Iron overload

Iron Load Toxicity in Medicine: From Molecular and Cellular Aspects to Clinical Implications

Iron is essential for all organisms and cells. Diseases of iron imbalance affect billions of patients, including those with iron overload and other forms of iron toxicity. Excess iron load is an adverse prognostic factor for all diseases and can cause serious organ damage and fatalities following chronic red blood cell transfusions in patients of many conditions, including hemoglobinopathies, myelodyspasia, and hematopoietic stem cell transplantation. Similar toxicity of excess body iron load but at a slower rate of disease progression is found in idiopathic haemochromatosis patients. Excess iron deposition in different regions of the brain with suspected toxicity has been identified by MRI T2* and similar methods in many neurodegenerative diseases, including Alzheimer's disease and Parkinson's disease. Based on its role as the major biological catalyst of free radical reactions and the Fenton reaction, iron has also been implicated in all diseases associated with free radical pathology and tissue damage. Furthermore, the recent discovery of ferroptosis, which is a cell death program based on free radical generation by iron and cell membrane lipid oxidation, sparked thousands of investigations and the association of iron with cardiac, kidney, liver, and many other diseases, including cancer and infections. The toxicity implications of iron in a labile, non-protein bound form and its complexes with dietary molecules such as vitamin C and drugs such as doxorubicin and other xenobiotic molecules in relation to carcinogenesis and other forms of toxicity are also discussed. In each case and form of iron toxicity, the mechanistic insights, diagnostic criteria, and molecular interactions are essential for the design of new and effective therapeutic interventions and of future targeted therapeutic strategies. In particular, this approach has been successful for the treatment of most iron loading conditions and especially for the transition of thalassemia from a fatal to a chronic disease due to new therapeutic protocols resulting in the complete elimination of iron overload and of iron toxicity.

Chemotherapy: a double-edged sword in cancer treatment

Chemotherapy is a well-known and effective treatment for different cancers; unfortunately, it has not been as efficient in the eradication of all cancer cells as been expected. The mechanism of this failure was not fully clarified, yet. Meanwhile, alterations in the physiologic conditions of the tumor microenvironment (TME) were suggested as one of the underlying possibilities. Chemotherapy drugs can activate multiple signaling pathways and augment the secretion of inflammatory mediators. Inflammation may show two opposite roles in the TME. On the one hand, inflammation, as an innate immune response, tries to suppress tumor growth but on the other hand, it might be not powerful enough to eradicate the cancer cells and even it can provide appropriate conditions for cancer promotion and relapse as well. Therefore, the administration of mild anti-inflammatory drugs during chemotherapy might result in more successful clinical results. Here, we will review and discuss this hypothesis. Most chemotherapy agents are triggers of inflammation in the tumor microenvironment through inducing the production of senescence-associated secretory phenotype (SASP) molecules. Some chemotherapy agents can induce systematic inflammation by provoking TLR4 signaling or triggering IL-1B secretion through the inflammasome pathway. NF-kB and MAPK are key signaling pathways of inflammation and could be activated by several chemotherapy drugs. Furthermore, inflammation can play a key role in cancer development, metastasis and exacerbation.

Use of dual-electron probes reveals the role of ferritin as an iron depot in ex vivo erythropoiesis

In the finely regulated process of mammalian erythropoiesis, the path of the labile iron pool into mitochondria for heme production is not well understood. Existing models for erythropoiesis do not include a central role for the ubiquitous iron storage protein ferritin; one model proposes that incoming endosomal Fe³⁺ bound to transferrin enters the cytoplasm through an ion transporter after reduction to Fe²⁺ and is taken up into mitochondria through mitoferrin-1 transporter. Here, we apply a dual three-dimensional imaging and spectroscopic technique, based on scanned electron probes, to measure Fe³⁺ in ex vivo human hematopoietic stem cells. After seven days in culture, we observe cells displaying a highly specialized architecture with anchored clustering of mitochondria and massive accumulation of nanoparticles containing high iron concentrations localized to lysosomal storage depots, identified as ferritin. We hypothesize that lysosomal ferritin iron depots enable continued heme production after expulsion of most of the cellular machinery.

TIM2 modulates retinal iron levels and is involved in blood-retinal barrier breakdown

Careful control of iron availability in the retina is central to maintenance of iron homeostasis, as its imbalance is associated with oxidative stress and the progression of several retinopathies. Ferritin, known for its role in iron storage and detoxification, has also been proposed as an iron-transporter protein, through its binding to Scara5 and TIM2 membrane receptors. In this study, the presence and iron-related functions of TIM2 in the mouse retina were investigated. Our results revealed for the first time the presence of TIM2 receptors in the mouse retina, mainly in Müller cells. Experimental TIM2 downregulation in the mouse retina promoted, probably due to a compensatory mechanism, Scara5 overexpression that increased retinal ferritin uptake and induced iron overload. Consecutive reactive oxygen species (ROS) overproduction and vascular endothelial growth factor (VEGF) overexpression led to impaired paracellular and transcellular endothelial transport characterized by tight junction degradation and increased caveolae number. In consequence, blood-retinal barrier (BRB) breakdown and retinal edema were observed. Altogether, these results point to TIM2 as a new modulator of retinal iron homeostasis and as a potential target to counteract retinopathy.

Iron mediated toxicity and programmed cell death: A review and a re-examination of existing paradigms

Iron is an essential micronutrient that is problematic for biological systems since it is toxic as it generates free radicals by interconverting between ferrous (Fe²⁺) and ferric (Fe³⁺) forms. Additionally, even though iron is abundant, it is largely insoluble so cells must treat biologically available iron as a valuable commodity. Thus elaborate mechanisms have evolved to absorb, re-cycle and store iron while minimizing toxicity. Focusing on rarely encountered situations, most of the existing literature suggests that iron toxicity is common. A more nuanced examination clearly demonstrates that existing regulatory processes are more than adequate to limit the toxicity of iron even in response to iron overload. Only under pathological or artificially harsh situations of exposure to excess iron does it become problematic. Here we review iron metabolism and its toxicity as well as the literature demonstrating that intracellular iron is not toxic but a stress responsive programmed cell death-inducing second messenger.

Acute iron overload leads to hypothalamic-pituitary-gonadal axis abnormalities in female rats

Iron plays a critical role in a mammal's physiological processes. However, iron tissue deposits have been shown to act as endocrine disrupters. Studies that evaluate the effect of acute iron overload on hypothalamic-pituitary-gonadal (HPG) axis health are particularly sparse. This study demonstrates that acute iron overload leads to HPG axis abnormalities, including iron accumulation and impairment in reproductive tract morphology. Female rats were treated with iron-dextran (Fe rats) to assess their HPG morphophysiology. The increasing serum iron levels due to iron-dextran treatment were positively correlated with higher iron accumulation in the HPG axis and uterus of Fe rats than in control rats. An increase in the production of superoxide anions was observed in the pituitary, uterus and ovary of Fe rats. Morphophysiological reproductive tract abnormalities, such as abnormal ovarian follicular development and the reduction of serum estrogen levels, were observed in Fe rats. In addition, a significant negative correlation was obtained between ovary superoxide anion and serum estrogen levels. Together, these data provide in vivo evidence that acute iron overload is toxic for the HPG axis, a finding that may be associated with the subsequent development of the risk of reproductive dysfunction.

Iron-rich ferritin in the hypoxia-tolerant rodent Spalax ehrenbergi: a naturally-occurring biomarker confirms the internalization and pathways of intracellular macromolecules

The discovery of pits/caveolae in the plasmalemma advanced the study of macromolecule internalization. "Transcytosis" describes the transport of macromolecular cargo from one front of a polarized cell to the other within membrane-bounded carrier(s), via endocytosis, intracellular trafficking and exocytosis. Clathrin-mediated transcytosis is used extensively by epithelial cells, while caveolae-mediated transcytosis mostly occurs in endothelial cells. The internalization pathways were monitored by various markers, including radioisotopes, nanoparticles, enzymes, immunostains, and fluorophores. We describe an internalization pathway identified using a naturally-occurring biomarker, in vivo assembled ferritin, containing electron-dense iron cores. Iron, an essential trace metal for most living species and iron homeostasis, is crucial for cellular life. Ferritin is a ubiquitous and highly conserved archeoprotein whose main function is to store a reserve iron supply inside the cytoplasm in a non-toxic form. Ferritin is present in all organisms which have a metabolic requirement for iron and in even in organisms whose taxonomic rank is very low. The newborns of the blind mole, Spalax ehrenbergi, are born and live in a hypoxic environment and have significant iron overload in their liver and heart, but their iron metabolism has not been previously studied. These newborns, which are evolutionarily adapted to fluctuations in the environmental oxygen, have a unique ability to sequester transplacental iron and store it in ferritin without any signs of iron toxicity. Using the ferrihydrite cores of ferritin, we were able to monitor the ferritin internalization from portals of its entry into the cytosol of hepatocytes and cardiomyocytes and into the lysosomes.

Iron storage diseases in birds

Parenteral iron is toxic to many species but, because the uptake of iron from the diet is regulated in the intestine, acute intoxication is not seen under natural conditions. Chronic ingestion of large amounts of absorbable iron in the diet can lead to the storage of iron in the liver in many species, including humans. The excess iron is stored within hepatocytes as haemosiderin and can be quantitatively assessed by liver biopsy or at necropsy using special stains such as Perls iron stain and/or biochemical tests. Iron may also be found within the Kupffer cells in the liver and the macrophage cells of the spleen especially where concurrent diseases are present such as haemolytic anaemia, septicaemia, neoplasia and starvation. Iron accumulation in the liver, also known as haemosiderosis, may not always be associated with clinical disease although in severe cases hepatic damage may occur. It is probable that concurrent disease conditions are largely responsible for the degree and nature of the pathological changes described in most cases of haemosiderosis. In some human individuals there may be a genetic predisposition to iron storage disease, haemochromatosis, associated with poor regulation of iron uptake across the intestine. In severe cases iron pigment will be found in the liver, spleen, gut wall, kidney and heart with subsequent development of ascites, heart failure and multisystem pathology. Clinical disease associated with accumulation of iron in the liver, and other tissues, has been reported in many species of bird although it is most commonly reported in Indian hill mynas ( Gracula religiosa ) and toucans ( Ramphastos sp ). It is likely that the tolerance to the build up of tissue iron varies in individual species of bird and that the predominant predisposing factors may differ, even within closely related taxonomic groups.

Ultrastructural aspects of iron storage, transport and metabolism

The iron storage proteins, ferritin and hemosiderin, enable electron microscopic visualization thanks to their electron-dense iron content, which is not present in other compounds involved in transport or metabolism of iron such as transferrin, lactoferrin, or hemoglobin. It is this electron density which contributed to the unraveling of stages in absorption, transport, deposition, storage, and release of iron. In recent years, additional methods of investigation have further supported the information achieved by the ultrastructural studies. Even while using new analytical methods, the seminal morphological observations remain valid for understanding the role of iron in health and disease. In this review, we will illustrate a few basic findings of electron microscopy in humans, experimental animals, and cell cultures. The importance of H chain ferritin as a transporter across the blood-brain barrier is just an example of a new role revealed for an "old" storage protein, explaining some controversial observations on the presence of iron in the brain.

Iron Loading and Overloading due to Ineffective Erythropoiesis

Erythropoiesis describes the hematopoietic process of cell proliferation and differentiation that results in the production of mature circulating erythrocytes. Adult humans produce 200 billion erythrocytes daily, and approximately 1 billion iron molecules are incorporated into the hemoglobin contained within each erythrocyte. Thus, iron usage for the hemoglobin production is a primary regulator of plasma iron supply and demand. In many anemias, additional sources of iron from diet and tissue stores are needed to meet the erythroid demand. Among a subset of anemias that arise from ineffective erythropoiesis, iron absorption and accumulation in the tissues increases to levels that are in excess of erythropoiesis demand even in the absence of transfusion. The mechanisms responsible for iron overloading due to ineffective erythropoiesis are not fully understood. Based upon data that is currently available, it is proposed in this review that loading and overloading of iron can be regulated by distinct or combined mechanisms associated with erythropoiesis. The concept of erythroid regulation of iron is broadened to include both physiological and pathological hepcidin suppression in cases of ineffective erythropoiesis.

A quantitative assessment of haemosiderosis in wild and captive birds using image analysis

An experimental model of haemosiderosis, using the chicken, was developed to examine the distribution of iron in the liver following an injection of iron dextran and to allow calibration of image analysis readings. Image analysis was used as a tool to quantify the stainable iron present in hepatic tissue obtained from wild and captive birds presented for necropsy. A retrospective study of 180 necropsy cases, representing 40 different species of bird, is described. Statistical evaluation of the amount and distribution of stainable iron in the liver tissue of birds from different taxonomic orders indicated that the concentration of iron measured in liver tissue was significantly different in different species of bird. The results of the study showed that hepatic haemosiderosis is a common histological finding in most avian species examined. Although not necessarily associated with overt liver disease, it is often associated with concurrent malignant and infectious diseases. The presence of excess stainable iron in the liver is probably a reflection of an altered iron metabolism associated with increased turnover of tissue iron. This alteration may occur following starvation or trauma.

Ferritin and ferritin isoforms I: Structure-function relationships, synthesis, degradation and secretion

Ferritin is the intracellular protein responsible for the sequestration, storage and release of iron. Ferritin can accumulate up to 4500 iron atoms as a ferrihydrite mineral in a protein shell and releases these iron atoms when there is an increase in the cell's need for bioavailable iron. The ferritin protein shell consists of 24 protein subunits of two types, the H-subunit and the L-subunit. These ferritin subunits perform different functions in the mineralization process of iron. The ferritin protein shell can exist as various combinations of these two subunit types, giving rise to heteropolymers or isoferritins. Isoferritins are functionally distinct and characteristic populations of isoferritins are found depending on the type of cell, the proliferation status of the cell and the presence of disease. The synthesis of ferritin is regulated both transcriptionally and translationally. Translation of ferritin subunit mRNA is increased or decreased, depending on the labile iron pool and is controlled by an iron-responsive element present in the 5'-untranslated region of the ferritin subunit mRNA. The transcription of the genes for the ferritin subunits is controlled by hormones and cytokines, which can result in a change in the pool of translatable mRNA. The levels of intracellular ferritin are determined by the balance between synthesis and degradation. Degradation of ferritin in the cytosol results in complete release of iron, while degradation in secondary lysosomes results in the formation of haemosiderin and protection against iron toxicity. The majority of ferritin is found in the cytosol. However, ferritin with slightly different properties can also be found in organelles such as nuclei and mitochondria. Most of the ferritin produced intracellularly is harnessed for the regulation of iron bioavailability; however, some of the ferritin is secreted and internalized by other cells. In addition to the regulation of iron bioavailability ferritin may contribute to the control of myelopoiesis and immunological responses.

Carbonyl-iron supplementation induces hepatocyte nuclear changes in BALB/CJ male mice

BACKGROUND/AIMS

In humans, chronic iron excess may induce hepatic fibrosis and/or hepatocellular carcinoma. This work was undertaken to investigate hepatic iron overload outcome in iron-overloaded mice.

METHODS

BALB/cJ male mice received supplements of 0, 0.5, 1.5 and 3% carbonyl-iron for 2, 4, 8 and 12 months. Histological staining, immunohistochemistry using ferritin antibodies and electron microscopic studies were performed on liver. Liver iron concentration was measured biochemically. Mitotic index and hepatocyte nuclear size were evaluated on Feulgen-stained slides.

RESULTS

Liver iron concentration was increased, reaching 13 times control value after 12 months in 3% iron-overloaded mice, and iron was found predominantly in hepatocytes, with a porto-centrolobular decreasing gradient. Neither hepatic fibrosis nor hepatocellular carcinoma was found. Perls' stain positive inclusions containing ferritin were found within hepatocyte nuclei in 3%-overloaded mice. Electron microscopy disclosed that inclusions consisted of ferritin particle aggregates without a limiting membrane. Mice overloaded with 3% iron for 12 months showed larger hepatocyte nuclei than control mice and a mitotic index increase with presence of abnormal tripolar mitotic figures. In addition, some iron-free hepatocytes were observed.

CONCLUSIONS

Carbonyl-iron supplementation produces significant iron overload in mice but does not result in liver fibrosis or hepatocellular carcinoma after 12 months. However, nuclear changes were produced in hepatocytes, and occasional iron-free hepatocytes were observed: these may represent preneoplastic changes caused by iron overload.

Interrelationships of alcohol and iron in liver disease with particular reference to the iron-binding proteins, ferritin and transferrin

It is known that the regular consumption of alcohol is responsible for the disruption of normal iron metabolism in humans, resulting in the excess deposition of iron in the liver in approximately one-third of alcoholic subjects. The mechanisms involved are largely unknown; however, it is likely that the two major proteins of iron metabolism, ferritin and transferrin are intimately involved in the process. Tissue damage in alcoholic liver disease and the inherited iron-overload disease, haemochromatosis, are caused by excess alcohol and iron, respectively. The mechanisms of this damage are believed to be similar in both disease conditions and involve free radical-mediated toxicity. A high proportion of haemochromatosis sufferers consume excessive amounts of alcohol and synergistic hepatotoxic events may occur leading to the earlier development of liver cirrhosis. This review describes briefly the role of ferritin and transferrin in normal iron metabolism and in iron overload disease and explores the possible involvement of these proteins in the pathophysiology of excess iron deposition in alcoholic subjects.

Effects of acute iron overload on Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss)

Distribution of radioiron to various tissues after intraperitoneal injections was examined in Atlantic salmon and rainbow trout. Liver and spleen were found to be the major iron storage tissues. Injections of 1 or 5 mg iron as ferric ammonium citrate led to a fall in hemoglobin levels in both species after 2 d. Hemoglobin levels returned to normal levels in rainbow trout after 8 d, but Atlantic salmon had not recovered, and Hb levels fell below 3 g/100 mL. In both species, the fall in Hb was associated with a raise in iron levels in spleen and liver, suggesting damage to erythrocytes. Atlantic salmon liver ferritin showed a two- to threefold increase, while rainbow trout showed a sixfold increase, and a more rapid response. The toxic effect of iron in fish appears to be different from the effect in other vertebrates.

Distribution of iron during embryogenesis and early larval life in sea lampreys (Petromyzon marinus)

The distribution of ferric iron was examined in eggs, embryos, and early larvae (up to 77 d post fertilization (PF)) of the sea lamprey, Petromyzon marinus, using the Prussian blue staining technique. Iron was not detected in eggs and embryos but was detected in the lumen of the oesophagus at 17 – 33 days PF, when the newly hatched larvae burrow and commence exogenous feeding. The posterior intestine and hindgut became the primary location of iron absorption by 36 days PF, and subsequent development (36 – 63 days PF) showed an iron distribution suggesting elimination of excess metal through exfoliation of epithelial cells of the posterior intestine. The deposition of iron in macrophages of the intestine and liver (Kupffer cells) by 40 days PF may be related to both erythrophagocytosis and erythropoiesis at this time. Iron deposits in the macrophages of the pronephros and the atrium of the heart at 42 – 44 days PF were likely a consequence of endocytosis/filtration of the metal from circulating plasma. The commencement of iron deposition in specific tissues of larval lampreys seems to be correlated with the time they begin filter-feeding. Macrophages play an important role in iron metabolism in early larval life of lampreys.

The hypotransferrinaemic mouse: ultrastructural and laser microprobe analysis observations

Homozygote hypotransferrinaemic mice (hpx/hpx) have cytopathological features similar to those of human congenital atransferrinaemia, genetic haemochromatosis, and neonatal haemochromatosis. These conditions all have in common high levels of cytotoxic non-transferrin-bound serum iron. This study describes the ultrastructural features of iron overload in liver, pancreas, heart, and small intestine of 2- and 12-month-old hypotransferrinaemic mice. Electron microscopic studies of unstained sections showed early parenchymal cell siderosis, with accumulation of numerous ferritin particles and clusters in the cytosol, as well as ferritin and haemosiderin in lysosomes (siderosomes). In the 12-month-old animals, iron was also found in Kupffer cells and macrophages in other tissues. In addition, there were conspicuous iron-containing compounds in the bile canaliculi, and marked iron deposition in the pancreas and heart. Laser microprobe mass analysis (LAMMA) enabled localization and relative quantitation of iron deposition in subcellular compartments providing in situ documentation of iron accumulation in siderosomes and contributed in assessing total cytosolic iron in various cell types. Moreover, it demonstrated the importance and magnitude of the biliary route for iron excretion in these animals.

T1 and T2 of ferritin at different field strengths: effect on MRI

Nuclear magnetic relaxation times T1 and T2 were measured in ferritin solutions at field strengths from 0.04 to 1.5 T. T1 was relatively constant, but 1/T2 increased linearly with field strength, in agreement with earlier MRI observations in the monkey brain. This finding supports the theory that ferritin is responsible for T2 shortening in brain nuclei containing iron. The linear dependence of 1/T2 on magnetic field is unique and not explained by present theories of the magnetic properties of ferritin.

The ultrastructural spectrum of lysosomal storage diseases

After the important advances that have been made in the diagnosis of inherited lysosomal disorders with the help of biochemical-enzymic methods, the importance of electron microscopy for the identification and study of these conditions has apparently declined. Nevertheless, numerous specimens continue to be submitted for ultrastructural examination when lysosomal storage diseases are suspected. The article summarizes the present role of electron microscopy in this area and depicts typical specific findings in comparison with suggestive and nonspecific lysosomal changes. It is concluded that ultrastructural examination remains a useful and occasionally compulsory diagnostic method. In addition, it contributes to the identification of new diseases, the study of animal models of storage diseases, and the assessment of novel therapeutic methods such as bone marrow transplantation.

Ferritin and hemosiderin in pathological tissues

The biological importance of iron for most living cells has been under increasing attention during recent years. In addition to iron deficiency, iron overload has been recognized as a significant metabolic abnormality with potentially damaging consequences. The iron-storing compounds ferritin and hemosiderin have the unique quality of being ultrastructurally recognizable because of the electron-density of the iron concentrated within their particles. In this review, the electron microscopic features of iron overload are discussed, as found in various subcellular compartments and different types of cells and tissues. Defensive mechanisms against iron overload are exhibited by most cell lines and include: (1) the capacity of synthesis of the protein apoferritin by most cells whenever the concentration of ambient iron increases, (2) the capacity to bind toxic inorganic iron within the hollow shell of apoferritin; the transfer of the assembled iron-rich ferritin molecules into siderosomes and (3) the capability of further iron segregation within siderosomes by degradation of ferritin to hemosiderin. The study provides examples of cytosiderosis in different types of cells and tissues, as well as iron-related ultrastructural pathological changes.

Biochemical and biophysical investigations of the ferrocene-iron-loaded rat. An animal model of primary haemochromatosis

Male Wistar rats fed with ferrocene had high hepatic iron loading (7.24 +/- 1.97 mg Fe/g tissue) after 6 weeks, principally located in lysosomes, which was comparable to the levels and distribution determined in human haemochromatosis. The two iron-storage proteins, ferritin and haemosiderin were isolated from the livers of the ferrocene-loaded rats and their iron cores were investigated by Mössbauer spectroscopy and inductively coupled plasma-emission spectrometry. Ferrihydrite was the predominant form of iron present in both ferritin and haemosiderin, while haemosiderin contained higher amounts of phosphorus, magnesium, calcium and barium, then either normal or ferrocene-loaded ferritin. Free-radical-mediated damage in the iron-loaded livers was inferred by the significant depletion of alpha-tocopherol in both the livers and subcellular hepatic lysosomal fraction, which inversely correlated with the increasing iron content (r = -0.61; P less than 0.05) and was associated with increased fragility of the lysosomal membranes.

Biological and ultrastructural aspects of iron overload: an overview

The morphologic aspects of iron overload have been studied in human subjects, mammals, and birds with spontaneous overload, in a variety of experimental animals, and also in cell cultures. Reviewed here are the contributions of electron microscopy to the understanding of the iron-loading process, as reported during the last 12 years. The electron-density of ferrihydrite cores located within the protein shell of the ferritin molecule enabled its identification within either cytosol or lysosomes (siderosomes) of iron-exposed cells. The process of (holo)ferritin assembly, its transfer into siderosomes, and its degradation to hemosiderin can be followed in various cells. Siderosomes display cell-line-specific ultrastructural features, and different cell types show varying iron-segregating capacity. The study of experimental animals and cultured cells show that an iron-rich milieu may be damaging, probably through iron-catalyzed lipid peroxidation. Recent ultrastructural studies stress the value of describing initial alterations as opposed to the irrelevant end-stage findings. Further efforts should be directed toward elucidating the origin of iron in neonatal hemochromatosis, the role of iron in infection and neoplasia, and the nature and role of brain iron.

The cytopathology of metal overload

Researchers have been able to demonstrate the cytotoxicity of copper overload in animal models. This has allowed them to not only localize the intracellular distribution of copper but also to study directly the subsequent organelle injury at the ultrastructural level. The lesions seen in copper overload appear to vary from species to species. In humans, marked mitochondrial abnormalities are seen in Wilson's disease while diet overloaded rats show nuclear destruction and various membranous abnormalities. Sequestration of copper within lysosomes appears to protect hepatocytes from its toxicity. However, the mechanism by which the metal is incorporated into lysosomes is not known.

Ultrastructural pathology of iron overload

The study of the ultrastructural changes in the iron-laden organism has indicated the presence of a number of 'defensive' features, best understood if examined according to a concept of biological levels. At the molecular level, the main features are: the increased capacity of cells to bind toxic, inorganic iron to a specific storage protein, apoferritin, which becomes visible due to its iron-containing, electron-opaque core; since iron itself is involved in the de-repression of apoferritin synthesis, the number of assembled ferritin molecules depends on the amount of unbound iron present in the cell; there is a maximal, cell line-specific concentration of cytosolic ferritin; ferritin particles have a variable iron content, with richer molecules having a tendency to form clusters. At the cellular level, the transport of ferritin into the lysosomal compartment with formation of ferritin and haemosiderin-containing siderosomes enables further segregation of iron and permits cell survival even in the new steady-state of cytosiderosis. When siderosomes increase beyond a cell line-specific concentration, signs of organelle alteration followed by cellular death are noted. At the tissue level, the contributory ultrastructural observations include finding of early intercellular fibrosis, atypical (amorphous) iron deposition, as well as the accessibility of a detailed assessment of iron distribution in various cell types, i.e. endothelium, parenchymal cells, RE cells. The ultrastructural study of material obtained from human subjects with various stages of iron overload and of experimental animals facilitates the understanding of the process of iron overload itself and of the ensuing cellular damage. The recent emphasis on iron as a putative contributory factor in infections, as well as its role in neoplasia, has provided new directions for research, both clinical and experimental. Ultrastructural observations, combined with biochemical, immunological and biophysical investigations, are mandatory for providing answers to the numerous pending questions related to iron metabolism. Sequential electron microscopic studies of iron-laden cells enable the evaluation of chelating agents in either clinical or experimental conditions.

Renal lesions and clinical findings in thalassemia major and other chronic anemias with hemosiderosis

Renal lesions found in 21 autopsied patients with hemosiderosis, 18 with beta-thalassemia, two with Blackfan-Diamond anemia, and one with aplastic anemia included: cellular glomeruli with increased mesangial matrix; hemosiderin deposit in visceral and parietal glomerular epithelial cells; greater hemosiderin deposit in terminal straight portions of proximal convoluted tubules and distal convoluted tubules than in connecting segments, or collecting tubules, connective tissue ferrugination; lipofuscin in tubular epithelium and vascular smooth muscle; infrequently, intimal or medial arterial thickening, and, in one patient with thalassemia, an infarct resulting from arterial thrombus. The progression of these lesions over the course of disease, and possible effects on the various lesions of high transfusion regimen, oral pancreatin, vitamin E supplementation, or treatment with intramuscular, subcutaneous, or intravenous desferrioxamine were evaluated. The results of urine and renal function studies of 4 of the autopsied patients (3 thalassemia, 1 Blackfan-Diamond anemia), and 14 patients with thalassemia and 4 with Blackfan-Diamond anemia who were not autopsied, are presented. Rarely significant until preterminal stages, the renal functional changes reflect distal more than proximal tubule dysfunction.

Release of iron from ferritin molecules and their iron-cores by 3-hydroxypyridinone chelators in vitro

Ferritin molecules contain 24 subunits forming a shell around an inorganic iron-core. Release of iron(III) from ferritin and its isolated iron-cores by a series of hydroxypyridinone chelators with high affinities for iron(III) has been compared. The results collectively suggest that the chelators act by penetrating the protein shell and interacting directly with the iron-core in ferritin. Iron(III) is probably removed bound to a single ligand, but once outside the protein shell, the trihydroxypyridinone iron(III) complex predominates. The order of effectiveness of a group of pyridinones found for iron removal from ferritin molecules in solution differs from that obtained with hepatocytes in culture or with whole animals, where membrane solubility and other factors may modulate the response.

Quantitative analysis of immunogold labelling for ferritin in liver from control and iron-overloaded rats

The distribution of ferritin antigenicity in control and iron-loaded rat hepatocytes was investigated with an immunogold-ferritin antibody technique. Antibody to horse spleen ferritin showed immunoreactivity as determined by dot blotting with immunogold/silver staining with purified rat liver ferritin but not with rat haemosiderin. The initial site of ferritin degradation was studied by analysing the density of gold labelling in the cytosol and lysosomes in combination with pre-embedding acid phosphatase cytochemistry. Immunoreactive ferritin was present in the cytosol, cytosolic clusters and lysosomes of normal hepatocytes. After iron-loading, the labelling density increased over tenfold in parenchymal cell cytosol with a smaller increase in Kupffer cells. Ferritin clusters contained substantially more immunoreactive ferritin than equivalent areas of lysosomes or cytosol. Analysis of the labelling density in hepatocyte lysosomes showed that, despite a striking increase in iron content, one-quarter of the lysosomes showed less immunolabelled ferritin than the cytosol. The existence of a wide range of ferritin labelling densities in the lysosomes with a large proportion unlabelled suggests that the ferritin protein shell is not degraded at a significant rate either in the cytosol or in clusters but only after incorporation into lysosomes.

Neuroblastomas contain iron-rich ferritin

The ultrastructure of neuroblastoma was examined using unstained sections so that ferritin particles could be identified by the electron density of their iron cores. Ferritin and hemosiderin were found in ten of 11 neuroblastomas that were examined when the patients first presented. The study was therefore expanded to an additional group of children, including some diagnosed by noninvasive procedures and given chemotherapy before the excision of their tumors. In this second group 12 of 20 specimens contained ferritin and hemosiderin in variable amounts. In both groups there was a tendency for patients with advanced disease to have increased amounts of iron compounds in the tumor tissue (Stage III and particularly Stage IV). Most Stage IV patients also had elevated serum ferritin levels. However, based on the available heterogenous material, no absolute relationship could be established between age, disease stage, tumoral storage iron, and the level of serum ferritin. The presence of ferritin in neuroblastoma may be linked to the elevated serum ferritin levels and may be implicated in tumorigenesis.

Exceptional iron concentrations in larval lampreys (Geotria australis) and the activities of superoxide radical detoxifying enzymes

This study aimed to elucidate the way in which larvae of the lamprey Geotria australis counteract the potential problems of the very high concentrations of non-haem iron they contain and thereby avoid the deleterious effects associated with iron overload in other vertebrates. Particular attention has been paid to ascertaining whether increasing concentrations of iron are accompanied by (i) change to a less readily available form of iron and (ii) an increase in the activity of those detoxifying enzymes responsible for minimizing the production of harmful hydroxyl radicals via the Haber-Weiss reaction. The mean concentrations of haemosiderin and ferritin in larval G. australis were each far higher in the nephric fold than in either the liver or intestine, but all these concentrations were much greater than those in rat liver. Since haemosiderin releases iron far more slowly than ferritin, the iron it contains is much less readily available to catalyse the Haber-Weiss reaction. It is thus relevant that (i) non-haem iron in the nephric fold occurred to a greater extent as large dense haemosiderin granules than as ferritin molecules and (ii) the proportion of iron in the form of haemosiderin rose with increasing concentration of total non-haem iron. A strong correlation was also recorded between the activity of superoxide dismutase in the nephric fold and the concentrations of total non-haem iron and its haemosiderin and ferritin components. This demonstrates that enzyme detoxification of O2.- rises with increasing amounts of iron. The exceptional iron concentrations in the nephric fold were not reflected by a greater measured activity of superoxide dismutase than that found in other tissues. However, the nephric fold was shown to contain an augmentation factor which is presumed to enhance the activity of this enzyme in vivo. The activity of catalase and glutathione peroxidase, which catalyse the breakdown of H2O2 to O2 and water, were each significantly correlated with the concentration of ferritin.

Studies on haemosiderin and ferritin from iron-loaded rat liver

Haemosiderin has been isolated from siderosomes and ferritin from the cytosol of livers of rats iron-loaded by intraperitoneal injections of iron-dextran. Siderosomal haermosiderin, like ferritin, was shown by electron diffraction to contain iron mainly in the form of small particles of ferrihydrite (5Fe2O3.9H2O), with average particle diameter of 5.36 +/- 1.31 nm (SD), less than that of ferritin iron-cores (6.14 +/- 1.18 nm). Mössbauer spectra of both iron-storage complexes are also similar, except that the blocking temperature, TB, for haemosiderin (23 K) is lower than that of ferritin (35 K). These values are consistent with their differences in particle volumes assuming identical magnetic anisotropy constants. Measurements of P/Fe ratios by electron probe microanalysis showed the presence of phosphorus in rat liver haemosiderin, but much of it was lost on extensive dialysis. The presence of peptides reacting with anti-ferritin antisera and the similarities in the structures of their iron components are consistent with the view that rat liver haemosiderin arises by degradation of ferritin polypeptides, but its peptide pattern is different from that found in human beta-thalassaemia haemosiderin. The blocking temperature, 35 K, for rat liver ferritin is near to that reported, 40 K, for human beta-thalassaemia spleen ferritin. However, the haemosiderin isolated from this tissue, in contrast to that from rat liver, had a TB higher than that of ferritin. The iron availability of haemosiderins from rat liver and human beta-thalassaemic spleen to a hydroxypyridinone chelator also differed. That from rat liver was equal to or greater, and that from human spleen was markedly less, than the iron availability from either of the associated ferritins, which were equivalent. The differences in properties of the two types of haemosiderin may reflect their origins from primary or secondary iron overload and differences in the duration of the overload.

Ultrastructural observations in the carbonyl iron-fed rat, an animal model for hemochromatosis

Rats fed a carbonyl iron-supplemented diet for 4-15 months were studied for iron content and morphologic changes in the liver, spleen, intestinal mucosa, pancreas and heart. All organs had an increased iron content measured by atomic absorption, with the highest concentrations in the liver and spleen. The periportal distribution of stored iron in the liver was similar to that in human hemochromatosis. In animals treated beyond 6 months Kupffer cells and sinusoidal lining cells also showed cytosiderosis. Electron microscopy provided information on ferritin and hemosiderin content and distribution within parenchymal and sinusoidal cells of the liver but no excessive fibrosis was found. Except for the spleen, the other organs showed less iron deposition. Iron-filled lysosomes (siderosomes) were found in macrophages in the intestinal lamina propria and pancreas, as well as in enterocytes, pancreatic acinar cells and heart muscle cells. Heavily iron-laden siderosomes had increased membrane instability which was demonstrated both morphologically and by measurements of latent lysosomal enzyme activities. Even though cirrhosis was not found, the distribution pattern of accumulated storage iron and lysosomal lability indicated that the carbonyl iron-fed rat is a suitable experimental model for human hemochromatosis.

Siderosomal ferritin. The missing link between ferritin and haemosiderin?

A minor electrophoretically fast component was found in ferritin from iron-loaded rat liver in addition to a major electrophoretically slow ferritin similar to that observed in control rats. The electrophoretically fast ferritin showed immunological identity with the slow component, but on electrophoresis in SDS it gave a peptide of 17.3 kDa, in contrast with the electrophoretically slow ferritin, which gave a major band corresponding to the L-subunit (20.7 kDa). Thus the electrophoretically fast ferritin resembles that reported by Massover [(1985) Biochim. Biophys. Acta 829, 377-386] in livers of mice with short-term parenteral iron overload. The electrophoretically fast ferritin had a lower iron content (2000 Fe atoms/molecule) than the electrophoretically slow ferritin (3000 Fe atoms/molecule). Removal and re-incorporation of iron was possible without effect on the electrophoretic mobility of either ferritin species. On subcellular fractionation the electrophoretically fast ferritin was enriched in pellet fractions and was the sole soluble ferritin isolated from iron-laden secondary lysosomes (siderosomes). The amount and relative proportion of the electrophoretically fast species increased with iron loading. Haemosiderin isolated from siderosomes was found to contain a peptide reactive to anti-ferritin serum and corresponding to the 17.3 kDa peptide of the electrophoretically fast ferritin species. Unlike the electrophoretically slow ferritin, the electrophoretically fast ferritin did not become significantly radioactive in a 1 h biosynthetic labelling experiment. We conclude that the minor ferritin is not, as has been suggested for mouse liver ferritin, 'a completely new species of smaller holoferritin that represents a shift in the ferritin phenotype' in response to siderosis, but a precursor of haemosiderin, in agreement with the proposal by Richter [(1984) Lab. Invest. 50, 26-35] concerning siderosomal ferritin.

Primary hyperoxaluria type I: ultrastructural observations in liver biopsies

The liver ultrastructure of four patients with the peroxisomal disease primary hyperoxaluria type I has been investigated. In all cases, peroxisomes of normal appearance were present in the parenchymal cells, except that they were somewhat reduced in number and size. In all patients, conspicuous lipofuscin was present, presumably resulting from the various metabolic disturbances to which the livers were subjected during the course of their disease. Considerable hepatocyte iron overloading was found in the three patients who had been hemodialyzed and/or had blood transfusions. Whether the relatively mild peroxisomal abnormalities (as compared to other peroxisomal diseases, such as Zellweger's syndrome) found in these hyperoxaluric patients are related directly to the peroxisomal deficiency of alanine:glyoxylate aminotransferase, or whether they are a secondary phenomenon, resulting from the consequent metabolic disturbance, remains to be elucidated.

Determination of iron to phosphorus ratios of iron storage compounds in patients with iron overload: a chemical and electron probe X-ray microanalysis

The amounts of phosphorus and iron in various isolated ferritin preparations were investigated by: chemical analysis on ferritin samples and electron probe X-ray microanalysis on ferritin particles from the same preparations. A high correlation was found between iron to phosphorus ratios obtained by both methods. Further investigation by electron probe X-ray microanalysis on lysosomes of hepatic cells of patients with idiopathic and secondary hemochromatosis revealed lysosomal iron to phosphorus ratios which were very similar in all parenchymal cells but different from ratios obtained in Kupffer cells. Lysosomal iron to phosphorus ratios in hepatocytes did not change after intensive phlebotomy treatment. It is postulated therefore that, during phlebotomy, iron and phosphorus are concomitantly lost from the hepatic lysosomes.

Successful reversal by chelation therapy of congestive cardiomyopathy due to iron overload

A patient who developed severe iron overload cardiomyopathy is described. Venesection could not be performed because the patient had chronic anemia. Deferoxamine mesylate, a chelating agent, was administered daily for more than 2 years and produced significant improvement in ventricular function which was associated with a biopsy-proven decrease in myocardial iron stores. This is the first reported case in which a severe cardiomyopathy due to iron overload was reversed by chelation therapy alone.

Studies of iron overload. Lysosomal proteolysis of rat liver ferritin

To learn more about pathological iron storage in the liver, two sorts of lysosomes were isolated from rat livers in Percoll - sucrose or sucrose gradients: siderosomes (= iron-loaded terminal lysosomes) and light lysosomes (secondary and terminal). Such cell fractions were obtained from acutely iron-loaded and control rat livers. After lysis with Triton X-100 the preparations were assayed for proteolytic activity against rat liver ferritin (RLF) and denatured bovine hemoglobin (DBH), for buffer-soluble ferritin protein content, total protein and non-heme iron. At pH 3.6 both fractions displayed considerable proteolytic activity (cathepsin D activity) against DBH and endogenous proteins but little activity against RLF. By contrast, proteolytic activity against RLF was maximal at the highest pH tested, 6.5, at which DBH was practically insusceptible. The behavior of proteolytic activity against ferritin at pH 6.5 makes it likely that a single enzyme was involved that acted by Michaelis-Menten kinetics. However, no more than 2.5% of endogenous ferritin protein in the organelles was buffer-soluble. 41 to 89 hours after an intramuscular dose of 50 mg Fe, given as iron dextran, the non-heme iron content of light lysosomes and siderosomes had increased markedly and the ratio of non-heme Fe to buffer-soluble ferritin protein also became much elevated in the organelles; but the ratio of buffer-soluble ferritin to total protein did not rise significantly. The rise in organellar non-heme Fe exceeded iron saturation of rat liver ferritin and thus reflected conversion of ferritin to hemosiderin, which is buffer-insoluble.(ABSTRACT TRUNCATED AT 250 WORDS)

Iron deficiency and iron overload

An up to date review of our knowledge of human iron metabolism is given including problems of iron balance, internal transport, and intracellular mechanisms. Current knowledge of the iron proteins is summarized and this background is used in discussing the pathophysiology of iron deficiency and overload, together with the internal derangements such as sideroblastic anemia which form much of the clinical practice associated with disorders of iron metabolism. The therapeutic approach to these problems will be described.

Iron overload of the liver in the baboon. An ultrastructural study

Liver biopsies from 4 baboons taken during 15 months of iron-polymaltose injections, were compared with specimens from 2 controls. A morphometric method was used to assess ferritin concentration in various cells. Initially, ferritin and siderosomes were conspicuous in reticuloendothelial cells but rare in hepatocytes. Unusual findings included intranuclear ferritin and coalesced ferritin within bile canaliculi. With advancing overload, ferritin and hemosiderin increased not only in sinusoidal cells, but also in hepatocytes, with concomitant elevation of transaminases. The hepatocytes now showed evidence of damage and excessive collagen was present mainly around portal spaces. A year after cessation of iron injections, hepatocyte ultrastructure was near normal while sinusoidal cells were still heavily overloaded. The baboon appeared to be a useful model for the study of iron overload. Although in this study most of the damage was reversible, it is suspected that more prolonged overload, a different route of administration or other, more toxic iron compounds, may lead to cirrhosis similar to that of the iron-loading anemias.