Zinc transport by respiratory epithelial cells and interaction with iron homeostasis

Despite recurrent exposure to zinc through inhalation of ambient air pollution particles, relatively little information is known about the homeostasis of this metal in respiratory epithelial cells. We describe zinc uptake and release by respiratory epithelial cells and test the postulate that Zn²⁺ transport interacts with iron homeostasis in these same cells. Zn²⁺ uptake after 4 and 8 h of exposure to zinc sulfate was concentration- and time-dependent. A majority of Zn²⁺ release occurred in the 4 h immediately following cell exposure to ZnSO₄. Regarding metal importers, mRNA for Zip1 and Zip2 showed no change after respiratory epithelial cell exposure to zinc while mRNA for divalent metal transporter (DMT)1 increased. Western blot assay for DMT1 protein supported an elevated expression of this transport protein following zinc exposure. RT-PCR confirmed mRNA for the metal exporters ZnT1 and ZnT4 with the former increasing after ZnSO₄. Cell concentrations of ferritin increased with zinc exposure while oxidative stress, measured as lipid peroxides, was decreased supporting an anti-oxidant function for Zn²⁺. Increased DMT1 expression, following pre-incubations of respiratory epithelial cells with TNF-α, IFN-γ, and endotoxin, was associated with significantly decreased intracellular zinc transport. Finally, incubations of respiratory epithelial cells with both zinc sulfate and ferric ammonium citrate resulted in elevated intracellular concentrations of both metals. We conclude that exposure to zinc increases iron uptake by respiratory epithelial cells. Elevations in cell iron can possibly affect an increased expression of DMT1 and ferritin which function to diminish oxidative stress. Comparable to other metal exposures, changes in iron homeostasis may contribute to the biological effects of zinc in specific cells and tissues.     

Hypoxic preconditioning increases iron transport rate in astrocytes

The mechanisms involved in the neuroprotection induced by hypoxic preconditioning (HP) have not been fully elucidated. The involvement of hypoxia-inducible factor-1 alpha (HIF-1alpha) in such neuroprotection has been confirmed. There is also evidence showing that a series of genes with important functions in iron metabolism, including transferrin receptor (TfR1) and divalent metal transporter 1 (DMT1), are regulated by HIF-1alpha in response to hypoxia in extra-neural organs or cells. We therefore hypothesized that HP is able to affect the expression of iron metabolism proteins in the brain and that changes in these proteins induced by HP might be associated with the HP-induced neuroprotection. We herein demonstrated for the first time that HP could induce a significant increase in the expression of HIF-1alpha as well as iron uptake (TfR1 and DMT1) and release (ferroportin1) proteins, and thus increase tansferrin-bound iron (Tf-Fe) and non-transferrin-bound iron (NTBI) uptake and iron release in astrocytes. Moreover, HP could lead to a progressive increase in cellular iron content. We concluded that HP has the ability to increase iron transport speed in astrocytes. Based on our findings and the importance of astrocytes in neuronal survival in hypoxic/ischemic preconditioning, we proposed that the increase in iron transport rate and cellular iron in astocytes might be one of the mechanisms associated with the HP-induced neuroprotection. We also demonstrated that ferroportin1 expression was significantly affected by HIF-1alpha in astrocytes, implying that the gene encoding this iron efflux protein might be a hypoxia-inducible one.     

Cellular iron transport

Iron has a split personality as an essential nutrient that also has the potential to generate reactive oxygen species. We discuss how different cell types within specific tissues manage this schizophrenia. The emphasis in enterocytes is on regulating the body's supply of iron by regulating transport into the blood stream. In developing red blood cells, adaptations in transport manage the body's highest flux of iron. Hepatocytes buffer the body's stock of iron. Macrophage recycle the iron from effete red cells among other iron management tasks. Pneumocytes provide a barrier to prevent illicit entry that, when at risk of breaching, leads to a need to handle the dangers in a fashion essentially shared with macrophage. We also discuss or introduce cell types including renal cells, neurons, other brain cells, and more where our ignorance, currently still vast, needs to be removed by future research.     

Human iron transporters

Human iron transporters manage iron carefully because tissues need iron for critical functions, but too much iron increases the risk of reactive oxygen species. Iron acquisition occurs in the duodenum via divalent metal transporter (DMT1) and ferroportin. Iron trafficking depends largely on the transferrin cycle. Nevertheless, non-digestive tissues have a variety of other iron transporters that may render DMT1 modestly redundant, and DMT1 levels exceed those needed for the just-mentioned tasks. This review begins to consider why and also describes advances after 2008 that begin to address this challenge.     

A brief history of iron metabolism

Summary A concise history of selected aspects of iron metabolism is presented. According to present understanding, the element is known to be required for transport and reduction of O₂, for reduction Of CO₂, N₂ and ribonucleotides, and for other essential cellular processes. The contributions of pioneers in the field, preeminent among them the cell physiologist and biochemist Otto Warburg, are recounted.     

Differential expression of iron metabolism proteins in diabetic and diabetic iron-supplemented rat liver

Diabetes mellitus is associated with altered iron homeostasis that can potentially effect reactive oxygen species generation and contribute to diabetes-related complications. We investigated, by quantitative polymerase chain reaction, whether the expression of liver hepcidin, ferritin, and TfR-1 is altered in diabetes. Rats in the control (C) group received a standard diet; control iron (CI) group received a standard diet supplemented with iron; diabetic (D) group received an injection of streptozotocin; and diabetic iron (DI) group received streptozotocin and the diet with iron. Animals of the D group showed higher levels of serum iron, increased concentration of carbonyl protein, and a decrease in antioxidant status. Group D rats showed increased hepatic expression of Trf-1 compared to the other groups. Iron supplementation reversed this increase. Hepcidin mRNA was 81% higher in DI than in C and CI rats. The results suggest that diabetes, with or without excess iron, can cause perturbations in iron status, hepcidin and Trf-1 expression.     

Superoxide dismutase 1 modulates expression of transferrin receptor

Copper–zinc superoxide dismutase (SOD1) plays a protective role against the toxicity of superoxide, and studies in Saccharomyces cerevisiae and in Drosophila have suggested an additional role for SOD1 in iron metabolism. We have studied the effect of the modulation of SOD1 levels on iron metabolism in a cultured human glial cell line and in a mouse motoneuronal cell line. We observed that levels of the transferrin receptor and the iron regulatory protein 1 were modulated in response to altered intracellular levels of superoxide dismutase activity, carried either by wild-type SOD1 or by an SOD-active amyotrophic lateral sclerosis (ALS) mutant enzyme, G93A-SOD1, but not by a superoxide dismutase inactive ALS mutant, H46R-SOD1. Ferritin expression was also increased by wild-type SOD1 overexpression, but not by mutant SOD1s. We propose that changes in superoxide levels due to alteration of SOD1 activity affect iron metabolism in glial and neuronal cells from higher eukaryotes and that this may be relevant to diseases of the nervous system.     

Lead and Cadmium Synergistically Enhance the Expression of Divalent Metal Transporter 1 Protein in Central Nervous System of Developing Rats

Divalent metal transporter 1 (DMT1) can transport a large range of ions, including toxic lead (Pb) and cadmium (Cd), across membranes. In this study, a total of 24 rats were divided into four groups for intragastrical perfusion treatment: control, Pb alone, Cd alone, and Pb + Cd. Pb and Cd contents in blood were detected, and the mRNA and protein levels of DMT1 were analyzed in the cerebellum, cortex, and hippocampus. Both Pb and Cd levels were elevated in all groups perfused with Pb and/or Cd, except for Pb level in the Cd-alone group (P < 0.05). The mRNA level of DMT1 did not differ among the four groups (P > 0.05). However, the DMT1 protein expression was significantly increased by 0.9-, 1.0-, and 1.1-fold in cerebellum, cortex, and hippocampus of the Pb + Cd group than in controls, respectively. Pb and Cd exposure can synergistically induce DMT1 protein synthesis and has implications for transportation of toxic ions in the developing rat’s brain. Chengwu Gu and Songjian Chen contributed equally to this work, they are joint first authors.     

COVID-19 compromises iron homeostasis: Transferrin as a target of investigation

ImportanceSince the beginning of the COVID-19 pandemic, numerous metabolic alterations have been observed in individuals with this disease. It is known that SARS-CoV-2 can mimic the action of hepcidin, altering intracellular iron metabolism, but gaps remain in the understanding of possible outcomes in other pathways involved in the iron cycle.ObjectiveTo profile iron, ferritin and hepcidin levels and transferrin receptor gene expression in patients diagnosed with COVID-19 between June 2020 and September 2020.Design, setting and participantsCross-sectional study that evaluated iron metabolism markers in 427 participants, 218 with COVID-19 and 209 without the disease.ExposuresThe primary exposure was positive diagnose to COVID-19 in general population of Santo André and São Bernardo cities. The positive and negative diagnose were determinate through RT-qPCR.Main outcomes and measuresDevido a evidências de alterações do ciclo do ferro em pacientes diagnosticados com COVID-19 e devido a corregulação entre hepcidina e receptor de transferrina, uma análise da expressão gênica deste último, poderia trazer insights sobre o estado de ferro celular. A hipótese foi confirmada, mostrando aumento da expressão de receptor de transferrina concomitante com redução do nível de hepcidina circulante.ResultsSerum iron presented lower values in individuals diagnosed with COVID-19, whereas serum ferritin presented much higher values in infected patients. Elderly subjects had lower serum iron levels and higher ferritin levels, and men with COVID-19 had higher ferritin values than women. Serum hepcidin was lower in the COVID-19 patient group and transferrin receptor gene expression was higher in the infected patient group compared to controls.Conclusions and relevanceCOVID-19 causes changes in several iron cycle pathways, with iron and ferritin levels being markers that reflect the state and evolution of infection, as well as the prognosis of the disease. The increased expression of the transferrin receptor gene suggests increased iron internalization and the mimicry of hepcidin action by SARS-CoV-2, reduces iron export via ferroportin, which would explain the low circulating levels of iron by intracellular trapping.     

肥胖对小鼠十二指肠 DMT1和 FPN1表达的影响

目的：观察肥胖对小鼠十二指肠二价金属离子转运体（divalent metal transporter 1,DMT1）mRNA、膜铁转运蛋白（ferroportin1,FPN1）mRNA及蛋白表达的变化,探讨肥胖影响铁吸收的机制。方法 C57BL／6J小鼠随机分为正常对照组和肥胖模型组,每组6只,通过喂养高脂饲料喂养建立肥胖模型,对照组采用普通饲料饲养,实验干预期14周。建模完成后,采用实时荧光定量PCR方法检测小鼠十二指肠DMT1、FPN1 mRNA 的表达,用Western blot检测小鼠十二指肠FPN1蛋白表达。结果与对照组小鼠相比,肥胖模型组小鼠十二指肠DMT1、FPN1 mRNA表达以及FPN1蛋白表达水平降低,差异具有统计学意义（ P ＜0.05）。结论肥胖会下调机体十二指肠DMT1、FPN1的表达,导致铁吸收不良,为进一步研究肥胖引起铁缺乏机制提供理论和实验依据。     

Role of extracellular Hydrogen peroxide in regulation of iron homeostasis genes in neuronal cells: Implication in iron accumulation

Iron accumulation and oxidative stress are associated with neurodegenerative disease. Labile iron is known to catalyze free radical generation and subsequent neuronal damage, whereas the role of oxidative stress in neuronal iron accumulation is less well understood. Here, we examined the effect of hydrogen peroxide (H₂O₂) treatment on cellular iron-uptake, -storage, and -release proteins in the neuroblastoma cell line SH-SY5Y. We found no detectable change in the iron-uptake proteins transferrin receptor-1 and divalent metal ion transporter. In contrast, H₂O₂ treatment resulted in significant degradation of the iron-exporter ferroportin (Fpn). A decrease in Fpn is expected to increase the labile iron pool (LIP), reducing the iron-regulatory protein (IRP)–iron-responsive element interaction and increasing the expression of ferritin-H (Ft-H) for iron storage. Instead, we detected IRP1 activation, presumably due to oxidative stress, and a decrease in Ft-H translation. A reduction in Ft-H mRNA was also observed, probably dependent on an antioxidant-response element present in the Ft-H enhancer. The decrease in Fpn and Ft-H upon H₂O₂ treatment led to a time-dependent increase in the cellular LIP. Our study reveals a complex regulation of neuronal iron-release and iron-storage components in response to H₂O₂ that may explain iron accumulation detected in neurodegenerative diseases associated with oxidative stress.     

细菌生产人铁蛋白的新功能及应用

铁蛋白(Ferritin)是一种广泛存在于生物体中的笼状蛋白,由24个亚基自组装形成的蛋白质外壳和铁内核两部分组成,是维持机体铁代谢平衡的重要蛋白。最新发现,人血清铁蛋白含量的变化与某些疾病相关,特别是发现利用大肠杆菌重组表达、仿生合成的磁性人铁蛋白具有双功能特性,即识别肿瘤并使其可视化。此外,铁蛋白独特的结构及理化性质使其成为理想的纳米载体,用于构筑多功能肿瘤成像和药物输送的平台。本文重点介绍人铁蛋白的新功能及其在疾病诊断和肿瘤靶向治疗中的应用前景。     

The Pivotal Role of Astrocytes in the Metabolism of Iron in the Brain

Iron is essential for the normal functioning of cells but since it is also capable of generating toxic reactive oxygen species, the metabolism of iron is tightly regulated. The present article advances the view that astrocytes are largely responsible for distributing iron in the brain. Capillary endothelial cells are separated from the neuropil by the endfeet of astrocytes, so astrocytes are ideally positioned to regulate the transport of iron to other brain cells and to protect them if iron breaches the blood-brain barrier. Astrocytes do not appear to have a high metabolic requirement for iron yet they possess transporters for transferrin, haemin and non-transferrin-bound iron. They store iron efficiently in ferritin and can export iron by a mechanism that involves ferroportin and ceruloplasmin. Since astrocytes are a common site of abnormal iron accumulation in ageing and neurodegenerative disorders, they may represent a new therapeutic target for the treatment of iron-mediated oxidative stress.     

Iron homeostasis and eye disease

Background

Iron is necessary for life, but excess iron can be toxic to tissues. Iron is thought to damage tissues primarily by generating oxygen free radicals through the Fenton reaction.

Methods

We present an overview of the evidence supporting iron's potential contribution to a broad range of eye disease using an anatomical approach.

Results

Iron can be visualized in the cornea as iron lines in the normal aging cornea as well as in diseases like keratoconus and pterygium. In the lens, we present the evidence for the role of oxidative damage in cataractogenesis. Also, we review the evidence that iron may play a role in the pathogenesis of the retinal disease age-related macular degeneration. Although currently there is no direct link between excess iron and development of optic neuropathies, ferrous iron's ability to form highly reactive oxygen species may play a role in optic nerve pathology. Lastly, we discuss recent advances in prevention and therapeutics for eye disease with antioxidants and iron chelators.

General significance

Iron homeostasis is important for ocular health.     

Ebselen inhibits iron-induced tau phosphorylation by attenuating DMT1 up-regulation and cellular iron uptake

Dysregulation of iron homeostasis is involved in the pathological process of Alzheimer's disease (AD). We have recently reported that divalent metal transporter 1 (DMT1) is upregulated in an AD transgenic mouse brain, and that silencing of DMT1, which reduces cellular iron influx, results in inhibition of amyloidogenesis in vitro, suggesting a potential target of DMT1 for AD therapy. In the present study, we tested the hypothesis that inhibition of DMT1 with ebselen, a DMT1 transport inhibitor, could affect tau phosphorylation. Human neuroblastoma SH-SY5Y cells were pre-treated with ebselen and then treated with ferrous sulfate (dissolved in ascorbic acid), and the effects of ebselen on tau phosphorylation and the relative signaling pathways were examined. Our results showed that ebselen decreased iron influx, reduced iron-induced ROS production, inhibited the activities of cyclin-dependent kinase 5 and glycogen synthase kinase 3β, and ultimately attenuated the levels of tau phosphorylation at the sites of Thr205, Ser396 and Thr231. The present study indicates that the neuroprotective effect of ebselen on AD is not only related to its antioxidant activity as reported previously, but is also associated with a reduction in tau phosphorylation by inhibition of DMT1.     

Insect ferritins: Typical or atypical?

Insects transmit millions of cases of disease each year, and cost millions of dollars in agricultural losses. The control of insect-borne diseases is vital for numerous developing countries, and the management of agricultural insect pests is a very serious business for developed countries. Control methods should target insect-specific traits in order to avoid non-target effects, especially in mammals. Since insect cells have had a billion years of evolutionary divergence from those of vertebrates, they differ in many ways that might be promising for the insect control field—especially, in iron metabolism because current studies have indicated that significant differences exist between insect and mammalian systems. Insect iron metabolism differs from that of vertebrates in the following respects. Insect ferritins have a heavier mass than mammalian ferritins. Unlike their mammalian counterparts, the insect ferritin subunits are often glycosylated and are synthesized with a signal peptide. The crystal structure of insect ferritin also shows a tetrahedral symmetry consisting of 12 heavy chain and 12 light chain subunits in contrast to that of mammalian ferritin that exhibits an octahedral symmetry made of 24 heavy chain and 24 light chain subunits. Insect ferritins associate primarily with the vacuolar system and serve as iron transporters—quite the opposite of the mammalian ferritins, which are mainly cytoplasmic and serve as iron storage proteins. This review will discuss these differences.     

Analysis of the rainbow trout solute carrier 11 family reveals iron import < or = pH 7.4 and a functional isoform lacking transmembrane domains 11 and 12

A Xenopus oocyte heterologous expression system was used to characterise iron transport properties of two members of the solute carrier 11 (slc11) protein family isolated from rainbow trout gills. One cDNA clone differed from the trout Slc11alpha containing an additional 52bp in the exon between transmembrane domains (TM) 10 and 11. The 52bp contained a stop codon, resulting in a novel isoform lacking the last two TM (termed slc11gamma). Slc11gamma and another isoform slc11beta, import Fe(2+) at external pHs < or = to 7.4. Trout slc11beta Fe(2+) import was more sensitive to inhibition by divalent metals. The novel vertebrate slc11gamma isoform functions without TM11 and 12.