The uptake of inorganic iron complexes by human melanoma cells

The human melanoma cell line, SK-MEL-28, expresses high levels of melanotransferrin. The uptake of inorganic iron (Fe) complexes compared to transferrin-bound Fe by these cells has been investigated to determine whether melanotransferrin has a role in Fe uptake. The mechanisms of Fe uptake have been characterised using 59Fe complexes of citrate, nitrilotriacetate, desferrioxamine, and 59Fe added to Eagle's minimum essential medium (MEM) and compared with human transferrin (Tf) labelled with 59Fe and iodine-125. Iron uptake from the Fe complexes of citrate, nitrilotriacetate and MEM were similar, and far greater than that from Tf at the same Fe concentration (2.5 microM). Ammonium chloride and a monoclonal antibody to the transferrin receptor (42/6), had no effect on the uptake of Fe from inorganic Fe complexes, suggesting that receptor-mediated endocytosis of Tf was not involved. The monoclonal antibody, 96.5, specific for melanotransferrin did not alter total Fe uptake but slightly increased the proportion of Fe internalised, possibly due to the modulation of the antigen by the antibody. However, from the time required for modulation to occur (approximately 2 h), the small increase in internalisation observed and the fact that no increase in total cell Fe occurred, it is suggested that melanotransferrin has little role in Fe uptake.     

The effect of desferrioxamine and ferric ammonium citrate on the uptake of iron by the membrane iron-binding component of human melanoma cells.

The effect of ferric ammonium citrate (FAC) and desferrioxamine (DFO) on membrane iron and transferrin uptake have been investigated using SK-MEL-28 human melanoma cells which express the membrane-bound transferrin homologue, melanotransferrin, at high concentrations. Exposure of melanoma cells to DFO increased membrane non-Tf-bound Fe uptake (putative melanotransferrin Fe-binding sites), suggesting upregulation of the membrane Fe-binding component. However, exposure to FAC did not result in down-regulation. Indeed, an increase in non-Tf-bound membrane Fe was apparent. Results suggested that non-Tf-bound membrane Fe uptake occurred by two processes corresponding to the specific and non-specific mechanisms of Fe uptake from Tf described previously (Richardson, D.R. and Baker, E. (1990) Biochim. Biophys. Acta 1053, 1-12).     

The human melanoma associated protein melanotransferrin promotes endothelial cell migration and angiogenesis in vivo

Melanotransferrin is a member of the transferrin family, which is comprised of serum transferrin, lactoferrin and ovotransferrin, and is highly expressed on melanoma cells compared to normal melanocytes. Since melanoma is an highly vascularized tumour that expresses melanotransferrin at high levels, we tested purified recombinant melanotransferrin for its capability to induce angiogenesis in the chick chorioallantoic membrane. Macroscopic and microscopic evaluation of the vascular density demonstrated that melanotransferrin exerts an angiogenic response quantitatively similar to that elicited by fibroblast growth factor-2. Overexpression of vascular endothelial growth factor-receptor-2 was observed in newly formed vessels, suggesting that the angiogenic activity of melanotransferrin may depend on activation of endogenous vascular endothelial growth factor. In addition, when antibodies against vascular endothelial growth factor were included in the assay, the angiogenic response was inhibited by 50%. In a Boyden chamber assay purified recombinant melanotransferrin induced chemotactic migration of vascular cells, which was decreased in the the presence of anti-vascular endothelial growth factor antibodies suggesting an involvement of vascular endothelial growth factor present in endothelial cells also in this assay. However, melanotransferrin was found not to directly bind to integrin alphavbeta3 or the vascular endothelial growth factor-receptor-2 as assessed in a BlAcore assay. A possible correlation between vascularization occurring during melanoma progression and the expression of melanotransferrin and vascular endothelial growth factor was established by immunolocalization of the two factors in sections of melanoma at different clinical steps of melanoma progression. These latter data strongly imply that melanotransferrin may participate in the vascularization of solid tumours and that inhibition of melanotransferrin could form the basis for intervention in tumours which use this pathway.     

Intermediate steps in cellular iron uptake from transferrin. Detection of a cytoplasmic pool of iron, free of transferrin.

The uptake of transferrin-bound iron by receptor-mediated endocytosis has been the subject of extensive experimental investigation. However, the path followed by iron (Fe) after release from transferrin (Tf) remains obscure. Once Fe is released from Tf within the endosome, it must be transported across the endosomal membrane into the cell. The present investigation describes the presence of a cytoplasmic Tf-free Fe pool which is detectable only when cells are detached from their culture dishes at low temperature, after initial incorporation of diferric transferrin at 37 degrees C. This cellular iron pool was greatly reduced if incubation temperatures were maintained at 37 degrees C or if cells were treated with pronase. Human melanoma cells (SK-MEL-28) in culture were prelabeled by incubation with human 125I-59Fe-transferrin for 2 h, washed, and reincubated at 4 degrees C or 37 degrees C in balanced salt solution in the presence or absence of pronase. The cells were then mechanically detached from the plates and separated into "internalized" and supernatant fractions by centrifugation. Approximately 90% of cellular 59Fe and 20% of 125I-Tf remained internalized when this reincubation procedure was carried out in balanced salt solution at 37 degrees C. However, at 4 degrees C, cellular internalized iron was reduced to approximately 50% of the initial value. The release of this component of cellular 59Fe (approximately 40% of total cell 59Fe) at 4 degrees C was completely inhibited in the presence of pronase and other general proteinases at 4 degrees C and at 37 degrees C, without affecting internalized transferrin levels. Similar results were obtained in fibroblasts and hepatoma cells, indicating that this phenomenon is not unique to melanoma cells. The characterization of this Tf-free cellular Fe pool which is detectable at low temperature may yield valuable insights into the metabolic fate of iron following its transport across the membrane of the endocytotic vesicle.     

A novel iron uptake mechanism mediated by GPI-anchored human p97.

The established process for iron uptake into mammalian cells involves transferrin and its receptor. Here, the role of the glycosyl-phosphatidylinositol (GPI)-linked transferrin homologue, melanotransferrin or p97, was studied using CHO cell lines defective in the transferrin receptor (TR) and transfected with human TR and/or human p97. The presence of p97 doubled the iron uptake, which could be explained by the binding of one atom of iron to one molecule of p97. The internalization of iron was shown to be temperature sensitive and saturated at a media iron concentration of 2.5 micrograms/ml with a Vmax of 0.1 pmol Fe/10(6) cell/min and a Km of 2.58 microM for p97. Treatment of the cells with either phosphatidylinositol-phospholipase C or monoclonal antibodies against p97 resulted in over a 50% reduction and a 47% increase in the iron uptake respectively. These data identify p97 as a unique cell surface GPI-anchored, iron binding protein involved in the transferrin-independent uptake of iron in mammals.     

Evidence for the functional equivalence of the iron-binding sites of rat transferrin

The role of the two iron-binding sites of rat transferrin in the exchange of iron with cells has been assessed using urea polyacrylamide gel electrophoresis to separate and quantitate the four possible molecular species of transferrin generated during the incubation of ¹²⁵I-labelled transferrin with rat reticulocytes and hepatocytes. Addition of diferric transferrin to reticulocytes led directly to the appearance of apotransferrin together with small and comparable amounts of the two monoferric transferrins. After 2 h 44.8% of the iron had been removed by the cells, and of the iron-depleted transferrin 71.8% was apotransferrin, the remainder being monoferric transferrin, 16.1% with N-terminal iron and 12.1% with C-terminal iron. A similar pattern emerged with hepatocytes, but the rate of iron removal was slower and the proportion of apotransferrin generated was lower. After 4 h 10.9% of the iron had been removed from the transferrin and the distribution of the iron-depleted protein was: apotransferrin 26.9% and monoferric (N-terminal) 39.2%, (C-terminal) 33.9%. The appearance of apotransferrin during each incubation and the generation of both monoferric transferrins suggest that both cell types are able to remove iron from differic transferrin in pairwise fashion and that they do not appreciably distinguish between the two iron-binding sites of the protein. Release of iron from hepatocytes to apotransferrin lead to the appearance of both monferric species and then to increasing amounts of diferric transferrin. The process of iron release did not seem to distinguish between the vacant iron-binding sites of transferrin.     

Role of plasma membrane phospholipids in the uptake and release of transferrin and its iron by reticulocytes

Summary The involvement of membrane phospholipids in the utilization of transferrinbound iron by reticulocytes was investigated using [⁵⁹Fe]- and [¹²⁵I]-labelled transferrin and rabbit reticulocytes which had been incubated with phospholipas A. Transferrin and iron uptake and release were all inhibited by phospholipas A which produced a marked decrease in the relative abundance of phosphatidylcholine and phosphatidylethanolamine and equivalent increases in their lyso-compounds in the reticulocyte plasma membrane. There was a close correlation between the iron uptake rate and the rate and amount of transferrin uptake and the amount of the lysophospholipids in the membrane. Incubation of the cells with exogenous lysophosphatidylethanolamine or lysophosphatidylcholine also produced inhibition of iron and transferrin uptake. The reduced uptake produced by phospholipase A could be reversed if the lyso-compounds were removed by fatty acid-free bovine serum albumin or by reincubation in medium 199. Treatment with phospholipase A was shown to increase the amount of transferrin bound by specific receptors on the reticulocyte membrane but to inhibit the entry of transferrin into the cells.The present investigation provides evidence that the phospholipid composition of the cell membrane influences the interaction of transferrin with its receptors, the processes of endocytosis and exocytosis whereby transferrin enters and leaves the cells, and the mechanism by which iron is mobilized between its binding to transferrin and incorporation into heme. In addition, the results indicate that phosphatidylethanolamine is present in the outer half of the lipid bilayer of reticulocyte membrane.     

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.     

Iron release from transferrin, its C-lobe, and their complexes with transferrin receptor: presence of N-lobe accelerates release from C-lobe at endosomal pH

Human transferrin, like other members of the transferrin class of iron-binding proteins, is a bilobal structure, the product of duplication and fusion of an ancestral gene during the course of biochemical evolution. Although the two lobes exhibit 45% sequence identity and identical ligand structures of their iron-binding sites (one in each lobe), they differ in their iron-binding properties and their responsiveness to complex formation with the transferrin receptor. A variety of interlobe interactions modulating these iron-binding functions has been described. We have now studied the kinetics of iron release to pyrophosphate from the isolated recombinant C-lobe and from that lobe in the intact protein, each free and bound to receptor. The striking finding is that the rates of iron release at the pH of the endosome to which transferrin is internalized by the iron-dependent cell are similar in the free proteins but 18 times faster from full-length monoferric transferrin selectively loaded with iron in the C-lobe than from isolated C-lobe when each is complexed to the receptor. The possibility that the faster release in the receptor complex of the full-length protein at endosomal pH contributes to the evolutionary advantage of the bilobal structure is considered.     

Overexpression of hemochromatosis protein, HFE, alters transferrin recycling process in human hepatoma cells

HFE is a MHC class 1-like protein that is mutated in hereditary hemochromatosis. In order to elucidate the role of HFE protein on cellular iron metabolism, functional studies were carried out in human hepatoma cells (HLF) overexpressing a fusion gene of HFE and green fluorescent protein (GFP). The expression of HFE-GFP was found to be localized on cell membrane and perinuclear compartment by fluorescent microscopy. By co-immunoprecipitation and Western blotting, HFE-GFP protein formed a complex with endogenous transferrin receptor and beta(2)-microglobulin, suggesting that this fusion protein has the function of HFE reported previously. We then examined the (59)Fe uptake and release, and internalization and recycling of (125)I-labeled transferrin in order to elucidate the functional roles of HFE in the cell system. In the transfectants, HFE protein decreased the rate of transferrin receptor-dependent iron ((59)Fe) uptake by the cells, but did not change the rate of iron release, indicating that HFE protein decreased the rate of iron influx. Scatchard analysis of transferrin binding to HFE-transfected cells showed an elevation of the dissociation constant from 1.9 to 4. 3 nM transferrin, indicating that HFE protein decreased the affinity of transferrin receptor for transferrin, while the number of transferrin receptors decreased from 1.5x10(5)/cell to 1. 2x10(5)/cell. In addition, the rate of transferrin recycling, especially return from endosome to surface, was decreased in the HFE-transfected cells by pulse-chase study with (125)I-labeled transferrin. Our results strongly suggest an additional role of HFE on transferrin receptor recycling in addition to the decrease of receptor affinity, resulting in the reduced cellular iron.     

Differences between human and rabbit transferrins

Rabbit reticulocyte incorporation of iron from rabbit transferrin was independent of transferrin iron saturation but uptake from human transferrin was saturation dependent. Unlike human transferrin, rabbit transferrin does not surrender its iron from any unique preferred iron-binding site and can be described as functionally homogeneic.The two proteins also differ in their acid-base iron-binding properties. One human transferrin iron binding site retains an ability to bind iron at somewhat acid pH but this property is not shared by rabbit transferrin.     

Laminin increases the release of type IV collagenase from malignant cells

We have studied the effect of laminin on type IV collagenolytic activity elaborated by malignant cells in culture. Laminin (at concentrations of 4-8 micrograms/ml) added to serum-free culture supernatants of subconfluent A2058 human melanoma cells significantly increased the release of the type IV collagenolytic activity (200-300%). The induction of type IV collagenase was more pronounced (580%) using a fragment of laminin which binds to the cell surface laminin receptor. A monoclonal antibody against the human laminin receptor blocked the effect of laminin on type IV collagenase, suggesting that occupation of the laminin receptor may be necessary for the effect. Increase in the type IV collagenolytic activity mediated by laminin was also demonstrated in two other malignant cell lines, HT fibrosarcoma (168%) and mouse melanoma (B16-F10) (271%). The increase in type IV collagenase was found to be specific for laminin because another cell-binding matrix protein, fibronectin, did not have any effect, and epidermal growth factor and transferrin actually decreased the type IV collagenase in human melanoma culture medium (epidermal growth factor, 50% at 20 ng/ml; and transferrin, 20% at 10 micrograms/ml). These studies suggest that tumor cell binding to laminin, which comprises the first step of basement membrane invasion, will induce the second step, namely the collagenolytic dissolution of the basement membrane.     

Calorimetric studies of melanotransferrin (p97) and its interaction with iron

The mammalian molecule melanotransferrin (mTf), also called p97, is a member of the transferrin family of molecules. It exists in both secreted and glycosylphosphatidylinositol-anchored forms and is thought to play a role in angiogenesis and in transporting iron across the blood brain barrier. The binding affinity of iron to this molecule has not been formally established. Here, the binding of ferric ion (chelated with a 2-fold molar ratio of nitrilotriacetate) to mTf has been studied using isothermal titration calorimetry and differential scanning calorimetry. One iron-binding site was determined for mTf with similar binding characteristics to other transferrins. In the absence of bicarbonate, binding occurs quickly with an apparent association constant of 2.6 x 10(7) M(-1) at 25 degrees C. The presence of bicarbonate introduces kinetic effects that prevent direct determination of the apparent binding constant by isothermal titration calorimetry. Differential scanning calorimetry thermograms of mTf unfolding in the presence and absence of iron were therefore used to determine the apparent binding constant in the bicarbonate-containing system; at pH 7.5 and 25 degrees C, iron binding occurs in a 1:1 ratio with a K(app) of 4.4 x 10(17) M(-1). This affinity is intermediate between the high and low affinity lobes of transferrin and suggests that mTf is likely to play a significant role in iron transport where the high affinity lobe of transferrin is occupied or where transferrin is in proportionally low concentrations.     

Transferrin and iron uptake by rat reticulocytes

The uptake of transferrin labeled with 3H and 59Fe by rat reticulocytes was studied to clarify the characteristics of the uptake process and intracellular transport. Rat reticulocytes took up transferrin in a saturable, time- and temperature-dependent manner. Scatchard analysis of the binding parameters indicated that transferrin molecules were bound to cell-surface receptors with high affinity. Monodansyl- cadaverine, a potent inhibitor of transglutaminase, reduced the amount of internalized transferrin but has no effect on the total amount of cell-associated transferrin, suggesting that transferrin is taken up by rat reticulocytes via receptor-mediated endocytosis. About 50% of the internalized 3H label was released from the cells after reincubation for 1 h in fresh medium. In contrast, no release of 59Fe label was observed. By immunoprecipitation and subsequent SDS-PAGE the released 3H-labeled product was identified as apotransferrin. Lysosomotropic reagents and a proton ionophore reduced the uptake of 59Fe. These results indicated that iron was removed from transferrin at an intracellular site in an acidic environment. The released iron was found not to associate with any intermediate ligands before it was utilized for heme synthesis in mitochondria.     

Release of iron from the two iron-binding sites of transferrin by cultured human cells: modulation by methylamine

We have investigated the effect of increasing concentrations of methylamine (5, 10, and 25 mM) on the removal of iron from the two iron-binding sites of transferrin during endocytosis by human erythroleukemia (K562) cells. The molecular forms of transferrin released from the cells were analyzed by polyacrylamide gel electrophoresis in 6 M urea. Endocytosis of diferric transferrin was efficient since greater than 10% of surface-bound protein escaped endocytosis and was released in the diferric form. Although transferrin exocytosed from control cells had been depleted of 80% of its iron and contained 65-70% apotransferrin, iron-bearing species were also released (15% C-terminal monoferric; 10% N-terminal; 10% diferric). The ratio of the two monoferric species (C/N) was 1.32 +/- 0.12 (mean +/- SD; n = 4), suggesting that iron in the N-terminal site was more accessible to cells. In the presence of methylamine there was a concentration-dependent increase in the proportion of diferric transferrin release (less than 80% at 25 mM) and a concomitant decrease in apotransferrin. Small amounts of the iron-depleted species, especially apotransferrin, appeared before diferric transferrin, suggesting that these were preferentially released from the cells. The discrepancy between the proportions of the monoferric transferrin species noted with control cells was enhanced at all concentrations of methylamine, most markedly at 10 mM when the C/N ratio was 2.4. The N-terminal site of transferrin loses its iron at a higher pH than the C-terminal site, and so by progressively perturbing the pH of the endocytic vesicle we have increased the difference between the two sites observed with control cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Differences between human and rabbit transferrins.

Rabbit reticulocyte incorporation of iron from rabbit transferrin was independent of transferrin iron saturation but uptake from human transferrin was saturation dependent. Unlike human transferrin, rabbit transferrin does not surrender its iron from any unique preferred iron-binding site and can be described as functionally homogeneic. The two proteins also differ in their acid-base iron-binding properties. One human transferrin iron binding site retains an ability to bind iron at somewhat acid pH but this property is not shared by rabbit transferrin.     

High-affinity binding by the periplasmic iron-binding protein from Haemophilus influenzae is required for acquiring iron from transferrin

The periplasmic iron-binding protein, FbpA (ferric-ion-binding protein A), performs an essential role in iron acquisition from transferrin in Haemophilus influenzae. A series of site-directed mutants in the metal-binding amino acids of FbpA were prepared to determine their relative contribution to iron binding and transport. Structural studies demonstrated that the mutant proteins crystallized in an open conformation with the iron atom associated with the C-terminal domain. The iron-binding properties of the mutant proteins were assessed by several assays, including a novel competitive iron-binding assay. The relative ability of the proteins to compete for iron was pH dependent, with a rank order at pH 6.5 of wild-type, Q58L, H9Q>H9A, E57A>Y195A, Y196A. The genes encoding the mutant FbpA were introduced into H. influenzae and the resulting strains varied in the level of ferric citrate required to support growth on iron-limited medium, suggesting a rank order for metal-binding affinities under physiological conditions comparable with the competitive binding assay at pH 6.5 (wild-type=Q58L>H9Q>H9A, E57A>Y195A, Y196A). Growth dependence on human transferrin was only obtained with cells expressing wild-type, Q58L or H9Q FbpAs, proteins with stability constants derived from the competition assay >2.0x10(18) M(-1). These results suggest that a relatively high affinity of iron binding by FbpA is required for removal of iron from transferrin and its transport across the outer membrane.     

A transferrin-like GPI-linked iron-binding protein in detergent- insoluble noncaveolar microdomains at the apical surface of fetal intestinal epithelial cells

A GPI-anchored 80-kD protein was found to be the major component of detergent-insoluble complexes, prepared from fetal porcine small intestine, constituting about 25% of the total amount of protein. An antibody was raised to the 80-kD protein, and by immunogold electron microscopy of ultracryosections of mucosal tissue, the protein was localized to the apical surface of the enterocytes, whereas it was absent from the basolateral plasma membrane. Interestingly, it was mainly found in patches of flat or invaginated apical membrane domains rather than at the surface of microvilli. Caveolae were not found in association with these labeled microdomains. In addition, the 80-kD protein was seen in apical endocytic vacuoles and in tubulo-vesicular structures, suggesting that the apical microdomains are involved in endocytosis of the 80-kD protein. By its NH2-terminal amino acid sequence, iron-binding capacity and partial immunological cross- reactivity with serum transferrin, the 80-kD protein was shown to belong to the transferrin family, and it is probably homologous to melanotransferrin, a human melanoma-associated antigen. The 80-kD iron- binding protein was fully detergent-soluble immediately after synthesis and only became insoluble after gaining resistance to endo H, supporting a mechanism for exocytic delivery to the apical cell surface by way of detergent-insoluble glycolipid "rafts" that fuse with the plasmalemma at restricted sites devoid of microvilli.     

Failure of rabbit reticulocytes to incorporate conalbumin or lactoferrin iron.

Despite the remarkable molecular similarity of human lactoferrin and human transferrin, the results of this investigation indicate that human lactoferrin was unable to furnish rabbit reticulocytes with iron for heme synthesis. Although conalbumin closely resembles transferrin in many of its properties, conalbumin iron-binding differs from human transferrin iron-binding. There are conflicting reports in the literature regarding conalbumin's ability to furnish iron to reticulocytes. In this study, small amounts of lactoferrin or conalbumin were adsorbed to mature and immature cell surfaces but neither of these iron-binding proteins surrendered iron intracellularly to reticulocytes for heme synthesis.     

Pumping iron in the '90s

The role o f iron in cell division, cell death and human disease has recently gained increased attention. The best studied process for iron uptake into mammalian cells involves traps ferrin and its receptor. This review discusses evidence supporting the existence of other routes by which iron can enter mammalian cells. Specifically, iron uptake by the cell-surface GPI-linked traps ferrin homologue, melanotransferrin or p97, is described and possible functions of this traps ferrin-independent pathway are proposed.