Mechanism of production of the serum transferrin receptor.
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Amino Acid Sequence↗
Biomedical subjects
R D Baynes
Publications and source records attributed to R D Baynes.
The physiological significance of circulating transferrin receptors.
A soluble truncated form of the tissue transferrin receptor has been recently identified in human serum. The concentration of this serum receptor appears to reflect the total mass of tissue receptor and is consequently elevated with tissue iron deficiency and enhanced red cell production. When coupled with the serum ferritin, the serum transferrin receptor concentration provides a sensitive, quantitative index of iron status over a wide spectrum. While the physiological significance of the circulating receptor is still unknown, this new laboratory measurement will play an important role in the clinical and epidemiological detection of iron deficiency anemia.
Erythropoiesis↗
Identification of the membrane remnants of transferrin receptor with domain-specific antibodies.
Tissue culture studies with K562 and HL60 cells have demonstrated the production of a soluble form of transferrin receptor identical to that identified in human serum. The present study was undertaken to search for membrane remnants of the truncated receptor with peptide antibodies specific for the extracellular and cytoplasmic domain of transferrin receptor. In cell membranes, a 105K remnant was identified that is consistent with truncation of one extracellular domain monomer of the transferrin receptor. In the exosomal fraction of the culture supernatant, a smaller 20K remnant consistent with truncation of both extracellular domains was also demonstrated. These findings provide evidence that soluble receptor is the product of proteolytic cleavage of intact membrane-bound transferrin receptor. Prior studies showing that the concentration of the extracellular domain in exosomes remained stable during incubation in culture supernatant suggest that this cleavage possibly occurs intracellularly.
Amino Acid Sequence↗
Serum form of the erythropoietin receptor identified by a sequence-specific peptide antibody.
The present investigation was undertaken to search for soluble forms of the erythropoietin receptor in human serum using polyclonal antibody against an amino terminal peptide sequence in the extracellular domain. This sequence was located adjacent to the amino terminus at residues 25-38. When this antibody was used for Western blots of solubilized membranes from nucleated bone marrow cells, a protein consistent with native erythropoietin receptor was seen. Purified soluble ectodomain of the erythropoietin receptor displayed appropriate reactivity with this antibody. When sera from normal subjects and patients with a range of hematologic disorders were examined by Western blotting, a protein with a molecular mass of 34 Kd was detected in sera from patients with enhanced erythropoiesis including sickle cell anemia, thalassemia, and megaloblastic anemia. This protein was rarely detected in normal serum but appeared when normal subjects were treated with recombinant erythropoietin and disappeared after full treatment of patients with megaloblastic anemia due to vitamin B12 deficiency. The protein was not detected after myeloablation for bone marrow transplantation but appeared with marrow engraftment. Reactivity of this protein with the peptide antibody was competitively inhibited by the amino terminal peptide sequence. An additional 48 Kd protein was detected that showed minimal variation in intensity with differing degrees of erythropoietic activity. Detection of this protein could not be inhibited by the addition of synthetic peptide. Our findings indicate the presence of a soluble form of the erythropoietin receptor related to the extracellular domain that is highly correlated with enhanced erythropoiesis.
Amino Acid Sequence↗
Characterization and quantitation of the circulating forms of serum transferrin receptor using domain-specific antibodies.
To characterize the nature of the immunoreactive transferrin receptor in human serum, antisera were developed to peptide sequences of the extracellular domain of human transferrin receptor between amino acids 107 and 120 and the intracellular domain between amino acids 40 and 54. Antisera against the extracellular domain exhibited reactivity against both purified intact receptor and immunopurified circulating receptor, whereas antisera against the intracellular domain reacted only with intact receptor. Using competitive binding enzyme-linked immunosorbent assays, transferrin receptor in ultracentrifuged sera from normal subjects and patients with sickle cell anemia could be detected with antisera against the extracellular but not the intracellular domain. When the pellet obtained by ultracentrifugation of these sera was assayed after solubilization in 1% teric (polyoxyethylene-9-lauryl ether), only 0.6% of total serum receptor was detected in normal subjects and 3.8% in subjects with sickle cell disease. Roughly equal amounts of this pelleted immunoactivity were detected with antibodies against the extracellular and intracellular domains. These results indicate that less than 1% of transferrin receptor in normal human sera is intact receptor consistent with an exosomal origin and that virtually all circulating transferrin receptor is in the form of a truncated extracellular domain.
Amino Acid Sequence↗
Serum transferrin receptor.
The transferrin receptor plays a critical role in iron metabolism by precisely controlling the flow of transferrin iron into body cells. A soluble truncated form of the receptor can be detected in human serum using sensitive immunoassays, and the initial clinical experience with this new measurement indicates that it reflects the total body mass of tissue receptor. Serum receptor levels rise significantly with tissue iron deficiency and the heightened demand for iron associated with expansion of the erythroid marrow. The serum receptor provides a quantitative measure of functional iron deficiency and distinguishes the associated anemia from that of chronic disease. If iron deficiency is excluded, the serum receptor provides a quantitative measure of total erythropoiesis that is more sensitive and less invasive than bone marrow examination currently used to assess red cell precursor mass. Performed in conjunction with serum ferritin measurements, the serum receptor will be useful in establishing the true prevalence of iron deficiency anemia in population studies.
Anemia↗
Production of the serum form of the transferrin receptor by a cell membrane-associated serine protease.
Recent investigations have demonstrated that the soluble form of the transferrin receptor in human serum is an 85-kDa fragment of intact receptor containing most of the extracellular domain. The recent demonstration of a remnant of the truncated receptor in cell membranes suggests that the soluble form arises from proteolytic cleavage of intact receptor. In the present investigation, domain-specific antibodies were used to further examine the subcellular location and nature of this proteolysis. HL60 cells were used in the investigation because the cells release an 85-kDa soluble form of the receptor to the culture supernatant that is identical to that found in serum. When intact, purified transferrin receptor from human placenta was incubated with the culture supernatant, no proteolytic activity could be demonstrated. However, when purified membrane fractions from HL60 cells were used in this incubation system, the 85-kDa fragment was produced. This activity was inhibited by serine protease inhibitors indicating that cell membrane fractions contain a serine protease capable of producing the serum form of the transferrin receptor.
Amino Acid Sequence↗
Anaemia, iron-related measurements and erythropoietin levels in untreated patients with active leprosy.
The mechanisms responsible for anaemia in leprosy were studied prior to the institution of therapy in 56 patients with active disease. Haematological indices, iron-related measurements, inflammatory markers and erythropoietin levels were assessed, with bone-marrow studies being performed on anaemic patients. Anaemia was more common in the patients with lepromatous leprosy (85.7%) than it was in the rest of the group (19%). The lepromatous group exhibited the disordered iron transport of the anaemia of chronic disorders in that they had a significantly lower mean serum iron level (P less than 0.05), and a mildly raised serum ferritin concentration. Anaemic lepromatous patients also showed a blunted erythropoietin response compared with controls with non-inflammatory anaemia. A subgroup of five anaemic subjects displayed apparently adequate transport of iron to the erythroid marrow (normal percentage transferrin saturations and appropriate sideroblast counts) and the blunted erythropoietin response appeared to be the dominant factor in the pathogenesis of their anaemia. Analysis of inflammatory markers revealed that while the erythrocyte sedimentation rate was very high in the lepromatous subjects, there was no concomitant rise in C-reactive protein concentration. This suggests the presence of a disordered cytokine-mediated acute phase response in the condition.
Adolescent↗
Serum transferrin receptor distinguishes the anemia of chronic disease from iron deficiency anemia.
Recent studies have shown that the serum transferrin receptor is a sensitive, quantitative measure of tissue iron deficiency. This study was undertaken to determine the serum transferrin receptor's ability to distinguish iron-deficiency anemia from the anemia of chronic inflammation and to identify iron deficiency in patients with liver disease. The mean transferrin receptor level in 17 normal controls was 5.36 +/- 0.82 mg/L compared with 13.91 +/- 4.63 mg/L in 17 patients with iron-deficiency anemia (p less than 0.001). The mean serum receptor level was normal in all 20 patients with acute infection, including five with acute hepatitis, and was also normal in 8 of 10 anemic patients with chronic liver disease. Receptor levels were in the normal range in all but 4 of 41 patients with anemia of chronic disease. We conclude that unlike serum ferritin levels, which are disproportionately elevated in relation to iron stores in patients with inflammation or liver disease, the serum transferrin receptor level is not affected by these disorders and is therefore a reliable laboratory index of iron deficiency anemia.
Anemia↗
Maternal and fetal iron measurements in a hemochromatotic pregnancy.
The findings in the cord blood sample of an infant from a treated hemochromatotic mother of a raised transferrin saturation (88%) and a raised ferritin concentration (250.2 micrograms/L) together with elevated maternal values (66% and 91.6 micrograms/L, respectively) yet a normal total placental iron content (26.9 mg) suggested that in common with gastrointestinal mucosal cells and reticuloendothelial cells in hemochromatosis, the placental cell may exhibit an abnormality of iron storage and transport.
Adult↗
Ascorbic acid prevents the dose-dependent inhibitory effects of polyphenols and phytates on nonheme-iron absorption.
The effects of maize-bran phytate and of a polyphenol (tannic acid) on iron absorption from a white-bread meal were tested in 199 subjects. The phytate content was varied by adding different concentrations of phytate-free and ordinary maize bran. Iron absorption decreased progressively when maize bran containing increasing amounts of phytate phosphorous (phytate P) (from 10 to 58 mg) was given. The inhibitory effect was overcome by 30 mg ascorbic acid. The inhibitory effects of tannic acid (from 12 to 55 mg) were also dose dependent. Studies suggested that greater than or equal to 50 mg ascorbic acid would be required to overcome the inhibitory effects on iron absorption of any meal containing greater than 100 mg tannic acid. Our findings indicate that it may be possible to predict the bioavailability of iron in a diet if due account is taken of the relative content in the diet of the major promoters and inhibitors of iron absorption.
Absorption↗
Production of soluble transferrin receptor by K562 erythroleukaemia cells.
The present study was undertaken to examine the production of soluble transferrin receptor by K562 erythroleukaemia cells under controlled experimental conditions. The concentrations of soluble and cellular transferrin receptor were measured by immunoassay employing monoclonal antibodies. Cellular ferritin was also measured as an index of iron supply. With incubation up to 48 h there was a progressive increase in the concentration of soluble transferrin receptor. Manipulating iron supply by adding iron chelators or diferric transferrin to the incubation medium produced marked alterations in cellular receptor and ferritin content. Under all such conditions examined, the relationship between soluble and cellular receptor remained highly constant. These findings support clinical studies of serum receptor suggesting that over a broad spectrum of haematological disorders there is a fixed relationship between serum receptor and tissue receptor mass.
Antibodies, Monoclonal↗
Characterization of transferrin receptor released by K562 erythroleukemia cells.
A soluble form of transferrin receptor has been detected in human serum and has been shown recently to be a truncated form of the intact membrane bound receptor. Mechanisms governing the release of transferrin receptor by cells are poorly understood and could be better defined by tissue culture. The present investigation was undertaken to characterize the transferrin receptor released by K562 erythroleukemic cells. In contrast with maturing sheep reticulocytes, which have been shown to release transferrin receptor in small vesicles termed exosomes, we demonstrated, with a monoclonal enzyme-linked immunoassay, that less than 30% of the transferrin receptor released by K562 cells in log phase growth was in a particulate form. The relative amounts of soluble and particulate receptor released to the supernatant did not change significantly during 48 hr of incubation. Soluble receptor was purified by immunoaffinity chromatography. On polyacrylamide gel electrophoresis, its mobility was the same (85 kDa) as that of the truncated monomeric form recently identified in human serum. Further evidence that serum and soluble receptors released by K562 cells are identical was provided by amino acid sequence analysis, which demonstrated that 16 of the first 19 residues of the N-terminal sequence of soluble K562 receptor are homologous with the serum receptor. The remaining three were not identifiable. K562 cells provide a useful in vitro model for studying the production of membrane-bound and soluble forms of released transferrin receptor.
Amino Acid Sequence↗
Effects of alpha-interferon on iron-related measurements in human subjects.
Subcutaneous administration of alpha-interferon to normal volunteers caused hypoferraemia and hyperferritinaemia. There was, however, no concomitant rise in other components of the acute-phase response, including the serum C-reactive protein value, the granulocyte count and the plasma lactoferrin concentration. In fact, the latter two dropped significantly. The hypoferraemic response could be prolonged when a second dose was given 48 hours after the initial one. This hypoferraemic response may play a role similar to that induced by interleukin-1, which limits the proliferation of invading micro-organisms or neoplastic cells. The present findings may therefore have relevance to the clinical mechanism of action of the interferons.
C-Reactive Protein↗
Serum transferrin receptor is a truncated form of tissue receptor.
Recent studies have provided immunological evidence for the existence of transferrin receptor in human serum and have revealed that its concentration is a sensitive measure of erythropoiesis and iron deficiency. The present study was undertaken to establish the molecular identity of this immunoreactive component. Purification from human serum was accomplished by immunoaffinity chromatography using, as the ligand, monoclonal antitransferrin receptor antibody. The receptor preparation contained two major components with Mr of 75,000 and 85,000, which were identified as transferrin and transferrin receptor, respectively. The physicochemical and immunochemical properties of the 85,000 serum receptor were compared with those established for intact placental transferrin receptor. The serum receptor exhibited an apparent Mr = 85,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis under non-reducing conditions, as compared with 190,000 for placental transferrin receptor. Upon reduction, the Mr of serum receptor was unaltered, whereas, the 190,000 placental receptor dimer decreased to the expected monomer value of 95,000. Amino-terminal amino acid sequence analysis revealed that residues 1-19 of serum receptor were identical to residues 101-119 of intact receptor. These findings provide physicochemical evidence for the existence of transferrin receptor in human serum, establish its molecular identity as a truncated form lacking the cytoplasmic and transmembrane domains (residues 1-100) of intact receptor, and demonstrate that it exists as a transferrin-receptor complex in serum.
Amino Acid Sequence↗
Dietary iron overload in southern African rural blacks.
A survey conducted in rural southern African black subjects indicated that dietary iron overload remains a major health problem. A full blood count, erythrocyte sedimentation rate, serum concentrations of iron, total iron-binding capacity, ferritin, C-reactive protein (CRP), gamma-glutamyltransferase (GGT) and serological screening for hepatitis B and human immunodeficiency virus (HIV) infections were carried out in 370 subjects (214 inpatients and 156 ambulatory Mozambican refugees). The fact that the geometric mean (SD range) serum ferritin concentration was much higher in the male hospital patients than in subjects living in the community [1,581 micrograms/l (421-5,944 micrograms/l) and 448 micrograms/l (103-1,945 micrograms/l) respectively] suggested that dietary iron overload was not the only factor raising the serum ferritin concentration. The major additional factor appeared to be inflammation, since the geometric mean (SD range) serum CRP was significantly higher in male hospital patients [21 mg/l (8-53 mg/l)] than in subjects in the community [3 mg/l (1-5 mg)]. Alcohol ingestion, as judged by history and by serum GGT concentrations, was also associated with significantly raised serum ferritin concentrations. This finding was ascribed to the fact that traditional brews are not only associated with alcohol-induced hepatic damage but are also a very rich source of highly bio-available iron. The role of iron overload in the genesis of the raised serum ferritin concentrations are confirmed in the diagnostic liver biopsy study. The majority of biopsies showed heavy siderosis, with varying degrees of hepatic damage.(ABSTRACT TRUNCATED AT 250 WORDS)
Adolescent↗
Effect of traditional oriental soy products on iron absorption.
The effect of a variety of traditional oriental unfermented and fermented soy products on iron absorption was evaluated in 242 Indian women. When compared with a soy-flour meal containing an equal amount of protein, iron absorption was found to be significantly improved with silken tofu, natto, tempeh, rice miso, barley miso, and soybean miso. This improvement could not be adequately explained except with reference to changes in the protein composition of the products. The protein fraction profiles of the soy products were obtained by size-exclusion high-performance liquid chromatography. An inverse relationship between food iron absorption and the high-molecular-weight fraction of the soy products was demonstrated (r = 0.66, p = 0.01). However, anomalous results obtained with three products (sufu, tempeh, and fully hydrolyzed isolated soy protein) did not make it possible to reach firm conclusions concerning the effect of the protein fraction of soy on iron bioavailability.
Biological Availability↗