Structure and expression of ferritin genes in a human promyelocytic cell line that differentiates in vitro

Transcriptome analysis of the white body of the squid Euprymna tasmanica with emphasis on immune and hematopoietic gene discovery

In the mutualistic relationship between the squid Euprymna tasmanica and the bioluminescent bacterium Vibrio fischeri, several host factors, including immune-related proteins, are known to interact and respond specifically and exclusively to the presence of the symbiont. In squid and octopus, the white body is considered to be an immune organ mainly due to the fact that blood cells, or hemocytes, are known to be present in high numbers and in different developmental stages. Hence, the white body has been described as the site of hematopoiesis in cephalopods. However, to our knowledge, there are no studies showing any molecular evidence of such functions. In this study, we performed a transcriptomic analysis of white body tissue of the Southern dumpling squid, E. tasmanica. Our primary goal was to gain insights into the functions of this tissue and to test for the presence of gene transcripts associated with hematopoietic and immune processes. Several hematopoiesis genes including CPSF1, GATA 2, TFIID, and FGFR2 were found to be expressed in the white body. In addition, transcripts associated with immune-related signal transduction pathways, such as the toll-like receptor/NF-κβ, and MAPK pathways were also found, as well as other immune genes previously identified in E. tasmanica's sister species, E. scolopes. This study is the first to analyze an immune organ within cephalopods, and to provide gene expression data supporting the white body as a hematopoietic tissue.

Molecular cloning and expression analysis of ferritin, heavy polypeptide 1 gene from duck (Anas platyrhynchos)

H-ferritin is a core subunit of the iron storage protein ferritin, and is related to the pathogenesis of malignant diseases. A differential expressed sequence tag of the ferritin, heavy polypeptide 1 gene (FTH1) was obtained from our previously constructed suppression subtractive cDNA library from 3-day-old ducklings challenged with duck hepatitis virus type I (DHV-1). The expression and function of FTH1 in immune defense against infection remains largely unknown in ducks. In this study, the full-length duFTH1 cDNA was obtained using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends. It consisted of 153 basepairs (bp) 5'untranslated region (UTR), 183 bp 3'UTR, and 546 bp open reading frame that encodes a single protein of 181 amino acid residues. duFTH1 shares high similarity with FTH1 genes from other vertebrates. The amino acid sequence possesses the conserved domain of typical ferritin H subunits, including seven metal ligands in the ferroxidase center, one iron binding region signature, and a potential bio-mineralization residue (Thy(29)). Moreover, in agreement with a previously reported ferritin H subunit, we identified an iron response element in the 5'UTR. RT-PCR analyses revealed duFTH1 mRNA is widely expressed in various tissues. Real-time quantitative polymerase chain reaction analyses suggested that duFTH1 mRNA is significantly up-regulated in the liver after DHV-1 injection or polyriboinosinic polyribocytidylic acid (polyI:C) treatment, reaching a peak 4 h post-infection, and dropping progressively and returning to normal after 24 h. Our findings suggest that duFTH1 functions as an iron chelating protein subunit in duck and contributes to the innate immune responses against viral infections.

Sulfide increases labile iron pool in RD4 cells

A linkage between sulfur and iron metabolism has been suggested since sulfide has the ability to release iron from ferritin in the presence of iron acceptors in vitro. Nevertheless, this linkage is still lacking evidence in vivo as well as in cellular models. In this study we have treated human RD4 skeletal muscle cells with sodium sulfide and measured the level of the labile iron pool (LIP) as well as the intracellular sulfide concentration. We have also detected the amounts of L-ferritin protein as well as the iron regulatory protein 2 (IRP2). The sulfide treatment resulted in a 100% increase in the amount of LIP after 1 and 2 h. We also found that the raise of the LIP levels was coupled to an elevation of the amounts of intracellular sulfide that increased by 60%. The bioavailability of the released iron was confirmed by a 100% increase in L-ferritin protein as well as a 60% decrease of the IRP2 protein levels. These results suggest that there is a linkage between sulfur metabolism and intracellular iron regulation in mammalian cells.

The ferritin family of iron storage proteins

The ferritins are a family of proteins produced in a variety of amounts and types depending on the state of development of an animal, or the state of differentiation of a particular cell type, or the environment. Iron storage is the main function of the ferritins when iron is needed for intracellular use (housekeeping) for iron proteins such as ribonucleotide reductase, cytochromes, oxidases, nitrogenases, or photosynthetic reaction centers or for extracellular use by other cells (specialized). Under abnormal conditions, such as the breach of transferrin-receptor-controlled incorporation of iron, ferritin can also serve to detoxify excess intracellular iron. The structure of ferritin is very complex, consisting of a protein coat of 24 polypeptide subunits, approximately 20 kDa, which surrounds an inorganic phase of hydrous ferric oxide. The polypeptide subunits, bundles of four alpha helices, display remarkable conservation of sequence among plants and animals, which is probably related to the necessity of forming the hollow sphere pierced by 14 channels through which iron may pass. In spite of the conserved regions of sequence, there are multiple genes for ferritin polypeptide subunits within an organism; at the moment three distinct subunit types, H H'(or M), and L, have been identified which are expressed in a cell-specific fashion. How many different subunit types exist, the influence on function, and the number of genes required to encode them are currently being actively investigated. Not only does the protein coat of ferritin display variations, the inorganic phase of ferritin can vary as well. For instance, differences can occur in the number of Fe atoms (up to 4500), as well as in the phosphorus content and in the degree of hydration and order. Such observations have depended on the use of a variety of physical techniques such as X-ray diffraction, EXAFS, and Mössbauer spectroscopy. The same approaches, as well as EPR spectroscopy, have been used to monitor the path taken by Fe as it passes from mononuclear Fe(II) outside the protein coat to polynuclear Fe(III) inside the protein coat. Both mononuclear Fe(II) and Fe(III) have been observed, as well as dimeric Fe(II)-O-Fe(III), and Fe(III)-oxo bridged clusters attached to the protein. A possible protein site for the Fe(III) cluster is a groove on the inner surface of the dimeric interface, suggested by the structure and from the affect of natural cross-links between subunit pairs.(ABSTRACT TRUNCATED AT 400 WORDS)

Ferritin subunits in CSF are decreased in restless legs syndrome

Restless legs syndrome (RLS) is a neurological disorder that may be related to iron misregulation at the level of the central nervous system. Evidence that iron is involved in RLS comes from magnetic resonance imaging data, autopsy studies, analyses of cerebrospinal fluid (CSF), and correlations of symptoms with serum ferritin. To further examine the possibility that brain iron status is insufficient in RLS, we determined ferritin levels in the CSF. Specifically, we differentiated between the H- and L-subunits of ferritin, because these peptides are expressed from different chromosomes and have different functions. We measured H- and L-ferritin subunit levels in control and RLS human CSF using immunoblot analysis and found that both H- and L-ferritin are significantly decreased in early but not late-onset RLS. Additionally, we quantified total protein in each CSF sample to establish that the decrease in ferritin subunits in RLS did not reflect a decrease in total protein in CSF. Furthermore, we used equal amounts of total CSF protein in the immunoblot analyses, in contrast to previously published studies that provided only volumetric data, to determine which approach was more accurate for quantifying the amount of ferritin relative to other proteins in CSF. Our results establish a protein standard in RLS, provide a comparative analysis of protein-controlled versus volumetric immunoblot techniques, and argue for a profound loss of iron storage capacity in the brain in RLS, specifically in the early onset RLS phenotype. These data suggest that CSF ferritin levels may provide a biomarker for assisting in the diagnosis of RLS.

JunD activates transcription of the human ferritin H gene through an antioxidant response element during oxidative stress

Ferritin is the major intracellular iron storage protein that sequesters excess free iron to minimize generation of iron-catalysed reactive oxygen species. We previously demonstrated that expression of ferritin heavy chain (ferritin H) was induced by pro-oxidants, which is a part of cellular antioxidant response to protect cells from oxidative damage. In this study, we have identified that the antioxidant/electrophile response element (ARE) located 4.5 kb upstream to the human ferritin H transcription initiation site is responsible for the oxidant response. The human ferritin H ARE comprises two copies of bidirectional AP1 motifs. Mutations in each AP1 motif significantly impaired protein binding and the function of the ARE, indicating that both of the AP1 motifs are required for pro-oxidant-mediated activation of the ferritin H gene. We identified that JunD, an AP1 family basic-leucine zipper (bZip) transcription factor, is one of the ferritin H ARE binding proteins and activates ferritin H transcription in HepG2 hepatocarcinoma cells. Gel retardation assay demonstrated that H2O2 (hydrogen peroxide) or t-BHQ (tert-butylhydroquinone) treatment increased total protein binding as well as JunD binding to the ferritin H ARE. Chromatin immunoprecipitation assay showed that H2O2 treatment induced JunD binding to the ferritin H ARE. Both H2O2 and t-BHQ induced phosphorylation of JunD at Ser-100, an activated form of JunD. Furthermore, overexpression of JunD induced endogenous ferritin H protein synthesis. Since JunD has recently been demonstrated to protect cells from several stress stimuli including oxidative stress, these results suggest that, in addition to NFE2-related factor 2 (Nrf2) as a major ARE regulatory protein, JunD is another ARE regulatory protein for transcriptional activation of the human ferritin H gene and probably other antioxidant genes containing the conserved ARE sequences by which JunD may confer cytoprotection during oxidative stress.

Screening and cloning of genes coding for leukocyte proteins interacting with NS5ATP9 by yeast-two hybrid technique

AIM: To investigate the biological functions of NS5ATP9, and to screen proteins in leukocytes interacting NS5ATP9 protein by yeast-two hybrid.

METHODS: The NS5ATP9 gene was amplified by polymerase chain reaction (PCR) and NS5ATP9 bait plasmid was constructed by using yeast-two hybrid system 3, and the yeast AH109 was then transformed. The transformed yeast mated with yeast Y187 containing leukocytes cDNA library plasmid in 2×YPDA medium. Diploid yeast was plated on synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) and synthetic dropout nutrient medium (SD/-Trp-Leu-His-Ade) containing X--gal for selecting two times and screening. After extracting and sequencing of plasmid DNA from blue colonies, we underwent analysis by bioinformatics.

RESULTS: Forty six colonies were sequenced, among which thirteen colonies were Homo sapiens immunoglobulin light chain, ten ubiquitin, two ferritin heavy chain, eleven Homo sapiens rearranged immunoglobulin lambda light chain, one 14-3-3 family protein, one Meningococcus PorA protein, three RNA polymerase III, one tobacco mitogen activated protein kinase, two cytochrome P450 II, one SLIT2 protein, and one dependent-protein kinase catalylic subunit.

CONCLUSION: Genes of NS5ATP9 interacting proteins in leukocytes are successfully cloned and the results bring some new clues for studying the biological functions of NS5ATP9 and associated proteins.

Aedes aegypti ferritin

Diseases transmitted by hematophagous (blood-feeding) insects are responsible for millions of human deaths worldwide. In hematophagous insects, the blood meal is important for regulating egg maturation. Although a high concentration of iron is toxic for most organisms, hematophagous insects seem unaffected by the iron load in a blood meal. One means by which hematophagous insects handle this iron load is, perhaps, by the expression of iron-binding proteins, specifically the iron storage protein ferritin. In vertebrates, ferritin is an oligomer composed of two types of subunits called heavy and light chains, and is part of the constitutive antioxidant response. Previously, we found that the insect midgut, a main site of iron load, is also a primary site of ferritin expression and that, in the yellow fever mosquito, Aedes aegypti, the expression of the ferritin heavy-chain homologue (HCH) is induced following blood feeding. We now show that the expression of the Aedes ferritin light-chain homologue (LCH) is also induced with blood-feeding, and that the genes of the LCH and HCH are tightly clustered. mRNA levels for both LCH- and HCH-genes increase with iron, H2O2 and hemin treatment, and the temporal expression of the genes is very similar. These results confirm that ferritin could serve as the cytotoxic protector in mosquitoes against the oxidative challenge of the bloodmeal. Finally, although the Aedes LCH has no iron responsive element (IRE) at its 5'-untranslated region (UTR), the 5'-UTR contains several introns that are alternatively spliced, and this alternative splicing event is different from any ferritin message seen to date.

Identification and mapping of the promoter for the gene encoding the ferritin heavy-chain homologue of the yellow fever mosquito Aedes aegypti

Mosquitoes are responsible for the transmission of numerous human diseases. The recent development of transgenic mosquitoes provides a new tool to examine molecular interactions between insect vectors and the pathogens they transmit. One focus in generating transgenic mosquito lies on expressing anti-pathogenic proteins at primary sites of pathogenic invasions, specifically the mosquito gut. Promoters that direct the expression of anti-pathogenic proteins in the mosquito gut are thus sought after because they may provide ways to hinder pathogenic development in the mosquito. Here, we report the identification and mapping of a strong promoter from the Aedes aegypti ferritin heavy-chain homologue (HCH) gene. All known insect ferritin HCH genes are expressed in the gut and inducible by an iron overload. Our transfection assays and DNase I footprinting analyses show that the mosquito ferritin HCH-gene contains regulatory elements both upstream and downstream of the transcriptional start site. The promoter of this gene contains a CF2 site, two GATA-binding sites, an E2F site, a TATA-box, an AP-1 site and a C/EBP binding site.

Overexpression of H ferritin and up-regulation of iron regulatory protein genes during differentiation of 3T3-L1 pre-adipocytes

The role of iron-dependent oxidative metabolism in protecting the oxidable substrates contained in mature adipocytes is still unclear. Because differentiation increases ferritin formation in several cell types, thereby leading to an accumulation of H-rich isoferritins, we investigated whether differentiation affects iron metabolism in 3T3-L1 pre-adipocytes. To this aim, we evaluated the expression of the genes coding for the H and L ferritin subunits and for cytoplasmic iron regulatory protein (IRP) during the differentiation of 3T3-L1 cells in adipocytes induced by the addition of isobutylmethylxanthine, insulin, and dexamethasone. Differentiation enhanced ferritin formation and caused overexpression of the H subunit, thus altering the H/L subunit ratio. Northern blot analysis showed increased levels of H subunit mRNA. A gel retardation assay of cytoplasmic extract from differentiated cells, using an iron-responsive element as a probe, revealed enhanced an RNA binding capacity of IRP1, which correlated with the increase of IRP1 mRNA. The observed correlation between differentiation and iron metabolism in adipocytes suggests that an accumulation of H-rich isoferritin may limit the toxicity of iron in adipose tissue, thus exerting an antioxidant function.

Thyrotropin-releasing hormone and epidermal growth factor regulate iron-regulatory protein binding in pituitary cells via protein kinase C-dependent and -independent signaling pathways

Intracellular iron homeostasis is regulated, in part, by interactions between iron-regulatory proteins (IRP1 and IRP2) and iron-responsive elements (IREs) in ferritin and transferrin receptor mRNAs. In addition to iron, cellular oxidative stress induced by H(2)O(2), nitric oxide, and hypoxia, and hormonal activation by thyroid hormone and erythropoeitin have each been shown to regulate IRP binding to IREs. Hormonal signals, in particular mediated through protein kinase C (PKC), play a central role in the modulation of IRP/IRE interactions since phorbol esters were shown to activate IRP binding (Eisenstein, R. S., Tuazon, P. T., Schalinske, K. L., Anderson, S. A., and Traugh, J. A. (1993) J. Biol. Chem. 268, 27363-27370). In pituitary thyrotrophs (TtT97), we found that thyrotropin releasing hormone (TRH) and epidermal growth factor (EGF) increased IRP binding to a ferritin IRE, dependent on PKC and mitogen-activated protein kinase (MAPK) activity. In contrast, TRH and EGF decreased IRP binding in pituitary lactotrophs (GH3), despite activation of PKC and MAPK. IRP1 and IRP2 levels remained constant and IRP2 binding was predominant throughout. TRH and EGF markedly decreased IRP binding in MAPK kinase inhibitor-treated GH3 cells, whereas, they increased IRP binding in phosphatase inhibitor-treated GH3 cells. IRE-dependent CAT reporter translational expression closely reflected IRP binding to the ferritin IRE in both GH3 and TtT97 cells. Interestingly, ferritin protein levels were regulated similarly by TRH in both cell lines. These data link two different cell receptor systems to common signaling pathways that regulate IRP binding and ferritin expression. Remarkably, for TRH and EGF, these effects may be PKC-dependent or -independent determined by the cell type.

Glucocorticoids and dietary iron regulate postnatal intestinal heavy and light ferritin expression in rats

To cope with increasing dietary iron exposure, the intestinal epithelium of weaning rats must control intracellular labile iron pools. Intestinal expression of heavy (H) and light (L) ferritin subunits during early weaning and after cortisone administration and/or iron feeding was investigated. Changes in H and L ferritin gene expression were determined by nuclear runoff transcriptional assay, Northern blot analysis, and metabolic labeling of protein synthesis. H ferritin mRNA levels did not change between days 12 and 15, doubled on day 18, and tripled on day 24. L ferritin mRNA was reduced by 50% on days 18 and 24. The protein level of the H and L subunits paralleled the change in mRNAs. Cortisone treatment on day 12 induced a precocious increase of H and decrease of L mRNA expression on day 15. Nuclear runoff assays showed that cortisone did not change H and reduced L ferritin gene transcription. The increased level of H mRNA by cortisone was not translated, unless the rats were fed an iron-fortified diet, which reduced iron regulatory protein activity and stimulated a three- to sixfold increase of ferritin synthesis. Thus changes in intestinal H and L ferritin expression in weaning rats are modulated by glucocorticoids and iron; the former stabilizes H mRNA and suppresses L ferritin gene transcription, and the latter derepresses translation of ferritin mRNA.

Effect of altered thyroid hormone status on rat brain ferritin H and ferritin L mRNA during postnatal development

The iron binding protein ferritin is a heterogeneous mix of 24 heavy (H) and light (L) subunits. The H subunit is associated with iron utilization, while the L subunit is responsible for iron storage. Examination of the developmental pattern of mRNA abundance in rat brain revealed that ferritin L mRNA is highest at birth and declines during the first postnatal week. A similar decline was seen in ferritin H mRNA, but was followed by an increase in ferritin H mRNA in the second postnatal week which continued through postnatal day 21. The pattern of H mRNA regulation is similar to that in previous reports of total ferritin protein in the developing rat brain and is consistent with the fact that brain ferritin is predominately ferritin H. The effect of thyroid hormone on the developmental regulation of ferritin mRNAs was examined by the subcutaneous injection of a single dose of exogenous thyroxine (T(4); 2 microg/g) on postnatal day 1. Hypothyroidism was induced in pregnant dams with propylthiouracil (PTU; 0.05% in drinking water) from gestational day 7. Northern analysis from postnatal days 2-21 showed that T(4) increased ferritin H mRNA throughout development, while ferritin L mRNA was decreased compared to age-matched controls. PTU treatment decreased ferritin H and increased L mRNA in the later stages (days 14-21) of development. Given the distinct functions of ferritin H and L this suggests a role for thyroid hormone in the ability of the brain to regulate stored vs. utilizable iron during critical periods of development.

Differential expression of multiple unexpected genes during U937 cell and macrophage differentiation detected by suppressive subtractive hybridization

OBJECTIVE

The objective of this study was to identify new markers of myelomonocytic differentiation using a sensitive technique that permits detection of rare differential gene expression.

MATERIALS AND METHODS

[corrected] Suppressive subtractive hybridization (SSH) was performed between the human myelomonocytic U937 cell line and 1 alpha, 25-dihydroxyvitamin D3 and transforming growth factor beta 1 differentiated U937 cells. cDNA clones with significant increased expression in differentiated U937 cells over nondifferentiated U937 cells were characterized by sequencing. [corrected] The pattern of differential gene expression obtained by SSH was confirmed by cDNA Southern and Northern blots on the undifferentiated vs. differentiated U937 cells, and by reverse transcriptase polymerase chain reaction on undifferentiated human CD34(+) stem cells isolated from bone marrow vs. peripheral blood CD14(+) mature monocytes.

RESULTS

Seven cDNAs never associated with in vitro U937 cell myelomonocytic differentiation (prolactin, 11-beta hydroxysteroid dehydrogenase [11 beta-HSD)] haptoglobin alpha (2FS)-beta precursor, GLIPR, RTVP, the RNA helicase P68, and spermidine-spermine N1-acetyltransferase) were identified. The first five of these genes previously were associated with immune function and the last two are important for intermediary metabolism. Differential expression was confirmed in CD34(+)/CD14(+) monocyte differentiation for all genes but 11 beta-HSD.

CONCLUSIONS

We identified six new markers of U937 cell differentiation, which also are differentially expressed during normal human myelomonocytic differentiation.

Targeting mRNA to regulate iron and oxygen metabolism

A family of non-coding sequences in the mRNA (iso-IREs [iron-responsive elements]) regulate synthesis of key proteins in animal iron and oxidative metabolism such as ferritin and mitochondrial aconitase. Differential recognition between iso-IREs and iso-IRPs (iron regulatory proteins) regulates the translation or degradation of the IRE-containing mRNAs. IREs are hairpin loop structures with an internal loop/bulge or bulge that influence the binding of the iso-IRPs. The iso-IRPs have sequence homology to the aconitases and at least one IRP can be converted to an aconitase. Signals that target the iso-IRE/iso-IRP interactions in mRNA include environmental iron, O2, nitric oxide, H2O2, ascorbate, growth factors, and protein kinase C-dependent IRP phosphorylation. Iso-IRE structural specificity suggests a means of pharmacologically targeting mRNA function with chemicals such as Fe-bleomycin and other transition metal complexes that could be extended to other mRNAs with specific structures. With the iso-IRE/iso-IRP system, nature has evolved coordinated combinatorial control of iron and oxygen metabolism that may exemplify control of mRNAs in other metabolic pathways, viral reproduction, and oncogenesis.

Transcriptional control is relevant in the modulation of mosquito ferritin synthesis by iron

In yellow fever mosquito cells (Aag2 clone), iron treatment induces a threefold increase in ferritin message (fer mRNA) and protein (ferritin) by 16 h. These data contrast with work in mammalian hepatocytes and fibroblasts in which fer mRNA levels do not change with iron stimulation, but ferritin levels increase 50-fold. Pretreatment of the Aag2 cells with actinomycin D blocks induction of fer mRNA and reduces the ferritin subunit synthesis, suggesting that iron induction of ferritin subunit synthesis is subjected to transcriptional control. A putative iron-regulatory protein has also been identified in cytoplasmic extracts from Aag2 cells.

Post-transcriptional regulation of H-ferritin mRNA. Identification of a pyrimidine-rich sequence in the 3'-untranslated region associated with message stability in human monocytic THP-1 cells

We have previously demonstrated that phorbol myristate acetate (PMA) up-regulates H-ferritin gene expression in myeloid cells by stabilization of its message. In the present report, we showed that insertion of the 3'-untranslated region (3'-UTR) of H-ferritin mRNA at the 3'-end of luciferase coding sequence significantly reduced the stability of luciferase mRNA in human monocytic THP-1 cells. However, the half-life of the chimeric transcript was markedly prolonged after PMA treatment. A cytosolic protein factor from THP-1 cells was found to specifically bind to H-ferritin 3'-UTR. PMA treatment of THP-1 cells resulted in the reduction of the RNA binding activity in a time-dependent manner. Deletion analysis and RNase T1 mapping revealed a pyrimidine-rich sequence within the 3'-UTR which interacts with the protein factor. Competition experiments with homoribopolymers further demonstrated the importance of uridines for the binding activity. Point mutations in uridines of the pyrimidine-rich sequence reduced the protein binding to 3'-UTR, while increasing the stability of the chimeric luciferase transcript. Together, these results demonstrate that the pyrimidine-rich sequence in the 3'-UTR is involved in post-transcriptional regulation of H-ferritin gene expression in myeloid cells.

Translational control of specific genes during differentiation of HL-60 cells

Eukaryotic gene expression can be regulated through selective translation of specific mRNA species. Nevertheless, the limited number of known examples hampers the identification of common mechanisms that regulate translation of specific groups of genes in mammalian cells. We developed a method to identify translationally regulated genes. This method was used to examine the regulation of protein synthesis in HL-60 cells undergoing monocytic differentiation. A partial screening of cellular mRNAs identified five mRNAs whose translation was specifically inhibited and five others that were activated as was indicated by their mobilization onto polysomes. The specifically inhibited mRNAs encoded ribosomal proteins, identified as members of the 5'-terminal oligopyrimidine tract mRNA family. Most of the activated transcripts represented uncharacterized genes. The most actively mobilized transcript (termed TA-40) was an untranslated 1.3-kilobase polyadenylated RNA with unusual structural features, including two Alu-like elements. Following differentiation, a significant change in the cytoplasmic distribution of Alu-containing mRNAs was observed, namely, the enhancement of Alu-containing mRNAs in the polysomes. Our findings support the notion that protein synthesis is regulated during differentiation of HL-60 cells by both global and gene-specific mechanisms and that Alu-like sequences within cytoplasmic mRNAs are involved in such specific regulation.

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.

Role of H and L subunits in mouse ferritin

Ferritin is an iron-binding protein composed of two subunits, H and L. Twenty-four of these subunits assemble to form apoferritins whose subunit composition varies in a characteristic way in different tissues. Using recombinant proteins, we have assessed the role of H and L subunits in mouse ferritin function and compared these to human ferritin subunits. We report that mouse ferritin subunits exhibit considerable functional similarity to their human counterparts, including a prominent role of the H subunit in the facilitation of rapid iron uptake, and a key role of amino acid residues Glu-62 and His-65 in this process. In addition, amino acid residues important to assembly of the protein are conserved between mouse and human, permitting the formation of fully functional hybrid proteins containing both mouse and human subunits. However, murine and human ferritin H subunits also evidenced substantial functional differences; murine ferritin H showed a consistent reduction in iron uptake activity relative to human ferritin H. Creation of chimeric human/mouse ferritin H subunits by "helix swapping" mapped the domain of the protein critical to this activity difference to the DE helix. These findings suggest a novel functional role for carboxyl-terminal domains of the ferritin H subunit.

Identification of differentially expressed genes in rat aortic allograft vasculopathy

Graft vasculopathy is an important complication of long-surviving organ transplants, but its pathogenesis has remained elusive. We investigated rat aortic transplants with vasculopathy, aortic transplants without vasculopathy, and normal aortas for differentially expressed mRNA transcripts to gain further insight into the molecular mechanisms involved. Aortic transplants were performed in allogeneic or syngeneic recipients followed by removal after 1 or 5 months, RNA isolation, and differential display to identify mRNA transcripts the expression of which was modulated in conjunction with the transplant procedure and the development of vasculopathy. Using 80 random primers, 57 differentially displayed polymerase chain reaction products were identified, 18 of which were found in allografts but not in syngeneic grafts or normal vessels, whereas 15 were expressed in normal vessels and syngeneic grafts but not in allografts. Of the differentially displayed amplicons, 13 were successfully reamplified and used as probes for Northern analysis; differential expression was confirmed in 6 instances. DNA sequence analysis of these PCR products revealed identity with the immunoglobulin J chain in 2 instances, the ferritin heavy chain, a sequence related but not identical with Ras, and an established sequence tag recently isolated from a human fetal heart library; 1 sequence was not related to any known gene. To assess whether differential mRNA expression of the J-chain gene, a gene expressed in cells of B lymphocyte lineage, was associated with infiltration of the graft by B lymphocytes, tissue sections were stained with an antibody against the B cell marker CD45RA. Although the number of CD45RA-positive cells was low, there was a significant increase in the number of CD45RA-positive cells in the adventitia and intima of grafts with vasculopathy. Furthermore, immunostaining with anti-ferritin antiserum confirmed the presence of ferritin-positive cells within the inner layer of the graft vessel wall and dispersed in the intima, media, and adventitia. The question remains as to which of these genes are critically relevant in the pathogenesis of graft vasculopathy and whether they serve as targets for therapeutic interventions.

Induction of ferritin synthesis in cells infected with Mengo virus

We have recently identified ferritin as a cellular protein particle whose synthesis is stimulated in mouse or human cells infected by the picornavirus Mengo. Immunoprecipitation of the particle from infected murine L929 cells showed a 4- and 6-fold increase in the intracellular concentrations of H and L apoferritin subunits, respectively. This differential expression altered the H/L subunit ratio from 3.0 in uninfected cells to 2.2 in Mengo virus-infected cells. The induction is not due to an increase in transcription of the apoferritin L and H genes, nor is it due to an increase in stability of the apoferritin mRNAs. At the level of translation, the iron regulatory protein (IRP) remained intact, with similar amounts being detected in uninfected and infected cells. The Mengo virus RNA genome does not compete with the iron regulatory element (IRE) for the binding of IRP, and sequence analysis confirmed that there are no IREs in the virus RNA. The IRE binding activity of IRP in infected cells decreased approximately 30% compared with uninfected cells. The decrease in binding activity could be overcome by the addition of Desferal (deferoxamine mesylate; CIBA) an intracellular iron chelator, which suggests that virus infection causes an increase in intracellular free iron. Electron paramagnetic resonance (EPR) studies have confirmed the increase in free iron in Mengo virus infected cells. The permeability of cells for iron does not change in virus infected cells, suggesting that the induction of ferritin by Mengo virus is due to a change in the form of intracellular iron from a bound to a free state.

Phosphorylation and activation of both iron regulatory proteins 1 and 2 in HL-60 cells

Iron regulatory proteins (IRPs) are RNA-binding proteins that post-transcriptionally regulate synthesis of iron uptake (transferrin receptor) and storage (ferritin) proteins. Our previous work demonstrating that IRP1 is phosphorylated by protein kinase C supported the hypothesis that factors in addition to iron modulate IRP function. We have investigated changes in activity and expression of both IRP1 and IRP2 during phorbol 12-myristate 13-acetate (PMA)-induced differentiation of HL-60 cells. In contrast to IRP1, IRP2 was highly phosphorylated in untreated cells. PMA stimulated phosphorylation of IRP1 and IRP2 by at least 2-3-fold without affecting incorporation of [35S]methionine into the proteins. IRP1 and IRP2 isolated from PMA-treated cells displayed different phosphopeptides. Phosphorylation of IRPs was associated with a 2-fold increase in high affinity RNA binding activity without altering KD, and this was accompanied by a 50% increase in transferrin receptor mRNA abundance. PMA acted on a latent pool of binding activity that is present in a nonaconitase oxidized form and is largely composed of a stable but inactive species of IRP2. Desferal and hemin modulated iron-responsive element binding activity in HL-60 cells without affecting the phosphorylation state of IRP1. Hemin appeared to reduce the abundance of phosphorylated IRP2. Thus, multiple factors affect the function of both IRPs and indicate that extracellular agents may program changes in cellular iron metabolism by altering the phosphorylation state of these regulatory RNA-binding proteins.

Structure and differential expression of two maize ferritin genes in response to iron and abscisic acid

In plants, synthesis of the iron-storage protein ferritin in response to iron is not regulated at the translational level; this is in contrast to ferritin synthesis in animals. Part of the response is mediated through a transduction pathway which involves the plant hormone abscisic acid. In this work, we report the cloning and sequencing of two maize ferritin genes (ZmFer1 and ZmFer2) coding for members of the two ferritin mRNA subclasses, FM1 and FM2, respectively. Although plant and animal ferritins are closely related proteins, a major difference is observed between the organisation of the genes. Both maize ferritin genes are organised as eight exons and seven introns, the positions of which are identical within the two genes, while animal ferritin genes are interrupted by three introns, at positions different from those found in maize genes. Sequence divergence between the 3' untranslated regions of these genes has allowed the use of specific probes to study the accumulation of FM1 and FM2 transcripts in response to various environmental cues. Such probes have shown that FM1 and FM2 transcripts accumulate with differential kinetics in response to iron; FM1 mRNA accumulate earlier than FM2 mRNA and only FM2 transcripts accumulate in response to exogenous abscisic acid or water stress. Mapping of the transcriptional initiation region of these two genes defined their 5' upstream regions and allowed a sequence comparison of their promoters, which appeared highly divergent. This raises the possibility that the differential accumulation of FM1 and FM2 mRNAs in response to iron, abscisic acid and drought could be due to differential transcription of ZmFer1 and ZmFer2.

The iron-binding protein ferritin is expressed in cells of the osteoblastic lineage in vitro and in vivo

Ferritin, a metal-binding protein responsible for maintaining the bioavailability of iron, has been demonstrated in cells of the osteoblastic lineage. Messenger RNAs encoding the light and heavy chain subunits of ferritin were detected in ROS 17/2.8, ROS 25/1, and UMR106 rat osteosarcoma cell lines, in fetal rat calvaria, and in primary cultures of rat calvarial osteoblast-like cells. In vivo, the expression of ferritin light-chain mRNA was observed in both active osteoblasts and in osteocytes. A 450-kD iron-binding protein was immunoprecipitated from ROS 17/2.8 cells by an antiferritin antiserum. This protein comigrated with native ferritin, and could be dissociated into subunits comigrating with ferritin light and heavy chains. Addition of extracellular Fe59-transferrin to cultures of ROS 17/2.8 cells resulted in the sequestration of the iron in intracellular ferritin. These observations demonstrate that cells of the osteoblastic lineage possess a functional ferritin-based iron uptake and storage system capable of regulating metal homeostasis in bone.

Iron metabolism in inflammation

Alteration in iron metabolism is one of the proposed mechanisms underlying the anaemia of inflammation and chronic disease, the most common disorder in hospitalized patients. Iron metabolism parameters in inflammatory disease are characterized by blockage of tissue iron release, decreased serum iron and total iron binding capacity and an elevated serum ferritin level, reflecting augmented ferritin synthesis as part of the acute-phase response. The altered iron metabolism in inflammation is proposed to be a part of the host defence mechanism against invading pathogens and tumor cells and is suggested to be mediated by inflammatory cytokines and NO.

Transcriptional regulation of the human H ferritin-encoding gene (FERH) in G418-treated cells: role of the B-box-binding factor

We have analysed the molecular basis underlying the increase in ferritin heavy-chain mRNA (FERH) levels in cells exposed to the antibiotic Geneticin (G418). Transient transfection experiments demonstrate that this increase is paralleled by an enhanced transcription driven by the promoter (pFERH) for the human FERH gene, in which the most proximal promoter element (B-box) appears to play a key role. This region is conserved in human and rat, and binds an unknown factor. The DNA-protein complex composed of B-box-binding factor and its cis-element becomes more abundant in the G418-treated cells, as compared with the untreated ones.

A negative cis-acting G-fer element participates in the regulation of expression of the human H-ferritin-encoding gene (FERH)

Ferritin (Fer) is the major iron storage protein in man. Its synthesis is regulated both at the translational and transcriptional levels. In previous studies on transcriptional regulation of the human H-ferritin-encoding gene (FERH), a 160-bp promoter segment was analyzed [Bevilacqua et al., Gene 111 (1992) 255-260]. In order to obtain a more complete view of the elements involved in the transcriptional regulation of FERH, we have studied, in a further upstream region of the human FERH promoter (pFERH), a sequence between -272 and -291, named G-fer, because it contains a stretch of ten G, which binds a nuclear factor present in different cell types. DNA-binding assays and competition experiments suggest that the factor binding to G-fer has binding properties very similar to inhibitory factor-1 (IF-1), an ubiquitous factor that interacts with G-rich elements in the promoters of the mouse type-I collagen genes. DNA transfection experiments in HeLa cells, using either a wild-type or mutated pFERH fused to a reporter gene, showed that a 3-bp substitution mutation, that abolished the binding of the specific factor to G-fer, increased the promoter activity, thus suggesting an inhibitory role for the G-fer element and its cognate trans-acting factor.

H and L ferritin gene expression in U937 cells induced to macrophage differentiation

Ferritin is an ubiquitous protein that has been shown to regulate cell differentiation in several experimental systems. In this study we have investigated the expression of ferritin genes encoding the heavy (H) and light (L) chains in t'B U937 cell line, induced to differentiate to macrophage-like cells by 12-O-tetradecanoylphorbol-13-acetate (TPA), retinoic acid (RA) or 1-beta-D-arabinofuranosylcytosine (Ara-C). An increase in the level of H ferritin mRNA was detected in U937 cells that had been incubated with Ara-C. Treatment of U937 cells with Actinomycin D suggested that the H ferritin mRNA increase was mediated by post-transcriptional mechanisms. The L ferritin mRNA level increased only following stimulation of U937 cells with RA. Immunophenotypic and cytochemical analyses showed that Ara-C was the strongest inducer of the macrophagic differentiation of U937 cells. These results suggest that the increase of H ferritin mRNA expression may represent a sensitive marker of myeloid cells differentiating along the monocyte-macrophage lineage.

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.

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.

The ferritin genes: their response to iron status

Iron is a required nutrient which, at high concentrations, can peroxidize cell lipids and other cellular components. To prevent excess iron from damaging cells, it is stored in ferritin, which consists of a shell of protein subunits of two related types, H (heavy) and L (light), surrounding a cavity in which the iron can be deposited. In order to prepare for a rapid increase in ferritin in response to a rise in cellular iron, a large number of dormant ferritin mRNAs are accumulated in the cytoplasm. These can be rapidly activated to yield a large population of ferritin subunits. Regulation is achieved through a 28-nucleotide "stem-and-loop" structure near the beginning of the H- and L-ferritin mRNAs. When this structure is associated with a binding protein (iron regulatory element binding protein, IRE-BP), translation of the ferritin mRNA cannot proceed. However, when intracellular iron accumulates, IRE-BP releases its hold and translation of the mRNA then takes place. IRE-BP has been identified as a cytosolic form of aconitase, containing several fourfold iron-sulfur clusters. Within each cluster one iron atom is labile; this may be the mechanism by which IRE-BP responds to intracellular iron levels. Finally, transcription of the L- and H-genes shows that L is preferentially transcribed in response to increased iron intake, whereas H responds to cell differentiation and other factors. More work is needed to define independent transcription of the individual genes, including regulation of components other than the 28-nucleotide segment.

Ferritin (mRNA, protein) and iron concentrations during soybean nodule development

To study how iron-rich nodules concentrate and store iron, ferritin (mRNA, protein) was analyzed in developing soybean nodules and compared to nitrogenase (mRNA/activity) and leghemoglobin (mRNA, protein, heme). Both ferritin mRNA and protein concentrations increased early in nodulation. Later in nodulation ferritin protein declined, in contrast to the mRNA, as nitrogenase (mRNA and activity) increased and leghemoglobin (mRNA and protein) accumulated. A precursor/product relationship between iron stored in ferritin and iron in nitrogenase or leghemoglobin is suggested. The uncoordinated changes in ferritin mRNA and protein during nodulation contrast with nitrogenase mRNA and nitrogenase activity suggesting possible translational and posttranscriptional effects on ferritin expression.

Promoter for the human ferritin heavy chain-encoding gene (FERH): structural and functional characterization

We conducted a functional analysis of the promoter for the human ferritin heavy chain-encoding gene (pFERH) in HepG2 and HeLa cells. The activity of pFERH is equivalent in both cell types, despite their different ferritin (Fer) isotypes. Transfections of a series of 5'-deletion mutants indicate that pFERH activity is essentially dependent on two motifs. One of them, accounting for about 50% of the total transcriptional activity, is recognized by the RNA polymerase II transcription factor, Sp1, and the other by a low-affinity factor present in both the cell types analyzed.

Effect of iron and retinoic acid on the control of transferrin receptor and ferritin in the human promonocytic cell line U937

The effect of changes in iron availability and induction of differentiation on transferrin receptor expression and ferritin levels has been examined in the promonocytic cell line U937. Addition of iron (as 200 micrograms/ml saturated transferrin) or retinoic acid (1 microM) both caused approx. 70% reduction in the average number of surface transferrin receptors, while the iron chelator desferrioxamine caused an 84% increase. Comparable changes also occurred in the levels of transferrin receptor mRNA. Neither iron nor retinoic acid significantly altered the half-life of transferrin receptor mRNA in the presence of actinomycin D (approx. 75 min) but a 10-fold increase in stability occurred in the presence of desferrioxamine. Iron and retinoic acid both caused an increase in intracellular ferritin levels (approx. 4-and 3-fold, respectively), while desferrioxamine reduced ferritin levels by approx. two-thirds. The effect of iron and retinoic acid added together did not differ greatly from that of each agent alone. None of the treatments greatly affected levels of L-ferritin mRNA. Virtually no H-ferritin mRNA was detected in U937 cells. These results show that changes in ferritin and transferrin receptor caused by treatment with retinoic acid are similar to those induced by excess iron, and suggest that changes in these proteins during cell differentiation are due to redistribution of intracellular iron into the regulatory pool(s), rather than to iron-independent mechanisms.

Molecular cloning and expression of ferritin mRNA in heavy metal-poisoned Xenopus laevis cells

In a search for genes transcriptionally regulated by metal ions, we have isolated a Xenopus laevis ferritin cDNA clone, XL2-17, from cadmium-poisoned XL2 cells. The large size of the corresponding ferritin mRNA (1.4 kb) is due to the presence of a 629-nucleotide 5'-untranslated region. The Xenopus ferritin sequence is highly isologous with other vertebrate ferritins. In particular, there is a complete sequence identity for the iron-responsive element (IRE) located in the 5'-untranslated region in both XL2-17 and Rana catesbeiana ferritin mRNAs. The position of this IRE is unusual since it is located 489 nucleotides from the 5' end of the ferritin mRNA. Our analysis of phylogenetic relationships among ferritins indicates that all amphibian ferritins thus far sequenced would be more closely related to the mammalian H-type ferritin than to the L-type. The level of ferritin mRNA in XL2 cells rises 10- to 15-fold following exposure of cells to cadmium or copper. This increase is due to both transcriptional and translational regulation. A 10-fold increase was also found at the protein level. These results suggest that ferritin may be a primary detoxification response to heavy metals in Xenopus cells.

Murine ferritin heavy chain: isolation and characterization of a functional gene

A mouse liver genomic library screened with a full-length cDNA encoding murine ferritin heavy chain (mFHC) [Torti et al., J. Biol. Chem. 263 (1988) 12638-12644] yielded a functional genomic clone mFHC. The genomic clone isolated included a region of approximately 3 kb containing four exons and three introns. Sequence comparisons of the mouse genomic clone with other genomic clones from rat, human and chicken showed a high degree of similarity among species in the coding regions. Introns and flanking sequences were less conserved. However, comparison of mFHC promoter elements with FHC genes from other species revealed common elements. Analysis of the genomic structure of FHC suggested the presence of pseudogenes. S1 nuclease analysis, however, confirmed that this mouse clone, when transfected into human MRC-5 fibroblasts, was transcribed, indicating that this clone contains an FHC functional gene.

Transcriptional regulation of ferritin heavy chain messenger RNA expression by thyroid hormone

The effect of 3,5,3'-triiodo-L-thyronine (T3) on the steady state levels of ferritin heavy chain (ferritin H) mRNA in cultured rat glioma C6 cells and various rat tissues was examined. Addition of T3 to cultured C6 cells showed the time and dose-dependent increase in the steady-state level of ferritin H mRNA. In vitro nuclear run-on assay revealed that the stimulatory effect was due to the increase in the transcription rate of ferritin H gene. T3 had no effect on the half life of ferritin H mRNA. In hyperthyroid rats, the level of ferritin H mRNA in the kidney was elevated. On the contrary, that was decreased in hypothyroid rats. The results suggest the involvement of T3 in the regulation of ferritin H gene expression.