The transferrin receptor: the cellular iron gate

Hereditary hemochromatosis (HH) is a common autosomal-recessive disorder of iron metabolism. More than 80% of HH patients are homozygous for a point mutation in a major histocompatibility complex (MHC) class I type protein (HFE), which results in a lack of HFE expression on the cell surface. A previously identified interaction of HFE and the transferrin receptor suggests a possible regulatory role of HFE in cellular iron absorption. Using an HeLa cell line stably transfected with HFE under the control of a tetracycline-sensitive promoter, we investigated the effect of HFE expression on cellular iron uptake. We demonstrate that the overproduction of HFE results in decreased iron uptake from diferric transferrin. Moreover, HFE expression activates the key regulators of intracellular iron homeostasis, the iron-regulatory proteins (IRPs), implying that HFE can affect the intracellular “labile iron pool.” The increase in IRP activity is accompanied by the downregulation of the iron-storage protein, ferritin, and an upregulation of transferrin receptor levels. These findings are discussed in the context of the pathophysiology of HH and a possible role of iron-responsive element (IRE)-containing mRNAs.

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Exposure of K562 cells to anti-receptor monoclonal antibody OKT9 results in rapid redistribution and enhanced degradation of the transferrin receptor.

The Journal of Cell Biology ◽

10.1083/jcb.102.3.951 ◽

1986 ◽

Vol 102 (3) ◽

pp. 951-958 ◽

Cited By ~ 60

Author(s):

A M Weissman ◽

R D Klausner ◽

K Rao ◽

J B Harford

Keyword(s):

Monoclonal Antibody ◽

Transferrin Receptor ◽

Half Life ◽

Iron Uptake ◽

K562 Cells ◽

Lower Percentage ◽

Cellular Iron ◽

Cellular Iron Uptake ◽

Small Change ◽

Transferrin Cycle

When the human erythroleukemia cell line K562 is treated with OKT9, a monoclonal antibody against the transferrin receptor, effects on receptor dynamics and degradation ensue. The apparent half-life of the receptor is decreased by greater than 50% as a result of OKT9 treatment. The transferrin receptor is also rapidly redistributed in response to OKT9 such that a lower percentage of the cellular receptors are displayed on the cell surface. OKT9 treatment also leads to a decrease in the total number of receptors participating in the transferrin cycle for cellular iron uptake. The reduction in iron uptake that results from the loss of receptors from the cycle leads to enhanced biosynthesis of the receptor. Receptors with bound OKT9 continue to participate in multiple cycles of iron uptake. However, OKT9 treatment appears to result in a relatively small increase per cycle in the departure of receptors from participation in iron uptake to a pathway leading to receptor degradation. Radiolabeled OKT9 is itself degraded by K562 cells and this degradation is inhibitable by leupeptin or chloroquine. In the presence of leupeptin, OKT9 treatment results in the enhanced intracellular accumulation of transferrin. Because the time involved in the transferrin cycle is shorter (12.5 min) than the normal half-life of the receptor (8 h), a small change in recycling efficiency caused by OKT9 treatment could account for the marked decrease in receptor half-life. In this paper the implications of these findings are discussed as they relate to systems in which receptor number is regulated by ligand.

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Enhanced erythropoiesis in Hfe-KO mice indicates a role for Hfe in the modulation of erythroid iron homeostasis

Blood ◽

10.1182/blood-2010-09-307462 ◽

2011 ◽

Vol 117 (4) ◽

pp. 1379-1389 ◽

Cited By ~ 31

Author(s):

Pedro Ramos ◽

Ella Guy ◽

Nan Chen ◽

Catia C. Proenca ◽

Sara Gardenghi ◽

...

Keyword(s):

Iron Overload ◽

Iron Uptake ◽

Iron Homeostasis ◽

Hereditary Hemochromatosis ◽

Dietary Iron ◽

Erythroid Cells ◽

Hepcidin Expression ◽

Stress Erythropoiesis ◽

Cellular Iron ◽

Cellular Iron Uptake

Abstract In hereditary hemochromatosis, mutations in HFE lead to iron overload through abnormally low levels of hepcidin. In addition, HFE potentially modulates cellular iron uptake by interacting with transferrin receptor, a crucial protein during erythropoiesis. However, the role of HFE in this process was never explored. We hypothesize that HFE modulates erythropoiesis by affecting dietary iron absorption and erythroid iron intake. To investigate this, we used Hfe-KO mice in conditions of altered dietary iron and erythropoiesis. We show that Hfe-KO mice can overcome phlebotomy-induced anemia more rapidly than wild-type mice (even when iron loaded). Second, we evaluated mice combining the hemochromatosis and β-thalassemia phenotypes. Our results suggest that lack of Hfe is advantageous in conditions of increased erythropoietic activity because of augmented iron mobilization driven by deficient hepcidin response. Lastly, we demonstrate that Hfe is expressed in erythroid cells and impairs iron uptake, whereas its absence exclusively from the hematopoietic compartment is sufficient to accelerate recovery from phlebotomy. In summary, we demonstrate that Hfe influences erythropoiesis by 2 distinct mechanisms: limiting hepcidin expression under conditions of simultaneous iron overload and stress erythropoiesis, and impairing transferrin-bound iron uptake by erythroid cells. Moreover, our results provide novel suggestions to improve the treatment of hemochromatosis.

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Heterotypic interactions between transferrin receptor and transferrin receptor 2

Blood ◽

10.1182/blood-2002-09-2742 ◽

2003 ◽

Vol 101 (5) ◽

pp. 2008-2014 ◽

Cited By ~ 31

Author(s):

Todd M. Vogt ◽

Aaron D. Blackwell ◽

Anthony M. Giannetti ◽

Pamela J. Bjorkman ◽

Caroline A. Enns

Keyword(s):

Transferrin Receptor ◽

Iron Uptake ◽

Iron Homeostasis ◽

Critical Role ◽

Hereditary Hemochromatosis ◽

Metabolic Labeling ◽

Western Blots ◽

Cellular Iron ◽

Transferrin Receptor 2

Cellular iron uptake in most tissues occurs via endocytosis of diferric transferrin (Tf) bound to the transferrin receptor (TfR). Recently, a second transferrin receptor, transferrin receptor 2 (TfR2), has been identified and shown to play a critical role in iron metabolism. TfR2 is capable of Tf-mediated iron uptake and mutations in this gene result in a rare form of hereditary hemochromatosis unrelated to the hereditary hemochromatosis protein, HFE. Unlike TfR, TfR2 expression is not controlled by cellular iron concentrations and little information is currently available regarding the role of TfR2 in cellular iron homeostasis. To investigate the relationship between TfR and TfR2, we performed a series of in vivo and in vitro experiments using antibodies generated to each receptor. Western blots demonstrate that TfR2 protein is expressed strongest in erythroid/myeloid cell lines. Metabolic labeling studies indicate that TfR2 protein levels are approximately 20-fold lower than TfR in these cells. TfR and TfR2 have similar cellular localizations in K562 cells and coimmunoprecipitate to only a very limited extent. Western analysis of the receptors under nonreducing conditions reveals that they can form heterodimers.

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HFE Downregulates Iron Uptake From Transferrin and Induces Iron-Regulatory Protein Activity in Stably Transfected Cells

Blood ◽

10.1182/blood.v94.11.3915 ◽

1999 ◽

Vol 94 (11) ◽

pp. 3915-3921 ◽

Cited By ~ 66

Author(s):

H.D. Riedel ◽

M.U. Muckenthaler ◽

S.G. Gehrke ◽

I. Mohr ◽

K. Brennan ◽

...

Keyword(s):

Transferrin Receptor ◽

Iron Uptake ◽

Iron Homeostasis ◽

Hereditary Hemochromatosis ◽

Regulatory Protein ◽

Intracellular Iron ◽

Iron Storage ◽

Iron Regulatory Proteins ◽

Cellular Iron

Abstract Hereditary hemochromatosis (HH) is a common autosomal-recessive disorder of iron metabolism. More than 80% of HH patients are homozygous for a point mutation in a major histocompatibility complex (MHC) class I type protein (HFE), which results in a lack of HFE expression on the cell surface. A previously identified interaction of HFE and the transferrin receptor suggests a possible regulatory role of HFE in cellular iron absorption. Using an HeLa cell line stably transfected with HFE under the control of a tetracycline-sensitive promoter, we investigated the effect of HFE expression on cellular iron uptake. We demonstrate that the overproduction of HFE results in decreased iron uptake from diferric transferrin. Moreover, HFE expression activates the key regulators of intracellular iron homeostasis, the iron-regulatory proteins (IRPs), implying that HFE can affect the intracellular “labile iron pool.” The increase in IRP activity is accompanied by the downregulation of the iron-storage protein, ferritin, and an upregulation of transferrin receptor levels. These findings are discussed in the context of the pathophysiology of HH and a possible role of iron-responsive element (IRE)-containing mRNAs.

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CBIO-10. REDUCED IRON EXPORT FUNCTIONS IN A CELL INTRINSIC MANNER TO DRIVE GLIOBLASTOMA GROWTH

Neuro-Oncology ◽

10.1093/neuonc/noaa215.070 ◽

2020 ◽

Vol 22 (Supplement_2) ◽

pp. ii17-ii17

Author(s):

Katie Troike ◽

Erin Mulkearns-Hubert ◽

Daniel Silver ◽

James Connor ◽

Justin Lathia

Keyword(s):

Tumor Cells ◽

Iron Uptake ◽

Iron Homeostasis ◽

Iron Status ◽

Tumor Grade ◽

Hfe Gene ◽

Iron Release ◽

Female Patients ◽

Cellular Iron

Abstract Glioblastoma (GBM), the most common primary malignant brain tumor in adults, is characterized by invasive growth and poor prognosis. Iron is a critical regulator of many cellular processes, and GBM tumor cells have been shown to modulate expression of iron-associated proteins to enhance iron uptake from the surrounding microenvironment, driving tumor initiation and growth. While iron uptake has been the central focus of previous investigations, additional mechanisms of iron regulation, such as compensatory iron efflux, have not been explored in the context of GBM. The hemochromatosis (HFE) gene encodes a transmembrane glycoprotein that aids in iron homeostasis by limiting cellular iron release, resulting in a sequestration phenotype. We find that HFE is upregulated in GBM tumors compared to non-tumor brain and that expression of HFE increases with tumor grade. Furthermore, HFE mRNA expression is associated with significantly reduced survival specifically in female patients with GBM. Based on these findings, we hypothesize that GBM tumor cells upregulate HFE expression to augment cellular iron loading and drive proliferation, ultimately leading to reduced survival of female patients. To test this hypothesis, we generated Hfe knockdown and overexpressing mouse glioma cell lines. We observed significant alterations in the expression of several iron handling genes with Hfe knockdown or overexpression, suggesting global disruption of iron homeostasis. Additionally, we show that knockdown of Hfe in these cells increases apoptosis and leads to a significant impairment of tumor growth in vivo. These findings support the hypothesis that Hfe is a critical regulator of cellular iron status and contributes to tumor aggression. Future work will include further exploration of the mechanisms that contribute to these phenotypes as well as interactions with the tumor microenvironment. Elucidating the mechanisms by which iron effulx contributes to GBM may inform the development of next-generation targeted therapies.

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Intermediate steps in cellular iron uptake from transferrin. Detection of a cytoplasmic pool of iron, free of transferrin.

Journal of Biological Chemistry ◽

10.1016/s0021-9258(19)36621-9 ◽

1992 ◽

Vol 267 (30) ◽

pp. 21384-21389

Author(s):

D.R. Richardson ◽

E Baker

Keyword(s):

Iron Uptake ◽

Cellular Iron ◽

Cellular Iron Uptake

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Bifunctional Chelating Supramolecular Polymer and Its Application in Downregulation of Cellular Iron Uptake

Biomacromolecules ◽

10.1021/acs.biomac.7b00765 ◽

2017 ◽

Vol 18 (9) ◽

pp. 2830-2838 ◽

Cited By ~ 2

Author(s):

Fan Xu ◽

Jing Jing ◽

Xiuman Zhou ◽

Yanwu Zhang

Keyword(s):

Iron Uptake ◽

Supramolecular Polymer ◽

Cellular Iron ◽

Cellular Iron Uptake

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Overexpression of mitochondrial ferritin causes cytosolic iron depletion and changes cellular iron homeostasis

Blood ◽

10.1182/blood-2004-07-2722 ◽

2005 ◽

Vol 105 (5) ◽

pp. 2161-2167 ◽

Cited By ~ 97

Author(s):

Guangjun Nie ◽

Alex D. Sheftel ◽

Sangwon F. Kim ◽

Prem Ponka

Keyword(s):

Iron Homeostasis ◽

Rna Binding ◽

Binding Activity ◽

Intracellular Iron ◽

Free Radical Damage ◽

Iron Regulatory Proteins ◽

Cellular Iron ◽

Cellular Iron Uptake ◽

A Cell ◽

Cellular Iron Homeostasis

AbstractCytosolic ferritin sequesters and stores iron and, consequently, protects cells against iron-mediated free radical damage. However, the function of the newly discovered mitochondrial ferritin (MtFt) is unknown. To examine the role of MtFt in cellular iron metabolism, we established a cell line that stably overexpresses mouse MtFt under the control of a tetracycline-responsive promoter. The overexpression of MtFt caused a dose-dependent iron deficiency in the cytosol that was revealed by increased RNA-binding activity of iron regulatory proteins (IRPs) along with an increase in transferrin receptor levels and decrease in cytosolic ferritin. Consequently, the induction of MtFt resulted in a dramatic increase in cellular iron uptake from transferrin, most of which was incorporated into MtFt. The induction of MtFt caused a shift of iron from cytosolic ferritin to MtFt. In addition, iron inserted into MtFt was less available for chelation than that in cytosolic ferritin and the expression of MtFt was associated with decreased mitochondrial and cytosolic aconitase activities, the latter being consistent with the increase in IRP-binding activity. In conclusion, our results indicate that overexpression of MtFt causes a dramatic change in intracellular iron homeostasis and that shunting iron to MtFt likely limits its availability for active iron proteins.

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