Structure, function and clinical significance of transferrin receptors

Soluble transferrin receptors and tissue oxygenation in non anaemic cystic fibrosis patients

BACKGROUND

Chronic pulmonary disease and progressive tissue hypoxia are major causes of morbidity and mortality in cystic fibrosis (CF). Normally the body adapts to tissue hypoxia by increasing the red cell mass and decreasing the Hb-O(2) affinity. These adaptations are commonly observed in patients with cyanotic heart disease and individuals living at high altitude. However, patients with CF not only have an impaired erythroid response to hypoxia, but also are frequently anaemic.

METHODS

In order to evaluate erythroid marrow activity and tissue oxygenation in 37 patients with CF we measured: the haematological and blood chemistry parameters; including red cell indices, ferritin, erythropoietin (Epo) and soluble transferrin receptors (sTfR) levels; arterial blood gases, P(50) and oxygen release to the tissues (O(2)(R)) and the 2,3-BPG levels.

RESULTS

The main results showed that a) patients with CF have a mild degree of tissue hypoxia which is expressed by the moderately decreased of P(50) and O(2)(R) values and the relative increase of Epo level, b) 2,3-BPG synthesis in patients with CF is normal and c) sTfR levels are significantly increased (3-fold normal) in patients with CF compared to normal controls.

CONCLUSIONS

The above observations indicate that erythroid marrow activity in patients with CF is increased.

The soluble transferrin receptor (sTfR)-ferritin index is a potential predictor of celiac disease in children with refractory iron deficiency anemia

The soluble transferrin receptor (sTfR) distinguishes iron deficiency anemia from other types of anemia. Refractory iron deficiency anemia is often the onset symptom in malabsorption-induced celiac disease. We evaluated whether sTfR levels distinguish celiac disease-associated iron deficiency anemia from iron deficiency anemia of other origin. To this aim we measured sTfR and ferritin levels and their ratio (the sTfR/ferritin index) and other hematological parameters in 42 anemic children (20 with and 22 without celiac disease) vs. 22 non-anemic children with celiac disease and 31 healthy controls (age range 4-12 years). Hemoglobin parameters, mean cell volume, and serum iron and ferritin levels were decreased to a similar extent in the anemic patients (celiac and non-celiac). The sTfR level in non-anemic celiac patients was similar to that of normal controls (1.7+/-0.35 mg/L), whereas it was significantly increased in non-celiac and celiac anemic patients (2.2+/-0.5 mg/L, p<0.05 and 2.7+/-1.2 mg/L, p<0.001, respectively). The sTfR/ferritin index was also increased more in the anemic celiac patients (mean 4.4, range 1.5-12.0) than in anemic non-celiac children (mean 2.6, range 1.4-4.0) compared with non-anemic children (mean 1.2, range 0.7-2.0). Differences were more pronounced when ferritin was <5 ng/mL. Thus, the sTfR/ferritin index may be a predictive measure in discriminating anemic patients with celiac disease from those without celiac disease.

Immunological study of complex formation between soluble transferrin receptor and transferrin

Transferrin receptor (TfR) is a dimeric transmembrane protein that provides iron transport from plasma to cells by binding and internalization of iron-loaded transferrin (Tf). Soluble transferrin receptor (sTfR) is an extracellular part of the TfR molecule that is truncated from the cell surface and released into the blood stream. Using monoclonal antibodies (HyTest Ltd., Turku, Finland), immunofluorescent methods for sTfR and sTfR-Tf complex determination were developed. Soluble TfR was isolated from human plasma, and complex formation between sTfR and Tf was studied by stepwise complex construction and by FPLC gel filtration. It was found that sTfR could bind two Tf molecules step by step when the sTfR-Tf complex is constructed in the plate wells. FPLC gel filtration of sTfR-Tf mixtures and analysis of sTfR and sTfR-Tf immunological activities in collected fractions showed that sTfR can form different complexes with TF depending upon the ratios between them: a 291-kDa compound is assumed to be a 2:1 sTfR/Tf complex, and a 345-kDa compound is assumed to be a 2:2 sTfR/Tf complex. Isolated sTfR eluted as a 237-kDa protein. FPLC gel filtration of serum revealed that all sTfR in serum is bound to Tf in a 2:2 complex, and no isolated sTfR can be found in serum. This raises the question as to the nature of the bonds that hold two molecules of sTfR together to form a dimer.

Iron status influences trace element levels in human blood and serum

Food is the main source of trace elements for the general population. The gastrointestinal absorption of certain trace elements, e.g., cadmium, is strongly influenced by iron (Fe) status. This factor may also be relevant for the bioavailability of other trace elements. Therefore, we investigated relationships between Fe status indicators and trace element concentrations in blood and serum of 234 boys and girls at ages 15 and 17 years. Fe status was measured using serum ferritin (S-Ft), soluble transferrin receptor in serum (sTfR), and the ratio sTfR/S-Ft. The trace elements we investigated were, in blood, cadmium, cobalt, copper, zinc, selenium, rubidium, mercury, and lead, and, in serum, cobalt, copper, zinc, selenium, rubidium, tungsten, mercury, and lead. We found inverse correlations between Fe status and blood cadmium, blood or serum cobalt, or blood copper. There were positive correlations between Fe status and mercury concentrations. Selenium was positively correlated with sTfR. The relationships between Fe status and lead were equivocal. There were fewer correlations for serum than for blood, but the inverse relationships between Fe status and cobalt were equally strong in serum and blood. We found only occasional, and perhaps spurious, correlations with zinc, rubidium, and tungsten. In conclusion, previous indications that cadmium, cobalt, and copper are absorbed by transport mechanisms similar to that of Fe are supported by this study. Strong positive correlations between Fe status and mercury concentrations remain to be explained.

Defective regulation of iron transporters leading to iron excess in the polymorphonuclear leukocytes of patients on maintenance hemodialysis

BACKGROUND

Although hemodialysis (HD) patients are suspected of having defectively regulated iron metabolism, intracellular iron status has never been investigated thoroughly. To clarify the iron metabolism of HD patients, proteins involved in iron import (transferrin receptor [TfR]), as well as export (ferroportin 1), were investigated in polymorphonuclear leukocytes (PMNLs). Relations between iron status and several PMNL functions also were tested.

METHODS

Seventeen HD patients and 17 controls were recruited. Relative quantitative polymerase chain reaction was used to measure ferroportin 1 and TfR messenger RNA (mRNA), and ferroportin 1 and TfR expression were semiquantified by means of Western blot analysis or immunohistochemistry. PMNL functions also were examined.

RESULTS

Serum iron levels were significantly lower in HD patients than controls, and serum ferritin levels, as well as PMNL ferritin and iron content, were elevated in HD patients. Ferroportin 1 mRNA levels were substantially lower in PMNLs from HD patients, whereas TfR mRNA levels were higher. Western blot analysis and immunohistochemistry confirmed that expression of the corresponding proteins paralleled those of the mRNAs. PMNL phagocytic and bactericidal activity did not differ between HD patients and controls. Chemotactic peptide f-Met-Leu-Phe-stimulated degranulation activity of lactoferrin (Lf) was decreased significantly in HD patients, whereas those of myeloperoxidase and elastase were accelerated. Lf release correlated negatively with intracellular ferritin level.

CONCLUSION

We show for the first time that increased iron levels in PMNLs of HD patients were associated with downregulation of ferroportin 1 and upregulation of TfR, which might be linked to hypercytokinemia.

Soluble transferrin receptor and transferrin receptor-ferritin index in iron deficiency anemia and anemia in rheumatoid arthritis

The aim of the study was to evaluate the clinical efficiency of soluble transferrin receptor and transferrin receptor-ferritin index (sTfR/logF) in the diagnosis of iron deficiency anemia, as well as the differential diagnosis of iron deficiency anemia and anemia in rheumatoid arthritis. The study included 96 patients with anemia and 61 healthy volunteers as a control group. In healthy subjects there were no significant sex and age differences in the parameters tested. The study results showed these parameters to be reliable in the diagnosis of iron deficiency anemia, as well as in the differential diagnosis of iron deficiency anemia and anemia of chronic disease. The results indicate that sTfR/logF could be used to help differentiate coexisting iron deficiency in patients with anemia of chronic disease. Receiver operating characteristic analysis showed a higher discriminating power of transferrin receptor-ferritin index vs. soluble transferrin receptor in the diagnosis of iron deficiency anemia, as well as in the differential diagnosis between iron deficiency anemia and anemia of chronic disease. In patients with anemia in rheumatoid arthritis, the parameters tested showed no significant differences with respect to C-reactive protein concentration. These results suggested that the parameters tested are not affected by acute or chronic inflammatory disease.

Friedreich's ataxia, no changes in mitochondrial labile iron in human lymphoblasts and fibroblasts: a decrease in antioxidative capacity?

Friedreich's ataxia (FRDA) is caused by low expression of frataxin, a small mitochondrial protein. Studies with both yeast and mammals have suggested that decreased frataxin levels lead to elevated intramitochondrial concentrations of labile (chelatable) iron, and consequently to oxidative mitochondrial damage. Here, we used the mitochondrion-selective fluorescent iron indicator/chelator rhodamine B-[(1,10-phenanthrolin-5-yl)aminocarbonyl]benzylester (RPA) to determine the mitochondrial chelatable iron of FRDA patient lymphoblast and fibroblast cell lines, in comparison with age- and sex-matched control cells. No alteration in the concentration of mitochondrial chelatable iron could be observed in patient cells, despite strongly decreased frataxin levels. Uptake studies with (55)Fe-transferrin and iron loading with ferric ammonium citrate revealed no significant differences in transferrin receptor density and iron responsive protein/iron regulatory element binding activity between patients and controls. However, sensitivity to H(2)O(2) was significantly increased in patient cells, and H(2)O(2) toxicity could be completely inhibited by the ubiquitously distributing iron chelator 2,2'-dipyridyl, but not by the mitochondrion-selective chelator RPA. Our data strongly suggest that frataxin deficiency does not affect the mitochondrial labile iron pool or other parameters of cellular iron metabolism and suggest a decreased antioxidative defense against extramitochondrial iron-derived radicals in patient cells. These results challenge current concepts favoring the use of mitochondrion-specific iron chelators and antioxidants to treat FRDA.

Evaluation of immunoassays for the measurement of soluble transferrin receptor as an indirect biomarker of recombinant human erythropoietin misuse in sport

Soluble transferrin receptor (sTfR) has been proposed as an indirect biomarker of the misuse of recombinant human erythropoietin in sport. An extended validation of four commercially available immunoassays for its measurement in serum is presented. Two ELISA techniques (ELISA1: Orion Diagnostica; ELISA2: R&D Systems), an immunoturbidimetric technique (Turbid: Roche Diagnostics), and a nephelometric technique (Nephel: Dade Behring) were investigated. Intra-laboratory precision better than 3% and correct accuracies were obtained for the Turbid and Nephel techniques using autoanalysers. Slightly worse precision (but always better than 11%) and correct accuracies were also obtained in almost all cases for the two ELISA techniques. Inter-laboratory results showed higher concordances for the ELISA procedures (intraclass correlation coefficients of 0.848 for ELISA1 and 0.973 for ELISA2 which was clearly better). Inter-technique correlations were good for the four techniques with lower dispersions found for the techniques using autoanalysers, i.e. Turbid and Nephel. While Turbid and ELISA1 results (expressed in mg/l) were comparable, results obtained with Nephel were approximately 2.7 times lower. The relationship between those three techniques was maintained when compared with ELISA2, which uses different units (nmol/l). We conclude that ELISA2 and Nephel in our hands were the most suitable techniques in terms of sensitivity, precision and accuracy, and adequacy of the calibration curve for the measurement of sTfR in real serum samples. Discrepancies observed in the results obtained with the different sTfR techniques showed that different reference standards were used and harmonization is recommended in order to obtain comparable results.

Iron assessment tests: transferrin receptor vis-à-vis zinc protoporphyrin

OBJECTIVES

To review and compare the biochemical, analytical, and clinical features of two relatively new tests for assessing iron status and diagnosing iron disorders, namely, the serum transferrin receptor concentration (sTfR) and the erythrocyte zinc protoporphyrin/heme ratio (ZnPP/H). To consider the merits of each test for the diagnosis of iron disorders with emphasis on iron-deficient erythropoiesis, especially in the clinically important preanemia stage of iron depletion.

CONCLUSIONS

Although the basic biochemical mechanisms underlying the two tests are very different, both of these tests are noteworthy because they are considered to reflect iron status in the bone marrow. The principal advantage to serum transferrin receptor is the lack of a response to anemia of chronic disease (ACD), which affects other iron status indicators, for example, ferritin and transferrin saturation. The principal advantage to erythrocyte zinc protoporphyrin is low cost, but point-of-care testing and simplicity can also be advantages. Both serum transferrin receptor and erythrocyte zinc protoporphyrin have been demonstrated to be useful in a variety of clinical situations. Serum transferrin receptor can be best used in diagnosing iron disorders, especially for patients with pathologies that may affect iron metabolism. Erythrocyte zinc protoporphyrin can be best used as a primary screening test for assessing iron status, especially in patients likely to have uncomplicated iron deficiency. While these applications are primary, both tests are more broadly useful, for example, in monitoring iron therapy.

Expression of epithelial cell iron-related genes upon infection by Neisseria meningitidis

Infection by the obligate human pathogens Neisseria meningitidis (MC) and Neisseria gonorrhoeae (GC) reduces the expression of host epithelial cell transferrin receptor 1 (TfR-1) (Bonnah et al., 2000, Cellular Microbiology 2: 207-218). In addition, the rate and pattern of TfR-1 cycling is altered, leading to diminished uptake of Tf-iron by infected host cells. As Tf-iron is important for maintaining iron homeostasis in the eukaryotic cell, these findings raised the possibility that Neisseria infection might affect further pathways of epithelial cell iron metabolism. We used a specialized cDNA microarray platform, the 'IronChip', to investigate the expression of genes involved in iron transport, storage and regulation. We show that mRNA expression of several host genes involved in iron homeostasis is altered. Surprisingly, the general mRNA expression profile of infected cells closely resembled that of uninfected cells grown in an iron-limited environment. An important exception to this profile is TfR-1, the mRNA level of which is strongly reduced. Low TfR-1 expression may be explained in part by decreased activity of the iron-regulatory proteins (IRPs) in MC-infected cells, which may result in the destabilization of TfR-1 mRNA. Intriguingly, low IRP activity contrasts with the decrease in H-ferritin protein levels in infected cells. This finding suggests that low IRP activity may be responsible in part for the decrease in TfR-1 mRNA levels. A discussion of these novel findings in relation to MC infection and virulence is provided.

Soluble transferrin receptor in Aboriginal children with a high prevalence of iron deficiency and infection

OBJECTIVES

Aboriginal children in tropical Australia have a high prevalence of both iron deficiency and acute infections, making it difficult to differentiate their relative contributions to anaemia. The aims of this study were to compare soluble transferrin receptor with ferritin in iron deficiency anaemia (IDA), and to examine how best to distinguish the effect of iron deficiency from infection on anaemia.

METHODS

We conducted a prospective study of 228 admissions to Royal Darwin Hospital in children from 6 to 60 months of age. Transferrin receptor concentrations were measured by a particle-enhanced immunoturbidimetric assay and ferritin by a microparticle enzyme immunoassay.

RESULTS

On multiple regression, the best explanatory variables for haemoglobin differences (r2=33.7%, P<0.001) were mean corpuscular volume (MCV), red cell distribution width (RDW) and C-reactive protein (CRP); whereas transferrin receptor and ferritin were not significant (P>0.4). Using > or =2 abnormal indices (MCV, RDW, blood film)+haemoglobin <110 g/l as the reference standard for IDA, transferrin receptor produced a higher area under the curve on receiver operating characteristic curve analysis than ferritin (0.79 vs. 0.64, P<0.001) or the transferrin receptor-ferritin index (0.77). On logistic regression, the effect of acute infection (CRP) on haemoglobin was significant (P<0.001) at cut-offs of 105 and 110 g/l, but not at 100 g/l when only iron deficiency indicators (MCV, RDW, blood film) were significant.

CONCLUSIONS

Transferrin receptor does not significantly improve the diagnosis of anaemia (iron deficiency vs. infection) over full blood count and CRP, but in settings with a high burden of infectious diseases and iron deficiency, it is a more reliable adjunctive measure of iron status than ferritin.

Single value of serum transferrin receptor is not diagnostic for the absence of iron stores in anaemic patients with rheumatoid arthritis

Serum transferrin receptor (sTfR) concentrations were measured in anaemic patients with rheumatoid arthritis (RA). Serum transferrin receptor concentrations were positively correlated with the percentage of hypochromic cells and negatively correlated with MCH. There was a weak correlation with serum ferritin (sFn) concentration but not with reticulocyte count. Thus, high concentrations of sTfR indicate iron-deficient erythropoiesis rather than levels of storage iron in the tissues. Patients were divided into three groups on the basis of sFn concentration: those with probable tissue iron deficiency, those with adequate iron stores and those with intermediate values of sFn which did not allow classification. The median sTfR concentration was significantly higher in the iron-deficient group than in the other two groups but because of overlap between the three groups, a single sTfR value was of limited value in determining the level of storage iron in an individual with RA.

Plasma levels of soluble transferrin receptors and Clara cell protein (CC16) during bipolar mania and subsequent remission

Clara cell protein (CC16) and transferrin receptor (TfR) have been reported as possible biological markers for major depression and schizophrenia. However, the alternations of plasma TfR and CC16 levels and the influences of numerous clinical variables on them during bipolar mania are not sufficiently described. We investigated the immune function of 36 bipolar I, manic (DSM-IV) patients with Young Mania Rating Scale (YMRS) scores > or =26 as well as during the subsequent remission (YMRS < or =12) and age- and sex- matched healthy controls. The plasma TfR levels were increased during acute mania along with subsequent remission and were independent of medication status, individual variations, clinical and erythrocyte variables. Among inflammatory parameters and haematological variables, the plasma TfR levels merely had significant and negative relationship with the percentage of monocyte in circulating leukocyte counts despite of elevated plasma soluble interleukin-2 receptors levels during bipolar mania. The plasma levels of CC16 of bipolar patients did not significantly alter during acute mania, whereas smoking, body mass index, and co-existing psychotic features collectively contributed 42% of the plasma levels of CC16. We provide additional evidence to indicate the pathophysiological role of the immune systems in affective disorders. It is suggested that the elevation of plasma TfR levels might be a trait phenomenon in bipolar disorder.

Serum transferrin receptor is a marker of iron deficiency in patients included in programs for preoperative autologous blood donation

Recently programs for preoperative autologous blood donation (PABD) have expanded to reduce the need for allogenic blood transfusion. Nevertheless, the ability of the patients's bone marrow to replace the red blood cells (RBCs) mass reduced by phlebotomies determines the efficacy of PABD. In mild anemia, known as iron-deficient erythropoiesis (IDE) or iron deficiency without anemia, precipitated by PABD, the marrow response is suboptimal and needs adjuvant therapy. The aim of this study was to evaluate the use of the serum transferrin receptor (sTfR) for the assessment of IDE in patients undergoing PABD.

METHODS

Two autologous blood units from 50 consecutive patients scheduled for elective orthopedic surgery were collected preoperatively. Serial measurements of RBCs, haematocrit (Hct), haemoglobin (Hb), serum iron, serum ferritin, reticulocyte count, reticulocyte maturity index (RMI), endogenous erythropoietin (EPO) and sTfR were performed throughout the phlebotomy program.

RESULTS

RBC, Hct, Hb and serum iron significantly decreased although within the normal range. There was no change in serum ferritin levels. Reticulocytes, RMI and EPO significantly increased as did sTfR which significantly exceeds the normal range.

CONCLUSIONS

These results demonstrate that the sTfR is a reliable laboratory marker for detecting mild anemia or IDE. In patients undergoing PABD increased sTfR levels may suggest a treatment with recombinant human EPO (rh-EPO) or iron to improve the bone marrow performance.

Malarial anemia leads to adequately increased erythropoiesis in asymptomatic Kenyan children

Malarial anemia is associated with a shift in iron distribution from functional to storage compartments. This suggests a relative deficit in erythropoietin production or action similar to that observed in other infections. Our study in Kenyan children with asymptomatic malaria aimed at investigating whether malaria causes increased erythropoiesis, and whether the erythropoietic response appeared appropriate for the degree of resulting anemia. Longitudinal and baseline data were used from a trial with a 2 x 2 factorial design, in which 328 anemic Kenyan children were randomly assigned to receive either iron or placebo, and sulfadoxine-pyrimethamine or placebo. Erythropoiesis was evaluated by serum concentrations of erythropoietin and soluble transferrin receptor. Prospectively collected data showed that malarial infection resulted in decreased hemoglobin concentrations, and increased serum concentrations of erythropoietin and transferrin receptor. Conversely, disappearance of malarial antigenemia resulted in increased hemoglobin concentrations, and decreased concentrations of these serum indicators. Additionally, our baseline data showed that current or recent malarial infection is associated with increased serum concentrations of erythropoietin and transferrin receptor, and that these were as high as or perhaps even higher than values of children without malarial infection and without inflammation. Our findings indicate that in asymptomatic malaria, the erythropoietic response is adequate for the degree of anemia, and that inflammation probably plays no or only a minor role in the pathogenesis of the resulting anemia. Further research is needed to demonstrate the role of deficient erythropoietin production or action in the pathogenesis of the anemia of symptomatic malaria.

Stunting may determine the severity of malaria-associated anemia in African children

OBJECTIVE

Evidence from previous studies that malnourished children are protected against malaria is controversial. In individuals repeatedly exposed to malaria, immunity may develop first against severe disease, then against pyrogens, and last, against parasites. If this is true, this would suggest that reduced immune function that may exist in stunted children exacerbates the severity of malarial signs and symptoms, rather than the occurrence of parasitemia. On the other hand, several studies have suggested that malnourished children are protected to some degree against malaria. Our aim was to evaluate whether observational data support the hypothesis that nutritional inadequacies that cause stunting modify the associations between malaria and hematologic indicators such as hemoglobin concentration and serum concentrations of C-reactive protein and soluble transferrin receptor (sTfR). We showed earlier that increased serum concentrations of these receptors in asymptomatic malaria may be explained, at least in part, by increased erythropoiesis to compensate for malaria-induced hemolysis.

METHODOLOGY

Community-based cluster survey among Kenyan children aged 2 to 36 months asymptomatic for malaria or anemia (n = 318).

RESULTS

When adjusted for age and wasting, the malaria-associated decrease in mean hemoglobin concentration was 8.5 g/L and 15.8 g/L in nonstunted and stunted children, respectively. The malaria-associated increase in geometric mean serum concentrations of sTfR was 1.1-fold and 1.8-fold in nonstunted and stunted children, respectively. The malaria-associated increase in geometric mean serum concentrations of C-reactive protein was 1.4-fold and 2.3-fold in nonstunted and stunted children, respectively. Thus, children with malaria and those who were stunted suffered from more severe anemia and had higher serum concentrations of C-reactive protein and sTfR than would be expected from the combined effect of the 2 working independently.

CONCLUSIONS

Our results are consistent with the notion that the nutritional inadequacies causing stunting also impair host immunity, thus increasing the degree to which malaria is associated with decreased concentrations of hemoglobin, with increased inflammation, and with increased iron demand in developing erythroblasts. Increased intake of micronutrients may not only reduce stunting and nutritional anemia, but also reduce malaria-associated anemia.

Effects of exercise on soluble transferrin receptor and other variables of the iron status

BACKGROUND

Soluble transferrin receptor (sTfr) is a new marker of iron status and erythropoietic activity. It has been included in multivariable blood testing models for the detection of performance enhancing erythropoietin misuse in sport.

OBJECTIVE

To evaluate the effect of different types and volumes of physical activity on sTfr concentration, variables of iron status (ferritin, transferrin, iron, and protein), and haematological indices.

METHODS

Thirty nine subjects were divided into three groups: 1, untrained (n = 12); 2, moderately trained (n = 14); 3, highly trained (n = 13, seven men, six women). Groups 1 and 2 carried out two exercise tests: an incremental running test until exhaustion (test A) and a 45 minute constant speed running test at 70% VO(2)MAX (test B). Group 3 performed three days (women) or four days (men) of prolonged aerobic cycling exercise. The above variables together with haemoglobin and packed cell volume were analysed in venous blood samples before and after exercise. Changes in blood and plasma volume were estimated.

RESULTS

sTfr levels were slightly increased in trained and untrained subjects immediately after test A. Test B and aerobic exercise had no significant effect on sTfr. Ferritin levels were increased after the laboratory tests for trained and untrained subjects and after prolonged aerobic exercise in male cyclists. Transferrin was increased significantly in trained and untrained subjects after both laboratory tests, but remained unchanged after prolonged exercise. Plasma and blood volumes were decreased after the laboratory tests but increased after aerobic exercise. No differences in the variables were observed between trained and untrained subjects with respect to response to exercise.

CONCLUSION

The changes in sTfr and the variables of iron status can be mainly attributed to exercise induced changes in volume. Taking these limitations into account, sTfr can be recommended as a marker of iron deficiency in athletes.

Multicenter evaluation of a fully mechanized soluble transferrin receptor assay on the Hitachi and cobas integra analyzers. the determination of reference ranges

Soluble transferrin receptor (sTfR) is reported to be a reliable marker for the diagnosis of iron deficiency, especially when iron metabolism is influenced by inflammatory disorders such as infection, chronic inflammation and cancer-related anemia. In the present multicenter study the analytical performance of a recently introduced, latex-enhanced immunoturbidimetric assay for the determination of soluble transferrin receptor (Tina quant [a] sTfR, Roche Diagnostics) on different fully mechanized analyzers such as Hitachi 917 and 911, and Cobas Integra 400 and 700 was evaluated. Within-run and between-run imprecision showed good results (CV<5% and <7%, respectively). The assay was found to be linear over a wide measuring range (0.4-35 mg/l). Endogenous substances did not interfere with the test results. Comparison of serum sTfR concentrations with those of heparinized plasma revealed good correlation (r>0.976). Method comparison with an existing fully mechanized method as well as with ELISA tests for sTfR showed very good correlation (r>0.987). Because of the lack of international standardization the results differed from each other up to 2.5-fold. The 95% of serum levels in healthy individuals ranged from 1.9 to 4.4 mg/l (n=427). However, the reference ranges should be reported in a sex-dependent manner, as 2.2-5.0 mg/l for men (n=211) and as 1.9-4.4 mg/l for premenopausal (n=216) and postmenopausal (n=45) women. The Tina quant [a] sTfR assay enables the precise, accurate, rapid and convenient determination of sTfR concentrations for routine clinical chemistry purposes.

Regulation of transferrin-induced endocytosis by wild-type and C282Y-mutant HFE in transfected HeLa cells

The hereditary hemochromatosis protein HFE is known to complex with the transferrin receptor; however, its function regarding endocytosis of transferrin is unclear. We performed patch-clamp capacitance measurements in transfected HeLa cells carrying wild-type or C282Y-mutant HFE cDNA under the control of a tetracycline-sensitive promoter. Whole cell experiments in cells with suppressed expression of wild-type HFE revealed a decrease in membrane capacitance, reflecting predominance of endocytosis in the presence of transferrin. Cells overexpressing C282Y-mutant HFE displayed less intense capacitance decreases, whereas no significant decrease was observed in cells overexpressing wild-type HFE. The formation of single endocytic vesicles in cells with suppressed expression of wild-type HFE was greatly increased in the presence of transferrin as revealed by cell-attached recordings. According to their calculated diameters, many of these vesicles corresponded to clathrin-coated vesicles. These results suggest that wild-type HFE negatively modulates the endocytic uptake of transferrin. This inhibitory effect is attenuated in cells expressing C282Y-mutant HFE. Time-resolved measurements of cell membrane capacitance provide a powerful tool to study transferrin-induced endocytosis in single cells.

Subclinical anaemia of chronic disease in adult patients with cystic fibrosis

Patients with chronic hypoxaemia develop secondary polycythaemia that improves oxygen-carrying capacity. Therefore, normal haemoglobin and haematocrit values in the presence of chronic arterial hypoxaemia in cystic fibrosis constitute 'relative anaemia'. We sought to determine the cause of this relative anaemia in patients with cystic fibrosis. We studied haematological indices and oxygen saturation in healthy volunteers (n=17) and in adult patients with cystic fibrosis (n=15). Patients with cystic fibrosis had lower resting arterial oxygen saturation when compared with normal volunteers (P<0.0001), and exercise led to a greater reduction in arterial oxygen saturation (P<0.0001). However, haemoglobin and haematocrit values in patients with cystic fibrosis did not significantly differ from normal volunteers. Serum iron (P=0.002), transferrin (P=0.02), and total iron-binding capacity (P=0.01) were lower in patients with cystic fibrosis. There were no significant differences in serum ferritin, percentage iron saturation, serum erythropoietin or red cell volume between the groups. The data presented demonstrate a characteristic picture of anaemia of chronic disease in adult patients with cystic fibrosis, except for normal haemoglobin and haematocrit values. Normal haemoglobin and haematocrit values in patients with cystic fibrosis appear to represent a combination of the effects of arterial hypoxaemia promoting polycythaemia, counterbalanced by chronic inflammation promoting anaemia of chronic disease.

A prospective, cross-sectional study of anaemia and peripheral iron status in antiretroviral naïve, HIV-1 infected children in Cape Town, South Africa

BACKGROUND

Anaemia is a common manifestation of paediatric HIV infection. Although there are many causes, anaemia of chronic diseases is the most frequent type. In poor countries iron deficiency is widespread. It is probable that many HIV-infected children in these countries are also iron deficient. This study describes the relationship between paediatric HIV infection and anaemia, and documents the peripheral iron status of antiretroviral naive, HIV-infected children.

METHODS

Sixty children were evaluated prospectively. Investigations included CD4+ count, haemoglobin concentration (Hb), red blood cell (RBC) morphology, and iron studies.

RESULTS

Anaemia was present in 73% of children. Compared to mild HIV infection, median Hb was lower in children with moderate clinical infection (104 g/L v 112 g/L, p = 0.04) and severe clinical infection (96 g/L v 112 g/L, p = 0.006), and more children with severe infection were anaemic (92% v 58%, 0.04). There was a significant relationship between immunological status and Hb. 68% had abnormal RBC morphology. Significantly more children with moderate and severe disease, and severe immunosuppression had abnormal RBC morphology. 52% were iron-depleted, 20% had iron-deficient erythropoiesis and 18% iron deficiency anaemia (IDA). 16% (7/44) of anaemic children had microcytosis and hypochromia. Median soluble transferrin receptor concentration was significantly higher in those with microcytic hypochromic anaemia (42.0 nmol/L v 30.0 nmol/L, p = 0.008).

CONCLUSIONS

Both the proportion of anaemic children and the median Hb were associated with disease status. Iron depletion and IDA are major problems in HIV-infected children in South Africa.

Identification of the hemoglobin scavenger receptor/CD163 as a natural soluble protein in plasma

The hemoglobin scavenger receptor (HbSR/CD163) is an interleukin-6- and glucocorticoid-regulated macrophage/monocyte receptor for uptake of haptoglobin-hemoglobin complexes. Moreover, there are strong indications that HbSR serves an anti-inflammatory function. Immunoprecipitation and immunoblotting enabled identification of a soluble plasma form of HbSR (sHbSR) having an electrophoretic mobility equal to that of recombinant HbSR consisting of the extracellular domain (scavenger receptor cysteine-rich 1-9). A sandwich enzyme-linked immunosorbent assay was established and used to measure the sHbSR level in 130 healthy subjects (median, 1.87 mg/L; range, 0.73-4.69 mg/L). To evaluate the sHbSR levels in conditions with increased leukocyte stimulation and proliferation, 140 patients admitted to a hematological department were screened. Several patients, with a broad spectrum of diagnoses, had a level of sHbSR above the range of healthy persons. Patients with myelomonocytic leukemias and pneumonia/sepsis exhibited the highest levels (up to 67.3 mg/L). In conclusion, sHbSR is an abundant plasma protein potentially valuable in monitoring patients with infections and myelomonocytic leukemia.

Serum transferrin receptor concentration indicates increased erythropoiesis in Kenyan children with asymptomatic malaria

BACKGROUND

Serum transferrin receptor concentrations indicate both erythropoietic activity and the deficit of functional iron in the erythron. In contrast with serum ferritin concentrations, serum transferrin receptor concentrations are not or are only marginally influenced by the inflammatory response to infection.

OBJECTIVE

We assessed iron status and examined the relation between serum transferrin receptor concentrations and malaria in children aged 2-36 mo who were asymptomatic for malaria.

DESIGN

This was a community-based cluster survey (n = 318).

RESULTS

Prevalences of malaria, anemia (hemoglobin concentration <110 g/L), iron deficiency (serum ferritin concentration <12 microg/L), and iron deficiency anemia were 18%, 69%, 53%, and 46%, respectively. Malaria was associated with lower mean hemoglobin concentrations (92.7 compared with 104.1 g/L; P = 0.0001) and higher geometric mean serum concentrations of transferrin receptor (11.4 compared with 7.8 mg/L; P = 0.005), ferritin (21.6 compared with 11.9 microg/L; P = 0.05), and C-reactive protein (12.5 compared with 6.8 mg/L; P = 0.004). There was no evidence for an association between serum concentrations of C-reactive protein and transferrin receptor. Children with malaria had higher serum transferrin receptor concentrations than expected for the degree of anemia, even after adjustment for inflammation indicated by serum C-reactive protein concentration quartiles (P = 0.02).

CONCLUSIONS

Our findings are consistent with the notion that malaria-induced hemolysis is accompanied by increased erythropoiesis. Serum transferrin receptor concentration is not useful for detecting iron deficiency in individuals with malaria. Individuals with high concentrations of serum C-reactive protein or similar acute phase reactants should be excluded from analysis if serum ferritin concentrations <12 microg/L are to be used to measure iron deficiency in malaria-endemic areas.

Molecular aspects of iron absorption and HFE expression

Hereditary hemochromatosis, a disease of iron overload, occurs in about 1 in 200-400 Caucasians. The gene mutated in this disorder is termed HFE. The product of this gene, HFE protein, is homologous to major histocompatibility complex class I proteins, but HFE does not present peptides to T cells. Based on recent structural, biochemical, and cell biological studies, transferrin receptor (TfR) is a ligand for HFE. This association directly links HFE protein to the TfR-mediated regulation of iron homeostasis. Although evidence is accumulating that binding of HFE to TfR is critical for the effects of HFE, the final pieces in the HFE puzzle have not been established. This review focuses on recent advances in HFE research and presents a hypothetical model of HFE function.

The effects of iron deficiency on lymphocyte cytokine production and activation: preservation of hepatic iron but not at all cost

Worldwide, over 40% of children have iron deficiency anaemia, frequently associated with infections. Certain cytokines are involved in both immune activation/response to infection and iron transport/metabolism. We therefore assessed the relations among iron deficiency, cytokine production and lymphocyte activation markers in 142 hospitalized Malawian children. We examined peripheral blood lymphocyte antigens/cytokine production using four- colour flow cytometry and serum transferrin receptor (TfR) levels, an inverse measure of iron status unaffected by acute illness or infection, with an enzyme-linked immunosorbent assay. Wilcoxon rank sum tests and logistic regression analyses (LRA) were performed. Iron deficiency (TfR > or = 10 microg/ml) versus TfR < 10 microg/ml, was associated with higher percentages of lymphocytes producing: (a) induced or spontaneous IL-6 (medians: induced, 15.9% for iron-deficient children versus 8.8% for iron-replete children, P = 0.002; spontaneous, 24.4% versus 13.0%, P < 0.001) and (b) induced IFN-gamma (medians:18.4% versus 12.4%, P = 0.006). The percentages of CD8(+) T cells spontaneously producing IL-6 and of all lymphocytes producing induced TNF-alpha and IFN-gamma in the same cell had the strongest relationships to iron deficiency (b = + 0.0211, P = 0.005 and b = + 0.1158, P = 0.012, respectively, LRA) and were also positively related to the co-expression of the T cell activation markers HLA DR and CD38. Severe iron deficiency (TfR > or = 30 microg/ml) was associated with the percentage of lymphocytes producing induced IL-4 (medians: 0.5% versus 1.6%, P < 0.010). The cytokine patterns associated with iron deficiency in our study would preserve iron stores but also preferentially retain the activation capabilities of T cells, albeit not necessarily other immune cells, until a critical level of iron depletion is reached.

Soluble transferrin receptor (sTfR): biological variations and reference limits

The aim of this study was to establish soluble serum transferrin receptor (sTfR) reference limits. sTfR was measured in 885 healthy subjects from 3 to 91 years old (433 men, 409 women), without hematological abnormalities, using an immunonephelometric assay. The sTfR median concentrations in our population decreased gradually from the group aged 3-10 years to the group aged 21-40 years, then there were no changes in the older groups except for the females >60 years of age. The interindividual variability ranged from 12.6% to 30.3% among different age groups, and the analytical variability was 5%. Biological factors and other factors associated with sTfR concentration variation were examined and accounted for 35% of the sTfR variability in men aged 20 years or less, and 18% in those older than 20 years. Also, they accounted for 45% of the variability in women aged 20 years or less and 14% in those older than 20 years. The main factors statistically associated with sTfR concentration in males were ferritin, orosomucoid, hemoglobin, and tobacco in all age groups and only mean corpuscular volume (MCV) in males less than 20 years old. In the females the main factors were age, orosomucoid, and hemoglobin in all age groups, MCV and tobacco in females less than 20 years old, and ferritin and physical activity in females more than 20 years old. These factors were used to define the exclusion and partition criteria for obtaining the reference samples. Medians for reference values were: 1.60 mg/l in the 3-10-year old group (males and females); 1.42 mg/l in males between 11 and 20 years of age, and 1.33 mg/l in females of the same age. In the other age groups, the median of the reference values was 1.16 mg/l, except in females over 60 years old, for whom it was 1.26 mg/l.

The effect of altitude on cycling performance: a challenge to traditional concepts

Acute exposure to moderate altitude is likely to enhance cycling performance on flat terrain because the benefit of reduced aerodynamic drag outweighs the decrease in maximum aerobic power [maximal oxygen uptake (VO2max)]. In contrast, when the course is mountainous, cycling performance will be reduced at moderate altitude. Living and training at altitude, or living in an hypoxic environment (approximately 2500 m) but training near sea level, are popular practices among elite cyclists seeking enhanced performance at sea level. In an attempt to confirm or refute the efficacy of these practices, we reviewed studies conducted on highly-trained athletes and, where possible, on elite cyclists. To ensure relevance of the information to the conditions likely to be encountered by cyclists, we concentrated our literature survey on studies that have used 2- to 4-week exposures to moderate altitude (1500 to 3000 m). With acclimatisation there is strong evidence of decreased production or increased clearance of lactate in the muscle, moderate evidence of enhanced muscle buffering capacity (beta m) and tenuous evidence of improved mechanical efficiency (ME) of cycling. Our analysis of the relevant literature indicates that, in contrast to the existing paradigm, adaptation to natural or simulated moderate altitude does not stimulate red cell production sufficiently to increase red cell volume (RCV) and haemoglobin mass (Hb(mass)). Hypoxia does increase serum erthyropoietin levels but the next step in the erythropoietic cascade is not clearly established; there is only weak evidence of an increase in young red blood cells (reticulocytes). Moreover, the collective evidence from studies of highly-trained athletes indicates that adaptation to hypoxia is unlikely to enhance sea level VO2max. Such enhancement would be expected if RCV and Hb(mass) were elevated. The accumulated results of 5 different research groups that have used controlled study designs indicate that continuous living and training at moderate altitude does not improve sea level performance of high level athletes. However, recent studies from 3 independent laboratories have consistently shown small improvements after living in hypoxia and training near sea level. While other research groups have attributed the improved performance to increased RCV and VO2max, we cite evidence that changes at the muscle level (beta m and ME) could be the fundamental mechanism. While living at altitude but training near sea level may be optimal for enhancing the performance of competitive cyclists, much further research is required to confirm its benefit. If this benefit does exist, it probably varies between individuals and averages little more than 1%.

Cord blood transferrin receptors to assess fetal iron status

OBJECTIVE

To study iron status at different gestational ages using cord blood serum transferrin receptors (STfRs).

METHODS

STfRs, iron, ferritin, total iron binding capacity, haemoglobin, and reticulocytes were measured in 144 cord blood samples. The babies were divided into three groups according to gestation (26 very preterm (24-29 weeks); 50 preterm (30-36 weeks); 68 term (37-41 weeks)).

RESULTS

Serum iron, ferritin, and total iron binding capacity were highest at term, whereas reticulocytes were highest in the very preterm. STfR levels were not influenced by gestation. Haemoglobin (r = 0.46; p < 0.0001) and reticulocytes (r = 0.42; p < 0.0001) were the only indices that independently correlated with STfR levels.

CONCLUSIONS

STfR levels in cord blood are not directly influenced by gestation and probably reflect the iron requirements of the fetus for erythropoiesis.

Reticulocyte subpopulations and reticulocyte maturity index (RMI) rise as body iron status falls

To investigate the influence of body iron status on reticulocyte subpopulations and reticulocyte maturity index (RMI), we measured serum iron markers, fluorescent intensity of reticulocytes, and serum transferrin receptor (sTfR) concentrations in 374 females aged 14-19 years. Reticulocyte subpopulations were analyzed by flow cytometry, and sTfR concentration was measured by enzyme immunoassay. There were no significant differences in the values of reticulocyte subpopulations and RMI between ferritin alone-depleted group and healthy controls. However, middle- and high-fluorescence reticulocytes and RMI were significantly higher in both the serum iron- and serum ferritin-depleted groups than in the ferritin alone-depleted group. Middle-fluorescence reticulocytes and RMI increased gradually as the body iron store was depleted and were 3.4- and 3.6-fold higher, respectively, than normal controls, when the subjects attained a frank iron-deficiency anemia. There were no significant changes in the values of red blood cells or total reticulocyte counts during iron-depleted states. The mean value of sTfR (3.98 mg/l) in the subjects with RMI > or = 1.5% was significantly higher than that (2.26 mg/l) in the subjects with RMI < 1.5% (P < 0.01). The sTfR concentration correlated significantly with RMI (r = 0.61, P < 0.01) and middle-fluorescence reticulocytes (r = 0.59, P < 0.01). In short, body iron depletion induces elevation of immature reticulocyte fractions and RMI.

Identification of Blood Erythroid Markers Useful in Revealing Erythropoietin Abuse in Athletes

Recombinant human erythropoietin (rEpo) is being used with increasing frequency by endurance athletes to improve aerobic potential. Although rEpo administration has been banned by the International Olympic Committee, no methods are available to unequivocally detect its abuse in sports. Prompted by these considerations, we evaluated the main hematological and biochemical modifications measured in the blood of 18 volunteers upon rEpo administration. Different rEpo regimens, iron, folic acid, and vitamin B12 administration did not significantly modify the percentage increase in hematocrit. However, a significant decrease in circulating ferritin (fr) and an increase in the soluble transferrin receptor (sTfr) were not found in athletes receiving low (30 IU/kg) doses of rEpo. Thus, an increase in the sTfr/fr ratio cannot be used as an indicator of rEpo abuse, at least when the hormone is administered at low concentrations. In contrast, the amounts of beta-globin mRNA detected by quantitative competitive (RT)-PCR in whole blood samples significantly increased above the threshold levels in all of the treatments investigated. Taken together, these data suggest that hematocrit value, reticulocyte count, soluble transferrin receptor content, and concentration of beta-globin mRNA, when included in a new multiparametric formula, can detect rEpo abuse in 57.5% of the samples examined with a confidence interval of 99.99%. Thus, the method reported in this paper could significantly improve the tests currently available, which in similar experiments allowed the detection of rEpo abuse in only 7.6% of the samples examined.

Iron homeostasis: new tales from the crypt

The enterocyte is a highly specialized cell of the duodenal epithelium that coordinates iron uptake and transport into the body. Until recently, the molecular mechanisms underlying iron absorption and iron homeostasis have remained a mystery. This review focuses on the proteins and regulatory mechanisms known to be present in the enterocyte precursor cell and in the mature enterocyte. The recent cloning of a basolateral iron transporter and investigations into its regulation provide new insights into possible mechanisms for iron transport and homeostasis. The roles of proteins such as iron regulatory proteins, the hereditary hemochromatosis protein (HFE)–transferrin receptor complex, and hephaestin in regulating this transporter and in regulating iron transport across the intestinal epithelium are discussed. A speculative, but testable, model for the maintenance of iron homeostasis, which incorporates the changes in the iron-related proteins associated with the life cycle of the enterocyte as it journeys from the crypt to the tip of the villous is proposed.

Iron homeostasis: new tales from the crypt

The enterocyte is a highly specialized cell of the duodenal epithelium that coordinates iron uptake and transport into the body. Until recently, the molecular mechanisms underlying iron absorption and iron homeostasis have remained a mystery. This review focuses on the proteins and regulatory mechanisms known to be present in the enterocyte precursor cell and in the mature enterocyte. The recent cloning of a basolateral iron transporter and investigations into its regulation provide new insights into possible mechanisms for iron transport and homeostasis. The roles of proteins such as iron regulatory proteins, the hereditary hemochromatosis protein (HFE)–transferrin receptor complex, and hephaestin in regulating this transporter and in regulating iron transport across the intestinal epithelium are discussed. A speculative, but testable, model for the maintenance of iron homeostasis, which incorporates the changes in the iron-related proteins associated with the life cycle of the enterocyte as it journeys from the crypt to the tip of the villous is proposed.

Measurement of soluble transferrin receptor by immunoturbidimetry and immunonephelometry

OBJECTIVE

To evaluate and compare two new commercially available immunoturbidimetric and immunonephelometric assays for measuring soluble transferrin receptor (sTfR) in serum. To adapt the immunonephelometric assay to an automated chemistry analyzer.

DESIGN AND METHODS

Total imprecisions and detection limits were calculated and compared. Fifty-six samples were used for methods comparison. The nephelometric assay was adapted to the Hitachi 911 analyzer.

RESULTS

Both methods displayed acceptable imprecisions and detection limits. Their correlation was good but a proportional bias was observed. The adaptation of the nephelometric assay to the Hitachi 911 was successful.

CONCLUSIONS

These two new immunoassays are analytically acceptable and more practicable than previously developed methods. The differences observed between both methods underscores the need for standardization. The nephelometric assay was easily adaptable to an automated chemistry analyzer.

Alteration of epithelial cell transferrin-iron homeostasis by Neisseria meningitidis and Neisseria gonorrhoeae

Iron is an essential element for nearly all organisms. In mammals, iron is transported to body tissues by the serum glycoprotein transferrin. Transferrin-iron is internalized by binding to specific receptors followed by endocytosis. In vitro, Neisseria meningitidis and Neisseria gonorrhoeae can use iron from a variety of iron-containing compounds, including human transferrin. In vivo, transferrin is an important source of iron for N. gonorrhoeae: a mutant that is unable to bind and use transferrin-iron is unable to colonize the urethra of men or initiate disease at this site. As pathogenic Neisseria and its human host derive much of their iron from transferrin, we reasoned that a competition may exist between microbe and host epithelial cells for transferrin-iron at certain stages of infection. We therefore tested the hypothesis that N. meningitidis and N. gonorrhoeae may actively interfere with host transferrin-iron metabolism. We report that Neisseria-infected human epithelial cells have reduced levels of transferrin receptor messenger RNA and cycling transferrin receptors. The ability of infected cells to internalize transferrin receptor is also reduced. Finally, the relative distribution of surface and cycling transferrin receptors is altered in an infected cell. We conclude that Neisseria infection alters epithelial cell transferrin-iron homeostasis at multiple levels.

Laboratory screening for erythropoietin abuse in sport: an emerging challenge

The growing diffusion of banned practice to improve the athletic performances is forcing clinical laboratories to identify and standardize reliable assays to detect potential unfairness. Among the doping practices, the use of recombinant human erythropoietin is becoming fairly popular, due to simplicity and safeties of administration and troublesome detection. The heterogeneous response rate, the presence of a little but significant amount of naturally occurring hormone, the short half-life exhibited by recombinant human erythropoietin and the lack of standardization of commercial assays appear the main problems to overcome. Aim of the present article is to provide a critical review of some of the more widespread laboratory techniques currently available for the screening for erythropoietin abuse in sport.