Dcytb (Cybrd1) functions as both a ferric and a cupric reductase in vitro

Copper homeostasis in chronic kidney disease and its crosstalk with ferroptosis

Chronic kidney disease (CKD) has become a global public health problem with high morbidity and mortality. Renal fibrosis can lead to end-stage renal disease (ESRD). However, there is still no effective treatment to prevent or delay the progression of CKD into ESRD. Therefore, exploring the pathogenesis of CKD is essential for preventing and treating CKD. There are a variety of trace elements in the human body that interact with each other within a complex regulatory network. Iron and copper are both vital trace elements in the body. They are critical for maintaining bodily functions, and the dysregulation of their metabolism can cause many diseases, including kidney disease. Ferroptosis is a new form of cell death characterized by iron accumulation and lipid peroxidation. Studies have shown that ferroptosis is closely related to kidney disease. However, the role of abnormal copper metabolism in kidney disease and its relationship with ferroptosis remains unclear. Here, our current knowledge regarding copper metabolism, its regulatory mechanism, and the role of abnormal copper metabolism in kidney diseases is summarized. In addition, we discuss the relationship between abnormal copper metabolism and ferroptosis to explore the possible pathogenesis and provide a potential therapeutic target for CKD.

Copper in Gynecological Diseases

Copper (Cu) is an essential micronutrient for the correct development of eukaryotic organisms. This metal plays a key role in many cellular and physiological activities, including enzymatic activity, oxygen transport, and cell signaling. Although the redox activity of Cu is crucial for enzymatic reactions, this property also makes it potentially toxic when found at high levels. Due to this dual action of Cu, highly regulated mechanisms are necessary to prevent both the deficiency and the accumulation of this metal since its dyshomeostasis may favor the development of multiple diseases, such as Menkes' and Wilson's diseases, neurodegenerative diseases, diabetes mellitus, and cancer. As the relationship between Cu and cancer has been the most studied, we analyze how this metal can affect three fundamental processes for tumor progression: cell proliferation, angiogenesis, and metastasis. Gynecological diseases are characterized by high prevalence, morbidity, and mortality, depending on the case, and mainly include benign and malignant tumors. The cellular processes that promote their progression are affected by Cu, and the mechanisms that occur may be similar. We analyze the crosstalk between Cu deregulation and gynecological diseases, focusing on therapeutic strategies derived from this metal.

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.

The mechanism of copper transporters in ovarian cancer cells and the prospect of cuproptosis

Copper transporters can not only carry copper (Cu) to maintain the homeostasis of Cu in cells but also transport platinum-based chemotherapy drugs. The effect of copper transporters on chemosensitivity has been demonstrated in a variety of malignancies. In addition, recent studies have reported that copper transporters can act as vectors to induce cuproptosis. Therefore, copper transporters can act on cells through different mechanisms to achieve different purposes. This review mainly describes the current research progress of the intracellular transport mechanism of copper transporters and cuproptosis, and prospects for the application of them in the treatment of ovarian cancer (OC).

Iron accumulation and lipid peroxidation: implication of ferroptosis in diabetic cardiomyopathy

Diabetic cardiomyopathy (DC) is a serious heart disease caused by diabetes. It is unrelated to hypertension and coronary artery disease and can lead to heart insufficiency, heart failure and even death. Currently, the pathogenesis of DC is unclear, and clinical intervention is mainly symptomatic therapy and lacks effective intervention objectives. Iron overdose mediated cell death, also known as ferroptosis, is widely present in the physiological and pathological processes of diabetes and DC. Iron is a key trace element in the human body, regulating the metabolism of glucose and lipids, oxidative stress and inflammation, and other biological processes. Excessive iron accumulation can lead to the imbalance of the antioxidant system in DC and activate and aggravate pathological processes such as excessive autophagy and mitochondrial dysfunction, resulting in a chain reaction and accelerating myocardial and microvascular damage. In-depth understanding of the regulating mechanisms of iron metabolism and ferroptosis in cardiovascular vessels can help improve DC management. Therefore, in this review, we summarize the relationship between ferroptosis and the pathogenesis of DC, as well as potential intervention targets, and discuss and analyze the limitations and future development prospects of these targets.

Mitochondrial copper in human genetic disorders

Copper is an essential micronutrient that serves as a cofactor for enzymes involved in diverse physiological processes, including mitochondrial energy generation. Copper enters cells through a dedicated copper transporter and is distributed to intracellular cuproenzymes by copper chaperones. Mitochondria are critical copper-utilizing organelles that harbor an essential cuproenzyme cytochrome c oxidase, which powers energy production. Mutations in copper transporters and chaperones that perturb mitochondrial copper homeostasis result in fatal genetic disorders. Recent studies have uncovered the therapeutic potential of elesclomol, a copper ionophore, for the treatment of copper deficiency disorders such as Menkes disease. Here we review the role of copper in mitochondrial energy metabolism in the context of human diseases and highlight the recent developments in copper therapeutics.

Mineral metabolism and ferroptosis in non-alcoholic fatty liver diseases

Nonalcoholic fatty liver disease (NAFLD) has become the most prevalent chronic liver disease worldwide. Minerals including iron, copper, zinc, and selenium, fulfil an essential role in various biochemical processes. Moreover, the identification of ferroptosis and cuproptosis further underscores the importance of intracellular mineral homeostasis. However, perturbation of minerals has been frequently reported in patients with NAFLD and related diseases. Interestingly, studies have attempted to establish an association between mineral disorders and NAFLD pathological features, including oxidative stress, mitochondrial dysfunction, inflammatory response, and fibrogenesis. In this review, we aim to provide an overview of the current understanding of mineral metabolism (i.e., absorption, utilization, and transport) and mineral interactions in the pathogenesis of NAFLD. More importantly, this review highlights potential therapeutic strategies, challenges, future directions for targeting mineral metabolism in the treatment of NAFLD.

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.

Expression profiles of iron transport molecules along the duodenum

Duodenal biopsies are considered a suitable source of enterocytes for studies of dietary iron absorption. However, the expression level of molecules involved in iron absorption may vary along the length of duodenum. We aimed to determine whether the expression of molecules involved in the absorption of heme and non-heme iron differs depending on the location in the duodenum. Analysis was performed with samples of duodenal biopsies from 10 individuals with normal iron metabolism. Samples were collected at the following locations: (a) immediately post-bulbar, (b) 1-2 cm below the papilla of Vater and (c) in the distal duodenum. The gene expression was analyzed at the mRNA and protein level using real-time PCR and Western blot analysis. At the mRNA level, significantly different expression of HCP1, DMT1, ferroportin and Zip8 was found at individual positions of duodenum. Position-dependent expression of other molecules, especially of FLVCR1, HMOX1 and HMOX2 was also detected but with no statistical significances. At the protein level, we observed statistically significantly decreasing expression of transporters HCP1, FLVCR1, DMT1, ferroportin, Zip14 and Zip8 with advancing positions of duodenum. Our results are consistent with a gradient of diminishing iron absorption along the duodenum for both heme and non-heme iron.

Functional significance and physiological regulation of essential trace metals in fish

Trace metals such as iron, copper, zinc and manganese play essential roles in various biological processes in fish, including development, energy metabolism and immune response. At embryonic stages, fish obtain essential metals primarily from the yolk, whereas in later life stages (i.e. juvenile and adult), the gastrointestine and the gill are the major sites for the acquisition of trace metals. On a molecular level, the absorption of metals is thought to occur at least in part via specific metal ion transporters, including the divalent metal transporter-1 (DMT1), copper transporter-1 (CTR1), and Zrt- and Irt-like proteins (ZIP). A variety of other proteins are also involved in maintaining cellular and systemic metal homeostasis. Interestingly, the expression and function of these metal transport- and metabolism-related proteins can be influenced by a range of trace metals and major ions. Increasing evidence also demonstrates an interplay between the gastrointestine and the gill for the regulation of trace metal absorption. Therefore, there is a complex network of regulatory and compensatory mechanisms involved in maintaining trace metal balance. Yet, an array of factors is known to influence metal metabolism in fish, such as hormonal status and environmental changes. In this Review, we summarize the physiological significance of iron, copper, zinc and manganese, and discuss the current state of knowledge on the mechanisms underlying transepithelial metal ion transport, metal-metal interactions, and cellular and systemic handling of these metals in fish. Finally, we identify knowledge gaps in the regulation of metal homeostasis and discuss potential future research directions.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

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.

Dietary Iron Intake in Excess of Requirements Impairs Intestinal Copper Absorption in Sprague Dawley Rat Dams, Causing Copper Deficiency in Suckling Pups

Physiologically relevant iron-copper interactions have been frequently documented. For example, excess enteral iron inhibits copper absorption in laboratory rodents and humans. Whether this also occurs during pregnancy and lactation, when iron supplementation is frequently recommended, is, however, unknown. Here, the hypothesis that high dietary iron will perturb copper homeostasis in pregnant and lactating dams and their pups was tested. We utilized a rat model of iron-deficiency/iron supplementation during pregnancy and lactation to assess this possibility. Rat dams were fed low-iron diets early in pregnancy, and then switched to one of 5 diets with normal (1×) to high iron (20×) until pups were 14 days old. Subsequently, copper and iron homeostasis, and intestinal copper absorption (by oral, intragastric gavage with ⁶⁴Cu), were assessed. Copper depletion/deficiency occurred in the dams and pups as dietary iron increased, as evidenced by decrements in plasma ceruloplasmin (Cp) and superoxide dismutase 1 (SOD1) activity, depletion of hepatic copper, and liver iron loading. Intestinal copper transport and tissue ⁶⁴Cu accumulation were lower in dams consuming excess iron, and tissue ⁶⁴Cu was also low in suckling pups. In some cases, physiological disturbances were noted when dietary iron was only ~3-fold in excess, while for others, effects were observed when dietary iron was 10–20-fold in excess. Excess enteral iron thus antagonizes the absorption of dietary copper, causing copper depletion in dams and their suckling pups. Low milk copper is a likely explanation for copper depletion in the pups, but experimental proof of this awaits future experimentation.

Relationship between Down-Regulation of Copper-Related Genes and Decreased Ferroportin Protein Level in the Duodenum of Iron-Deficient Piglets

In mammals, 2 × 10¹² red blood cells (RBCs) are produced every day in the bone marrow to ensure a constant supply of iron to maintain effective erythropoiesis. Impaired iron absorption in the duodenum and inefficient iron reutilization from senescent RBCs by macrophages contribute to the development of anemia. Ferroportin (Fpn), the only known cellular iron exporter, as well as hephaestin (Heph) and ceruloplasmin, two copper-dependent ferroxidases involved in the above-mentioned processes, are key elements of the interaction between copper and iron metabolisms. Crosslinks between these metals have been known for many years, but metabolic effects of one on the other have not been elucidated to date. Neonatal iron deficiency anemia in piglets provides an interesting model for studying this interplay. In duodenal enterocytes of young anemic piglets, we identified iron deposits and demonstrated increased expression of ferritin with a concomitant decline in both Fpn and Heph expression. We postulated that the underlying mechanism involves changes in copper distribution within enterocytes as a result of decreased expression of the copper transporter—Atp7b. Obtained results strongly suggest that regulation of iron absorption within enterocytes is based on the interaction between proteins of copper and iron metabolisms and outcompetes systemic regulation.

L-Cysteine-mediated modulation of copper trafficking in prostate cancer cells: an in vitro and in vivo investigation with 64Cu and 64Cu-PET

Copper imbalance is implicated in many diseases, including cancer. Copper in blood is mainly transported by carrier proteins but a small fraction is bound to low molecular weight species, possibly amino acids. Their roles in cellular copper delivery are unknown. Our aim was to test whether accumulation of 64Cu into cancer-derived cells can be influenced by copper-binding serum amino acids. In vitro cellular accumulation of 64Cu was measured in Hank's Balanced Salt Solution in the presence of 100 μM l-histidine, l-methionine, l-cysteine and l-threonine. l-Cysteine markedly increased 64Cu accumulation and retention in DU145, PC3 and SK-OV-3 cells, while some other cell lines did not show an effect. This effect was not due to 64Cu delivery in the form of a 64Cu-cysteine complex, nor to reduction of 64Cu(ii) to 64Cu(i) by l-cysteine. Pre-incubation of cells with l-cysteine increased 64Cu accumulation, even if l-cysteine was removed from HBSS before 64Cu was added. The effect of l-cysteine on 64Cu accumulation was not mediated by increased glutathione synthesis. Despite the demonstrable in vitro effect, pre-injection of l-cysteine precursor N-acetyl-cysteine (NAC) in vivo did not enhance 64Cu delivery to DU145 xenografts in mice. Instead, it decreased 64Cu accumulation in the DU145 tumour and in brain, as assessed by PET imaging. We conclude that 64Cu is not delivered to DU145 cancer cells in vitro as a complex with amino acids but its cellular accumulation is enhanced by l-cysteine or NAC influx to cells. The latter effect was not demonstrable in vivo in the DU145 xenograft.

Cu Homeostasis in Bacteria: The Ins and Outs

Copper (Cu) is an essential trace element for all living organisms and used as cofactor in key enzymes of important biological processes, such as aerobic respiration or superoxide dismutation. However, due to its toxicity, cells have developed elaborate mechanisms for Cu homeostasis, which balance Cu supply for cuproprotein biogenesis with the need to remove excess Cu. This review summarizes our current knowledge on bacterial Cu homeostasis with a focus on Gram-negative bacteria and describes the multiple strategies that bacteria use for uptake, storage and export of Cu. We furthermore describe general mechanistic principles that aid the bacterial response to toxic Cu concentrations and illustrate dedicated Cu relay systems that facilitate Cu delivery for cuproenzyme biogenesis. Progress in understanding how bacteria avoid Cu poisoning while maintaining a certain Cu quota for cell proliferation is of particular importance for microbial pathogens because Cu is utilized by the host immune system for attenuating pathogen survival in host cells.

The molecular mechanisms of copper metabolism and its roles in human diseases

Copper is an essential element in cells; it can act as either a recipient or a donor of electrons, participating in various reactions. However, an excess of copper ions in cells is detrimental as these copper ions can generate free radicals and increase oxidative stress. In multicellular organisms, copper metabolism involves uptake, distribution, sequestration, and excretion, at both the cellular and systemic levels. Mammalian enterocytes take in bioavailable copper ions from the diet in a Ctr1-dependent manner. After incorporation, cuprous ions are delivered to ATP7A, which pumps Cu⁺ from enterocytes into the blood. Copper ions arrive at the liver through the portal vein and are incorporated into hepatocytes by Ctr1. Then, Cu⁺ can be secreted into the bile or the blood via the Atox1/ATP7B/ceruloplasmin route. In the bloodstream, this micronutrient can reach peripheral tissues and is again incorporated by Ctr1. In peripheral tissue cells, cuprous ions are either sequestrated by molecules such as metallothioneins or targeted to utilization pathways by chaperons such as Atox1, Cox17, and CCS. Copper metabolism must be tightly controlled in order to achieve homeostasis and avoid disorders. A hereditary or acquired copper unbalance, including deficiency, overload, or misdistribution, may cause or aggravate certain diseases such as Menkes disease, Wilson disease, neurodegenerative diseases, anemia, metabolic syndrome, cardiovascular diseases, and cancer. A full understanding of copper metabolism and its roles in diseases underlies the identification of novel effective therapies for such diseases.

Body distribution of stable copper isotopes during the progression of cholestatic liver disease induced by common bile duct ligation in mice

Patients with chronic liver disease from different aetiologies show a light serum Cu isotopic composition compared to the reference population, with the enrichment in the 63Cu isotope correlating with the severity of the disease. However, the mechanisms underlying Cu isotope fractionation at the onset and during progression of the disease are still unclear. In this work, a common bile duct ligation (CBDL) murine model was used to investigate the effect of cholestasis-induced liver disease on the Cu isotopic composition. Wild type male and female mice underwent surgical ligation of the common bile duct and were sacrificed 2, 4 and 6 weeks, and 4, 6 and 8 weeks after the surgical intervention, respectively. The age- and gender-matched control mice underwent sham surgery. Disease progression was evaluated using serum bilirubin levels, hepatic pro-inflammatory chemokine levels and Metavir fibrosis score. CBDL-operated mice show an overall body enrichment in the light isotope 63Cu. The Cu isotopic composition of organs, bone and serum becomes gradually lighter compared to the sham-operated mice with increasing severity of the disease. The light Cu isotopic composition of the CBDL-operated mice might result from an altered Cu intake and/or excretion. As the intestinal uptake of dietary Cu is largely mediated by transporters of Cu(i), mRNA and protein expression levels of two major metal transporters (CTR1 and DMT1) and Cu reductases (STEAP proteins and duodenal cytochrome B) were examined in the duodenal tissues as potential factors inducing Cu isotope fractionation. However, no significant differences in protein expression levels were observed between the CBDL- and sham-operated mice.

The cbb 3-type cytochrome oxidase assembly factor CcoG is a widely distributed cupric reductase

Copper (Cu)-containing proteins execute essential functions in prokaryotic and eukaryotic cells, but their biogenesis is challenged by high Cu toxicity and the preferential presence of Cu(II) under aerobic conditions, while Cu(I) is the preferred substrate for Cu chaperones and Cu-transport proteins. These proteins form a coordinated network that prevents Cu accumulation, which would lead to toxic effects such as Fenton-like reactions and mismetalation of other metalloproteins. Simultaneously, Cu-transport proteins and Cu chaperones sustain Cu(I) supply for cuproprotein biogenesis and are therefore essential for the biogenesis of Cu-containing proteins. In eukaryotes, Cu(I) is supplied for import and trafficking by cell-surface exposed metalloreductases, but specific cupric reductases have not been identified in bacteria. It was generally assumed that the reducing environment of the bacterial cytoplasm would suffice to provide sufficient Cu(I) for detoxification and cuproprotein synthesis. Here, we identify the proposed cbb ₃-type cytochrome c oxidase (cbb ₃-Cox) assembly factor CcoG as a cupric reductase that binds Cu via conserved cysteine motifs and contains 2 low-potential [4Fe-4S] clusters required for Cu(II) reduction. Deletion of ccoG or mutation of the cysteine residues results in defective cbb ₃-Cox assembly and Cu sensitivity. Furthermore, anaerobically purified CcoG catalyzes Cu(II) but not Fe(III) reduction in vitro using an artificial electron donor. Thus, CcoG is a bacterial cupric reductase and a founding member of a widespread class of enzymes that generate Cu(I) in the bacterial cytosol by using [4Fe-4S] clusters.

Mobilization of iron from ferritin: new steps and details

Much evidence indicates that iron stored in ferritin is mobilized through protein degradation in lysosomes, but concerns about this process have lingered, and the mechanistic details of its aspects are lacking. In the studies presented here, ⁵⁹Fe-labeled ferritin was induced by preloading hepatic (HepG2) cells with radiolabeled Fe. Placing these cells in a medium containing desferrioxamine resulted in the loss of ferritin-⁵⁹Fe, but adding high concentrations of reducing agents or modulating the internal GSH concentration failed to alter the rates of ferritin-⁵⁹Fe release. Confocal microscopy showed that Fe deprivation increased the movement of ferritin into lysosomes and hyperaccumulation was observed when lysosomal proteolysis was inhibited. It also resulted in the rapid movement of DMT1 to lysosomes, which was inhibited by bafilomycin. Ferrihydrite crystals isolated from purified rat liver/spleen ferritin were solubilized at pH 5 and 7 by GSH, ascorbate, citrate and lysosomal fluids obtained from livers and J774a.1 macrophages. The inhibition of DMT1/Nramp2 and siRNA knockdown of Nramp1 each reduced the transfer of ⁵⁹Fe from lysosomes to the cytosol; and hepatocyte-specific knockout of DMT1 in mice prevented the release of Fe from the liver responding to EPO treatment, but did not inhibit lysosomal ferritin degradation. We conclude that ferritin-Fe mobilization does not occur through changes in cellular concentrations of reducing/chelating agents but by the coordinated movement of ferritin and DMT1 to lysosomes, where the ferrihydrite crystals exposed by ferritin degradation dissolve in the lysosomal fluid, and the reduced iron is transported back to the cytosol via DMT1 in hepatocytes, and by both DMT1 and Nramp1 in macrophages, prior to release into the blood or storage in ferritin.

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.

Long-Term High-Fat Diet Decreases Hepatic Iron Storage Associated with Suppressing TFR2 and ZIP14 Expression in Rats

High-fat diet-induced obesity is known to disturb hepatic iron metabolism in a time-dependent manner. The mechanism of decreased hepatic iron deposits induced by long-term high-fat diet needs to be further investigated. In this study, 24 6-week-old male Sprague-Dawley rats were given a 16-week high-fat diet and hepatic iron metabolism was examined. High-fat diet feeding considerably decreased hepatic iron contents, enhanced transferrin expression, and reduced the expression of ferritin heavy chain, ferritin light chain, and hepatic iron uptake-related proteins (transferrin receptor 2, TFR2, and ZRT/IRT-like protein 14, ZIP14) in rats. Impaired expression of hepatic TFR2 coincided with DNA hypermethylation on the promoter and repressed expression of transcription factor hepatocyte nuclear factor 4α (HNF4α). miR-181 family expression was markedly increased and verified to regulate Zip14 expression by the dual-luciferase reporter system. Taken together, long-term high-fat diet decreases hepatic iron storage, which is closely linked to inhibition of liver iron transport through the TFR2 and ZIP14-dependent pathway.

Copper Metabolism of Newborns Is Adapted to Milk Ceruloplasmin as a Nutritive Source of Copper: Overview of the Current Data

Copper, which can potentially be a highly toxic agent, is an essential nutrient due to its role as a cofactor for cuproenzymes and its participation in signaling pathways. In mammals, the liver is a central organ that controls copper turnover throughout the body, including copper absorption, distribution, and excretion. In ontogenesis, there are two types of copper metabolism, embryonic and adult, which maintain the balance of copper in each of these periods of life, respectively. In the liver cells, these types of metabolism are characterized by the specific expression patterns and activity levels of the genes encoding ceruloplasmin, which is the main extracellular ferroxidase and copper transporter, and the proteins mediating ceruloplasmin metalation. In newborns, the molecular genetic mechanisms responsible for copper homeostasis and the ontogenetic switch from embryonic to adult copper metabolism are highly adapted to milk ceruloplasmin as a dietary source of copper. In the mammary gland cells, the level of ceruloplasmin gene expression and the alternative splicing of its pre-mRNA govern the amount of ceruloplasmin in the milk, and thus, the amount of copper absorbed by a newborn is controlled. In newborns, the absorption, distribution, and accumulation of copper are adapted to milk ceruloplasmin. If newborns are not breast-fed in the early stages of postnatal development, they do not have this natural control ensuring alimentary copper balance in the body. Although there is still much to be learned about the neonatal consequences of having an imbalance of copper in the mother/newborn system, the time to pay attention to this problem has arrived because the neonatal misbalance of copper may provoke the development of copper-related 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.

The teleos of metallo-reduction and metallo-oxidation in eukaryotic iron and copper trafficking

Eukaryotic cells, whether free-living or organismal, rely on metallo-reductases to process environmental ferric iron and cupric copper prior to uptake. In addition, some free-living eukaryotes (e.g. fungi and algae) couple ferri-reduction to ferro-oxidation, a process catalyzed by a small cohort of multi-copper oxidases; in these organisms, the ferric iron product is a ligand for cell iron uptake via a ferric iron permease. In addition to their support of iron uptake in lower eukaryotes, ferroxidases support ferrous iron efflux in Chordata; in this process the release of the ferrous iron from the efflux transporter is catalyzed by its ferroxidation. Last, ferroxidases also catalyze the oxidation of cuprous copper and, as metallo-oxidases, mirror the dual activity of the metallo-reductases. This Perspective examines the teleos of the yin-yang of this redox cycling of iron and copper in their metabolism.

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.

Mathematical Modeling of Intestinal Iron Absorption Using Genetic Programming

Iron is a trace metal, key for the development of living organisms. Its absorption process is complex and highly regulated at the transcriptional, translational and systemic levels. Recently, the internalization of the DMT1 transporter has been proposed as an additional regulatory mechanism at the intestinal level, associated to the mucosal block phenomenon. The short-term effect of iron exposure in apical uptake and initial absorption rates was studied in Caco-2 cells at different apical iron concentrations, using both an experimental approach and a mathematical modeling framework. This is the first report of short-term studies for this system. A non-linear behavior in the apical uptake dynamics was observed, which does not follow the classic saturation dynamics of traditional biochemical models. We propose a method for developing mathematical models for complex systems, based on a genetic programming algorithm. The algorithm is aimed at obtaining models with a high predictive capacity, and considers an additional parameter fitting stage and an additional Jackknife stage for estimating the generalization error. We developed a model for the iron uptake system with a higher predictive capacity than classic biochemical models. This was observed both with the apical uptake dataset used for generating the model and with an independent initial rates dataset used to test the predictive capacity of the model. The model obtained is a function of time and the initial apical iron concentration, with a linear component that captures the global tendency of the system, and a non-linear component that can be associated to the movement of DMT1 transporters. The model presented in this paper allows the detailed analysis, interpretation of experimental data, and identification of key relevant components for this complex biological process. This general method holds great potential for application to the elucidation of biological mechanisms and their key components in other complex systems.

Orchestration of dynamic copper navigation – new and missing pieces

A general principle in all cells in the body is that an essential metal – here copper – is taken up at the plasma membrane, directed through cellular compartments for use in specific enzymes and pathways, stored in specific scavenging molecules if in surplus, and finally expelled from the cells.

Acute Copper and Ascorbic Acid Supplementation Inhibits Non-heme Iron Absorption in Humans

The objective of the study is to determine the effect of copper (Cu) plus the reducing agent ascorbic acid (AA) on the absorption of non-heme iron (Fe). Experimental study with block design in which each subject was his own control. After signing an informed consent, 14 adult women using an effective method of contraception and negative pregnancy test received 0.5 mg Fe, as ferrous sulfate, alone or with Cu, as copper sulfate, plus ascorbic acid (AA/Cu 2/1 molar ratio) at 4/1; 6/1 and 8/1 Cu/Fe molar ratios as an aqueous solution on days 1, 2, 14, and 15 of the study. Fe absorption was assessed by erythrocyte incorporation of iron radioisotopes (55)Fe and (59)Fe. Geometric mean (range ± SD) absorption of Fe at 4/1 and 6/1 Cu/Fe molar ratios (and AA/Cu 2/1 molar ratio) and Fe alone was 57.4 % (35.7-92.1 %), 64.2 % (45.8-89.9 %), and 38.8 % (20.4-73.8 %), respectively (ANOVA for repeated measures p < 0.001; post hoc test Scheffé, p < 0.05). This is attributable to the enhancing effect of AA on non-heme Fe absorption; however, Fe absorption at Cu/Fe 8/1 molar ratio was 47.3 % (27.7-80.8) (p = NS compared with Fe alone). It was expected that Fe absorption would have been equal or greater than at 4/1 and 6/1 molar ratios. Copper in the presence of ascorbic acid inhibits non-heme Fe absorption at Cu/Fe 8/1 molar ratio.

Red wine activates plasma membrane redox system in human erythrocytes

In the present study, we report that polyphenols present in red wine obtained by a controlled microvinification process are able to protect human erythrocytes from oxidative stress and to activate Plasma Membrane Redox System (PMRS). Human plasma obtained from healthy subjects was incubated in the presence of whole red wine at a concentration corresponding to 9.13-73 μg/ml gallic acid equivalents to verify the capacity to protect against hypochlorous acid (HOCl)-induced plasma oxidation and to minimize chloramine formation. Red wine reduced hemolysis and chloramine formation induced by HOCl of 40 and 35%, respectively. PMRS present on human erythrocytes transfers electrons from intracellular molecules to extracellular electron acceptors. We demonstrated that whole red wine activated PMRS activity in human erythrocytes isolated from donors in a dose-dependent manner with a maximum at about 70-100 μg/ml gallic acid equivalents. We also showed that red wine increased glutathione (GSH) levels and erythrocytic antioxidant capacity, measured by 2,2-diphenyl-1-picrylhydrazyl (DPPH) quenching assay. Furthermore, we reported that GSH played a crucial role in regulating PMRS activity in erythrocytes. In fact, the effect of iodoacetamide, an alkylating agent that induces depletion of intracellular GSH, was completely counteracted by red wine. Bioactive compounds present in red wine, such as gallic acid, resveratrol, catechin, and quercetin were unable to activate PMRS when tested at the concentrations normally present in aged red wines. On the contrary, the increase of PMRS activity was associated with the anthocyanin fraction, suggesting the capacity of this class of compounds to positively modulate PMRS enzymatic activity.

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.

Responses in Micro-Mineral Metabolism in Rainbow Trout to Change in Dietary Ingredient Composition and Inclusion of a Micro-Mineral Premix

Responses in micro-mineral metabolism to changes in dietary ingredient composition and inclusion of a micro-mineral premix (Fe, Cu, Mn, Zn and Se) were studied in rainbow trout. In a 2 x 2 factorial design, triplicate groups of rainbow trout (initial weight: 20 g) were fed over 12 weeks at 17°C a fishmeal-based diet (M) or a plant-ingredient based diet (V), with or without inclusion of a mineral premix. Trout fed the V vs. M diet had lower feed intake, growth, hepato-somatic index, apparent availability coefficient (AAC) of Fe, Cu, Mn and Zn and also lower whole body Se and Zn concentration, whereas whole body Fe and Cu and plasma Fe concentrations were higher. Feeding the V diet increased intestinal ferric reductase activity; at transcriptional level, hepatic hepcidin expression was down-regulated and ferroportin 1 was up-regulated. Transcription of intestinal Cu-transporting ATPases and hepatic copper transporter1 were higher in V0 compared to other groups. Among the hepatic metalo-enzyme activities assayed, only Se-dependent glutathione peroxidase was affected, being lower in V fed fish. Premix inclusion reduced the AAC of Fe, Cu and Zn; increased the whole body concentration of all micro- minerals; up-regulated hepatic hepcidin and down-regulated intestinal ferroportin 1 transcription; and reduced the transcription of Cu-transporting ATPases in the intestine. Overall, the regulation of micro-mineral metabolism in rainbow trout, especially Fe and Cu, was affected both by a change in ingredient composition and micro-mineral premix inclusion.

Behind the Link between Copper and Angiogenesis: Established Mechanisms and an Overview on the Role of Vascular Copper Transport Systems

Angiogenesis critically sustains the progression of both physiological and pathological processes. Copper behaves as an obligatory co-factor throughout the angiogenic signalling cascades, so much so that a deficiency causes neovascularization to abate. Moreover, the progress of several angiogenic pathologies (e.g. diabetes, cardiac hypertrophy and ischaemia) can be tracked by measuring serum copper levels, which are being increasingly investigated as a useful prognostic marker. Accordingly, the therapeutic modulation of body copper has been proven effective in rescuing the pathological angiogenic dysfunctions underlying several disease states. Vascular copper transport systems profoundly influence the activation and execution of angiogenesis, acting as multi-functional regulators of apparently discrete pro-angiogenic pathways. This review concerns the complex relationship among copper-dependent angiogenic factors, copper transporters and common pathological conditions, with an unusual accent on the multi-faceted involvement of the proteins handling vascular copper. Functions regulated by the major copper transport proteins (CTR1 importer, ATP7A efflux pump and metallo-chaperones) include the modulation of endothelial migration and vascular superoxide, known to activate angiogenesis within a narrow concentration range. The potential contribution of prion protein, a controversial regulator of copper homeostasis, is discussed, even though its angiogenic involvement seems to be mainly associated with the modulation of endothelial motility and permeability.

Intestinal DMT1 is critical for iron absorption in the mouse but is not required for the absorption of copper or manganese

Divalent metal-ion transporter-1 (DMT1) is a widely expressed iron-preferring membrane-transport protein that serves a critical role in erythroid iron utilization. We have investigated its role in intestinal metal absorption by studying a mouse model lacking intestinal DMT1 (i.e., DMT1(int/int)). DMT1(int/int) mice exhibited a profound hypochromic-microcytic anemia, splenomegaly, and cardiomegaly. That the anemia was due to iron deficiency was demonstrated by the following observations in DMT1(int/int) mice: 1) blood iron and tissue nonheme-iron stores were depleted; 2) mRNA expression of liver hepcidin (Hamp1) was depressed; and 3) intraperitoneal iron injection corrected the anemia, and reversed the changes in blood iron, nonheme-iron stores, and hepcidin expression levels. We observed decreased total iron content in multiple tissues from DMT1(int/int) mice compared with DMT1(+/+) mice but no meaningful change in copper, manganese, or zinc. DMT1(int/int) mice absorbed (64)Cu and (54)Mn from an intragastric dose to the same extent as did DMT1(+/+) mice but the absorption of (59)Fe was virtually abolished in DMT1(int/int) mice. This study reveals a critical function for DMT1 in intestinal nonheme-iron absorption for normal growth and development. Further, this work demonstrates that intestinal DMT1 is not required for the intestinal transport of copper, manganese, or zinc.

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.

Molecular mechanisms of non-transferrin-bound and transferring-bound iron uptake in primary hippocampal neurons

The molecular mechanisms of iron trafficking in neurons have not been elucidated. In this study, we characterized the expression and localization of ferrous iron transporters Zip8, Zip14 and divalent metal transporter 1 (DMT1), and ferrireductases Steap2 and stromal cell-derived receptor 2 in primary rat hippocampal neurons. Steap2 and Zip8 partially co-localize, indicating these two proteins may function in Fe(3+) reduction prior to Fe(2+) permeation. Zip8, DMT1, and Steap2 co-localize with the transferrin receptor/transferrin complex, suggesting they may be involved in transferrin receptor/transferrin-mediated iron assimilation. In brain interstitial fluid, transferring-bound iron (TBI) and non-transferrin-bound iron (NTBI) exist as potential iron sources. Primary hippocampal neurons exhibit significant iron uptake from TBI (Transferrin-(59) Fe(3+)) and NTBI, whether presented as (59) Fe(2+) -citrate or (59) Fe(3+) -citrate; reductase-independent (59) Fe(2+) uptake was the most efficient uptake pathway of the three. Kinetic analysis of Zn(2+) inhibition of Fe(2+) uptake indicated that DMT1 plays only a minor role in the uptake of NTBI. In contrast, localization and knockdown data indicate that Zip8 makes a major contribution. Data suggest also that cell accumulation of (59) Fe from TBI relies at least in part on an endocytosis-independent pathway. These data suggest that Zip8 and Steap2 play a major role in iron accumulation from NTBI and TBI by hippocampal neurons. Analysis of the expression and localization of known iron uptake transporters demonstrated that Zip8 makes a major contribution to iron accumulation in primary cultures of rat embryonic hippocampal neurons. These cells exhibit uptake pathways for ferrous and ferric iron (non-transferrin-bound iron, NTBI in figure) and for transferrin-bound iron; the ferrireductases Steap2 and SDR2 support the uptake of ferric iron substrates. Zip8 and Steap2 are strongly expressed in the plasma membrane of both soma and processes, implying a crucial role in iron accumulation from NTBI and transferrin-bound iron (TBI) by hippocampal neurons.

STEAP4 and insulin resistance

Obesity is a multifactorial disease that caused by the interactions between genetic susceptibility genes and environmental cues. Obesity is considered as a major risk factor of insulin resistance. STEAP4 is a novel anti-obesity gene that is significantly down-regulated in adipose tissue of obese patients. Over-expression of STEAP4 can improve glucose uptake and mitochondrial function, and increase insulin sensitivity. STEAP4 expression is regulated by a variety of inflammatory cytokines, hormones, or adipokines. In this review, we discuss function of STEAP4 in regulating insulin resistance in adipose tissue in vivo, as well as in adipocytes in vitro.

Iron transport across the blood-brain barrier: development, neurovascular regulation and cerebral amyloid angiopathy

There are two barriers for iron entry into the brain: (1) the brain-cerebrospinal fluid (CSF) barrier and (2) the blood-brain barrier (BBB). Here, we review the literature on developmental iron accumulation by the brain, focusing on the transport of iron through the brain microvascular endothelial cells (BMVEC) of the BBB. We review the iron trafficking proteins which may be involved in the iron flux across BMVEC and discuss the plausible mechanisms of BMVEC iron uptake and efflux. We suggest a model for how BMVEC iron uptake and efflux are regulated and a mechanism by which the majority of iron is trafficked across the developing BBB under the direct guidance of neighboring astrocytes. Thus, we place brain iron uptake in the context of the neurovascular unit of the adult brain. Last, we propose that BMVEC iron is involved in the aggregation of amyloid-β peptides leading to the progression of cerebral amyloid angiopathy which often occurs prior to dementia and the onset of Alzheimer's disease.

RNA-seq analysis of the effect of kanamycin and the ABC transporter AtWBC19 on Arabidopsis thaliana seedlings reveals changes in metal content

Plants are exposed to antibiotics produced by soil microorganisms, but little is known about their responses at the transcriptional level. Likewise, few endogenous mechanisms of antibiotic resistance have been reported. The Arabidopsis thaliana ATP Binding Cassette (ABC) transporter AtWBC19 (ABCG19) is known to confer kanamycin resistance, but the exact mechanism of resistance is not well understood. Here we examined the transcriptomes of control seedlings and wbc19 mutant seedlings using RNA-seq analysis. Exposure to kanamycin indicated changes in the organization of the photosynthetic apparatus, metabolic fluxes and metal uptake. Elemental analysis showed a 60% and 80% reduction of iron uptake in control and wbc19 mutant seedlings respectively, upon exposure to kanamycin. The drop in iron content was accompanied by the upregulation of the gene encoding for FERRIC REDUCTION OXIDASE 6 (FRO6) in mutant seedlings but not by the differential expression of other transport genes known to be induced by iron deficiency. In addition, wbc19 mutants displayed a distinct expression profile in the absence of kanamycin. Most notably the expression of several zinc ion binding proteins, including ZINC TRANSPORTER 1 PRECURSOR (ZIP1) was increased, suggesting abnormal zinc uptake. Elemental analysis confirmed a 50% decrease of zinc content in wbc19 mutants. Thus, the antibiotic resistance gene WBC19 appears to also have a role in zinc uptake.

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.

Iron in Eukarya

Iron is essential for the normal physiological function of all organisms. In humans it is required for a plethora of biochemical roles including the transport of oxygen in the blood and energy production in the mitochondria. However, iron is also highly cytotoxic when present at high levels as it readily participates in oxidation-reduction reactions that lead to the generation of reactive oxygen species. One unique feature of iron biology is the lack of excretory mechanisms to remove excess iron from the body. Therefore, the concerted action of several genes and proteins working together to regulate the movement of iron across cell membranes, its storage in peripheral tissues and its physiological utilization in the body is essential for maintaining iron homeostasis. Humans are exposed to iron in a number of chemical forms (haem or non-haem; ferric or ferrous). This chapter will describe how humans acquire iron from their diet; the subsequent delivery of iron to its sites of utilization and storage; and how iron is recycled from effete erythrocytes for re-use in metabolism. Mutations in a number of the genes controlling iron metabolism have been identified and study of the pathological consequences of these mutations has allowed us to gain a greater understanding of how the body senses changes in iron status and coordinates its transport, storage and utilization to maintain homeostasis.

Quercetin inhibits intestinal iron absorption and ferroportin transporter expression in vivo and in vitro

Balancing systemic iron levels within narrow limits is critical for maintaining human health. There are no known pathways to eliminate excess iron from the body and therefore iron homeostasis is maintained by modifying dietary absorption so that it matches daily obligatory losses. Several dietary factors can modify iron absorption. Polyphenols are plentiful in human diet and many compounds, including quercetin--the most abundant dietary polyphenol--are potent iron chelators. The aim of this study was to investigate the acute and longer-term effects of quercetin on intestinal iron metabolism. Acute exposure of rat duodenal mucosa to quercetin increased apical iron uptake but decreased subsequent basolateral iron efflux into the circulation. Quercetin binds iron between its 3-hydroxyl and 4-carbonyl groups and methylation of the 3-hydroxyl group negated both the increase in apical uptake and the inhibition of basolateral iron release, suggesting that the acute effects of quercetin on iron transport were due to iron chelation. In longer-term studies, rats were administered quercetin by a single gavage and iron transporter expression measured 18 h later. Duodenal FPN expression was decreased in quercetin-treated rats. This effect was recapitulated in Caco-2 cells exposed to quercetin for 18 h. Reporter assays in Caco-2 cells indicated that repression of FPN by quercetin was not a transcriptional event but might be mediated by miRNA interaction with the FPN 3'UTR. Our study highlights a novel mechanism for the regulation of iron bioavailability by dietary polyphenols. Potentially, diets rich in polyphenols might be beneficial for patients groups at risk of iron loading by limiting the rate of intestinal iron absorption.

Iron in neurodegenerative disorders of protein misfolding: a case of prion disorders and Parkinson's disease

SIGNIFICANCE

Intracellular and extracellular aggregation of a specific protein or protein fragments is the principal pathological event in several neurodegenerative conditions. We describe two such conditions: sporadic Creutzfeldt-Jakob disease (sCJD), a rare but potentially infectious and invariably fatal human prion disorder, and Parkinson's disease (PD), a common neurodegenerative condition second only to Alzheimer's disease in prevalence. In sCJD, a cell surface glycoprotein known as the prion protein (PrP(C)) undergoes a conformational change to PrP-scrapie, a pathogenic and infectious isoform that accumulates in the brain parenchyma as insoluble aggregates. In PD, α-synuclein, a cytosolic protein, forms insoluble aggregates that accumulate in neurons of the substantia nigra and cause neurotoxicity.

RECENT ADVANCES

Although distinct processes are involved in the pathogenesis of sCJD and PD, both share brain iron dyshomeostasis as a common associated feature that is reflected in the cerebrospinal fluid in a disease-specific manner.

CRITICAL ISSUES

Since PrP(C) and α-synuclein play a significant role in maintaining cellular iron homeostasis, it is important to understand whether the aggregation of these proteins and iron dyshomeostasis are causally related. Here, we discuss recent information on the normal function of PrP(C) and α-synuclein in cellular iron metabolism and the cellular and biochemical processes that contribute to iron imbalance in sCJD and PD.

FUTURE DIRECTIONS

Improved understanding of the relationship between brain iron imbalance and protein aggregation is likely to help in the development of therapeutic strategies that can restore brain iron homeostasis and mitigate neurotoxicity.

Molecular mediators governing iron-copper interactions

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

The families of zinc (SLC30 and SLC39) and copper (SLC31) transporters

The solute carriers families 30 (SLC30; ZnT), 39 (SLC39; ZIP), and 31 (SLC31; CTR) are involved in the essential maintenance of cellular zinc (Zn²⁺) and copper (Cu²⁺) homeostasis, respectively. ZnTs mediate Zn²⁺ extrusion from cells (SLC30A1) or transport Zn²⁺ into organelles and secretory vesicles/granules (SLC30A2-SLC30A8). SLC39 family members are predominantly localized to the cell membrane where they perform Zn²⁺ uptake and increase the availability of cytosolic Zn²⁺. SLC39A1 is ubiquitously expressed, whereas other ZIP transporters (e.g., SLC39A2 and SLC39A3) show a more tissue-restricted expression consistent with organ-specific functions of these proteins. The members A1 (CTR1) and A2 (CTR2) of the SLC31 family of solute carriers belong to a network of proteins that acts to regulate the intracellular Cu²⁺ concentration within a certain range. SLC31A1 is predominantly localized to the plasma membrane, whereas SLC31A2 is mainly found in intracellular membranes of the late endosome and lysosome. The specific function of SLC31A2 is not known. SLC31A1 is ubiquitously expressed and has been characterized as a high-affinity importer of reduced copper (Cu⁺). Cu²⁺ transport function of CTR proteins is associated with oligomerization; SLC31A1 trimerizes and thereby forms a channel-like structure enabling Cu²⁺ translocation across the cell membrane. The molecular characteristics and structural details (e.g., membrane topology, conserved Zn²⁺, and Cu²⁺ binding sites) and mechanisms of translational and posttranslational regulation of expression and/or activity have been described for SLC30 and SLC39 family members, and for SLC31A1. For SLC31A1, data on tissue-specific functions (e.g., in the intestine, heart, and liver) are also available. A link between SLC31A1, immune function, and disorders such as Alzheimer's disease or cancer makes the protein a candidate therapeutic target. In secretory tissues (e.g., the mammary gland and pancreas), Zn²⁺ transporters of SLC families 30 and 39 are involved in specific functions such as insulin synthesis and secretion, metallation of digestive proenzymes, and transfer of nutrients into milk. Defective or dysregulated Zn²⁺ metabolism in these organs is associated with disorders such as diabetes and cancer, and impaired Zn²⁺ secretion into milk.

Divalent metal transporter 1 (Dmt1) mediates copper transport in the duodenum of iron-deficient rats and when overexpressed in iron-deprived HEK-293 cells

Intracellular copper-binding proteins (metallothionein I/II) and a copper exporter (Menkes copper-transporting ATPase) are upregulated in duodenal enterocytes from iron-deficient rats, consistent with copper accumulation in the intestinal mucosa. How copper enters enterocytes during iron deficiency is, however, not clear. Divalent metal transporter 1 (Dmt1), the predominant iron importer in the mammalian duodenum, also transports other metal ions, possibly including copper. Given this possibility and that Dmt1 expression is upregulated by iron deprivation, we sought to test the hypothesis that Dmt1 transports copper during iron deficiency. Two model systems were utilized: the Belgrade (b) rat, expressing mutant Dmt1, and an inducible Dmt1-overexpression cell culture system. Mutant rats (b/b) were fed a semipurified, AIN93G-based control diet and phenotypically normal littermates (+/b) were fed control or iron-deficient diets for ~14 wk. An everted gut sleeve technique and a colorimetric copper quantification assay were utilized to assess duodenal copper transport. The control diet-fed +/b rats had normal hematological parameters, whereas iron-deprived +/b and b/b rats were iron deficient and Dmt1 mRNA and protein levels increased. Importantly, duodenal copper transport was similar in the control +/b and b/b rats; however, it significantly increased (~4-fold) in the iron-deprived +/b rats. Additional experiments in Dmt1 overexpressing HEK-293 cells showed that copper ((64)Cu) uptake was stimulated (∼3-fold) in the presence of an iron chelator. Dmt1 transcript stabilization due to a 3' iron-responsive element was also documented, likely contributing to increased transport activity. In summary, these studies suggest that Dmt1 enhances copper uptake into duodenal enterocytes during iron deprivation.

Citric acid mediates the iron absorption from low molecular weight human milk fractions

Previously, we have demonstrated increased iron absorption from low molecular weight (LMW) human milk whey fractions. In the present study, we investigated the effect of heat denaturation, zinc (a competitor of iron), duodenal cytochrome b (DcytB) antibody neutralization and citrate lyase treatment on LMW human milk fraction (>5 kDa referred as 5kF) induced ferric iron reduction, solubilization, and uptake in Caco-2 cells. Heat denaturation and zinc inhibited the 5kF fraction induced ferric iron reduction. In contrast, zinc but not heat denaturation abrogated the ferric iron solubilization activity. Despite inhibition of ferric iron reduction, iron uptake in Caco-2 cells was similar from both native and heat denatured 5kF fractions. However, iron uptake was higher from native compared to heat denatured 5kF fractions in the cells preincubated with the DcytB antibody. Citrate lyase treatment inhibited the ferric iron reduction, solubilization, and uptake in Caco-2 cells. These findings demonstrate that citric acid present in human milk solubilizes the ferric iron which could be reduced by other heat labile components leading to increased uptake in intestinal cells.

Prion protein regulates iron transport by functioning as a ferrireductase

Prion protein (PrPC) is implicated in the pathogenesis of prion disorders, but its normal function is unclear. We demonstrate that PrPC is a ferrireductase (FR), and its absence causes systemic iron deficiency in PrP knock-out mice (PrP-/-). When exposed to non-transferrin-bound (NTB) radioactive-iron (59FeCl3) by gastric-gavage, PrP-/- mice absorb significantly more 59Fe from the intestinal lumen relative to controls, indicating appropriate systemic response to the iron deficiency. Chronic exposure to excess dietary iron corrects this deficiency, but unlike wild-type (PrP+/+) controls that remain iron over-loaded, PrP-/- mice revert back to the iron deficient phenotype after 5 months of chase on normal diet. Bone marrow (BM) preparations of PrP-/- mice on normal diet show relatively less stainable iron, and this phenotype is only partially corrected by intraperitoneal administration of excess iron-dextran. Cultured PrP-/- BM-macrophages incorporate significantly less NTB-59Fe in the absence or presence of excess extracellular iron, indicating reduced uptake and/or storage of available iron in the absence of PrPC. When expressed in neuroblastoma cells, PrPC exhibits NAD(P)H-dependent cell-surface and intracellular FR activity that requires the copper-binding octa-peptide-repeat region and linkage to the plasma membrane for optimal function. Incorporation of NTB-59Fe by neuroblastoma cells correlates with FR activity of PrPC, implicating PrPC in cellular iron uptake and metabolism. These observations explain the correlation between PrPC expression and cellular iron levels, and the cause of iron imbalance in sporadic-Creutzfeldt-Jakob-disease brains where PrPC accumulates as insoluble aggregates.