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PubMed1999

The transferrin receptor: role in health and disease.

The transferrin receptor is a membrane glycoprotein whose only clearly defined function is to mediate cellular uptake of iron from a plasma glycoprotein, transferrin. Iron uptake from transferrin involves the binding of transferrin to the transferrin receptor, internalization of transferrin within an endocytic vesicle by receptor-mediated endocytosis and the release of iron from the protein by a decrease in endosomal pH. With the exception of highly differentiated cells, transferrin receptors are probably expressed on all cells but their levels vary greatly. Transferrin receptors are highly expressed on immature erythroid cells, placental tissue, and rapidly dividing cells, both normal and malignant. In proliferating nonerythroid cells the expression of transferrin receptors is negatively regulated post-transcriptionally by intracellular iron through iron responsive elements (IREs) in the 3' untranslated region of transferrin receptor mRNA. IREs are recognized by specific cytoplasmic proteins (IRPs; iron regulatory proteins) that, in the absence of iron in the labile pool, bind to the IREs of transferrin receptor mRNA, preventing its degradation. On the other hand, the expansion of the labile iron pool leads to a rapid degradation of transferrin receptor mRNA that is not protected since IRPs are not bound to it. However, some cells and tissues with specific requirements for iron probably evolved mechanisms that can override the IRE/IRP-dependent control of transferrin receptor expression. Erythroid cells, which are the most avid consumers of iron in the organism, use a transcriptional mechanism to maintain very high transferrin receptor levels. Transcriptional regulation is also involved in the receptor expression during T and B lymphocyte activation. Macrophages are another example of a cell type that shows 'unorthodox' responses in terms of IRE/IRP paradigm since in these cells elevated iron levels increase (rather than decrease) transferrin receptor mRNA and protein levels. Erythroid cells contain the highest mass of the total organismal transferrin receptors which are released from reticulocytes during their maturation to erythrocytes. Hence, plasma contains small amounts of transferrin receptors which represent a soluble fragment of the extracellular receptor domain. Measurements of serum transferrin receptor concentrations are clinically useful since their levels correlate with the total mass of immature erythroid cells.

Animals↗
PubMed1995

Non-iron mediated alteration in hepatic transferrin gene expression in the nephrotic rat.

Both transferrin and the iron it carries are lost in the urine in the nephrotic syndrome. Patients may develop hypochromic microcytic anemia and synthesis of transferrin, a protein regulated in large part by iron availability, is increased. Transferrin synthesis has also been reported to be increased in liver slices from rats with hereditary analbuminemia, and their plasma transferrin levels are increased, suggesting that transferrin synthesis may be stimulated by processes other than iron depletion in this hypoalbuminemic condition. Transferrin metabolism was studied in rats with Heymann nephritis (HN), in a strain of Sprague-Dawley (SD) rats with hereditary analbuminemia [Nagase analbuminemic rats (NAR)], and in normal SD rats. Plasma transferrin concentration and mass was decreased significantly in HN, but increased in NAR. Transferrin synthesis was increased both in NAR (measured either as the disappearance of [125I] labeled transferrin or as the incorporation of [3H] phenylalanine) and in HN (incorporation of [3H] phenylalanine). The fractional rate of transferrin catabolism was unchanged in NAR. Thus transferrin mass was increased in NAR entirely as a consequence of increased synthesis. Transferrin and albumin synthesis correlated with one another in both HN and SD (P < 0.001). Transferrin mRNA was increased in both HN and NAR and was unaffected by administration of iron to HN. Hepatic transferrin and albumin mRNA levels were also correlated positively in HN and SD, suggesting that increased hepatic synthesis of both proteins might be responding to the same stimuli. Transferrin gene transcription was increased in both HN and NAR and was unaffected by administration of iron to HN. Transferrin mRNA was not increased in the testis in either HN or NAR, suggesting that augmentation in transferrin gene expression is driven by a non-iron dependent process and is confined to the liver.

Albumins↗
PubMed2000

Transferrin microheterogeneity in human perilymph.

OBJECTIVES/HYPOTHESIS: Assay for beta2-(asialo-) transferrin has been advocated for use in diagnosis of cerebrospinal fluid (CSF) leak or perilymphatic fistula based on the fact that it is present in these fluids but not in serum. Quantitation of the sensitivity of transferrin assays has not been reported previously. The present study was undertaken to quantify the sensitivity of a microelectrophoretic assay of beta2-transferrin and assess its potential applicability to clinical diagnosis of perilymphatic fistula. STUDY DESIGN: The initial part of the study was a prospective bench biochemistry assessment of assay sensitivity and reliability. Subsequent application of the assay was a blinded prospective clinical trial. METHODS: Transferrin is a ubiquitous monomeric glycoprotein consisting of 679 amino acids, two iron-binding sites, and two N-linked complex glycan chains. The N-glycan chains branch in variable degree, carrying from zero to eight sialic acid residues. This variation in sialylation has been termed "microheterogeneity." When both iron-binding sites are saturated, the microheterogeneity of sialic acid content results in isoelectric points ranging from pH 5 to pH 6. Thus these nine transferrin variants can be distinguished by isoelectric focusing. Samples of transferrin solution or body fluids (serum, CSF, and perilymph) were incubated in iron-loading buffer to saturate both iron-binding sites and then subjected to isoelectric focusing (IEF). The separated proteins were immunoprecipitated in the IEF gel and silver stained for visualization. Serial dilutions of pure transferrin solution were used to determine assay sensitivity. Neuraminidase was used to digest sialic acid side chains from pure transferrin in solution, and the reaction product was used as a reference standard for comparison to assay of unknown fluids. Patient inner ear fluid samples obtained during stapedectomy or cochlear implantation were used to assess clinical applicability of the assay. RESULTS: This microelectrophoretic technique, using only 0.3 microL of iron-loaded sample, was able to consistently detect less than 250 pg of transferrin in solution and separate the different sialylation variants based on their isoelectric points. Assay of patient serum samples clearly demonstrated transferrin microheterogeneity. Assay of CSF consistently showed the predicted beta2-(asialo-) transferrin band. Assay of inner ear fluid samples also demonstrated transferrin microheterogeneity. However, no inner ear fluid samples had detectable levels of beta2-transferrin. Presumably, perilymph sample dilution during iron loading and by admixture with serum, local anesthetic, or middle ear secretions lowered the beta2-transferrin concentration below the detection limen of the assay. CONCLUSIONS: Microelectrophoretic assay of iron-loaded transferrin can detect as little as 250 pg of protein and can identify microheterogeneity in serum, CSF, and perilymph. However, dilutional effects of sample handling and preparation can lower the beta2-transferrin concentration of inner ear fluid samples below the detection limen of the assay. Thus, depending on the relative amounts of serum and perilymph (or CSF) in a mixed sample, electrophoretic separation of transferrin variants may not be diagnostic.

Fistula↗
PubMed1988

Killing of K562 cells with conjugates between human transferrin and a ribosome-inactivating protein (SO-6).

Cellular iron uptake is mediated by binding of transferrin with specific surface receptors and internalization of the Fe-transferrin-receptor complex. This has been examined as a possible pathway for carrying into leukaemic cells a ribosome-inactivating protein (RIP), SO-6, derived from Saponaria officinalis. Purified human differic transferrin was conjugated with SO-6 and a pool of proteins was obtained, with variable numbers of SO-6 molecules linked to a single transferrin molecule. Human erythroleukaemic K562 cells were grown in the presence of human transferrin, SO-6 and human transferrin conjugated with SO-6. The conjugate was found to be internalized via binding with transferrin receptor. Whereas the presence of unconjugated human transferrin and SO-6 in the medium did not significantly influence K562 cell growth, the conjugated proteins displayed an inhibitory activity on cell proliferation. This was maximal after 72 h at a transferrin concentration of 10(-9) M, with about 50% of cells being killed. Bovine transferrin, present in fetal calf serum, did not appear to compete with human diferric transferrin in binding to K562 cells in suspension culture. In a clonogenic assay, colony formation by leukaemic cells was not influenced by free SO-6 or transferrin, whereas the conjugated proteins were markedly inhibitory (about 100% at 10(-9) M). Our findings indicate that SO-6 can be efficiently carried into mammalian cells via the transferrin-transferrin receptor cycle and exert its ribosome inactivating activity. This is in keeping with the existence of an alternative pathway of transferrin endocytosis in addition to the classic acidic endosome pathway. From a practical viewpoint, conjugates between transferrin and SO-6 can be useful tools for studying the expression of transferrin receptors, and deserve also to be investigated for a possible use in cancer therapy.

Humans↗
PubMed1982

Transferrin receptors and gallium-67 uptake in vitro.

The relationship was studied between the number of transferrin-receptor positive cells and in vitro uptake of 67Ga and 125I-labeled transferrin in human cell lines, including two normal cell lines (WI-38 and foreskin fibroblasts), two transformed cell lines (AV-3 amnionic cells and Chang liver cells), and two neoplastic cell lines (HEp-2, larynx cancer and HeLa, cervical cancer). Transferrin receptors were determined by an indirect immunofluorescence technique based on their ability to bind purified human transferrin. Gallium-67 uptake was determined after a 24-h incubation of cells with Ga-67 in the presence and absence of transferrin (0-2.5 mg/ml). 125I-labeled transferrin uptake was also obtained after a 24-h incubation. The fraction of cells with transferrin receptors was low in normal cell lines (3% and 9%), intermediate with transformed cell lines (33% and 61%). Transferrin stimulated 67Ga uptake by all six cell lines. However, there was poor correlation between the number of transferrin-receptor positive cells and 67Ga uptake either in the presence (r = 0.21) or absence (r = 0.46) of human transferrin. Likewise, there was poor correlation between the number of transferrin-receptor positive cells and 125I-labeled transferrin uptake (r = 0.35). In contrast, the correlation between 67Ga uptake (in the presence of transferrin) and 125I-labeled transferrin uptake was highly significant (r = 0.96). These results suggest that human cell lines in culture are capable of both transferrin-dependent and transferrin-independent uptake of 67Ga.

Amnion↗
PubMed1984

Immunoregulation by low density lipoproteins in man. Inhibition of mitogen-induced T lymphocyte proliferation by interference with transferrin metabolism.

Human low density lipoprotein (LDL, d = 1.020-1.050 g/ml) inhibits mitogen-stimulated T lymphocyte DNA synthesis. Because both LDL and transferrin bind to specific cell surface receptors and enter cells by the similar means of receptor-mediated endocytosis, and because transferrin is necessary for lymphocyte DNA synthesis, we investigated the possibility that LDL may inhibit mitogen-stimulated lymphocyte responses by interfering with transferrin metabolism. LDL inhibited mitogen-stimulated lymphocyte [3H]thymidine incorporation in a concentration-dependent manner. The degree of inhibition was most marked in serum-free cultures, but was also observed in serum-containing cultures. The addition of transferrin not only augmented mitogen-induced lymphocyte [3H]thymidine incorporation in serum-free medium but also completely reversed the inhibitory effect of LDL in both serum-free and serum-containing media. Similar results were obtained when lymphocyte proliferation was assayed by counting the number of cells in culture. Transferrin also reversed the inhibition of lymphocyte responses caused by very low density lipoproteins and by cholesterol. The ability of transferrin to reverse the inhibitory effect of lipoproteins was specific, in that native but not denatured transferrin was effective whereas a variety of other proteins were ineffective. These results indicate that LDL inhibits mitogen-stimulated lymphocyte responses by interfering with transferrin metabolism. LDL only inhibited lymphocyte responses after a 48-h incubation if present from the initiation of the culture. By contrast, transferrin reversed inhibition when added after 24 h of the 48-h incubation. LDL did not inhibit lymphocyte responses by nonspecifically associating with transferrin. In addition, the acquisition of specific lymphocyte transferrin receptors was not blocked by LDL. Moreover, transferrin did not prevent the binding and uptake of fluorescent-labeled LDL by activated lymphocytes. Furthermore, LDL did not prevent the binding of transferrin to its receptor. Finally, LDL inhibition did not require specific high affinity cell surface receptors for cholesterol transport by LDL because similar inhibition and reversal by transferrin were observed with lymphocytes from a patient with homozygous familial hypercholesterolemia. Thus, LDL alters lymphocyte responses in a non-LDL receptor-mediated way by interfering with transferrin metabolism after specific binding of transferrin to receptors on activated lymphocytes.

Adult↗
PubMed2000

Amonabactin-mediated iron acquisition from transferrin and lactoferrin by Aeromonas hydrophila: direct measurement of individual microscopic rate constants.

The effectiveness and mechanism of iron acquisition from transferrin or lactoferrin by Aeromonas hydrophila has been analyzed with regard to the pathogenesis of this microbe. The ability of A. hydrophila's siderophore, amonabactin, to remove iron from transferrin was evaluated with in vitro competition experiments. The kinetics of iron removal from the three molecular forms of ferric transferrin (diferric, N- and C-terminal monoferric) were investigated by separating each form by urea gel electrophoresis. The first direct determination of individual microscopic rates of iron removal from diferric transferrin is a result. A. hydrophila 495A2 was cultured in an iron-starved defined medium and the growth monitored. Addition of transferrin or lactoferrin promoted bacterial growth. Growth promotion was independent of the level of transferrin or lactoferrin iron saturation (between 30 and 100%), even when the protein was sequestered inside dialysis tubing. Siderophore production was also increased when transferrin or lactoferrin was enclosed in a dialysis tube. Cell yield and growth rate were identical in experiments where transferrin was present inside or outside the dialysis tube, indicating that binding of transferrin was not essential and that the siderophore plays a major role in iron uptake from transferrin. The rate of iron removal from diferric transferrin shows a hyperbolic dependence on amonabactin concentration. Surprisingly, amonabactin cannot remove iron from the more weakly binding N-terminal site of monoferric transferrin, while it is able to remove iron from the more strongly binding C-terminal site of monoferric transferrin. Iron from both sites is removed from diferric transferrin and it is the N-terminal site (which does not release iron in the monoferric protein) that releases iron more rapidly! It is apparent that there is a significant interaction of the two lobes of the protein with regard to the chelator access. Taken together, these results support an amonabactin-dependent mechanism for iron removal by A. hydrophila from transferrin and lactoferrin. The implications of these findings for an amonabactin-dependent mechanism for iron removal by A. hydrophila from transferrin and lactoferrin are discussed.

Aeromonas hydrophila↗
PubMed1985

Maternal transferrin uptake by and transfer across the visceral yolk sac of the early postimplantation rat conceptus in vitro.

Uptake and transfer of maternal transferrin by rat embryos during organogenesis in vitro was investigated using radiolabelled rat transferrin and rocket immunoelectrophoresis. Colloidal gold to which rat transferrin was adsorbed was used as an electron microscopical marker in order to follow the route taken by internalised transferrin across the visceral yolk sac. Culture of rat conceptuses from 9.5 to 11.5 days of gestation in rat or human sera resulted in the passage of rat or human transferrin from the culture medium into the extraembryonic coelom as determined by quantitative immunoelectrophoretic analysis of exo-coelomic fluid. The concentration of human transferrin which was transferred to the exo-coelomic fluid of conceptuses cultured in whole human serum at 10.5 days and 11.5 days of gestation was similar to the concentration of rat transferrin in the fluid of conceptuses cultured in rat serum which had been diluted with Hanks' saline to 50% in order to match the levels of transferrin found in human serum. Growth of rat embryos in 50% rat serum was identical to embryonic growth in 100% rat serum. Uptake of radiolabelled rat transferrin by the visceral yolk sac at 11.5 days of gestation, following culture for 60 min in radiolabelled medium, was much greater than nonspecific uptake of radiolabelled bovine serum albumin. Accumulation of radiolabelled transferrin by the embryo was reduced by the inclusion of unlabelled transferrin into the culture medium. Uptake of transferrin adsorbed 18 nm gold particles was mediated by attachment to coated pits on the apical cell surface of the extraembryonic endoderm. Transferrin-adsorbed gold colloid was internalised via coated vesicles and found in cisternal structures of the peripheral and juxtanuclear areas, as well as in smooth and coated vesicles deep within the cell. The intercellular presence of gold particles in the endodermal layer of the visceral yolk sac and their presence in the mesoderm after 60 min of incubation suggested that passage of transferrin was rapid and mediated by vesicular evagination from the extraembryonic endoderm. These findings suggest that maternal transferrin is the primary source of transferrin for the early rat embryo and its passage to the exo-coelom and embryo is mediated by specific receptors on the apical surface of the extraembryonic endoderm.

Animals↗
PubMed1979

Three types of human asialo-transferrin and their interactions with the rat liver.

Three types of asialo-transferrin were obtained from immunologically pure human transferrin by chromatography on DEAE-cellulose, followed by desialylation and affinity chromatography on a column of the immobilized asialo-glycoprotein-binding hepatic lectin from rabbit liver. Of the asialo-transferrins, type 1 was derived from the principal DEAE-cellulose chromatographic component of transferrin, i.e. the one that contains two biantennary glycans. The two other asialo-transferrins (types 2 and 3) were derived from a minor DEAE-chromatographic transferrin component, which is assumed to possess one biantennary and one triantennary glycan. The three asialo-transferrin types were indistinguishable by electrophoretic mobility, but they were readily distinguished on the basis of their binding strengths to the hepatic lectin in intact rats. Glycan structures responsible for the difference in binding strengths between asialo-transferrin types 2 and 3 are not known. Metabolic studies in rats showed that none of the individual asialo-transferrin types was capable of generating a signal for endocytosis at low doses (<1mug/100g body wt.) and, consequently, most of the injected protein was recoverable with the plasma and the liver 35min after injection. However, endocytosis and catabolism of each asialo-transferrin type was readily induced by injecting a larger dose (50-250mug/100g body wt.) of unlabelled asialo-transferrin of the same type or of a different type a short interval after the labelled dose. These findings support the view that the dose-dependent uptake of human asialo-transferrin by the hepatocyte, as established in an earlier study with asialo-transferrin made from whole transferrin [Regoeczi, Taylor, Hatton, Wong & Koj (1978) Biochem. J.174, 171-178], also holds for these asialo-transferrin subfractions. Furthermore, the present studies indicate that asialo-transferrins of different carbohydrate compositions are capable of synergistically promoting endocytosis of each other.

Animals↗
PubMed1986

Uptake and subcellular processing of 59Fe-125I-labelled transferrin by rat liver.

The uptake of transferrin and iron by the rat liver was studied after intravenous injection or perfusion in vitro with diferric rat transferrin labelled with 125I and 59Fe. It was shown by subcellular fractionation on sucrose density gradients that 125I-transferrin was predominantly associated with a low-density membrane fraction, of similar density to the Golgi-membrane marker galactosyltransferase. Electron-microscope autoradiography demonstrated that most of the 125I-transferrin was located in hepatocytes. The 59Fe had a bimodal distribution, with a larger peak at a similar low density to that of labelled transferrin and a smaller peak at higher density coincident with the mitochondrial enzyme succinate dehydrogenase. Approx. 50% of the 59Fe in the low-density peak was precipitated with anti-(rat ferritin) serum. Uptake of transferrin into the low-density fraction was rapid, reaching a maximal level after 5-10 min. When livers were perfused with various concentrations of transferrin the total uptakes of both iron and transferrin and incorporation into their subcellular fractions were curvilinear, increasing with transferrin concentrations up to at least 10 microM. Analysis of the transferrin-uptake data indicated the presence of specific transferrin receptors with an association constant of approx. 5 X 10(6) M-1, with some non-specific binding. Neither rat nor bovine serum albumin was taken up into the low-density fractions of the liver. Chase experiments with the perfused liver showed that most of the 125I-transferrin was rapidly released from the liver, predominantly in an undegraded form, as indicated by precipitation with trichloroacetic acid. Approx. 40% of the 59Fe was also released. It is concluded that the uptake of transferrin-bound iron by the liver of the rat results from endocytosis by hepatocytes of the iron-transferrin complex into low-density vesicles followed by release of iron from the transferrin and recycling of the transferrin to the extracellular medium. The iron is rapidly incorporated into mitochondria and cytosolic ferritin.

Animals↗
PubMed1994

Modulation of transferrin secretion by epidermal growth factor in immature rat Sertoli cells in vitro.

The modulation of transferrin secretion by FSH and epidermal growth factor (EGF) was studied in highly pure, primary cultures of immature rat Sertoli cells grown on a reconstituted basement membrane (Matrigel) in bicameral chambers. Sertoli cell purity was assessed by (1) morphometry, (2) alkaline phosphatase cytochemistry (a specific marker enzyme for peritubular cells) and (3) immunocytochemistry for the alpha-isoform of smooth muscle actin in contaminating peritubular cells. Results revealed a less than 0.5% peritubular cell contamination. During initial periods of culture with EGF or FSH alone or in combination, both EGF and FSH alone maintained transferrin secretion over basal values and their effects were additive. At subsequent times, EGF alone maintained transferrin secretion, but to less extent than did FSH alone, and inhibited significantly the ability of FSH to maintain transferrin secretion. The ratio of polarized transferrin secretion in response to FSH, EGF, or in combination was also examined. FSH significantly reversed the polarity of transferrin secretion, whereas EGF, although significantly reducing the ratio of apical to basal transferrin secretion, did not lead to a preferential basal secretion of transferrin. The change in the apical:basal transferrin secretion ratio, however, was not due to a reversal of the apically secreted transferrin towards a basal direction, but rather to an increase in the total basally secreted transferrin. The effects of cell density effects on transferrin secretion were then examined. At low cell density, the relative ability of EGF and FSH together to maintain transferrin secretion was greater than at high cell density, but overall transferrin secretion was greater as cell density increased. The inhibition of FSH by EGF on transferrin secretion was also density dependent: EGF significantly inhibited FSH effects at low cell density, but failed to do so at high cell density. These results suggest that regulation of transferrin secretion by Sertoli cells appears to be under dual control, involving both FSH and EGF and may provide an explanation for the mechanism by which EGF exerts a regulatory role in spermatogenesis.

Animals↗
PubMed1990

Influence of cellular iron status on the release of soluble transferrin receptor from human promyelocytic leukemic HL60 cells.

We have previously shown that human leukemic HL60 cells release from their surface a soluble form of the transferrin receptor. Because of the regulatory role of iron in transferrin receptor expression, we have now examined the relationship between iron and the release of soluble transferrin receptor from HL60 cells. Cells grown in serum-free, transferrin-free medium containing iron-pyridoxal isonicotinoyl hydrazone (Fe-PIH) displayed approximately 70% less iodine 125-labeled transferrin surface binding and released 60% less soluble transferrin receptor than cells grown in serum-supplemented medium. Incubation of cells with increasing concentrations of Fe-PIH resulted in a progressive decrease in the release of soluble transferrin receptor over 18 hours of incubation. In contrast, receptor release was increased after incubation of cells with the iron chelator deferoxamine. This effect was completely blocked by cycloheximide. Transferrin receptor release from cells over 2 hours was unaffected by the presence of transferrin-iron, suggesting that transferrin receptor release occurs independent of the cellular handling of its ligand. Exposure of cells to phorbol myristate acetate resulted in a decrease in cell surface transferrin receptor and a decrease in the release of soluble transferrin receptor. Our studies show that transferrin receptor release from HL60 cells changes during iron excess or iron deficiency and that these changes are the result of alterations in cell surface transferrin receptor density. Our studies suggest that elevated serum transferrin receptor levels seen in clinical iron deficiency reflect corresponding increases in transferrin receptors at the cellular level.

Cell Membrane↗
PubMed1996

Evaluation of human transferrin radiolabeled with N-succinimidyl 4-[fluorine-18](fluoromethyl) benzoate.

UNLABELLED: Iron metabolism plays a key role in cell proliferation and survival in rapidly growing cancer cells. Uptake is mediated by the carrier protein transferrin. The increased need for iron has been used as a method to target tumors and there is well-documented evidence that certain tumors can be imaged with tracers such as 67Ga, that mimic transferrin-mediated iron uptake. To obtain a tracer that would be better able to quantitate transferrin kinetics and indirectly evaluate iron metabolism, we have labeled human transferrin with the positron emitter, 18F, with a one-step high-specific activity method developed in our laboratory. METHODS: We measured the binding affinities of [18F]diferric (holo-) and iron-free (apo-) transferrin on two human cell lines. We also compared cellular uptake of [18F]holo-transferrin and [67Ga]citrate in various conditions, and washout of label incorporated into cells. RESULTS: The binding affinity of [18F]holo-transferrin was found to be the same as that reported for [125I]holo-transferrin. In our hands there was no significant difference in binding affinity between diferric holo-transferrin and iron-free apo-transferrin. [18F]holo-transferrin uptake rapidly reaches a steady-state equilibrium between the intracellular and extracellular environment, while gallium accumulation linearly increases with time. [18F]holo-transferrin is rapidly recycled out of the cell with similar kinetics to those reported for [125I]holo-transferrin. CONCLUSION: [18F]holo-transferrin displays the properties of native transferrin and appears suitable for quantitative evaluation of transferrin kinetics in vivo.

Benzoates↗
PubMed1988

Transferrin synthesis by small cell lung cancer cells acts as an autocrine regulator of cellular proliferation.

Since transferrin is required for cellular proliferation, we investigated transferrin synthesis by a small cell lung cancer line (NCI-H510) that survives in serum-free media without added transferrin. Immunoassays for human transferrin demonstrated that these cells contained immunoreactive human transferrin. Immunofluorescence studies showed that the protein is expressed on the surface of cells, presumably bound to transferrin receptor. Media conditioned by NCI-H510 cells support proliferation of human leukemic cells that would not survive in media lacking transferrin. [35S]Methionine incorporation documented transferrin synthesis by NCI-H510 cells as well as three other small cell lines. Transferrin synthesis by NCI-H510 cells increased more than 10-fold when cells entered active phases of the cell cycle, and this increase was seen before large increases in transferrin-receptor expression. Further experiments examining the effects of agents that affect iron metabolism show that the addition of transferrin-iron or hemin to the media is associated with a more rapid initial rate of proliferation and lower rates of transferrin synthesis than control cells. Gallium salts, which inhibit iron uptake, inhibited proliferation of these cells. If the cells recovered from this effect, transferrin synthesis remained greatly increased compared to control. We conclude that transferrin synthesis by these malignant cells is ultimately related to an iron requirement for cellular proliferation. It appears that this synthesized transferrin acts as part of an important autocrine mechanism permitting proliferation of these cells, and perhaps permitting tumor cell growth in vivo in areas not well vascularized.

Carcinoma, Small Cell↗
PubMed1992

Transferrin: a potential source of iron for oxygen free radical-mediated endothelial cell injury.

The ability of transferrin to potentiate oxygen free radical-mediated endothelial cell injury was assessed. 51Cr-labeled endothelial cells derived from rat pulmonary arteries (RPAECs) were incubated with hydrogen peroxide (H2O2) in the presence and absence of holosaturated human transferrin, and the effect of transferrin on H2O2-mediated endothelial cell toxicity was determined. Addition of holosaturated transferrin potentiated H2O2-mediated RPAEC cytotoxicity at concentrations of H2O2 greater than 10 microM, suggesting that transferrin may provide a source of iron for free radical-mediated endothelial cell injury. Free radical-mediated injury is dependent on non-protein-bound iron. The ability of RPAECs to facilitate the release of iron from transferrin was assessed. We determined that RPAECs facilitate the release of transferrin-derived iron by reduction of transferrin-bound ferric iron (Fe3+) to ferrous iron (Fe2+). The reduction and release of transferrin-derived Fe2+ were inhibited by apotransferrin and chloroquine, indicating a dependence on receptor-specific binding of transferrin to the RPAEC cell surface, with subsequent endocytosis, acidification, and reduction of transferrin-bound Fe3+ to Fe2+. The release of transferrin-derived Fe2+ was potentiated by diethyldithiocarbamate, an inhibitor of intracellular superoxide dismutase (SOD). In contrast, exogenous SOD did not alter iron release, suggesting that intracellular superoxide anion (O2-) may play an important role in mediating the reduction and release of transferrin-derived iron. Results of this study suggest that transferrin may provide a source of iron for oxygen free radical-mediated endothelial cell injury and identify a novel mechanism by which endothelial cells may mediate the reduction and release of transferrin-derived iron.

Animals↗
PubMed1995

Reversal by transferrin of growth-inhibitory effect of suramin on hormone-refractory human prostate cancer cells.

BACKGROUND: The second leading cause of cancer-related deaths in American men is metastatic hormone-refractory adenocarcinoma of the prostate, for which there is currently no effective treatment. Transferrin is abundant in bone stroma and has been found to stimulate models of hormone-refractory metastatic prostate cancer. Suramin, a compound that has been used to treat metastatic prostate cancer, has been demonstrated to antagonize the binding of transferrin to the transferrin receptor and to suppress uptake of iron by hematopoietic cells. PURPOSE: The purpose of our study was to determine whether transferrin may reverse the inhibitory action of suramin on metastatic prostate-derived cell lines. METHODS: Five human prostate cell lines (PC-3, PC-3M, DU-145, TSU-Pr1, and LNCaP) derived from metastatic deposits were examined for response to growth stimulation by apotransferrin, for the presence of transferrin receptors by binding of 125I-labeled transferrin, and for relative transferrin receptor messenger RNA (mRNA) content by ribonuclease protection assays. We measured the amount of growth inhibition by suramin in low serum assays to demonstrate maximal inhibition over the apotransferrin to reverse the inhibition of suramin in these tumors. RESULTS: The results clearly demonstrate that the androgen-insensitive metastatic cell lines (PC-3, PC-3M, DU-145, and TSU-Pr1) demonstrate increased cell numbers when exposed to holotransferrin or apotransferrin, while the androgen-sensitive cell line (LNCaP) did not show any increase. All cell lines demonstrated a similar number of transferrin receptors and transferrin receptor mRNA. We used these maximally inhibitory, but clinically relevant, concentrations of suramin to determine whether transferrin could reverse the inhibition, and it did, but only in the androgen-insensitive metastatic lines. Indeed, in the PC-3 cells, inhibition turned to stimulation with the addition of transferrin, and even at the highest concentration of suramin tested, 400 microM, a concentration that would be toxic to patients, the amount of inhibition by suramin was still reduced by more than 50% by transferrin in TSU-Pr1 cells. In the androgen-sensitive LNCaP cells, however, transferrin had limited ability to block the inhibitory activity of suramin. CONCLUSIONS: Concentrations of tumor-stimulating factors, such as transferrin, in the metastatic microenvironment need to be taken into consideration in the use of suramin and suramin-like derivatives. Novel strategies need to be identified that will negate the action of transferrin on androgen-insensitive cells.

Androgens↗
PubMed1996

Transferrin increases adherence of iron-deprived Neisseria gonorrhoeae to human endometrial cells.

OBJECTIVE: Our purpose was to study the effects of iron deprivation with and without human transferrin supplementation on the adherence and invasion of Neisseria gonorrhoeae to human endometrial cells. STUDY DESIGN: N. gonorrhoeae grown with our without iron was placed in media alone or media containing 2.5 mg/ml saturated human transferrin or unsaturated transferrin. N. gonorrhoeae was inoculated onto polarized human endometrial carcinoma cell (HEC 1-B) monolayers, and at various intervals monolayers were washed and incubated with media containing gentamicin or media alone. Colony-forming units per milliliter of N. gonorrhoeae associated with HEC 1-B cells were then determined. N. gonorrhoeae strains tested included both a transferrin receptor-positive (wild-type) and a transferrin receptor-negative mutant. Differences in percent of original inoculum remaining at varying time points were analyzed by the Mann-Whitney U test. Transmission electron microscopy using a primary endometrial cell line was used to verify findings. RESULTS: Iron-negative N. gonorrhoeae exhibited less adherence than did iron-positive N. gonorrhoeae. No difference in HEC 1-B adherence was seen when either saturated transferrin or unsaturated transferrin was added to the iron-positive N. gonorrhoeae. With iron-negative N. gonorrhoeae addition of either saturated transferrin or unsaturated transferrin significantly increased N. gonorrhoeae adherence although unsaturated transferrin did not permit growth of iron-negative N. gonorrhoeae in tissue culture media alone. Transmission electron microscopy confirmed increased adherence of iron-negative N. gonorrhoeae supplemented with unsaturated transferrin. An iron-negative N. gonorrhoeae mutant lacking the transferrin receptor exhibited no adherence regardless of addition of saturated transferrin or unsaturated transferrin. Invasion could not be quantitated reliably because of persistence of gentamicin effect. CONCLUSION: Iron and transferrin increased attachment of N. gonorrhoeae to human endometrial cells.

Bacterial Adhesion↗
PubMed1993

Posttranscriptional regulation of chimeric human transferrin genes by iron.

Transferrin, the transferrin receptor, and ferritin are integral to the body's management of iron, an element required for life but highly toxic when present in excess. The transferrin receptor and ferritin are regulated posttranscriptionally by iron: the transferrin receptor by mRNA stability and ferritin by mRNA translation. Results described here indicate that transferrin, like ferritin, is regulated by iron at the level of translation. Chimeric genes introduced into the mouse genome were composed of the human transferrin 5' regulatory region fused to the chloramphenicol acetyl transferase (CAT) reporter gene. Iron administration to transgenic mice resulted in a significant decrease of transferrin-directed CAT enzyme activity and CAT protein in liver, but no significant decrease in human transferrin-CAT mRNA levels. Binding of specific RNA iron regulatory elements by proteins in cytoplasmic extracts have been shown to regulate ferritin and transferrin receptor synthesis. Similar results have been obtained with transferrin mRNA. A decreased binding of human transferrin 5'-untranslated region RNA by factors in cytoplasmic extracts of livers from mice receiving iron was found when compared to extracts from control mice. A human transferrin RNA-protein complex migrated electrophoretically with the same mobility as a ferritin iron responsive element RNA-iron responsive element binding protein complex. The ferritin iron responsive element RNA also competed with the human transferrin 5'-untranslated region RNA-protein complexes formed and vice versa. Therefore, iron modulation of human transferrin may share a factor common or similar to that observed in ferritin and transferrin receptor iron modulation.

Animals↗