Expression of the neuronal transferrin receptor is age dependent and susceptible to iron deficiency

In order to characterize the mechanism by which Iron (Fe) is taken up by neurons, we examined the neuronal expression of transferrin receptor (TR) in rats during development and iron (Fe) deficiency by using immunohistochemistry, in vitro receptor autoradiography and in situ hybridization. In contrast to the continuous expression of TR in brain capillary endothelial cells regardless of the age of the animals studied, the expression of neuronal TR was almost absent at late embryonic and early postnatal ages but increased with increasing age to reach a plateau from postnatal (P) 21 through adulthood as verified by immunohistochemical staining. Reducing the Fe stores potentiated the expression of TR immunoreactivity in neurons of both young and adult rats in several grey matter regions. Increased TR immunoreactivity was also observed in neuronal extensions of neurons of the medial habenular nucleus, reticular neurons of the brainstem, and fibers projecting to the area postrema. TR immunoreactivity was never observed in white matter regions, except for that recorded in brain capillaries. In vitro receptor autoradiography verified the increased capacity for transferrin binding during Fe deficiency. By contrast, TR mRNA expression was not affected by Fe deficiency. These findings demonstrate that the expression of the neuronal TR protein is age dependent and susceptible to Fe deficiency.

Transport mechanisms for iron and other transition metals in rat and rabbit erythroid cells

1. Earlier studies have shown that Fe2+ transport into erythroid cells is inhibited by several transition metals (Mn2+, Zn2+, Co2+, Ni2+) and that Fe2+ transport can occur by two saturable mechanisms, one of high affinity and the other of low affinity. Also, the transport of Zn2+ and Cd2+ into erythroid cells is stimulated by NaHCO3 and NaSCN. The aim of the present investigation was to determine whether all of these transition metals can be transported by the processes described for Fe2+, Zn2+ and Cd2+ and to determine the properties of the transport processes. 2. Rabbit reticulocytes and mature erythrocytes and reticulocytes from homozygous and heterozygous Belgrade rats were incubated with radiolabelled samples of the metals under conditions known to be optimal for high- and low-affinity Fe2+ transport and for the processes mediated by NaHCO3 and NaSCN. 3. All of the metals were transported by the high- and low-affinity Fe2+ transport processes and could compete with each other for transport. The Km and Vmax values and the effects of incubation temperature and metabolic inhibitors were similar for all the metals. NaHCO3 and NaSCN increased the uptake of Zn2+ and Cd2+ but not the other metals. 4. The uptake of all of the metals by the high-affinity process was much lower in reticulocytes from homozygous Belgrade rats than in those from heterozygous animals, but there was no difference with respect to low-affinity transport. 5. It is concluded that the high- and low-affinity 'iron' transport mechanisms can also transport several other transition metals and should therefore be considered as general transition metal carriers.

Kinetics and distribution of [59Fe-125I]transferrin injected into the ventricular system of the rat

We examined the kinetics and distribution of [59Fe-125I] rat Tf and unlabelled human Tf injected into a lateral cerebral ventricle (i.c. v. injection) in the rat. [56Fe-131I]Tf injected intravenously served as a control of blood-brain barrier (BBB) integrity. In CSF of adult rats, 59Fe and [125I]Tf decreased to only 2.5% of the dose injected after 4 h. In brain parenchyma, [125I]Tf had disappeared after 24 h, whereas approximately 18% of i.c.v.-injected 59Fe was retained even after 72 h. The elimination pattern of [125I]Tf from the CSF corresponded to that of [131I]albumin injected i.c.v., suggesting a nonselective washout of CSF proteins. [131I]Tf was hardly detectable in the brain, reflecting an unimpaired BBB during the experiments. Morphologically, 59Fe and i.c.v. injected human Tf were confined to the ventricular surface and meningeal areas, whereas grey matter regions at distances more than 2-3 mm from the ventricles and the subarachnoid space were unlabelled. However, accumulation of 59Fe was observed in the anterior thalamic and the medial habenular nuclei, and in brain regions with synaptic communications to these areas. In the newborn rats aged 7 days (P7) injected i.c.v. with [59Fe-125I]Tf and examined after 24 h, the amounts of [125I]Tf in CSF were approximately 3.5 times higher than in adult rats collected after the same time interval, whereas the amounts of 59Fe in CSF were at the same level in P7 and adult rats. In the brain tissue of the i.c.v. injected P7 rats, both [125I]Tf and 59Fe were retained to a significantly higher degree compared to that seen in adult brains. The rapid washout and lack of capability for i.c.v. injected [125I]Tf to penetrate deeply into the brain parenchyma of the adult brain question the importance of Tf of the CSF, and choroid plexus-derived Tf, for Fe neutralization and delivery of Fe-Tf to TfR-containing neurons and other cells in the CNS. However, it may serve these functions in young animals due to a lower rate of turnover of CSF.

Ferric citrate uptake by cultured rat hepatocytes is inhibited in the presence of transferrin

Diseases associated with iron overload occur worldwide. In subjects suffering from these conditions, transferrin is likely to be fully saturated and excess plasma iron must be complexed to other molecules. Consequently, the liver, which is the major site of iron storage, will be presented with iron in both transferrin-bound and non-transferrin-bound forms and these forms may compete for uptake by hepatocytes. The endogenous low-molecular-mass iron chelator, citrate, is considered to be a major contributing molecule to non-transferrin iron transport. This study was conducted to investigate the effects of transferrin on the uptake of citrate and iron citrate by hepatocytes in culture. Rat hepatocytes were incubated with 100 microM [14C]citrate and 1.0 microM 55Fe in the presence or absence of various forms of transferrin. Binding and internalisation of both citrate and iron were inhibited in a dose-dependent manner with increasing concentration of diferric transferrin, with iron uptake decreasing to less than 5% of control values. Apotransferrin was markedly more effective in blocking citrate and iron uptake, reaching the same levels of inhibition at a 15-fold lower concentration of protein. The binding of citrate to the cell membrane was not affected significantly by changing the iron saturation of transferrin but internalisation decreased with decreasing saturation. In contrast, both the binding and internalisation of iron decreased with decreasing saturation. Incubations carried out using 55Fe-labelled citrate in the presence of 59Fe-labelled diferric transferrin indicated that citrate-mediated iron binding by the cells decreased with increasing diferric transferrin concentrations but the citrate iron was not replaced by iron from transferrin during the 15-min incubation period. Instead, total iron uptake decreased. These data suggest that citrate-mediated iron uptake by hepatocytes shares at least one common pathway with transferrin-mediated iron uptake.

Characterisation of non-transferrin-bound iron (ferric citrate) uptake by rat hepatocytes in culture

Under conditions of iron overload plasma transferrin can be fully saturated and the plasma can transport non-transferrin-bound Fe which is rapidly cleared by the liver. Much of this Fe is complexed by citrate. The aim of the present work was to characterise the mechanisms by which Fe-citrate is taken up by hepatocytes using a rat hepatocyte cell culture model. The cells, after one day in culture, were incubated with 59Fe-labelled Fe-citrate for varying time periods, then washed and Fe uptake to the membrane and intracellular compartments of the cell was determined by radioactivity measurements. Maximal rates of internalisation of Fe occurred at a Fe:citrate molar ratio of 1:100 or greater, a pH of approximately 7.4 and an extracellular Ca2+ concentration of 1.0 mM. Fe uptake showed Michaelis-Menten kinetics and was a temperature-dependent process. The K(m) and Vmax for Fe internalisation by the cells at 37 degrees C were approximately 7 microM and 2 nmol/mg DNA/min (25 x 10(6) atoms/cell/min), respectively; and the Arrhenius activation energy was 35 kJ/mol. The transition metals, Zn2+, Co2+ and Ni2+, inhibited Fe uptake when used at 10 and 100 times the concentration of Fe. The rate of Fe internalisation from Fe-citrate was found to be approximately 20 times as great as that from Fe-transferrin with Fe concentrations of 1 and 2.5 microM for both forms of Fe. The rate of Fe uptake by iron-loaded hepatocytes obtained from rats which had been fed carbonyl Fe was not significantly different from that by normal hepatocytes. These experiments show that rat hepatocytes in primary culture have a high capacity to take up non-transferrin-bound Fe in the form of Fe-citrate and that uptake occurs by facilitated diffusion. The iron transport process does not appear to be regulated by cellular Fe levels.

Characterization of isolated duodenal epithelial cells along a crypt-villus axis in rats fed diets with different iron content

The intestinal mucosa is characterized by cell proliferation, commitment, differentiation, digestion and absorption. These processes occur at specified locations along the crypt to villus axis. A technique is reported for the isolation of cells along this axis which allows the study of any one of these processes in an enriched population of cells. As an example, the uptake of transferrin-bound iron by enterocytes was studied. Rats were fed diets normal, high (30% carbonyl iron) or low in iron for 12 days. Cells from either the duodenum or ileum were isolated by incubating in a Ca(2+)-, Mg2+-free, cation chelating solution for varying periods. The incorporation of thymidine into DNA was measured in these cells as a marker of the crypt region, while alkaline phosphatase and sucrase activities marked mature enterocytes. The in vivo uptake of transferrin-bound 59Fe was measured in cells isolated either 2 or 4 h after intravenous injection. This procedure resulted in the isolation of 10 fractions of viable cells. Earlier fractions were enriched at least 10-fold in villus cells and the last fractions in crypt cells. Cells in intermediate fractions were at various stages of development. Uptake of transferrin-bound iron into enterocytes was highest with feeding an iron-loaded diet compared with control or iron-deficient diets. However, with all diets uptake was highest in crypt cells and this fell at the crypt-villus junction to be only 25%, as high at the villus tip as the crypt. A technique for the reproducible isolation of viable enterocytes along a crypt-villus axis is described. Transferrin receptor activity changes with maturation of the enterocyte.

Haemoglobin synthesis in erythropoietin-stimulated J2E cells does not require increased numbers of transferrin receptors

Changes in transferrin-receptor numbers and iron utilisation were monitored during erythropoietin-induced maturation of J2E erythroid cells. Uptake of transferrin and iron doubled 24 h after exposure to erythropoietin, due to a twofold rise in surface transferrin receptors. In addition, a tenfold increase in iron incorporation into haem was observed after erythropoietin stimulation, as iron taken up from transferrin was directed towards haem biosynthesis and away from storage in ferritin. The rise in iron chelation into haem correlated extremely well with haemoglobin synthesis. However, the increase in numbers of transferrin receptors was not essential for haemoglobin synthesis; rather, it was linked with a burst in proliferation stimulated by erythropoietin. We have shown previously that amiloride blocks erythropoietin-enhanced proliferation of J2E cells, but potentiates maturation [Callus, B. A., Tilbrook, P. A., Busfield, S. J. & Klinken, S. P. (1995) Exp. Cell Res. 219, 39-46]. Here we demonstrate that amiloride suppressed the hormone-induced increase in transferrin receptors, whereas the enhanced incorporation of iron into haem was not inhibited. Similarly, when sodium butyrate was used to induce differentiation of J2E cells, proliferation ceased and surface transferrin receptors remained unaltered, while haemoglobin production was accelerated. It was concluded from these experiments that the erythropoietin-stimulated rise in transferrin receptors during the final stages of J2E cell maturation is linked to cell division, and is not essential for haemoglobin synthesis.

Ferritin gene expression and transferrin receptor activity in intestine of rats with varying iron stores

Expression of transferrin receptor and ferritin genes has been shown previously to be under transcriptional and posttranscriptional regulation, the latter being reciprocally regulated according to cellular iron levels. This study examined transferrin receptor function and ferritin gene expression along the crypt-villus axis of the intestinal tract in rats with varying iron stores. Altered iron stores were produced by feeding a control diet and diets low or high in iron (2% carbonyl iron) for 8-10 wk. Expression and activity of the ferritin genes were assessed by in situ hybridization and immunohistochemical localization, respectively. Transferrin receptor activity was determined by the uptake of intravenously injected transferrin-bound iron and was shown to increase with the level of iron loading. In all iron status groups, ferritin mRNA was seen at highest levels in the epithelial cells of the crypt and macrophages within the lamina propria and at lower levels in villus epithelial cells. In all groups, ferritin protein was not seen in the crypt region but was seen with increasing staining in the apical two-thirds of the villus cells of control and iron-loaded, but not iron-deficient, rats. Ferritin staining increased with iron loading. We conclude that in undifferentiated crypt cells ferritin genes are transcribed, but the message is not translated. After differentiation, these genes appear to be controlled posttranscriptionally by cellular iron stores.

Effect of dietary cadmium on iron metabolism in growing rats

Little is known regarding the interactions between iron and cadmium during postnatal development. This study examined the effect of altered levels of dietary iron and cadmium loading on the distribution of cadmium and iron in developing rats ages 15, 21, and 63 days. The uptake of iron, transferrin, and cadmium into various organs was also examined using 59Fe, [125I]transferrin, and 109Cd. Dietary cadmium loading reduced packed cell volume and plasma iron and nonheme iron levels in the liver and kidneys, evidence of the inducement of an iron deficient state. Dietary iron loading was able to reverse these effects, suggesting that they were the result of impaired intestinal absorption of iron. Cadmium loading resulted in cadmium concentrations in the liver and kidneys up to 20 microg/g in rats age 63 days, while cadmium levels in the brain reached only 0.16 microg/g, indicating that the blood-brain barrier restricts the entry of cadmium into the brain. Iron loading had little effect on cadmium levels in the organs and cadmium feeding did not lower tissue iron levels in iron loaded animals. These results suggest that cadmium inhibits iron absorption only at low to normal levels of dietary iron and that at high levels of intake iron and cadmium are largely absorbed by other, noncompetitive mechanisms. It was shown that 109Cd is removed from the plasma extremely quickly irrespective of iron status and deposits mainly in the liver. One of the most striking effects of cadmium loading on iron metabolism was increased uptake of [125I]transferrin by the heart, possibly by disrupting the process of receptor-mediated endocytosis and recycling of transferrin by heart muscle.

Manganese metabolism is impaired in the Belgrade laboratory rat

Homozygous Belgrade rats have a hypochromic anaemia due to impaired iron transport across the cell membrane of immature erythroid cells. This study aimed at investigating whether there are also abnormalities of Mn metabolism in erythroid and other types of cells. The experiments were performed with homozygous (b/b) and heterozygous (+/b) Belgrade rats and Wistar rats and included measurements of Mn uptake by reticulocytes in vitro, Mn absorption from in situ closed loops of the duodenum, and plasma clearance and uptake by several organs after intravenous injection of radioactive Mn bound to transferrin (Tf) or mixed with serum. Similar measurements were made with 59Fe-labelled Fe in several of the experiments. Mn uptake by reticulocytes and absorption from the duodenum was impaired in b/b rats compared with +/b or Wistar rats. The plasma clearance of Mn-Tf was much slower than Mn-serum, but both were faster than the clearance of Fe-Tf. Uptake of 54Mn by the kidneys, brain and femurs was less in b/b than Wistar or /+b rats, but uptake by the liver was greater in b/n rats. Similar differences were found for 59Fe uptake by kidneys, brain and femurs but is concluded that the genetic abnormality present in b/b rats affects Mn metabolism as well as Fe metabolism and that Mn and Fe share similar transport mechanisms in the cells of erythroid tissue, duodenal mucosa, kidney and blood-brain barrier.

Effects of dietary iron loading with carbonyl iron and of iron depletion on intestinal growth, morphology, and expression of transferrin receptor in the rat

BACKGROUND

The intestine has one of the highest cell turnovers of the body, which is characterised by cell proliferation and differentiation occurring at specified locations along the crypt to the villus axis. These processes require iron for the synthesis of iron-dependent proteins, the supply of which is mediated through the transferrin receptor. In this study, we varied dietary iron intake to determine whether this affected the pattern of transferrin receptor expression and activity on intestinal cell turnover and cell differentiation.

METHODS

Variations in iron stores were produced by feeding a control diet and diets high (2% carbonyl iron) or low in iron for 8-10 weeks. Total tissue DNA and the incorporation of thymidine into DNA, and RNA and protein were used as indices of hyperplasia and hypertrophy, respectively. Transferrin receptor expression and activity in the intestinal mucosa were assessed by using in situ hybridisation and the uptake of transferrin-bound 55Fe.

RESULTS

Iron loading caused mucosal hypertrophy in the small and large intestines. With all levels of dietary iron transferrin- receptor expression and activity were present within the progenitor and differentiating regions of the mucosa but ceased upon cellular maturation.

CONCLUSIONS

Feeding carbonyl iron leads to mucosal hypertrophy. Expression of transferrin receptor mRNA and activity is dependent upon proliferation and differentiation of the mucosal epithelium, regardless of the cellular iron stores within these cells.

Cellular iron processing

Iron is transported in the blood plasma, mainly bound to transferrin, but in abnormal conditions other iron containing compounds may become important. These include ferritin, haemopexin-haem, haptoglobin-haemoglobin and non-specific non-transferrin-bound iron, all of which are taken up from the circulation by the liver. Transferrin-bound iron can be used by all types of cells in amounts that depend on their complement of transferrin receptors. Immature erythroid cells are the most active in this function. Investigations using reticulocytes as an example of erythroid cells have demonstrated the presence of two mechanisms for the uptake of ferrous iron. One, a high affinity process disappears as reticulocytes mature. It probably represents the mechanism by which iron derived from transferrin is transported into the cytosol after receptor-mediated endocytosis of the iron-transferrin complex. The other mechanism has a lower affinity for iron, is retained when reticulocytes mature and is probably associated with Na+ transport across the cell membrane. The physiological characteristics of the two iron transport processes and the evidence for the above conclusions are summarized in the present paper.

Effects of dietary iron loading with carbonyl iron and of iron depletion on intestinal growth, morphology, and expression of transferrin receptor in the rat

BACKGROUND

The intestine has one of the highest cell turnovers of the body, which is characterised by cell proliferation and differentiation occurring at specified locations along the crypt to the villus axis. These processes require iron for the synthesis of iron-dependent proteins, the supply of which is mediated through the transferrin receptor. In this study, we varied dietary iron intake to determine whether this affected the pattern of transferrin receptor expression and activity on intestinal cell turnover and cell differentiation.

METHODS

Variations in iron stores were produced by feeding a control diet and diets high (2% carbonyl iron) or low in iron for 8-10 weeks. Total tissue DNA and the incorporation of thymidine into DNA, and RNA and protein were used as indices of hyperplasia and hypertrophy, respectively. Transferrin receptor expression and activity in the intestinal mucosa were assessed by using in situ hybridisation and the uptake of transferrin-bound 55Fe.

RESULTS

Iron loading caused mucosal hypertrophy in the small and large intestines. With all levels of dietary iron transferrin- receptor expression and activity were present within the progenitor and differentiating regions of the mucosa but ceased upon cellular maturation.

CONCLUSIONS

Feeding carbonyl iron leads to mucosal hypertrophy. Expression of transferrin receptor mRNA and activity is dependent upon proliferation and differentiation of the mucosal epithelium, regardless of the cellular iron stores within these cells.

Effects of iron deficiency and iron overload on manganese uptake and deposition in the brain and other organs of the rat

Manganese (Mn) is an essential trace element at low concentrations, but at higher concentrations is neurotoxic. It has several chemical and biochemical properties similar to iron (Fe), and there is evidence of metabolic interaction between the two metals, particularly at the level of absorption from the intestine. The aim of this investigation was to determine whether Mn and Fe interact during the processes involved in uptake from the plasma by the brain and other organs of the rat. Dams were fed control (70 mg Fe/kg), Fe-deficient (5-10 mg Fe/kg), or Fe-loaded (20 g carbonyl Fe/kg) diets, with or without Mn-loaded drinking water (2 g Mn/L), from day 18-19 of pregnancy, and, after weaning the young rats, were continued on the same dietary regimens. Measurements of brain, liver, and kidney Mn and nonheme Fe levels, and the uptake of 54Mn and 59Fe from the plasma by these organs and the femurs, were made when the rats were aged 15 and 63 d. Organ nonheme Fe levels were much higher than Mn levels, and in the liver and kidney increased much more with Fe loading than did Mn levels with Mn loading. However, in the brain the increases were greater for Mn. Both Fe depletion and loading led to increased brain Mn concentrations in the 15-d/rats, while Fe loading also had this effect at 63 d. Mn loading did not have significant effects on the nonheme Fe concentrations. 54Mn, injected as MnCl2 mixed with serum, was cleared more rapidly from the circulation than was 59Fe, injected in the form of diferric transferrin. In the 15-d-rats, the uptake of 54Mn by brain, liver, kidneys, and femurs was increased by Fe loading, but this was not seen in the 63-d rats. Mn supplementation led to increased 59Fe uptake by the brain, liver, and kidneys of the rats fed the control and Fe-deficient diets, but not in the Fe-loaded rats. It is concluded that Mn and Fe interact during transfer from the plasma to the brain and other organs and that this interaction is synergistic rather than competitive in nature. Hence, excessive intake of Fe plus Mn may accentuate the risk of tissue damage caused by one metal alone, particularly in the brain.

The effects of iron loading and iron deficiency on the tissue uptake of 64Cu during development in the rat

This study examined the uptake of 64Cu by the brain, liver and other organs during development in rats aged 15, 21 and 63 days fed low, normal and high iron diets, using either a solution of 64CuCl2 chelated with nitrilo-triacetic acid (NTA) or 64Cu-ceruloplasmin (64Cu-Cp). 64Cu-NTA uptake was higher in the brain, spleen, kidneys, femurs and red cells at 15 days than at the later ages, while the liver took up most of the 64Cu in 63-day-old rats over the 2 h of the study. The brain had similar levels of 64Cu-NTA uptake at 15 and 21 days, even though liver uptake significantly increased, suggesting that Cu-NTA uptake by the brain increases from 15 to 21 days. The brain took up a greater percent of the injected dose of 64Cu-Cp than 64Cu-NTA yet, in either case, brain uptake was lower than that of the other organs. Iron loaded rats had significantly higher uptake of non-ceruloplasmin-bound 64Cu in all the organs examined, for at least one of the three ages, when compared with control rats. However, iron deficiency produced little change. Iron loading has a greater effect on 64Cu-Cp uptake than 64Cu-NTA, decreasing 64Cu uptake in the brain, liver, kidneys and femurs. Iron deficiency only increased 64Cu-Cp uptake in the liver. These results suggest that the mechanism of copper uptake by the liver is still maturing during suckling in the rat, and that ceruloplasmin receptor numbers are down regulated by iron loading, thus providing evidence of a new link between iron and copper metabolism.

Use of inhibitors of ion transport to differentiate iron transporters in erythroid cells

Iron uptake by rabbit reticulocytes and mature erythrocytes was investigated using 4 incubation systems: 1. Fe-transferrin in NaCl at pH 7.4, 2. Fe-transferrin in sucrose at pH 5.9, 3. Fe(II)-sucrose in sucrose at pH 6.5, and 4.Fe(II)-sucrose in KCl at pH 7.0. These systems were compared with respect to their magnitude and response to many membrane transport inhibitors and modifying agents. Iron uptake via the first 3 systems had many similar features that were quite distinct from those of iron uptake in the fourth system. On the basis of these results, it is concluded that erythroid cells contain two iron transport mechanisms, one with high affinity and relatively low capacity for iron transport, which can be studied using incubation systems 1-3, and the other of low affinity but high capacity (incubation system 4). High-affinity transport is present only in immature erythroid cells, is relatively sensitive to inhibition by N-ethylmaleimide (NEM), N,N1- dicyclohexylcarbodiimide (DCCD), and 7-chloro-4-nitrobenz-2-oxa-1,3 diazole (NBD), and is probably the mechanism by which iron, released from transferrin within endosomes, is transported across the endosomal membrane into the cytosol. DCCD and NBD are also inhibitors of the endosomal H(+)-ATPase, which is in keeping with the hypothesis that this ATPase functions as the iron transporter in endosomal membranes. However, the more-specific inhibitor of this enzyme, bafilomycin A1, inhibited iron uptake only in incubation system 1, where its action can be attributed to inhibition of endosomal acidification. Hence, it is unlikely that the ATPase also functions as the iron transporter. The low-affinity uptake mechanism is sensitive to inhibition by amiloride, valinomycin, quinidine, imipramine, quercetin, and diethylstilbestrol (to all of which high-affinity transport is relatively resistant), and is present in mature erythrocytes as well as reticulocytes.

Mediation of iron uptake and release in erythroid cells by photodegradation products of nifedipine

The effects of five Ca2+ channel antagonists on iron uptake by erythroid cells were investigated using rabbit reticulocytes and erythrocytes, and transferrin-bound iron and non-transferrin-bound iron (Fe(II)). All of the antagonists except nifedipine inhibited iron uptake, but only at relatively high concentrations (10-100 microM). Nifedipine markedly stimulated the uptake of Fe(II) but not transferrin-bound iron, but only after it had been photodegraded to its nitrosophenylpyridine derivative. This compound was found to mediate Fe(II) exchange between the cytosol and extracellular medium in both directions with both reticulocytes and erythrocytes, but not by the known iron transport processes. The effect could be reversed by washing the cells with ice-cold NaCl solution. It appeared to be relatively specific for Fe(II) since photodegraded nifedipine had little effect on the uptake of Fe(III) or Mn2+. It is suggested that the nitrosopyridine derivative of nifedipine can act as an Fe(II) ionophore and may be of use as an adjuvant in chelator therapy with desferrioxamine in conditions of iron overload.

Iron transport into erythroid cells by the Na+/Mg2+ antiport

Rabbit erythroid cells can take up non-transferrin-bound iron by a high-affinity and a low-affinity transport mechanism (Hodgson et al. (1995) J. Cell. Physiol. 162, 181-190). The latter process, which is present in mature erythrocytes as well as reticulocytes, was investigated in this study using rabbit reticulocytes and erythrocytes. Iron uptake was optimal in isotonic KCI (pH 7.0), was shown to be much greater for Fe(II) than Fe(III), to be saturable with a Km value of approx. 15 microM Fe(II), temperature-dependent and inhibited by inhibitors of cell energy metabolism, by Na+ and many divalent cations and by several known inhibitors of membrane cation transport mechanisms. Uptake was more rapid with rabbit than with rat or human erythrocytes. The Fe(II) transport process was much more sensitive to inhibition by Mg2+ than by Ca2+ and the inhibition by both Mg2+ and Na+ was of competitive type. Cells depleted of intracellular Mg2+ by the use of the ionophore A23187 had low rates of Fe(II) uptake. High rates of uptake could be achieved by replenishment of intracellular Mg2+, and the Mg(2+)-dependent uptake of Fe(II) was inhibited by the same reagents which reduced the uptake by untreated cells. Many features of the Fe(II) transport process are very similar to those of the previously described Na+/Mg2+ antiport. These features, plus the stimulation of Fe(II) uptake by intracellular Mg2+ and inhibition by extracellular Mg2+ or Na+, strongly suggest that the iron is transported into the cells by the antiport.

Effects of overexpression of the transferrin receptor on the rates of transferrin recycling and uptake of non-transferrin-bound iron

The possibilities that the recycling of the transferrin receptor is a rate-limiting step in the efflux of endocytosed transferrin, and that the receptor functions as a trans-membrane Fe transporter were investigated in untransfected Ltk- cells and in cells transfected with different levels of DNA for wild-type, mutant and chimeric human transferrin receptors. The uptake of transferrin-bound Fe and non-transferrin-bound Fe(II), and the surface binding, endocytosis and recycling of transferrin were measured. In cells that expressed increasing numbers of surface transferrin receptors, the rate of Fe uptake increased at a slower rate than the number of receptors. By measurement of the rates of endocytosis and recycling of transferrin it was shown that this effect was not due to a deficiency of endocytosis, but to a slower rate of recycling as the receptor numbers increased. Hence, a restricted recycling rate of the transferrin receptor appeared to be responsible for the slower rate of Fe uptake by cells with high receptor numbers, presumably because one or more cytosolic components required for recycling were in limited supply. The rate of uptake of non-transferrin-bound Fe(II) was not influenced by the number of transferrin receptors present on the surface of the cells even though this varied more than 20-fold between the different cell lines. Hence, this investigation does not support the hypothesis that the receptors play a direct role in the transport of Fe(II) across cell membranes, as has been proposed previously [Singer, S. J. (1989) Biol. Cell 65, 1-5].

Interactions between tissue uptake of lead and iron in normal and iron-deficient rats during development

Environmental lead intoxication, which frequently causes neurological disturbances, and iron deficiency are clinical problems commonly found in children. Also, iron deficiency has been shown to augment lead absorption from the intestine. Hence, there is evidence for an interaction between lead and iron metabolism which could produce changes in lead and iron uptake by the brain and other tissues. These possibilities were investigated using 15-, 21-, and 63-old rats with varying nutritional iron and lead status. Dams were fed diets containing 0 or 3% lead-acetate and 0.2% lead-acetate in the drinking water. After weaning, 0.2% lead-acetate in the drinking water became the sole source of dietary lead. Measurements were made of tissue lead and nonheme iron levels and the uptake of 59Fe after intravenous injection of transferrin-bound 59Fe. Iron deficiency was associated with increased intestinal absorption of lead as indicated by blood and kidney lead levels in rats exposed to dietary lead. However, iron deficiency did not increase lead deposition in the brain, and in all rats brain lead levels were relatively low (< 0.1 microgram/g). Lead concentrations in the liver were below 2 micrograms/g, whereas kidneys had almost 20 times this concentration. Animals with iron deficiency had lower liver iron levels and had increased brain 59Fe uptake in comparison to control rats. However, iron levels in brain and kidneys were unaffected by lead intoxication regardless of the animal's iron status. 59Fe uptake rates were also unaffected by lead, but increased rates of uptake were apparent in iron-deficient rats. Lead did increase liver iron levels in all iron-adequate rats, but iron deficiency had little effect. It is concluded that, compared with other tissues, the blood-brain barrier largely restricts lead uptake by the brain and that the uptake that does occur is unrelated to the iron status of the animal. Also, the level of lead intoxication produced in this investigation did not influence iron uptake by the brain and kidneys, but liver iron stores could be increased if iron levels were already adequate.

Mechanisms of manganese transport in rabbit erythroid cells

1. The mechanisms of manganese transport into erythroid cells were investigated using rabbit reticulocytes and mature erythrocytes and 54Mn-labelled MnCl2 and Mn2-transferrin. In some experiments iron uptake was also studied. 2. Three saturable manganese transport mechanisms were identified, two for Mn2+ (high and low affinity processes) and one for transferrin-bound manganese (Mn-Tf). 3. High affinity Mn2+ transport occurred in reticulocytes but not erythrocytes, was active only in low ionic strength media such as isotonic sucrose and had a Km of 0.4 microM. It was inhibited by metabolic inhibitors and several metal ions. 4. Low affinity Mn2+ transport occurred in erythrocytes as well as in reticulocytes and had Km values of approximately 20 and 50 microM for the two types of cells, respectively. The rate of Mn2+ transport was maximal in isotonic KCl, RbCl or CsCl, and was inhibited by NaCl and by amiloride, valinomycin, diethylstilboestrol and other ion transport inhibitors. The direction of Mn2+ transport was reversible, resulting in Mn2+ efflux from the cells. 5. The uptake of transferrin-bound manganese occurred only with reticulocytes and depended on receptor-mediated endocytosis of Mn-Tf. 6. The characteristics of the three saturable manganese transport mechanisms were similar to corresponding mechanisms of iron uptake by erythroid cells, suggesting that the two metals are transported by the same mechanisms. 7. It is proposed that high affinity manganese transport is a surface representation of the process responsible for the transport of manganese across the endosomal membrane after its release from transferrin. Low affinity transport probably occurs by the previously described Na(+)-Mg2+ antiport, and may function in the regulation of intracellular manganese concentration by exporting manganese from the cells.

Defective iron uptake by the duodenum of Belgrade rats fed diets of different iron contents

Homozygous Belgrade rats have an inherited hypochromic, microcytic anemia that is due to impaired iron transport into immature erythrocytes. There is also evidence for abnormal iron transport in other tissues such as the intestine. This study was aimed at investigating the intestinal defect in rats that had been fed diets for 12 days that are normal, low, or high in iron. The duodenal uptake, transfer, and absorption of Fe(III)-nitrilotriacetate and Fe(II)-ascorbate were studied using in vivo tied-off gut sacs in genetically normal rats and in heterozygous or homozygous Belgrade rats. In normal and heterozygous Belgrade rats, the handling of Fe(III) and Fe(II) was similar; uptake, transfer, and absorption of Fe(III) and Fe(II) changed inversely with the iron content of the diet. In contrast, in homozygous Belgrade rats the uptake of both Fe(III) and Fe(II) was markedly reduced and absorption of Fe(III) did not change when animals were fed an iron-deficient diet. Since absorption of Fe(II) was similar to Fe(III), there is no evidence that the defect in iron absorption is due to failure of a mechanism for reduction of Fe(III). The lowered uptake of Fe(III) and Fe(II) in homozygous Belgrade rats probably involves a defective iron carrier associated with the microvillous membrane of the duodenum.

Effect of lipid peroxidation on transferrin-free iron uptake by rabbit reticulocytes

The relationship between lipid peroxidation and uptake of transferrin- free iron, Fe(II), by reticulocytes in an experimental system for studying membrane transport of Fe(II) was investigated by using free radical scavengers: BHA (butylated hydroxyanisole), BHT (butylated hydroxytoluene), superoxide dismutase, alpha-tocopherol, propyl gallate and DPPD (N,N-diphenyl-1,4-phenylenediamine), and producers: t-butyl hydroperoxide, cumene hydroperoxide, H2O2 and aluminium carbonate. Measurements were made of MDA (malondialdehyde) and the rate of Fe(II) uptake from a sucrose solution buffered at pH 6.5 by Pipes. Most scavengers and producers used could increase or decrease only slightly the rate of Fe(II) uptake and some of them had no effect on Fe(II) uptake and MDA could not be detected at iron concentration of lower than 10 microM and incubation time of 20 min. At iron concentration of higher than 100 microM and incubation time of 4 h, there was the production of MDA which increased with the increment of iron concentration of incubation medium and BHT could inhibit the production of MDA. In addition, no difference was found in the rates of Fe(II) uptake in three experimental groups whose incubation medium was buffered by Pipes, Mops and Mes respectively. The results suggested that iron could induce free radical reaction under experimental conditions, especially at high concentration of iron and longer incubation time; however, at low concentration of iron (<10 microM) and the usual incubation time (20 min) free radical reaction was very slight and the extent of the reaction was not enough to damage the integrity and function of the membrane of reticulocytes, and that Fe(II) uptake by reticulocytes was not the result of free radical reaction and lipid peroxidation. It was therefore concluded that iron could not initiate its own membrane transport in rabbit reticulocytes by free radical reaction and lipid peroxidation and that the experimental system we used for studying membrane transport of Fe(II) is valid.

Iron and copper interact during their uptake and deposition in the brain and other organs of developing rats exposed to dietary excess of the two metals

This study examined the effect of iron and copper loading on rat brain, liver, kidney, femur, blood and plasma concentrations of these metals and iron transport into the organs during development. Dams were fed control diets or iron-loaded diets (20 g/kg carbonyl iron) with either distilled water or copper-loaded water (350 mg/L) beginning at d 20 of pregnancy. The weanlings also had access to the diets and water supply and were examined at 15, 21 and 63 d of age. The iron content of the liver was 17- to 30-fold greater in iron-loaded rats than in controls, whereas liver, kidney and plasma copper levels generally were lower. Iron loading alone did not increase brain iron concentrations, suggesting the blood-brain barrier is already developed at birth. However, dual loading of iron and copper resulted in elevated concentrations of brain non-heme iron and copper in 15- and 63-d-old rats compared with animals loaded with iron alone. These results suggest that brain iron uptake mechanisms may be different when excess copper is present. Liver non-heme iron was also greater in copper-loaded rats, irrespective of iron status. However, kidney iron concentrations generally were not affected by dietary copper. In rats fed the copper-containing diet, the uptake of iron into brain and liver was significantly lower than in those fed the control diet, suggesting that copper loading can decrease iron uptake into organs. It is concluded that combined dietary supplementation with iron and copper can alter the metabolism of each metal. These changes are age and organ dependent. Developing rats may be very susceptible to these combined overload states because significant effects are seen in early adulthood.