Molecular and functional roles of duodenal cytochrome B (Dcytb) in iron metabolism

Understanding the role of iron/heme metabolism in the anti‑inflammatory effects of natural sulfur molecules against lipopolysaccharide‑induced inflammation

Iron transport and heme synthesis are essential processes in human metabolism, and any dysregulation in these mechanisms, such as inflammation, can have deleterious effects. Lipopolysaccharide (LPS)‑induced inflammatory responses can result in a number of adverse effects, including cancer. Natural mineral sulfur, methylsulfonylmethane (MSM) and nontoxic sulfur (NTS) suppress inflammatory responses. The present study hypothesized that MSM and NTS may inhibit LPS‑induced inflammatory responses in THP‑1 human monocytes. Reverse transcription‑quantitative PCR and western blotting assays were performed to analyze the molecular signaling pathways associated with sulfur‑treated and untreated cells. A comet assay was used to evaluate DNA damage, flow cytometry was performed to analyze cell surface receptors and chromatin immunoprecipitation was used to examine molecular interactions. Notably, LPS‑induced inflammation increased iron/heme metabolism, whereas MSM and NTS inhibited this effect. Furthermore, LPS treatment activated the Toll‑like receptor 4/NF‑κB signaling axis, which was downregulated by NTS and MSM. These sulfur compounds also suppressed the nuclear accumulation of LPS‑induced NF‑κB, which could induce the production of proinflammatory cytokines, such as TNF‑α, IL‑1β and IL‑6. Finally, MSM and NTS inhibited LPS‑induced reactive oxygen species generation and DNA damage in THP‑1 monocytic leukemia cells. These results suggested that natural sulfur molecules may be considered promising candidates for anti‑inflammation studies.

Iron and ferroptosis in kidney disease: molecular and metabolic mechanisms

Maintaining iron homeostasis is necessary for kidney functioning. There is more and more research indicating that kidney disease is often caused by iron imbalance. Over the past decade, ferroptosis' role in mediating the development and progression of renal disorders, such as acute kidney injury (renal ischemia-reperfusion injury, drug-induced acute kidney injury, severe acute pancreatitis induced acute kidney injury and sepsis-associated acute kidney injury), chronic kidney disease (diabetic nephropathy, renal fibrosis, autosomal dominant polycystic kidney disease) and renal cell carcinoma, has come into focus. Thus, knowing kidney iron metabolism and ferroptosis regulation may enhance disease therapy. In this review, we discuss the metabolic and molecular mechanisms of iron signaling and ferroptosis in kidney disease. We also explore the possible targets of ferroptosis in the therapy of renal illness, as well as their existing limitations and future strategies.

Plasma and tissue transferrin and ferritin, and gene expression of ferritin, transferrin, and transferrin receptors I and II in channel catfish Ictalurus punctatus fed diets with different concentrations of inorganic or organic iron

Ferritin, transferrin, and transferrin receptors I and II play a vital role in iron metabolism, health, and indication of iron deficiency anaemia in fish. To evaluate the use of high-iron diets to prevent or reverse channel catfish (Ictalurus punctatus) anaemia of unknown causes, we investigated the expression of these iron-regulatory genes and proteins in channel catfish fed plant-based diets. Catfish fingerlings were fed five diets supplemented with 0 (basal), 125, and 250 mg/kg of either inorganic iron or organic iron for 2 weeks. Ferritin, transferrin, and transferrin receptor I and II mRNA and protein expression levels in fish tissues (liver, intestine, trunk kidney, and head kidney) and plasma were determined. Transferrin (iron transporter) and TfR (I and II) genes were generally highly expressed in fish fed the basal diet compared to those fed the iron-supplemented diets. In contrast, ferritin (iron storage) genes were more expressed in the trunk kidney of fish fed the iron-supplemented diets than in those fed the basal diet. Our results demonstrate that supplementing channel catfish plant-based diets with iron from either organic or inorganic iron sources affected the expression of the iron-regulatory genes and increased body iron status in the fish.

Mechanism of Iron Ion Homeostasis in Intestinal Immunity and Gut Microbiota Remodeling

Iron is a vital trace element that plays an important role in humans and other organisms. It plays an active role in the growth, development, and reproduction of bacteria, such as Bifidobacteria. Iron deficiency or excess can negatively affect bacterial hosts. Studies have reported a major role of iron in the human intestine, which is necessary for maintaining body homeostasis and intestinal barrier function. Organisms can maintain their normal activities and regulate some cancer cells in the body by regulating iron excretion and iron-dependent ferroptosis. In addition, iron can modify the interaction between hosts and microorganisms by altering their growth and virulence or by affecting the immune system of the host. Lactic acid bacteria such as Lactobacillus acidophilus (L. acidophilus), Lactobacillus rhamnosus (L. rhamnosus), and Lactobacillus casei (L. casei) were reported to increase trace elements, protect the host intestinal barrier, mitigate intestinal inflammation, and regulate immune function. This review article focuses on the two aspects of the iron and gut and generally summarizes the mechanistic role of iron ions in intestinal immunity and the remodeling of gut microbiota.

Comparison of insect and human cytochrome b561 proteins: Insights into candidate ferric reductases in insects

Cytochrome b561 (cytb561) proteins comprise a family of transmembrane oxidoreductases that transfer single electrons across a membrane. Most eukaryotic species, including insects, possess multiple cytb561 homologs. To learn more about this protein family in insects, we carried out a bioinformatics-based investigation of cytb561 family members from nine species representing eight insect orders. We performed a phylogenetic analysis to classify insect cytb561 ortholog groups. We then conducted sequence analyses and analyzed protein models to predict structural elements that may impact the biological functions and localization of these proteins, with a focus on possible ferric reductase activity. Our study revealed three ortholog groups, designated CG1275, Nemy, and CG8399, and a fourth group of less-conserved genes. We found that CG1275 and Nemy proteins are similar to a human ferric reductase, duodenal cytochrome b561 (Dcytb), and have many conserved amino acid residues that function in substrate binding in Dcytb. Notably, CG1275 and Nemy proteins contain a conserved histidine and other residues that play a role in ferric ion reduction by Dcytb. Nemy proteins were distinguished by a novel cysteine-rich cytoplasmic loop sequence. CG8399 orthologs are similar to a putative ferric reductase in humans, stromal cell-derived receptor 2. Like other members of the CYBDOM class of cytb561 proteins, these proteins contain reeler, DOMON, and cytb561 domains. Drosophila melanogaster CG8399 is the only insect cytb561 with known ferric reductase activity. Our investigation of the DOMON domain in CG8399 proteins revealed a probable heme-binding site and a possible site for ferric reduction. The fourth group includes a subgroup of proteins with a conserved "KXXXXKXH" non-cytoplasmic loop motif that may be a substrate binding site and is present in a potential ferric reductase, human tumor suppressor cytochrome b561. This study provides a foundation for future investigations of the biological functions of cytb561 genes in insects.

Cytochrome b561 regulates iron metabolism by activating the Akt/mTOR pathway to promote Breast Cancer Cells proliferation

Breast cancer (BC) is the leading cause of cancer-related mortality in women, necessitating the development of novel therapeutic targets. While cytochrome b561 (CYB561) expression is associated with poor prognosis in BC, the precise role of CYB561 in BC and its potential mechanisms remain unclear. In the present study, we found that CYB561 plays an essential role in BC growth. CYB561 expression was up-regulated in surgically resected cancerous tissues and in six BC cell lines. Lentivirus-mediated CYB561 knockdown in BC cells significantly reduced their proliferation, migration, and invasiveness. CYB561 participates in the regulation of iron metabolism in BC. CYB561 knockdown reduced total iron content, increased ferrous iron content, and down-regulated the expression of proteins associated with iron metabolism (transferrin receptor 1, divalent metal transporter 1, and ferritin heavy chain 1). Conversely, up-regulation of CYB561 through co-incubation with exogenous iron (ferric ammonium citrate) produced contrary outcomes. Additionally, CYB561 activated the protein kinase B/mammalian target of rapamycin (Akt/mTOR) signaling pathway in BC cells. Down-regulation of CYB561 expression inhibited the Akt/mTOR signaling pathway activity. The application of an mTOR agonist (MHY1485) rescued this negative effect, as well as the inhibitory effect of CYB561 knockdown on cell proliferation. Importantly, the dual mTOR inhibitor MLN0128 (50 nM, 48 h) down-regulated CYB561 expression and the iron metabolism-related proteins transferrin receptor, divalent metal transporter 1, and ferritin heavy chain 1, whereas the mTOR agonist MHY1485 rescued the down-regulation of CYB561 knockdown on iron metabolism-related proteins. We conclude that CYB561 promotes the proliferation of BC cells by regulating iron metabolism through the activation of the Akt/mTOR signaling pathway.

Ferroptosis, a Rising Force against Renal Fibrosis

Ferroptosis is a type of programmed cell death characterized by iron overload, oxidative stress, imbalance in lipid repair, and mitochondria-specific pathological manifestations. Growing number of molecular mechanisms and signaling pathways have been found to be involved in ferroptosis progression, including iron metabolism, amino acid metabolism, lipid metabolism, and energy metabolism. It is worth noting that ferroptosis is involved in the progression of fibrotic diseases such as liver cirrhosis, cardiomyopathy, and idiopathic pulmonary fibrosis, and inhibition of ferroptosis has acquired beneficial outcomes in rodent models, while studies on ferroptosis and renal fibrosis remains limited. Recent studies have revealed that targeting ferroptosis can effectively mitigate chronic kidney injury and renal fibrosis. Moreover, myofibroblasts suffer from ferroptosis during fiber and extracellular matrix deposition in the fibrotic cascade reaction and pharmacological modulation of ferroptosis shows great therapeutic effect on renal fibrosis. Here, we summarize the latest molecular mechanisms of ferroptosis from high-quality studies and review its therapeutic potential in renal fibrosis.

Expression and clinical prognostic value of CYB561 in breast cancer

PURPOSE

The expression of cytochrome B561 (CYB561) and its role in breast cancer (BC) prognosis remain unclear. We analyzed the differential expression and prognostic value of CYB561 using online databases and a clinical cohort through bioinformatics and immunohistochemistry.

METHODS

The differential expression of CYB561 and its association with BC were analyzed using the tumor immune estimation resource (TIMER), gene expression profiling interaction analysis2 (GEPIA2), Human Protein Atlas, Cancer Cell Line Encyclopedia, and Kaplan-Meier Plotter website. Important pathways of CYB561 enrichment were explored using gene set enrichment analysis. Immunohistochemistry detected CYB561 expression in normal breast, breast hyperplasia, ductal carcinoma in situ (DCIS), para-cancer, and invasive BC groups. Association between CYB561 expression and BC prognosis was analyzed using Kaplan-Meier and Cox regression analyses.

RESULTS

CYB561 mRNA expression was higher in GEPIA and TIMER BC patients than in para-cancer tissues. CYB561 was expressed in the glandular epithelium and myoepithelium, with positive localization in the cytoplasm and cell membrane. CYB561 protein expression significantly differed among the groups. CYB561 expression was correlated with ERBB2/HER2 and infiltrating CD4+ T cells in GEPIA and TIMER BC patients and associated with HER2 status, histological grade, and molecular subtypes in the clinical cohort but not related to tumor-infiltrating lymphocytes. CYB561 mRNA overexpression predicted reduced recurrence-free survival and overall survival in BC. Patients with CYB561 expression had significantly reduced overall survival and increased risk of death.

CONCLUSION

CYB561 can serve as an effective clinical prognostic biomarker for BC.

A Bioinformatics-Assisted Review on Iron Metabolism and Immune System to Identify Potential Biomarkers of Exercise Stress-Induced Immunosuppression

The immune function is closely related to iron (Fe) homeostasis and allostasis. The aim of this bioinformatics-assisted review was twofold; (i) to update the current knowledge of Fe metabolism and its relationship to the immune system, and (ii) to perform a prediction analysis of regulatory network hubs that might serve as potential biomarkers during stress-induced immunosuppression. Several literature and bioinformatics databases/repositories were utilized to review Fe metabolism and complement the molecular description of prioritized proteins. The Search Tool for the Retrieval of Interacting Genes (STRING) was used to build a protein-protein interactions network for subsequent network topology analysis. Importantly, Fe is a sensitive double-edged sword where two extremes of its nutritional status may have harmful effects on innate and adaptive immunity. We identified clearly connected important hubs that belong to two clusters: (i) presentation of peptide antigens to the immune system with the involvement of redox reactions of Fe, heme, and Fe trafficking/transport; and (ii) ubiquitination, endocytosis, and degradation processes of proteins related to Fe metabolism in immune cells (e.g., macrophages). The identified potential biomarkers were in agreement with the current experimental evidence, are included in several immunological/biomarkers databases, and/or are emerging genetic markers for different stressful conditions. Although further validation is warranted, this hybrid method (human-machine collaboration) to extract meaningful biological applications using available data in literature and bioinformatics tools should be highlighted.

New Insights into the Pivotal Role of Iron/Heme Metabolism in TLR4/NF-κB Signaling-Mediated Inflammatory Responses in Human Monocytes

Iron metabolism and heme biosynthesis are essential processes in cells during the energy cycle. Alteration in these processes could create an inflammatory condition, which results in tumorigenesis. Studies are conducted on the exact role of iron/heme metabolism in induced inflammatory conditions. This study used lipopolysaccharide (LPS)- or high-glucose-induced inflammation conditions in THP-1 cells to study how iron/heme metabolism participates in inflammatory responses. Here, we used iron and heme assays for measuring total iron and heme. We also used flow cytometry and Western blotting to analyze molecular responses. Our results demonstrated that adding LPS or high-glucose induced iron formation and heme synthesis and elevated the expression levels of proteins responsible for iron metabolism and heme synthesis. We then found that further addition of heme or 5-aminolevulinic acid (ALA) increased heme biosynthesis and promoted inflammatory responses by upregulating TLR4/NF-κB and inflammatory cytokine expressions. We also demonstrated the inhibition of heme synthesis using succinylacetone (SA). Moreover, N-MMP inhibited LPS- or high-glucose-induced inflammatory responses by inhibiting TLR4/NF-κB signaling. Hence, iron/heme metabolism checkpoints could be considered a target for treating inflammatory conditions.

A holistic view of mammalian (vertebrate) cellular iron uptake

Cell iron uptake in mammals is commonly distinguished by whether the iron is presented to the cell as transferrin-bound or not: TBI or NTBI. This generic perspective conflates TBI with canonical transferrin receptor, endosomal iron uptake, and NTBI with uptake supported by a plasma membrane-localized divalent metal ion transporter, most often identified as DMT1. In fact, iron uptake by mammalian cells is far more nuanced than this somewhat proscribed view suggests. This view fails to accommodate the substantial role that ZIP8 and ZIP14 play in iron uptake, while adhering to the traditional premise that a relatively high endosomal [H+] is thermodynamically required for release of iron from holo-Tf. The canonical view of iron uptake also does not encompass the fact that plasma membrane electron transport - PMET - has long been linked to cell iron uptake. In fact, the known mammalian metallo-reductases - Dcytb and the STEAP proteins - are members of this cohort of cytochrome-dependent oxido-reductases that shuttle reducing equivalents across the plasma membrane. A not commonly appreciated fact is the reduction potential of ferric iron in holo-Tf is accessible to cytoplasmic reducing equivalents - reduced pyridine and flavin mono- and di-nucleotides and dihydroascorbic acid. This allows for the reductive release of Fe2+ at the extracellular surface of the PM and subsequent transport into the cytoplasm by a neutral pH transporter - a ZIP protein. What this perspective emphasizes is that there are two TfR-dependent uptake pathways, one which does and one which does not involve clathrin-dependent, endolysosomal trafficking. This raises the question as to the selective advantage of having two Tf, TfR-dependent routes of iron accumulation. This review of canonical and non-canonical iron uptake uses cerebral iron trafficking as a point of discussion, a focus that encourages inclusion also of the importance of ferritin as a circulating 'chaperone' of ferric iron.

The Cross-Link between Ferroptosis and Kidney Diseases

Acute and chronic kidney injuries result from structural dysfunction and metabolic disorders of the kidney in various etiologies, which significantly affect human survival and social wealth. Nephropathies are often accompanied by various forms of cell death and complex microenvironments. In recent decades, the study of kidney diseases and the traditional forms of cell death have improved. Nontraditional forms of cell death, represented by ferroptosis and necroptosis, have been discovered in the field of kidney diseases, which have reshuffled the role of traditional cell death in nephropathies. Although interactions between ferroptosis and acute kidney injury (AKI) have been continuously explored, studies on ferroptosis and chronic kidney disease (CKD) remain limited. Here, we have reviewed the therapeutic significance of ferroptosis in AKI and anticipated the curative potential of ferroptosis for CKD in the hope of providing insights into ferroptosis and CKD.

Effects of circadian iron administration on iron bioavailability and biological rhythm in pigs

BACKGROUND

Iron supplements are limited by their poor absorption and low efficacy. A circadian feeding schedule would affect the circadian rhythm and improve nutrient metabolism. In this study, 18 iron-deficient piglets were randomly assigned to three groups: a control group receiving a constant diet with mid-iron (MI), a 'HL' group receiving a high-iron (HI) diet at 8:00 h and a low-iron (LI) diet at 18:00, and an 'LH' group receiving a LI diet at 8:00 and a HI diet at 18:00. The effects of circadian iron administration on iron absorption, iron status, and biological rhythm in iron-deficient piglets were investigated.

RESULTS

Serum iron and hemoglobin improved significantly (P < 0.05) but did not significantly differ in the circadian iron-feeding groups (P > 0.05). Iron concentration in the liver and spleen was significantly higher in the LH group than in the HL group (P < 0.05), and mRNA expression of divalent metal transport 1 (DMT1), cytochrome B (CYBRD1) and ferroportin (FPN) genes in the duodenum was significantly elevated in the LH group (P < 0.05). The clock-related genes showed differential expression in the duodenum, with greater mRNA expression for period (Per2) and cryptochrome (Cry1 and Cry2) in the LH group (P < 0.05).

CONCLUSION

Circadian iron administration affected iron absorption and iron storage in pigs. Iron supplementation in the evening might be a more effective pattern for iron utilization. The rhythmic system in the intestine, driven by the time, played an important role in this process. © 2020 Society of Chemical Industry.

Supplemental Microalgal Iron Helps Replete Blood Hemoglobin in Moderately Anemic Mice Fed a Rice-Based Diet

Iron deficiency anemia affects 1.2 billion people globally. Our objectives were to determine if (1) supplemental iron extracted from defatted microalgae (Nannochloropsis oceanica, DGM) and (2) a combination of minute amount of plant phytase and inulin could help replete hemoglobin in anemic mice. Mice (7 weeks old) were fed a control diet (6 mg Fe/kg). After 10 weeks, the mice were assigned to three treatments: control, control + DGM iron (Fe-DGM, 39 mg Fe/kg), or control + 1% inulin + 250 units of phytase/kg (INU−PHY, 6 mg Fe/kg). The mice had free access to diets and water for 6 weeks. The Fe-DGM group had elevated blood hemoglobin (p < 0.01) and a two-fold greater (p < 0.0001) liver non-heme iron over the control. Strikingly, the INU-PHY group had 34% greater non-heme iron than the control, despite the same concentrations of iron in their diets. Fe-DGM group had altered (p < 0.05) mRNA levels of hepcidin, divalent metal transporter 1, transferrin and transferrin receptor 1. Iron extracted from defatted microalgae seemed to be effective in alleviating moderate anemia, and INU-PHY enhanced utilization of intrinsic iron present in the rice diet. Our findings may lead to a novel formulation of these ingredients to develop safer and bioavailable iron supplements for iron-deficient populations.

Iron Oversupplementation Causes Hippocampal Iron Overloading and Impairs Social Novelty Recognition in Nursing Piglets

BACKGROUND

Iron oversupplementation in healthy term infants may adversely affect growth and cognitive development.

OBJECTIVE

We hypothesized that early-life iron excess causes systemic and central nervous system iron overload, and compromises social behavior.

METHODS

The nursing pig was used as a translational model in a completely randomized study. On postnatal day (PD) 1, 24 pigs (1.57 ± 0.28 kg mean ± standard deviation body wt) were assigned to the following treatment groups: 1) nonsupplemented iron-deficient group (NON); 2) control group (CON), intramuscularly injected with iron dextran (100 mg Fe) on PD2; 3) moderate iron group (MOD), orally administered ferrous sulfate at 10 mg Fe · kg body wt-1 · d-1; and 4) high iron group (HIG), orally administered ferrous sulfate at 50 mg Fe · kg-1 · d-1. Piglets were nursed by sows during the study from PD1 to PD21. Tissue iron was analyzed by atomic absorption spectrophotometry. Messenger RNA and protein expression of iron regulator and transporters were analyzed by quantitative reverse transcriptase-polymerase chain reaction and Western blot. A sociability test was performed on PD19-20.

RESULTS

Both MOD and HIG treatments (5.51 and 9.85 µmol/g tissue), but not CON (0.54 µmol/g), increased hepatic iron as compared with NON (0.25 µmol/g, P < 0.05). Similarly, the hippocampal iron concentrations in the MOD and HIG groups were 14.9% and 31.8% higher than that of NON, respectively (P < 0.05). In comparison with NON, MOD and HIG treatment repressed DMT1 in duodenal mucosa by 4- and 46-fold, respectively (P < 0.05); HIG drastically induced HAMP in liver by 540-fold (P < 0.05); iron-supplemented groups reduced TFRC in the hippocampus by <1-fold (P < 0.05). However, duodenal expression of ferroportin, the predominant transporter in basal membrane, was not affected by treatment. Despite normal sociability, the MOD and HIG pigs displayed deficits in social novelty recognition (P = 0.004).

CONCLUSIONS

Duodenal ferroportin was hyporesponsive to iron excess (MOD and HIG), which caused hippocampal iron overload and impaired social novelty recognition in nursing pigs.

Mutation in Cytochrome B gene causes debility and adverse effects on health of sheep

Cytochrome B is the mitochondrial protein, which functions as part of the electron transport chain and is the main subunit of transmembrane cytochrome bc1 and b6f complexes affecting energy metabolism through oxidative phosphorylation. The present study was conducted to study the effect of mutation of Cytochrome B gene on the health condition of sheep, which the first report of association of mitochondrial gene with disease traits in livestock species. Non-synonymous substitutions (F33 L and D171N) and Indel mutations were observed for Cytochrome B gene, leading to a truncated protein, where anemia, malfunctioning of most of the vital organs as liver, kidney and mineral status was observed and debility with exercise intolerance and cardiomyopathy in extreme cases were depicted. These findings were confirmed by bioinformatics analysis, haematological and biochemical data analysis, and other phenotypical physiological data pertaining to different vital organs. The molecular mechanism of cytochrome B mutation was that the mutant variant interferes with the site of heme binding (iron containing) domain and calcium binding essential for electron transport chain. Mutation at amino acid site 33 is located within transmembrane helix A, a hydrophobic environment at the Qi site and close to heme binding domain, and mutation effects these domain and diseases occur. Thermodynamic stability was also observed to decrease in mutant variant. Sheep Cytochrome B being genetically more similar to the human, it may be used as a model for studying human diseases related to cytochrome B defects. Future prospect of the study includes the therapeutic application of recombinant protein, gene therapy and marker-assisted selection of disease-resistant livestock.

New Perspectives on Iron Uptake in Eukaryotes

All eukaryotic organisms require iron to function. Malfunctions within iron homeostasis have a range of physiological consequences, and can lead to the development of pathological conditions that can result in an excess of non-transferrin bound iron (NTBI). Despite extensive understanding of iron homeostasis, the links between the “macroscopic” transport of iron across biological barriers (cellular membranes) and the chemistry of redox changes that drive these processes still needs elucidating. This review draws conclusions from the current literature, and describes some of the underlying biophysical and biochemical processes that occur in iron homeostasis. By first taking a broad view of iron uptake within the gut and subsequent delivery to tissues, in addition to describing the transferrin and non-transferrin mediated components of these processes, we provide a base of knowledge from which we further explore NTBI uptake. We provide concise up-to-date information of the transplasma electron transport systems (tPMETSs) involved within NTBI uptake, and highlight how these systems are not only involved within NTBI uptake for detoxification but also may play a role within the reduction of metabolic stress through regeneration of intracellular NAD(P)H/NAD(P)⁺ levels. Furthermore, we illuminate the thermodynamics that governs iron transport, namely the redox potential cascade and electrochemical behavior of key components of the electron transport systems that facilitate the movement of electrons across the plasma membrane to the extracellular compartment. We also take account of kinetic changes that occur to transport iron into the cell, namely membrane dipole change and their consequent effects within membrane structure that act to facilitate transport of ions.

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.

Disorders of Iron Metabolism and Anemia in Chronic Kidney Disease

Dysregulated iron homeostasis plays a central role in the development of anemia of chronic kidney disease (CKD) and is a major contributor toward resistance to treatment with erythropoiesis-stimulating agents. Understanding the underlying pathophysiology requires an in-depth understanding of normal iron physiology and regulation. Recent discoveries in the field of iron biology have greatly improved our understanding of the hormonal regulation of iron trafficking in human beings and how its alterations lead to the development of anemia of CKD. In addition, emerging evidence has suggested that iron homeostasis interacts with bone and mineral metabolism on multiple levels, opening up new avenues of investigation into the genesis of disordered iron metabolism in CKD. Building on recent advances in our understanding of normal iron physiology and abnormalities in iron homeostasis in CKD, this review characterizes how anemia related to disordered iron metabolism develops in the setting of CKD. In addition, this review explores our emerging recognition of the connections between iron homeostasis and mineral metabolism and their implications for the management of altered iron status and anemia of CKD.

New perspectives on the regulation of iron absorption via cellular zinc concentrations in humans

Iron deficiency is the most prevalent nutritional deficiency, affecting more than 30% of the total world's population. It is a major public health problem in many countries around the world. Over the years various methods have been used with an effort to try and control iron-deficiency anemia. However, there has only been a marginal reduction in the global prevalence of anemia. Why is this so? Iron and zinc are essential trace elements for humans. These metals influence the transport and absorption of one another across the enterocytes and hepatocytes, due to similar ionic properties. This paper describes the structure and roles of major iron and zinc transport proteins, clarifies iron-zinc interactions at these sites, and provides a model for the mechanism of these interactions both at the local and systemic level. This review provides evidence that much of the massive extent of iron deficiency anemia in the world may be due to an underlying deficiency of zinc. It explains the reasons for predominance of cellular zinc status in determination of iron/zinc interactions and for the first time thoroughly explains mechanisms by which zinc brings about these changes.

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.

DCYTB is a predictor of outcome in breast cancer that functions via iron-independent mechanisms

BACKGROUND

Duodenal cytochrome b (DCYTB) is a ferrireductase that functions together with divalent metal transporter 1 (DMT1) to mediate dietary iron reduction and uptake in the duodenum. DCYTB is also a member of a 16-gene iron regulatory gene signature (IRGS) that predicts metastasis-free survival in breast cancer patients. To better understand the relationship between DCYTB and breast cancer, we explored in detail the prognostic significance and molecular function of DCYTB in breast cancer.

METHODS

The prognostic significance of DCYTB expression was evaluated using publicly available microarray data. Signaling Pathway Impact Analysis (SPIA) of microarray data was used to identify potential novel functions of DCYTB. The role of DCYTB was assessed using immunohistochemistry and measurements of iron uptake, iron metabolism, and FAK signaling.

RESULTS

High DCYTB expression was associated with prolonged survival in two large independent cohorts, together totaling 1610 patients (cohort #1, p = 1.6e-11, n = 741; cohort #2, p = 1.2e-05, n = 869; log-rank test) as well as in the Gene expression-based Outcome for Breast cancer Online (GOBO) cohort (p < 1.0e-05, n = 1379). High DCYTB expression was also associated with increased survival in homogeneously treated groups of patients who received either tamoxifen or chemotherapy. Immunohistochemistry revealed that DCYTB is localized on the plasma membrane of breast epithelial cells, and that expression is dramatically reduced in high-grade tumors. Surprisingly, neither overexpression nor knockdown of DCYTB affected levels of ferritin H, transferrin receptor, labile iron or total cellular iron in breast cancer cells. Because SPIA pathway analysis of patient microarray data revealed an association between DCYTB and the focal adhesion pathway, we examined the influence of DCYTB on FAK activation in breast cancer cells. These experiments reveal that DCYTB reduces adhesion and activation of focal adhesion kinase (FAK) and its adapter protein paxillin.

CONCLUSIONS

DCYTB is an important predictor of outcome and is associated with response to therapy in breast cancer patients. DCYTB does not affect intracellular iron in breast cancer cells. Instead, DCYTB may retard cancer progression by reducing activation of FAK, a kinase that plays a central role in tumor cell adhesion and metastasis.

Mechanism of Copper Uptake from Blood Plasma Ceruloplasmin by Mammalian Cells

Ceruloplasmin, the main copper binding protein in blood plasma, has been of particular interest for its role in efflux of iron from cells, but has additional functions. Here we tested the hypothesis that it releases its copper for cell uptake by interacting with a cell surface reductase and transporters, producing apoceruloplasmin. Uptake and transepithelial transport of copper from ceruloplasmin was demonstrated with mammary epithelial cell monolayers (PMC42) with tight junctions grown in bicameral chambers, and purified human (64)Cu-labeled ceruloplasmin secreted by HepG2 cells. Monolayers took up virtually all the (64)Cu over 16h and secreted half into the apical (milk) fluid. This was partly inhibited by Ag(I). The (64)Cu in ceruloplasmin purified from plasma of (64)Cu-injected mice accumulated linearly in mouse embryonic fibroblasts (MEFs) over 3-6h. Rates were somewhat higher in Ctr1+/+ versus Ctr1-/- cells, and 3-fold lower at 2 °C. The ceruloplasmin-derived (64)Cu could not be removed by extensive washing or trypsin treatment, and most was recovered in the cytosol. Actual cell copper (determined by furnace atomic absorption) increased markedly upon 24h exposure to holoceruloplasmin. This was accompanied by a conversion of holo to apoceruloplasmin in the culture medium and did not occur during incubation in the absence of cells. Four different endocytosis inhibitors failed to prevent 64Cu uptake from ceruloplasmin. High concentrations of non-radioactive Cu(II)- or Fe(III)-NTA (substrates for cell surface reductases), or Cu(I)-NTA (to compete for transporter uptake) almost eliminated uptake of (64)Cu from ceruloplasmin. MEFs had cell surface reductase activity and expressed Steap 2 (but not Steaps 3 and 4 or dCytB). However, six-day siRNA treatment was insufficient to reduce activity or uptake. We conclude that ceruloplasmin is a circulating copper transport protein that may interact with Steap2 on the cell surface, forming apoceruloplasmin, and Cu(I) that enters cells through CTR1 and an unknown copper uptake transporter.

Prion protein functions as a ferrireductase partner for ZIP14 and DMT1

Excess circulating iron is stored in the liver, and requires reduction of non-Tf-bound iron (NTBI) and transferrin (Tf) iron at the plasma membrane and endosomes, respectively, by ferrireductase (FR) proteins for transport across biological membranes through divalent metal transporters. Here, we report that prion protein (PrP(C)), a ubiquitously expressed glycoprotein most abundant on neuronal cells, functions as a FR partner for divalent-metal transporter-1 (DMT1) and ZIP14. Thus, absence of PrP(C) in PrP-knock-out (PrP(-/-)) mice resulted in markedly reduced liver iron stores, a deficiency that was not corrected by chronic or acute administration of iron by the oral or intraperitoneal routes. Likewise, preferential radiolabeling of circulating NTBI with (59)Fe revealed significantly reduced uptake and storage of NTBI by the liver of PrP(-/-) mice relative to matched PrP(+/+) controls. However, uptake, storage, and utilization of ferritin-bound iron that does not require reduction for uptake were increased in PrP(-/-) mice, indicating a compensatory response to the iron deficiency. Expression of exogenous PrP(C) in HepG2 cells increased uptake and storage of ferric iron (Fe(3+)), not ferrous iron (Fe(2+)), from the medium, supporting the function of PrP(C) as a plasma membrane FR. Coexpression of PrP(C) with ZIP14 and DMT1 in HepG2 cells increased uptake of Fe(3+) significantly, and surprisingly, increased the ratio of N-terminally truncated PrP(C) forms lacking the FR domain relative to full-length PrP(C). Together, these observations indicate that PrP(C) promotes, and possibly regulates, the uptake of NTBI through DMT1 and Zip14 via its FR activity. Implications of these observations for neuronal iron homeostasis under physiological and pathological conditions are discussed.

Duodenal cytochrome b (DCYTB) in iron metabolism: an update on function and regulation

Iron and ascorbate are vital cellular constituents in mammalian systems. The bulk-requirement for iron is during erythropoiesis leading to the generation of hemoglobin-containing erythrocytes. Additionally, both iron and ascorbate are required as co-factors in numerous metabolic reactions. Iron homeostasis is controlled at the level of uptake, rather than excretion. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance non-heme iron absorption in the gut, ascorbate regulates iron homeostasis. The involvement of ascorbate in dietary iron absorption extends beyond the direct chemical reduction of non-heme iron by dietary ascorbate. Among other activities, intra-enterocyte ascorbate appears to be involved in the provision of electrons to a family of trans-membrane redox enzymes, namely those of the cytochrome b₅₆₁ class. These hemoproteins oxidize a pool of ascorbate on one side of the membrane in order to reduce an electron acceptor (e.g., non-heme iron) on the opposite side of the membrane. One member of this family, duodenal cytochrome b (DCYTB), may play an important role in ascorbate-dependent reduction of non-heme iron in the gut prior to uptake by ferrous-iron transporters. This review discusses the emerging relationship between cellular iron homeostasis, the emergent “IRP1-HIF2α axis”, DCYTB and ascorbate in relation to iron metabolism.

Age-dependent expression of duodenal cytochrome b, divalent metal transporter 1, ferroportin 1, and hephaestin in the duodenum of rats

BACKGROUND AND AIM

The body's requirement for iron is different at different developmental stages. However, the molecular mechanisms of age-dependent iron metabolism are poorly understood. In the present study, we investigated the expression of iron transport proteins in the duodenum of Sprague-Dawley rats at five different age stages.

METHODS

Male Sprague-Dawley rats at postnatal week (PNW) 1, 3, 12, 44, and 88 were employed in the study. Serum iron status and tissue non-heme iron concentrations in the spleen, liver, bone marrow, heart, kidney, duodenal epithelium, and gastrocnemius were examined at each age stage. The expression of duodenal cytochrome b (DcytB), divalent metal transporter 1 (DMT1), ferroportin 1 (FPN1), hephaestin, and hepcidin were measured by real-time polymerase chain reaction or Western blot.

RESULTS

The levels of serum iron and transferrin saturation were higher in the rats at PNW1 and 3 than in those at PNW12, 44, and 88. Non-heme iron contents decreased from PNW1 to PNW3 and then increased thereafter. Duodenal DcytB, DMT1, and FPN1 increased to the highest level at PNW3 and then decreased from PNW12 to 88. The hepatic hepcidin mRNA level decreased to the lowest level at PNW3 and then increased with age.

CONCLUSION

Our findings showed that age had a significant effect on body iron status. The increased duodenal DcytB, DMT1, and FPN1 expression can enhance intestinal iron absorption to meet the high iron requirements in infants. Hepcidin or enterocyte iron levels may be involved in the regulation of age-dependent FPN1, DMT1, and DcytB expression in the duodenum.

Iron clad: iron homeostasis and the diagnosis of hereditary iron overload

Iron is an ubiquitous metal of vital importance to the normal physiologic processes of many organisms. Over the last 2 decades, the discovery of mutations in genes leading to hereditary disorders of iron overload, iron deficiency, and iron maldistribution have accelerated our understanding of human iron homeostasis. This chapter provides an updated overview of the human iron cycle, regulation of iron homeostasis, and how perturbations in these homeostatic mechanisms lead to iron overload disease and provides strategies for the diagnosis of hereditary iron overload.

A nanoparticulate ferritin-core mimetic is well taken up by HuTu 80 duodenal cells and its absorption in mice is regulated by body iron

BACKGROUND

Iron (Fe) deficiency anemia remains the largest nutritional deficiency disorder worldwide. How the gut acquires iron from nano Fe(III), especially at the apical surface, is incompletely understood.

OBJECTIVE

We developed a novel Fe supplement consisting of nanoparticulate tartrate-modified Fe(III) poly oxo-hydroxide [here termed nano Fe(III)], which mimics the Fe oxide core of ferritin and effectively treats iron deficiency anemia in rats.

METHODS

We determined transfer to the systemic circulation of nano Fe(III) in iron-deficient and iron-sufficient outbread Swiss mouse strain (CD1) mice with use of (59)Fe-labeled material. Iron deficiency was induced before starting the Fe-supplementation period through reduction of Fe concentrations in the rodent diet. A control group of iron-sufficient mice were fed a diet with adequate Fe concentrations throughout the study. Furthermore, we conducted a hemoglobin repletion study in which iron-deficient CD1 mice were fed for 7 d a diet supplemented with ferrous sulfate (FeSO4) or nano Fe(III). Finally, we further probed the mechanism of cellular acquisition of nano Fe(III) by assessing ferritin formation, as a measure of Fe uptake and utilization, in HuTu 80 duodenal cancer cells with targeted inhibition of divalent metal transporter 1 (DMT1) and duodenal cytochrome b (DCYTB) before exposure to the supplemented iron sources. Differences in gene expression were assessed by quantitative polymerase chain reaction.

RESULTS

Absorption (means ± SEMs) of nano Fe(III) was significantly increased in iron-deficient mice (58 ± 19%) compared to iron-sufficient mice (18 ± 17%) (P = 0.0001). Supplementation of the diet with nano Fe(III) or FeSO4 significantly increased hemoglobin concentrations in iron-deficient mice (170 ± 20 g/L, P = 0.01 and 180 ± 20 g/L, P = 0.002, respectively). Hepatic hepcidin mRNA expression reflected the nonheme-iron concentrations of the liver and was also comparable for both nano Fe(III)- and FeSO4-supplemented groups, as were iron concentrations in the spleen and duodenum. Silencing of the solute carrier family 11 (proton-coupled divalent metal ion transporter), member 2 (Slc11a2) gene (DMT1) significantly inhibited ferritin formation from FeSO4 (P = 0.005) but had no effect on uptake and utilization of nano Fe(III). Inhibiting DCYTB with an antibody also had no effect on uptake and utilization of nano Fe(III) but significantly inhibited ferritin formation from ferric nitrilotriacetate chelate (Fe-NTA) (P = 0.04). Similarly, cellular ferritin formation from nano Fe(III) was unaffected by the Fe(II) chelator ferrozine, which significantly inhibited uptake and utilization from FeSO4 (P = 0.009) and Fe-NTA (P = 0.005).

CONCLUSIONS

Our data strongly support direct nano Fe(III) uptake by enterocytes as an efficient mechanism of dietary iron acquisition, which may complement the known Fe(II)/DMT1 uptake pathway.

The active role of vitamin C in mammalian iron metabolism: much more than just enhanced iron absorption!

Ascorbate is a cofactor in numerous metabolic reactions. Humans cannot synthesize ascorbate owing to inactivation of the gene encoding the enzyme l-gulono-γ-lactone oxidase, which is essential for ascorbate synthesis. Accumulating evidence strongly suggests that in addition to the known ability of dietary ascorbate to enhance nonheme iron absorption in the gut, ascorbate within mammalian systems can regulate cellular iron uptake and metabolism. Ascorbate modulates iron metabolism by stimulating ferritin synthesis, inhibiting lysosomal ferritin degradation, and decreasing cellular iron efflux. Furthermore, ascorbate cycling across the plasma membrane is responsible for ascorbate-stimulated iron uptake from low-molecular-weight iron-citrate complexes, which are prominent in the plasma of individuals with iron-overload disorders. Importantly, this iron-uptake pathway is of particular relevance to astrocyte brain iron metabolism and tissue iron loading in disorders such as hereditary hemochromatosis and β-thalassemia. Recent evidence also indicates that ascorbate is a novel modulator of the classical transferrin-iron uptake pathway, which provides almost all iron for cellular demands and erythropoiesis under physiological conditions. Ascorbate acts to stimulate transferrin-dependent iron uptake by an intracellular reductive mechanism, strongly suggesting that it may act to stimulate iron mobilization from the endosome. The ability of ascorbate to regulate transferrin iron uptake could help explain the metabolic defect that contributes to ascorbate-deficiency-induced anemia.

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.

Iron transport through ferroportin is induced by intracellular ascorbate and involves IRP2 and HIF2α

A few tightly regulated transport proteins mediate iron absorption across the intestinal epithelium. At the basolateral border of intestinal cells there is one identified transporter, ferroportin, for the transfer of intracellular iron to the vascular system. Here, we investigate the effects of ascorbate (vitamin C) on the regulation of ferroportin in human intestinal Caco-2 cells using ELISA and Western Blot analyses. The results indicate that ferroportin protein levels peak at 100 μM of added ascorbate with an increase of 274% (p = 0.02). At 150 μM of ascorbate, the increase was only 28% (p = 0.04), and at 200 μM there was no significant change from the baseline control. In addition, the ascorbate-induced, (at 150 μM) up-regulated ferroportin levels were associated with increased ⁵⁵Fe transport across the basolateral border (19%, p = 0.03). Ascorbate-induced up-regulation of cellular ferroportin levels (no added iron) was associated with increased levels of the iron regulatory protein IRP2 (230%, p = 0.0009), and the hypoxia-inducible factor HIF2α (69%, p = 0.03). Thus, iron transport across the basal border via ferroportin is influenced by the intracellular status of ascorbate and IRP2 and HIF2α are involved. We discuss possible reasons for the ascorbate-effects and the dependence of cellular growth conditions for iron transport-related protein expression.

Effect of genotype on duodenal expression of nutrient transporter genes in dairy cows

BACKGROUND

Studies have shown clear differences between dairy breeds in their feed intake and production efficiencies. The duodenum is critical in the coordination of digestion and absorption of nutrients. This study examined gene transcript abundance of important classes of nutrient transporters in the duodenum of non lactating dairy cows of different feed efficiency potential, namely Holstein-Friesian (HF), Jersey (JE) and their F1 hybrid. Duodenal epithelial tissue was collected at slaughter and stored at -80°C. Total RNA was extracted from tissue and reverse transcribed to generate cDNA. Gene expression of the following transporters, namely nucleoside; amino acid; sugar; mineral; and lipid transporters was measured using quantitative real-time RT-PCR. Data were statistically analysed using mixed models ANOVA in SAS. Orthogonal contrasts were used to test for potential heterotic effects and spearman correlation coefficients calculated to determine potential associations amongst gene expression values and production efficiency variables.

RESULTS

While there were no direct effects of genotype on expression values for any of the genes examined, there was evidence for a heterotic effect (P < 0.05) on ABCG8, in the form of increased expression in the F1 genotype compared to either of the two parent breeds. Additionally, a tendency for increased expression of the amino acid transporters, SLC3A1 (P = 0.072), SLC3A2 (P = 0.081) and SLC6A14 (P = 0.072) was also evident in the F1 genotype. A negative (P < 0.05) association was identified between the expression of the glucose transporter gene SLC5A1 and total lactational milk solids yield, corrected for body weight. Positive correlations (P < 0.05) were also observed between the expression values of genes involved in common transporter roles.

CONCLUSION

This study suggests that differences in the expression of sterol and amino acid transporters in the duodenum could contribute towards the documented differences in feed efficiency between HF, JE and their F1 hybrid. Furthermore, positive associations between the expression of genes involved in common transporter roles suggest that these may be co-regulated. The study identifies potential candidates for investigation of genetic variants regulating nutrient transport and absorption in the duodenum in dairy cows, which may be incorporated into future breeding programmes.

The ferric enterobactin transporter Fep is required for persistent Salmonella enterica serovar typhimurium infection

Most bacterial pathogens require iron to grow and colonize host tissues. The Gram-negative bacterium Salmonella enterica serovar Typhimurium causes a natural systemic infection of mice that models acute and chronic human typhoid fever. S. Typhimurium resides in tissues within cells of the monocyte lineage, which limit pathogen access to iron, a mechanism of nutritional immunity. The primary ferric iron import system encoded by Salmonella is the siderophore ABC transporter FepBDGC. The Fep system has a known role in acute infection, but it is unclear whether ferric iron uptake or the ferric iron binding siderophores enterobactin and salmochelin are required for persistent infection. We defined the role of the Fep iron transporter and siderophores in the replication of Salmonella in macrophages and in mice that develop acute followed by persistent infections. Replication of wild-type and iron transporter mutant Salmonella strains was quantified in cultured macrophages, fecal pellets, and host tissues in mixed- and single-infection experiments. We show that deletion of fepB attenuated Salmonella replication and colonization within macrophages and mice. Additionally, the genes required to produce and transport enterobactin and salmochelin across the outer membrane receptors, fepA and iroN, are needed for colonization of all tissues examined. However, salmochelin appears to be more important than enterobactin in the colonization of the spleen and liver, both sites of dissemination. Thus, the FepBDGC ferric iron transporter and the siderophores enterobactin and salmochelin are required by Salmonella to evade nutritional immunity in macrophages and cause persistent infection in mice.

Modulation of Dcytb (Cybrd 1) expression and function by iron, dehydroascorbate and Hif-2α in cultured cells

BACKGROUND

Duodenal cytochrome b (Dcytb) is a mammalian plasma ferric reductase enzyme that catalyses the reduction of ferric to ferrous ion in the process of iron absorption. The current study investigates the relationship between Dcytb, iron, dehydroascorbate (DHA) and Hif-2α in cultured cell lines.

METHODS

Dcytb and Hif-2α protein expression was analysed by Western blot technique while gene regulation was determined by quantitative PCR. Functional analyses were carried out by ferric reductase and (59)Fe uptake assays.

RESULTS

Iron and dehydroascorbic acid treatment of cells inhibited Dcytb mRNA and protein expression. Desferrioxamine also enhanced Dcytb mRNA level after cells were treated overnight. Dcytb knockdown in HuTu cells resulted in reduced mRNA expression and lowered reductase activity. Preloading cells with DHA (to enhance intracellular ascorbate levels) did not stimulate reductase activity fully in Dcytb-silenced cells, implying a Dcytb-dependence of ascorbate-mediated ferrireduction. Moreover, Hif-2α knockdown in HuTu cells led to a reduction in reductase activity and iron uptake.

CONCLUSIONS

Taken together, this study shows the functional regulation of Dcytb reductase activity by DHA and Hif-2α.

GENERAL SIGNIFICANCE

Dcytb is a plasma membrane protein that accepts electrons intracellularly from DHA/ascorbic acid for ferrireduction at the apical surface of cultured cells and enterocytes.

Mammalian iron homeostasis in health and disease: uptake, storage, transport, and molecular mechanisms of action

Iron is a crucial factor for life. However, it also has the potential to cause the formation of noxious free radicals. These double-edged sword characteristics demand a tight regulation of cellular iron metabolism. In this review, we discuss the various pathways of cellular iron uptake, cellular iron storage, and transport. Recent advances in understanding the reduction and uptake of non-transferrin-bound iron are discussed. We also discuss the recent progress in the understanding of transcriptional and translational regulation by iron. Furthermore, we discuss recent advances in the understanding of the regulation of cellular and systemic iron homeostasis and several key diseases resulting from iron deficiency and overload. We also discuss the knockout mice available for studying iron metabolism and the related human conditions.

Expression and localization of duodenal cytochrome b in the rat hippocampus after kainate-induced excitotoxicity

Brain iron accumulation and oxidative stress are common features of many neurodegenerative diseases, and could be due in part to increased iron influx across the blood-brain interface. The iron transport protein, divalent metal transporter 1 (DMT1) is found in reactive astrocytes of the lesioned hippocampal CA fields after excitotoxicity induced by the glutamate analog kainate (KA), but in order for iron to be transported by DMT1, it must be converted from the ferric to the ferrous form. The present study was carried out to investigate the expression of a ferric reductase, duodenal cytochrome b (DCYTB), in the rat hippocampus after KA injury. Quantitative reverse transcriptase-polymerase chain reaction showed significant increases in DCYTB mRNA expression of 2.5, 2.7, and 5.2-fold in the hippocampus at 1week, 2weeks and 1month post-KA lesions respectively compared to untreated controls, and 3.0-fold compared to 1month post-saline injection. DCYTB-positive cells were double labeled with glial fibrillary acidic protein, and electron microscopy showed that the DCYTB-positive cells had dense bundles of glial filaments, characteristic of astrocytes, and were present as end-feet around unlabeled brain capillary endothelial cells. DMT1 labeling in astrocytes and increased iron staining were also observed in the lesioned hippocampus. Together, the present findings of DCYTB and DMT1 localization in astrocytes suggest that DCYTB is a ferric reductase for reduction of ferric iron, for transport by DMT1 into the brain. We postulate that the coordinated action of these two proteins could be important in iron influx across the blood-brain interface, in areas undergoing neurodegeneration.

Regulatory effects of Cu, Zn, and Ca on Fe absorption: the intricate play between nutrient transporters

Iron is an essential nutrient for almost every living organism because it is required in a number of biological processes that serve to maintain life. In humans, recycling of senescent erythrocytes provides most of the daily requirement of iron. In addition, we need to absorb another 1–2 mg Fe from the diet each day to compensate for losses due to epithelial sloughing, perspiration, and bleeding. Iron absorption in the intestine is mainly regulated on the enterocyte level by effectors in the diet and systemic regulators accessing the enterocyte through the basal lamina. Recently, a complex meshwork of interactions between several trace metals and regulatory proteins was revealed. This review focuses on advances in our understanding of Cu, Zn, and Ca in the regulation of iron absorption. Ascorbate as an important player is also considered.

Intestinal dehydroascorbic acid (DHA) transport mediated by the facilitative sugar transporters, GLUT2 and GLUT8

Intestinal vitamin C (Asc) absorption was believed to be mediated by the Na(+)-dependent ascorbic acid transporter SVCT1. However, Asc transport across the intestines of SVCT1 knock-out mice is normal indicating that alternative ascorbic acid transport mechanisms exist. To investigate these mechanisms, rodents were gavaged with Asc or its oxidized form dehydroascorbic acid (DHA), and plasma Asc concentrations were measured. Asc concentrations doubled following DHA but not Asc gavage. We hypothesized that the transporters responsible were facilitated glucose transporters (GLUTs). Using Xenopus oocyte expression, we investigated whether facilitative glucose transporters GLUT2 and GLUT5-12 transported DHA. Only GLUT2 and GLUT8, known to be expressed in intestines, transported DHA with apparent transport affinities (Km) of 2.33 and 3.23 mm and maximal transport rates (Vmax) of 25.9 and 10.1 pmol/min/oocyte, respectively. Maximal rates for DHA transport mediated by GLUT2 and GLUT8 in oocytes were lower than maximal rates for 2-deoxy-d-glucose (Vmax of 224 and 32 pmol/min/oocyte for GLUT2 and GLUT8, respectively) and fructose (Vmax of 406 and 116 pmol/min/oocyte for GLUT2 and GLUT8, respectively). These findings may be explained by differences in the exofacial binding of substrates, as shown by inhibition studies with ethylidine glucose. DHA transport activity in GLUT2- and GLUT8-expressing oocytes was inhibited by glucose, fructose, and by the flavonoids phloretin and quercetin. These studies indicate intestinal DHA transport may be mediated by the facilitative sugar transporters GLUT2 and GLUT8. Furthermore, dietary sugars and flavonoids in fruits and vegetables may modulate Asc bioavailability via inhibition of small intestinal GLUT2 and GLUT8.

Duodenal reductase activity and spleen iron stores are reduced and erythropoiesis is abnormal in Dcytb knockout mice exposed to hypoxic conditions

Duodenal cytochrome b (Dcytb, Cybrd1) is a ferric reductase localized in the duodenum that is highly upregulated in circumstances of increased iron absorption. To address the contribution of Dcytb to total duodenal ferric reductase activity as well as its wider role in iron metabolism, we first measured duodenal ferric reductase activity in wild-type (WT) and Dcytb knockout (Dcytb(-/-)) mice under 3 conditions known to induce gut ferric reductase: dietary iron deficiency, hypoxia, and pregnancy. Dcytb(-/-) and WT mice were randomly assigned to control (iron deficiency experiment, 48 mg/kg dietary iron; hypoxia experiment, normal atmospheric pressure; pregnancy experiment, nonpregnant animals) or treatment (iron deficiency experiment, 2-3 mg/kg dietary iron; hypoxia experiment, 53.3 kPa pressure; pregnancy experiment, d 20 of pregnancy) groups and duodenal reductase activity measured. We found no induction of ferric reductase activity in Dcytb(-/-) mice under any of these conditions, indicating there are no other inducible ferric reductases present in the duodenum. To test whether Dcytb was required for iron absorption in conditions with increased erythropoietic demand, we also measured tissue nonheme iron levels and hematological indices in WT and Dcytb(-/-) mice exposed to hypoxia. There was no evidence of gross alterations in iron absorption, hemoglobin, or total liver nonheme iron in Dcytb(-/-) mice exposed to hypoxia compared with WT mice. However, spleen nonheme iron was significantly less (6.7 ± 1.0 vs. 12.7 ± 0.9 nmol · mg tissue(-1); P < 0.01, n = 7-8) in hypoxic Dcytb(-/-) compared with hypoxic WT mice and there was evidence of impaired reticulocyte hemoglobinization with a lower reticulocyte mean corpuscular hemoglobin (276 ± 1 vs. 283 ± 2 g · L(-1); P < 0.05, n = 7-8) in normoxic Dcytb(-/-) compared with normoxic WT mice. We therefore conclude that DCYTB is the primary iron-regulated duodenal ferric reductase in the gut and that Dcytb is necessary for optimal iron metabolism.

Pharmacology of iron transport

Elucidating the molecular basis for the regulation of iron uptake, storage, and distribution is necessary to understand iron homeostasis. Pharmacological tools are emerging to identify and distinguish among different iron transport pathways. Stimulatory or inhibitory small molecules with effects on iron uptake can help characterize the mechanistic elements of iron transport and the roles of the transporters involved in these processes. In particular, iron chelators can serve as potential pharmacological tools to alleviate diseases of iron overload. This review focuses on the pharmacology of iron transport, introducing iron transport membrane proteins and known inhibitors.

Incidence patterns of small bowel cancer in a population-based study in Sweden: increase in duodenal adenocarcinoma

BACKGROUND

Increasing incidences of malignant small bowel tumours have been reported, but data from European populations are limited. This study aimed to clarify the incidence patterns of malignant small bowel tumours in Sweden.

METHODS

Patients with a first and primary malignant small bowel tumour were identified from the Swedish Cancer Register during the study period 1960-2009. Sex-specific and age-standardised incidence rates of these tumours were calculated by their anatomical location and histological type in different time periods. Figures were plotted to show the proportions and incidence rates over time, and joinpoint loglinear regression models were estimated to assess any time trends.

RESULTS

A total of 6604 patients with malignant small bowel tumours were identified. The age-standardised incidence of all malignant small bowel tumours increased from 14.2 to 19.7 per 1,000,000 person-years during the study period. The incidence of duodenal cancer increased more than 3-fold (from 1.6 to 5.4 per 1,000,000 person-years), which was mainly expained by a dramatical rising trend of adenocarcinoma of the duodenum (from 0.7 to 4.2 per 1,000,000 person-years). Malignant tumours of small bowel with unspecified anatomical locations showed a slight increase (from 7.0 to 7.9 per 1,000,000). The incidence of small bowel tumours in other locations or of other histological types was more stable.

CONCLUSIONS

The incidence of small bowel malignancies has increased during the period 1960-2009. Among the specific types of small bowel cancer, a particularly rapid increase was found for duodenal adenocarcinoma.

High-yield production, purification and characterization of functional human duodenal cytochrome b in an Escherichia coli system

Human duodenal cytochrome b (Dcytb) is a transmembrane hemoprotein found in the duodenal brush border membrane and in erythrocytes. Dcytb has been linked to uptake of dietary iron and to ascorbate recycling in erythrocytes. Detailed biophysical and biochemical characterization of Dcytb has been limited by difficulties in expressing sufficient amounts of functional recombinant protein in yeast and insect cell systems. We have developed an Escherichia coli Rosetta-gami B(DE3) cell system for production of recombinant His-tagged human Dcytb with a yield of ∼26 mg of purified, ascorbate-reducible cytochrome per liter of culture. The recombinant protein is readily solubilized with n-dodecyl-β-D-maltoside and purified to electrophoretic homogeneity by one-step chromatography on cobalt affinity resin. The purified recombinant Dcytb has a heme to protein ratio very close to the theoretical value of 2 and retains functional reactivity with ascorbate, as assessed by spectroscopic and kinetic measurements. Ascorbate showed a marked kinetic selectivity for the high-potential heme center over the low-potential heme center in purified Dcytb. This new E. coli expression system for Dcytb offers ∼7-fold improvement in yield and other substantial advantages over existing expression systems for reliable production of functional Dcytb at levels suitable for biochemical, biophysical and structural characterization.

A cytochrome b561 with ferric reductase activity from the parasitic blood fluke, Schistosoma japonicum

BACKGROUND

Iron has an integral role in numerous cellular reactions and is required by virtually all organisms. In physiological conditions, iron is abundant in a largely insoluble ferric state. Ferric reductases are an essential component of iron uptake by cells, reducing iron to the soluble ferrous form. Cytochromes b561 (cyts-b561) are a family of ascorbate reducing transmembrane proteins found in most eukaryotic cells. The identification of the ferric reductase duodenal cytochrome b (dcytb) and recent observations that other cyts-b561 may be involved in iron metabolism have opened novel perspectives for elucidating their physiological function.

METHODOLOGY/PRINCIPAL FINDINGS

Here we have identified a new member of the cytochrome b561 (Sjcytb561) family in the pathogenic blood fluke Schistosoma japonicum that localises to the outer surface of this parasitic trematode. Heterologous expression of recombinant Sjcyt-b561 in a Saccharomyces cerevisiae mutant strain that lacks plasma membrane ferrireductase activity demonstrated that the molecule could rescue ferric reductase activity in the yeast.

SIGNIFICANCE/CONCLUSIONS

This finding of a new member of the cytochrome b561 family further supports the notion that a ferric reductase function is likely for other members of this protein family. Additionally, the localisation of Sjcytb561 in the surface epithelium of these blood-dwelling schistosomes contributes further to our knowledge concerning nutrient acquisition in these parasites and may provide novel targets for therapeutic intervention.