Studies on the mechanism of transferrin iron uptake by rat reticulocytes

Interleukin-6 enhances hepatic transferrin uptake and ferritin expression in rats

To explore a mechanism of interleukin (IL)-6-induced hypoferremia in rats, iron metabolism was investigated both in vivo and in vitro. Recombinant IL-6 was intraperitoneally administered to male Wistar rats and the serial change of parameters related to iron metabolism was examined. After administration of IL-6, plasma IL-6 concentration increased rapidly, reached its maximum in 1 hr and thereafter decreased quickly. Plasma IL-6 3 hr after IL-6 injection (50 micrograms/kg) was 3 units/ml, which is a concentration capable of inducing hepatic 125I-labeled transferrin uptake in vitro using isolated hepatocytes. Plasma iron concentration and transferrin saturation had decreased to approximately one third of the initial level within 3 hr and then recovered. Total iron binding capacity remained unchanged for 6 hr, then began to decrease. Red blood cell count and hemoglobin concentration showed no remarkable changes during this period. By ferrokinetic study with plasma that contained iron 59-labeled transferrin, the plasma iron disappearance half time, calculated from the disappearance curve, was significantly shortened from 55 min to 22 min by IL-6 treatment (p < 0.01). The ferritin concentration in the liver was increased significantly after the administration of IL-6 (p < 0.001), but transiently decreased in the spleen. The plasma ferritin showed a gradual increase during the 6-hr period after IL-6 injection. The uptake of 125I-labeled diferric transferrin by isolated hepatocytes was increased by IL-6 treatment and this increment was inhibited by addition of 100-fold excess unlabeled transferrin. On the other hand, no significant increment of 125I-labeled diferric transferrin uptake was observed in Kupffer cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Transferrin-mediated cellular iron uptake

The basic model for cellular uptake of iron relies on the iron-chelating protein transferrin (Tf), which is capable of binding iron under one set of conditions and releasing it under another set of conditions. Tf has specific membrane receptors on the surface of the cells that require iron. Tf-receptor binding is followed by internalization through a system of coated pits and vesicles. The rapid decline of pH of these vesicles leads to release and sequestration of iron by the cell. Apotransferrin-receptor complex returns to the cell surface, where, under neutral pH conditions, apotransferrin is dissociated. Other models for cellular uptake of iron include extraction of iron from Tf on the cell surface without internalization, uptake by adsorptive mechanism, and fluid-phase endocytosis. Recent advances in cellular and molecular biology, gene cloning, and monoclonal antibody technique have elucidated many features of these processes at a molecular level. These advances are reviewed and prospects for future work discussed.

Transferrin receptor on rat Kupffer cells in primary culture

Kupffer cells may play a role in the turnover of iron in acute viral hepatitis. The transferrin receptor of rat Kupffer cells in primary culture was therefore investigated in this study. Daily specific bindings on 125I-diferric transferrin (Tf) to rat Kupffer cells in primary culture from day 3 to day 6 of culture were 1.64 +/- 0.08%, 4.16 +/- 0.05%, 4.34 +/- 0.07% and 2.63 +/- 0.07%, respectively. The specificity of the Tf binding sites was examined by competition studies showing that galactose (30 mmol x l-1) and ovalbumin (90 mumol x l-1) did not compete for the binding sites, but human lactoferrin (50 mumol x l-1) competed for the binding sites by about 30%. The affinity and capacity of Tf receptor on rat Kupffer cells in 5-day culture were analyzed according to the method of Scatchard. A single class of 125I-diferric Tf binding sites with an affinity constant of 1.65 x 10(7) l x l-1) and a capacity of 6.86 x 10(6) sites/cell was found. After zymosan (500 micrograms/ml) preincubation for 30 min, the binding capacity increased about 1.7-fold, and this increase depended upon the increase of the affinity of Tf receptor. These data suggest that Kupffer cells in the activated state accelerate the removal of elevated serum iron.

Discordance between transferrin receptor expression and susceptibility to lysis by natural killer cells

Expression of the transferrin receptor on target cell lines has recently been implicated as a determinant of susceptibility to cytolysis by natural killer (NK) lymphocytes. We have examined this proposed relationship in several ways. First, K562 (a cell line highly vulnerable to NK lysis) cells were grown for 24 h in the iron chelator desferrioxamine. Under these conditions, the cells doubled their surface transferrin receptor expression as determined both by radioligand binding and surface binding of the OK-T9 monoclonal anti-transferrin receptor antibody. In contrast, cells grown for the same period of time in hemin halved their receptor expression. This fourfold change in transferrin receptor expression between the desferrioxamine-treated and hemin-treated cells produced no change in susceptibility to NK cytolysis. Second, HeLa (a cell line which in its native state is very resistant to NK cytolysis) cells were compared with K562 cells with respect to surface transferrin receptor expression. The difference in NK susceptibility of the two cell lines was not reflected in differences in transferrin receptor expression: the K562 cells expressed approximately 1.5 X 10(5) receptors per cell while HeLa cells expressed 2.0 X 10(5) receptors/cell. Third, infection of HeLa cells by measles virus greatly increased their susceptibility to NK lysis but produced no change in surface transferrin receptor expression. Furthermore, when measles-infected HeLa cells were grown for 6 d in medium supplemented with iron-saturated human transferrin they underwent a 50% reduction in receptor expression but no change in NK susceptibility. Finally, possible alterations in the surface expression of NK target antigens on modified cells were further assayed by their ability to serve as cold-target inhibitors of cytolysis of NK-sensitive target cells. We examined two groups of cells in which transferrin receptor expression was reduced. These were the transferrin-treated, measles-infected HeLa cells with the 50% receptor reduction, and K562 cells grown in medium containing hemin and iron salts where the reduction was five- to sixfold relative to control. In neither case was there a change in the apparent expression of NK target antigen(s). We conclude that there is a discordance between transferrin receptor expression and susceptibility to NK cytolysis in the model systems examined. Therefore, it is unlikely that the transferrin receptor per se is the target recognition structure for human NK cells, although a role in concert with other, as yet undefined molecules, cannot be excluded.

Separation of haem compounds by reversed-phase ion-pair high-performance liquid chromatography and its application in the assay of ferrochelatase activity

The separation of haems and porphyrins was achieved in a reversed-phase ion-pair high-performance liquid chromatography system using tetrabutylammonium hydrogen sulphate as the pairing ion. The concentration of methanol and pH in the mobile phase were determinative parameters for the elution pattern of the compounds. Two isocratic systems--one for the assay of protohaem IX and one for deuterohaem IX--were developed. The chromatographic systems were applied to the assay of ferrochelatase activity in mitochondria using either protoporphyrin or deuteroporphyrin as the substrate. The ferrochelatase activity was also measured in reticulocytes, which contain high levels of endogenous haem.

Availability, distribution and kinetics of the transferrin receptors of rabbit reticulocytes

The binding of transferrin by reticulocytes was studied at 3 degrees C and 37 degrees C. It was demonstrated that 50% of the total number of receptors are in an exposed position on the outer side of the plasma membrane and 50% of them are in a masked position. All of the receptors can take either of the positions. The transition between these positions is temperature dependent and can be observed as the progressive phase of the transferrin binding. It is suggested that the transferrin receptors are in dimers, one of the subunits being in the exposed position the other in the masked position. The subunits change position simultaneously. This movement of the receptors is independent of the presence of transferrin. In the light of these results a new model for transferrin-cell cycle is proposed.

Receptor-mediated endocytosis of transferrin and recycling of the transferrin receptor in rat reticulocytes

At 4 degrees C transferrin bound to receptors on the reticulocyte plasma membrane, and at 37 degrees C receptor-mediated endocytosis of transferrin occurred. Uptake at 37 degrees C exceeded binding at 4 degrees C by 2.5-fold and saturated after 20-30 min. During uptake at 37 degrees C, bound transferrin was internalized into a trypsin-resistant space. Trypsinization at 4 degrees C destroyed surface receptors, but with subsequent incubation at 37 degrees C, surface receptors rapidly appeared (albeit in reduced numbers), and uptake occurred at a decreased level. After endocytosis, transferrin was released, apparently intact, into the extracellular space. At 37 degrees C colloidal gold-transferrin (AuTf) clustered in coated pits and then appeared inside various intracellular membrane-bounded compartments. Small vesicles and tubules were labeled after short (5-10 min) incubations at 37 degrees C. Larger multivesicular endosomes became heavily labeled after longer (20-35 min) incubations. Multivesicular endosomes apparently fused with the plasma membrane and released their contents by exocytosis. None of these organelles appeared to be lysosomal in nature, and 98% of intracellular AuTf was localized in acid phosphatase-negative compartments. AuTf, like transferrin, was released with subsequent incubation at 37 degrees C. Freeze-dried and freeze-fractured reticulocytes confirmed the distribution of AuTf in reticulocytes and revealed the presence of clathrin-coated patches amidst the spectrin coating the inner surface of the plasma membrane. These data suggest that transferrin is internalized via coated pits and vesicles and demonstrate that transferrin and its receptor are recycled back to the plasma membrane after endocytosis.

Ultrastructural localization of the transferrin receptor and transferrin on marrow cell surfaces

The monoclonal antibody OKT9 (a known transferrin receptor antibody) and a monoclonal antibody to transferrin (ATfn) were used to localize the transferrin receptor and transferrin on marrow cells. After incubation of cell suspensions with the antibody, the cells were reacted with an affinity purified Fab fragment of goat anti-mouse IgG conjugated to horseradish peroxidase (GAM-HRP), which was in turn visualized by reaction with 3,3'-diaminobenzidine (DAB). Erythroblast cell surfaces stained intensely with OKT9-GAM-HRP-DAB, weaker staining was observed on reticulocyte surfaces, whereas erythrocyte surfaces lacked staining. Staining was present on surface caveolae, which often contained endogenous ferritin particles. Moderate to strong OKT9 staining was observed on less than 10% of presumed lymphoid cells. Monocytes, macrophages, promyelocytes, granulocytes, megakaryocytes and platelets consistently lacked OKT9 staining. ATfn-GAM-HRP-DAB staining of erythroid cells was similar to that observed with OKT9 staining; however, in contrast to the latter staining, ATfn stained the surfaces of megakaryocytes, platelets, monocytes and most lymphocytes. Promyelocytes stained weakly, whereas late granulocytes lacked staining. These results indicate that the T9 transferrin receptor (1) is largely confined to erythroid cells in marrow, (2) is diffusely distributed on the surface of early erythroid cells, (3) decreases with cell maturation, and (4) is lost when haemoglobin synthesis is completed. Transferrin appears in a similar distribution on erythroid cell surfaces but also appears to bind to some cell surfaces lacking the T9 receptor.

Receptor-mediated endocytosis and exocytosis of transferrin in Concanavalin A-stimulated rat lymphoblasts

Iron-loaded transferrin has been shown to be necessary for the support of cell proliferation in culture. This function depends upon interaction of transferrin with a specific high-affinity cell surface receptor. The present report is directed toward determining the consequences of the interaction of transferrin with this receptor on Concanavalin A-stimulated rat lymphocytes. Three specific questions have been posed: a) Is transferrin endocytosed following binding to its specific receptor in a temperature-dependent fashion? b) Following endocytosis, is the carrier protein released from the cell in a structurally and functionally intact form? and c) Is the cell surface transferrin receptor also endocytosed following ligand binding? The results provide affirmative answers to all questions. Using two independent probes of the cell surface versus intracellular location of transferrin we observed that cell-bound transferrin moved from the cell surface to the inside of the cell and subsequently back to the medium. This process occurred in a temperature-dependent fashion. When cells containing only intracellular transferrin were further incubated at 37 degrees C approximately 80% of cell-bound transferrin was released to the medium. Nearly all of this material retained reactivity with antibody to transferrin. In addition, endocytosed transferrin exhibited qualitatively and quantitatively equivalent binding reactivity with the transferrin receptor and showed identical electrophoretic mobility on SDS gel electrophoresis. Finally, using similar methodology to that employed with transferrin itself, we provide evidence that the specific receptor is also endocytosed.

Biphasic uptake of iron-transferrin complex by L1210 murine leukemia cells and rat reticulocytes

The kinetics of the cellular uptake of iron-transferrin complex was studied in L1210 murine leukemia cells and rat reticulocytes using 125I-transferrin. Saturation of transferrin with iron was necessary for optimal uptake. Following the incubation of cells with the radiolabeled complex a biphasic pattern of uptake was observed. The initial phase was rapid and relatively temperature-independent and was not altered by ethylamine, an inhibitor of transglutaminase activity which is necessary for receptor-mediated endocytosis. This phase was considered to result from receptor-ligand interaction which could be reversed to a great degree by replacement with unlabeled transferrin. A plateau was then reached, indicating a saturation of receptors. After 30 min a second phase of uptake was indicated by the second rise in the curve. This phase was slow, relatively temperature-dependent and could be abolished by ethylamine. It was interpreted as evidence of internalization of the ligand. Analysis of the data from competition studies with unlabeled transferrin indicated that the first phase might itself comprise a reversible and an irreversible step with a ratio of 5 to 1.4 for bound transferrin. Thus, the cellular uptake of iron-transferrin complex may consist of a reversible ligand-receptor interaction. Conformational changes may render this interaction irreversible and the internalization of the ligand may then follow.

Non-enzymic glycosylation of horse spleen and rat liver ferritins

Incubation of horse spleen ferritin and rat liver ferritin with [14C]glucose, [14C]mannose or [14C]fucose resulted in the covalent incorporation of the sugar into ferritin. The rate of reaction depended on the concentrations of ferritin and sugar and time of incubation. The order of this nonenzymic incorporation was glucose greater than mannose greater than or equal to fucose. Glucosylated or mannosylated ferritin was not retained by concanavalin A. The plasma half-life of rat liver ferritin and apoferritin was found to be 20 min. This value remained unaffected by in vitro glucosylation or mannosylation of ferritin. It is suggested that varying degrees of glycosylation might account for the occurrence of isoferritins.

Influence of zinc on iron uptake by monolayer cultures of rat hepatocytes and the hepatocellular ferritin

Monolayer cultures of isolated rat hepatocytes on collagen gels were used to study the effect of zinc on (a) 59Fe uptake by the hepatocytes; (b) 59Fe uptake by the hepatocellular ferritin and (c) biosynthesis of ferritin and total proteins. While deposition of iron into the cellular ferritin was inhibited by zinc in a dose-dependent manner, iron uptake by the hepatocytes was inhibited significantly only at high concentrations of zinc. Biosynthesis of total proteins or ferritin was not affected by zinc. Since only a small fraction of the cellular 65Zn was found to be associated with ferritin, it was concluded that zinc was not deposited in ferritin. Intracellular distribution of 59Fe revealed that newly accumulated iron went into an iron pool the size of which could be increased by the presence of zinc.