Iron transport and the kidney

Iron Metabolism and Inflammatory Mediators in Patients with Renal Dysfunction

Chronic kidney disease (CKD) affects around 850 million people worldwide, posing significant challenges in healthcare due to complications like renal anemia, end-stage kidney disease, and cardiovascular diseases. This review focuses on the intricate interplay between iron metabolism, inflammation, and renal dysfunction in CKD. Renal anemia, prevalent in CKD, arises primarily from diminished erythropoietin (EPO) production and iron dysregulation, which worsens with disease progression. Functional and absolute iron deficiencies due to impaired absorption and chronic inflammation are key factors exacerbating erythropoiesis. A notable aspect of CKD is the accumulation of uremic toxins, such as indoxyl sulfate (IS), which hinder iron metabolism and worsen anemia. These toxins directly affect renal EPO synthesis and contribute to renal hypoxia, thus playing a critical role in the pathophysiology of renal anemia. Inflammatory cytokines, especially TNF-α and IL-6, further exacerbate CKD progression and disrupt iron homeostasis, thereby influencing anemia severity. Treatment approaches have evolved to address both iron and EPO deficiencies, with emerging therapies targeting hepcidin and employing hypoxia-inducible factor (HIF) stabilizers showing potential. This review underscores the importance of integrated treatment strategies in CKD, focusing on the complex relationship between iron metabolism, inflammation, and renal dysfunction to improve patient outcomes.

Role of the SLC22A17/lipocalin-2 receptor in renal endocytosis of proteins/metalloproteins: a focus on iron- and cadmium-binding proteins

The transmembrane protein SLC22A17 [or the neutrophil gelatinase-associated lipocalin/lipocalin-2 (LCN2)/24p3 receptor] is an atypical member of the SLC22 family of organic anion and cation transporters: it does not carry typical substrates of SLC22 transporters but mediates receptor-mediated endocytosis (RME) of LCN2. One important task of the kidney is the prevention of urinary loss of proteins filtered by the glomerulus by bulk reabsorption of multiple ligands via megalin:cubilin:amnionless-mediated endocytosis in the proximal tubule (PT). Accordingly, overflow, glomerular, or PT damage, as in Fanconi syndrome, results in proteinuria. Strikingly, up to 20% of filtered proteins escape the PT under physiological conditions and are reabsorbed by the distal nephron. The renal distal tubule and collecting duct express SLC22A17, which mediates RME of filtered proteins that evade the PT but with limited capacity to prevent proteinuria under pathological conditions. The kidney also prevents excretion of filtered essential and nonessential transition metals, such as iron or cadmium, respectively, that are largely bound to proteins with high affinity, e.g., LCN2, transferrin, or metallothionein, or low affinity, e.g., microglobulins or albumin. Hence, increased uptake of transition metals may cause nephrotoxicity. Here, we assess the literature on SLC22A17 structure, topology, tissue distribution, regulation, and assumed functions, emphasizing renal SLC22A17, which has relevance for physiology, pathology, and nephrotoxicity due to the accumulation of proteins complexed with transition metals, e.g., cadmium or iron. Other putative renal functions of SLC22A17, such as its contribution to osmotic stress adaptation, protection against urinary tract infection, or renal carcinogenesis, are discussed.

Dysregulated Iron Homeostasis as Common Disease Etiology and Promising Therapeutic Target

Iron is irreplaceably required for animal and human cells as it provides the activity center for a wide variety of essential enzymes needed for energy production, nucleic acid synthesis, carbon metabolism and cellular defense. However, iron is toxic when present in excess and its uptake and storage must, therefore, be tightly regulated to avoid damage. A growing body of evidence indicates that iron dysregulation leading to excess quantities of free reactive iron is responsible for a wide range of otherwise discrete diseases. Iron excess can promote proliferative diseases such as infections and cancer by supplying iron to pathogens or cancer cells. Toxicity from reactive iron plays roles in the pathogenesis of various metabolic, neurological and inflammatory diseases. Interestingly, a common underlying aspect of these conditions is availability of excess reactive iron. This underpinning aspect provides a potential new therapeutic avenue. Existing hematologically used iron chelators to take up excess iron have shown serious limitations for use but new purpose-designed chelators in development show promise for suppressing microbial pathogen and cancer cell growth, and also for relieving iron-induced toxicity in neurological and other diseases. Hepcidin and hepcidin agonists are also showing promise for relieving iron dysregulation. Harnessing iron-driven reactive oxygen species (ROS) generation with ferroptosis has shown promise for selective destruction of cancer cells. We review biological iron requirements, iron regulation and the nature of iron dysregulation in various diseases. Current results pertaining to potential new therapies are also reviewed.

Correlating metal exposures and dietary habits with hyperuricemia in a large urban elderly cohort by artificial intelligence

Epidemiological studies using conventional statistical methods have reported an association between individual metal exposure and hyperuricemia (HUA). There is also evidence that diet may influence HUA development, although the available data are inconsistent. We therefore used an elastic net regression (ENR) model to screen the usefulness of various environmental and dietary factors as predictors of HUA in a large sample cohort. This study included 6217 subjects drawn from the Shenzhen Aging Related Disorder Cohort. We obtained information on the subjects' dietary habits via face-to-face interviews and used inductively coupled plasma mass spectrometry (ICP-MS) to measure the urinary concentrations of 24 metals to which elderly persons in large urban areas may be exposed. An elastic net regression (ENR) model was generated to screen the utility of the metals and dietary factors as predictors of HUA, and we demonstrated the superiority of the ENR model by comparing it to a traditional logistic regression model. The identified predictors were used to create a clinically usable nomogram for identifying patients at risk of developing HUA. The area under curve (AUC) value of the final model was 0.692 for the training set and 0.706 for the test set. Important predictors of HUA were Zn, As, V, and Fe as well as consumption of wheat, beans, and rice; the corresponding estimated odds ratios and 95% confidence intervals were 1.091 (0.932,1.251), 1.190 (1.093,1.286), 0.924 (0.793,1.055), 0.704 (0.626,0.781), 0.998 (0.996,1.001), 0.993 (0.989,0.998), and 1.001 (0.998,1.002), respectively. In contrast to previous studies, we found that both urinary metal concentrations and dietary habits are important for predicting HUA risk. Exposure to specific metals and consumption of specific foods were identified as important predictors of HUA, indicating that the incidence of this disease could be reduced by reducing exposure to these metals and promoting improved dietary habits.

Deferasirox and vitamin D3 co-therapy mitigates iron-induced renal injury by enhanced modulation of cellular anti-inflammatory, anti-oxidative stress, and iron regulatory pathways in rat

BACKGROUND

Chronic iron overload could induce nephropathy via oxidative stress and inflammation, and chelating therapy has limited efficacy in removing excess intracellular iron. Although vitamin D (VD) has shown potent antioxidant and anti-inflammatory effects, as well contribute to iron homeostasis, none of the previous studies measured its potential remedial effects against chronic iron toxicity.

AIMS

To measure the alleviating effects of deferasirox (DFX) and/or vitamin D (VD) single and combined therapies against nephrotoxicity induced by chronic iron overload.

METHODS

Forty male rats were divided into negative (NC) and positive (PC) controls, DFX, VD, and DFX/VD groups. The designated groups received iron for six weeks followed by DFX and/or VD for another six weeks. Then, the expression pattern of renal genes and proteins including hepcidin, ferroportin (FPN), megalin, transferrin receptor 1 (TfR1), ferritin heavy and light chains, VD receptor (VDR), VD synthesizing (Cyp27b1) and catabolizing (Cyp24a1) enzymes were measured alongside serum markers of renal function and iron biochemical parameters. Additionally, several markers of oxidative stress (MDA/H₂O₂/GSH/SOD1/CAT/GPx4) and inflammation (IL-1β/IL-6/TNF-α/IL-10) together with renal cell apoptosis and expression of caspase-3 (Casp-3) were measured.

RESULTS

The PC rats showed pathological iron and renal biochemical markers, hypovitaminosis D, increased renal tissue iron contents with increased Cyp24a1/Megalin/ferritin-chains/hepcidin, and decreased Cyp27b1/VDR/TfR1/FPN expression than the NC group. The PC renal tissues also showed abnormal histology, increased inflammatory (IL-1β/IL-6/TNF-α), oxidative stress (MDA/H₂O₂), and apoptosis markers with decreased IL-10/GSH/SOD1/CAT/GPx4. Although DFX monotherapy reduced serum iron levels, it was comparable to the PC group in renal iron concentrations, VD and iron-homeostatic molecules, alongside markers of oxidative stress, inflammation, and apoptosis. On the other hand, VD monotherapy markedly modulated renal iron and VD-related molecules, reduced renal tissue iron concentrations, and preserved renal tissue relative to the PC and DFX groups. However, serum iron levels were equal in the VD and PC groups. In contrast, the best significant improvements in serum and renal iron levels, expression of renal iron-homeostatic molecules, oxidative stress, inflammation, and apoptosis were seen in the co-therapy group.

CONCLUSIONS

iron-induced nephrotoxicity was associated with dysregulations in renal VD-system together with renal oxidative stress, inflammation, and apoptosis. While DFX reduced systemic iron, VD monotherapy showed better attenuation of renal iron concentrations and tissue damage. Nonetheless, the co-therapy approach exhibited the maximal remedial effects, possibly by enhanced modulation of renal iron-homeostatic molecules alongside reducing systemic iron levels.

AVAILABILITY OF DATA AND MATERIALS

All data generated or analysed during this study are included in this published article [and its Supplementary information files].

Evaluating renal iron overload in diabetes mellitus by blood oxygen level-dependent magnetic resonance imaging: a longitudinal experimental study

BACKGROUND

Iron overload plays a critical role in the pathogenesis of diabetic nephropathy. Non-invasive evaluation of renal iron overload in diabetes in the management and intervention of diabetic nephropathy is of great significance. This study aimed to explore the feasibility of blood oxygen level-dependent (BOLD) magnetic resonance imaging (MRI) in evaluating renal iron overload in diabetes using a rabbit model.

METHODS

The rabbits were randomly divided into control, iron-overload (I), diabetes (D), and diabetes with iron-overload (DI) groups (each n = 19). The diabetes models were generated by injecting intravenous alloxan solution, and the iron-overload models were generated by injecting intramuscular iron-dextran. BOLD MRI was performed immediately (week 0) and at week 4, 8, and 12 following modeling. The differences in renal cortex (CR2*) and outer medulla R2* (MR2*) and the ratio of MR2*-CR2* (MCR) across the different time points were compared.

RESULTS

Iron was first deposited in glomeruli in the I group and in proximal tubular cells in renal cortex in the D group. In the DI group, there was iron deposition in both glomeruli and proximal tubular cells at week 4, and the accumulation increased subsequently. The degree of kidney injury and iron overload was more severe in the DI group than those in the I and D groups at week 12. At week 8 and 12, the CR2* and MR2* in the DI group were higher than those in the I and D groups (all P < 0.05). The MCR in the I, D, and DI groups decreased from week 0 to 4 (all P < 0.001), and that in the I group increased from week 8 to 12 (P = 0.034). CR2* and MR2* values displayed different trends from week 0-12. Dynamic MCR curves in the D and DI groups were different from that in the I group.

CONCLUSION

It presents interactions between diabetes and iron overload in kidney injury, and BOLD MRI can be used to evaluate renal iron overload in diabetes.

Effect of Cisplatin on Renal Iron Homeostasis Components: Implication in Nephropathy

Cisplatin is an important chemotherapeutic drug for the treatment of solid tumors but often causes nephropathy as part of the off-target toxicity. Iron accumulation and related damage were implicated in cisplatin-induced kidney injury. However, the role of cisplatin in the renal iron sensing mechanism and its target genes responsible for iron uptake, storage, and release have not been investigated. Cellular iron homeostasis is controlled by the interaction of iron regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) present in the untranslated regions of iron transport and storage components. Here, we report that cisplatin does not influence the expressions of IRP targets such as transferrin receptor-1 (TfR1), divalent metal transporter-1 (DMT1), and ferroportin in renal cells despite the increased heme oxygenase-1 (HO-1) level. Ferritin subunits (Ft-H and Ft-L) are elevated in different magnitudes due to the increased mRNA expression. Intriguingly, a higher expression of Ft-L mRNA is detected than that of Ft-H mRNA. The inability of cisplatin in altering the IRE-IRP interaction is confirmed by examining IRE-containing luciferase activity, RNA electrophoretic mobility shift assay, and activation of IRPs. The labile iron pool is depleted but reversed by silencing of either Ft-H or Ft-L, suggesting increased iron storage by ferritin. Silencing of Ft-H or Ft-L promotes cell death, suggesting that ferritin acts to protect the renal cells from cisplatin-mediated toxicity. A differential increase of transcripts and equivalent increase of proteins of Ft-H and Ft-L and unaltered TfR1 and DMT1 transcripts are found in the kidneys of cisplatin-treated rats along with iron accumulation. Our results reveal that cisplatin does not influence the IRE-IRP interaction despite alteration of the cellular iron pool in renal cells. This insensitivity of the IRE-IRP system may be implicated in the accumulation of iron to contribute to cisplatin-induced nephropathy.

Therapeutic Implications of Ferroptosis in Renal Fibrosis

Renal fibrosis is a common feature of chronic kidney disease (CKD), and can lead to the destruction of normal renal structure and loss of kidney function. Little progress has been made in reversing fibrosis in recent years. Ferroptosis is more immunogenic than apoptosis due to the release and activation of damage-related molecular patterns (DAMPs) signals. In this paper, the relationship between renal fibrosis and ferroptosis was reviewed from the perspective of iron metabolism and lipid peroxidation, and some pharmaceuticals or chemicals associated with both ferroptosis and renal fibrosis were summarized. Other programmed cell death and ferroptosis in renal fibrosis were also firstly reviewed for comparison and further investigation.

In vivo growth of Staphylococcus lugdunensis is facilitated by the concerted function of heme and non-heme iron acquisition mechanisms

Staphylococcus lugdunensis has increasingly been recognized as a pathogen that can cause serious infection indicating this bacterium overcomes host nutritional immunity. Despite this, there exists a significant knowledge gap regarding the iron acquisition mechanisms employed by S. lugdunensis, especially during infection of the mammalian host. Here we show that S. lugdunensis can usurp hydroxamate siderophores and staphyloferrin A and B from Staphylococcus aureus. These transport activities all required a functional FhuC ATPase. Moreover, we show that the acquisition of catechol siderophores and catecholamine stress hormones by S. lugdunensis required the presence of the sst-1 transporter-encoding locus, but not the sst-2 locus. Iron-dependent growth in acidic culture conditions necessitated the ferrous iron transport system encoded by feoAB. Heme iron was acquired via expression of the iron-regulated surface determinant (isd) locus. During systemic infection of mice, we demonstrated that while S. lugdunensis does not cause overt illness, it does colonize and proliferate to high numbers in the kidneys. By combining mutations in the various iron acquisition loci (isd, fhuC, sst-1, and feo), we demonstrate that only a strain deficient for all of these systems was attenuated in its ability to proliferate to high numbers in the murine kidney. We propose the concerted action of heme and non-heme iron acquisition systems also enable S. lugdunensis to cause human infection.

Renal hypoxia-HIF-PHD-EPO signaling in transition metal nephrotoxicity: friend or foe?

The kidney is the main organ that senses changes in systemic oxygen tension, but it is also the key detoxification, transit and excretion site of transition metals (TMs). Pivotal to oxygen sensing are prolyl-hydroxylases (PHDs), which hydroxylate specific residues in hypoxia-inducible factors (HIFs), key transcription factors that orchestrate responses to hypoxia, such as induction of erythropoietin (EPO). The essential TM ion Fe is a key component and regulator of the hypoxia–PHD–HIF–EPO (HPHE) signaling axis, which governs erythropoiesis, angiogenesis, anaerobic metabolism, adaptation, survival and proliferation, and hence cell and body homeostasis. However, inadequate concentrations of essential TMs or entry of non-essential TMs in organisms cause toxicity and disrupt health. Non-essential TMs are toxic because they enter cells and displace essential TMs by ionic and molecular mimicry, e. g. in metalloproteins. Here, we review the molecular mechanisms of HPHE interactions with TMs (Fe, Co, Ni, Cd, Cr, and Pt) as well as their implications in renal physiology, pathophysiology and toxicology. Some TMs, such as Fe and Co, may activate renal HPHE signaling, which may be beneficial under some circumstances, for example, by mitigating renal injuries from other causes, but may also promote pathologies, such as renal cancer development and metastasis. Yet some other TMs appear to disrupt renal HPHE signaling, contributing to the complex picture of TM (nephro-)toxicity. Strikingly, despite a wealth of literature on the topic, current knowledge lacks a deeper molecular understanding of TM interaction with HPHE signaling, in particular in the kidney. This precludes rationale preventive and therapeutic approaches to TM nephrotoxicity, although recently activators of HPHE signaling have become available for therapy.

Endothelin A receptor antagonist attenuated renal iron accumulation in iron overload heme oxygenase-1 knockout mice

Progressive iron accumulation and renal impairment are prominent in both patients and mouse models of sickle cell disease (SCD). Endothelin A receptor (ETA) antagonism prevents this iron accumulation phenotype and reduces renal iron deposition in proximal tubules of SCD mice. To better understand the mechanisms of iron metabolism in the kidney and the role of ETA receptor in iron chelation and transport, we studied renal iron handling in a non-sickle cell iron overload model, heme oxygenase-1 (Hmox-1-/-) knockout mice. We found that Hmox-1-/- mice had elevated plasma endothelin-1 (ET-1), cortical ET-1 mRNA expression, and renal iron content compared to Hmox-1+/+ controls. The ETA receptor antagonist, ambrisentan, attenuated renal iron deposition, without any changes to anemia status in Hmox-1-/- mice. This was accompanied by reduced urinary iron excretion. Finally, ambrisentan had an important iron recycling effect by increasing expression of cellular iron exporter, ferroportin-1 (FPN-1) and circulating total iron levels in Hmox-1-/- mice. These findings suggest the ET-1/ETA signaling pathway contributes to in renal iron trafficking in a murine model of iron overload.

Iron Is Filtered by the Kidney and Is Reabsorbed by the Proximal Tubule

The aim of this study was to determine the iron (Fe) concentration profile within the lumen of the S2 renal proximal convoluted tubule (PCT) and to resolve whether this nephron segment transported Fe. To do this, we performed in vivo renal micropuncture on Wistar rats, collected PCT tubular fluid from superficial nephrons, and measured Fe concentration. The Fe concentration profile along the S2 PCT suggested significant Fe reabsorption. Proximal tubules were also microperfused in vivo with physiological solutions containing Fe and Zn, Cu, Mn, or Cd. PCTs perfused with 12μmol.l^{−1 55}FeCl₃ reabsorbed 105.2±12.7 fmol.mm⁻¹.min⁻¹ Fe, 435±52pmol.mm^-1.min⁻¹ Na, and 2.7±0.2nl.mm⁻¹.min⁻¹ water (mean ± SEM; n=19). Addition of ascorbate (1mmol.l⁻¹) to the perfusate did not significantly alter Fe, Na, or water reabsorption. Supplementing the control perfusate with 60μmol.l⁻¹ FeSO₄ significantly decreased ⁵⁵Fe uptake. Recalculating for the altered molar activity following addition of unlabeled Fe revealed a three-fold increase in Fe flux. Addition to the perfusate 12μmol.l⁻¹ CuSO₄, MnSO_4, CdSO_4, or ZnSO₄ did not affect Fe, Na, or water flux. In conclusion, (1) in vivo, S2 PCTs of rat reabsorb Fe and (2) Fe is reabsorbed along the PCT via a pathway that is insensitive to Cu, Mn, Cd, or Zn. Together, these data demonstrate for the first time the hitherto speculated process of renal Fe filtration and subsequent tubular Fe reabsorption in a living mammal.

Role of hepcidin in oxidative stress and cell death of cultured mouse renal collecting duct cells: protection against iron and sensitization to cadmium

The liver hormone hepcidin regulates systemic iron homeostasis. Hepcidin is also expressed by the kidney, but exclusively in distal nephron segments. Several studies suggest hepcidin protects against kidney damage involving Fe2+ overload. The nephrotoxic non-essential metal ion Cd2+ can displace Fe2+ from cellular biomolecules, causing oxidative stress and cell death. The role of hepcidin in Fe2+ and Cd2+ toxicity was assessed in mouse renal cortical [mCCD(cl.1)] and inner medullary [mIMCD3] collecting duct cell lines. Cells were exposed to equipotent Cd2+ (0.5-5 μmol/l) and/or Fe2+ (50-100 μmol/l) for 4-24 h. Hepcidin (Hamp1) was transiently silenced by RNAi or overexpressed by plasmid transfection. Hepcidin or catalase expression were evaluated by RT-PCR, qPCR, immunoblotting or immunofluorescence microscopy, and cell fate by MTT, apoptosis and necrosis assays. Reactive oxygen species (ROS) were detected using CellROX™ Green and catalase activity by fluorometry. Hepcidin upregulation protected against Fe2+-induced mIMCD3 cell death by increasing catalase activity and reducing ROS, but exacerbated Cd2+-induced catalase dysfunction, increasing ROS and cell death. Opposite effects were observed with Hamp1 siRNA. Similar to Hamp1 silencing, increased intracellular Fe2+ prevented Cd2+ damage, ROS formation and catalase disruption whereas chelation of intracellular Fe2+ with desferrioxamine augmented Cd2+ damage, corresponding to hepcidin upregulation. Comparable effects were observed in mCCD(cl.1) cells, indicating equivalent functions of renal hepcidin in different collecting duct segments. In conclusion, hepcidin likely binds Fe2+, but not Cd2+. Because Fe2+ and Cd2+ compete for functional binding sites in proteins, hepcidin affects their free metal ion pools and differentially impacts downstream processes and cell fate.

Effects of mine waste water on rat: bioaccumulation and histopathological evaluation

The highlighting of the bioaccumulation capacity of metals in the internal organs, the mode of distribution at the level of internal organs, the interactions between them, respectively, and the histological changes occurred at the level of the liver and kidneys are the main aspects addressed in the present study. The experiment was performed on 4 groups of Wistar rats: 3 groups which were administered water from rivers located in the vicinity of the Bor mining operation and 1 control group. The determination of the metal content in the administered water samples and in the internal organs was performed using the flame atomic absorption spectrophotometer. Tissue alterations were assessed by histological technique and hematoxylin-eosin staining. The metal retention capacity in the internal organs differs depending on the metal concentration in the administered water sample but also on the organ in which the determination was made. Also, correlations were established between the concentrations of metals at the level of the organs, showing (a) positive and significant correlations-at the level of the heart between Zn and Cu, Fe, and Mn and at the level of the lungs between Mn and Cd-but the most numerous were reported in the testicle; (b) moderate correlations at liver level between Fe and Zn, at spleen level between Cu and Mn and Cd and at the level of the kidneys between Pb and Zn, Cu, and Fe; (c) negative correlations at renal level between Pb and Mn; and (d) insignificant correlations between Pb and Fe. The histological changes identified at the level of the liver and kidney become more obvious, and their aggravation is registered with the increase of the metal content.

Renal Iron Accumulation and Oxidative Injury With Aging: Effects of Treatment With an Iron Chelator

Dysregulation of iron metabolism in the kidney may contribute to age-related increases in renal oxidative stress and dysfunction. This study assessed the effects of short-term iron chelation on markers of iron status, oxidative stress, inflammation, and autophagy in the kidneys of old rats. Old Fischer 344 rats (24 months) were treated with deferoxamine (DFO; 200 mg/kg, twice daily for 4.5 days); saline-treated young (6 months) and old rats served as controls. Renal nonheme iron was significantly higher in the old rats, with iron localized in the renal cortex. Ferritin levels were elevated in the kidneys of old rats, while expression of several antioxidant enzymes and mitochondrial proteins were reduced and protein carbonyls increased compared to young rats. DFO treatment significantly reduced ferritin levels, and increased transferrin receptor-1 protein, but did not affect nonheme iron content or protein carbonyls, nor did it reverse age-related changes in antioxidant enzymes and mitochondrial proteins. Although short-term DFO treatment did not mitigate the age-related increase in iron content and oxidative damage, this work demonstrates that old rats respond appropriately to DFO, suggesting that optimization of iron chelation regimens could be useful in improving renal homeostasis with aging.

Iron and Cadmium Entry Into Renal Mitochondria: Physiological and Toxicological Implications

Regulation of body fluid homeostasis is a major renal function, occurring largely through epithelial solute transport in various nephron segments driven by Na⁺/K⁺-ATPase activity. Energy demands are greatest in the proximal tubule and thick ascending limb where mitochondrial ATP production occurs through oxidative phosphorylation. Mitochondria contain 20-80% of the cell's iron, copper, and manganese that are imported for their redox properties, primarily for electron transport. Redox reactions, however, also lead to reactive, toxic compounds, hence careful control of redox-active metal import into mitochondria is necessary. Current dogma claims the outer mitochondrial membrane (OMM) is freely permeable to metal ions, while the inner mitochondrial membrane (IMM) is selectively permeable. Yet we recently showed iron and manganese import at the OMM involves divalent metal transporter 1 (DMT1), an H⁺-coupled metal ion transporter. Thus, iron import is not only regulated by IMM mitoferrins, but also depends on the OMM to intermembrane space H⁺ gradient. We discuss how these mitochondrial transport processes contribute to renal injury in systemic (e.g., hemochromatosis) and local (e.g., hemoglobinuria) iron overload. Furthermore, the environmental toxicant cadmium selectively damages kidney mitochondria by "ionic mimicry" utilizing iron and calcium transporters, such as OMM DMT1 or IMM calcium uniporter, and by disrupting the electron transport chain. Consequently, unraveling mitochondrial metal ion transport may help develop new strategies to prevent kidney injury induced by metals.

Local hepcidin increased intracellular iron overload via the degradation of ferroportin in the kidney

BACKGROUND

Hepcidin is a key regulator of iron homeostasis. Some studies showed that exogenous hepcidin decreased the expression of divalent metal transporter (DMT1) rather than ferroportin(FPN1) to regulate renal iron metabolism. This study explored the effects of hepcidin synthesized by the kidney and its mechanism of iron regulation.

METHODS

In the in vivo experiments, mice were divided into a unilateral ureter obstruction (UUO) model group and a sham operation group, and mice in the UUO model group were sacrificed on days 1, 3, 5 and 7. The expression of renal hepcidin, FPN1, DMT1 and the retention of renal iron were studied. In the in vitro experiments, we overexpressed hepcidin in HK-2 cells. Then we tested the expression of renal hepcidin, FPN1, DMT1 and observed the production of intracellular ferrous ions.

RESULTS

Renal hepcidin expression was consistently higher in the UUO group than in the sham group from the first day. The expression of FPN1 gradually decreased, and the expression of DMT1 gradually increased in the UUO model. Intracellular ferrous ions significantly increased on the first day of the UUO model. In hepcidin overexpressed HK-2 cells, the expression of FPN1 was decreased, while the expression of DMT1 has no significant change. In addition, production of intracellular ferrous ions increased.

CONCLUSION

local hepcidin can regulate iron metabolism in the kidney by adjusting the expression of FPN1.

Role of the kidneys in the redistribution of heme-derived iron during neonatal hemolysis in mice

Moderate intravascular hemolysis is a common condition in newborns. It is followed by the accumulation of bilirubin, which is a secondary product of the activity of heme oxygenase-1, an enzyme that catalyzes the breakdown of heme released from disrupted erythrocytes and taken up by hepatic macrophages. Although these cells are a major site of enzymatic heme breakdown in adults, we show here that epithelial cells of proximal tubules in the kidneys perform the functions of both heme uptake and catabolism in mouse neonates. A time-course study examining mouse pups during the neonatal period showed a gradual recovery from hemolysis, and concomitant decreases in the expression of heme-related genes and non-heme iron transporters in the proximal tubules. By adjusting the expression of iron-handling proteins in response to the disappearance of hemolysis in mouse neonates, the kidneys may play a role in the detoxification of iron and contribute to its recirculation from the primary urine to the blood.

Iron Homeostasis in Healthy Kidney and its Role in Acute Kidney Injury

Iron is required for key aspects of cellular physiology including mitochondrial function and DNA synthesis and repair. However, free iron is an aberration because of its ability to donate electrons, reduce oxygen, and generate reactive oxygen species. Iron-mediated cell injury or ferroptosis is a central player in the pathogenesis of acute kidney injury. There are several homeostatic proteins and pathways that maintain critical balance in iron homeostasis to allow iron's biologic functions yet avoid ferroptosis. Hepcidin serves as the master regulator of iron homeostasis through its ability to regulate ferroportin-mediated iron export and intracellular H-ferritin levels. Hepcidin is a protective molecule in acute kidney injury. Drugs targeting hepcidin, H-ferritin, and ferroptosis pathways hold great promise to prevent or treat kidney injury. In this review we discuss iron homeostasis under physiological and pathologic conditions and highlight its importance in acute kidney injury.

Ferritin in Kidney and Vascular Related Diseases: Novel Roles for an Old Player

Iron is at the forefront of a number of pivotal biological processes due to its ability to readily accept and donate electrons. However, this property may also catalyze the generation of free radicals with ensuing cellular and tissue toxicity. Accordingly, throughout evolution numerous pathways and proteins have evolved to minimize the potential hazardous effects of iron cations and yet allow for readily available iron cations in a wide variety of fundamental metabolic processes. One of the extensively studied proteins in the context of systemic and cellular iron metabolisms is ferritin. While clinicians utilize serum ferritin to monitor body iron stores and inflammation, it is important to note that the vast majority of ferritin is located intracellularly. Intracellular ferritin is made of two different subunits (heavy and light chain) and plays an imperative role as a safe iron depot. In the past couple of decades our understanding of ferritin biology has remarkably improved. Additionally, a significant body of evidence has emerged describing the significance of the kidney in iron trafficking and homeostasis. Here, we briefly discuss some of the most important findings that relate to the role of iron and ferritin heavy chain in the context of kidney-related diseases and, in particular, vascular calcification, which is a frequent complication of chronic kidney disease.

Proximal tubule transferrin uptake is modulated by cellular iron and mediated by apical membrane megalin-cubilin complex and transferrin receptor 1

Receptor-mediated endocytosis is responsible for reabsorption of transferrin (Tf) in renal proximal tubules (PTs). Although the role of the megalin-cubilin receptor complex (MCRC) in this process is unequivocal, modalities independent of this complex are evident but as yet undefined. Here, using immunostaining and Tf-flux assays, FACS analysis, and fluorescence imaging, we report localization of Tf receptor 1 (TfR1), the cognate Tf receptor mediating cellular holo-Tf (hTf) acquisition, to the apical brush border of the PT, with expression gradually declining along the PT in mouse and rat kidneys. In functional studies, hTf uptake across the apical membrane of cultured PT epithelial cell (PTEC) monolayers increased in response to decreased cellular iron after desferrioxamine (DFO) treatment. We also found that apical hTf uptake under basal conditions is receptor-associated protein (RAP)-sensitive and therefore mediated by the MCRC but becomes RAP-insensitive under DFO treatment, with concomitantly decreased megalin and cubilin expression levels and increased TfR1 expression. Thus, as well as the MCRC, TfR1 mediates hTf uptake across the PT apical brush border, but in conditions of decreased cellular iron, hTf uptake is predominated by augmented apical TfR1. In conclusion, both the MCRC and TfR1 mediate hTf uptake across apical brush border membranes of PTECs and reciprocally respond to decreased cellular iron. Our findings have implications for renal health, whole-body iron homeostasis, and pathologies arising from disrupted iron balance.

Iron uptake by ZIP8 and ZIP14 in human proximal tubular epithelial cells

In patients with iron overload disorders, increasing number of reports of renal dysfunction and renal iron deposition support an association between increased iron exposure and renal injury. In systemic iron overload, elevated circulating levels of transferrin-bound (TBI) and non-transferrin-bound iron (NTBI) are filtered to the renal proximal tubules, where they may cause injury. However, the mechanisms of tubular iron handling remain elusive. To unravel molecular renal proximal tubular NTBI and TBI handling, human conditionally immortalized proximal tubular epithelial cells (ciPTECs) were incubated with ⁵⁵Fe as NTBI and fluorescently labeled holo-transferrin as TBI. Ferrous iron importers ZIP8 and ZIP14 were localized in the ciPTEC plasma membrane. Whereas silencing of either ZIP8 or ZIP14 alone did not affect ⁵⁵Fe uptake, combined silencing significantly reduced ⁵⁵Fe uptake compared to control (p < 0.05). Furthermore, transferrin receptor 1 (TfR1) and ZIP14, but not ZIP8, colocalized with early endosome antigen 1 (EEA1). TfR1 and ZIP14 also colocalized with uptake of fluorescently labeled transferrin. Furthermore, ZIP14 silencing decreased ⁵⁵Fe uptake after ⁵⁵Fe-Transferrin exposure (p < 0.05), suggesting ZIP14 could be involved in early endosomal transport of TBI-derived iron into the cytosol. Our data suggest that human proximal tubular epithelial cells take up TBI and NTBI, where ZIP8 and ZIP14 are both involved in NTBI uptake, but ZIP14, not ZIP8, mediates TBI-derived iron uptake. This knowledge provides more insights in the mechanisms of renal iron handling and suggests that ZIP8 and ZIP14 could be potential targets for limiting renal iron reabsorption and enhancing urinary iron excretion in systemic iron overload disorders.

Cohen L, Lifshitz L, Magid Gold I, Zhang DL, Truman-Rosentsvit M, Leichtmann-Bardoogo Y, Nyska A, Addadi S, Rouault TA, Meyron-Holtz EG. Orchestrated regulation of iron trafficking proteins in the kidney during iron overload facilitates systemic iron retention

The exact route of iron through the kidney and its regulation during iron overload are not completely elucidated. Under physiologic conditions, non-transferrin and transferrin bound iron passes the glomerular filter and is reabsorbed through kidney epithelial cells, so that hardly any iron is found in the urine. To study the route of iron reabsorption through the kidney, we analyzed the location and regulation of iron metabolism related proteins in kidneys of mice with iron overload, elicited by iron dextran injections. Transferrin Receptor 1 was decreased as expected, following iron overload. In contrast, the multi-ligand hetero-dimeric receptor-complex megalin/cubilin, which also mediates the internalization of transferrin, was highly up-regulated. Moreover, with increasing iron, intracellular ferritin distribution shifted in renal epithelium from an apical location to a punctate distribution throughout the epithelial cells. In addition, in contrast to many other tissues, the iron exporter ferroportin was not reduced by iron overload in the kidney. Iron accumulated mainly in interstitial macrophages, and more prominently in the medulla than in the cortex. This suggests that despite the reduction of Transferrin Receptor 1, alternative pathways may effectively mediate re-absorption of iron that cycles through the kidney during parenterally induced iron-overload. The most iron consuming process of the body, erythropoiesis, is regulated by the renal erythropoietin producing cells in kidney interstitium. We propose, that the efficient re-absorption of iron by the kidney, also during iron overload enables these cells to sense systemic iron and regulate its usage based on the systemic iron state.

Physiological functions of ferroportin in the regulation of renal iron recycling and ischemic acute kidney injury

Renal iron recycling preserves filtered iron from urinary excretion. However, it remains debated whether ferroportin (FPN), the only known iron exporter, is functionally involved in renal iron recycling and whether renal iron recycling is required for systemic iron homeostasis. We deleted FPN in whole nephrons by use of a Nestin-Cre and in the distal nephrons and collecting ducts, using a Ksp-Cre, and investigated its impacts on renal iron recycling and systemic iron homeostasis. FPN deletion by Nestin-Cre, but not by Ksp-Cre, caused excess iron retention and increased ferritin heavy chain (FTH1) specifically in the proximal tubules and resulted in the reduction of serum and hepatic iron. The systemic iron redistribution was aggravated, resulting in anemia and the marked downregulation of hepatic hepcidin in elderly FPN knockout (KO)/Nestin-Cre mice. Similarly, in iron-deficient FPN KO/Nestin-Cre mice, the renal iron retention worsened anemia with the activation of the erythropoietin-erythroferrone-hepcidin pathway and the downregulation of hepatic hepcidin. Hence, FPN likely located at the basolateral membrane of the proximal tubules to export iron into the circulation and was required for renal iron recycling and systemic iron homeostasis particularly in elderly and iron-deficient mice. Moreover, FPN deletion in the proximal tubules alleviated ischemic acute kidney injury, possibly by upregulating FTH1 to limit catalytic iron and by priming antioxidant mechanisms, indicating that FPN could be deleterious in the pathophysiology of ischemic acute kidney injury (AKI) and thus may be a potential target for the prevention and mitigation of ischemic AKI.

Acute and Chronic Iron Overloading Differentially Modulates the Expression of Cellular Iron-homeostatic Molecules in Normal Rat Kidney

Little is known about the renal responses to acute iron overloading. This study measured the renal tubular expression of transferrin receptor-1 (TfR1), cubilin/megalin receptors, hepcidin, ferroportin, and ferritin chains following subacute intoxication of 40 male Wistar rats with a single oral dose of ferrous iron (300 mg/kg). The animals were randomly subdivided into 4 equal subgroups at the time of necropsy (1, 2, 4, and 8 hr). The results were compared with the controls ( n=15) and with the chronic group ( n=15), which received iron for 4 weeks (75 mg/kg/day; 5 days/week). Although both toxicity models inhibited TfR1, they upregulated the cubilin/megalin receptors and hepcidin, and triggered iron deposition in tubular cells. The ferritin heavy-chain and ferroportin were downregulated in the 2-hr and 4-hr acute subgroups, whereas chronic toxicity promoted their expression, compared with controls. Moreover, the 4-hr and 8-hr subgroups had higher intracellular Fe⁺² and marked cell apoptosis compared with the chronic group. In conclusion, the kidney appears to sustain iron reabsorption in both intoxication models. However, the cellular iron storage and exporter proteins were differentially expressed in both models, and their inhibition post-acute toxicity might contribute toward the intracellular accumulation of Fe⁺², oxidative stress, and ferroptosis.

Urinary Iron Excretion is Associated with Urinary Full-Length Megalin and Renal Oxidative Stress in Chronic Kidney Disease

BACKGROUND/AIMS

Megalin mediates the uptake of glomerular-filtered iron in the proximal tubules. Urinary full length megalin (C-megalin) excretion has been found to be increased in association with megalin-mediated metabolic load to the endo-lysosomal system in proximal tubular epithelial cells (PTECs) of residual nephrons. In the present study, we investigated the association between urinary iron and C-megalin in chronic kidney disease (CKD) patients, and the possible harmful effect of iron in renal tubules.

METHODS

Urinary levels of iron and C-megalin were measured in 63 CKD patients using automatic absorption spectrometry and a recently-established sandwich ELISA, respectively.

RESULTS

Although both urinary C-megalin and urinary total protein levels were correlated with urinary iron (C-megalin: ρ = 0.574, p <0.001; total protein: ρ = 0.500, p <0.001, respectively), urinary C-megalin alone emerged as an independent factor positively associated with urinary iron (β = 0.520, p <0.001) (R2 = 0.75, p <0.001). Furthermore, urinary iron was significantly and positively associated with urinary 8-hydroxydeoxyguanosine, an oxidative stress marker, while no association with other markers of renal tubular injury, i.e., β2-microglobulin and N-acetyl-β-D-glucosaminidase, was noted.

CONCLUSIONS

Our findings suggest that renal iron handling may be associated with megalin-mediated endo-lysosomal metabolic load in PTECs of residual nephrons and oxidative stress in renal tubules.

Change in iron metabolism in rats after renal ischemia/reperfusion injury

Previous studies have indicated that hepcidin, which can regulate iron efflux by binding to ferroportin-1 (FPN1) and inducing its internalization and degradation, acts as the critical factor in the regulation of iron metabolism. However, it is unknown whether hepcidin is involved in acute renal ischemia/reperfusion injury (IRI). In this study, an IRI rat model was established via right renal excision and blood interruption for 45 min in the left kidney, and iron metabolism indexes were examined to investigate the change in iron metabolism and to analyze the role of hepcidin during IRI. From 1 to 24 h after renal reperfusion, serum creatinine and blood urea nitrogen were found to be time-dependently increased with different degrees of kidney injury. Regular variations in iron metabolism indexes in the blood and kidneys were observed in renal IRI. Renal iron content, serum iron and serum ferritin increased early after reperfusion and then declined. Hepcidin expression in the liver significantly increased early after reperfusion, and its serum concentration increased beginning at 8 h after reperfusion. The splenic iron content decreased significantly in the early stage after reperfusion and then increased time-dependently with increasing reperfusion time, and the hepatic iron content showed a decrease in the early stage after reperfusion. The early decrease of the splenic iron content and hepatic iron content might indicate their contribution to the increase in serum iron in renal IRI. In addition, the duodenal iron content showed time-dependently decreased since 12 h after reperfusion in the IRI groups compared to the control group. Along with the spleen, the duodenum might contribute to the decrease in serum iron in the later stage after reperfusion. The changes in iron metabolism indexes observed in our study demonstrate an iron metabolism disorder in renal IRI, and hepcidin might be involved in maintaining iron homeostasis in renal IRI. These findings might suggest a self-protection mechanism regulating iron homeostasis in IRI and provide a new perspective on iron metabolism in attenuating renal IRI.

Nifedipine Increases Iron Content in WKPT-0293 Cl.2 Cells via Up-Regulating Iron Influx Proteins

Nifedipine was reported to enhance urinary iron excretion in iron overloaded mice. However, it remains unknown how nifedipine stimulates urinary iron excretion in the kidney. We speculated that nifedipine might inhibit the TfR1/ DMT1 (transferrin receptor 1/divalent metal transporter1)-mediated iron uptake by proximal tubule cells in addition to blocking L-type Ca2+ channels, leading to an increase in iron in lumen-fluid and then urinary iron excretion. To test this hypothesis, we investigated the effects of nifedipine on iron content and expression of TfR1, DMT1 and ferroportin1 (Fpn1) in WKPT-0293 Cl.2 cells of the S1 segment of the proximal tubule in rats, using a graphite furnace atomic absorption spectrophotometer and Western blot analysis, respectively. We demonstrated for the first time that nifedipine significantly enhanced iron content as well as TfR1 and DMT1 expression and had no effect on Fpn1 levels in the cells. We also found that ferric ammonium citrate decreased TfR1 levels, increased Fpn1 expression and had no effect on DMT1 content, while co-treatment with nifedipine and FAC increase TfR1 and DMT1 expression and also had no effect on Fpn1 levels. These findings suggest that the nifedipine-induced increase in cell iron may mainly be due to the corresponding increase in TfR1 and DMT1 expression and also imply that the effects of nifedipine on iron transport in proximal tubule cells can not explain the increase in urinary iron excretion.

Targeting Iron Homeostasis in Acute Kidney Injury

Iron is an essential metal involved in several major cellular processes required to maintain life. Because of iron's ability to cause oxidative damage, its transport, metabolism, and storage is strictly controlled in the body, especially in the small intestine, liver, and kidney. Iron plays a major role in acute kidney injury and has been a target for therapeutic intervention. However, the therapies that have been effective in animal models of acute kidney injury have not been successful in human beings. Targeting iron trafficking via ferritin, ferroportin, or hepcidin may offer new insights. This review focuses on the biology of iron, particularly in the kidney, and its implications in acute kidney injury.

Hephaestin and ceruloplasmin facilitate iron metabolism in the mouse kidney

Multicopper ferroxidases (MCFs) play an important role in cellular iron homeostasis. However, the role of MCFs in renal metabolism remains unclear. We used Hephaestin (Heph) and Ceruloplasmin (Cp) single or double (Heph/Cp) knockout (KO) mice to study the roles of MCFs in the kidney. Renal iron levels and the expression of iron metabolism genes were examined. The non-heme iron content both in the renal cortex and medulla of Heph/Cp KO mice was significantly increased. Perls' Prussian blue staining showed iron accumulation on the apical side of renal tubular cells in Heph/Cp KO mice. A significant increase in ferritin protein expression was also observed in the renal medulla and cortex of Heph/Cp KO mice. Both DMT1 and TfR1 protein expression were significantly decreased in the renal medulla of Heph/Cp KO mice, while the expression of DMT1 protein was significantly increased in the renal cortex of these animals. Significant increase in proteinuria and total urinary iron was observed in the double knockout mice, and this was associated with compromised structural integrity. These results suggest that KO of both the HEPH and CP genes leads to kidney iron deposition and toxicity, MCFs could protect kidney against a damage from iron excess.

Iron transport in the kidney: implications for physiology and cadmium nephrotoxicity

The kidney has recently emerged as an organ with a significant role in systemic iron (Fe) homeostasis. Substantial amounts of Fe are filtered by the kidney, which have to be reabsorbed to prevent Fe deficiency. Accordingly Fe transporters and receptors for protein-bound Fe are expressed in the nephron that may also function as entry pathways for toxic metals, such as cadmium (Cd), by way of "ionic and molecular mimicry". Similarities, but also differences in handling of Cd by these transport routes offer rationales for the propensity of the kidney to develop Cd toxicity. This critical review provides a comprehensive update on Fe transport by the kidney and its relevance for physiology and Cd nephrotoxicity. Based on quantitative considerations, we have also estimated the in vivo relevance of the described transport pathways for physiology and toxicology. Under physiological conditions all segments of the kidney tubules are likely to utilize Fe for cellular Fe requiring processes for metabolic purposes and also to contribute to reabsorption of free and bound forms of Fe into the circulation. But Cd entering tubule cells disrupts metabolic pathways and is unable to exit. Furthermore, our quantitative analyses contest established models linking chronic Cd nephrotoxicity to proximal tubular uptake of metallothionein-bound Cd. Hence, Fe transport by the kidney may be beneficial by preventing losses from the body. But increased uptake of Fe or Cd that cannot exit tubule cells may lead to kidney injury, and Fe deficiency may facilitate renal Cd uptake.

Dietary-induced gestational iron deficiency inhibits postnatal tissue iron delivery and postpones the cessation of active nephrogenesis in rats

Gestational iron deficiency (ID) can alter developmental programming through impaired nephron endowment, leading to adult hypertension, but nephrogenesis is unstudied. Iron status and renal development during dietary-induced gestational ID (<6 mg Fe kg-1 diet from Gestational Day 2 to Postnatal Day (PND) 7) were compared with control rats (198 mg Fe kg-1 diet). On PND2-PND10, PND15, PND30 and PND45, blood and tissue iron status were assessed. Nephrogenic zone maturation (PND2-PND10), radial glomerular counts (RGCs), glomerular size density and total planar surface area (PND15 and PND30) were also assessed. Blood pressure (BP) was measured in offspring. ID rats were smaller, exhibiting lower erythrocyte and tissue iron than control rats (PND2-PND10), but these parameters returned to control values by PND30-PND45. Relative kidney iron (µg g-1 wet weight) at PND2-PND10 was directly related to transport iron measures. In ID rats, the maturation of the active nephrogenic zone was later than control. RGCs, glomerular size, glomerular density, and glomerular planar surface area were lower than control at PND15, but returned to control by PND30. After weaning, the kidney weight/rat weight ratio (mg g-1) was heavier in ID than control rats. BP readings at PND45 were lower in ID than control rats. Altered kidney maturation and renal adaptations may contribute to glomerular size, early hyperfiltration and long-term renal function.

Iron-dependent turnover of IRP-1/c-aconitase in kidney cells

The kidney plays an important role in iron homeostasis and actively reabsorbs citrate. The bifunctional iron-regulatory protein IRP-1 potentially regulates iron trafficking and participates in citrate metabolism as a cytosolic (c-) aconitase. We investigated the role of cellular iron status in determining the expression and dynamics of IRP-1 in two renal cell types, with the aim of identifying a role of the protein in cellular ROS levels, citrate metabolism and glutamate production. The effects of iron supplementation and chelation on IRP-1 protein and mRNA levels and protein turnover were compared in cultured primary rat mesangial cells and a porcine renal tubule cell line (LLC-PK1). Levels of ROS were measured in both cell types, and c-aconitase activity, glutamate, and glutathione were measured in LLC-PK1 cells, with and without IRP-1 silencing and in glutamine-supplemented or nominally glutamine-free medium. Iron supplementation decreased IRP-1 levels (e.g., approx. 40% in mesangial cells treated with 10 μg ml(-1) iron for 16 h) and increased ubiquitinated IRP-1 levels in both cells types, with iron chelation having the opposite effect. Although iron increased ROS levels (three-fold with 20 μg ml(-1) iron in mesangial cells and more modestly by about 30% with 50 μg ml(-1) in LLC-PK1 cells, both after 24 h), protein degradation was not ROS-dependent. In LLC-PK1 cells, 10 μg ml(-1) iron (24 h) increased both aconitase activity (30%) and secreted glutamate levels (65%). Silencing did not remove the glutamate response to iron but decreased the c-aconitase activity of the residual protein independent of iron loading (37% and 46% of control levels, without and with iron treatment, respectively). However, in glutamine-free medium, glutamate was still increased by iron, even in IRP-1-silenced cells, and did not correspond to c-aconitase. Silencing decreased the amount of ferritin measured in response to iron loading, decreased the affect of iron on total glutathione by 48%, and increased the response of ROS to iron loading by 38%. We conclude that iron increases turnover of IRP-1 in kidney cells, while increasing aconitase activity, suggesting that the apoprotein (aconitase-inactive) form is not exclusively responsible for turnover. Iron increases glutamate levels in tubule epithelial cells, but this appears to be independent of c-aconitase activity or the availability of extracellular glutamine. IRP-1 protein levels are not regulated by ROS, but IRP-1-dependent ferritin expression may decrease ROS and increase total glutathione levels, suggesting that ferritin levels are more important than citrate metabolism in protecting renal cells against iron.

Prion protein promotes kidney iron uptake via its ferrireductase activity

Brain iron-dyshomeostasis is an important cause of neurotoxicity in prion disorders, a group of neurodegenerative conditions associated with the conversion of prion protein (PrP(C)) from its normal conformation to an aggregated, PrP-scrapie (PrP(Sc)) isoform. Alteration of iron homeostasis is believed to result from impaired function of PrP(C) in neuronal iron uptake via its ferrireductase activity. However, unequivocal evidence supporting the ferrireductase activity of PrP(C) is lacking. Kidney provides a relevant model for this evaluation because PrP(C) is expressed in the kidney, and ∼370 μg of iron are reabsorbed daily from the glomerular filtrate by kidney proximal tubule cells (PT), requiring ferrireductase activity. Here, we report that PrP(C) promotes the uptake of transferrin (Tf) and non-Tf-bound iron (NTBI) by the kidney in vivo and mainly NTBI by PT cells in vitro. Thus, uptake of (59)Fe administered by gastric gavage, intravenously, or intraperitoneally was significantly lower in PrP-knock-out (PrP(-/-)) mouse kidney relative to PrP(+/+) controls. Selective in vivo radiolabeling of plasma NTBI with (59)Fe revealed similar results. Expression of exogenous PrP(C) in immortalized PT cells showed localization on the plasma membrane and intracellular vesicles and increased transepithelial transport of (59)Fe-NTBI and to a smaller extent (59)Fe-Tf from the apical to the basolateral domain. Notably, the ferrireductase-deficient mutant of PrP (PrP(Δ51-89)) lacked this activity. Furthermore, excess NTBI and hemin caused aggregation of PrP(C) to a detergent-insoluble form, limiting iron uptake. Together, these observations suggest that PrP(C) promotes retrieval of iron from the glomerular filtrate via its ferrireductase activity and modulates kidney iron metabolism.

No evidence of persisting unrepaired nuclear DNA single strand breaks in distinct types of cells in the brain, kidney, and liver of adult mice after continuous eight-week 50 Hz magnetic field exposure with flux density of 0.1 mT or 1.0 mT

BACKGROUND

It has been hypothesized in the literature that exposure to extremely low frequency electromagnetic fields (50 or 60 Hz) may lead to human health effects such as childhood leukemia or brain tumors. In a previous study investigating multiple types of cells from brain and kidney of the mouse (Acta Neuropathologica 2004; 107: 257-264), we found increased unrepaired nuclear DNA single strand breaks (nDNA SSB) only in epithelial cells of the choroid plexus in the brain using autoradiographic methods after a continuous eight-week 50 Hz magnetic field (MF) exposure of adult mice with flux density of 1.5 mT.

METHODS

In the present study we tested the hypothesis that MF exposure with lower flux densities (0.1 mT, i.e., the actual exposure limit for the population in most European countries, and 1.0 mT) shows similar results to those in the previous study. Experiments and data analysis were carried out in a similar way as in our previous study.

RESULTS

Continuous eight-week 50 Hz MF exposure with 0.1 mT or 1.0 mT did not result in increased persisting unrepaired nDNA SSB in distinct types of cells in the brain, kidney, and liver of adult mice. MF exposure with 1.0 mT led to reduced unscheduled DNA synthesis (UDS) in epithelial cells in the choroid plexus of the fourth ventricle in the brain (EC-CP) and epithelial cells of the cortical collecting duct in the kidney, as well as to reduced mtDNA synthesis in neurons of the caudate nucleus in the brain and in EC-CP.

CONCLUSION

No evidence was found for increased persisting unrepaired nDNA SSB in distinct types of cells in the brain, kidney, and liver of adult mice after continuous eight-week 50 Hz magnetic field exposure with flux density of 0.1 mT or 1.0 mT.

Malondialdehyde, antioxidant enzymes, and renal tubular functions in children with iron deficiency or iron-deficiency anemia

We aimed to investigate the effects of iron deficiency (ID) or iron-deficiency anemia (IDA) on oxidative stress and renal tubular functions before and after treatment of children. A total of 30 children with a diagnosis of IDA constituted the IDA group and 32 children with a diagnosis of ID constituted the ID group. Control group consisted 38 age-matched children. Serum ferritin, soluble transferrin receptor (sTfR), serum, and urinary sodium (Na), potassium (K), calcium (Ca), phosphorus (P), creatinine (Cr), uric acid (UA), urinary N-acetyl-β-D-glucosaminidase (NAG) levels, and intra-erythrocyte malondialdehyde (MDA), catalase (CAT), superoxide dismutase (SOD), and glutathione peroxidase (GSH-Px) levels were measured before and after iron therapy in the IDA and ID groups, whereas it was studied once in the control group. We have divided the study group in groups according to age (infants <2 years, children 3-9 years, and adolescents 10-15 years). Patients with IDA (infant, adolescent) and ID (infant, children, and adolescent) had a significantly high level of MDA in post-treatment period in comparison to those of healthy control. Patients with IDA (children, adolescent) and ID (infant, children) had a significantly high level of pre-treatment GSH-Px than controls. Post-treatment SOD was lower in IDA (children and adolescent) groups than control and post-treatment CAT was lower in IDA and ID (adolescent) groups than control. These findings show that ferrous sulfate used in the treatment of ID or IDA could lead to oxidative stress; however, a marked deterioration of in proximal renal tubular functions was not seen.

Ovine fetal renal development impacted by multiple fetuses and uterine space restriction

Intrauterine growth restriction (IUGR) from uteroplacental dysfunction causes impaired nephrogenesis and ultimately hypertension, but it is unknown whether IUGR caused by insufficient space for placental development seen in uterine anomalies and/or multifetal gestation exerts the same effects. Fetal renal development and metabolism were studied in an ovine space-restriction model by combining unilateral horn surgical ligation and/or multifetal gestation. Reduced placental attachment sites and placental weight per fetus defined space-restricted (USR) v. control nonrestricted (NSR) fetuses. Space-restricted fetuses exhibited evidence for decreased plasma volume, with higher hematocrit and plasma albumin at gestational day (GD) 120, followed by lower blood pO2, and higher osmolarity and creatinine at GD130, P < 0.05 for all. By combining treatments, fetal kidney weight relative to fetal weight was inversely related to both fetal weight and plasma creatinine levels, P < 0.05 for both. At GD130, space-restricted fetal kidney weights, cortical depths and glomerular generations were decreased, P < 0.05 for all. Space-restricted kidneys underwent an adaptive response by prolonging active nephrogenesis and increasing maculae densa number, P < 0.05 for both. The major renal adaptations in space-restricted IUGR fetuses included immaturity in both development and endocrine function, with evidence for impaired renal excretory function.

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.

Pathophysiology of the Belgrade rat

The Belgrade rat is an animal model of divalent metal transporter 1 (DMT1) deficiency. This strain originates from an X-irradiation experiment first reported in 1966. Since then, the Belgrade rat’s pathophysiology has helped to reveal the importance of iron balance and the role of DMT1. This review discusses our current understanding of iron transport homeostasis and summarizes molecular details of DMT1 function. We describe how studies of the Belgrade rat have revealed key roles for DMT1 in iron distribution to red blood cells as well as duodenal iron absorption. The Belgrade rat’s pathology has extended our knowledge of hepatic iron handling, pulmonary and olfactory iron transport as well as brain iron uptake and renal iron handling. For example, relationships between iron and manganese metabolism have been discerned since both are essential metals transported by DMT1. Pathophysiologic features of the Belgrade rat provide us with a unique and interesting animal model to understand iron homeostasis.

Evidence for mitochondrial localization of divalent metal transporter 1 (DMT1)

In mammalian cells, mitochondria receive most incoming iron, yet no entry pathway for iron at the outer mitochondrial membrane (OMM) has been characterized. Our results show that the divalent metal transporter 1 (DMT1) occurs in the OMM. Immunoblots detected DMT1 in mitochondria from a pneumocyte cell model in their OMM. Using the split-ubiquitin yeast 2-hybrid system, we found that cytochrome c oxidase subunit II (COXII) and the translocase of OMM 6-kDa subunit (Tom6) homologue interact with DMT1. COXII coimmunoprecipitates with DMT1. There are 4 DMT1 isoforms that differ at the N and C termini. Using HEK293 cells that inducibly express all of the 4 ends of DMT1, we found all of them in the OMM, as detected by immunoblots after cell fractionation, and in isolated mitochondria, as detected by immunofluorescence. Immunoblot analysis of purified cell fractions from rat renal cortex confirmed and extended these results to the kidney, which expressed high levels of DMT1. Immunogold labeling detected DMT1 colocalization in mitochondria with the voltage-dependent anion-selective channel protein-1, which is expressed in the OMM. We suggest that DMT1 not only exports iron from endosomes, but also serves to import the metal into the mitochondria.

Fungal iron availability during deep seated candidiasis is defined by a complex interplay involving systemic and local events

Nutritional immunity – the withholding of nutrients by the host – has long been recognised as an important factor that shapes bacterial-host interactions. However, the dynamics of nutrient availability within local host niches during fungal infection are poorly defined. We have combined laser ablation-inductively coupled plasma mass spectrometry (LA-ICP MS), MALDI imaging and immunohistochemistry with microtranscriptomics to examine iron homeostasis in the host and pathogen in the murine model of systemic candidiasis. Dramatic changes in the renal iron landscape occur during disease progression. The infection perturbs global iron homeostasis in the host leading to iron accumulation in the renal medulla. Paradoxically, this is accompanied by nutritional immunity in the renal cortex as iron exclusion zones emerge locally around fungal lesions. These exclusion zones correlate with immune infiltrates and haem oxygenase 1-expressing host cells. This local nutritional immunity decreases iron availability, leading to a switch in iron acquisition mechanisms within mature fungal lesions, as revealed by laser capture microdissection and qRT-PCR analyses. Therefore, a complex interplay of systemic and local events influences iron homeostasis and pathogen-host dynamics during disease progression.

Microbial pathogens must assimilate essential micronutrients to establish infections. During bacterial infection, mammals limit the availability of micronutrients to inhibit the growth of the pathogen – a phenomenon termed ‘nutrient immunity.’ Nutrient immunity has not been examined during disseminated candidiasis. Yet micronutrient assimilation, and iron assimilation in particular, is required for fungal virulence, and life-threatening disseminated fungal infections are recognised as a major medical threat for patients with compromised immune systems. We show that nutrient immunity operates during disseminated Candida albicans infections in mice. Over time immune cells congregate around the fungal lesions in the kidney cortex, driving nutrient immunity and reducing iron availability for the pathogen. The fungus responds by tuning its iron assimilation strategies to the reduced iron levels. Paradoxically, iron levels increase in other parts of the kidney as Candida infections progress. We show that the fungal infection disturbs global iron homeostasis in the host by perturbing red blood cell recycling in the spleen and this is associated with increased iron storage in the kidney medulla. Therefore, fungal infection exerts system-wide effects upon iron homeostasis in the mammalian host, whilst triggering local nutrient immunity to limit the infection.

Proximal tubule H-ferritin mediates iron trafficking in acute kidney injury

Ferritin plays a central role in iron metabolism and is made of 24 subunits of 2 types: heavy chain and light chain. The ferritin heavy chain (FtH) has ferroxidase activity that is required for iron incorporation and limiting toxicity. The purpose of this study was to investigate the role of FtH in acute kidney injury (AKI) and renal iron handling by using proximal tubule-specific FtH-knockout mice (FtH(PT-/-) mice). FtH(PT-/-) mice had significant mortality, worse structural and functional renal injury, and increased levels of apoptosis in rhabdomyolysis and cisplatin-induced AKI, despite significantly higher expression of heme oxygenase-1, an antioxidant and cytoprotective enzyme. While expression of divalent metal transporter-1 was unaffected, expression of ferroportin (FPN) was significantly lower under both basal and rhabdomyolysis-induced AKI in FtH(PT-/-) mice. Apical localization of FPN was disrupted after AKI to a diffuse cytosolic and basolateral pattern. FtH, regardless of iron content and ferroxidase activity, induced FPN. Interestingly, urinary levels of the iron acceptor proteins neutrophil gelatinase-associated lipocalin, hemopexin, and transferrin were increased in FtH(PT-/-) mice after AKI. These results underscore the protective role of FtH and reveal the critical role of proximal tubule FtH in iron trafficking in AKI.

Ovine uterine space restriction alters placental transferrin receptor and fetal iron status during late pregnancy

BACKGROUND

Fetal growth restriction is reported to be associated with impaired placental iron transport. Transferrin receptor (TfR) is a major placental iron transporter in humans but has not been studied in sheep. TfR is regulated by both iron and nitric oxide (NO), the molecule produced by endothelial nitric oxide synthase (eNOS). We hypothesized that limited placental development downregulates both placental TfR and eNOS expression, thereby lowering fetal tissue iron.

METHODS

An ovine surgical uterine space restriction (USR) model, combined with multifetal gestation, tested the extremes of uterine and placental adaptation. Blood, tissues, and placentomes from non-space restricted (NSR) singletons were compared with USR fetuses at gestational day (GD) 120 or 130.

RESULTS

When expressed proportionate to fetal weight, liver iron content did not differ, whereas renal iron was higher in USR vs. NSR fetuses. Renal TfR protein expression did not differ, but placental TfR expression was lower in USR fetuses at GD130. Placental levels of TfR correlated to eNOS. TfR was localized throughout the placentome, including the hemophagous zone, implicating a role for TfR in ovine placental iron transport.

CONCLUSION

Fetal iron was regulated in an organ-specific manner. In USR fetuses, NO-mediated placental adaptations may prevent the normal upregulation of placental TfR at GD130.

Ferroportin expression in haem oxygenase 1-deficient mice

HO1 (haem oxygenase 1) and Fpn (ferroportin) are key proteins for iron recycling from senescent red blood cells and therefore play a major role in controlling the bioavailability of iron for erythropoiesis. Although important aspects of iron metabolism in HO1-deficient (Hmox1-/-) mice have already been revealed, little is known about the regulation of Fpn expression and its role in HO1 deficiency. In the present study, we characterize the cellular and systemic factors influencing Fpn expression in Hmox1-/- bone marrow-derived macrophages and in the liver and kidney of Hmox1-/- mice. In Hmox1-/- macrophages, Fpn protein was relatively highly expressed under high levels of hepcidin in culture medium. Similarly, despite high hepatic hepcidin expression, Fpn is still detected in Kupffer cells and is also markedly enhanced at the basolateral membrane of the renal tubules of Hmox1-/- mice. Through the activity of highly expressed Fpn, epithelial cells of the renal tubules probably take over the function of impaired system of tissue macrophages in recycling iron accumulated in the kidney. Moreover, although we have found increased expression of FLVCR (feline leukaemia virus subgroup C receptor), a haem exporter, in the kidneys of Hmox1-/- mice, haem level was increased in these organs. Furthermore, we show that iron/haem-mediated toxicity are responsible for renal injury documented in the kidneys of Hmox1-/- mice.

Suppression of hepcidin expression and iron overload mediate Salmonella susceptibility in ankyrin 1 ENU-induced mutant

Salmonella, a ubiquitous Gram-negative intracellular bacterium, is a food borne pathogen that infects a broad range of hosts. Infection with Salmonella Typhimurium in mice is a broadly recognized experimental model resembling typhoid fever in humans. Using a N-ethyl-N-nitrosurea (ENU) mutagenesis recessive screen, we report the identification of Ity16 (Immunity to Typhimurium locus 16), a locus responsible for increased susceptibility to infection. The position of Ity16 was refined on chromosome 8 and a nonsense mutation was identified in the ankyrin 1 (Ank1) gene. ANK1 plays an important role in the formation and stabilization of the red cell cytoskeleton. The Ank1^Ity16/Ity16 mutation causes severe hemolytic anemia in uninfected mice resulting in splenomegaly, hyperbilirubinemia, jaundice, extramedullary erythropoiesis and iron overload in liver and kidneys. Ank1^Ity16/Ity16 mutant mice demonstrated low levels of hepcidin (Hamp) expression and significant increases in the expression of the growth differentiation factor 15 (Gdf15), erythropoietin (Epo) and heme oxygenase 1 (Hmox1) exacerbating extramedullary erythropoiesis, tissue iron deposition and splenomegaly. As the infection progresses in Ank1^Ity16/Ity16, the anemia worsens and bacterial load were high in liver and kidneys compared to wild type mice. Heterozygous Ank1^+/Ity16 mice were also more susceptible to Salmonella infection although to a lesser extent than Ank1^Ity16/Ity16 and they did not inherently present anemia and splenomegaly. During infection, iron accumulated in the kidneys of Ank1^+/Ity16 mice where bacterial loads were high compared to littermate controls. The critical role of HAMP in the host response to Salmonella infection was validated by showing increased susceptibility to infection in Hamp-deficient mice and significant survival benefits in Ank1^+/Ity16 heterozygous mice treated with HAMP peptide. This study illustrates that the regulation of Hamp and iron balance are crucial in the host response to Salmonella infection in Ank1 mutants.