Regulation of the hepatic transferrin receptor in hereditary hemochromatosis

Serum or plasma ferritin concentration as an index of iron deficiency and overload

BACKGROUND

Reference standard indices of iron deficiency and iron overload are generally invasive, expensive, and can be unpleasant or occasionally risky. Ferritin is an iron storage protein and its concentration in the plasma or serum reflects iron stores; low ferritin indicates iron deficiency, while elevated ferritin reflects risk of iron overload. However, ferritin is also an acute-phase protein and its levels are elevated in inflammation and infection. The use of ferritin as a diagnostic test of iron deficiency and overload is a common clinical practice.

OBJECTIVES

To determine the diagnostic accuracy of ferritin concentrations (serum or plasma) for detecting iron deficiency and risk of iron overload in primary and secondary iron-loading syndromes.

SEARCH METHODS

We searched the following databases (10 June 2020): DARE (Cochrane Library) Issue 2 of 4 2015, HTA (Cochrane Library) Issue 4 of 4 2016, CENTRAL (Cochrane Library) Issue 6 of 12 2020, MEDLINE (OVID) 1946 to 9 June 2020, Embase (OVID) 1947 to week 23 2020, CINAHL (Ebsco) 1982 to June 2020, Web of Science (ISI) SCI, SSCI, CPCI-exp & CPCI-SSH to June 2020, POPLINE 16/8/18, Open Grey (10/6/20), TRoPHI (10/6/20), Bibliomap (10/6/20), IBECS (10/6/20), SCIELO (10/6/20), Global Index Medicus (10/6/20) AIM, IMSEAR, WPRIM, IMEMR, LILACS (10/6/20), PAHO (10/6/20), WHOLIS 10/6/20, IndMED (16/8/18) and Native Health Research Database (10/6/20). We also searched two trials registers and contacted relevant organisations for unpublished studies.

SELECTION CRITERIA

We included all study designs seeking to evaluate serum or plasma ferritin concentrations measured by any current or previously available quantitative assay as an index of iron status in individuals of any age, sex, clinical and physiological status from any country.

DATA COLLECTION AND ANALYSIS

We followed standard Cochrane methods. We designed the data extraction form to record results for ferritin concentration as the index test, and bone marrow iron content for iron deficiency and liver iron content for iron overload as the reference standards. Two other authors further extracted and validated the number of true positive, true negative, false positive, false negative cases, and extracted or derived the sensitivity, specificity, positive and negative predictive values for each threshold presented for iron deficiency and iron overload in included studies. We assessed risk of bias and applicability using the Quality Assessment of Diagnostic Accuracy Studies (QUADAS)-2 tool. We used GRADE assessment to enable the quality of evidence and hence strength of evidence for our conclusions.

MAIN RESULTS

Our search was conducted initially in 2014 and updated in 2017, 2018 and 2020 (10 June). We identified 21,217 records and screened 14,244 records after duplicates were removed. We assessed 316 records in full text. We excluded 190 studies (193 records) with reasons and included 108 studies (111 records) in the qualitative and quantitative analysis. There were 11 studies (12 records) that we screened from the last search update and appeared eligible for a future analysis. We decided to enter these as awaiting classification. We stratified the analysis first by participant clinical status: apparently healthy and non-healthy populations. We then stratified by age and pregnancy status as: infants and children, adolescents, pregnant women, and adults. Iron deficiency We included 72 studies (75 records) involving 6059 participants. Apparently healthy populations Five studies screened for iron deficiency in people without apparent illness. In the general adult population, three studies reported sensitivities of 63% to 100% at the optimum cutoff for ferritin, with corresponding specificities of 92% to 98%, but the ferritin cutoffs varied between studies. One study in healthy children reported a sensitivity of 74% and a specificity of 77%. One study in pregnant women reported a sensitivity of 88% and a specificity of 100%. Overall confidence in these estimates was very low because of potential bias, indirectness, and sparse and heterogenous evidence. No studies screened for iron overload in apparently healthy people. People presenting for medical care There were 63 studies among adults presenting for medical care (5042 participants). For a sample of 1000 subjects with a 35% prevalence of iron deficiency (of the included studies in this category) and supposing a 85% specificity, there would be 315 iron-deficient subjects correctly classified as having iron deficiency and 35 iron-deficient subjects incorrectly classified as not having iron deficiency, leading to a 90% sensitivity. Thresholds proposed by the authors of the included studies ranged between 12 to 200 µg/L. The estimated diagnostic odds ratio was 50. Among non-healthy adults using a fixed threshold of 30 μg/L (nine studies, 512 participants, low-certainty evidence), the pooled estimate for sensitivity was 79% with a 95% confidence interval of (58%, 91%) and specificity of 98%, with a 95% confidence interval of (91%, 100%). The estimated diagnostic odds ratio was 140, a relatively highly informative test. Iron overload We included 36 studies (36 records) involving 1927 participants. All studies concerned non-healthy populations. There were no studies targeting either infants, children, or pregnant women. Among all populations (one threshold for males and females; 36 studies, 1927 participants, very low-certainty evidence): for a sample of 1000 subjects with a 42% prevalence of iron overload (of the included studies in this category) and supposing a 65% specificity, there would be 332 iron-overloaded subjects correctly classified as having iron overload and 85 iron-overloaded subjects incorrectly classified as not having iron overload, leading to a 80% sensitivity. The estimated diagnostic odds ratio was 8.

AUTHORS' CONCLUSIONS

At a threshold of 30 micrograms/L, there is low-certainty evidence that blood ferritin concentration is reasonably sensitive and a very specific test for iron deficiency in people presenting for medical care. There is very low certainty that high concentrations of ferritin provide a sensitive test for iron overload in people where this condition is suspected. There is insufficient evidence to know whether ferritin concentration performs similarly when screening asymptomatic people for iron deficiency or overload.

Dietary and prophylactic iron supplements : Helpful or harmful?

Mild hypoferremia represents an aspect of the ability of the body to withhold iron from pathogenic bacteria, fungi, and protozoa, and from neoplastic cells. However, our iron-withholding defense system can be thwarted by practices that enhance iron overload such as indiscriminate iron fortification of foods, medically prescribed iron supplements, alcohol ingestion, and cigarette smoking. Elevated standards for normal levels of iron can be misleading and even dangerous for individuals faced with medical insults such as chronic infection, neoplasia, cardiomyopathy, and arthritis. We are becoming increasingly aware that the wide-spread hypoferremia in human populations is a physiological response to insult rather than a pathological cause of insult, and that attempts to correct the condition by simply raising iron levels may not only be misguided but may actually impair host defense.

Accelerated transferrin degradation in HFE-deficient mice is associated with increased transferrin saturation

HFE, a major histocompatibility complex class I-related protein, is implicated in the iron overload disease, hereditary hemochromatosis. Whereas patients with hereditary hemochromatosis have low serum transferrin levels, little is known about transferrin turnover in HFE deficiency states. We injected mice intravenously with radioiodinated transferrin and compared plasma transferrin decay and steady-state endogenous transferrin concentration in the plasma between HFE-deficient and wild-type C57BL/6 mouse strains. HFE-deficient mice degraded transferrin faster than normal (P < 0.001) and had lower plasma transferrin concentrations (P < 0.001). Both HFE-deficient and wild-type mice were then fed diets with 3 different iron concentrations that we designated deficient (2-5 mg/kg of iron), control (0.2 g/kg), and overload (20 g/kg) for 6 wk immediately after weaning to create a range of serum iron concentrations and resultant transferrin saturations ranging from 16 to 78%. We found an inverse correlation between transferrin saturation and transferrin half-life (P < 0.0001, r = -0.839) for both HFE-deficient and wild-type mice, which suggests that HFE does not have a direct effect on transferrin catabolism; rather, HFE may influence transferrin half-life indirectly through its effect on transferrin saturation, which in turn enhances transferrin decay in HFE-deficient mice.

Recycling, degradation and sensitivity to the synergistic anion of transferrin in the receptor-independent route of iron uptake by human hepatoma (HuH-7) cells

To secure iron from transferrin, hepatocytes use two pathways, one dependent on transferrin receptor (TfR 1) and the other, of greater capacity but lower affinity, independent of TfR 1. To clarify further similarities and differences of the two pathways, we have suppressed TfR 1 by 75-80% in human hepatoma-derived HuH-7 cells co-transfected with vectors bearing full-length TfR 1 cDNA or its first 100 bases in antisense orientation. Suppression of TfR 1 does not lead to down regulation of TfR 2, a recently described second transferrin receptor of as yet uncertain function. Both pathways depend on acidification of the compartments in which iron release from transferrin takes place. Recycling of transferrin is a feature of both pathways, but is substantially more efficient in the receptor-dependent route. Degradation of transferrin occurs only in the receptor-independent route, in the first example of a specific catabolic pathway of transferrin. Linkage of cellular iron uptake to release of the synergistic anion (without which iron is not bound by transferrin) is particularly evident in the receptor-independent pathway. Although the relative importance of the two pathways in normal and deranged hepatic iron metabolism remains to be determined, the receptor-independent route is a substantial accessory for iron uptake to the better-known receptor-dependent track.

Effects of cell proliferation on the uptake of transferrin-bound iron by human hepatoma cells

The effects of cellular proliferation on the uptake of transferrin-bound iron (Tf-Fe) and expression of transferrin receptor-1 (TfR1) and transferrin receptor-2 (TfR2) were investigated using a human hepatoma (HuH7) cell line stably transfected with TfR1 antisense RNA expression vector to suppress TfR1 expression. At transferrin (Tf) concentrations of 50 nmol/L and 5 micromol/L, when Tf-Fe uptake occurs by the TfR1- and TfR1-independent (NTfR1)-mediated process, respectively, the rate of Fe uptake by proliferating cells was approximately 250% that of stationary cells. The maximum rate of Fe uptake by the TfR1- and NTfR1-mediated process by proliferating cells was increased to 200% and 300% that of stationary cells, respectively. The maximum binding of Tf by both TfR1- and NTfR1-mediated processes by proliferating cells was increased significantly to 160% that of stationary cells. TfR1 and TfR2-alpha protein levels expressed by proliferating cells was observed to be approximately 300% and 200% greater than the stationary cells, respectively. During the proliferating growth phase, expression of TfR1 messenger RNA (mRNA) increased to 300% whereas TfR2-alpha mRNA decreased to 50% that of stationary cells. In conclusion, an increase in Tf-Fe uptake by TfR1-mediated pathway by proliferating cells was associated with increased TfR1 mRNA and protein expression. An increase in Tf-Fe uptake by NTfR1-mediated pathway was correlated with an increase in TfR2-alpha protein expression but not TfR2-alpha mRNA. In conclusion, TfR2-alpha protein is likely to have a role in the mediation of Tf-Fe uptake by the NTfR1 process by HuH7 hepatoma cell in proliferating and stationary stages of growth.

Increased duodenal DMT-1 expression and unchanged HFE mRNA levels in HFE-associated hereditary hemochromatosis and iron deficiency

HFE-associated hereditary hemochromatosis is characterized by imbalances of iron homeostasis and alterations in intestinal iron absorption. The identification of the HFE gene and the apical iron transporter divalent metal transporter-1, DMT-1, provide a direct method to address the mechanisms of iron overload in this disease. The aim of this study was to evaluate the regulation of duodenal HFE and DMT-1 gene expression in HFE-associated hereditary hemochromatosis. Small bowel biopsies and serum iron indices were obtained from a total of 33 patients. The study population comprised 13 patients with hereditary hemochromatosis (C282Y homozygous), 10 patients with iron deficiency anemia, and 10 apparently healthy controls, all of whom were genotyped for the two common mutations in the HFE gene (C282Y and H63D). Total RNA was isolated from tissue and amplified via RT-PCR for HFE, DMT-1, and the internal control GAPDH. DMT-1 protein expression was additionally assessed by immunohistochemistry. Levels of HFE mRNA did not differ significantly between patient groups (P = 0.09), specifically between C282Y homozygotes and iron deficiency anemic patients, when compared to controls (P = 0.09, P = 0.9, respectively). In contrast, DMT-1 mRNA levels were at least twofold greater in patients with hereditary hemochromatosis and iron deficiency anemia when compared to controls (P = 0.02, P = 0.01, respectively). Heightened DMT-1 protein expression correlated with mRNA levels in all patients. Loss of HFE function in hereditary hemochromatosis is not derived from inhibition of its gene expression. DMT-1 expression in C282Y homozygote subjects is consistent with the hypothesis of a "paradoxical" duodenal iron deficiency in hereditary hemochromatosis. The observed twofold upregulation of the DMT-1 is consistent with the slow but steady increase in body iron stores observed in those presenting with clinical features of hereditary hemochromatosis.

Haemochromatosis in the new millennium

Hereditary haemochromatosis (HHC) is a common inherited disorder of iron metabolism characterised by progressive iron loading of parenchymal cells of the liver, pancreas, heart and other organs ultimately leading to cirrhosis and organ failure. Despite HLA studies which localised the defective gene to the short arm of chromosome 6, the haemochromatosis gene remained elusive until 1996, when the gene was identified by a massive positional cloning effort. The haemochromatosis gene (HFE) encodes a novel nonclassical MHC class-1-like molecule. Two missense mutations have been identified in patients with HHC, a G to A at nucleotide 845, resulting in a substitution of tyrosine for cysteine at amino acid 282 (referred to as the C282Y mutation) and a C to G at nucleotide 187, resulting in a substitution of aspartate for histidine at amino acid 63 (H63D). An average of 85-90% of patients with typical clinical features of HHC are homozygous for the C282Y mutation. H63D is not associated with the same degree of iron loading as C282Y. Clinical expression is variable depending on environmental (dietary) iron, physiological and pathological blood loss and as yet unidentified modifying genetic factors. One recent Australian study indicates that only about 50% of homozygous subjects are fully expressing and symptomatic and that about 30% show no clinical or biochemical expression. Genetic tests for identifying mutations in the HFE gene provide precise means for diagnosis, family testing and population screening and have led to re-evaluation of the indications for liver biopsy in this disease. At the present time, however, the most practical and cost-effective method of screening is for phenotypic expression by transferrin saturation or unsaturated iron binding capacity measurement. In the future, population screening by genotype should be feasible once the relevant technical, legal and ethical issues are resolved.

Concentration of iron and distribution of iron and transferrin after experimental iron overload in rat tissues in vivo: study of the liver, the spleen, the central nervous system and other organs

The purpose of this study was to estimate the iron concentration in the liver, spleen and brain of control rats and rats overloaded with iron and to determine the distribution of iron and of transferrin (TF). Iron was administered to Wistar rats by food supplemented with 3% carbonyl iron for 3 months, or intraperitoneally, or intraveneously as iron polymaltose for 4 months (total administered dose: 300 or 350 mg/rat, respectively). Iron concentration was estimated by atomic absorption spectrophotometry and iron- and TF-distribution histochemically and immunohistochemically, respectively. In control rats the organ with the highest iron content was the spleen, followed by the liver and brain. After iron loading the increase of iron in the liver was greater than that of the spleen; iron concentration in the brain did not change significantly. Distribution of iron in the liver was in Kupffer cells throughout the lobule and in hepatocytes at its periphery. No difference in the number of positive cells or staining intensity for TF was observed between control rats and iron overloaded animals in the liver or central nervous system (CNS); the spleen was negative for TF. Distribution of TF in the liver showed a centrilobular localisation in hepatocytes. TF reaction in the brain occurred in oligodendrocytes, vessel walls, choroid plexus epithelial cells and some neurons. In conclusion, experimental iron overload in rats leads to iron uptake mainly by reticuloendothelial (RE) cells and hepatocytes, indicating that hepatocytes are of particular importance for iron metabolism. Iron uptake by the brain was not significant, probably because the brain is protected against iron overload. Iron overload did not influence location and quantity of TF in the liver and CNS, whereas the visualisation of iron and TF did not coincide. This indicates that TF may have other functions beyond iron transport.

Transferrin receptor 2: continued expression in mouse liver in the face of iron overload and in hereditary hemochromatosis

Hereditary hemochromatosis (HH) is a common autosomal recessive disorder characterized by excess absorption of dietary iron and progressive iron deposition in several tissues, particularly liver. Liver disease resulting from iron toxicity is the major cause of death in HH. Hepatic iron loading in HH is progressive despite down-regulation of the classical transferrin receptor (TfR). Recently a human cDNA highly homologous to TfR was identified and reported to encode a protein (TfR2) that binds holotransferrin and mediates uptake of transferrin-bound iron. We independently identified a full-length murine EST encoding the mouse orthologue of the human TfR2. Although homologous to murine TfR in the coding region, the TfR2 transcript does not contain the iron-responsive elements found in the 3' untranslated sequence of TfR mRNA. To determine the potential role for TfR2 in iron uptake by liver, we investigated TfR and TfR2 expression in normal mice and murine models of dietary iron overload (2% carbonyl iron), dietary iron deficiency (gastric parietal cell ablation), and HH (HFE -/-). Northern blot analyses demonstrated distinct tissue-specific patterns of expression for TfR and TfR2, with TfR2 expressed highly only in liver where TfR expression is low. In situ hybridization demonstrated abundant TfR2 expression in hepatocytes. In contrast to TfR, TfR2 expression in liver was not increased in iron deficiency. Furthermore, hepatic expression of TfR2 was not down-regulated with dietary iron loading or in the HFE -/- model of HH. From these observations, we propose that TfR2 allows continued uptake of Tf-bound iron by hepatocytes even after TfR has been down-regulated by iron overload, and this uptake contributes to the susceptibility of liver to iron loading in HH.

Peripheral blood erythrocyte parameters in hemochromatosis: evidence for increased erythrocyte hemoglobin content

We studied peripheral blood erythrocyte parameters and HFE genotypes in 94 hemochromatosis probands and 132 white, normal control subjects. Mean red blood cell counts in probands and control subjects were not significantly different. However, mean values of hemoglobin, hematocrit, mean corpuscular volume (MCV), mean corpuscular hemoglobin (MCH), and mean corpuscular hemoglobin concentration (MCHC) were significantly higher in C282Y/C282Y probands (n = 60) than in wild-type control subjects (n = 65). Probands with other HFE genotypes also had increased mean erythrocyte parameters (other than red blood cell count). Peripheral blood smears prepared before therapeutic phlebotomy revealed that erythrocytes in many probands had increased diameters and were well filled with hemoglobin. Erythrocyte parameters were similar in C282Y/C282Y probands with and without hepatomegaly, elevated serum concentrations of hepatic enzymes, hepatic cirrhosis, diabetes mellitus, arthropathy, or hypogonadism. Among C282Y/C282Y probands, significantly greater values of MCV (but not other erythrocyte parameters) occurred among those who had transferrin saturation values of 75% or greater or iron overload at diagnosis. After iron depletion, the mean MCV, MCH, and MCHC values of C282Y/C282Y probands decreased but remained significantly greater than values in wild-type control subjects. Mean values of prephlebotomy MCH and MCHC concentrations were lower in HLA-A3-positive than in HLA-A3-negative C282Y/C282Y probands. We conclude that increased values of mean hemoglobin, hematocrit, MCV, MCH, and MCHC in hemochromatosis probands are caused primarily by increased iron uptake and hemoglobin synthesis by immature erythroid cells. Mechanisms of iron uptake by erythrocytes that could explain these results are discussed.

Mechanism of increased iron absorption in murine model of hereditary hemochromatosis: increased duodenal expression of the iron transporter DMT1

Hereditary hemochromatosis (HH) is a common autosomal recessive disorder characterized by tissue iron deposition secondary to excessive dietary iron absorption. We recently reported that HFE, the protein defective in HH, was physically associated with the transferrin receptor (TfR) in duodenal crypt cells and proposed that mutations in HFE attenuate the uptake of transferrin-bound iron from plasma by duodenal crypt cells, leading to up-regulation of transporters for dietary iron. Here, we tested the hypothesis that HFE-/- mice have increased duodenal expression of the divalent metal transporter (DMT1). By 4 weeks of age, the HFE-/- mice demonstrated iron loading when compared with HFE+/+ littermates, with elevated transferrin saturations (68.4% vs. 49.8%) and elevated liver iron concentrations (985 micrograms vs. 381 micrograms). By using Northern blot analyses, we quantitated duodenal expression of both classes of DMT1 transcripts: one containing an iron responsive element (IRE), called DMT1(IRE), and one containing no IRE, called DMT1(non-IRE). The positive control for DMT1 up-regulation was a murine model of dietary iron deficiency that demonstrated greatly increased levels of duodenal DMT1(IRE) mRNA. HFE-/- mice also demonstrated an increase in duodenal DMT1(IRE) mRNA (average 7.7-fold), despite their elevated transferrin saturation and hepatic iron content. Duodenal expression of DMT1(non-IRE) was not increased, nor was hepatic expression of DMT1 increased. These data support the model for HH in which HFE mutations lead to inappropriately low crypt cell iron, with resultant stabilization of DMT1(IRE) mRNA, up-regulation of DMT1, and increased absorption of dietary iron.

Inappropriately High Iron Regulatory Protein Activity in Monocytes of Patients With Genetic Hemochromatosis

In genetic hemochromatosis (GH), excess iron is deposited in parenchymal cells, whereas little iron is found in reticuloendothelial (RE) cells until the later stages of the disease. As iron absorption is inversely related to RE cells stores, a failure of RE to retain iron has been proposed as the basic defect in GH. In RE cells of GH subjects, we examined the activity of iron regulatory protein (IRP), a reliable indicator of the elusive regulatory labile iron pool, which modulates cellular iron homeostasis through control of ferritin (Ft) and transferrin receptor gene expression. RNA-bandshift assays showed a significant increase in IRP activity in monocytes from 16 patients with untreated GH compared with 28 control subjects (1.5-fold) and five patients with secondary hemochromatosis (SH) with similar iron burden (fourfold). In 17 phlebotomy-treated GH patients, IRP activity did not differ from that of control subjects. In both GH and SH monocyte-macrophages, Ft content increased by twofold and the L subunit-rich isoferritin profile was unchanged as compared with controls. IRP activity was still upregulated in vitro in monocyte-derived macrophages of GH subjects but, following manipulations of iron levels, was modulated normally. Therefore, the sustained activity of monocyte IRP found in vivo in monocytes of GH patients is not due to an inherent defect of its control, but is rather the expression of a critical abnormality of iron metabolism, eg, a paradoxical contraction of the regulatory iron pool. By preventing Ft mRNA translation, high IRP activity in monocytes may represent a molecular mechanism contributing to the inadequate Ft accumulation and insufficient RE iron storage in GH.

A duodenal mucosal abnormality in the reduction of Fe(III) in patients with genetic haemochromatosis

BACKGROUND

Previous in vitro studies have shown that the uptake of Fe(III) by freshly isolated duodenal mucosal biopsy specimens is increased in patients with genetic haemochromatosis. Moreover, in the mouse it has recently been found that reduction of Fe(III) to Fe(II) is a prerequisite for iron uptake by the proximal intestine.

AIMS/METHODS

This study used the in vitro technique to investigate the rates of reduction and uptake of 59Fe(III) by duodenal mucosal biopsy specimens obtained at endoscopy from treated and untreated patients with genetic haemochromatosis.

RESULTS

The rate of reduction of iron in the medium was proportional to the incubation time and was not caused by the release of reducing factors from the tissue fragments. Ferrozine, a specific Fe(II) chelator and ferricyanide, a non-permeable oxidising agent, inhibited uptake of 59Fe showing that reduction of Fe(III) precedes uptake. The rates (all values given as pmol/mg/min) of reduction (152 (49) v 92 (23)) and uptake (8.3 (4.0) v 3.6 (1.3), mean (SD)), were significantly increased in biopsy specimens from the untreated group (n = 6) compared with those from 10 control subjects (p < 0.04). Furthermore, the reduction and uptake rates were still increased in five patients in whom iron stores were normal after venesection treatment.

CONCLUSIONS

These results show that there is a persistent abnormality in the reduction and uptake of iron by the intestine in genetic haemochromatosis.

Modulation by iron loading and chelation of the uptake of non-transferrin-bound iron by human liver cells

Hepatic non-transferrin-bound Fe (NTBI) flux and its regulation were characterized by measuring the uptake of Fe from [59Fe]/nitrilotriacetate (NTA) complexes in control and Fe-loaded cultures of human hepatocellular carcinoma cells (HepG2). Exposure to ferric ammonium citrate (FAC) for 1 to 7 days resulted in a time- and dose-dependent increase in the rate of NTBI uptake. In contrast to previous studies showing a dependence of the rate of Fe uptake on extracellular Fe, this was positively correlated with total cellular Fe content. The Fe3+ chelating agents deferoxamine (DFO), 1,2-dimethyl-3-hydroxypyrid-4-one (CP 020) and 1,2-diethyl-3-hydroxypyrid-4-one (CP 094) prevented or diminished the increase in NTBI transport when present during Fe loading and reversed the stimulation in pre-loaded cells in relation to their abilities to decrease intracellular iron. Although saturation of the Fe uptake process was not achieved in control cells, kinetic modelling to include linear diffusion-controlled processes yielded estimated parameters of Km = 4.3 microM and Vmax = 2.6 fmol/micrograms protein/min for the underlying process. There was a significant increase in the apparent Vmax (31.2 fmol/micrograms protein per min) for NTBI uptake in Fe-loaded cells, suggesting that Fe loading increases the number of a rate-limiting carrier site for Fe. Km also increased to 15.2 microM, comparable to values reported when whole liver is perfused with FeSO4. We conclude that HepG2 cells possess a transferrin-independent mechanism of Fe accumulation that responds reversibly to a regulatory intracellular Fe pool.

Etiologies, consequences, and treatment of iron overload

From a global perspective, severe systemic iron overload occurs predominantly in individuals affected by geographically specific genetic mutations that permit the daily absorption from the diet of more iron than is physiologically needed. Two main types of hereditary iron overload are well recognized: (1) HLA-linked hemochromatosis in populations derived from Europe and (2) iron overload complicating thalassaemia major and intermedia syndromes in Southeast Asia, the Middle East, and the Mediterranean. Another very common form of iron overload occurs in Africa and is clearly related to high dietary iron content; recent evidence suggests that a genetic predisposition may also contribute to the pathogenesis. Patients with iron overload may develop multiorgan system toxicity; aggressive therapy with phlebotomy or iron chelation to remove excess iron from the body prevents organ damage and prolongs life.

Regulation of ferritin and transferrin receptor expression by iron in human hepatocyte cultures

HepG2 cell cultures and human hepatocyte primary cultures were used to develop appropriate hepatocytic in vitro models of iron load in order to further understand the pathophysiological mechanisms occurring in the liver of patients with hemochromatosis. The first step of this study was to obtain an efficient iron supply in conditions of minimal toxicity. It was demonstrated that iron complexed to citrate entered efficiently into HepG2 cells and human hepatocytes. This iron load was obtained with minimal toxicity in both culture models as evaluated by the intracellular LDH activity and the total protein content. The second step was to study the effect of iron on ferritin and transferrin receptor expression. In HepG2 cell cultures, intracellular and extracellular ferritin concentrations were strikingly increased by iron in dose- and time-dependent manners. However, the relative amounts of H and L ferritin mRNAs were not significantly affected by iron, suggesting that ferritin regulation occurred at a translational level. On the other hand, in human hepatocyte cultures, the increase of intracellular and extracellular ferritin concentrations was accompanied by an increase in the amounts of H and L ferritin mRNAs. In this model, iron-induced ferritin biosynthesis seemed to be more complex than in HepG2 cells and to be governed by transcriptional and/or post-transcriptional regulatory mechanisms. However, an additional translational level of regulation could not be excluded. In contrast, transferrin receptor expression was decreased by iron in HepG2 cells as well as in human hepatocyte cultures. This decrease was associated with a decrease in the mRNA steady-state level. In both culture models, transferrin receptor regulation seemed to occur at a transcriptional or post-transcriptional level. These results demonstrate that normal human hepatocytes in primary culture respond to iron in a manner close to that observed in vivo and thereby provide a promising experimental model for further understanding pathophysiological mechanisms involved in human hemochromatotic liver.

Persistent iron overload 4 years after inadvertent transplantation of a haemochromatotic liver in a patient with primary biliary cirrhosis

An iron-loaded liver from a 20-year-old man with occult haemochromatosis was inadvertently transplanted into a 64-year-old lady with primary biliary cirrhosis. Increased hepatic iron storage was observed in biopsy specimens undertaken at 18, 30 and 36 months after transplantation. Serum iron levels and transferrin iron saturation increased and remained elevated 4 years after surgery. Ferritin levels were also consistently increased. Iron absorption test, performed at 4 years after transplant, was increased at 38%. Our data suggest that an intrahepatic defect in haemochromatosis cannot be excluded, and casts some doubt on the assertion that a precirrhotic haemochromatotic liver can be transplanted without sequelae.

Saturability of hepatic iron deposits in genetic hemochromatosis

The relationship of pretreatment serum ferritin and hepatic iron concentration to body iron removed by venesections was evaluated in 33 patients with genetic hemochromatosis. The median values of the three variables considered were 1,950 micrograms/L (range = 255 to 10,000), 1,175 micrograms/100 mg dry weight (range = 270 to 4,310) and 10 gm (range = 2 to 41), respectively. At basal liver biopsy 18 patients had cirrhosis, 6 patients had fibrosis and 9 patients had a normal pattern; siderosis was degree 3 in 6 patients and degree 4 in 27 patients. The results of fitting a polynomial regression of second degree showed that the curve of serum ferritin on iron removed was a straight line (R2 = 0.79, with a significant coefficient of linearity, p less than 0.01, and a nonsignificant coefficient of curvature), whereas that of hepatic iron concentration on iron removed showed a curvature (R2 = 0.62, with significant coefficient of linearity and curvature, p less than 0.01) and reached a plateau. The sigmoid model fit the curve of hepatic iron concentration on iron removed (R2 = 0.61), which suggested a saturation of hepatic iron storage capability; the asymptote corresponded to a hepatic iron concentration of about 2,000 micrograms/100 mg. In alcoholic patients (17 cases) the location of the sigmoid was greater than in nonalcoholic patients. Our results suggest that iron deposition occurs in the liver before other organs are involved and that with massive iron overload hepatic deposits reach saturation, after which hepatic iron concentration does not always reflect the amount of total stores. Alcohol consumption could slow the saturation of hepatic iron deposits.

Rapid induction of hepatic fibrosis in the gerbil after the parenteral administration of iron-dextran complex

The parenteral administration of iron-dextran complex to gerbils caused hepatic hemosiderosis and fibrosis after 6 wk. Type I and III collagen synthesis in the liver developed from perisinusoidal stellate cells that are often referred to as myofibroblasts. Immunohistologically these cells were shown to have large intracellular deposits of ferritin. The hepatic fibrosis appeared to be associated with aggregates of these cells rather than the aggregates of Kupffer cells, which also occur in hemosiderosis in the liver. No appreciable necrosis of hepatocytes to trigger the fibrotic response was found, so that the fibrosis appeared to be related to the accumulation of ferritin in the perisinusoidal stellate cells. In contrast, rats and mice did not accumulate ferritin in their perisinusoidal cells or develop hepatic fibrosis in response to parenterally administered iron, although they accumulated similar or greater amounts of total iron in their livers. The rapid induction of hepatic fibrosis in gerbils in response to parenterally administered iron will provide a model to investigate the mechanism of induction of collagen deposition in response to iron overload and a means of quickly evaluating therapeutic treatments for iron overload-induced fibrosis in vivo using iron-chelating drugs.

Differential expression of transferrin receptor in duodenal mucosa in iron overload. Evidence for a site-specific defect in genetic hemochromatosis

In genetic hemochromatosis, metabolic studies have demonstrated inappropriately increased iron absorption by cells of the duodenal mucosa. It is not clear whether this reflects an intrinsic abnormality of iron homeostasis at this site or is a consequence of a more generalized defect in cellular iron metabolism particularly involving the liver. We have previously used the expression of iron-related proteins as markers of iron homeostasis and have demonstrated normal regulation of the transferrin receptor and ferritin in the liver in this condition. In the present study we used immunohistochemical techniques to study transferrin-receptor expression in the gastrointestinal epithelium in normal subjects and patients with iron overload. In untreated genetic hemochromatosis and normal subjects, villus epithelial cells expressed receptor in the basolateral, subnuclear region. In contrast, in patients with secondary iron overload, receptor staining was absent in villus epithelial cells. The cells in the duodenal crypts showed intense staining for the transferrin receptor in all subjects investigated, a finding consistent with the known behavior of this receptor in proliferating cells. Given that body iron stores in both types of iron overload were comparable, these findings indicating a failure of down-regulation of the villus enterocyte transferrin receptor in genetic hemochromatosis may reflect the presence of a regulatory defect associated with the inability to control iron absorption in this condition.

Transferrin receptor distribution and regulation in the rat small intestine. Effect of iron stores and erythropoiesis

A combination of biochemical quantitation and immunohistochemistry has been used to examine in detail transferrin receptor distribution and expression in the rat small intestine and its relationship to iron absorption. Receptor numbers were quantitated by transferrin binding to preparations of basolateral or brush-border membranes. Receptors were demonstrated on the basolateral membranes of the gut cells, but not on the brush-border fraction. Apotransferrin demonstrated little binding to basolateral membranes at physiological pH. Dietary or parenteral iron loading of animals produced a significant decline in transferrin binding, whereas binding was increased in iron deficiency. These data were confirmed by immunohistochemical studies using a monoclonal antibody to the transferrin receptor. When iron absorption was increased threefold following acute hemolysis and without a decrease in body iron stores, there was no change in transferrin receptor number. These data indicate that intestinal transferrin receptors may be regulated by body iron stores but suggest that they are not directly involved in iron absorption.

Transferrin receptor expression in rat liver: immunohistochemical and biochemical analysis of the effect of age and iron storage

Hepatic transferrin receptors were studied in normal male rats at 1 to 59 wk after weaning, using immunohistochemical and biochemical techniques. The number of transferrin receptors measured and the intensity of the staining in situ decreased rapidly during the first 10 wk of life and more slowly thereafter. Immunohistochemistry further demonstrated changes in the topographical and (sub)cellular localization of the transferrin receptor. In the young rat livers, staining was almost exclusively present on hepatocytes in acinar zone 2 + 3 in a honeycomb to sinusoidal pattern. With aging, a panacinar heterogeneous and mainly sinusoidal staining of hepatocytes was more frequent. Kupffer cell positivity was more obvious as compared with the young rat livers. The observed changes in transferrin receptor expression may partly be explained by age-dependent alterations in DNA synthesis and proliferative potential of the liver cells. A series of rats were iron loaded with carbonyl iron up to 39 wk and "unloaded" by administration of a normal diet during 20 wk. In these animals, serial histochemical studies showed predominantly parenchymal (7 to 14 wk), mixed parenchymal and reticuloendothelial (39 wk) and almost exclusive reticuloendothelial siderosis (59 wk). In the siderotic livers transferrin receptor numbers tended to be lower than in the controls with significant differences after 14 and 39 wk. Immunohistochemistry showed decreased parenchymal but increased reticuloendothelial transferrin receptor expression with iron load. After the period of unloading, parenchymal transferrin receptors were virtually absent despite the negligible siderosis of these cells. In contrast, siderotic reticuloendothelial cells were intensely positive. These findings support down-regulation of parenchymal transferrin receptor resulting from iron storage. However, the positivity of siderotic reticuloendothelial cells and the absence of re-emergence of parenchymal receptors in conditions of minimal parenchymal and prominent reticuloendothelial siderosis need further elucidation.