium (which did not contain a high-affinity iron chelator). However, under these conditions, the uptake of 59Fe was dramatically inhibited as a result of prolongation of the transferrin-transferrin-receptor complex recycling time. These results demonstrate that the release of iron from BeWo cells is independent of cellular ATP levels, whereas iron uptake is ATP-dependent. Rates of both 59Fe release and 59Fe uptake were temperature-dependent. Analysis of these data via an Arrhenius plot suggests a single rate-limiting step for the release and uptake processes between 0 and 37 degrees C. The apparent energies of activation of these processes are very similar (approx. 59.0 kJ/mol for iron release and 50.6 kJ/mol for iron uptake), which raises the possibility that the release and uptake of iron share a common thermodynamically rate-limiting step. Possible mechanisms involved in iron release out of the cell and out of the endosome are discussed.     

Iron uptake and metabolism by hepatocytes

The hepatocytes form part of the iron storage system of the body. In serving this function they exchange iron bidirectionally with the plasma iron transport protein transferrin (Tf). Iron uptake involves binding of the iron-Tf complex to cell membrane receptors and endocytosis into low-density vesicles, where the iron is released from its carrier protein before the Tf is returned undegraded to the extracellular medium. Two components of the iron uptake process can be distinguished, one saturable at low concentrations of diferric Tf and the other not saturable by increasing the Tf concentration. Both result in net uptake of iron by the cells and both appear to depend on specific binding to the cell membrane and endocytosis. Hepatocytes also obtain some iron from haptoglobin-hemoglobin, heme-hemopexin, and ferritin (Fn), in each case by interaction with membrane receptors and endocytosis. Within the cell iron from all sources enters one or more transit pools, where it is available for exchange with the iron storage protein Fn, and for release from the cell to plasma Tf or to iron chelators administered therapeutically or experimentally. Chelator-mediated iron release occurs to the plasma and/or to the bile, depending on the nature of the chelator and the source of the iron.     

Dengue haemorrhagic fever and dengue shock syndrome: are they tumour necrosis factor-mediated disorders?

Growth of Bacteroides fragilis under anaerobic conditions in the presence of either haemin or protoporphyrin IX was inhibited by the ferrous iron chelator bipyridyl. The ferric-iron chelator desferrioxamine inhibited growth in the presence of protoporphyrin but not haemin, suggesting that even under anaerobic conditions Fe3+ is involved in uptake of non-haem iron, which is required in the absence of haemin. However, the ferric iron chelators 1,2-dimethyl-3-hydroxy-pyrid-4-one (L1) and pyridoxal isonicotinoyl hydrazone (PIH) were only weakly inhibitory. Apotransferrin, which also binds Fe3+, inhibited growth, but this was not simply due to binding of iron in the medium, as under the reducing conditions present, transferrin was unable to bind iron. This study suggests that even under anaerobic conditions, uptake of non-haem iron by B. fragilis may involve conversion of Fe2+ to Fe3+.     

Growth of human tumor cell lines in transferrin-free,low-iron medium

Summary Iron is essential for tumor cell growth. Previous studies have demonstrated that apart from transferrin-bound iron uptake, mammalian cells also possess a transport system capable of efficiently obtaining iron from small molecular weight iron chelates (Sturrock et al., 1990). In the present study, we have examined the ability of tumor cells to grow in the presence of low molecular weight iron chelates of citrate. In chemically defined serum-free medium, most human tumor cell lines required either transferrin (5 μg/ml) or a higher concentration of ferric citrate (500 μM) as an iron source. However, we have also found that from 13 human cell lines tested, 4 were capable of long-term growth in transferrin-free medium with a substantially lower concentration of ferric citrate (5 μM). When grown in medium containing transferrin, both regular and low-iron dependent cell lines use transferrin-bound iron. Growth of both cell types in transferrin medium was inhibited to a certain degree by monoclonal antibody 42/6, which specifically blocks the binding of transferrin to the transferrin receptor. On the contrary, growth of low-iron dependent cell lines in transferrin-free, low-iron medium (5 μM ferric citrate) could not be inhibited by monoclonal antibody 42/6. Furthermore, no autocrine production of transferrin was observed. Low-iron dependent cell lines still remain sensitive to iron depletion as the iron(III) chelator, desferrioxamine, inhibited their growth. We conclude that low-iron dependent tumor cells in transferrin-free, low-iron medium may employ a previously unknown mechanism for uptake of non-transferrin-bound iron that allows them to efficiently use low concentrations of ferric citrate as an iron source. The results are discussed in the context of alternative iron uptake mechanisms to the well-characterized receptor-mediated endocytosis process.     

Factors influencing survival of mammalian cells exposed to hypothermia. IV. Effects of iron chelation

Survival of V-79 Chinese hamster cells was assessed by colony growth assay after hypothermic exposure in the presence of iron chelators. At 5 degrees C, maximum protection from hypothermic damage was achieved with a 50 microM concentration of the intracellular ferric iron chelator Desferal. A 3-hr prehypothermic incubation with 50 microM Desferal followed by replacement with chelator-free medium at 5 degrees C also provided some protection. This was not observed when the extracellular chelator DETA-PAC (50 microM) was used prior to cold storage. Treating 5 degrees C-stored cells with Desferal just prior to rewarming was ineffective, but treating cells with Desferal during hypothermia exposure after a significant period of unprotected cold exposure ultimately increased the surviving fraction. Submaximal protection during hypothermia was achieved to various degrees with extracellular chelators at 5 degrees C, including 50 microM DETAPAC and 110 microM EDTA. EGTA (110 microM) had little effect. The sensitization of cells at 5 degrees C with 200 microM FeCl3 could be reduced or eliminated with Desferal in accordance with a 1:1 binding ratio. At 10 degrees C, 50 microM Desferal, 50 microM DETAPAC, and 110 microM EDTA were as or less effective in protecting cells than at 5 degrees C. An Arrhenius plot of cell inactivation rates shows a break at 7-8 degrees C, corresponding to maximum survival for control cells and cells in 50 microM Desferal; however, the amount of protection offered by the chelator increases with decreasing temperature below about 19 degrees C, and sensitization increases above that point. It has not previously been shown that iron chelators protect against cellular hypothermia damage which is uncomplicated by previous or simultaneous ischemia. This may be relevant to the low-temperature storage of transplant organs, in which iron of intracellular origin and in the perfusate may be active and damaging.     

Iron uptake from transferrin and transferrin endocytic cycle in Friend erythroleukemia cells

Several aspects of iron metabolism were studied in cultured Friend erythroleukemia cells before and after induction of hemoglobin synthesis by dimethyl sulfoxide. The maximal rate of iron uptake from 59Fe-labeled transferrin, 1.5 X 10(6) atoms of Fe/cell per 30 min in uninduced cells, increased to 3 X 10(6) atoms/cell after 5 days of induction. The increase in iron uptake was not accompanied by a proportional increase in the number of transferrin receptors detected by 125I-labeled transferrin binding, suggesting a more efficient iron uptake by transferrin receptors in induced cells, with the rate of about 26 iron atoms per receptor per hour, compared to 15 atoms in uninduced cells. In agreement with this conclusion are results of the study of cellular 125I or 59Fe labeled transferrin kinetics. In the induced cells transferrin endocytosis and release proceeded with identical rates and all the endocytosed iron was retained inside the cell. On the other hand, transferrin release by uninduced cells was significantly slower and a substantial part of internalized 59Fe was released. On the basis of these results, different efficiency of iron release from internalized transferrin, accompanied by changes in cellular transferrin kinetics, is proposed as one of the factors determining the rate of iron uptake by developing erythroid cells.     

Effects of chelators on iron uptake and release by the brain in the rat

The iron chelators desferrioxamine (DFO), pyridoxal isonicotinoyl hydrazone (PIH), 2,2-bipyridine, diethylenetriamine penta-acetic acid (DTPA) and 1,2 dimethyl-3-hydroxy pyrid-4-one (CP20) were analysed for their ability to change⁵⁹Fe uptake and release from the brain of 15- and 63-day rats either during or after intravenous injection of⁵⁹Fe-¹²⁵I-transferrin. DTPA was the only chelator unable to significantly reduce iron uptake into the brain of 15-day rats. This indicates that iron is not released from transferrin at the luminal surface of brain capillary endothelial cells. CP20 was able to reduce iron uptake in the brain by 85% compared to 28% with DFO. Only CP20 was able to significantly reduce brain iron uptake in 63 day rats. Once⁵⁹Fe had entered the brain no chelator used was able to mediate its release. All of the chelators except CP20 had similar effects on femur iron uptake as they did on brain uptake, suggesting similar iron uptake mechanisms. It is concluded that during the passage of transferrin-bound iron into the brain the iron is released from transferrin within endothelial cells after endocytosis of transferrin.     

The effect of desferrioxamine on transferrin receptors, the cell cycle and growth rates of human leukaemic cells.

The effect of the iron chelator, desferrioxamine, on transferrin binding, growth rates and the cell cycle was investigated in the human leukaemic cell line, K562. At all concentrations of the chelator (2-50 microM) binding of 125I-transferrin was increased by 24 h and reached a maximum at 72-96 h. Maximum binding (6-8-fold increased) occurred in cells treated with 20 microM-desferrioxamine, in contrast with control cells which, at 96 h, showed a 50% decrease over initial binding. Scatchard analysis at 4 degrees C showed that this increased binding was due to an increase in the number of receptors, as the Kd was similar in induced (1.8 nM) and control (1.5 nM) cells. After 96 h cells, cultured with 20 and 50 microM-desferrioxamine accumulated 59Fe from bovine transferrin at over twice the rate found with control cells, reflecting the increase in transferrin receptors. Although iron uptake was unimpaired by the chelator there was a dose-dependent inhibition of cell growth, with control cells completing three divisions in 96 h and those in 10 microM-desferrioxamine only two divisions. At the highest concentration (50 microM), cell division was abrogated although cell viability was maintained (85%). In contrast, DNA synthesis was not markedly affected, except at 50 microM-desferrioxamine when incorporation of [3H]thymidine was 52% of that in control cells. Flow cytometry revealed that there was a progressive accumulation of the cells in the active phases of their cycle (S, G2 + M). Desferrioxamine may increase transferrin receptors in two ways: by chelating a regulatory pool of iron within the cell, and by arresting cells in S phase when receptors are maximally expressed.     

Uptake of iron from transferrin by isolated hepatocytes

Isolated rat hepatocytes containing 0.56-1.79 micrograms iron/10(6) cells and with an intracellular ATP concentration of 3-4 mM, accumulate iron from transferrin linearly with time for at least 3 h. At 37 degrees C the rate of uptake amounts to 0.3-0.7 pmol/mg cell protein per min. The uptake reaches a saturation level of 21-40 pmol/mg cell protein per h at 2.2 microM iron. At 5 degrees C the uptake does not increase over the time of incubation. Uptake of iron, but not binding of transferrin is increased 4-5-fold at oxygen concentrations 10-20 microM. At oxygen concentrations beyond these limits iron uptake is decreased. Iron taken up at low oxygen concentrations can be chelated by bathophenanthroline and bathophenanthroline disulphonate , but only if the chelators are present during the uptake experiments. The results suggest that iron uptake from transferrin by hepatocytes in suspension involves reductive removal of iron.     

Receptor-mediated endocytosis of transferrin in K562 cells

Human diferric transferrin binds to the surface of K562 cells, a human leukemic cell line. There are about 1.6 X 10(5) binding sites per cell surface, exhibiting a KD of about 10(-9) M. Upon warming cells to 37 degrees C there is a rapid increase in uptake to a steady state level of twice that obtained at 0 degree C. This is accounted for by internalization of the ligand as shown by the development of resistance to either acid wash or protease treatment of the ligand-cell association. After a minimum residency time of 4-5 min, undegraded transferrin is released from the cell. Internalization is rapid but is dependent upon cell surface occupancy; at occupancies of 20% or greater the rate coefficient is maximal at about 0.1-0.2 min-1. In the absence of externally added ligand only 50% of the internalized transferrin completes the cycle and is released to the medium with a rate coefficient of 0.05 min-1. The remaining transferrin can be released from the cell only by the addition of ligand, suggesting a tight coupling between cell surface binding, internalization, and release of internalized ligand. There is a loss of cell surface-binding capacity that accompanies transferrin internalization. At low (less than 50%) occupancy this loss is monotonic with the extent of internalization. Even at saturating levels of transferrin, the loss of surface receptors upon internalization never exceeds 60-70% of the initial binding capacity. This suggests that receptors enter the cell with ligand but are replaced so as to maintain a constant, albeit reduced, receptor number on the cell surface. In the absence of ligand, the cell surface receptor number returns at 37 degrees C. Neither sodium azide nor NH4Cl blocks internalization of ligand. However, they both prevent the release of transferrin from the cell thus halting the transferrin cycle. Excess ligand can overcome the block due to NH4Cl but not azide although the cycle is markedly slower. Iron is delivered to these cells by transferrin at 37 degrees C with a rate coefficient of 0.15 to 0.2 min-1. The iron is released from the transferrin and the majority is found in intracellular ferritin. There is a large internal receptor pool comprising 70 to 80% of the total cell receptors and this may be involved in maintaining the steady state iron uptake.     

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.     

Transferrin binding and iron uptake in mouse hepatocytes

Methods were developed for obtaining highly viable mouse hepatocytes in single cell suspension and for maintaining the hepatocytes in adherent static culture. The characteristics of transferrin binding and iron uptake into these hepatocytes was investigated. (1) After attachment to culture dishes for 18–24 h hepatocytes displayed an accelerating rate of iron uptake with time. Immediately after isolation mouse hepatocytes in suspension exhibited a linear iron uptake rate of 1.14·10⁵molecules/cell per min in 5 μM transferrin. Iron uptake also increased with increasing transferrin concentration both in suspension and adherent culture. Pinocytosis measured in isolated hepatocytes could account only for 10–20% of the total iron uptake. Iron uptake was completely inhibited at 4°C. (2) A transferrin binding component which saturated at 0.5 μM diferric transferrin was detected. The number of specific, saturable diferric transferrin binding sites on mouse hepatocytes was 4.4·10⁴±1.9·10⁴ for cells in suspension and 6.6·10⁴±2.3·10⁴ for adherent cultured cells. The apparent association constants were 1.23·10⁷ 1·mol^?1 and 3.4·10⁶ 1·mol^?1 for suspension and cultured cells respectively. (3) Mouse hepatocytes also displayed a large component of non-saturable transferrin binding sites. This binding increased linearly with transferrin concentration and appeared to contribute to iron uptake in mouse hepatocytes. Assuming that only saturable transferrin binding sites donate iron, the rate of iron uptake is about 2.5 molecules iron/receptor per min at 5 μM transferrin in both suspension and adherent cells and increases to 4 molecules iron/receptor per min at 10 μM transferrin in adherent cultured cells. These rates are considerably greater than the 0.5 molcules/receptor per min observed at 0.5 μM transferrin, the concentration at which the specific transferrin binding sites are fully occupied. The data suggest that either the non-saturable binding component donates some iron or that this component stimulates the saturable component to increase the rate of iron uptake. (4) During incubations at 4°C the majority of the transferrin bound to both saturable and nonsaturable binding sites lost one or more iron atoms. Incubations including 2 mM α,α′-dipyridyl (an Fe¹¹ chelator) decreased the cell associated ⁵⁹Fe at both 4 and 37°C while completely inhibiting iron uptake within 2–3 min of exposure at 37°C. These observations suggest that most if not all iron is loosened from transferrin upon interaction of transferrin with the hepatocyte membrane. There is also greater sensitivity of ⁵⁹Fe uptake compared to transferrin binding to pronase digestion, suggesting that an iron acceptor moiety on the cell surface is available to proteolysis.     

Iron metabolism in K562 erythroleukemic cells

Iron delivery to K562 cells is enhanced by desferrioxamine through induction of transferrin receptors. Experiments were performed to further characterize this event with respect to iron metabolism and heme synthesis. In control cells, up to 85% of the iron taken up from iron-transferrin was incorporated into ferritin, 7% into heme, and the remainder into compartments not yet identified. In cells grown with desferrioxamine, net accumulation of intracellular desferrioxamine (14-fold) was observed and iron incorporation into ferritin and heme was inhibited by 86% and 75%, respectively. In contrast, complete inhibition of heme synthesis in cells grown with succinylacetone had no effect on transferrin binding or iron uptake. Exogenous hemin (30 microM) inhibited transferrin binding and iron uptake by 70% and heme synthesis by 90%. These effects were already evident after 2 h. Thus, although heme production could be reduced by desferrioxamine, succinylacetone, and hemin, cell iron uptake was enhanced only by the intracellular iron chelator. The effects of exogenous heme are probably unphysiologic and the greater inhibition of iron flow into heme can be explained by effects on early steps of heme synthesis. We conclude that in this cell model a chelatable intracellular iron pool rather than heme synthesis mediates regulation of iron uptake.     

Transferrin receptors and transferrin iron uptake by cultured human blood monocytes

Transferrin receptors have been previously found on human macrophages and it has also been shown that transferrin iron is taken up by these cells. It has therefore been inferred that the uptake is receptor mediated and involves an endocytic pathway. The subject was addressed directly in the present study in which the transferrin-iron-receptor interaction was characterized in cultured human blood monocytes. Specific, saturable diferric transferrin binding was demonstrated, with a kDa of 3.6 X 10(-8) M and a calculated receptor density of 1.25-2.5 X 10(5) receptors per cell. Incubation at 4 degrees C markedly reduced transferrin binding and completely inhibited iron uptake. Chase experiments confirmed progressive cellular loading of iron, with concomitant loss of transferrin. Inhibitors of endocytic vesicle acidification (ammonium chloride and 2,4-dinitrophenol) inhibited iron unloading from endocytosed diferric transferrin, while microtubular inhibitors (colchicine and vindesine) and a microfilament inhibitor (cytochalasin B) reduced diferric transferrin uptake but had little effect on the iron unloading pathway. A similar effect was noted with a calcium ion antagonist (verapamil) and with 2 calmodulin antagonists (chlorpromazine and imipramine). These latter findings suggest the importance of cytoskeleton-membrane interactions via a calcium, calmodulin and protein kinase C mediated system. Endocytosed iron accumulated progressively as ferritin within the cultured monocytes.     

Utilization and cell-surface binding of hemin by Histoplasma capsulatum

Histoplasma capsulatum, a dimorphic fungus capable of causing severe respiratory illness in immunocompromised individuals, resides in macrophages during mammalian infection. Previous studies suggest that siderophore-mediated iron transport may be important for the acquisition of iron from transferrin while the organism resides in macrophages. However, iron is also present as hemin in the intracellular environment of the macrophage and may serve as a major source of iron during infection. Thus the ability of H. capsulatum to use hemin and heme-containing compounds was examined. Histoplasma capsulatum G217B was iron-starved by adding the iron chelator deferoxamine mesylate to the culture. The addition of 10 microM hemin in the presence of deferoxamine mesylate restored growth to the levels seen in the absence of the chelator. Histoplasma capsulatum was also cultivated in an iron-limited, chemically defined medium without the addition of chelators and it was determined that the organism could also use hemoglobin as a sole source of iron. The method of iron internalization from heme was examined by measuring hemin binding to the yeast-cell surface. The ability of H. capsulatum to bind hemin was related to the nutritional status of the cells. Cells grown under iron-limited conditions bound more heme to the cell surface than did cells grown in medium without chelator. Pretreatment of iron-starved cells with proteinase K eliminated the ability of the organism to bind hemin. Additionally, the pre-incubation of iron-starved H. capsulatum with hemin eliminated the ability of these cells to remove hemin from the solution, although pre-incubation of cells with the iron-free form of hemin, protoporphyrin IX, only modestly affected the ability of the organism to bind hemin. These results suggest that H. capsulatum uses hemin as a sole source of iron and that one mechanism of iron acquisition involves a cell-surface receptor for hemin.