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PubMed1987

Regulation of transferrin receptor cycling by protein kinase C is independent of receptor phosphorylation at serine 24 in Swiss 3T3 fibroblasts.

Treatment of Swiss 3T3 fibroblasts with tumor-promoting phorbol diester or with platelet-derived growth factor caused the phosphorylation of the transferrin receptor by protein kinase C (Ca2+/phospholipid-dependent enzyme) at serine 24 and increased the cell surface expression of the transferrin receptor. The hypothesis that the regulation of transferrin receptor cycling by protein kinase C is causally related to the phosphorylation of the receptor at serine 24 was critically tested. Site-directed mutagenesis of the human transferrin receptor cDNA was used to substitute serine 24 with threonine or alanine residues in order to create phosphorylation defective receptors. Wild-type and mutated transferrin receptors were expressed in Swiss 3T3 fibroblasts using the retrovirus vector pZipNeoSV (X). These receptors were functionally active and caused the receptor-mediated endocytosis of diferric transferrin. Incubation of the fibroblasts with phorbol diester caused the phosphorylation of the wild-type (Ser-24) human transferrin receptor, but this treatment did not result in the phosphorylation of the mutated (Ala-24 and Thr-24) receptors. The cycling of the phosphorylation defective receptors was regulated by phorbol diester and platelet-derived growth factor in a manner similar to that observed for the wild-type receptor. We conclude that the regulation of transferrin receptor cycling by protein kinase C is independent of receptor phosphorylation at serine 24 in Swiss 3T3 fibroblasts.

Animals↗
PubMed1986

Changes in the transferrin requirement of cultured chick embryo mesoderm cells during early differentiation.

Mesodermal tissue from the chick embryo at various stages of early differentiation was cultured in hydrated gels of type I collagen in the presence and absence of transferrin. Primary mesoderm explants from primitive-streak-stage embryos responded to the presence of avian transferrin by significantly improved outgrowth which appeared to be related to the ability of the cells to attach to, and migrate in, the collagen. No evidence was obtained which suggested that this observation was dependent on increased cell proliferation. This outgrowth enhancement was not duplicated by transferrin of human origin. The avian transferrin did not produce this effect on cells cultured on plastic substrata, suggesting that the species-specific effect involves modulation by the extracellular matrix. Mesoderm explants from somite stages of development showed no increase in outgrowth in the presence of either avian or human transferrin as judged by counting the number of outwandering cells. Ultrastructural immunocytochemistry indicated surface binding of transferrin by cells in the gels, and the presence of endogenous transferrin on the surfaces of mesoderm cells in situ and in their extracellular environment. It is suggested that by binding to cell surface receptors, transferrin may be able to influence the strength of cellular adhesion to collagen and hence the capacity for cell locomotion.

Animals↗
PubMed1985

Transferrin in foetal and adult mouse tissues: synthesis, storage and secretion.

Transferrin is an important growth-promoting serum glycoprotein synthesized chiefly in the liver in adults. The transferrin found in the mouse foetus is thought to be wholly a product of the foetus itself and its synthesis starts at lest as early as the 7th day of gestation. The major sites of synthesis in mouse foetuses are the visceral yolk sac (VYS) and liver (Adamson, 1982). We now report that other murine foetal tissues synthesize readily detectable amounts, namely lung, spleen, spinal cord and rib cage. Very low levels are also synthesized by the brain, muscle and pancreas. We can detect no synthesis of transferrin in late foetal thymus, heart or skin although mid-gestation foetal skin may make a very small amount. No synthesis of transferrin can be detected in adult brain, lung and spleen, but approximately equal rates of synthesis are detected in adult liver and adult ear pinna. Transferrin is accumulated by foetal and adult tissues in widely varying amounts and these have been measured by enzyme-linked immunosorbent assays of extracts. In addition to VYS and liver, high levels of transferrin are found in foetal skin, lung and rib cage with lower amounts in spinal cord, spleen and muscle tissues. Tissues of the 15th day foetus accumulate the highest concentrations of transferrin. A role for the mediation of transferrin in the stimulation of growth and differentiation by interaction tissues is discussed.

Animals↗
PubMed1979

The metabolism of transferrin in burned patients.

The metabolism of transferrin was studied in 17 selected burned patients. Transferrin metabolism also was studied in 5 of the patients after healing. The concentration of plasma transferrin in burned patients, measured by radial immunodiffusion, was reduced to 300 mg% for the first week following injury. Thereafter it rose steadily reaching the normal concentration of 350-360 mg% 35 days after injury. Traces of transferrin were present in urine a week after injury. Transferrin was labeled by injecting 14C-glucosamine intravenously following injury and again after healing was complete. Labeled transferrin was isolated from serum by specific immunoprecipitation. The specific activity of transferrin reached a peak 3-6 hr after injury and 9-12 hr after healing. The half-life time was 4.5 days during injury and 9.6 days after healing. The data indicated that both synthesis and catabolism of transferrin were accelerated in injury.

Burns↗
PubMed1981

Plasma transferrin and the relation to iron status in patients with chronic uremia.

The relations between plasma transferrin, iron status and plasma albumin were studied in 76 patients with chronic uremia (23 non-dialyzed, 18 peritoneal dialyzed and 35 hemodialyzed). Patients with reduced (grade O) hemosiderin marrow iron had higher plasma transferrin (P less than 0.001), lower serum iron (P less than 0.01), transferrin saturation (P less tha 0.001) and serum ferritin (P less than 0.001), and higher iron absorption (P less than 0.001) than patients with "normal" (grade 1+) marrow iron. There were no significant differences between transferrin levels in the three groups of uremic subjects, when patients with identical marrow iron grade were compared. Plasma transferrin was correlated both to serum ferritin (r = -0.59, P less than 0.001) and iron absorption (r = 0.56, P less than 0.001). Patients with grade O marrow iron had normal transferrin levels compared with a healthy control group of 75 subjects, while levels in patients with grade 1+ marrow iron were lower than in controls (P less than 0.001). Plasma albumin displayed significant variations between the groups, being almost normal in non-dialyzed, slightly lower in hemodialyzed and lowest in peritoneal dialyzed patients. There was no correlation between plasma transferrin and plasma albumin. Due to its close association with iron metabolism, plasma transferrin is unsuitable as a marker of protein depletion, whereas plasma albumin seems to be a better indicator of protein status in uremic subjects.

Absorption↗
PubMed1982

Unfolded transferrin polypeptide chain is immunologically crossreactive with similar derivatives of serum albumin and alpha-fetoprotein.

Radioimmunoassays utilizing reduced and carboxymethylated (RC) proteins as antigens reveal a cross-reactivity between alpha-fetoprotein (AFP) and albumin. Similar assays were used to study the relationships of AFP and albumin to other serum proteins. Of the several serum proteins tested, transferrin showed the most similarity with AFP and albumin. There was no cross-reactivity of the native proteins, but antisera prepared against RC-albumin and RC-AFP bound 125I-labeled RC-transferrin at high titers, and antiserum to RC-transferrin bound labeled RC-AFP but not RC-albumin. Inhibition assays utilizing binding of 125I-RC-AFP or 125I-RC-transferrin to anti-RC-albumin showed that the RC derivatives of AFP, albumin, and transferrin were equally efficient inhibitors, whereas other serum proteins inhibited much less. The serum vitamin D carrier protein (Gc protein) showed intermediate reactivity. The reactivities of the antisera to RC-albumins with RC-transferrin and RC-Gc protein were corroborated by immunostaining of proteins separated by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and transferred to nitrocellulose filters. These antisera stained the bands formed by RC derivatives of albumin, AFP, transferrin, and Gc protein, but not other proteins tested. AFP and albumin are known to have amino acid sequence homology. Our results suggest that transferrin and possibly also Gc protein may be structurally related to AFP and albumin.

Amino Acid Sequence↗
PubMed1983

[Binding of 67 Ga and 59 Fe to ferritin or transferrin].

The bindings of 67Ga and 59Fe to ferritin or transferrin in vitro has been investigated. Affinity constants have been measured using the equilibrium dialysis, and the results have been obtained as follows: 1 Apo-ferritin could not bind to 67Ga until it was transformed into ferritin in presence of Fe-citrate. On the contrary, the affinity of 67Ga to ferritin was reduced when Fe was released from ferritin; thus indicating that Fe-core has been required for the binding of 67Ga to ferritin. 2 Binding of 67Ga to ferritin was inhibited with apo-transferrin, and this was also shown in the case of 59Fe. In the presence of NaHCO3 or citrate, more remarkable inhibitions were observed. NaHCO3 or citrate was found to give a synergistic effect on the binding of 67Ga to transferrin, as well as Fe-transferrin. Therefore, both 67Ga and 59Fe could not bind to ferritin in the state of 67Ga- or 59Fe-transferrin. 3 The release of 59Fe from 59Fe-transferrin was enhanced with adenosine triphosphate (ATP), citrate, or ascorbic acid, while any of these reagents did not affect the release of 67Ga from 67Ga-transferrin. The comparison of 59Fe and 67Ga through their bindings to ferritin or transferrin has suggested one of points to distinguish 67Ga from 59Fe in the cell.

Apoferritins↗
PubMed1983

An electron-microscope autoradiographic study of transferrin endocytosis by immature erythroid cells.

The receptor-mediated endocytosis of 125I-transferrin by immature erythroid cells was studied using the technique of quantitative electron microscope autoradiography. Morphometric analysis of the grain distribution in erythroid cells from the foetal rat liver revealed that the 125I-transferrin radioactivity was localized mainly to intracellular vesicles (61%) and the cell membrane (25%) after 20 min incubation at 37 degrees C. No activity was found associated with the nucleus or mitochondria and only a small amount with the cytosol (13%). In erythroid cells which possessed a prominent Golgi complex, most of the autoradiographic grains were associated with vesicles located in this region, giving rise to a polar distribution of the 125I-transferrin. Uptake of transferrin was found to be maximal at the basophilic normoblast stage of development and then declined progressively during maturation to the reticulocyte. The kinetics of endocytosis of 125I-transferrin by rabbit reticulocytes was also studied by electron microscope autoradiography. Up to 30% of the cell-bound transferrin was internalized almost immediately upon incubation at 37 degrees C. After 30 sec incubation, 42% of the cell-bound 125I-transferrin was estimated to be internal and this rose to almost 70% at steady state between the binding and release of transferrin after 12 min incubation.

Animals↗
PubMed1983

The primary structure of human serum transferrin. The structures of seven cyanogen bromide fragments and the assembly of the complete structure.

The amino acid sequences of seven cyanogen bromide fragments of human serum transferrin have been determined, and the primary structure of transferrin established by determining the order of these and three additional fragments (Sutton, M. R., MacGillivray, R. T. A., and Brew, K. (1975) Eur. J. Biochem. 51, 43-48) in the polypeptide chain. The order of the fragments was deduced from peptides that overlap methionyl residues which were obtained by thermolysin digestion of performic acid-oxidized transferrin or by partial peptic hydrolysis of unmodified transferrin, together with other evidence. The polypeptide chain of transferrin contains 679 amino acid residues, which together with the two N-linked oligosaccharide chains gives a calculated molecular weight of 79,570. Transferrin consists of two homologous domains (residues 1-336, 337-679), each associated with a single Fe-binding site, with both sites of glycosylation in the carboxyl-terminal domain at positions 413 and 611. Consideration of the primary structure in relation to previously published results provides information concerning the evolutionary development of transferrins and related proteins, and the locations of metal-binding residues in the transferrin molecule.

Amino Acid Sequence↗
PubMed1982

Iron transfer between the purple phosphatase uteroferrin and transferrin and its possible role in iron metabolism of the fetal pig.

Uteroferrin, a purple-colored, iron-containing phosphatase which is induced by progesterone in the porcine uterus, has been proposed to be an intermediary in iron transfer between the mother and conceptus in the pig. Along with a number of other uterine proteins of maternal origin, it accumulates in the allantoic fluid during mid-pregnancy. When [59Fe]uteroferrin was introduced into the allantoic sacs of five fetuses at Day 60 of pregnancy, its iron was transferred to another protein, identified as transferrin. The half-life of iron loss from uteroferrin was approximately 24 h and the kinetics suggested an approximately second order process. 59Fe was also distributed to the fetus and was recovered at high specific activity in the fetal red blood cells (as hemoglobin), liver, and spleen. Introduction of [59Fe] transferrin to allantoic fluid resulted in an almost identical distribution of isotope in the fetus as was observed with [59Fe]uteroferrin. Whereas [125I]transferrin has been shown to be capable of leaving the allantoic sac in intact form, [125I]uteroferrin either has only a limited capacity to exit or else is almost immediately degraded upon entering the fetal circulation. Following loss of its iron to transferrin in allantoic fluid, uteroferrin rapidly loses immunological reactivity and is cleaved proteolytically into small peptides. The transfer of iron between uteroferrin and transferrin has also been followed in vitro. Iron on uteroferrin is relatively stable to pH and is not readily lost to transferrin at around neutral pH unless low molecular iron chelators such as citrate, pyrophosphate, ATP, or ascorbate are present. Iron transfer was promoted at ascorbate concentrations (approximately 50 microM) and at pH values (approximately 6.8) approximating those found in allantoic fluid. Results are consistent with a model in which maternal uteroferrin can transfer its iron to fetal transferrin via a low molecular weight intermediary in the allantoic sac. Transferrin is then responsible for iron distribution to the fetus.

Acid Phosphatase↗
PubMed1993

Cloning and characterization of Atlantic salmon (Salmo salar) serum transferrin cDNA.

Characterization of Atlantic salmon serum transferrin cDNA representing a near full-length transferrin mRNA revealed a 2,070-bp open reading frame encoding a protein of 690 amino acids. The predicted protein contains an 18 amino acids' long signal sequence and shows 49% amino acid positional identity with Xenopus laevis transferrin and human serum transferrin. On the basis of sequence differences obtained from different salmon transferrin cDNA clones, 2 distinct classes of transferrin mRNA were identified. Both variants were present together in genomic DNA from haploid embryos, demonstrating that Atlantic salmon have 2 transferrin genes per haploid complement. Salmon transferrin is expressed mainly in the liver.

Amino Acid Sequence↗
PubMed1998

Transferrin production by the ciliary body of rabbits: a biochemical and immunocytochemical study.

PURPOSE: We have previously reported that transferrin is one of several glycoproteins synthesized within the eye and secreted into the vitreous. The present investigation was designed to determine the role of the ciliary body in the production of this vitreous transferrin. METHODS: Isolated ciliary body-iris were incubated with 3H-fucose, 3H-tyrosine or 35S-methionine and afterwards the culture media were processed for affinity chromatography using columns of Sepharose conjugated with antibody to rabbit plasma transferrin. Reverse transcription-polymerase chain reaction (RT-PCR) was carried out using total RNA extracted from fresh ciliary body-iris and primers constructed on the basis of the known sequence of transferrin mRNA from rabbit liver. The fragment obtained was employed as a probe in northern-blots of total RNA of ciliary body-iris. Furthermore, paraffin sections of eyes were treated for immunocytochemical visualization of transferrin. RESULTS: A labeled polypeptide, specifically eluted from the antitransferrin columns, was detected in the incubation medium, transferrin mRNA was found in extracts of whole ciliary body-iris, and transferrin antigenicity was identified in the ciliary and iridial epithelial cells by immunocytochemistry. CONCLUSIONS: These results demonstrate the ciliary epithelium as one of the sources of the vitreous transferrin.

Animals↗
PubMed2001

Entamoeba histolytica: transferrin binding proteins.

Entamoeba histolytica trophozoites depend on iron for their growth; thus, they must use some host iron-containing molecules to fulfill this requirement. In this work we report that amoebas are able to utilize human holo-Tf as iron source and to recognize it through transferrin binding proteins. By use of an anti-human transferrin antiserum in an immunoblotting assay, two main polypeptides with apparent molecular masses of 70 and 140 kDa were found in total extract of trophozoites cultured in vitro. However, when a monoclonal anti-human transferrin receptor antibody was used, only one band with molecular mass of 140 kDa was observed. Both the human transferrin and the monoclonal antibody recognized a protein on the amoebic surface, demonstrated by confocal microscopy. Furthermore, the complex transferrin-transferrin binding protein was internalized by an endocytic process and probably dissociated inside the cell. This mechanism could be one manner in which E. histolytica acquires iron from the human host transferrin.

Animals↗
PubMed1999

Non-transferrin-bound iron and hepatic dysfunction in African dietary iron overload.

BACKGROUND: Circulating iron is normally bound to transferrin. Non-transferrin-bound iron (NTBI) has been described in most forms of iron overload, but has not been studied in African dietary iron overload. This abnormal iron fraction is probably toxic, but this has not been demonstrated. METHODS: High-pressure liquid chromatography was used to assay serum NTBI in 25 black African subjects with iron overload documented by liver biopsy and in 170 relatives and neighbours. Levels of NTBI were correlated with indirect measures of iron status and conventional liver function tests. RESULTS: Non-transferrin-bound iron (> 2 micromol/L) was present in 43 people, 22 of patients of whom underwent liver biopsy and 21 relatives and neighbours. All but four of these had evidence of iron overload on the basis of either liver biopsy or elevated transferrin and serum ferritin concentrations. Among all 195 subjects, the presence of NTBI in serum was independently related to elevations in alanine and aspartate aminotransferase activity and bilirubin concentration. This relationship between serum NTBI and hepatic dysfunction was confirmed in the subgroup of 25 subjects with iron overload documented by liver biopsy. Non-transferrin-bound iron correlated significantly with elevations in alanine and aspartate aminotransferase activities after adjustment for hepatic iron grades, inflammation and diet. CONCLUSIONS: Non-transferrin-bound iron was found to be commonly present in African patients with dietary iron overload and to correlate with transferrin saturation and serum ferritin concentration. The independent relationship between NTBI and elevated liver function tests suggests that it may be part of a pathway leading to hepatic injury.

Aspartate Aminotransferases↗
PubMed2003

Rhodococcus equi and genetic susceptibility: assessing transferrin genotypes from paraffin-embedded tissues.

Rhodococcus equi is a bacterial pathogen, ubiquitous in the soil, that infects many foals and is lethal to some. Transferrin is an iron-binding protein that has bacteriostatic properties in the blood. Transferrin is also highly polymorphic in most species, with 15 variants identified for horses using biochemical methods, and may be responsible for variation in susceptibility to bacterial pathogens. The objectives of this study were 1) to compare biochemical typing with DNA typing of transferrin, 2) to determine if transferrin DNA from archival paraffin-embedded tissue samples can be recovered and typed with molecular methods, and 3) to determine if there was an association between foal death caused by R. equi and transferrin type. Comparing biochemical methods and DNA sequencing for 41 horses demonstrated correspondence between the typing methods. The allele frequency of archival paraffin-embedded tissue samples from 34 Thoroughbred foals that succumbed to R. equi showed an excess of the F allele and a deficiency of the D allele (P < 0.05). Year of collection (P > 0.8), age of foal (P > 0.3), and sex of foal (P > 0.6) were not statistically associated with transferrin type. The archival material was successfully transferrin typed using DNA sequencing, and there may be an association between foal death caused by R. equi and transferrin type.

Actinomycetales Infections↗
PubMed1996

Transferrin and its receptor in the development of genetically determined neural tube defects in the mouse embryo.

The iron-binding growth factor transferrin is taken up and localised in the hindgut of midgestation mouse embryos. We investigated whether the distribution of transferrin may be disturbed in mutant curly tail embryos, a proportion of which exhibit a cell proliferation defect affecting the hindgut endoderm, as part of the pathogenetic sequence leading to development of neural tube defects. Immunostaining revealed a reduction in the binding and/or uptake of transferrin by hindgut epithelial cells in affected curly tail embryos compared with their unaffected littermates. There was no apparent difference between the two embryo types, however, in the distribution or level of expression of the transferrin receptor. The receptor is expressed specifically in the hindgut endoderm of the 10.5-day embryo, although its mRNA is present in all tissues of the posterior neuropore region, suggesting posttranscriptional control of gene expression. These findings may indicate a role for transferrin binding and/or uptake in the regulation of cell proliferation in the hindgut endoderm, with a defect in this process in the curly tail mutant. However, an alternative explanation is suggested by our finding that transferrin immunostaining is more intense in the hindgut of unaffected curly tail embryos than in nonmutant CBA/Ca and CD-1 embryos. Thus, mutant embryos may increase their uptake of transferrin in an attempt to compensate for defective cell proliferation in the hindgut resulting from a defect in another pathway. Only a proportion of embryos are able to mount this compensatory response leading to the observed partial penetrance of developmental defects in the curly tail mutant mouse.

Animals↗
PubMed1994

Receptor-independent uptake of transferrin-bound iron by reticulocytes.

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

Animals↗
PubMed1993

Transferrin receptors associate with drug resistance in cancer cells.

Transferrin mediates its growth promoting properties through transferrin receptors, which are increased on cancer cells. We have studied drug-sensitive and drug-resistant cancer cells by using quantitative flow cytomerty with fluorochrome labelled transferrin and antibody to transferrin receptor, and we have found that drug-resistant cells have more transferrin receptors than drug-sensitive cells. This finding was confirmed and extended by using isotopically labelled ligand and by correcting for differences in cell surface areas. We also found that the number of transferrin receptors could be down-regulated by calcium channel blockers, and that such down-regulation diminished drug resistance, suggesting a hitherto unrecognised role for transferrin receptors in drug resistance.

ATP Binding Cassette Transporter, Subfamily B, Mem↗