Hepatic Iron Uptake Research Articles

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

High fat (HF) diet‐induced obesity is known to disturb hepatic iron metabolism in a time‐dependent manner. The mechanism of decreased hepatic iron store induced by long‐term high fat diet remains unclear. In this study, twenty‐four 6‐week‐old male Sprague‐Dawley rats were fed a high fat diet for 16 weeks and hepatic iron metabolism was examined. HF diet feeding considerably decreased hepatic iron contents, enhanced transferrin (TF) expression, reduced ferritin heavy chain (FTH), ferritin light chain (FTL), transferrin receptor 2 (TFR2) and ZRT/IRT‐like protein 14 (ZIP14) expression in the liver of rats. Down‐regulation of hepatic TFR2 coincided with DNA hypermethylation on the promoter and repressing expression of transcription factor hepatocyte nuclear factor 4α (HNF4α). MiR‐181 family expression was markedly increased and verified to regulate Zip14 expression using the dual‐luciferase reporter assay. These results indicate that the long‐term high fat diet decreases hepatic iron content, which is closely associated with inhibition of hepatic iron uptake via TFR2 and ZIP14‐dependent pathway.Support or Funding InformationThis work was supported by grants from the National Key Research and Development Program of China (2016YFD0500500), the National Natural Science Foundation of China (31302053), the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Jiangsu Collaborative Innovation Center of Meat Production and Processing, Quality and Safety Control.This abstract is from the Experimental Biology 2019 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

Regulation of Iron Uptake in Primary Culture Rat Hepatocytes

Decreased serum and increased hepatic iron uptake is the hallmark of acute-phase (AP) response. Iron uptake is controlled by iron transport proteins such as transferrin receptors (TfRs) and lipocalin 2 (LCN-2). The current study aimed to understand the regulation of iron uptake in primary culture hepatocytes in the presence/absence of AP mediators. Rat hepatocytes were stimulated with different concentrations of iron alone (0.01, 0.1, 0.5 mM) and AP cytokines (interleukin 6 [IL-6], IL-1β, tumor necrosis factor α) in the presence/absence of iron (FeCl3: 0.1 mM). Hepatocytes were harvested at different time points (0, 6, 12, 24 h). Total mRNA and proteins were extracted for reverse transcriptase-polymerase chain reaction (RT-PCR) and Western blot. A significant iron uptake was detected with 0.1 mM iron administration with a maximum (133.37 ± 4.82 µg/g of protein) at 24 h compared with control and other iron concentrations. This uptake was further enhanced in the presence of AP cytokines with a maximum iron uptake (481 ± 25.81 µg/g of protein) after concomitant administration of IL-6 + iron to cultured hepatocytes. Concomitantly, gene expression of LCN-2 and ferritin subunits (light- and heavy-chain ferritin subunits) was upregulated by iron or/and AP cytokines with a maximum at 24 h both at mRNA and protein levels. In contrast, a decreased TfR1 level was detected by IL-6 and iron alone, whereas combination of iron and AP cytokines (mainly IL-6) abrogated the downregulation of TfR1. An increase in LCN-2 release into the supernatant of cultured hepatocytes was observed after addition of iron/AP cytokines into the medium. This increase in secretion was further enhanced by combination of IL-6 + iron. In conclusion, iron uptake is tightly controlled by already present iron concentration in the culture. This uptake can be further enhanced by AP cytokines, mainly by IL-6.

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. 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.

Ezetimibe Increases Hepatic Iron Levels in Mice Fed a High-Fat Diet

Accumulating evidence suggests that ezetimibe may be a promising agent for treatment of nonalcoholic fatty liver disease and steatohepatitis (NAFLD/NASH). Phlebotomy and dietary iron restriction reduce serum transaminase in NAFLD/NASH patients. Recent studies have shown that a mutual effect exists between lipid metabolism and iron metabolism. Accordingly, we examined the effect of ezetimibe on iron metabolism in mice fed a high-fat diet with or without iron. We fed C57BL/6 mice the following diets for 12 weeks. Experiment 1 comprised [1] a control diet (C), [2] C plus ezetimibe (0.3 mg/day; 4 weeks) (CE), [3] a high-fat diet (H), and [4] H plus ezetimibe (HE). Experiment 2 comprised [1] C containing carbonyl iron (average; 22.4 mg/day; 6 weeks) (CI), [2] CI plus ezetimibe (CIE), [3] H containing carbonyl iron (HI), and [4] HI plus ezetimibe (HIE). Blood, livers, and duodenum were removed after 12 weeks. In experiment 1, the hepatic iron levels were higher in HE than H, whereas there was no difference between C and CE. Hepatic mRNA expression of transferrin receptor 1 and 2, ferritins, and hepcidin were increased more in CE than C, and more in HE than H. In the duodenum, divalent metal transporter 1, ferritin H, and hephaestin mRNA levels were increased in CE compared with C. In experiment 2, hepatic iron concentrations were higher in HIE than HI. Hepatic mRNA expression of ferritin L and hepcidin were increased in HIE compared with HI. In duodenum, ferritin L mRNA was increased in HIE compared with CIE. Ezetimibe induced hepatic iron uptake transporter expression in mice fed a high-fat diet, causing increased hepatic iron concentrations.

Insulin promotes iron uptake in human hepatic cell by regulating transferrin receptor-1 transcription mediated by hypoxia inducible factor-1

Hepatic iron is known to regulate insulin signaling pathways and to influence insulin sensitivity in insulin resistance (IR) patients. However, the role of insulin on hepatic iron homeostasis remains unexplored. Here, we report that insulin promotes transferrin-bound iron uptake but shows no influence on non transferrin-bound iron uptake in human hepatic HepG2 cells. As a mechanism we detected increased transferrin receptor-1 (TfR1) expression both at protein and mRNA levels. Unaltered stability of protein and transcript of TfR1 suggested the regulation at transcriptional level that was confirmed by promoter activity. Involvement of transcription factor hypoxia inducible factor-1 (HIF-1) was shown by mutational analyses of the TfR1 promoter region and by electrophoretic mobility shift assay. When HepG2 cells were transfected with specific siRNA targeted to 3′UTR of HIF-1α, the regulatory subunit of HIF-1; insulin-induced TfR1 expression and iron uptake were inhibited. Transfection of cDNA expressing stable form of HIF-1α reversed the increased TfR1 expression and iron uptake. These results suggest a novel role of insulin in hepatic iron uptake by a HIF-1 dependent transcriptional regulation of TfR1.

Simvastatin downregulates the expression of hepcidin and erythropoietin in HepG2 cells

Statin therapy may improve responsiveness to erythropoietin-stimulating agents in patients with end-stage renal disease. Although statins increase hepatic iron uptake and storage capacity in cholestatic rats, the underlying mechanisms are unclear. Therefore, we examined the effects of a statin (simvastatin) on the expression of hepcidin, erythropoietin receptor (EPOR) and erythropoietin (EPO) in cultured HepG2 cells. HepG2 cells (6-6.5 × 10(5) cells) were seeded in 6-cm dishes and incubated overnight. The cells were then treated with 0, 0.5, 1, 3, 5, or 10 μM simvastatin, and the mRNA expression of hepcidin, EPOR, and EPO was determined. Data were collected from three independent experiments. The cDNA extracted from the cells was used as a template for real-time polymerase chain reaction, and each sample was tested in duplicate. Significant differences (P < 0.05) among groups were determined using one-way analysis of variance with Fisher's least significant difference post hoc test. Data were adjusted using Bonferroni's method. The relative mRNA expression of hepcidin in HepG2 cells treated with 0.5, 1, 3, 5, and 10 μM simvastatin, relative to the control group, was 0.7273, 0.3303, 0.2418, 0.4131, and 0.4064, respectively. The relative mRNA expression of EPOR was 0.5196, 0.2319, 0.2398, 0.4253, and 0.1245, respectively, while that of EPO was 0.9751, 0.4712, 0.4613, 0.4875, and 0.1654. There was a reverse dose-dependent effect of simvastatin. These results suggest that statins increase erythropoiesis by targeting hepcidin and iron regulatory pathways, independent of erythropoietin.

Modification of hepatic iron metabolism induced by pravastatin during obstructive cholestasis in rats

Aims To evaluate iron biochemistry and contributing liver mechanisms during obstructive cholestasis and pravastatin treatment in rats. Main methods A rat model of cholestasis induced by bile duct ligation (BDL) was used for the study. The detection of iron and the expression of relevant molecules were performed one week after surgery in the control, and cholestatic animals after treatment with either saline or pravastatin (1 mg/kg/day). Key findings Saline-administered BDL rats showed, in comparison to sham-operated animals, a significant increase in plasma iron concentration, increased liver protein content of heme oxygenase-1 (HO-1) and a decline in the expression of hepcidin. Ferroportin 1 expression was increased with a simultaneous reduction in intrahepatic iron concentration. The administration of pravastatin to BDL animals attenuated proliferation changes in liver parenchyma, prevented HO-1 induction, restored hepatic mRNA hepcidin expression to control levels and induced the expression of ferritin, transferrin receptors (TfR1/2) and divalent metal transporter-1. This was accompanied by an increased content of intrahepatic iron when compared to the BDL animals, and a reduction of hyperbilirubinemia. Significance Cholestasis-induced increase in plasma and decrease in hepatic iron levels were associated with up-regulation of liver HO-1 and ferroportin 1. Pravastatin alleviated cholestatic liver impairment and raised liver iron content by modulation of heme catabolism and an increase of hepatic iron uptake and storage capacity.

Effect of epigallocatechin-3-gallate on iron overload in mice with alcoholic liver disease

Iron has long been related to the pathological process of alcoholic liver disease (ALD). Liver iron overload is known to accelerate the development of ALD. In the present study we aimed to examine the effect of epigallocatechin-3-gallate (EGCG) on iron overload of ALD and to explore the potential mechanisms involved in its protection against ALD in mice. Male C57BL/6J mice were given alcohol by intragastric administration for 12 weeks. At the end of 8th week, ALD mice were treated for 4 weeks for 10, 20 and 30 mg kg(-1) EGCG by intraperitoneal injection. Liver injuries were assessed by histopathologic examination and Serum Alanine Aminotransferase (ALT) levels. Serum iron content, hepatic iron concentration and liver malondialdehyde (MDA) contents were examined. In addition, hepcidin mRNA levels and transferrin (Tf) and transferrin receptor 1 (TfR1) protein levels of liver tissue were also evaluated. Compared with model group, treatment of ALD mice with EGCG ameliorated liver injuries, decreased serum iron level, hepatic iron levels and liver MDA contents, increased hepcidin mRNA level and decreased Tf and TfR1 protein expression in the liver. The results of our study explain a new point of view that the protective effect of EGCG on ALD is associated with its iron-chelating property. The possible mechanisms are that EGCG affects hepatic iron uptake and inhibits iron absorption in the small intestinal.

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

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. 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. 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. 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 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. 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.

Dysregulation of systemic iron metabolism in alcoholic liver diseases

Alcoholic liver diseases (ALD) are frequently associated with iron overload. Until recently, the effects of ethanol in hepatic iron uptake and intestinal iron absorption have not been clarified in detail. Two possible mechanisms for iron overload are the uptake of iron into hepatocytes in a specific manner through the increased expression of transferrin receptor (TfR) 1; and increased intestinal iron absorption by the lowering of hepcidin. It is worthwhile to examine whether a similar mechanism is present in the development of steatosis and non-alcoholic steatohepatitis (NASH). Hepatocytes have several iron uptake pathways. Ethanol increases transferrin (Tf)-mediated uptake via a receptor-dependent manner, but downregulates the non-Tf-bound iron uptake. According to immunohistochemical study, TfR1 was increased in hepatocytes in 80% of hepatic tissues of patients with ALD, but was not detected in normal hepatic tissues. In an experimental model, ethanol exposure to the primary cultured-hepatocytes in the presence of iron increased TfR1 expression and (59)Fe-labeled Tf uptake. In patients with ALD, intestinal iron absorption is increased by oral iron uptake assay. The regulatory hormone for iron homeostasis, hepcidin is downregulated in ethanol-loaded mice liver. As well as ALD, a similar mechanism was present in the mouse model fed with a high-fat diet, a model of the initial phenomenon of steatosis. The common mechanism for hepatic iron deposition and the triggering role of iron may be present in the development of ALD and non-alcoholic fatty liver disease/NASH.

Hepcidin Is Down‐Regulated in Alcohol Loading

It is common for alcoholic patients to have excess iron accumulation in the liver, which may contribute to the development of alcoholic liver disease (ALD). However, the mechanism of hepatic iron uptake in ALD is still obscure. Recently, a novel iron-regulatory hormone hepcidin was found that suppresses the absorption of iron from the small intestine and the release of iron from macrophages. To elucidate the contribution of hepcidin toward the hepatic excess iron accumulation in ALD, we examined whether alcohol loading affects hepcidin expression both in ALD patients and in an ethanol-fed mouse model. Serum prohepcidin concentration was quantified by enzyme-linked immunosorbent assay. Hepatic hepcidin-1 and hepcidin-2 mRNA expressions in mouse liver were evaluated by quantitative real-time reverse-transcriptase polymerase chain reaction method. The protein expression of prohepcidin in mouse liver was examined immunohistochemically by rabbit antimouse prohepcidin antibody. Serum prohepcidin concentration in ALD was significantly lower than that in healthy subjects (p<0.001). Especially, serum prohepcidin concentrations were decreased in the patients whose serum ferritin value was high. In the ethanol-fed mouse model, hepatic hepcidin-1 mRNA expression was significantly lower than that in control (p=0.04). Prohepcidin was expressed in the cytoplasm of hepatocytes of mice liver tissue sections, and its expression was decreased after ethanol loading. Alcohol loading down-regulates hepatic hepcidin expression and leads to the increase of iron absorption from the intestine.

DMT1-Independent Uptake of Non-Transferrin Bound Iron into Human Hepatoma Cells Is Selectively Modulated by T-Type Voltage-Dependent Calcium Channel Inhibitors.

The liver is the major clearance organ for non-transferrin bound iron (NTBI) but the mechanisms of uptake are unclear. Although divalent metal transporter 1 (DMT1) has been identified as a key iron transporter in enterocytes, its role in hepatic iron uptake and regulation is uncertain. The HuH7 human hepatoma cell line provides an appropriate model to study hepatic iron uptake as these cells retain differentiated functions of liver cells in culture.Using radiolabelled iron-citrate species as a model for NTBI uptake into HuH7 cells we have shown that NTBI uptake involves a calcium-dependent, non-selective divalent cation transport mechanism which is up-regulated by iron loading with ferric ammonium citrate (FAC) and down-regulated by deferoxamine (DFO). In these cells, blocking antibody to DMT1 failed to inhibit iron uptake at low pH (5.5) or high pH (7.4), in contrast to the significant blocking observed in differentiated CaCo2 cells at low pH. Internalized iron uptake was optimal at pH 7.4 in HuH7 cells and at pH 5.5 in CaCo2 cells. RT-PCR analysis showed DMT1(±IRE) mRNA levels in HuH7 cells were not up-regulated in response to iron loading, suggesting the FAC-stimulated rate is not dependent on increased DMT1 expression. These results are consistent with a DMT1-independent mechanism of transmembrane iron uptake in HuH7 cells, suggesting alternative iron transport mechanism(s) are available.A common feature of tissues susceptible to iron overload, such as heart, pancreas and anterior pituitary, is the large number and activity of voltage-dependent calcium channels (VDCC). Recently, high voltage L-type calcium channels were shown to provide an alternative mechanism of iron entry into cardiomyocytes (Oudit et al, Nat Med , 2003, 9:1187–94). Whilst L-type VDCC are predominately found in excitable tissues, low voltage dependent (T-type) calcium channel expression is more diverse, including non-excitable cells such as hepatocytes. Hence, we investigated the role of VDCC in mediating iron uptake into HuH7 cells with selective modulators of L- and T-type VDCC. T-type channel blockers (mibefradil, NNC 55-0396 and pimozide) inhibited NTBI uptake by 10–55% at micromolar concentrations, in contrast to L-type channel antagonists (verapamil, nifedipine and (+)-Bay K8644) which had little or no effect on NTBI uptake. Loading cells with FAC in the presence of 10μM mibefradil strongly attenuated the FAC-stimulated iron uptake rate, whereas verapamil had no effect. Furthermore, stimulation of NTBI uptake by KCl was attenuated by mibefradil but not by verapamil, suggesting membrane depolarization enhances iron uptake by activation of T- but not L-type channels in hepatocytes. RT-PCR analysis showed T-type channel expression was up-regulated by iron chelation with DFO and unaffected by iron loading with FAC, whilst the L-type channel was down-regulated by iron loading. Thus, stimulation of hepatocyte NTBI uptake by FAC loading appears to occur at a post-transcriptional level possibly involving activation of VDCC.These data show that a component of hepatic iron uptake is mediated selectively by T-rather than L-type VDCC, and that this pathway becomes more significant under conditions of iron overload. Consequently, L-type channel blockers, which are in widespread clinical use for the treatment of hypertension and angina, are unlikely to inhibit hepatic NTBI clearance.

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.

166 Oxidative stress mediates hepatic iron uptake in vitro and in vivo

166 Oxidative stress mediates hepatic iron uptake in vitro and in vivo

Effects of alcohol, carbon tetrachloride, and choline deficiency on iron metabolism in the rat.

The effects of alcohol on hepatic iron uptake and intestinal iron transport were studied in rats fed a nutritionally replete liquid diet containing varying quantities of ethanol. Results were compared with those from animals exposed to carbon tetrachloride (CCl4) to produce hepatocellular necrosis or a choline-deficient diet to produce steatosis and cirrhosis. A high ethanol intake for 4 or 10 weeks produced hepatic steatosis. CCl4 produced hepatocellular necrosis. Choline deficiency was associated with steatosis +/- cirrhosis. Intestinal iron transport was unaffected by ethanol, CCl4, or choline deficiency. Hepatic iron uptake was significantly depressed in rats consuming 11.7 g/kg/day ethanol (p < 0.01) for 4 weeks. Choline-deficient animals studied at 14 weeks also had significantly decreased hepatic iron uptake (p < 0.01); results were similar in the cirrhotic and noncirrhotic animals. Conversely, CCl4 exposure produced a significant 5-fold increase in hepatic iron uptake (p < 0.001). Results suggest that ethanol consumption, fatty liver, and cirrhosis are not responsible for any increase in iron absorption or of hepatic iron uptake in the rat model. Acute hepatocellular injury is followed by increased hepatic iron uptake.

NAD(P)H:Ferric Iron Reductase in Endosomal Membranes from Rat Liver

Endosomes isolated from rat liver, characterized by high enrichment of endocytosed ligand after liver perfusion, displayed ferric reductase activity with higher affinity for NADH (1.7 μM) than for NADPH (7.1 μM). The ferric-NTA complex was reduced by NADH with a molar stoichiometry of 2:1 for the iron complex to pyridine nucleotide ratio under near anaerobic conditions. Superoxide radicals were not apparently involved in the reduction of ferric iron under these conditions, despite measurable generation of superoxide under aerobic atmosphere. The reaction was inhibited by sulfhydryl reagents, was heat labile, and may account for reduction of ferric to ferrous iron during hepatic iron uptake from transferrin or from other iron sources.

Lactoferrin interferes with uptake of iron from transferrin and asialotransferrin by the rat liver

Intravenous injection of bovine or human lactoferrin (6.25 x 10(-2) mumol/100 g body wt) in rats resulted in marked reduction of hepatic iron uptake from transferrin and asialotransferrin. The effect was dose dependent, saturable at approximately 5 mg/100 g body wt, and independent of lactoferrin's iron content. At this dose level, iron uptake from transferrin was reduced by 28% and from asialotransferrin by 43% in experiments lasting 90 min. Bovine lactoperoxidase, another basic protein, was similarly effective. The clearance of asialofetuin and pinocytosis of polyvinylpyrrolidone remained unaffected. Perfusion of isolated rat livers at 4 degrees C showed a strong reduction in asialotransferrin binding in the presence of lactoferrin. Chromatography of hepatic heparan sulfate proteoglycan on immobilized lactoferrin, lactoperoxidase, asialotransferrin, and transferrin showed that it possessed affinity for each of these proteins, more for the first two than the latter two. Heparan sulfate proteoglycan binding and efficacy in reducing hepatic iron uptake were also studied after selective modifications of positively charged amino acids in these proteins. The data obtained are compatible with the hypothesis that lactoferrin and other proteins with similarly high affinity for hepatic heparan sulfate exert their negative effect on iron uptake by preventing transferrin binding to the proteoglycan. The possibility is thus raised that the large number of low-affinity transferrin binding sites reported by earlier investigators for the liver may be heparan sulfate molecules.

Reduced hepatic iron uptake from rat aglycotransferrin

Rat aglycotransferrin (rAgTf) was produced from the disialosyl diantennary fraction of rat transferrin (rTf) by treatment with peptide: N-glycosidase F. Following removal of the enzyme by gel filtration and isolation of the deglycosylated protein by lectin chromatography, rAgTf was compared to rTf both in vitro and in vivo. No significant differences were found between the two proteins with respect to affinity for iron and kinetics of Fe release from the N-lobe and C-lobe. The fluorescence emission spectrum of apo-rTf was red-shifted by approximately 3 nm relative to diferric rTf; however, no spectral difference was detected between rTf and rAgTf when the analogous forms (apo or diferric) were compared. Plasma clearance of radioactive iron administered to rats as either rTf or rAgTf was comparable. Reticulocytes took up iron from rAgTf slightly faster than from rTf. In contrast, Fe acquisition by the liver from rAgTf was significantly reduced relative to rTf. This finding contrasts sharply with earlier observations with asialotransferrin (rAsTf) and provides a basis for discounting charge loss as the mechanism of enhanced hepatic Fe uptake from rAsTf. It is suggested that the glycan complement of rTf, while unimportant for interaction of the protein with specific receptors, probably plays a role in the interaction with low-affinity hepatic binding sites.

Specificity of hepatic iron uptake from plasma transferrin in the rat

1. The role of specific interaction between transferrin and its receptors in iron uptake by the liver in vivo was investigated using 59Fe- 125I-labelled transferrins from several animal species, and adult and 15-day rats. Transferrin-free hepatic uptake of 59Fe was measured 2 or 0.5 hr after intravenous injection of the transferrins. 2. Rat, rabbit and human transferrins gave high and approximately equal levels of hepatic iron uptake while transferrins from a marsupial ( Sentonix brachyurus), lizard, crocodile, toad and fish gave very low uptake values. Chicken ovotransferrin resulted in higher uptake than with any other species of transferrin. 3. Iron uptake by the femurs (as a sample of bone marrow erythroid tissue) and, in another group of 19-day pregnant animals by the placentas and fetuses, was also measured, for comparison with the liver results. The pattern of uptake from the different transferrins was found to be similar to that of iron uptake by the liver except that with femurs, placentas and fetuses ovotransferrin gave low values comparable to those of the other non-mammalian species. 4. It is concluded that iron uptake by the liver from plasma transferrin in vivo is largely or completely dependent on specific transferrin-receptor interaction. The high hepatic uptake of iron from ovotransferrin was probably mediated by the asialoglycoprotein receptors on hepatocytes.