Discovery of a cytosolic/soluble ferroxidase in rodent enterocytes

The biology of mammalian multi-copper ferroxidases

The mammalian multicopper ferroxidases (MCFs) ceruloplasmin (CP), hephaestin (HEPH) and zyklopen (ZP) comprise a family of conserved enzymes that are essential for body iron homeostasis. Each of these enzymes contains six biosynthetically incorporated copper atoms which act as intermediate electron acceptors, and the oxidation of iron is associated with the four electron reduction of dioxygen to generate two water molecules. CP occurs in both a secreted and GPI-linked (membrane-bound) form, while HEPH and ZP each contain a single C-terminal transmembrane domain. These enzymes function to ensure the efficient oxidation of iron so that it can be effectively released from tissues via the iron export protein ferroportin and subsequently bound to the iron carrier protein transferrin in the blood. CP is particularly important in facilitating iron release from the liver and central nervous system, HEPH is the major MCF in the small intestine and is critical for dietary iron absorption, and ZP is important for normal hair development. CP and HEPH (and possibly ZP) function in multiple tissues. These proteins also play other (non-iron-related) physiological roles, but many of these are ill-defined. In addition to disrupting iron homeostasis, MCF dysfunction perturbs neurological and immune function, alters cancer susceptibility, and causes hair loss, but, despite their importance, how MCFs co-ordinately maintain body iron homeostasis and perform other functions remains incompletely understood.

Expression of soluble, active, fluorescently tagged hephaestin in COS and CHO cell lines

Hephaestin (Hp) is a trans-membrane protein, which plays a critical role in intestinal iron absorption. Hp was originally identified as the gene responsible for the phenotype of sex-linked anaemia in the sla mouse. The mutation in the sla protein causes accumulation of dietary iron in duodenal cells, causing severe microcytic hypochromic anaemia. Although mucosal uptake of dietary iron is normal, export from the duodenum is inhibited. Hp is homologous to ceruloplasmin (Cp), a member of the family of multi copper ferroxidases (MCFs) and possesses ferroxidase activity that facilitates iron release from the duodenum and load onto the serum iron transport protein transferrin. In the present study, attempts were made to produce biologically active recombinant mouse hephaestin as a secretory form tagged with green fluorescent protein (GFP), Hpsec-GFP. Plasmid expressing Hpsec-GFP was constructed and transfected into COS and CHO cells. The GFP aided the monitoring expression in real time to select the best conditions to maximise expression and provided a tag for purifying and analysing Hpsec-GFP. The protein had detectable oxidase activity as shown by in-gel and solution-based assays. The methods described here can provide the basis for further work to probe the interaction of hephaestin with other proteins using complementary fluorescent tags on target proteins that would facilitate the fluorescence resonance energy transfer measurements, for example with transferrin or colocalisation studies, and help to discover more about hephaestin works at the molecular level.

Intersection of Iron and Copper Metabolism in the Mammalian Intestine and Liver

Iron and copper have similar physiochemical properties; thus, physiologically relevant interactions seem likely. Indeed, points of intersection between these two essential trace minerals have been recognized for many decades, but mechanistic details have been lacking. Investigations in recent years have revealed that copper may positively influence iron homeostasis, and also that iron may antagonize copper metabolism. For example, when body iron stores are low, copper is apparently redistributed to tissues important for regulating iron balance, including enterocytes of upper small bowel, the liver, and blood. Copper in enterocytes may positively influence iron transport, and hepatic copper may enhance biosynthesis of a circulating ferroxidase, ceruloplasmin, which potentiates iron release from stores. Moreover, many intestinal genes related to iron absorption are transactivated by a hypoxia-inducible transcription factor, hypoxia-inducible factor-2α (HIF2α), during iron deficiency. Interestingly, copper influences the DNA-binding activity of the HIF factors, thus further exemplifying how copper may modulate intestinal iron homeostasis. Copper may also alter the activity of the iron-regulatory hormone hepcidin. Furthermore, copper depletion has been noted in iron-loading disorders, such as hereditary hemochromatosis. Copper depletion may also be caused by high-dose iron supplementation, raising concerns particularly in pregnancy when iron supplementation is widely recommended. This review will cover the basic physiology of intestinal iron and copper absorption as well as the metabolism of these minerals in the liver. Also considered in detail will be current experimental work in this field, with a focus on molecular aspects of intestinal and hepatic iron-copper interplay and how this relates to various disease states. © 2018 American Physiological Society. Compr Physiol 8:1433-1461, 2018.

Copper supplementation reverses dietary iron overload-induced pathologies in mice

Dietary iron overload in rodents impairs growth and causes cardiac hypertrophy, serum and tissue copper depletion, depression of serum ceruloplasmin (Cp) activity and anemia. Notably, increasing dietary copper content to ~25-fold above requirements prevents the development of these physiological perturbations. Whether copper supplementation can reverse these high-iron-related abnormalities has, however, not been established. The current investigation was thus undertaken to test the hypothesis that supplemental copper will mitigate negative outcomes associated with dietary iron loading. Weanling mice were thus fed AIN-93G-based diets with high (>100-fold in excess) or adequate (~80 ppm) iron content. To establish the optimal experimental conditions, we first defined the time course of iron loading, and assessed the impact of supplemental copper (provided in drinking water) on the development of high-iron-related pathologies. Copper supplementation (20 mg/L) for the last 3 weeks of a 7-week high-iron feeding period reversed the anemia, normalized serum copper levels and Cp activity, and restored tissue copper concentrations. Growth rates, cardiac copper concentrations and heart size, however, were only partially normalized by copper supplementation. Furthermore, high dietary iron intake reduced intestinal ⁶⁴Cu absorption (~60%) from a transport solution provided to mice by oral, intragastric gavage. Copper supplementation of iron-loaded mice enhanced intestinal ⁶⁴Cu transport, thus allowing sufficient assimilation of dietary copper to correct many of the noted high-iron-related physiological perturbations. We therefore conclude that high- iron intake increases the requirement for dietary copper (to overcome the inhibition of intestinal copper absorption).

Knockdown of copper-transporting ATPase 1 (Atp7a) impairs iron flux in fully-differentiated rat (IEC-6) and human (Caco-2) intestinal epithelial cells

Intestinal iron absorption is highly regulated since no mechanism for iron excretion exists. We previously demonstrated that expression of an intestinal copper transporter (Atp7a) increases in parallel with genes encoding iron transporters in the rat duodenal epithelium during iron deprivation (Am. J. Physiol.: Gastrointest. Liver Physiol., 2005, 288, G964-G971). This led us to postulate that Atp7a may influence intestinal iron flux. Therefore, to test the hypothesis that Atp7a is required for optimal iron transport, we silenced Atp7a in rat IEC-6 and human Caco-2 cells. Iron transport was subsequently quantified in fully-differentiated cells plated on collagen-coated, transwell inserts. Interestingly, ⁵⁹Fe uptake and efflux were impaired in both cell lines by Atp7a silencing. Concurrent changes in the expression of key iron transport-related genes were also noted in IEC-6 cells. Expression of Dmt1 (the iron importer), Dcytb (an apical membrane ferrireductase) and Fpn1 (the iron exporter) was decreased in Atp7a knockdown (KD) cells. Paradoxically, cell-surface ferrireductase activity increased (>5-fold) in Atp7a KD cells despite decreased Dcytb mRNA expression. Moreover, increased expression (>10-fold) of hephaestin (an iron oxidase involved in iron efflux) was associated with increased ferroxidase activity in KD cells. Increases in ferrireductase and ferroxidase activity may be compensatory responses to increase iron flux. In summary, in these reductionist models of the mammalian intestinal epithelium, Atp7a KD altered expression of iron transporters and impaired iron flux. Since Atp7a is a copper transporter, it is a logical supposition that perturbations in intracellular copper homeostasis underlie the noted biologic changes in these cell lines.

Consumption of a High-Iron Diet Disrupts Homeostatic Regulation of Intestinal Copper Absorption in Adolescent Mice

High-iron feeding of rodents has been commonly used to model human iron-overload disorders. We recently noted that high-iron consumption impaired growth and caused severe systemic copper deficiency in growing rats, but the mechanism by which this occurred could not be determined due to technical limitations. In the current investigation, we thus utilized mice; first to determine if the same phenomenon occurred in another mammalian species, and secondly since we could assess in vivo copper absorption in mice. We hypothesized that excessive dietary iron impaired intestinal copper absorption. Weanling, male mice were thus fed AIN-93G-based diets containing high (HFe) (~8800 ppm) or adequate (AdFe) (~80 ppm) iron in combination with low (~0.9 ppm), adequate (~9 ppm) or high (~180 ppm) copper for several weeks. Iron and copper homeostasis was subsequently assessed. Mice consuming the HFe diets grew slower, were anemic, and had lower hepatic copper levels and serum ceruloplasmin activity. These physiologic perturbations were all prevented by higher dietary copper, demonstrating that copper depletion was the underlying cause. Furthermore, homeostatic regulation of copper absorption was noted in the mice consuming the AdFe diets, with absorption increasing as dietary copper decreased. HFe-fed mice did not have impaired copper absorption (disproving our hypothesis), but homeostatic control of absorption was disrupted. There were also noted perturbations in the tissue distribution of copper in the HFe-fed mice, suggesting that altered storage and thus bioavailability contributed to the noted copper deficiency. Dietary iron loading thus antagonizes copper homeostasis leading to pathological symptoms of severe copper depletion.

Large scale expression and purification of secreted mouse hephaestin

Hephaestin is a large membrane-anchored multicopper ferroxidase involved in mammalian iron metabolism. Newly absorbed dietary iron is exported across the enterocyte basolateral membrane by the ferrous iron transporter ferroportin, but hephaestin increases the efficiency of this process by oxidizing the transported iron to its ferric form and promoting its release from ferroportin. Deletion or mutation of the hephaestin gene leads to systemic anemia with iron accumulation in the intestinal epithelium. The crystal structure of human ceruloplasmin, another multicopper ferroxidase with 50% sequence identity to hephaestin, has provided a framework for comparative analysis and modelling. However, detailed structural information for hephaestin is still absent, leaving questions relating to metal coordination and binding sites unanswered. To obtain structural information for hephaestin, a reliable protocol for large-scale purification is required. Here, we present an expression and purification protocol of soluble mouse hephaestin, yielding milligram amounts of enzymatically active, purified protein using the baculovirus/insect cell system.

Intestinal hephaestin potentiates iron absorption in weanling, adult, and pregnant mice under physiological conditions

Regulation of intestinal iron absorption is crucial to maintain body iron levels because humans have no regulated iron-excretory system. Elucidating molecular events that mediate intestinal iron transport is thus important for the development of therapeutic approaches to modify iron absorption in pathological states. The process of iron uptake into duodenal enterocytes is relatively well understood, but less is known about the functional coupling between the iron exporter ferroportin 1 and the basolateral membrane iron oxidase hephaestin (Heph). Initial characterization of intestine-specific Heph knockout (Heph^int) mice demonstrated that adult male mice were mildly iron deficient; however, the specific role of intestinal Heph has not been determined in weanling mice, in female mice, or during physiological states which stimulate iron absorption. Furthermore, because ferroportin 1-mediated iron export from some tissues (eg, liver) is impaired in the absence of the Heph homolog, ceruloplasmin, we hypothesized that Heph is rate limiting for intestinal iron absorption, especially when iron demands increase. Our experimental approach was to assess various physiological parameters and iron (⁵⁹Fe) absorption and tissue distribution in weanling, adult, and pregnant Heph^int mice (and controls) under physiological conditions and in adult Heph^int mice after dietary iron deprivation or acute hemolysis. Results demonstrate that intestinal Heph is essential for optimal iron transport in weanlings and adults of both sexes and during pregnancy, but not in adult mice with iron-deficiency or hemolytic anemia. Moreover, activation of unidentified, intestinal ferroxidases was noted, which may explain why intestinal Heph is not always required for optimal iron absorption.

Mechanistic and regulatory aspects of intestinal iron absorption

Iron is an essential trace mineral that plays a number of important physiological roles in humans, including oxygen transport, energy metabolism, and neurotransmitter synthesis. Iron absorption by the proximal small bowel is a critical checkpoint in the maintenance of whole-body iron levels since, unlike most other essential nutrients, no regulated excretory systems exist for iron in humans. Maintaining proper iron levels is critical to avoid the adverse physiological consequences of either low or high tissue iron concentrations, as commonly occurs in iron-deficiency anemia and hereditary hemochromatosis, respectively. Exquisite regulatory mechanisms have thus evolved to modulate how much iron is acquired from the diet. Systemic sensing of iron levels is accomplished by a network of molecules that regulate transcription of the HAMP gene in hepatocytes, thus modulating levels of the serum-borne, iron-regulatory hormone hepcidin. Hepcidin decreases intestinal iron absorption by binding to the iron exporter ferroportin 1 on the basolateral surface of duodenal enterocytes, causing its internalization and degradation. Mucosal regulation of iron transport also occurs during low-iron states, via transcriptional (by hypoxia-inducible factor 2α) and posttranscriptional (by the iron-sensing iron-regulatory protein/iron-responsive element system) mechanisms. Recent studies demonstrated that these regulatory loops function in tandem to control expression or activity of key modulators of iron homeostasis. In health, body iron levels are maintained at appropriate levels; however, in several inherited disorders and in other pathophysiological states, iron sensing is perturbed and intestinal iron absorption is dysregulated. The iron-related phenotypes of these diseases exemplify the necessity of precisely regulating iron absorption to meet body demands.

The multicopper ferroxidase hephaestin enhances intestinal iron absorption in mice

Hephaestin is a vertebrate multicopper ferroxidase important for the transfer of dietary iron from intestinal cells to the blood. Hephaestin is mutated in the sex-linked anemia mouse, resulting in iron deficiency. However, sex-linked anemia mice still retain some hephaestin ferroxidase activity. They survive, breed, and their anemia improves with age. To gain a better understanding of the role of hephaestin in iron homeostasis, we used the Cre-lox system to generate knockout mouse models with whole body or intestine-specific (Villin promoter) ablation of hephaestin. Both types of mice were viable, indicating that hephaestin is not essential and that other mechanisms, multicopper ferroxidase-dependent or not, must compensate for hephaestin deficiency. The knockout strains, however, both developed a microcytic, hypochromic anemia, suggesting severe iron deficiency and confirming that hephaestin plays an important role in body iron acquisition. Consistent with this, the knockout mice accumulated iron in duodenal enterocytes and had reduced intestinal iron absorption. In addition, the similarities of the phenotypes of the whole body and intestine-specific hephaestin knockout mice clarify the important role of hephaestin specifically in intestinal enterocytes in maintaining whole body iron homeostasis. These mouse models will serve as valuable tools to study the role of hephaestin and associated proteins in iron transport in the small intestine and other tissues.

Molecular mediators governing iron-copper interactions

Given their similar physiochemical properties, it is a logical postulate that iron and copper metabolism are intertwined. Indeed, iron-copper interactions were first documented over a century ago, but the homeostatic effects of one on the other has not been elucidated at a molecular level to date. Recent experimental work has, however, begun to provide mechanistic insight into how copper influences iron metabolism. During iron deficiency, elevated copper levels are observed in the intestinal mucosa, liver, and blood. Copper accumulation and/or redistribution within enterocytes may influence iron transport, and high hepatic copper may enhance biosynthesis of a circulating ferroxidase, which potentiates iron release from stores. Moreover, emerging evidence has documented direct effects of copper on the expression and activity of the iron-regulatory hormone hepcidin. This review summarizes current experimental work in this field, with a focus on molecular aspects of iron-copper interplay and how these interactions relate to various disease states.

Silencing the Menkes copper-transporting ATPase (Atp7a) gene in rat intestinal epithelial (IEC-6) cells increases iron flux via transcriptional induction of ferroportin 1 (Fpn1)

The Menkes copper-transporting ATPase (Atp7a) gene is induced in rat duodenum during iron deficiency, consistent with copper accumulation in the intestinal mucosa and liver. To test the hypothesis that ATP7A influences intestinal iron metabolism, the Atp7a gene was silenced in rat intestinal epithelial (IEC-6) cells using short hairpin RNA (shRNA) technology. Perturbations in intracellular copper homeostasis were noted in knockdown cells, consistent with the dual roles of ATP7A in pumping copper into the trans-Golgi (for cuproenzyme synthesis) and exporting copper from cells. Intracellular iron concentrations were unaffected by Atp7a knockdown. Unexpectedly, however, vectorial iron ((59)Fe) transport increased (∼33%) in knockdown cells grown in bicameral inserts and increased further (∼70%) by iron deprivation (compared with negative control shRNA-transfected cells). Additional experiments were designed to elucidate the molecular mechanism of increased transepithelial iron flux. Enhanced iron uptake by knockdown cells was associated with increased expression of a ferrireductase (duodenal cytochrome b) and activity of a cell-surface ferrireductase. Increased iron efflux from knockdown cells was likely mediated via transcriptional activation of the ferroportin 1 gene (by an unknown mechanism). Moreover, Atp7a knockdown significantly attenuated expression of an iron oxidase [hephaestin (HEPH); by ∼80%] and membrane ferroxidase activity (by ∼50%). Cytosolic ferroxidase activity, however, was retained in knockdown cells (75% of control cells), perhaps compensating for diminished HEPH activity. This investigation has thus documented alterations in iron homeostasis associated with Atp7a knockdown in enterocyte-like cells. Alterations in copper transport, trafficking, or distribution may underlie the increase in transepithelial iron flux noted when ATP7A activity is diminished.

Copper deficiency leads to anemia, duodenal hypoxia, upregulation of HIF-2α and altered expression of iron absorption genes in mice

Iron and copper are essential trace metals, actively absorbed from the proximal gut in a regulated fashion. Depletion of either metal can lead to anemia. In the gut, copper deficiency can affect iron absorption through modulating the activity of hephaestin - a multi-copper oxidase required for optimal iron export from enterocytes. How systemic copper status regulates iron absorption is unknown. Mice were subjected to a nutritional copper deficiency-induced anemia regime from birth and injected with copper sulphate intraperitoneally to correct the anemia. Copper deficiency resulted in anemia, increased duodenal hypoxia and Hypoxia inducible factor 2α (HIF-2α) levels, a regulator of iron absorption. HIF-2α upregulation in copper deficiency appeared to be independent of duodenal iron or copper levels and correlated with the expression of iron transporters (Ferroportin - Fpn, Divalent Metal transporter - Dmt1) and ferric reductase - Dcytb. Alleviation of copper-dependent anemia with intraperitoneal copper injection resulted in down regulation of HIF-2α-regulated iron absorption genes in the gut. Our work identifies HIF-2α as an important regulator of iron transport machinery in copper deficiency.

Multiple Menkes copper ATPase (Atp7a) transcript and protein variants are induced by iron deficiency in rat duodenal enterocytes

The Menkes copper ATPase (Atp7a) pumps copper into the trans-Golgi for cuproenzyme synthesis, and translocates to the basolateral membrane of enterocytes for copper export. Recent studies demonstrated that three 5' end splice variants of the Atp7a transcript exist in rat duodenum, all of which are strongly induced during iron deprivation. To explore a possible role for Atp7a (and copper) in intestinal iron absorption, the current studies were undertaken to test the hypothesis that multiple Atp7a transcript and protein variants exist in intestinal epithelial cells. Northern blot analyses using probes generated from the full-length Atp7a cDNA revealed several specific hybridization bands, all of which were more intense in RNA samples extracted from duodenal enterocytes isolated from iron-deficient rats. A PCR-based approach, using forward primers specific for the alternative 5' end splice variants and a reverse primer in exon 23, demonstrated that 3 full-length transcripts exist in rat IEC-6 cells. To identify possible Atp7a protein variants, three distinct polyclonal antisera were utilized. The specificity of the antisera was first established by western blotting and immunoprecipitation studies using samples derived from isolated rat enterocytes and Atp7a knockdown IEC-6 cells. Several specific immunoreactive bands were documented, and a unique Atp7a protein distribution in cytosolic vesicle-like structures was noted. In conclusion, multiple Atp7a transcript and protein variants exist in rodent intestinal epithelial cells and are induced by dietary iron deprivation. Further studies will be designed to determine the subcellular distribution of Atp7a protein variants and possible unique functions of each.

Immunoreactive hephaestin and ferroxidase activity are present in the cytosolic fraction of rat enterocytes

Discovered over a decade ago, hephaestin (Heph) has been implicated as a ferroxidase (FOX) vital for intestinal iron absorption. Stringent structural or kinetic data derived from purified, native protein is however lacking, leading to the hypothesis that an alternate, undiscovered form of Heph could exist in mammalian enterocytes. This possibility was tested using laboratory rodent and cell culture models. Cytosolic and membrane fractions were obtained from rat enterocytes and purity of the fractions was assessed. Western blot analyses revealed Heph in cytosol obtained by three different methods, ruling out the possibility of a method-induced artifact being the major contributor to this observation. Absence of two different membrane-proteins, ferroportin 1 and Menke's copper ATPase in cytosol, and the absence of lipids in representative cytosolic samples tested by thin layer chromatography, eliminated significant membrane contamination of cytosol. Further, immunohisto- and immunocyto-chemical analyses identified Heph in rat enterocytes and in two intestinal epithelial cell lines, IEC-6 and Caco-2, intracellularly. Additionally, cytosolic Heph increased upon iron-deprivation but more important, decreased significantly upon copper-deprivation, mimicking the response of membrane-bound Heph. Moreover, FOX activity was present in rat cytosol, and was partly inhibited by anti-Heph antibody. Finally, lack of immunodetectable ceruloplasmin (Cp) by western blot precluded Cp as an underlying cause of this activity. These data demonstrate that rat enterocytes contain a soluble/cytosolic form of Heph possibly contributing to the observed FOX activity.