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PubMed1986

Immunocytochemical distribution of transferrin and its receptor in the developing chicken nervous system.

Transferrin is the plasma protein responsible for iron transport in all vertebrates. While transferrin is known to have growth-promoting activity on a variety of cells in culture, the role of transferrin and its membrane receptor in neuronal development is unknown. Using antibodies to transferrin and transferrin receptors, we studied the immunocytochemical localization of transferrin and its receptor in developing chicken neural tissues by the peroxidase-antiperoxidase method. In 5-day-old embryonic brain, germinal cells of the ventricular zone showed a positive reaction for transferrin receptors but were negative for transferrin. By 6-7 days, transferrin-positive cells were seen in the inner layer of the ventricular zone and a few 'patches' of transferrin-positive cells were also seen in the adjacent area. By 10 days, large neurons throughout the brain were strongly positive for transferrin. By 11-16 days, all neurons in the brain showed a strong positive reaction for the protein. Thereafter, the transferrin-positive reaction became gradually weaker in neurons whereas the walls of blood capillaries showed a positive reaction for transferrin. In the adult brain, neurons showed very weak or negative staining. A similar staining pattern for transferrin was observed in the developing spinal cord and dorsal root ganglia (DRG). By 10-12 days, both spinal cord neurons and DRG neurons showed strong reactions for transferrin. Thereafter, the transferrin-positive reaction gradually diminished in older spinal cord neurons and completely disappeared from DRG neurons. Cultured cerebral hemisphere, spinal cord, and DRG neurons showed positive staining reactions for both transferrin and its receptor. Our results suggest that: transferrin is initially taken up by developing neurons from cerebrospinal fluid via receptor-mediated endocytosis; the accumulation of transferrin ultimately reaches a maximum level within immunoreactive neurons and then declines just prior to hatching; in contrast to other CNS neurons, DRG neurons accumulate transferrin only briefly and then become negative for transferrin by immunocytochemistry; and after closure of the blood-brain barrier, transferrin may reach neurons by transport across capillaries into the 'paravascular' spaces. In view of these results, transferrin may play some important but unrecognized role in early neuronal development in vivo as well as in vitro.

Animals↗
PubMed1962

Studies on the transferrins of adult serum, cord serum, and cerebrospinal fluid. The effect of neuraminidase.

Nine of the twelve known variants of human transferrin have been resolved by the action of neuraminidase into stepwise patterns of four additional slower moving components whose relative intensities depended upon the concentration of enzyme. These components appeared to represent the stepwise removal of the four sialic acid residues from the transferrin molecule, and at large enzyme concentrations, almost all of the transferrin was reduced to the position of the slowest moving component. In contrast, the electrophoretic mobilities of haptoglobin, ceruloplasmin, and alpha(2)-macroglobulin showed a gradual decrease with increasing neuraminidase concentration. The transferrins of chimpanzees, rhesus and cynomolgus monkeys, and cattle were resolved by neuraminidase into two slower moving components. These experiments suggested that the primate and cattle transferrins contained only two sialic acid residues accessible to the enzyme. Transferrins C, B(2), and D(1) and a cynomolgus monkey transferrin were purified from serum by starch block electrophoresis and cellulose chromatography. Ultracentrifugal analysis could detect no difference in sedimentation rate between transferrin C, the primate transferrin, and neuraminidase-treated transferrin C. The human transferrins showed no variation in amino acid composition, but the cynomolgus transferrin was approximately 20 per cent higher in serine content and 50 per cent lower in glucosamine than human transferrin C. Reactions of antigenic identity were obtained among five human transferrin variants but a reaction of only partial identity was obtained between transferrin C and the cynomolgus transferrin. The transferrin pattern of cord blood showed a prominent band in the position of transferrin C, accompanied by four faint slower moving bands which coincided with the four transferrin components produced by the action of neuraminidase on transferrin C. The transferrin pattern of cerebrospinal fluid in individuals homozygous for serum transferrin C showed two principal components, one of which appeared to contain no sialic acid. Haptoglobin, ceruloplasmin, and alpha(2)-macroglobulin were also present in cerebrospinal fluid.

Adult↗
PubMed1996

Functional analysis of human/chicken transferrin receptor chimeras indicates that the carboxy-terminal region is important for ligand binding.

Chimeric human/chicken transferrin receptors have been constructed using the polymerase chain reaction. Different regions of the 671-residue external domain of the human transferrin receptor were replaced by the corresponding sequences from the chicken transferrin receptor. As chicken transferrin receptors do not bind human transferrin, functional analysis of such chimeric receptors provides an approach to define the ligand-binding site of the human transferrin receptor. Four of 16 chimeric human/chicken transferrin receptors expressed in chick embryo fibroblasts were efficiently transported to the plasma membrane and displayed on the cell surface. Studies of the four chimeric receptors indicated that binding of human transferrin was abolished if the carboxy terminal 192 amino acids of the human transferrin receptor (residues 569-760) were replaced with the corresponding region from the chicken transferrin receptor. Further, a chimeric receptor in which the carboxy-terminal 72 residues were derived from the chicken transferrin receptor exhibited a 16-fold decrease in binding affinity for human transferrin. In contrast, analysis of the other two chimeric receptors showed that 340 amino acids of the human transferrin receptor external domain more proximal to the transmembrane region (residues 151-490) could be replaced with the corresponding region from the chicken transferrin receptor without loss of high-affinity ligand binding. In contrast, two mAbs against the human transferrin receptor external domain, B3/25 and D65.3, that do not compete with transferrin binding, do not bind the chimeric transferrin receptors in which the membrane proximal part is replaced by chicken sequences, while they do bind the two other chimeric transferrin receptors with high affinity. These data indicate that sequence differences in the carboxy-terminal region of human and chicken transferrin receptor external domains are important for the species specificity of transferrin binding and imply that this portion of the human transferrin receptor is critical for ligand binding.

Animals↗
PubMed2000

Transferrin in patients with multiple sclerosis: a comparison among various subgroups of multiple sclerosis patients.

OBJECTIVES: To compare cerebrospinal fluid (CSF) and serum transferrin (Tf) concentrations, transferrin quotient and index in various subgroups of MS patients. MATERIAL AND METHODS: CSF and serum transferrin concentrations, transferrin quotient QTf (i.e. CSF transferrin/serum transferrin x 10(3)) and index (QTf/Qalbumin) were determined in a group of 51 patients with clinically definite or probable multiple sclerosis (MS). Patients were subdivided according to the disease form (relapsing-remitting = RR, secondary progressive = SP, primary progressive = PP; patients with RR form were further subdivided into those in the attack and those in remission), disease severity (EDSS 0-5.5, EDSS 6.0-10.0), its treatment (non-treated - including patients treated with vitamins and/ or vasodilators only, treated - i.e. glucocorticoids and/or immunosuppressants and/or (exceptionally) beta-interferon), disease duration (0-2 years, >2-10 years, > 10 years) and sex. Correlation of transferrin values with age was also performed. RESULTS: Serum transferrin was somewhat lower and significantly more frequently subnormal in PP patients in comparison with the SP form and the RR form in remission. Transferrin index was significantly higher in the PP form than in the RR as well as the SP form. Transferrin quotient was significantly more frequently subnormal in patients in remission compared to those in the attack of the RR disease. CSF transferrin as well as transferrin quotient were more frequently subnormal in patients with short disease duration (0-2 years) than in patients with longer disease duration; these parameters, however, correlated also significantly with age. CSF transferrin and transferrin quotient were higher in male than in female patients. CONCLUSION: The authors conclude that evaluation of transferrin in MS patients - along with albumin - may help to differentiate among various MS subgroups, since there are significant differences among RR, SP and PP forms. For this purpose, however, other CSF protein fractions should be evaluated in parallel in order to obtain more complex information and to establish a panel of examinations enabling multiple statistical analyses. Transferrin evaluation in MS may also be of significant theoretical interest, since transferrin is known to be involved in the regulation of iron metabolism and it may have a protective role against the oxidative stress. Moreover, transferrin is a growth factor important for proliferation of activated T lymphocytes. By means of the use of transferrin quotient and especially transferrin index, it may be possible to estimate the proportion of intra-CNS-synthesized transferrin and/or rate of specific transferrin transport across the blood-CSF barrier. Further studies are, however, needed for such an evaluation.

Age Factors↗
PubMed1980

Transferrin-binding and iron-binding proteins of rabbit reticulocyte plasma membranes. Three distinct moieties.

1. Transferrin-membrane complexes and iron-binding membrane complexes were solubilized with sodium dodecyl sulfate from the plasma membranes of reticulocytes that had been incubated with (59Fe,125I)-labeled transferrin. Gel filtration of solubilized material demonstrated 125I-labeled transferrin complexed to two moieties, a minor component (Peak I) of apparent molecular weight 435,000 and a major component (Peak II) of apparent molecular weight 200,000. Most of the membrane 59Fe was located in Peak I. 2. Sepharose-bound anti-transferrin was used to purify the 125I-labeled transferrin-membrane complexes. The 59Fe/125I ratio in the transferrin complex purified from Peak I was the same as in the original transferrin and thus contained membrane-bound transferrin to which the 59Fe was still attached. The 59Fe/125I ratio in the purified Peak II transferrin complex was 0.33 times that of the original transferrin, indicating that more than 60% of its 59Fe had been delivered to the reticulocyte. 3. The purified transferrin complexes analyzed by SDS-polyacrylamide gel electrophoresis demonstrated a single band of apparent molecular weight 78,000 both by Coomassie blue stain for protein and by 125I radioactivity. The specific activity of this material was 0.27 and 0.56 times that of the original transferrin for Peak I and Peak II, respectively, indicating that transferrin in Peak I and II was bound to a membrane component with a molecular weight similar to that of transferrin. 4. The isoelectric focusing pattern of the Peak II transferrin complex showed isoelectric points of pH 6.7 and 6.2 compared to pH 5.4 for transferrin. 5. On the basis of these studies we propose that transferrin is first bound to a membrane protein and then delivers iron to a membrane component distinct and separate from the transferrin-binding moiety. Prior to its release, transferrin markedly depleted of iron is still bound to a component in the plasma membrane.

Animals↗
PubMed1990

Effect of aluminium on iron uptake and transferrin-receptor expression by human erythroleukaemia K562 cells.

Incubation of human erythroleukaemia K562 cells with Al-transferrin inhibited iron uptake from 59Fe-transferrin by about 80%. The inhibition was greater than that produced by a similar quantity of Fe-transferrin. Preincubation of cells for 6 h with either Al-transferrin or Fe-transferrin diminished the number of surface transferrin receptors by about 40% compared with cells preincubated with apo-transferrin. Al-transferrin did not compete significantly with Fe-transferrin for transferrin receptors and, when cells were preincubated for 15 min instead of 6 h, the inhibitory effect of Al-transferrin on receptor expression was lost. Both forms of transferrin also decreased the level of transferrin receptor mRNA by about 50%, suggesting a common regulatory mechanism. Aluminium citrate had no effect on iron uptake or transferrin-receptor expression. AlCl3 also had no effect on transferrin-receptor expression, but at high concentration it caused an increase in iron uptake by an unknown, possibly non-specific, mechanism. Neither Al-transferrin nor AlCl3 caused a significant change in cell proliferation. It is proposed that aluminium, when bound to transferrin, inhibits iron uptake partly by down-regulating transferrin-receptor expression and partly by interfering with intracellular release of iron from transferrin.

Aluminum↗
PubMed1998

Receptor-mediated recognition and uptake of iron from human transferrin by Staphylococcus aureus and Staphylococcus epidermidis.

Staphylococcus aureus and Staphylococcus epidermidis both recognize and bind the human iron-transporting glycoprotein, transferrin, via a 42-kDa cell surface protein receptor. In an iron-deficient medium, staphylococcal growth can be promoted by the addition of human diferric transferrin but not human apotransferrin. To determine whether the staphylococcal transferrin receptor is involved in the removal of iron from transferrin, we employed 6 M urea-polyacrylamide gel electrophoresis, which separates human transferrin into four forms (diferric, monoferric N-lobe, and monoferric C-lobe transferrin and apotransferrin). S. aureus and S. epidermidis but not Staphylococcus saprophyticus (which lacks the transferrin receptor) converted diferric human transferrin into its apotransferrin form within 30 min. During conversion, iron was removed sequentially from the N lobe and then from the C lobe. Metabolic poisons such as sodium azide and nigericin inhibited the release of iron from human transferrin, indicating that it is an energy-requiring process. To demonstrate that this process is receptor rather than siderophore mediated, we incubated (i) washed staphylococcal cells and (ii) the staphylococcal siderophore, staphyloferrin A, with porcine transferrin, a transferrin species which does not bind to the staphylococcal receptor. While staphyloferrin A removed iron from both human and porcine transferrins, neither S. aureus nor S. epidermidis cells could promote the release of iron from porcine transferrin. In competition binding assays, both native and recombinant N-lobe fragments of human transferrin as well as a naturally occurring human transferrin variant with a mutation in the C-lobe blocked binding of 125I-labelled transferrin. Furthermore, the staphylococci removed iron efficiently from the iron-loaded N-lobe fragment of human transferrin. These data demonstrate that the staphylococci efficiently remove iron from transferrin via a receptor-mediated process and provide evidence to suggest that there is a primary receptor recognition site on the N-lobe of human transferrin.

Animals↗
PubMed1984

Role of transferrin in branching morphogenesis, growth and differentiation of the embryonic kidney.

Our previous work has suggested that transferrin is an important serum component for differentiation of the kidney. In this study we have analysed more closely the response of cultured mouse embryonic kidney to exogenous transferrin and the dependence of kidney tubule induction on transferrin. Our results show that transferrin causes a dose-dependent increase in cell proliferation in the differentiating kidney mesenchyme, but no stimulation of cell proliferation in the inductor tissue used, the embryonic spinal cord. In cultures of whole kidney rudiments a remarkable increase in the amounts of DNA and protein are caused by transferrin but not by other serum components present in a transferrin-depleted serum. The morphology of the explants was similar when cultured in the presence of human serum and in the transferrin-depleted serum supplemented with transferrin. In transferrin-containing chemically-defined medium the explants flattened and spread out, but the morphology of the kidney tubules was similar as in explants cultured in the presence of serum. Examination of the cultured explants by electron microscopy showed that in all transferrin-containing culture media the mesenchymal cells had differentiated into kidney tubules consisting of epithelial cells lined by a basement membrane. The experiments with the transferrin-depleted serum demonstrate that the main mitogen for kidney development is transferrin, and that other serum factors are mainly required for maintenance of tissue compactness. Our earlier studies have shown that exogenous transferrin is not needed for certain changes preceding overt tubule formation in the kidney mesenchyme, and we suggested that transferrin responsiveness is acquired during the induction of kidney mesenchyme. Our present results do not contradict the postulate, although they demonstrate that the acquisition of the responsiveness is more complicated than previously thought. When the mesenchyme is exposed to inductor tissue for 24 h without transferrin, and then subcultured without the inductor in the presence of transferrin, morphogenesis fails and there is no proliferation of the mesenchyme. The experiment shows that the inductor, the mesenchyme and transferrin must all three be simultaneously present for the acquisition of the transferrin responsiveness. Other experiments show that the induced mesenchyme can be a direct target tissue, since it can proliferate in response to transferrin also in the absence of the inductor. It is evident that the inductor is required for the acquisition of the responsiveness, as suggested.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Transferrin-receptor-independent but iron-dependent proliferation of variant Chinese hamster ovary cells.

The purpose of this study is to clarify the role of iron, transferrin, an iron-binding protein in vertebrate plasma, and transferrin receptors in cell proliferation. Transferrin, which is indispensable for most cells growing in tissue culture, is frequently referred to as a "growth factor". Proliferating cells express high numbers of transferrin receptors, and the binding of transferrin to their receptors that is needed for cells to initiate and maintain their DNA synthesis is sometimes regarded as analogous to other growth factor-receptor interactions. Although numerous previous experiments strongly indicate that the only function of transferrin in supporting cell proliferation is supplying cells with iron, they did not completely rule out some direct or signaling role transferrin receptors could play in cell proliferation. To address this issue, we exploited transferrin-receptor-deficient mutant Chinese hamster ovary (CHO) cells (McGraw, T. E., Greenfield, L., and Maxfield, F. R., 1987, J. Cell. Biol. 105, 207-214) in which various aspects of iron and transferrin metabolism in relation to their capacity to proliferate were investigated. Variant cells neither specifically bind transferrin nor do their extracts contain any detectable functional transferrin receptors, yet they proliferate and synthesize DNA with rates comparable to those observed with parent CHO cells. Desferrioxamine, an iron chelating agent, inhibits growth and DNA synthesis of both variant and control CHO cells. This inhibition can be fully alleviated, in both cell types, by ferric pyridoxal isonicotinoyl hydrazone, which can supply cells with a utilizable form of iron by a pathway not requiring transferrin and their receptors. Studies of 59Fe uptake and 125I-transferrin binding revealed that parent cells can take up iron by at least three mechanisms: from transferrin by receptor-dependent and -independent (nonspecific, nonsaturable, not requiring acidification) pathways and from inorganic iron salts (initially present in the medium as FeSO4). Although variant CHO cells are unable to acquire transferrin iron via the receptor pathway, two remaining mechanisms provide these cells with sufficient amounts of iron for DNA synthesis and cell proliferation. In conclusion, although transferrin receptors are dispensable in terms of their absolute requirement for proliferating cells, a supply of iron is still needed for their DNA synthesis. Transferrin-receptor-deficient CHO cells may be a useful model for investigating receptor-independent iron uptake from transferrin and nontransferrin iron sources.

Animals↗
PubMed1989

Transferrin in FRTL5 cells: regulation of its receptor by mitogenic agents and its role in growth.

Transferrin, a serum iron-binding protein, delivers iron to the cell after binding to specific receptors on the cell surface and is an important component of culture medium for virtually all cell lines, including the FRTL5 line of rat thyroid follicular cells. Therefore, we undertook studies in FRTL5 cells to examine the regulation of the transferrin receptor, the effects of transferrin on growth and differentiated functions, and the interactions of transferrin with several mitogenic pathways. FRTL5 cells possess one class of saturable transferrin receptors (Ka, 0.7 x 10(9) M-1). Binding of 125I-labeled transferrin was highest in actively growing cells and declined progressively, reaching minimal values when confluence was achieved. Removal of transferrin from culture medium caused a rapid increase in transferrin binding. TSH, acting within 5 min, induced a modest increase in transferrin binding, due to a cycloheximide-resistant increase in binding sites. Binding of transferrin after a 24-h incubation was also increased by other mitogenic agents, (Bu)2cAMP, forskolin (FK), insulin, insulin-like growth factor-I (IGF-I), and the phorbol ester TPA. Transferrin alone stimulated growth only minimally, but enhanced the mitogenic effect of TSH, (Bu)2cAMP, and FK, all of which act through the cAMP pathway. In contrast, transferrin did not alter the cAMP-independent mitogenic effects of insulin and IGF-I. Transferrin did not affect TSH-induced cAMP generation. Desferoxamine, an iron chelator, inhibited the mitogenic effects of all of the agents tested. Desferoxamine had no significant effect on TSH-induced cAMP accumulation. We conclude that FRTL5 cells contain saturable receptors for transferrin whose abundance varies with the rate of cell replication. Transferrin down-regulates its own receptors, while stimulation of growth by various mitogens is accompanied by increased binding of transferrin. Transferrin enhances the mitogenic effect of the cAMP-dependent mitogens, TSH, (Bu)2cAMP, and FK, without modifying basal or stimulated cAMP generation. In contrast, transferrin fails to affect the mitogenic responses to IGF-I and insulin, which are cAMP independent. Iron is required for the mitogenic response to various mitogens, especially those that are cAMP dependent.

Animals↗
PubMed1991

Effect of epidermal and insulin-like growth factors on vectorial secretion of transferrin by rat Sertoli cells in vitro.

Within the seminiferous tubules, the Sertoli cells create an impermeable blood-testis barrier and an unique intratubular microenvironment that fosters the development of spermatozoa. The functional differentiation of spermatozoa therefore requires vectorial secretion by Sertoli cells of substances that cannot cross the blood-testis barrier. We investigated the role of epidermal (EGF) and insulin-like growth factors I and II (IGF-I and IGF-II) in the regulation of vectorial secretion of transferrin by Sertoli cells. In order to study the regulation of vectorial transferrin secretion, we modified culture conditions in the twin chamber culture system to maximise gradients of transferrin secretion. Sertoli cells were plated at high density (3-4 x 10(6) cells/well) into chambers of near equal volume, cultured at 37 degrees C and maintained in simple, fully defined media omitting standard supplements (insulin, EGF, FSH) which affect vectorial transferrin secretion. Using this optimised culture system, maximum gradients of transferrin secretion occurred between days 2 and 3 of culture with preferential secretion (mean ratio 3.7 +/- 0.2) directed towards the apical compartment. The transferrin ratio (ratio of transferrin secreted into the upper over the lower chamber) was decreased by insulin and FSH but not by retinoic acid or testosterone, yet all four stimuli increased total transferrin secretion. IGF-I and IGF-II were effective at physiological concentrations (ED50 = 1 ng/ml) in lowering transferrin ratio and were 100-fold more potent than insulin suggesting that insulin effects on vectorial transferrin secretion by Sertoli cells is mediated through type 1 IGF receptors. EGF also reduced the transferrin ratio (ED50 = 50 ng/ml) as well as stimulating total transferrin secretion. The hormonally mediated reduction in transferrin ratio was consistently due to enhanced secretion of transferrin into the lower chamber. In the first demonstration of a highly polarised response of Sertoli cells to hormonal stimuli, the effects of insulin, FSH and EGF on vectorial transferrin secretion were effected primarily via the basal membrane of the Sertoli cell and operated independent of mechanisms controlling total transferrin secretion. These results establish a potential role for epidermal and insulin-like growth factors in the paracrine regulation of vectorial secretion by the Sertoli cell, in particular the developmental regulation of vectorial transferrin secretion by Sertoli cells. These findings also indicate that previous studies which included insulin and EGF routinely in culture media have systematically underestimated apically directed transferrin secretion.

Animals↗