Iron transferrin receptor complex endocytosis

Many of the investigations into endosomal pathways have concentrated on receptor-mediated endocytosis, as in the iron-transferrin-receptor complex, and it is not clear how the systems vary depending on whether or not the pathway is clathrin-dependent or clathrin-independent [54],... 

The uptake of iron into the cell could follow several pathways. The iron could be released from the transferrin at the receptor site and be carried into the cell. Alternatively, the whole transferrin-receptor complex could be taken into the cell via endocytosis, and passed into an acidic compartment, where the iron is released, passed out of the compartment, and stored in ferritin. 

In humans, iron is transported across the gut by a series of poorly defined processes. Fe(III), ferric ion, is absorbed via a J03 integrin and mobilferrin, whereas ferrous ion enter the cells via Nramp. Once inside the body, Fe(III) is transported through the serum by transferrin, a protein of molecular weight 63,000 Da. Fe(III) transferrin is recognized by a receptor protein on the cell surface. Via a process known as cell-mediated endocytosis, the Fe(III) transferrin/receptor complex induces the external cell membrane to pucker and eventually form a clatharin-coated vesicle in the cytoplasm. After removal of the clatharin, the vesicle (known as an endosome) becomes... 

Another example is uptake of the iron-containing protein, transferrin, which circulates in the blood. It binds to its receptor to form a complex that enters the cell via endocytosis. The iron is then released from the endosome for use in the cell (e.g. haemoglobin formation for erythrocyte production or cytochrome production in proliferating cells). The number of transferrin receptors in the plasma membrane increases in proliferating cells and the number in the liver is increased by cytokines during infection. This results in a lower concentration of iron in the blood which decreases the proliferation of invading pathogens (Chapters 15 and 18). 

Transferrin is a single-chain glycoprotein that binds 2 g-atoms of ferric iron per mole of protein. The iron is chelated via tyrosyl and histidyl residues, and the complex is extremely stable at physiologic pH. The function of transferrin is to transport iron throughout the human organism, especially to the immature red cells, which cannot effectively acquire iron for the biosynthesis of hemoglobin unless it is presented to them in combination with transferrin. Specific transferrin receptors are present on the surface of such immature red cells as well as in all other tissues. Receptor-mediated endocytosis (see Chapter 9) is believed to be the main means of transferrin-bound iron entry into cells. Transferrin is also believed to be antimicrobial because it withholds iron from microorganisms. 

In the bloodstream, ferric iron binds tightly to circulating plasma transferrin (TF) to form diferric transferrin (FeTF). Absorption of iron into erythrocytes depends on basolateral membrane receptor-mediated endocytosis of FeTF by transferrin receptor 1 (TfR 1). FeTF binds to TfR 1 on the surface of erythroid precursors. These complexes invaginate in pits on the cell surface to form endosomes. Proton pumps within the endosomes lower pH to promote the release of iron into the cytoplasm from transferrin. Once the cycle is completed,TF and TfR 1 are recycled back to the cell surface. TF and TfR 1 play similar roles in iron absorption at the basolateral membrane of crypt enterocytes (Parkilla et al., 2001 Pietrangelo, 2002). 

The bulk of transferrin iron is delivered to immature erythroid cells for utilization in heme synthesis. Iron in excess of this requirement is stored as ferritin and hemosiderin. Unloading of iron to immature erythroid cells is by receptor-mediated endocytosis. The process begins in the clathrin-coated pits with the binding of diferric transferrin to specific plasma membrane transferrin receptors that are associated with the HFE protein complex. The next step is the internalization of the transferrin-transferrin receptor-HFE protein complex with formation of endosomes. The iron transporter DMTl present in the cell membrane is also internalized into the endosomes. In the endosomes, a proton pump acidifies the complex to pH 5.4, and by altering conformation of proteins, iron is released from transferrin bound to transferrin receptor... 

Early experiments showed that a transferrin-polycation complex transported bacterial DNA into cells [12]. Ions are taken up by cells as an iron-transferrin complex by receptor-mediated endocytosis. Protamine or poly-lysine ligated by disulfide bonds to transferring and mixed with a lu-ciferase-encoding plasmid may bind the DNA because of the cationic properties of the complex [12]. Subsequently, avian ery-throblasts and human K-562 cells were incubated with the transferrin-polycation peptide-DNA complex, and the complexes were recognized and transported into the cells by receptor-mediated endocytosis and taken up into endosome-Hke intracellular vesicles [12]. Treatment with chloroquine (an agent that affects the endosomal pH) enhanced the uptake considerably. In contrast to other transfection methods, the transfection of cells with transferrin-mediated endocytosis did not cause significant cell death, because of the physiologi-... 

Internal exchange of iron is accomplished by the plasma protein transferrin. This 76 kDa /Ij-glycoprotein has 2 binding sites for ferric iron. Iron is delivered from transferrin to intracellular sites by means of specific transferrin receptors in the plasma membrane. The iron-transferrin complex binds to the receptor, and the ternary complex is taken up by receptor-mediated endocytosis. Iron subsequently dissociates in the acidic, intracellular vesicular compartment (the endosomes), and the receptor returns the apotransferrin to the cell surface, where it is released into the extracellular environment. Cells regulate their expression of transferrin receptors and intracellular ferritin in response to the iron supply. Apoferritin synthesis is regulated post-transcriptionally by 2 cytoplasmic binding proteins (IRP-1 and lRP-2) and an iron-regulating element on its mRNA (IRE). 

Iron can be drawn from ferritin stores, transported in the blood as transferrin, and taken up via receptor-mediated endocytosis by cells that require iron (e.g., by reticulocytes that are synthesizing hemoglobin). When excess iron is absorbed from the diet, it is stored as hemosiderin, a form of ferritin complexed with additional iron that cannot be readily mobilized. 

Using cultured mammalian sarcoma cells, it has been found that transferrin is necessary in the growth medium for galllum-67 uptake to occur (95,96.97). A "transferrin receptor" on EMT-6 sarcoma cells for 25i iabeled transferrin was characterized by Scatchard analysis to have an average association constant K = 4.54 X 10 1/mole and approximately (with variation) 500,000 receptors per cell ( ). It was proposed that tumor accumulation of galllum-67 can occur only if the metal is complexed with transferrin so that it can interact with the receptors of tumor, as well as non-mallgnant cells (.33). The complex then enters the cell via an "adsorptive endocytosis" process (95.96.97.98.99) similar to the manner in which iron is taken up by reticulocytes and bone marrow cells (100.101). These transferrin receptors are saturable (that is, a plot of 125i transferrin uptake versus extracellular transferrin concentration reaches a peak ( at about 200 u g/ml) as more carrier transferrin is added to the medium) (95). Since uptake is also proportional to the fraction of... 

The transferrin receptor was one of the first to be exploited for receptor-mediated gene delivery. All actively metabolising cells require iron that is internalised by the cell as a transferrin-iron complex by means of receptor mediated endocytosis. To exploit this ubiquitous and efficient transport mechanism for introducing DNA into cells, conjugates of chicken or human transferrin with polycations (polylysine or protamine) were synthesised and used to form complexes with plasmid DNA. The number of transferrin molecules attached to each polylysine molecule varies according to the molecular weight of the polymer, but is generally around 1 transferrin molecule for every 50 lysine residues. ... 

The haem molecule would be incomplete without iron so this must be delivered to the progenitor red cells. Iron is toxic so it is carried in the plasma bound to a specific protein named transferrin (Tf). Uptake of iron is via a Tf receptor, of which there are approximately 300 000 per cell. The whole iron/Tf complex is taken into the cell by endocytosis where the iron is released and made available for incorporation into the porphyrin ring by ferrochelatase. 

The protein that transports iron around the body in blood and lymph, and indeed within the cell, is transferrin (500 kDa). It has two binding sites for iron (Fe " ) when no iron is bound, it is known as apotransferrin. Transferrin picks up not only the iron absorbed from the intestine but also that released from the macrophages and then transports it to the cells that require it, which is primarily the cells in the bone marrow but also other cells that are proliferating. For uptake into the cells, the transferrin binds to a receptor on the plasma membrane and then the complex enters the cell by endocytosis. The iron is released from the complex in the cytosol where it is bound by the intra-... 

A few substances are so large or impermeant that they can enter cells only by endocytosis, the process by which the substance is bound at a cell-surface receptor, engulfed by the cell membrane, and carried into the cell by pinching off of the newly formed vesicle inside the membrane. The substance can then be released inside the cytosol by breakdown of the vesicle membrane. Figure 1-5D. This process is responsible for the transport of vitamin B12, complexed with a binding protein (intrinsic factor) across the wall of the gut into the blood. Similarly, iron is transported into hemoglobin-synthesizing red blood cell precursors in association with the protein transferrin. Specific receptors for the transport proteins must be present for this process to work. 

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