Human hephaestin expression is not limited to enterocytes of the gastrointestinal tract but is also found in the antrum, the enteric nervous system, and pancreatic {beta}-cells

Induction of Hepcidin Expression in the Renal Cortex of Sickle Cell Disease Mice

In patients with sickle cell disease (SCD), chronic hemolysis and frequent blood transfusions cause iron overload and accumulation in the kidneys. The iron deposition is found in the renal cortex and correlates with the severity of hemolysis. In this study, we observed a significant accumulation of iron in the renal cortex of a mouse model of SCD, and assessed the expression of the proteins involved in maintaining renal iron homeostasis. Despite the intracellular iron accumulation, the levels of the transferrin receptor in the kidneys were increased, but the levels of the iron exporter ferroportin were not altered in SCD mice. Ferroportin is regulated by hepcidin, which binds to it and promotes its degradation. We found reduced serum hepcidin levels but increased renal hepcidin production in SCD mice. Furthermore, we observed significant macrophage infiltration and increased expression of intercellular adhesion molecule 1 in the endothelial cells of the kidneys in SCD mice. These observations correlated with elevated levels of proinflammatory cytokines IL-1β and IL-6, which can potentially stimulate hepcidin expression. Taken together, our results demonstrate that in individuals with SCD, a renal inflammation state induces renal hepcidin production that blocks the upregulation of ferroportin levels, resulting in dysregulation of iron homeostasis in the kidney and iron deposition in the renal cortex.

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.

Lactoferrin: from the structure to the functional orchestration of iron homeostasis

Iron is by far the most widespread and essential transition metal, possessing crucial biological functions for living systems. Despite chemical advantages, iron biology has forced organisms to face with some issues: ferric iron insolubility and ferrous-driven formation of toxic radicals. For these reasons, acquisition and transport of iron constitutes a formidable challenge for cells and organisms, which need to maintain adequate iron concentrations within a narrow range, allowing biological processes without triggering toxic effects. Higher organisms have evolved extracellular carrier proteins to acquire, transport and manage iron. In recent years, a renewed interest in iron biology has highlighted the role of iron-proteins dysregulation in the onset and/or exacerbation of different pathological conditions. However, to date, no resolutive therapy for iron disorders has been found. In this review, we outline the efficacy of Lactoferrin, a member of the transferrin family mainly secreted by exocrine glands and neutrophils, as a new emerging orchestrator of iron metabolism and homeostasis, able to counteract iron disorders associated to different pathologies, including iron deficiency and anemia of inflammation in blood, Parkinson and Alzheimer diseases in the brain and cystic fibrosis in the lung.

Impact of maternal iron deficiency anaemia on the expression of the newly discovered multi-copper ferroxidase, Zyklopen, in term placentas

In the present study, we investigated the effect of maternal iron deficiency anaemia (IDA) on expression of the newly discovered iron transporter, Zyklopen in term placenta, in 200 pregnant women. Placental expression of Zyklopen was studied by mRNA analysis and immunohistochemistry for the protein. In addition neonatal anthropometric parameters were also analysed. 58.8% of 200 subjects were anaemic. Both Zyklopen mRNA as well as protein expression in the placenta showed a statistically significant increase with increasing severity of anaemia. Although all the neonatal anthropometric parameters were lower in newborns of anaemic mothers, none showed any statistical significance. Zp mRNA levels did not show any significant correlation with newborn and placental parameters (except newborn skinfold thickness and head circumference). Similar to mRNA expression, Zp IHC expression correlated positively, albiet non-significantly, with newborn length and Hb levels, the correlation was however negative with birth weight, head circumference, mid-arm circumference unlike the mRNA expression, where it positively correlated with the above parameters. Our study for the first time demonstrated a definite increase in expression of Zyklopen at both mRNA and protein levels in term placenta, in maternal IDA.IMPACT STATEMENTWhat is already known on this subject? Iron deficiency anaemia (IDA) in a pregnant mother can lead to anaemia in the developing foetus; which is frequently observed to be of lesser severity than that in the mother. Recently a copper-containing oxidase called Zyklopen was discovered which was involved in iron efflux in BeWo cells. The gene encoding Zyklopen has been identified with a putative C-terminal membrane-spanning sequence and high sequence identitical to hephaestin (Heph) and ceruloplasmin (Cp), the other known vertebrate multicopper ferroxidase (MCF). Protein expression of this new MCF was observed in multiple diverse mouse tissues, including placenta and mammary gland.What do the results of this study add? Zyklopen protein immunohistochemical expression showed a statistically significant increase with increasing severity of anaemia. Similarly, placental mRNA expression of the Zyklopen gene was observed to be higher in anaemic mothers when compared to non-anaemic mothers. Our study for the first time demonstrated a definite increase in expression of Zyklopen at both protein and mRNA levels in term placenta, in maternal IDA.What are the implications of these findings for clinical practice and/or further research? This study will help us to understand better, the increased potential for influx of iron from mother to foetus in the condition of maternal iron deficiency. This study will help to determine how placental iron transport proteins can be regulated in response to maternal and neonatal iron status and will further our existing knowledge on relationships between maternal and neonatal iron status and mechanisms by which placental iron transport is modified in relation to these parameters.

Role of endolysosome function in iron metabolism and brain carcinogenesis

Iron, the most abundant metal in human brain, is an essential microelement that regulates numerous cellular mechanisms. Some key physiological roles of iron include oxidative phosphorylation and ATP production, embryonic neuronal development, formation of iron-sulfur clusters, and the regulation of enzymes involved in DNA synthesis and repair. Because of its physiological and pathological importance, iron homeostasis must be tightly regulated by balancing its uptake, transport, and storage. Endosomes and lysosomes (endolysosomes) are acidic organelles known to contain readily releasable stores of various cations including iron and other metals. Increased levels of ferrous (Fe2+) iron can generate reactive oxygen species (ROS) via Fenton chemistry reactions and these increases can damage mitochondria and genomic DNA as well as promote carcinogenesis. Accumulation of iron in the brain has been linked with aging, diet, disease, and cerebral hemorrhage. Further, deregulation of brain iron metabolism has been implicated in carcinogenesis and may be a contributing factor to the increased incidence of brain tumors around the world. Here, we provide insight into mechanisms by which iron accumulation in endolysosomes is altered by pH and lysosome membrane permeabilization. Such events generate excess ROS resulting in mitochondrial DNA damage, fission, and dysfunction, as well as DNA oxidative damage in the nucleus; all of which promote carcinogenesis. A better understanding of the roles that endolysosome iron plays in carcinogenesis may help better inform the development of strategic therapeutic options for cancer treatment and prevention.

Iron Metabolism in Pancreatic Beta-Cell Function and Dysfunction

Iron is an essential element involved in a variety of physiological functions. In the pancreatic beta-cells, being part of Fe-S cluster proteins, it is necessary for the correct insulin synthesis and processing. In the mitochondria, as a component of the respiratory chain, it allows the production of ATP and reactive oxygen species (ROS) that trigger beta-cell depolarization and potentiate the calcium-dependent insulin release. Iron cellular content must be finely tuned to ensure the normal supply but also to prevent overloading. Indeed, due to the high reactivity with oxygen and the formation of free radicals, iron excess may cause oxidative damage of cells that are extremely vulnerable to this condition because the normal elevated ROS production and the paucity in antioxidant enzyme activities. The aim of the present review is to provide insights into the mechanisms responsible for iron homeostasis in beta-cells, describing how alteration of these processes has been related to beta-cell damage and failure. Defects in iron-storing or -chaperoning proteins have been detected in diabetic conditions; therefore, the control of iron metabolism in these cells deserves further investigation as a promising target for the development of new disease treatments.

The Ferroxidase Hephaestin in Lung Cancer: Pathological Significance and Prognostic Value

Hephaestin (HEPH) belongs to a group of exocytoplasmic ferroxidases which contribute to cellular iron homeostasis by favouring its export. Down-regulation of HEPH expression, possibly by stimulating cell proliferation due to an increase in iron availability, has shown to correlate with poor survival in breast cancer. The lung is particularly sensitive to iron-induced oxidative stress, given the high oxygen tension present, however, HEPH distribution in lung cancer and its influence on prognosis have not been investigated yet. In this study we explored the prognostic value of HEPH and its expression pattern in the most prevalent histotypes of lung cancers, namely lung adenocarcinoma and lung squamous cell carcinoma. In silico analyses, based on UALCAN, Gene Expression Profiling Interactive Analysis (GEPIA) and Kaplan–Meier plotter bioinformatics, revealed a significant correlation between higher levels of HEPH expression and favorable prognosis, in both cancer histotypes. Moreover, TIMER web platform showed a statistically significant association between HEPH expression and cell elements belonging to the tumor microenvironment identified as endothelial cells and a subpopulation of cancer-associated fibroblasts, further confirmed by double immunohistochemical labeling with cell type specific markers. Taken together, these data shed a light on the complex mechanisms of local iron handling lung cancer can exploit to support tumorigenesis.

Copper nutrition and biochemistry and human (patho)physiology

The essential trace mineral copper plays important roles in human physiology and pathophysiology. Disruption of copper homeostasis may underlie the development of ischemic heart disease, and connective tissue and neurodegenerative disorders. Copper also likely participates in the host response to bacterial infection and is further implicated more broadly in regulating immunity. Recent studies further associate copper with disruption of lipid homeostasis, as is frequently seen in, for example, non-alcoholic fatty liver disease (NAFLD). Moreover, continuing investigation of copper chaperones has revealed new roles for these intracellular copper-binding proteins. Despite these (and many other) significant advances, many questions related to copper biology remain unanswered. For example, what are the most sensitive and specific biomarkers of copper status, and which ones are useful in marginal (or "sub-clinical" copper deficiency)? Further research on this topic is required to inform future investigations of copper metabolism in humans (so the copper status of study participants can be fully appreciated). Also, are current recommendations for copper intake adequate? Recent studies suggest that overt copper deficiency is more common than once thought, and further, some have suggested that the copper RDAs for adults may be too low. Additional human balance and interventional studies are necessary and could provide the impetus for reconsidering the copper RDAs in the future. These and myriad other unresolved aspects of copper nutrition will undoubtedly be the focus of future investigation.

ATP7A-Regulated Enzyme Metalation and Trafficking in the Menkes Disease Puzzle

Copper is vital for numerous cellular functions affecting all tissues and organ systems in the body. The copper pump, ATP7A is critical for whole-body, cellular, and subcellular copper homeostasis, and dysfunction due to genetic defects results in Menkes disease. ATP7A dysfunction leads to copper deficiency in nervous tissue, liver, and blood but accumulation in other tissues. Site-specific cellular deficiencies of copper lead to loss of function of copper-dependent enzymes in all tissues, and the range of Menkes disease pathologies observed can now be explained in full by lack of specific copper enzymes. New pathways involving copper activated lysosomal and steroid sulfatases link patient symptoms usually related to other inborn errors of metabolism to Menkes disease. Additionally, new roles for lysyl oxidase in activation of molecules necessary for the innate immune system, and novel adapter molecules that play roles in ERGIC trafficking of brain receptors and other proteins, are emerging. We here summarize the current knowledge of the roles of copper enzyme function in Menkes disease, with a focus on ATP7A-mediated enzyme metalation in the secretory pathway. By establishing mechanistic relationships between copper-dependent cellular processes and Menkes disease symptoms in patients will not only increase understanding of copper biology but will also allow for the identification of an expanding range of copper-dependent enzymes and pathways. This will raise awareness of rare patient symptoms, and thus aid in early diagnosis of Menkes disease patients.

Vitamin D Rescues Pancreatic β Cell Dysfunction due to Iron Overload via Elevation of the Vitamin D Receptor and Maintenance of Ca2+ Homeostasis

SCOPE

Accumulating evidence indicates that micronutrients are related to metabolic diseases. However, comparatively less attention has been devoted to their influence on each other during the development of metabolic diseases. To investigate the underlying mechanisms, the effects of iron and vitamin D on pancreatic β cell functions are examined.

METHODS AND RESULTS

Iron overload is induced in INS-1 rat insulinoma pancreatic β cells and it is found that iron overload dramatically reduce expression of the vitamin D receptor (VDR). Iron overload-induced β cell dysfunction is rescued by 1,25-dihydroxyvitamin D3 (1,25(OH)2 D3 ) cotreatment via restoration of VDR level and the consequent maintenance of Ca2+ homeostasis. Iron accumulation is also observed in the islets of 22-month-old C57BL/6 mice fed with a chow diet (1000 IU vitamin D3 per kg). In contrast, islet iron accumulation and hyperinsulinemia are ameliorated in mice fed with a vitamin D3 -supplemented diet (20 000 IU kg-1 ).

CONCLUSION

The authors show that functional failure of β cells due to iron accumulation is rescued by 1,25(OH)2 D3 , and iron overload significantly reduces VDR levels in β cells. These results suggest that iron and vitamin D inversely influence pancreatic β cell function.

The response of glandular gastric transcriptome to T-2 toxin in chicks

This study was conducted to determine the effect of T-2 toxin on the transcriptome of the glandular stomach in chicks using RNA-sequencing (RNA-Seq). Four groups of 1-day-old Cobb male broilers (n = 4 cages/group, 6 chicks/cage) were fed a corn-soybean-based diet (control) and control supplemented with T-2 toxin at 1.0, 3.0, and 6.0 mg/kg, respectively, for 2 weeks. The histological results showed that dietary supplementation of T-2 toxin at 3.0 and 6.0 mg/kg induced glandular gastric injury including serious inflammation, increased inflammatory cells, mucosal edema, and necrosis and desquamation of the epithelial cells in the glandular stomach of chicks. RNA-Seq analysis revealed that there were 671, 1393, and 1394 genes displayed ≥2 (P < 0.05) differential expression in the dietary supplemental T-2 toxin at 1.0, 3.0, and 6.0 mg/kg, respectively, compared with the control group. Notably, 204 differently expressed genes had shared similar changes among these three doses of T-2 toxin. GO and KEGG pathway analysis results showed that many genes involved in oxidation-reduction process, inflammation, wound healing/bleeding, and apoptosis/carcinogenesis were affected by T-2 toxin exposure. In conclusion, this study systematically elucidated toxic mechanisms of T-2 toxin on the glandular stomach, which might provide novel ideas to prevent adverse effects of T-2 toxin in chicks.

Brain iron transport

Brain iron is a crucial participant and regulator of normal physiological activity. However, excess iron is involved in the formation of free radicals, and has been associated with oxidative damage to neuronal and other brain cells. Abnormally high brain iron levels have been observed in various neurodegenerative diseases, including neurodegeneration with brain iron accumulation, Alzheimer's disease, Parkinson's disease and Huntington's disease. However, the key question of why iron levels increase in the relevant regions of the brain remains to be answered. A full understanding of the homeostatic mechanisms involved in brain iron transport and metabolism is therefore critical not only for elucidating the pathophysiological mechanisms responsible for excess iron accumulation in the brain but also for developing pharmacological interventions to disrupt the chain of pathological events occurring in these neurodegenerative diseases. Numerous studies have been conducted, but to date no effort to synthesize these studies and ideas into a systematic and coherent summary has been made, especially concerning iron transport across the luminal (apical) membrane of the capillary endothelium and the membranes of different brain cell types. Herein, we review key findings on brain iron transport, highlighting the mechanisms involved in iron transport across the luminal (apical) as well as the abluminal (basal) membrane of the blood-brain barrier, the blood-cerebrospinal fluid barrier, and iron uptake and release in neurons, oligodendrocytes, astrocytes and microglia within the brain. We offer suggestions for addressing the many important gaps in our understanding of this important topic, and provide new insights into the potential causes of abnormally increased iron levels in regions of the brain in neurodegenerative disorders.

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.

Organ‑specific expression of the divalent ion channel proteins NCKX3, TRPV2, CTR1, ATP7A, IREG1 and HEPH in various canine organs

Transmembrane cation channels include those for calcium, copper and iron ion transport. Each channel has physiological significance, and all have been associated with disease. However, the comparative study of transcriptional‑translational levels in canine organs has not been previously reported. In the present study, organ‑specific expression of calcium channels, including sodium/potassium/calcium exchanger 3 (NCKX3) and transient receptor potential cation channel subfamily V member 2 (TRPV2), copper channels, including high affinity copper uptake protein 1 (CTR1) and copper‑transporting ATPase 1 (ATP7A), and iron channels, including iron‑regulated transporter 1 (IREG1) and hephaestin (HEPH) proteins and their mRNAs were examined in the canine duodenum, kidney, spleen and liver. NCKX3 protein expression was highest in the kidney, moderate in the duodenum, and lowest in the spleen and liver, whereas TRPV2 protein was highly expressed in the kidney, duodenum and liver, and was low in the spleen. The CTR1 protein expression level was highest in the liver, followed (in descending order) by the duodenum, kidney and spleen. The ATP7A protein expression level was highest in the duodenum and lowest in the spleen. The IREG1 protein expression level was highest in the liver, followed (in descending order) by the kidney, duodenum and spleen. The HEPH protein level was high in liver, moderate in the duodenum and kidney, and low in the spleen. The results of the immunohistochemistry analysis demonstrated ion channel protein localizations. These results suggested that cation channel proteins are differentially expressed among canine organs, and they may be involved in organ‑specific functions associated with the maintenance of physiological homeostasis.

Ceruloplasmin and hephaestin jointly protect the exocrine pancreas against oxidative damage by facilitating iron efflux

Little is known about the iron efflux from the pancreas, but it is likely that multicopper ferroxidases (MCFs) are involved in this process. We thus used hephaestin (Heph) and ceruloplasmin (Cp) single-knockout mice and Heph/Cp double-knockout mice to investigate the roles of MCFs in pancreatic iron homeostasis. We found that both HEPH and CP were expressed in the mouse pancreas, and that ablation of either MCF had limited effect on the pancreatic iron levels. However, ablation of both MCFs together led to extensive pancreatic iron deposition and severe oxidative damage. Perls’ Prussian blue staining revealed that this iron deposition was predominantly in the exocrine pancreas, while the islets were spared. Consistent with these results, plasma lipase and trypsin were elevated in Heph/Cp knockout mice, indicating damage to the exocrine pancreas, while insulin secretion was not affected. These data indicate that HEPH and CP play mutually compensatory roles in facilitating iron efflux from the exocrine pancreas, and show that MCFs are able to protect the pancreas against iron-induced oxidative damage.

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.

Targeting iron metabolism in drug discovery and delivery

Iron fulfils a central role in many essential biochemical processes in human physiology; thus, proper processing of iron is crucial. Although iron metabolism is subject to relatively strict physiological control, numerous disorders, such as cancer and neurodegenerative diseases, have recently been linked to deregulated iron homeostasis. Consequently, iron metabolism constitutes a promising and largely unexploited therapeutic target for the development of new pharmacological treatments for these diseases. Several iron metabolism-targeted therapies are already under clinical evaluation for haematological disorders, and these and newly developed therapeutic agents are likely to have substantial benefit in the clinical management of iron metabolism-associated diseases, for which few efficacious treatments are currently available.

Mechanisms Linking Glucose Homeostasis and Iron Metabolism Toward the Onset and Progression of Type 2 Diabetes

OBJECTIVE

The bidirectional relationship between iron metabolism and glucose homeostasis is increasingly recognized. Several pathways of iron metabolism are modified according to systemic glucose levels, whereas insulin action and secretion are influenced by changes in relative iron excess. We aimed to update the possible influence of iron on insulin action and secretion and vice versa.

RESEARCH DESIGN AND METHODS

The mechanisms that link iron metabolism and glucose homeostasis in the main insulin-sensitive tissues and insulin-producing β-cells were revised according to their possible influence on the development of type 2 diabetes (T2D).

RESULTS

The mechanisms leading to dysmetabolic hyperferritinemia and hepatic overload syndrome were diverse, including diet-induced alterations in iron absorption, modulation of gluconeogenesis, heme-mediated disruption of circadian glucose rhythm, impaired hepcidin secretion and action, and reduced copper availability. Glucose metabolism in adipose tissue seems to be affected by both iron deficiency and excess through interaction with adipocyte differentiation, tissue hyperplasia and hypertrophy, release of adipokines, lipid synthesis, and lipolysis. Reduced heme synthesis and dysregulated iron uptake or export could also be contributing factors affecting glucose metabolism in the senescent muscle, whereas exercise is known to affect iron and glucose status. Finally, iron also seems to modulate β-cells and insulin secretion, although this has been scarcely studied.

CONCLUSIONS

Iron is increasingly recognized to influence glucose metabolism at multiple levels. Body iron stores should be considered as a potential target for therapy in subjects with T2D or those at risk for developing T2D. Further research is warranted.

Pleiotropic actions of iron balance in diabetes mellitus

As an essential element, iron plays a central role in many physiological processes, including redox balance, inflammation, energy metabolism, and environment sensing. Perturbations in iron homeostasis are associated with several conditions, including hyperglycemia and diabetes, both of which have been studied in patients and animal models. To clarify the pleiotropic role of iron homeostasis in diabetes development, the early studies on diseases with iron-overload, studies on clinical iron depletion therapies, associations between iron-related genetic polymorphisms and diabetes, and etiological mechanisms underlying iron perturbations-impaired insulin secretion and insulin sensitivity were carefully reviewed and discussed. Hereditary hemochromatosis, transfusion-dependent thalassemia, and excess heme iron intake can increase the risk of developing diabetes. Genetically modified mice and mice fed a high-iron diet present with discrepant phenotypes due to differences in tissue iron distribution. Moreover, several genetic polymorphisms related to iron homeostasis have been associated with the risk of developing diabetes. Tightly controlled iron metabolism is essential for insulin secretion and insulin sensitivity, and iron overload in pancreatic islets alters reactive oxygen species (ROS) generation, as well as hypoxia-inducible factor-1α (HIF-1α) stability and adenosine triphosphate (ATP) synthesis, thereby impairing the function and viability of β-cells. Decreased levels of adiponectin, macrophage-mediated inflammation, and ROS-mediated liver kinase B1 (LKB1)/adenosine monophosphate-activated protein kinase (AMPK) activation can contribute to iron overload-induced insulin resistance, whereas iron deficiency could also participate in obesity-related inflammation, hypoxia, and insulin resistance. Because iron homeostasis is closely correlated with many metabolic processes, future studies are needed in order to elucidate the finely tuned network among iron homeostasis, carbohydrate and lipid metabolism, inflammation, and hypoxia.

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.

Effects of iron overload on chronic metabolic diseases

Iron can affect the clinical course of several chronic metabolic diseases such as type 2 diabetes, obesity, non-alcoholic fatty liver disease, and atherosclerosis. Iron overload can affect major tissues involved in glucose and lipid metabolism (pancreatic β cells, liver, muscle, and adipose tissue) and organs affected by chronic diabetic complications. Because iron is a potent pro-oxidant, fine-tuned control mechanisms have evolved to regulate entry, recycling, and loss of body iron. These mechanisms include the interplay of iron with transferrin, ferritin, insulin, and hepcidin, as well as with adipokines and proinflammatory molecules. An imbalance of these homoeostatic mechanisms results in systemic and parenchymal siderosis that contributes to organ damage (such as β-cell dysfunction, fibrosis in liver diseases, and atherosclerotic plaque growth and instability). Conversely, iron depletion can exert beneficial effects in patients with iron overload and even in healthy frequent blood donors. Regular assessment of iron balance should be recommended for patients with chronic metabolic diseases, and further research is needed to produce guidelines for the identification of patients who would benefit from iron depletion.

The amyloid precursor protein (APP) does not have a ferroxidase site in its E2 domain

The ubiquitous 24-meric iron-storage protein ferritin and multicopper oxidases such as ceruloplasmin or hephaestin catalyze oxidation of Fe(II) to Fe(III), using molecular oxygen as oxidant. The ferroxidase activity of these proteins is essential for cellular iron homeostasis. It has been reported that the amyloid precursor protein (APP) also has ferroxidase activity. The activity is assigned to a ferroxidase site in the E2 domain of APP. A synthetic 22-residue peptide that carries the putative ferroxidase site of E2 domain (FD1 peptide) has been claimed to encompass the same activity. We previously tested the ferroxidase activity of the synthetic FD1 peptide but we did not observe any activity above the background oxidation of Fe(II) by molecular oxygen. Here we used isothermal titration calorimetry to study Zn(II) and Fe(II) binding to the natural E2 domain of APP, and we employed the transferrin assay and oxygen consumption measurements to test the ferroxidase activity of the E2 domain. We found that this domain neither in the presence nor in the absence of the E1 domain binds Fe(II) and it is not able to catalyze the oxidation of Fe(II). Binding of Cu(II) to the E2 domain did not induce ferroxidase activity contrary to the presence of redox active Cu(II) centers in ceruloplasmin or hephaestin. Thus, we conclude that E2 or E1 domains of APP do not have ferroxidase activity and that the potential involvement of APP as a ferroxidase in the pathology of Alzheimer’s disease must be re-evaluated.

Multi-copper oxidases and human iron metabolism

Multi-copper oxidases (MCOs) are a small group of enzymes that oxidize their substrate with the concomitant reduction of dioxygen to two water molecules. Generally, multi-copper oxidases are promiscuous with regards to their reducing substrates and are capable of performing various functions in different species. To date, three multi-copper oxidases have been detected in humans—ceruloplasmin, hephaestin and zyklopen. Each of these enzymes has a high specificity towards iron with the resulting ferroxidase activity being associated with ferroportin, the only known iron exporter protein in humans. Ferroportin exports iron as Fe²⁺, but transferrin, the major iron transporter protein of blood, can bind only Fe³⁺ effectively. Iron oxidation in enterocytes is mediated mainly by hephaestin thus allowing dietary iron to enter the bloodstream. Zyklopen is involved in iron efflux from placental trophoblasts during iron transfer from mother to fetus. Release of iron from the liver relies on ferroportin and the ferroxidase activity of ceruloplasmin which is found in blood in a soluble form. Ceruloplasmin, hephaestin and zyklopen show distinctive expression patterns and have unique mechanisms for regulating their expression. These features of human multi-copper ferroxidases can serve as a basis for the precise control of iron efflux in different tissues. In this manuscript, we review the biochemical and biological properties of the three human MCOs and discuss their potential roles in human iron homeostasis.

Iron and diabetes risk

Iron overload is a risk factor for diabetes. The link between iron and diabetes was first recognized in pathologic conditions-hereditary hemochromatosis and thalassemia-but high levels of dietary iron also impart diabetes risk. Iron plays a direct and causal role in diabetes pathogenesis mediated both by β cell failure and insulin resistance. Iron also regulates metabolism in most tissues involved in fuel homeostasis, with the adipocyte in particular serving an iron-sensing role. The underlying molecular mechanisms mediating these effects are numerous and incompletely understood but include oxidant stress and modulation of adipokines and intracellular signal transduction pathways.

Functional role of the putative iron ligands in the ferroxidase activity of recombinant human hephaestin

Hephaestin is a multicopper ferroxidase expressed mainly in the mammalian small intestine. The ferroxidase activity of hephaestin is thought to play an important role during iron export from intestinal enterocytes and the subsequent iron loading of the blood protein transferrin, which delivers iron to the tissues. Structurally, the ectodomain of hephaestin is predicted to resemble ceruloplasmin, the soluble ferroxidase of blood. In this study, the human hephaestin ectodomain was expressed in baby hamster kidney cells and purified to electrophoretic homogeneity. Ion exchange chromatography of purified recombinant human hephaestin (rhHp) resulted in the isolation of hephaestin fractions with distinct catalytic and spectroscopic properties. The fraction of rhHp with the highest enzymatic activity also showed an enhanced molar absorptivity at 600 nm, characteristic of type 1 copper sites. Kinetic analysis revealed that rhHp possesses both high-affinity and low-affinity binding sites for ferrous iron. To investigate the role of particular residues in iron specificity of hephaestin, mutations of putative iron ligands were introduced into rhHp using site-directed mutagenesis. Kinetic analysis of ferroxidation rates of wild-type rhHp and mutants demonstrated the important roles of hephaestin residues E960 and H965 in the observed ferroxidase activity.

Recent advances in small bowel diseases: Part II

As is the case in all areas of gastroenterology and hepatology, in 2009 and 2010 there were many advances in our knowledge and understanding of small intestinal diseases. Over 1000 publications were reviewed, and the important advances in basic science as well as clinical applications were considered. In Part II we review six topics: absorption, short bowel syndrome, smooth muscle function and intestinal motility, tumors, diagnostic imaging, and cystic fibrosis.

An X chromosome association scan of the Norfolk Island genetic isolate provides evidence for a novel migraine susceptibility locus at Xq12

Migraine is a common and debilitating neurovascular disorder with a complex envirogenomic aetiology. Numerous studies have demonstrated a preponderance of women affected with migraine and previous pedigree linkage studies in our laboratory have identified susceptibility loci on chromosome Xq24-Xq28. In this study we have used the genetic isolate of Norfolk Island to further analyse the X chromosome for migraine susceptibility loci.An association approach was employed to analyse 14,124 SNPs spanning the entire X chromosome. Genotype data from 288 individuals comprising a large core-pedigree, of which 76 were affected with migraine, were analysed. Although no SNP reached chromosome-wide significance (empirical α = 1 × 10(-5)) ranking by P-value revealed two primary clusters of SNPs in the top 25. A 10 SNP cluster represents a novel migraine susceptibility locus at Xq12 whilst a 11 SNP cluster represents a previously identified migraine susceptibility locus at Xq27. The strongest association at Xq12 was seen for rs599958 (OR = 1.75, P = 8.92 × 10(-4)), whilst at Xq27 the strongest association was for rs6525667 (OR = 1.53, P = 1.65 × 10(-4)). Further analysis of SNPs at these loci was performed in 5,122 migraineurs from the Women's Genome Health Study and provided additional evidence for association at the novel Xq12 locus (P<0.05).Overall, this study provides evidence for a novel migraine susceptibility locus on Xq12. The strongest effect SNP (rs102834, joint P = 1.63 × 10(-5)) is located within the 5'UTR of the HEPH gene, which is involved in iron homeostasis in the brain and may represent a novel pathway for involvement in migraine pathogenesis.

Discovery of a cytosolic/soluble ferroxidase in rodent enterocytes

Hephaestin (Heph), a membrane-bound multicopper ferroxidase (FOX) expressed in duodenal enterocytes, is required for optimal iron absorption. However, sex-linked anemia (sla) mice harboring a 194-amino acid deletion in the Heph protein are able to absorb dietary iron despite reduced expression and mislocalization of the mutant protein. Thus Heph may not be essential, and mice are able to compensate for the loss of its activity. The current studies were undertaken to search for undiscovered FOXs in rodent enterocytes. An experimental approach was developed to investigate intestinal FOXs in which separate membrane and cytosolic fractions were prepared and FOX activity was measured by a spectrophotometric transferrin-coupled assay. Unexpectedly, FOX activity was noted in membrane and cytosolic fractions of rat enterocytes. Different experimental approaches demonstrated that cytosolic FOX activity was not caused by contamination with membrane Heph or a method-induced artifact. Cytosolic FOX activity was abolished by SDS and heat (78 °C), suggesting protein-mediated iron oxidation, and was also sensitive to Triton X-100. Furthermore, cytosolic FOX activity increased ∼30% in iron-deficient rats (compared with controls) but was unchanged in copper-deficient rats (in contrast to the reported dramatic reduction of Heph expression and activity during copper deficiency). Additional studies done in sla, Heph-knockout, and ceruloplasmin-knockout mice proved that cytosolic FOX activity could not be fully explained by Heph or ceruloplasmin. Therefore rodent enterocytes contain a previously undescribed soluble cytosolic FOX that may function in transepithelial iron transport and complement membrane-bound Heph.

Oxidation of organic and biogenic amines by recombinant human hephaestin expressed in Pichia pastoris

Hephaestin is a multicopper ferroxidase involved in iron absorption in the small intestine. Expressed mainly on the basolateral surface of duodenal enterocytes, hephaestin facilitates the export of iron from the intestinal epithelium into blood by oxidizing Fe(2+) into Fe(3+), the only form of iron bound by the plasma protein transferrin. Structurally, the human hephaestin ectodomain is predicted to resemble ceruloplasmin, the major multicopper oxidase in blood. In addition to its ferroxidase activity, ceruloplasmin was reported to oxidize a wide range of organic compounds including a group of physiologically relevant substrates (biogenic amines). To study oxidation of organic substrates, the human hephaestin ectodomain was expressed in Pichia pastoris. The purified recombinant hephaestin has an average copper content of 4.2 copper atoms per molecule. The K(m) for Fe(2+) of hephaestin was determined to be 3.2μM which is consistent with the K(m) values for other multicopper ferroxidases. In addition, the K(m) values of hephaestin for such organic substrates as p-phenylenediamine and o-dianisidine are close to values determined for ceruloplasmin. However, in contrast to ceruloplasmin, hephaestin was incapable of direct oxidation of adrenaline and dopamine implying a difference in biological substrate specificities between these two homologous ferroxidases.

Serum ceruloplasmin protein expression and activity increases in iron-deficient rats and is further enhanced by higher dietary copper intake

Increases in serum and liver copper content are noted during iron deficiency in mammals, suggesting that copper-dependent processes participate during iron deprivation. One point of intersection between the 2 metals is the liver-derived, multicopper ferroxidase ceruloplasmin (Cp) that is important for iron release from certain tissues. The current study sought to explore Cp expression and activity during physiologic states in which hepatic copper loading occurs (eg, iron deficiency). Weanling rats were fed control or low iron diets containing low, normal, or high copper for ∼ 5 weeks, and parameters of iron homeostasis were measured. Liver copper increased in control and iron-deficient rats fed extra copper. Hepatic Cp mRNA levels did not change; however, serum Cp protein was higher during iron deprivation and with higher copper consumption. In-gel and spectrophotometric ferroxidase and amine oxidase assays demonstrated that Cp activity was enhanced when hepatic copper loading occurred. Interestingly, liver copper levels strongly correlated with Cp protein expression and activity. These observations support the possibility that liver copper loading increases metallation of the Cp protein, leading to increased production of the holo enzyme. Moreover, this phenomenon may play an important role in the compensatory response to maintain iron homeostasis during iron deficiency.

Advances in iron chelation: an update

INTRODUCTION

Oxidative stress (caused by excess iron) can result in tissue damage, organ failure and finally death, unless treated by iron chelators. The causative factor in the etiology of a variety of disease states is the presence of iron-generated reactive oxygen species (ROS), which can result in cell damage or which can affect the signaling pathways involved in cell necrosis-apoptosis or organ fibrosis, cancer, neurodegeneration and cardiovascular, hepatic or renal dysfunctions. Iron chelators can reduce oxidative stress by the removal of iron from target tissues. Equally as important, removal of iron from the active site of enzymes that play key roles in various diseases can be of considerable benefit to the patients.

AREAS COVERED

This review focuses on iron chelators used as therapeutic agents. The importance of iron in oxidative damage is discussed, along with the three clinically approved iron chelators.

EXPERT OPINION

A number of iron chelators are used as approved therapeutic agents in the treatment of thalassemia major, asthma, fungal infections and cancer. However, as our knowledge about the biochemistry of iron and its role in etiologies of seemingly unrelated diseases increases, new applications of the approved iron chelators, as well as the development of new iron chelators, present challenging opportunities in the areas of drug discovery and development.

Identification of zyklopen, a new member of the vertebrate multicopper ferroxidase family, and characterization in rodents and human cells

We previously detected a membrane-bound, copper-containing oxidase that may be involved in iron efflux in BeWo cells, a human placental cell line. We have now identified a gene encoding a predicted multicopper ferroxidase (MCF) with a putative C-terminal membrane-spanning sequence and high sequence identity to hephaestin (Heph) and ceruloplasmin (Cp), the other known vertebrate MCF. Molecular modeling revealed conservation of all type I, II, and III copper-binding sites as well as a putative iron-binding site. Protein expression was observed in multiple diverse mouse tissues, including placenta and mammary gland, and the expression pattern was distinct from that of Cp and Heph. The protein possessed ferroxidase activity, and protein levels decreased in cellular copper deficiency. Knockdown with small interfering RNA in BeWo cells indicates that this gene represents the previously detected oxidase. We propose calling this new member of the MCF family "zyklopen."