Iron transport mechanisms in reticulocytes and mature erythrocytes

The mechanism of iron transport into erythroid cells was investigated using rabbit reticulocytes and mature erythrocytes incubated with 59Fe-labelled Fe(II) in isotonic sucrose or in solutions in which the sucrose was replaced with varying amounts of isotonic NaCl or KCl. Iron uptake was inhibited at all concentrations of NaCl, in a concentration-dependent manner, but with KCl inhibition occurred only at concentrations up to 10 mM. Higher KCl concentrations stimulated iron uptake to the cytosol of the cells, but inhibited its incorporation into heme. This effect became more marked as the iron concentration was raised. It was found that KCl inhibits iron incorporation into heme and stimulates iron uptake by mature erythrocytes, as well as by reticulocytes. It is concluded that erythroid cells can take up nontransferrin-bound Fe(II) by two mechanisms. One is a high-affinity mechanism that is limited to reticulocytes, saturates at a low iron concentration, and is inhibited by metabolic inhibitors. The other is a low-affinity process that is found in both reticulocytes and erythrocytes, becomes more prominent at higher iron concentrations, and is stimulated by KCl, as well as RbCl, LiCl, CsCl, and choline Cl. The KCl stimulation is inhibited by amiloride, but not by metabolic inhibitors, and its operation is not dependent on changes in cell volume or membrane potential, but it does require the presence of a permeant extracellular anion. Iron uptake by this process appears to occur by facilitated transport and is possibly associated with exchange of Na+. A further aspect of this study was a comparison of iron uptake by reticulocytes from Fe(II)-sucrose and Fe(II)-ascorbate using a variety of incubation conditions. No major differences were observed.

Role of membrane surface potential and other factors in the uptake of non-transferrin-bound iron by reticulocytes

Reticulocytes suspended in low ionic strength media such as isotonic sucrose solution efficiently take up non-transferrin-bound iron and utilize it for heme synthesis. The present study was undertaken to determine how such media facilitate iron utilization by the cells. The effects of changes in membrane surface potential, membrane permeability, cell size, transmembrane potential difference, oxidation state of the iron, and lipid peroxidation were investigated. Iron uptake to heme, cytosol, and stromal fractions of cells was measured using rabbit reticulocytes incubated with 59Fe-labelled Fe(II) in 0.27 M sucrose, pH 6.5. Suspension of the cells in sucrose led to increased membrane permeability, loss of intracellular K+, decreased cell size, and increased transmembrane potential difference. However, none of these changes could account for the high efficiency of iron uptake which was observed. The large negative membrane surface potential which occurs in sucrose was modified by the addition of mono-, di-, tri-, and polyvalent cations to the solution. This inhibited iron uptake to a degree which for many cations varied with their valency. Other cations (Mn2+, Co2+, Ni2+, Zn2+) were also very potent inhibitors, probably due to direct action on the transport process. Ferricyanide inhibited iron uptake, while ferrocyanide and ascorbate increased the uptake of Fe(III) but not Fe(II). It is concluded that the high negative surface potential of reticulocytes suspended in sucrose solution facilitates iron uptake by aiding the approach of iron to the transport site on the cell membrane. The iron is probably transported into the cell in the ferrous form.

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 change 59Fe uptake and release from the brain of 15- and 63-day rats either during or after intravenous injection of 59Fe-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. Once 59Fe 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.

Receptor-independent uptake of transferrin-bound iron by reticulocytes

Under physiological conditions the uptake of transferrin-bound iron by reticulocytes involves transferrin binding to membrane receptors followed by endocytosis, release of iron from the transferrin within endosomes, and recycling of apotransferrin to the cell surface. However, as shown in the present work, iron uptake and incorporation into heme will also occur if the cells are incubated in low-ionic-strength media such as isotonic sucrose. This process has a pH optimum of 5.9, is not inhibited by inactivation of the transferrin receptors, and does not involve transferrin endocytosis, but is inhibited by addition of various salts, ferricyanide, and low concentrations of ferric iron chelators, including apotransferrin, to the incubation medium. Iron uptake is temperature dependent, has a high activation energy and is inhibited by a variety of metabolic inhibitors. It is also saturable with an apparent Km of approximately 0.2 microM transferrin-Fe. It is concluded that under these incubation conditions iron is released from transferrin at the external surface of the cell and is transported into the cell by a facilitated, possibly active, transport process. This may occur via the iron carrier which normally functions in the membrane lining of endosomes. Reduction of the iron to the ferrous state is probably necessary for its transport into the cell.

Iron and transferrin uptake by brain and cerebrospinal fluid in the rat

Iron and transferrin uptake into the brain, CSF and choroid plexus, and albumin uptake into the CSF and choroid plexus, were determined after the intravenous injection of [59Fe-125I]transferrin and [131I]albumin into control rats aged 15, 21 and 63 days and 21-day iron-deficient rats. Iron uptake by the brain was unidirectional, greatly exceeded that of transferrin and was equivalent to 39 and 36% of the plasma iron pool per day in the 15-day control and 21-day iron-deficient rats. The rate of transferrin catabolism in the rats was only about 20% of the plasma pool per day. Iron and transferrin uptake into the brain and CSF decreased with increasing age and was greater in the iron-deficient than in the control 21-day rats. The quantity of 125I-transferrin recovered in the CSF could account for only a small proportion of the iron taken up by the brain. Albumin transfer to the CSF also decreased with age but was lower than that of transferrin and was not affected by iron deficiency. Similarly, the plasma: CSF concentration ratios of transferrin and albumin, as determined immunologically, decreased with age and were greater for transferrin than albumin. It is concluded that iron uptake by the brain is dependent on iron release from transferrin at the cerebral capillary endothelial cells with recycling of transferrin to the plasma and transfer of the iron into the brain interstitium. Only a small fraction of the transferrin bound by brain capillaries is transcytosed into the brain and CSF, this being one source of CSF transferrin while other sources are local synthesis and transfer from the plasma by the choroid plexuses.

Iron uptake in relation to transferrin degradation in brain and other tissues of rats

The possibility that iron uptake by the brain involves transcytosis of the iron-transferrin complex across the brain capillaries, followed by degradation of the transferrin (Tf) within the brain, was investigated using diferric 125I-[59Fe]Tf and [59Fe]Tf coupled to 125I-tyramine cellobiose (TC). The radiolabeled catabolic products of proteins labeled with 125I-TC remain in the cells where degradation occurs. The TCTf complex behaved normally with respect to its ability to donate iron to rat reticulocytes in vitro or to the brain, liver, kidneys, and femurs in vivo. In the brain there was little difference in the uptake of 125I derived from Tf and TCTf, and the amounts were equivalent to only a small fraction of the 59Fe uptake. Hence, the rate of Tf catabolism in the brain was insufficient to account for the rate of accumulation of iron from plasma Tf. It was concluded that Tf recycles to the plasma after delivering its iron to the brain. The uptake of 125I from TCTf by the liver and kidneys accounted for 40-50% of the total rate of Tf catabolism. This indicated that they were important but not the only sites of degradation of this protein.

Changes in the uptake of transferrin-free and transferrin-bound iron during reticulocyte maturation in vivo and in vitro

The uptake of non-transferrin-bound iron, Fe(II), transferrin-bound iron, Tf-Fe and transferrin was studied in reticulocytes from anaemic rabbits during maturation and then synchronized regeneration in vivo (following injection of actinomycin D) and while maturing during in vitro incubation. The uptake of Fe(II) and Tf-Fe decreased in parallel with each other and with the reticulocyte count and transferrin uptake during maturation in vivo and in vitro. Only during the early phase of reticulocyte regeneration in vivo was there a significant difference between the rates of Fe(II) and Tf-Fe uptake. These results suggest that a membrane carrier for iron and the transferrin receptor are lost at the same rate during reticulocyte maturation, possibly because they are associated with each other in the cell membrane. During reticulocyte maturation the rate of Fe(II) uptake into heme declined more rapidly than uptake into the total cellular cytosol. The loss of transferrin receptors and the uptake of iron from transferrin during reticulocyte maturation was not associated with a change in the affinity of the receptors for transferrin, in the relative distribution of the receptors between the outer cell membrane and intracellular sites or in the ability of the transferrin molecule to donate two iron atoms to the cell with each intracellular cycle, but the average duration of the cycle increased.

Diminished iron acquisition by cells and tissues of Belgrade laboratory rats

Iron uptake from transferrin by a variety of cells and tissues of homozygous Belgrade laboratory rats was compared with heterozygotes, and normal and iron-deficient Wistar rats. In all cases the results for homozygous Belgrade rats were lower than for the other animals. The maximal rate of iron uptake by fibroblasts cultured in vitro and iron passage to homozygous fetuses in utero was less than 60% of control values. In vivo studies of 15-day-old Belgrade rats revealed a defect in the homozygotes with reduced iron transfer to heart, liver, brain, and femurs. In addition, adult Belgrade laboratory rats had impaired intestinal iron absorption compared with the genetically normal animals. It is concluded that the defect in iron metabolism in the Belgrade laboratory rat is a ubiquitous one that affects transport of iron across membranes of many types of cells, resulting in low intracellular iron levels. This suggests that the genetic defect leads to a widely expressed abnormality in the structure and/or function of a membrane carrier for iron.

Uptake of transferrin-bound and nontransferrin-bound iron by reticulocytes from the Belgrade laboratory rat: comparison with Wistar rat transferrin and reticulocytes

The mechanism underlying the impaired uptake of iron from transferrin by reticulocytes from the Belgrade laboratory rat was investigated using 125I- and 59Fe-labeled transferrin isolated from homozygous Belgrade rats and from Wistar rats, nontransferrin-bound Fe(II) in an isotonic sucrose solution, and reticulocytes from Belgrade and Wistar rats. The Belgrade rat transferrin had the same molecular weight and net charge as Wistar rat transferrin, donated iron equally well to both types of reticulocytes, and competed equally for transferrin binding sites on the cells. Hence, the defect in iron uptake by Belgrade rat reticulocytes could not be attributed to an abnormality of the transferrin molecule. The rate of uptake of Fe(II) from sucrose into the cytosolic and stromal fractions of Belgrade rat reticulocytes was only about 35% as great as that by Wistar rat reticulocytes. With both types of cells, the uptake process was saturable, suggesting the presence of a carrier-mediated process. It was therefore concluded that the defect in iron uptake by Belgrade rat erythroid cells is probably the consequence of a deficiency in a membrane carrier for iron.

Transferrin and iron uptake by the brain: effects of altered iron status

Transferrin (Tf) and iron uptake by the brain were measured in rats using 59Fe-125I-Tf and 131I-albumin (to correct for the plasma content of 59Fe and 125I-Tf in the organs). The rats were aged from 15 to 63 days and were fed (a) a low-iron diet (iron-deficient) or, as control, the same diet supplemented with iron, or (b) a chow diet with added carbonyl iron (iron overload), the chow diet alone acting as its control. Iron deficiency was associated with a significant decrease and iron overload with a significant increase in brain nonheme iron concentration relative to the controls. In each dietary treatment group, the uptake of Tf and iron by the brain decreased as the rats aged from 15 to 63 days. Both Tf and iron uptake were significantly greater in the iron-deficient rats than in their controls and lower in the iron-loaded rats than in the corresponding controls. Overall, iron deficiency produced about a doubling and iron overload a halving of the uptake values compared with the controls. In contrast to that in the brain, iron uptake by the femurs did not decrease with age and there was relatively little difference between the different dietary groups. 125I-Tf uptake by the brains of the iron-deficient rats increased very rapidly after injection of the labelled proteins, within 15 min reaching a plateau level which was maintained for at least 6 h. The uptake of 59Fe, however, increased rapidly for 1 h and then more slowly, and in terms of percentage of injected dose reached much higher values than did 125I-Tf uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of transferrin in iron uptake by the brain: a comparative study

The role of specific transferrin (Tf) and Tf receptor interaction on brain capillary endothelial cells in iron transport from the plasma to the brain was investigated by using Tf from several species of animals labeled with 59Fe and 125I, and 15-day and adult rats. The rate of iron transfer was much greater in the 15-day rats. It was greatest with Tf from the mammals, rat, rabbit and human, but much lower with chicken ovotransferrin and quokka (a marsupial), toad, lizard, crocodile, and fish Tf. The uptake of Tf by the brain showed a similar pattern, except for a very high uptake of ovotransferrin (ovoTf). Iron uptake by the femurs (a source of bone marrow) was also high with Tf from the mammalian species and low with the other types of Tf, but showed little change with aging of the animals. It is concluded that iron transport into the brain is dependent on the function of Tf receptors, probably on capillary endothelial cells, and that these receptors show the same type of species specificity as the receptors on immature erythroid cells. Also, the decrease in iron uptake by the brain as rats age from 15 days to adulthood is specific for the brain and is not a general effect of the aging process.

Effect of metabolic inhibitors on uptake of non-transferrin-bound iron by reticulocytes

The relationship between transferrin-free iron uptake and cellular metabolism was investigated using rabbit reticulocytes in which energy metabolism was altered by incubation with metabolic inhibitors (antimycin A, 2,4-dinitrophenol, NaCN, NaN3 and rotenone) or substrates. Measurements were made of cellular ATP concentration and the rate of uptake of Fe(II) from a sucrose solution buffered at pH 6.5. There was a highly significant correlation between the rate of iron uptake into cytosolic and stromal fractions of the cells and ATP levels. Iron transport into the cytosol showed saturation kinetics. The metabolic inhibitors all reduced the Vmax but had no effect on the Km values for this process. It is concluded that the uptake of transferrin-free iron by reticulocytes is dependent on the cellular concentration of ATP and that it crosses the cell membrane by an active, carrier-mediated transport process. Additional studies were performed using transferrin-bound iron. The metabolic inhibitors also reduced the uptake of this form of iron but the inhibition could be accounted for entirely by reduction in the rate of transferrin endocytosis.

Specificity of hepatic iron uptake from plasma transferrin in the rat

1. The role of specific interaction between transferrin and its receptors in iron uptake by the liver in vivo was investigated using 59Fe-125I-labelled transferrins from several animal species, and adult and 15-day rats. Transferrin-free hepatic uptake of 59Fe was measured 2 or 0.5 hr after intravenous injection of the transferrins. 2. Rat, rabbit and human transferrins gave high and approximately equal levels of hepatic iron uptake while transferrins from a marsupial (Sentonix brachyurus), lizard, crocodile, toad and fish gave very low uptake values. Chicken ovotransferrin resulted in higher uptake than with any other species of transferrin. 3. Iron uptake by the femurs (as a sample of bone marrow erythroid tissue) and, in another group of 19-day pregnant animals by the placentas and fetuses, was also measured, for comparison with the liver results. The pattern of uptake from the different transferrins was found to be similar to that of iron uptake by the liver except that with femurs, placentas and fetuses ovotransferrin gave low values comparable to those of the other non-mammalian species. 4. It is concluded that iron uptake by the liver from plasma transferrin in vivo is largely or completely dependent on specific transferrin-receptor interaction. The high hepatic uptake of iron from ovotransferrin was probably mediated by the asialoglycoprotein receptors on hepatocytes.

Effect of cellular iron concentration on iron uptake by hepatocytes

The effect of intracellular iron content on transferrin and iron uptake by cultured hepatocytes isolated from fetal rat liver was examined with ferric ammonium citrate and the iron chelator desferrioxamine (DFO). Incubation of the cells with ferric ammonium citrate for 24 h significantly increased the cellular nonheme iron level, whereas the number of transferrin binding sites and the uptake of transferrin and iron were reduced. In contrast, when iron-treated cells were incubated with DFO for 24 h, the cellular nonheme iron level was not altered, but the number of transferrin binding sites was increased. Treatment of the cells with exogenous iron and/or DFO did not affect the uptake of transferrin and iron by the nonsaturable processes. These results indicated that, in cultured hepatocytes, transferrin receptor expression and the subsequent uptake of transferrin and iron are regulated by the size of an intracellular, chelatable iron pool, whereas the uptake of iron by the nonsaturable processes is dependent on the extracellular transferrin concentration.

Effect of lead on the transport of transferrin-free and transferrin-bound iron into rabbit reticulocytes

The effects of Pb on iron transport into rabbit reticulocytes was investigated using two sources of iron, non-transferrin-bound ferrous iron, Fe(II), and transferrin-bound iron, and fractionating the cells into haem, cytosolic and stromal fractions. Uptake of Fe(II) into all three fractions was inhibited by low concentrations of Pb, 50% inhibition of uptake to the cytosol (IC50) occurring at 1 microM Pb. Fe(II) uptake could be divided into saturable and non-saturable components. The saturable component was inhibited at lower concentrations of Pb than the non-saturable component. Pb reduced the Vmax and increased the Km values for saturable Fe(II) transport. The effects of Pb on Fe(II) transport were reversible and were observed with PbCl2 and Pb (NO3)2 as well as with lead acetate. Pb also inhibited the uptake of transferrin-bound iron but at higher concentrations (IC50, 4 microM) and the inhibition was less readily reversible. The effect was attributable to inhibition of transferrin endocytosis which resulted in a redistribution of transferrin receptors from intracellular to cell surface sites. These results show that Pb can inhibit transferrin endocytosis and iron transport across the cell membrane of reticulocytes and raise the possibility that these effects may contribute to the hypochromic anaemia associated with Pb poisoning, in addition to the previously established inhibition of enzymes of the haem synthesis pathway.

Developmental changes in transferrin and iron uptake by the brain in the rat

The uptake of transferrin and iron by the brain, liver and femurs was investigated in rats using 125I-59Fe-transferrin (Tf), and 131I-albumin in order to measure the plasma content of the organs. Measurements in rats ranging in age from birth to 70 days revealed that the rate of iron uptake by the brain increased rapidly over the first 15 days of life, peaking at 15 days and thereafter declining. A similar pattern occurred in the uptake of 125I-Tf. These changes were accompanied by rapid growth of the brain up to 15 days and a decrease in the concentration of non-haem iron. The turnover of 59Fe and 125I-Tf in the brain was also determined by measuring radioactivity in the brain of 15-day rats at various times after injection from 15 min to 13 days. The amount of 59Fe in the brain increased over the first 4 h and thereafter remained constant. By contrast, the 125I-Tf values increased rapidly during the first 15 min to reach a relatively constant level which was maintained for at least 6 h after which it declined. The patterns of uptake by the brain were different from those found in the liver and femurs, indicating that the changes in the brain were specific for that organ.(ABSTRACT TRUNCATED AT 250 WORDS)

Uptake and distribution of transferrin and iron in perfused, iron-deficient rat liver

Uptake of transferrin and iron by the rat liver was investigated by perfusion in vitro with 125I-59Fe-labeled rat transferrin and subcellular fractionation on sucrose density gradients. Most of the 125I-transferrin was located in a low-density vesicle fraction. The 59Fe was in three peaks, of lower, the same, and higher densities than the transferrin peak. Iron deficiency resulted in a large increase in transferrin and iron uptake into all subcellular fractions. When livers were perfused with increasing concentrations of transferrin the uptake into the different peaks of transferrin and iron increased in a curvilinear fashion, which indicated that uptake occurred by saturable and nonsaturable processes, both of which increased in iron deficiency. In contrast, the uptake of 131I-labeled rat serum albumin increased linearly with concentration, and there was no difference between control and iron-deficient livers. It is concluded that iron deficiency leads to an increase in the number of high-affinity transferrin receptors and receptor-mediated endocytosis of transferrin. It also increases a nonsaturable transferrin uptake process that is probably due to adsorptive, but selective, endocytosis of transferrin.

Calcium chelators induce association with the detergent-insoluble cytoskeleton and functional inactivation of the transferrin receptor in reticulocytes

Incubation of reticulocytes with EDTA, EGTA (ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid) and BAPTA (1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid), but not with desferrioxamine B, at temperatures above 20 degrees C resulted in the loss of their ability to take up iron in a temperature-, time- and concentration-dependent manner. No inhibition of transferrin or iron uptake occurred if the incubations were performed at 20 degrees C or below. At higher temperatures, the inhibition was attributable to loss of functional transferrin receptors, not to altered affinity or endocytosis of the remaining receptors. The changes could not be reversed by washing the cells and reincubation in the presence of Ca2+, Mg2+ or Zn2+. However, they could be completely prevented by performing the initial incubation with chelators in the presence of diferric transferrin and partly prevented by the use of apotransferrin. Incubation with the chelators resulted in much less reduction in the ability of the cells to bind anti-transferrin receptor immunoglobulin than transferrin. The fate of the receptor was studied by polyacrylamide gel electrophoresis of reticulocyte membrane proteins before and after extraction with Triton X-100, and by immunological staining of Western blots for the transferrin receptor. Treatment of the cells with EDTA led to a loss of the ability of Triton X-100 to solubilize the receptor and its retention in the Triton-insoluble cytoskeletal matrix of the cells. It is concluded that incubation of reticulocytes with the chelators at temperatures above 20 degrees C causes an altered interaction of the transferrin receptor with the cytoskeleton. This change, which is probably due to chelation of Ca2+ in the cell membrane, is accompanied by an irreversible loss of the receptor's ability to bind transferrin.

Transformation-induced changes in transferrin and iron metabolism in myogenic cells

The uptake of transferrin and iron by cultured myogenic cells transformed with a temperature-sensitive strain of the Rous sarcoma virus (tsLA24) was compared with that of normal developing myogenic cells which were proliferating at the same rate as the transformed cells. The mechanism of transferrin and iron uptake was the same in the transformed cells as in normal myogenic cells and involved receptor-mediated endocytosis of transferrin. However, there were differences in transferrin receptor numbers and receptor function. The number of receptors in transformed cells was more than twice as great as in the normal cells largely due to increased surface receptor numbers. Despite this, the rate of iron uptake increased by only 20% in the transformed cells due to less efficient cycling of the transferrin receptors and less efficient release of iron from transferrin to intracellular sites. Some internalized iron was released from the transformed cells still bound to transferrin. A fast and a slow rate of transferrin exocytosis were identified in transformed cells, as in normal cells, indicating that there were at least two intracellular pathways for transferrin. The fast pathway predominated in the transformed cells, compared with an equal importance of the two pathways in the normal cells.

Transferrin endocytosis and iron uptake in developing myogenic cells in culture: effects of microtubular and metabolic inhibitors, sulphydryl reagents and lysosomotrophic agents

The experiments described in this study were designed to investigate receptor-mediated endocytosis of transferrin and its role in iron uptake by cultured chick presumptive myoblasts (dividing and non-dividing) and myotubes. The effects of a variety of inhibitors on the internalization of transferrin and iron were investigated and three main effects were found: (i) sulphydryl reagents and microtubular inhibitors reduced the rate of transferrin and iron internalization to similar degrees, (ii) metabolic inhibitors reduced the rate of iron uptake more than that of transferrin endocytosis, and (iii) lysosomotrophic agents almost completely abolished iron accumulation by the cells without any effect on the rate of transferrin internalization. The results suggest that metabolic energy is required not only for the endocytosis of transferrin but also for subsequent steps in the iron uptake process, and that iron release from transferrin occurs in acidified endosomes. Overall, these experiments show that all or virtually all of the iron taken up by developing muscle cells from transferrin occurs as a consequence of receptor-mediated endocytosis of the protein.

Differences in transferrin receptor function between normal developing and transformed myogenic cells as revealed by differential effects of phorbol ester on receptor distribution and rates of iron uptake

The effects of the tumor promotor, 4 beta-phorbol 12 beta-myristate 13 alpha-acetate (PMA), on the intra- and extracellular distribution of transferrin receptors and rates of iron uptake were studied in normal developing myogenic cells and myogenic cells transformed with a temperature-sensitive strain of the Rous sarcoma virus. In normal developing cells PMA was found to increase the rate of iron uptake by 15-30%. There was, however, no effect on transferrin receptor distribution, suggesting that the increase in iron uptake was due to stimulation of the rate of receptor cycling. In contrast, in transformed myogenic cells, PMA had no effect even at concentrations 10 times those effective in normal myogenic cells. The specificity of PMA was demonstrated by comparison with 4 alpha-phorbol which had no effect compared with the control cells which were incubated with dimethyl sulfoxide, the solvent used to dissolve the phorbols. These results indicate a functional difference in the transferrin receptor between normal and transformed myogenic cells. The data for normal myogenic cells are similar to those previously reported for normal erythroid cells, but differ from those for some transformed cell lines in which phorbol esters were shown to cause internalization of transferrin receptors.

The effects of an antibody to the rat transferrin receptor and of rat serum albumin on the uptake of diferric transferrin by rat hepatocytes

The role of high-affinity specific transferrin receptors and low-affinity, non-saturable processes in the uptake of transferrin and iron by hepatocytes was investigated using fetal and adult rat hepatocytes in primary monolayer culture, rat transferrin, rat serum albumin and a rabbit anti-rat transferrin receptor antibody. The intracellular uptake of transferrin and iron occurred by saturable and non-saturable mechanisms. Treatment of the cells with the antibody almost completely eliminated the saturable uptake of iron but had little effect on the non-saturable process. Addition of albumin to the incubation medium reduced the endocytosis of transferrin by the cells but had no significant effect on the intracellular accumulation of iron. The maximum effect of rat serum albumin was observed at concentrations of 3 mg/ml and above. At a low incubation concentration of transferrin (0.5 microM), the presence of both rat albumin and the antibody decreased the rate of iron uptake by the cells to about 15% of the value found in their absence, but to only 40% when the diferric transferrin concentration was 5 microM. These results confirm that the uptake of transferrin-bound iron by both fetal and adult rat hepatocytes in culture occurs by a specific, receptor-mediated process and a low-affinity, non-saturable process. The low-affinity process increases in relative importance as the iron-transferrin concentration is raised.

Membrane transport of non-transferrin-bound iron by reticulocytes

The transport of non-transferrin-bound iron into rabbit reticulocytes was investigated by incubating the cells in 0.27 M sucrose with iron labelled with 59Fe. In most experiments the iron was maintained in the reduced state, Fe(II), with mercaptoethanol. The iron was taken up by cytosolic, haem and stromal fractions of the cells in greater amounts than transferrin-iron. The uptake was saturable, with a Km value of approx. 0.2 microM and was competitively inhibited by Co2+, Mn2+, Ni2+ and Zn2+. It ceased when the reticulocytes matured into erythrocytes. The uptake was pH and temperature sensitive, the pH optimum being 6.5 and the activation energy for iron transport into the cytosol being approx. 80 kJ/mol. Ferric iron and Fe(II) prepared in the absence of reducing agents could also be transported into the cytosol. Sodium chloride inhibited Fe(II) uptake in a non-competitive manner. Similar degrees of inhibition was found with other salts, suggesting that this effect was due to the ionic strength of the solution. Iron chelators inhibited Fe(II) uptake by the reticulocytes, but varied in their ability to release 59Fe from the cells after it had been taken up. Several lines of evidence showed that the uptake of Fe(II) was not being mediated by transferrin. It is concluded that the reticulocyte can transport non-transferrin-bound iron into the cytosol by a carrier-mediated process and the question is raised whether the same carrier is utilized by transferrin-iron after its release from the protein.