The effect of the flavonol rutin on serum and liver iron content in a genetic mouse model of iron overload

The flavonol rutin has been shown to possess antioxidant and iron chelating properties in vitro and in vivo. These dual properties are beneficial as therapeutic options to reduce iron accumulation and the generation of reactive oxygen species (ROS) resultant from excess free iron. The effect of rutin on iron metabolism has been limited to studies performed in wildtype mice either injected or fed high-iron diets. The effect of rutin on iron overload caused by genetic dysregulation of iron homoeostasis has not yet been investigated. In the present study we examined the effect of rutin treatment on tissue iron loading in a genetic mouse model of iron overload, which mirrors the iron loading associated with Type 3 hereditary haemochromatosis patients who have a defect in Transferrin Receptor 2 (TFR2). Male TFR2 knockout (KO) mice were administered rutin via oral gavage for 21 continuous days. Following treatment, iron levels in serum, liver, duodenum and spleen were assessed. In addition, hepatic ferritin protein levels were determined by Western blotting, and expression of iron homoeostasis genes by quantitative real-time PCR. Rutin treatment resulted in a significant reduction in hepatic ferritin protein expression and serum transferrin saturation. In addition, trends towards decreased iron levels in the liver and serum, and increased serum unsaturated iron binding capacity were observed. This is the first study to explore the utility of rutin as a potential iron chelator and therapeutic in an animal model of genetic iron overload.

TFR2-related haemochromatosis in the Netherlands: a cause of arthralgia in young adulthood

BACKGROUND

Type 3 hereditary haemochromatosis (HH) is a rare iron overload disorder caused by variants in the transferrin 2 receptor (TFR2) gene. We aim to present characteristics of patients diagnosed with TFR2-HH in the Netherlands, in order to increase knowledge and awareness of this disease.

METHODS

We collected clinical, biochemical and genetic data from four patients from three families diagnosed with HH type 3 in the Netherlands between 2009 and 2016.

RESULTS

Three women and one man diagnosed with HH type 3 presented with arthralgia and elevated ferritin levels and transferrin saturation (TSAT) at ages 25-41 years. The hepcidin/ferritin ratio as measured in three patients was low. Liver iron content in two patients as assessed by MRI or liver biopsy was highly increased (250 and 362.7 μmol iron/g dry weight, respectively, reference < 35 μmol/g). DNA analysis revealed four different TFR2 pathogenic variants: one nonsense, one splicing and two missense variants, of which three are novel. Phlebotomy decreased the serum iron parameters but did not relieve the arthralgia.

CONCLUSION

In patients with a combination of elevated TSAT and ferritin in the absence of anaemia, and after exclusion of HFE-related HH, rare forms of HH should be considered. In these cases, presentation with arthralgia in young adulthood, low hepcidin/ferritin ratio and/or liver iron content > 100 μmol/g form an indication for analysis of the TFR2 gene. Although type 3 HH is extremely rare, awareness of the disease among physicians is important in order to achieve an early diagnosis and prevent complications, such as liver damage.

An unusual case of iron deficiency anemia is associated with extremely low level of transferrin receptor

A case study of a female patient, diagnosed with iron deficiency anemia, was unresponsive to oral iron treatment and only partially responsive to parenteral iron therapy, a clinical profile resembling the iron-refractory iron deficiency anemia (IRIDA) disorder. However, the patient failed to exhibit microcytic phenotype, one of the IRIDA hallmarks. Biochemical assays revealed that serum iron, hepcidin, interluekin 6, and transferrin saturation were within the normal range of references or were comparable to her non-anemic offspring. Iron contents in serum and red blood cells and hemoglobin levels were measured, which confirmed the partial improvement of anemia after parenteral iron therapy. Strikingly, serum transferrin receptor in patient was almost undetectable, reflecting the very low activity of bone-marrow erythropoiesis. Our data demonstrate that this is not a case of systemic iron deficiency, but rather cellular iron deficit due to the low level of transferrin receptor, particularly in erythroid tissue.

Bone morphogenetic proteins 2, 4, and 9 stimulate murine hepcidin 1 expression independently of Hfe, transferrin receptor 2 (Tfr2), and IL-6

Recently, it has been suggested that hepcidin, a peptide involved in iron homeostasis, is regulated by bone morphogenetic proteins (BMPs), apparently by binding to hemojuvelin (Hjv) as a coreceptor and signaling through Smad4. We investigate the role of Hfe, Tfr2 (transferrin receptor 2), and IL-6 in BMP2-, BMP4-, and BMP9-stimulated up-regulation of murine hepcidin, because these molecules, like Hjv, are known to be involved in hepcidin signaling. We show that the BMP signaling pathway acts independently of Hfe, Tfr2, and IL-6: The response to BMP2, BMP4, and BMP9 is similar in isolated hepatocytes of wild-type, Hfe(-/-), IL-6(-/-), and Tfr2(m) mutant mice. The potency of different human BMPs in stimulating hepcidin transcription by murine primary hepatocytes is BMP9 > BMP4 > BMP2. However, in human HepG2 cells, BMP4 and BMP9 are equally potent, whereas BMP2 requires a higher dose to become an effective hepcidin activator. Moreover, all of the tested BMPs are more potent regulators of hepcidin than IL-6 and thus are the most potent known stimulators of hepcidin transcription.

Expression of hepcidin is down-regulated in TfR2 mutant mice manifesting a phenotype of hereditary hemochromatosis

Transferrin receptor 2 (TfR2) is a membrane glycoprotein that mediates cellular iron uptake from holotransferrin. Homozygous mutations of this gene cause one form of hereditary hemochromatosis in humans. We recently reported that homozygous TfR2(Y245X) mutant mice, which correspond to the TfR2(Y250X) mutation in humans, showed a phenotype similar to hereditary hemochromatosis. In this study, we further analyzed the phenotype as well as iron-related gene expression in these mice by comparing the TfR2-mutant and wild-type siblings. Northern blot analyses showed that the levels of expression of hepcidin mRNA in the liver were generally lower, whereas those of duodenal DMT1, the main transporter for uptake of dietary iron, were higher in the TfR2-mutant mice as compared to the wild-type siblings. Expression of hepcidin mRNA in the TfR2 mutant mice remained low even after intraperitoneal iron loading. In isolated hepatocytes from both wild-type and TfR2 mutant mice, interleukin-6 and lipopolysaccharide each induced expression of hepcidin mRNA. These results suggest that up-regulation of hepcidin expression by inflammatory stimuli is independent of TfR2 and that TfR2 is upstream of hepcidin in the regulatory pathway of body iron homeostasis. (Blood. 2005;105:376-381)

The hereditary hemochromatosis protein, HFE, lowers intracellular iron levels independently of transferrin receptor 1 in TRVb cells

Hereditary hemochromatosis (HH) is an autosomal recessive disease that leads to parenchymal iron accumulation. The most common form of HH is caused by a single amino acid substitution in the HH protein, HFE, but the mechanism by which HFE regulates iron homeostasis is not known. In the absence of transferrin (Tf), HFE interacts with transferrin receptor 1 (TfR1) and the 2 proteins co-internalize, and in vitro studies have shown that HFE and Tf compete for TfR1 binding. Using a cell line lacking endogenous transferrin receptors (TRVb cells) transfected with different forms of HFE and TfR1, we demonstrate that even at low concentrations Tf competes effectively with HFE for binding to TfR1 on living cells. Transfection of TRVb cells or the derivative line TRVb1 (which stably expresses human TfR1) with HFE resulted in lower ferritin levels and decreased Fe2+ uptake. These data indicate that HFE can regulate intracellular iron storage independently of its interaction with TfR1. Earlier studies found that in HeLa cells, HFE expression lowers Tf-mediated iron uptake; here we show that HFE lowers non-Tf-bound iron in TRVb cells and add to a growing body of evidence that HFE may play different roles in different cell types.

The IL-6- and lipopolysaccharide-induced transcription of hepcidin in HFE-, transferrin receptor 2-, and beta 2-microglobulin-deficient hepatocytes

The antimicrobial peptide hepcidin appears to play a central role in the regulation of iron homeostasis. In intact animals, iron overload or the injection of lipopolysaccharide (LPS) stimulates transcription of HAMP, the gene that encodes hepcidin. In isolated hepatocytes, IL-6, an inflammatory cytokine the production of which is stimulated by LPS, up-regulates transcription of hepcidin. In contrast, iron has no stimulatory effect on hepcidin expression in isolated hepatocytes. There is apparently a signaling pathway, activated by iron, that is present in the intact animal but not in isolated hepatocytes. Studies in humans and mice have shown that this iron-dependent pathway requires the presence of Hfe, hemojuvelin, and probably transferrin receptor 2 (tfr-2). To determine whether activation of hepcidin transcription by IL-6 also requires Hfe and tfr-2, we have studied mice homozygous for targeted disruption of HFE, beta(2)-microglobulin, and for a truncating mutation of TFR-2. We show that these mutant mice react normally to injection of endotoxin and that their isolated hepatocytes react normally to IL-6. This indicates that the signaling pathway activated by IL-6 does not require either Hfe or tfr-2. Mice with disruption of the gene encoding IL-6 seem to have a blunted response to LPS, but the statistical significance of the small response documented is borderline. It is therefore not clear whether LPS stimulates secretion of cytokines other than IL-6 that may stimulate hepcidin transcription.

Type 3 hemochromatosis and β -thalassemia trait

Type 3 hemochromatosis is a rare autosomal recessive disorder due to mutations of the TFR2 gene. We describe clinical, biochemical and histopathologic findings of a patient with type 3 hemochromatosis at presentation and during a follow-up of more than 20 yr and we evaluate the effect of an associated beta-thalassemia trait on phenotypic expression. At the age of 33 yr the patient showed a marked iron overload and severe iron-related complications. After removal of 26 g of iron by subcutaneous deferoxamine infusion a marked clinical improvement was observed. Liver biopsies, performed at the age of 34 and 49 yr, indicate that in type 3 hemochromatosis there is a progressive hepatocellular iron accumulation from Rappaport's zone 1-3 and that iron loading in sinusoidal and portal macrophages occurs only in the more advanced stage. As observed in HFE hemochromatosis, the beta-thalassemia trait seems to aggravate the clinical picture of patients lacking TFR2, favoring higher rates of iron accumulation probably by activation of the erythroid iron regulator.

Can defects in transferrin receptor 2 and hereditary hemochromatosis genes account for iron overload in HbH disease?

Iron overload was found to be the major cause of disability in Chinese HbH disease patients although they were not on regular blood transfusion. The transferrin receptor 2 (TFR2) and hereditary hemochromatosis (HFE) genes were examined to see if inheritance of these gene defects may be a possible cause of iron overload in 45 HbH patients. A novel intronic (IVS6 (+6) T-->A) mutation of the TFR2 gene was identified in one patient, and six others were found to carry a known missense mutation (exon 5, I238M) that was also present in one normal control subject. One HbH patient and one normal control carried the H63D mutation of the HFE gene. Since only eight out of 45 iron-overloaded HbH patients carry a defect in the TFR2 or HFE gene in the heterozygote state and their iron loading status was comparable to the matched controls without such defects, it would appear that the accumulation of excess iron in HbH disease is more likely a result of increase dietary absorption secondary to ineffective erythropoiesis.

Hemochromatosis due to mutations in transferrin receptor 2

A rare recessive disorder which leads to iron overload and severe clinical complications similar to those reported in HFE-related hemochromatosis has been delineated and sometimes called hemochromatosis type 3. The gene responsible is Transferrin Receptor 2 (TFR2), which maps to chromosome 7q22. The TFR2 gene presents a significative homology to transferrin receptor (TFRC) gene, encodes for a transmembrane protein with a large extracellular domain, is able to bind transferrin, even if with lower affinity than TFRC. The TFR2 function is still unclear. The transcript does not contain IRE elements and is not modified by the cellular iron status. At variance with TFRC, interactions between TFR2 and HFE do not occur, at least in their soluble forms. TFR2 is spliced in two alternative forms, alfa and beta. The alfa form is strongly expressed in the liver. The beta form, codified from a start site in exon 4 of the alpha, has a low and ubiquitous expression. Using anti-TFR2 monoclonal antibodies we have confirmed expression of the protein in the liver but also in duodenal epithelial cells, and studied the protein functional behaviour in cell lines, in response to iron addition, iron deprivation and olo-transferrin exposure. Our results suggest a regulatory role of TFR2 in iron metabolism. Five TFR2 homozygous mutations have been documented in HFE3 patients: a nonsense mutation (Y250X); a C insertion that causes a frameshift and a premature stop codon (E60X); a missense mutation (M172K); a 12 basepair deletion in exon 16, that causes 4 aminoacid loss (AVAQ 594-597del) in the extracellular domain of TFR2; a missense mutation in exon 17 (Q690P). The mutation analysis supports the hypothesis that all are private mutations. The pathogenetic role of TFR2 in hemochromatosis has been recently further demonstrated through the targeted expression of the Y250X human mutation in mice, which develop sings of iron overload identical to the human disease. Although the rarity of TFR2 mutations limits their usefulness in diagnostic/screening programs, their study can contribute to a better understanding of the protein function.

Rare causes of hereditary iron overload

Iron is a vitally important element in mammalian metabolism because of its unsurpassed versatility as a biologic catalyst. However, when not appropriately shielded or when present in excess, iron plays a key role in the formation of extremely toxic oxygen radicals, which ultimately cause peroxidative damage to vital cell structures. Organisms are equipped with specific proteins designed for iron acquisition, export, transport, and storage as well as with sophisticated mechanisms that maintain the intracellular labile iron pool at an appropriate level. These systems normally tightly control iron homeostasis but their failure can lead to iron deficiency or iron overload and their clinical consequences. This review describes several rare iron loading conditions caused by genetic defects in some of the proteins involved in iron metabolism. A dramatic decrease in the synthesis of the plasma iron transport protein, transferrin, leads to a massive accumulation of iron in nonhematopoietic tissues but virtually no iron is available for erythropoiesis. Humans and mice with hypotransferrinemia have a remarkably similar phenotype. Homozygous defects in a recently identified gene encoding transferrin receptor 2 lead to iron overload (hemochromatosis type 3) with symptoms similar to those seen in patients with HFE-associated hereditary hemochromatosis (hemochromatosis type 1). Transferrin receptor 2 is primarily expressed in the liver but it is unclear how mutant forms cause iron overload. Mutations in the gene encoding the iron exporter, ferroportin 1, cause iron overload characterized by iron accumulation in macrophages yet normal plasma iron levels. Plasma iron, together with dominant inheritance, discriminates iron overload due to ferroportin mutations (hemochromatosis type 4) from hemochromatosis type 1. Heme oxygenase 1 is essential for the catabolism of heme and in the recycling of hemoglobin iron in macrophages. Homozygous heme oxygenase 1 deletion in mice leads to a paradoxical accumulation of nonheme iron in macrophages, hepatocytes, and many other cells and is associated with low plasma iron levels, anemia, endothelial cell damage, and decreased resistance to oxidative stress. A similar phenotype occurred in a child with severe heme oxygenase 1 deficiency. Recently, a mutation in the L-subunit of ferritin has been described that causes the formation of aberrant L-ferritin with an altered C-terminus. Individuals with this mutation in one allele of L-ferritin have abnormal aggregates of ferritin and iron in the brain, primarily in the globus pallidus. Patients with this dominantly inherited late-onset disease present with symptoms of extrapyramidal dysfunction. Mice with a targeted disruption of a gene for iron regulatory protein 2 (IRP2), a translational repressor of ferritin, misregulate iron metabolism in the intestinal mucosa and the central nervous system. Significant amounts of ferritin and iron accumulate in white matter tracts and nuclei, and adult IRP2-deficient mice develop a movement disorder consisting of ataxia, bradykinesia, and tremor. Mutations in the frataxin gene are responsible for Friedreich ataxia, the most common of the inherited ataxias. Frataxin appears to regulate mitochondrial iron (or iron-sulfur cluster) export and the neurologic and cardiac manifestations of Friedreich ataxia are due to iron-mediated mitochondrial toxicity. Finally, patients with Hallervorden-Spatz syndrome, an autosomal recessive, progressive neurodegenerative disorder, have mutations in a novel pantothenate kinase gene (PANK2). The cardinal feature of this extrapyramidal disease is pathologic iron accumulation in the globus pallidus. The defect in PANK2 is predicted to cause the accumulation of cysteine, which binds iron and causes oxidative stress in the iron-rich globus pallidus.