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

Resveratrol ameliorates iron overload induced liver fibrosis in mice by regulating iron homeostasis

This study is intended to explore the protective effects of resveratrol (RES) on iron overload-induced liver fibrosis and its mechanism. Iron dextran (50 mg/kg) was injected intraperitoneally in all groups except the control group. Mice in the L-RES, M-RES and H-RES groups were gavaged with RES solution at 25, 50 mg/kg and 100 mg/kg, respectively, 4 h before injection of iron dextran every day; mice in the deferoxamine (DFO) group were injected with DFO intraperitoneally (100 mg/kg); mice in the control group received isovolumetric saline. After seven weeks of RES administration, serum alanine aminotransferase (ALT), aspartate aminotransferase (AST) activities and liver hydroxyproline (Hyp) levels were reduced; the malondialdehyde (MDA) activities decreased and the levels of superoxide dismutase (SOD) and glutathione (GSH) were raised. Hematoxylin and eosin (H&E), Prussian, and Masson staining indicated that RES treatment could improve cell damage and reduce hepatic iron deposition and collagen deposition in iron-overload mice. The expression of Bcl-2 was increased, the expression levels of Bax and caspase-3 were decreased under RES treatment. Moreover, RES reduced the expression of hepcidin, ferritin (Ft), divalent metal transporter-1 (DMT-1), transferrin receptor-2 (TFR-2), and raised the expression of ferroprotein-1 (FPN-1). In conclusion, RES could ameliorate iron overload-induced liver fibrosis, and the potential mechanisms may be related to antioxidant, anti-inflammatory, anti-apoptotic, and more importantly, regulation of iron homeostasis by reducing iron uptake and increasing iron export.

Leucine zipper transcription factor-like 1 binds adaptor protein complex-1 and 2 and participates in trafficking of transferrin receptor 1

LZTFL1 participates in immune synapse formation, ciliogenesis, and the localization of ciliary proteins, and knockout of LZTFL1 induces abnormal distribution of heterotetrameric adaptor protein complex-1 (AP-1) in the Lztfl1-knockout mouse photoreceptor cells, suggesting that LZTFL1 is involved in intracellular transport. Here, we demonstrate that in vitro LZTFL1 directly binds to AP-1 and AP-2 and coimmunoprecipitates AP-1 and AP-2 from cell lysates. DxxFxxLxxxR motif of LZTFL1 is essential for these bindings, suggesting LZTFL1 has roles in AP-1 and AP-2-mediated protein trafficking. Since AP-1 and AP-2 are known to be involved in transferrin receptor 1 (TfR1) trafficking, the effect of LZTFL1 on TfR1 recycling was analyzed. TfR1, AP-1 and LZTFL1 from cell lysates could be coimmunoprecipitated. However, pull-down results indicate there is no direct interaction between TfR1 and LZTFL1, suggesting that LZTFL1 interaction with TfR1 is indirect through AP-1. We report the colocalization of LZTFL1 and AP-1, AP-1 and TfR1 as well as LZTFL1 and TfR1 in the perinuclear region (PNR) and the cytoplasm, suggesting a potential complex between LZTFL1, AP-1 and TfR1. The results from the disruption of adaptin recruitment with brefeldin A treatment suggested ADP-ribosylation factor-dependent localization of LZFL1 and AP-1 in the PNR. Knockdown of AP-1 reduces the level of LZTFL1 in the PNR, suggesting that AP-1 plays a role in LZTFL1 trafficking. Knockout of LZTFL1 reduces the cell surface level and the rate of internalization of TfR1, leading to a decrease of transferrin uptake, efflux, and internalization. However, knockout of LZTFL1 did not affect the cell surface levels of epidermal growth factor receptor and cation-independent mannose 6-phosphate receptor, indicating that LZTFL1 specifically regulates the cell surface level of TfR1. These data support a novel role of LZTFL1 in regulating the cell surface TfR1 level by interacting with AP-1 and AP-2.

The Functional Versatility of Transferrin Receptor 2 and Its Therapeutic Value

Iron homeostasis is a tightly regulated process in all living organisms because this metal is essential for cellular metabolism, but could be extremely toxic when present in excess. In mammals, there is a complex pathway devoted to iron regulation, whose key protein is hepcidin (Hepc), which is a powerful iron absorption inhibitor mainly produced by the liver. Transferrin receptor 2 (Tfr2) is one of the hepcidin regulators, and mutations in TFR2 gene are responsible for type 3 hereditary hemochromatosis (HFE3), a genetically heterogeneous disease characterized by systemic iron overload. It has been recently pointed out that Hepc production and iron regulation could be exerted also in tissues other than liver, and that Tfr2 has an extrahepatic role in iron metabolism as well. This review summarizes all the most recent data on Tfr2 extrahepatic role, taking into account the putative distinct roles of the two main Tfr2 isoforms, Tfr2α and Tfr2β. Representing Hepc modulation an effective approach to correct iron balance impairment in common human diseases, and with Tfr2 being one of its regulators, it would be worthwhile to envisage Tfr2 as a therapeutic target.

Crossing the Iron Gate: Why and How Transferrin Receptors Mediate Viral Entry

Because both the host and pathogen require iron, the innate immune response carefully orchestrates control over iron metabolism to limit its availability during times of infection. Nutritional iron deficiency can impair host immunity, while iron overload can cause oxidative stress to propagate harmful viral mutations. An emerging enigma is that many viruses use the primary gatekeeper of iron metabolism, the transferrin receptor, as a means to enter cells. Why and how this iron gate is a viral target for infection are the focus of this review.

Transferrin Receptors TfR1 and TfR2 Bind Transferrin through Differing Mechanisms

Hereditary hemochromatosis (HH), a disease marked by chronic iron overload from insufficient expression of the hormone hepcidin, is one of the most common genetic diseases. One form of HH (type III) results from mutations in transferrin receptor-2 (TfR2). TfR2 is postulated to be a part of signaling system that is capable of modulating hepcidin expression. However, the molecular details of TfR2's role in this system remain unclear. TfR2 is predicted to bind the iron carrier transferrin (Tf) when the iron saturation of Tf is high. To better understand the nature of these TfR-Tf interactions, a binding study with the full-length receptors was conducted. In agreement with previous studies with truncated forms of these receptors, holo-Tf binds to the TfR1 homologue significantly stronger than to TfR2. However, the binding constant for Tf-TfR2 is still far above that of physiological holo-Tf levels, inconsistent with the hypothetical model, suggesting that other factors mediate the interaction. One possible factor, apo-Tf, only weakly binds TfR2 at serum pH and thus will not be able to effectively compete with holo-Tf. Tf binding to a TfR2 chimera containing the TfR1 helical domain indicates that the differences in the helical domain account for differences in the on rate of Tf, and nonconserved inter-receptor interactions are necessary for the stabilization of the complex. Conserved residues at one possible site of stabilization, the apical arm junction, are not important for TfR1-Tf binding but are critical for the TfR2-Tf interaction. Our results highlight the differences in Tf interactions with the two TfRs.

A specialized pathway for erythroid iron delivery through lysosomal trafficking of transferrin receptor 2

Erythroid progenitors are the largest consumers of iron in the human body. In these cells, a high flux of iron must reach the mitochondrial matrix to form sufficient heme to support hemoglobinization. Canonical erythroid iron trafficking occurs via the first transferrin receptor (TfR1)-mediated endocytosis of diferric-transferrin into recycling endosomes, where ferric iron is released, reduced, and exported to the cytosol via DMT1. However, mice lacking TfR1 or DMT1 demonstrate residual erythropoiesis, suggesting additional pathways for iron use. How iron moves from endosomes to mitochondria is incompletely understood, with both cytosolic chaperoning and "kiss and run" interorganelle transfer implicated. TfR2, in contrast to its paralog TfR1, has established roles in iron sensing, but not iron uptake. Recently, mice with marrow-selective TfR2 deficiency were found to exhibit microcytosis, suggesting TfR2 may also contribute to erythroid hemoglobinization. In this study, we identify alternative trafficking, in which TfR2 mediates lysosomal transferrin delivery. Imaging studies reveal an erythroid lineage-specific organelle arrangement consisting of a focal lysosomal cluster surrounded by a nest of mitochondria, with direct contacts between these 2 organelles. Erythroid TfR2 deficiency yields aberrant mitochondrial morphology, implicating TfR2-dependent transferrin trafficking in mitochondrial maintenance. Human TFR2 shares a lineage- and stage-specific expression pattern with MCOLN1, encoding a lysosomal iron channel, and MFN2, encoding a protein mediating organelle contacts. Functional studies reveal these latter factors to be involved in mitochondrial regulation and erythroid differentiation, with Mfn2 required for mitochondrial-lysosomal contacts. These findings identify a new pathway for erythroid iron trafficking involving TfR2-mediated lysosomal delivery followed by interorganelle transfer to mitochondria.

N-linked glycosylation is required for transferrin-induced stabilization of transferrin receptor 2, but not for transferrin binding or trafficking to the cell surface

Transferrin receptor 2 (TfR2) is a member of the transferrin receptor-like family of proteins. Mutations in TfR2 can lead to a rare form of the iron overload disease, hereditary hemochromatosis. TfR2 is proposed to sense body iron levels and increase the level of expression of the iron regulatory hormone, hepcidin. Human TfR2 (hTfR2) contains four potential Asn-linked (N-linked) glycosylation sites on its ectodomain. The importance of glycosylation in TfR2 function has not been elucidated. In this study, by employing site-directed mutagenesis to remove glycosylation sites of hTfR2 individually or in combination, we found that hTfR2 was glycosylated at Asn 240, 339, and 754, while the consensus sequence for N-linked glycosylation at Asn 540 was not utilized. Cell surface protein biotinylation and biotin-labeled Tf indicated that in the absence of N-linked oligosaccharides, hTfR2 still moved to the plasma membrane and bound its ligand, holo-Tf. However, without N-linked glycosylation, hTfR2 did not form the intersubunit disulfide bonds as efficiently as the wild type (WT). Moreover, the unglycosylated form of hTfR2 could not be stabilized by holo-Tf. We further provide evidence that the unglycosylated hTfR2 behaved in manner different from that of the WT in response to holo-Tf treatment. Thus, the putative iron-sensing function of TfR2 could not be achieved in the absence of N-linked oligosaccharides. On the basis of our analyses, we conclude that unlike TfR1, N-linked glycosylation is dispensable for the cell surface expression and holo-Tf binding, but it is required for efficient intersubunit disulfide bond formation and holo-Tf-induced stabilization of TfR2.

Nanocarriers enhance Doxorubicin uptake in drug-resistant ovarian cancer cells

Resistance to anthracyclines and other chemotherapeutics due to P-glycoprotein (pgp)-mediated export is a frequent problem in cancer treatment. Here, we report that iron oxide-titanium dioxide core-shell nanocomposites can serve as efficient carriers for doxorubicin to overcome this common mechanism of drug resistance in cancer cells. Doxorubicin nanocarriers (DNC) increased effective drug uptake in drug-resistant ovarian cells. Mechanistically, doxorubicin bound to the TiO(2) surface by a labile bond that was severed upon acidification within cell endosomes. Upon its release, doxorubicin traversed the intracellular milieu and entered the cell nucleus by a route that evaded pgp-mediated drug export. Confocal and X-ray fluorescence microscopy and flow cytometry were used to show the ability of DNCs to modulate transferrin uptake and distribution in cells. Increased transferrin uptake occurred through clathrin-mediated endocytosis, indicating that nanocomposites and DNCs may both interfere with removal of transferrin from cells. Together, our findings show that DNCs not only provide an alternative route of delivery of doxorubicin to pgp-overexpressing cancer cells but also may boost the uptake of transferrin-tagged therapeutic agents.

Hepatocytes internalize trophic receptors at large endocytic "Hot Spots"

Clathrin-mediated endocytosis in mammalian epithelial cells is believed to require the synergistic action of structural coat proteins and mechanochemical enzymes to deform and sever the plasma membrane (PM) into discreet vesicles. It is generally believed that the formation of clathrin-coated pits in epithelial cells occurs randomly along the apical and basolateral plasma membranes. In this study we visualized the endocytic machinery in living hepatocytes using green fluorescent protein (GFP)-tagged dynamin, a large mechanochemical guanosine triphosphate (GTP)ase implicated in the liberation of nascent vesicles from the plasma membrane and a variety of internal membrane compartments. Confocal microscopy of living cells expressing the epithelial isoform of GFP-tagged dynamin [Dyn2-GFP] revealed a distribution along the ventral PM in discrete vesicle-like puncta or in large (2-10 μm) tubuloreticular plaques. Remarkably, these large structures are dynamic as they form and then disappear, while generating large numbers of motile endocytic vesicles with which dynamin associates. Inhibiting dynamin function by microinjection of purified dynamin antibodies increases the number and size of the tubuloreticular plaques. Importantly, these "hot spots" sequester specific trophic receptors and cognate ligands such as transferrin receptor 1 (TfR1), but not TfR2.

CONCLUSION

These findings suggest that hepatocytes sequester or prerecruit both structural and enzymatic components of the clathrin-based endocytic machinery to functional hot spots, from which large numbers of coated pits form and vesicles are generated. This process may mimic the endocytic organization found at the synapse in neuronal cells.

Hepatocyte-targeted HFE and TFR2 control hepcidin expression in mice

Hereditary hemochromatosis is caused by mutations in the hereditary hemochromatosis protein (HFE), transferrin-receptor 2 (TfR2), hemojuvelin, hepcidin, or ferroportin genes. Hepcidin is a key iron regulator, which is secreted by the liver, and decreases serum iron levels by causing the down-regulation of the iron transporter, ferroportin. Mutations in either HFE or TfR2 lower hepcidin levels, implying that both HFE and TfR2 are necessary for regulation of hepcidin expression. In this study, we used a recombinant adeno-associated virus, AAV2/8, for hepatocyte-specific expression of either Hfe or Tfr2 in mice. Expression of Hfe in Hfe-null mice both increased Hfe and hepcidin mRNA and lowered hepatic iron and Tf saturation. Expression of Tfr2 in Tfr2-deficient mice had a similar effect, whereas expression of Hfe in Tfr2-deficient mice or of Tfr2 in Hfe-null mice had no effect on liver or serum iron levels. Expression of Hfe in wild-type mice increased hepcidin mRNA and lowered iron levels. In contrast, expression of Tfr2 had no effect on wild-type mice. These findings suggest that Hfe is limiting in formation of the Hfe/Tfr2 complex that regulates hepcidin expression. In addition, these studies show that the use of recombinant AAV vector to deliver genes is a promising approach for studying physiologic consequences of protein complexes.

The role of transferrin receptor 1 and 2 in transferrin-bound iron uptake in human hepatoma cells

Transferrin receptor (TFR) 1 and 2 are expressed in the liver; TFR1 levels are regulated by cellular iron levels while TFR2 levels are regulated by transferrin saturation. The aims of this study were to 1) determine the relative importance of TFR1 and TFR2 in transferrin-bound iron (TBI) uptake by HuH7 human hepatoma cells and 2) characterize the role of metal-transferrin complexes in the regulation of these receptors. TFR expression was altered by 1) incubation with metal-transferrin (Tf) complexes, 2) TFR1 and TFR2 small interfering RNA knockdown, and 3) transfection with a human TFR2 plasmid. TBI uptake was measured using (59)Fe-(125)I-labeled Tf and mRNA and protein expression by real-time PCR and Western blot analysis, respectively. Fe(2)Tf, Co(2)Tf, and Mn(2)Tf increased TFR2 protein expression, indicating that the upregulation was not specifically regulated by iron-transferrin but also other metal-transferrins. In addition, Co(2)Tf and Mn(2)Tf upregulated TFR1, reduced ferritin, and increased hypoxia-inducible factor-1alpha protein expression, suggesting that TFR1 upregulation was due to a combination of iron deficiency and chemical hypoxia. TBI uptake correlated with changes in TFR1 but not TFR2 expression. TFR1 knockdown reduced iron uptake by 80% while TFR2 knockdown did not affect uptake. At 5 microM transferrin, iron uptake was not affected by combined TFR1 and TFR2 knockdown. Transfection with a hTFR2 plasmid increased TFR2 protein expression, causing a 15-20% increase in iron uptake and ferritin levels. This shows for the first time that TFR-mediated TBI uptake is mediated primarily via TFR1 but not TFR2 and that a high-capacity TFR-independent pathway exists in hepatoma cells.

Mammalian iron transport

Iron is essential for basic cellular processes but is toxic when present in excess. Consequently, iron transport into and out of cells is tightly regulated. Most iron is delivered to cells bound to plasma transferrin via a process that involves transferrin receptor 1, divalent metal-ion transporter 1 and several other proteins. Non-transferrin-bound iron can also be taken up efficiently by cells, although the mechanism is poorly understood. Cells can divest themselves of iron via the iron export protein ferroportin in conjunction with an iron oxidase. The linking of an oxidoreductase to a membrane permease is a common theme in membrane iron transport. At the systemic level, iron transport is regulated by the liver-derived peptide hepcidin which acts on ferroportin to control iron release to the plasma.

Transferrin-directed internalization and cycling of transferrin receptor 2

Transferrin receptor 2 (TfR2) is a homologue of transferrin receptor 1 (TfR1) but has distinct functions from TfR1 in iron homeostasis. In keeping with its proposed role in iron sensing, previous studies showed that TfR2 has a short half-life and that holo-Tf stabilizes TfR2 by redirecting it from a degradative pathway to a recycling pathway. In this study, we characterized how the endocytosis, recycling and degradation of TfR2 relates to its function and differs from TfR1. TfR2 endocytosis was adaptor protein-2 (AP-2) dependent. Flow cytometry analysis showed that TfR1 and TfR2 utilized the same endocytic pathway only in the presence of holo-Tf, indicating that holo-Tf alters the interaction of TfR2 with the endocytic machinery. Unlike TfR1, phosphofurin acidic cluster sorting protein 1 (PACS-1) binds to the cytoplasmic domain of TfR2 and data suggest that PACS-1 is involved in the TfR2 recycling. Depletion of TSG101 by siRNA or expression of a dominant negative Vps4 inhibited TfR2 degradation, indicating that TfR2 degradation occurs through a multivesicular body (MVB) pathway. TfR2 degradation is not mediated through ubiquitination on the single lysine (K31) in the cytoplasmic domain or on the amino terminal residue. No ubiquitination of TfR2 by HA-ubiquitin was detected, indicating a lack of direct TfR2 ubiquitination involvement in its degradation.

Analysis of hepatic genes involved in the metabolism of fatty acids and iron in nonalcoholic fatty liver disease

AIMS

Hepatic steatosis and iron cause oxidative stress, thereby progressing steatosis to steatohepatitis. We quantified the expression of genes involved in the metabolism of fatty acids and iron in patients with nonalcoholic fatty liver disease (NAFLD).

METHODS

The levels of transcripts for the following genes were quantified from biopsy specimens of 74 patients with NAFLD: thioredoxin (Trx), fatty acid transport protein 5 (FATP5), sterol regulatory element-binding protein 1c (SREBP1c), fatty acid synthase (FASN), acetyl-coenzyme A carboxylase (ACAC), peroxisome proliferative activated receptor alpha (PPARalpha), cytochrome P-450 2E1 (CYP2E1), acyl-coenzyme A dehydrogenase (ACADM), acyl-coenzyme A oxidase (ACOX), microsomal triglyceride transfer protein (MTP), transferrin receptor 1 (TfR1), transferrin receptor 2 (TfR2) and hepcidin. Twelve samples of human liver RNA were used as controls. Histological evaluation followed the methods of Brunt.

RESULTS

The levels of all genes were significantly higher in the NAFLD patients than in controls. The Trx level increased as the stage progressed. The levels of FATP5, SREBP1c, ACAC, PPARalpha, CYP2E1, ACADM and MTP significantly decreased as the stage and grade progressed (P < 0.05). Hepatic iron score (HIS) increased as the stage progressed. The TfR1 level significantly increased as the stage progressed (P < 0.05), whereas TfR2 level significantly decreased (P < 0.05). The ratio of hepcidin mRNA/ferritin (P < 0.001) or hepcidin mRNA/HIS (P < 0.01) was significantly lower in NASH patients than simple steatosis patients.

CONCLUSIONS

Steatosis-related metabolism is attenuated as NAFLD progresses, whereas iron-related metabolism is exacerbated. Appropriate therapies should be considered on the basis of metabolic changes.

The role of Hfe in transferrin-bound iron uptake by hepatocytes

HFE-related hereditary hemochromatosis results in hepatic iron overload. Hepatocytes acquire transferrin-bound iron via transferrin receptor (Tfr) 1 and Tfr1-independent pathways (possibly Tfr2-mediated). In this study, the role of Hfe in the regulation of hepatic transferrin-bound iron uptake by these pathways was investigated using Hfe knockout mice. Iron and transferrin uptake by hepatocytes from Hfe knockout, non-iron-loaded and iron-loaded wild-type mice were measured after incubation with 50 nM (125)I-Tf-(59)Fe (Tfr1 pathway) and 5 microM (125)I-Tf-(59)Fe (Tfr1-independent or putative Tfr2 pathway). Tfr1 and Tfr2 messenger RNA (mRNA) and protein expression were measured by real-time polymerase chain reaction and western blotting, respectively. Tfr1-mediated iron and transferrin uptake by Hfe knockout hepatocytes were increased by 40% to 70% compared with iron-loaded wild-type hepatocytes with similar iron levels and Tfr1 expression. Iron and transferrin uptake by the Tfr1-independent pathway was approximately 100-fold greater than by the Tfr1 pathway and was not affected by the absence of Hfe. Diferric transferrin increased hepatocyte Tfr2 protein expression, resulting in a small increase in transferrin but not iron uptake by the Tfr1-independent pathway.

CONCLUSION

Tfr1-mediated iron uptake is regulated by Hfe in hepatocytes. The Tfr1-independent pathway exhibited a much greater capacity for iron uptake than the Tfr1 pathway but it was not regulated by Hfe. Diferric transferrin up-regulated hepatocyte Tfr2 protein expression but not iron uptake, suggesting that Tfr2 may have a limited role in the Tfr1-independent pathway.

HFE association with transferrin receptor 2 increases cellular uptake of transferrin-bound iron

Mutations in either HFE or transferrin receptor 2 (TfR2) cause decreased expression of the iron regulatory hormone hepcidin and hemochromatosis. HFE and TfR2 were recently discovered to form a stable complex at the cell membrane when co-expressed in heterologous cell lines. We analyzed the functional consequences of the co-expression of these proteins using transfected TRVb cells, a Chinese hamster ovary derived cell line without endogenous HFE or transferrin receptor. The co-expression of TfR2 in TRVb cells expressing HFE led to accelerated HFE biosynthesis and late-Golgi maturation, suggesting interaction prior to cell surface localization. The co-expression of HFE in cells expressing TfR2 led to increased affinity for diferric transferrin, increased transferrin-dependent iron uptake, and relative resistance to iron chelation. These observations indicate that HFE influences the functional properties of TfR2, and suggests a model in which the interaction of these proteins might influence signal transduction to hepcidin.

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.

Transferrin receptor 2 mediates uptake of transferrin-bound and non-transferrin-bound iron

BACKGROUND/AIMS

Transferrin receptor 2 appears to have dual roles in iron metabolism; one is signalling, the other is iron transport. It is sensitive to high levels of diferric transferrin, which is associated with disorders of iron overload. Also present in these disorders are increased levels of plasma non-transferrin-bound iron. This study sought to clarify the role of transferrin receptor 2 in the uptake of transferrin-bound and non-transferrin-bound iron.

METHODS

Variant Chinese Hamster Ovary (CHO) cells, transfected with transferrin receptor 2, were incubated with radio-labelled transferrin-bound or non-transferrin-bound iron. Competition studies were performed in the presence of unlabelled dimetallic transferrin; knockdown was performed using specific siRNA.

RESULTS

Cells expressing transferrin receptor 2 bound and internalised transferrin and transferrin-bound iron. Transferrin recycling occurred with an average cycling time of 11-15 min. Interestingly, the presence of transferrin receptor 2 was also associated with uptake of non-transferrin-bound iron which was inhibited by unlabelled transferrin-bound metals. Knockdown reduced transferrin-bound and non-transferrin-bound iron uptake by approximately 60%.

CONCLUSIONS

Transferrin receptor 2 mediates transferrin-bound iron uptake by receptor-mediated endocytosis. It is also involved in the uptake of non-transferrin-bound iron and the inhibition of non-transferrin-bound iron uptake by diferric transferrin in CHO cells.

Liver iron transport

The liver plays a central role in iron metabolism. It is the major storage site for iron and also expresses a complex range of molecules which are involved in iron transport and regulation of iron homeostasis. An increasing number of genes associated with hepatic iron transport or regulation have been identified. These include transferrin receptors (TFR1 and 2), a ferrireductase (STEAP3), the transporters divalent metal transporter-1 (DMT1) and ferroportin (FPN) as well as the haemochromatosis protein, HFE and haemojuvelin (HJV), which are signalling molecules. Many of these genes also participate in iron regulatory pathways which focus on the hepatic peptide hepcidin. However, we are still only beginning to understand the complex interactions between liver iron transport and iron homeostasis. This review outlines our current knowledge of molecules of iron metabolism and their roles in iron transport and regulation of iron homeostasis.

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 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.

Iron absorption and hepatic iron uptake are increased in a transferrin receptor 2 (Y245X) mutant mouse model of hemochromatosis type 3

Hereditary hemochromatosis type 3 is an iron (Fe)-overload disorder caused by mutations in transferrin receptor 2 (TfR2). TfR2 is expressed highly in the liver and regulates Fe metabolism. The aim of this study was to investigate duodenal Fe absorption and hepatic Fe uptake in a TfR2 (Y245X) mutant mouse model of hereditary hemochromatosis type 3. Duodenal Fe absorption and hepatic Fe uptake were measured in vivo by 59Fe-labeled ascorbate in TfR2 mutant mice, wild-type mice, and Fe-loaded wild-type mice (2% dietary carbonyl Fe). Gene expression was measured by real-time RT-PCR. Liver nonheme Fe concentration increased progressively with age in TfR2 mutant mice compared with wild-type mice. Fe absorption (both duodenal Fe uptake and transfer) was increased in TfR2 mutant mice compared with wild-type mice. Likewise, expression of genes participating in duodenal Fe uptake (Dcytb, DMT1) and transfer (ferroportin) were increased in TfR2 mutant mice. Nearly all of the absorbed Fe was taken up rapidly by the liver. Despite hepatic Fe loading, hepcidin expression was decreased in TfR2 mutant mice compared with wild-type mice. Even when compared with Fe-loaded wild-type mice, TfR2 mutant mice had increased Fe absorption, increased duodenal Fe transport gene expression, increased liver Fe uptake, and decreased liver hepcidin expression. In conclusion, despite systemic Fe loading, Fe absorption and liver Fe uptake were increased in TfR2 mutant mice in association with decreased expression of hepcidin. These findings support a model in which TfR2 is a sensor of Fe status and regulates duodenal Fe absorption and liver Fe uptake.

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.

TfR2 localizes in lipid raft domains and is released in exosomes to activate signal transduction along the MAPK pathway

Transferrin receptor 2 (TfR2) possesses a YQRV motif similar to the YTRF motif of transferrin receptor 1 (TfR1) responsible for the internalization and secretion through the endosomal pathway. Raft biochemical dissection showed that TfR2 is a component of the low-density Triton-insoluble (LDTI) plasma membrane domain, able to co-immunoprecipitate with caveolin-1 and CD81, two structural raft proteins. In addition, subcellular fractionation experiments showed that TfR1, which spontaneously undergoes endocytosis and recycling, largely distributed to intracellular organelles, whereas TfR2 was mainly associated with the plasma membrane. Given the TfR2 localization in lipid rafts, we tested its capability to activate cell signalling. Interaction with an anti-TfR2 antibody or with human or bovine holotransferrin showed that it activated ERK1/ERK2 and p38 MAP kinases. Integrity of lipid rafts was required for MAPK activation. Co-localization of TfR2 with CD81, a raft tetraspanin exported through exosomes, prompted us to investigate exosomes released by HepG2 and K562 cells into culture medium. TfR2, CD81 and to a lesser extent caveolin-1, were found to be part of the exosomal budding vesicles. In conclusion, the present study indicates that TfR2 localizes in LDTI microdomains, where it promotes cell signalling, and is exported out of the cells through the exosome pathway, where it acts as an intercellular messenger.

A compartmental model of iron regulation in the mouse

A simple compartmental model is developed for investigating the mechanism of iron homeostasis. In contrast to previous mathematical models of iron metabolism, the liver is included as a key site of iron regulation. Compartments for free iron in blood, diferric transferrin (Tf) in blood, hepatocytes, red blood cells, and macrophages are included, and their roles in iron regulation are explored. The function of hepcidin in regulating iron absorption is modeled through an inverse relationship between hepatocyte transferrin receptor 2 (TfR2) levels and the rate of iron export processes mediated by ferroportin (Fpn). Simulations of anemia and erythropoiesis stimulation support the idea that the iron demands of the erythroid compartment can be communicated through diferric Tf. The iron-responsive element of Fpn is found to be important for stabilizing intracellular iron stores in response to changing iron demands and allowing proper iron regulation through diferric Tf. The contribution of iron dysregulation to the pathogenesis of iron overload disorders is also investigated. It is shown that the characteristics of HFE hemochromatosis can be reproduced by increasing the setpoint of iron absorption in the duodenum to a level where the system cannot downregulate iron absorption to meet the iron excretion rate.

Role of transferrin receptor 2 in hepatic accumulation of iron in patients with chronic hepatitis C

BACKGROUND AND AIM

Iron deposition in the liver is a common finding in patients with chronic hepatitis C (CH-C). The mechanism of this hepatic accumulation of iron is not completely understood. This study assessed if the protein expression of transferrin receptor 2 (TfR2) is upregulated in the liver of patients with CH-C and if TfR2 protein mediates iron accumulation during hepatitis C virus (HCV) infection.

METHOD

Liver specimens from patients with CH-C that underwent interferon (IFN) therapy (n=23) and from patients with CH-B (n=18) were evaluated. Hepatic expression of TfR2 protein was analyzed by immunohistochemistry. Total hepatic iron score (THIS) was evaluated by Prussian blue staining.

RESULTS

TfR2 protein was expressed in the cell membrane and cytosol of hepatocytes. Cytosol TfR2 protein was found to co-localize with Tf. THIS (P=0.0198) and hepatic TfR2 (P=0.0047) expression were significantly higher in CH-C than in CH-B. The change in THIS values (rho=0.580, P=0.0079) and the grade of histological activity (rho=0.444, P=0.0373) were significantly correlated with changes in TfR2 expression after IFN therapy.

CONCLUSIONS

The protein expression of TfR2 is significantly associated with iron deposition in the liver in patients with CH-C. HCV infection may affect the hepatic expression of TfR2, leading to iron accumulation in the liver.

Single-cell FRET imaging of transferrin receptor trafficking dynamics by Sfp-catalyzed, site-specific protein labeling

Fluorescence imaging of living cells depends on an efficient and specific method for labeling the target cellular protein with fluorophores. Here we show that Sfp phosphopantetheinyl transferase-catalyzed protein labeling is suitable for fluorescence imaging of membrane proteins that spend at least part of their membrane trafficking cycle at the cell surface. In this study, transferrin receptor 1 (TfR1) was fused to peptide carrier protein (PCP), and the TfR1-PCP fusion protein was specifically labeled with fluorophore Alexa 488 by Sfp. The trafficking of transferrin-TfR1-PCP complex during the process of transferrin-mediated iron uptake was imaged by fluorescence resonance energy transfer between the fluorescently labeled transferrin ligand and TfR1 receptor. We thus demonstrated that Sfp-catalyzed small molecule labeling of the PCP tag represents a practical and efficient tool for molecular imaging studies in living cells.

Understanding iron homeostasis through genetic analysis of hemochromatosis and related disorders

Genetic analysis of hemochromatosis has led to the discovery of a number of genes whose mutations disrupt iron homeostasis and lead to iron overload. The introduction of molecular tests into clinical practice has provided a tool for early diagnosis of these conditions. It has become clear that hemochromatosis includes a spectrum of disorders that range from simple biochemical abnormalities to chronic asymptomatic tissue damage in midlife to serious life-threatening diseases in young subjects. Molecular studies have identified the systemic loop that controls iron homeostasis and is centered on the hepcidin-ferroportin interaction. The complexity of this regulatory pathway accounts for the genetic heterogeneity of hemochromatosis and related disorders and raises the possibility that genes encoding components of the pathway may be modifiers of the main genotype. Molecular diagnosis has improved the classification of the genetic conditions leading to iron overload and identified novel entities, characterized by both iron loading and variable degrees of anemia. Despite the progress in the diagnosis, classification, and mechanisms of iron overload disorders, the treatment of affected patients continues to rely on regular phlebotomy. Understanding the molecular circuitry of iron control may lead to the identification of potential therapeutic targets for novel treatment strategies to be used in association with or as an alternative to phlebotomy.