The effects of iron deficient and high iron diets on SARS-CoV-2 lung infection and disease

Introduction

The severity of Coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is often dictated by a range of comorbidities. A considerable literature suggests iron deficiency and iron overload may contribute to increased infection, inflammation and disease severity, although direct causal relationships have been difficult to establish.

Methods

Here we generate iron deficient and iron loaded C57BL/6 J mice by feeding standard low and high iron diets, with mice on a normal iron diet representing controls. All mice were infected with a primary SARS-CoV-2 omicron XBB isolate and lung inflammatory responses were analyzed by histology, immunohistochemistry and RNA-Seq.

Results

Compared with controls, iron deficient mice showed no significant changes in lung viral loads or histopathology, whereas, iron loaded mice showed slightly, but significantly, reduced lung viral loads and histopathology. Transcriptional changes were modest, but illustrated widespread dysregulation of inflammation signatures for both iron deficient vs. controls, and iron loaded vs. controls. Some of these changes could be associated with detrimental outcomes, whereas others would be viewed as beneficial.

Discussion

Diet-associated iron deficiency or overload thus induced modest modulations of inflammatory signatures, but no significant histopathologically detectable disease exacerbations.

New insights on neurodegeneration triggered by iron accumulation: Intersections with neutral lipid metabolism, ferroptosis, and motor impairment

Brain iron accumulation constitutes a pathognomonic indicator in several neurodegenerative disorders. Metal accumulation associated with dopaminergic neuronal death has been documented in Parkinson's disease. Through the use of in vivo and in vitro models, we demonstrated that lipid dysregulation manifests as a neuronal and glial response during iron overload. In this study, we show that cholesterol content and triacylglycerol (TAG) hydrolysis were strongly elevated in mice midbrain. Lipid cacostasis was concomitant with the loss of dopaminergic neurons, astrogliosis and elevated expression of α-synuclein. Exacerbated lipid peroxidation and markers of ferroptosis were evident in the midbrain from mice challenged with iron overload. An imbalance in the activity of lipolytic and acylation enzymes was identified, favoring neutral lipid hydrolysis, and consequently reducing TAG and cholesteryl ester levels. Notably, these observed alterations were accompanied by motor impairment in iron-treated mice. In addition, neuronal and glial cultures along with their secretomes were used to gain further insight into the mechanism underlying TAG hydrolysis and cholesterol accumulation as cellular responses to iron accumulation. We demonstrated that TAG hydrolysis in neurons is triggered by astrocyte secretomes. Moreover, we found that the ferroptosis inhibitor, ferrostatin-1, effectively prevents cholesterol accumulation both in neurons and astrocytes. Taken together, these results indicate that lipid disturbances occur in iron-overloaded mice as a consequence of iron-induced oxidative stress and depend on neuron-glia crosstalk. Our findings suggest that developing therapies aimed at restoring lipid homeostasis may lead to specific treatment for neurodegeneration associated with ferroptosis and brain iron accumulation.

The microbiota and the host organism switch between cooperation and competition based on dietary iron levels

The microbiota significantly impacts digestive epithelium functionality, especially in nutrient processing. Given the importance of iron for both the host and the microbiota, we hypothesized that host-microbiota interactions fluctuate with dietary iron levels. We compared germ-free (GF) and conventional mice (SPF) fed iron-containing (65 mg/Kg) or iron-depleted (<6 mg/Kg) diets. The efficacy of iron privation was validated by iron blood parameters. Ferritin and Dmt1, which represent cellular iron storage and transport respectively, were studied in tissues where they are abundant: the duodenum, liver and lung. When the mice were fed an iron-rich diet, the microbiota increased blood hemoglobin and hepcidin and the intestinal ferritin levels, suggesting that the microbiota helps iron storage. When iron was limiting, the microbiota inhibited the expression of the intestinal Dmt1 transporter, likely via the pathway triggered by Hif-2α. The microbiota assists the host in storing intestinal iron when it is abundant and competes with the host by inhibiting Dmt1 in conditions of iron scarcity. Comparison between duodenum, liver and lung indicates organ-specific responses to microbiota and iron availability. Iron depletion induced temporal changes in microbiota composition and activity, reduced α-diversity of microbiota, and led to Lactobacillaceae becoming particularly more abundant after 60 days of privation. By inoculating GF mice with a simplified bacterial mixture, we show that the iron-depleted host favors the gut fitness of Bifidobacterium longum.

NMDA receptor activation induces damage of alveolar type II cells and lung fibrogenesis through ferroptosis

Ferroptosis, a newly discovered type of regulated cell death, has been implicated in numerous human diseases. Idiopathic pulmonary fibrosis (IPF) is a progressive and ultimately fatal interstitial lung disease with poor prognosis and limited treatment options. Emerging evidence has linked ferroptosis and glutamate-determined cell fate which is considered a new light on the etiology of pulmonary fibrosis. Here, we observed that N-methyl d-aspartate receptor (NMDAR) activation promoted cell damage and iron deposition in MLE-12 cells in a dose-, time-, and receptor-dependent manner. This mediated substantial Ca2+ influx, upregulated the expression levels of nNOS and IRP1, and affected intracellular iron homeostasis by regulating the expression of iron transport-related proteins (i.e., TFR1, DMT1, and FPN). Excessive iron load promoted the continuous accumulation of total intracellular and mitochondrial reactive oxygen species, which ultimately led to ferroptosis. NMDAR inhibition reduced lung injury and pulmonary fibrosis in bleomycin-induced mice. Bleomycin stimulation upregulated the expression of NMDAR1, nNOS, and IRP1 in mouse lung tissues, which ultimately led to iron deposition via regulation of the expression of various iron metabolism-related genes. NMDAR activation initiated the pulmonary fibrosis process by inducing iron deposition in lung tissues and ferroptosis of alveolar type II cells. Our data suggest that NMDAR activation regulates the expression of iron metabolism-related genes by promoting calcium influx, increasing nNOS and IRP1 expression, and increasing iron deposition by affecting cellular iron homeostasis, ultimately leading to mitochondrial damage, mitochondrial dysfunction, and ferroptosis. NMDAR activation-induced ferroptosis of alveolar type II cells might be a key event to the initiation of pulmonary fibrosis.

Iron cycle disruption by heme oxygenase-1 activation leads to a reduced breast cancer cell survival

Heme oxygenase-1 (HO-1), which catalyzes heme degradation releasing iron, regulates several processes related to breast cancer. Iron metabolism deregulation is also connected with several tumor processes. However the regulatory relationship between HO-1 and iron proteins in breast cancer remains unclear. Using human breast cancer biopsies, we found that high HO-1 levels significantly correlated with low DMT1 levels. Contrariwise, high HO-1 levels significantly correlated with high ZIP14 and prohepcidin expression, as well as hemosiderin storage. At mRNA level, we found that high HO-1 expression significantly correlated with low DMT1 expression but high ZIP14, L-ferritin and hepcidin expression. In in vivo experiments in mice with genetic overexpression or pharmacological activation of HO-1, we detected the same expression pattern observed in human biopsies. In in vitro experiments, HO-1 activation induced changes in iron proteins expression leading to an increase of hemosiderin, ROS levels, lipid peroxidation and a decrease of the growth rate. Such low growth rate induced by HO-1 activation was reversed when iron levels or ROS levels were reduced. Our findings demonstrate an important role of HO-1 on iron homeostasis in breast cancer. The changes in iron proteins expression when HO-1 is modulated led to the iron accumulation deregulating the iron cell cycle, and consequently, generating oxidative stress and low viability, all contributing to impair breast cancer progression.

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

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

Iron overload impairs renal function and is associated with vascular calcification in rat aorta

Vascular calcification (VC) has been associated with a risk of cardiovascular diseases. Iron may play a critical role in progressive VC. Therefore, we investigated the effects of iron overload on the aorta of rats. A rat model of iron overload was established by intraperitoneal injection of Iron-Dextran. The levels of iron, calcium, and ALP activity were detected. Von Kossa staining and Perl's staining were conducted. The expression of iron metabolism-related and calcification related factors were examined in the aortic tissue of rats. The results showed serum and aortic tissue iron were increased induced by iron overload and excessive iron induced hepatic and renal damage. In iron overload rats, the expression of divalent metal transporter 1 (DMT1) and hepcidin were higher, but ferroportin1 (FPN1) was lower. Von Kossa staining demonstrated calcium deposition in the aorta of iron overload rats. The calcium content and ALP activity in serum and aortic tissue were increased and iron level in aortic tissue highly correlated with calcium content and ALP activity. The expressions of the osteogenic markers were increased while a decrease of Alpha-smooth muscle actin (α-SMA) in the aortic tissue of iron overload rats. IL-24 was increased during the calcification process induced by iron. Overall, we demonstrated excessive iron accumulation in the aortic tissue and induced organs damage. The iron metabolism-related factors were significantly changed during iron overload. Moreover, we found that iron overload leads to calcium deposition in aorta, playing a key role in the pathological process of VC by mediating osteoblast differentiation factors.

Iron overload and mitochondrial dysfunction orchestrate pulmonary fibrosis

Pulmonary fibrosis (PF) is a chronic, progressive heterogeneous disease of lung tissues with poor lung function caused by scar tissue. Due to our limited understanding of its mechanism, there is currently no treatment strategy that can prevent the development of PF. In recent years, iron accumulation and mitochondrial damage have been reported to participate in PF, and drugs that reduce iron content and improve mitochondrial function have shown significant efficacy in animal experimental models. Excessive iron leads to mitochondrial impairment, which may be the key cause that results in the dysfunction of various kinds of pulmonary cells and further promotes PF. As an emerging research hotspot, there are few targeted effective therapeutic strategies at present due to limited mechanistic understanding. In this review, the roles of iron homeostasis imbalance and mitochondrial damage in PF are summarized and discussed, highlighting a promising direction for finding truly effective therapeutics for PF.

Prolonged exposure to welding fumes as a novel cause of systemic iron overload

BACKGROUND & AIMS

Inhalation of welding fume may cause pulmonary disease known as welder's lung. At our centre we came across a number of welders with systemic iron overload and prolonged occupational history and we aimed at characterizing this novel clinical form of iron overload.

METHODS

After exclusion of other known causes of iron overload, 20 welders were fully evaluated for working history, hepatic, metabolic and iron status. MRI iron assessment was performed in 19 patients and liver biopsy in 12. We included 40 HFE-HH patients and 24 healthy controls for comparison.

RESULTS

75% of patients showed lung HRCT alterations; 90% had s-FERR > 1000 ng/mL and 60% had TSAT > 45%. Liver iron overload was mild in 8 and moderate-severe in 12. The median iron removed was 7.8 g. Welders showed significantly lower TSAT and higher SIS and SIS/TIS ratio than HFE-HH patients. Serum hepcidin was significantly higher in welders than in HFE-HH patients and healthy controls. At liver biopsy, 50% showed liver fibrosis that was mild in four, and moderate-severe in two. Liver staging correlated with liver iron overload.

CONCLUSIONS

Welders with prolonged fume exposure can develop severe liver iron overload. The mechanism of liver iron accumulation is quite different to that of HFE-HH suggesting that reticuloendothelial cells may be the initial site of deposition. We recommend routine measurement of serum iron indices in welders to provide adequate diagnosis and therapy, and the inclusion of prolonged welding fume exposure in the list of acquired causes of hyperferritinemia and iron overload.

Clioquinol Attenuates Pulmonary Fibrosis through Inactivation of Fibroblasts via Iron Chelation

Strict control of iron homeostasis is critical for the maintenance of normal lung function. Iron accumulates in the lungs of patients with idiopathic pulmonary fibrosis (PF), but the characteristics of iron metabolism in the pathogenesis of PF and related targeting therapeutics are not well studied. In this study, we investigated the cellular and molecular characteristics of iron metabolism in fibrotic lungs and further explored the efficacy of clioquinol (CQ) for the treatment of PF as well as its functional mechanism. Iron aggregates accumulated in the lungs of patients with idiopathic PF, and FTL (ferritin light chain) transcripts were increased in their pulmonary fibroblasts. In the bleomycin (BLM)-induced PF (BLM-PF) mouse model, pulmonary iron accumulation is a very early and concomitant event of PF. Labile iron pool levels in both fibroblasts and macrophages from the BLM-PF model were elevated, and iron metabolism was dysregulated. CQ attenuated PF induced by BLM and FITC, and iron-saturated CQ did not alleviate BLM-PF. Furthermore, CQ inhibited the activation of fibroblasts, including proliferation, fibrotic differentiation, proinflammatory cytokine secretion, and migration. In conclusion, our study demonstrated that CQ, acting as an iron chelator, attenuates experimental PF through inactivation of fibroblasts, providing support for targeting iron metabolism as a basis for PF treatment.

Critical role for iron accumulation in the pathogenesis of fibrotic lung disease

Increased iron levels and dysregulated iron homeostasis, or both, occur in several lung diseases. Here, the effects of iron accumulation on the pathogenesis of pulmonary fibrosis and associated lung function decline was investigated using a combination of murine models of iron overload and bleomycin-induced pulmonary fibrosis, primary human lung fibroblasts treated with iron, and histological samples from patients with or without idiopathic pulmonary fibrosis (IPF). Iron levels are significantly increased in iron overloaded transferrin receptor 2 (Tfr2) mutant mice and homeostatic iron regulator (Hfe) gene-deficient mice and this is associated with increases in airway fibrosis and reduced lung function. Furthermore, fibrosis and lung function decline are associated with pulmonary iron accumulation in bleomycin-induced pulmonary fibrosis. In addition, we show that iron accumulation is increased in lung sections from patients with IPF and that human lung fibroblasts show greater proliferation and cytokine and extracellular matrix responses when exposed to increased iron levels. Significantly, we show that intranasal treatment with the iron chelator, deferoxamine (DFO), from the time when pulmonary iron levels accumulate, prevents airway fibrosis and decline in lung function in experimental pulmonary fibrosis. Pulmonary fibrosis is associated with an increase in Tfr1⁺ macrophages that display altered phenotype in disease, and DFO treatment modified the abundance of these cells. These experimental and clinical data demonstrate that increased accumulation of pulmonary iron plays a key role in the pathogenesis of pulmonary fibrosis and lung function decline. Furthermore, these data highlight the potential for the therapeutic targeting of increased pulmonary iron in the treatment of fibrotic lung diseases such as IPF. © 2020 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Iron in Lung Pathology

The lung presents a unique challenge for iron homeostasis. The entire airway is in direct contact with the environment and its iron particulate matter and iron-utilizing microbes. However, the homeostatic and adaptive mechanisms of pulmonary iron regulation are poorly understood. This review provides an overview of systemic and local lung iron regulation, as well as the roles of iron in the development of lung infections, airway disease, and lung injury. These mechanisms provide an important foundation for the ongoing development of therapeutic applications.

Iron Homeostasis in the Lungs-A Balance between Health and Disease

A strong mechanistic link between the regulation of iron homeostasis and oxygen sensing is evident in the lung, where both systems must be properly controlled to maintain lung function. Imbalances in pulmonary iron homeostasis are frequently associated with respiratory diseases, such as chronic obstructive pulmonary disease and with lung cancer. However, the underlying mechanisms causing alterations in iron levels and the involvement of iron in the development of lung disorders are incompletely understood. Here, we review current knowledge about the regulation of pulmonary iron homeostasis, its functional importance, and the link between dysregulated iron levels and lung diseases. Gaining greater knowledge on how iron contributes to the pathogenesis of these diseases holds promise for future iron-related therapeutic strategies.

The Multiple Faces of the Metal Transporter ZIP14 (SLC39A14)

The SLC39A family of metal transporters was identified through homologies with the Zrt- and Irt-like (ZIP) proteins from yeast and plants. Of all the ZIP transporters, ZIP14 is arguably the most robustly characterized in terms of function at the integrative level. Mice with a global knockout of Zip14 are viable, thus providing the opportunity to conduct physiologic experiments. In mice, Zip14 expression is highly tissue specific, with the greatest abundance in the jejunum > liver > heart > kidney > white adipose tissue > skeletal muscle > spleen > pancreas. A unique feature of Zip14 is its upregulation by proinflammatory conditions, particularly increased interleukin 6 (IL-6) and nitric oxide. The transcription factors AP-1, ATF4, and ATF6α are involved in Zip14 regulation. ZIP14 does not appear to be zinc-regulated. The Zip14 knockout phenotype shows multiple sites of ZIP14 function, including the liver, adipose tissue, brain, pancreas, and bone. A prominent feature of the Zip14 ablation is a reduction in intestinal barrier function and onset of metabolic endotoxemia. Many aspects of the phenotype are accentuated with age and accompany increased circulating IL-6. Studies with 65Zn, 59Fe [nontransferrin-bound iron (NTBI)] and 54Mn show that ZIP14 transports these metals. At a steady state, the plasma concentrations of zinc, NTBI, and manganese are such that zinc ions are the major substrate available for ZIP14 at the cell surface. Upregulation of ZIP14 accounts for the hypozincemia and hepatic zinc accumulation associated with acute inflammation and sepsis and is required for liver regeneration and resistance to endoplasmic reticulum (ER) stress. Zip14 ablation in mice produces a defect in manganese excretion that leads to excess manganese accumulation in the brain that produces characteristics of Parkinsonism.

Maternal serum levels and placental expression of hepcidin in preeclampsia

Preeclampsia (PE) is a multifactorial pregnancy-induced syndrome and infection could have a role in its etiopathogenesis. Hepcidin, central regulator of iron homeostasis, is an antimicrobial peptide induced by inflammatory/infective stimuli. Therefore, hepcidin could be a good nonspecific marker of infection in PE. In a cross-sectional study, we assessed maternal serum levels (ELISA) and placental expression (Real-Time PCR and ELISA) of hepcidin in PE and normal pregnancies. In a prospective study, hepcidin maternal serum levels were assessed in early pregnancy before PE onset and in age matched controls. Hepcidin protein and gene expressions were significantly decreased in PE placentae with normal fetal growth compared to controls and PE with Fetal Growth Restriction (FGR), respectively. In contrast, we did not find significant differences in maternal serum hepcidin levels in PE vs gestational age-matched controls. Hepcidin serum levels in the first half of pregnancy were found significantly higher in women who subsequently developed PE compared to mothers having a physiological pregnancy until term. Altered hepcidin expression in PE placentae could be explained by direct infective/inflammatory stimuli. Furthermore, high hepcidin levels in maternal serum could be an early marker of PE, further emphasizing the role of inflammatory status before symptoms onset in PE.

Pulmonary Iron Homeostasis in Hepcidin Knockout Mice

Pulmonary iron excess is deleterious and contributes to a range of chronic and acute inflammatory diseases. Optimal lung iron concentration is maintained through dynamic regulation of iron transport and storage proteins. The iron-regulatory hormone hepcidin is also expressed in the lung. In order to better understand the interactions between iron-associated molecules and the hepcidin-ferroportin axis in lung iron balance, we examined lung physiology and inflammatory responses in two murine models of systemic iron-loading, either hepcidin knock-out (Hepc KO) or liver-specific hepcidin KO mice (Hepc KOliv), which do (Hepc KOliv) or do not (Hepc KO) express lung hepcidin. We have found that increased plasma iron in Hepc KO mice is associated with increased pulmonary iron levels, consistent with increased cellular iron uptake by pulmonary epithelial cells, together with an increase at the apical membrane of the cells of the iron exporter ferroportin, consistent with increased iron export in the alveoli. Subsequently, alveolar macrophages (AM) accumulate iron in a non-toxic form and this is associated with elevated production of ferritin. The accumulation of iron in the lung macrophages of hepcidin KO mice contrasts with splenic and hepatic macrophages which contain low iron levels as we have previously reported. Hepc KOliv mice with liver-specific hepcidin deficiency demonstrated same pulmonary iron overload profile as the Hepc KO mice, suggesting that pulmonary hepcidin is not critical in maintaining local iron homeostasis. In addition, the high iron load in the lung of Hepc KO mice does not appear to enhance acute lung inflammation or injury. Lastly, we have shown that intraperitoneal LPS injection is not associated with pulmonary hepcidin induction, despite high levels of inflammatory cytokines. However, intranasal LPS injection stimulates a hepcidin response, likely derived from AM, and alters pulmonary iron content in Hepc KO mice.

Potential Toxicity and Underlying Mechanisms Associated with Pulmonary Exposure to Iron Oxide Nanoparticles: Conflicting Literature and Unclear Risk

Fine/micron-sized iron oxide particulates are incidentally released from a number of industrial processes, including iron ore mining, steel processing, welding, and pyrite production. Some research suggests that occupational exposure to these particulates is linked to an increased risk of adverse respiratory outcomes, whereas other studies suggest that iron oxide is biologically benign. Iron oxide nanoparticles (IONPs), which are less than 100 nm in diameter, have recently surged in use as components of novel drug delivery systems, unique imaging protocols, as environmental catalysts, and for incorporation into thermoplastics. However, the adverse outcomes associated with occupational exposure to IONPs remain relatively unknown. Relevant in vivo studies suggest that pulmonary exposure to IONPs may induce inflammation, pulmonary fibrosis, genotoxicity, and extra-pulmonary effects. This correlates well with in vitro studies that utilize relevant dose, cell type(s), and meaningful end points. A majority of these adverse outcomes are attributed to increased oxidative stress, most likely caused by particle internalization, dissolution, release of free iron ions, and disruption of iron homeostasis. However, because the overall toxicity profile of IONPs is not well understood, it is difficult to set safe exposure limit recommendations that would be adequate for the protection of at-risk workers. This review article will focus on known risks following IONPs exposure supported by human, animal, and cell culture-based studies, the potential challenges intrinsic to IONPs toxicity assessment, and how these may contribute to the poorly characterized IONPs toxicity profile.

Markers of oxidative/nitrosative stress and inflammation in lung tissue of rats exposed to different intravenous iron compounds

Iron deficiency anemia is a frequent complication in clinical conditions such as chronic kidney disease, chronic heart failure, inflammatory bowel disease, cancer, and excessive blood loss. Given the ability of iron to catalyze redox reactions, iron therapy can be associated with oxidative stress. The lung is uniquely susceptible to oxidative stress, and little is known about the effects of intravenous iron treatment in this organ. This study characterized changes in markers of oxidative/nitrosative stress and inflammation in the lung of non-iron deficient, non-anemic rats treated with five weekly doses (40 mg iron per kg body weight) of low molecular weight iron dextran (LMWID), iron sucrose (IS), ferric carboxymaltose (FCM), ferumoxytol (FMX), iron isomaltoside 1000 (IIM), or saline (control). Rats treated with LMWID, FMX, or IIM showed significant changes in most measures of oxidative/nitrosative stress, inflammation, and iron deposition compared to the saline-treated controls, with greatest changes in the LMWID treatment group. Increases in products of lipid peroxidation (thiobarbituric acid reactive substances) and protein nitrosation (nitrotyrosine) were consistent with increases in the activity of antioxidant enzymes (catalase, Cu,Zn-SOD, GPx), decreases in antioxidative capacity (reduced:oxidized GSH ratio), increased levels of transcription factors involved in the inflammatory pathway (NF-κB, HIF-1α), inflammatory cytokines (TNF-α, IL-6), adhesion molecules (VCAM-1), markers of macrophage infiltration (ED-1), and iron deposition (Prussian blue, ferritin). Since changes in measured parameters in FCM- or IS-treated rats were generally modest, the results suggest that FCM and IS have a low propensity to induce lung inflammation. The relevance of these findings to clinical safety profiles of the tested intravenous iron products requires further investigation.

Role of iron in the pathogenesis of respiratory disease

Iron is essential for many biological processes, however, too much or too little iron can result in a wide variety of pathological consequences, depending on the organ system, tissue or cell type affected. In order to reduce pathogenesis, iron levels are tightly controlled in throughout the body by regulatory systems that control iron absorption, systemic transport and cellular uptake and storage. Altered iron levels and/or dysregulated homeostasis have been associated with several lung diseases, including chronic obstructive pulmonary disease, lung cancer, cystic fibrosis, idiopathic pulmonary fibrosis and asthma. However, the mechanisms that underpin these associations and whether iron plays a key role in the pathogenesis of lung disease are yet to be fully elucidated. Furthermore, in order to survive and replicate, pathogenic micro-organisms have evolved strategies to source host iron, including freeing iron from cells and proteins that store and transport iron. To counter these microbial strategies, mammals have evolved immune-mediated defence mechanisms that reduce iron availability to pathogens. This interplay between iron, infection and immunity has important ramifications for the pathogenesis and management of human respiratory infections and diseases. An increased understanding of the role that iron plays in the pathogenesis of lung disease and respiratory infections may help inform novel therapeutic strategies. Here we review the clinical and experimental evidence that highlights the potential importance of iron in respiratory diseases and infections.

Disruption of the Hepcidin/Ferroportin Regulatory System Causes Pulmonary Iron Overload and Restrictive Lung Disease

Emerging evidence suggests that pulmonary iron accumulation is implicated in a spectrum of chronic lung diseases. However, the mechanism(s) involved in pulmonary iron deposition and its role in the in vivo pathogenesis of lung diseases remains unknown. Here we show that a point mutation in the murine ferroportin gene, which causes hereditary hemochromatosis type 4 (Slc40a1^C326S), increases iron levels in alveolar macrophages, epithelial cells lining the conducting airways and lung parenchyma, and in vascular smooth muscle cells. Pulmonary iron overload is associated with oxidative stress, restrictive lung disease with decreased total lung capacity and reduced blood oxygen saturation in homozygous Slc40a1^C326S/C326S mice compared to wild-type controls. These findings implicate iron in lung pathology, which is so far not considered a classical iron-related disorder.

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Ferroportin resistance to hepcidin binding leads to pulmonary iron overload.

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Lung iron accumulation is restricted to specific cell types.

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Iron overload causes restrictive lung disease and decreased blood oxygen saturation.

Pulmonary iron accumulation is associated with a wide spectrum of lung diseases, such as chronic obstructive pulmonary disease and cystic fibrosis. Impaired lung function was further reported in patients with thalassemia major, a disease hallmarked by transfusional iron overload. So far, the mechanism(s) leading to pulmonary iron deposition and its role in disease onset and progression are still unknown. Our study shows that in a murine disease model, in which the control of systemic iron homeostasis is disrupted, iron accumulates in the lung and correlates with oxidative stress, restrictive lung disease and decreased blood oxygen saturation. These findings implicate iron overload in lung pathology, which is not considered a classical iron-related disorder.