Altered body iron distribution and microcytosis in mice deficient in iron regulatory protein 2 (IRP2)

Iron regulatory protein 2 (IRP2)-deficient mice have been reported to suffer from late-onset neurodegeneration by an unknown mechanism. We report that young adult Irp2-/- mice display signs of iron mismanagement within the central iron recycling pathway in the mammalian body, the liver-bone marrow-spleen axis, with altered body iron distribution and compromised hematopoiesis. In comparison with wild-type littermates, Irp2-/- mice are mildly microcytic with reduced serum hemoglobin levels and hematocrit. Serum iron and transferrin saturation are unchanged, and hence microcytosis is not due to an overt decrease in systemic iron availability. The liver and duodenum are iron loaded, while the spleen is iron deficient, associated with a reduced expression of the iron exporter ferroportin. A reduction in transferrin receptor 1 (TfR1) mRNA levels in the bone marrow of Irp2-/- mice can plausibly explain the microcytosis by an intrinsic defect in erythropoiesis due to a failure to adequately protect TfR1 mRNA against degradation. This study links a classic regulator of cellular iron metabolism to systemic iron homeostasis and erythropoietic TfR1 expression. Furthermore, this work uncovers aspects of mammalian iron metabolism that can or cannot be compensated for by the expression of IRP1.     

Increased plasma transferrin, altered body iron distribution, and microcytic hypochromic anemia in ferrochelatase-deficient mice

Patients with deficiency in ferrochelatase (FECH), the last enzyme of the heme biosynthetic pathway, experience a painful type of skin photosensitivity called erythropoietic protoporphyria (EPP), which is caused by the excessive production of protoporphyrin IX (PPIX) by erythrocytes. Controversial results have been reported regarding hematologic status and iron status of patients with EPP. We thoroughly explored these parameters in Fechm1Pas mutant mice of 3 different genetic backgrounds. FECH deficiency induced microcytic hypochromic anemia without ringed sideroblasts, little or no hemolysis, and no erythroid hyperplasia. Serum iron, ferritin, hepcidin mRNA, and Dcytb levels were normal. The homozygous Fechm1Pas mutant involved no tissue iron deficiency but showed a clear-cut redistribution of iron stores from peripheral tissues to the spleen, with a concomitant 2- to 3-fold increase in transferrin expression at the mRNA and the protein levels. Erythrocyte PPIX levels strongly correlated with serum transferrin levels. At all stages of differentiation in our study, transferrin receptor expression in bone marrow erythroid cells in Fech(m1Pas) was normal in mutant mice but not in patients with iron-deficiency anemia. Based on these observations, we suggest that oral iron therapy is not the therapy of choice for patients with EPP and that the PPIX-liver transferrin pathway plays a role in the orchestration of iron distribution between peripheral iron stores, the spleen, and the bone marrow.     

Nitric oxide-mediated modulation of iron regulatory proteins: implication for cellular iron homeostasis

Iron regulatory proteins (IRP1 and IRP2) control the synthesis of transferrin receptors (TfR) and ferritin by binding to iron-responsive elements (IREs) that are located in the 3' untranslated region (UTR) and the 5' UTR of their respective mRNAs. Cellular iron levels affect binding of IRPs to IREs and consequently expression of TfR and ferritin. Moreover, NO(.), a redox species of nitric oxide that interacts primarily with iron, can activate IRP1 RNA-binding activity resulting in an increase in TfR mRNA levels and a decrease in ferritin synthesis. We have shown that treatment of RAW 264.7 cells (a murine macrophage cell line) with NO(+) (nitrosonium ion, which causes S-nitrosylation of thiol groups) resulted in a rapid decrease in RNA-binding of IRP2, followed by IRP2 degradation, and these changes were associated with a decrease in TfR mRNA levels and a dramatic increase in ferritin synthesis. Moreover, we demonstrated that stimulation of RAW 264.7 cells with lipopolysaccharide (LPS) and interferon-gamma (IFN-gamma) increased IRP1 binding activity, whereas RNA-binding of IRP2 decreased and was followed by a degradation of this protein. Furthermore, the decrease of IRP2 binding/protein levels was associated with a decrease in TfR mRNA levels and an increase in ferritin synthesis in LPS/IFN-gamma-treated cells, and these changes were prevented by inhibitors of inducible nitric oxide synthase. These results suggest that NO(+)-mediated degradation of IRP2 plays a major role in iron metabolism during inflammation.     

Microcytic and hypochromic anemias

In the majority of cases, microcytosis is the result of impaired hemoglobin synthesis. Disorders of iron metabolism and protoporphyrin and heme synthesis, as well as impaired globin synthesis, lead to defective hemoglobin production and to the generation of microcytosis and microcytic anemia. Iron deficiency anemie, anemia of chronic diseases, thalassemias, congenital sideroblastic anemias and homozygous HbE disease are the main representatives of microcytosis and microcytic anemias. Serum iron, total iron binding capacity, transferrin saturation, serum ferritin, serum transferrin receptor, transferrin receptor-ferritin index, and zinc-protoporhyrin concentration in erythrocytes are tests used for assessment of iron deficiency. The convention laboratory test for diagnosing iron deficiency is the measurement of serum ferritin. The most precise method for evaluating body iron stores is the examination for iron on aspirated bone marrow or marrow biopsy. Increased content of Hb A2 over 3.5% is diagnostic for beta-thalassemia. Presence of ringed sideroblasts is characteristic of sideroblastic anemias. Hemoglobin electrophoresis is required for the diagnosis of hemoglobinopathy E. The optimal therapeutic regimen in iron deficiency anemia used in this country is to administer 100 mg of elemental iron twice daily separately from meals. Ferrous sulphate (Ferronat Retard tbl. or Sorbifer Dulures tbl.) which are slow-releasing iron formulations are preferred because of their low cost, high bioavailability and low side-effects. Parenteral iron therapy is justified only in patients who cannot absorb iron, who have blood losses that exceed the maximal absorptive capacity of their intestinal tract or who are totally intolerant of oral iron. However, parenteral iron therapy may be associated with serious and even fatal side-effects.     

Defective iron supply for erythropoiesis and adequate endogenous erythropoietin production in the anemia associated with systemic-onset juvenile chronic arthritis

Systemic-onset juvenile chronic arthritis (SoJCA) is associated with high levels of circulating interleukin-6 (IL-6) and is frequently complicated by severe microcytic anemia whose pathogenesis is unclear. Therefore, we studied 20 consecutive SoJCA patients with hemoglobin (Hb) levels <12 g/dL, evaluating erythroid progenitor proliferation, endogenous erythropoietin production, body iron status, and iron supply for erythropoiesis. Hb concentrations ranged from 6.5 to 11.9 g/dL. Hb level was directly related to mean corpuscular volume (r = .82, P < .001) and inversely related to circulating transferrin receptor (r = - .81, P < .001) suggesting that the severity of anemia was directly proportional to the degree of iron-deficient erythropoiesis. Serum ferritin ranged from 18 to 1,660 microgram/L and was unrelated to Hb level. Bone marrow iron stores wore markedly reduced in the three children investigated, and they also showed increased serum transferrin receptor and normal-to-high serum ferritin. All 20 patients had elevated IL-6 levels and normal in vitro growth of erythroid progenitors. Endogenous erythropoietin (epo) production was appropriate for the degree of anemia as judged by both the observed to predicted log (serum epo) ratio 10.95 +/- 0.12) and a comparison of the serum epo- Hb regression found in these subjects with that of thalassemia patients. Multiple regression analysis showed that serum transferrin receptor was the parameter most closely related to hemoglobin concentration: variation in circulating transferrin receptor explained 61% of the variation in Hb level (P < .001). In 10 severely anemic patients, amelioration of anemia following intravenous iron administration resulted in normalization of serum transferrin receptor. Defective iron supply to the erythron rather than blunted epo production is the major cause of the microcytic anemia associated with SoJCA. A true body-iron deficiency caused by decreased iron absorption likely complicates long-lasting inflammation in the most anemic children, and this can be recognized by high serum transferrin receptor levels. Although oral iron is of no benefit, intravenous iron saccharate is a safe and effective means for improving iron availability for erythropoiesis and correcting this anemia. Thus, while chronically high endogenous IL-6 levels do not appear to blunt epo production, they are probably responsible for the observed abnormalities in iron metabolism. Anemia of chronic disease encompasses a variety of anemic conditions whose peculiar features may specifically correlate with the type of cytokine(s) predominantly released.     

Erythroblast iron metabolism in sideroblastic and sideropenic states

Iron appears to exert self-regulatory control over erythroblast iron uptake, iron storage and its incorporation into haem. It does this via iron regulatory proteins (IRPs) which bind reversibly to the iron responsive elements (IREs) on the mRNA of transferrin receptor (TfR), erythroid 5-aminolaevulinic acid synthase (ALA-S2) and ferritin. Iron deficiency leads to the binding of IRP to IRE. This binding inhibits the translation of mRNA for ALA-S2 and ferritin but stabilizes mRNA for TfR expression. Sideroblastic erythropoiesis is highly ineffective and characterized by mitochondrial iron loading. The study of X-linked sideroblastic anaemia has shown that the entry of iron into the mitochondria is poorly controlled and able to occur when protoporphyrin production is reduced, as is seen with the ALA-S2 mutations, or when it is increased as has been seen with ABC7 transporter mutations. Sideropenia characterises both iron deficiency anaemia (IDA) and the anaemia of chronic disease (ACD). Erythroblasts in ACD seem doubly equipped to protect their iron supply with their ability to increase the efficiency of transferrin-iron uptake as well as to activate the IRP/IRE system to increase surface TfR production. This increase in efficiency restricts the need to increase surface TfR production and maintains serum soluble TfR (sTfR) values within the normal range in iron replete ACD. The coexistence of iron deficiency with chronic disease, however, is associated with an increase in both the efficiency and number and a highly significant rise in sTfR values.     

Microcytic anemia with iron malabsorption: An inherited disorder of iron metabolism

Two siblings were identified with severe hypoproliferative microcytic anemia and iron malabsorption, in the absence of any gastrointestinal disorder or blood loss. These children had severe microcytosis (MCV 48 fl, hemoglobin 7.5 g/dl) with decreased serum iron, elevated serum TIBC, and decreased serum ferritin, despite prolonged treatment with oral iron. An iron challenge study with an oral dose of 2 mg/kg elemental iron as ferrous sulfate documented iron malabsorption. After treatment with intravenous iron dextran, there was an absence of the expected reticulocytosis and only a partial correction of the hemoglobin, hematocrit, and microcytosis. The bone marrow was hypocellular with abnormal iron incorporation into erythroid precursor cells. This appears to be a rare form of inherited anemia characterized by iron malabsorption and disordered iron metabolism that only partially corrects after the administration of parenteral iron. These features resemble those found in the microcytic mouse (mk/mk), which also has severe microcytic anemia and iron malabsorption that partially responds to parenteral iron. © 1996 Wiley-Liss, Inc.     

Serum transferrin receptor

Transferrin receptors (TfRs) are the conventional pathway by which cells acquire iron for physiological requirements. Under iron-deficient conditions there is an increased concentration of surface TfR, especially on bone marrow erythroid precursors, as a mechanism to sequester needed iron. TfRs are also present in the circulation, and the circulating serum TfR (sTfR) level reflects total body TfR concentration. Under normal conditions erythroid precursors are the main source of sTfR. Disorders of the bone marrow with reduced erythroid precursors are associated with low sTfR levels. The sTfR concentration begins to rise early in iron deficiency with the onset of iron-deficient erythropoiesis, and continues to rise as iron-deficient erythropoiesis progressively worsens, prior to the development of anemia. The sTfR level does not increase in anemia of chronic inflammation, but is increased when anemia of chronic inflammation is combined with iron deficiency. The sTfR level is also increased in patients with expanded erythropoiesis, including hemolytic anemias, myelodysplastic syndromes, and use of erythropoietic stimulating agents. The ratio of sTfR/ferritin can be used to quantify the entire spectrum of iron status from positive iron stores through negative iron balance, and is particularly useful in evaluating iron status in population studies. The sTfR/log ferritin ratio is valuable for distinguishing anemia of chronic inflammation from iron deficiency anemia, whether the latter occurs alone or in combination with anemia of chronic inflammation.     

Erythroblast Iron Metabolism in Sideroblastic and Sideropenic States

Iron appears to exert self-regulatory control over erythroblast iron uptake, iron storage and its incorporation into haem. It does this via iron regulatory proteins (IRPs) which bind reversibly to the iron responsive elements (IREs) on the mRNA of transferrin receptor (TfR), erythroid 5-aminolaevulinic acid synthase (ALA-S2) and ferritin. Iron deficiency leads to the binding of IRP to IRE. This binding inhibits the translation of mRNA for ALA-S2 and ferritin but stabilizes mRNA for TfR expression.

Sideroblastic erythropoiesis is highly ineffective and characterized by mitochondrial iron loading. The study of X-linked sideroblastic anaemia has shown that the entry of iron into the mitochondria is poorly controlled and able to occur when protoporphyrin production is reduced, as is seen with the ALA-S2 mutations, or when it is increased as has been seen with ABC7 transporter mutations.

Sideropenia characterises both iron deficiency anaemia (IDA) and the anaemia of chronic disease (ACD). Erythroblasts in ACD seem doubly equipped to protect their iron supply with their ability to increase the efficiency of transferrin-iron uptake as well as to activate the IRP/IRE system to increase surface TfR production. This increase in efficiency restricts the need to increase surface TfR production and maintains serum soluble TfR (sTfR) values within the normal range in iron replete ACD. The coexistence of iron deficiency with chronic disease, however, is associated with an increase in both the efficiency and number and a highly significant rise in sTfR values.     

Prediction of iron deficiency in chronic inflammatory rheumatic disease anaemia

Summary. We prospectively studied 45 anaemic patients (3 7 women, 8 men) with chronic inflammatory rheumatic diseases. The combination of serum ferritin and CRP (as well as ESR) in its predictive capacity for bone marrow iron stores was examined. The relationship between other iron-related measurements (transferrin, transferrin saturation, soluble transferrin receptor, erythrocyte porphyrins and percentage of hypochromic/microcytic erythrocytes) and bone marrow iron stores was also investigated. Stainable bone marrow iron was taken as the most suitable standard to separate iron-deficient from iron-replete patients. 14 patients (31%) were lacking bone marrow iron. Regression analysis showed a good correlation between ferritin and bone marrow iron (adjusted R ²=0.721, P<00001). The combination of ferritin and CRP (ESR) did not improve the predictive power for bone marrow iron (adjusted R ²=0.715) in this cohort of patients with low systemic inflammatory activity. With respect to the bone marrow iron content the best predictive cut-off value of ferritin was 30μg/l (86% sensitivity, 90% specificity). The other iron-related parameters both individually and when combined were less powerful in predicting bone marrow iron than ferritin alone. Only zinc bound erythrocyte protoporphyrin in combination with ferritin slightly improved prediction (adjusted R ²=0.731). A cut-off point of 11% hypochromic erythrocytes reached a high specificity (90%), but was less sensitive (77%).     

The diagnosis of iron deficiency anemia in sickle cell disease

We determined the prevalence and optimal methods for laboratory diagnosis of iron deficiency anemia in patients with sickle cell disease. Laboratory investigations of 38 nontransfused and 32 transfused patients included transferrin saturation, serum ferritin, mean corpuscular volume (MCV), and free erythrocyte protoporphyrin (FEP). Response to iron supplementation confirmed the diagnosis of iron deficiency anemia in 16% of the nontransfused patients. None of the transfused patients were iron deficient. All iron-deficient patients (mean age 2.4 yr) had a low MCV, serum ferritin less than 25 ng/ml, transferrin saturation less than 15%, and FEP less than 90 micrograms/dl RBC. Following therapy, all parameters improved and the hemoglobin concentration increased greater than 2 g/dl. A serum ferritin below 25 ng/ml was the most reliable screening test for iron deficiency. There were 13% false positive results with transferrin saturation, 3% with MCV, and 62% with FEP. FEP values correlated strongly with reticulocyte counts. The high FEP was in part due to protoporphyrin IX and not completely due to zinc protoporphyrin, which is elevated in iron deficiency. We conclude that iron deficiency anemia is a potential problem in young nontransfused sickle cell patients. Serum ferritin below 25 ng/ml and low MCV are the most useful screening tests.     

Iron regulatory protein 1 is not required for the modulation of ferritin and transferrin receptor expression by iron in a murine pro-B lymphocyte cell line

Iron regulatory proteins (IRPs) are cytoplasmic RNA binding proteins that are central components of a sensory and regulatory network that modulates vertebrate iron homeostasis. IRPs regulate iron metabolism by binding to iron responsive element(s) (IREs) in the 5′ or 3′ untranslated region of ferritin or transferrin receptor (TfR) mRNAs. Two IRPs, IRP1 and IRP2, have been identified previously. IRP1 exhibits two mutually exclusive functions as an RNA binding protein or as the cytosolic isoform of aconitase. We demonstrate that the Ba/F3 family of murine pro-B lymphocytes represents the first example of a mammalian cell line that fails to express IRP1 protein or mRNA. First, all of the IRE binding activity in Ba/F3-gp55 cells is attributable to IRP2. Second, synthesis of IRP2, but not of IRP1, is detectable in Ba/F3-gp55 cells. Third, the Ba/F3 family of cells express IRP2 mRNA at a level similar to other murine cell lines, but IRP1 mRNA is not detectable. In the Ba/F3 family of cells, alterations in iron status modulated ferritin biosynthesis and TfR mRNA level over as much as a 20- and 14-fold range, respectively. We conclude that IRP1 is not essential for regulation of ferritin or TfR expression by iron and that IRP2 can act as the sole IRE-dependent mediator of cellular iron homeostasis.     

Expression of transferrin receptor 2 in normal and neoplastic hematopoietic cells.

Iron is essential for cell proliferation, heme synthesis, and a variety of cellular metabolic processes. In most cells, transferrin receptor-mediated endocytosis is a major pathway for cellular iron uptake. Recently, transferrin receptor 2 (TfR2), another receptor for transferrin, was cloned. High levels of expression of TfR2 messenger RNA (mRNA) occur in the liver, as well as in HepG2 (a hepatoma cell line) and K562 (an erythroid leukemia cell line). In this study, TfR2 mRNA expression was analyzed in hematological cell lines, normal erythroid cells at various stages of differentiation, and leukemia and preleukemia cells. High levels of TfR2 expression occurred in all of the erythroid cell lines that were examined. Erythroid-specific expression of TfR2 protein in bone marrow cells was confirmed by immunohistochemical staining. Expression of TfR2 mRNA was high in normal CD34(+) erythroid precursor cells, and levels decreased during erythroid differentiation in vitro. Levels of expression of TfR2-alpha mRNA were significantly higher in erythroleukemia (M6) marrow samples than in nonmalignant control marrow samples. In addition, relatively higher levels of TfR2-alpha mRNA expression occurred in some samples of myelodysplastic syndrome that had erythroid hyperplasia in bone marrow, acute myelogenous leukemia M1, M2, and chronic myelogenous leukemia. Expression profiles of normal members of the erythroid lineage suggest that TfR2-alpha may be a useful marker of early erythroid precursor cells. The clinical significance of TfR2-alpha expression in leukemia cells remains to be determined.     

Factor V Leiden mutation carriership and venous thromboembolism in polycythemia vera and essential thrombocythemia

The aim of the present study is to evaluate in an elderly hospitalized population the diagnostic value of the serum transferrin receptor (sTfR) in distinguishing IDA (iron deficiency anemia) from ACD (anemia of chronic disease) as compared to conventional laboratory tests of iron metabolism, especially serum ferritin. In a prospective study, 34 patients with IDA and 38 patients with ACD (a chronic disorder in 23 and an acute infection in 15) were evaluated using iron status tests including serum transferrin receptor assay. The iron stores were assessed by bone marrow examination. sTfR levels were elevated (>28.1 nmol/L) in 68% of the IDA patients but also in 43% of the patients with ACD-chronic inflammation and 33% with ACD-acute infection. Serum ferritin was the best test to differentiate IDA from ACD patients. We conclude that serum ferritin is a more sensitive and specific parameter than the sTfR assay to predict the bone marrow iron status in an elderly anemic population.     

The human counterpart of zebrafish shiraz shows sideroblastic-like microcytic anemia and iron overload

Inherited microcytic-hypochromic anemias in rodents and zebrafish suggest the existence of corresponding human disorders. The zebrafish mutant shiraz has severe anemia and is embryonically lethal because of glutaredoxin 5 (GRLX5) deletion, insufficient biogenesis of mitochondrial iron-sulfur (Fe/S) clusters, and deregulated iron-regulatory protein 1 (IRP1) activity. This leads to stabilization of transferrin receptor 1 (TfR) RNA, repression of ferritin, and ALA-synthase 2 (ALAS2) translation with impaired heme synthesis. We report the first case of GLRX5 deficiency in a middle-aged anemic male with iron overload and a low number of ringed sideroblasts. Anemia was worsened by blood transfusions but partially reversed by iron chelation. The patient had a homozygous (c.294A>G) mutation that interferes with intron 1 splicing and drastically reduces GLRX5 RNA. As in shiraz, aconitase and H-ferritin levels were low and TfR level was high in the patient's cells, compatible with increased IRP1 binding. Based on the biochemical and clinical phenotype, we hypothesize that IRP2, less degraded by low heme, contributes to the repression of the erythroblasts ferritin and ALAS2, increasing mitochondrial iron. Iron chelation, redistributing iron to the cytosol, might relieve IRP2 excess, improving heme synthesis and anemia. GLRX5 function is highly conserved, but at variance with zebrafish, its defect in humans leads to anemia and iron overload.