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

Unraveling the Molecular Regulation of Ferroptosis in Respiratory Diseases

Ferroptosis, a type of programmed cell death that relies on iron, is distinct in terms of its morphological, biochemical and genetic features. Unlike other forms of cell death, such as autophagy, apoptosis, necrosis, and pyroptosis, ferroptosis is primarily caused by lipid peroxidation. Cells that die due to iron can potentially trigger an immune response which intensifies inflammation and causes severe inflammatory reactions that eventually lead to multiple organ failure. In recent years, ferroptosis has been identified in an increasing number of medical fields, including neurological pathologies, chronic liver diseases and sepsis. Ferroptosis has the potential to cause an inflammatory tempest, with many of the catalysts and pathological indications of respiratory ailments being linked to inflammatory reactions. The growing investigation into ferroptosis in respiratory disorders has also garnered significant interest to better understand the mechanism of ferroptosis in these diseases. In this review, the recent progress in understanding the molecular control of ferroptosis and its mechanism in different respiratory disorders is examined. In addition, this review discusses current challenges and prospects for understanding the link between respiratory diseases and ferroptosis.

Evaluation of the level of selected iron-related proteins/receptors in the liver of rats during separate/combined vanadium and magnesium administration

BACKGROUND

The current knowledge about the effects of vanadium (V) on iron (Fe)-related proteins and Fe homeostasis (which is regulated at the systemic, organelle, and cellular levels) is still insufficient.

OBJECTIVE

This fact and our earlier results prompted us to conduct studies with the aim to explain the mechanism of anemia accompanied by a rise in hepatic and splenic Fe deposition in rats receiving sodium metavanadate (SMV) separately and in combination with magnesium sulfate (MS).

RESULTS

We demonstrated for the first time that SMV (0.125 mg V/mL) administered to rats individually and in conjunction with MS (0.06 mg Mg/mL) for 12 weeks did not cause significant differences in the hepatic hepcidin (Hepc) and hemojuvelin (HJV) concentrations, compared to the control. In comparison with the control, there were no significant changes in the concentration of transferrin receptor 1 (TfR1) in the liver of rats treated with SMV and MS alone (in both cases only a downward trend of 14% and 15% was observed). However, a significant reduction in the hepatic TfR1 level was found in rats receiving SMV and MS simultaneously. In turn, the concentration of transferrin receptor 2 (TfR2) showed an increasing trend in the liver of rats treated with SMV and/or MS.

CONCLUSIONS

The experimental data suggest that the pathomechanism of the SMV-induced anemia is not associated with the effect of V on the concentration of Hepc in the liver, as confirmed by the unaltered hepatic HJV and TfR1 levels. Therefore, further studies are needed in order to check whether anemia that developed in the rats at the SMV administration (a) results from the inhibitory effect of V on erythropoietin (EPO) production, (b) is related to the effect of V on the induction of matriptase-2 (TMPRSS6) expression, or (c) is associated with the influence of this metal on haem synthesis.

Age-Related Changes and Sex-Related Differences in Brain Iron Metabolism

Iron is an essential element that participates in numerous cellular processes. Any disruption of iron homeostasis leads to either iron deficiency or iron overload, which can be detrimental for humans' health, especially in elderly. Each of these changes contributes to the faster development of many neurological disorders or stimulates progression of already present diseases. Age-related cellular and molecular alterations in iron metabolism can also lead to iron dyshomeostasis and deposition. Iron deposits can contribute to the development of inflammation, abnormal protein aggregation, and degeneration in the central nervous system (CNS), leading to the progressive decline in cognitive processes, contributing to pathophysiology of stroke and dysfunctions of body metabolism. Besides, since iron plays an important role in both neuroprotection and neurodegeneration, dietary iron homeostasis should be considered with caution. Recently, there has been increased interest in sex-related differences in iron metabolism and iron homeostasis. These differences have not yet been fully elucidated. In this review we will discuss the latest discoveries in iron metabolism, age-related changes, along with the sex differences in iron content in serum and brain, within the healthy aging population and in neurological disorders such as multiple sclerosis, Parkinson's disease, Alzheimer's disease, and stroke.

Repeated Restraint Stress Enhances Hepatic TFR2 Expression and Induces Hepatic Iron Accumulation in Rats

Abnormal hepatic iron metabolism is detrimental to health. The objective of this study was to detect repeated restraint stress on liver iron metabolism in rats. Twenty-four male rats aged 7 weeks were randomly divided into 2 groups: control group (Con) and repeated restraint stress group (RS). Rats were subjected to 6 h of daily restraint stress for 14 consecutive days in the repeated restraint stress group. The results showed that repeated restraint stress exposure decreased growth performance including impaired final weight (P = 0.07), reducing average daily gain (P = 0.01), and average daily feed intake (P = 0.00) during the 14-day experimental period. Repeated restraint stress exposure did not affect hemoglobin content and plasma iron parameters except downregulated unsaturated iron-binding capacity (P = 0.04). Repeated restraint stress exposure inhibited liver development (P = 0.03) and induced liver iron accumulation (P = 0.05). In addition, repeated restraint stress downregulated the expression of transferrin (TF) and transferrin receptor 2 (TFR2) at the mRNA level (P < 0.01), but upregulated at the protein level (P = 0.03 for TF; P = 0.00 for TFR2). These results indicated that repeated restraint stress induces hepatic iron accumulation, which is closely related to higher expression of hepatic TFR2 protein in rats.

Role of Divalent Cations in HIV-1 Replication and Pathogenicity

Divalent cations are essential for life and are fundamentally important coordinators of cellular metabolism, cell growth, host-pathogen interactions, and cell death. Specifically, for human immunodeficiency virus type-1 (HIV-1), divalent cations are required for interactions between viral and host factors that govern HIV-1 replication and pathogenicity. Homeostatic regulation of divalent cations' levels and actions appear to change as HIV-1 infection progresses and as changes occur between HIV-1 and the host. In people living with HIV-1, dietary supplementation with divalent cations may increase HIV-1 replication, whereas cation chelation may suppress HIV-1 replication and decrease disease progression. Here, we review literature on the roles of zinc (Zn2+), iron (Fe2+), manganese (Mn2+), magnesium (Mg2+), selenium (Se2+), and copper (Cu2+) in HIV-1 replication and pathogenicity, as well as evidence that divalent cation levels and actions may be targeted therapeutically in people living with HIV-1.

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.

MR molecular imaging of HCC employing a regulated ferritin gene carried by a modified polycation vector

Purpose: Early diagnosis is essential for reducing liver cancer mortality, and molecular diagnosis by magnetic resonance imaging (MRI) is an emerging and promising technology. The chief aim of the present work is to use the ferritin gene, modified by the alpha-fetoprotein (AFP) promoter, carried by a highly safe vector, to produce signal contrast on T2-weighted MR imaging as an endogenous contrast agent, and to provide a highly specific target for subsequent therapy.

Methods: Polyethyleneimine-β-cyclodextrin (PEI-β-CD, PC) was synthesized as a novel vector. The optimal nitrogen/phosphorus ratio (N/P) of the PC/plasmid DNA complex was determined by gel retardation, biophysical properties and transmission electron microscopy morphological analysis. The transfection efficiency was observed under a fluorescence microscope and analyzed by flow cytometry. Cellular iron accumulation caused by ferritin overexpression was verified by Prussian blue staining, and the resulting contrast imaging effect was examined by MRI.

Results: The modified cationic polymer PC was much safer than high molecular weight PEI, and could condense plasmid DNA at an N/P ratio of 50 with suitable biophysical properties and a high transfection efficiency. Overexpression of ferritin enriched intracellular iron. The short-term iron imbalance initiated by AFP promoter regulation only occurred in hepatoma cells, resulting in signal contrast on MRI. The specific target TfR was also upregulated during this process.

Conclusion: These results illustrate that the regulated ferritin gene carried by PC can be used as an endogenous contrast agent for MRI detection of hepatocellular carcinoma (HCC). This molecular imaging technique may promote safer early diagnosis of HCC, and provide a more highly specific target for future chemotherapy drugs.

The involvement of iron in chemerin induced cell cycle arrest in human hepatic carcinoma SMMC7721 cells

Chemerin exhibits a tumor-inhibitory role in hepatocellular carcinoma. However, the effect of chemerin on essential metal elements in hepatic cells remains unclear. In our study, the contents of six important metal ions, including potassium, calcium, sodium, magnesium, iron and zinc, were detected in human hepatoma SMMC7721 and immortal hepatic QSG7701 cells by ICP-AES. The data showed that chemerin only decreases the content of intracellular iron in SMMC7721 cells. The reduction was due to the blockage of iron entry through the decrease in the mRNA levels of divalent metal transporter 1, iron regulatory proteins and transferrin receptors. Furthermore, the reduction of the cellular iron content induced alterations of p53-p27-p21 signaling to arrest the cell cycle at S phase in SMMC7721 cells treated by chemerin. Conversely, iron addition led to recovery from the inhibitory effect of chemerin on SMMC7721 cells. The results suggest that chemerin plays an important role in inhibiting the cell proliferation of hepatocellular carcinomas by interfering with cellular iron homeostasis in this type of tumors.

Long-Term High-Fat Diet Decreases Hepatic Iron Storage Associated with Suppressing TFR2 and ZIP14 Expression in Rats

High-fat diet-induced obesity is known to disturb hepatic iron metabolism in a time-dependent manner. The mechanism of decreased hepatic iron deposits induced by long-term high-fat diet needs to be further investigated. In this study, 24 6-week-old male Sprague-Dawley rats were given a 16-week high-fat diet and hepatic iron metabolism was examined. High-fat diet feeding considerably decreased hepatic iron contents, enhanced transferrin expression, and reduced the expression of ferritin heavy chain, ferritin light chain, and hepatic iron uptake-related proteins (transferrin receptor 2, TFR2, and ZRT/IRT-like protein 14, ZIP14) in rats. Impaired expression of hepatic TFR2 coincided with DNA hypermethylation on the promoter and repressed expression of transcription factor hepatocyte nuclear factor 4α (HNF4α). miR-181 family expression was markedly increased and verified to regulate Zip14 expression by the dual-luciferase reporter system. Taken together, long-term high-fat diet decreases hepatic iron storage, which is closely linked to inhibition of liver iron transport through the TFR2 and ZIP14-dependent pathway.

Reabsorption of iron into acutely damaged rat liver: A role for ferritins

AIM

To studied iron metabolism in liver, spleen, and serum after acute liver-damage, in relation to surrogate markers for liver-damage and repair.

METHODS

Rats received intraperitoneal injection of the hepatotoxin thioacetamide (TAA), and were sacrificed regularly between 1 and 96 h thereafter. Serum levels of transaminases and iron were measured using conventional laboratory assays. Liver tissue was used for conventional histology, immunohistology, and iron staining. The expression of acute-phase cytokines, ferritin light chain (FTL), and ferritin heavy chain (FTH) was investigated in the liver by qRT-PCR. Western blotting was used to investigate FTL and FTH in liver tissue and serum. Liver and spleen tissue was also used to determine iron concentrations.

RESULTS

After a short initial decrease, iron serum concentrations increased in parallel with serum transaminase (aspartate aminotransferase and alanine aminotransferase) levels, which reached a maximum at 48 h, and decreased thereafter. Similarly, after 48 h a significant increase in FTL, and after 72h in FTH was detected in serum. While earliest morphological signs of inflammation in liver were visible after 6 h, increased expression of the two acute-phase cytokines IFN-γ (1h) and IL-1β (3h) was detectable earlier, with maximum values after 12-24 h. Iron concentrations in liver tissue increased steadily between 1 h and 48 h, and remained high at 96 h. In contrast, spleen iron concentrations remained unchanged until 48 h, and increased mildly thereafter (96 h). Although tissue iron staining was negative, hepatic FTL and FTH protein levels were strongly elevated. Our results reveal effects on hepatic iron concentrations after direct liver injury by TAA. The increase of liver iron concentrations may be due to the uptake of a significant proportion of the metal by healthy hepatocytes, and only to a minor extent by macrophages, as spleen iron concentrations do not increase in parallel. The temporary increase of iron, FTH and transaminases in serum is obviously due to their release by damaged hepatocytes.

CONCLUSION

Increased liver iron levels may be the consequence of hepatocyte damage. Iron released into serum by damaged hepatocytes is obviously transported back and stored via ferritins.

The liver in regulation of iron homeostasis

The liver is one of the largest and most functionally diverse organs in the human body. In addition to roles in detoxification of xenobiotics, digestion, synthesis of important plasma proteins, gluconeogenesis, lipid metabolism, and storage, the liver also plays a significant role in iron homeostasis. Apart from being the storage site for excess body iron, it also plays a vital role in regulating the amount of iron released into the blood by enterocytes and macrophages. Since iron is essential for many important physiological and molecular processes, it increases the importance of liver in the proper functioning of the body's metabolism. This hepatic iron-regulatory function can be attributed to the expression of many liver-specific or liver-enriched proteins, all of which play an important role in the regulation of iron homeostasis. This review focuses on these proteins and their known roles in the regulation of body iron metabolism.

Long-Term Dexamethasone Exposure Down-Regulates Hepatic TFR1 and Reduces Liver Iron Concentration in Rats

Exposure to stress is known to cause hepatic iron dysregulation, but the relationship between prolonged stress and liver iron metabolism is not yet fully understood. Thirty 13-week-old female Sprague-Dawley rats were randomly divided into two groups, as follows: the control group (saline-injection) and the dexamethasone group (Dexamethasone (Dex)-injection 0.1 mg/kg/day). After the 21-day stress trial, the results showed that chronic Dex administration not only impaired serum corticosterone (p = 0.00) and interleukin-6 (IL-6) (p = 0.01) levels, but also decreased white blood cell counts (p = 0.00), and reduced blood lymphocyte counts (p = 0.00). The daily Dex-injection also significantly reduced body weight (p < 0.01) by inhibiting food intake. Consecutive Dex administration resulted in decreased iron intake (p = 0.00), enhanced serum iron levels (p = 0.01), and increased the serum souble transferrin receptor (sTfR) content (p = 0.00) in rats. Meanwhile, long-term Dex exposure down-regulated duodenal cytochrome b (DCYTB) (p = 0.00) and the divalent metal transporter 1 (DMT1) (p = 0.04) protein expression, but up-regulated ferroportin (FPN) protein expression (p = 0.04). Chronic Dex administration reduced liver iron concentration (p = 0.02) in rats. Hepatic transferrin receptor 1 (TFR1) expression was lowered at the protein level (p = 0.03), yet with uncoupled mRNA abundance in Dex-treated rats. Enhanced iron-regulatory protein (IRP)/iron-responsive element (IRE) binding activity was observed, but did not line up with lowered hepatic TFR1 protein expression. This study indicates that long-term Dex exposure reduces liver iron content, which is closely associated with down-regulated hepatic TFR1 protein expression.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

Correlates of hepcidin and NTBI according to HFE status in patients referred to a liver centre

BACKGROUND/AIMS

Innately low hepcidin levels lead to iron overload in HFE-associated hereditary haemochromatosis.

METHODS

This study compared hepcidin and non-transferrin bound iron (NTBI) levels in untreated iron-loaded and non-iron-loaded C282Y homozygotes to levels in C282Y/H63D compound heterozygotes and individuals with other HFE genotypes associated with less risk of iron overload.

RESULTS

As the genotypic risk for iron overload increased, transferrin saturation and serum NTBI levels increased while serum hepcidin levels decreased. Overweight and obese male C282Y homozygotes had significantly higher hepcidin levels than male C282Y homozygotes with a normal BMI. Pearson product-moment analysis showed that serum hepcidin levels significantly correlated with HFE status, serum ferritin, age, NTBI, transferrin saturation, gender and BMI. Subsequent multiple regression analysis showed that HFE status and serum ferritin were significant independent correlates of serum hepcidin levels.

CONCLUSIONS

In summary, this study has shown that while serum ferritin and HFE status are the most important determinants of hepcidin levels, factors such age, gender, BMI, transferrin saturation and NTBI all interact closely in the matrix of homeostatic iron balance.

Variability of the transferrin receptor 2 gene in AMD

Oxidative stress is a major factor in the pathogenesis of age-related macular degeneration (AMD). Iron may catalyze the Fenton reaction resulting in overproduction of reactive oxygen species. Transferrin receptor 2 plays a critical role in iron homeostasis and variability in its gene may influence oxidative stress and AMD occurrence. To verify this hypothesis we assessed the association between polymorphisms of the TFR2 gene and AMD. A total of 493 AMD patients and 171 matched controls were genotyped for the two polymorphisms of the TFR2 gene: c.1892C>T (rs2075674) and c.-258+123T>C (rs4434553). We also assessed the modulation of some AMD risk factors by these polymorphisms. The CC and TT genotypes of the c.1892C>T were associated with AMD occurrence but the latter only in obese patients. The other polymorphism was not associated with AMD occurrence, but the CC genotype was correlated with an increasing AMD frequency in subjects with BMI < 26. The TT genotype and the T allele of this polymorphism decreased AMD occurrence in subjects above 72 years, whereas the TC genotype and the C allele increased occurrence of AMD in this group. The c.1892C>T and c.-258+123T>C polymorphisms of the TRF2 gene may be associated with AMD occurrence, either directly or by modulation of risk factors.

Hepatocyte divalent metal-ion transporter-1 is dispensable for hepatic iron accumulation and non-transferrin-bound iron uptake in mice

Divalent metal-ion transporter-1 (DMT1) is required for iron uptake by the intestine and developing erythroid cells. DMT1 is also present in the liver, where it has been implicated in the uptake of transferrin-bound iron (TBI) and non-transferrin-bound iron (NTBI), which appears in the plasma during iron overload. To test the hypothesis that DMT1 is required for hepatic iron uptake, we examined mice with the Dmt1 gene selectively inactivated in hepatocytes (Dmt1(liv/liv) ). We found that Dmt1(liv/liv) mice and controls (Dmt1(flox/flox) ) did not differ in terms of hepatic iron concentrations or other parameters of iron status. To determine whether hepatocyte DMT1 is required for hepatic iron accumulation, we crossed Dmt1(liv/liv) mice with Hfe(-) (/) (-) and hypotransferrinemic (Trf(hpx/hpx) ) mice that develop hepatic iron overload. Double-mutant Hfe(-) (/) (-) Dmt1(liv/liv) and Trf(hpx/hpx) ;Dmt1(liv/liv) mice were found to accumulate similar amounts of hepatic iron as did their respective controls. To directly assess the role of DMT1 in NTBI and TBI uptake, we injected (59) Fe-labeled ferric citrate (for NTBI) or (59) Fe-transferrin into plasma of Dmt1(liv/liv) and Dmt1(flox/flox) mice and measured uptake of (59) Fe by the liver. Dmt1(liv/liv) mice displayed no impairment of hepatic NTBI uptake, but TBI uptake was 40% lower. Hepatic levels of transferrin receptors 1 and 2 and ZRT/IRT-like protein 14, which may also participate in iron uptake, were unaffected in Dmt1(liv/liv) mice. Additionally, liver iron levels were unaffected in Dmt1(liv/liv) mice fed an iron-deficient diet.

CONCLUSION

Hepatocyte DMT1 is dispensable for hepatic iron accumulation and NTBI uptake. Although hepatocyte DMT1 is partially required for hepatic TBI uptake, hepatic iron levels were unaffected in Dmt1(liv/liv) mice, suggesting that this pathway is a minor contributor to the iron economy of the liver.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

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.

Transferrin iron uptake is stimulated by ascorbate via an intracellular reductive mechanism

Although ascorbate has long been known to stimulate dietary iron (Fe) absorption and non-transferrin Fe uptake, the role of ascorbate in transferrin Fe uptake is unknown. Transferrin is a serum Fe transport protein supplying almost all cellular Fe under physiological conditions. We sought to examine ascorbate's role in this process, particularly as cultured cells are typically ascorbate-deficient. At typical plasma concentrations, ascorbate significantly increased (59)Fe uptake from transferrin by 1.5-2-fold in a range of cells. Moreover, ascorbate enhanced ferritin expression and increased (59)Fe accumulation in ferritin. The lack of effect of cycloheximide or the cytosolic aconitase inhibitor, oxalomalate, on ascorbate-mediated (59)Fe uptake from transferrin indicate increased ferritin synthesis or cytosolic aconitase activity was not responsible for ascorbate's activity. Experiments with membrane-permeant and -impermeant ascorbate-oxidizing reagents indicate that while extracellular ascorbate is required for stimulation of (59)Fe uptake from (59)Fe-citrate, only intracellular ascorbate is needed for transferrin (59)Fe uptake. Additionally, experiments with l-ascorbate analogs indicate ascorbate's reducing ene-diol moiety is necessary for its stimulatory activity. Importantly, neither N-acetylcysteine nor buthionine sulfoximine, which increase or decrease intracellular glutathione, respectively, affected transferrin-dependent (59)Fe uptake. Thus, ascorbate's stimulatory effect is not due to a general increase in cellular reducing capacity. Ascorbate also did not affect expression of transferrin receptor 1 or (125)I-transferrin cellular flux. However, transferrin receptors, endocytosis, vacuolar-type ATPase activity and endosomal acidification were required for ascorbate's stimulatory activity. Therefore, ascorbate is a novel modulator of the classical transferrin Fe uptake pathway, acting via an intracellular reductive mechanism.

Iron transport machinery of human cells: players and their interactions

Organisms, like cells, maintain tight control of iron. In humans as well as other mammals, control is achieved through the regulation of iron uptake into the body rather than through the excretion of iron. The mechanisms by which humans and mice regulate both iron uptake and the distribution of iron within the body and cells are reviewed. Special emphasis is given to the iron transporters involved in this process.

The intracellular trafficking pathway of transferrin

BACKGROUND

Transferrin (Tf) is an iron-binding protein that facilitates iron-uptake in cells. Iron-loaded Tf first binds to the Tf receptor (TfR) and enters the cell through clathrin-mediated endocytosis. Inside the cell, Tf is trafficked to early endosomes, delivers iron, and then is subsequently directed to recycling endosomes to be taken back to the cell surface.

SCOPE OF REVIEW

We aim to review the various methods and techniques that researchers have employed for elucidating the Tf trafficking pathway and the cell-machinery components involved. These experimental methods can be categorized as microscopy, radioactivity, and surface plasmon resonance (SPR).

MAJOR CONCLUSIONS

Qualitative experiments, such as total internal reflectance fluorescence (TIRF), electron, laser-scanning confocal, and spinning-disk confocal microscopy, have been utilized to determine the roles of key components in the Tf trafficking pathway. These techniques allow temporal resolution and are useful for imaging Tf endocytosis and recycling, which occur on the order of seconds to minutes. Additionally, radiolabeling and SPR methods, when combined with mathematical modeling, have enabled researchers to estimate quantitative kinetic parameters and equilibrium constants associated with Tf binding and trafficking.

GENERAL SIGNIFICANCE

Both qualitative and quantitative data can be used to analyze the Tf trafficking pathway. The valuable information that is obtained about the Tf trafficking pathway can then be combined with mathematical models to identify design criteria to improve the ability of Tf to deliver anticancer drugs. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The role of lysosomes in iron metabolism and recycling

Iron is the most abundant transition metal in the earth's crust. It cycles easily between ferric (oxidized; Fe(III)) and ferrous (reduced; Fe(II)) and readily forms complexes with oxygen, making this metal a central player in respiration and related redox processes. However, 'loose' iron, not within heme or iron-sulfur cluster proteins, can be destructively redox-active, causing damage to almost all cellular components, killing both cells and organisms. This may explain why iron is so carefully handled by aerobic organisms. Iron uptake from the environment is carefully limited and carried out by specialized iron transport mechanisms. One reason that iron uptake is tightly controlled is that most organisms and cells cannot efficiently excrete excess iron. When even small amounts of intracellular free iron occur, most of it is safely stored in a non-redox-active form in ferritins. Within nucleated cells, iron is constantly being recycled from aged iron-rich organelles such as mitochondria and used for construction of new organelles. Much of this recycling occurs within the lysosome, an acidic digestive organelle. Because of this, most lysosomes contain relatively large amounts of redox-active iron and are therefore unusually susceptible to oxidant-mediated destabilization or rupture. In many cell types, iron transit through the lysosomal compartment can be remarkably brisk. However, conditions adversely affecting lysosomal iron handling (or oxidant stress) can contribute to a variety of acute and chronic diseases. These considerations make normal and abnormal lysosomal handling of iron central to the understanding and, perhaps, therapy of a wide range of diseases.

Non-transferrin bound iron: a key role in iron overload and iron toxicity

BACKGROUND

Besides transferrin iron, which represents the normal form of circulating iron, non-transferrin bound iron (NTBI) has been identified in the plasma of patients with various pathological conditions in which transferrin saturation is significantly elevated.

SCOPE OF THE REVIEW

To show that: i) NTBI is present not only during chronic iron overload disorders (hemochromatosis, transfusional iron overload) but also in miscellaneous diseases which are not primarily iron overloaded conditions; ii) this iron species represents a potentially toxic iron form due to its high propensity to induce reactive oxygen species and is responsible for cellular damage not only at the plasma membrane level but also towards different intracellular organelles; iii) the NTBI concept may be expanded to include intracytosolic iron forms which are not linked to ferritin, the major storage protein which exerts, at the cellular level, the same type of protective effect towards the intracellular environment as transferrin in the plasma.

MAJOR CONCLUSIONS

Plasma NTBI and especially labile plasma iron determinations represent a new important biological tool since elimination of this toxic iron species is a major therapeutic goal.

GENERAL SIGNIFICANCE

The NTBI approach represents an important mechanistic concept for explaining cellular iron excess and toxicity and provides new important biochemical diagnostic tools. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

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.

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.

A general map of iron metabolism and tissue-specific subnetworks

Iron is required for survival of mammalian cells. Recently, understanding of iron metabolism and trafficking has increased dramatically, revealing a complex, interacting network largely unknown just a few years ago. This provides an excellent model for systems biology development and analysis. The first step in such an analysis is the construction of a structural network of iron metabolism, which we present here. This network was created using CellDesigner version 3.5.2 and includes reactions occurring in mammalian cells of numerous tissue types. The iron metabolic network contains 151 chemical species and 107 reactions and transport steps. Starting from this general model, we construct iron networks for specific tissues and cells that are fundamental to maintaining body iron homeostasis. We include subnetworks for cells of the intestine and liver, tissues important in iron uptake and storage, respectively, as well as the reticulocyte and macrophage, key cells in iron utilization and recycling. The addition of kinetic information to our structural network will permit the simulation of iron metabolism in different tissues as well as in health and disease.

Hereditary hemochromatosis in the post-HFE era

Following the discovery of the HFE gene in 1996 and its linkage to the iron overload disorder hereditary hemochromatosis (HH) there have been profound developments in our understanding of the pathogenesis of the biochemical and clinical manifestations of a number of iron overload disorders. This article provides an update of recent developments and key issues relating to iron homeostasis and inherited disorders of iron overload, with emphasis on HFE-related HH, and is based on the content of the American Association for the Study of Liver Diseases Single-Topic Conference entitled "Hemochromatosis: What has Happened After HFE?" which was held at the Emory Convention Center in Atlanta, September 7-9, 2007.

Melatonin controls oxidative stress and modulates iron, ferritin, and transferrin levels in adriamycin treated rats

AIM

Chemotherapy with adriamycin (ADR) is limited by its iron-mediated pro-oxidant toxicity. Because melatonin (MLT) is a broad spectrum antioxidant, we investigated the ability of MLT to control iron, its binding proteins, and the oxidative damage induced by ADR.

MAIN METHODS

ADR was given as single i.p. dose of 10 mg kg(-1) body weight into male rats. MLT at a dose of 15 mg kg(-1) was injected daily for 5 days before ADR treatment followed by another injection for 5 days. Biochemical methods were used for this investigation.

KEY FINDINGS

ADR injection caused elevations in plasma creatine kinase isoenzyme, lactic dehydrogenase, and aminotransferases, iron, ferritin, and transferrin. These changes were associated with increases in lipid peroxidation and protein oxidation as well as decreases in glutathione (GSH) levels and glutathione-S-transferase (GST) activity, while glutathione peroxidase (GSH-Px), and catalase (CAT) activity were elevated in the heart and liver of ADR treated rats. In the MLT+ADR group, the cardiac and hepatic function parameters and the levels of iron, transferrin and ferritin in plasma were normalized to control levels. The rats that were subjected to MLT+ADR had normalized CAT and GSH-Px activity and decreased TBARS and protein carbonyl levels compared the group only treated with ADR. GST activity and GSH concentration in the heart and liver were normalized when MLT accompanied ADR treatment.

SIGNIFICANCE

MLT ameliorated oxidative stress by controlling iron, and binding protein levels in ADR treated rats demonstrating the usefulness of adriamycin in cancer chemotherapy and allowing a better management of iron levels.

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.