Human ferritin gene expression

Three ferritin subunit analogs in Chinese giant salamander (Andrias davidianus) and their response to microbial stimulation

Ferritin, an evolutionarily conserved iron-binding protein, plays important roles in iron storage and detoxification and in host immune response to invading stimulus as well. In the present study, we identified three ferritin subunit analog cDNAs from Chinese giant salamander (Andrias davidianus). All the three ferritin subunit cDNAs had a putative iron responsive element in the 5'-untranslated region. Two deduced ferritin subunits (designated as cgsFerH and cgsFerM) had the highest identity of 90% to H type subunit of vertebrate ferritins, while another deduced ferritin subunit (designated as cgsFerL) had the highest identity of 84% to L type subunit of vertebrate ferritins. The Chinese giant salamander ferritin (cgsFer) was widely expressed in various tissues, with highest expression for cgsFerH and cgsFerL in liver and highest expression for cgsFerM in spleen. Infection of Chinese giant salamander with A. davidianus ranavirus showed significant induction of cgsFer expression. Both lipopolysaccharide and iron challenge drastically augmented cgsFer expression in the splenocytes and hepatocytes from Chinese giant salamander. In addition, recombinant cgsFers bound to ferrous iron in a dose-dependent manner, with significant ferroxidase activity. Furthermore, the recombinant cgsFer inhibited the growth of the pathogen Vibrio anguillarum. These results indicated that cgsFer was potential candidate of immune molecules involved in acute phase response to invading microbial pathogens in Chinese giant salamander possibly through its regulatory roles in iron homeostasis.

Disturbance of iron metabolism in Parkinson’s disease — ultrasonography as a biomarker

A central role of iron in the pathogenesis of Parkinson's disease (PD) has been discussed for many years. So far, however, a biomarker indicating increased iron levels in the substantia nigra (SN) in PD patients has been missing. Performing transcranial ultrasound we detected an increased area of SN echogenicity as a typical echofeature in PD, visible already in the early stages of the disease and in subjects with subclinical impairment of the nigrostriatal system. Animal studies and post mortem analyses of human brain tissue revealed that this echofeature is associated with increased iron levels of the substantia nigra as well as a reduced neuromelanin content. The apparently autosomal dominant inheritance of this echofeature in relatives of patients with idiopathic PD indicates a primary role of disturbances of iron metabolism in PD. Consequently performed mutation analyses in genes involved in brain iron metabolism lead to the discovery of specific mutations in the ferritin-H, IRP2 and HFE gene in single PD patients. Moreover, variations in the ceruloplasmin gene were found to be associated with PD or SN hyperechogenicity. Functional relevance of some of these mutations for iron metabolism could be proven. Therefore, SN hyperechogenicity can be regarded as biomarker for both: impairment of the nigrostriatal system and increased iron levels of the SN. Future studies aim at substantiating the hypothesis that healthy subjects with SN hyperechogenicity indeed represent a population at risk for nigrostriatal degeneration, which would have a significant impact on therapeutical options.

Heme regulation in traumatic brain injury: relevance to the adult and developing brain

Intracranial bleeding is one of the most prominent aspects in the clinical diagnosis and prognosis of traumatic brain injury (TBI). Substantial amounts of blood products, such as heme, are released because of traumatic subarachnoid hemorrhages, intraparenchymal contusions, and hematomas. Despite this, surprisingly few studies have directly addressed the role of blood products, in particular heme, in the setting of TBI. Heme is degraded by heme oxygenase (HO) into three highly bioactive products: iron, bilirubin, and carbon monoxide. The HO isozymes, in particular HO-1 and HO-2, exhibit significantly different expression patterns and appear to have specific roles after injury. Developmentally, differences between the adult and immature brain have implications for endogenous protection from oxidative stress. The aim of this paper is to review recent advances in the understanding of heme regulation and metabolism after brain injury and its specific relevance to the developing brain. These findings suggest novel clinical therapeutic options for further translational study.

Distribution of ferritin in the rat hippocampus after kainate-induced neuronal injury

A gradual increase in iron occurs in the lesioned hippocampus after neuronal injury induced by the excitotoxin kainate, and the present study was carried out to investigate whether this increase in iron might be associated with changes in expression of the iron binding protein, ferritin. An increase in ferritin immunoreactivity was observed in glial cells of the hippocampus, as early as three days after intracerebroventricular injections of kainate. The number of ferritin positive cells peaked four weeks after the kainate injection, and decreased eight and twelve weeks after injection. They were found to be mostly microglia and oligodendrocytes by double immunofluorescence labeling with glial markers. A number of ferritin-labeled endothelial cells were also observed via electron microscopy. The decline in ferritin immunoreactivity four weeks after the injection of kainate is accompanied by an increase in the number of ferric and ferrous iron positive cells in the lesioned tissue. A substantial non-overlap between ferritin and iron-containing cells was observed. In particular, spherical ferric or ferrous iron-laden cells in the degenerating hippocampus were unlabeled for ferritin for long time periods after the kainate injection. An increase in iron, together with a reduced expression of iron binding proteins such as ferritin at long time intervals after kainate lesions, could result in a relative decrease in ferritin-induced ferroxidase activity and the presence of some of the iron in the ferrous form. It is postulated that this may contribute to chronic neuronal injury, following acute kainate-induced neurodegeneration.

Iron in neurodegenerative disorders

Increasing evidence implicates a role of iron in the pathogenesis of numerous neurodegenerative diseases due to its capacity to enhance production of toxic reactive radicals and to induce protein aggregation. The underlying mechanism of iron accumulation in areas of the brain specific for the respective disease, however, is still unknown. Recent molecular and biochemical studies provide new insights into the consequences of impairment of brain iron metabolism. This review summarizes our understanding of the regulation of iron in the brain and defines the current knowledge on the involvement of iron metabolism in neurodegenerative diseases with genetically determined iron accumulation in the brain.

Brain iron pathways and their relevance to Parkinson's disease

A central role of iron in the pathogenesis of Parkinson's disease (PD), due to its increase in substantia nigra pars compacta dopaminergic neurons and reactive microglia and its capacity to enhance production of toxic reactive oxygen radicals, has been discussed for many years. Recent transcranial ultrasound findings and the observation of the ability of iron to induce aggregation and toxicity of alpha-synuclein have reinforced the critical role of iron in the pathogenesis of nigrostriatal injury. Presently the mechanisms involved in the disturbances of iron metabolism in PD remain obscure. In this review we summarize evidence from recent studies suggesting disturbances of iron metabolism in PD at possibly different levels including iron uptake, storage, intracellular metabolism, release and post-transcriptional control. Moreover we outline that the interaction of iron with other molecules, especially alpha-synuclein, may contribute to the process of neurodegeneration. Because many neurodegenerative diseases show increased accumulation of iron at the site of neurodegeneration, it is believed that maintenance of cellular iron homeostasis is crucial for the viability of neurons.

Specific repression of beta-globin promoter activity by nuclear ferritin

Developmental hemoglobin switching involves sequential globin gene activations and repressions that are incompletely understood. Earlier observations, described herein, led us to hypothesize that nuclear ferritin is a repressor of the adult beta-globin gene in embryonic erythroid cells. Our data show that a ferritin-family protein in K562 cell nuclear extracts binds specifically to a highly conserved CAGTGC motif in the beta-globin promoter at -153 to -148 bp from the cap site, and mutation of the CAGTGC motif reduces binding 20-fold in competition gel-shift assays. Purified human ferritin that is enriched in ferritin-H chains also binds the CAGTGC promoter segment. Expression clones of ferritin-H markedly repress beta-globin promoter-driven reporter gene expression in cotransfected CV-1 cells in which the beta-promoter has been stimulated with the transcription activator erythroid Krüppel-like factor (EKLF). We have constructed chloramphenicol acetyltransferase reporter plasmids containing either a wild-type or mutant beta-globin promoter for the -150 CAGTGC motif and have compared the constructs for susceptibility to repression by ferritin-H in cotransfection assays. We find that stimulation by cotransfected EKLF is retained with the mutant promoter, whereas repression by ferritin-H is lost. Thus, mutation of the -150 CAGTGC motif not only markedly reduces in vitro binding of nuclear ferritin but also abrogates the ability of expressed ferritin-H to repress this promoter in our cell transfection assay, providing a strong link between DNA binding and function, and strong support for our proposal that nuclear ferritin-H is a repressor of the human beta-globin gene. Such a repressor could be helpful in treating sickle cell and other genetic diseases.

Differential regulation of H- and L-ferritin messenger RNA subunits, ferritin protein and iron following focal cerebral ischemia-reperfusion

Iron may catalyse the production of reactive oxygen species during post-ischemic reoxygenation and subsequently lead to brain damage. Ferritin, an iron sequestering and storage protein, can also be a source of iron after ischemic insult. However, its role in ischemia-reperfusion has not been carefully investigated. In the present study, we examined the temporal and spatial induction profiles of both H- and L-ferritin messenger RNA and protein in a well-defined focal cerebral ischemia model. Results of northern blot analysis showed a delayed and prolonged induction of both H- and L-ferritin messenger RNA in the ischemic cortex of rats subjected to 60min ischemic insult. A significant induction of both H- and L-ferritin messenger RNA was observed at 12h and remained elevated for up to 336h after the onset of reperfusion. At the peak level, quantitative analysis of the blot indicated a 2.5-fold and a six-fold increase in H- and L-ferritin messenger RNA, respectively, compared with the sham-operated controls. No apparent change in the levels of either messenger RNA was observed in the contralateral side. Results of in situ hybridization studies revealed constitutive expression of both H- and L-ferritin messenger RNA throughout the brain in sham-operated animals, in particular the hippocampus and the piriform cortex. Nevertheless, the signal intensity of H-ferritin messenger RNA was much higher than that of L-ferritin messenger RNA. Seventy-two hours after 60min ischemia, marked expression of H-ferritin messenger RNA was observed in the area surrounding the middle cerebral artery irrigated cortex, the medial part of the caudoputamen and in the subfield of the CA1 hippocampal region of the ipsilateral hemisphere. Similarly, a large induction of L-ferritin messenger RNA was also noted in several areas, including the middle cerebral artery irrigated cortex, the lateral part of the caudoputamen and the stratum pyramidale of the CA1 hippocampal region, which were totally different from areas where H-ferritin messenger RNA was found. At 336h after ischemia, increased expression of H-ferritin messenger RNA was observed in the peri-necrosis and ipsilateral thalamus regions, while L-ferritin messenger RNA was noted exclusively at the edge within the necrosis. Results of immunohistochemical study further revealed that ferritin immunoreactivity was present in the same areas where increased ferritin messenger RNA was found. Sixty-minute ischemia also led to iron deposition in discrete areas. Iron deposition was highly associated with the induction of ferritin, particularly in the macrophage- and microglia-positive areas where cell death or tissue necrosis was noted.In summary, our initial findings indicate that ischemic insult leads to induction of both H- and L-ferritin messenger RNA. In the present study, although the temporal induction profiles were similar, the major expression areas for these two genes were totally different. Ferritin immunoreactivity was observed in the same areas where increased ferritin messenger RNA was found. Ischemia also resulted in iron deposition, which highly associated with the ferritin immunoreactivity. The exact regulatory mechanism and pathological significance for the differential expression of H- and L-ferritin genes following ischemia/reperfusion remain to be clarified.

Structure and location of a ferritin gene of the yellow fever mosquito Aedes aegypti

We have isolated and sequenced a genomic clone encoding the 24- and 26-kDa ferritin subunits in the mosquito Aedes aegypti (Rockefeller strain). The A. aegypti gene differs from other known ferritin genes in that it possesses an additional intron and an unusually large second intron. The additional intron is located within the 5' untranslated region, between the CAP site and the start codon. The second intron contains numerous putative transposable elements. In addition, unlike the human and rat ferritin genes, the A. aegypti ferritin gene is a single copy gene, located at 88.3% FLpter on the q-arm of chromosome 1. Primer extension analysis indicates that the A. aegypti ferritin gene has multiple transcriptional start sites. A differential usage of these sites is observed with varied cellular iron concentrations.

High level of ferritin light chain mRNA in lens

Ferritin is of particular interest with regard to cataract because (i) cataract occurs in individuals with hereditary hyperferritinemia cataract syndrome (HHCS), a condition in which ferritin light chain (L-ferritin) protein is overexpressed systemically, and (ii) ferritin is an important regulator of oxidative stress, a primary factor in the etiology of aging-related cataract. From gene array analysis two novel observations were made with respect to ferritin gene expression: first, lenses from guinea pigs and humans have disproportionately high levels of L-ferritin mRNA relative to the amounts of ferritin protein present, and second, L-ferritin message increased markedly in lenses from guinea pigs with hereditary nuclear cataract. The human lens L-ferritin sequence was identical to previous data from human liver; the guinea pig sequence was 86% identical to the human sequence at the amino acid level. Despite mRNA levels similar to those of major lens crystallins, lens ferritin was undetectable by Western blot techniques.

Characterization of hepatic iron overload following dietary administration of dicyclopentadienyl iron (Ferrocene) to mice: cellular, biochemical, and molecular aspects

A unique organic form of iron (dicyclopentadienyl iron; ferrocene) has been used to further elucidate specific hepatic histopathologic, biochemical, and molecular parameters associated with dietary iron overload. Male C57BL/6Ibg mice fed a diet containing 0.04-0.2% w/w ferrocene for 115 days displayed severe hepatic siderosis of hepatocytes accompanied by a 15-fold induction of nonheme iron content compared to control mice receiving a diet with normal amounts of iron. The ferrocene treatment led to significant increases in hepatocellular necrosis as measured by plasma alanine aminotransferase activity. Histological assessment of hepatic fibrosis revealed mild increases in collagen deposition localized with accumulations of hemosiderin primarily in centrilobular hepatocytes. Hepatic fibrosis was confirmed by measurement of hepatic hydroxyproline content that was increased 4-fold in ferrocene-fed animals compared to control animals not ingesting ferrocene. Hepatic siderosis was accompanied by significant increases in hepatic malondialdehyde content suggesting the ferrocene-induced iron burden initiated lipid peroxidation in vivo. Expression of the heavy-chain isoform of ferritin mRNA and protein measured in liver after ferrocene feeding was increased approximately 8- and 2-fold, respectively, compared to the appropriate controls. These results, using an organic form of iron fed to genetically well-characterized inbred mice, provide new additional insight into the specific molecular and biochemical events that occur in association with histopathologic changes initiated by iron-induced liver injury. These data support the hypothesis that peroxidation of cellular membrane lipids is an important mechanism involved in the toxicity of excess hepatic iron and possibly the initiation of liver fibrogenesis. The results presented here also provide novel in vivo evidence documenting the cellular modulation of ferritin in response to the toxic effects of hepatic iron overloading and iron-mediated oxidative stress.

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.

A sequence-ready high-resolution physical map of the best macular dystrophy gene region in 11q12-q13

Best disease, an autosomal dominant inherited macular degenerative disorder, was previously localized between D11S1765 and UGB (uteroglobin) in 11q13 by genetic linkage analysis. Since this region was found to be refractory to cloning in YAC (yeast artificial chromosome)-based vectors, a P1 artificial chromosome (PAC) contig was assembled. Gridded PAC libraries representing a 16-fold genome equivalent were screened by hybridization using PCR products representing STSs derived from YAC end sequences, markers binned to 11q13, and PAC-derived insert ends. A highly marker dense approximately 1.7-Mb PAC contig that encompassed the disease gene region was constructed, allowing us to order accurately the markers throughout the region and to provide the most precise estimate of its physical size. Using this contig, thus far we have mapped seven anonymous ESTs and five known genes into this region. This high-resolution physical map will facilitate the isolation of polymorphic markers for refinement of the disease gene region, as well as the identification of candidate genes by exon trapping, cDNA selection, and gene prediction from PAC-derived genomic sequence.

The molecular mechanisms of the metabolism and transport of iron in normal and neoplastic cells

Iron uptake by mammalian cells is mediated by the binding of serum Tf to the TfR. Transferrin is then internalized within an endocytotic vesicle by receptor-mediated endocytosis and the Fe released from the protein by a decrease in endosomal pH. Apart from this process, several cell types also have other efficient mechanisms of Fe uptake from Tf that includes a process consistent with non-specific adsorptive pinocytosis and a mechanism that is stimulated by small-Mr Fe complexes. This latter mechanism appears to be initiated by hydroxyl radicals generated by the Fe complexes, and may play a role in Fe overload disease where a significant amount of serum non-Tf-bound Fe exists. Apart from Tf-bound Fe uptake, mammalian cells also possess a number of mechanisms that can transport Fe from small-Mr Fe complexes into the cell. In fact, recent studies have demonstrated that the membrane-bound Tf homologue, MTf, can bind and internalize Fe from 59Fe-citrate. However, the significance of this Fe uptake process and its pathophysiological relevance remain uncertain. Iron derived from Tf or small-Mr complexes is probably transported into mammalian cells in the Fe(II) state. Once Fe passes through the membrane, it then becomes part of the poorly characterized intracellular labile Fe pool. Iron in the labile Fe pool that is not used for immediate requirements is stored within the Fe-storage protein, ferritin. Cellular Fe uptake and storage are coordinately regulated through a feedback control mechanism mediated at the post-transcriptional level by cytoplasmic factors known as IRP1 and IRP2. These proteins bind to stem-loop structures known as IREs on the 3 UTR of the TfR mRNA and 5 UTR of ferritin and erythroid delta-aminolevulinic acid synthase mRNAs. Interestingly, recent work has suggested that the short-lived messenger molecule, NO (or its by-product, peroxynitrite), can affect cellular Fe metabolism via its interaction with IRP1. Moreover, NO can decrease Fe uptake from Tf by a mechanism separate to its effects on IRP1, and NO may also be responsible for activated macrophage-mediated Fe release from target cells. On the other hand, the expression of inducible NOS which produces NO, can be stimulated by Fe chelators and decreased by the addition of Fe salts, suggesting that Fe is involved in the control of NOS expression.

Manduca sexta hemolymph ferritin: cDNA sequence and mRNA expression

A cDNA clone encoding a subunit of the tobacco hornworm Manduca sexta (Ms) hemolymph (serum) ferritin (Fer) has been identified and sequenced. The deduced amino acid (aa) sequence shows approx. 50% similarity to vertebrate Fer subunit sequences, and the nucleotide sequence contains a stem-loop structure in the 5' untranslated region that could serve as an iron-responsive element (IRE). The stem-loop of this putative IRE exhibits high identity to vertebrate IRE that play an essential role in the control of Fer synthesis. The Ms Fer subunit lacks one of the three Tyr residues required for the rapid biomineralization of iron shown in vertebrate heavy-chain Fer. In addition, aa residues that comprise the putative ferroxidase centers generally are not conserved, suggesting that the Ms Fer subunit more closely resembles the vertebrate light-chain subunit. Northern blot analyses indicate that the fer mRNA is expressed in the midgut, fat body and hemocytes, with the greatest expression in the midgut.

An immuno-histochemical study of ferritin in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced hemiparkinsonian monkeys

Iron is increased in the substantia nigra of patients with Parkinson's disease, but the mechanism of its accumulation is unknown. We report the distribution of ferritin in the basal ganglia of hemiparkinsonian monkeys made by MPTP. We stereotactically injected MPTP unilaterally into the caudate nucleus of four monkeys, and the substantia nigra and the basal ganglia regions were stained for L-ferritin by an immunohistochemical method. The ferritin immuno-staining was most intense in the pallidum and the pars reticulata of the substantia nigra on both injected and non-injected sides. No significant difference was noted in the immunostaining for ferritin in the pars compacta of the substantia nigra between the injected and the non-injected side. Iron was increased in the pars compacta of the substantia nigra of this hemiparkinsonian monkeys in our previous study. Normal ferritin immunostaining on the injected side would indicate that iron accumulation is not related to altered metabolism of L-ferritin in this model.

Novel properties of L-type polypeptide subunits in mouse ferritin molecules

Properties of the L- and H-type polypeptide subunits forming ferritin 24-mer molecules in mice were investigated, using the products of in vitro transcription and translation from the two cloned genes, and recombinant ferritin molecules (H24L0 or H0L24) produced by transformation in Escherichia coli. Several different conditions for analytical electrophoresis reproducibly show that the relative migration position of the two mouse ferritin subunits is reversed from that reported for ferritin H- and L-subunits in all other mammals; since mouse and human H-polypeptides almost co-migrate, this unusual relative mobility is due largely to novel properties of the murine L-subunit. This unusual electrophoretic property of the mouse L-subunit has led to conflicting reports about the subunit composition of natural mouse ferritin. Here, we show that the single major electrophoretic band given by liver ferritin purified from mice having a short-term iron overload matches that produced by the genetically defined L-polypeptide and that some bona fide H-subunits are also detected. In conclusion, it is reasonable to assume that, when mouse ferritin samples will be analyzed under the same conditions as those described here, the slower species will correspond to the L-type subunit. However, when dealing with ferritin from species other than human or mouse, it should be kept in mind that upon electrophoretic analysis of ferritin polypeptide, the designation of an electrophoretic band as being H- or L-type subunits will be very uncertain without corroboration from genetic, immunological, or amino acid sequencing data.

A link between ferritin gene expression and ribonucleotide reductase R2 protein, as demonstrated by retroviral vector mediated stable expression of R2 cDNA

We have constructed a retroviral expression vector for the mammalian ribonucleotide reductase R2 component. Stable infectants, which express a myc epitope tagged R2 protein from the vector cDNA were obtained and described for the first time. Cells containing the recombinant protein exhibited increased ribonucleotide reductase activity, and were resistant to the antitumour agent hydroxyurea, which targets the R2 component of ribonucleotide reductase. Furthermore, a direct link between ferritin gene expression and R2 protein was observed, since cells containing vector expressed recombinant R2 protein exhibited increased H-chain and L-chain ferritin gene expression.

Ferritin excretion and iron balance in humans

Under normal circumstances, most of the lumenal iron taken into the intestinal mucosal cell is stored within the cell as ferritin and subsequently is lost in the faeces when the cell exfoliates at the end of its lifespan. To evaluate whether faecal iron proteins reflect mucosal cell iron as well as whole body iron and to examine further the kinetics of gastrointestinal iron transport, faecal H-rich and L-rich ferritin were measured in normal subjects and patients with iron deficiency and genetic haemochromatosis. In normal and iron-deficient subjects, the concentration of L-rich but not H-rich faecal ferritin correlated closely with body iron status. In genetic haemochromatosis, the faecal L-rich and H-rich ferritin concentrations were lower than expected for their body iron status. The administration of oral iron to normal subjects led to a rise in L-rich ferritin. Administration of oral or parenteral iron to patients with iron deficiency led to a prompt rise in both forms of faecal ferritin, although the relative increase of L-rich ferritin was greater than that of H-rich ferritin with oral iron administration. Faecal ferritin correlated closely with iron stores in normals and patients with iron deficiency but faecal ferritin levels were lower than expected in genetic haemochromatosis, similar to that previously noted in the duodenal mucosal cells of these patients.

Induction of ferritin synthesis by oxidative stress. Transcriptional and post-transcriptional regulation by expansion of the "free" iron pool

Ferritin, by regulating the "free" intracellular iron pool, controls iron-catalyzed generation of reactive oxygen species, but its role in oxidative damage is still unclear. We show that ferritin synthesis is significantly stimulated in the liver of rats subjected to oxidative stress by treatment with phorone, a glutathione-depleting drug. RNA-bandshift assays document reduced activity of iron regulatory factor, in particular of IRFB, the cytoplasmic protein that post-transcriptionally controls ferritin mRNA translation. Furthermore, Northern blot analysis shows increased accumulation of H and L subunit mRNAs, and nuclear run-on experiments provide evidence of transcriptional activation. Direct measurements of intracellular free iron levels by EPR indicate that the increased ferritin synthesis can be mediated by an expansion of the free iron pool. An early drop of ferritin content after phorone treatment indicates that part of the iron that fuels the free pool might derive from ferritin degradation. Present data seem to suggest that, under conditions of oxidative stress, liver ferritin can represent either a pro- or an anti-oxidant in a time-dependent manner. In fact, its early degradation contributes to expand the intracellular free iron pool that, later on, activates multiple molecular mechanisms to reconstitute ferritin content, thus limiting the pro-oxidant challenge of iron.

Isoforms of ferritin have a specific cellular distribution in the brain

Ferritin is the major iron storage protein and accounts for the majority of the iron in the brain. Thus, ferritin is a key component in protecting the brain from iron induced oxidative damage. The high lipid content, high rate of oxidative metabolism, and high iron content combine to make the brain the organ most susceptible to oxidative stress. The role of oxidative damage and disruption of brain iron homeostasis is considered clinically important to normal aging and a potential pathogenic component of a number of neurologic disorders including Alzheimer's disease and Parkinson's disease. Little is known, however, of the mechanism by which the brain maintains iron homeostasis at either the whole organ or cellular level. In this study we report the cellular distribution of the two isoforms of ferritin in the brain of adult subhuman primates. A subset of neurons immunolabel specifically for the H-chain ferritin protein, whereas cells resembling microglia are immunolabeled only after exposure to the L-chain ferritin antibody. Only one cell type immunostains for both H- and L-chain ferritin; these cells are morphologically similar and have the same distribution pattern as oligodendrocytes. Neither ferritin isoform is usually detected in astrocytes. These data indicate considerable differences in iron sequestration and use between neurons and glia and among neuronal and glial subtypes. This information will be essential in determining the role of each of these cells in maintaining general brain iron homeostasis and the relative abilities of these cells to withstand oxidative stress.

Differential processing of the ferritin heavy chain mRNA in human liver and adult human brain

Northern blot analyses of the poly(A)+ RNAs from human brain and liver, using a human brain ferritin heavy chain (FTH) cDNA as the probe, shows the presence of two transcripts of 1.4 and 1.1 kb. The larger, 1.4-kb RNA, is expressed predominantly in the brain, whereas the smaller, 1.1 kb, is expressed abundantly in the liver. Screening of two normal human brain cDNA libraries yielded two types of human brain FTH cDNAs. One type corresponds to the previously characterized 1.1-kb RNA from liver and lymphocytes. The other is also identical to the previously characterized FTH cDNA except that it contains an additional 279-bp sequence at the 3' untranslated region. This additional sequence shows 94.1%, 62.5%, and 58.9% identity to the 3' flanking sequence of the human liver and mouse and rat FTH genomic clones, respectively. A fragment of a genomic clone containing the 279-bp sequence was also isolated and sequenced. These data suggest that differential processing of the primary transcript for the FTH mRNA in human brain and liver could generate two mature mRNAs of 1.4 and 1.1 kb. This could be due to the use of alternative polyadenylation sites in the pre-mRNA.

Regulation of ferritin and transferrin receptor expression by iron in human hepatocyte cultures

HepG2 cell cultures and human hepatocyte primary cultures were used to develop appropriate hepatocytic in vitro models of iron load in order to further understand the pathophysiological mechanisms occurring in the liver of patients with hemochromatosis. The first step of this study was to obtain an efficient iron supply in conditions of minimal toxicity. It was demonstrated that iron complexed to citrate entered efficiently into HepG2 cells and human hepatocytes. This iron load was obtained with minimal toxicity in both culture models as evaluated by the intracellular LDH activity and the total protein content. The second step was to study the effect of iron on ferritin and transferrin receptor expression. In HepG2 cell cultures, intracellular and extracellular ferritin concentrations were strikingly increased by iron in dose- and time-dependent manners. However, the relative amounts of H and L ferritin mRNAs were not significantly affected by iron, suggesting that ferritin regulation occurred at a translational level. On the other hand, in human hepatocyte cultures, the increase of intracellular and extracellular ferritin concentrations was accompanied by an increase in the amounts of H and L ferritin mRNAs. In this model, iron-induced ferritin biosynthesis seemed to be more complex than in HepG2 cells and to be governed by transcriptional and/or post-transcriptional regulatory mechanisms. However, an additional translational level of regulation could not be excluded. In contrast, transferrin receptor expression was decreased by iron in HepG2 cells as well as in human hepatocyte cultures. This decrease was associated with a decrease in the mRNA steady-state level. In both culture models, transferrin receptor regulation seemed to occur at a transcriptional or post-transcriptional level. These results demonstrate that normal human hepatocytes in primary culture respond to iron in a manner close to that observed in vivo and thereby provide a promising experimental model for further understanding pathophysiological mechanisms involved in human hemochromatotic liver.

Sequence of a cDNA encoding the ferritin H-chain from an 11-week-old human fetal brain

A cDNA library in lambda Charon BS(-) from 11-week-old human fetal brain (FB) was screened using a human liver ferritin (FTH)-encoding cDNA as a probe. The complete sequence of the positive clone, cFB1, showed that the coding region and a part of the 5' and 3' untranslated regions (UTR) are identical to the corresponding published sequence of the liver cDNA. However, a particularly noteworthy difference is the presence of 279 bp of additional sequence in the FB 3'-UTR. Northern blot analysis of FB poly(A)+RNA showed it to be a part of the FTH transcript. Comparison of the 279-bp sequence with the GenBank and EMBL databases showed it to be 94.1, 62.5, and 58.9% similar to segments from human, mouse and rat FTH genomic sequences, respectively. However, in all these cases, only a part of this 279-bp sequence has been found in the nontranscribed region. We therefore conclude that in FB, the 279-bp sequence is a part of the mature FTH mRNA. Sequence analysis also suggests a differential poly(A) site selection in the production of FTH mRNA in FB and liver.

Modulation of ferritin H-chain expression in Friend erythroleukemia cells: transcriptional and translational regulation by hemin

The mechanisms that regulate the expression of the H chain of the iron storage protein ferritin in Friend erythroleukemia cells (FLCs) after exposure to hemin (ferric protoporphyrin IX), protoporphyrin IX, and ferric ammonium citrate (FAC) have been investigated. Administration of hemin increases the steady-state level of ferritin mRNA about 10-fold and that of ferritin protein expression 20-fold. Experiments with the transcriptional inhibitor actinomycin D and transfection studies demonstrate that the increment in cytoplasmic mRNA content results from enhanced transcription of the ferritin H-chain gene and cannot be attributed to stabilization of preexisting mRNAs. In addition to transcriptional effects, translational regulation induces the recruitment of stored mRNAs into functional polyribosomes after hemin and FAC administration, resulting in a further increase in ferritin synthesis. Administration of protoporphyrin IX to FLCs produces divergent transcriptional and translational effects. It increases transcription but appears to suppress ferritin mRNA translation. FAC treatment increases the mRNA content slightly (about twofold), and the ferritin levels rise about fivefold over the control values. We conclude that in FLCs, hemin induces ferritin H-chain biosynthesis by multiple mechanisms: a transcriptional mechanism exerted also by protoporphyrin IX and a translational one, not displayed by protoporphyrin IX but shared with FAC.

Modulation of ferritin H-chain expression in Friend erythroleukemia cells: transcriptional and translational regulation by hemin

The mechanisms that regulate the expression of the H chain of the iron storage protein ferritin in Friend erythroleukemia cells (FLCs) after exposure to hemin (ferric protoporphyrin IX), protoporphyrin IX, and ferric ammonium citrate (FAC) have been investigated. Administration of hemin increases the steady-state level of ferritin mRNA about 10-fold and that of ferritin protein expression 20-fold. Experiments with the transcriptional inhibitor actinomycin D and transfection studies demonstrate that the increment in cytoplasmic mRNA content results from enhanced transcription of the ferritin H-chain gene and cannot be attributed to stabilization of preexisting mRNAs. In addition to transcriptional effects, translational regulation induces the recruitment of stored mRNAs into functional polyribosomes after hemin and FAC administration, resulting in a further increase in ferritin synthesis. Administration of protoporphyrin IX to FLCs produces divergent transcriptional and translational effects. It increases transcription but appears to suppress ferritin mRNA translation. FAC treatment increases the mRNA content slightly (about twofold), and the ferritin levels rise about fivefold over the control values. We conclude that in FLCs, hemin induces ferritin H-chain biosynthesis by multiple mechanisms: a transcriptional mechanism exerted also by protoporphyrin IX and a translational one, not displayed by protoporphyrin IX but shared with FAC.

Increased expression of cytokeratin and ferritin-H genes in tumorigenic clones of the SW 613-S human colon carcinoma cell line

Subclones of the SW 613-S human colon carcinoma cell line differ by their ability to induce tumors in nude mice and by their level of amplification of the c-myc gene. Clones with a high level of amplification are tumorigenic in nude mice whereas those with a low level are not. Genes overexpressed in the tumorigenic clones as compared to the nontumorigenic ones were searched by differential screening of a cDNA library. Two cDNA clones corresponding to cytokeratin K18 and ferritin-H chain were isolated. The steady state level of the corresponding mRNAs is higher in cells of all tumorigenic clones. The level of cytokeratin K8 mRNA, the specific partner of cytokeratin K18 in intermediate filaments of epithelial cells, is also elevated in these cells. For all three genes, this is mainly due to an increase in the transcription rate, as shown by a nuclear run-on assay. Immunoblotting experiments showed that cytokeratins K8, K18, and K19 are more abundant in cells of tumorigenic clones. The mRNA of the other subunit of apo-ferritin (ferritin-L chain) is expressed at the same level in both types of clones. The mRNAs of cytokeratins K18 and K8 and of ferritin-H chain are also overexpressed in cells of nontumorigenic clones which have acquired a tumorigenic phenotype after transfection of c-myc gene copies.

Ferritin mRNAs on rat liver membrane-bound polysomes synthesize ferritin that does not translocate across membranes

Ferritin is a typical intracellular protein but small amounts are also present in serum and other biological fluids. The source and physiological significance of serum ferritin are still obscure. The presence of ferritin mRNAs on polysomes bound to endoplasmic reticulum (ER) could be relevant for the secretion of ferritin. By Northern blot analysis we found significant amounts of both L and H subunit mRNAs on rat liver membrane-bound polysomes. Immunoprecipitation of translational products of membrane-bound polysomes with anti-rat liver ferritin antibody showed that ferritin is actually synthesized on ER membranes. Analysis of RNA extracted from salt-washed rat liver microsomes demonstrated that ferritin mRNAs are translated by polysomes tightly bound to ER membranes. Following iron treatment, both the amount of H and L subunit mRNAs and ferritin synthesis increased sharply in both free and bound polysomal fractions. Translation of membrane-bound polysomes in the presence of microsomal membranes indicated that ferritin is not processed by signal sequence cleavage or glycosylation and is not translocated into ER membranes. Ferritin mRNAs found on membrane-bound polysomes are associated with ER in a specific way, however, their products do not seem to follow the classic secretory pathway and therefore the significance of the large amount of ferritin mRNAs in the bound ribosome fraction remains unclear.

Regulation of transferrin, transferrin receptor, and ferritin genes in human duodenum

To gain insights at the molecular level into the expression of iron-regulated genes [transferrin (Tf), transferrin receptor (TfR), and ferritin H and L subunits] in human intestinal areas relevant to iron absorption, the steady-state levels of specific messenger RNAs (mRNAs) were analyzed in gastric and duodenal samples obtained from 6 normal subjects, or 10 patients with anemia, 14 patients with untreated iron overload, and 8 patients with various gastrointestinal disorders. No Tf mRNA was detected in human gastroduodenal tissue, confirming earlier findings in the rat. In normal subjects, although higher levels of ferritin H- and L-subunit mRNAs were consistently found in duodenal than in gastric samples, no differences in the content of TfR transcripts were detected. However, a dramatic increase in TfR mRNA levels was specifically found in duodenal samples from subjects with mild iron deficiency but severe anemia. This response of the TfR gene is presumably secondary to decreased cellular iron content due to its accelerated transfer into the bloodstream, as also indicated by the low levels of ferritin subunit mRNAs found in the same tissue samples, and is not linked to faster growth rate of mucosal cells because no changes in duodenal expression of histone, a growth-related gene, were detected. In patients with secondary iron overload, a down-regulation of duodenal TfR gene expression and a concomitant increase in ferritin mRNA content were documented. On the contrary, a lack of TfR gene down-regulation and an abnormally low accumulation of ferritin H- and L-subunit mRNAs were detected in the duodenums of subjects with idiopathic hemochromatosis. Whether these molecular abnormalities in idiopathic hemochromatosis are relevant to the metabolic defect(s) of the disease is presently unknown.

Na+/K(+)-ATPase activity and its alpha II subunit gene expression in rat skeletal muscle: influence of diabetes, fasting, and refeeding

We have examined the effects of diabetes, fasting, and refeeding on Na+/K(+)-adenosine triphosphatase (ATPase) activity and its catalytic alpha II subunit gene expression in skeletal muscle. Two hypoinsulinemic states, streptozotocin-induced diabetes and 48-hour fasting caused a significant decrease (P less than .05) in skeletal muscle Na+/K(+)-ATPase activity and a marked increase (P less than .01) in the levels of alpha II subunit mRNA. A decrease in enzyme activity was observed on the 2nd and the 14th day of diabetes, whereas an increase in alpha II mRNA levels was found only on the 14th day. The levels of alpha I mRNA were not affected, while the levels of mRNA of the structural beta subunit were decreased on the 14th day of diabetes. Correction of hyperglycemia with insulin restored enzyme activity and alpha II isoform mRNA levels toward normal in diabetic animals. Refeeding for 48 or 72 hours restored these parameters to normal in skeletal muscle of previously fasting rats. These observations suggest that a decrease in muscle Na+/K(+)-ATPase activity may lead to a compensatory increase in its alpha II subunit gene expression. The levels of insulin and not of glycemia appear to be critical in modulating Na+/K(+)-ATPase activity and gene expression.

Expression of the genes for the ferritin H and L subunits in rat liver and heart. Evidence for tissue-specific regulations at pre- and post-translational levels

The proportion of ferritin light-chain and heavy-chain subunits (L and H) present in the ferritin multimeric shell varies between different tissues. To identify the regulatory mechanisms responsible for the greater amount of L in liver than in heart isoferritins, we analysed ferritin-gene expression at the RNA and protein levels in these two tissues of the rat. In the heart the ratio between the amount of L and H, at the level both of synthesis and accumulation, is about 1 and is the same as the ratio between their respective mRNAs. In contrast, in the liver, the ratio between the L- and H-mRNAs is approx. 2 and cannot entirely explain the large predominance of L in isoferritins in this tissue. Since in the liver the L-mRNA is neither preferentially associated with polyribosomes nor translated more efficiently than its H- counterpart, it seems that the liver-specific isoferritin profile is determined by a combination of pre- and post-translational mechanisms, whereas in heart the post-translational regulation does not seem to be relevant and the tissue-specific pattern is determined at the level of mRNA accumulation.

The theoretical implications and experimental and clinical results of radiolabeled antiferritin

Ferritin is produced in malignant and normal tissues. It acts both as an immunosuppressant and as an iron storage protein. As a tumor associated protein, it is related to virally induced tumors, and selective tumor targeting by radiolabeled antiferritin antibodies has led to its use in clinical trials. In patients with advanced Hodgkin's disease who have failed conventional therapy, 131I antiferritin produced partial remissions, while 90Y antiferritin led to complete remissions and a demonstrable dose-response relationship. Combining the variable low-dose radiation patterns produced by radiolabeled antibody therapy with chemotherapy in the treatment of hepatocellular cancer has led to enhanced tumor cytotoxicity and, in some cases, the conversion of non-resectable hepatoma to resectable. Further, the potential for clinical and laboratory investigation of radiolabeled antibody therapy is discussed in light of new findings.

An overview of iron metabolism at a molecular level

Over the last 10 years there has been steady progress in our understanding of the structure of the iron-binding proteins transferrin and ferritin, and the transferrin receptor. In the last few years there have been very rapid developments in understanding of the genetics of these proteins and the regulation of synthesis. This review includes a description of gene localization and structure, the regulation of protein synthesis and the structure of proteins of the transferrin family, the transferrin receptor and the iron storage protein ferritin.

Iron-binding proteins

The structure and properties of the iron-binding proteins transferrin, lactoferrin and transferrin are reviewed. Transferrin and lactoferrin are structurally similar, consisting of a single polypeptide chain and reversibly binding two iron atoms per molecule. Transferrin is found mainly in serum, whereas lactoferrin is found in neutrophils and in external secretions. Transferrin functions mainly as a donor of iron to cells, but there is no established iron-transport role for lactoferrin. Both these proteins may have antimicrobial activity as a result of their ability to sequester iron. Lactoferrin may act principally as a scavenger of iron in conditions where transferrin may not bind iron well, e.g. at low pH. Ferritin is a multisubunit protein capable of binding up to 4,000 iron atoms and serves principally as an iron-storage protein, though it may also serve to detoxify iron. In iron-rich tissues ferritin is largely degraded and the iron is converted to haemosiderin.