Transferrin receptor 2 protein is not expressed in normal erythroid cells

The Fate of Iron in The Organism and Its Regulatory Pathways

Iron is an essential element involved in many life-necessary processes. Interestingly, in mammals there is no active excretion mechanism for iron. Therefore iron kinetics has to be meticulously regulated. The most important step for regulation of iron kinetics is absorption. The absorption takes place in small intestine and it is implicated that it requires several proteins. Iron is then released from enterocytes into the circulation and delivered to the cells. Iron movement inside the cell is only partially elucidated and its traffic to mitochondia is not known. Surprisingly, the regulation of various proteins related to iron kinetics and energy metabolism at the molecular level is better described. On contrary, the complex control of iron absorption cannot be fully explicated with present knowledge.

Transferrin receptor 2 is a component of the erythropoietin receptor complex and is required for efficient erythropoiesis

Erythropoietin (Epo) is required for erythroid progenitor differentiation. Although Epo crosslinking experiments have revealed the presence of Epo receptor (EpoR)-associated proteins that could never be identified, EpoR is considered to be a paradigm for homodimeric cytokine receptors. We purified EpoR-binding partners and identified the type 2 transferrin receptor (TfR2) as a component of the EpoR complex corresponding to proteins previously detected in cross-linking experiments. TfR2 is involved in iron metabolism by regulating hepcidin production in liver cells. We show that TfR2 and EpoR are synchronously coexpressed during the differentiation of erythroid progenitors. TfR2 associates with EpoR in the endoplasmic reticulum and is required for the efficient transport of this receptor to the cell surface. Erythroid progenitors from TfR2(-/-)mice show a decreased sensitivity to Epo and increased circulating Epo levels. In human erythroid progenitors, TfR2 knockdown delays the terminal differentiation. Erythroid cells produce growth differentiation factor-15, a cytokine that suppresses hepatic hepcidin production in certain erythroid diseases such as thalassemia. We show that the production of growth differentiation factor-15 by erythroid cells is dependent on both Epo and TfR2. Taken together, our results show that TfR2 exhibits a non hepatic function as a component of the EpoR complex and is required for efficient erythropoiesis.

Iron uptake from plasma transferrin by a transferrin receptor 2 mutant mouse model of haemochromatosis

BACKGROUND & AIMS

Hereditary haemochromatosis type 3 is caused by mutations in transferrin receptor (TFR) 2. TFR2 has been shown to mediate iron transport in vitro and regulate iron homeostasis. The aim of this study was to determine the role of Tfr2 in iron transport in vivo using a Tfr2 mutant mouse.

METHODS

Tfr2 mutant and wild-type mice were injected intravenously with (59)Fe-transferrin and tissue (59)Fe uptake was measured. Tfr1, Tfr2 and ferroportin expression was measured by real-time PCR and Western blot. Cellular localisation of ferroportin was determined by immunohistochemistry.

RESULTS

Transferrin-bound iron uptake by the liver and spleen in Tfr2 mutant mice was reduced by 20% and 65%, respectively, whilst duodenal and renal uptake was unchanged compared with iron-loaded wild-type mice. In Tfr2 mutant mice, liver Tfr2 protein was absent, whilst ferroportin protein was increased in non-parenchymal cells and there was a low level of expression in hepatocytes. Tfr1 expression was unchanged compared with iron-loaded wild-type mice. Splenic Tfr2 protein expression was absent whilst Tfr1 and ferroportin protein expression was increased in Tfr2 mutant mice compared with iron-loaded wild-type mice.

CONCLUSIONS

A small reduction in hepatic transferrin-bound iron uptake in Tfr2 mutant mice suggests that Tfr2 plays a minor role in liver iron transport and its primary role is to regulate iron metabolism. Increased ferroportin expression due to decreased hepcidin mRNA levels is likely to be responsible for impaired splenic iron uptake in Tfr2 mutant mice.

TfR2 expression in human colon carcinomas

Different proteins regulate iron metabolism at the level of various tissues. Among these is a second transferrin receptor (TfR2) that seems to play a key role in the regulation of iron homeostasis. Although TfR2 expression in normal tissues is restricted at the level of the liver, we observed that TfR2 is frequently expressed in cancer cell lines. Taking advantage of this observation we investigated TfR2 expression in primary colon cancers, and showed that this receptor is expressed in about 26% of cases. TfR2 expression in colon cancer is not related to histological grade, but is preferentially associated with mucinous tumors. In colon cancer cell lines, TfR2 is localized in membrane lipid rafts, induces ERK1/ERK2 phosphorylation, when activated by its ligand transferring, and is preferentially expressed during S-M phases of the cell cycle. The presence of TfR2 on the membrane of colon cancer cells may contribute the growth advantage to these cells.

The regulation of cellular iron metabolism

While iron is an essential trace element required by nearly all living organisms, deficiencies or excesses can lead to pathological conditions such as iron deficiency anemia or hemochromatosis, respectively. A decade has passed since the discovery of the hemochromatosis gene, HFE, and our understanding of hereditary hemochromatosis (HH) and iron metabolism in health and a variety of diseases has progressed considerably. Although HFE-related hemochromatosis is the most widespread, other forms of HH have subsequently been identified. These forms are not attributed to mutations in the HFE gene but rather to mutations in genes involved in the transport, storage, and regulation of iron. This review is an overview of cellular iron metabolism and regulation, describing the function of key proteins involved in these processes, with particular emphasis on the liver's role in iron homeostasis, as it is the main target of iron deposition in pathological iron overload. Current knowledge on their roles in maintaining iron homeostasis and how their dysregulation leads to the pathogenesis of HH are discussed.

Defective trafficking and localization of mutated transferrin receptor 2: implications for type 3 hereditary hemochromatosis

Transferrin receptor 2 (TfR2), a homologue of transferrin receptor 1 (TfR1), is a key molecule involved in the regulation of iron homeostasis. Mutations in TfR2 result in iron overload with similar features to HFE-associated hereditary hemochromatosis. The precise role of TfR2 in iron metabolism and the functional consequences of disease-causing mutations have not been fully determined. We have expressed wild-type and various mutant forms of TfR2 that are associated with human disease in a mouse liver cell line. Intracellular and surface analysis shows that all the TfR2 mutations analyzed cause the intracellular retention of the protein in the endoplasmic reticulum, whereas the wild-type protein is expressed in endocytic structures and at the cell surface. Our results indicate that the majority of mutations that cause type 3 hereditary hemochromatosis are a consequence of the defective localization of the protein.

Recent advances in adult T-cell leukemia therapy: focus on a new anti-transferrin receptor monoclonal antibody

HTLV-I is an endemic retrovirus responsible for the adult T-cell leukemia/lymphoma (ATLL). This aggressive lymphoid proliferation is associated with a bad prognosis due to the resistance of HTLV-I-infected cells to most classical chemotherapeutic agents. Here we review recent advances in ATLL immunotherapy. We particularly focus on promising data from our group, characterizing a new mouse monoclonal antibody (mAb A24) against the human transferrin receptor (TfR-1). Monoclonal antibodies to target cell differentiation markers on ATLL cells have already been proposed as therapeutic agents. However, in clinical trials acute forms of ATLL were resistant to these immunotherapies. A24 binds TfR-1 (K(d) 2.7 nM) and competes with transferrin for receptor binding. It blocks the proliferation of malignant cells (TfR-1(high)), such as HTLV-I-infected T cells but not of resting cells. A24 induces TfR-1 endocytosis in lysosomal compartments where the receptor is degraded leading to intracellular iron deprivation. In HTLV-I-infected cells, A24 targets and induces apoptosis of both chronic and acute ATLL forms, independent of antibody aggregation, antibody-dependent cellular cytotoxicity and/or complement addition. The antibody efficacy was confirmed in animal models. We are currently developing strategies to use A24 in clinical trials.

Molecular regulation of hepatic expression of iron regulatory hormone hepcidin

Iron is a micronutrient that is an essential component that drives many metabolic reactions. Too little iron leads to anemia and too much iron increases the oxidative stress of body tissues leading to inflammation, cell death, and system organ dysfunction, including cancer. Maintaining normal iron balance is achieved by rigorous control of the amount absorbed by the intestine, that released from macrophages following erythrophagocytosis of effete red cells and by either release or uptake from hepatocytes. Hepcidin is a recently characterized molecule that appears to play a key role in the regulation of iron efflux from enterocytes, macrophages, and hepatocytes. It is produced by hepatocytes under basal conditions, in response to alterations in increased iron stores or reduced requirement for erythropoiesis and by inflammation. The proteins that regulate hepcidin expression are presently being defined, albeit that our present understanding is still far from complete. This review focuses on the molecules which regulate hepcidin expression. The subsequent characterization of these proteins using molecular, cellular, and physiological approaches also is discussed along with inflammatory signals and receptors involved in hepcidin expression.

Transferrin receptor 2 is frequently expressed in human cancer cell lines

Different proteins ensure the fine control of iron metabolism at the level of various tissues. Among these proteins, it was discovered a second transferrin receptor (TfR2), that seems to play a key role in the regulation of iron homeostasis. Its mutations are responsible for type 3 hemochromatosis (Type 3 HH). Although TfR2 expression in normal tissues was restricted at the level of liver and intestine, we observed that TfR2 was frequently expressed in tumor cell lines. Particularly frequent was its expression in ovarian cancer, colon cancer and glioblastoma cell lines; less frequent was its expression in leukemic and melanoma cell lines. Interestingly, in these tumor cell lines, TfR2 expression was inversely related to that of receptor 1 for transferrin (TfR1). Experiments of in vitro iron loading or iron deprivation provided evidence that TfR2 is modulated in cancer cell lines according to cellular iron levels following two different mechanisms: (i) in some cells, iron loading caused a downmodulation of total TfR2 levels; (ii) in other cell types, iron loading caused a downmodulation of membrane-bound TfR2, without affecting the levels of total cellular TfR2 content. Iron deprivation caused in both conditions an opposite effect compared to iron loading. These observations suggest that TfR2 expression may be altered in human cancers and warrant further studies in primary tumors. Furthermore, our studies indicate that, at least in tumor cells, TfR2 expression is modulated by iron through different biochemical mechanisms, whose molecular basis remains to be determined.

Transferrin receptor 2 is emerging as a major player in the control of iron metabolism

Our knowledge of mammalian iron metabolism has advanced dramatically over recent years. Iron is an essential element for virtually all living organisms. Its intestinal absorption and accurate cellular regulation is strictly required to ensure the coordinated synthesis of the numerous iron-containing proteins involved in key metabolic processes, while avoiding the uptake of excess iron that can lead to organ damage. A range of different proteins exist to ensure this fine control within the various tissues of the body. Among these proteins, transferrin receptor (TFR2) seems to play a key role in the regulation of iron homeostasis. Disabling mutations in TFR2 are responsible for type 3 hereditary hemochromatosis (Type 3 HH). This review describes the biological properties of this membrane receptor, with a particular emphasis paid to the structure, function and cellular localization. Although much information has been garnered on TFR2, further efforts are needed to elucidate its function in the context of the iron regulatory network.

Targeted treatment of cancer with artemisinin and artemisinin-tagged iron-carrying compounds

Artemisinin is a chemical compound that reacts with iron to form free radicals which can kill cells. Cancer cells require and uptake a large amount of iron to proliferate. They are more susceptible to the cytotoxic effect of artemisinin than normal cells. Cancer cells express a large concentration of cell surface transferrin receptors that facilitate uptake of the plasma iron-carrying protein transferrin via endocytosis. By covalently tagging artemisinin to transferrin, artemisinin could be selectively picked up and concentrated by cancer cells. Futhermore, both artemisinin and iron would be transported into the cell in one package. Once an artemisinin-tagged transferrin molecule is endocytosed, iron is released and reacts with artemisinin moieties tagged to transferrin. Formation of free radicals kills the cancer cell. The authors have found that artemisinin-tagged transferrin is highly selective and potent in killing cancer cells. Thus, artemisinin and artemisinin-tagged iron-carrying compounds could be developed into powerful anticancer drugs.

Targeted disruption of the hepatic transferrin receptor 2 gene in mice leads to iron overload

BACKGROUND & AIMS

Transferrin receptor 2 (TfR2) plays a key role in the regulation of iron metabolism. Mutations of TfR2 in humans cause type 3 hereditary hemochromatosis. Although highly expressed in liver, several studies have reported TfR2 expression in other tissues. To determine the contribution of liver expressed TfR2 in iron homeostasis, we have generated and characterized a liver-specific TfR2-knockout (KO) mouse.

METHODS

Liver-specific TfR2-KO mice were generated by crossing TfR2-floxed mice with transgenic albumin-Cre mice. Tissue and serum from homozygous TfR2-floxed mice with and without albumin-Cre were analyzed. Serum transferrin saturation, hepatic, and splenic iron concentrations were determined. The expression of iron-related mRNA transcripts was analyzed by real-time PCR. Levels of the iron-related proteins TfR1, TfR2, ferritin, and prohepcidin were analyzed by immunoblotting.

RESULTS

Liver-specific TfR2-KO mice develop significant iron overload comparable to complete TfR2-KO mice. At all ages studied, transferrin saturation, hepatic iron concentration, and hepatic ferritin were significantly elevated. Hepatic TfR2 mRNA and protein were absent in the livers of liver-specific TfR2-KO mice, and TfR1 expression was reduced consistent with liver iron loading. At 5 weeks of age, hepcidin1 mRNA, and prohepcidin protein were decreased in liver-specific TfR2-KO compared to control mice.

CONCLUSIONS

The significant iron loading and modulation of expression of iron-related genes in liver-specific TfR2-KO mice demonstrates that the liver is the primary site for TfR2 expression and activity and that liver-expressed TfR2 is required for the regulation of hepcidin1.

Transferrin receptor 2: evidence for ligand-induced stabilization and redirection to a recycling pathway

Transferrin receptor 2 (TfR2) is a homologue of transferrin receptor 1 (TfR1), the protein that delivers iron to cells through receptor-mediated endocytosis of diferric transferrin (Fe(2)Tf). TfR2 also binds Fe(2)Tf, but it seems to function primarily in the regulation of systemic iron homeostasis. In contrast to TfR1, the trafficking of TfR2 within the cell has not been extensively characterized. Previously, we showed that Fe(2)Tf increases TfR2 stability, suggesting that trafficking of TfR2 may be regulated by interaction with its ligand. In the present study, therefore, we sought to identify the mode of TfR2 degradation, to characterize TfR2 trafficking, and to determine how Fe(2)Tf stabilizes TfR2. Stabilization of TfR2 by bafilomycin implies that TfR2 traffics to the lysosome for degradation. Confocal microscopy reveals that treatment of cells with Fe(2)Tf increases the fraction of TfR2 localizing to recycling endosomes and decreases the fraction of TfR2 localizing to late endosomes. Mutational analysis of TfR2 shows that the mutation G679A, which blocks TfR2 binding to Fe(2)Tf, increases the rate of receptor turnover and prevents stabilization by Fe(2)Tf, indicating a direct role of Fe(2)Tf in TfR2 stabilization. The mutation Y23A in the cytoplasmic domain of TfR2 inhibits its internalization and degradation, implicating YQRV as an endocytic motif.

Molecular Evolution of the Transferrin Receptor/Glutamate Carboxypeptidase II Family

The transferrin receptor family is represented by at least seven different homologous proteins in primates. Transferrin receptor (TfR1) is a type II membrane glycoprotein that, as a cell surface homodimer, binds iron-loaded transferrin as part of the process of iron transfer and uptake. Other family members include transferrin receptor 2 (TfR2), glutamate carboxypeptidase II (GCP2 or PSMA), N-acetylated alpha-linked acidic dipeptidase-like protein (NLDL), N-acetylated alpha-linked acidic dipeptidase 2 (NAALAD2), and prostate-specific membrane antigen-like protein (PMSAL/GCPIII). We compared 86 different sequences from 24 different species, from mammals to fungi. Through this comparison, we have identified several highly conserved residues specific to each family not previously associated with clinical mutations. The evolutionary history of the TfR/GCP2 family shows repeated episodes of duplications consistent with recent theories that nondispensable, slowly evolving genes are more likely to form multiple gene families.

The transferrin receptor part I: Biology and targeting with cytotoxic antibodies for the treatment of cancer

The transferrin receptor (TfR) is a cell membrane-associated glycoprotein involved in the cellular uptake of iron and in the regulation of cell growth. Iron uptake occurs via the internalization of iron-loaded transferrin (Tf) mediated by the interaction with the TfR. In addition, the TfR may also contain other growth regulatory properties in certain normal and malignant cells. The elevated levels of TfR in malignancies, its relevance in cancer, and the extracellular accessibility of this molecule make it an excellent antigen for the treatment of cancer using antibodies. The TfR can be targeted by monoclonal antibodies specific for the extracellular domain of the receptor. In this review, we summarize advancements in the basic physiology of the TfR including structure, function, and expression. We also discuss the efficacy of targeting the TfR using cytotoxic antibodies that inhibit cell growth and/or induce apoptosis in targeted malignant cells.

Engagement of transferrin receptor by polymeric IgA1: evidence for a positive feedback loop involving increased receptor expression and mesangial cell proliferation in IgA nephropathy

IgA nephropathy (IgAN), the most common primary glomerulonephritis in the world, is characterized by IgA immune complex-mediated mesangial cell proliferation. The transferrin receptor (TfR) was identified previously as an IgA1 receptor, and it was found that, in biopsies of patients with IgAN, TfR is overexpressed and co-localizes with IgA1 mesangial deposits. Here, it is shown that purified polymeric IgA1 (pIgA1) is a major inducer of TfR expression (three- to four-fold increase) in quiescent human mesangial cells (HMC). IgA-induced but not cytokine-induced HMC proliferation is dependent on TfR engagement as it is inhibited by both TfR1 and TfR2 ectodomains as well as by the anti-TfR mAb A24. It is dependent on the continued presence of IgA1 rather than on soluble factors released during IgA1-mediated activation. In addition, pIgA1-induced IL-6 and TGF-beta production from HMC was specifically inhibited by mAb A24, confirming that pIgA1 triggers a TfR-dependent HMC activation. Finally, upregulation of TfR expression induced by sera from patients with IgAN but not from healthy individuals was dependent on IgA. It is proposed that deposited pIgA1 or IgA1 immune complexes could initiate a process of auto-amplification involving hyperexpression of TfR, allowing increased IgA1 mesangial deposition. Altogether, these data unveil a functional cooperation between pIgA1 and TfR for IgA1 deposition and HMC proliferation and activation, features that are commonly implicated in the chronicity of mesangial injuries observed in IgAN and that could explain the recurrence of IgA1 deposits in the mesangium after renal transplantation.

Regulation of transferrin receptor 2 protein levels by transferrin

Transferrin receptor 2 (TfR2) plays a critical role in iron homeostasis because patients carrying disabling mutations in the TFR2 gene suffer from hemochromatosis. In this study, iron-responsive regulation of TfR2 at the protein level was examined in vitro and in vivo. HepG2 cell TfR2 protein levels were up-regulated after exposure to holotransferrin (holoTf) in a time- and dose-responsive manner. ApoTf or high-iron treatment with non–Tf-bound iron failed to elicit similar effects, suggesting that TfR2 regulation reflects interactions of the iron-bound ligand. Hepatic TfR2 protein levels also reflected an adaptive response to changing iron status in vivo. Liver TfR2 protein levels were down- and up-regulated in rats fed an iron-deficient and a high-iron diet, respectively. TfR2 was also up-regulated in Hfe-/- mice, an animal model that displays liver iron loading. In contrast, TfR2 levels were reduced in hypotransferrinemic mice despite liver iron overload, supporting the idea that regulation of the receptor is dependent on Tf. This idea is confirmed by up-regulation of TfR2 in β-thalassemic mice, which, like hypotransferrinemic mice, are anemic and incur iron loading, but have functional Tf. Based on these combined results, we hypothesize that TfR2 acts as a sensor of iron status such that receptor levels reflect Tf saturation.

Diferric transferrin regulates transferrin receptor 2 protein stability

Transferrin receptor 2 (TfR2) is a type 2 transmembrane protein expressed in hepatocytes that binds iron-bound transferrin (Tf). Mutations in TfR2 cause one form of hereditary hemochromatosis, a disease in which excessive absorption of dietary iron can lead to liver cirrhosis, diabetes, arthritis, and heart failure. The function of TfR2 in iron homeostasis is unknown. We have studied the regulation of TfR2 in HepG2 cells. Western blot analysis shows that TfR2 increases in a time- and dose-dependent manner after diferric Tf is added to the culture medium. In cells exposed to diferric Tf, the amount of TfR2 returns to control levels within 8 hours after the removal of diferric Tf from the medium. However, TfR2 does not increase when non-Tf-bound iron (FeNTA) or apo Tf is added to the medium. The response to diferric Tf appears to be hepatocyte specific. Real-time quantitative reverse transcription-polymerase chain reaction (qRT-PCR) analysis shows that TfR2 mRNA levels do not change in cells exposed to diferric Tf. Rather, the increase in TfR2 is attributed to an increase in the half-life of TfR2 protein in cells exposed to diferric Tf. Our results support a role for TfR2 in monitoring iron levels by sensing changes in the concentration of diferric Tf.

Transferrin receptor 2 mediates a biphasic pattern of transferrin uptake associated with ligand delivery to multivesicular bodies

The physiological role of transferrin (Tf) receptor 2 (TfR2), a homolog of the well-characterized TfR1, is unclear. Mutations in TfR2 result in hemochromatosis, indicating that this receptor has a unique role in iron metabolism. We report that HepG2 cells, which endogenously express TfR2, display a biphasic pattern of Tf uptake when presented with ligand concentrations up to 2 muM. The apparently nonsaturating pathway of Tf endocytosis resembles TfR1-independent Tf uptake, a process previously characterized in some liver cell types. Exogenous expression of TfR2 but not TfR1 induces a similar biphasic pattern of Tf uptake in HeLa cells, supporting a role for TfR2 in this process. Immunoelectron microscopy reveals that while Tf, TfR1, and TfR2 are localized in the plasma membrane and tubulovesicular endosomes, TfR2 expression is associated with the additional appearance of Tf in multivesicular bodies. These combined results imply that unlike TfR1, which recycles apo-Tf back to the cell surface after the release of iron, TfR2 promotes the intracellular deposition of ligand. Tf delivered by TfR2 does not appear to be degraded, which suggests that its delivery to this organelle may be functionally relevant to the storage of iron in overloaded states.