|Figure 1: Schematic mechanism for the uptake of iron by a cell. TfR expressed on the cell surface preferentially binds Transferrin (Tf) that has two atoms of iron. The Tf-TfR complex is internalized into endosomes. The pH of endosomes decreases, facilitationg iron dissociation and increasing the affinity of the receptor for apoTf. The iron moves to the cytosol, and the endosome recycles to the plasma membrane with the Tf-TfR complex. At the higher pH of the extracellular fluid, apo-Tf dissociates and is replaced by diferric Tf, and the cycle repeats.|
First printed in R&D Systems' 1998 Catalog.
Iron is transported in plasma complexed with transferrin, an 85-kDa protein with two iron-binding sites (reviewed in references 1 and 2). The uptake of iron by cells is mediated by a cell-surface transferrin receptor (TfR). The amount of transferrin receptor expressed on a cell is proportional to the cell's need for iron. A proteolytic product of the transferrin receptor circulates in plasma as soluble transferrin receptor (sTfR). The concentration of circulating sTfR is proportional to the total concentration of cellular TfR. Since most cellular iron utilization is by erythroid precursor cells, circulating sTfR is proportional to erythroid precursor mass (i.e., rate of erythropoiesis), and it is elevated in iron deficiency, when cells must be more competitive for obtaining their iron requirement.
The mechanism of iron uptake by TfR involves a selective affinity of the receptor for diferric transferrin.1 The TfR-transferrin complex is internalized and cycled through endosomes, where the iron is released to the cytosol, with the TfR-apotransferrin complex returning to the extracellular surface (Fig. 1). At the surface the apotransferrin dissociates to be replaced by diferrictransferrin, repeating the cycle.
Cellular need for more iron causes increased expression of TfR, regulated by a mechanism involving Iron-Responsive Elements (IRE) of mRNA and Iron-Binding Proteins (IBP).1,16-19 IBPs change conformation on binding iron; the iron-free conformation binds to IREs, modifying mRNA in ways that depend on the location of the IRE. In the case of TfR mRNA, binding of IBPs stabilizes the mRNA, thereby increasing the steady-state concentration and increasing the rate of synthesis of TfR. Thus, a low concentration of intracellular iron leads to increased expression of TfR.
80% of metabolic iron is for the synthesis of hemoglobin by erythroid precursors,2 and as a consequence, 80% of total TfR is on erythroid precursor cells. Thus, total TfR is roughly proportional to the erythroid precursor mass (i.e., the rate of erythropoiesis). As a consequence, sTfR is low in patients with hypoplastic anemias, and high is patients with hyperplastic anemias such as RBC hemolysis or chronic blood loss, in which an accelerated rate of anemia is insufficient.20-26 Similarly, plasma sTfR is high in patients who are iron deficient.27-29
Transferrin receptor is a transmembrane, disulfide-linked dimer of identical 85-kDa polypeptides1-7 (Fig 2). Each polypeptide chain is glycosylated, is palmitoylated at the inner membrane and has an intracellular phosphorylation site. The two polypeptide chains are covalently linked by disulfide bonds at residues 89 and 98, immediately extracellular of the membrane.
An Arg-Leu bond just distal to the second disulfide bond is susceptible to proteolysis8 to yield a short transmembrane protein with only 11 residues extending from the outside of the membrane plus a 660-amino-acid protein, sTfR, that circulates in plasma. There are conflicting reports about the physiological location of this proteolysis and the nature of the protease,9-14 and there is virtually nothing known about the half-time of circulation of sTfR or about its ultimate fate. It has been reported that the level of circulating sTfR is proportional to the amount of total cellular TfR15 and that nearly all transferrin receptor in plasma is the proteolyzed product.13
|Figure 2: Schematic of the TfR molecule showing the change leading to sTfR. Standard one-letter abbreviations for amino acids are used (C, cysteine; E, glutamate; L, leucine; M, methionine; F, phenylalanine; R, arginine). Arrows indicate the site of proteolytic cleavage.|
Most anemias are a consequence of either chronic disease (e.g., an inflammatory, infectious or malignant disease) or iron deficiency. These two causes of anemias have similar manifestations, and they can be differentiated only by ruling out iron deficiency. The importance of the differentiation is because iron deficiency is frequently the first symptom of serious gastrointestinal blood loss, possibly due to malignancy or ulcers. Because chronic disease and iron deficiency each affect the traditional laboratory measures of iron status in either the same or opposite direction, there have been no reliable laboratory tests to identify iron deficiency in patients with chronic disease. It has, however, been established that serum or plasma sTfR is elevated by iron deficiency in patients with or without chronic disease but is not affected by chronic disease in the absence of iron deficiency.30-34 In published clinical trials as well as in unpublished clinical trials sponsored by R&D Systems, it was demonstrated that elevated sTfR was highly correlated with absence of iron in a marrow aspirate,34 the "gold standard" of iron deficiency.
In addition to diagnosis of iron deficiency, serum sTfR has been used to monitor the rate of erythropoiesis.20-26 It has been used to follow marrow engraftment after bone-marrow transplant and to categorize anemias as either hyperplastic or hypoplastic. R&D Systems offers an ELISA for sTfR for research use or for the diagnosis of iron deficiency (Catalog # DTFR1).