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PubMed1991

A mutated transferrin receptor lacking asparagine-linked glycosylation sites shows reduced functionality and an association with binding immunoglobulin protein.

The function of the transferrin receptor is to transport iron-bound transferrin into the cell. In order to function properly, this dimeric glycoprotein must be expressed on the cell surface and be able to bind transferrin. Site-directed mutagenesis was performed to abolish the three asparagine-linked glycosylation consensus sequences of the human transferrin receptor. The DNA encoding the mutated transferrin receptor was stably transfected into mouse fibroblasts. This form of the human transferrin receptor shows reduced transferrin binding, reduced intersubunit bond formation, and reduced cell surface expression, indicating that the transferrin receptor which lacks asparagine-linked glycosylation is not fully functional. In addition, the mutated form of the receptor is not processed as quickly. It shows an association with an endoplasmic reticular chaperone protein, binding immunoglobulin protein, leading to the hypothesis that the mutated transferrin receptor experiences increased retention in the endoplasmic reticulum.

Asparagine↗
PubMed1991

Regulation of the transferrin-independent iron transport system in cultured cells.

Mammalian cells accumulate iron via the binding of transferrin to high affinity surface receptors, or through a transferrin-independent pathway which involves the uptake of iron-organic anion chelates by a membrane-based transport system. Previously we determined that the transferrin-independent transport system was present on a wide variety of cultured cells (Sturrock, A., Alexander, J., Lamb, J., Craven, C. M., and Kaplan, J. (1990) J. Biol. Chem. 265, 3139-3145). In this communication we demonstrate that the transferrin-independent iron uptake system is regulated differently than the transferrin-mediated pathway. The activity of the transferrin-independent system was unaffected by changes in cellular growth rate, induction of DNA synthesis and cell division, or depletion of cellular iron. Exposure of cells to ferric or ferrous iron, however, resulted in a time-dependent increase in transport activity, due to a change in Vmax with no change in Km. Increased transport activity was seen in a variety of cultured cell types, occurred in the presence of cycloheximide, and persisted for hours after removal of iron. The ability of other transition metals to induce changes in transport, or to compete with iron for accumulation by the transferrin-independent uptake system, was critically dependent on the composition of the media in which the cells were incubated. Metals such as Cu2+ or Zn2+, but not Cd2+ or Mn2+, when dissolved in a balanced salt solution buffered with 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid, induced changes in the transferrin-independent iron transport system. The same metals which induced changes in transport were ineffective in media containing amino acids, ascorbate, or N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine. The Vmax of the transferrin-independent iron transport system was also elevated by increases in intracellular Ca2+. The effect of iron on transport activity, however, did not result from an iron-induced release of intracellular Ca2+. These results suggest a novel form of regulation in which the presence of extracellular iron induces the appearance of previously cryptic transporters and thus accelerates the clearance of potentially toxic molecules.

Biological Transport↗
PubMed1991

Tumor necrosis factor-alpha and interleukin 1-alpha regulate transferrin receptor in human diploid fibroblasts. Relationship to the induction of ferritin heavy chain.

We have studied transferrin receptor expression in MRC5 human fibroblasts in response to tumor necrosis factor-alpha (TNF, cachectin) or interleukin 1-alpha (IL-1). Treatment of exponentially growing MRC5 cells with these cytokines led to a 3-4-fold increase in transferrin receptor mRNA and a coordinate increase in transferrin receptor protein by 24 h. Under these conditions, stimulation of [3H]thymidine incorporation was minimal, suggesting that the induction of transferrin receptor by TNF and IL-1 is mediated by a growth-independent regulatory mechanism. A study of the time course of this response showed that cytokine-mediated increases in transferrin receptor mRNA and protein proceeded after a lag of 12-24 h. A simultaneous analysis of the effects of TNF and IL-1 on ferritin in MRC5 cells was also performed. Ferritin L mRNA levels were unchanged. However, induction of ferritin H mRNA was seen within 4 h, preceding the induction of the transferrin receptor. The synthesis of ferritin H (but not ferritin L) protein peaked at 8 h after TNF or IL-1 treatment, followed by a rapid decrease in both ferritin H and L protein synthesis. As ferritin H synthesis declined, levels of transferrin receptor protein increased, reaching a maximum by 24 h. These results suggest that the cytokine-dependent induction of ferritin H and subsequent increase in the transferrin receptor are related and possibly interdependent events. This study demonstrates that the complex role of TNF and IL-1 in iron homeostasis includes modulation of the transferrin receptor.

Cell Division↗
PubMed1986

Expression of transferrin receptors in phytohemagglutinin-stimulated human T-lymphocytes. Evidence for a three-step model.

Resting human T-lymphocytes show an elevated intracellular concentration of ferritin, whereas transferrin receptors are not detectable. Stimulation by phytohemagglutinin markedly lowers their ferritin content, while inducing the synthesis of transferrin receptors. Addition of iron salts (ferric ammonium citrate) in activated T-lymphocyte cultures causes a marked enhancement of both [3H]uridine and [3H]thymidine incorporation. Nevertheless, it also induces a concentration-dependent decrease in transferrin receptor synthesis, associated with a marked rise of ferritin production. Hemin treatment exerts the same effects. Addition of picolinic acid in phytohemagglutinin-stimulated cultures causes a decrease of [3H]thymidine incorporation, whereas transferrin expression is markedly enhanced. The action of iron salts and chelators is specific for transferrin receptors, since the expression of other membrane markers of activated human T-lymphocytes (interleukin-2 receptor, insulin receptor, and HLA-DR antigen) is not modified by treatment with iron or picolinic acid. These observations suggest that expression of transferrin receptors in activated T-lymphocytes is specifically modulated by their intracellular iron level, rather than their proliferative rate. Addition of picolinic acid to resting T-lymphocytes in the absence of mitogen induces a marked decrease of their ferritin content, but not the appearance of transferrin receptors. On the basis of these results, we suggest a three-step model: (a) in resting T-lymphocytes, the gene for transferrin receptor is apparently "closed," in that it is not expressed under both normal conditions and following iron deprivation. (b) After mitogen stimulus, T-lymphocytes are reprogrammed into cell cycle progression, which necessarily entails synthesis of transferrin receptors (c) Expression of these receptors is modulated by the intracellular iron level, rather than the rate of proliferation per se.

Ferric Compounds↗
PubMed1988

Localization of transferrin within the developing vertebrate nervous system.

Transferrin is one of several serum proteins localized within neurons during development of the nervous system. The expression of transferrin receptors appears to precede the active accumulation of transferrin by neurons. The first cells immunoreactive for transferrin appear adjacent to the ventricles or to the central canal of the spinal cord. These cells then appear to migrate from this site. These neurons become progressively more immunoreactive for transferrin, attain a peak of reactivity and then lose their reaction to antitransferrin antibodies. Thus, a "window" of transferrin immunoreactivity is found. As neurons lose their reactivity to antitransferrin antibodies, glia and the walls of capillaries become positive. In the rat nervous system, the gradual decrease in intraneuronal transferrin is accompanied by an increase in mitochondrial malate dehydrogenase, an enzyme of the tricarboxylic acid cycle. Thus, the accumulation of transferrin appears to closely precede the ontogeny of oxidative metabolism in the brain. As transferrin appears transiently in all neurons, this protein may be involved in a number of other important developmental events such as the expression of dopamine D2 receptors and the period of "programmed" cell death in the spinal cord.

Animals↗
PubMed1988

Transferrin iron interactions with cultured hepatocellular carcinoma cells (PLC/PRF/5).

Hepatocellular carcinoma cells of the PLC/PRF/5 cell line had 1.9 x 10(5) transferrin receptors per tumor cell with a Kd of 1.5 x 10(-8) M. At high concentrations of transferrin the binding was not saturable. Transferrin internalization by hepatoma cells was shown by time and temperature-dependent binding studies and by pronase experiments. Transferrin recycling was confirmed by the demonstration of a progressive increase in the cellular molar ratios of iron to transferrin and by chase experiments. Ammonium chloride interfered with iron unloading. The vinca alkaloid vincristine inhibited iron and transferrin uptake. The hepatocarcinoma cells appeared to lack asialoglycoprotein receptors and therefore internalized partially desialated transferrin by the regular route. Iron uptake from transferrin was markedly inhibited by the hydrophobic ferrous chelator 2,2' bipyridine but was relatively unaffected by the hydrophilic ferric chelator desferroxamine. The implication that ferrous iron was involved in postendocytic transvesicular membrane iron transport was supported by a study in which hepatoma cells were shown to take up large amounts of ferrous iron suspended in 270 mM sucrose at pH 5.5. The interaction at this pH between surface labeled hepatoma cell extracts and ferrous iron on a Sephacryl S-300 column suggested that the postendocytic transvesicular transport of iron through the membrane was in part protein mediated. The endocytosed iron in hepatoma cells was found in association with ferritin (33%), transferrin (31%) and a low molecular weight fraction (21%).

2,2'-Dipyridyl↗
PubMed1987

Progressive desialidation of human transferrin.

Transferrin is a serum glycoprotein which contains four sialic acid residues located at the end of two branched carbohydrate structures. The presence of these four acidic residues influences the electrophoretic mobility of the transferrin molecule. Alterations in the electrophoretic mobility of transferrin may be encountered in forensic science case work, particularly in association with postmortem samples. These altered transferrins usually appear in a highly stylized "ladder" banding pattern. To determine whether these altered transferrins are the result of sialic acid removal, serum samples of known transferrin type were treated with neuraminidase. These experiments support the hypothesis that the "ladder" banding pattern of transferrin observed in some case samples is due to the removal of sialic acid residues by bacterial or endogenous neuraminidase. These studies also demonstrate that partially desialidated transferrin variants cannot be clearly typed until the sialic acid is completely stripped from the transferrin molecule. Reliable typing of partially desialidated samples can be accomplished by treating these samples with neuraminidase.

Electrophoresis, Agar Gel↗
PubMed1984

[Dynamics and significance of transferrin receptors in various human placental cell membranes].

It has been postulated that the process of iron transfer to the human fetus begins with the binding of maternal transferrin-iron complexes to placental transferrin receptors. This presentation deals with the dynamics of transferrin receptors on the cell membrane of trophoblastic cells including normal placenta cells, molar trophoblasts and choriocarcinoma cell lines (GCH-1, GCH-2, ENAMI). Results were as follows. Receptors for transferrin were observed on all the trophoblastic cell membranes tested. The transferrin receptors increased with the gestational weeks. Transferrin receptors on molar trophoblasts were approximately one-half to twice as numerous as those on normal trophoblasts. The transferrin receptors on choriocarcinoma cell lines were five to ten times as numerous as those on normal trophoblast. Thymidine uptake and native hCG secretion in choriocarcinoma cell lines increased with transferrin-iron addition to these mediums (but no increase in the beta-hCG secretion in the medium was observed). On the other hand, the transferrin receptors on these membranes decreased.

Cell Line↗
PubMed1983

Transferrin, derived from an OKT8-positive subpopulation of T lymphocytes, suppresses the production of granulocyte-macrophage colony-stimulatory factors from mitogen-activated T lymphocytes.

Purified human transferrin, when saturated with iron or zinc, decreased the production of granulocyte-macrophage colony-stimulating factors (GM-CSF) by human T lymphocytes that had been stimulated by phytohemagglutin or concanavalin-A. The iron-saturated transferrin was more active than the zinc-saturated transferrin. This effect was not seen for copper-saturated transferrin or for apotransferrin, and the inhibitory effect was seen whether production of GM-CSF occurred in the absence or presence of serum. If the lymphocytes were pretreated with monoclonal antibody against transferrin receptors, no suppressive effect with transferrin was seen. Transferrin did not have a direct effect on the granulocyte-macrophage colony or cluster-forming cells (CFU-GM) or on preformed GM-CSF. Transferrin-inhibitory activity was produced and released only from a subpopulation of T lymphocytes that had the OKT8+ antigenic phenotype. Release of this activity from OKT8+ lymphocytes, into culture medium at 37 degrees C, was first detected after 6-17 hr, but the capacity of the GM-CSF-producing lymphocytes to respond to transferrin-inhibitory activity was apparent only within the first 3 hr of placing the lymphocytes at 37 degrees C. These studies demonstrate feedback interactions confined to cells of the T-lymphocyte lineage that may be of relevance to the regulation of myelopoiesis.

Antibodies, Monoclonal↗
PubMed1982

[Biosynthesis and post-translational maturation of transferrin in subcellular fractions of rat liver].

Using [14C]leucine pulse label in vivo, the dynamics of transferrin synthesis and its intracellular transport in rat liver and the changes in the molecular sizes of the newly formed transferrin chains during this transport were investigated. The transferrin radioactivity peaks in the rough and smooth endoplasmic reticulum and in Golgi vesicles were observed on the 15th, 20th and 20-30th min after injection of the pulse label. In blood stream the [14C]transferrin was detected 30 min after the label administration, the maximum of radioactivity being observed on the 60th min. The intracellular transport of transferrin was accompanied by changes in its molecular weight. The molecular weights of the transferrin precursors from the rough and smooth endoplasmic reticulum, of immunoreactive transferrin from Golgi vesicles and of circulating transferrin are 86 000-88 000, 84 000 and 77 000, respectively. These variations in molecular weights are indicative of a complex reaction sequence of posttranslational maturation of transferrin in the liver, which includes in all probability proteolysis and glycosylation.

Animals↗
PubMed1981

The formation of Fe3+-transferrin-CO3(2-) via the binding and oxidation of Fe2+.

This paper examines the reaction pathway in which Fe2+ is bound by apotransferrin and subsequently oxidized by O2 to yield Fe3+-transferrin-CO3(2-). The time course of the reaction follows a curved first order function suggesting somewhat different reactivities of the two transferrin binding sites. The initial velocity of the oxidation reaction follows saturation kinetics with regard to apotransferrin, Fe2+, and NaHCO3. We suggest an equilibrium between these components and Fe2+-transferrin-CO3(2-). The initial velocity is a linear function of O2 concentration. This is consistent with the rate-limiting step of the overall reaction being the oxidation of the Fe2+-transferrin-CO3(2-). A second order rate constant of approximately 4 X 10(3) M-1 s-1 was estimated for the oxidation of Fe2+-transferrin-CO3(2-) by O2. Oxidation by H2O2 is about 30 times faster. The reaction velocity increases with increasing pH between pH 6.0 and 7.5 Fe3+-transferrin-anion complexes are formed by the binding and oxidation of Fe2+ iun the presence of O2 and synergistic anions. The anion is found to have a strong effect on the reaction rate and provides additional evidence for the proposed reaction route. The presence of chelating agents also strongly affects the rate of Fe3+-transferrin-CO3(2-) formation. EDTA and N-(2-hydroxyethyl)ethylenediaminetriacetic acid severely depress the rate, while other chelating reagents have a moderately inhibiting effect. Thioglycolate is found to enhance the reaction by a factor of 9. The formation of a quaternary complex consisting of thioglycolate-Fe2+-transferrin-CO3(2-) is suggested. The results are correlated with an earlier study on the reductive release of iron from transferrin (Kojima, N., and Bates, G. W. (1979) J. Biol. Chem. 254, 8847-8854).

Carbonates↗
PubMed1993

Transferrin synthesis by mouse lymph node and peritoneal macrophages: iron content and effect on lymphocyte proliferation.

Transferrin is an essential requirement for lymphocyte proliferation, because it supplies activated lymphocytes with iron needed for cell proliferation. However, during inflammation or an immune response, the iron content of circulating transferrin, which is of hepatic origin, decreases. It is hypothesized that activated lymphocytes may therefore obtain transferrin-iron from an alternative source, and we have investigated the possibility that transferrin is synthesized locally in lymphoid tissues. It was found that lymph node cells from mice stimulated in vivo with Freund's complete adjuvant were able to synthesize transferrin, and this was because of the macrophage rather than the lymphocyte population. Transferrin synthesized by mouse lymph node or peritoneal macrophages contained iron and was able to promote mouse lymphocyte proliferation. Peritoneal macrophages activated in vivo synthesized more transferrin, released more transferrin-bound iron, and were more effective than resident macrophages at enhancing lymphocyte proliferation. These results suggest that transferrin synthesized by macrophages acts in a paracrine manner to support lymphocyte proliferation, thus eliminating possible detrimental effect of hypoferremia on the immune system.

Animals↗
PubMed1998

Serum transferrin receptors are decreased in the presence of iron overload.

To test the hypothesis that the quantities of circulating transferrin receptors are reduced in iron overload, we studied serum transferrin receptors and indirect measures of iron status in 150 subjects from rural Zimbabwe. We found significant inverse correlations between serum concentrations of transferrin receptors and ferritin, the ratio of ferritin to aspartate aminotransferase, and transferrin saturation (r > or = 0.44; P < 0.001). The mean +/- SD concentration of serum transferrin receptors in 23 subjects classified as having iron overload (ferritin > 300 microg/L and transferrin saturation > 60%) was 1.55 +/- 0.61 mg/L, significantly lower than the 2.50 +/- 0.62 mg/L in 75 subjects with normal iron stores (ferritin 20-300 microg/L and transferrin saturation 15-55%; P < 0.0005) and the 2.83 +/- 1.14 mg/L in 8 subjects with iron deficiency (ferritin < 20 microg/L; P = 0.001). In keeping with the regulation of transferrin receptor expression at the cellular level, our findings suggest that serum transferrin receptors are decreased in the presence of iron overload.

Adolescent↗
PubMed1990

Transferrin receptor expression in rat liver: immunohistochemical and biochemical analysis of the effect of age and iron storage.

Hepatic transferrin receptors were studied in normal male rats at 1 to 59 wk after weaning, using immunohistochemical and biochemical techniques. The number of transferrin receptors measured and the intensity of the staining in situ decreased rapidly during the first 10 wk of life and more slowly thereafter. Immunohistochemistry further demonstrated changes in the topographical and (sub)cellular localization of the transferrin receptor. In the young rat livers, staining was almost exclusively present on hepatocytes in acinar zone 2 + 3 in a honeycomb to sinusoidal pattern. With aging, a panacinar heterogeneous and mainly sinusoidal staining of hepatocytes was more frequent. Kupffer cell positivity was more obvious as compared with the young rat livers. The observed changes in transferrin receptor expression may partly be explained by age-dependent alterations in DNA synthesis and proliferative potential of the liver cells. A series of rats were iron loaded with carbonyl iron up to 39 wk and "unloaded" by administration of a normal diet during 20 wk. In these animals, serial histochemical studies showed predominantly parenchymal (7 to 14 wk), mixed parenchymal and reticuloendothelial (39 wk) and almost exclusive reticuloendothelial siderosis (59 wk). In the siderotic livers transferrin receptor numbers tended to be lower than in the controls with significant differences after 14 and 39 wk. Immunohistochemistry showed decreased parenchymal but increased reticuloendothelial transferrin receptor expression with iron load. After the period of unloading, parenchymal transferrin receptors were virtually absent despite the negligible siderosis of these cells. In contrast, siderotic reticuloendothelial cells were intensely positive. These findings support down-regulation of parenchymal transferrin receptor resulting from iron storage. However, the positivity of siderotic reticuloendothelial cells and the absence of re-emergence of parenchymal receptors in conditions of minimal parenchymal and prominent reticuloendothelial siderosis need further elucidation.

Aging↗
PubMed1990

Characterization of the synergistic effect of insulin and transferrin and the regulation of their receptors on a human colon carcinoma cell line.

The human colon carcinoma cell line, HCT 116, can be grown in chemically defined media in the absence of exogenous growth factors. The addition of transferrin and insulin will significantly stimulate growth. The interaction of these growth factors with their receptors was studied to determine whether the synergistic action of insulin and transferrin on growth involved alterations in the growth-factor receptors. Redistribution of the transferrin receptor occurred in the presence of transferrin or transferrin plus insulin. The presence of insulin in the growth media resulted in occupation of cell-surface insulin receptors without a reduction in total insulin binding. Addition of transferrin with insulin resulted in a decrease in insulin binding to its receptor, with no alteration in receptor affinity. It appears that transferrin plays a role in regulating the insulin receptor and that this may contribute to the synergistic effect of insulin and transferrin on growth.

Carcinoma↗
PubMed1985

Receptor-mediated endocytosis of human transferrin and its cell surface receptor.

We have studied the process of transferrin endocytosis in human erythromyeloid cell line K562 using fluorescein (FL) and rhodamine (RD) labeled iron-saturated transferrin (FeTF), and a fluorescein labeled monoclonal antibody to the transferrin receptor (FL-mAB). Because the antireceptor antibody and FeTF bind to different sites on the TF receptor molecule, it is possible to simultaneously and independently follow receptor and ligand. We have measured the relative amounts of transferrin or antireceptor antibody bound in the presence or absence of proteolytic enzymes using a cell sorter. At 4 degrees C almost all of the FL-TF and the FL-mAB is surface bound in a diffuse pattern. Within minutes of elevating the temperature to 37 degrees C surface aggregates form and the FL-TF is internalized. At this time about one sixth of the transferrin is still surface bound and accessible to papain digestion. The remainder localizes in a perinuclear cluster of vesicles. Monoclonal antibody binds to the cell surface transferrin receptor but is not internalized at 4 degrees C or 37 degrees C. When unlabeled diferric transferrin is added, it promotes the uptake of the Fl-mAB. The addition of goat anti-mouse immunoglobulin also promotes FL-mAB uptake. These studies support the concept that a specific trigger is required for transferrin receptor endocytosis.

Animals↗
PubMed1996

Tritrichomonas foetus: iron acquisition from lactoferrin and transferrin.

Acquisition of iron from lactoferrin and transferrin by a parasitic protozoon Tritrichomonas foetus has been studied in vitro. Specific, time-dependent, and saturable binding of iodinated ligands to the outer membrane of T. foetus at 4 degrees C was demonstrated for 125I-labeled lactoferrin only. About 1.7 x 10(5) binding sites of a single class with Kd approximately equal to 3.6 microM was estimated by means of Scatchard analysis. Internalization of the bound lactoferrin was observed at 37 degrees C. The cell-associated radioactivity after 30 min incubation of the parasite with 125I-lactoferrin at 37 degrees C was about 3.5-fold higher than the amount bound at 4 degrees C. The majority of internalized 125I-lactoferrin was released within 15 min of cell reincubation at 37 degrees C in the presence of a 100-fold excess of nonlabeled lactoferrin. Released lactoferrin displayed unchanged mobility on autoradiography. In contrast to lactoferrin, binding of 125I-transferrin was nonspecific and did not display saturable kinetics. The growth of T. foetus in iron-restricted media was stimulated by both lactoferrin and transferrin. The ability of the cells to remove and accumulate iron from both proteins was therefore examined using 59Fe-saturated lactoferrin and transferrin. It was found that trichomonads acquired a comparable amount of iron from both lactoferrin and transferrin during 60 min incubation at 37 degrees C (495 and 577 pmole Fe/mg of protein, respectively). The pH of the assay medium (PBS) decreased from pH 7.4 to 5.6 after incubation with trichomonads. At this pH, marked release of iron from transferrin (up to 47%) but not from lactoferrin (4%) was determined in cell-free media. These results indicate that T. foetus is able to utilize both lactoferrin and transferrin to cover its iron requirements. However, mechanisms of iron acquisition from these host proteins appear to be different. Specific binding and internalization of lactoferrin suggests the possible involvement of receptor-mediated endocytosis in the acquisition of lactoferrin-bound iron, while retrieval of iron from transferrin may depend on the extracellular release of iron from this ligand.

2,2'-Dipyridyl↗
PubMed1993

The effect of lead on iron uptake from transferrin in human erythroleukemia (K562) cells.

The effect of lead on cellular iron metabolism has been investigated using human erythroleukemia (K562) cells. When the cells were cultured with 100 microM Pb2+ for 48 h, the rate of cellular iron uptake from transferrin decreased to 46% of that in untreated cells. Scatchard analysis of the binding data revealed that this reduction was the result of a decrease in the number of transferrin receptors rather than an alteration in ligand-receptor affinity. The results of immunoprecipitation of transferrin receptors on the cell surface also confirmed the decreased expression of transferrin receptors by lead-treated cells. The down-regulation of transferrin receptors by treatment with lead did not result from a decrease in the total amount of the receptor, as determined by immunoblotting. Moreover, the biosynthesis of the receptor was unaffected by lead treatment. Thus, the down-regulation of surface transferrin receptors in lead-treated cells might be due to a redistribution of receptors rather than an actual loss of receptors from the cell. Using kinetic analysis, it was shown that redistribution of the receptor did not result from the alteration in the rates of transferrin receptor recycling. A comparison of the amounts of transferrin receptor on the cell surface and in the cycling pool revealed that the sequestration of the receptor from normal flow through the cycle might cause down-regulation of the surface receptor.

Biological Transport↗