Expression of SFT (stimulator of Fe transport) is enhanced by iron chelation in HeLa cells and by hemochromatosis in liver

Iron, meat and health

This article is a summary of the publication "Iron and Health" by the Scientific Advisory Committee on Nutrition (SACN) to the U.K. Government (2010), which reviews the dietary intake of iron and the impact of different dietary patterns on the nutritional and health status of the U.K. population. It concludes that several uncertainties make it difficult to determine dose-response relationships or to confidently characterize the risks associated with iron deficiency or excess. The publication makes several recommendations concerning iron intakes from food, including meat, and from supplements, as well as recommendations for further research.

Developmental, regional, and cellular expression of SFT/UbcH5A and DMT1 mRNA in brain

Brain iron has marked developmental, regional, and cellular distribution patterns. To characterize better the potential mechanisms for iron transport into and within the brain, we have analyzed expression patterns of two factors: divalent metal transporter 1 (DMT1) and stimulator of Fe transport (SFT). DMT1 is known to participate in brain iron uptake although functional information is lacking. Even less clear is the possible role of SFT, which is related to a member of the ubiquitin-conjugating E2 family UbcH5A, but previous studies have found SFT/Ubc5Ha mRNA expressed abundantly in mouse brain. Like DMT1, SFT function has been implicated in transferrin and nontransferrin-bound iron uptake. Comparative Northern analysis indicates that SFT/UbcH5A mRNA levels are threefold higher in 3-day-old mice than at later ages, whereas levels of DMT1 mRNA do not change. In situ analysis of neonatal mouse brain reveals prominent SFT/UbcH5A mRNA expression in epithelial and ependymal cells in the choroid plexus and neurons of the olfactory bulb, hippocampus, and cortex. Adult mouse brain expresses SFT/UbcH5A mRNA mainly in white matter of the cerebellum and pons. Using a multiple tissue expression (MTE) array containing 20 different human brain regions, the highest levels of both SFT/UbcH5A and DMT1 mRNA are detected in the corpus callosum and cerebellum. The significantly elevated levels of SFT/UbcH5A mRNA in the neonatal mouse and its localization to choroid plexus, a major site of brain iron acquisition, suggest that this factor may contribute to the rapid rate of brain iron uptake that occurs in the early postnatal period.

UbcH5A, a member of human E2 ubiquitin-conjugating enzymes, is closely related to SFT, a stimulator of iron transport, and is up-regulated in hereditary hemochromatosis

SFT, a stimulator of iron (Fe) transport, has been described as a transmembrane protein that facilitates the uptake of ferrous and ferric iron in mammalian cells. This study was initiated to investigate the 5' regulatory region of SFT and its role in the etiology of hereditary hemochromatosis. Sequence analyses of the putative 5' regulatory region revealed that the SFT cDNA sequence corresponds to intron 6/exon 7 of UbcH5A, a member of E2 ubiquitin-conjugating enzymes, which is involved in the iron-dependent ubiquitination of the hypoxia-inducible factor (HIF) by the von Hippel-Lindau tumor suppressor (pVHL) E3 ligase complex. Further mRNA expression studies using a sequence-specific reverse transcriptase-polymerase chain reaction (RT-PCR) assay showed that UbcH5A is significantly up-regulated in the liver of iron-overloaded patients with hereditary hemochromatosis, as previously published for SFT. However, in vitro studies on HepG2 cells failed to demonstrate any significant UbcH5A regulation in response to iron loading or iron chelation. In conclusion, in vivo mRNA expression data previously obtained for SFT might be attributed to UbcH5A. The role of UbcH5A and the ubiquitination pathway in the etiology of hereditary hemochromatosis remains to be elucidated further.

Transferrin receptor 1 (TfR1) and putative stimulator of Fe transport (SFT) expression in iron deficiency and overload: an overview

Transferrin Receptor 1 (TfR1) and putative Stimulator of Fe Transport (SFT) represent two different proteins involved in iron metabolism in mammalian cells. The expression of TfR1 in the duodenum of subjects with normal body iron stores has been mainly localized in the basolateral portion of the cytoplasm of crypt cells, supporting the idea that this molecule may be involved in the sensing of body iron stores. In iron deficiency anemia TfR1 expression demonstrated an inverse relationship with body iron stores as assessed by immunohistochemistry with anti-TfR1 antibodies. In iron overload, TfR1 expression in the duodenum differed according to the presence or absence of the C282Y mutation in the HFE gene, being increased in HFE-related hemochromatosis and similar to controls in non-HFE-related iron overload. SFT is characterized by its ability to increase iron transport both through the transferrin dependent and independent uptake, and could thus affect iron absorption in the intestine. Immunohistochemistry using anti-SFT antibodies which recognize a putative stimulator of Fe transport of approximately 80 KDa revealed a localization of this protein in the apical part of the cytoplasm of enterocytes localized at the tip of the villi. The expression of the protein recognized by these antibodies was increased in iron deficiency, as well as in patients carrying the C282Y HFE mutation. Thus, the increased expression of both proteins only in patients with HFE-related hemochromatosis suggests that other factors should be involved in determining non-HFE-related iron overload.

Iron metabolism in mammalian cells

Most living things require iron to exist. Iron has many functions within cells but is rarely found unbound because of its propensity to catalyze the formation of toxic free radicals. Thus the regulation of iron requirements by cells and the acquisition and uptake of iron into tissues in multicellular organisms is tightly regulated. In humans, understanding iron transport and utility has recently been advanced by a "great conjunction" of molecular genetics in simple organisms, identifying genes involved in genetic diseases of metal metabolism and by the application of traditional cell physiology approaches. We are now able to approach a rudimentary understanding of the "iron cycle" within mammals. In the future, this information will be applied toward modulating the outcome of therapies designed to overcome diseases involving metals.

The iron metabolism of neoplastic cells: alterations that facilitate proliferation?

For many years it has been known that neoplastic cells express high levels of the transferrin receptor 1 (TfR1) and internalize iron (Fe) from transferrin (Tf) at a tremendous rate. Considering the high requirement of neoplastic cells for Fe, understanding its metabolism is vital in terms of devising potential new therapies. Apart from TfR1, a number of molecules have been identified that may have roles in Fe metabolism and cellular proliferation. These molecules include transferrin (Tf), the oestrogen-inducible transferrin receptor-like protein, transferrin receptor 2 (TfR2), melanotransferrin (MTf), ceruloplasmin, and ferritin. In the present review these latter molecules are discussed in terms of their potential functions in tumour cell Fe metabolism and proliferation. Further studies are essential to determine the specific roles of these proteins in the pathogenesis of cancer.

Inactivation of the hemochromatosis gene differentially regulates duodenal expression of iron-related mRNAs between mouse strains

BACKGROUND & AIMS

Hfe knockout mice, like patients with hereditary hemochromatosis, have augmented duodenal iron absorption and increased iron deposition in hepatic parenchymal cells. The goals of the present study were to gain further insight into the control of iron absorption by comparing the transcript levels of iron-related genes in the duodenum of DBA/2 Hfe-/- mice, susceptible to iron loading, and wild-type controls, and to test whether variations in the duodenal expression of these messengers contribute to the DBA/2 and C57BL/6 strain differences in the severity of hepatic iron loading.

METHODS

Expression of the different transcripts was quantified by real-time polymerase chain reaction.

RESULTS

The 2 strains differ strikingly, not only in the severity of hepatic iron loading, but also in the duodenal expression of iron-related genes. In DBA/2 Hfe-/- mice, increased intestinal iron absorption results from the concomitant up-regulation of the Dcytb, DMT1, and FPN1 messengers. No increase in the expression of these messengers is seen in C57BL/6 Hfe-/- mice.

CONCLUSIONS

The up-regulation of these transcripts suggests that an inappropriate iron-deficiency signal is sensed by the duodenal enterocytes, leading to an enhanced ferric reductase activity and the increase of duodenal iron uptake and transfer to the circulation. The genes modifying the hemochromatosis phenotype probably act by modifying the expression of these 3 messengers.

Cellular location of proteins related to iron absorption and transport

K562 erythroleukemia cells and IEC6 rat cells were examined using confocal microscopy and antibodies raised against DMT-1 (Nramp-2, DCT-1), transferrin receptor (CD71), beta(3) integrin (CD61), mobilferrin (calreticulin), and Hephaestin. The cellular location of each of these proteins was identified by immunofluorescence in both saponin-permeabilized and non-permeabilized cells. Fluorescent reactivity was observed on or near the cell surface of each of these proteins, suggesting that they might participate in surface membrane transport of iron. Fluorescence was observed in the region of the cytoplasm with each antibody to include beta(3) integrin and transferrin receptor. It was pronounced in cells incubated with mobilferrin, Hephaestin, and DMT-1 antibodies. Speckled nuclear fluorescence was observed in cells incubated with anti-DMT-1. While these observations are descriptive, they demonstrate that there are significant concentrations of DMT-1, mobilferrin, and Hephaestin in the cytoplasmic region of cells. This suggests that there may be intracellular roles for these proteins in addition to their serving to transit iron across the cell surface membrane.

Duodenal expression of a putative stimulator of Fe transport and transferrin receptor in anemia and hemochromatosis

BACKGROUND & AIMS

Stimulator of Fe Transport (SFT) and transferrin receptor (TfR) are proteins involved in iron transport. This study evaluated iron metabolism protein expression in duodenal biopsy specimens from controls and patients with abnormal iron metabolism.

METHODS

Twelve controls, 8 patients with iron deficiency anemia, 7 with HFE-related hemochromatosis, and 6 with non-HFE-related iron overload were studied. Immunohistochemistry was performed on duodenal biopsy specimens with anti-TfR and anti-SFT antibodies which recognize a putative stimulator of Fe transport of ~80 kilodaltons.

RESULTS

In controls, the putative stimulator of Fe transport was expressed in the middle and distal part of the villi in the subapical cytoplasmatic region. Its expression increased in anemics and, to a lesser degree, in HFE-related hemochromatotics, whereas it was reduced in patients with non-HFE-related iron overload. TfR expression showed a crypt-to-tip gradient in controls, but not in anemics, in whom it was uniformly overexpressed. TfR expression was intermediate in HFE-related hemochromatotics and similar to controls in non-HFE-related iron overload.

CONCLUSIONS

Expression of the putative stimulator of Fe transport and TfR increases in iron deficiency. Increased expression of both proteins is present only in HFE-related hemochromatotics suggesting that other factors may be involved in determining non-HFE-related iron overload phenotype.

Expression of stimulator of Fe transport is not enhanced in Hfe knockout mice

Hfe knockout (-/-) mice recapitulate many of the biochemical abnormalities of hereditary hemochromatosis (HH), but the molecular mechanisms involved in the etiology of iron overload in HH remain poorly understood. It was found previously that livers of patients with HH contained 5-fold higher SFT (stimulator of Fe transport) mRNA levels relative to subjects without HH. Because this observation suggests a possible role for SFT in HH, we investigated SFT mRNA expression in Hfe(-/-) mice. The 4- and 10-wk-old Hfe(-/-) mice do not have elevated levels of hepatic SFT transcripts relative to age-matched Hfe(+/+) mice, despite having 2.2- and 3.3-fold greater hepatic nonheme iron concentrations, respectively. Northern blot analyses of various mouse tissues revealed that SFT is widely expressed. The novel observation that SFT transcripts are abundant in brain prompted a comparison of SFT transcript levels and nonheme iron levels in the brains of Hfe(+/+) and Hfe(-/-) mice. Neither SFT mRNA levels nor nonheme iron levels differed between groups. Further comparisons of Hfe(-/-) and Hfe(+/+) mouse tissues revealed no significant differences in SFT mRNA levels in duodenum, the site of increased iron absorption in HH. Important distinctions between Hfe(-/-) mice and HH patients include not only differences in the relative rate and magnitude of iron loading but also the lack of fibrosis and phlebotomy treatment in the knockout animals.

Expression study of genes involved in iron metabolism in human tissues

Iron is required in all organisms for crucial functions, as a number of proteins need iron for activity. Mutations of the genes encoding proteins involved in iron uptake, transport, and utilization result in various human disorders or animal models with very different clinical presentations and organ involvement. However, little is known concerning the expression of iron metabolism genes in various human tissues and their eventual concerted regulation. We therefore examined the expression levels of various genes involved in iron uptake, reduction, and storage, in Fe-S protein biogenesis, in mitochondrial electron transport chain, plus the two SOD genes, in human adult tissues by Northern blot analysis. We observed that most of these genes were ubiquitously expressed, but that their transcript showed strongly different levels in the various tissues investigated denoting different mechanisms for iron utilization in various organs. However, surprisingly, no correlation could be made between expression pattern of these genes and the clinical presentation resulting in their mutations.

Intestinal metal ion absorption: an update

Recent progress in the field of metal ion transport has significantly advanced our understanding of the mechanisms of intestinal metal ion absorption under normal and pathological conditions. In this brief review, we focus on the key proteins involved in intestinal absorption of iron, zinc, and copper. Following the initial description of the apical iron transporter, DCT1, the basolateral transporter complex has been identified, which consists of the metal transporter IREG1/MTP1 and the multicopper oxidase, hephaestin. Novel zinc and copper transporters have been identified as well, mostly based on their homology to yeast and plants transporters. The identification of a variety of copper and zinc transporters is consistent with the importance of copper and zinc in a wide variety of enzymatic reactions, free radical scavenging, and transcriptional control.

The roles of iron in health and disease

Iron is vital for almost all living organisms by participating in a wide variety of metabolic processes, including oxygen transport, DNA synthesis, and electron transport. However, iron concentrations in body tissues must be tightly regulated because excessive iron leads to tissue damage, as a result of formation of free radicals. Disorders of iron metabolism are among the most common diseases of humans and encompass a broad spectrum of diseases with diverse clinical manifestations, ranging from anemia to iron overload and, possibly, to neurodegenerative diseases. The molecular understanding of iron regulation in the body is critical in identifying the underlying causes for each disease and in providing proper diagnosis and treatments. Recent advances in genetics, molecular biology and biochemistry of iron metabolism have assisted in elucidating the molecular mechanisms of iron homeostasis. The coordinate control of iron uptake and storage is tightly regulated by the feedback system of iron responsive element-containing gene products and iron regulatory proteins that modulate the expression levels of the genes involved in iron metabolism. Recent identification and characterization of the hemochromatosis protein HFE, the iron importer Nramp2, the iron exporter ferroportin1, and the second transferrin-binding and -transport protein transferrin receptor 2, have demonstrated their important roles in maintaining body's iron homeostasis. Functional studies of these gene products have expanded our knowledge at the molecular level about the pathways of iron metabolism and have provided valuable insight into the defects of iron metabolism disorders. In addition, a variety of animal models have implemented the identification of many genetic defects that lead to abnormal iron homeostasis and have provided crucial clinical information about the pathophysiology of iron disorders. In this review, we discuss the latest progress in studies of iron metabolism and our current understanding of the molecular mechanisms of iron absorption, transport, utilization, and storage. Finally, we will discuss the clinical presentations of iron metabolism disorders, including secondary iron disorders that are either associated with or the result of abnormal iron accumulation.

Functional and molecular responses of human intestinal Caco-2 cells to iron treatment

BACKGROUND

Divalent metal transporter 1 (DMT1), HFE, and stimulator of iron transport (SFT) are transmembrane proteins that have been implicated in the regulation of iron homeostasis.

OBJECTIVE

The objective of this study was to investigate whether absorption and transepithelial movement of iron correlated with gene expression of DMT1, HFE, and SFT in an experimental model of human absorptive enterocytes.

DESIGN

Caco-2 cells were exposed to iron-supplemented media in either the presence or the absence of serum for 24, 72, and 168 h. At each time point, the uptake and transepithelial movement of iron were examined and gene expression of DMT1, HFE, and SFT was measured. Manganese and zinc absorption was also examined at 168 h.

RESULTS

Iron treatment in the presence or absence of serum reduced the uptake and transepithelial movement of iron by approximately 50% after 72 and 168 h. No effect was observed at 24 h. The uptake and transepithelial movement of manganese were similar to those of iron at 168 h, whereas the effects on zinc were less pronounced. In the absence of serum, iron treatment was associated with a reduction of DMT1 expression by 50% at 72 and 168 h. HFE expression was dependent on serum, but iron treatment did not alter HFE expression. SFT expression was not affected by iron.

CONCLUSIONS

Iron treatment decreased cellular uptake of iron, manganese, and zinc, suggesting that these metals may utilize the same apical transporter. The transepithelial movement of iron and manganese, but not of zinc, was reduced across iron-treated Caco-2 cells, suggesting that iron and manganese are regulated by the same mechanism at the basolateral membrane. The gene expression of DMT1, HFE, and SFT did not fully correlate with the functional responses of Caco-2 cells. This may have been a result of posttranscriptional regulation of these genes or regulation of other genes involved in the uptake and transepithelial movement of iron in Caco-2 cells.

Iron absorption and transport-an update

Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intracellularly, iron is released from heme by heme oxygenase so that iron leaves the enterocyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a beta(3) integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from enterocytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Enterocytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake.

Regulation of mammalian iron homeostasis

Iron homeostasis is regulated with respect to uptake, storage and utilization. Newer work is presented that defines proteins responsible for iron transport, sequestration and sensing, and that addresses their regulation at the cellular and organismal levels by ambient iron concentrations, demand for erythropoiesis, body iron burden, and redox stimuli.

Genetic disorders affecting proteins of iron metabolism: clinical implications

Remarkable progress is being made in understanding the molecular basis of disorders of human iron metabolism. Recent work has uncovered unanticipated relationships with the immune and nervous systems, intricate interconnections with copper metabolism, and striking homologies between yeast and human genes involved in the transport of transition metals. This review examines the clinical consequences of new insights into the pathophysiology of genetic abnormalities affecting iron metabolism. The proteins recently found to be involved in the absorption, transport, utilization, and storage of iron are briefly described, and the clinical manifestations of genetic disorders that affect these proteins are discussed. This chapter considers the most common inherited disorder in individuals of European ancestry (hereditary hemochromatosis), a widespread disease in sub-Saharan populations for which the genetic basis is still uncertain (African dietary iron overload), and several less frequent or rare disorders (juvenile hemochromatosis, atransferrinemia, aceruloplasminemia, hyperferritinemia with autosomal dominant congenital cataract, Friedreich's ataxia, and X-linked sideroblastic anemia with ataxia).

Targeted gene disruption reveals an essential role for ceruloplasmin in cellular iron efflux

Aceruloplasminemia is an autosomal recessive disorder of iron metabolism. Affected individuals evidence iron accumulation in tissue parenchyma in association with absent serum ceruloplasmin. Genetic studies of such patients reveal inherited mutations in the ceruloplasmin gene. To elucidate the role of ceruloplasmin in iron homeostasis, we created an animal model of aceruloplasminemia by disrupting the murine ceruloplasmin (Cp) gene. Although normal at birth, Cp(-/-) mice demonstrate progressive accumulation of iron such that by one year of age all animals have a prominent elevation in serum ferritin and a 3- to 6-fold increase in the iron content of the liver and spleen. Histological analysis of affected tissues in these mice shows abundant iron stores within reticuloendothelial cells and hepatocytes. Ferrokinetic studies in Cp(+/+) and Cp(-/-) mice reveal equivalent rates of iron absorption and plasma iron turnover, suggesting that iron accumulation results from altered compartmentalization within the iron cycle. Consistent with this concept, Cp(-/-) mice showed no abnormalities in cellular iron uptake but a striking impairment in the movement of iron out of reticuloendothelial cells and hepatocytes. Our findings reveal an essential physiologic role for ceruloplasmin in determining the rate of iron efflux from cells with mobilizable iron stores.

Iron metabolism

The understanding of iron metabolism at the molecular level has been enormously expanded in recent years by new findings about the functioning of transferrin, the transferrin receptor and ferritin. Other recent developments include the discovery of the hemochromatosis gene HFE, identification of previously unknown proteins involved in iron transport, divalent metal transporter 1 and stimulator of Fe transport, and expanded insights into the regulation and expression of proteins involved in iron metabolism. Interactions among principal participants in iron transport have been uncovered, although the complexity of such interactions is still incompletely understood. Correlated efforts involving techniques and concepts of crystallography, spectroscopy and molecular biology applied to cellular processes have been, and should continue to be, particularly revealing.