Transferrin, biochemistry, physiology and clinical significance

Role of iron in the tubulo-interstitial injury in nephrotoxic serum nephritis

We studied the possibility that tubule fluid iron could be involved in the pathogenesis of the tubulo-interstitial injury associated with primary glomerular disease. Tubule fluid iron is determined by the magnitude of the glomerular leak for transferrin and the iron saturation of transferrin. To minimize tubule fluid iron in an experimental model of glomerulonephritis, iron deficiency was induced in rats prior to the induction of nephrotoxic serum nephritis. Iron deficiency did not effect the development of glomerular disease as determined by proteinuria, but had a marked effect on preventing the development of tubulo-interstitial disease and renal functional deterioration. There was also a strong correlation between the amount of functional deterioration and extent of tubulo-interstitial disease and urinary iron excretion in both the control and iron deficient animals. It is proposed that injury results from iron being dissociated from transferrin at the more acid pH of the tubule fluid. Iron, a transition element, is able to catalyze the Haber-Weiss reaction with the formation of free hydroxyl radicals which causes renal tubule cell injury. This tubulo-interstitial injury is the major determinate of progressive renal functional deterioration in this experimental model of glomerulonephritis.

Iron-binding proteins

The structure and properties of the iron-binding proteins transferrin, lactoferrin and transferrin are reviewed. Transferrin and lactoferrin are structurally similar, consisting of a single polypeptide chain and reversibly binding two iron atoms per molecule. Transferrin is found mainly in serum, whereas lactoferrin is found in neutrophils and in external secretions. Transferrin functions mainly as a donor of iron to cells, but there is no established iron-transport role for lactoferrin. Both these proteins may have antimicrobial activity as a result of their ability to sequester iron. Lactoferrin may act principally as a scavenger of iron in conditions where transferrin may not bind iron well, e.g. at low pH. Ferritin is a multisubunit protein capable of binding up to 4,000 iron atoms and serves principally as an iron-storage protein, though it may also serve to detoxify iron. In iron-rich tissues ferritin is largely degraded and the iron is converted to haemosiderin.

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.

Transferrin receptor and ferritin levels during murine mammary gland development

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

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.

Incorporation of myristate and palmitate into the sheep reticulocyte transferrin receptor: evidence for identical sites of labeling

The ability of sheep reticulocytes and plasma membranes isolated from them to incorporate fatty acids into the transferrin receptor has been examined using both [3H]palmitate and [3H]myristate. Both fatty acids, when incorporated into the transferrin receptor, can be released by treating the protein with 1 M hydroxylamine at pH 7.0. After treatment of the 3H-acylated receptor with borohydride, an 3H-labeled alcohol is released, suggesting that the receptor-bound fatty acid is in thioester linkage. With both [3H]myristate and [3H]palmitate, Cleveland maps from immunoprecipitates of the transferrin receptor labeled in intact cells and isolated membranes show that identical peptides are labeled. No evidence was obtained for qualitatively different labeling with the two fatty acids. In intact reticulocytes, incorporation of [3H]palmitate into the transferrin receptor is approximately 3.5 times greater than the incorporation of [3H]myristate from equivalent concentrations of the labeled fatty acids. However, in isolated reticulocyte plasma membranes, there is much less difference between palmitate and myristate incorporation (with ATP) or between their acyl-CoA derivatives. The reason for the discrepancy between cells and membranes is unknown but may be due to the presence in intact cells of more than one enzyme for activating the fatty acids. Acylation of the receptor in isolated plasma membranes is fourfold greater with the CoA derivatives than with the free fatty acids. The fatty acid activating enzyme(s) as well as the acyltransferase(s) appear to be membrane bound in reticulocytes.

A stem-loop in the 3' untranslated region mediates iron-dependent regulation of transferrin receptor mRNA stability in the cytoplasm

Expression of the human transferrin receptor (hTR) and its mRNA is strongly induced by iron deprivation. By measuring transcription elongation rates, levels of hTR-specific nuclear RNA, and mRNA half-lives, we found this regulation to occur posttranscriptionally in the cytoplasm. Analysis of hTR cDNA mutants with deletions in the 3' untranslated region revealed the existence of two distinct domains, both of which are essential for regulation in mouse L cells. The regulated phenotype correlates with the presence of a stem-loop structure predicted by a computer algorithm. Expression of point and deletion mutants affecting the stem-loop confirmed the requirement for this secondary structure in regulation. The 3' untranslated region of hTR cDNA was sufficient to confer iron-dependent regulation on another gene.

Evidence that transferrin supports cell proliferation by supplying iron for DNA synthesis

Transferrin is essential for cell proliferation and it was suggested that it may trigger a proliferative response following its interaction with receptors, serving as a growth factor. However, since the only clearly defined function of transferrin is iron transport, it may merely serve as an iron donor. To further clarify this issue, we took advantage of an iron chelate, ferric salicylaldehyde isonicotinoyl hydrazone (Fe-SIH), which we developed and previously demonstrated to efficiently supply iron to cells without using physiological transferrin receptor pathway. As expected, we observed that blocking monoclonal antibodies against transferrin receptors inhibited proliferation of both Raji and murine erythroleukemia cells. This inhibited cell growth was rescued upon the addition of Fe-SIH which was also shown to deliver iron to Raji cells in the presence of blocking anti-transferrin receptor antibodies. Moreover, blocking anti-transferrin receptor antibodies inhibited [3H]thymidine incorporation into DNA and this inhibition could be overcome by added Fe-SIH. In addition, Fe-SIH slightly stimulated, while SIH (an iron chelator) significantly inhibited, DNA synthesis in phytohemagglutinin-stimulated peripheral blood lymphocytes. Taken together, these results indicate that the only function of transferrin in supporting cell proliferation is to supply cells with iron.

Regulatory aspects of placental iron transfer--a comparative study

Receptor-mediated endocytosis is generally assumed to be the process by which the haemochorial placenta takes up iron from transferrin. The involvement of an additional nonendocytic process cannot, however, be excluded. It appears from a study of iron transport mechanisms and of the maturation of the transfer process that placental ferritin is involved in the transfer of iron from mother to fetus. The metabolic relationship between the ferritin pool and the placental transfer pool remains to be elucidated. There is no evidence for short-term regulation of placental transfer capacity in response to changes in the maternal iron supply or to changes in the trophoblastic iron content. This cannot yet be said of fetal feedback control of placental iron uptake because the experiments performed so far do not permit conclusions on this point. The capacity for iron uptake and transfer seems to increase in accordance with the ontogenetically determined placental growth pattern. This does not exclude long-range adaptive modifications of the transfer capacity in response to early maternal or fetal disturbances. The results obtained from studying placental maturation suggest that a possible long-term regulatory interaction between growing placenta and fetus may occur. Clinical evidence so far is inconclusive. The relatively moderate reductions in the fetal iron stores which are generally associated with severe iron-deficient pregnancies might be seen as an argument in favour of long-term placental control. The marked impact of pregnancy on maternal iron metabolism in rodents, as compared to other mammals, is possibly met by means of direct placental control of mucosal iron uptake. In primates, mucosal iron uptake during pregnancy seems to be governed by factors related to systemic iron deficiency only.

Reduction of diferric transferrin by SV40 transformed pineal cells stimulates Na+/H+ antiport activity

Transplasmalemma electron transport by HeLa and pineal cells to reduce external ferricyanide is associated with proton release from the cells. Diferric transferrin also acts as an electron acceptor for the transmembrane oxidoreductase. We now show that reduction of external diferric transferrin by RPNA-209-1 SV40 transformed pineal cells is accompanied by proton release from the cells. The stoichiometry of proton release to electron transfer is much greater than would be expected from aniostropic electron flow across the membrane through protonated carriers. The proton release is not stimulated by apotransferrin and the diferric transferrin-stimulated activity is inhibited by apotransferrin. Apotransferrin also inhibits reduction of diferric transferrin by these cells. The proton release is dependent on external sodium ions and is inhibited by amiloride, which indicates that the proton release is mediated by the Na+/H+ antiport and that this antiport is activated by electron transport through the transmembrane dehydrogenase. Growth stimulation by diferric transferrin or other external oxidants can be based in part on activation of the Na+/H+ antiport.

Species specificity of transferrin binding, endocytosis and iron internalization by cultured chick myogenic cells

The ability of unlabelled heterologous transferrin to interact with transferrin receptors on developing chick myogenic cells was investigated by measuring their capacity to inhibit the surface-binding and internalization of 125I- and 59Fe-labelled ovotransferrin. Transferrins from rat, rabbit, human, and a species of kangaroo (Macropus fuliginosus) were unable to inhibit either surface-binding or internalization of labelled ovotransferrin even at concentrations ten times the molar concentration of the ovotransferrin. Transferrins isolated from the serum of a toad (Bufo marinus) and a lizard (Teliqua rugosa), when added at high concentrations, were found to reduce surface-binding of 125I-Tf by 20-25% but did not inhibit internalization of either 125I-Tf or 59Fe. This suggests that the effects of toad and lizard transferrins are due to non-specific binding to the myogenic cells. In contrast, inhibition of both surface-binding and internalization of labelled ovotransferrin was found when myogenic cells were incubated in the presence of the homologous transferrin (ovotransferrin). The species-specificity of transferrin binding, endocytosis and iron internalization did not vary with the state of proliferation or differentiation of the myogenic cells. However, the intracellular iron utilization was found to differ between differentiating presumptive and terminally differentiated myotubes. Internalized 59Fe was fractioned by gel filtration. In dividing and non-dividing presumptive myoblasts 59Fe was found to elute in three peaks, two with elution volumes corresponding to ferritin and transferrin and one at greater elution volume than that of myoglobin.(ABSTRACT TRUNCATED AT 250 WORDS)

Transferrin and the growth-promoting effect of nerves

In addition to its role in the activity of specialized proteins such as hemoglobin and myoglobin, iron is required as a cofactor in several important enzymes common to most animal cells. One such enzyme, ribonucleotide reductase, which regulates the production of deoxyribonucleotides during DNA synthesis, requires a continuous supply of iron to maintain its activity throughout the process of DNA replication. The mechanism by which animal cells normally acquire iron involves receptor-mediated uptake of iron-loaded transferrin, followed by release of apotransferrin. The density of transferrin receptors on the cell surface is greatly increased in rapidly dividing normal and neoplastic cells. Various mitogens and certain organogenic tissue interactions have been shown to induce the appearance of transferrin receptors, signalling the onset of DNA replication. Interference with this process of iron delivery causes the rapid arrest of cell cycling, frequently during the S phase itself, which underscores the importance of iron for DNA replication. Although most circulating transferrin is synthesized in the liver and embryonic yolk sac, smaller quantities are produced in several other embryonic organs and certain other adult tissues. It has been suggested that local synthesis and/or release of transferrin supplies the iron required by rapidly growing cells in situations where the cells do not have ready access to adequate amounts of plasma transferrin due to incomplete development of the vasculature or the presence of blood-tissue barriers (Ekblom and Thesleff, 1985; Meek and Adamson, 1985). Oligodendrocytes and Schwann cells have been shown to synthesize and/or contain high concentrations of transferrin and these cells therefore may constitute a local source of this factor for neurons, whose growth and survival in vitro require transferrin. Transferrin in central and peripheral nervous tissues may be significant for the trophic or growth-promoting effect neurons exert on cells of certain tissues. Transferrin duplicates the activity of neural tissue or neural extracts on growth and development of cultured skeletal myoblasts from chick embryos and on proliferation of mesenchymal cells in blastemas from regenerating amphibian limbs, two systems that have been widely used in investigations of the growth-promoting influence of nerves. Moreover, removal of active transferrin from neural extracts, either with antibodies to transferrin or chelation of the iron, inhibits reversibly the effect of the extract in these developing systems. While the physiological significance of the extract in these developing systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Transferrin: an early marker of oligodendrocytes in culture

In this study, the developmental pattern of transferrin expression, the iron transporting glycoprotein, was investigated morphologically and immunocytochemically in mixed glial cultures as well as pure cultures of mature oligodendrocytes, both derived from newborn rat brain. Double immunofluorescent labeling of pure oligodendrocyte cultures revealed that transferrin co-localizes with the oligodendroglial marker, myelin basic protein. During early development in mixed glial cultures, the presence of transferrin was detected at 3 days in vitro in small round process-bearing cells lying on top of astrocytes. These cells were galactocerebroside negative. However, at 7 days these process-bearing cells began to express galactocerebrosides and transferrin co-localized with the oligodendroglial marker. Transferrin did not co-localize with any neuronal or astroglial markers at any time. These results indicate that transferrin is an oligodendrocyte-specific marker which is expressed earlier than galactocerebroside.

Effect of osmolar and ionic composition of the extracellular fluid on transferrin endocytosis and exocytosis and iron uptake by reticulocytes

The effects of osmolar and ionic factors on endocytosis and exocytosis were investigated using rabbit reticulocytes and 125I-59Fe labelled transferrin. Endocytosis and exocytosis of transferrin and the uptake of iron were inhibited by increasing the osmolality or decreasing the ionic strength or pH of the cell incubation medium. However, elevation of the pH above 8.0 inhibited endocytosis but not exocytosis. Replacement of the NaCl in the incubation medium by Nal, NaF, NaSCN, NaClO4, Na2SO4, Na phosphate, or Na Hepes inhibited endocytosis and iron uptake but only Nal, NaF, and NaSCN inhibited exocytosis. Transferrin exocytosis was insensitive to inhibitors of anion or cation transport, but endocytosis and iron uptake were inhibited by several anion transport inhibitors. Overall, transferrin endocytosis was more sensitive than exocytosis to most of the factors which were investigated, and the effects on the rates of endocytosis and iron uptake were quantitatively very similar. The results provide strong support for the concept that transferrin endocytosis is a necessary step in iron uptake by reticulocytes. They do not support the chemiosmotic models of exocytosis in their present formulations, but do not rule out the possible role of an osmotic event in exocytosis.

Iron-binding properties and amino acid composition of marsupial transferrins: comparison with eutherian mammals and other vertebrates

1. Some physicochemical properties of transferrin from three marsupials, viz a possum (Trachosurus vulpecula), a kangaroo (Macropus fuliginosus) and the quokka (Setonix brachyurus) were studied and compared with those of transferrins from mammalian and non-mammalian vertebrate species. 2. The molecular weight of the marsupial transferrins fell within the range of 76,000-79,000 daltons. 3. The marsupial transferrins were similar to the transferrins of eutherian mammals with respect to optical spectral properties, iron binding capacity and the pH-dependence of iron binding, and iron release mediated by 2,3-DPG. 4. The amino acid compositions of the marsupial transferrins were compared with each other and with the transferrins from the other vertebrate species. The compositions of the marsupial transferrin were closely related to each other, and also showed similarities with transferrins from eutherian mammals and chicken ovotransferrin.

Apotransferrin receptors and the delivery of iron from cultured human blood monocytes

A study was done to find out whether apotransferrin receptors are involved in the release of iron from reticuloendothelial cells. To this end, human macrophages which had been obtained by culturing blood monocytes for 7 days were incubated with either diferric or apotransferrin at the physiological pH of 7.4 or at an acidic pH (6.0). While specific diferric transferrin receptors (Kd 1.3 X 10(-8) M) were demonstrated at pH 7.4, no apotransferrin receptors were found. In contrast, both diferric receptors (Kd 2.1 X 10(-8) M) and apotransferrin receptors (Kd 6.8 X 10(-9) M) were found at pH 6.0. The findings of specific apotransferrin binding at acidic pH fits in with the current understanding of iron uptake by cells, in which the iron-transferrin complex is endocytosed and the iron is released at acidic pH. The present results suggest that the apotransferrin remains attached to its receptor in the endocytosed vesicle at this acidic pH but that it becomes detached at the cell surface where the pH is neutral. No evidence was found to indicate that iron is transported out of macrophages via apotransferrin receptors at the physiological pH.

Transferrin and ferritin endocytosis and recycling in guinea-pig reticulocytes

Transferrin and ferritin endocytosis and exocytosis by guinea-pig reticulocytes were studied using incubation with pronase at 4 degrees C to distinguish internalized and membrane-bound protein. Internalization of both transferrin and ferritin occurred in a time- and temperature-dependent fashion. Transferrin endocytosis was more rapid than that of ferritin. Transferrin binding to receptors was not altered, but transferrin endocytosis was decreased in the presence of ferritin. Iron accumulation from transferrin was inhibited by ferritin to a greater extent than could be accounted for by the decreased rate of endocytosis. In pulse-chase experiments, almost all of the transferrin was released intact from reticulocytes, but only about 50% of the total internalized ferritin was released, of which 85% was intact. The endocytosis of transferrin by rabbit reticulocytes was 2- to 2.5-times faster than guinea-pig reticulocytes. These data suggest that ferritin and transferrin are internalized by receptor-mediated endocytosis, possibly involving the same coated pits and vesicles, but that the proteins are recycled only partly in common.

Endocytosis of the transferrin receptor requires the cytoplasmic domain but not its phosphorylation site

The transferrin receptor (TR) mediates cellular iron uptake by bringing about the endocytosis of transferrin. We investigated whether the cytoplasmic domain of 65 N-terminal amino acids or phosphorylated sites within this domain constitute a structure that is required for TR endocytosis. To test this hypothesis, we modified the cytoplasmic serine residues or introduced a deletion of 36 amino acids by in vitro mutagenesis of a cDNA expression vector for human TR. Upon expression in transfected mouse Ltk- cells, both the wild-type and phosphorylation site mutant receptors mediated transferrin internalization, whereas the truncated receptor did not. These results provide evidence that the cytoplasmic domain, or part of it, is essential for internalization of the TR, but argue against a role for receptor phosphorylation in endocytosis.

Transferrin is a major mouse milk protein and is synthesized by mammary epithelial cells

We have identified a major mouse milk protein as transferrin (Tf) using immunoprecipitation, 2-dimensional electrophoresis, Ouchterlony diffusion and V-8 protease digests. We show that Tf is synthesized by mammary epithelial cells themselves and that its synthesis and secretion is regulated distinctly from that of other milk proteins. In culture, the kinetics of Tf synthesis and secretion are distinct from that of beta-casein; furthermore, Tf is relatively insensitive to lactogenic hormones whereas beta-casein is hormone-dependent. In vivo, however, Tf is regulated by pregnancy. While the virgin gland produces small amounts of Tf, its production is greatly increased during pregnancy and lactation. Thus, Tf synthesis in the mammary gland is modulated by as yet unknown factors in vivo. These observations are discussed in terms of Tf's possible role in mammary gland growth, differentiation and function.

Vinblastine but not other microtubule inhibitors block transferrin endocytosis and iron uptake by reticulocytes

The antimicrotubule reagents, colchicine, griseofulvin, nocodazole, podophylotoxin and taxol had no effect on transferrin endocytosis or iron uptake by rabbit or rat reticulocytes but were inhibitory when used at high concentrations with rat fetal liver erythroid cells. The results imply that microtubules do not have a role in endocytosis and iron uptake by reticulocytes but may have a permissive role in the fetal cells. The only reagent found to inhibit iron uptake by reticulocytes was vinblastine. It was shown to act by inhibiting the endocytosis of transferrin. It is concluded that this effect is not the result of an interaction with microtubules, but may result from a non-specific action on the cell membrane or a more specific effect, such as inhibition of calmodulin.

Transferrin-receptor interaction and iron uptake by reticulocytes of vertebrate animals--a comparative study

Transferrin-receptor interactions and iron uptake were studied in eleven different species of vertebrate animals (3 eutherian mammals, 3 marsupials, 2 reptiles and 1 bird, amphibian and bony fish). In the initial experiments it was shown that the uptake of transferrin-bound iron by immature erythroid cells from marsupial and reptilian species occurs by receptor-mediated endocytosis as in other vertebrate animals. Reticulocytes were incubated with 125I-59Fe-labelled transferrins from heterologous species and the results for iron and transferrin uptake compared with those obtained with the homologous protein. Cells from eutherian mammals were able to take up transferrin and iron from other eutherians and from the bob-tailed lizard but not from marsupials and other submammalian species. With marsupials and reptiles a similar specificity was observed, and the marsupial cells could also utilize chicken transferrin but not vice versa. The results were extended by performing competition experiments in which the cells were incubated with radiolabelled homologous transferrin in the presence of increasing concentrations of non-radioactive heterologous transferrins. From the ability of the heterologous proteins to inhibit uptake of the homologous protein relative association constants (Ka1) for the transferrin-receptor interactions could be calculated. These Ka1 values reflected the patterns observed in the first series of experiments. These studies demonstrate that, although specificity exists in transferrin-receptor interactions throughout the range of vertebrate animals, in several instances reactivity between widely divergent species is also observed. Hence, structural similarities have been maintained throughout evolution. Nevertheless, no evidence of interaction between transferrin and its receptor from the two divisions of the Mammalia, the eutherians and the marsupials, was observed.

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

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

Vanadium complexes of transferrin and ferritin in the rat

Vanadium associates with serum transferrin of rats administered vanadyl(IV) sulfate or ammonium metavanadate(V) by gastric intubation. Low molecular weight species account for only 3% of the vanadium present in plasma. The element distributes between the two major isotransferrins in proportion to their concentrations. Rat apotransferrin binds both vanadium(IV) and vanadium(V), forming 2:1 metal-protein complexes in both instances. Although the two isotransferrins apparently differ in their physiological properties, they exhibit identical vanadyl(IV) (VO2+) EPR spectra, indicating identical or very similar metal binding sites for both proteins. In contrast to other transferrins, the two sites of the rat protein are spectroscopically indistinguishable and exhibit a VO2+ EPR spectrum similar to that of the C-terminal metal binding site of human serum transferrin. VO2+ EPR signals are observed with liver, spleen, and kidney tissue samples from animals maintained on a vanadium-supplemented diet. These signals arise from a specific intracellular VO2+ complex with the iron storage protein ferritin.

Transplasmalemma electron transport from cells is part of a diferric transferrin reductase system

Intact cells are known to reduce external, impermeable electron acceptors. We now show that cells can reduce the iron in diferric transferrin at the cell surface and that this reduction reaction depends on the transferrin receptor as well as the transmembrane electron transport system. Reduction of external diferric transferrin is accompanied by oxidation of internal NADH which indicates that the transmembrane enzyme is an NADH diferric transferrin reductase. Highly purified liver plasma membranes have NADH diferric transferrin reductase activity which shows properties similar to the diferric transferrin reductases activity of intact cells. Cell growth stimulation by diferric transferrin and other impermeable oxidants which can react with the diferric transferrin reductase can be based on electron transport through he plasma membrane.

Depression of iron uptake from transferrin by isolated hepatocytes in the presence of ethanol is a pH-dependent consequence of ethanol metabolism

Incubation of freshly isolated rat hepatocytes with highly purified radiolabeled rat transferrin in weakly buffered medium in the presence of 10 mM ethanol resulted in a marked diminution of iron uptake by these cells, associated with a greater pH depression than in ethanol-free control studies. This effect on iron uptake persisted, even when the cells were preincubated for 90 min with ethanol before the addition of transferrin. Increasing the buffering capacity of the system or the addition of a metabolic inhibitor of alcohol dehydrogenase (4-methylpyrazole) returned iron uptake to control values. Acetaldehyde, acetate, lactate (products of ethanol metabolism), and 3-butanol (an alcohol not metabolized by alcohol dehydrogenase) had no influence on iron uptake. Further investigation of iron uptake over the pH range 6-8.5 revealed a marked dependency of iron uptake on the extracellular pH. Leucine incorporation into cell protein was also found to be pH dependent. It is suggested that, in the light of current understanding of transferrin recycling by other cell types, the disturbances of iron homeostasis observed in alcoholics can be partially accounted for by alterations in their acid-base metabolism.

A comparative study on iron sources for mitochondrial haem synthesis including ferritin and models of transit pool species

The rates of reaction of various exogenic iron(III) complexes with deuteroporphyrin IX in isolated mitochondria to form deuterohaem were measured. Ferritin was shown to supply iron readily for haem synthesis if the ferritin iron was reductively mobilised by the mitochondrial respiratory chain with succinate as substrate and FMN as mediator. In contrast, polynuclear complexes of iron(III) were able to form deuterohaem without added FMN. Rates of haem formation are about five times higher for the lowest polynuclear units than for ferritin. Sorbitol, gluconate, and bovine serum albumin were used as scavengers for polynuclear complexes with restricted size. Strong chelators of iron(II) compete favourably for deuterohaem formation, which supports the multistep mechanism for haem formation suggested by a priori arguments. Rates of deuterohaem formation were measured in homologous and heterologous systems of ferritins and mitochondria. Slightly differing rates of haem formation were shown to originate in different rates of iron mobilisation from the ferritins. The lack of species specificity in the interaction of ferritin with mitochondria also shows up in the linear dependence of ferritin binding on its bulk concentration as measured using 3H-labeled ferritin. Rates of haem formation are virtually the same in mitoplasts and mitochondria which indicates insignificant influences of the outer membrane. The hypothesis of low polynuclears as major components of the intracellular transit iron pool implies that both ferritin and transit iron pool species are largely equivalent sources of iron for mitochondrial haem synthesis.

Subcellular localization of transferrin protein and iron in the perfused rat liver. Effect of Triton WR 1339, digitonin and temperature

The subcellular localization of 3H-labelled 59Fe-loaded transferrin accumulated by the liver has been studied by means of cell fractionation techniques. More than 96% of the 59Fe present in the liver of rats perfused with 59Fe-labelled transferrin is recovered in the parenchymal cells. Rat livers were perfused with 10 micrograms/ml 3H-labelled 59Fe-saturated transferrin, homogenized separated in nuclear (N), mitochondrial (M), light mitochondrial (L), microsomal (P) and supernatant (S) fractions; M, L and P fractions were further analysed by isopycnic centrifugation in sucrose gradients. 3H label distributes essentially around densities of 1.13-1.14 g/ml overlapping to a large extent with the distribution of galactosyltransferase, the marker enzyme of the Golgi complex. However, after treatment with low concentrations of digitonin the 3H label dissociates from galactosyltransferase and is shifted to higher densities, suggesting an association of transferrin with cholesterol-rich endocytic vesicles which could derive from the plasma membrane. 59Fe is mostly found in the supernatant fraction largely in the form of ferritin, as indicated by its reaction with antiferritin antibodies. In the mitochondrial fraction the density distribution of 59Fe suggests an association with lysosomes and/or mitochondria. In contrast to the lysosomal enzyme cathepsin B, the density distribution of 59Fe was only slightly affected by pretreatment of the rats with Triton WR 1339, suggesting its association with the mitochondria. At 15 degrees C, 59Fe and 3H labels are recovered together in low-density endocytic vesicles. On the basis of our results we suggest that, at low extracellular transferrin concentration, iron uptake by the liver involves endocytosis of the transferrin protein. The complex is interiorized in low-density acidic vesicles where iron is released. The iron passes into the cytosol, where it is incorporated into ferritin and into the mitochondria. The iron-depleted transferrin molecule would then be returned to the extracellular medium during the recycling of the plasma membrane.

Hematopoiesis in the rat: quantitation of hematopoietic progenitors and the response to iron deficiency anemia

To determine the quantitative effects of iron deficiency on erythropoiesis and to assess the response of erythroid progenitors to sustained anemia, we developed quantitative assays for various hematopoietic progenitors in the adult, Sprague-Dawley rat including erythroid colony- and burst-forming cells (CFU-E and BFU-E), granulocyte/macrophage colony-forming cells (CFU-GM), and megakaryocytic colony-forming cells (CFU-Meg). CFU-E were cultured in methylcellulose and grew best in the presence of fetal calf serum. CFU-GM, BFU-E, and CFU-Meg grew better in normal rat plasma and required the presence of pokeweed mitogen-stimulated rat spleen cell conditioned medium. The numbers of progenitors and nucleated erythroblasts in total marrow were estimated by the ratios of radioactivity in the humerus to the total skeleton as determined by radioiron dilution. The numbers of progenitors and erythroblasts in the spleen were measured by simple dilution. Sustained anemia was brought about through chronic iron deficiency. The response to iron deficiency anemia (IDA) was monitored by the numbers of the various progenitors and their cell cycle characteristics as measured by the tritiated thymidine suicide technique. With IDA, the number of CFU-F in the body (marrow plus spleen) was increased to 3.5 times control, whereas the numbers of BFU-E and CFU-GM were unchanged. There was no difference in the percentage of CFU-E, BFU-E, and CFU-GM in DNA synthesis (68%, 19.4%, and 18.8%, respectively). With iron therapy of IDA, CFU-E numbers in marrow began to decrease by day 1 and fell in a manner reciprocal to changes in the hematocrit. Marrow and spleen erythroblasts, 1.7 times control in IDA, increased further to 3.9 times control by the fourth day after iron administration. There was no change in BFU-E or CFU-GM numbers in response to iron repletion, although the fraction of progenitors increased in the spleen. Thus, IDA does not limit the increase in CFU-E seen with anemia, but does restrict erythroid maturation. Furthermore, the increase in CFU-E and the state of chronic anemia occur without detectable changes in the number of cell cycle state of the more primitive BFU-E.

A simple and sensitive solid-phase radioimmunoassay for measuring the transferrin content of human biological fluids: its application to seminal plasma

A highly sensitive and reproducible radioimmunoassay was established to detect transferrin in human fluids. By this technique, applied to seminal fluid, transferrin levels (micrograms/ml) were found in normozoospermic individuals (64.49 +/- 25.41) at level higher than in oligozoospermic (38.93 +/- 21.35), azoospermic (19.49 +/- 10.23), or vasectomized (19.61 +/- 8.95) subjects. A relationship between transferrin and spermatozoid concentration in sperm was shown. These results reinforce previous findings that seminal transferrin can be used as a reliable clinical marker of Sertoli Cell function.

Transferrin receptors on circulating monocytes in hereditary haemochromatosis

In patients with hereditary haemochromatosis (HH) abnormal functional properties of the macrophage system have been observed. The present study is a preliminary report of increased transferrin receptor expression on monocytes from 12 patients with HH. There was no correlation between the degree of iron overload and the transferrin receptor expression on the monocytes. The results obtained thus indicate that the observed increase in transferrin receptors is not a secondary phenomenon due to systemic iron overload but could be an expression of a primary inborn error of iron metabolism in HH. The functional aspects of the receptors were not evaluated as they were analyzed by means of monoclonal antibody technique.

Iron deficiency and iron overload

An up to date review of our knowledge of human iron metabolism is given including problems of iron balance, internal transport, and intracellular mechanisms. Current knowledge of the iron proteins is summarized and this background is used in discussing the pathophysiology of iron deficiency and overload, together with the internal derangements such as sideroblastic anemia which form much of the clinical practice associated with disorders of iron metabolism. The therapeutic approach to these problems will be described.

Uptake and metabolism of transferrin and albumin by rat yolk sac placenta

The uptake of radiolabeled albumin and transferrin by the rat yolk sac and their subsequent transport to the embryo were studied. Transferrin uptake increases with incubation time whether the results are expressed in terms of the total amount accumulated or per milligram embryo or yolk sac protein, whereas albumin levels increase only in absolute terms. The fate of transferrin and albumin was examined by partitioning 125I into low- and high-molecular-weight fractions. Nearly all the embryonic radioiodine originally derived from transferrin is in the low-molecular-weight fraction, compared with only 60% albumin. These results have been extended by examining the uptake and hydrolysis of transferrin and albumin by the isolated yolk sac. Transferrin is taken up more rapidly than albumin. The release of hydrolyzed transferrin to the incubation medium occurred 40 min after the initiation of incubation, compared with 20 min for albumin. Transferrin uptake by the yolk sac at different transferrin concentrations showed an initial rapid phase followed by a slower linear phase, whereas albumin uptake increased linearly with concentration. There was no competition between the two proteins for uptake. Transferrin was released from the yolk sacs at approximately twice the rate of albumin. Results demonstrate at least two uptake mechanisms in the rat visceral yolk sac, one for transferrin, which probably involves receptor-mediated endocytosis, and one for albumin, by which transferrin can also be transported, which probably involves pinocytotic mechanisms.

Heme inhibits transferrin endocytosis in immature erythroid cells

The inhibitory effect of heme on iron uptake from transferrin by rat and rabbit reticulocytes and erythroid cells from the fetal rat liver was studied in vitro. Addition of hemin was shown to cause a decrease in the rate of transferrin endocytosis, the degree of inhibition being proportional to the reduction in iron uptake. The heme synthesis inhibitors, isoniazid and succinylacetone, stimulated the rate of transferrin endocytosis by 15-30% and caused a proportional increase in the rate of iron uptake, possibly by reducing the intracellular free heme concentration. It is concluded from these results that heme affects iron uptake by influencing the rate of transferrin endocytosis and recycling.

The control of transferrin receptor synthesis in mitogen-stimulated human lymphocytes

Human lymphocytes cultured in the presence of the plant mitogenic lectin phytohemagglutin in (PHA) become activated and leave the G0 phase of the cell cycle. In the presence of PHA and lymphokines produced in situ the cells will enter S phase and undergo cell division. We have determined the time course of appearance for the receptor for transferrin as an initial attempt to understand the molecular mechanisms regulating the onset of lymphocyte differentiation and proliferation in the 48 h following PHA addition. Using three different assay methods we have shown that the increase in the number of surface receptor molecules is due to the accumulation of newly synthesized receptor and not to the redistribution of a previously existing pool of receptor molecules. The total amount of transferrin receptor increased at least four-fold. In vitro translation of RNA from activated lymphocytes indicates that the new receptor synthesis is due, at least in part, to increased availability of mRNA encoding the transferrin receptor.

Expression of the transferrin gene during development of non-hepatic tissues: high level of transferrin mRNA in fetal muscle and adult brain

Using a cloned rat transferrin cDNA probe, we looked for transferrin mRNA in the various rat tissues during development. In all the cases the mRNA detected seemed to be the same and to be product of a single gene. The transferrin gene is early expressed at a high level during liver differentiation. In the muscle and other non-hepatic and non-nervous tissues, the gene expression is maximal just before birth (19-20th day of gestational age), then markedly decreases during the postnatal development, the mRNA level being very low in the adult tissues. In brain, by contrast, transferrin mRNA level is very low before birth, then gradually increases during the postnatal development and reaches a plateau in the adult. Maximal mRNA concentration in fetal muscle (2 days before birth) and adult brain is about 1:7 to 1:10 of that obtained in adult liver. These results are analyzed in the light of the evidence that transferrin is not only an iron-binding protein, but also a factor involved in cell proliferation and differentiation, and particularly in nerve control of muscle differentiation.

Intravesicular pH and iron uptake by immature erythroid cells

The intravesicular pH of intact rabbit reticulocytes was measured by two methods; one based on the intracellular:extracellular distribution of DMO (5, 5, dimethyl + oxazolidin-2,4-dione), methylamine, and chloroquine and the other by quantitative fluorescence microscopy of cell-bound transferrin. The latter method was also applied to nucleated erythroid cells from the fetal rat liver. A pH value of approximately 5.4 was obtained with both methods and in both types of cells. Treatment of the cells with lysosomotrophic agents, metabolic inhibitors, and ionophores elevated the intravesicular pH and inhibited iron uptake from transferrin. When varying concentrations of NH4Cl were used, a close correlation was observed between the inhibition of iron uptake and elevation of the intravesicular pH. At pH 5.4 iron release from rabbit iron-bicarbonate transferrin in vitro was much more rapid than from iron-oxalate transferrin. The bicarbonate complex donates its iron to rabbit reticulocytes approximately twice as quickly as the oxalate complex. It is concluded that the acidic conditions within the vesicles provide the mechanism for iron release from the transferrin molecule after its endocytosis and that the low vesicular pH is dependent on cellular metabolism.